Effects of Surface Anchoring Groups (Carboxylate vs Phosphonate) in Ruthenium-Complex-Sensitized TiO2 on Visible Light Reactivity in Aqueous Suspensions

Abstract

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.