JP2006281168A - Catalyst for manufacturing double-layer carbon nanotube, and manufacturing method of double-layer carbon nanotube using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a catalyst for manufacturing a double-layer nanotube excellent in field emission characteristics with a high selectivity and yield, and its manufacturing method. <P>SOLUTION: The catalyst is employed to manufacture the double-layer carbon nanotube comprising at least one kind of representative elements of 0.01-5 mol parts selected from zinc, copper, and tin, and sulfur of 0.01-5 mol parts to at least one kind of transition metal elements of 1 mol part selected from iron, cobalt, and nickel. Further, the manufacturing method of the double-layer carbon nanotube comprises the steps of vaporizing the catalyst in the atmosphere consisting of at least one kind of inert gases selected from hydrogen and carbon hydride together with carbon, and thereafter, of condensing it. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a catalyst for producing a double-walled carbon nanotube and a method for producing a double-walled carbon nanotube using the same, and more particularly, to a double-walled carbon nanotube having excellent field emission characteristics by an arc discharge method. The present invention relates to a catalyst that can be selectively produced and a method for producing double-walled carbon nanotubes using such a catalyst. Furthermore, this invention relates to the electron emission material which consists of a double-walled carbon nanotube obtained in this way.

  The carbon nanotube is a carbon substance having a structure in which a sheet made of graphite (hereinafter referred to as a graphite layer) is wound in a hollow cylindrical shape, and the diameter of the cylinder made of the graphite layer is about 1 to 30 nm. . Such carbon nanotubes are generally divided into single-walled nanotubes having a single cylindrical structure (for example, see Patent Document 1) and multi-walled nanotubes in which two or more graphite layers are concentrically overlapped (for example, see Patent Document 2). Research into application is progressing in various fields because of its unique structure and characteristics. In particular, carbon nanotubes have a field emission characteristic, and have been attracting attention as an electron emission material for a display using this characteristic.

  However, as this electron emission material, single-walled nanotubes have excellent initial characteristics but low durability. On the other hand, multi-walled nanotubes have high durability, but have high electron emission voltage, both of which are practical. There was a problem. In contrast, double-walled nanotubes emit electrons at the same low voltage as single-walled nanotubes, and have a long life and excellent durability. Therefore, they are attracting attention as a new material for the practical application of field emission displays. ing.

  Under such circumstances, there has been a demand for a carbon nanotube production method capable of controlling the number of layers, and recently several selective production methods for double-walled nanotubes have been found (for example, Patent Document 3). And 4).

  Conventionally, various methods for producing carbon nanotubes are known (see, for example, Non-Patent Document 1). Basically, carbon is evaporated in a gas phase (possibly together with a catalyst). Can be obtained by condensing. Specifically, for example, a laser evaporation method using a pulse laser, an arc discharge method in which an arc discharge such as direct current, alternating current, or pulse is caused between carbon electrodes to evaporate carbon from an anode, a high temperature furnace using a carbon source as a gas. A gas phase method to be introduced into the inside is known.

  According to such a method, the carbon nanotube is obtained as a substance contained in the soot mixed with amorphous carbon, graphite-like nanoparticles, metal fine particles used for the catalyst, and the like. However, the appearance of the carbon nanotube itself is a bowl-like substance.

However, according to the conventional method for producing the double-walled carbon nanotube as described above, the catalyst is mainly composed of an expensive metal such as nickel, cobalt, yttrium, or a noble metal such as palladium or platinum. (For example, refer to Patent Document 5), and after the reaction, these metal components are mixed into the obtained nanotubes, causing a decrease in physical properties such as heat resistance and field emission characteristics. However, it is usually not easy to remove such a metal component by acid treatment or the like, and when the acid treatment is repeatedly performed to remove a considerable amount of the metal component, the nanotube itself is also lost. There was a problem that the yield was drastically reduced, and further, the field emission characteristics and the like of the nanotube were also lowered.
Japanese Patent Laid-Open No. 08-091816 Japanese Patent Laid-Open No. 05-146592 Japanese translation of PCT publication No. 02-503334 JP 2003-034515 A JP 2000-095509 A “Hideki Inakura,“ Carbon Nanotube ”, Material Integration (published by T.I.C.), Vol. 17, No. 6, pp. 11-14, 2004)

