WO2015157280A1 - Eco-friendly production of highly-conductive and clean graphene dispersions - Google Patents

Abstract

Disclosed is a novel eco-friendly, rapid approach to directly produce highly-conductive and "clean" graphene sheets without involving any toxic regents or metal-containing compounds, and without generating toxic byproducts, thus enabling a broad spectrum of applications by solution processing techniques of low cost. In one embodiment, the approach relies on the synergy of piranha solution, intercalated molecular oxygen, and microwave heating that enables controlled oxidation of graphite particles, leading to rapid and direct generation of highly-conductive, clean graphene sheets without releasing any toxic gases and/or any potentially toxic aromatic byproducts.

Description

ECO-FRIENDLY PRODUCTION OF HIGHLY- CONDUCTIVE AND CLEAN

GRAPHENE DISPERSIONS

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims 35 U.S.C. § 119(e) priority of U.S. Provisional Patent Application Serial No. 61/976,305 filed on April 7, 2014, the disclosure of which is incorporated herein by reference in its entirety.

STATEMENT OF GOVERNMENT LICENSE RIGHTS

The present invention was made with government support under grant number 1346496 awarded by National Science Foundation. The United States government has certain rights in this invention.

FIELD OF THE INVENTION

The invention relates to a novel eco-friendly, rapid approach to directly produce highly-conductive and "clean" graphene sheets of different lateral sizes without involving any toxic regents or metal-containing compounds, and without generating toxic byproducts, thus enabling a broad spectrum of applications, by solution processing techniques of low cost. In one embodiment, the approach relies on the synergy of piranha solution, intercalated molecular oxygen, and microwave heating that enables controlled oxidation of graphite particles, leading to rapid and direct generation of highly-conductive, clean graphene sheets of different lateral sizes without releasing any toxic gases and/or any potentially toxic aromatic byproducts. BACKGROUND

Graphene is a flat monolayer of carbon atoms tightly packed into a two- dimensional (2D) honeycomb lattice, and is a basic building block for graphitic materials of all other dimensionalities. It can be wrapped up into 0D fullerenes, rolled into ID nanotubes or stacked into 3D graphite. Due to its excellent electronic, thermal and mechanical properties, and its large surface area and low mass, graphene holds great potential for a range of applications. Except for ultrahigh speed electronics, most of the proposed applications require large quantities of high-quality, low cost large graphene sheets (preferably solution-processable) for practical industrial scale applications. Examples include energy and hydrogen storage devices, inexpensive flexible macroelectronic devices, and mechanically reinforced conductive coatings including films for electromagnetic interference (EMI) shielding in aerospace applications.

While extensive efforts have focused on enabling cost effective mass production of conductive and solution processible graphene sheets, such as reduced graphene oxide (r-GO) and graphene nanoplatelets, most approaches known in the art rely on lengthy methods, such as those of Staudenmaier, Hofmann, Hummers or Hummers modified methods, or Tour's method. The individual graphene oxide (GO) sheets in the graphite oxide were dispersed into solutions, followed by reduction via thermal and/or chemical methods to recover the properties of graphene. However, these processes irreparably destroyed the honeycomb structures, so only a fraction of the remarkable properties of intrinsic graphene is recovered. Furthermore, trace amounts of reducing agents and metal ions involving in these approaches can participate in unwanted reactions and can be detrimental to many applications. Therefore, extensive cleaning and purification steps are required, making industrial scale production expensive and time- consuming. Finally, these processes use toxic oxidation or reduction reagents, metal-containing compounds or release toxic substances as byproducts, causing potential hazard to the environ- ment, thereby increasing economic, safety and environmental costs for large scale production.

Alternatively, mass production of graphene nanoplatelets mainly starts with expandable graphite flakes, which are essentially H₂SO₄-HNO₃ graphite intercalation compounds (GIC). Quick high temperature heating of these GIC leads to expansion and exfoliates these graphite flakes due to generation of large amount of gas, including toxic N0₂. Furthermore, the highly reduced GO sheets and graphene nanoplatelets cannot be directly dispersed into water, the most useful, clean and eco-friendly solvent. They can only be dispersed either in special organic solvents, such as N-methyl-pyrrolidinone (NMP), or in aqueous solutions with the help of surfactants for stabilization. However, these solvents and/or surfactant are difficult to completely remove from the graphene surfaces, which results in poor electronic contacts between graphene sheets in multisheet graphene films. As a consequence, high conductivity is difficult to reach without submitting the products to a high-temperature annealing process.

It has been reported that graphene nanoplatelets can be directly produced from graphite particles and dispersed into these special organic solvents or aqueous solutions with the help of surfactants for stabilization. The issue of releasing toxic gases is solved using these approaches, however the production needs a lengthy sonication process and the yield is too low for industrial applications. Recently, scalable production of large-size pristine few-layer graphene was achieved via an interlayer catalytic exfoliation (ICE) approach. FeCl₃ was intercalated, and acted as an interlayer catalyst for decomposition of H₂0₂, with the generation of a large amount of oxygen and water which facilitates graphite exfoliation. This approach is environmentally friendly and can be used for mass production of large amounts of high quality graphene dispersion in NMP, but not in aqueous solution due to the lack of oxygen-containing groups on the basal plane of graphene sheets. A similar metallic salt intercalation approach was recently reported, which can scalably produce non-defective graphene sheets that can be dispersed in pyridine, but not in aqueous solution. Therefore, the issues associated with trace amounts of metal ions and high boiling solvents still exist.

Recently, a fast, scalable oxidation approach was described in U.S. Patent Publication No. 2013/0266501 to He et al. (incorporated herein by reference in its entirety). The approach can be employed to directly and controllably produce highly-conductive graphene sheets of different lateral sizes without using any metallic compounds. Thus produced graphene sheets can be dispersed in both aqueous and organic solvents without the requirement of surfactants. In this approach, KMn0₄ (as is used in Hummer's methods) is intentionally excluded and nitronium aromatic oxidation combined with microwave heating (fast and local heating) was exploited. The mixture of H₂S0₄/HN0₃, which generates nitronium ions (N0²⁺) could intercalate into the graphite layers, accompanying oxidation of the graphene sheets in graphite particles. The unique process leads to a controllable oxidation of randomly positioned carbon atoms across an entire graphene sheet. The oxidation is controlled by the concentration of nitronium ions and the microwave irradiation power, and/or time of microwave heating, such that only the minimum density of oxygen atoms is incorporated into graphene sheets to enable exfoliation of graphite powders. The separated graphene sheets, also known as microwave-enabled low oxygen graphene (ME-LOGr), are highly conductive and do not require further reduction to achieve the desired properties of graphene. Even in the absence of polymeric or surfactant stabilizers, high concentrations of clean and well separated ME-LOGr can be obtained in both aqueous and organic solvents. However, due to the utilization of nitronium ions for graphite oxidation, toxic gas (N0₂) is released during the production process, which can cause contamination of the environment.

