Noble metal-oxide combined nanoparticle, and, method of producing the same with high purity

Abstract

A method of producing a composite nanoparticle (M-A.sub.xO.sub.y), having: generating, in an inert gas, an alloy (A-M) nanoparticle, which contains 0.1 at. % to 30 at. % of a noble metal (M), with the balance being a base metal (A) and inevitable impurities, and which has a particle size of 1 nm to 100 nm, to heat the alloy (A-M) nanoparticle and to bring the alloy (A-M) nanoparticle into contact with a supplied oxidizing gas during transportation of the alloy (A-M) nanoparticle with the inert gas, to oxidize the base metal component (A) in the floating alloy (A-M) nanoparticle, and to phase separate into the thus-oxidized base metal component (A.sub.xO.sub.y) and the noble metal component (M), to thereby obtain a composite nanoparticle (M-A.sub.xO.sub.y) having one noble metal particle (M) combined to the surface of a particulate base metal oxide (A.sub.xO.sub.y).

Claims

1. A method of producing a composite nanoparticle (M-A.sub.xO.sub.y), comprising: generating, in an inert gas, an alloy (A-M) nanoparticle, which contains 0.1 at. % to 30 at. % of a noble metal (M), with the balance being a base metal (A) and inevitable impurities, and which has a particle size of 1 nm to 100 nm; heating the alloy (A-M) nanoparticle and bringing the alloy (A-M) nanoparticle into contact with a supplied oxidizing gas during transportation of the alloy (A-M) nanoparticle with the inert gas; oxidizing the base metal component (A) in the floating alloy (A-M) nanoparticle, and phase-separating into the thus-oxidized base metal component (A.sub.xO.sub.y) and the noble metal component (M), to thereby obtain a composite nanoparticle (M-A.sub.xO.sub.y) having one noble metal particle (M) combined to the surface of one base metal oxide (A.sub.xO.sub.y) particle, wherein the one noble metal particle and the one base metal oxide particle are present at a 1:1 particle number ratio, and wherein a thermal oxidation of the alloy (A-M) nanoparticle is carried out at a temperature of 400 C. or higher.

2. The method of producing a composite nanoparticle (M-A.sub.xO.sub.y) according to claim 1, wherein the composite nanoparticle (M-A.sub.xO.sub.y) is obtained in an independently dispersed state.

3. The method of producing a composite nanoparticle (M-A.sub.xO.sub.y) according to claim 1, wherein the thermal oxidation treatment of the alloy (A-M) nanoparticle is carried out in a gas phase, at a temperature of 400 C. or higher, for a treatment time period of 10 seconds or less.

4. The method of producing a composite nanoparticle (M-A.sub.xO.sub.y) according to claim 1, wherein the oxidizing gas is supplied before the heating of the alloy (A-M) nanoparticle, thereby heating it with a mixed gas of the oxidizing gas and the inert gas, or the oxidizing gas is supplied during the heating of the alloy (A-M) nanoparticle in the inert gas.

5. The method of producing a composite nanoparticle (M-A.sub.xO.sub.y) according to claim 1, wherein the generating of the alloy (A-M) nanoparticle is conducted by any one of an inert-gas evaporation method, a laser ablation method, a sputtering method, an arc plasma method, and an atmospheric pressure plasma method.

6. The method of producing a composite nanoparticle (M-A.sub.xO.sub.y) according to claim 1, wherein the base metal (A) of the base metal oxide (A.sub.xO.sub.y) component is one or more selected from Cu, Sn, Ti, V, Cr, Mn, Co, Fe, Ni, Zn, Al, Y, Zr, Mo, In, Mg, La, Ce, Nd, Sm, Eu, Gd, Si, Ge, Pb and Bi; and wherein the noble metal (M) is one or more selected from Au, Pt, Pd, Rh, Ag, Ru and Ir.