  The present invention has been made to solve the above-mentioned problems in the production of carbon nanotubes, and provides a method for producing double-walled nanotubes having excellent field emission characteristics with high selectivity and yield. The purpose is to do. A further object of the present invention is to provide an electron emission material comprising double-walled carbon nanotubes produced by such a method.

  According to the present invention, with respect to 1 mol part of at least one transition metal element selected from iron, cobalt and nickel, 0.01 to 5 mol parts of at least one typical element selected from zinc, copper and tin, and A catalyst for producing a double-walled carbon nanotube comprising 0.01 to 5 mole parts of sulfur is provided.

  According to the present invention, there is also provided a double-walled carbon nanotube characterized in that the catalyst is evaporated in an atmosphere composed of at least one selected from hydrogen and hydrocarbons together with carbon and an inert gas, and then condensed. A manufacturing method is provided.

  In particular, as a preferred embodiment of the present invention, in a vacuum reaction vessel, an atmosphere containing at least one selected from hydrogen and hydrocarbons and an inert gas, wherein the anode containing the catalyst and carbon is opposed to the cathode made of carbon. A method for producing a double-walled carbon nanotube is provided in which arc discharge is performed between the anode and the cathode, and the anode is evaporated and then condensed.

  Furthermore, according to the present invention, an electron emission source comprising a double-walled carbon nanotube obtained by the above-described method is provided.

  According to a preferred embodiment of the catalyst according to the present invention, iron is used as a transition metal, which is a combination of a certain typical element and sulfur. In high yields. Moreover, according to the present invention, the double-walled carbon nanotube obtained by using such a catalyst has excellent heat resistance, and has little loss (decrease) or deterioration of characteristics due to heating. For example, an electron-emitting material for display Can be suitably used.

  In addition, by treating the soot containing the double-walled nanotubes with an acid, the above-mentioned metal components mixed in the soot can be easily dissolved and removed, so that the acid loss can be avoided without losing the nanotubes or degrading their properties. The obtained double-walled nanotube can be easily purified by removing the metal component by the combustion and burning and removing amorphous carbon in the air. Furthermore, the manufacture of a field emission display includes a step of firing the substrate at a temperature of several hundred degrees, but even in such a case, the double-walled carbon nanotube obtained by the method according to the present invention has little deterioration in characteristics. .

  The catalyst for producing the double-walled carbon nanotube according to the present invention has at least one typical selected from zinc, copper and tin with respect to 1 mol part of at least one transition metal element selected from iron, cobalt and nickel. It consists of 0.01 to 5 mole parts of element and 0.01 to 5 mole parts of sulfur. Preferably, the catalyst according to the present invention has at least one typical element selected from zinc, copper and tin in an amount of 0.05 to 1 mole part of at least one transition metal element selected from iron, cobalt and nickel. 3 mol parts, particularly preferably in the range of 0.1-1 mol parts, and sulfur in the range of 0.05-3 mol parts, particularly preferably in the range of 0.1-1 mol parts.

  According to the present invention, the transition metal element constituting the catalyst, the typical element and the catalyst composed of sulfur may be partially or entirely in the form of sulfide.

  According to the present invention, iron is particularly preferable among the transition metal elements, and zinc and copper are preferable among the typical elements, and zinc is particularly preferable. Therefore, in particular, according to the present invention, at least one selected from zinc and copper, or 0.05 to 1 mol part of zinc and 0.05 to 1 mol part of sulfur, with respect to 1 mol part of iron. Preferably, a catalyst comprising at least one selected from zinc and copper, or 0.1 to 0.5 mole parts of zinc and 0.1 to 0.5 mole parts of sulfur is most preferable with respect to 1 mole part of iron. . In this case, at least one selected from cobalt and nickel may be used in combination with iron in a range of up to 90 mol% in total.

