U.S. patent application number 12/829658 was filed with the patent office on 2010-10-28 for production of metal nanoparticles.
This patent application is currently assigned to CABOT CORPORATION. Invention is credited to James Caruso, Chuck Edwards, Mark J. Hampden-Smith, Scott Thomas Haubrich, Anthony R. James, Hyungrak Kim, Toivo T. Kodas, Mark H. Kowalski, Klaus Kunze, Allen B. Schult, Aaron D. Stump, Karel Vanheusden.
Application Number | 20100269634 12/829658 |
Document ID | / |
Family ID | 36678236 |
Filed Date | 2010-10-28 |
United States Patent
Application |
20100269634 |
Kind Code |
A1 |
Vanheusden; Karel ; et
al. |
October 28, 2010 |
PRODUCTION OF METAL NANOPARTICLES
Abstract
A process for the production of metal nanoparticles. The process
comprises a rapid mixing of a solution of at least about 0.1 mole
of a metal compound that is capable of being reduced to a metal by
a polyol with a heated solution of a polyol and a substance that is
capable of being adsorbed on the nanoparticles.
Inventors: |
Vanheusden; Karel; (Los
Altos, CA) ; Kunze; Klaus; (Manchester, NM) ;
Kim; Hyungrak; (Albuquerque, NM) ; Stump; Aaron
D.; (Albuquerque, NM) ; Schult; Allen B.;
(Albuquerque, NM) ; Hampden-Smith; Mark J.;
(Albuquerque, NM) ; Edwards; Chuck; (Albuquerque,
NM) ; James; Anthony R.; (Rio Rancho, NM) ;
Caruso; James; (Albuquerque, NM) ; Kodas; Toivo
T.; (Albuquerque, NM) ; Haubrich; Scott Thomas;
(Albuquerque, NM) ; Kowalski; Mark H.;
(Albuquerque, NM) |
Correspondence
Address: |
Patent Administrator;Cabot Corporation
Two Seaport Lane, Suite 1300
Boston
MA
02210-2019
US
|
Assignee: |
CABOT CORPORATION
Boston
MA
|
Family ID: |
36678236 |
Appl. No.: |
12/829658 |
Filed: |
July 2, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11331230 |
Jan 13, 2006 |
7749299 |
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12829658 |
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60643577 |
Jan 14, 2005 |
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60643629 |
Jan 14, 2005 |
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60643578 |
Jan 14, 2005 |
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Current U.S.
Class: |
75/343 |
Current CPC
Class: |
B22F 1/0022 20130101;
Y10S 977/896 20130101; Y02P 10/20 20151101; C22B 23/0453 20130101;
B22F 9/24 20130101; B22F 1/0018 20130101; B22F 2998/10 20130101;
C09D 11/30 20130101; C09D 11/38 20130101; C22B 3/20 20130101; C09D
11/101 20130101; H01B 1/22 20130101; B82Y 30/00 20130101; H01L
21/288 20130101; Y02P 10/234 20151101; H05K 1/097 20130101; C22B
11/04 20130101; H05K 2203/013 20130101; H05K 3/125 20130101; Y10S
977/777 20130101; B22F 2998/10 20130101; B22F 9/24 20130101; B22F
1/0022 20130101; B22F 1/0018 20130101 |
Class at
Publication: |
75/343 |
International
Class: |
B22F 1/00 20060101
B22F001/00 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
[0002] This invention was made with Government support under
Agreement No. MDS972-93-2-0014 or DAAD1919-02-3-0001 awarded by the
Army Research Laboratory ("ARL"). The Government has certain rights
in the invention.
Claims
1-271. (canceled)
272. A process for purifying a nanoparticle dispersion, comprising
the step of: filtering a nanoparticle dispersion comprising
nanoparticles, a liquid phase and impurities and/or contaminants
through a membrane having a pore size of from about 0.01 .mu.m to
about 1 .mu.m and/or a lumen of from about 0.1 mm to about 5 mm,
wherein the membrane retains substantially all the nanoparticles in
the liquid phase of the nanoparticle dispersion passes through the
membrane.
273. The process of claim 272, wherein the nanoparticles comprise
metal nanoparticles.
274. The process of claim 273, wherein the metal nanoparticles
comprise a metal selected from the group consisting of gold,
silver, copper, nickel, cobalt, palladium, platinum, iridium,
osmium, rhodium, ruthenium, rhenium, vanadium, chromium, manganese,
niobium, molybdenum, tungsten, tantalum, iron and cadmium.
275. The process of claim 272, wherein at least 70% of impurities
and/or contaminants in the liquid phase of the nanoparticle
suspension pass through the membrane.
276. The process of claim 272, wherein at least 80% of impurities
and/or contaminants in the liquid phase of the nanoparticle
suspension pass through the membrane.
277. The process of claim 272, wherein at least 90% of impurities
and/or contaminants in the liquid phase of the nanoparticle
suspension pass through the membrane.
278. The process of claim 272, wherein at least 95% of impurities
and/or contaminants in the liquid phase of the nanoparticle
suspension pass through the membrane.
279. The process of claim 272, further comprising the step of
washing the membrane retaining substantially all of the
nanoparticles with a washing liquid to remove impurities and/or
contaminants from the surfaces of the nanoparticles.
280. The process of claim 279, wherein the step of washing is
repeated more than once.
281. The process of claim 279, wherein the washing liquid is water
and/or an organic solvent.
282. The process of claim 279, further comprising the step of
removing the washing liquid.
283. The process of claim 282, further comprising the step of
drying the metal nanoparticles.
284. The process of claim 283, wherein the dried nanoparticles are
capable of being redispersed in a liquid phase.
285. The process of claim 272, wherein the nanoparticles have an
average particle size of about 10 nm to about 80 nm.
286. The process of claim 272, wherein the membrane has a pore size
of from about 0.01 .mu.m to about 1 .mu.m.
287. The process of claim 272, wherein the membrane comprises a
polymeric material.
288. The process of claim 287, wherein the polymeric material
comprises at least one of a polysulfone, a polyethersulfone, a
sulfonated polysulfone, a polyamide, and a cellulose ester.
289. The process of claim 272, wherein the membrane comprises a
ceramic material.
290. The process of claim 289, wherein the ceramic material
comprises an oxide of at least one of titanium, zirconium, silicon
and aluminum.
291. The process of claim 272, wherein the membrane has a molecular
weight cutoff in the range of from about 10,000 to about
1,000,000.
292. The process of claim 272, further comprising the step of
treating the surfaces of the nanoparticles of the nanoparticle
dispersion with an absorptive substance prior to the step of
filtering.
293. The process of claim 272, further comprising the step of
increasing the concentration of the nanoparticles in the
nanoparticle dispersion prior to the filtering step.
294. The process of claim 293, wherein the step of increasing the
concentration of the nanoparticles is performed by drawing the
nanoparticle dispersion though a membrane.
295. The process of claim 272, wherein the nanoparticles are
produced by rapid mixing of a solution of at least about 0.1 mole
of a metal compound that is capable of being reduced to a metal by
a polyol with a heated solution that comprises a polyol and a
substance that is capable of being adsorbed on the
nanoparticles.
296. The process of claim 295, further comprising the step of
precipitating the nanoparticles in the nanoparticle dispersion
prior to the filtering step.
297. The process of claim 272, wherein the nanoparticle dispersion
further comprises one or more adhesion promoters and/or
humectants.
298. The process of claim 272, further comprising the step of
creating a printing formulation, ink or paste from the filtered
nanoparticle dispersion.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 11/331,230, filed Jan. 13, 2006, which claims
the benefit of U.S. Provisional Application Ser. Nos. 60/643,577;
60/643,629; and 60/643,578, all filed on Jan. 14, 2005, the
entireties of which are all incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0003] 1. Field of the Invention
[0004] The present invention relates to a process for the
production of metal nanoparticles. In particular, it relates to a
process which affords better control of the size, size distribution
and/or shape of the particles than the so-called polyol
process.
[0005] 2. Discussion of Background Information
[0006] The production of metal particles by the polyol process is
known from, e.g., U.S. Pat. No. 4,539,041 to Figlarz et al., the
entire disclosure whereof is expressly incorporated by reference
herein. In the polyol process, a metal compound is reduced at an
elevated temperature by a polyol to afford the corresponding metal
in the form of particles (usually in the micron and nanometer size
range). A number of metal compounds and in particular, a number of
transition metal compounds can be converted to metal particles by
this process. In a typical procedure, a solid metal compound is
suspended in a polyol and the suspension is heated until the
reduction of the metal compound is substantially complete.
Thereafter, the formed particles are isolated by separating them
from the liquid phase, e.g., by centrifugation.
[0007] A modification of this method is described in, e.g., P.-Y.
Silvert et al., "Preparation of colloidal silver dispersions by the
polyol process" Part 1--Synthesis and characterization, J. Mater.
Chem., 1996, 6(4), 573-577; and Part 2--Mechanism of particle
formation, J. Mater. Chem., 1997, 7(2), 293-299. According to the
Silvert et al. articles, the entire disclosures whereof are
expressly incorporated by reference herein, the polyol process is
carried out in the presence of a polymer, i.e.,
polyvinylpyrrolidone (PVP). In particular, the PVP is dissolved in
the polyol and helps to control the size and the dispersity of the
metal particles. In a typical experiment, about 10 g of PVP was
dissolved at room temperature in 75 ml of ethylene glycol and 2.4
mmole (400 mg) of silver nitrate was added to this solution. The
resultant suspension was stirred at room temperature until the
silver nitrate had dissolved completely, whereafter the system was
heated to 120.degree. C. and the reaction was conducted at this
temperature for several hours. After cooling and dilution with
water, the reaction mixture afforded silver particles having a mean
particle size of 21 nm with a standard deviation of 16%.
[0008] While the reported results are desirable, the present
inventors have found that when the modified polyol process is
scaled up and conducted with a significantly larger amount of metal
compound such as, e.g., 0.1 mole of metal compound or more, in
order to produce metal nanoparticles in commercially significant
amounts, the size and shape of the particles becomes non-uniform
and the formation of large chunks, needle-like particles and the
like is observed in addition to the formation of sphere-like
particles. Accordingly, it would be advantageous to have available
a process of the type described by Silvert et al. which affords
satisfactory results in terms of particle size, shape and/or size
distribution even when it is conducted on a significantly larger
scale than that reported by Silvert et al. Corresponding
nanoparticles would be useful in variety of applications. For
example, there exists a need for compositions for the fabrication
of electrically conductive features for use in electronics,
displays, and other applications. Accordingly, nanoparticles
produced by a process that affords commercially significant amounts
of substantially non-agglomerated, redispersible metal
nanoparticles with a substantially uniform shape and size could,
for example, be used for the manufacture of printing inks and in
particular, into inks for the printing of electrically conductive
features.
SUMMARY OF THE INVENTION
[0009] The present invention provides an improved polyol process
for the production of metal nanoparticles. This process comprises
the rapid mixing of a solution of at least about 0.1 mole of a
metal compound that is capable of being reduced to a metal by a
polyol with a heated solution of a polyol and a substance that is
capable of being adsorbed on the nanoparticles.
[0010] The present invention also provides a process for the
production of metal nanoparticles which comprises a rapid mixing of
a solution of at least about 0.25 mole of a compound of at least
one metal selected from gold, silver, rhodium, palladium, platinum,
copper, nickel and cobalt with a heated solution that comprises a
polyol and a polymer that is capable of substantially preventing an
agglomeration of the nanoparticles.
[0011] The present invention further provides a process for the
production of silver nanoparticles which comprises a rapid mixing
of a solution of at least about 0.5 mole of a silver compound with
a heated solution which comprises a polyol and a vinyl pyrrolidone
polymer.
[0012] The present invention further provides a process for the
production of silver nanoparticles which comprises a one-shot
addition of a solution of at least about 0.75 mole of a silver
compound in ethylene glycol and/or propylene glycol to a heated
solution of polyvinylpyrrolidone in ethylene glycol and/or
propylene glycol. The solution of the silver compound is at a
temperature of not higher than about 30.degree. C., the
polyvinylpyrrolidone solution is at a temperature of at least about
110.degree. C. The total volume of the solution of the silver
compound and the polyvinylpyrrolidone solution is from about 3 to
about 4 liters per one mole of the silver compound and the volume
ratio of the polyvinylpyrrolidone solution and the solution of the
silver compound is from about 4:1 to about 6:1. The molar ratio of
vinyl pyrrolidone units in the polyvinylpyrrolidone and the silver
compound is from about 5:1 to about 15:1.
[0013] The present invention also provides a process for the
production of metal nanoparticles which affords at least about 50 g
of substantially non-agglomerated metal nanoparticles in a single
run.
[0014] The present invention further provides a plurality of
substantially non-agglomerated metal nanoparticles which are
obtainable by the processes of the present invention, as well as a
dispersion of these nanoparticles which has a metal content of at
least about 50 g/L and comprises the nanoparticles in a
substantially non-agglomerated state.
[0015] The present invention also provides process for producing an
ink for ink-jet printing, which process comprises combining these
nanoparticles with a liquid vehicle.
[0016] The present invention also provides a composition which is
suitable for the fabrication of an electrically conductive feature
by using a direct-write tool. This composition comprises (a) the
above metal nanoparticles and (b) a vehicle that is capable of
forming a dispersion with the metal nanoparticles.
[0017] The present invention also provides for the use of a
composition for the fabrication of an electrically conductive
feature, wherein the composition comprises (a) metal nanoparticles
and (b) a vehicle that is capable of forming a dispersion with the
metal nanoparticles.
[0018] The present invention also provides a composition for the
fabrication of a conductive feature by ink-jet printing. This
composition comprises (a) at least about 5 weight percent of silver
nanoparticles which are obtainable by one of the processes of the
present invention and (b) a vehicle which comprises one or more
organic solvents. The composition has a surface tension at
20.degree. C. of from about 20 dynes/cm to about 40 dynes/cm and a
viscosity at 20.degree. C. of from about 5 cP to about 15 cP.
[0019] The present invention also provides a composition for
ink-jet printing. The composition comprises metal nanoparticles
metal particles which are obtainable by one of the processes of the
present invention, and is capable of being deposited in not more
than two passes of an ink-jet printing head on a substrate as a
line that can be rendered electrically conductive.
[0020] The present invention further comprises a composition for
providing a substrate with a metal structure. The composition
comprises (a) at least about 10 weight percent of silver
nanoparticles which are obtainable by one of the processes of the
present invention and have polyvinylpyrrolidone thereon and (b) a
vehicle which comprises an organic solvent that is capable of
dissolving polyvinylpyrrolidone. The composition has a surface
tension at 20.degree. C. of not more than about 50 dynes/cm and a
viscosity at 20.degree. C. of not higher than about 30 cP.
[0021] The present invention also provides a method for the
fabrication of a conductive feature on a substrate, which method
comprises forming the feature by applying a composition of the
present invention to the substrate and subjecting the feature to
heat, pressure and/or radiation to render it conductive. The
present invention also provides a conductive feature made by this
process.
DETAILED DESCRIPTION OF THE PRESENT INVENTION
[0022] The particulars shown herein are by way of example and for
purposes of illustrative discussion of the embodiments of the
present invention only and are presented in the cause of providing
what is believed to be the most useful and readily understood
description of the principles and conceptual aspects of the present
invention. In this regard, no attempt is made to set forth the
present invention in more detail than is necessary for the
fundamental understanding of the present invention, the description
making apparent to those skilled in the art how the several forms
of the present invention may be embodied in practice.
[0023] According to the process of the present invention, a
solution of at least about 0.1 mole of a metal compound that is
capable of being reduced to a metal by a polyol (or other
reductant) is rapidly mixed with a heated solution of a polyol and
a substance that is capable of being adsorbed on the nanoparticles.
