*2.3. Characterization*

The X−ray diffractometer (XRD, Ultima IV, Rigaku Corporation, Akishima, Japan) was selected to collect the diffraction patterns of materials using Cu Kα radiation (λ = 0.15406 nm, 40 KV, 1.64 mA) in the ranges of 10◦ to 80◦ of 2-theta angle. The morphologies of oxides were recorded with a scanning electron microscope (SEM, JSM-6010 PLUS/LA, JEOL Ltd., Tokyo, Japan). Elemental analysis of CuO/ZnO was examined using a JSM-7900F SEM attached with EDX. The chemical structures of CuO/ZnO were studied with a Fourier transform infrared (FTIR) spectrophotometer (IR Prestige-21, SHIMADZU, Kyoto, Japan).

## *2.4. Evaluation of Photocatalytic Activity*

The photocatalytic experimental conditions are presented in Table 1. Briefly, the photocatalytic performance of CuO/ZnO composites was investigated in aqueous MB solution using sunlight. Experiments were conducted under similar conditions on a sunny day between 11:00 and 14:00. In photodegradation, 20 mg of photocatalyst was added into 30 mL aqueous MB of 10 mg/L in the beakers. To equilibrate the suspension, a magnetic stirrer was used in the dark for 30 min. Then, the suspensions in the beakers were kept in sunlight for different time intervals. About 3 mL MB solution was withdrawn and separated with an Advantec membrane filter 0.45 μm. The MB concentration was calculated using a UV–visible spectrometry (UV-1700 Pharma Spec, SHIMADZU, Kyoto, Japan). The relative MB concentration (C/C0) was determined at the relative absorbance (A/A0) of λ = 662 nm, according to the Beer–Lambert law, where A0 and A were the absorbance of aqueous MB at a starting time (*t*0) of photodegradation and at any time t, respectively.

The total organic carbon (TOC) was studied using Shimadzu TOC analyzer (TOC–VCPH, Kyoto, Japan). The oxidation and titration with potassium permanganate method was applied for the measurement of chemical oxygen demand (COD).


**Table 1.** Experimental conditions.
