PREDICTION OF TiO2 AND WO3 NANOPOWDERS SURFACE CHARGE BY THE EVALUATION OF POINT OF ZERO CHARGE (PZC)

Point of zero charge (PZC) is defined as the condition at which the surface charge density of a material equals to zero and, for most of the solids it is strongly pH dependent.The PZC can be controlled by tuning the solid surface properties (crystalline structure, morphology) and by a proper environment control, thus it is important to evaluate the PZC for a certain type of material, in given working conditions, for enhancing the efficiency in the targeted application (e.g.layer by layer thin films deposition, adsorption including (photo)catalysis).TiO 2 and WO 3 are semiconductor materials with tremendous potential in environmental applications such as photocatalysis process used for water treatment.This paper presents the results obtained in PZC evaluation, by pH-titration, for the WO 3 nanopowder (~30 nm) and the state of the art -TiO 2 Deggusa P25.The pH titration was done using different amounts of TiO 2 and WO 3 nanopowders, in 0.1 M NaOH solution, by adding μL portions of 0.1 M HCl.Considering the WO x Pourbaix diagram, experiments were also done using NH 4 OH, for eliminating the Na 2 WO 4 formation at the WO 3 surface.The PZC was evaluated on the derivative of the pH titration curve, at the significant peak(s).The PZC values were found strongly dependent on the chemical surface composition and only slightly dependent on the crystalline structure and morphology.The results proved that, in PZC reporting it is important to specify the experimental conditions, especially for highly pH-reactive solids like WO 3 .