Note: Descriptions are shown in the official language in which they were submitted.
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MULTICOMPONENT NANOPARTICLES
FORMED USING A DISPERSING AGENT
BACKGROUND OF THE INVENTION
1. The Field of the Invention
The invention is in the field of nanoparticles and/or catalysts that
incorporate
such nanoparticles. More particularly, the present invention relates to multi-
component nanoparticles made using a dispersing agent that helps bring
together and
distribute different (e.g., dissimilar) components within the nanoparticles.
1o 2. The Relevant Technology
Nanoparticles are becoming increasingly more important in many industrial
processes and products. Nanoparticles find use in a variety of applications,
including
catalysis and nanomaterials. Catalytic applications include uses for both
supported
and unsupported nanoparticles of various components, including precious
metals,
base metals, and oxides. Nanomaterial applications include uses for light
blocking,
pigmentation, UV absorption, antimicrobial activity, chemical mechanical
polishing,
and others.
While useful nanoparticles may include only a single component (element or
coinpound), it may be the case that advantageous properties can be achieved if
the
nanoparticles were to contain two or more distinct components to form a
multicomponent nanoparticle. In general, combinations of two or more metals
can
have a variety of beneficial effects. In the case of catalysts, the use of
different
elements can modify the catalytic activity to improve an important performance
parameter such as activity or selectivity, or they may make the catalyst
particle or
crystal more resistant to some deleterious effect, such as chemical poisoning
or
mechanical attrition. In the case of nanomaterials, the inclusion of two or
more
components would be expected to add additional functionality to the particles,
such as
combining light blocking function with W absorption or anti-microbial
activity.
Alternatively, additional components might be expected to stabilize or
strengthen the
nanoparticles.
While there is a strong motivation for producing multicomponent
nanoparticles, it is difficult, if not impossible, to manufacture particles
that contain
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two or more dissimilar components. This problem is particularly true of small
nanoparticles. Recently, academia and industry have made significant
advancements
toward making very small particles. In some cases, the sizes of the particles
are near
or below 1 nanometer.
While nanometer sized particles are very advantageous for producing desired
properties such as increased catalytic activity and unique material
properties, the very
smallness of such particles makes it difficult, if not impossible, to create
multicomponent nanoparticles that include dissimilar components or elements
within
the same nanoparticle. One reason for this difficulty is that similar or like
elements
or compounds have a greater affinity for each other than to dissimilar
materials. This
same-component attraction means each component has a propensity to combine and
form particles with itself rather than forming a mixture with other,
dissimilar
components. As a result, multicomponent nanoparticle mixtures are largely
heterogeneous, composed of two or more distinct particle compositions, each
relatively rich in one component and largely depleted or devoid of the other
dissimilar
components.
In general, the composition of particles, including the distribution of
different
components among and between the particles, is driven by tlzermodynamics. The
chance of finding multiple components in any given particle depends to a large
extent
on the size of the particles being formed. Where the particles are relatively
large, the
probability is higher that two dissimilar components can be compounded within
a
single particle and/or form an alloy. As the size of the particles decreases,
however,
the likelihood of finding multiple components within a single particle
decreases
dramatically. At the nanometer scale, it is virtually impossible to
consistently and
predictably compound two or more dissimilar elements within a single
nanoparticle
using known procedures. Small nanoparticles tend to be all of one component or
another.
Part of the problem with forming multicomponent nano-sized particles is that
conventional methods used to form nano-sized particles are performed at
relatively
low temperatures since high temperatures can causes nanoparticles to
undesirably
sinter or agglomerate together to form larger particles. Unfortunately, at
such low
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temperatures, the thermodynamics of nanoparticle formation favors formation of
single-component particles, as described above. On the other hand, raising the
temperature sufficiently to overcome thermodynamic barriers to multicomponent
formation causes agglomeration of smaller to larger particles. Consequently,
conventional particle formation methods are not able to form nano-sized
particles in
which a substantial portion of the nanoparticles contain two or more
components in
each particle.
Another factor that significantly affects the uniformity of multicomponent
particles is the dissimilarity of the components. For example, two noble
metals such
1o as palladium and platinum are typically more easily combined together
within
particles because their electronic and chemical properties are similar. In
contrast, a
noble metal such as platinum and a base metal such as iron have different
electronic
and chemical properties and are thus much more difficult, if not impossible,
to
compound together in a single nanoparticle using conventional manufacturing
methods. In many cases, compounding dissimilar components does not produce a
viable nanoparticle system because of ,the lack of uniformity in the
distribution of the
components throughout the nanoparticles. This is particularly true in the case
of
catalyst particles that require both catalyst components to be in close
proximity and/or
to be alloyed together to generate the desired catalytic activity.
R.W.J. Scott et al., JACS Cominunications, 125 (2003) 3708, state: "... at
present there are no methods for preparing nearly monodisperse, bimetallic
nanoparticles that are catalytically active . . . ." X. Zhang and K.Y. Chan,
Chem.
Mater., 15 (2003) 451, teach: "A number of techniques have been used for
producing
nanoparticles, including vapor phase techniques, sol-gel methods, sputtering,
and
coprecipitation. The synthesis of mixed metal nanoparticles is attracting a
lot of
recent interest for their catalytic properties.... The synthesis of mixed
metal
nanoparticles is a complex problem because of the composition control in
addition to
size and size distribution control. Platinum-ruthenium bimetallic catalysts
have been
prepared by co-impregnation methods but without good control of particle size,
particle size distribution, and chemical composition." R.W.J. Scott et al.,
JACS
Communications 127 (2005), 1380, disclose: "Most other methods for preparing
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supported bimetallic nanoparticles in the < 5 nm size range lead to phase
segregation
of the two metals and thus poor control over the composition of individual
particles."
K. Hiroshima et al, Fuel Cells, 2 (2002) 31, teach: "The preparation of a
highly
dispersed alloy catalyst typically requires heat treatment, which is necessary
to form
an alloy but promotes particle aggregation. As a result, alloy catalysts
usually have
lower surface areas."
Therefore, what are needed are multicomponent nanoparticles that include
different components that are more evenly dispersed among the particles.
