Structure Of Graphitic Carbon Nitride Research Articles

Ionic Salt-Mediated Tuning of the Morphology and Band Structure of Graphitic Carbon Nitride for NO Removal under Visible Light

Graphitic carbon nitride (g-C3N4, CN) as a popular photocatalyst has been investigated for the removal of air pollutants, but its photocatalytic efficiency and selectivity for ionic species (Si) can be enhanced through chemical or microstructural modification. The combinatorial study of six salts, Na+, K+, and NH4+ (per)sulfate, as mediators in the melamine precursor for the CN synthesis indicates that the thermal decomposition behaviors of the salts affect the CN polymerization process and the functional groups on CN samples, though all of them are found to increase the surface area. The mediation with Na+ and K+ (per)sulfate increases the dangling −C≡N bonds and surface moieties at a higher oxidation state. The sodium and ammonia salts caused an upward shift of the conduction band energy for the final photocatalyst, while K2SO4 and K2S2O8 induced a downward shift of the conduction band. The NO removal efficiency of CN with sodium persulfate and sulfate reached 80% and near 100%, respectively, much higher than that of the pristine CN (60%). Moreover, the samples with persulfates showed higher Si than those with sulfate salts. The high surface area and the keen hierarchical pores down to 2 nm in the salt-mediated CN samples increased the residence time of the gaseous species and the resultant NO removal efficiency, and Si. NO2 can still be produced in the dark for 15 min after the photocatalysis, indicating that there are long-lived species related to H2O2 produced in the prior illumination. The systematic investigation of the effects of inorganic salt addition was presented on CN in terms of the microstructure, band structure, and correlated photocatalytic oxidation efficiency, pathway, and product selectivity.

Sulfuric Acid Treated g-CN as a Precursor to Generate High-Efficient g-CN for Hydrogen Evolution from Water under Visible Light Irradiation

Modifying the physical, chemical structures of graphitic carbon nitride (g-CN) to improve its optoelectronic properties is the most efficient way to meet a high photoactivity for clean and sustainable energy production. Herein, a higher monomeric precursor for synthesizing improved micro-and electronic structure possessing g-CN was prepared by high-concentrated sulfuric acid (SA) treatment of bulk type g-CN (BCN). Several structural analyses show that after the SA treatment of BCN, the polymeric melon-based structure is torn down to cyameluric or cyanuric acid-based material. After re-polycondensation of this material as a precursor, the resulting g-CN has more condensed microstructure, carbon and oxygen contents than BCN, indicating that C, O co-doping by corrosive acid of SA. This g-CN shows a much better visible light absorption and diminished radiative charge recombination by the charge localization effect induced by heteroatoms. As a result, this condensed C, O co-doped g-CN shows the enhanced photocatalytic hydrogen evolution rate of 4.57 µmol/h from water under the visible light (>420 nm) by almost two times higher than that of BCN (2.37 µmol/h). This study highlights the enhanced photocatalytic water splitting performance as well as the provision of the higher monomeric precursor for improved g-CN.

Copper-oxygen synergistic electronic reconstruction on g-C3N4 for efficient non-radical catalysis for peroxydisulfate and peroxymonosulfate

Due to the selectivity and environmental tolerance of non-radical reaction reactions, they have emerged as a promising way to treat special water bodies. Herein, we proposed a new non-radical reaction system that used a Cu and O co-doped g-C3N4 catalyst (CuO-CN) activated by peroxydisulfate (PDS) or peroxymonosulfate (PMS). In CuO-CN, Cu and O atoms were introduced into the structure of graphitic carbon nitride (g-C3N4) in an innovative configuration, resulting in a differentiated charge distribution around the Cu and O centres. The PDS/CuO-CN and PMS/CuO-CN systems could selectively degrade organic pollutants (e.g., bisphenol A, BPA) over a wide pH range (3–9), and the maximum BPA removal could reach 99%. For the PDS/CuO-CN system, the mechanism was hypothesized to involve the effective removal of BPA via electron transfer, and the PMS/CuO-CN system exploited the synergistic effect of singlet oxygen (1O2) and electron transfer. This study describes a novel process for effective PMS or PDS activation by CuO-CN to efficiently degrade organic pollutants via a non-radical pathway.

