Patent Publication Number: US-2012027955-A1

Title: Reactor and method for production of nanostructures

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Patent Application No. 60/978,673, filed Oct. 9, 2007, which is hereby incorporated by reference. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     The invention was supported, in whole or in part, by a grant, No. W9113M-04-C-0024, from Nanowire Technology for Missile Defense of the U.S. Army Space Missile Defense Command; a grant, No. DE-FG36-05G085013A, from the U.S. Department of Energy/Kentucky Rural Energy Consortium; and a grant, No. DE-FG02-05ER64071, from the U.S. Department of Energy which supports the Institute for Advanced Materials and Renewable Energy at the University of Louisville. The Government has certain rights in the invention. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to the field of nanotechnology, and more particularly to a reactor and method for the production of nanostructures, such as nanowires and nanoparticles. 
     INTRODUCTION 
     Nanostructures, such as nanowires and nanoparticles, can have unique applications and are beginning to be used in electronics, optoelectronics, electrochemical cells, nanoelectromechanical devices, catalysis, and several other fields. Unique properties of nanowires include high aspect ratio, low conductivity, high surface to volume ratio and enhanced material characteristics due to quantum confinement effects. Synthesis of bulk quantities of nanowires with controlled composition, crystallinity, and morphology is important to continued development and commercialization of nanowire technology. For many applications, nanowire quantities of several grams or more are needed. Similarly, bulk production of nanoparticles are needed. 
     Metal oxide nanowires have been synthesized in a variety of ways. Some of these methods include (i) direct plasma and thermal oxidation using hydrogen and oxygen-containing gas phase of low-melting metal melts supplied through the gas phase onto a substrate; (ii) chemical vapor transport of metal using hot-filaments onto substrates using chemical vapor deposition in low oxygen-containing atmospheres; (iii) exposure of metal foils to low-pressure, weakly ionized, fully dissociated, cold oxygen plasmas; (iv) chemical vapor deposition of metal oxides in the presence of catalysts, e.g., iron metal particles; (v) thermal evaporation synthesis of zinc oxide nanowires; and (vi) synthesis of zinc oxide nanowires using a radio-frequency (RF), high power plasma. 
     Many of the previously-described approaches involve nanowire synthesis on a substrate. Other approaches have used catalysts or high temperature evaporation of a precursor. It can be difficult, time consuming, and expensive to produce large quantities of nanowires using these methods. 
     Other approaches, such as synthesis of zinc oxide nanowires using an RF, high power plasma, have not proven the ability to produce nanowires in a consistent, efficient, and cost-effective manner. See Peng, et al., “Plasma Synthesis of Large Quantities of Zinc Oxide Nanorods,”  J. Phys. Chem.,  111, 194-200 (2000). Attempts to use RF, high power plasmas to produce nanowires suffer the drawbacks of requiring high power input, high gas flow rates, and careful control of reaction temperature gradients. See id. Alternatives to nanowire synthesis which overcome the limitations of the known processes are needed. Similarly, alternatives to nanoparticle synthesis which overcome the limitations of the known processes are needed. 
     SUMMARY 
     The present invention includes a reactor and method for production of nanostructures, for example, metal oxide nanowires and nanoparticles. 
     The present invention includes a reactor for producing metal oxide nanostructures, such as nanowires and nanoparticles. In one embodiment, the reactor comprises a metal powder delivery system wherein the metal powder delivery system includes a funnel in communication with a dielectric tube; a plasma-forming gas inlet also in communication with the dielectric tube, whereby a plasma-forming gas is delivered substantially longitudinally into the dielectric tube; a sheath gas inlet also in communication with the dielectric tube, whereby a sheath gas is delivered into the dielectric tube; and a microwave energy generator coupled to the dielectric tube, whereby microwave energy is delivered into the dielectric tube and to the plasma-forming gas. In one embodiment, the reactor further includes a recycle system to recycle unreacted metal to a plasma formed in the dielectric tube. 
     The present invention also includes a method for producing metal oxide nanostructures, such as nanowires and nanoparticles. In some embodiments, the method comprises delivering a plasma-forming gas substantially longitudinally into a dielectric tube; delivering a sheath gas into the dielectric tube; forming a plasma from the plasma-forming gas by applying microwave energy to the plasma-forming gas; delivering a metal powder into the dielectric tube; and reacting the metal powder within the plasma at a certain microwave energy level to form metal oxide nanowires or metal oxide nanoparticles. In one embodiment, the method further includes delivering a bulk of the metal powder substantially into the center of the plasma. 
     The present invention produces bulk quantities of nanostructures, such as nanowires and nanoparticles quickly and at a fraction of the cost of known processes for making nanostructures. Practice of the present invention produces bulk quantities of highly pure metal oxide nanostructures using a high throughput plasma reactor. By using the reactors and methods described herein, nanostructures can be produced very quickly. In some embodiments, reacting metal powders into metal oxides nanostructures can take less than one second. For example, it can take only about one minute to produce about 10 grams of nanostructures. The reactor and methods described herein can be used to produce nanostructures in quantities of a kilogram, or more, per day. 
     The present invention can be used to produce highly pure nanostructure products. Since there is no need for any catalyst, substrate, or template to produce nanostructures, foreign material contamination of the nanostructure product can be avoided or minimized. In contrast, nanostructure products made using known synthesis methods often contain materials other than the nanostructure such as catalyst particles. Because the nanostructure products produced by the present invention are highly pure, expensive and time consuming purification processes can be minimized or even avoided completely. 
     The present invention can be used to produce nanostructures more cost effectively than known synthesis methods. For example, the present invention does not use high power or high temperatures which are associated with known processes for preparing nanostructures. Reactor designs described herein can be continuously operated for extended periods of time without significant heating of the reactor. Thus, the present invention can avoid the expenses associated with high power and high temperature operation. In addition, the present invention does not use catalysts, substrates, or templates and thus can achieve cost savings over known processes that require such materials. Further, the present invention can produce nanostructures without using expensive precursor materials such as, for example, precursor materials used in thermal evaporation processes. The present invention has demonstrated, in one embodiment, reaction efficiency of about 90% when 100 nm metal powder particles were used. 
     In some embodiments, the present invention uses lower gas volumes than known processes for making nanostructures in the gas phase. A lower gas volume can reduce waste disposal expenses and can also simplify separation procedures used to recover nanostructure products from process gases. Lower gas volumes can also reduce the amount of heat input that is necessary to provide appropriate conditions for making nanostructures. 
     The reactor of the present invention can be modular and can be easily adapted or modified to suit production needs. Further, because the reactor can be modular, the reactor can be easily serviced, for example, by swapping reactor components as needed. 
