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Articles in press have been peer-reviewed and accepted, which are not yet assigned to volumes/issues, but are citable by Digital Object Identifier (DOI).
Chemical looping reforming of methane to syngas (CO and H2) is one of the most promising routes for methane utilization, where the further reaction of CO on oxygen carrier surfaces is a primary determinant of CO selectivity. In this work, the effects of oxygen vacancy (VO) on CO desorption, CO oxidation, and CO dissociation are systematically studied by using density functional theory calculations. Our calculated results reveal that increasing VO concentration can weaken CO desorption at Fe sites due to the enhanced localization of electrons in the Fe atoms. Also, the increase in VO concentration from 1/12 ML to 1/6 ML leads to a dramatic increase of activation energy in the CO oxidation from 0.64 eV to 1.10 eV. Moreover, the increase in VO concentration is conducive to CO dissociation, but the dissociation is still almost impossible due to the high reaction energies (large than 3.00 eV). Considering these three reaction paths, CO desorption can proceed spontaneously at reaction temperatures above 900 K. Increasing VO concentration can improve the selectivity of syngas production due to the less favorable CO oxidation compared with CO desorption at high VO concentrations (1/6 ML). This work reveals the microscopic mechanism that CO selectivity rises in the CLRM as the degree of Fe2O3 reduction increases.
The exciton polariton is a kind of quasiparticles and provides a promising opportunity to explore fundamental quantum phenomena for photonic applications. Transition-metal dichalcogenide (TMD) materials provide the platform of nanophotonics that supports the propagative exciton polaritons even at room-temperature. Previously, real space studies on thin flakes of TMDs by scattering-type scanning nearfield optical microscopy (s-SNOM) were limited to waveguide thickness down to 30 nm. In this work, we present the nano-optical imaging of ordinary transverse electric modes of exciton polaritons in MoS2 and WSe2 down to a few atomic layers, measured by atomic force microscope-based s-SNOM. Surprisingly, the interference fringe patterns can be observed clearly at the prepared ultrathin TMD flakes with thickness down to ~3 nm (4 ML) and ~8 nm (12 ML) for MoS2 and WSe2, respectively, which breaks greatly the previous measurement limitation. The wavevectors stay around 1.6–1.7k0 constantly when the thickness approaching to a few MLs, instead of 1k0 according to the theory. These modes are supported by the nearly-freestanding TMD microflakes in the form of three-layer symmetric waveguide to confine the exciton polaritons. Our results provide in-depth understanding and open new avenues to explore the polaritonic devices operating at the near infrared region based on ultrathin TMD materials.
In the low-energy electron impacts with a liquid beam of methanol, we detect the anionic yields using a linear time-of-flight mass spectrometer. With help of quantum chemistry calculations, we further identify these anionic products as the hydrogen-bonding clusters, i.e., CH3OH·CH3O and CH3OH·CH3OH·CH3O, and their singly occupied molecular orbitals show the valence-bound feature. In contrast, CH3OH·CH3OH and CH3OH·CH3OH·CH3OH are unfavored in energetics with respect to the dehydrogenated ones, although they also correspond to the minima on the potential energy surfaces. Furthermore, the singly occupied molecular orbitals of CH3OH·CH3OH and CH3OH·CH3OH·CH3OH indicate the typical feature of aqueous-solvated electron.
We study the photodissociation dynamics of the hydrogen sulfide cations (H2S+) using the time-sliced velocity map ion imaging (VMI) technique and high-accuracy calculations. High-resolution ion images of the S+(4S) products were measured at four photolysis wavelengths from 325.158 nm to 325.307 nm, which correspond to the excitation to the A2A1(0, 13, 0) K = 1 state of H2S+. Rotational state-resolved total kinetic energy releases and angular distributions have been derived as a function of the photolysis wavelengths. Notably, photolysis wavelength dependent product rotational state and anisotropy parameter distributions have been clearly observed. Full-dimensional potential energy surface characterization suggests that nonadiabatic coupling between A2A1 and B2B2 states at C2v configurations, as well as relaxation of the symmetry to Cs in the conical intersection region between the two states, plays a key role in the photodissociation process.
The reactivity of chromium oxide cluster anions (CrO<sub>3</sub>)<sub>1−4</sub>O<sup>−</sup> towards low carbon (C<sub>1</sub>−C<sub>4</sub>) alkanes has been experimentally investigated at 298 K by employing a homemade ship-lock-type reactor coupled with a time-of-flight mass spectrometer. The results demonstrate that while CrO<sub>4</sub><sup>−</sup> and Cr<sub>2</sub>O<sub>7</sub><sup>−</sup> clusters could abstract a hydrogen atom from C<sub>2</sub>H<sub>6</sub> and CH<sub>4</sub>, respectively, Cr<sub>3</sub>O<sub>10</sub><sup>−</sup> and Cr<sub>4</sub>O<sub>13</sub><sup>−</sup> clusters were found to be inert towards <i>n</i>-C<sub>4</sub>H<sub>10</sub> within the detection limit. Theoretical results reveal that CrO<sub>4</sub><sup>−</sup> and Cr<sub>2</sub>O<sub>7</sub><sup>−</sup> clusters possess delocalized chromium-bonded oxygen radicals (Cr−O<sup>−•</sup>), which rationalizes the hydrogen atom abstraction reactions between (CrO<sub>3</sub>)<sub>1,2</sub>O<sup>−</sup> clusters and alkanes. However, the active sites of (CrO<sub>3</sub>)<sub>3,4</sub>O<sup>−</sup> clusters evolve to peroxide species (O<sub>2</sub><sup>2−</sup>), which exhibit inferior activity compared to O<sup>−•</sup> radicals. The increase of Cr-3d orbital energy driven by the more negative charge around CrO<i><sub>y</sub></i> unit formed via downsizing the cluster size has been proposed to account for favorable reduction of O<sub>2</sub><sup>2−</sup> and selective generation of reactive O<sup>−•</sup> radicals in small-sized (CrO<sub>3</sub>)<sub>1,2</sub>O<sup>−</sup> clusters. This study not only enriches the chemistry of metal-bonded O<sup>−•</sup> radicals toward alkane activation under mild conditions, but also provides a new insight into the conversion between O<sub>2</sub><sup>2−</sup> and O<sup>−•</sup> radicals over metal oxides.
