*2.1. Chemicals*

The chemicals used were: cupric chloride dihydrate, zinc sulfate heptahydrate, sodium hydroxide, ascorbic acid, urea, C2H6O, methyl red, and distilled water. The chemicals were analytical grade and used throughout without further purification. Distilled water was used in all the experiments.

#### *2.2. Synthesis of Photocatalysts*

The undoped Cu2O nanoparticles were prepared from CuCl2·2H2O as a precursor via precipitation and reduction techniques. 0.01 moles of CuCl2·2H2O was dissolved in 100 mL deionized water. The solution was adjusted in its pH level by adding 30 mL of 2 M NaOH drop by drop with continuous stirring until 12.2. During the addition of the NaOH aqueous solution, a deep blue precipitate was formed quickly. After being stirred for 30 min, 2.2 g of C6H8O6 was added into the above solution to reduce Cu2<sup>+</sup> into Cu<sup>+</sup> and stirred for an extra 30 min. The color of the precipitate then changed to brick red indicating the formation of Cu2O. The brick-red precipitate was filtered and washed four times using deionized water and dried at 100 ◦C in an oven [27]. To show high crystallinity and remove organic impurities, the powders were calcined at 500 ◦C in a furnace for 2 h.

ZnO nanoparticles were prepared using ZnSO4·7H2O as a precursor through a precipitation method [27]. To prepare ZnO nanoparticles, 0.01 moles of ZnSO4·7H2O dissolved in 100 mL deionized water. While stirring, 30 mL of 2 M NaOH was dropped carefully into the aqueous solution of zinc sulfate until the pH of the solution reached 12.4. Upon addition of NaOH aqueous solution, a white precipitate was formed and stirred for 30 min. The white product was separated by filtration and washed using deionized water repeatedly to remove impurities and dried at 100 ◦C in an oven. Finally, the powders were calcined at 500 ◦C in a furnace for 2 h.

The Cu2O/ZnO nanocomposite was synthesized from CuCl2·2H2O and ZnSO4·7H2O via the co-precipitation method followed by reduction using ascorbic acid; 0.01 moles of CuCl2·2H2O and 0.01 moles of ZnSO4·7H2O were dissolved in 100 mL deionized water and stirred using a magnetic stirrer; 30 mL of 2 M NaOH dropped slowly into the mixture of cupric chloride and zinc sulfate aqueous solutions with continuous stirring and the pH of the solution became 11.8. A light blue precipitate was formed and stirred for 30 min. Once the solution has been stirred for half an hour, the copper reduced from +2 to +1 oxidation states after 2.2 g of C6H8O6 added while stirring for an additional 30 min. The color of the precipitate changed from light blue into yellow as soon as the ascorbic acid was added as a reducing agent. Lastly, the yellow precipitate was filtered, washed, repeatedly, using deionized water and ethanol, and dried at 100 ◦C in the oven [27]. The nanocomposite was then calcined at 500 ◦C in a furnace for 2 h.

To prepare the N-Cu2O/ZnO nanocomposite, 2.681 g of CO (NH2)2 dissolved in ethanol and 25 g of uncalcined Cu2O/ZnO nanocomposites was added to the solution. The mixture was stirred up to mix well and then dried. Finally, the powder was calcined at 500 ◦C for 2 h and the nitrogen-doped Cu2O/ZnO nanocomposite was obtained [25].
