Silica nanocoils

Article abstract of DOI:10.1021/jp0607519

We report here the results of nanoparticle-catalyzed synthesis of a variety of silica nanocoils (SiNCs) under similar growth conditions to those of making regular straight silica/silicon nanowires (SiNWs). Synthesis of individual SiNCs with alternating coiled and straight segments was achieved with cobalt nanoparticle catalysts at alternating temperatures. In addition, a variety of SiNCs, including multiple SiNCs sharing the same nanoparticle catalyst, SiNC double helices, and individual SiNCs with varying pitches and diameters were made.

Full text of DOI:10.1021/jp0607519

8296
J. Phys. Chem. B 2006, 110, 8296-8301
Silica Nanocoils
Yongquan Qu, Joshua D. Carter, and Ting Guo*
Department of Chemistry, One Shields AVenue, UniVersity of California, DaVis, California 95616
ReceiVed: February 4, 2006; In Final Form: March 8, 2006
We report here the results of nanoparticle-catalyzed synthesis of a variety of silica nanocoils (SiNCs) under
similar growth conditions to those of making regular straight silica/silicon nanowires (SiNWs). Synthesis of
individual SiNCs with alternating coiled and straight segments was achieved with cobalt nanoparticle catalysts
at alternating temperatures. In addition, a variety of SiNCs, including multiple SiNCs sharing the same
nanoparticle catalyst, SiNC double helices, and individual SiNCs with varying pitches and diameters were
made.
I. Introduction
turn can be used to produce novel nanostructures by controlling
the catalytic growth conditions and the catalysts themselves.
The intricate and rich interplays between nanoparticle cata-
lysts and reactants in various growth environments have led to
the discovery of many novel nanomaterials, including single-
walled carbon nanotubes (SWNTs) and metal and semiconductor
These results presented here suggest that even the same
nanoparticles under different external physical conditions may
possess totally different catalytic functions and hence can give
rise to totally different end products. Such evidence shows that
the size of premade nanoparticle catalysts is not the only factor
to control the outcome of synthesis. Other properties such as
surface composition and morphology, overall composition and
structure, and physical state of all the nanomaterials involved
are important. It is thus crucial to carefully design and control
the catalytic growth environment in order to synthesize complex
new nanostructures.
nanowires. These discoveries have been the driving forces
behind the rapid advancement of nanotechnology.1-6 Recently,
nanostructures such as nanobelts and nanosprings have also been
made using nanoparticles catalysts.7-9 It is certain that many
more novel nanostructures will be produced catalytically.
The catalysts and growth conditions have been known to
greatly influence the outcome of the synthesis of these nano-
and micromaterials. Carbon microcoils, for example, have been
made when metal particle catalysts are used to make carbon
fibers.10-14
II. Experimental Section
Single- and double-walled carbon nanotubes can also
15
be made from different nanoparticle catalysts. In another
example, large diameter SWNTs are made with the addition of
sulfur or other elements in the catalysts.16 Silica nanosprings
with tight turns and triangular cross-sections have been made
when a trace amount of carbon is present, which is different
from another kind of nanosprings made from laser vaporization
Cobalt nanoparticles have been used to produce SWNTs and
2,3,17-20
SiNWs.
In this work we chose Co nanoparticles as the
catalysts to produce SiNCs. Two kinds of Co nanoparticles were
made and used. All the chemicals used here were purchased
from Aldrich without further treatment.
One type of Co nanoparticle (type A) was made via a high
temperature thermal decomposition method from Co2(CO)8 in
7
8
of Si powder at 1200 °C that have round cross sections. The
growth of silica nanocoils (SiNCs) with round cross sections
using a different synthetic method has also been recently
a mixture of oleic acid (OA), trioctylphosphine oxide (TOPO),
21,22
and trioctylamine (TOA).
Depending on the relative amounts
1
7
reported. These examples have clearly demonstrated that
controlling the very productive and yet complicated catalytic
processes involving nanoparticle catalysts and nanostructures
is the key to making new nanomaterials. Moreover, many of
the growth parameters are interdependent. For example, pa-
rameters such as growth temperature can be used not only to
control the rate of growth but also the geometry and the
structures of the nanoparticle catalysts and the structure of
nanomaterials thus produced. High temperatures and trace
elements present in the growth environment may further
complicate the phase diagram and the growth processes. In some
sense, the task of fully describing a nanoparticle catalyst is
similar to that of defining a macromolecule in biology: it can
only be fully characterized by its whole structures for a specific
catalytic function under certain conditions.
of the surfactants in the mixture, Co nanoparticles of different
sizes were made. For example, this method produced 3-12 nm
Co nanoparticles in anhydrous dichlorobenzene (σ-DCB) solu-
tions.
