Kaining Ding

Density functional theory study on the electronic and optical properties of three crystalline phases of BiVO4

The differences in the photocatalytic activity of bismuth vanadate in three crystalline phases have been investigated through calculating their electronic structures and optical properties based on density functional theory. Our results indicate that zircon-tetragonal BiVO4 has a direct and wide band gap, while monoclinic BiVO4 and scheelite-tetragonal BiVO4 has indirect and narrow band gaps. The density of states and atom populations of monoclinic and scheelite-tetragonal phases are similar, but slightly different from those of zircon-tetragonal phase. Among three phases, the monoclinic BiVO4 possesses the largest dipole moment and the lightest effective mass of carriers, which can promote the generation and separation of photo-induced carriers, and subsequently may improve photocatalytic activity.

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.

Solvothermal synthesis of two new coordination polymers: in situ heterocycle conversion and N-alkylation, network topologies and luminescence properties

Two new copper complexes [CuI4(L2)I5]n (1, L2 = 3,5-bis(1-ethylpyridinium-4-yl)-1,2,4-triazol-4-ide) and [CuI6(L3)3Br3]n (2, L3 = 3,5-bis(4-pyridyl)-1,2,4-triazolate) were obtained from the solvothermal reactions of different copper halides with 2,5-bis(4-pyridyl)-1,3,4-oxadiazole (L1) in the presence of aqueous ammonia. The X-ray diffraction, IR spectrum and elemental analyses of 1 and 2 clearly show that the 1,3,4-oxadiazole ligand L1 has transformed into the triazolate ligands L2 and L3. Most fascinatingly, 1 represents the first example of an integrated reaction system involving heterocyclic conversion from oxadiazole to triazolate, N-alkylation, and further self-assembly of the in situ generated ligand L2 with metal cations to form a coordination polymer in one solvothermal spot. The possible formation mechanism of 1 is proposed, and in addition, the interesting topologies and luminescence properties of 2 are studied based on the results of the DFT calculations.

Electronic properties of red and black phosphorous and their potential application as photocatalysts

To explore the photocatalytic performance of mono-elemental semiconductors, the electronic structure and optical properties of red and black phosphorous were investigated using first-principles calculations. Interestingly, although red phosphorous (rP) in the bulk form is a typical indirect semiconductor, it transforms into a direct semiconductor when thinned to a monolayer. The increased band gap still spans the redox potential levels of water with stronger oxidizing capacity. Additionally, the lighter charge carrier mobility is hardly affected by the smaller electrostatic potential in the plane, which favors photocatalysis. Black phosphorous (bP) in the bulk form is a narrow band gap semiconductor with high electronic mobility. Its band gap can be tuned as the number of layers is reduced and the interlayer distance is widened. In monolayer bP, the high efficiency of charge carrier mobility is retained, and its band gap increases to 1.67 eV, which indicates an opportune response to visible light irradiation. The redox potentials of the valence band and conduction band edges are suitable for the catalysis of the water splitting reaction.

Effects of Ti doping at the reduced SnO 2 (110) surface with different oxygen vacancies: a first principles study

A series of Ti-doped SnO2(110) surfaces with different oxygen vacancies have been investigated by means of first principles DFT calculations combined with a slab model. Three kinds of defective SnO2(110) surfaces are considered, including the formations of bridging oxygen (Ob) vacancy, in-plane oxygen (Oi) vacancy, and the coexistence of Ob and Oi vacancies. Our results indicate that Ti dopant prefers the fivefold-coordinated Sn site on the top layer for the surface with Ob or Oi vacancy, while the replacement of sublayer Sn atom becomes the most energetically favorable structure if the Ob and Oi vacancies are presented simultaneously. Based on analyzing the band structure of the most stable configuration, the presence of Ti leads to the variation of the band gap state, which is different for three defective SnO2(110) surfaces. For the surface with Ob or Oi vacancy, the component of the defect state is modified, and the reaction activity of the corresponding surface is enhanced. Hence, the sensing performance of SnO2 may be improved after introducing Ti dopant. However, for the third kind of reduced surface with the coexistence of Ob and Oi vacancies, the sublayer doping has little influence on the defect state, and only in this case, the Ti doping state partly appears in the band gap of SnO2(110) surface.

