U.S. patent application number 14/378921 was filed with the patent office on 2016-01-21 for hollow nanoparticles with hybrid double layers.
The applicant listed for this patent is The Administrators of the Tulane Educational Fund. Invention is credited to Vijay JOHN, Gary MCPHERSON.
Application Number | 20160015652 14/378921 |
Document ID | / |
Family ID | 48984831 |
Filed Date | 2016-01-21 |
United States Patent
Application |
20160015652 |
Kind Code |
A1 |
JOHN; Vijay ; et
al. |
January 21, 2016 |
HOLLOW NANOPARTICLES WITH HYBRID DOUBLE LAYERS
Abstract
The present invention discloses the morphology of hollow,
double-shelled submicrometer particles generated through a rapid
aerosol-based process. The inner shell is an essentially
hydrophobic carbon layer of nanoscale dimension (5-20 nm), and the
outer shell is a hydrophilic silica layer of approximately 5-40 nm,
with the shell thickness being a function of the particle size. The
particles are synthesized by exploiting concepts of salt bridging
to lock in a surfactant (CTAB) and carbon precursors together with
iron species in the interior of a droplet. This deliberate negation
of surfactant templating allows a silica shell to form extremely
rapidly, sealing in the organic species in the particle interior.
Subsequent pyrolysis results in a buildup of internal pressure,
forcing carbonaceous species against the silica wall to form an
inner shell of carbon. The incorporation of magnetic iron oxide
into the shells opens up applications in external
stimuli-responsive nanomaterials.
Inventors: |
JOHN; Vijay; (Destrehan,
LA) ; MCPHERSON; Gary; (Mandeville, LA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
The Administrators of the Tulane Educational Fund |
New Orleans |
LA |
US |
|
|
Family ID: |
48984831 |
Appl. No.: |
14/378921 |
Filed: |
February 19, 2013 |
PCT Filed: |
February 19, 2013 |
PCT NO: |
PCT/US13/26745 |
371 Date: |
August 14, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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61599788 |
Feb 16, 2012 |
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61610798 |
Mar 14, 2012 |
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61621642 |
Apr 9, 2012 |
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Current U.S.
Class: |
424/490 ; 264/12;
429/221; 429/527; 502/183; 502/184; 502/185; 502/241; 502/242;
502/245; 502/253; 502/255; 502/257; 502/258; 502/259; 502/261;
502/262; 502/5 |
Current CPC
Class: |
C01P 2006/16 20130101;
B01J 35/002 20130101; H01M 4/9041 20130101; Y02E 60/10 20130101;
C09B 67/0008 20130101; B82Y 40/00 20130101; A61K 9/5192 20130101;
B01J 35/1066 20130101; C01P 2006/14 20130101; C01P 2006/62
20130101; A61K 9/501 20130101; C09B 11/24 20130101; B82Y 30/00
20130101; C01P 2002/72 20130101; B01J 21/063 20130101; B01J 37/08
20130101; H01M 10/052 20130101; C01P 2004/64 20130101; B01J 35/0033
20130101; B01J 35/023 20130101; C01P 2006/12 20130101; C09B 67/0097
20130101; A61K 9/5089 20130101; H01M 4/8605 20130101; C09B 61/00
20130101; B01J 35/1061 20130101; H01G 9/2031 20130101; H01M 10/0525
20130101; B01J 37/0072 20130101; C09C 1/3607 20130101; H01M 4/587
20130101; B01J 35/004 20130101; C01P 2004/03 20130101; Y02E 60/50
20130101; B01J 13/14 20130101; B01J 13/22 20130101; B01J 35/026
20130101; C01P 2004/34 20130101; C09C 1/24 20130101; A61K 9/51
20130101; B01J 21/18 20130101; B01J 35/0013 20130101; C01P 2004/62
20130101; C09C 1/407 20130101; H01M 4/366 20130101; B01J 23/745
20130101; C01P 2004/04 20130101; C09C 1/30 20130101; A61K 9/5094
20130101; B01J 35/1057 20130101; H01M 4/8657 20130101 |
International
Class: |
A61K 9/50 20060101
A61K009/50; B01J 37/00 20060101 B01J037/00; B01J 37/08 20060101
B01J037/08; B01J 35/02 20060101 B01J035/02; H01M 10/0525 20060101
H01M010/0525; B01J 35/00 20060101 B01J035/00; B01J 35/10 20060101
B01J035/10; B01J 21/18 20060101 B01J021/18; H01M 4/36 20060101
H01M004/36; H01M 4/90 20060101 H01M004/90; A61K 9/51 20060101
A61K009/51; B01J 23/745 20060101 B01J023/745 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
[0003] Funding was received from the US Department of Energy (grant
DE-FG02-05ER46243), the National Science Foundation (grants
0933734, 1034175, and 1236089), and the Gulf of Mexico Research
Initiative. The United States government has certain rights in this
invention.
Claims
1. A method of producing hollow double shelled particles with an
outer layer of ceramic, and an inner layer of carbon by an
aerosol-based method, comprising the steps of: a) preparing a
precursor solution comprising (1) a ceramic source, (2) a carbon
source, (3) an iron source, and (4) a surfactant; b) passing the
precursor solution through a nozzle for atomization to form aerosol
droplets; c) passing the aerosol droplets through a heating zone
and a drying zone for evaporation, wherein the iron source binds
with the surfactant in the interior of the droplet and the ceramic
source is on the surface of the droplet; d) collecting the
particles on a filter; and e) passing the particles for pyrolysis
under the flow of an inert gas, wherein the carbon source located
in the interior of the particle generates an inner layer from the
inside of the particle.
2. The method of claim 1, wherein the ceramic is from the group
consisting of silica, titania, zirconia, alumina, yttria, ceria,
and mixtures thereof, and silica in combination with titania,
zirconia, alumina, yttria, or ceria, and mixtures thereof, and the
ceramic source from the group consisting of a source of silica,
titania, zirconia, alumina, yttria, ceria, and mixtures
thereof.
3. The method of claim 1, wherein the iron source is from the group
consisting of ferric halides, and mixtures thereof.
4. The method of claim 1, wherein the inert gas is from the group
consisting of nitrogen, argon, and mixtures thereof.
5. The method of claim 1, wherein the ceramic source is tetraethyl
orthosilicate (TEOS).
6. The method of claim 1, wherein the carbon source is a
monosaccharide or polysaccharide.
7. The method of claim 1, wherein the carbon source is from the
group consisting of sucrose, glucose, cellulose, and cyclodextrins,
and mixtures thereof.
8. The method of claim 7, wherein the carbon source is sucrose.
9. The method of claim 1, wherein the surfactant is from the group
consisting of cetyltrimethyl ammonium bromide (CTAB),
cetyltrimethyl ammonium chloride (CTAC), cetyltrimethyl ammonium
iodide (CTAI), cetyltrimethyl ammonium fluoride (CTAF), and
cetyltrimethyl ammonium astatide (CTAA), and mixtures thereof.
10. The method of claim 1, wherein the surfactant is cetyltrimethyl
ammonium bromide (CTAB).
11. The method of claim 1, wherein 0.8 g-1.9 g of the iron source
is added, 0.1 g-2.2 g of the surfactant is added, 1.0 mL-9 mL of
the ceramic source is added, and 0.01 g-3 g of the carbon source is
added.
12. The method of claim 1, wherein 0.95 g of the iron source is
added, 1.1 g of the surfactant is added, 4.5 mL of the ceramic
source is added, and 1.0 g of the carbon source is added.
13. The method of claim 1, wherein silica condensation and sucrose
dehydration occur in step "c".
14. The method of claim 1, wherein the particles are 50 nm to 5000
nm in diameter.
15. The method of claim 1, wherein the particles are 100 nm to 1000
nm in diameter.
16. The method of claim 1, wherein the outer layer is 5 nm to 100
nm thick.
17. The method of claim 1, wherein the inner layer is 5 nm to 100
nm thick.
18. The method of claim 1, wherein the inner layer is 50 nm to 5000
nm in diameter.
19. The method of claim 1, wherein the inner layer is 100 nm to
1000 nm in diameter.
20. The method of claim 1, wherein the outer layer is
hydrophilic.
21. The method of claim 1, wherein the inner layer is
hydrophobic.
22. The method of claim 1, wherein the outer layer is nonporous
after step "e".
23. The method of claim 1, wherein during step "e" the carbon
source forms said inner layer adjoining the outer layer and leaving
a fully hollow interior.
24. The method of claim 1, further comprising the step of etching
out the outer silica layer.
25. The method of claim 24, wherein the etching is done by a highly
acidic solution.
26. The method of claim 25, wherein the highly acidic solution is
from the group consisting of HF, HCl, and sulfuric acid.
27. The method of claim 24, wherein the etching is done by a highly
basic solution.
28. The method of claim 27, wherein the highly basic solution is
from the group consisting of NaOH and ammonium hydroxide.
29. The method of claim 1, further comprising the step of removing
the inner carbon layer by calcination.
30. The method of claim 1, wherein the precursor solution further
comprises a second metal.
31. The method of claim 30, wherein the second metal is from the
group consisting of tin, copper, palladium, chromium, zinc,
rhodium, ruthenium, molybdenum, manganese, nickel, and
aluminum.
32. Hollow double shelled particles, comprising: a) a silica layer
having an interior; b) a carbon layer attached to the interior of
the silica layer; and c) iron particles incorporated in the silica
layer.
33. The particles of claim 32, wherein the particles are 50 nm to
5000 nm in diameter.
34. The particles of claim 32, wherein the particles are 100 nm to
1000 nm in diameter.
35. The particles of claim 32, wherein the silica layer is 5 nm to
100 nm thick.
36. The particles of claim 32, wherein the carbon layer is 5 nm to
100 nm thick.
37. The particles of claim 32, wherein the carbon layer is 50 nm to
5000 nm in diameter.
38. The particles of claim 32, wherein the carbon layer is 100 nm
to 1000 nm in diameter.
39. The particles of claim 32, wherein the silica layer is
hydrophilic.
40. The particles of claim 32, wherein the carbon layer is
hydrophobic.
41. The particles of claim 32, wherein the silica layer is
nonporous.
42. The particles of claim 32, wherein the particles have a fully
hollow interior.
43. Particles produced by etching the silica layer out of the
particles of claim 32.
44. The particles of claim 43, wherein the etching is done by a
highly acidic solution.
45. The particles of claim 44, wherein the highly acidic solution
is from the group consisting of HF, HCl, and sulfuric acid.
46. The particles of claim 43, wherein the etching is done by a
highly basic solution.
47. The particles of claim 46, wherein the highly basic solution is
from the group consisting of NaOH and ammonium hydroxide.
48. Particles produced by removing by calcination the carbon layer
of the particles of claim 32.
49. The particles of claim 32, wherein the particles further
comprise a second metal.
50. The particles of claim 49, wherein the second metal is from the
group consisting of tin, copper, palladium, chromium, zinc,
rhodium, ruthenium, molybdenum, manganese, nickel, and aluminum,
and mixtures thereof.
51. The particles of claim 32, wherein the particles are prepared
by an aerosol-based method.
52. The particles of claim 51, wherein the aerosol-based method
comprises the steps of: a) preparing a precursor solution
comprising (1) a ceramic source, (2) a carbon source, (3) an iron
source, and (4) a surfactant; b) passing the precursor solution
through a nozzle for atomization to form aerosol droplets; c)
passing the aerosol droplets through a heating zone and a drying
zone for evaporation, wherein the iron source binds with the
surfactant in the interior of the droplet and the ceramic source is
on the surface of the droplet; d) collecting the particles on a
filter; and e) passing the particles for pyrolysis under the flow
of an inert gas, wherein the carbon source located in the interior
of the particle generates an inner layer from the inside of the
particle.
53. The particles of claim 52, wherein the ceramic is from the
group consisting of silica, titania, zirconia, alumina, yttria,
ceria, and mixtures thereof, and silica in combination with
titania, zirconia, alumina, yttria, or ceria, and mixtures thereof,
and the ceramic source from the group consisting of a source of
silica, titania, zirconia, alumina, yttria, ceria, and mixtures
thereof.
54. The particles of claim 52, wherein the iron source is from the
group consisting of ferric halides, and mixtures thereof.
55. The particles of claim 52, wherein the inert gas is from the
group consisting of nitrogen, argon, and mixtures thereof.
56. The particles of claim 52, wherein the ceramic source is
tetraethyl orthosilicate (TEOS).
57. The particles of claim 52, wherein the carbon source is a
monosaccharide or polysaccharide.
58. The particles of claim 52, wherein the carbon source is from
the group consisting of sucrose, glucose, cellulose, and
cyclodextrins, and mixtures thereof.
59. The particles of claim 52, wherein the carbon source is
sucrose.
60. The particles of claim 52, wherein the surfactant is from the
group consisting of cetyltrimethyl ammonium bromide (CTAB),
cetyltrimethyl ammonium chloride (CTAC), cetyltrimethyl ammonium
iodide (CTAI), cetyltrimethyl ammonium fluoride (CTAF), and
cetyltrimethyl ammonium astatide (CTAA), and mixtures thereof.
61. The particles of claim 52, wherein the surfactant is
cetyltrimethyl ammonium bromide (CTAB).
62. The particles of claim 52, wherein 0.8 g-1.9 g of the iron
source is added, 0.1 g-2.2 g of the surfactant is added, 1.0 mL-9
mL of the ceramic source is added, and 0.01 g-3 g of the carbon
source is added.
63. The particles of claim 52, wherein 0.95 g of the iron source is
added, 1.1 g of the surfactant is added, 4.5 mL of the ceramic
source is added, and 1.0 g of the carbon source is added.
64. The particles of claim 52, wherein silica condensation and
sucrose dehydration occur in step "c".
65. A method of producing hollow carbon particles by an
aerosol-based method, comprising the steps of: a) preparing a
precursor solution comprising (1) a ceramic source, (2) a carbon
source, (3) an iron source, and (4) a surfactant; b) passing the
precursor solution through a nozzle for atomization to form aerosol
droplets; c) passing the aerosol droplets through a heating zone
and a drying zone for evaporation, wherein the iron source binds
with the surfactant in the interior of the droplet and the ceramic
source is on the surface of the droplet; d) collecting the
particles on a filter; and e) passing the particles for pyrolysis
under the flow of an inert gas, wherein the carbon source located
in the interior of the particle generates an inner layer from the
inside of the particle; and f) etching out the outer layer.
66. The method of claim 65, wherein the ceramic is from the group
consisting of silica, titania, zirconia, alumina, yttria, ceria,
and mixtures thereof, and silica in combination with titania,
zirconia, alumina, yttria, or ceria, and mixtures thereof, and the
ceramic source from the group consisting of a source of silica,
titania, zirconia, alumina, yttria, ceria, and mixtures
thereof.
