Patent Application
20080048152
One preferred specific embodiment of the present invention is a process for producing a nano-scaled graphene plate (NGP) material that is essentially composed of a sheet of graphene plane or multiple sheets of graphene plane stacked and bonded together. Each graphene plane, also referred to as a graphene sheet or basal plane, comprises a two-dimensional hexagonal structure of carbon atoms. Each plate has a length and a width parallel to the graphite plane and a thickness orthogonal to the graphite plane. The thickness of an NGP is 100 nanometers (nm) or smaller. The length and width of a NGP could exceed 1 μm. Preferably, however, both length and width are smaller than 1 μm. Graphite is but one of the many examples of laminar or layered materials that can be exfoliated to become nano-scaled platelets. A layered inorganic compound may be selected from (a) clay; (b) bismuth selenides or tellurides; (c) transition metal dichalcogenides; (d) sulfides, selenides, or tellurides of niobium, molybdenum, hafnium, tantalum, tungsten or rhenium; (e) layered transition metal oxides; (f) graphite or graphite derivatives; (g) pre-intercalated compounds, or a combination thereof. For instance, both un-intercalated and intercalated graphites are commercially available, which can be exfoliated with the presently invented pressure reduction process, with or without heat. The presently invented process works for all of these classes of laminar materials.
Generally speaking, a process has been developed for exfoliating a layered or laminar material to produce nano-scaled platelets having a thickness smaller than 100 nm. The process comprises: (a) placing a layered material to a gaseous environment at a first temperature and first pressure sufficient to cause gas species to penetrate (into the interstitial space) between layers of the layered material, forming a gas-intercalated layered material; and (b) subjecting the gas-intercalated layered material to a second pressure, or a second pressure and a second temperature, allowing gas species to pressurize or expand the interstitial space, thereby exfoliating the layered material to produce the platelets.
In a preferred embodiment, referring to
The pressurized gas may be supplied from a gas cylinder 24 through a tubing 18, with the gas pressure controlled by a gas regulator 22 and a pressure gauge 20. The gas species can penetrate into the interstitial space between layers of the laminar material and stay therein under a pressure. The amount (solubility) of gas species that can reside in the interstitial space at a given temperature increases with the increasing pressure. After a duration of gas intercalation time (typically from minutes to hours), the excess pressurized gas is released (e.g., through a gas release valve 16) and the gas-intercalated layered material is removed from the vessel (e.g., by removing the cover 14 first). The gas-intercalated material is now at a second pressure (e.g., 1 atm in room air), which is lower than the first pressure (typically greater than 1 atm, typically up to 10 atm, but could be higher). The material is quickly placed in a furnace (with the second temperature being typically in the range of 50° C. to 1,500° C., but more typically between 100° C. to 500° C.), allowing the gas species to exfoliate the layered material by way of pressurizing, expanding, and/or escaping.
It is of great interest to note that prior art exfoliation processes normally involve intercalating laminar materials with liquid or solid intercalants, which are heated to pressurize the interstitial space through vaporization (as a result of chemical decomposition or phase transition). Heating is absolutely required in these prior art processes to achieve exfoliation. In contrast, it is surprising to observe that by simply reducing the surrounding pressure of the laminar material (containing super-saturated gas species residing in the interlayer spaces) in an abrupt or quick manner one could readily exfoliate layered materials. Additional heat is not required. However, optionally and preferably, this pressure reduction step is immediately followed by a step to rapidly expose the gas-intercalated material to a higher temperature that generally produces a high pressure in the interstitial space, leading to a larger expansion ratio (final exfoliated sample thickness/original sample thickness).
