Inducing Multiporous Hematite Nanorod Photoanode by Zirconium Oxide Overlayer and Niobium Oxide Underlayer for Efficient Solar Water Splitting

Overlayer and underlayer have been proven to be effective for the design of efficient photoelectrodes. Here, a facile formation of multiporous hematite nanorods is demonstrated through hybrid microwave annealing (HMA)-induced treatment of zirconium oxide overlayer and niobium oxide underlayer, simultaneously enhancing the photocurrent density and reducing the turn-on voltage, which originates from the passivation of surface states, reduction of hole migration distance, and facilitation of electron conduction. Interestingly, multiporous hematite nanorods are achieved by HMA instead of conventional thermal annealing due to a coupling effect that high thermal stability and Vickers hardness of zirconium oxide serve as a strong and rigid framework during rapid and high-temperature annealing. As a result, the optimized Nb₂O₅/Fe₂O₃@ZrO_x photoanode achieves a photocurrent density of 2.56 mA cm^-2, a cathodic shift onset potential of ≈110 mV, and a significantly improved stability in photoelectrochemical water splitting at 1.23 V_RHE under 100 mW cm^-2 solar irradiation, which represents a remarkable improvement over bare hematite and already reported overlayer and/or underlayer-based hematite photoanodes. This work has nicely demonstrated a new strategy to construct multiporous nanostructure for designing efficient metal oxide photoelectrodes in the application of solar energy conversion.