Nickel, an important transition metal element, is one of the trace elements for human body and has a crucial impact on life and health. Some evidences show the excess exposure to metal ions might be associated with neurological diseases. Herein, we applied Raman spectroscopy to study the Ni(II) ion effect on kinetics of amyloid fibrillation of hen egg white lysozyme (HEWL) in thermal and acidic conditions. Using the well-known Raman indicators for protein tertiary and secondary structures, we monitored and analyzed the concentration effect of Ni(II) ions on the unfolding of tertiary structures and the transformation of secondary structures. The experimental evidence validates the accelerator role of the metal ion in the kinetics. Notably, the additional analysis of the amide I band profile, combined with thioflavin-T fluorescence assays, clearly indicates the inhibitory effect of Ni(II) ions on the formation of amyloid fibrils with organized β-sheets structures. Instead, a more significant promotion influence is affirmed on the assembly into other aggregates with disordered structures. The present results provide rich information about the specific metal-mediated protein fibrillation.
Machine learning (ML) approaches like neural networks have been widely used in chemical researches for fast estimating chemical properties. Generating ML models of good precision requires datasets of high quality, which can be difficult to obtain. In this work, we trained graph neural network (GNN) models from different datasets and verified transferring of the models to other datasets. Our result shows that cross-dataset evaluation can give less accurate but still correlative prediction results on different datasets. Errors are mainly due to systematic errors. The value range of prediction result is highly related to the range of training set. The precisions of different bonds show different distributions. C–H bond constantly gets the highest precision in the tested bonds.
We report here scanning tunneling microscopy (STM) observations of bridge-bonded oxygen vacancies (OVs) on the TiO2(110) surface diffusing under the influence of 266 nm ultraviolet (UV) laser irradiation. OV pairs, and even OV trimers, were formed as a result of UV light-induced OV diffusion. There are two stable STM representations of the OV-pair defects, which are interchangeable during scanning. An extended irradiation time (68 min) can lead to the formation of a TiO2(110) surface with predominant OV-pair point defects. Our results enrich the understanding of OV behavior upon UV irradiation, and future photocatalytic studies on reduced rutile TiO2(110) surfaces involving 266 nm UV light can benefit from the knowledge of the observed diffusion of OVs and the formation of OV oligomers. We also provide a plausible way to prepare an OV-pair abundant TiO2(110) surface, a requisite for further investigations of the otherwise unapproachable defects.
A narrow-pulsed and velocity-controlled molecular beam system is constructed by using a high-speed chopper and a temperature adjustable pulsed valve. The duration of the hydrogen molecular beam pulse is reduced to approximately 6.3 μs and characterized using resonance-enhanced multiphoton ionization (REMPI) at a distance of ~193 mm downstream from the nozzle. To precisely determine the velocity of the hydrogen molecules, a pump-probe technique combining stimulated Raman pumping (SRP) and REMPI is employed at a fixed distance (193 mm) with nanosecond lasers. By adjusting the temperature of the pulsed valve, the velocity of the hydrogen beam is continuously varied from 1290 m/s to 3550 m/s. The system exhibits potential for multiple applications in the field of chemical reaction dynamics, including its potential to integrate with ion traps and surface scattering experiments.
We present a quantum dynamics study on the isotope effects of hydrogen transfer isomerization in the formic acid dimer, and this is achieved by multidimensional dynamics calculations with an efficient quantum mechanical theoretical scheme developed by our group, on a full-dimensional neural network ab initio potential energy surface. The ground-state and fundamental tunneling splittings for four deuterium isotopologues of formic acid dimer are considered, and the calculated results are in very good general agreement with the available experimental measurements. Strong isotope effects are revealed, the mode-specific fundamental excitation effects on the tunneling rate are evidently influenced by the deuterium substitution of H atom with the substitution on the OH bond being more effective than on the CH bond. Our studies are helpful for acquiring a better understanding of isotope effects in the double-hydrogen transfer processes.