Furthermore, what is needed are compositions and processes that can be used to
bring
together and compound different (e.g., dissimilar) components together in
individual
nanoparticles without destroying the nanometer size of the particles.
BRIEF SUMMARY OF THE INVENTION
The present invention relates to nanoparticle compositions that overcome the
limitations of the prior art by providing "nano" sized particles that are
composed of
two or more components in a desired distribution. During manufacture, a
dispersing
agent binds the two or more components and maintains them in close proximity
during nanoparticle formation in order to control the arrangement and/or
distribution
of the components in the nanoparticle material.
In an exemplary embodiment, the multicomponent compositions of the present
invention include a plurality of nanoparticles having a size less than about
100 nm.
According to one embodiment, the plurality of nano,particles includes at least
two
dissimilar nanoparticle components selected from different ones of the
following
groups: noble metals, base transition metals, alkali metals, alkaline earth
metals, rare
earth metals, and nonmetals. In an alternative embodiment, the multicomponent
composition is made from two dissimilar nanoparticle components selected from
two
or more different groups of the periodic table of elements. The components
that form
the nanoparticles can be elements or compounds such as elemental metals or
metal
oxides.
Preferably, at least about 50% of the nanoparticles include two or more
dissimilar components. More preferably, at least about 75% of the
nanoparticles
include two or more dissimilar components, even more preferably at least about
85%
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of the nanoparticles include two or more dissimilar components, and most
preferably
at least about 95% of the nanoparticles include two or more dissimilar
components. It
is within the scope of the invention for at least about 99% (or essentially
all) of the
nanoparticles to include two or more dissimilar components.
5 The present invention also includes a method to produce the uniform
multicomponent nanoparticles. In general, the process includes preparing first
and
second solutions of dissimilar components and mixing them together with a
dispersing agent to form a component complex. The molecules of the dispersing
agent bind to at least a portion of the molecules of the first and second
components to
sufficiently overcome the same-component attractions such that the components
can
be arranged randomly or according to the molecular arrangement of the
dispersing
agent within the suspension. In some cases the component complex forms a
suspension of nanoparticles. In other cases, the component complex is a
precursor to
the formation of nanoparticles (e.g., which may be formed by attaching the
component complex to a support and/or removing at least a portion of the
dispersing
agent from the component complex).
In one embodiment, a suspension of nanoparticles can be used as an active
catalyst while remaining in suspension form. In another embodiment, the
nanoparticles can be attached to or formed on a solid support by suitable
impregnation
or attachment methods. The nanoparticles can also be separated from some or
all of
the liquid to form a concentrate of nanoparticles or a dry powder. As needed,
the
suspension can be chemically modified to stabilize the nanoparticles (e.g.,
prevent
agglomeration), adjust pH, or otherwise adjust coinposition to suit an end use
application. In one embodiment, the nanoparticles can be isolated by removing
the
dispersing agent from the nanoparticles, such as under reducing conditions
(e.g., by
reducing under H2 gas or using strong reducing catalysts such as lithium
aluminum
hydride, sodium hydride, sodium borohydride, sodium bisulfite, sodium
thiosulfate,
hydroquinone, methanol, aldehydes, and the like, or by oxidation such as by
using
molecular oxygen, hydrogen peroxide, organic peroxides, and the like).
In an exemplary embodiment, the nanoparticles of the present invention are
also of a substantially uniform size such that the particle size distribution
(or
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deviation) is extremely narrow. The substantially uniform particle size
distribution
produces a nanoparticle material with more consistent properties and activity
throughout the material.
The nanoparticles and methods of the present invention provide many
advantages for making novel nanomaterials such as catalysts and/or for
improving the
activity and performance of existing nanomaterials. Novel nanomaterials are
possible
because dissimilar components, which typically do not form uniform particles,
can be
combined using one or more dispersing agents such that most or all of the
particles
have the two or more components in each particle. Because each nanoparticle
contains a mixture or alloy of the two or more components, each nanoparticle
has the
intended or desired characteristic needed to produce the properties of the
multicomponent material.
Unlike the nanoparticles of the prior art, the dissimilar components in the
nanoparticles of the present invention are evenly dispersed among the
nanoparticles.
The dispersing agent overcomes the tendency for like components to agglomerate
and
form homogeneous particles but instead helps form multicomponent particles. In
many cases, the functionality of the material depends on forming heterogeneous
(i.e.
multicomponent) particles rather than forming a heterogeneous mixture of
homogeneous (i.e., single component) particles, as is typically seen in the
prior art.
The proper dispersing and mixing of the two or more components according to
the
present invention imparts beneficial characteristics, such as those described
above.
Another advantage of the present invention is that the dispersing agents are
readily available and relatively inexpensive. Still another advantage of the
inventive
process is that it is highly flexible in that it works well with a variety of
components
and thus can be used to improve many new and existing catalysts and
nanomaterials.
Furthermore, existing and novel catalysts can be stabilized thereby providing
opportunities to use the nanoparticles in new processes or improve the
resistance of
the nanoparticles to degradation.
These and other advantages and features of the present invention will become
more fully apparent from the following description and appended claims as set
forth
hereinafter.
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DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS
1. INTRODUCTION AND DEFINITIONS
The present iiivention is directed to nanoparticles and nanoparticle materials
made from two or more different components. The multicomponent nanoparticles
are
formed using a dispersing agent. In an exemplary embodiment, the dispersing
agent
binds to the coinponents and determines in part the molecular arrangement of
the
components. The dispersing agent is able to ensure that two or more different
components are distributed between and among nanoparticles in a desired
distribution. Nanoparticles according to the invention can be used as
catalysts with
improved and/or novel catalytic activity and/or to form nanomaterials having
superior
properties.
For purposes of this invention, the terin "nanoparticles" or "nano-sized
particles," means particles with a diameter of less than about 100 nanometers
(nm).
The term "component complex" refers to a solution, colloid, or suspension in
which a bond or coordination complex is formed between a dispersing agent and
one
or more different types of particle atoms: The "bond" between the control
agent and
particle atoms can be ionic, covalent, electrostatic, or it can involve other
bonding
forces such as coordination with nonbonding electrons, van der Waals forces,
and the
like.
