U.S. patent application number 11/701974 was filed with the patent office on 2008-01-03 for methods for production of silver nanostructures.
This patent application is currently assigned to University of Washington. Invention is credited to Sang-Hyuk Im, Yun Tack Lee, Yugang Sun, Benjamin Wiley, Younan Xia.
Application Number | 20080003130 11/701974 |
Document ID | / |
Family ID | 38876856 |
Filed Date | 2008-01-03 |
United States Patent
Application |
20080003130 |
Kind Code |
A1 |
Xia; Younan ; et
al. |
January 3, 2008 |
Methods for production of silver nanostructures
Abstract
Methods for producing silver nanostructures with improved
dimensional control, yield, purity, monodispersity, and scale of
synthesis.
Inventors: |
Xia; Younan; (St. Louis,
MO) ; Im; Sang-Hyuk; (Pohang-Shi, KR) ; Lee;
Yun Tack; (Seattle, WA) ; Sun; Yugang;
(Naperville, IL) ; Wiley; Benjamin; (Boston,
MA) |
Correspondence
Address: |
STOEL RIVES LLP
900 SW FIFTH AVENUE
SUITE 2600
PORTLAND
OR
97204-1268
US
|
Assignee: |
University of Washington
Seattle
WA
98105-4608
|
Family ID: |
38876856 |
Appl. No.: |
11/701974 |
Filed: |
February 1, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60764541 |
Feb 1, 2006 |
|
|
|
Current U.S.
Class: |
420/501 ;
420/507; 977/762; 977/810 |
Current CPC
Class: |
B22F 2998/10 20130101;
B22F 9/24 20130101; B82Y 40/00 20130101; B22F 2001/0037 20130101;
B22F 2304/05 20130101; C22C 5/06 20130101; B22F 2301/255 20130101;
C22B 11/04 20130101; C30B 29/02 20130101; C30B 7/00 20130101; B22F
2009/245 20130101; C30B 29/60 20130101; B82Y 30/00 20130101; C22C
5/02 20130101; B22F 1/0025 20130101 |
Class at
Publication: |
420/501 ;
420/507; 977/762; 977/810 |
International
Class: |
C22C 5/06 20060101
C22C005/06; C22C 5/02 20060101 C22C005/02 |
Goverment Interests
FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] This work has been supported in part by ONR
(N-00014-01-1-0976), a research grant from the NIH (R01
CA120480-01), by the NSF (DMR-0451788).
Claims
1. A method of producing silver nanostructures, the method
comprising: heating ethylene glycol; forming a mixture by mixing a
halide with the heated ethylene glycol; mixing a silver nitrate
(AgNO.sub.3) solution with the mixture; mixing a polyvinyl
pyrrolidone (PVP) solution with the mixture; and isolating silver
nanostructures from the mixture.
2. The method of claim 1, wherein the halide is a chloride.
3. The method of claim 2, wherein the chloride comprises HCl.
4. The method of claim 1, wherein the chloride included in the
mixture is in a concentration ranging from about 0.1 mM to about
0.4 mM.
5. The method of claim 1, wherein the isolated silver
nanostructures comprise single-crystal silver nanocubes.
6. The method of claim 1, wherein the ethylene glycol is heated to
a temperature ranging from about 130.degree. C. to about
150.degree. C.
7. The method of claim 1, wherein the ethylene glycol is heated to
a temperature up to about 140.degree. C.
8. The method of claim 1, wherein the mixture is maintained at a
temperature ranging from about 130.degree. C. to about 150.degree.
C.
9. The method of claim 1, wherein the mixture is maintained at a
temperature up to about 140.degree. C.
10. The method of claim 1, wherein the mixture comprises AgNO.sub.3
in a range from about 20 mM to about 30 mM.
11. The method of claim 1, wherein the mixture comprises PVP in a
range from about 30 mM to about 40 mM (calculated in terms of the
PVP repeating unit).
12. A method of producing silver nanostructures, the method
comprising: heating ethylene glycol; forming a mixture by mixing a
chloride with the heated ethylene glycol; adding a silver nitrate
(AgNO.sub.3) solution and a polyvinyl pyrrolidone (PVP) solution to
the mixture, wherein the PVP comprises an iron salt; and isolating
silver nanostructures from the mixture.
13. The method of claim 12, wherein the isolated silver
nanostructures comprise silver nanowires.
14. The method of claim 12, wherein the isolated silver
nanostructures comprise single-crystal silver nanocubes.
15. The method of claim 12, wherein the iron salt is a ferrous salt
or a ferric salt.
16. The method of claim 12, wherein the iron salt is iron (III)
acetylacetonate (Fe(acac).sub.3).
17. The method of claim 12, wherein the iron salt is iron (II)
acetylacetonate (Fe(acac).sub.2).
18. The method of claim 12, wherein the iron salt is iron (III)
chloride (FeCl.sub.3).
19. The method of claim 12, wherein the chloride comprises sodium
chloride (NaCl)
20. The method of claim 12, wherein the chloride is included in the
mixture in a concentration ranging from about 0.5 mM to about 0.25
mM.
21. The method of claim 12, wherein the isolated silver
nanostructures comprise single-crystal silver nanocubes or silver
nanowires depending of the concentration of the iron salt.
22. The method of claim 12, wherein the iron salt is included in
the mixture in a concentration ranging from about 0.1 .mu.M to
about 25 .mu.M.
23. The method of claim 12, wherein the concentration of the iron
salt in the mixture ranges from about 0.44 .mu.M to about 25 .mu.M
and produces silver nanostructures comprising nanowires.
24. The method of claim 12, wherein the concentration of the iron
salt is less than or equal to about 0.44 .mu.M and produces silver
nanostructures comprising nanowires.
25. The method of claim 12, wherein the ethylene glycol is heated
to a temperature in the range from about 150.degree. C. to about
170.degree. C.
26. The method of claim 12, wherein the ethylene glycol is heated
to a temperature up to about 160.degree. C.
27. The method of claim 12, wherein the mixture is maintained at a
temperature ranging from about 150.degree. C. to about 170.degree.
C.
28. The method of claim 12, wherein the mixture is maintained at a
temperature up to about 160.degree. C.
29. A method of producing right bypyramid silver nanostructures,
the method comprising: heating ethylene glycol; forming a mixture
by mixing a bromide with the heated ethylene glycol; adding a
silver nitrate (AgNO.sub.3) solution to the mixture; adding a
polyvinyl pyrrolidone (PVP) solution to the mixture; and isolating
right bypyramid silver nanostructures from the mixture.
30. The method of claim 25, wherein the bromide comprises HBr.
31. The method of claim 25, wherein the PVP solution comprises
HBr.
32. A right bypyramid synthesized according to the method of claim
25.
33. A method of producing silver nanocubes, the method comprising:
heating ethylene glycol; forming a mixture by mixing a sulfide with
the heated ethylene glycol; adding a silver nitrate (AgNO.sub.3)
solution to the mixture; adding a polyvinyl pyrrolidone (PVP)
solution to the mixture; and isolating silver nanocubes from the
mixture.
34. The method of claim 33, wherein the sulfide comprises a sodium
sulfide (Na.sub.2S) solution.
35. The method of claim 33, wherein the sulfide comprises a sodium
hydrogen sulfide solution (NaHS).
36. The method of claim 33, wherein the sulfide is included in the
mixture in a concentration ranging from about 25 82 M to about 35
.mu.M.
37. The method of claim 33, wherein the mixture is maintained at a
temperature of about 135.degree. C. to about 165.degree. C.
38. The method of claim 33, wherein the PVP solution and the
AgNO.sub.3 solution are added to the mixture such that the molar
ratio between the PVP (in terms of the PVP repeating unit) and the
AgNO.sub.3 (PVP: AgNO.sub.3) in the mixture ranges from about
0.75:1 to 2.7:1.
