Photocatalytic activity of ZnO + CuO thin films deposited by dip coating: coupling effect between oxides

ZnO + CuO thin films were obtained by the dip-coating method on glass substrates. The precursor solution was obtained from the mixture of ZnO and CuO independent precursor solutions prepared at room temperature (RT). The atomic percentage of Cu in solution (X at.%), in steps of 10 at.%, was varied in the full range. The number of coatings was fixed to obtain thicknesses of ~250 nm for all films, which were sintered in an open atmosphere at 450 °C for 1 h. A detriment of crystalline quality was observed for ZnO as the X value was increased up to 50, decreasing its crystallite size (CS) from 20 to 12 nm. For CuO, CS remained almost constant (~10 nm). No presence of other compounds was observed in X-ray diffraction measurements. EDS measurements showed that Cu and Zn contents in the films (Y) are in accordance with the nominal concentration set in solution, being both uniformly distributed in the films. SEM images showed dependence of the texture on the X value: (i) a homogeneous distribution of spherical particles of higher size for X = 0 (ZnO) and X = 50 (CuO + ZnO), and (ii) particles of irregular shape–size for other X values. The direct band gap for ZnO (3.28 eV) and, indirect band gap for CuO (1.2 eV) were calculated from transmission spectra. A shift of edge absorption to higher wavelengths was observed as the Cu content increased. The photocatalytic activity was quantified by the photobleaching of methylene blue. ZnO films showed the highest photocatalytic activity, but also another maximum was obtained for the ZnO + CuO films with X = 50 at.%. This result has been attributed to the coupling effect among both oxides and a higher packing density of the crystallites.