U.S. patent application number 10/765015 was filed with the patent office on 2004-08-12 for silicon based nanospheres and nanowires.
Invention is credited to Gole, James L., Stout, John D., White, Mark G..
Application Number | 20040157414 10/765015 |
Document ID | / |
Family ID | 27393102 |
Filed Date | 2004-08-12 |
United States Patent
Application |
20040157414 |
Kind Code |
A1 |
Gole, James L. ; et
al. |
August 12, 2004 |
Silicon based nanospheres and nanowires
Abstract
A nanowire, nanosphere, metallized nanosphere, and methods for
their fabrication are outlined. The method of fabricating nanowires
includes fabricating the nanowire under thermal and non-catalytic
conditions. The nanowires can at least be fabricated from metals,
metal oxides, metalloids, and metalloid oxides. In addition, the
method of fabricating nanospheres includes fabricating nanospheres
that are substantially monodisperse. Further, the nanospheres are
fabricated under thermal and non-catalytic conditions. Like the
nanowires, the nanospheres can at least be fabricated from metals,
metal oxides, metalloids, and metalloid oxides. In addition, the
nanospheres can be metallized to form metallized nanospheres that
are capable as acting as a catalyst.
Inventors: |
Gole, James L.; (Atlanta,
GA) ; Stout, John D.; (Atlanta, GA) ; White,
Mark G.; (Woodstock, GA) |
Correspondence
Address: |
THOMAS, KAYDEN, HORSTEMEYER & RISLEY, LLP
100 GALLERIA PARKWAY, NW
STE 1750
ATLANTA
GA
30339-5948
US
|
Family ID: |
27393102 |
Appl. No.: |
10/765015 |
Filed: |
January 26, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10765015 |
Jan 26, 2004 |
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09820413 |
Mar 29, 2001 |
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6720240 |
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60192846 |
Mar 29, 2000 |
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60192844 |
Mar 29, 2000 |
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Current U.S.
Class: |
438/487 |
Current CPC
Class: |
B01J 23/72 20130101;
B82Y 30/00 20130101; H01L 29/16 20130101; C23C 14/22 20130101; B22F
1/0547 20220101; C23C 14/223 20130101; H01L 29/0669 20130101; Y10S
438/94 20130101; B22F 2998/00 20130101; C30B 29/605 20130101; Y10S
438/931 20130101; Y10S 438/962 20130101; Y10S 977/762 20130101;
H01L 29/413 20130101; C23C 14/228 20130101; Y10S 977/90 20130101;
B01J 35/023 20130101; B22F 1/054 20220101; C30B 23/00 20130101;
C30B 23/00 20130101; C30B 29/605 20130101; B22F 2998/00 20130101;
B22F 1/17 20220101; B22F 2998/00 20130101; B22F 1/17 20220101 |
Class at
Publication: |
438/487 |
International
Class: |
C30B 001/00 |
Claims
Therefore, having thus described the invention, at least the
following is claimed:
1. A method of preparing a nanostructure, comprising the step of
forming a nanowire under thermal conditions and under non-catalytic
conditions.
2. The method of claim 1, wherein the step of forming the nanowire
under thermal conditions comprises the step of forming a nanowire
in the temperature range of about 800.degree. C. to about
1500.degree. C.
3. The method of claim 1, wherein the step of forming the nanowire
comprises the step of forming a metal nanowire.
4. The method of claim 3, wherein the step of forming the metal
nanowire, comprises the step of forming a metal nanowire, wherein
the metal is selected from the group consisting of: tin, chromium,
iron, nickel, silver, titanium, cobalt, zinc, platinum, palladium,
osmium, gold, lead, iridium, molybdenum, vanadium, and
aluminum.
5. The method of claim 3, wherein the step of forming the metal
nanowire, comprises the step of forming a metal oxide nanowire,
wherein the metal oxide is selected from the group consisting of:
tin dioxide, chromia, iron oxide, nickel oxide, silver oxide,
titanium oxide, cobalt oxide, zinc oxide, platinum oxide, palladium
oxide, vanadium oxide, molybdenum oxide, and lead oxide.
6. The method of claim 1, wherein the step of forming the nanowire
comprises the step of forming a metalloid nanowire.
7. The method of claim 6, wherein the step of forming the metalloid
nanowire, comprises the step of forming a silicon dioxide sheathed
crystalline silicon nanowire, where the axis of the crystalline
silicon nanowire core is substantially parallel to a <111>
plane and substantially free of defects.
8. The method of claim 7, wherein the step of forming the silicon
dioxide sheathed silicon nanowire that is substantially free of
defects further comprises the step of forming a silicon dioxide
sheathed silicon nanowire that is substantially free of twinning,
substantially free of high order grain boundaries, and
substantially free of stacking faults.
9. A method of preparing a nanostructure, comprising the step of
forming a plurality of substantially monodisperse nanospheres.
10. The method of claim 9, wherein the step of forming the
plurality of nanospheres comprises the step of forming a plurality
of substantially monodisperse metal nanospheres.
11. The method of claim 10, wherein the step of forming the metal
nanosphere, comprises the step of forming the metal nanosphere
where the metal is selected from the group consisting of: tin,
chromium, iron, nickel, silver, titanium, cobalt, zinc, platinum,
palladium, osmium, gold, lead, iridium, molybdenum, vanadium, and
aluminum.
12. The method of claim 9, wherein the step of forming the
plurality of nanospheres, comprises the step of forming a plurality
of substantially monodisperse metal oxide nanospheres.