Using graphite as an example, the first step may involve preparing a laminar material powder containing fine graphite particulates (granules) or flakes, short segments of carbon fiber (including graphite fiber), carbon or graphite whiskers, graphite nano-fibers, or their mixtures. The length and/or diameter of these graphite particles are preferably less than 0.2 mm (200 μm), further preferably less than 0.01 mm (10 μm), and most preferably smaller than 1 μm. The graphite particles are known to typically contain micron- and/or nanometer-scaled graphite crystallites with each crystallite being composed of one sheet or several sheets of graphite plane. Preferably, large graphite particles are pulverized, chopped, or milled to become small particles or short fiber segments before being sealed in a pressurized chamber. The reduced particle sizes facilitate fast diffusion or migration of an intercalating gas into the interstices between graphite planes in graphite crystallites.
The advantage of having small-sized starting materials may be further illustrated as follows: The diffusion coefficient D of an intercalant between two graphite planes is known to be typically in the range of 10−12 to 5×10−9 cm2/sec at room temperature [e.g., M. D. Levi and D. Aurbach, J. Phys. Chem. B, 101 (1997) 4641-4647]. The required diffusion time T to achieve a desired diffusion path λ is given approximately by τ=λ2/D. Assume that D=10−10 cm2/sec and λ=100 μm (graphite particle size=100 μm), then the required diffusion time will be τ=(100×10−4 cm)2/(10−10 cm2/sec)=106 sec (277 hours or 11.5 days). This implies a very lengthy gas intercalation time if the particle size is too large. However, the diffusion time can be reduced if the diffusion temperature T is raised substantially to increase the diffusion coefficient since D=Do exp(−Q/RT), where Q is the activation energy for the diffusion process and R is the universal gas constant. By contrast, if the particle size is λ=1 μm, then τ=(1×10−4 cm)2/(10−10 cm2/sec)=100 sec at room temperature. This is a very reasonable diffusion time. In the worst case scenario, where D=10−12 cm2/sec (instead of 10−10 cm2/sec), the required intercalation time will be 10,000 sec=2.78 hours for graphite particles with a lateral dimension of 1 μm. This processing time of less than 3 hours can be further reduced by increasing the pressure vessel temperature for interaction.
The second step involves exfoliating the graphite crystallites in the graphite particles by simply releasing the vessel pressure, or by releasing the pressure and quickly placing the gas-intercalated laminar graphite in an oven at a pre-set temperature (typically in the range of 50° C.-1,500° C., but more typically lower than 500° C.). Since the de-intercalation time is also 100 sec or longer, we have sufficient time to transfer the gas-intercalated material from the vessel to the oven. At a much reduced pressure (now 1 atm), the gas solubility in a laminar material (e.g., graphite flakes) is much lower and the excess gas species would want to expand or escape. Surprisingly, even without additional heat, the escaping gas species appear to be capable of overcoming weak van der Waal's forces between layers, thereby delaminating or fully separating graphene planes in a graphite crystallite. This observation could also be theorized as follows: When the laminar material is subjected to a high gas pressure, gas molecules penetrate into the interstitial spaces to the extent that the internal pressure (inside the interstitial spaces) is balanced by the chamber pressure of the sealed vessel. When the chamber pressure is suddenly reduced by releasing the excess gas in the chamber, the gas molecules inside the interstitial spaces find themselves under a high pressure and wanting to expand. This pressure is sufficient to overcome the relatively weak van der Waal's forces between layers, producing separated layers.
Furthermore, when a gas-intercalated material is quickly placed in a pre-heated oven, the interstitial gas pressure is increased dramatically, which is effective to bring about an almost instantaneous and maximum expansion of the laminar particles. Typically, the substantially complete and full expansion of the particles is accomplished within a time of from about a fraction of a second to about 2 minutes, more typically from 1 second to 20 seconds. This can be conducted by pre-heating a furnace to a temperature in the range of 50°-1,500° C., but most preferably in the range of 100° C.-500° C.
Microwave heating was found to be particularly effective and energy-efficient in heating to exfoliate fine graphite particles. Although the presence of some moisture appears to promote exfoliation of minute graphite particles, it is not a necessary requirement when the intercalated sample is microwave-heated. It may take minutes for a microwave oven to heat and exfoliate a treated graphite sample, as opposed to seconds for the cases of pre-heating a furnace to an ultra-high temperature (e.g., 1,500° C.). However, the amount of energy required is much smaller for microwave heating.
