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2023, Volume 36,  Issue 2

Chinese Abstracts
Chinese Abstracts
2023, 36(2): ⅰ-ⅱ.
2023, 36(2): ⅲ-ⅳ.
The catalytic performance of metal oxide surface mainly depends on its atomic surface structure, which usually changes under various treatment conditions and during catalytic reactions. Therefore, it is quite important to acquire the atomic geometries of the surfaces under different treatments for further understanding the catalytic mechanisms in the surfaces with complicated reconstructions. Here, we report the investigation on the evolution of surface geometries of the Ar+-ion-sputtered anatase TiO$ _2 $(001) films followed by heating treatments at various temperatures, characterized using variable-temperature scanning tunneling microscopy. Our experimental results reveal the different surface morphologies at different heating temperatures. During the heating treatment, the migrations of O atoms from the bulk to the surface of TiO$ _2 $(001) play an important role in the reoxidation of the Ti$ ^{2+} $ and Ti$ ^{3+} $ states for the formation of (1×4) reconstruction. The atomic-resolution images of the ridges show asymmetric features, which well support the fully oxidized structural model of the reconstructed TiO$ _2 $(001)-(1×4) surface.
Bimetallic nanoparticle (NP) catalysts have attracted long-standing attentions for their wide applications in a broad range of chemical reactions. Their catalytic performance tightly relies on the structure of bimetallic NPs. Atomic-level understanding of their structural thermostability is of great importance for developing advanced bimetallic catalysts with high stability. Here we precisely fabricated Au@Pt and Au@Pd core-shell catalysts on a SiO2 support with an identical Au core size of ~5.1 nm and a similar shell thickness of ~2 monolayers via selective atomic layer deposition. Spectroscopic characterizations were employed to compare their structural thermostability at elevated temperatures in a hydrogen reducing atmosphere. We revealed that the Au@Pt/SiO2 core-shell catalyst exhibited a considerably higher structural thermostability against atom inter-diffusion to alloys than that of Au@Pd/SiO2. Meanwhile, these two catalysts both preserved the particle size without any visible aggregation even after reduction at 550 ℃. Higher structural thermostability of Au@Pt/SiO2 core-shell catalyst might mainly stem from the distinctly higher melting point of Pt shell and their relatively smaller Au-Pt lattice mismatch. Such direct comparison of the structural thermostability of two different core-shell catalysts but with identical structures provides a valuable insight into the nature of thermodynamic behavior of bimetallic NPs at elevated temperatures.

Catalytic hydrolysis of ammonia borane for dehydrogenation is a promising way for generation and storage of hydrogen energy. Catalysts with reduced utilization of costly noble metals while high activity and stability are highly desired. Herein we show that the catalytic activity of the prototypical Pt/SiO2 catalysts towards ammonia borane hydrolysis could be significantly improved by the presence of a layer of Co(OH)2 beneath the supported Pt nanoparticles. By changing the Pt: Co molar ratio in the Pt-Co(OH)2/SiO2 catalysts, the hydrogen generation rates from ammonia borane hydrolysis show a volcano-type curve, with the maximum catalytic activity at the Pt: Co molar ratio of 1:11. The highest turnover frequency value of 829 molH2·molPt-1·min-1 at room temperature outperforms most of the reported Pt-based catalysts, and the apparent activation energy is drastically decreased to 36.2 kJ/mol from 61.6 kJ/mol for Pt/SiO2. The enhanced catalytic performance of Pt-Co(OH)2/SiO2 is attributed to the electrons donation from Co atoms on Co(OH)2 to Pt occurring at the metal-hydroxide interface, which is beneficial for optimizing the oxidation cleavage of the O-H bond of attacked H2O.

Polymers are routinely used as embedding matrices for organic molecular phosphors to substantially reduce the non-radiative decay rate and promote room-temperature phosphorescence (RTP). However, most previous studies focus on how glass transition temperature and free volume of various polymers influence RTP efficiency; very little is known on how electronic coupling between the matrix and the phosphor can modulate organic RTP. In the current investigation, we attempt to address the issue by synthesizing a monomeric version of an aromatic ketone phosphor and copolymerizing the ketone with four different types of matrix monomers. The resulting copolymers exhibit clear matrix-dependent RTP efficiency: a gradual decrease of RTP quantum yield from 22% to nearly 0 can be observed when the electronic conjugation of the matrix increases, suggesting that energy dissipation can occur in the triplet excited state via electron exchange when the triplet state of the matrix is close to that of the phosphor. The study provides a guiding principle on regulating the lifetime of triplet-excited states for organic dyes.