  According to the present invention, the catalyst is evaporated in an atmosphere composed of at least one selected from hydrogen and hydrocarbons together with carbon and an inert gas, and then condensed to selectively obtain double-walled carbon nanotubes. be able to.

  Nitrogen, argon, helium or the like is used as the inert gas, and among them, argon is preferably used. As the hydrocarbon, any hydrocarbon can be used as long as it is gaseous at room temperature, in particular, if it has 1 to 4 carbon atoms, regardless of whether it is saturated or unsaturated. Examples thereof include methane, ethane, ethylene, propylene, acetylene and the like. However, according to the present invention, an atmosphere composed of argon and hydrogen is particularly preferably used. The ratio of at least one selected from hydrogen and hydrocarbons / inert gas is usually at least one selected from hydrogen and hydrocarbons, preferably 10 to 100% by volume of hydrogen. The atmospheric pressure is usually in the range of 100 to 760 Torr.

  According to a preferred aspect of the present invention, an atmosphere comprising at least one selected from hydrogen and hydrocarbons and an inert gas, in a vacuum reaction vessel, the anode containing the catalyst and carbon is opposed to the cathode made of carbon. A double-walled carbon nanotube can be selectively obtained by performing arc discharge between the anode and the cathode under a predetermined pressure, evaporating the anode, and then condensing in the gas phase.

  FIG. 1 shows a vacuum reactor that can be suitably used to obtain double-walled carbon nanotubes by the method as described above. In this vacuum reactor, the above-described anode 2 containing the catalyst and carbon and the cathode 3 made of carbon are disposed in the reaction vessel 1 so as to face each other, and the anode is preferably made of graphite, A hole is formed along the central axis from the surface facing the surface, and the catalyst is filled in the hole. The anode is attached to the position adjusting means 4 so that the distance between the anode and the cathode can be freely adjusted while being opposed to the cathode and can be moved little by little toward the cathode. The electrode and catalyst are not limited to the above examples in the form and structure of the carbon material used in them, and the catalyst is not limited to the above examples. It may be formed into a shape.

  To be precise, the anode is, for example, a metal electrode including a catalyst and carbon attached to the tip of a metal electrode, and the cathode is similarly a carbon electrode attached to the tip of the metal electrode. The anode is connected to the positive terminal of the arc power source 6 via the positive terminal 5 attached to the reaction vessel. Similarly, the cathode is connected to the negative terminal 7 attached to the reaction vessel, and is connected to the negative terminal of the arc power source through this terminal. The reaction vessel is connected to an evacuation device (not shown) by an exhaust pipe 8 and to a gas introduction pipe 9 having a flow rate adjusting valve in order to make a required vacuum inside. A required atmospheric gas is introduced into the evacuated reaction vessel at a predetermined flow rate to obtain a required atmospheric pressure. A pressure sensor 10 for measuring the pressure in the reaction vessel is attached to the reaction vessel.

  In order to selectively produce double-walled carbon nanotubes using such a vacuum reactor, first, after evacuating the inside of the reaction vessel, the required atmospheric gas is introduced to the required pressure, and then the anode and After setting the distance from the cathode to a predetermined value, a DC voltage is applied between the anode and the cathode from the arc power source, and a DC arc discharge is generated between the anode and the cathode, so that the anode evaporates from its tip. Begin to. The anode thus decreases in length as it evaporates, during which time the evaporated catalyst and carbon condense in the gas phase and grow in columns toward the anode at the cathode tip. However, it accumulates to form a deposit, and as soot, soot adheres to the inner wall of the reaction vessel together with the metal fine particles.

