U.S. patent application number 10/412971 was filed with the patent office on 2003-11-06 for synthesis of substantially monodispersed colloids.
Invention is credited to Klabunde, Kenneth J., Sorensen, Christopher, Stoeva, Savka.
Application Number | 20030207949 10/412971 |
Document ID | / |
Family ID | 25525567 |
Filed Date | 2003-11-06 |
United States Patent
Application |
20030207949 |
Kind Code |
A1 |
Klabunde, Kenneth J. ; et
al. |
November 6, 2003 |
Synthesis of substantially monodispersed colloids
Abstract
A method of forming ligated nanoparticles of the formula
Y(Z).sub.x where Y is a nanoparticle selected from the group
consisting of elemental metals having atomic numbers ranging from
21-34, 39-52, 57-83 and 89-102, all inclusive, the halides, oxides
and sulfides of such metals, and the alkali metal and alkaline
earth metal halides, and Z represents ligand moieties such as the
alkyl thiols. In the method, a first colloidal dispersion is formed
made up of nanoparticles solvated in a molar excess of a first
solvent (preferably a ketone such as acetone), a second solvent
different than the first solvent (preferably an organic aryl
solvent such as toluene) and a quantity of ligand moieties; the
first solvent is then removed under vacuum and the ligand moieties
ligate to the nanoparticles to give a second colloidal dispersion
of the ligated nanoparticles solvated in the second solvent. If
substantially monodispersed nanoparticles are desired, the second
dispersion is subjected to a digestive ripening process. Upon
drying, the ligated nanoparticles may form a three-dimensional
superlattice structure.
Inventors: |
Klabunde, Kenneth J.;
(Manhattan, KS) ; Stoeva, Savka; (Manhattan,
KS) ; Sorensen, Christopher; (Manhattan, KS) |
Correspondence
Address: |
Hovey Williams LLP
Suite 400
2405 Grand Boulevard
Kansas City
MI
64108
US
|
Family ID: |
25525567 |
Appl. No.: |
10/412971 |
Filed: |
April 14, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10412971 |
Apr 14, 2003 |
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09977838 |
Oct 15, 2001 |
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6562403 |
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Current U.S.
Class: |
516/9 |
Current CPC
Class: |
Y10S 977/773 20130101;
B01J 13/0043 20130101; Y10S 977/896 20130101; B82Y 30/00
20130101 |
Class at
Publication: |
516/9 |
International
Class: |
B01F 003/00 |
Goverment Interests
[0002] This invention was made with government support under Grant
No. NAG8-1687 awarded by NASA. The government has certain rights in
the invention.
Claims
We claim:
1. A colloidal nanoparticle dispersion comprising nanoparticles
saturated in a molar excess of a first solvent, a second solvent
different than the first solvent, and a quantity of ligand
moieties.
2. The dispersion of claim 1, said first solvent having a boiling
point at least about 25.degree. C. below the boiling point of the
second solvent.
3. The dispersion of claim 2, the first solvent having a boiling
point at least about 40.degree. C. below the boiling point of the
second solvent.
4. The dispersion of claim 1, wherein said first solvent is
selected from the group consisting of ketones of the formula 3where
R.sub.1 and R.sub.2 are independently and respectively selected
from the group consisting of straight and branched chain C1-C5
alkyl and alkenyl groups, and the C1-C5 straight and branched chain
alcohols.
5. The dispersion of claim 3, said first solvent being acetone.
6. The dispersion of claim 1, said second solvent being an aryl
organic solvent.
7. The dispersion of claim 6, said second solvent selected from the
group consisting of solvents of the formula 4where X.sub.1 and each
X.sub.2 are each independently and respectively selected from the
group consisting of H and C1-C5 straight and branched chain alkyl
and alkenyl groups, n is from 0 to 3, and each X.sub.2 may be
independently located at any unoccupied ortho, meta or para
position relative to X.sub.1.
8. The dispersion of claim 7, said second solvent being
toluene.
