Rapid and Direct Conversion of Graphite Crystals into High-Yielding, Good-Quality Graphene by Supercritical Fluid Exfoliation

Transition Metal Dichalcogenides Nanoscrolls: Preparation and Applications

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) nanosheets have shown extensive applications due to their excellent physical and chemical properties. However, the low light absorption efficiency limits their application in optoelectronics. By rolling up 2D TMDCs nanosheets, the one-dimensional (1D) TMDCs nanoscrolls are formed with spiral tubular structure, tunable interlayer spacing, and opening ends. Due to the increased thickness of the scroll structure, the light absorption is enhanced. Meanwhile, the rapid electron transportation is confined along the 1D structure. Therefore, the TMDCs nanoscrolls show improved optoelectronic performance compared to 2D nanosheets. In addition, the high specific surface area and active edge site from the bending strain of the basal plane make them promising materials for catalytic reaction. Thus, the TMDCs nanoscrolls have attracted intensive attention in recent years. In this review, the structure of TMDCs nanoscrolls is first demonstrated and followed by various preparation methods of the TMDCs nanoscrolls. Afterwards, the applications of TMDCs nanoscrolls in the fields of photodetection, hydrogen evolution reaction, and gas sensing are discussed.

Influence of Al-O and Al-C Clusters on Defects in Graphene Nanosheets Derived from Coal-Tar Pitch via Al4C3 Precursor

By treating Al₄C₃ as the precursor and growth environment, graphene nanosheets (GNs) can efficiently be derived from coal-tar pitch, which has the advantages of simple preparation process, high product quality, green environmental protection, low equipment requirements and low preparation cost. However, the defects in the prepared GNs have not been well understood. In order to optimize the preparation process, based on density functional theory calculations, the influence mechanism of Al-O and Al-C clusters on defects in GNs derived from coal-tar pitch via Al₄C₃ precursor has been systematically investigated. With minute quantities of oxygen-containing defects, Al-O and Al-C clusters have been realized in the prepared GNs from X-ray photoelectron spectroscopy analysis. Therefore, the influences of Al-O and Al-C clusters on graphene with vacancy defects and oxygen-containing defects are systematically explored from theoretical energy, electron localization function and charge transfer analysis. It is noted that the remaining Al-O and Al-C clusters in GNs are inevitably from the thermodynamics point of view. On the other hand, the existence of defects is beneficial for the further adsorption of Al-O and Al-C clusters in GNs.

Advances in Studies of Boron Nitride Nanosheets and Nanocomposites for Thermal Transport and Related Applications

Hexagonal boron nitride (h-BN) and exfoliated nanosheets (BNNs) not only resemble their carbon counterparts graphite and graphene nanosheets in structural configurations and many excellent materials characteristics, especially the ultra-high thermal conductivity, but also offer other unique properties such as being electrically insulating and extreme chemical stability and oxidation resistance even at elevated temperatures. In fact, BNNs as a special class of 2-D nanomaterials have been widely pursued for technological applications that are beyond the reach of their carbon counterparts. Highlighted in this article are significant recent advances in the development of more effective and efficient exfoliation techniques for high-quality BNNs, the understanding of their characteristic properties, and the use of BNNs in polymeric nanocomposites for thermally conductive yet electrically insulating materials and systems. Major challenges and opportunities for further advances in the relevant research field are also discussed.

Supercritically exfoliated Bi2Se3 nanosheets for enhanced photocatalytic hydrogen production by topological surface states over TiO2

Owing to the unique electronic properties of layered materials, topological insulators have interestingly grabbed much attention in the field of photocatalytic water splitting. Nowadays, 2D layered materials were composited with semiconductor photocatalysts, encourage much as it provides enormous active sites and also significantly prevent photogenerated charge recombination. Especially, Bi₂Se₃ possesses exceptional properties like topologically preserved conducting surface states with bulk insulating behavior and high surface area, which provides unconventional electron dynamics, resulting in facile electron transport and effective charge separation to photocatalyst. So far, several methods have been attempted to synthesize few-layered Bi₂Se₃ nanosheets from its bulk crystals. Here, a unique attempt is made and succeeded to exfoliate bulk Bi₂Se₃ to few layered nanosheets via surfactant free supercritical fluid processing using N-Methyl-2-pyrrolidone (NMP) as an exfoliating agent, with a short reaction time of 15 min. The exfoliation of Bi₂Se₃ crystal was confirmed by several characterization techniques, such as XRD, SEM, Raman, and HR-TEM. Furthermore, different weight percentages of exfoliated Bi₂Se₃ sheets/anatase TiO₂ nanoparticles were prepared and examined the photocatalytic activity using glycerol as a hole scavenger. Among them, 15 wt.% Bi₂Se₃ coupled TiO₂ nanocomposite showed enormous hydrogen evolution rate of 84.9 mmol h^-1g^-1_cat, which is 80 times higher than that of TiO₂ nanoparticles. In addition, the photostability of the nanocomposite was also verified, where it retains 94% of activity even after 4 cycles of continuous experiments. The improved rate of H₂ production was understood by theoretical calculations that topologically preserved conducting surface states of co-catalyst, Bi₂Se₃ nanosheets is supported to high mobile and scatter free electrons. It mediates the transport of electrons with TiO₂ nanoparticles that helped the effective charge separation. Thus, it proves a promising candidate for photocatalytic hydrogen production.

