CN1304631C - Technology for preparing nano tube of carbon by direct current glow plasma chemical vapour phase deposition process - Google Patents

Abstract

The process for preparing carbon nanotubes by the DC glow plasma chemical vapor deposition method of the present invention belongs to the process for preparing carbon nanotubes. A silicon wafer or a quartz wafer with a catalyst layer on the surface is used as a substrate, and a mixed gas of methane and hydrogen is used as a raw material, and carbon nanotubes are grown by chemical vapor deposition. Firstly, the vacuum degree in the vacuum chamber is about 2Pa, and the distance between the electrodes is 4-5mm, and the raw material gas is introduced. When the pressure reaches 200-500Pa, the discharge current is maintained at about 2A. When the gas pressure reaches 5-10KPa, the discharge current is adjusted to 5-10A. The distance between the electrodes is 20-40mm, and the deposition takes 5-600 seconds. Since the plasma generated by the DC glow discharge is stable, the ionization rate of the gas is higher, and the surface state of the substrate is uniform, the carbon nanotubes with high purity and uniform diameter can be uniformly prepared in a large area. Forming discrete nanoscale catalyst particles on the surface of the substrate can deposit ordered upright arrays of carbon nanotubes.

Description

Preparation of Carbon Nanotubes by DC Glow Plasma Chemical Vapor Deposition

technical field

The invention belongs to a process for preparing carbon nanotubes, in particular to a preparation process for carbon nanotubes with high purity and uniform diameter, and a preparation process for orderly and vertically arranged carbon nanotubes.

Background technique

Carbon nanotubes are tubular fiber materials composed of carbon elements, and their diameters range from a few nanometers to tens of nanometers. Carbon nanotubes have excellent electronic and mechanical properties, are a good field emission material, a new generation of electronic device materials, excellent hydrogen storage materials and new fiber materials. Therefore, people have established a variety of Methods to prepare carbon nanotubes.

It is generally believed that the necessary conditions for chemical vapor deposition of carbon nanotubes are: ① There is a carbon-containing gas participating in the reaction. ②Decompose carbon-containing gas to produce carbon ions or atoms and their active groups. ③ There is a catalyst, and the general catalyst is iron series elements or some rare earth elements. ④Appropriate substrate temperature, the substrate temperature is generally between 650°C and 900°C. The main difference between various chemical vapor deposition methods is the different means of decomposition of the reaction gas.

A method close to the present invention is the plasma-assisted hot filament decomposition method.

The preparation of carbon nanotubes by hot filament decomposition method is a chemical vapor deposition (CVD) method. Reactive gases such as methane and hydrogen are introduced into the vacuum chamber. The gas is decomposed under the high-temperature hot filament and carbon nanotubes are deposited on the substrate containing the catalyst. . In order to prepare oriented carbon nanotubes, Z.P.Huang, Z.F.Ren, et al. (Growth of highly oriented carbon nanotubes by plasma-enhanced hotfilament chemical vapor deposition, APPLIED PHYSICS LETTERS, VOLUME73, NUMBER26, 1998.) in filament and substrate A method of plasma-assisted hot filament deposition of carbon nanotubes is formed by applying a bias voltage between them.

However, due to the arrangement of the filaments, the state and discharge of the substrate surface are not uniform, and the carbon nanotubes grown on the substrate surface cannot guarantee a large area and uniform diameter, which affects the quality of the carbon nanotubes.

Contents of the invention

The technical problem to be solved by the present invention is: to prepare carbon nanotubes by means of direct current glow plasma CVD with flat electrodes, and to deposit carbon nanotubes on substrates coated with catalyst materials by establishing an optimized experimental process.

In order to prepare carbon nanotubes with uniform diameter, high purity and orderly vertical arrangement, the inventor established a process for preparing carbon nanotubes by DC glow discharge plasma chemical vapor deposition method. From the perspective of gas decomposition, the plasma generated by the DC glow discharge method is more stable, the ionization rate of the gas is higher, and the state of the substrate surface is uniform, which has greater advantages in the preparation of high-quality carbon nanotubes .

