CN Materials Research Articles

Condensation of Supramolecular Assemblies at Low Temperatures as a Tool for the Preparation of Photoactive C3N3O Materials

AbstractWe report the synthesis of new photoactive carbon nitride polymers with high oxygen content (CNO) at low calcination temperatures (300–400 °C). To do so, we designed new carbon‐ and oxygen‐rich supramolecular assemblies as reactants. The starting monomers’ composition allows the formation of photoactive CNO semiconductors with extended absorption and ordered morphologies. Interestingly, at these reaction temperatures, the chemical structure of the starting monomers is partially retained, opening a path for the transfer of properties from the molecular level to the final CN materials after condensation. The new CNO polymers exhibit good photoactivity for the hydrogen evolution reaction, CO2 reduction and rhodamine B dye degradation.

One-step preparation and characterization of nanosized Sn–CN materials

Sn-based N-doped carbon composites (Sn–CN) were simply fabricated via a one-pot thermal reaction. This experiment uses SnCl4·5H2O, resorcinol, formaldehyde and melamine as raw material. In order to improving the theoretical capacitance and cycle performance, the tin-carbon composite material can be modified to obtain an excellent electrochemical properties. Meanwhile, the introduction of an appropriate amount of nitrogen can increase the electrical conductivity of the material. This article mainly analyzes the properties of the material from X-ray diffraction, scanning electron microscopy and cyclic voltammetry curve.

Carbon Nitride Materials for Water Splitting Photoelectrochemical Cells.

Graphitic carbon nitride materials (CNs) have emerged as suitable photocatalysts and heterogeneous catalysts for various reactions thanks to their tunable band gap, suitable energy-band position, high stability under harsh chemical conditions, and low cost. However, the utilization of CN in photoelectrochemical (PEC) and photoelectronic devices is still at an early stage owing to the difficulties in depositing high-quality and homogenous CN layer on substrates, its wide band gap, poor charge-separation efficiency, and low electronic conductivity. In this Minireview, we discuss the synthetic pathways for the preparation of various structures of CN on substrates and their underlying photophysical properties and current photoelectrochemical performance. The main challenges for CN incorporation into PEC cell are described, together with possible routes to overcome the standing limitations toward the integration of CN materials in PEC and other photoelectronic devices.

Construction of novel Sr0.4H1.2Nb2O6·H2O/g-C3N4 heterojunction with enhanced visible light photocatalytic activity for hydrogen evolution

Construction of heterojunction is an effective strategy to conquer the severe charge carrier recombination limitation of single component g-C3N4 photocatalyst. In the present work, novel heterojunctions composed of g-C3N4 nanosheets and Sr0.4H1.2Nb2O6·H2O nanooctahedrons were constructed via a simple hydrothermal method. The as-prepared Sr0.4H1.2Nb2O6·H2O/g-C3N4 (HSN/CN) heterojunction showed high photocatalytic activity in the water splitting reactions. Specially, it is found that the developed 20 wt%-HSN/CN heterojunction shows high water splitting activity with H2 evolution rate up to 469.4 μmol g−1, which was much higher than that of bare CN. This enhanced photocatalytic activity for H2 evolution can be mainly attributed to the matched energy level and heterojunction structure which could improve the photo-generated charge carriers separation and transfer. This work implies that construction of heterojunctions with a wide band gap semiconductor is a feasible strategy for enhancement of photocatalytic activity of CN materials.

Carbon and Nitrogen Based Nanosheets as Fluorescent Probes with Tunable Emission.

2D carbon and nitrogen based semiconductors (CN) have attracted widespread attention for their possible use as low-cost and environmentally friendly materials for various applications. However, their limited solution-dispersibility and the difficulty in preparing exfoliated sheets with tunable photophysical properties restrain their exploitation in imaging-related applications. Here, the synthesis of carbon and nitrogen organic scaffolds with highly tunable optical properties, excellent dispersion in water and DMSO, and good bioimaging properties is reported. Tailored photophysical and chemical properties are acquired by the synthesis of new starting monomers containing different substituent chemical groups with varying electronic properties. Upon monomer condensation at moderate temperature, 350 °C, the starting chemical groups are fully preserved in the final CN. The low condensation temperature and the effective molecular-level modification of the CN scaffold lead to well-dispersed photoluminescent CN thin sheets with a wide range of emission wavelengths. The good bioimaging properties and the tunable fluorescence properties are exemplified by in situ visualization of giant unilamellar vesicles in a buffered aqueous solution as a model system. This approach opens the possibility for the design of tailor-made CN materials with tunable photophysical and chemical properties toward their exploitation in various fields, such as photocatalysis, bioimaging, and sensing.

