KR101568145B1 - Graphitization of nanodiamonds by photochemical reaction and carbon nano materials fabricated by the same - Google Patents

Abstract

It provides a simple way to graphitize nanodiamonds without expensive vacuum equipment and high energy consumption. In the graphitization method of the present invention, the nanodiamond is dispersed in a solvent to form a colloid solution, and the colloidal solution is irradiated with a pulsed laser to induce a photochemical reaction to graphitize the nanodiamond. According to the present invention, a diamond structure is converted to a graphite structure by photochemical reaction of a nanodiamond and a solvent on a colloid solution. Because it can be carried out without using expensive vacuum equipment and large energy consumption by using solvent and laser, it is possible to graphitize nano diamond simply and at low cost.

Description

TECHNICAL FIELD The present invention relates to a method for graphitizing nanodiamonds using a photochemical reaction, and a method for producing graphitization of nanodiamonds by photochemical reaction and carbon nano materials fabricated by the same method,

The present invention relates to a method for graphitizing a nano-diamond, and more particularly, to a method for converting part or all of nano-diamond into graphite to convert it into a carbon nanomaterial having a part or all of graphite structure.

Carbon-based nanomaterial technology is applicable to a wide range of fields such as physics, advanced materials engineering, biotechnology, electricity, electronics, energy conversion and storage. In particular, the importance of carbon-based nanomaterials technology in energy depletion and environmental pollution is highly evaluated based on price of cheap raw materials, various application possibilities through chemical change, and excellent electrical characteristics.

As a crystal of carbon, two types of diamond and graphite are known. Continued research after the discovery of C60 (aka fullerene) has found carbon nanomaterials of various graphite structures, such as nanotubes, graphene, and onion-like carbon. Since these graphitic carbon nanomaterials exhibit various properties such as superconductivity, conductivity, semiconducting properties, insulating properties, ferromagnetism, super-atomic properties, and the like, it is possible to use nanotransistors o Diodes, electron emitters, single electronic devices, It can be applied to various applications such as super high speed devices, superconducting magnets, solar cells, solid lubricants, three dimensional nonlinear optical devices, ultra high speed optical switching devices, lightweight permanent magnets, gas storage materials (oxygen, hydrogen and argon) have.

Of these, onion-like carbon has a concentric spherical carbon structure, which is also called an onion-structure carbon nano-spheres. Its size is very small and light on the order of several nanometers and is excellent in resistance to radiation and high temperature. High elasticity is expected, Have been studied in terms of application to meter-level mechanical materials. Further, it has been studied for use as a hydrogen storage carbon material for medicines, cosmetics, and fuel cells. Recently, a team of researchers from the University of Toulouse (France) and Drexel University (USA) has produced a super capacitor electrode using onion-like carbon to confirm notable improvements in energy and power characteristics. 5, pp. 651-654).

As a method for synthesizing onion-like carbon, there is a method of producing a molded body made of glassy carbon by heat treatment at 2000 to 3000 占 폚 under a pressure of 1000 to 3000 atm by hot isostatic pressing (HIP) A method of graphitizing by heating at 1600 to 1800 占 폚 has been disclosed. In particular, due to the stable supply of detonation nanodiamonds formed by the explosion method, graphitization by thermal annealing of nano-diamonds is becoming the mainstream of the synthesis of onion-like carbon.

However, the graphitization reaction of nanodiamonds as known so far requires a high reaction temperature of about 1600 to 1800 ° C, so that not only expensive vacuum equipment but also a large energy is consumed. Therefore, it is necessary to develop a method that can replace such an uneconomical method.

The problem to be solved by the present invention is to provide a simple method for graphitizing nanodiamonds without expensive vacuum equipment and large energy consumption.

Another problem to be solved by the present invention is to provide a carbon nanomaterial derived from a nanodiamond having controlled degree of graphitization.

In order to solve the above problems, in the method of graphitizing nanodiamonds according to the present invention, a nanodiamond is dispersed in a solvent to form a colloid solution, and then a pulse laser is irradiated to the colloid solution to induce a photochemical reaction, Graphitize nanodiamonds.

The wavelength of the pulsed laser may be 1064 nm, 532 nm, or 355 nm. The pulsed laser may be an Nd-YAG pulsed laser. The pulse laser irradiation time may be 10 minutes to 5 hours. The solvent may be water, methanol, ethanol, propanol, hexanol, acetone, butanone, acetonitrile, hexane, benzene or toluene.