  Since the temperature in the reaction vessel rises during the arc discharge, cooling water is preferably circulated through the walls of the reaction vessel to cool it, and the anode and cathode also cool the metal electrode portion. To do. In addition, the deposit tip on the cathode is maintained at a constant interval so that the tip of the anode that grows on the cathode as described above is consumed by evaporation and the length of the anode tip decreases. Along with the growth, the anode is moved by the position adjusting means, and the distance between the electrodes is kept constant, and the discharge is stably performed. The distance between the electrodes is usually about 0.5 to 2 mm. In this way, normally, after arc discharge until the anode is consumed, the discharge and introduction of atmospheric gas are stopped, a sufficient cooling time is allowed, and the inside of the reaction vessel is evacuated as necessary. After exhausting, open to atmosphere.

  In general, various fullerenes and nanotubes can be obtained by changing the component composition of the catalyst or adjusting the composition and pressure of the atmosphere. Is contained in the soot attached to the wall of the reaction vessel together with amorphous carbon and metal fine particles derived from the catalyst, and by collecting this soot, a double-walled nanotube can be obtained. According to the present invention, if carbon nanotubes are produced by the arc discharge method using the above catalyst as described above, the high-concentration double-walled nanotubes are collected in a cloth shape. Single-wall nanotubes can be obtained with high yield. In fact, it was confirmed by TEM observation that most of the carbon nanotubes contained in the cloth-like cocoons are double-walled nanotubes. Therefore, if the cocoon packed in the cloth shape is purified, higher-purity double-walled nanotubes can be obtained. On the other hand, the deposit on the cathode mainly includes two or more multi-walled nanotubes.

  As described above, in particular, according to the present invention, the catalyst is preferably composed of iron, zinc and sulfur, and the soot containing double-walled nanotubes obtained using such a catalyst is treated with an acid. By doing so, the metal component derived from the catalyst can be easily dissolved and removed. Furthermore, when the soot contains metal fine particles coated with carbon, it is difficult to dissolve and remove the metal component even if the soot is acid-treated. Since the carbon covering the metal component can be burned and removed by firing at a temperature of 300 to 600 ° C. in the air, for example, according to the present invention, after firing the soot in this way, By performing the acid treatment again, a high-purity double-walled carbon nanotube can be obtained.

  The double-walled nanotube obtained by the method of the present invention is less susceptible to nanotube loss and deterioration of its characteristics even when purification is performed in this manner. On the other hand, when a soot containing carbon nanotubes using a conventional catalyst is heated and fired at a temperature of 300 ° C. or higher, the loss of the nanotubes is remarkably reduced, and the characteristics are also deteriorated.

  EXAMPLES The present invention will be described below with reference to examples, but the present invention is not limited to these examples.

(Production of double-walled carbon nanotubes and evaluation of their thermal properties)
Example 1
A soot containing double-walled carbon nanotubes was obtained by a direct current arc discharge method using the above-described apparatus. That is, a 3 mm diameter hole is drilled along a central axis in a graphite rod having a diameter of 9 mm and a length of 220 mm, and an iron / zinc / sulfur molar ratio of 3/1/1 iron powder, zinc powder and sulfur is formed in the hole. A mixture filled with a 1: 1 weight ratio mixture of the mixture and graphite powder was used as the anode, and a graphite rod was used as the cathode. The atmosphere gas in the vacuum reaction vessel was a mixed gas of argon: hydrogen = 4: 1 (volume ratio), the pressure was 350 Torr, and the whole wall of the vacuum reaction vessel was water-cooled during discharge. After the reaction, the soot deposited on the inner wall excluding the bottom of the reaction vessel where the slag is deposited and around the cathode is treated with a sieve (500 μm). As described above, the soot gathered in a cloth shape with a high content of double-walled nanotubes. It was collected as a soot containing nanotubes. As described above, it was confirmed by observation with a TEM (transmission electron micrograph) that most of the carbon nanotubes contained in the cloth-like bag were double-walled nanotubes.