9. The dispersion of claim 1, said colloidal nanoparticles selected
from the group consisting of the elemental metals having atomic
numbers ranging from 21-34, 39-52, 57-83 and 89-102, all inclusive,
the halides, oxides and sulfides of such metals, and the alkali
metal and alkaline earth metal halides.
10. The dispersion of claim 1, said colloidal nanoparticle selected
from the group consisting of elemental gold and silver.
11. The dispersion of claim 1, said dispersion formed by vaporizing
a solid substance and said first solvent, depositing the vaporized
atoms or molecules and first solvent onto a cold surface, to give a
mixture, warming the mixture and forming nanoparticles by
aggregation of the atoms or molecules, allowing the nanoparticles
and first solvent mixture to mix with said second solvent and
ligand moieties.
Description
RELATED APPLICATION
[0001] This application is a division of Ser. No. 09/977,838, filed
Oct. 15, 2001.
BACKGROUND OF THE INVENTION
[0003] 1. Field of the Invention
[0004] The present invention is concerned with methods of forming
large quantities of ligated nanoparticles which can be deposited in
two- and three-dimensional superlattices. Broadly speaking, the
method involves initially forming a first colloidal dispersion made
up of nanoparticles solvated in a molar excess of a first solvent,
a second solvent different from the first solvent, and a quantity
of ligand moieties. Thereupon, a substantial part of the first
solvent is removed and the ligand moieties are caused to ligate to
the nanoparticles to give a second colloidal dispersion.
Preferably, the second dispersion is subjected to a heat and reflux
digestive ripening process to give substantially monodispersed
colloidal particles. The invention also pertains to the ligated
nanoparticle colloidal dispersions and to the final products.
[0005] 2. Description of the Prior Art
[0006] It is known that high surface area nanoparticles can be
formed by a vaporization-co-condensation process sometimes referred
to as the solvated metal atom dispersion (SMAD) method. The latter
involves vaporization of a metal under vacuum and codeposition of
the metal atoms with the vapors of a solvent on the walls of a
reactor cooled to 77 K (liquid nitrogen temperature). After
warm-up, nanoparticles are stabilized both sterically (by
solvation) and electrostatically (by incorporation of negative
charge). The SMAD technique was first disclosed in 1986 by Klabunde
and co-workers, and is also described in U.S. Pat. No. 4,877,647. A
major advantage of the SMAD process is that no biproducts of metal
salt reduction are present, and pure metal colloids are formed.
Additionally, the SMAD process lends itself to industrial-scale
operations, as opposed to other competing processes such as the
inverse micelle and reductive procedures for metal colloid
preparation.
[0007] Organization of nanoparticles into two and three-dimensional
structures (nanocrystalline superlattices, NCSs) leads to the
formation of materials characterized by very different properties
compared to those of the discrete species. The manifestation of
novel and technologically attractive properties is due to the
collective interactions of the particles, as well as to the finite
number of atoms in each crystalline core. Synthesis and
characterization of such materials are interesting from both
fundamental and industrial points of view. Regularly arranged
nanosized particles find applications in the development of optical
and electronic devices, and magnetic recording media, for example.
Nanoparticles of gold and other noble metals have attracted
significant attention not only because of ease of preparation, but
also due to their potential application in nano and
microelectronics. Heretofore the challenge has been to form a
structure of a planar array of small metal islands separated by
tunnel barriers for use in electronics. Gold nanoparticles are
excellent candidates in this respect.
[0008] Numerous methods for synthesis of particles arranged in 2D-
and 3D-NCSs have been reported in the literature. The most common
procedures include reduction of a suitable metal salt in the
presence of different stabilizing agents. In all methods, the most
important requirement is the ability to produce monodispersed
particles that can order over a long-range. Crystalline arrays of
particles covered by organic molecules have become of great
interest, especially since the improved synthesis of
thiol-stabilized gold nanoparticles has been developed (Brust, et
al., J. Chem. Soc., Chem. Commun., 1994, 801-802). Their advantage
is that they behave as simple chemical compounds in respect that
they can be dissolved, precipitated, and redispersed without change
in properties, much as molecular crystals can.
