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 constructi...

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

Semiconductor-based photocatalysis is considered to be an attractive way for solving the worldwide energy shortage and environmental pollution issues. Since the pioneering work in 2009 on graphitic carbon nitride (g-C3N4) for visible-light photocatalytic water splitting, g-C3N4 -based photocatalysis has become a very hot research topic. This review summarizes the recent progress regarding the design and preparation of g-C3N4 -based photocatalysts, including the fabrication and nanostructure design of pristine g-C3N4 , bandgap engineering through atomic-level doping and molecular-level modification, and the preparation of g-C3N4 -based semiconductor composites. Also, the photo-catalytic applications of g-C3N4 -based photocatalysts in the fields of water splitting, CO2 reduction, pollutant degradation, organic syntheses, and bacterial disinfection are reviewed, with emphasis on photocatalysis promoted by carbon materials, non-noble-metal cocatalysts, and Z-scheme heterojunctions. Finally, the concluding remarks are presented and some perspectives regarding the future development of g-C3N4 -based photocatalysts are highlighted.

Polymeric Photocatalysts Based on Graphitic Carbon Nitride

A review on g-C3N4-based photocatalysts

Photocatalysis is considered as one of the promising routes to solve the energy and environmental crises by utilizing solar energy. Graphitic carbon nitride (g-C3N4) has attracted worldwide attention due to its visible-light activity, facile synthesis from low-cost materials, chemical stability, and unique layered structure. However, the pure g-C3N4 photocatalyst still suffers from its low separation efficiency of photogenerated charge carriers, which results in unsatisfactory photocatalytic activity. Recently, g-C3N4-based heterostructures have become research hotspots for their greatly enhanced charge carrier separation efficiency and photocatalytic performance. According to the different transfer mechanisms of photogenerated charge carriers between g-C3N4 and the coupled components, the g-C3N4-based heterostructured photocatalysts can be divided into the following categories: g-C3N4-based conventional type II heterojunction, g-C3N4-based Z-scheme heterojunction, g-C3N4-based p–n heterojunction, g-C3N4/metal heterostructure, and g-C3N4/carbon heterostructure. This review summarizes the recent significant progress on the design of g-C3N4-based heterostructured photocatalysts and their special separation/transfer mechanisms of photogenerated charge carriers. Moreover, their applications in environmental and energy fields, e.g., water splitting, carbon dioxide reduction, and degradation of pollutants, are also reviewed. Finally, some concluding remarks and perspectives on the challenges and opportunities for exploring advanced g-C3N4-based heterostructured photocatalysts are presented.

g-C3N4-Based Heterostructured Photocatalysts

A facile synthetic strategy for nitrogen-deficient graphitic carbon nitride (g-C3 Nx ) is established, involving a simple alkali-assisted thermal polymerization of urea, melamine, or thiourea. In situ introduced nitrogen vacancies significantly redshift the absorption edge of g-C3 Nx , with the defect concentration depending on the alkali to nitrogen precursor ratio. The g-C3 Nx products show superior visible-light photocatalytic performance compared to pristine g-C3 N4 .

Alkali-Assisted Synthesis of Nitrogen Deficient Graphitic Carbon Nitride with Tunable Band Structures for Efficient Visible-Light-Driven Hydrogen Evolution.

https://www.rsc.org/suppdata/cc/c2/c2cc32181e/c2cc32181e.pdf

Carbon self-doping induced high electronic conductivity and photoreactivity of g-C3N4

Hydrogen peroxide (H2O2) is of great significance in biological and environmental processes as well as in chemical industry. Even though anthraquinone autoxidation (AO) process has been the major artificial way to produce H2O2, its energy cost and non-green nature have been motivating people to develop more efficient, economic and green technologies as alternatives. Here we demonstrated that photocatalytic H2O2 production at g-C3N4 could be improved by as much as 14 times in the absence of organic scavenger through a carbon vacancy-based strategy. Both the experimental and theoretical calculation results indicated that the creation of carbon vacancies could reduce the symmetry of g-C3N4 and produce the effect of electron delocalization. This will allow g-C3N4 to possess more excitable electrons and a narrower band gap. On the other hand, carbon vacancies provided more sites to adsorb molecular oxygen and thereby help electrons transfer from g-C3N4 to the surface adsorbed O2. More interestingly, the presence of carbon vacancies changed the H2O2 generation pathway from a two-step single-electron indirect reduction to an one-step two-electron direct reduction. This study could not only develop a novel strategy to improve the H2O2 production activity of semiconductors, but also shed light on the deep understanding of the role played by surface defect structure on photocatalytic activity of semiconductor photocatalysts.

/pdf/effective-photocatalytic-h2o2-production-under-visible-light-1cctx1ljj7.pdf

Effective photocatalytic H2O2 production under visible light irradiation at g-C3N4 modulated by carbon vacancies

We demonstrate that graphitic carbon nitride can photoreduce CO2 to CO in the presence of water vapor and exhibit interesting porous structure dependent reactivity on photoreduction and photooxidation under visible light (λ > 420 nm). Graphitic carbon nitride was synthesized by directly heating the inexpensive melamine and the replacement of melamine with melamine hydrochloride could result in porousification in the final graphitic carbon nitride with much higher surface area (39 times) and more abundant pores, accompanied by a band gap increase of 0.13 eV. The porousification could significantly enhance the photoreactivity of graphitic carbon nitride in rhodamine B photooxidation by 9.4 times, but lower its activity in CO2 photoreduction by 4.6 times. The reasons for the porous structure dependent photoreactivity were investigated in detail. These new findings could shed light on the design of efficient photocatalysts and the tuning of their photoreactivity for environmental and energy applications.

Porous structure dependent photoreactivity of graphitic carbon nitride under visible light

Photoreduction of N2 is among the most interesting and challenging methods for N2 fixation under mild conditions. In this study, we determined that nitrogen vacancies (NVs) could endow graphitic carbon nitride (g-C3N4) with the photocatalytic N2 fixation ability. Photocatalytic N2 fixation induced by NVs is free from the interference of other gases. The reason is that NVs could selectively adsorb and activate N2 because NVs have the same shape and size as the nitrogen atom in N2. In addition to this, NVs could also improve many other properties of g-C3N4 to support photocatalytic N2 fixation. These new findings could elucidate the design of efficient photocatalysts for photocatalytic N2 fixation.

Selective photocatalytic N2 fixation dependent on g-C3N4 induced by nitrogen vacancies

Photoreduction of N₂ is among the most interesting and challenging methods for N₂ fixation under mild conditions. In this study, we determined that nitrogen vacancies (NVs) could endow graphitic carbon nitride (g-C₃N₄) with the photocatalytic N₂ fixation ability. Photocatalytic N₂ fixation induced by NVs is free from the interference of other gases. The reason is that NVs could selectively adsorb and activate N₂ because NVs have the same shape and size as the nitrogen atom in N₂. In addition to this, NVs could also improve many other properties of g-C₃N₄ to support photocatalytic N₂ fixation. These new findings could elucidate the design of efficient photocatalysts for photocatalytic N₂ fixation.

Guohui Dong

Papers

Carbon self-doping induced high electronic conductivity and photoreactivity of g-C3N4

Effective photocatalytic H2O2 production under visible light irradiation at g-C3N4 modulated by carbon vacancies

Porous structure dependent photoreactivity of graphitic carbon nitride under visible light

Selective photocatalytic N2 fixation dependent on g-C3N4 induced by nitrogen vacancies

Selective photocatalytic N₂ fixation dependent on g-C₃N₄ induced by nitrogen vacancies