13. The method of claim 12, wherein the step of forming the metal
oxide nanospheres comprises the step of forming a metal oxide
nanospheres, wherein the metal oxide is selected from the group
consisting of: tin dioxide, chromia, iron oxide nickel oxide,
silver oxide, titanium oxide, cobalt oxide, zinc oxide, platinum
oxide, palladium oxide, vanadium oxide, molybdenum oxide, and lead
oxide.
14. The method of claim 12, wherein the step of forming the
plurality of substantially monodisperse metal oxide nanospheres,
includes the step of forming a plurality of substantially disperse
tin dioxide nanospheres.
15. The method of claim 9, wherein the step of forming the
plurality of nanospheres, includes the step of forming a plurality
of substantially monodisperse metalloid oxide nanospheres.
16. The method of claim 15, wherein the step of forming the
plurality of substantially monodisperse metalloid oxide
nanospheres, includes a step of forming a plurality of
substantially monodisperse metalloid oxide nanospheres, wherein the
metalloid oxide is silicon dioxide.
17. The method of claim 16, wherein the step of forming the
plurality of substantially monodisperse metalloid oxide
nanospheres, wherein the metalloid oxide is silicon dioxide
comprises the step of forming an amorphous silicon dioxide
nanosphere.
18. The method of claim 16, wherein the step of forming the
plurality of substantially monodisperse metalloid oxide
nanospheres, wherein the metalloid oxide is silicon dioxide
comprises the step of forming a plurality of substantially disperse
metalloid oxide nanospheres with a diameter range of about 8
nanometers to about 45 nanometers.
19. The method of claim 9, wherein the step of forming the
nanosphere, further comprises the step of forming a nanosphere
under thermal conditions.
20. The method of claim 9, wherein the step of forming a
nanosphere, further includes the step of forming a nanosphere under
non-catalytic conditions.
21. A method of fabricating catalytic nanostructures, comprising
the step of metallizing a nanosphere.
22. The method of claim 21, wherein the step of metallizing the
nanosphere, includes the step of producing at least a gram of
nanospheres.
23. The method of claim 21, wherein the step of metallizing the
nanosphere, includes the step of metallizing a metal
nanosphere.
24. The method of claim 22, wherein the step of metallizing the
metal nanosphere, includes the step of metallizing a metal
nanosphere, wherein the metal is selected from the group consisting
of: tin, chromium, iron, nickel, silver, titanium, cobalt, zinc,
platinum, palladium, osmium, gold, lead, iridium, molybdenum,
vanadium, and aluminum.
25. The method of claim 21, wherein the step of metallizing the
nanosphere, includes the step of metallizing a metalloid oxide
nanosphere, wherein the metalloid oxide is silicon dioxide.
26. The method of claim 21, wherein the step of metallizing the
nanosphere, includes the step of metallizing a metal oxide
nanosphere.
27. The method of claim 12, wherein the step of metallizing the
metal oxide nanosphere, includes the step of metallizing a metal
oxide nanosphere, wherein the metal oxide is selected from the
group consisting of: tin dioxide, tin dioxide, chromia, iron oxide
nickel oxide, silver oxide, titanium oxide, cobalt oxide, zinc
oxide, platinum oxide, palladium oxide, vanadium oxide, molybdenum
oxide, and lead oxide.
28. The method of claim 26, wherein the step of metallizing the
metal oxide nanosphere, includes the step of metallizing a metal
oxide nanosphere, wherein the metal oxide is tin dioxide.
29. The method of claim 21 wherein the step of metallizing the
nanosphere, includes metallizing a nanosphere with a second
metal.
30. The method of claim 26, wherein the step of metallizing the
nanosphere with the second metal, includes the step of metallizing
a nanosphere with a second metal selected from the group consisting
of: copper, tin, and aluminum.
31. A nanostructure, comprising a metal nanowire.
32. The nanostructure of claim 31, wherein the metal nanowire
comprises a metal wherein the metal is selected from the group
consisting of: chromium, iron, nickel, silver, titanium, cobalt,
zinc, platinum, palladium, osmium, gold, lead, iridium, molybdenum,
vanadium, and aluminum.
33. The nanostructure of claim 31, wherein the metal nanowire
comprises a metal oxide nanowire, wherein the metal oxide is
selected from the group consisting of: tin dioxide, chromia, iron
oxide nickel oxide, silver oxide, titanium oxide, cobalt oxide,
zinc oxide, platinum oxide, palladium oxide, vanadium oxide,
molybdenum oxide, lead oxide.
34. The nanostructure of claim 33, wherein the metal oxide nanowire
is a tin dioxide nanowire.
35. A nanostructure, comprising a metalloid nanowire.
36. The nanostructure of claim 35, wherein the metalloid nanowire
includes a silicon dioxide sheathed crystalline silicon nanowire,
where the axis of the crystalline silicon nanowire core is
substantially parallel to a <111> plane and substantially
free of defects.
37. A nanostructure, comprising a metal nanosphere.
38. The nanostructure of claim 37, including a plurality of
substantially monodisperse metal nanospheres.
39. The nanostructure of claim 37, wherein the metal is selected
from the group consisting of: chromium, iron, nickel, silver,
titanium, cobalt, zinc, platinum, palladium, osmium, gold, lead,
iridium, molybdenum, vanadium, and aluminum.
40. The nanostructure of claim 37, wherein the metal nanosphere
includes a metal oxide nanosphere, wherein the metal oxide is
selected from the group consisting of: tin dioxide, chromia, iron
oxide nickel oxide, silver oxide, titanium oxide, cobalt oxide,
zinc oxide, platinum oxide, palladium oxide, vanadium oxide,
molybdenum oxide, and lead oxide.
