*3.3. Synthesis of ZnO Nanoparticles and g-C3N<sup>4</sup> Nanosheet*

Both the ZnO nanoparticles (ZnO-NPs) and g-C3N<sup>4</sup> nanosheet were prepared through a solvent-free modified thermal method. For ZnO-NPs, 5 g of Zn(CH3COO)2.2H2O was poured into a quartz crucible and covered with a lid. The crucible was then positioned in a stainless-steel chamber (SUS314; diameter = 35 mm; length = 60 mm), which was subsequently sealed with an oxygen-free copper gasket (OFHC). The chamber was put into a furnace (KSL-1100X-S-UL-LD, Richmond, CA, USA) and heated at 450 ◦C for 7 h with a heat-up rate of 5 ◦C min−<sup>1</sup> . After thermolysis, the furnace was kept undisturbed to cool slowly to ambient temperature to induce the uniform and smooth formation of ZnO-NPs. Next, the same described procedure for ZnO-NPs was used to prepare a g-C3N<sup>4</sup> nanosheet; the differences were (i) 2 g of urea was used to prepare g-C3N<sup>4</sup> nanosheet and (ii) the urea was heated at 520 ◦C for 380 min with a heating rate of 4 ◦C min−<sup>1</sup> .
