U.S. patent application number 12/303668 was filed with the patent office on 2010-10-14 for nanostructured metal oxides comprising internal voids and methods of use thereof.
This patent application is currently assigned to CORNELL RESEARCH FOUNDATION, INC.. Invention is credited to Lynden A. Archer, Xiong Wen Lou.
Application Number | 20100258759 12/303668 |
Document ID | / |
Family ID | 39314702 |
Filed Date | 2010-10-14 |
United States Patent
Application |
20100258759 |
Kind Code |
A1 |
Archer; Lynden A. ; et
al. |
October 14, 2010 |
Nanostructured Metal Oxides Comprising Internal Voids and Methods
of Use Thereof
Abstract
The present invention relates to nano structures of metal oxides
having a nanostructured shell (or wall), and an internal space or
void. Nanostructures may be nanoparticles, nanorod/belts/arrays,
nanotubes, nanodisks, nanoboxes, hollow nanospheres, and mesoporous
structures, among other nanostructures. The nanostructures are
composed of polycrystalline metal oxides such as SnO2. The
nanostructures may have concentric walls which surround the
internal space of cavity. There may be two or more concentric
shells or walls. The internal space may contain a core such ferric
oxides or other materials which have functional properties. The
invention also provides for a novel, inexpensive, high-yield method
for mass production of hollow metal oxide nanostructures. The
method may be template free or contain a template such as silica.
The nanostructures prepared by the methods of the invention provide
for improved cycling performance when tested using rechargeable
lithium-ion batteries.
Inventors: |
Archer; Lynden A.; (Ithaca,
NY) ; Lou; Xiong Wen; (Ithaca, NY) |
Correspondence
Address: |
AXINN, VELTROP & HARKRIDER LLP;Attn. Michael A. Davitz
114 West 47th Street
New York
NY
10036
US
|
Assignee: |
CORNELL RESEARCH FOUNDATION,
INC.
Ithaca
NY
|
Family ID: |
39314702 |
Appl. No.: |
12/303668 |
Filed: |
June 6, 2007 |
PCT Filed: |
June 6, 2007 |
PCT NO: |
PCT/US2007/070553 |
371 Date: |
December 23, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60804031 |
Jun 6, 2006 |
|
|
|
Current U.S.
Class: |
252/62.56 ;
252/518.1; 252/520.3; 252/62.51R; 423/592.1; 423/618; 427/58;
428/34.1 |
Current CPC
Class: |
H01M 4/48 20130101; Y10T
428/2991 20150115; H01M 2004/021 20130101; H01M 4/485 20130101;
H01M 10/0525 20130101; Y02E 60/10 20130101; Y10T 428/2993 20150115;
C01P 2004/64 20130101; H01G 9/2027 20130101; H01M 2004/027
20130101; C01P 2006/40 20130101; C01G 19/02 20130101; C01B 33/12
20130101; Y10T 428/13 20150115; C01G 1/02 20130101; Y10T 428/2984
20150115; B82Y 30/00 20130101; H01M 4/366 20130101 |
Class at
Publication: |
252/62.56 ;
252/62.51R; 252/518.1; 252/520.3; 423/592.1; 423/618; 427/58;
428/34.1 |
International
Class: |
C01G 49/08 20060101
C01G049/08; H01B 1/08 20060101 H01B001/08; C01B 13/14 20060101
C01B013/14; C01G 19/02 20060101 C01G019/02; B05D 5/12 20060101
B05D005/12; B29D 22/04 20060101 B29D022/04 |
Goverment Interests
STATEMENT OF GOVERNMENT RIGHTS
[0002] This invention was made with Government support under
contract number DMR 0404278, awarded by the National Science
Foundation. The government has certain rights in this invention.
Claims
1. A nanostructure comprising at least one metal oxide
nanostructured shell defining an inner void.
2. The nanostructure of claim 1 comprising at least two metal oxide
nanostructured concentric shells, wherein the shells are not
covalently linked to each other.
3. The nanostructure of claim 2, comprising two metal oxide
nanostructured shell concentric shells, wherein the shells are not
covalently linked to each other.
4. The nanostructure of claim 1 further comprising a solid core
within the inner void.
5. The nanostructure of claim 4 wherein, the solid core comprises
materials selected from the group consisting of Au, Ag, Pt and
Pd.
6. The nanostructure of claim 5, wherein the solid core comprises
materials with magnetic properties.
7. The nanostructures of claim 6, wherein the materials with
magnetic properties comprise Fe, Co or Ni.
8. The nanostructures of claim 7, wherein the materials with
magnetic properties comprise ferric oxides.
9. The nanostructures of claim 6, wherein the materials with
magnetic properties are selected from the group consisting of ZnO,
CuO, and Cu.sub.2O.
10. The nanostructures of claim 6, wherein the materials with
magnetic properties comprise metal sulfide semiconductors.
11. The nanostructures of claim 1, wherein at least one metal oxide
nanostructured shell has one or more pores.
12. The nanostructures of claim 8, wherein the pore is an
opening.
13. The nanostructure of claim 1, wherein the nanostructure is
spherical, cylindrical or polyhedral.
14. The nanostructure of claim 1, wherein the nanostructure is
bowl-shaped.
15. The nanostructure of claim 1, wherein the nanostructure is
selected from the group consisting of nanoparticles, nanorods,
nanobelts, nano-arrays, nanotubes, nanodisks, nanoboxes,
nanospheres, nanocylinders, nanococoons, and nanospindles.
16. The nanostructure of claim 1, wherein the metal oxide comprises
a metal selected from the group consisting of titanium, zirconium,
aluminum, tin, germanium, indium, gallium, hafnium, vanadium,
tantalum, zinc, copper, iron, cobalt, nickel, chromium, and
manganese.
17. The nanostructure of claim 16, wherein the metal oxide is tin
oxide.
18. The nanostructure of claim 1, wherein the nanostructure is
polycrystalline.
19. The polycrystalline nanostructure of claim 18, wherein the
average size of each crystal is between 1.0 and 50.0 nm.
20. The polycrystalline nanostructure of claim 19, wherein the
average size of each crystal is between 10.0 and 20.0 nm.
21. The nanostructure of claim 1, wherein the maximum average
cross-sectional distance of the nanostructure is between 50.0 and
1,000.0 nm.
22. The nanostructure of claim 21, wherein the maximum average
cross-sectional distance of the nanostructure is between 100.0 and
500.0 nm.
23. The nanostructure of claim 22, wherein the maximum average
cross-sectional distance of the nanostructure is between 75.0 and
150.0 nm.
24. The nanostructure of claim 1, wherein the thickness of the
nanostructured shell defining the inner space is between 5.0 and
50.0 nm.
25. The nanostructure of claim 24, wherein the thickness of the
nanostructured shell defining the inner space is between 10.0 and
40.0 nm.
26. The nanostructure of claim 25, wherein the thickness of the
nanostructured shell defining the inner space is between 20.0 and
30.0 nm.
27. The nanostructure of claim 4, wherein the average diameter of
the solid core within the inner space is between 5.0 and 500.0
nm.
28. The nanostructure of claim 27, wherein the average diameter of
the solid core within the inner space is between 30.0 and 300.0
nm.
29. The nanostructure of claim 28, wherein the average diameter of
the solid core within the inner space is between 50.0 and 150.0
nm.
30. The nanostructure of claim 11, wherein the average diameter of
the pores in one or more of the nanostructured shells is between
1.0 and 100.0 nm.
31. The nanostructure of claim 30, wherein the average diameter of
the pores in one or more of the nanostructured shells is between
3.0 and 30.0 nm.
32. The nanostructure of claim 1, wherein the nanostructure is
rutile tetragonal SnO.sub.2.
33. The nanostructure of claim 29, wherein the corresponding
ring-like selected-area electron diffraction pattern of the
nanostructure reveals diffraction rings from inside to outside
which are indexed to (110, (101), (200), (211), and (112) planes of
rutile SnO.sub.2 respectively.
