Abstract:
The present disclosure is directed to methods for producing indium nanoparticles. The methods comprise dissolving indium chloride in a solution that includes a solvent and a surfactant, adding a reducing agent to the reaction mixture to form an agglomerate of In nanoparticles, and exposing the reaction mixture to a gas including oxygen to disperse the agglomerate into a plurality of individual indium nanoparticles.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation application of U.S. patent application Ser. No. 14/794,931 filed Jul. 9, 2015 for “METHOD FOR CONTINUOUS AND CONTROLLABLE PRODUCTION OF SINGLE WALLED CARBON NANOTUBES”, which claims priority to U.S. Provisional Patent Application Ser. No. 62/022,398 filed Jul. 9, 2014 for “METHOD FOR CONTINUOUS AND CONTROLLABLE PRODUCTION OF SINGLE WALLED CARBON NANOTUBES”, the disclosures of which are hereby incorporated by reference in their entirety. 
     
    
     BACKGROUND 
       [0002]    The present disclosure relates to methods for producing a single-walled carbon nanotube in a chemical vapor deposition (CVD) reactor comprising contacting liquid catalyst droplets and a carbon source in the reactor, and forming a single walled carbon nanotube at the surface of the liquid catalyst droplets. 
         [0003]    Single-walled carbon nanotubes (SWNT) are increasingly becoming of interest for various applications in nanotechnology because of their unique electronic structures, which gives them exceptional thermal, mechanical, and electrical properties. For example, SWNT can be used in electronics, energy devices, medicine, and composite materials in order to obtain desirable physical and chemical properties. These uses require methods for producing significant quantities of SWNT. 
         [0004]    Various processes are used to produce SWNT including physical methods (e.g., electrical arc, laser ablation) and chemical methods (e.g., pyrolysis, chemical vapor deposition). Bench production yield from these methods is low and expensive. In addition, the product is not homogenous and contains tubes with broad diameter distributions and various helicity and thereby various electrical and mechanical properties, which limit their broader applications. Accordingly, there is a need in the art for methods for controllably producing SWNT with narrow distributions of structural helices, and high yields. 
       BRIEF SUMMARY 
       [0005]    In one aspect, a method for producing indium (In) nanoparticles is provided. The method includes dissolving indium chloride in a solution that includes a solvent and a surfactant to form a reaction mixture. The method also includes adding a reducing agent to the reaction mixture to form an agglomerate of In nanoparticles within the reaction mixture. The method further includes exposing the reaction mixture to a gas including oxygen to disperse the agglomerate into a plurality of individual In nanoparticles. 
         [0006]    In another aspect, a method for producing indium (In) nanoparticles is provided. The method includes dissolving indium chloride in a solution that includes tetrahydrofuran and trioctylphosphine to form a reaction mixture. The method also includes adding superhydride solution to the reaction mixture to form an agglomerate of In nanoparticles within the reaction mixture. The method further includes exposing the reaction mixture to air to disperse the agglomerate into a plurality of individual In nanoparticles. 
         [0007]    The features, functions, and advantages described herein may be achieved independently in various embodiments of the present disclosure or may be combined in yet other embodiments, further details of which may be seen with reference to the following description and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  shows an example implementation of a vertical CVD reactor utilized in the methods described herein. 
           [0009]      FIG. 2  shows an example implementation of a horizontal CVD reactor utilized in the methods described herein. 
           [0010]      FIG. 3A  shows a solid catalyst particle with facets or surface defects, and  FIG. 3B  shows a liquid catalyst droplet of the present method. 
           [0011]      FIGS. 4A and 4B  show example implementations of vertical CVD reactors utilized in the methods described herein. 
           [0012]      FIG. 5  shows a flow chart representation of an exemplary method of producing SWNT in a CVD reactor. 
           [0013]      FIG. 6  shows a flow chart representation of an exemplary method of producing SWNT in a CVD reactor using colloidal solid catalyst particles as a catalyst precursor. 
           [0014]      FIG. 7  shows a flow chart representation of an exemplary method of producing SWNT in a CVD reactor using a catalyst metalorganic precursor. 
           [0015]      FIG. 8  shows a TEM image of hcp Ru nanoparticles suitable for use as a catalyst using the methods described herein. 
           [0016]      FIG. 9  shows a TEM image of In nanoparticles suitable for use as a catalyst using the methods described herein. 
           [0017]      FIG. 10  shows a TEM image of nanotubes formed using gallium (III) acetylacetonate in ethanol as a catalyst precursor at a growth temperature of about 900° C. using the methods described herein. 
           [0018]      FIG. 11  shows a TEM image of nanotubes formed using gallium (III) acetylacetonate in ethanol as a catalyst precursor at a growth temperature of about 925° C. using the methods described herein. 
           [0019]      FIG. 12  shows Ramen spectra of nanotubes formed using the methods described herein. 
           [0020]      FIG. 13  shows Ramen spectra of nanotubes formed using the methods described herein, and showing the radial breathing modes. 
       
    
    
       [0021]    Although specific features of various implementations may be shown in some drawings and not in others, this is for convenience only. Any feature of any drawing may be referenced and/or claimed in combination with any feature of any other drawing. 
       DETAILED DESCRIPTION 
       [0022]    The subject matter described herein relates generally to methods for producing a single-walled carbon nanotube by a chemical vapor deposition (CVD) method. Generally, the methods include contacting liquid catalyst droplets and a carbon source in the reactor, and forming a single walled carbon nanotube at the surface of the liquid catalyst droplets. The following detailed description illustrates implementations of the subject matter described in this application by way of example and not by way of limitation. 
