KR102338793B1 - Multiple elements co-doped graphene quantum dot and manufacturing method thereof - Google Patents

Abstract

A multi-element co-doped graphene quantum dot according to one embodiment of the present invention comprises: a graphene quantum dot; and two or more elements doped onto the graphene quantum dot. According to embodiments of the present invention, the graphene quantum dot co-doped with heterogeneous elements exhibits excellent catalytic activity and supercapacitor properties, so that it can be utilized as an electrode catalyst for water electrolysis or a supercapacitor electrode.

Description

Graphene quantum dots co-doped with multiple elements and manufacturing method thereof

The present invention relates to graphene quantum dots, and more particularly, to graphene quantum dots co-doped with various elements and a method for manufacturing the same.

Graphene is composed of sp ² hybrid carbon structure, so it has excellent physical properties such as electrical, thermal and mechanical properties. However, graphene, as a zero bandgap semiconductor, can be converted into graphene quantum dots to give optical properties for use in various industrial fields.

Graphene quantum dots (GQDs) are graphene fragments having a crystal structure of several to tens of nanometers and exhibit different electrical and optical properties from sheet-type graphene. By varying the size and shape of graphene quantum dots, Its bandgap can be adjusted.

These graphene quantum dots can be used in electrochemical energy applications such as batteries and batteries to replace fossil fuels in modern society, where interest in green energy is increasing due to air pollution issues.

In the conventional field of electrochemical energy, noble metals such as Pd, Pt, Au and Ru have been used as electrodes to increase efficiency for fuel cells, water electrolysis and batteries.

However, since these metals are expensive, development of non-metal electrode materials that can replace them are being made, and graphene quantum dots can be used as a substitute for them.

That is, graphene quantum dots are becoming a major substitute for noble metals due to their tunable optical and electronic properties from quantum confinement effects, and research to increase their activity is continuously needed to act as more efficient catalysts.

Republic of Korea Patent Publication No. 10-2014-0065275

An object of the present invention is to provide a graphene quantum dot co-doped with multiple elements and a method for manufacturing the same.

Another object of the present invention is to provide an electrocatalyst for water electrolysis including graphene quantum dots co-doped with multiple elements.

Another object is to provide a graphene quantum dot supercapacitor electrode co-doped with multiple elements.

The technical problems to be achieved by the present invention are not limited to the technical problems mentioned above, and other technical problems not mentioned can be clearly understood by those of ordinary skill in the art to which the present invention belongs from the description below. There will be.

In order to achieve the above technical object, an embodiment of the present invention provides a graphene quantum dot co-doped with multiple elements.

In an embodiment of the present invention, a multi-element co-doped graphene quantum dot includes: a graphene quantum dot; It includes two or more elements doped into the graphene quantum dots.

In an embodiment of the present invention, the two or more elements may be two or more elements selected from the group consisting of nitrogen (N), boron (B), oxygen (O), and fluorine (F).

In an embodiment of the present invention, the content of the two or more elements may be each independently 0.5at% to 30at% relative to the total atomic percent.

In an embodiment of the present invention, the average diameter of the multi-element co-doped graphene quantum dots may be 1 nm to 30 nm.

In an embodiment of the present invention, the spacing between each layer of the multi-element co-doped graphene quantum dots may be 5 nm or less.

In order to achieve the above technical object, another embodiment of the present invention provides an electrocatalyst for water electrolysis including multi-element co-doped graphene quantum dots.

In order to achieve the above technical object, another embodiment of the present invention provides a supercapacitor electrode including multi-element co-doped graphene quantum dots.

In order to achieve the above technical object, another embodiment of the present invention provides a method for manufacturing multi-element co-doped graphene quantum dots.

In an embodiment of the present invention, nitrogen and fluorine are co-doped graphene quantum dots manufacturing method, comprising the steps of preparing graphene quantum dots; doping the graphene quantum dots with a first element; and doping a second element to the graphene quantum dots doped with the first element. includes

In an embodiment of the present invention, the doping of the first element may be performed by chemical vapor deposition (CVD) using a gas containing the first element.

In an embodiment of the present invention, the doping of the second element may be performed by plasma treatment in a gas atmosphere containing the second element.

In an embodiment of the present invention, the content of the first element doped into the graphene quantum dots may be 0.5at% to 30at% relative to the total atomic percent.

In an embodiment of the present invention, the content of the second element doped in the graphene quantum dots may be 0.5at% to 30at% relative to the total atomic percent.

According to an embodiment of the present invention, when graphene quantum dots co-doped with multiple elements are used as electrode catalysts, hydrogen evolution reaction (HER) and oxygen evolution reaction compared to conventional graphene quantum dots , OER) shows a low overvoltage, indicating improved catalytic activity.

In addition, when the graphene quantum dots co-doped with the multiple elements are used as a capacitor electrode, the positive induced by higher electronegativity helps activate the electrochemical reaction due to the co-doping effect of the multiple elements. It may exhibit excellent supercapacitor characteristics in the potential range.

