Graphite Phase Research Articles

Preparation and Photocatalytic Degradation Performance of Amine-Rich Carbon Nitride with Structural Modulation of the Precursor.

The photocatalytic performance of graphitic phase carbon nitride (g-C3N4) is strongly influenced by its own microstructure as well as the precursor structure that causes the microstructure changes. In this paper, a composite precursor of sodium chloride and cyanamide (NaCl/CA-2) was obtained by freeze-drying, which possess an aggregated state different from that of the non-freeze-drying method. This new aggregation state with the introduction of sodium ions into the cyanamide lattice results in a higher activation energy of NaCl/CA-2 in the thermal polycondensation process of the molten salt-assisted preparation of g-C3N4, which prevented the condensation of two cyanamides to one dicyandiamide, ultimately obtaining FD-CN with an amino-rich structure. The nitrogen atoms on the amino group can provide the photocatalyst with more unpaired electrons that can participate in the photoexcitation process, further improving its electron-hole separation ability and charge transfer efficiency, thus effectively enhancing its photocatalytic activity. Compared to the original g-C3N4, the photocatalytic activity of FD-CN for the degradation of methylene blue increased 2.19 times.

Understanding of the adhesive strength enhancement mechanisms of bilayer diamond film at nanoscale

In virtue of its outstanding comprehensive performances, the development of multilayer diamond holds significant potential for expanding the range of applications for diamond. However, there still is a major challenge to synthesize multilayer diamond with high adhesive strength. In this work, a bilayer diamond film with enhanced adhesive strength was successfully prepared through depositing nanocrystalline diamond film on plasma-etched polycrystalline diamond substrate using chemical vapor deposition. The adhesion strength was evaluated by the critical load when film failed during scratch test. The nanocrystalline diamond film deposited on plasma-etched substrate exhibited a critical load of 121.6 N, which is higher by 57 % than that deposited on substrate without plasma etching. TEM observation demonstrated that the nanocrystalline diamond has a preference for nucleating at the bottom of pits on the substrate produced by plasma etching, constructing a unique interface with a mortise and tenon joint structure. Moreover, Raman results verified the removal of the graphite phase in substrate surface during plasma etching. Our findings reveal the mechanisms responsible for the enhanced adhesive strength of the developed bilayer diamond film at nanoscale. The improved adhesive strength can be attributed to the combination of the mechanical interlocking interface and the removal of the graphite phase. In addition, the method presented in this work can also be employed on chemical vapor deposition of other multilayer materials beyond diamond, offering the potential to enhance their adhesive strength.

Porous Graphitic phase carbon nitride/graphene oxide hydrogel microspheres for efficient and recyclable degradation of aflatoxin B1 in peanut oil.

Removal of aflatoxin is an urgent issue in agricultural products. A porous graphitic carbon nitride/graphene oxide hydrogel microsphere (CN/GO/SA) was synthesized and used to degrade AFB1 in peanut oil. CN/GO/SA was characterized by scanning electron micrograph (SEM), X-ray diffraction (XRD) and FT-IR. The introduction of GO significantly improved the adsorption capacity and visible light activity of photocatalysts. About 98.4% AFB1 in peanut oil was removed by 20% CN/GO/SA under visible light for 120min. ‧O2- and h+ were the main active species during photoreaction, and five degradation products were identified by UPLC-Q-Orbitrap MS analysis. At the same time, the quality of treated peanut oil was still acceptable. More importantly, CN/GO/SA showed excellent cycle stability, and the degradation rate of AFB1 in peanut oil remained above 95% after five-time recycling. This work provides a practical way for developing efficient and sustainable photocatalysts to degrade mycotoxins in edible oil.

Preparation and Thermal Properties of Propyl Palmitate-Based Phase Change Composites with Enhanced Thermal Conductivity for Thermal Energy Storage.

Phase change materials (PCMs), which can absorb and release large amounts of latent heat during phase change, have been extensively studied for heat storage and thermal management. However, technical bottlenecks regarding low thermal conductivity and leakage have hindered practical applications of PCMs. In this paper, a simple, economical, and scalable absorption polymerization technique is proposed to prepare the polymethyl methacrylate/propyl palmitate/expanded graphite (MPCM/EG) phase change composites by constructing the microencapsulated phase change materials (polymethyl methacrylate/propyl palmitate, MPCM) with core-shell structures in the three-dimensional (3D) EG networks, taking propyl palmitate as the PCM core, polymethyl methacrylate (PMMA) as the shell, and long-chain "worm-like" EG as the thermally conductive networks. This technique proved to be a more appropriate combinatorial pathway than direct absorption of MPCM via EG. The MPCM/EG composites with high thermal conductivity, high enthalpy, excellent thermal stability, low leakage, and good thermal cycle reliability were prepared. The results showed that the MPCM-80/EG-10 composite demonstrated a high thermal conductivity of 3.38 W/(m·K), a phase change enthalpy up to 152.0 J/g, an encapsulation ratio of 90.3%, outstanding thermal stability performance, and long-term thermal cycle reliability when the EG loading is 10% and propyl palmitate is 80%. This research offers an easy and efficient approach for designing and fabricating phase change composites with promising applications in diverse energy-saving fields, such as renewable energy collection, building energy conservation, and microelectronic devices thermal protection.