41. The nanostructure of claim 40, wherein the metal nanosphere is
a tin dioxide nanosphere.
42. A nanostructure, comprising silicon dioxide nanosphere.
43. The nanostructure of claim 42, wherein the silicon dioxide
nanosphere has a diameter from about 8 to about 45 nanometers.
44. The nanostructure of claim 42, wherein the silicon dioxide
nanosphere is metallized with 3 weight percent copper.
45. A method of metallizing a nanostructure, comprising the steps
of: forming a nanosphere; metallizing the nanosphere with a metal;
and forming a metallized nanosphere that has been metallized with
the metal.
46. The method of claim 45, wherein the step of metallizing the
nanosphere with the metal, includes metallizing a nanosphere with
copper.
47. The method of claim 45, wherein the step of forming the
metallized nanosphere, includes the step of forming a metallized
copper nanosphere that has been metallized with about 3 weight
percent copper.
48. The method of claim 45, wherein the step of metallizing the
nanosphere with a metal, includes the step of metallizing a
nanosphere with a metal selected from the group consisting of:
copper, tin, aluminum, silver, platinum, palladium, iron, cobalt,
and nickel.
49. The method of claim 45, wherein the step of forming the
metallized nanosphere, includes the step of forming a metallized
metal nanosphere, wherein the metal is selected from the group
consisting of: copper, tin, aluminum, silver, platinum, palladium,
iron, cobalt, and nickel.
50. The method of claim 45, wherein forming the nanosphere includes
the step of forming a nanosphere under thermal conditions.
51. The method of claim 50, wherein the step of forming the
nanowire under thermal conditions comprises the step of forming a
nanowire in the temperature range of about 800.degree. C. to about
1500.degree. C.
52. The method of claim 45, wherein forming the nanosphere includes
the step of forming a nanosphere under non-catalytic
conditions.
53. A method of dehydrogenating ethanol, comprising the steps of:
introducing gaseous ethanol to 3 weight percent copper metallized
silicon dioxide nanosphere; and producing at least 6 percent
conversion/mg copper for the selective dehydrogenation of ethanol
to acetaldehyde.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims priority to copending U.S.
Provisional application entitled, "Silicon Based Nanowires and
Nanospheres", filed with the United States Patent and Trademark
Office on Mar. 29, 2000, and assigned Serial No. 60/192,846, and
U.S. provisional application entitled "New Cu/SiO.sub.2 Based
Catalyst for Selective Ethanol-Acetaldehyde Conversion", filed with
the United States Patent and Trademark Office on Mar. 29, 2000, and
assigned Serial No. 60/192,844, which are both entirely
incorporated herein by reference.
TECHNICAL FIELD
[0002] The present invention is generally related to nanostructures
and, more particularly, is related to nanowires and nanospheres and
methods for their preparation and use.
BACKGROUND OF THE INVENTION
[0003] Semiconductor nanostructures, nanoagglomerates, and
nanowires have attracted considerable attention because of their
potential applications in mesoscopic research, the development of
nanodevices, and the potential application of large surface area
structures. For several decades, the vapor-liquid-solid (VLS)
process, where gold particles act as a mediating solvent on a
silicon substrate forming a molten alloy, has been applied to the
generation of silicon whiskers. The diameter of the whisker is
established by the diameter of the liquid alloy droplet at its tip.
The VLS reaction generally leads to the growth of silicon whiskers
epitaxially in the <111> direction on single crystal silicon
<111> substrates. In addition, laser ablation techniques have
been performed on metal-containing (iron or gold) silicon targets,
producing bulk quantities of silicon nanowires. Further, thermal
techniques have been used to produce a jumble of silicon dioxide
(SiO.sub.2) coated crystalline nanowires that have their axes
parallel to the <112> direction. Further these nanowires are
deficient because of twinning, high order grain boundaries, and
stacking faults.
[0004] Recently, national lab researchers, in an effort to begin an
ongoing dialogue to forecast the direction of environmental science
and technology, ranked the top ten environmental technology
breakthroughs for 2008. Not surprisingly, molecular design is
expected to play an important role in the development of advanced
materials. Included in this framework is the design of
nano-assembled and non-stoichiometric catalysts designed for the
efficient control of chemical processes.
[0005] Heterogeneous catalysts are typically prepared by decorating
high surface area solids such as silica or alumina with active
metals or metal ions from precursor materials such as cation
complexes [M.sup.n+(L.sup.m-.sub.x].sup.(n-xm), anion complexes
(e.g., [Pt.sup.4+F.sub.6].sup.2- or neutrals such as copper (II)
acetylacetonate (Cu(AcAc).sub.2)). These processes typically use
starting reagents and produce products that are harmful to the
environment (e.g. solvents, metal halides, strong acids, or other
environmentally aggressive reagents and or products). A
high-surface-area support is needed to provide the proper
dispersion of the active ingredients so that the high intrinsic
activity of these catalytic metals or ions can be realized in
practice. Without this support, many catalytic agents show very
little active surface area. Often, the intrinsic catalytic activity
of the supported metals or metal ions is changed by interaction
with the support metal ions or oxygen atoms. Thus, some supports
are not benign towards the catalytic agents. Moreover, the
catalytic properties of these agents are often compromised as a
result of the efforts to synthesize supported catalysts having high
dispersions of the active ingredient. These uniquely assembled
catalysts might then be used to more efficiently control combustion
processes, and reactions such as hydrocarbon reforming.
[0006] Thus, a heretofore unaddressed need exists in the industry
to address the aforementioned deficiencies and inadequacies.
