U.S. patent application number 13/601496 was filed with the patent office on 2013-09-05 for copper nanostructures and methods for their preparation.
This patent application is currently assigned to WASHINGTON UNIVERSITY in St. Louis. The applicant listed for this patent is Mingshang Jin, Younan Xia. Invention is credited to Mingshang Jin, Younan Xia.
Application Number | 20130230717 13/601496 |
Document ID | / |
Family ID | 49043000 |
Filed Date | 2013-09-05 |
United States Patent
Application |
20130230717 |
Kind Code |
A1 |
Xia; Younan ; et
al. |
September 5, 2013 |
COPPER NANOSTRUCTURES AND METHODS FOR THEIR PREPARATION
Abstract
Copper nanostructures with relatively small dimensions and
method for producing such structures are discloses. The ratios of
the various reaction products may be adjusted to produce pentagonal
nanowires and other structures such as tadpole shaped nanowires,
nanocubes or pentagonal bi-pyramids.
Inventors: |
Xia; Younan; (Atlanta,
GA) ; Jin; Mingshang; (Xi ' an, CN) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Xia; Younan
Jin; Mingshang |
Atlanta
Xi ' an |
GA |
US
CN |
|
|
Assignee: |
WASHINGTON UNIVERSITY in St.
Louis
|
Family ID: |
49043000 |
Appl. No.: |
13/601496 |
Filed: |
August 31, 2012 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
61530734 |
Sep 2, 2011 |
|
|
|
Current U.S.
Class: |
428/397 ;
428/401; 75/343; 75/370 |
Current CPC
Class: |
B82Y 40/00 20130101;
B22F 1/0025 20130101; Y10T 428/2973 20150115; C22B 15/00 20130101;
B22F 9/24 20130101; B82Y 30/00 20130101; Y10T 428/298 20150115;
D02G 3/22 20130101; B22F 1/0044 20130101; D01F 9/08 20130101 |
Class at
Publication: |
428/397 ;
428/401; 75/343; 75/370 |
International
Class: |
D02G 3/22 20060101
D02G003/22; C22B 15/00 20060101 C22B015/00; D01F 9/08 20060101
D01F009/08 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH &
DEVELOPMENT
[0002] The claimed subject matter was developed with Government
support under NSF Grant Nos. 0804088, 1104614 and ECS-0335765,
awarded by the National Science Foundation. The Government has
certain rights in the claimed subject matter.
Claims
1. A method of producing a copper nanostructure, the method
comprising: forming a reaction mixture in a reaction vessel, the
reaction mixture comprising a copper-containing compound, a capping
agent and a reducing agent; and reducing the copper-containing
compound with the reducing agent to cause copper to form a copper
nanostructure, wherein (1) the pressure in the reaction vessel is
less than about 190 kPa and/or (2) the temperature of the reaction
mixture is less than about 115.degree. C.
2. The method as set forth in claim 1 wherein the pressure in the
reaction vessel is about atmospheric pressure.
3. The method as set forth in claim 1 wherein the capping agent is
an alkylamine.
4. The method as set forth in claim 3 wherein the alkylamine has
less than about 25 carbon atoms.
5. The method as set forth in claim 1 wherein the reaction mixture
comprises water as a solvent.
6. The method as set forth in claim 1 wherein the temperature of
the reaction mixture during formation of the nanostructure is less
than about 115.degree. C.
7. The method as set forth in claim 1 wherein the copper-containing
compound is selected from the group consisting of copper (II)
sulfate, copper (II) chloride, copper (II) hydroxide and copper
(II) nitrate, copper (II) acetate and copper (II)
trifluoroacetate.
8. The method as set forth in claim 1 wherein the reducing agent is
selected from the group consisting of glucose and ascorbic
acid.
9. The method as set forth in claim 1 wherein the capping agent is
selected from the group consisting of hexadecylamine,
octadecylamine and oleylamine.
10. The method as set forth in claim 1 wherein the capping agent is
hexadecylamine.
11. The method as set forth in claim 1 wherein the concentration of
at least one of the reducing agent and the capping agent in the
reaction mixture is controlled to produce a copper nanostructure in
the shape of (1) a nanowire, (2) a pentagonal bi-pyramid, (3) a
nanowire having a tad-pole shaped portion or (4) a nanocube.
