METHODS FOR PRODUCING GRAPHENE WITH TUNABLE PROPERTIES BY A MULTI-STEP THERMAL REDUCTION PROCESS

Abstract

A process for the preparation of graphene includes the steps of a) providing flash thermal treatment of a graphite oxide at a temperature up to 700° C. sufficient to produce exfoliation and under inert atmosphere and b) cooling the material obtained in the previous step below 90° C. The method further includes the step of c) heating the material resulting from the previous step under inert atmosphere at a temperature which is higher than the temperature of step a), wherein the heating rate is between 1 and 15° C./min. The graphenes obtained from the process exhibit excellent physico-chemical properties.

Claims

1. A process for the preparation of graphene, the process including the following steps: a) flash thermal treatment under inert atmosphere of a graphite oxide at a temperature up to −700° C. sufficient to produce exfoliation; b) cooling the material obtained in the previous step below 90° C.; and c) heating the material resulting from the previous step under inert atmosphere at a temperature which is higher than the temperature of step a), wherein the heating rate is between 1° C./min and 15° C./min.

2. The process for the preparation of graphene according to claim 1, wherein the heating rate is between 2° C./min and 10° C./min.

3. The process for the preparation of graphene according to claim 1, wherein the material is cooled in step b) at a temperature below 40° C.

4. The process for the preparation of graphene according to claim 1, further including the following steps: a) flash thermal treatment under inert atmosphere of a graphite oxide at a temperature comprised between 90° C. and up to 700° C. sufficient to produce exfoliation; b) cooling the material obtained in the previous step below 40° C.; and c) heating the material obtained in the previous step above 700° C. and up 3000° C. at a graphene oxide reducing temperature under inert atmosphere at a heating rate between 2° C./min and 10° C./min.

5. The process for the preparation of graphene according to claim 1, wherein flash thermal treatment is carried out at a temperature greater than 90° C.

6. The process for the preparation of graphene according to claim 5, wherein flash thermal treatment is carried out at a temperature between 300° C. and 600° C.

7. The process for the preparation of graphene according to claim 2, wherein cooling in step b) is carried out down to ambient temperature.

8. The process for the preparation of graphene according to claim 1, wherein step c) is carried out at a temperature comprised between 800° C. and 2,800° C.

9. The process for the preparation of graphene according to claim 1, wherein step c) is carried out at a temperature comprised between 400° C. and 1200° C.

10. The process for the preparation of graphene according to claim 1, wherein the graphite oxide is prepared from graphite by oxidation.

11. The process for the preparation of graphene according to claim 10, wherein graphite is obtained from coke by graphitization.

12. The process for the preparation of graphene according to claim 1, wherein the reactor volume/graphene mass is 20-100 cm.sup.3/g.

13. The process for the preparation of graphene according to claim 2, wherein the material is cooled in step b) at a temperature below 40° C.

14. The process for the preparation of graphene according to claim 2, further including the following steps: a) flash thermal treatment under inert atmosphere of a graphite oxide at a temperature comprised between 90° C. and up to 700° C. sufficient to produce exfoliation; b) cooling the material obtained in the previous step below 40° C.; and c) heating the material obtained in the previous step above 700° C. and up 3000° C. at a graphene oxide reducing temperature under inert atmosphere at a heating rate between 2° C./min and 10° C./min.

15. The process for the preparation of graphene according to claim 4, wherein flash thermal treatment is carried out at a temperature greater than 90° C.

16. The process for the preparation of graphene according to claim 6, wherein cooling in step b) is carried out down to ambient temperature.

17. The process for the preparation of graphene according to 4, wherein step c) is carried out at a temperature comprised between 800° C. and 2,800° C.

18. The process for the preparation of graphene according to claim 4, wherein step c) is carried out at a temperature comprised between 400° C. and 1200° C.

19. The process for the preparation of graphene according to claim 4, wherein the graphite oxide is prepared from graphite by oxidation.

20. The process for the preparation of graphene according to claim 19, wherein graphite is obtained from coke by graphitization.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] FIG. 1: Images of different graphene samples obtained by the multi-step thermal process of the disclosure (MS) and a single-step process (SS). FIG. 1a compares the appearance of the MS-GO-C-300/1000 (Example 1) with the appearance of the SS-GO-C-1000 sample (Example 2). FIG. 1b compares the appearance of the MS-GO-P-460/700 (Example 3) with the appearance of the SS-GO-P-700 sample (Example 4) and that of the pseudo MS-GO-P-460 ramp/700 (example 5). FIG. 1c compares the appearance of MS-GO-P-300/700 (example 6) with the appearance of MS-GO-P-400/700 (example 7). FIG. 1d compares the appearance of the MS-GO-S-300/1000 (Example 8) with the appearance of the SS-GO-S-1000 sample (Example 9).

