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 el ectrode were measured using an atomic force microscope in an aqueous e lectrolyte solution. These interactions were a strong function of the solution pH, the presence of specifically adsorbed anions, and the TiO 2 electrode potential. For a series of pH values, a strong electrostat ic 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 in teraction when TiO2 was held at negative electrode potentials, which t ransformed to an attractive force at positive potentials. The potentia l at which the interaction passed through a minimum, called the potent ial of zero force (pzf), corresponded closely to the flat-band potenti al (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 i n good agreement with photoelectrochemical measurements made under sim ilar conditions. At pH values deviating from the iep, the pzf and V-fb were not equivalent. This was illustrated by potential-and pH-depende nt force curves taken at the same n-TiO2 electrode in the presence of the polymeric anion hexametaphosphate (HMP), which is known to specifi cally adsorb on TiO2. An increase in negative surface charge due to ad sorbed HMP was observed by an increase in the repulsive force with res pect to the silica probe at open circuit for a specific pH value. Pote ntial-dependent force measurements determined that the pzf shifted tow ard 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, w hich could not be differentiated by simply measuring the diffuse doubl e-layer charge.