A solution that overcomes the above-described inadequacies and shortcomings in the process of producing graphene is desired. In particular, it would be desirable to produce highly-conductive and clean graphene dispersions at low cost yet in an eco-friendly manner. SUMMARY OF THE INVENTION

Having recognized the shortcomings of the prior art, in one embodiment, a novel eco-friendly method is disclosed for direct and scalable production of highly-conductive graphene sheets from graphite particles. Accordingly, the present invention includes highly- conductive amphiphilic graphene sheets, which can be directly and rapidly fabricated from cheap and abundant graphite particles without post reduction processing, and can be dispersed in aqueous or organic solvents without stabilizers. Also, the present invention includes monodispersed graphene nanosheets that are directly and rapidly fabricated from cheap and abundant graphite particles without post- separation and reduction processes, and also can be dispersed in aqueous or organic solvents without stabilizers. Collectively, the disclosed methods offer many advantages for mass production of high quality graphene dispersions, such as (1) the process is eco-friendly - toxic agents are not used, toxic gas or potentially toxic aromatic byproducts are not generated or released; (2) the process is a rapid and low energy fabrication process; (3) production of graphene sheets with different lateral sizes is possible without a post-reduction; (4) fabricated sheets have lower level of oxygen-containing groups, which ensures outstanding electrical and optical properties; (5) high temperature annealing process is not required; (6) high-concentration dispersions both in aqueous and organic solvents (without requiring polymeric or surfactant stabilizers); (7) dramatically reduced waste from purification steps; and (8) byproducts can be reused to produce soil fertilizers. All these advantages ensure mass production of high quality graphene dispersions with low environmental footprints and at a much lower-cost.

Thus, in accordance with the present invention, there is provided simple and scalable methods for quickly and directly producing highly-conductive low-oxygen graphene sheets and monodispersed low-oxygen graphene nanosheets. These methods exploit carbon oxidation chemistry by piranha solution (a mixture of H₂SO₄ and H₂O₂) and do not release any toxic gases and/or generate any toxic byproducts. In one embodiment, these methods include production of reversible graphite intercalation compounds ("GIC"), oxygen purging, and the microwave irradiation of the oxygen-purged GICs in a piranha solution. Without wishing to be bound by any particular theory, it is believed that the synergy of piranha solution, the intercalated oxygen, and microwave heating enables controlled oxidation of graphite particles, which leads to rapid (e.g. , 60 seconds) and direct generation of highly-conductive graphene sheets.

Specifically, one embodiment of this invention includes a method for making Eco-friendly Microwave Enabled Low Oxygen Graphene Sheets ("Eco-ME-LOGr"), which includes preparing reversible graphite intercalation compounds ("GIC"), followed by a short period of oxygen purging, and microwave irradiation in piranha solution until a finely dispersed suspension of Eco-ME-LOGr sheets is formed in said solution. The Eco-ME-LOGr sheets can be readily dispersed into various solvents, such as aqueous solvents, without surfactants, thus making a "clean" graphene surface achievable. For the purposes of the present application, the term "clean" means that the graphene sheets or surfaces have a level of surface contamination, such as, for example, from residual reducing agents, metal ions or surfactants, which is below a level that produces un-wanted reactions or is detrimental to the desired applications, for example a level that reduces the conductivity unacceptably.

By formation of reversible GIC, the distance between graphene sheets is increased, which provides enough space for 0₂ intercalation. At the same time, the positive charges on the graphene sheets formed within the HSO₄ -GIC also induces a strong attractive driving force for its intercalation. The interaction between the positive charges and O₂ also helps stabilize the intercalated O₂ and/or HSO₄ ^" ions against de-intercalation upon introduction of piranha solution. The existence of the intercalated O₂ not only maintains the inter-sheet distance for piranha to access and oxidize the inner parts of graphite particles, but also acts as a mild oxidant to generate more oxygen-containing groups on the graphene sheets, which facilitate graphene sheet dispersion into aqueous solutions. The synergy of the piranha- generated oxygen radicals, the intercalated O₂, and microwave heating enables rapid (60 seconds), direct and controllable fabrication of highly-conductive graphene sheets of different lateral sizes without requirement of post reduction procedure. The intrinsic oxidation mechanism of the disclosed method determines that no small aromatic toxic molecules are generated, and no toxic gas is released. Finally, the unique microwave heating not only dramatically speeds up the fabrication process, it also facilitates large graphene sheet fabrication, compared to those fabricated via traditional heating. Another embodiment of this invention further includes the steps of controlling the microwave power and irradiation time, oxygen purging and the ratio of Η₂0₂ and H₂S0₄ in the piranha solution to allow the fabrication control of graphene sheets from tens of micrometers down to several tens of nanometers.

One embodiment of this invention includes an Eco-ME-LOGr product produced according to the disclosed method.

Another embodiment of this invention includes graphene sheets with an average lateral size between about 0.5 micrometer and about 100 micrometers fabricated by oxidation of graphite particles.

Another embodiment of this invention includes graphene nanosheets with an average lateral size between about 3 nanometers and about 200 nanometers fabricated by oxidation of graphite particles.

