Patent Application
20100112349
This application claims priority to Taiwan Patent Application No. 097142632 filed on Nov. 5, 2008, the disclosures of which are incorporated herein by reference in their entirety.
Not applicable.
1. Field of the Invention
The present invention provides a nanomaterial with a core-shell structure. In particular, the present invention provides a nanocomposite material comprising a non-stoichiometric compound.
2. Descriptions of the Related Art
Generally, a material is deemed as a nanomaterial if at least one of its length, width, and height is in a range of nonameter scale (usually ranging from 1 nm to 100 nm). The nanomaterials can be generally divided into three types. The zero-dimensional nanomaterial has the length, width, and height in the range of nanometer scale. The one-dimensional nanomaterial has the width and height in the range of nanometer scale. The two-dimensional nanomaterial has the height in the range of nanometer scale only. For example, the zero-dimensional nanomaterial includes particle forms such as nanopowders; the one-dimensional nanomaterial includes thin bar forms such as nanowires or nanobars; and the two-dimensional nanomaterial includes plane layer forms such as nanofilms. As compared to the zero- and two-dimensional nanomaterials, the one-dimensional nanomaterial exhibits unique and excellent properties such as good field emission property due to its high specific surface area and high aspect ratio. Therefore, the one-dimensional nanomaterials have been highly valued by researchers, and it can be seen in T. Ruecks et al. Science, 289, 84 (2000) and M. H. Huang et al. Science, 292, 1897 (2001).
When the size of a well known bulk material is narrowed down to a nanoscale, its original chemical properties and physical properties, such as thermal, optical, electric, magnetic, or mechanical properties will change greatly, thereby opening a door to the field of nanomaterial applications. For example, the melting point of pure gold has a constant value (about 1064°), but as its particle size is narrowed down to a nanoscale, the melting point is no longer constant (see Ph. Buffat and J. P. Borel, Phys. Rev. A, 13, 2287 (1976)). Moreover, for example, the tungsten oxide (WO3) nanomaterial in saturated state is very advantageous for use in electrochromatic techniques, gas sensing techniques, and photocatalysts due to its unique properties. The related teachings can be found in E. B. Franke et al. J. App. Phys. 88, 5777 (2000) and H. Kominami et al. J. Mat. Chem. 11, 3222 (2001). Furthermore, there is a research showing that the tungsten oxide material in unsaturated state (i.e., non-stoichiometric) such as W18O49 has a unique structure and improved properties induced by oxygen defects, while the tungsten oxide material in unsaturated state exhibits better performances in practical applications as compared with that in saturated state (see G. L. Frey et al. J. Solid State Chemistry, 162, 200 (2001)). The whole contents of the above documents are all incorporated hereinto by reference.
Although it is possible to change the size and structure of the single nanomaterial to adjust its properties, the changing level is very limited due to the original properties of the nanomaterial. Therefore, the research of composite nanomaterials (referred to as “nanocomposite materials” hereinafter) has gradually become a trend in nanotechnology developments for improving the applicability of nanomaterials.
Furthermore, the nanocomposite material combines two or more nanomaterials with different properties, and the overall characteristics of the resulting composite material can be adjusted to a certain level by selecting the amounts and species of different materials (e.g., blending inorganic materials, organic materials, crystalline materials and/or amorphous materials) and the conditions of the preparation process. The nanocomposite material not only has the original characteristics of composition materials but also exhibits more novel functions than each composition material especially in optic, electricity, and magnetism, thus greatly improving the applicability. For example, it has been reported that the nanocomposite material containing tungsten oxide (WO3) can provide improved applicability as compared with the nanomaterial only composed of tungsten oxide (WO3). It can be seen in P. S. Patil et al. Applied Surface Science, 252, 1643 (2005) and Md. M. H. Bhuiyan et al. JJAP, 45, 8469 (2006), which are incorporated hereinto by reference.