         [0023]    Carbon nanotubes are allotropes of carbon, typically with a substantially cylindrical nanostructure. Carbon nanotubes are categorized as single-walled nanotubes (SWNT) and multi-walled nanotubes (MWNT). SWNT have a single layer of carbon comprising its microstructure while MWNT are comprised of several layers. While described herein with respect to SWNT, it should be understood that the methods of the present disclosure are applicable to the formation of both SWNT and MWNT. 
         [0024]    Catalysts used in the methods of the present disclosure are particularly suitable to initiate and increase the rate of chemical reactions which produce SWNT. More specifically, the catalysts of the present disclosure interact with a carbon source, further described below, to form SWNT. The catalysts serve as the formation surface for SWNT. Any catalysts and catalyst mixtures having a melting point lower than the SWNT synthesis temperature may be used in the present disclosure. More specifically, the catalysts described herein may include materials including iron, nickel, cobalt, copper, chromium, indium, gallium, platinum, manganese, cerium, europium, ytterbium, silver, gold, zinc, cadmium, and lanthanum, any other catalysts known in the art, and compounds and combinations thereof. In particularly suitable embodiments, the catalysts include indium, gallium, and combinations thereof. 
         [0025]    The catalysts used in the methods of the present disclosure may be prepared in many different forms. Non-limiting examples include a catalyst precursor material that is formed as a solid catalyst particle, a catalyst precursor material that is formed as a solid that is dispersed in a liquid, or a catalyst precursor material that is dissolved in a solvent. In an illustrative example, the catalyst precursor material is formed in a colloidal solid catalyst particle. The colloidal solid catalyst particle may be dispersed in a liquid. In a non-limiting example, the liquid may be an organic solution. In a non-limiting example, the catalyst precursor material is a colloidal solid catalyst particle, and the liquid has a decomposition temperature lower than the vaporization temperature of the catalyst. 
         [0026]    In another non-limiting example, the catalyst precursor materials may be a catalyst metalorganic precursor (e.g., metallocene). Metallocene precursors of the present disclosure typically include catalyst metals bonded to organic material (e.g., cyclopentadienyl anions). Non-limiting examples of suitable catalyst metalorganic precursors include: gallium acetylacetonate, indium acetylacetonate, ruthenium acetylacetonate, gallium acetate, indium acetate, ruthenium acetate, gallium nitrate, indium nitrate, ruthenium nitrate, gallium sulfate, indium sulfate, ruthenium sulfate, gallium chloride, indium chloride, ruthenium chloride, and any combination thereof. It is to be understood that the metallocene precursor may also be provided as a solid powder, as a solid dispersed in a liquid, or dissolved in a solvent. 
         [0027]    Catalysts used in the methods of the present disclosure, described more fully above, are configured to form liquid catalyst droplets that provide surfaces for the growth of SWNT during catalytic decomposition of a carbon source. The carbon source used in the methods of the present disclosure include elemental carbon and any carbon containing source capable of providing elemental carbon for the formation of SWNT. In general, any carbon containing gas that does not pyrolize at temperatures up to 500° C. to 1000° C. can be used. Non-limiting examples include carbon monoxide, aliphatic hydrocarbons, both saturated and unsaturated, such as methane, ethane, propane, butane, pentane, hexane, acetylene, ethylene, and propylene; oxygenated hydrocarbons such as acetone, methanol, and ethanol; aromatic hydrocarbons such as benzene, toluene, and naphthalene; and mixtures of the above, for example carbon monoxide and methane. The carbon source may optionally be mixed with a diluent gas such as hydrogen, helium, argon, neon, krypton and xenon or a mixture thereof. 
         [0028]    The catalyst droplets in the described methods provides for a decomposition reaction of the carbon source to elemental carbon with a lower activation energy. Still in another example, the catalyst droplets provide for a formation reaction of elemental carbon to SWNT with a lower activation energy. As shown in  FIG. 3A , solid catalytic material  302  includes facets or defects  304  on the surface where the nanotube  306 A/ 306 B grows during synthesis. Such surface variations result in a heterogeneous nanotube product, as evidenced by the variation in size between nanotubes  306 A and  306 B. In contrast, a support free liquid catalyst droplet  308  as used in the methods of the present disclosure and as shown in  FIG. 3B  provides a smooth, spherical shape  310  that is not disturbed by various facets and surface defects inherent in solid catalytic material and supports. Therefore, it provides a more uniform surface for nanotube growth that produces a more homogeneous nanotube product, as evidenced by the uniform sizes of nanotubes  306 C and  306 D formed on the free liquid catalyst droplet  308 . 
         [0029]    In some embodiments of the present disclosure, the reactants used in the chemical process of forming SWNT include a carrier gas In methods of the present disclosure, the carrier gas is typically a gas used as a mass transport mechanism but may also be used to dilute the reactants or slow the reaction rate. The carrier gas used in methods of the present disclosure may include one or more inert gases (e.g., nitrogen, helium, neon, argon, etc.) or any other gas that may be suitable for improving the formation of SWNT as known in the art. Non-limiting examples of other suitable gases include: hydrogen, carbon dioxide, ammonia, and any combination thereof. 
         [0030]    In some embodiments of the present disclosure, the carrier gas may transport the reactants through the various zones of the reactor, promote mixing of the liquid catalyst droplets and carbon source, inhibit the introduction of unwanted gaseous reactants including, but not limited, to air or oxygen, and/or maintain the reactants in a suitable position within the zones of the reactor. By way of non-limiting example, the carrier gas may offset the effect of gravity on the various particulate reactants within a vertically-oriented reactor, thereby maintaining the particulate reactants at a suitable vertical position within the reactor. Non-limiting examples of particulate reactants maintained in a suitable position within the reactor by the carrier gas include: catalyst particles, liquid catalyst droplets, and SWNT produced by the reactor. 
