Pristine Graphitic Carbon Nitride Research Articles

Carbonyl Linked Carbon Nitride Loading Few Layered MoS2 for Boosting Photocatalytic Hydrogen Generation

Pristine graphitic carbon nitride g-C3N4 materials, as a novel metal-free photocatalyst with moderate activity, have attracted intense interest. However, its fast photogenerated carriers recombination always induces a relative low performance. Herein, we for the first time report one new ═C═O group linked g-C3N4 (CO-C3N4) through CO2-assisted thermal polymerization of urea. It is found that the edge ═C═O groups work as the photogenerated electrons collection sites and then promote the carriers separation. The visible-light phototatalytic hydrogen evolution performance of our synthesized samples shows 1.85 times higher than that of the reference g-C3N4. To get a considerable visible-light driven photocatalytic hydrogen generation, a new few layered MoS2 with a small size (ca. 20 nm) is prepared through a liquid exfoliation and then is loaded onto the CO-C3N4. The optimal MoS2/CO-C3N4 sample gives the photocatalytic hydrogen evolution of 1990 and 1440 μmol/(g*h) under the λ > 400 and 420 illumination, highe...

Improving the Visible-Light Photocatalytic Activity of Graphitic Carbon Nitride by Carbon Black Doping.

Hydrogen production by water splitting and the removal of aqueous dyes by using a catalyst and solar energy are an ideal future energy source and useful for environmental protection. Graphitic carbon nitride can be used as the photocatalyst with visible light irradiation. However, it typically suffers from the high recombination of carriers and low electrical conductivity. Here, we have developed a facile mix-thermal strategy to prepare carbon black-modified graphitic carbon nitrides, which possess high electrical conductivity, a wide adsorption range of visible light, and a low recombination rate of carriers. With the help of carbon black, highly crystallized graphitic carbon nitrides with built-in triazine and heptazine heterojunctions are obtained. Improved photocatalytic activities have been achieved in carbon black-modified graphitic carbon nitride. The dye removal rate can be three times faster than that of pristine graphitic carbon nitride and the photocatalytic H2 generation is 234 μmol h–1 g–1 under visible light irradiation.

Alteration of the electronic structure and the optical properties of graphitic carbon nitride by metal ion doping.

The photoluminescence quenching of graphitic carbon nitride (GCN) was systematically investigated with the doping of transition metal ions. The photoluminescence spectra of metal doped and pristine GCN were monitored and the trend of quenching efficiency was found to be Cu2+ > Co2+ > Mn2+. Interestingly, with the increasing doping concentration of different metal ions simultaneous red shift and luminescence quenching was determined in the photoluminescence spectra as well as increased absorption tail in longer wavelength hence enhancement in the bandgap. The change in the optical properties could be mainly due to structural reconstruction and doping induced electronic redistribution is discussed.

Salt-template-assisted construction of honeycomb-like structured g-C3N4 with tunable band structure for enhanced photocatalytic H2 production

Honeycomb-like structured graphitic carbon nitride (g-C3N4) with tunable band structure was prepared through a one-step salt-template assisted approach. The honeycomb structure, cyano group defects and Na ion doping result in enhanced light absorption, improved photoexcited charge carrier separation, and up-shift of conduction band. As a consequence, the photocatalytic H2 evolution rate is about 5.2 times higher than that of the pristine graphitic carbon nitride. This newly developed approach provides a promising way to modify other functional materials through nanostructure design coupled with electronic modulation strategy simultaneously.

Fabrication of nanoscale graphitic carbon nitride/copper oxide hybrid composites coated solid-phase microextraction fibers coupled with gas chromatography for determination of polycyclic aromatic hydrocarbons