39. The method of claim 33, wherein the PVP solution and the
AgNO.sub.3 solution both comprise ethylene glycol.
40. At least one gold nanocage synthesized by using at least one
silver nanocube, produced by the method of claim 33, as a
sacrificial template.
Description
PRIORITY CLAIMS
[0001] This application claims the benefit of U.S. Provisional
Application No. 60/764,541, filed Feb. 1, 2006, the entirety of
which is incorporated herein by reference.
TECHNICAL FIELD
[0003] The present disclosure relates to nanotechnology and the
production of nanostructures. More particularly, this disclosure
relates to the production of silver nanostructures.
BRIEF DESCRIPTION OF THE DRAWINGS
[0004] Understanding that drawings depict only typical embodiments
of the invention and are not therefore to be considered to be
limiting of its scope, the invention will be described and
explained with specificity and detail through the use of the
accompanying drawings as listed below.
[0005] FIG. 1 is schematic illustration of the mechanism by which
single-crystal silver nanocubes are produced.
[0006] FIGS. 2A-2B are transmission electron microscope (TEM)
images of silver nanoparticles produced at t=4 min (2A), 46 min
(2B), 103 min (2C), and 15 h (2D).
[0007] FIG. 3 UV-Visible extinction spectra take from aqueous
solutions that contained silver nanocubes with different edge
lengths (30, 45, and 60 nm).
[0008] FIGS. 4A-4D are scanning electron microscope (SEM) images of
silver nanoparticles synthesized at different HCl concentrations:
(4A) 0.125, (4B) 0.25, and, (4C) 0.375 mM, respectively. The
reaction solution contained 23.5 mM of AgNO.sub.3 and 36.7 mM of
PVP (calculated in terms of the repeating unit), and was heated in
an oil bath held at 140.degree. C. (D) Silver nanoparticles
obtained under the same condition except that the 0.25 mM HCl was
replaced with 0.25 mM HNO.sub.3.
[0009] FIGS. 5A-5C are Typical SEM images of the as-synthesized
silver nanocubes. The 5A inset is an SEM image illustrating the
sharp corners and edges of these nanocubes. (5B) An X-ray
diffraction (XRD) pattern of the same batch of silver nanocubes.
(5C) is a TEM image of the silver nanocubes. The inset gives an
electron diffraction pattern recorded by directing the electron
beam perpendicular to the (100) facet of a silver nanocube.
[0010] FIG. 6 is an illustration of the mechanism by which Fe(II)
removes atomic oxygen from the surface of silver nanostructures.
Reduction by ethylene glycol (EG) competes with oxidation by atomic
oxygen to form an equilibrium between Fe(III) and Fe(II).
[0011] FIGS. 7A-7B show TEM (7A) and SEM (7B) images of samples
taken from reactions at t=10 and t=60 min, respectively. The
reaction solution contained 2.2 .mu.M of Fe(acac).sub.3.
[0012] FIGS. 7C-7C show TEM (7C) and SEM (7D) images of samples
taken from reactions at t=10 and t=60 min, respectively. The
reaction solution contained 2.2 .mu.M of Fe(NO.sub.3).sub.3.
[0013] FIGS. 8A-8F show TEM (left) and SEM (right) images of
samples taken from reactions at t=10 min, and t=60 min,
respectively. Each solution contained (8A, 8B) 2.2, (8C, 8D) 22,
and (8E, 8F) 220 .mu.M of Fe(acac).sub.2. The inset in (F) shows
the etched sample at higher magnification.
[0014] FIGS. 9A-9D show TEM images of samples taken from the
reaction solution at different times: (9A) 10 min, (9B) 2 h, (9C) 6
h, and (9D) 8.5 h. The reaction solution contained 0.44 .mu.M
Fe(acac).sub.2.
[0015] FIG. 9E shows an SEM image of isolated nanocubes taken at
t=15 h. The reaction solution contained 0.44 .mu.M
Fe(acac).sub.2.
[0016] FIGS. 10A-10B show SEM images of bipyramids approximately
(10A) 150 nm and (10B) 75 nm in edge length. The inset in (10B)
shows the electron diffraction pattern obtained from a single
bipyramid, indicating it may be bounded by (100) facets.
[0017] FIGS. 10C-10D show comparisons of the UV-Vis-NIR spectra
taken from the bipyramids in FIGS. 10A and 10B with DDA
calculations performed on bipyramids of the same size, but with
corners truncated by (111) facets 14 and 12 nm in edge length,
respectively.
[0018] FIG. 11 shows a comparison of SEM images of a single right
bipyramid with CAD models and the bipyramid is viewed under SEM
from above and with a 45.degree. tilt at four different
rotations.
[0019] FIG. 12A shows TEM of a right bipyramid .about.40 nm in edge
length taken from the reaction at t=2.5 hours.
[0020] FIG. 12B shows the twin plane which bisects the bipyramid is
clearly visible after tilting the particle from FIG. 12A by
45.degree..
[0021] FIG. 12C shows single twinned seeds approximately 25 nm in
diameter observed at t=1.5 hours.
[0022] FIG. 12D shows a high resolution transmission electron
microscope (HRTEM) image of a twinned seed showing the lattice
fringes reflecting across the (111) twin plane.
[0023] FIG. 12E is a model of the seed in FIG. 12C showing (111)
truncation of corners, reentrant (111) surfaces at the twin
boundary and reentrant (100) surfaces at the twin boundary
corners.
[0024] FIG. 13 shows a SEM of isolated nanostructures at t=5 hours
when NaBr was not added to the preheated EG before the Ag
precursor.
[0025] FIG. 14 shows a XRD pattern obtained from 150 nm right
bipyramids on a glass slide.
[0026] FIGS. 15A-15B show comparisons of the UV-Vis-NIR spectra
taken from the bipyramids in FIGS. 10A and 10B with DDA
calculations performed on right bipyramids 150 and 75 nm in edge
length without any truncation.
[0027] FIG. 16 shows a comparison of the UV-Vis-NIR spectra taken
from aqueous solutions of silver nanocubes and bipyramids
demonstrating the extinction peak of a 75 nm right bipyramid is
red-shifted 50 nm from that of nanocubes of similar size.
[0028] FIG. 17 shows a 3D model of a bipyramid seed obtained by
truncating a model of the bipyramid till its outline matched that
of the seed.
[0029] FIGS. 18A-18C showing SEM images of samples taken at
different stages of a sulfide-assisted polyol synthesis: (18A) 3
minutes after the injection of silver nitrate; (18Bb) the growth of
the cubes after 7 minutes into the reaction when the average edge
length was increased to 45 nm; (18C) if the reaction was allowed to
continue (here after 30 minutes) etching would become dominant,
stopping the growth and rounding the corners of the cubes.
[0030] FIGS. 19A-19D show SEM images of nanostructures formed with
a sodium sulfide concentration of 26 .mu.M at different
temperatures (145 to 160.degree. C.).
[0031] FIGS. 19E-19H show SEM images of nanostructures formed with
a sodium sulfide concentration of 28 .mu.M at different
temperatures (145 to 160.degree. C.).
[0032] FIGS. 19I-19L show SEM images of nanostructures formed with
a sodium sulfide concentration of 30 .mu.M at different
temperatures (145 to 160.degree. C.).
[0033] FIGS. 19M-19P show SEM images of nanostructures formed with
a sodium sulfide concentration of 32 .mu.M at different
temperatures (145 to 160.degree. C.).
[0034] FIGS. 20A-20F SEM images of reactions containing increasing
molar ratios between the repeating unit of PVP and silver nitrate.
The ratios of PVP to silver nitrate were (20A) 0.77, (20B) 1.15,
(20C) 1.5, (20D) 1.9, (20E) 2.3, and (20F) 0.7.
