METHODS FOR FABRICATING CARBON NANOTUBES USING

SILICON MONOXIDE

FIELD OF THE INVENTION

The present invention relates generally to methods for fabricating nanotubes using catalytic chemical vapor deposition. More specifically, it relates to techniques for producing carbon nanotubes, silicon carbide nanotubes, and mixtures thereof.

BACKGROUND OF THE INVENTION

A well-known method for fabricating carbon nanotubes (CNTs) or nanofibers (CNFs) involves catalytic chemical vapor deposition (CVD) of hydrocarbons (e.g., methane) on support materials (e.g., powders or films of MgO, Siθ2, AI2O3) impregnated with catalyst (e.g., Co, Ni, Fe metal salts). In some applications, it is desirable to obtain pure CNTs with little or none of the support material that is used in fabrication. One known technique uses a strong acid (e.g., HNO ₃ , HCl, HF) or a strong alkali (e.g., NaOH) to dissolve the support. The use of acids or alkalis, however, can damage the CNTs and reduce the yield. Moreover, after the acid or alkali treatment, a neutralization treatment (e.g., washing with deionized water) is needed to bring the pH value up or down to 6 or 7, followed by drying. In a commercial process, these procedures add undesired complexity and expense to the fabrication. Also, some acid or alkali would likely remain in the product, even after neutralization. Other impurities in the product may include metal catalysts undissolved by the acid or alkali due to encapsulation of metal particles in the nanotubes or polyhedral graphitic nanoparticles. Due to these and other disadvantages of fabricating CNTs using acid or alkali treatment, it would be an advance in the art to provide improved methods for CNT fabrication.

SUMMARY OF THE INVENTION

According to one aspect of the invention, a method of fabricating CNTs uses CVD to synthesize CNTs on a silicon monoxide (SiO) support material impregnated with metal catalysts. The SiO

support material has the advantage that it can be at least partly removed by sublimation at a temperature above the CNT synthesis temperature in order to increase the purity of the CNTs. According to a preferred embodiment, CNTs are formed on the SiO particles impregnated with metal catalysts (e.g., Fe, Ni, Fe/Ni, Mo, or Co) using CVD of a hydrocarbon gas (e.g., methane, ethylene, or acetylene) at an elevated CNT synthesis temperature (e.g., from about 500 ⁰ C to about 1000 °C). The CNTs with SiO substrate may potentially be used as a final product. Alternatively, after CNT formation, the product may be raised to a sublimation temperature (e.g., above 1200 ⁰ C) to cause complete or partial sublimation of the SiO substrate, producing individual CNTs with reduced SiO. If silicon carbide nanotubes (SiCNTs) are desired (or a mixture of silicon carbide nanotubes and carbon nanotubes), reaction of SiO vapor with the CNTs may be induced by raising the sublimation temperature higher, (e.g., above 1350 ⁰ C). For CNTs, a temperature between 1200 ⁰ C and 1350 ⁰ C is preferred.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a flow chart illustrating steps in a method for fabricating carbon nanotubes according to an embodiment of the present invention.

FIG. 2 is a schematic illustration of a single substrate powder particle with several catalyst impregnation sites on its surface according to an embodiment of the invention.

FIG. 3 illustrates a single substrate powder particle with three nanotubes grown on its surface as produced by one embodiment of the invention.

FIG. 4 is a schematic illustration of three carbon nanotubes produced by another embodiment of the invention.

FIG. 5 is a schematic diagram of three silicon carbide nanotubes produced by yet another embodiment of the invention.

DETAILED DESCRIPTION

A method of fabricating CNTs avoids the use of acids or alkalis entirely by using a support material that can be removed by sublimation. In general, any support material suitable for CVD formation of nanotubes may take advantage of the techniques of the present invention provided that the support material sublimates at temperatures above the temperatures used in the CVD

process. For example, the substrate may be composed of SiO mixed with one or more impurities or other materials, or the substrate may be composed of another material altogether.