Due to the preparation of carbon nanotubes by this method, it is very difficult to directly ignite the glow in the pressure range of a few KPa to more than a dozen KPa, and it takes a long time to stabilize the discharge from the lower pressure to the predetermined discharge result. During this period of time, non-carbon nanotube deposits will be produced on the surface of the substrate, which will pollute the substrate surface. In the subsequent discharge, even if the discharge conditions meet the deposition of carbon nanotubes, no carbon nanotubes will be obtained on the substrate. Tube.

The equipment and raw materials used in the preparation of carbon nanotubes of the present invention are:

Vacuum system: In the vacuum chamber, there are circular flat cathodes and anodes opposite to each other, which are indirectly water-cooled by cooling water respectively. The cathode is on the top and the anode is on the bottom. The chip is placed on the water-cooled anode copper base, and the substrate can be lifted and lowered with the anode copper base under the action of the lifting system to adjust the distance between the electrodes. A suitable heat shield is placed between the substrate and the anode copper seat to allow the substrate to reach a predetermined deposition temperature.

Substrate: carbon nanotubes are prepared by using a substrate coated with a catalyst layer. A silicon wafer or a quartz wafer is used as a substrate, on which a catalyst layer of 5 nanometers to 60 nanometers is plated. Iron, cobalt or nickel, or their alloys can be used as the catalyst material, and a nickel/gold composite membrane material can also be used as the catalyst layer. The catalyst layer on the surface of the substrate can be a catalyst film layer or a catalyst particle layer with nanoscale particles on the surface.

Raw material: The mixed gas of methane and hydrogen is used as raw material.

The technology that carbon nanotube of the present invention prepares is:

1. Wipe the substrate clean and place it on the anode copper seat in the vacuum chamber, and cover the vacuum chamber.

2. Pump the vacuum in the vacuum chamber to about 1-3Pa, and adjust the distance between the cathode and the anode to be 4-5mm.

3. Close the main pumping valve of the vacuum system, and feed the mixed gas of hydrogen and methane into the vacuum chamber in a certain proportion. When the pressure of the vacuum chamber reaches 200-500Pa, apply a DC voltage between the cathode and the anode to discharge the gas, and maintain the discharge current at 1.5-2.5A. When the gas pressure in the vacuum chamber reaches between 5 and 10KPa, open the fine-tuning needle valve for pumping air and adjust the needle valve to maintain a constant air pressure in the vacuum chamber. In this process, there is a discharge phenomenon between the electrodes, but there is no plasma, and the discharge gas is hardly decomposed. Therefore, carbon nanotubes or other deposits will not be deposited on the substrate, and the temperature is very low, and the substrate will not have Any change, the discharge state is shown in Figure 1.

4. Increase the discharge current and adjust the distance between the electrodes to increase the discharge current to 5-10A and the distance between the electrodes to 20-40mm. At the same time, the temperature of the substrate can reach 600-900°C, and a glow is generated between the cathode and the anode. Photodischarged plasma, as shown in Figure 2. At this time, carbon nanotubes will be deposited on the surface of the substrate, and this deposition state is maintained for 5 seconds to 10 minutes.

5. Turn off the discharge power supply, and get carbon nanotubes on the substrate after shutdown.

During the growth process of carbon nanotubes, gas pressure, discharge current, substrate temperature, catalyst type and thickness on the substrate surface, distance between electrodes, etc. are the main process parameters. Adjusting these process parameters within a certain range can prepare various types of carbon nanotubes. carbon nanotubes.

The certain range of process parameters mentioned above refers to: the gas pressure range is 5KPa ~ 12KPa; the discharge current is 5 ~ 10A; the substrate temperature is 600°C ~ 900°C; the distance between electrodes is 20mm ~ 40mm; the gas flow ratio is: hydrogen Gas flow rate: methane gas flow rate is between 100:10 and 100:40; the type and thickness of the catalyst on the surface of the substrate refers to: iron, cobalt, nickel, and their alloys can be selected respectively, and nickel/gold composite film can also be used etc. as a catalyst film layer, the thickness of the catalyst film layer coated on the surface of the substrate is generally between 5nm and 60nm.

Preparation of carbon nanotubes with smaller diameters: A substrate with a thin catalyst film layer can be selected and grown at a lower temperature. The conditions for preparing carbon nanotubes with thicker tube diameters are opposite to the above.

