Crystalline Carbon Research Articles

Acid protonation promoted different crystal phase structure silicon carbide-based carbon nitride composites to enhance the photocatalytic degradation of dye wastewater

Acid-protonated crystalline silicon carbide-supported carbon nitride photocatalytic composites were successfully prepared by the impregnation-heat treatment method (P-g-C3N4/β-SiC and P-g-C3N4/α-SiC).

General outlook of the elaboration of nitrogen doped graphenic materials from the reaction between an amino alcohol and metallic sodium

Nitrogen-doped graphenic materials (N-Gr) are attracting increasing interest in the field of electrocatalysis, where their applications as noble metal-free catalysts or as catalyst supports are explored worldwide. Solvothermal-based processes are an efficient way to produce large quantities of N-Gr, without compromising their valuable properties. Reported in earlier publications, our elaboration route is based on a solvothermal reaction between various organic alcohols, e.g. cyclohexanol, ethanolamine, 1-(2-hydroxyethyl)piperidine, and metallic sodium, followed by a pyrolysis treatment under nitrogen flow. Rarely investigated in the literature mainly due to their complex mechanisms, the understanding of such processes opens many paths to tailor the properties of N-Gr, leading to high porosity (>2200 m2/g), good crystallinity, high purity, etc. The present article focuses on the influence of the solvothermal reaction experimental parameters on the final N-Gr, i.e. temperature, pressure, and sodium content. The elaborated materials are studied through multi-scale and complementary characterization techniques, i.e. Raman spectroscopy, thermogravimetric analysis, X-ray photoelectron spectroscopy, N2 adsorption at 77 K. An overview of the whole process follows the experimental part, giving quick access to optimized experimental parameters depending on the desired N-Gr properties, e.g. yield, crystallinity or porosity. By way of illustration, some trends were evidenced, such as (i) the larger conversion rate of solvent into crystalline carbon material as the reaction temperature is increased (300–380 °C), (ii) the increase of the surface area and the larger nitrogen content with increasing pressure (100–200 bar), and (iii) the beneficial impact of the sodium content on the yield and the material crystallinity (Na/solvent ratio 1–2).

Phase junction crystalline carbon nitride nanosheets modified with CdS nanoparticles for photocatalytic CO2reduction

PTI–melon molecular junctions anchored with CdS nanoparticles enable efficient CO2photoreduction, affording a high AQE of 10.8% at 420 nm.

Electron-rich pyrimidine rings enabling crystalline carbon nitride for high-efficiency photocatalytic hydrogen evolution coupled with benzyl alcohol selective oxidation

With electron-rich pyrimidine rings introduced, the obtained crystalline PCN is favored with rationally modulated band and electronic structures, resulting in efficient photocatalytic hydrogen evolution and benzyl alcohol selective oxidation.

Construction of π-conjugated crystalline carbon dots with carbon nitride nanofragments for efficient photocatalytic H2 evolution.

Crystalline carbon dots (CCDs) embedded in carbon nitride (CN) nanofragments (CCDs-CN) have been developed through facile molten salt treatment. Molten salt treatment not only reconstructs CN layered sheets to form nanofragments, but also promotes the crystallization of CDs-CN. The π-conjugated electric field between CCDs and CN accelerates charge carrier separation for efficient photocatalytic H2 evolution.

Realizing the potential of hydrophobic crystalline carbon as a support for oxygen evolution electrocatalysts

Introduction of a hydrophobic crystalline carbon support enhances the performance of AEMWE and improves the corrosion resistance of carbon by reducing its interaction with water. This demonstrates the promising potential of utilizing a carbon support.

Efficient generation of highly crystalline carbon quantum dots via electrooxidation of ethanol for rapid photodegradation of organic dyes

Crystalline Carbon Quantum Dots (CQDs) simply obtained by the electrooxidation of ethanol on Ni foam used for fast and efficient photodegradation of organic dyes.

