Nano-carbon materials

Abstract

To provide nano-carbon materials with enhanced reactivity.SOLUTION: The nano-carbon materials have a curved outer peripheral surface in which graphite edge planes are exhibited at a density of 10 or more pieces per 10 nm.SELECTED DRAWING: Figure 6

Description

[Technical Field]

[0001] The present invention relates to nanocarbon materials. [Background technology]

[0002] Carbon materials with nanometer-sized structures, such as multi-walled carbon nanotubes and graphene nanosheets, are used as adsorbents and the like. Graphite is a low-dimensional substance with basal planes in which carbon atoms are arranged in a honeycomb pattern and edges located at the ends of the basal planes. Graphite edges have a much higher surface energy than basal planes, making them easier to chemically modify (Non-Patent Document 1). Therefore, the more graphite edges exposed in a carbon material, the higher its reactivity with molecules in the gas or liquid phase, resulting in a material with excellent adsorption capabilities. For example, in Non-Patent Document 1, the reactivity of multi-walled carbon nanotubes is enhanced by exposing edges on both the inner and outer walls of hollow multi-walled carbon nanotubes.

[0003] However, there is room for further improvement in carbon materials such as multi-walled carbon nanotubes in terms of increasing their reactivity. [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] David Burton and two others, "Physical Properties and Applications of Carbon Nanofibers (CNF)," [Retrieved September 20, 2023], Internet <https: / / www.sigmaaldrich.com / JP / ja / technical-documents / technical-article / materials-science-and-engineering / batteries-supercapacitors-and-fuel-cells / carbon-nanofibers> Summary of the Invention [Problem to be solved by the invention]

[0005] In one aspect, the present invention aims to increase the reactivity of nanocarbon materials. [Means for solving the problem]

[0006] The first nanocarbon material of the present invention is a nanocarbon material having a curved outer peripheral side surface on which graphite edge faces are exposed at a density of 10 / 10 nm or more.

[0007] The second nanocarbon material of the present invention is the first nanocarbon material, characterized in that the angle between the outer peripheral side surface and the basal plane of the graphite is 30° or more and 90° or less.

[0008] The third nanocarbon material of the present invention is a nanocarbon material characterized in that, in the first or second nanocarbon material, the outer side surface is the outer side surface of a fiber having a plurality of edge surfaces of the graphite.

[0009] A fourth nanocarbon material of the present invention is the third nanocarbon material, wherein the fiber has a spiral shape.

[0010] The fifth nanocarbon material of the present invention is the nanocarbon material of the fourth nanocarbon material, characterized in that the thickness of the fiber is 10 nm or more and 500 nm or less, the length of the fiber is 1 μm or more, and the pitch angle of the spiral shape is 30° or more and 90° or less.

[0011] A sixth nanocarbon material of the present invention is the fifth nanocarbon material, characterized in that each end of the two fibers is fixed to one catalyst particle.

[0012] The seventh nanocarbon material of the present invention is a nanocarbon material characterized in that, in any one of the third to sixth nanocarbon materials, the end opposite to the end of the fiber fixed to the catalyst particle is a free end. [Effects of the Invention]

[0013] According to the present invention, the reactivity of a nanocarbon material can be increased. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 1 is a schematic diagram of graphite. [Figure 2] FIG. 1 is a cross-sectional view taken along the axial direction of a carbon nanofiber made of graphite. [Figure 3] 3(a) is a partial cross-sectional view of a solid catalyst, and FIG. 3(b) is a schematic diagram of a catalytic reaction occurring on the crystal surface of the solid catalyst. [Figure 4] A scanning electron microscope image of a nanocarbon material (Figure 4(a)) and a transmission electron microscope image of a nanocarbon material (Figure 4(b)). [Figure 5] FIG. 10 is a diagram showing the results of element mapping. [Figure 6] FIG. 1 is a transmission electron microscope image (high resolution) of the vicinity of the surface of a nanocarbon material. [Figure 7] 1 is a schematic diagram of a nanocarbon material. [Figure 8] FIG. 2 is a partially enlarged view of the helical structure of a nanocarbon material. [Figure 9] FIG. 2 is a diagram showing an edge surface of a nanocarbon material. DETAILED DESCRIPTION OF THE INVENTION

[0015] Prior to describing the present embodiment, the matters considered by the inventors of the present invention will be described. Fig. 1 is a schematic diagram of graphite. As shown in Fig. 1, graphite 1 is a two-dimensional material in which carbon atoms are arranged in a honeycomb pattern, and has basal planes 1b and edge planes 1e.

