PARTIALLY OXIDIZED GRAPHENE AND METHOD FOR PREPARING SAME

Abstract

The present invention relates to a partially oxidized graphene and a method for preparing the same. Since the partially oxidized graphene is prepared by subjecting the partially oxidized graphite to a high pressure homogenization, the exfoliation efficiency is excellent, the inherent characteristics of graphene are maintained even without using a reduction step after exfoliation, and the dispersibility thereof in organic solvents is excellent, and thus the invention can be applied to various fields.

Claims

1. A partially oxidized graphene having: an oxygen/carbon (O/C) atomic ratio of 5 to 20%, an average size (lateral size) of 100 nm to 20 μm, and a thickness of 0.34 nm to 30 nm.

2. The partially oxidized graphene according to claim 1, wherein the partially oxidized graphene has a ratio of D/G in the Raman spectra of 0.12 to 0.5.

3. A method for preparing a partially oxidized graphene, comprising a step of passing a feed solution including partially oxidized graphite through a high-pressure homogenizer including an inlet, an outlet, and a micro-channel that connects between the inlet and the outlet and has a diameter in a micrometer scale, wherein the partially oxidized graphene has an oxygen/carbon (O/C) atomic ratio of 5 to 20%.

4. The method for preparing graphene according to claim 3, wherein the partially oxidized graphite may be prepared by oxidizing a pristine graphite with at least one acidic solution selected from the group consisting of nitric acid and sulfuric acid.

5. The method for preparing graphene according to claim 4, wherein the oxidation time is 2 to 30 hours.

6. The method for preparing graphene according to claim 3, wherein the concentration of the partially oxidized graphite in the feed solution is 0.05 to 100 mg/mL.

7. The method for preparing graphene according to claim 3, wherein the solvent of the feed solution is one or more selected from the group consisting of water, NMP (N-methyl-2-pyrrolidone), acetone, DMF (N,N-dimethylformamide), DMSO (dimethyl sulfoxide), CHP (cyclohexyl-pyrrolidinone), N12P (N-dodecyl-pyrrolidone), benzyl benzoate, N8P (N-octyl-pyrrolidone), DMEU (dimethyl-imidazolidinone), cyclohexanone, DMA (dimethylacetamide), NMF (N-methyl formamide), bromobenzene, chloroform, chlorobenzene, benzonitrile, quinoline, benzyl ether, ethanol, isopropyl alcohol, methanol, butanol, 2-ethoxyethanol, 2-butoxyethanol, 2-methoxypropanol, THF (tetrahydrofuran), ethylene glycol, pyridine, N-vinylpyrrolidone, methyl ethyl ketone (butanone), alpha-terpineol, formic acid, ethyl acetate and acrylonitrile.

8. The method for preparing graphene according to claim 3, wherein the partially oxidized graphite in the feed solution is exfoliated while passing through a micro-channel under application of a shear force, thereby preparing a graphene.

9. The method for preparing graphene according to claim 3, wherein the micro-channel has a diameter of 50 to 300 μm.

10. The method for preparing graphene according to claim 3, wherein the feed solution is introduced in the inlet of the high-pressure homogenizer under application of a pressure of 500 to 3000 bar and passed through the micro-channel.

11. The method for preparing graphene according to claim 3, wherein the step of passing the material recovered in the inlet through a micro-channel is additionally repeated once to 9 times.

12. The method for preparing graphene according to claim 3, wherein the graphene prepared has an average thickness of 0.34 nm to 30 nm.

13. The method for preparing graphene according to claim 3, wherein the graphene prepared has a lateral size of 100 nm to 20 μm.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0059] FIG. 1 illustrates the measurement results of XRD spectra of the raw material BNB90 (FIGS. 1(a) and (b)), and the prepared poGF-75-20 (FIGS. 1(c) and (d)), poGF 90-16 (FIGS. 1(e) and (d)) and poGF 95-3 (FIGS. 1(g) and (h)) used in Example of the present invention.

[0060] FIG. 2 shows SEM images of the raw material BNB90 (FIGS. 2(a) and (b)) and the prepared GF-75-20 (FIGS. 2(c) and (d)) and poGF-95-3 (FIGS. 2(e) and (d)) used in Example of the present invention.

[0061] FIG. 3 shows SEM images of G-10 (FIGS. 3(a) and (b)), poGF-75-20-10 (FIGS. 3(c) and (d)), poGF-85-20-10 (FIGS. 3(e) and (f)), poGF-95-20-10 (FIGS. 3(g) and (h)), and poGF-95-3-10 (FIGS. 3(i) and (j)) prepared in Comparative Example and Example of the present invention.