In this work, a novel nanoscale graphitic carbon nitride hybridized with copper oxide (nano-g-C3N4/CuO) composite was prepared and used as the coating of solid-phase microextraction (SPME) for the first time. The coating was applied to adsorption and preconcentration of polycyclic aromatic hydrocarbons (PAHs) including naphthalene, acenaphthene, fluorene, phenanthrene, anthracene, and pyrene from water and soil prior to gas chromatography (GC) analysis. Compared with pristine nanoscale graphitic carbon nitride (nano-g-C3N4) or copper oxide (CuO), the as-prepared nano-g-C3N4/CuO composite as the coating of SPME provided the best adsorption affinity for PAHs. Moreover, the extraction efficiencies of the nano-g-C3N4/CuO coated fibers were much higher than that of bulk-g-C3N4/CuO hybrids. The developed method exhibited excellent linearity for target analytes in the range of 0.1–1000 ng/mL with determination coefficient (R2) not lower than 0.9944. The limits of detection (LODs) of the established method for the analysis of PAHs were achieved between 0.025 and 0.40 ng/mL. The relative standard deviations (RSDs) for three replicate extractions with one fiber were determined from 2.5% to 7.3%. The fiber-to-fiber reproducibility (n = 3) was 5.4–12.8%. The prepared coating also demonstrated excellent thermal and chemical stability. The method was successfully applied to analysis of target analytes in water and soil samples and the corresponding contents were calculated between 0.07 and 1.58 μg/g.

Electrospun one-dimensional graphitic carbon nitride-coated carbon hybrid nanofibers (GCN/CNFs) for photoelectrochemical applications

Coupling of graphitic carbon nitride (GCN) with electrospun carbon nanofibers (CNFs) enhanced the photoelectrochemical (PEC) performance of a pristine GCN photoanode. Polyacrylonitrile (PAN) was electrospun to form fibers that were then carbonized to form one-dimensional (1D) CNFs, which were then used to fabricate the GCN structure. The optimum GCN/CNFs hybrid structure was obtained by controlling the amount of GCN precursors (urea/thiourea). The surface morphology of the hybrid structure revealed the coating of GCN on the CNFs. Additionally, X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy, and X-ray diffraction confirmed the phases of the GCN/CNFs hybrids. PEC results showed a higher photocurrent of 3 μA for the hybrid compared with that of 1 μA for the pristine GCN. The high photocurrent for the hybrid structures indicated the formation of heterojunctions that resulted from a lower recombination rate of charge carriers. Moreover, UTh0.075 (0.075 g of urea and 0.075 g of thiourea) hybrid sample showed the highest performance of hydrogen generation with its numerical value of 437 μmol/g, compared to those of UTh0.1(0.1 g of urea and 0.1 g of thiourea) and UTh0.05 (0.05 g of urea and 0.05 g of thiourea) composite samples. This higher hydrogen production could be explained again with successful formation of heterojunctions between GCN and CNFs. Overall, we report a new approach for obtaining 1D hybrid structures, having better PEC performance than that of pristine GCN. These hybrids could potentially be used in energy-related devices.

The electronic and optical properties of carbon nitride derivatives: A first principles study

It is well known that pristine graphitic carbon nitride (g-C3N4) exhibits low carrier mobility and low visible response, thus hindering its application to some extent. The derivatization on g-C3N4 derivatives is the way to tune its band gap and thus modify its electronic and optical properties. In present work, the geometric, electronic and optical properties of g-C3N4 and its derivatives (C6N7)n, [C6N7(C2)1.5]n, [C6N7(C4)1.5]n, [C6N7(C3N3)]n and [C6N7(N2)1.5]n were investigated by first-principles calculations. Compared to g-C3N4, the band gaps of five carbon nitride derivatives decrease. The work function of carbon nitrides derivatives are larger than g-C3N4. The strong absorption peak of g-C3N4 is around 350 nm, while the absorption spectra extents of five carbon nitride derivatives increase, the absorption wavelength ranges extend to the visible region. For five carbon nitride derivatives, the HOMOs and LUMOs show significant delocalization compared to that of g-C3N4, which are helpful to facilitate the enhancement of carrier mobility. It infers that the photogenerated e−/h+ pairs in five carbon nitride derivatives effectively separate, which can improve the photocatalytic efficiency. The present results are expected to give a guide for design and application of the g-C3N4 derivatives in further experimental and theoretical investigations.

Rapid high-temperature treatment on graphitic carbon nitride for excellent photocatalytic H2-evolution performance

Graphitic carbon nitride is considered as one of the most potential photocatalysts for utilizing solar energy. Herein, an efficient route of rapid high-temperature treatment (at 800 °C in air for 0–20 min) was developed to modify pristine graphitic carbon nitride. It was found that, after rapid high-temperature treatment, the obtained photocatalysts showed greatly enhanced photocatalytic H2-evolution activities (around 25.5 times) under visible-light irradiation attributed to the following reasons. More active sites for photocatalytic reaction were provided by the significant increase of surface areas. The generation, separation and transfer of photo-generated carriers were effectively promoted by the quantum confinement effect of thinner graphitic carbon nitride sheets and by the generated nitrogen vacancies in CN heterocyclic units. Moreover, stronger driving force for photocatalytic H2-evolution reaction was obtained to enhance the ability of water reduction reaction.

