PROCESS FOR PRODUCING NANOPARTICLES

Abstract

The present invention is in the field of processes for the production of nanoparticles. It relates to a process for the preparation of nanoparticles comprising heating a water-free solution containing (a) a metal-organic compound containing an alkaline earth metal and a group 4 metal, and (b) a stabilizer to at least 150 C. for at least 30 minutes.

Claims

1. A process for of preparing nanoparticles, the process comprising heating a water-free solution containing (a) a metal-organic compound containing an alkaline earth metal and a group 4 metal, (b) a stabilizer, and (c) a solvent to a temperature of at least 150 C. for a period of at least 30 minutes.

2. The process according to claim 1, wherein the alkaline earth metal is Sr.

3. The process according to claim 1, wherein the group 4 metal is Ti.

4. The process according to claim 1, wherein the metal-organic compound contains an alcoholate.

5. The process according to claim 1, wherein the metal-organic compound contains a C.sub.1 to C.sub.10 alkyl alcoholate or an oligoether alcoholate.

6. The process according to claim 1, wherein a molar ratio of the alkaline earth metal and the group 4 metal in the metal-organic compound is 0.5 to 2.

7. The process according to claim 1, wherein the metal-organic compound is a compound of formula (I) or formula (II)
M.sup.1(OR.sup.1).sub.2M.sup.1[M.sup.2(OR.sup.2).sub.5].sub.2(I)
M.sup.1M.sup.2(OR.sup.2).sub.6(II) where M.sup.1 is an alkaline earth metal, M.sup.2 is a group 4 metal, and R.sup.1 and R.sup.2 are each independently an alkyl, alkenyl, aryl or oligoether group.

8. The process according to claim 1, wherein the heating is performed via microwave irradiation.

9. The process according to claim 1, wherein the stabilizer is a C.sub.6 to C.sub.22 carboxylic acid.

10. The process according to claim 1, wherein the solvent is a C.sub.1 to C.sub.12 alcohol.

11. The process according to claim 1, wherein a concentration of the metal-organic compound in the solution is 10 to 200 mmol/l.

12. The process according to claim 1, wherein the nanoparticles are stabilized by an organic compound containing at least a phosphoric acid group and an ester group or at least two carboxylic acid groups.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0067] FIG. 1 shows the X-Ray diffractogram (XRD) of the nanoparticles obtained in example 1.

[0068] FIG. 2 shows the dynamic light scattering diagram of the particles obtained in example 1.

[0069] FIG. 3, 5-8 show the transmission electron microscopy images of the nanoparticles obtained in the examples 2-6.

[0070] FIG. 4 shows the XRD of the nanoparticles obtained in example 2.

[0071] FIG. 9 shows the X-Ray diffractogram (XRD) of the nanoparticles obtained in comparative example 2.

EXAMPLES

Example 1

[0072] Sr(MEE).sub.2Ti(MEE).sub.4, wherein MEE stand for (2-methoxyethoxy)ethoxide, was added to trioctylphosphine oxide (TOPO) at 100 C. to form a 0.2 M solution. The mixture was heated to 300 C. and left to react for 2 hours 20 minutes. The resulting mixture was dark orange. After precipitation in acetone twice, the precipitate was suspended in toluene to yield a turbid suspension which became immediately clear upon addition of some oleic acid.

Example 2

[0073] A 10 mL microwave vial was charged with 4 mL benzyl alcohol. Sr(MEE).sub.2Ti(MEE).sub.4 was added under vigorous stirring to obtain a clear, orangish solution with a concentration of 0.08 M. This vials was subjected to microwave heating for 4 hours using a 2.45 GHz Discover SP CEM Microwave at 270 C. A white precipitate and a supernatant formed. The precipitate was collected by centrifugation (4000 rpm, 3 min) and washed two times with ethanol and diethyl ether to remove excess of organic byproducts. 4 mL of toluene and 0.2 mmol oleic acid were added, whereupon a transparent suspension was obtained instantly. A transmission electron microscopy image of the obtained nanoparticles is shown in FIG. 3. A powder X-ray diffractogram is shown in FIG. 4.

Example 3

[0074] The procedure according to example 2 was performed with Sr(ME).sub.2Ti(ME).sub.4, wherein ME stands for 2-methoxyethoxide. Upon suspension after the synthesis a transparent suspension was obtained instantly. A transmission electron microscopy image of the obtained nanoparticles is shown in FIG. 5.