[0016] The basal plane 1b is a plane on which each carbon atom is arranged, while the edge plane 1e is the end of the basal plane 1b.

[0017] Fig. 2 is a cross-sectional view along the axial direction 3a of a carbon nanofiber made of graphite 1. As shown in Fig. 2, carbon nanofiber 3 is a hollow fiber extending along the axial direction 3a, and has an inner circumferential side surface 3b and an outer circumferential side surface 3c. A plurality of graphite particles 1 are stacked with their basal planes 1b tilted from the axial direction 3a, and edge surfaces 1e of the graphite particles 1 are exposed on each side surface (inner circumferential side surface 3b, outer circumferential side surface 3c).

[0018] The reactivity of the carbon nanofiber 3 is determined by the number of edge surfaces 1e exposed on each side (inner peripheral side surface 3b, outer peripheral side surface 3c), and the greater the number, the higher the reactivity. Hereinafter, the number of edge surfaces 1e exposed per unit length N along the axial direction 3a of the multilayer carbon nanofiber 3 will be referred to as the exposed density of edge surfaces 1e per unit length N.

[0019] 2, the density of exposed edge surfaces 1e per unit length N is proportional to the sine (sin θ) of the angle θ between the axial direction 3a and the basal plane 1b. Therefore, it is thought that the closer the angle θ is to 90°, the higher the density of exposed edge surfaces 1e, thereby increasing the reactivity of the multilayer carbon nanofiber 3.

[0020] However, when the angle θ approaches 90°, the morphology of the carbon nanofiber 3 changes from fibrous to flake-like, lowering the surface energy of the system. Because the basal planes cover most of the surface of flake-shaped carbon nanomaterials, the density of exposed edges decreases, resulting in reduced reactivity. For example, in the structure shown in Figure 2, the angle θ cannot exceed 30° and remains at approximately 20°, resulting in an exposed density of edge planes 1e of approximately 8–9 per 10 nm. This tendency is rooted in the properties of graphite, the molecular unit that constitutes the material. Therefore, it is difficult to produce carbon materials with an angle θ exceeding 30° using conventional material design techniques.

[0021] (Present embodiment) One of the features of the nanocarbon material of the present invention is that the density of exposed edge faces is 10 / 10 nm or more. Nanocarbon materials with this feature are expected to have superior reactivity compared to conventional carbon materials. Below, the manufacturing method and structure of the nanocarbon material will be explained based on experimental examples.

[0022] Nanocarbon materials can be produced by catalytic reactions in a mixed gas atmosphere containing carbon monoxide (CO) and hydrogen (H2). Specifically, by introducing the mixed gas onto the surface of a heated solid catalyst, they can be precipitated and produced via the CO disproportionation reaction (2CO → CO2 + C) and / or the CO hydrogenation reaction (CO + H2 → H2O + C). The production procedure for nanocarbon materials can include three steps: preparing a solid catalyst (step 1), heating the solid catalyst to a predetermined temperature and supplying the mixed gas (step 2), and recovering the nanocarbon material from the surface of the solid catalyst (step 3).

[0023] A composite oxide material can be used as a solid catalyst. An experimental example of the preparation procedure for the composite oxide material is given below. First, an alloy material consisting of iron and tantalum (or titanium) was heat-treated in an oxygen gas-containing atmosphere to a temperature of 600°C or higher. The heat treatment caused a composite oxide phase consisting of a mixture of iron and tantalum (or titanium) oxides to form on the surface of the alloy material. This composite oxide phase (complex oxide material) was used as a solid catalyst.

[0024] The above is just one example, and the solid catalyst may be an oxide of iron or an alloy of iron and a non-ferrous metal. Non-ferrous metals are not particularly limited, but include metals from Groups 3 to 12 (excluding iron), aluminum, gallium, indium, thallium, tin, lead, bismuth, and polonium. The non-ferrous metals may be used alone or in combination of two or more. At least one of the non-ferrous metals is preferably selected from metals from Groups 4 to 12, and more preferably from metals from Groups 4 to 6.

[0025] The shape and size of the solid catalyst are not particularly limited. FIG. 3(a) is a partial cross-sectional view (schematic diagram) of the solid catalyst. The synthesis reaction of the nanocarbon material occurs on the crystal surfaces 11a and 11b of the solid catalyst 11. The size of the catalytic reaction site (active site) is preferably 1 nm or more and 200 nm or less, for example. However, the size of the active site does not limit the overall size of the solid catalyst to a specific range.

