Photoactivation of peroxymonosulfate (PMS) over a CuO–ZnO p–n heterojunction for the selective C2 trimerization of indoles

Abstract

In this study, a CuO–ZnO p–n heterojunction was found to be an unprecedented photocatalyst for the selective C2 trimerization of indoles. The hetero-conjunction of monoclinic CuO with the wurtzite lattice structure of ZnO was evident from TEM and Mott–Schottky analysis. Upon irradiation with UV light, the CuO–ZnO photocatalyst with a band gap of 3.1 eV activated peroxymonosulfate (PMS) forming highly active hydroxyl (˙OH) and sulphate (SO₄⁻˙) radicals. Both these radicals actively participated in the selective C2 trimerization of indoles. Mechanistic investigations revealed an S-scheme catalytic pathway for the reaction. Photo-luminescence (PL) spectroscopic analysis indicated the involvement of radical species in the reaction. The presence of suitable band-edge potential along with the capacity to reduce electron (e⁻)–hole (h⁺) recombination make CuO–ZnO a highly promising low-cost photocatalyst. The present protocol is advantageous in terms of reaction time, product selectivity, substrate scope and the recyclability of the catalyst. The reaction occurred within a short time span of 15–45 min without employing any harsh conditions or additives. Four new molecules, including di-substituted indoles and indoles with a –COOH functional group, were isolated in 76–87% yields. The partial and total density of states (PDOS and TDOS) were evaluated to understand the contribution of individual metals to the band structure and mechanistic aspects.