# Patent

## Title
Synthesis of unit cell sized oxide particulates

## Abstract

A method of making unit cell sized oxide particulates comprising preparing a water solution of a metal or ceramic salt or methanol solution of Pt, adding a 2-fold molar excess of KO2 to the water solution and forming a reaction solution, spinning down the reaction solution, and creating oxide nanoparticles.

## Background
This application claims priority to and the benefits of U.S. Patent Application No. 61/904,211 filed on Nov. 14, 2013, the entirety of which is herein incorporated by reference.


BACKGROUND
Oxide nanoparticles are the subject of intense research efforts due to the wide variety of applications for which they are suited. Nanometer sized oxide particulates have been used in a wide variety of applications: electronic and magnetic devices, energy storage and generating systems and even medical applications. In general, there is a myriad of different synthetic techniques depending on the type of particle being prepared.
However, none of these techniques have demonstrated broad-spectrum applicability for oxide nanoparticle synthesis. The prior art techniques only produce micrograms of material.
Nanometer sized oxide particles were first considered theoretically in the mid 1980's, and synthesized a few years later. One of the first uses for metal oxide nanoparticles was Fe2O3 nanoparticles (≈50 nM) in magnetic data storage. Since that time, numerous other applications have been developed such as the Gretzel Solar cell, which uses dye-coated TiO2 particles to absorb incident radiation. Additionally, metal oxide nanoparticles have been explored as high-energy cathode materials for lithium batteries.
The critical performance aspect of these nanoparticles relates to their very small size, which corresponds to increased surface area.
As a general rule, this reduction in size and increase in surface area significantly increase the desired interactions either by enhancing energy adsorption in the Gretzel cell, or increasing the amount and rate at which Li+ can be intercalated and de-intercalated into the cathode.
Additionally, nanoparticles in general allow for new and varied approaches to creating nanometer to micron sized electrical components by creating transferable inks of these nanometer-sized particles. These techniques, such as laser printing, laser direct write printing, and 3-d printing take advantage to two key features of nanoparticles: small size allows for great flexibility and compactness in component design. The very small size of the particles also results in far greater reactivity. As such, annealing of an ink such as one composed of nanoparticulates of CeO2 can create stable ceramic films at temperatures well below the melting point of CeO2.
Unfortunately, one of the greatest problems facing the production of oxide-based nanoparticulates is the difficulty in rapidly synthesizing significant quantities of material. Typically, production requires several different steps to control particles size and to coat the nanoparticles to prevent them from aggregating, resulting in increasing particle size over time. These syntheses result in the formation of only micrograms of the oxide nanoparticulates.
SUMMARY OF THE INVENTION
Here we show for the first time a solution based flash oxidative synthesis of oxide nanoparticles through the use of KO2 that produces oxide nanometer sized particles down to 0.5 nM, which corresponds to unit cell sized particles.
This synthetic technique described herein also produces these particles in gram scale quantities, which is an order of magnitude greater than previous techniques.
This synthetic approach not only works for metals, but also many different cations such as Mg, and Ce. Additionally, this synthetic technique can prepare nanoparticulate blends as precursors to many interesting compounds, such as ferroelectric, multiferroic, ferromagnetic or superconducting compounds.
These metal oxide particulates also show great promise for a multitude of electrochemical processes, from novel cathode materials to solar cells.
Also, because of their small size and the ability to prepare bulk quantities of these metal oxide particulates, they will also serve as excellent precursors for the formation of metallic nanoparticles of very small size.
Presented here is a description demonstrating and quantifying the synthesis and behavior of these nanoparticles and nanoparticulate blends.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an illustration of a basic synthesis of the oxide particulates.
FIG. 2 is an illustration of JOEL SEM EDAX showing no remaining potassium, sulfate, chlorine or acetate.
FIG. 3 is an illustration of a transmission electron microscopic image of ZnO2 nanoparticulates. Average size estimate 1.7 nM.
FIG. 4 is an illustration of X-ray diffraction patterns of ZnO2.
FIG. 5 is an illustration of magnetic data of some of the oxides prepared. A. illustrates Mn3O4; B. illustrates Ni(OH)2; C. illustrates Co3O4; and D. illustrates Cu(OH)2.



