Supported metal catalyst for synthesizing carbon nanotubes by low-temperature thermal chemical vapor deposition and method of synthesizing carbon nanotubes using the same

Abstract

The present invention discloses a supported metal catalyst useful in synthesizing carbon nanotubes by low-temperature (<600° C.) thermal chemical vapor deposition (CVD), which contains particles of a noble metal having a diameter of 0.1-10 microns as a support and a metal catalyst deposited on the support. The metal catalyst is iron, cobalt, nickel or an alloy thereof. The weight ratio of the metal catalyst to the support ranges from 0.1:100 to 10:100. The present invention also discloses a method of synthesizing carbon nanotubes directly on a substrate by low-temperature thermal CVD, wherein the support is not needed to be removed from the substrate after growth of carbon nanotubes.

Description

FIELD OF THE INVENTION

[0001] The present invention relates to a process for producing carbon nanotubes, and particularly to a supported metal catalyst for synthesizing carbon nanotubes by a low temperature thermal chemical vapor deposition (CVD).

BACKGROUND OF THE INVENTION

[0002] Carbon nanotubes have very special properties, such as low density, high strength, high toughness, high flexibility, high surface area, high surface curvature, high thermal conductivity, and excellent electric conductivity, etc. That is why carbon nanotubes have attracted many researchers to study on the possible applications of the carbon nanotubes which include: composite material, microelectronic components, flat displays, radio communication, fuel cells, and lithium cells, etc. Carbon nanotube field emission displays (CNT-FED) are novel flat displays that have a great potential. Usually, a process for producing a large CNT-FED comprises: mixing carbon nanotubes with a conductive paste; coating the paste mixture on the surface of a conductive glass substrate by a screen printing technique, or the like; sintering the composite at 450-550° C. to remove the polymeric material in the paste mixture, thereby forming an electron emissive film having a good electrical conductivity. Such a CNT-FED production process requires several steps and uses a technique that is somehow cumbersome. Furthermore, the carbon nanotubes are difficult to be uniformly distributed in said conductive paste.

[0003] At present, the processes for producing nanotubes for use in the CNT-FED include: arc discharge, laser vaporization, and thermal CVD, etc. The carbon nanotube products prepared by the arc discharge process and the laser vaporization process not only are difficult to be controlled as to the length and the diameter thereof, but they are produced in a rather low yield. Furthermore, those processes will generate a large amount of graphite particles, so that further purification treatments are required. Moreover, these processes require a fabrication temperature exceeding 1000° C. such that carbon nanotubes can not be produced directly on a glass substrate. Therefore, it is widely recognized that a thermal CVD has the best possibility for producing carbon nanotubes at a lower temperature.

[0004] In the past, a process for producing carbon nanotubes by a thermal CVD uses an active metal catalyst deposited on a porous support such as silica, zeolite, alumina or magnesium oxide by impregnation. The main reason in selecting the abovementioned supports is that such supports are stable inert oxides, and they will not react with the active metal catalyst inadvertently during a heating process, so that the active metal catalyst can catalyze a synthesis reaction of the carbon nanotubes as desired. The active metal mainly comprises: Fe, Co or Ni, and a minor quantity of other metals, such as Cu, Mo, Mn, Zn or Pt, etc., for adjusting the catalytic activities. The reaction conditions of using an active metal catalyst, which is deposited on a support, to catalyze a carbon accumulation reaction for forming carbon nanotubes include: introducing an inert gas (He, Ar, or N2), hydrogen and a carbon source gas into a reactor at a reaction temperature of 650-1000° C. and a pressure of 1-2 atm for a reaction time of 1-120 min. The carbon source used includes: a hydrocarbon or carbon monoxide (CO). Upon completion of the reactions, the support needs to be removed by acid washing in order to obtain purer carbon nanotubes for use in CNT-FED or other applications.

