A universal method for the synthesis of metal nanoparticles via scanning probe lithography

Abstract

A method for forming metal or metal oxide nanoparticles on a substrate may include depositing a precursor ink onto a hydrophobic surface of the substrate to form a nanoreactor on the hydrophobic surface. The precursor ink includes a metal precursor and a non-coordinating polymer dissolved in a solvent. The process may then include evaporating the solvent from the nanoreactor, whereupon the solvent evaporates, the polymer and metal precursor phase separate and the metal precursor aggregates on the surface of the nanoreactor. After evaporating the solvent, the polymer is removed, thereby leaving the aggregated metal precursor in contact with the hydrophobic surface. The resulting aggregated metal precursor is then annealed to form metal or metal oxide nanoparticles.

Description

[Technical field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS Benefit of priority is claimed herein to U.S. Provisional Patent Application No. 63 / 312,711, filed February 22, 2022, the disclosure of which is incorporated herein by reference in its entirety.

[0002] The present disclosure relates to methods for forming metal and / or metal oxide nanoparticles, and more particularly, to a universal method for forming metal and / or metal oxide nanoparticles that can occur without regard to element-specific chemistry. [Background technology]

[0003] Throughout history, materials that have been used and relied upon have evolved over time, slowly becoming more complex. The progression from the stone tools used by early humans to the composite synthetic materials used today took centuries due to the enormous parameter space that materials encompass. For example, when considering the 91 metallic elements of the periodic table and all their possible combinations, there is a nearly infinite number of possible materials. This is especially true at the nanoscale, where even with a fixed chemical composition, small changes in size or shape can dramatically change the properties of a material. Summary of the Invention

[0004] The ability to rapidly synthesize and subsequently screen materials for desired properties is required. Nanoscale scanning probe lithography approaches have been developed that allow the preparation of as many as 5 billion positionally encoded "mega-libraries" of nanomaterials with unique chemistries, including metal or ionic nanoparticles and perovskites, through deposition and thermal annealing in polymeric nanoreactors. These libraries can be tailored to encompass a wide variety of alloys and phase-separated nanoparticles composed of as many as seven different elements with up to four phases and six interfaces. Importantly, one mega-library contains more new, well-defined inorganic materials than chemists have cumulatively generated and characterized to date, and can be used to identify new materials and catalysts for important chemical transformations. In addition, important insights have been gained into how thermodynamic phases form in multi-element nanoparticles, and design rules have been established for engineering heterostructures in multi-element nanoparticles. New high-throughput structural, catalytic, and luminescent characterization techniques have been developed to match the unprecedented speed of mega-library synthesis. [Brief description of the drawings]

[0005] [Figure 1A] FIG. 1A is a schematic diagram of a conventional method for nanoparticle synthesis. [Figure 1B] FIG. 1B is a schematic diagram of the method of the present disclosure. [Figure 1C] FIG. 1C includes HAADF images of precursor aggregates on the nanoreactor surface for a nanoreactor array, a single nanoreactor, precursor aggregates, and high-resolution images of amorphous aggregates. Inset: FFT of the corresponding ABF images. Scale bars: 2 μm, 100 nm, 20 nm, and 5 nm, respectively. [Figure 1D] FIG. 1D includes HAADF images of precursor aggregates after polymer removal in high-resolution ABF images of nanoreactor arrays, single nanoreactors, precursor aggregates, and partial crystalline aggregates. Inset: FFT. Scale bars: 2 μm, 100 nm, 20 nm, and 5 nm, respectively. [Figure 1E] FIG. 1E includes HAADF images of the resulting nanoparticles in the nanoreactor array, a single nanoparticle, a zoomed-in image of a nanoparticle, and a high-resolution image of a crystalline particle with an amorphous oxide surface resulting from air exposure. Inset: FFT of the corresponding ABF image. Scale bars: 2 μm, 100 nm, 20 nm, and 5 nm, respectively. [Diagram 2] FIG. 2 is a diagram with representative images of various single-component nanoparticles that can be synthesized using the methods of the present disclosure. White outlines indicate elements that can be synthesized as both metals and oxides. Scale bar: 10 nm. [Figure 3A-3E] 3A-3E are HAADF images and EDS maps of multicomponent nanoparticles formed by the methods of the present disclosure. [Figure 4A-4C] Figures 4A-C are HAADF images of Pt particles formed by the conventional SPCBL method in which particles are formed in a polymer nanoreactor, showing (A) formation with PEO-b-P2VP, (B) formation with P2VP, and (C) formation with PEG400. [Diagram 5] FIG. 5 is a HAADF image of Pt nanoparticles synthesized by the method of the present disclosure using polystyrene in the precursor ink. [Figure 6A] FIG. 6A contains HAADF and ABF images of W after annealing at 240° C. using the method of the present disclosure and compared to the conventional method using P2VP as the polymer, showing that in both cases, W remains amorphous. [Figure 6B] FIG. 6B contains HAADF and ABF images of the W nanoparticles of FIG. 6A after annealing at 600° C., showing that the W was polycrystalline. [Figure 6C] FIG. 6C includes TEM images and SAED patterns confirming that W nanoparticles were formed by each method. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0006] Referring to Figure 1A, conventional processes for forming nanoparticles using nanoreactors formed using lithographic methods generally involve the formation of nanoparticles within the nanoreactor. For example, as shown in Figure 1A, a nanoreactor precursor ink is deposited on a substrate and then subjected to a two-stage heating process to first aggregate the metal components within the nanoreactor and then convert the aggregated metal components into nanoparticles, all formed within the nanoreactor. Typically, such processes utilize coordination polymers, such as the block copolymer PEO-b-P2VP.

