Coordination of Ti4+ Sites in Nanocrystalline TiO2 Films Used for Photoinduced Electron Conduction: Influence of Nanoparticle Synthesis and Thermal Necking

Nanocrystalline titania (TiO2) anatase films are widely utilized as substrates for electron conduction in photoelectrochemical devices. In this paper, we subjected the lattice disorder of TiO2 anatase nanoparticles and the resulting nanocrystalline films to analysis with X-ray absorption fine structure spectroscopy. The TiO2 nanoparticles were synthesized from dehydration of a titanate and from a conventional sol-gel method. Although both specimens had similar first shell Ti4+ coordination numbers (CNs) of ca. 5.7, the titanate-derivative TiO2 was shown to be phase-pure anatase and the sol-gel TiO2 contained a minute amount of brookite impurity. After nanoparticle necking into films, the former TiO2 exhibited a negligible decrease in the CN, whereas the latter showed a significant decrease to a value of ca. 4.9. As a result, the titanate-derivative film is more efficient than the sol-gel one in transmitting electrons injected from a photoexcited dye. Significant lattice distortion near the grain boundaries of films are believed to occur during necking of the nanoparticles containing impurities. We have demonstrated that the synthesis of phase-pure nanoparticles is essentially important in fabricating films with a minimal degree of lattice disorder.