2021, 34(1): 1-14.
doi: 10.1063/1674-0068/cjcp2102027
In this review, we present a brief overview on the recent advances in Ångström-resolved tip-enhanced Raman spectromicroscopy. We first introduce the theoretical understanding of the confinement of light at the atomistic scale, and explain how the Raman scattering from a single molecule happens under the "illumination" of such an atomically confined light. Then we describe the latest developments on Ångström-resolved tip-enhanced Raman spectromicroscopy, particularly on a new methodology called "scanning Raman picoscopy" for visually constructing the chemical structure of a single molecule in real space. Finally, we give a perspective of this technique in various applications where identifying the chemical structures of materials at the chemical bond level is required.![]()
2021, 34(3): 263-272.
doi: 10.1063/1674-0068/cjcp2009171
Ruthenium (Ru) serves as a promising catalyst for ammonia synthesis via the Haber-Bosch process, identification of the structure sensitivity to improve the activity of Ru is important but not fully explored yet. We present here density functional theory calculations combined with micro-kinetic simulations on nitrogen molecule activation, a crucial step in ammonia synthesis, over a variety of hexagonal close-packed (hcp) and face-center cubic (fcc) Ru facets. Hcp \begin{document}$\left\{ {21\overline 3 0} \right\}$\end{document} \begin{document}$_2$\end{document} \begin{document}$_2$\end{document} \begin{document}$\{211\}$\end{document} \begin{document}$_2$\end{document} \begin{document}$\{311\}$\end{document} \begin{document}$\left\{ {21\overline 3 0} \right\}$\end{document} \begin{document}$\{211\}$\end{document} \begin{document}$_2$\end{document} \begin{document}$_2$\end{document}
2021, 34(4): 373-380.
doi: 10.1063/1674-0068/cjcp2103045
Fast and accurate quantitative detection of 14CO2 has important applications in many fields. The optical detection method based on the sensitive cavity ring-down spectroscopy technology has great potential. But currently it has difficulties of insufficient sensitivity and susceptibility to absorption of other isotopes/impurity molecules. We propose a stepped double-resonance spectroscopy method to excite 14CO2 molecules to an intermediate vibrationally excited state, and use cavity ring-down spectroscopy to probe them. The two-photon process significantly improves the selectivity of detection. We derive the quantitative measurement capability of double-resonance absorption spectroscopy. The simulation results show that the double-resonance spectroscopy measurement is Doppler-free, thereby reducing the effect of other molecular absorption. It is expected that this method can achieve high-selectivity detection of 14CO2 at the sub-ppt level.
2021, 34(1): 71-80.
doi: 10.1063/1674-0068/cjcp2012219
In this study, we report the design and simulation of an electrostatic ion lens system consisting of 22 round metal plates. The opening of the extractor plate is covered with metal mesh, which is for shielding the interaction region of the lens system from the high DC voltages applied to all other plates than the repeller and extractor plates. The Simion simulation shows that both velocity-mapping and time focusing can be achieved simultaneously when appropriate voltages are applied to each of the plates. This makes the ion lens system be able to focus large ionic volumes in all three dimensions, which is an essential requirement for crossed ion-molecule scattering studies. A three-dimensional ion velocity measurement system with multi-hit and potential multi-mass capability is built, which consists of a microchannel plate (MCP), a P47 phosphor screen, a CMOS camera, a fast photomultiplier tube (PMT), and a high-speed digitizer. The two velocity components perpendicular to the flight axis are measured by the CMOS camera, and the time-of-flight, from which the velocity component along the flight axis can be deduced, is measured by the PMT. A Labview program is written to combine the two measurements for building the full three-dimensional ion velocity in real time on a frame-by-frame basis. The multi-hit capability comes from the fact that multiple ions from the camera and PMT in the same frame can be correlated with each other based on their various intensities. We demonstrate this by using the photodissociation of CH3I at 304 nm.![]()
In the pioneering work by R. A. Marcus, the solvation effect on electron transfer (ET) processes was investigated, giving rise to the celebrated nonadiabatic ET rate formula. In this work, on the basis of the thermodynamic solvation potentials analysis, we reexamine Marcus' formula with respect to the Rice-Ramsperger-Kassel-Marcus (RRKM) theory. Interestingly, the obtained RRKM analogue, which recovers the original Marcus' rate that is in a linear solvation scenario, is also applicable to the nonlinear solvation scenarios, where the multiple curve-crossing of solvation potentials exists. Parallelly, we revisit the corresponding Fermi's golden rule results, with some critical comments against the RRKM analogue proposed in this work. For illustration, we consider the quadratic solvation scenarios, on the basis of physically well-supported descriptors.
2021, 34(4): 406-412.
doi: 10.1063/1674-0068/cjcp2103037
The pharmaceutically active compound atenolol, a kind of $\beta$-blockers, may result in adverse effects both for human health and ecosystems if it is excreted to the surface water resources. To effectively remove atenolol in the environment, both direct and indirect photodegradation, driven by sunlight play an important role. Among indirect photodegradation, singlet oxygen (1O2), as a pivotal reactive species, is likely to determine the fates of atenolol. Nevertheless, the kinetic information on the reaction of atenolol with singlet oxygen has not been well investigated and the reaction rate constant is still ambiguous. Herein, the reaction rate constant of atenolol with singlet oxygen is investigated directly through observing the decay of the 1O2 phosphorescence at 1270 nm. It is determined that the reaction rate constant between atenolol and 1O2 is 7.0×105 (mol/L)$^{-1}\cdot$s-1 in D2O, 8.0×106 (mol/L)$^{-1}\cdot$s-1 in acetonitrile, and 8.4×105 (mol/L)$^{-1}\cdot$s-1 in EtOH, respectively. Furthermore, the solvent effects on the title reaction were also investigated. It is revealed that the solvents with strong polarity and weak hydrogen donating ability are suitable to achieve high rate constant values. These kinetics information on the reaction of atenolol with singlet oxygen may provide fundamental knowledge to the indirect photodegradation of $\beta$-blockers.
Understanding the influence of nanoparticles on the formation of protein amyloid fibrillation is crucial to extend their application in related biological diagnosis and nanomedicines. In this work, Raman spectroscopy was used to probe the amyloid fibrillation of hen egg-white lysozyme in the presence of silver nanoparticles (AgNPs) at different concentrations, combined with atomic force microscopy and thioflavin T (ThT) fluorescence assays. Four representative Raman indicators were utilized to monitor transformation of the protein tertiary and secondary structures at the molecular level: the Trp doublet bands at 1340 and 1360 cm-1, the disulfide stretching vibrational peak at 507 cm-1, the N-C$\alpha$-C stretching vibration at 933 cm-1, and the amide Ⅰ band. All experimental results confirmed the concentration-dependent influence of AgNPs on the hen egg-white lysozyme amyloid fibrillation kinetics. In the presence of AgNPs at low concentration (17 μg/mL), electrostatic interaction of the nanoparticles stabilizes disulfide bonds, and protects the Trp residues from exposure to hydrophilic environment, thus leading to formation of amorphous aggregates rather than fibrils. However, with the action of AgNPs at high concentration (1700 μg/mL), the native disulfide bonds of hen egg-white lysozyme are broken to form Ag-S bonds owing to the competition of electrostatic interaction from a great deal of nanoparticles. As for providing functional surfaces for protein to interact with, AgNPs play a bridge role in direct transformation from $\alpha$-helices to organized $\beta$-sheets. The present investigation sheds light on the controversial effects of AgNPs on the kinetics of hen egg-white lysozyme amyloid fibrillation.
KSSOLV (Kohn-Sham Solver) is a MATLAB (Matrix Laboratory) toolbox for solving the Kohn-Sham density functional theory (KS-DFT) with the plane-wave basis set. In the KS-DFT calculations, the most expensive part is commonly the diagonalization of Kohn-Sham Hamiltonian in the self-consistent field (SCF) scheme. To enable a personal computer to perform medium-sized KS-DFT calculations that contain hundreds of atoms, we present a hybrid CPU-GPU implementation to accelerate the iterative diagonalization algorithms implemented in KSSOLV by using the MATLAB built-in Parallel Computing Toolbox. We compare the performance of KSSOLV-GPU on three types of GPU, including RTX3090, V100, and A100, with conventional CPU implementation of KSSOLV respectively and numerical results demonstrate that hybrid CPU-GPU implementation can achieve a speedup of about 10 times compared with sequential CPU calculations for bulk silicon systems containing up to 128 atoms.
We propose a method for calculating the nonradiative decay rates for polyatomic molecules including anharmonic effects of the potential energy surface (PES) in the Franck-Condon region. The method combines the n-mode representation method to construct the ab initio PES and the nearly exact time-dependent density matrix renormalization group method (TD-DMRG) to simulate quantum dynamics. In addition, in the framework of TD-DMRG, we further develop an algorithm to calculate the final-state-resolved rate coefficient which is very useful to analyze the contribution from each vibrational mode to the transition process. We use this method to study the internal conversion (IC) process of azulene after taking into account the anharmonicity of the ground state PES. The results show that even for this semi-rigid molecule, the intramode anharmonicity enhances the IC rate significantly, and after considering the two-mode coupling effect, the rate increases even further. The reason is that the anharmonicity enables the C-H vibrations to receive electronic energy while C-H vibrations do not contribute on the harmonic PES as the Huang-Rhys factor is close to 0.
LASP (large-scale atomistic simulation with neural network potential) software developed by our group since 2018 is a powerful platform (www.lasphub.com) for performing atomic simulation of complex materials. The software integrates the neural network (NN) potential technique with the global potential energy surface exploration method, and thus can be utilized widely for structure prediction and reaction mechanism exploration. Here we introduce our recent update on the LASP program version 3.0, focusing on the new functionalities including the advanced neural network training based on the multi-network framework, the newly-introduced \begin{document}$ S^7 $\end{document} \begin{document}$ S^8 $\end{document} \begin{document}$ - $\end{document} \begin{document}$ - $\end{document} \begin{document}$ - $\end{document} \begin{document}$ S^7 $\end{document} \begin{document}$ S^8 $\end{document}
2021, 34(5): 621-627.
doi: 10.1063/1674-0068/cjcp2012220
After binding to human serum albumin, bilirubin could undergo photo-isomerization and photo-induced cyclization process. The latter process would result the formation of a product, named as lumirubin. These photo induced behaviors are the fundamental of clinical therapy for neonatal jaundice. Previous studies have reported that the addition of long chain fatty acids is beneficial to the generation of lumirubin, yet no kinetic study has revealed the mechanism behind. In this study, how palmitic acid affects the photochemical reaction process of bilirubin in Human serum albumin (HSA) is studied by using femtosecond transient absorption and fluorescence up-conversion techniques. With the addition of palmitic acid, the excited population of bilirubin prefers to return to its hot ground state (S0) through a 4 ps decay channel rather than the intrinsic ultrafast decay pathways (< 1 ps). This effect prompts the Z-Z to E-Z isomerization at the S$_0$ state and then further increases the production yield of lumirubin. This is the first time to characterize the promoting effect of long chain fatty acid in the process of phototherapy with femtosecond time resolution spectroscopy and the results can provide useful information to benefit the relevant clinical study.
2021, 34(5): 605-612.
doi: 10.1063/1674-0068/cjcp2108135
MXenes, a new family of two-dimensional (2D) materials, have received extensive interest due to their fascinating physicochemical properties, such as outstanding light-to-heat conversion efficiency. However, the photothermal conversion mechanism of MXenes is still poorly understood. Here, by using femtosecond visible and mid-infrared transient absorption spectroscopy, the electronic energy dissipation dynamics of MXene (Ti3C2Tx) nanosheets dispersed in various solvents are carefully studied. Our results indicate that the lifetime of photoexcited MXene is strongly dependent on the surrounding environment. Especially, the interfacial electron-vibration coupling between the MXene nanosheets and the adjacent solvent molecules is directly observed following the ultrafast photoexcitation of MXene. It suggests that the interfacial interactions at the MXene-solvent interface play a critical role in the ultrafast energy transport dynamics of MXene, which offers a potentially feasible route for tailoring the light conversion properties of 2D systems.
2020, 33(1): 1-7.
doi: 10.1063/1674-0068/cjcp1910173
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2020, 33(1): 13-22.
doi: 10.1063/1674-0068/cjcp1910191
Experimental vibrational spectra of heavy light XH stretching vibrations of simple molecules have been analyzed using the local mode model. In addition, the bond dipole approach, which assumes that the transition dipole moment (TDM) of the XH stretching mode is aligned along the XH bond, has helped analyze experimental spectra. We performed theoretical calculations of the XH stretching vibrations of HOD, HND\begin{document}$^-$\end{document} \begin{document}$^-$\end{document} \begin{document}$^-$\end{document} \begin{document}$^-$\end{document}
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The development of acidic-available noble-metal-free oxygen reduction reaction (ORR) catalysts with high activity and good long-term durability is of significant importance for efficient proton exchange membrane fuel cells (PEMFC), but is still very challenging. Herein, we develop originally a facile wet-chemical-adsorption, pyrolysis and post-etching strategy to effectively intercalate Fe clusters among two nitrogen-doped carbon (NC) layers, forming unique hollow spherical nanostructures with sandwiched NC/Fe/NC shells as an active ORR catalyst. Thanks to the sandwiched nanostructure and the active Fe-N species, this as-prepared hollow sandwiched NC/Fe/NC catalyst could present superior ORR activity in an acidic medium, with a nice onset potential of 0.92 V and decent diffusion-limited current density of ~5.1 mA/cm2. The NC/Fe/NC catalyst manifests strong methanol tolerance and outstanding durability during a long-term acidic ORR operation, being a promising alternative to Pt-based catalysts toward efficient proton exchange membrane fuel cells. Synchrotron radiation characterizations and X-ray photoemission spectroscopy reveal at the atomic-level that the abundant robust Fe−N bonds presented in the sandwiched shells of hollow NC/Fe/NC spherical catalyst contribute substantially to high electrochemical activity and superior corrosion-resistance for efficient 4e− ORR in acidic electrolyte.
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doi: 10.1063/1674-0068/cjcp2203055
With the bloom of deep learning algorithms, various models have been widely utilized in quantum chemistry calculation to design new molecules and explore molecular properties. However, limited studies focus on multi-task molecular property prediction, which offers more efficient ways to simultaneously learn different but related properties by leveraging the inter-task relationship. In this work, we apply the hard parameter sharing framework and advanced loss weighting methods to multi-task molecular property prediction. Based on the performance comparison between single-task baseline and multi-task models on several task sets, we find that the prediction accuracy largely depends on the inter-task relationship, and hard parameter sharing improves the performance when the correlation becomes complex. In addition, we show that proper loss weighting methods help achieve more balanced multi-task optimization and enhance the prediction accuracy. Our additional experiments on varying amount of training data further validate the multi-task advantages and show that multi-task models with proper loss weighting methods can achieve more accurate prediction of molecular properties with much less computational cost.
, Available online ,
doi: 10.1063/1674-0068/cjcp2204058
The superalkali cations and superhalogen anions commonly have different type of core moieties. Based on the previous reports that Be2H3L′ 2+ (L′=NH3 and noble gases Ne−Xe) are superalkali cations, we have demonstrated in the present work through designing the superhalogen anions Be2H3L2− (L=CH3 and halogens F−I) that the both superalkali cations and superhalogen anions can be constructed using Be2H3 as the core moiety. The newly designed Be2H3L2− species are much more stable than their isoelectronic cationic counterparts Be2H3L′ 2+, as can be reflected by the highly exergonic substitution reaction of L′ ligand in Be2H3L′ 2+ with isoelectronic L− to give Be2H3L2−. These anionic species possess the well-defined electronic structure, which can be proven by their large HOMO−LUMO gaps of 4.69 eV to 5.38 eV. It is remarkable that Be2H3L2− can be regarded as the hyperhalogen anions due to the extremely high vertical detachment energies (5.38 eV to 6.06 eV) and the Be−Be distances in these species (1.776 Å to 1.826 Å) are rare in ultrashort metal-metal distances (defined as dM−M<1.900 Å) between main group metals. In five small model species designed, three of them, i.e. Be2H3L2− (L=CH3, Cl, and Br), are kinetical viable global energy minima, which are the promising target for generation and characterization in anion photoelectron spectroscopy. The analogue molecule [t-Bu−Be2H3−t-Bu]− with bulky protecting tert-butyl (t-Bu) groups is designed as a possible target for synthesis and isolation in condensed states.
, Available online ,
doi: 10.1063/1674-0068/cjcp2205084
Low-dimensional materials have excellent properties which are closely related to their dimensionality. However, the growth mechanism underlying tunable dimensionality from 2D triangles to 1D ribbons of such materials is still unrevealed. Here, we establish a general kinetic Monte Carlo model for transition metal dichalcogenides (TMDs) growth to address such an issue. Our model is able to reproduce several key findings in experiments, and reveals that the dimensionality is determined by the lattice mismatch and the interaction strength between TMDs and the substrate. We predict that the dimensionality can be well tuned by the interaction strength and the geometry of the substrate. Our work deepens the understanding of tunable dimensionality of low-dimensional materials and may inspire new concepts for the design of such materials with expected dimensionality.
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Peltier effect is an important thermoelectric phenomenon which stands for the generation of temperature gradient of the interface between two dissimilar conductors by the electric current flowing through them. In this paper, we investigate the Peltier effect in serially coupled noninteracting double quantum dot system under a bias voltage. By means of an accurate hierarchical equations of motion approach, we first demonstrate that the local temperatures of the two dots differ from each other by applying an electric current through the dots. We then analyze theoretically the influence of interdot and dot-lead coupling on the thermopower and electric current. Finally, we elucidate the variation of Peltier heat and Joule heat with the interdot coupling and dot-lead coupling, which leads to the changes in the local temperature of the quantum dots.
We investigate the reaction probability, integral cross section and energy efficiency of the OH<sup>─</sup> + CH<sub>3</sub>I reaction using time-dependent, quantum dynamics wave packet method. A four-degree-of-freedom dynamics model is developed for this study due to the synchronized bond-breaking and formation S<sub>N</sub>2 mechanism. We find that the reaction probability decreases as a function of the collision energy, which is a typical character of the reaction with a negative energy barrier. The ground-state integral cross section calculated using this model is in excellent agreement with the quasi-classical trajectory results. The integral cross-section ratios of the vibrational excitations over the ground state, at the same equal amount of total energy, indicate that the vibrational motion of the CH<sub>3</sub>-I is more efficient in enhancing the reactivity than the translational motion, which, in turn, has a bigger contribution to the reactivity than the C-H<sub>3</sub> vibrational motion. The above order of the energy efficiency role on the reactivity is confirmed by the Sudden Vector Model prediction.
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In this work, we investigated the methanol steam reforming (MSR) reaction (CH3OH + H2O → CO2 + 3H2) catalyzed by α-MoC by means of density functional theory calculations. The adsorption behavior of the relevant intermediates and the kinetics of the elementary steps in the MSR reaction are systematically investigated. The results show that, on the α-MoC(100) surface, the O-H bond cleavage of CH3OH leads to CH3O, which subsequently dehydrogenates to CH2O. Then, the formation of CH2OOH between CH2O and OH is favored over the decomposition to CHO and H. The sequential dehydrogenation of CH2OOH results in a high selectivity for CO2. In contrast, the over-strong adsorption of the CH2O intermediate on the α-MoC(111) surface leads to its dehydrogenation to CO product. In addition, we found that OH species, which is produced from the facile water activation, help the O-H bond breaking of intermediates by lowering the reaction energy barrier. This work not only reveals the catalytic role played by α-MoC(100) in the MSR reaction, but also provides theoretical guidance for the design of α-MoC-based catalysts.
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Symmetric covalent organic framework (COF) photocatalysts generally suffer from inefficient charge separation and short-lived photoexcited states. By performing density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations, we find that partial substitution with one or two substituents (N or NH2) in the linkage of the representative symmetric COF (N0-COF) gives rise to the separation of charge carriers in the resulting COFs (i.e., N1-COF, N2-COF, (NH2)1-N0-COF, and (NH2)2-N0-COF). Moreover, we also find that the energy levels of the highest occupied crystal orbital (HOCO) and the lowest unoccupied crystal orbital (LUCO) of the N0-COF can shift away from or toward the vacuum level, depending on the electron-withdrawing or electron-donating character of the substituent. Therefore, we propose that partial substitution with carefully chosen electron-withdrawing or electron-donating substituents in the linkages of symmetric COFs can lead to efficient charge separation as well as appropriate HOCO and LUCO positions of the generated COFs for specific photocatalytic reactions. The proposed rule can be utilized to further boost the photocatalytic performance of many symmetric COFs.
