Patent Application
20170050856
Graphene is an allotrope of carbon with a two-dimensional atomic-scale hexagonal lattice structure in which each carbon atom forms a vertex in the lattice structure. Each carbon atom has four bonds, one σ bond with each of its three in-plane neighbors and one π bond that is oriented out of plane.
Due to its unique atomically thin two-dimensional lattice structure constructed with sp2-bonded carbons, graphene can exhibit extraordinary properties, such as high charge carrier mobility (over 2×105 cm2·V−1·s−1 at an electron density of 2×1011 cm−2), high thermal conductivity (over 3000 W·m−1·K−1), and exceptional Young modulus values (over 0.5 TPa). Furthermore, its high surface area, theoretically predicted as being over 2600 m2·g−1 and experimentally measured to be 400-700 m2-g−1 has also made graphene an attractive material. Such properties render graphene advantageous for use in numerous emerging applications in a broad range of fields, such as flexible electronics, photonics, energy conversion and storage, electrically/thermally conductive inks, and functional polymer composites.
For this reason, great efforts have been spent in the production of graphene. One route has been the development of defect-free, single-layer graphene sheets with the largest possible lateral size. Toward this goal, graphene can be obtained in the form of very-high-quality sheets produced in limited quantities using bottom-up methods, including chemical vapor deposition, annealing SiC substrates, and building up graphene from molecular building blocks. On the other hand, top-down methods for generating graphene from graphite still dominate in large-volume production of graphene in the scale from grams to kilograms to tons. These methods generate exfoliated graphene and have been widely used in making graphene composites.
The exfoliation of graphite into graphene requires counteracting the enormous van der Waals attraction between graphite layers, which is equivalent to an interlayer binding energy of about 1.65 eV·nm−2. Methods for achieving exfoliation include ultra-sonication or shearing-mixing-assisted exfoliation in organic solvent or surfactant solution; electrochemical exfoliation of graphite in electrolyte; and chemical reduction of exfoliated graphite oxide, with defect concentrations from low to high. One of the most well-known methods is to oxidize and exfoliate graphite into graphene oxide, then reduce the graphene oxide sheet to obtain graphene. Graphene produced via this process is call reduced graphene oxide (r-GO). However, compared to pristine graphene that is free of defects, graphene obtained from oxidation shows significantly reduced electrical properties owing to the considerable disruption by high concentration of defects, e.g., dangling bonds and out-of-plane sp3-carbon bonds.
A colloidal solution with pristine graphene that is free of defects, however, is intrinsically unstable. The exfoliation of graphite into graphene requires counteracting the enormous van der Waals attraction between graphite layers, which is equivalent to an interlayer binding energy of ˜1.65 eV·nm−2. After exfoliation, the solvent-graphene interaction needs to balance the inter-sheet attractive forces, or the graphene layers tend to aggregate in order to re-establish the graphitic structure and to minimize surface free energy. As shown in
The precipitation process follows a multi-component exponential decay (as shown by the plots of light absorbance through the bulk of 30-weight-% IPA aqueous solution 16 and through 30-weight-% aqueous solution NMP aqueous solution 18 over time in
Efforts have been spent on slowing down the re-stacking of graphene solution. Recently, Smith, et al., “The Importance of Repulsive Potential Barriers for the Dispersion of Graphene Using Surfactants,” 12 New J. Phys. 125008 (2010), investigated the dispersion of graphene using 12 ionic and non-ionic surfactants; they found that a larger absolute zeta potential is critical for better dispersion and slower aggregation. From a reaction kinetics point of view, this re-stacking process is also dependent on graphene concentration, where a lower graphene concentration leads to less collision and slower restacking. Ultra-sonication produces a graphene solution with concentration up to 15 mg/ml. This concentration may be too low for using the solution as a precursor for making graphene composites. Concentrating the solutions to a concentration more than 10 times greater or even drying the solutions into graphene dry powder is essential in most cases for making graphene composites.
Concentrating or drying graphene solution makes the re-stacking effect significant, which cancels the great effort spent on the exfoliation processes. Once this happens, long-term ultra-sonication is applied again to transfer the re-stacked graphene into monolayer or few-layer graphene. The aggregation of graphene in solution is more problematic for industrial storage and transport. On one hand, a solution with a low graphene concentration demands significantly more transportation efforts, with the solvent being the majority of what is being transported. On the other hand, long-term storage or transportation destroys the value of the graphene products due to the aggregation of graphene into multi-layer or even graphite.
