Density Functional Theory Calculation Of Titanium-based Perovskite Type Semiconductor Photocatalysts

Currently,over 80%of the world’s energy requirements are supplied by the combustion of fossil fuels,so there is an urgent need for clean,renewable and cheap new energies to replace them.Solar energy provides more than 100 times the energy of the world’s known fossil fuel reserves each year.Semiconductor photocatalytic decomposition of water is one of the important ways to produce H₂by using solar energy.So far,hundreds of photocatalysts based on metal oxides have been reported,which have different structures and compositions.However,many semiconductor photocatalysts have low efficiency for water splitting due to their wide band gap,easy photocorrosion and low quantum efficiency.In order to overcome the drawbacks,people have tried to improve the photocatalytic activity of these semiconductor materials through doping,surface modification and construction of heterostructures.A type of compound with the defined structure of perovskite（ABX₃）has become a promising semiconductor photocatalytic material because of its simple structure,rich components,good stability and good photocatalytic performance.Among them,titanium based perovskite materials（ATi O₃）show many excellent photocatalytic activities,are widely studied and have the following advantages:（1）it can be composed of a variety of elements and has a variety of crystal structures,which is convenient for synthesis and modification;（2）The bulk phase structure of ATi O₃type has characteristics,and the surface properties can be inferred from the related properties of their bulk structures;（3）Their stoichiometric number and intrinsic vacancies are so rich,which provides a variety of means for the regulation of their properties;（4）There has been a good accumulation of research on their physical and solid-state chemical properties.Codoping with cation and anion is found to be an effective approach to tailor the electronic structures of semiconductor-based photocatalysts.In this work,a systematic hybrid density functional study has been carried out for cubic BaTiO₃with a wide bandgap,codoped with TM and X（TM=V,Nb,Ta,Mo and W,X=N and C）,with the objective of improving its photocatalytic activity for water splitting under visible light irradiation.Co-doped BaTiO₃systems are more thermodynamically stable than TM-monodoped systems,which is beneficial to the synthesis of（TM+X）co-doped materials,and less deformation of BaTiO₃crystal structure corresponds to the more favorable formation of（TM+X）-codoped systems.The band alignments for all the codoped systems are well positioned for the feasibility of both photooxidation and photoreduction of water.It is found that there are seven investigated systems with the bandgaps smaller than 3 e V.Among them,codoped systems including Mo/W with N,have been found to be the most effective for narrowing the bandgap（E_gis in the range of 1.55 e V～2.16 e V）,increasing the mobilities of photoinduced carriers and improving the photocatalytic activity of BaTiO₃under visible light.These materials are promising photocatalysts for overall water splitting under visible-light irradiation and further experimental investigations are highly demanded to explore their potential applications the photocatalytic field.Based on the investigations of bulk BaTiO₃,we have further studied the influence of co-doping metal cations and nonmetal anions on the stability,electronic structure,and photocatalytic activity of tetragonal BaTiO₃（001）surface using density functional theory calculations adopting hybrid functionals.The results show that all the metal-nonmetal co-doped BaTiO₃（metal=V,Nb,Ta,Mo,W and nonmetal=N,C）are thermodynamically stable.Most of co-doped surfaces have remarkable narrower bandgaps than the pristine BaTiO₃（001）surface,which favors the movement of absorption spectra to the longer-wavelength region.Co-doped systems have better affinity toward H₂O than the pure surface.The dissociative adsorption of water is energetically more favorable than the molecular adsorption.The HER and OER on co-doped systems tend to occur at the O site bonded to the M dopant and the Ti site adjacent to the M dopant,respectively.For pure BaTiO₃,the computed value of?_His 0.68 e V and the overpotential of OER is 1.13 V.（V+N）,（Nb+N）,（Ta+N）,（W+C）,（Mo+2N）and（W+2N）co-doped systems exhibit high activities toward the HER with calculated values of|?_H|less than 0.15 e V.（Mo+N）,（W+N）,（Mo+C）,（Mo+2N）and（W+2N）co-doped BaTiO₃are promising candidates for the OER with lower overpotentials of 0.59～0.74 V.This work sheds light on the potential application of BaTiO₃modified by surface co-doping as efficient photocatalysts for overall water splitting.SrTiO₃loaded with NiO cocatalyst is one of representative photocatalysts for overall water splitting,which have attracted considerable attention.However,the function of NiO particles in the photocatalytic system is still underbate.Herein,we present a theoretical investigation based on density functional theory calculations to reveal the origin of the high photocatalytic performance achieved by loading NiO onto SrTiO₃surface.By considering two terminations of SrTiO₃（001）,the interfacial geometries,electronic structures,charge densiand optical absorptions of NiO（001）/SrTiO₃（001）have been calculated.The interface including Sro-termination connecting with NiO（001）（NiO/Sr O）has a stronger covalent interaction than the one including Ti O₂-termination connecting with NiO（001）（NiO/Ti O₂）.The analyses on electronic properties and charge redistributions indicat that the formation of interfacial structures reduces the effective masses of photoinduced carriers and results type-II band alignment,which is conducive to separate the photogenerated carriers into two components of the interface.The electrons gathering on SrTiO₃side will take part in the hydrogen evolution reaction,while the holes accumulating on NiO side will participate in the oxygen evolution reaction.It is found that the geometrical reconstruction around the interface has no influence on the extension of absorption edge due to the characteristic of heterostructures with indirect band gap.This study reveals that NiO functions as the water oxidation cocatalyst suppress the recombination of photoinduced electron and hole pairs,which leads to the enhancement of photocatalytic activity in SrTiO₃system.