[0035] FIG. 21A shows an SEM image of silver nanocubes synthesized
under the mediation of sodium sulfide (scale bar is 100 nm).
[0036] FIG. 21B shows a TEM image and electron diffraction pattern
(inset) taken from a single nanocube, indicating that it is single
crystal and enclosed by {100} facet (scale bar is 50 nm).
[0037] FIG. 21C an XRD pattern taken from the same batch of silver
nanocubes in FIG. 21B.
[0038] FIG. 21D is a UV-Vis spectra taken from silver nanocubes of
different sizes that were all synthesized under the mediation of
sodium sulfide showing that the main peak blue-shifted as the size
was reduced.
[0039] FIG. 22A shows SEM and TEM (inset) images of a sample of
gold nanocages produced from silver nanocubes.
[0040] FIG. 22B shows UV-Vis extinction spectra taken from the
silver nanocubes and gold nanocages.
[0041] FIG. 23 shows the sulfide species present in aqueous
solutions with respect to the pH of the solution.
DETAILED DESCRIPTION
[0042] It will be readily understood that the components and
methods of the embodiments as generally described and illustrated
in the figures herein could be arranged and designed in a wide
variety of different configurations. Thus, the following more
detailed description of various embodiments, as represented in the
figures, is not intended to limit the scope of the invention, as
claimed, but is merely representative of various embodiments. While
the various aspects of the embodiments are presented in drawings,
the drawings are not necessarily drawn to scale unless specifically
indicated.
[0043] Shape control of metal nanoparticles has received
considerable attention in recent years because of the strong
correlation between the shape and the chemical, physical,
electronic, optical, magnetic, and catalytic properties of a
nanoparticle. Silver and gold, in particular, have been intensively
studied due to their numerous applications that may include surface
plasmonics, surface-enhanced Raman scattering (SERS), as well as
chemical and biological sensing. A wealth of chemical methods have
been developed for synthesizing silver and gold nanostructures
having well-controlled shapes, and typical examples include
triangular plates, cubes, belts, wires, rods, and branched
multipods. Most of these methods, however, still require
improvement in terms of yield, purity, monodispersity, and scale of
synthesis before they will find use in commercial applications.
[0044] The polyol process is well known by those in the art as a
means to synthesize silver and gold nanoparticles with controllable
shapes in relatively large quantities. (See, e.g. Y. Sun, Y. Xia,
Science 2002, 298, 2176; R. Jin, S. Egusa, N. F. Scherer, J. Am.
Chem. Soc. 2004, 126, 9900; T. K. Sau, C. J. Murphy, J. Am. Chem.
Soc. 2004, 126, 8648; F. Kim, S. Connor, H. Song, T. Kuykendall, P.
Yang, Angew. Chem. Int. Ed. 2004, 43, 3673; Y. Sun, B. Mayers, Y.
Xia, Nano. Lett. 2003, 5, 675; Y. Sun, B. Mayers, T. Herricks, Y.
Xia, Nano Lett. 2003, 3, 955; Y. Sun, Y. Yin, B. Mayers, T.
Herricks, Y. Xia, Chem. Mater. 2002, 14, 4736; and Y. Sun, B.
Mayers, Y. Xia, Adv. Mater. 2003, 15, 641, each of which are
incorporated in their entirety by reference herein). The following
experimental examples disclose various embodiments of silver
nanostructure production.
EXAMPLE 1
[0045] It is know by those of skill in the art that single-crystal
cubes and tetrahedrons of silver with truncated corners/edges may
be prepared through the selective etching and dissolution of
twinned seeds by chloride ions and oxygen (from air). In one
possible embodiment hydrochloric acid may be used as a mediator for
the production of single-crystal nanocubes.
[0046] In a typical polyol synthesis, silver atoms may be obtained
by reducing AgNO.sub.3 with ethylene glycol (EG) through the
following reactions:
2HOCH.sub.2CH.sub.2OH.fwdarw.2CH.sub.3CHO+2H.sub.2O (1)
2Ag.sup.++2CH.sub.3CHO.fwdarw.CH.sub.3CHO--OHCCH.sub.3+2Ag+2H.sup.+
(2) Once the concentration of silver atoms has reached the
supersaturation value, they may start to nucleate and grow into
nanoparticles. At the same time, nitric acid generated in situ
activates a backward reaction that dissolves the solid silver
initially formed: 4HNO.sub.3+3Ag.fwdarw.3AgNO.sub.3+NO+2H.sub.2O
(3) Through the introduction of HCl, reaction (3) can be driven
further to the right due to the formation of more HNO.sub.3 from
HCl and AgNO.sub.3.
[0047] On the basis of these reactions single-crystal silver
nanocubes may be formed. Upon addition of silver nitrate and PVP to
the hot EG solution, both twined and single-crystal seeds of silver
can be formed through homogeneous nucleation, with the twinned
particles being the most abundant morphology as a result of their
relatively lower surface energies. These initially formed
nanoparticles are dissolved due to the relatively high
concentration of HNO.sub.3 present in the early stages of the
reaction. As the reaction continues, HNO.sub.3 is gradually
consumed and a second round of nucleation occurs. At very small
sizes the crystal structure of these nuclei may fluctuate. When the
nanoparticles grow in dimension, they can be locked into either a
single crystalline or twinned morphology. While the twinned
particles have a lower overall surface energy, this comes at the
expense of significant lattice distortion and surface defects.
Thus, twinned particles are expected to exhibit a stronger
reactivity and susceptibility towards etching. Since no lattice
distortion is required to form a single crystal, these seeds should
be relatively more stable in this environment, and can continue to
grow. Consequently, through the selective etching of twinned seeds
by HNO.sub.3, high yields of single-crystal nanocubes result. FIG.
1 shows a schematic illustration summarizing this mechanism.
[0048] While selective etching is the dominant mechanism for
producing nanocubes, there are additional elements that contribute
to its success. In one embodiment of the present invention, both
the proton and the chloride ion of HCl may play a significant role.
In addition to its function in increasing the concentration of
HNO.sub.3, the proton can greatly reduce the net reaction rate
according to Le Chatelier's principle. At the same time, chloride
ions likely adsorb onto the surfaces of silver seeds, and thereby
prevent agglomeration through electrostatic stabilization. In yet
another embodiment, the reaction temperature may be reduced from
160.degree. C. to 140.degree. C. to further slow down the net
reaction rate in an effort to increase the efficiency of selective
etching by nitric acid. This combination of factors may promote the
production of silver nanocubes at high yields and with
monodispersed sizes.
[0049] The primary stages of the reaction can be recognized by
their distinctive colors (not shown). For example, the solution may
progress from clear to milky white (up until t=4 min) after the
injection of the AgNO.sub.3 and PVP solutions. This color suggests
the presence of AgCl precipitate due to the relatively high
concentration of chloride ions in the reaction mixture (0.25 mM).
FIG. 2A shows a TEM image of a sample taken from the solution at
t=4 min. The chemical composition of these particles was AgCl, as
confirmed by both XRD and EDX analyses. At t=45 min, the solution
became light yellow in color (not shown), indicating that the AgCl
particles had dissolved and only Ag nanoparticles remained in the
reaction mixture. As shown by the TEM of FIG. 2B, Ag nanoparticles
of two different sizes (.about.17 and .about.5 nm) coexisted in the
solution at this time. The light yellowish color gradually faded,
and the solution appeared transparent and colorless around t=105
min (not shown), suggesting the complete dissolution of all large
Ag nanoparticles. This picture is also consistent with TEM
observations shown by FIG. 2C where only very few Ag nanoparticles
of very small size (.about.6 nm) could be found.