In another embodiment of this invention, the graphene sheets fabricated from graphite have a carbon-to-oxygen ratio between about 10: 1 and about 50: 1, and preferably between 20: 1 and 40: 1, more preferably 30: 1. Yet another embodiment of this invention includes the as- synthesized graphene sheets fabricated from graphite can assemble to graphene films with a conductivity between 7,000 S/m and 50,000 S/m. Low temperature annealing (meaning without requirement of high temperature annealing, i.e., below 300 °C under argon) can further improve the conductivity into the range of 20,000 S/m to 200,000 S/m. In one embodiment the conductivity is about 75,000 to about 200,000 S/m. In another embodiment the conductivity is about 100,000 to about 200,000 S/m. The electrical performance of the Eco-ME-LOGr films significantly outperforms the ME-LOGr films fabricated via nitronium microwave oxidation. The present objectives, features and advantages of the disclosed invention will be apparent from the following detailed description of the invention, which is to be read in conjunction with the accompanying drawings. The scope of the invention will be pointed out in the claims. The following drawings, taken in conjunction with the subsequent description, are presented to enable one of ordinary skill in the art to make and use the invention and to incorporate it in the context of particular applications.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic representation of the graphene production processes with the disclosed eco-friendly approach. FIG. 2A-2C are representative STEM, SEM and AFM images of the Eco-ME-

LOGr fabricated via 5 min 0₂ purging of freshly prepared GIC, followed by 60 seconds microwave irradiation in piranha solution. 2D is an AFM image of Eco-ME-LOGr sheets fabricated from graphite particles without formation of GIC and 0₂ purging before microwave irradiation. FIG. 3A is a UV- Visible-Near Infrared spectrum of the Eco-ME-LOGr dispersion in water. 3B is a Raman shift spectrum of the Eco-ME-LOGr films on an alumina anodic membrane.

FIG. 4 is a plot showing electronic percolation of the Eco-ME-LOGr films prepared by simple vacuum filtration. FIG. 5A-5D are XPS spectra of Eco-ME-LOGr films (5 A, 5B) and ME-LOGr films (5C, 5D) on Au substrates. 5A and 5C show the C_ls signal, and 5B and 5D show the 0_2p signal.

FIG. 6A-6D are AFM images of graphene sheets prepared from fresh GIC (6A) without 0₂ purging; (6B) GIC purged with 20 minutes 0₂; (6C) GIC with 5 minutes 0₂ purging, but longer microwave irradiation (75 second, instead of 60 seconds); and (6D) GIC with 5 minutes 0₂ purging with traditional heating instead of microwave heating. DETAILED DESCRIPTION

In one embodiment, the disclosed invention provides highly-conductive (the as- synthesized graphene sheets have a conductivity above 7000 S/m), low oxygen containing (C:0 ratio higher than 10: 1) amphiphilic graphene sheets which can be dispersed in aqueous or organic solvents without stabilizers and/or a reduction process, and simple and scalable methods for quickly and directly producing said highly-conductive amphiphilic graphene sheets in an ecologically friendly manner. In general terms, as illustrated in FIG. 1, the method for producing highly-conductive graphene sheets includes at least three steps: (1) the production of reversible graphite intercalation compounds ("GICs"), (2) the 0₂ purge, and (3) the microware irradiation in a piranha solution. The present invention demonstrates that the piranha solution generates oxygen radicals (0· ) having similar functions as the N0₂ ⁺ generated by the mixture of H₂S0₄/HN0₃ described in U.S. Pat. Publication No. 2013/0266501, yet without producing any toxic agents or byproducts. However, as disclosed herein, the direct replacement of HN0₃/H₂S0₄ with piranha solution for the oxidation of graphite particles does not provide the desired graphene because piranha solution has limited capability to reach and oxidize the internal sites of the graphite particles. Most of the dispersed sheets generated by simple replacement are smaller than 200 nm and the concentration of the dispersed graphene nanosheets is low (0.1 mg/ml), suggesting the oxidized sheets are quickly cut to small pieces as illustrated in FIG. 2D. The majority of the graphite particles are precipitated out and only a small fraction of the graphene surfaces, which are located on the surface of the graphite particles, can be oxidized.

Having recognized the shortcomings of the piranha solution, in one embodiment, in order to provide the piranha oxidant access to the inner parts of graphite particles, the distance between graphene sheets is enlarged before microwave oxidation in piranha by exposing graphite powders to a mixture of ammonium persulfate ((NH₄)₂S₂08) and sulfuric acid (H₂S0₄) to produce reversible graphite intercalation compounds ("GICs"). In one embodiment, the distance between graphene sheets is enlarged by exposing graphite powders to a mixture of sulfuric acid (H₂S0₄, 98%) and ammonium persulfate ((NH₄)₂S₂0s) at a weight ratio beween 1: 1 and 100: 1, preferably between 10: 1 and 50: 1, and more preferably between 15: 1 and 30: 1. The ratio for sulfuric acid and graphite is between 500: 1 and 10: 1, preferably between 300: 1 and 30: 1, more preferably between 150: 1 and 50: 1 to produce reversible sulfuric acid-based GICs. Preferably, the graphite powder is exposed to such a mixture at room- temperature for 2 to 48 hours (e.g. , about 24 hours in one embodiment).

Nonetheless, as illustrated in FIG. 6, the GICs prepared in such manner are subjected to microwave irradiation in a piranha solution. The product is very similar to those obtained without any intercalation process. The concentration is slightly increased (0.17 mg/ml), while the size of the sheets is still very small (< 100 nm). It is believed that the formed GICs (e.g., from (NH₄)₂S₂0₈) are reversible since there are no C-0 bonds formed and rapid de- intercalation occurs. For instance, with water washing the intercalated HS0₄ ^" and H₂S0₄ in the GICs formed from (NH₄)₂S₂0₈ rapidly de-intercalate. Since the H₂0₂ solution in piranha contains large amount (65-70 wt %) of water, the majority of HS0₄ ^" and H₂S0₄ groups are believed to be de-intercalated before the O- radicals reach the inner graphite particles.

Having recognized the shortcomings of the reversible GICs, in one embodiment, to keep the enlarged distance in the GICs for oxygen radical internalization, 0₂ gas is purged through the freshly prepared GICs. The rate and/or the duration of the 0₂ purge can be varied to optimize stabilization of the GIC against deintercalation. In some embodiments, the GICs are 0₂- purged at the rate of 50-500 ml/min, preferably 50-200 ml/min and more preferably 70-90 ml/min, for a duration sufficient to stabilize the GICs against deintercalation. In one embodiment, the GICs are 0₂-purged at the rate of 79-84 ml/min for 1 to 120 minutes before putting them into the piranha solution. In another embodiment, the GICs are 0₂-purged at the rate of 79-84 ml/min for about 5 minutes before putting them into the piranha solution.