67. The method of claim 65, wherein the iron source is from the
group consisting of ferric halides, and mixtures thereof.
68. The method of claim 65, wherein the inert gas is from the group
consisting of nitrogen, argon, and mixtures thereof.
69. The method of claim 65, wherein the ceramic source is
tetraethyl orthosilicate (TEOS).
70. The method of claim 65, wherein the carbon source is a
monosaccharide or polysaccharide.
71. The method of claim 65, wherein the carbon source is from the
group consisting of sucrose, glucose, cellulose, and cyclodextrins,
and mixtures thereof.
72. The method of claim 65, wherein the carbon source is
sucrose.
73. The method of claim 65, wherein the surfactant is from the
group consisting of cetyltrimethyl ammonium bromide (CTAB),
cetyltrimethyl ammonium chloride (CTAC), cetyltrimethyl ammonium
iodide (CTAI), cetyltrimethyl ammonium fluoride (CTAF), and
cetyltrimethyl ammonium astatide (CTAA), and mixtures thereof.
74. The method of claim 65, wherein the surfactant is
cetyltrimethyl ammonium bromide (CTAB).
75. The method of claim 65, wherein 0.8 g-1.9 g of the iron source
is added, 0.1 g-2.2 g of the surfactant is added, 1.0 mL-9 mL of
the ceramic source is added, and 0.01 g-3 g of the carbon source is
added.
76. The method of claim 65, wherein 0.95 g of the iron source is
added, 1.1 g of the surfactant is added, 4.5 mL of the ceramic
source is added, and 1.0 g of the carbon source is added.
77. The method of claim 65, wherein silica condensation and sucrose
dehydration occur in step "c".
78. The method of claim 65, wherein the inner layer is 5 nm to 100
nm thick.
79. The method of claim 65, wherein the inner layer is 50 nm to
5000 nm in diameter.
80. The method of claim 65, wherein the inner layer is 100 nm to
1000 nm in diameter.
81. The method of claim 65, wherein the etching is done by a highly
acidic solution.
82. The method of claim 81, wherein the highly acidic solution is
from the group consisting of HF, HCl, and sulfuric acid.
83. The method of claim 65, wherein the etching is done by a highly
basic solution.
84. The method of claim 83, wherein the highly basic solution is
from the group consisting of NaOH and ammonium hydroxide.
85. Thin hollow silica shelled particles, comprising: a) a silica
outer layer having a thickness of 5 to 20 nm; and b) iron particles
incorporated in the silica layer.
86. The particles of claim 85, wherein the diameter of the
particles is 100 nm to 3000 nm.
87. The particles of claim 86, wherein the diameter of the
particles is 200 nm to 1000 nm.
88. The particles of claim 85, wherein the silica later has a
thickness of 7 nm to 20 nm.
89. The particles of claim 85, wherein the silica later has a
thickness of 7 nm to 15 nm.
90. The particles of claim 85, wherein the silica later has a
thickness of 10 nm to 15 nm.
91. The particles of claim 85, wherein the particles have a fully
hollow interior.
92. The particles of claim 85, wherein the particles further
comprise a second metal.
93. The particles of claim 92, wherein the second metal is from the
group consisting of tin, copper, palladium, chromium, zinc,
rhodium, ruthenium, molybdenum, manganese, nickel, and
aluminum.
94. The particles of claim 85, wherein the particles are fractured
by ultrasonication.
95. The particles of claim 85, wherein the porosity of the
particles is increased by adding sodium chloride.
96. The particles of claim 95, wherein 0.01-1.0 g of sodium
chloride is added.
97. The particles of claim 95, wherein 0.4 g of sodium chloride is
added.
98. The particles of claim 96 or 97, wherein the pore size is 0.5
nm to 100 nm in diameter.
99. The particles of claim 96 or 97, wherein the pore size is 10 nm
in diameter.
100. The particles of claim 85, wherein the particles are prepared
by increasing the amount of surfactant.
101. The particles of claim 85, wherein the particles are prepared
by an aerosol-based method.
102. The particles of claim 101, wherein the aerosol-based method
comprises the steps of: a) preparing a precursor solution
comprising (1) a ceramic source, (2) an iron source, and (3) a
surfactant; b) passing the precursor solution through a nozzle for
atomization to form aerosol droplets; c) passing the aerosol
droplets through a heating zone and a drying zone for evaporation,
wherein the iron source binds with the surfactant in the interior
of the droplet and the ceramic source is on the surface of the
droplet; d) collecting the particles on a filter; and e) passing
the particles for pyrolysis under the flow of an inert gas.
103. The particles of claim 102, wherein the ceramic is from the
group consisting of silica, titania, zirconia, alumina, yttria,
ceria, and mixtures thereof, and silica in combination with
titania, zirconia, alumina, yttria, or ceria, and mixtures thereof,
and the ceramic source from the group consisting of a source of
silica, titania, zirconia, alumina, yttria, ceria, and mixtures
thereof.
104. The particles of claim 102, wherein the iron source is an iron
source from the group consisting of ferric halides, and mixtures
thereof.
105. The particles of claim 102, wherein the inert gas is from the
group consisting of nitrogen, argon, and mixtures thereof.
106. The particles of claim 102, wherein the ceramic source is
tetraethyl orthosilicate (TEOS).
107. The particles of claim 102, further comprising a carbon source
that is a monosaccharide or polysaccharide.
108. The particles of claim 107, wherein the carbon source is from
the group consisting of sucrose, glucose, cellulose, and
cyclodextrins, and mixtures thereof.
109. The particles of claim 108, wherein the carbon source is
sucrose.
110. The particles of claim 102, wherein the surfactant is from the
group consisting of cetyltrimethyl ammonium bromide (CTAB),
cetyltrimethyl ammonium chloride (CTAC), cetyltrimethyl ammonium
iodide (CTAI), cetyltrimethyl ammonium fluoride (CTAF), and
cetyltrimethyl ammonium astatide (CTAA), and mixtures thereof.
111. The particles of claim 102, wherein the surfactant is
cetyltrimethyl ammonium bromide (CTAB).
112. The particles of claim 102, wherein 0.8 g-1.9 g of the iron
source is added, 0.1 g-2.2 g of the surfactant is added, and 1.0
mL-9 mL of the ceramic source is added.
113. The particles of claim 102, wherein 0.95 g of the iron source
is added, 1.1 g of the surfactant is added, and 4.5 mL of the
ceramic source is added.
114. The particles of claim 107, wherein 0.01 g-3 g of the carbon
source is added.
115. The particles of claim 107, wherein 1.0 g of the carbon source
is added.
116. The particles of claim 102, wherein the size of the shell
thickness decreases with decreasing amounts of the silica
source.
117. The particles of claim 85, wherein the particles are
nonporous.
118. The particles of claim 102, further comprising the step of
adding sodium chloride.
119. The particles of claim 118, wherein 0.01-1.0 g of sodium
chloride is added.
120. The particles of claim 118, wherein 0.4 g of sodium chloride
is added.
121. The particles of claim 119 or 120, wherein the pore size is
0.5 nm to 100 nm in diameter.
122. The particles of claim 119 or 120, wherein the pore size is 10
nm in diameter.
123. The particles of claim 1, 32 or 85, wherein the particles
encapsulate a compound.
124. The particles of claim 123, wherein the compound is a
pharmaceutical agent.
125. The particles of claim 1 or 32, wherein the particles are used
to stabilize emulsions.
126. The particles of claim 1, 32 or 85, wherein the particles are
used as a catalytic support.
127. The particles of claim 1, 32 or 85, wherein the particles are
used for drug delivery.
128. The particles of claim 1 or 32, wherein the particles are used
for Li-ion batteries.
129. The particles of claim 1 or 32, wherein the particles are used
for fuel cell catalysts.
130. The particles of claim 1, 32 or 85, wherein the particles are
magnetically responsive.
131. Hollow double shelled particles, comprising: a) a silica outer
layer; b) a titania inner layer attached to the silica outer layer;
and c) iron particles incorporated in the silica layer.
132. The particles of claim 131, wherein the silica outer layer is
etched out.
133. The particles of claim 132, wherein the etching is done by HF
solution.
134. The particles of claim 132, wherein the etching is done by
NaOH solution.
135. The particles of claim 131, wherein the particles are prepared
by an aerosol-based method.
136. The particles of claim 135, wherein the aerosol-based method
comprises the steps of: a) preparing a precursor solution
comprising a silica source, a titania source, ferric chloride and a
surfactant; b) passing the precursor solution through a nozzle for
atomization to form aerosol droplets; c) passing the aerosol
droplets through a heating zone and a drying zone for evaporation,
wherein the ferric chloride binds with the surfactant in the
interior of the droplet and the silica source is on the surface of
the droplet; d) collecting the particles on a filter; and e)
passing the particles under the flow of nitrogen gas for
pyrolysis.
137. The particles of claim 136, wherein the silica source is
tetraethyl orthosilicate (TEOS).
138. The particles of claim 136, wherein the titania source is
titanium isopropoxide.
139. The particles of claim 136, wherein the surfactant is from the
group consisting of cetyltrimethyl ammonium bromide (CTAB),
cetyltrimethyl ammonium chloride (CTAC), cetyltrimethyl ammonium
iodide (CTAI), cetyltrimethyl ammonium fluoride (CTAF), and
cetyltrimethyl ammonium astatide (CTAA), and mixtures thereof.
140. The particles of claim 136, wherein the surfactant is
cetyltrimethyl ammonium bromide (CTAB).
141. The particles of claim 131 or 132, wherein the particles are
used for photocatalysis.
142. The particles of claim 131 or 132, wherein the particles are
used in oil spill mitigation technologies.
143. A method of making hollow shelled particles, comprising: a)
forming a silica shell due to a silica condensation reaction along
a gas-liquid interface of an aerosol droplet; and b) forming an
iron-surfactant rich core by coagulation of ferric species in the
presence of a surfactant.
144. A method of producing thin shelled cage-like particles by an
aerosol-based method, comprising the steps of: a) preparing a
precursor solution comprising (1) a ceramic source, (2) a carbon
source, (3) an iron source, and (4) a surfactant; b) passing the
precursor solution through a nozzle for atomization to form aerosol
droplets; c) passing the aerosol droplets through a heating zone
and a drying zone for evaporation, wherein the iron source binds
with the surfactant in the interior of the droplet and the ceramic
source is on the surface of the droplet; d) collecting the
particles on a filter; and e) passing the particles for
calcination.
145. The method of claim 144, wherein the ceramic is from the group
consisting of silica, titania, zirconia, alumina, yttria, ceria,
and mixtures thereof, and silica in combination with titania,
zirconia, alumina, yttria, or ceria, and mixtures thereof, and the
ceramic source from the group consisting of a source of silica,
titania, zirconia, alumina, yttria, ceria, and mixtures
thereof.
146. The method of claim 144, wherein the iron source is from the
group consisting of ferric halides, and mixtures thereof.
147. The method of claim 144, wherein the inert gas is from the
group consisting of nitrogen, argon, and mixtures thereof.
148. The method of claim 144, wherein the ceramic source is
tetraethyl orthosilicate (TEOS).
149. The method of claim 144, wherein the carbon source is a
monosaccharide or polysaccharide.
150. The method of claim 144, wherein the carbon source is from the
group consisting of sucrose, glucose, cellulose, and cyclodextrins,
and mixtures thereof.
151. The method of claim 150, wherein the carbon source is
sucrose.
152. The method of claim 144, wherein the surfactant is from the
group consisting of cetyltrimethyl ammonium bromide (CTAB),
cetyltrimethyl ammonium chloride (CTAC), cetyltrimethyl ammonium
iodide (CTAI), cetyltrimethyl ammonium fluoride (CTAF), and
cetyltrimethyl ammonium astatide (CTAA), and mixtures thereof.
153. The method of claim 144, wherein the surfactant is
cetyltrimethyl ammonium bromide (CTAB).
154. The method of claim 144, wherein the thickness of outer shell
is 5 nm to 20 nm.
155. A method of producing hollow double shelled particles with an
outer layer of ceramic from the group consisting of silica,
titania, zirconia, alumina, yttria, ceria, and mixtures thereof,
and silica in combination with titania, zirconia, alumina, yttria,
or ceria, and mixtures thereof, and an inner layer of carbon by an
aerosol-based method, comprising the steps of: a) preparing a
precursor solution comprising (1) a ceramic source from the group
consisting of a source of silica, titania, zirconia, alumina,
yttria, ceria, and mixtures thereof, (2) a carbon source, (3) a
source of magnetic metal, such as an iron source from the group
consisting of ferric halides, and mixtures thereof, and (4) a
surfactant; b) passing the precursor solution through a nozzle for
atomization to form aerosol droplets; c) passing the aerosol
droplets through a heating zone and a drying zone for evaporation,
wherein the magnetic metal source binds with the surfactant in the
interior of the droplet and the ceramic source is on the surface of
the droplet; d) collecting the particles on a filter; and e)
passing the particles for pyrolysis under the flow of an inert gas
from the group consisting of nitrogen, argon, and mixtures thereof,
wherein the carbon source located in the interior of the particle
generates an inner layer from the inside of the particle.
156. The method of claim 155, wherein the ceramic source is
tetraethyl orthosilicate (TEOS).
157. The method of claim 155, wherein the carbon source is a
monosaccharide or polysaccharide.
158. The method of claim 155, wherein the carbon source is from the
group consisting of sucrose, glucose, cellulose, and cyclodextrins,
and mixtures thereof.
159. The method of claim 158, wherein the carbon source is
sucrose.
160. The method of claim 155, wherein the surfactant is from the
group consisting of cetyltrimethyl ammonium bromide (CTAB),
cetyltrimethyl ammonium chloride (CTAC), cetyltrimethyl ammonium
iodide (CTAI), cetyltrimethyl ammonium fluoride (CTAF), and
cetyltrimethyl ammonium astatide (CTAA), and mixtures thereof.
161. The method of claim 155, wherein the surfactant is
cetyltrimethyl ammonium bromide (CTAB).
162. The method of claim 155, wherein 0.8 g-1.9 g of the iron
source is added, 0.1 g-2.2 g of the surfactant is added, 1.0 mL-9
mL of the ceramic source is added, and 0.01 g-3 g of the carbon
source is added.
163. The method of claim 155, wherein 0.95 g of the iron source is
added, 1.1 g of the surfactant is added, 4.5 mL of the ceramic
source is added, and 1.0 g of the carbon source is added.
164. The method of claim 155, wherein silica condensation and
sucrose dehydration occur in step "c".