In short, graphene flakes in their colloidal solution tend to restack and precipitate during their storage, leading to inconsistency in their following applications. Moreover, the solvent occupies the major weight and volume of the solution and, therefore, significantly increases the transportation cost.
A re-dispersible dry graphene powder and methods for its production are described herein, where various embodiments of the compositions and methods may include some or all of the elements, features and steps described below.
As described herein, a re-dispersible dry graphene powder can be produced by a method comprising producing a solution of graphene sheets in solvent; adding surfactant to the solution; and then drying the solution to produce dry graphene sheets coated with surfactant that stabilizes the dry graphene sheets.
The re-dispersible surfactant-stabilized dry graphene powder can remedy the problem of re-stacking, described above, while allowing for storage transport of the graphene in a very concentrated form. Moreover, the graphene powder can be quickly and easily re-dispersed (e.g., with just one ultra-sonication treatment). Additionally, the surfactant can act simultaneously both as the exfoliation agent and as the stabilization agent. Further still, the re-dispersible graphene dry powder offers not only better weight economy for storage and transport, but also better usability for making composites.
In the accompanying drawings, like reference characters refer to the same or similar parts throughout the different views; and apostrophes are used to differentiate multiple instances of the same or similar items sharing the same reference numeral. The drawings are not necessarily to scale; instead, emphasis is placed upon illustrating particular principles in the exemplifications discussed below.
The foregoing and other features and advantages of various aspects of the invention(s) will be apparent from the following, more-particular description of various concepts and specific embodiments within the broader bounds of the invention(s). Various aspects of the subject matter introduced above and discussed in greater detail below may be implemented in any of numerous ways, as the subject matter is not limited to any particular manner of implementation. Examples of specific implementations and applications are provided primarily for illustrative purposes.
Unless otherwise herein defined, used or characterized, terms that are used herein (including technical and scientific terms) are to be interpreted as having a meaning that is consistent with their accepted meaning in the context of the relevant art and are not to be interpreted in an idealized or overly formal sense unless expressly so defined herein. For example, if a particular composition is referenced, the composition may be substantially (though not perfectly) pure, as practical and imperfect realities may apply; e.g., the potential presence of at least trace impurities (e.g., at less than 1 or 2%) can be understood as being within the scope of the description; likewise, if a particular shape is referenced, the shape is intended to include imperfect variations from ideal shapes, e.g., due to manufacturing tolerances. Percentages or concentrations expressed herein can represent either by weight or by volume. Processes, procedures and phenomena described below can occur at ambient pressure (e.g., about 50-120 kPa—for example, about 90-110 kPa) and temperature (e.g., −20 to 50° C.—for example, about 10-35° C.) unless otherwise specified.
Although the terms, first, second, third, etc., may be used herein to describe various elements, these elements are not to be limited by these terms. These terms are simply used to distinguish one element from another. Thus, a first element, discussed below, could be termed a second element without departing from the teachings of the exemplary embodiments.
Spatially relative terms, such as “above,” “below,” “left,” “right,” “in front,” “behind,” and the like, may be used herein for ease of description to describe the relationship of one element to another element, as illustrated in the figures. It will be understood that the spatially relative terms, as well as the illustrated configurations, are intended to encompass different orientations of the apparatus in use or operation in addition to the orientations described herein and depicted in the figures. For example, if the apparatus in the figures is turned over, elements described as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, the exemplary term, “above,” may encompass both an orientation of above and below. The apparatus may be otherwise oriented (e.g., rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly.
Further still, in this disclosure, when an element is referred to as being “on,” “connected to,” “coupled to,” “in contact with,” etc., another element, it may be directly on, connected to, coupled to, or in contact with the other element or intervening elements may be present unless otherwise specified.
The terminology used herein is for the purpose of describing particular embodiments and is not intended to be limiting of exemplary embodiments. As used herein, singular forms, such as “a” and “an,” are intended to include the plural forms as well, unless the context indicates otherwise. Additionally, the terms, “includes,” “including,” “comprises” and “comprising,” specify the presence of the stated elements or steps but do not preclude the presence or addition of one or more other elements or steps.