It is believed that positive charges are generated in the graphene sheets during GIC formation and the charges are balanced with intercalated HS0₄ ^" ions. It is believed that the distance between graphene sheets in the GIC is large enough for 0₂ intercalation. Due to the high electronegativity of 0₂, a strong attractive interaction between 0₂ and the positive charges on the graphene sheets exists, which facilitates 0₂ intercalation and prevents its de-intercalation when the GIC is exposed to an aqueous environment. The weight of GICs with and without 0₂ purging after water cleaning was examined to confirm that intercalation of 0₂ helps to stabilize the GIC against deintercalation. The weight of the one with 0₂ purging is much greater than the one without (see Table 1). However, since there are no positive charges on graphene sheets in pristine graphite particles, there is no driving force for 0₂ to internalize without forming a GIC in the first place. On the other hand, purging with 0₂ for a longer time and/or increasing the microwave irradiation time causes a significant decrease in the lateral sizes of the graphene sheets, or even dramatic carbon lost, possibly due to over-oxidation induced cutting and etching (FIG. 6).

Table 1: Weights of Graphite and GICs with and without 5-min 0₂ purge after wash.

Reaction mixtures Initial graphite Weight of GlCafter

weight(mg) washing(mg)

Fresh GIC 40.1mg 40.2mg

Fresh GIC purged 40mg 41.6mg

with 02

Once graphite intercalation compounds ("GICs") are formed and purged with 0₂ to prevent its de-intercalation, in one embodiment, the oxidation of graphite particles occurs on both internal and external graphene sheets upon addition of piranha solution followed by microwave irradiation. The piranha solution, as used herein, is a mixture of sulfuric acid (H₂SO₄) and hydrogen peroxide (H₂0₂). Typically, the amount of sulfuric acid and hydrogen peroxide mixed together is selected to provide oxygen radicals to controllably oxidize graphene sheets in graphite particles. Relative amounts of sulfuric acid and hydrogen peroxide should be mixed together for a given quantity of graphite to achieve the desired amount of oxygen radicals. For purposes of the present invention piranha solution is defined as a mixture of H₂SO₄ and H₂0₂ solutions in a volume ratio between about 20: 1 and about 1: 1, inclusive of water in the H₂SO₄ (95 to 98% by weight) and H₂0₂ (30 to 35%, by weight) solutions. According to one embodiment the ratio is between about 7: 1 and about 1: 1. In one exemplary embodiment, the ratio is about 3: 1.

If the reaction does not stop in a timely manner, subsequent oxidation will lead to the formation of oxides, vacancies, larger holes, and ultimately cutting of the graphene into small pieces. Therefore, the key to directly produce large, conductive graphene sheets by piranha solution treatment is to quickly produce the low concentration of oxygen moieties that is required for the separation of individual graphene sheets, and then quench the reaction before holes and/or vacancies formation. To satisfy this requirement, microwave heating is exploited. Indeed, it was observed that the majority of the products prepared via conventional heating were nanosheets (heating for 7 hours at 100 °C) with lateral dimensions smaller than 100 nm (FIG. 6D), which is much smaller than the ones obtained via microwave irradiation (several μιη).

Specifically, the process used in accordance with the present invention heats a sample of 0₂-purged GICs, sulfuric acid and hydrogen peroxide (i.e., piranha solution) by irradiating the sample with electromagnetic radiation in the microwave or radio frequency range. The frequency of the electromagnetic radiation used ranges from 10 8 to 1011 Hertz (Hz). The input power is selected to provide the desired heating rate. In one embodiment, the preferred output power is up to about 10 kilowatts. In one exemplary embodiment, the output power is between 200 W and 900 W. In another exemplary embodiment, the output power is about 300 W.

The electromagnetic radiation may be pulsed or continuous. When using pulsed radiation, any arrangement of pulse duration and pulse repetition frequency which allows for the dissipation of adverse heat buildup may be used in the present invention. The pulse duration may be varied, from 1 to 100 microseconds and the pulse repetition frequency from 2 to 1000 pulses per second. The sample may be irradiated for any period of time sufficient to oxide the graphite particles that allows dispersion of the graphene sheets with the desired lateral sizes into solvents. These conditions can be readily determined by one of ordinary skill in the art without undue experimentation. The time required to achieve the result will be shorter for higher power settings.

When continuous radiation is utilized, the sample is heated for a time sufficient to oxidize the graphite particles that allows dispersion of the graphene sheets with desired lateral sizes into solvents. Generally, the irradiation time is at least about 30 seconds. Preferably, the irradiation time ranges from about 30 seconds to 5 minutes in order to achieve the desired extent of oxidation. As with pulsed radiation, the time, and power input can be routinely adjusted to achieve the desired result, which can be readily determined by one of ordinary skill in the art without undue experimentation.

Typically, continuous radiation is first employed to attain the desired reaction temperature, after which, pulsed radiation is employed to maintain the desired temperature. Accordingly, the duration of continuous radiation, pulse radiation duration, and radiation frequency can be readily adjusted by one having ordinary skill in the art to achieve the desired result based on simple calibration experiments. The extent of graphite oxidation may be confirmed, by conventional analytical techniques.

Irradiation of the sample may be conducted in any microwave and/or radio frequency heating device which is capable of continuous or pulsed radiation and has the power requirements necessary to thermally induce the conversion to graphene. Suitable heating devices include microwave ovens, wave guides, resonant cavities, and the like. Suitable heating devices are well known in the art, and are commercially available.

One preferred device for performance of the present invention is a single-mode resonant cavity. Any available mode for heating in this device can be used in the present invention. However, the present invention is not to be limited to use of this device but can be performed in any microwave or radio frequency heating equipment.

In general, the process of the present invention is carried out by placing the sample inside a microwave or radio frequency device and applying the appropriate input power. The present invention may be applied as either a batch or continuous process. Thus, to create large graphene sheets, the mixture of oxygen-purged GIC and piranha solution is heated using microware irradiation for 10 to 90 seconds, or longer. In one exemplary embodiment, the mixture is heated using microware irradiation for less than 90 seconds. In another exemplary embodiment, the mixture is heated using microware irradiation for about 60 seconds. However, since microwave heating is volumetric heating, the irradiation time and/or power increases as the amount of reactant increases, especially for large scale production. Thus, those of ordinary skill in the art could readily adjust the power and duration to compensate for the increased amount of the reactant.