The term "minority component" means the coinponent in a multicomponent
nanoparticle with the lesser concentration within the particle. In the case
where two
or more components have essentially the same concentration within the
particle,
evidenced by the fact that the determination of a minority is statistically
impractical,
then either component is considered to be the minority component.
For purposes of disclosure and the appended claims, the term "Number Ratio"
or "NR" is equal to NA/NB where NA is the number (or moles) of atoms of a more
numerous component A in a given nanoparticle or set of nanoparticles, and NB
is the
number (or moles) of atoms of a less numerous component B in the nanoparticle
or
set of nanoparticles. For a particular nanoparticle i, NR can be expressed as
the
specific value (NR;). The average NR for all of the nanoparticles in a given
set of
nanoparticles is expressed as the average value (NRa~g).
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Ibi most cases, the individual NR values corresponding to the various
particles
within a given sample or set of nanoparticles do not equal a single discrete
value but
fall within a range of NR values (i.e., the "Range of NR"). The Range of NR
for a
given sample of set of nanoparticles having at least two different
nanoparticle
components within each particle has an upper value NR,,,a,, and a lower value
NR,,,
II. MULTICOMPONENT NANOPARTICLE COMPOSITIONS
A. Nanoparticle Forming Component Complexes
As discussed above, two or more dissimilar atoms, molecules or components
are joined together into multicoinponent nanoparticles by means of a
dispersing agent.
The dissimilar components and the dispersing agent form one or more types of
component complexes from which the multicomponent nanoparticles are formed.
Thus, component complexes include one or more different types of component
atoms
complexed with one or more different types of dispersing agents. When so
complexed, the component atoms are arranged in such a manner that the
components
either (i) form dispersed nanoparticles in solution or (ii) that upon or after
contact
with a support, the component complex forms dispersed nanoparticles. In either
case,
the dispersing agent can form a component complex to produce nanoparticles
that are
dispersed, stable, uniform, and/or desirably sized. In the case where the
component
complex has not yet resulted in the fonnation of nanoparticles, it may be
proper to
refer to this complex as a nanoparticle-forming intermediate complex.
1. Particle Component Atoms or Molecules
Any two or more elements or groups of elements that can form catalysts or
nanomaterials can be used to form component complexes according to the present
invention. As the primary component, metals or metal oxides are preferred.
Exemplary metals can include base transition metals, rare earth metals, noble
metals,
and rare earth metals. Nanoparticles may also comprise non-metal atoms, alkali
metals and alkaline earth metals. A catalyst compound comprising two or more
different types of atoms is referred to as a molecule. Where catalytic
activity is
desired, elements or groups of elements can be selected that exhibit primary
catalytic
activity, as well as promoters and modifiers.
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Examples of base transition metals include, but are not limited to, chromium,
manganese, iron, cobalt, nickel, copper, zirconium, tin, zinc, tungsten,
titanium,
molybdenum, vanadium, and the like. These can be used in various coinbinations
with each other, and/or in combinations with other different and/or dissimilar
metals
such as noble metals, alkali metals, alkaline earth metals, rare earth metals,
or non-
metals.
Molecules such as ceramics and metal oxides can also be used in the
nanoparticles of the present invention. Examples include iron oxide, vanadium
oxide,
aluminum oxide, silica, titania, yttria, zinc oxide, zirconia, cerium oxide,
and the like.
Examples of noble metals, also referred to as precious metals, include
platinum, palladium, iridium, gold, osmium, ruthenium, rhodium, rhenium, and
the
like. Noble metals can be used in combination with other different and/or
dissimilar
elements, such as base transition metals, alkali metals, alkaline earth
metals, rare earth
metals, or non-metals.
Examples of rare earth metals include, but are not limited to, lanthanum and
cerium. These can be used alone, in various combinations with each other,
and/or in
combinations with other different and/or dissimilar elements, such as base
transition
metals, noble metals, alkali metals, alkaline earth metals, or non-metals.
Examples of non-metals include, but are not limited to, phosphorus, oxygen,
sulfur, antimony, arsenic, and halides, such as chlorine, bromine and
fluorine. At
least some of the foregoing are typically included as functionalizing agents
for one or
more metals, such as those listed above.
When added to an appropriate solvent or carrier to form a suspension, as
described below, component atoms can be added in elemental form; however, the
component atoms are typically in ionic form so as to more readily dissolve or
disperse
within the solvent or carrier. For example, metal coinponents can be added in
the
form of salts or other compounds. Components that are compounds themselves,
such
as oxides, can be added to a liquid medium in the appropriate compound form,
or may
be in a different chemical form that is converted to the appropriate chemical
form
during nanoparticle formation. In the case of a metallic component, the atoms
may be
in the form of a metal halide, nitrate or other appropriate salt that is
readily soluble in
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the solvent or carrier, e.g., metal phosphates, sulfates, tungstates,
acetates, citrates, or
glycolates.
2. Dissimilar Components
In an exemplary embodiment, the nanoparticles of the present invention
5 include two or more dissimilar components. Two components are dissimilar
where
the unique electronic configuration of each component creates same-conlponent
attractions that, absent a dispersing agent according to the present
invention,
significantly affect or dominate the thermodynamics of particle formation
and/or
arrangement. For example, iron is dissimilar from platinum. When forming
10 nanoparticles of platinum and iron using conventional methods, most, if not
all, of the
platinum atoms form homogeneous particles with other platinum atoms, and most,
if
not all, of the iron atoms form homogeneous particles with other iron atoms.
Absent
the use of a dispersing agent according to the present invention, the
dissimilarity of
iron and platinum atoms creates same-component attractions that predominate
over
other thermodynamic forces during particle formation or arrangement. The
result is
generally a heterogeneous mixture of largely homogeneous nanoparticles. In
contrast,
the use of one or more dispersing agents as disclosed herein overcomes such
thermodynamic barriers and causes dissimilar components to be compounded
together
so as to yield multicomponent nanoparticles that include two or more
dissimilar
components in each of a substantial portion, if not essentially all, of the
nanoparticles.
According to one embodiment, the dissimilar components comprise one or
more components selected from each of at least two groups comprising (i) noble
metals, (ii) base transition metals, (iii) alkali metals, (iv) alkaline earth
metals, (v) rare
earth metals, and (vi) non metals. That is, the dissimilar components
according to this
embodiment comprise at least one component (a) selected from one of -groups
(i)-(vi)
and at least one other component (b) selected from at least one other of
groups (i)-(vi).
