Apparent zeta potential of nano-porous Al₂O₃ film deposited on different substrates in streaming potential method

Al₂O₃ films were coated on SUS304L stainless steel and fused silica substrates using chemical solution deposition. Continuous pores with a diameter of approximately 2 nm were observed through the measurement of the pore diameter distribution in the Al₂O₃ films using N₂ gas adsorption. The zeta potential of the Al₂O₃ film was measured using the streaming potential method, and the effect of the substrate material on the zeta potential was investigated. Initially, the measured zeta potential of the Al₂O₃ films was + 40 to + 50 mV, which was the same for both the SUS304L and fused silica substrates. However, the zeta potential of the Al₂O₃ film on the fused silica substrate decreased significantly with repeated measurements. Elemental analysis of the Al₂O₃ film in the depth direction using dynamic secondary ion mass spectroscopy showed that both K and Cl contents increased after zeta potential measurements were taken. Moreover, the zeta potential of a specimen impregnated with KCl electrolyte solution under vacuum exhibited no dependence on the number of measurements taken. It was thereby considered that the decrease in the zeta potential with repeated measurements was caused by the gradual penetration of the electrolyte solution into the pores, which eventually reached the fused silica substrate. This is a characteristic phenomenon observed when the zeta potential of a film that contains continuous pores is measured using the streaming potential method.