Another type of Co nanoparticle (type B) was also made.23
Specifically, 0.5 g of Co(CH3COO)2‚4H2O and 0.75 mL of oleic
acid were dissolved in 20 mL of diphenyl ether. The mixture
was gradually heated to 200 °C in an Ar atmosphere, at which
point 1 mL of trioctylamine was added to the mixture and the
temperature was then raised to 250 °C. Then 2 g of reducing
agent (1,2-dodecanediol) dissolved in 5 mL of diphenyl ether
in a separate flask and heated to 80 °C in a water bath was
quickly injected into the mixture. The system was held at 250
°
C for 30 min. After the solution was allowed to cool to room
temperature, ethanol was added to precipitate the nanoparticles.
The precipitate was recovered via centrifugation and washed
with ethanol three times. The nanoparticles were soluble in
organic solvents such hexane, dichlorobenzene (σ-DCB), and
chloroform (CHCl3). As shown later, the nanoparticles made
In this report we will use experimental evidence to further
illustrate the complex nature of catalytic processes, which in
*
Corresponding author. Phone: (530)-754-5283. E-mail: tguo@
ucdavis.edu.
1
0.1021/jp0607519 CCC: $33.50 © 2006 American Chemical Society
Published on Web 04/05/2006

Silica Nanocoils
J. Phys. Chem. B, Vol. 110, No. 16, 2006 8297
Figure 1. TEM image of type B Co nanoparticles. The average size
is 12 ( 4 nm. The scale bar is 50 nm.
Figure 2. SEM images of SiNCs and SiNWs. (A) SEM image of
SiNCs made at 1035 °C for 30 s, the lowest temperature at which SiNCs
can be grown with relatively good yield. Curly short nanowires were
seen with metal nanoparticles at their tips. SEM image of SiNCs made
at 1035 °C for 45 min (B) and SiNWs made at 1100 °C for 45 min
in this report (12 nm diameter) were slightly smaller than that
originally reported (15 nm diameter).
Piranha [1:4 H2O2 (30%):H2SO4 (concentrated)] cleaned Si-
(100) wafers (Virginia Semiconductor) were used as substrates
(C). Scale bars are 1 µm.
for nanoparticle deposition and also as the source of silicon for
making SiNCs. The wafers were immersed in σ-DCB solutions
of Co nanoparticles for 15 min. They were then removed from
the solution, dried with argon gas, and placed in an airtight
quartz tube in a high-temperature furnace (Lindburg/Mini-Mite).
Hydrogen (99.95%) and argon (99.997%) gas were fed through
the quartz tube. The total pressure was maintained at ∼1 psi
above the ambient pressure, and the gas flow rates were held at
at the ends of the nanowires suspended in space. Such
observation implied that the growth followed a tip-growth
mechanism. Figure 2A shows the result of SEM investigation
after a short growth time (30 s at 1035 °C) using type A Co
nanoparticles. Metal-containing nanoparticles, shown here as
bright spots, were seen at the tips of the leading sections of
short nanowires. Short SiNCs began to develop, as shown by
the curved nanowires in Figure 2A. In many cases, crystalline
cobalt silicide nanoparticles wrapped in a sheath of amorphous
SiO2 were observed with the high-resolution TEM. Similar
280 sccm for Ar and 210 sccm for H2. Temperature and reaction
time were adjusted using the furnace controller. The gases were
continuously fed during temperature changes, and the rate of
temperature change between 1000 and 1100 °C was ap-
proximately 25 °C/min. The time specified in the growth
diagram did not include any residence time.
The grown samples were inspected with a scanning electron
microscope (SEM, FEI XL-30 SFEG) at 5 keV of electron
acceleration voltage. Transmission electron microscopy (TEM)
was used to identify the Co nanoparticles and the structure of
nanowires and nanocoils. High resolution (HR) TEM (Philips
CM-300) at the National Center for Electron Microscopy
2
4
results were observed in other growth conditions. In general,
the overall size of the catalyst heads at the tip of SiNCs was
1
0-100% larger than the average diameter of nanowires. The
size of the metal-containing core in these heads was smaller,
as revealed by high-resolution TEM. It is worth noting that
because the samples were inspected after they were cooled to
room temperature, these structural identifications may not reflect
the true structure of the metal nanoparticles and their immediate
environment during the growth. Future in situ studies will be
needed in order to fully understand the structure of nanoparticle
catalysts during the growth of SiNCs.