12. A population of copper nanowire structures, each structure
having a length and a diameter, the average diameter of the copper
nanowire structures being less than about 40 nm and the average
ratio of length to diameter of the copper nanowire structures being
at least about 10:1.
13. The population as set forth in claim 12 wherein the average
ratio of length to diameter of the copper nanowire structures is at
least about 50:1.
14. The population as set forth in claim 12 wherein the average
diameter of the copper nanowire structures is less than about 30
nm.
15. The population as set forth in claim 12 wherein the copper
nanowire structures comprise at least about 60 wt % copper.
16. The population as set forth in claim 12 wherein the population
comprises at least about 100 copper nanowires.
17. A copper nanowire structure, the structure comprising at least
about 60 wt % copper and being characterized by a penta-twinned
shape.
18. The copper nanowire structure as set forth in claim 17 wherein
the nanowire structure comprises at least about 60 wt % copper.
19. The copper nanowire structure as set forth in claim 17 wherein
the ratio of the length to the diameter of the copper nanowire
structure is at least about 10:1.
20. The copper nanowire structure as set forth in claim 17 wherein
the diameter of the structure is less than about 40 nm.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims the benefit of U.S. Provisional
Patent Application No. 61/530,734, filed Sep. 2, 2011, which is
incorporated herein by reference in its entirety.
BACKGROUND
[0003] Copper nanostructures have increasingly been found to have
significant utility in the microelectronics and catalysis fields.
For example, copper nanowires (e.g., polycrystalline wires that are
usually fabricated by lithographic techniques) are currently used
as interconnects in computer chips. Copper nanostructures hold
great promise for use in microelectronics including low-cost
flexible displays, light-emitting diodes and thin film solar cells.
Copper nanostructures have also been found to exhibit localized
surface plasmon resonance (LSPR) peaks in the visible region.
Copper nanoparticles have been widely used as catalysts for
water-gas shift and gas detoxification reactions.
[0004] Metal nanostructures in the shape of nanowires are believed
to find widespread use in applications such as the fabrication of
transparent electrodes for flexible electronic and display devices.
They are also useful in formulating conductive coatings for
electrostatic discharging and electromagnetic shielding. Research
has conventionally focused on use of silver nanowires for use in
such applications. Compared to silver, copper is several orders of
magnitude more abundant and is significantly less expensive. Copper
nanowires with reduced sizes (i.e., reduced diameters) exhibit
increased transmittance of visible light making them even more
ideal for electronics use.
[0005] A continuing need exists for copper nanostructures that are
suitable for use in various applications such as microelectronics
and catalysis and for methods for producing them. A particular need
exists for copper nanowires with relatively small diameters and
methods for producing such nanowires.
SUMMARY
[0006] One aspect of the present disclosure is directed to a method
for producing a copper nanostructure. A reaction mixture is formed
in a reaction vessel. The reaction mixture includes a
copper-containing compound, a capping agent and a reducing agent.
The copper-containing compound is reduced with the reducing agent
to cause copper to form a copper nanostructure. The pressure in the
reaction vessel is less than about 190 kPa and/or the temperature
of the reaction mixture is less than about 115.degree. C. during
formation of the nanostructure.
[0007] A further aspect of the present disclosure is directed to a
population of copper nanowire structures. Each structure has a
length and a diameter. The average diameter of the copper nanowire
structures is less than about 40 nm and the average ratio of length
to diameter of the copper nanowire structures is at least about
10:1.
[0008] Another aspect of the present disclosure is directed to a
copper nanowire structure. The structure includes at least about 60
wt % copper and is characterized by a penta-twinned shape.