7. A method of producing a composite nanoparticle (M-A.sub.xO.sub.y), comprising: generating, in an inert gas, an alloy (A-M) nanoparticle, which contains 0.1 at. % to 30 at. % of a noble metal (M), with the balance being a base metal (A) and inevitable impurities, and which has a particle size of 1 nm to 200 nm; heating the alloy (A-M) nanoparticle and bringing the alloy (A-M) nanoparticle into contact with a supplied oxidizing gas during transportation of the alloy (A-M) nanoparticle with the inert gas; oxidizing the base metal component (A) in the floating alloy (A-M) nanoparticle, and phase-separating into the thus-oxidized base metal component (A.sub.xO.sub.y) and the noble metal component (M), to thereby obtain a composite nanoparticle (M-A.sub.xO.sub.y) composed of a region of one base metal oxide (A.sub.xO.sub.y) particle and a region of one noble metal (M) particle, wherein the one noble metal particle and the one base metal oxide particle are present at a 1:1 particle number ratio, wherein a thermal oxidation of the alloy (A-M) nanoparticle is carried out at a temperature of 400 C. or higher, wherein the base metal (A) is one or more selected from Cu, Sn, Al, Ni, Co, Ti, Zr, In, Si, La, Ce and Eu, and wherein the noble metal (M) is one or more selected from Au, Pt, Pd, Rh and Ag.

8. A composite nanoparticle (M-A.sub.xO.sub.y), having one noble metal particle (M) with a particle size of 1 nm to 10 nm combined to the surface of one base metal oxide (A.sub.xO.sub.y) particle with a particle size of 1 nm to 100 nm, wherein the one noble metal particle and the one base metal oxide particle are present at a 1:1 particle number ratio, wherein the base metal (A) of the base metal oxide (A.sub.xO.sub.y) component is one or more selected from the group consisting of Cu, Sn, Ti, V, Cr, Mn, Co, Ni, Zn, Al, Y, Zr, Mo, In, Mg, La, Ce, Nd, Sm, Eu, Gd, Si, Ge, Pb and Bi, provided that the case where the base metal oxide is ZnO or MnO is excluded, and wherein the noble metal (M) is one or more selected from the group consisting of Au, Pt, Pd, Rh, Ag, Ru and Ir.

9. The composite nanoparticle (M-A.sub.xO.sub.y) according to claim 8, wherein the composite nanoparticle (M-A.sub.xO.sub.y) is AuCu.sub.2O or AuSnO.sub.2.

10. A composite nanoparticle (M-A.sub.xO.sub.y), having a region of one base metal oxide (A.sub.xO.sub.y) particle, with a size of 1 nm to 200 nm and a region of one noble metal (M) particle with a size of 1 nm to 100 nm, wherein the one noble metal particle and the one base metal oxide particle are present at a 1:1 particle number ratio, wherein the base metal (A) of the base metal oxide (A.sub.xO.sub.y) component is one or more selected from the group consisting of Cu, Sn, Al, Ni, Co, Ti, Zr, In, Si, La, Ce and Eu, and wherein the noble metal (M) is one or more selected from the group consisting of Au, Pt, Pd, Rh and Ag.

11. The composite nanoparticle (M-A.sub.xO.sub.y) according to claim 10, wherein the base metal (A) of the base metal oxide (A.sub.xO.sub.y) component is one or more selected from the group consisting of Cu, Sn and Al, and wherein the noble metal (M) is one or more selected from the group consisting of Au and Pt.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a diagram illustrating a production process for the noble metal-oxide combined nanoparticles of the present invention (FIG. 1 is illustrated such that an oxidizing gas is supplied between G and H, but instead, the oxidizing gas may also be supplied in any arbitrary internal position in H).

(2) FIG. 2 is a TEM micrograph of a sample in which AuCu.sub.2O composite nanoparticles are sparsely deposited onto an amorphous carbon film.

(3) FIG. 3(a) is an electron diffraction pattern of the sample in which the AuCu.sub.2O composite nanoparticles are sparsely deposited onto the amorphous carbon film; and FIG. 3(b) is a diagram illustrating the electron diffraction pattern of FIG. 3(a) converted into an intensity distribution as a function of the wave vector s.

(4) FIG. 4(a) and FIG. 4(b) are high resolution TEM micrographs of 17-nm AuCu.sub.2O composite nanoparticles observed along <100> and <110> orientations, respectively; and FIG. 4(c) and FIG. 4(d) are high resolution TEM micrographs of 14-nm and 7-nm AuCu.sub.2O composite nanoparticles, respectively.

(5) FIG. 5 is a diagram illustrating a catalytic activity (a temperature dependence of a conversion of CO to CO.sub.2) of a sample containing AuCu.sub.2O composite nanoparticles.