It has been found that the control of the size and/or the size
distribution and/or the shape of the nanoparticles formed from the
metal compound can be significantly improved when substantially the
entire metal compound is employed in dissolved form and contacted
with the heated polyol within a relatively short period such as,
e.g., within seconds. Compared to the standard polyol process with
its gradual dissolution/reaction of a solid metal compound in a
polyol, the advantages associated with the way of contacting the
metal compound with the polyol according to the present invention
are particularly pronounced when the amount of metal compound is
relatively large, i.e., where the required volume of the liquid
phase and/or the relatively high amount of metal compound per
volume of the liquid phase makes it difficult, if not impossible,
to avoid local concentration gradients in the course of the
dissolution of the metal compound and/or to avoid inhomogeneous
reaction conditions. Accordingly, the advantages associated with
the process of the present invention in terms of, e.g., particle
size, particle size distribution, and/or particle shape in
comparison with the standard polyol process will usually become
particularly pronounced if the amount of employed metal compound is
about 0.1 mole or higher, e.g., at least about 0.25 mole, at least
about 0.5 mole, at least about 0.75 mole, or at least about 1 mole
and/or if the (initial) concentration of metal compound in the
reaction mixture is at least about 0.1 mole, e.g., at least about
0.2 mole, at least about 0.3 mole, or at least about 0.4 mole per
liter of reaction mixture.
Metal Compound
[0024] The metal compounds that may be used in the process of the
present invention include all metal compounds that a polyol can
reduce to the corresponding metal (oxidation state=0). Non-limiting
examples of such metals include main group metals such as, e.g.,
lead, tin, antimony and indium, and transition metals such as,
e.g., gold, silver, copper, nickel, cobalt, palladium, platinum,
iridium, osmium, rhodium, ruthenium, rhenium, vanadium, chromium,
manganese, niobium, molybdenum, tungsten, tantalum, iron and
cadmium. Examples of preferred metals include gold, silver, copper
and nickel, in particular, silver, copper and nickel. Silver is a
particularly preferred metal for the purposes of the present
invention.
[0025] Since the metal compound is to be employed in dissolved form
it should be soluble to at least some extent in at least one
solvent, preferably a polyol and/or a solvent that is substantially
miscible with the heated solution. Also, the metal compound will
usually be soluble to at least some extent in the polyol(s) of the
heated solution so that there is no substantial precipitation or
other separation of the metal compound from the liquid phase when
the solution of the metal compound is contacted with the heated
solution.
[0026] Non-limiting examples of suitable metal compounds include
metal oxides, metal hydroxides (including hydrated oxides), metal
salts of inorganic and organic acids such as, e.g., nitrates,
nitrites, sulfates, halides (e.g., fluorides, chlorides, bromides
and iodides), carbonates, phosphates, azides, borates (including
fluoroborates, pyrazolylborates, etc.), sulfonates, carboxylates
(such as, e.g., formates, acetates, propionates, oxalates and
citrates), substituted carboxylates (including halogenocarboxylates
such as, e.g., trifluoroacetates, hydroxycarboxylates,
aminocarboxylates, etc.) and salts and acids wherein the metal is
part of an anion (such as, e.g., hexachloroplatinates,
tetrachloroaurate, tungstates and the corresponding acids).
[0027] Further non-limiting examples of suitable metal compounds
for the process of the present invention include alkoxides, complex
compounds (e.g., complex salts) of metals such as, e.g.,
beta-diketonates (e.g., acetylacetonates), complexes with amines,
N-heterocyclic compounds (e.g., pyrrole, aziridine, indole,
piperidine, morpholine, pyridine, imidazole, piperazine, triazoles,
and substituted derivatives thereof), aminoalcohols (e.g.,
ethanolamine, etc.), amino acids (e.g., glycine, etc.), amides
(e.g., formamides, acetamides, etc.), and nitriles (e.g.,
acetonitrile, etc.). Non-limiting examples of preferred metal
compounds include nitrates, formates, acetates, trifluoroacetates,
propionates, oxalates and citrates, particularly nitrates and
acetates. Especially for applications of the metal nanoparticles
wherein a high electrical conductivity is a desired property, the
metal compound is preferably such that the reduction by-product is
volatile and/or can be decomposed into a volatile by-product at a
relatively low temperature. By way of non-limiting example, the
reduction of a metal nitrate will usually result in the formation
of nitrogen oxide gases as the only by-products.
[0028] Non-limiting examples of specific metal compounds for use in
the process of the present invention include silver nitrate, silver
nitrite, silver oxide, silver fluoride, silver hydrogen fluoride,
silver carbonate, silver oxalate, silver azide, silver
tetrafluoroborate, silver acetate, silver propionate, silver
butanoate, silver ethylbutanoate, silver pivalate, silver
cyclohexanebutanoate, silver ethylhexanoate, silver neodecanoate,
silver decanoate, silver trifluoroacetate, silver
pentafluoropropionate, silver heptafluorobutyrate, silver
trichloroacetate, silver
6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedionate, silver
lactate, silver citrate, silver glycolate, silver glyconate, silver
benzoate, silver salicylate, silver phenylacetate, silver
nitrophenylacetate, silver dinitrophenylacetate, silver
difluorophenylacetate, silver 2-fluoro-5-nitrobenzoate, silver
acetylacetonate, silver hexafluoroacetylacetonate, silver
trifluoroacetylacetonate, silver tosylate, silver triflate, silver
trispyrazolylborate, silver tris(dimethylpyrazolyl)borate, silver
amine complexes, trialkylphosphine and triarylphosphine derivatives
of silver carboxylates, silver beta-diketonates, silver
beta-diketonate olefin complexes and silver cyclopentadienides;
nickel oxide, nickel hydroxide, nickel chloride, nickel nitrate,
nickel sulfate, nickel amine complexes, nickel tetrafluoroborate,
nickel oxalate, nickel isopropoxide, nickel methoxyethoxide, nickel
acetylacetonate, nickel formate, nickel acetate, nickel octanoate,
nickel ethylhexanoate, and nickel trifluoroacetate; platinum
formate, platinum acetate, platinum propionate, platinum carbonate,
platinum nitrate, platinum perchlorate, platinum benzoate, platinum
neodecanoate, platinum oxalate, ammonium hexafluoroplatinate,
ammonium tetrachloroplatinate, sodium hexafluoroplatinate,
potassium hexafluoroplatinate, sodium tetrachloroplatinate,
potassium hexabromoplatinate, hexachloroplatinic acid,
hexabromoplatinic acid, dihydrogen hexahydroxoplatinate, diamine
platinum chloride, tetraamine platinum chloride, tetraamine
platinum hydroxide, tetraamine platinum tetrachloroplatinate,
platinum(II) 2,4-pentanedionate, diplatinum
trisdibenzylideneacetonate, platinum sulfate and platinum
divinyltetramethyldisiloxane; gold(III) acetate, gold(III)
chloride, tetrachloroauric acid, gold azide, gold isocyanide, gold
acetoacetate, imidazole gold ethylhexanoate and gold hydroxide
acetate isobutyrate; palladium acetate, palladium propionate,
palladium ethylhexanoate, palladium neodecanoate, palladium
trifluoracetate, palladium oxalate, palladium nitrate, palladium
chloride, tetraamine palladium hydroxide, tetraamine palladium
nitrate and tetraamine palladium tetrachloropalladate; copper
oxide, copper hydroxide, copper nitrate, copper sulfate, copper
chloride, copper formate, copper acetate, copper neodecanoate,
copper ethylhexanoate, copper methacrylate, copper
trifluoroacetate, copper acetoacetate and copper
hexafluoroacetylacetonate; cobalt oxide, cobalt hydroxide, cobalt
chloride and cobalt sulfate; ruthenium(III) chloride,
ruthenium(III) acetylacetonate, ruthenium(III) acetate, ruthenium
carbonyl complexes, ruthenium perchlorate, and ruthenium amine
complexes; rhodium(III) chloride, rhenium(II) chloride, tin(II)
oxide, iron(II) acetate, sodium tungstate and tungstic acid. The
above compounds may be employed as such or optionally in the form
of solvates and the like such as, e.g., as hydrates.
[0029] Examples of preferred metal compounds for use in the present
invention include silver nitrate, silver acetate, silver
trifluoroacetate, silver oxide, copper oxide, copper hydroxide,
copper sulfate, nickel oxide, nickel hydroxide, nickel chloride,
nickel sulfate, nickel acetate, cobalt oxide, cobalt hydroxide,
cobalt chloride and cobalt sulfate.
[0030] The use of mixtures of different compounds, e.g., different
salts, of the same metal and/or the use of mixtures of compounds of
different metals and/or of mixed metal compounds (e.g., mixed salts
and/or mixed oxides) are also contemplated by the present
invention. Accordingly, the term "metal compound" as used herein
includes both a single metal compound and any mixture of two or
more metal compounds. Depending, inter alia, on the metal compounds
and reaction conditions employed, the use of more than one metal in
the process of the present invention will result in a mixture of
nanoparticles of different metals and/or in nanoparticles which
comprise different metals in the same nanoparticle, for example, in
the form of an alloy or a mixture of these metals. Non-limiting
examples of alloys include Ag/Ni, Ag/Cu, Pt/Cu, Ru/Pt, Ir/Pt and
Ag/Co.
Polyol
[0031] The polyol for use in the present invention may be a single
polyol or a mixture of two or more polyols (e.g., three, four or
five polyols). In the following description, whenever the term
"polyol" is used, this term is meant to include both a single
polyol and a mixture of two or more polyols. The polyol may have
any number of hydroxyl groups (but at least two) and carbon atoms.
Also, the polyol may comprise heteroatoms (such as, e.g., O and N),
not only in the form of hydroxyl groups, but also in the form of,
e.g., ether, ester, amine and amide groups and the like (for
example, it may be a polyester polyol, a polyether polyol, etc.).
Preferably, the polyol comprises from about 2 to about 6 hydroxy
groups (e.g., 2, 3 or 4 hydroxy groups). Also, the preferred polyol
comprises from 2 to about 12 carbon atoms, e.g., up to about 3, 4,
5 or 6 carbon atoms. A particularly preferred group of polyols for
use in the present invention are the (alkylene) glycols, i.e.,
compounds which comprise two hydroxyl groups bound to adjacent
(aliphatic or cycloaliphatic) carbon atoms. Usually these glycols
will comprise up to about 6 carbon atoms, e.g., 2, 3 or 4 carbon
atoms. Ethylene glycol, propylene glycol and the butylene glycols
are non-limiting examples of preferred glycols for use in the
present invention.
[0032] The polyglycols constitute another group of preferred
polyols for use in the present invention. Specific and preferred
examples thereof are compounds which comprise up to about 6
alkylene glycol units, e.g., up to 4 alkylene glycol units, such
as, e.g., diethylene glycol, triethylene glycol, tetraethylene
glycol, dipropylene glycol and tripropylene glycol.
[0033] Non-limiting examples of other specific compounds which may
advantageously be used as the or a polyol in the process of the
present invention include 1,3-propanediol, 1,3-butanediol,
1,4-butanediol, glycerol, trimethylolpropane, pentaerythritol,
triethanolamine and trihydroxymethylaminomethane.
[0034] Of course, it also is possible to use other polyols than
those mentioned above, either alone or in combination. For example,
sugars and sugar alcohols can form at least a part of the polyol
reactant of the process of the present invention. While polyols
that are solid or semi-solid at room temperature may be employed,
it is preferred that the employed polyol or at least the employed
mixture of polyols is liquid at room temperature, although this is
not mandatory. Further, it is also possible to use one or more
other reducing agents in conjunction with the polyol(s), for
example, in order to reduce the required reaction time and/or the
reaction temperature. For instance, the substance that is capable
of being adsorbed on the nanoparticles may exhibit a reducing
effect with respect to the metal compound. A non-limiting example
of such a substance is polyvinylpyrrolidone. Non-limiting examples
of other reducing agents which may be employed in accordance with
the present invention include hydrazine and derivatives thereof,
hydroxylamine and derivatives thereof, aldehydes such as, e.g.,
formaldehyde, hypophosphites, sulfites, tetrahydroborates (such as,
e.g., the tetrahydroborates of Li, Na, K), LiAlH.sub.4,
polyhydroxybenzenes such as, e.g., hydroquinone, alkyl-substituted
hydroquinones, catechols and pyrogallol; phenylenediamines and
derivatives thereof; aminophenols and derivatives thereof; ascorbic
acid and ascorbic acid ketals and other derivatives of ascorbic
acid; 3-pyrazolidone and derivatives thereof; hydroxytetronic acid,
hydroxytetronamide and derivatives thereof; bisnaphthols and
derivatives thereof; sulfonamidophenols and derivatives thereof;
and Li, Na and K.
Adsorptive Substance
[0035] One of the functions of the substance that is capable of
being adsorbed on the nanoparticles (hereafter frequently referred
to as "the adsorptive substance") will usually and preferably be to
help prevent a substantial agglomeration of the nanoparticles. Due
to their small size and the high surface energy associated
therewith, the metal nanoparticles exhibit a strong tendency to
agglomerate and form larger secondary particles (for example, soft
agglomerates). The adsorptive substance will shield (e.g.,
sterically and/or through charge effects) the nanoparticles from
each other to at least some extent and thereby substantially reduce
or prevent a direct contact between the individual nanoparticles.
The term "adsorbed" as used herein and in the appended claims
includes any kind of interaction between the adsorptive substance
and a nanoparticle surface (e.g., the metal atoms on the surface of
a nanoparticle) that manifests itself in an at least weak bond
between the adsorptive substance and the surface of a nanoparticle.
Preferably, the bond is strong enough for the
nanoparticle-adsorptive substance combination to withstand a
washing operation with a solvent for the adsorptive substance. In
other words, merely washing the nanoparticles with the solvent at
room temperature will preferably not remove more than minor amounts
(e.g., less than about 10%, less than about 5%, or less than about
1%) of that part of the adsorptive substance that is in direct
contact with (and (weakly) bonded to) the nanoparticle surface. Of
course, adsorptive substance that is not in direct contact with a
nanoparticle surface and is merely associated with the bulk of the
nanoparticles as a contaminant, i.e., without any significant
interaction with the nanoparticles, is preferably removable from
the nanoparticles by washing the latter with a solvent for the
adsorptive substance. Further, it is also preferred for the bond
between the adsorptive substance and nanoparticle to be not too
strong and, in particular, to not be a covalent bond.
[0036] While the adsorptive substance will usually be a single
substance or at least comprise substances of the same type, the
present invention also contemplates the use of two or more
different types of adsorptive substances. For example, a mixture of
two or more different low molecular weight compounds or a mixture
of two or more different polymers may be used, as well as a mixture
of one or more low molecular weight compounds and one or more
polymers. The terms "substance that is capable of being adsorbed on
the nanoparticles" and "adsorptive substance" as used herein
include all of these possibilities. One of skill in the art will
understand that volatile components of the mixture such as, e.g.,
the polyol and/or solvent may also have a tendency of being
adsorbed on the nanoparticle surface. These substances do not
qualify as "adsorptive substances" within the meaning of this term
as used herein.
[0037] The adsorptive substance should preferably be compatible
with the polyol in the heated solution, i.e., it preferably does
not react with the polyol to any significant extent, even at the
elevated temperatures that will often be employed in the process of
the present invention. If the heated solution does not comprise any
other solvent for the adsorptive substance, the substance should
also dissolve in the polyol to at least some extent. The adsorptive
substance will usually have a solubility at room temperature of at
least about 1 g per liter of solvent (including solvent mixtures),
e.g., at least about 5 g, at least about 10 g, or at least about 20
g per liter of solvent. Preferably, the adsorptive substance has a
solubility of at least about 100 g, e.g., at least about 200 g, or
at least about 300 g per liter of solvent.
