Synthesis and up-to-date applications of 2D microporous g-C3N4 nanomaterials for sustainable development

Proceeding of catalytic water splitting on Cu/Ce@g-C3N4 photocatalysts: an exceptional approach for sunlight-driven hydrogen generation

Increasing energy demands and environmental problems require carbon-free and renewable energy generation systems. For this purpose, we have synthesized efficient photocatalysts (i.e., g-C3N4, Cu@g-C3N4, Ce@g-C3N4 and Cu/Ce@g-C3N4) for H2 evolution from water splitting. Their optical, structural and electrochemical properties were investigated by UV-Vis-DRS, PL, XRD, FTIR, Raman and EIS methods. Their surface morphologies were evaluated by AFM and SEM analyses. Their chemical characteristics, compositions and stability were assessed using XPS, EDX and TGA techniques. Photoreactions were performed in a quartz reactor (150 mL/Velp-UK), whereas hydrogen generation activities were monitored using a GC-TCD (Shimadzu-2014/Japan). The results depicted that Cu/Ce@g-C3N4 catalysts are the most active catalysts that deliver 23.94 mmol g-1 h-1 of H2. The higher rate of H2 evolution was attributed to the active synergism between Ce and Cu metals and the impact of surface plasmon electrons (SPEs) of Cu that were produced during the photoreaction. The rate of H2 production was optimized by controlling various factors, including the catalyst amount, light intensity, pH, and temperature of the reaction mixture. It has been concluded that the current study holds promise to replace the conventional and costly catalysts used for hydrogen generation technologies.

Progress of charge carrier dynamics and regulation strategies in 2D CxNy-based heterojunctions

Two-dimensional carbon nitrides (CxNy) have gained significant attention in various fields including hydrogen energy development, environmental remediation, optoelectronic devices, and energy storage owing to their extensive surface area, abundant raw materials, high chemical stability, and distinctive physical and chemical characteristics. One effective approach to address the challenges of limited visible light utilization and elevated carrier recombination rates is to establish heterojunctions for CxNy-based single materials (e.g. C2N3, g-C3N4, C3N4, C4N3, C2N, and C3N). The carrier generation, migration, and recombination of heterojunctions with different band alignments have been analyzed starting from the application of CxNy with metal oxides, transition metal sulfides (selenides), conductive carbon, and Cx'Ny' heterojunctions. Additionally, we have explored diverse strategies to enhance heterojunction performance from the perspective of carrier dynamics. In conclusion, we present some overarching observations and insights into the challenges and opportunities associated with the development of advanced CxNy-based heterojunctions.

Exploring the Remarkably High Photocatalytic Efficiency of Ultra-Thin Porous Graphitic Carbon Nitride Nanosheets

Graphitic carbon nitride (g-C3N4) is a metal-free photocatalyst used for visible-driven hydrogen production, CO2 reduction, and organic pollutant degradation. In addition to the most attractive feature of visible photoactivity, its other benefits include thermal and photochemical stability, cost-effectiveness, and simple and easy-scale-up synthesis. However, its performance is still limited due to its low absorption at longer wavelengths in the visible range, and high charge recombination. In addition, the exfoliated nanosheets easily aggregate, causing the reduction in specific surface area, and thus its photoactivity. Herein, we propose the use of ultra-thin porous g-C3N4 nanosheets to overcome these limitations and improve its photocatalytic performance. Through the optimization of a novel multi-step synthetic protocol, based on an initial thermal treatment, the use of nitric acid (HNO3), and an ultrasonication step, we were able to obtain very thin and well-tuned material that yielded exceptional photodegradation performance of methyl orange (MO) under visible light irradiation, without the need for any co-catalyst. About 96% of MO was degraded in as short as 30 min, achieving a normalized apparent reaction rate constant (k) of 1.1 × 10-2 min-1mg-1. This represents the highest k value ever reported using C3N4-based photocatalysts for MO degradation, based on our thorough literature search. Ultrasonication in acid not only prevents agglomeration of g-C3N4 nanosheets but also tunes pore size distribution and plays a key role in this achievement. We also studied their performance in a photocatalytic hydrogen evolution reaction (HER), achieving a production of 1842 µmol h-1 g-1. Through a profound analysis of all the samples' structure, morphology, and optical properties, we provide physical insight into the improved performance of our optimized porous g-C3N4 sample for both photocatalytic reactions. This research may serve as a guide for improving the photocatalytic activity of porous two-dimensional (2D) semiconductors under visible light irradiation.