         [0031]    In some embodiments of the present disclosure, the carrier gas may be injected or inserted into the reactor at a flow rate ranging from about 500 sccm (standard cubic cm/min.) to about 2000 sccm. In various other embodiments of the present disclosure, the carrier gas may be injected or inserted into the reactor at a flow rate ranging from about 500 sccm to about 600 sccm, 550 sccm to about 650 sccm, 600 sccm to about 700 sccm, 650 sccm to about 750 sccm, 700 sccm to about 800 sccm, 750 sccm to about 850 sccm, 800 sccm to about 900 sccm, 850 sccm to about 950 sccm, 900 sccm to about 1000 sccm, 950 sccm to about 1050 sccm, 1000 sccm to about 1200 sccm, 1100 sccm to about 1300 sccm, 1200 sccm to about 1400 sccm, 1300 sccm to about 1500 sccm, 1400 sccm to about 1600 sccm, 1500 sccm to about 1700 sccm, 1600 sccm to about 1800 sccm, 1700 sccm to about 1900 sccm, and from about 1800 sccm to about 2000 sccm. 
         [0032]    The liquid catalyst droplets and carbon source are generally contacted to form the carbon nanotubes in any type of reactor known in the nanotube formation art. Generally, carbon nanotubes are formed by a number of different chemical processes including pyrolysis and chemical vapor deposition (CVD) in a number of different types of reactors. Such reactors are suitable for methods of the present disclosure. More particularly, reactors of the present disclosure may include a plurality of zones including one or more of an injection zone, a decomposition zone, a size selection zone, a growing zone, a cooling zone, and a collection zone. It should be understood by one skilled in the art that the reactors used in the methods of the present disclosure may include any combination of these zones in any suitable orientation without departing for the scope of the present disclosure. Reactors used in the methods of the present disclosure are configured both horizontally and vertically.  FIGS. 1 and 2  provide non-limiting examples of CVD reactors that can be used in the methods of the present disclosure. 
         [0033]      FIG. 1  is a schematic illustration of an exemplary vertically-oriented CVD reactor  100  in one embodiment. The CVD reactor  100  may include an injection zone  102  to introduce the catalyst material and carbon source to the CVD reactor  100 , a decomposition zone  104  to partially or completely decompose one or more of the reagents within the CVD reactor  100 , a growing zone  106  to grow SWNT, a cooling zone  108  to cool the reactants and/or products in the CVD reactor  100 , and a collection zone  110  to collect products and unreacted reactants from the CVD reactor  100 . 
         [0034]    As illustrated in  FIG. 1 , the injection zone  102  may be configured to inject or insert the catalyst material or catalyst precursor material  124 , and the carbon source  126  into the reactor  100 . In particularly suitable embodiments, the injection zone  102  includes one or more feed mechanisms (e.g., a syringe, pump, atomizer, nozzle, etc.), which are capable of controlling a number of different variables with respect to the catalyst or catalyst precursor  124 , and carbon source  126  entering the reactor  100  including, but not limited to: volume, ratio, flow rate, and pressure. The injection zone  102  may also operate to increase the surface area of the reactants as they enter the reactor and/or contact the catalyst  124  and the carbon source  126 . In one embodiment, the injection zone may include an injector  112  to inject the catalyst or catalyst precursor  124  into the injection zone  102  for subsequent transport to other zones of the reactor  100  including, but not limited to, the decomposition zone  104 . 
         [0035]    The decomposition zone  104  of the reactor  100  used in the methods of the present disclosure may be configured to partially or completely decompose one or more of the catalyst precursor material  124 , carbon source  126  and any liquids entering or exiting the reactor  100 . For example, the decomposition zone  104  may be configured to evaporate or decompose a liquid or solvent in which the catalyst precursor  124  may be dispersed or otherwise dissolved, transform the catalyst precursor  124  to liquid catalyst droplets, melt solid catalyst material  128  introduced to the decomposition zone  104  to form liquid catalyst droplets  130 , maintain liquid catalyst droplets  130  introduced to the decomposition zone  104 , or any combination thereof. The decomposition zone  104  may be maintained at optimal temperature, pressure, and/or pH conditions to facilitate such processes. The decomposition zone  104  may be maintained at a temperature above the melting point of the solid catalyst material  124  used in the methods, but below the vaporization temperature of the resulting liquid catalyst droplets  130 . Accordingly, the temperature of the decomposition zone may be adjusted to accommodate the selected catalyst material  124  and carbon source  126 . The decomposition zone  104  may be heated with any type of heating mechanism without limitation. In one non-limiting example, the decomposition zone  104  may include a plasma (not illustrated) to transform the catalyst precursor  124  to catalyst particles  128 . 
         [0036]    In one embodiment, the decomposition zone  104  may be maintained at a decomposition temperature ranging from about 150° C. to about 400° C. In another embodiment, the decomposition zone  104  may be maintained at a decomposition temperature ranging from about 200° C. to about 300° C. In other embodiments, the decomposition zone  104  may be maintained at a decomposition temperature ranging from about 150° C. to about 170° C., from about 160° C. to about 180° C., from about 170° C. to about 190° C., from about 180° C. to about 200° C., from about 190° C. to about 210° C., from about 200° C. to about 220° C., from about 210° C. to about 230° C., from about 220° C. to about 240° C., from about 230° C. to about 250° C., from about 240° C. to about 260° C., from about 250° C. to about 270° C., from about 260° C. to about 280° C., from about 270° C. to about 290° C., from about 280° C. to about 300° C., from about 290° C. to about 310° C., from about 300° C. to about 320° C., from about 310° C. to about 330° C., from about 320° C. to about 340° C., from about 330° C. to about 350° C., from about 340° C. to about 360° C., from about 350° C. to about 370° C., from about 360° C. to about 380° C., from about 370° C. to about 390° C., and from about 380° C. to about 400° C. 