[0038] A preferred and non-limiting example of an adsorptive
substance for use in the process of the present invention includes
a substance that is capable of electronically interacting with a
metal atom of a nanoparticle. Usually, a substance that is capable
of this type of interaction will comprise one or more atoms (e.g.,
at least two atoms) with one or more free electron pairs such as,
e.g., oxygen, nitrogen and sulfur. By way of non-limiting example,
the adsorptive substance may be capable of a dative interaction
with a metal atom on the surface of a nanoparticle and/or of
chelating the metal atom. Particularly preferred adsorptive
substances comprise one or two O and/or N atoms. The atoms with a
free electron pair will usually be present in the substance in the
form of a functional group such as, e.g., a hydroxy group, a
carbonyl group, an ether group and an amino group, or as a
constituent of a functional group that comprises one or more of
these groups as a structural element thereof. Non-limiting examples
of suitable functional groups include --COO--, --O--CO--O--,
--CO--O--CO--, --C--O--C--, --CONR--, --NR--CO--O--,
--NR.sup.1--CO--NR.sup.2--, --CO--NR--CO--, --SO.sub.2--NR-- and
--SO.sub.2--O--, wherein R, R.sup.1 and R.sup.2 each independently
represent hydrogen or an organic radical (e.g., an aliphatic or
aromatic, unsubstituted or substituted radical comprising from
about 1 to about 20 carbon atoms). Such functional groups may
comprise the above (and other) structural elements as part of a
cyclic structure (e.g., in the form of a cyclic ester, amide,
anhydride, imide, carbonate, urethane, urea, and the like).
[0039] In one aspect of the process of the present invention, the
adsorptive substance is or comprises a substance that is capable of
reducing the metal compound, i.e., in addition to the reduction by
the polyol used. A specific, non-limiting example of such a
substance is polyvinylpyrrolidone (PVP).
[0040] The adsorptive substance may comprise a low molecular weight
compound, preferably a low molecular weight organic compound, e.g.,
a compound having a molecular weight of not higher than about 500,
more preferably not higher than about 300, and/or may comprise an
oligomeric or polymeric compound having a (weight average)
molecular weight (in Daltons) of at least about 1,000, for example,
at least about 3,000, at least about 5,000, or at least about
8,000, but preferably not higher than about 500,000, e.g., not
higher than about 200,000, or not higher than about 100,000. Too
high a molecular weight may give rise to an undesirably high
viscosity of the solution at a desirable concentration of the
adsorptive substance and/or cause flocculation. Also, the most
desirable molecular weight may be dependent on the metal. By way of
non-limiting example, in the case of polyvinylpyrrolidone, a
particularly preferred weight average molecular weight is in the
range of from about 3,000 to about 60,000, in particular if the
metal comprises silver.
[0041] In general, it is preferred for the adsorptive substance to
have a total of at least about 10 atoms per molecule which are
selected from C, N and O, e.g., at least about 20 such atoms or at
least about 50 such atoms. More preferably, the adsorptive
substance has a total of at least about 100 C, N and O atoms per
molecule, e.g., at least about 200, at least about 300, or at least
about 400 C, N and O atoms per molecule. In the case of polymers
these numbers refer to the average per polymer molecule.
[0042] Non-limiting examples of the low molecular weight adsorptive
substance for use in the present invention include fatty acids, in
particular, fatty acids having at least about 8 carbon atoms.
Non-limiting examples of oligomers/polymers for use as the
adsorptive substance in the process of the present invention
include homo- and copolymers (including polymers such as, e.g.,
random copolymers, block copolymers and graft copolymers) which
comprise units of at least one monomer which comprises one or more
O atoms and/or one or more N atoms. A non-limiting class of
preferred polymers for use in the present invention is constituted
by polymers which comprise at least one monomer unit which includes
at least two atoms which are selected from O and N atoms.
Corresponding monomer units may, for example, comprise at least one
hydroxyl group, carbonyl group, ether linkage and/or amino group
and/or one or more structural elements of formula --COO--,
--O--CO--O--, --CO--O--CO--, --C--O--C--, --CONR--, --NR--CO--O--,
NR.sup.2--, --CO--NR--CO--, --SO.sub.2--NR-- and --SO.sub.2--O--,
wherein R, R.sup.1 and R.sup.2 each independently represent
hydrogen or an organic radical (e.g., an aliphatic or aromatic,
unsubstituted or substituted radical comprising from about 1 to
about 20 carbon atoms).
[0043] Non-limiting examples of corresponding polymers include
polymers which comprise one or more units derived from the
following groups of monomers:
(a) monoethylenically unsaturated carboxylic acids of from about 3
to about 8 carbon atoms and salts thereof. This group of monomers
includes, for example, acrylic acid, methacrylic acid,
dimethylacrylic acid, ethacrylic acid, maleic acid, citraconic
acid, methylenemalonic acid, allylacetic acid, vinylacetic acid,
crotonic acid, fumaric acid, mesaconic acid and itaconic acid. The
monomers of group (a) can be used either in the form of the free
carboxylic acids or in partially or completely neutralized form.
For the neutralization alkali metal bases, alkaline earth metal
bases, ammonia or amines, e.g., sodium hydroxide, potassium
hydroxide, sodium carbonate, potassium carbonate, sodium
bicarbonate, magnesium oxide, calcium hydroxide, calcium oxide,
ammonia, triethylamine, methanolamine, diethanolamine,
triethanolamine, morpholine, diethylenetriamine or
tetraethylenepentamine may, for example, be used; (b) the esters,
amides, anhydrides and nitriles of the carboxylic acids stated
under (a) such as, e.g., methyl acrylate, ethyl acrylate, methyl
methacrylate, ethyl methacrylate, n-butyl acrylate, hydroxyethyl
acrylate, 2- or 3-hydroxypropyl acrylate, 2- or 4-hydroxybutyl
acrylate, hydroxyethyl methacrylate, 2- or 3-hydroxypropyl
methacrylate, hydroxyisobutyl acrylate, hydroxyisobutyl
methacrylate, monomethyl maleate, dimethyl maleate, monoethyl
maleate, diethyl maleate, maleic anhydride, 2-ethylhexyl acrylate,
2-ethylhexyl methacrylate, acrylamide, methacrylamide,
N,N-dimethylacrylamide, N-tert-butylacrylamide, acrylonitrile,
methacrylonitrile, 2-dimethylaminoethyl acrylate,
2-dimethylaminoethyl methacrylate, 2-diethylaminoethyl acrylate,
2-diethylaminoethyl methacrylate and the salts of the
last-mentioned monomers with carboxylic acids or mineral acids and
the quaternized products; (c) acrylamidoglycolic acid,
vinylsulfonic acid, allylsulfonic acid, methallylsulfonic acid,
styrenesulfonic acid, 3-sulfopropyl acrylate, 3-sulfopropyl
methacrylate and acrylamidomethylpropanesulfonic acid and monomers
containing phosphonic acid groups, such as, e.g., vinyl phosphate,
allyl phosphate and acrylamidomethylpropanephosphonic acid; and
esters, amides and anhydrides of these acids; (d) N-vinyllactams
such as, e.g., N-vinylpyrrolidone, N-vinyl-2-piperidone and
N-vinylcaprolactam; (e) vinyl acetal, vinyl butyral, vinyl alcohol
and ethers and esters thereof (such as, e.g., vinyl acetate, vinyl
propionate and methylvinylether), allyl alcohol and ethers and
esters thereof; N-vinylimidazole, N-vinyl-2-methylimidazoline, and
the hydroxystyrenes.
[0044] Corresponding polymers may also contain additional monomer
units, for example, units derived from monomers without functional
group, halogenated monomers, aromatic monomers etc. Non-limiting
examples of such monomers include olefins such as, e.g., ethylene,
propylene, the butenes, pentenes, hexenes, octenes, decenes and
dodecenes, styrene, vinyl chloride, vinylidene chloride,
tetrafluoroethylene, etc. Further, the polymers for use as
adsorptive substance in the process of the present invention are
not limited to addition polymers, but also comprise other types of
polymers, for example, condensation polymers such as, e.g.,
polyesters, polyamides, polyurethanes and polyethers, as well as
polysaccharides such as, e.g., starch, cellulose and derivatives
thereof, etc.
[0045] Other non-limiting examples of polymers which are suitable
for use as adsorptive substance in the present invention are
disclosed in e.g., U.S. Patent Application Publication 2004/0182533
A1, the entire disclosure whereof is expressly incorporated by
reference herein.
[0046] Preferred polymers for use as adsorptive substance in the
present invention include those which comprise units derived from
one or more N-vinylcarboxamides of formula (I)
CH.sub.2.dbd.CH--NR.sup.3--CO--R.sup.4 (I)
wherein R.sup.3 and R.sup.4 independently represent hydrogen,
optionally substituted alkyl (including cycloalkyl) and optionally
substituted aryl (including alkaryl and aralkyl) or heteroaryl
(e.g., C.sub.6-20 aryl such as phenyl, benzyl, tolyl and phenethyl,
and C.sub.4-20 heteroaryl such as pyrrolyl, furyl, thienyl and
pyridinyl).
[0047] R.sup.3 and R.sup.4 may, e.g., independently represent
hydrogen or C.sub.1-12 alkyl, particularly C.sub.1-6 alkyl such as
methyl and ethyl. R.sup.3 and R.sup.4 together may also form a
straight or branched chain containing from about 2 to about 8,
preferably from about 3 to about 6, particularly preferably from
about 3 to about 5 carbon atoms, which chain links the N atom and
the C atom to which R.sup.3 and R.sup.4 are bound to form a ring
which preferably has about 4 to about 8 ring members. Optionally,
one or more carbon atoms may be replaced by heteroatoms such as,
e.g., oxygen, nitrogen or sulfur. Also optionally, the ring may
contain a carbon-carbon double bond.
[0048] Non-limiting specific examples of R.sup.3 and R.sup.4 are
methyl, ethyl, isopropyl, n-propyl, n-butyl, isobutyl, sec-butyl,
tert-butyl, n-hexyl, n-heptyl, 2-ethylhexyl, n-octyl, n-decyl,
n-undecyl, n-dodecyl, n-tetradecyl, n-hexadecyl, n-octadecyl and
n-eicosyl. Non-limiting specific examples of R.sup.3 and R.sup.4
which together form a chain are 1,2-ethylene, 1,2-propylene,
1,3-propylene, 2-methyl-1,3-propylene, 2-ethyl-1,3-propylene,
1,4-butylene, 1,5-pentylene, 2-methyl-1,5-pentylene, 1,6-hexylene
and 3-oxa-1,5-pentylene.
[0049] Non-limiting specific examples of N-vinylcarboxamides of
formula (I) are N-vinylformamide, N-vinylacetamide,
N-vinylpropionamide, N-vinylbutyramide, N-vinylisobutyramide,
N-vinyl-2-ethylhexanamide, N-vinyldecanamide, N-vinyldodecanamide,
N-vinylstearamide, N-methyl-N-vinylformamide,
N-methyl-N-vinylacetamide, N-methyl-N-vinylpropionamide,
N-methyl-N-vinylbutyramide, N-methyl-N-vinylisobutyramide,
N-methyl-N-vinyl-2-ethylhexanamide, N-methyl-N-vinyldecanamide,
N-methyl-N-vinyldodecanamide, N-methyl-N-vinylstearamide,
N-ethyl-N-vinylformamide, N-ethyl-N-vinylacetamide,
N-ethyl-N-vinylpropionamide, N-ethyl-N-vinylbutyramide,
N-ethyl-N-vinylisobutyramide, N-ethyl-N-vinyl-2-ethylhexanamide,
N-ethyl-N-vinyldecanamide, N-ethyl-N-vinyldodecanamide,
N-ethyl-N-vinylstearamide, N-isopropyl-N-vinylformamide,
N-isopropyl-N-vinylacetamide, N-isopropyl-N-vinylpropionamide,
N-isopropyl-N-vinylbutyramide, N-isopropyl-N-vinylisobutyramide,
N-isopropyl-N-vinyl-2-ethylhexanamide,
N-isopropyl-N-vinyldecanamide, N-isopropyl-N-vinyldodecanamide,
N-isopropyl-N-vinylstearamide, N-n-butyl-N-vinylformamide,
N-n-butyl-N-vinylacetamide, N-n-butyl-N-vinylpropionamide,
N-n-butyl-N-vinylbutyramide, N-n-butyl-N-vinylisobutyramide,
N-n-butyl-N-vinyl-2-ethylhexanamide, N-n-butyl-N-vinyldecanamide,
N-n-butyl-N-vinyldodecanamide, N-n-butyl-N-vinylstearamide,
N-vinylpyrrolidone, N-vinyl-2-piperidone and
N-vinylcaprolactam.
[0050] Particularly preferred polymers for use in the present
invention include polymers which comprise monomer units of one or
more unsubstituted or substituted N-vinyllactams, preferably those
having from about 4 to about 8 ring members such as, e.g.,
N-vinylcaprolactam, N-vinyl-2-piperidone and N-vinylpyrrolidone.
These polymers include homo- and copolymers. In the case of
copolymers (including, for example, random, block and graft
copolymers), the N-vinyllactam (e.g., N-vinylpyrrolidone) units are
preferably present in an amount of at least about 10 mole-%, e.g.,
at least about 30 mole-%, at least about 50 mole-%, at least about
70 mole-%, at least about 80 mole-%, or at least about 90 mole-%.
By way of non-limiting example, the comonomers may comprise one or
more of those mentioned in the preceding paragraphs, including
monomers without functional group (e.g., ethylene, propylene,
styrene, etc.), halogenated monomers, etc.
[0051] If the vinyllactam (e.g., vinylpyrrolidone) monomers (or at
least a part thereof) carry one or more substituents on the
heterocyclic ring, non-limiting examples of such substituents
include alkyl groups (for example, alkyl groups having from 1 to
about 12 carbon atoms, e.g., from 1 to about 6 carbon atoms such
as, e.g., methyl, ethyl, propyl and butyl), alkoxy groups (for
example, alkoxy groups having from 1 to about 12 carbon atoms,
e.g., from 1 to about 6 carbon atoms such as, e.g., methoxy,
ethoxy, propoxy and butoxy), halogen atoms (e.g., F, Cl and Br),
hydroxy, carboxy and amino groups (e.g., dialkylamino groups such
as dimethylamino and diethylamino) and any combinations of these
substituents.
[0052] Non-limiting specific examples of vinyllactam polymers for
use in the present invention include homo- and copolymers of
vinylpyrrolidone which are commercially available from, e.g.,
International Specialty Products (www.ispcorp.com). In particular,
these polymers include
(a) vinylpyrrolidone homopolymers such as, e.g., grades K-15 and
K-30 with K-value ranges of from 13-19 and 26-35, respectively,
corresponding to average molecular weights (determined by
GPC/MALLS) of about 10,000 and about 67,000; (b) alkylated
polyvinylpyrrolidones such as, e.g., those commercially available
under the trade mark GANEX.RTM. which are
vinylpyrrolidone-alpha-olefin copolymers that contain most of the
alpha-olefin (e.g., about 80% and more) grafted onto the
pyrrolidone ring, mainly in the 3-position thereof; the
alpha-olefins may comprise those having from about 4 to about 30
carbon atoms; the alpha-olefin content of these copolymers may, for
example, be from about 10% to about 80% by weight; (c)
vinylpyrrolidone-vinylacetate copolymers such as, e.g., random
copolymers produced by a free-radical polymerization of the
monomers in a molar ratio of from about 70/30 to about 30/70 and
having weight average molecular weights of from about 14,000 to
about 58,000; (d) vinylpyrrolidone-dimethylaminoethylmethacrylate
copolymers; (e) vinylpyrrolidone-methacrylamidopropyl
trimethylammonium chloride copolymers such as, e.g., those
commercially available under the trade mark GAFQUAT.RTM.; (f)
vinylpyrrolidone-vinylcaprolactam-dimethylaminoethylmethacrylate
terpolymers such as, e.g., those commercially available under the
trade mark GAFFIX.RTM.; (g) vinylpyrrolidone-styrene copolymers
such as, e.g., those commercially available under the trade mark
POLECTRON.RTM.; a specific example thereof is a graft emulsion
copolymer of about 70% vinylpyrrolidone and about 30% styrene
polymerized in the presence of an anionic surfactant; (h)
vinylpyrrolidone-acrylic acid copolymers such as, e.g., those
commercially available under the trade mark ACRYLIDONE.RTM. which
are produced in the molecular weight range of from about 80,000 to
about 250,000.
