Graphene Flakes Research Articles

Impact of solvent temperature on graphite shear exfoliation efficiency

Liquid phase exfoliation (LPE) of graphite is a promising pathway for graphene flakes (GF) due to its scalability and cost-effectiveness. However, the method has significant limits at the industrial scale, such as low yield of GF and long processing times. In this study, we investigate the effect of organic solvents such as N-methyl-2-pyrrolidone (NMP) and methyl-5-(dimethylamino)-2-methyl-5-oxopentanoate (PolarClean) temperature on shear-induced exfoliation. The GF concentration was successfully obtained 0.62 mg/ml in NMP and 0.68 mg/ml in PolarClean within just 2 h at 263 K, without surfactants or any additional additives. These results revealed the improved exfoliation efficiency due to changes in solvent polarity, surface tension, and dispersion stability with increased viscosity. In addition, we investigate the optimization conditions required for liter-scale shear-induced exfoliation of graphite in PolarClean. As-prepared GF has significant potential as an effective nanofiller for enhancing the mechanical strength of commercial polymers. This study not only advances the understanding of the LPE mechanisms but also paves the way for the industrial application of this method in the green synthesis of graphene-based nanocomposites.

Sustainable graphene production for solution-processed microsupercapacitors and multipurpose flexible electronics

The growing demand for portable and wearable electronics, Internet of Things microdevices, and wireless sensor networks has led to the development of miniaturized energy storage devices, such as microsupercapacitors (mSCs). With excellent electrical conductivity and high surface area in a layered structure, graphene materials are ideal for mSCs, but current manufacturing methods still hinder their widespread integration. Here, we propose a sustainable approach for the rapid and eco-friendly production of few-layer graphene flakes based on the exfoliation of graphite in water by a combination of high-shear mixing and a high-pressure airless spray. An all-carbon composite paste with high electrical conductivity and tunable viscosity was designed to fabricate planar, interdigitated mSCs on polyethylene terephthalate (PET). The flexible, metal-free mSCs achieved a Coulombic efficiency close to 100%, with areal and volumetric capacitances of 6.16 mF cm−2 and 2.46 F cm−3, respectively. The maximum energy density exceeds 200 μWh cm−3 with 91.5% capacitance retention after 10000 galvanostatic chargedischarge cycles. The mSCs retain the same performance when subjected to a wide bending range and can be easily modularized to adjust the voltage and capacitance outputs. Finally, high-performance coatings for electromagnetic interference shielding and wearable strain sensors are also fabricated to demonstrate the multipurpose applicability of the graphene-based paste.

Controlling Thermoelectric Properties of Laser-Induced Graphene on Polyimide

In the field of wearable thermoelectric generators, graphene-based materials have attracted attention as suitable candidates due to their low material costs and tunable electronic properties. However, their high thermal conductivity poses significant challenges. Low thermal conductivity due to porous structure of the laser-induced graphene, combined with its affordability and scalability, positions it as a promising candidate for thermoelectric applications. In this study, thermoelectric properties of the laser-induced graphene (LIG) on polyimide and their dependence on structural modifications of LIG were investigated. Furthermore, it was shown that increasing the laser scribing power on polyimide results in larger graphene flakes and a higher degree of graphitization. Electrical conductivity measurements indicated an increase with increasing laser power, due to a higher degree of graphitization, which enhances charge carrier mobility. Our findings reveal that LIG exhibits p-type semiconducting behavior, characterized by a positive Seebeck coefficient. It was shown that increasing laser power increased the Seebeck coefficient and electrical conductivity simultaneously, which is attributed to a charge carrier energy filtering effect arising from structures occurred on the graphene flakes. Moreover, the porous structure of LIG contributes to its relatively low thermal conductivity, ranging between 0.6 W/m·K and 0.85 W/m·K, which enhances the thermoelectric performance of LIG. It has been observed that with increasing laser power, the figure of merit for laser-induced graphene can be enhanced by nearly 10 times, which holds promising applications for laser-induced graphene due to the tunability of its thermoelectric performance by changing laser parameters.