(NCEM) and another HR-TEM (JEOL, JEM-2500SE) at UC
Davis as well as a lower resolution TEM (Philips CM-12)
located at UC Davis were employed to inspect the samples.
Small area electron diffraction (SAED) was used to inspect the
crystallinity of the silica nanowires in SiNCs.
SiNWs of different morphologies can be made from type A
nanoparticles. For example, many SiNCs were visible when the
samples were grown at 1035 °C. An example is shown in Figure
2
B. These SiNCs were mixed with straight SiNWs. For the
III. Results
results shown in Figure 2B, most SiNCs were on the top of a
thick layer of nanowires, which also contained a large quantity
of straight SiNWs. It was difficult to directly estimate the yield
of SiNCs in this case. On the other hand, predominantly (over
99%) straight SiNWs were made at 1100 °C (Figure 2C), and
almost no SiNCs were detected.
The results of type A Co nanoparticles were shown earlier.18
The average size of these spherical Co nanoparticles was 3 (
1
and 12 ( 2 nm, indicating that the sample contained
nanoparticles of two sizes. A TEM (CM-12) image of type B
Co nanoparticles is shown in Figure 1. The average size is 12
(
4 nm. Some nanoparticles were darker than others under
Although overwhelmingly straight SiNWs were grown at
1100 °C, more SiNCs were made between 1035 and 1050 °C
using type A nanoparticle catalysts at a high deposition density
of nanoparticles, as shown in Figure 2B. To further investigate
the growth of SiNCs as a function of temperature, synthesis
TEM, implying that they were crystalline. Most nanoparticles
adopted nonspherical shapes.
There was strong experimental evidence that suggested that
the growth of SiNCs was caused by the nanoparticle catalysts

8298 J. Phys. Chem. B, Vol. 110, No. 16, 2006
Qu et al.
Figure 4. High-resolution TEM images of SiNCs. Round cross section
(A) and regular noncircular cross section (B) SiNCs are shown.
pitches: Segment 3 was slightly larger than segment 5. This
difference was possibly caused by subtle changes in the
nanoparticle catalyst during temperature cycling. An SEM image
of another hybrid nanowire grown under a similar condition is
shown in Figure 3C, and again coiled and straight sections are
clearly observed.
We found that while many of the cobalt nanoparticle catalysts
at the tips of SiNCs after growth were crystalline (HR-TEM),
the SiNCs were amorphous (HR-TEM and SAED). Energy-
dispersive X-ray analyses (EDX) using both SEM and TEM
were used to identify the composition of the samples, and the
results showed that they were nearly SiO2. Furthermore, the
cross-section of these SiNCs may adopt the conventional
cylindrical or other patterned shapes, as shown in Figure 4A,B.
This could imply that the as-grown SiNCs were crystalline, and
they were subsequently oxidized to SiO2 when they were
exposed in air. However, as shown by other work, even
crystalline Si nanowires could be perfectly straight, demonstrat-
ing that it is more than the crystallinity of the nanowires that
Figure 3. SEM image of a SiNW made from low-density type A Co
nanoparticle deposition (A). The growth temperatures and times are
shown in B (1050 °C for 10 min, 1100 °C for 10 min, 900 °C for a
few min; repeated three times).The last growth was done at 1100 °C,
and the last segment is a straight nanowire (#6). This was preceded by
a coiled segment (#5) and then a straight segment (#4) and another
highly coiled segment (#3). The first two segments of the nanowire
were less visible (#2 and #1). The scale bar is 1 µm. Another coil-
straight-coil SiNW is shown in C, as the result of a similar temperature
varying growth experiment.
2
5,26
controls their morphology.
The average diameter of SiNCs (53 ( 10 nm) was generally
larger than that of straight nanowires (40 ( 5 nm) when type
A Co nanoparticles were used. The yield of SiNCs made with
type A nanoparticles at low deposition density was still low,
generally on the order of 10%. When type B Co nanoparticle
stock material was used under low nanoparticle deposition
conditions and at ∼1050 °C, the yield of SiNCs was over 50%.
