We introduce a simple strategy to unintentional heterogeneous Ti4+/Sn4+ doping and surface passivation of hematite via TiO2 underlayers at high temperature quenching. The effects of the controlled TiO2 underlayer thickness and high temperature quenching process on the interfacial diffusion of Ti4+/Sn4+ and TiO2 passivation of hematite nanorod arrays have been carefully studied. The improved photoelectrochemical water oxidation performance of the TiO2 underlayered hematite nanorod photoanodes after high-temperature quenching (800 °C for 10 min) suggests enhanced interfacial Ti4+ diffusion, blocking of electron back transfer, and reduced interfacial charge recombination. The TiO2 underlayers led to more inclined growth of hematite (α-Fe2O3) nanorods on the fluorine-doped tin oxide (FTO) substrates. Ti4+ and Sn4+ diffusion and formation of the TiO2 passivation layer on the α-Fe2O3 surface are confirmed by HRTEM and X-ray photoelectron spectroscopy (XPS) analyses. As a result, the TU2 photoanode displayed higher donor density and enhanced photocurrent density of (1.45 mA·cm–2) than the pristine hematite photoelectrode (1.0 mA·cm–2). The improved photoelectrochemical performance of TU2 is attributed to the high separation efficiency of photoinduced carriers via TiO2 underlayer, the Ti4+/Sn4+ diffusion, and surface passivation of hematite at high-temperature annealing. The thickness of the TiO2 underlayer has great influence on the surface passivation as well as resistance on FTO/hematite interfaces than the diffused Sn.

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