Facile two-step treatment of carbon nitride for preparation of highly efficient visible-light photocatalyst

Tailoring the microstructure of graphitic carbon nitride (GCN) is an effective way to improve its photocatalytic activities. Protonation of GCN has been extensively studied, while few researches focused on modifying GCN with alkali. Herein, we report a facile two-step hydrothermal-calcination method to modify GCN. After hydrothermal treatment with ammonia solution and subsequently calcination under argon atmosphere, the as-synthesized GCN-xh-Ar (x stands for hydrothermal time) samples exhibit drastically improved photocatalytic activities towards photocatalytic degradation of RhB and evolution of hydrogen. The GCN-2h-Ar sample shows the highest hydrogen evolution rate of 736 μmol h−1 g−1, which is nearly 11 times of pristine GCN (67 μmol h−1 g−1). The improved activities of samples can be attributed to the higher surface area, larger pore volume and enhanced visible light absorption. This approach sheds light on the synthesis of highly efficient photocatalysts.

Cross-Linked Graphitic Carbon Nitride with Photonic Crystal Structure for Efficient Visible-Light-Driven Photocatalysis.

Highly cross-linked graphitic carbon nitride has been prepared by a thermal copolymerization of dicyanodiamide with tetramethylammonium salts. The cross-linking can be evidenced by (i) increased C/N ratio without new carbon species, (ii) decreased specific surface area, and (iii) Tyndall effect after dissolution in concentrated sulfuric acid. The cross-linked graphitic carbon nitride with photonic crystal structure has highly efficient photocatalytic activity for water splitting under visible light due to the synergistic enhancement by the greatly suppressed photoluminescence, red-shifted absorption edges, strong inner reflections, and effective PCs stop band overlaps. It exhibits an enhanced photodegradation kinetic of methyl orange and a high visible-light-driven hydrogen-evolution rate of 166.9 μmol h-1 (25 times higher than that of the pristine graphitic carbon nitride counterpart). This work presents a facile method for designing and developing high-performance graphitic carbon nitride photocatalysts, providing a broad range of application prospects in the fields of electronics and energy conversion.

Water soluble graphitic carbon nitride with tunable fluorescence for boosting broad-response photocatalysis

A one-step salt melt assisted thermal polymerization for the synthesis of nitrogen-rich water soluble graphitic carbon nitride (WS-GCN) is presented. The as-prepared WS-GCN shows strong tunable excitation-emission pathways that have multi-colored photoluminescence from blue to green with extremely high absolute quantum yields (AQY) of 22%, which is almost 6 fold of pristine graphitic carbon nitride (GCN) nanosheets. It also shows significant red-shift of response edge benefiting from the narrowed band energy, due to the feature of N-defected surface and N-rich inner of WS-GCN. After being decorated onto TiO2, the WS-GCN endows a great enhancement of NO-photodegradation under visible light irradiation, 5.5-fold increase to comparison with what the pristine GCN does. Further, the starting edge of photocatalysis is enlarged to about 650 nm for TiO2/WS-GCN from about 450 nm for TiO2/GCN. This is ascribed to both enhanced light-response range and efficiently restraining the radiative transition probability of WS-GCN owing to the migration of photoinduced electrons from the CB of WS-GCN to TiO2. This multi-functional material may provide a way for the development of bio-imaging, file secrecy and broad absorption photocatalysts.

Distinctive defects engineering in graphitic carbon nitride for greatly extended visible light photocatalytic hydrogen evolution

Defects modulation usually has great influence on the electronic structures and activities of photocatalysts. Here, porous structured graphitic carbon nitride materials with large amount of defects are obtained through facile 5min thermal treatment in air without additional reactants. The resultant materials show remarkably extended light absorption in the visible light region. Theoretical calculations indicate that the distinctive origin of red-shifted intrinsic light absorption edge and newly occurred light absorption edge are attributed to cyano groups and nitrogen vacancies, respectively. Compared to pristine graphitic carbon nitride, the optimally modified material shows greatly enhanced photocatalytic hydrogen evolution rate by 21.5 times under λ > 440nm and the responsive wavelength is extended from 450nm to 650nm. This work is expected to provide guidance for rational design of graphitic carbon nitride and inspire similar attempts for the modification of nanomaterials.

