WO2011041108A1

WO2011041108A1 - Production of bare nanoparticles-on-nanoparticles

Info

Publication number: WO2011041108A1
Application number: PCT/US2010/048955
Authority: WO
Inventors: Desiderio Kovar; Michael F. Becker; John W. Keto; Ignacio F. Gallardo; Manuj Nahar
Original assignee: Board Of Regents, The University Of Texas System
Priority date: 2009-10-02
Filing date: 2010-09-15
Publication date: 2011-04-07

Abstract

A nanocomposite structure and methods for producing such a nanocomposite structure. The nanocomposite structure includes a core of a larger nanoparticle or a group of nanoparticles that are unagglomerated and free of organic surface layers. The nanocomposite structure further includes distributions of isolated or groups of smaller nanoparticles on the surface of the oxide core which are also free of organic surface layers. By forming such a nanocomposite structure, the surface area as well as the catalytic activity of the nanostructure may be improved.

Description

PRODUCTION OF BARE NANOPARTICLES-ON-NANOP ARTICLES

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is related to the following commonly owned co-pending U.S. Patent Application:

Provisional Patent Application Serial No. 61/247,971, "Production of Bare Nanoparticle- On-Nanoparticle Surfaces," filed October 2, 2009, and claims the benefit of its earlier filing date under 35 U.S.C. § 119(e).

GOVERNMENT INTERESTS

[0002] The U.S. Government may own certain rights in this invention pursuant to the terms of the National Science Foundation Grant No. CBET-0708779.

TECHNICAL FIELD

[0003] The present invention relates to the production of nanoparticles, and more particularly to the production of small, bare nanoparticles on the surface of larger, bare nanoparticles to increase the surface area as well as possibly improving the catalytic activity of the resulting nanostructure.

BACKGROUND

[0004] An aspect of nanotechnology is the vastly increased ratio of surface area-to-volume present in many nanoscale materials that makes possible new quantum mechanical effects, for example, the "quantum size effect" where the electronic properties of solids are altered with great reductions in particle size. This effect is not realized at macroscale to microscale dimensions but becomes pronounced when the nanometer size range is reached. Materials that are reduced to the nanoscale can have very different properties compared to what they exhibit on a macroscale, enabling unique properties. For instance, metals, such as gold, which are chemically inert at macro or micro scales, can serve as potent chemical catalysts at the nanoscale level.

[0005] The catalytic properties of metals, such as gold or silver, depend critically upon the particle morphology (i.e., size and shape) and the substrate surface. Further, in order to be used as catalysts, the size of the metal catalysts needs to be stable at the temperature they are used. To improve the catalysis of a metal, such as gold, a thin layer or a sub-layer of gold nanoparticles may be placed on a flat oxide surface, such as titania (Ti0₂). It is believed that the high catalytic activity of nanoscale gold supported on titania is due to the electronic interactions between the gold and the titania that are unique at the nanoscale level.

[0006] Currently, the gold nanoparticles supported on titania are not composed as a single nanocomposite structure. Further, current nanoparticle technologies usually require the use of an organic layer on the surface of the nanoparticles to prevent agglomeration. Such organic layers can impair the catalytic function of the nanoparticle and may have to be removed by heating before the particles can be used for catalysis. Heating may cause sintering of the nanoparticles into larger structures, with a concomitant loss in catalytic activity.

[0007] If, however, metal or metal alloy nanoparticles could be placed on the surface of oxide nanoparticles in a single nanocomposite structure, then the surface area as well as the catalytic functionality of the metal may be improved.

[0008] Therefore, there is a need in the art for producing nanoparticles-on-nanoparticles that are free of organic layers and that are thermally stable.

BRIEF SUMMARY

[0009] In one embodiment of the present invention, a nanocomposite structure comprises a core comprising a nanoparticle or a group of nanoparticles. The nanocomposite structure further comprises a distribution of isolated nanoparticles or islands of nanoparticles smaller in size than the core, where the isolated nanoparticles or islands of nanoparticles are attached to the surface of the core. The islands of nanoparticles are free of organic ligands.

[0010] In one embodiment of the present invention, a method for producing nanoparticles- on-nanoparticles comprises aerosolizing powders of a first composition of a micrometer size. The method further comprises ablating the aerosolized powders thereby generating nanoparticles of the first composition. Additionally, the method comprises aerosolizing powders of micrometer sized particles of a second composition. Furthermore, the method comprises mixing the nanoparticles of the first composition with the aerosolized powders of the second composition. In addition, the method comprises ablating the mixture in such a manner as to generate nanoparticles of the second composition, where the nanoparticles of the second composition are formed on the surface of the nanoparticles of the first composition in such a manner as to form nanoparticles-on-nanoparticles comprising nanoparticles of the second composition on the surface of a core of nanoparticles of said the composition. The nanoparticles of the second composition are smaller in size than the core of nanoparticles.

