Paving the road toward the use of β-Fe2O3 in solar water splitting: Raman identification, phase transformation and strategies for phase stabilization

Although beta-Fe2O3 has a high theoretical solar-to-hydrogen efficiency because of its narrow band gap, the study of beta-Fe2O3 photoanodes for water splitting is elusive as a result of their metastable nature. Raman identification of beta-Fe2O3 is theoretically and experimentally investigated in this study for the first time, thus clarifying the debate about its Raman spectrum in the literature. Phase transformation of beta-Fe2O3 to alpha-Fe2O3 was found to potentially take place under laser and electron irradiation as well as annealing. Herein, phase transformation of beta-Fe2O3 to alpha-Fe2O3 was inhibited by introduction of Zr doping, and beta-Fe2O3 was found to withstand a higher annealing temperature without any phase transformation. The solar water splitting photocurrent of the Zr-doped beta-Fe2O3 photoanode was increased by 500% compared to that of the pure beta-Fe2O3 photoanode. Additionally, Zr-doped beta-Fe2O3 exhibited very good stability during the process of solar water splitting. These results indicate that by improving its thermal stability, metastable beta-Fe2O3 film is a promising photoanode for solar water splitting. The use of element doping to prevent the phase transition of metastable semiconductors has been approved for the application of photoelectrochemcial hydrogen production via water splitting for the first time.