Engineering 2D g-C3N4 for enhanced photocatalytic water purification: the impact of post-processing on activity enhancement

Abstract

In recent decades, g-C₃N₄ has emerged as a potential candidate among the 2D materials for revolutionizing the use of materials in water remediation applications. g-C₃N₄ is an accessible semiconductor with excellent conductivity but faces challenges in its bulk state due to fast recombination rates, few active sites for adsorption, and poor light absorption capacity. The prime focus of this work is to address these drawbacks and develop an efficient metal-free photocatalyst that only uses g-C₃N₄. The main aim is the engineering of metal-free, pristine g-C₃N₄ ultrathin nanosheets (NS) and partially exfoliated nanosheets (PS) from the bulk form via two scalable post-processing methods (thermal exfoliation and mechano-hydrothermal treatment) in the absence of dopants, heterojunctions, or co-catalysts. The engineered g-C₃N₄ sheets help to conduct a straight comparison between the photocatalytic enhancement that exclusively arises from different tailored morphologies, thus enabling precise benchmarking of the adsorption, charge dynamics and ROS-mediated degradation properties of different morphologies. The formed bulk g-C₃N₄ (BS), ultrathin nanosheets (NS) and partially exfoliated form (PS) were rigorously studied with analytical tools to understand their structure–property–activity relationships. Remarkably, NS had the highest degradation efficiency (97%) toward methylene blue dye under visible-light illumination within 60 min followed by PS (82%) and BS (62%). EPR analysis and scavenging test showed that super oxide radicals and singlet oxygen are responsible for the enhanced and rapid degradation of MB. The intermediates involved in MB breakdown and the degradation pathway were outlined through LC-MS studies. Experimental UV–Vis DRS analysis was correlated with DFT calculated bandgaps, thus enabling a rare dual insight into band structure evolution. In addition, a clear quantitative relationship was recognized between the surface area, porosity and degradation efficiency. The developed photocatalysts demonstrated excellent stability over six cycles of reuse, and were used in the degradation of a complex water matrix with a mixture of various dyes, where an outcome of 80.9% mineralization emphasizes their practical relevance. The use of plant growth (green gram seeds) as a proxy for toxicity suggested that the growth rate (root length 16 cm; shoot length 14 cm) is similar for plants grown using distilled water and dye-containing water treated with the NS photocatalyst, further substantiating the environmental sustainability of the catalyst. This research underlines the potential of morphology controlled, metal-free, scalable g-C₃N₄ as an excellent green photocatalyst for sustainable water treatment.