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 TiO2 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@TiO2 was investigated through quantum yields determinations in nonscattering sols of TiO2 modified nanoparticles. In contrast with the behavior observed in TiO2 microparticulated systems, the chemisorbed ligand has a high stability under aerated visible light irradiation. The quantum yield for alizarin red oxidation Phi(-AR) = 4 x 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@TiO2 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, Phi(-Cr(VI)), approaches 37% at [Cr(VI)(0) = 200 mu M. The interactions between Cr(VI)/AR and Cr(VI)/TiO2 are analyzed in detail. Spectroscopic evidence is presented for the first time that Cr(VI) forms a charge-transfer complex with TiO2 nanoparticles that could be excited by visible light (lambda <= 440 nm). The environmental implications of the above findings are briefly discussed.