[0005] Generally speaking, the strain temperature of the calcined temperature resistant glass can reach up to 650° C., while the strain temperature of the sodium glass is about 550° C. or lower. Therefore, if the thermal CVD is used to directly grow carbon nanotubes on the surface of the glass substrate, the thermal CVD temperature can not exceed the strain temperature of the glass substrate, i.e. preferably lower than 650° C. However, the thermal CVD temperature cannot be too low, since the catalytic activity of the thermal CVD catalyst will be reduced and become insufficient for use in the synthesis of the carbon nanotubes. Therefore, it is necessary to develop a high catalytic activity catalyst system which can be used in synthesizing the carbon nanotubes at a temperature lower than 650° C.

[0006] European Patent Application No. 1061041 A1 discloses a low temperature CVD device and a method for synthesizing carbon nanotubes using such a device. The method comprises dividing a reaction pipe in the device into a space adjacent to the gas input part, a first zone for pyrolyzing the input gases, and a space adjacent to the gas exhaust part, a second zone for synthesizing carbon nanotubes by using the resulting pyrolyzed gases; and maintaining the temperatures of the two zones so that the temperature of the second zone is lower than the temperature of the first zone. Two different catalyst substrates are used in the synthesizing zone of carbon nanotubes, wherein one substrate has an assist catalyst such as Pd, Cr and Pt, etc., which is mainly used to accelerate the pyrolysis of acetylene; the other substrate is deposited with a catalyst layer containing Fe, Co, Ni or an alloy thereof, which is a catalyst for synthesizing the carbon nanotubes. Said other catalyst substrate having a catalyst membrane containing Fe, Co, Ni or an alloy thereof is corroded by an etching gas to form nano-grade catalytic particles. The abovementioned device is used to pyrolyze a carbon source gas in the first zone by the assist catalyst. Then, in the second zone, the carbon source gas, which has been decomposed, is used to grow perpendicularly aligned carbon nanotubes on each isolated nano-grade catalytic particle on the substrate by the thermal CVD at a temperature equal to or lower than the strain temperature of the substrate. This prior art technique, in addition to using a low temperature reaction zone of 450-650° C., still needs to pyrolyze the carbon source gas (first zone) at a high temperature of 700-1000° C., and is not a pure low temperature process. This prior art technique also needs to use a special CVD reactor. Furthermore, in this prior art technique, it is necessary to form two types of metal catalyst layers on two substrates, and the two substrates are mounted in the thermal CVD such that the two metal layers are facing each other at a clearance. Obviously, this prior art technique is complex, costly, and difficult to be implemented.

[0007] European Patent Application No. 1061043 A1 discloses a method for synthesizing carbon nanotubes at a low temperature by using a metal catalyst layer, which comprises: forming a metal catalyst layer on a substrate, wherein said metal catalyst layer is etched to form isolated nano-grade catalytic metal particles; and growing perpendicularly aligned carbon nanotubes on each isolated nano-grade catalytic particle on the substrate by a thermal CVD by passing a pyrolyzed carbon source gas at a temperature equal to or lower than the strain temperature of the substrate. Said pyrolyzed carbon source gas is formed by using a carbon-source-gas decomposing metal catalyst layer. In this prior art technique, it is necessary to form two different metal catalyst layers on two substrates, and then the two substrates are mounted in a thermal CVD reactor such that the metal layers are facing each other at a clearance. Obviously, this prior art technique is an improvement to the process disclosed in the above-mentioned EP1061041 A1. The major improvement comprises modifying a two-staged heating system into a one-staged heating system. However, this prior art technique has no conspicuous improvement over the catalyst system, which still requires the use of two different catalyst systems on two substrates.

SUMMARY OF THE INVENTION

[0008] A primary objective of the present invention is to provide a supported metal catalyst for synthesizing carbon nanotubes by low-temperature thermal chemical vapor deposition (CVD), which can be prepared easily.

[0009] Another objective of the present invention is to provide a supported metal catalyst for synthesizing carbon nanotubes by low-temperature thermal CVD, which has the advantages of easy adjustment and control of the catalyst composition thereof.

[0010] Still another objective of the present invention is to provide a process of direct low-temperature growth of carbon nanotubes on a substrate, which is free of the drawbacks of the abovementioned prior art.

[0011] A further objective of the present invention is to provide a process of direct low-temperature growth of carbon nanotubes on a substrate, which has an advantage of easy preparation of the catalyst system thereof.