34. A process to produce the nanostructures of claim 1, comprising:
mixing a metal oxide precursor with a solvent, and optionally one
or more yield enhancing additives to form a mixture; and heating
said mixture for at least about 3 hours.
35. The process of claim 34, wherein the nanostructure is selected
from the group consisting of nanoparticles, nanorods, nanobelts,
nano-arrays, nanotubes, nanodisks, nanoboxes, nanospheres,
nanocylinders, nanococoons, and nano spindles.
36. The process of claim 34, wherein the metal oxide precursor is a
precursor of metal oxides selected from the group consisting of
titanium, zirconium, aluminum, tin, germanium, indium, gallium,
hafnium, vanadium, tantalum, zinc, copper, iron, cobalt, nickel,
chromium, and manganese.
37. The process of claim 36, wherein the metal oxide precursor is a
precursor of tin oxide.
38. The process of claim 37, wherein the precursor is an alkali
metal precursor salt.
39. The process of claim 38, wherein the alkali metal is potassium
or sodium.
40. The process of claim 39, wherein the alkali metal is
potassium.
41. The process of claim 34, wherein the solvent is a polar
solvent.
42. The process of claim 41, wherein the polar solvent is an
alcohol and water mixture.
43. The process of claim 42, wherein the alcohol is any C.sub.2 to
C.sub.10 alcohol.
44. The process of claim 34, wherein the solvent is a non-polar
solvent seeded with a polar entity.
45. The process of claim 44, wherein the polar entities are chosen
from the group consisting of surfactants, polymers, charged
oligomers and particulate materials.
46. The process of claim 34, wherein the yield enhancing additives
are selected from the group consisting of urea compounds and --NH2
compounds.
47. The process of claim 46, wherein the urea compounds are chosen
from the group consisting of biuret, thiourea, thiobiuret, alkyl
substituted ureas, aryl substituted ureas, alkyl substituted
thioureas, aryl substituted thioureas, alkylene ether ureas,
arylene ether ureas, alkylene ether thioureas and arylene ether
thioureas.
48. The process of claim 34, wherein the mixture is heated at a
temperature of between about 140.degree. C. and about 200.degree.
C. for between about 3 to about 24 hours.
49. The process of claim 48, further comprising the step of cooling
the nanostructure formed after heating.
50. The process of claim 49, further comprising the step of washing
the cooled nanostructure.
51. The process of claim 34, wherein the nanostructure is formed by
inside-out Ostwald ripening.
52. The process of claim 34, wherein the nanostructure formed has
enhanced electrochemical properties when used as an anode material
in lithium ion batteries.
53. The process of claim 34, wherein the nanostructure formed has
initial discharge capacities between about 1000.0 mAh/g and 1,300,0
mAh/g.
54. The process of claim 34, wherein the nanostructure formed can
undergo at least about 25 discharge cycles while still functioning
effectively at a discharge capacity of at least 300.0 mAh/g.
55. The process of claim 34, wherein the nanostructure formed can
undergo between 25 and 50 cycles while still functioning
effectively at a discharge capacity of at least 300.0 mAh/g.
56. The nanostructure of claim 1, further comprising a magnetic
core.
57. The nanostructure of claim 56, wherein the magnetic core
comprises a ferric-based material.
58. The nanostructure of claim 57, wherein the ferric based
material is a ferric oxide.
59. The nanostructure of claim 58, wherein the ferric oxide is
Fe.sub.3O.sub.4.
60. The nanostructure of claim 1, further comprising a core having
electrical properties.
61. The nanostructure of claim 1, further comprising a core having
semiconductor properties.
62. The nanostructure of claim 1, further comprising a core bearing
noble metals.
63. The nanostructure of claim 62, wherein the noble metals are
chosen from the group consisting of Au, Ag, Pt and Pd.
64. A process to manufacture the nanostructure of claim 60,
comprising: (a) coating a material capable of being magnetized with
a silica-based material to form a particle in which said material
is coated with the silica-based material; (b) coating the particle
of step (a) with metal oxide such that the nanotemplate is
surrounded by at least one metal oxide nanostructured shell; (c)
heating the nanotemplate of step (b) at a temperature between 400
to 1000.degree. C. to cause annealing; (d) suspending said annealed
structure of step (c) in a solvent suitable to dissolve the
silica-based material.
65. The process of claim 64, wherein the silica based material is
amorphous silicon dioxide.
66. The process of claim 64, wherein the material capable of being
magnetized is .alpha.-Fe.sub.2O.sub.3.
67. The process of claim 66, wherein the process further comprises
a step in which the .alpha.-Fe.sub.2O.sub.3 is magnetized by
H.sub.2 reduction to Fe.sub.3O.sub.4.
68. The nanostructure of claim 60, further comprising an outermost
shell comprising a semiconductor material.
69. The nanostructure of claim 68, wherein the semiconductor
material comprises carbon black.
70. The nanostructure of claim 68, wherein the semiconductor
material comprises a propylenevinylidenefluoride compound.
71. The nanostructure of claim 11, wherein the pores are formed by
treatment with a poregen material followed by heating at a
temperature between 100.degree. C. to 1000.degree. C.
72. The nanostructure of claim 11, wherein the pores are formed by
treatment with a poregen material followed by lyophilization at a
temperature less than 0.degree. C.
73. A process to produce the nanostructure of claim 1, comprising:
(a) hydrothermally depositing a metal oxide precursor on a template
to form a nanostructure comprising a metal oxide nanostructured
shell on said template; and (b) treating the nanostructure of step
(a) with a solvent to dissolve the template material to create an
internal void in said nanostructure.
74. A process to produce the nanostructure of claim 2, comprising:
(a) hydrothermally depositing a metal oxide precursor at least two
times on a template to form a nanostructure comprising a plurality
of metal oxide nanostructured shells on said template; and (b)
treating the nanostructure of step (a) with a solvent to dissolve
the template material to create an internal void in said
nanostructure.
75. The process of claim 73, wherein the metal oxide is
SnO.sub.2.
76. The process of claim 73, wherein the template is a silica-based
compound.
Description
PRIORITY CLAIM
[0001] The present application claims priority to U.S. Provisional
Pat. App. 60/804,031 filed on Jun. 6, 2006, the contents of which
are hereby incorporated by reference in their entirety.
FIELD OF INVENTION
[0003] The present invention relates to nanostructures and more
particularly to nanostructures comprising nanostructured metal
oxide shells enclosing internal voids, as well as methods for
making and using the same.
BACKGROUND
[0004] Recently, hollow inorganic micro- and nanostructures have
attracted considerable attention because of their promising
applications as nanoscale chemical reactors, catalysts, drug
delivery carriers, semiconductors, and photonic building blocks. X.
W. Lou, C. Yuan, Q. Zhang, L. A. Archer, Angew. Chem. Int. Ed.
2006, 45, 3825. In particular, nanostructures comprising metal
oxides may be used as semiconductors in applications such as gas
sensors and lithium rechargeable batteries. Y. Wang, X. Jiang, Y.
Xia, J. Am. Chem. Soc. 2003, 125, 16176; M. Law, H. Kind, B.
Messer, F. Kim, P. Yang, Angew. Chem. Int. Ed. 2002, 41, 2405; Y.
Idota, T. Kubota, A. Matsufuji, Y. Maekawa, T. Miyasaka, Science,
1997, 276, 1395. However, although metal oxides (for example, tin
oxide (SnO.sub.2)) generally have a much higher theoretical
specific lithium storage capacity area than more traditionally used
materials such as graphite, the large volume changes in metal oxide
nanostructures during charging/discharging processes result in poor
cyclability, thus limiting their use in such applications. S. Han,
B. Jang, T. Kim, S. M. Oh, T. Hyeon, Adv. Funct. Mater. 2005, 15,
1845; K. T. Lee, Y. S. Jung, S. M. Oh, J. Am. Chem. Soc. 2003, 125,
5652; Y. Wang, H. C. Zeng, J. Y. Lee, Adv. Mater. 2006, 18, 645.