         [0037]    The decomposition zone  104  of the reactor  100  may also serve as the area for contacting the liquid catalyst droplets  130  and the carbon source  126 . Therefore, the decomposition zone  104  may be maintained at a decomposition temperature selected to allow for the catalytic decomposition of the carbon source  126  and subsequent growth of the single-walled carbon nanotubes (SWNT)  120  on the liquid catalyst droplets  130  in the growing zone  106 . 
         [0038]    The growing zone  106  of the reactor  100  used in the methods of the present disclosure is typically configured to grow SWNT  120 . To facilitate SWNT growth, the growing zone  106  may be maintained at optimal temperature and/or pressure conditions. In one embodiment, the growing zone  106  may be maintained at a synthesis temperature ranging from about 500° C. to about 1300° C. In another embodiment, the growing zone  106  may be maintained at a synthesis temperature ranging from about 800° C. to about 1200° C. In an additional embodiment, the growing zone  106  may be maintained at a synthesis temperature ranging from about 900° C. to about 1050° C. 
         [0039]    In other additional embodiments, the growing zone  106  may be maintained at a synthesis temperature ranging from about 500° C. to about 600° C., from about 550° C. to about 650° C., from about 600° C. to about 700° C., from about 650° C. to about 750° C., from about 700° C. to about 800° C., from about 750° C. to about 850° C., from about 800° C. to about 900° C., from about 850° C. to about 950° C., from about 900° C. to about 1000° C., from about 950° C. to about 1050° C., from about 1000° C. to about 1100° C., from about 1050° C. to about 1150° C., from about 1100° C. to about 1200° C., from about 1150° C. to about 1250° C., and from about 1200° C. to about 1300° C. 
         [0040]    The cooling zone  108  of the reactor  100  used in the methods of the present disclosure is typically configured to cool the reactants and/or products in the reactor  100 . In particularly suitable embodiments, the cooling zone  108  is maintained at a cooling temperature below the solidification temperature of one or more reactants or products so as to initiate solidification, freezing, or deposition. 
         [0041]    The collection zone  110  of the reactor  100  used in the methods of the present disclosure is typically configured to collect the SWNT  120  produced within the growth zone  106  as well as any unreacted reactants  132  from the reactor  100 . In particularly suitable embodiments, the collection zone  110  is situated at an end of the reactor  100  opposite the injection zone  102 . 
         [0042]      FIG. 2  is a schematic illustration of an exemplary horizontally-oriented CVD reactor  100 A in one embodiment. The CVD reactor  100  may include an injection zone  102  to introduce the carbon source  116  and a carrier gas  202  to the CVD reactor  100 , a decomposition zone  104  to partially or completely decompose one or more of the reagents within the CVD reactor  100 , a growing zone  106  to grow SWNT, a cooling zone  108  to cool the SWNT products, and a collection zone  110  to collect the SWNT products and unreacted reactants from the CVD reactor  100 . 
         [0043]    The injection zone  102  of the reactor  100 A may be configured to inject or insert the carbon source  126  and a carrier gas  202  into the reactor  100 A. In particularly suitable embodiments, the injection zone  102  includes one or more feed mechanisms (e.g., a syringe, pump, atomizer, nozzle, etc.), which are capable of controlling a number of different variables with respect to the carbon source  126  and the carrier gas  202  entering the reactor  100 A including, but not limited to: volume, ratio, flow rate, and pressure. As illustrated in  FIG. 2 , one feed mechanism  204  may include an elongated conduit  208  to introduce the carbon source  116  into one end of the growing zone  106  of the reactor  100 A. In another aspect, a second feed mechanism  206  may be configured to introduce the carrier gas  202  used to transport the reactants through the reactor  100 A into the decomposition zone  104 . 
         [0044]    As illustrated in  FIG. 2 , the decomposition zone  104  may be configured to receive a powdered catalyst metallocene precursor  210  in a carrier  212 . The decomposition zone  104  may be further configured to partially or completely decompose the powdered catalyst metallocene precursor  210  and to form catalyst liquid droplets  130 . The decomposition zone  104  may be maintained at optimal temperature, pressure, and/or pH conditions to facilitate such processes. The decomposition zone  104  may be maintained at a temperature above the melting point of the solid catalyst material  124  used in the methods, but below the vaporization temperature of the resulting liquid catalyst droplets  130 . Accordingly, the temperature of the decomposition zone may be adjusted to accommodate the selected catalyst material  124  and carbon source  126 . 
         [0045]    In one embodiment, the decomposition zone  104  may be maintained at a decomposition temperature ranging from about 200° C. to about 300° C. In other embodiments, the decomposition zone 104 may be maintained at a decomposition temperature ranging from about 200° C. to about 220° C., from about 210° C. to about 230° C., from about 220° C. to about 240° C., from about 230° C. to about 250° C., from about 240° C. to about 260° C., from about 250° C. to about 270° C., from about 260° C. to about 280° C., from about 270° C. to about 290° C., and from about 280° C. to about 300° C. 