[0012] In order to accomplish the aforesaid objection of the present invention, a supported metal catalyst useful in synthesizing carbon nanotubes by low-temperature thermal chemical vapor deposition (CVD) prepared according to the present invention comprises:

[0013] particles of a noble metal with diameters of 0.01-10 microns; and

[0014] a metal catalyst deposited on the noble metal particles, wherein said metal catalyst is selected from the group consisting of iron, cobalt, nickel and an alloy thereof, and the weight ratio of said metal catalyst to said noble metal particles ranges from 0.1:100 to 10:100.

[0015] Preferably, said noble metal particles are silver, gold, platinum, palladium, copper, or an alloy thereof, and more preferably are silver.

[0016] Preferably, the supported metal catalyst of the present invention is prepared by mixing said noble metal particles with a solution of a salt of said metal catalyst; and heating the resulting mixture to evaporate solvent therein, so that said metal catalyst is deposited on said noble metal particles. Said salt solution of the metal catalyst is preferably a nitrate solution and a sulfate solution, and said salt solution of the metal catalyst is preferably an aqueous or an alcohol solution.

[0017] Alternatively, the supported metal catalyst of the present invention is prepared by a deposition precipitation method comprising the following steps:

[0018] a) Dispersing said noble metal particles in a solvent;

[0019] b) Adding a salt solution of said metal catalyst to the resulting dispersion of said noble metal particles from Step (a);

[0020] c) Heating the resulting mixture from Step (b) and adding a precipitation agent to the heated mixture, so that said metal catalyst is deposited on said noble metal particles; and

[0021] d) Adding a reducing agent to the resulting mixture from Step (c) to reduce ions of said metal catalyst.

[0022] Preferably, said solvent in Step (a) is water or an alcohol.

[0023] Preferably, the precipitation agent in Step (c) is ammonia or sodium hydrocarbonate.

[0024] Preferably, the reducing agent in Step (d) is hydrazine, formaldehyde, phosphite, or benzaldehyde.

[0025] The present invention also discloses a method of synthesizing carbon nanotubes by low-temperature thermal CVD, which comprises the following steps:

[0026] A) Dispersing the supported metal catalyst of claim 1 on a substrate; and

[0027] B) Growing carbon nanotubes on said supported metal catalyst by using a carbon source gas and through a thermal CVD.

[0028] Preferably, said Step (A) of the method of the present invention comprises: dispersing said supported metal catalyst in an adhesive containing a polymer and an organic solvent; coating the obtained dispersion on said substrate; and heating the obtained coating to remove the polymer and the organic solvent contained therein. A suitable heating temperature for removing the polymer and the organic solvent is 350-500° C.

[0029] Preferably, said Step (A) of the method of the present invention comprises: adding said supported metal catalyst into an organic solvent; dispersing said supported metal catalyst in said organic solvent by an ultrasonic oscillation for a certain period of time; pouring the obtained dispersion on a quartz boat substrate to form a thin layer thereon; and drying said thin layer by heating.

[0030] Said substrate in Step (A) preferably is selected from the group consisting of ITO conductive glass, reinforced glass, sodium glass, quartz, silicon oxide, silicon wafer, aluminum, and metal sheets.

[0031] Said thermal CVD in Step (B) of the method of the present invention can be carried out at a reaction temperature of 400-600° C.

[0032] Preferably, said thermal CVD in Step (B) of the method of the present invention is carried out at a pressure of 0.5-2 atm for a reaction time of 1-120 minutes; and said carbon source gas comprises a hydrocarbon or carbon monoxide. Said hydrocarbon may contain 1-6 carbons. Preferably said carbon source gas is methane, acetylene, or carbon monoxide.