Because of the ecologically friendly manner in which the graphene sheets are produced and their high electrical conductivity, they are herein referred to as Ecologically Friendly Microwave-Enabled Low-Oxygen Graphene ("Eco-ME-LOGr"). Specifically, the chemistry to fabricate these high quality graphene sheets does not produce/release toxic gases and/or generate toxic byproducts. While it is expected that the gas released during reaction is colorless, since there are no nitronium ions involved in the disclosed piranha/0₂ oxidation method, it was surprisingly discovered that the filtrate is also colorless, which qualitatively suggests that there are no small aromatic compounds generated as byproducts during the oxidation reaction. The results from gas chromatography-mass spectroscopy (GC-MS) data show that the majority of the components are 0₂ and a small amount of C0₂, but no toxic S0₂ or CO were detected.

Without wishing to be bound by any particular theory, it is believed that the initiation oxidation mechanism using piranha solution differs from the oxidization pathways of H₂S0₄/HN0₃ and N0₂ ⁺. It is known that nitronium ions not only attack the existing defects on the graphene, but also randomly attack the relatively inert defect-free graphene basal planes, generating multiple oxygen-containing groups, such as -OH and epoxy, across the graphene sheets. In the following oxidation step, N0₂ ⁺ continues to attack the already oxidized carbon atoms (results in etching and generating vacancies and holes) and carbon atoms far away from those already oxidized (producing more oxygen-containing groups). Associated with the etching step, C0₂ and CO were released, and a large amount of small aromatic molecules were also generated as byproducts which were filtrated out during cleaning.

On the other hand, it is believed that the route of piranha oxidation is the generation of atomic oxygen, which directly attacks a carbon in a graphene sheet to form a carbonyl group. The formation of the carbonyl group simultaneously disrupts the bonds of the neighboring carbon atoms. With further oxidation, the initial carbonyl group can be converted into C0₂ (carbon lost) and at the same time, a new carbonyl group is created on the neighboring carbon atoms whose bonds are disrupted. It is possible that the intercalated molecular oxygen is involved in generation of epoxy groups on the graphene surfaces and etching (carbon lost as C0₂) upon microwave heating. As a positive outcome of these oxidation pathways, there would be no small aromatic molecules produced, in contrast to what is observed with the nitronium oxidation approach. If these oxidation mechanisms are correct, the present process should provide more epoxy groups, which would locate on the basal planes, and fewer carbonyl groups, which would mainly exist on the edges of Eco-ME-LOGr sheets. To reveal their chemical functionalities, the Eco-ME-LOGr sheets were further studied using X-Ray photoelectron spectroscopy (XPS) and a control experiment was also performed with nitronium oxidation for comparison. The results support the proposed oxidation mechanisms (see Examples). For the ME-LOGr sheets, the C_ls spectrum is similar to that of Eco-ME-LOGr sheets, while the 0_2p spectrum is very different (see FIG. 5). The quantity of epoxide functional group (C-O-C) and -OH groups connected with aromatic structures, which should be the major functional groups of the products from the first oxidation step via nitronium oxidation, is surprisingly low. However, the quantity of oxygen-containing groups connected with aliphatic carbons is extremely high (see FIG. 5D). These results unambiguously indicate that dramatic further oxidation has occurred after the first step oxidation. The experimental results show that a large amount of small aromatic molecules were generated as byproducts, which is in line with the XPS analysis. Therefore the intrinsically different oxidation mechanisms determines that the piranha/0₂ approach is eco-friendly, while all the nitronium oxidation related approaches were accompanied by releasing of toxic gases, such as, N0₂, and generating small aromatic molecules as byproducts, which could cause potential contamination to the environment and ground water system if they are not treated properly.

The examples set forth below also serve to further define the disclosed invention, but are not meant in any way to restrict the scope of the invention.

EXAMPLES

Example 1

A graphite intercalation compound with HS0₄ ^" was achieved by following the process described in Dimiev, A. M. et al. ACS Nano 2012, 6, 7842 (incorporated herein by reference in its entirety). In brief, 1000 mg of ammonium per sulfate [(NH₄)₂S₂0₈] (reagent grade 98%; Sigma Aldrich) was dissolved in 10 ml of H₂S0₄ (98%; Pharmaco Aaper). The obtained mixture solution was stirred for 5-10 min and then 200 mg of synthetic graphite powder (size <20μιη; Sigma Aldrich) was added. The obtained mixture was stirred for 24 hrs, which led to the formation of Graphite intercalation Compound (reversible S0₄ -GIC). In the GIC-S0₄ solution oxygen was purged for 5 min at a rate of 79-84 ml/min using extra dry grade 0₂.

Thereafter, 1 ml of the 0₂-purged GIC-HS0₄ ^" solution was taken and mixed with 9 ml of Piranha solution (H₂S0₄: H₂0₂ = 3: 1, v/v), which was microwaved at 300W for 60 sec in a CEM discover microwave vessel. The H₂0₂ in the Piranha solution was a laboratory grade solution with a concentration of 35% (by weight) obtained from BDH. The reaction was initially quenched with 200 ml deionized water. Similar to the nitronium oxidation approach, the reaction also resulted in a finely dispersed suspension that was significantly easier to purify and handle than the sticky paste obtained from Hummer's method.

The obtained slurry was washed via vacuum filtration through a polycarbonate membrane with a pore size of 0.8 μιη with 200 ml water each for four times. The final product was dispersed in 40 ml deionized water by sonication in a bath sonicator for 30 min. With the aid of bath sonication, the cleaned filtration cake on the filter paper can be re-dispersed in a wide range of solvents to form colloidal solutions without the use of surfactants or stabilizers (see Table 2). The solution was allowed to settle for 3-5 days; the supernatant solution obtained contains large graphene sheets. The filtrate was collected and then extracted with THF to study the byproducts via gas chromatography-mass spectrometry (GC-MS).

Table 2: Different concentration and production yield of the Eco-ME-LOGr in various solvents.

