Tailoring the electronic landscape of TiO₂ through Cu doping: XPS insights into defect states and charge compensation mechanisms

Transition metal doping has emerged as an effective strategy to improve the photocatalytic performance of TiO₂ by tailoring its surface electronic structure and defect chemistry. Since the concentration and nature of these defects play a crucial role in governing the functional properties of TiO₂, understanding their evolution with dopant concentration is essential. In this context, Cu-doped triphasic TiO₂ materials with varying Cu concentrations were systematically investigated to elucidate the concentration dependent evolution of surface chemistry and electronic structure using X-ray photoelectron spectroscopy (XPS). Survey spectra confirmed the successful incorporation of Cu into the TiO₂ lattice without detectable secondary impurity phases. High-resolution Ti 2p, O 1s, and Cu 2p spectra revealed that Cu doping promotes the generation of Ti³⁺ species and oxygen vacancies, with their relative concentrations strongly dependent on Cu loading. An optimal defect-mediated electronic structure was observed at 1.5 wt.% Cu, characterized by enhanced defect-related oxygen contributions and significant Ti³⁺ formation arising from strong Cu–Ti–O electronic interactions. At higher Cu loadings, Cu species were predominantly stabilized in surface associated CuOₓ-like states, accompanied by pronounced shake-up satellite features, indicating weakened interaction between oxygen vacancies and the TiO₂ lattice. This systematic XPS investigation demonstrates that controlled Cu incorporation effectively tailors the surface electronic structure and defect chemistry of triphasic TiO₂, providing new mechanistic insights into concentration dependent defect engineering in functional oxide materials.