Boosted Charge Separation in Core@Shell N-Ta₂O₅@a-Fe₂O₃ Heterojunctions via Band Structure Engineering for Enhanced Visible-Light-Driven Photoelectrocatalytic Water Oxidation

Photoelectrochemical (PEC) water oxidation is considered to be a promising approach to converting solar energy into clean chemical fuels. a-Fe2O3 is a potential photoanode material for water oxidation with a high theoretical photocurrent density (12.6 mA cm-2) driven by visible light. Constructing a-Fe2O3-based heterojunction with other semiconductors is a promising strategy to improve the actual photoelectrocatalytic activity. In this work, nitrogen-doped Ta2O5 is introduced to synthesize N-Ta2O5-h@a-Fe2O3 nanorod heterojunction with different band structures. At the interface, electrons transfer from N-Ta2O5-h to a-Fe2O3, which forms a built-in electric field. The heterojunction structure contributes to inhibiting the severe recombination of photogenerated electron-hole pairs. By controlling the duration of mild heat treatment, the N-Ta2O5-h nanorods with different Fermi energy levels and contents of oxygen vacancies are obtained. A mild heat treatment in N2 is proved to enlarge the & UDelta;E by leveling up the Fermi level and to promote the content of oxygen vacancies for the N-Ta2O5-h@a-Fe2O3 heterojunction as the heat-treatment time increases. The PEC water oxidation performance of the N-Ta2O5-h@a-Fe2O3 heterojunction increases when the mild-heat-treatment time is 2 h and decreases for 6 h treatment. Eventually, N-Ta2O5-2@a-Fe2O3 exhibits an enhanced photocurrent density of 3.10 mA cm-2 (4.4 times higher than that of bare a-Fe2O3), an efficiently boosted charge separation (2.0 times higher than bare a-Fe2O3), and a high chemical stability after a long-term test.