(6) FIG. 6 is a TEM micrograph of AuSnO.sub.2 composite nanoparticles.

(7) FIG. 7 is a diagram illustrating a catalytic activity (a temperature dependence of a conversion of CO to CO.sub.2) of a sample containing AuSnO.sub.2 composite nanoparticles.

(8) FIG. 8 is a TEM micrograph of AuAl.sub.2O.sub.3 composite nanoparticles.

(9) FIG. 9 is a TEM micrograph of PtCu.sub.2O composite nanoparticles.

(10) FIG. 10 is a TEM micrograph of PtAl.sub.2O.sub.3 composite nanoparticles.

MODE FOR CARRYING OUT THE INVENTION

(11) Hereafter, some modes for carrying out the present invention are described.

(12) An example of the method of producing composite nanoparticles of the present invention is illustrated in FIG. 1. The method of producing composite nanoparticles of the present invention includes a step, for example, of: generating, in an inert gas, alloy (A-M) nanoparticles, which contain 0.1 at. % to 30 at. % of a noble metal (M), with the balance being a base metal (A) and inevitable impurities, and which have a particle size of 1 nm to 200 nm (preferably, 1 nm to 100 nm), to heat the alloy (A-M) nanoparticles and to bring the alloy nanoparticles into contact with a supplied oxidizing gas during transportation of the alloy nanoparticles with the inert gas, to thereby oxidize the base metal component (A) in the floating alloy (A-M) nanoparticles, and to phase separate the alloy nanoparticles into the oxidized base metal component (A.sub.xO.sub.y) and the noble metal component (M).

(13) In this production method, it is not necessary to use noble metals and base metals in the form of soluble compounds [for example, HAuCl.sub.4, H.sub.2PtCl.sub.6, and Fe(CO).sub.5], and use can be made of any of noble metals and base metals as long as they are capable of generating alloy (A-M) nanoparticles in an inert gas. Thus, a wide variety of noble metals and base metals other than Au, Pt, Cu, Sn, and Al used in the Examples described below can be employed, since the present invention is free from limitations to the types of noble metal and base metal elements which are, for example, caused by the restrictions on compounds in the case of liquid phase synthesis.

(14) The production apparatus for the composite nanoparticles, which is used to carry out this production method, is constituted of connection in series of: an apparatus for generating noble metal-base metal alloy nanoparticles as a raw material (G; hereinafter, also referred to as alloy nanoparticle generation apparatus), a high temperature thermal oxidation reactor (H), a collector (C), and an exhaust pump (P). The exhaust pump (P) is employed; to exhaust the inert gas supplied into the alloy nanoparticle generation apparatus (G), and the oxidizing gas supplied into an intermediate position between the alloy nanoparticle generation apparatus (G) and the high temperature thermal oxidation reactor (H) or into a high temperature zone in the high temperature thermal oxidation reactor; and to control the pressure conditions of the alloy nanoparticle generation apparatus (G).

(15) There are no limitations on the base metal A that constitutes the raw material, and the base metal may be one or more selected from, for example, Cu, Sn, Ti, V, Cr, Mn, Co, Fe, Ni, Zn, Al, Y, Zr, Mo, In, Mg, La, Ce, Nd, Sm, Eu, Gd, Si, Ge, Pb, and Bi. Preferably, the base metal A may be one or more selected from Cu, Sn, Al, Ni, Co, Ti, Zr, In, Si, La, Ce, and Eu.

(16) There are no limitations on the noble metal M that constitutes the raw material, and the noble metal may be one or more selected from, for example, Au, Pt, Pd, Rh, Ag, Ru, and Ir. Preferably, the noble metal M may be one or more selected from Au, Pt, Pd, Rh, and Ag.