Selective hydrogenation of phenol to cyclohexanone over Pd@CN (N-doped porous carbon): Role of catalyst reduction method

Selective phenol hydrogenation is a green and sustainable technology to produce cyclohexanone. The work focused on investigating the role of catalyst reduction method in the liquid-phase phenol hydrogenation to cyclohexanone over Pd@CN (N-doped porous carbon). A series of reduction methods including flowing hydrogen reduction, in-situ reaction reduction and liquid-phase reduction were designed and performed. The results highlighted that the reduction method significantly affected the catalytic performance of Pd@CN in the liquid-phase hydrogenation of phenol to cyclohexanone, and the liquid-phase reduction with the addition of appropriate amount of phenol was highly efficient to improve the catalytic activity of Pd@CN. The influence mechanism was explored by a series of characterizations. The results of TEM, XPS and CO chemisorption confirmed that the reduction method mainly affected the size, surface composition and dispersion of Pd in the CN material. The addition of phenol during the liquid-phase reduction could inhibit the aggregation of Pd NPs and promote the reduction of Pd (2+), and then improved the catalytic activity of Pd@CN. The work would aid the development of high-performance Pd@CN catalysts for selective phenol hydrogenation.

Enhanced photocatalytic hydrogen production of restructured B/F codoped g-C3N4 via post-thermal treatment

Restructured B/F codoped carbon nitride (r-CN-B/F) with enhanced photocatalytic H2 evolution was prepared by post-thermal treatment of B/F codoped carbon nitride (CN-B/F), which is obtained from direct condensation doping using ionic liquid as doping dopant. Characterization results show that B and F doped in the skeleton and surface layer of CN material respectively. It is found that the post-annealing strategy makes for compact interlayer accumulation and large surface area for doped CN. In particular, decreased surface graphitic carbon, increased B doping content and optimized conjugate aromatic ring structure of catalyst after post-sintering treatment are favorable for the photo-generated charge transfer and separation, thereby, the photocatalytic H2 production of r-CN-B/F was much enhanced. This study demonstrates a facile way for optimizing the polymeric structure of heteroatom doped semiconductor materials.

Novel Two-Dimensional Liquid Chromatography–Tandem Mass Spectrometry for the Analysis of Twenty Antibiotics Residues in Dairy Products

A novel two-dimensional hydrophilic interaction chromatography/reversed phase liquid chromatography–tandem mass spectrometry system was developed to analyze seven categories of antibiotics residues in dairy products. The seven categories were β-lactams, tetracyclines, macrolides, aminoglycosides, amphenicols, quinolones, and sulphonamides, and involved 20 antibiotics. Samples of milk powder or milk samples were extracted by mixed matrix solid phase dispersion using C18 and CN material. The analytes were eluted by acetonitrile and water, and were dried with rotary evaporation. Finally, the residues were dissolved with mobile phase and then analyzed. Under sample pretreatment conditions, chromatographic and mass spectrometric parameters were optimized. Twenty analytes showed good linearities with correlation coefficients of 0.9945 ~ 0.9998. The limits of detection and quantification for milk powder and milk samples were 0.10 ~ 2.40 and 0.33 ~ 7.92 μg/kg, respectively. The proposed method has been applied to the determination of antibiotics residues in milk powder and milk samples.

Pd nanoparticles supported on N-doped porous carbons derived from ZIF-67: Enhanced catalytic performance in phenol hydrogenation

Abstract We reported a Pd@CN catalyst made of Pd nanoparticles supported on a N-doped carbon derived from ZIF-67. The CN materials with micro/mesoporous structures were well developed for the loading of Pd nanoparticles and improving the catalytic performances. With the increase of calcination temperature of ZIF-67, the BET surface of CN materials initially increased and then reduced. Moreover, the higher calcination temperature would lead to lower nitrogen content in CN materials, causing lower selectivity of phenol hydrogenation to cyclohexanone. Our Pd@CN 600 catalyst achieved a conversion of ca. 95% and a selectivity of ca. 95%, and exhibited excellent stability during recycling experiments.

Facile preparation of SBA-15-supported carbon nitride materials for high-performance base catalysis

SBA-15-supported carbon nitride materials were prepared using dicyandiamide as a precursor through a facile impregnation method. The synthesized CND/SBA-15 materials were characterized by various techniques, including N2 adsorption–desorption, small angle X-ray scattering, transmission electron microscopy, Fourier transform infrared (FT-IR), CO2 temperature-programmed desorption, etc. The results indicated that CND/SBA-15 retained the well-ordered mesoporous structure of SBA-15. The introduction of CN materials enhanced the overall basic quantity of SBA-15 and the basic sites were owing to the nitrogen-containing sites of CN materials, as revealed by FT-IR results. As solid base catalysts, CND/SBA-15 materials featuring easy preparation as well as high catalytic performances demonstrated their potential application in both Knoevenagel condensation and transesterification reactions.

