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

Graphitic carbon nitride (g-C3N4), as an intriguing earth-abundant visible light photocatalyst, possesses a unique two-dimensional structure, excellent chemical stability and tunable electronic structure. Pure g-C3N4 suffers from rapid recombination of photo-generated electron–hole pairs resulting in low photocatalytic activity. Because of the unique electronic structure, the g-C3N4 could act as an eminent candidate for coupling with various functional materials to enhance the performance. According to the discrepancies in the photocatalytic mechanism and process, six primary systems of g-C3N4-based nanocomposites can be classified and summarized: namely, the g-C3N4 based metal-free heterojunction, the g-C3N4/single metal oxide (metal sulfide) heterojunction, g-C3N4/composite oxide, the g-C3N4/halide heterojunction, g-C3N4/noble metal heterostructures, and the g-C3N4 based complex system. Apart from the depiction of the fabrication methods, heterojunction structure and multifunctional application of the g-C3N4-based nanocomposites, we emphasize and elaborate on the underlying mechanisms in the photocatalytic activity enhancement of g-C3N4-based nanocomposites. The unique functions of the p–n junction (semiconductor/semiconductor heterostructures), the Schottky junction (metal/semiconductor heterostructures), the surface plasmon resonance (SPR) effect, photosensitization, superconductivity, etc. are utilized in the photocatalytic processes. Furthermore, the enhanced performance of g-C3N4-based nanocomposites has been widely employed in environmental and energetic applications such as photocatalytic degradation of pollutants, photocatalytic hydrogen generation, carbon dioxide reduction, disinfection, and supercapacitors. This critical review ends with a summary and some perspectives on the challenges and new directions in exploring g-C3N4-based advanced nanomaterials.

Graphitic carbon nitride based nanocomposites: a review

Novel WO3/g-C3N4 composite photocatalysts were prepared by a calcination process with different mass contents of WO3. The photocatalysts were characterized by thermogravimetric analysis (TG), powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDS), high-resolution transmission electron microscopy (HRTEM), UV-vis diffuse reflection spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL) and electrochemical impedance spectroscopy (EIS). The photocatalytic activity of the photocatalysts was evaluated by degradation of methylene blue (MB) dye and 4-chlorophenol (4-CP) under visible light. The results indicated that the WO3/g-C3N4 composite photocatalysts showed higher photocatalytic activity than both the pure WO3 and pure g-C3N4. The optimum photocatalytic activity of WO3/g-C3N4 at a WO3 mass content of 9.7% under visible light irradiation was up to 4.2 times and 2.9 times as high as that of the pure WO3 and pure g-C3N4, respectively. The remarkably increased performance of WO3/g-C3N4 was mainly attributed to the synergistic effect between the interface of WO3 and g-C3N4, including enhanced optical absorption in the visible region, enlarged specific surface areas and the suitable band positions of WO3/g-C3N4 composites.

Visible-light-induced WO3/g-C3N4 composites with enhanced photocatalytic activity

The novel two-dimensional (2D) porous ultrathin oxygen-doped g-C3N4 nanosheets (PUOCNs) were prepared. The comprehensive characterization methods were used to study morphology, microstructure, crystal structure, chemical states and photocatalytic performance of PUOCNs. 2D porous ultrathin structure and the introduction of oxygen are beneficial to the enhancement of the photocatalytic activity of PUOCNs. The average H2 evolution rate of PUOCNs is ∼189.3 μmol h−1, which is ∼5.2 times higher than that of the bulk g-C3N4 and the degradation efficiency of organic dye methyl orange (MO) is almost 71 times higher than that of the bulk g-C3N4. The enhanced photocatalytic activity is due to the more adsorption sites and more active sites, the enhanced redox ability and improved electron transport ability, which leads to less recombination and a more efficient separation of photogenerated electron and hole pairs. Through XPS VB and ESR analysis, the photocatalytic mechanism was also researched in detail.

Template-free synthesis of 2D porous ultrathin nonmetal-doped g-C3N4 nanosheets with highly efficient photocatalytic H2 evolution from water under visible light

Cerium dioxide/graphitic carbon nitride (CeO2/g-C3N4) visible-light-induced photocatalysts were successfully prepared by a simple mixing-calcination technique. The CeO2/g-C3N4 nanocomposites were characterized by thermogravimetric analysis (TG), powder X-ray diffraction (XRD), transmission electron microscopy (TEM), UV-vis diffuse reflection spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL) and the photocurrent-time (PT) measurements. Photocatalytic activities of the prepared samples were examined by studying the degradation of methylene blue (MB) and 4-chlorophenol (4-CP) under visible light irradiation (>400 nm). The CeO2/g-C3N4 composites showed higher photocatalytic activity than that of CeO2 and g-C3N4. The optimum photoactivity of CeO2/g-C3N4 (13.0%) exhibited the highest photocatalytic activity in pollutant treatment and an enhanced photocurrent under visible light. These enhanced activities could be attributed to the synergetic effect between g-C3N4 and CeO2. The increasing photocatalytic activity mechanism was also discussed.

Synthesis and characterization of CeO2/g-C3N4 composites with enhanced visible-light photocatatalytic activity

In this paper, a simple preparation method was utilized to study for the optimum calcination temperature of graphitic carbon nitride (g-C3N4). The g-C3N4 prepared at different temperatures were characterized by X-ray diffraction (XRD), UV-vis diffuse reflectance spectroscopy (DRS) and so on. The results demonstrated that g-C3N4 could not be formed fully until the calcination temperature was higher than 500 °C. From DRS and photoluminescence (PL) spectrum, a red shift of absorption peaks with increasing calcination temperatures was found. This could enhance the visible light absorption, and then the photocatalytic activity would be improved. The photocatalytic activity was evaluated via the photodegradation of methylene blue (MB) and 4-chlorophenol (4-CP), respectively. Moreover, due to the gradual decrement trend of photocurrent intensity with addition of MB, g-C3N4 prepared at 650 °C could be used as a photoelectrochemical sensor to detect the existence of MB and estimate the concentration of MB. Hence g-C3N4 will become a promising candidate for photoelectrochemical applications.

Yeping Li

Papers

Visible-light-induced WO3/g-C3N4 composites with enhanced photocatalytic activity

Template-free synthesis of 2D porous ultrathin nonmetal-doped g-C3N4 nanosheets with highly efficient photocatalytic H2 evolution from water under visible light

Synthesis and characterization of CeO2/g-C3N4 composites with enhanced visible-light photocatatalytic activity

Synthesis of g-C3N4 at different temperatures for superior visible/UV photocatalytic performance and photoelectrochemical sensing of MB solution

Synthesis of g-C3N4/Ag3VO4 composites with enhanced photocatalytic activity under visible light irradiation