Co-dependency of TiO₂ underlayer and ZrO₂ top layer in sandwiched microwave-assisted Zr-Fe₂O₃ photoanodes for photoelectrochemical water splitting

A variety of electron/hole recombination pathways limit the photoelectrochemical capabilities of hematite (Fe2O3) photoanodes. To address the recombination issue in hematite photoanodes, a microwave-assisted deposition technique with a systematic surface and underlayer modification is presented. The TiO2 under-layered microwave-assisted Zr-Fe2O3 is quenched at high temperatures, and ex situ ZrO2 formation, as well as in situ Zr doping into the hematite lattice, significantly reduced bulk and surface recombinations. High-resolution scanning electron microscopy (HRSEM) analysis reveals the influence of the TiO2 underlayer on microwave-assisted Zr-Fe2O3 photoanodes. After high-temperature quenching of microwave-assisted Zr-Fe2O3, TEM, and XPS studies confirmed the coexistence of Zr, Ti co-doping, and ZrO2 surface decorations. Significantly, the combination of a TiO2 underlayer, microwave-assisted Fe2O3 formation, and in situ and ex situ ZrO2 deposition effectively achieved a photocurrent density of 1.49 mA cm(-2) at 1.23 V-RHE applied potential in a TiO2/alpha-Fe2O3/ZrO2 (TZF2ZQ) photoanode. Furthermore, electrochemical analyses confirm that Zr doping, ZrO2 surface passivation, and the TiO2 underlayer reduce the bulk and surface resistances (R-1 and R-2) in the TZF2ZQ photoanode. The PEC water splitting experiments reveal 144 and 71 mu mol of H-2 and O-2 evolution over the optimum TZF2ZQ photoanode within 5 hours. The consecutive role of the ZrO2 top layer and TiO2 underlayer in sandwiched Zr-Fe2O3 photoanodes is well clarified with the proposed charge transfer mechanism.