Solvent for Metal Compound
[0053] The solvent for the metal compound may be a single solvent
or a mixture of two or more solvents/diluents (collectively
referred to herein as "solvent" or "solvent for the metal
compound"). The solvent is preferably capable of dissolving a
significant amount of the metal compound at room temperature and/or
at the temperature that the solution of the metal compound is
intended to have when it is combined with the heated polyol
solution. Usually the solvent will dissolve the metal compound at
room temperature in an amount of at least about 1 g/l, e.g., at
least about 5 g/l, or at least about 10 g/l. Preferably, the metal
compound dissolves in the solvent in an amount of at least about 50
g/l, e.g., at least about 100 g/l, at least about 200 g/l, or at
least about 300 g/l. In this regard, it is to be appreciated that
one or more components of the solvent may be poor solvents for the
metal compound as long as the entirety of the solvent, i.e., all
components thereof, are capable of dissolving the metal compound to
the desired extent. The solvent for the metal compound should
preferably also be miscible with the polyol of the heated solution
to at least some extent. Also, the solvent for the metal compound
is preferably of high purity. This applies also to any other
liquids/solvents that are used in the process of the present
invention.
[0054] It is to be understood that the solution of the metal
compound may still contain some undissolved metal compound,
although this is not preferred. By way of non-limiting example, not
more than about 20 weight percent, e.g., not more than about 10
weight percent, or not more than about 5 weight percent, of the
metal compound may be present in undissolved form. Preferably, the
amount of undissolved metal compound is not higher than about 1
weight percent, even more preferred not higher than about 0.1
weight percent. Most preferably, the solution is substantially free
of undissolved metal compound (e.g., not higher than about 0.01
weight percent, or not higher than about 0.001 weight percent of
undissolved metal compound). If undissolved metal compound is
present, it is preferred for it to be dissolved substantially
immediately upon contact with the heated solution, for example,
within less than about one minute (e.g., less than about 30
seconds). Especially in cases where the temperature difference
between the solution of the metal compound and the heated solution
is large (e.g., larger than about 40.degree. C.)., it may be
particularly advantageous to employ a highly concentrated and in
particular, a substantially saturated solution of the metal
compound (preferably in a good solvent therefor) in order to keep
the temperature drop relative to the temperature of the heated
solution upon combining the solutions small.
[0055] In a preferred aspect of the method of the present
invention, the solvent for the metal compound is or at least
comprises one or more polyols, preferably the same polyol(s) that
is/are present in the heated solution. It is noted however, that
the use of one or more polyols for dissolving the metal compound is
not mandatory. Other solvents may be used as well, such as, e.g.,
protic and aprotic polar solvents. Non-limiting examples of such
solvents include aliphatic, cycloaliphatic and aromatic alcohols
(the term "alcohol" as used herein is used interchangeably with the
terms "monoalcohol" and "monohydric alcohol") such as, e.g.,
ethanol, propanol, butanol, pentanol, cyclopentanol, hexanol,
cyclohexanol, octanol, decanol, isodecanol, undecanol, dodecanol,
benzyl alcohol, butyl carbitol and the terpineols, ether alcohols
such as, e.g., the monoalkyl ethers of diols such as, e.g., the
C.sub.1-6 monoalkyl ethers of C.sub.1-6 alkanediols and
polyetherdiols derived therefrom (e.g., the monomethyl, monoethyl,
monopropyl and monobutyl ethers of ethylene glycol, diethylene
glycol, triethylene glycol, propylene glycol, dipropylene glycol,
1,3-propanediol, and 1,4-butanediol such as, e.g, 2-methoxyethanol,
2-ethoxyethanol, 2-propoxyethanol and 2-butoxyethanol),
aminoalcohols such as, e.g., ethanolamine, amides such as, e.g.,
dimethylformamide, dimethylacetamide 2-pyrrolidone and
N-methylpyrrolidone, esters such as, e.g., ethyl acetate and ethyl
formate, sulfoxides such as, e.g., dimethylsulfoxide, ethers such
as, e.g., tetrahydrofuran and tetrahydropyran, and water. These and
other suitable solvents may be used alone or as a mixture of two or
more thereof and/or in combination with one or more polyols. By the
same token, it is possible for the heated solution to comprise one
or more solvents in addition to the one or more polyols included
therein. By way of non-limiting example, the heated solution may
additionally comprise solvents such as those that may be present in
the solution of the metal compound. However, in the combined
solutions (solution of metal compound and heated solution) the
concentration of polyol(s) should be sufficiently high to bring
about a reduction of at least a substantial portion (and
preferably, substantially all) of the metal compound within a
commercially acceptable period of time (for example, within not
more than about 24 hours, preferably not more than about 12 hours
or not more than about 6 hours) when the mixture is heated to a
temperature that does not cause a substantial decomposition of any
of the components that are present in the mixture.
Temperature
[0056] The temperature of the heated solution is preferably at
least about 60.degree. C., for example, at least about 70.degree.
C., at least about 80.degree. C., at least about 85.degree. C., at
least about 90.degree. C., at least about 100.degree. C., at least
about 110.degree. C., or at least about 120.degree. C. On the other
hand, the temperature of the heated solution will usually be not
higher than about 180.degree. C., e.g., not higher than about
170.degree. C., not higher than about 160.degree. C., not higher
than about 150.degree. C., or not higher than about 140.degree. C.
The most suitable temperature of the heated solution is at least in
part determined by factors such as the boiling point of the
solvent(s) included therein (i.e., the boiling point of at least
the polyol), the thermal stability of the adsorptive substance, the
reactivities of the metal compound and the polyol, and the
temperature of the solution of the metal compound and the volume
thereof relative to the heated solution.
[0057] The temperature of the second solution used in the process
of the present invention, i.e., the solution of the metal compound,
will usually be not higher than that of the heated solution and
will frequently be not higher than about 50.degree. C., e.g., not
higher than about 40.degree. C., or not higher than about
30.degree. C. On the other hand, too low a temperature may increase
the viscosity of the solution and/or reduce the solubility of the
metal compound to an undesirable degree. Usually, the temperature
of the solution will be about room temperature. Particularly in
cases where the metal compound is dissolved in a solvent that
comprises one or more polyols which are capable of reducing the
metal compound, it is advantageous to keep the solvent at a
temperature at which the rate of the reaction between the metal
compound and the polyol is low in order to substantially prevent a
reduction of the metal compound by the polyol component while the
metal compound or at least a part thereof is still present in the
solid state. Even if all or almost all of the metal compound is
present in dissolved form, a substantial reaction thereof with the
polyol component in the absence of the adsorptive substance is
undesirable, wherefore the temperature of a polyol containing
solution of the metal compound prior to the combination thereof
with the heated solution should preferably be kept low, in
particular when highly reactive polyols and/or highly reactive
metal compounds are present. One of skill in the art will
appreciate that if the solution of the metal compound does not
comprise a substance (solvent) that will reduce or otherwise react
with the metal compound to any substantial extent even at an
elevated temperature, the temperature of the solution of the metal
compound may even be higher than that of the heated solution and
will preferably be close to (e.g., within about 10.degree. C.), for
example, about the same as the temperature of the heated solution
so that upon mixing these two solution there is substantially no
temperature change in the system.
[0058] In a preferred aspect of the process of the present
invention, the temperature of the solution of the metal compound
will be not substantially higher than about 40.degree. C. and the
heated solution will be at a temperature of at least about
80.degree. C.
Mixing
[0059] The rate at which the solution of the metal compound and the
heated solution are combined in the process of the present
invention is preferably as high as possible. By way of non-limiting
example, the two solutions will usually be completely combined
within not more than about 5 minutes, preferably within not more
than about 2 minutes, e.g., within not more than about 1 minute,
within not more than about 30 seconds, within not more than about
15 seconds, or within not more than about 5 seconds. Most
preferably, the solutions are combined virtually instantaneously,
such as by a one-shot addition of one of the solutions to the other
solution, e.g., by a one-shot addition of the solution of the metal
compound to the heated solution.
[0060] It is also preferred according to the present invention to
promote the complete mixing of the two solutions, for example, by
agitation such as, e.g., by (preferably vigorous) stirring, shaking
and/or sonication of the combined solutions.
Ratio and Total Volume of Solutions
[0061] The total volume of the solution of the metal compound and
the heated solution are not particularly limited. However, with
increasing amounts of the employed metal compound it will become
increasingly desirable to keep the total volume of the solutions
small so as to keep the volume of the reaction mixture and, thus
the required size of the reaction vessel as small as possible and
also to keep the amount of liquids that need to be
discarded/recycled after the reaction is completed at a minimum.
Accordingly, provided the solubility of the metal compound in the
selected liquid components of the reaction mixture is high enough,
the combined volume of the two solutions per one mole of employed
metal compound will usually be not larger than about 10 liters,
e.g., not larger than about 8 liters, not larger than about 6
liters, not larger than about 5 liters, or not larger than about 4
liters. On the other hand, for reasons of, inter alia, solubility,
the combined volume will usually be not smaller than about 1 liter,
e.g., not smaller than about 2 liters, or not smaller than about 3
liters per one mole of employed metal compound. Of course,
(considerably) smaller or larger combined volumes than those
indicated herein may sometimes be more desirable and/or
advantageous.
[0062] The volume ratio of the two solutions is not particularly
limited, either. The most desirable volume ratio is influenced by
several factors such as, e.g., the volume of solvent that is to
dissolve the metal compound, the temperature difference between the
solutions, and the desire to keep the total volume of the liquid
phase as low as possible (e.g., for the reasons stated above). In
many cases the volume ratio of the heated solution and the solution
of the metal compound will be not higher than about 10:1, e.g., not
higher than about 8:1, or not higher than about 6:1. On the other
hand, the volume ratio will often be not lower than about 1:1,
e.g., not lower than about 2:1, not lower than about 3:1, or not
lower than about 4:1. Of course, there may be situations where
lower or higher volume ratios than those indicated herein may be
more advantageous and/or desirable. Especially in cases where the
temperature difference between the solution of the metal compound
and the heated solution is large (e.g., higher than about
40.degree. C.), it will often be advantageous to use a relatively
high volume ratio in order to keep the temperature drop relative to
the temperature of the heated solution upon combining the solutions
small. This may be accomplished, for example, by employing a
concentrated (e.g., saturated) solution of the metal compound in a
good solvent therefor.
Ratio of Metal Compound and Adsorptive Substance
[0063] The most desirable ratio of the metal compound and the
adsorptive substance is a function of a variety of factors. In this
regard, it is to be appreciated that the adsorptive substance will
generally have multiple functions. These functions include, of
course, a substantial prevention of an agglomeration of the
nanoparticles and, as a result thereof, facilitating an isolation
of the nanoparticles from the reaction mixture, ensuring a
substantial redispersibility of the isolated nanoparticles and a
stabilization of dispersions comprising these nanoparticles.
Another function of the adsorptive substance usually comprises
assisting in the control of the size and shape of nanoparticles
during the reduction of the metal compound. For example, if the
amount of adsorptive substance is not sufficient to shield the
growing nanoparticles completely, the formation of particles with a
high aspect ratio such as, e.g., nanorods and/or nanowires and/or
irregularly shaped particles may be observed. Also, under otherwise
identical conditions, the average size of the formed nanoparticles
will usually decrease with increasing molar ratio of adsorptive
substance and metal compound. It has been found that under
otherwise identical conditions, the rapid mixing of the solution of
the metal compound and the heated solution according to the process
of the present invention allows to obtain substantially the same
results with respect to the control of the size, the size
distribution and/or the shape of the particles as the known method
with its gradual dissolution/reaction of the metal compound in the
presence of the adsorptive compound, but at a (substantially) lower
molar ratio of the adsorptive compound and the metal compound than
required in the known method. At any rate, the adsorptive substance
should be present in at least the amount that is sufficient to
substantially prevent an agglomeration of the nanoparticles. This
amount is at least in part dependent on the size of the metal cores
of the formed nanoparticles.
[0064] By way of non-limiting example, a batch of dry nanoparticles
will usually require a minimum of surface passivation or surface
coverage by the adsorptive substance in order to be redispersible.
A simple rule of thumb is that the smaller the particle size the
larger the surface area and thus, the more adsorptive substance is
required for complete coverage. Depending on the adsorptive
substance, one can make some simple assumptions regarding the
thickness of a monolayer of adsorptive substance that is adsorbed
on the surface of the metal cores of the nanoparticles. In
addition, one may also assume that a minimum of one monolayer of
adsorptive substance around a metal core is needed to allow for
complete dispersibility of dry particles. Usually, not more than
about 10 monolayers (and often not more than about 5 monolayers or
even not more than about 2 monolayers) of adsorptive substance are
needed to redisperse and stabilize a metal nanoparticle in
solution. With this simple model one may make a rough estimate of
the amount of adsorptive substance that is needed to (re)disperse
metal nanoparticles of any size. For example, for PVP as the
adsorptive substance, one may assume that the thickness of a
monolayer thereof is about 1 nm. Based on this model and using the
densities of Ag (10.5 g/cm.sup.3) and PVP (1.0 g/cm.sup.3) one can
calculate that for a PVP coated sphere-shaped silver core having a
diameter of 20 nm the minimum amount of PVP needed to disperse a
dry particle is about 3.2% by weight (one monolayer). Preferably,
not more than 10 monolayers or 41% by weight of PVP will be
present. Most preferably, about 4 to about 8 monolayers or about
14.8% to about 32.5% by weight of PVP will be used. For a 50 nm
PVP-coated sphere-shaped Ag core, a minimum of about 1.3% by weight
of PVP will be needed to cover the nanoparticle completely with a
monolayer. No more than about 14.8% by weight or 10 monolayers will
usually be needed. Most preferably about 5.3 to about 11.5% by
weight of PVP is used (for about 4 to about 8 monolayers).
[0065] If the adsorptive substance is or comprises a low molecular
weight compound (i.e., one or more low molecular weight compounds,
collectively referred to herein as a single compound), the molar
ratio of the low molecular weight compound and the metal in the
reaction mixture will often be at least about 3:1, e.g., at least
about 5:1, or at least about 10:1. While there is no particular
upper limit for this ratio, for practical reasons and reasons of
economic efficiency one will usually avoid a substantially higher
amount of adsorptive substance than the amount that is needed for
obtaining particles in the desired size range and/or for
substantially preventing an agglomeration of the nanoparticles.
[0066] If the adsorptive substance is or comprises a polymer (i.e.,
one or more polymers, collectively referred to herein as a single
polymer), the molar ratio in the reaction mixture of the monomer
units of the polymer (and preferably of only those monomer units
that are capable of being adsorbed on the nanoparticles), and the
metal will often be at least about 3:1, e.g., at least about 5:1,
at least about 6:1, at least about 8:1, or at least about 10:1.
However, for practical reasons (in particular in view of the
viscosity increasing effect of certain polymers) and for reasons of
economic efficiency (excess adsorptive substance, i.e., substance
that will not be adsorbed may have to be removed and
discarded/recycled later) this ratio will usually be not higher
than about 100:1, e.g., not higher than about 80:1, not higher than
about 50:1, not higher than about 30:1, or not higher than about
20:1.