Influence on solar PV performance integrated with heat sinks and nano-enhanced phase change material

The inevitable advancement of solar photovoltaic (PV) system performance highly relies on temperature control, which enhances electrical efficiency. The objective of this study is to enhance electrical efficiency by incorporating phase change material of HS36 (PCM) and nano enhanced PCM (NePM). The thermal properties of PCM were improved by adding graphene flakes (GF) into PCM to enhance the thermal transfer rate between the PV panel and PCM. The phase change properties and thermal conductivity of PCM with different concentrations of GF were assessed. The results indicated that the 0.9 wt.% NePCM exhibited the highest thermal conductivity enhancement of 47% in solid states and a significant reduction in latent heat by 9.4% during freezing compared to pure PCM. The effect of cooling on PV performance was studied using a reference panel, integrated with a heat sink containing PCM (PV/PCM) and NePCM (PV/NePCM). Outdoor experiments were conducted using a 10 W panel, and the results for the use of PCM showed a reduction in temperature and an improvement in efficiency. The findings revealed that attaching a heat sink and integrating the PV panel with pure PCM and NePCM reduced the temperature by 8.65°C and 10.04°C, respectively, and improved efficiency by 11.65% and 12.15%, respectively. It can be concluded that the proposed thermal management method for PV systems offers a viable solution for increasing daily average electrical generation with maximum efficiency.

Development of non-oxidized graphene flakes (NOGF)/polyvinylidene fluoride (PVDF) composites for high-performance EMI shielding efficiency

As numerous communication devices use multiband electromagnetic waves in modern society, electromagnetic interference (EMI) shielding materials become increasingly important due to the harmful effects of electromagnetic waves. While metal-based materials have shown high shielding performance, they have many drawbacks such as high density, high processing cost, and low corrosion resistance. Conductive polymer composites (CPCs), which can overcome these shortcomings of metal-based materials, have become increasingly popular in recent years. In this study, non-oxidized graphene flakes (NOGFs) are synthesized by exfoliating the ternary graphite intercalant compound (t-GIC) and used as fillers for CPCs. The electrical conductivity and EMI shielding effiency (SE) of NOGF/polyvinylidene fluoride (PVDF), graphite/PVDF and reduced graphene oxide (rGO)/PVDF composites are investigated. NOGF/PVDF composites showed notably enhanced electrical conductivity and EMI SE than the others. This is mostly due to the non-oxidative fabrication method that does not destroy the sp2 structure of graphene sheet, thus preserving the unique electrical properties of graphene.

Reliable Lift‐Off Patterning of Graphene Dispersions for Humidity Sensors

AbstractDispersion‐based graphene materials are promising candidates for various sensing applications. They offer the advantage of relatively simple and fast deposition via spin‐coating, Langmuir–Blodgett deposition, or inkjet printing. Film uniformity and reproducibility remain challenging in all of these deposition methods. Here, a scalable structuring method is demonstrated, characterized, and successfully applied for graphene dispersions. The method is based on a standard lift‐off process, is simple to implement, and increases the film uniformity of graphene devices. It is also compatible with standard semiconductor manufacturing methods. Two different graphene dispersions are investigated via Raman spectroscopy and Atomic Force Microscopy and observe no degradation of the material properties by the structuring process. Furthermore, high uniformity of the structured patterns and homogeneous graphene flake distribution is achieved. Electrical characterizations show reproducible sheet resistance values correlating with material quantity and uniformity. Finally, repeatable humidity sensing is demonstrated with van der Pauw devices, with sensing limits of less than 1% relative humidity.

Spatial Graphene Structures with Potential for Hydrogen Storage

Spatial graphene is a 3D structure of a 2D material that preserves its main features. Its production can be originated from the water solution of graphene oxide (GO). The main steps of the method include the crosslinking of flakes of graphene via treatment with hydrazine, followed by the reduction of the pillared graphene oxide (pGO) with hydrogen overpressure at 700 °C, and further decoration with catalytic metal (palladium). Experimental research achieved the formation of reduced pillared graphene oxide (r:pGO), a porous material with a surface area equal to 340 m2/g. The transition from pGO to r:pGO was associated with a 10-fold increase in pore volume and the further reduction of remaining oxides after the action of hydrazine. The open porosity of this material seems ideal for potential applications in the energy industry (for hydrogen storage, in batteries, or in electrochemical and catalytic processes). The hydrogen sorption potential of the spatial graphene-based material decorated with 6 wt.% of palladium reached 0.36 wt.%, over 10 times more than that of pure metal. The potential of this material for industrial use requires further refining of the elaborated procedure, especially concerning the parameters of substrate materials.

