Sp2-hybridized Carbon Research Articles

Biomass-Derived Carbon With Large Interlayer Spacing for Anode of Potassium Ion Batteries.

Potassium-ion batteries (PIBs) are emerging as powerful candidate for grid-oriented energy storage owing to their potentially low cost. Carbon is considered the promising anode for PIBs on the basis of its high conductivity and abundant sources. The biggest challenge confronted by carbon anodes lies in insufficient cycle life as well as rate capability, resulting from the limited interlayer spacing of the sp2-hybrid carbon incompatible with the large-radius potassium. Herein, a biomass-derived carbon with a large interlayer spacing of 0.44nm is fabricated via a zinc-assisted pyrolysis synthesis. The unique structure endows the carbon with superior capacity, rate capability, and cycle durability. The large interlayer spacing of carbons can promote fast potassium diffusion and alleviate the volume expansion during potassiation, conferring those rate capabilities and cycleability. The interconnected network structure is also able to shorten both the transport distances of electrons and ions. The demonstration exemplifies an advanced carbon for anodes of PIBs for energy storage applications.

Se-functionalized metal-free catalysts rich in sp3-hybridized carbon enable efficient oxygen electroreduction

Carbon-based metal-free materials are emerging as leading candidates to replace noble-metal catalysts in the oxygen reduction reaction (ORR). Herein, we introduce a facile secondary carbonation technique for fabricating Se and N co-doped metal-free catalysts using a zeolite imidazole framework (ZIF-8) as the precursor. The optimal electrocatalyst, designated SeNC-900, exhibited good ORR performance under both alkaline and acidic conditions, with half-wave potentials of 0.864 V and 0.731 V (vs. RHE), respectively. Density functional theory (DFT) calculations reveal that the enhanced activity of SeNC-900 originates from Se doping, which triggers an increase in the intrinsic defects of sp3-hybridized C. Concurrently, the sp3-hybridized C, in concert with Se dopant, modulates the electronic structure of the active C atoms. This work not only underscores the significance of tuning the electronic structure to boost catalytic performance by enriching intrinsic defects but also presents a fresh insight into the effect of heteroatom doping on carbon-based materials for electrocatalysis.

A Negatively Curved Nanographene with Four Embedded Heptagons.

Negatively curved nanographenes are considered as cutouts of three-dimensional fully sp2-hybridized carbon allotropes such as Schwarzites. Here we present the synthesis of a C76 cut-out of the Schwarzite 8-4-1-p proposed by Lenosky et al. and investigate its optical as well as electrochemical properties. Furthermore, supramolecular interactions with fullerenes C60 and C70 were studied.

Exploring the graphitization transformation mechanism of deposited carbon in molten salt electrolysis: A novel insight from molecular structure models

Molten salt electrolysis graphitization is a novel method for the electrochemical graphitization, offering significant technological advantages. Researchers have shown keen interest in preparing graphite materials through molten salt strategies. However, there is still a lack of comprehensive research methodologies at the molecular scale for the removal of heteroatoms and rearrangement of carbon atoms in different amorphous carbon conversion systems. To address the above issue, the deposited carbon (DC, a coking byproduct) was converted into graphitized material (electrolysis products, EP) through the electrochemical method in this study. By processing Transmission Electron Microscope images with Python programming, aromatic skeleton structure information was quantitatively extracted. The evolution of the structure of DC was studied using Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. Based on information about elemental content, aromatic skeletal structure, and heteroatom functional groups, average chemical structure models were constructed for DC and EP, and relevant chemical bond parameters were statistically analyzed. Then, the influence of the electric field on the structural transformation of amorphous carbon in molten salts was investigated. The research results show that the electric field significantly affects the graphitization transformation process of amorphous carbon in molten salts, enhancing the interaction between calcium ions and DC molecules, increasing the diffusion coefficient of particles in the system, and promoting the formation of more sp2-hybridized carbon atoms, thus forming a more compact aromatic ring network. This study provides a new research method and molecular/atomic-level insights for a deeper understanding of the mechanism of graphitization transformation of amorphous carbon.