[0021] FIG. 2: Images of different graphene sample suspensions in DMSO obtained by the multi-step thermal process of the disclosure (MS) and a single-step process (SS). FIG. 2a illustrates the suspensions in DMSO at t=0 of SS-GO-C-1000 (left vial) and MS-GO-C-300/1000 (right vial) and the same suspensions after 5 min (t=5). See also Examples 1 and 2. FIG. 2b compares the DMSO suspension of the MS-GO-P-460/700 (Example 3) with the DMSO suspension of the SS-GO-P-700 sample (Example 4) and that of the pseudo MS-GO-P-460 ramp/700 (example 5). FIG. 1c compares the suspension of MS-GO-P-300/700 (example 6) with the DMSO suspension of MS-GO-P-400/700 (example 7). FIG. 1d compares the DMSO suspension of the MS-GO-S-300/1000 (Example 8) with the DMSO suspension of the SS-GO-S-1000 sample (Example 9).

[0022] FIG. 3: Nitrogen adsorption isotherm of MS-GO-C-300/1000 and SS-GO-C-1000. See also Examples 1 and 2.

[0023] FIG. 4: shows an embodiment of a reactor of the present disclosure, for carrying out the step a) of flash thermal treatment of graphite oxide.

DETAILED DESCRIPTION OF THE DRAWINGS

[0024] Throughout the present disclosure, graphenes are labeled according to the nomenclature Y-GO-X-T, wherein Y is either SS (single step) or MS (multistep), GO indicates Graphite Oxide. X indicates the origin of the graphite, wherein S indicates “synthetic”, C indicates “commercial”, CC indicates “combustion coke” and P is “petroleum coke”. T indicates the temperature at which graphene is obtained. For single step graphenes (SS), only one temperature is indicated, and for multi-step processes (MS) a first temperature is indicated corresponding to the first heating temperature, followed by “/” and the second heating temperature.

[0025] According to a particular embodiment, the process for the preparation of graphene further comprises a cooling step a′), between steps a) and b). In another particular embodiment, the process for the preparation of graphene of the present disclosure comprises:

[0026] a) flash thermal treatment of a graphite oxide at a temperature comprised between 90° C. and up to 700° C. sufficient to produce exfoliation and under inert atmosphere;

[0027] b) cooling the material obtained in the previous step at a temperature below 40° C.; and

[0028] c) heating the material obtained in the previous step above 700° C. and up 3000° C. under inert atmosphere at a heating rate between 2° C./min and 10° C./min.

[0029] The step of cooling the graphene oxide b) is carried out to a temperature below the temperature reached in the previous step a). The cooling temperature may vary always being less than disclosure 90° C. Typically, cooling can be done down to ambient temperature. In the present disclosure ambient temperature refers to a temperature typically below 40° C., for example between 10° C. and 30° C.

[0030] The term “flash thermal treatment” throughout the present disclosure is understood as a treatment wherein the sample is put in contact with a system preheated to the temperature indicated. For example, if treated in an oven, the flash thermal treatment according to the present disclosure comprises heating the entire oven volume to the temperature indicated and then introducing the sample so that is suddenly presented with such temperature.

[0031] According to a particular embodiment, the flash thermal treatment is carried out at a temperature greater than 90° C., preferably greater than 120° C. Exfoliation does not usually take place in a linear fashion with temperature, but there is typically a critical temperature (“explosion temperature”) at which the gases formed by the decomposition of graphite oxide are released suddenly resulting in most of the exfoliation of the material. Different graphite oxides have different explosion temperatures which can be determined by the skilled person. As the temperature of the flash thermal treatment increases with respect to that of the explosion temperature, a graphene with higher BET values is usually obtained. Thus, the process of the disclosure can lead at will to graphenes with high BET values and low reduction, as well as to graphenes with low BET values and high reduction, while providing at the same time excellent stability of dispersions thereof and processability. In another particular embodiment the flash thermal treatment is carried out at a temperature greater than 300° C., more particularly greater than 450° C. In a particular embodiment the flash thermal treatment is carried out at 460° C. In a further particular embodiment, the temperature in step a) is set between 100 and 700° C., particularly, between 300 and 600° C. In a further particular embodiment, the temperature in step a) is set between 100 and 250° C.