Synthesis of Direct Z-Scheme MnWO4/g-C3N4 Photocatalyst with Enhanced Visible Light Photocatalytic Activity

In this study, direct Z-scheme MnWO4/g-C3N4 photocatalyst was fabricated via facile hydrothermal method. Compared with pristine graphitic carbon nitride (g-C3N4) and manganous tungstate (MnWO4), the prepared MnWO4/g-C3N4 photocatalyst showed obviously enhanced photocatalytic activity for Rhodamine B (RhB) degradation under visible light irradiation. The MnWO4/g-C3N4 photocatalyst prepared with 10% MnWO4 (MC10%) showed the highest photocatalytic activity among all samples, which is about 2.3 and 12.7 times than that of pristine g-C3N4 and MnWO4, respectively. This enhancement is due to the strong light absorption and efficient electron–hole separation of direct Z-scheme MnWO4/g-C3N4 photocatalyst. Electron spin resonance (ESR) experiments and active species trapping experiments revealed that [Formula: see text]OH and [Formula: see text] are the main active species in the photocatalytic process. This work may be beneficial for designing MnWO4-based Z-scheme photocatalyst for application in environmental remediation.

Significantly enhanced photocatalytic hydrogen generation over graphitic carbon nitride with carefully modified intralayer structures

Graphitic carbon nitride has been widely investigated as promising photocatalyst for solar-driven photocatalytic hydrogen production. In this study, the intralayer properties of graphitic carbon nitride was modified by a facile hydrothermal treatment with appropriate amount of ammonium hydroxide to promote its photocatalytic H2 production performance. Transition from intralayer modification to bulk structural transformation occurred once the concentration of NH3·H2O is over certain concentration. The destruction of heptazine units resulted in the partial oxidation of C atoms in g-C3N4 during the hydrothermal treatment process and the introduction of OH and CO can effectively enhance the interaction between g-C3N4 with reactants. Band structure measurement revealed that the thus obtained g-C3N4 possessed elevated conduction band bottom indicative of improved reduction capacities. The significantly enhanced redox ability on the surface reaction sites overcame the deceased light absorption and higher charge recombination efficiency of charge carriers, and finally induced a significantly improved photocatalytic H2 evolution (707.58μmol·h−1·g−1). The highest visible light photocatalytic H2 evolution rate was 7.5 times higher than that of pristine graphitic carbon nitride (94.46μmol·h−1·g−1), with an apparent quantum yield of 5.18% at 420nm. This study demonstrates a facile and effective strategy to tune the electronic and photocatalytic properties of carbon nitride for improved solar energy conversion efficiency which are expected to be extended to the modifications of other photocatalytic materials.

Enhanced visible-light photocatalytic nitrogen fixation over semicrystalline graphitic carbon nitride: Oxygen and sulfur co-doping for crystal and electronic structure modulation

Oxygen and sulfur co-doped semicrystalline graphitic carbon nitride (HGCNOS) was synthesized by a one-pot hydrothermal method and applied in visible-light photocatalytic nitrogen fixation. Remarkably, HGCNOS exhibited a higher photocatalytic activity than pristine graphitic carbon nitride (GCN). Oxygen doping caused semicrystalline structure, making exciton dissociated at the order-disorder interfaces of HGCNOS and releasing more electrons and holes. Furthermore, the conduction band position of HGCNOS was elevated by sulfur doping, promoting the reduction ability of HGCNOS. Thus, the special electronic and physicochemical structure of HGCNOS contributed to the enhanced photocatalytic activity, facilitating its application on nitrogen photofixation.