165. The method of claim 155, wherein the particles are 50 nm to
5000 nm in diameter.
166. The method of claim 155, wherein the particles are 100 nm to
1000 nm in diameter.
167. The method of claim 155, wherein the outer layer is 5 nm to
100 nm thick.
168. The method of claim 155, wherein the inner layer is 5 nm to
100 nm thick.
169. The method of claim 155, wherein the inner layer is 50 nm to
5000 nm in diameter.
170. The method of claim 155, wherein the inner layer is 100 nm to
1000 nm in diameter.
171. The method of claim 155, wherein the outer layer is
hydrophilic.
172. The method of claim 155, wherein the inner layer is
hydrophobic.
173. The method of claim 155, wherein the outer layer is nonporous
after step "e".
174. The method of claim 155, wherein during step "e" the carbon
source forms said inner layer adjoining the silica outer layer and
leaving a fully hollow interior.
175. The method of claim 155, further comprising the step of
etching out the outer silica layer.
176. The method of claim 175, wherein the etching is done by a
highly acidic solution.
177. The method of claim 176, wherein the highly acidic solution is
from the group consisting of HE HCl, and sulfuric acid.
178. The method of claim 175, wherein the etching is done by a
highly basic solution.
179. The method of claim 178, wherein the highly basic solution is
from the group consisting of NaOH and ammonium hydroxide.
180. The method of claim 155, further comprising the step of
removing the inner carbon layer by calcination.
181. The method of claim 155, wherein the precursor solution
further comprises a second metal.
182. The method of claim 181, wherein the second metal is from the
group consisting of tin, copper, palladium, chromium, zinc,
rhodium, ruthenium, molybdenum, manganese, nickel, and aluminum.
Description
CROSS-REFERENCES TO RELATED APPLICATIONS
[0001] This is a non provisional patent application of U.S.
Provisional Patent Application Ser. No. 61/599,788, filed 16 Feb.
2012; U.S. Provisional Patent Application Ser. No. 61/610,798,
filed 14 Mar. 2012; and U.S. Provisional Patent Application Ser.
No. 61/621,642, filed 9 Apr. 2012.
[0002] Priority of U.S. Provisional Patent Application Ser. No.
61/599,788, filed 16 Feb. 2012; U.S. Provisional Patent Application
Ser. No. 61/610,798, filed 14 Mar. 2012; and U.S. Provisional
Patent Application Ser. No. 61/621,642, filed 9 Apr. 2012, each of
which is hereby incorporated herein by reference, is hereby
claimed.
COMPACT DISK SUBMISSION
[0004] Not Applicable.
BACKGROUND OF INVENTION
[0005] 1. Field of the Invention
[0006] The present invention relates to hollow nanoparticles. More
particularly, the present invention relates to methods for
fabricating double layer hollow nanoparticles.
[0007] The discovery of mesoporous silica templated by surfactants
has led to tremendous interest in developing new classes of porous
materials. It is now well-known now that the introduction of a
templating surfactant such as cetyl trimethylammonium bromide
(CTAB) into a solution containing a silica precursor such as
tetraethyl orthosilicate (TEOS) leads to the formation of ordered
mesoporous silica. Of significance is the rapid synthesis of such
materials by the aerosol based method of making porous materials by
incorporating surfactants into precursor solutions of a silica
precursor. The aerosol method is advantageous because it is
continuous, effective and economical. A very recent and
comprehensive review (X. W. Lou, Lynden A. Archer, and Zichao Yang,
Hollow Micro-/Nanostructures: Synthesis and Applications, Adv.
Mater. 2008, 20, 3987-4019) describes the versatility and use of
the process to produce functional inorganic materials.
[0008] This invention discloses the use of similar methods to
produce unexpected non-mesoporous materials.
[0009] 2. Description of Related Art
[0010] Hollow particles are of considerable interest because of
their wide range of applications in encapsulation, catalysis,
biomolecule separation, controlled drug release, and sensor
technologies/biosensors. Typically, the preparation of such hollow
particles requires building a desirable material layer around a
core, followed by removal of the core by dissolution or high
temperature calcination. These synthesis approaches generally
involve multistep operations and complex components, leading to
difficulties in scale up to commercially viable quantities.
[0011] The present invention discloses a scalable, rapid
aerosol-based process for fabricating hollow submicrometer
particles with novel morphology, one where the shell is made up of
two thin layers: an outer layer of silica (or other ceramic) and an
inner layer of carbon. The exposed surfaces therefore have
contrasting physical characteristics, with the outer surface being
hydrophilic and the inner surface being hydrophobic. Additionally,
the particles contain iron nanoparticles, making them magnetically
responsive.
[0012] Double-shelled hollow particles have recently been pioneered
through template-based methods to form SnO.sub.2 layers for lithium
storage enhancements, and these advances point to important
applications of such materials.
[0013] A recent and comprehensive review of the aerosol process
describes the versatility and use of the process to produce
functional inorganic materials, and we refer the readers to this
article for an excellent background of the process (See Boissiere,
C.; Grosso, D.; Chaumonnot, A; Nicole, L.; Sanchez, C. Aerosol
route to functional nanostructured inorganic and hybrid porous
materials. Adv. Mater. 2011, 23, 599-623). The concept behind the
present work is an important aspect of the aerosol process that has
hitherto not been explored. It is well known now that the
introduction of a templating surfactant such as cetyl
trimethylammonium bromide (CTAB) into a solution containing a
silica precursor such as tetraethyl orthosilicate (TEOS) leads to
the formation of ordered mesoporous silica. In our recent work,
however, we have surprisingly found that the inclusion of ferric
chloride into the precursor solution completely negates the
templating effect. Rather, the inclusion of the ferric salt leads
to a binding of the CTAB and a phase segregation where the iron
salt and CTAB become occluded within the interior of a rapidly
forming shell of silica during the passage of the aerosol droplets
through the heating zone of a tube furnace. Subsequent calcination
of these particles leads to the burnoff of the organic surfactant
species, leaving behind hollow silica particles containing magnetic
iron oxides. The present invention is based on a new extension of
this concept. If a rapid shell of silica is formed, can this shell
act as a seal to prevent the escape of material from the interior
of the particle? Specifically, if carbon precursors (sucrose) are
introduced into the precursor solution, can the carbonization of
sucrose be conducted in the interior of such thin-shelled silica
particles? The first part of the schematic in FIG. 4 (I and II)
illustrates the concept of aerosolization and the incorporation of
carbon into the interior of the particle. The latter parts of FIG.
4 are related to the results that are described herein.
[0014] Various other methods have been developed to synthesize
single layer hollow nanoparticles, including hard template, soft
template, dual template, Ostwald ripening, as well as Kerkendall
effect, but these preparation schemes generally involve multistep
operations, complex components, and hence are less economical.
[0015] One-step aerosol-assisted process is an efficient approach
to prepare single layer hollow nanoparticles. However, to our
knowledge fabrication of double layer nanoparticles through a
simple and effective aerosol assisted process have never been
reported before.
[0016] Incorporated herein by reference are the following
references: [0017] X. W. Lou, Lynden A. Archer, and Zichao Yang,
Hollow Micro-/Nanostructures: Synthesis and Applications, Adv.
Mater. 2008, 20, 3987-4019. [0018] Hu Wang, Jin-Gui Wang, Hui-Jing
Zhou, Yu-Ping Liu, Ping-Chuan Sun and Tie-Hong Chen, Facile
fabrication of noble metal nanoparticles encapsulated in hollow
silica with radially oriented mesopores: multiple roles of the
N-lauroylsarcosine sodium surfactant, Chem. Commun., 2011, 47,
7680-7682. [0019] Yinqquin Wang, Bhanukiran Sunkara, Jinjing Zhan,
Jibao He, Ludi Miao, Gary L. McPherson, Vijay T. John, and Leonard
Spinu, Synthesis of Submicrometer Hollow Particles with Nanoscale
Double-Layer Shell Structure, Langmuir 2012, 28, 13783-13787.
BRIEF SUMMARY OF THE INVENTION
[0020] The present invention provides bilayer hollow nanoparticles
and a method of making the same.
[0021] In a preferred embodiment, a double layered nanoparticle is
fabricated in a one-step aerosol-assisted synthesis method. In one
embodiment, the outer layer is silica and the inner layer is
carbon.
[0022] In another embodiment of the present invention, an outer
silica layer of a bilayer nanoparticle may be etched away to
fabricate hollow carbon spheres. In one embodiment a hollow sphere
may encapsulate a substance. In yet another embodiment, the
substance encapsulated may be a pharmaceutical compound.
[0023] In another embodiment of the present invention an inner
carbon layer of hollow bilayer nanoparticles may be burnt away to
fabricate silica spheres.
[0024] Another embodiment of the present invention may be to
manufacture hollow bilayer nanoparticles with magnetic
nanoparticles. In one embodiment, the magnetic nanoparticles may be
iron. In addition to iron, it is possible to insert a variety of
other metallic nanoparticles (tin, copper, palladium, chromium,
zinc, rhodium, ruthenium, molybdenum--the whole series of
transition metal oxides). In another embodiment, the magnetic
nanoparticles may be used for drug delivery.
[0025] In another embodiment, bilayer particles may be used as
amphiphilic particles to stabilize emulsions. In one embodiment,
bilayer particles may be used in Pickering emulsions.
[0026] In another embodiment, bilayer particles may be used as
catalytic materials. In one embodiment carbon and silica within
bilayer particles may function as supports for catalytic
materials.
[0027] In accordance with this invention, it is an object of this
invention to incorporate iron oxide into the shell of the bilayer
structure to make it magnetically responsive. The inner void allows
entrapment of a high concentration of a drug agent which may be
magnetically guided for targeted drug delivery.
[0028] In accordance with this invention, it is an additional
object to etch away the silica layer to make hollow carbon spheres
with applications to fuel cell technologies as electrode for fuel
cells, for using carbon as a catalyst.
[0029] In accordance with this invention, it is an additional
object to burn away carbon to make hollow silica spheres with
application in drug delivery and as catalyst supports.
[0030] Some embodiments of the invention include eggshell type
nanoparticles that are particles with an extremely thin outer layer
that can crack or break upon a slight impact or ultrasonication. In
some embodiments these eggshell particles may have a shell of 10-15
nm (though even a shell as thin as 5-7 nm and up to 20 nm thick can
be useful). Some embodiments include methods of producing said
eggshell particles comprising sending a precursor solution
comprising a surfactant, a silica precursor, and a metal precursor
such as a metal salt, through a heating zone. Some embodiments
comprise a precursor solution with less silica precursor than a 1
to 8 ratio of metal salt to silica precursor.
[0031] Some embodiments of the invention include bilayer
nanoparticles with protuberances referred to here as "nanohorns."
In some embodiments, there may be at least one nanohorn with the
nanoparticle comprising an outer layer of silica and an inner layer
of carbon. Some embodiments include methods of producing said
nanoparticles with nanohorns comprising sending a precursor
solution comprised of a surfactant, a silica precursor, a metal
precursor such as a metal salt, and a carbon precursor through a
heating zone, and then pyrolizing the particles. Some embodiments
may further include calcination of the particles to remove the
carbon layer, or etching of the particles to remove the silica
layer.
[0032] Some embodiments of the invention include nanoparticles made
with metal based precursors in place of a carbon based precursor.
Some embodiments may use a titania precursor, such as titanium
isopropoxide in place of a carbon precursor. In some embodiments,
the silica may be etched away leaving titania nanospheres. In some
embodiments, light is expected to penetrate the titania
nanospheres.
[0033] Hollow nano and microparticles have a variety of
applications in encapsulation, catalysis, energy storage, chemical
sensing and controlled drug release. Typically they are prepared by
forming a layer of the desired materials over a template which is
then selectively removed by dissolution or burn-off to create a
hollow core. In the present invention, a new method for
manufacturing ceramic particles where a shell is created extremely
rapidly, locking in chemical constituents in the interior. This is
done using an aerosol based process where we have exploited salt
bridging concepts to lock a surfactant (CTAB) and carbon precursors
together with iron oxides in the interior of a droplet while a
silica shell is allowed to form on the droplet surface. Subsequent
pyrolysis results in a buildup of internal pressure forcing carbon
formation as a second layer attached to the silica shell. Thus we
have developed bilayer "amphiphilic" ceramic particles with a
hollow interior. This new assembly method is expected to be a
general approach to fabricate various hybrid double layer hollow
particles with unique potential properties. In addition, the
incorporation of magnetic iron oxide into the shells opens up
opportunities in external stimuli responsive materials.
[0034] The present invention describes novel nanoparticles and
methods of producing the same. The novel aspects of the present
invention are the following: (1) the iron chloride ties up the
surfactant (e.g., CTAB) so that the silica cannot grow inwards from
the surface of the drop--this is why hollow particles are
generated; and (2) when the carbon precursor is also enclosed in
the interior, the pressure build up leads to the second shell being
generated from the inside (as opposed to building shells from the
outside through a layer-by-layer method). These are important
differences from prior art and lead to the ability to be able to
generate large quantities of hollow and double shelled
particles.
BRIEF DESCRIPTION OF DRAWINGS
[0035] The following drawings form part of the present
specification and are included to further demonstrate certain
aspects of the present invention. The invention may be better
understood by reference to one or more of these drawings in
combination with the description of specific embodiments presented
herein.
[0036] FIG. 1 illustrates an exemplary schematic of an aerosol
based process to make submicron particles.
[0037] FIG. 2 illustrates exemplary TEM images of calcined
particles synthesized with 1.1 g CTAB and 0 g (a), 0.15 g (b), 0.8
g (c) and 0.9 g (d) of FeCl.sub.3. The scale bars are 20 nm in all
the images.
[0038] FIG. 3 illustrates one example of the carbon source.
[0039] FIG. 4 illustrates an exemplary method for the formation of
silica/carbon double layer hollow particles.
[0040] FIG. 5 illustrates an exemplary mechanism for formation of
silica hollow sphere and encapsulation of iron oxide inside the
nanoparticle sphere. The preferential partitioning of CTAB to the
ferric species leads to depletion of CTAB from silicate regions and
a segregation of dense silica as the shell and the ferric species
in the core.
[0041] FIG. 6 shows representative electron micrographs of hollow
nanoparticles with double silica/carbon layer shell: (a) TEM image
of as-synthesized nanoparticles; (b) TEM image of pyrolyzed
nanoparticles at low magnification; (c) TEM image of pyrolyzed
nanoparticles at high magnification; (d) High resolution TEM image
of a pyrolyzed nanoparticle; (e) Cross section TEM image of a
pyrolyzed nanoparticle; (f) SEM image of a pyrolyzed nanoparticle
at high magnification; (g) Cross section SEM image of a pyrolyzed
nanoparticle with partial removal of the outer layer; (h) Cross
section SEM image of a pyrolyzed nanoparticle.