[0032] According to a particular embodiment, flash thermal treatment can take place in an especially designed reactor as the one illustrated in FIG. 4. This reactor is part of a device for carrying out the process of the disclosure which constitutes a further aspect of the disclosure. As explained below, it is especially suited to perform step a) of the present disclosure. In a further aspect the disclosure relates to a device comprising a reactor especially designed for carrying out the flash thermal treatment of the graphite oxide. The reactor according to a particular embodiment is represented in FIG. 4, wherein the reactor comprises an inlet (1), and inlet valve (8), a preheated volume (2) comprising and oven (3) and a discharge valve (4), heating means (5), a gas inlet (6) and a gas outlet (7).

[0033] This reactor allows carrying out step a) of the process of the disclosure, the exfoliation of graphite oxide into graphene oxide, in a semicontinous mode.

[0034] According to an embodiment, said reactor comprises an inlet (1) suitable for the addition of grafite oxide, said inlet (1) being separated from a preheated volume (2), which comprises an oven (3) connected to a discharge valve (4), by an inlet valve (8), and wherein said heated volume (2) is connected to a gas inlet (6) and a gas outlet (7), and in contact with heating means (5), said discharge valve (4) being connected to discharge means (9) [not shown] where the exfoliated material is recovered, and optionally allowed to cool. The reactor is under inert atmosphere in order in order to avoid undesired fire.

[0035] The graphene oxide is introduced in the inlet (1) of the reactor and through the inlet valve (8) reaching the preheated volume (2) where the flash thermal exfoliation takes place. The inventors have observed that in order to achieve an adequate exfoliation, it is important to introduce the graphite oxide in a preheated area, so that the graphite oxide is suddenly submitted to a temperature of up to 700° C. Also the relation between the mass (grams) of graphene oxide introduced in the reactor and the volume (cm.sup.3) of the preheated volume (2) can be advantageously controlled. The preheated volume (2) is preheated by conventional means in the oven (3). The nature of said heating means is not critical and typically involves an electric heating system. After the exfoliation, the graphene oxide is discharged through a discharge valve (4) ready to be submitted to steps b) and c). Residence times in the reactor are typically between about 2 and about 10 minutes depending on the applied temperature, in particular about 5 min.

[0036] According to a particular embodiment, step c) is carried out at a temperature comprised between 800° C. and 2,800° C., more particularly between 900° C. and 2500° C., even more particularly between 1000° C. and 2000° C. In a further embodiment the temperature of step c) is carried out at a temperature comprised between 400° C. and 1200° C., more particularly between 450° C. and 1050° C., more particularly between 500° C. and 1000° C. and even more particularly between 600° C. and 1000° C. This step can be carried out in a graphite oven with heating rates that vary between 1 and 15° C./min, more particularly between 2 and 10° C./min, for example 3° C./min, 4° C./min, 5° C./min, 6° C./min or 7° C./min.

[0037] The process of the disclosure is carried under inert atmosphere which may be achieved by introducing any inert gas like N.sub.2, Ne, Ar, etc. In a particular embodiment step a) is carried out under a N.sub.2 atmosphere, with flows between 50 and 300 mL/min, and step c) is carried out in an Ar atmosphere with flows between 2 and 4 L/min.

[0038] According to a particular embodiment, the ratio between the preheated volume and the graphite oxide added is comprised between 20-100 cm.sup.3/g. Such particular ranges provide the mass of graphite oxide inside the preheated volume of enough space to guarantee optimum exfoliation of the graphite oxide, minimizing the risk of an excessive stacking of the resulting graphene oxide.

[0039] In principle, any graphite oxide can be used as starting material in the process of the disclosure. According to a preferred embodiment, the graphite oxide is prepared from graphite by oxidation. Oxidation can be carried out by any of the well-known methods in the art like the Hummer's method (Hummers W S, Offeman R E. Preparation of graphitic oxide. Journal of the American Chemical Society 1958; 80: 1339-40) or a modified Hummer's method. It can be also prepared by other methodologies such as Brodie method (Brodie B C. Sur le poids atomique du graphite. Annales de chimie et de physique 1860; 59: 466-72), Staudenmaier method (L. Staudenmaier, Ber. Dtsch. Chem. Ges., 1898, 31, 1481), mild oxidation of graphite with different reactant (H.sub.2O.sub.2, etc.) (US patent application 20090028777). In a preferred embodiment, the graphite oxide is prepared by the Hummer's method or any modified Hummer's method, as illustrated in the Examples.

