Impact of adding cadmium to zinc oxide thin films on the structural, morphological, optoelectronic and photocatalytic properties for ecological applications

Abstract

Transparent ZnO and Cd-doped ZnO thin films were fabricated on glass substrates utilising the sol-gel process applied by dip coating. This work explores the effects of cadmium adding on the structural, surface morphological, topographical, and optoelectronic characteristics of zinc oxide thin films as well as possible photocatalytic uses for them. X-ray diffraction analyses verified preferential development along the (002) plane, indicative of the wurtzite structure, with no supplementary phases present. The addition of Cd to ZnO resulted in a decrease in crystallite size from 42 nm to 37 nm at a concentration of 3 wt%, followed by an increase to 39 nm and 42 nm at 7 and 10 wt%, respectively. FESEM micrographs of the undoped, 7 wt%, and 10 wt% Cd-doped ZnO thin films reveal a uniform, homogeneous surface morphology characterised by evenly distributed spherical nanoparticles. In contrast, the 3 wt% Cd-doped sample exhibits a distinct granular structure punctuated by micropores. The EDS examination confirmed the successful incorporation of Cd into the ZnO lattice of the wurtzite-structured film. Atomic force microscopy (AFM) findings demonstrate that the RMS roughness diminishes from 115 nm to 99 nm at a doping level of 3 wt%, then escalates to 159 nm and 124 nm at concentrations of 7 wt% and 10 wt%, respectively. UV-Vis spectroscopy data demonstrate that the integration of Cd into the ZnO lattice results in a gradual reduction of the band gap energy, decreasing from 3.35 eV for undoped ZnO to 3.26 eV, 2.25 eV, and 3.22 eV for the Cd-doped ZnO samples doped at 10 wt%, 7 wt%, and 3 wt%, respectively. The photodetection study demonstrated that the incorporation of Cd markedly enhanced the devices’ sensitivity, reactivity, detectivity, and the rise and fall times. The ZnO sample doped with 3 wt% Cd exhibited enhanced efficiency in the degradation of methylene blue dye under solar radiation, achieving 98.35% degradation. The thin layer demonstrated improved stability and reusability after six recycling cycles. Furthermore, the trapping test results reveal that photogenerated holes (h⁺) are the primary active species, succeeded by hydroxyl radicals (•OH), with superoxide radicals (•O₂⁻) playing a secondary role in the photocatalytic process. The decomposition rate of MB under sunlight rose from 98.35% after 120 min to 99.99% after 60 min at H₂O₂ concentrations of 5 mM., These results demonstrate that Cd-doped ZnO possesses considerable potential for wastewater removal, thereby affirming its relevance for ecological uses.