         [0046]    The growing zone  106  of the reactor  100 A used in the methods of the present disclosure is typically configured to grow SWNT  120 . To facilitate SWNT growth, the growing zone  106  may be maintained at an optimal temperature and/or pressure conditions. Any form of heating may be provided to maintain the growing zone  106  at the desired synthesis temperature. In one embodiment, the growing zone  106  may be maintained at a synthesis temperature ranging from about 500° C. to about 1300° C. In another embodiment, the growing zone  106  may be maintained at a synthesis temperature ranging from about 800° C. to about 1200° C. In an additional embodiment, the growing zone  106  may be maintained at a synthesis temperature ranging from about 900° C. to about 1050° C. 
         [0047]    In other additional embodiments, the growing zone  106  may be maintained at a synthesis temperature ranging from about 500° C. to about 600° C., from about 550° C. to about 650° C., from about 600° C. to about 700° C., from about 650° C. to about 750° C., from about 700° C. to about 800° C., from about 750° C. to about 850° C., from about 800° C. to about 900° C., from about 850° C. to about 950° C., from about 900° C. to about 1000° C., from about 950° C. to about 1050° C., from about 1000° C. to about 1100° C., from about 1050° C. to about 1150° C., from about 1100° C. to about 1200° C., from about 1150° C. to about 1250° C., and from about 1200° C. to about 1300° C. 
         [0048]    The cooling zone  108  of the reactor  100 A used in the methods of the present disclosure is typically configured to cool the reactants and/or products in the reactor  100 A. In particularly suitable embodiments, the cooling zone  108  is maintained at a cooling temperature below the solidification temperature of one or more reactants or products so as to initiate solidification, freezing, or deposition. 
         [0049]    The collection zone  110  of the reactor  100 A used in the methods of the present disclosure is typically configured to collect the SWNT  120  produced within the growth zone  106  as well as any unreacted reactants  132  from the reactor  100 A. In particularly suitable embodiments, the collection zone  110  is situated at an end of the reactor  100 A opposite the injection zone  102 . 
         [0050]    In various other embodiments, the CVD reactor  100  may further include a size selection zone  402 , as illustrated in  FIGS. 4A and 4B . The optional size selection zone  402  is configured to separate out a catalyst material or catalyst precursor material  128  with a desired particle size distribution. Accordingly, the size selection zone  402  can be provided at any point in the method prior to the growth of nanotubes on the liquid catalyst droplets  130 . In a non-limiting example, the size selection zone or step can be performed by using a cylindrical differential mobility analyzer (DMA) and an ultrafine condensation particle counter. For instance, the particle size and distribution could be determined by on-line aerosol size classification using the DMA and the ultrafine condensation particle counter as described in Lenggoro et al., “Sizing of Colloidal Nanoparticles by Electrospray and Differential Mobility Analyzer Methods,” Langmuir, 2002, 18, 4584-4591, which is hereby incorporated by reference to the extent it is consistent herewith. The DMA allows for the isolation of catalyst material or catalyst precursor material particles that fall within a desired range of electrical mobility from a steady stream of charged particles suspended in a gas. The narrow range of electrical mobilities in the aerosol that is classified by the DMA directly translates to a narrow range of particle size. Such controlled particle size distribution provides for a more homogenous growth environment, and the resulting nanotube product. 
         [0051]    Without being limited to any particular theory, colloidal chemistry allows for the production of catalyst precursor materials having a narrow particle size distribution, thereby resulting in liquid catalyst droplets having a narrow particle size distribution. Therefore, the use of a size selection step for colloidal catalyst material may be optional. In a non-limiting example, the size selection step may be performed before introduction of the colloidal catalyst particles into the reactor. 
         [0052]    However, colloidal catalyst precursors may be expensive to produce. Accordingly, the size selection step allows the use of less expensive catalyst material or catalyst material precursors production methods that would otherwise produce a broad particle size distribution for the liquid catalyst droplets and catalyst material precursors, such as catalyst metalorganic precursors (e.g., metallocene). As shown in  FIGS. 4A and 4B , a catalyst material precursor  124 , such as a catalyst metallocene precursor  204 , is introduced into the reactor  100 . The catalyst precursor  124  may be in the form of a solid powder, a solid dispersed in a liquid, or the catalyst material may be dissolved in a solvent  404 . In the event the catalyst precursor  124  is dispersed in a liquid or dissolved in a solvent  404 , the liquid or solvent  404  is evaporated when introduced into the injection zone  102  and/or decomposition zone  104  of the reactor  100 . In addition, if the catalyst material precursor  124  is provided in the form of a catalyst metallocene precursor  204 , the catalyst metallocene precursor  204  may be pyrolized to form catalyst particles  128 . In some embodiments, the evaporated solvent  404  may be separated from the catalyst particles  128  and ejected from the reactor  100 , as illustrated in  FIG. 4A  and  FIG. 4B . In other embodiments, the solvent may be retained along with the catalyst particles  128 . By way of non-limiting example, the evaporated solvent  404  may be a carbon-containing solvent such as ethanol that may be evaporated and used as a carbon source  126  in the growing zone  106 . 
         [0053]    As shown in  FIGS. 4A and 4B , the catalyst particles  128  enter the size selection zone  402  so that catalyst particles  128  within the desired particle size distribution are allowed to enter the growing zone. Although  FIGS. 4A and 4B  illustrate catalyst particles  128  entering the size selection zone  402 , the catalyst may alternatively be in the form of liquid catalyst droplets  130  when entering and exiting the size selection zone (not illustrated). The growing zone  106  is maintained at a synthesis temperature that is above the melting point of the catalyst particles  128  so that the catalyst is maintained in the form of liquid catalyst droplets  130 . The carbon source  126  is introduced in the growing zone  106  so that nanotubes (not illustrated) are formed on the surface of the liquid catalyst droplets  130 . 