[0033] Preferably, the thermal CVD in Step (B) of the method of the present invention is carried out in the presence of hydrogen gas; and said carbon source gas comprises a hydrocarbon or carbon monoxide.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0034] The present invention discloses a supported metal catalyst for synthesizing carbon nanotubes by low-temperature (less than 600° C.) thermal chemical vapor deposition (CVD), which is different from the consuming-type growth of nano-metal catalyst disclosed in the abovementioned EP applications. The present invention uses an addition method to prepare the catalyst. Firstly, a support that can be used together with the carbon nanotubes in a downstream product is selected. That is the support will not affect the downstream process and the product thereof. Take a CNT-FED as an example. The silver particles in the silver paste can be used as a catalyst support. Since the silver paste is a necessary surface adhesive in the CNT-FED fabrication process, the support does not need to be removed and can be directly used in the CNT-FED fabrication process. Then, an active metal catalyst is implanted on the surface of said support by a deposition precipitation method or an impregnation method. Said supported metal catalyst is then dispersed or coated on the surface of the substrate. Later on carbon nanotubes can be grown by the thermal CVD. The carbon nanotubes can still be synthesized in a large quantity while the reaction temperature is controlled at below 600° C.

[0035] The catalyst support according to the present invention is a noble metal (such as Au, Ag, Cu, Pd, Pt) particle with a particle distribution of 0.01-10 microns. There are two methods of preparing said active catalyst: The first is an impregnation method, and the second is a deposition precipitation method. Both methods require dispersing the noble metal particles in water.

[0036] An impregnation method comprises subjecting an aqueous solution containing silver particles to an ultrasonic oscillation for a certain period of time, e.g. 10 min.; adding a solution containing an active metal salt into said aqueous solution, in which said active metal includes transitional metal elements, such as Fe, Co, Ni, etc., and said salt includes nitrate or sulfate (e.g. an aqueous solution of nickel nitrate); uniformly mixing the two solutions; concentrating the mixture solution by heating in order to remove the solvent such that the active metal catalyst is distributed on the support to form a supported metal catalyst.

[0037] A deposition precipitation method comprises: adding an alkali solution (such as ammonia water) into an aqueous solution containing silver particles to adjust the pH value thereof to be 8˜9; boiling the solution for a period of time, e.g. 30 min., in order to modify the surface of the support into alkaline; adding an aqueous solution containing an active metal salt into the solution; mixing the solution and adding a precipitation agent (e.g. ammonia water) and a chemical reducing agent (e.g. formaldehyde) into the solution in order to precipitate and reduce the active metal; filtering off the solution to obtain a supported metal catalyst.

[0038] A method of synthesizing carbon nanotubes directly by low-temperature thermal CVD according to the present invention comprises: dispersing the abovementioned supported metal catalyst on a substrate; and growing carbon nanotubes on said supported metal catalyst by using a carbon source gas and through a thermal CVD.

[0039] One method for dispersing the abovementioned supported metal catalyst on the substrate comprises: preparing a substrate; immersing said substrate in acetone; cleaning said substrate by oscillating acetone with an ultrasonic oscillator for about 10 minutes; and removing said substrate from acetone. This procedure is a pre-treatment, which can increase the adhesion of the catalyst system on the surface of said substrate. The abovementioned substrate can be a silicon wafer, a quartz glass, a reinforced glass, a sodium glass, an ITO conductive glass, a metal sheet, or a silicon oxide. The supported metal catalyst prepared according to the present invention is uniformly mixed with a polymeric adhesive, wherein the mixing ratio (weight) of said supported metal catalyst to said polymeric adhesive is 1:10 to 3:1. A suitable polymeric adhesive includes 35 wt % of a cellulose resin, 50 wt % of dl-α-terpineol as a solvent, 10 wt % of sodium phosphate as a dispersant, and 15 wt % of a glass powder. The function of said glass powder is to increase the bonding property. The mixture of the supported metal catalyst and the polymeric adhesive is coated on the surface of the substrate by screen-printing. The composite is oven-dried at 110° C. for 30 minutes, and sintered in air at 350˜500° C. for 30 min. to remove the polymeric adhesive. Another method for dispersing the supported metal catalyst on the substrate according to the present invention comprises: adding said supported metal catalyst into an organic solvent, e.g. ethanol; dispersing said supported metal catalyst in said organic solvent by subjecting the mixture to an ultrasonic oscillation for a certain period of time, e.g. 10 min.; pouring the mixture on a quartz boat substrate; and oven-drying the mixture at 110° C. for 30 min.