The ratio of the diamond structure to the graphite structure in the nanodiamonds can be controlled by varying the irradiation time of the pulse laser and the type of the solvent.

After the graphitized nanodiamonds are separated from the solvent, an oxygen reduction catalyst can be prepared by further performing a heat treatment for doping nitrogen (N) or sulfur (S) elements. The nitrogen or sulfur element doping is performed by heat treating the graphitized nanodiamond in an NH ₃ or H ₂ S gas atmosphere.

The graphitized nanodiamonds according to the present invention are new carbon nanomaterials that can be utilized as an electrocatalyst. The carbon nanomaterial may be in the form of a diamond-graphite core-shell nanostructure, an onion-like carbon, or a mesoporous carbon sphere depending on the ratio of the diamond structure and the graphite structure in the nanodiamond.

According to the present invention, a diamond structure is converted to a graphite structure by photochemical reaction of a nanodiamond and a solvent on a colloid solution. Because it can be carried out without using expensive vacuum equipment and large energy consumption by using solvent and laser, it is possible to graphitize nano diamond simply and at low cost.

If the chemical reaction rate is controlled by changing the type of solvent or controlling the laser irradiation time, the thickness of the graphite layer can be controlled to graphitize the nanodiamonds, and thus, all or a part of the carbon nanomaterials Can be synthesized. For example, it is possible to synthesize various types of carbon nanomaterials such as onion-like carbon and porous carbon spheres converted to graphite layers as well as spherical diamond-graphite core-shell nanoparticles with controlled ratio of diamond structure and graphite structure have.

Accordingly, carbon nanomaterials having controlled graphitization degree in a form suitable for the application field can be environmentally synthesized. According to the present invention, graphitized nanodiamonds and carbon nanomaterials doped with sulfur or nitrogen can be used as an electrocatalyst having excellent activity in an oxygen reduction reaction.

1 is a schematic view of a graphitization method of a nanodiamond using a photochemical reaction according to the present invention.
2 is a transmission electron microscope (TEM) image of a nanodiamond used in the experiment.
FIGS. 3 to 5 are photographs showing the color change of the nanodiamond dispersed solution with the elapse of the pulse laser irradiation time, wherein the solvent of FIG. 3 is water, the solvent of FIG. 4 is ethanol, and the solvent of FIG. 5 is hexanol.
FIG. 6 is a graph showing the change in color of the solution with UV-Vis spectroscopy when water is used as a solvent.
FIGS. 7 and 8 are transmission electron micrographs of nanodiamonds in the ethanol solvent according to the laser irradiation time. FIG.
FIG. 9 is a schematic diagram showing a step of progressing graphitization of nanodiamonds over time when ethanol is used as a solvent. FIG.
10 is a graph for evaluating the electrocatalytic performance of graphitized nanodiamonds according to the present invention.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. However, the embodiments of the present invention can be modified into various other forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. Embodiments of the present invention are provided to more fully describe the present invention to those skilled in the art. Therefore, the shapes and the like of the elements in the drawings are exaggerated in order to emphasize a clearer explanation.

The method according to the present invention is a method of dispersing nano-diamonds in various simple solvents such as water or ethanol and inducing graphitization through photochemical reaction between solvent and nano diamond by irradiating pulsed laser at room temperature, And is differentiated from the onion-like carbon synthesis method by annealing. In addition, it includes a step of adjusting the ratio of the diamond structure and the graphite structure through the change of the solvent type and the laser irradiation time, thereby differentiating it from the conventional methods in that it can synthesize carbon nanomaterials having various structures.

Hereinafter, a graphitizing method of a nanodiamond using the photochemical reaction according to the present invention will be described in detail.

1 is a schematic view of a graphitization method of a nanodiamond using a photochemical reaction according to the present invention.

Referring to FIG. 1, a colloid solution 10 is prepared by dispersing a nanodiamond in a solvent, and the colloidal solution 10 is introduced into a pyrex-made reactor 20.

The nanodiamond can be a commercial nanodiamond synthesized by a method in which a raw material of an explosive containing carbon atoms is exploded in a high-temperature and high-pressure environment to bond carbon atoms together to form diamond crystals. Commercial products that are most readily available and readily available have an average diameter of 5 nm and a surface area (Brunauer-Emmett-Teller, BET) of about 300 m ² / g.