Alkali metals induced stacking phase transition of graphite

It is well known that alkali metals can intercalate into graphite to form graphite intercalation compounds but the structural change of graphite induced by alkali metals has been paid less attention. Herein, we report that alkali metals (Li, Na, K) induce the phase transition from ABC-stacking (or 3R phase) to AB-stacking (or 2H phase) of graphite. Specifically, for the graphite powder containing ∼45% 3R phase in a mixed phase, the temperature needed to ensure a complete stacking phase transition is 600 °C, 500 °C or 350 °C, for Li, Na or K, respectively, much lower than that needed in bare annealing (1800 °C) while without these alkali metals. Simulations based on density functional theory (DFT) suggest that the charge transfer from alkali metals to graphene distinctly lowers the energy barrier of transition, explaining the different phase transition temperatures for these alkali metals.

Enhanced power factor of expanded graphite phase change thermoelectric cement composites by metal oxides

Cement-based materials with thermoelectric effect can realize the conversion of waste heat in building environment, which is of great significance in energy conversion and utilization, and has become a research hotspot in recent years. In this study, three kinds of metal oxides were added into the phase change modified expanded graphite cement-based composites to investigate the impact of metal oxide content on the thermoelectric characteristics of cement composites. The findings shown that the thermoelectric properties of cement-based materials can be enhanced by the addition of metal oxides. The power factor increased to 3.15 μW·m−1·K−2, 3.60 μW·m−1·K−2, and 2.53 μW·m−1·K−2 after mixing ZnO, CuO, and TiO2. This is because the incorporation of metal oxides introduced a large number of interfaces into the cement, increased carrier scattering, strengthened the Seebeck coefficient of cement, and improved the power factor. This work presented a novel strategy to improving the thermoelectric properties of cement-based materials.

Research on the chemical barrier and failure behavior of WS2 and WS2/Ti coatings under high-temperature conditions and the effects on the lifespan of diamond-coated cutting tools

With its outstanding performance as a solid lubricant, WS2 has found applications in cutting tools. Diamond-coated tools often experience the formation of a graphite phase during machining of ferrous metals. The WS2 coating effectively hinders the thermochemical reaction between the tool-chip and tool-work, thereby extending the tool's operational lifespan. However, WS2 is susceptible to oxidation at high temperatures, leading to the loss of its lubricating properties. Therefore, improvements in its high-temperature resistance are necessary. This study proposes a WS2/Ti multilayer coating structure, which enables the development of solid lubricating coatings (SLC) that are better suited for high-temperature environments. Through the application of negative bias, Ti atoms are propelled into the gaps within the WS2 dendritic structure at high velocities, thereby forming metallic bonds between the layered structures. This process reduces the transfer film effect and hinders the rapid degradation of the SLC. Additionally, the Ti layer forms a sealing barrier on the surface of WS2, effectively impeding contact between WS2 and O2, minimizing WS2 oxidation, and prolonging the SLC's operational lifespan at elevated temperatures.

Hydrogen production on g-C3N4 nanoflakes via photoelectrochemical water splitting

Graphitic carbon nitride (g-C3N4) is an emerging non oxide wide band gap semiconductor which has been investigated for hydrogen production through photoelectrochemical (PEC) water splitting. In this study, g-C3N4 nanoflakes film was fabricated by the thermal pyrolysis process and its PEC water splitting performance was investigated. The nanoflakes morphology and graphitic phase of C3N4 with an average crystallite size of 8.49 nm were confirmed with FESEM and XRD analysis respectively. A UV–visible spectrophotometer was used to estimate the band gap of the fabricated photoanode film (2.7 eV). Furthermore, the fabricated g-C3N4 film has shown a significant photocurrent density of 0.87 µA/cm2 under UV light illumination.

Recent Developments in Direct C-H Functionalization of Quinoxalin-2(1H)-Ones via Heterogeneous Catalysis Reactions.

