High-Concentration Graphene Dispersions with Minimal Stabilizer: A Scaffold for Enzyme Immobilization for Glucose Oxidation

When graphene meets ionic liquids: a good match for the design of functional materials

Graphene is an attractive material that is characterized by its exceptional properties (i.e. electrical, mechanical, thermal, optical, etc.), which have pushed researchers to attach high interest to its production and functionalization processes to meet applications in different fields (electronics, electromagnetics, composites, sensors, energy storage, etc.). The synthesis (bottom-up) of graphene remains long and laborious, at the same time expensive, and it is limited to the development of this material in low yield. Hence, the use of graphite as a starting material (top-down through exfoliation or oxidation) seems a promising and easy technique for producing a large quantity of graphene or graphene oxide (GO). On the one hand, GO has been extensively studied due to its ease of synthesis, processing and chemical post-functionalization. One the other hand, "pristine" graphene sheets, obtained through exfoliation, are limited in processability but present enhanced electronic properties. Both types of materials have been of great interest to design functional nanomaterials. Ionic liquids (ILs) are task-specific solvents that exhibit tunable physico-chemical properties. ILs have many advantages as compared with conventional solvents, such as high thermal and chemical stability, low volatility, excellent conductivity and inherent polarity. In the last decade, ILs have been widely employed for the preparation and stabilization of various nanomaterials. In particular, the combination of ILs and graphene, including GO and pristine graphene sheets, has been of growing interest for the preparation, processing and functionalization of hybrid nanomaterials. Understanding the structure and properties of the graphene/IL interface has been of considerable interest for a large panel of applications ranging from tribology to energy storage.

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.

Coupled Heterostructure of Mo-Fe Selenide Nanosheets Supported on Carbon Paper as an Integrated Electrocatalyst for Efficient Hydrogen Evolution

Hydrogen evolution reaction (HER) driven by high-performance and low-cost electrocatalysts has been well-identified as one of the most promising technologies to explore sustainable power sources. In this work, the authors demonstrated a series of Fe-Mo selenide composite nanomaterials with adjustable HER catalytic activities by a simple one-step hydrothermal reaction. The heterostructured catalyst exhibited significant enhancement in HER activity compared to the pristine MoSe₂ and FeSe₂ catalysts. It is found that the optimized Mo-Fe selenide can drive the HER at a current density of 10 mA cm^-2 by an overpotential of 86.9 mV in acidic solution. Also, it possesses outstanding kinetics (Tafel slope of 57.7 mV dec^-1) of the electrochemical reaction. Besides the synergistic effect of different chemicals, the coupled Mo-Fe selenide achieved the electronic modulation of the heterointerface, where electrons were accelerated to transfer from dispersed FeSe₂ to the active edges on 1T-MoSe₂ and further activated the electrochemical performance.

New solvent-stabilized few-layer black phosphorus for antibacterial applications

Discovering highly efficient, environmentally friendly, and low-cost exfoliating media that can both disperse and protect black phosphorus (BP) remains a challenge. Herein, we demonstrate such a new molecule, N,N'-dimethylpropyleneurea (DMPU), for effective exfoliation and dispersion of two-dimensional BP nanosheets. A very high exfoliation efficiency of up to 16% was achieved in DMPU, significantly surpassing other good solvents. Exfoliated flakes are free from structural disorder or oxidation. Nanosheets retain high stability in DMPU even after addition of 25 vol% of common solvents. The solvation shell appears to protect the nanosheets from reacting with water and air, more remarkably than the best solvent N-cyclohexyl-2-pyrrolidone reported so far. Molecular dynamics simulations of the exfoliation process show that DMPU is among the effective solvents, although energetically it does not appear as favorable as some other amides. We also demonstrate that our exfoliated BP nanosheets exhibit excellent antimicrobial activities against both Escherichia coli and Staphylococcus aureus, outperforming other common two-dimensional materials of graphene and MoS2, suggesting promise in biomedical applications.

Efficient exfoliation of layered materials by waste liquor

Based on their unique material properties, two-dimensional (2D) nanomaterials such as graphene, molybdenum disulfide (MoS₂), and boron nitride (BN) have been attracting increased research interest. The potential of 2D materials, in the form of nanoplatelets that are used as new materials, will be important to both nanomaterials and advanced materials. Water is usually considered to be the ideal dispersed medium, and the essential hydrophobicity and limitations to mass production of 2D nanoplatelets have become quite serious obstacles to their usage in various fields. In this paper, pulping black liquor was used as dispersant, with high concentration of lignin to get single- and few-layered nanoplatelets. The whole process required only the high-shear mixing of 2D layered materials and pulping waste liquor. This method was not only simple and efficient but also environmentally friendly and resource-recycling. Moreover, the fabricated single- or few-layered nanoplatelets possessed good solubility in aqueous solution due to their edge functionalization, and could be well dispersed in water at concentrations (10 mg ml^-1 for graphene, 6.3 mg ml^-1 for MoS₂, and 6.0 mg ml^-1 for BN) which were much higher than that of other methods. The dispersions of graphene, MoS₂, and BN nanosheets were highly stable over several months, which allowed us to easily prepare graphene, MoS₂, and BN films through simple vacuum filtration or spraying. These results indicated that pulping black liquor can be used as a material or reagent, and the mass production of 2D material is possible in a simple and fast method.

