Honggang Sun

ZnWO4/BiOI heterostructures with highly efficient visible light photocatalytic activity: the case of interface lattice and energy level match

ZnWO4/BiOI heterostructures with different constituents are synthesized via a chemical bath approach under mild conditions by tuning the Zn/Bi molar ratios. The obtained ZnWO4/BiOI heterostructures display high photocatalytic activities in degradation of MO and photocurrent response under visible light irradiation. Combining the experimental findings, first-principles calculations are used to investigate the surface geometry structures and the work functions of the (011) and (010) surfaces of the ZnWO4 phase and the (001) surface of the BiOI phase. The results show that the lattice and energy levels between the ZnWO4 and BiOI phases match well with each other to be capable of forming efficient ZnWO4/BiOI p–n heterojunction structures. This match promotes the separation and transfer of photoinduced electron–hole pairs at the interface, resulting in the excellent photocatalytic performance of the ZnWO4/BiOI heterostructures. Our findings show that the formation of a heterostructure would possess the excellent photocatalytic activities only if the lattice and energy level match between the two semiconductors was satisfied, which is of great importance for designing and developing more efficient heterostructured photocatalysts.

Surface dependence of CO2 adsorption on Zn2GeO4.

An understanding of the interaction between Zn(2)GeO(4) and the CO(2) molecule is vital for developing its role in the photocatalytic reduction of CO(2). In this study, we present the structure and energetics of CO(2) adsorbed onto the stoichiometric perfectly and the oxygen vacancy defect of Zn(2)GeO(4) (010) and (001) surfaces using density functional theory slab calculations. The major finding is that the surface structure of the Zn(2)GeO(4) is important for CO(2) adsorption and activation, i.e., the interaction of CO(2) with Zn(2)GeO(4) surfaces is structure-dependent. The ability of CO(2) adsorption on (001) is higher than that of CO(2) adsorption on (010). For the (010) surface, the active sites O(2c)···Ge(3c) and Ge(3c)-O(3c) interact with the CO(2) molecule leading to a bidentate carbonate species. The presence of Ge(3c)-O(2c)···Ge(3c) bonds on the (001) surface strengthens the interaction of CO(2) with the (001) surface, and results in a bridged carbonate-like species. Furthermore, a comparison of the calculated adsorption energies of CO(2) adsorption on perfect and defective Zn(2)GeO(4) (010) and (001) surfaces shows that CO(2) has the strongest adsorption near a surface oxygen vacancy site, with an adsorption energy -1.05 to -2.17 eV, stronger than adsorption of CO(2) on perfect Zn(2)GeO(4) surfaces (E(ads) = -0.91 to -1.12 eV) or adsorption of CO(2) on a surface oxygen defect site (E(ads) = -0.24 to -0.95 eV). Additionally, for the defective Zn(2)GeO(4) surfaces, the oxygen vacancies are the active sites. CO(2) that adsorbs directly at the Vo site can be dissociated into CO and O and the Vo defect can be healed by the oxygen atom released during the dissociation process. On further analysis of the dissociative adsorption mechanism of CO(2) on the surface oxygen defect site, we concluded that dissociative adsorption of CO(2) favors the stepwise dissociation mechanism and the dissociation process can be described as CO(2) + Vo → CO(2)(δ-)/Vo → CO(adsorbed) + O(surface). This result has an important implication for understanding the photoreduction of CO(2) by using Zn(2)GeO(4) nanoribbons.

First-principles studies of electronic, optical, and vibrational properties of LaVO4 polymorph

First-principles calculations of electronic, optical, and vibrational properties of LaVO4 polymorph were performed with the density functional theory plane-wave pseudopotential method. The results of the electronic structure reveal that the different coordinated structure for monoclinic LaVO4 leads to an indirect band gap, while tetragonal LaVO4 has a direct band gap. Besides, the analysis of the electronic structure shows ionic nature in La–O bonds and covalent nature in V–O bonds. From further study in chemical bonding behavior, we find that the V–O covalent bonds have four types: σ bonding, π bonding, π∗ antibonding, and σ∗ antibonding states. Various optical properties, including the dielectric function, reflectivity, absorption coefficient, refractive index, and the energy-loss spectrum as functions of the photon energy were calculated. Our calculations indicate that monoclinic LaVO4 has excellent dielectric properties along [0 0 1] direction. In the optical-frequency (ω→∞) contributed from electrons...

Microcrystalline sodium tungsten bronze nanowire bundles as efficient visible light-responsive photocatalysts

Microcrystalline sodium tungsten bronze nanowire bundles were obtained via a facile hydrothermal synthesis, and were applied in water purification as visible-light-driven photocatalysts for the first time.

Optical Properties of the High-Pressure Phases of SnO2: First-Principles Calculation

We present a detailed investigation on the optical properties, including dielectric function, reflectivity, absorption, refractive index, and electron energy-loss spectrum, of the high-pressure phase SnO(2) in the rutile, pyrite, fluorite, and cotunnite structures by using the density functional theory (DFT) plane-wave pseudopotential method. The results indicate that with the increasing of pressure the band gaps become larger, the density of states are broader, so the curves of optical properties have a little blue shift. Except that the fluorite phase has some metallic properties, the other three phases exhibit excellent dielectric behavior. Interestingly, the fluorite and cotunnite SnO(2) phases always have some special characteristics, such as higher plasma frequency, which need further fundamental and application research.

Synergistic effects in La/N codoped TiO2 anatase (101) surface correlated with enhanced visible-light photocatalytic activity.