In recent years, Web of Science has published nearly one hundred reports per year on quinoxalin-2(1H)-ones, which have attracted great interest due to their wide applications in pharmaceutical and materials fields, especially in recyclable heterogeneous catalytic reactions for direct C-H functionalisation. This review summarises for the first time the methods and reaction mechanisms of heterogeneous catalytic reactions of quinoxalin-2(1H)-ones, including six major types of heterogeneous catalysts involved. The heterogeneous reactions of quinoxalin-2(1H)-ones are summarised by classifying different types of catalytic materials (graphitic phase carbon nitride, MOF, COF, ion exchange resin, piezoelectric materials, and microsphere catalysis). In addition, this review discusses the future development of heterogeneous catalytic reactions of quinoxalin-2(1H)-ones, including the construction of C-B/Si/P/RF/X/Se bonds by heterogeneous catalytic reactions, the enrichment of heterogeneous catalysts such as metal oxides, graphene-based composites, doped metal nanoparticles, and molecular sieve-based porous materials, asymmetric synthesis, and other areas. The aim of this review is to contribute to the development of green and sustainable heterogeneous reaction methods for quinoxalin-2(1H)-ones with applications in materials chemistry and pharmacology.

Unusual material removal characteristics of the novel graphite/diamond composite in mechanical polishing

Novel graphite/diamond composite (Gradia) is ultrahard, tough and semiconductive. Surface finishing of Gradia is difficult because of the disparity in mechanical properties between diamond and graphite components. In this work, material removal characteristics of Gradia in mechanical polishing were studied with experiments and molecular dynamic simulations. First, the Gradia surface was well-finished. Then, microstructural investigation showed the main mechanism of phase transformation was amorphization during polishing. The graphite and diamond phases of Gradia underwent interconversion under the mechanical action. In addition, initial and diamond-transformed graphitic phases experienced distortion, twist and reconstruction, producing graphite film. Finally, a Gradia tool was created and demonstrated a lower wear rate than single-crystal diamond tool, suggesting an unusual self-lubricating characteristic with abrasion-induced generation of graphite.

Graphite–hexagonal diamond hybrid with diverse properties

The recently discovered graphite–diamond hybrid materials (Gradia) with mixed sp2- and sp3-hybridizations have opened up a new direction in carbon allotropes research. Herein, we reported Gradia-HZ, constituted by interfaced graphite and hexagonal diamond parts in the unit cell, which demonstrates distinct electronic and mechanical properties. With the modulation of graphite width, Gradia-HZ exhibits unexpected topological nodal-line semimetal, semiconductor, and normal metal integrating with a distinctive Quasi-1D electronic transport capability based on first-principles calculations. More interestingly, pressure-induced graphite phase transformation might be an implementable and effective method to regulate the structure and physical properties of Gradia-HZ. The discovery of rich and peculiar physical properties in Gradia-HZ, e.g., high-conductivity metals, semiconductors with variable bandgap, and topological semimetals, will arouse great research interest to graphite–diamond hybrid materials, to promote their development and application in advanced devices.

Fe doped NiS2 derived from metal-organic framework embedded into g-C3N4 for efficiently oxygen evolution reaction

The exploitation of the superior oxygen evolution reaction (OER) electrocatalysts is of great paramount importance to the investigation of developing hydrogen production from water splitting to alleviate the energy crisis. Herein, Fe3+ ion doped NiS2 for OER electrocatalysts obtained from a metal-organic framework (MOF) as a self-template, with the graphitic phase carbon nitride (g-C3N4) as a chain armor, was prepared by the two-step solvothermal-pyrolysis method. The MOF-derived Fe–NiS2@g-C3N4 composite achieves a current density of 10 mA cm−2 at a tiny OER overpotential (280 mV) and shows good stability in duration tests over 60 h. Compared with the undoped sample, the overpotential is significantly lower by 60 mV and the stability is substantially improved. In this study, two strategies, iron ion doping and g-C3N4 coating, were implemented to target the defects of NiS2, promoting its catalytic activity and achieving a significant improvement in stability at the cost of a bit of activity. Balancing activity and stability maximized has been achieved by our work. This study presents a potential approach to improve the catalyst activity and stability in an integrated strategy.

On the tribological behaviors of Cu matrix composites with different Cu-coated graphite content

Graphite/Cu composite is widely used in pantograph slide plates. To further understand its lubrication behaviors, the tribological properties of graphite/Cu composites with different graphite content were studied using ball-on-disk wear tests. The results reveal that the composites contain only graphite and Cu phases. The tribological results show that the friction coefficient and wear rate of 7 wt.% graphite/Cu composite were decreased 7.7% and 12.7%, respectively, compared with 5 wt.% graphite/Cu. However, both friction coefficient and wear rate increased with the graphite content exceeding 7 wt.%. The excellent tribological properties of the composites were mainly attributed to the synergistic action of the oxidative trib o-layer and graphite lubricant film, which is composed of graphite, Cu, CuO, and Fe2O3. The wear mechanism of the graphite/Cu composites was mainly abrasive wear, accompanied by adhesion and oxidation wear.