In addition, the graphene quantum dots co-doped with the multiple elements can form a passivation film on the electrode in an aqueous system to prevent corrosion from aqueous solution, thereby suppressing deterioration of electrode performance and enabling long-term operation.

It should be understood that the effects of the present invention are not limited to the above-described effects, and include all effects that can be inferred from the configuration of the invention described in the detailed description or claims of the present invention.

1 is a schematic diagram of multi-element co-doped graphene quantum dots according to an embodiment of the present invention.
2 is a flowchart of a method for manufacturing multi-element co-doped graphene quantum dots, according to an embodiment of the present invention.
3 is a schematic diagram of a method for manufacturing multi-element co-doped graphene quantum dots, according to an embodiment of the present invention.
4 is an atomic force microscope (AFM) image and a height graph of N,F-GQDs according to an embodiment of the present invention.
5 is a TEM image and size graph of N,F-GQDs according to an embodiment of the present invention.
6 is an FT-IR spectrum and XPS graph of GQDs, N-GQDs and N,F-GQDs according to an embodiment of the present invention.
7 is a current density curve of GQDs, N-GQDs and N,F-GQDs according to an embodiment of the present invention.
8 is a polarization curve and a CA curve according to an embodiment of the present invention.
9 is a CV curve and a GCD curve according to an embodiment of the present invention.

Hereinafter, the present invention will be described with reference to the accompanying drawings. However, the present invention may be embodied in several different forms, and thus is not limited to the embodiments described herein. And in order to clearly explain the present invention in the drawings, parts irrelevant to the description are omitted, and similar reference numerals are attached to similar parts throughout the specification.

Throughout the specification, when a part is said to be “connected (connected, contacted, coupled)” with another part, it is not only “directly connected” but also “indirectly connected” with another member interposed therebetween. "Including cases where In addition, when a part "includes" a certain component, this means that other components may be further provided without excluding other components unless otherwise stated.

The terminology used herein is used only to describe specific embodiments, and is not intended to limit the present invention. The singular expression includes the plural expression unless the context clearly dictates otherwise. In the present specification, terms such as “comprise” or “have” are intended to designate that a feature, number, step, operation, component, part, or combination thereof described in the specification exists, but one or more other features It should be understood that this does not preclude the existence or addition of numbers, steps, operations, components, parts, or combinations thereof.

A multi-element co-doped graphene quantum dot according to an embodiment of the present invention will be described.

1 is a schematic diagram of a multi-element co-doped graphene quantum dot 1 according to an embodiment of the present invention.

Referring to FIG. 1 , a multi-element co-doped graphene quantum dot 1 according to an embodiment of the present invention includes: a graphene quantum dot; and two or more elements doped into the graphene quantum dots.

First, the graphene quantum dots are materials made in the form of dots with a size of about several to several tens of nanometers in order to make graphene, which is a conductor material, into a semiconductor form. When the particle is several tens of nanometers or less, electrons are trapped by the space wall, so that the conductor material specifically has semiconductor properties. In addition, graphene quantum dots have attracted considerable attention due to their tunable optical and electronic properties from quantum confinement effects.

As a water electrolysis (water decomposition) catalyst, noble metals such as platinum or iridium have been mainly used in the prior art, but noble metal catalysts are expensive and have poor stability, so more stable and economical graphene quantum dots can be used as water electrolysis catalysts.

The graphene quantum dots may be oxidized. Oxygen-rich functional groups at the edges of graphene quantum dots have unique properties such as luminescence upon excitation, a tunable bandgap, water solubility, electrochemical properties, and biocompatibility. affects By doping the oxidized graphene quantum dots with a heterogeneous material, its catalytic activity may be improved.

The two or more elements doped into the graphene quantum dots may be two or more elements selected from the group consisting of nitrogen (N), boron (B), oxygen (O), and fluorine (F).

For example, N and B, B and O, B and F or N, B and F may be co-doped into graphene quantum dots.

The two or more elements may be doped by being substituted or added to the graphene quantum dots.

For example, the multi-element co-doped graphene quantum dots 1 may be graphene quantum dots (N,F-GQDs) co-doped with nitrogen and fluorine, which include: graphene quantum dots; nitrogen (N) atoms 10 doped into the graphene quantum dots; and a fluorine (F) atom 20 bonded to some carbons in the graphene quantum dots; including (Fig. 1).

The content of the two or more elements may each independently be 0.5at% to 30at% relative to the total atomic percent.

If the content of the two or more elements is less than 0.5 at% of the total atomic percent, the effect of element doping may not be obtained and may not be obtained, and if it is more than 30 at%, the synergistic effect due to joint doping may be insignificant.

For example, the two or more elements may be nitrogen and fluorine. Again, for example, the content of the nitrogen atoms 10 may be 11 at% relative to the total atomic percent, and the content of the fluorine atoms 20 may be 9.6 at% relative to the total atomic percent.

