Multifunctional carbon nitride nanoarchitectures for catalysis

Catalysis is at the heart of modern-day chemical and pharmaceutical industries, and there is an urgent demand to develop metal-free, high surface area, and efficient catalysts in a scalable, reproducible and economic manner. Amongst the ever-expanding two-dimensional materials family, carbon nitride (CN) has emerged as the most researched material for catalytic applications due to its unique molecular structure with tunable visible range band gap, surface defects, basic sites, and nitrogen functionalities. These properties also endow it with anchoring capability with a large number of catalytically active sites and provide opportunities for doping, hybridization, sensitization, etc. To make considerable progress in the use of CN as a highly effective catalyst for various applications, it is critical to have an in-depth understanding of its synthesis, structure and surface sites. The present review provides an overview of the recent advances in synthetic approaches of CN, its physicochemical properties, and band gap engineering, with a focus on its exclusive usage in a variety of catalytic reactions, including hydrogen evolution reactions, overall water splitting, water oxidation, CO2 reduction, nitrogen reduction reactions, pollutant degradation, and organocatalysis. While the structural design and band gap engineering of catalysts are elaborated, the surface chemistry is dealt with in detail to demonstrate efficient catalytic performances. Burning challenges in catalytic design and future outlook are elucidated.

Simultaneous layer exfoliation and defect activation in g-C3N4 nanosheets with air-water interfacial plasma: spectroscopic defect probing with tailored optical properties

Defect-activated ultrathin graphitic carbon nitride nanosheets (g-C₃N₄) show an enhanced visible light absorption, better charge-separation, and facile charge transport properties. These are requisites for the designing of an active photocatalyst. Conventional methods used for layer exfoliation and defect activation require strong acids, reducing agents, or ultrasonic treatment for a sufficiently long duration. Furthermore, single-step approaches for layer exfoliation and defect incorporation have hardly been reported. Herein, we have shown atmospheric plasma enabled fabrication of g-C₃N₄ nanosheets. This approach is simple, low-cost, less time-consuming, and a green approach to exfoliate layers and activate multiple defects concurrently. The protocol involves plasma discharging at an air-water interface at 5 kV for 30-150 min. Atomic force microscopy (AFM) reveals a layer thickness of 96.27 nm in bulk g-C₃N₄. The thickness becomes 3.78 nm after 150 min of plasma treatment. The exfoliated layers emerge with nitrogen-vacancy sites and self-incorporated defects as probed by positron annihilation spectroscopy (PAS) and X-ray photoelectron spectroscopy (XPS). The defect activated layers show visible light absorption extended up to 600 nm. It is demonstrated that a non-uniform change in the band gap with the plasma treatment time results from quantum confinement in thin layers and Urbach tailing due to defects acting in opposition. Further, steady-state and time-resolved spectroscopy shows the contribution of multiple defect sites for a prolonged lifetime of photoinduced carriers. These defect-activated ultrathin nanosheets of CN serve as an active photocatalyst in the degradation of rhodamine B (RhB) under white LED illumination.

Retina-Inspired Carbon Nitride-Based Photonic Synapses for Selective Detection of UV Light

Photonic synapses combine sensing and processing in a single device, so they are promising candidates to emulate visual perception of a biological retina. However, photonic synapses with wavelength selectivity, which is a key property for visual perception, have not been developed so far. Herein, organic photonic synapses that selectively detect UV rays and process various optical stimuli are presented. The photonic synapses use carbon nitride (C₃ N₄ ) as an UV-responsive floating-gate layer in transistor geometry. C₃ N₄ nanodots dominantly absorb UV light; this trait is the basis of UV selectivity in these photonic synapses. The presented devices consume only 18.06 fJ per synaptic event, which is comparable to the energy consumption of biological synapses. Furthermore, in situ modulation of exposure to UV light is demonstrated by integrating the devices with UV transmittance modulators. These smart systems can be further developed to combine detection and dose-calculation to determine how and when to decrease UV transmittance for preventive health care.