         [0054]    As shown in  FIG. 4A , all of the steps of the method may occur in a single reactor  100 , or they may be performed in separate reactor stages  406 / 408  as shown in  FIG. 4B . Therefore, the catalyst particles  128  may be formed at a location separate from the size selection device  402  and the growing zone  106 . Alternatively, the catalyst particles  128  may be formed and undergo the size selection step at one location, and introduced to the growing zone  106  at another location. 
         [0055]      FIG. 5  is a flow chart of one embodiment of a method  500  of producing single-walled carbon nanotubes (SWNT) in a CVD reactor. Any embodiment of the CVD reactor  100 / 100 A illustrated in  FIGS. 1, 2, 4A, and 4B  may be used to effectuate this method  500 . As illustrated in  FIG. 5 , the method  500  may include contacting liquid catalyst droplets and a carbon source at a synthesis temperature at step  502 . Generally, the method for producing SWNT in a CVD reactor of the present disclosure includes contacting the liquid catalyst droplets described herein above and a carbon source described herein above within the growing zone  106  of the reactor  100 / 100 A described herein above and illustrated in  FIGS. 1 and 2 . The synthesis temperature may range from about 500° C. to about 1300° C. or may be selected from one of the synthesis temperature ranges provided herein above. As described herein previously, the synthesis temperature may be selected based on any one of at least several factors including but not limited to: the composition of the liquid catalyst drops and the composition of the carbon source. 
         [0056]    Referring again to  FIG. 5 , the method may further include forming the SWNT at the surface of the liquid catalyst droplets at step  504 . As described herein above, the SWNT are formed as a result of a catalytic decomposition of the carbon source to elemental carbon at the surface of the liquid catalyst droplets. The SWNT formed at step  504  may be collected for subsequent use (step not shown). 
         [0057]      FIG. 6  is a flow chart of another embodiment of a method  600  of producing single-walled carbon nanotubes (SWNT) in a CVD reactor. The method  600  includes introducing colloidal solid catalyst particles into the reactor at a decomposition temperature to form liquid catalyst droplets at step  602 . By way of non-limiting example, step  602  may be conducted by injecting colloidal solid catalyst particles into an injection zone  102  at one end of a reactor  100  as described herein above and as illustrated in  FIG. 1 . The colloidal solid catalyst particles may move into a decomposition zone  104  of the reactor  100  as illustrated in  FIG. 1  by gravity alone or by any other method of moving particles within a reactor as known in the art. The decomposition zone  104  is maintained at the decomposition temperature. The decomposition temperature may be any decomposition temperature provided herein above, including but not limited to a decomposition temperature ranging from about 200° C. to about 300° C. 
         [0058]    The liquid catalyst droplets may be formed at step  602  by melting the colloidal solid catalyst particles as they move through a decomposition zone  104  as illustrated in  FIG. 1 . Depending on the form in which the colloidal solid catalyst particles are provided, the colloidal solid catalyst particles may be subjected to additional processes. If the colloidal solid catalyst particles are provided as a suspension within a solution, the solution surrounding the colloidal solid catalyst particles may be thermally removed at step  604  as illustrated in  FIG. 6 . In some embodiments, the solution surrounding the colloidal solid catalyst particles may be evaporated or thermally decomposed in the decomposition zone  104  of the reactor  100  as illustrated in  FIG. 1 . In a non-limiting example, the thermal decomposition of the solution surrounding the colloidal solid catalyst particles may occur before the melted catalyst (i.e., liquid catalyst droplets) contact the carbon source. 
         [0059]    In some embodiments, the liquid catalyst droplets may undergo size selection at step  606 . Any known method may be used to subject the liquid catalyst droplets to size selection at step  606  including, but not limited to size selection within a size selection zone  402  of a reactor  100  as illustrated in  FIGS. 4A and 4B . By way of non-limiting example, the liquid catalyst droplets may be selected for size using a cylindrical differential mobility analyzer as described herein above. In an embodiment, the catalyst may undergo size selection before contacting the carbon source to promote homogeneous catalyst particle or droplet size and uniform SWNT growth on the droplets. 
         [0060]    Referring again to  FIG. 6 , the method  600  may further include contacting the liquid catalyst droplets and a carbon source at a synthesis temperature at step  608  and forming the SWNT at the surface of the liquid catalyst droplets at step  610 . Step  608  and step  610  of method  600  correspond to steps  502  and  504  of method  500  as described herein above. According, steps  608  and  610  may be conducted as described herein above in connection with steps  502  and  504  of method  500 , respectively. As illustrated in  FIG. 1 , the liquid catalyst droplets  130  move through the reactor  100  and contact a carbon source  126 , typically in the growing zone  106 . However, it should be understood that this reaction can take place in one or more other zones of the reactor  100  as described herein above without departing from the scope of the present disclosure. As described herein above, the SWNT are produced in the reactor by a decomposition reaction catalyzed along the surface of the liquid catalyst droplets. 
         [0061]    The method  600  illustrated in  FIG. 6  may further include cooling the liquid catalyst droplets to a solidification temperature at step  612 . As illustrated in  FIG. 1 , the liquid catalyst droplets with attached SWNT  120  formed in the growing zone  106  may be transported into the cooling zone  108  of the reactor  100  using gravity or any other known means of transporting particles within a reactor. Within the cooling zone  108 , the liquid catalyst droplets may be solidified, and the solidified catalyst with SWNT thereon may enter the collection zone  110  of the reactor, where the catalyst with SWNT is collected (step not illustrated). To facilitate the solidification of the liquid catalyst drops, the cooling zone  108  may be maintained at a cooling temperature as described herein above. In one embodiment, the cooling temperature may be selected to be a temperature at or below the solidification temperature of the catalyst. In other embodiments, the cooling temperature may be selected to be below the solidification temperature of the catalyst. In these other embodiments, lower cooling temperatures may result in more rapid cooling rates of the liquid catalyst droplets within the cooling zone  108 . 
