Semiconducting properties of ZnO/TiO₂ composites by electrochemical measurements and their relationship with photocatalytic activity

ZnO core/TiO₂ shell nanocomposite powders with different ZnO:TiO₂ molar ratios (01:99, 03:97, 06:94, 12:88 and 20:80) were synthesized by sol-gel procedure, using ZnO rods and titanium(IV) alkoxide as precursors. This synthesis method allows obtaining core/shell systems comprising multiple cores. Morphology, crystalline phase, particle size, specific surface area, and optical band gap energy of as-prepared photocatalyst powders were determined by FE-SEM, HR-TEM coupled with energy-dispersive spectroscopy (EDS), XRD, nitrogen adsorption isotherms and diffuse reflectance UV-visible spectroscopy (DRS). The semiconducting properties of the composites were obtained in the dark by cyclic voltammetry (CV) and the Mott-Schottky analysis (capacitance measurements). The band edge energy levels of all samples were estimated through measurements of flat band potentials and forbidden band gaps. The photodegradation of 4-chlorophenol in aqueous solution using the nanocomposite powders showed the highest photocatalytic activity for 06 ZnO:94 TiO₂ molar ratio. Correlations between the energetic positions of band edges, density of donors and the extent of 4-chlorophenol oxidation allowed explaining this high photocatalytic activity. The presence of surface states, due to grain boundary appearing at the ZnO/TiO₂ interface, can affect the band bending by modifying the transport (and separation) of charge carriers, thereby changing the photocatalytic activity of the samples.