An investigation into the modification of microwave-assisted oxidation in the macromolecular structure of coal via XRD and Raman spectroscopy

The macromolecular structure of coal governs the generation and expansion of micro/mesopores, contributing significantly to the storage and transport of coalbed methane (CBM). To reveal the modification mechanism of microwave-assisted oxidation at molecular scale, this paper presents a comprehensive analysis of the evolutionary characteristics of macromolecular structure of coal using X-ray diffraction and Raman spectroscopy tests. Results show that microwave irradiation triggers the conversion of ordered graphitic crystalline carbon to amorphous carbon, as well as the condensation and heterogeneous spatial arrangement of aromatic clusters. The oxidation of NaClO or Na2S2O8 destroys the initial aromatic structure and reduces the aromaticity of coal, whereas Fenton's reagent oxidation causes the cyclocondensation of macromolecular structure and contributes to the graphitization and crystallization of coal. Microwave-assisted oxidation not only facilitates the directional growth of aromatic rings and the cross-linking of aliphatic chains, but also induces the rotation, displacement, stacking and gradual parallel alignment of aromatic lamellae, thus improving the integrity of three-dimensional lattice of graphite structure and the orderliness of macromolecular structure. As the average size of microcrystalline structural units increases and align directionally to form molecularly oriented domains, the pore size and volume subsequently increases, thereby enhancing the gas transport capacity of coal and the extraction of CBM. This study provides theoretical guidance for the optimisation of microwave-assisted oxidant modification and the paramount consideration of CBM development strategy.

Production of graphitic carbons from plant-based SiC/C nanocomposites for Li-ion batteries

Barley husks are a byproduct of grain processing produced in large volumes globally. They contain a large amount of nanostructured silica (nSiO2) which can be converted via magnesiothermic reduction into carbon-rich nanostructured silicon carbide composite (nSiC/C). The nSiC/C can be further utilized as a precursor for the production of graphitic carbons (GCs) under atmospheric pressure at 2400 °C, typically utilized as active material in Li-ion battery (LIB) anodes. However, the current synthesis of graphite utilizes fossil-based precursors and requires temperatures up to 3000 °C. Therefore, we propose the sustainable and renewable plant-based GCs as an alternative anode material for LIBs. The synthesized GCs demonstrated a fully crystalline 3D carbon structure with stacking faults between graphene layers. Particle morphology, carbon structure, and elemental composition of the GCs were not influenced by the amount of free carbon in the precursors but the yield (11–41 wt%) of the GCs increased with increasing content of free carbon (1–26 wt%) in the SiC/C composite. The GC material had a specific capacity of 294 mAh g−1 as an active anode material in LIBs, demonstrated significantly better capacity retention at high current rates (1 and 2 C) compared with commercial graphite, and withheld 80% of the specific capacity over 175 cycles at 1 C. Synthesis of GCs from barley husks may provide an advantageous route of valorization of this underutilized sidestream.

Simple and straightforward method to prepare highly dispersed Ni sites for selective nitrobenzene coupling to Azo/Azoxy compounds

Azo and azoxy compounds are essential substances for the chemical industry. The development of catalysts to produce these molecules directly from nitroarenes is highly desirable. Nevertheless, this reaction is extremely challenging since the catalyst should not only activate nitro-compounds but also avoids the complete reduction to amines. Beyond noble-metals, such as Au, nickel-containing catalysts have shown attention for nitroarenes coupling to produce azo/azoxy compounds. Recently, outstanding results were reported for the nitrobenzene coupling using Ni-N4 sites obtained via pyrolysis of Ni(phen). However, pyrolysis methods lack on the control of metal loading and morphology of the metal sites, clearly, a mix of nickel nanoparticles and atomic dispersed metal sites were obtained. Here, we present a simple and straightforward method to synthesize highly dispersed nickel sites stabilized on highly crystalline carbon nitrides with poly(heptazine imide) structure (PHI). This method allows fine control of the metal loading and leads to mostly highly dispersed nickel sites, enabling to investigate in depth the coordination sphere of the Ni-sites during the reaction. The synthesized catalysts (Ni-PHI) were applied to nitrobenzene coupling reactions, showing high yields towards azo/azoxybenzene (98 %) and the highest TOF (435 h−1) for this reaction in the literature under mild conditions. Raman spectroscopy analyses show that nickel sites are crucial to adsorb nitrobenzene molecules through N atoms, favoring the production of nitrosobenzene. Furthermore, Ni-PHI exhibit high stability under different cycles and can yield azo/azoxy molecules with a broad scope, revealing the wide versatility of this catalyst.

Single LiCl-driven synthesis of heptazine-/triazine-based crystalline carbon nitride homojunction for enhanced solar hydrogen evolution

LiCl-based molten salts have been widely used to induce the polymerization of melamine-derived monomer or intermediate into triazine-based or heptazine-based crystalline carbon nitride. Nevertheless, the role of LiCl beyond lowering melting point has not been fully revealed, and pure LiCl was exclusively reported to induce the formation of triazine-based crystalline counterpart. Herein, we firstly found that single LiCl can promote the deep deamination of melon-based pristine carbon nitride (PCN) to heptazine-based crystalline counterpart. As a result, a heptazine-/triazine-based crystalline carbon nitride homojunction was in situ formed by single LiCl-induced deamination and phase transition of PCN. It exhibits a favorable photocatalytic H2 evolution reaction rate 18.3 times higher than PCN, which benefits from the promoted charges separation between heptazine and triazine units in phase homojunction. This work emphasizes that single LiCl can realize diverse modulation of carbon nitride framework with advanced photocatalytic activity.