Solvent Concentration Total weight in Initial

(mg/ml) the solution weight(mg)

Ethylene 0.40 16.1 20 80.6

glycol

NMP 0.29 11.4 20 57.2

Water 0.22 10.3 20 51.4

DMF 0.20 8.0 20 40.1

Chloroform 0.19 7.6 20 37.8

THF 0.071 2.8 20 14.2

Acetone 0.026 1.0 20 5.3

It is noteworthy that graphene sheets can be dispersed in water, N-methyl-pyrrolidinone (NMP), and chloroform in similar concentrations. Specifically, the graphene sheets can be dispersed both in water (220 mg/L), which is commonly used to disperse graphene oxide (GO), and NMP (290 mg/L), Ν,Ν-dimethylformamide (DMF) (200 mg/L), which are well known solvents to disperse intrinsic graphene sheets and platelets. Even in a nonpolar solvent, such as chloroform, in which neither GO, r-GO, nor graphene platelets can be dispersed, Eco-ME-LOGr can be dispersed with a concentration of 190 mg/L. The high dispersability in both aqueous and organic, and both polar and nonpolar solvents without requiring surfactants or stabilizers indicates the unique molecular structures of the graphene sheets, which is quite different from previously reported GO, r-GO or graphene nanoplatelets, as well as the ME-LOGr sheets produced by the nitronium microwave oxidation approach.

Example 2

To demonstrate that this approach can be scaled up for mass production, 10ml of graphite intercalated solution, which is purged with 0₂ for 5 min, was prepared and to it was added 90 ml of Piranha solution (H₂S0₄: H₂0₂- 3: 1, v/v). The solution was then microwaved at 900W for 60 sec with a SYNTHWAVE™ device (from Milestone). The solution was initially quenched with 500 ml deionized water and filtered, and the solid was washed four times with 200 ml deionized water each. The final product was dispersed in 200 ml deionized water and sonicated in bath sonicator for 30 min. The microwave enabled nitronium oxidation approach graphene synthesis was conducted according to the procedure described in U.S. Pat. Pub. No. 2013/0266501 (incorporated herein by reference in its entirety). However the starting material was the graphite intercalated compound. For comparison, in the inventive method heating of the piranha trial was conducted by heating 1 ml of 0₂-purged GIC with 9 ml of Piranha solution at 100°C for 7 hrs and then quenching the reaction mixture with 200 ml deionized water and washing the collected product four times with 200 ml water each.

Example 3

The lateral size and thickness of the dispersed Eco-ME-LOGr sheets were characterized by scanning transmission electron microscopy (STEM), scanning electron microscopy (SEM), and atomic force microscopy (AFM).

The AFM samples were prepared by dropping 1-2μ1 of the dispersed graphene solution onto a freshly cleaved mica surface and then allowing it to dry. After drying, the sample was washed with water drop by drop to remove the dirt on the sample and was again dried. This sample was scanned using a Nanoscope Ilia multimode SPM (Digital Instruments) with a J scanner for small scan size and G scanner for larger scan size operated in "Tapping mode". The AFM tips for imaging were 160 μιη long rectangular silicon cantilever/tip assembly from AppNano, which was used with a resonance frequency of 160 kHz and a spring constant of approximately 7.7N/m with a tip radius of less than 10 nm.

SEM samples were prepared by dropping 1-2μ1 of the sample onto a cleaned silicon substrate. The silicon substrate was cleaned initially with piranha solution and then water, and then dried with N₂ gas. The sample solution was dropped onto silicon substrate and then allowed to dry for 2-3 min and then dried in N₂ gas to spread the sample throughout the substrate. The SEM images were captured using a Hitachi S-4800 Field Emission Scanning Electron Microscope (FE-SEM, Hitachi Co. Ltd.) under an accelerating voltage of 1-2KV and a probe current of 10μΑ to obtain images with high contrast.

SEM samples were prepared by dropping Ιμΐ of the sample on a Cu TEM grid. After the samples were dried in air, they were imaged with a Hitachi S-4800 FE SEM under high accelerating voltage of 30KV and a probe current of 10-15μΑ with a working distance of 15mm.

The Eco-ME-LOGr sheets have a lateral size of up to several tens of micrometers with an average of one to two μιη. The thickness of the Eco-ME-LOGr sheets is between 0.7-3 nm, corresponding to one to a few layers as shown in FIG. 2. The color of the Eco-ME-LOGr suspensions is grayish-black, which is very similar to the previously reported r-GO and ME- LOGr suspensions, but quite different from the typical brown GO solutions.

Example 4

The optical properties of the graphene dispersions were measured by UV-VIS NIR spectroscopy. The spectra were obtained on a Cary-5000 Ultraviolet- Visible-Near Infrared Spectroscopy operated in double beam mode with 200-1000 nm wavelength range. The UV- Vis- near infrared (NIR) spectrum of the Eco-ME-LOGr solution displayed a plasmon band absorption maximum at 268 nm and strong absorption in the visible and NIR region, qualitatively suggesting that the as-prepared Eco-ME-LOGr sheets already contain a large amount of intact graphene domains without the requirement for a post-reduction procedure. Example 5

Raman spectroscopy was utilized to characterize the size of the intact graphene domains. Raman spectra of graphene sheets were collected with a Kaiser Optical Systems Raman Microprobe with a 785 nm solid state diode laser, the collection time is 60 sec for each spectrum and average three times.

The typical features of the G band, defect D band, and 2D band are shown in the Raman spectrum of Eco-ME-LOGr film prepared on an anodic filter membrane via vacuum filtration (see FIG. 3B). The intensity ratio of D to G band (I_D/I_G) is 0.75, and is much lower than those of GO and r-GO. Using the empirical Tuinstra-Koenig relation, the size of the intact graphene domains was ca. 6.0 nm in Eco-ME-LOGr, slightly smaller than those in ME-LOGr, while much larger than those in GO(l-3.5 nm). The reported I_D I_G ratios for r-GO are similar to or even higher than GO, which was explained by the fact that chemical reduction preferentially generates a greater number of smaller crystalline domains rather than increasing the size of existing graphitic domains. Therefore, even though the apparent electronic structure of the Eco- ME-LOGr sheets is similar to that of r-GO, as demonstrated by its color and UV-Vis spectrum, the Eco-ME-LOGr sheets have unique molecular structures that differ from both GO and r-GO. It has been demonstrated that large sp -domain sizes that are minimally interrupted by defects are essential for achieving high conductivity and high charge mobility in reduced GO. The large domain sizes in the as-produced graphene demonstrate the high quality of the graphene sheets fabricated by this simple method. Further, the Eco-ME-LOGr sheets also show a strong 2D band, suggesting that the Eco-ME-LOGr sheets are also clean, without adsorbent-induced surface contamination. The existence of large intact graphene domains and minimal contamination ensure high conductivity of graphene films. Example 6

The electrical properties of the Eco-ME-LOGr sheets without any chemical and thermal reduction processes were studied. Graphene films of different thicknesses were prepared from the Eco-ME-LOGr aqueous suspension by vacuum filtration through an anodic filter membrane (Whatman Ltd) with 0.2 μιη pores. These films were dried in vacuum for 1 day to remove the residual solvent before conductivity measurements. The sheet resistance was measured by a manual four point resistivity probe (from Lucas Laboratories, model 302).