[0009] Various refinements exist of the features noted in relation
to the above-mentioned aspects of the present disclosure. Further
features may also be incorporated in the above-mentioned aspects of
the present disclosure as well. These refinements and additional
features may exist individually or in any combination. For
instance, various features discussed below in relation to any of
the illustrated embodiments of the present disclosure may be
incorporated into any of the above-described aspects of the present
disclosure, alone or in any combination.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 is an XRD pattern of a copper nanowire produced
according to Example 1;
[0011] FIGS. 2-3 are SEM images of copper nanowire structures
produced according to Example 1;
[0012] FIG. 4 is a TEM image of copper nanowire structures produced
according to Example 1;
[0013] FIG. 5 is a graph showing the distribution of diameters of
copper nanowires produced according to Example 1;
[0014] FIG. 6 is a TEM image of a portion of a copper nanowire
produced according to Example 1;
[0015] FIG. 7 is a high-resolution TEM image of the region marked
by the box in FIG. 6;
[0016] FIG. 8 is a TEM image of a second portion of a copper
nanowire produced according to Example 1;
[0017] FIG. 9 is a high-resolution TEM image of the region marked
by the box in FIG. 8;
[0018] FIG. 10 is a UV-vis spectra of an aqueous suspension of
copper nanowires having an average diameter of about 24 nm and of
silver nanowires having an average diameter of about 80 nm;
[0019] FIG. 11 is a SEM image of copper bi-pyramids that formed
after 30 minutes of reaction as produced according to Example 3
with an inset showing the SEM image of a tilted sample showing the
pentagonal cross-section of the nanocrystals;
[0020] FIG. 12 is a SEM image of copper bi-pyramids that formed
after 1 hour of reaction as produced according to Example 3;
[0021] FIG. 13 is a SEM image of copper bi-pyramids that formed
after 3 hours of reaction as produced according to Example 3;
[0022] FIG. 14 is a SEM image of copper bi-pyramids that formed
after 6 hours of reaction as produced according to Example 3 with
an inset showing the SEM image of a tilted sample showing the
pentagonal cross-section of the nanocrystals;
[0023] FIG. 15 is a TEM image of the copper nanowire of FIG.
14;
[0024] FIG. 16 is a high-resolution TEM image of the region marked
by the box in FIG. 15;
[0025] FIG. 17 is a UV-vis spectra of the aqueous suspension of
copper nanostructures of FIG. 11;
[0026] FIG. 18 is a SEM image showing one type of pentagonal
bi-pyramid;
[0027] FIG. 19 is a geometric model of the bi-pyramid of FIG.
18;
[0028] FIG. 20 is a SEM image showing a second type of pentagonal
bi-pyramid;
[0029] FIG. 21 is a geometric model of the bi-pyramid of FIG.
20;
[0030] FIG. 22 is a SEM image showing a third type of pentagonal
bi-pyramid;
[0031] FIG. 23 is a geometric model of the bi-pyramid of FIG.
22;
[0032] FIG. 24 is a SEM image of copper nanocubes that formed after
30 minutes of reaction as produced according to Example 4;
[0033] FIG. 25 is a SEM image of copper nanocubes that formed after
1 hour of reaction as produced according to Example 4;
[0034] FIG. 26 is a SEM image of copper nanocubes that formed after
6 hours of reaction as produced according to Example 4;
[0035] FIG. 27 is a XRD pattern of the copper nanocubes produced
according to Example 4;
[0036] FIG. 28 is a TEM image of a copper nanocube produced
according to Example 4;
[0037] FIG. 29 is high-resolution TEM image of the region marked by
the box in FIG. 28;
[0038] FIG. 30 is the UV-vis spectra of three separate aqueous
suspensions of 50 nm, 100 nm and 200 nm copper nanocubes; and
[0039] FIG. 31 is a schematic of the reaction pathways used to
produce various copper nanostructures according to Examples
1-4.
DETAILED DESCRIPTION
[0040] The field of the disclosure relates to copper nanostructures
and, more particularly, to copper nanostructures with relatively
small dimensions and methods for producing such structures. The
ratios of the various reaction products may be adjusted to produce
other structures such as tad-pole shaped nanowires, nanocubes or
pentagonal bi-pyramids.
[0041] Provisions of the present disclosure are directed to copper
nanostructures (e.g., nanowires) and methods for producing copper
nanostructures. Without being held to any particular theory, it has
been found that copper nanostructures formed at relatively low
pressures (e.g., atmospheric pressure) and/or low temperatures
(e.g., 100.degree. C. or less) have a relatively small diameter.
Further it has been found that by adjusting the concentration of
the components of the reaction mixture and/or adjusting the
respective ratios of the components, the shape of the resulting
nanostructure may be changed.
