**1. Introduction**

It is evident that water pollution is a global environmental problem now due to the presence of different types of hazardous pollutants in it [1]. Textile wastewater, which contains huge amounts of pollutants that are very harmful to the environment, is of special research interest. The release of textile wastewater to the environment causes aesthetic problems due to the changed color of the water bodies [2]. Nanocomposites, containing oxides of metal, are of great research interest at the moment due to their high photocatalytic degradation efficiency, sustainable development characteristics, and lack of secondary pollution for water pollution treatment [3,4]. Recently, different types of metal oxide nanocomposites have been synthesized, such as ZnO–Mg [5], CuO/ZnO [6], ZnO–NiO [7], Co3O4–ZnO [8], and CeO2–ZnO [9]. Saravanan et al. prepared CuO/ZnO, V2O5/ZnO, and ZnO/γ-Mn2O3 nanocomposites using the thermal decomposition method [10–12]. They also studied the photocatalytic degradation of rhodamine B under sunlight irradiation using CuO/ZnO nanocomposite. Li et al. [13] synthesized CuO/ZnO nanocomposites using the thermal decomposition method and investigated the visible light-driven photocatalytic degradation of methylene blue and methyl orange. Kuriakose et al. [14] prepared CuO/ZnO nanocomposites using the carbothermal

evaporation method and evaluated the photocatalytic degradation of methylene blue and methyl orange dyes under sunlight irradiation. Wang el al. [15] prepared MnO@MnOx microspheres through the solvothermal process and reported the degradation of levofloxacin under simulated sunlight irradiation. Nanocomposite photocatalytic technology can be considered a green technology and provides the advantages of abundance, including postpone electron-hole recombination, higher photocatalytic activity, and the ability to convert solar energy to chemical energy, which eventually realizes the solution of energy and environmental issues [16–21].

ZnO is an n-type semiconductor, having a conductivity of about 10<sup>−</sup>7–10−<sup>3</sup> S/cm. It has a relatively large binding energy of 60 meV. However, the main disadvantages of ZnO are its fast electron-hole recombination rate and inefficient utilization of sunlight that lead to a reduction in photodegradation efficiency. The photodegradation performance of ZnO can be increased by modifying ZnO with transition metals [22]. Among the various transition metals, Cu-doped ZnO nanomaterials are of special interest due to the photocatalytic efficiency enhancement that creates defects in the lattice and reduces the recombination of photogenerated charge carriers [23]. Cu also provides plenty of advantages, such as low cost, more electronegativity than zinc, and a similar atomic size to that of zinc, and leads to better doping efficiency [24]. CuO is a natural p-type semiconductor with a narrow band gap, having a conductivity of 10−<sup>4</sup> S/cm, and it can be applied in photodegradation reactions [25]. Among the various metal oxide nanocomposites, researchers are paying more attention to CuO/ZnO because of its non-toxicity, economical benefits, and availability. It possesses high energy density and good electrical and piezoelectric properties [26,27]. CuO/ZnO nanocomposites improve physicochemical properties, compared to pure ZnO and CuO nanostructures [14]. The formation of a CuO/ZnO heterojunction also enhances the optical and electronic properties, which are considered to be promising applications in photocatalysis [10].

Herein, the various proportions of CuO/ZnO nanocomposites are prepared using the mechanochemical combustion method. The synthesized composites are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and Fourier transform infrared (FTIR). Methylene blue (MB) is a non-biodegradable and hazardous organic compound intensively used in textile industries and, so, it is selected as the degradation target to evaluate the photocatalytic performances of the composites using sunlight. The results show that CuO/ZnO exhibit increased photocatalytic activities, compared with ZnO. The degradation mechanism and mineralization of the improved photocatalytic performance are also discussed.
