Ternary ZnCdSO composite photocatalyst for efficient dye degradation under visible light retaining Z-scheme of migration pathways for the photogenerated charge carriers

Abstract

Simple co-precipitation method followed by calcination was adopted to synthesize ZnCdSO ternary semiconductor photocatalyst. In the composite material highly crystalline and hexagonal wurtzite phase of ZnO along with hexagonal CdS and cubic CdO phases were separately identified. Oxygen vacancies (VO) in ZnO network and CdO phase resulted in narrowing the optical band gap, facilitating increased visible light absorption. CdS–CdO phase mediated surface defects increased significantly over the core/bulk defects in pristine ZnO, which simultaneously acted as trap-sites for charge carriers and active adsorption-sites for dyes for photocatalytic degradation. In photocatalytic Z-scheme charge transfer mechanism CdS with a higher negative potential conduction band acted as efficient reduction site and created superoxide radical anions (O2•−), whereas ZnO and CdO both having lower valence band potential acted as efficient oxidation sites and created hydroxyl radicals (•OH). Spatial separation of photo-generated charge carriers enhanced their lifetime and facilitated creating superoxide and hydroxyl radicals which, by virtue of their powerful and respective reducing and oxidizing properties, efficiently degraded the dye molecules. The hydroxyl radicals (•OH) and holes (h+) dominantly contributed in sequence, in the dye degradation process, while electrons (e−) and superoxide radicals (O2•−) had gradually lesser inputs to the phenomenon, as evidently identified via radical scavenging experiment with optical absorption study. Thus, photoactive ternary ZnO–CdS–CdO composite, produced via formation of CdO phase upon low temperature (300 °C) calcination, offered a significantly photostable superior photocatalyst for efficient dye degradation under visible light via a double Z-scheme charge transfer mechanism.