Dissociative charge exchange reactions between Ar+ ion and trans-/cis-dichloroethylene (trans-/cis-C2H2Cl2) are investigated with the ion-velocity imaging technique. The dechlorinated product C2H2Cl+ is the predominant, and most of this product show the spatial distribution around the target, implying that the dissociation occurs in the large impact-parameter collision and via the energy resonant charge transfer. Meanwhile, a few C2H2Cl+ locate around the center-of-mass, which is attributed to the fragmentation of intimate association between C2H2Cl2 and Ar+ or in the small impact-parameter collision. The product C2HCl+ exhibits the velocity distribution features similar to those of C2H2Cl+. The rarest product C2HCl2+ shows the distributions around the molecular target, due to the quick dehydrogenation after the energy-resonant charge transfer in the large impact-parameter collision.
The interaction of proteins with salt ions plays an important role in life activities. We used butyramide as a model molecule to investigate the interaction of protein backbones with cations. The experiment was performed in an aqueous solution of metal chloride using UV Raman spectroscopy. It was found that well-hydrated metal cations (Ca2+, Mg2+) tend to bind to C=O in the amide bond, resulting in redistribution of the amide I band peaks. Specifically, the peak intensity ratio of 1655 cm−1 to 1610 cm−1 increases significantly with increasing concentrations. However, this phenomenon is not obviously observed in NaCl solution. Furthermore, we studied the effect of salt ions on the water structures. The addition of Ca2+ and Mg2+ is beneficial to the enhancement of the water signal at the 3400 cm−1 position, while the Na+ at the same concentration is not obvious. The results have shown that the interaction between cations and amides satisfies the following order: Ca2+>Mg2+>Na+, which conforms to the Hofmeister series.
Fluorescent labels are widely used in the characterizations of DNA-based reaction network operations. We systematically studied the effects of commonly used fluorescent pairs on thermal stabilities of signal-substrate duplex and the strand displacement kinetics. It is demonstrated that the modifications of duplex with fluorescent pairs stabilize DNA duplex by up to 3.5 °C, and the kinetics of DNA strand displacement circuit is also evidently slowed down. These results highlight the importance of fluorescent pairs towards the kinetic modulation in designing nucleic acid probes and complex DNA dynamic circuits.
As a widely-used sunscreen compound, the caffeic acid (CA) shows the strong UV absorption, while the photoinduced reaction mechanisms behind its photoprotection ability are not fully understood. We try to investigate the photoinduced internal conversion dynamics of CA in order to explore the photoprotection mechanism. The most stable CA isomer is selected to examine its nonadiabatic dynamics using the on-the-fly surface hopping simulations at the semi-empirical level of electronic-structure theory. The dynamics starting from different electronic states are simulated to explore the dependence of the photoinduced reaction channels on the excitation wavelengths. Several S1/S0 conical intersections, driven by the H-atom detachments and the ring deformations, have been found to be responsible for the nonadiabatic decay of the CA. The simulation results show that the branching ratios towards these intersections are modified by the light with different excitation energies. This provides the valuable information for the understanding of the photoprotection mechanism of the CA compound.
Directional synthesis of bio-based light olefins has great significance for promoting sustainable development of chemical industry. Present work proves that light olefins can be selectively prepared from the cellulose-derived acetone-butanol-ethanol. This transformation has been achieved by coupling cellulose fermentation and acetone-butanol-ethanol catalytic dehydration over the Ce@SAPO-34 catalyst. The active sites and reusability of the catalyst were investigated. High acetone-butanol-ethanol conversion (91.9%) and high olefin selectivity (86.1%) are achieved. Based on the study of the individual components in acetone-butanol-ethanol, the reaction pathways are put forward.
Two soy protein 11S fractions with different surface sulfhydryl contents were prepared. Utilizing analytical ultracentrifugation, the effects of storage time and hydrogen peroxide at different concentrations (0.5–100 mmol/L) on the two 11S fractions were investigated. Results show that after removing 2-mercaptoethanol (2-ME) by size exclusion chromatography, the 11S fraction with high surface sulfhydryl content (2.0 mol sulfhydryl/mol 11S) progressively formed 15S and 21S in dilute solutions during storage at 4 °C for 82 days. While, the 11S fraction with low surface sulfhydryl content (0.2 mol sulfhydryl/mol 11S) was stable under the same condition. Moreover, after treating the 11S with high surface sulfhydryl content with 1 mmol/L H2O2, the weight percentage of 15S reached the maximum value of 20%. The 15S induced by air and H2O2 could be totally converted to 11S with the addition of 10 mmol/L 2-ME, which could be attributed to that the disulfide bond linking two 11S molecules is on the surface of the 15S and easily accessible to the reducing agent 2-ME. This study helps us to deeply understand the formation mechanism of 15S and the stability of 11S.
The pressure-induced structural phase transition of XeO3 is studied by first principle calculations. The transition from P212121 to Pnma accompanied by a drastic reduction of volume is found at 2.18 GPa. The symmetrilized Xe–O2 bonds give rise to the better symmetry of high pressure phase. O-hopping between different possible local minima and the motion of Xe along the y axis may be responsible for phase transition. Results of electron localization function indicate that three ipsilateral Xe–O bonds lead to a lone-pair contour of Xe6+.