The solvent is water, methanol (CH ₃ OH), ethanol (C ₂ H ₅ OH), propanol (C ₃ H ₇ OH), cyclohexanol (C ₆ H ₁₃ OH), acetone _{[CH 3 C (O) CH} 3 ], butanone _{[CH 3 C (O) CH} 2 CH 3], acetonitrile (CH ₃ CN), hexane (C ₆ H _14), benzene (C ₆ H _6), toluene (C ₆ H ₅ CH ₃ ), And various solvents that are readily available, inexpensive, and easy to handle can be used.

Next, a pulsed laser 30 is irradiated on the colloidal solution 10 to induce the multiphoton absorption of the nanodiamonds having a band gap of 5.5 eV. The pulsed laser 30 induces a photochemical reaction by irradiating 10 Hz, for example, with an intensity of 800 mJ per pulse. If the irradiation intensity is too low, graphitization is difficult to occur. If the irradiation intensity is too high, the decomposition of the generated graphite layer is accelerated, and it is difficult to control the degree of graphitization. The width of the pulsed laser is also considered. If the pulse width is too narrow, the amount of energy to be injected is reduced, and the degree of graphitization is reduced. If the pulse width is too wide, the energy to be supplied is excessive, which is rather undesirable for graphitization.

When the pulsed laser 30 is irradiated to the colloidal solution 10, it is conceivable that graphitization occurs due to instantaneous heating and pressurization at the interface between the nanodiamond and the solvent depending on the energy input. However, it is a feature of the present invention that electrons and holes generated due to multiphoton absorption cause a solvent and a photochemical reaction to cause graphitization. The wavelength of the pulsed laser 30 may be near-infrared (1064 nm), visible (532 nm) or ultraviolet (355 nm) wavelengths. It is also possible to use a wavelength of 532 nm or 355 nm which can be made by combining a 1064 nm fundamental wavelength. And the pulse laser 30 may be an Nd-YAG pulse laser.

It is preferable to continue stirring by using a stirrer 40 using a magnet rod for uniform reaction during irradiation of the pulsed laser 30. [ The colloidal solution 10 may be heated to an appropriate temperature, but heating is not necessarily required, and graphite can be also obtained by treatment at room temperature if the pulsed laser 30 irradiation intensity is appropriate. The irradiation time of the pulsed laser 30 may be 10 minutes to 5 hours, and it is possible to control the ratio of the diamond structure and the graphite structure according to the type of the solvent for dispersing the nano diamond and the pulse laser irradiation time. As the intensity of the pulse laser 30 increases, the time required for graphitization decreases in inverse proportion to the square. Also, as shown in the experimental example to be described later, the point at which the nano diamond graphitization reaction is terminated varies depending on the solvent.

After the pulsed laser 30 is irradiated, the colloidal solution 10 is put into a centrifuge to separate the solvent and the graphitized nanodiamond, and then dried in an oven to obtain a powdery product.

Subsequently, the graphitized nanodiamond can be placed in a quartz tube and treated for 2 hours at 800 ° C with flowing hydrogen. At this time, in order to further doping nitrogen or sulfur, NH ₃ or H ₂ S gas may be supplied instead of hydrogen. The doping concentration can be controlled by controlling the gas flow rate and the heat treatment temperature. Graphitized nanodiamonds doped with nitrogen or sulfur can be used as an electrical catalyst for an oxygen reduction reaction (ORR).

Experimental Example

The amount of the nanodiamond used in the graphitization was 5 mg, and the solvent was 5 mL. The nanodiamond is also a commercial product (JSC diamond center, 99% purity) with a size of about 5 nm.

The pulsed laser 30 was irradiated to the specific point of time at which the graphitization reaction was terminated with different kinds of solvents, and the results shown in Table 1 were obtained. The pulse laser 30 had a wavelength of 532 nm.

In Table 1, it can be seen that as the length of the carbon chain of the solvent increases, the termination time of the graphitization reaction of the nanodiamond increases, and when methanol or ethanol is used as the solvent, it is faster than other hydrocarbon solvents such as water The graphitization of the nanodiamond proceeds and is a very effective solvent for the graphitization reaction. Alcohols such as methanol and ethanol are more effective than graphite in water. The reason for this is considered to be related to the viscosity of the solvent, boiling point, and the like. Photochemical oxidation of solvents accelerates the graphitization of nanodiamonds.