Determination of molecular structures of two dimensional (2D) organic-inorganic hybrid perovskite (OIHPs) nanocrystals at the single-nanocrystal and ensemble levels is essential to understanding the mechanisms responsible for their size-dependent optoelectronic properties and the nanocrystal assembling process, but its detection is still a bit challenge. In this study, we demonstrate for the first time that femtosecond sum frequency generation vibrational spectroscopy can provide a highly sensitive tool for probing the molecular structures of nanocrystals with a size comparable to the Bohr diameter (~10 nm) at the single-nanocrystal level. The SFG signals are monitored using the spectral features of the phenyl group in (R-MBA)2PbBr4 and (R-MBA)2PbI4 nanocrystals (MBA: methyl-benzyl-ammonium). It is found that the SFG spectra exhibit a strong resonant peak at 3067(±3) cm-1 (ν2 mode) and a weak shoulder peak at 3045 (±4) cm-1 (ν7a mode) at the ensemble level, whereas a peak of the ν2 mode and a peak at 3025 (±3) cm-1 (ν20b mode) at the single-nanocrystal level. The nanocrystals at the single-nanocrystal level tend to lie down on the surface. They stand up as the ensemble number and the averaged sizes increase. This finding may provide valuable information on the structural origins for size-dependent photo-physical properties and photoluminescence blinking dynamics in nanocrystals.
Cr2O3 has been recognized as key oxide component in bifunctional catalysts to produce bridging intermediate, e.g., methanol, from syngas. By combining density functional theory calculations and microkinetic modeling, we computationally studied the surface structures and catalytic activities of bare Cr2O3 (001) and (012) surfaces, and two reduced (012) surfaces covered with dissociative hydrogens or oxygen vacancies. The reduction of (001) surface is much more difficult than that of (012) surface. The stepwise or the concerted reaction pathways were explored for the syngas to methanol conversion, and the hydrogenation of CO or CHO is identified as rate-determining step. Microkinetic modeling reveals that (001) surface is inactive for the reaction, and the rates of both reduced (012) surfaces (25-28 s-1) are about five times higher than bare (012) surface (4.3 s-1) at 673 K. These theoretical results highlight the importance of surface reducibility on the reaction and may provide some implications on the design of individual component in bifunctional catalysis.
Hydroxyaromatic compounds have a wide range of applications in catalytic synthesis and biological processes due to their enhanced acidity upon photo-excitation. Most hydroxyaromatic compounds with a medium excited state acidity are unable to deprotonate in non-aqueous solvents such as alcohol due to their short-lived excited singlet states. The nitro group in 4-hydroxy-4’-nitrobiphenyl (NO2-Bp-OH) increases the spin orbital coupling between excited singlet states and the triplet manifold, resulting in ultrafast intersystem crossing and the formation of the long-lived lowest excited triplet state (T1) with a high yield. Using transient absorption spectroscopy and kinetic analysis, we discovered that, despite its moderate acidity, the T1 state of NO2-Bp-OH (3NO2-Bp-OH) is able to transfer proton to methanol. Following the formation of the hydrogen-boned complex between 3NO2-Bp-OH and three methanol molecules in a consecutive process, proton transfer occurs very fast. This finding suggests that the long lifetime of the photoacid excited state allows for the formation of alcohol oligomers with sufficient basicity to induce photoacid deprotonation.
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The rotationally resolved spectrum of the A2A'' – X2A'' 000 band of jet-cooled 1-indanyl near 473 nm is recorded by laser induced fluorescence with a spectral resolution of ~0.014 cm-1. Accurate spectroscopic constants for both A2A'' and X2A'' states of 1-indanyl are determined from rotational analysis of the experimental spectrum. These indicative spectroscopic parameters are applied to test the calculated structure of 1-indanyl. The calculations show good agreement with the experimental data. Based on the computational molecular orbitals and spin densities for 1-indanyl, the delocalization of unpaired pπ electron that stabilizes the 1-indanyl radical has been discussed.
Bimetallic nanoparticle (NP) catalysts have attracted long-standing attentions for their wide applications in a broad range of chemical reactions. Their catalytic performance tightly relies on the structure of bimetallic NPs. Atomic-level understanding of their structural thermostability is of great importance for developing advanced bimetallic catalysts with high stability. Here we precisely fabricated Au@Pt and Au@Pd core-shell catalysts on a SiO<sub>2</sub> support with an identical Au core size of ~5.1 nm and a similar shell thickness of ~2 monolayers via selective atomic layer deposition. Spectroscopic characterization were employed to compare their structural thermostability at elevated temperatures in a hydrogen reducing atmosphere. We revealed that the Au@Pt/SiO<sub>2</sub> core-shell catalyst exhibited a considerably higher structural thermostability against atom inter-diffusion to alloys than that of Au@Pd/SiO<sub>2</sub>. Meanwhile, these two catalysts both preserved the particle size without any visible aggregation even after reduction at 550 <sup>o</sup>C. Higher structural thermostability of Au@Pt/SiO<sub>2</sub> core-shell catalyst might mainly stem from the distinctly higher melting point of Pt shell and their relatively smaller Au-Pt lattice mismatch. Such direct comparison of the structural thermostability of two different core-shell catalysts but with identical structures provides a valuable insight into the nature of thermodynamic behavior of bimetallic NPs at elevated temperatures.
In a previous work [{\it J. Chem. Phys.} {\bf 140}, 174105 (2014)], we have shown that a mixed quantum classical (MQC) rate theory can be derived to investigate the quantum tunneling effects in the proton transfer reactions. However, the method is based on the high temperature approximation of the hierarchical equation of motion (HEOM) with the Debye-Drude spectral density, and results in a multi-state Zusman type of equation. We now extend this theory to include quantum effects of the bath degrees of freedom. By writing the full HEOM into a multidimensional partial differential equation in phase space, we can define a new reaction coordinate, and the previous method can be generalized to the full quantum regime. The validity of the new method is demonstrated by using numerical examples including the spin-Boson model, and the double well model for proton transfer reaction. The new method is found to resolve some key problems of the previous theory based on high temperature approximation, including possible numerical instability in long time simulation and wrong rate constant at low temperatures.
The design of low-cost and robust electrocatalysts with rich active sites remains challenging for improving the efficiency of water oxidation. Herein, ternary Ni-Co-Mo oxide films were uniformly synthesized on Cu foil via simple electrochemical deposition method. After surface reconstruction, the robust amorphous-crystalline (a-c) Ni(Co) heterostructures with rich oxygen vacancies were achieved. Accordingly, the as-obtained surface-reconstructed heterostructure catalysts exhibited a superior OER activity with overpotential at 20 mA/cm2 as low as 308 mV and a small Tafel slope of 90 mV/dec. Moreover, a negligible activity degradation was observed for the heterostructure catalyst continuously catalyzing OER process over 24 h, highlighting the structural robustness of the self-reconstructed Ni-Co-Mo catalyst for practically electrocatalytic applications.
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The commonly used oxide-supported metal catalysts are usually prepared in aqueous phase, which then often need to undergo calcination before usage. Therefore, the surface hydration and dehydration of oxide supports are critical for the realistic modeling of supported metal catalysts. In this work, by ab initio molecular dynamics (AIMD) simulations, the initial anhydrous monoclinic ZrO2 (-111) surfaces (or formally written as ("1" ̅"11" )) are evaluated within explicit solvents in aqueous phase at mild temperatures. During the simulations, all the two-fold-coordinated O sites will soon be protonated to form the acidic hydroxyls (OLH), remaining the basic hydroxyls (HO*) on Zr. The basic hydroxyls (HO*) can easily diffuse on surfaces via the active proton exchange with the undissociated adsorption waters (H2O*). Within the temperatures ranging from 273K to 373K, in aqueous phase a certain representative equilibrium hydrated m-ZrO2 (-111) surface is obtained with the coverage (θ) of 0.75 on surface Zr atoms. Later, the free energies on the stepwise surface water desorption are calculated by density functional theory (DFT) to mimic the surface dehydration under the mild calcination temperatures lower than 800K. By obtaining the phase diagrams of surface dehydration, the representative partially hydrated m-ZrO2 (-111) surfaces (0.25≤θ<0.75) at various calcination temperatures are illustrated. These hydrated m-ZrO2 (-111) surfaces can be crucial and readily applied for the more realistic modeling of ZrO2 catalysts and ZrO2-supported metal catalysts.
Three-coordinate Au(I) complexes with thermally activated delayed fluorescence (TADF) have recently gained experimental attention. However, its luminescence mechanism is elusive. Herein, we have employed the density functional theory (DFT), time-dependent DFT (TD-DFT), and QM/MM methods to investigate the excited-state and emission properties of this Au(I) complex in both gas and crystal phases. In both environments, the S1 and T1 emitting states mainly involve HOMO and LUMO and show clear MLCT and ILCT characters. The good spatial separation of HOMO and LUMO minimizes the S1-T1 energy gap, which benefits the reverse intersystem crossing (rISC) from T1 to S1. At 300 K, the rISC rate is faster than the T1 phosphorescence emission, which enables the TADF emission. However, at 77 K, such rISC process is blocked and TADF disappears; instead, only phosphorescence is recorded experimentally. Importantly, this work highlights the importance of environments in regulating luminescence properties and contributes to understanding the TADF emission of organometallic complexes.
As a direct wide bandgap semiconductor, CsPbCl3 has great potential applications in the field of near-ultraviolet photodetectors, lasers and higher-order multiphoton fluorescent detectors. In this paper, we systematically explored the technology to synthesize CsPbCl3 micro/nanocrystals by vapor deposition method with CsCl and PbCl2 powders as the source materials. It was confirmed that the formation of CsPbCl3 perovskite through the chemical reaction of CsCl with PbCl2 occurred in the quartz boat before the source evaporation, not in vapor or on substrate surface. The evaporated CsPbCl3 can form micro/nanocrystals on substrate surfaces in appropriate conditions. Various morphologies including irregular polyhedrons, rods and pyramids could be observed at lower temperature, while stable and uniform CsPbCl3 single crystal microplatelets were controllably synthesized at 450 ℃. Prolonging the growth time could modulate the size and density of the microcrystals, but could not change the morphology. Substrate types made little difference to the morphology of CsPbCl3 crystals. The photoluminescence spectra indicated that the crystallinity and morphology of CsPbCl3 micro/nanocrystals have significant effects on their optical properties. The above results are expected to be helpful to the development of optoelectronic devices based on individual CsPbCl3 microcrystal.
The one-step conversion of ethanol to 1,3-butadiene has achieved a breakthrough with the development of beta zeolite supported dual metal catalysts. However, the reaction mechanism from ethanol to butadiene is complex and has not yet been fully elucidated, and no catalyst screening efforts has been done based on central metal atoms. In this work, density functional theory (DFT) calculations were employed to study the mechanism of one-step conversion of ethanol to butadiene over Zn-Y/BEA catalyst. The results show that ethanol dehydrogenation prefers to proceed on Zn site with a reaction energy in the rate-determining step of 0.77 eV, and the aldol con-densation to produce butadiene prefers to proceed on Y site with a reaction energy in the rate-determining step of 0.69 eV. Based on the mechanism revealed, six ele-ments were selected to replace Y for screening superior combination of Zn-M/BEA (M = Sn, Nb, Ta, Hf, Zr, Ti) for this reaction. As a result, Zn-Y/BEA (0.69 eV) is proved to be the most preferring catalyst compared with the other six ones, and Zn-Zr/BEA (0.85 eV), Zn-Ti/BEA (0.87eV) and Zn-Sn/BEA (0.93eV) can be poten-tial candidates for the conversion of ethanol to butadiene. This work not only pro-vides mechanistic insights into one-step catalytic conversion of ethanol to butadiene over Zn-Y/BEA catalyst but also offers more promising catalyst candidates for this reaction.
Catechol adsorbed on TiO2 is one of the simplest models to explore the relevant properties of dye-sensitized solar cells. However, the effects of water and defects on the electronic levels and the excitonic properties of the catechol/TiO2 interface have been rarely explored. Here, we investigate four catechol/TiO2 interfaces aiming to determine the influence of coverage, water, and defects on the electronic levels and the excitonic properties of the catechol/TiO2 interface through the first-principles many-body Green’s function theory. We find that the adsorption of catechol on the rutile (110) surface increases the energies of both the TiO2 valence band maximum (VBM) and conduction band minimum (CBM) by approximately 0.7 eV. The increasing coverage and the presence of water can reduce the optical absorption of charge-transfer excitons with maximum oscillator strength. Regarding the reduced hydroxylated TiO2 substrate, the CBM decreases greatly, resulting in a sub-bandgap of 2.51 eV. The exciton distributions in the four investigated interfaces can spread across several unit cells, especially for the hydroxylated TiO2 substrate. Although the hydroxylated TiO2 substrate leads to a lower open-circuit voltage, it may increase the separation between photogenerated electrons and holes and may therefore be beneficial for improving the photovoltaic efficiency by controlling its concentration. Our results may provide guidance to the design of highly efficient solar cells in future.
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The catalytic performance of metal oxide surface mainly depends on its atomic surface structure, which usually changes under various treatment conditions and during catalytic reactions. Therefore, it is quite important to acquire the atomic geometries of the surfaces under different treatments for further understanding the catalytic mechanisms in the surfaces with complicated reconstructions. Here, we report the investigation on the evolution of surface geometries of the Ar+-ion-sputtered anatase TiO<sub>2</sub>(001) films followed by heating treatments at various temperatures, characterized using variable-temperature scanning tunneling microscopy (VT-STM). Our experimental results revealed the different surface morphologies at different heating temperatures. During the heating treatment, the migrations of O atoms from the bulk to the surface of TiO<sub>2</sub>(001) play an important role for the reoxidation of the Ti<sup>2+</sup> and Ti<sup>3+</sup> states for the formation of (1×4) reconstruction. The atomic-resolution images of the ridges showed asymmetric features, which well support the fully oxidized structural model of the reconstructed TiO<sub>2</sub>(001)-(1×4) surface.
Density functional theory (DFT) has been established as a powerful research tool for heterogeneous catalysis research in obtaining key thermodynamic and/or kinetic parameters like adsorption energies, enthalpies of reaction, activation barriers and rate constants. Understanding of density functional exchange-correlation approximations is essential to reveal the mechanism and performance of a catalyst. In the present work, we reported the influence of six exchange-correlation density functionals, including PBE, RPBE, BEEF+vdW, optB86b+vdW, SCAN and SCAN+rVV10, on the adsorption energies, reaction energies and activation barriers of carbon hydrogenation and carbon-carbon couplings during the formation of methane and ethane over Ru(0001) and Ru(10"1" ̅1) surfaces. We found the calculated reaction energies are strongly dependent on exchange-correlation density functionals due to the difference in coordination number between reactants and products to surfaces. The deviation of the calculated reaction energies can be accumulated to a large value for chemical reaction involving multiple steps and vary considerably with different exchange-correlation density functionals calculations. The different exchange-correlation density functionals is found to influence considerably the selectivity of Ru(0001) surface for methane, ethylene and ethane formation determined by the adsorption energies of intermediates involved. However, the influence on the barriers of the elementary surface reactions and the structural sensitivity of Ru(0001) and Ru(10"1" ̅1) are modest. Our work highlights the limitation of exchange-correlation density functionals on computational catalysis and the importance of choosing a proper exchange-correlation density functional in correctly evaluating the activity and selectivity of a catalyst.
Understanding the interaction mechanism between divalent metal ions with amino acids is of great significance to underatanding the interaction between metal ions with proteins. In this study, the interaction mechanism of Mg2+, Ca2+,and Zn2+ with amino acid side chain analogs in water were systematically studied by combining neural network potential energy surface, molecular dynamics simulation and umbrella sampling. The calculated potential mean forces not only reveal the binding process of each ion and amino acid, the most stable coordination structure, but also show the difference between different ions. In addition, we also use the neural network based potential of mean force as a standard to benchmark classical force fields, which is also meaningful for the development of force fields targeting metal ions.
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1,3,5-triazines represent a class of molecules that may have been prebiotic information carriers in a primordial soup in early Earth and their excited state dynamics has received attention in recent years. In our previous study, one component with lifetime longer than 100 ps was discovered in 2-amino-1,3,5-trainzine (2-AT), but its nature has not been revealed. In this study, excited state dynamics of 2-AT is studied in different solvents by using femtosecond time-resolved transient absorption and fluorescence up-conversion spectroscopy. Interestingly, an equilibrium state consists of the bright ππ* and dark nπ* states in 2-AT is directly observed in aqueous solution and its dynamics is solvent sensitive. The whole picture of the excited state deactivation mechanism of 2-AT is proposed based on our spectroscopy results.
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Glycerol is an important raw material in the chemical industry and dehydroxylation of glycerol would produce 1,2-propanediol and 1,3-propanediol. Here we studied glycerol dehydroxylation with ab initio molecular dynamics simulations on Pt(111) and Pt(211) surfaces at 453 K. The free energies obtained on Pt show that dehydroxylation is more likely to occur at the terminal carbon than the central carbon, and 1,2-propanediol would be produced preferentially, which is consistent with the selectivity observed experimentally. We found a linear relationship between the free energy barrier and the difference of average distances between O atoms at the initial states and transition states. Although a high correlation between the stability of gaseous glycerol and the number of formed hydrogen bonds was determined from density functional theory calculations, the hydrogen bonds formed within surface structures play a negligible role in determining the free energy barriers of dehydroxylation.
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Utilizing the exact diagonalization method, the optical absorption spectra of two π-conjugated molecules, anthracene and pyrene are calculated in various dielectric environments. In a confined quantum system with an anisotropic geometry, it is commonly believed that the 1<i>st</i> excited state is localized along the elongated direction. In the meantime, the dipole approximation says that the transitions to those states localized along the elongated direction shall generally have higher intensities. In this work we report that anthracene and pyrene would respectively fail these intuitive expectations. It is found that the 1st active transition in anthracene is always polarized along its short axis direction. For pyrene, it is revealed that the 2<i>nd</i> active transition polarized along the short axis direction of the molecule has almost the highest intensity. Furthermore, the 1<i>st</i> excited state in either anthracene or pyrene is often found to be optically inactive, which is successfully attributed to the short-range interactions by examining the energy spectra in varying interaction environments.
THz absorption is a fingerprint property of materials, due to the underlying low-frequency vibration/phonon modes being strongly dependent on the chemical constitutions and microscopic structures. The low excitation energies (0.414 ~ 41.4 meV) are related to two intrinsic properties of THz vibrations: the potential energy surfaces (PESs) are shallow, and the vibrationally excited states are usually populated via thermal fluctuations. The shallow PESs make the vibrations usually anharmonic, leading to redshifted vibrational excited state absorption; combined with considerable vibrational excited states population, characteristic THz signals are usually redshifted and congested with varying degrees at different temperatures. Combining existing experimental THz spectra at low temperatures, first principles vibration analysis, and the Morse potential, we developed a semi-empirical model to evaluate the anharmonicity of the low-frequency modes. The model was benchmarked with purine molecular crystal to generate THz spectra at different temperatures that are consistent with experiments. The good agreement suggests this model would facilitate the application of THz spectroscopy in molecular crystal characterization.
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The rovibronic spectra of PbS in the range of 19 520 - 22 900 cm<sup>-1</sup> were investigated using the laser ablation - laser induced fluorescence method. The spectra in this range were assigned as the transitions of A-X and B-X according to the spectral analyzation. The upper electronic state of the transition in the range of 19 520 - 21 090 cm<sup>-1</sup> was analyzed and discussed, and it is concluded that the upper state, A, is a mixture of <sup>3</sup> Π<sub>0<sup>+</sup></sub>and <sup>3</sup>Σ<sup>-</sup><sub>0<sup>+</sup></sub> states, the <sup>3</sup>Π<sub>0<sup>+</sup></sub> state is in domination. The spectrum in the range of 22 025 - 22 900 cm<sup>-1</sup> was assigned as the B<sup>3</sup>Π<sub>1</sub>-X<sup>1</sup>Σ<sup>+</sup> transition. The molecular constants of these two transitions were derived from the observed spectra. The Frank-Condon factors (FCFs) of these two transitions were also calculated using the RKR/LEVEL method. All the results were compared with the reported theoretical and experimental results.
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Molybdenum trioxide (MoO3) with layered structures adopts exotic physical features, which has evoked an extensive interest in electronic and photoelectronic devices. Here, we report a low-cost, simple-handle, atmospheric-pressure, and rapid-synthesis technique for growing large-scale MoO3 crystals, i.e., a modified hot plate method. The growth rate and morphology of the MoO3 crystals were well controlled by changing source temperatures and substrates. Complementary measurements, including optical microscope, atomic force microscope, X-ray diffraction, Raman spectroscope, and scanning near-field optical microscope, were used to investigate the structural and physical properties. The results reveal that large-scale MoO3 crystals with well-defined crystallinity have been obtained. Meanwhile, surface hyperbolic phonon polaritons on as-prepared MoO3 crystal planes have also been observed, which may provide an attractive insight into nanoelectronic and nanophotonic devices.