(17) Based on the reason described below, the base metal A-noble metal M alloy of the raw material is preferably such that the content of the noble metal M in the A-M alloy nanoparticles is about 0.1 at. % to 30 at. % (more preferably, 1 at. % to 15 at. %, and even more preferably, 2 at. % to 10 at. %), and the content of the noble metal M is preferably determined, in consideration of the vapor pressures of the base metal A and the noble metal M, and the like. In the inert gas evaporation method, the noble metal content of the base metal A-noble metal M alloy of the raw material may vary depending on the type of the base metallic element, but, for example, the noble metal content can be set to a range of 1 at. % to 60 at. %. When the base metal element is Cu or Sn, it is appropriate to set the noble metal content to 30 at. % to 60 at. % (preferably, 35 at. % to 55 at. %, and more preferably 40 at. % to 50 at. %). In the laser ablation method, since composition of the raw material alloy and composition of the alloy nanoparticles produced becomes almost identical, the noble metal content in the base metal A-noble metal M alloy of the raw material may be set to about 0.1 at. % to 30 at. % (more preferably 1 at. % to 15 at. %, and even more preferably 2 at. % to 10 at. %).

(18) For the gas phase generation of the A-M alloy nanoparticles of the base metal A and the noble metal M that served as raw materials, use can be made of the generation apparatus operable under a pressure range from a low pressure (for example, about 0.1 kPa to 10 kPa) up to about the atmospheric pressure (101.3 kPa). For example, use can be made of an inert gas evaporation method, a laser ablation method, a sputtering method, an arc plasma method, and an atmospheric pressure plasma method, and in addition to these, various methods for generating nanoparticles in a gas phase can be appropriately used. An inert gas, such as helium, argon and nitrogen, is used, to generate noble metal-base metal alloy nanoparticles having a size of about 1 nm to 200 nm (preferably 1 nm to 100 nm, more preferably 2 nm to 80 nm, and even more preferably 5 nm to 60 nm). The particle size of the alloy nanoparticles generally increases as the temperature of the evaporation source (i.e. the energy input to the evaporation source) is higher, or as the pressure of the inert gas is higher and the flow rate is slower. Thus, the particle size can be appropriately controlled while the experiment results are checked.

(19) In the case that secondary nanoparticles are formed by aggregation of primary nanoparticles on the production of noble metal-base metal alloy nanoparticles, the secondary nanoparticles are heated and sintered into isolatedly dispersed particles in an inert atmosphere by means of, for example, a preheating mechanism or the like, before being transferred into the high temperature thermal oxidation reactor (H) for the thermal oxidation treatment.

(20) The thus-formed alloy nanoparticles are mixed with the oxidizing gas during transporting with the inert gas stream, where the oxidizing gas is provided at a position upstream to or at a position inside the high temperature thermal oxidation reactor. As the oxidizing gas, use can be made, for example, of oxygen gas alone, or a mixture of air or oxygen gas and an inert gas. In the case of supplying the oxidizing gas at a position upstream to the high temperature thermal oxidation reactor, the alloy nanoparticles are caused to flow into the high temperature thermal oxidation reactor, being partially oxidized together with a mixed gas of the inert gas and the oxidizing gas. Thus, only the base metal element constituting the alloy nanoparticles is completely oxidized by the oxygen gas in the mixed gas heated to a high temperature. On the other hand, in the case of supplying the oxidizing gas at a position inside the high temperature thermal oxidation reactor, the alloy nanoparticles are caused to flow into the high temperature thermal oxidation reactor together with the inert gas, to be exposed to oxygen at a high temperature, where the nanoparticles just before oxidation is in a state of being heated to a high temperature (a high temperature solid or a molten state). Thus, only the base metal element is rapidly oxidized completely. The velocity of oxidation and the extent of evaporation of the base metal and the noble metal, which is caused by heat of oxidation, depend on the supplying position of the oxidizing gas. However, in any cases, the noble metal and the base metal oxide undergo nano-scale phase separation during the heating, to generate composite nanoparticles (M-A.sub.xO.sub.y) each having one noble metal particle (M) combined to the surface of the base metal oxide particle (A.sub.xO.sub.y). On the phase separation, a uniform heterointerface is occasionally created in individual particles, by a simultaneous heat treatment at high temperatures.

(21) The alloy nanoparticles in the high temperature thermal oxidation reactor can be indirectly heated with the mixed gas heated to a high temperature, using, for example, a tubular electric furnace equipped with a quartz tube as a furnace core tube, or alternatively the nanoparticles can be directly heated, using induction heating, microwave heating, or the like.