An SEM image of one such sample is displayed in Figure 5.
Even more interesting was that SiNCs of various helicities were
made. In Figure 5, all eight coiled SiNCs had different helicities
and coil diameters. The pitch of the tightest coil (#1) was 20
nm, with a 20-nm coil diameter. The diameter of the nanowire
in this SiNC was less than 10 nm. This was one of the most
tightly coiled SiNCs we observed so far. We did not observe a
strong correlation between the coil diameter and the helicity,
although it seemed that thicker nanowires are more tightly
coiled, as sample #1 in Figure 5 may have suggested. SiNCs
may also adopt a varying helicity within the same nanowire, as
indicated by sample #8. Nanoparticles were visible on some
SiNCs, as shown by the nanoparticle at the end of coil #4. When
type B nanoparticles were used, the difference between the
diameters of nanowires in coiled and straight nanowires was
less than 5%.
was performed at a much lower nanoparticle deposition density
and varying growth temperatures. Figure 3A shows the result
of the growth of a single SiNC produced at alternating
temperatures between 1050 and 1100 °C. The temperature
history is plotted in Figure 3B. The 700 °C cooling periods of
2
-3 min were inserted into the growth process to make sure
that growth was totally quenched after each heating cycle at
1
100 °C.
Because nanoparticle catalysts were observed at the tips of
SiNCs, isolated from the wafer surface and other Co nanopar-
ticles, they should remain unchanged during the whole growth
period. Consequently, the same nanoparticle catalyst should be
responsible for the growth of the whole nanowire shown in
Figure 3A. Because it was the same nanoparticle, one may
speculate that only the growth rate of the nanowire would
change as the reaction temperature was lowered or raised.
However, this was clearly not the case. As shown in Figure
3A, there are two distinct morphologies in this nanowire: two
straight (#4 and #6) and two coiled (#3 and #5) segments,
following closely the profile of the temperature history. Seg-
ments 1 and 2, grown during the first temperature cycle, were
less visible. These results suggested that the same Co or cobalt
silicide nanoparticle catalyst (shown at the end of #6 in Figure
It is also possible to produce various shaped SiNCs. As shown
in Figure 6, there are several shapes that are worth mentioning.
We observed nanowires sharing a nanoparticle catalyst, resem-
bling a thermal couple tip (Figure 6A). There were also SiNC
double helices, one of which is shown in Figure 6B. Figure 6C
shows a clean single SiNC that was several microns long.
Samples of this format can facilitate mechanical property
measurements using atomic force microscopy (AFM). Figure
3
1
A) catalyzed the growth of coiled segments (#3 and #5) at
050 °C and straight ones at 1100 °C (#4 and #6). Furthermore,
even the coiled segments of this nanowire had two different

Silica Nanocoils
J. Phys. Chem. B, Vol. 110, No. 16, 2006 8299
Figure 7. Four isolated individual SiNCs on a Si wafer grown from
type B Co nanoparticles.
Figure 5. SEM image of eight SiNCs on a Si wafer. Type B Co
nanoparticles were used. The scale bar is 200 nm.
Figure 8. Weight (fg) and length (mm) of SiNCs as a function of
growth time (min). Both exhibit a near-linear relationship.
temperature. A small kink is visible near the transition point in
Figure 6D, indicating a possible structural change at that place.
We also used very low density deposition of type B
nanoparticles to limit the coalescence of Co nanoparticles on
the surface. In this case, only isolated SiNCs were visible, as
shown in Figure 7. Each SiNC again possessed a different coil
diameter and helicity or pitch. This growth configuration was
suited for making prototype nanoscale devices from these
SiNCs.
The length of the SiNCs may not be a good parameter to
define the growth rate because of the differences in pitch, coil,
and nanowire diameters. Therefore, we used other parameters
such as the length and volume of the nanowires in those SiNCs
or the overall weight (assuming the density is the same as regular
SiO2) of the SiNCs to calibrate the growth rate. Figure 8 shows
the relationship between the growth time and the nanowire
length and weight of the SiNCs. Both length and weight seemed
to follow a near-linear relationship with the growth time.
IV. Discussion
We performed yield measurements and found that almost no
nanowires could be produced at temperatures below 1030 °C.