Scanning tunneling microscope induced luminescence can be used to study various optoelectronic phenomena of single molecules and to understand the fundamental photophysical mechanisms involved. To clearly observe the molecule-specific luminescence, it is important to improve the quantum efficiency of molecules in the metallic nanocavity. In this paper, we investigate theoretically the influence of an atomic-scale protrusion on the substrate on the emission properties of a point dipole oriented parallel to the substrate in a silver plasmonic nanocavity by electromagnetic simulations. We find that an atomic-scale protrusion on the substrate can strongly enhance the quantum efficiency of a horizontal dipole emitter, similar to the situation with a protrusion at the tip apex. We also consider a double-protrusion junction geometry in which there is an atomic-scale protrusion on both the tip and substrate, and find that this geometry does provide significantly enhanced emission compared with the protrusion-free situation, but does not appear to improve the quantum efficiency compared to the mono-protrusion situation either at the tip apex or on the substrate. These results are believed to be instructive for future STM induced electroluminescence and photoluminescence studies on single molecules.
Transition metal carbides have been shown to exhibit good catalytic performance that depends on their compositions and morphologies, and understanding such catalytic properties requires knowledge of their precise geometry, determination of which is challenging, particularly for clusters formed by multiple elements. In this work, we investigate the geometric and electronic structures of binary VnC3− (n = 1–6) clusters and their neutrals employing photoelectron spectroscopy and density functional theory calculations. The adiabatic detachment energies of VnC3−, or equally, the electron affinities of VnC3, have been determined from the measured photoelectron spectra. Theoretical calculations reveal that the carbon atoms become separate with increasing number of V atoms in the clusters, i.e., the C–C interactions present in small clusters being replaced by V–C and/or V–V interactions in larger ones. We further explored the composition dependent formation of cubic or cube-like structures in 8-atom VnCm (n + m = 8) clusters.
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Our recent theoretical studies have screened out CuCs-doped Ag-based as promising catalysts for ethylene epoxidation (ACS Catalysis, 2021, 3371-3383). The theoretical results were based on surface modeling, while in the actual reaction process Ag catalysts are particle shaped. In this work, we combine Density functional theory (DFT), Wulff construction theory, and microkinetic analysis to study the catalytic performance of Ag catalysts at the particle model. It demonstrates that the CuCs-doped Ag catalysts are superior to pure Ag catalysts in terms of selectivity and activity in ethylene oxide production, which is further proved by our experimental results. The characterization analysis finds that both Cu and Cs dopants promote particle growth as well as particle dispersion, resulting in a grain boundary-rich Ag particle. Besides, CuCs also facilitates electrophilic atomic oxygen formation on catalyst surface, which is benefit for ethylene oxide formation and desorption. Our work provides a case study of Ag-based catalyst design for ethylene epoxidation by combining theory and experiment.
Integration of non-noble transition metal oxides with graphene is known to construct high-activity electrocatalysts for oxygen evolution reduction (OER). In order to avoid the complexity of traditional synthesis process, for the first time, a facile electrochemical method is elaborately designed to engineer efficient WO3-x/graphene (photo-)electrocatalyst for OER by a two-electrode electrolysis system, where graphite cathode is exfoliated into graphene and tungsten wire anode evolves into VO-rich WO3-x profiting from formed reductive electrolyte solution. Among as-prepared samples, WO3-x/G-2 shows the best electrocatalytic performance for OER with an overpotential of 320 mV (without iR compensation) at 10 mA•cm−2, superior to commercial RuO2 (341 mV). With introduction of light illumination, the activity of WO3-x/G-2 is greatly enhanced and its overpotential decreases to 290 mV, benefited from additional reaction path produced by photocurrent effect and extra active sites generated by photogenerated carriers (h+). Characterization results indicate that both VO-rich WO3-x and graphene contribute to the efficient OER performance. The activity of WO3-x for OER is decided by the synergistic effect between VO concentration and particle size. The graphene could not only disperse WO3-x nanoparticles, but also improve the holistic conductivity and promote electron transmission. This work supports a novel method for engineering WO3-x/graphene composite for highly efficient (photo-)electrocatalytic performance for OER.
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Metal-organic frameworks (MOFs) draw more and more attentions due to their abundant properties and potential applications in materials science. Developing new MOFs structures often gets unexpected material properties. Herein, we report the properties of a copper [2,2]paracyclophane dicarboxylate MOF (CuCP-MOF). The magnetic properties of both CuCP-MOF and activated CuCP-MOF are investigated. CuCP-MOF shows the triplet state EPR spectrum at room temperature due to the antiferromagnetic coupling of the copper(II) paddlewheel (Cu-PW) dimer centers. The MOF has strong intramolecular antiferromagnetic interactions inside the paddlewheel dimer centers and very weak intermolecular interactions, while activated CuCP-MOF exhibits strong intramolecular and intermolecular interactions due to the existence of unpaired Cu(II) centers. We also investigate the electronic structure and semiconductor behavior of CuCP-MOF. The MOF is assigned to direct bandgap semiconductors. Moreover, CuCP-MOF could selectively adsorb cationic organic dyes. By utilizing the synergistic effects of adsorption and photodegradation, we successfully apply CuCP-MOF to organic dye removal.
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Cell membrane fusion is a fundamental biological process involving in a number of cellular living functions. Regarding this, divalent cations can induce fusion of the lipid bilayers through binding and bridging of divalent cations to the charged lipids, thus leading to the cell membrane fusion. However, the elaborate mechanism of cell membrane fusion induced by divalent cations is still needed to be elucidated. Here, surface/interface sensitive sum frequency generation vibrational spectroscopy (SFG-VS) and dynamic light scattering (DLS) were applied in this research to study the responses of phospholipid monolayer to the exposure of divalent metal ionsi.e. Ca2+ and Mg2+. According to the particle size distribution results measured by DLS experiments, it was found that Ca2+ could induce inter-vesicular fusion while Mg2+ could not. An Octadecyltrichlorosilane self-assembled monolayer (OTS-SAMs)/lipid monolayer system was designed to model the cell membrane for the SFG-VS experiment. We found, Ca2+ can interact with the lipid PO2- head groups more strongly, resulting in cell membrane fusion more easily, in comparison to Mg2+. No specific interaction between the two metal cations and the C=O groups was observed. However, the C=O orientations changed more after Ca2+ -PO2- binding than Mg2+ mediation on lipid monolayer. Meanwhile, Ca2+ can induce dehydration of the lipids (which should be related to the strong Ca2+ - PO2- interaction), leading to the reduced hindrance for cell membrane fusion. .
The oxygen reduction reaction (ORR) by the nitrogen-doped fullerene (C59N) catalyst demonstrates an excellent activity in hydrogen fuel cells. However, the intermediates and catalytic active sites in pathways have not been directly characterized, hindering the understanding of the enhanced activity mechanism for ORR on C59N. By taking the inhomogeneity of spatially confined plasmon (SCP) into account, we theoretically propose that the high-resolution tip-enhanced Raman scattering (TERS) can effectively identify different intermediate configurations of ORR on C59N. With the modulation of the focused SCP center position, vibrational modes that are directly related to site-specific O2−C59N interactions in ORR can be lighted up and then selected out by TERS spectra. Furthermore, the vibration-resolved TERS images for the selected modes of different intermediate configurations give spatial hot spot around the adsorption site, providing the in-situ details of catalytic active sites of ORR on C59N. These findings serve as good references for future high-resolution TERS experiments on probing catalytic systems at the molecular scale.
氮掺杂富勒烯(C59N)催化剂在氢燃料电池的氧还原反应(ORR)中表现出良好的活性。然而，C59N上发生的ORR反应路径的中间体和催化活性位点尚未被直接表征，阻碍了我们对C59N催化剂在ORR中活性增强机制的理解。通过在模拟计算中考虑空间限制等离子体(SCP)的不均匀分布，我们从理论上提出高空间分辨针尖增强拉曼散射(TERS)可以有效地识别C59N上ORR的不同中间体构型。通过调整聚焦的SCP位置，ORR中与O2−C59N相互作用有关联的振动模式可以被TERS光谱直接选择出来，并且得到增强。此外，选择出来的振动模式对应的TERS图像在吸附位点周围给出了拉曼热点，提供了ORR在C59N上催化活性位点的原位观测细节。这些发现为今后通过高分辨率TERS技术在分子尺度上探索催化系统提供了良好的参考。
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The industrial pollutant NO is a potential threat to the environment and to human health. Thus, selective catalytic reduction (SCR) of NO into harmless N2, NH3, and/or N2O gas is of great interest. Among many catalysts, metal Pd has been demonstrated to be most efficient for selectivity, reducing NO to N2. However, the reduction mechanism of NO on Pd, especially the route of N-N bond formation, remains unclear, impeding the development of new, improved catalysts. We report here the elementary reaction steps in the reaction pathway of reducing NO to NH3, N2O, and N2, based on density functional theory (DFT)-based quantum mechanics calculations. We show that the formation of N2O proceeds through an Eley-Rideal (E-R) reaction pathway that couples one adsorbed *NO with one non-adsorbed NO from the solvent or gas phase. This reaction requires high NO* surface coverage, leading first to the formation of the trans-(NO)2* intermediate with a low N-N coupling barrier (0.58 eV). Notably, we found that trans-(NO)2* will continue to react with NO in the solvent to form N2O that has not been reported. With the consumption of NO and the formation of N2O* in the solvent, the L-H mechanism will dominate at this time, and N2O* will be reduced by hydrogenation at a low chemical barrier (0.42 eV) to form N2. In contrast, NH3 is completely formed by the L-H reaction, which has a higher chemical barrier (0.87 eV). Our predicted E-R reaction has not previously been reported, but it explains existing experimental observations. In addition, we examine how catalyst activity might be improved by doping a single metal atom (M) at the NO* adsorption site to form M/Pd and show its influence on the barrier for forming the N-N bond to provide control over the product distribution.
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Understanding organic photovoltaic (OPV) work principles and the materials’ optoelectronic properties is fundamental for developing novel heterojunction materials with the aim of improving power conversion efficiency (PCE) of organic solar cells. Here, in order to understand more than 13% of PCE achieved by OPV device composed of the non-fullerene acceptor fusing naphtho[1,2-b:5,6-b']dithiophene with two thieno[3,2-b]thiophene (IDCIC) and the polymer donor fluorobenzotriazole (FTAZ), by the aid of extensive quantum chemistry calculations, we investigated the geometries, molecular orbitals, excitations, electrostatic potentials (ESP), transferred charges and charge transfer (CT) distances of FTAZ, IDCIC and their complexes with face-on configurations, which was constructed as heterojunction interface model. The results indicate that, the prominent OPV performance of FTAZ:IDCIC heterojunction is caused by co-planarity between the donor and acceptor fragments in IDCIC, the CT and hybrid excitations of FTAZ and IDCIC, the complementary optical absorptions in visible region, and the large ESP difference from FTAZ to IDCIC. The electronic structures and excitations of FTAZ/IDCIC complexes suggest that exciton dissociation (ED) can fulfill through the decay of local excitation exciton in acceptor by means of hole transfer, which is quite different from the OPVs based on fullerenes acceptor. The rates of ED, charge recombination and CT processes, which were evaluated by Marcus theory, support the efficient ED that is also responsible for good photovoltaic performance.
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The ethoxycarbonyl isothiocyanate has been investigated by using supersonic jet Fourier transform microwave spectroscopy. Two sets of rotational spectra belonging to conformers TCC (with the backbone of C-C-O-C, C-O-C=O, and O-C(=O)-NCS being trans, cis, and cis arranged, respectively) and GCC (gauche, cis, and cis arrangement of the C-C-O-C, C-O-C=O, and O-C(=O)-NCS) have been measured and assigned. The measurements of 13C, 15N and 34S mono-substituted species of the two conformers were also performed. The comprehensive rotational spectroscopic investigations generate accurate values of rotational constants and 14N quadrupole coupling constants, which lead to structural determinations of the two conformers of ethoxycarbonyl isothiocyanate. For conformer TCC, the values of Pcc keep constant upon isotopic substitution, indicating that the heavy atoms of TCC effectively be located in the ab plane.
In this work, mixed polymer brushes coating based on poly(2-methyl-2-oxazoline) (PMOXA)/poly(4-vinyl pyridine) (P4VP) was prepared by simultaneously grafting amine-terminated PMOXA and thiol-terminated P4VP onto poly(dopamine) (PDA)-modified substrates. The coatings were characterized by X-ray photoelectron spectroscopy (XPS), ellipsometry, zeta potential measurements, and the static water contact angle (WCA) tests. The results demonstrated that the coating based mixed PMOXA/P4VP brushes with desired surface composition could be obtained by simply maintaining their percentage in the mixture of PMOXA and P4VP solutions. Moreover, the zeta potential and the WCA of mixed brushes modified surfaces could be tuned by changing the environmental pH value and surface compositions. Finally, the switchable behavior of PMOXA/P4VP based coatings toward pepsin adsorption was investigated by fluorescein isothiocyanate-labelled pepsin assay and surface plasmon resonance. The results showed that by adjusting the fraction of PMOXA or P4VP, the PMOXA/P4VP mixed brushes coated surfaces could adsorb large amounts of pepsin at pH 3, but more than 92% of the adsorbed protein could be desorbed at pH 7.
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Crystallographic group is an important character to describe the crystal structure, but it is difficult to identify the crystallographic group of crystal only given chemical composition. Here, we present a machine-learning method to predict the crystallographic group of crystal structure from its chemical formula. 34528 stable compounds in 230 crystallographic groups are investigated, of which 72% of data set are used as training set, 8% as validation set, and 20% as test set. Based on the results of machine learning, we present a model which predicts crystallographic group among the top-1, top-5, and top-10 groups of compounds with the estimated accuracy of 60.8%, 76.5%, and 82.6%, respectively. In particular, the performance of deep-learning model presents high generalization through comparison between validation set and test set. Additionally, 230 crystallographic groups are classified into 19 new labels, denoting 18 heavily represented crystallographic groups with each containing more 400 compounds and one combination group of remaining compounds in other 212 crystallographic groups. A deep-learning model trained on 19 new labels yields a promising result to identify crystallographic group with the estimated accuracy of 72.2%. Our results provide a promising approach to identify crystallographic group of crystal structures only from their chemical composition.
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Deriving reaction coordinates (RC) for the characterization of chemical reactions has long been a demanding task. In our previous article (ACS Central Science 2017, 3(5), 407-414), the reaction coordinate of a (retro-) Claisen rearrangement in aqueous solution optimized through a Bayesian measure, 0.82d(C1-C6) - 0.18d(O3- C4), was judged to be optimal among all trails. Here, considering the nonlinearity of the transition state, we used Isometric Mapping ( Isomap) and Locally Linear Embedding (LLE) to obtain one reaction coordinate which is composed of a few collective variables. With this method, we have found a more reasonable and powerful one-dimensional reaction coordinate, which can well describe the reaction progression. To explore the reaction mechanism, we analyzed the contribution of intrinsic molecular properties and the solvent-solute interactions to the nonlinear reaction coordinate. Furthermore, a second coordinate is identified to characterize the heterogeneity of reaction mechanisms.
Revealing the relationship between electronic structures and the decomposition mechanism is the key to explore novel primary explosives. A systematic investigation on electronic structures and microscopic decomposition pathways of 4-amino-5-mercapto-1,2,4-triazole (AMTA) and 4-amino-5-mercapto-3-nitro-1,2,4- triazole (AMNTA) in the ground, charged, and excited states (S0→T1) has been analyzed with density functional theory (DFT). The effect of electrifying molecules and exciting electrons on the decomposition mechanism has been clarified by thermodynamics and kinetics. This study shows that the neutral amino dissociation from the triazole ring has an advantage among different substituents dissociation. For AMTA, electrifying the molecule can make the ring cleavage occur easily at the "N4-C5" site, and exciting electrons makes the triazole ring decompose directly and release 3.3 kcal/mol of heat. For AMNTA, positively electrifying the molecule makes CONO isomerization become the dominant reaction and hinders the H-transfer reaction. When the molecule is electrified negatively or its electrons are excited, H-transfer will take place preferentially. This work sheds light on how to control the decomposition pathways of novel primary explosives at the electronic structure level by the means of electrifying molecules and exciting electrons.
The kinetics of U(IV) produced by hydrazine reducting U(VI) with platinum as catalyst in nitric acid media was studied for revealing the reaction mechanism and optimizing the reaction process. Electron spin resonance (ESR) was used to determine the influence of nitric acid oxidation. The influence of nitric acid, hydrazine, U(VI) concentration, catalyst dosage and temperature on the reaction were studied. Concurrently, the simulation of the reaction process was performed using density functional theory (DFT). The results show that the influence of oxidation on the main reaction is limited when the concentration of nitric acid was below 0.5 mol/L. The reaction kinetics equation below the concentration of 0.5 mol/L is found as follows: –dc(UO22+)/dt) = kc0.5323(UO22+)c0.2074(N2H5+)c−0.2009(H+). When the temperature is 50°C, and the solid/liquid ratio r is 0.0667g/mL, the reaction kinetics constant is k = 0.00199 (mol/L) 0.4612/min. Between 20℃ and 80℃, with the increase of temperature, the reaction rate is gradually accelerated, and the reaction changes from chemically controlled to diffusion-controlled. The reaction process is simulated by DFT, and the influence of various factors on the reaction process is deduced. The reaction process and mechanism are determined according to the reaction kinetics and simulation results finally.
The general application of antibiotics has brought a series of negative impacts on human health and the environment, which has aroused widespread public attention to their removal from aqueous systems. In this study, a chitosan (CS)-linked graphene oxide (GO) composite (GO-CS) was synthesized by a modified Hummers/solvothermal method. It was separated from the mixed aqueous phase by low-speed centrifugation, thereby endowing the GO with high separation efficiency in water. The adsorption of tetracycline (TC), norfloxacin (NOR), and sulfadiazine (SDZ) by GO-CS were then studied by experimental techniques and theoretical calculations. In batch experiments at 298 K and optimal pH, the adsorption capacities of TC, NOR, and SDZ were 597.77, 388.99, and 136.37 mg/g, respectively, which were far better than those for pristine graphene oxide. The spectra results illustrated that the adsorption process was mainly contributed by the interactions between antibiotics and functional groups (carboxyl, hydroxyl, and amino groups) of GO-CS. Furthermore, density functional theory calculations showed that electrostatic interaction and hydrogen bonds are of vital importance for the uptake of the antibiotics; the former is extremely important for TC adsorption. This research will provide theoretical references for the removal of antibiotics by graphene-based composite materials, thus offering their promising application in environmental remediation.
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The anionic carbonyl complexes of groups IV and V metals are prepared in the gas phase using a laser vaporation-supersonic expansion ion source. The infrared spectra of the TM(CO)<sub>6,7</sub><sup>-</sup> (TM=Ti, Zr, Hf, V, Nb, Ta) anion complexes in the carbonyl stretching frequency region are measured by mass-selected infrared photodissociation spectroscopy. The six-coordinated TM(CO)<sub>6</sub><sup>-</sup> anions are determined to be the coordination saturate complexes for both the group IV and group V metals. The TM(CO)<sub>6</sub><sup>-</sup> complexes of group IV metals (TM=Ti, Zr, Hf) are 17-electron complexes having a <sup>2</sup>A<sub>1g</sub> ground state with D<sub>3d</sub> symmetry, while the TM(CO)<sub>6</sub><sup>-</sup> complexes of group V metals (TM=V, Nb, Ta) are 18-electron species with a closed-shell singlet ground state possessing O<sub>h</sub> symmetry. The energy decomposition analyses indicate that the metal-CO covalent bonding are dominated by TM-(d) → (CO)<sub>6</sub> π-backdonation and TM-(d)←(CO)<sub>6</sub> σ-donation interactions.
A better understanding of the photophysical processes occurring within organic semiconductors is important for designing and fabricating organic solar cells (OSCs). In this paper, we investigated the triplet‒triplet annihilation process in CuPc thin films with different molecular stacking configurations. The ultrafast transient absorption measurements indicate that the primary annihilation mechanism is one-dimensional exciton diffusion collision destruction. The decay kinetics shows a clearly time-dependent annihilation rate constant with γ ∝ t−1/2. Annihilation rate constants were determined to be γ0 = (2.87 ± 0.02) × 10−20 and (1.42 ± 0.02) × 10−20 cm3·s−1/2 for upright and lying-down configurations, respectively. Compared to the CuPc thin film with an upright configuration, the thin film with a lying-down configuration shows a longer exciton lifetime and a higher absorbance, which are beneficial for OSCs. The results in this paper have important implications on the design and mechanistic understanding of organic optoelectronic devices.