(22) The high temperature thermal oxidation treatment may vary depending on types of the noble metal and the base metal, but in general, the high temperature thermal oxidation treatment can be carried out at a temperature of 400 C. or higher (preferably 500 C. to 1,200 C., and more preferably 600 C. to 1,100 C.) for a treatment time period of 10 seconds or less. When the treatment temperature is raised, the treatment time period can be shortened. In the high temperature thermal oxidation reactor, the heating zone is configurated along the transportation path of the alloy nanoparticles as the alloy nanoparticles can be heated during such a treatment time period.

(23) The noble metal-base metal oxide composite nanoparticles flowed out from the high temperature thermal oxidation reactor are cooled down to around ambient temperature by natural cooling or by use of an appropriate cooler, followed by collecting them inside the collector. Regarding the collecting method, use can be appropriately made of any of dry methods and wet methods that are used in aerosol collection techniques.

(24) In the process described above, in order to separate the noble metal M phase completely by thermal oxidation of the A-M alloy nanoparticles in a gas phase, the composition of the noble metal M is about 0.1 at. % to 30 at. % (more preferably, 1 at. % to 15 at. %, and even more preferably 2 at. % to 10 at. %), and the oxygen concentration in the mixed gas is preferably 10% or more.

(25) Furthermore, in the applications, such as electronic devices and catalysts, high-purity production of the composite nanoparticles is very important, therefore it is desirable that the A-M alloy nanoparticles have impurities as small contents as possible. However, the A-M alloy nanoparticles may contain the impurities that originate from impurities in the noble metal or base metal of the raw material or the like. It is desirable to select the raw material such that the content of the impurities is in the range to the extent that the intended functions are not significantly impaired (for example, less than 0.01 at. %, preferably less than 0.001 at. %).

(26) The noble metal-base metal oxide composite nanoparticles (M-A.sub.xO.sub.y) of the present invention are continuously and very cleanly produced via the process step as described above, and made of one noble metal particle (M) of size 1 nm to 100 nm (preferably 1 nm to 10 nm) combined to the surface of the base metal oxide particle (A.sub.xO.sub.y) of size 1 nm to 200 nm (preferably 1 nm to 100 nm), in which the base metal (A) of the base metal oxide (A.sub.xO.sub.y) component is one or more selected from Cu, Sn, Ti, V, Cr, Mn, Co, Ni, Zn, Al, Y, Zr, Mo, In, Mg, La, Ce, Nd, Sm, Eu, Gd, Si, Ge, Pb and Bi (provided that the case where the base metal oxide is ZnO or MnO is excluded), and in which the noble metal (M) is one or more selected from Au, Pt, Pd, Rh, Ag, Ru and Ir. The composite nanoparticles can be defined also to be composed of one region of the base metal oxide (A.sub.xO.sub.y) having a size of 1 nm to 200 nm, and one region of the noble metal (M) having a size of 1 nm to 100 nm.

(27) The particle size of the base metal oxide is 1 nm to 200 nm, preferably 1 nm to 100 nm, more preferably 2 nm to 50 nm, and even more preferably 5 nm to 30 nm. The particle size of the noble metal particles is 1 nm to 100 nm, preferably 1 nm to 10 nm, more preferably 1 nm to 8 nm, and even more preferably 1 nm to 5 nm. The particle size of the base metal oxide depends not only on that of the noble metal-base metal alloy nanoparticles before oxidation, but also on evaporation of the base metal caused by the heat of oxidation in the high temperature thermal oxidation treatment. Thus, in use of a base metal having high evaporability, the particle size of the base metal oxide can be controlled by adjusting the supplying position of the oxidizing gas as mentioned above.

(28) In the present invention, the particle size is defined as the average value of largest and smallest diameters measured through the gravitational center of the particle image obtained by TEM (transmission electron microscope). The size of the region of the base metal oxide or noble metal is defined as the average value of largest and smallest diameters measured through the gravitational center of the TEM image of the base metal oxide region or the noble metal region, respectively, where each region can be regarded as particle.

