Elucidation of augmented visible-light photocatalysis in surface-modified coloured rutile TiO2

The prevalent TiO₂ photocatalysts have some glaring limitations, viz., (i) their inactivity under visible light irradiation and (ii) the irreversible temperature-dependent transformation of photocatalytically active anatase phase into inactive rutile. In this study, we employed a facile method to develop visible-light sensitive coloured rutile photocatalysts to tackle these issues. Here, we executed a surface modification technique on sol-gel derived, crystalline rutile TiO₂ using sodium borohydride (NaBH₄). This was done in favourable atmospheric conditions, rendering it cost-effective and scalable. A series of rutile TiO₂ samples, with colour varied from white to black, were prepared by changing the amount of NaBH₄. Surface-modified samples exhibited crystalline core and disordered shell heterostructure. The pristine rutile sample was rich in Ti³⁺ and doubly charged oxygen vacancy V_o⁺⁺, respectively corresponding to shallow and deep defect states. In contrast, the surface-modified samples are rich in photocatalytically active shallow-trapped surface Ti³⁺ and singly charged oxygen vacancy (V_o⁺) defects, which act as colour centers. Hence, the photocatalytic activities of the surface-modified rutile TiO₂ enhanced significantly upon white LED irradiation. Amongst a series of surface-modified samples, rR3 (treated with 0.1 g of NaBH₄) showed the highest visible-light photocatalytic degradation rate constant (10.35 × 10⁻³ min⁻¹) for methylene blue (MB) dye. In addition to MB, sample rR3 showed better degradation of rhodamine B (RhB) and phenol than the pure rutile and commercial photocatalyst Degussa P25 under visible-light irradiation. Since rutile is the thermodynamically stable form of TiO₂, surface-modified samples could be effective in high-temperature applications, especially without the assistance of regularly utilized UV radiation.