A review on the synthesis, properties, and characterizations of graphitic carbon nitride (g-C3N4) for energy conversion and storage applications

Graphitic carbon nitride (g-C3N4) is a non-metallic semiconductor, that has received enormous interest in the research area of energy conversion and storage due to its several exceptional characteristics such as moderate bandgap, high thermal and chemical stability, cost-effectiveness, and perfect conduction and valence band position. Nevertheless, the catalytic performance of g-C3N4 is considerably constrained owing to its low solar light absorption, small surface area, and quick photoinduced charge recombination. Enormous studies have been dedicated to the optimization of photoresponsive efficiency through structure engineering, resulting in a variety of techniques for achieving productive photoresponsive applications based on a perspective of the photoexcitation mechanisms and structural characteristics of the exciting polymeric framework. This review examines the impact of vacancy defects within g-C3N4 such as carbon vacancies (CVs), nitrogen vacancies (NVs), amino vacancies, a combination of cyano groups with narrow optical gap materials, bandgap engineering, upconversion materials, plasmonic materials, and photosensitizers have been briefly described. In addition, numerous analyzing techniques such as microscopic, elemental, and computational characterizations have been summed up and investigated. Significantly, this review also describes the physicochemical features of g-C3N4 and highlights the synthetic procedures for g-C3N4 morphology including thermal oxidation etching, chemical exfoliation, ultrasonication-assisted liquid phase exfoliation, chemical vapor deposition, etc. Moreover, the universal optimization methodologies for the synthesis of advanced photoresponsive materials, the optimistic light between structural variables, and photoexcitation mechanisms in g–C3N4–based materials have been explored. Furthermore, the rational development and sustainable fabrications of g–C3N4–based materials for a wide range of applications in energy conversion and storage, such as photocatalytic H2 evolution, photocatalytic O2 evolution, photocatalytic overall water splitting, photoreduction of CO2 source, electrocatalytic H2 evolution, O2 evolution, O2 reduction, alkali-metal ion batteries, lithium-metal batteries, lithium-sulfur batteries, metal-air batteries, and supercapacitors have been discussed in detail. In the end, the future challenges and opportunities with a brief discussion on the developments of novel g–C3N4–based materials have been concluded.