[0040] In turn, the graphite can be natural or synthetic graphite. In a particular embodiment the graphite used as starting material is commercial graphite, petrochemical graphite or semisynthetic graphite. The synthetic graphite can be obtained for example by graphitization of carbochemical, petrochemical or synthetic graphitizable carbonaceous precursors, such as coke, pitch or polymerized aromatic compounds. According to a preferred embodiment, graphite is obtained from coke by graphitization.

[0041] The graphene of the disclosure has been characterized by different methods as follows:

[0042] Elemental Analysis:

[0043] The carbon, hydrogen, sulphur and nitrogen contents of the samples were determined with a LECO-CHNS-932 microanalyzer. The oxygen content was obtained directly using a LECO-VTF-900 furnace coupled to the micro-analyzer. The proximate analysis was carried out following the ASTM D3174-04 Standard Test Method. All of the analyses were made using 1 mg of sample ground and sieved to <0.2 mm. The results were quoted as a mean of values from four determinations. In all cases, the standard deviation was found to be <0.5% of the absolute value. The results vary within certain ranges depending on facts like the type of graphite oxide used, the type of graphite, the reaction conditions for preparing the graphite oxide, and the thermal treatment temperatures.

[0044] Surface BET: The textural characteristics of the different samples were analyzed using N.sub.2 adsorption at 77 K. These analyses were performed in an ASAP 2020 Micromeritics equipment. The measurements were carried out following the ASTM standard D-6556 with a modification: before the experiments, the samples were outgassed at 350° C. for 10 h under vacuum (pressure below 10.sup.−3 Pa), using around 100 mg of sample in each experiment. The apparent surface area was determined from the N.sub.2-adsorption isotherm using the BET equation. The increased surface area of the graphene obtained by the process of the disclosure has been determined to be at least 100 m.sup.2/g, preferably 200 m.sup.2/g, more preferably 300 m.sup.2/g, even more preferably 400 m.sup.2/g and most preferably 500 m.sup.2/g.

[0045] Stability of dispersion of the graphenes in solvent: stability of dispersions of the different graphenes was evaluated by dispersing 10 mg of the graphene in 10 mL of DMSO at room temperature and 1 atm. After introducing in the ultrasound for 15 min, the suspension was left at room temperature and the presence of precipitate qualitatively evaluated. The corresponding graphene was considered to form stable dispersions if after 5 minutes no appreciable precipitation was observed.

[0046] Volume of the sample obtained: For comparative purposes of the volume occupied for a certain amount of graphene, digital images of the volume occupied for 22.5 grams of the obtained graphenes are shown in FIG. 1.

[0047] The graphene of the disclosure presents among other characteristics, a controlled surface area, mainly as a factor of the temperature of initial flash pyrolysis step, while the subsequent step in ramp to the final temperature required to obtain the reduction desired does increase the surface area but not substantially. As a consequence of the control of the surface area, parameters as the volume occupying the sample or the possibility to disperse the sample in certain solvents are also established. These parameters (and their control) are crucial for diverse applications, like in batteries.

[0048] Even further, the graphenes obtainable by the process of the disclosure can form very stable dispersions in solvents, and therefore provide improved processability compared to other graphenes described in the literature. This is a very important feature for the preparation of pastes of graphene and to form homogeneous mixtures with other components, such as polymers, in order to provide materials with improved properties, a technology area currently of key interest.

[0049] For example, such processability can provide improved mixtures with spinels and other fillers in order to prepare improved and more homogeneous materials destined to store energy (e.g. batteries, supercondensers).The following examples are non-limiting and are merely representative of various aspects of the disclosure.

EXAMPLES

[0050] The present disclosure will be better understood from the following examples and more specifically by comparing the characterization of the samples produced by the preferred (but not limiting) embodiments of the multi-step procedure developed herein (examples 1, 3, 6, 7 and 8) and those resulted in the characterization of the same set of samples by thermally treatment at up to the same final temperature but by means of standard procedures described in the state of the art and reproduced herein (examples 2, 4, 5 and 9).

Example 1 (According to the Disclosure)

Preparation of Graphene from Commercial Graphite with a First Flash Thermal Treatment at 300° C. and a Second Thermal Treatment at 1000° C. (MS-GO-C-300/1000)

[0051] MS-GO-300/1000 was prepared by using as starting material a graphite oxide (GO-C) prepared from a commercial graphite using the modified Hummers' method described below.

