Bismuth-based Semiconductors:Investigation On Photocatalytic Water Splitting

Artificial photosynthesis of solar energy into chemical fuels is regarded as one of the "Holy Grail" reactions with potentially promising applications and significant scientific interests.The development of novel visible-light-responsive photocatalysts with suitable band structures is always important and desired.Bismuth-based semiconductors have gained extensive attentions in the field of photocatalysis owing to the special properties of crystal and electronic structures.In this thesis,the main work has been focused on a serious of novel visible-light-responsive layered perovskite materials,bismuth tantalum oxyhalogen as photocatalysts for water splitting.Strategies like morphology tailoring and surface charge modulation have been employed to optimize the performance of photogenerated charge transfer and separation,to promote the efficiency of photocatalytic water splitting.Furthermore,the evolution of surface structures of semiconductors under reaction conditions has also been in situ investigated to unveil the intrinsic mechanism of photocatalytic reaction.The achieved findings are summarized as follows:1.A serious of novel visible-light-responsive semiconductor photocatalysts,bismuth tantalum oxyhalogen,Bi4TaO8X(X=Cl,Br)were developed via introducing halogen atoms into layers of perovskite bismuth-based semiconductors.By electrochemical test and theoretical calculation,the valence band and conduction band positions of the Bi4TaO8X(X=Cl,Br)were found to satisfy the thermodynamic requirements of proton reduction and water oxidation reaction.Furthermore,the Bi4TaO8X(X=Cl,Br)with microplatelet morphology and high-crystallinity could be successfully prepared by the flux synthetic method.The photocatalysts based on these materials were found to be capable of both water reduction and oxidation reactions,especially showing an apparent quantum efficiency as high as 20%at 420 nm for water oxidation.In addition,a Z-scheme system coupling Bi4TaO8Br with SrTiO3:Rh could be successfully achieved for overall water splitting with a stoichiometric ratio of H2 and O2 evolutions.2.Given the band structure of Bi4TaO8X(X=Cl,Br),MoO3 with a large work function was introduced on the surface of Bi4TaO8X(X=Cl,Br)to modulate the charge transfer properties of the semiconductor.Owing to the difference of large work function between MoO3 and Bi4TaO8X(X=Cl,Br),the electrons would flow from Bi4TaO8X(X=Cl,Br)to MoO3,leaving holes to accumulate at the surface of Bi4TaO8X(X=Cl,Br).After reaching an equilibrium,a strong upward band bending at the semiconductor surface was consequently established which would cause an enhanced electric field at the interface.The enhanced band bending and built-in electric field were found to promote the photogenerated charge separation,thus facilitating the photocatlaytic water oxidation reaction.3.Taking Bi4TaO8X(X=Cl,Br)as an example,we unraveled a commonly overlooked photoinduced surface activation phenomenon,in which the intrinsic active sites generate and dominate the enhanced photocatalytic activity under reaction conditions.We demonstrated that a thin layer of amorphous tantalum oxides species were in situ generated on the surface of Bi4TaO8X(X=Cl,Br)photocatalyst at the initial stage of the photocatalytic reaction,and such amorphous species were proven to be the active sites which can significantly facilitate the photocatalytic water splitting reactions.The investigation focusing on surface activation of photocatalysts under reaction conditions revealed the catalytic reactive states of semiconductor photocatalysts,and could help to understand the intrinsic mechanism of photocatalysis.