A METHOD TO FORM COPPER NANOPARTICLES

Abstract

The invention relates to a method to form copper nanoparticles. The method comprises heating a solution comprising a copper precursor comprising at least one neat copper carboxylate in a concentration of at least 0.2 M, a stabilizer comprising an amine in a concentration equal or larger than the concentration of the copper precursor and optionally a solvent to a temperature T1 to form metallic copper. The solution is then heated to a temperature T2, with the temperature T2 being at least 10° C. higher than the temperature T1. The solution is heated from temperature T1 to temperature T2 with an average rate of at least 2 degrees per minute.

The invention further relates to copper nanoparticles obtainable by such method and to formulations comprising such nanoparticles.

Claims

1.-15. (canceled)

16. A method of forming copper nanoparticles, the method comprising the steps of: a) providing a solution at temperature T0, the solution comprising i. a copper precursor comprising at least one neat copper carboxylate; the neat copper carboxylate not being coordinated by a complexing agent or ligand other than carboxylate; ii. a stabilizer comprising at least one amine; and iii. optionally a solvent; wherein the concentration of the copper precursor in the solution is at least 0.2 M and the concentration of the stabilizer in the solution is greater than or equal to the concentration of the copper precursor; b) heating the solution of step a) to a temperature T1 at which temperature the copper precursor starts to decompose to form metallic copper (Cu0); and c) heating the solution of step b) to a temperature T2 to form copper nanoparticles having a size of at least 4 nm, the temperature T2 being at least 10° C. higher than temperature T1, wherein the temperature of the solution is increased from temperature T1 to temperature T2 with an average rate of at least 2 degrees per minute.

17. The method according to claim 16, wherein the neat copper carboxylate is selected from the group consisting of copper formate, copper acetate, copper lactate, copper oxalate, and copper citrate.

18. The method according to claim 16, wherein the neat copper carboxylate does not comprise amine ligands.

19. The method according to claim 16, wherein the amine comprises a C.sub.1-C.sub.30 alkylamine.

20. The method according to claim 16, wherein the amine consists of oleylamine present in the solution of step a) at a concentration of at least 0.2 M.

21. The method according to claim 16, wherein the solvent is an apolar solvent.

22. The method according to claim 16, wherein the concentration of the copper precursor in the solution of step a) is at least 0.5 M.

23. The method according to claim 16, wherein the stabilizer's concentration in the solution of step a) is between 1 and 10 times the copper precursor's concentration.

24. The method according to claim 16, wherein the temperature of the solution is increased from the temperature T1 to the temperature T2 with an average rate of at least 10 degrees per minute.

25. The method according to claim 16, the method further comprising: an additional step d) and/or an additional step e), wherein the additional step d) comprises maintaining the solution at temperature T2 for at least 5 minutes to form copper particles; and wherein the additional step e) comprises collecting copper nanoparticles from a reaction mixture obtained in step c).

26. The method according to claim 16, wherein the method is performed under an atmosphere comprising oxygen.

27. Copper nanoparticles produced by the method of claim 1.

28. A formulation comprising: the copper nanoparticles of claim 27.

29. A conductive printing ink comprising the formulation of claim 28.

30. A method of forming a copper film, the method comprising: a) applying the formulation of claim 28 onto a substrate so as to provide a copper film, and b) optionally subjecting the copper film to a heat treatment.

31. The method according to claim 26, wherein the method is performed under atmospheric conditions.

32. A method of forming copper nanoparticles, the method comprising: (a) heating a solution from a temperature T0 to a temperature T1, wherein at temperature T1 a copper precursor starts to decompose to form metallic copper (CuO), wherein the solution comprises: i. the copper precursor comprising at least one neat copper carboxylate that is not coordinated by a complexing agent or ligand other than carboxylate; ii. a stabilizer comprising at least one amine; and iii. an optional solvent, wherein the concentration of the copper precursor in the solution is at least 0.2 M and the concentration of the stabilizer in the solution is greater than or equal to the concentration of the copper precursor; and (b) heating the resulting solution to a temperature T2 to form copper nanoparticles having a size of at least 4 nm, wherein the temperature T2 is at least 10° C. greater than temperature T1, and wherein the temperature of the solution is increased from temperature T1 to temperature T2 at an average rate of at least 2 degrees per minute so as to form copper nanoparticles.