The average diameter of the multi-element co-doped graphene quantum dots 1 of the present invention may be 1 nm to 30 nm, for example, 8.7 nm.

The spacing between each layer of the multi-element co-doped graphene quantum dots 1 of the present invention may be 5 nm or less, for example, 1.5 nm.

The multi-element co-doped graphene quantum dots 1 of the present invention may be configured by stacking three graphene layers. This can be confirmed with a three-dimensional AFM image, and it can be confirmed that the co-doped graphene quantum dot is composed of approximately three layers by estimating the height. When graphene quantum dots are formed through chemical exfoliation, the average number of layers may vary depending on the processing time.

By doping the graphene quantum dots with two or more elements, more catalytically active carbon parts are provided near the dopant, thereby providing excellent catalytic properties.

The multi-element co-doped graphene quantum dot (1) of the present invention is a catalyst for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) due to the synergistic effect of hetero doping between heterogeneous elements. can be used

For example, when nitrogen and fluorine co-doped graphene quantum dots are used as electrode catalysts, due to the synergistic effect of N, F hetero doping, hydrogen evolution reaction (HER) compared to conventional graphene quantum dots For 0.13V and oxygen evolution reaction (OER), a low overvoltage of 0.4V at a current density of ^{10mA cm -2 may be exhibited, thereby exhibiting improved catalytic activity.}

In addition, the multi-element co-doped graphene quantum dots (1) of the present invention can be utilized as a supercapacitor electrode.

A super capacitor is an electric double layer capacitor (EDLC) that stores energy by adsorption and desorption of reversible charges by electrostatic attraction in the electric double layer near the electrode/electrolyte interface and at the electrode/electrolyte interface. It is divided into an inductive capacitor (psedudo capacitor) that stores energy by a reversible faradaic oxidation/reduction reaction.

In general, carbon-based materials have a fast and long cycle life due to the physical adsorption and desorption of electrolyte ions without chemical reaction. To take advantage of these advantages, multi-element co-doped graphene quantum dots can be applied as supercapacitor electrodes.

The multi-type element may be two or more kinds of elements.

The two or more elements may be two or more elements selected from the group consisting of nitrogen (N), boron (B), oxygen (O), and fluorine (F).

The content of the two or more elements may each independently be 0.5at% to 30at% relative to the total atomic percent.

For example, when nitrogen and fluorine are co-doped graphene quantum dots (N,F-GQDs), the content of nitrogen atoms 10 may be 0.5at% to 30at% relative to the total atomic percent, preferably , may be 1 at% to 15 at%. In addition, the content of the fluorine atom 20 may be 0.5at% to 30at%, preferably, 1at% to 15at%, based on the total atomic percent.

The multi-element co-doped graphene quantum dot (1) supercapacitor electrode according to an embodiment of the present invention may have similar capacitive properties.

In general, the capacitance increases as the measured current density increases. The Faraday-like capacitance occurs simultaneously with the formation of the electrochemical double layer and depends on the chemical affinity of the electrode material for the structure of the electrolyte ions and multi-element co-doped graphene quantum dots. .

For example, the N,F-GQDs electrode may have a capacitance of 245.15 F/g at a scan rate ^{of 10 mV s −1 .}

The supercapacitor electrode including the multi-element co-doped graphene quantum dots 1 may be applied to a supercapacitor.

The supercapacitor may include a pair of electrodes disposed to face each other; an electrolyte provided between the pair of electrodes; and a separator provided on the pair of electrodes to suppress an electrical short circuit. may include

The pair of electrodes disposed opposite to each other may include multi-element co-doped graphene quantum dots 1 .

The electrolyte may be one selected from the group consisting of an acid-based electrolyte containing sulfuric acid, an alkali-based electrolyte containing potassium hydroxide, and a neutral electrolyte containing sodium sulfate, but is not limited thereto. For example, the electrolyte may be 1M KOH.

The separator may be a nonwoven fabric, polytetrafluoroethylene (PTFE), porous film, kraft paper, cellulosic electrolyte, rayon fiber, or the like, but is not limited thereto.

For example, when the polyatomic elements are nitrogen and fluorine, due to the co-doping effect of N,F, N,F-GQDs are positively induced by higher electronegativity between the fluorine and carbon electrodes. It shows excellent supercapacitor properties in the potential range, indicating that doping helps the activation of the electrochemical reaction.

A multi-element co-doped graphene quantum dot manufacturing method according to another embodiment of the present invention will be described.

2 is a flowchart of a method for manufacturing multi-element co-doped graphene quantum dots, according to an embodiment of the present invention.

3 is a schematic diagram of a method for manufacturing multi-element co-doped graphene quantum dots, according to an embodiment of the present invention.

