Actuation triggered exfoliation of graphene oxide at low temperature for electrochemical capacitor applications

An actuation triggered thermal exfoliation process was realized at a low temperature of 200 °C and atmospheric pressure. Densely packed graphene oxide (GO) paper was transformed into airy-like expanded architectures. The underlying mechanism was found to be similar to corn popping and attributed to the thermally-stimulated-actuation and molecules escape. Scanning electron microscopy, X-ray photoelectron spectra and electrochemical characterization showed that after the exfoliation process, the resultant popped graphene oxide (P-GO) exhibited highly expanded structures; the oxygen-containing groups were effectively removed; and the P-GO demonstrated good electrochemical capacitance performance with excellent stability.     

Towards scale‐up of graphene production via nonoxidizing liquid exfoliation methods

Graphene, the two‐dimensional form of carbon, has received a great deal of attention across academia and industry due to its extraordinary electrical, mechanical, thermal, chemical, and optical properties. In view of the potential impact of graphene on numerous and diverse applications in electronics, novel materials, energy, transport, and healthcare, large‐scale graphene production is a challenge that must be addressed. In the past decade, top–down production has demonstrated high potential for scale‐up. This review features the recent progress made in top–down production methods that have been proposed for the manufacturing of graphene‐based products. Fabrication methods such as liquid‐phase mechanical, chemical and electrochemical exfoliation of graphite are outlined, with a particular focus on nonoxidizing routes for graphene production. Analysis of exfoliation mechanisms, solvent considerations, key advantages and issues, and important production characteristics including production rate and yield, where applicable, are outlined. Future challenges and opportunities in graphene production are also highlighted. © 2018 American Institute of Chemical Engineers AIChE J, 64: 3246–3276, 2018     

低温氧化石墨制备技术研究

采用Hummers法、硫酸/磷酸法和反应釜法3种化学氧化法制备氧化石墨,在超声情况下,将氧化石墨分散于水中制备氧化石墨烯。采用X衍射(XRD)、扫描电镜(SEM)对所得氧化石墨、氧化石墨烯分别进行表征。并对制备氧化石墨的3种化学氧化过程进行对比分析,结果表明,Hummers法在高温阶段有红色刺激性气体放出,且发生喷溅现象。硫酸/磷酸法和反应釜法是在低温条件下进行,制备过程比Hummers法安全、环保。氧化石墨XRD结果表明,3种化学氧化法中氧化程度最好的是硫酸/磷酸法,其次是反应釜法,最后是Hummers法。氧化石墨烯SEM结果表明,氧化石墨经超声处理后剥离成为氧化石墨烯纳米片。     

Tri-layer graphene films produced by mechanochemical exfoliation of graphite

We report a method for the production of transparent thin films composed in majority (59%) by tri-layer graphene, in which both the synthesis and processing are combined in one single step. Starting from graphite, two mechanical exfoliation processes are performed, followed by chemical/thermal exfoliation in a water/oil two phase system, which yields few layer graphene flakes spontaneously self-assembled at the water/oil interface as a thin film that is easily transferred to an arbitrary substrate.     

Cationic surfactant mediated exfoliation of graphite into graphene flakes

A simple and effective method for the preparation of a few layered graphene nanoflakes directly from graphite has been successfully demonstrated. Mild ultrasonication of highly ordered pyrolytic graphite, in presence of a cationic surfactant cetyltrimethylammonium bromide and acetic acid yielded graphene nanoflakes, which formed a stable colloidal suspension in organic solvent such as N,N-dimethyl formamide. Scanning and transmission electron microscopic analyses showed that the dispersed phase consist of mainly few layered graphene nanoflakes. Average thickness of the flakes was found to be ∼1.18 nm. Energy dispersive X-ray analysis indicated the absence of graphene oxide. Field emission measurements for the nanoflakes showed a turn on voltage of 7.5 V/μm and emission current densities of 0.15 mA/cm².     

Molecular simulation of interfacial mechanics for solvent exfoliation of graphene from graphite

The interfacial force for exfoliating a graphene monolayer from graphite in solvent media was studied by restrained molecular dynamics simulations. Three solvents, N-methylpyrrolidone (NMP), dimethyl sulfoxide (DMSO), and water, were considered. The interfacial structures show that NMP and DMSO have a stronger affinity with graphene surfaces. In the solvent media, there exists an inherent attractive force hindering the exfoliation, which is almost exclusively determined by the interaction between graphene sheets. Along the perpendicular exfoliation direction (relative to the graphene plane), the initial exfoliation is less dependent upon solvent conditions, and the subsequent exfoliation can be facilitated by the solvent-induced interaction. However, along the shift direction parallel to the graphene plane, the organic solvent provides a favorable driving force to assist the exfoliation, whereas water offers obstructing effect. The parallel shift of graphene requires less external power than the perpendicular shift for our simulated systems. The confined solvent molecules between graphene sheets play an important role in exfoliating and stabilizing graphene in solvent media. This result provides a microscopic understanding of the function of solvent-induced interaction in the solvent exfoliation of graphene.     