One possible strategy to mitigate this problem and further enhance
structural stability is to use hollow nanostructures. Such hollow
nanostructures have higher surface to volume ratios which allow for
greater charge capacities than solid nanostructures. Hollow
nanostructures may be designed in various shapes and sizes, and
commonly include nanoparticles, nanorods/belts/arrays, nanotubes,
nanocylinders, nanococoons, nanodisks, nanoboxes, nanospheres,
among others.
[0005] At present, there exist numerous methods for the preparation
of hollow metal oxide nanostructures. One approach involves the use
of various removable or sacrificial templates, including, for
example, monodispersed silica, carbon or polymer latex spheres,
reducing metal nanoparticles, and even emulsion droplets/micelles
and gas bubbles. Q. Peng, Y. Dong, Y. Li, Angew. Chem. Int. Ed.
2003, 42, 3027-3030. Although conceptually simple and versatile,
such methods are often burdened with the challenge of uniformly
depositing metal oxides (or their precursors) on templates, a
problem which has traditionally been dealt by prior surface
modification, itself a tedious process. M. Yang, J. Ma, C. Zhang,
Z. Yang, Y. Lu, Angew. Chem. Int. Ed. 2005, 44, 6727. Other
templating methods such as templating sol-gel precursors with
colloidal crystals or their replicas (often called the "lost-wax
approach") result in amorphous and structurally fragile
nanostructures upon crystallization at high temperatures and upon
template removal. Z. Zhong, Y. Yin, B. Gates, Y. Xia, Adv. Mater.
2000, 12, 206-209.
[0006] Other approaches, which are template free, employ mechanisms
such as corrosion-based inside out evacuation and the nanoparticle
Kirkendall effect. Y. D. Yin, R. M. Rioux, C. K. Erdonmez, S.
Hughes, G. A. Somorjai, A. P. Alivisatos, Science 2004, 304, 711.
However, many such existing methods for the production of
nanostructures are often cumbersome involving multiple steps that
are often difficult to control, and are cost-prohibitive which
prevent them from being used in large-scale applications. Moreover,
many of these existing methods also result in poor yields of
mono-disperse, hollow nanostructures, producing mixed hollow and
solid nanostructures, or nanostructures with large-size
distributions.
[0007] Thus, there exists a need for improved hollow metal oxide
nanostructures comprising a relatively high surface to volume ratio
(and thus a large number of potential active sites) and physical
stability. There also exists a need for viable industrially
scaleable methods of producing such hollow metal oxide
nanostructures at low cost, high yields, narrow-size dispersions,
geometric stability and homogeneous morphologies.
SUMMARY OF THE INVENTION
[0008] In one aspect, the present invention relates to
nanostructures of metal oxides, comprising a nanostructured shell
defining an internal space, internal cavity, or "internal void."
The nanostructures may be nanoparticles, nanorods/belts/arrays,
nanotubes, nanocylinders, nanodisks, nanoboxes, nanospheres,
nanococoons, nanospindles, among other nanostructures. In one
embodiment, the nanostructure comprises a metal oxide nanosphere
comprising an outer shell defining a completely hollow cavity. In
another embodiment, the nanostructure comprises a inner hollow
cavity comprising an internal core. In yet another embodiment, the
nanostructures comprise two or more nanostructured metal oxide
concentric shells surrounding an inner cavity, while in another
embodiment, such nanostructured metal oxide concentric shells
surrounding the inner space are in turn separated by intervening
spaces. In yet another embodiment, the nanostructures may comprise
a plurality of pores in one or more of the nanostructured walls. In
other embodiments, the nanostructures comprise additional materials
in the inner space, for example, materials bearing magnetic or
electrical properties, for example ferric oxides including, but not
limited to, Fe.sub.2O.sub.3, Fe.sub.3O.sub.4, CoFe.sub.2O.sub.4, as
well as other metal oxides like CO.sub.3O.sub.4, ZnO, CuO,
Cu.sub.2O, or even metal sulfide semiconductors like CdS, ZnS, etc.
Materials with other functional properties, including noble metals,
like Au, Ag, Pt, Pd, etc., may also be enclosed within the inner
space of the nanostructures of the invention. When the
nanostructures are synthesized using templates comprising silica,
latex, carbon, or gels, the inner space of the nanostructure may
comprise cores of varying sizes of the template material used. In
preferred embodiments, the nanostructures are made of tin oxides,
but may also comprise oxides of titanium, zirconium, boron,
aluminum, germanium, indium, gallium, hafnium, silicon, vanadium,
or tantalum, zinc, copper, iron, nickel, copper and combinations
thereof.
[0009] In another aspect, the present invention is directed to
novel inexpensive, viable, high-yield methods for large-scale and
industrial mass production of such hollow metal oxide
nanostructures. In one embodiment, the invention comprises a
template-free "one-pot" method, based on an inside-out Oswald
ripening mechanism. This method comprises hydrothermal treatment of
a metal-oxide precursor in a mixed solvent, usually a polar solvent
in and water, and the mediation of general ionic and non-ionic
surfactants, polymers, or crystal modifiers such as urea-based
compounds at temperatures above about 140.degree. C. and up to
about 200.degree. C. that allow for high-yield mass production of
hollow metal oxide nanostructures with controllable sizes in the
range of 200.0-500.0 nm.
[0010] In yet another aspect, the invention is drawn to a novel
hydrothermal shell-by-shell templating strategy suitable for the
preparation of polycrystalline monodisperse metal oxide hollow
nanostructures with nanostructured metal oxide shells. Unlike
existing methods, the hydrothermal shell-by-shell templating method
of the invention requires no prior surface modification of the
template, making this method both easier to use and less costly
than existing methods. In one embodiment, the metal oxide hollow
nanostructures are multi-shelled which leads to greater physical
stability. In another embodiment, these single or multi-shelled
nanostructures are functionalized with other functional
nanoparticles (e.g., materials with magnetic properties, materials
with good heat and electrical conductivity properties, etc.). The
deposition protocol taught by the invention can be applied to a
variety of template materials, including both silica and non-silica
templates, and to templates having varying shapes, sizes and
symmetries. In one embodiment, the nanostructures produced by this
method may be coated with additional magnetic or semiconductor
materials.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] Certain embodiments of the present invention are illustrated
by the accompanying figures. It should be understood that the
figures are not necessarily to scale and that details not necessary
for an understanding of the invention or that render other details
difficult to perceive may be omitted. It should be understood, of
course, that the invention is not necessarily limited to the
particular embodiments illustrated herein.
[0012] FIGS. 1A, B, and C depicts a cross-sectional schematic
diagram of nanostructures comprising various shell-like structures
enclosing a hollow internal void.
[0013] FIG. 2A depicts the ring-like selected area electron
diffraction pattern ("SAED") pattern shown by the SnO.sub.2 in the
nanostructured shell. FIG. 2B depicts the rutile tetragonal
structure of SnO.sub.2. FIG. 2C is a magnified energy filtered
Transmission electron microscope ("TEM") image of a SnO.sub.2
nanosphere.
[0014] FIG. 3 illustrates cross-sectional diagrams of the
inside-out ripening mechanism that forms the basis of the one-pot
template-free synthesis of a metal oxide hollow nanostructure.
[0015] FIG. 4 illustrates the shell-by-shell template-based method
of preparing a SnO.sub.2 nanosphere.
[0016] FIG. 5 depicts cavity functionalization of a nanosphere with
an Au nanoparticle.
[0017] FIGS. 6A and 6B depict TEM images of Au/silica core/shell
particles and double-shelled Au/SnO.sub.2 hollow colloids prepared
via steps I and II of FIG. 5, respectively; FIG. 6C shows
Au/SnO.sub.2 hollow colloids obtained by HF etching the colloids
shown in FIG. 5B; FIG. 6D depicts the EDX spectrum of the particles
shown in FIG. 5.
