A mini-review on the synthesis and structural modification of g-C <sub>3</sub>N <sub>4</sub>-based materials, and their applications in solar energy conversion and environmental remediation

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Abstract

Recently, graphitic carbon nitride (g-C ₃N ₄) as a metal-free conjugated polymer has emerged as a photocatalyst showing catalytic activity for water splitting, CO ₂ photoreduction and degradation of organic pollutants under visible light irradiation.

Abstract

In recent years, graphitic carbon nitride (g-C ₃N ₄) has shown promising performance as a metal-free conjugated polymer containing graphitic stacking of C ₃N ₄ layers, which are constructed from tri- s-triazine units connected by planar amino groups. Pure g-C ₃N ₄ with a band gap energy of 2.7 eV (460 nm) makes it an applicable and efficient visible-active photocatalyst for energy conversion and environmental applications. This is because of its appropriate electronic band structures, efficient optical absorption, and extraordinary thermal and chemical stability. Currently, the major focus in photocatalysis research is on the design and development of visible-light-driven photocatalysts that are stable and highly efficient for practical applications. In this current mini-review, a comprehensive survey is conducted on graphitic carbon nitride (g-C ₃N ₄) as a metal-free novel visible-light photocatalyst and the progress in the synthesis and design of high-efficiency g-C ₃N ₄-based nanomaterial photocatalysts with controllable structures and morphologies is discussed in detail. Various approaches for modifying the electronic band structure and visible-light harvesting, including metal (cation), non-metal (anion) doping, noble metal loading, and composite formation with other semiconductors are discussed in detail. Photocatalytic applications of g-C ₃N ₄ as metal-free catalysts in photocatalytic water splitting for hydrogen production, CO ₂ photoreduction, organic pollutant degradation, and bacterial disinfections are introduced and discussed in detail. Finally, this critical mini-review summarizes some outlooks on the ongoing challenges in the development of efficient and low cost metal-free g-C ₃N ₄ systems, which provide a good complement to the literature on g-C ₃N ₄ based metal-free nanomaterial photocatalysts.

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A metal-free polymeric photocatalyst for hydrogen production from water under visible light.

Xinchen Wang, Kazuhiko Maeda, Arne Thomas … (2009)

The production of hydrogen from water using a catalyst and solar energy is an ideal future energy source, independent of fossil reserves. For an economical use of water and solar energy, catalysts that are sufficiently efficient, stable, inexpensive and capable of harvesting light are required. Here, we show that an abundant material, polymeric carbon nitride, can produce hydrogen from water under visible-light irradiation in the presence of a sacrificial donor. Contrary to other conducting polymer semiconductors, carbon nitride is chemically and thermally stable and does not rely on complicated device manufacturing. The results represent an important first step towards photosynthesis in general where artificial conjugated polymer semiconductors can be used as energy transducers.

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

Wee-Jun Ong, Lling-Lling Tan, Yun Ng … (2016)

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.