Diffusion and adsorption of dye molecules in mesoporous TiO2 photoelectrodes studied by indirect nanoplasmonic sensing

In this study, we used Hidden Interface-Indirect Nanoplasmonic Sensing (HI-INPS) for real time monitoring of dye impregnation (adsorption-diffusion process) of mesoporous TiO2 electrodes of the kind used in dye-sensitized solar cells. We measured the dye percolation time (i.e. the time to diffuse to the bottom of a TiO2 photoelectrode film) for dye Z907 in a 1 : 1 volume mixture of acetonitrile and tert-butanol for different dye concentrations and for different thicknesses of the TiO2 film, while the total amount of adsorbed dye was simultaneously measured by optical absorption spectroscopy. The experimental data for the impregnation process were analyzed by employing a diffusion-front model, combining diffusion and Langmuir type adsorption, which allows extraction of the effective diffusion coefficient for the system. The latter value is about 15 mu m(2) s(-1) for the combined adsorption-diffusion movement of dye molecules through the TiO2 structure, which is an order of magnitude or more smaller than that for "free" diffusion of dye molecules in bulk solvents.