[0050] This transparent state may lasted for approximately 1 hour,
whereupon the solution acquired a reddish tint of increasing
intensity over a period of several hours. By t=15 h, the solution
had become indisputably red (not shown), implying the formation of
small Ag nanocubes. Referring to the TEM image of FIG. 2D, the Ag
nanocubes appear symmetrical in shape and .about.30 nm in edge
length. These Ag nanocubes could further grow into larger sizes if
the reaction was allowed to continue. The solution became ocher in
color at approximately t=26 h (not shown). The Ag nanocubes
contained in this solution were also symmetrical in shape, with
their edge length having increased to .about.130 nm. This
observation implies that it will be possible to control the size,
and therefore the optical properties of Ag nanocubes, simply by
varying reaction time. FIG. 3 shows the size-dependent extinction
spectra of the cubes. The number of peaks and relative positions
are consistent with theoretical calculations.
[0051] In yet another embodiment, the dependence of morphology on
the concentration of HCl was also examined. FIG. 4A shows the SEM
image of a product obtained at t=25 h when the concentration of HCl
in the final mixture was 0.125 mM. This sample contained a mixture
of polydisperse silver nanocubes, tetrahedrons, and nanowires. It
is believed that the wires and irregular particles formed due to
the incomplete etching of twined seeds by the lower concentration
of HNO.sub.3. FIG. 4B shows an SEM image of the final product
obtained at t=26 h when the concentration of HCl was increased to
0.25 mM (i.e., the synthesis described in FIGS. 2A-2D). The
solution contained only monodisperse silver nanocubes of .about.130
nm in edge length. As the concentration of HCl was further
increased to 0.375 mM, a different morphology was observed (FIG.
4C). This product, obtained at t=22 h, was characterized by a
mixture of relatively thick wires and irregular particles. In this
case, it was found that the reaction mixture did not become
transparent, indicating that the twinned seeds were not dissolved
to cut short their growth into nanowires and irregular particles.
The chloride ions may slow down the etching of twined seeds by
selectively blocking the twin sites through surface adsorption.
Alternatively, it is possible that an increase in the HCl
concentration may result in the formation of more AgCl precipitate
at the initial stage of a synthesis, and that some of these AgCl
colloids may survive and serve as seeds for the subsequent growth
of twinned particles.
[0052] In order to separate the roles played by the proton and
chloride, the 0.25 mM HCl was replaced with 0.25 mM HNO.sub.3. The
NO.sub.3-- ions from HNO.sub.3 should have a negligible effect on
the synthesis because the concentration of HNO3 was extremely low
as compared to the concentration of AgNO.sub.3. FIG. 4D shows a SEM
image of the product at t=30 h, which contained a mixture of small
silver nanocubes (.about.40 nm in edge length) and some irregular
particles. This observation implies that HNO.sub.3 may be able to
induce the etching and dissolution of twined seeds, and thus
channel the product into single crystal nanocubes. However, due to
the absence of Cl-- in the solution, the single-crystal seeds could
not be stabilized, and therefore might agglomerate into larger,
irregularly shaped particles. It is also expected that a
combination of Cl-- and O.sub.2 will facilitate the etching and
dissolution of twined seeds to further reduce the percentage of
twined particles in the final product.
[0053] In yet another embodiment, volumes of all the solutions were
multiplied by five times. In this case, the synthesis followed the
same pattern of color changes, suggesting the nucleation and growth
mechanisms did not change as the reaction volume was increased.
FIG. 5A shows a SEM image of this sample, indicating that all
particles were cubic in shape with an average edge length of 125
nm. The inset is a tilted SEM image at higher magnification, which
clearly displays the sharp corners and edges of these nanocubes.
FIG. 5b shows an XRD pattern recorded from the same batch of silver
nanocubes. The abnormal intensity of the (200) peak may suggest
that the sample is made of nanocubes that were preferentially
oriented with their (100) planes parallel to the supporting
substrate. FIG. 5C shows a typical TEM image of the silver
nanocubes. The inset shows an electron diffraction pattern recorded
by directing the electron beam perpendicular to the (100) facet of
an individual nanocube, confirming that the particles are single
crystals.
[0054] In summary, monodispersed nanocubes of silver have been
synthesized in large quantities by introducing a small amount of
hydrochloric acid to the conventional polyol synthesis. Based on
color changes and electron microscopy studies, it is believed that
hydrochloric acid plays an important role in selectively etching
and dissolving twinned silver nanoparticles. In addition, the
presence of protons may slows down the reduction reaction, and
thereby facilitate the formation of single-crystal seeds.
Experimental
[0055] In a polyol synthesis, 5 mL of ethylene glycol (EG, J. T.
Baker, 9300-01) was placed in a 20-mL vial, capped, and heated in
an oil bath at 140.degree. C. for 1 h. Thereafter, 1 mL of a 3 mM
HCl solution in EG was quickly added, and the vial was recapped.
After 10 min, 3 mL of an EG solution of AgNO3 (94 mM, Aldrich,
209139-100G) and 3 mL of an EG solution of poly(vinyl pyrrolidone)
(PVP, M.W..apprxeq.55,000, Aldrich, 856568-100G, 147 mM in terms of
the repeating unit) were simultaneously added with a two-channel
syringe pump (KDS-200, Stoelting, Wood Dale, Ill.) at a rate of 45
mL per hour. The vial was then capped and heated at 140.degree. C.
Magnetic stirring was applied throughout the synthesis. Upon
injection of the AgNO.sub.3 solution, the reaction mixture went
through a series of colors that included milky white, light yellow,
transparent, red, and ocher. In order to separate the roles of the
proton and chloride, a synthesis was performed under the same
conditions except the substitution of HCl by HNO.sub.3. For the
scale-up synthesis, the vial was replaced with a 100-mL flask and
the volumes of all solutions were increased by five times.
[0056] Silver nanostructure samples that were isolated for
morphology and structure analysis were washed with acetone and then
with water to remove excess EG and PVP. SEM images were taken using
a field emission scanning electron microscope (FEI, Sirion XL)
operated at an accelerating voltage of 10-20 kV. The transmission
electron microscopy (TEM) images and diffraction patterns were
obtained using a JEOL microscope (1200EX II) operated at 80 kV.
X-ray diffraction (XRD) studies were performed on a Philips-1820
diffractometer with a scanning rate of 0.2 degrees per minute in
the range of 20 to 90 degrees. UV-visible extinction spectra were
taken at room temperature on a Hewlett-Packard 8452 spectrometer
(Palo Alto, Calif.) using a quartz cuvette with an optical path of
1 cm.
EXAMPLE 2
[0057] The properties and applications of metallic nanostructures
depend on their shapes. For silver nanostructures, shape control
enables optimization of the surface plasmon resonance (SPR)
features, and of the local electric field strength for chemical
sensing or surface-enhanced Raman scattering (SERS). The loading of
silver required in thermally and electrically conductive polymer
composites can also be greatly reduced if the silver flakes
typically employed are replaced with nanorods of high
aspect-ratios. Furthermore, silver nanowires hold great promise as
interconnects and sensors in nanoscale devices due to their
extremely high electrical conductivity and chemical
sensitivity.
[0058] Since a quasi-spherical nanoparticle has the lowest possible
surface energy and is therefore favored by thermodynamics, the
growth kinetics of a seed must be carefully controlled to obtain a
shape that does not represent an energy minimum. Factors that
influence the growth kinetics of a solution-phase synthesis
include: i) the concentration of metal precursor; ii) the rate of
reduction (the concentration and power of reductant); iii) the
presence of a soft template or capping agent; and iv) the specific
adsorption of a capping agent to a particular crystallographic
plane. Those of skill in the art have employed such kinetic
controls to generate triangular and circular nanoplates of silver
in a number of different solvent systems. In the polyol synthesis,
silver nanocubes, nanowires, and quasi-spheres have been obtained
by controlling the ratio of the capping agent, poly(vinyl
pyrrolidone) (PVP), to the silver precursor, AgNO.sub.3. Specific
adsorption of PVP to the {100} facets of the seeds governed their
growth into either nanocubes (for single-crystal seeds) or
nanowires (for multiply twinned seeds with a decahedral shape).