[0007] In contrast, as shown in FIG. 1B, the disclosed method provides a universal method for particles to be formed on the surface of a nanoreactor as opposed to within the nanoreactor. The disclosed precursor ink comprises a non-coordinating polymer or polymer mixture, a metal precursor, and a polar non-volatile solvent or solvent mixture. The components of the precursor ink should be selected such that the polymer or polymer mixture and the metal precursor are soluble in the solvent or solvent mixture. The precursor ink can be non-aqueous. References herein to a polymer, metal precursor, and solvent should be understood to include a single polymer, a single metal precursor, and / or a single solvent, and / or a polymer mixture, a mixture of metal precursors containing the same or different metals, and a mixture of solvents.

[0008] A non-coordinating polymer is a polymer that does not coordinate to the metal precursor used. A non-coordinating polymer suitable for use in the disclosed method is soluble in a solvent or solvent mixture, either by itself or in a polymer mixture, and is immiscible with the metal precursor in the absence of a solvent. The non-coordinating polymer can be, for example, polystyrene (PS) or polystyrene-based. For example, the polymer can be a mixture of 500 Da PS and 600 Da sulfonic acid terminated PS. This polymer mixture was found to be soluble in sulfolane-DMSO mixtures up to at least 200 mg / ml when the proportion of sulfonic acid terminated PS is at least 2% of the total amount of polymer.

[0009] The non-coordinating polymer may be present in the precursor ink in a total amount of about 10 mg / mL to about 200 mg / mL, about 100 mg / mL to about 200 mg / mL, about 10 mg / mL to about 50 mg / mL, about 35 mg / mL to about 125 mg / mL, or about 15 mg / mL to about 75 mg / mL. Other suitable amounts include about 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190, and 200 mg / mL, and any value therebetween, or ranges defined by these values. For example, the precursor ink may include about 50 mg / mL of polymer.

[0010] The metal precursor can be, for example, a metal salt. For example, a metal nitrate or metal halide of the desired metal to be formed into nanoparticles can be used. The metal precursor can include, for example, a combination of metal nitrates and metal halides with different metals to form multicomponent metal nanoparticles. For example, the metal contained in the metal precursor can be one or more of Mg, Al, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, Sr, Y, Zr, Mo, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Ba, Lu, Hf, Ta, W, Re, Ir, Pt, Au, Pb, Bi, La, Ce, Pr, Nd, Sm, Eu, Gd, Th, Dy, Ho, Er, Tm, Yb. All of the above can form metal oxide nanoparticles. In addition, all elements that can be reduced by hydrogen and are non-volatile in their metallic state, i.e., Fe, Co, Ni, Cu, Mo, Ru, Rh, Pd, Ag, Sn, W, Re, Ir, Pt, Au, Pb, Bi, can form metallic particles, so that metal, metal oxide, and mixed metal / metal oxide nanoparticles can be prepared. The disclosed method can also be used with metal precursors containing Be, Tc, Os, Hg, Tl, Ra, or any actinide.