2 and 3, the multi-element co-doped graphene quantum dot manufacturing method according to an embodiment of the present invention includes the steps of preparing the graphene quantum dots (S100); doping the graphene quantum dots with a first element (S200); and doping a second element to the graphene quantum dots doped with the first element (S300); includes

In the first step, graphene quantum dots are prepared (S100).

The graphene quantum dots (GQD) may be manufactured by a top-down method or a bottom-up method.

The top-down method is a method of forming quantum dots by physically and chemically cutting a relatively large carbon mass, such as a graphene sheet, carbon nanotube, carbon fiber, or graphite, into small pieces. For example, it may include a chemical exfoliation method, electrochemical synthesis, oxygen plasma treatment, hydrothermal treatment, microwave assistance, nano-cutting assistance, or laser fragmentation method. does not

The bottom-up method may be a method of forming carbon quantum dots by applying heat to carbohydrates such as glucose or organic acid to cause carbonization, and is not limited thereto, as long as it is a technology capable of manufacturing the graphene quantum dots.

Again, for example, the graphene quantum dots may be prepared by a chemical exfoliation method, and more specifically, may be prepared by a modified Hummer's method.

The modified Hummus method is a method for producing graphene by oxidizing graphite. For example, the modified Hummers method can be performed by adding chopped carbon fibers to an acid mixture, stirring, and heating. For example, first, 5 g of chopped carbon fibers _{are placed in an acid mixture of H 2} SO ₄ (800 mL, 95%) and HNO ₃ (200 mL, 60%), heated in an oil bath at 90 °C with vigorous stirring for 48 h, then , the solution is cooled to room temperature, diluted with deionized water to 10 times the volume, purified using a dialysis bag to pH 6, and dried using a vacuum evaporator to obtain graphene quantum dot (GQDs) powder.

In the second step, the graphene quantum dots are doped with a first element (S200).

The first element and the second element may be an element selected from the group consisting of nitrogen (N), boron (B), oxygen (O), and fluorine (F).

The doping of the first element may be performed by chemical vapor deposition (CVD) using a gas containing the first element, but is not limited thereto.

Chemical vapor deposition is a reaction that forms a thin film through chemical reactions such as thermal decomposition, photolysis, redox reaction, and substitution on the surface of the substrate by supplying a gas containing an element to be formed on the substrate.

For example, it may be performed by chemical vapor deposition (CVD) using a gas containing nitrogen. The nitrogen-containing gas may be ammonia gas, but is not limited thereto.

Again, for example, nitrogen-doping may be performed by injecting a gas containing nitrogen and heating it. The heating temperature may be 100° C. to 1000° C., but is not limited thereto.

Also, for example, graphene formed on a metal material may be doped with the first element by applying plasma including the first element to the graphene. For example, by applying nitrogen plasma, the formation of defects and N-doping in the graphene quantum dots may be increased.

In the third step, a second element is doped into the graphene quantum dots doped with the first element (S300).

The doping of the second element may be performed by plasma treatment in a gas atmosphere containing the second element.

For example, when the second element is fluorine, plasma treatment may be performed in a gas atmosphere containing fluorine.

The fluorine-containing gas is nitrogen trifluoride (NF ₃ ), fluorine (F ₂ ), fluorocarbon (C _x H _y F _z ), hydrogen fluoride (HF), carbon tetrafluoride (CF ₄ ), and sulfur hexafluoride ( SF ₆ ) and any one selected from the group consisting of combinations thereof, methane (CH ₄ ), acetylene (C ₂ H ₂ ), hydrazine (N ₂ H ₂ ), hydrogen (H ₂ ), carbon tetrachloride (CCl ₄ ) , chlorine dioxide (ClO ₂ ), chlorine (Cl ₂ ), hydrogen bromide (HBr), and bromine (Br ₂ ) It may be a gas mixed with at least one gas.

For example, in the doping of the second element, the first element-doped graphene quantum dots may be F-passivated by generating plasma using the fluorine-containing gas.

The plasma treatment may utilize a dielectric barrier discharge (DBD) discharge technology among dielectric film discharge technologies, but is not limited thereto.

In addition, in order to uniformly plasma-treat the entire area of the powder, the first element-doped graphene quantum dots may be placed on a moving stage that reciprocates in the horizontal direction, rotates, or vibrates, and plasma treatment may be performed.

It may be passivated by the plasma treatment to form a passivation film to have corrosion resistance.

In addition, the plasma treatment may be to uniformly dope the surface by direct contact with the plasma jet while mixing well through the process of floating and dropping the first element-doped graphene quantum dots through a plasma jet.

The content of the first element doped in the graphene quantum dots may be 0.5at% to 30at% relative to the total atomic percent. When the content of the first element is less than 0.5 at%, the effect of doping the first element may not be obtained.

For example, when the first element is nitrogen, the content of the nitrogen atom may be 1at% to 15at%, for example, 11at% relative to the total atomic percent.