[0007] An embodiment of the present invention provides for a
nanowire and method of fabrication thereof. The method includes
fabricating the nanowires under thermal and non-catalytic
conditions. The nanowires can be fabricated from at least metals,
metal oxides, metalloids, and metalloid oxides. A preferred
embodiment of the present invention includes, but is not limited
to, the fabrication of a silicon dioxide sheathed crystalline
silicon nanowire, where the axis of the crystalline silicon
nanowire core is substantially parallel to a <111> plane and
is substantially free of defects.
[0008] Another embodiment of the present invention provides for a
nanosphere and method of fabrication thereof. The method includes
fabricating the substantially monodisperse nanospheres under
thermal and non-catalytic conditions. The nanospheres can at least
be fabricated from metals, metal oxides, metalloids, and metalloid
oxides. A preferred embodiment of the present invention includes,
but is not limited to, fabricating amorphous silicon dioxide
nanospheres.
[0009] Still another embodiment of the present invention provides
for a metallized nanosphere and method of fabrication thereof. The
method includes fabricating the subtantially monodisperse
nanospheres under thermal and non-catalytic conditions. The
nanospheres can be fabricated from at least metals, metal oxides,
metalloids, and metalloid oxides. The nanospheres can be metallized
to form metallized nanospheres that are capable of having catalytic
properties. In addition, the formation of the nanospheres and
metallization of the nanospheres can be performed substantially in
one step. A preferred embodiment of the present invention includes
fabricating amorphous silicon dioxide nanospheres and depositing
three weight percent (%) copper onto the nanosphere.
[0010] Still a further embodiment of the present invention provides
for a method of the dehydrogenation of ethanol. The method includes
introducing gaseous ethanol to three weight percent metallized
silicon dioxide nanospheres to produce at least a three percent
conversion/mg copper for the selective dehydrogenation of ethanol
into acetaldehyde.
[0011] Other systems, methods, features, and advantages of the
present invention will be or become apparent to one with skill in
the art upon examination of the following drawings and detailed
description. It is intended that all such additional systems,
methods, features, and advantages be included within this
description; be within the scope of the present invention, and be
protected by the accompanying claims.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0012] Embodiments of the present invention provide for
nanostructures, catalytic nanostructures, and methods of
preparation of same. Nanostructures include, but are not limited
to, nanowires, nanospheres, nanoagglomerates, nanotubes, etc. More
specifically, exemplary embodiments of the present invention
provide a nanowire and methods of preparation thereof. Another
exemplary embodiment provides a nanosphere and methods of
preparation thereof. Still another exemplary embodiment provides a
catalytic nanosphere and methods of preparation thereof (e.g., a
metallized nanosphere with catalytic activity). The nanostructures
can be made of materials such as, but not limited to, metals, metal
oxides, metalloids, metalloid oxides, combinations of metals,
combinations of metal oxides, combinations of metalloids,
combinations of metalloid oxides, combinations of metals and metal
oxides, combinations of metalloid and metalloid oxides, or any
other appropriate combination. Further, the nanostructures can be
metallized to form catalytic nanostructures that can be used to
enhance reaction kinetics and reaction efficiency.
[0013] A. Nanowires and Nanospheres
[0014] One exemplary embodiment of the present invention provides
for a nanowire prepared under thermal and non-catalytic conditions.
The thermal conditions include, but are not limited to, the range
of 800.degree. C. to 1500.degree. C. The term non-catalytic
conditions means, for the purposes of this disclosure, that an
additional catalyst is unnecessary for the nanostructures to be
fabricated. In an exemplary embodiment, the nanowire can be
fabricated to form metal, metal oxide, metalloid, metalloid oxide,
or combinations thereof nanowires. In a preferred embodiment, the
nanowires include silicon dioxide sheathed crystalline silicon
nanowires where the axis of the crystalline silicon nanowire core
is substantially parallel to a <111> plane. In addition, the
silicon nanowires are substantially defect free. That is, the
silicon nanowires are substantially free of twinning, high order
grain boundaries, and stacking faults. Non-limiting examples of
metals from which the nanowires can be fabricated include, but are
not limited to, tin (Sn), chromium (Cr), iron (Fe), nickel (Ni),
silver (Ag), titanium (Ti), cobalt (Co), zinc (Zn), platinum (Pt),
palladium (Pd), osmium (Os), gold (Au), lead (Pb), iridium (Ir),
molybdenum (Mo), vanadium (V), aluminum (Al), or combinations
thereof. In addition, non-limiting examples of metal oxides which
the nanowires can be fabricated into include, but not limited to,
tin dioxide (SnO.sub.2), chromia (Cr.sub.2O.sub.3), iron oxide
(Fe.sub.2O.sub.3, Fe.sub.3O.sub.4, or FeO) nickel oxide (NiO),
silver oxide (AgO), titanium oxide (TiO.sub.2), cobalt oxide
(Co.sub.2O.sub.3, Co.sub.3O.sub.4, or CoO), zinc oxide (ZnO),
platinum oxide (PtO), palladium oxide (PdO), vanadium oxide
(VO.sub.2), molybdenum oxide (MoO.sub.2), lead oxide (PbO), and
combinations thereof. In addition, a non-limiting example of a
metalloid includes, but is not limited to, silicon or germanium.
Further, a non-limiting example of a metalloid oxide includes, but
is not limited to, silicon monoxide, silicon dioxide, germanium
monoxide, and germanium dioxide.
