Control of surface and ζ potentials on nanoporous TiO2 films by potential-determining and specifically adsorbed ions

The effect of a specifically adsorbed ion, phosphate, on the electrochemical response and adsorption properties of nanocrystalline TiO2 is examined. Phosphate is known to affect the zeta potential, as measured by electrophoretic mobility, by changing the charge of the oxide surface. The adsorption of a cationic probe molecule, thionine, onto TiO2 was monitored with an in-situ cell using UV-vis spectroscopy. The adsorption of the cationic dye molecule was found to be governed by changes in the zeta potential, whether the zeta potential was modified by pH or by changes in phosphate concentration. Onset potential measurements were used to estimate the flat-band potential of a Ti/TiO2 electrode. The flat-band potential results for the electrode showed a nearly Nernstian response to changes in the pH for a broad pH range. The addition of phosphate had no effect on the onset potential or on the shape of the photocurrent/potential curve. Flat-band potentials determined by Mott-Schottky analysis in the absence of phosphate were Nernstian only for pH 3-7, matching the pH dependence of the electrophoretic mobility results. With the addition of phosphate, impedance spectroscopy results showed additional space charge capacitance, peaking at potentials 150 mV positive of the flat-band potential. UV irradiation also resulted in an additional space charge capacitance. For both cases, the additional space charge capacitance was accompanied by a decrease in the resistance of the electrode, as shown in Nyquist plots. The change in film conductivity is believed to affect the space charge layer capacitance. Similarly, a decrease in film resistance was also seen with lower pH values. Currently, this change in TiO2 film conductivity with surface acidity is being investigated in our laboratory for application in fuel cell electrolytes.