[0011] In another embodiment of the present invention, a method for producing nanoparticles-on-nanoparticles comprises aerosolizing a mixture of powders of oxide of a micrometer size and metal or metal alloy micrometer sized particles. The method further comprises ablating the aerosolized mixture to generate nanoparticles of oxide and nanoparticles of metal or metal alloy. The nanoparticles of metal or metal alloy are formed on the surface of the nanoparticles of oxide in such a manner as to form nanoparticles-on- nanoparticles comprising a distribution of individual metallic nanoparticles or islands of metallic nanoparticles on the surface of a core of the oxide nanoparticles.

[0012] The foregoing has outlined rather generally the features and technical advantages of one or more embodiments of the present invention in order that the detailed description of the present invention that follows may be better understood. Additional features and advantages of the present invention will be described hereinafter which may form the subject of the claims of the present invention.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0013] A better understanding of the present invention can be obtained when the following detailed description is considered in conjunction with the following drawings, in which:

[0014] Figure 1 is a flowchart of a method for producing nanoparticles-on-nanoparticles in accordance with an embodiment of the present invention;

[0015] Figure 2 illustrates an embodiment of the present invention of a nanocomposite structure formed using the methods of the present invention; and

[0016] Figure 3 is a flowchart of an alternative method for producing nanoparticles-on- nanoparticles in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION

[0017] The present invention comprises a nanocomposite structure and methods for producing said nanocomposite structure that includes a core of larger, bare nanoparticle(s). The nanocomposite structure further includes a distribution of isolated, smaller, bare nanoparticles on the surface of the larger core. By forming such a nanocomposite structure, the surface area as well as the catalytic activity of the nanostructure may be improved.

[0018] While the following discusses the present invention in connection with forming a nanocomposite structure that includes "islands" of nanoparticles (either isolated or grouped together) on the surface of a core of nanoparticles, where these islands of nanoparticles contain the same element (e.g., metal, semi-metal), the principles of the present invention may be applied to forming islands of nanoparticles that contain different elements (e.g., alloys, compounds). Embodiments covering such permutations would fall within the scope of the present invention.

[0019] As stated in the Background Information section, the catalytic properties of metals, such as gold or silver, depend critically upon the particle morphology (i.e., size, shape and thickness). Further, in order to be used as catalysts, the size of the metal needs to be stable at their use temperature. To improve the catalysis of a metal, such as gold, a thin layer of gold nanoparticles may be placed on a flat oxide surface, such as titania (Ti0₂). It is believed that the high catalytic activity of gold nanoparticles supported on titania is due to the interactions between the gold nanoparticles and the titania. Currently though the gold nanoparticles supported on titania are not composed as a single nanocomposite structure. If, however, metal or metal alloy nanoparticles could be placed on the surface of oxide nanoparticles in a single nanocomposite structure, then the surface area as well as the catalytic activity of the metal may be improved. Therefore, there is a need in the art for producing nanoparticles-on- nanoparticles that are thermally stable.

[0020] Nanoparticles-on-nanoparticles are produced using various apparatuses as disclosed in U.S. Patent No. 7,527,824, which is hereby incorporated herein by reference in its entirety. A process for producing thermally stable nanoparticles-on-nanoparticles is discussed below in connection with Figure 1. Figure 2 illustrates the resulting thermally stable nanoparticles- on-nanoparticles that are produced using the process described in Figure 1. Figure 3 is an alternative process for producing thermally stable nanoparticles-on-nanoparticles, as depicted in Figure 2.

[0021] Referring to Figure 1, Figure 1 is a flowchart of a method 100 for producing nanoparticles-on-nanoparticles using the apparatus for producing nanoparticles as disclosed in Figures 3 and 4 of U.S. Patent No. 7,527,824 in accordance with an embodiment of the present invention.

[0022] In step 101, powders of oxide (e.g., silicon dioxide (Si0₂), titania (Ti0₂)) of a micrometer size (e.g., 0.5 micrometers to tens of micrometers) are aerosolized. In one embodiment, the powders of oxide may include metal oxides that are in their reduced state (e.g., SiO_x, TiO_x, where x is less than 2).

