Graphitic Carbon Nitride (g-C₃N₄)-Based Photocatalysts for Artificial Photosynthesis and Environmental Remediation: Are We a Step Closer To Achieving Sustainability?

Abstract

As a fascinating conjugated polymer, graphitic carbon nitride (g-C3N4) has become a new research hotspot and drawn broad interdisciplinary attention as a metal-free and visible-light-responsive photocatalyst in the arena of solar energy conversion and environmental remediation. This is due to its appealing electronic band structure, high physicochemical stability, and "earth-abundant" nature. This critical review summarizes a panorama of the latest progress related to the design and construction of pristine g-C3N4 and g-C3N4-based nanocomposites, including (1) nanoarchitecture design of bare g-C3N4, such as hard and soft templating approaches, supramolecular preorganization assembly, exfoliation, and template-free synthesis routes, (2) functionalization of g-C3N4 at an atomic level (elemental doping) and molecular level (copolymerization), and (3) modification of g-C3N4 with well-matched energy levels of another semiconductor or a metal as a cocatalyst to form heterojunction nanostructures. The construction and characteristics of each classification of the heterojunction system will be critically reviewed, namely metal-g-C3N4, semiconductor-g-C3N4, isotype g-C3N4/g-C3N4, graphitic carbon-g-C3N4, conducting polymer-g-C3N4, sensitizer-g-C3N4, and multicomponent heterojunctions. The band structures, electronic properties, optical absorption, and interfacial charge transfer of g-C3N4-based heterostructured nanohybrids will also be theoretically discussed based on the first-principles density functional theory (DFT) calculations to provide insightful outlooks on the charge carrier dynamics. Apart from that, the advancement of the versatile photoredox applications toward artificial photosynthesis (water splitting and photofixation of CO2), environmental decontamination, and bacteria disinfection will be presented in detail. Last but not least, this comprehensive review will conclude with a summary and some invigorating perspectives on the challenges and future directions at the forefront of this research platform. It is anticipated that this review can stimulate a new research doorway to facilitate the next generation of g-C3N4-based photocatalysts with ameliorated performances by harnessing the outstanding structural, electronic, and optical properties for the development of a sustainable future without environmental detriment.

CO2 Reduction: From the Electrochemical to Photochemical Approach

“…In 2009, Domen and co‐workers first reported its photocatalytic activity for hydrogen production under visible light 164. Great efforts have been invested on C 3 N 4 to explore its potential for photocatalytic CO 2 reduction since 2013 165, 166, 167, 168. Various reduction products such as CO, CH 4 , C 2 H 6 , HCOOH, and CH 3 OH are measured 169, 170, 171.…”

Recent Progress in Energy‐Driven Water Splitting

“…On the other hand, the crystalline photocatalysts with smaller particle size have the shorter distance for the photogenerated electrons and holes to quickly migrate to the active reaction sites on the surface as compared to those with bigger particle size, thus lowering the recombination probability before water splitting. Meanwhile, different shapes such as two dimensional nanostructures are expected to lower the recombination rate of photogenerated electron–hole pairs for giving higher photocatalytic activity 10. In case that the photogenerated electrons and holes possess thermodynamically sufficient potentials for water splitting, the recombination may still occur when there is lack of suitable active sites on the surface for water splitting.…”

Tuning the Intrinsic Properties of Carbon Nitride for High Quantum Yield Photocatalytic Hydrogen Production

“…It has suitable energy levels (band gap ≈2.7 eV; LUMO ≈ −1.1 V vs SHE; HOMO ≈ +1.6 V vs SHE9) straddling the redox potential required (i.e., 0.0 V vs SHE for proton reduction and +1.23 V vs SHE for water oxidation2) for water splitting 8, 15, 31. These HOMO and LUMO values are predictions from computational results using DFT.…”