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Paper # 070HE-0199 Topic: Heterogeneous combustion, sprays & droplets
8th
U. S. National Combustion Meeting
Organized by the Western States Section of the Combustion Institute
and hosted by the University of Utah
May 19-22, 2013
Morphological Control of Metal Oxide Nanowire Heterostructures
Synthesized by Sol-flame Method
Runlai Luo1, In Sun Cho
2, Yunzhe Feng
1, Lili Cai
2, Pratap M. Rao
2, and Xiaolin Zheng
2
1Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305,
United States 2 Department of Mechanical Engineering, Stanford University, Stanford, CA 94305, United States
Abstract: Nanowire heterostructures, such as core/shell nanowires and nanoparticle-decorated
nanowires, are versatile building blocks for a wide range of applications because they integrate
dissimilar materials at the nanometer scale to achieve unique functionalities. Our group has recently
developed a simple and general sol-flame method that combines solution chemistry and rapid flame
annealing to decorate nanowire arrays with other materials in the form of shells or chains of
nanoparticles. In this report, we investigate the fundamental aspects of morphology control of the
nanowire heterostructures synthesized by the sol-flame method. We use copper (II) oxide (CuO)
nanowires decorated by cobalt (II,III) oxide (Co3O4) as a model system and study the effects of
various solution parameters on the morphology of the decorated Co3O4. In a typical sol-flame
synthesis, CuO nanowires are dip-coated with a cobalt salt precursor solution, then air dried, and
subsequently heated in the post-flame region of a premixed co-flow flame at a typical temperature of
990 o
C for only 5s. We find that the final morphology of Co3O4 on the CuO nanowires is closely
connected to the properties of both the solvent and the cobalt salt in the cobalt-precursor solution.
First, the gaseous products generated by solvent combustion are responsible for the formation of
Co3O4 nanoparticle chains. The gases generated by the solvent combustion blow the cobalt salts
radially outward from the nanowire as the cobalt salts precipitate and decompose to form Co3O4.
Larger amount of gas generation leads to higher degree of Co3O4 nanoparticle branching. Second,
when most of the solvent is removed before the flame annealing step, no Co3O4 nanoparticle chain,
but a Co3O4 shell, is formed instead due to the lack of gas blowing effect. Finally, when a cobalt salt
with high solubility in the solvent is used as a precursor, precipitation does not occur until most of
the solvent has evaporated and combusted. Hence, a Co3O4 shell is formed again due to the lacking
of gas blowing effect. We believe that the new understanding will facilitate the application of the
sol-flame method for the synthesis of nanowire heterostructures with tailored morphologies to satisfy
the needs of diverse applications such as catalysis, sensors, solar cells, Li-ion batteries and
photosynthesis.
1. Introduction
Nanowire (NW) heterostructures, such as radially modulated core/shell NWs, axially modulated
NWs, nanoparticle (NP) -decorated NWs and branched NWs, are of great interest for diverse
applications because they integrate dissimilar materials at the nanometer length scale to achieve
unique and unprecedented functionalities [1]. Nanowire heterostructures have already their potential
2
in applications such as photosynthesis [2,3], gas sensing [4,5], batteries [2,6] and catalysis [7]. For
instance, ZnO/ZnSe heterostructure nanowires yielded a significantly higher photocurrent than the
pristine ZnO nanowire arrays for photoelectrochemical water splitting due to light absorption
enhancement by the ZnSe nanostructures [8]. In addition, SnO2/CdS nanowire-QDs heterostructures
showed a nearly 109% enhancement of photocatalytic activity with respect to the SnO2 nanowires
because CdS increases the light absorption in the visible region [9]. Furthermore, Co3O4/Co(OH)2
core/shell nanowire arrays exhibited over two times as much specific capacitance as Co3O4 nanowire
arrays, and also better cycling stability [10]. Existing synthesis methods for nanowire
heterostructures include sol-gel method [11], hydrothermal method [12], physical/chemical vapor
deposition [13] and self-assembly [14], and they require either complex procedures or sophisticated
equipments, hindering the broad applications of nanowire heterostructures. Our group has recently
developed a simple and general sol-flame method, as illustrated in Fig. 1a, which combines solution
chemistry and rapid flame annealing to decorate nanowire arrays with other materials in the form of
shells or chains of nanoparticles [15]. However, for the sol-flame method, there is lack of
fundamental understanding on the controlling factors that determine the final morphologies of the
formed nanowire heterostructures. Herein, we systematically investigate the major factors that
control the morphology of the nanowire heterostructures synthesized by the sol-flame method. We
use copper (II) oxide (CuO) nanowires decorated by cobalt (II,III) oxide (Co3O4) as a model system
and study the relationship between the metal salt precursor, the solvent and the final morphology of
the decorated Co3O4 on CuO nanowires.