(29) The base metal oxide (A.sub.xO.sub.y) in the composite nanoparticles (M-A.sub.xO.sub.y) produced by the present invention may be an oxide of a single base metal element having the same valency [in that case, x and y in the formula of A.sub.xO.sub.y each represent a positive integer, and x and y satisfy the formula: xn=2y (wherein n represents the valency of the base metal atom A)]. Alternatively, the base metal oxide may be an oxide of a single base metal element having a different valency (mixed valence oxide), or a composite oxide of plural kinds of base metal elements [in that case, the following relationships are satisfied: A.sub.x=A.sub.1x.sub.1 . . . A.sub.ix.sub.i; x=x.sub.i; x.sub.in.sub.i=2y (wherein A.sub.i represents the elemental component of the same base metal with different valences, or different base metals; x.sub.i represents the mole number of A.sub.i; and n.sub.i represents the valency of A.sub.i)]. Furthermore, alternatively, the base metal oxide may also be a mixture of plural kinds of base metal oxides.

(30) The noble metal (M) in the composite nanoparticles (M-A.sub.xO.sub.y) that are produced by the present invention may be a noble metal of one kind, or may be a mixture of plural kinds of noble metals. The purity of the noble metal (M) is brought to the extent of a noble metal raw material (generally about 99.99% to 99.999%) or higher (for example, 99.9999% or higher), through the processes of alloy nanoparticle generation and phase separation from the base metal oxide component (A.sub.xO.sub.y).

EXAMPLES

(31) Hereafter, the present invention will be more specifically described by way of Examples, but the present invention is not intended to be limited by these Examples, and various adjustments in the setting or modifications in design can be made to the extent that the gist of the present invention is maintained.

Example 1

Formation of AuCu2O Composite Nanoparticles, and Measurement of CO Oxidation Catalytic Activity

(32) As a source for generating alloy nanoparticles, use was made of a generation apparatus according to an inert gas evaporation method. Helium was supplied at an inlet rate of 0.4 L/min into the apparatus, and the pressure inside the apparatus was maintained at a reduced pressure of 2 kPa using an oil rotary pump. A Cu-46 at. % Au ingot was placed in a crucible of pBN (pyrolytic boron nitride) coupled with a carbon crucible, followed by heating to 1,200 C. by high-frequency heating. The above experimental conditions enabled to generate Cu-4 at. % Au alloy nanoparticles, in helium gas, in which the Au content of the nanoparticles was decreased from that of the raw alloy ingot due to the higher vapor pressure of Cu than Au. The alloy nanoparticles thus generated were transported into a quartz tube heated to 1,100 C., together with helium gas and oxygen gas which was mixed at an inlet rate of 0.2 L/min, followed by subjecting to a high temperature oxidation treatment. The time period for the thermal oxidation treatment was about 0.1 seconds. The AuCu.sub.2O composite nanoparticles formed after the oxidation treatment were deposited sparsely on an amorphous carbon film settled inside the collector, to obtain a sample for electron microscopy observations. Another sample for catalytic activity measurements was obtained by collecting together with a silica nanopowder (Sigma-Aldrich, 637238) as buffer particles.

(33) FIG. 2 shows a TEM micrograph of the AuCu.sub.2O composite nanoparticles having an average particle size of 9.84.6 nm. It can be seen that a very small region with dark contrast is present partially in each of the particles. FIG. 3(a) is an electron diffraction pattern image of the same sample, and FIG. 3(b) is an intensity distribution obtained by converting the electron diffraction pattern as a function of the wave vector s. All of the clear peaks shown in FIG. 3(b) were due to diffraction by Cu.sub.2O crystal planes. Among these, very weak peaks originating from Au (shown with arrows) were confirmed. By a detailed analysis, the lattice constants of Cu.sub.2O and Au were determined to be 0.42710.0004 nm and 0.40700.0034 nm, respectively, and it was verified, within an error range, that the substances were Cu.sub.2O (literature value: 0.42696 nm) and Au (literature value: 0.4079 nm). It was thus confirmed that the very small region with high contrast in each of the particles in FIG. 2 was Au, while the other large region was Cu.sub.2O. Furthermore, according to the results of an ICP (inductively coupled plasma) analysis, the molar composition ratio of Au:Cu of the composite nanoparticles was 5:95, which was almost the same as that ratio of the alloy nanoparticles before subjected to the oxidation.

