Enhanced charge transfer with tuning surface state in hematite photoanode integrated by niobium and zirconium co-doping for efficient photoelectrochemical water splitting

Niobium and zirconium co-doping was introduced into a hematite (Fe₂O₃) photoanode by a facile two-step synthesis. The hydrothermally prepared zirconium-doped photoanode shows a reduction in the crystallite size of hematite, with H(104) being the dominant photoactive phase. The incorporation of niobium ions by drop-casting and high-temperature annealing does not alter the crystallinity. The core 3d spin-orbit splitting shows the Nb⁴⁺ oxidation state forming NbO₂ in the hematite lattice. The Nb⁴⁺–Zr⁴⁺ co-doped hematite photoanode, prepared on a fluorine-tin oxide glass substrate, shows an enhanced photocurrent density of 2.05 mA cm⁻² with no co-catalyst. This enhanced performance is attributed to the Zr⁴⁺ doping, which improves the bulk charge transfer in hematite, and Nb⁴⁺ suppressed charge recombination in the surface state holes at the electrode–electrolyte interface. This synergistic improvement of bulk and surface properties leads to stable water splitting at the water oxidation potential (1.23 V_RHE) of the Nb–Zr co-doped hematite photoanode.