[0011] The metal precursors may be present in the precursor ink in a total amount of about 5 mM to about 200 mM. Other concentrations are contemplated herein so long as the metal precursors remain soluble in the solvent. For example, the precursor ink may include 30 mM of the metal precursor. The metal precursor may be one or more metal precursors. For example, the metal precursor may include two or more metal precursors, each containing a different metal, to form multi-component nanoparticles. In precursor inks having two or more metal precursors, each may be included in a relative amount determined by the desired multi-component nanoparticles to be formed.

[0012] The solvent is polar and non-volatile. The solvent is selected to dissolve the metal precursor and the polymer. The solvent can be, for example, 1,3-dimethyl-2-imidazolidone (DMI), propylene carbonate (PC), or sulfolane (SF). For example, the solvent can be sulfolane with 1-10% dimethyl sulfoxide (DMSO) to reduce the melting point of sulfolane to below room temperature.

[0013] The precursor ink is deposited on a hydrophobic surface to form nanoreactors on the surface. For example, the precursor ink can be deposited on a hydrophobic surface by coating it on a hydrophilic scanning probe lithography tip and repeatedly contacting the hydrophobic surface with the scanning probe lithography tip to form a pattern of nanoreactors on the hydrophobic surface. The residence time can be used to control the amount of precursor ink transferred to the substrate, thereby controlling the resulting size of the nanoparticles. The hydrophobic surface can be, for example, a hydrophobic coating deposited on the substrate without a coating or the hydrophobic substrate itself. For example, the hydrophobic surface can be a substrate functionalized with perfluorinated phosphonic acid. For example, the hydrophobic surface can be a phosphonic acid modified surface or substrate. The substrate can be any substrate that is thermally stable, flat, and hydrophobic or can be modified with a hydrophobic coating. For example, the substrate can have a surface modified by plasma polymerizing perfluorinated carbon thereon. For example, the substrate can be TiO2, Al2O3, or Ta2O5 functionalized with phosphonic acid. The substrate can be any thermally stable metal oxide. For example, the substrate can be ITO. For example, the substrate can be a carbon-based surface modified with a perfluorinated polymer film.

[0014] Once deposited, the solvent in the nanoreactor evaporates, causing a phase separation between the polymer or polymer mixture and the metal precursor, leading to the agglomeration of the metal precursor on the nanoreactor surface. The agglomeration allows the metal precursor to be converted into a single nanoparticle. Advantageously, it has been observed that this agglomeration and conversion into a single nanoparticle can occur using the methods of the present disclosure, regardless of any element specific chemistry. The agglomeration process can be carried out at room temperature up to below the boiling point of the maximum solvent. For example, the temperature can be up to 80° C. The agglomeration process can be carried out using exposure to a solvent vapor, such as toluene. For example, the agglomeration process can be accelerated by heating, for example, at 60° C. in toluene vapor. The polymer can be removed, for example, by solvent annealing. For example, THF can be used.

[0015] FIG. 1C is a HAADF image of precursor aggregates on the nanoreactor surface showing high-resolution images of a nanoreactor array, a single nanoreactor, precursor aggregates, and amorphous aggregates. The inset is an FFT of the corresponding ABF image.

[0016] The polymer is then removed so that the metal precursor aggregates are in direct contact with the hydrophobic surface. The polymer can be removed by a variety of processes depending on the thermal and chemical stability of the substrate and / or metal precursor. For example, a plasma treatment can be used to remove the polymer while reducing or oxidizing the surface layer of the precursor aggregates. Suitable treatment times and plasma powers for complete removal of the polymer without etching the metal precursor aggregates are determinable within the knowledge of one skilled in the art. For example, the plasma treatment to remove the polymer and reduce the surface layer of the precursor aggregates can be a 5 minute treatment of 100W H2 plasma. For example, the plasma treatment to remove the polymer layer and oxidize the surface layer of the precursor aggregates can be a 1 minute treatment of 100W O2 plasma, which can be useful for preparing metal oxide nanoparticles. Alternatively, a very gentle plasma (e.g., 5W for 5-10 seconds) may be used for sensitive surfaces or substrates, and the remainder of the polymer can be removed by high vacuum (<10 -6 The metal precursor may be removed at 60° C. in 1000 torr. Other temperatures and vacuum (pressure) conditions are contemplated herein and are generally selected based on the relative vapor pressures of the polymer and metal precursor such that the polymer can be evaporated but the metal precursor cannot. Generally, lower temperatures can be used at lower pressures.

