Abstract:
Few-walled carbon nanotubes (FWNTs) can be synthesized in a simple chemical vapor deposition (CVD) system using a mixture of methanol and ethanol as the carbon source. In preferred embodiments, the present invention uses an ethanol/methanol mixture as the carbon source so that few walled nanotubes (FWNTs) with high purity can be prepared following a simplified purification process. Under the growth conditions of the present invention, ethanol is believed to act as the carbon source while methanol is believed to act as a “carbonaceous impurity remover” to remove the impurities deposited on a support (e.g., MgO) and thereby hinder the formation of such impurities.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The present application is based on and claims domestic priority benefits under 35 USC §119(e) from U.S. Provisional Application Ser. No. 60/856,309 filed on Nov. 3, 2006, the entire content of which is expressly incorporated hereinto by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to nanotube structures. In especially preferred embodiments, the present invention relates to few-walled nanotubes (FWNTs) which are made using alcohol CVD techniques with a methanol and ethanol mixture as a carbonaceous source. 
       BACKGROUND AND SUMMARY OF THE INVENTION 
       [0003]    Since their discovery in 1991, carbon nanotubes (CNTs) have attracted extensive attention due to their unique electrical and mechanical properties and potential applications 1 . Most of the research has focused on two kinds of CNTs, single-walled carbon nanotubes (SWNTs) 2, 3  that consist of only one layer of graphite sheet, and multi-walled carbon nanotubes (MWNTs) 4  that consist of up to dozens of layers of sidewalls. Recently a new kind of CNTs, few-walled carbon nanotubes (FWNTs) have attracted much attention due to their unique structure that can potentially solve some existing problems associated with either SWNTs and MWNTs 5 . Strictly speaking, FWNTs are a special kind of MWNTs that include between about 2 to 6 layers of sidewalls and have the structural perfection comparable that of SWNTs. FWNTs can be considered as an intermediate between SWNTs and MWNTs. A specific example of FWNTs include conventional double-walled carbon nanotubes (DWNTs) 6-8 . 
         [0004]    Due to their unique structure, FWNTs have special properties different from SWNTs and MWNTs. For example, FWNTs can retain the remarkable mechanical and electronic properties of the inner layers when the outmost layer is functionalized to improve the solubility of the materials in various solvent and improve the interaction between the nanotubes and the polymer matrix around them in high performance composites. As compared with MWNTs, especially MWNTs made from chemical vapor deposition (CVD) methods, FWNTs generally have much better structural perfection, making them better candidates in applications like composites and field emission. FWNTs also form bundles with smaller diameter than those of SWNT bundles. In applications related to field emission, this has been proven to be advantageous in lowering the voltage threshold and improving the stability at high emission current 5 . 
         [0005]    Among the three common methods in producing CNTs, namely, arc discharge 1-4, 9, 10 , laser ablation 11-14  and CVD 15-17 , CVD was already proven to be the best choice in producing large amount of CNTs at relatively low cost. Various carbon sources have been used in CVD, such as hydrocarbons 18, 19 , carbon monoxide  20, 21  and alcohols 22, 23 . Alcohols are particularly attractive because they are safe to handle and were observed to produce less impurity in the synthesis of SWNTs. The etching effect by oxygen containing radicals generated during alcohol decomposition is believed to be the cause of improved purity 24 . Currently, almost all of the reported results on CNT synthesis using alcohol CVD used pure alcohol as carbon sources and CNTs were synthesized at relatively low temperature, below 900° C. 25-27 . 
         [0006]    It has been discovered that FWNTs can be synthesized using alcohol CVD at higher temperature (1000° C. or higher). However, carbonaceous impurities always form simultaneously with CNTs, thereby making it difficult to purify the sample which limits the application of the product. Although CVD methods can produce large amount of CNTs at relatively low cost, it is the purification that limits the ability to obtain obtaining large amounts of highly purified CNTs. Currently, the purification is becoming the rate limiting step for many of the bulk applications using nanotubes. It is relatively easy to obtain milligram or even gram quantities of highly purified nanotubes using many of the published purification processed developed over the last few years 28-33 . However, the purity and yield drop dramatically when handling large amounts of samples. 
         [0007]    Impurities in raw CNT product can be divided into two groups, these being non-carbonaceous and carbonaceous impurities. Non-carbonaceous impurities can be easily removed by treating with acid or base, since they are comprised mainly metal catalyst and support. However, it is much more difficult to remove carbonaceous impurities, including amorphous carbon, carbon fiber and MWNTs (in the case of SWNT and FWNT synthesis) because they are made of carbon and their chemical properties are similar to those of CNTs. The common purification method relies on selective oxidation of carbonaceous impurities. The key to purification therefore is to control the oxidization reaction so that only carbonaceous impurities are removed while CNTs remain unreacted. However, due to the similarity in the chemical reactivity of the impurities and the CNTs, a large portion of the CNTs is lost during the purification step. One technique to solve this problem is to continuously improve the selectivity of the purification process. Another technique to solve this problem is to reduce the percentage of non-tube carbonaceous species in the raw sample. 
         [0008]    The present invention involves employing an alcohol mixture as the carbon source for CNT synthesis. In this regard, it has been identified that the majority of the impurities in the process of the present invention are due to carbon deposition on the surface of the catalyst support. The use of an ethanol/methanol mixture as the carbon source thereby increases the FWNT percentage in the raw samples by reducing the carbonaceous species deposited on the catalyst support to almost zero. A systematic study of the effect of ethanol/methanol ratio has revealed the mechanism for the improved purity of the sample. Under the growth conditions studied, ethanol acted as the carbon source while methanol acted as a “carbonaceous impurity remover” to remove the impurities deposited on the MgO support of catalyst, and hindered the formation of such impurities. 
         [0009]    These and other aspects and advantages will become more apparent after careful consideration is given to the following detailed description of the preferred exemplary embodiments thereof. 
     