An overview of a fabrication method according to an embodiment of the invention is shown in the flowchart of FIG. 1. A suitable support material is obtained or fabricated in step 100 and passivated if desired. In step 102 the support material is impregnated with a metal salt or mixture of metal salts. The impregnated support material is oxidized to convert the metal salt or metal salt mixture to a metal oxide or metal oxide mixture in step 103. The impregnated support material is then pretreated in an ammonia gas stream, as shown in step 104. Using chemical vapor deposition synthesis, carbon nanotubes are grown on the support material in hydrocarbon gas at elevated temperature, as shown in step 106. In one embodiment, the carbon nanotubes with support material are used as a final product. In an alternative embodiment, the support material is partly or completely removed from the carbon nanotubes by heating the product above the sublimation temperature of the support material, as shown in step 107. The resulting purified carbon nanotubes are then used as a final product. Alternatively, a higher temperature may be used, as shown in step 108, to both partly or completely remove the substrate from the nanotubes as well as convert some or all of the carbon nanotubes to silicon carbide nanotubes. These steps will now be illustrated in more detail in the following description of a preferred embodiment of the invention.

SUPPORT MATERIAL

The support material preferably sublimates at temperatures above the temperatures used in the CVD process. In a preferred embodiment, the support material is SiO. In the context of the present description an SiO substrate is understood to refer to either pure SiO or SiO with one or more trace impurities (e.g., slight oxidation on the surface, trace contaminants, and the like). In a preferred embodiment, the CNTs are fabricated using an SiO support impregnated with a catalyst such as an Fe catalyst, Ni catalyst, or Fe/Ni mixed catalyst. The SiO support preferably takes the form of a powder, but may take various other forms as well. A powder of SiO may be formed, for example, by grinding SiO particles. Alternatively, nano-sized SiO powder may be obtained by heating SiO particles to a temperature above 1200 ⁰ C to sublimate SiO, or a mixture of Si and Siθ2 powders to a temperature above 1200 °C, to produce SiO vapor. Precipitation of the SiO vapor at lower temperatures then yields nano-sized SiO powder. For example, SiO particles may be put in an alumina tray, which is then placed in an argon stream flowing through a horizontal

tubular flow reactor heated in a furnace. SiO vapor is collected on the reactor wall where the temperature drops and by micro-filters mounted in the outlet gas line from the reactor. A vertical tubular flow reactor may also be used, which has a perforated-plate as a distributor. Alumina wool is placed on the perforated distributor, on which SiO particles are placed to form a bed of SiO particles. An argon gas stream flowing downward through the tube carries SiO vapor out of the SiO bed, which condenses on the reactor wall where temperature drops and is also collected by micro-filters mounted in the exit gas line. Because the surface of the nano-sized powder is very reactive, prior to catalyzation it is preferable to passivate the powder in an air stream at 450- 480 ⁰ C to slightly oxidize the SiO surface.

CATALYZATION

Prior to CVD, the SiO powder is impregnated with a catalyst as follows. For an Fe catalyst, 25 ml of a solution made by dissolving 1.00 g Fe(NO3)3 ^• 9H ₂ O into 100 ml methanol is placed in a mortar. Then 1.00 g of SiO particles are added to the mortar where it is ground and stirred while the mixture is heated to evaporate the methanol. Alternatively, the SiO nano-sized powder is impregnated with a catalyst as follows. For an Fe catalyst, 25 ml of a solution made by dissolving 1.00 g Fe(NO ₃ ) ₃ ^• 9H ₂ O into 100 ml methanol is placed in a beaker. Then 0.50 g of SiO nano-sized powder is added to the beaker where it is first sonicated for 2-5 min and then stirred while the mixture is heated to evaporate the methanol. In order to oxidize the Fe(NO ₃ ) ₃ ^• 9H ₂ O to form Fe ₂ O ₃ , the SiO powder impregnated with Fe(NO ₃ ) ₃ ^• 9H ₂ O is placed in an alumina tray, and the tray is inserted in a 40 mm inner diameter alumina tube through which air is flowing. It is then heated in an air stream at 10 °C/min to 450 ⁰ C, maintained at that temperature for 2-3 hours, and then cooled down at 30 °C/min to room temperature. The mass ratio of Fe to SiO in the impregnated powder may be adjusted in the range from about 0.01 to 0.43, which is equivalent to between 0.19 and 7.7 millimole of Fe per gram of SiO.