[0017] FIG. 1D includes HAADF images of the precursor aggregates after polymer removal showing high-resolution ABF images of the nanoreactor array, a single nanoreactor, the precursor aggregates, and partial crystalline aggregates. Insets are FFTs of the corresponding ABF images.

[0018] Once the polymer is removed, the precursor aggregates are converted to single nanoparticles by thermal annealing. For example, for metal nanoparticles, the metal can be reduced by H2 during annealing. It has been observed that higher temperatures can improve yields. The temperature of annealing to form metal nanoparticles can be selected to be at least higher than the decomposition temperature of the metal precursor and lower than the temperature at which the metal precursor evaporates. For precursor inks with multiple metal precursors, annealing can be performed at least higher than the highest one of the decomposition temperatures of the metal precursors and at least lower than the lowest evaporation temperature of the metal precursors. For example, for many metal precursors and substrates, thermal annealing to form metal nanoparticles can be from about 400°C to about 800°C. For example, non-volatile metal particles can be formed from metal precursor aggregates by annealing in H2 at 700°C for 12 hours. Consideration for the selection of the annealing temperature should also be given to the thermal stability of the substrate, and temperatures at which the substrate is unstable should be avoided.

[0019] The annealing process to form metal oxide nanoparticles may include a first annealing at a temperature at or near the decomposition temperature of the metal precursor, and a second annealing at a temperature higher than the decomposition temperature but lower than the evaporation temperature of the metal precursor and / or lower than the thermal stability limit of the substrate. The temperature at or near the decomposition temperature of the metal precursor may be, for example, equal to 20%, or 10%, or 5%, or 2%, or 1% of the decomposition temperature of the metal precursor, or up to (higher or lower than) those temperatures, and any value therebetween. Since the precursor ink includes multiple metal precursors, the highest decomposition temperature of the metal precursor and the lowest evaporation temperature of the metal precursor may be considered in selecting suitable first and second annealing temperatures. For example, metal oxide nanoparticles may be formed from metal precursor aggregates by first annealing in O2 at 400°C for 6 hours, then at 700°C for 18 hours, followed by cooling to room temperature at a rate of 50°C / h. For example, the second stage annealing can be at the highest temperature possible without exceeding the evaporation temperature of the metal precursor and / or the thermal stability limit of the substrate, for example, the second annealing temperature can be from about 600° C. to about 800° C.

[0020] The total annealing time may depend on the temperature used. Generally, at lower temperatures, longer times are required. If the time is too short, the particles formed will have irregular shapes. Longer times have not been observed to affect the results.

[0021] It has been observed that improved formation of nanoparticles can be achieved by using the fastest possible heating rate, for example, the furnace used for the heating step can be preheated to the target temperature to obtain a fast heating rate.

[0022] Figure 1E includes HAADF images of the resulting nanoparticles showing a nanoparticle array, a single nanoparticle, a zoomed-in image of a nanoparticle, and a high-resolution image of a crystalline nanoparticle with an amorphous oxide surface resulting from air exposure. The inset is an FFT on the corresponding ABF image.

[0023] Using the methods of the present disclosure, nanoparticles have been synthesized from nearly every stable metal in the periodic table. Figure 2 shows metals that have been synthesized into nanoparticles using the methods of the present disclosure, with those that can be synthesized as both metals and metal oxides outlined in white.