Higher concentrations of PVP resulted in isotropic coverage of the
seed surface and the formation of quasi-spherical particles.
[0059] It has been shown that etching of silver by O.sub.2/Cl--
selectively removed the twinned seeds involved in the polyol
synthesis of silver nanostructures. Such etching resulted in high
yields of monodispersed single-crystal seeds that grew to form
truncated nanocubes and tetrahedra. Because the concentration of
O.sub.2 in ethylene glycol is difficult to control, it would be
advantageous to have other etchants of silver that could be added
to the reaction in a controllable fashion. Fe(III) is a
well-established wet etchant for silver and other noble metals. In
a recent demonstration, addition of Fe(III) to the polyol reduction
of H.sub.2PtCl.sub.4 at 110.degree. C. reduced the level of
supersaturation by oxidizing Pt(0) atoms back into Pt(II).8 This
led to slow growth during which Pt atoms preferentially added to
{111} facets, resulting in uniform Pt nanowires as the final
product.
[0060] In yet another embodiment of the present invention, Fe(III)
may be used as an oxidative etchant to the polyol synthesis of
silver nanostructures. Contrary to the synthesis of platinum
nanowires, addition of Fe(III) to the polyol synthesis of silver
nanostructures consistently accelerated the rate of reduction.
Further, the function of Fe(III) was similar to that of Fe(II), and
both produced results that were concentration dependent. At
approximately 2.2 .mu.M, uniform nanowires with pentagonal
cross-sections were obtained as the product. At <0.44 .mu.M,
high yields of single-crystal nanocubes were produced in about 1/5
the time previously required. By simply adjusting the concentration
of Fe(II) or Fe(III), it was possible to obtain pure nanocubes or
nanowires.
[0061] FIG. 7A shows TEM and FIG. 7B shows SEM images of samples
taken from a polyol synthesis to which 0.06 mM of NaCl and 2.2
.mu.M of Fe(acac).sub.3 were added. Fe(acac).sub.3 instead of
FeCl.sub.3 may be use so that the concentration of iron can be
changed independently from the concentration of chloride. At 10 min
(FIG. 7A), the majority of the nanoparticles were twinned. These
twinned particles then grew into nanowires tens of micrometers long
by t=60 min (FIG. 7B). Previously, when the same reaction was
performed without addition of Fe(III), twinned silver nanoparticles
that formed by 10 min were etched by oxygen in a matter of hours.
Instead of accelerating the etching of silver, addition of Fe(III)
seemed to prevent the etching process entirely. To confirm that the
(acac)--anion played no role in this synthesis, Fe(acac).sub.3 was
replaced with Fe(NO.sub.3).sub.3. Again, the twinned particles
present at t=10 min (FIG. 7C) grew into nanowires by t=60 min (FIG.
7D).
[0062] The different behaviors of Fe(III) in the polyol syntheses
of platinum and silver nanostructures may be due to the difference
in reaction temperature: 110.degree. C. for platinum versus
148.degree. C. for silver. It has been shown that metals which are
difficult to reduce by ethylene glycol at low temperatures can be
reduced if the temperature is raised. Thus, the higher reaction
temperature in the silver synthesis may reduce Fe(III) to Fe(II).
As Fe(II) cannot oxidize silver, this would explain why addition of
Fe(III) does not result in faster etching of twinned seeds.
However, it does not explain why addition of Fe(III) prevented
oxygen from etching silver nanoparticles. Interestingly, nanowires
at high yields were also obtained when the same reaction was
performed under argon without adding Fe(III). Between .about.200 K
and .about.500 K, molecular oxygen (O.sub.2) is known to adsorb and
dissociate to atomic oxygen (Oa) on a silver surface, contributing
to silver's function in the catalytic oxidation of methanol to
aldehyde, ethylene to ethylene oxide, as well as its effectiveness
as a bactericide. As nanowires could be produced either in the
absence of oxygen, or in the presence of oxygen and iron ions, it
is likely that Fe(II) reacted with and removed the adsorbed atomic
oxygen that would otherwise etch twinned seeds, and block
self-catalytic addition of silver atoms. The proposed reaction
mechanism is illustrated in FIG. 6.
[0063] The same reaction with Fe(acac).sub.2 in place of
Fe(acac).sub.3 produced twinned nanoparticles at 10 min (FIG. 8A)
and nanowires by 60 min (FIG. 8B). When the concentration of
Fe(acac).sub.2 was increased by another ten times to 22 .mu.M,
nearly the same result was obtained: the mixture of twinned and
single-crystal nanoparticles present at the early stage (t=10 min)
of the reaction had an average diameter of .about.25 nm (FIG. 8C),
and a high yield of nanowires was observed at t=60 min (FIG.
8D).
[0064] When the concentration of Fe(acac).sub.2 was increased again
by ten times to 220 .mu.M, the particles present in a sample taken
at t=10 min were larger in size (FIG. 8E). FIG. 8F shows that,
rather than producing nanowires, this reaction produced a mix of
irregular particles and rods with rough surfaces that appeared to
be etched. It is possible that the relatively large amount of
Fe(III) in the reaction solution (resulting from the oxidation of
Fe(II) by Oa, see FIG. 6) etched these silver nanostructures. This
hypothesis is supported by the observation that no etching was
observed when the same reaction was performed under argon. Without
oxygen, Fe(II) could not be oxidized to Fe(III) and no etching took
place.
[0065] In yet another embodiment, the ability of Fe(II) to remove
oxygen may provide a means to control the oxidative etching of
silver. FIGS. 9A-9D show TEM images and FIG. 9E shows a SEM images
of samples taken from a synthesis to which 0.06 mM NaCl and 0.44
.mu.M Fe(acac).sub.2 were added. At t=10 min (FIG. 9A), both
twinned and single-crystal nanoparticles were present. By t=2 h
(FIG. 9B), etching by the Cl--/O.sub.2 pair had plainly reduced the
average particle size. As etching continued to t=6 h (FIG. 9C), the
average particle size continued to decrease from .about.10 nm to
.about.5 nm. In the TEM image of this sample, it appeared that
there were a large number of very small particles (.about.3 nm),
albeit the resolution limit of the microscope did not allow us to
clearly resolve them. The structure of these small particles might
fluctuate at the reaction temperature (148.degree. C.) until they
grew to larger sizes and became locked into either a single
crystalline or twinned morphology. Cl--/O.sub.2 would then
preferentially etch twinned particles due to the higher density of
defects on their surfaces. This selective etching is responsible
for the high yield of single-crystal nanoparticles obtained at
t=8.5 h (FIG. 9D). Such single-crystal nanoparticles could
subsequently grow to form truncated nanocubes (FIG. 9E), whose
average size was mainly controlled by the reaction time. The corner
and edge truncation on these nanocubes relative to those produced
by O.sub.2/Cl.sup.- etching (but without the addition of Fe(II))
was reduced, likely due to their faster growth rate, and thus a
kinetically favored shape. The faster growth of silver seeds in
this new protocol may be due to removal of adsorbed oxygen from
active surface sites. This polyol synthesis illustrates that
addition of a small amount of Fe(II) mitigated the effect of
oxidation such that selective etching of twinned particles still
occurred, but the formation and growth of single-crystal nanocubes
took place over a much shorter period of time (8.5 h versus 44
h).
[0066] Varying the concentration of iron ions, nanocubes or
nanowires can be obtained with the same synthetic procedure. Polyol
reduction maintained the iron species in the reduced, Fe(II) form,
which in turned removed oxygen from the surface of seeds. At
relatively low concentrations of Fe(II) or Fe(III), oxygen was
partially removed, and high yields of single-crystal nanocubes were
produced in about 1/5 the time previously required. Higher
concentrations of iron species prevented selective oxidation of
twinned seeds, which rapidly grew to form nanowires.