The Hydroxylation of Aromatics with Oxygen by Vanadium Catalysts Supported on N-doped Carbon Materials

Vanadium catalysts supported on N-doped carbon materials (CN) were prepared and the catalyst VOSiW showed high activity for the hydroxylation of various aromatics with O2. Aromatics with electron-withdrawing groups such as CN, NO2, COOH, CF3, COCH3 and aromatic halides (F, Cl and Br) were oxygenated to the corresponding phenols in considerable yields. The CN materials were not only indispensable for the reusability of the vanadium catalyst VOSiW, but also favorable to the high catalytic activity. The VOSiW catalyst is ready for the hydroxylation of aromatics with O2 as both V4+ and V5+ species are coexisted in the VOSiW catalyst.

The reactivity of mesoporous silica modified with acidic sites in the production of biodiesel

This work has studied a range of acidic catalysts which have been prepared through the modification of mesoporous silica materials. Specifically we have prepared –SO 3H modified materials (in the presence and absence of a hydrophobic component) and–COOH and –CN materials. We have characterized these catalysts using TEM, TGA, TPD of probe molecules and IR and we have analysed their activity in the esterification of octanoic acid and the transesterification of triacetin. Activity in the esterification reaction (in which all catalysts are active) is not determined by the acid site concentration of the catalyst while the activity of active catalysts in the transesterification reaction (in which only –SO 3H materials are active) appears related to acid site concentration.

Preparation and characterization of novel microporous carbon nitride with very high surface area via nanocasting technique

A novel microporous carbon nitride material (Mic-CN-1) with very high surface area and pore volume has been prepared for the first time by nanocasting technique using MCM-22 zeolite as a template through a simple polymerization reaction between ethylenediamine and carbon tetrachloride. The material has been thoroughly characterized by XRD, N 2 adsorption, TGA, HRSEM, HRTEM, XPS, FT-IR, and UV–vis spectroscopy. It has been found that the surface area of the Mic-CN-1 is much higher than that of the parent template and the mesoporous carbon nitride. We suppose that the novel CN material with microporous structure could offer great potential for the applications, such as catalytic supports, gas storage, lubricants, the size selective adsorption of small organic molecules such as amino acids and vitamins.

Stability of Small Carbon-Nitride Heterofullerenes

Carbon nitride nanostructures have been produced by the arc-discharge technique and analyzed by mass spectrometry. A series of structured peaks in the region of masses from 480 up to 600 suggests the existence of heterofullerenes C(n-x)Nx(40 < or = n < or = 50). The structure and stability of these small fullerenes were theoretically investigated by quantum chemical calculations. The obtained heats of formation indicate that C(n) molecules stabilize upon nitrogen substitution. Two C(n-x)Nx cages are quite stable, with heats of formation per atom approaching that of C60. These molecules could be the seeds of onion-like structures seen in CN materials [Phys. Rev. Lett. 87, 225503 (2001)]].

Electronic structure of hydrogenated carbon nitride films

Hydrogen-induced changes on the electronic and structural properties of amorphous carbon nitride (a-CNx:H) prepared by ion beam assisted deposition are investigated by x-ray photoelectron, ultraviolet photoelectron, infrared, and Raman spectroscopies. Two series of specimen are studied: films with a constant nitrogen content (C/N=26%) grown at 150 °C using different hydrogen partial pressures between 0% and 70% and samples deposited at different substrate temperatures (150–500 °C) with fixed H2 partial pressure of 60%. The pronounced changes of the N 1s and C 1s core level spectra on increasing hydrogen incorporation (up to 17 at. %) are interpreted as due to the formation of terminating NH and CH bonds accompanied by modifications of the local C–N bonding structure. Corresponding changes are observed in the He II valence band spectra showing a recession of the leading edge of more than 0.9 eV while the optical band gap widens from 0 to more than 1 eV. Consistent with these results, the information obtained from the infrared and Raman spectra suggests a hydrogen induced transformation of the disordered sp2/sp3 network into a polymerlike structure. With increasing substrate temperature a reversed process takes place. At 700 °C an increasing graphitization of the films is observed, but the effect of hydrogen on the structure influencing the growth kinetics is still present at this temperature. The nitrogen concentration (N/C) of about 30% indicates high thermal stability of the CN material.

