Precursor concentration induced nanostructural evolution of electrodeposited ZnO thin films and its effect on their optical and photocatalytic properties

A series of ZnO thin films has been synthesized using an electrodeposition system on Indium tin oxide (ITO) coated glass substrate by varying zinc nitrate concentration. For the synthesized thin films, the composition, morphology, crystalline structure, and optical properties are characterized using Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), grazing incidence X-ray diffraction spectroscopy (GIXRD), UV-Vis absorption spectroscopy, and photoluminescence spectroscopy (PL). The formation of ZnO bonds and the increment of nitrate and -OH groups with increasing zinc nitrate concentration are revealed by FTIR spectroscopy. The GIXRD results show a slight shift towards the higher 2 theta angle for all ZnO-related diffraction peaks, confirming the presence of micro-strain in all electrodeposited thin films. The estimated strain values increase with increasing precursor concentration, justifying the increment in crystal defects. SEM images exhibit an evolution of surface morphologies from nanosphere-like to nanosheet or nanoleaf-like structures with the zinc nitrate concentration increment. UV-Vis spectra reveal a red shift in band edge absorption from the lowest to highest zinc nitrate concentration sample; however, the edge again exhibits a blue shift for the moderate zinc nitrate concentration thin film. The calculated bandgap values for all samples are lie within the range of 3.07-2.81 eV. The PL spectra declare an enhancement in emission intensity with increasing precursor concentration. This variation in emission intensity is explained by the change in nanostructures that eventually tuned the defect states and subsequently the luminescence. A UV-light-driven photocatalytic degradation of MO dye in water has been studied to assess the photocatalytic activities of ZnO thin films using UV-Vis absorption spectroscopy. The results show the best photocatalytic efficiency (70.43%) for moderate precursor concentration thin film, achieved by an optimum concentration of surface oxygen vacancy states. However, due to a huge surface oxygen vacancy, the highest precursor concentration thin film exhibits the minimum catalytic efficiency (32.36%). It declares that an excessive amount of surface oxygen vacancy can create bulk oxygen vacancies, which eventually ruins the catalytic activity and reduces the photocatalytic performance.

Automated summary

A series of ZnO thin films were synthesized on ITO coated glass substrate by varying zinc nitrate concentration through an electrodeposition system. The characterization of the thin films revealed changes in composition, morphology, crystalline structure, and optical properties. The increase in zinc nitrate concentration led to the formation of ZnO bonds and higher presence of nitrate and -OH groups, influencing the surface morphology evolution.