     In some embodiments, the plasma is formed at pressures at or near atmospheric pressure. Practice of the present invention at or near atmospheric pressure can produce nanostructures without the use of expensive vacuum components. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic representation of a reactor for producing nanostructures according to one embodiment of the present invention. 
         FIG. 2  is a schematic representation of a helical gas path in a dielectric tube according to one embodiment of the present invention. 
         FIG. 3  is a partial schematic representation of a reactor for producing nanostructures according to another embodiment of the present invention. 
         FIG. 4  is a schematic representation of an example of a metal powder and gas delivery system. 
         FIGS. 5A to 5C  are schematic views of an example of a gas delivery system. 
         FIGS. 6A and 6B  are schematic views of an example of a cooling jacket and powder delivery system. 
         FIG. 7  is a schematic view of an example of a cooling jacket cover and plasma-forming gas inlet system. 
         FIG. 8  is a partial schematic representation of an example of a recycle system in communication with a dielectric tube. 
         FIG. 9  is a schematic representation of an example of a nanostructure product collector. 
         FIGS. 10A to 10E  are photomicrographs of tin oxide nanowires produced from tin metal powder according to one embodiment of the invention and as described in Example 1. 
         FIG. 11  is a Raman spectrum of tin oxide nanowires produced from tin metal powder according to one embodiment of the invention and as described in Example 1. 
         FIGS. 12A to 12F  are photomicrographs of zinc oxide nanowires produced from zinc metal powder according to one embodiment of the invention and as described in Example 2. 
         FIGS. 13A to 13B  are photomicrographs of titanium dioxide nanowires produced from titanium metal powder according to one embodiment of the invention and as described in Example 3. 
         FIGS. 14A to 14B  are photomicrographs of copper-zinc oxide nanowires/nanobelts produced from copper-zinc metal powder according to one embodiment of the invention and as described in Example 4. 
         FIGS. 15A to 15F  are photomicrographs of tin oxide nanowires produced from tin metal powders according to one embodiment of the invention and as described in Example 5. 
         FIGS. 16A to 16C  are photomicrographs of tin oxide nanowires produced from tin metal powders according to one embodiment of the invention and as described in Example 6. 
         FIGS. 17A to 17B  are photomicrographs of aluminum oxide nanowires produced from aluminum metal powder according to one embodiment of the invention and as described in Example 7. 
         FIGS. 18A to 18B  are photomicrographs of aluminum oxide (alumina) nanoparticles ( 18 B) produced from aluminum metal powder ( 18 A) according to one embodiment of the invention and as described in Example 8. 
         FIGS. 19A to 19B  are photomicrographs of titanium oxide (titania) nanoparticles ( 19 B) produced from titanium metal powder ( 19 A) according to one embodiment of the invention and as described in Example 9. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     A description of example embodiments of the invention follows. 
     Nanostructures can be described in terms of their longest and shortest dimensions. For example, the aspect ratio of a nanostructure is the ratio of a nanostructure&#39;s longest dimension to the nanostructure&#39;s shortest dimension. Generally, a nanoparticle is a nanostructure having an aspect ratio of 1. In some embodiments, a nanoparticle is a nanostructure having a diameter of the nanoscale, that is, from 1 nanometer to hundreds of nanometers, but below 1 micron. Generally, a nanowire is a nanostructure that has an aspect ratio greater than 1, i.e., the nanoparticle&#39;s longest dimension is greater than the particle&#39;s shortest dimension. As used herein, the term “nanowire” refers to a nanostructure that has an aspect ratio greater than 1. In some embodiments, the nanowires of the present invention have an aspect ratio, e.g., an individual or an average aspect ratio, of at least 1.5 such as at least about 2. In other embodiments, the nanowires of the present invention have an aspect ratio e.g., an individual or an average aspect ratio, of at least about 10, at least about 50, or at least about 75, for example, the nanowires can have an aspect ratio of about 10 to about 150 or about 50 to about 125, such as about 100. In some instances, the nanowires can have a length of about 1 to about 20 microns such as, for example, about 10 microns and a diameter of about 20 to about 200 nanometers (nm) such as, for example, about 100 nanometers. 
     As the term is used herein, “nanowires” can include individually separate nanowires as well as intertwined or connected nanowires. For example, in one embodiment of the invention, nanowires are joined together or agglomerated to form a star-burst shaped mass. See, for example,  FIGS. 12B and 12C , described infra. 
     As the term is used herein, “nanoparticles” can include individually separate nanoparticles as well as connected nanoparticles. For example, in one embodiment of the invention, nanoparticles are joined together or agglomerated. See, for example,  FIGS. 18B and 19B , described infra. 
     Unless otherwise indicated, all numbers expressing quantities of ingredients, reaction conditions, and so forth used in the specification and claims are to be understood as being modified in all instances by the term “about”. Accordingly, unless indicated to the contrary, the numerical parameters set forth in this specification and attached claims are approximations that can vary depending upon the desired properties sought to be obtained by the presently disclosed subject matter. 
     As used herein, the term “about,” when referring to a value or to an amount of mass, weight, time, volume, concentration or percentage is meant to encompass variations of in some embodiments ±20%, in some embodiments ±10%, in some embodiments ±5%, in some embodiments ±1%, in some embodiments ±0.5%, and in some embodiments ±0.1% from the specified amount, as such variations are appropriate to perform the disclosed method. 
     The present invention includes a reactor for producing metal oxide nanostructures. In one embodiment, the reactor includes a metal powder delivery system wherein the metal powder delivery system includes a funnel in communication with a dielectric tube; a plasma-forming gas inlet also in communication with the dielectric tube, whereby a plasma-forming gas is delivered substantially longitudinally into the dielectric tube; a sheath gas inlet also in communication with the dielectric tube, whereby a sheath gas is delivered into the dielectric tube; and a microwave energy generator coupled to the dielectric tube, whereby microwave energy is delivered into the dielectric tube and to the plasma-forming gas. 
     As the term is used herein, “longitudinally” or “longitudinal” means “along the major (or long) axis” as opposed to latitudinal which means “along the width, transverse, or across.” For example, in one embodiment of the invention, plasma-forming gas is delivered substantially into and along the length of the dielectric tube. 
       FIG. 1  is a schematic representation of one embodiment of a reactor. Reactor  10  includes metal powder and plasma-forming gas delivery system  12 , dielectric tube  14 , sheath gas inlets  16  and  18 , and microwave energy generator  20 . In one embodiment, metal powder and plasma-forming gas delivery system  12  includes a funnel in communication with dielectric tube  14 . The funnel of metal powder and plasma-forming gas delivery system  12  can be, for example, a conical funnel. In some embodiments, described more fully infra, metal powder and plasma-forming gas delivery system  12  is cooled, for example, the metal powder and plasma-forming gas delivery system is liquid cooled. Metal powder and plasma-forming gas delivery system  12  can also include a plasma-forming gas inlet in communication with dielectric tube  14 . The plasma-forming gas inlet can be configured to deliver plasma-forming gas substantially longitudinally into dielectric tube  14 . 