A series of model peptides (Trp–Pron–Tyr, WPnY, n=0, 1, 2, 3, 5), which contain tryptophan (Trp, W), tyrosine (Tyr, Y), and proline (Pro, P), have been studied under three typical pH conditions (3, 7, and 10) by steady-state absorption and fluorescence spectroscopy, nanosecond time-resolved fluorescence and femtosecond time-resolved transient absorption spectroscopy. When the peptide’s chain length is increased, Trp fluorescence quenching is expected to be gradually weakened. However, Trp fluorescence in WPY is strongly quenched and reveals even stronger quenching with increasing pH values, whose hypochromicity is clearly different from other model peptides. Transient absorption spectra also demonstrate that the excited state decay of WPY is much faster than that of other model peptides, especially at pH = 10. It is attributed to the efficient proton coupled electron transfer (PCET) between Trp and Tyr. Moreover, due to the very short distance between Trp and Tyr in WPY, this PCET process could be achieved by “direct transfer”, contrasted with the slow and long-range PCET process in other model peptides. Our results of the dipeptides WY and WP further suggest that Trp may also have more complex interactions with the peptide backbone or proline in those peptides. This work provides an experimental evidence for the electron transfer mechanism in WY dyads, which can help ones to understand how to reduce Trp radicals in proteins.
With the capability of quantitative identifying surface species and measuring desorption kinetics, temperature-programmed desorption (TPD) is widely used in heterogeneous catalysis and surface science fields. Minimum gas load during adsorption, fast substrate replacement, and comprehensive temperature control are of great significance for efficient and high quality TPD experiments. Unfortunately, these requirements usually cannot be met at the same time for the existing apparatuses in surface science. In order to increase the universality, a TPD spectrometer combining minimum gas load, fast substrate replacement, and comprehensive temperature control in our laboratory has been built. By using an automatically controlled microcapillary array-based effusive molecular beam gas doser, optimizing the thermal contact at the sample stage, using liquid nitrogen transfer line and designing thermocouple connection, controllable and reproducible molecule adsorption, minimum gas load, fast substrate replacement, rapid cooling, accurate temperature measuring and excellent linear heating are achieved simultaneously. Capabilities of the TPD spectrometer, for example, determination of desorption energy and desorption order, quantitative measurements of surface species and binding sites, and investigation of surface photochemical reactions, are demonstrated by measuring the desorption of water from highly oriented pyrolytic graphite and TiO2(110) and photocatalyzed oxidation of methoxy anions on TiO2(110). The apparatus described here will contribute effectively to the high throughput measurements.
UV-B (280–320 nm) sunscreening is crucial for lives on Earth. Examining the role of surrounding of UV-B screening molecules can help us better understand UV-B absorption. Water is ubiquitous in cells as the solvent, but its impacts on UV-B absorption of sunscreen agents are underexplored. Herein, we report a first-principle study on UV-B absorption of sinapate esters (SM) and relevant molecular species, with a focus on the solvent effect of water. The capability of UV-B screening by anions of SM, the proposed species for actual sunscreening, is shaped by water. Both the implicit water providing the dielectric environment of solvation and the explicit water molecules forming hydrogen bonding to SM anion can appreciably alter the nature of transition orbitals responsible to the UV-B absorption of the anion. Finally, we find the molecular dipole moment of an organic UV-B screening agent can be an indicator of its UV-B screening sensitivity to the surrounding water. Our work may serve as a starting point of developing new water-soluble UV-B screening agent.
Single-molecule spin logic gates provide fundamental functions and are of importance in the field of molecular spintronics. Here, by using the first-principles method, the effects of ambient gas molecules (CO2, O2, N2, or H2O) on the functional stability of the investigated single-molecule spin logic gate consisting of two serially connected cobalt dibenzotetraaza[14]annulene (CoDBTAA) molecules between single-walled carbon nanotubes (SWCNTs) electrodes, have been theoretically investigated. The calculated results suggest that the investigated spin logic gate can realize AND, NOR, or XNOR logic functions depending on the definition of the input and output signals. It is found that these logic functions are not affected by CO2 adsorption. On the contrary, these logic functions are no longer retained upon O2, N2, or H2O adsorption. Further analysis reveals that the interaction between the CoDBTAA molecule and the CO2 adsorbate is very weak while it is strong for O2, N2, or H2O molecules. Therefore, the electronic states of the logic gate around Fermi energy (EF) are almost unchanged for CO2 adsorption. While the adsorption of O2, N2, or H2O obviously modifies the electronic states around EF. The strong interaction between CoDBTAA and these three gas adsorbates drives the conductive electronic states to move far away from EF, resulting in the blocking of both spin-up and spin-down currents and further voiding the logic functions. This work suggests that ambient air has an important effect on the functional stability of single-molecule devices and should be carefully evaluated in the future design of functional single-molecule devices.
A case study has been performed on the adaptability of electron-localization strategy in designing clusters with planar tetracoordinate nitrogen (ptN) through the detailed comparison between ptN clusters NLi3E+ (E = N, P, As) and their ptC counterparts CLi3E (E = N, P, As). The results revealed that NLi3E+ clusters possessed similar planar geometries to CLi3E, which are both determined by the existence of a localized π bond. Nevertheless, NLi3E+ clusters possess the obviously higher electronic, thermodynamic, and dynamic stabilities than CLi3E clusters, as reflected by wider HOMO–LUMO gaps (4.58–4.68 eV versus 2.10–2.74 eV), the overall lower-lying positions on potential energy surfaces verified at the CCSD(T)/aug-cc-pVTZ level, and better rigidity during the molecular dynamic simulations at the PBE/DZVP level. Therefore, our results suggest that electron-localization strategy may be more suitable for designing the clusters, whose central atom possesses relatively higher electronegativity and more favours the localized bond. In addition, due to the good stability, the cationic NLi3E+ clusters designed in this work are suitable for gas phase generation, mass-selection, and spectroscopic characterization.