Eliminating the use of solvent for graphene storage and transport and, instead, producing and maintaining the graphene in a dry-powder form represents a significant advancement. Whenever needed, the dry graphene powders 20, as described herein, can be easily redispersed into colloidal solution 12″ with a quality and thickness distribution similar to that of the initial colloidal solution 12′ (as shown in
Electrochemically exfoliated (EE) graphene was chosen as a sample material in this study because it balances production yield (32 g per hour), defect concentration (C/O=17.2) and lateral size (several micrometers). The results and conclusions are potentially applicable to graphene ink prepared by other methods. The electrochemically exfoliated graphene was prepared based on a previously reported method [C. Y. Su, et al., “High-Quality Thin Graphene Films from Fast Electrochemical Exfoliation,” 5 ACS Nano 2332-2339 (2011)].
Briefly, graphite flakes (99.9%, #43319 from Alfa Aesar of Ward Hill, Mass., US) were electrolyzed in an aqueous solution with a mixture of 0.2M K2SO4 and 0.1M KOH. An alternating bias between +10 and −10 V was applied, each for 5 seconds. The resulting mixture was vacuum filtrated and re-dispersed four times in water first and then twice in 30 wt % isopropyl alcohol (IPA). Finally, the mixture was centrifuged twice at 1000 g for 10 minutes to get rid of bulk graphite. The concentration of the resulting electrolyzed graphene solution 12′ in 30 wt % IPA was adjusted to 1 mg/ml. 30-wt % IPA was used as the solvent due to its low toxicity and high vapor pressure. One tenth of the solution 12′ was oven dried and weighed to obtain the weight concentration, c, of the graphene solution. Absorbance of the solution at 650 nm was measured; and the absorption coefficient, α650, of the graphene solution was determined to be 2,230 L g−1 m−1 using the Beer-Lambert law.
The optical microscopic image of
Two different techniques were used to remove the solvent and to thereby transform the graphene into dry powders 20. The techniques include vacuum filtration and solvent thermal evaporation. In a typical re-dispersion experiment, after sonication and before centrifugation, 3 mL of the pristine graphene solution 12′ was transferred to a 15-mL centrifuge tube and frozen in liquid nitrogen. The centrifuge tube was left open, and the frozen solution was then dried using a freeze dry system (from Labconco of Kansas City, Mo., US) working at −80° C. and 0.1 mbar. The pristine graphene solution 12′ was also dried using thermal solvent evaporation in an oven at 60° C., 1 atmosphere for comparison. Scanning electron microscopic images reveal the difference between the resulting dry graphene powders 20. Dry graphene that is produced via freeze-drying exhibits meso-pores with pore sizes in the range of several micrometers (e.g., less than 10 μm, as shown in
The resulting dry powder 20 was added with the same amount of 3-mL 30-wt % IPA; next, ultra-sonication was performed for one minute. The dispersion was centrifuged at a relative centrifugation force of 500 g for 20 minutes, and the supernatant was collected as the re-dispersed graphene solution 12″, as shown in
The low re-dispersion of the graphene indicates that the majority of the graphene remains as aggregates after the one minute of ultra-sonication. Extending the sonication period helps to re-disperse the graphene powder but is un-desirable because the sonication fractures the graphene sheets and reduces their aspect ratio, while at the same time increasing processing duration and cost. For the re-dispersed lyophilized graphene powder, the atomic force microscopy (AFM) measurements reveal a maximum thickness probability of 3 nm with a narrow thickness distribution from 1 to 6 nm, as shown in
Compared with solvent thermal evaporation, freeze drying offers a better re-dispersion ratio of 0.5. This ratio, however, is still not ideal for practical application. To further improve the re-dispersion of the graphene dry powder 20, surfactant molecules were added to the graphene solution 12′ before drying. The solution with surfactant was ultra-sonicated for one minute and then kept still for ten minutes to enable surfactant adsorption on the graphene to reach equilibrium. The solution 12′ was then dried by freeze-drying, and the dry graphene powder 20 was re-dispersed. The surfactant/graphene weight ratio was varied from ⅛ to 2 to study the re-dispersion as a function of the mass ratio between the surfactant and the graphene. Typically, re-dispersible graphene with a lower amount of stabilizer is more desirable.
