PREPARATION AND PHOTOELECTROCHEMICAL CHARACTERIZATION OF THIN SNO(2) NANOCRYSTALLINE SEMICONDUCTOR-FILMS AND THEIR SENSITIZATION WITH BIS(2,2'-BIPYRIDINE)(2,2'-BIPYRIDINE-4,4'-DICARBOXYLIC ACID)RUTHENIUM(II) COMPLEX

Thin, transparent films of SnO2 semiconductor have been prepared from 30-Angstrom-diameter colloids. Scanning electron microscopy and X-ray powder diffraction studies indicate a three-dimensional network of SnO2 nanocrystallites of particle diameter less than or equal to 50 Angstrom. These thin nanocrystalline films exhibit reversible electrochromic effects. The electron trapping process in SnO2 particles has been investigated by both spectroelectrochemical and laser flash photolysis techniques. These electrodes are photoelectrochemically active in the UV region with incident photon-to-photocurrent conversion efficiency of 20% at 280 nm. The photocurrent increases with increasing film thickness but attains a limiting value at thickness greater than 0.75 mu m. The migration of charge across the grain boundaries is a limiting factor for the photocurrent generation in thicker films. These SnO2 films are highly porous and exhibit strong affinity for adsorption of sensitizer molecules such as bis(2,2'-bipyridine)(2,2'-bipyridine-4,4'-dicarboxylic acid)ruthenium(II) perchlorate (Ru(bpy)(2)(dcbpy)(2+)). SnO2 films modified with Ru(bpy)(2)(dcbpy)(2+) exhibit excellent photoelectrochemical response in the visible with a power conversion efficiency of similar to 1% at 470 nm. The rate constant for the charge injection process as measured from the analysis of luminescence decay of Ru(bpy)(2)(dcbpy)(2+*) on a SnO2 surface is in the range 7.6 x 10(7)-0.82 X 10(7) s(-1).