[0023] In step 102, the aerosolized powders of oxide are ablated thereby generating nanoparticles of oxide. In one embodiment, the aerosolized powders of oxide are introduced to an ablation chamber in a carrier gas. The carrier gas consists of a coaxial gas surrounding the center flow to ensure a laminar flow of the aerosol through an ablation point. The aerosolized powders of oxide are irradiated by a laser beam having a wavelength in a range between 0.15 and 11 micrometers. The laser fluence is either below or above the breakdown threshold of these particles. If it is below the threshold for breakdown, then evaporation of the oxide occurs as the particles are heated, followed by condensation of the evaporate into nanoparticles. If the fluence is greater than the breakdown threshold, then plasma breakdown occurs that results in the generation of a Shockwave. This Shockwave compresses and heats these particles above the critical point. Condensation of nanoparticles occurs in the low pressure region behind the Shockwave, as expansion and cooling occurs. The expansion of the heated material is determined by the gas atmosphere and pressure, which determines the rate of coalescence of the vapor, and thus the nanoparticle size.

[0024] In step 103, a metal (e.g., gold, silver, cobalt, yttrium, lanthanum, tantalum, palladium, copper, iron, titanium, magnesium, manganese, aluminum or platinum) or a metal alloy (e.g., two or more of the following: gold, silver, cobalt, cerium, gadolinium, yttrium, lanthanum, tantalum, palladium, copper, iron, titanium, magnesium, manganese, aluminum and platinum) of a micrometer size (e.g., 0.5 micrometers to tens of micrometers) is aerosolized. In another embodiment, a metal compound may be used instead of a metal or metal alloy that includes two or more of the following: silicon, titanium, tantalum, carbon and nitrogen. In another embodiment, a metal oxide may be used instead of a metal or metal alloy that includes two or more of the following: lanthanum, manganese, cerium, gadolinium, strontium, magnesium, iron, cobalt, copper, titanium, tantalum and aluminum.

[0025] In step 104, the nanoparticles of oxide are mixed with the aerosolized micrometer- sized particles of metal, metal alloy.

[0026] In step 105, the aerosolized micrometer sized particles of metal, metal alloy, or along with the nanoparticles of oxides are ablated thereby generating nanoparticles of metal, metal alloy. In one embodiment, the mixture is irradiated by a laser beam having a wavelength in a range between 0.15 and 11 micrometers. Because the absorption depth for the laser is larger than the oxide nanoparticles, the oxide nanoparticles are heated uniformly but do not produce a Shockwave. As a result, the oxide nanoparticles are reduced slightly in size by evaporation but otherwise remain intact. In contrast, a Shockwave is generated in the larger metal/metal alloy microparticles that results in their ablation, and then, in step 106, metal/metal alloy nanoparticles nucleate are formed on the surfaces of the oxide nanoparticles in a manner resulting in the formation of a distribution of isolated nanoparticles or of "islands" of metallic nanoparticles on the surface of an oxide core as shown in Figure 2.

[0027] Figure 2 illustrates an embodiment of the present invention of a nanocomposite structure 200 that is formed using method 100 of Figure 1. Referring to Figure 2, in conjunction with Figure 1, nanocomposite structure 200 is formed as the result of the metal/metal alloy nanoparticles forming on the surfaces of the oxide nanoparticles. Nanocomposite structure 200 includes a core 201 of an oxide nanoparticle or nanoparticles (e.g., silicon dioxide (Si0₂), titania (Ti0₂)). Nanocomposite structure 200 further includes smaller metallic nanoparticles 202A-E (e.g., gold, silver, gold-copper alloy, gold-platinum alloy) either isolated individual nanoparticles or "islands" or "groups" of nanoparticles formed on the surface of oxide core 201. Metallic nanoparticles 202A-E may collectively or individually be referred to as metallic nanoparticles 202 or metallic nanoparticle 202, respectively. Both oxide core 201 and metallic nanoparticles 202 are said to be bare (i.e., free of organic surface layers). While Figure 2 illustrates metallic nanoparticles 202 A-E forming on the surfaces of oxide core 201, any number of metallic, metal alloy, metal compound, semiconducting (e.g. carbides such as SiC or TiC), or oxide nanoparticles 202 may be formed on the surface of a metal, metal alloy, metal compound, semiconducting, or oxide core 201. Furthermore, while the description herein discusses forming nanocomposite structure 200 that includes isolated nanoparticles or "islands" of nanoparticles 202 on the surface of a core 201 of nanoparticles, where these nanoparticles 202 contain the same element (e.g., metal, metal alloy), the principles of the present invention may be applied to nanoparticles 202 that contain different elements. For example, a powder of a first material could be ablated and then mixed with nanoparticles of the ablated material along with microparticles of a second and third material. The mixture could then be ablated thereby forming mixtures of nanoparticles 202 some of which would be comprised of the second material and others would be comprised of the third material if the aerosol density in the second ablation were low enough. In another embodiment, the first material could be ablated and then mixed with microparticles of the second material. The mixture may then be ablated a second time before mixing with the microparticles of a third material. The subsequent mixture may then be ablated for the third time.