DETAILED DESCRIPTION
The general synthetic approach is a simple two step process, as shown in FIG. 1.
Here we show for the first time a solution based flash oxidative synthesis of oxide nanoparticles that produces oxide nanometer sized particles.
This synthetic technique described herein also produces these particles in gram scale quantities, which is an order of magnitude greater than previous techniques.
This synthetic approach not only works for metals, but also many different cations such as Mg, and Ce. Additionally, this synthetic technique can prepare nanoparticulate blends as precursors to many interesting compounds, such as ferroelectric, multiferroic, ferromagnetic or superconducting compounds.
Example 1
A water solution of the metal or ceramic salt is prepared.
To this rapidly stirring solution, a 2-fold molar excess of KO2 is added quickly.
The solution stirs for approximately 10 minutes until it is cool enough to place into the plastic centrifuge tubes.
The solution is spun down to remove excess water, and this process is repeated until the remaining solution is pH neutral (typically 5 repetitions).
As a final cleansing of the nanoparticles, the material is re-suspended in methanol and spun down to remove the remaining water and methanol.
The samples are dried under a dynamic vacuum (10−3 torr) at room temperature to avoid heating the material, which could result in increasing the particle size.
To date, this approach has made oxide nanoparticles using solutions of chlorides, sulphates or acetates; all of which resulted in the formation of oxide nanoparticles. This approach can work for many different types of solutions, including 2 row transition metals, Lanthanides, Actinides, semi-metals such as Bi or Sn, as well as other simple cations such as Ca, Sr, Ba, Sc, or Y.
Control of the final product was achieved by two methods, reaction time or temperature of the solution prior to adding the KO2. Varying the length of time the solution was allowed to stir and react affected both the final oxidation and the particle size.
Example 2
Minimum particle size for each material was achieved when the reaction was quenched 1-2 minutes after the KO2 was added by diluting the reaction mixture by 50% with methanol. For some reactions, this did not alter the final product for Mg(OH)2, TiO2, CeO2, and ZnO2. However, for the Mn, Fe, Co, Ni and Cu reactions, this quenching resulted in the formation of metal hydroxides. For Mn, Fe and Co, allowing the reaction to either proceed for approximately 20 minutes resulted in the formation of the M3O4 phase of each compound. For the Ni and Cu systems, prolonged reaction time resulted in the formation of NiO—OH and CuO, respectively. Prolonged reaction times also resulted in an increase in particle size into the hundreds of nanometer scale.
Example 3
By heating the solution before adding the KO2, a mix of higher oxidation state compounds and larger particle sizes were observed for several systems. The Mn, Fe, Co, Ni and Cu systems formed mixes of Mn3O4 and MnO2, Fe3O4 and Fe2O3, for example. However, in the case of the reaction of SnCl2 with KO2, SnO2 was only formed when the reaction was heated to 90 C, and allowed to stir for 1 hour.
Example 4
The Pt system proved to be challenging. Oxidation in water resulted in the formation of a mix of insoluble Pt compounds that were primarily an amorphous brown color, possibly indicating PtO2, or other mixed valence Pt compounds. In order to prepare a single phase Pt-0 compound, significant changes were made to the process.
Rather than performing the reaction in water, the reaction was carried out in methanol. In this instance, the KO2 was added slowly due to the potential flammability of methanol.
However, since the oxidation of Pt is not exothermic, all of the KO2 was added. As the reaction proceeded, the solution turned first clear yellow, then light brown, which then slowly turned black over 1 hour.
After rinsing and drying, x-ray analysis indicated the formation of a Pt2O, with very small particle size.
Tables 1 and 2 detail the experimental results, including X-ray analysis. Table 1 lists the initial and final product, the average particle size, and the average number of unit cells per particle, assuming the particles are spherical. The number of unit cells was rounded to the nearest whole number.







TABLE 1







Particle Synthesis Analyses















Particle 

Unit Cells 


Starting 


Size 

per 


Material
Product
Color
(nM)
Volume
Particle















MgSO4
Mg(OH)2
White
0.990
40.840
24


Ti(SO4)2
TiO2 
Light Yellow
0.610
130.8
2



(anatase)