The graphene films were then transferred onto Si surfaces after etching the alumina anodic membrane in a strong base (NaOH, 4M), followed by washing with water until the pH of the solution became neutral. After transferring to Si surfaces, the samples were dried in vacuum and then the average thickness for each graphene film was measured with Rutherford Backscattering Spectroscopy (RBS) using a 2 MeV He²⁺ ion beam produced in a tandem accelerator with an ionic current of 2-3 nA. Spectra were collected in the back scattering geometry and simulations were performed using the SIMNRA program. The conductivity of the films was calculated from the sheet resistance and thickness by the formula:

i

Conductivity =

/Sheet resistancexthickness

This formula can be used to measure the films with thickness not more than half of the probe spacing (the distance between two probes of the four point probe instrument). The error in this case is less than 1%.

Similar to previously reported solution-dispersed graphene sheets, the Eco-ME- LOGr films show percolation-type electronic behavior. The sheet resistance of the Eco-ME- LOGr film decreases with increasing film thickness, as shown in FIG. 4.

The Eco-ME-LOGr sheets reached percolation at a thickness of 88 nm, which has a sheet resistance of 0.5 kQ/ square. This corresponds to a DC conductivity of 22,560 S/m. Upon annealing the Eco-ME-LOGr film at 300°C in Ar for 2 hours, the conductivity was further increased to 74,433 S/m. These conductivity values dramatically outperformed our previous ME- LOGr films fabricated via nitronium microwave oxidation (6600 S/m for as-prepared films and 19,200 S/m after 2-hour annealing at 300°C). To the best of our knowledge, these films have achieved the highest conductivity values reached the highest conductivity compared to all the previous reported paper-like graphene films prepared by vacuum filtration from solution dispersed graphene sheets (see Table 3).

Table 3: Conductivity of various graphene samples

Graphene dispersion technique Annealing temperature(°C) Conductivity(S/m)

Eco-ME-LOGr Air dry 22,560

300 °C for 2hrs with Ar 74,433

ME-LOGr Air dry 6,600

300°C for 2 hrs with Ar 19,200

Graphene nanoplatelets Air dry

in NMP

300°C for 2 hrs with Ar 5,000

250°C for 2 hrs with Ar/H₂ 6,500

Graphene nanoplatelets dispersed in Air dry 35 aqueous solution via sonication with

sodium dodecyl benzene sulfate

Graphene nanoplatelets dispersed in Air dry 1,900-2,150 aqueous solution via sonication with

Pyrene derivatives

Reduced graphene oxide via hydrazine in Air dry 200 the presence of Pyrene derivatives

Reduced graphene oxide via hydrazine at Air dry 7,200 basic conditions

220°C for 2hrs with Ar 11,800

Sulfonyl modified Reduced Graphene Air dry 17 oxide in aqueous

Electrochemical reduction of graphene Air dry 3,500 oxide

Solvothermal reduction of graphene oxide Air dry 374 in NMP

250°C for 2hrs 1,380

Reduced GO in variety of organic solvent Air dry 1,700 mixures

150°C for 12hrs 16,000

Reduced K-modified reduced GO air 690

Flash reduced GO Air dry 1,000 Example 7

Gas chromatography-mass spectroscopy (GC-MS) was used to carefully study the composition of the gas phase released, and the byproducts in the filtrates collected during cleaning the microwave oxidized products. The gas phase collected during the microwave oxidation was directly injected into a GC-MS. Specifically, the gas evolved during the reaction process was carefully collected through a syringe and 1ml of the gas sample taken from the headspace (total headspace volume: 5 mL) was injected into an Agilent HP6890 system which was equipped with a HP-5-MS capillary column. For the filtrate, 1 μΐ of the THF extract was injected into the same GC-MS system by sampling through the septum of one of four vials containing: (1) THF extract of the filtrate from the nitronium oxidation approach, (2) the filtrate from the present Eco-friendly approach, (3) the filtrate from a control experiment via the Eco- friendly approach without adding graphite particles, and (4) pure THF solvent. A temperature program was performed, starting at 50 °C held for 1 min, followed by temperature ramping at a rate of 10 °C/min to a final temperature of 300 °C and held for an additional 1 min. The results show that the majority of the components are 0₂ with a small amount of C0₂, while no toxic S0₂ or CO were detected.

Example 8

To study the components in the cleaning filtrate, the filtrate was mixed with a polar and low-melting organic solvent, such as tetrahydrofuran (THF), before injection. For comparison, the filtrates from a nitronium oxidation and a blank solution (obtained by microwave irradiation of the same amount of (NH₄)₂S₂0₈ and piranha solution but without graphite particles) were also studied for comparison. The filtrate from nitronium oxidations shows several peaks at retention time of 1.5 min, 4.17 min, 7.49 min, and 11.78 min.

The mass spectrum (MS) for each of the peaks was determined. The molecular structures were identified based on the score (max score is 1.00) of the MS spectrum compared to the spectra in a mass bank database. A peak at 1.5 min is mainly from THF, and peaks at 4.17 min are most possibly from flavanol derivatives. While the peaks at 7.49 min, and 11.78 min were related to some relatively high molecular weight compounds like cyanine or 1,1'- dianthrimide. Detailed molecular structures and their score compared to the mass spectra in the mass bank database are given in Table 4. In contrast, the GC spectrum of the filtrate obtained from the piranha oxidation approach is similar to the spectra of THF and the blank. No peak was observed in the GC spectra of the filtrate except the peaks from the solvent itself (THF alone), demonstrating this new piranha/0₂ oxidation approach is indeed eco-friendly without releasing any toxic gases and generating toxic aromatic byproducts.

Table 4: Detailed molecular structures and their score compared to the mass spectra in the mass bank database.

