Effect of Adsorbed Water Molecules on Light Harvesting\nand Electron Injection Processes in Dye-Sensitized Nanocrystalline\nTiO₂ Films

Abstract

We\nstudied the effects of adsorbed water molecules on light harvesting\nand electron injection processes in nanocrystalline TiO₂ films sensitized with Ru-complex dyes. We found that the absorption\nspectra of adsorbed dyes containing SCN groups were sensitive to humidity\n(that is, the dyes exhibited vapochromism) and that the absorption\nspectra of dyes adsorbed on nanocrystalline films differed slightly\nfrom the spectra of the same dyes adsorbed on TiO₂ single\ncrystals, suggesting that the orientation of the dye molecules on\nthe surface was affected by surface-adsorbed water molecules. The\ndynamics of electron injection from the excited dye molecules into\nthe TiO₂ were studied by transient absorption spectroscopy.\nTransient absorption signal intensities decreased with decreasing\nhumidity, indicating that electron injection efficiency was reduced\nby loss of water molecules from the surface. IR spectroscopy indicated\nthat only a small amount of physisorbed water was lost upon drying,\nand the use of an indicator dye showed that the proton concentration\naround the dye molecules on the surface was markedly increased by\ndrying. On the basis of these findings, we discuss possible mechanisms\nfor the vapochromism and for the reduction in electron injection efficiency\ncaused by the loss of water molecules from the surface.