Synergistic optimization of mechanical and tribological properties of TiC modified copper-graphite composites by direct current in-situ sintering

The application of Cu-graphite composites in the field of friction materials is limited by the poor wettability between Cu and graphite and weakened mechanical properties. In this work, in-situ TiC layers were generated by interfacial resistance sintering with direct current to manipulate the interfacial bonding of the composites and enhance their comprehensive properties. The Ti added to the composites would react with graphite at the interface to generate TiC layers and form strong Cu–TiC-graphite interfaces due to interfacial reactions. When the added Ti content is 6 wt%, the composite demonstrates the most excellent mechanical properties and tribological characteristics, i.e., yield strength (168 MPa) and wear rate (2.7 × 10−10 m2/N) are 93.1% higher and 29.7% lower than those of the Cu-graphite composite without Ti addition, respectively. The dense TiC layer induces the strengthening of the Cu matrix and serves as the reinforcing phase to optimize the interfacial bonding and stress transfer, which not only greatly enhances the mechanical properties of the composite but also enables the composite to take full advantage of the hard TiC and graphite phases to obtain stable friction coefficient and low wear rate. This work provides a simpler technique to prepare modified Cu-graphite composites with excellent performance and contributes to the in-depth understanding of the enhancement mechanism of hard ceramic layers on the mechanical and tribological properties of composites.

Onion skin–derived sorbent for the sequestration of methylparaben in contaminated aqueous medium

Carbon-based adsorbents were produced from onion skin waste for the adsorption of methylparaben from contaminated water. The biomass-derived carbon was characterized using various established analytical techniques. The microscopic examinations revealed micro- and mesoporous structures with a partially disordered network of the graphenic carbon-like multilayer structure, confirmed by XPS and Raman spectra. XRD analysis revealed that the biomass-derived carbon is largely amorphous with the graphitic phase also confirmed. Aside from the prominence of sp2 hybridized carbon, FTIR analysis shows the existence of moieties and functional groups that may facilitate the sorption of methylparaben or other organic pollutants if explored. The adsorption isotherm revealed that the multilayer adsorption model (Freundlich) best fits experimental data with an SSE value of 0.454. A complex adsorption process is suspected between methylparaben and OSDC, and the physicochemical properties of the sorbate and sorbent played a huge role in the sorption process. The plausible interactions include van der Waals, hydrophobic bonding, hydrogen bonding, π-π stacking, and pore-filling mechanisms, leading to a hysteretic sorption process. The optimal removal efficiency and adsorption maxima of ~ 100% and ~ 8200 mg/g are obtainable at optimum process conditions. Therefore, waste valorization and adsorption performance achieved in this study suggest a sustainable and cost-effective pathway for pollution remediation.

Erythritol/expanded graphite form-stable phase change materials with excellent thermophysical properties

Organic phase change materials possess the merits of high latent heat storage, small temperature changes, high chemical/thermal stability, low corrosion and reusability, and thus have attracted extensive attention in the research of low and medium-temperature energy storage media. However, pure phase change material has the deficiency of low thermal conductivity and easy leakiness, which seriously hinder their practical large-scale applications. In this work, the expanded graphite was used as the skeleton to encapsulate erythritol to form the composite phase change material. The composite had remarkably boosted thermal conductivity (5.97 ± 0.30 W/m·K compared with pure erythritol of 0.60 ± 0.03 W/m·K) and high melting latent heat (290.52 J/g) and solidification latent heat (239.52 J/g) at a loading rate of 95 wt%. The supercooling degree was reduced from 92.41 °C to 43.75 °C, facilitating the release of latent heat. The developed composite phase change material exhibits extraordinary heat transfer and storage characteristics, which has great application prospects in the fields of solar photothermal conversion utilization.

