Sol-gel synthesis of porous cobalt-doped ZnO thin films leading to rapid and large scale Orange-II photocatalysis

Abstract

Herein, a sol-gel dip-coating scheme has been employed to prepare the high-quality ZnO thin films with various loadings of cobalt (Co) on porous ceramics substrates. The impact of cobalt element on the crystal phase, surface morphology, optical and chemical properties of films were investigated using different spectroscopic techniques including PXRD, FE-SEM, FT-IR,and UV-Vis respectively. The X-rays diffraction pattern showed the synthesis of monophasic ZnO at all Co contents without appearance of any secondary phase; the grain size is decreased from 40 to 12 nm as Co loading scales from 0 to 3%. Scanning electron microscopy study reveals that the existence of Co doping in ZnO impacts its morphology change, and its size is reduced and presented in homogeneous distribution as well as Co content increases, in FT-IR results no secondary phase was observed in the spectra corresponding to Co. In addition, the mean value of reflectance as deduced from UV–Vis spectra lies 86–95% in the visible range, showing Co doping drastically reduces the band gap values from 3.14 to 2.93 eV. The practical applicability was investigated in the removal of Orange II (OII) dye on exposure to ultra violet radiations. In comparison to other samples, the thin films with 5 wt% Co show the highest rate of decomposition in the order of 92% for 4 h irradiation time.

Highlights

This paper makes an elaborate study on Co-doped ZnO thin films synthesized by sol-gel (dip-coating) method on small porous ceramic support. The different physio-chemical features of the prepared materials are reported here in this issue.

The bandgap was calculated by knowing the maximum wavelength from the absorption derivative and applying the following relationship [16], the results show that with the doping concentration, the optical bandgap of the film gradually decreases as indicated in Table 2, this may be due to the Co²⁺ introduced in the lattice of ZnO.

When the dopant concentration (Co) is increased, it is observed that there is an enhancement in the density while the grains size reduces compared to pure ZnO. It is found that when the doping concentration is 5%, the grain size is the most uniform and the film density is best, the grain size is around 20 nm.