Luminescent Crystalline Carbon- and Nitrogen-Centered Organic Radicals Based on N-Heterocyclic Carbene-Triphenylamine Hybrids.

Developing luminescent radicals with tunable emission is a challenging task due to the limitation of alternative skeletons. Herein, a series of carbene-triphenylamine hybrids were prepared by the direct C2-arylation of N-heterocyclic carbenes with 4-bromo-N,N-bis(4-methoxyphenyl)aniline. These hybrids showed multiple redox-active properties and could be converted to carbon-centered luminescent radicals with blue-to-cyan emissions (λmax : 436-486 nm) or nitrogen-centered luminescent radicals with orange emissions (λmax : 590-623 nm) through chemical reduction or oxidation, respectively. The radical species were characterized by electron paramagnetic resonance spectroscopy, ultraviolet-visible spectroscopy, and single-crystal X-ray diffractometry analysis. Notably, the corresponding nitrogen-centered radicals exhibited good stability in atmospheric air, and their thermal decomposition temperatures were determined to be above 200 °C. In addition, spectral and theoretical calculations indicate that all radicals exhibit anti-Kasha emissions.

Fe2P/N-doped biocarbon composite derived from mycelial pellet for bisphenol AF removal through peroxymonosulfate activation

In this work, microorganism of Phanerochaete chrysosporium with iron-tolerating and accumulating abilities was cultivated and harvested as mycelial pellets, which could serve as a sustainable and environmental-friendly precursor for iron phosphide/N-doped biocarbon composite preparation. Through one-step pyrolysis process, the inherent Fe and P elements in mycelial pellet enabled the formation of well crystallized Fe2P particles, loading on N-doped biocarbon matrix. Benefiting from the porous structure, high specific surface area, N doping-induced defects and CO groups of biocarbon, as well as the good electron-donating property of Fe2P, the as-obtained Fe2P/BC composite performed well for bisphenol pollutant (e.g., BPAF) removal through peroxymonosulfate (PMS) activation. Specifically, almost complete BPAF removal was achieved within 30 min adsorption and 10 min catalysis process with an apparent reaction rate constant of 0.486 min−1. The good crystallinity and protective carbon layer wrapping on Fe2P enabled the wide pH adaptability and low iron leaching of Fe2P/BC in catalytic reaction. Inorganic anions and humic acid brought only slight inhibition for BPAF removal, which could be overcome by prolonging reaction time to 30 min. Further mechanism studies indicated that reactive oxygen species, e.g., SO4-•, •OH and 1O2 were generated through both radical and non-radical pathways and contributed to BPAF degradation. After treatment by Fe2P/BC-PMS system, the total organic carbon (TOC) of original BPAF solution (20 mg L−1) was reduced by 51.6%. Fortunately, the residual degradation intermediate products showed much reduced ecotoxicity in both predicted (ECOSAR program) and experimental results (growth inhibition test using Raphidocelis subcapitata as ecological indicator).

Pyrolysis-reforming of cellulose to simultaneously produce hydrogen and heavy organics

The small molecular organics in bio-oil could not be effectively transformed into hydrocarbons with long aliphatic chains or aromatic rings via hydrotreatment, but they could be reformed with steam to generate hydrogen for the hydrotreatment. In this study, the pyrolysis of cellulose coupled with steam reforming of volatiles of small molecular size were performed at 400–600 °C over Ni/Al2O3 catalyst, which is termed as a pyro-reforming process for simultaneous production of hydrogen and heavy organics for further production of biofuels. The results indicated that the effective reforming of small volatiles became dominant at 600 °C. The coke formed at the low temperatures was mainly polymeric coke of aliphatic nature with low thermal stability, low carbon crystallinity, high reactivity towards oxidation and high hydrophilicity. Increase of temperature to 600 °C reduced the formation of coke (from maximum of 15.0%–12.8%) and suppressed the formation of polymeric coke by promoting gasification of precursors of coke. This enhanced formation of catalytic coke with amorphous morphology as well as defective ring structures. However, homogeneous polymerization of volatiles also took place, forming amorphous coke between catalyst particles. Formation of polymeric coke should be tackled for the successful implementation of pyro-reforming process.

