Green synthesis of g-C3N4/CuONP/LDH composites and derived g-C3N4/MMO and their photocatalytic performance for phenol reduction from aqueous solutions

Abstract

A facile, environmentally friendly route for the synthesis of ZnAlLDH/g-C3N4/CuONP composite was demonstrated by exploiting the specific behaviour of the freshly calcined ZnAlLDH/melamine hybrids in Cu(OAc)2 aqueous solution. ZnAlLDH was synthesised and thereafter calcined at 550°C in air, in the absence or in presence of melamine, to obtain the mixtures of metal oxides (ZnAlMMO and ZnAlMMO/g-C3N4, respectively). Furthermore, by structural reconstruction of the freshly calcined ZnAlLDH in the aqueous solution of Cu(OAc)2 we transformed ZnAlMMO and ZnAlMMO/g-C3N4 into ZnAlLDH/CuONP and ZnAlLDH/g-C3N4/CuONP, respectively. The effects of coupling g-C3N4 with ZnAlLDH, but also the effects of the formation of CuONP on the surface of g-C3N4/ZnAlLDH on the morphology, structural, optical, electrochemical and thermal properties were investigated by powder X-ray diffraction (XRD), thermogravimetric analysis (TGA), UV–Vis spectroscopy, scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS). The obtained results revealed a synergic heterojunction between the semiconductor components coupled with ZnAlLDH, with a tuned position of energy bands in the g-C3N4/ZnAlLDH and a good dispersion of CuONP in the composite. Furthermore, the heterojunction between g-C3N4 and calcined LDH was proved in ZnAlMMO/g-C3N4, which was an intermediate in the green synthesis of the reconstructed hydrotalcites. When tested for phenol photodegradation under irradiation with UV and Vis light the best efficiency was obtained for the ZnAlMMO/g-C3N4 sample (100% phenol conversion after 4h irradiation under UV, though 75% conversion was reached after 24h irradiation with Vis light). The mechanism of charge separation and the electron transfer in the heterojunction of g-C3N4 with CuONP and ZnAlLDH, was also discussed.