Advanced Materials for Energy-Water Systems: The Central Role of Water/Solid Interfaces in Adsorption, Reactivity, and Transport

The structure, chemistry, and charge of interfaces between materials and aqueous fluids play a central role in determining properties and performance of numerous water systems. Sensors, membranes, sorbents, and heterogeneous catalysts almost uniformly rely on specific interactions between their surfaces and components dissolved or suspended in the water-and often the water molecules themselves-to detect and mitigate contaminants. Deleterious processes in these systems such as fouling, scaling (inorganic deposits), and corrosion are also governed by interfacial phenomena. Despite the importance of these interfaces, much remains to be learned about their multiscale interactions. Developing a deeper understanding of the molecular- and mesoscale phenomena at water/solid interfaces will be essential to driving innovation to address grand challenges in supplying sufficient fit-for-purpose water in the future. In this Review, we examine the current state of knowledge surrounding adsorption, reactivity, and transport in several key classes of water/solid interfaces, drawing on a synergistic combination of theory, simulation, and experiments, and provide an outlook for prioritizing strategic research directions.

Recent Advances in Transition Metal Dichalcogenide Cathode Materials for Aqueous Rechargeable Multivalent Metal-Ion Batteries

The generation of renewable energy is a promising solution to counter the rapid increase in energy consumption. Nevertheless, the availability of renewable resources (e.g., wind, solar, and tidal) is non-continuous and temporary in nature, posing new demands for the production of next-generation large-scale energy storage devices. Because of their low cost, highly abundant raw materials, high safety, and environmental friendliness, aqueous rechargeable multivalent metal-ion batteries (AMMIBs) have recently garnered immense attention. However, several challenges hamper the development of AMMIBs, including their narrow electrochemical stability, poor ion diffusion kinetics, and electrode instability. Transition metal dichalcogenides (TMDs) have been extensively investigated for applications in energy storage devices because of their distinct chemical and physical properties. The wide interlayer distance of layered TMDs is an appealing property for ion diffusion and intercalation. This review focuses on the most recent advances in TMDs as cathode materials for aqueous rechargeable batteries based on multivalent charge carriers (Zn2+, Mg2+, and Al3+). Through this review, the key aspects of TMD materials for high-performance AMMIBs are highlighted. Furthermore, additional suggestions and strategies for the development of improved TMDs are discussed to inspire new research directions.

A Review on van der Waals Boron Nitride Quantum Dots

Boron nitride quantum dots (BNQDs) have gained increasing attention for their versatile fluorescent, optoelectronic, chemical, and biochemical properties. During the past few years, significant progress has been demonstrated, started from theoretical modeling to actual application. Many interesting properties and applications have been reported, such as excitation-dependent emission (and, in some cases, non-excitation dependent), chemical functionalization, bioimaging, phototherapy, photocatalysis, chemical, and biological sensing. An overview of this early-stage research development of BNQDs is presented in this article. We have prepared un-bias assessments on various synthesis methods, property analysis, and applications of BNQDs here, and provided our perspective on the development of these emerging nanomaterials for years to come.

Graphene and Reproduction: A Love-Hate Relationship

Since its discovery, graphene and its multiple derivatives have been extensively used in many fields and with different applications, even in biomedicine. Numerous efforts have been made to elucidate the potential toxicity derived from their use, giving rise to an adequate number of publications with varied results. On this basis, the study of the reproductive function constitutes a good tool to evaluate not only the toxic effects derived from the use of these materials directly on the individual, but also the potential toxicity passed on to the offspring. By providing a detailed scientometric analysis, the present review provides an updated overview gathering all the research studies focused on the use of graphene and graphene-based materials in the reproductive field, highlighting the consequences and effects reported to date from experiments performed in vivo and in vitro and in different animal species (from Archea to mammals). Special attention is given to the oxidized form of graphene, graphene oxide, which has been recently investigated for its ability to increase the in vitro fertilization outcomes. Thus, the potential use of graphene oxide against infertility is hypothesized here, probably by engineering the spermatozoa and thus manipulating them in a safer and more efficient way.