FeCl2
Fe3O4
Black
0.530
544.080
1


Mn(Ac)3
Mn3O4
Black
0.540
314.400
1


CoSO4
Co3O4
Black
1.790
516.370
11


Ni(Ac)2
Ni(OH)2
Light Green
1.460
38.500
86


Cu(Ac)2
Cu(OH)2
Blue-green
2.800
164.400
50


*Zn(Ac)2
ZnO2
White
1.630
115.084
38


*Zn(Ac)2
ZnO2
White
0.420
114.024
1


CeCl3
CeO2
Yellow-orange
0.750
181.700
2


SnCl2
SnO2
Faint yellow
0.850
71.550
9


H2PtCl6
Pt2O
black
0.290
59.996
1
















TABLE 2







Detailed X-ray Data















Space








Product
Group
a
b
c
Beta
Re %
S

















Mg(OH)2
Fd-3m
3.130(7)
3.130(7)
4.81(5)
90.
7.75
1.5225


TiO2
I41/amd
 3.806(17)
 3.806(17)
 9.03(11)
90.000
5.55
1.4342


Fe3O4
Fd-3m
8.163(7)
8.163(7)
8.163(7) 
90.000
2
1.2425


Mn3O4
141/amd
 5.759(12)
 5.759(12)
9.48(2)
90.000
7.5
1.602


Co3O4
Fd-3m
8.023
8.023
8.023
90.000
0.92
1.8092


Ni(OH)2
P-3m1
3.105(7)
3.105(7)
4.61(3)
90.
2.74
3.7594


Cu(OH)2
Cmc21
2.954(3)
10.569(17)
5.266(4) 
90.000
5.19
1.4517


{circumflex over ( )}ZnO2
Pa-3
4.862(3)
4.865(3)
4.865(3) 
90.000
5.49
1.7804


*ZnO2
Pa-3
4.849(4)
4.849(4)
4.849(4) 
90.000
3.67
1.7168


CeO2
Fm-3m
 5.664(11)
 5.664(11)
5.664(11)
90.000
6.49
1.3617


SnO2
P42/mnm
 4.738(15)
 4.738(15)
3.183(17)
90.000
9.1
1.1884


Pt2O
Pn-3m
3.9141(7) 
3.9141(7) 
3.9141(7) 
90.000
5.93
1.7317





*Prepared by quenching the oxidation by adding 50 mL of methanol after a reaction time of 2 minutes.


{circumflex over ( )}This data was reproduced after the sample was left dry at room temperature for 7 days. TEM images were also taken from this aged samples.






Elemental analysis was performed on a JOEL SEM in EDAX mode. Because the metal salts either have chlorine, acetate or sulfate as the anion, EDAX data was collected on the Fe3O4 made from FeCl2, Co3O4 made from CoSO4 and the Ni(OH)2 made from nickel acetate. As seen in FIG. 2, EDAX showed only the presence of the metal and oxygen.
Particle size analysis was primarily performed on the Rigaku SmartLab using the PDXL2 program. Peaks were fit via a Split Psuedo-Voigt function and refined to minimize % Re and S, which provide a measure of the accuracy of the degree of fit to the x-ray diffraction pattern. For the ZnO2 size analysis, TEM images were also taken in order to confirm the X-ray analyses. As shown in FIG. 3, the TEM indicates that the ZnO2 particles were approximately 1.7 nM, which is in good agreement with the X-ray analysis, which calculated an average particle size of 1.63 nM. This falls within the expected deviation of +5%, as reported in literature when comparing X-ray particle size analysis to TEM particle size analysis.
One additional advantage of this approach is that the oxide particles are very stable over time, even when dry. The TEM images were taken of a week old sample of nanometer sized ZnO2 particles. As shown in FIG. 4 where A is the initial ZnO2 sample, B and C are the same sample of 1 and then 2 weeks of aging. Pattern D shows the ZnO2 pattern for the sample with an average particle size of 0.42 nM, which is within experimental effort of the size of a single ZnO2 unit cell.
Magnetic Data was collected to characterize the magnetic behavior of the metal oxide nanoparticles. Behavior observed was very similar to that predicted in the literature. As shown in FIG. 5A, the Mn3O4 shows a ferromagnetic transition at approximately 42 K, as expected, as well as significant differences between the field cooled and zero field cooled magnetism. Again confirming the nanoparticulates nature of the oxides prepared.
FIG. 5b shows the low temperature onset of ferromagnetism of Ni(OH)2 nanoparticles as shown in literature. FIG. 5C shows similar behavior for the Co3O4 nanoparticles, with a ferromagnetic onset at 25 K, as expected from literature. FIG. 5D shows the paramagnetic behavior of Cu(OH)2. It is important to note that there is a very minimal upturning at low temperature indicating the high purity of the copper hydroxide product.
Another additional advantage is the use of these nanomaterials as precursors to even smaller metal particles. Typically, metal particles are formed by the reduction of a metal salt in a solution using a reducing agent such as NaBH4. A good general method for this is called the polyol method, when ethylene glycol, or other type of glycol is used as the solvent.
Here we prepared metal nanoparticles from the Ni(OH)2 and Cu(OH)2 nanoparticles. Typically, 3 grams of material were dissolved in 100 mL of ethylene glycol. In order to limit the formation of large particles, the NaBh4 was added in small amounts until a few flake like crystals could be seen in solution. This was determined to be the end point in the reduction, and the solution was further diluted by the addition of 50 mL of water. This was then spun down and centrifuged several times in methanol to remove the water and ethylene glycol. The metal particles were then dried under a dynamic vacuum for 24 hours and then characterized by x-ray diffraction. Initial tests on reduction of Ni(OH)2 showed that nanoparticles of 12 nM were produced.
As described herein, this is a simple, one step method for creating oxide nanoparticles. The oxide nanoparticles are as small as an individual unit cell.
This method is applicable for producing oxides nanoparticles for most of the metals or ceramic precursors in the periodic table.
The synthesis technique does not require the use of expensive capping agents or complex chemicals to perform.
The synthesis produced grams of material and is scalable, whereas other synthetic methods can only generate micrograms of material.
This approach can also be used to create blends of metal oxide particulates suitable for inks in various types of 3-d and 2-d processing techniques, including metal oxide inks, as well as ceramic inks.
Additionally, blends of different oxides, such as oxide particles of yttrium, barium and copper could be combined to create a nanoparticulate precursor to the YBa2Cu3O7−x superconductor.
Many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that the claimed invention may be practiced otherwise than as specifically described. Any reference to claim elements in the singular, e.g., using the articles “a,” “an,” “the,” or “said” is not construed as limiting the element to the singular.