    
     
       BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS 
         [0010]    Reference will hereinafter be made to the accompanying drawings, wherein like reference numerals throughout the various FIGURES denote like structural elements, and wherein; 
           [0011]      FIG. 1  is a high resolution TEM image of FWNTs made according to the present invention; 
           [0012]      FIGS. 2   a - 2   e  are photographs and TGA weight loss plots of FWNT materials prepared at varying volume percentages of methanol and ethanol as the carbonaceous source; 
           [0013]      FIG. 3  is a TEM image of the material of  FIG. 2   a  showing no CNTs therein; 
           [0014]      FIG. 4  is a plot of the first order differentiation of the weight loss of the samples shown in  FIGS. 2   a - 2   e  versus temperature; and 
           [0015]      FIGS. 5   a  and  5   b  are TEM images of purified FWNT samples prepared using higher volume percentage of ethanol and showing the presence of impurities therein. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]    According to the present invention, high purity few-walled carbon nanotubes are prepared by a chemical vapor deposition method with ethanol/methanol mixture as carbon source. Preferably, Co/Mo/MgO is employed as catalyst. A high resolution TEM image of FWNTs made according to the present invention is shown in  FIG. 1 . 
         [0017]    As compared to the samples prepared from pure ethanol as a carbon source, samples from an ethanol/methanol mixture are of higher purity and can be more easily purified. It is suspected that the oxidative radicals generated by methanol decomposition hindered the formation of carbonaceous impurities on MgO support. Thus, the produced raw materials contain less carbonaceous impurities and the carbonaceous impurities have more defects. Such impurities have lower burning temperatures under airflow compare to impurities prepared from pure ethanol during nanotube growth, making the purification with air oxidation more effective. 
         [0018]    FWNTs were prepared using the following synthesis technique. 
         [0019]    I. Synthesis of FWNTs 
         [0020]    A catalyst was prepared by a combustion method 5 . Specifically, a clear solution was first prepared by dissolving Mg(NO 3 ) 2 .6H 2 O, (NH 4 ) 6 Mo 7 O 24 .4H 2 O, Co(NO 3 ) 2 .6H 2 O, citric acid and glycine in deionized (DI) water. The atomic ratio between metals was Co:Mo:Mg=1:3:48. The solution was then heated to evaporate the solvent. After all the solvent was evaporated, the residue was combusted. The catalyst was obtained by collecting the ashes and annealing them at 500° C. for 1 hour. MgO blank catalyst was prepared by the same procedure with Mg(NO 3 ) 2 , glycine and citric acid. 
         [0021]    FWNTs were then synthesized in a one-inch quartz tube heated in a tube furnace. The carbon source used for FWNTs synthesis was an ethanol/methanol mixture with different ratios. (As used herein, all ratios are volume ratios unless specifically identified to the contrary.) In a typical experiment, the catalyst was first heated from room temperature to 1100° C. under in a 1500 sccm Ar and 500 sccm H 2  mixture. After the temperature reached 1100° C., H 2  was turned off, Ar was increased to 2000 sccm, and the methanol/ethanol mixture was added to the system using a syringe pump at a controlled rate of 20 ml/hr. After 20 min, the methanol/ethanol was turned off and the system was cooled down to room temperature. 
         [0022]    To purify the sample, the raw material was first heated under 1:4 air/Ar mixture (350 sccm air and 1400 sccm Ar) at 575□ for 2.5 hr, and then stirred in 6M HCl to remove the Co/Mo catalyst and MgO support. The sample was rinsed with diluted KOH aqueous solution and deionized water (DI water). 