[0052] Preparation of Graphite Oxide

[0053] This method makes use of the Hummers' reagents with additional amounts of NaNO.sub.3 and KMnO.sub.4. Concentrated H.sub.2SO.sub.4 (360 mL) was added to a mixture of graphite (7.5 g) and NaNO.sub.3 (7.5 g), and the mixture was cooled down to 0° C. by means of an ice bath. KMnO.sub.4 (45 g) was added slowly in small doses to keep the reaction temperature below 20° C. The solution was heated up to 35° C. and stirred for 3 h, at which point 3% H.sub.2O.sub.2 (1.5 L) was slowly added, giving rise to a pronounced exothermal effect up to 98° C. The reaction mixture was stirred for 30 min and, finally, the mixture was centrifuged (3700 rpm for 30 min), the supernatant being decanted away. The remaining solid material was then washed with 600 mL of water and centrifuged again, this process being repeated until the pH was neutral.

[0054] Preparation of MS-GO-C-300/1000

[0055] In a first step of flash thermal treatment, 0.3 g of GO-C were introduced in an oven having a volume of 25 cm.sup.3 previously heated at 300° C. under an atmosphere of N.sub.2 (100 mL min.sup.−1). The sample was then allowed to cool to room temperature. In a second step of thermal treatment the sample previously obtained was introduced in an oven and heated up to 1000° C. under an atmosphere of Ar (3 L min.sup.−1) at a heating rate of 5° C. min.sup.−1 to 700° C., the sample being kept at this temperature for 1 h. The sample so obtained was labeled as MS-GO-C-300/1000.

[0056] Characterization of MS-GO-C-300/1000

[0057] The obtained sample exhibits a fluffy appearance (FIG. 1a right). The suspension of MS-GO-C-300/1000 in DMSO is homogeneous and stable after 5 min (FIG. 2, right sample on both pictures). The SBET surface area calculated for this sample was ≈430 m.sup.2g.sup.−1 (Table 1). The C/O ratio, calculated by elemental analysis, is 130 (Table 1), well in the range of the samples thermally treated at 1000° C., but with a very high SBET and excellent stability in dispersion.

Example 2 (Comparative)

Preparation of Graphene from Commercial Graphite by a Single-Step Thermal Treatment at 1000° C. with Heating Rate of 5° C./min (SS-GO-C-1000)

[0058] Preparation of SS-GO-C-1000

[0059] SS-GO-C-1000 was prepared by using as starting material the graphite oxide (GO-C) prepared in example 1. GO was thermally treated in an oven and heated up to 1000° C. under an atmosphere of Ar (3 L min.sup.−1) at a heating rate of 5° C./min, the sample being kept at this temperature for 1 h. The sample was labeled as SS-GO-C-1000.

[0060] Characterization of SS-GO-C-1000

[0061] SS-GO-C-1000 exhibits an appearance completely different form MS-GO-C-300/1000 (FIG. 1a, left sample). The image illustrates the volume occupied for 22.5 mg of each sample, volume which is much lower in the case of the sample obtained in a single step than by a multi-step MS-GO-C-300/1000.

[0062] The suspension of the sample in DMSO is not as homogeneous as MS-GO-C-300/1000 and it precipitates even after 5 min (FIG. 2 left sample in both pictures). The SBET surface area calculated for SS-GO-C-1000 is ≈100 m.sup.2g.sup.−1 (Table 1), much less than that obtained for MS-GO-C-300/1000.

[0063] The C/O ratio, calculated by elemental analysis (Table 1) is 69, in the range of the expected for thermally treated samples at 1000° C. This value is a bit larger that for the sample obtained by a multiple-step.

[0064] The above results demonstrate the validity of the hypothesis that a multi-step procedure up to the same final temperature led to a reduced graphene with modified characteristics. Particularly a more stable suspension is obtained and the sample exhibit and enhanced SBET surface area. The reduction of the sample, although a bit larger in the case of the multi-step procedure, is in the range expected.