Reaction Time and Temperature
[0067] The reaction between the metal compound and the polyol will
usually take some time (e.g., one or more hours) before a
substantial percentage of the employed metal compound has been
converted to metal nanoparticles. The reaction rate depends, inter
alia, on the temperature at which the mixed solutions are kept. It
will usually be advantageous to heat the mixed solutions to an
elevated temperature (if they are not at the desired temperature
already) and to keep them at this temperature for a sufficient
period to convert at least a substantial portion of, and preferably
substantially the entire metal compound (e.g., at least about 90%,
or at least about 95% thereof) to metal nanoparticles. The
temperature that is needed to achieve a desired degree of
conversion within a predetermined period of time depends, inter
alia, on the reactivities and concentrations of the reactants. Of
course, the reaction temperature should not be so high as to cause
a more than insignificant decomposition of the various components
of the reaction mixture (e.g., of the adsorptive substance). Also,
the temperature will usually be not significantly higher than the
boiling point of the lowest-boiling component of the reaction
mixture, although this is not mandatory, especially if the reaction
mixture is kept under a higher than atmospheric pressure, e.g., in
an autoclave. In many cases, the reaction mixture will be heated
to/kept at a temperature of at least about 80.degree. C., e.g., at
least about 90.degree. C., at least about 100.degree. C., at least
about 110.degree. C., or at least about 120.degree. C. On the other
hand, it will usually be advantageous for the temperature of the
reaction mixture to not exceed about 200.degree. C., e.g., to not
exceed about 180.degree. C., to not exceed about 160.degree. C., to
not exceed about 150.degree. C., or to not exceed about 140.degree.
C.
[0068] Especially (but not only) in cases where the volume of the
liquid phase is kept relatively small relative to the amount of
components dissolved therein and/or the reaction temperature is
relatively close to the boiling point of the liquid phase or a
component thereof, respectively, and no reflux is provided for, it
may be advantageous to add additional solvent, in particular, a
polyol to the reaction mixture.
[0069] One of skill in the art will understand that the process of
the present invention can be carried out batch-wise, and also
semi-continuously or continuously. By way of non-limiting example,
in the case of a continuous process, separate feeds of the solution
of the metal compound and the heated solution may be introduced
continuously into a constant temperature reactor. Reaction product
is continuously withdrawn at the same rate as the rate at which the
two feed streams are introduced into the reactor. Alternatively,
the feed streams may be premixed before they enter the reactor as a
single feed stream. The required residence time in the reactor can
be calculated on the basis of the reaction rate at the selected
temperature, the desired degree of conversion, etc.
Optional Further Processing
[0070] Once the desired degree of conversion of the metal compound
is achieved the reaction mixture is preferably cooled to room
temperature. Cooling can be accomplished in any suitable manner,
e.g., by cooling the reaction vessel with a cooling medium such as,
e.g., water (forced cooling). Further, the reaction mixture may
simply be allowed to cool to room temperature in the ambient
atmosphere.
[0071] Preferably after the cooling of the reaction mixture to room
temperature the formed nanoparticles may be separated from the
liquid phase of the reaction mixture. This can be accomplished, for
example, in the various ways of separating a solid from a liquid
that are known to those of skill in the art. Non-limiting examples
of suitable separation methods include filtration, centrifugation,
chromatographic methods, electrophoretic techniques, etc. Because
the nanoparticles have a very small mass, do not substantially
agglomerate and have adsorptive substance thereon that will usually
interact with the components of the liquid phase, the nanoparticles
will often not settle, i.e., separate from the liquid phase by
themselves, at least not to a sufficient extent and/or within a
desirably short period of time. A preferred method of achieving a
separation of the nanoparticles from the liquid phase of the
reaction mixture comprises the addition of one or more
nanoparticle-precipitating substances to the reaction mixture. The
suitability of a substance for causing a precipitation of the
nanoparticles will usually depend, inter alia, on the nature of the
adsorptive substance. A non-limiting example of a
nanoparticle-precipitating substance includes a substance that
interferes with the (electronic and/or steric) interaction between
the adsorptive substance that is adsorbed on the surface of the
nanoparticles and one or more components of the liquid phase. A
preferred example of such a nanoparticle-precipitating substance is
a solvent in which the adsorptive substance is substantially
insoluble or at least only poorly soluble. The
nanoparticle-precipitating substance is preferably substantially
soluble in and/or miscible with the liquid phase of the reaction
mixture, in particular, the polyol(s). Often this substance will
comprise an aprotic solvent, preferably a polar aprotic solvent.
The term "aprotic" characterizes a solvent that is not capable of
releasing (dissociating into) protons. Non-limiting examples of
such solvents include ethers (e.g., diethyl ether, tetrahydrofuran,
tetrahydropyran, etc.), sulfonyl compounds and particularly,
carbonyl compounds such as, e.g., ketones, esters and amides,
especially ketones. Preferred ketones comprise from 3 to about 8
carbon atoms such as, e.g., acetone, butanone, the pentanones, the
hexanones, cyclopentanone and cyclohexanone. Of course, mixtures of
aprotic solvents may be used as well.
[0072] The nanoparticle-precipitating substance(s) will usually be
employed in an amount which is sufficient to cause a precipitation
of at least a substantial portion of the nanoparticles that are
present in the reaction mixture, e.g., at least about 90%, at least
about 95%, or at least about 98% of the nanoparticles.
[0073] While the addition of a nanoparticle-precipitating substance
in a sufficient quantity may result in a precipitation, the
precipitation will frequently be unsatisfactory, particularly in
cases where the volume of the liquid phase of the reaction mixture
is substantial (e.g., at least about 1 liter, at least about 2
liters, or at least about 3 liters) and/or the concentration of the
adsorptive substance and/or the nanoparticles in the liquid phase
is relatively high. For example, the addition of the
nanoparticle-precipitating substance in a sufficient quantity to
cause a precipitation of substantially all of the nanoparticles may
frequently cause a concomitant precipitation of at least a
substantial portion of the excess (unbound) adsorptive substance
that is present in the reaction mixture. The precipitated
adsorptive substance may form an oil which prevents or at least
significantly interferes with (slows down) a settling of the
nanoparticles, thereby making the separation of the nanoparticles
from the liquid phase difficult, if not impossible.
[0074] It has been found that the required settling times can be
shortened and/or the formation of oily precipitates can be
significantly reduced or completely eliminated if before and/or
during and/or after the addition of the nanoparticle-precipitating
substance a protic solvent is added to the reaction mixture. The
term "protic" characterizes a solvent that is capable of releasing
(dissociating into) protons. Preferably, the protic solvent
comprises a hydroxyl-containing compound, in particular, an alcohol
such as, e.g., ethanol, propanol, butanol and the like, and/or a
polyol, e.g., any of the polyols that may be used as a reactant in
the process of the present invention. Water may also be used as the
(or a part of the) protic solvent. Preferably, the protic solvent
is or comprises one or more of the polyols that are present in the
reaction mixture. Details regarding the addition of the protic
solvent may be taken from U.S. Provisional Application Ser. No.
60/643,629 entitled "Separation of Metal Nanoparticles," the entire
disclosure whereof is expressly incorporated by reference
herein.
[0075] According to a preferred aspect of the process of the
present invention, the precipitated nanoparticles are isolated by
removing the liquid phase of the reaction mixture therefrom. Any
process that is suitable for removing a liquid from a solid can be
used for this purpose. Non-limiting examples of such a process
include decantation, filtration, centrifugation and any
combinations thereof. Preferably, the nanoparticles are isolated by
centrifugation (including, for example, continuous centrifugation),
filtration (including ultrafiltration, diafiltration etc.) or a
combination of two or more of these processes.
[0076] With respect to continuous centrifugation, this can be
accomplished in different ways. For example, one may use a unit
(centrifuge) which is optimized for affording at least three
different products in different sections of the unit, for example,
a supernatant in a top section, undesirably large particles in a
bottom section and desired product (e.g., nanoparticles in the
desired particle size range) in a middle section. Each of these
three products may be withdrawn continuously from the centrifuge
while a fresh mixture for separation is continuously introduced
into the centrifuge. According to another alternative of the
continuous centrifugation, two or more centrifuges may be arranged
in series, each of them being optimized for the removal of one kind
of separation product, e.g., supernatant, undersized particles,
particles in the desired particle size range, oversized particles,
etc.
[0077] Regarding the ultrafiltration/diafiltration of nanoparticles
reference may be made, for example, to U.S. Pat. Nos. 6,328,894,
5,879,715, 6,245,494 and 6,811,885, the entire disclosures whereof
are incorporated by reference herein. Briefly, ultrafiltration and
diafiltration use a filtration under pressure through a membrane
which allows only components of a certain maximum size to pass
therethrough. In the present case, the metal nanoparticles will be
retained by the membrane while preferably a major part or
substantially all of the contaminants (e.g., dissolved inorganic
matter, excess adsorptive substance, etc.) and the like will be
able to pass through the membrane. Any size of membrane as well as
any channel (pore) size thereof can be used as long as the process
permits a preferably substantial removal of contaminants and the
like while retaining substantially all of the nanoparticles. In a
preferred aspect, the membrane may vibrate to substantially reduce
clogging and/or to permit a higher permeate flow rate. Also, the
ultrafiltration/diafiltration may be pressure-driven (i.e.,
involving pressing through the membrane) or vacuum-driven (i.e.,
involving sucking through the membrane). Membrane configurations
include, but are not limited to, flat sheet membranes, cross flow
membranes, spiral wound tubes, or hollow fiber tubes. For example,
a three compartment through-flow cell comprising two membranes may
be used. Non-limiting examples of membrane materials include
polymeric and ceramic materials such as, e.g., polysulfone,
polyethersulfone, sulfonated polysulfone, polyamide, cellulose
ester (e.g., cellulose acetate), and metal oxides such as the
oxides of titanium, zirconium, silicon, aluminum and combinations
of two or more thereof. By way of non-limiting example, the
membrane may have a molecular weight cutoff (MWCO) in the range of
from about 10,000 to about 1,000,000, e.g., about 50,000, about
100,000, about 200,000 or about 500,000, and/or a pore size of from
about 0.01 .mu.m to about 1 .mu.m (preferably at least about 0.02
.mu.m and not higher than about 0.5 .mu.m) and/or a lumen of from
about 0.1 mm to about 5 mm (preferably at least about 2 mm and not
more than about 3 mm).
[0078] Any type of ultrafiltration/diafiltration process may be
used as long as the process is capable of removing a substantial
portion of the contaminants and the like (e.g., at least about 70%,
at least about 80%, at least about 90%, or at least about 95%) and
in particular, that part of the adsorptive substance that is not
adsorbed on the surface of the nanoparticles while retaining
substantially all of the nanoparticles. By way of non-limiting
example, a cross-flow separation process may be used.
[0079] The nanoparticles that have been separated from the liquid
phase are preferably subjected to a washing operation to remove at
least a substantial portion of the impurities that may still be
associated therewith such as, e.g., materials that are not adsorbed
on the surface of the nanoparticles to any significant extent. For
example, these impurities may include inorganic salts formed during
the reduction of the metal compound, residual solvent(s) from the
precipitation step and excess adsorptive substance, i.e.,
adsorptive substance that is merely present as an impurity without
being adsorbed on the nanoparticles. The washing liquid used for
the washing operation preferably is or comprises a solvent that is
capable of dissolving the impurities associated with the
nanoparticles, in particular, excess adsorptive substance. By way
of non-limiting example, the washing liquid may comprise water
and/or an organic solvent, for example, a polar organic solvent.
One of skill in the art will appreciate that the most desirable
washing liquid(s) for a specific case will to a large extent depend
on the nature of the impurities to be removed, e.g., polar vs.
apolar, inorganic vs. organic, etc. In some cases it may be
advantageous to use two or more different washing liquids
(preferably successively or with a concentration gradient) for
obtaining the best results. Non-limiting examples of a washing
liquids include liquids which comprise or consist of water and/or
one or more protic organic solvents such as, e.g., a
hydroxyl-containing compound, preferably, an alcohol and/or a
polyol. Illustrative and non-limiting examples of alcohols and
polyols that may be used for the washing operation include
aliphatic alcohols having from 1 to about 12 carbon atoms such as,
e.g., methanol, ethanol, propanol, isopropanol, butanol, pentanol,
cyclopentanol, hexanol, cyclohexanol, octanol, decanol, dodecanol
and the like, and polyols having from 1 to about 4 hydroxyl groups
and from 2 to about 12 carbon atoms such as, e.g., ethylene glycol,
propylene glycol, glycerol and the like. A preferred solvent for
use in the washing operation includes ethanol, which may be used
alone or in combination with other solvents (e.g., water). Of
course, non-protic solvents may also be useful for the washing
operation. Non-limiting examples thereof include ketones such as,
e.g., acetone and butanone, ethers such as, e.g., diethylether and
tetrahydrofuran, esters such as, e.g., ethyl acetate, amides such
as, e.g., dimethylformamide and dimethylacetamide, sulfoxides such
as, e.g., dimethyl sulfoxide, and optionally halogenated
hydrocarbons such as, e.g., hexane, cyclohexane, heptane, octane,
petrol ether, methylene chloride, chloroform, toluene, the xylenes,
etc. Combinations of two or more of these solvents may, of course,
also be used.
[0080] The washing operation may, for example, be carried out by
dispersing the isolated crude nanoparticles obtained after, e.g., a
filtration (including, e.g., a diafiltration/ultrafiltration)
and/or centrifugation of the reaction mixture in a washing liquid,
followed by a separation of the particles from the washing liquid
by, e.g., filtration and/or centrifugation. This process may
optionally be repeated one or more times. The washed (purified)
nanoparticles may thereafter be dried (e.g., under reduced pressure
and/or at a temperature that does not adversely affect the
adsorptive substance to any significant extent) and thereafter
stored and/or shipped. Even after storage for extended periods the
dry particles can be redispersed in a desired liquid to form a
dispersion (e.g., a printing ink) that is substantially stable over
several days or even weeks (for example, wherein not more than
about 20%, e.g., not more than about 10%, or not more than about 5%
of the nanoparticles have settled after storing the dispersion at
room temperature for at least one day, e.g., at least two days, or
at least one week).
[0081] In a preferred aspect of the present invention, the further
processing of the nanoparticle containing reaction mixture obtained
after carrying out the reduction of the metal compound may be
carried out by using substantially only
ultrafiltration/diafiltration. In particular, the use of
ultrafiltration/dialfiltration makes it possible to dispense with
the addition of a nanoparticle-precipitating substance to the
reaction mixture and even allows combining the separation and
washing operations of the nanoparticles in a single operation.
Further, at least the last liquid that is added to the
nanoparticles before the ultrafiltration/diafiltration thereof is
completed may be selected to be the vehicle of a desired dispersion
of the nanoparticles (for example, of a printing ink) or a
component thereof, thereby making it possible to convert the
nanoparticle containing reaction mixture into the desired
nanoparticle containing product in a single unit/operation. Also,
one or more additives may be incorporated in the washing liquid
and/or the liquid that is intended to be the vehicle of the desired
dispersion or a component thereof. For example, in order to keep
the dissolution of adsorbed adsorptive substance at a minimum it
may be advantageous to add some adsorptive substance to, e.g., the
washing liquid. Also, one or more additives whose presence may be
desirable in the final nanoparticle containing product (e.g., a
printing ink) may be incorporated into the liquid used in the final
step(s) of a diafiltration operation (such as, e.g., adhesion
promoters, humectants, etc.).