Because more SiNCs were grown at a temperature close to the
lowest possible temperature to produce any nanowires, we
speculate that the growth of SiNCs was caused by anisotropy
of the nanoparticles (metal or metal silicides) at near melting
point (1035 °C). Since Si-Co bulk binary alloys have a melting
point of 1239 °C, a ∼200 °C decrease in melting temperature
indicates a moderate quantum effect for the nanoparticle
Figure 6. SEM images of several different SiNCs, including multiple
SiNCs grown from the same nanoparticles (A), SiNC double helix (B),
an isolated long SiNC (C), and a SiNC with two pitches (D). The scale
bars are 500 nm in A and B and 1 µm in C and D.
27
catalysts used in this work. Because catalytic growth of SiNWs
2
8-30
requires the migration of Si through catalysts,
the result of
no nanowires below 1030 °C implies that the rate of migration
might be too low, thus suggesting that the melting point was
below but very close to 1035 °C for most catalytic nanoparticles
6D shows another hybrid SiNC. In this case, the pitch changes
from tight to loose in the middle of the growth even at the same

8300 J. Phys. Chem. B, Vol. 110, No. 16, 2006
Qu et al.
in the size range of 10-80 nm. As the temperature approaches
the melting point, nanoparticles may not take the form of molten
spheres to produce straight nanowires (symmetric with respect
to the growth axis). Instead, the inhomogeneity in these
nanoparticles at near melting point results in anisotropic growth,
which makes the nanowires coil. As a result, nanowires
produced at near melting point are more coiled, and those
produced at a higher temperature at which the nanoparticle
catalysts are totally molten are straight. For smaller nanopar-
ticles, the melting temperatures are more likely lower than 1035
As mentioned before, the nanoparticles observed at the tips
of these short nanowires were most likely not the same as those
originally deposited on the Si wafer, because those nanoparticles
might migrate, sinter, and break up on the surface en route to
high-temperature growth conditions. Future work is needed to
study the synthesis and anchoring of uniformly sized nanopar-
ticle catalysts to guarantee controlled growth of only SiNCs.
The growth mechanisms will have to be studied with in situ
methods such as in situ TEM or X-ray absorption spectroscopy
because the structures of nanoparticles and SiNCs at the growth
3
3,34
°C, due to a larger quantum effect, and hence they only catalyze
temperature are needed.
the growth of straight nanowires. However, one cannot rule out
the possibility of impurities in those small nanoparticles, which
may result in an increased melting temperature. This may
explain why some SiNCs were made of small diameter
nanowires. Other possibilities include the contact angle anisot-
ropy (CAA), which may control the growth even when the
nanoparticles are molten.31
The coiled and straight hybrid silicon-based nanowires with
varying pitches can be used as composite nanomaterials or
templates for making other nanomaterials. SiNCs of different
tensile strengths can be used in nanoscale mechanical, photonic,
and electronic devices. Furthermore, functionalized small coil
diameter SiNCs may be developed to mimic the function of
biological macromolecules.
The improvement of type B Co nanoparticle over type A in
catalyzing the growth of SiNCs may be understood as follows.
When type A nanoparticles were used, many of them had to
undergo coalescence to form larger nanoparticles in order to
make SiNCs at ∼1050 °C because of the requirement for
anisotropy at near melting temperature. Coalescence could
happen when the substrate temperature was raised above 500
Acknowledgment. This work is partially supported by NSF
CAREER award (CHE0135132) and University of California
Campus-Laboratory Exchange grants. We thank the National
Center for Electron Microscopy (NCEM) for their support of
this work. We thank Drs. C. Song and V. Radmilovic at NCEM
for their assistance. The NCEM is a DOE facility. We thank D.
Masiel, Dr. C. Mitterbauer, and Prof. N. Browning at UC Davis
for their assistance in TEM measurements. J.D.C. thanks Tyco
for the Tyco Electronic Fellowship for Research in Fundamental
Materials.
32
°C, as shown by an in situ TEM study. As a result, uncoalesced
small nanoparticles in the sample (having a lower melting
temperature) catalyzed the growth of small diameter straight
SiNWs at 1035 °C. Coalesced larger nanoparticles, on the other
hand, catalyzed the growth of SiNCs at 1035 °C, as shown in
Figure 2. At 1100 °C, all the nanoparticles catalyzed the growth
of straight SiNWs. For type B Co nanoparticles used in this
work, few nanoparticles were smaller than 10 nm. Therefore,
the yield of SiNCs using type B Co nanoparticles was relatively
higher.
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