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

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

The formation of semiconductor composites comprising multicomponent or multiphase heterojunctions is a very effective strategy to design highly active photocatalyst systems. This review summarizes the recent strategies to develop such composites, and highlights the most recent developments in the fi eld. After a general introduction into the different strategies to improve photocatalytic activity through formation of heterojunctions, the three different types of heterojunctions are introduced in detail, followed by a historical introduction to semiconductor heterojunction systems and a thorough literature overview. Special chapters describe the highly-investigated carbon nitride heterojunctions as well as very recent developments in terms of multiphase heterojunction formation, including the latest insights into the anatase-rutile system. When carefully designed, semiconductor composites comprising two or three different materials or phases very effectively facilitate charge separation and charge carrier transfer, substantially improving photocatalytic and photoelectrochemical effi ciency.

https://faculty.missouri.edu/~glaserr/3700s15/Marschall_Strategies_adfm201303214.pdf

Semiconductor Composites: Strategies for Enhancing Charge Carrier Separation to Improve Photocatalytic Activity

A novel and simple synthetic approach toward core–shell Ag@C3N4 nanocomposites is developed. Ag@C3N4 core–shell nanostructures were formed via reflux treatment of Ag nanoparticles with graphitic C3N4 nanosheets in methanol. The core–shell hybrid photocatalysts showed dramatic photoinduced electron–hole separation efficiency and photocatalytic activity under visible light irradiation. The photocurrent intensity, photocatalytic activity for the photodegradation of methylene blue (MB) and hydrogen evolution reaction of Ag@C3N4 were about 4, 1.8 and 30 times as that of pure C3N4 sample, respectively. The enhanced photocatalytic activity for core–shell Ag@C3N4 originated from a combined result of the localized surface plasmon resonance (LSPR) effect for Ag and hybrid effect from C3N4, resulting in the coupling interaction of the enhanced light absorption intensity, high separation efficiency of photogenerated electrons–holes, longer lifetime of charge carriers and its favorable adsorptivity.

Enhancement of visible photocatalytic activity via Ag@C3N4 core–shell plasmonic composite

C 60 modified graphitic carbon nitride (g-C 3 N 4 ) composite photocatalysts C 60 /g-C 3 N 4 were prepared by a facile thermal treatment at 550 °C in atmosphere involving polymerization of dicyandiamide in the presence of C 60 without adding any other reagent. By incorporating C 60 into the matrix of g-C 3 N 4 , the valance band (VB) of g-C 3 N 4 shifts to lower energy position, and thus gives a strong photo-oxidation capability under visible light. The as-prepared sample shows enhanced degradation of phenol and methylene blue (MB) under visible light ( λ  > 420 nm). The C 60 /g-C 3 N 4 composites present considerably high photocatalytic degradation activities on phenol and MB, as well as photocurrent response, under visible light irradiation. They are about 2.9, 3.2 and 4.0 times as high as those of bulk g-C 3 N 4 , respectively. Such greatly enhanced photocatalytic activity was originated from the holes and • OH, which can be ascribed to strong interaction of conjugative π-bond between C 60 and g-C 3 N 4 .

Enhanced oxidation ability of g-C3N4 photocatalyst via C60 modification

Bi2WO6 photocatalyst was hybridized by graphite-like C3N4via facile chemisorption. After hybridization with C3N4, the photocatalytic activity of Bi2WO6 was obviously enhanced. A C3N4/Bi2WO6 photocatalyst hybridized with monolayer C3N4 exhibited the highest photocatalytic activity which was 68.9% higher than that of pure Bi2WO6. The enhanced photocatalytic activity of the C3N4/Bi2WO6 photocatalysts could be attributed to the synergetic effect between C3N4 and Bi2WO6. The photogenerated holes on the valence band of Bi2WO6 could transfer to the highest occupied molecular orbital of C3N4via the well developed interface, causing rapid photoinduced charge separation and enhancing the photocatalytic activity.

Enhancement of photocatalytic activity of Bi2WO6 hybridized with graphite-like C3N4

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

ZnO photocatalyst with surface oxygen vacancies was obtained via vacuum deoxidation method. The concentration of surface oxygen vacancies could be controlled by tuning the temperature and time in vacuum, which governs the production of visible activity and the enhanced level of photocatalytic activity. The optimum UV photocatalytic activity and photocurrent of vacuum ZnO are almost 1.7 and 2.4 times as high as that of pure ZnO, respectively. Interestingly, the dramatic visible photocatalytic activity and distinct photocurrent both are generated due to the introducing of oxygen vacancies on ZnO surface. The enhancement in performance is attributed to the high separation efficiency of photogeneration electron–hole pairs due to the broadening of the valance band (VB) induced by surface oxygen-vacancies states. The production of visible photoactivity is demonstrated to be the narrow of energy band gap resulting from the rising of VB.

Xiaojuan Bai

Papers

Enhancement of visible photocatalytic activity via Ag@C3N4 core–shell plasmonic composite

Enhanced oxidation ability of g-C3N4 photocatalyst via C60 modification

Enhancement of photocatalytic activity of Bi2WO6 hybridized with graphite-like C3N4

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

Production of visible activity and UV performance enhancement of ZnO photocatalyst via vacuum deoxidation