WO2024071514A1 - Reduced graphene oxide with improved antibacterial properties and production method therefor - Google Patents

Abstract

A method for producing reduced graphene oxide of an embodiment includes: performing hydrothermal synthesis by additionally adding a maximum of 300 urea and a maximum of 7 hydrazine (N2H4) based on 1 graphene oxide (GO) in terms of an amount of the urea to the hydrazine (N2H4) added; and washing the synthesized nitrogen-doped reduced graphene oxide (N-rGO).

Description

REDUCED GRAPHENE OXIDE WITH IMPROVED ANTIBACTERIAL PROPERTIES AND PRODUCTION METHOD THEREFOR

The present disclosure relates to reduced graphene oxide (rGO) having excellent antibacterial properties and stability to heat and ultraviolet rays.

Graphene is a material with a two-dimensional structure composed of carbon atoms. Graphene has high thermal conductivity (5,300W/m.K) and low electrical resistance (10^(-8)Ω.m). Graphene, which has only a single-layer structure, has a transmittance of 90% or more, and thus may be used in various places such as transparent touch panels, solar cells, thermoelectric materials, and electrode materials.

Graphene oxide powder produced through chemical treatment of graphite contains various chemical functional groups on the surface, so reactive oxygen species generated from these functional groups have been reported to have excellent resistance (or antibacterial properties) to microorganisms.

However, it is very vulnerable to external stimuli (UV, heat), and is easily reduced to generate reduced graphene oxide. Reduced graphene oxide has few chemical functional groups, so it has excellent resistance to external stimuli (heat, UV), but has low antibacterial activity.

An embodiment of the present disclosure intends to secure antibacterial strength (or antibacterial activity) through the modification of graphene, which is a biocompatible material composed only of carbon.

In addition, an embodiment of the present disclosure is intended to provide a plastic product having antibacterial properties by producing a plastic composite using reduced graphene oxide having a stable structure and increased antibacterial properties.

A method for producing reduced graphene oxide of an embodiment includes: performing hydrothermal synthesis by additionally adding a maximum of 300 urea and a maximum of 7 hydrazine (N2H4) based on 1 graphene oxide (GO) in terms of an amount of the urea to the hydrazine (N2H4) added; and washing the synthesized nitrogen-doped reduced graphene oxide (N-rGO).

The hydrothermal synthesis is carried out at 140°C for 12 hours, and the urea is preferably in a solid phase.

The washing includes a process of adding the nitrogen-doped reduced graphene oxide (N-rGO) to metallic ionized water (or antibacterial additive) with antibacterial strength in a mechanical mixer for stirring.

The washing includes a process of washing the synthesized nitrogen-doped reduced graphene oxide (N-rGO) into a washing solution to which an metallic antibacterial additive is added.

The metallic antibacterial additive is at least one of Ag, Cu, and Zn, preferably Ag.

The washing includes a process of putting distilled water containing 50-200 ppm of Ag and the synthesized nitrogen-doped reduced graphene oxide (N-rGO) into a mechanical mixer rotating at a speed of 500 rpm and stirring the same within 30-60 minutes.

In another embodiment, the reduced graphene oxide includes nitrogen.

Carbon, oxygen, and the nitrogen, which are main components, are included in contents of carbon: 70 to 80 at%, oxygen: 10 to 20 at%, and nitrogen: 0 to 10 at%.

The nitrogen is included in a content of 6 at% to 10 at%, and a particle size of the powder is 100 nm to 30 um.

Nitrogen-doped oxidized graphene of an embodiment is doped with nitrogen in rGO, and thus has high antibacterial properties even though an amount of functional groups containing oxygen is extremely low.

In addition, the nitrogen-doped reduced graphene oxide of an embodiment is thermally stable because there is almost no functional group containing oxygen.

The accompanying drawings, which are included to provide a further understanding of the present disclosure and are incorporated into and constitute a part of this specification, illustrate embodiments of the present disclosure and together with the description serve to explain the principles of the present disclosure.