Vacuum ultraviolet photodissociation dynamics of N2O + hv → N2(X1Σg+) + O(1S0) in the short wavelength tail of D1Σ+ band have been performed using the time-sliced velocity-mapped ion imaging technique by probing the images of the O(1S0) photoproducts at a set of photolysis wavelengths from 121.47 to 123.95 nm. The product total kinetic energy release distributions, vibrational state distributions of the N2(X1Σg+) photofragments and angular anisotropy parameters have been obtained through analyzing the raw O(1S0) images. It is noted that an additional vibrationally excited photoproducts (3≤v≤8) with a Boltzmann-like feature start to appear except for the non-statistical component as the photolysis wavelength decreases to 123.25 nm, and the corresponding populations become more pronounced with decreasing of the photolysis wavelength. Furthermore, the vibrational state specific β-value at each photolysis wavelength exhibits a drastic fluctuation near β=1.75 at v<8, and decreases to a minimum as the vibrational quantum number further increases. While the overall β-value for the N2(X1Σg+) + O(1S0) channel presents a roughly monotonically increasing from the value of 1.63 at 121.47 nm to 1.95 at 123.95 nm. The experimental observations suggest that there is at least one fast nonadiabatic pathway from initially prepared D1Σ+ state to the dissociative state with bent geometry dominating to generate the additional vibrational structures at high photoexcitation energies.
Catalytic hydrolysis of ammonia borane for dehydrogenation is a promising way for generation and storage of hydrogen energy. Catalysts with reduced utilization of costly noble metals while high activity and stability are highly desired. Herein we show that the catalytic activity of the prototypical Pt/SiO2 catalysts towards ammonia borane hydrolysis could be significantly improved by the presence of a layer of Co(OH)2 beneath the supported Pt nanoparticles. By changing the Pt:Co molar ratio in the Pt-Co(OH)2/SiO2 catalysts, the hydrogen generation rates from ammonia borane hydrolysis show a volcano-type curve, with the maximum catalytic activity at the Pt:Co molar ratio of 1:11. The highest turnover frequency value of 829 molH2·molPt−1·min−1 at room temperature outperforms most of the reported Pt-based catalysts, and the apparent activation energy is drastically decreased to 36.2 kJ/mol from 61.6 kJ/mol for Pt/SiO2. The enhanced catalytic performance of Pt-Co(OH)2/SiO2 is attributed to the electrons donation from Co atoms on Co(OH)2 to Pt occurring at the metal-hydroxide interface, which is beneficial for optimizing the oxidation cleavage of the O−H bond of attacked H2O.
Herein, we present the decoration of NiFeCoAlOOH nanoparticles onto titanium doped nanoporous hematite (Ti-PH) utilizing a simple electroless ligand-controlled oxidation method for photoelectrochemical water splitting. Owing to the improved oxygen evolution reaction kinetics and reduced charge transfer resistance, the resulting Ti-PH/NiFeCoAlOOH photoanode presents an excellent photocurrent density of 2.46 mA/cm<sup>2</sup> at 1.23V vs. RHE and good stability compared to Ti-PH or bare hematite (H). Furthermore, the onset potential of the photocurrent density is shifted cathodically by ~ 60 mV with reference to the titanium doped nanoporous hematite. This work offers a promising method for designing high-performance, stable, and inexpensive catalysts for photoelectrochemical (PEC) applications.
Although lead-based halide perovskites have promising applications in optoelectronic devices, these applications are limited by the toxicity of the materials. Therefore, it is necessary to develop lead-free all-inorganic substitute such as tin-based halide perovskites in spite of the enormous challenges in their controllable synthesis and stability. Here, we report the controlled growth of high quality CsSnBr<sub>3</sub> microcrystals on SiO<sub>2</sub>/Si substrates by chemical vapor deposition method. The as-prepared products predominantly show the morphology of triangle star and nail-like rod and the structure of cubic phase. The control of nucleation density and size of CsSnBr<sub>3</sub> microcrystals has been realized by varying the growth temperature. The results of air-exposed samples provide direct evidences for explaining the structural instability of the tin-based perovskites, which is attributed to the production of SnO. The power and temperature dependent photoluminescence spectra reveal that CsSnBr<sub>3</sub> microcrystals with different morphologies possess different exciton binding energies and produce different photoexcitation species due to the quantum confinement effect that changes the electron-hole effect.
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Ultra-wide-bandgap semiconductors have tremendous potential to advance electronic devices, as device performance improves nonlinearly with increasing gap. In this work we employ density-functional theory with the accurate screened-hybrid functional to evaluate the electronic and optical properties of NaYO2 in two different phases. The electronic structure calculation results show that both monoclinic and trigonal phases of NaYO2 exhibit direct bandgaps of 5.6 and 5.4 eV, respectively, offering a physically realistic material platform to derive the semiconductor industry beyond the well-established diamond, and GaN semiconducting materials. Next, we investigate the optical properties and reveal that both phases of NaYO2 are transparent in the infrared and visible regions, thereby, these materials can be used as infrared window materials.
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Polymers are routinely used as embedding matrices for organic molecular phosphors to substantially reduce the non-radiative decay rate and promote room-temperature phosphorescence (RTP). However, most previous studies focus on how glass transition temperature and free volume of various polymers influence RTP efficiency; very little is known on how electronic coupling between the matrix and the phosphor can modulate organic RTP. The current investigation attempts to address the problem by synthesizing a monomeric version of an aromatic ketone phosphor and copolymerizing the ketone with four different types of matrix monomers. The resulting copolymers exhibit clear matrix-dependent RTP efficiency: a gradual decrease of RTP quantum yield from 22% to nearly 0 can be observed when the electronic conjugation of the matrix increases, suggesting that energy dissipation can occur in the triplet excited state via electron exchange when the triplet state of the matrix is close to that of the phosphor. The study provides a guiding principle on regulating the lifetime of triplet-excited states for organic dyes.
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A novel electrochemical non-enzymatic glucose sensor based on three-dimensional Au/MXene nanocomposites was developed. MXenes were prepared using the mild etched method, and the porous foam of Au nanoparticles was combined with the MXene by means of in situ synthesis. By controlling the mass of MXene in the synthesis process, porous foam with Au nanoparticles was obtained. The three-dimensional foam structure of nanoparticles was confirmed by scanning electron microscopy (SEM). Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were used to study the electrochemical performance of the Au/MXene nanocomposites. The Au/MXene nanocomposites acted as a fast redox probe for non-enzymatic glucose oxidation and showed good performance, including a high sensitivity of 22.45 μA mM−1 cm−1 and a wide linear range of 1–12 mM. Studies have shown that MXene as a catalyst-supported material is beneficial to enhance the conductivity of electrons and increase the loading rate of the catalyst materials. The foam structure with Au nanoparticles can provide a larger surface area, increase the contact area with the molecule in the catalytic reaction, and enhance the electrochemical reaction signal. In summary, this study showed that Au/MXene nanoparticles have the potential to be used in non-enzymatic glucose sensors.
The yolk-shell MIL-125/TiO<sub>2</sub>/Pt/CdS and hollow TiO<sub>2</sub>/Pt/CdS visible-light catalysts were successfully synthesized from MIL-125 by γ-ray irradiation. What is interesting is that during the reduction process by γ-ray irradiation, MOFs are partially or completely hydrolyzed to TiO<sub>2</sub> nanosheets, forming the unique yolk-shell or hollow structure. The hydrogen production rate is 2983.5 μmol·g<sup>-1</sup>·h<sup>-1</sup> for yolk-shell structures and 1934.2 μmol·g<sup>-1</sup>·h<sup>-1</sup> for hollow structures under visible-light illumination, which is 7.9 and 5.1 times higher than that of CdS, respectively. The excellent properties of these photocatalysts may be attributed to the effective absorption and utilization of the light, the porous yolk-shell or hollow structure derived from MIL-125 to facilitate mass transfer, and close contact among CdS nanoparticles, TiO<sub>2</sub> nanosheets and Pt nanoparticles to improve the separation of electron-hole pairs. This research can provide a simple and new method for the construction of high efficiency photocatalysts derived from MOFs using the γ-ray irradiation.
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In this work, we firstly elucidated the ultraviolet light protection dynamics mechanism of the typical hemicyanines of Hemicy and DHemicy by combining the theoretical calculation method and the transient absorption spectra. It was theoretically and experimentally demonstrated that both of Hemicy and DHemicy have strong absorption in UVC, UVB, and UVA regions. Moreover, after absorbing energy, Hemicy and DHemicy can jump into the excited states. Subsequently, the Hemicy and DHemicy will relax to S0 states from S1 states rapidly by the non-adiabatic transition at the conical intersection (CI) point between the potential energy curves of S1 and S0 states and accompany by the trans-cis photoisomerism. The transient absorption spectra showed that the trans-cis photoisomerization will occur within a few picoseconds. Thus, the ultraviolet energy absorbed by Hemicy and DHemicy could be relaxed ultrafast by the non-adiabatic trans-cis photoisomerization processes.
Specific energy and self-discharge are two important performances of electrochemical capacitors. In this work, we have fabricated the composite electrodes by complexing the negatively charged carboxylated multi-walled carbon nanotubes (cMWCNT) with the redox active units-containing positively charged random copolymers. 2,2,6,6-Tetramethylpiperidinyl-N-oxyl and viologen are employed as model redox active units to exemplify the strategy of the concurrent increase of specific energy and suppression of self-discharge of a two-electrode device. The enhanced specific energy is mainly attributed to the increased electrolyte decomposition window induced by the faster redox reactions than those of the hydrogen and oxygen evolution reactions. The improved performance of self-discharge is due to the suppression of the cross-diffusion and redox shuttling of the redox couples induced by the complexation between the cMWCNT and the copolymers. By employing the redox active units-containing charged copolymers, this work provides a convenient and universal strategy to concurrently increase specific energy and suppress self-discharge of electrochemical capacitors with the carbon-based electrodes.
It is important to identify non-planar deformations of porphyrin macrocycle in metallo-porphyrin proteins due to their functional relevance. The relationship between non-planar deformations of porphyrin macrocycle and low frequency Raman spectral bands of Ni(II) Meso-tetraphenyl porphyrin (NiTPP), with different coordination numbers , was studied by density functional theory (DFT) , normal coordinate structural decomposition (NSD) method and Raman experiments. The results show that the crystal of four-coordinate NiTPP has two major kinds of nonplanar deformations: ruffling and saddling. The non-planar deformations of ruffling and saddling for NiTPP are 1.473 Å and 0.493 Å determined by DFT calculation. The ruffling and saddling deformations can be identified by using the low frequency Raman characteristic peaks (r12 ,r13 ) and (r16 ,r17 ), respectively. When four-coordinate NiTPP is transformed to the six-coordinate bis(pyrrolidine) NiTPP (NiTPP(Pyr)2), the large non-planar distortion of the porphyrin macrocycle almost disappears, with the non-planar deformation of saddling only about 0.213 Å estimated by DFT calculation. Experimentally, we can make use of the characteristic peaks of low frequency Raman spectra to identify the saddling deformation beyond 0.25 Å.
Though poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) has been widely adopted as hole transport material (HTM) in flexible perovskite solar cells (PSCs), arising from high optical transparency, good mechanical flexibility, and high thermal stability, its acidity and hygroscopicity would inevitably hamper the long-term stability of the PSCs and its energy level does not match well with that of perovskite materials which would lead to a relatively low open-circuit voltage (VOC). In this investigation, p-type delafossite CuCrO2 nanoparticles synthesized through hydrothermal method have been employed as an alternative HTM for triple cation perovskite [(FAPbI3)0.87(MAPbBr3)0.13]0.92[CsPbI3]0.08 (possessing better photovoltaic performance and stability than conventional CH3NH3PbI3) based inverted architecture PSCs. The average VOC of PSCs has increased from 908 mV of the devices with PEDOT:PSS HTM to 1020 mV of the devices with CuCrO2 HTM. Ultraviolet photoemission spectroscopy measurement demonstrates the energy band alignment between CuCrO2 and perovskite is better than that between PEDOT:PSS and perovskite, the electrochemical impedance spectroscopy indicates CuCrO2 based PSCs exhibit larger recombination resistance and longer charge carrier lifetime, which contribute to the high VOC of CuCrO2 HTM based PSCs.
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The burgeoning two-dimensional (2D) layered materials provide a powerful strategy to realize efficient light-emitting devices. Among them, Gallium telluride (GaTe) nanoflakes, emerging strong photoluminescence (PL) emission from multilayer to bulk crystal, relax the stringent fabrication requirements of nanodevices. However, detailed knowledge on the optical properties of GaTe varied as layer thickness is still missing. Here we perform thickness-dependent PL and Raman spectra, as well as temperature-dependent PL spectra of GaTe nanoflakes. Spectral analysis reveals a spectroscopic signature for the coexistence of both the monoclinic and hexagonal phases in GaTe nanoflakes. To understand the experimental results, we propose a crystal structure where the hexagonal phase is on the top and bottom of nanoflakes while the monoclinic phase is in the middle of the nanoflakes. On the basis of temperature-dependent PL spectra, the optical gap of the hexagonal phase is determined to 1.849 eV, which can only survive under a temperature higher than 200 K with the increasing phonon population. Furthermore, the exciton-phonon interaction of the hexagonal phase is estimated to be 1.24 meV/K. Our results prove the coexistence of dual crystalline phases in multilayer GaTe nanoflakes, which may provoke further exploration of phase transformation in GaTe materials, as well as new applications in 2D light-emitting diodes and heterostructure-based optoelectronics.
We report a measurement of electron momentum distributions of valence orbitals of cyclopentene employing symmetric noncoplanar (e, 2e) kinematics at impact energies of 1200 and 1600 eV plus the binding energy. Experimental momentum profiles for individual ionization bands are obtained and compared with theoretical calculations considering nuclear dynamics by harmonic analytical quantum mechanical and thermal sampling molecular dynamics approaches. The results demonstrate that molecular vibrational motions including ring-puckering of this flexible cyclic molecule have obvious influences on the electron momentum profiles for the outer valence orbitals, especially in the low momentum region. For π*-like molecular orbitals 3a'' and 2a''+3a' , the impact-energy dependence of the experimental momentum profiles indicates a distorted wave effect.
Red phosphorus (RP) has attracted more attention as a promising sodium storage material due to its ultra-high theoretical capacity, suitable sodiation potential. However, the low intrinsic electrical conductivity and large volume change of pristine RP lead to high polarization and fast capacity fading during cycling. Herein, surface synergistic protections on red phosphorus composite is successfully proposed by conductive poly (3, 4-ethylenedioxythiophene) (PEDOT) coating and electrolyte strategy. Nanoscale RP is confined in porous carbon skeleton and the outside is packaged by PEDOT coating via in-situ polymerization. Porous carbon provides rich access pathways for rapid Na+ diffusion and empty spaces accommodate the volume expansion of RP; PEDOT coating isolates the direct contact between electrolyte and active materials to form a stable solid electrolyte interphase. In addition, the reformulated electrolyte with 3 wt% SbF3 additive can stabilize the electrode surface and thus enhance the electrochemical performance, especially cycling stability and rate capability (433 mAh g-1 at high current density of 10 A g-1).
Metal-halide perovskite solar cells (PSCs) have attracted considerable attention during the past decade. However, due to the existence of non-radiative recombination losses, the best power conversion efficiency (PCE) is still lower than the theoretical limit defined by shockley-Queser theory. In this work, we investigate1,2,3-oxathiazin-4(3h)-one,6-methyl-2,2-dioxide (Acesulfame Potassium, abbreviated as AK) as a additional dopant for the 2,2′,7,7′-Tetrakis(N,N-di-p-methoxyphenyl-amine)-9,9′-spirobifluorene (Spiro-OMeTAD) and fabricate PSCs in the air. It is found that 12 mol % fraction of AK relative to lithium bis((trifluoromethyl)sulfonyl)-amide (Li-TFSI) reduced the non-radiative recombination from 86.05 % to 69.23 %, resulting in an average 0.08 V enhancement of Voc. The champion solar cell gives a PCE up to 21.9% and over 84% retention of the initial value during 720 h aging in dry air with 20%~30% humidity.
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We have investigated the adsorption of 9 different adatoms on the (111) and (100) surfaces of
Iridium (Ir) using first principles density functional theory. The study explores surface functionalization
of Ir which would provide important information for further study towards investigating its
functionality in catalysis and other surface applications. The adsorption energy, stable geometry,
density of states and magnetic moment are the physical quantities of our interest. Strong hybridization
between the adsorbates and the substrate electronic states revealed to impact the adsorption,
while the magnetic moment of the adsorbates found to be suppressed. In general, stronger binding
is observed on the (100) surface.
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Based on density functional theory (DFT), a new silicon allotrope (C2-Si) is proposed in this work. The mechanical stability and dynamic stability of C2-Si are examined based on the elastic constants and phonon spectrum. According to the BH/GH values, C2-Si has ductility under ambient pressure; compared with Si64, Si96, I4/mmm and h-Si6, C2-Si is less brittle. Within the Heyd-Scuseria-Ernzerhof (HSE06) hybrid functional, C2-Si is an indirect narrow band gap semiconductor, and the band gap of C2-Si is only 0.716 eV, which is approximately two-thirds that of c-Si. The ratios of the maximum and minimum values of the Young's modulus, shear modulus and Poisson's ratio in their 3D spatial distributions for C2-Si are determined to characterize the anisotropy. In addition, the anisotropy in different crystal planes is also investigated via 2D representations of the Young’s modulus, shear modulus and Poisson’s ratio. In addition, among more than ten silicon allotropes, C2-Si has the strongest absorption ability for visible light.
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In order to reduce the impact of CdS photogenerated electron-hole recombination on its photocatalytic performance, a narrow band gap semiconductor MoS2 and organic macromolecular cucurbit[n]urils(Q[n]) were used to modify CdS. Q[n]/CdS-MoS2 (n=6, 7, 8) composite photocatalysts were synthesized by hydrothermal method. Infrared spectroscopy (FT-IR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (SEM), ultraviolet-visible (UV-Vis) and photoluminescence spectrum (PL) were used to characterize the structure, morphology and optical properties of the products, and the catalytic degradation of the solutions of methylene blue, rhodamine B and crystal violet by Q[n]/CdS-MoS2 composite catalyst was investigated. The results showed that the Q[n] played a regulatory role on the growth and crystallization of CdS-MoS2 particles, Q[n]/CdS-MoS2 (n=6, 7, 8) formed flower clusters with petal-like leaves, the flower clusters of petal-like leaves increased the surface area and active sites of the catalyst, the Q[n]/CdS-MoS2 barrier width decreased, the electron-hole pair separation efficiency was improved in the Q[6]/Cds-MoS2. Q[n] makes the electron-hole pair to obtain better separation and migration. The Q[6]/CdS-MoS2 and Q[7]/CdS-MoS2 have good photocatalytic activity for methylene blue, and the catalytic process is based on hydroxyl radical principle.
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The laser-induced fluorescence excitation spectra of UF have been recorded in the range of 17000−19000 cm-1 using two-dimensional spectroscopy. High resolution dispersed fluorescence spectra and fluorescence decay curves time-resolved fluorescence spectroscopy were also recorded. Three rotationally resolved bands have been intensively analyzed, and all bands were found to be derived from the ground state X(1)4.5 with a rotational constant of 0.23421 cm-1. The low-lying electronic states have been observed near 435 and 651 cm-1 in the dispersed fluorescence spectra, which were assigned as Ωʹ = 3.5, and 2.5, respectively. The vibrational constants for the X(1)4.5 and X(1)3.5 states have been calculated. The branching ratios of the dispersed fluorescence spectra for the [18.62]3.5, [17.72]4.5, and [17.65]4.5 states were reported. Radiative lifetimes of 332(9), 825(49), and 433(15) ns for the [18.62]3.5, [17.72]4.5, and [17.65]4.5 states were obtained by fitting the fluorescence decay curves time-resolved fluorescence spectroscopy, respectively. Transition dipole moments were performed using the branching ratios and the radiative lifetimes.
Dimension-controllable supramolecular organic frameworks (SOFs) with aggregation-enhanced fluorescence are hierarchically fabricated through the host-guest interactions of CB[8] and coumarin-modified tetraphenylethylene derivatives (TPEC). The three-dimensional (3D) layered SOFs could be constructed from the gradual stacking of two-dimensional (2D) mono-layered structures via simply regulating the self-assembly conditions including the culturing time and concentration. Upon light irradiation under the wavelength of 350 nm, the photodimerization of coumarin moieties occurred, which resulted in the transformation of the resultant TPECn/CB[8]4n 2D SOFs into robust covalently-connected 2D polymers with molecular thickness. Interestingly, the supramolecular system of TPECn/CB[8]4n exhibited intriguing multicolor fluorescence emission from yellow to blue from 0 to 24 hours at 365 nm irradiation, possessing potential applicability for cell imaging and photochromic fluorescence ink.