Example 3 (According to the Disclosure)

Preparation of Graphene from Petrochemical Graphite with a First Flash Thermal Treatment at 460° C. and a Second Thermal Treatment at 700° C. (MS-GO-P-460/700)

[0065] Preparation of MS-GO-P-460/700

[0066] MS-GO-P-460/700 was prepared by using as starting material a graphite oxide (GO-P) prepared by oxidation, using the Hummers method described in example 1, of a graphite, which itself was obtained by graphitization at 2800° C. of a petroleum coke. In a first step of flash thermal treatment, 0.3 g of GO-P were introduced in an oven having a volume of 25 cm.sup.3 previously heated at 460° C. under an atmosphere of N.sub.2 (100 mL min.sup.−1). The sample was then cooled to room temperature. In a second step of thermal treatment the sample previously obtained was introduced in an oven and heated up to 700° C. under an atmosphere of Ar (3 L min.sup.−1) at a heating rate of 5° C. min.sup.−1, the sample being kept at this temperature for 1 h. The sample so obtained was labeled as MS-GO-P-460/700

[0067] Characterization of MS-GO-P-460/700

[0068] The obtained sample exhibits a fluffy appearance (FIG. 1b right). The suspension of MS-GO-P-460/700 in DMSO is homogeneous and stable after 5 min (FIG. 2b right). The SBET surface area calculated for this sample was ≈530 m.sup.2g.sup.−1 (Table 1). The C/O ratio is 16.3 (Table 1), in the range of the samples thermally treated at 700° C., but with a very high SBET and excellent stability in dispersion.

Example 4 (Comparative)

Preparation of Graphene from Petrochemical Graphite by a Single-Step Thermal Treatment at 700° C. with Heating Rate of 5° C./min (SS-GO-P-700)

[0069] Preparation of SS-GO-P-700

[0070] SS-GO-P-700 was prepared by using as starting material the graphite oxide (GO-P) prepared in example 3. GO-P was thermally treated in an oven and heated up to 700° C. under an atmosphere of Ar (3 L min.sup.−1) at a heating rate of 5° C. min.sup.−1, the sample being kept at this temperature for 1 h. The sample was labeled as SS-GO-P-700.

[0071] Characterization of SS-GO-P-700

[0072] SS-GO-P-700 (FIG. 1b, middle) exhibits an appearance completely different from MS-GO-C-460/700. The image illustrates the volume occupied for 22.5 mg of each sample, volume which is visually much lower in the case of the sample obtained in a single step (SS-GO-P-700) than in the multi-step MS-GO-C-460/700. The suspension of the sample in DMSO is not as homogeneous as MS-GO-P-460/700 and it precipitates even after 5 min (FIG. 2b, middle). The SBET surface area calculated for SS-GO-P-700 is ≈210 m.sup.2g.sup.−1, much less than the obtained for MS-GO-P-460-700 (530 m.sup.2g.sup.−1). The C/O ratio is 15 (Table 1), in the range of the samples thermally treated at 700° C. as MS-GO-C-460/700.

[0073] From the results showed in examples 3 and 4 it can be seen that the process of the disclosure leads to a reduced graphene forming more stable suspensions and having larger SBET surface area.

[0074] We also demonstrate that the initial temperature of the initial thermal treatment can be modified to certain point to control de SBET surface area obtained.

Example 5: (Comparative)

Preparation of Graphene from Petrochemical Graphite by a First Thermal Treatment with Ramp of 5° C./min Up to 460° C., Cooling to 300° C., and a Second Thermal Treatment Up to 700° C. with Ramp of 5° C./min (PseudoMS-GO-P-460Ramp/700)

[0075] Preparation of PseudoMS-GO-P-460Ramp/700

[0076] pseudoMS-GO-P-460ramp/700 was prepared by using as starting material the graphite oxide (GO-P) prepared in example 3. GO-P was thermally treated in an oven and heated up to 460° C. under an atmosphere of Ar (3 L min.sup.−1) at a heating rate of 5° C. min.sup.−1. The sample was then cooled to room temperature. In a second step of thermal treatment the sample previously obtained was introduced in an oven and heated up to 700° C. under an atmosphere of N.sub.2 (3 L/min) at a heating rate of 5° C./min, the sample being kept at this temperature for 1 h.

[0077] Characterization of PseudoMS-GO-P-460Ramp/700

[0078] pseudoMS-GO-P-460ramp/700 (FIG. 1b, left) exhibits an appearance completely different from MS-GO-C-460/700 and similar to that of SS-GO-P-700. The image illustrates the volume occupied for 22.5 mg of each sample, volume which is much lower than in the multi-step MS-GO-C-460/700. The suspension stability of the sample in DMSO is also similar to that of SS-GO-P-700 (FIG. 2b left), as well as its C/O ratio (15, Table 1) and SBET surface area calculated ≈200 m.sup.2g.sup.−1 (Table 1), much less than the obtained for MS-GO-P-460-700 (530 m.sup.2g.sup.−1).

