A comparative study of Cu-based nanoparticles and their spin-coated films: photocatalytic degradation mechanisms and efficiencies towards malachite green and neutral red azo dyes

In this work, a comparison of the photocatalytic activity of free-standing Cu-based nanoparticle mixtures and spin-coated nanoparticle films under visible-light radiation is conducted. Herein, Cu₂O, Cu₂O-Cu, Cu₂O-Cu₃N-Cu, and Cu₃N-Cu nanoparticle mixtures were successfully synthesized by a non-aqueous sol–gel route and then deposited on a glass substrate by spin-coating. The surface chemistry of the nanoparticles studied by X-ray photoelectron spectroscopy (XPS) allowed elucidating the nanoparticle synthesis mechanism. The UV–Vis absorption spectroscopy illustrates that photocatalytic activity is attributed to the high specific surface of the nanoparticles and their wider absorption range region from 500 to 1100 nm. Unlike the free-standing photocatalysts, the photocatalytic effect of spin-coated nanoparticle films enabled their facile reclamation, which solves a key issue for practical applications of the photocatalysts. The photocatalytic performances on neutral red and malachite green organic dyes were influenced by the type of visible light sources, i.e., solar simulator and natural sunlight. The results indicate that photodegradation efficiency is the highest for Cu₂O nanoparticles, reaching values of 82% for neutral red and 94% for malachite green. We also demonstrate that the degradation of cationic neutral red undergoes a photoconversion to its neutral form during the degradation process, which in turn, lowers its degradation efficiency. On the other hand, higher degradation efficiency was observed on malachite green owing to its unique cationic form, soluble in aqueous solutions.