Functional inks and extrusion-based 3D printing of 2D materials: a review of current research and applications

Graphene and related 2D materials offer an ideal platform for next generation disruptive technologies and in particular the potential to produce printed electronic devices with low cost and high throughput. Interest in the use of 2D materials to create functional inks has exponentially increased in recent years with the development of new ink formulations linked with effective printing techniques, including screen, gravure, inkjet and extrusion-based printing towards low-cost device manufacturing. Exfoliated, solution-processed 2D materials formulated into inks permits additive patterning onto both rigid and conformable substrates for printed device design with high-speed, large-scale and cost-effective manufacturing. Each printing technique has some sort of clear advantages over others that requires characteristic ink formulations according to their individual operational principles. Among them, the extrusion-based 3D printing technique has attracted heightened interest due to its ability to create three-dimensional (3D) architectures with increased surface area facilitating the design of a new generation of 3D devices suitable for a wide variety of applications. There still remain several challenges in the development of 2D material ink technologies for extrusion printing which must be resolved prior to their translation into large-scale device production. This comprehensive review presents the current progress on ink formulations with 2D materials and their broad practical applications for printed energy storage devices and sensors. Finally, an outline of the challenges and outlook for extrusion-based 3D printing inks and their place in the future printed devices ecosystem is presented.

Exfoliation of 2D Materials for Energy and Environmental Applications

The fascinating properties of single-layer graphene isolated by mechanical exfoliation have inspired extensive research efforts toward two-dimensional (2D) materials. Layered compounds serve as precursors for atomically thin 2D materials (briefly, 2D nanomaterials) owing to their strong intraplane chemical bonding but weak interplane van der Waals interactions. There are newly emerging 2D materials beyond graphene, and it is becoming increasingly important to develop cost-effective, scalable methods for producing 2D nanomaterials with controlled microstructures and properties. The variety of developed synthetic techniques can be categorized into two classes: bottom-up and top-down approaches. Of top-down approaches, the exfoliation of bulk 2D materials into single or few layers is the most common. This review highlights chemical and physical exfoliation methods that allow for the production of 2D nanomaterials in large quantities. In addition, remarkable examples of utilizing exfoliated 2D nanomaterials in energy and environmental applications are introduced.

Exfoliation and stabilization mechanism of graphene in carbon dioxide expanded organic solvents: molecular dynamics simulations

CO₂ expanded organic solvents possess significant advantages in liquid-phase exfoliation to obtain monolayer/few-layer graphene from graphite. Further insights into the mechanism of graphene exfoliation in such solvents are essential to explore liquid-phase dispersion of graphene as a more potent alternative to chemical vapor deposition. In this study, dynamic processes of exfoliation and stabilization of graphene in CO₂-N,N-dimethylformamide (DMF), CO₂-N-methylpyrrolidone (NMP), CO₂-dimethyl sulfoxide (DMSO), and CO₂-ethanol (EtOH) were investigated using molecular dynamics simulations. The origin of the effect of each solvent on graphene exfoliation was analyzed quantitatively through potential mean force simulations. It has been found that the organic solvent in a CO₂ expanded solvent should be chosen with proper surface tension, and there exist two different graphene exfoliation processes in the effective solvents, which can be described as "burger dissociation" and "extrusion-taking away" processes, respectively. In the former process, a characteristic "super-burger-like" conformation with a semi-exfoliated structure was formed, which was the deciding factor to obtain high ratio of monolayer/few-layer graphene in dispersion product. A theoretical explanation has also been provided at the molecular level to the earlier experimental phenomena. A predicted simulation of the CO₂-3,3'-iminobis(N,N-dimethylpropylamine) (DMPA) system is also calculated. This investigation helps to avoid incompatible CO₂ expanded organic solvents employed in the experimental studies and provides theoretical clues to understand the mechanism of exfoliation and stabilization of graphene in such solvents.

Investigation of and mechanism proposal for solvothermal reaction between sodium and 1-(2-hydroxyethyl)piperidine as the first step towards nitrogen-doped graphenic foam synthesis

Solvothermal reaction involving 1-(2-hydroxyethyl)piperidine and sodium: a promising step in the synthesis of high surface area N-doped graphenic materials.

Dissipation enhancement effect from titania semiconductor modulation of graphene-based electromagnetic absorbing composites

A scheme of semiconductor modulation of electromagnetic (EM) absorbing materials is proposed. Homogeneous composites as ideal media are obtained by uniformly mixing fillers using supercritical fluid (SCF) processing. Based on the ideal media, titania modulation design prediction of surface shield materials is confirmed for the EM composites of graphene nanosheets and manganese oxides. A low ratio of titania between 10 percent and 50 percent gives rise to dissipation enhancement in the absorbers. Simulation of the involved cases displays the EM fields in the composite absorbers, demonstrating EM energy loss in the absorbers. Frequency modulation using titania improves the electromagnetic compatibility (EMC) of the absorbing materials in the X and K_u bands. Rapid exfoliation and full premixing of the components by SCF contribute to the modulation scheme confirmation with uniform media. Semiconductor titania modulation of EM-absorbing composites is effective.

Eddy current occupying contribution to EMC absorbers with semiconductor modulation due to uniformity from SCF-assisted mixing.