[0018] FIG. 7A depicts a first cycle charge-discharge curve for
SnO.sub.2 hollow nanospheres. FIG. 7B illustrates the comparative
cycling performance of various nanostructures.
[0019] FIG. 8 illustrates cyclic voltammograms of hollow SnO.sub.2
nanospheres (first and second cycle). The scan range of 0-1.5 V
(vs. Lithium) was swept at the rate of 0.1 mV/s).
DETAILED DESCRIPTION
[0020] The hollow nanostructures of the present invention comprise
one or more polycrystalline metal oxide nanostructured shells
enclosing an inner cavity or "internal void." As used herein, the
terms cavity and void are interchangeable. Various metal oxides may
be employed in the invention, including, but not restricted to
oxides of titanium, zirconium, boron, aluminum, germanium, indium,
gallium, hafnium, silicon, vanadium, tin, tantalum, iron, nickel,
cobalt, zinc, copper and combinations thereof. In preferred
embodiments, the metal oxides may be tin oxide (SnO.sub.2),
titanium dioxide (TiO.sub.2), and zirconium oxide (ZrO.sub.2). More
preferably, the metal oxide is SnO.sub.2.
[0021] The hollow nanostructures as taught by the invention exhibit
various configurations, including, but not limited to,
nanoparticles, nanorod/belts/arrays, nanotube, nanodisks,
nanoboxes, nanocylinders, nanospheres, nanococoons and
nanospindles, among others. In one embodiment of the invention, the
hollow nanostructures of the invention are symmetrical, in that
they comprise at least one axis of symmetry; in other embodiments,
the hollow nanostructures may be asymmetrical.
[0022] As depicted in FIGS. 1A, B, and C by way of example of a
hollow nanosphere, the nanostructures of the invention comprise
shell-like structures enclosing a hollow internal void. These
nanostructures include, but are not limited to: [0023] (i) a
nanostructure comprising a single nanostructured shell 300
enclosing a hollow internal void 304 (FIG. 1A); [0024] (ii) a
nanostructure comprising a single nanostructured shell 300
enclosing a hollow internal void 304, in which is contained a solid
or nanostructured inner core 306 (FIG. 1B); [0025] (iii) a
multiple-shelled nanostructure comprising a single nanostructured
shell 300 and at least one additional concentric nanostructured
shell 308 enclosing a hollow internal void 304, which optionally
comprises a solid or nanostructured inner core 306 (FIG. 1C). As
defined herein, the term "concentric" is not limited to circular
shells, or shells that share a common center, but is meant to
define shells that are "one inside the other."
[0026] In the embodiment in which the nanostructure comprises
multiple nanostructured shells, the shells may be in direct contact
with each other at one or more points on their respective surfaces,
in full contact at all points on their respective adjacent
surfaces, or may be separated by an intervening void. The
intervening void may be completely hollow or may comprise an
additional material. As stated above, the basic cross-sectional
elements of the nanostructures as depicted are applicable to
varying cross-sectional shapes, forms, and sizes.
[0027] Typically, the average cross-sectional distance of the
nanostructure, depending on its shape and morphology, may vary
between about 50.0 nm to about 1,000.0 nm, more particularly
between about 100.0 nm and about 700.00 nm, and still more
particularly between about 200.0 nm and about 500.0 nm. The average
thickness of the nanostructured shells may vary from about 10.0 nm
and 50.0 nm, more particularly between about 15.0 nm and 40.0 nm,
and still more particularly between about 20.0 nm and about 30.0
nm. In those embodiments of the nanostructures comprising an inner
core, the core typically comprises an average diameter of between
about 50.0 nm and 500.0 nm, more particularly between about 200.0
nm and about 400.0 nm, and still more particularly between about
75.0 nm and about 125.0 nm.
[0028] In one embodiment, the nanostructures of the invention may
comprise a single pore or a plurality of pores within one or more
of the nanostructured shells. Unless stated herein, the pores may
be present in any nanostructured shell anywhere on said shell. As
discussed herein, the term pore may be construed as a hole of any
shape or size across the entire thickness of a nanostructured
shell. For those nanostructures comprising multiple nanostructured
shells, the pores may be present in only one nanostructured shell,
all nanostructured shells, or a subset of the nanostructured
shells. In one embodiment of the invention, the pores are present
in only the outermost shell. In another embodiment, the pores are
present in every shell. In another embodiment, a pore in one
nanostructured shell may be contiguous with a pore in an adjacent
nanostructured shell. In yet another embodiment, a pore across each
nanostructured shell is contiguous with a pore in the adjacent
concentric shell such that the contiguous pores create an opening
from the exterior of the nanostructure to the internal void.
[0029] In keeping with IUPAC notation (J. Rouquerol et al., Pure
& Appl. Chem., 66 (1994) 1739-1758), pores in the
nanostructured shells may be macroporous pores with pore dimensions
less than about 2.0 nm, macroporous with pore dimensions typically
greater than about 50.0 nm, or mesoporous with pore dimensions
between 2.0 and 50.0 nm. Typically, the pores in the nanostructured
shells diameters between about 1.0 nm and about 10.0 nm, more
particularly between about 3.0 nm and about 5.0 nm. In one
embodiment, a single-shelled nanostructure may comprise a single
macropore of sufficient diameter such that the resulting
nanostructure resembles a cup or a bowl. In another embodiment, the
presence of contiguous macropores in a multi-shelled nanostructure
may result in a similar cup or bowl-like nanostructure. In yet
another embodiment of the invention, the pores are formed by
treating the nanostructure with a pore-forming material (porogen)
followed by heating between 100 and 1,000.degree. C. C. V. Nguyen
et al., Chem. Mater. 11, 3080 (1999); C. Nguyen et al.,
Macromolecules 33, 4281 (2000). Heat may be applied at high
temperatures in an autoclave. Alternately, the porogen-treated
nanostructure may be dried at low temperatures (<0.degree. C.)
in a lyophilizer.
[0030] The nanostructured shells of the invention are
polycrystalline, typically comprising a plurality of tin oxide
primary metal oxide "crystallites," which may vary in size. The
average size of these primary crystallites is between about 1.0 nm
and about 50.0 nm, more particularly between about 10.0 nm and
about 20.0 nm.
[0031] X-ray diffraction patterns ("XRD") conducted on a
nanostructure comprising SnO.sub.2 nanostructured shells confirm
the polycrystalline nature of the nanostructured shells. The XRD
patterns determined can be assigned to tetragonal rutile SnO.sub.2
(cassiterite, JCPDS card No. 41-1445, space group: P4.sub.2/mnm,
a.sub.o=4.738 A, c.sub.o=3.187 A) as is to be expected for
polycrystalline SnO.sub.2. The SAED pattern shown in FIG. 2A
indicates that the nanostructures are polycrystalline and the
diffraction rings from inside to outside can be indexed to (110),
(101), (200), (211) and (112) planes of rutile SnO.sub.2,
respectively. The diffraction rings come from different crystal
planes, with each crystal plane in a primary crystal generating a
point in the SAED pattern. The rutile tetragonal structure of
SnO.sub.2 typically looks like the structure depicted in FIG. 2B,
where the Sn atom is octahedrally surrounded by oxygen atoms,
which, in turn, are surrounded by planar triangles of Sn atoms.
[0032] The cross-structural morphology of the metal oxide
nanostructures of the invention are further illustrated by a low
magnification transmission electron microscopy ("TEM") image as
shown in FIG. 2C. The SnO.sub.2 nanospheres clearly depict the
nanostructured outer shell 300, the inner space 304 and an internal
core 306. The dark contrast inside each shell as shown in FIG. 2C
reveals that the inner space is in fact filled with a solid core.