An optional third step includes subjecting the exfoliated material to a mechanical attrition treatment to further reduce the particle sizes for producing the desired nano-scaled platelets. With this treatment, either the individual graphene planes (one-layer NGPs) or stacks of graphene planes bonded together (multi-layer NGPs) are reduced to become nanometer-sized in width and/or length. In addition to the thickness dimension being nano-scaled, both the length and width of these NGPs could be reduced to smaller than 100 nm in size if so desired. In the thickness direction (or c-axis direction normal to the graphene plane), there may be a small number of graphene planes that are still bonded together through the van der Waal's forces that commonly hold the basal planes together in a natural graphite. Typically, there are less than 20 layers (often less than 5 layers) of graphene planes, each with length and width smaller than 100 nm, that constitute a multi-layer NGP material produced after mechanical attrition.
Attrition can be achieved by pulverization, grinding, ultrasonication, milling, etc., but the most effective method of attrition is ball-milling. High-energy planetary ball mills were found to be particularly effective in producing nano-scaled graphene plates. Since ball milling is considered to be a mass production process, the presently invented process is capable of producing large quantities of NGP materials cost-effectively. This is in sharp contrast to the production and purification processes of carbon nano-tubes, which are slow and expensive.
The ball milling procedure, when down-sizing the particles, tend to produce free radicals at peripheral edges of graphene planes. These free radicals are inclined to rapidly react with non-carbon elements in the environment. These non-carbon atoms may be selected to produce desirable chemical and electronic properties. Of particular interest is the capability of changing the dispersibility of the resulting nano-scaled platelets in a liquid or matrix material for the purpose of producing nanocomposites. Non-carbon atoms typically include hydrogen, oxygen, nitrogen, sulfur, and combinations thereof.
Another embodiment of the present invention is a process as described above, but the pressurizing gas is produced by placing a controlled amount of a volatile but benign liquid (liquid with a low vaporization temperature such as water, ethanol, and methanol) inside the vessel and implementing a heating element in the vessel to heat and vaporize the liquid. The resulting water and/or alcohol vapor is capable of intercalating interlayer galleries of a range of laminar materials such as graphite oxide and transition metal dichalcogenides. The intercalated compounds may be cooled back to room temperature before the vapor is released, or the vessel may be opened while the intercalant is still in the vaporous state. The water/alcohol-intercalated laminar material is then transferred to a heating zone for a further exfoliation treatment.
Carbon materials can assume an essentially amorphous structure (glassy carbon), a highly organized crystal (graphite), or a whole range of intermediate structures that are characterized in that various proportions and sizes of graphite crystallites and defects are dispersed in an amorphous matrix. Typically, a graphite crystallite is composed of a number of graphene plates (sheets of graphene planes or basal planes) that are bonded together through van der Waals forces in the c-direction, the direction perpendicular to the basal plane. These graphite crystallites are typically micron- or nanometer-sized. The graphite crystallites are dispersed in or connected by crystal defects or an amorphous phase in a graphite particle, which can be a graphite flake, carbon/graphite fiber segment, or carbon/graphite whisker or nano-fiber. In the case of a carbon or graphite fiber segment, the graphene plates may be a part of a characteristic “turbostratic structure.”
When a graphite flake sample is sealed in a vessel containing oxygen at an elevated temperature (200-500° C.), chemical reactions between oxygen and graphite could occur, resulting in the formation of a partially oxidized graphite or graphite oxide. The vessel temperature may be reduced to below 60° C. (so that the sample can be more easily handled) while still under a relatively high pressure. By releasing the pressurizing oxygen gas, one obtains well-exfoliated graphite oxide platelets. This example illustrates the potential of permitting a pressurizing gas to take part in a benign chemical reaction to produce an exfoliated product that is chemically different from the starting laminar material.
