No abstract

The conversion of light into chemical fuels in photochemical devices equipped with semiconductor electrodes (e.g. of n‐CdS) is associated with a serious problem: “holes” produced in the valence band of the semidonductor upon irradiation migrate to the surface where photocorrosion occurs. In mi‐croheterogenous CdS systems this is remedied by a thin layer of RuO2. A CdS sol prepared in the presence of maleic anhydride/styrene copolymer has now been loaded with RuO2 and Pt; these CdS microelectrodes are surprisingly active catalysts for cleavage of H2O and H2S.

By using the redox indicators o-dianisidine and o-toluidine as reagents for quantitative analysis, it is shown that in colloidal Ti02-Pt dispersions surface-bound peroxo titanium complexes are formed during photolytic water cleavage in closed systems and as a result of oxygen photo-uptake.

Irradiation of acidic (pH 2) solutions of RuLb+ 2C1-(L = diisopropyl 2,2'-bipyridine-4,4'-dicarboxylate) in the presence of TiO, at 100°C leads to the loss of one bipyridyl ligand and the chemical fixation of RuL, at the surface of the TiO, particles through formation of Ru-0-Ti bonds. These surface complexes are very stable and shift the absorption onset of TiO, beyond 600 nm. Efficient sensitization of H,-generation is achieved with this system beginning in the wavelength domain between 590 and 665 nm. Preliminary water cleavage experiments with the bifunctional TiO,/Pt/RuO, redox catalyst are reported.Introduction. -Spectral sensitization of large band-gap semiconductors is widely applied in silver halide [l] and electrophotography [2]. Optimum results are achieved with systems where intimate contact between dye and semiconductor is established. Thus, in the case of ZnO, a successful strategy for sensitization, applied in patents by Ada-Gevaert [3] and Lee [4] and established through the pioneering work of Zollinger et al. [5] and Haufle [6], consists of the chelation of Zn2+ ions at the particle surface by suitable chromophores. Surface modification of semiconductors such as TiO, and SnO, by adsorbed or covalently linked photoactive dyes is also a subject of increasing investigation [7-91 in view of the potential applications to solar energy conversion. In both photoelectrochemical cells [7] and H,-generating photochemical systems [8] (91, the phenomenon of photosensitized electron injection is used to effect charge separation with light of less than band-gap energy. Very interesting in this regard is our recent observation [9] that complexation of Ti4+ by 8-hydroxyquinoline on the surface of TiO, leads to in situ formation of an efficient sensitizer for H, generation by visible light. A drawback of the latter system, making necessary the use of sacrificial electron donors, is the inability of the oxidized sensitizer to produce oxygen from water. Here, we report on a new method for surface derivatization of TiO, by a more suitable chromophore which, we believe, should be of great importance for the design of reliable catalytic systems affording water cleavage by visible light with no assistance from sacrificial donors or acceptors.