Imported: 10 Mar '17 | Published: 27 Nov '08
USPTO - Utility Patents
Metal powder (such as tin, titanium, or tungsten powder) is heated in a flowing stream of an inert gas, such as argon, containing a small abundance of oxygen at a temperature to produce metal vapor. The metal reacts with the oxygen to form and deposit one-dimensional nanostructures of oxygen-containing metal on the metal powder (in the case of Ti and W) or on a suitable nearby substrate in the case of the lower melting tin. The metal oxides are not necessarily stoichiometric compounds. Water may be introduced into the flowing inert gas to increase or control the oxygen content. Sulfur vapor or a carbon source may be introduced to dope the nanostructures with sulfur or carbon. Reaction conditions may be modified to vary the shapes of the one-dimensional nanostructures.
This application claims priority based on provisional application 60/824,910, titled One Dimensional Metal and Metal Oxide Nanostructures, filed Sep. 8, 2006 and which is incorporated herein by reference.
This disclosure pertains to one-dimensional metal and metal oxide nanostructures (for example, structures of tin, tin oxide, titanium oxide, and tungsten oxide) with variable compositions and morphologies. This disclosure also pertains to methods for making and doping nanostructures in forms such as wires, rods, needles, and flowers.
Nanostructured materials (i.e., structures with at least one dimension in the range of 1-100 nm) have attracted steadily growing interest due to their unique, properties and potential applications complementary to three-dimensional bulk materials. Dimensionality plays a critical role in determining the properties of materials due to, for example, the different ways that electrons interact in three-dimensional (3D), two-dimensional (2D), one-dimensional (1D), and zero-dimensional (0D) structures. Compared with 0D nanostructures (so-called quantum dots or nano-particles) and 2D nanostructures (thin films), 1D nanostructures (including carbon nanotubes (CNTs) and nanowires (NWs)) are ideal as model systems for investigating the dependence of electronic transport, optical, and mechanical properties on size confinement and dimensionality as well as for various potential applications, including composite materials, electrode materials, field emitters, nanoelectronics, and nanoscale sensors.
Nanowires are a class of newer one-dimensional nanomaterials with a high aspect ratio (length-to-diameter typically greater than 10). They have interest separately from carbon nanotubes. Nanowires can be made of various compositions of materials in addition to carbon. Nanowires have demonstrated superior electrical, optical, mechanical and thermal properties. For example, the ultrahigh-strength of gold nanowires has recently been demonstrated. The significant increase in strength is due to reduced defects in the crystal structure and a smaller number of grains crossing the diameter of the nanowires. The broad choice of various crystalline materials and doping methods makes the properties (e.g. electrical) of nanowires tunable with a high degree of freedom and precision.
Nanowires consist of a variety of inorganic materials including elemental semiconductors (Si, Ge, and B), Group III-V semiconductors (GaN, GaAs, GaP, InP, InAs), Group II-VI semiconductors (ZnS, ZnSe, CdS, CdSe), and metal oxides (ZnO, MgO, SiO2, Al2O3, SnO2, WO3, TiO2). Among them, metal oxide nanowires have obvious advantages for some special applications due to their unique properties such as strong chemical interaction with metallic components. This phenomenon is sometimes explained as strong metal-support interaction. Significant progress has been reported in the use of metal oxide nanowires and nanobelts as sensors and in other electronic applications.
Substantially one-dimensional nanostructures (e.g. nanowires, nanorods, and nanobelts having a length much larger than thickness) are a new class of nanomaterials. Synthesis methods for such nanostructures usually fall into two categories: vapor-phase deposition or solution-based crystal growth. While solution-based syntheses generally offer better control of processing conditions and are easy to achieve higher productivity, vapor deposition often yields higher aspect ratios (for example, length-to-width or length-to-diameter ratios) and excellent crystallinity due to the higher growth temperatures. However, one of the prominent current challenges is in controlling the synthesis of metal oxide nanostructures in ways that allow variation in their morphology. This would permit exploration of different potential materials applications of the nanostructures as their shapes are changed.