[0042] FIG. 7 shows representative electron micrographs of hollow
particles with a double silica/carbon layer shell: (a) TEM image of
as-synthesized particles; (b) TEM image of pyrolyzed particles at
low magnification showing a wide range of particle sizes; (c) TEM
image of a pyrolyzed particle at high magnification; (d) HRTEM
image of a pyrolyzed particle showing the two layers; (e) Cross
section TEM image of a pyrolyzed particle with a detached carbon
layer; (f) SEM image of a pyrolyzed particle; (g) Cross section SEM
image of a pyrolyzed particle where sectioning leaves an intact
inner shell. (h) Cross section SEM image of a pyrolyzed particle
where sectioning cuts across both shells to reveal the hollow
interior.
[0043] FIG. 8 shows representative electron micrographs of hollow
microspheres with double layer silica/carbon shell: (a)-(b), TEM
images of pyrolyzed microspheres; (c) Cross section TEM image of a
pyrolyzed microsphere; (d)-(e), SEM images of pyrolyzed
microspheres; (f)-(g), Cross section SEM images of pyrolyzed
microspheres.
[0044] FIG. 9 shows further representative electron micrographs of
hollow microspheres with double layer silica/carbon shell: (a)-(b),
TEM images of calcined microspheres; (c)-(d), SEM images of
calcined microspheres; (e)-(f), Cross section SEM images of
calcined microspheres.
[0045] FIG. 10 shows further representative electron micrographs of
hollow microspheres with double layer silica/carbon shell: (a)-(b),
TEM images of etched microspheres; (c)-(d), SEM images of etched
microspheres; (e)-(f), Cross section SEM images of etched
microspheres.
[0046] FIG. 11 illustrates energy dispersive spectroscopy (EDS) of
exemplary double layer pyrolyzed hollow nanoparticles: (a) the
carbon layer of cross section nanoparticles; (b) the silica layer
of cross section nanoparticles.
[0047] FIG. 12 illustrates a vibrating sample magnetometry
hysteresis loop of exemplary double silica carbon layer hollow
nanoparticles.
[0048] FIG. 13 shows representative electron micrographs of hollow
particles with single silica layer shell: (a) TEM image of calcined
particles at low magnification; (b) TEM image of a calcined
particle at high magnification; (c) HRTEM image of a calcined
particle; (d) Cross section TEM image of a pyrolyzed particle,
locations of iron species (black dots) indicated by the arrows; (e)
HRTEM image of iron species (small white box in panel d shows the
location) on hollow particle inner surface; (f) SEM image of a
calcined particle; (g) Cross section SEM image of a hollow calcined
particle showing a crack-like interior surface.
[0049] FIG. 14 shows representative electron micrographs of hollow
nanoparticles with single silica layer shell: (a) TEM image of
calcined nanoparticles at low magnification; (b) TEM image of a
calcined nanoparticle at high magnification; (c) High resolution
TEM image of a calcined nanoparticle; (d) Cross section TEM image
of a pyrolyzed nanoparticle; (e) SEM image of a calcined
nanoparticle; (f) Cross section SEM image of a calcined
nanoparticle.
[0050] FIG. 15 illustrates EDS of calcined hollow particles: the
cross section TEM images show the locations where EDS is acquired.
(a) the circle focuses on a small particles represented by the tiny
black dots (b) the circle focuses on the silica matrix.
[0051] FIG. 16 illustrates representative XRD patterns of
nanoparticles after different treatments: solid line identified as
A for double layer nanoparticles without calcinations; solid line
identified as B for nanoparticles treated with calcination at
400.degree. C.; solid line identified as C for nanoparticles
treated with calcination 500.degree. C. (the XRD pattern can be
indexed as Hematite (.alpha.-Fe.sub.2O.sub.3).
[0052] FIG. 17 shows representative electron micrographs of hollow
nanoparticles with single carbon layer shell: (a) TEM image of an
etched nanoparticles at low magnification; (b) TEM image of an
etched nanoparticle at high magnification; (c) High resolution TEM
image of an etched nanoparticle; (d) Cross section TEM of an etched
nanoparticle; (e) SEM image of an etched nanoparticle at high
magnification; (f) Cross section SEM image of an etched
nanoparticle at low magnification.
[0053] FIG. 18 shows representative electron micrographs of hollow
particles with a single carbon layer shell: (a) TEM image of etched
particles at low magnification; (b) TEM image of an etched particle
at high magnification; (c) HRTEM image of an etched particle; (d)
Cross section TEM of an etched particle; (e) SEM image of an etched
particle; (f) Cross section SEM image of an etched particle.
[0054] FIG. 19 illustrates EDS of etched hollow particles. Inserted
cross section TEM image shows the location (black dot) where EDS is
acquired.
[0055] FIG. 20 illustrates XRD patterns (a) double layer particles
pyrolyzed at 500.degree. C. for 3 h; (b) particles calcined at
500.degree. C. for 3 h and additional 1000.degree. C. for 2 h; (c)
calcined particles are further reduced at 400.degree. C. for 2 h
under the flow of H.sub.2/N.sub.2 (9% H.sub.2).
[0056] FIG. 21 illustrates nitrogen adsorption-desorption
isotherms: (a) pyrolyzed hollow particles (black solid dots
(.cndot.), BET surface area 12.5 m.sup.2/g, the corresponding BJH
desorption pore volume 0.0279 cm.sup.3/g); (b) etched hollow
particles (black solid diamonds (.diamond-solid.), BET surface area
104 m.sup.2/g, the corresponding BJH desorption pore volume 0.162
cm.sup.3/g); (c) calcined hollow particles (white hollow circles
(.smallcircle.), BET surface area 180 m.sup.2/g, the corresponding
BJH desorption pore volume 0.112 cm.sup.3/g).
[0057] FIG. 22 shows representative electron micrographs of
calcined hollow silica microspheres: (a) TEM image at Fe:Si molar
ratio of 1:13; (b) TEM image at Fe:Si molar ratio of 1:8; (c) TEM
image at Fe:Si molar ratio of 1:6; (d) TEM image at Fe:Si molar
ratio of 1:2.7.
[0058] FIG. 23 shows representative electron micrographs of
calcined hollow silica microspheres with Fe:Si molar ratio of 1:2.7
after ultrasonication treatment: (a) SEM image at low
magnification; (b) SEM image at high magnification for a
microsphere; (c) TEM image of a microsphere.
[0059] FIG. 24 shows exemplary nanoparticles with protrusions where
increasing the sucrose concentration leads to the development of
protrusions in the particles
[0060] FIG. 25 shows exemplary silica-titanate hollow spheres.
[0061] FIG. 26 shows representative TEM images of single titania
layer hollow microspheres after etching and calcination treatment:
(a) at low magnification; (b) HR TEM of spindle area.
[0062] FIG. 27 shows an illustration that the addition of
FeCl.sub.3 leads to very weakly crystalline silicas and the
negation of the templating effect of CTAB.
[0063] FIG. 28 shows an exemplary illustration of the transition
from spherical CTAB micelles to long wormlike micelles upon
incorporation of interfacially active phenols.
[0064] FIG. 29 illustrates representative electron micrographs of
calcined hollow silica microspheres: (a) low magnification TEM
image at Fe:Si molar ratio of 1:13; (b) low magnification TEM image
at Fe:Si molar ratio of 1:8; (c) low magnification TEM image at
Fe:Si molar ratio of 1:6; (d) low magnification TEM image at Fe:Si
molar ratio of 1:2.7; (e) TEM image of an as-synthesized
microsphere at Fe:Si molar ratio of 1:2.7; (f) HRTEM of a calcined
microsphere at Fe:Si molar ratio of 1:2.7.
[0065] FIG. 30 illustrates representative electron micrographs of
calcined hollow silica microspheres: (a) TEM image at Fe:Si molar
ratio of 1:13; (b) TEM image at Fe:Si molar ratio of 1:8; (c) TEM
image at Fe:Si molar ratio of 1:6; (d) TEM image at Fe:Si molar
ratio of 1:2.7.
[0066] FIG. 31 shows SEM images of calcined microspheres before
ultrasonic treatment with Fe:Si molar ratio of (a) 1:13, (b) 1:8,
(c) 1:6 and (d) 1:2.7; SEM images of calcined microspheres after
ultrasonic treatment with Fe:Si molar ratio of (e) 1:13, (f) 1:8,
(g) 1:6 and (h) 1:2.7.
[0067] FIG. 32 illustrates representative electron micrographs of
calcined hollow silica particles after ultrasonication treatment:
(a) SEM image at low magnification with Fe:Si molar ratio of 1:13;
(b) SEM image at high magnification with Fe:Si molar ratio of 1:13;
(c) TEM image of a calcined particle with Fe:Si molar ratio of
1:13; (d) SEM image at low magnification with Fe:Si molar ratio of
1:2.7; (e) SEM image at high magnification with Fe:Si molar ratio
of 1:2.7; (0 TEM image of a microsphere with Fe:Si molar ratio of
1:2.7.
[0068] FIG. 33 illustrates representative TEM of calcined hollow
silica microspheres: (a) Na:Fe molar ratio of 0.6:1 washed with DI
water; (b) Na:Fe molar ratio of 1:1 washed with DI water; (c) cut
section TEM at low magnification; (d) cut section TEM at high
magnification.
[0069] FIG. 34 illustrates representative SEM of calcined silica
microspheres: (a) Na:Fe molar ratio of 2:1 washed with water; (b)
Na:Fe molar ratio of 2:1 washed with water; (c) high resolution
SEM.
[0070] FIG. 35 shows Nitrogen adsorption-desorption isotherms: (a)
silica particles without NaCl loading (black solid triangles, BET
surface area 13.8 m.sup.2/g, the corresponding BJH desorption pore
volume 0.043 cm.sup.3/g); (b) the silica particles with NaCl after
washing (black diamonds, BET surface area 33.3 m.sup.2/g, the
corresponding BJH desorption pore volume 0.127 cm.sup.3/g). (Na:Fe
molar ratio is 2:1)
[0071] FIG. 36 shows Fluorescent micrographs of silica microspheres
(a) Bright field image of silica microspheres without NaCl; (b)
Fluorescence image of silica microspheres without NaCl; (c) Bright
field image of silica microspheres with 0.4 g NaCl; (d)
Fluorescence image of silica microspheres with 0.4 g NaCl.
[0072] FIG. 37 illustrates magnetic hysteresis loops of pyrolyzed
particles (sample A), etched particles (sample B) and particles
reduced after calcination to form magnetitie (sample C).
[0073] FIG. 38 illustrates wide open highly porous titania.
[0074] FIG. 39 shows single layer hollow silica particles washed
with deionized water, wherein adding salt (NaCl) in the precursor
makes the shells porous. Molar ratio of NaCl/FeCl.sub.3=1/1.
[0075] FIG. 40 shows nitrogen adsorption-desorption isotherm with a
precursor of 2 ml TEOS and no sucrose after calcination.
[0076] FIG. 41(a)-(b) shows TEM images of particles with a
precursor of 2 ml TEOS and 0.5 g sucrose after calcination.
[0077] FIG. 42 shows nitrogen adsorption-desorption isotherm with a
precursor of 2 ml TEOS and 0.5 g sucrose after calcination.
[0078] FIG. 43(a)-(b) shows TEM images of particles with a
precursor of 1 ml TEOS and 0.5 g sucrose after calcination.
[0079] FIG. 44 shows nitrogen adsorption-desorption isotherm with a
precursor of 1 ml TEOS and 0.5 g sucrose after calcination.
[0080] FIG. 45(a)-(b) shows TEM images of particles with a
precursor of 0.75 ml TEOS and 0.5 g sucrose after calcination.
[0081] FIG. 46 shows SEM images of particles with a precursor of
0.75 ml TEOS and 0.5 g sucrose after calcination.
[0082] FIG. 47 shows nitrogen adsorption-desorption isotherm with a
precursor of 0.75 ml TEOS and 0.5 g sucrose after calcination.
[0083] FIG. 48(a)-(d) shows TEM images of particles with a
precursor of 1 ml TEOS and 0.75 g sucrose. FIGS. 48(a) and (b) are
images of aerosolized particles. FIGS. 48(c) and (d) are images of
calcined particles.
[0084] FIG. 49(a)-(d) shows TEM images of particles with a
precursor of 1 ml TEOS and 1 g sucrose. FIGS. 49(a) and (b) are
images of aerosolized particles. FIGS. 49(c) and (d) are images of
calcined particles.
[0085] FIG. 50(a)-(c) shows SEM images of aerosolized particles
with a precursor of 0.75 ml TEOS and 1 g sucrose.
[0086] FIG. 51(a)-(b) shows TEM images of aerosolized particles
with a precursor of 0.75 ml TEOS and 1 g sucrose.
[0087] FIG. 52(a)-(b) shows cut section-TEM images of aerosolized
particles with a precursor of 0.75 ml TEOS and 1 g sucrose.
[0088] FIG. 53(a)-(b) shows SEM images of calcined particles with a
precursor of 0.75 ml TEOS and 1 g sucrose.
[0089] FIG. 54(a)-(b) shows TEM images of calcined particles with a
precursor of 0.75 ml TEOS and 1 g sucrose.
[0090] FIGS. 55(a)-(b) shows cut section-TEM images of calcined
particles with a precursor of 0.75 ml TEOS and 1 g sucrose.
DETAILED DESCRIPTION OF THE INVENTION
[0091] Detailed descriptions of one or more preferred embodiments
are provided herein. It is to be understood, however, that the
present invention may be embodied in various forms. Therefore,
specific details disclosed herein are not to be interpreted as
limiting, but rather as a basis for the claims and as a
representative basis for teaching one skilled in the art to employ
the present invention in any appropriate manner.
[0092] The hollow particles of the claimed invention can be made of
(a) silica; (b) silica-carbon double shelled; (c) silica-titania
mixed shell; or (d) silica-titania-carbon with the outer shell
being silica-titania and the inner shell being carbon. The silica
in these particles can be etched out leaving (a) carbon hollow
particles; (b) titania shelled hollow particles; or (c)
titania-carbon particles.