Understanding the interaction mechanism between divalent metal ions with amino acids is of great significance to understand the interaction between metal ions with proteins. In this study, the interaction mechanisms of Mg2+, Ca2+, and Zn2+ with amino acid side chain analogs in water were systematically studied by combining neural network potential energy surface, molecular dynamics simulation and umbrella sampling. The calculated potential mean forces not only reveal the binding process of each ion and amino acid, the most stable coordination structure, but also show the difference between different ions. In addition, we also use the neural network based potential of mean force as a standard to benchmark classical force fields, which is also meaningful for the development of force fields targeting metal ions.

We investigate the reaction probability, integral cross section, and energy efficiency of the OH-+CH3I reaction using the time-dependent quantum dynamics wave packet method. A four-degree-of-freedom dynamics model is developed for this study due to the synchronized SN2 bond-breaking and formation mechanism. We find that the reaction probability decreases as a function of the collision energy, which is a typical character of reactions with a negative energy barrier. The ground-state integral cross section calculated using this model is in excellent agreement with the quasi-classical trajectory results. The integral cross-section ratios of the vibrational excitations over the ground state, at the same equal amount of total energy, indicate that the vibrational motion of the CH3-I is more efficient in enhancing the reactivity than the translational motion, which, in turn, has a bigger contribution to the reactivity than the C-H3 vibrational motion. The energy efficacy order in the reactivity is confirmed by the sudden vector model prediction.