[0015] Another exemplary embodiment of the present invention
provides for a plurality of nanospheres that are substantially
monodisperse and a method of preparation thereof. In addition, the
nanospheres can be fabricated in gram quantities under thermal and
non-catalytic conditions. The thermal condition includes, but is
not limited to, the range of 800.degree. C. to 1500.degree. C. The
term non-catalytic conditions means that an additional catalyst is
unnecessary for the nanostructures to be fabricated. Further, the
nanospheres can be fabricated to form metal, metal oxide,
metalloid, metalloid oxide, or combinations thereof nanospheres.
Non-limiting examples of metals from which the nanospheres can be
fabricated include, but are not limited to, tin (Sn), chromium
(Cr), iron (Fe), nickel (Ni), silver (Ag), titanium (Ti), cobalt
(Co), zinc (Zn), platinum (Pt), palladium (Pd), osmium (Os), gold
(Au), lead (Pb), iridium (Ir), molybdenum (Mo), vanadium (V),
aluminum (Al), and combinations thereof. In addition, non-limiting
examples of metal oxides from which the nanospheres can be
fabricated include, but not limited to, tin dioxide (SnO.sub.2),
chromia (Cr.sub.2O.sub.3), iron oxide (Fe.sub.2O.sub.3,
Fe.sub.3O.sub.4, or FeO), nickel oxide (NiO), silver oxide (AgO),
titanium oxide (TiO.sub.2), cobalt oxide (Co.sub.2O.sub.3,
Co.sub.3O.sub.4, or CoO), zinc oxide (ZnO), platinum oxide (PtO),
palladium oxide (PdO), vanadium oxide (VO.sub.2), molybdenum oxide
(MoO.sub.2), lead oxide (PbO), and combinations thereof. In
addition, a non-limiting example of a metalloid includes, but is
not limited to, silicon and germanium. Further, a non-limiting
example of a metalloid oxide includes, but is not limited to,
silicon monoxide, silicon dioxide, germanium monoxide, and
germanium dioxide. The nanospheres can range in diameter from a few
nanometers to on the order of hundreds of nanometers. More
particularly, silicon dioxide nanospheres are amorphous, have no
dangling bonds, and range in diameter from about 8-45 nanometers
(run). Further, the method of fabricating nanospheres and nanowires
using thermal techniques can be similar. In this regard, both
nanospheres and nanowires can be fabricated using similar
fabrication steps. Modifications in fabrication parameters,
disclosed hereinafter, can be used to control the quality and
quantity of the fabricated nanospheres and nanowires.
EXAMPLE 1
[0016] The following is a non-limiting illustrative example of an
embodiment of the present invention that is described in more
detail in Gole, et al., Appl. Phys. Lett., 76, 2346 (2000), which
is incorporated herein by reference. This example is not intended
to limit the scope of any embodiment of the present invention, but
rather is intended to provide specific experimental conditions and
results. Therefore, one skilled in the art would understand that
many experimental conditions can be modified, but it is intended
that these modification are within the scope of the embodiments of
the present invention.
[0017] The apparatus to fabricate silicon based nanostructures
includes a double concentric alumina tube combination that can be
heated to the desired temperature in a Lindberg Scientific tube
furnace configuration. The inner alumina tube is vacuum sealed by
two water cooled stainless steel end pieces which are attached to
the alumina tube and tightly lock-press fit against custom viton
o-rings. At one end of the furnace, ultra-high purity argon (Ar)
enters through the upstream stainless steel end piece and passes
through a matched set of zirconia insulators to the central region
of the inner tube oven. Here the entraining argon flows over a
crucible containing the sample mixture of interest, which may be
either a silicon-silica (Si/SiO.sub.2) mixture or powdered silicon
monoxide, at a flow rate of 100 standard cubic centimeter per
minute (sccm) controlled by a flow controller. It should be noted
that other sample mixtures can be used that correspond to the
metals listed hereinabove.
[0018] The total tube pressure in the inner tube can range from 200
to 650 Torr as measured by a Baratron differential pressure
transducer, but is typically about 225 Torr. The pressure in the
inner tube can be controlled by a mechanical pump or other
appropriate pump attached to the inner alumina tube through the
downstream stainless steel end piece. This end piece is
mechanically attached to a "water cooled" cold plate, with an
adjustable temperature system, through a matching set of insulating
zirconia blocks. Depending on the desired temperature range of
operation, the crucibles used to contain the silicon/silicon oxide
based mixtures were either commercially available quartz
(1200-1350.degree. C.) or alumina (1400-1500.degree. C.) or were
machined from low porosity carbon (1500.degree. C.). The parameters
that can be controlled in this experiment were (1) gas flow rate,
(2) total tube gas pressure, (3) central region temperature and
temperature gradients to the end regions, and (4) cold plate
temperature. The ultra-high purity argon was not heated before it
enters the inner furnace tube, although it could be heated. The
condensation of silicon-based nanowires produced dark brown
deposits in a narrow region on the wall of the inner alumina tube,
close to the defining end points of the Lindberg oven shell, which
corresponds to a temperature in the range approximately
900-1000.degree. C. Large quantities (e.g. gram quantities) of
SiO.sub.2 nanospheres were deposited on the temperature controlled
cold plate.
[0019] In an exemplary embodiment, virtually uniform and straight
nanowires were generated from a 50/50 Si/SiO.sub.2 equimolar
mixture heated to a temperature of about 1400.degree. C. at a total
pressure of about 225 Torr for about 12 hours. The central
crystalline silicon core for the nanowire is about 30 nm in
diameter, whereas the outer SiO.sub.2 sheathing is about 15 nm in
thickness, as exemplified in Gole et al., Appl. Phys. Lett. 76,
2346 (2000), which is incorporated herein by reference. However,
nanowires with much smaller and larger diameter central crystalline
cores and different sheathing thickness have been obtained. The
axis of the SiO.sub.2 clad crystalline silicon nanowire core is
substantially parallel to the <111> plane. This is distinct
from the results obtained by Lee et al.; MRS Bulletin, 36 (1999)
whose wires have their axis parallel to <112> plane as they
display twinning, high order grain boundaries, and stacking faults.