[0082] By way of non-limiting example, the
diafiltration/ultrafiltration may be carried out by placing the
nanoparticle containing reaction mixture in a diafiltration unit
and concentrating the reaction mixture therein to a predetermined
fraction of the original volume by pressing (application of
pressure) or drawing (application of vacuum) the reaction mixture
through one or more ultrafiltration membranes of suitable MWCO/pore
size. Thereafter, a first liquid that is capable of dissolving
impurities and contaminants present in the reaction mixture (in
particular, excess adsorptive substance) may be added to the
concentrated reaction mixture (e.g., in an amount sufficient to
restore the originally employed volume of the reaction mixture) and
the resulting mixture may be concentrated in the same way as the
originally employed reaction mixture. A second liquid which is
capable of dissolving impurities and contaminants and which may be
the same as or different from the first liquid may be added to the
resulting concentrate and the resulting mixture may be concentrated
again. This process may be repeated as often as necessary with a
third, fourth, etc. liquid. Alternatively, before concentrating the
original reaction mixture a predetermined amount of the first
liquid may be added thereto and the resulting mixture may be
concentrated, e.g., until the original volume of the reaction
mixture is reached again. Then the second liquid may be added and a
second concentration operation may be carried out, etc. Of course,
any combination of the two alternatives described above may be used
as well. For example, the original reaction mixture may be
concentrated first and then the first liquid may be added in an
amount which results in a volume of the resultant mixture which
exceeds the volume of the original reaction mixture, whereafter the
resultant mixture may be concentrated to the volume of the original
reaction mixture, whereafter a second liquid may be added, etc. At
the end of each of these alternative ways of isolating/purifying
the metal nanoparticles by ultrafiltration/diafiltration the liquid
may be removed partially or completely by ultrafiltration, leaving
behind the purified substantially non-agglomerated metal
nanoparticles with the adsorptive substance thereon, or a
concentrated and stable dispersion thereof. The nanoparticles may
then optionally be dried to form a powder batch of dry
nanoparticles. Alternatively, the liquids that are used for the
diafiltration operation may be selected such that at least at the
end of the diafiltration operation the purified nanoparticles are
combined with a liquid which is the vehicle or at least a part of
the vehicle of a desired dispersion of the metal nanoparticles
(e.g., a printing ink). The liquids which may be used for carrying
out the diafiltration/ultrafiltration include those which have been
mentioned above in the context of the separation of the
nanoparticles from the liquid phase and the washing of the
separated nanoparticles.
[0083] It is to be appreciated that the use of
ultrafiltration/diafiltration is advantageous not only for the
separation and/or purification of metal nanoparticles that have
been produced by the process of the present invention but is a
procedure which is of general applicability for separating
inorganic nanoparticles and, in particular, metal nanoparticles
that have a polymeric substance (e.g., an anti-agglomeration
substance as used in the process of the present invention) adsorbed
on their surface from a liquid which comprises a dissolved
polymeric substance (either the same as or different from the
adsorbed polymeric substance) and for purifying (e.g., washing)
such nanoparticles and for formulating such separated/purified
nanoparticles into a desired product (e.g., a dispersion).
Metal Nanoparticles
[0084] Due to the particular way of combining the metal compound
and the polyol according to the process of the present invention,
it is possible to control the size, the size distribution and/or
the shape of the nanoparticles even on a large scale. For example,
particles which exhibit a high degree of uniformity in size and/or
shape may be produced by the process of the present invention. For
example, the process of the present invention is capable of
affording particles with a substantially spherical shape. In one
aspect of the present invention, at least about 90%, e.g., at least
about 95% of the nanoparticles formed by the process of the present
invention may be of a substantially spherical shape. In another
aspect, the particles may be substantially free of micron-size
particles (i.e., particles having a size of about 1 .mu.m or
above). Even more preferably, the nanoparticles may be
substantially free of particles having a size (=largest dimension,
e.g., diameter in the case of substantially spherical particles) of
more than about 500 nm, e.g., of more than about 200 nm, or of more
than about 100 nm. In this regard, it is to be understood that
whenever the size and/or dimensions of the nanoparticles are
referred to herein and in the appended claims, this size and these
dimensions refer to the nanoparticles without adsorptive substance
thereon, i.e., the metal cores of the nanoparticles. Depending on
the type and amount of adsorptive substance, an entire
nanoparticle, i.e., a nanoparticle which has the adsorptive
substance thereon, may be significantly larger than the metal core
thereof. Also, the term "nanoparticle" as used herein and in the
appended claims encompasses particles having a size/largest
dimension of the metal cores thereof of up to about 900 nm,
preferably of up to about 500 nm. By way of non-limiting example,
not more than about 5%, e.g., not more than about 2%, not more than
about 1%, or not more than about 0.5% of the particles that are
formed by the process of the present invention may be particles
whose largest dimension (e.g., diameter) is larger than about 200
nm, e.g., larger than about 150 nm, or larger than about 100 nm. In
a particularly preferred aspect, at least about 95% of the
nanoparticles may have a size of not larger than about 80 nm and/or
at least about 80% of the nanoparticles may have a size of from
about 30 nm to about 70 nm.
[0085] In another aspect, the nanoparticles formed by the process
of the present invention may have an average particle size
(expressed as number average) of at least about 10 nm, e.g., at
least about 20 nm, or at least about 30 nm, but preferably not
higher than about 80 nm, e.g., not higher than about 70 nm, not
higher than about 60 nm, or not higher than about 50 nm. The
average particle sizes and particle size distributions referred to
herein may be measured by conventional methods such as, e.g., by
scanning electron microscopy (SEM) or tunneling electron microscopy
(TEM) and refer to the metal cores.
[0086] In yet another aspect of the process of the present
invention, at least about 80 volume percent, e.g., at least about
90 volume percent of the nanoparticles formed by the process of the
present invention may be not larger than about 2 times, e.g., not
larger than about 1.5 times the average particle size (volume
average).
[0087] The reduction process of the present invention and the
optional further processing of the reaction mixture obtained
thereby are capable of affording large, commercially useful
quantities of substantially non-agglomerated, dispersed or
redispersable metal nanoparticles in a single run. For example, in
batch-wise operation the process of the present invention can be
carried out on a scale at which at least about 30 g, e.g., at least
about 40 g, at least about 50 g, or at least about 60 g of
substantially non-agglomerated, dispersed or redispersable metal
(e.g., silver) nanoparticles (expressed as pure metal without
adsorptive substance) are produced in a single run. In a preferred
aspect, a single run will afford at least about 100 g, at least
about 200 g, or at least about 500 g of substantially
non-agglomerated, dispersed or redispersable metal
nanoparticles.
[0088] Further, in one aspect of the present invention a
concentrated batch of metal nanoparticles--a so-called
"masterbatch"--may be produced which may be a liquid or solid at
room temperature and comprises a high concentration of metal
nanoparticles and may be stored for an extended period of time and
subsequently redispersed by adding solvents and/or diluents. By way
of non-limiting example, the masterbatch may comprise adsorptive
substance and metal nanoparticles alone, or the masterbatch may
comprise metal nanoparticles, adsorptive substance and
solvents/diluents. In another embodiment of this invention, a
concentrated metal nanoparticle batch may be further concentrated
by removing the solvent to produce a batch of dry metal
nanoparticles comprised primarily of metal nanoparticles and
adsorptive substance, which batch can be substantially completely
redispersed to form a stable dispersion (e.g., a desired printing
ink) on adding a suitable solvent or diluent liquid. Due to, for
example, the reduced volume thereof, a dry masterbatch is
particularly advantageous for shipping and storage over an extended
period of time. In this regard, it is to be appreciated that the
adsorptive substance on the surface of the metal nanoparticles will
usually not only substantially prevent an (irreversible)
agglomeration of the nanoparticles, but also increase the shelf
life of the nanoparticles and in particular, of dry nanoparticles
by shielding to at least some extent the surfaces of the metal
cores of these nanoparticles from an attack by oxygen (oxidation),
heat, harmful radiation (e.g., UV rays) and the like.
Preferred Aspects
[0089] A preferred aspect of the process of the present invention
comprises the production of silver nanoparticles by the rapid
mixing a solution of at least about 0.1 mole of a silver compound
with a heated solution of a polyol and a vinyl pyrrolidone polymer.
In the following description this aspect will be discussed with
respect to particularly advantageous features thereof.
[0090] Non-limiting examples of silver compounds for use in the
preferred aspect of the process of the present invention include
silver nitrate, silver acetate, silver trifluoroacetate and silver
oxide. Particularly preferred is silver nitrate. Preferably, the
silver compound comprises a single silver compound, although
mixtures of different silver compounds and mixtures of one or more
silver compounds with one or more compounds of other metals may be
used as well.
[0091] The polyol(s) used in combination with the silver compound
will usually have from 2 to about 4 hydroxy groups and/or from 2 to
about 6 carbon atoms such as, e.g., ethylene glycol, diethylene
glycol, triethylene glycol, propylene glycol, dipropylene glycol,
1,3-propanediol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol,
2,3-butanediol, glycerol, triethanolamine and trimethylolpropane.
Usually, the polyol(s) will include on or more glycols, preferably
at least one glycol having from 2 to about 4 carbon atoms.
Preferred examples of such glycols include ethylene glycol and
propylene glycol. Particularly preferred is ethylene glycol.
[0092] The vinyl pyrrolidone polymer for use in combination with
the silver compound may be a vinyl pyrrolidone homopolymer (PVP)
and/or a vinyl pyrrolidone copolymer. Preferably, the polymer is or
at least comprises a vinyl pyrrolidone homopolymer.
[0093] The vinyl pyrrolidone polymer (and particularly, the vinyl
pyrrolidone homopolymer) will usually have a weight average
molecular weight of up to about 60,000, e.g., up to about 50,000,
up to about 40,000, or up to about 30,000, and preferably, of not
less than about 3,000, e.g., not less than about 4,000, not less
than about 5,000, not less than about 8,000, or not less than about
10,000. An example of a particularly preferred vinyl pyrrolidone
polymer is a vinyl pyrrolidone homopolymer (=PVP) having a weight
average molecular weight of about 10,000.
[0094] The solution of the silver compound preferably comprises one
or more polyols. Even more preferably, at least one of the one or
more polyols is identical with at least one of the one or more
polyols present in the heated solution. Most preferably, both the
solution of the silver compound and the heated solution comprise
ethylene glycol, which may be the only polyol used in the process
of the present invention.
[0095] In another aspect of the preferred process of the present
invention, no solvent other than the polyol(s) may be present in
the solution of the silver compound and/or in the heated solution.
Preferably, both the solution of the metal compound and the heated
solution will comprise one or more polyols (e.g., ethylene glycol
and/or propylene glycol) as the only liquid component.
[0096] In yet another aspect of the preferred process of the
present invention, the heated solution may be at a temperature of
not higher than about 150.degree. C., preferably not higher than
about 130.degree. C., and not lower than about 80.degree. C.,
preferably, not lower than about 100.degree. C. A particularly
preferred temperature of the heated solution is about 120.degree.
C.
[0097] In a still further aspect of the preferred process of the
present invention, the temperature of the solution of the silver
compound may be not higher than about 40.degree. C., e.g., not
higher than about 30.degree. C., or not higher than about
25.degree. C.
[0098] In another aspect of the preferred process of the present
invention, the rapid mixing may comprise combining the solutions
within not more than about 30 seconds, preferably with agitation
such as, e.g., stirring, shaking and/or sonication. Preferably, the
solutions are combined by a one-shot addition of one of the
solutions to the other solution, in particular, by adding the
solution of the silver compound to the heated solution.
[0099] In another aspect of the preferred process of the present
invention, the solution of the silver compound may comprise at
least about 0.5 mole of the silver compound, e.g., at least about
0.75 mole of the silver compound, or at least about 1 mole of the
silver compound.
[0100] In yet another aspect of the preferred process of the
present invention, the combined volume of the solution of the
silver compound and the heated solution per one mole of the silver
compound may not be larger than about 5 liters, e.g., not larger
than about 4 liters, and may often be not smaller than about 3
liters, e.g., not smaller than about 2 liters per one mole the of
silver compound.
[0101] In yet another aspect of the preferred process of the
present invention, the volume ratio of the heated solution and the
solution of the silver compound may be not higher than about 8:1,
e.g., not higher than about 7:1, or not higher than about 6:1, and
not lower than about 2:1, e.g., not lower than about 3:1, or not
lower than about 4:1. A particularly preferred volume ratio is
about 5:1. By way of non-limiting example, the solution of the
silver compound (e.g., silver nitrate) may comprise about 1.5 mole
of silver compound per liter of solvent (e.g., ethylene glycol) and
the ratio of the heated solution and the solution of the silver
compound may be about 5:1.
[0102] In a still further aspect of the preferred process of the
present invention, the molar ratio of the vinyl pyrrolidone units
in the vinyl pyrrolidone polymer (which preferably comprises or
consists of a vinyl pyrrolidone homopolymer) and the silver
compound will preferably be not lower than about 3:1, e.g., not
lower than about 6:1, or not lower than about 10:1, and will
preferably be not higher than about 100:1, e.g., not higher than
about 50:1, not higher than about 20:1, or not higher than about
15:1. By way of non-limiting example, the molar ratio of vinyl
pyrrolidone units of a PVP and the silver compound may be about
12:1.
[0103] In one aspect, the preferred process of the present
invention may further comprise keeping the mixed solutions at a
temperature of at least about 100.degree. C., e.g., at least about
110.degree. C., for a sufficient period to convert a substantial
portion (preferably at least about 90%, and more preferably at
least about 95%, and most preferably at least about 98%) of the
silver compound to silver nanoparticles. In a preferred aspect, the
mixed solutions are heated at a temperature of about 120.degree. C.
for about 1 hour and/or until substantially all of silver compound
is converted to nanoparticles.
[0104] In another aspect, the preferred process of the present
invention may further comprise the addition of a polyol to the
mixed solutions. The polyol preferably comprises or is a glycol,
preferably a glycol that is already present in the mixed solutions.
The amount of glycol will preferably be at least about the amount
that compensates any losses of polyol due to evaporation during the
heating of the mixed solutions. The polyol may be added, for
example, continuously, in two or more portions or in one shot.
[0105] In another aspect, the preferred process of the present
invention may further comprise allowing the heated mixture to cool
to room temperature.
[0106] In yet another aspect, the preferred process of the present
invention may further comprise a precipitation of formed silver
nanoparticles. This precipitation preferably comprises the addition
of a sufficient amount of a nanoparticle-precipitating liquid that
is miscible with the one or more polyols that are present in the
reaction mixture to precipitate at least a substantial portion
(preferably at least about 90%) of the silver nanoparticles.
[0107] The nanoparticle-precipitating liquid will usually comprise
a polar aprotic solvent such as, e.g., a ketone. A non-limiting
example of a ketone is acetone.
[0108] In a preferred aspect of the precipitation of the silver
nanoparticles, before and/or during and/or after the addition of
the nanoparticle-precipitating liquid a polyol is added in a
sufficient amount to improve the precipitation of the nanoparticles
and/or the separation thereof from the liquid phase. This polyol
may be or may comprise a polyol that is already present in the
reaction mixture. For example, the polyol may comprise a glycol
such as ethylene glycol and/or propylene glycol. By way of
non-limiting example, the nanoparticle-precipitating liquid may
comprise acetone and the polyol may comprise ethylene glycol.
[0109] In yet another aspect, the preferred process of the present
invention may further comprise an isolation of the (precipitated)
silver nanoparticles. This isolation can be accomplished by any
means that are suitable for removing a liquid from a solid.
However, the separation preferably includes a centrifugation
(including, e.g., a continuous centrifugation) and/or an
ultrafiltration and/or a diafiltration.
[0110] In a still further aspect, the preferred process of the
present invention may further comprise subjecting the isolated
silver nanoparticles to a washing operation with a liquid that is
capable of dissolving the vinyl pyrrolidone polymer. Preferably,
the liquid comprises an aliphatic alcohol such as, e.g., ethanol
and/or water. The washing operation will usually remove at least a
major part of the excess vinyl pyrrolidone polymer that is not
adsorbed on the nanoparticles (i.e., is present merely as a
contaminant). While the presence of major quantities of excess
polymer may be acceptable for many applications for which the
silver nanoparticles are currently intended, there are also
applications in which the presence of significant amounts of excess
polymer should be avoided.