In an alternative embodiment, dissimilar components are selected from
different groups of the periodic table of elements (i.e., different columns of
the
periodic table). The dissimilar components according to this embodiment
comprise at
least one component (a') selected from one column of the periodic table and at
least
one other component (b') selected from at least one other colunm of the
periodic table.
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Components selected from different groups of the periodic table are often
dissimilar
because of the difference in the number of valence electrons. As a non-
limiting
example of components formed from different groups of the periodic table,
uniform
nanoparticles may be composed of a mixture of titania and zinc oxide.
It is within the scope of the invention for the dissimilar components to
comprise different base transition metals. Although sometimes categorized
together
for simplicity, different base transition metals often exhibit dissimilar
properties.
These dissimilarities often create same-component attractions, which make
different
base transition metals difficult to combine or alloy in a dispersed manner.
Likewise
metal oxides can be difficult to combine. Those skilled in the art are
familiar with
atoms and molecules that are difficult or impossible to combine or alloy due
to
dissimilarities in the two components.
3. Dispersing Agents
One or more types of dispersing agents are selected to promote the formation
of multicomponent nanoparticles that have a desired composition or
distribution.
Dispersing agents within the scope of the invention include a variety of
organic
molecules, polymers, and oligomers. The dispersing agent comprises individual
molecules that mediate in the formation of the multicomponent nanoparticles.
In general, useful dispersing agents include organic compounds that can form
a complex with the component atoms or molecules used to make nanoparticles in
the
presence of an appropriate solvent or carrier, and optionally promoters and/or
support
materials. The dispersing agent is able to interact and complex with particle
component atoms or molecules that are dissolved or dispersed within an
appropriate
solvent or carrier through various mechanisms, including ionic bonding,
covalent
bonding, van der Waals interaction, hydrogen bonding, or coordination bonding
involving non-bonding electron pairs.
To provide the interaction between the dispersing agent and the particle
component atoms or molecules, the dispersing agent includes one or more
appropriate
functional groups. In one embodiment, the functional groups comprise a carbon
atom
bonded to at least one electron-rich atom that is more electronegative than
the carbon
atom and that is able to donate one or more electrons so as to form a bond or
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attraction with a particle component atom. Preferred dispersing agents include
functional groups which either have a negative charge, one or more lone pairs
of
electrons, or a positive charge that can be used to complex or bond to a
particle
component atom. These functional groups allow the dispersing agent to have a
strong
binding interaction with dissolved particle component atoms or molecules,
which, in
the case of metals, are preferably in the form of positively charged ions in
solution.
The dispersing agent may be a natural or synthetic compound. In the case
where the nanoparticle component atoms are metals and the dispersing agent is
an
organic compound, the complex so formed is an organometallic complex.
In one embodiment, the functional groups of the dispersing agent comprise
carboxyl groups, either alone or in combination with other types of functional
groups.
In other embodiments, the functional groups may include one or more of a
hydroxyl, a
carboxyl, a carbonyl, an amine, a thiol, an ester, an amide, a nitrile, a
nitrogen with a
free lone pair of electrons, a ketone, an aldehyde, a sulfonic acid, an acyl
halide, a
sulfonyl halide, and combinations of these. Examples of suitable dispersing
agents
include glycolic acid, oxalic acid, malic acid, maleic acid, citric acid,
pectins, amino
acids, celluloses, combinations of these, and salts of any of these.
Suitable polymers and oligomers within the scope of the invention include, but
are not limited to, polyacrylates, polyvinylbenzoates, polyvinyl sulfate,
polyvinyl
sulfonates including sulfonated styrene, polybisphenol carbonates,
polybenzimidizoles, polypyridine, sulfonated polyethylene terephthalate. Other
suitable polymers include polyvinyl alcohol, polyethylene glycol,
polypropylene
glycol, and the like. The dispersing agent can also be an inorganic compound
(e.g.,
silicon-based) or a salt of any of the foregoing.
It may be advantageous to provide an amount of dispersing agent so as to
provide an excess of functional groups relative to the number of particle
component
atoms or molecules. Including an excess of functional groups helps ensure that
all or
substantially all of the particle component atoms or molecules are complexed
by the
dispersing agent, which is particularly beneficial in the case where at least
one of the
nanoparticle components is expensive, such as in the case of noble metals.
Providing
an excess of dispersing agent can also help ensure the availability of
functional groups
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for bonding the nanoparticle complex to a support where a supported
nanoparticle is
desired. It is also believed that employing an excess of functional groups
helps yield
nanoparticles that are more evenly dispersed in the particle system. Excess
dispersing
agent molecules are believed to intervene and maintain spacing between
dispersing
agent molecules. The excess dispersing agent molecules can increase spacing
and
dispersion in a suspension as well as aid in spacing nanoparticles upon
deposition to a
support surface.
In addition to the foregoing, it may also be useful to express the molar ratio
of
dispersing agent to the particle component atoms in a nanoparticle suspension.
In one
1o embodiment, the molar ratio of dispersing agent molecules to particle
component
atoms is in the range of about 0.01:1 to about 40:1. Preferably, the molar
ratio of
dispersing agent molecules to particle component atoms is in a range of about
0.1:1 to
about 35:1, most preferably in a range of about 0.5:1 to about 30:1.
In some cases, a more useful measurement is the molar ratio between
.15 dispersing agent functional groups and particle component atoms. For
example, in the
case of a divalent metal ion two molar equivalents of a monovalent functional
group
would be necessary to provide the theoretical stoichiometric ratio. It may be
desirable
to provide an excess of dispersing agent functional groups to (1) ensure that
all or
substantially all of the particle component atoms are complexed, (2) bond the
20 nanoparticles to a support, and (3) help keep the nanoparticles segregated
so that they
do not clump or agglomerate together. In general, it will be preferable to
include a
molar ratio of dispersing agent functional groups to particle component atoms
in a
range of about 0.5:1 to about 40:1, more preferably in a range of about 1:1 to
about
35:1, and most preferably in a range of about 3:1 to about 30:1.
25 As discussed below, the nanoparticles can be supported on a support
surface.