[0093] All these hollow particles can be made to contain iron
nanoparticles. In addition to iron, it is possible to also insert a
variety of other metallic nanoparticles (tin, copper, palladium,
chromium, zinc, rhodium, ruthenium, molybdenum, manganese, nickel,
aluminum--in fact the whole series of transition metal oxides). It
is noted that these particles are in addition to iron; for example,
the nanoparticles within the hollow particles are either (a) iron;
or (b) iron plus a second metal. The second metal can be inserted
into the hollow particles through multiple pathways, such as for
example, (a) it can be added to the precursor solution prior to
aerosolization and thus gets incorporated into the hollow
particles; or (b) it can be allowed to diffuse using the metal salt
through the pores of premade hollow particles. Tin (Sn) is
especially important as it can be used effectively for Li-ion
batteries. We also note that the metal inside the hollow particles
is loose and not attached to the shell. This qualifies denoting the
particles as "rattle" type particles.
[0094] FIG. 1 illustrates an aerosol based process to make
submicron particles. In one embodiment, the precursor solution may
be aerosolized through a nozzle and the drops may pass through a
furnace where solvent evaporation may occur together with chemical
reaction. This so called "chemical reaction in an evaporating
droplet" may result in small particles which may be collected on a
filter. The process is easy to scale up to a semicontinuous
production method where the product from the furnace can be
directed towards a second filter set up when the first one is
disconnected for particulate retrieval.
[0095] Normally, this process results in mesoporous particles,
however, in one embodiment as FIG. 2 illustrates, increasing the
amount of iron salt (FeCl.sub.3) in solution unexpectedly leads to
a loss of order in the mesoporous material and finally the
formation of hollow particles. Embodiments describes here include a
very rapid method of producing hollow particles, which may be used
in encapsulation, catalysis, biomolecule separation, controlled
drug release, and biosensor technologies. Typically the preparation
of such hollow particles requires building a layer around a core
followed by removing the core by dissolution or high temperature.
Such schemes generally contain multistep operations and complex
components, involving difficulties in scale up to commercially
viable quantities. Developing a rapid technique to make such
particles in reasonably large quantities would therefore be of
technological significance.
[0096] As illustrated in FIG. 3, double layer hollow nanoparticles
may be prepared using tetraethyl orthosilicate (TEOS) as a silica
source and a monosaccharide or polysaccharide, such as sucrose
(most preferable), glucose, cellulose, or cyclodextrins as a carbon
source in the presence of ferric chloride (or other iron salt such
as other ferric halides, or mixtures of iron salts) and surfactant,
such as cetyltrimethyl ammonium bromide (CTAB, a directing agent),
cetyltrimethyl ammonium chloride (CTAC) or other CTA-halides, such
as cetyltrimethyl ammonium iodide (CTAI), cetyltrimethyl ammonium
fluoride (CTAF), cetyltrimethyl ammonium astatide (CTAA). Also,
instead of the silica source, zirconia, alumina or titania can be
used. The precursor solution may be initially atomized to form
aerosol droplets, which may then be sent through a drying zone and
heating zone where preliminary solvent evaporation, silica
condensation and sucrose dehydration may occur.
[0097] Typically, CTAB templates highly ordered hexagonal
mesoporous silica through the aerosol process. However FeCl.sub.3
in the precursor solution disrupts the formation of mesoporous
silica due to preferential partitioning of the surfactant CTAB on
ferric species, therefore the dense, low porous silica layer may be
formed during the aerosol process. The iron chloride ties up the
surfactant CTAB so that the silica cannot grow inwards from the
surface of the drop--this is why hollow particles are
generated.
[0098] As depicted in the schematic of FIG. 5, the hollow particle
formation mechanism may involve two key and novel processes: 1) the
formation of the silica shell due to the preferred silica
condensation reaction along the gas-liquid interface of an aerosol
droplet; and 2) the formation of an iron-surfactant rich core by
the coagulation of ferric species in the presence of the
surfactant. Upon mixing FeCl.sub.3, TEOS and CTAB in an acidic
ethanol/water solution, the hydrolysis of FeCl.sub.3 generates
ferric colloids that are stabilized by the electrical double layers
of chloride anions. The added CTAB, however, serves as an
indifferent electrolyte, promoting ferric colloid coagulation
visualized by a translucent solution appearance. The coagulation
effect is further supported by the fact that increased CTAB loading
leads to larger CTAB/Fe cores and thinner silica shells. Higher
indifferent electrolyte concentration in the precursor promotes the
coagulation of ferric colloids, resulting in larger CTAB/Fe
colloids. Subsequent atomization of the mixture generates aerosol
droplets containing silicates, CTAB, the coagulated ferric species,
ethanol and water. Solvent evaporation enriches these nonvolatile
components and promotes faster silicate condensation along the
gas/liquid interface, resulting in the formation of a silica-rich
shell. Calcination of the particles removes the CTAB and converts
the ferric core into an iron oxide nanoparticle possibly due to the
sintering and/or ripening processes confined by the silica
shell.
[0099] The schematic in FIG. 4 illustrates an embodiment where a
carbon precursor is added to the precursor solution. The precursor
solution (I) which now contains a carbon precursor (e.g., sucrose)
is atomized to form aerosol droplets, which are then sent through a
heating and a drying zone where preliminary solvent evaporation,
silica condensation and sucrose dehydration occur.
[0100] The morphology of the hollow nanoparticles was evaluated by
scanning electron microscopy (SEM) and transmission electron
microscopy (TEM). The synthesized nanoparticles have well-defined
spherical structures and the size is in the range of 100 to 1000
nm, which is consistent with characteristic droplet size
distribution through aerosol process. The hollowness of synthesized
nanoparticles is supported by TEM. The representative images of
synthesized double layer hollow nanoparticles are shown in FIG.
6.
[0101] The nanoparticles have interesting morphology transition
from ill-defined hollow structure (FIG. 6a) to well-defined double
layer hollow structure after pyrolysis treatment (FIGS. 6b and 6c).
The as-synthesized nanoparticles may have hollow irregular inner
surface and the surface may become spherical after pyrolysis
treatment, indicating the pyrolysis of nanoparticles effectively
removes CTAB and the remaining sucrose. The thickness of two layers
is variable with the size of nanoparticle. The silica shell layer
can be about 5 nm to 100 nm thick and 50 nm to 5,000 nm in outer
diameter. For example, the silica shell layer outer diameter can be
about 100 nm to 1000 nm. The carbon layer can be about 5 nm to 100
nm thick and 50 to 5000 nm, 100 nm to 1,000 nm in outer diameter.
For example, the carbon layer outer diameter can be about 100 nm to
1,000 nm. Based on TEM observation (FIG. 6d), it is clear that the
thickness of the outer layer and inner layer are approximately 30
nm and 50 nm, respectively. Cross section TEM and SEM images (FIGS.
6e, 6g and 6h) further reveal the double layer nature of the hollow
nanoparticles. The cross section TEM images show noodle like inner
layer structure and fractured outer layer (FIG. 6e), which may be
caused by the cutting procedure of cross section TEM sample
preparation. The cross section SEM images (FIGS. 6g and 6h) of a
synthesized nanoparticle further confirm the appearance of two
distinct layers of the hollow particle.
[0102] The present invention demonstrates that in the presence of
sucrose the much slower carbonization of the sugar would occur in
the interior of a particle with a silica shell. FIG. 7a shows the
nature of the particles obtained immediately after the aerosol
process (II in FIG. 4). The particles are as yet poorly defined,
although they do show evidence of significant internal void
space.
[0103] In one embodiment, the as-synthesized particles are then
pyrolyzed at 500.degree. C. for 3 h in a tube furnace under flowing
nitrogen gas to generate carbon species through full dehydration
and carbonization. Instead of nitrogen, one could use a different
inert gas, such as argon or any gas which one of ordinary skill in
the art would know or discover through routine experimentation. The
resultant observation is remarkable, as the carbon forms as a
discrete second layer adjoining the silica shell leaving a fully
hollow interior (IV in FIG. 1 and FIG. 7b). When the carbon
precursor is enclosed in the interior, the pressure build up leads
to the second shell being generated from the inside, as opposed to
building shells from the outside through a layer-by-layer method.
FIG. 7b provides a panoramic image of multiple particles showing a
large size distribution, but also showing that every particle is
hollow. The higher resolution images of FIGS. 7c and 7d shows the
evidence of a double layer at progressively increasing resolution
of the images. As FIG. 7d illustrates, the outer layer is
approximately 40 nm thick, while the inner layer is less than 20
nm. The cross section TEM of a particle (FIG. 7e) shows that the
sectioning process destroys the integrity of the layers. The
striations seen in the outer layer are not pores but jagged edges
created during thin (70 nm thickness) sectioning. It is clear that
the broken inner layer also becomes separated from the outer layer
upon sectioning. FIG. 7f illustrates the external morphology of a
particle through SEM. FIGS. 7g and 7h were obtained by embedding
the particles in epoxy resin, making just one cut to create a thick
section which was then imaged through SEM. FIG. 7g shows a particle
where the outer layer was cut away exposing an intact inner layer
surface, while FIG. 7h illustrates a particle where both layers
were cut revealing the interior voidage. We note that the particles
shown in FIGS. 7e through 7h represent those at the upper end of
the size distribution where clarity of the microstructure is
achieved after the sectioning process.
[0104] The present invention demonstrates that reason for the
generation of these double-layer particles is that the silica shell
seals in the carbon precursors during the aerosolization process.
During pyrolysis, the off gases generated build up a high internal
pressure and push the carbonaceous species to the inner surface of
the silica shell. Assuming the silica shell is impermeable until
pressures are built up to force out the pyrolysis gases, the
internal pressures generated can be as high as 175-200 atm. The
pyrolysis gases are essentially forced out through micropores in
the silica layer. The pyrolysis step is depicted through the
schematic (III) in FIG. 4, and the final pyrolyzed material (IV)
illustrates the generation of the double-layer particles.
[0105] Estimation of internal pressures generated during
pyrolysis--Assumptions:
[0106] a) the outer silica layer is formed extremely rapidly to
seal in carbon precursors;
[0107] b) an average particle size of 190 nm with a silica shell of
35 nm;
[0108] c) precursor concentrations in a droplet are the same as
that in the precursor feed solution.
[0109] With these assumptions, the molar concentration of sucrose
inside a droplet is 0.137M. If we assume a droplet dimension also
of 190 nm, the droplet (and eventually the particle) contains
3.94.times.10.sup.-18 moles of sucrose.
The dehydration reaction of sucrose during pyrolysis is
C.sub.12H.sub.22O.sub.11.fwdarw.12 C+11 H.sub.2O (g) Hence, the
moles of generated gas species (superheated steam) is
n=11.times.3.94.times.10.sup.-18 moles=4.33.times.10.sup.-17 moles
Appling the ideal gas law, the internal pressure can be
estimated:
P = nRT / V = ( 4.33 .times. 10 - 17 mole ) .times. ( 8.314 J K - 1
Mole - 1 ) .times. ( 500 + 273 ) K / ( 4 / 3 .pi. ( 190 - 35 ) nm )
3 = 1.79 .times. 10 7 Pa = 177 atm ##EQU00001##
[0110] Thus, the formation mechanism of double-layer hollow
particles involves two steps: the generation of the silica layer
due to the preferred silica condensation reaction along the
gas-liquid interface of an aerosol droplet and the formation of the
carbon layer by the dehydration and carbonization of dissolved
sucrose during the subsequent pyrolysis.
[0111] Electron Dispersive Spectroscopy (EDS) indicates that the
silica is confined to the outer layer (with a carbon background
from the TEM grid), and carbon to the inner layer. Elemental
analysis obtained by X-ray energy dispersive spectroscopy (EDS) of
exemplary cross section sample reveals that the atomic ratio of
C:O:Si:Fe of the inner carbon layer is 99.1:0.9:0:0, while that of
the outer layer is 93.1:4.3:2.2:0.4 (FIG. 11). In both cases, the
carbon level is incidental as the particles are placed on a carbon
grid. The noteworthy aspect of the EDS analysis is the lack of
silica and iron in the inner layer and the significant presence of
silica in the outer layer. Considering the results of EDS, we
believe that the inner layer is a carbon layer and the outer layer
is a silica layer. The EDS results also indicate that iron is
incorporated in the silica layer of the hollow nanoparticles. From
the magnetization curve (FIG. 12), it is clear that the double
layer nanoparticles display a hysteretic behavior which is
consistent with the ferromagnetism given by Fe nanoparticles.
[0112] To understand the structural characteristics of these
particles further and to prove that the outer layer is silica and
the inner layer is carbon, calcination and etching treatments were
conducted to selectively remove the inner and outer layers,
respectively. The resulting nanoparticles were characterized with
SEM and TEM.
[0113] To remove the carbon layer completely, the pyrolyzed
particles were calcined at 500.degree. C. for 3 h and an additional
2 h at 1000.degree. C. The removal of the carbon layer from
pyrolyzed particles leads to the morphology transition from
double-layer particles to single silica-layer particles (FIG.
13a-c). The cross-sectional TEM (FIG. 13d) shows small dots in the
inner periphery of the silica shell with a few within the silica
shell. High-resolution TEM (HRTEM) (FIG. 13e) and energy-dispersive
spectroscopy (EDS) of these dots (see FIG. 11) indicate these are
iron oxide nanoparticles.
[0114] X-ray diffraction for the double layer particles (FIG. 20,
line (a)) implies that the iron species are nanoparticles with an
insufficient number of diffraction planes for indexing. After
calcination, the pattern (FIG. 20, line (b)) reveals that the peaks
of incorporated calcined particles are reasonably consistent with
hematite (.alpha.-Fe.sub.2O.sub.3), the most thermodynamically
stable polymorph of iron oxide. The peaks at 33.2, 35.6, 40.9, 49.5
2.theta. correspond to the (104), (110), (113) and (024) planes of
hematite, respectively. The XRD pattern of particles after
additional reduction at 400.degree. C. for 2 h with 9% H.sub.2 flow
(FIG. 20, line (c)) can be indexed as magnetite
(Fe.sub.3O.sub.4).
[0115] A simple calculation assuming that the composition of the
precursor solution is reflected in the relative silicon and iron
atomic ratio in the bilayer particles and that the iron oxide
particles are approximately 10 nm in diameter indicates that there
are approximately 300 iron oxide nanoparticles in each hollow
calcined particle. FIGS. 13f, 13g illustrate the external
morphology of the particles and the interior of the cut particles,
respectively. The system of cracks in the interior (shown by the
arrow) may indicate pathways for the egress of CO.sub.2 upon
calcination of the carbon inner layer. The reduced particles
exhibit ferromagnetic hysteretic behavior given by magnetite
(Fe.sub.3O.sub.4). The magnetic properties of the samples with
three different treatments were investigated at 300 K (FIG. 37).