Rational designs of electrocatalytic active sites and architectures are of great importance to develop cost-efficient non-noble metal electrocatalysts towards efficient oxygen reduction reaction (ORR) for high-performance energy conversion and storage devices. In this paper, active amorphous Fe-based nanoclusters (Fe NC) are elaborately embedded at the inner surface of balloon-like N-doped hollow carbon (Fe NC/C<sub>h</sub> sphere) as an efficient ORR electrocatalyst with an ultrathin wall of about 10 nm. When evaluated for electrochemical performance, Fe NC/C<sub>h</sub> sphere exhibits decent ORR activity with a diffusion-limited current density of ~5.0 mA cm<sup>-2</sup> and a half-wave potential of ~0.81 V in alkaline solution, which is comparable with commercial Pt/C and superior to Fe nanoparticles (NP) supported on carbon sheet (Fe NP/C sheet) counterpart. The electrochemical analyses combined with electronic structure characterizations reveal that robust Fe-N interactions in amorphous Fe nanoclusters are helpful for the adsorption of surface oxygen-relative species, and the strong support effect of N-doped hollow carbon is benefit for accelerating the interfacial electron transfer, which jointly contributes to improved ORR kinetics for Fe NC/C<sub>h</sub> sphere.
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In order to search for high energy density materials, various 4, 8-dihydrodifurazano[3,4-b,e]pyrazine based energetic materials were designed. Density functional theory was employed to investigate the relationships between the structures and properties. The calculated results indicated that the properties of these designed compounds were influenced by the energetic groups and heterocyclic substituents. The –N3 energetic group was found to be the most effective substituent to improve the heats of formation of the designed compounds while the tetrazole ring/–C(NO2)3 group contribute much to the values of detonation properties. The analysis of bond orders and bond dissociation energies showed that the addition of –NHNH2, –NHNO2, –CH(NO2)3 and –C(NO2)3 groups will decrease the bond dissociation energies remarkably. Compounds A8, B8, C8, D8, E8, and F8 were finally screened as the potential candidates for high energy density materials since these compounds possess excellent detonation properties and acceptable thermal stabilities. Additionally, the electronic structures of the screened compounds were calculated.
The photochemical reaction of potassium ferrocyanide (K4Fe(CN)6}) exhibits excitation wavelength dependence and non-Kasha rule behavior. In this study, the excited-state dynamics of K4Fe(CN)6 were studied by transient absorption spectroscopy. Excited state electron detachment (ESED) and phtoaquation reactions were clarified by comparing the results of 260 , 320 , 340 , and 350 nm excitations. ESED is the path to generate a hydrated electron (eaq-). ESED energy barrier varies with the excited state, and it occurrs even at the first singlet excited state (1T1g). The 1T1g state shows ~0.2 ps lifetime and converts into triplet [Fe(CN)6]4- by intersystem crossing. Subsequently, 3[Fe(CN)5]3- appears after one CN- ligand is ejected. In sequence, H2O attacks [Fe(CN)5]3- to generate [Fe(CN)5H2O]3- with a time constant of approximately 20 ps. The 1T1g state and eaq- exhibit strong reducing power. The addition of UMP to the K4Fe(CN)6 solution decreased the yield of eaq- and reduced the lifetimes of the eaq- and 1T1g state. The obtained reaction rate constant of 1T1g state and UMP was 1.7×1014 M-1 s-1, and the eaq- attachment to UMP was ~8×109 M-1 s-1. Our results indicate that the reductive damage of K4Fe(CN)6 solution to nucleic acids under ultraviolet irradiation cannot be neglected.
Rhodium-catalyzed cycloaddition reaction was calculated by density functional theory (DFT) M06-2X method to directly synthesize benzoxepine and coumarin derivatives. In this paper, we conducted a computational study of two competitive mechanisms in which the carbon atom of acetylene or carbon monoxide attacked and inserted from two different directions of the six-membered ring reactant to clarify the principle characteristics of this transformation. The calculation result reveal (1) the insertion process of alkyne or carbon monoxide is the key step of the reaction; (2) For the (5 + 2) cycloaddition reaction of acetylene, higher energy is required to break the Rh-O bond of the reactant, and the reaction tends to complete the insertion from the side of the Rh-C bond; (3) For the (5 + 1) cycloaddition of carbon monoxide, both reaction paths have lower activation free energy, and the two will generate a competition mechanism.
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Methyl 2-furoate (FAME2) is a renewable biofuel with the development of the new synthesis method of dimethyl furan-2,5-dicarboxylate. The potential energy surfaces (PES) of H-abstractions and OH-additions between FAME2 and hydroxyl radical (OH) were studied at the CCSD(T)/CBS//M062X/cc-pVTZ level. The following isomerization and decomposition reactions were also determined for the main radicals produced. The results show that the H-abstraction on the branch methyl group is the dominant channel and that the OH-addition reactions on the furan ring has a significant pressure dependency. The current rate coefficients provide important kinetic data for the further improving of the combustion mechanism of FAME2, which bring a trusty reference for further research on practical fuels.
The vacuum ultraviolet (VUV) photodissociation of OCS via the F 31Π Rydberg states was investigated in the range of 134–140 nm, by means of the time-sliced velocity map ion imaging technique. The images of S (1D2) products from the CO (X1Σ+) + S (1D2) dissociation channel were acquired at five photolysis wavelengths, corresponding to a series of symmetric stretching vibrational excitations in OCS (F 31Π, v1=0-4). The total translational energy distributions, vibrational populations and angular distributions of CO (X1Σ+, v) coproducts were derived. The analysis of experimental results suggests that the excited OCS molecules dissociate to CO (X1Σ+) and S (1D2) products via non-adiabatic couplings between the upper F 31Π states and the lower-lying states both in the C∞v and Cs symmetry. Furthermore, strong wavelength dependent behavior has been observed: the greatly distinct vibrational populations and angular distributions of CO (X1Σ+, v) products from the lower (v1=0-2) and higher (v1=3,4) vibrational states of the excited OCS (F 31Π, v1) demonstrate that very different mechanisms are involved in the dissociation processes. This study provides evidence for the possible contribution of vibronic coupling and the crucial role of vibronic coupling on the VUV photodissociation dynamics.
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The hierarchical stochastic Schrödinger equations (HSSE) are a kind of numerically exact wavefunction-based approaches suitable for the quantum dynamics simulations in a relatively large system coupled to a bosonic bath. Starting from the influence-functional description of open quantum systems, this review outlines the general theoretical framework of HSSEs and their concrete forms in different situations. The applicability and efficiency of HSSEs are exemplified by the simulations of ultrafast excitation energy transfer processes in large-scale systems.
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Stars with masses between 1 and 8 solar masses (M⊙) lose large amounts of material in the form of gas and dust in the late stages of stellar evolution, during their Asymptotic Giant Branch phase. Such stars supply up to 35% of the dust in the interstellar medium and thus contribute to the material out of which our solar system formed. In addition, the circumstellar envelopes of these stars are sites of complex, organic chemistry with over 80 molecules detected in them. We show that internal ultraviolet photons, either emitted by the star itself or from a close-in, orbiting companion, can significantly alter the chemistry that occurs in the envelopes particularly if the envelope is clumpy in nature. At least for the cases explored here, we find that the presence of a stellar companion, such as a white dwarf star, the high flux of UV photons destroys H2O in the inner regions of carbon-rich AGB stars to levels below those observed and produces species such as C+ deep in the envelope in contrast to the expectations of traditional descriptions of circumstellar chemistry.
Two-photon fluorescence dyes have shown promising applications in biomedical imaging. However, the substitution site effect on geometric structures and photophysical properties of fluorescence dyes is rarely illustrated in detail. In this work, a series of new lipid droplets detection dyes are designed and studied, molecular optical properties and non-radiative transitions are analyzed. The intramolecular weak interaction and electron-hole analysis reveal its inner mechanisms. All dyes are proved to possess excellent photophysical properties with high fluorescence quantum efficiency and large stokes shift as well as remarkable TPA cross section. Our work reasonably elucidates the experimental measurements and the effects of substitution site on two-photon absorption and excited states properties of Lipid droplets detection NAPBr dyes are highlighted, which could provide a theoretical perspective for designing efficient organic dyes for lipid droplets detection in biology and medicine fields.
Smart functional microgels hold great potential in a variety of applications, especially in drug transportation. However, current drug carriers based on physiological internal stimuli cannot efficiently orientate to designated locations. Therefore, it is necessary to introduce the self-propelled particles to the drug release of the microgels. In order to study self-propulsion of microgels induced by light, it is also a challenge to prepare micron-sized microgels so that they can be observed directly under optical microscopes. This paper presents a method to prepare phototactic microgels with photoresponsive properties. The microgel particles is observed by confocal laser scanning microscopy (CLSM). The photoresponsive properties of microgels are fully investigated by various instruments. Light can also regulate the state of the microgel solution, making it switch between turbidity and clarity. The phototaxis of particles irradiated by UV light was studied, which may be used for microgels enrichment and drug transportation and release.
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Our experimental progresses on the reaction dynamics of dissociative electron attachment (DEA) to carbon dioxide (CO2) are summarized in this review. First, we introduce some fundamentals about the DEA dynamics and provide an epitome about the DEAs to CO2. Second, our development on the experimental techniques is described, in particular, on the high-resolution velocity map imaging apparatus in which we put a lot of efforts during the past two years. Third, our findings about the DEA dynamics of CO2 are surveyed and briefly compared with the others’ work. At last, we give a perspective about the applications of the DEA studies and highlight the inspirations in the production of molecular oxygen on Mars and the catalytic transformations of CO2.
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Si (111) electrode has been widely used in electrochemical and photoelectrochemical studies. The potential dependent measurements of the second harmonic generation (SHG) were performed to study Si (111) electrolyte interface. At different azimuthal angles of the Si (111) and under different polarization combinations, the curve of the intensity of SHG with extern potential have different form of line or parabola. A quantitative analysis showed that this differences of the potential-dependence can be explained by the isotropic and anisotropic contribution of the Si (111) electrode. The change in isotropic and anisotropic contribution of the Si (111) electrode may be attributed to the increase in doping concentration of Si (111) electrodes.
Reactions of gas-phase species with small molecules are being actively studied to understand the elementary steps and mechanistic details of related condensed-phase processes. Activation of the very inert N≡N triple bond of dinitrogen molecule by isolated gas-phase species has attracted considerable interest in the past few decades. Apart from molecular adsorption and dissociative adsorption, interesting processes such as C–N coupling and degenerate ligand exchange were discovered. The present review article focuses on the recent progress on adsorption, activation, and functionalization of N2 by gas-phase species (particularly metal cluster ions) using mass spectrometry, infrared photo-dissociation spectroscopy, anion photoelectron spectroscopy, and quantum chemical calculations including density functional theory and high-level ab-initio calculations. Recent advances including characterization of adsorption products, dependence of clusters' reactivity on their sizes and structures, and mechanisms of N≡N weakening and splitting have been emphasized and prospects have been discussed.
Defect-mediated processes in two-dimensional transition metal dichalcogenides have a significant influence on their carrier dynamics and transport properties, however, the detailed mechanisms remain poorly understood. Here, we present a comprehensive ultrafast study on defect-mediated carrier dynamics in ion exchange prepared few-layer MoS<sub>2</sub> by femtosecond time-resolved Vis-NIR-MIR spectroscopy. The broadband photobleaching feature observed in the near-infrared transient spectrum discloses that the mid-gap defect states are widely distributed in few-layer MoS<sub>2</sub> nanosheets. The processes of fast trapping of carriers by defect states and the following nonradiative recombination of trapped carriers are clearly revealed, demonstrating the mid-gap defect states play a significant role in the photoinduced carrier dynamics. The positive to negative crossover of the signal observed in the mid-infrared transient spectrum further uncovers some occupied shallow defect states distributed at less than 0.24 eV below the conduction band minimum. These defect states can act as effective carrier trap centers to assist the nonradiative recombination of photo-induced carriers in few-layer MoS<sub>2</sub> on the picosecond time scale.
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Silicon bulk etching is an important part of micro-electro-mechanical system (MEMS) technology. In this work, a novel etching method is proposed based on the vapor from TMAH solution heated up to boiling point. The monocrystalline silicon wafer is positioned over the solution surface and can be anisotropically etched by the produced vapor. This etching method does not rely on the expensive vacuum equipment used in dry etching. Meanwhile, it presents several advantages like low roughness, high etching rate and high uniformity compared with the conventional wet etching methods. The etching rate and roughness can reach 2.13 μm/min and 1.02 nm, respectively. To our knowledge, this rate is the highest record for the wet etching based on TMAH. Furthermore, the diaphragm structure and Al-based pattern on the non-etched side of wafer can maintain intact without any damage during the back-cavity fabrication. Finally, an etching mechanism has been proposed to illustrate the observed experimental phenomenon. It is suggested that there is a water thin film on the etched surface during the solution evaporation. It is in this water layer that the ionization and etching reaction of TMAH proceed, facilitating the desorption of hydrogen bubble and the enhancement of molecular exchange rate.
Methyl vinyl ketone oxide (MVCI), an unsaturated four-carbon Criegee intermediate produced from the ozonolysis of isoprene has been recognized to play a key role in determining the tropospheric OH concentration. It exists in four configurations (anti_anti, anti_syn, syn_anti and syn_syn) due to two different substituents of saturated methyl and unsaturated vinyl groups. In this study, we have carried out the electronic structure calculation at the multi-configurational CASSCF and multi-state MS-CASPT2 levels, as well as the trajectory surface-hopping (TSH) nonadiabatic dynamics simulation at the CASSCF level to reveal the different fates of syn/anti configurations in photochemical process. Our results show that the dominant channel for the S1-state decay is a ring closure, isomerization to dioxirane, during which, the syn(C-O) configurations with an intramolecular hydrogen bond show slower nonadiabatic photoisomerization. More importantly, it has been found for the first time in photochemistry of Criegee intermediate that the cooperation of two heavy groups (methyl and vinyl) leads to an evident pyramidalization of C3 atom in MVCI, which then results in two structurally-independent minimal-energy crossing points (CIs) towards the syn(C-O) and anti(C-O) sides, respectively. The preference of surface hopping for a certain CI is responsible for the different dynamics of each configuration.
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The photodissociation dynamics of AlO at 193 nm is studied using time-sliced ion velocity mapping. Two dissociation channels are found through the speed and angular distributions of aluminum ions: one is one photon dissociation of the neutral AlO to generate Al(2Pu) + O(3Pg), and the other is two-photon ionization and then dissociation of AlO+ to generate Al+(1Sg) + O(3Pg). Each dissociation channel includes the contribution of AlO in the vibrational states v = 0-2. The anisotropy parameter of the neutral dissociation channel is more dependent on the vibration state of AlO than the ion dissociation channel.
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We report a study on photo-ionization of benzene and aniline with incidental subsequent dissociation by the customized reflection time-of-flight mass spectrometer utilizing a deep ultraviolet (DUV) 177.3 nm laser. Highly efficient ionization of benzene is observed with a weak C4H3+ fragment formed by undergoing disproportional C−C bond dissociation. In comparison, a major C5H6+• fragment and a minor C6H6+• radical are produced in the DUV ionization of aniline pertaining to the removal of CNH* and NH* radicals, respectively. First-principles calculation is employed to reveal the photo-dissociation pathways of these two molecules having a structural difference of just an amino group. It is demonstrated that hydrogen atom transfer (HAT) plays an important role in the cleavage of C−C or C−N bonds in benzene and aniline ions. This study helps understand the underlying mechanisms of chemical bond fracture of benzene ring and related aromatic molecules.
Recent experiments report the rotation of FA (FA= HC[NH2]2+) cations significantly influence the excited-state lifetime of FAPbI3. However, the underlying mechanism remains unclear. Using ab initio nonadiabatic (NA) molecular dynamics combined with time-domain density functional simulations, we have demonstrated that eeorientation of partial FA cations significantly inhibits nonradiative electron-hole recombination with respect to the pristine FAPbI3 due to the decreased NA coupling by localizing electron and hole in different positions and the suppressed atomic motions. Slow nuclear motions simultaneously increase the decoherence time but which is overcomed by the reduced NA coupling, extending electron-hole recombination time scales to several nanoseconds and being about 3.9 times longer than that in pristine FAPbI3, which occurs within sub-nanosecond and agrees with experiment. Our study established the mechanism for the experimentally reported prolonged excited-state lifetime, providing rational strategy for design of high performance of perovskite solar cells and optoelectronic devices.
Sum frequency generation vibrational spectroscopy (SFG-VS) is a powerful technique for determining molecular structures at both buried interface and air surface. Distinguishing the contribution of SFG signals from buried interface and air surface is crucial to the applications in devices such as microelectronics and bio-tips. Here we demonstrate that the SFG spectra from buried interface and air surface can be differentiated by controlling the film thickness and employment of surface-plasmon enhancement. Using substrate-supported PMMA films as a model, we have visualized the variations in the contribution of SFG signals from buried interface and air surface. By monitoring carbonyl and C-H stretching groups, we found that SFG signals are dominated by the moieties (-CH2, -CH3, -OCH3 and C=O) segregated at the PMMA/air surface for the thin films while they are mainly contributed by the groups (-OCH3 and C=O) at the substrate/PMMA buried interface for the thick films. At the buried interface, the tilt angle of C=O decreases from 65° to 43° as the film preparation concentration increases; in contrast, the angles at the air surface fall in the range between 38° and 21°. Surface plasmon generated by gold nanorod can largely enhance SFG signals, particularly the signals from the buried interface.
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Inspired by the branching corrected surface hopping (BCSH) method [J. Xu and L. Wang, J. Chem. Phys. 150, 164101 (2019)], we present two new decoherence time formulas for trajectory surface hopping. Both the proposed linear and exponential formulas characterize the decoherence time as functions of the energy difference between adiabatic states and correctly capture the decoherence effect due to wave packet reflection as predicted by BCSH. The relevant parameters are trained in a series of 200 diverse models with different initial nuclear momenta and the exact quantum solutions are utilized as references. As demonstrated in the three standard Tully models, the two new approaches exhibit significantly higher reliability than the widely used counterpart algorithm while holding the appealing efficiency, thus promising for nonadiabatic dynamics simulations of general systems.
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A fundamental study on C?C coupling that is the crucial step in the Fischer-Tropsch synthesis (FTS) process to obtain multi-carbon products is of great importance to tailor catalysts and then guide a more promising pathway. It has been demonstrated that the coupling of CO with the metal carbide can represent the early stage in the FTS process, while the related mechanism is elusive. Herein, the reactions of the CuC3H– and CuC3– cluster anions with CO have been studied by using mass spectrometry and theoretical calculations. The experimental results showed that the coupling of CO with the C3H– moiety of CuC3H– can generate the exclusive ion product COC3H–. The reactivity and selectivity of this reaction are greatly higher than that on the reaction of CuC3– with CO, and this H-assisted C?C coupling process was rationalized by theoretical calculations.
Photo-induced proton coupled electron transfer (PCET) is essential in the biological, photosynthesis, catalysis and solar energy conversion processes. Recently, p-nitrophenylphenol (HO-Bp-NO<sub>2</sub>) has been used as a model compound to study the photo-induced PCET mechanism using ultrafast spectroscopy. In transient absorption spectra both singlet and triplet exhibited PCET behavior. When we focused on the PCET in the triplet state, a new sharp band attracted us. This band had not been observed for p-nitrobiphenyl which is without hydroxyl substitution. To assign the new band, acidic solutions were used as an additional proton donor. Based on results in strong (~10<sup>-1</sup> M) and weak (~10<sup>-4</sup> M) acidic solutions, the new band is identified as the open shell singlet O-Bp-NO<sub>2</sub>H, which is generated through protonation of nitro O in <sup>3</sup>HO-Bp-NO<sub>2</sub> followed by deprotonation of hydroxyl. Kinetics analysis indicates the formation of radical •O-Bp-NO<sub>2</sub> competes with O-Bp-NO<sub>2</sub>H in the way of concerted electron-proton transfer and/or proton followed electron transfers and is responsible for the low yield of O-Bp-NO<sub>2</sub>H. The results in the present work will make it clear that how the <sup>3</sup>HO-Bp-NO<sub>2</sub> deactivates in aprotic polar solvents and provide a solid benchmark for the deeply studying the PCET mechanism in triplets of analogous aromatic nitro compound.