Methods for Producing Copper Nanostructures
[0042] Generally the copper nanostructures of the present
disclosure are produced by forming a reaction mixture that contains
a copper-containing compound, a capping agent and a reducing agent.
The copper-containing compound is reduced by the reducing agent to
produce elemental copper that forms the nanostructure. During
reduction, the pressure and/or temperature of the reaction vessel
may be maintained relatively low (e.g., a pressure of less than
about 190 kPa and/or a temperature of less than about 115.degree.
C.) such that nanowires with a relatively small diameter may be
produced.
[0043] Suitable copper-containing compounds that may be included in
the reaction mixture include any compounds from which elemental
copper)(Cu.sup.0 is formed upon contact with a reducing agent or
during electrolysis or an electroless deposition method, or upon
decomposition. Exemplary copper-containing compounds include copper
(II) nitrate (Cu(NO.sub.3).sub.2, anhydrous or hydrated), copper
(II) sulfate (CuSO.sub.4, anhydrous or hydrated), copper (II)
chloride (CuCl.sub.2, anhydrous or hydrated), copper (II) hydroxide
(Cu(OH).sub.2, anhydrous or hydrated), copper (II) acetate
(Cu(CH.sub.3COO).sub.2, anhydrous or hydrated), and copper (II)
trifluoroacetate (Cu(CF.sub.3COO).sub.2, anhydrous or hydrated).
Suitable copper-containing compounds may also include various
ligands and/or chelates that contain copper without limitation.
[0044] The reducing agent that is combined with the
copper-containing compound is any compound (or ligand or chelate)
that reduces copper ions into elemental copper to deposit as a
nanostructure seed or as part of the growing copper nanostructure.
Suitable reducing agents include glucose (a or (3 form) and
ascorbic acid.
[0045] In addition to the copper-containing compound and the
reducing agent, a capping agent is included in the reaction
mixture. The capping agent stabilizes the resulting nanostructure
(e.g., by changing the surface energies of different facets) and
prevents aggregation between the structures. The capping agent
becomes incorporated into the matrix during formation of the copper
nanostructure-based composites. Suitable capping agents include
alkylamines. Alkylamines have the general structure of Formula (I)
shown below
##STR00001##
wherein R.sub.1 is an alkyl group (or substituted alkyl group) and
R.sub.2 and R.sub.3 are either hydrogen or an alkyl group (or
substituted alkyl group). In some embodiments, the alkyl group of
R.sub.1 has 25 carbon atoms or less. One particularly preferred
alkylamine is hexadecylamine ("HDA"). HDA has been found to be an
effective capping agent for copper and has a strong selectivity
toward the {100} facets of the nanostructure. In some particular
embodiments, HDA is used as a capping agent and glucose is used as
a reducing agent. In such embodiments, copper nanostructures may be
produced in relatively large quantities with high purity and good
uniformity. Other alkylamines of Formula (I) that may be used
include octadecylamine and oleylamine.
[0046] Generally, the components that form the reaction mixture are
dissolved in water; however in some embodiments an organic solvent
may be used or even a two-solvent system may be used. The
copper-containing compound, the reducing agent and capping agent
may be added to any suitable reaction vessel in any manner suitable
to those of skill in the art (e.g., as solids or in solution form
and in any order of addition). Suitable vessels may be lab scale
(e.g., reaction vials) or may be commercial-scale (e.g., steel
vessels which may be polymer-lined). Preferably the reaction vessel
is agitated during formation of the copper nanostructures. The
nanostructures may be produced batch-wise or in a continuous manner
(e.g., a continuous-stirred tank reactor (CSTR)).
[0047] Upon formation of the reaction mixture, the reaction
contents are heated. Generally, the reaction mixture is heated to a
temperature less than about 115.degree. C. In some embodiments, the
reaction mixture is heated to a temperature less than about
110.degree. C. or less than about 105.degree. C. Preferably, the
reaction mixture is heated to a temperature of 100.degree. C. or
less to prevent the reaction mixture from boiling causing the
pressure of the reaction contents to increase as in pressurized
vessel systems. It is preferred that the reaction mixture be
maintained at about ambient pressure (101 kPa) or less. However in
some embodiments, the pressure is maintained to be below about 190
kPa, less than about 150 kPa, less than about 125 kPa or less than
about 105 kPa.