3D macroporous architecture of self-assembled defect-engineered ultrathin g-C3N4 nanosheets for tetracycline degradation under LED light irradiation

Herein, the facile fabrication of a novel 3D macroporous structure of graphitic carbon nitride composed of self-assembled 2D nitrogen vacancy engineered nanosheets have been reported. The synthesis of the defect-engineered structure was achieved upon the addition of ethylene glycol during the solid-state synthesis process. The photocatalyst was characterized through a wide range of characterization techniques. The macroporous structure exhibited superior photocatalytic efficiency towards tetracycline antibiotic degradation under LED light illumination. The defect-engineered material (4EGCN) exhibited a 3.3-fold superior degradation rate coefficient of 0.010 min−1, compared to the degradation rate coefficient (0.003 min−1) of the bulk counterpart. This superior photocatalytic performance could be accredited to the extended visible light extraction efficiency, increased suppression efficacy of charge carriers, and porous structure. Moreover, the photocatalyst showed excellent stability even after five cycles of photocatalytic tetracycline degradation. Further, a probable degradation mechanism has been proposed according to the observations of the radical scavenging experiments.

Modification of Graphitic Carbon Nitride with Hydrogen Peroxide.

Graphitic carbon nitride (GCN) was synthetized by heating melamine and then it was thermally exfoliated for 1–3 h in air. Both bulk and exfoliated GCN nanomaterials were treated in the 10–30% aqueous solutions of H2O2 for us to study their modification. The light absorption properties were observed by the reddish color and the red-shifts of their UV-Vis spectra. The content of oxygen increased and hydrogen peroxide was supposed to partially oxidize C-OH groups to C=O ones and to form C-O-C groups instead of edge C-NH-C ones. The GCN structure changes were not observed. However, a surface modification of the GCN materials was recognized by their changed photocatalytic activities tested by means of Acid Orange 7 (AO7) and Rhodamines B (RhB), zeta-potentials, and neutralization titration curves.

Influence of High Temperature Synthesis on the Structure of Graphitic Carbon Nitride and Its Hydrogen Generation Ability.

Graphitic carbon nitride (g-C3N4) was obtained by thermal polymerization of dicyandiamide, thiourea or melamine at high temperatures (550 and 600 °C), using different heating rates (2 or 10 °C min−1) and synthesis times (0 or 4 h). The effects of the synthesis conditions and type of the precursor on the efficiency of g-C3N4 were studied. The most efficient was the synthesis from dicyandiamide, 53%, while the efficiency in the process of synthesis from melamine and thiourea were much smaller, 26% and 11%, respectively. On the basis of the results provided by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), infrared spectroscopy (FTIR), ultraviolet–visible spectroscopy (UV–vis), thermogravimetric analysis (TGA), elemental analysis (EA), the best precursor and the optimum conditions of synthesis of g-C3N4 were identified to get the product of the most stable structure, the highest degree of ordering and condensation of structure and finally the highest photocatalytic activity. It was found that as the proton concentration decreased and the degree of condensation increased, the hydrogen yields during the photocatalytic decomposition of water–methanol solution were significantly enhanced. The generation of hydrogen was 1200 µmol g−1 and the selectivity towards hydrogen of more than 98%.