Such nanostructures can be prepared using a template-free as well
as the template-based methods taught by the invention and discussed
herein.
[0033] As stated herein, controlled synthesis of the foregoing
nanostructures may be carried out by a simple "one-pot"
template-free manner or by using templates and employing a
shell-by-shell deposition of the metal oxide. The one-pot
template-free method based on a novel and unusual "inside-out
Ostwald ripening mechanism" comprises the steps of combining metal
oxide precursor with a solvent to create a mixture, optionally
adding one or more yield-enhancing additives (fore example,
urea-based additives) to this mixture, and heating this mixture for
a predetermined reaction times. These steps may be followed by
cooling, centrifugation and washing with deionized water. This
one-pot, template-free method taught by the invention is highly
advantageous over existing methods in that it involves fewer steps
than existing methods suitable for high-yield mass production of
mono-disperse, structurally sound metal oxide nanospheres. By way
of example of the production of SnO.sub.2 nanospheres using a
potassium stannate precursor, this template-free method has been
shown to give high yield mass production of SnO.sub.2 hollow
nanospheres as measured by the % of Sn units in the stannate
precursor converted to SnO.sub.2 crystallites in the end-product,
and at nearly 100% morphological yield (i.e., the percentage of
nanospheres displaying the same size, shape and cross-sectional
characteristics).
[0034] Preferably, the precursor salt of the metal oxide is an
alkali metal precursor salt, more preferably the alkali metal
precursor salt is a sodium or a potassium salt, most preferably the
potassium salt. Thus, when the desired metal oxide nanostructure is
a SnO.sub.2 nanostructure, the metal precursor salts may be sodium
stannate or potassium stannate. The mixed solvent typically
comprises a polar solvent in water, preferably a highly polar
solvent with some degree of hydrophobic/surfactant character, more
preferably, any C.sub.2 to C.sub.10 alcohol which may be employed
as a single component or in mixture with other additives.
Typically, the yield-enhancing additives used in the method
comprise urea compounds (such as urea and thiourea), --NH.sub.2
compounds (including, but not limited to ammonia, ethylenediamine,
surfactants amines, diamines, ammonia, and hydrazine), or
surfactants, both ionic and non-ionic (for example,
cetyltrimethylammortium bromide ("CTAB"), Tween-based compounds,
dodecyl sulfates, etc.). Preferably, yield-enhancing additives are
urea-based catalytic additives such as urea and thiourea.
Additional yield-enhancing additives, including ammonia,
ethylenediamine and surfactants such as cetyltrimethylammortium
bromide ("CTAB") may be used during synthesis. In addition, ammonia
may be used as a pH modifier. Ethylenediamine and the CTAB
surfactant may be used to effect the shape and size of the end
product. Typically, the mixture of the metal-oxide precursor in the
mixed solvent and the yield enhancing additives is heated to a
temperature of between 140 and 200.degree. C., preferably between 3
and 24 hours
[0035] By way of example, describing the formation of a hollow
SnO.sub.2 nanosphere by heating a mixture of potassium stannate in
a mixed ethanol/water solvent in the presence of urea, the
mechanism for the template-free formation of hollow nanostructures
may be described. During the first stage of reaction, solid
nanospheres are formed by hydrolysis of the stannate. With
prolonged hydrothermal treatment, these solid nanospheres are
transformed by the "inside-out Ostwald ripening" or "core
evacuation" of the formed solid nanosphere in which the metal oxide
crystals inside the solid nanosphere are forced out of the solid
nanosphere to create an internal void. Ostwald ripening, first
described by Wilhelm Ostwald in 1896, is a well-known particle
(crystal) growth mechanism, in which the growth of large particles
occurs at the expense of small ones while trying to minimize the
surface energy (because of the smaller surface to volume ratio in
large particles). In the case of the nanostructures of the
invention, inside-out Ostwald ripening is understood to be the
process by which the metal oxide crystals in the interior of the
nanostructures dissolve preferentially because of the
crystallization of the metal oxides on the surface of the forming
nanostructures, even as the more amorphous material within the
interior of the nanostructure amorphous tend to dissolve. L.
Archer, X. W. Lou, Adv. Mater. 2006 18, 2325-2329; H. G. Yang, H.
C. Zeng, J. Phys. Chem. B 2004, 108, 3492; J. Li H. C. Zeng, Angew.
Chem. 2005, 117, 4416.
[0036] This inside-out ripening process may initiate at regions
either just below the external surface of the solid nanosphere, or
around the nanostructure center, depending on the reaction
conditions. Core evacuation by either mechanism yields,
respectively, hollow nanostructures or hollow nanostructures in
which the internal void comprises a solid core. Since the solid
nanospheres typically formed first are not typically
single-crystalline but comprise many tin oxide primary
crystallites, the tin oxide particles in the inner region are
packed more loosely than those in the outer layer resulting in
larger surface energies which can lead to the tendency to dissolve.
Moreover, as the hollowing process is typically accompanied by
crystallization of the particles, the primary particles in the
outer surfaces are relatively easy to crystallize facilitated by
surrounding solvent, and therefore less crystalline particles in
the inner region are easy to dissolve.
[0037] FIG. 3A illustrates cross-sectional diagrams of the proposed
inside-out ripening mechanism: inside-out evacuation can initiate
from just below surface of the shell 300 to form nanostructure (i)
in which the hollow inner void 304 is beginning to be formed; then
evolve to form a nanostructure (ii) with a well-defined inner void
304 further comprising an inner core 306; and further to hollow
nanostructure (iii). Alternately, inside-out ripening mechanism may
start around the central region of the solid sphere and evolve to
particle (iii) directly.
[0038] By carefully controlling certain key reaction parameter in
the template-free method of the invention the shape, size,
morphology, and yield of the resulting metal oxide nanostructure
may be determined. These parameters include, but are not limited
to, the type and concentration of the metal-oxide precursor, the
properties of the polar solvent and the polarity of the resultant
polar solvent/water mixture, the type and amount of yield-enhancing
additives used, the temperature at which the mixing reaction is
conducted, and the length of time for which this reaction is
performed.
[0039] In one embodiment of the invention, changes in precursor
concentration affect both the morphology and size of the
end-product. By way of example of the production of SnO.sub.2
nanospheres, it was determined that when the precursor
concentration of potassium stannate was reduced from 8.4 mM to 4.7
mM, the product comprised discrete hollow nanospheres with
diameters in the range of 50.0 nm to 200.0 nm and a shell thickness
of 10.0 nm. In comparison, keeping all other reaction parameters
the same, a higher precursor concentration (e.g. 22 mM) produced
nanospheres comprising a core spaced apart from and surrounded by a
shell at approximately the same thickness (10.0 nm), but with
larger diameters, for example, between about 300.0 and about 400.0
nm as well as a highly defined inner core within the internal
void.
[0040] Apart from precursor concentration, solvent characteristics
are also relevant in affecting control of the end product. As
stated previously, the solvent is preferably a highly polar solvent
with some degree of hydrophobic/surfactant character, more
preferably, any C.sub.2 to C.sub.10 alcohol which may be employed
as a single component or in mixture with other additives.
Typically, the polar solvents are dissolved in water to create a
mixed solvent Typically, the ratio of C.sub.2 to C.sub.10 alcohol
to water used is 1:1, but ratios may range from 5:1 to 1:4 to
produce similar results. By way of example of the hydrothermal
template-free production of SnO.sub.2 nanospheres in a mixed
ethanol ("EtOH")/water solvent, it was shown that the r value (EtOH
v/v % in the solvent of EtOH/H.sub.2O) or polarity of the mixed
solvent is a parameter which has an effect on the resultant metal
oxide nanostructures. Typically, the preferred r values are in the
range of 30-50%. In general, well-defined hollow nanospheres are
difficult to produce when r is lower than 30%. In order to
synthesize well-defined, mono-dispersed, hollow SnO.sub.2
nanospheres at r values <30%, the precursor concentrations must
be increased than what are required for higher r values. If the r
value remains fixed at less than 30%, and often even at less than
40%, aggregated clumps of small SnO.sub.2 crystallites form,
instead well-defined SnO.sub.2 microspheres. For example, while
keeping the r value fixed at 37.5% while changing the precursor
potassium stannate concentrations, the products comprise mixtures
of single hollow nanospheres (100-250 nm) and aggregated
nanospheres at a precursor concentration of 13.5 mM, well-defined
hollow nanospheres (300-550 nm) at a precursor concentration of
16.5 mM, and bowl-like large hollow spheres (450-750 nm) at a
precursor concentration 19.5 mM, respectively.