The flow rate of hydrogen gas can be selected as 50-500 sccm, and the flow rate of methane gas can be selected as 5-200 sccm accordingly.

Since the plasma generated by the DC glow discharge method is more stable, the ionization rate of the gas is higher, and the state of the substrate surface is uniform, the carbon nanotubes prepared by this process are characterized by high purity and uniform tube diameter. , can uniformly prepare carbon nanotubes on a large-area substrate.

The surface of the substrate used to prepare orderly and vertically arranged carbon nanotubes has discrete nanoscale particle layers, and the catalyst is iron, cobalt, nickel or alloys thereof. The substrate with the nanoscale particle layer can be obtained by the following method, and the substrate coated with a catalyst coating of 5 to 20 nanometers is pretreated with glow plasma of nitrogen, and the method for the substrate pretreatment is:

1. Wipe the substrate plated with a catalyst coating of 5 to 20 nanometers clean, place it on the anode copper seat in the vacuum chamber, and cover the vacuum chamber.

2. Pump the vacuum in the vacuum chamber to about 1-3Pa, and adjust the distance between the cathode and the anode to be 4-5mm.

3. Close the main pumping valve of the vacuum system, feed hydrogen gas into the vacuum chamber to 200-500Pa, stop the hydrogen gas flow, apply DC voltage between the electrodes, ignite the glow, and maintain the discharge current at 1.5-2.5A.

4. Introduce nitrogen into the vacuum chamber to 5KPa, increase the discharge current and the distance between the electrodes to 5A and 30mm respectively, and maintain this state for 5-10 minutes.

5. A substrate with a separated layer of nanoscale particles can be obtained after shutdown.

Discrete nanoscale catalyst particles are formed on the surface of the substrate, and then carbon nanotubes arranged in an orderly upright manner can be deposited according to the aforementioned carbon nanotube preparation process. Carbon nanotubes with ordered vertical alignment have broad application prospects in field emission.

Description of drawings

Fig. 1 is a discharge state in which deposits do not grow in the vacuum chamber of the present invention.

Fig. 2 is the discharge state during the process of plasma growth of carbon nanotubes in which glow discharge is generated in the vacuum chamber of the present invention.

Detailed ways

Example 1 gives an example of specific preparation of carbon nanotubes.

A silicon wafer coated with nickel with a thickness of 5nm is used as the deposition substrate, placed on the anode base in the vacuum chamber, the vacuum degree in the vacuum chamber is evacuated to 2Pa, and the distance between the cathode and the anode is adjusted to 4mm. Close the main pumping valve of the vacuum system, feed the mixed gas of hydrogen and methane into the vacuum chamber, the hydrogen flow rate is 500 sccm, and the methane flow rate is 200 sccm. Discharge, maintain the discharge current at 2A, and when the gas pressure in the vacuum chamber reaches 5KPa, keep the air pressure in the vacuum chamber constant. The discharge current was increased to 5A, while the distance between the electrodes was increased to 30mm, the substrate temperature was reached to 800°C, and this deposition state was maintained for 30 seconds. After shutdown, carbon nanotubes with high purity and uniform diameter are obtained on the surface of the substrate.

Example 2 gives an example of specific preparation of carbon nanotubes.

A silicon wafer plated with a nickel film with a thickness of 60nm or 30nm was used as the deposition substrate, and the process of Example 1 was repeated to obtain carbon nanotubes with high purity and uniform diameter.

Example 3 gives an example of specific preparation of carbon nanotubes.

The working pressures were respectively 7KPa and 10KPa, and the process of Example 1 was repeated to obtain carbon nanotubes on the substrate.

Example 4 gives an example of specific preparation of carbon nanotubes.

The discharge currents for maintaining the deposited carbon nanotubes were 7A and 9A respectively, and the experimental process of Example 1 was repeated to obtain carbon nanotubes on the substrate.

Example 5 gives an example of specific preparation of carbon nanotubes.

The deposition temperature of the substrate was adjusted to 600° C. or 900° C., and the experimental process of Example 1 was repeated to obtain carbon nanotubes on the substrate.

Example 6 gives an example of specific preparation of carbon nanotubes.