Experimental
[0067] In a polyol synthesis, 5 mL ethylene glycol (EG, J. T.
Baker, 9300-01) was first immersed and then heated in an oil bath
set at 160.degree. C. for 10 min under a light nitrogen flow. This
was done to quickly remove water. Heating was continued under air
for another 50 min. A syringe pump (KDS-200, Stoelting, Wood Dale,
Ill.) then regulated the simultaneous injection of two 3-mL EG
solutions into the hot EG at a rate of 45 mL per hour. One of the
solutions contained 94 mM silver nitrate (Aldrich, 209139-100G),
and the other contained 144 mM poly(vinyl pyrrolidone) (PVP,
Mw=55,000, Aldrich, 856568-100G, the concentration was calculated
in terms of the repeating unit), and 0.22 mM NaCl (Fisher,
S271-500). A small amount of NaCl was added to each synthesis to
prevent aggregation of seeds. Varying concentrations of iron
compounds were also added to the PVP solution to analyze their
effect on the synthesis. The ethylene glycol used for every
experiment contained trace amounts of chloride (3 .mu.M) and iron
(0.4 .mu.M). Magnetic stirring was applied throughout the entire
synthesis. A set of samples were taken in the course of each
synthesis using a glass pipette. To minimize temperature
perturbations during sampling, the glass pipette was held just
above the solution and preheated for 30 sec before immersion. The
samples were washed with acetone and then with water to remove most
of the EG and PVP. During the washing process, the suspension was
centrifuged at 16,000 rpm for 10 min or 30 min (depending on
whether acetone or water was used) to make sure that most of the
silver particles taken from the reaction were recovered. Finally,
the sample was dispersed in water for further characterization.
[0068] For the scanning electron microscopy (SEM) and transmission
electron microscopy (TEM) studies, a drop of the aqueous suspension
of particles was placed on a piece of silicon wafer or
carbon-coated copper grid (Ted Pella, Redding, Calif.),
respectively, and dried in the fume hood. After that, the sample
was transferred into a gravity-fed flow cell, and washed for 1 h
with deionized water to remove the remaining PVP. Finally, the
sample was dried and stored in a vacuum. SEM images were taken
using a FEI field-emission microscope (Sirion XL) operated at an
accelerating voltage of 20 kV. TEM and electron diffraction studies
were performed with a Phillips 420 microscope operated at 120
kV.
EXAMPLE 3
[0069] Plasmon excitation within a silver nanostructure not only
gives it color in far-field imaging, but also greatly magnifies the
electric field near its surface. This intense near-field has
recently enabled nanoscale waveguiding and localization of light
for nanolithography and optical devices, and has long been used to
enhance the Raman scattering from adsorbed molecules. Theoretical
calculations predict a 50-nm silver sphere provides a maximum
electric field enhancement on the order of 102, but this can be
improved 100-fold by a nanostructure with sharp corners, such as a
tetrahedron. Indeed, the theoretical field enhancement provided by
the sharp corners of a tetrahedron is on the same order of
magnitude as the "hot spots" between nanoparticles thought to allow
single-molecule surface enhanced Raman scattering (SERS)
detection.
[0070] Another embodiment of the invention includes the synthesis
of silver right bipyramids--nanoparticles with sharp corners
similar to those of a tetrahedron. This remarkable shape, which is
essentially two right tetrahedra symmetrically placed base-to-base,
was proposed over 20 years ago by Harris in studying sulfur-induced
faceting of platinum nanoparticles in the gas phase, but there have
been no further reports, and it has never before been selectively
produced in high yield.
[0071] To synthesize right bipyramids, 3 ml of two EG solutions,
one containing 94 mM AgNO.sub.3, the other containing 144 mM
poly(vinyl pyrrolidone) (PVP) and 0.11 mM NaBr, were added dropwise
via a two channel syringe pump into 5 ml of EG heated in an oil
bath at 160.degree. C. A 30 .mu.L drop of 10 mM NaBr was also added
to the preheated EG before the Ag precursor, as this was found to
prevent formation of small spherical nanoparticles that otherwise
contaminated the final product, as shown in FIG. 13. The reaction
solution turned yellow 30 seconds after addition of AgNO.sub.3 and
PVP, indicating the formation of silver nanoparticles. The yellow
color faded in intensity after 10 minutes due to oxidative etching,
15 and maintained a light yellow color for approximately two hours
before turning light brown and gray as the nanoparticles increased
in size.
[0072] FIG. 10A shows a SEM image of the right bipyramids obtained
after 5 hours. Their average edge length was 150 nm, and their
corners had an average radius of curvature R=11 nm. If the reaction
was stopped after 3 hours, right bipyramids 75 nm in edge length
were obtained as shown by FIG. 10B. Although the corners of the 75
nm right bipyramids appear less sharp, they were actually more so,
with R=8 nm. The inset of FIG. 10B shows an electron diffraction
pattern obtained from a single right bipyramid, with the beam
perpendicular to the substrate. The spot array is characteristic of
diffraction from the (100) zone axis, and indicates the bipyramid
lies with a (100) facet flush with the substrate. Because every
facet on the bipyramid is identical, it can be concluded that the
bipyramid is bounded by (100) facets, similar to a single-crystal
nanocube. The X-ray diffraction (XRD) pattern of FIG. 14 obtained
from the right bipyramids gives further evidence that they
preferentially orient with the (100) plane parallel to the
substrate. The (111), (200), and (220) diffraction peaks in FIG. 14
are characteristic of face-centered-cubic silver, but the ratio of
the (200) to (111) peak is three times that of a powder sample (1.2
versus 0.4). The relatively small contribution to diffraction from
the (111) plane may come from the randomly oriented right
bipyramids in multilayers as may be seen in FIG. 10A.
[0073] The unique shape of the right bipyramid gives it an
ultra-violet-visible-near-infrared (UV-Vis-NIR) extinction spectrum
distinct from that of any previously reported silver nanostructure.
To determine if the experimental spectrum of the bipyramids agrees
with the theoretically predicted one, Maxwell's equations were
solved with the discrete dipole approximation (DDA) for bipyramids
150 nm and 75 nm in edge length, with each composed of 2600
dipoles. The spectrum calculated with a perfect bipyramid model has
more peaks than the experimental spectrum, and the maximum peak is
further red-shifted as seen in FIGS. 15A-15B. Better agreement
between the experimental and calculated spectra was achieved when
the corners of the 150- and 75-nm bipyramids were truncated with
(111) facets 14 and 12 nm in edge length, respectively. As shown in
FIGS. 10C and 10D, this truncation reduced the number of peaks and
caused the most intense peaks to blue-shift.
[0074] In spite of this truncation, the extinction peak of a 75 nm
right bipyramid is red-shifted 50 nm from that of nanocubes of a
similar size as shown by FIG. 16. This suggests that the three
corners at the junction between the two tetrahedrons of the
bipyramid are significantly sharper than the corners of the cube,
and thus should generate greater localized field enhancement. In
addition, because one of these sharp corners will always point up
and away from the substrate no matter how the bipyramid lie, it
will be free from possible dampening of its localized field through
electromagnetic coupling to induced charges in the substrate. The
top corners of a silver nanocube (those not in contact with a
substrate) have already been shown to have a figure of merit
.about.1.8 times that of triangular nanoprisms for single
nanoparticle sensing applications; the bipyramid may further
improve upon these results.