[0111] In particular, the silver nanoparticles of the present
invention may be used in the formulation of printing inks for
various purposes and for various printing techniques such as, e.g.,
ink-jet printing, screen printing, intaglio printing, roll
printing, lithographic printing and gravure printing. Major fields
of application for these inks include electronics (e.g., for the
printing of electrically conductive features and the like by, for
example, ink-jet printing), graphics (e.g., decorative purposes),
security features and the like. In many of these applications, the
excess polymer is either acceptable or can be removed by, e.g.,
thermal means, for example, by heating the deposited ink to a
temperature above the decomposition temperature of the polymer
(which at the same time will remove the polymer that is adsorbed on
the nanoparticles). The removal of excess polymer by the
application of thermal energy is particularly suitable for
applications which involve heat-resistant materials (substrates)
such as, e.g., glass, ceramic, metal and heat-resistant
polymers.
[0112] Especially in the field of electronics and, in particular in
applications which involve temperature-sensitive substrates (e.g.,
polymers, cellulose-based materials etc.) the presence of
significant amounts of excess polymer is undesirable because on the
one hand the excess polymer may have a substantial adverse effect
on the conductivity of the electrically conductive features printed
on the substrates and on the other hand the removal of the excess
polymer by thermal means will usually not be possible without
severely damaging or destroying the substrate. Accordingly, in
these cases the silver nanoparticles should be substantially free
of excess polymer. While even the nanoparticles which are free of
excess polymer still have vinyl pyrrolidone polymer adsorbed on
their surfaces it has unexpectedly been found that with the
adsorbed polymer alone substantial electric conductivity can be
achieved at sintering temperatures of the deposited ink which are
as low as about 100.degree. C. Without wishing to be bound by any
theory, it is assumed that already at these low sintering
temperatures the adsorbed vinyl pyrrolidone polymer becomes mobile
enough to at least partially and sufficiently expose the silver
cores of the individual nanoparticles to allow a direct contact
between (and sintering of) these silver cores, whereby electrical
conductivity is established. Details regarding the formulation and
properties of printable inks comprising vinyl pyrrolidone polymers
are disclosed in U.S. Provisional Application Ser. No. 60/643,577
entitled "Metal Nanoparticle Compositions," the entire disclosure
whereof is incorporated by reference herein.
[0113] In a further aspect of the preferred process of the present
invention, at least about 90%, e.g., at least about 95% of the
silver nanoparticles may be of a substantially spherical shape,
and/or at least about 90%, preferably at least about 95% of the
nanoparticles may have a diameter of not more than about 80 nm
and/or at least about 90% of the nanoparticles may have a diameter
of from about 10 nm to about 70 nm and/or at least about 80 volume
percent of the nanoparticles may be not larger than about 1.5 times
the average particle size. Preferably, the silver nanoparticles
will be substantially free of micron-size (and larger) particles.
Even more preferably, they will be substantially free of particles
having a size of more than about 200 nm, e.g., of more than about
150 nm.
EXAMPLES
[0114] The present invention will be further illustrated by the
following non-limiting examples.
Example 1
[0115] In a mixing tank a solution of 1000 g of PVP (M.W. 10,000,
Aldrich) in 2.5 L of ethylene glycol is prepared and heated to
120.degree. C. In a second mixing tank, 125 g of silver nitrate is
dissolved in 500 ml of ethylene glycol at 25.degree. C. These two
solutions are rapidly combined (within about 5 seconds) in a
reactor, in which the combined solutions (immediately after
combination at a temperature of about 114.degree. C.) are stirred
at 120.degree. C. for about 1 hour. The resultant reaction mixture
is allowed to cool to room temperature and about 0.25 L of ethylene
glycol is added thereto to replace evaporated ethylene glycol. This
mixture is stirred at high speed for about 30 minutes to resuspend
any particles that have settled during the reaction. The resultant
mixture is transferred to a mixing tank where 12 L of acetone and
about 1 L of ethylene glycol are added. The resultant mixture is
stirred thoroughly and then transferred to a centrifuge where it is
centrifuged for about 20 minutes at 1,500 g to separate the silver
nanoparticles from the liquid phase. This affords 70 g of
nanoparticles which have PVP adsorbed thereon. The particles are
subsequently suspended in 2,000 ml of ethanol to remove, inter
alia, excess PVP, i.e., PVP that is not adsorbed on the
nanoparticles but is present merely as a contaminant. At this
point, the ethanol suspension of particles is preferably filtered
through a 1.5 .mu.m nylon filter, thus filtering out particles that
are larger than 1.5 .mu.m. The filtrate is subsequently centrifuged
and the resulting cake is dried in a vacuum oven at about
35.degree. C. and about 10-2 torr to afford dry nanoparticles.
These nanoparticles exhibit a PVP content of about 4 to about 8
weight percent, depending on the time the nanoparticles have been
in contact with the ethanol.
[0116] Alternatively, the ethanol suspension may be centrifuged
without first filtering through a 1.5 .mu.m nylon filter. The
resultant filter cake of nanoparticles is dried in a vacuum oven at
about 35.degree. C. and about 10.sup.-2 torr to afford dry
nanoparticles. These nanoparticles, like the particles obtained
after filtering through a 1.5 .mu.m filter, exhibit a PVP content
of about 4 to about 8 weight percent, depending on the time the
nanoparticles have been in contact with the ethanol.
[0117] ICP (inductively coupled plasma) data indicates that the
longer the particles are in contact with the ethanol, the more of
the acetone and ethylene glycol present in the PVP matrix is
displaced by ethanol, resulting in particles with an increasingly
higher silver content.
[0118] It is believed that several characteristics of the reagents
that are used in the process described above may ultimately affect
the particle size distribution (PSD) of the nanoparticles produced
by the process. At least four such characteristics are the water
content of the PVP, the 2-pyrrolidone content of the PVP (PVP
inherently contains 2-pyrrolidone), the formic acid content of the
PVP and the water content of the ethylene glycol used.
[0119] Although the reasons are not entirely clear at this time, it
has been observed that the water content of the PVP may have an
effect on the PSD of the nanoparticles produced by the process
described above. PVP is a hygroscopic substance that typically has
a residual amount of water contained therein. In some cases, the
presence of water in the PVP leads to the production of
nanoparticles with a desirable PSD. In other cases, however, when
the water content in the PVP is reduced, the same result is
observed. Thus, it is not entirely clear if it is preferable to
have PVP where the water content has been reduced or PVP where the
residual water content is not reduced. In one embodiment, the water
content of the PVP is about 1-10% by weight, or about 1-8% by
weight, or about 2-10% by weight or preferably about 2-7% by
weight. The water content of the PVP may be reduced by heating the
PVP at about 70-80.degree. C. overnight (e.g., 8-14 hours). The PVP
may optionally be stirred while it is heated. In addition, the
heating may be optionally performed under an inert atmosphere
(e.g., argon and nitrogen) or in vacuo.
[0120] Although the reasons are not entirely clear at this time, it
has also been observed that the presence of 2-pyrrolidone in the
PVP may have an effect on the PSD of the nanoparticles produced by
the process described above. 2-pyrrolidone is a contaminant
comprised in the PVP that results from the synthesis of PVP. In one
embodiment, the 2-pyrrolidone content of the PVP is about 1-10% by
weight, or about 1-8% by weight, or about 1-5% by weight or
preferably about 5% by weight.
[0121] In addition, although the reasons are not entirely clear at
this time, it has also been observed that the presence of formic
acid in the PVP may have an effect on the PSD of the nanoparticles
produced by the process described above. Formic acid is a
contaminant that may be comprised in the PVP that results from the
synthesis of PVP. In some cases, the presence of formic acid in the
PVP leads to the production of nanoparticles with a desirable PSD.
In other cases, however, when the formic acid content in the PVP is
reduced, the same result is observed. Thus, it is not clear if it
is preferable to have PVP where the formic acid content has been
reduced or PVP where the formic acid content is not reduced. In one
embodiment, the formic acid content of the PVP is about 0-2% by
weight, or about 0-1% by weight, or about 0-0.8% by weight or
preferably about 0.3% by weight. The formic acid content of the PVP
may be reduced by heating the PVP at about 70-80.degree. C.
overnight (e.g., 8-14 hours). The PVP may optionally be stirred
while it is heated. In addition, the heating may be optionally
performed under an inert atmosphere (e.g., argon and nitrogen) or
in vacuo.
[0122] Finally, although the reasons are not entirely clear at this
time, it has also been observed that the presence of water in the
ethylene glycol may have an effect on the PSD of the nanoparticles
produced by the process described above. It is possible that some
water must be present in the ethylene glycol in order to produce
nanoparticles with a desirable PSD. Without being bound by any
particular theory, it is possible that the presence of water
content in ethylene glycol influences the equilibrium between
ethylene glycol and its decomposition product, acetaldehyde,
according to the reaction shown below:
##STR00001##
[0123] Even though it is possible that it is produced in an
extremely small quantity (i.e., the equilibrium favors ethylene
glycol), the acetaldehyde produced, in turn, acts as a reducing
agent for the silver nitrate, thereby effecting the reduction of
silver ions in silver nitrate to silver metal. The silver metal, in
turn, can precipitate out of the ethylene glycol solution prior to
mixing the silver nitrate/ethylene glycol solution with the
PVP/ethylene glycol solution. It is possible that the precipitation
of silver metal in the silver nitrate/ethylene glycol solution,
prior to mixing with the PVP/ethylene glycol solution, could lead
to the production of nanoparticles once the solutions are mixed,
where the nanoparticles have an undesirable PSD (e.g., a
polydisperse powder fraction with an increased amount of large
particles and agglomerates). In one embodiment, the water content
of the ethylene glycol is about 0.01-0.10% by weight, or about
0.01-0.05% by weight, or about 0.01-0.04% by weight or preferably
about 0.04% by weight.
Example 2
Effect of PVP Molecular Weight on Resistivity and Curing
Temperature
[0124] Silver nanoparticles can be synthesized using 58,000 g/mol
PVP using a process substantially the same as the process used to
synthesize silver nanoparticles using 10,000 g/mol PVP by mixing a
PVP/ethylene glycol solution with a silver nitrate/ethylene glycol
solution. Once the reaction is complete, 250 mL of ethylene glycol
is mixed into the reaction solution after cooling down. The
resulting solution is equally divided into five 4-L Nalgene.RTM.
bottles. 200 mL of ethylene glycol and 3 L of acetone are added to
each plastic bottle. The bottles are sealed with lids and shaken up
to create a black precipitate suspension in the solvent mixture.
The suspensions are transferred to centrifuge bottles and
centrifuged at 2,200 RPM for 10 minutes. The clear, light orange
supernatants are discarded to leave behind a reflective, silver
cake. More black suspension is added to the same bottles and the
separation steps are repeated until all of the silver nanoparticles
are caked in the bottom of the centrifuge bottles. 600 mL of
ethanol are added to each centrifuge bottle to remove, inter alia,
excess PVP. At this point, the ethanol suspension of particles is
preferably filtered through a 1.5 .mu.m nylon filter, thus
filtering out particles that are larger than 1.5 .mu.m. The
filtrate is subsequently centrifuged and the resulting cake is
dried in vacuo for 2-3 hours.
[0125] Alternatively, the ethanol suspension may be centrifuged
without filtering through a 1.5 .mu.m nylon filter. In this case,
the bottles are centrifuged at 2,200 RPM overnight, and the clear,
dark orange supernatants are discarded to leave behind a highly
reflective, bluish-silver cake. The silver nanoparticles are then
dried in vacuo for 2-3 hours to form gold-colored granules.
[0126] Silver nanoparticle inks are spin coated onto glass and
tested to ascertain the electrical properties. Liquid crystal
display (LCD) glass slides are cut into 2'' squares and given 5
minutes of UV-Ozone treatment. This is followed by wiping the
slides with a lint-free cloth wetted with acetone and then another
lint-free cloth wetted by denatured ethanol. In succession, each
slide is secured to a 2'' vacuum chuck on the spin coater. The
slide is spun at 500 RPM for 10 seconds while 0.8 mL of silver ink
is dispensed using a micropipette, after which the slide is spun at
1,000 RPM for 20 seconds. After spin coating, slides are placed in
a pre-heated Despatch oven set to either 100 or 120.degree. C. for
60 minutes to cure the silver ink films. Then the silver-coated
slides are removed and allowed to cool. Each slide is placed on the
four-point probe station, and five resistance measurements are
taken near the center of the silver films. After the measurements
are taken, a 1-2 mm wide, 0.5-1 cm long scratch is made in the
center of each film using a stainless steel scalpel. When making
the scratch, enough pressure is applied to remove the ink layer but
not enough to significantly scratch the glass substrate. Then the
layer thickness is measured across five locations along the
scratch, using the scratch as a reference, on the Zygo
profilometer. Finally, the resistance and thickness measurements
are used to calculate the resistivity for each silver film.
[0127] The electrical testing of six silver inks formulated from
different silver nanoparticles made with the 10,000 g/mol PVP it
was observed that the average resistivity of the samples cured at
100.degree. C. is 101.times. bulk silver over a range of
22.1-234.times. bulk silver. For curing at 120.degree. C., the
average resistivity of the samples is 17.8.times. bulk silver over
a range of 14.3-24.2.times. bulk silver. Unexpectedly, for the ink
sample made with silver nanoparticles formed in the presence of
58,000 g/mol PVP, the average resistivity is 12.8.times. bulk
silver when cured at 100.degree. C. and 7.20.times. bulk silver
when cured at 120.degree. C. It is evident from the data given
above that, relative to inks that employ 10,000 g/mol PVP, the ink
that employs 58,000 g/mol PVP results in superior electrical
performance at a relatively low curing temperature. The ability of
achieving such superior electrical performance at relatively low
curing temperatures with inks comprising 58,000 g/mol PVP
facilitates printing such inks on flexible substrates.
Example 3
[0128] To demonstrate the effect of the rate of addition and the
temperature of the silver nitrate solution, a reaction of the type
described in Example 1 was conducted with different addition rates
and temperatures of the solutions. The results are summarized in
Table 1 below.
TABLE-US-00001 TABLE 1 Temperature of AgNO.sub.3/ Temperature of
PVP/ Experiment ethylene glycol ethylene glycol No. solution
[.degree. C.] solution [.degree. C.] Addition Comments 1 RT 120
Half&half/20 min Larger particles, good size distribution, long
needles (up to 100 .mu.m) 2 RT 120 Drop-wise/20 min No size and
shape control 3 120 120 Half&half/20 min Some large chunks,
broad size distribution 4 120 120 Drop-wise/20 min No control,
large chunks 5 120 120 One-shot Good control of size and shape,
narrow size distribution 6 RT RT One-shot Good control of size and
shape, relatively broad distribution 7 RT 120 One-shot Good control
of size and shape, narrow size distribution; slightly better than
Exp. 5 RT = room temperature
[0129] From the results summarized in Table 1 it can be seen that
the drop-wise addition of the silver nitrate solution to the PVP
solution (which is similar to dissolving solid silver nitrate in
the PVP solution) does not allow any control of the size and shape
of the silver nanoparticles. Adding the silver nitrate solution in
two portions 20 minutes apart affords a somewhat better but still
unsatisfactory result. The one-shot addition of the silver nitrate
solution to the PVP solution at room temperature (Experiment 6)
results in a relatively broad particle size distribution, while it
is satisfactory in terms of particle size and shape. The one-shot
addition of a silver nitrate solution to a 120.degree. C. PVP
solution affords a good control of particle size and shape and a
narrow size distribution, with slightly better results in the case
of a silver nitrate solution at room temperature compared to a
solution at 120.degree. C.
Example 4
[0130] To 4 L of a particle suspension composed of 1.0 kg of
unbound PVP, 3 L of ethylene glycol and about 71 g of silver
nanoparticles having PVP adsorbed thereon (prepared according to a
process similar to that described in Example 1) is added 12 L of
acetone. The resultant mixture is subjected to centrifugation at
1,500 g. An oil suspension is formed during centrifugation. The
particle oil suspension is difficult to break and complete
separation of the particles from the liquid phase cannot be
accomplished.
Example 5
[0131] Example 3 is repeated, except that 1.25 L of ethylene glycol
is added before carrying out the centrifugation. The extra ethylene
glycol prevents the formation of an oil. The particles form a cake
and complete separation of the particles from the liquid phase can
be accomplished.