Method of ultrasound-assisted liquid-phase exfoliation to prepare graphene

Graphene is a two-dimensional material with unique structure and excellent properties. After first being successfully prepared in 2004, it rapidly became a research hotspot in the fields of materials, chemistry, physics, and engineering. Currently, there are many methods for preparing graphene, such as ball milling method, chemical oxidation-reduction, chemical vapor deposition, and liquid-phase exfoliation. Among these methods, liquid-phase exfoliation is the most important preparation method. In this paper, ultrasound-assisted liquid-phase exfoliation is systematically studied. The output power and frequency of the ultrasonic crusher used in the experiment are 100 W and 20 kHz, respectively. Results show that ultrasonic waves can affect the size and thickness distribution of graphene sheets; ultrasound-assisted deoxycholic acid sodium aqueous solution has a good exfoliation effect. In addition, the effects of the 3 liquid-phase systems on preparing graphene are studied, including organic solvent system, aqueous surfactant system, and ionic liquids system; the improvement efforts for ultrasound-assisted liquid-phase exfoliation method are discussed including the exploration of new solvents and optimization of exfoliation process. The application of auxiliary agent-assisted liquid-phase exfoliation method is also discussed.

Synergistic effect of supercritical CO2 and organic solvent on exfoliation of graphene: experiment and atomistic simulation studies

In this work, experiments and molecular dynamics (MD) simulations are carried out to explore the synergistic effect of supercritical CO2 (scCO2) and organic solvent on intercalation and exfoliation of graphene. Experimental characterizations via transmission electron microscopy, atomic force microscopy and Raman spectroscopy indicate that by combining scCO2 and organic solvent (N-methylpyrrolidone, NMP), few-layer graphene is successfully exfoliated from graphite, among which over 30% is 1-4 layers, and 55% is 5-8 layers. Systematic experiments have shown that compared with pure scCO2 or NMP, the mixed scCO2 and NMP can significantly increase the amount of graphene and the rate of few-layer graphene, and the optimum volume fraction of NMP is 25%. Parallel MD simulations indicate that the scCO2 molecules first diffuse into the interlayer of graphite, and then the larger NMP molecules insert as wedges and further expand interlayer spacing, promoting intercalation and exfoliation. The iteration of scCO2 diffusion and the NMP wedge can generate positive feedback to improve the exfoliation productivity and efficiency. This work explores the synergistic effect of scCO2 and NMP on the exfoliation of graphene, which may provide useful insights for exfoliation of other two dimensional materials.

Supercritical Fluid-Facilitated Exfoliation and Processing of 2D Materials

Since the first intercalation of layered silicates by using supercritical CO2 as a processing medium, considerable efforts have been dedicated to intercalating and exfoliating layered two-dimensional (2D) materials in various supercritical fluids (SCFs) to yield single- and few-layer nanosheets. Here, recent work in this area is highlighted. Motivating factors for enhancing exfoliation efficiency and product quality in SCFs, mechanisms for exfoliation and dispersion in SCFs, as well as general metrics applied to assess quality and processability of exfoliated 2D materials are critically discussed. Further, advances in formation and application of 2D material-based composites with assistance from SCFs are presented. These discussions address chemical transformations accompanying SCF processing such as doping, covalent surface modification, and heterostructure formation. Promising features, challenges, and routes to expanding SCF processing techniques are described.

Supercritical water assisted preparation of recyclable gold nanoparticles and their catalytic utility in cross-coupling reactions under sustainable conditions

Preparation of gold nanoparticles (AuNPs) in environmentally friendly water without using any reducing agents under supercritical conditions is demonstrated. PXRD, XPS, FE-SEM and HR-TEM analysis confirmed the formation of phase-pure and crystalline AuNPs of the size of ∼10-30 nm. The catalytic potential of AuNPs was manifested through a generalized green procedure that could accommodate both Sonogashira as well as Suzuki coupling under aqueous conditions at low catalytic loading (0.1 mol%). The AuNP catalyst was found to be recuperated after the reaction and reused for up to six catalytic cycles with no leaching out of gold species as confirmed through ICP-OES analysis. With no confinement of AuNP catalysis to cross-coupling reaction, synthetic extension to one-flask preparation of π-conjugated semiconductors (4 examples) and their optoelectronic properties were also investigated. Other significant features of the present work include short reaction time, site-selectivity, wide substrate scope, high conversion, good chemical yields and applicability in gram-scale synthesis. Overall, the results of this paper signify an operationally sustainable supercritical fluid processing method for the synthesis of AuNPs and their catalytic application towards cross-coupling reactions in green media.

Liquid-Phase Exfoliation of Graphene: An Overview on Exfoliation Media, Techniques, and Challenges

Graphene, a two-dimensional (2D) carbon nanomaterial, has attracted worldwide attention owing to its fascinating properties. One of critical bottlenecks on some important classes of applications, such as printed electronics, conductive coatings, and composite fillers, is the lack of industrial-scale methods to produce high-quality graphene in the form of liquid suspensions, inks, or dispersions. Since 2008, when liquid-phase exfoliation (LPE) of graphene via sonication was initiated, huge progress has been made in the past decade. This review highlights the latest progress on the successful preparation of graphene in various media, including organic solvents, ionic liquids, water/polymer or surfactant solutions, and some other green dispersants. The techniques of LPE, namely sonication, high-shear mixing, and microfluidization are reviewed subsequently. Moreover, several typical devices of high-shear mixing and exfoliation mechanisms are introduced in detail. Finally, we give perspectives on future research directions for the development of green exfoliation media and efficient techniques for producing high-quality graphene. This systematic exploratory study of LPE will potentially pave the way for the scalable production of graphene, which can be also applied to produce other 2D layered materials, such as BN, MoS₂, WS₂, etc.

