The promotion of photoinduced charge separation to enhance the efficiency of photocatalytic reactions has been, for example, performed using noble metals such as platinum or gold deposited on semiconductor nanoclusters (1). The development of heterogeneous photocatalysis, based on nanosized, nanostructured, or nanoparticulated semiconductor materials has now been placed in forefront (2). In this respect, we have very recently synthesised novel materials based on transition metal compounds (within cluster or framework structures) in colloidal form (3). Some characteristics of this type of compounds can briefly be outlined: (i) a highly monodispersitivity was obtained; (ii) the presence of the chemical stabiliser, during synthesis, does not perturb the chemical nature of the novel compound, therefore, a similar stoichiometry (Ru_xSe_y, in which x =89 2 and y =3D 1) a=s well as a similar atomic co-ordination distance was obtained, as revealed by RBS and EXAFS measurements, respectively; and (iii) the easiness of depositing this colloidal material, e.g. by dipping on a conducting substrate (glassy carbon), allowed us to verify its selectivity and electrocatalytic activity towards the oxygen reduction. This last property is interesting in the development of selective cathodes for fuel cell. A further exploration was, however, undertaken aiming at using such novel compounds, when deposited on a large band gap semiconductor with a huge projected area, to (photo)sensitize surface reactions catalytically. The substrate element of choice was TiO₂ anatase layers (particle size about 10 nm) produced with colloidal precursors. TiO₂ surfaces were modified with Ru_xSe_y nanocristals (3 nm). Figures 1 and 2 show the (photo)electrochemical behaviour of the modified anatase layers. In the former, the photoeffect of charge injection, although very small in comparison to the ruthenium dye (4), is apparent. However, as the Figure 2 reveals, the majority carrier capture is enhanced on the modified TiO₂ surface. The electrochemical curves obtained in 0.5M H₂SO₄ show mainly two faradic processes: proton (Fig. 2A) and molecular oxygen reduction (Fig. 2B). In the presence of molecular oxygen, to increase the photooxidation process, one key issue is to convey efficiently the photogenerated electrons to the adsorbed oxygen species. The modified interface: n-TiO₂/Ru_xSe_y/electrolyte, appears to fulfil this requirement. This statement will be discussed and substantiated with on-line mass spectroscopy experiments.

References

1. P.V. Kamat, B. Shanghavi, J. Phys. Chem. B, 101 (1997) 7675.
2. Book of Abstracts, IPS-12, Photochemical Conversion and Storage of Solar Energy, Berlin, August 9-14, 1998.
3. N. Alonso-Vante, German patent DE 196 44 628 A1.
4. M. Nazeeruddin, A. Kay, I. Rodicio, R. Humphry-Baker, E. Müller, P. Liska, N. Vlachopoulos and M. Grätzel, J. Am. Chem. Soc., 115 (1993) 6382.

Figure 1. Action spectra of= naked and Ru_xSe_y-surface modified TiO₂ using 0.5M H₂SO₄ electrolyte.

Figure 2. Current-voltage characteristics of naked and Ru_xSe_y-surface modified TiO₂ using argon saturated (A) and oxygen saturated (B) 0.5M H₂SO₄ electrolyte.

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