Graphitic carbon nitride/graphene oxide/reduced graphene oxide nanocomposites for photoluminescence and photocatalysis

The study presents a modification of graphitic carbon nitride (g-C3N4) with graphene oxide (GO) and reduced graphene oxide (rGO) and investigation of photoluminescent and photocatalytic properties. The influence of GO and rGO lateral sizes used for the modification was investigated. The nanomaterials were characterized with atomic force microscopy (AFM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), diffuse reflectance UV–vis spectroscopy (DR-UV-vis) and photoluminescence spectroscopy (PL). PL revealed that pristine graphitic carbon nitride and its nanocomposites with GO and rGO emitted up-converted photoluminescence (UCPL) which could contribute to the improvement of photocatalytic activity of the materials. The photoactivity was evaluated in a process of phenol decomposition under visible light.A hybrid composed of rGO nanoparticles (rGONPs, 4–135nm) exhibited the highest photoactivity compared to rGO with size of 150nm–7.2μm and graphene oxide with the corresponding sizes. The possible reason of the superior photocatalytic activity is the most enhanced UCPL of rGONPs, contributing to the emission of light with higher energy than the incident light, resulting in improved photogeneration of electron-hole pairs.

Oxygen functional groups in graphitic carbon nitride for enhanced photocatalysis

Metal-free semiconductors offer a new opportunity for environmental photocatalysis toward a potential breakthrough in high photo efficiency with complete prevention of metal leaching. In this study, graphitic carbon nitride (GCN) modified by oxygen functional groups was synthesized by a hydrothermal treatment of pristine GCN at different temperatures with H2O2. Insights into the emerging characteristics of the modified GCN in photocatalysis were obtained by determining the optical properties, band structure, electrochemical activity and pollutant degradation efficiency. It was found that the introduction of GCN with oxygen functional groups can enhance light absorption and accelerate electron transfer so as to improve the photocatalytic reaction efficiency. The photoinduced reactive radicals and the associated photodegradation were investigated by in situ electron paramagnetic resonance (EPR). The reactive radicals, O2− and OH, were responsible for organic degradation.

Dispersing molecular cobalt in graphitic carbon nitride frameworks for photocatalytic water oxidation.

The development of water oxidation catalysts (WOCs) to cooperate with light-energy transducers for solar energy conversion by water splitting and CO2 fixation is a demanding challenge. The key measure is to develop efficient and sustainable WOCs that can support a sustainable photocatalyst to reduce over-potentials and thus to enhance reaction rate of water oxidation reaction. Cobalt has been indentified as active component of WOCs for photo/electrochemical water oxidation, and its performance relies strongly on the contact and adhesion of the cobalt species with photoactive substrates. Here, cobalt is homogeneously engineered into the framework of pristine graphitic carbon nitride (g-C3 N4 ) via chemical interaction, establishing surface junctions on the polymeric photocatalyst for the water oxidation reaction. This modification promotes the surface kinetics of oxygen evolution reaction by the g-C3 N4 -based photocatalytic system made of inexpensive substances, and further optimizations in the optical and textural structure of Co-g-C3 N4 is envisaged by considering ample choice of modification schemes for carbon nitride materials.

Bandgap engineering and mechanism study of nonmetal and metal ion codoped carbon nitride: C+Fe as an example.

Bandgap narrowing and a more positive valence band (VB) potential are generally considered to be effective methods for improving visible-light-driven photocatalysts because of the significant enhancement of visible-light absorption and oxidation ability. Herein, an approach is reported for the synthesis of a novel visible-light-driven high performance polymer photocatalyst based on band structure control and nonmetal and metal ion codoping, that is, C and Fe-codoped as a model, by a simple thermal conversion method. The results indicate that compared to pristine graphitic carbon nitride (g-C3 N4 ), C+Fe-codoped g-C3 N4 shows a narrower bandgap and remarkable positively shifted VB; as a result the light-absorption range was expanded and the oxidation capability was increased. Experimental results show that the catalytic efficiency of C+Fe-codoped g-C3 N4 for photodegradation of rhodamine B (RhB) increased 14 times, compared with pristine g-C3 N4 under visible-light absorption at λ>420 nm. The synergistic enhancement in C+Fe-codoped g-C3 N4 photocatalyst could be attributed to the following features: 1) C+Fe-codoping of g-C3 N4 tuned the bandgap and improved visible-light absorption; 2) the porous lamellar structure and decreased particle size could provide a high surface area and greatly improve photogenerated charge separation and electron transfer; and 3) both increased electrical conductivity and a more positive VB ensured the superior electron-transport property and high oxidation capability. The results imply that a high-performance photocatalyst can be obtained by combining bandgap control and doping modification; this may provide a basic concept for the rational design of high performance polymer photocatalysts with reasonable electronic structures for unique photochemical reaction.