         [0062]      FIG. 7  is a flow chart of an additional embodiment of an exemplary method  700  of producing single-walled carbon nanotubes (SWNT) in a CVD reactor. The method  700  includes injecting a catalyst metalorganic precursor into the reactor at a decomposition temperature at step  702 . The catalyst metalorganic precursor may be provided in any form described herein previously including, but not limited to: a solid powder, a solid dispersed in a liquid, or dissolved in a solvent. In the event the catalyst metalorganic precursor is dispersed in a liquid or dissolved in a solvent, the liquid or solvent may be evaporated when introduced into the injection zone  102  and/or decomposition zone  104  of the reactor  100  as described herein previously and illustrated in  FIGS. 4A and 4B . In some embodiments, the catalyst metalorganic precursor may be provided in the form of a metallocene as described herein above. 
         [0063]    Referring again to  FIG. 7 , the method may further include decomposing the catalyst metalorganic precursor to form liquid catalyst droplets at step  704 . In various embodiments, the catalyst metalorganic precursor may be pyrolized to remove the organic material within the metallocene to form catalyst particles as described herein previously. The resulting catalyst particles may then be melted to form the liquid catalyst droplets. In this method  700 , the decomposition temperature may be selected to be higher than the melting temperature of the catalyst, as well as higher than the pyrolysis temperature of the organic materials within the catalyst metalorganic precursor. In various embodiments, the decomposition temperature may range from about 200° C. to about 300° C., or range within any of the other decomposition temperature ranges described herein above. 
         [0064]    In some embodiments, the liquid catalyst droplets may undergo size selection at step  706  using any known method including, but not limited to size selection within a size selection zone  402  of a reactor  100  as illustrated in  FIGS. 4A and 4B . By way of non-limiting example, the liquid catalyst droplets may be selected for size using a cylindrical differential mobility analyzer as described herein above. In an embodiment, the catalyst may undergo size selection before contacting the carbon source to promote homogeneous catalyst particle or droplet size and uniform SWNT growth on the droplets. 
         [0065]    Referring again to  FIG. 7 , the method  700  may further include contacting the liquid catalyst droplets and a carbon source at a synthesis temperature at step  708  and forming the SWNT at the surface of the liquid catalyst droplets at step  710 . Step  708  and step  710  of method  700  correspond to steps  502  and  504  of method  500  and may be conducted as described herein above in connection with steps  502  and  504  of method  500 , respectively. As illustrated in  FIG. 1 , the liquid catalyst droplets  130  move through the reactor  100  and contact a carbon source  126 , typically in the growing zone  106 . However, it should be understood that this reaction can take place in one or more other zones of the reactor  100  as described herein above without departing from the scope of the present disclosure. As described herein above, the SWNT are produced in the reactor by a decomposition reaction catalyzed along the surface of the liquid catalyst droplets. 
         [0066]    The method  700  illustrated in  FIG. 7  may further include cooling the liquid catalyst droplets to a solidification temperature at step  712 . In this embodiment, step  712  of method  700  corresponds to step  612  of method  600  and may be conducted as described herein above in connection with step  612  of method  600 . 
         [0067]    It should be understood by one skilled in the art that while the instant disclosure discusses many of the steps of the method in one or more zones of the reactor, any of the steps can be performed in any one or combination of the zones as commonly known in the CVD reactor art without departing from the scope of the present disclosure. 
         [0068]    In another method (not illustrated) of the present disclosure, a SWNT is produced in a vertical CVD reactor by injecting a catalyst metallocene precursor into the injection zone at one end of the reactor. The catalyst metallocene precursor enters the decomposition zone of the reactor, wherein the organic material within the metallocene is decomposed. Typically, the decomposition zone has a temperature of from about 200-300° C. Catalyst moving through the decomposition zone is contiguously melted into liquid catalyst droplets. Thereafter, the catalyst droplets moving through the reactor contact a carbon source. SWNT are produced accordingly in the growing zone of the reactor along the surface of the catalyst droplets. The catalyst droplets and SWNT move into the cooling zone of the reactor, which solidifies the catalyst droplets. Solidified catalyst and SWNT enter the collection zone of the reactor. 
         [0069]    In still another method of the present disclosure (not illustrated), a SWNT may be produced in a horizontal CVD reactor  100 A by placing powdered catalyst metallocene precursor  210  into a carrier  212  as illustrated in  FIG. 2 . The carrier  212  is then placed in the decomposition zone  104  of the reactor  100 A, wherein the organic material within the metallocene is decomposed. In a non-limiting example, the decomposition zone  104  is maintained at a decomposition temperature ranging from about 200° C. to about 300° C. The catalyst metallocene precursor may be contiguously melted or evaporated. A carrier gas, including but not limited to the gases described herein above, transports the catalyst particles  130  to a growing zone  106  in the reactor  100 A maintained at a synthesis temperature higher than the melting point of the catalyst. The resulting liquid catalyst droplets contact the carbon source within the growing zone  106 , and SWNT are produced accordingly along the surface of the liquid catalyst droplets. The liquid catalyst droplets and SWNT enter the cooling zone  108  of the reactor  100 A, which solidifies the catalyst droplets. Solidified catalyst and SWNT enter the collection zone  110  of the reactor  100 A. 
         [0070]    Although described herein with respect to vertical and horizontal reactors configured perpendicular to each other, it is to be understood that the reactors are not limited to such configurations and that they can be oriented at any angle with respect to horizontal and/or vertical. In addition, any form of catalyst or catalyst precursor material may be used in the reactors, as the reactor configurations are not limited to the use of only solids, solids dispersed in liquids, or liquids. 