Preparation and application of yttrium oxide with a large specific surface area through moderate carbonation in the presence of carbon dioxide

Yttrium oxide with a large specific surface area (SSA) (hereafter called LSSA Y2O3) has high-porosity structure, relatively large interface, and relatively abundant active surface sites, and its optical, chemical and thermal stability properties are greatly improved compared with ordinary yttrium oxide. As a result, LSSA Y2O3 has been applied in various fields as a luminescent, catalytic, and adsorbent material, showing enormous market potential. This study creatively presents a process designed to prepare LSSA Y2O3 powders through moderate carbonation in the presence of CO2. Experimentally, CO2 was used to carbonate a yttrium hydroxide [Y(OH)3] slurry. During the initial stage of carbonation, crystalline yttrium carbonate encapsulated Y(OH)3 through heterogeneous nucleation on its surface. This encapsulation considerably improved the filterability of the carbonation product while allowing it to retain the phase structure and high-porosity morphology of Y(OH)3. Further calcination of the carbonation product produced LSSA Y2O3 with an SSA of approximately 84 m2/g. This Y2O3 powder exhibited a relatively high adsorption capacity for methyl orange and was easy to recycle and reuse, thus showing potential for use as an adsorbent. The process developed in this study for preparing LSSA Y2O3 powders through carbonation in the presence of CO2 is advantageous because it requires only moderate conditions, causes no pollution, produces products with a uniform granularity and morphology, and is easy to scale up to meet industrial demands. Therefore, this process can effectively increase the added value and market competitiveness of Y2O3 powders and provide an experimental basis and theoretical guidance for the synthesis of other rare earth (RE) compounds with large SSAs through carbonation.

Green surface treatment of graphite felt using modified TEMPO mediated oxidation for use in vanadium redox flow batteries

MTMO (modified TEMPO((2,2,6,6-Tetramethylpiperidin-1-yl)oxyl) mediated oxidation) process was suggested as a green surface treatment to functionalize carboxylic acid group (COOH) on lower crystallinity carbon materials (LCCMs). With the proposed treatment, COOH contents on the surface of graphite felt (GF) was increased twice that of conventional heat-treated GF (CH–GF), resulting in high hydrophilicity without defects on the surface of LCCMs. These are attributed to the mild process feature of LCCMs using pre-activation of TEMPO to N-ammonium TEMPO, which changes hydroxyl groups to COOH without damaging carbon structure even on the LCCMs. Moreover, the MTMO process has environmental-friendly features such as a recyclable oxidizing agent, non-toxic effluents (NaCl and water), and room-temperature (25℃) aqueous process. Benefitting from these advantages, the maximum current densities toward vanadium ion redox reaction (VIRR) improved, and the charge transfer resistance for VIRR also decreased to 38.3 % of CH–GF. The energy efficiency for using MTMO–GF electrodes on both sides also improved by 119.4 % (200 mA cm−2) of using CH–GF during the cycling test of vanadium redox flow battery (VRFB). Furthermore, the VRFB using MTMO–GF electrodes exhibited stable operation for 500 cycles (200 mA cm−2), while VRFB using CH–GF failed at around 300th cycle.

High-yield and crystalline graphitic carbon nitride photocatalyst: One-step sodium acetate-mediated synthesis and improved hydrogen-evolution performance

To avoid the drawbacks (such as multi-step operations and causing big quality loss) of currently reported molten salt-assisted strategy for the preparation of crystalline graphitic carbon nitride (g-C3N4) photocatalysts, in this study, an innovative and one-step sodium acetate (CH3COONa)-mediated synthesis strategy has been designed to synthesize a high-yield and crystalline g-C3N4 photocatalyst. It is found that CH3COONa can strongly combine with dicyandiamide (DCDA) to availably prevent the massive sublimation of DCDA and the following intermediates, causing the high-efficiency transformation of DCDA into g-C3N4 with a high yield (52.2 wt%). In addition to the promoted denitrification and quick polymerization of DCDA via CH3COONa, the produced Na2CO3 from CH3COONa decomposition at a higher temperature can further accelerate the polymerization reaction of 3-s-triazine units, leading to the final production of highly ordered and crystalline g-C3N4. Consequently, the resultant high-yield and crystalline g-C3N4 shows an obviously strengthened hydrogen (H2)-evolution rate, about 2.4 times higher than that of bulk g-C3N4, which is due to the synergetic function of highly crystalline structure, reduced band gap and cyano-groups. The current one-step CH3COONa-mediated synthesis strategy may open a novel horizon for the facile preparations and various applications of crystalline g-C3N4 materials.