  Table 1 shows the yield of the cocoons in the above-mentioned cloth shape including the double-walled carbon nanotubes and the content of the double-walled nanotubes in the cocoons. In Table 1, “yield” means the weight of the above-mentioned cloth-like soot containing the double-walled nanotubes with respect to the consumption (weight) of the graphite rod filled with the catalyst, expressed as a percentage. , Which means the proportion of nanotubes in the total carbonaceous matter in the cloth containing the double-walled nanotubes, and the amount of fibrous material by the nanotubes in the observation by SEM (scanning electron micrograph) When it was 20% or more, it was H (high), when it was 10% or more, and less than 20%, it was M (medium), and when it was less than 10%, L (low).

  Moreover, the SEM photograph of the soot containing the double-walled carbon nanotube obtained in this example is shown in FIG. 2, and the SEM photograph after firing at 400 ° C. for 10 minutes in the air is shown in FIG. There was little aggregation of particles and loss of nanotubes.

Furthermore, the cloth-like soot containing the double-walled nanotubes obtained in this example was dispersed in acetone, filtered using a filter paper, and deposited in a sheet form on the filter paper. The sheet-like material is affixed to a disc-shaped sample stage with a diameter of 30 mm in a vacuum vessel of a field emission characteristic measuring apparatus with a conductive epoxy resin as a cathode, and a stainless steel cylinder with a diameter of 5 mm facing the anode is used as an anode. The inside of the vacuum vessel was set to a degree of vacuum of 10 ⁻⁶ Pa, and a direct-current voltage was applied between these electrodes, and the field emission characteristics of the soot that was bundled in a cloth shape containing the above-mentioned double-walled nanotubes were measured. The results are shown in FIG. 4 as IV (current-voltage) curves. In FIG. 4, the horizontal axis represents applied voltage / interelectrode distance (V / μm), and the vertical axis represents current density due to field emission, that is, current / anode area (μA / cm ² ). As shown in FIG. 4, the cloth-like soot containing the double-walled nanotube according to the present invention emits field emission electrons with a low applied voltage.

  Next, the wrinkles gathered in the cloth shape containing the above-mentioned double-walled nanotubes were baked at 400 ° C. for 10 minutes, and the field emission characteristics were measured in the same manner. The results are shown in FIG. The soot containing the double-walled nanotubes according to the present invention has little reduction in field emission characteristics even after firing.

  Further, the double-walled nanotubes obtained in this example were baked in air at 400 ° C. for 10 minutes, and then observed with a state before firing and an SEM photograph to examine their heat resistance. The case where there was almost no decrease in the nanotubes after firing was indicated as ◯, the case where the nanotubes were slightly reduced after firing, and the case where there was agglomeration of particles and a significant decrease in the nanotubes after firing. The results are shown in Table 1.

Examples 2-11
In Example 1, except that the catalyst shown in Table 1 was used, the soot collected in a cloth shape containing the double-walled nanotubes was recovered, and the yield of the soot and the content of the double-walled nanotubes in this soot were collected. The rate and heat resistance are shown in Table 1. According to the present invention, if a double-walled nanotube is produced using a catalyst that is a combination of a transition metal and sulfur and a typical element, the resulting double-walled nanotube is a conventionally known catalyst composed of a transition metal and sulfur. Compared to the double-walled nanotubes obtained by using them, the heat resistance is improved.

  The field emission characteristics of the soot containing the double-walled nanotube obtained in Example 2 were measured in the same manner as in Example 1. The results are shown in FIG. 4 as an IV curve. Further, in the same manner as in Example 1, the soot containing this double-walled nanotube was baked at 400 ° C. for 10 minutes, and the field emission characteristics were measured in the same manner. The results are shown in FIG. The soot containing the double-walled nanotubes according to the present invention has little reduction in field emission characteristics even after firing.

