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

As a fascinating conjugated polymer, graphitic carbon nitride (g-C3N4) has been the hotspot in the materials science as a metal-free and visible-light-responsive photocatalyst. Pure g-C3N4 suffers from the insufficient sunlight absorption, low surface area and the fast recombination of photo-induced electron-hole pairs, resulting in low photocatalytic activity. Element doping is known to be an efficient method to tune the unique electronic structure and band gap of g-C3N4, which considerably broaden the light responsive range and enhance the charge separation. This review summarizes the recent progress in the development of efficient and low cost doped g-C3N4 systems in various realms such as photocatalytic hydrogen evolution, reduction of carbon dioxide, photocatalytic removal of contaminants in wastewater and gas phase. Typically, metal doping, nonmetal doping, co-doping and heterojunction based on doped g-C3N4 have been explored to simultaneously tune the crystallographic, textural and electronic structures for improving photocatalytic activity by enhancing the light absorption, facilitating the charge separation and transportation and prolonging the charge carrier lifetime. Finally, the current challenges and the crucial issues of element doped g-C3N4 photocatalysts that need to be addressed in future research are presented. This review presented herein can pave a novel avenue and add invaluable knowledge to the family of element doped g-C3N4 for the develop of more effective visible-light-driven photocatalysts.

Doping of graphitic carbon nitride for photocatalysis: A reveiw

Novel porous P-doped graphitic carbon nitride (g-C3N4) nanosheets were for the first time fabricated by combining P doping and thermal exfoliation strategies. The as-prepared P-doped g-C3N4 nanosheets show a high visible-light photocatalytic H2-production activity of 1596 μmol h−1 g−1 and an apparent quantum efficiency of 3.56% at 420 nm, representing one of the most highly active metal-free g-C3N4 nanosheet photocatalysts. This outstanding photocatalytic performance originates from the P-doped conjugated system and novel macroporous nanosheet morphology. Particularly, the empty midgap states (−0.16 V vs. standard hydrogen electrode) created by P doping are for the first time found to greatly extend the light-responsive region up to 557 nm by density functional theory and experimental studies, whilst the novel macroporous structure promotes the mass-transfer process and enhances light harvesting. Our study not only demonstrates a facile, eco-friendly and scalable strategy to synthesize highly efficient porous g-C3N4 nanosheet photocatalysts, but also paves a new avenue for the rational design and synthesis of advanced photocatalysts by harnessing the strong synergistic effects through simultaneously tuning and optimizing the electronic, crystallographic, surface and textural structures.

Porous P-doped graphitic carbon nitride nanosheets for synergistically enhanced visible-light photocatalytic H2 production

The potassium doped graphitic carbon nitride (K-C 3 N 4 ) photocatalysts were prepared via thermal polymerization of dicyandiamide and KI in atmosphere. The valence band (VB) position of g-C 3 N 4 was decreased via potassium doping, resulting in enhanced separation and immigration of photogenerated carriers under visible light. The optimum K-C 3 N 4 exhibited obviously enhanced photocatalytic activities for phenol and MB degradation, which were about 3.3 and 5.8 times as high as those of bulk g-C 3 N 4 , respectively. The photocatalytic activity of K-C 3 N 4 decreased with excessive KI mass fraction in the precursor due to the incomplete polymerization of g-C 3 N 4 .

Enhanced catalytic activity of potassium-doped graphitic carbon nitride induced by lower valence position

The series porous graphitic carbon nitride (pg-C3N4) materials have been prepared by pyrolysis of dicyandiamide in air using urea as bubble template. The surface area of as-prepared pg-C3N4 was increased from 5.4 to 60 m2 g−1 by increasing the mass ratio of urea/dicyandiamide and calcination temperature. We have found that thus-produced nanostructures can display efficient photocatalytic performance for the photodegradation of methylene blue (MB) and phenol pollutant when energized with visible light. The photocurrent and photocatalytic activities of as-synthesized pg-C3N4 were increased with the higher proportion of urea and calcination temperature under visible light irradiation. Compared with the bulk g-C3N4, the optimum photocatalytic activity of our synthesized pg-C3N4 displays a significant enhancement by a factor of 2.1 and 2.8 for MB and phenol, respectively. The separation and migration efficiency of photogenerated electron–hole pairs were improved by the porous structure, which resulted in the enhancement of photocatalytic performances.

Enhancement of visible light photocatalytic activities via porous structure of g-C3N4

A porous ZnO nanosheet with a near-rectangular morphology has been successfully prepared through a simple solvothermal-annealing method using Zn5(OH)6(CO3)2 as a pore-directing agent. Moreover, the features of ZnO can be easily tuned by changing the annealing temperature. The evolution of defects along with the increase of annealing temperature has been revealed as follows: the content of surface oxygen vacancy of the as-prepared samples first increases and then decreases, however, the content of impurities decreases gradually. A clear relationship between the defects and photoluminescence/photocatalytic characteristics of ZnO is observed. The defect-related emission mechanism of the visible photoluminescence (PL) for the as-prepared ZnO samples has been proposed. In addition, the samples also show good activities for photo-degradation of phenol under UV light irradiation. ZnO-500 °C (annealed at 500 °C) exhibits the best photocatalytic activity, which is superior to that of commercial ZnO. The photocatalytic activity can be greatly influenced by the relative concentration ratio of surface defects to bulk defects, crystallization performance and specific surface area.

Defect-related photoluminescence and photocatalytic properties of porous ZnO nanosheets

A novel Bi4O5Br2/Bi24O31Br10/Bi2SiO5 heterostructure photocatalyst was synthesized successfully by in situ ion exchange reaction using BiOBr microspheres as the starting material for the first time. The heterostructured semiconductor showed superior photocatalytic activity in visible light degradation of phenol compared to pure BiOBr and other reported Bi-based photocatalysts. The exceptional photocatalytic activity of the heterostructured system can be attributable to the well-aligned straddling band-gap structures which can promote the flow of charge carriers via the heterojunction, extended light absorbance and a large Brunauer–Emmett–Teller (BET) specific surface area. The present work also proves that engineering of heterojunctions with high matching of lattices and well-aligned straddling band-structures is a significantly efficient way to achieve photocatalysts with high photocatalytic activity.

Enhanced visible light photocatalytic performance of a novel heterostructured Bi4O5Br2/Bi24O31Br10/Bi2SiO5 photocatalyst

A polyaniline (PANI) hydrogel with 3D network nanostructure is synthesized by polymerizing aniline. TiO2 nanoparticles are located onto the 3D matrix of the PANI hydrogel. This PANI/TiO2 composite 3D hydrogel is efficient in removing organic contaminants owing to the synergistic effect of adsorption-enrichment and the succedent in situ photocatalytic degradation. In this process, organic contaminant is first adsorbed and enriched onto the surface of the TiO2 nanoparticles through 3D network nanostructure of PANI hydrogel. The organic contaminant is then in situ degraded. The 3D network structured PANI/TiO2 composite hydrogel has the advantage of high reactivity in nano-sized materials and possesses the characteristics of separation-free in bulk materials, indicating that the composite hydrogel is ease of recycling. Overall, this work provides new insights into the fabrication of hydrogel-based nanocomposite materials and facilitates their potential application for the synergistic removal of various aquatic pollutants in the field of water purification.

Separation-Free Polyaniline/TiO2 3D Hydrogel with High Photocatalytic Activity

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