At the Si--SiO.sub.2 interface for the material obtained in the
present synthesis the crystal planes are best described as {211}.
The nanowires synthesized are so perfect that slight undulations of
the crystalline silicon core, due to strain induced by measuring
devices, can be observed.
[0020] Other distinguishing characteristics of the nanowires
include the pinch off of the crystalline silicon core at the
beginning of the wire growth, suggesting a distinctly different
formation mechanism than that suggested by Lee et al. for their
wires generated using a similar source and by Hu et al., Acc. Chem.
Res. 32, 435 (1999) for their iron-catalyzed wire formation from
Fe/Si mixtures generated using laser ablation. While Lee et al.
find evidence for a growth mechanism along <111> with which
they associate a complex process involving SiO.sub.2 formation, the
observed structures generated using the described thermal source
likely indicate that the mechanism for these nanowires is a close
analogy to the VLS mechanism, albeit with an apparent self-assembly
of the silicon in the absence of a metal catalyst. Further, the
outer SiO.sub.2 sheath of the nanowire has significant strength.
Finally, a comparison to the transmission electron micrograph (TEM)
micrographs of Hu et al., which show the clear termination of their
nanowires at larger --nearly spherical FeSi.sub.2 nanoclusters,
offers yet an additional contrast suggesting further alternate
mechanisms for the wire formation. The mechanism for formation of
the nanowires in the present study would appear to be distinct and
possess both the attributes of the Si/SiO.sub.2 reaction mechanism
presented by Lee et al. and of the VLS growth method.
[0021] Nearly monodisperse SiO.sub.2 nanospheres in the diameter
range of 8-45 nm can be generated as a deposit in gram quantities
on the cold plate of the described apparatus. Nanospheres can be
generated in the same apparatus that produced the nanowires. By
adjusting the flow parameters and temperature, it is possible to
generate nanospheres ranging in diameter from 8-45 nm in virtually
monodisperse distributions. It is possible to generate these
nanospheres not only from Si/SiO.sub.2 mixtures but also from SiO
powders, albeit at somewhat higher temperatures.
[0022] Judicious manipulation of the high temperature system
including reactant mixture stoichemistry, flow conditions
(kinetics), and temperature range, may yield more than would have
been previously anticipated by others skilled in the art. The
results suggest that additional mechanisms which are analogs not
only of the VLS mechanism on the nanoscale but also represent some
crystalline silicon self-assembly may be operative. Further, Lee et
al. produce a jumble of uniform SiO.sub.2 coated crystalline
silicone nanowires of various sizes which, when straight, have
their axes parallel to <111>. These wires, however, display
twining, high order grain boundaries, and defect sites (stacking
faults). In contrast, embodiments of the present invention are
capable of producing nanowires where the axis of the nanowire core
is substantially parallel to a <111> plane, virtually defect
free, and demonstrate no twining. Given the high temperature
synthesis of alternate combinations of metal/metal oxide nanowire
configurations, embodiments of the present invention appear to be
well suited to photonic waveguide applications.
[0023] B. Nanosphere Catalysts
[0024] Still another exemplary embodiment of the present invention
provides a catalytic nanosphere (e.g., metallized nanosphere) and
method of preparation thereof. The nanosphere of this embodiment
can be formed in a manner similar to the preparation of nanospheres
described earlier and includes the same properties as those
nanospheres. After the nanospheres are fabricated, the nanospheres
can be metallized to form metallized nanospheres that are capable
of having catalytic properties. One of many advantages of this
embodiment is that the nanosphere and metallized nanosphere can be
fabricated in one step rather than multiple steps, as required by
present techniques in the art.
[0025] Non-limiting examples of metals from which the nanospheres
can be fabricated include, but are not limited to, tin (Sn),
chromium (Cr), iron (Fe), nickel (Ni), silver (Ag), titanium (Ti),
cobalt (Co), zinc (Zn), platinum (Pt), palladium (Pd), osmium (Os),
gold (Au), lead (Pb), iridium (Ir), molybdenum (Mo), vanadium (V),
aluminum (Al), and combinations thereof. In addition, non-limiting
examples of metal oxides from which the nanospheres can be
fabricated include, but not limited to, tin dioxide (SnO.sub.2),
chromia (Cr.sub.2O.sub.3), iron oxide (Fe.sub.2O.sub.3,
Fe.sub.3O.sub.4, or FeO), nickel oxide (NiO), silver oxide (AgO),
titanium oxide (TiO.sub.2), cobalt oxide (Co.sub.2O.sub.3,
Co.sub.3O.sub.4, or CoO), zinc oxide (ZnO), platinum oxide (PtO),
palladium oxide (PdO), vanadium oxide (VO.sub.2), molybdenum oxide
(MoO.sub.3), lead oxide (PbO), and combinations thereof. In
addition, a non-limiting example of a metalloid includes, but is
not limited to, silicon and germanium. Further, a non-limiting
example of a metalloid oxide includes, but is not limited to,
silicon monoxide, silicon dioxide, germanium monoxide, and
germanium dioxide. The nanospheres can range in diameter. More
particularly, silicon dioxide nanospheres are amorphous, have no
dangling bonds, and range in diameter from about 8-45
nanometers.