Comparative Examples 1-7
In Example 1, except that the catalyst shown in Table 1 was used, the soot deposited on the inner wall and the cathode except for the bottom of the reaction vessel where slag was deposited was collected as soot containing nanotubes after the reaction. Table 1 shows the yield of the soot and the content of the double-walled nanotube in the soot. However, in Table 1, in Comparative Examples 5 and 6, since the total amount of soot passed through the sieve, the yield of soot containing carbon nanotubes was set to zero. Further, it was confirmed that the carbon nanotubes were not contained in the basket.

  FIG. 6 shows an SEM photograph of the soot containing the double-walled carbon nanotubes obtained in Comparative Example 1, and FIG. 7 shows an SEM photograph after baking in air at 400 ° C. for 10 minutes in the same manner as in Example 1. Thus, after firing, significant loss of nanotubes was observed along with particle aggregation.

  The field emission characteristics of the soot containing the double-walled nanotube obtained in Comparative Example 1 were measured in the same manner as in Example 1. The results are shown in FIG. Further, in the same manner as in Example 1, the soot containing this double-walled nanotube was baked at 400 ° C. for 10 minutes, and the field emission characteristics were measured in the same manner. The results are shown in FIG. In the soot containing the double-walled nanotube, the field emission characteristics after firing are significantly reduced.

It is sectional drawing which shows an example of the vacuum manufacturing apparatus for manufacturing a double-walled carbon nanotube according to the method of this invention. 2 is a SEM photograph of a cocoon containing double-walled nanotubes obtained in Example 1. It is a SEM photograph after baking the soot containing the double-walled nanotube obtained in Example 1. It is a current-voltage curve which shows the field emission characteristic of the soot containing a double-walled nanotube. It is a current-voltage curve which shows the field emission characteristic after baking the soot containing a double-walled nanotube. 4 is a SEM photograph of a cocoon containing double-walled nanotubes obtained in Comparative Example 1. It is a SEM photograph after baking the soot containing the double-walled nanotube obtained in Comparative Example 1.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Reaction container 2 ... Anode 3 ... Cathode 4 ... Position adjustment means 6 of an anode ... Arc power supply

Claims (6)

  0.01 to 5 mole parts of at least one typical element selected from zinc, copper and tin and 0.01 to 5 sulfur with respect to 1 mole part of at least one transition metal element selected from iron, cobalt and nickel A catalyst for producing double-walled carbon nanotubes composed of molar parts.   0.01 to 5 mole parts of at least one typical element selected from zinc, copper and tin and 0.01 to 5 sulfur with respect to 1 mole part of at least one transition metal element selected from iron, cobalt and nickel A method for producing a double-walled carbon nanotube, comprising evaporating a catalyst composed of a molar part in an atmosphere composed of at least one selected from hydrogen and hydrocarbons together with carbon and an inert gas, and then condensing the catalyst.   In a vacuum reaction vessel, at least one kind of typical element selected from zinc, copper and tin is 0.01 to 5 parts by mole with respect to 1 part by mole of at least one kind of transition metal element selected from iron, cobalt and nickel, and An anode containing 0.01 to 5 mole parts of sulfur and a carbon-containing anode is opposed to a cathode made of carbon, and the anode and cathode are in an atmosphere containing at least one selected from hydrogen and hydrocarbons and an inert gas. A method for producing a double-walled carbon nanotube, characterized in that arc discharge is performed between the electrode and the anode is evaporated and then condensed.   The method for producing a double-walled carbon nanotube according to claim 2 or 3, wherein the atmosphere in the vacuum reaction vessel comprises 10 to 100% by volume of at least one selected from hydrogen and hydrocarbons and 90 to 0% by volume of an inert gas.   The method for producing a double-walled carbon nanotube according to any one of claims 2 to 4, wherein the atmosphere in the vacuum reaction vessel is maintained at a pressure of 100 to 760 Torr. An electron emitting material comprising a double-walled carbon nanotube obtained by the method according to claim 2.