Theoretical Studies On Charges Separation And Transport And Oxygen Evolution Reaction On TaON And Ta₃N₅

Photocatalytic water splitting on semiconductor-based photocatalysts is one of the most efficient ways to obtain clean energy hydrogen for solar energy utilization.There are mainly three basic processes:（1）Electrons and holes are generated in semiconductor by photons which the energy are larger than the band gap energy of the material;（2）Electrons and holes are separated and transferred to the surface;（3）Electrons and holes are consumed by surface catalysic reactions.Charge separation and transfer and surface catalytic reaction play an important role in the photocatalytic water splitting.Tantalum oxynitride（TaON）and tantalum nitride（Ta₃N₅）are widely used in the field of photocatalysis due to the appropriate energy band structure,and they are very promising materials for photocatalytic water splitting.However,they are limited by self-oxidation easily and low photocatalytic activity,and the charge transfer mechanism and water oxidation reaction process are not well studied.Therefore,in this work,TaON and Ta₃N₅ were selected as examples to study the charge separation and transport and the surface reaction mechanism.This work includes following three parts:（1）Theoretical calculation of charge separation and transport in Ta₂O₅,TaON,Ta₃N₅Using DFT + U,the electron and hole small polaron transport properties in Ta₂O₅ and TaON was calculated,combining the results of the Ta₃N₅ in literature,the charge separation and transport process of Ta₂O₅,TaON,Ta₃N₅ was systematically analyzed.The results showed that both electrons and holes are well localized on selected atoms to form small polarons in Ta₃N₅ and TaON,and the charge transfer mechanism in these materials is small polaron hopping.We found that the activation energy of electron transfer in TaON is smaller than in Ta₂O₅.Although the activation energy of the hole small polaron transfer between O atoms in TaON is higher than that in Ta₂O₅,the activation energy of the hole transfer between N atoms in TaON is extremely low,it is much smaller than that of Ta₂O₅.The charge transfer mechanism in Ta₃N₅ is band transport,which is faster than Ta₂O₅ and TaON,indicating that tantalum oxide is better for charge transfer after nitriding.（2）Theoretical study for the effect of O and N vacancies on the water oxidation reaction（OER）on TaON and Ta₃N₅Using the DFT theory,the OER process on TaON and Ta₃N₅ surface was systematically studied.The structures of the intermediate species in the OER process on the surface of TaON and Ta₃N₅（100）with and without defects was constructed.By analyzing the water oxidation reaction process,it was found that there are unique Ta-N-O-Ta and Ta-O-O-Ta intermediate species in OER for O* adsorption.In addition,it is found that for the TaON（100）clean surface,the overpotential which is 0.60 V through the O3*（Ta-O-O-Ta）path is the lowest by analyzing the step energy and free energy of OER.When N vacancy exists,the overpotential which is 0.65 V passing through O1 *（dangling O*）is the lowest.The overpotential is higher with the presence of O vacancy.The water oxidation reaction overpotentials on Ta₃N₅ are high.The minimum overpotential is 0.78 V on the clean surface,and the overpotential is higher when N vacancies exists.All the calculations shows then N vacancies are good for OER on TaON but negative for OER on Ta₃N₅.Through Bader charge analysis,it was found that the excess electrons generated by the vacancies were not transferred to the OER intermediates,but increased the negative charge of N atoms in the solid,making N atoms more easily oxidized by photogenerated holes.This may be the main reason why TaON and Ta₃N₅ are unstable in the water oxidation reaction.（3）Theoretical study for the OER on cocatalyst Co₄O₄ loaded on TaON and Ta₃N₅ surfaceIn order to study the effect of co-catalysts for the surfaces OER processes,Co₄O₄ co-catalyst clusters were supported on the TaON and Ta₃N₅（100）surface.In the process of water oxidation reaction,the active sites are chosen on the Ta atom and Co atom,respectively.When N defects exist,after loaded co-catalyst,TaON（100）surface is more conducive to water oxidation,and its overpotential is only 0.28 V.But the overpotential on Ta₃N₅（100）surface is still high.According to Bader charge analysis,it is found that,unlike on clean surface,after loaded co-catalyst Co₄O₄,the excess electrons generated in the vacancies are partially transferred to the intermediate species of water oxidation reaction,and reduced the overpotential of the reaction,which is good for OER.