Metals and metal oxides are formed in substantially one-dimensional nanostructures having a variety of shapes such as wires, rods, needles, belts, and flowers (wires or rods joined at a center and extending like radiating petals). Nanowires and rods have a uniform diameter from end to end while nanoneedles have sharp tips. These nanostructures have a high aspect ratio (e.g., ratio of length to diameter or thickness of ten or greater) where the smaller dimension is in the range of one to one hundred nanometers. Examples of metals that may be processed by practices disclosed in this specification include tin, titanium, and tungsten.
In a first embodiment, a powder of the metal is heated in a stream of an inert gas such as argon. Suitable temperatures are often in the range of about 700 C. to about 1000 C. The metal powder is heated to a suitable temperature at which it produces a quantity of vapor (e.g., tin), and the inert gas contains a small amount of oxygen molecules (for example in parts per million). Oxygen reacts with the hot metal vapor to form oxidized metal which is solid at the reaction temperature and deposits as one-dimensional nanostructures on a nearby substrate. When the metal produces little vapor at the temperature of the chamber (e.g., titanium and tungsten) the one-dimensional nanostructures may be grown directly on the metal powder.
The oxygen content of the inert gas stream may be low enough such that non-stoichiometric metal oxides are formed. For example, oxygen-containing metal compositions such as SnOx, TiOx and WOx may be formed where x is not an integer reflecting a common oxide such as SnO, SnO2, TiO2, WO3, etc.
Oxygen for oxidation of the metal powder particles is obtained from one or more several sources, including: oxygen initially absorbed on the starting metal material; the controlled addition of oxygen into the flowing inert gas; leakage of air into the inert gas; and/or humidification of the inert gas to add water molecules, some of which dissociate to oxygen and hydrogen in the hot reaction medium.
The metal oxide nanostructures may be doped with sulfur, carbon, or the like by adding the doping element to the inert gas stream flowing over the vapor-generating metal powder. Sulfur may be vaporized into the inert gas upstream of the metal powder. A hydrocarbon gas such as methane, ethylene or acetylene may be added to the inert gas upstream of the metal powder.
A tubular flow reactor heated by an enclosing electric furnace may provide a suitable reaction system. In this embodiment a container for the powder is paced within the heated zone of the tubular reactor and a suitable substrate for nanostructure growth provided at a suitable location near the metal powder. The one-dimensional nanostructure shape taken by the condensing oxygen-containing metal is found to depend on the parameters of the reacting system, including the location of substrate material and nature of dopant material if it is employed.
In another embodiment of the invention, composites of carbon nanotubes containing tin nanowires are formed when a stream of argon containing, for example, about 2% ethylene is passed over tin powder in a reaction vessel at 900 C. The composite of one-dimensional nanostructures may be formed on a substrate such as carbon fiber paper. In this embodiment, the low-melting tin vapor is deposited as liquid droplets on the carbon fiber substrate. The tin droplets catalyze the decomposition of ethylene and the formation of carbon nanotubes. Upon cooling of the reactor the liquid tin droplets solidify and shrink as one-dimensional tin nanowires within the carbon nanotubes.
The resulting one dimensional metal-carbon composite nanostructures and one-dimensional metal-oxygen nanostructures have utility, for example, in electrical/electronic applications and as catalyst supports.
Other objects and advantages of the invention will be apparent from descriptions of preferred embodiments which are presented in the following specification.
The following experimental procedures were followed in the preparation of one-dimensional nanostructures of the oxides of tin (SnOx), titanium (TiOx), and tungsten (WOx), respectively, starting with commercial powders of tin, titanium, and tungsten as obtained from Aldrich Chemical Company.
Nanostructures, nanowires, nanorods, nanobelts and nanoneedles, of metal oxide SnOx, WOx and TiOx were synthesized by a chemical vapor deposition method. Here, the values of x are usually greater than zero and up to, for example, three or greater, depending on the oxidation state of the metal. The values are not necessarily integer values and do not necessarily represent stoichiometric compounds. In some instances nanostructures of the metal were obtained.