Three-coordinate Au(Ⅰ) complexes with thermally activated delayed fluorescence (TADF) have recently gained experimental attention. However, its luminescence mechanism is elusive. Herein, we have employed density functional theory (DFT), time-dependent DFT (TD-DFT), and QM/MM methods to investigate the excited-state and emission properties of the Au(Ⅰ) complex in both gas and crystal phases. In both environments, the S1 and T1 emitting states mainly involve HOMO and LUMO and show clear metal-ligand charge transfer and intra-ligand charge transfer characters. The good spatial separation of HOMO and LUMO minimizes the S1−T1 energy gap, which benefits the reverse intersystem crossing (rISC) from T1 to S1. At 300 K, the rISC rate is faster than the T1 phosphorescence emission, which enables the TADF emission. However, at 77 K, such a rISC process is blocked and TADF disappears; instead, only phosphorescence is recorded experimentally. Importantly, this work highlights the importance of environments in regulating luminescence properties and contributes to understanding the TADF emission of organometallic complexes.
Ultra-wide-bandgap semiconductors have tremendous potential to advance electronic devices, as device performance improves nonlinearly with increasing gap. In this work we employ density-functional theory with the accurate screened-hybrid functional to evaluate the electronic and optical properties of NaYO2 in two different phases. The electronic structure calculation results show that both monoclinic and trigonal phases of NaYO2 exhibit direct bandgaps of 5.6 eV and 5.4 eV, respectively, offering a physically realistic material platform to derive the semiconductor industry beyond the well-established diamond and GaN semiconducting materials. Next, we investigate the optical properties and reveal that both phases of NaYO2 are transparent in the infrared and visible regions, thereby, these materials can be used as infrared window materials.
Understanding organic photovoltaic (OPV) work principles and the materials' optoelectronic properties is fundamental for developing novel heterojunction materials with the aim of improving power conversion efficiency (PCE) of organic solar cells. Here, in order to understand the PCE performance (> 13%) of OPV device composed of the non-fullerene acceptor fusing naphtho[1, 2-b: 5, 6-b$ ' $]dithiophene with two thieno[3, 2-b]thiophene (IDCIC) and the polymer donor fluorobenzotriazole (FTAZ), with the aid of extensive quantum chemistry calculations, we investigated the geometries, molecular orbitals, excitations, electrostatic potentials, transferred charges and charge transfer distances of FTAZ, IDCIC and their complexes with face-on configurations, which was constructed as heterojunction interface model. The results indicate that, the prominent OPV performance of FTAZ: IDCIC heterojunction is caused by co-planarity between the donor and acceptor fragments in IDCIC, the the charge transfer (CT) and hybrid excitations of FTAZ and IDCIC, the complementary optical absorptions in visible region, and the large electrostatic potential difference between FTAZ and IDCIC. The electronic structures and excitations of FTAZ/IDCIC complexes suggest that exciton dissociation can fulfill through the decay of local excitation exciton in acceptor by means of hole transfer, which is quite different from the OPVs based on fullerenes acceptor. The rates of exciton dissociation, charge recombination and CT processes, which were evaluated by Marcus theory, support the efficient exciton dissociation that is also responsible for good photovoltaic performance.
The general application of antibiotics has brought a series of negative impacts on human health and the environment, which has aroused widespread public attention to their removal from aqueous systems. In this study, a chitosan (CS)-linked graphene oxide (GO) composite (GO-CS) was synthesized by a modified hummers/solvothermal method. It was separated from the mixed aqueous phase by low-speed centrifugation, thereby endowing the GO with high separation efficiency in water. The adsorption of tetracycline (TC), norfloxacin (NOR), and sulfadiazine (SDZ) by GO-CS were then studied by experimental techniques and theoretical calculations. In batch experiments at 298 K and optimal pH, the adsorption capacities of TC, NOR, and SDZ were 597.77, 388.99, and 136.37 mg/g, respectively, which were far better than those of pristine graphene oxide. The spectra results illustrated that the adsorption process was mainly contributed by the interactions between antibiotics and functional groups (carboxyl, hydroxyl, and amino groups) of GO-CS. Furthermore, density functional theory calculations showed that electrostatic interaction and hydrogen bonds were of vital importance for the uptake of the antibiotics; the former was extremely important for TC adsorption. This research provides theoretical references for the removal of antibiotics by graphene-based composite materials, thus offering their promising application in environmental remediation.
Metal-halide perovskite solar cells (PSCs) have attracted considerable attention during the past decade. However, due to the existence of non-radiative recombination losses, the best power conversion efficiency (PCE) is still lower than the theoretical limit defined by Shockley-Queser theory. In this work, we investigate 1, 2, 3-oxathiazin-4(3h)-one, 6-methyl-2, 2-dioxide (acesulfame potassium, abbreviated as AK) as a additional dopant for the 2, 2$ ' $, 7, 7$ ' $-tetrakis(N, N-di-p-methoxyphenyl-amine)-9, 9'-spirobifluorene (Spiro-OMeTAD) and fabricate PSCs in the air. It is found that 12 mol% fraction of AK relative to lithium bis((trifluoromethyl)sulfonyl)-amide (Li-TFSI) reduces the non-radiative recombination from 86.05% to 69.23%, resulting in an average 0.08 V of Voc enhancement. The champion solar cell gives a perovskite solar cells up to 21.9% and over 84% retention of the initial value during 720 h aging in dry air with 20%?30% humidity.
Molybdenum trioxide (MoO3) with layered structures adopts exotic physical features, which has evoked an extensive interest in electronic and photoelectronic devices. Here, we report a low-cost, simple-handle, atmospheric-pressure, and rapid-synthesis technique for growing large-scale MoO3 crystals, i.e., a modified hot plate method. The growth rate and morphology of the MoO3 crystals were well controlled by changing source temperatures and substrates. Complementary measurements, including optical microscope, atomic force microscope, X-ray diffraction, Raman spectroscope, and scanning near-field optical microscope, were used to investigate the structural and physical properties. The results reveal that large-scale MoO3 crystals with well-defined crystallinity have been obtained. Meanwhile, surface hyperbolic phonon polaritons on as-prepared MoO3 crystal planes have also been observed, which may provide an attractive insight into nanoelectronic and nanophotonic devices.
Two-dimensional thermoelectric materials is of special interest in recent years. Here, we studied the electronic and thermoelectric properties of two semiconducting carbon allotropes, γ-graphyne and its derivative, based on first-principles calculations. The small band gaps and long relaxation times of carriers benefit the thermal transport. We found that the thermoelectric efficiency in both materials is quite large, and reaches the maximum value around 900 K, with carrier concentration in the order of 1021 cm−3. Our research suggests that these two allotropes are promising candidates for the thermoelectric materials applications.
The superalkali cations and superhalogen anions commonly have different type of core moieties. Based on the previous reports that Be2H3L′ 2+ (L′=NH3 and noble gases Ne−Xe) are superalkali cations, in the present work, we designed the superhalogen anions Be2H3L2 (L=CH3 and halogens F−I), and both superalkali cations and superhalogen anions can be constructed using Be2H3 as the core moiety. The newly designed Be2H3L2 species are much more stable than their isoelectronic cationic counterparts Be2H3L′ 2+, as can be reflected by the highly exergonic substitution reaction of L′ ligand in Be2H3L′ 2+ with isoelectronic L to give Be2H3L2. These anionic species possess the well-defined electronic structure, which can be proven by their large HOMO−LUMO gaps of 4.69 eV to 5.38 eV. It is remarkable that Be2H3L2 can be regarded as the hyperhalogen anions due to the extremely high vertical detachment energies (5.38 eV to 6.06 eV) and the Be−Be distances in these species (1.776 Å to 1.826 Å) are short in ultrashort metal-metal distances (defined as dM−M<1.900 Å) between main group metals. In the designed five small model species, three of them, i.e. Be2H3L2 (L=CH3, Cl, and Br), are kinetical viable global energy minima, which are the promising target for generation and characterization in anion photoelectron spectroscopy. The analogue molecule [t-Bu−Be2H3t-Bu] with bulky protecting tert-butyl (t-Bu) groups is designed as a possible target for synthesis and isolation in condensed states.