Effect of polyelectrolyte dispersants on the preparation of silica-coated zinc oxide particles in aqueous media

Sodium silicate was utilized to obtain a SiO2 coating on ZnO particles to prevent a photocatalytic reaction between ZnO and phenol. During the coating process, pH control is important to avoid dissolution of the ZnO as well as to obtain a good dispersion. Two kinds of polyelectrolyte dispersants were used to control the surface charge of the ZnO particles in aqueous media. As a result, poly(ethylenimine) (PEI) shifted the isoelectric point of ZnO from pH 9 to pH 10, whereas poly(ammonium acrylate) (PAA) made the surface charge of ZnO negative between pH 6 and pH 11. The change in the ZnO surface charge produced by adding polyelectrolyte dispersants makes it possible to obtain uniform silica-coated ZnO particle in aqueous media. UV-irradiation experiments showed that PEI, which can make the surface charge opposite to that of SiO2, is more effective in obtaining a thick silica coating on ZnO.