     Dielectric tube  14  can be made of any one of several dielectric materials known to those of ordinary skill in the art. For example, in one embodiment, dielectric tube  14  is a quartz tube or a tube of a related material. In other embodiments, dielectric tube is a ceramic or a related material. Dielectric tube  14  can have an inside diameter, for example, of about 1 millimeter (mm) to about 60 mm such as about 5 to about 10 mm, about 10 to about 65 mm, about 10 to about 50 mm, about 10 to about 40 mm, about 15 to about 25 mm, about 15 to about 35 mm, about 20 to about 25 mm, or about 20 to about 30 mm. Without being held to any particular theory, it is believed that the diameter of the dielectric tube is important so that the plasma (described in more detail infra) distributes uniformly within the tube. Preferably, the plasma should occupy a large portion of the tube&#39;s cross section without touching or melting the tube. It is thought that a dielectric tube that is substantially larger in diameter than the plasma formed within can result in substantial quantities of unreacted metal powder during operation of the reactor. 
     In some instances, the diameter of the dielectric tube changes as a function of the tube&#39;s length. For example, the diameter of the dielectric tube can be smaller in the section of the tube in which a plasma is generated and larger downstream of the plasma. In one embodiment, the inside diameter of the dielectric tube is about 22 mm in the section of the tube in which a plasma is generated and is about 10 centimeters (cm) in diameter further downstream. It is thought that by increasing the diameter of the dielectric tube downstream of the plasma, wall deposition of particles can be reduced. In some instances, however, the diameter of the dielectric tube can be chosen to encourage deposition of particles on the walls of the dielectric tube. For example, relatively small dielectric tube diameters are believed to contribute to increased particle deposition on the walls of the dielectric tube during operation of the reactor. 
     Dielectric tube  14  can have a length, for example, of about 20 centimeters (cm) to about 200 cm. In one particular embodiment, dielectric tube  14  has a length of about 75 cm. Proper orientation of dielectric tube  14  can be determined depending on the particular process requirements. In one instance, dielectric tube  14  can be vertical. In other instances, dielectric tube  14  can be angled or horizontal. 
     Sheath gas inlets  16  and  18  are in communication with dielectric tube  14  and can be used to deliver a sheath gas to dielectric tube  14 . In another embodiment, sheath gas inlets  16  and  18  can be configured to deliver either a sheath gas or a plasma-forming gas, or both a sheath gas and a plasma-forming gas, to dielectric tube  14 . Sheath gas inlet  16  and sheath gas inlet  18  can be angled with respect to a longitudinal axis of the dielectric tube. In some instances, only one of sheath gas inlet  16  or sheath gas inlet  18  is angled with respect to a longitudinal axis of the dielectric tube. For example, one or both gas inlets can be angled at less than 90° such as at about 10° to about 80°, about 15° to about 75°, about 20° to about 70°, about 25° to about 65°, about 30° to about 60°, about 40° to about 50°, such as about 45°, about 42°, about 44°, about 46°, or about 48°, with respect to a longitudinal axis of the dielectric tube. In some embodiments, the angle of a gas inlet can produce a helical gas path in the dielectric tube when gas is delivered through the gas inlet. For example, the angle of a gas inlet can produce a helical sheath gas path in the dielectric tube when sheath gas is delivered through the gas inlet. A helical sheath gas path in the dielectric tube can help to contain the plasma and keep the dielectric tube cool during operation of the reactor. 
     Microwave energy generator  20  can include, for example, magnetron  22 , circulator  24 , power detector  26  (e.g., a forward and reflected power detector), tuner  28  (e.g., a three stub tuner), and load  30 . Microwave energy generator  20  can be coupled to dielectric tube  14  via coupler  32 . In one embodiment, coupler  32  is a tapered waveguide which surrounds dielectric tube  14 . Microwave energy produced by microwave energy generator  20  is delivered to dielectric tube  14  via coupler  32 . In some embodiments, microwave energy generator  20  produces microwave energy at 2.45 gigahertz (GHz). 
     Microwave energy produced by microwave energy generator  20  is delivered to the plasma-forming gas contained in dielectric tube  14  to produce plasma  34 . With reference to  FIG. 2 , in some embodiments, a device such as holder  36  is used to hold dielectric tube  14 . 
     Referring again to  FIG. 1 , in one embodiment, reactor  10  includes a recycle. For example, reactor  10  can include recycle system  38  in communication with dielectric tube  14 . In one embodiment, recycle system  38  is also in communication with a plasma-forming gas inlet. Recycle system  38  can also include a nanostructure separator. A nanostructure separator such as, for example, a cyclone, can be used to remove, completely or partially, nanostructures from a reaction product stream exiting the bottom of dielectric tube  14  before unreacted metal is recycled to the plasma. 
     Reactor  10  can also include a nanostructure product collector such as product gathering cup  40 . In some embodiments, the nanostructure product collector contains a baffle or other device to slow gas velocity and disentrain nanostructure product from the reaction product stream. In another embodiment, the nanostructure product collector is a powder collecting cup wherein the diameter of the powder collecting cup is less than the inner diameter of the dielectric tube so that gases can escape from the bottom of the powder collecting cup. In additional embodiments, a powder collecting cup is porous to the gases so that the gases can escape through the powder collecting cup. Excesses gases can be vented, for example, via exhaust line  42 . 
     In one embodiment, reactor  10  includes inlet port  44  for introducing a precursor feed for downstream reaction. For example, inlet port  44  can be used to introduce a precursor feed for thin film formation. 
     In one embodiment, reactor  10  does not contain any additional heating elements or any additional heat insulating materials. For example, in some embodiments, dielectric tube  14  is not covered with heat insulation. In additional embodiments, reactor  10  does not contain any igniters to ignite the plasma. For example, reactor  10  does not contain any ignition device to ignite the plasma. In one particular embodiment, microwave energy produced by microwave energy generator  20  is delivered to dielectric tube  14  via coupler  32  and the microwave energy is capable of igniting the plasma. In another embodiment, a metal ignition rod with pointed ends (not illustrated) is used to ignite the plasma. 
       FIG. 2  is a schematic representation of one embodiment of the present invention having helical gas path  46  within dielectric tube  14 . Sheath gas inlet  16  and sheath gas inlet  18  are shown angled with respect to a longitudinal axis of the dielectric tube. The angle of a gas inlet can produce a helical gas path in the dielectric tube when gas is delivered through the gas inlet. For example, the angle of a sheath gas inlet can produce a helical sheath gas path in the dielectric tube when sheath gas is delivered through the sheath gas inlet. In another embodiment, the angle of a sheath gas inlet can produce a helical sheath gas and plasma-forming gas path in the dielectric tube when sheath gas and plasma-forming gas are delivered through the sheath gas inlet. 