[0048] In this regard, it has been found that by utilizing a
reduced temperature (e.g., less than about 115.degree. C. and
preferably less than about 100.degree. C.) and/or a reduced
pressure (e.g., less than about 190 kPa and preferably 101 kPa or
less) copper nanostructures and, in particular, copper nanowires
are produced with a relatively small diameter (e.g., less than
about 40 nm, less than about 30 nm or even less than about 25 nm).
Without being bound to any particular theory, it is believed the
reduced temperature and/or pressure influences nucleation of the
copper nanostructure. It is believed that the seeds that are
produced at such reduced temperatures and pressures have a
decahedral shape which allows nanowires having a penta-twinned
structure to be produced. Such penta-twinned copper structures have
a relatively small diameter compared to conventional copper
nanostructures.
[0049] Generally the reaction is substantially complete after 6
hours or less. Other reaction times may be used depending on the
concentration of components added, the desired structure of the
nanomaterial and the desired conversion. Reaction times may be at
least about 30 minutes, at least about 1 hour, at least about 3
hours, at least about 5 hours, from about 30 minutes to about 6
hours or from about 30 minutes to about 3 hours.
[0050] The nanostructure that forms as a result of the process of
embodiments of the present disclosure depends on the relative
reaction rates and, in particular, the amount of reducing agent
and/or capping agent present in the reaction mixture. At relatively
low reaction rates, decahedral seeds are nucleated and form
penta-twinned nanowires with relatively uniform diameter due to
anisotropic growth (FIGS. 2-4). At greater reaction rates,
isotropic growth is promoted during early stage of growth. As the
reaction continues, the reaction rate becomes smaller and the
structure narrows to form a pentagonal bi-pyramid (FIG. 11). As the
reaction proceeds, an even smaller reaction rate results and the
pentagonal bi-pyramid further grows into tadpole-shaped nanowires
(FIGS. 12-15).
[0051] In some embodiments, the reaction conditions are controlled
such that single crystal seeds are nucleated rather than decahedral
seeds. This allows nanocubes (FIGS. 24-26) to form rather than
nanowires and/or bi-pyramids.
[0052] In this regard, copper nanowires have been found to be
produced without formation of bi-pyramids (FIGS. 2-4) at relatively
low concentrations of reducing agent and relatively high
concentrations of capping agent. If the concentration of reducing
agent is increased, pentagonal bi-pyramids form and the bi-pyramids
taper off to form nanowires as the reaction proceeds. In contrast,
if the concentration of capping agent is lowered, nanocubes form.
Without being bound to any particular theory, it is believed that
nanocubes may form due to oxidative etching. The oxidative etching
causes single crystal seeds to form which results in growth of
nanocubes. Such oxidative etching is blocked by the capping and
protective effect of the capping agent (e.g., HDA) at higher
concentrations of capping agent allowing multiply twinned copper
seeds to form.
[0053] The relative molar concentrations between copper, reducing
agent and capping agent that may result in formation of the various
structures are shown in Table 1 below. Generally these ratios were
used in Examples 1-4 described below.
TABLE-US-00001 TABLE 1 Relative amounts of components used to grow
various copper nanostructures. NANO-BI- NANOWIRES NANOCUBES
PYRAMIDS Concentration (mol/l) Copper 0.012 0.012 0.012 Capping
Agent 0.075 0.037 0.075 Reducing Agent 0.028 0.028 0.055 Molar
Ratios Capping Agent/ 6.1 3.0 6.1 Copper Reducing Agent/ 2.3 2.3
4.5 Copper Capping Agent/ 2.7 1.3 1.3 Reducing Agent
[0054] In this regard, the relative amounts of the components may
be adjusted to produce the desired structure as appreciated by
those of skill in the art.