Synthesis of Hexabenzocoronene-Cored Graphdiyne Nanosheets through Dehydrogenative Coupling on Au(111) Surface.

Thermally-induced dehydrogenative coupling of polyphenylenes on metal surfaces is an important technique to synthesize -conjugated carbon nanostructures with atomic precision. However, this protocol has rarely been utilized to fabricate structurally defined carbon nanosheets composed of sp- and sp2-hybridized carbon atoms. Here, we present the synthesis of butadiyne-linked hexabenzocoronenes (HBCs) on Au(111) surfaces as core-expanded graphdiynes. The reaction started from hexa(4-ethylphenyl)benzene, which undergoes dehydrogenation toward hexa(4-vinylphenyl)benzene, followed by planarization to hexabenzocoronene, coupling between the vinyl groups, and further dehydrogenation. In addition to butadiyne linkages, benzene groups were also found as another type of linker. The reaction sequences were monitored by scanning tunneling microscopy and bond-resolved non-contact atomic force microscopy, which disclose the structures of intermediates and final products. In combination with density functional theory simulations, the key steps from ethyl substituents to butadiyne and benzene linkers were elucidated. This is a new on-surface synthesis of core-expanded graphdiynes with unprecedented electronic properties.

Superconductivity in Monolayer Carbon Allotropes with High Thermal Stability.

Intrinsic superconductivity is rarely discovered in sp2-hybridized monolayer carbon allotropes. Here we design a carbon monolayer configured of pentagon, heptagon, and hexagon rings with p2 plane group symmetry. Full-sp2 hybridization is proposed to favor thermal metastability on a low Gibbs free energy. The extremely small thermal expansion coefficient is predicted to the turn negative value to positive with elevating temperature. Carbon polygon structures remain intact at a high thermal temperature of 3,000 K. The high specific surface area is found to approach 2,700 m2/g, with O2-adsorption being advantageous over pristine graphene. We reveal electronic Fermi surfaces mediated by phonon modes of carbon out-of-plane vibrations. By calculating the Eliashberg equation, we evaluate intrinsic superconductivity with a large electron-phonon coupling coefficient. The superconducting transition temperature is estimated to reach 20 K under a high logarithmic average frequency. These first-principles calculations shall stimulate experimentalists' interest in exploring low-dimensional carbon superconductors with gas sensitivity.

Pushing Theoretical Potassium Storage Limits of MXenes through Introducing New Carbon Active Sites.

Surface-driven capacitive storage enhances rate performance and cyclability, thereby improving the efficacy of high-power electrode materials and fast-charging batteries. Conventional defect engineering, widely-employed capacitive storage optimization strategy, primarily focuses on the influence of defects themselves on capacitive behaviors. However, the role of local environment surrounding defects, which significantly affects surface properties, remains largely unexplored for lack of suitable material platform and has long been neglected. As proof-of-concept, typical Ti3C2Tx MXenes are chosen as model materials owing to metallic conductivity and tunable surface properties, satisfying the requirements for capacitive-type electrodes. Using density functional theory (DFT) calculations, the potential of MXenes with modulated local atomic environment is anticipated and introducing new carbon sites found near pores can activate electrochemically inert surface, attaining record theoretical potassium storage capacities of MXenes (291 mAh g-1). This supposition is realized through atomic tailoring via chemical scissor within sublayers, exposing new sp3-hybridized carbon active sites. The resulting MXenes demonstrate unprecedented rate performance and cycling stability. Notably, MXenes with higher carbon exposure exhibit a record-breaking capacity over 200 mAh g-1 and sustain a capacity retention higher than 80% after 20 months. These findings underscore the effectiveness of regulating defects' neighboring environment and illuminate future high-performance electrode design.

