Processing induced morphology change in ZnO-TiO2 multilayer thin films and its effect on their photocatalytic activity under visible light irradiation

Abstract

Multilayer films with alternate layers of zinc and titanium sols on glass and quartz, when prepared following two different protocols, showed significantly altered morphology after calcination. On glass, after calcination, pairs of layer of zinc-on-titanium sols deposited and dried at 180 °C, referred as TlZ, showed a network of vertical walls with sharp boundaries, whereas samples with each sol layer dried before next coating, referred as TsZ, had wrinkled boundaries. On quartz we prepared ZlT and ZsT films with order in the pair switched. Upon calcination at 900 °C, ZlT showed flat polycrystalline films, and ZsT vertically stacked crystallites. Quartz samples showed formation of rhombohedral zinc-titanate due to higher temperature, and photocatalytic activity towards Methylene Blue degradation under visible light illumination, but a higher photocatalytic activity was observed in ZsT films due to vertically stacked crystallites offering higher area. Difference in sol-viscosity and surface energies of crystalline phases influence the morphology.