Efficient photocatalytic H2 production of g-C3N4 using nickel (II) functional complex as promoter

With the increasingly prominent energy and environmental problems, the construction of sustainable development, green environmental protection and new efficient energy system has become the focus of the world's attention. Photocatalytic hydrogen production technology is an important way to solve energy and environmental problems. The key to realizing the visible light catalytic water hydrogen production technology effectively lies in the selection of photocatalytic materials, graphite phase carbon nitride (g-C3N4) is an important photocatalyst. However, the photocatalytic performance of g-C3N4 is severely limited by the high recombination rate of photogenerated electron-hole pairs. Functional complex has a matching band gap with carbon nitride to form heterojunction structure, which can effectively reduce the photogenic carrier recombination and improve the catalytic hydrogen production performance. For this reason, the functional complex [Ni2(Medpq)2(BPDA)·2H2O] (BPDA = 4,4′-Biphthalic Anhydride, Medpq = 2-methyldipyrido[3,2-f:2′,3′-h]quinoxaline) was synthesized by a hydrothermal process. The matching band gap between [Ni2(Medpq)2(BPDA)·2H2O] and g-C3N4 forms heterojunction structure, which effectively solve the defect of low utilization rate of light energy and improve the catalytic hydrogen production performance of g-C3N4. The experiment proved that complex/g-C3N4 photocatalytic rates of hydrogen production 2054.13 μmol·h−1·g−1, about 5.04 times that of pure g-C3N4 (407.56 μmol·h−1·g−1), thus high-performance of the hydrogen production from water decomposition was realized.

B-doped nanodiamond composite hard coatings deposited on cemented carbide: Mechanical, structural, and tribological properties

Herein, the effects of B doping on the mechanical, structural, and tribological properties of nanodiamond composite (NDC) coatings deposited on cemented tungsten carbide-based Co binder substrates via coaxial arc plasma deposition are studied. The results reveal decreasing films hardness 35, 27, and 25 GPa with increasing B contents of 1, 5, and 10 at.%, respectively, which is attributed to film graphitisation. Graphitisation is induced by the catalytic effects of Co atoms, which diffuse proportionally from the substrate surface into the films with increasing B content. However, Co diffusion hardly occurs during the deposition of undoped films. The 1 at.% B-doped films with an undoped NDC buffer layer exhibit enhanced hardness from 35 to 58.3 GPa owing to a decrease in the catalytic effects of Co, thereby resulting in the formation of a high amount of CC sp3 bonds at the expense of CC sp2 bonds. The adhesion of the doped films is improved by a factor of three compared with the undoped films owing to enhanced film hardness and toughness as B doping tends to release internal stress by forming long CB bonds. The sliding behaviour remains at a friction coefficient of approximately 0.08 owing to the presence of sacrificial B2O sliding layers and graphitic self-lubrication phases. The B-doped NDC films are promising hard coatings for machining difficult-to-cut materials.

Tailoring optical emission of silicon-vacancy centers in two-dimensional diamond nanosheets via a two-step oxidation approach

Incorporating two-dimensional (2D) diamond nanosheets with fluorescent color centers exhibits great potential in the application of quantum sensing. However, color centers always show poor optical emission in chemical-vapor-deposited (CVD) diamond nanomaterials. To address this issue, Si doped diamond/graphite hybrid films were successfully fabricated in microwave-plasma CVD device. The films consist of diamond-core/graphite-shell nanosheets with high amount of diamond nanocrystalline particles. Two post treatments of acid oxidation and annealing in air were used to tailor photoluminescence (PL) of silicon-vacancy (SiV) centers. The SiV centers in the oxidized samples exhibit small PL increase compared with the as-deposited samples with SiV PL quenching. It is found that the graphite phase is selectively etched away with the presence of nanocrystalline diamond particles during the treatment of acid oxidation while the nanocrystalline diamond particles are efficiently removed with the presence of graphite using the air annealing method. Based on this result, a two-step approach of acid oxidation followed by air annealing was conducted to etch the non-diamond phase, forming diamond nanosheets. The SiV centers exhibit significant PL enhancement with a maximum value of 28 folds, compared with the single-step oxidized samples. The Raman and XPS results reveal that such PL increase originates from direct bonding of oxygen on the sp 3 carbon. Therefore, our work provides a feasible approach to prepare 2D diamond nanosheets with high-brightness color centers.

High-pressure phase of cold-compressed bulk graphite and graphene nanoplatelets

We have studied the high-pressure vibrational and structural behavior of bulk graphite and graphene nanoplatelets at room temperature by means of high-pressure Raman spectroscopic and x-ray diffraction probes. We have detected a clear pressure-induced structural transition in both materials, evidenced by the appearance of new Bragg peaks and Raman features, deviating from the starting hexagonal graphitic structure. The high-pressure phase is identified as a partially disordered orthorhombic structure, consisting of mixed $s{p}^{2}$- and $s{p}^{3}$-type bonding. Our experimental findings serve as direct evidence for the existence of a metastable transient modification in cold compressed carbon, lying between the $s{p}^{2}$-type graphite and ${sp}^{3}$-type diamond allotropes.