There are many known methods for preparing a nanocomposite material, and they can be substantially divided into two types, i.e., physical methods and chemical methods. Generally, the physical methods include, for example, chemical mechanical polishing methods and high-energy ball milling methods. The chemical methods include, for example, chemical vapor deposition methods, sol-gel methods, hydrothermal synthesis methods and template synthesis methods. The chemical mechanical polishing method and the high-energy ball milling method of the physical methods usually can make the material nanoscale with a high energy impact in a short time. However, the resulting nanomaterial has the drawbacks such as larger size, irregular shape, overly wide particle size distribution, and low purity. The chemical methods can prepare a nanomaterial with higher purity, but the preparation is time-consuming. Up to now, the chemical methods are more useful in preparing a nanocomposite material in saturated state, and most methods focus on the preparation of a nanocomposite material in the form of film. Nanocomposite materials in unsaturated state and the preparation methods thereof have not yet been disclosed.
Due to the developments of nanotechnology and the shortages of the substance characteristics of single nanomaterials, a novel nanocomposite material is highly desired in industry.
The objective of the present invention is to provide a nanomaterial with a core-shell structure, comprising a shell and a core, wherein the shell is located on at least a portion of the core; the shell is substantially composed of a first metal oxide; and the core is substantially composed of a second metal oxide and the second metal oxide is a non-stoichiometric compound.
The detailed technology and preferred embodiments implemented for the subject invention are described in the following paragraphs accompanying the appended drawings for people skilled in this field to well appreciate the features of the claimed invention.
The nanomaterial of the present invention has a core-shell structure comprising a shell and a core. The shell is located on at least a portion of the surface of the core. According to the present invention, the shell is substantially uniformly distributed over the surface of the core. Preferably, the shell is substantially uniformly distributed over all surface of the core. The shell is substantially composed of a first metal oxide. The first metal oxide may be, for example, titanium oxide, zinc oxide, vanadium oxide, tin oxide, or combinations thereof; and titanium oxide is preferred. Generally, the first metal oxide of the shell is in saturated state, i.e., a metal oxide with a formula such as TiO2, ZnO, V2O5, or SnO2.
As implied by the term, a nanomaterial with a core-shell structure according to the present invention further comprises a core in addition to the shell. The core is substantially composed of a second metal oxide which is a non-stoichiometric compound. In other words, the ratio of the metal atoms of the second metal oxide to its oxygen atoms fails to conform to the law of definite proportions, and thus, the second metal oxide is in unsaturated state. For example, FeO1.05 is a non-stoichiometric compound (in unsaturated state), and TiO2 is a stoichiometric compound (in saturated state).
The second metal oxide useful in the present invention may be, for example, a non-stoichiometric tungsten oxide, a non-stoichiometric molybdenum oxide, a non-stoichiometric manganese oxide, or combinations thereof. According to one embodiment of the present invention, the second metal oxide is tungsten oxide with a formula WaOb, wherein the ratio b/a is between 2 and 3 (excluding the end-points 2 and 3) and preferably ranges from about 2.6 to about 2.9. For example, W24O68 or W18O49, preferably W18O49, can be used as the second metal oxide of the present invention, i.e., constituting the core structure of the nanomaterial.
According to one preferred embodiment of the present invention, the shell is composed of titanium oxide (TiO2), while the core is composed of non-stoichiometric tungsten oxide (W18O49). Titanium oxide is a semiconductor material that has recently been used. Because the optical and chemical properties of titanium oxide are very stable, titanium oxide is widely used in photocatalysts and fuel cells. However, titanium oxide has an energy gap of about 3.2 eV (about 387 nm in wavelength), and thus, its applications are mostly limited to the range of UV light, thereby reducing the application efficiency. Tungsten oxide is also a widely used semiconductor material and primarily used in the electrochromatic technique, gas sensing technique, and photocatalyst. However, the performance properties of tungsten oxide cannot be further improved to correspond to the requirement of high accuracy. The W18O49—TiO2 core-shell nanomaterial of the present invention has the characteristics of both titanium oxide and tungsten oxide and their drawbacks also can be retrieved by each other. Therefore, the whole application performance is improved which can be further demonstrated in the following examples.