Improvement Synthesis of Graphene Oxide Yield in Two Steps of Intercalation and Oxidation of Flexible Graphite Foil by Electrochemical Exfoliation

Synthesis of high quality and quantity of graphene by cost-effective methods are highly desirable for various application. In electrochemical exfoliation, graphite foil has been used as carbon source for the synthesis of high yield of graphene flakes. Electrochemical exfoliation is one of the faster and cheaper method to synthesize graphene sheets. In this work, five different types of concentration of sulphuric acid were used for electrochemical exfoliation. The electrochemical cell design where graphite foil as anode and copper foil as cathode which were connected to DC power supply of 5V. To examine the morphology scanning electron microscope (SEM) was employed in which the sheet structures with large lateral dimension and thin graphene flakes. Furthermore, X-ray diffraction (XRD) revealed that exfoliated graphene samples showed a significant peak at about 2θ = 26º corresponded to graphite.

Effects of carbon concentration on the local atomic structure of amorphous GST.

Ge-Sb-Te (GST) alloys are leading phase-change materials for data storage due to the fast phase transition between amorphous and crystalline states. Ongoing research aims at improving the stability of the amorphous phase to improve retention. This can be accomplished by the introduction of carbon as a dopant to Ge2Sb2Te5, which is known to alter the short- and mid-range structure of the amorphous phase and form covalently bonded C clusters, both of which hinder crystallization. The relative importance of these processes as a function of C concentration is not known. We used molecular dynamics simulation based on density functional theory to study how carbon doping affects the atomic structure of GST-C. Carbon doping results in an increase in tetrahedral coordination, especially of Ge atoms, and this is known to stabilize the amorphous phase. We observe an unexpected, non-monotonous trend in the number of tetrahedral bonded Ge with the amount of carbon doping. Our simulations show an increase in the number of tetrahedral bonded Ge up to 5 at.% C, after which the number saturates and begins to decrease above 14 at.% C. The carbon atoms aggregate into clusters, mostly in the form of chains and graphene flakes, leaving less carbon to disrupt the GST matrix at higher carbon concentrations. Different degrees of carbon clustering can explain divergent experimental results for recrystallization temperature for carbon doped GST.

Operando Characterization and Molecular Simulations Reveal the Growth Kinetics of Graphene on Liquid Copper During Chemical Vapor Deposition.

In recent years, liquid metal catalysts have emerged as a compelling choice for the controllable, large-scale, and high-quality synthesis of two-dimensional materials. At present, there is little mechanistic understanding of the intricate catalytic process, though, of its governing factors or what renders it superior to growth at the corresponding solid catalysts. Here, we report on a combined experimental and computational study of the kinetics of graphene growth during chemical vapor deposition on a liquid copper catalyst. By monitoring the growing graphene flakes in real time using in situ radiation-mode optical microscopy, we explore the growth morphology and kinetics over a wide range of CH4-to-H2 pressure ratios and deposition temperatures. Constant growth rates of the flakes' radius indicate a growth mode limited by precursor attachment, whereas methane-flux-dependent flake shapes point to limited precursor availability. Large-scale free energy simulations enabled by an efficient machine-learning moment tensor potential trained to density functional theory data provide quantitative barriers for key atomic-scale growth processes. The wealth of experimental and theoretical data can be consistently combined into a microkinetic model that reveals mixed growth kinetics that, in contrast to the situation at solid Cu, is partly controlled by precursor attachment alongside precursor availability. Key mechanistic aspects that directly point toward the improved graphene quality are a largely suppressed carbon dimer attachment due to the facile incorporation of this precursor species into the liquid surface and a low-barrier ring-opening process that self-heals 5-membered rings resulting from remaining dimer attachments.