Interestingly, the re-dispersion of graphene powder without surfactant depends intensively on the initial concentration of the graphene solution (as shown in
To better understand the re-dispersion of graphene dry powder, we look into the interlayer binding energy between graphene sheets. The specific binding energy per unit area between parallel graphene sheets can be expressed as the Lennard-Jones potential, E, as follows:
wherein d is the interlayer distance. The differences in the exponents compared to the traditional Lennard-Jones potential, E=c12/d12−c6/d6, compensate for the two-dimensional planar atomic structure of the graphene. For pristine graphene, as shown in
The magnitude of this binding energy represents the energy that is needed to peel these two graphene sheets 20 apart. In other words, if an easy separation of two graphene sheets 20 is expected, efforts are needed to reduce the interlayer binding energy. Since the van der Waals attraction is a short range interaction and is strongly dependent on the interlayer distance, a small increase in the interlayer distance can dramatically reduce the magnitude of the van der Waals attraction force. This can be achieved by artificially inserting a steric layer between the adjacent graphene sheets 20, which is also called intercalation. We demonstrate this point theoretically by adding a steric thickness parameter, ds, into the steric repulsion term in Equation 1.
The interaction can then be written as follows:
where Es, is the specific binding energy between two graphene layers.
By using this equation, we assume (1) that the steric layer has steric repulsive interaction with both graphene sheets and (2) that the steric layer does not simultaneously have van der Waals interaction with both graphene sheets. In other words, the steric layer mediates the steric repulsion but does not mediate the van der Waals attraction. In this case, the interlayer distance, d, in the steric repulsion term is defined to be larger than ds. It is worth notice that increasing the steric layer thickness drastically decreases the interlayer binding energy (as shown in
For example, a steric layer thickness of only 0.55 nm can diminish the binding energy by two orders of magnitude. Consequently, binding energy can be used as a quantitative parameter for evaluating the re-dispersity of graphene; i.e., a smaller binding energy indicates easier re-dispersion.
To quantify the minimum energy input required to fully disperse one gram of graphene dry powder into graphene colloidal solution, the total energy per gram of graphene powder, Etotal) is calculated using the specific binding energy, Es, and the surface area, A, of the graphene powder, as follows:
wherein 2600 m2 is the theoretical maximum surface area, A, per gram of graphene. The specific surface area of the graphene powder can be estimated experimentally from a gas adsorption/desorption isotherm; and the specific binding energy, Es, can be estimated from the interlayer distance, d. The total binding energy, Etotal) provides a practical way to quantify the energy input that is necessary to disperse the graphene from a powder state. It is advantageous to produce graphene dry powder with a larger specific surface area and smaller binding energy. Not only sodium deoxycholate (SDOC), but other surfactant molecules that can effectively reduce the interlayer binding energy between graphene flakes can also be used to produce re-dispersible graphene dry powder. To further illustrate the genericity of this method, eight different surfactants were systematically compared (see
Before each batch of ultra-sonication treatment, the sonicator was refilled with 1 L of distilled water at 20±1° C. The water temperature rose to 40±2° C. at the end of the one-hour sonication. After sonication, the solution was centrifuged (using an EPPENDORF 5804 R microcentrifuge) at a relative centrifugal force of 500 g (1700 round per minute) for 20 minutes, the supernatant was carefully collected as the graphene product without disturbing the sediment. The absorbance of the obtained graphene solution using SDOC as the surfactant is plotted as dots 32 versus the surfactant concentration in
As the stabilizer for graphene, the surfactant typically has a dual role. First, the surfactant lowers the liquid-vapor interfacial energy, also indicated as the surface tension, of the solution, e.g., to an optimum range corresponding to the energy required to separate the sheets beyond the range of the van der Waals forces. That is, the work of cohesion of the aqueous phase and the sheets of graphene within the graphite solid are comparable. Second, the charged surfactant adsorbs onto the exfoliated graphene sheets, creating an extra electrostatic repulsive term that prevents the re-aggregation of the sheets in the solution. Starting from pure water with a surface tension of ˜73 mN·m−1, addition of surfactant decreases the surface tension until it reaches the optimum value of ˜40 ml·m−1 for graphene dispersion.
Moreover, the surfactant also increases the surface charging of graphene due to adsorption of more surfactant ions on graphene. This absorption explains the rise of graphene production when surfactant concentration increases from zero to the optimum concentration. Further increases in surfactant concentration induces two negative affects to graphene dispersion. On one hand, surface tension is hence reduced to such an extent that exfoliation is not preferable because the energies are no longer matched. On the other hand, for ionic surfactants, addition of surfactant increases the ionic strength of the solution, thus compressing the Debye length of the electrical double layer on the graphene surface, and screening the graphene surface charge that is a consequence of the adsorption of surfactant ions. The compressed Debye length reduces the range of electrostatic repulsion, and the screened surface charge lowers the electrostatic barrier height. These effects explain the drop of graphene production when the concentration is further increased to higher than the optimum concentration. From the results reported here, all of the surfactants followed the volcano-shaped surfactant performance as a function of concentration.
