Enhanced Charge Separation through ALD‐Modified Fe 2 O 3 /Fe 2 TiO 5 Nanorod Heterojunction for Photoelectrochemical Water Oxidation

Hematite suffers from poor charge transport and separation properties for solar water splitting. This paper describes the design and fabrication of a 3D Fe 2 O 3 /Fe 2 TiO 5 heterojunction photoanode with improved charge separation, via a facile hydrothermal method followed by atomic layer deposition and air annealing. A highly crystallized Fe 2 TiO 5 phase forms with a distinct interface with the underlying Fe 2 O 3 core, where a 4 nm Fe 2 TiO 5 overlayer leads to the best photoelectrochemical performance. The favorable band offset between Fe 2 O 3 and Fe 2 TiO 5 establishes a type‐II heterojunction at the Fe 2 O 3 /Fe 2 TiO 5 interface, which drives electron–hole separation effectively. The Fe 2 O 3 /Fe 2 TiO 5 composite electrode exhibits a dramatically improved photocurrent of 1.63 mA cm −2 at 1.23 V versus reversible hydrogen electrode (RHE) under simulated 1 sun illumination (100 mW cm −2 ), which is 3.5 times that of the bare Fe 2 O 3 electrode. Decorating the Fe 2 O 3 /Fe 2 TiO 5 heterojunction photoanode with earth‐abundant FeNiO x cocatalyst further expedites surface reaction kinetics, leading to an onset potential of 0.8 V versus RHE with a photocurrent of 2.7 mA cm −2 at 1.23 V and 4.6 mA cm −2 at 1.6 V versus RHE. This sandwich photoanode shows an excellent stability for 5 h and achieves an overall Faradaic efficiency of 95% for O 2 generation. This is the best performance ever reported for Fe 2 O 3 /Fe 2 TiO 5 photoanodes.