The pyrolyzed particles (sample A) display a combination of
paramagnetism and weak ferromagnetic behavior (details shown in the
inset). The etched particles (sample B) show virtually no
magnetization, consistent with the observation that there are
essentially no iron species associated with the carbon layer. The
particles that are calcined and then reduced to form magnetite show
a clear but weak ferromagnetic behavior (sample C). The
corresponding remnant magnitization (M.sub.r) and coercivity
(H.sub.C) of these particles are 0.341 emu/g and 331 Oe,
respectively (FIG. 37).
[0116] During calcinations treatment, oxygen molecules react with
inner carbon layer of the nanoparticles and the oxidation product
CO.sub.2 diffuses out of the hollow nanoparticles. The complete
removal of carbon layer of hollow nanoparticles was confirmed by
TEM. The removal of carbon layer from pyrolyzed nanoparticles leads
to the morphology transition from double layers nanoparticles to
single silica layer nanoparticles (FIGS. 14a and 14b). From TEM
observation (FIG. 14c), it is clear that the thickness of the
single silica layer is approximately 60 nm, which is consistent
with double layer nanoparticles. The cross section TEM images also
show that the particle has single layer nature with few black dots
located at both inner and outer surface (FIGS. 14d and 14e). The
cross section SEM image (FIG. 14f) provides further confirmation of
the single layer hollow nanoparticle after calcination.
[0117] In addition, the EDS results of calcined particles (FIG. 15)
demonstrate that the atomic ratio of iron to silicon of the black
dots on the inner surface is 19.8:11.9, while that of other area is
6.74:25.4. EDS result (FIG. 15) of these dots area demonstrates
that they have significantly higher Fe atomic percentage (19.7%)
than other area (6.74%), indicating that the calcination process
converts the ionic iron into incorporated iron oxide nanoparticles.
Considering these results, we conclude that the tiny black dots are
iron oxide particles. Again, we discount the carbon levels as the
particles arc on a carbon grid.
[0118] X-ray diffraction (FIG. 16) reveals that the peaks of the
incorporated nanoparticles are well consistent with hematite
(.alpha.-Fe.sub.2O.sub.3), the most thermodynamically stable
polymorph of iron oxide. Meanwhile the strongest peak from the (1 0
4) plane of .alpha.-Fe.sub.2O.sub.3 is centered at
2.theta.=33.degree. with d-spacing of 2.70 {acute over (.ANG.)}.
The average size of hematite crystallites deduced from Sherrer's
equation for the most intense peak is found to be 16 nm.
[0119] To remove the silica layer from the double layer hollow
particles, the pyrolyzed nanoparticles may be etched using 10%
(v/v) HF solution (or other highly acidic solution such as HCl,
sulfuric acid, or other highly acidic solution that a person having
ordinary skill in the art would know or discover through routine
experimentation) for 48 h. The silica layer of hollow nanoparticles
reacts with HF solution, giving rise to H.sub.2SiF.sub.6, which can
be washed out using deionized water. Silica can also be etched out
also using a highly basic solution of for example NaOH, though one
could use other highly basic solutions such as ammonium hydroxide,
or other highly basic solution that a person having ordinary skill
in the art would know or discover through routine experimentation.
The representative TEM images (FIGS. 17a-c, 18a-c) of hollow
nanoparticles after etching clearly demonstrate that the particles
only have single layer structure, implicating the removal of silica
layer by HF solution and preservation of hollow carbon
microspheres. Additionally, the low magnification TEM image (FIG.
17a) indicates that the single layer nanoparticles are
polydisperse, which is associated with aerosol synthesis process.
EDS results (FIG. 19) of these nanoparticles demonstrate 100%
carbon atomic percentage, indicating the complete removal of silica
layer and its incorporated iron by etching treatment. There is no
evidence of iron particles on the electron micrograph, and XRD also
does not show the presence of iron species, indicating that the
iron is confined to the silica and the silica-carbon interface. To
further confirm these findings, the etched hollow nanoparticles
were cut using Leica Microtome and their corresponding cross
section SEM and TEM images were acquired (see FIGS. 17d and 18d).
They show that the nanoparticles are hollow in nature with only
single layer. Meanwhile, the regular SEM images indicate that the
etched hollow carbon nanoparticles have some observable buds on
their surface, which may be resulted from the gradually sucrose
diffusion during high temperature aerosol process.
[0120] The cross section TEM shown in panels 17d and 18d illustrate
an almost intact ring, as it is difficult to section without
breaking the particle. Panels 17e and 18e show the SEM of the
external morphology of an etched particle and panels 17f and 18f
indicate the SEM of the cross section of a particle after a single
cut to demonstrate that the particles are indeed hollow.
[0121] FIG. 21 shows the N.sub.2 adsorption isotherm obtained from
the pyrolyzed double layer hollow particles. The surface area for
these particles at 77 K was calculated using Brunauer-Emmet-Teller
(BET) method. The BET surface area of double-layer hollow particles
was found to be 12.5 m.sup.2/g and the corresponding
Barret-Joyner-Halenda (BJH) desorption pore volume was determined
to be 0.0279 cm.sup.3/g. Upon calcination to silica hollow
particles or etching to the carbon hollow particles, the surface
areas increase to 180 m.sup.2/g (single silica layer) and 104
m.sup.2/g (single carbon layer), respectively (FIG. 21). The low
surface area of the double-layer hollow particles implies the
minimal porosity of the double-layer particles with pores opening
up as the carbon is burnt away or the silica is etched away. Our
interpretation is that the pores in the silica are essentially
clogged with carbonaceous species during the pyrolysis step to
produce the bilayer particles. Upon calcination, the burnoff of
carbon exposes the silica pores. Etching away the silica exposes
the intrinsic porosity of carbon. The lack of high porosity in the
solid is to be expected since the materials are not templated to
highly porous structures by the surfactant.
[0122] It should be pointed out that the dense, low-porosity silica
outer layer of nanoparticles is due to the fact that the relative
amounts of free CTAB is not high enough compared to TEOS to
template mesoporous silica. The dense silica outer layer of
nanoparticles also suggests a possible mechanism for inner carbon
layer formation. During pyrolysis, the escape of organic gases in
the nanoparticles through the dense silica structure may force the
carbonaceous species to the silica wall resulting in the formation
of the inner carbon layer of hollow nanoparticles.
[0123] The double-layer hollow particles that contain an internal
carbon layer and an outer silica layer have been fabricated by a
simple and effective aerosol-based process. The preparation is
based on the concept of rapidly forming a silica shell that retains
carbon precursors within the interior of the particle. Subsequent
pyrolysis jams the carbon as a second layer against the silica
shell.
[0124] The generation of a silica shell by negating the templating
effect of the surfactant is expected to be quite general, allowing
the encapsulation of a variety of other components in the interior
of the particle. In addition to the generation of a new class of
hybrid materials using the aerosol technique, the fact that these
systems contain iron makes them magnetically responsive.
[0125] It is also possible to add layers to the double shelled
particles. Silica is hydrophilic and carbon is hydrophobic,
creating an ampiphilic particle. Building additional layers can be
done by adding a carbon layer to the silica shell, and then adding
a silica coating on the carbon layer. The building of additional
layers is done by known layer-by-layer techniques.
[0126] Another embodiment of the present invention seeks to control
and exploit particle properties through modulating layer thickness
as described herein. These materials are expected to have multiple
applications because they are able to incorporate the benefits of
both carbon and silica and additionally include magnetic materials.
Their uses as catalytic materials and in stabilizing emulsions are
distinct directions of continuing research.
Applications
[0127] The following are some specific applications of these
materials. All these applications are elaborated upon in the Lou et
al. publication, which is incorporated herein by reference, which
is a review of hollow particles. [0128] 1. Their use as electrode
materials in Li-ion batteries. [0129] 2. Their use in catalysis and
sensing. [0130] 3. Biomedical Applications of drug delivery. We
particularly note the photothermic applications. [0131] 4. Fuel
cell catalysts. [0132] 5. Photocatalysis. [0133] 6. Self-healing
applications. [0134] 7. Materials for stabilizing interfaces, such
as Pickering emulsions.
[0135] The present invention is able to make the hollow particles
in large quantities because of the nature of the aerosol
process.
[0136] None of the prior art processes listed in the literature is
able to produce the double shelled particles. Because of the double
shell, the present invention has a system that is hydrophilic on
the exterior and hydrophobic on the interior. There is a distinct
possibility that such systems will provide enhanced properties in
catalysis and gas sensing.
[0137] The present invention is able to modulate the porosity of
the shell particularly in systems with a single shell, going from
an entirely nonporous to a porous system.
[0138] In addition to the applications listed in the Lou et al.
publication, we propose there are environmental applications. There
are potential applications to the environmental remediation of
chlorinated compounds, of arsenic, and other chemicals, due to the
catalytic materials in the hollow particles. Since the particles
are hollow, they may have some extremely important applications in
the remediation of oil spills. They can be filled with dispersants
and sprayed onto oil spills. Such controlled delivery of
dispersants can be efficacious in breaking up oil spills and
dispersing the droplets. Additionally, the particles can be
stabilized at an oil water interface to stabilize emulsion
droplets. Finally the iron oxide within the particles makes them
magnetically responsive and it may be possible to recover the oil
through the formation of Pickering emulsions.
[0139] The hollow particles can be used to store agricultural
pesticides which can be sprayed onto plants for controlled
release.
[0140] The hollow particles can be used to store fertilizers which
can be injected into the ground for controlled release. They can be
temperature tuned by coating them with a wax that melts as the
temperature increases in the growing season, releasing the
fertilizer contents.
[0141] The hollow particles can be used to store enzymes for the
biological breakdown of organophosphorous compounds. In
application, these could be used against nerve agents.
Experimental Procedure
Synthesis of Hollow Silica-Carbon Bilayer Nanoparticles.
[0142] All chemicals are commercially available and were used as
received. TEOS is used as the silica source and sucrose is used as
the carbon source, together with ferric chloride and the surfactant
(CTAB). Alternatively, the carbon source can be a monosaccharide or
polysaccharide, such as sucrose (most preferable), glucose,
cellulose, or cyclodextrins. Alternatively, the surfactant can be
cetyltrimethyl ammonium bromide (CTAB), cetyltrimethyl ammonium
chloride (CTAC) or other CTA-halides. Alternatively, instead of the
silica source, zirconia, alumina or titania can be used.
[0143] In a typical synthesis, about 0.8 g-1.9 g, preferably 0.95
g, of FeCl.sub.3.6H.sub.2O was first dissolved in 15 mL ethanol
under vigorous stirring. Then about 0.1 g-2.2 g, preferably 1.1 g,
of CTAB and about 1.0 mL-9.0 mL, preferably 4.5 mL, of TEOS were
added to the solution, followed by 1.8 mL of a 0.1 M HCl solution
and the dissolution of about 0.01 g-3 g, preferably 1.0 g, of
sucrose. The resulting solution was then aged for 0.5 h and
atomized using a commercial atomizer (Model 3076, TSI Inc.) to form
aerosol droplets, which were passed through a quartz tube placed in
a furnace (FIG. 1). The temperature of the heating zone was held at
400.degree. C., and the entering gas pressure of N.sub.2 was
adjusted to yield a droplet residence time of about 15 seconds
through the furnace. During passage through the heating zone,
sucrose carbonization and silica condensation reactions occurred.
The as-synthesized particles were collected by a filter system and
then pyrolyzed at 500.degree. C. for 3 h under the flow of N.sub.2
gas.
[0144] It is noted that during passage through the heating zone,
the coassembly of CTAB with silicate and the formation of
mesoporous silica are disrupted by the preferential partitioning of
CTAB on intermediate iron species such as FeO(OH). This salt
bridging between the iron salt and CTAB locks the CTAB within the
interior of a rapidly forming silica shell.
Synthesis of Hollow Silica Nanoparticles.
[0145] To obtain the single silica layer particles, the pyrolyzed
particles were calcined at 500.degree. C. for 3 h and additional
1000.degree. C. for 2 h in air.
Synthesis of Hollow Carbon Nanoparticles.
[0146] To obtain the single carbon layer particles, the pyrolyzed
particles were incubated with 10% HF solution for 48 h to remove
silica layer.
Characterization
[0147] The morphology of the particles was characterized by field
emission scanning electron microscopy (SEM, Hitachi S-4700,
operated at 20 kV) and transmission electron microscopy (TEM, JEOL
2010, operated at 200 kV). The crystal phases present in the
particles were identified using X-ray diffraction (XRD, Siemens, D
500, using Cu .alpha.K radiation at 1.54 .ANG.). The cross section
samples for SEM and TEM were prepared by embedding particles within
a resin (Embed 812) in 70.degree. C. for 48 h and cut by a Leica
ultracuts Microtome. Magnetic properties were characterized using a
superconducting quantum interference device (SQUID, MPMS Quantum
Design Inc.). The BET surface area of the particles was measured
using the nitrogen sorption technique at 77K (Micromeritics, ASAP
2010).
[0148] While the double layer hollow particles constitute the key
finding, there other aspects based on expanding on the conjecture
that we can rapidly create a silica shell by preventing the
templating effect of CTAB through salt bridging with
FeCl.sub.3.
Thin Silica Shells--Template-Free Synthesis of Ultrasound
Responsive Hollow Silica Microspheres with Ultrathin
Nanometer-Scale Shell Structures
[0149] Novel ultrathin hollow silica microspheres have been
synthesized using aerosol based process with reduced silica
precursor loading (tetraethyl orthosilicate, TEOS). Hollow silica
microspheres with ultrathin silica shell about 5 nm to 20 nm; or 7
nm to 20 nm, or 7 nm to 15 nm, for example 10 nm-15 nm are also
conveniently cracked using ultrasonic treatment, which is one of
the most promising external triggers. The ultrathin calcined hollow
silica microspheres are presumably fractured by the transient
cavitation, a well-known phenomenon of ultrasonication. In
addition, the pore size of hollow silica microspheres can be
uniquely adjusted by introducing sodium chloride into precursor
solution. For example, pore sizes of 0.5 nm to 100 nm, for example
10 nm, in diameter can be obtained by with a NaCl:precursor
solution ratio of 0.1:1.0 to 10:1.0. The microspheres with
locked-in magnetic iron oxide open up further opportunities in
magnetic stimuli responsive applications.
[0150] In some embodiments, with decreased levels of tetraethoxy
silane (TEOS) in the precursor, the silica shells become
progressively thinner till we get shells that are about 5 nm-20 nm
thin, for example 10-15 nm thin as shown in panel d of FIG. 22.
Below this thickness it is difficult to prepare particles with the
requisite consistency. Ultrasonication (150 watts power) easily
results in the breakage of the particles as observed in FIG. 23.
These "eggshell" type particles in principle can be developed into
systems that release their contents as a burst through ultrasound
induced rupture. We note that the systems of FIGS. 22 and 23 do not
include a carbon precursor (sucrose). In some embodiments, a carbon
precursor may be included for a double layer particle.
