Theoretical Studies On Charges Transport And Water Oxidation Reaction On ATiO₃?A=Sr,Ba,Pb? Semiconductor Photocatalysts

Hydrogen is considered as a potential energy carrier due to its high energy capacity,renewable and environment-friendly characteristics.The produce of hydrogen from overall water splitting by solar photocatalysis is one of the most promising new energy technologies.There are three steps in overall water splitting on semiconductor photocatalyst: the semiconductor absorbs photon energy greater than the band-gap energy of the material and generates photoexcited electron-hole pairs;the photoexcited charges are separated and migrated to the surface;water is oxidized and reduced with the photogenerated holes and electrons to produce O2 and H2,respectively.Perovskite oxide has rich content,high photochemical?electrochemical?stability,flexible composition and structure which make it possible to become a catalyst with high catalytic activity and good sunlight absorption capacity.SrTiO₃ is a typical perovskite oxide.The details of water splitting on SrTiO₃ are not clear.The lattice constants of BaTiO₃ and PbTiO₃ are similar to SrTiO₃,which are also used for photocatalysis water splitting.In this work,taking ATiO₃?A = Sr,Ba,Pb?perovskite as models,the photo-generated charge separation and surface water oxidation for photocatalytic water splitting were investigated and the main results are summarized as follows.1.In SrTiO₃,OERs on clean and oxygen deficient?100?,?110?,and?111?surfaces are performed by density functional theory.We found that,on the clean?100?surface,the overpotential is only 0.66 V,which is small enough for OER taking place on this surface.The excess electron left by Vo can transfer to dangling O1* and OOH* species,and the overpotential becomes larger,but the OER can still occur.Combining with Mu's experimental results,we believe the reason for no photoactivity of water splitting on?100?surfaces is due to the photogenerated holes not being able to migrate to the surfaces.On the?110?surfaces,we find the OOH* species is a dissociated O* plus OH*.These factors lead to the overpotential on the?110?surface being very high with a value of 8.62 V.Therefore,OER can't taking place on this surface.On the?111?surface,the overpotential is 2.57 V,OER can't taking place on this surface either.The Vo has very little effect on both?110?and?111?surfaces.OER cannot occur on?110?and?111?surfaces due to the high overpotential.2.Using small polaron hopping model and effective mass approximation to study the transport of ATiO₃?A = Sr,Ba and Pb?carriers.The results show that the small polaron model can describe the transport mechanism of electrons and holes in SrTiO₃ and BaTiO₃,and electrons in PbTiO₃.The calculated electron and hole mobility in SrTiO₃ are smaller than those in the experiments,due to the model used in the calculation is the perfect crystal without consider the defects ect.The activation energy of electron small polaron in SrTiO₃ is smaller than that in BaTiO₃,smaller than that in PbTiO₃.The activation energy of hole small polaron in SrTiO₃ is smaller than that in BaTiO₃.The hole of PbTiO₃ can not be localized on the oxygen atom which is consistent with other's theoretical results,and its charge transfer mechanism may be band model.We also used the effective mass approximation method to study the charge transport in SrTiO₃,BaTiO₃ and PbTiO₃.It is found that the charge of SrTiO₃ and BaTiO₃ is relatively easy to separate in [110] and [111] directions.The electron mobility of PbTiO₃ is similar in three directions,and the hole mobility in [111] direction is larger.