The dissociative adsorption of H2 on Cu19 and defective graphene-supported Cu19 clusters are investigated using ab initio molecular dynamics. The molecular-level trajectories show that, on Cu19, the preferred adsorption site is the bridge-hollow site, where the two H atoms are adsorbed at the bridge and hollow sites beside a Cu atom, with an adsorption energy of –0.74 eV. In contrast, on the defective graphene-supported Cu19 cluster, the favorite adsorption site is located where the two H atoms are adsorbed at hollow-hollow sites with an adsorption energy of –1.27 eV. In general, the average adsorption energy on the defective graphene-supported Cu19 cluster is –1.07 eV, which is about 84% larger than that of –0.58 eV on the Cu19 cluster. This indicates that the adsorption capacity is greatly enhanced for the dissociative adsorption of H2 on the defective graphene-supported Cu19 cluster. The d-band center shifts to the Fermi level, illustrating the enhanced adsorption capacity on the defective graphene-supported Cu19 cluster. The integrated crystal orbital Hamilton population analysis reveals that stronger bond interactions between hydrogen atoms with their bonded Cu atoms lead to much larger adsorption energies on the defective graphene-supported Cu19 cluster compared to the Cu19 cluster.
Lithium has been paid great attention in recent years thanks to its significant applications for battery and lightweight alloy. Developing a potential model with high accuracy and efficiency is important for theoretical simulation of lithium materials. Here, we build a deep learning potential (DP) for elemental lithium based on a concurrent-learning scheme and DP representation of the density-functional theory (DFT) potential energy surface (PES), the DP model enables material simulations with close-to DFT accuracy but at much lower computational cost. The simulations show that basic parameters, equation of states, elasticity, defects and surface are consistent with the first principles results. More notably, the liquid radial distribution function (RDF) based on our DP model is found to match well with experiment data. Our results demonstrate that the developed DP model can be used for the simulation of lithium materials.
Plasmonic catalysis, which is driven by the localized surface plasmon resonance of metal nanoparticles, has become an emerging field in heterogeneous catalysis. The microscopic mechanism of this kind of reaction, however, remains controversial partly because of the inaccuracy of temperature measurement and the ambiguity of reagent adsorption state. In order to investigate the kinetics of plasmonic catalysis, an online mass spectrometer-based apparatus has been built in our laboratory, with emphases on dealing with temperature measurement and adsorption state identification issues. Given the temperature inhomogeneity in the catalyst bed, three thermocouples are installed compared with the conventional design with only one. Such a multiple-point temperature measuring technique enables the quantitative calculation of equivalent temperature and thermal reaction contribution of the catalysts. Temperature-programmed desorption is incorporated into the apparatus, which helps to identify the adsorption state of reagents. The capabilities of the improved apparatus have been demonstrated by studying the kinetics of a model plasmon-induced catalytic reaction, i.e., H2+D2→HD over Au/TiO2. Dissociative adsorption of molecular hydrogen at Au/TiO2 interface and non-thermal contribution to HD production have been confirmed.
Accurate potential energy surface (PES) calculation is the basis of molecular dynamics research. Using deep learning (DL) methods can improve the speed of PES calculation while achieving competitive accuracy to ab initio methods. However, the performance of DL model is extremely sensitive to the distribution of training data. Without sufficient training data, the DL model suffers from overfitting issues that lead to catastrophic performance degradation on unseen samples. To solve this problem, based on the message passing paradigm of graph neural networks, we innovatively propose an edge-aggregate-attention mechanism, which specifies the weight based on node and edge information. Experiments on MD17 and QM9 datasets show that our model not only achieves higher PES calculation accuracy but also has better generalization ability, compare with Schnet, which demonstrates that edge-aggregate-attention can better capture the inherent features of equilibrium and non-equilibrium molecular conformations.
Spinel-type CuFe2O4 nanoparticles were synthesized by a solvothermal method using ethylene glycol as solvent and polyvinylpyrrolidone (PVP) as dispersant. The characterization results showed that the average diameter of the hollow-spherical CuFe2O4 was approximately 100 nm with homogeneous morphology and negligible agglomeration. CuFe2O4 was used as the active electrode material to explore its supercapacitive properties in different concentrations of KOH electrolytes. It was found that the CuFe2O4 hollow-spherical nanoparticles exhibit potential electronic performance in supercapacitor, with a specific capacitance of 368.2 F/g and capacitance stability retention of 91.0% after 2000 cycles at the current density of 5 A/g in 3 mol/L KOH electrolyte. The present findings demonstrate that the CuFe2O4 electrode materials can have important implications with practical prospects in energy storage systems.