In the present invention, the graphitization of nanodiamonds is induced by using photochemical reaction. By controlling the thickness of the graphite layer by controlling the type of solvent and controlling the pulse laser irradiation time, various types of shape control as well as the ratio of the diamond structure to the graphite structure Can be effectively controlled.

FIG. 2 shows a transmission electron microscope image of a nanodiamond used in the experiment. The size of the nanodiamond is about 5 nm and the crystal structure of the sp3 hybrid bond having a (111) interplanar spacing of 2.1 Å is shown by a lattice analysis.

FIGS. 3 to 5 are photographs showing the color change of the nanodiamond dispersed solution with the elapse of the pulse laser irradiation time. FIG.

FIG. 3 shows the case where the solvent is water. Referring to FIG. 3, it can be seen that the color of the solution slowly changes gradually to black for 3 hours. This is due to the slowly changing color of the solution as the nanodiamond is graphitized.

4 shows a case where the solvent is ethanol. Referring to FIG. 4, after 10 minutes of laser irradiation, the color of the solution changed to black. As a result, it can be seen that the time for graphitization of nanodiamonds due to a solvent and a photochemical reaction is much shorter than that in FIG. When the pulse laser irradiation time is 40 to 50 minutes, the graphitized structure is destroyed after the graphitization reaction is terminated, and the color of the solution changes to a transparent yellow. Yellow is the result of polyyne (C _n H ₂ ) molecule formation.

FIG. 5 shows the solution change when hexanol was used as a solvent. Referring to FIG. 5, it can be seen that the time point at which the graphitization reaction is terminated is longer than that of ethanol and takes 90 minutes.

FIG. 6 is a graph showing the change in color of the solution with UV-Vis spectroscopy when water is used as a solvent. Referring to FIG. 6, it can be seen that the graphitization progresses with time and the absorbance of the long wavelength region increases, while the absorbance of the short wavelength region absorbed by the nano diamond decreases.

Structural changes of nanodiamonds with laser irradiation time were confirmed by transmission electron microscopy using ethanol as a solvent. FIG. 7 is a transmission electron microscope photograph of a 10 minutes laser irradiation time in an ethanol solvent. FIG. Referring to FIG. 7, it can be seen that the surface of a part of the nanodiamonds is changed to a graphite layer. (i) and (ii) enlarged image and FFT (Fast-Fourier Transform) image analysis results are also shown in FIG. As a result of analyzing the crystal structure change by performing FFT image analysis, there is a (111) lattice pattern of nano diamond sp3 hybrid structure in (i) and (ii).

FIG. 8 is a transmission electron micrograph of a time when a laser irradiation time of 30 minutes has elapsed in an ethanol solvent. Referring to FIG. 8, it can be seen that when the pulse laser irradiation time is increased to 30 minutes, most of the nano-diamonds are graphitized, and the thickness of the graphite layer on the surface is further increased to the graphite layer to the inside. (iii) and (iv) enlarged image and FFT image analysis results are also shown in FIG. As a result of analyzing the crystal structure change by performing FFT image analysis, in (iii) and (iv), the lattice pattern of the nano diamond sp3 hybrid structure (111) disappears and the sp2 hybrid structure (002) lattice pattern is formed.

The lattice resolution transmission electron microscopic image analysis of FIGS. 7 and 8 shows that the thickness of the graphite layer of the nanodiamond (i) increases with the increase of the pulse laser irradiation time and the morphology changes from graphite core-shell nanostructure (ii) (Iii), and the porous carbon spheres (iv). As a result, it can be seen that the sp3 hybrid bond inherent in the nano-diamond decreases with an increase in the graphite layer, and conversely, the sp2 hybrid bond increases due to graphitization. As a result of analysis of the crystal structure by FFT image analysis, it was confirmed that the lattice pattern of the nano diamond sp3 hybrid structure (111) disappears and the sp2 hybrid structure (002) lattice pattern is formed as the pulse laser irradiation time passes.

FIG. 9 is a schematic diagram showing a step of progressing graphitization of nanodiamonds over time when ethanol is used as a solvent. FIG. As the time increases, the graphite layer thickness of the nanodiamonds increases. When the irradiation time is short, a graphite layer is formed only on the surface of the nanodiamond to form a diamond-graphite core-shell nanostructure. When the irradiation time is reached within a certain time, the onion-like carbon in which the nanodiamond is changed into a graphite layer can be obtained. If the irradiation time is longer than this, the graphitized structure may be destroyed to form porous carbon spheres.