Copper Nanowires
[0055] Copper nanowires produced in accordance with the present
disclosure are characterized by a relatively small diameter and a
high aspect ratio. Generally, the population of copper nanowire
structures that are produced according to embodiments of the
present disclosure have an average diameter of less than about 40
nm. In some embodiments the population has an average diameter of
less than about 30 nm, less than about 25 nm, from about 10 nm to
about 40 nm, from about 10 nm to about 30 nm from about 15 nm to
about 40 nm, from about 15 nm to about 30 nm, from about 20 nm to
about 40 nm or from about 20 nm to about 30 nm. The average length
of the copper nanowire structures produced according to embodiments
of the present disclosure may be at least about 10 nm, at least
about 100 nm or even at least about 1 mm. In some embodiments, the
average aspect ratio (i.e., the average ratio of length to diameter
of the copper nanowire structures) is at least about 10:1. In other
embodiments, the aspect ratio is at least about 50:1, at least
about 100:1, at least about 1000:1, at least about 10,000:1 or even
at least about 25,000:1.
[0056] The population of nanowires contains copper and amounts of
organic material (e.g., the capping agent). In this regard, the
amount of copper in the population of nanowires (and in each
nanowire) by at least about 60 wt % copper or, as in other
embodiments, at least about 70 wt % copper, at least about 80 wt %
copper, from about 60 wt % to about 99 wt % copper or from about 70
wt % to about 95 wt % copper.
[0057] In this regard, the properties applied above may be an
average of the population of copper nanowires that is produced or
of individual nanowires. Populations of copper nanowires may
include at least about 100 copper nanowires, at least about 1000
copper nanowires, at least about 10,000 copper nanowires, at least
about 1.times.10.sup.6 copper nanowires or even at least about
1.times.10.sup.9 copper nanowires.
[0058] The copper nanowires of the present disclosure have been
found to have a penta-twinned structure (i.e., five single
crystallites bound together). It is believed the penta-twinned
structure is bound by ten {111} facets at the two ends and five
{100} side faces. It should be noted that the copper nanowires are
not constructed on a template or membrane. In contrast, metallic
copper atoms themselves give the nanowire its structural
characteristics.
Other Nanostructures
[0059] As discussed above, other structures may be produced by
varying the reaction conditions. In some embodiments, a tadpole
shaped nanostructure may be produced in which a bi-pyramid
structure tapers from a base of about 200 nm (FIG. 11). If the
reaction is allowed to continue, the reaction slows and a nanowire
with a radius less than about 40 nm extends from the point of the
bi-pyramid (FIGS. 12-15). In some embodiments, the reaction
conditions are controlled such that copper nanocubes are formed. In
the initial stage of reaction (e.g., at about 1 hour), the cube
sides are about 50 nm in size (FIG. 25). If the reaction is allowed
to continue (e.g., for about 6 hours) the edges of the cube grow to
about 200 nm in size (FIG. 26).
EXAMPLES
[0060] The reaction conditions were varied in Examples 1-4 to
produce various structures as shown in FIG. 31. It should be noted
that other reaction conditions (e.g., component concentrations) may
be used to produce the desired nanostructures and the recited
conditions are exemplary and should not be considered in a limiting
sense.
Example 1
Production of Copper Nanowires and Images Collected from Same
[0061] To produce copper nanowires, CuCl.sub.2.2H.sub.2O (0.021 g),
HDA (0.18 g) and glucose (0.05 g) were dissolved in water (10 ml)
in a vial (22.2 ml, borosilicate glass vial, with a black phenolic
molded screw cap and polyvinyl-faced pulp liner, VWR International
(Radnor, Pa.)) at room temperature. After the vial had been capped,
the solution was magnetically stirred at room temperature
overnight. The capped vial was then transferred into an oil bath
and heated at 100.degree. C. for 6 hours under magnetic stirring.
As the reaction proceeded, the solution changed its color from blue
to brown and finally red-brown. All the chemicals were obtained
from Sigma-Aldrich (St. Louis, Mo.) and used as received.
[0062] To prepare samples for electron microscopy
characterizations, the as-prepared aqueous suspensions were
directly dropped onto silicon substrates (for SEM) or carbon-coated
copper grids (for TEM and high-resolution TEM) and then dried under
the ambient conditions of a chemical laboratory. The silicon
substrates or copper grids were then rinsed with hot water (about
60.degree. C.) to remove the excess HDA and glucose, followed by
another round of drying. The products could have alternatively been
collected as powders by use of centrifugation processes.