Tuning the electronic band structure of graphitic carbon nitride by breaking intramolecular bonds: A simple and effective approach for enhanced photocatalytic hydrogen production

In this work, the intramolecular structure of a pristine graphitic carbon nitride (GCN) was modified to investigate how it would affect the photocatalytic activity towards hydrogen evolution reaction (HER) under simulated solar irradiation. The intramolecular bonding of GCN was broken by subjecting it to a simple thermal treatment under a controlled environment. Both experimental and computational studies revealed that removal of inter-heptazine CN(H)C groups from the pristine structure induced an amorphous phase and additional energy bands to GCN. It was also observed that a mid-gap state was formed between the conduction band (CB) of the amorphous carbon nitride (ACN) and the H+/H2 reduction potential. The presence of the mid-gap state not only served as an additional reduction site for HER, but also acted as a buffer to reduce the recombination rate of photogenerated charge carriers. The optimum ACN sample displayed an enhanced photocatalytic performance achieving a HER rate of 789 µmol/gcat, which was about 2-fold higher than that of pristine GCN. This could be ascribed to the synergistic effect of improved light absorption, increased surface area, suppressed charge recombination and increased charge carrier densities for the ACN samples to drive the overall reduction–oxidation process.

Fabrication of spherical-graphitic carbon nitride via hydrothermal method for enhanced photo-degradation ability towards antibiotic

Graphitic-carbon nitride with spherical morphology (SCN) was synthesized via very simple one step solvothermal method. Prepared photocatalyst was carefully analyzed by different techniques such as FTIR-infrared spectra, XPS, X-ray diffraction, UV–Vis spectroscopy, and transmission electron microscopy. These analysis results indicated that the synthesized material possess the structure of graphitic carbon nitride with a novel morphology. The degradation ability of spherical g-C3N4 was checked by degrading an antibiotic (i.e. Tetracycline hydrochloride, TC-HCl). SCN shows the degradation rate (0.0357 min−1) much higher as compare to traditionally prepared graphitic carbon nitride (0.0047 min−1). Moreover, recycling tests showed that the SCN possess good stability and reusability.

Construction of sponge-like graphitic carbon nitride and silver oxide nanocomposite probe for highly sensitive and selective turn-off fluorometric detection of hydrogen peroxide

In the present work, spongy graphitic carbon nitride (GCN) and silver oxide nanocomposites were prepared through a facile hydrothermal method at 160 °C for 4 h using GCN, silver nitrate, and dipotassium hydrogen phosphate as the starting materials. The prepared samples were characterized by scanning electron microscopy, energy dispersive X-ray spectrometry, Brunauer-Emmett-Teller method, X-ray diffraction, X-ray photoelectron spectroscopy (XPS), UV–Visible diffuse reflectance spectroscopy, Fourier transform infrared spectroscopy, Raman, and photoluminescence techniques. SEM images showed Ag2O loaded GCN nanocomposite has a sponge-like structure due to the interconnecting of the enormous layer on the planar structure of GCN. XRD of samples showed the diffraction planes due to the hexagonal structure of carbon nitride with a decrease in intensity of peaks with increasing silver oxide (Ag2O) in the nanocomposite. Further, the addition of silver oxide improved the electrical properties of the nanocomposite by reducing the recombination of electron and hole pairs as shown by photoluminescence spectra. XPS spectra have confirmed the oxidation state of Ag as well as the coexistence of Ag2O and GCN in the nanocomposite. BET results indicated the increase in surface area for Ag2O/GCN-4.2% nanocomposite as compared to GCN. FTIR study indicated that the graphitic structure in GCN remained intact with the loading of Ag2O. A fluorescent quenching based H2O2 sensor was constructed by simultaneous oxidation of Rhodamine B in the presence of hydrogen peroxide and nanocomposite as the catalyst. In phosphate buffer saline at room temperature, Rhodamine B displayed a strong fluorescence emission peak around 577 nm under an excitation wavelength of 554 nm. This fluorescence signal of Rhodamine B was quenched in the presence of H2O2 and nanocomposite. The catalytic fluorescence quenching was increased with the increase in H2O2 concentration in the reaction system. The detection conditions of the prepared sensor were optimized as a reaction temperature of 25 °C, Rhodamine B concentration as 66 ng mL−1 and nanocomposite concentration as 56 µg mL−1. The catalytic fluorescence quenching response of the biosensor exhibited a linear range and limit of detection for H2O2 as 30–300 nM and 22 nM respectively.