To reveal their chemical functionalities, the Eco-ME-LOGr sheets were further studied using X- Ray photoelectron spectroscopy (XPS) and a control experiment was also performed with nitronium oxidation for comparison. XPS was pursued by depositing graphene solution onto a gold film of l x l cm surface area. The deposited film had a thickness roughly 50 nm. XPS data was acquired using a Thermo Scientific Ka system with a mo nochro mated Al Ka X-ray source( hv=1486.7ev).

The spectrum of C_ls of the Eco-ME-LOGr sheets shows a main peak of oxygen- free carbon and a shoulder of oxygen-containing carbon (see FIG. 5A). The oxygen- free carbon makes up 76% of the spectrum, comparable to the spectrum of rGO and ME-LOGr obtained with nitronium oxidation. The oxygen free carbon is mainly derived from the C_ls peak of aromatic rings (284.2 eV, 61.8%), and that of the aliphatic rings and/or linear alkylinic carbon chains (284.7 eV, 13.9%). The 0_2p spectra complement the information provided by the C_ls spectra and the peak was deconvoluted into three peaks (see FIG. 5B), which have been assigned to three types of oxygen: oxygen in aromatic epoxide functional group (C-O-C ) and/or -OH groups connected with an aromatic structures (533.0 eV; 62%), 17% of the oxygen atoms were in C=0 connected with an aromatic structures, and the remaining constituted aliphatic ether, ester, carbonyl, and -OH groups ( 21%). Considering the trivial ratio between the carbons on the edges to those in the basal plane of a graphene sheet, the lower quantity of C=0 groups and the relatively higher amount of epoxy groups on the Eco-ME-LOGr are reasonable, supporting the proposed oxidation mechanisms. For the ME-LOGr sheets, the C_ls spectrum is similar to that of Eco-ME-LOGr sheets, while the 0_2p spectrum is very different. The quantity of aromatic epoxide functional group (C-O-C) and -OH groups connected with aromatic structures, which should be the major functional groups of the products from the first oxidation step via nitronium oxidation, is surprisingly low. However, the quantity of oxygen-containing groups connected with carbons with aliphatic configurations is extremely high (FIG. 5D). These results unambiguously indicate that dramatic further oxidation has occurred after the first step oxidation. These experimental results demonstrate that a large amount of small aromatic molecules was generated as byproducts, which is in line with the XPS analysis. Therefore the intrinsically different oxidation mechanisms demonstrates that the piranha/0₂ approach is eco-friendly, while all the nitronium oxidation related approaches were accompanied by the release of toxic gases, such as, N0₂, and the generation of small aromatic molecules as byproducts, which could cause potential contamination to the air and ground water system if they are not treated properly.

Example 9

To examine how the size and yield of the graphene sheets depends on the ratio of the piranha solution, microwave power and exposure duration, the dispersed graphene was prepared under various conditions.

In the tested power range, increasing microwave power resulted in efficient oxidation and more graphite staying in solution.

The duration of the microwave exposure should be optimized to maximum the extent of oxidation of graphite particles while avoiding over oxidation- induced gasification (as evidenced by carbon loss).

The dispersed graphene solution obtained at 3: 1 ratio of H₂SO₄ to ¾(¾ of the piranha solutions generated more graphene sheets leaving fewer unreacted graphite particles.

It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described. Rather, the scope of the present invention is defined by the claims which follow. It should further be understood that the above description is only representative of illustrative examples of embodiments. The description has not attempted to exhaustively enumerate all possible variations. Alternate embodiments may not have been presented for a specific portion of the invention, and may result from a different combination of described portions, or other un-described alternate embodiments may be available for a portion, which is not to be considered a disclaimer of those alternate embodiments. It will be appreciated that many of those un-described embodiments are within the literal scope of the following claims, and others are equivalent. Furthermore, all references, publications, U.S. Patents, and U.S. Patent Application Publications cited throughout this specification are incorporated by reference as if fully set forth in this specification.

Claims

CLAIMS What is claimed is:

1. A method for making highly-conductive graphene sheets of low oxygen content, comprising the steps of: a) exposing graphite powder to a mixture of ammonium persulfate and sulfuric acid to produce a reversible graphite intercalation compound;

b) purging the reversible graphite intercalation compound with oxygen gas to provide an oxygenated graphite intercalation compound;

c) mixing the oxygenated graphite intercalation compound with a piranha solution to provide a piranha oxidation mixture; and

d) subjecting the piranaha oxidation mixture to electromagnetic radiation until the graphene particles have been oxidized to the extent that highly-conductive graphene sheets of low oxygen content are obtained, wherein the graphene sheets are dispersible in various solvents without the use of special organic solvents or surfactants.

2. The method of claim 1, wherein the electromagnetic radiation is microwave radiation.

3. The method of claim 1 or 2, wherein the piranha solution has a volume ratio of sulfuric acid to hydrogen peroxide solution between about 20: 1 (v/v) and about 1: 1 (v/v) inclusive of water in the sulfuric acid and hydrogen peroxide solutions.

4. The method of claim 3, wherein the volume ratio of sulfuric acid and hydrogen peroxide solution in the piranha solution is between about 7:1 (v/v) and about 1: 1 (v/v).

5. The method of claim 4, wherein the volume ratio of sulfuric acid and hydrogen peroxide solution in the piranha solution is about 3: 1 (v/v).

6. The method of any one of claims 2 to 5, wherein said sulfuric acid is 95 to 98% H₂SO₄ by weight, with the remainder being water.

7. The method of any one of claims 2 to 6, wherein said hydrogen peroxide solution is 30 to 35% H₂O₂ by weight, with the remainder being water.

8. Highly-conductive low oxygen-containing graphene sheets produced according to the method of any one of claims 1 to 7.

9. The graphene sheets of claim 8, wherein the average lateral size of a single sheet is between about 3 nanometers and about 100 nanometers.

10. The graphene sheets of claim 8, wherein the average lateral size of a single sheet is between about 1.0 μιη and about 100 μιη.

11. The graphene sheets of any one of claims 8 to 10, wherein the graphene has a

conductivity between about 20,000 and about 200,000 S/m.

12. The graphene sheets of claim 11, wherein the graphene has a conductivity between about 75,000 and about 200,000 S/m.

13. The graphene sheets of claim 8, having a carbon-to-oxygen ratio between about 10: 1 and about 50: 1.

14. The method of any one of claims 1 to 7, wherein said graphene sheets have a

conductivity above 7000 S/m and a C:0 ratio greater than 10: 1.