[0026] Further, the method of metallization is capable of
depositing a second metal onto the nanosphere. The term "second
metal" is used here to differentiate the material (e.g. metal,
metalloid, or oxides thereof) that the nanosphere may be fabricated
into, and refers to the metal that is deposited upon the nanosphere
during a metallization process. The second metals that can be
deposited during the metallization process include, but are not
limited to, copper, tin, aluminum, silver, platinum, palladium,
iron, cobalt, nickel, combinations thereof, and other appropriate
metallization metals.
EXAMPLE 2
[0027] The following is a non-limiting illustrative example of an
embodiment of the present invention that is described in more
detail in Gole, et al., submitted to J. Appl. Phys., Gole et al.
submitted to Chemistry of Materials, which are herein incorporated
by reference. This example is not intended to limit the scope of
any embodiment of the present invention, but rather is intended to
provide specific experimental conditions and results. Therefore,
one skilled in the art would understand that many experimental
conditions can be modified, but it is intended that these
modifications are within the scope of the embodiments of the
present invention.
[0028] Silica nanospheres, of about a 30 nm diameter, can be
prepared at elevated temperature (e.g. 800-1500.degree. C.) from an
Si/SiO.sub.2 mixture. Under ambient conditions, the high population
of surface hydroxyl groups on these nanospheres, confirmed by FTIR
spectroscopy, is probed by decorating the surfaces of the spheres
with the metal complex copper (II) acetylacetonate: Cu(AcAc).sub.2.
These metal complexes are known in the art to be anchored by the
surface SiOH species, and can be converted into an active catalyst
by thermolysis of the ligands. The resulting monatomic copper
distribution forms a selective catalyst whose conversion efficiency
appears to be at least comparable to, if not better than, Cu/fumed
silica described in Kenvin, et al., J. Catal. 135, 81 (1992). In
contrast to the fumed silica, however, the preparation of this
catalyst support is environmentally benign.
[0029] Dispersed nanospheres have been fabricated without the use
of solvents and without producing byproducts, such as hydrochloric
acid gas, to compromise the environment. The synthesis technique
uses a mixture of silicon and silicon dioxide, heated under a flow
of ultra high purity argon at elevated temperature for a specified
duration. The synthesis method can produce silica nanospheres,
having nearly monodisperse particle size of about 30 nm. These
nanospheres, as demonstrated by high-resolution transmission
electron microscopy and x-ray diffraction, are amorphous. Further,
as elaborated in more detail in this example, the silica nanosphere
has surface properties that demonstrate the presence of surface
silanol groups (--SiOH) which can be used to sequester active Cu
sites for the selective conversion of ethanol to acetaldehyde. A
surface population of --SiOH groups on silica can influence the
bonding of metal complexes to the surface. The loading of the metal
complexes and the resulting morphology of the supported metal ions
is influenced by the --SiOH groups on the surface. Silica
nanospheres are contacted with Cu(AcAc).sub.2 in acetonitrile in
sufficient concentration to produce silica nanospheres that contain
about 3 wt % Cu. This same procedure has been used to make
monatomic dispersions of Cu ions on fumed amorphous silica
manufactured and commercially available from the Cabot Corporation
(Cab-O-Sil.TM.) Alpharetta, Ga.
[0030] The products of the ethanol dehydrogenation reaction depend
upon the ensemble size of supported Cu ions. Isolated copper ions
catalyze only the dehydrogenation to acetaldehyde whereas multiple
Cu ensembles show high yields of ethyl acetate in addition to
acetaldehyde. Thus, the ethanol/acetaldehyde probe reaction can be
used to define the presence of monatomic dispersions of Cu ion from
an examination of the product distribution.
[0031] TEM micrographs indicate that nearly monodisperse SiO.sub.2
nanospheres of diameter of close 30 nm can be generated in gram
quantities on the cold plate of the high temperature synthesis
device described earlier. As described earlier, the apparatus
includes a double concentric alumina tube combination heated to the
desired temperature in a Lindberg Scientific tube furnace
configuration. The inner alumina tube is vacuum sealed by two water
cooled stainless steel end pieces which are attached to the alumina
tube and tightly lock-press fit against custom viton.TM. o-rings.
At one end of the furnace, ultra high purity argon enters thru the
upstream stainless steel end piece and passes through a matched set
of zirconia insulators to the central region of the inner tube
oven. The entraining argon then flows over a crucible containing
the sample mixture of interest, which is either a silicon-silica
(Si/SiO.sub.2) mixture or powdered silicon monoxide, at a flow rate
of 100 sccm controlled a flow controller.
[0032] The total tube pressure in the inner tube can range from 200
to 650 Torr but is typically about 225 Torr. This pressure can be
controlled by a mechanical pump or other appropriate pump attached
to the inner alumina tube through the downstream stainless steel
end piece. This end piece is mechanically attached to a water
cooled cold plate, which has as adjustable temperature system,
through a matching set of insulating zirconia blocks. Depending on
the desired temperature range of operation, the crucibles used to
contain the silicon/silicon oxide based mixtures are either
commercially available quartz (1200-1350.degree. C.) or alumina
(1400-1500.degree. C.) or are machined from low porosity carbon
(1500.degree. C.). The controlled parameters may include for
example, but not limited to, (1) gas flow rate, (2) total tube gas
pressure, (3) central region temperature and temperature gradients
to the end regions, and (4) cold plate temperature. It is to be
noted that, at least for the experimental results reported here, no
attempt was made to heat the ultra high purity argon before it
enters the inner furnace tube. Large quantities of SiO.sub.2
nanospheres were deposited on the temperature controlled cold
plate.