It is believed that when a support material is added to a suspension of
nanoparticles,
the dispersing agent acts to uniformly disperse the complexed component atoms
and/or suspended nanoparticle complexes onto the support material.
In addition to the foregoing, the dispersing agent can be selected in order to
30 act as an anchor between the nanoparticles and a support material or
substrate.
Preferably, the support substrate has a plurality of hydroxyl or other
functional groups
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on the surface thereof which are able to chemically bond to one or more
functional
groups of the dispersing agent, such as by way of a condensation reaction. One
or
more additional functional groups of the dispersing agent are also bonded to
one or
more atoms within the nanoparticle, thereby anchoring the nanoparticle to the
substrate.
While the dispersing agent has the ability to inhibit particle agglomeration
in
the absence of being anchored to a support, chemically bonding the
nanoparticle to
the substrate surface through the dispersing agent is an additional and
particularly
effective mechanism for preventing particle agglomeration since the
nanoparticles
1o thereby become fixed in space.
B. Solvents and Carriers
A solvent or carrier may be used as a vehicle for the particle component atoms
(typically in the form of an ionic salt) and/or the dispersing agent. The
solvent may
be an organic solvent, water or a combination thereof. Organic solvents that
can be
used include alcohols, ethers, glycols, ketones, aldehydes, nitriles, and the
like.
Preferred solvents are liquids with sufficient polarity to dissolve the metal
salts. They
include water, methanol, ethanol, normal and isopropanol, acetonitrile,
acetone,
tetrahydrofuran, ethylene glycol, dimethylformamide, dimethylsulfoxide,
methylene
chloride, and mixtures thereof.
Other chemical modifiers may also be included in the liquid mixture. For
example, acids or 'bases may be added to adjust the pH of the mixture.
Surfactants
may be added to adjust the surface tension of the mixture, or to stabilize the
nanoparticles.
The solvent for the nanoparticle components may be a neat solvent, but it is
preferable to include an acid to yield an acidic solution, as acids aid in the
dissolution
of the nanoparticle components. The solvent solution may be acidified with any
suitable acid, including organic and inorganic acids. Preferred acids include
mineral
acids such as sulfuric, phosphoric, hydrochloric, nitric, and the like, or
combinations
thereof. While it is possible to use an acid in a wide range of
concentrations, it is
generally only necessary to use relatively dilute solutions to accomplish the
desired
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solubility enhancement. Moreover, concentrated acid solutions may present
added
hazard and expense. Thus, dilute acid solutions are currently preferred.
C. Supports and Support Materials
As discussed above, it is within the scope of the invention for the
5 nanoparticles to be isolated on a support surface. The support material may
be
organic or inorganic. According to one embodiment, the supported nanoparticles
may
function as a catalyst. In the case of a supported catalyst, the support
material can be
chemically inert in the cheinical reaction environment, or the support
material may
itself serve a catalytic function complementary to the function of the
supported
10 nanocatalyst particles.
Any solid support material known to those skilled in the art as useful
nanoparticle supports can be used as supports for the dispersed nanoparticles
of the
present invention. The support may be selected from a variety of physical
forms.
Exemplary supports may be porous or non-porous. They may be 3-dimensional
15 structures, such as a powder, granule, tablet, extrudate, or the like.
Supports may be
in the form of 2-dimensional structures, such as a film, membrane, coatings,
or the
like. It is even conceivable for the support to be a 1-dimensional structure,
such as
ultra thin fibers or filaments.
A variety of materials, alone or in combination, can comprise the support.
One exeinplary class of support materials preferred for some applications
includes
porous inorganic materials. These include, but are not limited to, alumina,
silica,
silica gel, titania, kieselguhr, diatomaceous earth, bentonite, clay,
zirconia, magnesia,
as well as the oxides of various other metals, alone or in combination. They
also
include the class of porous solids collectively known as zeolites, natural or
synthetic,
which have ordered porous structures.
Another useful class of exemplary supports includes carbon-based materials,
such as carbon black, activated carbon, graphite, fluoridated carbon, and the
like.
Other useful classes of support materials include organic solids (e.g.,
polymers),
metals and metal alloys.
In the case where the nanoparticles are attached to a support, the
nanoparticles
can be deposited in a wide range of loadings on the support material. The
loading can
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16
range from 0.01% to 90% by weight of the total weight of the supported
nanoparticles. The preferred loading will depend on the application. In the
case
where porous solids are used as the support material, it is preferred that the
surface
area of the support be at least 20 m2/g, and more preferably more than 50
m2/g.
D. Distribution of Components Within the Nanoparticles
At least a portion of the nanoparticles within a preparation of nanoparticles
manufactured according to the invention will include two or more (e.g., both)
of the
nanoparticle components. Iil a preferred embodiment, at least about 50% of the
nanoparticles include two or more of the nanoparticle components. More
preferably,
at least about 75% of the nanoparticles within the preparation include two or
more of
the nanoparticle components, even more preferably at least about 85% of the
nanoparticles include two or more of the nanoparticle components, and most
preferably at least about 95% of the nanoparticles within the preparation
include two
or more of the nanoparticle components. It is within the scope of the
invention for at
least about 99% (i.e., essentially all) of the nanoparticles within a
preparation
according to the invention to include two or more of the nanoparticle
components.
Because a substantial proportion of the nanoparticles prepared according to
the
invention include two or more nanoparticle components, the benefits derived
from
having the components in a single particle are more uniformly distributed
throughout
the nanoparticles compared to heterogeneous mixtures of homogeneous particles.
Consequently, the overall nanoparticle material or catalyst has an increased
display of
these beneficial properties.
According to another aspect of the invention, the degree of dispersion of the
two or more components within nanoparticles prepared according to the
invention can
be measured by the Number Ratio (NR) or Range of NR for a given set of
nanoparticles having two or more components. As mentioned above, the Number
Ratio = NA/NB, where NA is the number (or moles) of atoms of a more numerous
component A within a nanoparticle or set of nanoparticles according to the
invention,
and NB is the number (or moles) of atoms of a less numerous component B within
the
nanoparticle or set of nanoparticles. The value of NR can be expressed as an
average
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17
value (NRa~g) for all of the nanoparticles in a given set or as the specific
value (NR;)
for a particular nanoparticle i.
