Electronic Structure And Photocatalytic Hydrogen Evolution Of Hydrogenated Titanium Dioxide

Hydrogenated Ti2(H-TiO2)has been demonstrated to be an efficient photocatalyst for water splitting under visible-light irradiation,and research has continued since the first report in 2011.However,the involvement of oxygen vacancies in the electronic and structural properties,and photocatalytic activity of H-TiO2 for water splitting are currently under debated.Herein,by controlling the hydrogenation time,a series of H-TiO2 samples are obtained.We take advantage of the synchrotron radiation X-ray technique to characterize the electronic structure of H-TiO2 samples.Combined with DFT calculation and other conventional characterization methods,the process for the formation of oxygen vacancies in H-TiO2 is illustrated,and the effects of oxygen vacancies on the electronic structure and photocatalytic activity of TiO2 are proposed.The main conclusions are summarized as below:(1)The process for the formation of oxygen vacancies in H-TiO2 is illustrated for the first time by using surface-sensitive soft X-ray technique and bulk-information-included hard X-ray technique for O K-edges and Ti K-edges,respectively.We find that oxygen vacancies are firstly generated on the surface of TiO2 and then extend into the bulk phase after a high degree of hydrogenation.Moreover,oxygen vacancies contribute to the surface and bulk disorder of TiO2.(2)DFT calculation results show that oxygen vacancies can introduce a defect state that overlaps with the conduction band(CB),which leads to a band tail,and this results in visible-light absorption and enhancement of H2 production of H-TiO2.However,at a high degree of hydrogenation,the tail of the CB extends below the H+/H2 redox potential.Although the absorption of visible light is enhanced and more electrons can be excited to the CB,a substantial part of them have to relax to the bottom of the CB tail(below the H+/H2 redox potential),and thus the H2 production is decreased.This work promotes a better understanding of the possible negative consequence of hydrogenated semiconductors in H2 production apart from the positive effect on visible-light response.