(34) FIG. 4(a) and FIG. 4(b) are high resolution TEM micrographs of 17-nm AuCu.sub.2O composite nanoparticles. Clear lattice images are observed in the Cu.sub.2O regions, and their crystallographic orientations were determined from lattice spacings and relative angle between lattice fringes to be <100> for FIG. 4(a) and <110> for FIG. 4(b). Clear lattice images are also observed in the Au region, and their crystallographic orientations were completely coherent with those of the Cu.sub.2O regions. Since the lattice fringes between the two phases are completely connected, the two phases are heteroepitaxially joined with each other. FIG. 4(c) and FIG. 4(d) are high resolution TEM micrographs of 14-nm and 7-nm AuCu.sub.2O composite nanoparticles, respectively. It can also be seen that the two phases in these particles are very well heterojoined. In the every cases, a half of the Au region is embedded in the Cu.sub.2O region. By changing the heat treatment temperature under the above experimental conditions, the phase separated structure was obtained at a temperature of 400 C. or higher, while such the structure could not be obtained at a temperature of 300 C. or lower.

(35) A sample for measuring a catalytic activity was prepared by mixing the AuCu.sub.2O composite nanoparticles with a silica powder. The total amount of the silica powder and the AuCu.sub.2O composite nanoparticles used for the measurement was 81 mg, and the content of the AuCu.sub.2O composite nanoparticles was 0.65 wt. %. The catalytic activity for CO oxidation was measured, using a fixed bed flow reactor. A mixed gas of CO (1%)+O.sub.2 (20%)+He was flowed at a flow rate of 0.1 L/min. The temperature was increased at a rate of 1 C./min. FIG. 5 shows the temperature dependence of the CO conversion to CO.sub.2. The CO conversion rapidly increased above about 200 C., and the temperature for 50% conversion (T.sub.50%) was 215 C. The reaction rate per unit weight of the AuCu.sub.2O composite nanoparticles (excluding the silica powder) was calculated, and the result in an Arrhenius plot was compared with the activity of catalysts previously reported. It was revealed that the catalytic activity of the composite nanoparticles is comparable to that of, for example, alumina-supported gold catalysts prepared by a liquid phase method (deposition-precipitation method). This catalytic activity comes from condition of the strong bonding between Au and Cu.sub.2O forming a heterointerface, as shown in FIG. 4, which was generated by subjecting individual particles to a high temperature heat treatment in the process of the present invention.

Example 2

Formation of AuSnO2 Composite Nanoparticles, and Measurement of CO Oxidation Catalytic Activity

(36) Sn-5 at. % Au alloy nanoparticles (raw material) were obtained in helium gas by heating a Sn-50 at. % Au ingot at 1,180 C., by the inert gas evaporation method in the same manner as in Example 1. The alloy nanoparticles were subjected to a high temperature thermal oxidation at 700 C. for about 0.1 seconds in gas phase, to obtain AuSnO.sub.2 composite nanoparticles. Other experimental conditions were almost the same as in Example 1. FIG. 6 shows a TEM micrograph of the AuSnO.sub.2 composite nanoparticles. Analyzing the electron diffraction pattern, it was confirmed that the regions of lighter contrast were SnO.sub.2 phase. Since there were so many peak positions of SnO.sub.2 and Au in the diffraction pattern are overlapped with each other, peaks originating only from Au were impossible to separate. However, the regions of dark contrast are considered to be Au without Sn. The micrograph shows composite nanoparticles, in which one nanoparticle of Au and one nanoparticle of SnO.sub.2 were combined by one interface.

(37) A sample for the catalytic activity measurement was obtained by collecting the composite nanoparticles together with a silica powder in the same manner as in Example 1. The total amount of the silica powder and the AuSnO.sub.2 composite nanoparticles (the content: 0.64 wt. %) used in the measurement was 50 mg. The temperature dependence of the conversion of CO to CO.sub.2 was measured, in the same manner as in Example 1. FIG. 7 shows that the AuSnO.sub.2 composite nanoparticles also exhibited a catalytic activity for oxidation of CO. The conversion rapidly increased above 300 C., and T.sub.50% was 345 C.