The experimental setup (see FIGS. 1A-1C) included a horizontal tube furnace with an electrically resistance heated zone of about 30 cm in length. A quartz tube, 60 cm long, was centered along its length and enclosed in the furnace. Provision was made for the controlled flow of argon gas into one end of the tube (right end in FIGS. 1A, 1B, and 1C) and through the quartz tube to provide an atmosphere with relatively low oxygen content for the formation and growth of oxygen-containing metal nanostructures.
In a typical procedure, the metal powder (Sn, Ti, or W powder) was placed in an alumina boat (labeled in FIG. 1A) and located at the mid-point of the quartz tube and tube furnace. High purity argon (99.999%) was flowed through the quartz tube at a rate of 50 sccm (standard cubic centimeters per minute) for 15 min to remove O2 and other gases from the quartz tube chamber. The argon, initially at an ambient temperature, was rapidly heated within the hot tube furnace and partially enclosed quartz tube. The vapor deposition chamber thus provided in the quartz tube, and the argon gas flowing through it, were heated from room temperature to relatively high temperatures (700-1000 C.). Tin is liquid at these temperatures while titanium and tungsten are solid.
A small amount of oxygen was required to react with the metal powder, or vapor from the hot metal, and slowly produce the respective particulate metal oxide materials, which were not necessarily stoichiometric compounds. The metal particles (typically about 99.8% by weight of the respective metal) inherently initially contained a thin adsorbed coating of oxygen molecules. Additional oxygen was obtained to grow the metal oxide nanostructures from the very small residual oxygen content of the argon and from a low rate of oxygen leakage from the ends of the quartz tube. These small oxygen sources were sufficient to slowly oxidize the metal particles and oxygen-containing metal nanostructures were formed. They typically formed on powder particles (e.g., Ti and W) in the ceramic boat, on the sides of the ceramic boat, or on another nearby substrate provided for nanoscale particle growth (e.g., in the case of the liquid tin).
After growing the metal or metal oxide nanostructures for a period of time (e.g., 1-4 hrs), the furnace was cooled down to room temperature as the flow of argon was continued.
Carbon paper was used as a substrate for growth of SnOx nanostructures. The commercially-used carbon paper is a class of electrode materials for various applications such as fuel cells and sensors. A piece of the carbon paper was paced on the ceramic boat shown in FIG. 1A.
WOx and TiOx nanostructures, in the form of free-standing powder, could be synthesized directly from and on their powder as a result of their relatively high melting points and relatively low vapor pressure at reaction conditions.
In order to increase and further control the amount of oxygen during the synthesis, water was introduced with argon gas during some of the metal oxide synthesis experiments. In the water-assisted oxidation reactions, the argon flow from its storage tank was bubbled through a hot water bath (80 C.) and then flowed into the quartz tube so that water vapor was continuously carried into the reaction zone. The water dissociated (partially) in the hot tube to provide additional oxygen for synthesis of the one-dimensional nanostructures. The control of water amount was managed by the temperature of the water and/or the flow rate and dispersion of the bubbled inert gas. The water-assisted oxidation reaction is very effective for the growth of WOx and TiOx nanostructures from their metal powders.
In-Situ Doping of the Metal Oxide Nanostructures with Sulfur
In some experiments the nanostructures were doped with sulfur to modify electrical properties of nanostructures and/or the interaction of the metal oxide nanoparticles with subsequently deposited catalyst particles. In-situ sulfur-doping was conducted by placing a container of sulfur powder upstream of the metal powder (with respect to the direction of flow of the inert gas) as shown in FIG. 1B. The sulfur readily vaporized in the heated quartz reactor and was carried in the flowing argon into contact with the oxidized (and oxidizing) metal particles. The sulfur vapor diffused into the metal oxide nanostructures as a dopant element.
Sulfur-doped WOx nanostructures were typically obtained by the CVD process with the furnace at 760 C. and an argon stream (flow rate, 100 sccm, initially bubbled through hot water at 80 C.) passed through the heated quartz tube for 1-4 hrs. A ratio of sulfur to WOx (molar ratio W/S) was about 3:1.
In-Situ Doping of Carbon into Nanostructures.
