Platinum Metals Rev., 1975, 19, (1), 21
Photocatalytic Properties of Materials
The Photoreduction of Palladium Ions on Titanium Dioxide
The predominant position occupied by the metal silver in photographic processes often obscures the fact that other metals and materials possess photographic properties. Both scientific and economic reasons have prompted recent studies of these alternatives with varying degrees of success.
It has been known since 1920 that titanium dioxide TiO2 exhibits photocatalytic properties, since it is darkened by ultra-violet radiation. Both electrons and holes produced by light excitation may be involved. More recently, the photoreduction of palladium ions Pd+ at TiO2 has been studied as an initial step towards a photographic process with TiO2. Photoreduction of a few metal ions forms the initial image in a photographic process and this image has subsequently to be developed during a second stage to make it visible and to retain it. Any photocatalytic reduction of Pd+ ions on TiO2 is, of course, accompanied by a corresponding oxidation process at the TiO2.
Simultaneous studies of the photocatalytic reduction of Pd+ and of the electrochemical properties of TiO2 have now been reported by F. Möllers, H. J. Tolle and R. Memming of Philips Forschungslaboratorium Hamburg G.m.b.H. (J. Electrochem. Soc., 1974, 121, (9), 1160–1167). They carried out electrochemical and photocurrent measurements on vapour-deposited TiO2 films on conducting substrates (SnO2 on glass, or titanium sheet).
It was shown that the primary step in this process is the generation of an anodic photocurrent, i.e. evolution of oxygen, which catalyses the cathodic deposition (reduction) of palladium under open-circuit conditions.
The nature of the photocatalytic deposition of palladium on TiO2 was interpreted on the basis of the photovoltaic effect of the semiconducting TiO2. Further studies included an examination of the various parameters affecting the phenomenon, e.g. space charge effects, film thickness and doping. The results are clearly illustrated by a series of graphs depicting the various electrochemical data and the whole is presented as a useful and stimulating paper.