**1. Introduction**

Synthetic dyes are commonly used by various industries, especially textile ones. These physically and chemically stable compounds are harmful to the environment. Synthetic dyes are recalcitrant compounds that exhibit high solubility in water and accumulate in both wastewater and industrial effluents [1,2]. Currently, water pollution is one of the major challenges of the modern world, and the recovery of wastewater by conventional methods is not suitable for emergent pollutants [3,4]. Heterogeneous photocatalysis can be satisfactorily applied for the decontamination of natural samples through the photocatalytic degradation of toxic pollutants from complex matrices, such as river water and wastewater [5]. All renewable technologies have become a promising alternative for both energy generation and wastewater treatments. Solar photocatalysis is a suitable option to degrade recalcitrant

pollutants from water [6,7]. Different catalysts have been reported to exhibit photocatalytic activity (e.g., TiO<sup>2</sup> [8], Fe2O<sup>3</sup> [9], ZnO [10], CuO [11], CdS [12], WO3, and SnO<sup>2</sup> [13]). ZnO has been used as a photocatalyst, but its high bandgap value (~3.3 eV) is one of its major drawbacks. ZnO is not photocatalytically active at longer wavelengths of the electromagnetic spectrum. As a consequence, using solar irradiation as the main energy source to develop practical applications is a challenge for ZnO [14]. Some alternative strategies to extend ZnO photoresponse in the visible light region are: (a) ZnO doping [15–18], (b) co-doping [19,20], (c) coupling with lower band gap semiconductors [21,22], (d) surface plasmon resonance [23–26], (e) quantum dots [27], and (f) sensitization with natural and synthetic dyes [28–31]. With the doping process, optical and catalytic properties can be tuned by doping [32], and the ZnO bandgap generates intragap electronic states inside the semiconductor [33]. Some transition metals (e.g., Co2+, Ag+, Cu2+, Mn2+) have been used to enhance the properties of ZnO [34–37]. Among these, copper is an economical option. Because the ionic size of Cu<sup>2</sup> is close to Zn2+, Cu2<sup>+</sup> ions can replace Zn2<sup>+</sup> ions to modify the absorption spectrum [38]. Kuriakose et al. reported that Cu-doped ZnO nanostructures photodegraded organic dyes. Their analysis associated the enhanced photocatalytic activity to the combined effects of: (i) the separation of charge carriers and (ii) the optimal Cu doping load [39]. Another transition element is cobalt, as the ionic size of the Co2<sup>+</sup> ion is close to that of Zn2+. Co2<sup>+</sup> ions can replace Zn2<sup>+</sup> and generate minimal distortion in the crystalline lattice [35,40]. Yongchun et al. reported the synthesis of cobalt-doped ZnO nanorods and reported an improvement in the performance of alizarin red photocatalytic degradation [41]. In addition, Kuriakose et al. synthesized Co-doped ZnO nanodisks and nanorods, and reported photocatalytic activity improvement as the by-effect of the doping process due to: (i) the charge separation efficiency and (ii) the surface area [42]. Poornaprakash et al. reported 66.5% efficiency in photocatalytic degradation of Rhodamine B under artificial solar light illumination on Co-doped ZnO nanorods [43]. The incorporation of these kinds of metals inside the ZnO structure can modify optical properties by extending the photodegradation ability towards the largest wavelength of the electromagnetic spectrum [44,45]. In this work, we report a facile wet chemical method for the synthesis of Cu and Co-doped ZnO thin films with highly enhanced photocatalytic activity. The metal doping process leads to highly efficient visible light photocatalytic degradation of methylene blue.