SUMMARY OF THE INVENTION
[0009] The present invention is broadly concerned with methods of
forming ligated nanoparticle colloidal dispersions and recovered
ligated nanoparticles which may be in superlattice form. In
general, the method involves initially forming a first colloidal
dispersion comprising nanoparticles solvated in a molar excess of a
first solvent, a second solvent different than the first solvent,
and a quantity of ligand moieties. Next, a substantial part of the
first solvent is removed and the ligand moieties are caused to
ligate to the nanoparticles to give a second colloidal dispersion
comprising the ligated nanoparticles solvated in the second
solvent. If desired, the ligated nanoparticles may then be
recovered as a dry product which, depending upon the nature of the
nanoparticles and ligands selected, may assume a superlattice
configuration.
[0010] Preparation of the first colloidal dispersion is preferably
accomplished by vaporizing the solid substance (e.g., metal or
metal salt) and first solvent in a reactor to give vaporized atoms
or molecules and depositing the vaporized atoms or molecules and
first solvent onto a cold surface. Upon subsequent warming of this
mixture, nanoparticles are formed by aggregation of the atoms or
molecules, and these nanoparticles and first solvent are allowed to
mix with a second solvent and ligand moieties. Thereupon, the first
solvent is removed by vacuum, which substantially completely
eliminates the first solvent and also, to a limited degree, some of
the second solvent.
[0011] In a particularly preferred technique, the second colloidal
dispersion is subjected to a digestive ripening process so that the
variation in particle size of the ligated nanoparticles is reduced;
this ripening process is advantageously carried out until the
second colloid is essentially monodispersed. This ripening process
is also important if a superlattice dry product is desired.
[0012] The nanoparticles useful in the invention are generally
selected from the group consisting of the elemental metals having
atomic numbers ranging from 21-34, 39-52, 57-83 and 89-102, all
inclusive, the halides, oxides and sulfides of such metals, and the
alkali metal and alkaline earth metal halides. Elemental gold and
silver are particularly preferred, with elemental gold being the
single most preferred nanoparticle material. The nanoparticles
should have an average diameter of from about 2-50 nm, and more
preferably from about 3-15 nm. Similarly, the nanoparticles should
have a BET surface area of from about 15-500 m.sup.2/g, and more
preferably from about 50-300 m.sup.2/g.
[0013] The first and second solvents should be selected so that the
first solvent may be readily removed by vacuum distillation or
other techniques from the initial colloid. In practice, the first
solvent should have a boiling point at least about 25.degree. C.
(more preferably at least about 40.degree. C.) below the boiling
point of the second solvent. Of course, the first and second
solvents must also have the ability to solvate the nanoparticles
and ligated nanoparticles, respectively.
[0014] Although a wide variety of solvents may be employed,
preferably the first solvent is a ketone, and especially a ketone
selected from the group consisting of ketones of the formula 1
[0015] where R.sub.1 and R.sub.2 are independently and respectively
selected from the group consisting of straight and branched chain
C1-C5 alkyl and alkenyl groups, and the C1-C5 straight and branched
chain alcohols. The single most preferred first solvent is acetone.
The first solvent should be used at a level so that it is in molar
excess relative to the nanoparticles, and preferably a molar excess
of from about 50-1000 should be established.
[0016] The second solvent is preferably an aryl organic solvent
such a toluene or xylene. More broadly, the solvent is
advantageously selected from the group consisting of solvents of
the formula 2
[0017] where X.sub.1 and each X.sub.2 are each independently and
respectively selected from the group consisting of H and C1-C5
straight and branched chain alkyl and alkenyl groups, n is from 0
to 3, each X.sub.2 may be independently located at any unoccupied
ortho, meta or para position relative to X.sub.1.