CO2 -Assisted Conversion of Crystal Two-Dimensional Molybdenum Oxide to Amorphism with Plasmon Resonances

Localized surface plasmon resonances (LSPRs) of ultra-thin two-dimensional (2D) nanomaterials have opened up a new regime in plasmonics in the last several years. 2D plasmonic materials are currently concentrated on the crystal structure, with amorphous materials hardly being reported because of their limited preparation methods rather than undesired plasmonic properties. Taking molybdenum oxides as an example, herein, we elaborate the 2D amorphous plasmons prepared with the assistance of supercritical CO₂ . In brief, we examine the reported characteristic plasmonic properties of molybdenum oxides, and applications of supercritical CO₂ in formations of 2D layer materials as well as introduced phase and disorder engineering based on our research. Furthermore, we propose our perspective on the development of 2D plasmons, especially for amorphous layer materials in the future.

High-Efficiency Production of Graphene by Supercritical CO2 Exfoliation with Rapid Expansion

In this study, direct nonequilibrium molecular dynamics simulations based on the density-functional tight-binding potential were performed to investigate the mechanism of graphite exfoliation by supercritical CO₂ in the depressurization process. We found that the graphite peeling rate and the graphene yield depended on the number of inserted CO₂ molecules in our simulations, and the appropriate pressure or density of CO₂ is a prerequisite to achieve graphite exfoliation. Our theoretical results proposed that the graphite peeling occurred till the pressure or the density of CO₂ was larger than 12.2 MPa or 0.21 g/cm³. This is confirmed by the experimental observations. Furthermore, we declared that the essential effect of the pressure or density of CO₂ was attributed to the competition between the van der Waals attraction in the graphite interlayer and repulsion of CO₂ and graphite, which resulted from the steric hinder effect. The current theoretical observations provide potential scientific evidence to control graphite exfoliation by supercritical CO₂.

Ultrasound coupled with supercritical carbon dioxide for exfoliation of graphene: Simulation and experiment

Ultrasound coupled with supercritical CO₂ has become an important method for exfoliation of graphene, but behind which a peeling mechanism is unclear. In this work, CFD simulation and experiment were both investigated to elucidate the mechanism and the effects of the process parameters on the exfoliation yield. The experiments and the CFD simulation were conducted under pressure ranging from 8MPa to 16MPa, the ultrasonic power ranging from 12W to 240W and the frequency of 20kHz. The numerical analysis of fluid flow patterns and pressure distributions revealed that the fluid shear stress and the periodical pressure fluctuation generated by ultrasound were primary factors in exfoliating graphene. The distribution of the fluid shear stress decided the effective exfoliation area, which, in turn, affected the yield. The effective area increased from 5.339cm³ to 8.074cm³ with increasing ultrasonic power from 12W to 240W, corresponding to the yield increasing from 5.2% to 21.5%. The pressure fluctuation would cause the expansion of the interlayers of graphite. The degree of the expansion increased with the increase of the operating pressure but decreased beyond 12MPa. Thus, the maximum yield was obtained at 12MPa. The cavitation might be generated by ultrasound in supercritical CO₂. But it is too weak to exfoliate graphite into graphene. These results provide a strategy in optimizing and scaling up the ultrasound-assisted supercritical CO₂ technique for producing graphene.

Scalable exfoliation and dispersion of two-dimensional materials - an update

The preparation of dispersions of single- and few-sheet 2D materials in various solvents, as well as the characterization methods applied to such dispersions, is critically reviewed. Motivating factors for producing single- and few-sheet dispersions of 2D materials in liquids are briefly discussed. Many practical applications are expected for such materials that do not require high purity formulations and tight control of donor and acceptor concentrations, as required in conventional Fab processing of semiconductor chips. Approaches and challenges encountered in exfoliating 2D materials in liquids are reviewed. Ultrasonication, mechanical shearing, and electrochemical processing approaches are discussed, and their respective limitations and promising features are critiqued. Supercritical and more conventional liquid and solvent processing are then discussed in detail. The effects of various types of stabilizers, including surfactants and other amphiphiles, as well as polymers, including homopolymeric electrolytes, nonionic polymers, and nanolatexes, are discussed. Consideration of apparent successes of stabilizer-free dispersions indicates that extensive exfoliation in the absence of dispersing aids results from processing-induced surface modifications that promote stabilization of 2D material/solvent interactions. Also apparent paradoxes in "pristineness" and optical extinctions in dispersions suggest that there is much we do not yet quantitatively understand about the surface chemistry of these materials. Another paradox, emanating from modeling dilute solvent-only exfoliation by sonication using polar components of solubility parameters and surface tension for pristine graphene with no polar structural component, is addressed. This apparent paradox appears to be resolved by realizing that the reactivity of graphene to addition reactions of solvent radicals produced by sonolysis is accompanied by unintended polar surface modifications that promote attractive interactions with solvent. This hypothesis serves to define important theoretical and experimental studies that are needed. We conclude that the greatest promise for high volume and high concentration processing lies in applying methods that have not yet been extensively reported, particularly wet comminution processing using small grinding media of various types.