[0026] In the second step, a mixed gas containing carbon monoxide and hydrogen is supplied to a solid catalyst heated to a predetermined temperature. Specifically, the solid catalyst placed in a reaction vessel is heated to a predetermined temperature, and the mixed gas is passed through the solid catalyst.

[0027] The composition of the mixed gas is not particularly limited. The volume ratio of the hydrogen content to the carbon monoxide content in the mixed gas (hydrogen / carbon monoxide) is preferably 0.01 or more and 1.00 or less. Considering the above-mentioned reaction mechanism, from the viewpoint of more efficiently proceeding with the reaction, it is more preferable that the volume ratio of hydrogen / carbon monoxide is 0.05 or more and 0.50 or less.

[0028] Methods for providing a mixed gas to a solid catalyst include a method in which two or more raw material gases are mixed in advance and the resulting mixed gas is provided to the solid catalyst, and a method in which two or more raw material gases are provided in predetermined amounts (e.g., at predetermined flow rates) to the solid catalyst. Examples of raw material gases include gases containing carbon monoxide as a main component and gases containing hydrogen as a main component. The mixed gas and raw material gas may contain components other than carbon monoxide and hydrogen. However, it is preferable that the content of components other than carbon monoxide and hydrogen in the mixed gas and raw material gas be as low as possible. Specifically, it is preferable that the content of carbon monoxide and hydrogen gas in the raw material gas is 99% or more by volume.

[0029] The temperature of the solid catalyst when the mixed gas is supplied is not particularly limited, but is preferably 500° C. or higher and 700° C. or lower in order to allow the catalytic reaction to proceed more efficiently.

[0030] For example, a mixed gas flow (growth gas) containing carbon monoxide and hydrogen is passed through the solid catalyst under heating conditions of 500°C or higher, whereby a fibrous nanocarbon material can be precipitated from the surface of the solid catalyst using carbon monoxide as a carbon source. In other words, the nanocarbon material can be recovered from the surface of the solid catalyst (third step).

[0031] In an experimental example, 100 mg of the solid catalyst was kept at 500°C or higher and 700°C or lower in a quartz reaction vessel. A mixed gas consisting of carbon monoxide and hydrogen, both with a purity of 99% or higher, was supplied to the vessel at a carbon monoxide:hydrogen ratio of 100 ml / min:10 ml / min for 1 hour. 20 atomic % of the carbon component constituting the supplied carbon monoxide was recovered as the solid nanocarbon material of this embodiment.

[0032] 3(b) is a schematic diagram of a catalytic reaction occurring on the crystal surfaces 11a and 11b of the solid catalyst 11. When a mixed gas is supplied as a growth gas to the solid catalyst heated to a predetermined temperature, a CO disproportionation reaction (2CO → CO2 + C) and / or a CO hydrogenation reaction (CO + H2 → H2O + C) occurs on the crystal surfaces 11a and 11b. As a result, a fibrous nanocarbon material 13 is formed from the crystal surfaces 11a and 11b. In other words, a reaction occurs in which the nanocarbon material 13 extends from one end (root 13a) on the crystal surfaces 11a and 11b toward the other free end (tip 13b).

[0033] Next, the structure of the nanocarbon material of this embodiment will be described. First, the procedure and results of the structural analysis of the experimentally obtained nanocarbon material will be described.

[0034] The nanocarbon material was mechanically peeled off and collected from the surface of the composite oxide material using a spatula. The collected nanocarbon material was characterized using X-ray diffraction, Raman spectroscopy, infrared spectroscopy, scanning electron microscope, and transmission electron microscope. The analytical results of X-ray diffraction, Raman spectroscopy, and infrared spectroscopy are omitted here.

[0035] Samples for observation with electron microscopes (scanning electron microscopes, transmission electron microscopes) were prepared according to the following procedure. First, 1 mg of nanocarbon material was dispersed in 10 ml of ethanol to prepare a dispersion. Next, this dispersion was dropped onto a collodion film-coated copper grid and dried. This was used as a sample for observation with an electron microscope.

[0036] The transmission electron microscope used was a JEOL JEM-ARM200F. The measurement conditions were an electron beam acceleration voltage of 200 keV and a convergent electron beam diameter of less than 0.1 nm. Under these conditions, a quantitative evaluation of the density of exposed graphite edge planes on the surface of the nanocarbon material was carried out.