Kink effect on the lattice properties of one-dimensional carbyne nanocrystals under high temperature

Carbyne nanocrystal (CN), as a kind of one-dimensional (1D) sp-hybridized carbon allotrope, has attracted much attention due to its excellent optical and transport properties. However, the lattice properties of 1D CNs under high temperature remain unclear. In our work, we investigate the stability and lattice properties of 1DCNs based on the atomic-bond-relaxation (ABR) approach and first-principles calculations. We find the change of transverse vibration frequencies and bond kinks induced by the thermal effect plays a significant role in the mechanical properties of 1D CNs. We establish a relationship between kink angle and phonon vibration modes, giving a deep inside into high frequency and low-frequency vibration behavior of 1D CNs. Our results indicate the phonon vibration modes modulated by kinks under applied temperatures can lead to a negative thermal expansion behavior in the axial direction of 1D CNs, suggesting an effective way to control the thermal properties of 1D CNs for practical applications.

Ultrahigh Pressure Generation at High Temperatures in a Walker-Type Large-Volume Press and Multiple Applications

Ultrahigh pressure generation at high temperatures is technologically challenging for large sample volumes. In this study, we successfully generated pressures of 37.3–40.4 GPa at 1900–2100 K in a Walker-type large-volume press (LVP). Expansion of the pressure range at high temperatures was achieved by adapting newly designed ZK01F tungsten carbide (WC) anvils with tapered surfaces and using cell assemblies with an ∼1 mm3 sample volume and hard materials, as well as by applying certain adjustments to the apparatus. The pressure efficiencies of the different types of WC anvils and cell assemblies were also studied. Using the above-mentioned techniques, we successfully synthesized and characterized bulk samples of nearly pure sp3-hybridized ultrahard amorphous carbon, core–shell nanocrystals with high Néel temperatures, as well as large-sized single crystals of lower-mantle minerals. The developed LVP techniques presented here could enable the exploration of the chemical and physical properties of novel materials and Earth’s interior.

Effect of sp2-Hybridized Carbon Inclusions in Diamond Films on the Sensor Performance Toward Synchrotron Radiation

Effect of sp2-Hybridized Carbon Inclusions in Diamond Films on the Sensor Performance Toward Synchrotron Radiation

Synthesis of 5-(4-Formylphenyl)barbituric Acid to Access Enolizable Chromophoric Barbituric Acids

AbstractBarbituric acids mono-substituted at the 5-position show keto–enol tautomerism. In the keto form, conjugation to an aryl substituent is interrupted due to the sp³-hybridized carbon atom at the 5-position of barbituric acid. The enol form generates a conjugated π-system to the aryl substituent and acts as an electron-donating group. If the aryl substituent is electron-deficient, a push-pull system is generated that shows typical UV/Vis absorption. These types of compounds are difficult to access synthetically due to their intrinsic convertibility. The synthesis of barbituric acids with a 4-formylphenyl functionality at the 5-position is reported. This compound, 5-(4-formylphenyl)barbituric acid, could be used to introduce extended π-systems with electron-withdrawing groups in great variety by simple condensation reactions. We demonstrate this by a Horner–Wadsworth–Emmons reaction that forms the enolizable dye (E)-5-(4-(4-nitrostyryl)phenyl)barbituric acid.

Electronic structure engineering of N-doped carbon nanozyme via incorporating Cl and sp3-hybridized defected carbon for organophosphorus pesticides assay