In recent decades, materials science has experienced rapid development and posed increasingly high requirements for the characterizations of structures, properties, and performances. Herein, we report on our recent establishment of a multi-domain (energy, space, time) high-resolution platform for integrated spectroscopy and microscopy characterizations, offering an unprecedented way to analyze materials in terms of spectral (energy) and spatial mapping as well as temporal evolution. We present several proof-of-principle results collected on this platform, including in-situ Raman imaging (high-resolution Raman, polarization Raman, low-wavenumber Raman), time-resolved photoluminescence imaging, and photoelectrical performance imaging. It can be envisioned that our newly established platform would be very powerful and effective in the multi-domain high-resolution characterizations of various materials of photoelectrochemical importance in the near future.
Hydrogels show versatile properties and are of great interest in the fields of bioelectronics and tissue engineering. Understanding the dynamics of the water molecules trapped in the three-dimensional polymeric networks of the hydrogels is crucial for us to elucidate their mechanical and swelling properties at the molecular level. In this report, the poly(DMAEMA-co-AA) hydrogels were synthesized and characterized by the macroscopic swelling measurements under different pH conditions. Furthermore, the microscopic structural dynamics of pH stimulus responsive hydrogels were studied using FTIR and ultrafast IR spectroscopies from the viewpoint of the SCN-anionic solute as the local vibrational reporter. Ultrafast IR spectroscopic measurements showed the time constants of the vibrational population decay of SCN- were increased from 14±1 ps to 20±1 ps when the pH of the hydrogels is varied from 2.0 to 12.0. Rotational anisotropy measurements further revealed that the rotation of SCN- anionic probe was restricted by the three-dimensional network formed in the hydrogels and the rotation of SCN- anionic probe can’t decay to zero especially at the pH of 7.0. These results presented in this study are expected to provide molecular level understanding of the microscopic structure of the cross-linked polymeric network in the pH stimulus-responsive hydrogels.
Among various photocatalytic materials, Z-scheme photocatalysts have drawn tremendous research interest due to the high photocatalytic performance in solar water splitting. Here, we perform extensive hybrid density functional theory calculations to explore electronic structures, interfacial charge transfer, electrostatic potential profile, optical absorption properties, and photocatalytic properties of a proposed two-dimensional small-lattice-mismatched GaTe/Bi2Se3 heterostructure. Theoretical results clearly reveal that the examined heterostructure with a small direct band gap can effectively harvest the broad spectrum of the incoming sunlight. Due to the relative strong interfacial built-in electric field in the heterostructure and the small band gap between the valence band maximum of GaTe monolayer and the conduction band minimum of Bi2Se3 nanosheet with slight band edge bending, the photogenerated carriers transfer via Z-scheme pathway, which results in the photogenerated electrons and holes effectively separating into the GaTe monolayer and the Bi2Se3 nanosheet for the hydrogen and oxygen evolution reactions, respectively. Our results imply that the artificial 2D GaTe/Bi2Se3 is a promising Z-scheme photocatalyst for overall solar water splitting.
The IRMPD spectrum of the protonated heterodimer of ProPheH+, in the range of 2700-3700 cm-1, has been obtained with a Fourier-transform ion cyclotron mass spectrometer combined with an IR OPO laser. The experimental spectrum shows one peak at 3560 cm?1 corresponding to the free carboxyl O-H stretching vibration, and two broad peaks centered at 2935 and 3195 cm?1. Theoretical calculations were performed on the level of M062X/6-311++G(d,p). Results show that the most stable isomer is characterized by a charge-solvated structure in which the proton is bound to the unit of Pro. Its predicted spectrum is in good agreement with the experimental one, although the coexistence of salt-bridged structures cannot be entirely excluded.
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A Mn3O4 coating is approved to modify the surface of LiNi0.5Mn1.5O4 particles by a simple wet grinding method for the first time, which realize an great improvement in electronic conductivity from 1.53?10-7 S cm-1 to 3.15?10-5 S cm-1 after 2.6% Mn3O4 coating. The electrochemical test resualts demonstrate that the Mn3O4 coating dramatically enhances both the rate performance and cycling capability (at 55 °C) of LiNi0.5Mn1.5O4. Among the samples, 2.6% Mn3O4-coated LiNi0.5Mn1.5O4 not only exhibits excellent rate capability (a large capacity of 108 mAh g-1 at 10 C rate) but also keep 78% capacity retention at 55 °C and 1 C rate after 100 cycles.
Formaldehyde and hydrogen peroxide are two important realistic molecules in atmospheric chemistry. We implement path integral Liouville dynamics (PILD) to calculate the dipole-derivative autocorrelation function for obtaining the infrared (IR) spectrum. In comparison to exact vibrational frequencies, PILD faithfully capture most nuclear quantum effects in vibrational dynamics as temperature changes and as the isotopic substitution occurs.
Two non-ionic hydro-fluorocarbon hybrid surfactants with and without hydroxyl groups were synthesized and compared. They exhibited good thermal stability and superior surface activity. It was observed that the hydroxyl group had a profound effect on modifying the surface tension of their solutions and the morphology of the formed micelles. This effect may be attributed to the rearranging of the alkane group from above the air/aqueous surface to below it and the disrupting of the interfacial water structure induced by the hydroxyl groups. This work provides a strategy to weaken the immiscibility between hydrocarbon and fluorocarbon chains by modifying their orientational structure at the interface, thus is helpful for the design of surfactants with varied interfacial properties.
Empirical potential structure refinement (EPSR) is a neutron scattering data analysis algorithm and a software package. It was developed by the British spallation neutron source (ISIS) Disordered Materials Group in 1980s, and aims to construct the most-probable atomic structures of disordered materials in the field of chemical physics. It has been extensively used during the past decades, and has generated reliable results. However, it implements a shared-memory architecture with Open Multi-Processing (OpenMP). With the extensive construction of supercomputer clusters and the widespread use of graphics processing unit (GPU) acceleration technology, it is now possible to rebuild the EPSR with these techniques in the effort to improve its calculation speed. In this study, an open source framework NeuDATool is proposed. It is programmed in the object-oriented language C++, can be paralleled across nodes within a computer cluster, and supports GPU acceleration. The performance of NeuDATool has been tested with water and amorphous silica neutron scattering data. The test shows that the software can reconstruct the correct microstructure of the samples, and the calculation speed with GPU acceleration can increase by more than 400 times, compared with CPU serial algorithm at a simulation box that consists about 100 thousand atoms. NeuDATool provides another choice to implement simulation in the (neutron) diffraction community, especially for experts who are familiar with C++ programming and want to define specific algorithms for their analysis.
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In this paper, the effect of channel length and width on the large and small-signal parameters of the Graphene Field Effect Transistor (GFET) have been explored using an analytical approach. In the case of faster saturation as well as extremely high transit frequency GFET shows outstanding performance. From the transfer curve, it is observed that there is a positive shift of Dirac point from the voltage of 0.15 V to 0.35 V because of reducing channel length from 440 nm to 20 nm and this curve depicts that graphene shows ambipolar behavior. Besides, it is found that because of widening channel the drain current increases and the maximum current is found approximately 2.4 mA and 6 mA for channel width 2μm and 5μm respectively. Furthermore, an approximate symmetrical capacitance-voltage (C–V) characteristic of GFET is obtained and found that capacitance reduces when the channel length decreases but the capacitance can be increased by raising the channel width. In addition, a high transconductance of 6.4 mS at channel length 20 nm and 4.45 mS at channel width 5 μm along with a high transit frequency of 3.95 THz has been found that demands high-speed radio frequency (RF) applications.
Two thin-film 2D organic–inorganic hybrid perovskites, i.e., 2-phenylethylammonium lead iodide (PEPI) and 4-phenyl-1-butylammonium lead iodide (PBPI) were synthesized and investigated by steady-state absorption, temperature-dependent photoluminescence, and temperature-dependent ultrafast transient absorption spectroscopy. PBPI has a longer organic chain (via introducing extra ethyl groups) than PEPI, thus its inorganic skeleton can be distorted bringing on structural disorder. The comparative analyses of spectral profiles and temporal dynamics revealed that the greater structural disorder in PBPI results in more defect states serving as trap states to promote exciton dynamics. In addition, the fine-structuring of excitonic resonances was unveiled by temperature-dependent ultrafast spectroscopy, suggesting its correlation with inorganic skeleton rather than organic chain. Moreover, the photoexcited coherent phonons were observed in both PEPI and PBPI, pointing to a subtle impact of structural disorder on the low-frequency Raman-active vibrations of inorganic skeleton. This work provides valuable insights into the optical properties, excitonic behaviors and dynamics, as well as coherent phonon effects in 2D hybrid perovskites.
In this study, we have developed a high-sensitivity, near-infrared photodetector (NIRPD)
based on PdSe2/GaAs heterojunction, which was made by transferring a multilayered PdSe2
film onto a planar GaAs. The as-fabricated PdSe2/GaAs heterojunction device exhibited
obvious photovoltaic behavior to 808 nm illumination, indicating that the NIRPD can be
used as a self-driven device without external power supply. Further device analysis showed
that the hybrid heterojunction exhibited a high on/off ratio ratio of 1.16×105 measured at
808 nm under zero bias voltage. The responsivity and specific detectivity of photodetector
were estimated to be 171.34 mA/W and 2.36×1011 Jones, respectively. Moreover, the device showed excellent stability and reliable repeatability. After 2 months, the photoelectric
characteristics of the NIRPD hardly degrade in air, attributable to the good stability of the
PdSe2. Finally, the PdSe2/GaAs-based heterojunction device can also function as a NIR
light sensor.
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Over the last few years, machine learning is gradually becoming an essential approach for the investigation of heterogeneous catalysis. As one of the important catalysts, binary alloys have attracted extensive attention for the screening of bifunctional catalysts. Here we present a holistic framework for machine learning approach to rapidly predict adsorption energies on the surfaces of metals and binary alloys. We evaluate different machine-learning methods to understand their applicability to the problem and combine a tree-ensemble method with a compressed-sensing method to construct decision trees for about 60,000 adsorption data. Compared to linear scaling relations, our approach enables to make more accurate predictions lowering predictive root-mean-square error by a factor of two and more general to predict adsorption energies of various adsorbates on thousands of binary alloys surfaces, thus paving the way for the discovery of novel bimetallic catalysts.
The simple homodinuclear M-M single bonds for Group II and XII elements are difficult to obtain as a result of the fulfilled s2 electronic configurations, consequently, a dicationic prototype is often utilized to design the M+-M+ single bond. Existing studies generally use sterically bulky organic ligands L- to synthesize the compounds in an L--M+-M+-L- manner. However, here we report the design of Mg-Mg and Zn-Zn single bonds in two ligandless clusters, Mg2B7- and Zn2B7-, using density functional theory methods. The global minima of both of the clusters are in the form of M22+(B73-), where the M-M single bonds are positioned above a quasi-planar hexagonal B7 moiety. Chemical bonding analyses further confirm the existence of Mg-Mg and Zn-Zn single bonds in these clusters, which are driven by the unusually stable B73- moiety that is both σ and π aromatic. Vertical detachment energies of Mg2B7- and Zn2B7- are calculated to be 2.79 eV and 2.94 eV, respectively, for the future comparisons with experimental data.
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We carried out first-principles calculations to investigate the electronic properties of the monolayer blue phosphorene (BlueP) decorated by the group-IVB transition-metal adatoms (Cr, Mo and W), and found that the Cr-decorated BlueP is a magnetic half metal, while the Mo- andW-decorated BlueP are semiconductors with band gaps smaller than 0.2 eV. Compressive biaxial strains make the band gaps close and reopen and band inversions occur during this process, which induces topological transitions in the Mo-decorated BlueP (with strain of ??5:75%) and W-decorated BlueP (with strain of ??4:25%) from normal insulators to topological insulators (TIs). The TI gap is 94 meV for the Mo-decorated BlueP and 218 meV for the W-decorated BlueP. Such large TI gaps demonstrate the possibility to engineer topological phases in the monolayer BlueP with transition-metal adatoms at high temperature.
We constructed two types of copper-doped metal–organic framework (MOF), i.e., Cu@UiO-66-NH2 and Cu-UiO-66-NH2. In the former, Cu2+ ions are impregnated in the pore space of the amine-functionalized, Zr-based UiO-66-NH2; while in the latter, Cu2+ ions are incorporated to form a bimetal-center MOF with Zr4+ being partially replaced by Cu2+ in the Zr–O oxo-clusters. Ultrafast spectroscopy revealed that the photoinduced relaxation kinetics associated with the ligand-to-cluster charge-transfer state are promoted for both Cu-doped MOFs relative to undoped one, but in a sequence of Cu-UiO-66-NH2 > Cu@UiO-66-NH2 > UiO-66-NH2. Such a sequence turned to be in line with the trend observed in the visible-light photocatalytic hydrogen evolution activity tests on the three MOFs. These findings highlighted the subtle effect of copper-doping location in this Zr-based MOF system, further suggesting that rational engineering of the specific metal-doping location in alike MOF systems to promote the photoinduced charge separation and hence suppress the detrimental charge recombination therein is beneficial for achieving improved performances in MOF-based photocatalysis.
In this work, p-type Co3O4 decorated n-type ZnO (Co3O4/ZnO) nanocomposite was designed with the assistance of bacterial cellulose template. Phase composition, morphology and element distribution were investigated by XRD, SEM, HRTEM, EDS mapping and XPS. Volatile organic compounds (VOCs) sensing measurements indicated a noticeable improvement of response and decrease of working temperature for Co3O4/ZnO sensor, in comparison with pure ZnO, i.e., the response towards 100 ppm acetone was 63.7 (at a low working temperature of 180 °C), which was 26 times higher than pure ZnO (response of 2.3, at 240 °C). Excellent VOCs response characteristics could be ascribed to increased surface oxygen vacancy concentration (revealed by defect characterizations), catalytic activity of Co3O4 and the special p-n heterojunction structure, and bacterial cellulose provides a facile template for designing diverse functional heterojunctions for VOCs detection and other applications.
The special mass shift coefficient, ΔKSMS, and field parameter factor, Ful of four multiples, 3〖s 〗^4 P→3〖p 〗^4 P^°, 3〖s 〗^4 P→3〖p 〗^4 D^°, 3〖s 〗^2 D→5〖p 〗^2 D^°, and 3〖s 〗^2 P→3〖p 〗^2 P^°, of 14N and 15N were studied using the multi-configuration Dirac–Hartree–Fock method and the relativistic configuration interaction approach. The normal mass shift, special mass shift, field shift, and isotope shift of N I were derived from the theoretical calculated ΔKSMS, ΔKSMS and Ful, and compared with the reported experimental measurements and theoretical results.
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The structures and electronic properties of the gaseous M2Pt20/? clusters (M represents the alkaline earth metal) are investigated using the density functional theory (B3LYP and PBE0) and wave function theory (SCS-MP2, CCSD and CCSD (T)). The results show that the D2h isomers with the planar structures are more stable than the C2V isomers with smaller dihedral angles and shorter Pt-Pt bond lengths. In this work we show that the mutual competition of M(s, p)-Pt(5d) interaction and Pt-Pt covalent bonding contributes to the different stabilizations of the two kinds of isomers. The M(s, p)-Pt(5d) interaction favors the planar isomers with D2h symmetry, while the Pt-Pt covalent bonding leads to the C2V isomers with bending structures. Two different crossing points are determined in the potential energy curves of Be2Pt2 with the singlet and triplet states. But there is just one crossing point in potential energy curves of Ra2Pt2 and Ca2Pt2? because of flatter potential energy curves of Ra2Pt2 with the triplet state or Ca2Pt2? with quartet state. The results reveal a unique example of dihedral angle-bending isomers with the smallest number of atoms and may help the understanding of the bonding properties of other potential angle-bending isomers.
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From the organization of animal ocks to the emergence of swarming behav- iors in bacterial suspension, populations of motile organisms at all scales display coherent collective motion. Recent studies showed the anisotropic interaction between the active particles plays a key role on the phase behaviors. Here we investigate the collective behaviors of active Janus particles that experience an anisotropic interaction that is opposite to the active force by using Langevin dynamics simulations in two dimensional space. Interestingly, the system shows emergence of collective swarming states upon increasing the total area fraction of particles, which is not observed for systems without anisotropic interaction or activity. The threshold value of area fraction c decreases with particle ac- tivity or interaction strength. We have also performed basic kinetic analysis to reproduce the essential features of the simulation results. Our results demon- strate that anisotropic interactions at the individual level are sucient to set homogeneous active populations into stable directed motion.
Photocatalytic degradation of organic pollutants has become a hot research topic because of its low energy consumption and environmental-friendly characteristics. Bismuth oxide (Bi2O3) nanocrystals with a bandgap ranging between 2.0-2.8 eV has attracted increasing attention due to high activity of photodegradation of organic pollutants by utilizing visible light. Though several methods have been developed to prepare Bi2O3-based semiconductor materials over recent years, it is still difficult to prepare highly active Bi2O3 catalysts in large-scale with a simple method. Therefore, developing simple and feasible methods for the preparation of Bi2O3 nanocrystals in large-scale is important for the potential applications in industrial wastewater treatment. In this work, we successfully prepared porous Bi2O3 in large scale via etching commercial BiSn powders, followed by thermal treatment with air. The acquired porous Bi2O3 exhibited excellent activity and stability in photocatalytic degradation of methylene blue (MB). Further investigation of the mechanism witnessed that the suitable band structure of porous Bi2O3 allowed the generation of reactive oxygen species, such as O2-? and ?OH, which effectively degraded MB.
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The structure-property relationship of DAE-derivative (C5F-4Py) molecular isomers which involve ring-closed status and ring-open status is investigated by employing non-equilibrium Green’s function formalism combined with density functional theory. Molecular junctions are formed by the isomers connecting to Au (111) electrodes through the flanked pyridine groups. The difference of electronic structures caused by different geometry structures for the two isomers, especially the alternative single bond and double bond in ring-closed molecule, contributes the remarkable different low-bias conductance values. The LUMO orbitals of isomers are mainly channels to transport electron. In addition, the more electrons transferred to ring-closed molecular junction in equilibrium condition drop down the LUMO orbitals closer to the Fermi energy which may be to contribute larger conductance value at Fermi level. Our findings are help to understand the mechanism of the low-bias conducting mechanism of and are conductive to design of high performance molecular switching based on DAE or DAE-derivatives molecules.
Multinanoparticles interacting with the phospholipid membranes in solution were studied by dissipative particle dynamics simulation. The nanoparticles selected have spherical or cylindrical shapes, and they have various initial velocities in the dynamical processes. Several translocation modes are defined according to their characteristics in the dynamical processes, in which the phase diagrams are constructed based on the interaction strengths between the particles and membranes and the initial velocities of particles. Furthermore, several parameters, such as the system energy and radius of gyration, are investigated in the dynamical processes for the various translocation modes. Results elucidate the effects of multiparticles interacting with the membranes in the biological processes.
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Surface passivation is one valuable approach to tune the properties of nanomaterials. The piezo- electric properties of hexagonal [001] ZnO nanowires with four kinds of surface passivations were investigated using the rst-principles calculations. It is found that in the 50% H(O), 50% Cl(Zn); 50% H(O), 50% F(Zn) passivations, the volume and surface e ects both enhance the piezoelectric coecient. This di ers from the unpassivated cases where the surface e ect was the sole source of piezoelectric enhancement. In the 100% H; 100% Cl passivations, the piezoelectric enhancement is not possible since the surface e ect is screened by surface charge with weak polarization. The study reveals that the competition between the volume e ect and surface e ect in uences the iden- ti cation of the diameter-dependence phenomenon of piezoelectric coecients for ZnO nanowires in experiments. Moreover, the results suggest that one e ective means of improving piezoelectricity of ZnO nanowires is shrinking axial lattice or increasing surface polarization through passivation.
A distributed feedback (DFB) laser with a wavelength of 2.8 m was used to measure the species produced by water vapor glow discharge. Only the absorption spectra of OH radicals and transient H2O molecules were observed using concentration modulation (CM) spectroscopy. The intensities and orientations of the absorption peaks change with the demodulation phase, but the direction of one absorption peak of H2O is always opposite to the other peaks. The different spectral orientations of OH and H2O reflect the increase or decrease of the number of particles in the energy levels. If more transient species can be detected in the discharge process, the dynamics of excitation, ionization and decomposition of H2O can be better studied. This study shows that the demodulation phase relationship of CM spectrum can be used to study the population change of molecular energy levels.