Shear-Assisted Production of Few-Layer Boron Nitride Nanosheets by Supercritical CO2 Exfoliation and Its Use for Thermally Conductive Epoxy Composites

Boron nitride nanosheets (BNNS) hold the similar two-dimensional structure as graphene and unique properties complementary to graphene, which makes it attractive in application ranging from electronics to energy storage. The exfoliation of boron nitride (BN) still remains challenge and hinders the applications of BNNS. In this work, the preparation of BNNS has been realized by a shear-assisted supercritical CO₂ exfoliation process, during which supercritical CO₂ intercalates and diffuses between boron nitride layers, and then the exfoliation of BN layers is obtained in the rapid depressurization process by overcoming the van der Waals forces. Our results indicate that the bulk boron nitride has been successfully exfoliated into thin nanosheets with an average 6 layers. It is found that the produced BNNS is well-dispersed in isopropyl alcohol (IPA) with a higher extinction coefficient compared with the bulk BN. Moreover, the BNNS/epoxy composite used as thermal interface materials has been prepared. The introduction of BNNS results in a 313% enhancement in thermal conductivity. Our results demonstrate that BNNS produced by supercritical CO₂ exfoliation show great potential applications for heat dissipation of high efficiency electronics.

Enhanced Reduction of Few-Layer Graphene Oxide via Supercritical Water Gasification of Glycerol

A sustainable and effective method for de-oxygenation of few-layer graphene oxide (FLGO) by glycerol gasification in supercritical water (SCW) is described. In this manner, reduction of FLGO and valorization of glycerol, in turn catalyzed by FLGO, are achieved simultaneously. The addition of glycerol enhanced FLGO oxygen removal by up to 59% due to the in situ hydrogen generation as compared to the use of SCW only. Physicochemical characterization of the reduced FLGO (rFLGO) showed a high restoration of the sp²-conjugated carbon network. FLGO sheets with a starting C/O ratio of 2.5 are reduced by SCW gasification of glycerol to rFLGO with a C/O ratio of 28.2, above those reported for hydrazine-based methods. Additionally, simultaneous glycerol gasification resulted in the concurrent production of H₂, CO, CH₄ and valuable hydrocarbons such as alkylated and non-alkylated long chain hydrocarbon (C12–C31), polycyclic aromatic hydrocarbons (PAH), and phthalate, phenol, cresol and furan based compounds.

Graphene Ingestion and Regrowth on "Carbon-Starved" Metal Electrodes

The interaction between graphene and various metals plays a central role in future carbon-based device and synthesis technologies. Herein, three different types of metal nanoelectrodes (W, Ni, Au) were employed to in situ study the graphene-metal interfacial kinetic behaviors in a high-resolution transmission electron microscope. The three metals exhibit distinctly different interactions with graphene when driven by a heating current. Tungsten tips, the most carbon-starved ones, can ingest a graphene sheet continuously; nickel tips, less carbon starved, typically "eat" graphene only by taking a "bite" from its edge; gold, however, is nonactive with graphene at all, even in its molten state. The ingested graphene atoms finally precipitate as freshly formed graphitic shells encapsulating the catalytic W and Ni electrodes. Particularly, we propose a periodic extension/thickening graphene growth scenario by atomic-scale observation of this process on W electrodes, where the propagation of the underlying tungsten carbide (WC) dominates the growth dynamics. This work uncovers the complexity of carbon diffusion/segregation processes at different graphene/metal interfaces that would severely degrade the device performance and stability. Besides, it also provides a detailed and insightful understanding of the sp² carbon catalytic growth, which is vital in developing efficient and practical graphene synthetic routes.