[0062] FIG. 4 shows TEM images of poGF-75-20-10 (FIGS. 4(a) and (b)), poGF-85-20-10 (FIGS. 4(c) and (d)), poGF-95-20-10 (FIGS. 4(e) and (f)) and poGF-95-3-10 (FIGS. 4(g) and (h)) prepared in Example of the present invention.

[0063] FIG. 5 shows the AFM measurement results of poGF-75-20-10 prepared in Example of the present invention.

[0064] FIG. 6 shows the XPS measurement results of G-10 (FIG. 6(a)) and poGF-75-20-10 (FIG. 6(b)) prepared in Comparative Example and Example of the present invention.

[0065] Further, FIG. 6(c) is a table showing the atomic ratios for each carbon and oxygen.

[0066] FIG. 7 shows the measurement results of the Raman spectra of poGF-75-20-10 (FIG. 7(a)), poGF-85-20-10 (FIG. 7(b)), poGF-95-20-10 (FIG. 7(c)) and poGF-95-3-10 (FIG. 7(d)) prepared in Example of the present invention.

[0067] FIG. 8 shows SEM images of poGF-75-20-1 (FIGS. 8(a) and 8(b)), poGF-75-20-3 (FIGS. 8(c) and 8(d)), poGF-75-20-5 (FIGS. 8(e) and 8(f)), poGF-75-20-7 (FIGS. 8(g) and 8(h)), poGF-75-20-10 (FIGS. 8(i) and 8(j)) and G-10 (FIGS. 8(k) and 8(l)) prepared in Comparative Example and Example of the present invention.

[0068] FIG. 9 shows the results of the analysis of graphene lateral sizes of poGF-75-20-1, poGF-75-20-3, poGF-75-20-5, poGF-75-20-7, poGF-75-20-10, GP-1, GP-3, GP-5, GP-7 and GP-10 prepared in Example and Comparative Example of the present invention.

[0069] FIG. 10 shows the results of visually observing the degree of redispersion in various solvents of poGF-85-20-10 (FIG. 10(a)), poGF-95-20-10 (FIG. 10(b)) and poGF-95-3-10 (FIG. 10(c)) prepared in Example of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0070] Hereinafter, preferred examples are presented to aid in understanding of the invention. However, the following examples are provided only for illustrative purposes, and the scope of the present invention is not limited thereto.

EXAMPLE

[0071] Step 1

[0072] 2.5 g of pristine graphite (BNB90) was added to 262.5 mL of a mixed solution of ice-cooled sulfuric acid and nitric acid (volume ratio of sulfuric acid to nitric acid=3:1) and stirred at about 500 rpm. This was reacted in an oil bath at the temperature and time shown in Table 1 below to partially oxidize pristine graphite. After completion of the reaction, the reaction mixture was cooled to room temperature and slowly added to 1 L of ice-cooled distilled water. The reaction solution diluted with distilled water was filtered under vacuum to recover the partially oxidized graphite and dried in an oven at 100° C. overnight. Partially oxidized graphite produced according to reaction temperature and reaction time was named ‘poGF-(reaction temperature)-(reaction time)’ as shown in Table 1 below.

TABLE-US-00001 TABLE 1 Example Reaction Temperature Reaction time poGF-75-20 75° C. 20 hours poGF-85-20 85° C. 20 hours poGF-90-16 90° C. 16 hours poGF-95-20 95° C. 20 hours poGF-95-3 95° C. 3 hours

[0073] Step 2

[0074] Each of the partially oxidized graphite prepared in the step 1 was dispersed in 500 mL of distilled water to prepare a partially oxidized graphite feed solution having a concentration of 5 mg/mL. The feed solution was fed to the inlet of the high pressure homogenizer. The high-pressure homogenizer has a structure including an inlet of the raw material, an outlet of the exfoliated product, and a micro-channel that connects between the inlet and the outlet and has a diameter in a micrometer scale. The feed solution was introduced in the inlet while applying high-pressure of 1,600 bar, and a high shear force was applied while passing through a micro-channel having a diameter of 75 μm.

[0075] A certain amount of sample was taken from the outlet, and the remainder except for the above sample was reintroduced into the inlet of the high pressure homogenizer and the high pressure homogenization process was repeated. This process was repeated, and the sample taken from the outlet was named ‘poGF-(reaction temperature)-(reaction time)-(number of times of passage through high pressure homogenizer)’. For example, when the high pressure homogenization process was repeated five times with poGF-75-20, the sample taken from the outlet was named ‘poGF-75-20-5’.

