Visible-light-induced and long-lived charge separation in a transparent nanostructured semiconductor membrane modified by an adsorbed electron donor and electron acceptor

The preparation of transparent nanostructured TiO₂ (anatase) membranes is described. Detailed characterization shows these membranes to be 50 μm thick nanoporous-nanocrystalline structures with associated values for porosity and surface roughness of 50% and 5000, respectively. Modification of these membranes by coadsorption of a ruthenium complex, bis[(4,4′-dicarboxy-2,2′-bipyridine)(4,4′-dimethyl-2,2′- bipyridine)-ruthenium(II)] dichloride (I), and of a viologen, 1-ethyl-1′-[(4-carboxy-3-hydroxyphenyl)methyl]-4,4′-bipyridinium perchlorate (II), is also described. Detailed studies show that visible-light-induced electron transfer by electronically excited I to the conduction band of the nanostructured TiO₂ membrane is followed by membrane mediated electron transfer to coadsorbed II. Detailed studies also show that, as a consequence of the rectifying properties of the semiconducting membrane, charge separation is long-lived. The possible significance of these findings for the development of a practical water splitting device is considered.