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

Although β-FeO has a high theoretical solar-to-hydrogen efficiency because of its narrow band gap, the study of β-FeO photoanodes for water splitting is elusive as a result of their metastable nature. Raman identification of β-FeO 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 β-FeO to α-FeO was found to potentially take place under laser and electron irradiation as well as annealing. Herein, phase transformation of β-FeO to α-FeO was inhibited by introduction of Zr doping, and β-FeO was found to withstand a higher annealing temperature without any phase transformation. The solar water splitting photocurrent of the Zr-doped β-FeO photoanode was increased by 500% compared to that of the pure β-FeO photoanode. Additionally, Zr-doped β-FeO exhibited very good stability during the process of solar water splitting. These results indicate that by improving its thermal stability, metastable β-FeO film is a promising photoanode for solar water splitting.