[0079] It can thus be seen that the multi-step procedure of the disclosure comprising an initial flash pyrolysis step) leads to graphenes with improved characteristics.

Example 6 (According to the Disclosure)

Preparation of Graphene from Petrochemical Graphite with a First Flash Thermal Treatment at 300° C. and a Second Thermal Treatment at 700° C. (MS-GO-P-300/700)

[0080] Preparation of MS-GO-P-300/700

[0081] MS-GO-P-300/700 was prepared by using as starting material the graphite oxide (GO-P) prepared in example 3. In a first step of flash thermal treatment, 0.3 g of GO-P were introduced in an oven having a volume of 25 cm.sup.3 previously heated at 300° C. under an atmosphere of N.sub.2 (100 mL min.sup.−1). The sample was then cooled to room temperature. In a second step of thermal treatment the sample previously obtained was introduced in an oven and heated up to 700° C. under an atmosphere of Ar (3 L min.sup.−1) at a heating rate of 5° C. min.sup.−1, the sample being kept at this temperature for 1 h. The sample so obtained was labeled as MS-GO-P-300/700.

[0082] Characterization of MS-GO-P-300/700

[0083] The obtained sample exhibits a fluffy appearance (FIG. 1c right). The suspension of MS-GO-P-300/700 in DMSO is homogeneous and stable after 5 min, although less than MS-GO-P-460/700 (example 3) (FIG. 2c right). The SBET surface area calculated for this sample was ≈270 m.sup.2g.sup.−1 (Table 1). This value is lower than that of MS-GO-P-460/700. The C/O ratio however is 13 (Table 1), in the range of MS-GO-P-460/700. Again the process according to the disclosure leads to graphene with improved SBET, controlled oxygen content and excellent dispersion stability and processability.

Example 7 (According to the Disclosure)

Preparation of Graphene from Petrochemical Graphite with a First Flash Thermal Treatment at 400° C. and a Second Thermal Treatment at 700° C. (MS-GO-P-400/700)

[0084] Preparation of MS-GO-P-400/700

[0085] MS-GO-P-400/700 was prepared by using as starting material the graphite oxide (GO-P) prepared in example 3. In a first step of flash thermal treatment, 0.3 g of GO-P were introduced in an oven having a volume of 25 cm.sup.3 previously heated at 400° C. under an atmosphere of N.sub.2 (100 mL min.sup.−1). The sample was then cooled to room temperature. In a second step of thermal treatment the sample previously obtained was introduced in an oven and heated up to 700° C. under an atmosphere of Ar (3 L min.sup.−1) at a heating rate of 5° C. min.sup.−1, the sample being kept at this temperature for 1 h. The sample so obtained was labeled as MS-GO-P-400/700

[0086] Characterization of MS-GO-P-400/700

[0087] The obtained sample exhibits a fluffy appearance (FIG. 1c left). The suspension of MS-GO-P-400/700 in DMSO is homogeneous and stable after 5 min, similar to MS-GO-P-460/700 (example 3) (FIG. 2c left). The SBET surface area calculated for this sample was ≈450 m.sup.2g.sup.−1 (Table 1). This value is lower than that of MS-GO-P-460/700 (experiment 3). The C/O ratio however is 15 (Table 1), in the range of MS-GO-P-460/700. Comparing experiments 3, 6 and 7 we confirm that the contribution of the initial flash pyrolysis steps to the final surface area is crucial, while the second step is required to control this surface area as well as to achieve the desired reduction degree. The process according to the disclosure again leads to graphene with improved SBET, controlled oxygen content and excellent dispersion stability and processability.

Example 8 (According to the Disclosure)

Preparation of Graphene from Synthetic Graphite with a First Flash Thermal Treatment at 300° C. and a Second Thermal Treatment at 1000° C. (MS-GO-S-300/1000)

[0088] Preparation of MS-GO-S-300/1000

[0089] MS-GO-S-300/1000 was prepared by using as starting material a graphite oxide (GO-S) prepared from a synthetic graphite, using the Hummers method described in example 1. In a first step of flash thermal treatment, 0.3 g of GO-S was introduced in an oven having a volume of 25 cm.sup.3 previously heated at 300° C. under an atmosphere of N.sub.2 (100 mL min−1). The sample was then cooled to room temperature. In a second step of thermal treatment the sample previously obtained was introduced in an oven and heated up to 1000° C. under an atmosphere of Ar (3 L min.sup.−1) at a heating rate of 5° C. min.sup.−1, the sample being kept at this temperature for 1 h. The sample so obtained was labeled as MS-GO-S-300/1000