Water-based and tannin-assisted liquid-phase exfoliation for a sustainable production of graphene

Graphene is a material with high potential in the development of advanced applications, such as 3D printing. In the current scenario, it is obvious that technological progress should align with sustainability, therefore, it is necessary to search for efficient and sustainable alternatives to the liquid phase exfoliation of graphene. In this work, chestnut tannins were employed as surfactants for graphene exfoliation in aqueous media, via ultrasound exfoliation and high-shear exfoliation. The effect of the initial concentration of graphite, the initial concentration of tannin and the exfoliation time over the concentration of the obtained graphene suspensions and mass yield was assessed, showing that comparable values to NMP based exfoliation were obtained. Obtained graphene flakes were morphologically characterized by Raman spectroscopy and Transmission Electron Microscopy, and results showed that high-shear exfoliation resulted in graphene flakes with the highest ID/IG ratio and more homogenous thickness. Life Cycle Assessment of the exfoliation processes revealed the environmental benefits of water-based exfoliation processes compared to traditional process based on NMP solvent. A sensitivity analysis showed that environmental impacts could be reduced if renewable energy sources are considered. High-shear exfoliation with initial concentration of graphite of 5 mg·mL−1, tannin concentration of 1 mg·mL−1 and shearing time of 15 min emerged as the most balanced choice between quality and sustainability.

ZZPolyCalc: An open-source code with fragment caching for determination of Zhang-Zhang polynomials of carbon nanostructures

Determination of topological invariants of graphene flakes, nanotubes, and fullerenes constitutes a challenging task due to its time-intensive nature and exponential scaling. The invariants can be organized in a form of a combinatorial polynomial commonly known as the Zhang-Zhang (ZZ) polynomial or the Clar covering polynomial. We report here a computer program, ZZPolyCalc, specifically designed to compute ZZ polynomials of large carbon nanostructures. The curse of the exponential scaling is avoided for a broad class of nanostructures by employing a sophisticated bookkeeping algorithm, in which each fragment appearing in the recursive decomposition is stored in the cache repository of molecular fragments indexed by a hash of the corresponding adjacency matrix. Although exponential scaling persists for the remaining nanostructures, the computational time is reduced by a few orders of magnitude owing to efficient use of hash-based fragment bookkeeping. The provided benchmark timings show that ZZPolyCalc allows for treating much larger carbon nanostructures than previously envisioned. Program summaryProgram Title: ZZPolyCalcCPC Library link to program files:https://doi.org/10.17632/vmnr5xfb6t.1Developer's repository link:https://github.com/quantumint/zzpolycalcLicensing provisions: GPLv3Programming language: Fortran 2008, CNature of problem: Calculations of Zhang-Zhang (ZZ) polynomials for carbon nanostructures have traditionally been performed by the application of a recursive algorithm, which scales exponentially with the size of the system. For large nanostructures, the problem often becomes intractable due to prohibitive computational cost. Consequently, ZZ polynomials were reported so far only for small or model systems, for which analytical formulas could be determined; larger systems, such as graphene flakes, fullerenes, or nanotubes, have been intractable with the existing implementations.Solution method: The process of determination of ZZ polynomials, based on recursive decomposition of a given carbon nanostructure, produces many identical fragments. In ZZPolyCalc, we avoid redundant calculations for such fragments by storing all the intermediate data in a hash table and reusing it efficiently whenever necessary. This new algorithm is particularly effective for elongated graphene flakes or carbon nanotubes, where the scaling becomes polynomial, but it also greatly reduces the computational time for other, less regular nanostructures.

Transitions of electrical conduction mechanism in graphene flake van der Waals thin film

The electrical conduction mechanism in 2D van der Waals (vdW) thin film made of graphene flakes can be characterized by the variable range hopping (VRH) over a wide range of temperature, followed by a transition into an Arrhenius type conduction at the higher temperature. Regarding the nature of the subsequent Arrhenius type conduction, a lack of consensus exists between the nearest neighbor hopping (NNH) and the band conduction (BC). We discuss how to assess the transition in conduction mechanism based on the transient behavior of the reduced activation energy (W) and the structure of electronic density of states (DOS) of the sample. With a multi-channel conduction model, we show that the transition from VRH to BC always accompany an increase in W, which is absent in the transition from VRH to NNH. Also discussed are the peculiarities of VRH in vdW thin film and what they imply towards the properties of defects. In most vdW flake thin films, the spread of defect band in DOS is governed by the intrinsic defect energy not by the Coulomb interaction between defects. It can further be deduced that the gap in DOS near Fermi level associated with Efros-Shklovskii VRH observed in vdW film is not a Coulomb gap as found in weakly doped crystalline semiconductor, but inherent in the distribution of defect intrinsic energy. For the graphene flake thin film we synthesized, Mott VRH and BC are identified.