The content of the second element doped in the graphene quantum dots may be 0.5at% to 30at% relative to the total atomic percent. When the content of the second element is less than 0.5 at%, the effect of doping the second element may not be obtained.

For example, when the first element is fluorine, the content of the fluorine atom may be 1at% to 15at%, for example, 9.6at% relative to the total atomic percent.

doping the second element; then doping a third element into the graphene quantum dots doped with the first element and the second element; can be done additionally.

The third element may be an element selected from the group consisting of nitrogen (N), boron (B), oxygen (O), and fluorine (F).

The doping of the third element may be performed by plasma treatment in a gas atmosphere containing the third element.

The plasma treatment method in the gas atmosphere containing the third element may be the same as the method used in the step of doping the second element, and the doping of the second element may be the same as the method of doping the first element.

Preparation Example 1- Synthesis of N,F-GQDs

1. Synthesis of GQDs

GQDs were prepared by a modified Hummer's method.

First, 5 g of chopped carbon fibers _{were placed in an acid mixture of H 2} SO ₄ (800 mL, 95%) and HNO ₃ (200 mL, 60%), and heated in an oil bath with vigorous stirring at 90 °C for 48 h. Next, the solution was cooled to room temperature, diluted to 10 volumes with deionized water, and purification was performed using a dialysis bag to pH 6. Next, after drying using a vacuum evaporator, GQDs powder was obtained.

2. Functionalization of GQDs with N

GQDs were functionalized by a chemical vapor deposition (CVD) system using ammonia (NH _{3 ) gas.} First, N-GQDs were prepared by putting N-GQDs in a CVD reactor and heating at 300° C. for 2 hours while flowing _{30 sccm NH 3 and 70 sccm Ar gas.}

3. Functionalization of N-GQDs as F

Next, N-GQDs were fluorinated using a dielectric barrier discharge (DBD) plasma reactor under atmospheric pressure. For uniform treatment during plasma generation, N-GQDs were placed on a three-step moving stage that vibrated horizontally ^{at 0.7 mms −1 , and the input power was maintained at 300 W.} After plasma treatment, N, F-GQDs were collected in the output chamber.

Preparation Example 2- Synthesis of N,B-GQDs

In Preparation Example 1, the first step of synthesizing GQDs and the second step of functionalizing GQDs with N were performed in the same manner to prepare N-GQDs. Next, N,B-GQDs were prepared by further functionalizing B using a plasma reactor under atmospheric pressure.

Preparation Example 3 Synthesis of B,O-GQDs

Step 1 of synthesizing GQDs in Preparation Example 1 was performed identically, and then, after doping B through a CVD (chemical vapor deposition) system, O was additionally functionalized using a plasma reactor under atmospheric pressure to further functionalize B, O -GQDs were prepared.

Preparation Example 4- Synthesis of N,B,F-GQDs

In Preparation Example 1, the first step of synthesizing GQDs and the second step of functionalizing GQDs with N were performed in the same manner to prepare N-GQDs. Next, after additionally functionalizing B using a plasma reactor under atmospheric pressure, the same three steps of functionalizing N-GQDs as F in Preparation Example 1 were performed to further functionalize F to obtain N,B,F-GQDs prepared.

Comparative Example 1- Synthesis of GQDs

GQDs were prepared by performing step 1 in Preparation Example 1.

Comparative Example 2 - Synthesis of N-GQDs

Steps 1 and 2 were performed in Preparation Example 1 to prepare N-GQDs.

Experimental example

Nickel foam was used as a substrate for all electrochemical experiments. Samples prepared as homogeneous slurries of GQDs, N-GQDs and N,F-GQDs were loaded onto nickel foam by brush coating. Active materials (GQDs, N-GQDs and N,F-GQDs), carbon black, and polyvinylidene difluoride were mixed with 0.4 mL of N-methyl 2-pyrrolidine in an 8:1:1 ratio. Then, the coated electrode was dried overnight at 80° C. in a vacuum oven. Electrochemical experiments were performed in a three-electrode configuration using a Pt wire as a counter electrode, Hg/HgO as a reference electrode and an electrode fabricated as a working electrode. A symmetric water splitting cell was composed of N,F-GQDs and N,F-GQDs as anode and cathode, respectively. All electrochemical experiments are performed under basic conditions using 1M KOH aqueous electrolyte.

4 is an atomic force microscope (AFM) image and a height graph of N,F-GQDs according to an embodiment of the present invention.

Referring to Figure 4, N,F-GQDs were prepared by directly applying fluorine plasma to N-GQDs powder at ambient pressure, and drop casting was used to study the surface morphology of N,F-GQDs. was dispersed in the Si substrate. (a) and (b) AFM is a device that measures the atomic phase of a surface by measuring the force between atoms by bringing a fine probe close to the surface of a material to the size of an atom. The height of the image shows that the homogeneous N,F-GQDs particles are uniformly placed on Si. (c) and (d) Also, the height profile of N,F-GQDs corresponding to dashed lines A and B in high magnification and 3D AFM images shows an average height of 1.5 nm or less, which indicates that the number of layers of N,F-GQDs is less than 1.5 nm. It indicates that it is about 3 layers.