       FIG. 3  is a partial schematic representation of a reactor for producing nanostructures according to another embodiment of the present invention. Reactor  48  includes metal powder and gas delivery system  50 , dielectric tube  14 , sheath gas lines  52  and  54 , sheath gas source  56 , and microwave energy generator  20 . Microwave energy generator  20  can include, for example, magnetron  22 , circulator  24 , power detector  26  (e.g., a forward and reflected power detector), tuner  28  (e.g., a three stub tuner), and load  30 . In one instance, sheath gas lines  52  and  54  can be configured to deliver a sheath gas and a plasma-forming gas to dielectric tube  14 . For example, sheath gas source  56  can be configured to mix and deliver a sheath gas and a plasma-forming gas to sheath gas lines  52  and  54 . 
       FIG. 4  is a schematic representation of an example of a metal powder and gas delivery system. Metal powder and gas delivery system  50  includes plasma-forming gas inlet  58 , coolant inlet  60 , coolant outlet  62 , and sheath gas inlets  64  and  66 . In some embodiments, coolant inlet  60  and coolant outlet  62  are for liquid coolant, e.g., cooling water. Metal powder and gas delivery system  50  can include gas delivery system  68 , cooling jacket and powder delivery system  70 , and cooling jacket cover and plasma-forming gas inlet system  72 . 
       FIGS. 5A to 5C  are schematic views of an example of a gas delivery system. Gas delivery system  68  includes sheath gas inlets  64  and  66 .  FIG. 5A  is a trimetric view of gas delivery system  68  showing sheath gas inlets  64  and  66  and central tube  74 . Sheath gas inlets  64  and  66  can be configured to deliver sheath gas, plasma-forming gas, or both sheath gas and plasma-forming gas to central tube  74 .  FIG. 5B  is a top view of gas delivery system  68  showing sheath gas inlets  64  and  66  tangential to central tube  74 .  FIG. 5C  is a side view of gas delivery system  68  showing sheath gas inlet  64  at an angle with respect to a longitudinal axis of the dielectric tube. As illustrated, gas inlet  64  is at a 45° angle with respect to the longitudinal axis of dielectric tube  14 . In one embodiment, gas delivery system  68  helps to protect the dielectric tube from heat that may result in a high power plasma discharge. For example, by delivering sheath gas via gas delivery system  68 , the plasma can be confined near the center of the tube and contact of the plasma with the dielectric tube can be avoided and also peripherally-located sheath gas can minimize or prevent transmission of heat from the plasma to the dielectric tube. 
       FIGS. 6A and 6B  are schematic views of an example of a cooling jacket and metal powder delivery system  70 .  FIG. 6A  is a trimetric view and  FIG. 6B  is a cross-sectional view of a cooling jacket and metal powder delivery system  70 . Cooling jacket and metal powder delivery system  70  can include coolant inlet  60  and coolant outlet  62 . In some embodiments, coolant inlet  60  and coolant outlet  62  are for liquid coolant, e.g., cooling water. Cooling jacket and powder delivery system  70  can also include conical funnel  76  through which powder can be made to flow into dielectric tube  14 . In other embodiments, a cooling jacket and metal powder delivery system can include a non-conical funnel such as, for example, a pyramidal funnel. Cooling jacket and metal powder delivery system  70  includes cooling jacket  78  wherein coolant can circulate to reduce or maintain temperature in the metal powder delivery system  70 . 
       FIG. 7  is a schematic view of an example of a cooling jacket cover and plasma-forming gas inlet system. Cooling jacket cover and plasma-forming gas inlet system  72  can include cooling jacket cover  80  and plasma-forming gas inlet  58 . Plasma-forming gas inlet  58  can be configured to deliver plasma-forming gas substantially longitudinally into the dielectric tube. In one embodiment, cooling jacket cover  80  is transparent to permit viewing of the metal powder during feeding of the metal powder to metal powder and gas delivery system  50  (shown in  FIG. 3 ). 
       FIG. 8  is a partial schematic representation of an example of a recycle system  90  in communication with dielectric tube  14 . Partial recycle system  90  includes tee  92  and separator  94 . During operation, reaction product stream  96  is separated in two parts through tee  92 . Reaction product stream  96  is split into heavy particle stream  98  and fine particle stream  100 . Heavier and mostly unreacted particles are directed downwards where they are entrained by high velocity gas  102 . High velocity gas  102  flowing through a small diameter tube  104  can entrain the lower velocity particles of heavy particle stream  98  to form entrained particle stream  106 . Entrained particle stream  106  can be in communication with plasma-forming gas inlet  58 , shown in  FIGS. 4 and 7 . Fine particle stream  100  can be in communication with a separator  94  such as, for example, a cyclone separator, wherein product stream  108  is collected and exhaust gases  110  are removed. 
       FIG. 9  is a schematic representation of an example of a nanostructure product collector. nanostructure product collector  112  is in communication with dielectric tube  114  and includes powder collecting cup  116 . In one embodiment, powder collecting cup  116  is made of quartz. During operation, reaction product stream  118  flows from dielectric tube  114  and into powder collecting cup  116 . The reaction products can settle in powder collecting cup  116  and exhaust gas  120  can flow out through exhaust  122 . 
     In one aspect, the present invention also includes a reactor for forming nanostructures from a precursor such as, for example, a metal organic precursor or a carbon nanotube precursor. For example, a reactor for producing nanostructures from a precursor can comprise: a precursor delivery system, wherein the precursor delivery system includes a funnel in communication with a dielectric tube; a plasma-forming gas inlet also in communication with the dielectric tube, whereby a plasma-forming gas is delivered substantially longitudinally into the dielectric tube; a sheath gas inlet also in communication with the dielectric tube, whereby a sheath gas is delivered into the dielectric tube; and a microwave energy generator coupled to the dielectric tube, whereby microwave energy is delivered into the dielectric tube and to the plasma-forming gas. Suitable components and configuration for such a reactor are described supra with respect to the reactor for producing metal oxide nanostructures. In one embodiment, the precursor delivery system can be substantially the same as the metal powder delivery system described herein. 
     The present invention also includes a method for producing metal oxide nanostructures. In some embodiments, the method includes delivering a plasma-forming gas substantially longitudinally into a dielectric tube; delivering a sheath gas into the dielectric tube; forming a plasma from the plasma-forming gas by applying microwave energy to the plasma-forming gas; delivering a metal powder into the dielectric tube; and reacting the metal powder within the plasma to form metal oxide nanostructures. 
