*3.4. Synthesis of ZnO/CuO/g-C3N<sup>4</sup> Nanocomposite*

The ZnO/CuO/g-C3N<sup>4</sup> nanocomposite was prepared by an efficient co-crystallization process. Initially, as-synthesized CuO, ZnO, and g-C3N<sup>4</sup> at a 2:2:1 weight ratio were transferred into a beaker containing 40 mL distilled water as a solvent. Then, the resultant mixture was homogenized using a bath sonication for 30 min. After that, the aqueous solution was stirred vigorously for 2 h at 600 rpm using a magnetic stirrer at ambient temperature (26 ◦C) to produce a nanocomposite in which the constituent components were well distributed. The resultant mixture was then left untouched for 1 day for cocrystallization and kept undisturbed for an additional 2 days to allow the precipitate to settle. The upper 50% of solution (20 mL) was removed, and the remaining mixture was subjected to evaporation at 60 ◦C in the oven to drive off the solvent molecules completely. The collected, dried powder of the prepared composite was then ground smoothly and stored in a sealed vial for further use. The nanocomposite is denoted as ZCG.

#### *3.5. Instrumentation*

An X-ray diffractometer (X'Pert PRO, PANalytical, Lelyweg, Almelo, The Netherlands) equipped with a Cu Kα (*λ* = 1.5406 Å) radiation source was applied to inspect the crystal structure of the compounds. The chemical composition, oxidation state, and binding energies of the sample were analyzed by X-ray photoelectron spectroscopy (XPS, NEXSA, Thermo Fisher Scientific, East Grinstead, West Sussex, UK). The functional groups of the materials were examined by Fourier transform infrared (FTIR) spectroscopy (Thermo Fisher Scientific, Nicolet iS5). The morphological properties of the composite were evaluated by field-emission scanning electron microscopy (FE-SEM, SU8230, Hitachi, Minato-ku, Tokyo, Japan) and high-resolution transmission electron microscopy (HR-TEM, JEM-2200FS, JEOL, Akishima, Tokyo, Japan). To assess the optical features and bandgaps of the materials, a UV–vis spectrophotometer (Perkin Elmer Lambda 25, Ayer Rajah, Singapore) was used.

#### *3.6. Measurements of Photocatalytic Performance*

The photocatalytic proficiency of all the compounds was assessed by measuring the degradation of MB as a target contaminant. The assessments were driven using a xenon lamp (300 W) as a source of visible light. Additionally, a UV cut-off filter (≥400 nm) was engaged to avoid UV irradiation. In a typical experiment, 0.020 g of sample was added into 100 mL of dye solution (20 mg L−<sup>1</sup> ). The resultant suspension was then ultrasonicated for 60 min to confirm the adsorption of dye molecules onto the catalyst surface. Further, this mixture was kept in dark conditions for 30 min to acquire an adsorption–desorption equilibrium. The photodegradation reaction was then started by contact with the assistance of a light source. Before and after the solar irradiation, the change in the dye concentration during the reaction was observed by UV–vis spectrophotometer. For measuring the absorbance of the samples, a constant volume of solution was measured at regular intervals. In addition, the photodegradation proficiency for CR was tested under identical conditions. The photocatalytic investigation of all the samples was executed under uniform experimental conditions. The recyclability of the ZCG nanocomposite was evaluated using a similar procedure in each set. This was achieved by collecting the precipitated sample (after centrifugation) and washing it several times with distilled water. The sample was then dried at 100 ◦C for approximately 5 h in a preheated furnace for successive evaluation of its recycling performance. Following this, the dry sample was used for the next cycle of the photocatalytic reaction.

#### **4. Conclusions**

ZnO/CuO/g-C3N<sup>4</sup> nanocomposite was successfully synthesized through a costeffective, facile co-crystallization method. The prepared nanocomposite was used to treat wastewater containing organic pollutants through visible light illumination and the product's durability and recycling capability was evaluated. The structural and morphological observation confirmed the nanocomposite was perfectly constructed. The top-most photocatalytic activity was observed for the ZCG nanocomposite, which induced 97.46% photodegradation of MB under visible light within 50 min. The ZCG nanocomposite showed 753%, 392%, 156%, and 130% higher photocatalytic efficiency compared with photolysis, g-C3N4, CuO-NPs, and ZnO-NPs, respectively. The photodeterioration of MB followed pseudo-first-order reaction kinetics and the *k* value of the ZCG composite was 23.6, 11.6, 3.4, and 2.4 times greater than those of photolysis, g-C3N4, CuO-NPs, and ZnO-NPs, respectively. The pHPZC of our as-synthesized ZCG affirmed the anionic nature of the surface at a higher pH and correspondingly showed a cationic nature at lower pH. In addition, the photocatalytic activity and reaction kinetics of the ZCG composite for the deterioration of CR were investigated and the results were found to be remarkable. The outstanding photocatalytic performance was observed due to heterojunction formation among the g-C3N4, CuO-NPs, and ZnO-NPs compounds, which minimized the photogenerated *e* −-*h* <sup>+</sup> pair recombination, and increased the electron flow rate. After six consecutive runs, the ZCG product showed excellent stability and recycling performance, confirmed by

employing XRD and FTIR analysis of the reused ZCG sample. Therefore, the overall results indicate that the ZCG nanocomposite could be practically used for visible-light-driven wastewater treatment; this sheds new light on the field of catalysts.

**Supplementary Materials:** The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/catal12020151/s1, Figure S1: Linear plots of modified Scherrer equation (a) CuO-NPs, (b) ZnO-NPs, and (c) ZCG nanocomposite.

**Author Contributions:** Conceptualization, M.A.H. and J.A.; methodology, M.A.H.; software, M.A.H.; formal analysis, M.A.H., J.A. and Y.S.K.; investigation and data curation, M.A.H.; writing original draft preparation, M.A.H.; writing—review and editing, M.A.H., J.A., Y.S.K., H.G.K., J.R.H. and L.K.K.; visualization, H.G.K., J.R.H. and L.K.K.; supervision, L.K.K.; project administration, L.K.K.; funding acquisition, L.K.K. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2016R1A6A1A03012069, 2018R1D1A1B07050752). This research was also supported by the Korean Government (NRF-2021R1I1A3045310).

**Data Availability Statement:** The data are available by corresponding author.

**Conflicts of Interest:** The authors declare no conflict of interest.

## **References**