5 is a TEM image and size graph of N,F-GQDs according to an embodiment of the present invention.

Referring to Fig. 5, the atomic structures of N,F-GQDs were investigated by low-resolution and high-resolution transmission electron microscopy (TEM) as shown in (a) and (c). The N,F-GQDs solution was removed and dried in CVD to increase the single-layer-graphene-supported TEM grid. (b) Most of the N,F-GQDs dispersed in the graphene sheet show a size distribution of 2 to 10 nm with an average size of 8.7 nm. The lattice structure shown in the high-resolution TEM images indicates that the N,F-GQDs are highly deterministic.

6 is an FT-IR spectrum and XPS graph of GQDs, N-GQDs and N,F-GQDs according to an embodiment of the present invention.

The nitrogen, fluorine and oxygen functional groups of GQDs were identified by Fourier transform infrared spectroscopy (FT-IR), X-ray photon spectroscopy (XPS) and scanning electron microscopy (SEM).

FT-IR is a device that changes the light of the light source from non-dispersive to interference light, passes through the sample, and then performs Fourier transform to obtain an infrared spectrum to check which functional groups exist. It is a device that analyzes whether the distribution has a certain amount of binding energy.

Referring to FIG. 6 , in (a), strong peaks corresponding to ^{oxygen functional groups CO (1725 cm −1} ) and C—OH (1245 cm ^{−1 ) were confirmed.} Unlike GQDs, a peak at ^{1629 cm -1 and three peaks from 2850 cm -1} to 3000 cm ^-1 can be assigned to the CC inplane and CH vibration modes, respectively. In the case of N-GQDs, the signals of COO and COOH are weakly detected, indicating that the functions of COO and COOH can be changed to CN, and NH waging is observed at ^{870 cm -1 for NCO, respectively.} In addition, N,F-GQDs show the presence of carbon, nitrogen, oxygen, and fluorine at 282.4, 400.8, 532.8, and 686.8 eV in half-ion CF, covalent CF, and CF-strained wide-scan XPS spectra. (b) is an XPS graph, and it can be seen that the peaks of N and F are well represented according to doping, and thus it can be confirmed that the functionalization is well performed. (c) High-resolution XPS measurements were performed to better understand the chemical bonding composition of the three samples. The C 1s spectrum of ^{GQDs shows peaks at 284.3 and 284.8 eV corresponding to sp 2} and sp ³ mixed carbon, respectively, and peaks at 285.3 and 289.0 eV indicate CO and OCO bonds, respectively. This indicates that GQDs contain oxygen-rich functional groups without nitrogen and fluorine species. On the other hand, according to the C 1s spectrum of N-GQDs and N,F-GQDs, a peak appears at 286.4 eV due to CN bonding, whereas the strength of carboxyl bonding at 289.0 eV is relatively decreased. (d) shows three peaks of N 1s peaks of 398.5 (pyridinic N), 399.8 (pyrrolic N) and 401.1 eV (graphitic N), respectively, which were successfully functionalized in N-GQDs and N,F-GQDs. show that it has been (e) In the F 1s spectrum, CF bonds composed of covalent CF bonds (688.6 eV) and half-ion CF bonds (686.2 eV) were observed only in N,F-GQDs. SEM images show that all samples of oxygen functional groups were present, nitrogen functional groups were only found in N-GQDs and N,F-GQDs, and fluorine functional groups were detected only in N,F-GQDs. The CF bonds of N,F-GQDs consist of 56.64 covalent bonds and 43.36% antiionic bonds, respectively, and these peaks were observed only in N,F-GQDs, suggesting that fluorine functional groups were effectively doped.

7 is a current density curve of GQDs, N-GQDs and N,F-GQDs according to an embodiment of the present invention. Graphene quantum dots double-doped in two steps were used as catalysts for hydrogen generation and oxygen generation, which are water decomposition reactions. Each reaction was operated in a three-electrode system, and an electrode made by placing N,F-GQDs on a Ni foam substrate was used as a working electrode, Hg/HgO as a reference electrode, and a Pt wire as a counter electrode.

