WO2013062306A2 - High purity graphene synthesis melted in water through photocatalytic reaction - Google Patents

Abstract

The present invention relates to a method for manufacturing reduced graphene oxide, and more specifically, to a method for manufacturing high purity graphene oxide by reducing graphene oxide with a photocatalyst. A method according to the present invention is capable of producing the reduced high purity graphene oxide in bulk.

Description

Synthesis of high purity graphene soluble in water through photocatalytic reaction

The present invention relates to a method for synthesizing high purity graphene, and more particularly, to a method for synthesizing high purity graphene dissolved in water by reducing oxidized graphene through a photocatalytic reaction.

Graphene is a two-dimensional planar structure in which carbon is connected to each other in a hexagonal honeycomb shape, with excellent electromagnetic and optical properties, high thermal conductivity (~ 5,000 W / mK) and mechanical strength (~ 200 times stronger than steal). ) And has a large surface area (2,630 m ² / g calculated).

Since 2004, Novoselov et al. ( Science, 2004 , 306, 666-669) have separated graphene in one layer by using the scotch tape method, many research groups have been studying graphene. Chemical Vapor Growth (CVD) is one of the representative methods of synthesizing graphene, and is a method of obtaining very high quality graphene to a desired area. However, the CVD process still requires a high temperature, and the manufacturing cost is still high because the Ni or Cu layer must be scraped off every time graphene is made. Graphene made with this synthesis is limited to materials whose application is mainly to replace indium tin oxide (ITO) or fluorine-doped tin oxide (FTO) electrodes.

If graphene can be effectively reduced in solution, graphene oxide (GO) can be synthesized in large quantities (more than tons), so that graphene can be obtained in cheaper quantities. Of course, when GO is reduced in solution, since graphite is oxidized once and then reduced, the quality is somewhat lower than that of general graphite. Nevertheless, reduced graphene oxide (RGO) can be utilized in many areas such as photocatalysts, fuel cells, cells, supercapacitors, hydrogen storage, etc., unlike CVD methods, RGO can be easily obtained in solution or powder state. However, aggregation of graphenes is the biggest problem in solution synthesis because of van der Waals interactions between graphenes.

Li et al. ( Nat. Nanotech., 2008 , 3, 101-105) solve this problem by inducing negative charge of graphene by reducing GO through hydrazine in base solution and dissolving it in water. The advantage of this method is that graphene can be dissolved in water without the use of any surfactants or stabilizers. The disadvantage of this method, however, is that hydrazine is very unhealthy, explosive and relatively expensive. In addition, there is a limit in increasing conductivity due to nitrogen impurities, and the remaining hydrazine in the graphene solution makes it difficult to handle and process it.

Meanwhile, Williams et al.Langmuir,2009, 25, 13869-13873) is a TiO₂And graphene conjugates were photochemically synthesized through ultraviolet light. Ethanol is used here as a hole scavenger (hole removing material) and GO is TiO₂Reduced in the conduction band of. This way is always TiO₂And a conjugate of graphene.

In Korea, Patent No. 1048490 issued to Sungkyunkwan University has been developed a method for reducing graphene oxide using a HI reducing agent containing a halogen element and a reducing agent containing a weaker acid. However, graphene produced through this method is insoluble in water. Therefore, there is still a need for a method of obtaining a large amount of reduced graphene oxide more easily.

It is an object of the present invention to provide a process for producing high purity reduced graphene oxide in large quantities.

Another object of the present invention is to provide a novel graphene oxide reduction method.

It is another object of the present invention to provide a method for separating reduced graphene oxide and a reduction catalyst.

The present invention provides a method for producing a reduced graphene oxide solution by producing a reduced graphene oxide by irradiating light to a solution containing an anionic photocatalyst and graphene oxide and separating the photocatalyst.

The present invention also provides a method of reducing the graphene oxide by mixing an anionic photocatalyst solution with the graphene oxide solution and irradiating light.

In the present invention, the graphene oxide is an oxide obtained by oxidizing graphene, and various oxidizing groups such as epoxy groups, carboxyl groups, and alcohol groups exist on the surface thereof. Graphene oxide may be prepared by a method of oxidizing graphene, and commercially available for use.

In the present invention, the term "solution" means a liquid contained in a liquid in which a photocatalyst, graphene and / or graphene oxide are dissolved or dispersed.

In the present invention, the photocatalyst may reduce the graphene oxide by absorbing light, and is preferably a catalyst particle having an anion so as to have a repulsive force between the anion-containing graphene oxide and the ions. As the photocatalyst, it is preferable to use a catalyst capable of absorbing infrared rays or visible light to reduce graphene oxide, for example, WO ₃ , Fe ₂ O ₃ , BiVO _4, and the like. In a preferred embodiment of the invention, as the photocatalyst, TiO ₂ particles which form an isoelectric point at pH 6 and have an anion in a pH environment higher than pH 6 are used.

