A Few Atomic FeNbO4 Overlayers on Hematite Nanorods: Microwave-Induced High Temperature Phase for Efficient Photoelectrochemical Water Splitting

Orthorhombic iron niobate (FeNbO4) has a band structure to form an effective heterojunction with hematite to make an efficient photoanode for photoelectrochemical water splitting. However, this high temperature phase is difficult to synthesize by conventional thermal annealing (CTA) without damaging F:SnO2 (FTO) substrate. In contrast, hybrid microwave annealing (HMA) selectively forms a few atomic overlayers of highly crystalline orthorhombic FeNbO4 phase covering hematite nanorods in an extremely short time (2 min) without any FTO damage forming a core–shell-type Fe2O3@FeNbO4 nanorod heterojunction on FTO. At the same time, hematite is codoped naturally with Nb and Sn during the HMA synthesis by diffusion from FeNbO4 and FTO, respectively. The optimized Nb,Sn:Fe2O3@FeNbO4/FTO electrode loaded with NiFeO x cocatalyst achieves a stable photocurrent density of 2.71 mA cm–2 at 1.23 VRHE under simulated sunlight (100 mW cm–2) with ∼100% faradaic efficiency of hydrogen production, which is ∼3.4 times higher than that of bare hematite prepared by CTA (0.8 mA cm–2).