Ag decorated G‐C3N4/black titanium oxides composite for the destruction of environmental pollutant under solar irradiation

Research on the synthesis of advanced composite materials is an important area of research on the visible‐active photodegradation of environmental pollutants. Novel hybridized structures of graphitic carbon nitride (g‐C3N4) are synthesized by a facile one‐step in‐situ method. Black titanium oxides, TiO, Ti2O 3, a mixture of TiO + Ti2O3, and a reference material, white TiO2 P25, are used for the synthesis of the hybridized g‐C3N4 and further decorated with Ag nanoparticles. Physicochemical and optical properties of the obtained materials are characterized by various techniques. XRD patterns revealed that the structural regularity of g‐C3N4 and titanium oxide(s) is maintained in the composites. However, the main diffraction peak for g‐C3N4 is prominently shifted to a lower 2‐theta along with a decrease in the intensity, indicating the formation of g‐C3N4 nanosheet heterojunction. Vibrational spectra of the composites showed signature IR peaks for g‐C3N4 and demonstrated an enhancement in the vibrations after modifications, and further supported the XRD results. A distinct red‐shift in the optical spectra of g‐C3N4 upon modifications with black titanium oxide(s) demonstrated an enhanced absorbance of visible light, yet on the contrast white TiO2 P25 showed a blue‐shift. Under the experimental conditions of simulated solar light, pristine g‐C3N4, and its composites g‐C3N4/TiO and g‐C3N4/Ti2O3 showed moderate activity for the photodegradation of an organic pollutant, rhodamine B. Interestingly, a synergistic enhancement in the photoactivity is demonstrated by the g‐C3N4 modified with a mixture of black titanium oxides (g‐C3N4/TiO + Ti2O3). On the other hand, the composite synthesized by a post‐synthesis method is found less effective in photodegradation reaction.

Effects of polymeric- and electronic-structure of graphitic carbon nitride (g-C3N4) on oxidative photocatalysis

Characteristics of polymeric- and electronic structure of graphitic carbon nitride (g-C3N4) responsible for the photocatalytic activity were analyzed with oxidation reaction of nitrogen monoxide (NO) and various kinds of spectroscopies. The photocatalytic activity per surface area was increased with increasing the preparation temperature up to 520 °C, and was decreased above 520 °C while the visible light absorbance and the relative surface area was increased with the preparation temperature up to 650 °C. Laser desorption/ionization mass spectrometry (LDI-MS) showed that the content of cyano group connected with tri-s-triazine unit in g-C3N4 increases with increasing the preparation temperature and that g-C3N4 with higher activity per surface area contained smaller amount of cyano group. Additionally, the degree of polymerization and the planarity of carbon-nitride layer possibly contributed for formation of mid-gap levels that increases the number of unpaired electron. The formation of the mid-gap level was not effective for the photocatalytic activity, but rather deactivated g-C3N4.

Localized π-conjugated structure and EPR investigation of g-C3N4 photocatalyst

The π-conjugated structure of graphitic carbon nitride (g-C3N4) is particularly vital to many photocatalytic reactions. Herein, the hybrid structure of tri-s-triazine unit in g-C3N4 framework is chemically analyzed and expounded according to the hybrid orbital theory. The localized π-conjugated structure of g-C3N4 is also monitored by the electron paramagnetic resonance (EPR) or electron spin resonance (ESR) technique. The experimental results indicate that this π-conjugated structure is attributed to the orbital overlapping of the hybrid carbon and nitride atoms in their 2pz orbits. Unlike graphene with the nonlocalized π-conjugated structure, this orbital overlapping in the whole two-dimensional plane of g-C3N4 is separated by the electrons pairs in 2pz orbits of the bridging nitride atoms, leading to the localized π-conjugated structure. Therefore, the g-C3N4 exhibits the typical features of a semiconductor with band gap and visible-light response. Meanwhile, the EPR or ESR technique can be acted as the ideal tool to indirectly evaluate the yield of photoelectrons by detecting the superoxide radicals (O2−) in g-C3N4-based photocatalytic reactions.