There is also interest in doping small metal oxide particles with carbon with the goal of modifying their properties, for example their electrical and anti-corrosion properties. Here, carbon-doping was carried out in three different ways: (i) a hydrocarbon gas (e.g. C2H4, CH4, C2H2) was mixed with argon, as shown in FIG. 1C; (ii) a carbon-containing liquid (e.g. acetone, methanol and ethanol) was mixed with the argon stream by a bubbler (a similar procedure to water); and (iii) a solid (e.g. graphite and carbon nanotubes) were mixed with metal powder.
For example, carbon-doped SnO2 was prepared by introducing ethylene gas (C2H4) into flowing argon (200 sccm) for 2-4 hrs with the furnace at 900 C. Carbon-doped WO3 was obtained from 100 sccm Ar with 2 sccm C2H4 at 760 C. for 1-4 hrs. The argon was also bubbled through water at 80 C. Carbon-doped TiOx was prepared by introducing both water and 30 volume % acetone as a carbon source with 100-200 sccm Ar at 800-900 C. for 2-4 hrs.
The metal oxide nanostructures produced were characterized using a transmission electron microscope (TEM), high-resolution TEM (HRTEM), with electron energy-loss spectroscopy (EELS), energy dispersed X-ray spectroscopy (EDX), as well as with a scanning electron microscope (SEM) and field-emission scanning electron microscope (FE-SEM) with EDX.
One-dimensional nanostructures of three kinds of metal oxides (SnO, SnO2, WO3 and TiOx), non-doped and doped, were synthesized by the vapor deposition methods described above. The detailed results will be presented according to the following order (i) SnOx, (ii) WOx and (iii) TiOx.
Tin oxide nanostructures with varying amounts of oxygen were prepared. These materials were identified as SnO and SnO2.
(1) SnO and SnO2 nanostructures were grown on carbon paper and Al2O3 ceramic substrates. FIG. 2A is a SEM image of commercially-used and bare carbon paper substrate. The carbon paper is widely used as an electrode material for electrochemical applications such as fuel cell backing and sensors. The carbon paper consists of small carbon fibers of 5-10 m in diameter and a small piece of the paper was placed on the ceramic (alumina) boat overlying the tin powder in the boat. SnO and SnO2 nanostructures were first synthesized on carbon fibers of the carbon paper substrate at 900 C. under a flow of oxygen-containing argon gas at a flow rate of 200 sccm for 2 hrs.
FIG. 2B portrays SnO, SnO2 nanowires, grown on carbon fibers of carbon paper substrate. The SEM image reveals a high density of the tin-oxygen nanowires that cover the whole surface of carbon fibers. A TEM image inset into a corner of FIG. 2B shows wire-like particles of nanoscale tin oxide particles. Obviously, such a nanostructure will have an appreciable and useful specific surface area.
The morphology of nanostructures can be controlled by changing the distance of sample substrates from powder or molten metal sources. Generally, most growth areas close to the molten Sn source grew wire-like nanostructures. However, some areas more removed from the Sn liquid produced needle-like (pointed ends) nanostructures on carbon paper, as shown in FIG. 2C. In some areas, the second generation of nanoneedles grown on top of the first generation of nanowires in a one-step synthesis process was also observed (FIG. 2D). Further, special orientations between the first and the second generation of nanowires were observed. The needle-like and second generation structures should be associated with role of carbon from carbon paper and relative higher growth temperature.
When the ceramic boat alone provided the support for the oxidized tin, it was found that the oxygen-containing nanoparticles formed as nanobeltsvery thin and 10 m wideas illustrated in FIG. 2E.
To improve electrical properties and corrosion resistance of SnOx nanostructures as well as to understand the role of carbon during growth of nanostructures, an experiment was conducted to produce in-situ carbon-doped SnOx structures by introducing ethylene gas (C2H4) in the flowing argon. But composites of tin nanowires in carbon nanotubes were obtained. This is a new finding.
While keeping the same vapor deposition conditions as described in the above section (1), 900 C. and 200 sccm argon flow rate, different amounts (0.5 sccm-12 sccm) of carbon were introduced with the argon gas flow. The results showed that the different amounts of carbon resulted in different morphologies and structure of tin and carbon nanostructures on carbon paper as shown in FIGS. 3A through 3G.