In general, the shape of the nanomaterial according to the present invention is not particularly limited. The nanomaterial may be in the form of such as a particle, a wire, a bar, a tube, a needle, a film, or a cube.
The shell/core ratio of the nanomaterial in accordance with the present invention is not particular limited in principle, and usually depends on the factors such as the shape, size, and use of the desired nanomaterial. For example, the shell/core ratio of the W18O49—TiO2 core-shell nanomaterial may range from about 1:1 to about 1:8 in volume.
According to one embodiment of the present invention, the nanomaterial with a core-shell structure can be prepared by a plasma arc gas condensation method. Accordingly, the method may be carried out by using an apparatus schematically depicted in
More specifically, the atoms produced by heating and evaporating the target material 6 collide with inert gas atoms and rapidly lose their energy. A uniform nucleation then proceeds. Due to the convection of inert gas, the nucleating substances gradually move close to the cooling bar 7 and then accumulate at its surface to form nanomaterials. Eventually, the nanomaterials are collected into a collecting vessel 12 by using the scraping plate 9.
The suitable components and their proportions of the target material 6, and the ratio of the inert gas to the oxygen gas provided by the air-blowing system 10 depends on the requirements of the prepared nanomaterials. The ratio of the inert gas to the oxygen gas usually ranges from about 1:1 to about 100:1. For example, tungsten oxide-titanium oxide nanomaterial can be prepared by using tungsten powder and titanium oxide as the target material 6. Meanwhile, the nanocomposite materials of tungsten oxide with different oxygen contents can be prepared by controlling the ratio of the inert gas and oxygen gas provided by the air-blowing system 10. It can be seen from the above that the use of the plasma arc gas condensation method is flexible and is easy to control the experimental parameters, and thus, the method can provide various choices for preparing nanocomposite materials.
Furthermore, the core-shell nanomaterial prepared by the plasma arc gas condensation method according to the present invention has a relatively stable combining structure between the shell and the core as compared with that prepared by the general chemical method. Because the nanocomposite material prepared by the plasma arc gas condensation method has a stronger bonding and a clear interface, the composite material prepared by the process according to the present invention has the original properties of single material and also new properties thus derived.
As described above, the nanomaterial with a core-shell structure according to the present invention can provide excellent application properties. The following will further describe the gas sensing properties and the electron field emission properties of the nanomaterial in accordance with the present invention.
Metal oxide semiconductors (MOS) are widely used in detecting toxic gases and combustible and explosive gases because they have the properties, such as good heat resistance, good corrosion resistance and short responding time, and are easy to prepare as elements and easy to fabricate with microprocessors to form a gas sensing system or a portable detector.
When MOS is used in gas sensing detectors to detect gas, the estimation is conducted primarily by the changing level of the electrical resistance value based on the Madeling model. The related contents can be found in J. D. Levine and P. Mark Phys. Rev, 144, 751 (1996). In brief, when the gas molecules to be detected attach to the metal oxide crystal grain, a space-charge layer is formed on its surface. The higher the concentration of the gas to be detected, the thicker the space-charge layer is formed. Therefore, the electrical resistance value will be increased as the resistance for transporting electrons among the crystal grains is increased, and vice versa. Consequently, the species and the concentration of the gas to be detected can be known from the changing amount of the electrical resistance.
Thus, the properties of the gas sensing detector such as sensitivity, stability, selectivity, and reproducibility are all influenced by the factors such as the species and crystal grain properties of the used gas sensing material (i.e., MOS). The latter includes the size of crystal grain, the structure of grain boundary, and the state and defect of the crystal. Because the one-dimensional nanomaterial has a high specific surface area that increases the contact area with the gas to be detected, the one-dimensional nanomaterial is particularly useful in improving the properties of the gas sensing detector and increasing the sensitivity thereof. Therefore, the nanomaterial in the form of a nanowire in accordance with the present invention is particularly useful in the gas sensing applications.