We have investigated the formation of ammonia (NH3) from atomic N and water (H2O) on a rutile(R)-TiO2(110) surface using the temperature-programed desorption method. The formation of NH3 can be detected after coadsorption of atomic N and H2O on the R-TiO2(110) surface, desorbing from the 5-fold coordinated Ti4+ (Ti5c) sites at about 400 K, demonstrating that the NH3 formation on R-TiO2(110) is feasible at low surface temperature. During the process, both hydroxyl groups at the bridging oxygen rows and H2O on the Ti5c sites contribute to the formation of NH3, which are affected by H2O coverage. At low H2O coverage, the direct hopping of hydrogen atoms may be the dominant process for hydrogen transfer; while H2O-assisted hydrogen atoms diffusion may be preferred at high H2O coverage. Our result will be of significant help to get a deeper insight into the fundamental understandings of hydrogenation processes during the NH3 synthesis.
Partial genetically encoded 4-hydroxybenzylidene-imidazolinone (HBI)-type chromophores are new promising fluorescent probes, which are suitable for imaging and detection of living cells. However, the lack of infrared chromophores hinders the development seriously. Here more than 30 HBI-type chromophores with regular structure modifications were employed and typical spectral redshift change laws and mechanisms were investigated by quantum methods. Results show that both one-photon spectrum (OPS, absorption/emission) and two-photon absorption (TPA) can achieve large redshift via either extending conjugated lengths of frag-3 or enlarging conjugated areas of frag-1 of HBI skeleton. Spectral redshifts of all chromophores are highly related to intramolecular charge transfer (ICT), but neutral ones are closely related to the total ICT or electron-accepting-numbers of frag-3, and the high correlative factor of anions is the aromaticity of frag-2 bridge. The frag-2 bridge with high aromaticity can open a reverse charge transfer channel in anion relative to neutral, obtaining significant redshift. Based on analysis, a new 6-hydroxyl-naphthalene-imidazolinone (HNI) series, which have larger conjugated area in frag-1, are predicted. The OPS and TPA of anionic HNI ones acquire about 76−96 nm and 119−146 nm redshift relative to traditional HBI series respectively as a whole. The longest emission of anionic HNI-4 realizes more 244 nm redshift relative to HBI-1. Our work clarifies worthy spectral regularities and redshift mechanisms of HBI-type chromophores and provide valuable design strategy for infrared chromophores synthesis in experiment.
Bis(15-crown-5)-stilbenes containing crown ether parts have been widely used in a variety of chemical applications, such as cation detectors, because of their ability to selectively bind to alkali metal cations, Bis(15-crown-5)-stilbenes and its derivatives with complexation of one- or two-alkali metal cation (Li+, Na+ and K+) have been theoretically investigated by quantum chemistry methods. The coordination of alkali cations results in partial shrinkage of crown ethers, which directly affected natural distribution analysis charges and molecular orbital energy levels. The number of alkali metal ions has significant effects on absorption spectra and mean second hyperpolarizability. When one alkali metal ion was added to the anticonformer of bis(15-crown-5)-stilbene, the absorption spectra were obviously redshifted and the mean second hyperpolarizability values were slightly increased; while two alkali metal ions were added to bis(15-crown-5)-stilbene, the absorption spectra were obviously blue shifted and the mean second hyperpolarizability values decreased. On the other hand, as the radius of the alkali ions increaseed, the mean second hyperpolarizability values of the compounds increased gradually. It indicated that the mean second hyperpolarizability value was sensitive to the number and radius of the alkali metal cations, thus the third order nonlinear optical response can be used as a signal to detect the number and type of alkali metal ions.
Bromodomain-containing protein 4 (BRD4) is critical in cell cycle regulation and has emerged as a potential target for treatment of various cancers. BRD4 contains two bromodomains, namely BD1 and BD2. Research suggests that selectively inhibiting BD1 or BD2 may provide more effective treatment options. Therefore, understanding the selective mechanism of inhibitor binding to BD1 and BD2 is essential for development of high selective inhibitors to BD1 and BD2. Multiple replica molecular dynamics (MRMD) simulations are utilized to investigate the binding selectivity of inhibitors SG3-179, GSK778, and GSK620 for BD1 and BD2. The results show that BD1 has stronger structural flexibility than BD2, moreover BD1 and BD2 exhibit different internal dynamics. The analyses of free energy landscapes reveal significant differences in the conformational distribution of BD1 and BD2. Binding free energy predictions suggest that entropy changes, electrostatic interactions, and van der Waals interactions are key factors in the selective binding of BD1 and BD2 by SG3-179, GSK778, and GSK620. The calculations of the energy contributions of individual residues demonstrate that residues (W81, W374), (P82, P375), (Q85, K378), (V87, V380), (L92, L385), (N93, G386), (L94, L387), (C136, C429), (N140, N433), (K141, P434), (D144, H437) and (I146, V439) corresponding to (BD1, BD2) generate significant energy difference in binding of SG3-179, GSK778, and GSK620 to BD1 and BD2, and they can serve as effective targets for development of high selective inhibitors against BD1 or BD2. The related information may provide significant theoretical guidance for improving the selectivity of inhibitors for BD1 and BD2.