[0024] Certain metals, such as alkali metals, have insufficient thermal stability to be converted from precursor aggregates into nanoparticles as single-component systems. However, they can be stabilized in polymetallic crystals. In general, this method allows the synthesis of any multicomponent nanoparticle from single-component building blocks, with the composition being adjustable by adjusting the metal precursor concentration in the ink. Figure 3 shows various multicomponent nanoparticles synthesized by the method of the present disclosure. (A) Mo 47 Fe 28 Ni 25 , (B)Y 75 Yb 21 Er4, (C)Ir 42 Co 48 W 10 (D)Ir 44 W 10 Ru 23 Fe 23 , (E)Pt 13 Pd 12 Au3Cu 40 Co 14 Ni 15HAADF images and EDS maps of In4. Scale bars in Fig. 3 are each 5 nm. In each case, the precursor ink contained a 50 mg / mL polymer mixture of 2 wt% 600 Da sulfonic acid terminated PS in 500 Da PS. The solvent was 1% DMSO in SF in each case. The total metal precursor concentration in each was 30 mM. Their relative metal precursor ratios were as follows: (A) 47% (Mo(OAc)2)2, 28% FeCl3, 25% Ni(NO3)2; (B) 75% Y(NO3)3, 21% Yb(NO3)3, and 4% Er(NO3)3; (C) 42% H2IrCl6, 48% CoCl2, 10% WCl4; (D) 44% H2IrCl6, 10% WCl4, 23% RuCl3, 23% FeCl3; (E) 13% H2PtCl6, 12% Pd(NO3)2, 3% HAuCl4, 40% Cu(NO3)2, 14% CoCl2, 15% Ni(NO3)2, 4% InCl3. Particles in A, C, D, and E were annealed at 700 °C in H2 for 12 h. The particles in B were annealed in O2 at 400 °C for 6 h and 700 °C for 18 h.

[0025] Advantageously, the disclosed method can provide a nanoscale scanning probe lithography approach that allows the preparation of as many as 5 billion positionally encoded "mega-libraries" of nanomaterials with unique chemical properties, including metal or ionic nanoparticles and perovskites. These libraries can be tailored to encompass a wide variety of alloys and phase-separated nanoparticles composed of as many as seven different elements with up to four phases and six interfaces. Importantly, one mega-library contains more new, well-defined inorganic materials than chemists have cumulatively produced and characterized to date, and can be used to identify new materials and catalysts for important chemical transformations. In addition, important insights have been gained into how thermodynamic phases form in multi-element nanoparticles, and design rules have been established for engineering heterostructures in multi-element nanoparticles. New high-throughput structural, catalytic, and luminescent characterization techniques have been developed to match the unprecedented speed of mega-library synthesis.

[0026] 4A-C show the formation of Pt nanoparticles using a conventional method of forming nanoparticles in a nanoreactor. The previously described system using the block polymer PEO-b-P2VP in HO is shown in FIG. 4A. FIGS. 4B and 4C show the same process used in FIG. 4A, but with the individual components of the block polymer included in the precursor. As can be seen in FIG. 4B, no particle formation occurred with P2VP. Particle formation was found to occur in PEG, but PEG alone was observed to have limited applicability to precious metals and not universally applicable compared to the disclosed method.

[0027] FIG. 5 shows Pt nanoparticle formation using polystyrene in the disclosed method. HADDF images were taken after heat treatment of the nanoreactor in H2 at 240°C for 12 hours. Polymers that interact with the metal precursor, such as P2VP, were found to inhibit the formation of single particles. Using polymers that have very strong interactions with the metal (e.g., poly(acrylic acid), poly(acrylonitrile), or poly(4-cyanostyrene) inhibits particle formation for all metals, including Au. The disclosed method intentionally uses non-coordinating polymers to enhance particle formation. It was also found that using polymers such as PS that are incompatible with the metal precursor results in the described aggregation of the metal precursor upon solvent evaporation. This results in a fundamentally different nanoparticle formation process that relies on thermal sintering on the substrate surface, as opposed to the nucleation and growth process within the nanoreactor reported in previous techniques.

[0028] 6A and 6B, it was observed that W required annealing at 600° C. to form polycrystalline structures in both the conventional process using P2VP and the process according to the present disclosure using PS. The nanostructures formed after annealing at 400° C. remained amorphous. As shown in FIG. 6C, both P2VP polymer and PS20K were capable of producing W nanoparticles.

[0029] Since modifications within the scope of the present disclosure may be apparent to those skilled in the art, the foregoing description has been given only for clarity of understanding, and no unnecessary limitations should be understood therefrom.

[0030] All patents, patent applications, government publications, government regulations, and literature references cited herein are incorporated by reference in their entirety. In case of conflict, the present specification, including definitions, will control.