Metal-free carbon-based nanozymes often exhibit superior chemical stability and detection reliability compared to their metal-doped counterparts. However, their catalytic activity remains an area ripe for further enhancement. Herein, we successfully prepared a chlorine (Cl)-modified, metal-free, and porous N-doped carbon nanozyme (Clx-pNC) via NaCl molten etching. The incorporation of Cl induced an increase in the intrinsic defects of sp3-hybridized carbon within Clx-pNC and optimized the electronic structure of the N-connected carbon atoms. Remarkably, the peroxidase (POD)-like activity of Clx-pNC was enhanced twelvefold compared to porous N-doped carbon (pNC). Theoretical simulations highlighted that the introduction of Cl not only promoted H2O2 adsorption but also lowered the energy barrier for its decomposition, facilitating the generation of active intermediates and thus boosting POD-like activity. Based on the POD mimic activity of Clx-pNC, we developed a colorimetric platform for OPs detection utilizing a cascade amplification strategy. This work provides insights into the rational design of carbon-based nanozymes and the development of nanozyme-based colorimetric biosensors.

Absorption, distribution, metabolism and excretion of carbon nanostructures

The interaction of any substance with the body is determined by several parameters, namely: its penetration, distribution, transformation and excretion, in other words, ADME properties. Naturally, this fully applies to such a class of compounds as carbon nanostructures (CN). They are mainly formed by sp2-hybridized carbon atoms (with the exception of nanodiamonds formed by sp3-hybridized atoms). However, their properties differ markedly from each other. This review is devoted to examining these differences. This review includes fullerenes, nanoionions, carbon nanotubes, carbon nanohorns, graphene and its derivatives, and nanodiamonds.

Comparison of Corrosion Behavior of a-C Coatings Deposited by Cathode Vacuum Arc and Filter Cathode Vacuum Arc Techniques

This study explores the utilization of cathodic vacuum arc (CVA) technology to address the limitations of magnetron sputtering technology in preparing amorphous carbon (a-C) coatings, such as having a low ionization rate, low deposition rate, and insufficiently dense structure. Specifically, a-C coatings were prepared by the cathodic vacuum arc (CVA)and the filtered cathodic vacuum arc (FCVA) technology,, one with embedded carbon particles and one without, both having closely related carbon structures. Research is currently underway on bipolar plate coatings for fuel cells. The corrosion behavior of the prepared a-C coatings was examined through Tafel polarization analysis under simulated fuel cell operating conditions as well as potentiostatic analysis at 0.6 V under normal conditions and 1.6 V under start–stop conditions for 7200 s. The coatings before and after corrosion are characterized using scanning electron microscopy, energy-dispersive X-ray spectroscopy, Raman spectroscopy, and infrared spectroscopy. The results reveal that the incorporation of conductive graphite-like particles in the coatings reduces their contact resistance. However, the gaps between these particles and the coatings act as pathways for corrosive solution, exacerbating the corrosion of the coatings. After corrosion at 0.6 V, both sets of coatings with sp2-hybridized carbon structures are contaminated by elements such as hydrogen and oxygen, leading to an increase in their contact resistance. Under high potential conditions (1.6 V), large corrosion pits and defects appear at the locations of graphite-like carbon particles. Furthermore, both sets of samples exhibit more severe oxygen contamination and a transformation of broken carbon bonds from sp3- to sp2-hybridized forms, irrespective of whether embedded graphite particles are present.

Pyrene Funtionlized Hollow Porous Carbon/S-Dib Composite As a Cathode for Lithium-Sulfur Batteries