[0018] A variety of ligands may be used in the invention, and can
be atoms, ions, or compounds. As used herein "ligand moieties"
refers to all such ligand species. The preferred class of ligands
are thiol compounds selected from the group consisting of
compounds
R.sub.3SH
[0019] where R.sub.3 is a C5-C20 straight or branched chain alkyl
or alkenyl group. More preferably, R.sub.3 is a C10-C15 straight or
branched alkyl or alkenyl group; an especially preferred ligand is
dodecanethiol.
[0020] The digestive ripening process comprises the step of heating
and refluxing the second colloidal dispersion, preferably at a
temperature of from about 60-180.degree. C. under an inert gas such
as argon for a period of from about 10-400 minutes. The goal of
digestive ripening is to reduce the particle size variation in the
ligated nanoparticles; preferably, this process is carried out to
achieve a ligated nanoparticle surface area of up to about 20%
above and below the mean surface area of the ligated
nanoparticles.
[0021] The final ligated nanoparticles in general have the formula
Y(Z).sub.x where Y is the nanoparticle and Z is the ligand; x is
variable depending upon the nanoparticle and ligand selected. In
the case of the preferred Au(dodecanethiol) ligated nanoparticles,
x would typically range from about 300-10,000, with a ligand
density on the gold nanoparticle surface ranging from about 1-10
ligand moieties per square nanometer of nanoparticle surface
area.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] FIG. 1 is a flow diagram illustrating the synthetic steps in
a preferred method for the preparation of nanocrystal superlattice
products;
[0023] FIG. 2 is an TEM micrograph of gold particles in an
Au-acetone-toluene-thiol colloid (colloid 1) described in the
Example;
[0024] FIG. 3A is a TEM micrograph of gold particles in an
Au-toluene-thiol colloid (colloid 2) described in the Example;
[0025] FIG. 3B is another TEM similar to that of FIG. 3A, but
viewing another area of the TEM grid;
[0026] FIG. 4 is a graph depicting the UV/VIS absorption spectra of
as-prepared colloid 2 (solid line) and the digestively ripened
colloid (dotted line);
[0027] FIG. 5A is a TEM micrograph of gold particles after the
digestive ripening step in the Example, where sampling was done
from the hot colloidal dispersion;
[0028] FIG. 5B is another TEM photograph similar to that of FIG.
5A;
[0029] FIG. 6A is TEM micrograph of gold particles after the
digestive ripening step of the Example (sampled from hot
dispersion);
[0030] FIG. 6B is a histogram derived from the measurement of 400
particles which corresponds to the gold particle TEM micrograph of
FIG. 6A;
[0031] FIG. 7A is a TEM micrograph of gold particles 15 minutes
after the completion of the digestive ripening process of the
Example;
[0032] FIG. 7B is a TEM micrograph similar to that of 7A, but
depicting the gold particles one day after the completion of the
digestive ripening process of the Example;
[0033] FIG. 7C is another TEM micrograph similar to that of 7B, and
depicting the gold particles one day after the completion of the
digestive ripening process of the Example; and
[0034] FIG. 7D is a TEM micrograph similar to that of 7A, but
depicting the gold particles approximately two months after the
completion of the digestive ripening process of the Example.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0035] The following example sets forth presently preferred methods
for the preparation of ligated nanoparticle superlattices in
accordance with the invention. It is to be understood, however,
that this Example is provided by way of illustration and nothing
therein should be taken as a limitation upon the overall scope of
the invention.
[0036] FIG. 1 is a flow diagram of the most preferred preparation
of gold-containing nanocrystalline superlattice products. This
method is also explained in detail below.
EXAMPLE
[0037] A) Preparation of Au-acetone-toluene-thiol colloid (colloid
1).
[0038] A stationary reactor described in Klabunde, et al., Inorg.
Syn., Shriver, D., ed., 19, (1979), 59-86 was used for the
synthesis of Au-acetone-toluene-thiol colloid. Acetone and toluene
solvents were purchased from Fisher. Acetone was dried over
molecular sieve. Both acetone and toluene were degassed five times
by the standard freeze-thaw procedure prior to the reaction.