The sensing properties of gas sensing materials can usually be determined by gas sensitivity. The gas sensitivity is defined as follows:
Sensitivity=[(Rgas−Rair)/Rair]×100%
wherein, Rair represents the electrical resistance value of the sensing material in the air and Rgas represents the electrical resistance of the sensing material in the gas to be detected. For example, the nanomaterial of the present invention has a gas sensitivity of about 1.48 for sensing about 1 ppm NO2 gas at 200° C. Also, for example, the nanomaterial of the present invention has a gas sensitivity of about 4.18 for sensing about 4 ppm NO2 gas at 200° C.
The electron field emission is a phenomenon that under the action of a strong electric field, the energy band that transforms the electrons of a substance surface from bonded electrons into free electrons will be curved, and thus induces a quantum mechanical tunneling phenomenon of electron at the solid surface. If the applied electric field is strong enough, the electrons will tunnel through the energy barrier of the substance surface into the vacuum level. This phenomenon is called the electron field emission and can be applied in, for example, field emission displays.
The electron field emission theory was proposed by R. H. Fowler and L. W. Nordheim in 1928 at the earliest, and its theory can be seen in
As described above, the intensity of the applied electric field will directly influence the magnitude of the field emission current. When applied in field emission displays, the electric field at the metal-vacuum interface must be increased to obtain the adequate current. However, the operation voltage of the components also has to be increased that diverges from the low operation voltage desired in industry. Therefore, for effectively saving energy, a material that can easily emit electrons with the action of a not very strong applied electric field under vacuum condition is highly desired in the industry.
For example, the W18O49—TiO2 core-shell nanomaterial of the present invention has a field emission turn-on field of less than about 2.5 V/μm and a field emission threshold field of less than about 3.5 V/μm under 2×10−6 torr. In this text, the term “turn-on field” represents the electric field required for generating a current density of 10 μA/cm2, and the term “threshold field” represents the electric field required for generating a current density of 10 mA/cm2.
The examples below are illustrated to further describe the present invention. However, the present invention may be embodied in other embodiments or other examples, and should not be limited to the examples provided herein.
The nanomaterial with a core-shell structure was prepared by using the apparatus shown in
A W18O49—TiO2 nanowire (referred to as “nanowire” hereinafter) with a diameter distribution of 20 nm to 100 nm was prepared, and its length could be up to several micrometers and its core-shell structure is shown in
First, the sensing substrate plated with the nanowire prepared by the example was placed into a rapid thermal annealing (RTA) furnace, and then heated to 300° C. with a rate of 5° C./minute. The temperature was held for 48 hours to facilitate the stability of the nanowire structure. The substrate was then placed into a gas sensing cavity with a flowing gas for the test.
The test for electron field emission property was conducted by a transparent-anode technique and measured in a vacuum chamber with about 2×10−6 torr at room temperature. The method used the nanowire prepared by the example as a field emission source (cathode) and used an ITO conductive glass as an anode, and the distance between the cathode and the anode was set to be 250 micrometers. A field emission voltage-current curve diagram was prepared by scanning using the voltage of 0 V to 1000 V as shown in
It is shown from
The W18O49 nanowire, however, had a field emission turn-on field of about 4.63 V/μm and a field emission threshold field of about 6.36 V/μm. Therefore, the W18O49—TiO2 nanowire according to the present invention had lower field emission turn-on field and field emission threshold field than that of the W18O49 nanowire, and exhibited excellent field emission properties.
According to the above tests, the composite nanomaterial of the present invention indeed has improved properties and exhibits a high applicability.
The above disclosure is related to the detailed technical contents and inventive features thereof. People skilled in this field may proceed with a variety of modifications and replacements based on the disclosures and suggestions of the present invention as described without departing from the characteristics thereof. Nevertheless, although such modifications and replacements are not fully disclosed in the above descriptions, they have substantially been covered in the following claims as appended.
| Number | Date | Country | Kind |
|---|---|---|---|
| 097142632 | Nov 2008 | TW | national |