In an ideal case, the value NR; for each nanoparticle i in a given set of
inventive nanoparticles equals NRavg. In this case, each particle i has an
equal
distribution of components A and B. The present invention also contemplates
controlling the dispersion of components in bi- or multi-component
nanoparticles
such that the Range of NR values for all of the nanoparticles in a particular
sample is
within a desired range. As mentioned above, the Range of NR has an upper value
NR,,,ax and a lower value NR,,,;,,. As NRmax and NR;,, deviate less from
NRavg, the
Range of NR becomes narrower, which indicates that the nanoparticles are more
uniform.
In a preferred embodiment, the value of NRmax does not exceed about 5 times
the value of NRavg, more preferably does not exceed about 3 times the value of
NR,g,
and most preferably does not exceed about 2 times the value of NRavg.
Conversely, the value of NR;,, is preferably at least about 0.2 times the
value
of NRa,,g, more preferably at least about 0.33 times the value of NRavg, and
most
preferably at least about 0.5 times the value of NRavg.
Given the foregoing, the Range of NR is therefore preferably about 0.2 to
about 5 times the value of NR,g, more preferably about 0.33 to about 3 times
the
value of NRavg, and most preferably about 0.5 to about 2 times the value of
NRa,,g. It
will be appreciated that the foregoing ranges do not count "outliers" (i.e.,
particles
that do not form correctly and that excessively deviate from NRa,,g as to be
outside the
Range of NR). Whereas the NR of the "outliers" may in some cases count toward
the
NRavg, they do not fall within the "Range of NR" by definition.
In a preferred embodiment, at least about 50% of the individual nanoparticles
in a given preparation will have an NR; within the Range of NR. More
preferably, at
least about 75% of the individual nanoparticles within the preparation will
have an
NR within the Range of NR, even more preferably at least about 85% of the
individual nanoparticles within the preparation will have an NR within the
Range of
NR, and most preferably at least about 95% of the individual nanoparticles
within the
preparation will have an NR; within the Range of NR. It is within the scope of
the
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invention for at least about 99% of the individual nanoparticles within a
preparation
according to the invention to have an NR; within the Range of NR.
In contrast to the relatively narrow Range of NR for nanoparticles made
according to the present invention, nanoparticles in the art, to the extent
they can be
made as all, have very wide Ranges of NR;, in some cases ranging from zero to
infinity, indicating that some particles have essentially none of one
component, and
other particles have essentially none of the other component.
The following two simple numerical examples provide non-limiting examples
of nanoparticles of the present invention having desired Ranges of NR.
Consider a
lo case where component B comprises 1% of a bimetallic nanoparticle mixture,
and
component A comprises the balance in a given set of nanoparticles. In this,
case the
NRa,,g for the set of nanoparticles is approximately 100. The preferred Range
of NR
for the set nanoparticles is tllus 20 to 500, which translates to a range of
0.2% to 5%
of component B in the individual nanoparticles that contain both components.
The
more preferred range for NR is 33 to 300, translating to a coinposition range
of 0.33%
to 3% of component B in the individual nanoparticles that contain both
components.
The most preferred range for NR; is 50 to 200, or a composition range of 0.5%
to 2%
component B in the individual nanoparticles that contain both components.
In a second simple numerical example, consider a case where component A
and component B are each present in equal quantities of 50% of the total, such
that
the overall NRa~g is 1. In this case, the preferred range of NR; is 0.2 to 5,
corresponding to a composition range of 16% to 83% of component B in the
individual nanoparticles that contain both components. The more preferred
range of
NR; is 0.33 to 3, corresponding to a composition range of 25% to 75% component
B
in the individual nanoparticles that contain both components. Finally, the
most
preferred range of NR; is 0.5 to 2, or a composition range of 33% to 67%
component
B in the individual nanoparticles that contain both components.
As discussed above, the dispersing agents according to the present invention
are used to provide the desired dispersion and uniformity that is
characteristic of the
nanoparticles of the present invention. Using the dispersing agents according
to the
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present invention, the above-mentioned uniformity as defined by the Range of
NR can
be obtained.
In one embodiment, the dispersing agent remains as a constituent of the
nanoparticles. The inventors of the present invention have found by infrared
spectroscopy that characteristic features attributable to the dispersing agent
can be
present in the final nanoparticle product, indicating that the dispersing
agent persists
beyond the nanoparticle production steps. In one embodiment, the dispersing
agent is
believed to be a stabilizing component in the final catalyst or nanoparticle
material.
For example, the dispersing agent can provide a desirable anchoring effect of
the
particle to a support which prevents migration and agglomeration of
nanoparticles,
even under relatively severe operating conditions. However, even where the
dispersing agent is not used as an anchor to a support material (e.g., in the
absence of
a support material or where the dispersing agent does not bond to the support
material), the dispersing agent can have a stabilizing effect.
While it is possible that the multicomponent nanoparticles may contain a true
multicomponent compound, alloy, or crystal structure in which the components
are in
an ordered arrangement, this is not required. In one embodiment, each
nanoparticle
can be coinposed of a mixture of components regardless of how they are
combined or
arranged. The components can be present as relatively isolated atoms, as small
atomic clusters, or decorated. They can also be present as amorphous
particles. The
components can be present as crystallites including alloys. Component crystals
can
have relatively random crystal face exposures; or they can have a controlled
or
selective exposure of particular crystal faces.
The statistical distribution or uniformity made possible by the dispersing
agent
of the present invention allows for nanocatalysts and nanomaterials with new
and/or
improved materials and/or catalytic properties. Maximizing multicomponent
catalyst
and nanomaterial properties may depend on the proximity of the two components.
The substantially uniform distribution of components between and among
nanoparticles provides a greater possibility for different components to come
into
proximity with one another to provide a desired functionality or property.
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The dispersing agent also makes it possible to select very precise ratios of
coiuponents by controlling the average percent composition. Because the
individual
multicomponent nanoparticles have a percent composition that varies very
little from
the average composition, the percent composition of the individual
nanoparticles can
5 be more precisely controlled by adjusting the starting materials to control
the average
percent composition.
