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 e 440 nm). The environmental implications of the above findings are briefly discussed.
Electron accumulation in TiO 2 ethanolic sols prepared by the HCl hydrolysis of a titanium alkoxide has been scrutinized by UV−vis and EPR spectroscopy. Unexpectedly, Ti(III) centers, g = 1.9551, formed af ter controlled monochromatic irradiation of the sols could be detected at room temperature by EPR spectroscopy. The yield of the paramagnetic signal and the number of accumulated reducing species, detected in dark titration experiments, increase as the water to titanium molar ratio, h, used in the synthesis diminishes. A 3.8% Ti(III) production efficiency was estimated for h = 6.5. Bidentate ethoxide coordination to the titanium dioxide surface and the replacement of surface hydroxylic groups by chloride ions is directly inferred by FTIR and EPR spectroscopies. Both findings are proposed to account for the room temperature detection of the Ti(III) species, and the higher electron storage capacity of the colloids prepared with lower h values.
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