Dodecanthiol was purchased from Aldrich and used as received. All
glassware was rigorously cleaned before use.
[0039] A W-Al.sub.2O.sub.3 crucible was assembled in the SMAD
reactor and the whole system was pumped down. This was followed by
a step-wise heating of the crucible and the pressure was allowed to
reach 4.times.10.sup.-3 Torr at each heating step. The crucible was
heated to red in about half an hour, then the heating was decreased
and the whole reactor was left under vacuum overnight while the
crucible was gently heated. This process ensured no contamination
of the crucible. After the overnight treatment, the reactor was
filled with air and the crucible was charged with .about.0.3 g Au
metal. At the same time 8 ml (6.8 g, 3.4.times.10.sup.2mol) of
dodecanethiol was placed in the bottom of the reactor chamber
together with a stirring bar. Degassed acetone and toluene solvents
were placed in Schlenk tubes and attached to the SMAD reactor. The
whole system was evacuated and a liquid nitrogen filled Dewar
placed around the vessel. Dodecanethiol was frozen in this way in
the bottom of the reactor. When the vacuum reached
4.times.10.sup.-3 Torr, 40 ml of toluene was evaporated in
.about.15 min and frozen on the walls of the reactor. The liquid
nitrogen Dewar was removed and toluene allowed to melt undisturbed
and fall to the bottom of the reactor. The liquid nitrogen Dewar
was again put in place, and Au vapor (0.27 g,
1.4.times.10.sup.-3mol) and acetone (100 ml) were codeposited over
a period of 3 hours. During this time, the pressure was maintained
at about 4.times.10.sup.-3 Torr. The frozen matrix had a deep red
color at the end of the deposition. After the process was complete
the liquid nitrogen Dewar was removed and the matrix allowed to
warm slowly over a period of .about.1 hour. During the warmup
process argon gas was allowed to fill the reactor system. Upon
melting the Au-acetone matrix mixed with the toluene and the color
became deep brown. When the dodecanethiol started to melt, stirring
was started and the whole solution was agitated for another 45 min.
The as-prepared dark brown Au-acetone-toluene-thiol colloid
(colloid 1) was syphoned under argon into a Schlenk tube.
[0040] B) Preparation of Au-toluene-thiol colloid (colloid 2).
[0041] The Schlenk tube containing the as-prepared
Au-acetone-toluene-thio- l colloid (colloid 1) was connected to a
vacuum line and the acetone was evaporated until a constant
1.times.10.sup.-2 Torr pressure was reached (the more volatile
acetone was removed along with some of the toluene). At this time
the Au-toluene-thiol colloid was diluted to 80 ml by addition of
degassed toluene. Thus the total volume of the final dark brown
Au-toluene-thiol colloid was 80 ml containing about 0.20 g of
gold.
[0042] C) Digestive Ripening.
[0043] The digestive ripening process is an important step for
formation of a monodispersed colloid from the polydisperse
Au-toluene-thiol colloid (colloid 2). The procedure involved
heating under reflux of a certain amount of Au-toluene-thiol
colloid for 1.5 hours. The heating temperature is the boiling point
of the colloidal solution (.about.120.degree. C.). The digestive
ripening was carried out under an argon atmosphere.
[0044] D) Isolation of a Dry Product.