     The methods for producing metal oxide nanostructures described herein involve the production of nanostructures directly in the vapor phase without the need for any catalyst, substrate, or template. Nanostructures can be formed of metal oxides such as, for example, tin oxide, zinc oxide, tungsten oxide, titanium dioxide, iron oxide, gallium oxides, indium oxides, bismuth oxides, niobium pentoxide, aluminum oxides, vanadium pentoxide, cooper oxides, alloy oxides, and the like, and combinations thereof, by using the appropriate metal feed. The methods and reactor described herein can also be used to produce sulfide and nitride nanostructures using, for example, an appropriate gas-phase chemistry feed. In addition, carbon nanotubes (CNT) can be formed using the methods and reactor described herein, for example, using iron and hydrocarbon species in a vapor phase feed. 
     A method for producing metal oxide nanostructures can include delivering a plasma-forming gas into a dielectric tube. In one embodiment, a method for producing metal oxide nanostructures includes delivering a plasma-forming gas substantially longitudinally into a dielectric tube. Delivering the plasma-forming gas substantially longitudinally into a dielectric tube can help to keep the plasma centered in the dielectric tube. In some embodiments, the plasma-forming gas is delivered in a helical gas path into the dielectric tube. The plasma-forming gas can include, for example, argon gas. The plasma-forming gas can also include an oxidative gas such as oxygen. In some instances, the plasma-forming gas can include water vapor. In some embodiments, the plasma-forming gas can include hydrogen gas. 
     In some embodiments, the plasma-forming gas is delivered into the dielectric tube at a flow rate of less than about 10 slpm, for example, about 1 to about 5 slpm, about 2 to about 4 slpm, or about 2 slpm. In one embodiment, the diameter of the dielectric tube is about 22 mm in diameter, thus, in some embodiments, the plasma-forming gas is delivered into the dielectric tube to produce a plasma-forming gas velocity within the dielectric tube of less than about 26.3 meters/min (m/min), for example, about 2.6 to about 13.2 m/min, about 5.3 to about 10.5 m/min, or about 5.3 m/min at standard conditions. In some instances, the plasma-forming gas is delivered into the dielectric tube to produce a plasma-forming gas velocity within the dielectric tube of less than about 30 m/min, for example, about 2 to about 15 m/min, about 5 to about 10 m/min, or about 5 m/min at standard conditions. The plasma-forming gas can include an oxidative gas such as, for example, oxygen gas. In some embodiments, an oxidative gas is delivered into the dielectric tube at a flow rate of equal to or less than about 500 sccm, for example, about 10 to about 500 sccm, 20 to about 400 sccm, 30 to about 300 sccm, about 50 to about 200 sccm, about 75 to about 150 sccm, 50 to about 150 sccm, or about 100 sccm. In one embodiment, the diameter of the dielectric tube is about 22 mm in diameter, thus, in some embodiments, the oxidative gas is delivered into the dielectric tube to produce a oxidative gas velocity within the dielectric tube of less than about 1.3 m/min, for example, about 0.03 to about 1.3 m/min, about 0.1 to about 0.5 m/min, about 0.2 to about 0.4 m/min, or about 0.26 m/min at standard conditions. In some instances, the oxidative gas is delivered into the dielectric tube to produce an oxidative gas velocity within the dielectric tube of less than about 2 m/min, for example, about 0.01 to about 1.5 m/min, about 0.1 to about 0.5 m/min, or about 0.2 to about 0.4 m/min at standard conditions. 
     Suitable dielectric tubes for use in the method are described supra. In one particular embodiment, the dielectric tube is made of quartz. 
     A method for producing metal oxide nanostructures can further include delivering a sheath gas into the dielectric tube. Use of a sheath gas can allow the operation of a plasma inside the dielectric tube for extended periods of time. The sheath gas can include, for example, air or nitrogen. In one particular embodiment, the sheath gas is air. The sheath gas can be delivered into the dielectric tube to form a helical sheath gas path. A helical sheath gas path in the dielectric tube can help to contain the plasma and keep the dielectric tube cool during operation of the reactor. Examples of suitable apparatus for producing a helical sheath gas path are described supra. 
     In some embodiments, the sheath gas is delivered into the dielectric tube at a flow rate of less than about 10 slpm, for example, about 1 to about 8 slpm, about 3 to about 6 slpm, about 4 to about 5 slpm, or about 5 slpm. In one embodiment, the diameter of the dielectric tube is about 22 mm in diameter, thus, in some embodiments, the sheath gas is delivered into the dielectric tube to produce a sheath gas velocity within the dielectric tube of less than about 26.3 m/min, for example, about 2.6 to about 21 m/min, about 7.9 to about 15.8 m/min, about 10.5 to about 13.2 m/min, or about 13.2 m/min at standard conditions. In some instances, the sheath gas is delivered into the dielectric tube to produce a sheath gas velocity within the dielectric tube of less than about 30 m/min, for example, about 1 to about 25 m/min, about 5 to about 20 m/min, or about 10 to about 15 m/min at standard conditions. 
     In addition, a plasma-forming gas can be delivered to the dielectric tube concurrently with a sheath gas. For example, a plasma-forming gas and a sheath gas can be mixed and delivered into the dielectric tube to form a helical sheath gas path via, for example, angled sheath gas inlets. Alternatively, a plasma-forming gas and a sheath gas can be delivered into the dielectric tube separately to form concurrent helical sheath gas paths via, for example, separate angled gas inlets. Examples of suitable apparatus for producing a helical sheath gas path are described supra. 
     A method for producing metal oxide nanostructures can further include forming a plasma from the plasma-forming gas by applying microwave energy to the plasma-forming gas. In one particular embodiment, the microwave energy is 2.45 GHz. In some embodiments, the power of microwave energy applied to the plasma-forming gas is less than about 15 kilowatts (kW), less than about 10 kW, or less than 8 kW. For example, the power of microwave energy applied to the plasma-forming gas can be about 300 watts (W) to about 8 kW such as about 500 W to about 2 kW, or about 1 kW to about 1.5 kW. When microwave energy is applied to the plasma-forming gas, a plasma, e.g., a plasma jet, can form in the dielectric tube. In one particular embodiment, microwave energy is delivered to the dielectric tube via a coupler and the microwave energy is used to ignite the plasma. In another embodiment, a metal ignition rod with pointed ends is used to ignite the plasma. 
     Without being held to any particular theory, it is believed that the metal oxide nanowires are formed under molten conditions and not vaporization conditions, while nanoparticles are formed when the feed metal is vaporized. Under molten conditions, the metal particles are reacted with the plasma at temperatures close to the metal&#39;s melting point. The molten metal forms metal oxide nanowires when oxygen reacts with the molten metal. By increasing the microwave power to increase the temperature in the reactor, vaporization conditions favorable to forming metal oxide nanoparticles occur. In other words, at higher temperatures, the metal particles are vaporized into very small nuclei (of a few nanometers) and during their fall in the quartz tube, where the temperature decreases, they begin to condense, form solid metal oxide nanoparticles, and also agglomerate. 