[0037] The observation area is an arbitrary 50 × 50 nm area on the surface of the nanocarbon material. 2 The size of this range was selected taking into consideration a length of more than 100 times the spacing of the basal planes of graphite (0.335 nm). When calculating the surface density, the obtained transmission electron microscope images were processed using a predetermined procedure. Specifically, periodic components with wavelengths more than 10 times and less than one-tenth the spacing of the basal planes of graphite (0.335 nm) were first regarded as noise and removed using numerical calculations. The images were then binarized.

[0038] A striped pattern consisting of alternating black and white lines was extracted from the binarized transmission electron microscope image, and each black line was confirmed to correspond to a single graphite basal plane constituting the nanocarbon material. The boundary between the area occupied by the striped pattern and the background area where the pattern is not visible corresponds to the projection image of the tangent between the surface of the material and a plane perpendicular to the electron beam, and this projection image was defined as the outer side surface of the material. The number of black lines in the striped pattern cutting the outer side surface per unit length was defined as the density of exposed graphite edge planes on the material surface. The angle between the stripe and the outer side surface was defined as the angle α between the basal plane and the outer side surface. When calculating the density of exposed edge planes, fractional quotients were truncated. That is, the fractional quotient obtained by dividing the spacing between the basal planes of graphite by the length of the outer side surface was truncated. The density of exposed edge planes was quantified without averaging within the above-mentioned observation range, including arbitrarily selected points on the outer side surface.

[0039] Next, the results of the structural analysis will be explained. Figure 4(a) shows a scanning electron microscope image of the nanocarbon material, Figure 4(b) shows a transmission electron microscope image of the nanocarbon material, Figure 5 shows the results of element mapping, and Figure 6 shows a transmission electron microscope image (high resolution) of the vicinity of the surface of the nanocarbon material.

[0040] In Figure 4(a), the bright areas are the fibrous structure (also called "fibers") of the nanocarbon material. Figure 4(a) shows that the nanocarbon material is made up of a fibrous structure with a diameter of several tens of nanometers and a length of 1 μm or more. It can also be seen that this fibrous structure is periodically twisted along the longitudinal direction, forming a helical shape. In other words, it can be seen that the fibrous structure extends in the longitudinal direction while rotating three-dimensionally. The scale is shown at the bottom of the image.

[0041] In Figure 4(a), the area (inside) defined by the bright outline due to contrast difference is the fibrous structure of the nanocarbon material. Figure 4(b) shows that the nanocarbon material is stretched and folded back at a certain angle in a zigzag pattern. The scale bar is shown at the bottom of the image.

[0042] Figure 5 shows the results of elemental mapping of the image in Figure 4(b), focusing on the darker area where two fibrous structures intersect. Figure 5(a) shows the distribution of iron atoms. The bright areas due to contrast differences indicate the locations of specific atoms (iron atoms in this figure). Figure 5(b) shows the distribution of carbon atoms, Figure 5(c) shows the distribution of oxygen atoms, and Figure 5(d) shows the distribution of non-ferrous metal atoms. From the results in Figures 5(a) to 5(d), it can be seen that the particularly dark areas in the image in Figure 4(b) are images derived from the solid catalyst. It can also be seen that the fibrous structures extending from the surface of the solid catalyst in the direction of the solid catalyst's surface (with the direction of the surface being the longitudinal direction) are composed of carbon atoms. These results reveal that one end (root) of each of the two fibrous structures is attached to one solid catalyst (particle). It is also apparent that the tips of the two fibrous structures (the other ends in the longitudinal direction relative to the base) are free ends (exposed) that are not fixed to the solid catalyst.

[0043] From Figure 6, it can be seen that the basal planes of the graphite constituting the nanocarbon material are aligned at a predetermined angle relative to the outer peripheral side surface 13d. The angle α between the basal plane and the outer peripheral side surface was measured to be 31±4°. The density of exposed edge surfaces calculated using the above method was 15±2 per 10 nm, which is consistent with the actual count of the number of striped patterns along the outer peripheral side surface: 13 / 10 nm. The density of exposed edge surfaces of the nanocarbon material of this embodiment is 10 per 10 nm or more, preferably 10 to 30 per 10 nm, more preferably 1 to 30 per 10 nm, even more preferably 12 to 30 per 10 nm, and particularly preferably 13 to 30 per 10 nm.

[0044] The experimentally confirmed structure of the nanocarbon material will be described in detail. Figure 7 is a schematic diagram of the nanocarbon material 13. Figure 8 is an enlarged view of a portion of the spiral structure of the nanocarbon material 13. Figure 9 is a diagram showing the edge surface of the nanocarbon material.