FIGS. 3 (A-G) show SEM images of carbon-doped tin nanostructures on carbon paper synthesized in 900 C, 200 sccm Ar and carbon amounts (0.5-12 sccm). In the case of 0.5 sccm ethylene (FIG. 3A), tin and carbon nanostructures grown on a carbon fiber are not very dense and are about 15 m in length. With the increase of carbon amounts (1-5 sccm) in the argon, the density of tin and carbon nanostructures significantly increase and totally cover the carbon fibers (FIGS. 3B-3D). But the length is still about 15 m. When the carbon compound flow rates in the argon flow reach 7 and 10 sccm, very short nanoparticles (1-5 m) were obtained (FIGS. 3E and 3F). For 12 sccm carbon compound flow rate, there was no significant change in the structure of the particles; only more spherical nanostructures were observed (FIG. 3G).
A change of nanostructure morphologies is associated with structural features and composition of carbon-doped SnOx. The detailed analysis will be illustrated in FIGS. 4A through 4F. Basically, carbon graphitic layers were formed on the surface of the intended SnOx nanostructures and metallic Sn nanostructures were obtained. When a sufficient amount of carbon was introduced into the argon gas, the growth of a Sn nanostructure was promoted. When the amount of carbon is too high (12 sccm), carbon limits the growth of the nanostructures. From these results it was recognized that a new nanostructure was obtained when carbon amounts are in a range of 2-5 sccm. Details of these tin and carbon composite structures will be presented in a further description of FIGS. 4A-4F which is presented in the following section of this specification.
(3). Single-Crystalline Tin Nanowires Encapsulated by Carbon Nanotubes Grown on a Substrate of Carbon Fibers
As mentioned above, a high-density of nanostructure wires was obtained on a carbon fiber substrate when ethylene at flow rates of 2-5 sccm was introduced with the flow of argon. The SEM images of FIGS. 4A and 4B reveal their morphologies. Individual tin fibers are seen in FIG. 4A while the tin fibers are clustered like petals of a flower in some local areas as illustrated in FIG. 4B. However, close observations from SEM and HRTEM (FIGS. 4C and 4D) showed that the nanostructure actually consists of carbon nanotubes (about 30 nm thick) at the tip and bottom of the structures, with tin nanowires in the middle of the fiber-like nanostructures. TEM images in FIGS. 4E and 4F gave more detailed information. In some cases, a carbon nanotube portion (white region labeled as C nanotube in FIG. 4E) appears between nanowire portions, labeled as Sn nanowire. In other cases, hollow nanotubes were found only in the tip of nanostructures (FIG. 4F). The compositional analysis by EDX (inset in FIG. 4E) showed that darker area of the nanowire is composed of tin covered by carbon layers. The tubular portion of the fiber is composed mostly of carbon.
As discussed in the section 4 (below), the growth mechanisms of WO3 nanostructures are different from the mechanisms for SnO, and SnO2 nanostructures. The growth of SnOx nanostructures follows vaporization of Sn powder followed by oxidation of the tin and condensation of the non-stoichiometric oxide on a substrate like carbon paper. The mechanism is a vapor-to-solid (VS) mechanism. In contrast, WO3 (and TiOx) nanostructures appear to grow directly on W (or Ti) powders. A difference in synthesis of WOx nanostructure is that the water-assisted process oxidation process was used. In this case, the argon flow, prior to entering the chamber, was bubbled through a hot water bath (80 C.) so that H2O vapor was continuously transferred into the reaction zone. The introduction of water serves to provide more oxygen (maybe H2 as well) during the reaction. The control of water amount was carried out by water bubbling. The water-assisted process is very effective for the growth of WOx and TiOx nanostructures. FIGS. 5A-5F show SEM images of WOx, sulfur-doped WOx and carbon-doped WOx nanostructures.