Comparative Example

[0076] Pristine graphite (BNB 90) was dispersed in 500 mL of distilled water to prepare a graphite feed solution having a concentration of 5 mg/mL. The feed solution was subjected to a high pressure homogenization process in the same manner as in step 2 of the above example, and each sample was named ‘G-(number of times of passage through high pressure homogenizer)’.

[0077] Pristine graphite (BNB 90) was dispersed in 500 mL of distilled water containing 0.5 g of PVP (polyvinylpyrrolidone, weight average molecular weight: 58 K) to prepare a graphite feed solution having a concentration of 5 mg/mL. The feed solution was subjected to high pressure homogenization in the same manner as in step 2 of the above Example, and each sample was named ‘GP-(number of times of passage through high pressure homogenizer)’.

Experimental Example 1

[0078] XRD spectra of the raw material BNB90, and the prepared poGF-75-20, poGF 90-16 and poGF 95-3 used in the step 1 of Example were measured and the results were shown in FIG. 1.

[0079] Pristine graphite was partially oxidized in a mixed solution of sulfuric acid and nitric acid, and the basal plane and edge of graphite were partially oxidized to introduce an oxygen-functional group into graphite. As a result, as shown in FIG. 1, a peak shift was observed at low angle at the initial peak position (26.35°) of BNB90 due to the introduced oxygen-functional group or acid ion intercalation, etc. and broadening of FWHM was also observed. Further, as the reaction temperature increased from 75° C. (FIGS. 1(c) and 1(d)) to 90° C. (FIGS. 1 (e) and 1(f)), the degree of partial oxidation increased and thereby more peak shift also occurred. When the reaction time was shortened from 20 hours (FIGS. 1 (e) and 1(f)) to 3 hours (FIGS. 1(g) and 1(h)), the degree of partial oxidation decreased and thereby less peak shift occurred.

[0080] Therefore, it can be confirmed from the above results that the inter-sheet spacing of graphite could be increased by introducing an oxygen-functional group by partial oxidation, thereby reducing the attraction between graphite sheets.

Experimental Example 2

[0081] SEM images of the raw material BNB90 and the prepared GF-75-20 and poGF-95-3 used in the step 1 of Example were observed, and the results were shown in FIG. 2.

[0082] As shown in FIG. 2, the partially oxidized graphite (FIG. 2(c) to FIG. 2(f)) exhibited a slightly expanded state as compared with BNB90 (FIGS. 2(a) and 2(b)). Therefore, it can be confirmed that the inter-sheet spacing of the partially oxidized graphite was widened, similarly to the results of Experimental Example 1.

Experimental Example 3

[0083] Each of G-10, poGF-75-20-10, poGF-85-20-10, poGF-95-20-10, and poGF-95-3-10 prepared in Comparative Example and Example was subjected to drop-casting on Si wafer and dried, and then SEM images thereof were observed. The results were shown in FIG. 3.

[0084] As shown in FIGS. 3(a) and 3(b), when pristine graphite was applied to high pressure homogenization without a dispersant, the surface roughness was observed to be high due to the less exfoliated graphite chunk. On the other hand, as shown in FIGS. 3(c) to 3(j), the partially oxidized graphite had a reduced attraction between the sheets, so that the exfoliation due to high-pressure homogenization was facilitated and the less exfoliated graphite chunks were reduced, and thereby the surface roughness was observed to be low.

[0085] In the case of G-10, a re-aggregation phenomenon was observed due to the absence of oxygen-functional group on the surface of the exfoliated graphene, but in the case of using partially oxidized graphite, stable dispersion state was maintained without re-aggregation due to the repulsive force by the oxygen-functional group.

Experimental Example 4

[0086] Each of poGF-75-20-10, poGF-85-20-10, poGF-95-20-10, and poGF-95-3-10 prepared in the Example was diluted 10-fold, and then drop casted on Lacey carbon TEM Cu grid followed by drying, and TEM images thereof was observed. The results were shown in FIG. 4.

[0087] As shown in FIG. 4, it was confirmed that a graphene having a few layers of thickness was produced by subjecting the partially oxidized graphite to high-pressure homogenization.

Experimental Example 5

[0088] The poGF-75-20-10 prepared in the above Example was diluted 5-fold, subjected to oxygen-plasma treatment, followed by spin-coating on Si wafer, and AFM was measured. The results were shown in FIG. 5.

[0089] The thicknesses of graphene measured at positions 1, 2 and 3 shown in FIG. 5 were measured to be 6.052 nm, 5.260 nm and 4.363 nm, respectively. From this, the overall thickness of graphene is expected to be about 5-10 nm.