In prior art processes, for the purpose of exfoliating graphene plane layers, the chemical treatment of the graphite powder involves subjecting particles of a wide range of sizes to a chemical solution for periods of time ranging from about one minute to about 48 hours. The chemical solution was selected from a variety of oxidizing solutions maintained at temperatures ranging from about room temperature to about 125° C. Commonly used intercalation compounds are H2SO4, HNO3, KMnO4, and FeCl3, ranging from about 0.1 normal to concentrated strengths. These strong acids are undesirable and, hence, the resulting exfoliated material has to be thoroughly washed, which is an expensive and lengthy process. In contrast, the presently invented process involves the utilization of environmentally benign intercalation gases such as hydrogen, inert gases, or oxygen. No potentially hazardous chemical is required.
Morrison, et al. [U.S. Pat. No. 4,822,590, Apr. 18, 1989] have disclosed a method of preparing single-layer materials of the form MX2, where MX2 is a transition metal layer-type dichalcogenide such as MoS2, TaS2; WS2, and the like. The process involved intercalating the MX2 with an alkali metal (e.g., lithium or sodium) in a dry environment for sufficient time to enable the lithium or sodium to substantially intercalate the MX2. The lithium- or sodium-intercalated MX2 is then immersed in water. The water reacts with the intercalated lithium or sodium and forms hydrogen gas between the layers of MX2. The pressure of the evolved hydrogen gas causes the layers of MX2 to exfoliate into single layers. This single layer MX2 material may be useful as a coating and a lubricant. However, pure lithium and sodium must be handled with extreme care in an absolutely dry environment. With a melting point of 180.7° C., lithium will have to be intercalated into MX2 at a high temperature in a completely water-free environment, which is not very conducive to mass production of exfoliated products. In contrast, the presently invented process does not involve a highly explosive chemical or a violent chemical reaction such as 2 Li+2 H2O→H2+2Li++2OH−.
Once the nano platelets are produced, the platelets may be dispersed in a liquid to form a suspension or in a monomer- or polymer-containing solvent to form a nanocomposite precursor suspension. The process may include a step of converting the suspension to a mat or paper, or converting the nanocomposite precursor suspension to a nanocomposite solid. If the platelets in a suspension comprise graphite oxide platelets, the process may further include a step of partially or totally reducing the graphite oxide after the formation of the suspension.
Alternatively, the resulting platelets may be mixed with a monomer to form a mixture, which can be polymerized to obtain a nanocomposite solid. The platelets can be mixed with a polymer melt to form a mixture that is subsequently solidified to become a nanocomposite solid.
One hundred grams of natural graphite flakes ground to approximately 20 μm or less in sizes were sealed in a hydrogen gas-filled steel container (schematically shown in
The procedure was similar to that used in Example 1, but the starting material was carbon fibers chopped into segments with 0.2 mm or smaller in length prior to the gas intercalation treatment. The diameter of carbon fibers was approximately 12 μm. No significant exfoliation was observed immediately after pressure release, but great expansions were achieved after rapid exposure to heat at 600° C.
A powder sample of graphitic nano-fibers was prepared by introducing an ethylene gas through a quartz tube pre-set at a temperature of approximately 800° C. A small amount of Cu—Ni powder was positioned on a crucible to serve as a catalyst, which promoted the decomposition of the hydrocarbon gas and growth of GNFs. Approximately 2.5 grams of GNFs (diameter of 10 to 80 nm) were intercalated with hydrogen gas at 10 atm. After pressure gas release, the intercalated particles were found to be exfoliated to a great extent (without an expansion ratio measurement). The sample was then rapidly heated to approximately 250° C. to further promote expansion.
Same as in Example 3, but heating was accomplished by placing the intercalated sample in a microwave oven using a high-power mode for 3-10 minutes. Very uniform exfoliation was obtained.