[0045] Isolation of a dry product was done after the
gold-toluene-thiol colloid (colloid 2) was subjected to digestive
ripening for 1.5 h. After cooling down to room temperature, 10 ml
of the digested colloid (containing 0.025 g Au) was precipitated
with 50 ml of absolute ethanol. After overnight treatment, the
precipitation was complete and the supernatant was carefully
removed by sucking out with a Pasteur pipette. The remaining
precipitate together with a small amount of leftover toluene, thiol
and ethanol was dried under vacuum until constant pressure
(5.times.10.sup.-3 Torr). After drying, the color of the product
was brown-red and it had the appearance of a wet paste. An
additional 3 ml of ethanol was added and the system was left
undisturbed overnight. The supernatant was then removed and the
sediment again was dried under vacuum at constant pressure. After
drying the precipitate (0.0214 g) was a powder with small
shiny-dark crystals. It was washed again with 3 ml of ethanol, left
overnight, the supernatant removed and dried under vacuum. After
drying, the precipitate was 0.0207 g and no change of the mass was
recorded after additional washing with ethanol and drying under
vacuum. The yield was 84% based on gold. If the adsorbed thiol is
taken into account, the yield was .about.73%.
[0046] The final dry product was in the form of soft, shiny dark
crystals, which are readily soluble in toluene or hexane. After
addition of the solvent, the crystals immediately dissolved giving
wine-red colored colloidal solution. However, the crystals are not
soluble in ethanol or acetone.
[0047] E) UV-VIS Spectroscopy.
[0048] UV/VIS absorption spectra were obtained using a Fiber Optic
CCD Array UV-VIS Spectrophotometer of Spectral Instruments,
Inc.
[0049] F) Transmission Electron Microscopy (TEM).
[0050] TEM studies were performed on a PHILIPS CM100 operating at
100 kV. The TEM samples were prepared by placing a 3 .mu.l drop
from the colloidal solution onto a carbon coated formvar copper
grid. The grids were allowed to dry in air for 1 hour and left
undisturbed at ambient conditions.
[0051] Results and Discussion
[0052] Since the first report in 1986 (Lin, et al., Langmuir, 2,
(1986), 259-260) of the synthesis of nonaqueous colloidal gold
solutions by the SMAD method, considerable work has been carried
out on the preparation and characterization of several non-aqueous
metal nanoscale particles (Franklin, et al., High-Energy Processes
in Organometallic Chemistry; Suslick, K. S., ed., ACS Symposium
series, (1987), 246-259; Trivino, et al., Langmuir, 3, 6, (1987),
986-992). Colloidal solutions of gold in acetone have been one of
the most intensively studied and well-understood systems. Acetone,
as a polar solvent, solvates the metal atoms and clusters during
the warmup stage. In this way steric stabilization is achieved and
gold colloids are stable for months.
[0053] These earlier results were the motivation for choosing
acetone as an initial solvent in the present example. Preliminary
attempts to improve size-distribution of particles from pure
acetone solutions using the digestive ripening procedure turned out
unsuccessful, and it was discovered that an additional stabilizing
agent like dodecanethiol was needed. However, when only acetone was
used as the solvent, addition of dodecanethiol did not allow the
formation of a stable colloid. For example, the precipitate formed
after addition of dodecanethiol to Au-acetone colloids, when
separated and dried under vacuum, was only partially redispersable
in toluene. Digestive ripening of the partially redispersed
Au-colloids led to the size improvement of only those particles
that were redispersed. The particles that remained in the sediment
did not change their shape and size during this procedure.
Therefore, it was found that a combination of solvents such as
acetone and toluene was needed during the SMAD reaction and
subsequent cluster growth and ligation by the thiol. The role of
acetone was found to be stabilization of the gold nanoparticles in
a preliminary way.
[0054] The size and shape changes of nanoparticles in the different
samples were investigated by TEM. Representative transmission
electron micrographs of the gold colloids at each step of the
preparative procedure of the monodispersed colloid are shown in the
Figures. A flow diagram of the major synthetic steps is given in
FIG. 1. The results from the separate preparative stages are
discussed below.
[0055] Formation of Monodispersed thiol-protected Au-colloid
[0056] A) Au-acetone-toluene-thiol colloid (colloid 1).