[0031] Throughout this specification, compounds, compositions, methods, and / or processes are described as including components, steps, or materials, but unless otherwise stated, it is contemplated that the compounds, compositions, methods, and / or processes may also include, consist essentially of, or consist of any combination of the listed components or materials.Unless otherwise indicated, component concentrations may be expressed in terms of weight concentration.Combinations of components are contemplated to include homogeneous and / or heterogeneous mixtures, as would be understood by one of skill in the art in light of the foregoing disclosure. References WO2011 / 068960 US11,534,831

Claims

1. 1. A method for forming metal or metal oxide nanoparticles on a substrate, comprising: depositing a precursor ink onto a hydrophobic surface of a substrate to form nanoreactors on the hydrophobic surface, the precursor ink comprising a metal precursor and a non-coordinating polymer dissolved in a solvent; evaporating the solvent from the nanoreactor, wherein upon evaporation of the solvent, the polymer and the metal precursor phase separate and the metal precursor precipitates on the surface of the nanoreactor; removing the polymer from the nanoreactor, thereby leaving the aggregated metal precursor in contact with the hydrophobic surface; annealing the aggregated metal precursor to form nanoparticles of the metal or metal oxide.

2. 2. The method of claim 1, wherein the solvent is one or more of 1,3-dimethyl-2-imidazolidinone (DMI), propylene carbonate (PC), and sulfolane (SF).

3. 3. The method of claim 1 or 2, wherein the polymer is polystyrene or polystyrene-based.

4. The method of claim 1 or 2, wherein the metal precursor comprises a metal nitride and / or a metal halide.

5. 3. The method of claim 1 or 2, wherein the metal of the metal precursor is one or more of Mg, Al, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, Sr, Y, Zr, Mo, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Ba, Lu, Hf, Ta, W, Re, Ir, Pt, Au, Pb, Bi, La, Ce, Pr, Nd, Sm, Eu, Gd, Th, Dy, Ho, Er, Tm, and Yb.

6. The method of claim 1 or 2, wherein the metal precursor is present in the precursor ink in an amount of about 5 mM to about 20 mM.

7. The method of claim 1 or 2, wherein the polymer is present in the precursor ink in an amount of from about 10 mg / ml to about 200 mg / ml.

8. 3. The method of claim 1, wherein depositing the precursor ink comprises contacting the hydrophobic surface with a scanning probe lithography tip coated in the precursor ink.

9. The method of claim 8 , wherein the hydrophobic surface is repeatedly contacted with the scanning probe lithography tip to deposit the nanoreactors on the hydrophobic surface.

10. The method of claim 1 or 2, wherein evaporating the solvent comprises heating the nanoreactor and is carried out at room temperature.

11. The method of claim 1 or 2, wherein evaporating the solvent comprises heating the nanoreactor to a temperature below the boiling point of the solvent.

12. The method of claim 1 or 2, wherein evaporating the solvent comprises heating the nanoreactor to a temperature between room temperature and about 80°C.

13. 3. The method of claim 1, wherein evaporating the solvent is performed with exposure to solvent vapor.

14. 14. The method of claim 13, wherein the solvent vapor is toluene or THF.

15. The method of claim 1 or 2, wherein removing the polymer comprises treating the nanoreactor with a plasma.

16. The nanoreactor is heated to H ２ 16. The method of claim 15, comprising treating with a plasma in

17. The plasma is O ２ The method of claim 15, wherein the plasma is a plasma.

18. 16. The method of claim 15, further comprising heating under vacuum to remove the polymer.

19. 3. The method of claim 1 or 2, wherein annealing the aggregated metal precursor comprises heating to a temperature at least above the decomposition temperature of the metal precursor and below the temperature at which the metal precursor vaporizes.

20. The method of claim 1 or 2, wherein annealing the aggregated metal precursor comprises heating to a temperature of about 400°C to about 800°C.

21. Annealing the aggregated metal precursor comprises: ２ 3. The method of claim 1 or 2, comprising annealing in a solution of 1000 ppm or less to reduce the metal precursor to metal nanoparticles.

22. Annealing the aggregated metal precursor comprises ２ 3. The method of claim 1 or 2, comprising annealing the metal precursor in a solution containing 20% ​​or more of ammonia to convert the metal precursor into metal oxide nanoparticles.

23. O ２ 23. The method of claim 22, wherein heating in comprises a first annealing at a first temperature at or near the decomposition temperature of the metal precursor, and a second annealing at a second temperature above the decomposition temperature of the metal precursor and below a temperature at which the metal precursor vaporizes.

24. 3. The method of claim 1 or 2, wherein the metal precursor comprises at least two or more metal precursors, each having a decomposition temperature, and the annealing is carried out at a temperature equal to or greater than the highest decomposition temperature of the two or more metal precursors.

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