Direct measurement of diffuse double-layer forces at the semiconductor/electrolyte interface using an atomic force microscope

The forces between a silica probe and an n-type TiO2 single-crystal electrode were measured using an atomic force microscope in an aqueous electrolyte solution. These interactions were a strong function of the solution pH, the presence of specifically adsorbed anions, and the TiO2 electrode potential. For a series of pH values, a strong electrostatic repulsion was seen at high pH and decreased as the pH was reduced. At pH values below 5.5, the interaction became attractive. A series of force measurements between SiO2 and n-type TiO2 showed a repulsive interaction when TiO2 was held at negative electrode potentials, which transformed to an attractive force at positive potentials. The potential at which the interaction passed through a minimum, called the potential of zero force (pzf), corresponded closely to the flat-band potential (V-fb) of the TiO2 electrode under conditions where the solution pH was held at the isoelectric point (iep) of titania. The V-fb measured by this method gave a value near -0.4 V vs SCE at pH 5.5, which was in good agreement with photoelectrochemical measurements made under similar conditions. At pH values deviating from the iep, the pzf and V-fb were not equivalent. This was illustrated by potential-and pH-dependent force curves taken at the same n-TiO2 electrode in the presence of the polymeric anion hexametaphosphate (HMP), which is known to specifically adsorb on TiO2. An increase in negative surface charge due to adsorbed HMP was observed by an increase in the repulsive force with respect to the silica probe at open circuit for a specific pH value. Potential-dependent force measurements determined that the pzf shifted toward more positive values in the presence of HMP, in direct opposition to the negative shift in V-fb. This apparent discrepancy was caused by the presence of both adsorbed and potential-induced surface charge, which could not be differentiated by simply measuring the diffuse double-layer charge.