Nitroxide-functionalized graphene oxide from graphite oxide

A facile method for preparing functionalized graphene oxide single layers with nitroxide groups is reported herein. Highly oxidized graphite oxide (GO = 83.1%) was obtained, slightly modifying an improved Hummer’s method. Oxoammonium salts (OS) were investigated to introduce nitroxide groups to GO, resulting in a one-step functionalization and exfoliation. The mechanisms of functionalization/exfoliation are proposed, where the oxidation of aromatic alcohols to ketone groups, and the formation of alkoxyamine species are suggested. Two kinds of functionalized graphene oxide layers (GOFT1 and GOFT2) were obtained by controlling the amount of OS added. GOFT1 and GOFT2 exhibited a high interlayer spacing (d₀₀₀₁ = 1.12 nm), which was determined by X-ray diffraction. The presence of new chemical bonds C–N (∼9.5%) and O–O (∼4.3%) from nitroxide attached onto graphene layers were observed by X-ray photoelectron spectroscopy. Single-layers of GOFT1 were observed by HRTEM, exhibiting amorphous and crystalline zones at a 50:50 ratio; in contrast, layers of GOFT2 exhibited a fully amorphous surface. Fingerprint of GOFT1 single layers was obtained by electron diffraction at several tilts. Finally, the potential use of these materials within Nylon 6 matrices was investigated, where an unusual simultaneous increase in tensile stress, tensile strain and Young’s modulus was observed.     

Microwave assisted exfoliation and reduction of graphite oxide for ultracapacitors

We report a simple yet versatile method to simultaneously achieve the exfoliation and reduction of graphite oxide. By treating graphite oxide powders in a commercial microwave oven, reduced graphite oxide materials could be readily obtained within 1 min. Extensive characterizations showed that the as-prepared materials consisted of crumpled, few-layer thick and electronically conductive graphitic sheets. Using the microwave exfoliated graphite oxide as electrode material in an ultracapacitor cell, specific capacitance values as high as 191 F/g have been demonstrated with KOH electrolyte.     

The production of concentrated dispersions of few-layer graphene by the direct exfoliation of graphite in organosilanes

We report the formation and characterization of graphene dispersions in two organosilanes, 3-glycidoxypropyl trimethoxysilane (GPTMS) and phenyl triethoxysilane (PhTES) as new reactive solvents. The preparation method was mild and easy and does not produce any chemical modification. The dispersions, which exhibit the Tyndall effect, were characterized by TEM and Raman spectroscopy to confirm the presence of few-layer graphene. Concentrations as high as 0.66 and 8.00 mg/ml were found for PhTES and GPTMS, respectively. The latter is one of the highest values reported for a dispersion of graphene obtained by any method. This finding paves the way for the direct synthesis of polymer nanofiller-containing composites consisting of graphene and reactive silanes to be used in sol–gel synthesis, without any need for solvent removal, thus preventing graphene reaggregation to form graphite flakes.     

Vacuum-assisted microwave reduction/exfoliation of graphite oxide and the influence of precursor graphite oxide

One-step synthesis of high quality graphene at gram-scale quantities is important for industrial applications, e.g. in electrochemistry for sensing and energy storage. Currently, thermal reduction/exfoliation of graphite oxide (GO) is a typical method of choice. However, it has the drawback of requiring specialized equipment for rapid thermal shock. A recent alternative method, microwave-assisted exfoliation, usually suffers from poor reduction of graphite oxide and thus low C/O ratios. Herein we show that vacuum-assisted microwave reduction/exfoliation of graphite oxide in a closed system leads to high C/O ratios and partial hydrogenation of graphene (2.6 at.% of H). Microwave irradiation of graphite oxide in vacuum leads to outgassing from GO and the creation of plasma which aids temperature distribution and hydrogenation. This plasma is quickly extinguished by further dramatic evolution of gases from GO and consequent pressure increase. We assess the influence of precursor graphite oxide, prepared by Hummers, Staudenmaier, and Hofmann methods, upon the materials properties of microwave exfoliated graphene. We show that microwave-exfoliated graphenes prepared from different graphite oxides show very fast heterogeneous electron transfer rates, with similar electrochemical behaviour to thermally reduced graphene oxide.     