[0041] In another embodiment, the effect of urea-based catalytic
additives such as urea and thiourea was found to increase both the
product yield (measured by the % of Sn units in the stannate
precursor converted to SnO.sub.2 crystallites in the end-product)
and morphological yield to nearly 100%. As an example, hollow
SnO.sub.2 nanospheres produced in the presence of urea, keeping all
other reaction parameters the same are well dispersed with good
monodispersity (usually 80-200 nm in diameter and an average shell
thickness of about 20 nm).
[0042] In another embodiment of the invention, temperature of the
reaction can be varied to affect changes in shape, size and
morphology. By way of example, in the case of hollow SnO.sub.2
nanospheres, it was shown that product synthesized at 150.degree.
C. comprises monodispersed hollow nanospheres with sizes in the
range of 350-460 nm, while those prepared at 180.degree. C. assume
a bowl-like shape at a similar size. In another embodiment of the
invention by way of example of hollow SnO.sub.2 nanospheres, the
hydrothermal reaction period was shown to affect the final product.
While the size of the SnO.sub.2 nanospheres obtained in the
presence of thiourea at different hydrothermal reaction times do
not typically increase with longer reaction times, their shell
thicknesses may decrease from .about.50 nm to .about.20 nm,
illustrating the continuing evacuation of SnO.sub.2 nanospheres
thus increasing the size of the inner void, while at the same time
decreasing the thickness of the nanostructured shell. Other
time-dependent experiments in which all other synthesis conditions
except reaction times were kept identical, the size distribution of
the nanospheres appeared largely unaltered, while the morphology
changed. Specifically, the product comprised solid nanospheres
after a reaction time of 6 hours, and hollow nanospheres when
allowed to react for 24 hours.
[0043] The present invention also teaches a novel template based
shell-by-shell nanostructure synthesis. Unlike existing methods of
preparing nanostructures, the shell by shell synthesis taught by
the invention creates nearly mono-disperse, narrow-size
distribution, hollow nanostructures.
[0044] Unlike existing methods taught in the prior art where the
templates are pre-treated, the method as taught by the invention
requires no prior surface modification of the template resulting in
both time and cost-savings over existing methods for producing
nanostructures. In particular, the advantages of this method allow
this method to be used in large-scale and industrial scale
applications. The deposition protocol involves direct treatment of
the template with the metal oxide precursor in the presence of a
mixed solvent and a catalyst which creates a polycrystalline
nanostructured metal oxide shell over the template. In one
embodiment of the invention, the method may be used to create a
single nanostructured shell, while in other embodiments, the
deposition protocol is successively repeated to create two or more
polycrystalline nanostructured shells such that each newly created
shell surrounds the prior formed shell. The shell-by-shell
deposition protocol as taught by the invention may be used on any
suitable template including, but not restricted to, templates made
of silica, latex, carbon, gels, micelles, or combinations thereof.
The shell-by-shell deposition protocol may also be used on
templates of any shape, including, but not restricted to,
spherical, polyhedral, cylindrical templates as well as any other
symmetrical or asymmetrical templates. It has also been shown that
the shell-by-shell synthesis as taught by the invention can work on
template sizes in which the size of the template as measured by its
longest cross-sectional distance is between 50.0 to 1,000.0 nm,
preferably between 150.0 to 500M nm, and more preferably between
200.0 to 300.0 nm.
[0045] By way of example only, the shell-by-shell synthesis of the
invention may be illustrated by the formation of polycrystalline
SnO.sub.2 nanospheres as depicted in FIG. 4. The SnO.sub.2
precursor is first hydrothermally deposited on silica nanospheres
to form a single nanostructured shell 300 as shown in Step 1. This
single nanostructured shell then serves as an interim nanotemplate
for further deposition. This deposition step may be repeated to
form double shells 308 as shown in Step 2. As formed, the
double-shelled nanostructures still contain partially dissolved
silica cores 306, the sizes of which may be controlled by "etching"
(or dissolution) with dilute HF (typically about 1-2% wt %) to form
either a hollow inner void 304 with an internal core 306 comprising
the unetched template material (Step 3), or proceed to form a
completely hollow inner void 306 (Step 4). Double-shelled hollow
nanostructures can also be formed in a single step by increasing
the relative amount of silica template (Step 5). By increasing the
extent of HF etching, the double-shelled hollow nanostructures can
be transformed into a single shell hollow nanostructure (Step 6).
Typically, the hydrothermal deposition of SnO.sub.2 is carried out
under basic conditions at a pH between 9 and 12, more preferably
between 10 and 11.
[0046] As with the one-pot template-free method described herein,
preferably, the precursor salt of the metal oxide is an alkali
metal precursor salt, more preferably the alkali metal precursor
salt is a sodium or a potassium salt, most preferably the alkali
metal precursor salt is a potassium salt. Thus, when the desired
metal oxide nanostructure is a SnO.sub.2 nanostructure, the metal
precursor salts may be sodium stannate or potassium stannate. The
mixed solvent typically comprises a polar solvent in water,
preferably a highly polar solvent with some degree of
hydrophobic/surfactant character, more preferably, any C.sub.2 to
C.sub.10 alcohol which may be employed as a single component or in
mixture with other additives. Typically, the yield-enhancing
additives used in the method comprise urea compounds (such as urea
and thiourea), --NH.sub.2 compounds (including, but not limited to
ammonia, ethylenediamine, surfactants amines, diamines, ammonia,
and hydrazine), or surfactants, both ionic and non-ionic (for
example, cetyltrimethylammortium bromide ("CTAB"), Tween-based
compounds, dodecyl sulfates, etc.) Preferably, yield-enhancing
additives are urea-based additives such as urea and thiourea. In
addition, ammonia may also be used as a pH modifier.
Ethylenediamine and the CTAB surfactant may be used to effect the
shape and size of the end product. Typically, the mixture of the
metal-oxide precursor and the template in the mixed solvent (and
optionally the yield enhancing additive)s is heated in an autoclave
to a temperature of between 100 and 1000.degree. C., preferably
between 120 and 200.degree. C., most preferably between 140 and
180.degree. C.
[0047] Various parameters are known to affect the shell-by-shell
synthesis of the metal oxide nanostructures. For instance,
depending on the kind of metal oxide nanostructure desired, the
synthesis was found to have an optimal precursor concentration. By
way of example, using the production of SnO.sub.2 nanospheres, and
keeping the precursor concentration at a concentration of 16 mM, it
was determined that using silica template amounts of 100 to 160 mg,
more preferably 120 to 140 mg result in single-shelled
nanostructures. When the amount of silica template is reduced from
120 mg to 80 mg, some double-shelled nanostructural formation is
observed, but only in the form of irregular SnO.sub.2 crystallites.
Only when the amount of silica template is further reduced to
between 30 and 50 mg, more preferably 40 to 45 mg, are well-formed
double-shelled hollow nanostructures observed.
[0048] The size of the inner cavity may be controlled by the extent
of HF etching. For instance, for those double-shelled
nanostructures bearing an internal silica core, continuous HF
etching leads to the greater dissolution of the inner silica core
and the simultaneous enlargening of the inner cavity, while also
causing the dissolution of the nanostructured double-shell into a
single shell. L. Archer, X. W. Lou, Small 2006 0000, 00, No. 0 1-5
(Manuscript available at www.small-journal.com).