         [0023]    Both the raw and purified products were characterized with thermogravimetric analyzer (TGA), transmission electron microscopy (TEM) and X-ray diffraction (XRD). All the TGA measurements were done under diluted air (20 vol % air and 80 vol % Ar). TEM samples were prepared by dispersing the FWNT samples in ethanol under assistance of sonication and drop-drying the mixture on copper grids (Ted Pella) at room temperature. 
         [0024]    II. Results and Discussion 
         [0025]    Other than FWNTs, there are always carbonaceous impurities in FWNT raw material (see  FIG. 1A ). These non-CNT carbonaceous species make purification of FWNTs rather difficult and inefficient. The impurities not only form due to the inhomogeneity of catalyst particles but also form due to deposition of carbon on the MgO support. Blank control samples were prepared by treating pure MgO powder at exactly the same condition as FWNT synthesis described above with ethanol as the carbon source. Photograph and TGA weight loss plot of the MgO blank control sample is shown in  FIG. 2   a . The black color of control sample of  FIG. 2   a  suggests that there are significant amounts of carbonaceous species deposited on the MgO even without the catalyst nanoparticles under such a condition. The TGA weight loss plot shows the carbon yield is approximately 54% while the TEM image ( FIG. 3 ) shows there are no CNTs in the sample. It is important to prevent the formation of these non-CNT carbon species on the MgO support to improve the FWNT purity in raw material. 
         [0026]    To reduce the formation of these carbonaceous impurities, an ethanol/methanol mixture was explored as the carbon source for FWNT synthesis.  FIGS. 2   b  through  2   d  show photographs and TGA weight loss plots of a series of control samples using ethanol/methanol mixtures of different volume ratios as carbon sources on blank MgO supports. One same shown in  FIG. 2   e  was prepared using 100% methanol (0% ethanol). 
         [0027]    The five samples shown in  FIGS. 2   a - 2   e  can be divided into two groups according to their colors. Specifically, samples made from 100 and 80 vol % ethanol belong to one group which are black ( FIG. 2   a  and  2   b ) while the samples made from 50, 20 and 0 vol % ethanol belong to another group which are white or light gray ( FIG. 2   c - 2   e ). Carbon yields of these blank samples can be deduced from the TGA weight loss plots depicted. The two black samples show a significant amount of carbon deposited; while lighter colored samples show almost no carbonaceous species. These results suggest that addition of methanol to ethanol in a volume ratio range of methanol to ethanol between about 20:80 to about 80:20 reduces the carbon deposit on a pure MgO support. This reduction in carbon deposit on a pure MgO support is most notable when the volume percentage of methanol is greater than 50%, that is at a volume ratio of methanol to ethanol of about 50:50 or greater. 
         [0028]    Accompanying  FIG. 4  shows the first order differentiation of the weight loss plots of the blank samples. First order differentiation of the TGA weight loss plot can be used to evaluate the composition of the samples. Peaks at higher temperatures indicate the existence of carbonaceous species with higher burning temperatures and less defective graphite. The number and position of the peaks depend on TGA parameters such as heating speed, carrier gas composition and flow rate as well as the amount and ratio of carbonaceous species in the sample. Burning temperatures of carbonaceous impurities are important since selective oxidation of carbonaceous impurities under diluted air is the most efficient method so far for FWNT sample purification. Higher burning temperature of the carbonaceous impurities results in less effective purification and/or very low purification yield. 
         [0029]    Peak positions in  FIG. 4  are summarized in Table 1 below. 
         [0000]    
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Peak position in FIG. 4 (° C.) 
               