Few-layer graphene production through graphite exfoliation in pressurized CO2 assisted by natural surfactant

Graphene research has captivated researchers worldwide, propelling innovation across diverse industries. Through the liquid-phase exfoliation methodology of graphite powder, we have demonstrated a rapid route for obtaining few-layer and multi-layer graphene using a natural surfactant, cardanol. Aqueous phase exfoliation of graphite in the presence of cardanol as a surfactant was conducted to obtain pre-exfoliated graphite suspensions. The influence of different ultrasonication times, 10, 20, and 30 min, and contact times with the surfactant, 1 and 60 min, on the stability and concentration of dispersed exfoliated graphite was evaluated. Results indicate that ultrasonication for 20 min resulted in improved stability and reduced graphene flake sizes, making it suitable for scalable graphene production. Subsequently, the most stable dispersions of exfoliated graphite were subjected to CO2-pressurized treatment. Promising results were obtained when employing cardanol at its critical micelle concentration. The graphene exhibited good structural quality, low defect density, and small stacking, with an average size of 15 nm, where 40 % of the stacked graphene was smaller than 5 nm. The findings provide valuable recommendations for the scalable production of graphene with multilayers and a few layers (FLG/MLG), using cardanol, a friendly surfactant, and a novel method of exfoliation utilizing supercritical CO2. This technology represents an innovative approach, with potential applications in supercapacitors, solar cells, biosensors, polymer composites, and advanced materials.

Investigating the interplay between charge transfer and CO2 insertion in the adsorption of a NiFe catalyst for CO2 electroreduction on a graphite support through DFT computational approaches.

This article describes a density functional theory (DFT) study to explore a bio-inspired NiFe complex known for its experimental activity in electro-reducing CO2 to CH4 when adsorbed on graphite. The coordination properties of the complex are investigated in isolated form and when physisorbed on a graphene surface. A comparative analysis of DFT approaches for surface modeling is conducted, utilizing either a finite graphene flake or a periodic carbon surface. Results reveal that the finite model effectively preserves all crucial properties. By examining predicted structures arising from CO2 insertion within the mono-reduced NiFe species, whether isolated or adsorbed on the graphene flake, a potential species for subsequent electro-reduction steps is proposed. Notably, the DFT study highlights two positive effects of complex adsorption: facile electron transfers between graphene and the complex, finely regulated by the complex state, and a lowering of the thermodynamic demand for CO2 insertion.

Field emission properties of LIG/ZnO heterojunction prepared by ultrafast laser direct writing

The facile in situ synthesis of graphene-coated metal-oxide nanoparticles and their potential applications in optoelectronic devices are highly anticipated. In this study, we prepared three-dimensional graphene flakes decorated with ZnO nanoparticles (LIG/ZnO) using femtosecond laser-induced zinc salt-containing cork. The LIG/ZnO heterojunction was formed by carbonizing the cork and decomposing the zinc salt in situ to coat LIG during laser irradiation. The correlation between the zinc salt concentration and the morphology, structure, and field-emission characteristics of LIG/ZnO was systematically investigated. The optimized LIG/ZnO-5 emitters demonstrate a decreased threshold electric field (Eth) of 1.07 V/μm, an enhanced field enhancement factor (β) value of 20,559, and a maximum current density of 18.07 mA/cm2 at 1.96 V/μm, representing the most superior performance reported to date. The enhanced field-emission performance of LIG/ZnO can be attributed to the larger number of effective emission sites from ZnO nanoparticles and LIG edges, as well as to the energy band variation induced by the heterostructures. Density functional theory (DFT) calculations show the migration of electrons from the LIG to the ZnO after the formation of the heterojunction, which elevates the Fermi level of emitters. In addition, the accumulation of electrons on the surface of ZnO nanoparticles in the presence of a strong electric field results in a downward shift of the energy band, thereby enhancing the likelihood of electron-vacuum tunneling.

