Yong-Fan Zhang

CO adsorption on MgO(001) surface with oxygen vacancy and its low-coordinated surface sites: embedded cluster model density functional study employing charge self-consistent technique

Abstract The adsorption of CO on regular and defect sites of MgO(0xa00xa01) surface has been studied theoretically using embedded cluster models by DFT/B3LYP method. The value of embedded point charges is determined by the charge self-consistent technique. The calculated results indicate that CO adsorption energy on the regular site of MgO(0xa00xa01) surface can agree well with the recent experimental data. The frequency shifts of CO for regular five-coordinated terrace, low four-coordinated edge and three-coordinated corner sites, via C bound down on cationic centers of MgO(0xa00xa01) surface are also satisfactorily close to the values of experiment. At the same time, the adsorption of CO on MgO(0xa00xa01) surface with neutral and charged oxygen vacancies, Fs, Fs+ and Fs2+ centers, has been investigated whose results show that MgO(0xa00xa01) surface with neutral oxygen vacancy has probably the good catalysis structure for CO adsorptive-decomposition. This is consistent with our previous study using a different method. By analyzing the bond component of Mg–C, it is found that the essential reasons why C–O bond strength is weaken or strengthen are the competitive results of 4σ* lone pair electrons and 5σ electrons of CO transferring from the adsorbate to substrate simultaneously.

Structural characterizations and electronic properties of Ti-doped SnO2(110) surface: A first-principles study

The Ti-doped SnO2(110) surface has been investigated by using first-principles method with a slab model. The geometrical optimizations and band-structure calculations have been performed for four possible doping models. Our results indicate that the substitution of Ti for sixfold-coordinated Sn atom at the top layer is most energetically favorable. Compared to the undoped surface, those Sn and O atoms located above Ti atom tend to move toward the bulk side. Besides the surface relaxations, the doping of Ti has significant influences on the electronic structures of SnO2(110) surface, including the value and position of minimum band gap, the components of valence and conduction bands, the distributions of the charge densities, and the work function of the surface. Furthermore, the effects introduced by the substitution of Ti atom observed in the experiments can be well explained when the sixfold-coordinated Sn atom at the first layer is replaced by Ti atom.

The adsorption and dissociation of Cl2 on the MgO (001) surface with vacancies: Embedded cluster model study

The adsorption of Cl(2) at a low-coordinated oxygen site (edge or corner site) and vacancy site (terrace, edge, corner F, F(+), or F(2+) center) has been studied by the density functional method, in conjunction with the embedded cluster models. First, we have studied the adsorption of Cl(2) at the edge and corner oxygen sites and the results show that Cl(2), energetically, is inclined to adsorb at the corner oxygen site. Moreover, similar to the most advantageous adsorption mode for Cl(2) on the MgO (001) perfect surface, the most favorable adsorption occurs when Cl(2) approaches the corner oxygen site along the normal direction. A small amount of electrons are transferred from the substrate to the antibonding orbital of the adsorbate, leading to the Cl-Cl bond strength weakened a little. Regarding Cl(2) adsorption at the oxygen vacancy site (F, F(+), or F(2+) center), both large adsorption energies and rather much elongation of the Cl-Cl bond length have been obtained, in particular at the corner oxygen vacancy site, with concurrently large amounts of electrons transferred from the substrate to the antibonding orbital of Cl(2). It suggests, at the oxygen vacancy site, that Cl(2) prefers to dissociate into Cl subspecies. And the potential energy surface indicates that the dissociation process of molecular Cl(2) to atomic Cl is virtually barrierless.

Computational design of inorganic nonlinear optical crystals based on a genetic algorithm

Based on the results of density functional theory calculations, a theoretical method to design inorganic nonlinear optical (NLO) crystals for second harmonic generation (SHG) is presented. In this method, a specialized genetic algorithm (GA) is developed to search the stable structures of the inorganic crystal with known compositions and study the noncentrosymmetric stable structures and the second-order nonlinear optical properties by calculating the corresponding SHG coefficients. Unlike normal GA techniques, the main feature of the present method is that the coordination fashions of the building units are introduced to construct the structures of individuals during the GA procedure, which can obviously improve the efficiency and success rate of obtaining the stable structure of the inorganic crystals. As typical examples, two ternary compounds, AgGaS2 and LiAsSe2 crystals are considered, and besides the structures observed experimentally, the geometries and optical performance of other metastable (or more stable) phases have been explored. Our results clearly demonstrate that the present method can provide a feasible way to design and optimize new inorganic NLO crystals.