Combination effect of growth enhancers and carbon sources on synthesis of single-walled carbon nanotubes from solid carbon growth seeds

In the synthesis of highly crystalline single-walled carbon nanotubes (SWCNTs), high growth temperatures are preferred, while the formation of impurity amorphous carbon (a-C) causes the termination of SWCNT growth and degrades its properties. To remove this by-product, H2O and CO2 have been employed in metal-catalyzed SWCNT growth systems because of their oxidizing ability. Recently, nonmetallic nanoparticles have become one of the growth seed candidates because of their high melting points and fewer metal impurities. In this study, by using nanodiamond-derived carbon nanoparticles as the solid growth seeds, we investigated the effect of CO2 and H2O on high-temperature SWCNT growth with two types of carbon-source supplies: C2H2 and C2H4. In this growth system, H2O showed oxidizing ability to etch a-C with either carbon sources. CO2 exhibited a similar a-C formation-preventing role in C2H4-supplied growth and achieved higher-purity SWCNTs with higher concentration of C2H4 than the case of H2O. However, in contrast to the other combinations, CO2 injection in the C2H2-supplied growth significantly enhanced the formation of a-C rather than the removal of it while the yield of SWCNTs was also increased, indicating the occurrence of the dehydration reaction between CO2 and C2H2. The present findings will lead to efficient growth of high-quality SWCNTs from nonmetallic growth seeds with the use of growth enhancers.

Highly crystalline carbon nitride with small-sized sea urchin-like structure for efficient photocatalytic hydrogen production under visible-light irradiation

Molten-salt method has been deemed as a feasible strategy to increase the crystallinity of graphitic carbon nitride (abbreviated as CN), thereby improving its photocatalytic activity. However, highly crystalline CN prepared by molten-salt method using bulk carbon nitride (BCN) as the precursor still suffers from a small specific surface area, which is unfavorable to the separation of photogenerated carriers. Here, we report a facile approach to obtain highly crystalline CN with a large specific surface area using tubular carbon nitride (TCN) as raw material, and the final sample is named as tubular carbon nitride-molten salt (TCN-M). Compared with bulk carbon nitride-molten salt (BCN-M) sample, the obtained TCN-M sample not only retains the advantage of high crystallinity, but also exhibits a smaller-sized sea urchin-like structure owing to the CN precursor pretreatment process. The experiment results demonstrate that TCN-M displays an excellent hydrogen production activity of 4.9 mmol g -1 h -1 under visible-light, and its hydrogen production performance can be further enhanced to 14.7 mmol g -1 h -1 after adding 0.5 M K 2 HPO 4 . Thus, this work may supply valuable ideas to further optimize the photocatalytic activity of highly crystalline CN prepared by molten-salt method from the perspective of CN precursor pretreatment. • A highly crystalline carbon nitride with small-sized sea urchin-like structure was successfully prepared. • Large specific surface area and the presence of cyano groups greatly improve the photocatalytic H 2 production activity. • Optimizing the photocatalytic activity of highly crystalline CN from the perspective of CN precursor pretreatment. • A typical H + -reduction cycle can be constructed by introducing K 2 HPO 4 into the reaction system.

Oxygen dissociation on the C3N monolayer: A first-principles study

The oxygen dissociation and the oxidized structure on the pristine C3N monolayer in exposure to air are the inevitably critical issues for the C3N engineering and surface functionalization yet have not been revealed in detail. Using the first-principles calculations, we have systematically investigated the possible O2 adsorption sites, various O2 dissociation pathways and the oxidized structures. It is demonstrated that the pristine C3N monolayer shows more O2 physisorption sites and exhibits stronger O2 adsorption than the pristine graphene. Among various dissociation pathways, the most preferable one is a two-step process involving an intermediate state with the chemisorbed O2 and the barrier is lower than that on the pristine graphene, indicating that the pristine C3N monolayer is more susceptible to oxidation than the pristine graphene. Furthermore, we found that the most stable oxidized structure is not produced by the most preferable dissociation pathway but generated from a direct dissociation process. These results can be generalized into a wide range of temperatures and pressures using ab initio atomistic thermodynamics. Our findings deepen the understanding of the chemical stability of 2D crystalline carbon nitrides under ambient conditions, and could provide insights into the tailoring of the surface chemical structures via doping and oxidation.