33. The method according to claim 32, the method further comprising: c) maintaining the solution at temperature T2 for at least 5 minutes to form copper particles; and/or d) collecting copper nanoparticles from a reaction mixture obtained in step b).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0110] The present invention will be discussed in more detail below, with reference to the attached drawings, in which:

[0111] FIG. 1a, FIG. 1b and FIG. 1c show Transmission Electron Microscope (TEM) images of copper nanoparticles synthesized by the method according to the present invention;

[0112] FIG. 2 shows an X-ray diffraction (XRD) pattern of copper nanoparticles synthesized according to example 1 at a copper formate and oleylamine concentration of 1M.

DESCRIPTION OF EMBODIMENTS

[0113] The present invention will be described with respect to particular embodiments and with reference to certain drawings but the invention is not limited thereto but only by the claims. The drawings are only schematic and are non-limiting. The size of some of the elements in the drawing may be exaggerated and not drawn on scale for illustrative purposes. The dimensions and the relative dimensions do not correspond to actual reductions to practice of the invention.

[0114] When referring to the endpoints of a range, the endpoints values of the range are included.

[0115] When describing the invention, the terms used are construed in accordance with the following definitions, unless indicated otherwise.

[0116] The term ‘and/or’ when listing two or more items, means that any one of the listed items can by employed by itself or that any combination of two or more of the listed items can be employed.

[0117] The term ‘complex’ or ‘coordinating complex’ refers to a central (metal) atom surrounded by molecules or anions, known as ligands.

[0118] The term ‘ligands’ or ‘complexing agents’ refers to neutral molecules or ions bonding to a central (metal) atom or ion to form a complex or coordinating complex.

EXAMPLES

First Series of Examples

[0119] In a first series of examples, solutions were prepared by mixing copper(II) formate tetrahydrate and oleylamine in n-dodecane at room temperature (T0) in concentrations as shown in Table 1. The reagents were stirred for 1 hour at 60° C. to ensure that copper(II) formate tetrahydrate is fully dissolved (deep blue color). The temperature was increased to 60° C. to accelerate the dissolution. It is however important to not surpass 100° C. to avoid decomposition of Cu(II) formate tetrahydrate.

[0120] Subsequently, the temperature of the solution was increased to a temperature of 120° C. (T1) and the solution turned to dark red/brown. Subsequently, the temperature of the solution was increased to a temperature of 140° C. (T2). The temperature was increased from 120° C. (T1) to a 140° C. (T2) with an average rate of 10° C./minute. The solution was then maintained at a temperature of 140° C. for at least 10 minutes to ensure that all copper(II) formate has been reduced. Finally, the resulting solution was washed multiple times using acetonitrile, methanol or ethanol as non-solvents and hexane or toluene as solvents.

[0121] Transmission Electron Microscope (TEM) images of the obtained copper nanoparticles are shown in FIG. 1a (Examples A1-A3), FIG. 1b (Examples B1-B3) and FIG. 1c (Examples C1-C3).

[0122] The size of the nanoparticles can be influenced by changing the ratio of the concentration of the stabilizer over the concentration of the copper precursor. The higher the concentration of the stabilizer, the smaller the size of the obtained copper nanoparticles. Preferably, the ratio of the concentration of the stabilizer over the concentration of the copper precursor ranges between 1 and 8, for example between 1 and 3 as shown in Table 1.

[0123] Also the initial concentration of copper(II) formate in the solution may influence the particle size. The lower the concentration of the copper(II) formate, the larger the size of the obtained copper nanoparticles. The concentration of copper(II) formate can for example be increased to 1 M or above 1M.

[0124] All examples of the first series were performed under an atmosphere comprising oxygen.

[0125] FIG. 2 shows a X-ray diffraction pattern of copper nanoparticles synthesized according to example 1 starting from copper formate in a concentration of 1 M and oleylamine in a concentration of 1 M.

