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?

Graphitic (g)-C3N4 with a layered structure has the potential of forming graphene-like nanosheets with unusual physicochemical properties due to weak van der Waals forces between layers. Herein is shown that g-C3N4 nanosheets with a thickness of around 2 nm can be easily obtained by a simple top-down strategy, namely, thermal oxidation etching of bulk g-C3N4 in air. Compared to the bulk g-C3N4, the highly anisotropic 2D-nanosheets possess a high specific surface area of 306 m2 g-1, a larger bandgap (by 0.2 eV), improved electron transport ability along the in-plane direction, and increased lifetime of photoexcited charge carriers because of the quantum confinement effect. As a consequence, the photocatalytic activities of g-C3N4 nanosheets have been remarkably improved in terms of OH radical generation and photocatalytic hydrogen evolution.

Graphene-Like Carbon Nitride Nanosheets for Improved Photocatalytic Activities

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

Polymeric graphitic carbon nitride materials (for simplicity: g-C(3)N(4)) have attracted much attention in recent years because of their similarity to graphene. They are composed of C, N, and some minor H content only. In contrast to graphenes, g-C(3)N(4) is a medium-bandgap semiconductor and in that role an effective photocatalyst and chemical catalyst for a broad variety of reactions. In this Review, we describe the "polymer chemistry" of this structure, how band positions and bandgap can be varied by doping and copolymerization, and how the organic solid can be textured to make it an effective heterogenous catalyst. g-C(3)N(4) and its modifications have a high thermal and chemical stability and can catalyze a number of "dream reactions", such as photochemical splitting of water, mild and selective oxidation reactions, and--as a coactive catalytic support--superactive hydrogenation reactions. As carbon nitride is metal-free as such, it also tolerates functional groups and is therefore suited for multipurpose applications in biomass conversion and sustainable chemistry.

Polymeric Graphitic Carbon Nitride as a Heterogeneous Organocatalyst: From Photochemistry to Multipurpose Catalysis to Sustainable Chemistry

Graphitic carbon nitride nanosheets are extracted, produced via simple liquid-phase exfoliation of a layered bulk material, g-C3N4. The resulting nanosheets, having ≈2 nm thickness and N/C atomic ratio of 1.31, show an optical bandgap of 2.65 eV. The carbon nitride nanosheets are demonstrated to exhibit excellent photocatalytic activity for hydrogen evolution under visible light.

Exfoliated Graphitic Carbon Nitride Nanosheets as Efficient Catalysts for Hydrogen Evolution Under Visible Light

Sulfur-mediated synthesis has been developed to modify the texture, optical and electronic properties, as well as the photocatalytic functions of a carbon nitride semiconductor. The water oxidation reaction has been achieved at a moderate rate with only photocatalysts without the aid of co-factors.

Sulfur-mediated synthesis of carbon nitride: Band-gap engineering and improved functions for photocatalysis

Semiconducting carbon nitride materials were successfully prepared via a thermal poly-condensation of dicyandiamide as a precursor at >500 °C. The resulting materials were investigated as metal-free catalysts for the activation of H2O2 with visible light under mild conditions, using the decomposition of Rhodamine B (RhB) in aqueous solution as a model reaction. Results revealed that carbon nitride catalysts can activate H2O2 to generate reactive oxy-radicals under visible light irradiation without employment of any metal additives, leading to the mineralization of the dye. Factors affecting the degradation of organic compounds are pH values and the concentration of H2O2. Recycling of the catalyst indicated no obvious deactivation during the entire catalytic reaction, indicating good (photo)chemical stability of metal-free polymeric carbon nitride photocatalysts for environmental purification. This study demonstrated a promising approach for the activation of green oxidant, hydrogen peroxide, by the newly-developed polymer photocatalysts for environmental remediation and oxidation catalysis.

Metal-free activation of H2O2 by g-C3N4 under visible light irradiation for the degradation of organic pollutants

Graphitic carbon nitride was synthesized by direct thermal polymerization of ammonium thiocyanate as the precursor. The transfer of this simple thermal-induced polymerization onto hard-templates with various nanoarchitectures enables the fabrication of nanostructured carbon nitridesvia a soft-chemical synthesis, while the involvement of a sulfur species within the reaction cascade offers additional chemical control of the texture and the electronic structures. The catalysts were subjected to several characterizations, and the results obtained revealed that nanoporous carbon nitrides can be obtained by templating with nanosized silica and SBA-15. Photocatalytic activity was evaluated toward hydrogen evolution from proton solution with visible light. Results show that g-C3N4 synthesized from ammonium thiocyanate exhibited improved photoactivity in comparison with g-C3N4 obtained from dicyandiamide. Further improvement in the activity was achieved by creating the nanostructures in g-C3N4. This is due to the enhanced surface area obtained which is favorable for light-harvesting and mass-transfer, as well as to the increased redox potential.

Synthesis of bulk and nanoporous carbon nitride polymers from ammonium thiocyanate for photocatalytic hydrogen evolution

TiO2 bilayer films with a normal surface (Ns-TiO2), surface defects (Sd-TiO2), and interface defects (Id-TiO2) were successfully prepared by a combination of cold plasma treatment (CPT) and sol−gel dip-coating technology. The photodegradation of rhodamine B (RhB) over these as-prepared TiO2 films was investigated via UV−vis irradiation. Results indicate that the three kinds of films exhibit very different photodegradation processes for RhB. A mainly N-deethylation reaction over the Ns-TiO2 films, whereas an efficient degradation (cycloreversion) of RhB occurs over the Sd-TiO2 films. In the RhB/Id-TiO2 system, however, efficient N-deethylation concomitant with the highly efficient cycloreversion of RhB is observed. The efficiency of the complete mineralization of RhB dye follows the order of Id-TiO2 > Sd-TiO2 > Ns-TiO2. It is proposed that the defect sites at the surface or the interface of TiO2 films promote the separation of photogenerated electron−holes, leading to a higher photoactivity of defective Ti...

Photocatalytic Degradation of RhB over TiO2 Bilayer Films: Effect of Defects and Their Location

ZnO nanocrystals with different oxygen defects (type and concentration) are successfully prepared via the templating method in Nafion membranes. Photoluminescence and infrared characterizations reveal that the concentration of oxygen defects (oxygen vacancy and interstitial oxygen) is quite different for the samples prepared via different synthesis processes. Photocatalytic activity measurements show that the sample with a higher amount of oxygen defects exhibits excellent activity toward the degradation of rhodamine B. Compared with bulk ZnO, ZnO nanocrystals embedded in Nafion membranes have an antiphotocorrosion property. The possible photocatalytic mechanism is discussed in detail, and the oxygen defects are proposed to be the active sites of the ZnO photocatalyst.

Ping Liu

Papers

Sulfur-mediated synthesis of carbon nitride: Band-gap engineering and improved functions for photocatalysis

Metal-free activation of H2O2 by g-C3N4 under visible light irradiation for the degradation of organic pollutants

Synthesis of bulk and nanoporous carbon nitride polymers from ammonium thiocyanate for photocatalytic hydrogen evolution

Photocatalytic Degradation of RhB over TiO2 Bilayer Films: Effect of Defects and Their Location

Relationship between Oxygen Defects and the Photocatalytic Property of ZnO Nanocrystals in Nafion Membranes