Synergetic effect of carboxylic acid functional groups and fractal surface characteristics for efficient dye sensitization of titanium oxide

FTIR spectroscopy has been used to investigate the surface interaction of TiO2 powder particles sensitized with two of the most efficient sensitizers, the mono-nuclear complex [RuL2(SCN)(2) and the tri-nuclear complex RuL2[mu-(CN)Ru(CN)L-2'](2) {L = 4,4'-dicarboxylic acid-2,2'-bipyridine and L' = 2,2'-bipyridine}, both having ligands substituted with carboxylic acid (-COOH) functional groups. C=O band peak frequency shifts of about 20 cm(-1) higher than that of bulk complexes and disappearance of the CN- and SCN- band vibrations were observed on derivative samples. The C=O shift was attributed to chemical adsorption of the dyes onto the oxide particles via ester-like binding between the carboxylic acid groups and the OH moieties on TiO2. Incident light-electricity conversion efficiencies approaching unity with the above two ruthenium-bpy complexes attached on transparent nanocrystalline TiO2 films were reported. The morphology and the surface characteristics of the films studied by AFM showed the presence of a fractal network of interconnected grains whose diameter were between 15 and 25 nm. The height distribution of surface features has a maximum around 60 nm. On such large surface area electrodes efficient adsorption of the Ru-bpy complexes via ester linkages leads to extremely high photoconversion yields. (C) 1998 Elsevier Science B.V. All rights reserved.