Dynamical Performance of Graphene Aerogel with Ductile and Brittle Characteristics

AbstractCurrent research regarding the efficiency of ultra‐light graphene aerogel (GA) energy dissipation is limited to quasi‐static tests and simulations. The lack of direct dynamical experiments has impeded its utilization in fields of energy dissipation. Therefore, in this study, the high dynamic energy dissipation capability of GA with ultra‐low density is obtained directly from the experiment. It is found that the porous and anisotropic properties of GA render the projectile deflected hierarchically and further induce gradually cascaded failure with asymmetry expansion in the GA. This feature, taking advantage of ductile materials, facilitates energy dissipation capability. Failure morphologies of rippled graphene flakes involve brittle features such as micron‐size cracks and local broken flakes. In addition, these coarse‐grained molecular dynamics (CGMD) simulation results imply kinetic energy changes due to movement, and fluctuations of graphene flakes are effective ways to dissipate energy. Moreover, the stiffness increase of graphene flakes plays a weakened role in energy dissipation because reduced contact area impedes the effectiveness of stress wave and thermal transfer while also increasing the brittle characteristics of GA. Combining the failure characteristics of brittle materials with the benefits of ductile network materials, GA shows great promise in impact protection applications.

Influence of Defects and Charges on the Colloidal Stabilization of Graphene in Water.

Mastering graphene preparation is an essential step to its integration into practical applications. For large-scale purposes, full graphite exfoliation appears as a suitable route for graphene production. However, it requires overpowering attractive van der Waals forces demanding large energy input, with the risk of introducing defects in the material. This difficulty can be overcome by using graphite intercalation compounds (GICs) as starting material. The greater inter-sheet separation in GICs (compared with graphite) allows the gentler exfoliation of soluble graphenide (reduced graphene) flakes. A solvent exchange strategy, accompanied by the oxidation of graphenide to graphene, can be implemented to produce stable aqueous graphene dispersions (Eau de graphene, EdG), which can be readily incorporated into many processes or materials. In this work, we prove that electrostatic forces are responsible for the stability of fully exfoliated graphene in water, and explore the influence of the oxidation and solvent exchange procedures on the quality and stability of EdG. We show that the amount of defects in graphene is limited if graphenide oxidation is carried out before exposing the material to water, and that gas removal of water before the incorporation of pre-oxidized graphene is advantageous for the long-term stability of EdG.

Characterization of a Novel Cost-efficient and Environmentally Friendly Graphene-enhanced Thermal Interface Material

With the continuous development of electronic devices, effective heat dissipation has become a major factor affecting service life. Thermal interface materials (TIM) play a key role in controlling heat dissipation of electronic devices and have thus attracted widespread attention. In this study, we used graphene flakes (GF) derived from graphene film that is wasted during the preparation of commercial large-scale graphene-enhanced TIMs as thermally conductive fillers to formulate a new TIM. The thermal conductivity of the developed TIM is 50% higher with GFs than without. Furthermore, the TIM has a tensile strength of 0.46 MPa with an elongation at break of 1225%, a maximum compression strength of 0.64 MPa at 50% compression, and high mechanical cycle stability. This report provides a cost-efficient and environmentally friendly approach to producing high-performance TIMs for electronic cooling applications.

Multiscale analysis for characterization of tensile properties in extrusion-based 3D printed carbon nanocomposites

In this study, we proposed a multiscale modeling methodology to predict tensile properties of carbon-based nanocomposites produced through extrusion-based 3D printing. This stepwise method integrates results across scales, from the nanoscale to the macroscale. The stochastic continuum model, informed by molecular dynamics, captures the direction-dependent stiffness of nanocomposites at the atomic level, integrating the transverse isotropic properties and morphological variations, along with process-induced uncertainties in a microscale domain. Considering the variable periodic layered filament shape determined by the printing conditions, we defined the geometry of the representative volume element and derived its homogenized equivalent effective properties. We validated the results of our model by comparing them with experimental stress-strain curves. Using this approach, we simulated the nonlinear response and analyzed the off-axis tensile characteristics of the specimens under various printing conditions. The alignment and orientation of the graphene flakes played pivotal roles in determining the mechanical properties of the nanocomposites. The proposed approach effectively assessed the overall behavior of nanocomposites produced using an extrusion-based 3D printing process, helping to accurately predict the mechanical properties and investigate the effects of the parameters on the macroscopic nonlinear response in applications using 3D printed nanocomposites.