Insight into the kinetics and mechanism of visible-light photocatalytic degradation of dyes onto the P doped mesoporous graphitic carbon nitride

In this study, Phosphorus-doped mesoporous graphitic carbon nitride (P-mpg-C3N4) was successfully prepared by a facile simple thermal method. Microstructure and chemical properties of catalyst were studied by a series of characterization methods, the results showed that the P atoms were successfully doped on the structure of graphitic carbon nitride and uniformly distributed on the whole mesoporous nanostructure. And specific surface area of P-mpg-C3N4 can reach 198.3 m2/g, which is almost 10.7 times more than that of g-C3N4 (18.6 m2/g), pore diameter of P-mpg-C3N4 is mainly distributed in 2–20 nm and the average pore size is 11.3 nm. Photocatalytic degradation performance of the catalyst for brilliant ponceau 5R (BP-5R) in solution were studied under dark and visible light irradiation. The photocatalytic activity of P-mpg-C3N4 is about 31.1 times higher compared with that of g-C3N4, and about 3.9 times and 2.4 times than that of mpg-C3N4 and P-g-C3N4, respectively. Mechanism of its enhanced photocatalytic performance was mainly attributed to the synergistic effect of P-doping and fragmented mesoporous structure, on the one hand, which can extend visible light absorption range. On the other hand, it's can also promote the separation of photo-generated electron-hole pairs and accelerate the transfer of photo-generated electrons.

Atomic palladium on graphitic carbon nitride as a hydrogen evolution catalyst under visible light irradiation

Developing single-atom catalysts is extremely attractive for maximizing atomic efficiency and activity. However, the properties a nd roles of atomic catalysts in catalyzing water splitting reactions remain unclear. Here we report atomic palladium on graphitic carbon nitride with low palladium loading (0.1 wt%). The hydrogen evolution of this graphitic carbon nitride increases from 1.4 to 728 µmol g−1 h−1 under visible light irradiation, which is also 10 times higher than that of palladium nanoparticles (3 wt%) counterpart. The electronic structure of graphitic carbon nitride is modified after isolated palladium is introduced, which results in efficient charge separation, appropriate sites for adsorption for hydrogen, as well as accumulation of photoinduced electrons. Our results suggest that the pyridine nitrogen in the adjacent cavity to the palladium rather than the isolated palladium site is the active site which differs to that of the palladium nanoparticle counterpart.

The charge regulation of electronic structure and optical properties of graphitic carbon nitride under strain.

The electronic structure of the graphitic carbon nitride (g-C6N6) under strain was obtained using the hybrid density functional HSE06 with a larger computational workload. The g-C6N6 could withstand 12% of the applied tensile strain. The electronic structure of g-C6N6 could be changed effectively under the tensile force. The band gap changed from direct to indirect under the strain and could be tuned in the range of 3.16 eV to 3.75 eV. At approximately 4% of the applied strain, there was a transition of the valence band maximum (VBM). A wider range of light absorption could be obtained under the strain. Our results provide a prospect for the future applications of two-dimensional materials in electronic and optoelectronic devices.

Manipulation structure of carbon nitride via trace level iron with improved interfacial redox activity and charge separation for synthetic enhancing photocatalytic hydrogen evolution

Manipulation structure of graphitic carbon nitride (GCN) to tune its photophysical and interfacial redox activity is one of fundamental issues for understanding and improving its photocatalytic hydrogen evolution (PHE) activity. In this work, we report our recent progress on manipulating structure of GCN via an easy and effective approach. Trace level iron is found to have increased atomic concentration and tuned bond state of carbon in structure manipulated GCN (M-GCN) along with porosity of it, which exhibits higher interfacial redox and photophysical activity than GCN. The improved interfacial redox and photophysical activities of M-GCN are found to have synthetic enhanced consumption of photo-generated electrons and holes during photocatalysis process and reduction of their recombination, which results in promotion of its PHE activity. PHE rate for M-GCN is found to be 2286 μmol−1 g−1 h−1, which is 2.82 time of GCN.