As a reference, our results for SC and SDBS show similar optimized surfactant concentrations of ˜0.1 g/L and ˜0.4 g/L, respectively, as the works reported in M. Lotya et al., “Liquid Phase Production of Graphene by Exfoliation of Graphite in Surfactant/Water Solutions,” 131J. Am. Chem. Soc. 3611-3620 (2009) and “High-Concentration Surfactant-Stabilized Graphene Dispersions”, 4 AC S Nano 3155-3162 (2010).
To investigate the re-dispersity of surfactant-stabilized graphene, the water was removed, and the exfoliated graphene sample was made into dry powder using the same two methods, described above. Then, the same amount of water was added, and the concentration of graphene that is re-dispersed is quantified. Each of the dried samples was added with 3 mL of de-ionized (DI) water, ultra-sonicated for one minute, and centrifuged at 500 g for 20 minutes; and the supernatant was collected for absorbance measurement. The absorbance of the re-dispersed graphene stabilized using different concentrations of SDOC was plotted with circles 34 for freeze-dried samples and with circles 36 for oven-dried samples in
To compare the re-dispersion performance for different surfactants, the highest absorbance of the exfoliated graphene was selected for each surfactant; and the data is plotted in
In conclusion, we have demonstrated a general guideline to make graphene dry powder that can be easily re-dispersed by, e.g., one-minute ultra-sonication. Freeze-dried graphene powder was found to re-disperse better than graphene powder obtained from thermal solvent evaporation. Adding surfactant will further increase the re-dispersity. The overall energy input needed to re-disperse the graphene flakes was modeled theoretically, from which the energy landscape can be simplified into two variables, the interlayer binding energy and the specific surface area. The less energy input that is needed, the easier the re-dispersion. The model confirms that graphene dry powder with larger specific surface area and smaller interlayer binding energy exhibit less overall binding energy and is, therefore, easier to disperse. The generality of the re-dispersion method was further compared experimentally by using surfactant molecules other than SDOC. The results shown that both SDOC and P123 triblock copolymer (HO(CH2CH2O)20(CH2CH(CH3)O)70(CH2CH2O)20H) perform well in stabilizing the graphene flakes and making them into re-dispersible powders.
Additional examples consistent with the present teachings are set out in the following numbered clauses:
In describing embodiments of the invention, specific terminology is used for the sake of clarity. For the purpose of description, specific terms are intended to at least include technical and functional equivalents that operate in a similar manner to accomplish a similar result. Additionally, in some instances where a particular embodiment of the invention includes a plurality of system elements or method steps, those elements or steps may be replaced with a single element or step; likewise, a single element or step may be replaced with a plurality of elements or steps that serve the same purpose. Further, where parameters for various properties or other values are specified herein for embodiments of the invention, those parameters or values can be adjusted up or down by 1/100th, ⅕th, 1/20th, 1/10th, ⅕th, ⅓rd, ½, ⅔rd, ¾th, ⅘th, 9/10th, 19/20th, 49/50th, 99/100th, etc. (or up by a factor of 1, 2, 3, 4, 5, 6, 8, 10, 20, 50, 100, etc.), or by rounded-off approximations thereof, unless otherwise specified. Moreover, while this invention has been shown and described with references to particular embodiments thereof, those skilled in the art will understand that various substitutions and alterations in form and details may be made therein without departing from the scope of the invention. Further still, other aspects, functions and advantages are also within the scope of the invention; and all embodiments of the invention need not necessarily achieve all of the advantages or possess all of the characteristics described above. Additionally, steps, elements and features discussed herein in connection with one embodiment can likewise be used in conjunction with other embodiments. The contents of references, including reference texts, journal articles, patents, patent applications, etc., cited throughout the text are hereby incorporated by reference in their entirety; and appropriate components, steps, and characterizations from these references may or may not be included in embodiments of this invention. Still further, the components and steps identified in the Background section are integral to this disclosure and can be used in conjunction with or substituted for components and steps described elsewhere in the disclosure within the scope of the invention. In method claims (or where methods are elsewhere recited), where stages are recited in a particular order—with or without sequenced prefacing characters added for ease of reference—the stages are not to be interpreted as being temporally limited to the order in which they are recited unless otherwise specified or implied by the terms and phrasing.
This application claims the benefit of U.S. Provisional Application No. 62/206,527, filed 18 Aug. 2015, the entire content of which is incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 62206527 | Aug 2015 | US |
