Construction Of Efficient And Stable α-Fe₂O₃-based Photoanode And Its Study On Photoelectrochemical Water Oxidation Performance

With the development of human society,non-renewable fossil fuels have been consumed in large quantities,resulting in energy crisis and environmental issues that should not be neglected.Therefore,finding and developing new energy which can replace fossil energy is the top priority of current social development.Solar energy,which has been widely used to alleviate the energy crisis,still possess huge potential compared with other energy.An effective way to efficiently utilize solar energy is to convert solar energy into storable chemical energy（such as hydrogen energy）through photoelectrochemical（PEC）water splitting system.However,the biggest challenge that must be faced of this technology is to find and develop a stable and efficient photoelectrode.Hematite（α-Fe₂O₃）,characterized by its abundant reserves,appropriate band edge position（2.2 e V）,nontoxicity and high photoelectrochemical stability,has been regarded as a greatly promising photoanode for PEC water splitting.However,the experimental photocurrent of hematite is much lower than the theoretical value(12.5 m A cm^-2),owing to its poor electrical conductivity,rapid charge recombination as well as sluggish oxygen-evolution kinetics.Besides,the bottom of the conduction band ofα-Fe₂O₃ is below the reduction potential of water,so an external bias is required to drive the photoelectrochemical water splitting reaction.These defects severely restrict the practical application ofα-Fe₂O₃ for photoelectrochemical water splitting.To solve above problems,we modifiedα-Fe₂O₃ by element doping,surfacepassivation and loading oxygen evolution cocatalyst to improve its performance for PEC water splitting.The main content of this thesis is as follows:1.We introduced Ti and Sn into the bottom and surface of Fe OOH,the precursor ofα-Fe₂O₃,successively.When Fe OOH was annealed to formα-Fe₂O₃ at high temperature,Ti and Sn were doped into its bulk successfully.Subsequently,an ultrathin layer of ZIF-67 with a thickness of about 2 nm was fabricated on the surface of Sn/Ti co-dopedα-Fe₂O₃ by a simple one-step solvothermal method.The photocurrent density of the resulting Sn-Ti-Fe₂O₃/ZIF-67 achieved an outstanding value of 2.00 m A cm^-2 at 1.23 V_RHE,corresponding to four folds of pristine Fe₂O₃.Additionally,the onset potential shifted negatively by 80 m V compared with that of Sn-Ti-Fe₂O₃.Detailed investigations manifested that Sn/Ti co-doping not only increased carrier density but also removed partial surface traps,while ZIF-67 coating not only expanded the optical-response range,but also accelerated the charge transfer at the semiconductor-electrolyte interface to facilitate water oxidation kinetics.The synergistic effect of Sn/Ti co-doping and ZIF-67 loading effectively overcame some inherent defects ofα-Fe₂O₃,leading to excellent photoelectrochemical performance of our Sn-Ti-Fe₂O₃/ZIF-67 photoanode.2.A layer of rutile TiO₂ was deposited on the surface ofα-Fe₂O₃ by a simplehydrothermal and annealing method.Then,a layer of amorphous Fe Ni OOH cocatalyst was electrodeposited on the surface of TiO₂ to obtain Fe₂O₃/TiO₂/Fe Ni OOH three-layer composite structure photoanode.The photocurrent density of the photoanode reached 1.78 m A cm^-2 at 1.23 V vs.RHE,which was 3.56 times that of pristineα-Fe₂O₃.After systematic testing and analyses,we found that the TiO₂loading not only increased the carrier density of the photoanode,but also passivated the surface states of the photoanode,thus greatly weakening the charge recombination.As a cocatalyst,Fe Ni OOH accelerated the charge transfer process at the interface and extracted the photogenerated holes from the valence bands ofα-Fe₂O₃ and TiO₂.By this means,the photogenerated holes could rapidly participate in the water oxidation reaction at the interface,thus effectively improving the water oxidation kinetics at the interface.The Fe₂O₃/TiO₂/Fe Ni OOH performed well for PEC water splitting owing to the synergistic effect of TiO₂ and Fe Ni OOH.