A new kind of phenyl-functionalized magnetic fibrous mesoporous silica (Fe3O4@SiO2@KCC-1-phenyl) was prepared by copolymerization as an efficient adsorbent for the magnetic extraction of phthalate esters from environmental water samples. The obtained Fe3O4@SiO2@KCC-1-phenyl showed monodisperse fibrous spherical morphology, fairly strong magnetic response (29 emu g–1), and an abundant π-electron system, which allowed rapid isolation of the Fe3O4@SiO2@KCC-1-phenyl from solutions upon applying an appropriate magnetic field. Several variables that affect the extraction efficiency of the analytes, including the type of the elution solvent, amount of adsorbent, extraction time and reusability, were investigated and optimized. Under optimum conditions, the Fe3O4@SiO2@KCC-1-phenyl was used for the extraction of four phthalate esters from environmental water samples followed by high-performance liquid chromatographic analysis. Validation experiments indicated that the developed method presented good linearity (0.1–20 ng mL-1), low limit of detection (7.5-29 μg L–1, S/N=3). The proposed method was applied to the determination of phthalate esters in different real water samples, with relative recoveries of 93-103.4% and RSDs of 0.8–8.3 %.
Developing low-cost and high-efficient noble-metal-free cocatalysts has been a challenge to achieve economic hydrogen production. In this work, molybdenum oxides (MoO3-x) were in-situ loaded on polymer carbon nitride (PCN) via a simple one-pot impregnation-calcination approach. Different from post-impregnation method, intimate coupling interface between high-dispersed ultra-small MoO3-x nanocrystal and PCN was successfully formed during the in-situ growth process. The MoO3-x-PCN-x photocatalyst without noble platinum (Pt) finally exhibited enhanced photocatalytic hydrogen performance under visible light irradiation (λ>420 nm), with the highest hydrogen evolution rate of 15.6 μmol/h, which was more than 3 times that of bulk PCN. Detailed structure-performance revealed that such improvement in visible-light hydrogen production activity originated from the intimate interfacial interaction between high-dispersed ultra-small MoO3-x nanocrystal and polymer carbon nitride as well as efficient charge carriers transfer brought by Schottky junction formed.
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Highly luminescent bulk two-dimensional covalent organic frameworks (COFs) attract much attention recently. Origin of their luminescence and their large Stokes shift is an open question. After first-principles calculations on two kinds of COFs using the GW method and Bethe-Salpeter equation, we find that monolayer COF has a direct band gap, while bulk COF is an indirect band-gap material. The calculated optical gap and optical absorption spectrum for the direct excitons of bulk COF agree with the experiment. However, calculated energy of the indirect exciton, in which the photoelectron and the hole locate at the conduction band minimum and the valence band maximum of bulk COF respectively, is too low compared to the fluorescence spectrum in experiment. This may exclude the possible assistance of phonons in the luminescence of bulk COF. Luminescence of bulk COF might result from exciton recombination at the defects sites. The indirect band-gap character of bulk COF originates from its AA-stacked conformation. If the conformation is changed to the AB-stacked one, the band gap of COF becomes direct which may enhance the luminescence.
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The geometric structures and vibration frequencies of para-chlorofluorobenzene (p-ClFPh) in the first excited state of neutral and ground state of cationic were investigated by resonance-enhanced multiphoton ionization (REMPI) and slow electron velocity-map imaging (SEVI). The infrared spectrum of S0 state and absorption spectrum for S1 ← S0 transition in p-ClFPh were also recorded. Based on the one-color resonant two-photon ionization (1C-R2PI) spectrum and two-color resonant two-photon ionization (2C-R2PI) spectrum, we measured the adiabatic excitation energy of p-ClFPh as 36302 ± 4 cm-1. In the 2C-R2PI SEVI spectra, the accurate adiabatic ionization potential (AIP) of p-ClFPh was extrapolated to be 72937 ± 8 cm?1 via a series of progressive measurements near threshold ionization region. In addition, Franck-Condon simulations were performed to help us confidently ascertain the main vibration modes in the S1 and D0 states. The wavenumber changes of vibration modes during the transition of S1 ← S0 and D0 ← S1 were discussed. Furthermore, the mixing of vibration modes both between S0 & S1 and S1 & D0 has been analyzed.
The geometric and electronic structures of several possible adsorption configurations of the pyrazine (C4H4N2) molecule covalently attached to Si(100) surface, which is of vital importance in fabricating functional nanodevices, have been investigated using X-ray spectroscopies. The Carbon K-shell (1s) X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy of predicted adsorbed structures have been simulated by density functional theory (DFT) with cluster model calculations. Both XPS and NEXAFS spectra demonstrate the structural dependence on different adsorption configurations. In contrast to the XPS spectra, it is found that the NEXAFS spectra exhibiting conspicuous dependence on the structures of all the studied pyrazine/Si(100) systems can be well utilized for structural identification, which has been discussed in detail in this article. In addition, according to the classification of carbon atoms, the spectral components of carbon atoms in different chemical environments have been investigated in the NEXAFS spectra as well.
Cancer is one of the most serious issues in human life. Blocking Programmed cell death protein 1 (PD-1) and programmed death ligand-1 (PD-L1) pathway is one of the great innovation on last few years, but a few numbers of inhibitors can be able to block it. (2-methyl-3-biphenylyl) methanol (MBPM) derivative is one of them. Here, the quantitative structure-activity relationship (QSAR) established twenty (2-methyl-3-biphenylyl) methanol (MBPM) derivatives as the programmed death ligand-1 (PD-L1) inhibitors. Density functional theory (DFT) at the B3LPY/6-31+G (d, p) level was employed to study the chemical structure and properties of the chosen compounds. Highest occupied molecular orbital energy EHOMO, lowest unoccupied molecular orbital energy ELUMO, total energy ET, dipole moment DM, absolute hardness η, absolute electronegativity χ, softness S, electrophilicity ω, energy gap ΔE, etc, properties were observed and determine. Principal component analysis (PCA), multiple linear regression (MLR) and multiple non-linear regression (MNLR) analysis were carried out to establish the QSAR. The proposed quantitative models and interpreted outcomes of the compounds were based on statistical analysis. Statistical results of MLR and MNLR exhibited the coefficient was 0.661 and 0.758, respectively. Leave-one-out cross-validation (LOO-CV), r2m metric, r2m test and “Golbraikh & Tropsha’s criteria” analyses were applied for the validation of MLR and MNLR, which indicate two models are statistically significant and well stable with data variation in the external validation towards PD-L1. The obtained values specified that the two different modelings can predict the bioactivity and may be helpful and supporting for evaluation of the biological activity of PD-L1 inhibitor.
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Zinc oxide is recently being used as magnetic semiconductor with introduction of magnetic elements in it. In this work, we report phase pure synthesis of Mg and Ni co-substituted ZnO to explore its structure, optical, magnetic and photo-catalytic properties. X-ray diffraction analysis reveal the formation of hexagonal wurtzite type structure having P63mc space group without any impurity phase. UV-Vis spectrophotometry demonstrate the variation in band gap with addition of Mg and Ni content in ZnO matrix. Magnetic measurements exhibit a clear boosted magnetization in Ni and Mg co-doped compositions with its stable value of band gap corroborating the structural stability and magnetic tuning for its advanced applications in modern day spintronic devices. Photo-catalytic measurements were performed using methyl green degradation demonstrate an enhanced trend of activity in Mg and Ni co-doped compositions.
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ATP-binding cassette (ABC) exporters transport many substrates out of cellular membranes via alternating between inward-facing (IF) and outward-facing (OF) conformations. Despite extensive research efforts over the past decades, understanding of the molecular mechanism remains elusive. As these large-scale conformational movements are global and collective, we have previously performed extensive coarse-grained molecular dynamics (CG-MD) simulations of the potential of mean force (PMF) along the conformational transition pathway [Z. Wang et. al JPCB, 119, 1295?1301 (2015)]. However, the occluded (OC) conformational state, in which both the internal and external gate are closed, was not determined in the calculated free energy profile. In this paper, we extend the above methods to the calculation of the free energy profile along the reaction coordinate, d1- d2, which are the COM distances between the two sides of the internal (d1) and the external gate (d2). The PMF is thus obtained to identify the transition pathway, along which several OF-, IF- and OC- state structures are predicted in good agreement with structural experiments. Our CG-MD free-energy simulations demonstrate that the internal gate is closed before the external gate is open during the IF to OF transition and vice versa during the IF to OF transition. Our results capture the unidirectional feature of substrate translocation via the exporter, which is functionally important in biology. This finding is different from the results, in which both the internal and external gates are open reported in an X-ray experiment [Ward A. et al, PNAS, 104, 19005?19010 (2007)]. Our study sheds light on the molecular mechanism of the state transitions in the ABC exporter.
The behavior of hydrogen production on ZnO electrode during the electrolytic reduction of water was investigated by cyclic voltammetry (CV) and cathode polarization experiments combining with in-situ Raman and photoluminescence spectroscopy. CV experiments indicate that hydrogen species prefers to diffuse into the ZnO bulk at negative potentials and occupies oxygen vacancies (Ho) and interstitial sites (Hi). Meanwhile, the H2O reduction is self-enhanced during the electroreduction process, as evidenced by the trace crossing of the CV curves and the chronoamperometric experiment. The influence of the H species on the ZnO electrode during the electrocatalytic processes was characterized by the in-situ Raman and photoluminescence spectroscopies. These results help us to understand the hydrogen-related catalytic or electrocatalytic processes on ZnO surfaces.
Accurate description of potential energy curves driven by non-bonded interactions remains a great challenge for pure density functional approximations (DFAs). It is because the R-6 decay behavior of dispersion cannot be intrinsically captured by the (semi)-local ingredients and the exact-exchange used in the popular hybrid DFAs. Overemphasizing the accuracy on the equilibrium region for the functional construction would likely deteriorate the overall performance on the other regions of potential energy surfaces. In consequence, the empirical dispersion correction becomes the standard component in DFAs to treat the non-bonded interactions. In this Letter, we demonstrate that without the use of empirical dispersion correction, doubly hybrid approximations, in particular two recently proposed revXYG3 and XYG7 functionals, hold the promise to have a balanced description of non-bonded interactions on the whole potential energy curves for several prototypes of π–π, CH/π, and SH/π interactions. The error of revXYG3 and XYG7 for non-bonded interactions is around 0.1 kcal/mol, and their potential energy curves almost coincide with the accurate CCSD(T)/CBS curves.
Single atom alloys (SAAs), composed of active metal dopants atomically dispersed on the Cu, Ag or Au host metals, have recently become a ‘rising star’ in single atom catalysis research. SAAs usually display unique catalytic behavior, mainly due to the anomalous electronic structure of isolated active sites, distinguishing from that of the parent metals. As the consequence, there is lack of robust yet reliable descriptor of catalytic properties of SAAs. In this work, we presented a systematically theoretical study on the first C-H bond activation of methane, propane and ethylbenzene over 15 SAAs comprising of Rh, Ir, Ni, Pd and Pt doping Cu(111), Ag(111) and Au(111) surfaces. Our DFT calculations demonstrated that not only the d-band centers but also the H atom adsorption energies could not correlate well with the activation barriers of alkyl C-H bond, while enhanced performance was achieved when using the reaction energy as a descriptor. We found that there existed orbital interaction similarity between C atom adsorption on top site and the transition states of C-H activation as both of them involved not only σ donation with dz2 orbital but also the π back-donation from dxy/dyz orbital(s). As the consequence, the C adsorption energies and C-H bond activation energies are very strongly correlated (R2>0.9), not only for methane but also for propane and ethylbenzene.
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The study of m-Xylene photoionization and dissociation photoionization using synchrotron vacuum ultraviolet (VUV) light and supersonic expanding molecular beam reflection time-of-flight mass spectrometer system. Photoionization efficiency curves (PIEs) of molecule ion C8H10+ and fragment ions C8H9+ and C7H7+ were observed, and the ionization energy (IE) and the appearance energies (AEs) of the fragment ions C8H10+, C8H9+ and C7H7+ are obtained to be 8.59eV, 11.74eV and 11.89eV, respectively. Optimized structures of transitional states, intermediates, and product ions were characterized at the B3LYP/6-311++ G (d, p) basis sets, and the energies were calculated using the G3 method. Based on the results, two major dissociative photoionization channels, C7H7++CH3 and C8H9++H have been calculated at the B3LYP/6-311++ G (d, p) level. With the combination of theoretical and experimental results, the dissociative photoionization pathways of m-Xylene are proposed. The predominant m-Xylene cleavage mechanism is hydrogen migration.
Au nanowires in 4H crystalline phase (4H NWs) are synthesized by colloid solution methods. The crystalline phase and surface structure as well as its performance toward electrochemical oxidation of CO before and after removing adsorbed Oleylamine molecules (OAs) introduced from its synthesis are evaluated by high-resolution transmission electron microscopy (HR-TEM), X-Ray Diffraction (XRD), underpotential deposition of Pb (Pb-upd) and cyclic voltammetry. Different methods, i.e. acetic acid cleaning, electrochemical oxidation cleaning, and diethylamine replacement, have been tried to remove the adsorbed OAs. For all methods, upon the removal of the adsorbed OAs, the morphology of 4H gold nanoparticles are found to gradually change from nanowires to large dumbbell-shaped nanoparticles, accompanying with a transition from the 4H phase to the face-centered cubic phase. On the other hand, the Pb-upd results show that the sample surfaces have almost the same facet composition before and after removal of the adsorbed OAs. After electrochemical cleaning with continuous potential scans up to 1.3 V, CO electro-oxidation activity of the 4H Au sample is significantly improved. The CO electro-oxidation activity is compared with results on the three basel Au single crystalline surfaces reported in the literature, possible origins for its enhancement are discussed.
The high reaction barrier of the oxygen evolution reaction (OER) has always been the bottleneck of the water decomposition reaction, low-cost, high-performance and stable catalysts are urgently needed currently. Herein, we designed an effective OER electrocatalyst BaCo0.6Fe0.2Ni0.2O3-δ (BCFN) by co-doping strategy. The overpotential of BCFN at a current density of 10 mA cm-2 reaches 310 mV, and possesses a Tafel slope of 50.2 mV dec-1. The catalytic capability of BCFN is much stronger than that of Fe-doped BaCo0.8Fe0.2O3-δ (BCF 360 mV), Ni-doped BaCo0.8Ni0.2O3-δ (BCN 375 mV) and benchmark for performance excellence IrO2 (329 mV). At the same time, BCFN is also a fairly stable alkaline OER catalyst. After 500-cycle scans, BCFN still shows high catalytic activity with no significant decrease in catalytic performance. Electrochemical experiments show that BCFN has the fastest reaction kinetics and the lowest charge transfer resistance among the materials in our study. In addition, a large amount of highly oxidative oxygen O22-/O- and hydroxyl groups OH- on the surface of BCFN are conducive to the occurrence of OER, thereby increasing the reaction rate. This work provides a universal strategy to develop high-performance electrocatalysts for electrochemical energy conversion technology.
Molecular ferroelectrics is a promising class of ferroelectrics, with environmental friendliness, flexibility and low cost. In this work, a set of characteristic molecular ferroelectrics were simulated by molecular dynamics (MD) with polarized crystal charge (PCC). From the simulated results, their ferroelectric switching mechanisms are elucidated, with their ferroelectric hysteresis loops. The PCC charge model, recently developed by our group, contains the quantum electric polarization effect, is suitable in nature for studying molecular ferroelectrics. The simulated systems include the typical molecular ionic ferroelectrics, di-isopropyl-ammonium halide (DIPAX, X = C (Cl), B (Br), and I), as well as a pair of newly validated organic molecular ferroelectrics, salicylideneaniline and (-)-camphanic acid. In total, there are five systems under investigation. Results demonstrate that the PCC MD method is efficient and reliable. It not only elucidates the ferroelectric switching mechanism of the studied molecular ferroelectrics, but also extends the application range of the PCC MD. In conclude, PCC MD provides an efficient protocol for extensive computer simulations of molecular ferroelectrics, with reliable ferroelectric properties and associated mechanisms, and would promote further exploration of novel molecular ferroelectrics.
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It is generally believed that the non-resonant tunneling decaying
through a molecular junction is exponentially dependent on the
properties of molecule and its contact to electrodes. Here we study
the dependence of electrode-materials on the transport for a
molecular junction with different graphene nanoribbon (GNR)
electrodes, in which the molecule consists of one, two, and three
phenyls are considered, respectively. By the \emph{ab initio}
calculations, we demonstrate that the current via a zigzag-edge GNR
(ZGNR)-electrode junction reaches to the magnitude of $\mu$A with a
convex-arc-shaped dependence under low bias, which is different from
that via the Au-electrode similar junction ($\sim$nA) with a linear
relationship over the same bias scope. Interestingly, the current
drops to $\sim$pA with a nonlinear dependence when the electrodes
are replaced by the same width conducting armchair-edge GNRs (AGNRs)
contacting the molecule at the insulating carbon-dimer chains, but
when the connection is moved to a conducting chain of AGNR the
current increases up to $\sim$nA. Further, the current through a
junction with chiral GNR-electrodes (more general case) is of
different orders of magnitude (few to hundreds of nA), but show the
similar \emph{I-V} characteristics to that for AGNR-electrode one.
Nonetheless, for all types of GNR-electrode junctions the
relationship between the resistance and the molecular length is
always a straight line in the semilog coordinate but with different
slopes, implying different decay factors. Therefore, although the
transport exponential decaying law itself is electrode-independent,
importantly, the decay factor is sensitively dependent not only on
the nature of electrode materials but also on the morphology of
electrode-molecule coupling. Our results here have demonstrated the
generality of the transport characteristics for molecular junctions.
Crayfish shell is an abundant natural waste meanwhile a potential biosorbent for pollutants if the release of heavy metals can be ignored. In this study, the safety of crayfish shell as a biosorbent was first accessed by the release experiments involving primary heavy metal ions, such as Cu2+, Zn2+, and Cr3+, in aqueous solution under environmental conditions. Although the release concentrations of heavy metals were dependent on pH, ionic strength, and humic acid, the concentrations were still lower than the national standard. The removal of Cu2+ and Pb2+ by crayfish shell in synthetic wastewater was investigated. The results indicated that crayfish shell is an excellent biosorbent for the removal of Cu2+ and Pb2+, and the removal process involved biosorption, precipitation, and complexation. Particularly, the precipitation step is calcium species-, pH- and temperature-dependent. The maximum removal capacity of Pb2+ and Cu2+ reached 676.20 and 119.98 mg g-1, respectively. The related precipitation and complex products include Cu2CO3(OH)2, Ca2CuO3, CuCO3, Pb2CO3(OH)2, CaPb3O4, and PbCO3. This study evaluated a recyclable pathway of crayfish shell waste and provides a new explanation for the biosorption of heavy metals by crayfish shell.
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We calculate the interaction strength of the van der Waals force between two Rydberg atoms by applying the applications of quantum information processing using for Rydberg blockade. The alkali metals (Cs and K) in states of principal quantum number n were used to calculate the interaction strength. We use some possible angular momentum channels involving s, p, and d states for measuring interaction strength. The obtained results were then generalized for all angular momentum channels which mostly have small interactions and therefore a poor candidate for blockade experiments. The interaction strength in the atoms of Cs and K dipole matrix elements was calculated first and then relevant energy levels were determined by using the quantum defects theory. The radial wave function was calculated numerically with the integration of the radial Schrödinger equation. Also, the interaction coefficients of van der Waals for various channels were calculated here.
Rotating disk electrode systems are widely used to study the kinetics of electrocatalytic reactions that may suffer from insufficient mass transfer of the reactants. Kinetic current density (jk) at certain overpotential calculated by K-L equation is commonly used as the metrics to evaluate the activity of electrocatalysts. However, it is frequently found that the jl is not correctly identified in the literatures. Instead of jK, the measured current density normalized by diffusion limiting current density (j/jL) has also been frequently under circumstance where its validity is not justified. By taking ORR/HER/HOR as examples, we demonstrate that identify the actualjL for the same reaction under otherwise identical conditions from the experimental data is essential to accurately deduce jk. Our analysis reveals that j/jL is a rough activity metric which can only be used to qualitatively compare the activity trend under conditions that the mass transfer conditions and the roughness factor of the electrode are exactly the same. In addition, if one wants to use j/jL to compare the intrinsic activity, the concentration overpotential should be eliminated.
Column
2022, 35(3): 407-412.
doi: 10.1063/1674-0068/cjcp2203053
Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) relies on the central molecular machine RNA-dependent RNA polymerase (RdRp) for the viral replication and transcription. Remdesivir at the template strand has been shown to effectively inhibit the RNA synthesis in SARS-CoV-2 RdRp by deactivating not only the complementary UTP incorporation but also the next nucleotide addition. However, the underlying molecular mechanism of the second inhibitory point remains unclear. In this work, we have performed molecular dynamics simulations and demonstrated that such inhibition has not directly acted on the nucleotide addition at the active site. Instead, the translocation of Remdesivir from +1 to -1 site is hindered thermodynamically as the post-translocation state is less stable than the pre-translocation state due to the motif B residue G683. Moreover, another conserved residue S682 on motif B further hinders the dynamic translocation of Remdesivir due to the steric clash with the 1'-cyano substitution. Overall, our study has unveiled an alternative role of motif B in mediating the translocation when Remdesivir is present in the template strand and complemented our understanding about the inhibitory mechanisms exerted by Remdesivir on the RNA synthesis in SARS-CoV-2 RdRp.