Electrochemical fabrication of shape-controlled Cu2O with spheres, octahedrons and truncated octahedrons and their electrocatalysis for ORR

Transition metal oxide catalysts have been the focus of recent research for the oxygen reduction reaction (ORR). However, the relationship between morphology and electrocatalysis of metal oxide catalysts has not been well studied or understood hitherto. In the present work, cuprous oxides (Cu2O) with three different morphologies (sphere, octahedron, and truncated octahedron) have been synthesized employing a potentiostatic electrodeposition method. The as-prepared Cu2O has been characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Three different shapes of Cu2O crystals were used to study the shape-dependent electrocatalytic properties for ORR in alkaline media. It was found that the surface-specific activity of Cu2O with truncated octahedron shape toward ORR was higher than that of Cu2O with a sphere or octahedron morphology. The morphology–activity relationship of Cu2O as a cathode catalyst was further studied through periodic spin density functional theory (DFT) calculations. Comprehensive analysis of electrocatalysis exiperiments and DFT calculations reveals that Cu2O as a truncated octahedron preferentially exposes the (100) crystal planes, which are beneficial to stronger O2 adsorption and easier activation of adsorbed O2 on the surface of Cu2O. This finding furthers our understanding of Cu2O catalysis and provides new light on the relevant catalyst design criteria.

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 .

Effects of ligand functionalization on the photocatalytic properties of titanium-based MOF: A density functional theory study

The first principle calculations have been performed to investigate the geometries, band structures and optical absorptions of a series of MIL-125 MOFs, in which the 1,4-benzenedicarboxylate (BDC) linkers are modified by different types and amounts of chemical groups, including NH2, OH, and NO2. Our results indicate that new energy bands will appear in the band gap of pristine MIL-125 after introducing new group into BDC linker, but the components of these band gap states and the valence band edge position are sensitive to the type of functional group as well as the corresponding amount. Especially, only the incorporation of amino group can obviously decrease the band gap of MIL-125, and the further reduction of the band gap can be observed if the amount of NH2 is increased. Although MIL-125 functionalized by NH2 group exhibits relatively weak or no activity for the photocatalytic O2 evolution by splitting water, such ligand modification can effectively improve the efficiency in H2 production because now the optical absorption in the visible light region is significantly enhanced. Furthermore, the adsorption of water molecule becomes more favorable after introducing of amino group, which is also beneficial for the water-splitting reaction. The present study can provide theoretical insights to design new photocatalysts based on MIL-125.The first principle calculations have been performed to investigate the geometries, band structures and optical absorptions of a series of MIL-125 MOFs, in which the 1,4-benzenedicarboxylate (BDC) linkers are modified by different types and amounts of chemical groups, including NH2, OH, and NO2. Our results indicate that new energy bands will appear in the band gap of pristine MIL-125 after introducing new group into BDC linker, but the components of these band gap states and the valence band edge position are sensitive to the type of functional group as well as the corresponding amount. Especially, only the incorporation of amino group can obviously decrease the band gap of MIL-125, and the further reduction of the band gap can be observed if the amount of NH2 is increased. Although MIL-125 functionalized by NH2 group exhibits relatively weak or no activity for the photocatalytic O2 evolution by splitting water, such ligand modification can effectively improve the efficiency in H2 production because now ...

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.

Pressure-tuning the nonlinear-optical properties of AgGaS 2 crystal: a first-principle study

A specialized genetic algorithm approach in combination with first-principles calculations is employed to predict the stable structures of AgGaS2 crystal at different pressures. The results show that the chalcopyrite structure first transforms to the monoclinic Cc phase, and then to a centrosymmetric structure that the second-harmonic generation (SHG) response of AgGaS2 is disappeared. The effects of external pressures, up to 7 GPa, on the linear and second-order nonlinear optical properties of AgGaS2 are explored systematically. Our work reveals that the resistance to laser-induced damage, the transparency range, and the phase matchability can be improved by the pressure-induced deformation of AgGaS2 crystal. Moreover, the feature of the strong SHG response of AgGaS2 crystal is still preserved in the whole IR region even under pressure up to 7 GPa.