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

•Core/shell materials with high photocatalytic activity were obtained by sol-gel.•Electrochemical techniques were used to uncover the ZnO and TiO2 interaction.•Surface states are generated in the ZnO/TiO2 interface acting as electron traps.•Photocatalytic activity is improved by favoring the charge transfer between oxides.•At high ZnO content the upward bending avoids charge transfer between ZnO and TiO2.

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.

Graphical abstractPhotocatalytic activity of ZnO@TiO2 core/shell nanomaterialsFigure optionsDownload full-size imageDownload as PowerPoint slide