         [0071]    It is also to be understood that the methods disclosed herein may be performed in a continuous manner characterized by the continuous introduction of the catalyst and the carbon source to the growing zone to continuously grow carbon nanotubes. Accordingly, a greater volume of carbon nanotubes may be produced as compared to conventional batch methods. 
         [0072]    Methods of producing SWNT in a reactor are described herein above in detail. The methods are not limited to the specific embodiments described herein, but rather, components of systems and/or steps of the method may be utilized independently and separately from other components and/or steps described herein. Each method step and each component may also be used in combination with other method steps and/or components. Although specific features of various embodiments may be shown in some drawings and not in others, this is for convenience only. Any feature of a drawing may be referenced and/or claimed in combination with any feature of any other drawing. 
       EXAMPLES 
     Example 1 
     Synthesis of Ru Nanoparticles 
       [0073]    To demonstrate a method of forming Ru nanoparticles suitable for use as a catalyst precursor in the methods described herein above, the following experiment was conducted. 
         [0074]    0.02 g of RuCl 3 .xH 2 O was dissolved in 25 mL of ethylene glycol. N 2  gas was bubbled through the precursor solution vigorously while maintaining the solution at a temperature of about 50° C. to remove air and moisture from the solution. After about ten minutes, the flow rate of the N 2  gas was decreased and the temperature of the precursor solution was increased to 105° C. for 7 min. At this temperature, 0.10 g of sodium acetate dissolved in 5 mL ethylene glycol was injected quickly into precursor solution, accompanied by a change in color of the precursor solution to black. The temperature of the precursor solution was further increased to 155° C. and subsequently 187° C. for an additional 1.5 hr. 
         [0075]    The precursor solution was cooled, resulting in the precipitation of Ru nanoparticles from the precursor solution. The Ru nanoparticles were separated by centrifugation and washed two times with ethanol. Half of the resulting precipitate of Ru nanoparticles was dispersed in 0.15 g of oleic acid and 0.1 g of oleylamine for hexagonal close-packed (hcp) Ru nanoparticles. This solution was sonicated for 30-60 min, stirred for 30 min, and again sonicated for 30 min. The precipitate of this solution was isolated by centrifugation and washed two times with ethanol. 
         [0076]      FIG. 8  is a TEM image of the hcp Ru nanoparticles formed as described above. The particle size of the hcp Ru nanoparticles was about 2 nm to about 2.5 nm with high dispersity, high purity, and narrow size distribution. 
         [0077]    The results of this experiment demonstrated a method of forming Ru nanoparticles suitable for use as a catalyst precursor in the methods described herein above. 
       Example 2 
     Synthesis of In Nanoparticles 
       [0078]    To demonstrate a method of forming In nanoparticles suitable for use as a catalyst precursor in the methods described herein above, the following experiment was conducted. 
         [0079]    0.17 g of InCl 3  was dissolved in 5 mL of TOP (trioctylphosphine) and 10 mL of THF (tetrahydrofuran) at room temperature. Once the indium chloride had completely dissolved, 4 mL of superhydride solution was injected into the precursor solution. Within a few minutes, large agglomerates formed within the solution. The solution was exposed to air and stirred for an additional 2-4 hours. The In nanoparticles were separated by centrifugation and washed two times with ethanol. 
         [0080]      FIG. 9  is a TEM image of the resulting In nanoparticles formed as described above. The particle size of the In nanoparticles was about  2  nm with high dispersity, high purity, and narrow size distribution. 
         [0081]    The results of this experiment demonstrated a method of forming In nanoparticles suitable for use as a catalyst precursor in the methods described herein above. 
       Example 3 
     Production of SWNT Using Liquid Catalyst Droplets 
       [0082]    To demonstrate the method of producing single-walled carbon nanotubes (SWNT) using the methods described herein above, the following experiments were conducted. 
         [0083]    A suspension of 0.4 wt % of gallium (III) acetylacetonate (Ga(acac) 3 ) in ethanol was introduced into a vertical CVD reactor similar to the reactor illustrated in  FIG. 1  at a liquid injection rate of about 6 ml/hr. In addition, argon was introduced into the reactor at a rate of about 450 sccm and hydrogen was introduced at a rate of 75 sccm as carrier gases. The acetylacetonate was pyrolized from the Ga(acac) 3  and the remaining gallium was melted to form liquid gallium catalyst droplets. The liquid gallium droplets were contacted with the ethanol carbon source for about 15 minutes at a synthesis temperature of about 900° C. to form the SWNT. The liquid gallium droplets with attached SWNT were cooled and collected for subsequent analysis. This experiment was repeated at a synthesis temperature of about 925° C. 
         [0084]      FIG. 10  is a TEM image of the SWNT formed using the method described above at a synthesis temperature of about 900° C., and  FIG. 11  is a TEM image of the SWNT formed using the method described above at a synthesis temperature of about 925° C. Both TEM images indicate the presence of nanotubes in the samples.  FIG. 12  is a graph summarizing typical Raman spectra obtained from samples of the SWNT, and  FIG. 13  is a zoomed-in graph of Raman spectra obtained from samples of SWNT. As illustrated in  FIG. 13 , Raman spectra are characterized by radial breathing modes indicative of SWNT. 
         [0085]    The results of these experiments demonstrated that SWNT may be produced using the methods described herein above. 
         [0086]    This written description uses examples to disclose the embodiments, including the best mode, and also to enable any person skilled in the art to practice the embodiments, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the disclosure is defined by the claims, and may include other examples that occur to those skilled in the art.