     Thus, a higher microwave power is needed to form nanoparticles compared to nanowire formation for the same feed metal. For example, to form titanium metal (titania) nanoparticles, the microwave power is required to be greater than about 1000 W, while a microwave power of less than about 1000, and more specifically, about 700 W, is required to form titanium oxide nanowires in the above described reactors. As another example, to form aluminum oxide (alumina) nanoparticles, the microwave power is required to be equal to or greater than about 1300 W, while a microwave power of less than about 1300 W, and more specifically, about 800 W, is required to form aluminum oxide nanowires in the above described reactors. 
     The gas pressure in the dielectric tube can range, for example, from a few torr to one atmosphere or more. In a specific embodiment, the gas pressure in the dielectric tube ranges from a few torr to about one atmosphere. The length of the plasma can be varied by changing the gas flow rates or by changing the microwave power. In some embodiments, the length of the plasma in the dielectric tube is about 1 to about 30 cm in length. The length of the plasma in the dielectric tube can be varied to alter the production of nanostructures in the plasma. The flame of the plasma can be stabilized by using a stub tuner and by adjusting the gas flow rates. Typically, the gases are introduced to the dielectric tube, the plasma is stabilized and the reflected power is minimized. In one aspect, the present method includes controlling the plasma uniformity inside the dielectric tube by adjusting the microwave power or the gas flow rates. By adjusting the plasma uniformity or length, it is believed that the morphology of the nanostructures and the efficiency of conversion can be adjusted. Generally, longer and more uniform plasmas are preferred. 
     In some embodiments, the temperatures of the gases in the reactor do not need to be carefully controlled. For example, in one embodiment, no heat insulation is used to cover the dielectric tube or to control the temperature of gases in the dielectric tube. Generally, the reaction of metal powder to metal oxide nanostructures occurs within the plasma and is complete, or substantially complete, upon exiting the plasma so that careful control of the gas temperature outside of the plasma can be unnecessary. 
     Examples of suitable apparatus for applying microwave energy to a plasma-forming gas are described supra. 
     In some instances, the plasma-forming gas can include hydrogen gas. In one embodiment, hydrogen gas is mixed with another plasma-forming gas such as argon and then fed to the dielectric tube. In another embodiment, hydrogen gas is concurrently fed to the dielectric tube along with another plasma-forming gas, such as argon. Without being held to any particular theory, it is believed that the introduction of hydrogen gas can reduce the microwave power needed to produce nanowires as compared to the same process which does not use hydrogen gas. It is also believed that hydrogen gas plasma can etch nanoparticles or form nanowires and thereby improve the production efficiency or quality of nanostructures. 
     In one embodiment, instead of, or in addition to, supplying sheath gas and a plasma-forming gas such as argon to the dielectric tube, water vapor can serve as the plasma-forming gas. For example, steam can be generated and introduced to the dielectric tube, for example, in a helical gas flow pattern. Water splitting into species such as H, O, OH, H 2 , and O 2  and also remaining or forming H 2 O can be used to produce high density plasma. In some embodiments, such a plasma can form thinner and higher quality nanostructures due to better etching properties of H 2  and OH. 
     A method for producing metal oxide nanostructures further includes delivering a metal powder (or metal-containing precursor) into the dielectric tube and reacting the metal powder within the plasma to form metal oxide nanostructures. Appropriate metal powders (or metal-containing precursors) can be selected based upon the desired composition of the nanostructures. Examples of metal powders suitable for use in this invention include tin, zinc, tungsten, titanium, iron, gallium, indium, bismuth, niobium, aluminum, vanadium, copper, alloys, and the like, and combinations thereof. In some embodiments, the powder consists of a particle having a particle diameter of less than about 20 microns such as less than about 15 microns, less than about 10 microns, less than about 5 microns, or less than about 1 micron. Generally, relatively small powders result in greater one pass efficiency in the production of nanowires. 
     In one embodiment, metal powder (or metal-containing precursor) is delivered into the dielectric tube via gravity feed and is conveyed into the plasma by gravity. Alternatively, the metal powder (or metal-containing precursor) can be delivered into the dielectric tube via pressure, e.g., by pressurized gas, or via a mechanical dispensing system. In one embodiment, the metal powder (or metal-containing precursor) is entrained within the plasma-forming gas. 
     In one embodiment, a bulk of the metal powder (or metal-containing precursor) is delivered substantially into the center of the plasma. For example, the metal powder can be delivered to the dielectric tube via a funnel such as a conical funnel. By using a funnel to deliver the metal powder, the metal powder can be directed into the center of the plasma. In one embodiment, the metal powder is delivered into the dielectric tube via a cooled metal powder delivery system. 
     Examples of suitable apparatus for delivering a metal powder (or metal-containing precursor) into a dielectric tube are also described supra. For example, an apparatus such as that shown in  FIGS. 4 ,  6 A-B, and  7  can be used to deliver a metal powder (or metal-containing precursor) into a dielectric tube. In one embodiment, powder is added to conical funnel  76  of cooling jacket and powder delivery system  70 , cooling jacket cover and plasma-forming gas inlet system  72  is placed over cooling jacket and powder delivery system  70 , and gas is supplied via plasma-forming gas inlet  58  and used to push the powder through conical funnel  76  into dielectric tube  14 . 
     In one embodiment, wherein a portion of the metal powder delivered to the dielectric tube does not react to form metal oxide nanostructures, the method of the present invention further includes separating nanostructures from a stream of nanostructures and unreacted metal powder. Examples of suitable apparatus for separating nanostructures from a stream of nanostructures and unreacted metal powder are described supra. 
     In one embodiment of the invention, wherein a portion of the metal powder delivered to the dielectric tube does not react to form metal oxide nanostructures, the method of the present invention further includes recycling unreacted metal powder into the plasma. In some embodiments, fresh metal powder feed can be added to the recycled metal powder before feeding the combined stream into the plasma. By recycling unreacted metal powder back to the plasma, efficiency of the process can be enhanced, waste materials can be reduced, continuous production of nanostructures can be achieved, and purity of the nanostructure product can be increased. 
     Examples of suitable apparatus for recycling unreacted metal powder into the plasma are described supra. 
     In one embodiment, a precursor feed can be added to the reaction product stream downstream of the plasma for further reaction. For example, a precursor feed can be added downstream of the plasma to promote thin film formation. 
