In-gap states enhance the hydroxylation of c(6 × 2) terminated SrTiO₃

Strontium titanate (SrTiO₃) has been established as a model perovskite for studying fundamental surface-molecule interactions and mechanisms in photo-electrocatalysis. Here, we investigated the electronic structure and water adsorption behaviour of Nb-doped, SrO-enriched SrTiO₃ surfaces prepared by HCl-HNO₃ etching followed by high-temperature annealing in air. We compared low-doped (0.05 wt% Nb) and high-doped (0.7 wt% Nb) samples under both oxidising and reducing conditions before and after water exposure. Using Low-Energy Electron Diffraction (LEED), a c(6 × 2) surface reconstruction characteristic of SrO surface segregation was identified for both doping concentrations. This was induced by the high-temperature air anneal after etching, and was observed independent of subsequent reducing or oxidising treatments. Synchrotron-based X-ray photoelectron spectroscopy (XPS) revealed that water adsorption is dissociative in all samples, leading to the formation of a hydroxylated surface. Furthermore, synchrotron-based XPS, X-ray absorption spectroscopy (XAS), and lab-based He II ultraviolet photoelectron spectroscopy (UPS) measurements identified Ti 3d-derived in-gap states below the Fermi level. We demonstrate that the combination of high Nb-doping and thermal reduction results in a Fermi level of 3.15 eV above the valence band maximum and Ti 3d states populate the bandgap close to the Fermi level. We propose that these near-Fermi level in-gap states serve as an electron reservoir that enhances the Lewis basicity of lattice oxygen anions by promoting Ti-O hybridisation, lowering the energy barrier for water dissociation, and stabilising surface hydroxyls.