Production of graphene quantum dots by ultrasound-assisted exfoliation in supercritical CO2/H2O medium

In this research, three kinds of graphene quantum dots (GQDs)-pristine graphene quantum dots (PGQDs), expanded graphene quantum dots (EGQDs) and graphene oxide quantum dots (GOQDs)-were produced from natural graphite, expanded graphite, and oxide graphite respectively in an ultrasound-assisted supercritical CO₂ (scCO₂)/H₂O system. The effects of aqueous solution content ratio, system pressure, and ultrasonic power on the yields of different kinds of GQDs were investigated. According to these experiment results, the combination of the intense knocking force generated from high-pressure acoustic cavitation in a scCO₂/H₂O system and the superior penetration ability of scCO₂ was considered to be the key to the successful exfoliation of such tiny pieces from bulk graphite. An interesting result was found that, contrary to common experience, the yield of PGQDs from natural graphite was much higher than that of GOQDs from graphite oxide. Based on the experimental analysis, the larger interlayer resistance of natural graphite, which hindered the insertion of scCO₂ molecules, and the hydrophobic property of natural graphite surface, which made the planar more susceptible to the attack of ultrasonic collapsing bubbles, were deduced to be the two main reasons for this result. The differences in characteristics among the three kinds of GQDs were also studied and compared in this research. In our opinion, this low-cost and time-saving method may provide an alternative green route for the production of various kinds of GQDs, especially PGQDs.

Enhanced Superhydrophobic Performance of BN-MoS2 Heterostructure Prepared via a Rapid, One-Pot Supercritical Fluid Processing

Fabrication of highly crystalline BN-MoS₂ heterostructure with >95% yield was demonstrated using one-pot supercritical fluid processing within 30 min. The existence of 20-50 layers of BN-MoS₂ in the prepared heterostructure was confirmed by AFM analysis. The HR-TEM imaging and mapping analysis revealed the well-melded BN and MoS₂ nanosheets in the heterostructure. The drastic reduction in XRD line intensities corresponding to the (002) plane and broadening of the peaks for the BN system over MoS₂ indicated the effective exfoliation and lateral size reduction in BN nanosheets during SCF processing. Also, the exfoliated MoS₂ nanosheets are preferentially exposed rather than BN nanosheets; consequently, the MoS₂ nanosheets sturdily covered BN nanosheets in the heterostructure. The exfoliated BN and MoS₂ nanosheets with nanoscale roughness make the surface highly hydrophobic in nature. As a result, the BN-MoS₂ heterostructure showed superior superhydrophobic performance with high water contact angle of 165.9°, which is much higher than the value reported in the literature.

Structure-Based Selective Adsorption of Graphene on a Gel Surface: Toward Improving the Quality of Graphene Nanosheets

Top-down graphene production via exfoliation from graphite produces a mass of graphene with structural variation in terms of the number of layers, sheet size, edge type, and defect density. All of these characteristics affect its electronic structure. To develop useful applications of graphene, structural separation of graphene is necessary. In this study, we investigate the adsorption behavior of different types of graphene fragments using a multicolumn gel chromatography system with a view to developing an efficient method for separating high-quality graphene. The graphene was dispersed in an aqueous sodium dodecyl sulfate (SDS) surfactant solution and flown through allyl-dextran-based gel columns connected in series. In the chromatographic operation, we observed that the small-sized or oxidized graphene fragments tended to bind to the gel and the relatively large-sized graphene with a low oxygen content eluted from the gel column. In this system, the adsorbed SDS molecules on the graphitic surface prevented graphitic materials from binding to the gel and the oxygen functional groups on the graphene oxide or at the abundant edge of small-sized graphene hindered SDS adsorption. We hypothesize that the reduced SDS adsorption density results in the preferential adsorption of small-sized or oxidized graphene fragments on the gel. This type of chromatographic separation is a cost-effective and scalable method for sorting nanomaterials. The structural separation of graphene based on the adsorption priority found in this study will improve the quality of graphene nanosheets on an industrial scale.

Exfoliated MoS2 and MoSe2 Nanosheets by a Supercritical Fluid Process for a Hybrid Mg-Li-Ion Battery

The ultrathin two-dimensional nanosheets of layered transition-metal dichalcogenides (TMDs) have attracted great interest as an important class of materials for fundamental research and technological applications. Solution-phase processes are highly desirable to produce a large amount of TMD nanosheets for applications in energy conversion and energy storage such as catalysis, electronics, rechargeable batteries, and capacitors. Here, we report a rapid exfoliation by supercritical fluid processing for the production of MoS₂ and MoSe₂ nanosheets. Atomic-resolution high-angle annular dark-field imaging reveals high-quality exfoliated MoS₂ and MoSe₂ nanosheets with hexagonal structures, which retain their 2H stacking sequence. The obtained nanosheets were tested for their electrochemical performance in a hybrid Mg-Li-ion battery as a proof of functionality. The MoS₂ and MoSe₂ nanosheets exhibited the specific capacities of 81 and 55 mA h g^-1, respectively, at a current rate of 20 mA g^-1.

Anchoring of ultrafine Co3O4 nanoparticles on MWCNTs using supercritical fluid processing and its performance evaluation towards electrocatalytic oxygen reduction reaction

Anchoring ultrafine Co₃O₄ nanoparticles on MWCNTs using a supercritical fluid showed high ORR performance.

Supercritical fluid processing for the synthesis of NiS2 nanostructures as efficient electrocatalysts for electrochemical oxygen evolution reactions

A facile supercritical fluid process was demonstrated for the synthesis of cubic NiS₂ nanostructures for efficient electrochemical oxygen evolution reactions.