Although not limited in theory, the photocatalyst has the same ions as graphene oxide or reduced graphene oxide, so that the repulsive force acts on each other, and can be reused through separation after the reaction. In the present invention, the reduced graphene oxide may be separated from the catalyst by centrifugation, and may be separated into a higher purity photocatalyst and reduced graphene oxide depending on the speed of centrifugation.

In the practice of the present invention, it is preferable that the solution contains ammonia water to maintain basic pH so that TiO ₂ and graphene oxide can be equally anionized. It is also possible to use known buffer solutions, but there is a fear that the metal ions contained in the buffer solution may aggregate the anions of the graphene to reduce the stability of the graphene solution.

In the present invention, the solution may further include a hole scavenger to help reduce the graphene oxide by the photocatalyst. As the hole scavenger, various known organic compounds such as formic acid, acetic acid, sucrose, salicylic acid, methanol, ethanol and butanol may be used. Although not limited in theory, it is particularly preferable to use methanol which emits electrons having a high reduction voltage to form graphene of high conductivity, and formaldehyde decomposed during the reaction does not produce impurities.

In the present invention, light can be selected and used in a wavelength band in which reduction reaction of graphene oxide by a photocatalyst occurs, and preferably infrared to visible light.

In another aspect, the present invention provides a method for providing a photocatalyst and a graphene oxide reducer by irradiating light to a liquid containing graphene oxide and a photocatalyst in a state in which mutual repulsive force acts.

In yet another aspect, the present invention provides a liquid in which the reduced graphene oxide and the photocatalyst are separated from each other in a state of repulsive separation. Here, "is separable" means that the reduced graphene oxide and the photocatalyst are not combined, so that they can be separated through the separation process in the usual purity in the same manner as precipitation, centrifugation, filtration and extraction. This does not mean that the photocatalyst is completely separated so that it is not included at all.

The present invention provides a new graphene oxide reduction method and high purity reduced graphene oxide. The reduction method according to the present invention is advantageous for commercialization in that the photocatalyst can be reused through a separation process, and the reduced graphene oxide can be dispersed in water, which is advantageous for storage and application.

1 is a schematic of photocatalytic synthesis of water-soluble graphene according to the present invention: a is TiO ₂ state at pH 6 or higher, b is RGO _UV state in basic solution, c is a basic solution of GO and TiO ₂ (pH> 10). ), D is the RGO _UV and TiO ₂ solution after 5 hours of UV irradiation, e is the RGO _UV solution after supercentrifugation, and the red laser shows the path through the RGO _UV solution (10x diluted).

2 is an AFM, and HRTEM images of RGO _UV: a is an AFM image of a _UV RGO, b is an AFM image of the RGO prepared by the method of Williams et al (ACS Nano, 2008 , 2, 1487-1491),. c is the SAED pattern corresponding to the HRTEM image.

3 is the UV-vis spectrum of RGO _UV : a is the RGO _UV UV-vis spectrum diluted 10-fold after supercentrifugation after each photocatalyst reduction time, and b is the color change during photocatalytic reduction.

FIG. 4 is an analysis of RGO _UV : a is a Carbon K-edge NEXAFS analysis of GO, RGO _H2N-NH2 and RGO _UV , b is Raman spectroscopy, c is XPS and d is IR spectroscopy.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail, referring drawings.

The inventors of the TiO₂Using photocatalyst and UV, TiO₂Not a graphene conjugate, but a single layer of graphene was dissolved in high purity water. No stabilizer or surfactant was used. TiO₂ The powder has the properties of acid and base at the same time, and has a negative charge above the isoelectric point, pH = 6 (FIG. 1A). Like RGO_UV(UV reduced graphene oxide) is also negatively charged at the base by the carboxy group (Fig. 1b). Therefore, in the water, the two push each other out and away from each other₂And RGO_UV exist.

1C to 1E schematically show a process of synthesizing graphene dissolved in high purity water. First, supersonication of P25 (commercial TiO ₂ ) -methanol / water solution containing a small amount of GO aqueous solution and a small amount of NH ₄ OH is performed, followed by mixing the two solutions in an optical reactor (FIG. 1C). In the absence of NH ₄ OH, the pH is not maintained as a base, so the aggregation of the negative charges of RGO _UV and TiO ₂ disappears. When the solution is irradiated with a 500 W Hg arc lamp, the color of the solution changes from light brown to dark gray. This means that GO effectively turned to graphene (FIG. 1D).