Example 3

Formation of AuAl2O3 Composite Nanoparticles

(38) As a source for generating alloy nanoparticles, use was made of a generation apparatus according to a laser ablation method. Helium gas was supplied at an inlet rate of 0.5 L/min into the apparatus, and the pressure inside the apparatus was maintained at a reduced pressure of 1.6 kPa using an oil rotary pump. A pellet (20 mm5 mm t) of Al-5 at. % Au alloy was used as a raw material target, and the second harmonic of Nd:YAG laser (wavelength: 532 nm, output power: 90 mJ/pulse, repetition frequency: 10 Hz) was concentrated and irradiated on the surface of the pellet, to instantaneously evaporate the target surface. Thus, aggregates of nanoparticles of the alloy were generated in helium gas. The thus-generated alloy nanoparticle aggregates moved with a helium gas stream through a preheating mechanism, followed by sintering into isolatedly dispersed particles. The particles were then transported into a quartz tube heated to 900 C., together with oxygen gas at an inlet rate of 0.25 L/min, followed by subjecting to a high temperature oxidation treatment. The time period for the thermal oxidation treatment was about 0.01 seconds. The AuAl.sub.2O.sub.3 composite nanoparticles formed by the oxidation treatment were naturally deposited sparsely on an amorphous carbon film settled inside the collector, to give a sample for electron microscopy observations.

(39) FIG. 8 shows a TEM micrograph of the AuAl.sub.2O.sub.3 composite nanoparticles. As shown in the micrograph, individual particles exhibit a morphology in which a small dark contrast region is joined with a large light contrast region. Analyzing the electron diffraction pattern, the Al oxide constituted the particles was -Al.sub.2O.sub.3 phase. Thus, the region of dark contrast in the particle is Au, while the region of light contrast is -Al.sub.2O.sub.3.

Example 4

Formation of PtCu2O Composite Nanoparticles

(40) The experiment was carried out with a pellet of Cu-5 at. % Pt alloy as a raw material target, using a laser ablation method, in the same manner as in Example 3. A laser light was concentrated and irradiated to the target, to instantaneously evaporate the target surface. Thus, aggregates of nanoparticles of the alloy were generated in helium gas. The aggregates were sintered through a preheating mechanism, followed by transporting into a quartz tube heated to 900 C., together with oxygen gas, and subjecting to a high temperature oxidation treatment, to give PtCu.sub.2O composite nanoparticles. The experimental conditions were the same as in Example 3. FIG. 9 shows a TEM micrograph of the PtCu.sub.2O composite nanoparticles. As shown in the micrograph, individual particles exhibit a morphology in which a small dark contrast region is joined with a large light contrast region. Analyzing the electron diffraction pattern, the Cu oxide constituted the particles was Cu.sub.2O phase. Thus, the region of dark contrast in the particle is Pt, while the region of light contrast is Cu.sub.2O.

Example 5

Formation of PtAl2O3 Composite Nanoparticles

(41) The experiment was carried out with a pellet of Al-5 at. % Pt alloy as a raw material target, using a laser ablation method, in the same manner as in Example 3. A laser light was concentrated and irradiated to the target, to instantaneously evaporate the target surface. Thus, aggregates of nanoparticles of the alloy were generated in helium gas. The aggregates were sintered by a preheating mechanism, followed by transporting into a quartz tube heated to 900 C., together with oxygen gas, and subjecting to a high temperature oxidation treatment, to give PtAl.sub.2O.sub.3 composite nanoparticles. The experimental conditions were the same as in Example 3. FIG. 10 shows a TEM micrograph of the PtAl.sub.2O.sub.3 composite nanoparticles. As shown in the micrograph, individual particles exhibit a morphology in which a small dark contrast region is joined with a large light contrast region. Analyzing the electron diffraction pattern, the Al oxide constituted the particles was -Al.sub.2O.sub.3 phase. Thus, the region of dark contrast in the particle is Pt, while the region of light contrast is -Al.sub.2O.sub.3.

INDUSTRIAL APPLICABILITY

(42) The present invention relates to a method of producing composite nanoparticles of a noble metal and an oxide, which is a method of forming heterojunction between two nanometer-scale phases. Furthermore, the present invention relates to composite nanoparticles of a high-purity noble metal and an oxide, in which the heterojunction is formed between two nanometer-scale phases. Thus, the nanometer-scale particles are to be given physical and chemical properties, such as chemical reactivity, electronic, magnetic and optical properties, which originate from the heterointerface between two phases having totally different properties. Thus, embodiments of the heterojoined noble metal-oxide composite nanoparticles according to the present invention are not limited to the catalyst applications in the Examples described above, and a quite wide variety of industrial applications thereof can be expected, by appropriately selecting the noble metal and the oxide.