Colloidal quantum dot (CQD) lasers show promising applications in flexible optoelectronic devices, due to their tunable emission wavelength, narrow spectrum bandwidth and high power intensity. However, fabricating a flexible CQD laser is challenging because of the difficulties in fabricating optical cavities on flexible substrates using traditional microfabrication technologies. Herein, we propose a one-step self-assembly approach to fabricate flexible CQD supraparticle lasers. The whole assembly approach is processed in a liquid environment without surfactants, and the formed spherical CQD supraparticles are featured with smooth surfaces, serving as high-quality-factor whispering-gallery mode cavities to support laser oscillation. A low lasing threshold of 54 µJ/cm2 is observed while exciting a CQD supraparticle with pulsed femtosecond lasers. The calculated cavity quality factor of 963 for CQD supraparticle lasers is twofold larger than that of CQD lasers assembled with surfactants. Moreover, the CQD supraparticles can serve as free-standing lasers, which allows them to be deposited on flexible substrates such as paper and cloth. Furthermore, our CQD lasers show high stability, after being continuously photoexcited above the threshold for 400 min, their lasing intensity remains at 85.7% of the initial value. As bright, free-standing and long-term stable light sources, the assembled CQD lasers proposed in this work show potential applications in wearable devices and medical diagnosis.
Electronic correlation is a fundamental topic in many-electron systems. To characterize this correlation, one may introduce the concept of exchange-correlation hole. In this paper, we first briefly revisit its definition and relation to electron and geminal densities, followed by their intimate relations to copula functions in probability theory and statistics. We then propose a copula-based approach to estimate the exchange-correlation hole from the electron density. It is anticipated that the proposed scheme would become a promising ingredient towards the future development of strongly correlated electronic structure calculations.
Effects of hydrogen bonds on two-photon absorption (TPA) of a new donor-acceptor type green fluorescent protein chromophore analogue are investigated by employing a combined molecular dynamics and quantum chemistry method. The probable configurations of the chromophore in water are extracted from molecular dynamics simulation and the TPA properties of more than twenty hydrogen bond complexes are computed by quadratic response theory. Thereby, the structure and property relations are established. Three types of hydrogen bonds including O···H−O, N−H···O and N···H−O can be formed between the chromophore and water molecules. The O···H−O induces a little decrease of TPA cross section with a red-shifted wavelength. The N−H···O gives rise to a great enhancement of TPA at a longer wavelength, while the N···H−O decreases TPA significantly and makes the wavelength blue-shifted. The reasons for these effects are rationalized well by using a two-state model analysis. The related molecular orbitals are also plotted to visualize the charge transfer characters. In addition, the averaged TPA spectrum is obtained by calculating the probabilities of various hydrogen bond complexes. Our research could provide a good insight into the design of two-photon materials by making use of hydrogen bond networks.
A minimum-modified Debye-Hückel (DH) theory for electrolytes with size asymmetry is developed. Compared with the conventional DH theory, the minimum-modified DH theory only introduces an extra surface charge density to capture the electrostatic effect of the size asymmetry of the electrolytes and hence facilitates a boundary element method for electrostatic potential calculation. This theory can distinguish the electrostatic energies and excess chemical potentials of ions with the same sizes but opposite charges, and is applied to a binary primitive electrolyte solution with moderate electrostatic coupling. Compared with the hyper-netted chain theory, the validity of this modified DH theory demonstrates significant improvement over the conventional DH theory.
Low photoluminescence (PL) quantum yield of molybdenum disulfide (MoS2) quantum dots (QDs) has limited practical application as potential fluorescent materials. Here, we report the intercalation of aluminum ion (Al3+) to enhance the PL of MoS2 QDs and the underlying mechanism. With detailed characterization and exciton dynamics study, we suggest that additional surface states including new emission centers have been effectively introduced to MoS2 QDs by the Al3+ intercalation. The synergy of new radiative pathway for exciton recombination and the passivation of non-radiative surface traps is responsible for the enhanced fluorescence of MoS2 QDs. Our findings demonstrate an efficient strategy to improve the optical properties of MoS2 QDs and are important for understanding the regulation effect of surface states on the emission of two dimensional sulfide QDs.
Zero-dimensional environmentally friendly carbon quantum dots (CQDs) combined with two-dimensional materials have a wide range of applications in optoelectronic devices. We combined steady-state and transient absorption spectroscopies to study the energy transfer dynamics between CQDs and molybdenum disulfide (MoS2). Transient absorption plots showed photoinduced absorption and stimulated emission features, which involved the intrinsic and defect states of CQDs. Adding MoS2 to CQDs solution, the lowest unoccupied molecular orbital of CQDs transferred energy to MoS2, which quenched the intrinsic emission at 390 nm. With addition of MoS2, CQDs-MoS2 composites quenched defect emission at 490 nm and upward absorption, which originated from another energy transfer from the defect state. Two energy transfer paths between CQDs and MoS2 were efficiently manipulated by changing the concentration of MoS2, which laid a foundation for improving device performance.
The 5-hydroxymethylfurfural (5-HMF) acts as an important chemical intermediate to bridge the biomass resources and industrial applications, which shows the potential for green development. However, the performance of biomass materials conversion to 5-HMF is still limited in the green solvent. Herein, an effective approach is reported to prepare the highly efficient solid acid catalysts, NbOx /WOy -ZrO2, to improve fructose conversion. It is found that the introduction of Nb results in the generation of the niobium oxides, which improves acid sites and tunes the ratios of Brønsted acid and Lewis acid on the surface of the WOy -ZrO2 support. With the acidity improvement and increasing acid sites of the NbOx /WOy -ZrO2, the highest fructose conversion is 99% in water. Meanwhile, the 5-HMF yield and the selectivity are also as high as 50.1% and 50.7% under the reaction temperature of 180 °C for a short reaction time of 30 min. The proposed NbOx /WOy -ZrO2 catalyst strategy will not only open a new way for designing the solid acid catalysts to achieve high performance of the 5-HMF in the water, but also promote the green production of biomass and sustainable development in the future.
