The Graphitic Carbon Nitride (g-C3N4): A promising material for photocatalytic CO2 reduction: g-C3N4 for photocatalytic reduction of CO2

The documented worldwide CO2 level in 2019 is 409.8 ± 0.1 ppm and reached 414 ppm in 2020. The CO2 emission has reached over 35 billion tons per year with 15% hike in the last era only [1-3]. The global consumption of CO2 is predicted to escalate momentarily which is 0.3 to 0.7 GT per year. To reduce the CO2 concentrations to recommended safe level (350 ppm), a number of novel techniques have been proposed for arrest of CO2, conversion, and storage. Among different state-of-art CO2 reduction techniques, the green energy forces like photo, electric, bio derived energy are prominent [4-6]. Artificial photocatalytic CO2 reduction is a fascinating system to harness clean, renewable and safe solar energy to convert into chemical energy. To synthesize a suitable catalyst for photocatalytic reduction of CO2 is challenging to entire scientific fraternity [7]. Among the catalytic materials selected for photocatalytic reduction, semiconductors based on graphenic materials and graphitic carbon nitride (g-C3N4) is emerged as the promising semiconductor for photocatalytic applications. These are mainly made up of tris-s-triazine (s-heptazine, C6N7) units linked with nitrogen atoms to produce a 2D graphitic structure [8-10]. These materials have acquired substantial attention due to its fascinating optical, electrical, physical and chemical characteristics. The constant recurrence of heptazine units facilitates band gap of 2.7 eV along with ECB = -1.1 eV and EVB= +1.6 eV and reduces it capability of arresting CO2 with the aid of sunlight. Till date g-C3N4 has explored its path on the photocatalytic reduction of CO2. Mao et al. used g-C3N4 for reduction of CO2 to CH3OH and C2H5OH in 2013 using photocatalysis route [19]. They have tested with two precursors: one with urea and the second one is melamine. g-C3N4 prepared through urea showed the best result on alteration of CO2 to CH3OH (6.28 µmol g-1 h-1) and C2H5OH (4.51 µmol g-1 h-1). Porous nitrogen rich g-C3N4 nanotube was fabricated by Zhao and his group for photocatalytic reduction of CO2 to CO. Increased surface area and Lewis basicity of porous g-C3N4 increases the adsorption cite for CO2 and consequently augments the photocatalytic reduction efficiency of CO2 to 103 µmol g-1h-1in comparisom to bulk g-C3N4 [11]. A schematic representation of removal of CO2 through g-C3N4 is given below. Last but not least, this review article summarizes invigorative perspective on the challenges and future direction towards the development of sustainability without environmental detriment.