In some cases, a higher CNT growth rate may be obtained by using a mixed Fe/Ni catalyst. An SiO powder formed by grinding SiO particles may be impregnated with a mixed Fe/Ni catalyst as follows. A mortar is filled with 14 ml of a solution made by dissolving 1.00 g Fe(NO ₃ ) ₃ ^• 9H ₂ O into 100 ml methanol and 10 ml of a solution made by dissolving 1.00 g Ni(NO ₃ )ι ^• 6H ₂ O into 100 ml methanol. Then 1.00 g of SiO powder is added to the mortar where it is ground and stirred while the mixture is heated to evaporate the methanol, just as with the Fe

catalyst. In order to oxidize the Fe(NO ₃ ) ₃ ^• 9H ₂ O and Ni(NO ₃ ) ₂ ^• 6H ₂ O to form a mixture of Fe ₂ O ₃ and NiO, the resulting powder is heated in an air stream at 10 °C/min to 450 ⁰ C, maintained at that temperature for 2-3 hours, and then cooled down at 30 °C/min to room temperature. Alternatively, SiO nano-sized powder obtained by sublimation may be impregnated with a mixed Fe/Ni catalyst as follows. A beaker is filled with 14 ml of a solution made by dissolving 1.00 g Fe(NO ₃ ) ₃ ^• 9H ₂ O into 100 ml methanol and 10 ml of a solution made by dissolving 1.00 g Ni(NO ₃ ) ₂ ^• 6H ₂ O into 100 ml methanol. Then 0.5O g of SiO powder is added to the beaker where it is sonicated and then stirred while the mixture is heated to evaporate the methanol, just as with the Fe catalyst. FIG. 2 is a schematic illustration of a single SiO powder particle 200 with several Fe/Ni impregnation sites on its surface, such as site 202.

CVD PROCESS

A small amount (10-100 mg) of the catalyst-impregnated SiO powder is placed in an alumina tray, and the tray is inserted directly into a dual alumina tube reactor heated in a stream of nitrogen to a temperature of about 1000 °C. The outer tube has a 40 mm inner diameter and is 1500 mm long. The inner tube is composed of a series of short alumina tubes, each with 30 mm inner diameter and 100 mm long. The short tubes absorb thermal shocks when the cold alumina tray is inserted into the heated reactor to protect the'outer tube from cracking.

Once in the reactor, the catalyst-impregnated SiO powder is pretreated in an NH ₃ /N ₂ mixture gas stream or NH ₃ stream for a short time period (e.g., 5-15 minutes). During this pretreatment the

Fe ₂ O ₃ or Fe ₂ O ₃ /NiO mixture on the SiO support is reduced to nano-clusters of Fe or a Fe/Ni composite or alloy. Pretreatment using ammonia or ammonia-nitrogen is preferable in most cases to using hydrogen or a nitrogen-hydrogen mixture since it results in faster CNT growth rate.

Alternatively, the ammonia may be combined with another inert gas, such as argon. However, nitrogen is preferable since it is the cheapest.

To initiate CNT formation by CVD synthesis, methane mixed with NH ₃ is then introduced in a heated reactor. To optimize catalyst activity and increase yield, the CVD synthesis temperature is most preferably in the range 975 ⁰ C to 1000 °C, although temperatures in a broader ranges (e.g., 950 ⁰ C to 1050 ⁰ C) may be useful as well. The ammonia content in the mixture with methane is selected in the range from zero to about 50 volume percent. Preferably, to optimize CNT yield, the ammonia to methane molar feed ratio is selected to be between 0.2 and 0.8, with the most

preferred ratio being between 0.3 and 0.5. The total flow rate of reactant gas mixture may be adjusted in the range from 200 cc/min to 500 cc/min. Instead of the ammonia/methane mixture, a mixture of ammonia with another hydrocarbon may be used. The synthesis temperature in such cases may be different.

Experiments by the inventors indicate that during an initial stage CVD synthesis lasting about 5 minutes the CNTs grow longitudinally, followed by radial growth at a slower rate. Thus, if radial growth is not desired for the CNTs, then the preferred CVD synthesis time is approximately 5-6 minutes or less. Otherwise, a longer CVD synthesis time may be used. At the end of the synthesis period, the methane-ammonia mixture is replaced by nitrogen gas to stop the reaction. FIG. 3 illustrates a single SiO powder particle 300 with three CNTs grown on its surface, CNTs 302, 304 and 306. The CNTs with SiO substrate attached may be used as an end product, or may be subjected to additional processing, as will be described below.