III. METHODS OF MAKING MULTICOMPONENT NANOPARTICLES
General processes for manufacturing multicomponent nanoparticles according
to the invention can be broadly summarized as follows. Two or more types of
particle
10 atoms and one or more types of dispersing agents are selected. The particle
atoms and
the dispersing agent are reacted or combined together to form a plurality of
-component complexes (collectively referred to as the "component complex").
The
component complex is generally formed by first dissolving the particle atoms
and
dispersing agent(s) in an appropriate solvent or carrier and then allowing the
15 dispersing agent to recombine the dissolved component atoms as the
component
complex so as to form a solution or suspension. In one embodiment,
multicomponent
nanoparticles form in the suspension. Alternatively, nanoparticles may form
upon or
after the component complex is disposed on a support surface. If desired, at
least a
portion of the dispersing agent can be removed to expose the multicomponent
20 nanoparticles. The dispersing agent may form a chemical bond with the
support
material in order to thereby anchor the nanoparticles to the support.
A more specific example for making multicomponent nanoparticles according
to the invention includes providing two or more types of particle component
atoms in
.solution (e.g., in the form of an ionic salt), providing a dispersing agent
in solution
(e.g., in the form of a carboxylic acid salt), and reacting the particle
component atoms
with the dispersing agent to form a component complex (i.e., a solution,
suspension or
colloid of component atoms complexed with the dispersing agent). The particle
component atoms can be provided in any form so as to be soluble or dispersible
in the
solvent or carrier that is used to form the component complex. In the case
where the
particle component atoms comprise one or more metals, salts of these metals
can be
formed that are readily soluble in the solvent or carrier. In the case where
the
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21
component atoms include noble metals, it may be advantageous to use noble
metal
chlorides and nitrates, since chlorides and nitrate of noble metals are more
readily
soluble than other salts. Chlorides and nitrates of other metal particle
atoms, such as
base transition metals and rare earth metals may likewise be used, since
chlorides and
nitrates are typically more soluble than other types of salts.
The component atoms can be added to the solvent or carrier singly or in
combination to provide final nanoparticles that comprise mixtures of various
types of
particle atoms. For example, a bimetallic iron/platinum catalyst can be formed
by
first forming a precursor solution into which is dissolved an iron salt, such
as iron
chloride, and a platinum salt, such as chloroplatinic acid. In general, the
composition
of the final nanoparticles will be determined by the types of particle
component atoms
used to form the component complex. Therefore, control of the amounts of
component atoms added to the solution, colloid or suspension provides a
convenient
method for controlling the relative concentrations of the different types of
component
atoms in the final multicomponent nanoparticles.
The dispersing agent is added to the solvent or carrier in a manner so as to
facilitate association with the particle component atoms in order to form the
coinponent complex. Some dispersing agents may themselves be soluble in the
solvent or carrier. In the case of dispersing agents that include carboxylic
acid
groups, it may be advantageous to form a metal salt of the acids (e.g., an
alkali or
alkaline earth metal salt). For example, polyacrylic acid can be provided as a
sodium
polyacrylate salt, which is both readily soluble in aqueous solvent systems
and able to
react with catalyst metal salts to form a metal-polyacrylate complex, which
may be
soluble or which may form a colloidal suspension within the solvent or
carrier. ,
In general, component complexes according to the invention comprise the
particle atoms and dispersing agent, exclusive of the surrounding solvent or
carrier.
Therefore, it is within the scope of the invention to create a component
complex in
solution, or as a colloid or suspension, and then remove the solvent or
carrier so as to
yield a dried component complex. The dried component complex can be used in
this
form, or it can be reconstituted as a solution, colloid or suspension by
adding an
appropriate solvent.
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In the case where the nanoparticles of the invention are to be formed on a
solid
support material, the component complex solution is physically contacted with
the
solid support. Contacting the component complex with the solid support is
typically
accomplished by means of an appropriate solvent within the component complex
solution, colloid or suspension in order to apply or impregnate the component
complex onto the support surface.
Depending on the physical form of the support material, the process of
contacting or applying the component complex to the support may be
accomplished
by a variety of methods. For example, the support may be submerged or dipped
into a
solution, colloid, or suspension comprising a solvent or carrier and the
component
complex. Alternatively, the solution, colloid, or suspension may be sprayed,
poured,
painted, or otherwise applied to the support material. Thereafter, the solvent
or carrier
is removed, optionally in connection with a reaction step that causes the
dispersing
agent to become chemically bonded or adhered to the support.
If desired, at least a portion of the nanoparticles can be exposed by removing
a
at least a portion of the dispersing agent, such as by reduction (e.g.,
hydrogenation) or
oxidation. Hydrogen is one preferred reducing agent. Instead of, or in
addition to,
using hydrogen as the reducing agent, a variety of other reducing agents may
be used,
including lithium aluminum hydride, sodium hydride, sodium borohydride, sodium
bisulfite, sodium thiosulfate, hydroquinone, methanol, aldehydes, and the
like. The
reduction process may be conducted at a temperature between "20 C and 500 C,
and
preferably between 100 C and 400 C.
In some cases, such as where it is desired for a portion of the dispersing
agent
to remain as an anchoring agent, oxidation may only be suitable when the
particle
atoms do not include noble metals, since noble metals might catalyze the
oxidation of
the entire dispersing agent, leaving none for anchoring. In such cases,
oxidation may
be more suitable, for example, in the case where the particle atoms comprise
transition metals and the support is non-combustible (e.g., silica or alumina
rather
than carbon black, graphite or polymer membranes). According to an exemplary
embodiment, oxidation may be carried out using oxygen, hydrogen peroxide,
organic
peroxides, and the like.
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In one embodiment, the process of removing the dispersing agent to expose
the particle atoms is carefully controlled to ensure that enough of the
dispersing agent
remains so as to reliably maintain a dispersed catalyst. Removing the
dispersing
agent to the extent that little. or none of it remains to disperse or anchor
the
nanoparticles has been found to reduce the stability of the nanoparticles,
particularly
when the catalyst is subjected to harsh reaction conditions during use.
Nevertheless,
it is within the scope of the invention to remove all or substantially all of
the
dispersing agent in order to yield free multicomponent nanoparticles that are
neither
anchored to a support or otherwise complexed with a dispersing agent to any
degree.