[0151] Hollow microspheres with controlled morphologies are of
extensive interest due to their wide applications including
encapsulation, biomolecule separation, catalysis, super-capacitors,
gas sensing, drug delivery and energy storage. Recently, the
research of hollow microspheres has been focused on design of
complex structures, such as double shelled, rattle like and yolk
shell structures. A variety of chemical strategies to synthesize
such hollow microspheres have been applied, including the
soft-template and hard-template processes. Particularly the hard
template method, the most common route, requires building a
desirable layer around a core and followed by the core removal by
chemical etching or high temperature calcination.
[0152] Ultrasound is one of the most promising external triggers
for encapsulated chemical release that can be accurately controlled
by parameters, such as frequency, power density as well as
duration. Although there are available methods for hollow
microspheres preparation, the synthesis of ultrathin hollow
microspheres that can be easily cracked by ultrasonic treatment is
seldom reported. Therefore the present invention discloses a novel
method to prepare ultrathin hollow microspheres and their potential
applications.
[0153] It is well-known that surfactant cetyl trimethylammonium
bromide (CTAB) typically templates highly ordered mesoporous silica
through aerosol method when precursor solution contains silica
source such as TEOS. However the present invention shows that the
introducing of ferric chloride into the precursor solution disrupts
the co-assembly of silicate and surfactant CTAB by preferential
partitioning of CTAB and more positively charged ferric chloride
under acidic condition. The iron chloride ties up the surfactant
CTAB so that the silica cannot grow inwards from the surface of the
drop, thereby generating hollow particles. The formation of silica
rich shell is due to faster silica condensation along the
gas-liquid interface of the aerosol droplets and subsequent high
temperature calcination remove surfactant CTAB and converts the
ferric species into iron oxides. However such synthesized hollow
microspheres have relatively thick silica shell that cannot be
conveniently ruptured by ultrasound irradiation. Based on this
concept, can ultrathin hollow silica microspheres be formed by
gradually decreasing silica precursor loading in the solution? Can
the synthesized ultrathin silica microspheres be easily cracked by
ultrasound treatment, so that encapsulated species can be released
to the surrounding?
[0154] In the present invention, a simple and efficient aerosol
based process is used to synthesize hollow silica microspheres with
ultrathin shell thickness (typically approximately 10-15 nm) that
can be easily cracked by external ultrasonic irradiation. In
addition, the present invention discloses a uniquely tuned pore
size on the hollow silica microspheres by conveniently adjusting
sodium chloride concentrations in precursor solution.
Experimental
Preparation of Ultrathin Hollow Silica Microspheres
[0155] All chemicals are commercially available and were used as
received. Alternatively, the surfactant can be cetyltrimethyl
ammonium bromide (CTAB), cetyltrimethyl ammonium chloride (CTAC) or
other CTA-halides. Alternatively, instead of the silica source,
zirconia, alumina, titania or some other ceramic source can be
used. In some embodiments, a carbon precursor may be included for a
double-layer particle. The carbon source can be a monosaccharide or
polysaccharide, such as sucrose (most preferable), glucose,
cellulose, or cyclodextrins.
[0156] In a typical synthesis about 0.8 g-1.9 g, preferably 0.95 g,
of FeCl.sub.3 was first dissolved in 15 mL ethanol (95%, v/v)
followed by the addition of about 0.1 g-2.2 g, preferably 1.1 g, of
cetyltrimethyl ammonium bromide (CTAB). Then various amounts of
TEOS (about 1.0 mL-9.0 mL, for example 10, 6, 4.5 and 2 mL) were
added to the above solution under vigorous stirring at room
temperature. 1.8 mL of 0.1M HCl solution was also added to the
solution after 3 minutes stirring. The resulting solution was aged
for 30 min and the precursor was then atomized to form aerosol
droplets, which were then sent through the dying zone and heating
zone of quartz tube. The temperature of the heating zone was held
at 400.degree. C. and the resulting particles were collected by a
filter system maintained at 80.degree. C. The as-synthesized
particles were then calcined at 500.degree. C. for 3 h.
Ultrasonic Treatments of Ultrathin Hollow Silica Microspheres
[0157] The parameters of ultrasonication used were set as 20 kHz
(frequency) and 150 W (power output). During the experiment period,
the ultrasonication probe was dipped into the sample solution. Each
sample solution has 5 mg synthesized microspheres dispersed in 1.5
mL deionized (DI) water and was exposed to 5 min treatment in a
sonic dismembrator (Fisher Scientific, model 550). The samples were
then centrifuged for 10 minutes at 12,000 rpm. The percentage of
damaged microspheres was then compared to evaluate the rupture
effect.
Tuning Porosity of Hollow Silica Microspheres
[0158] Similar to the synthesis of ultrathin hollow silica
particles, about 0.8 g-1.9 g, preferably 0.95 g, of FeCl.sub.3 was
first dissolved in 13.3 mL ethanol followed by the addition of
about 0.1 g-2.2 g, preferably 1.1 g, of CTAB. Then 1.0 mL-9 mL,
preferably 2 mL, TEOS were added to the above solution under
stirring at room temperature. 3.5 mL of 0.1M HCl solution with
about 0.01 g-1.0 g, preferably 0.4 g, NaCl was also added to the
solution after 3 minutes. The as-synthesized particles from aerosol
process were then calcined at 500.degree. C. for 3 h to remove
surfactant CTAB and convert iron species to iron oxides. FIG. 39
shows single layer hollow silica particles washed with deionized
water, wherein adding salt (NaCl) in the precursor makes the shells
porous. Molar ratio of NaCl/FeCl.sub.3=1/1.
Dye-Encapsulation Experiment
[0159] 1 mg hollow silica microspheres were dispersed into 1 mL
Rhodamine solution (0.6 mg/mL) and then dried in atmospheric
pressure. The dye-loaded microspheres were washed with DI water.
Optical and fluorescence images of these samples were examined
using inverted fluorescence microscope (model Olympus 1X71).
Hollow Silica Microspheres Characterization
[0160] The morphology and structures of the microspheres were
evaluated using field emission scanning electron microscopy (SEM,
Hitachi S-4700, operated at 20 kV), transmission electron
microscopy (TEM), high-resolution TEM (HRTEM) (JEOL 2010, operated
at 200 kV) and X-ray diffraction (XRD, Siemens, D 500, using Cu
.alpha.K radiation at 1.54{acute over (.ANG.)}). The specimens for
TEM examination were obtained by dispersing microspheres in ethanol
(95%, v/v) and drops of microsphere suspension were added onto a
copper grid for TEM microscope.
[0161] The cross section samples for SEM and TEM were prepared by
embedding silica microspheres within resin (Embed 812) in
70.degree. C. for 48 h and cut by Leica ultracuts Microtome. The
porosity of the microspheres was evaluated by the nitrogen sorption
technique at 77K (Micromeritics, ASAP 2010).
Results and Discussion
[0162] The synthesis of ultrathin hollow silica microsphere is
achieved through a simple and effective aerosol process, which is
illustrated in FIG. 29. Typically, the precursor solution was first
atomized to generate aerosol droplets, which were sent through the
heating zone where silica dehydration and condensation reactions
occurred. The morphology of synthesized hollow silica microspheres
was evaluated by TEM and HRTEM. The representative TEM images in
FIG. 29 show that these hollow silica microspheres are composed of
a large number of spheres with diameter of 100 nm to 3000 nm, and
more preferably 100-1000 nm. These images also reveal that these
microspheres are of well-defined hollow structures, which show the
bright center and dark trim. More importantly, the silica shell
thickness of hollow silica microspheres is decreasing with the
decreasing of TEOS loading in the precursor solution (FIG. 29a-d).
In addition, the breakage of small percentage of ultrathin hollow
silica microspheres prepared from the lowest TEOS loading (FIG.
29d) is noted after calcination treatment. This is reasonable
because ultrathin silica shells of some microspheres cannot
maintain their structures due to the decomposition of surfactant
CTAB during calcination process that generates high internal
pressure. It is also interesting to observe that the microspheres
have morphology transition from ill-defined hollow structure (FIG.
29e) to well-defined hollow structure (FIG. 29d) after calcination
treatment.
[0163] Based on TEM observation with high resolution (FIG. 290, the
ultrathin silica shell is not porous amorphous silica in nature.
The N.sub.2 adsorption desorption isotherm is then used to analyze
the porosity of synthesized microspheres. Surface area for these
microspheres at 77K is calculated using Brunauer-Emmet-Teller (BET)
method. The BET surface area is found to be 13.8 m.sup.2/g which is
close to its geometric surface area, indicating that these calcined
silica microspheres are almost nonporous.
[0164] In order to clearly demonstrate the TEOS loading effect on
silica shell thickness, TEM images of hollow silica microspheres
made from various TEOS loadings with comparable size (200 nm-1000
nm in diameter, more preferably .about.250 nm in diameter) are
acquired (FIG. 30a-d). Obviously, the TEM images demonstrate that
calcined hollow silica microspheres have decreased shell
thicknesses with the lowering of TEOS loading. The thinnest shell
thickness of silica microspheres from Fe:Si molar ratio of 1:2.7 is
measured to be approximately 15 nm, while the thickest one from
Fe:Si molar ratio of 1:2.7 is around 40 nm. The formation of silica
shell is the result of hydrolysis and condensation of TEOS during
the aerosol process. Therefore lower loading of silica precursor is
expected to result in thinner silica shell, which is consistent
with the observations in FIG. 30. Additionally, calcination
treatment of as-synthesized microspheres converts iron species to
iron oxides. X-ray diffraction (XRD) reveals that the peaks of
incorporated iron oxide nanoparticles are well consistent with
hematite (.alpha.-Fe.sub.2O.sub.3), the most thermodynamically
stable polymorph of iron oxide. Meanwhile the strongest peak from
the (1 0 4) plane of .alpha.-Fe.sup.2O.sup.3 is centered at
2.theta.=33.degree. with d-spacing of 2.70 {acute over
(.ANG.)}.
[0165] Silica hollow microspheres prepared in different Fe:Si molar
ratios (1:13 to 1:2.7) are subject to ultrasonic treatment to
evaluate the rupture properties. SEM images in FIG. 31 reveal the
external morphology of calcined silica microspheres before and
after ultrasonic treatments. It is clear that calcined silica
microspheres have well-defined spherical structures with the size
of 100 to 1000 nm (FIG. 31a-d), which is consistent with
characteristic droplet size distribution through aerosol process.
Comparing the morphology of these microspheres before and after
same ultrasonic treatments, it is interesting to observe that
calcined hollow silica microspheres with thicker thicknesses (Si:Fe
ratio.gtoreq.6:1) are intact after ultrasonic treatment (FIG.
31e-g), while most of microspheres with ultrathin shell thicknesses
(Si:Fe ratio=2.7:1) are ruptured (FIG. 31h) by ultrasound treatment
(20 kHz, 150 W). Upon ultrasonic irradiation, ultrathin hollow
silica microspheres keep cracking and eventually most of them
collapse. This transformation kinetics of ultrathin silica
microspheres is fast and it only take 5 minutes for ultrathin
microspheres to turn into pieces.
[0166] To further reveal the structure of ruptured silica
microspheres with ultrathin silica shell triggered by ultrasound,
SEM and TEM images of calcined microspheres prepared from Fe:Si
ratio of 1:13 and 1:2.7 are obtained and compared (FIG. 32a-f). The
hollow silica microspheres (Fe:Si=1:2.7) are stable in air or in
the aqueous solution before ultrasonic treatment. However, when an
ultrasonic treatment (20 kHz, 150 W) is applied, the hollow silica
structures are deconstructed. It is clear that most of silica
microspheres (Fe:Si=1:2.7) are ruptured by ultrasound treatment
(FIG. 32d-f), while silica microspheres of lower silica content
(Fe:Si=1:13) remain intact (FIG. 32a-c). FIGS. 32c and 32f
demonstrate that silica microspheres prepared from Fe:Si=1:13 have
apparently significant thicker thickness, which can maintain the
shell structure upon ultrasonic treatment. The ultrathin calcined
hollow silica microspheres are presumably fractured by the
transient cavitation, a well-known phenomenon of ultrasonication.
In cavitation, extreme conditions of local temperature and pressure
are known to exist at rapid heating and cooling rate, which can
lead to explosion by gas buildup inside the microspheres and
rupture the silica shell of microspheres. Additionally, the
ultrasound wave generated by cavitation may accelerate hollow
microspheres to high velocities and fracture the ultrathin silica
shells.
[0167] It appears that a Fe:Si molar ratio of 1:2.7 is ideal for
creating thin silica spheres. However, the Fe:Si molar ratio can
vary from 0.5:3 to 5:3 and still create acceptable thin silica
spheres for sonication destruction.
[0168] The pore size tunability of hollow microspheres plays an
important role in widening their applications. The porosity of
hollow silica microspheres can be adjusted by varying the amount of
sodium chloride loading while keeping all concentrations of other
chemical species same. As shown in FIG. 33a, the introduction of
about 0.01-1.0 g, preferably 0.4 g, sodium chloride results in
hollow silica microspheres with porous structures. HRTEM reveals
the detailed porous structure of the microspheres (FIG. 33b). To
further confirm these findings, the hollow silica microspheres were
cut using a Leica Microtone. The cut section TEM (FIGS. 33c and
33d) illustrates that these hollow microspheres have less dense
area along the rings. The embedding epoxy resin can also be
observed inside the particles, which indicates the silica shells
are porous enough to let resin diffuse inside these microspheres
during cut section TEM preparation.
[0169] The SEM images (FIG. 34) further provide the external
morphology of such porous hollow particles. It is clear that these
microspheres have irregular shape pores on silica shells, and the
size of these pores is in the range of 0.5-100 nm, for example 10
nm. The BET surface area and desorption pore volume of these
calcined hollow microspheres are calculated as 33.3 m.sup.2/g and
0.127 cm.sup.3/g respectively (FIG. 35), which are more than twice
of those of silica microspheres prepared without sodium chloride.
The surface area results demonstrate that introducing sodium
chloride in the precursor solution can increase porosity by
generating more channels along the silica shell. The mechanism of
creating pore channel along the silica shell may be that silica
condensation reaction and sodium chloride precipitation occur
simultaneously along aerosol droplets.
[0170] The XRD patterns of two groups of hollow silica
microspheres: with and without washing treatments are shown in FIG.
35. It can be seen that silica microspheres before washing have
both hematite and NaCl peak patterns, while silica microspheres
after washing only have hematite pattern. This clearly indicates
that washing procedure can remove NaCl particles on and inside the
silica microspheres and therefore open up more available pores.
