Shear Exfoliation Research Articles

Production of Graphene-Containing Suspensions and Concentrates by Cascade Exfoliation of Graphite

Various technologies and equipment for the production of graphene-containing suspensions by liquid-phase exfoliation of graphite are considered. The prospects of using liquid-phase graphite shear exfoliation in a continuous mode are shown. Taking into account the analyzed shortcomings of existing technologies, a technology for the production of graphene-containing suspensions and concentrates by cascade exfoliation of graphite and a device for its implementation are proposed. In particular, the following are proposed: a new method for two-stage dosing of graphite powder; new design of rod drum mill for mechanical activation of graphite; a new design of the rotary apparatus with compound movable blades, providing an increase in shear forces acting on graphite particles in the process of exfoliation. The kinetics of the exfoliation process has been studied. Experimental studies on the modification of concrete with graphene have been carried out and an increase in compressive strength of at least 33% has been proven.

Relationship between the shape of particles of initial graphite and graphene plates during liquid-phase shear exfoliation

Relationship between the shape of particles of initial graphite and graphene plates during liquid-phase shear exfoliation

Enhancing thermoelectric performance of Bi2O2Se by W-doping with the shear exfoliation- restacking process

In this study, W-doping with the shear exfoliation- restacking process was used to optimize the thermoelectric properties of Bi2O2Se. It is noted that W-doping with the shear exfoliation- restacking process can synergistically improve carrier concentration and mobility, thus a higher electrical conductivity was obtained. In addition, total thermal conductivity of Bi1.98W0.02O2Se was restricted to 0.6 Wm−1 K−1 at 770 K. Consequently, a maximum ZT value of Bi1.98W0.02O2Se is raised to 0.62 at 773 K. The results show that the thermoelectric properties of Bi2O2Se can be further optimized by W-doping with the shear exfoliation-restacking process.

Shear exfoliation of large-size GO sheets for high-performance films

Shear exfoliation of large-size GO sheets for high-performance films

Thermoelectric properties of Bi2-xTixO2Se with the shear exfoliation-restacking process

Bi2O2Se is a potential medium-temperature n-type thermoelectric material owing to its lower lattice thermal conductivity and larger Seebeck coefficient. However, very low electrical conductivity of pristine polycrystalline Bi2O2Se hinders further enhancement of its thermoelectric performance. In this work, the thermoelectric properties of Bi2O2Se were investigated by Ti-doping with the shear exfoliation- restacking process. The results show that both the electron content and mobility can be further improved by Ti-doping with the shear exfoliation-restacking process, thus a higher electrical conductivity can be obtained. (e.g., 368 S cm−1 of Bi1.99Ti0.01O2Se at room temperature) On the other hand, the thermal conductivity of Bi1.99Ti0.01O2Se is suppressed to 0.84 Wm−1 K−1 at 772 K. Finally, the ZT peak of Bi1.99Ti0.01O2Se is raised to 0.56, which is 1.2 times of that of un-doped sample. The results show that the thermoelectric properties of Bi2O2Se can be further optimized by Ti doping with the shear exfoliation-restacking process.

Very-few-layer graphene obtained from facile two-step shear exfoliation in aqueous solution