[0040] Carbon nanotubes can be grown on said supported metal catalyst by mounting said substrate, on which said supported metal catalyst is dispersed, in a reactor and performing a thermal CVD. An inert gas (e.g. helium, argon, nitrogen), hydrogen, and a carbon source gas are introduced a reaction chamber of the thermal CVD. The carbon source gas used includes a hydrocarbon or carbon monoxide. The reaction temperature is 400-600° C., the reaction time is 1-120 min., and the reaction pressure is 0.5-2 atm for the thermal CVD. After the reaction, carbon nanotubes with a diameter distribution of 1-200 nanometers are grown on the surface of the catalyst support.

EXAMPLE 1

[0041] One gram of silver particles having a particle size distribution of 1-5 microns was added with 50 ml of water. The mixture was subjected to an ultrasonic oscillation treatment for 10 min. The obtained silver aqueous dispersion was added with 1 g of an aqueous solution containing 1% of nickel nitrate while stirring. The resulting mixture was heated to remove the solvent contained therein in order to obtain 1.01 g of a silver supported catalyst semi-product containing 1% of nickel.

[0042] Example 2:

[0043] 1.0 gram of a silver powder having a particle size distribution of 0.05-0.1 micron was added with 50 ml of deionized water and agitated for 15 minutes. The mixture was then added with 0.05 g of 28% ammonia water, followed by agitation for 5 minutes. The resulting mixture was heated under reflux for 30 minutes, and then slowly added with 0.5 g of 10% aqueous solution of nickel nitrate. The resulting mixture was added with 0.08 g of 28% ammonia water. The resulting mixture was agitated continuously and boiled for 4 hours, and added with 0.44 g of 37% formaldehyde aqueous solution. The resulting mixture was boiled for another 30 minutes. After cooling, the mixture was filtered. The filtration cake was dried at 110° C. for 4 hours, thereby obtaining 1.05 g of a silver supported catalyst containing 5% of nickel.

[0044] Example 3:

[0045] A supported metal catalyst prepared from Example 1 was uniformly mixed with a polymeric adhesive at a mixing ratio (weight) of 1:1. The polymeric adhesive includes: 35 wt % of a cellulose resin, 50 wt % of dl-α-terpineol as a solvent, 10 wt % of sodium phosphate as a dispersant, and 15 wt % of a glass powder. Said coated substrate was dried at 110° C. for 30 minutes, then mounted in a thermal CVD reactor. The temperature of the reactor was then increased to 500° C. to sinter said substrate in air for 30 minutes in order to remove the resin and solvent. Argon gas was introduced at a flow rate of 1500 ml/min for 10 minutes in order to expel air from the reactor. Argon gas (500 ml/min) and hydrogen gas (75 ml/min) were introduced into the reactor for 5 minutes, and then acetylene (25 ml/min) was introduced to perform the thermal CVD reaction. After the reaction has conducted for 6 minutes, acetylene and hydrogen gas were turned off, and the electric power of heating was switched off. After the system temperature had reduced to below 100° C., argon gas was turned off and the substrate was removed. At this stage, black deposits were observed on the coated catalyst layer on said substrate. Through observation with an electronic microscope, said black deposits were carbon nanotubes having a diameter of 20-60 nanometers.

[0046] Example 4:

[0047] The procedure of Example 3 was repeated except that the supported metal catalyst prepared in Example 2 was used to replace the one prepared in Example 1. The surface of said substrate was grown with carbon nanotubes having a diameter of 20-60 nanometers.

[0048] Example 5:

[0049] 0.01 g of the supported metal catalyst prepared in Example 2 was dispersed on a quartz boat substrate. Next, said quartz boat substrate was mounted in a thermal CVD reactor to grow carbon nanotubes with the conditions and procedures identical to those used in Example 3. The surface of said substrate was grown with carbon nanotubes having a diameter of 20-60 nanometers.

