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/TiO2 photocatalysts (Pt/ TiO2/RuL3) on which ruthenium bipyridyl complexes (RuL3) are anchored through carboxylic (c-RuL3) or phosphonic (p-RuL3) acid groups. The photoreductive dechlorination of CCl4 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-RuL3 was lower than that of c-RuL3, but Pt/TiO2/p-RuL3 exhibited higher visible light activities than Pt/TiO2/c-RuL3 for all tested reactions. On the other hand, the steady-state photocurrent obtained with the Pt/TiO2/p-RuL3 (or TiO2/p-RuL3) electrode was lower than that obtained with the Pt/TiO2/ c-RuL3 (or TiO2/c-RuL3) electrode. As for the sensitizer adsorption, more p-RuL3 than c-RuL3 adsorbed on a TiO2 surface over a wide pH range: the adsorption of p-RuL3 decreased above pH 7, whereas that of c-RuL3 was reduced even at pH >4. Therefore, the photoreactivity of Pt/TiO2/p-RuL3 was higher than that of Pt/TiO2/c-RuL3 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/TiO2/c-RuL3 and Pt/TiO2/p-RuL3 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 TiO2 were slowly photodegraded under visible light, and both carboxylate and phosphonate linkages anchored on the TiO2 surface were hydrolyzed in the absence of light, which was supported by FT-IR analysis.