In Situ Synthesis of α-Fe₂O₃/Fe₃O₄ Heterojunction Photoanode via Fast Flame Annealing for Enhanced Charge Separation and Water Oxidation

Hematite (α-Fe2O3) is a promising photoanode material in photoelectrochemical (PEC) water splitting. To further improve the catalytic activity, a reasonable construction of heterojunction and surface engineering can effectively improve the photoanode PEC water-splitting performance via improving bulk carrier transport and interfacial charge-transfer efficiency. As Fe3O4 has an excellent conductivity and a suitable energy band position, α-Fe2O3/Fe3O4 heterojunction can be an ideal structure to improve the activity of α-Fe2O3. However, only few studies have been reported on α-Fe2O3/Fe3O4 heterojunctions as photoanodes. In this work, a holey nanorod Fe2O3/Fe3O4 heterojunction photoanode with oxygen vacancies was fabricated using a rapid and facile flame reduction treatment. Compared with pure Fe2O3, the water oxidation performance of the Fe2O3/Fe3O4 photoanode is improved by ninefold at 1.23 VRHE. Our study revealed that the porous nanorod structure providing more active sites and oxygen vacancies as the hole transfer medium, together improve the interface charge transfer performance of the photoanode. At the same time, Fe3O4 can form a Fe2O3/Fe3O4 heterojunction to improve the carrier separation efficiency. More importantly, Fe3O4 can serve as active sites, solving the slow water oxidation kinetic problem of hematite to enhance the catalytic activity. Our work shows that when flame acts on precursors containing oxygen or hydroxide, it is easy to form compounds with different microstructures or compositions in situ.