Referring to Figure 7, (a) shows the recorded comparative polarization curves of N,F-GQDs for N-GQDs, GQDs and bare nickel foam. The LSV curve clearly shows that N,F-GQDs show enhanced catalytic activity compared to other samples. The voltage required to show a 10mA cm ^-2 current density is called overvoltage. In N,F-GQDs electrocatalyst ^{, the lowest overvoltage is exhibited at 0.13V with a current density of 10mA cm -2} , and N-GQDs, GQDs and Ni foams Requires relatively high overpotentials of 0.25V, 0.26V and 0.29. The smallest overvoltage values of N,F-GQDs mean that they require less activation energy for the evolution of hydrogen compared to N-GQDs, GQDs and pure Ni foam. Electrocatalytic performance can be strongly influenced by the electronic structure of the material. The strong electron withdrawing effect of nitrogen and fluorine atoms can reduce the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of carbon points. It is possible to match the LUMO level, F dopant lowered by N, with the original HOMO level of the GQDs, which allows the holes in the LUMO to recombine with the electrons in the HOMO at the interface junction between the doped and original regions of the GQDs. Recombination creates useful electron hole pairs at the GQDs-water interface, where the doped region and the original domain each serve as n-type domains. The P-type region is capable of electron injection for hydrogen evolution. The LSV oxygen evolution reaction curve in (b) means that among the electrocatalysts, N,F-GQDs showed improved oxygen evolution reaction catalytic activity than others in the positive potential range between 0 V versus RHE and 1.8 V versus RHE. N,F-GQDs ^{required a low overvoltage of 0.4V to generate a current density of 10mA cm -2} , and N-GQDs and GQDs required a high overvoltage to obtain a similar current density. A previous analysis of chemical bond composition, N,F-GQDs, was identified as composed of covalent CF bonds and antiionic bonds. The carbon in the half-ion CF bond contributes to the oxygen evolution reaction activity due to the positive charge, which is why N,F-GQDs show the highest electrocatalytic performance for oxygen evolution reaction. (c) In addition, heteroatom doping verified doping-induced charge redistribution around them, which _{altered the O 2} chemical adhesion mode to lower the overvoltage and effectively weaken the OO bond, thus facilitating the oxygen evolution reaction in the heteroatom-doped carbon electrode. made it Stability is one of the important parameters to understand the ability of electrocatalysts. Besides the dynamic behavior, measurements for the stability of N,F-GQDs were performed at -0.3V vs. hydrogen evolution reaction and 1.6V vs. RHE. N,F-GQDs were 1.6 vs. RHE and -0.3Vvs. There is no current density loss for 80000 seconds for both reactions of RHE, indicating that the catalytic action of N,F-GQDs prevents electrode corrosion in the aqueous system and thus has high stability.

8 is a polarization curve and a CA curve according to an embodiment of the present invention.

Referring to Figure 8, (a) is a polarization curve showing the current density as a function of voltage. The cell system of N,F-GQDs requires a low cell voltage of 1.61V to generate a water split current density of ^{10mA cm -2 .} On the other hand, it shows that N-GQDs (1.66V) and bare nickel foam (1.8V) based systems require higher voltages to drive the same current density. (b) shows that N,F-GQDs were measured for stability in various potential ranges at 5-hour intervals under alkaline conditions of 1 molar KOH electrolyte. Overall, they showed low overvoltage, high current density, and long-term stability. This is a result of the graphene providing more catalytically active carbon moieties near the dopant as it is doped. In particular, the arrangement of pyridine N specified in the XPS result of FIG. 6(c) provides a good catalytically active site.

Intermediate formation with adsorbents such as H*, OH*, etc. during the catalytic process determines the rate. Hetero doping lowers the intermediate formation energy for hydrogen production reaction for graphene, allowing it to be in a favorable position for electrochemical reactions. The synergistic effect of N,F hetero doping shows the best electrocatalytic activity for hydrogen evolution reaction and oxygen evolution reaction.

9 is a CV curve and a GCD curve according to an embodiment of the present invention.

In general, carbon-based materials have a fast and long cycle life due to the physical adsorption and desorption of electrolyte ions without chemical reaction. To take advantage of these advantages, N,F-GQDs were applied as supercapacitor electrodes, and a three-electrode setup was used in 1M KOH aqueous electrolyte. Electrochemical capacitors can be generally classified into two types: electrochemical double-layer capacitors based on electrostatic adsorption of electrolyte ions on the surface of electrodes, and pseudo-capacitors in which energy is stored through a reduction reaction occurring at the electrode and electrolyte interface.