[0063] Scanning electron microscope (SEM) images were captured of
the copper nanowires dried on silicon substrates. All SEM images
were captured with a field-emission microscope (Nova NanoSEM 230,
FEI (Hilsboro, Oreg.)) operated at 15 kV. All transmission electron
microscope (TEM) images were conducted with a microscope (Tecnai G2
Spirit, FEI (Hilsboro, Oreg.)) operated at 120 kV. High-resolution
TEM imaging was performed using a microscope (2100F, JEOL (Tokyo,
Japan)) operated at 200 kV. Powder x-ray diffraction (XRD) patterns
were recorded using a diffractometer (DMAX/A, Rigaku (The
Woodlands, Tex.)) operated at 35 kV and 35 mA. The concentrations
of Cu (II)/Cu (I) left behind in the reaction solutions were
determined using an inductively-coupled plasma mass spectrometer
(ICP-MS, PerkinElmer (Waltham, Mass.)).
[0064] FIG. 1 shows the X-ray diffraction (XRD) pattern of a copper
nanowire. The three peaks at 20=43.5, 50.7, and 74.4.degree.
correspond to diffractions from {111}, {200}, and {220} planes,
respectively, of face-centered cubic copper (JCPDS #03-1018). No
other phases such as Cu.sub.2O and CuO were detected. The
concentrations of Cu.sup.2+/Cu.sup.+ ions left behind in the
reaction solution was measured using inductively-coupled plasma
mass spectrometry (ICP-MS). It was determined that the precursor
had been converted into atomic copper at a percentage of 93%.
[0065] The scanning electron microscopy (SEM) image shown in FIG. 2
demonstrates that copper nanowires could be prepared in high
purity, typically approaching 95%, without any post-synthesis
separation. Only a very small amount of copper nanocubes was found
to co-exist with the nanowires. In addition, the nanowires were
found to be highly flexible and some of them showed bending more
than 360 degrees without being broken. Both the SEM image at a
higher magnification (FIG. 3) and TEM image (FIG. 4) reveal that
the nanowires were uniform in diameter and tended to be aligned in
parallel to form bundles during sample preparation. The nanowires
had an average diameter of 24.+-.4 nm as calculated from 100
nanowires randomly selected from a number of TEM images (FIG. 5).
The lengths of the copper nanowires varied in the range of several
tens to hundreds of micrometers; some of them were as long as
several millimeters. The band-like contrast (see the box in FIG. 4)
observed on the TEM images can be attributed to strains caused by
bending or twisting.
[0066] FIGS. 6-9 show transmission electron microscopy (TEM) images
and the corresponding high-resolution TEM images taken from the
middle (FIGS. 6 and 7) and end portions (FIGS. 8 and 9) of two
different Cu nanowires, respectively. The insets in FIG. 7 and FIG.
9 schematically illustrate the orientations of the copper nanowires
relative to the incident electron beam (indicated by arrows). The
high-resolution TEM images (FIGS. 7 and 9) show the existence of
{111} twin planes parallel to the long axis of the copper nanowire.
When the direction of the e-beam was perpendicular to the bottom
side of the pentagonal nanowire (FIG. 7), two sets of fringes with
lattice spacing of 2.1 nm and 1.3 nm were observed, corresponding
to the {111} and {220} planes of copper, respectively. FIG. 9 shows
the high-resolution TEM image taken from a copper nanowire oriented
with one of its side faces parallel to the e-beam. The fringes with
lattice spacing of 2.1, 1.8, and 1.3 .ANG. could be indexed to the
{111}, {200}, and {220} planes of copper, respectively. Based on
the analysis of both SEM and high-resolution TEM images, it is
evident that the copper nanowires had a penta-twinned structure
bound by ten {111} facets at the two ends and five {100} side
faces, which are consistent with the results previously reported
for other metals (e.g., Ag, Au, and Pd).