We explore the transport properties of oligophenylene molecular junctions, where the center molecule containing 1, 2, or 3 phenyls is sandwiched between two graphene nanoribbons (GNR) with different edge shapes. According to the obtained results of the first-principles calculations combined with non-equilibrium Green's function method, we find that the molecular length-dependent resistance of all examined oligophenylene molecular junctions follows well the exponential decay law with different slopes, and the exponential decay factor is sensitive to the edge shape of GNRs and the molecule-electrode connecting configuration. These observations indicate that the current through the oligophenylene molecular junction can be effectively tuned by changing the edge shape of GNRs, the molecular length, and the molecular contacting configuration. These findings provide theoretical insight into the design of molecular devices using GNRs as electrodes.
Revealing the fundamental mechanisms governing reactant-induced disintegration of supported metal nanoparticles and their dependences on the metal component and reactant species is vital for improving the stability of supported metal nanocatalysts and single-atom catalysts. Here we use first-principles-based disintegration thermodynamics to study the CO- and OH-induced disintegration of Ag, Cu, Au, Ni, Pt, Rh, Ru, and Ir nanoparticles into metal-reactant complexes (M(CO)n, M(OH)n, n=1 and 2) on the pristine and bridge oxygen vacancy site of TiO2(110). It was found that CO has a stronger interaction with these considered transition metals compared to OH, resulting in lower formation energy and a larger promotion effect on the disintegration of nanoparticles (NPs). The corresponding reactant adsorption energy shows a linear dependence on the metal cohesive energy, and metals with higher cohesive energies tend to have higher atomic stability due to their stronger binding with reactant and support. Further disintegration free energy calculations of NPs into metal-reactant complexes indicate only CO-induced disintegration of Ni, Rh, Ru, and Ir nanoparticles is thermodynamically feasible. These results provide a deeper understanding of reactant-induced disintegration of metal nanoparticles into thermodynamically stable metal single-atom catalysts.
The hairpin element (HpE) near the start codon in the 5′ UTR was developed to tune the mRNA translation in mammalian cells. The parameters of HpEs including thermodynamic stability, GC content, and distance between HpEs and the 5′ cap were investigated. The parameters of HpEs including thermodynamic stability, GC content, and distance between HpEs and the 5′ cap were investigated, which influenced the mRNA expression level. In addition, the start codon and the upstream open reading frame sequestered within the structures of HpEs also reduced the translation initiation. In summary, this study shows that the simple engineering HpE structure can be efficiently adopted for gene expression regulation. The predictable controllability of this simple cloning strategy can potentially achieve precise gene expression regulation in different mammalian cell types.
The behavior of hydrogen production on ZnO electrode during the electrolytic reduction of water was investigated by cyclic voltammetry (CV) and cathode polarization experiments combined 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 and interstitial sites . 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.
The high reaction barrier of the oxygen evolution reaction (OER) has always been the bottleneck of the water decomposition reaction, so 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 a co-doping strategy. The overpotential of BCFN at a current density of 10 mA/cm2 reaches 310 mV, and possesses a Tafel slope of 50.2 mV/dec. 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 (375 mV), and benchmark IrO2 with excellent performance (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 without 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.
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.
Au nanowires in 4H crystalline phase (4H Au 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 is 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.
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 present 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 demonstrate 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 is achieved when using the reaction energy as a descriptor. We find that there existed orbital interaction similarity between C atom adsorption on top site and the transition states of C–H activation because both of them involve not only σ donation with dz2 orbital but also the π back-donation from dxy/dyz orbital(s). As a 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.
Understanding the mechanism of how micro-environments affect molecular rotors helps the design and development of molecular sensors. Here, we utilized femtosecond stimulated Raman spectroscopy, helped by quantum chemical calculation, to study the structural dynamics of 9-(2,2-dicyanovinyl) julolidine in cyclohexane, THF, and DMSO solvents. The obtained hydrogen out-of-plane (HOOP) mode and symmetric/anti-symmetric stretching of two nitriles (C≡N) indicate the rotation of the C7=C8 double bond and C4−C7 single bond in the excited-state which provide two non-radiative decay channels to effectively quench the excited-state population on local excited (LE) state via isomerization and twisted intramolecular charge transfer (TICT). In nonpolar solvent, the excited molecule in the LE state radiatively relaxes to the ground state or performs rotation motions via isomerization and TICT to deactivate fluorescence in the LE state. In the polar solvent, the isomerization plays a role to quench the LE state population; simultaneously, an ultrafast intramolecular charge transfer (ICT) from LE state to emissive ICT state was followed by an TICT between ICT state and dark ICT’ state.
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.
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.
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.
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.
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.
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.
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。
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.
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.
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.
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.

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.

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.
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.
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.
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.
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.
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.
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.
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.

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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.