Supercritical diethylamine facilitated loading of ultrafine Ru particles on few-layer graphene for solvent-free hydrogenation of levulinic acid to γ-valerolactone

We demonstrate a facile and versatile method to grow Ru particles uniformly on pristine few-layer graphene (FLG) in supercritical diethylamine. In particular, a large number of Ru subnanometer clusters less than 1.0 nm were observed. The particle size can be tuned by manipulating the loading content of Ru and controlling the reaction temperature. The resulting Ru/FLG showed remarkably high activity, selectivity, and reusability towards the hydrogenation of levulinic acid to γ-valerolactone. This method is flexible, and can be extended to the synthesis of a variety of other ultrafine metal particles supported on FLG.

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.

Build a Rigid-Flexible Graphene/Silicone Interface by Embedding SiO2 for Adhesive Application

An effective strategy was developed to enhance the adaptability of graphene/silicone matrices under external stimuli by embedding nanoscale SiO2 into the graphene/silicone interfaces as a buffer layer. Chemically reduced graphene (rGE) was first covered by SiO2 using an in situ preparation, forming sandwichlike rGE/SiO2 (rGES). Then, rGES was integrated into methyl vinyl polysiloxane, followed by vulcanization, producing the final rGES/silicone rubber (SR) nanocomposite. Such interfacial modification actually built a rigid-flexible SiO2 buffer layer between rGE and polysiloxane. Obvious improvements were seen in both thermal and mechanical properties due to improved interfacial interaction. In a vulcanized rGES/SR system, the addition of 30 wt % rGES (3 wt % rGE) yielded a tensile strength of 6.13 MPa (up to 25 times that of the unmodified rGE in filled SR), a tear strength of 18.08 kN/m, and an elongation at break of 267%, several times higher than those of an rGE/SR nanocomposite. Thermal analysis results indicated that the initial decomposition temperature of rGES/SR containing 5 wt % rGES (0.5 wt % rGE) increased by more than 98 and 288 °C compared to that of SiO2/SR and rGE/SR, respectively. The rGES/polysiloxane matrices showed a tensile shear adhesive strength of 1.78 MPa when used as an adhesive for aluminum sheets, which is higher than that of the rGE/polysiloxane matrix (0.93 MPa).

Preparation of CO2-switchable graphene dispersions and their polystyrene nanocomposite latexes by direct exfoliation of graphite using hyperbranched polyethylene surfactants

Herein the preparation of CO₂-switchable graphene dispersions by noncovalent exfoliation of graphite in water using a CO₂-switchable star copolymer surfactant is described.

Achieving extremely concentrated aqueous dispersions of graphene flakes and catalytically efficient graphene-metal nanoparticle hybrids with flavin mononucleotide as a high-performance stabilizer

The stable dispersion of graphene flakes in an aqueous medium is highly desirable for the development of materials based on this two-dimensional carbon structure, but current production protocols that make use of a number of surfactants typically suffer from limitations regarding graphene concentration or the amount of surfactant required to colloidally stabilize the sheets. Here, we demonstrate that an innocuous and readily available derivative of vitamin B2, namely the sodium salt of flavin mononucleotide (FMNS), is a highly efficient dispersant in the preparation of aqueous dispersions of defect-free, few-layer graphene flakes. Most notably, graphene concentrations in water as high as ∼50 mg mL(-1) using low amounts of FMNS (FMNS/graphene mass ratios of about 0.04) could be attained, which facilitated the formation of free-standing graphene films displaying high electrical conductivity (∼52000 S m(-1)) without the need of carrying out thermal annealing or other types of post-treatment. The excellent performance of FMNS as a graphene dispersant could be attributed to the combined effect of strong adsorption on the sheets through the isoalloxazine moiety of the molecule and efficient colloidal stabilization provided by its negatively charged phosphate group. The FMNS-stabilized graphene sheets could be decorated with nanoparticles of several noble metals (Ag, Pd, and Pt), and the resulting hybrids exhibited a high catalytic activity in the reduction of nitroarenes and electroreduction of oxygen. Overall, the present results should expedite the processing and implementation of graphene in, e.g., conductive inks, composites, and hybrid materials with practical utility in a wide range of applications.

Graphene based enzymatic bioelectrodes and biofuel cells

The excellent electrical conductivity and ease of functionalization make graphene a promising material for use in enzymatic bioelectrodes and biofuel cells. Enzyme based biofuel cells have attracted substantial interest due to their potential to harvest energy from organic materials. This review provides an overview of the functional properties and applications of graphene in the construction of biofuel cells as alternative power sources. The review covers the current state-of-the-art research in graphene based nanomaterials (physicochemical properties and surface functionalities), the role of these parameters in enhancing electron transfer, the stability and activity of immobilized enzymes, and how enhanced power density can be achieved. Specific examples of enzyme immobilization methods, enzyme loading, stability and function on graphene, functionalized graphene and graphene based nanocomposite materials are discussed along with their advantages and limitations. Finally, a critical evaluation of the performance of graphene based enzymatic biofuel cells, the current status, challenges and future research needs are provided.