Shallow Zirconia Diffusion Synergistically Enhances Surface Charge Transport in Ge-Doped Hematite for Efficient...

Iron oxide-based materials suffer from inherently low conductivity and severe charge carrier recombination, resulting in poor PEC performance and low solar-to-hydrogen (STH) conversion efficiency. In this study, we aim to enhance the intrinsic conductivity of pristine hematite through a dual co-doping strategy. Zr shallow doping was introduced into Ge-doped hematite. XPS-depth profiling confirms a gradient distribution of Zr, incorporated via surface diffusion during high-temperature thermal sintering. In conjunction with Ge-doping during nucleation reaction, which promotes the photoactive H(110) plane and suppresses FTO-induced Sn-diffusion, Zr plays a dual role in improving bulk electronic properties while mitigating surface recombination in hematite. This synergistic effect facilitates more efficient charge transport at the electrode-electrolyte interface, as evidenced by the lowest interfacial R₂ obtained from EIS, along with reduced bulk R₁, indicating enhanced conductivity. As a result, the photocurrent density increases from 2.06 to 2.41 mA cm^-2 at 1.23 V_RHE upon Zr co-doping, even in the absence of a cocatalyst. The synergistic lattice restructuring induced by elemental co-doping-through the selection of dopants with complementary roles-enhances carrier density without introducing excessive defect states. Consequently, bulk recombination is suppressed, and the surface becomes more favorable for hole transfer to the electrolyte.