Optical and photocatalytic properties of composite TiO2/ZnO thin films

Abstract

Pure TiO2 and ZnO thin films named Ti100 and Zn100 were prepared via sol–gel technique using Ti alkoxide and Zn acetate as metal precursors and dip coating deposition on quartz substrate. Different vol.% ratios of the initial Ti and Zn solutions were used to obtain composite TiO2/ZnO films nominated as Ti25, Ti50 and Ti75. The XRD patterns of pure TiO2 and ZnO films revealed the formation of anatase TiO2 and wurtzite hexagonal ZnO, respectively. The patterns of the composite film Ti75 were similar to the pure TiO2, while the composite films Ti25 and Ti50 revealed amorphous structure. The XPS analysis proved presence of TiO2 and ZnO oxide phases in the pure and composite films that is in accordance with XRD analysis. The UV–vis transmission spectra of the films showed that absorption edges of the composite films were blue shifted in comparison with the pure films. The largest shift was attained in the Ti25 composite film. The Forouhi–Bloomer (FB) and Tauc–Lorentz (TL) dispersion models were used for description of the complex refractive index of the films and for estimation of their thickness and energy band gap. The refractive index in the films’ transparency region and high energy region increased with the increase of TiO2 phase content. The thickness of the films also increased with the increase of TiO2 content. The pure TiO2 film was approximately 2.5 times thicker (145nm) than the pure ZnO film (60nm). The estimation of the energy gaps showed comparable gap value of ∼3.2eV for the pure oxide films. The largest value of ∼3.8eV was calculated for the Ti25 composite film. A good agreement between the two models was observed in the estimation of gap and thickness values. For pure ZnO film, better accordance between experimental and theoretical transmittance spectra was achieved via FB model simulations in the region above the band gap. The photocatalytic properties of the films in air purification were investigated via standard NO oxidation procedure. The Ti100 and Ti75 films exhibited improved performance in comparison to Ti25 and Zn100 films. The highest photocatalytic efficiency of 1.6×10−2mol/einstein in NO oxidation was recorded for pure TiO2 film. Also, the latter showed the highest hydrophylicity without and under UV illumination.