Two-dimensional (2D) B-C compounds possess rich allotropic structures with many applications. Obtaining new 2D B4C3 structures is highly desirable due to the novel applications of three-dimensional (3D) B4C3 in protections. In this work, we proposed a new family of 2D B4C3 from the first-principles calculations. Distinct from previous observations, this family of 2D B4C3 consists of bonded 2D B4C3 bilayers. Six different types of bilayers with distinct bonded structures are found. The phonon spectrum calculations and ab initio molecular dynamics simulations at room temperature demonstrate their dynamic and thermal stabilities. Low formation energies suggest the high possibility of realizing such structures in experiments. Rich electronic structures are found, and the predicted Young's moduli are even higher than those of the previous ones. It is revealed that the unique electronic and mechanical properties are rooted in the bonding structures, indicating the prompting applications of this family of 2D B4C3 materials in photovoltaics, nanoelectronics, and nanomechanics.

The assignment of experimental optical absorption spectra of protonated anthracene has been under debate for years. It is complicated by the presence of rich vibronic spectral features and the possible co-occurrence of two isomers, 9H-An(+) and 1H-An(+). In this study, the vibrationally resolved absorption spectra of 9H-An(+) and 1H-An(+) have been calculated using time-dependent density functional theory. The calculated vibronic spectra profiles of 9H-An(+) and 1H-An(+) are in excellent agreementwith the corresponding experimental results and provide unambiguously spectra assignments. It shows that the previously reported assignments based on vertical excitation energy are largelywrong. The onset located at 493.8 nmof the experimental spectrumcan be assigned to the S-0 -> S-1 transition of 9H-An(+), while the origin band located at 453.5 nmcorresponds to the S-0 -> S-2 transition of 1H-An(+).

Switchable trans-cis isomerization of azobenzene (AB) and its derivatives on metallic surfaces have offered rich possibilities to functionalize molecular devices. However, the lack of a good understanding of the isomerization pathway has severely limited our ability for rational design. One of the long-debated issues is the cis configuration of the parental AB on the Au(111) surface, for which the experimentally inferred structure differs from the theoretically predicted global minimum. Here, we theoretically identify a new in situ metastable configuration for cis-AB on Au(111) that can reproduce all the observations reported in the scanning tunneling microscopy experiments. It reveals that the bistability of AB on the Au(111) surface is attributed to the significantly increased kinetic stability of the newly discovered cis-AB isomer. A fascinating tumbling pathway that overcomes two energy barriers stimulated by tunneling electrons for the trans-cis AB isomerization on Au(111) has been verified, suggesting a new type of molecular motion based on the AB systems.

Photocatalytic synthesis of organic compounds has attracted more and more attention recently. In this work, we present a theoretical study on the molecular mechanism of the photocatalytic reduction of nitrobenzene to aniline on the rutile TiO2(110) surface. We have studied the adsorption and conversion of nitrobenzene at both the surface Ti site and the oxygen vacancy (O-v) site. The full reaction pathways at these two sites were calculated. The rate-limiting step and possible intermediates were identified. The results suggest that O-v is more active in the adsorption and conversion of nitrobenzene. Interestingly, we found that the chemistry of nitrobenzene on the rutile TiO2(110) surface, especially the breaking of the N-O bond, is closely related to the number of excess electrons available. Based on the calculation, we have proposed a full molecular mechanism which is compatible with the existing experiments. The results should be helpful for the design of more efficient photocatalysts for the conversion of nitrobenzene.

Solar-driven conversion of CO2 with H-terminated silicon has recently attracted increasing interest. However, the molecular mechanism of the reaction is still not well understood. A systematic study of the mechanism has been carried out with first-principles calculations. The formation energies of the intermediates are found to be insensitive to the structure of the surface. On the fully H-terminated Si(111) surface, several pathways for the conversion of CO2 into CO at a coordinatively saturated Si site are studied, including the conventional COOH* pathway and the direct insertion of CO2 into Si−H and Si−Si bonds. Although the barrier of the COOH* pathway is lowest among the three pathways, it is higher than that for OH* elimination, which suggests that CO2 should be converted by other types of active site. The reaction at the isolated and dual coordinatively unsaturated (CUS) Si sites, which can be generated by light illumination, heat, and Pd loading, are then studied. The results suggest that the most efficient pathway to convert CO2 is to convert it into CO and O* at an isolated CUS Si site before O* reacts with a terminating H* to form adsorbed OH* and generate new isolated CUS Si sites. Therefore, the CUS Si site catalyzes the reaction until all H* is converted into OH*. The results provide new insight into the mechanism of the reaction and should be helpful for the design of more efficient Si-based catalysts for CO2 conversion. 