FIG. 1 shows the thermal stability of each type of graphene oxide.

FIG. 2 shows the results of evaluating the antibacterial and antifungal strength of each type of graphene oxide.

FIG. 3 shows the change trend of antibacterial strength according to the nitrogen content.

FIG. 4 shows the UV stability of graphene oxide.

FIG. 5 shows the crystal structure of each type of graphene oxide.

FIG. 6 shows the composition of each type of graphene oxide.

FIG. 7 shows the surface shape of each type of graphene oxide.

FIG. 8 shows a production method of one embodiment.

FIG. 9 is a schematic diagram of a production method.

Hereinafter, various aspects and various embodiments of the present disclosure will be described in more detail.

In general, graphene oxide (GO) includes various chemical functional groups, and in particular, these oxygen functional groups have excellent resistance to microorganisms (hereinafter, antimicrobial properties). However, since graphene oxide is very vulnerable to external stimuli (ultraviolet rays, heat, etc.), it easily loses an oxygen functional group (or is reduced) and is converted into reduced graphene oxide.

Reduced graphene oxide (rGO) has few chemical functional groups containing oxygen, so it has excellent resistance to external stimuli (heat, UV), but has low antibacterial activity.

Accordingly, an embodiment of the present disclosure intends to develop a new antibacterial graphene material having excellent microbial resistance of graphene oxide and stability (heat, UV) of reduced graphene oxide, and the new antibacterial graphene material has high stability from external stimuli (heat or ultraviolet rays, UV) despite its excellent antibacterial properties.

To this end, an embodiment of the present disclosure includes: performing hydrothermal synthesis by additionally adding a maximum of 300 urea and a maximum of 7 hydrazine (N2H4) based on 1 graphene oxide (GO) in terms of an amount of urea to hydrazine (N2H4) added; and washing the synthesized nitrogen-doped reduced graphene oxide (N-rGO).

In addition, in an embodiment of the present disclosure, the doping concentration of nitrogen in the reduced graphene oxide is 6 at% to 10 at%.

In this regard, the description will be made with reference to the drawings.

FIG. 1 shows the thermal stability of each type of graphene oxide.

As shown in FIG. 1, graphene oxide has a progressive decrease in thermal stability (weight loss) starting at about 100°C, and radically lowers thermal stability at about 250°C as an inflection point. This is because the oxygen functional groups (C-OH, C-O-C, -COOH, etc.) occupying about 30-50 wt% of graphene oxide (GO) cause a rapid thermal decomposition reaction as the temperature rises.

In contrast, it may be seen that reduced graphene oxide (rGO) and nitrogen-doped reduced graphene oxide (N-rGO) have very few functional groups containing oxygen compared to GO, and thus have excellent high-temperature stability. In other words, even at a high temperature of 300°C, GO has a weight loss of 50% compared to its initial weight, but rGO and N-rGO may reduce the weight loss within 5%.

As such, it may be seen that the nitrogen-doped reduced graphene oxide of an embodiment has the same high thermal stability as rGO.

FIG. 2 shows the results of evaluating the antibacterial and antifungal strength of each type of graphene oxide.

Referring to FIG. 2, graphene oxide (GO) was evaluated to have excellent antibacterial and antifungal strength, but have low heat resistance. On the other hand, it was evaluated that reduced graphene oxide (rGO) exhibited a tendency to increase antibacterial/antifungal strength as the nitrogen (N) content increases.

Herein, the reduced graphene oxide (rGO) did not show a significant difference in heat resistance even when the content of nitrogen (N) was increased. As shown in FIG. 3, the antibacterial strength increases proportionally as the nitrogen content increases. However, it was evaluated that the antibacterial strength did not show a significant difference even when the nitrogen content increased beyond 10 at%.

As such, it may be seen that by doping the reduced graphene oxide with nitrogen (N), it is possible to simultaneously secure thermal stability and antibacterial/antifungal strength. In addition, it was evaluated that it was meaningless to dope the reduced graphene oxide with a nitrogen (N) content of 10 at% or more.