A defect-free graphene with 1–3 layers (very-few-layer graphene, VFLG) has been successfully produced using a facile and environmentally friendly two-step shear exfoliation method in rotating-blade mixer (kitchen blender) and in high shear rotor–stator mixer. An aqueous solution containing an inexpensive and safe anionic surfactant of sodium lauryl sulphate (SLS) was deployed as the working fluid. Raman spectroscopy (RS) and Raman imaging (RI), density functional theory (DFT) simulation, transmission electron microscopy and high-resolution transmission electron microscopy (TEM-HRTEM), as well as Fourier-transform infrared spectroscopy (FTIR) analyses were used to characterize the samples. Some configurations of mixer setting were applied, and the exfoliation mechanisms were discussed. RS analysis revealed that the exfoliation process in rotating-blade mixer continued by high shear in rotor–stator mixer for 3 h had produced VFLG dominated by a single layer of graphene. The Lorentzian peak fitting of 2D band was well-fitted by a single Lorentzian component at Raman shift of 2651.317 cm−1. The produced VFLG endured the lowest peak intensity ratio of D and G bands (ID/IG) of 0.146 which corresponded with the highest lateral size. RI analysis showed that produced VFLG had edge chirality of armchairs and zigzag terminations. TEM-HRTEM confirmed the presence of VFLG with the highest mean lateral size of ~375.4 nm. DFT calculations provided pertinent optical/electronic properties of graphene in terms of Raman Spectra, density of states (DOS) curve and nuclear magnetic resonance (NMR) shielding tensor. FTIR analysis showed that the exfoliation process did not cause any oxidation, sulphonation, or functional defects on the VFLG. The turbulence and hydrodynamic force in liquid were the critical factors in the exfoliation processes. High-quality VFLG, with a facile, inexpensive, and environmentally friendly synthesis process, rendered this process to be a promising route for large-scale high-quality graphene production.

From shear exfoliation of graphite in Coca-Cola® to few-layer graphene for smart ink

Liquid-phase exfoliation of few-layer graphene in Coca-Cola ® was performed by a kitchen blender for rapid preparation of strain/light/humidity sensitive ink. The advantages of Coca-Cola as an exfoliating medium have been identified from the low structural defect (ID/IG = 0.39) and, moderate carbon to oxygen ratio (C/O = 2.6) obtained by the resulting graphene. It is discovered that a deposition of graphene on photo paper allowed a facile fabrication of smart paper that is able to detect minor strain (0 – 2.0%) with GF of 18.7. Meanwhile, the multi-responsiveness of ink towards light/humidity had been validated from the 10 s response time after the exposure to stimulants with the maximum resulting curvature degree of 16° and 14° under light and humidity changes, respectively. Finally, it is shown that our actuator is able to vertically lift a load (545 mg) from its initial flat position upon irradiation of light (200 mW/cm2).

Combination of few-layer graphene and commercial cosmetic film for tetrahydrofuran-sensitive smart film

A shear exfoliation of graphite in Arabic gum solution is one of the interesting non-toxic methods for production of few-layer graphene. While the utilization of Arabic-based graphene is mainly as conductive ink for inkjet or screen printing of circuit for sensing or electronic device, it is noted that there is lack of works reporting on the role of this polysaccharide class-based graphene for smart vapour-driven sensing film. In this brief report, the effect of Arabic gum/graphene presence on the cosmetic oil control film (Clean and Clear®) for bending-unbending performance of vapour-driven film was investigated. It is discovered that the film was able to complete a bending cycle within 13 s while the maximum recorded bending radius was measured at 0.48 cm after exposure to tetrahydrofuran (THF) in controlled environment condition (50%rh and 25 °C). By controlling the distance between THF source and the smart film, we further show the crawling motion of this film that resembling leech with a speed of 0.5 cm/48 s.

Rational Electronic and Structural Designs Advance BiCuSeO Thermoelectrics

AbstractIn this work, a record high thermoelectric figure‐of‐merit ZT of 1.6 ± 0.2 at 873 K in p‐type polycrystalline Bi0.94Pb0.06CuSe1.01O0.99 by a synergy of rational band manipulation and novel nanostructural design is reported. First‐principles density functional theory calculation results indicate that the density of state at the Fermi level that crosses the valence band can be significantly reduced and the measured optical bandgap can be enlarged from 0.70 to 0.74 eV by simply replacing 1% O with 1% Se, both indicating a potentially reduced carrier concentration and in turn, an improved carrier mobility and a boosted power factor up to 9.0 µW cm−1 K−2. Meanwhile, comprehensive characterizations reveal that under Se‐rich condition, Cu2Se secondary microphases and significant lattice distortions triggered by Pb‐doping and Se‐substitution can be simultaneously achieved, contributing to a reduced lattice thermal conductivity of 0.4 W m−1 K−1. Furthermore, a unique shear exfoliation technique enables an effective grain refinement with higher anisotropy of the polycrystalline pellet, leading to a further improved power factor up to 10.9 µW cm−1 K−2 and a further reduced lattice thermal conductivity of 0.30 W m−1 K−1, which gives rise to record high ZT.