Centrifugation at 7000 rpm to recover TiO ₂ used in the reaction can recover relatively heavy TiO ₂ . In addition, centrifugation at 20000 rpm yields a high purity RGO _UV solution. In this case more than 90% of the RGO _UV can be recovered (˜0.25 mg / 1 mL) (FIG. 1E).

The concentration of _UV RGO in the solution may be filtered to RGO _UV solution of constant volume and Al by measuring the weight of the filter paper. The highest concentration was above 1 mg / 1 mL. The RGO _UV solution melted stably without aggregation after a month. We can clearly see the way the laser passes by the tyndall effect of the RGO _UV colloid (FIG. 1E). Likewise, since the RGO _UV colloid is negatively charged, a small amount of NaCl is added to settle the particles. The RGO _UV colloid + TiO ₂ and TiO ₂ solutions are similarly negative because they are negatively charged.

When the resulting RGO _UV solution was analyzed by atomic force microscope (AFM), the height of the RGO _UV was ~ 0.64 nm to obtain a general value (Fig. 2a). This is due to the unreduced epoxy and carbonyl groups attached to the RGO _UV , which is greater than 0.334 nm, the theoretical graphene thickness. We found that RGO _UV is present in pure graphene, but there is a very small amount of TiO ₂ . This was less than the measurement range of X-rays. Increasing the speed of centrifugation or removing small particles of TiO ₂ from P25 can remove most of the trace amounts of TiO ₂ . In the XPS analysis, 459.6 eV and 465.4 eV indicating Ti 2p _3/2 and Ti 2p _1/2 were not observed. Similarly, titanium was not detected by SEM-EDAX analysis.

Williams et al. When graphene is synthesized according to the method, almost all graphene is TiO₂It was composited with (P25) (FIG. 2B). TiO₂ Peaks were observed in XRD analysis. Whereas RGO according to the present invention_UV TiO in the XRD pattern of the sample₂ No peak was observed. RGO_UV In the XRD pattern of the sample, the peak of 2θ = ˜24.1 ° (d-spacing ˜3.69 μs) showed a value similar to the (002) plane of graphite, that is, 2θ = 26.7 ° or d-spacing ˜3.34 μs.

After diluting the RGO _UV solution, the resultant was placed on a holy carbon grid, dried in an oven at 80 ° C., and a transmission electron microscope (TEM) photograph was taken (FIG. 2C). Restriction field electron diffraction (SAED) patterns were obtained for two selected areas in the TEM image. Both parts were taken along the [001] zone axis. In the upper part of the first ring 12 strong points are visible, which corresponds to the reflection plane corresponding to the (1100) plane, which shows the hexagonal symmetry (hexagonal symmetry) of the refractive pattern. The twelve bright spots indicate that two relatively well-developed RGO _UV sheets are overlapped at 30 degrees. In the lower part, 24 dots are faintly observed, indicating that four graphenes overlap each other.

To find out the _UV RGO of the conductivity by a _UV RGO solution (~ 0.25 mg / 1 mL) in 0.5 mL and then dropped over a 18 x 18 mm glass drying curve for one hour at 100 ℃ oven made RGO _UV. The thickness of the obtained film was 300 nm. Sheet resistance and conductivity were measured using a four point probe method. As shown in Table 1 below, the conductivity of the RGO _UV film is about the same as the resistance of RGO _H2N-NH2 .

Formation of RGO _UV in water was observed through UV-vis spectroscopy. During UV irradiation, the absorption wavelength of GO is red-shifted at 231 nm and the absorbance becomes larger over time. This shows that the sp ² hybrid carbon network was recovered and formed. After 5 hours, there is no longer any increase in red shift or absorbance, indicating that the reaction is complete. The further reaction does not affect the change in the conductivity of the RGO _UV . All solutions measured here were diluted 10-fold after centrifugation. Since RGO _UV and TiO ₂ are separated from each other due to negative charges, decomposition of RGO _UV does not occur even after a certain time of irradiation.

Carbon K-edge (NEXAFS) analysis uses C 1s core electrons to excite an uncharged or partially filled graphene conduction. When GO is reduced to RGO _UV , the peak of 285.6 eV increases due to the transition to π ^* symmetry state around the M and L points of the graphene Brillouin zone. On the other hand, the peak near 288 eV corresponding to the peak combined with π ^* C = O symmetry is reduced (Fig. 4a). NEXAFS results are in line with Raman spectroscopy. The peak intensity of the G band (˜1600 cm ⁻¹ ) resulting from the E _2g mode of aromatic carbon is weakened, even though 6-fold aromatic rings are recovered because the graphene plates split during the reduction reaction. And the peak of the D band (~ 1354 cm ^-1 ) caused by disordered portions such as ripples, edges, and defects becomes stronger as the reduction occurs. Therefore, the I _D / I _G ratio is a good measure of the extent to which the sp ² hybridization recovers. As shown in Table 1, graphene has similar values when reduced to UV and hydrazine (FIG. 4B).

