Photoinduced Reactivity of Strongly Coupled TiO₂ Ligands under Visible Irradiation: An Examination of an Alizarin Red@TiO₂ Nanoparticulate System

Similarly to alizarin molecules, 3,4-dihydroxy-9,10-dioxo-2-anthracenesulfonate (alizarin red, AR), chelates TiO 2 nanoparticles through the catechol moiety, and shifts the absorption threshold of the semiconductor to the visible region. The photoinduced reactivity of the coupled system AR@TiO 2 was investigated through quantum yields determinations in nonscattering sols of TiO 2 modified nanoparticles. In contrast with the behavior observed in TiO 2 microparticulated systems, the chemisorbed ligand has a high stability under aerated visible light irradiation. The quantum yield for alizarin red oxidation Φ −AR = 4 × 10 −4 correlates with the negligible efficiency for oxygen reduction in the constrained environment of the smaller particles. Conversely, reduction of Cr(VI) to Cr(V) in the coupled AR@TiO 2 system, confirmed by electron paramagnetic resonance spectroscopy, utilizes a high fraction of the photogenerated electrons and induces the degradation of the complex. Quantum efficiencies for chromium(VI) disappearance, Φ −Cr(VI), approaches 37% at [Cr(VI)] 0 = 200 μM. The interactions between Cr(VI)/AR and Cr(VI)/TiO 2 are analyzed in detail. Spectroscopic evidence is presented for the first time that Cr(VI) forms a charge-transfer complex with TiO 2 nanoparticles that could be excited by visible light (λ ≤ 440 nm). The environmental implications of the above findings are briefly discussed.