     In one particular embodiment, production of metal oxide nanostructures is conducted at less than about 1000 W of plasma power in an atmosphere of about 5 slpm, about 2 slpm argon, and about 100 sccm oxygen. Metal powder or granules are allowed to fall under gravity through a plasma jet in a quartz tube, the metal granules are melted to form metal oxide nanowires, and the metal oxide nanowires are collected from the bottom of the dielectric tube. 
     In another particular embodiment, production of metal oxide nanoparticles is conducted at equal to or greater than about 1000 W of plasma power in an atmosphere of about 5 slpm, about 2 slpm argon, and about 100 sccm oxygen. Metal powder or granules are allowed to fall under gravity through a plasma jet in a quartz tube, the metal granules are vaporized within the plasma to form metal oxide nanoparticles, and the metal oxide nanoparticles are collected from the bottom of the dielectric tube. 
     In one embodiment, a method for producing nanostructures further includes delivering a precursor, e.g., a metal organic precursor such as a carbon nanotube precursor, into the dielectric tube and reacting the precursor within the plasma to form nanostructures. In each instance of the present disclosure, a metal-containing precursor such as a metal-containing organic precursor such as a carbon nanotube precursor, can be substituted for the metal powder in a reactor and method to form nanostructures from the precursor. For example, in some aspects, the present invention includes a method for producing nanostructures comprising: delivering a plasma-forming gas substantially longitudinally into a dielectric tube; delivering a sheath gas into the dielectric tube; forming a plasma from the plasma-forming gas by applying microwave energy to the plasma-forming gas; delivering a precursor into the dielectric tube; and reacting the precursor within the plasma to form nanostructures. The precursor can include a metal organic precursor such as a carbon nanotube precursor, e.g., an iron and hydrocarbon species in a vapor phase feed. 
     In one aspect, the method for producing nanostructures further includes depositing nanostructures in a thin film or in an array onto a suitable substrate, for example, using downstream plasma oxidation of metal film coated substrates or metal substrates. 
     The methods and apparatus described herein can be used in both batch and continuous processes for the production of nanostructures. In one embodiment, nanostructures are deposited on the sides of a dielectric tube and, after operation of the reactor for a period of time, the nanostructures are recovered from the sides of the dielectric tube. In other embodiments, nanostructures are continuously collected from the reactor during its operation. 
     The methods for producing nanowires described herein can be performed individually, in parallel with other nanostructure production processes, or in series with other nanostructure production processes. For example, in one embodiment, the products from one nanostructure production process can be fed to another nanostructure production process to form a continuous production route. 
     The reactor and methods of the present invention can be used to produce highly pure nanostructure products. In some embodiments, the nanostructure products do not contain any foreign material contamination such as, for example, catalyst, substrate, or template materials. In particular embodiments, the nanostructure products contain less than about 5%, less than about 1%, less than about 0.5%, less than about 0.1%, less than about 0.01%, or less than about 0.001% by weight foreign material contamination. For example, the nanostructure products can contain at least about 99%, at least about 99.9%, at least about 99.99%, or at least about 99.999% metal oxide by weight. In some preferred embodiments, highly pure nanostructure products are produced without additional purification or separation of the nanostructure products exiting the reactor. 
     EXAMPLES 
     The metal oxide nanowires of Examples 1 to 4 and 6 were produced using the reactor illustrated in  FIGS. 1-2  but without recycling system  38 . The reactor was operated at 1000 watts (W) in an atmosphere of 5 standard liters per minute (slpm) air sheath gas (fed through sheath gas inlets  16  and  18 ), and a plasma-forming gas of 2 slpm argon and 100 standard cubic centimeters per minute (sccm) of oxygen (fed through metal powder and plasma-forming gas delivery system  12 ) at atmospheric pressure. A metal ignition rod with pointed ends was used to ignite the plasma. The metal powder or the metal-containing precursor was supplied to the top of the dielectric tube into a microwave plasma jet. Gases and metals reacted at the center of the dielectric tube near the plasma flame head and simultaneously fell under gravity along the plasma flame length. The plasma flame length was about 10 centimeters in length. The dielectric tube was quartz and had a length of about 75 cm and an inside diameter of about 22 millimeters (mm) (about 25 mm outside diameter). Metal oxide nanowires were collected from the bottom of the dielectric tube. The efficiency of nanowire production was about 80 to 90% using about 100 nanometer (nm) diameter metal powder or granules but was less than 20% when metal granules with sizes greater than about 10 microns were used. 
     Example 1 
     Tin granules (separately, less than about 10 microns (tin powder, spherical, &lt;10 microns, 99%, Catalog No. 520373 from Sigma Aldrich) and then greater than about 100 nm (tin powder, APS approx. 0.1 micron, Catalog No. 43461 from Alfa Aesar)) were allowed to fall under gravity through the plasma jet in the quartz tube and nanowires were collected from the bottom of the tube. The obtained nanowires were tin oxide and had diameters ranging from about 50 to about 500 nanometers and lengths of about 1 to about 10 microns. 
     The products obtained using the two different tin metal diameter precursors (about 10 micron and about 100 nm) under the same operating conditions were imaged using SEM. The about 100 nm metal produced more uniform nanowires and about 90% conversion efficiency. The about 10 micron metal had less conversion efficiency (20-30%) and produced less uniform nanowires. Thus, smaller metal powders appeared to produce better results than larger metal powders. 
       FIGS. 10A to 10E  are photomicrographs of the tin oxide nanowires produced. The obtained nanowires had diameters ranging from about 50 to about 500 nanometers and lengths of about 5 to about 10 microns. Nanobeads were also observed as shown in one of the photomicrographs.  FIG. 11  is a Raman spectrum of the tin oxide nanowires. 
     Example 2 
     Zinc metal powder or granules (&lt;50 nm particle size, 99+%, Catalog No. 578002 from Sigma Aldrich) (observed to be greater than 100 nm under SEM) were allowed to fall under gravity through the plasma jet in the quartz tube and nanowires were collected from the bottom of the tube. The obtained nanowires were zinc oxide and had diameters ranging from about 100 to about 500 nm and lengths of about 1 to about 10 microns. 
       FIGS. 12A to 12F  are photomicrographs of the zinc oxide nanowires produced from the zinc metal powder or granules.  FIGS. 12B and 12C  show flowery-shaped zinc oxide nanowires with a high density of nanowires with uniform diameters.  FIG. 12D  shows a tripod structure, while  FIG. 12E  shows a nanobrush, and  FIG. 12F  shows a nanocomb (also shown in  FIG. 12C ) of ZnO nanowires. 
     Example 3 
     Titanium metal powder or granules (greater than about 10 microns) (titanium powder, spherical, 150 mesh, 99.9%, Catalog No. 41545 from Alfa Aesar) were allowed to fall under gravity through the plasma jet in the quartz tube and nanowires were collected from the bottom of the tube. The obtained nanowires were made of titania and had diameters from about 100 to about 500 nm and lengths of about 1 to about 10 microns. The microwave power for form titania nanowires was at less than about 1000 W, and more specifically, about 700 W. 