## Claims

1. A method of making unit cell sized oxide particulates, comprising:
preparing a water solution of a metal or ceramic salt;
stirring the water solution;
adding KO2 to the water solution and forming a reaction solution comprising metal oxide nanoparticles wherein the metal oxide nanoparticles are selected from the group consisting of Fe3O4, Mn3O4, ZnO2, and Pt2O;
stirring the reaction solution;
placing the reaction solution in a centrifuge tube;
spinning down the reaction solution; and
harvesting the metal oxide nanoparticles wherein the metal oxide nanoparticles are selected from the group consisting of Fe3O4, Mn3O4, ZnO2, and Pt2O.

2. The method of making unit cell sized oxide particulates of claim 1 wherein the step of stirring the reaction solution is for about 10 minutes to about 1 hour.

3. The method of making unit cell sized oxide particulates of claim 2 wherein the step of spinning down comprises removing excess water.

4. The method of making unit cell sized oxide particulates of claim 3 wherein the step of spinning down to remove excess water is repeated until the remaining solution is pH neutral.

5. The method of making unit cell sized oxide particulates of claim 1 further including the step of:
cleansing the oxide nanoparticles.

6. The method of making unit cell sized oxide particulates of claim 5 wherein the step of cleansing the oxide nanoparticles further comprises the steps of suspending the oxide nanoparticles in methanol and spinning down and removing the water and the methanol.

7. The method of making unit cell sized oxide particulates of claim 6 further comprising the step of:
drying the oxide nanoparticles under a dynamic vacuum at room temperature.

8. The method of making unit cell sized oxide particulates of claim 7 wherein the step of drying under a dynamic vacuum is at about 10−3 torr.

9. The method of making unit cell sized oxide particulates of claim 1 further comprising the step of:
diluting the reaction solution by 50% with methanol after the step of adding a 2-fold molar excess of KO2 to the water solution and forming a reaction solution.

10. The product of the process of preparing a water solution of a metal or ceramic salt; stirring the water solution; adding a 2-fold molar excess of KO2 to the water solution and forming a reaction solution; stirring the reaction solution; placing the reaction solution in a centrifuge tube; spinning down the reaction solution; and creating oxide nanoparticles wherein the oxide nanoparticles are unit cell sized and are selected from the group consisting of Fe3O4 Mn3O4, ZnO2, and Pt2O.