In this evaluation, antibacterial strength was made based on JIS Z 2801 (evaluation time: 24 hours), and antifungal strength was made based on ASTM G 21 (evaluation time: 4 weeks).

In addition, the antibacterial strength was evaluated based on the following criteria.

- Evaluation of antibacterial strength for non-porous specimens (plastic, film, iron plate, tile, ceramic, etc.) (sample size: 50 X 50 mm, 8 mm or less in height)

- Evaluation strain: Staphylococcus aureus, Escherichia coli

In addition, the antifungal strength was evaluated based on the following criteria.

- Evaluation of antifungal strength for non-porous specimens (plastic, film, iron plate, tile, ceramic, etc.)

- Evaluation strain: a spore solution mixed with 5 types of fungi of Aspergillus brasiliensis, Penicillium funiculosum, Chaetomium globosum, Trichoderma virens, Aureobasidium pullulans

- Evaluation criteria: 0 - The growth of the strain is not recognized in the inoculation part of the test piece, 1-4 - The growth area of the mycelium where the growth of the strain is recognized in the inoculation part of the test piece (1: less than 10%, 2: 10-30%, 3: 30-60%, 4: 60% or more)

FIG. 4 shows the UV stability of graphene oxide.

In FIG. 4, XPS analysis was performed for chemical bonding structure analysis of samples treated with 254 nm of UV and 70% of ethanol for sterilization treatment of graphene oxide.

As a result, the sample exposed to 70% of ethanol had no difference in chemical bonding structure with the sample before UV exposure.

However, as a result of exposure to 254 nm of UV for 2 hours, the C-O-C bonding structure was reduced (reduction of graphene oxide proceeded). Thus, it may be seen that graphene oxide was partially reduced only by UV exposure (in other words, UV stability was low).

FIG. 5 shows the crystal structure of each type of graphene oxide.

In FIG. 5, graphene oxide (GO) was evaluated to have high crystallinity by being produced through chemical reduction treatment of graphite with high crystallinity.

In the reduced graphene oxide (rGO) reduced graphene oxide (GO), a peak with a wide width in the (002) direction was observed near 22º. When nitrogen was doped into the reduced graphene oxide, the XRD peak in the (002) direction near 22 º was shifted to a high angle, and a peak in the (002) direction was observed near 26º.

In other words, it was evaluated that the crystal structure of rGO was maintained even when nitrogen (N) was doped into the reduced graphene oxide (rGO).

FIG. 6 shows the composition of each type of graphene oxide. The evaluation of FIG. 6 was performed by X-ray Photoelectron Spectroscopy (XPS).

Graphene oxide (GO) produced through the chemical treatment of graphite had the highest oxygen content of 33.5 at% because of its many functional groups containing oxygen.

In addition, it may be seen that in reduced graphene oxide (rGO) produced by thermally reducing graphene oxide (GO), the oxygen functional group is reduced by heat, so that the oxygen content is significantly reduced compared to GO to 12.4 at%.

In addition, as a result of reacting the reduced graphene oxide (rGO) with a nitrogen source (ammonia water or urea), it was found that nitrogen (N) was doped up to 4.5 to 9.5 at% depending on the synthesis conditions.

FIG. 7 shows the surface shape of each type of graphene oxide. The evaluation of FIG. 7 was performed through a scanning electron microscope (SEM).

As a result of analyzing samples by type of dried graphene for shape analysis of graphene synthesized in liquid phase, there was no difference in shape among graphene oxide (GO), reduced graphene oxide (rGO), and nitrogen-doped reduced graphene oxide (N-rGO).

In other words, even when N-rGO is synthesized, it means that there is no difference in shape from GO and rGO.

Hereinafter, a production method for synthesizing N-rGO will be described with reference to the accompanying drawings.

FIG. 8 shows a production method of one embodiment, and FIG. 9 is a schematic diagram of a production method.