Example 6
[0132] To 1 L of a particle suspension composed of 250 g of unbound
PVP, 750 mL of ethylene glycol and about 20 g of silver
nanoparticles having PVP adsorbed thereon (prepared according to a
process similar to that described in Example 1) is added 1 L of
deionized water and the resultant mixture is subjected to a
diafiltration in a tangential cross-flow manner. (membrane made of
polysulfone, pore size 50 nm, surface area 615 cm.sup.2, Spectrum
Laboratories, Inc.; applied pressure about 10 psi gauge to about 20
psi gauge). The diafiltration is carried out in concentration mode
and is performed until the retentate has a volume of about 100 mL.
To the retentate is added 1 L of deionized water and diafiltration
is again performed in concentration mode until the volume of the
retentate is about 100 mL. To the resultant second retentate is
added 250 mL of a mixture of 40% by weight of ethylene glycol, 25%
by weight of glycerol and 35% by weight of ethanol and
diafiltration is performed in concentration mode until the volume
of the retentate is about 100 mL. To the resultant third retentate
is added 103 mL of a mixture of 40% by weight of ethylene glycol,
25% by weight of glycerol and 35% by weight of ethanol and
diafiltration is performed in concentration mode until the volume
of the retentate is about 100 mL. The resultant fourth retentate is
passed through syringe filters (diameter 30 mm, pore size 1.5
.mu.m) to prepare an ink for ink-jet printing. This ink has
properties similar to those of an ink formulated from dry silver
nanoparticles which have been synthesized in the same manner as the
particles of the initial suspension.
Example 7
[0133] To 1 L of a particle suspension composed of 250 g of unbound
PVP, 750 mL of ethylene glycol and about 20 g of silver
nanoparticles having PVP adsorbed thereon (prepared according to a
process similar to that described in Example 1) is added 2 L of
deionized water and the resultant mixture is subjected to a
diafiltration in a tangential cross-flow manner. The diafiltration
is carried out in concentration mode and is performed until the
retentate has a volume of about 100 mL. To the retentate is added 1
L of deionized water and diafiltration is performed in
concentration mode until the volume of the retentate is about 100
mL. To the resultant second retentate is added 500 mL of ethanol
and diafiltration is performed in concentration mode until the
volume of the retentate is about 100 mL. The resultant third
retentate is filled in a container with a loose fitting top and
placed in a vacuum oven that is maintained at 35.degree. C. A
vacuum is applied on the oven and continuously drawn, maintaining
the pressure at about 845 mbar until the particles are dry. The
dried particles are analyzed by TGA (thermogravimetric analysis)
and ICP. TGA indicates that the particles contain about 90% by
weight of silver. ICP indicates the following composition (in % by
weight): Ag 86.54, N 0.85, C 5.48, H 0.85. The dried particles are
formulated into an ink for ink-jet printing.
Example 8
[0134] To 1 L of a particle suspension composed of 250 g of unbound
PVP, 750 mL of ethylene glycol and about 20 g of silver
nanoparticles having PVP adsorbed thereon (prepared according to a
process similar to that described in Example 1) is added 2 L of
deionized water and the resultant mixture is subjected to a
diafiltration in a tangential cross-flow manner. The diafiltration
is carried out in concentration mode and is performed until the
retentate has a volume of about 100 mL. To the retentate is added 2
L of deionized water and diafiltration is performed in
concentration mode until the volume of the retentate is about 100
mL. To the resultant second retentate is added 2 L of deionized
water and diafiltration is performed in concentration mode until
the volume of the retentate is about 100 mL. To the resultant third
retentate is added 1 L of ethanol and diafiltration is performed in
concentration mode until the volume of the retentate is about 100
mL. The resultant fourth retentate is filled in a container with a
loose fitting top and placed in a vacuum oven that is maintained at
35.degree. C. A vacuum is applied on the oven and continuously
drawn, maintaining the pressure at 25 inch Hg until the particles
are dry. The dried particles are analyzed by TGA (thermogravimetric
analysis) and ICP. TGA indicates that the particles contain about
95% by weight of silver. ICP indicates the following composition
(in % by weight): Ag 92.79, N 0.49, C 2.57, H 0.27. The dried
particles are formulated into an ink for ink-jet printing.
Example 9
[0135] Silver nanoparticles prepared according to the process
described in Example 1 (ranging from about 30 nm to about 50 nm in
size) are suspended in a solvent mixture composed of, in weight
percent based on the total weight of the solvent mixture, 40% of
ethylene glycol, 35% of ethanol and 25% of glycerol to produce an
ink for ink jet printing. The concentration of the silver particles
in the ink is 20% by weight.
[0136] The ink had the following properties:
TABLE-US-00002 Viscosity* (22.degree. C.) 14.4 cP Surface tension**
(25.degree. C.) 31 dynes/cm Density 1.24 g/cc *measured at 100 rpm
with a Brookfield DVII+ viscometer (spindle no. 18) **measured with
a KSV Sigma 703 digital tensiometer with a standard Du Nouy ring
method
[0137] The ink is chemically stable for 6 months, some
sedimentation occurring after 7 days at room temperature.
[0138] A Spectra SE 128 head (a commercial piezo ink-jet head) is
loaded with the above ink and the following optimized printing
parameters are established:
TABLE-US-00003 Optimized Jetting Parameters (at 22-23.degree. C.):
Pulse Voltage 120 Volts Pulse Frequency 500 Hz (for up to one 1
hour of continuous operation) Pulse Rise Time 2.5 .mu.s Pulse Width
12.0 .mu.s Pulse Fall Time 2.5 .mu.s Meniscus Vacuum 3.0 inches of
water Performance Summary: Drop Size 39 .mu.m (calculated volume 31
pL) Drop Velocity 0.33 m/s Spot Size (average) 70 .mu.m (on Kapton
.RTM.; measured using optical microscope)
Spot Size (average) 70 .mu.m (on Kapton.RTM.; measured using
optical microscope)
[0139] The deposited ink can be rendered conductive after curing in
air at temperatures as low as 100.degree. C. The ink exhibits a
high metal yield, allowing single pass printing.
[0140] Using the above optimized jetting parameters, the above ink
is deposited in a single pass with a Spectra SE 128 head on a
Kapton.RTM. substrate and on a glass substrate to print a line. The
line has a maximum width of about 140 .mu.m (Kapton.RTM.) and about
160 .mu.m (glass) and a parabolic cross-section. The thickness of
the line at the edges averages about 275 nm (Kapton.RTM.) and about
240 nm (glass) and the maximum height of the line is about 390 nm
(Kapton.RTM. and glass). The differences between Kapton.RTM. and
glass reflect the different wetting behavior of the ink on these
two types of substrate materials.
[0141] Single pass printing with the above ink affords a sheet
resistivity of from about 0.1 to about 0.5 .OMEGA./sq. The printed
material shows a bulk resistivity in the fully sintered state of
from about 4 to about 5 .mu..OMEGA.cm (about 2.5-3 times the bulk
resistivity of silver).
[0142] The polymer (polyvinylpyrrolidone (PVP)) on the surface of
the silver nanoparticles allows the sintering of a deposited ink at
very low temperatures, e.g., in the range of from about 100.degree.
C. to about 150.degree. C. The PVP does not volatilize or
significantly decompose at these low temperatures. Without wishing
to be bound by any theory, it is believed that at these low
temperatures the polymer moves out of the way, allowing the cores
of the nanoparticles to come into direct contact and sinter
together (necking). In comparison to its anti-agglomeration effect
in the printing ink prior to printing, the polymer in the deposited
and heat-treated ink assumes a new function, i.e., it promotes the
adhesion of the printed material to a range of polymeric substrates
such as, e.g., FR4 (fiberglass-epoxy resin) and Mylar.RTM.
(polyethylene terephthalate) and provides structural strength. As a
result of the low-temperature sintering mechanism a continuous
percolation network is formed that provides continuous channels for
the conduction of electrons to flow throughout the material without
obstacles. This is fundamentally different from the traditional
polymer thick film approach, where electrical conductivity is
established during thermal curing as a result of polymer matrix
shrinkage, inducing compressive stress on the flake particles and
causing a reduction in their large contact resistance.
[0143] When higher-temperature sintering is performed (at about
300.degree. C. to about 550.degree. C.), the polymer volatilizes.
As a result, sintering will occur and in comparison to
low-temperature sintering a much denser metal material is formed.
This leads to a better conductivity (close to the conductivity of
the bulk metal), better adhesion to substrates such as glass, and
better structural integrity and/or scratch resistance.
[0144] In the low temperature sintering range (from about
100.degree. C. to about 150.degree. C.), the present ink can
advantageously be employed for applications such as, e.g., printed
RF ID antennas and tags, digitally printed circuit boards, smart
packages, "disposable electronics" printed on plastics or paper
stock, etc. In the medium temperature range (from about 150.degree.
C. to about 300.degree. C.) the ink may, for example, be used for
printing interconnects for applications in printed logic and
printed active matrix backpanes for applications such as polymer
electronics, OLED displays, AMLCD technology, etc. In the high
temperature range (from about 300.degree. C. to about 550.degree.
C.) its good performance and adhesion to glass make it useful for
printed display applications such as, e.g., plasma display
panels.
Example 10
Coductivity Testing of Compositions on Various Paper Substrates
[0145] It was found that the Ag ink composition of Example 9 yields
ink jet printed lines on Epson Gloss IJ ink jet paper that exhibit
an electric resistance after annealing at 100.degree. C. which is
comparable to that of the same ink printed on Kapton and annealed
at 200.degree. C.
[0146] In one set of tests, the following experiments were carried
out: [0147] An aqueous silver ink was jetted onto glossy IJ photo
paper (Canon), producing three groups of 4 lines; 1 set as single
pass, 1 set as double pass, and 1 set as triple pass. All three
sets were annealed on a hot plate set to 200.degree. C. for 30
minutes. After the anneal, the lines were tested for electrical
conductivity; all lines failed to exhibit conductivity. [0148] The
solvent-based Ag ink of Example 9 was printed on EPSON S041286
Gloss photo paper to produce samples for comparison testing with a
commercially available Ag ink sample (Nippon Paint) printed on
Cannon gloss paper (model not known). Two samples were printed, 1
coupon with a single print pass and 1 coupon with a double print
pass. [0149] The double pass print was annealed at 100.degree. C.
for 60 minutes. [0150] The commercial Ag ink sample was cured at
100.degree. C. for 60 minutes. [0151] The single pass print was
annealed at 100.degree. C. for 110 minutes.
[0152] Both samples produced with the ink of Example 9 exhibited
very good conductivity, comparable to the same silver ink, printed
on Kapton, and annealed at 200.degree. C. for 30 minutes.
[0153] The commercially available ink yielded a conductivity much
worse than that of the ink samples according to the present
invention.
[0154] The ink of Example 9 was printed on four different
substrates: (a) Kapton HN-300, (b) Hammermill 05502-0 gloss color
copy paper, (c) Canon Bubblejet Gloss Photo Paper GP-301 and (d)
Epson Gloss Photo Paper for ink-jet S041286.
[0155] The results listed in the table below confirm the superior
performance of the Example 9 ink/Epson paper combination.
TABLE-US-00004 Cure Approx. Temp/ Resistivity pH of Ink Substrate
Time (.mu..OMEGA. *cm).sup.1 Substrate.sup.3 Example 9 Kapton
200.degree. C./ 21 N/A 30 min Example 9 Kapton 100.degree. C./ 180
N/A 60 min Example 9 Epson 100.degree. C./ 16 4-5 Photo Paper 60
min Example 9 Xerox 100.degree. C./ No N/A High Gloss 60 min
Conductivity Example 9 Canon 100.degree. C./ 525 6.5-7 Photo Paper
60 min Commercial Canon 100.degree. C./ 5400.sup.2 6.5-7 Photo
Paper 60 min N/A HP Premium N/A N/A 5.0 Satin Gloss N/A HP Premium
N/A N/A 5.0 Gloss N/A Kodak N/A N/A 5.5 Premium Gloss N/A Kodak
Ultima N/A N/A 5.0-5.5 N/A Canon PP101 N/A N/A 6.5 N/A Canon PR101
N/A N/A 6.5-7.0 N/A Fuji N/A N/A 7.0 .sup.1assuming 1-micron line
thickness .sup.2average based on fewer measurements than ink of
Example 9 .sup.3pH of the substrate determined using solution
indicator kit: VWR VW 5704-1 Universal Indicator solution
(isopropanol 50% (v/v), NaOH, H.sub.2O).
[0156] The results shown in the table above suggest that it is
possible that the pH of the substrate may have an effect on the
resistivity of the inks printed thereon. It is contemplated that an
ink that is printed on an acidic substrate will have a lower
resistivity than an ink printed on a relatively more basic
substrate (compare Example 9 ink printed on Epson photo paper
versus the same ink printed on Canon photo paper).
[0157] It is known that Canon photo paper is coated with basic
alumina, while Epson photo paper is coated with silica. See, e.g.,
Blum, A. E. and Eberl, E. E., Clays and Clay Minerals 52: 589-602
(2004), the contents of which are incorporated herein by reference
in their entirety. Other coatings, e.g., alumina doped silica, are
also known in the art. See, e.g., European Patent Application
EP0995718, the contents of which are incorporated herein by
reference in their entirety. It is possible that the silica coating
on the Epson paper imparts acidic properties in the paper.
Likewise, it is possible that the alumina coating on the Canon
paper imparts less acidic properties in that paper.
[0158] The reasons for the apparent effect of substrate pH on the
resistivity of the ink printed thereon are not entirely clear at
this time. It is possible that, in addition to the substrate pH,
other substrate characteristics, alone, or in conjunction with the
presence of certain materials found in the ink formulation, may be
the actual factors that cause inks printed on certain substrates to
have lower resistivities than when they are printed on other
substrates. Such characteristics may include the nature of coatings
on the substrate and substrate porosity.
[0159] As mentioned above, Epson paper is coated with silica, while
the Canon paper is coated with basic alumina. Without being bound
by any particular theory, it is possible that the PVP on the
nanoparticles adsorbs to some extent on the silica coating on the
Epson paper, thereby exposing the silver nanoparticle surface.
Adsorption of the PVP on silica may be expected, since it is known
that silica has a high affinity for PVP. See, e.g., Blum, A. E. and
Eberl, E. E., Clays and Clay Minerals 52: 589-602 (2004), the
contents of which are incorporated herein by reference in their
entirety. When the ink is subsequently cured, the exposed silver
nanoparticles may be in closer contact with one another and may
sinter to form a network with decreased resistivity.
[0160] It is also possible that the substrate porosity may lead to
inks with decreased resistivity upon curing. Without being bound by
any particular theory, it is possible that the substrate porosity,
in conjunction with the presence of plasticizers (e.g., glycerol)
in the ink formulation may be aiding in the adsorption of PVP into
the pores of the substrate, thereby exposing the silver of the
nanoparticles. Without being bound by any particular theory, it is
possible that plasticizers sufficiently soften PVP such that the
PVP can migrate onto the substrate and away from the silver
nanoparticles. When the ink is subsequently cured, the exposed
silver nanoparticles may be in closer contact with one another and
may sinter to form a network with decreased resistivity.
[0161] It is noted that the foregoing examples have been provided
merely for the purpose of explanation and are in no way to be
construed as limiting of the present invention. While the present
invention has been described with reference to an exemplary
embodiment, it is understood that the words that have been used are
words of description and illustration, rather than words of
limitation. Changes may be made, within the purview of the appended
claims, as presently stated and as amended, without departing from
the scope and spirit of the present invention in its aspects.
Although the invention has been described herein with reference to
particular means, materials and embodiments, the invention is not
intended to be limited to the particulars disclosed herein.
Instead, the invention extends to all functionally equivalent
structures, methods and uses, such as are within the scope of the
appended claims.
* * * * *