[0057] The initial Au-acetone-toluene-thiol colloid has a dark
brown color. TEM studies of this colloid (FIG. 2) illustrate
particles ranging from 5 to 40 nm with no definite geometrical
shapes. These particles are very similar to the ones obtained in
pure acetone solvent. As reported in the prior art, two types of
stabilization are characteristic for these systems:
[0058] 1) steric stabilization (by solvation with the acetone
molecules) and 2) electrostatic stabilization (by acquiring
electrons from the reaction vessel walls, electrodes, solvent
medium). Another indication that the gold particles are negatively
charged is the occasionally observed `blinking` in the electron
microscope due to the interaction of the particles with the
negatively charged electron beam. However, it should be pointed out
that in no case was change in the shape or morphology of the
particles observed under the influence of the electron beam. Both
stabilization processes take place during the warmup step, should
to be carried out slowly in order to ensure good stabilization.
[0059] B) Au-toluene-thiol colloid (colloid 2).
[0060] The Au-toluene-thiol colloid (colloid 2) was obtained by
vacuum evaporation of all the acetone from colloid 1. TEM
micrographs of two representative types of particles found in the
colloid are shown in FIGS. 3A and 3B. Drastic change of the size
and shape of the particles is characteristic at this stage. Nearly
spherical particles with sizes in the range of 1 to 5 nm are
dominant. There are also a small number of larger particles (10-40
nm) like those in the initial acetone-containing colloid.
[0061] UV/VIS absorption spectrum (FIG. 4) of colloid 2 is in
agreement with the sizes of the particles observed in TEM. It is
characterized by a broad plasmon absorption band with no definite
maximum.
[0062] One possible explanation for the change of size and shape of
the gold particles induced by the removal of acetone is the
following. In colloid 1 the amount of acetone is in great excess.
It strongly solvates the gold particles and the attachment of
dodecanethiol molecules on the particles' surface is suppressed. As
acetone is removed from the system, the ability for thiol
adsorption is increased. Thus acetone acts as a preliminary
stabilizing agent, which is substituted by dodecanethiol molecules
when acetone is evaporated. This ensures good dispersity of the
thiol-ligated gold particles in the toluene medium. The fact that
most of the particles in the Au-colloid after evaporation of
acetone have size in the region of 1 to 5 nm suggests that some
ripening has already taken place, presumably due to the strong
adsorption of dodecanethiol molecules on their surface. At this
stage the colloid is ready for digestive ripening.
[0063] C) Digestive Ripening of colloid 2 and Organization of the
Gold Particles.
[0064] Heating of colloid 2 under reflux results in a dramatic
narrowing of the particle size-distribution. TEM studies (FIGS. 5A
and 5B) of a hot colloidal solution show formation of spherically
shaped particles with sizes of about 4 nm. They have a tendency to
organize into 2D-layers. Some of the particles from the hot colloid
organize in nice 3D-structures. The remarkable effect of the
digestive ripening procedure is the great improvement of the
size-distribution. Practically polydisperse colloid containing
particles with sizes ranging from 1 to 40 nm are transformed into
an almost monodispersed colloid with particles' sizes of about
4-4.5 nm. A photograph taken at higher magnification (FIG. 6)
reveals that the shape of the particles is more polyhedral rather
than spherical. The average size diameter is 4.5 nm and the
size-distribution is log-normal as typical for colloidal systems.
The UV/VIS absorption spectrum of the colloid after cooling to room
temperature (FIG. 4) shows an appearance of a definite plasmon
absorption maximum at 513 nm, which is in agreement with the size
and monodispersity of the obtained particles.
[0065] The TEM micrographs of colloids cooled down for a different
amount of time are shown in FIGS. 7A-7D. The amazing result is that
the particles predominantly organize on the TEM grid in large
3D-structures in only about 15 min after the digestive ripening
process is finished (FIG. 7A). A small number of areas of
2D-arrangement is also observed.
[0066] Even larger 3D-structures (>3 .mu.m) are observed after 1
day (FIGS. 7B and 7C) and after .about.2 months (FIG. 7D). The
results suggest that the activation energy for 2D-organization is
lower compared to this of 3D-organization.
* * * * *