The interaction between implanted La, substitutional N, and an oxygen vacancy at TiO(2) anatase (101) surface has been investigated by means of first-principles density function theory calculations to investigate the origin of enhanced visible-light photocatalytic activity of La/N-codoped anatase observed in experiments. Our calculations suggest that both the adsorptive and substitutional La-doped TiO(2) anatase (101) surfaces are probably defective configurations in experiments. The h-Cave-adsorbed La doping decreases the formation energy for the substitutional N implantation and vice versa, while the charge compensation effects do not take effect between the adsorptive La and substitutional N dopants, resulting in some partially occupied states in the band gap acting as traps of the photoexcited electrons. The Ti(5c)-substituted La doping decreases the energy required for the substitutional N implantation, and the substitutional La and N codoping promotes the formation of an oxygen vacancy, which migrates from the O(sb-3c) site at the inner layer toward the surface O(b) site. For the substitutional La/N-codoped (Ti(5c)_O(3c-down)) surface, the charge compensation between the substitutional La and substitutional N leads to the formation of two isolated occupied N(s)-O π* impurity levels in the gap, while the excitation energy from the higher impurity level to the CBM decreases by about 0.89 eV. After further considering an oxygen vacancy on the Ti(5c)_O(3c-down) surface, the two electrons on the double donor levels (O(b) vacancy) passivate the same amount of holes on the acceptor levels (substitutional La and N), forming the acceptor-donor-acceptor compensation pair, which provides a reasonable mechanism for the enhanced visible-light photocatalytic activity of La/N codoped TiO(2) anatase. This knowledge may aid the further design and construction of new effective visible-light photocatalysts.

Electronic, optical and lattice dynamic properties of the novel diamond-like semiconductors Li2CdGeS4 and Li2CdSnS4.

Li(2)CdGeS(4) and Li(2)CdSnS(4) are novel quaternary diamond-like semiconductors (DLSs) which have been synthesized recently. We present first-principles calculations of their electronic, optical and lattice dynamic properties with the plane-wave pseudopotential method. We have found an indirect band gap of 2.78 eV for Li(2)CdGeS(4) and a direct band gap of 2.50 eV for Li(2)CdSnS(4). The serious stretching vibrations of the Ge/Sn-S and Li-S bonds may enhance their phonon energies, and cause them to exhibit high heat capacities and Debye temperatures, which are promising for nonlinear optical applications. Compared with Cu-based DLSs, Li plays a key role in enlarging the band gaps and increasing the lattice phonon energies, which would increase the thermal conductivity accompanied by an increase of the optical damage threshold.

First-principles study of the electronic, optical, and lattice dynamics properties of LiInSe2 polymorph

First-principles calculations of the electronic, optical, and lattice dynamics properties of LiInSe2 polymorph were performed with the density functional theory plane-wave pseudopotential method. The results of electronic structure reveal that the different coordinated structure for α-NaFeO2-type LiInSe2 leads to the broadening of density of states and the smaller band gap. Various optical properties, including the dielectric function, reflectivity, absorption coefficient, refractive index, and the energy-loss spectrum as functions of the photon energy were calculated and are found to be in good agreement with experiments. We also presented phonon dispersion relation, zone-center optical mode frequency, density of phonon states, and some thermodynamic properties, such as constant volume heat capacity and Debye temperature. The results show that the α-NaFeO2-type LiInSe2 does not only have special optical properties but also demonstrates special vibrational properties and thermodynamic properties, which ma...

Structural, electronic, and optical properties of α, β, and γ-TeO2

First-principles calculations of the structural, electronic, and optical properties of TeO2 polymorphs were performed with the density functional theory plane-wave pseudopotential method. The results reveal that all the three crystalline TeO2 phases are wide-gap semiconductors and the lone electron pairs have contributions near the Fermi energy level. The layer structure of β-TeO2 leads to the obvious anisotropy of the complex dielectric function. Considering the lattice contribution of dielectric constants, we predict the static dielectric constants of TeO2 polymorphs. For α-TeO2, the calculated values of 19.0 for e1⊥ and 25.3 for e1∥ agree well with the experimental value, and the β- and γ-phases also belong to the high dielectric constant materials. Besides, a special collective plasma resonance for γ-TeO2 has been found in lower energy. It corresponds to the small peak in the imaginary part of dielectric function, and reflects the abrupt reduction in the reflectivity spectrum.

Effects of surface chemistry on the morphology transformation of ZnWO4 nanocrystals: investigated from experiment and theoretical calculations

The morphology of the nanocrystals is a critical parameter for determining their performance in particular applications. In our experimental work, ZnWO4 nanocrystals with different morphologies are synthesized by the hydrothermal method. It has been found that the shape of ZnWO4 nanocrystals is strongly dependent upon the pH of the growth solution. Combining the experimental findings, first-principles calculations are used to investigate the microscopic mechanism of morphology transformation of ZnWO4 nanocrystals controlled by surface chemistry. Surface structures, surface energies and surface tensions of several low-index surfaces of ZnWO4 under the different surface passivated conditions are examined. The surface energies of (010) and (011) facets are increased when the fraction of hydroxyl in the adsorbates is increased. The (100) surface is consistently the highest energy surface except for the most-basic conditions. According to the Wulff law, the equilibrium shapes of ZnWO4 nanocrystals are obtained from the respective surface energies. Our results show that the effect of surface chemistry on the morphology of ZnWO4 nanocrystals is apparent. The aspect ratio of the nanocrystals is reduced gradually when the basicity of the surface conditions is increased, which is consistent with our experimental findings. A modified Wulff construction model which considers the effect of surface tension draws the same conclusions. Our investigations are meaningful to gain further understanding of how to achieve shape-controlled nanocrystal synthesis by tuning the surface chemistry.