Lithium-sulfur batteries (LiSBs) stand out as promising candidates for the next-generation energy storage system. However, challenges such as the “shuttling effects” of intermediate lithium polysulfides and sluggish conversion between sulfur and lithium sulfide have led to low specific capacity under high current and a shortened cycling life for LiSBs. Extensive research has explored porous carbon materials as hosts for sulfur in LiSBs cathodes. Nevertheless, overcoming the “shuttling effects” and ensuring the uniform deposition of lithium sulfide without using metals or metal-containing species remains a challenge.Taking advantage of its aromatic properties, which promotes robust π-π interactions with sp2-hybridized carbon, pyrene with various functionalized groups emerges as a viable solution for surface modification through non-covalent bonding to carbon. In this study, we present the synthesis of X-pyrene (X=OH, NH2, Cl) modified hollow porous carbon (HPC) serving as a micro-reactor for inverse vulcanization, resulting in the formation of S-DIB/X-HPC (X=OH, NH2, Cl) with a uniform structure. Abundant oxygen-, nitrogen- and chloride interfaces were designed to act as adsorption catalytic sites, accelerating conversion kinetics and mitigating shuttle effects.Hollow porous carbon (HPC) nanoparticles were prepared from ZIF-8 by carbonization at 950 °C under argon atmosphere. (Figure 1a). The cross-sectional view in Figure 1b reveals numerous individual hexagonal structures with abundant internal pores. This structure helps accommodate inverse vulcanization (IV) reactions and ensures the uniform distribution of the final product (S-DIB). To evaluate the adsorption quantity of pyrene on the carbon surface, standard solutions of varying pyrene concentrations in dimethyl sulfoxide (DMSO) were prepared and analyzed using Ultraviolet–visible spectroscopy (UV-VIS). A linear relationship is fitted between the intensity of these peaks with the standard pyrene solution. Therefore, the concentration in the filtrate is verified to be approximately 2 mM, in contrast to the initial concentration of 10 mM before adsorption. This observed concentration decrease implies an effective adsorption of pyrene molecules onto the HPC surface. To further investigate the catalytic effect of Cl-pyrene and NH2-pyrene for sulfur redox reactions, potentiostatic nucleation of Li2S on the HPC, Cl-HPC and NH2-HPC electrodes were conducted to clarify the involved multiphase transition process. As shown in Figure 3, the capacity of precipitated Li2S on Cl-HPC (160.62 mAh gS −1) and NH2-HPC (135.75 mAh gS −1) are both higher than that on HPC (99. 93 mAh gS −1). As a result, superior to S-DIB/HPC, the S-DIB/Cl-HPC cathode exhibits a higher initial specific capacity of 1208 mA h g−1 at 0.1 C and a capacity of 643 mA h g−1 at 3 C, and still retain 913 mAh g−1 while returning to 0.1 C. Furthermore, the S-DIB/NH2-HPC cathode possesses excellent cyclability with a high capacity retention of 652 mAh g−1 at 1 C over 300 cycles, yielding an average capacity decline of only 0.076% per cycle. Additionally, we investigate the impact of native surface groups on the deposition of lithium sulfide and the conversion of polysulfides. Figure 1

Laser-Induced Graphene for Electrochemical Analysis of Food and Agricultural Contaminants

Carbon-nanomaterial sensors offer promising routes toward low-cost, high-throughput electrochemical sensors. Carbon nanomaterials, such as graphene and carbon nanotube (CNT), have been studied with various fabrication methods to produce sensors, including high-quality thin film graphene and CNT by chemical vapor deposition. However, the complexity and cost associated with their fabrication have hindered their implementation into single-use test strip sensors. Even high-yield mechanical and/or chemical exfoliation of graphene from graphite followed by solution phase printing of electrodes can be costly due to the complexity of ink formulation and the need for post-print annealing requirements. However, laser-induced graphene (LIG) avoids the need to exfoliate graphene from graphite or to create a solution-phase graphene ink by converting sp3 carbon found in carbon-rich substrates into conductive sp2-hybridized carbon found in graphene through a laser scribing technique. The resulting LIG also exhibits a high surface area and is porous with a high density of edge plane sites, which are beneficial for biofunctionalization and heterogeneous charge transport during electrochemical sensing. Furthermore, we have demonstrated that the surface area and wettability of the LIG can be tuned to greatly improve the sensitivity of electrochemical enzymatic biosensors (detection limits down to the picomolar range) and reduce the water layer buildup between the ion-selective membrane and the electrode, which improves the accuracy of the sensor readings. The fabricated LIG electrodes have been used in a wide variety of electrochemical sensing applications, including in food safety (foodborne pathogens and other contaminants), environmental monitoring (agrochemicals), and health monitoring sensors (salivary potassium and lactate, and urinary potassium and ammonium). Different electroanalytical methods are used, including amperometric, potentiometric, coulometric, and impedimetric, to quantify the analytes. We demonstrate how the ISE (ion-selective electrodes) can be functionalized with PVC-based ion-selective membranes for potassium, nitrate, nitrite, ammonium, sodium, and pH in various biological solutions, including soil and water for nutrient/fertilizer monitoring in farm fields. Additionally, we demonstrate how the hydrophilic LIG graphene can be used to create other highly sensitive electrochemical biosensors including enzymatic pesticide biosensors and Salmonella spp. immunosensors. All these electroanalytical sensors were validated in real-world samples and demonstrated to have limits of detection and linear sensing ranges that are relevant to their sensing applications. Results demonstrate the utility of LIG-based sensors for rapid and sensitive electrochemical measurement of contaminants as a readily modified platform for scalable production of electroanalytical devices applied toward point-of-service environmental and food monitoring.