[0049] For those nanostructures with only one shell, the structural
integrity of a single-shelled nanostructure may be enhanced either
by the creation of one or more additional shells or by a
post-synthesis annealing process carried out at temperatures
between 400 to 600.degree. C., more preferably between 500 and
550.degree. C. Alternately, structural integrity may also be
enhanced by increasing the deposition temperature from a range of
about 140 to 160.degree. C. to between 180 to 200.degree. C.
[0050] The present invention allows for the exploitation of the
single or multi-shelled hollow nanostructures by functionalizing
the inner cavity with potentially useful nanoparticles. In one
embodiment of the invention, the inner cavities may be
functionalized with magnetic nanoparticles by first coating the
nanoparticle with the template material. Magnetic nanoparticles
include, but are not limited to, nanoparticles comprising ferric
oxides including, but not limited to, Fe.sub.2O.sub.3,
Fe.sub.3O.sub.4, CoFe.sub.2O.sub.4, as well as other metal oxides
like CO.sub.3O.sub.4, ZnO, CuO, Cu.sub.2O, or even metal sulfide
semiconductors like CdS, ZnS, etc. Materials with other functional
properties, including noble metals, like Au, Ag, Pt, Pd, etc., may
also be enclosed within the inner space of the nanostructures of
the invention. The template material may include any suitable
material that may be coated onto said magnetic nanoparticle.
Template materials include, but are not limited to, latex, silica,
gels, carbon, or combinations thereof. Such functionalized
nanostructures with magnetized functionalities may be used in
applications such as targeted drug delivery, separation methods,
biomedical imaging, and use in photovoltaic cells among others. In
one preferred embodiment of the invention, SnO.sub.2 is deposited
on an .alpha.-Fe.sub.2O.sub.3 nanosphere, nanococoon, or
nanospindle by the shell-by-shell synthesis taught by the invention
to form the corresponding uniform multi-shelled SnO.sub.2
nanostructure.
[0051] In yet another embodiment of the invention, noble metals
including, but not limited to Au, Ag, Pt, Pd, etc. are treated with
a template material and then subjected to a metal oxide
shell-by-shell nanostructure synthesis as taught by the invention
to create a mixed metal/metal oxide nanostructure functionalized to
serve a particular end use. In one preferred embodiment of the
invention, the functional metal is gold, while the metal oxide is
SnO2. Using the shell-by-shell synthesis of the invention, a mixed
Au--SnO2 metal-semiconductor composite nanostructure is created
that is suitable for use in applications such as sensors,
catalysts, controlled release drug delivery systems and
bioseparations. The process by which an Au nanoparticle is
functionalized within a hollow SnO.sub.2 nanostructure is shown in
FIG. 5. Step 1 involves the conformal coating of Au nanoparticles
310 with uniform shells of silica 312. The as-obtained Au/silica
nanoparticle are then used as the template for subsequent SnO.sub.2
deposition to form a SnO.sub.2 shell 300 in Step II. FIGS. 6A and
6B depict TEM images of Au/silica core/shell particles 400 and
double-shelled Au/SnO.sub.2 hollow colloids 402 prepared via steps
I and II of FIG. 5, respectively; FIG. 6C shows Au/SnO.sub.2 hollow
colloids obtained by HF etching the colloids shown in FIG. 5B; FIG.
6D depicts the energy dispersive X-ray ("EDX") spectrum of the
particles shown in FIG. 5B.
[0052] The polycrystalline metal oxide nanostructures of the
invention generated by the methods taught herein, may also be
coated with additional industrially useful materials. In one
embodiment, the metal oxide nanostructures of the invention may
serve as templates to be coated with materials bearing electrical
conductivity properties, where optionally the metal oxide
nanostructures bear electrical conductivity properties as well. In
a preferred embodiment, the metal oxide nanostructures are
SnO.sub.2 nanostructures that are coated with semiconductors
including, but not limited to, propylenevinylidenefluoride
("PVDF"), hexafluoropropylenevinylidenefluoride semiconductor
("PVDF-HFP"), and carbon-black filled PVDF or PVDF-HFP. For those
nanostructures used as semiconductors in new generation lithium
batteries, the added presence of such conductors on the metal oxide
nanostructures enhances cyclability by improving the contact
between the nanostructure and the battery. L. Archer, X. W. Lou,
Adv. Mater. 2006 18, 2325-2329; Xinlu Li, Feiyu Kang, and Wanci
Shen, Electrochem. Solid-State Lett., 2006, 9, A126-A129.
[0053] The metal oxide nanostructured materials of the present
invention may be used in a variety of applications. For example,
the materials may be utilized as nanoscale chemical reactors,
catalysis and photocatalysts, photovoltaic devices and as the anode
in lithium storage batteries.
[0054] When used as anode materials in lithium ion batteries,
nanostructured SnO.sub.2 made in accordance with the present
invention exhibits enhanced electrochemical properties. One
hindrance against use of Sn-based anode materials in lithium ion
batteries is the large volume change exhibited by Sn-based
particles, owing to particle shattering and agglomeration during
charging/discharging processes which leads to poor cyclability. The
hollow nanostructures of the invention mitigate this problem.
SnO.sub.2 nanostructures of the present invention exhibit initial
discharge capacities between about 1000.0 mAh/g and 1,300 mAh/g and
more particularly between about and about 1,100 and 1,200 mAh/g and
still more particularly greater than 1,100 mAh/'g. Additionally,
the nanostructures may undergo between about 25 and 45 cycles and
more particularly between about 30 and 40 cycles while still
functioning effectively at a discharge capacity of at least 300
mAh/g. It is believed that the superior lithium storage capacity is
associated with the unique structure of hollow nanospheres with
porous shells formed during the inside-out evacuation process.
EXAMPLES
[0055] The present invention is illustrated, but in no way limited
by the following examples.
Example 1
[0056] Hollow SnO.sub.2 nanoparticles were prepared by a
hydrothermal method in an ethanol/H.sub.2O mixed solvent. Potassium
stannate trihydrate (K.sub.2SnO.sub.3; 3H.sub.2O, Aldrich, 99.9%)
was added to 30 ml of ethanol/H.sub.2O mixture with an r value of
25-50%, to achieve potassium stannate concentrations of 4.7 mM-40
mM. After gentle shaking by hand for about 5 minutes, a slightly
white translucent or clear solution (depending on values of r and
c) was obtained, which was then transferred to a 40 mL Teflon-lined
stainless steel autoclave. In certain experiments, urea, thiourea
or ethyl diamine were also used as additives, typically with
overall concentrations of about 0.1 mM. After heating in an
electric oven at 150.degree. C. for a period of 3-48 hours, the
autoclave was gradually cooled down in air, or rapidly using tap
water. The white product was harvested by centrifugation and washed
with deionized water and ethanol before drying at 50.degree. C.
overnight.
[0057] The electrochemical properties of the hollow SnO.sub.2
nanospheres were characterized at room temperature. For these
measurements SnO.sub.2 nanospheres were used to form the negative
electrode in rechargeable lithium-ion batteries. The working
electrode comprised 80 wt % of the active material (hollow
SnO.sub.2), 10 wt % of conductivity agent (carbon black, Super-P),
and 10 wt % of binder (polyvinylidene difluoride, PVDF, Aldrich).
Lithium foil was used as the counter and reference electrodes. The
electrolyte was 1 M Li PF.sub.6, in a 50:50 w/w mixture of ethylene
carbonate and diethyl carbonate. Cell assembly was carried out in a
glove box with the concentrations of moisture and oxygen below 1
ppm. The room-temperature electrode activities were measured using
a Maccor-Series-2000 battery tester (Maccor, Inc. Tulsa, Okla.).