Exploration of the form factors of turbulence kinetic energy transfer for shear exfoliation of graphene

Mass production of defect-free and large-lateral-size 2D materials via cost-effective methods is very important. Recently, shear exfoliation has shown great promise for large-scale production due to its simple operation, environmental-benignity and wide adaptability. However, a long-standing challenge is that with the production of more nanosheets, a ceiling yield and shattered products are encountered, which significantly limits their wider application. The method and efficiency of energy transfer in fluid is undoubtedly the key point in determining exfoliation efficiency, yet its in-depth mechanism has not yet been described. Thus, a thorough investigation of turbulence energy transfer is critically necessary. Herein, we identify two main factors that critically determine the exfoliation yield and provide a statistical analysis of the relationship between these factors and the exfoliation yield. In the initial shearing process, the coexistence of the 2D nanosheets and raw particles is the dominant factor; as time passes, the dimensional change of raw materials gradually has a greater influence on the energy transfer. These factors together lead to attenuated efficiency and a power function relationship between yield and exfoliation time. This investigation gives a statistical explanation of shear exfoliation technology for 2D material preparation and provides valuable insights for mechanical exfoliating high-quality 2D materials.

Miscanthus as a carbon precursor for graphene oxide: A possibility influenced by pyrolysis temperature

This study investigates the potential of producing graphene oxide (GO) from biomass via green (comparatively) processing and the impact of graphitization temperature on GO quality. Our findings show that it is possible to convert biomass into highly pyrolytic biochar, followed by shear exfoliation to produce few-layer GO. However, pyrolysis temperature is key in ensuring that the biochar is suited for effective exfoliation. Low temperatures (<1000 °C) would preserve undesirable heterogenous, complex cellular structure of biomass whilst excessive temperatures (≥1300 °C) result in uncontrolled melting, coalescence and loss of functional groups. Results show 1200 °C to be the optimum graphitization temperature for miscanthus, where the resultant biochar is highly aromatic with sufficient functional groups to weaken van der Waals forces, thus facilitating exfoliation to form 6-layer GO with specific surface area of 545.3 m2g−1. This study demonstrates the potential of producing high quality, fit-for-purpose graphene materials from renewable sources.

Organic salt-assisted liquid-phase shear exfoliation of expanded graphite into graphene nanosheets

How to improve the efficiency of liquid-phase shear exfoliation (LPSE) for mass production of large-size graphene nanosheets still remains an ongoing challenge. In this work, we have developed a LPSE method using a rotor-stator mixer. It is quite simple and efficient by exfoliation of expanded graphite (EG) in N-methyl-2-pyrrolidone (NMP) with the assistance of organic salts including sodium citrate, potassium citrate and sodium tartrate. The LPSE of EG in NMP can provide improved yields, up to 6 times as high as values from exfoliation of natural flake graphite (NFG). The additive of organic salts in NMP can make a further improvement in graphene yields, 1.5 times higher than that obtained only in NMP. Remarkably, the yields of the as-exfoliated graphene are as high as 10% under optimal conditions, and up to 50% after multiple-cycle exfoliation. Organic salts in LPSE act as analogue grinding aids enhancing the applied shear forces and thus contributing to the improved efficiency of LPSE, but they do not intercalate into the interplanar spaces of graphite. This facile LPSE method should have excellent potential in the large scale production of graphene nanosheets for numerous applications.

Highly Concentrated, Conductive, Defect-free Graphene Ink for Screen-Printed Sensor Application

