2.1. Cu2O Synthesis

The Cu₂O nanocrystals were synthesized by modifying a previously published work [28]. Briefly, the synthesis consists of an ascorbic acid (C₆H₈O₆) and NaOH reduction route at room temperature. The addition of C₆H₈O favors the formation of Cu₂O, and the concentration of NaOH dictates the nanoparticle shape [34]. The reaction mechanism is as follows [34]:

Firstly, 0.1 mmol of CuCl₂ and 0.1 g of polyvinylpyrrolidone were dissolved in 40 mL. After a dropwise addition of 2.5 mL of 0.2 M NaOH aqueous solution, the mixture was stirred for 5 min. Then, 2.5 mL of 0.1 M aqueous ascorbic acid was added dropwise, and the solution was stirred for further 5 min. The Cu₂O crystals were recovered by centrifugation and washed two times with ethanol. The crystals were dried and suspended in deionized water (1 mg/mL) to be used for further experiments.