X-ray photoemission spectroscopy (XPS) and Fourier-transform infrared spectroscopy (FT-IR) spectra also tend to be similar to NEXAFS and Raman results. GO, RGO_H2N-NH2 And RGO_UVIn the XPS analysis, the peak positions of 284.6, 286.5, 287.8, and 289.1 eV are C, respectively._{1 s} Corresponds to C-C, C-O, C = O, and C (O) O of the peak. Among them, RGO, as the size of 286.5 (C-O) eV peaks are rapidly reduced in epoxy group and hydroxyl group,_H2N-NH2 And RGO_UV In both cases it is reduced (Figure 4c). Looking at the spectrum of GO in FT-IR spectroscopy, C-O (v_co. at 1064 cm^-One), C-O-C (v_coc. at 1250 cm^-One), C-OH (v_coc. at 1365 cm^-One), C = O (v_{c = o}. at 1728 cm^-One) And skeletal vibrations of unoxidized graphite domain (1616 cm)^-One) Exists. RGO_H2N-NH2 And RGO_UV In both cases it can be seen that all of these peaks are drastically reduced. This means GO is RGO_UVThis means that the graphic structure has been restored.

All NEXAFS, Raman, XPS and FT-IR results showed that GO was effectively converted to graphene by photocatalysts, and RGO _UV had properties similar to RGO _H2N-NH2 reduced from hydrazine.

Example 1

45 mg of graphene oxide prepared according to Hummers et al. ( J. Am. Chem. Soc. 1958 , 80, 1339) method was added to 45 mL of water using a high intensity ultrasonic processor (Autotune Series 750 Watt model). After 30 minutes of super sonication, MF 500 (Hanil) was centrifuged at 3000 rpm for 30 minutes. Independently, 500 mg of P25 (Agonics; TiO ₂ ) was added to a mixture of methanol and water and 250 µL of ammonia solution (~ 30%), followed by supersonication for 30 minutes. The two solutions were mixed in a photoreactor and carried out in an ice bath. After 30 minutes of N ₂ purging, photoreaction was performed using a 500-W Hg arc lamp (Oriel 61945) as a light source. The light source was irradiated to the photoreactor after passing through the IR filter. After more than 5 hours of reaction P25 was separated by centrifugation at 7000 rpm for 30 minutes using a super centrifuge (Mega 21R), and the remaining graphene solution was centrifuged at 20000 rpm to increase the purity of the RGO _UV solution. The resistance, conductivity, and Raman parameters of the prepared RGO _UV were measured and described in Table 1.

Example 2

Except for using ethanol instead of methanol in Example 1 it was carried out in the same manner. During the reaction, a small amount of acetaldehyde was formed. Conductivity was measured to a similar degree when using methanol.

Comparative Example 1

Graphene was prepared according to Williams et al ( ACS Nano , 2008 , 2, 1487-1491) and the conductivity of the RGO _hydrazine , Raman parameters were measured and listed in Table 1.

TABLE 1 Properties of graphene oxide (GO) and reduced graphene oxide (RGO)

Claims (14)

1. A method of preparing a high purity reduced graphene oxide solution by irradiating a solution containing an anionic photocatalyst and graphene oxide with light to reduce graphene oxide and separating the photocatalyst.
2. The method of claim 1 wherein the photocatalyst is TiO ₂ .
3. The method of claim 1 or 2, wherein the photocatalyst is separated using a centrifuge.
4. The method of any one of claims 1 to 3, wherein the solution has a pH of at least 7.
5. The method of any one of claims 1 to 4, wherein the solution comprises methanol.
6. The method of any one of claims 1 to 5, wherein the solution comprises ammonia water.
7. A method of providing a graphene oxide reducer by irradiating light to a liquid contained in the state in which the graphene oxide and the photocatalyst react with each other.
8. 8. The method of claim 7, wherein the repulsive force is inter-ion repulsive force.
9. The method of claim 7 or 8, wherein the solution is basic so that the photocatalyst, the graphene oxide and the reduced graphene oxide can carry anions.
10. 10. The method of any of claims 7-9, wherein the photocatalyst is TiO ₂ and the liquid comprises ammonia water and methanol.
11. A method of reducing graphene oxide by mixing an anion photocatalyst solution with a graphene oxide solution and irradiating with light.
12. The method of claim 11, wherein the anionic photocatalyst solution is prepared by dispersing TiO ₂ in a basic solution.
13. The method of claim 12, wherein the TiO ₂ is dispersed by ultrasonic waves.
14. 14. A method according to any one of claims 11 to 13, wherein said light is infrared or visible light.

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