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.

Sodium-doped carbon nitride nanotubes for efficient visible light-driven hydrogen production

Sodium-doped carbon nitride nanotubes (Na x -CNNTs) were prepared by a green and simple two-step method and applied in photocatalytic water splitting for the first time. Transmission electron microscopy (TEM) element mapping and X-ray photoelectron spectroscopy (XPS) measurements confirm that sodium was successfully introduced in the carbon nitride nanotubes (CNNTs), and the intrinsic structure of graphitic carbon nitride (g-C3N4) was also maintained in the products. Moreover, the porous structure of the CNNTs leads to relatively large specific surface areas. Photocatalytic tests indicate that the porous tubular structure and Na+ doping can synergistically enhance the hydrogen evolution rate under visible light (λ > 420 nm) irradiation in the presence of sacrificial agents, leading to a hydrogen evolution rate as high as 143 μmol·h−1 (20 mg catalyst). Moreover, other alkali metal-doped CNNTs, such as Li x -CNNTs and K x -CNNTs, were tested; both materials were found to enhance the hydrogen evolution rate, but to a lower extent compared with the Na x -CNNTs. This highlights the general applicability of the present method to prepare alkali metal-doped CNNTs; a preliminary mechanism for the photocatalytic hydrogen evolution reaction in the Na x -CNNTs is also proposed.

Improving the photocatalytic activity of graphitic carbon nitride by thermal treatment in a high-pressure hydrogen atmosphere

Extending visible light absorption range and suppressing the recombination of photogenerated charge carriers are always important topics in developing efficient solar-driven photocatalysts. In this study, the thermal treatment process at 400 °C in a high-pressure hydrogen atmosphere was applied to modify graphitic carbon nitride. Compared to the normal atmospheric hydrogen treatment process, this process has the merit of producing nitrogen deficient graphitic carbon nitride in high-yield. The optimal photocatalytic activity of modified graphitic carbon nitride was demonstrated by controlling the treatment duration in the hydrogen atmosphere. The changes in the crystal structure, microstructure and optical properties of carbon nitrides were investigated by several characterizations. The relationship between the photocatalytic activity and structures of graphitic carbon nitride was preliminarily established. The results obtained in this study could provide some new ways of improving the activity of graphitic carbon nitride based photocatalyst.

Modified graphitic carbon nitride prepared via a copolymerization route for superior photocatalytic activity

Molecular doping strategy has been applied for regulating the electronic band structure of graphitic carbon nitride (g-C3N4). By incorporating the electron-withdrawing impurity group of N,N′,N″-s-triazine-2,4,6-triyl-tri-p-aminobenzoic acid (H3TATAB) into the network of g-C3N4, a series of H3TATAB-doped g-C3N4 (TABCN) photocatalysts were synthesized via thermal copolymerization at 550 °C for 4 h. H3TATAB doping shifts the VB of TABCN from 1.78 eV (pristine g-C3N4) to a low position (2.07 eV for TCBCN0.8), which enhances the oxidation ability of the VB hole for producing hydroxyl radicals in aqueous solution, and, therefore, the UV photocatalytic activity of TABCN0.8 is estimated to be about twice as high as that of g-C3N4 for the degradation of AO7. Further, doping the H3TATAB impurity group into the g-C3N4 network induces plenty of defect levels below the CB of g-C3N4, which gives a significant absorbance enhancement in the visible region. As a result, the visible light photocatalytic activity of TABCN0.8 is 7.5 times higher than that of g-C3N4 for the degradation of AO7. This work may provide guidance on designing an efficient g-C3N4 polymer photocatalyst with desirable electronic structure.