Microwave-assisted in-Situ Synthesis of ZnS/g-C3N4 Heterojunction Composite for Efficient Photocatalytic Degradation of Malachite Green Dye

Abstract

The escalating impacts of climate change coupled with rapid population growth, and unsustainable consumption patterns, have created a global water crisis of unprecedented proportions. The availability of clean water is a fundamental human right, yet billions of people worldwide lack access to safe drinking water and basic sanitation. This necessitates the development of advanced wastewater treatment systems capable of producing high-quality effluent. In this study, we successfully synthesized highly efficient photocatalysts, specifically ZnS, bulk-g-C₃N₄, and bulk-g-C₃N₄/ZnS composites, using microwave-assisted technique. These materials were designed to serve as effective photocatalysts driven by visible light for environmental applications. The synthesized materials included ZnS, bulk-g-C₃N₄, and their composites at a 1:1, 1:2, and 1:3 weight ratios. Comprehensive characterization of the prepared composites using various techniques, including XRD, UV–Vis, FTIR, FESEM, EDS, HRTEM, and XPS was conducted. The cubic zinc blend structure and layered stacking arrangement for the ZnS, and bulk-g-C₃N₄ compounds were revealed by the composite material's XRD analysis; the sizes of ZnS, pure bulk-g-C₃N₄, and their composites with various ratios of ZnS/bulk-g-C₃N₄ (1:1, 1:2, 1:3) were 2.72 nm, 5.62 nm, 3.02 nm, 2.74 nm, and 2.69 nm, respectively. FTIR analysis revealed that the stretching vibrations of C = N and C≡N bonds were located inside certain spectrum regions. Peaks in the 1600–1800 cm⁻¹ range were seen for C = N bonds, while peaks in the 2350 cm⁻¹ range were observed for C≡N bonds. Moreover, the noticeable peaks observed between 1300 and 1570 cm⁻¹ are caused by the aromatic C-N stretching vibrations. The FESEM analysis showed that ZnS/bulk-g-C₃N₄ composites had a sheet-like nanohybrid morphology, whereas pure ZnS and bulk-g-C₃N₄ appeared as nanosheets and nanohybrids, respectively. The Zn, S, C, and N elements found in the produced materials were identified by EDS analysis, which also confirmed the lack of impurities. The HRTEM image of the ZnS/bulk-g-C₃N₄ (1:1) composite was used to quantify the interatomic distance between the ZnS atoms. The cubic zinc blend structure of ZnS was discovered to have a (111) plane that corresponds to a 0.31 nm lattice spacing. XPS revealed that Zn, S, C, and N were in the Zn 2p, S 2p, C 1 s, and N 1 s oxidation states. The photocatalytic performance of the different composites (e.g., 30 mg) was evaluated for the degradation of malachite green dye (e.g., 3 × 10⁵ M) in aqueous solution, utilizing a custom-built photocatalytic reactor equipped with a 250W halogen lamp under continuous magnetic stirring for 120 min. The findings indicated that the g-C₃N₄/ZnS composite photocatalysts exhibited superior degradation efficiency compared to the individual components, showing a degradation rates of 2% and 28% for pure bulk-g-C₃N₄ and ZnS, respectively. Remarkably, under visible light irradiation, the g-C₃N₄/ZnS composite with a 1:3 weight ratio demonstrated the highest photocatalytic efficiency, achieving 33.50%. The 1:1, and 1:2 weight ratios exhibited photocatalytic efficiencies of 16.79%, and 25.57%, respectively. Ultimately, these findings indicate that ZnS/bulk-g-C₃N₄ (1:3) can be regarded as an exceptionally effective photocatalyst for the removal and degradation of malachite green dye from wastewater.