Example 4

[0075] The procedure according to example 2 was performed with Sr(Oct).sub.2Sr[Ti(Oct).sub.5].sub.2, wherein Oct stands for 1-octanolate. Upon suspension after the synthesis a transparent suspension was obtained instantly. A transmission electron microscopy image of the obtained nanoparticles is shown in FIG. 6.

Example 5

[0076] The procedure according to example 2 was performed with Sr(OiPr).sub.2Sr[Ti(OiPr).sub.5].sub.2, wherein OiPr stands for isopropanolate. Upon suspension after the synthesis a transparent suspension was obtained instantly. A transmission electron microscopy image of the obtained nanoparticles is shown in FIG. 7.

Example 6

[0077] The procedure according to example 2 was performed with Sr(OBn).sub.2Sr[Ti(OBn).sub.5].sub.2, wherein OBn stands for benzyl alcoholate. Upon suspension after the synthesis a transparent suspension was obtained instantly. A transmission electron microscopy image of the obtained nanoparticles is shown in FIG. 8.

[0078] Characterization of the Nanocrystals

[0079] The nanoparticles obtained in examples 1 to 6 were dried at 60 C. The dried samples were mixed with 10 wt-% ZnO (internal standard) and side loaded to a standard sample holder (8 mm height and 0.8 mm depth) to reduce preferential orientation effects. These samples were subject to X-Ray Diffraction (XRD) using a Thermo Scientific ARL Xtra X-ray diffractometer with the Cu K.sub. line as the primary X-ray source. The crystallite size was calculated via the Scherrer equation using 0.95 as shape factor. Rietveld quantitative analysis was selected to determine the crystallinity. TOPAS-Academic V4.1 software was used for performing Rietveld refinement. The results are summarized in the following table.

[0080] The solvodynamic diameter was determined via dynamic light scattering (DLS) using a Malvern Nano ZS in backscattering mode(173) at a temperature of 25 C.

TABLE-US-00001 Crystal size Crystallinity Solvodynamic Example in nm in % diameter in nm 1 3.5 8.7 2 5.3 70 11.5 3 3.7 79 7.3 4 3.2 58 9.6 5 1.2 12 6 6.5 45 13.8

Comparative Example 1

[0081] Example 4 was performed with the difference that two moles of water with regard to the molar amount of the metal-organic compound were added. Complete resuspension was not possible.

Comparative Example 2 (Corresponds to Example 4 of U.S. Pat. No. 6,329,058)

[0082] 20 grams of the barium titanium ethoxide slurry was transferred into a 40 milliliter screw cap jar. The barium titanium ethoxide slurry was mixed with 0.73 gram hexanoic acid and 0.395 gram of deionized water. The mixture was shaken vigorously for approximately 1 minute and then transferred into an autoclave. The reactor head space was purged with dry nitrogen for 2 minutes. The autoclave was then heated to 225 C. for 2 hours. The slurry was collected and washed twice with acetone for XRD analysis. XRD analysis which is shown in FIG. 9 indicates the formation of cubic BaTiO.sub.3, yet also an additional reflection is present at a 20 value of about 28 indicating the presence of rutile TiO.sub.2. The crystallite size is about 10 nm. The crystallinity degree was determined using Rietveld refinement and indicated the presence of 16.8% crystalline cubic BaTiO.sub.3.

[0083] The BaTiO.sub.3 particles were stabilized in toluene as described in example 4 of U.S. Pat. No. 6,329,058 or methanol using stabilizer Ia, which is a compound of general formula (I) with a=0, b=6, c=5, n=2-3, or IIIa, which is a compound of general formula (III) with e=0 and f=5-6, directly after synthesis. Yet, from the DLS data it is clear that the stabilizer IIIa provides better stabilization in terms of solvodynamic diameter. Yet, all stabilization methods tend to show some agglomeration (higher Z-average and some tailing in the DLS data). The data are given in the following table.

TABLE-US-00002 Z-average of the Solvodynamic solvodynamic diameter in nm diameter in nm BaTiO.sub.3 - oleic acid 40 80 BaTiO.sub.3 - stabilizer Ia 21 65 BaTiO.sub.3 - stabilizer IIIa 8.1 66