HighlightsUltrathin and defect-free graphene ink is prepared through a high-throughput fluid dynamics process, resulting in a high exfoliation yield (53.5%) and a high concentration (47.5 mg mL−1).A screen-printed graphene conductor exhibits a high electrical conductivity of 1.49 × 104 S m−1 and good mechanical flexibility.An electrochemical sodium ion sensor based on graphene ink exhibits an excellent potentiometric sensing performance in a mechanically bent state.Real-time monitoring of sodium ion concentration in sweat is demonstrated.Conductive inks based on graphene materials have received significant attention for the fabrication of a wide range of printed and flexible devices. However, the application of graphene fillers is limited by their restricted mass production and the low concentration of their suspensions. In this study, a highly concentrated and conductive ink based on defect-free graphene was developed by a scalable fluid dynamics process. A high shear exfoliation and mixing process enabled the production of graphene at a high concentration of 47.5 mg mL−1 for graphene ink. The screen-printed graphene conductor exhibits a high electrical conductivity of 1.49 × 104 S m−1 and maintains high conductivity under mechanical bending, compressing, and fatigue tests. Based on the as-prepared graphene ink, a printed electrochemical sodium ion (Na+) sensor that shows high potentiometric sensing performance was fabricated. Further, by integrating a wireless electronic module, a prototype Na+-sensing watch is demonstrated for the real-time monitoring of the sodium ion concentration in human sweat during the indoor exercise of a volunteer. The scalable and efficient procedure for the preparation of graphene ink presented in this work is very promising for the low-cost, reproducible, and large-scale printing of flexible and wearable electronic devices.

Additive-Enhanced Exfoliation for High-Yield 2D Materials Production.

The success of van-der-Waals electronics, which combine large-scale-deposition capabilities with high device performance, relies on the efficient production of suitable 2D materials. Shear exfoliation of 2D materials’ flakes from bulk sources can generate 2D materials with low amounts of defects, but the production yield has been limited below industry requirements. Here, we introduce additive-assisted exfoliation (AAE) as an approach to significantly increase the efficiency of shear exfoliation and produce an exfoliation yield of 30%. By introducing micrometer-sized particles that do not exfoliate, the gap between rotor and stator was dynamically reduced to increase the achievable shear rate. This enhancement was applied to WS2 and MoS2 production, which represent two of the most promising 2D transition-metal dichalcogenides. Spectroscopic characterization and cascade centrifugation reveal a consistent and significant increase in 2D material concentrations across all thickness ranges. Thus, the produced WS2 films exhibit high thickness uniformity in the nanometer-scale and can open up new routes for 2D materials production towards future applications.

Real-time monitoring and hydrodynamic scaling of shear exfoliated graphene

Shear-assisted liquid exfoliation is a primary candidate for producing defect-free two-dimensional (2D) materials. A range of approaches that delaminate nanosheets from layered precursors in solution have emerged in recent years. Diverse hydrodynamic conditions exist across these methods, and combined with low-throughput, high-cost characterization techniques, strongly contribute to the wide variability in performance and material quality. Nanosheet concentration and production rate are usually correlated against operating parameters unique to each production method, making it difficult to compare, optimize and predict scale-up performance. Here, we reveal the shear exfoliation mechanism from precursor to 2D material and extract the derived hydrodynamic parameters and scaling relationship that are key to nanomaterial output and common to all shear exfoliation processes. Our investigations use conditions created from two different hydrodynamic instabilities—Taylor vortices and interfacial waves—and combine materials characterization, fluid dynamics experiments and numerical simulations. Using graphene as the prototypical 2D material, we find that scaling of concentration of few-layer nanosheets depends on local strain rate distribution, relationship to the critical exfoliation criterion, and precursor residence time. We report a transmission-reflectance method to measure concentration profiles in real-time, using low-cost optoelectronics and without the need to remove the layered precursor material from the dispersion. We show that our high-throughput, in situ approach has broad uses by controlling the number of atomic layers on-the-fly, rapidly optimizing green solvent design to maximize yield, and viewing live production rates. Combining the findings on the hydrodynamics of exfoliation with this monitoring technique, we unlock targeted process intensification, quality control, batch traceability and individually customizable 2D materials on-demand.