Flexible Graphene-Based Sensors for Electrochemical Analysis of Human Saliva and Sweat

Flexible carbon nanomaterial sensors offer a low-cost, facile fabrication approach toward producing sensors at an industrial scale for electrochemical sensing. Graphene and carbon nanotube (CNT) based sensors have been studied, with various fabrication methods available, including producing high-quality thin film graphene and CNTs via chemical vapor deposition (CVD). However, CVD is a high-temperature vacuum process that is low-yielding and consequently costly, hindering its implementation into single-use test strip sensors. Other methods, such as inkjet or aerosol printing, are more cost-effective as they print graphene inks obtained from liquid-phase exfoliation of graphite, which can be performed using large batch processing techniques. Additionally, the laser-induced graphene (LIG) technique directly converts sp3 carbon found in carbon-rich substrates, such as polyimide, into conductive sp2-hybridized carbon found in graphene through a laser scribing technique. Saliva and sweat offer promising biological fluids for precision health monitoring and diagnostics due to the non-invasive nature of their sampling (no need for a blood draw at a laboratory or a fingerstick at home) and due to the abundance of medically relevant analytes. Saliva can also be used for monitoring infections such as COVID-19 infection. Herein, we introduce graphene-based flexible sensors fabricated using scalable manufacturing based on aerosol jet printing (AJP) using custom-formulated graphene inks and laser-induced graphene via direct writing. We report a label-free, high-sensitivity, and rapid-response-time graphene-based electrochemical immunosensor for quantitatively detecting the SARS-CoV-2 Spike Receptor-Binding Domain (RBD) as well as SARS-CoV-2 Spike S1 protein. We demonstrate LIG-based electrochemical sensors for real-time monitoring of glucose, lactate, and sodium in sweat, and potassium and lactate in saliva. Different electrochemical methods are used, including amperometric, potentiometric, coulometric, and impedimetric, to quantify the analytes. All these electrochemical sensors were validated in human saliva and sweat and demonstrated to have limits of detection and linear sensing ranges that are relevant to their sensing applications. Flexibility studies were also performed to evaluate the ability of the sensors to operate under stresses caused by bending, such as those that would be encountered by skin and implanted oral sensors. Results demonstrate the utility of AJP-graphene and LIG for rapid and sensitive measurement of biological analytes and pathogen infection as simple methods for scalable production of non-invasive salivary and perspiration (sweat) health sensors.