The saturated vapor pressure of SiO at temperatures around 1000 ⁰ C is extremely low (0.39 Pa) and so sublimation of the SiO is negligible during the CVD process. Moreover, sublimation of SiO is inhibited during the CVD process due to the formation of a very thin film of silicon oxynitride on the surface of the SiO resulting from the reaction of SiO with nitrogen in the CVD environment. In addition, the SiO also reacts with ammonia in the CVD environment to form a thin silicon nitride film on the SiO surface, further inhibiting SiO sublimation during the CVD process.

SUBLIMATION

It may be desirable in many applications to reduce or eliminate the amount of SiO in the product. An important advantage of the use of a SiO substrate is that much or all of it can be removed from the CNT product through sublimation without the use of acid. Although the SiO is stable at the CVD process temperatures around 1000 ⁰ C, at higher temperatures the saturated vapor pressure of SiO is orders of magnitude larger than at 1000 ⁰ C. For example, the saturated vapor pressure of SiO is 31 Pa at 1200 ⁰ C and 412 Pa at 1350 ⁰ C. Thus, in one embodiment, after CNT formation using CVD synthesis as described above, the temperature of the product is raised to around 1200 ⁰ C or higher in a container placed in a stream of inert gas (e.g., argon) to induce sublimation of the SiO substrate, thereby reducing or eliminating the SiO attached to the CNTs. According to one technique, a small amount (50-100 mg) of CNT product is placed in an

alumina tray, and the tray is inserted into a horizontal alumina tube of 40 mm inner diameter at room temperature, through which argon is flowing. The tube is heated to 1200 ⁰ C at a rate of 10 °C/min, and the temperature is held for 2 hours, followed by cooling to room temperature. Alternatively, the sublimation of SiO from the CNT product may be carried out in the dual alumina tube reactor, in which the CNT product placed in an alumina a tray is heated directly to 1200 °C in an argon gas stream. Also, these two sublimation processes may take place in a vertical tube reactor. Although temperatures lower than 1200 °C may also be used for sublimation, temperatures at or above 1200 °C are preferred due to the higher corresponding sublimation rate. As temperatures approach 1350 ⁰ C, however, the reactivity of the sublimated SiO with the carbon becomes significant. Thus, when CNTs are the desired end product, it is preferable to use sublimation temperatures of lower than 1350 ⁰ C in order to keep this reactivity low. While lower sublimation temperatures ensure lower reactivity of sublimated SiO with the carbon nanotubes, higher sublimation temperatures provide faster sublimation rates. Thus, the specific temperature used is a trade-off between these two factors, and can be selected based on the particular fabrication requirements. A sublimation temperature of approximately 1200 ⁰ C is suitable in most cases for fabricating CNTs.

Typically, after two hours at the sublimation temperature, the SiO support material will be sufficiently sublimated to remove the majority of the SiO from the CNTs, thereby producing the purified CNTs. Once the sublimation is done, the temperature is reduced to room temperature. The product CNTs remain in the alumina tray where the original raw product was placed, which may be taken out of the tube. FIG. 4 is a schematic illustration of three product CNTs 402, 410, 406, where two have small amounts of SiO material 400, 408 attached and one nanotube has no SiO attached.

CONVERSION TO SILICON CARBIDE NANOTUBES

In an alternative embodiment, silicon carbide nanotubes may be produced by using a higher sublimation temperature. The technique is the same as that described above for fabricating CNTs, except that a sublimation temperature of 1350 ⁰ C or higher is used. At these higher temperatures, the sublimated SiO vapor reacts with the carbon in the nanotubes to form silicon carbide. In most cases, the resulting nanotubes will contain a mixture of carbon nanotubes and silicon carbide nanotubes, where the mixture ratio depends on the sublimation temperature and sublimation time. The higher sublimation temperature has the advantage that more, if not all, of

the SiO material is removed, resulting in highly purified nanotubes. FIG. 5 is a schematic diagram showing product silicon carbide nanotubes 502, 504, and a product carbon nanotube 506, with no SiO substrate attached to any of them. At high enough temperature, the product is almost pure silicon carbide nanotubes with no carbon nanotubes or SiO substrate. Note, however, that some catalyst metal or alloy clusters may remain in the nanotube product, even after complete removal of the substrate.

In other embodiments, alternate techniques may be used for CVD synthesis of CNTs, including the use of alternate metal salt catalysts, such as salts of Ni, Mo or Co, and/or an alternate hydrocarbon gas, such as ethylene or acetylene. Many other variations of the specific examples described above are possible and will be evident to those skilled in the art in view of the principles of the inventions.