One-step synthesis of Cu3N, Cu2S and Cu9S5 and photocatalytic degradation of methyl orange and methylene blue

Herein we report on a novel one-step synthesis of copper-rich Cu3N, Cu2S and Cu9S5 nanoparticles by using the surfactant dodecanethiol as a sulfur source to substitute the nitrogen in Cu3N using the Pearson acid-base concept. The Cu3N nanoparticles are indexed to a cubic anti-ReO3 phase, whilst the Cu2S nanoparticles are indexed to the alpha low cubic phase and Cu9S5 to a rhombohedral phase. The Williamson-Hall method is then used to estimate the sizes from the x-ray diffraction patterns. The estimated sizes are larger than the sizes observed from transmission electron microscopy. X-ray photoelectron spectroscopy confirms the formation of Cu3N, Cu2S and Cu9S5 nanoparticles. The oxidation state of copper in Cu3N and Cu2S is +1 with Cu9S5 having both Cu1+ and Cu2+ present. In addition the S 2p high resolution spectrum for Cu9S5 shows the presence of S2- and (S-2)(2-). The TEM shows the formation of cubic Cu3N nanoparticles (41 nm) as well as hexagonal Cu2S (27 nm), and elongated Cu9S5 nanoparticles (aspect ratio 1.8). These nanoparticles have direct band-gap transitions of 2.41 eV, 2.75 eV and 2.36 eV respectively. The blue-shifting of photoluminescence spectrum of the Cu3N nanoparticles is indicative of the presence of surface traps while Cu2S and Cu9S5 show near band-edge emission indicative of defect-free samples. The nanoparticles are then used as photocatalysts to degrade methyl orange and methylene blue dyes. The Cu3N photocatalyst degrades 89 % of 20 ppm methyl orange dye in a 1st order reaction with the degradation products being aniline and C5H6N2SO3- fragments whilst the Cu9S5 is the best catalyst at degrading 20 ppm methylene blue (79 %). The methylene blue degradation follows a 2nd order reaction and aniline is the only detectable degradation product.