       FIGS. 13A to 13B  are photomicrographs of the titanium dioxide nanowires produced from the titanium metal powder or granules. 
     Example 4 
     Copper-zinc alloy powder or granules (about 100 nm) (Catalog No. 593583 from Sigma Aldrich) were allowed to fall under gravity through the plasma jet in the quartz tube and the reaction product was collected from the bottom of the dielectric tube. The obtained product took the form of copper-zinc oxide nanowires/nanobelts and had diameters from about 100 to about 800 nm and lengths of about 10 to about 50 microns.  FIGS. 14A to 14B  are photomicrographs of the copper-zinc oxide nanowires/nanobelts. 
     Example 5 
     Using the reactor illustrated in  FIGS. 3-7  with an approximately about 75 cm long, about 22 mm inside diameter dielectric tube of quartz, tin metal powder or granules (100 nm) (tin powder, APS approx. 0.1 micron, Catalog No. 43461 from Alfa Aesar) were placed in conical funnel  76  and argon gas was delivered to push the metal through the funnel. A plasma-forming gas of about 500 sccm of O 2  and 2 slpm of argon were delivered to the quartz tube via gas inlet  58 . The plasma power was about 1500 watts. About 15 slpm of air was delivered through sheath gas inlets  64  and  66 . The powder delivery system was kept at less than about 100° C. by flowing cooling water into coolant inlet  60  and out of coolant outlet  62 . 
     Very high quality (with diameters less than about 100 nm, uniform size distribution, and a low percentage of other nanostructures) tin oxide nanowires were produced and collected in a nanowire product collector as shown in  FIG. 9 . The nanowires had diameters as low as about 15 nm with a mean diameter of about 40 nm and a maximum diameter of about 100 nm. The length of the tin oxide nanowires was about 5 microns. The efficiency of nanowire production was at least about 90%.  FIGS. 15A to 15F  are photomicrographs of the tin oxide nanowires at various magnifications. 
     Example 6 
     Using the reactor illustrated in  FIGS. 3-7  with an approximately about 75 cm long, about 22 mm inside diameter dielectric tube of quartz, tin metal powder or granules (about 100 nm) were placed in conical funnel  76  and argon gas was delivered to push the metal through the funnel. A plasma-forming gas of about 700 sccm of O 2 , 2 slpm of argon, and about 100 sccm of hydrogen gas were delivered to the quartz tube via gas inlet  58 . The plasma power was about 1500 watts. About 10 slpm of air was delivered through sheath gas inlets  64  and  66 . The powder delivery system was kept at less than about 100° C. by flowing cooling water into coolant inlet  60  and out of coolant outlet  62 . 
     Very high quality (with diameters less than about 100 nm, uniform size distribution, and a low percentage of other nanostructures) tin oxide nanowires were produced and collected in a nanowire product collector as shown in  FIG. 9 . The nanowires had diameters of about 20 nm to about 30 nm and a length of several microns. The efficiency of nanowire production was at least about 90%.  FIGS. 16A to 16C  are photomicrographs of the obtained tin oxide nanowires at various magnifications. 
     Example 7 
     Aluminum metal powder or granules (about 3-4.5 microns in size) (Aluminum powder, spherical, 97.5%, Catalog No. 41000 from Alfa Aesar) were allowed to fall under gravity through the plasma jet in the quartz tube and nanowires were collected from the bottom of the tube. The obtained nanowires were made of alumina and had diameters from about 100 to about 500 nm and lengths of about 1 to about 10 microns. 
       FIGS. 17A  (low resolution) to  17 B (high resolution) are photomicrographs of the aluminum dioxide nanowires produced from the aluminum metal powder or granules. The Al 2 O 3  nanowires tend to be inverted funnel shaped and protrudes out from the bulk metal in a flowery pattern. Further, straight and isolated Al 2 O 3  nanowires have also been observed. 
     Example 8 
     Aluminum metal powder or granules (about 3 to about 10 microns) (Aluminum metal powder, spherical, 97.5%, Catalog No. 41000 from Alfa Aesar) were allowed to fall under gravity through the plasma jet in the quartz tube and nanoparticles were collected from the bottom of the tube. In this case, the microwave power required to form nanoparticles is greater than that required to form nanowires. For example, to form alumina nanoparticles, the microwave power must be equal to or greater than about 1300 W with about 10 slpm air, about 2 slpm Argon, about 100 sccm of H 2  and about 500 sccm of O 2 . At lower microwave powers, such as less than about 1300 W, and more specifically, about 800 W, alumina nanowires were formed. The obtained nanoparticles were made of alumina and had diameters from about 50 to about 100 nm. Without being held to any particular theory, it is believed that nanoparticle formation occurs only under the vaporization conditions of the higher microwave power and not in molten conditions. 
       FIGS. 18A to 18B  are photomicrographs of the about 50 to about 100 nm size aluminum dioxide nanoparticles ( 18 B) produced from the 5-10 micron size aluminum metal powder or granules ( 18 A). 
     Example 9 
     Titanium metal powder or granules (greater than about 10 microns, about 20 to about 100 microns) (titanium powder, spherical, 150 mesh, 99.9%, Catalog No. 41545 from Alfa Aesar) were allowed to fall under gravity through the plasma jet in the quartz tube and nanoparticles were collected from the bottom of the tube. In this case, the microwave power required to form titania nanoparticles is about 1000 W with about 10 slpm air, about 2 slpm Argon, about 100 sccm of H 2  and about 500 sccm of O 2 . At lower microwave powers, such as less than about 1000 W, and more specifically, about 700 W, titanina nanowires were formed. The obtained nanoparticles were made of titania and had diameters from about 50 to about 100 nm. 
       FIGS. 19A to 19B  are photomicrographs of the about 50 to about 100 nm titanium dioxide nanoparticles produced from the about 20 to about 100 micron size titanium metal powder or granules. It appears that alumina nanoparticles have more uniform size distribution compared to titania nanoparticles. Without being held to any particular theory, it is believed that the more uniform alumina nanoparticles could be due to the smaller size of the alumina starting metal, with Al metal powder or granules being about 5 to about 10 microns compared with Ti metal powder or granules at about 20 to about 100 microns. 
     One of ordinary skill in the art will recognize that additional configurations are possible without departing from the teachings of the invention or the scope of the claims which follow. This detailed description, and particularly the specific details of the exemplary embodiments disclosed, is given primarily for completeness and no unnecessary limitations are to be imputed therefrom, for modifications will become obvious to those skilled in the art upon reading this disclosure and may be made without departing from the spirit or scope of the claimed invention.