The same sequence of steps can be utilized to form nano platelets from other layered compounds: gas intercalation and exfoliation, followed by milling, attrition, or sonication. Dichalcogenides, such as MoS2, have found applications as electrodes in lithium ion batteries and as hydro-desulfurization catalysts.
For instance, MoSe2 consisting of Se—Mo—Se layers held together by weak van der Waals forces can be exfoliated via the presently invented process. Intercalation can be achieved by placing MoSe2 powder in a sealed and pressurized chamber, allowing oxygen to completely intercalate into the van der Waals gap between Se—Mo—Se sheets. After pressure release and heating at 250° C. for less than 5 minutes, the resulting MoSe2 platelets were found to have a thickness in the range of approximately 1.4 nm to 13.5 nm with most of the platelets being mono-layers or double layers.
Other single-layer platelets of the form MX2 (transition metal dichalcogenide), including MoS2, TaS2, and WS2, were similarly intercalated and exfoliated. Again, most of the platelets were mono-layers or double layers. This observation clearly demonstrates the versatility of the presently invented process in terms of producing relatively uniform-thickness platelets.
Graphite oxide was prepared by oxidation of graphite flakes with KMnO4/H2SO4 followed by a chemical removal step according to the method of Lerf, et al. [J. Phys. Chem., B 102 (1998) 4477-4482]. The interlayer spacing of the resulting laminar graphite oxide was determined by the Debey-Scherrer X-ray technique to be approximately 0.73 nm (7.3 Å), which was found to be conducive to the intercalation by larger gas species such as oxygen and nitrogen molecules.
Selected samples of graphite oxide (particle sizes of approximately 4.2 μm) were sealed in an oxygen-filled chamber at a pressure of approximately 5 atm for two hours at room temperature. The chamber was then isolated from the gas-supplying cylinder with the gas release valve being opened to release the excess gas. The resulting oxygen-intercalated graphite oxide was then transferred to a furnace pre-set at 350° C. to allow for exfoliation. Well separated graphite oxide nano platelets were obtained.
It is of great interest to note that, when mixed with water and subjected to a mild ultrasonic treatment after mixing, these nano platelets were well-dispersed in water, forming a stable water dispersion (suspension). Upon removal of water, the nano platelets settled to form an ultra-thin nano-carbon film. Depending upon the volume fraction of nano platelets, the film could be as thin as one to ten graphite oxide layers (approximately 0.73 nm to 7.3 nm).
A small amount of water-soluble polymer (e.g., poly vinyl alcohol) was added to the nano platelet-water suspension with the polymer dissolved in water. The resulting nano platelet suspension with polymer-water solution as the dispersing medium was also very stable. Upon removal of water, polymer was precipitated out to form a thin coating on nano platelets. The resulting structure is a graphite oxide reinforced polymer nanocomposite.
A small amount of the nano platelet-water suspension was reduced with hydrazine hydrate at 100° C. for 24 hours. As the reduction process progressed, the brown-colored suspension of graphite oxides turned black, which appeared to become essentially graphite nano platelets or NGPs.
An attempt was made to carry out the reduction of graphite oxide nano platelets in the presence of poly(sodium 4-styrene sulfonate) (PSS with Mw=70,000 g/mole). A stable dispersion was obtained, which led to PSS-coated NGPs upon removal of water. This is another way of producing platelet-based nanocomposites.
Bentolite-L, hydrated aluminum silicate (bentonite clay) was obtained from Southern Clay Products. Bentolite clay (5 g) was subjected to room temperature intercalation by argon gas at 5 atm. Exfoliation temperature was 400° C. and the resulting clay nano platelets have a thickness in the range of approximately 1 to 25 nm. The technique used for nanocomposite preparation was melt mixing. The amounts of clay and epoxy were 0.1 g, and 0.9 g, respectively. The mixture was manually stirred for 30 min. When stirring, the sample was actually sheared or “kneaded” with a spatula or a pestle. A well dispersed clay nanoplatelet composite was obtained.
This invention is based on the research result of a DoE SBIR project. The US government has certain rights on this invention.