Electrochemical cycling and beyond: unrevealed activation of MoO3 for electrochemical hydrogen evolution reactions

Electrochemical cycling-induced reduction of α-MoO₃ to monoclinic molybdenum dioxide and molybdenum sub-oxides with excellent electrochemical HER activity has been demonstrated.

Molecular dynamics simulations of solvent-exfoliation and stabilization of graphene with the assistance of compressed carbon dioxide and pyrene–polyethylene glycol

Understanding the solvent-exfoliation and stabilization of graphene with cpCO₂and pyrene–polyethylene glycol from molecular dynamics simulations.

Simultaneous Graphite Exfoliation and N Doping in Supercritical Ammonia

We report the exfoliation of graphite and simultaneous N doping of graphene by two methods: supercritical ammonia treatment and liquid-phase exfoliation with NH₄OH. While the supercritical ammonia allowed N doping at a level of 6.4 atom % in 2 h, the liquid-phase exfoliation with NH₄OH allowed N doping at a level of 2.7 atom % in 6 h. The N doped graphene obtained via the supercritical ammonia route had few layers (<5) and showed large lateral flake size (∼8 μm) and low defect density (I_D/I_G < 0.6) in spite of their high level of N doping. This work is the first demonstration of supercritical ammonia as an exfoliation agent and N doping precursor for graphene. Notably, the N doped graphene showed electrocatalytic activity toward oxygen reduction reaction with high durability and good methanol tolerance compared to those of commercial Pt/C catalyst.

Supercritical Fluid Facilitated Disintegration of Hexagonal Boron Nitride Nanosheets to Quantum Dots and Its Application in Cells Imaging

Preparation of quantum dots (QDs) and exfoliation of two-dimensional layered materials have gathered significant attention in recent days. Though, there are number of attempts have been reported, facile and efficient methodology is yet to be explored. Here, we demonstrate supercritical fluid processing approach for rapid and facile synthesis of blue luminescent BN QDs from layered bulk material via in situ exfoliation followed by disintegration. The microscopic and AFM analysis confirmed the few layer BN QDs formation. The strong luminescent behavior of BN QDs is utilized to stain Gram-negative bacterial cells specifically in the presence of Gram-positive bacterial cells.

High Yield Synthesis of Aspect Ratio Controlled Graphenic Materials from Anthracite Coal in Supercritical Fluids

This paper rationalizes the green and scalable synthesis of graphenic materials of different aspect ratios using anthracite coal as a single source material under different supercritical environments. Single layer, monodisperse graphene oxide quantum dots (GQDs) are obtained at high yield (55 wt %) from anthracite coal in supercritical water. The obtained GQDs are ∼3 nm in lateral size and display a high fluorescence quantum yield of 28%. They show high cell viability and are readily used for imaging cancer cells. In an analogous experiment, high aspect ratio graphenic materials with ribbon-like morphology (GRs) are synthesized from the same source material in supercritical ethanol at a yield of 6.4 wt %. A thin film of GRs with 68% transparency shows a surface resistance of 9.3 kΩ/sq. This is apparently the demonstration of anthracite coal as a source for electrically conductive graphenic materials.

Prospects of Supercritical Fluids in Realizing Graphene-Based Functional Materials

Supercritical-fluids science and technology predate all the approaches that are currently established for graphene production by several decades in advanced materials design. However, it has only recently been proposed as a plausible approach for graphene processing. Since then, supercritical fluids have emerged into contention as an alternative to existing technologies because of their scalability and versatility in processing graphene materials, which include composites, aerogels, and foams. Here, an overview is presented of such materials prepared through supercritical fluids from an advanced materials science standpoint, with a discussion on their fundamental properties and technological applications. The benefits of supercritical-fluid processing over conventional liquid-phase processing are presented. The benefits include not only better performances for advanced applications but also environmental issues associated with the synthesis process. Nevertheless, the limitations of supercritical-fluid processing are also stressed, along with challenges that are still faced toward the achievement of the great expectations from graphene materials.

CO₂-Induced Phase Engineering: Protocol for Enhanced Photoelectrocatalytic Performance of 2D MoS₂ Nanosheets

Molybdenum disulfide (MoS2) is a promising non-precious-metal catalyst, but its performance is limited by the density of active sites and poor electrical transport. Its metallic 1T phase possesses higher photoelectrocatalytic activity. Thus, how to efficiently increase the concentration of the 1T phase in the exfoliated two-dimensiaonal (2D) MoS2 nanosheets is an important premise. In this work, we propose a strategy to prepare a 2D heterostructure of MoS2 nanosheets using supercritical CO2-induced phase engineering to form metallic 1T-MoS2. Theoretical calculations and experimental results demonstrate that the introduced CO2 in the 2H-MoS2 host can prompt the transformation of partial 2H-MoS2 lattices into 1T-MoS2. Moreover, the electrical coupling and synergistic effect between 2H and 1T phases can greatly facilitate the efficient electron transfer from the active sites of MoS2, which significantly improves the photocatalytic performance.