Example 2
Comparison of the UV-Vis Transmission Spectra Between the Copper
Nanowires of Example 1 and Silver Nanowires
[0067] UV-vis spectra were taken with a diode array
spectrophotometer (Cary 50, Varian (Palo Alto, Calif.)). FIG. 10
shows UV-vis transmission spectra recorded from aqueous suspensions
of the 24-nm copper nanowires of Example 1 and penta-twinned silver
nanowires of 80-nm in diameter (prepared according to the
literature) at roughly the same metal concentration (30 .mu.g/ml),
suggesting a slightly higher transmittance in the visible region
for the copper nanowires. This higher transmittance could be
attributed to the smaller diameter of the copper nanowires.
Example 3
Production of Copper Nanostructures with a Bi-Pyramid Shape and
Images Collected from Same
[0068] The preparation procedure of Example 1 was used to produce
copper nanocrystals but the concentration of glucose (i.e., the
reducing agent) was increased from 5 to 10 mg/ml. As can be seen
from FIGS. 11-16, tadpole-like copper nanostructures resulted from
the increased amount of reducing agent. In an effort to uncover the
growth mechanism, the products obtained at different reaction times
were analyzed as detailed in FIGS. 11-16. In the initial stage
(t=30 min), the solution changed its color from blue to red-brown
due to the formation of tapered copper nanocrystals whose diameter
gradually changed from 200 to 25 nm over a length of 0.5 to 1 .mu.m
(FIG. 11). The tapered cooper nanocrystals exhibited a UV-vis
absorption peak around 591 nm (FIG. 17) and are characterized by a
pentagonal bi-pyramid structure (see inset of FIG. 11 and FIGS.
18-23) formed by stretching apart the five-fold apices of a
decahedron.
[0069] After the reaction had proceeded to 1 hour (FIG. 12), thin
copper nanowires of about 24 nm in diameter started to appear from
the thinner end of a tapered nanocrystal. As the reaction was
continued for three hours (FIGS. 13 and 14), the copper nanowires
further grew along the long axes with almost no change to their
diameters. These results indicate that the tadpole-like copper
nanowires originated from the tapered nanocrystals. The SEM image
in the inset of FIG. 14 indicates that the tadpole-like copper
nanowires also had a pentagonal cross-section. The TEM and
high-resolution TEM images shown in FIGS. 15 and 16 further confirm
a tadpole-like morphology and a penta-twinned structure for the
copper nanowires.
Example 4
Production of Copper Nanostructures with a Cubic Shape and Images
Collected from Same
[0070] The preparation procedure of Example 1 was used to produce
copper nanocrystals but the concentration of HDA (i.e., the capping
agent) was decreased from 18 mg/ml to 9 mg/ml. Copper nanocubes
(FIGS. 24-26) formed rather than copper nanowires. FIGS. 24-26 are
SEM images of the products obtained after 0.5 h, 1 h, and 6 h of
reaction, respectively. FIG. 27 gives an XRD pattern of the
nanocubes obtained at 6 h. For bulk copper, the strongest XRD
diffraction is the (111) peak, followed by the (200), (220), and
(311) peaks. In contrast, the copper nanocubes tend to give (200)
diffraction as the strongest peak because of their preferential
orientation with {100} planes parallel to the substrate. The
high-resolution TEM image of an individual Cu nanocube viewed along
the <100> zone axis (FIGS. 28-29) clearly shows
well-resolved, continuous fringes with lattice spacing of 1.8
.ANG., corresponding to the {100} planes, indicating that the
nanocube was a single crystal bound by {100} facets.
[0071] FIG. 30 shows UV-vis absorption spectra taken from the
copper nanocubes dispersed in water. The copper nanocubes exhibited
a major SPR peak in the visible region, whose position was
red-shifted from 565 to 625 nm as the edge length of the nanocubes
was increased from 50 to 200 nm. Compared to silver nanocubes with
a similar size, the SPR peak of the copper nanocubes was positioned
at a much longer wavelength.
[0072] When introducing elements of the present disclosure or the
preferred embodiments(s) thereof, the articles "a", "an", "the" and
"said" are intended to mean that there are one or more of the
elements. The terms "comprising", "including" and "having" are
intended to be inclusive and mean that there may be additional
elements other than the listed elements.
[0073] As various changes could be made in the above apparatus and
methods without departing from the scope of the disclosure, it is
intended that all matter contained in the above description and
shown in the accompanying figures shall be interpreted as
illustrative and not in a limiting sense.
* * * * *