[0033] The Cu/silica catalysts were prepared through batch
impregnation of 1 g of the silica with sufficient Cu(AcAc).sub.2
metal complex to produce a sample having 3 wt % Cu. The complex was
added to 25 mL of acetonitrile solvent and allowed to reflux with
stirring for 24 h. The solid was separated by filtration and dried
at room temperature for 18 h. This solid was dried at 100.degree.
C. for 1 hour then placed in a microreactor tube.
[0034] The ethanol dehydrogenation reaction was completed in a
micro-catalytic reactor. Prior to the reaction, the nanosphere
catalyst was heated to about 350.degree. C. for about 1 h in
flowing helium, then cooled to the reaction temperature. The
reaction conditions were conducted at about 330.degree. C., 20 mL
per minute of He carrier gas flow over a 100 mg bed of catalyst
having a Cu loading of 3 weight percent. Five to term pL pulses of
ethanol were vaporized into the He carrier gas stream to create the
reactant feed. Pulses of unreacted ethanol and the products of
reaction were partitioned on a GC column and detected by a thermal
conductivity detector.
[0035] The silica nanospheres have been characterized by Fourier
Transform Infrared (FTIR) spectroscopy. The nanospheres were
scanned just after their introduction into the sample chamber at
25.degree. C. and 1 atm. Subsequently, the samples were evacuated
to <I milli-Torr at 25.degree. C. and their spectrum was
recorded. The nanospheres were then heated to 100, 200, and
300.degree. C. in vacuo and their spectra were recorded under these
conditions.
[0036] Under 1 atm pressure at room temperature, the sample shows a
large, broad peak between 3000 and 4000 cm.sup.-1 that is
characteristic of adsorbed, molecular water. This feature decreases
to a negligible level immediately upon evacuation at room
temperature. This result suggests that most of the water is only
weakly adsorbed to the silica nanospheres. Additional peaks are
present at 1800, 1600, 1200 and 800 cm.sup.-1. In vacuo at 25 C, a
sharp peak appears at 3700 cm.sup.-1 and a broad peak near 3400
cm.sup.-1. When the sample is heated to 300 C under vacuum, the
peak at 3700 cm.sup.-1 grows even sharper and the adjacent peak at
3400 cm.sup.-1 grows smaller, demonstrating further water removal.
With increased heating above 200 C, the peaks at 1200 and 800
cm.sup.-1 at first increase and then broaden and decrease in
intensity as a shift of intensity to higher frequency features is
apparent.
[0037] Flame-hydrolyzed, amorphous silica shows a signature for the
SiO--H vibration near 3743 cm.sup.-1 and a broad peak near 3400
cm.sup.-1 that corresponds to adsorbed water. Additionally. Si--O
vibrations are evident at 1800 and 1600 cm.sup.-1. It appears that
the surface functional groups found on the silica nanospheres are
similar to those found to be present on Cab-O-Sil.TM..
[0038] The effect of the Cu/silica nanocatalyst on the ethanol
dehydrogenation reaction is presented in Table 1. Acetaldehyde was
the only product observed. Forty five percent of the ethanol was
converted over about 3 mg of Cu in the 100 mg sample of Cu/silica
using the nano-silica sample. The conversion per mg of Cu in this
sample is 45%/3 mg or 15% conversion/mg Cu. Compare this to the
results reported by Kenvin et al. for a Cu/silica prepared from
Cab-O-Sil.TM. and operated under similar conditions (300 C, 5.1, mg
Cu ion +143 mg of silica, 15.5 mL/minute of He carrier, 1-2 L of
ethanol in liquid pulses). These authors observed 25% conversion
over 5.1, mg Cu for a 5.1% conversion/mg Cu. No other products were
observed.
[0039] These results demonstrate that the conversion efficiency for
the catalyst formed from the copper loaded silica nanospheres is at
least comparable to if not better than that formed from the fumed
silica (within the accuracy of the micro-catalytic technique for
determining catalyst activity). Moreover, the selectivity to form
acetaldehyde is the same for the two catalysts. Each solid
catalyzes the single reaction to form the simple dehydrogenation
product without the side reaction corresponding to ethyl acetate
coupling. The absence of the ethyl acetate forming reaction shows
that no large ensembles of Cu are present in either sample.
1 TABLE 1 SUMMARY OF RESULTS Nanosphere Fused Silica Species mol %
mol % EtOH 55 75 Acetaldehyde 45 25 Other products 0 0
[0040] The results obtained using the nanospheres clearly
demonstrate that only the products of mono-atomically dispersed Cu
(only acetaldehyde is observed) with an apparently improved
efficiency. It should be noted that the process for forming the new
catalyst suggests an additional advantage in that it might replace
the present technique for making fumed amorphous silica by a
process that is environmentally benign. The currently applied
process for making fumed silica burns silicon tetrachloride to make
silica and HCl. The present embodiment, which relies on an elevated
temperature synthesis involving only an Si/SiO.sub.2 mixture,
eliminates the need to handle silicon tetrachloride and it does not
produce the hydrochloric acid gas.
[0041] It should be emphasized that the above-described embodiments
of the present invention, particularly, any "preferred"
embodiments, are merely possible examples of implementations,
merely set forth for a clear understanding of the principles of the
invention. Many variations and modifications may be made to the
above-described embodiment(s) of the invention without departing
substantially from the spirit and principles of the invention. All
such modifications and variations are intended to be included
herein within the scope of this disclosure and the present
invention and protected by the following claims.
* * * * *