# Catalytic Degradation of Anionic Organic Dye on Greenly Synthesized CuO/ZnO Nanocomposites
###### Abstract  
CuO/ZnO nanocomposites were greenly prepared and tested for the catalytic degradation of methyl orange. The XRD analysis confirmed the existence of CuO and ZnO with crystallite sizes within the range of 15-30 nm. TEM and SEM images showed different morphological properties. The TGA analysis revealed a good thermal stability of the nanocomposite, with a total loss of less than 18% at a temperature of 700 \({}^{\circ}\)C. The nanocomposites were tested for the catalytic degradation of methyl orange under mild conditions with a catalyst mass/wastewater volume of 10 g/3 L, an initial dye concentration of 40 ppm, a pH of 4.5, and a degradation time of 3 h. The best efficiency of 49.1% was achieved by CuO nanoparticles (C), followed by 47.6%, which was obtained by 1C1Z. The degradation efficiency of ZnO (Z) was 16.4%, and it was increased by increasing the CuO precursor in the synthesis mixture, while adding ZnO to the CuO, resulting in a decrease in its catalytic performance.  
n  
Ca  
nanocomposites were greenly prepared and utilized for the adsorption of methyl orange, and reported a removal efficiency of 100% from 6.25 ppm [13].  
However, despite all the advantages of the adsorption technique for removing organic dyes from wastewater, it has some drawbacks, such as the separation of adsorbents from water bodies, safe disposal of the saturated adsorbent especially if they are single-use adsorbents, and the possibility of leaching the dye back into to the water system under certain conditions. Accordingly, there is a need for a more suitable technique to be developed. Photocatalytic degradation is considered very attractive, especially if a suitable catalyst is used under suitable conditions to produce non-harmful products [14]. The main advantages of photocatalytic degradation over other treating methods are that there is no need for further treatment or another separation process, there is a low consumption of chemicals, and the catalyst can be reused for many treatment cycles [15].  
For this purpose, semiconductor-based catalysts were developed and applied for the photodegradation of methyl orange [15]. The development of ZnO-based nanoparticles has attracted much attention due to their high chemical and physical stabilities, availability, nontoxicity, low cost, wide bandgap (3.37 eV), and very high photosensitivity [16; 17; 18]. However, it suffers from a fast recombination of the photo-generated electrons and the holes; also, it absorbs light only in the UV region and does not absorb the visible light. To overcome this shortage, ZnO can be improved by being mixed with CuO, as the oxide is a p-type semiconductor with a small bandgap (1.2-1.5 eV), to improve its photodegradation activity [19]. In addition to this, mixing metal oxides' nanoparticles also enhances the charge carriers' transfer (holes and electrons) and inhibits the recombination of these charge carriers [15].  
However, there are still some limitations of the photodegradation treatment, such as the fluctuation of light between days and nights, which leads to an instability of the processes [20]. Therefore, there is still a need to develop a catalyst material that can work under dark conditions without the need for exposure to any kind of light. Accordingly, this work aims to investigate CuO/ZnO greenly prepared nanocomposites' catalytic ability towards the degradation of methyl orange under mild conditions (dark and stagnant). The catalytic degradation of the dye was investigated by changing the composition of the nanocomposite and, also, with and without controlling the pH of the reaction mixture.