Referring to FIG. 9 , (a) is a CV curve of N,F-GQDs, N-GQDs, and bare nickel foam electrodes at a fixed scan rate of ^{10 mV s −1 .} Both the N-GQDs and N,F-GQDs electrodes show clear similar capacitive properties, with a pair of paradise reduction peaks. In general, the capacitance increases as the measured current density increases. The Faraday-like capacitance occurs at the same time as the electrochemical double layer is formed, and it depends on the chemical affinity of the electrode material for the electrolyte ions and the structure of N,F-GQDs and N-GQDs. . In particular, the N,F-GQDs electrode shows a capacitance of 245.15 F/g at a scan rate ^{of 10 mV s -1 .} It delivers over five times the electrochemical capacitance compared to N-GQDs with a capacitance of 47.3 F/g. (b) CV curves of N,F-GQDs electrodes prepared at variable scan rates ranging from 1 to 10 mV s ^{−1 .} The specific capacitance of the CV curve represents 270 F/g with a scan rate of ^{1 mV s −1 .} Positive and negative shifts with increasing scan rate are observed in the anode and cathode peaks to understand the diffusion-controlled reaction mechanism involved in the overall process. (c) 0V vs. Hg/HgO to 0.6V vs. Galvanostatic charge discharge (GCD) curves of N,F-GQDs electrodes in a potential range between Hg/Hg/HgO and ^{a current density of 3 to 10 mA cm -2 are shown.} The non-linear nature of the charge discharge profile shows typical quasi-capacitive behavior for both electrodes, consistent with the CV measurements. The specific capacitance of N,F-GQDs calculated based on the discharge curve is 244F g ^{-1 at} a current density of 3 A g ^-1 . Nitrogen doping is effective in increasing capacitance through surface Faraday reaction without compromising high capacity and long cycle life. In addition, the high electronegativity difference between the F and N doped carbons creates positively charged carbons, accelerating the capacitance into the positively charged range. In (d), it can be seen that the decrease in capacity is small as the cycle is repeated. it can be seen that These results indicate that N,F-GQDs are novel multifunctional catalysts for simultaneous water splitting and supercapacitors.

According to an embodiment of the present invention, when graphene quantum dots co-doped with multiple elements are used as electrode catalysts, hydrogen evolution reaction (HER) and oxygen evolution reaction compared to conventional graphene quantum dots , OER) shows a low overvoltage, indicating improved catalytic activity.

In addition, when the graphene quantum dots co-doped with the multiple elements are used as a capacitor electrode, the positive induced by higher electronegativity helps activate the electrochemical reaction due to the co-doping effect of the multiple elements. It may exhibit excellent supercapacitor characteristics in the potential range.

In addition, the graphene quantum dots co-doped with the multiple elements can form a passivation film on the electrode in an aqueous system to prevent corrosion from aqueous solution, thereby suppressing deterioration of electrode performance and enabling long-term operation.

The description of the present invention described above is for illustration, and those of ordinary skill in the art to which the present invention pertains can understand that it can be easily modified into other specific forms without changing the technical spirit or essential features of the present invention. will be. Therefore, it should be understood that the embodiments described above are illustrative in all respects and not restrictive. For example, each component described as a single type may be implemented in a dispersed form, and likewise components described as distributed may also be implemented in a combined form.

The scope of the present invention is indicated by the following claims, and all changes or modifications derived from the meaning and scope of the claims and their equivalents should be construed as being included in the scope of the present invention.

10: nitrogen atom 20: fluorine atom

Claims (12)

graphene quantum dots; and
Characterized in that it contains two or more elements doped in the graphene quantum dots,
The doped two or more elements include a first element and a second element,
The graphene quantum dots are doped with the second element by plasma treatment in a gas atmosphere containing the second element after the first element is doped, and the second element is the total area of the graphene quantum dots In order to uniformly plasma-process the pin quantum dots. According to claim 1,
The two or more elements are multi-element co-doped graphene quantum dots, characterized in that they are two or more elements selected from the group consisting of nitrogen (N), boron (B), oxygen (O) and fluorine (F). According to claim 1,
The content of the two or more elements is each independently multi-element co-doped graphene quantum dots, characterized in that 0.5at% to 30at% with respect to the total atomic percent. According to claim 1,
Multi-element co-doped graphene quantum dots, characterized in that the average diameter is 1 nm to 30 nm. According to claim 1,
A multi-element co-doped graphene quantum dot, characterized in that the spacing between each layer is 5 nm or less. The electrocatalyst for water electrolysis comprising the multi-element co-doped graphene quantum dots of claim 1 . A supercapacitor electrode comprising the multi-element co-doped graphene quantum dots of claim 1. preparing graphene quantum dots;
doping the graphene quantum dots with a first element; and
doping a second element to the graphene quantum dots doped with the first element; characterized in that it comprises,
The doping of the second element is characterized in that the plasma treatment is performed in a gas atmosphere containing the second element,
In the step of doping the second element, the graphene quantum dots doped with the first element on a moving stage that reciprocates in the horizontal direction, rotates, or vibrates in order to uniformly plasma-treat the entire area of the graphene quantum dots. A multi-element co-doped graphene quantum dot manufacturing method, characterized in that it is placed and plasma treated. 9. The method of claim 8,
The doping of the first element is a multi-element co-doped graphene quantum dot manufacturing method, characterized in that it is performed by chemical vapor deposition (CVD) using a gas containing the first element. delete 9. The method of claim 8,
A multi-element co-doped graphene quantum dot manufacturing method, characterized in that the content of the first element doped into the graphene quantum dots is 0.5 at% to 30 at% relative to the total atomic percent. 9. The method of claim 8,
Multi-element co-doped graphene quantum dot manufacturing method, characterized in that the content of the second element doped in the graphene quantum dots is 0.5at% to 30at% relative to the total atomic percent.

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