[0075] To visualize its shape and orientation more clearly, SEM
images were taken of a single right bipyramid from above and with a
45.degree. tilt at four different rotations. FIG. 11 compares this
set of images with models (drawn with a CAD program) of a right
bipyramid at identical orientations. It is very difficult to
imagine the nanostructure as a right bipyramid from the top view
alone, but from 45.degree. it becomes clear that one half of the
bipyramid (i.e., one of the two right tetrahedra) points away from
and is not in contact with the substrate. The other right
tetrahedron that makes up the bipyramid points toward and has one
of its sides in contact with the substrate. The reason the bottom
tetrahedron can not be seen from above is because the angles
between the faces of the bipyramid are all 90.degree.. Because one
facet of the bottom tetrahedron is parallel to the substrate, the
other two facets are perpendicular to the substrate and cannot be
viewed from above.
[0076] The mirror symmetry of the bipyramid, as well as the fact
that it is covered by (100) facets, suggests that a (111) twin
plane bisects its two tetrahedra halves. In a transmission electron
microscopy (TEM) image of a 40-nm bipyramid, as shown by FIG. 12A,
taken from the reaction at 2.5 hours, there is an internal
difference in contrast between its two halves, suggesting that it
is not a single crystal. The bisecting twin plane is apparent after
tilting the same bipyramid by 45.degree. as seen in FIG. 12B, which
aligns electron beam along the <110> axis, and parallel with
the twin plane. The presence of the twin plane is significant
because it differentiates the bipyramid from the nanocube, which is
a single crystal. The difference between the shapes of bipyramid
and cube is a direct result of the presence of this twin plane,
because it restricts the way in which the nanoparticle can
grow.
[0077] Just as nanocubes grow from smaller single-crystal seeds, it
is likely the bipyramid grew from a seed with a single twin plane.
Indeed, nanoparticle seeds with a single twin could readily be
found in TEM images of a sample taken from the reaction at t=1.5
hours, although the fact that their twin plane is only visible at
certain orientations makes it difficult to determine their yield.
Another example of the twinned seed is shown in FIG. 12C. The
reflection of the lattice fringes across the twin plane is evident
in a high resolution TEM image of the twinned seed shown in FIG.
12D. The fringe spacing indicates the seed is aligned along the
<110>, confirming that the twin is on the (111) plane. To
visualize the shape of the seed, a CAD model of the bipyramid was
truncated until its outline matched that of the seed as represented
by FIG. 17. FIG. 12E shows an image of the approximate seed
structure rendered in 3-dimensions. The differences between the
seed and the bipyramid are the (111) truncation of its corners, the
presence of reentrant (111) surfaces along the twin boundary, and
the presence of small reentrant (100) facets at the twin boundary
corners. All these changes lead to a reduction of surface area that
gives the seed a more sphere-like shape. By comparing the images of
the seed with those of larger bipyramids, it may be seen how the
shape evolved as the nanoparticle grew. The selective growth of
(100) surfaces on the twinned seed to make a right bipyramid is
consistent with a likely nanocube and nanowire syntheses, in which
it is hypothesized that selective capping of the (100) facet by PVP
led to growth of nanostructures bounded by (100) facets.
Experimental
[0078] In one embodiment of the synthesis of right bipyramids, 5 mL
ethylene glycol (EG, J. T. Baker, 9300-01) was first immersed and
then heated in an oil bath set at 160.degree. C. for 1 hour. A 30
.mu.L drop of 10 mM NaBr was then added to the preheated EG before
a syringe pump (KDS-200, Stoelting, Wood Dale, Ill.) regulated the
simultaneous injection of two 3-mL EG solutions into the hot EG at
a rate of 45 mL per hour. One of the solutions contained 94 mM
silver nitrate (Aldrich, 209139-100G), and the other contained 144
mM poly(vinyl pyrrolidone) (PVP, Mw.apprxeq.55,000, Aldrich,
856568-100G, the concentration was calculated in terms of the
repeating unit), and 0.11 mM NaBr (Fisher, S271-500). The ethylene
glycol used for every experiment contained trace amounts of
chloride (3 .mu.M) and iron (0.4 .mu.M). Magnetic stirring was
applied throughout the entire synthesis. A set of samples were
taken over the course of the synthesis using a glass pipette. To
minimize temperature perturbations during sampling, the glass
pipette was held just above the solution and preheated for 30 sec
before immersion. The samples were washed with acetone and then
with water to remove most of the EG and PVP. During the washing
process, the suspension was centrifuged at 16,000 rpm for 10 min or
30 min (depending on whether acetone or water was used) to make
sure that most of the silver nanoparticles taken from the reaction
were recovered. Finally, the sample was dispersed in water for
further characterization.
[0079] For the SEM and TEM studies, a drop of the aqueous
suspension of nanostructures was placed on a piece of silicon wafer
or carbon-coated copper grid (Ted Pella, Redding, Calif.),
respectively, and dried in the fume hood. After that, the sample
was transferred into a gravity-fed flow cell, and washed for 1 h
with deionized water to remove the remaining PVP. Finally, the
sample was dried and stored in a vacuum. SEM images were taken
using an FEI field-emission microscope (Sirion XL) operated at an
accelerating voltage of 20 kV. TEM and electron diffraction studies
were performed with a Phillips 420 microscope operated at 120 kV.
HRTEM images were taken using a JEOL 2010 LaB6 high-resolution
transmission electron microscope operated at 200 kV. UV-Vis-NIR
extinction spectra of aqueous solutions of bipyramids were taken at
room temperature with a Cary 5E (Varian) spectrophotometer using a
quartz cuvette with an optical path of 1 cm. XRD patterns were
recorded on a Philips 1820 diffractometer equipped with a
Cu--K.alpha. radiation source (.lamda.=1.54180 .ANG.).
EXAMPLE 4
[0080] In yet another example, the polyol synthesis may be used as
a simple, robust and versatile method for producing silver
nanocubes as monodispersed samples. In a typical polyol synthesis,
silver atoms are formed by reducing AgNO.sub.3 precursor with
ethylene glycol through the following mechanism:
HOCH.sub.2CH.sub.2OH.fwdarw.2CH.sub.3CHO+2H.sub.2O (1)
Ag++2CH.sub.3CHO.fwdarw.CH.sub.3CO--OCCH.sub.3+2Ag+2H+ (2)
[0081] Once the concentration of silver atoms has reached the
supersaturation level, they will begin to nucleate and grow into
silver nanostructures in the solution phase. Despite the
demonstration of various methods for controlling polyol reduction,
it is still a grand challenge to produce silver nanocubes on very
large scales because of the length of time required for the
formation of nanocubes from single-crystal seeds as well as the
variation of reaction time between different batches. In general, a
typical polyol synthesis may take anywhere from 16 to 26 hours to
form silver nanocubes.
[0082] In on particular embodiment, the production rate of silver
nanocubes may be improved by adding a trace amount of sodium
sulfide (Na.sub.2S) or sodium hydrosulfide (NaHS). Sulfide species
are known to interact quite strongly with silver, such as the
creation of Ag.sub.2S when silver exists at concentrations above
the .mu.M level with trace sulfides in aqueous systems.
Furthermore, Ag.sub.2S nanoparticles have been shown to catalyze
the reduction of Ag+ in a mechanism analogous to the autocatalytic
reduction of silver clusters by drastically reducing the reduction
potential compared to that of free Ag+. At this enhanced rate, the
evolution of silver nanocubes is dominated by the fast kinetic
growth of single-crystal seeds. As a result, it is possible to
effectively limit the formation of twinned seeds and minimize the
size distribution of resultant single-crystal cubes by creating a
more simultaneous nucleation event, allowing all silver nanocubes
to grow to the same size.