[0045] The nanocarbon material 13 can be generated so as to extend from the surface of the solid catalyst 11. In this case, the length L of the fibrous texture was confirmed to be 1 μm or more from the electron microscope image. Furthermore, the electron microscope image also confirmed the existence of a fibrous texture having a length L of 2 μm or more and a fibrous texture having a length L of 3 μm or more. The length L of the fibrous texture is preferably 1 μm or more and 20 μm or less, more preferably 2 μm or more and 20 μm or less, and even more preferably 3 μm or more and 20 μm or less.

[0046] The electron microscope image confirmed that the diameter D (thickness) of the fibrous structure of the nanocarbon material 13 was 5 nm or more and 200 nm or less. The electron microscope image also confirmed that the diameter D of the fibrous structure was 10 nm or more and 500 nm or less.

[0047] The electron microscope image confirmed that the nanocarbon material 13 was periodically twisted (curved) along its longitudinal direction, giving it a helical shape. As shown in Figure 8, the center line C of the fibrous structure of the nanocarbon material 13 was zigzag in side view. The pitch angle φ of the folded portion was 30° or more and 90° or less. The pitch angle φ was defined as 1 / 2 of the angle of the curved portion of the center line C (180° or less).

[0048] 9 is an enlarged view of outer peripheral side surface 13d of nanocarbon material 13. Edge surface 1e of graphite 1 is exposed along outer peripheral side surface 13d. Angle α (angle in side view) formed between basal plane 1b of graphite 1 and outer peripheral side surface 13d corresponds to angle θ formed between center line C and basal plane 1 of nanocarbon material 13. That is, in nanocarbon material 13, angle α formed between basal plane 1b and outer peripheral side surface 13d and angle θ formed between center line C and basal plane 1 are each 30° or more and 60° or less.

[0049] As described above, the nanocarbon material of this embodiment has a curved outer peripheral side surface on which graphite edge faces are exposed at a density of 10 / 10 nm or more, and is expected to have superior reactivity compared to conventional carbon materials, which have an exposed edge face density of about 8 / 10 nm.

[0050] In conventional carbon materials, the angle between the outer peripheral side surface and the basal plane of graphite is less than 30°. In contrast, nanocarbon materials in which the angle between the outer peripheral side surface and the basal plane of graphite is between 30° and 60° are expected to have superior reactivity.

[0051] Furthermore, if the outer side surface of the nanocarbon material is the outer side surface of a fiber having multiple graphite particles, it can be easily applied to applications similar to conventional carbon nanofibers and is expected to have better functionality.

[0052] Furthermore, when the fiber has a helical shape, the density of exposed edge surfaces tends to be higher compared to conventional straight carbon nanofibers, which is expected to result in further improved reactivity.

[0053] Furthermore, when the fiber thickness is 10 nm to 500 nm, the fiber length is 1 μm or more, and the helical pitch angle is 30° to 90°, the exposed density of the edge surface tends to be higher compared to conventional linear carbon nanofibers, which is expected to result in further improved reactivity.

[0054] Although the embodiments of the nanocarbon material have been described above, the nanocarbon material of the present invention is not limited to the above embodiments and can be appropriately modified within the scope of achieving the desired effects. [Explanation of symbols]

[0055] 1. Graphite 1b Basal side 1e Edge surface 3 Multi-layer carbon nanofiber 3a Axial direction 3b Inner side 3c Outer side 11 Solid catalysts 11a Crystal surface 11b Crystal surface 13 Nanocarbon materials 13a base 13b tip 13d outer surface 13d Peripheral side

Claims

1. A nanocarbon material having a curved outer peripheral side surface on which graphite edge faces are exposed at a density of 10 per 10 nm or more.

2. 2. The nanocarbon material according to claim 1, wherein the angle between the outer peripheral side surface and the basal plane of the graphite is 30° or more and 90° or less.

3. 3. The nanocarbon material according to claim 1, wherein the outer peripheral side surface is an outer peripheral side surface of a fiber having a plurality of edge surfaces of the graphite.

4. 4. The nanocarbon material according to claim 3, wherein the fiber has a helical shape.

5. The nanocarbon material according to claim 4, characterized in that the thickness of the fiber is 10 nm or more and 500 nm or less, the length of the fiber is 1 μm or more, and the pitch angle of the spiral shape is 30° or more and 90° or less.

6. 6. The nanocarbon material according to claim 3, wherein each end of the two fibers is fixed to one catalyst particle.

7. The nanocarbon material according to claim 6, wherein the end of the fiber opposite to the end fixed to the catalyst particle is a free end.

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