Supported nanoparticles can be optionally heat-treated to further activate the
nanoparticles. It has been found that, in some cases, subjecting the
nanoparticles to a
heat treatment process before initially using the nanoparticles causes the
nanoparticles
to be more active initially. The step of heat treating the nanoparticles may
be referred
to as "calcining" because it may act to volatilize certain components within
the
nanoparticles. The heat treatment process may be carried in inert, oxidizing,
or
reducing atmospheres.
In some cases it may be desirable to maintain at least some of the
nanoparticle
components in a non-zero oxidation state during the heat treatment process in
order to
increase the bond strength between the dispersing agent and the -
nanoparticles.
Increasing the bond between the dispersing agent and the nanoparticles is
believed to
increase the dispersion of the nanoparticles and/or the distribution of
components
within the particles by reducing the tendency of nanoparticles to migrate
and/or
agglomerate together when exposed to higher temperatures. This is particularly
true
in the case of supported multicomponent nanoparticles.
Where the nanoparticles are subjected to a heat treatment process, the process
is preferably carried out at a temperature in a range of about 50 C to about
300 C,
more preferably in a range of about 100 C to about 250 C, and most preferably
in a
range of about 125 C to about 200 C. The duration of the heat treatment
process is
preferably in a range of about 30 minutes to about 12 hours, more preferably
in a
range of about 1 hour to about 5 hours.
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An important feature of the heat treating step according to the present
invention is that it does not degrade the nanoparticles or reduce catalytic
activity. The
dispersing agent provides the stability needed to subject the nanoparticles to
higher
temperatures without destroying or partially destroying the nanoparticles.
Further
stability may be possible where the particle component atoms are bonded to the
dispersing agent and then maintained in a non zero-oxidation state, which
enhances
the bond between the component atoms and the active complexing groups of the
dispersing agent.
The following exemplary procedures where used to prepare iron-platinum
multicomponent nanoparticles according to the invention. By showing that iron
and
platinum can be compounded together to form heterogeneous multicomponent
nanoparticles, the examples demonstrate that two very dissimilar materials
having
very strong same-component attractions can, in fact, be compounded together
using a
dispersing agent. From this it may be expected that any two or more dissimilar
materials can be compounded together using the compositions and methods
described
herein.
Example 1: Nanoparticle Suspension
An Iron (III) solution was prepared by dissolving 2.32 g of FeC13 in 4 ml HCl
and 996 ml de-ionized water to produce a 0.08 wt% solution of Fe (III). A
Pt.solution
was prepared by dissolving 0.2614 .g H2PtC16 (from Strem Chemicals) in 1000 ml
de-
ionized water to make 0.01 wt% solution of Pt. To make a 6.75 wt% solution of
polyacrylate, 15 g of a 45 wt% poly acrylate solution (Aldrich with MW ca.
1,200)
was diluted to 100 grams with de-ionized water.
To prepare 2.4 grams of a 10% Fe and 0.2% Pt supported nanoparticles, 300
ml of the 0.08 wt% Fe solution was mixed with 48 ml of the 0.010 wt% Pt
solution
and 40 ml of the 6.75 wt% polyacrylate solution. The ratio of Fe:polyacrylate
was
1:1. The solution was then diluted to 4000 ml with de-ionized water. This
solution
was purged by 100 ml/min N2 for 1 hour. Then the N2 was replaced with 130
ml/min
H2 for 16 minutes. The flask was then held overnight. The Fe-Pt solution
resulted in
the formation of a suspension of nanoparticles.
Example 2
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Supported nanoparticles were prepared by first preparing a solution of Fe-Pt
particles according to Example 1. 24 g of Black Pearls 700 were impregnated by
4000 ml of the Fe-Pt solution or suspension prepared according to Example 1.
The
slurry was heated by an IR lamp under rotation until all the liquid*was
evaporated.
5 The obtained samples were kept in an oven at 100 C. The sample was packed in
a
reduction unit between two layers of glass-wool. The sample was then treated
by the
following procedure: purged by 100 ml/min N2 for 15 minute and then witll 100
ml/min H2 at the following temperatures and for the following amount of time:
25 C
(0.5h), then 90 C (2h), then 90 C (2h), then 300 C (17h). The sample was then
10 cooled to room temperature in 100 ml/min H2. It was then purged by 100
ml/min of
N2 for one hour.
Example 3
8.13g FeC13 was mixed with 16.5g 70 wt% glycolic acid and diluted with
water to 100 g. After oveniight agitation, the FeC13 was totally dissolved. To
this
15 solution 2.8 g 0.01 wt% Pt solution from Example 1 was added. This solution
was
used to impregnate 140 g CaCO3. After the same drying and activation procedure
as
for Example 1, an alloy sample with 2%Fe and 0.02% Pt was formed.
The multicomponent nanoparticle materials produced in examples 1, 2, and 3
had nanoparticles in which essentially all the nanoparticles included both
iron and
20 platinum, which would be virtually thermodynamically impossible using heat
compounding techniques.
Example 4
Any of Examples 1-3 is modified in order to compound together two or more
dissimilar components in which at least one of the components is selected from
one of
25 the following groups and at least one other of the components is selected
from another
of the following groups: noble metals, base transition metals, alkali metals,
alkaline
earth metals, rare earth metals, and nonmetals.
The dispersing agent may be one or more of any of the dispersing agents
described herein. A substantial portion of the nanoparticles manufactured
thereby
include two or more dissimilar components in each of the nanoparticles.
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Example 5
Any of Examples 1-3 is modified in order to compound together two or more
dissimilar components in which at least one of the components is selected from
one
group of the periodic table of elements and at least one other of the
coinponents is
selected from another group of the periodic table of elements.
The dispersing agent may be one or more of any of the dispersing agents
described herein. A substantial portion of the nanoparticles manufactured
thereby
include two or more dissimilar components in each of the nanoparticles.
The present invention may be embodied in other specific forms without
departing from its spirit or essential characteristics. The described
embodiments are
to be considered in all respects only as illustrative and not restrictive. The
scope of
the invention is, therefore, indicated by the appended claims rather than by
the
foregoing description. All changes which come within the meaning and range of
equivalency of the claims are to be embraced within their scope.