[0126] Starting from a solution comprising copper(II) formate in a concentration of at least 0.5 M and an amine (oleylamine) in a concentration of at least 0.5 M, metallic Cu nanoparticles can be obtained by the method according to the present invention even if the reaction is performed without a protective atmosphere.

[0127] TEM images confirmed that lowering the concentration of the copper formate leads to larger copper nanoparticles.

[0128] TEM images also confirmed that the ratio of the concentration of the stabilizer over the concentration of the copper precursor influences the size of the nanoparticles : a higher ratio results in smaller copper nanoparticles.

TABLE-US-00001 TABLE 1 Concentration Ratio concentration oleylamine/ copper formate concentration copper formate Example (M) ([oleylamine]:[copper formate]) A1 0.157M 1:1 A2 0.157M 2:1 A3 0.157M 3:1 B1 0.315M 1:1 B2 0.315M 2:1 B3 0.315M 3:1 C1 0.630M 1:1 C2 0.630M 2:1 C3 0.630M 3:1

Second Series of Examples

[0129] In a second series of examples copper nanoparticles (nanocrystals) were prepared following the method of example 1 using copper formate as copper precursor and using different stabilizers in a 2:1 ratio with the precursor : octylamine (example D), di-octylamine (example E), dodecylamine (example F), hexadecylamine (example G) as indicated in Table 2. Details of examples 2 to 5 are summarized in Table 2. For all examples the temperature T2 is equal to T1+20° C. The stabilizers of examples D to G were all suitable to obtain copper nanoparticles.

TABLE-US-00002 TABLE 2 concentration Cu.sup.0 copper formate nanoparticle (M) Stabilizer T1 formation? Example D 0.630M Octylamine 145° C. Yes 8-Am Example E 0.630M Dioctylamine 155° C. Yes D-8-Am Example F 0.630M Dodecylamine 136° C. Yes 12-Am Example G 0.630M Hexadecylamine 136° C. Yes 16-Am

Third Series of Examples

[0130] Copper nanoparticles (nanocrystals) were prepared following the method of example 1 using oleylamine as stabilizer but using different copper precursors: copper acetate (example H), copper acetylacetonate (example I) as indicated in Table 3. Oleylamine is present in a 2:1 ratio with the precursor. Table 3 further specifies the temperature T1 for the different examples. For all examples the temperature T2 is equal to T1+20° C. The copper precursor of example H was suitable to obtain copper nanoparticles. Copper nanoparticles could not be obtained using the copper precursor of example I.

TABLE-US-00003 TABLE 3 Cu.sup.0 Copper nanoparticle precursor Stabilizer T1 formation? Example H Copper Oleylamine 185° C. Yes acetate Example I Copper Oleylamine / No acetylacetonate

Fourth Series of Examples

[0131] Copper nanoparticles (nanocrystals) were prepared as described in the first series of examples but in larger concentrations and volumes. A 1 M solution of copper formate was prepared mixing the copper precursor with 32.9 mL of oleylamine (1 M) and 67.1 mL of dodecane. The temperature of the solution was raised following the same temperature scheme described in the first series of examples but with an average increase of the temperature of 5° C. per minute. The average diameter of the nanoparticles obtained was 120 nm.

[0132] The particles obtained by this method were washed twice by using a mixture of toluene and ethanol and centrifuged at 2000 rpm for 5 minutes. The resulting dry powder of Cu nanoparticles (50% in weight) were mixed with diacetone alcohol as solvent and Byk-333 (0.3%) and Disperbyk-180 (4%) as additives and sonicated for 1 hour. The resulting ink was deposited using a wire bar and sintered in a nitrogen-filled oven at 500° C. for 1 hour. The obtained copper films had a resistivity of 2.57 μΩ cm, corresponding to 66% of bulk Cu conductivity.

Fifth Series of Examples

[0133] Copper nanoparticles (nanocrystals) were prepared as described in the fourth series of examples but in even larger volume. A 1 M solution of copper formate was prepared mixing the copper precursor with 329 mL of oleylamine (1 M) and 671 mL of dodecane. The temperature of the solution was raised following the same temperature scheme described in the first series of examples but with an average increase of the temperature of 2.5° C. per minute. The average diameter of the nanoparticles obtained was 240 nm.