Adjust the distance between the substrate and the cathode to 20 mm or 40 mm, repeat the experimental process of Example 1, and obtain carbon nanotubes on the substrate.

Example 7 gives an example of specific preparation of carbon nanotubes.

The catalyst film layer of the nickel film is replaced by iron or cobalt, and the above examples 1-6 are repeated to obtain carbon nanotubes on the substrate.

Example 8 gives an example of specific preparation of carbon nanotubes.

The nickel/gold composite film is used as the catalyst film layer, and the thickness of the catalyst film layer on the surface of the substrate is: 20 nanometers of nickel and 20 nanometers of gold. Using such a substrate, the experimental process of Example 1 was repeated, and carbon nanotubes were obtained on the substrate.

Example 9 gives an example of the specific preparation of a substrate having discrete nanoscale catalyst particle surfaces.

Wipe the silicon wafer coated with a nickel film with a thickness of 10nm and place it on the anode copper seat in the vacuum chamber; pump the vacuum in the vacuum chamber to about 1-3Pa, and adjust the distance between the cathode and the anode to 4-5mm; Close the main pumping valve; feed H ₂ into the vacuum chamber, when the air pressure rises to 200-500Pa, apply a DC voltage between the cathode and the anode to discharge the gas, and maintain the discharge current at 1.5-2.5A; N ₂ gas to increase the pressure in the vacuum chamber to 2-10KPa; increase the discharge current to 7A, and increase the distance between electrodes to 30mm; maintain this discharge state for 8 minutes. _N2 decomposes and ionizes under the plasma, and etches the substrate surface. A substrate is prepared having discrete nanoscale catalyst particles on the surface.

Using the substrate with discrete nanoscale catalyst particles, according to the above preparation process of carbon nanotubes, highly oriented vertically arranged carbon nanotubes can be deposited.

Example 10 gives an example of specific preparation of carbon nanotubes arranged in an upright order.

The substrate prepared in Example 9 with discrete nanoscale catalyst particles was used. Placed on the anode seat, and according to the process of Examples 1-7, carbon nanotubes with ordered orientation and upright arrangement can be prepared.

Claims (3)

1. A process for preparing carbon nanotubes by a DC glow plasma chemical vapor deposition method, using a silicon wafer or a quartz wafer as a substrate, on which iron, cobalt or nickel or their alloys are plated or nickel/gold A catalyst layer made of a composite membrane; a mixed gas of methane and hydrogen is used as a raw material, and the method of chemical vapor deposition is used to grow carbon nanotubes. It is characterized in that the catalyst layer is a catalyst layer or a surface with nanoscale particles. The catalyst particle layer has a thickness of 5 nanometers to 60 nanometers; the process is to wipe the substrate clean and place it on the flat anode copper seat in the vacuum chamber, and cover the vacuum chamber; the vacuum degree in the vacuum chamber is pumped to 1~ 3Pa, and adjust the distance between the flat-plate cathode and the flat-plate anode to be 4-5mm; close the main pumping valve of the vacuum system, and feed a mixed gas of hydrogen and methane into the vacuum chamber, and the ratio of the gas flow rate is hydrogen flow:methane gas Flow rate = 100:10~40, when the pressure of the vacuum chamber reaches 200~500Pa, apply a DC voltage between the flat cathode and the flat anode, and maintain the discharge current at 1.5~2.5A, when the gas pressure in the vacuum chamber reaches 5~10KPa Keep the air pressure in the vacuum chamber constant; adjust the discharge current to 5-10A, increase the distance between the flat electrodes to 20-40mm, and the substrate temperature to 600-900°C, and turn off the discharge power after maintaining this state for 5-600 seconds , to obtain carbon nanotubes on the substrate. 2. The process for preparing carbon nanotubes according to the DC glow plasma chemical vapor deposition method according to claim 1, characterized in that the catalyst particle layer with nanoscale particles on the surface is plated with 5-20 nanometer particles The catalyst-coated substrates were obtained by pretreatment with nitrogen glow plasma. 3. According to the process for preparing carbon nanotubes according to the DC glow plasma chemical vapor deposition method described in claim 1 or 2, it is characterized in that the gas flow rate is 50 to 500 sccm for hydrogen, and 5 to 500 sccm for methane gas. 200 sccm.