[0090] Characterization of MS-GO-S-300/1000

[0091] The obtained sample exhibits a fluffy appearance (FIG. 1d right). The suspension of MS-GO-S-300/1000 in DMSO is homogeneous and stable after 5 min (FIG. 2d right). The C/O ratio calculated for this sample is 163 (Table 1), similar to other samples heated at this temperature. The SBET surface area calculated for this sample was ≈530 m.sup.2g.sup.−1 (Table 1).

Example 9 (Comparative)

Preparation of Graphene from Synthetic Graphite by a Single-Step Thermal Treatment at 1000° C. with Heating Rate of 5° C./min (SS-GO-S-1000)

[0092] Preparation of SS-GO-S-1000

[0093] SS-GO-S-1000 was prepared by using as starting material the graphite oxide (GO-S) prepared in example 8. GO-S was thermally treated in an oven and heated to 1000° C. under an atmosphere of Ar (3 L min.sup.−1) at a heating rate of 5° C. min.sup.−1, the sample being kept at this temperature for 1 h. The sample was labelled as SS-GO-S-1000.

[0094] Characterization of SS-GO-S-1000

[0095] SS-GO-S-1000 (FIG. 1d, left) exhibits an appearance completely different form MS-GO-S-300/1000. The image illustrates the volume occupied for 22.5 mg of each sample, volume which is much larger in the case of the sample obtained in a single step by a multi-step MS-GO-S-300/1000. The C/O ratio for this sample is 159 (Table 1). The SBET surface area calculated for SS-GO-S-1000 is ≈270 m.sup.2g.sup.−1, Table 1, much less than that obtained for MS-GO-S-300/1000 (530 m.sup.2g.sup.−1, Table 1). The suspension of the sample SS-GO-S-1000 in DMSO is not as homogeneous as MS-GO-S-300/1000 and it precipitates even after 5 min (FIG. 2d left).

Example 10 (According to the Disclosure)

Preparation of Graphene from Synthetic Graphite with a First Flash Thermal Treatment at 400° C. and a Second Thermal Treatment at 1000° C. (MS-GO-S-400/1000)

[0096] Preparation of MS-GO-S-400/1000

[0097] MS-GO-S-400/1000 was prepared by using as starting material a graphite oxide (GO-S) prepared from a synthetic graphite, using the Hummers method described in example 1. In a first step of flash thermal treatment, 0.3 g of GO-S was introduced in an oven having a volume of 25 cm.sup.3 previously heated at 400° C. under an atmosphere of N.sub.2 (100 mL min−1). The sample was then cooled to room temperature. In a second step of thermal treatment the sample previously obtained was introduced in an oven and heated up to 1000° C. under an atmosphere of Ar (3 L min.sup.−1) at a heating rate of 5° C. min.sup.−1, the sample being kept at this temperature for 1 h. The sample so obtained was labeled as MS-GO-S-400/1000

[0098] Characterization of MS-GO-S-400/1000

[0099] The obtained sample exhibits a fluffy appearance. The suspension of MS-GO-S-400/1000 in DMSO is homogeneous and stable after 5 min. The C/O ratio calculated for this sample is 129 (Table 1), similar to other samples heated at this temperature. The SBET surface area calculated for this sample was ≈355 m.sup.2g.sup.−1 (Table 1).

TABLE-US-00001 TABLE 1 Material Ex. Kind Flash BET C/O Dispersion MS-GO-C- 1 Disclosure yes 430 130 Stable 300/1000 SS-GO-C-1000 2 Comparative Ramp 5° C./min 100 69 Precipitates MS-GO-P-460/700 3 Disclosure yes 530 16 Stable SS-GO-P-700 4 Comparative Ramp 5° C./min 210 15 Precipitates pseudoMS-GO-P- 5 Comparative Ramp 5° C./min 200 15 Precipitates 460ramp/700 MS-GO-P-300/700 6 Disclosure yes 270 15 Stable MS-GO-P-400/700 7 Disclosure yes 450 15 Stable MS-GO-S-300/1000 8 Disclosure yes 530 163 Stable SS-GO-S-1000 9 Comparative Ramp 5° C./min 270 159 Precipitates MS-GO-S-400/1000 10 Disclosure yes 355 129 Stable