Non-blockage of atomic-scale active sites in photocatalytic TiO2 thin films deposited on silica-based substrates

The present work delivers an in-depth analysis on contamination and active site blockage of anatase thin films deposited on three types of SiO2-based glass substrates using high-resolution XPS and ICP-MS. Further, the photocatalytic performance of the thin films before and after purification were analysed by UV-Vis spectrophotometry. The results reveal that ionic contaminants including alkalies (Na), alkaline earths (Mg, Ca), Al, Si, and F (and probably B) diffuse from the substrates into the grain boundaries of the thin films and represent potential sources of active site blockage. The long-term leaching in aqueous-based medium showed the entire removal of the contaminants. It was also revealed that the kinetics of leaching are controlled not by the ionic sizes or valences of the contaminant ions but by their solubilities, which are confirmed by theoretical thermodynamic study and speciation diagrams for soluble species deriving from the three types of substrates.