[0171] Dye-encapsulation experiments were also conducted by
dispersing hollow silica microspheres (with and without sodium
chloride loading) into Rhodamine B solution (2 mg/mL). Phase
contrast (FIG. 36a, 36c) and fluorescence images (FIG. 36b, 36d)
are acquired. The fluorescence images of these microspheres readily
show that discrete red shapes (FIG. 36d) for microspheres prepared
with sodium chloride, suggesting that rhodamine B molecules are
entrapped inside hollow silica microspheres, which supports open
pore structure along the silica shell.
[0172] The ultrathin hollow silica particles are synthesized
through a simple and effective aerosol based process using reduced
TEOS loading in the precursor solution. These hollow microspheres
with ultrathin shell thickness can be easily ruptured by
ultrasonication treatment in a short time via cavitation mechanism,
which make them a promising material for ultrasound-triggered
release application. The porosity of silica hollow microspheres can
be conveniently tuned by introducing sodium chloride due to
simultaneous silica condensation and sodium chloride precipitation.
The fact that these novel microspheres have ultrathin silica shell
makes them ultrasound responsive and expected to have wide range of
applications where pulsatile encapsulated release is needed.
[0173] In another embodiment of the present invention, a dense
carbon particle with a net-like or cage-like thin silica shell can
be created. TEOS loading in the precursor solution is reduced to
make a thinner silica shell, as seen in FIGS. 40-47. Generally,
TEOS loading of 1 mL-2 mL will result in a relatively thick silica
shell (about 20-40 nm thick), while TEOS loading of 0.75 mL-1 mL
will result in a relatively thin silica shell (about 10-20 nm
thick), and TEOS loading of 0.5 mL-0.75 mL will result in a
relatively thin silica shell (about 5-7 nm thick), with a net-like
or cage-like structure.
[0174] FIG. 40 shows nitrogen adsorption-desorption isotherm with a
precursor of 2 ml TEOS and no sucrose after calcination. The BET
surface area is 13 m.sup.2/g.
[0175] FIGS. 41(a)-(b) and 42 show TEM images of particles and
nitrogen adsorption-desorption isotherm with a precursor of 2 ml
TEOS and 0.5 g sucrose after calcination. The BET surface area is
339 m.sup.2/g. As seen in FIG. 41, with the addition of carbon
source-sucrose in the precursor solution, some of the carbon source
are mixed within the silica layer during aerosolization. After
calcination, carbon is burnt off leaving many pores in the silica
shell, hence the particles have very high surface area.
[0176] FIGS. 43(a)-(b) and 44 show TEM images of particles and
nitrogen adsorption-desorption isotherm with a precursor of 1 ml
TEOS and 0.5 g sucrose after calcination. The BET surface area is
372 m.sup.2/g.
[0177] FIGS. 45(a)-(b), 46 and 47 show TEM and SEM images of
particles and nitrogen adsorption-desorption isotherm with a
precursor of 0.75 ml TEOS and 0.5 g sucrose after calcination. The
BET surface area is 323 m.sup.2/g.
TABLE-US-00001 T = 400 P = 10 psig 2 furnaces TEOS Sucrose Observe
2 ml 0 g Hollow thin shells after calcination. BET surface area is
13 m2/g. 2 ml 0.5 g Hollow thin shells after calcination (10-40
nm). BET surface area is 339 m.sup.2/g. molar ratio = 6:1 1 ml 0.5
g Hollow thin shells after calcination (10-20 nm). BET surface area
is 372 m.sup.2/g. molar ratio = 3:1 0.75 ml 0.5 g Hollow thin
shells after calcination (10-20 nm). BET surface area is 323
m.sup.2/g. molar ratio = 2.3:1 0.5 ml 0.5 g Cannot form
spheres.
[0178] With the same TEOS loading (1 ml), when the sucrose in the
precursor solution is increasing from 0.5 g to 1 g, the morphology
of the particles is changed from hollow to dense spheres. This is
because the concentration of sucrose in the aerosol droplet is too
high and it obstructs the diffusion of the silica source to the
gas-liquid interface. So the silica source is mixed with carbon
source and other species forming dense spheres.
[0179] FIG. 48(a-d) shows TEM images of particles with a precursor
of 1 ml TEOS and 0.75 g sucrose. FIGS. 48(a) and (b) are images of
aerosolized particles. FIGS. 48(c) and (d) are images of calcined
particles.
[0180] FIG. 49(a-d) shows TEM images of particles with a precursor
of 1 ml TEOS and 1 g sucrose. FIGS. 49(a) and (b) are images of
aerosolized particles. FIGS. 49(c) and (d) are images of calcined
particles.
[0181] FIG. 50(a-c) show SEM images of aerosolized particles with a
precursor of 0.75 ml TEOS and 1 g sucrose. FIG. 51(a-b) show TEM
images of aerosolized particles with a precursor of 0.75 ml TEOS
and 1 g sucrose. FIGS. 52(a-b) show cut section-TEM images of
aerosolized particles with a precursor of 0.75 ml TEOS and 1 g
sucrose.
[0182] FIG. 53(a-b) show SEM images of calcined particles with a
precursor of 0.75 ml TEOS and 1 g sucrose. FIG. 54(a-b) show TEM
images of calcined particles with a precursor of 0.75 ml TEOS and 1
g sucrose. FIGS. 55(a-b) show cut section-TEM images of calcined
particles with a precursor of 0.75 ml TEOS and 1 g sucrose.
TABLE-US-00002 TEOS Sucrose Observe 1 ml 0.5 g Hollow thin shells
after calcination (10-20 nm). BET surface area is 372 m2/g. molar
ratio = 3:1 1 ml 0.75 g After calcination, some (particles with
diameter below 300 nm) are hollow and particles with large diameter
are dense. There are pores inside the particles. molar ratio = 2:1
1 ml 1 g After calcination, some of the particles are hollow but
most particles are dense spheres. molar ratio = 1.5:1 0.75 ml 1 g
After synthesis, particles have cage-like structures. Most of the
particles are not hollow. molar ratio = 1:1 After calcination,
particles still have cage-like structures. There are many pores on
the particles. Most of the particles are not hollow.
Nanohorns
[0183] In some embodiments, the carbon precursor concentration in
the precursor solution may be increased, resulting in particles
that have long protrusions, leading to a term coined as
"nanohoms"
[0184] The pressure buildup during pyrolysis, rather than rupturing
the shells, leads to yielding and the formation of these long
protrusions some of which are longer than the particle diameter.
This is an interesting structural feature as it implies that the
hydrodynamics of such particles are significantly different from
the hydrodynamics of spherical particles. Additionally, the
protrusions may have significant consequences in the anchoring of
these particles at fluid interfaces and the formation of Pickering
emulsions. Such particles may not be able to easily rotate at an
interface leading to the possibility of preparing a variety of
Janus particles. An interesting aspect of these particles seems to
be that they are not hollow internally. In other words, the excess
carbon precursor loading leads to a yielding of the silica shell
and the inability to firmly compress the carbon onto the silica
shell.
[0185] In an exemplary synthesis, about 0.8 g-1.9 g, preferably 1.0
g, of FeCl.sub.3.6H.sub.2O is first dissolved in 15 mL of ethanol
followed by the addition of about 0.1 g-2.2 g, preferably 1.1 g, of
CTAB. To this solution, 1.0 mL-9 mL, preferably 4.2 g, of TEOS, 1.8
mL of 0.1 M HCl and 2.0 g sucrose are added. The resulting solution
is aged for 0.5 h under stirring. The precursor is first atomized
to form aerosol droplets, which are then sent through a drying zone
and heating zone where preliminary solvent evaporation and silica
condensation occur. The temperature of the heating zone is held at
400.degree. C. The resulting particles are collected by a filter
maintained at 80.degree. C. The as-synthesized particles are
pyrolyzed at 500.degree. C. for 3 h to generate nanohorn
structure.
Using a Titania Precursor
[0186] In some embodiments, materials other than carbon precursors
may be used in the precursor solution. In one embodiment, titanium
isopropoxide (TIP) rather than sucrose may be introduced to the
precursor solution. FIG. 25 illustrates exemplary results. There is
evidence of a two layer structure indicating the ability of silica
to form a rapid shell. However, there is no clear delineation
between the two layers as silica titanate bonds can form
(--Si--O--Ti--). In some embodiments, the silica may be etched out
(using NaOH to selectively remove silica) leads to the fragile
spherical structures of titania shown in FIG. 26. FIG. 38 shows
wide open highly porous titania.
[0187] Both the double layer silica-titania particles and the
etched particles have significant applications in photocatalysis.
Light may be able to penetrate easily through the shell allowing
efficient photocatalysis to take place. The buoyancy of these
hollow particles might make them especially suitable to be used in
oil spill mitigation technologies.
[0188] FIG. 27 illustrates that the addition of FeCl.sub.3 destroys
the hexagonally ordered structure of MCM-41 (ordered mesoporous
silica obtained by templating silicas with CTAB). This is clear
indication that the structure modifying effect of Fe (III) may also
be present in solution based experiments.
[0189] In an exemplary synthesis, an aerosol precursor is prepared
by mixing 3.5 mmol FeCl.sub.3.6H.sub.2O and 3.0 mmol CTAB in
ethanol (15 mL) first, followed by sonication for 5 min. To this
solution, 3.3 mmol TIP, 20.3 mmol TEOS and 1.8 mL of 0.1 M HCl are
added. The final precursor solution has a molar ratio of
FeCl.sub.3.6H.sub.2O:TIP:TEOS:CTAB:HCl:EtOH=1:0.94:5.8:0.86:0.05:74.
The solution is then aged for 30 min under magnetic stirring and
atomized to form aerosol droplets which were sent through a drying
zone and heating zone.
EXAMPLES
[0190] Examples and methods of use are described herein as a basis
for teaching one skilled in the art to employ the invention in any
appropriate manner. These examples disclosed herein are not to be
interpreted as limiting.
Example 1
The Development of New Photocatalysts
[0191] Some embodiments may utilize thin silica shells with an
inner backing primarily of titania indicates for photocatalysts
with buoyant properties in solution. This has tremendous
applications in cleaning and remediation technologies and in the
development of dye sensitized solar cells. The photocatalytic
activity of each TiO.sub.2 sample will be evaluated by the
degradation of Rhodamine B in deionized water. The reaction will be
carried out in a RPR-100 Rayonet reactor (1.65.times.108
photons/s/cm3) using emission at 254 nm. In a typical experiment,
10 mg of TiO2 is added to 50 mL of a 1.0.times.10-5 molL-1
Rhodamine B solution and magnetically stirred in the dark for 30
min prior to irradiation, to achieve adsorption equilibrium of
Rhodamine B with the catalyst. The samples are collected every 20
min by centrifugation to determine the degradation rate by UV-vis
adsorption (553.5 nm, Shimadzu UV 1700). Comparisons with the
standard photocatalyst (Degussa P25) will be made.
Example 2
The Development of New Classes of Colloidosomes
[0192] Embodiments can be used to solubilize/emulsify mutually
immiscible phases forming emulsions that are stable over extended
periods. Such surfactant free emulsions, also known as Pickering
emulsions, are characterized by the degree of wettability of the
particles by either the dispersed phase or the continuous phase as
determined by the contact angle (A), defined as
cos .theta. = .gamma. so - .gamma. sw .gamma. ow ##EQU00002##
where .gamma..sub.so, .gamma..sub.sw, and .gamma..sub.ow are the
interfacial tensions at the solid-oil, solid-water, and oil-water
interfaces, respectively. As a general rule of thumb, hydrophobic
particles)(.theta.>90.degree. preferentially disperse in the oil
phase and stabilize water-in-oil emulsions, while hydrophilic
particles wetted by water (.theta.<90.degree.) solubilize
oil-in-water emulsions. The contact angle of colloidal particles at
the interface is analogous to the hydrophilic-lipophilic balance of
surfactants, and the value of the contact angle typically
determines the nature of the emulsion (oil in water or water in
oil) in systems that have similar amounts of the two phases.
Mechanisms involved in the stabilization of particle based
emulsions and phase transitions have been extensively discussed by
Binks and coworkers and in recent years, particle stabilized
emulsions have been used to develop novel applications ranging from
the synthesis of Janus particles and colloidosomes to drug delivery
and catalysis at interfaces. The self assembly of micro and
nanoparticles at interfaces is also of much interest from the
perspective of creating building blocks for hierarchical
structures. In understanding the nature of Pickering emulsions and
self-assembly at interfaces, model systems typically used are
hydrophilic colloidal silicas or hydrophilic latex particles which
form oil-in-water emulsions. Water-in-oil Pickering emulsions are
usually studied through the use of hydrophobically modified
silicas. The rapid development of applications involving carbon
based materials has led to interest in the assembly of irregular
sized carbon black particles, graphene sheets, and carbon nanotubes
at interfaces.
[0193] The feasibility of forming water-in-trichloroethylene
emulsions using carbon microspheres has been shown. While the work
was done in relevance to environmental remediation of TCE, it is
straightforward to substitute an oil phase (e.g. octane) instead of
TCE. Cryo-SEM of water in TCE Pickering emulsions can illustrate
the assembly of particles at the interface.
[0194] The approach in the present invention will be to use the
bilayer particles to stabilize oil in water emulsions and then
connect up the particles through formation of the Si--O--Si bond
between particles. In one embodiment, the present invention can
create novel colloidosomes of bilayer particles. It is also
possible to individually use the silica shells to stabilize oil in
water emulsions and the carbon shells to stabilize water in oil
emulsions. One embodiment may comprise combination of the two
systems to lead to the formation of bicontinuous emulsions
stabilized by particles.
ACRONYMS
[0195] BET Brunauer-Emmet-Teller [0196] BJH Barret-Joyner-Halenda
[0197] CTAA cetyltrimethyl ammonium astatide [0198] CTAB cetyl
trimethylammonium bromide [0199] CTAF cetyltrimethyl ammonium
fluoride [0200] CTAI cetyltrimethyl ammonium iodide [0201] DI
deionized [0202] EDS energy dispersive spectroscopy [0203] Hc
coercivity [0204] HF Hydrogen fluoride [0205] HRTEM High-resolution
transmission electron microscopy [0206] MCM-41 Mobil Composition of
Matter No. 41 [0207] Mr magnitization [0208] SEM scanning electron
microscopy transmission electron microscopy [0209] SQUID
superconducting quantum interference device [0210] TEM transmission
electron microscopy [0211] TEOS tetraethyl orthosilicate [0212] TIP
titanium isopropoxide [0213] UV ultraviolet [0214] XRD X-ray
diffraction
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