Monolayer graphene from graphite oxide

Graphene, a new carbon material, is attracting presently an increasing research interest. It stems from the unique electrical and mechanical properties of graphene predicted by theory. Experimental studies of graphene are, however, severely curtailed by a lack of an appropriate technique for its preparation. Mechanical cleavage of graphite proved to be ineffective, since it yields only very small (a few microns in size) particles of monolayer graphene. The rapidly developing approach based on chemical exfoliation of graphite produces large-area coatings composed primarily of arbitrarily oriented multilayer graphene particles. We have developed a technique for preparation of monolayer graphene sheets involving liquid exfoliation of crystalline graphite, which includes synthesis of graphite oxide by deep oxidation as an intermediate stage. Electron diffraction traces, as well as the variation of diffracted intensities with local orientation of graphene sheets, AFM, and HRTEM images testify to a remarkably good monolayer structure of the graphite oxide particles obtained by our technique. These results open a way to setting up high-efficiency production of monolayer graphene sheets appropriate for electrical and optical measurements and fabrication of structures for use in the field of applications.     

Synthesis and exfoliation of isocyanate-treated graphene oxide nanoplatelets

A number of functionalized graphite oxides were prepared by treatment of graphite oxide (GO) with organic isocyanates. These isocyanate-treated GOs (iGOs) can then be exfoliated into functionalized graphene oxide nanoplatelets that can form a stable dispersion in polar aprotic solvents. Characterization of iGOs by FT-IR spectroscopy and elemental analysis suggested that the isocyanate treatment results in the functionalization of the carboxyl and hydroxyl groups in GO via formation of amides and carbamate esters, respectively. The degree of GO functionalization can be controlled via either the reactivity of the isocyanate or the reaction time. When used with functionalized isocyanates, the described methodology allows for the elaboration of graphene oxide nanoplatelets with different surface functional groups.     

Permanent adsorption of organic solvents in graphite oxide and its effect on the thermal exfoliation

The dispersion of graphite oxide (GO) in organic solvents followed by their evaporation at relative high temperature resulted in a strong adsorption of the solvent molecules in the graphitic interlayers as confirmed by ¹³C magic-angle-spinning NMR. Three series of solvents, alcohols (1-methanol, 1-propanol, 1-pentanol, 1-heptanol), aromatics (benzene, toluene, p-xylene, chlorobenzene) and chloride compounds (dichloromethane, chloroform, carbon tetrachloride) were studied to understand the interaction of graphite oxide with solvents. The distribution of basal interlayer spacing changed due to solvent intercalation and this distribution was particularly different for each solvent series. Even though there was on average 1 solvent molecule per 100 carbon atoms of GO, they had a profound effect on the thermal properties of the resulting GO. The exfoliation temperature was drastically reduced by the presence of the solvents due to the increase of the interlayer spacing and the reducing power of the solvent.     

High-yield exfoliation of graphite in acrylate polymers: A stable few-layer graphene nanofluid with enhanced thermal conductivity

High-yield exfoliation of pristine graphite in low boiling point alcohols was achieved using a set of acrylate polymers resulting in few-layer graphene concentrations of up to ∼4 mg mL⁻¹. The polymer showed superior dispersing capabilities for graphene compared to the best reported dispersants, including the solvent N-methyl-pyrrolidone, the surfactants sodium cholate and sodium taurodeoxycholate, and the polymer polyvinylpyrrolidone. The dispersions were stable regardless of freezing (−26 °C) or heating (70 °C) for 24 h, or dilution with water up to 80% volume ratio over 160 h. The as-obtained nanofluid exhibited an enhancement in thermal conductivity suggesting a great potential in coolant applications.     

A note on using expanded graphite for achieving energy- and time-efficient production of graphene nanoplatelets via liquid phase exfoliation

Although easily scalable, the production of graphene nanoplatelets (GNP) by the means of liquid-phase exfoliation of graphite flakes (GF) remains an energy- and time-intensive process. In this work, we demonstrate that significant time and energy can be saved in GNP production when employing expanded graphite (EG) in a surfactant-assisted liquid phase exfoliation process. Owing to its increased interlayer distance, the exfoliation of EG can be accomplished in a much shorter time (<30 min) compared to GF (approximately 7 h in the present case). Moreover, the energy required for the EG exfoliation is close to 80-fold lower than that for GF exfoliation. Monitoring of the mean lateral dimension, specific surface area, and graphite flake-to-GNP transition during exfoliation was performed experimentally using several analytical techniques. The EG-derived GNPs are produced much faster and require less energy for exfoliation compared to GF, thus making it a more efficient alternative technique.     