[0028] By producing such nanoparticles-on-nanoparticles structures 200 with metallic nanoparticles 202 on the surface of oxide core 201, the surface area as well as the catalytic activity of the metal may be improved. Such a nanocomposite structure may be used for chemical production (e.g., oxidation of carbon monoxide to carbon dioxide) or as catalysts (e.g., anode and cathode catalysts) in electrochemical cells, such as fuel cells. Other applications can include but are not limited to nonlinear optics, plasmonic devices, fluorescence markers for biology and medical applications, or energy absorbers for treatment of cancer. All of these applications can benefit from the tailoring of the optical, physical and chemical properties of the outer nanoparticles by the presence of the core particle.

[0029] Returning to Figure 1, in step 107, the nanoparticles-on-nanoparticles are collected into a liquid or onto a solid surface. In one embodiment, the metal/oxide nanoparticles are photo and thermally ionized by the laser during their formation which results in charging that allows them to be collected electrostatically.

[0030] The charging also serves to prevent agglomeration of the bare nanoparticles without the need for an organic layer on the surface of the nanoparticles. Such organic layers can interfere with the catalytic function of the nanoparticles and therefore may need to be removed before the particles are used for catalysis. The removal process may require substantial heating during which the nanoparticles many sinter into larger nanoparticles, which reduces their catalytic activity.

[0031] Method 100 may include additional steps that, for clarity, are not depicted. Further, method 100 may be executed in a different order than presented herein and the order presented in the discussion of Figure 1 is illustrative. Additionally, certain steps in method 100 may be executed in a substantially simultaneous manner or may be omitted.

[0032] An alternative method for producing nanocomposite structure 200 of Figure 2 is discussed below in connection with Figure 3.

[0033] Referring to Figure 3, Figure 3 is a flowchart of a method 300 for producing nanoparticles-on-nanoparticles using the apparatus for producing nanoparticles as disclosed in Figure 5 of U.S. Patent No. 7,527,824 in accordance with an embodiment of the present invention.

[0034] Referring to Figure 3, in conjunction with Figure 2, in step 301, a mixture of powders of oxide (e.g., silicon dioxide (Si0₂), titania (Ti0₂)) of a micrometer size and metal/metal alloy (e.g., gold, silver, gold-copper alloy, gold-platinum alloy) micrometer sized particles are aerosolized.

[0035] In step 302, the aerosolized mixture is ablated to generate nanoparticles of oxide and nanoparticles of metal/metal alloy. In one embodiment, the mixture is irradiated by a laser beam having a wavelength in a range between 0.15 and 11 micrometers. A Shockwave may be generated resulting in the ablation of the microparticles, and then, in step 303, metal/metal alloy nanoparticles are formed on the surfaces of the oxide nanoparticles in a manner resulting in the formation of a distribution of isolated metallic nanoparticles or "islands" of metallic nanoparticles 202 on the surface of an oxide core 201 as shown in Figure 2.

[0036] Returning to Figure 3, in step 304, the nanoparticles-on-nanoparticles are collected into a liquid or onto a solid surface. In one embodiment, the metal/oxide nanoparticles are photo and thermally ionized by the laser during their formation which results in charging that allows them to be collected electrostatically. [0037] Method 300 may include other and/or additional steps that, for clarity, are not depicted. Further, method 300 may be executed in a different order presented and the order presented in the discussion of Figure 3 is illustrative. Additionally, certain steps in method 300 may be executed in a substantially simultaneous manner or may be omitted.

[0038] Although the method and nanocomposite structure are described in connection with several embodiments, it is not intended to be limited to the specific forms set forth herein, but on the contrary, it is intended to cover such alternatives, modifications and equivalents, as can be reasonably included within the spirit and scope of the invention as defined by the appended claims.

