This study compares the visible light reactivities and properties of two sensitized Pt/TiO 2 photocatalysts (Pt/ TiO 2 /RuL 3 ) on which ruthenium bipyridyl complexes (RuL 3 ) are anchored through carboxylic (c-RuL 3 ) or phosphonic (p-RuL 3 ) acid groups. The photoreductive dechlorination of CCl 4 and trichloroacetate (TCA) and the hydrogen production in aqueous suspensions were used as probe reactions for the visible light reactivity. The molar absorptivity of p-RuL 3 was lower than that of c-RuL 3 , but Pt/TiO 2 /p-RuL 3 exhibited higher visible light activities than Pt/TiO 2 /c-RuL 3 for all tested reactions. On the other hand, the steady-state photocurrent obtained with the Pt/TiO 2 /p-RuL 3 (or TiO 2 /p-RuL 3 ) electrode was lower than that obtained with the Pt/TiO 2 / c-RuL 3 (or TiO 2 /c-RuL 3 ) electrode. As for the sensitizer adsorption, more p-RuL 3 than c-RuL 3 adsorbed on a TiO 2 surface over a wide pH range: the adsorption of p-RuL 3 decreased above pH 7, whereas that of c-RuL 3 was reduced even at pH >4. Therefore, the photoreactivity of Pt/TiO 2 /p-RuL 3 was higher than that of Pt/TiO 2 /c-RuL 3 in the whole pH region tested. However, both sensitizer systems were not stable in aqueous solutions not only under visible light illumination but also in the dark. Both Pt/TiO 2 /c-RuL 3 and Pt/TiO 2 /p-RuL 3 in water gradually lost their photoreactivities with time, although the reactivity of the latter was consistently higher than that of the former. Ruthenium complexes on TiO 2 were slowly photodegraded under visible light, and both carboxylate and phosphonate linkages anchored on the TiO 2 surface were hydrolyzed in the absence of light, which was supported by FT-IR analysis.

This study reports an example of visible-light photocatalyst based on TiO2 modified by ruthenium-complex sensitizers and noble metal deposits. The photodegradation of trichloroacetate (TCA) and carbon tetrachloride was used as a probe reaction for evaluating the visible light activity of the photocatalyst under the illumination of lambda > 420 nm. Photodeposition of platinum nanoparticles on dye-sensitized TiO2 (Pt/TiO2/Ru(II)L3) drastically enhanced the degradation rate of TCA and CCl4. The visible light reactivity of Pt/TiO2/Ru(II)L3 was optimal with [Ru(II)L3] = 10 microM, [TiO2] = 0.5 g/L, and Pt loading of about 0.2 wt %. Although no electron donors to regenerate the oxidized Ru-sensitizers were added in the aqueous suspension, the photoreductive dechlorination of perchlorinated compounds proceeded far beyond the stoichiometric limit of the initial sensitizer concentration. Water acted as an electron donor to regenerate the sensitizer with a concurrent production of dioxygen. On the other hand, Pt/TiO2/Ru(II)L3 was completely inactive in the presence of dissolved oxygen and the in-situ generated dioxygen gradually decelerated the dechlorination rate. Conduction band electrons transferred to O2 in preference to CCl4 and TCA on Pt deposits. Other noble metals (Ag, Au, and Pd) deposited on TiO2 showed a better oxygen-tolerance but less visible-light reactivity than PtTiO2/Ru(II)L3. Effects of metal loading on the visible light activity and its implications for the efficientvisible-light photocatalyst development are discussed.

The effects of the number of anchoring groups (carboxylate vs phosphonate) in Ru-bipyridyl complexes on their binding to TiO(2) surface and the photoelectrochemical performance of the sensitized TiO(2) electrodes were systematically investigated. Six derivatives of Ru-bipyridyl complexes having di-, tetra-, or hexacarboxylate (C2, C4, and C6) and di-, tetra-, or hexaphosphonate (P2, P4, and P6) as the anchoring group were synthesized. The properties and efficiencies of C- and P-complexes as a sensitizer depended on the number of anchoring groups in very different ways. Although C4 exhibited the lowest visible light absorption, C4-TiO(2) electrode showed the best cell performance and stability among C-TiO(2) electrodes. However, P6, which has the highest visible light absorption, was more efficient than P2 and P4 as a sensitizer of TiO(2). The surface binding (strength and stability) of C-complexes on TiO(2) is highly influenced by the number of carboxylate groups and is the most decisive factor in controlling the sensitization efficiency. A phosphonate anchor, however, can provide a stronger chemical linkage to TiO(2) surface, and the overall sensitization performance was less influenced by the adsorption capability of P-complexes. The apparent effect of the anchoring group number on the P-complex sensitization seems to be mainly related with the visible light absorption efficiency of each P-complex.

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