Chemical modification of a titanium (IV) oxide electrode to give stable dye sensitisation without a supersensitiser

Abstract

SEMICONDUCTING electrodes stable in aqueous solution during the photoelectrochemical oxidation of water have generally been large-bandgap oxides, and their photosensitisation to visible light is of continuing interest¹. In particular, n-type TiO₂ (rutile, with a bandgap of 3.0 eV) is a stable photoanode for the photoelectrolysis of water by uv light², and considerable effort has been devoted to its photosensitisation to sunlight¹. One approach to photosensitisation is the use of a dye that, on excitation by visible light, transfers an electron to the conduction band of the solid and subsequently returns to its reduced ground state by the oxidation of water. The three major problems to be avoided are (1) irreversible degradative oxidation of the dye³, (2) failure of the oxidised form of the dye to oxidise water, reduction being carried out by a supersensitiser, such as hydro-quinone, that is consumed by the reaction^4,5, and (3) inefficient electron transfer from the dye to the solid⁶. To circumvent the first two of these problems, we have selected an inorganic complex as the dye, a derivative of [Ru(BIPY)₃]²⁺ (BIPY = 2,2′-bipyridine). [Ru(BIPY)₃]³⁺ is known to oxidise water in suitable pH conditions evolving about 80% of the theoretical yield of oxygen⁷. The last problem was avoided by chemically attaching the complex to the electrode surface, a procedure recently demonstrated for organic dyes (such as rhodamine-B) that require the use of a supersensitiser^4,5. We report here that a single-crystal n-type TiO₂ electrode, chemically modified by the attachment of a monolayer of a derivative of [Ru(BIPY)₃]²⁺, will produce significant anodic photocurrents when irradiated with visible light in the absence of a supersensitiser.