             
          
           
               
                 EtOH 100% 
                 EtOH 80% 
                 EtOH 50% 
                 EtOH 20% 
                 EtOH 0% 
               
               
                   
               
               
                 736 
                 708 
                   
                   
                   
               
               
                 642 
                 598 
               
               
                   
                   
                 528 
                 515 
               
               
                 304 
                 301 
                 267 
                 268 
                 276 
               
               
                   
               
             
          
         
       
     
         [0030]    As shown in  FIG. 4 , there are up to four peaks in each of the plots. The highest peak is around 700° C. Only samples prepared with 100 vol % and 80 vol % ethanol show the peak at 700° C. All samples show the lowest peak which is around 280° C. Samples with higher ethanol percentage (100 vol % and 80 vol %) show a peak around 600□ and those with lower ethanol percentage (50 vol % and 20 vol %) show a peak around 520° C. Samples prepared with pure methanol do not show these intermediate peaks. Peak intensities of samples also vary considerably as a function of ethanol volume percentage. 
         [0031]    These results suggest that when the ethanol percentage in the mixture is less than 50 vol %, the amount of carbonaceous species deposited on MgO support is small and easily removed. Higher purity raw FWNT material with less carbonaceous impurities thus is expected from a carbon source with low ethanol percentage. This expectation is confirmed by the purification results. Specifically, as shown in  FIGS. 5   a  and  5   b , samples prepared using low ethanol percentage carbon sources are almost free of impurities after purification, while considerable carbonaceous impurities can still be found in the purified samples obtained from raw samples prepared with higher ethanol percentage carbon sources. 
         [0032]    Among all the reported results of CNT synthesis with alcohol as carbon source, most used ethanol 22, 25-27  as the carbon source and only a few used methanol 23, 34 . Oxidative radicals formed during alcohol decomposion are considered to play an important role in the CNT growth 22, 24 . The oxidative radicals have two effects on CNT material, one is etching carbonaceous impurities and the other is making carbonaceous impurities more defective so that they are easier to be removed by selective oxidation. 
         [0033]    According to the present invention, methanol and ethanol, the two simplest alcohol members have been discovered to be usefully employed as a carbon source for FWNT synthesis. However, surprisingly they each acted totally differently. Under the specific growth conditions, it was discovered that ethanol by itself can act as carbon source for FWNT growth, while methanol can not be used as a carbon source under the same conditions. Samples made from pure methanol do not show any carbon deposition. TGA and XRD results show no carbon signal and no CNTs were found under TEM. 
         [0034]    The effect of adding methanol to ethanol is not simply to dilute ethanol in the feeding gas stream. For example, when a carbon source of ethanol only was fed with a rate of 10 mL/hr, the carbon yield on pure MgO support was 38%. In contrast, a mixture of 50 vol % ethanol and 50 vol % methanol fed with 20 mL/hr deposits almost no carbon on an MgO support ( FIG. 2   c ). 
         [0035]    Without wishing to be bound to any particular theory, it is believed that oxidative radicals generated from methanol decomposition play an important role in the FWNT growth. The decomposition of methanol resulted in stronger oxidative environment during FWNT growth. Under such conditions, carbonaceous species with less stable structure may be removed. Similar effect has also been reported in the art 23, 34  and showed that there are fewer small diameter SWNTs in samples made from methanol than those from ethanol. Under the FWNT growth condition, the oxidative environment is strong enough to remove almost all carbon deposit on a MgO support when methanol was employed as the carbon source. This hypothesis is consistent with the decreased carbon yield of raw FWNT material as a function of the increased methanol percentage in the mixture. Moreover, although an oxidative environment can help to remove carbonaceous impurities during the FWNT growth, FWNTs may also be removed if the oxidizability of the environment is stronger than necessary. Thus, if only about 1% O 2  is added to Ar carrier gas, almost no FWNTs can be found in the sample. 
         [0036]    While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope thereof.
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