[0050] In comparison with the prior arts, the present invention has the following major advantages: (1) The supported metal catalyst according to the present invention can be used in the synthesis of carbon nanotubes by a low temperature (less than 600° C.) thermal CVD. (2) The supported metal catalyst according to the present invention can be used in the direct synthesis of carbon nanotubes on a substrate at a low temperature without the need of removing the catalyst support. (3) The present invention uses a single high activity catalyst system instead of two catalyst systems, thereby reducing the cost. (4) The present invention uses a one-stage low temperature heating without the need of using a front-stage high-temperature treatment on the carbon source gas. (5) An electrically conductive metal identical to the current thick film process of CNT-FED fabrication is used as the support of the supported metal catalyst of the present invention, so that the growth of carbon nanotubes according to the process of the present invention can be directly integrated into the CNT-FED fabrication process and has a great compatibility.

Claims

1. A supported metal catalyst for synthesizing carbon nanotubes by low-temperature thermal chemical vapor deposition (CVD) comprising: particles of a noble metal with diameters of 0.01-10 microns; and a metal catalyst deposited on the noble metal particles, wherein said metal catalyst is selected from the group consisting of iron, cobalt, nickel and their alloys, and the weight ratio of said metal catalyst to said noble metal particles ranges from 0.1:100 to 10:100.

2. The supported metal catalyst as claimed in claim 1, wherein said noble metal particles are selected from the group consisted of silver, gold, platinum, palladium, copper, and their alloys.

3. The supported metal catalyst as claimed in claim 2, wherein said noble metal particles are silver.

4. The supported metal catalyst as claimed in claim 1, which is prepared by mixing said noble metal particles with a solution of a salt of said metal catalyst; and heating the resulting mixture to evaporate solvent therein, so that said metal catalyst is deposited on said noble metal particles.

5. The supported metal catalyst as claimed in claim 4, wherein said salt solution of the metal catalyst is a nitrate solution or a sulfate solution.

6. The supported metal catalyst as claimed in claim 5, wherein said salt solution of the metal catalyst is an aqueous or an alcohol solution.

7. The supported metal catalyst as claimed in claim 1, which is prepared by a deposition precipitation method comprising the following steps: a) dispersing said noble metal particles in a solvent; b) adding a salt solution of said metal catalyst to the resulting dispersion of said noble metal particles from Step (a); c) heating the resulting mixture from Step (b) and adding a precipitation agent to the heated mixture, so that said metal catalyst is deposited on said noble metal particles; and d) adding a reducing agent to the resulting mixture from Step (c) to reduce ions of said metal catalyst.

8. The supported metal catalyst as claimed in claim 7, wherein said solvent in Step (a) is water or an alcohol.

9. The supported metal catalyst as claimed in claim 7, wherein the precipitation agent in Step (c) is ammonia or sodium hydrocarbonate.

10. The supported metal catalyst as claimed in claim 7, wherein the reducing agent in Step (d) is hydrazine, formaldehyde, phosphite, or benzaldehyde.

11. A method of synthesizing carbon nanotubes by low-temperature thermal CVD, which comprises the following steps: A) dispersing the supported metal catalyst of claim 1 on a substrate; and B) growing carbon nanotubes on said supported metal catalyst by using a carbon source gas and through a thermal CVD.

12. The method as claimed in claim 11, wherein said substrate in Step (A) is selected from the group consisting of ITO conductive glass, reinforced glass, sodium glass, quartz, silicon oxide, silicon wafer, aluminum, and metal sheets.

13. The method as claimed in claim 11, wherein said thermal CVD in Step (B) is carried out at a reaction temperature of 400-600° C.

14. The method as claimed in claim 11, wherein said thermal CVD in Step (B) is carried out at a pressure of 0.5-2 atm for a reaction time of 1-120 minutes; and said carbon source gas comprises a hydrocarbon or carbon monoxide.

15. The method as claimed in claim 14, wherein said hydrocarbon contains 1-6 carbons.

16. The method as claimed in claim 14, wherein said carbon source gas is methane, acetylene, or carbon monoxide.

17. The method as claimed in claim 11, wherein the thermal CVD in Step (B) is carried out in the presence of hydrogen gas; and said carbon source gas comprises a hydrocarbon or carbon monoxide.