Referring to FIGS. 8 and 9, a production method of one embodiment includes: performing hydrothermal synthesis by additionally adding, to a hydrothermal synthesizer, a maximum of 300 urea and a maximum of 7 hydrazine (N2H4) based on 1 graphene oxide (GO) in terms of an amount of urea to hydrazine (N2H4) added (S10); and washing the synthesized nitrogen-doped reduced graphene oxide (N-rGO) (S20).

Graphene oxide (GO) may be made through various well-known methods, for example, a production method such as Tour's method, Hummers' method, and Brodie's method, after grinding graphite (graphite) into a powder form.

After preparing graphene oxide of a powder state as such, hydrazine (N2H2) used as a reducing agent in a container is added up to 7 based on 1 graphene oxide (GO), and urea is added up to 300 based on 1 graphene oxide (GO).

Urea acts as a source for nitrogen doping. Since solid urea is used as a nitrogen source, it has the benefit of not emitting toxic gas compared to ammonia water. In addition, since it is possible to always supply solid urea in a certain amount regardless of water quality, it is possible to dope nitrogen (N) close to the maximum value by controlling the content of urea. For reference, as described above, as the doping amount of nitrogen increases, the antibacterial strength of the reduced graphene oxide also tends to increase.

For nitrogen (N) doping, the reaction time and the synthesis temperature have a correlation. When the reaction time is shortened at the same synthesis temperature, the nitrogen (N) content is reduced. When the synthesis temperature is lowered at the same reaction time, the nitrogen (N) doping content tends to decrease.

As nitrogen (N) sources, in addition to urea, solid chemical substances containing nitrogen such as melamine, ammonium nitrate, 5-aminotetrazole monohydrate, thiourea, sodium diethyldithiocarbamate, hexamethelenetetramine, 4-nitroaniline, 4-aminophenol, 4-nitro-o-phenylenediamine, trithiocyanuric acid, ammonium formate, and gycine may also be used.

The prepared material is put into a hydrothermal synthesizer and synthesized at 140°C for 12 hours.

Since an embodiment of the present disclosure is configured to perform hydrothermal synthesis using solid urea as a nitrogen source, it is possible to produce nitrogen-doped reduced graphene oxide at a high concentration for a relatively short synthesis time of 12 hours.

It is preferable that the carbon, oxygen, and nitrogen, which are the main components of the synthesized nitrogen (N)-doped reduced graphene oxide (N-rGO), are included in contents of carbon: 70 to 80 at%, oxygen: 10 to 20 at%, and nitrogen: 0 to 10 at%.

As the nitrogen (N) content increases, the antibacterial strength tends to increase. As reviewed above, the antibacterial strength is maximum at 10 at% of the nitrogen content.

Such that graphene oxide (GO) has high antibacterial strength due to many functional groups containing oxygen (O), also in the nitrogen-doped reduced graphene oxide (N-rGO), as the functional group containing nitrogen (N) increases, the antibacterial strength increases.

The nitrogen content in the nitrogen-doped reduced graphene oxide (N-rGO) is preferably 0.1 at% to 30 at%, more preferably 3 at% to 15 at%, and most preferably 6 at% to 10 at%.

When the nitrogen content is 30 at% or more, the crystal structure may be changed to graphitic carbon nitride (g-C3N4).

In addition, the particle diameter of the nitrogen-doped reduced graphene powder (N) preferably has a wide distribution from 100 nm to 100 um, more preferably a distribution of 100 nm to 50 um, and most preferably a distribution of 100 nm to 30 um.

When a particle size of graphene oxide is too small, graphene particles are easily agglomerated due to an increase in surface energy. When the particle size is too large, the surface area decreases and the antibacterial active site decreases, thereby reducing the antimicrobial strength.

In another preferred form, it is also possible to increase the antibacterial strength by adding a metallic antibacterial additive (Ag, Cu, Zn, etc.) to the nitrogen-doped reduced graphene oxide (N-rGO) (N).