The cells were charged and discharged at a constant current of
.about.0.2 C and the fixed voltage limits were between 2 V and 5
mV. Cyclic voltammetry was performed on an EG&G model 273
potentiostat/galvanostat (Princeton Applied Research, Oakridge,
Tenn.) using the active anode as the working electrode and lithium
as both the counter and reference electrodes. FIG. 7A shows the
first cycle charge-discharge curve of SnO.sub.2 hollow nanospheres
500. It is apparent from FIG. 7A that the SnO.sub.2 hollow
nanospheres display a very large initial discharge capacity of
about 1140 mAh/g. This value is more than 75% greater than that of
pristine SnO.sub.2 nanoparticles which display an initial discharge
capacity of about 645 mAh/g and also higher than, any other
previously reported SnO.sub.2 based hollow structures. FIG. 7B
compares the cycling performance of as-prepared SnO.sub.2 hollow
nanospheres of the invention (curve a) 502 with pristine SnO.sub.2
hollow nanospheres synthesized in accordance with the methods
outlined in J. Power Sources 2005, 144, 220 by Wang and Lee (curve
c) 506 and previous SnO.sub.2 hollow nanospheres synthesized in
accordance with the methods outlined in Y. Wang et al., Chem.
Mater, 2006, 18, 1347 (curve b) 504, which were all tested under
similar conditions. It is apparent from this figure that the
cycling performance of the SnO.sub.2 hollow nanospheres of the
present invention is superior.
[0058] Specifically, while all materials show some fall-off in
performance over time, the capacity of the SnO.sub.2 hollow
nanospheres of the invention is still comparable to the theoretical
capacity of SnO.sub.2 after more than 30 cycles; and much higher
than the theoretical capacity of graphite after more than 40
cycles.
[0059] Cyclic voltammograms, shown at FIG. 8, are consistent with
previously reported SnO.sub.2 anode materials, indicating the same
mechanism for lithium storage, nanopores in the shells and the
interior microcavities of hollow nanospheres should allow or at
least facilitate storage of the extra amount of lithium.
Specifically, FIG. 8 illustrates cyclic voltammograms of hollow
SnO.sub.2 nanospheres (first and second cycle). The scan range of
0-1.5 V (vs. Lithium) was swept at the rate of 0.1 mV/s).
Example 2
[0060] Monodisperse silica nanospheres with different sizes were
prepared from the well known Stobers method. W. Stober, A. Fink, E.
Bohn, J. Colloid Interface Sci. 1968, 26, 62-69. Polycrystalline
SnO.sub.2 shells were facilely deposited on silica templates
without any prior surface modification by a hydrothermal method.
Depending on the amount of nanostructures desired, 40-120 mg silica
nanospheres were first dispersed by ultrasonication in 30 ml of
ethanol/water (37.5 vol % ethanol) mixed solvent. To this white
suspension, urea (0.9 g or 0.5 M) and potassium stannate trihydrate
(.about.144 mg or 16 mM; K.sub.2SnO.sub.3.3H.sub.2O, Aldrich,
99.9%) were added. After shaking by hand for about 5 minutes until
the salt dissolved, the suspension was transferred to a 40 ml
Teflon-lined stainless-steel autoclave, which was then heated in an
airflow electric oven at 150-190.degree. C. for 36 hours. After the
autoclave cooled down naturally, the white product was harvested by
centrifugation and washed with deionized water. This hydrothermal
SnO.sub.2 deposition can be repeated to form double and even triple
shells. The silica core was etched in a dilute ethanol/water
solution of HF (.about.2%).
Example 3
[0061] The Au/silica core/shell particles were prepared as
described in detail by Liu et al. S. H. Liu, M. Y. Han, Adv. Funct.
Matter. 2005, 15, 961-967. After extensive washing with water, the
as-obtained Au/silica core/shell particles (.about.50 mg) were
directly used without drying as the template for the same SnO.sub.2
deposition.
Example 4
[0062] .alpha.-Fe.sub.2O.sub.3 spindles were prepared by aging a
solution of 0.02 M FeCl.sub.3 and 0.45 mM NaH.sub.2PO.sub.4 at
105.degree. C. for 48 hours as described by Ozaki et al. M. Ozaki,
S. Kratohvili, E. Matijevic, J. Colloid Interface Sci., 1984, 102,
146-151. For SiO.sub.2 coating, 63 mg .alpha.-Fe.sub.2O.sub.3
spindles were first dispersed by ultrasonication in a mixture
consisting 650 mL of 2-propanol and 65 mL of deionized water,
followed by 60 mL of ammonia (29.6%). Under magnetic stirring, 4 mL
of tetraethylorthosilicate ("TEOS") in 2-propanol (10% v/v) was
added, followed by another 3.5 mL of TEOS after 2 hours. The
reaction was then continued for 18 hours. The
.alpha.-Fe.sub.2O.sub.3/SiO.sub.2 particles were harvested by
centrifugation, and washed with ethanol and water several times
before being vacuum-dried at room temperature. For hydrothermal
SnO.sub.2 deposition, 118 mg .alpha.-Fe.sub.2O.sub.3/SiO.sub.2
particles were dispersed in 25 mL of ethanol/water (37.5 vol %
ethanol) mixed solvent. To this suspension, urea (0.75 g) and
potassium stannate trihydrate (113 mg; K.sub.2SnO.sub.3.3H.sub.2O,
Aldrich (St. Louis, Mo.), 99.9%) were added. After shaking by hand
for about 5 minutes until the salts dissolved, the suspension was
transferred to a 40 mL Teflon-lined stainless-steel autoclave,
which was then heated in an air flow electric oven at 170.degree.
C. for 36 hours. After the autoclave was allowed to cool down
naturally, the particles were washed with ethanol/water once before
the same hydrothermal deposition was repeated. For synthesis of
porous double-shelled nanococoons, about 34 mg
.alpha.-Fe.sub.2O.sub.3/SiO.sub.2 particles were used. After
annealing the particles at 550-600.degree. C. for 8 hours, the
silica was dissolved in 1 M NaOH solution at 50.degree. C. for
about 2 days (alternately, the silica is dissolved by etching in a
very diluted HF solution as SnO.sub.2 tends to partially dissolve
in NaOH solution when exposed to NaOH for extended periods of
time).
[0063] The nanococoons were characterized with X-ray powder
diffraction ("XRD") using a Scintag PAD X-ray Powder Diffractometer
(Scintag PAD X, Cu K.alpha., .lamda.=1.5406 .ANG.) (Scintag, Inc,
Cupertino, Calif.), transmission electron microscopy (TEM/SAED)
using a JEOL-1200EX transmission electron microscope (120 kV)
(JEOL, Tokyo, Japan), ultra-high vacuum scanning transmission
electron microscopy (UHV-STEM, 100 kV) (Fisons Instruments, Inc,
San Carlos, Calif.) equipped with energy dispersive X-ray analysis
(EDX), and field emission scanning electron microscopy (FE-SEM; LEO
1550) (Carl Zeiss, Oberkochen, Germany). Nitrogen adsorption and
desorption isotherms were measured using the Micromeritics ASAP
2020 Accelerated Surface Area and Porosimetry analyzer
(Micromeritics, Inc., Norcross, Ga.).
[0064] Numerous references, including patents and various
publications, are cited and discussed in the description of this
invention. The citation and discussion of such references is
provided merely to clarify the description of the present invention
and is not an admission that any reference is prior art to the
invention described herein. All references cited and discussed in
this specification are incorporated herein by reference in their
entirety.
[0065] Variations, modifications and other implementations of what
is described herein will occur to those of ordinary skill in the
art without departing from the spirit and scope of the invention.
While certain embodiments of the present invention have been shown
and described, it will be obvious to those skilled in the art that
changes and modifications may be made without departing from the
spirit and scope of the invention. The matter set forth in the
foregoing description and accompanying drawings is offered by way
of illustration only and not as a limitation. The actual scope of
the invention is intended to be defined in the following
claims.
* * * * *
References