A simple method for graphene production based on exfoliation of graphite in water using 1-pyrenesulfonic acid sodium salt

In this work we use a chemical approach based on supra-molecular and non-covalent interactions between graphene and 1-pyrenesulfonic acid sodium salt (Py–1SO₃) to obtain a stable dispersion of graphene by using only water as solvent. The material has been characterized by a combination of spectroscopic and microscopic techniques. In particular, an extensive Raman analysis shows that we have ∼70% few-graphene layers (<7). We also show that the exfoliation efficiency strongly depends on the number of functional groups by comparing suspensions obtained by Py–1SO₃ and 1,3,6,8-pyrenetetrasulfonic acid (Py–4SO₃). A strong decrease in the exfoliation yield has been observed by using pyrene with 4 sulphonic groups (Py–4SO₃), as compared to one sulphonic group (Py–1SO₃). Being completely water-based, these suspensions can be used as inks for printable tattoo-based electro-chemical sensors.     

The effect of heating rate on porosity production during the low temperature reduction of graphite oxide

We studied the relationship between the thermal conditions and the evolution of the porosity in thermally reduced graphite oxide (TH-rGO) and found that the heating rate, rather than reduction temperature and atmospheric condition, played a crucial role in the evolution of porosity during the thermal reduction of GO at low temperatures. Higher heating rates increased the porosity of the TH-rGO. A slow heating rate facilitated the evolution of H₂O and CO₂ whereas a higher heating rate released CO₂ and CO gases with the concurrent development of a folded and crumpled morphology. We further demonstrated that the higher heating rate resulted in a highly porous texture with lower reduction temperature (below 140 °C) and shorter reduction time (less than 5 min), indicating that the reduction time and temperature are found to be dependent on the heating rate.     

Size-controlled synthesis of graphite nanoflakes and multi-layer graphene by liquid phase exfoliation of natural graphite

Multi-layer graphene and graphite nanoflakes were produced through graphite liquid exfoliation using organic solvents. The nanoflakes size distribution was statistically analyzed, with the number of measured samples being high enough (from ∼200 to 900) for reliable evaluation of the statistical model. The nanoflakes size data were found to follow a log-normal distribution, with higher fraction of large size flakes as compared to a conventional normal distribution. The same kind of distribution was also obtained for nanoflakes thickness. Based on these findings, the detailed mechanism of the pristine polycrystalline graphite exfoliation in a liquid phase due to formation and collapse of cavitation bubbles was discussed. The high quality of nanoflakes was confirmed by Raman spectroscopy.     

Large scale production of highly conductive reduced graphene oxide sheets by a solvent-free low temperature reduction

A novel one-pot process that can produce freestanding reduced graphene oxide (RGO) sheets in large scale through a mechanochemical method is presented, which is based on a 1:1 adduct of hydrazine and carbon dioxide (H₃N⁺NHCO₂⁻, solid hydrazine). We were able to synthesize RGO sheets by grinding solid hydrazine with graphene oxide (GO), followed by storing the mixed powder at 50 °C for 10 min. No solvents, nor large vessels, nor post-annealing at high temperatures are required. The resulting RGO sample was characterized by elemental analysis, X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy, Brunauer–Emmett–Teller measurement, thermo gravimetric analysis, Fourier transform infrared spectroscopy, solid state nuclear magnetic resonance spectroscopy, and conductivity measurement. It exhibits excellent conductivity and possesses a high specific surface area. This reduction method was successfully applied for the fabrication of inkjet-printed RGO devices on a flexible substrate.     

Ammonia borane assisted solid exfoliation of graphite fluoride for facile preparation of fluorinated graphene nanosheets

Fluorinated graphene, which combines the unique properties of graphite fluoride and graphene, has attracted considerable attention in recent years. Here, we developed a facile, efficient, and scalable method for high-yield exfoliation of graphite fluoride into fluorinated graphene (fluorographene) nanosheets. The exfoliation approach consists of solid ball milling of graphite fluoride with ammonia borane and followed washing with ethanol to get rid of ammonia borane from the products. The majority of the as-synthesized fluorographene nanosheets consist of 1–6 atomic layers with grain sizes in the range of 0.3–1 μm. X-ray diffraction, Raman spectroscopy, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy demonstrated that fluorographene has the same structure as pristine graphite fluoride.