[0083] The presence of sulfide anions accelerates the polyol
synthesis of silver nanocubes due to a dramatic increase in the
reduction rate of silver ions. It is known that sulfide anions
exist in three states in an aqueous medium depending on the pH of
the solution as shown in FIG. 23. In one example, the reaction
medium was EG and the pH was close to 7 so both H.sub.2S and HS-
species should be present independent of whether Na.sub.2S or NaHS
was used. The purple-black color developed upon injection of silver
nitrate is indicative of Ag.sub.2S nanoparticles, a catalyst for
the reduction of Ag+ ions. During the synthesis, the progress of
the nanoparticle production may be monitored through its color
changes and shape using scanning electron microscopy (SEM). FIGS.
18A-18C show SEM images of samples taken at different stages of a
typical Na.sub.2S-mediated synthesis. The solution started
darkening from a bright yellow to a deep orange after 3 minutes
into the reaction due to the growth of small silver particles (not
shown). FIG. 18A indicates that these particles were primarily
cubic in profile with an average edge length of .about.25 nm.
Between 3 and 7 minutes, the cubes grow to 45 nm as shown in FIG.
18B, displaying a ruddy-brown color and opalescence. Once the cubes
had reached 45 nm, most of the silver in the solution had been
consumed and etching overtook growth as the dominant force in this
system, producing nanocubes with more rounded corners as shown in
FIG. 18C. At the same time, the reaction solution turned into
paler, whitish brown.
[0084] In order to better control the polyol synthesis for
mass-production of monodispersed silver nanocubes, both the
concentration of sulfide species and reaction temperature were
adjusted. FIGS. 19A-19P show a series of SEM images of silver
nanoparticles synthesized with different concentrations of
Na.sub.2S at various reaction temperatures demonstrating the
delicate nature of reaction conditions. In one example, 28-30 .mu.M
was a possible concentration of sulfide ions for the fast reduction
of Ag+ in the range of 150-155.degree. C. with the optimal etching
conditions (temperature dependent) to produce high quality,
monodispersed silver nanocubes. The concentration of sulfide
species available in the reaction is very important to the
synthesis of high quality small nanocubes. In this example, if the
concentration of sulfide species in the solution was under 28
.mu.M, the reduction was not fast enough to produce only the
single-crystal nanocubes as there would be some round and twinned
particles as seen in the images of FIGS. 19A-19D. When the
concentration is higher than 30 .mu.M, the rate of reduction may
have increased beyond that of etching, favoring the production of
multiply-twinned particles, wires, and agglomerated particles along
with some nanocubes (FIGS. 19M-19P).
[0085] In yet another embodiment, the sulfide-assisted synthesis
was further optimized by adjusting the molar ratio between the
repeating unit of PVP and silver. The series of reactions in FIGS.
20A-20F show a range of molar ratios from 0.75:1 to 2.7:1 for the
repeating unit of PVP to silver nitrate. PVP has been shown to
preferentially adsorb onto the {100} surface of silver particles
both stabilizing and protecting the small single-crystal seeds.
When there was not enough PVP to lower the {100} surface energy of
the small seeds sufficiently, the higher surface energy allowed for
the production of twin defects, which can bee seen as
single-twinned bipyramids depicted in FIG. 20A. Conversely, if
there was too much PVP in the system, it would indiscriminately
protect all facets of the initial seeds including the
thermodynamically favored multiply-twinned, quasi-spherical seeds.
These twinned species would end up as dominant products as the
ratio of PVP to silver atoms became larger as seen in FIG. 20F.
Therefore the ratio of PVP in the system may be balanced in order
to lower the surface energy of {100} facets without overwhelming
the seeds with a blanket layer that would prevent etching.
[0086] In addition to scanning electron microscopy (FIG. 21A), the
silver nanocubes were analyzed with transmission electron
microscopy (TEM), electron diffraction, x-ray diffraction (XRD),
and UV-Vis spectroscopy. FIG. 21B shows a TEM image, confirming
that the particles are single-crystal cubes and the electron
diffraction pattern in the inset indicates that the surface is
bounded by {100} planes as expected for a cube on a flat substrate.
In addition, the XRD pattern shown in FIG. 21C also suggests highly
crystalline silver. Note that the (200) peak is significantly
stronger relative to the (111) that dominates the JCPDS pattern
mainly because the cubes were bound by {100} facets and the powder
standard were overwhelmed by the lower energy {111} facets. This is
expected and indicative that most nanocubes were aligned flat on
the substrate with their {100} planes being oriented upward. The
UV-Vis spectra of FIG. 21D shows cubes produced via the
sulfide-assisted synthesis and there was a blue-shift as the edge
length of the cubes decreased and became closer to spherical.
[0087] In yet another embodiment, silver nanocubes may be used as
sacrificial template to generate gold nanocages. Although some
sulfur ions likely to adsorb to the surface of as-synthesized cubes
due to strong binding between sulfur and silver, this sulfur did
not interfere with the galvanic replacement reaction between Ag and
HAuCl4: Ag+HAuCl.sub.4.fwdarw.Au+HCl+3AgCl (3)
[0088] FIG. 22A shows typical SEM and TEM (inset) images of gold
nanocages produced by reacting the as-prepared Ag nanocubes with
HAuCl.sub.4 in an aqueous medium. It may be possible to precisely
tune the surface plasmon resonance (SPR) peaks of the hollow
nanostructures to any position in the visible and near-infrared
regions by controlling the volume of HAuCl.sub.4 added to the
reaction system. For example, FIG. 22B shows the spectra taken from
a set of samples, where the SPR peak had been tuned to 900 nm.
Experimental
[0089] 1. Synthesis of Silver Nanocubes
[0090] In a polyol synthesis, 6 mL ethylene glycol (EG, J. T.
Baker, 9300-03) was heated under stirring with a Teflon-coated
magnetic stirring bar for 1 hour in a 24-mL glass vial. While the
EG was heated, EG solutions containing AgNO.sub.3 (48 mg/mL,
Aldrich, 209139) and poly(vinyl pyrrolidone) (PVP, 20 mg/mL, MW
.about.55,000, Aldrich, 856568) were prepared. A 3 mM solution of
Na.sub.2S (Aldrich, 208043) or NaHS (Aldrich, 161527) in EG was
also prepared 45 minutes prior to injection. Shortly after
injecting 80 .mu.L of the sulfide solution, 1.5 mL and 0.5 mL of
the PVP and AgNO.sub.3 solutions were sequentially injected (all
with a micro-pipettor). As silver nitrate was added, the clear and
colorless solution immediately turned purple-black, followed
instantly by a transparent bright yellow color. The appearance of
yellow color indicates the formation of small silver particles.
After 2-3 minutes into the reaction, the solution darkened to an
orange-yellow color and some silver nanoparticles were observed to
deposit on the wall of the vial. After 6-8 minutes, the solution
changed to an opalescent ruddy-brown and concurrently became
opaque. If allowed to continue, the solution faded to a lighter,
whitish-brown color but remained opaque. The final produce was
diluted with acetone and collected by centrifugation, washed with
water, and then suspended in water (4 mL) for future use.
[0091] 2. Synthesis of Gold Nanocages
[0092] In a typical synthesis, a fixed amount (100 .mu.L) of the
as-synthesized silver nanocubes was dispersed in 5 mL water
containing 1 mg/mL PVP in a 50 mL flask under magnetic stirring and
then heated to boil for 10 minutes. A specific amount (3 mL) of 0.2
mM HAuCl.sub.4 aqueous solution was added to the flask through a
syringe pump at a rate of 45 mL/h under magnetic stirring. The
solution was heated for another 10 minutes until the color of the
system was stable. Once cooled down to room temperature, the sample
was centrifuged and washed with saturated NaCl solution to remove
AgCl and then with water several times to remove PVP and NaCl
before characterization by SEM and TEM.
[0093] It will be obvious to those having skill in the art that
many changes may be made to the details of the above-described
embodiments without departing from the underlying principles of the
invention. The scope of the present invention should, therefore, be
determined only by the following claims.
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