[Table 1] below is a table showing the change in antibacterial strength when Ag is added.

Section		Heat resistance	Ag addition	Antibacterial strength(%), JIS Z 2801
				E. coli	Staphylococcus aureus
N content doped in rGO	0at%(rGO)	O	X	75.0%	70.0%
		O	O	over 99.9%	over 99.9%
	10at%(N-rGO)	O	X	over 99.9%	over 99.9%
		O	O	over 99.9%	over 99.9%

As can be seen from Table 1, it may be seen that when a small amount of Ag is added to reduced graphene oxide (rGO) with low antibacterial strength, the E. coli antibacterial strength is improved from 75% to 99.9%.

The metallic antibacterial additive (Ag, Cu, Zn, etc.) may be supplied in the washing (S20).

In more detail, the washing (S20) is a process generally performed to remove impurities remaining in the synthesized reduced graphene oxide. In this embodiment, the metallic antibacterial additive is doped by washing the synthesized reduced graphene oxide using distilled water containing a high concentration of the metallic antimicrobial additive as a washing solution in a washing process.

For example, in the case of adding Ag, Ag may be additionally added by putting distilled water containing 50-200 ppm of Ag made through electrolysis and the synthesized nitrogen-doped reduced graphene oxide (N-rGO) into a low-speed mixer rotating at a speed of 500 rpm and stirring the same within 30 minutes.

The features, structures, and effects described in the above embodiments are included in at least one embodiment of the present disclosure and are not necessarily limited to one embodiment. Furthermore, features, structures, and effects shown in each embodiment may be combined or modified in other embodiments by those skilled in the art. Therefore, it should be interpreted that contents relating to such combination and modification are included in the range of the present disclosure.

Claims (13)

1. A method for producing reduced graphene oxide, the method including:

   performing hydrothermal synthesis by additionally adding a maximum of 300 urea and a maximum of 7 hydrazine (N2H4) based on 1 graphene oxide (GO) in terms of an amount of the urea to the hydrazine (N2H4) added; and

   washing the synthesized nitrogen-doped reduced graphene oxide (N-rGO).
2. The method of claim 1, wherein the hydrothermal synthesis is carried out at 140°C for 12 hours.
3. The method of claim 1, wherein the urea is in a solid phase.
4. The method of claim 1, wherein the washing includes a process of adding the nitrogen-doped reduced graphene oxide (N-rGO) to Ag ionized water in a mechanical mixer for stirring.
5. The method of claim 1, wherein the washing includes a process of washing the synthesized nitrogen-doped reduced graphene oxide (N-rGO) into a washing solution to which an metallic antibacterial additive is added.
6. The method of claim 5, wherein the metallic antibacterial additive is at least one of Ag, Cu, and Zn.
7. The method of claim 6, wherein:

   the metallic antibacterial additive is Ag; and

   the washing includes a process of putting distilled water containing 50-200 ppm of Ag and the synthesized nitrogen-doped reduced graphene oxide (N-rGO) into a low-speed mixer rotating at a speed of 500 rpm and stirring the same within 30 minutes.
8. A reduced graphene oxide, wherein

   the reduced graphene oxide includes nitrogen; and

   carbon, oxygen, and the nitrogen, which are main components, are included in contents of carbon: 70 to 80 at%, oxygen: 10 to 20 at%, and nitrogen: 0 to 10 at%.
9. The reduced graphene oxide of claim 8, wherein the nitrogen is included in a content of 6 at% to 10 at%.
10. The reduced graphene oxide of claim 8, wherein a particle size of nitrogen-doped reduced graphene oxide is 100 nm to 30 um.
11. The reduced graphene oxide of claim 8, wherein the nitrogen-doped reduced graphene oxide further includes a metallic antibacterial additive.
12. The reduced graphene oxide of claim 11, wherein the metallic antibacterial additive is at least one of Ag, Cu, and Zn.
13. The reduced graphene oxide of claim 12, wherein the metallic antibacterial additive is Ag.

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