Composition of SiCN crystals consisting of a predominantly carbon-nitride network

We report here on the synthesis of large crystals of Si-containing carbon nitride, consisting of a predominantly C–N network, by microwave CVD. The Si content in this material varies from crystal to crystal and also with the deposition conditions and has been observed to be as low as less than 5 at.% in some crystals, wherein the Si atoms are believed to substitute for some of the C sites only. This is the first time that such large and well-faceted crystals consisting almost entirely of carbon-nitride network have been synthesized. Moreover, there is no obvious deposition of amorphous CN material.

Growth of SiCN Crystals Consisting of a Predominantly Carbon Nitride Network

AbstractWe report here growth of thin films of Si-containing crystalline carbon-nitride by microwave plasma assisted CVD. The chemical composition of these films has been observed to be a strong function of the substrate temperature. A significant incorporation of Si into the CN network occurs for substrate temperatures exceeding 1000C. XPS analyses of these films indicate a multiple bonding structure between carbon and nitrogen and also indicate that a majority of the carbon atoms are tetrahedrally coordinated and covalently bonded with nitrogen. Investigations of the growth rate as a function of various deposition parameters revealed that Si substrates exhibit weight loss rather than deposition, suggesting that abstraction of Si atoms from the substrate surface by the chemically active radicals from the plasma is a dominant process. The species thus formed, i.e., the active radicals bonded with Si, appear to facilitate the subsequent reactions in the gas phase. A major achievement of this study is that there is no obvious deposition of any amorphous CN material.

Implications of a mixing origin for globular cluster bimodal CN variations

New spectroscopic data are presented that reveal that the red giants in the globular cluster NGC 6934 exhibit a bimodal CN distribution, as is common among intermediate-metallicity clusters. Clear evidence for an anticorrelation between the strengths of the lambda3883 CN and the lambda4300 CH bands is found. This observation suggests that the CN-rich NGC 6934 giants have mixed CN(O.)-processed material up into their atmospheres. An analytical formulation is then developed to investigate the behavior of the CN opacity as increasing amounts of CN-processed material are brought into the atmosphere. It is found that in the absence of significant CO formation the CN opacity reaches a maximum at a carbon depletion of approx.0.22 dex, a result that is confirmed by spectrum-synthesis calculations. The advent of significant CO formation in relatively cool (T/sub eff/< or approx. =4000 K) atmospheres causes the CN opacity to peak at smaller carbon depletions. These results are shown to imply that the CN-poor stars, in clusters that exhibit bimodal CN distributions, must be relatively unmixed. It is also found that a bimodal CN distribution within a globular cluster need not imply an underlying bimodal carbon-abundance distribution. In fact a monomodal carbon distribution, with a peak at themore » low carbon-depletion end, could produce a CN bimodality. The addition of reasonable amounts of both CN and ON-processed material into the atmosphere does not greatly alter the surface carbon depletion at which the CN opacity maximum occurs.« less

Approach to Equilibrium in the CNO Bi-Cycle

Differential equations of the CNO bi-cycle are solved to obtain the abundances of carbon, nitrogen, and oxygen nuclei as functions of protons consumed per initial nucleus for hydrogen burning in the temperature range 10-50 x 106 K. General features of the curves of abundance ratios for C12, C'3, N14, N'2, and 016 nuclei as functions of protons consumed are found to be insensitive to the temperature at which the reactions take place. The resonance at 65 keV in the O' (p, a) N'4 reaction is responsible for the temperature dependence of ratios involving O' nuclei. Analysis is made of the effect of convection with microscopic mixing on CN material passing through a hydrogen-burning zone. If a simple linear relationship or a sinusoidal relationship is assumed for the time dependence of temperature, the ratios of abundances attained at the surface in a given time are the same whether the time is spent in a single transit to the surface or in several transits back and forth from the bottom of the hydrogen-burning zone to the surface of the star. It is shown that the C"-C" ratio observed in typical carbon stars can be explained on the basis of the CNO bi-cycle if the time spent in the process is at least one-half the mean lifetime of C" for proton capture, but that, in order to explain the ratio of nitrogen to oxygen suggested by astronomers for these stars, it is necessary to assume that the time spent by the nuclei at the site of hydrogen burning must be no longer than the mean lifetime of C" for proton capture. In the Appendix a table is given of contributions of several resonant levels in the compound nuclei involved in the proton-capture reactions of C", C", and N'4. These contributions are presented in the form of equations for the temperature dependence of the mean lifetimes of the reactions.