Graphene-reinforced polymer matrix composites fabricated by in situ shear exfoliation of graphite in polymer solution: processing, rheology, microstructure, and properties

Effective methods are needed to fabricate the next generation of high-performance graphene-reinforced polymer matrix composites (G-PMCs). In this work, a versatile and fundamental process is demonstrated to produce high-quality graphene-polymethylmethacrylate (G-PMMA) composites via in situ shear exfoliation of well-crystallized graphite particles loaded in highly-viscous liquid PMMA/acetone solutions into graphene nanoflakes using a concentric-cylinder shearing device. Unlike other methods where graphene is added externally to the polymer and mixed, our technique is a single step process where as-exfoliated graphene can bond directly with the polymer with no contamination/handling. The setup also allows for the investigation of the rheology of exfoliation and dispersion, providing process understanding in the attainment of the subsequently heat injection-molded and solidified G-PMC, essential for future manufacturing scalability, optimization, and repeatability. High PMMA/acetone concentration correlates to high mixture viscosity, which at large strain rates results in very-high shear stresses, producing a large number of mechanically-exfoliated flakes, as confirmed by liquid-phase UV–visible spectral analysis. Raman spectroscopy and other imaging evince that single- and bi-layer graphene are readily achieved. Nevertheless, a limit is reached at high mixtures viscosities where the process becomes unstable as non-Newtonian fluid behavior (e.g. viscoelastic) dominates the system. Characterization of microstructure, morphology, and properties of this new class of nanostructured composites reveals interesting trends. Observations by transmission electron microscopy, scanning electron microscopy, and helium ion microscopy of the manufactured G-PMCs show uniform distributions of unadulterated, well-bonded, discontinuous, graphene nanoflakes in a PMMA matrix, which enhances stiffness and strength via a load-transfer mechanism. Elastic modulus of 5.193 GPa and hardness of 0.265 GPa are achieved through processing at 0.7 g ml−1 of acetone/PMMA for 1% wt. starting graphite loading when injected into a sample mold at 200 °C. Mechanical properties exhibit 31% and 28.6% enhancement in elastic modulus and hardness, respectively, as measured by nano-indentation.

Continuous and semi-continuous industrial production of lubricants modified with graphene nanostructures

This article proposes one of the options for the production of graphene-containing suspensions and their modification of lubricants. It is based on the technology of obtaining graphene nanostructures by the method of liquid-phase shear exfoliation of graphite. The proposed technological scheme can operate in both semi-continuous and continuous modes. The main advantages of this technology are simplicity and reliability of equipment, as well as environmental friendliness of production.

Production of Graphene-Containing Suspensions for Liquid-Phase Shear Exfoliation of Graphite

A method and device for obtaining graphene-containing suspensions by liquid-phase shear exfoliation of graphite in a continuous mode have been developed. The comparison of the intensities of the process of exfoliation of graphite in the periodic and continuous modes was carried out and it was found that the intensity of the exfoliation process in the continuous mode is higher than in the periodic one. The limiting concentration of graphene structures in a suspension is reached 1.5 times faster when using an apparatus operating in a continuous mode than in a periodic mode, and its numerical value is greater (by at least 25 %).

Improved efficiency of liquid-phase shear exfoliation of expanded graphite with mica plates as bifunctional additives

Liquid-phase shear exfoliation (LPSE) is a potential method of large-scale production of good-quality graphene.

Fast high-shear exfoliation of natural flake graphite with temperature control and high yield

High-shear exfoliation of graphite is a cost-effective liquid-phase exfoliation (LPE) method for producing few-layer graphene dispersions. However, the drawbacks of LPE such as low graphene concentrations, long processing times and solvent residuals hamper the scalability of the process and applicability of the dispersions. Here, we demonstrate a fast graphene synthesis where concentrations as high as 3 mg/ml with a 3% yield are produced only after 2 h of shear exfoliation in environmentally friendly aqueous medium using sodium cholate as surfactant. The produced graphene is of high quality and mostly less than 5 layers thick. The high concentration is attributed to a proper choice of effective mixing parameters, low exfoliation temperature (10 °C) and an appropriate surfactant concentration. In addition, we noticed that larger graphite flakes (250 μm-1 mm) increased the lateral dimensions of the exfoliated sheets while the yield and concentration were independent of the flake size. Furthermore, using larger graphite flakes improved the conductivity of the subsequent graphene films. Prior to film fabrication, dialysis removed excess surfactant from the dispersions improving the film conductivity (17 000 S/m) by ca 40%.