The carbon nanomaterial obtained by the graphitization method according to the present invention can be used as an electrical catalyst for an oxygen reduction reaction. The graphite layer of the carbon nanomaterial can be easily doped with other atoms such as nitrogen or sulfur, as described above, thereby further increasing the catalytic activity. These new carbon nanomaterials can replace expensive Pt-carbon catalysts currently used in fuel cells.

CV (cyclic voltammetry) measurement and RDE (rotating disk electrode) measurement were performed to evaluate the characteristics of ORR as an electrical catalyst.

10 (a) is a graph showing the relationship between a nanodiamond (hereinafter referred to as GND), nitrogen doped GND (hereinafter referred to as N-GND) and sulfur doped GND (Hereinafter referred to as S-GND) in 0.1 M KOH without oxygen and 0.1 M KOH saturated with oxygen. In the case of 0.1 M KOH without oxygen, there is a characteristic curve in the ORR potential range, but in the case of oxygen saturated 0.1 M KOH, it shows a distinct ORR peak near 0.25 V. This indicates the electrical catalytic activity in oxygen reduction. When nitrogen or sulfur is doped, the peak is further increased.

LSV (linear sweep voltammetry) measurement using RDE was also performed to compare the performance as an electrocatalyst. The LSV is an analytical method in which the potential of the working electrode is changed from the initial potential to the final potential at a constant rate and the corresponding current is recorded. The speed is set at 400 to 2500 rpm. 10 (b) shows LSV curves at a speed of 1600 rpm. The LSV curve of commercially available 20 wt% Pt / vanadium carbon (VC) was used as a reference.

10 (c) and 10 (d) show the LSV curves of N-GND and S-GND under various rotational speeds. The ORR onset potentials of GND, N-GND, and S-GND are 0.19, 0.08, and 0.16 V, which is higher than 0.01 V of Pt / VC. It can be seen that the ORR current is improved due to nitrogen doping and is close to Pt / VC.

Compared with the conventional method using high temperature annealing, the present invention using a pulsed laser is a simple method and easy to handle. Since the conventional method requires heating at a high temperature, it is expensive and requires expensive equipment for forming an inert gas atmosphere. In the present invention, a simple eco-friendly method of pulsed laser irradiation is used for a solvent that is easy to handle, and the graphitization can be accomplished at a room temperature by an extremely short time treatment, so that the process cost is reduced. The ratio of diamond to graphite can be easily controlled by changing the type of solvent and laser irradiation time.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is to be understood that the invention is not limited to the disclosed exemplary embodiments, but many variations and modifications can be made by those skilled in the art within the technical scope of the present invention. Is obvious. The embodiments of the present invention are to be considered in all respects as illustrative and not restrictive and cover the scope of the invention as defined by the appended claims rather than the detailed description thereto and the equivalents of the claims and all variations within the means .

Claims (10)

Dispersing the nanodiamides in a solvent to form a colloid solution; And
And graphitizing the nanodiamond by irradiating the colloidal solution with a pulsed laser at room temperature to induce a photochemical reaction. The graphitization method of claim 1, wherein the pulse laser has a wavelength of 1064 nm, 532 nm, or 355 nm. The method of claim 1, wherein the pulse laser is an Nd-YAG pulse laser. The method for graphitizing nanodiamonds according to claim 1, wherein the pulsed laser irradiation time is from 10 minutes to 5 hours. The method of claim 1, wherein the solvent is water, methanol, ethanol, propanol, hexanol, acetone, butanone, acetonitrile, hexane, benzene or toluene. The method for graphitizing nanodiamonds according to any one of claims 1 to 5, wherein the ratio of the diamond structure to the graphite structure in the nanodiamonds is controlled by varying the irradiation time of the pulse laser. The graphitization method of claim 1, wherein the graphitized nanodiamond is separated from the solvent and further subjected to a heat treatment for doping nitrogen or sulfur atoms. The graphitization method of claim 7, wherein the graphitized nanodiamonds are annealed in an NH ₃ or H ₂ S gas atmosphere to perform nitrogen or sulfur element doping. delete delete

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