*3.2. Synthesis of Nanomaterials*

Zinc oxide nanoparticles (ZnONPs) were synthesized by thermal decomposition of anhydrous zinc acetate. In a standard synthesis, 5 g of zinc acetate was introduced into an alumina crucible which was covered by a perforated alumina lid, and subsequently placed in a tube furnace. Next, the sample was subjected to heat treatment in air flow (300 mL/min) using a temperature ramp of 3 ◦C/min until reaching 450 ◦C. The sample was kept at this temperature for 2 h then proceeded to cool, maintaining an air flow of 100 mL/min and a ramp of −10 ◦C/min. The sample was recovered the next day. Graphitic carbon nitride was synthesized using the procedure described by Mo et al. [66]. For this, 2 g of melamine were introduced into a crucible, which was subsequently treated at 650 ◦C (heating rate of 2 ◦C/min) in flowing nitrogen (200 mL/min), for 4 h. After treatment, the synthesized material was subjected to exfoliation. For this, 200 mg of gC3N<sup>4</sup> were put in contact with 20 mL of concentrated H2SO<sup>4</sup> (with stirring for 6 h). The solution was then treated with ultrasound for 2 h. The mixture was decanted and centrifuged, washed with water several times until neutral, and finally rinsed with ethanol to facilitate drying. The gC3N<sup>4</sup> crystals obtained after the described process showed a characteristic white color.

The deposition of Au NPs has been described elsewhere [49]. In a typical synthesis, 1g of ZnONPs was dispersed in 100 mL of H2O and the mixture was sonicated for 30 min. After that, the desired amount of the gold precursor (HAuCl4·3H2O) was added to the reaction mixture and kept stirring for 30 min. Finally, a solution of NaBH<sup>4</sup> (10 mg in 10 mL of H2O) was added dropwise while maintaining stirring. After the reagent was added, the solution was kept stirring for an additional 30 min. The reaction product was separated by centrifugation, washed 4 times with deionized water, and dried overnight at 80 ◦C. The different compounds of Au@ZnONPs were identified as 1%Au@ZnONPs, 5%Au@ZnONPs and 10%Au@ZnONPs, where the numbers correspond to the percentage by weight of gold nanoparticles deposited in each sample. The material obtained was used for the following stages of preparation of the catalysts. Thus, 200 mg of ZnONPs containing the gold nanoparticles were dispersed in a solution containing 10 mL of ethanol and 40 mL of deionized water, and the mixture was vigorously stirred for 30 min. Subsequently, the reduced graphene oxide (rGO) was added, and the suspension was kept under stirring for 1 h. Subsequently, the product was separated from the solution by centrifugation, washed 4 times with deionized water and dried overnight at 80 ◦C. Next, and by same procedure described for rGO, the incorporation of graphitic carbon nitride (gC3N4) was carried out. Finally, the product was collected, sealed, and stored at room temperature. The different catalysts prepared, based on Au@ZnONPs, Au@ZnONPs-rGO, and Au@ZnONPs-rGOgC3N<sup>4</sup> were identified indicating the percentage of gold incorporated on the surface. In all cases, the amount of rGO and gC3N<sup>4</sup> used was 3% by weight.
