An efficient Z-scheme CdS/g-C3N4 nano catalyst in methyl orange photodegradation: Focus on the scavenging agent and mechanism

Abstract

The work focused on estimating the band gap energy (Eg) for various electronic transitions in the CdS/g-C3N4 coupled sample and the potential positions of the valence and conduction bands. Based on the data obtained and the effects of the scavenging agents, suitable energy diagrams were constructed for illustrating the photodegradation mechanism pathway. The coupled catalyst showed a red shift in Eg-value concerning the g-C3N4 alone (Eg (eV): CdS: 1.88, g-C3N4: 2.83, CdS/g-C3N4: 2.17). The pHpzc values of 5.1, 6.2, and 5.4 were estimated for CdS, g-C3N4, and CdS/g-C3N4, respectively. The highest methyl orange MO degradation was achieved at acidic pH 3–5. In this pH range, the zwitterion MO species is the predominant MO species in media which the catalyst can adsorb via hydrogen bonding by the MO sulfonic group. The photogenerated e- ≈ photogenerated h+ > superoxide radicals > hydroxyl radicals trend was obtained for the importance of the reactive species in MO photodegradation. In the proposed system, MO degradation can begin by both reduction and oxidation processes, and both are more efficient when the direct Z-scheme mechanism is considered. >85% of MO molecules can be degraded at conditions of catalyst dose: 0.5 g/L, pH: 3, CMO: 2 ppm, and irradiation time: 90 min. No considerable hydroxyl radicals can be produced by the system based on the potential positions of the species (E (eV): CdS-VB: 1.93, g-C3N4-VB: 1.63, OH /OH–: 1.9). The direct Z-scheme pathway can prove the H2-production and O2-evolution by the proposed system.