Aims. Due to the limitations of current computational technology, the fragmentation and isomerization products of vibrationally-excited polycyclic aromatic hydrocarbon (PAH) molecules and their derivatives have been poorly studied. In this work, we investigate the intermediate products of PAHs and their derivatives as well as the gas-phase reactions relevant to the interstellar medium, with coronene as a case study. Methods. Based on the semi-empirical method of PM3 as implemented in the CP2K program, molecular dynamics simulations were performed to model the major processes (e.g., vibrations, fragmentations, and isomerizations) of coronene and its derivatives (e.g., methylated coronene, hydrogenated coronene, dehydrogenated coronene, nitrogen-substituted coronene, and oxygen-substituted coronene) at temperatures of 3000 K and 4000 K. Results. We find that the anharmonic effects are crucial for the simulation of vibrational excitation. For the molecules studied here, H-2, CO, HCN, and CH2 are the major fragments. Following the dissociation of these small units, most of the molecules could maintain their ring structures, but a few molecules would completely break into carbon chains. The transformation from a hexagon to a pentagon or a heptagon may occur and the heteroatomic substitutions (e.g., N- or O-substitutions) would facilitate the transformation.

Molecular junctions hold great potential for future microelectronics, while the practical utilization has long been limited by the problem of conformational deformation during charge transport. Here we present a first-principles theoretical study on the surface-enhanced Raman spectroscopy (SERS) characterization of the p-terphenyl-4,4 ''-dithiol molecule and its 2,2',5',2 ''-tetramethylated analogue in gold junctions to investigate the molecular deformation mechanism. The effects of charge injection and external electric field were examined, both of which could change pi-conjugation by varying the dihedral angle between the central and ending rings (D-IPT). The induced significant structural deformations then change SERS responses. Only the SERS responses under an external electric field can account for the experimentally observed Raman spectra, and those of charge injections cannot. Moreover, applying a strong electric field could enlarge the conductivities of the two molecular junctions, agreeing well with experiments. This information not only elaborates that the electric field effect constitutes one important mechanism for molecular deformation but also provides useful insights for the control of charge transport in molecular junctions.

Dioxygen (O-2) activation is a vital step in many oxidation reactions, and a graphitic carbon nitride (g-C3N4) sheet is known as a famous semiconductor catalytic material. Here, we report that the atomic boron (B)-doped g-C3N4 (B/g-C3N4) can be used as a highly efficient catalyst for O-2 activation. Our first-principles results show that O-2 can be easily chemisorbed at the B site and thus can be highly activated, featured by an elongated O-O bond (similar to 1.52 angstrom). Interestingly, the O-O cleavage is almost barrier free at room temperatures, independent of the doping concentration. It is revealed that the B atom can induce considerable spin polarization on B/g-C3N4, which accounts for O-2 activation. The doping concentration determines the coupling configuration of net-spin and thus the magnitude of the magnetism. However, the distribution of net-spin at the active site is independent of the doping concentration, giving rise to the doping concentration-independent catalytic capacity. The unique monolayer geometry and the existing multiple active sites may facilitate the adsorption and activation of O-2 from two sides, and the newly generated surface oxygen-containing groups can catalyze the oxidation coupling of methane to ethane. The present findings pave a new way to design g-C3N4-based metal-free catalysts for oxidation reactions.

Tremendous advances in nanoscience have been made since the discovery of fullerenes. However, the short timescale of the growth process and high-energy conditions of synthesis result in severe constraints to investigation of the mechanism of fullerene formation. In this work, we attempted to reveal the formation process by analyzing the variation in the yield of fullerenes under different conditions. Experiments and theoretical analysis show that the formation of fullerenes could be affected by the addition of polycyclic aromatic compounds. It is proposed that the formation of C-60 during arc-discharge synthesis is fragment assembling, while the yield of C-2m (m=35, 38, 39) is strongly enhanced by building-block splicing. In addition, several features of the building blocks are put forward to predict the extent of their influence to the formation of larger fullerenes C-2n (n >= 42). This work not only provides essential insight into the formation process of fullerenes, but more importantly also paves the way to improving the yield of larger fullerenes selectively.

Conversion of CO2 and H2O into value-added organic molecules via artificial photosynthesis is a promising solution to current energy and environment problems. In the reaction, it is generally believed that CO2 is converted into organic molecules by photogenerated electrons and protons that result from photo-oxidation of H2O. In this work, we investigate the possibility that H2O, without being oxidized, directly donates protons to CO2 and other intermediates adsorbed at the oxygen vacancy on the anatase TiO2(101) surface. We found that this can greatly lower the barriers (by about 0.3 eV) for the hydrogenation of CO2, CO, H2CO, and CH3O because less energy is required to displace these adsorbates to accept the proton (in H2O). The OH- group produced in these reactions can recombine with a surface-adsorbed proton to form a new H2O molecule, making H2O a shuttling center of the adsorbed protons, or it can take part in the oxygen evolution reaction with a lower barrier. The results suggest that H2O can play multiple roles in artificial photosynthesis and the reduction and oxidation parts of the reaction may have synergistic effects.