FIGS. 5A (4,000) and 5B (12,000) show high-density clusters of WO3 monofilaments or nanowires formed on and covering underlying tungsten powder. The WO3 nanostructures are about 3-50 micrometers in length and about 100 nm in diameter. When a small amount of sulfur (molar ratio of W/S 3:1) was introduced in form of powder upstream of the tungsten powder boat (as shown in FIG. 1B), a significant increase of density and uniform has been obtained as illustrated in the SEM images of FIGS. 5C and 5D. For carbon-doping, a small amount of ethylene (2 sccm) is introduced with wet Ar. The carbon-doped WOx nanostructures have similar morphologies as WO3 and sulfur-doped WO3 as shown in FIGS. 5E and 5F. However, HRTEM images as presented in FIG. 6 showed that carbon-doped WOx nanostructures were covered a layer of amorphous carbon.
TiOx nanostructures were synthesized on Ti powder by chemical vapor deposition under conditions of 800-900 C. and Ar flow of 100-200 sccm. The results are illustrated in the SEM images of FIGS. 7A-7F.
FIG. 7A shows the morphology of the original Ti powder consisting of relatively large, generally spherical particles (10-15 micrometers in diameter). FIG. 7B is an SEM image of TiOx nanstructures synthesized and grown on the titanium powder particles under an oxygen-containing argon stream. It can be seen that the surfaces of the titanium particles are coarse and that they were oxidized into many polycrystalline grains. Both microwire and microbelt structures were found, although both of them are very sparse.
The process for forming TiOx particles on titanium powder was repeated using humidified argon. An argon stream was bubbled through a column of water at a rate of 30-80 bubbles per minutes. The humidified argon stream was then introduced into the quartz tube under the above specified conditions. As illustrated in the SEM images of FIGS. 7C (at 1000) and 7D (5000) very dense nanoneedles were obtained. The TiOx nanoneedles had sharp and long tips (FIG. 7D).
When the argon stream contained acetylene, TiOx nanostructures having a new feature (called nanowall) appeared as shown in FIGS. 7E and 7F. The nanowall structures were obtained by introducing into the flowing argon both water and 30 vol. % acetone as a carbon source at 800-900 C. for 2-4 hrs.
The growth mechanisms of SnOx nanostructures are different from the mechanisms for WOx and TiOx nanostructures.
The SnOx nanostructure growth is governed by the vapor-solid (VS) mechanism. During the heating, Sn vapor is generated from the molten Sn than combines with oxygen, which comes from three sources: residual oxygen in the reaction chamber, oxygen impurity in the Ar gas, and surface oxygen layers on metal powders. As a first step, Sn vapor and oxygen form SnO vapor. It is well-known that SnO is metastable and will decompose into SnO2 and Sn. The decomposition of SnO will result in the precipitation of SnO2 nanoparticles, which are carried by the flowing Ar gas and deposited on the walls of the alumina boat or carbon paper. The nanoparticles then act as nucleation sites and initiate the growth of SnO2 nanostructures via the VS mechanism. When SnO2 was formed on carbon paper, the mechanism is more complicated due to the presence of carbon on the paper surface. The reaction of carbon with oxygen in the hot environment may reduce supply of oxygen and, eventually, produce Sn or SnO.
The growth mechanisms of WOx and TiOx grown on their powders are not clearly understood. At present it is believed that the presence of water in the inert gas stream is necessary to obtain either WOx or TiOx nanostructures. The titanium and tungsten powders have higher melting points than tin, and the titanium and tungsten powder produces less vapor at the operating temperatures (700 C. to 1000 C.) of the reactor. The limited vapor production may account for the formation of their respective oxide nanostructures directly on their powders.
The growth of the composites of carbon nanotubes and tin nanowires is understood as follows. When heated at 900 C. under argon gas flow, the liquid tin (mp 232 C., bp 2270 C.) produces vapor transported downstream by the argon and deposited as growing liquid droplets on the graphitic fiber substrate. Two different nanostructure growth mechanisms occur simultaneously at the substrate-borne tin droplets. Ethylene vapor decomposes on the tin droplets and forms carbon nanotubes while the continued condensation of tin provides for the later formation of tin nanowires. When the reactor is cooled, the tin solidifies within the carbon nanotubes and shrinks to form the interesting carbon and tin composite nanostructures.
The practice of the invention has been illustrated by several specific examples but the scope of the invention is not limited by such illustrations.