S Adsorption at Regular and Defect Sites of the MgO (001) Surface: Cluster Model Study at DFT Level

We have studied the adsorption of sulfur at regular and defect sites of the MgO (001) surface using cluster models embedded in a large array of point charges by the density functional method. The calculated results indicate that it is a chemical adsorption regarding sulfur at both the regular site and the defect site of the MgO (001) surface. Especially for sulfur adsorbed at different oxygen vacancy sites (F, F+ and F2+ centers) and different magnesium vacancy sites (V, V- and V2- centers), it has very large adsorption energies, which reflects the fact that the MgO (001) surface with the vacancies is an excellent adsorbent for sulfur adsorption. Besides, we find that the adsorbed sulfur is almost inserted into the lattice for sulfur adsorbed at the magnesium vacancy site of the MgO (001) surface. The adsorption energy of sulfur on the MgO (001) surface with magnesium vacancies is much larger when compared to that on the MgO (001) surface with oxygen vacancies. At the same time, it is also found that the S behaves as an electron acceptor except that it is adsorbed at the magnesium vacancy site behaving as an electron donor.

THEORETICAL STUDY OF N2O ADSORPTION AND DECOMPOSITION AT REGULAR AND DEFECT SITES OF MgO (001) SURFACE

The adsorption of N2O at regular and defect sites of MgO (001) surface has been studied systematically using cluster models embedded in a large array of point charges by density functional method. The calculated results show that the MgO (001) surface with oxygen vacancies exhibits high catalytic reactivity toward N2O adsorptive-decomposition at variance with the regular MgO surface or the surface with magnesium vacancies. Much elongation of O–N bond of N2O after adsorption at oxygen vacancy site with O end of N2O down indicates that O–N bond has been broken with concurrent production of N2, leaving a regular site instead of the original oxygen vacancy site (F center). Besides, the MgO (001) surface with magnesium vacancies hardly exhibits catalytic reactivity. It can be concluded that N2O dissociation is likely occurred at MgO (001) surface oxygen vacancy sites, which is consistent with the generally accepted viewpoint in the experiment.

Different Atomic Terminations Affect the Photocatalytic Nitrogen Fixation of Bismuth Oxybromide: A First Principles Study

We have systematically investigated the electronic structures and activation capacities of BiOBr {001} facets with different atomic terminations by means of DFT methods. Our calculations reveal that oxygen vacancies (OVs) give a significant boost in band edges of the O-terminated BiOBr {001} facets, and excess electrons induced by OVs could exceed the reduction potentials of high-energy N2 intermediates. Interestingly, the Bi-terminated BiOBr {001} facets may be good candidates for photocatalytic nitrogen fixation due to the stronger activation ability of N2 molecules comparing with O-terminated BiOBr {001} facets with OVs. Moreover, the Bi-terminated BiOBr {001} facets may tend to yield NH3 instead of N2 H4 .

Why does F-doping enhance the photocatalytic water-splitting performance of mBiVO4? – a density functional theory study

By means of density functional theory (DFT) computations, we investigated the variations in the geometric structures and electronic properties, as well as the adsorption behavior of water on the (010) and (110) surfaces, introduced by an F dopant in a monoclinic BiVO4 system. For the bulk phase, F atoms are easier to substitute O atoms as they form a stable geometric structure. With F-doping, the band gap is narrowed and the separation efficiency of the photogenerated carriers is improved. On both (010) and (110) surfaces, F atoms prefer to substitute the two-coordinated O atoms at the outermost layer. Besides, F-doping on the surfaces can also reduce the band gap, which may enhance the visible light utilization. Due to hydrogen bonds between the F dopant and the O atom of water, the interactions between the F-doped surface and the absorbed water molecules are increased, which is favorable for water splitting under visible light.