The electrocatalytic carbon dioxide reduction reaction (CO2RR) producing HCOOH and CO is one of the most promising approaches for storing renewable electricity as chemical energy in fuels. SnO2 is a good catalyst for CO2-to-HCOOH or CO2-to-CO conversion, with different crystal planes participating the catalytic process. Among them, (110) surface SnO2 is very stable and easy to synthesisze. By changing the ratio of Sn: O for SnO2(110), we have two typical SnO2 thin films: fully oxidized (stoichiometric) and partially reduced. In this work, we are concerned with different metals (Fe, Co, Ni, Cu, Ru, Rh, Pd, Ag, Os, Ir, Pt, and Au)-doped SnO2(110) with different activity and selectivity for CO2RR. All these changes are manipulated by adjusting the ratio of Sn: O in (110) surface. The results show that stochiometric and reduced Cu/Ag doped SnO2(110) have different selectivity for CO2RR. More specifically, stochiometric Cu/Ag-doped SnO2(110) tends to generate CO(g). Meanwhile, the reduced surface tends to generate HCOOH(g). Moreover, we also considered the competitive hydrogen evolution reaction (HER). The catalysts SnO2(110) doped by Ru, Rh, Pd, Os, Ir, and Pt have high activity for HER, and others are good catalysts for CO2RR.
2022, 35(3): 422-430.
doi: 10.1063/1674-0068/cjcp2109163
Excited-state double proton transfer (ESDPT) is a controversial issue which has long been plagued with theoretical and experimental communities. Herein, we took 1, 8-dihydroxy-2-naphthaldehyde (DHNA) as a prototype and used combined complete active space self-consistent field (CASSCF) and multi-state complete active-space second-order perturbation (MS-CASPT2) methods to investigate ESDPT and excited-state deactivation pathways of DHNA. Three different tautomer minima of S1-ENOL, S1-KETO-1, and S1-KETO-2 and two crucial conical intersections of S1S0-KETO-1 and S1S0-KETO-2 in and between the S0 and S1 states were obtained. S1-KETO-1 and S1-KETO-2 should take responsibility for experimentally observing dual-emission bands. In addition, two-dimensional potential energy surfaces (2D-PESs) and linear interpolated internal coordinate paths connecting relevant structures were calculated at the MS-CASPT2//CASSCF level and confirmed a stepwise ESDPT mechanism. Specifically, the first proton transfer from S1-ENOL to S1-KETO-1 is barrierless, whereas the second one from S1-KETO-1 to S1-KETO-2 demands a barrier of ca. 6.0 kcal/mol. The linear interpolated internal coordinate path connecting S1-KETO-1 (S1-KETO-2) and S1S0-KETO-1 (S1S0-KETO-2) is uphill with a barrier of ca. 12.0 kcal/mol, which will trap DHNA in the S1 state while therefore enabling dual-emission bands. On the other hand, the S1/S0 conical intersections would also prompt the S1 system to decay to the S0 state, which could be to certain extent suppressed by locking the rotation of the C5$-$C8$-$C9$-$O10 dihedral angle. These mechanistic insights are not only helpful for understanding ESDPT but also useful for designing novel molecular materials with excellent photoluminescent performances.
2022, 35(3): 431-442.
doi: 10.1063/1674-0068/cjcp2110211
The product branching ratio between different products in multichannel reactions is as important as the overall rate of reaction, both in terms of practical applications (\emph{e.g}. models of combustion or atmosphere chemistry) in understanding the fundamental mechanisms of such chemical reactions. A global ground state potential energy surface for the dissociation reaction of deuterated alkyl halide CD$ _3 $CH$ _2 $F was computed at the CCSD(T)/CBS//B3LYP/aug-cc-pVDZ level of theory for all species. The decomposition of CD$ _3 $CH$ _2 $F is controversial concerning C$ - $F bond dissociation reaction and molecular (HF, DF, H$ _2 $, D$ _2 $, HD) elimination reaction. Rice-Ramsperger-Kassel-Marcus (RRKM) calculations were applied to compute the rate constants for individual reaction steps and the relative product branching ratios for the dissociation products were calculated using the steady-state approach. At the different energies studied, the RRKM method predicts that the main channel for DF or HF elimination from 1, 2-elimination of CD$ _3 $CH$ _2 $F is through a four-center transition state, whereas D$ _2 $ or H$ _2 $ elimination from 1, 1-elimination of CD$ _3 $CH$ _2 $F occurs through a direct three-center elimination. At 266, 248, and 193 nm photodissociation, the main product CD$ _2 $CH$ _2 $+DF branching ratios are computed to be 96.57%, 91.47%, and 48.52%, respectively; however, at 157 nm photodissociation, the product branching ratio is computed to be 16.11%. Based on these transition state structures and energies, the following photodissociation mechanisms are suggested: at 266, 248, 193 nm, CD$ _3 $CH$ _2 $F$ \rightarrow $absorption of a photon$ \rightarrow $TS5$ \rightarrow $the formation of the major product CD$ _2 $CH$ _2 $+DF; at 157 nm, CD$ _3 $CH$ _2 $F$ \rightarrow $absorption of a photon$ \rightarrow $D/F interchange of TS1$ \rightarrow $CDH$ _2 $CDF$ \rightarrow $H/F interchange of TS2$ \rightarrow $CHD$ _2 $CHDF$ \rightarrow $the formation of the major product CHD$ _2 $+CHDF.
2022, 35(3): 443-450.
doi: 10.1063/1674-0068/cjcp2201009
The dissociative chemisorption of N$ _2 $ is the rate-limiting step for ammonia synthesis in industry. Here, we investigated the role of initially vibrational excitation and rotational excitation of N$ _2 $ for its reactivity on the Fe(111) surface, based on a recently developed six-dimensional potential energy surface. Six-dimensional quantum dynamics study was carried out to investigate the effect of vibrational excitation for incidence energy below 1.6 eV, due to significant quantum effects for this reaction. The effects of vibrational and rotational excitations at high incidence energies were revealed by quasiclassical trajectory calculations. We found that raising the translational energy can enhance the dissociation probability to some extent, however, the vibrational excitation or rotational excitation can promote dissociation more efficiently than the same amount of translational energy. This study provides valuable insight into the mode-specific dynamics of this heavy diatom-surface reaction.
The nonadiabatic dynamics of methyl nitrate (CH$ _3 $ONO$ _2 $) is studied with the on-the-fly trajectory surface hopping dynamics at the ADC(2) level. The results confirmed the existence of the ultrafast nonadiabatic decay to the electronic ground state. When the dynamics starts from S$ _1 $ and S$ _2 $, the photoproducts are CH$ _3 $O+NO$ _2 $, consistent with previous results obtained from the experimental studies and theoretical dynamics simulations at more accurate XMS-CASPT2 level. The photolysis products are CH$ _3 $O+NO$ _2 $ at the ADC(2) level when the dynamics starts from S$ _3 $, while different photolysis products were obtained in previous experimental and theoretical works. These results demonstrate that the ADC(2) method may still be useful for treating the photolysis mechanism of CH$ _3 $ONO$ _2 $ at the long-wavelength UV excitation, while great caution should be paid due to its inaccurate performance in the description of the photolysis dynamics at the short-wavelength UV excitation. This gives valuable information to access the accuracy when other alkyl nitrates are treated at the ADC(2) level.
A new diabatic potential energy matrix (PEM) of the coupled $ ^1 $$ \pi\pi^* $ and $ ^1\pi\sigma^* $ states for the $ ^1\pi\sigma^* $-mediated photodissociation of thiophenol was constructed using a neural network (NN) approach. The diabatization of the PEM was specifically achieved by our recent method [Chin. J. Chem. Phys. 34 , 825 (2021)], which was based on adiabatic energies without the associated costly derivative couplings. The equation of motion coupled cluster with single and double excitations (EOM-CCSD) method was employed to compute adiabatic energies of two excited states in this work due to its high accuracy, simplicity, and efficiency. The PEM includes three dimensionalities, namely the S-H stretch, C-S-H bend, and C-C-S-H torsional coordinates. The root mean square errors of the NN fitting for the S$ _1 $ and S$ _2 $ states are 0.89 and 1.33 meV, respectively, suggesting the high accuracy of the NN method as expected. The calculated lifetimes of the S$ _1 $ vibronic 0$ ^0 $ and 3$ ^1 $ states are found to be in reasonably good agreement with available theoretical and experimental results, which validates the new EOM-CCSD-based PEM fitted by the NN approach. The combination of the diabatization scheme solely based on the adiabatic energies and the use of EOM-CCSD method makes the construction of reliable diabatic PEM quite simple and efficient.
Carbon capture and storage technology have been rapidly developed to reduce the carbon dioxide (CO$ _2 $) emission into the environment. It has been found that the amine-based organic molecules could absorb CO$ _2 $ efficiently and form the bicarbonate salts through hydrogen-bond (H-bond) interactions. Recently, the aqueous 1, 3-diphenylguanidine (DPG) solution was developed to trap and convert CO$ _2 $ to valuable chemicals under ambient conditions. However, how the DPG molecules interact with CO$ _2 $ in an aqueous solution remains unclear. In this work, we perform molecular dynamics simulations to explore the atomistic details of CO$ _2 $ in the aqueous DPG. The simulated results reveal that the protonated DPGH$ ^+ $ and the bicarbonate anions prefer to form complexes through different H-bond patterns. These double H-bonds are quite stable in thermodynamics, as indicated from the accurate density functional theory calculations. This study is helpful to understand the catalytic mechanism of CO$ _2 $ conversion in the aqueous DPG.
The H+CH$ _3 $OH reaction, which plays an important role in combustion and the interstellar medium, presents a prototypical system with multiple channels. In this work, mode specific dynamics of different product channels is investigated theoretically on a recently developed reliable potential energy surface based on a large number of data points calculated at the level of UCCSD(T)-F12a/AVTZ. It has been demonstrated that vibrational excitations of the O$ - $H stretching motion, the torsional motion, the C$ - $H stretching vibrations, show different influences on the four product channels, H$ _2 $+CH$ _3 $O, H$ _2 $+CH$ _2 $OH, H$ _2 $O+CH$ _3 $, and H+CH$ _3 $OH. This work is helpful for understanding the mode-specific dynamics and controlling the competition for complicated reactions with multiple product channels.
2022, 35(3): 488-498.
doi: 10.1063/1674-0068/cjcp2201023
The branching corrected surface hopping (BCSH) has been demonstrated as a robust approach to improve the performance of the traditional fewest switches surface hopping (FSSH) for nonadiabatic dynamics simulations of standard scattering problems [J. Chem. Phys. 150 , 164101 (2019)]. Here, we study how reliable populations of both adiabatic and diabatic states can be interpreted from BCSH trajectories. Using exact quantum solutions and FSSH results as references, we investigate a series of one-dimensional two-level scattering models and illustrate that excellent time-dependent populations can be obtained by BCSH. Especially, we show that different trajectory analysis strategies produce noticeable differences in different representations. Namely, the method based on active states performs better to get populations of adiabatic states, while the method based on wavefunctions produces more reliable results for populations of diabatic states.
2022, 35(3): 499-508.
doi: 10.1063/1674-0068/cjcp2203039
Based on the boron-containing thermally activated delayed fluorescence (TADF) compound p-AC (AC: acridine) 5, 9-dioxa-13b-boranaphtho [3, 2, 1-de] anthracene ( a ), a series of new TADF molecules b1 - b4 were designed via adding two nitrogen atoms at the AC donor part. Density functional theory and time-dependent density functional theory calculations were performed on the frontier orbital energy levels, emission spectra, singlet-triplet states energy gaps ($ \Delta E_{\rm{ST}} $), reverse intersystem crossing (RISC) rate constant ($ k_{\rm{RISC}} $) for compounds a and b1 - b4 . Our calculation results show that the maximum emission wavelengths of b1 - b4 are significantly blue-shifted by 47 - 125 nm compared with that of a . Molecules b1 and b3 exhibit dark-blue emission, while molecules b2 and b4 display light-blue emission, indicating that these four derivatives could be potential organic light-emitting diode (OLED) candidates with blue-light emitting. Moreover, we found the RISC processes in a , b2 , and b4 can occur not only from T1 state to S1 state, but also from T2 state to S1 state significantly, while the RISC processes in b1 and b3 mainly take place via the T2→S1 hot exciton way. Importantly, the T1→S1 $ k_{\rm{RISC}} $ values of b2 and b4 are predicted to be two to three times of that of a , indicating enhanced TADF property. Our results not only provide two promising boron-based TADF candidates ( b2 and b4 ), but also offer useful theoretical basis for the design of blue OLED materials.
Antifreeze proteins (AFPs) inhibit ice recrystallization by a mechanism remaining largely elusive. Dynamics of AFPs' hydration water and its involvement in the antifreeze activity have not been identified conclusively. We herein, by simulation and theory, examined the water reorientation dynamics in the first hydration layer of an AFP from the spruce budworm, Choristoneura fumiferana, compared with a protein cytochrome P450 (CYP). The increase of potential acceptor water molecules around donor water molecules leads to the acceleration of hydrogen bond exchange between water molecules. Therefore, the jump reorientation of water molecules around the AFP active region is accelerated. Due to the mutual coupling and excitation of hydrogen bond exchange, with the acceleration of hydrogen bond exchange, the rearrangement of the hydrogen bond network and the frame reorientation of water are accelerated. Therefore, the water reorientation dynamics of AFP is faster than that of CYP. The results of this study provide a new physical image of antifreeze protein and a new understanding of the antifreeze mechanism of antifreeze proteins.
The imaginary time path integral formalism offers a powerful numerical tool for simulating thermodynamic properties of realistic systems. We show that, when second-order and fourth-order decompositions are employed, they share a remarkable unified analytic form for the partition function of the harmonic oscillator. We are then able to obtain the expression of the thermodynamic property and the leading error terms as well. In order to obtain reasonably optimal values of the free parameters in the generalized symmetric fourth-order decomposition scheme, we eliminate the leading error terms to achieve the accuracy of desired order for the thermodynamic property of the harmonic system. Such a strategy leads to an efficient fourth-order decomposition that produces third-order accurate thermodynamic properties for general systems.
2022, 35(3): 537-541.
doi: 10.1063/1674-0068/cjcp2205090
In this work, electron energy spectroscopic mapping of surface plasmon of Ag nanostructures on highly oriented pyrolytic graphite is reported. Benefitting from the angular dispersive feature of the present scanning probe electron energy spectrometer, a multi-channel detection mode is developed. By scanning along one direction, the two-dimensional intensity distribution of Ag surface plasmon excitation due to the collision of electron emitted from the tip can be obtained in parallel. The spectroscopic spatial resolution is determined to be around 80 nm.
2022, 35(3): 542-550.
doi: 10.1063/1674-0068/cjcp2006101
OX$_2$ (X=halogen) molecules was studied theoretically. Calculation results show that delocalized $\pi_3^6$ bonds exist in their electronic structures and O atoms adopt the sp$^2$ type of hybridization, which violates the prediction of the valence shell electron pair repulsion theory of sp$^3$ type. Delocalization stabilization energy is proposed to measure the contribution of delocalized $\pi_3^6$ bond to energy decrease and proves to bring extra-stability to the molecule. These phenomena can be summarized as a kind of coordinating effect.
2022, 35(3): 551-561.
doi: 10.1063/1674-0068/cjcp2008142
We predict two novel group 14 element alloys Si$_2$Ge and SiGe$_2$ in $P$6$_2$22 phase in this work through first-principles calculations. The structures, stability, elastic anisotropy, electronic and thermodynamic properties of these two proposed alloys are investigated systematically. The proposed $P$6$_2$22-Si$_2$Ge and $P$6$_2$22-SiGe$_2$ have a hexagonal symmetry structure, and the phonon dispersion spectra and elastic constants indicate that these two alloys are dynamically and mechanically stable at ambient pressure. The elastic anisotropy properties of $P$6$_2$22-Si$_2$Ge and $P$6$_2$22-SiGe$_2$ are examined elaborately by illustrating the surface constructions of Young's modulus, the contour surfaces of shear modulus, and the directional dependence of Poisson's ratio; the differences with their corresponding group 14 element allotropes $P$6$_2$22-Si$_3$ and $P$6$_2$22-Ge$_3$ are also discussed and compared. Moreover, the Debye temperature and sound velocities are analyzed to study the thermodynamic properties of the proposed $P$6$_2$22-Si$_2$Ge and $P$6$_2$22-SiGe$_2$.
2022, 35(3): 562-569.
doi: 10.1063/1674-0068/cjcp2008149
The boom in ultra-thin electronic devices and the growing need for humanization greatly facilitated the development of wearable flexible micro-devices. But the technology to deposit electrode material on flexible substrate is still in its infancy. Herein, the flexible symmetric micro-supercapacitors made of carbon nanotubes (CNTs) on commercial printing paper as electrode materials were fabricated by combining tetrahedral preparator auxiliary coating method and laser-cutting interdigital configuration technique on a large scale. The electrochemical performance of the obtained micro-supercapacitors can be controlled and tuned by simple choosing different models of tetrahedral preparatory to obtain CNTs film of different thicknesses. As expected, the micro-supercapacitor based on CNTs film can deliver an areal capacitance up to 4.56 mF/cm$ ^2 $ at current of 0.02 mA. Even if, micro-supercapacitor undergoes continuous 10000 cycles, the performance of device can still remain nearly 100%. The as-demonstrated tetrahedral preparator auxiliary coating method and laser-cutting interdigital configuration technique provide new perspective for preparing microelectronics in an economical way. The paper electrode appended by CNTs achieves steerable areal capacitance, showing broad application prospect in fabricating asymmetric micro-supercapacitor with flexible planar configurations in the future.
2022, 35(3): 570-577.
doi: 10.1063/1674-0068/cjcp2005076
In this study, the application of bovine serum albumin (BSA) as a carrier to glucose-sensitive materials for the detection of glucose was proposed. Au-CuO bimetallic nanoclusters (Au-CuO/BSA) were prepared using BSA as a template, the new sensing material (Au-CuO/BSA/MWCNTs) was synthesized by mixing with multi-walled carbon nanotubes (MWCNT) and applied to non-enzymatic electrochemical sensors to detect glucose stably and effectively under neutral condition. The scanning electron microscopy was used to investigate the morphology of the synthesized nanocomposite. The electrochemical properties of the sensor were studied by cyclic voltammetry. Glucose detection experiments show that Au-CuO/BSA/MWCNTs/Au electrode has good glucose detection ability, stability, accuracy, repeatability, and high selectivity in neutral environment. Unlike existing glucose-sensitive materials, due to the use of BSA, the composite material is firmly fixed to the electrode surface without a Nafion solution, which reduces the current blocking effect on the modified electrode. The composite materials can be effectively preserved for extremely long periods, higher than 80% activity is maintained at room temperature in a closed environment for 3 to 4 months, due to the special effects of BSA. In addition, the feasibility of using BSA in glucose-sensitive materials is confirmed.
2022, 35(3): 578-588.
doi: 10.1063/1674-0068/cjcp2009154
Herein we present a facile approach for the preparation of a novel hierarchically porous carbon, in which seaweeds serve as carbon source and KOH as activator. The fabricated KOH-activated seaweed carbon (K-SC) displays strong affinity towards tetracycline with maximum uptake quantity of 853.3 mg/g, significantly higher than other tetracycline adsorbents. The superior adsorption capacity ascribes to large specific surface area (2614 m$ ^2 $/g) and hierarchically porous structure of K-SC, along with strong $ \pi $-$ \pi $ interactions between tetracycline and K-SC. In addition, the as-prepared K-SC exhibits fast adsorption kinetics, capable of removing 99% of tetracycline in 30 min. Meanwhile, the exhausted K-SC can be regenerated for four cycling adsorption without an obvious degradation in capacities. More importantly, pH and ionic strengths barely affect the adsorption performance of K-SC, implying electrostatic interactions hardly play any role in tetracycline adsorption process. Furthermore, the K-SC packed fixed-bed column (0.1 g of adsorbents) can continually treat 2780 mL solution spiked with 5.0 mg/g tetracycline before reaching the breakthrough point. All in all, the fabricated K-SC equips with high adsorption capacity, fast adsorption rate, glorious anti-interference capability and good reusability, which make it hold great feasibilities for treating tetracycline contamination in real applications.
