Effect of slight cobalt incorporation on the chemical, structural, morphological, optoelectronic, and photocatalytic properties of ZnO thin film

In this study, the effect of low-content cobalt doping on the chemical, structural, morphological, optoelectronic, and photocatalytic properties of ZnO was systematically investigated via XRD, XPS, AES, UPS, PL, UV-vis, and AFM techniques. In this regard, pure and Co (1 at%, 3 at%)-doped ZnO (Co-ZnO#1, Co-ZnO#3) thin films were synthesized using the ultrasonic spray pyrolysis technique. Similarly, the samples were used to coat the inner walls of transparent tubes for MB photocatalytic degradation. AFM-images reveal a high tendency for crystalline grains of both CoZnO surfaces to aggregate to form large ones, increasing the mean roughness. XRD, XPS, and AES, as sensitive and complementary techniques, substantiate the successful Co incorporation as a substitute for Zn, especially with low content (1 at%). UV-vis, PL, and UPS were adopted to study the influence of Co substituting Zn on the optoelectronic properties of ZnO, including the valence band electronic structure, bandgap, mid-bandgap states, excitonic emission, work function, Fermi level, and ionization potential. Besides, PL spectra of both Co-ZnO films reveal the participation of band-to-band transition in UV emissions and a useful intense red-emission at 685 nm, due to the Co incorporation. However, bandgap narrowing, Fermi level lowering, a redshift with considerable fluctuations in excitonic emission, and a perfect quenching of visible emission (400-640 nm) were observed only with Co(1 at %)-doping. These factors combined with the increased grain size, the decreased microstrain, and the low ionization potential (6.80 eV) are suggested to be responsible for improving the photocatalytic efficiency in Co-ZnO#1.(c) 2022 Elsevier B.V. All rights reserved.