Towards understanding the TiO₂ doping at the surface and bulk

Anatase titanium dioxide is a common photocatalyst with limited application scope due to its wide band gap energy. Valence and conduction band doping have long been used to narrow TiO₂ band gap energy. The effectiveness of the process is highly dependent on the procedure adopted, and its underlying mechanism remains largely unknown. Importantly, optical changes caused by bulk doping are not always represented at the surface where the catalysis occurs. Therefore, methodologies sensitive to bulk and surface modifications are key for a holistic understanding of the doping mechanism and its effect on photocatalysis quantum yield. Herein, it is proposed the use of X-ray absorption spectroscopy (XAS) with different probing depths to elucidate the chemical composition of bulk and surface. Soft XAS was employed to determine the chemical state of typical dopants (N and Cu) in TiO₂ thin films. The XAS measurements at N K-edge and Cu L_2,3-edge were performed in surface and bulk-sensitive detection modes, providing direct insight into the surface-bulk effects caused by the dopants. In the case of copper doping in the TiO₂ conduction band, the data show the formation of Cu²⁺ in the bulk states. In contrast, the surface states are dominated by Cu⁺ components resulting from surface termination geometries. Moreover, X-ray absorption spectra confirmed that the substitutional mechanism is the dominant process in Cu doping. The nitrogen doping was found to be interstitial, causing minor changes to the band structure but affecting the photoluminescence. The proposed methodology closes the gap in knowledge between materials design and photocatalytic performance by establishing atoms' disposition in bulk that affects electronic properties and on the surface that impacts the catalytic cycle.