Fullerol-Titania Charge-Transfer-Mediated Photocatalysis Working under Visible Light

Abstract

The development of visible-light-active photocatalysts is being investigated through various approaches. In this study, C-60-based sensitized photocatalysis that works through the charge transfer (CT) mechanism is proposed and tested as a new approach. By employing the water-soluble fullerol (C-60(OH)(x)) instead Of C-60, we demonstrate that the adsorbed fullerol activates TiO2 under visible-light irradiation through the "surface-complex CT" mechanism, which is largely absent in the C-60/TiO2 system. Although fullerene and. its derivatives have often been utilized in TiO2-based photochemical conversion systems as an electron transfer relay, their successful photocatalytic application as a visible-light sensitizer of TiO2 is not well established. Fullerol/TiO2 exhibits marked visible photocatalytic activity not only for the redox conversion of 4-chlorophenol, I-, and Cr-VI, but also for H-2 production. The photoclectrode of fullerol/TiO2 also generates an enhanced anodic photocurrent under visible light as compared with the electrodes of bare TiO2 and C60/TiO2, which confirms that the visible-light-induced electron transfer from fullerol to TiO2 is particularly enhanced. The surface complexation of fullerol/TiO2 induced a visible absorption band around 400-500 nm, which was extinguished when the adsorption of fullerol was inhibited by fluorination of the surface of TiO2. The transient absorption spectroscopic measurement gave an absorption spectrum ascribed to fullerol radical cations (fullerol(center dot+)) the generation of which should be accompanied by the proposed CT. The theoretical calculation regarding the absorption spectra for the (TiO2 cluster + fullerol) model also confirmed the proposed CT, which involves excitation from HOMO (fullerol) to LUMO (TiO2 cluster) as the origin of the visible-light absorption.