Experimental Example 6

[0090] The content of each element of poGF-75-20-10, poGF-85-20-10, poGF-95-20-10, and poGF-95-3-10 prepared in the above Example was analyzed, and the results are shown in Table 2 below.

TABLE-US-00002 TABLE 2 O/C C O atomic N S Average Deviation Average Deviation ratio Average Deviation Average Deviation poGF-75-20-10 86.96 86.96 7.39 <0.1 8.50% 0.22 0.004 0.31 0.44 poGF-85-20-10 86.36 86.36 9.46 0.25 10.95% 0.09 0.12 0.74 0.03 poGF-95-20-10 84.30 84.30 10.60 0.06 12.57% 0.16 0.01 0.86 0.02 poGF-95-3-10 89.12 89.12 7.37 0.05 8.27% 0.23 0.01 0.65 0.01

[0091] As shown in Table 2, it was confirmed that that as the reaction temperature increased, the degree of oxidation (O/C atomic ratio) increased from 8.50% to 12.57% by about 1.5 times. It was also confirmed that at the same reaction temperature (95° C.), as the reaction time increased, the degree of oxidation increased from 8.27% to 12.57%. From the above results, it was confirmed that the degree of oxidation can be easily adjusted by controlling the partial oxidation reaction conditions.

Experimental Example 7

[0092] From the XPS analysis of G-10 and poGF-75-20-10 prepared in the Comparative Example and the Example, the types of oxygen-functional groups produced and their degree of oxidation were analyzed. The results were shown in FIG. 6.

[0093] As shown in FIG. 6, it was confirmed that the oxygen-functional group mainly formed was an epoxide and a carboxyl group. In addition, similarly to Experimental Example 6, it was confirmed that the ratio of carbon atoms ((C2+C3)/C1) produced by partial oxidation exhibited about six times higher than G-10, and as a result, an oxygen-functional group was effectively introduced due to the partial oxidation.

Experimental Example 8

[0094] XPS quantitative elemental analysis of G-10, poGF-75-20-10, poGF-85-20-10, poGF-95-20-10 and poGF-95-3-10 prepared in the Comparative Example and the Example was carried out, and the results are shown in Table 3 below.

TABLE-US-00003 TABLE 3 poGF-75- poGF-85- poGF-95- poGF-95- G-10 20-10 20-10 20-10 3-10 C atomic % 98.1 89.9 90.5 89.2 91.2 O atomic % 1.9 10.1 9.5 10.8 8.8 O/C atomic 1.94% 11.23% 10.50% 12.11% 9.65% ratio

[0095] As shown in Table 3, as the oxidation reaction temperature increased, the oxygen-to-carbon atomic ratio increased from 1.94% to 12.11% of G-10 by about 5.4 to 6.2 times. Further, when the oxidation reaction time was reduced, the oxygen/carbon atomic ratio decreased from 12.11% to 9.65%. Therefore, it was confirmed that the degree of oxidation can be controlled according to the oxidation reaction conditions, similarly to the elemental analysis results of Experimental Example 6.

Experimental Example 9

[0096] Raman spectra of G-10, poGF-75-20-10, poGF-85-20-10, poGF-95-20-10, and poGF-95-3-10 prepared in the Comparative Examples and Examples were measured, and the results are shown in FIG. 7. The D/G ratios calculated therefrom are shown in Table 4 below.

TABLE-US-00004 TABLE 4 D/G ratio G-10 0.107 poGF-75-20-10 0.155 poGF-85-20-10 0.253 poGF-95-20-10 0.285 poGF-95-3-10 0.262

[0097] In the oxidized graphite produced by the conventionally known Hummer's process, many defects occurred such that the ratio of D/G in the Raman spectra was close to about 1.0, and such graphite oxide caused loses of electrical conductivity. However, as shown in Table 4, it was confirmed that when the partially oxidized graphite was homogenized at high pressure, it exhibited a very small D/G ratio as compared to graphite oxide, and thus the occurrence of defects was low. In addition, similarly to Experimental Example 12 to be described later, the film produced with such graphene causes an electric conductivity to be high.

Experimental Example 10

[0098] Each of poGF-75-20-1, poGF-75-20-3, poGF-75-20-5, poGF-75-20-7, poGF-75-20-10 and G-10 prepared in the Example and Comparative Example was drop-casted on a Si wafer and dried, and SEM images were observed. The results were shown in FIG. 8.

[0099] As shown in FIG. 8, when the partially oxidized graphite was exfoliated by high pressure homogenization, it was confirmed that a superior exfoliation effect was exhibited as compared with G-10 even when high pressure homogenization was applied once.