Claims

CLAIMS:

1. A nanocomposite structure comprising:

a core comprising one of a nanoparticle and a group of nanoparticles; and

one of a distribution of isolated nanoparticles and islands of nanoparticles smaller in size than said core, wherein one of said isolated nanoparticles and said islands of nanoparticles are attached to the surface of said core, wherein said islands of nanoparticles are free of organic ligands.

2. The nanocomposite as recited in claim 1, wherein said islands of nanoparticles comprise one of gold, silver, cobalt, yttrium, lanthanum, tantalum, palladium, copper, iron, titanium, magnesium, manganese, aluminum and platinum.

3. The nanocomposite as recited in claim 1, wherein said core of nanoparticles comprises one of silica and titania.

4. The nanocomposite as recited in claim 1, wherein said core of nanoparticles comprises metal oxides, wherein said metal oxides are in their reduced state.

5. The nanocomposite as recited in claim 1, wherein nanoparticles of one of said isolated nanoparticles and said islands of nanoparticles comprise a metal alloy comprising two or more of the following: gold, silver, cobalt, cerium, gadolinium, yttrium, lanthanum, tantalum, palladium, copper, iron, titanium, magnesium, manganese, aluminum and platinum.

6. The nanocomposite as recited in claim 1, wherein nanoparticles of one of said isolated nanoparticles and said islands of nanoparticles comprise a metal compound comprising two or more of the following: silicon, titanium, tantalum, carbon and nitrogen.

7. The nanocomposite as recited in claim 1, wherein nanoparticles of one of said isolated nanoparticles and said islands of nanoparticles comprise a metal oxide comprising two or more of the following: lanthanum, manganese, cerium, gadolinium, strontium, magnesium, iron, cobalt, copper, titanium, tantalum and aluminum.

8. The nanocomposite as recited in claim 1, wherein said core comprises a first element, wherein at least one nanoparticle of one of said isolated nanoparticles and said islands of nanoparticles comprises a second element, wherein at least one nanoparticle of one of said isolated nanoparticles and said islands of nanoparticles comprises a third element.

9. A method for producing nanoparticles-on-nanoparticles, the method comprising: aerosolizing powders of a first composition of a micrometer size;

ablating said aerosolized powders thereby generating nanoparticles of said first composition;

aerosolizing powders of micrometer sized particles of a second composition;

mixing said nanoparticles of said first composition with said aerosolized powders of said second composition; and

ablating said mixture in such a manner as to generate nanoparticles of said second composition, wherein said nanoparticles of said second composition are formed on the surface of said nanoparticles of said first composition in such a manner as to form nanoparticles-on-nanoparticles comprising nanoparticles of said second composition on the surface of a core of nanoparticles of said first composition, wherein said nanoparticles of said second composition are smaller in size than said core of nanoparticles.

10. The method as recited in claim 9 further comprising:

collecting said nanoparticles-on-nanoparticles into a liquid.

11. The method as recited in claim 9 further comprising:

collecting said nanoparticles-on-nanoparticles onto a surface.

12. The method as recited in claim 9, wherein said aerosolized powders of said first composition are ablated using a laser beam having a wavelength in a range between 0.15 and 11 micrometers.

13. The method as recited in claim 9, wherein said micrometer-sized particles of said second composition are ablated using a laser beam having a wavelength in a range between 0.15 and 11 micrometers, wherein an absorption depth of said laser beam is larger than a size of said nanoparticles of said first composition.

14. The method as recited in claim 9, wherein said core of nanoparticles comprises a first element, wherein at least one of said nanoparticles of said second composition comprises a second element, wherein at least one of said nanoparticles of said second composition comprises a third element.

15. A method for producing nanoparticles-on-nanoparticles, the method comprising: aerosolizing a mixture of powders of oxide of a micrometer size and one of metal and metal alloy micrometer sized particles; and

ablating said aerosolized mixture to generate nanoparticles of oxide and nanoparticles of one of metal and metal alloy;

wherein nanoparticles of said one of metal and metal alloy are formed on the surface of said nanoparticles of oxide in such a manner as to form nanoparticles-on-nanoparticles comprising one of a distribution of individual metallic nanoparticles and islands of metallic nanoparticles on the surface of a core of said oxide nanoparticles.

16. The method as recited in claim 15 further comprising:

collecting said nanoparticles-on-nanoparticles into a liquid.

17. The method as recited in claim 15 further comprising:

collecting said nanoparticles-on-nanoparticles onto a surface.