Shifting mechanisms in the initial stage of dye photodegradation by hollow TiO₂ nanospheres

Hollow TiO₂ (HT) sphere aggregates were prepared using carbon spheres as templates. The photocatalytic activity of HT was determined by degradation of two nitrogen-containing dyes, methylene blue (MB) and methyl orange (MO). The adsorption isotherms and the photocatalytic degradation kinetics of the two dyes were studied and compared using different concentrations of dyes for the pure, isopropanol-added, and KI-added systems. Isopropanol was used as a OH^• radical scavenger, while KI was added as a valance band hole scavenger. The results showed that the reaction mechanism of the photocatalytic process of MB was first governed by OH^• radicals, and then by valence band holes, whereas holes played a major role in the whole photodegradation process of MO. The photocatalytic adsorption constant K _V has a positive correlation with the reaction constant k _ov in all systems. The photodegradation efficiencies of the dyes were discussed considering the surface characteristics of HT and the structure of the dyes with different catalyst loads (0.25–2 g L⁻¹) and under different pH (3–10) conditions. Compared with solid TiO₂, HT exhibited enhanced performance in photocatalytic degradation of both MB and MO.