The Relationship Between Photoelectrochemical Performance,Interface Electronic Structure And Surface States Of Sn/Ti Doped α-Fe₂O₃

Photoelectrochemical(PEC)water splitting can convert water into hydrogen(H2),using solar energy,which is currently regarded as one of the ideal ways to solve global energy and environmental crisis.Hematite(α-Fe2O3)is considered to be one of the ideal photoanode for PEC decomposition of water due to a moderate band gap(2.0-2.2 eV),theoretical hydrogen production efficiency up to 15.5%,stability in aqueous solution and so on.However,as is well-known,its PEC performance is limited by the short carrier life time,poor minority charge carrier mobility,slower OER reaction kinetics and so on.Nanostructuring,form P-N junction,introduce surface passivation and element doping are considered to be effective methods.Among them,it has been widely reported that both Sn and Ti doping can enhance the PEC performance of hematite,while the underlying mechanism is elusive to date.The current literatures focus more on the effects of Sn and Ti doping on the optical absorption,conductivity,and carrier recombination of hematite,while ignoring the effects of Sn and Ti doping on the surface states at the interface between hematite and electrolyte.In response to the above controversy about the effect of Sn and Ti doping on the PEC performance of hematite,this thesis uses UV-Visible spectroscopy,X-ray photoelectron spectroscopy(XPS),X-ray absorption spectroscopy(XAS),electrochemical impedance spectroscopy(EIS)techniques and PEC method et al.Revealed the effects of Sn and Ti doping on the photoelectrochemical properties,electronic structure and surface states of hematite from the mechanism.The specific research works as follows.(1)We used sol-gel method to prepare Sn and Ti doped hematite films with different doping concentrations(0,0.5%,1%,5%)as the photoanode.We obtained the better PEC properties of doped hematite when the doping concentration was 0.5%.The improved PEC activity is partially attributed to an increased band bending potential which facilitates the charge separation at space charge region.However,when the doping concentration higher than 0.5%,due to the effect of doping elements on the surface charge transfer efficiency at th e interface between hematite and electrolyte,Sndoped samples showed a decreasing performance,while Ti-doped samples showed an increasing performance.XPS and O K-edge spectroscopy studies show that Sn and Ti doping gradually shifts the Fermi level(EF)of hematite toward the conduction band bottom(CBM),which is consistent with the tendency to move electron distribution to CBM.(2)We used EIS to investigate the different trends in the PEC performance of hematite caused by Sn and Ti doping.The results show that with the introduction of doping elements,the gap position of the surface states between the contact surface of the hematite semiconductor and the electrolyte can be adjusted.When the potential overlap between the surface states and the oxidation state of water is greater,holes at the interface are easier to transfer.This also explains the root cause of the difference in the PEC performance of hematite caused by Sn and Ti doping.