Exploring New Carbon Allotropes and Nanographenes on Surfaces

The quest for planar sp2-hybridized carbon allotropes other than graphene has stimulated substantial research efforts because of their predicted unique mechanical, electronic, and transport properties. However, the syntheses of these nonbenzenoid networks remain challenging due to the lack of reliable protocols for generating nonhexagonal rings. We have developed various on-surface synthesis strategies by which straight polymer chains are linked to form the nonbenzenoid carbon networks. In this way, we synthesized biphenylene network, a new carbon allotrope with 4-6-8-membered rings, which is metallic already at very small dimensions. Similarly, we generated phagraphene nanoribbons, which are based on 5-6-7-membered rings.Acenes are another interesting class of carbon materials that have fascinated chemists and physicists due to their potential for use in organic electronic applications. We show the synthesis of tridecacene (13ac) and pentadecacene (15ac), the longest acenes known to date, via multistep single-molecule manipulation of suitable precursor molecules on Au(111). We find antiferromagnetic open-shell ground state electron configurations for both acenes. 15ac shows a low-bias spin-excitation feature, indicating a singlet-triplet gap of around 124 meV. Investigation of 15ac complexes with up to 6 gold atoms suggest considerable multiradical contributions to the electronic ground state of 15ac.Altering the electronic and magnetic properties of carbon-based nanomaterials can also be achieved by doping with heteroatoms. We present an access to a variety of nitrogen-containing 0D and 1D carbon nanostructures including planar and curved cycloarenes with different cavities as well as N-doped graphene nanoribbons.

Evolution of macromolecular structure during coal oxidation via FTIR, XRD and Raman

The analysis of the macromolecular structure and morphology in coal during oxidation is the basis to explore the mechanism of spontaneous combustion. To explore the evolutionary rules of coal macromolecular structure during oxidation, Fourier Transform Infrared Spectroscopy (FTIR), X-ray Diffraction (XRD), and Raman Spectroscopy (Raman) were employed to analyze the coal samples with different oxidation degrees. The results revealed that the oxidation action led to the decrease of the aliphatic structures and aromatic hydroxyl groups in coal, while promoting the formation of oxygen-containing functional groups and aromatic structures. It also led to a relative increase of free hydroxyl groups linked to hydrogen bonds. The aromatic layer spacing (d002) decreased with increasing oxidation degree, while the microcrystal stacking height (Lc), the aromatic layer diameter (La), the average number of crystal stacking layers (n) generally increased. It indicated that small aromatic ring molecules in coal could undergo continuous polymerization during oxidation to form a single aromatic layer structure. The variation of Raman spectrum parameters exhibited a consistent decreasing trend in WD/WG, ID/IG, AD/AG, and A(GR+SL)/AG value, indicating an increase in the vibration of sp2 hybridization carbon atoms within the lattice structure of coal. Conversely, PG-D, AS/AD and A(GR+VL+VR)/AD value increased overall, suggesting that small aromatic rings decreased in content during oxidation while polymerizing into larger aromatic rings. The coal structure underwent a brief stage of disordered evolution during oxidation, followed by removal of impurity structures and condensation of aromatic structures due to increasing oxidation temperatures, ultimately leading to a highly ordered crystalline state. The oxidation process significantly influenced the development of coal's aromatic structure, particularly in less metamorphic coal. The research findings provide a theoretical basis for analyzing the underlying mechanism behind spontaneous combustion induced by coal oxidation.

Novel Superhard Tetragonal Hybrid sp3/sp2 Carbon Allotropes Cx (x = 5, 6, 7): Crystal Chemistry and Ab Initio Studies

Novel superhard tetragonal carbon allotropes C5, C6, and C7, characterized by the presence of sp3- and sp2-like carbon sites, have been predicted from crystal chemistry and extensively studied by quantum density functional theory (DFT) calculations. All new allotropes were found to be cohesive, with crystal densities and cohesive energies decreasing along the C5-C6-C7 series due to the greater openness of the structures resulting from the presence of C=C ethene and C=C=C propadiene subunits, and they were mechanically stable, with positive sets of elastic constants. The Vickers hardness evaluated by different models qualifies all allotropes as superhard, with Hv values ranging from 90 GPa for C5 to 79 GPa for C7. Phonon band structures confirm that the new allotropes are also dynamically stable. The electronic band structures reveal their metallic-like behavior due to the presence of sp2-hybridized carbon.