Experimental Example 11

[0100] The graphene particle size (lateral size) of each of poGF-75-20-1, poGF-75-20-3, poGF-75-20-5, poGF-75-20-7, poGF-75-20-10, GP-1, GP-3, GP-5, GP-7 and GP-10 prepared in the Example and Comparative Example was analyzed, and the results were shown in FIG. 9.

[0101] As shown in FIG. 9, when the high pressure homogenization was applied once to a pristine graphite, the bimodal particle distribution was shown, That is, since the interlayer attraction of pristine graphite was high, the graphene layer was exfoliated while being partially ruptured, and thereby not allowing the occurrence of uniform exfoliation. On the other hand, this phenomenon was not observed when high pressure homogenization was applied once to the partially oxidized graphite. This is because the interlayer attraction of the partially oxidized graphite became small and thus the graphene layer was exfoliated without partial rupture. In addition, when comparing the average particle size (lateral size) of graphene after applying the high pressure homogenization 10 times, it was confirmed that poGF-75-20-10 (5.84 μm) was significantly larger than GP-10 (1.89 μm).

[0102] From the above results, it was confirmed that the large area graphene could be exfoliated when the partially oxidized graphite was applied to high pressure homogenization.

Experimental Example 12

[0103] Each of poGF-75-20-10, poGF-85-20-10, poGF-95-20-10, poGF-95-3-10 and GP-10 prepared in the Example and Comparative Example was diluted such that graphene concentrations was 0.2 mg/mL, and 31.5 mL of the diluent was vacuum filtered through an AAO membrane (200 nm pore, diameter of 4.5 cm) and dried at 55° C. for 2 days. The sheet resistance was measured for AAO membrane using a 4-point probe, and the results were shown in Table 5 below.

TABLE-US-00005 TABLE 5 Rs(Ω/□) S.D. poGF-75-20-10 8.966 0.083 poGF-85-20-10 12.424 0.962 poGF-95-20-10 32.985 2.143 poGF-95-3-10 10.458 0.226 GP-10 34.557 2.305

[0104] Generally, peroxidized product graphene oxide prepared by the Hummer's method exhibits the characteristics of an insulator, and thus an additional thermal or chemical reduction process is required to impart an electrical conductivity. However, graphene exfoliated from the partially oxidized graphite as in the present invention could maintain a considerable part of the electrical conductivity, and as shown in Table 5 above, there was a difference depending on the degree of oxidation, but it exhibited generally low sheet resistance.

[0105] In addition, in the case of GP-10, the use of a dispersant is essential for producing a stable dispersion solution. The dispersant causes a contact resistance between graphenes to increase the sheet resistance of the graphene film. This could be confirmed from GP-10 shown in Table 5 above. On the other hand, graphene exfoliated from the partially oxidized graphite as in the present invention could produce a stable dispersion solution without using a dispersant, and therefore, the problem of contact resistance by the dispersant does not occur, thereby exhibiting low sheet resistance as shown in Table 5 above.

Experimental Example 13

[0106] Each of poGF-85-20-10, poGF-95-20-10 and poGF-95-3-10 prepared in the Example was filtered under vacuum to recover graphene and dried at 55° C. for 2 days. 1.0 g of each dried graphene was added to 3 mL of the solvent shown in Table 6, followed by bath sonication for 1 hour, and the degree of redispersion was observed by a naked eye. The criteria of judgment by a naked eye was determined through the residual amount of graphene not dispersed on the bottom after bath sonication, and the results were shown in FIG. 10 and Table 6 below.

TABLE-US-00006 TABLE 6 H.sub.2O EtOH IPA Acetone THF DMF DMSO NMP Toluene poGF-85-20-10 ◯ Δ X X ◯ ◯ ◯ Δ X poGF-95-20-10 ◯ ◯ Δ ◯ ◯ ◯ ◯ ◯ X poGF-95-3-10 ◯ ◯ ◯ X X ◯ X ◯ X ◯: well-dispersed, Δ: partially-dispersed, X: not-dispersed

[0107] As shown in FIG. 10 and Table 6, it was confirmed that the dispersibility in various polar organic solvents was increased according to the degree of oxidation. In particular, the dispersibility was excellent in polar organic solvents such as water, NMP, DMF and DMSO. In case of poGF-95-20-10, the dispersibility was excellent even in EtOH, IPA, acetone and THF.

[0108] From the above results, it was confirmed that the degree of oxidation can be adjusted by controlling the solvent dispersibility of graphene.