2. Experimental Methods
CuO NWs were synthesized by a thermal annealing method [16–18]. Briefly, Copper wires (wire
diameter: 0.0045 inches; McMaster) with a length of 1 cm were annealed at 550 oC for 12 hrs in air
in a tube furnace (Lindberg/Blue M). CuO NWs grew perpendicularly to the copper wire surface
with diameters in the range of 70 - 120 nm and an average length of 16 µm (Fig. 1b). The cobalt
precursor solutions with a typical concentration of 0.4M were prepared by mixing cobalt acetate
tetrahydrate (Co(CH3COO)2 .
4H2O, 99%, Sigma-Aldrich Chemicals) or cobalt nitrate hexahydrate
(Co(NO3)2 .
6H2O, 99%, Sigma-Aldrich Chemicals) with acetic acid (CH3COOH, 99.7%, EMD
Chemicals). Other solvents, including propionic acid (C2H6COOH, 99%, Mallinckrodt Chemicals),
xylene (C6H4(CH3)2, >96%, Sigma-Aldrich) and their 1:1 (v/v) mixture solution, were also used to
determine the effect of solvents on the final morphology of nanowire heterostructures. After mixing,
the precursor solutions were sonicated for 10 minutes to completely dissolve the cobalt salt and then
aged overnight at room temperature before use. Next, the CuO NWs were dipped into the prepared
cobalt precursor solution and subsequently dried in air. This dip-coating process was repeated three
times to form a conformal precursor shell on top of CuO NWs (Fig. 1c). Finally, the dip-coated CuO
NWs were heated in the post-flame region of a premixed co-flow flame (McKenna Burner) at a
typical temperature of 990 oC for 5 s, for which the post-flame region gas temperature was measured
by a K-type thermocouple (1/16 in. bead size, Omega Engineering, Inc.). The morphology, crystal
structures and element compositions of the prepared nanowire heterostructures were characterized
by scanning electron microscope (SEM, FEI XL30 Sirion, 5 kV), transmission electron microscope
(TEM, Philips CM20 FEG, 200 kV) and TEM-EDS (energy dispersive X-ray spectroscopy),
respectively.
3
3. Results and Discussion
3.1 Effects of solvent on the morphology of Co3O4 on CuO NWs
As discussed above, the as-grown CuO NWs are dip-coated with a cobalt precursor solution (cobalt
salt dissolved in a solvent) to form a shell of cobalt precursor on the NWs, and then dried in air prior
to flame annealing (Fig. 1c). Typically, the cobalt salt is cobalt acetate Co(OAc)2 and the solvent is
acetic acid HAc. To investigate the effect of solvent in the cobalt precursor on the final morphology
of Co3O4, we vary the amount of solvent residual on the NWs by varying the air drying conditions
prior to the flame annealing, i.e., (i) 0.4 h at 25 oC, (ii) 22 h at 25
oC, and (iii) 1.5 h at 130
oC. First,
when the dip-coated sample is dried at 25 oC for 0.4 h (highest amount of solvent residual), a Co3O4
NP chain morphology is formed on CuO NWs after flame annealing (Fig. 1d). Second, the longer
drying duration of 22 h at 25 °C leads to smaller amount of solvent residual and a monolayer coating
of Co3O4 NPs is formed after flame annealing (Fig. 1e). Finally, the amount of solvent residual is
minimized by drying at 130 oC, which is higher than the boiling temperature of acetic acid (118
oC)
but is lower than the decomposition temperature of Co(OAc)2 (230 oC) to avoid decomposition [19].
In this case, no particles are observed at all, but instead, a conformal and dense layer of Co3O4 is
coated onto CuO NWs (Fig. 1f). Importantly, after air drying the sample for 1.5 h at 130 oC, when
the solvent acetic acid is reapplied onto the sample by drop casting, the NP chain morphology is
formed again after flame annealing. According to these results, it is clear that the amount of the
solvent residual in the precursor coating layer is critical for the final morphology of Co3O4 on CuO
NWs. Larger amount of solvent residual leads to the formation of the NP-chain morphology, and
smaller amount of solvent residual leads to the formation of shells, or equivalently thin film coating.
In our previous study, we have proposed that the formation of the NP-chain morphology is due to the
generation of gas and heat during flame annealing [20]. In light of the results above, it suggests that
most of the gas and heat are generated from the evaporation and combustion of the residual solvent
rather than from the decomposition of the cobalt salt itself. To understand the effect of the amount of
gas and heat generation on the morphology of Co3O4, two other solvents, i.e., propionic acid (PA),
and mixture of propionic acid and xylene (PA:XL=1:1 (v/v)), are selected to compare with the acetic
acid (HAc). For all three solvents, the samples are dried for 0.4 h at 25 oC after dip-coating to leave
larger amount of solvent on CuO NWs before flame annealing. The amount of gas production for
each solvent (unit: mol/mL solvent) is calculated on the basis of the combustion products and is
listed together with heat of combustion (kJ/mL solvent) for each solvent in Table 1. Both gas
production and heat of combustion are normalized on the basis of unit volume of solvent because we
assume that the same volume of solvent is coated on CuO NWs after air drying. The resulting
morphology and size distribution of the Co3O4 NPs are compared in Fig. 2 and Table 2. First, as
shown in Fig. 2, all three solvents lead to the formation of Co3O4 NP-chain structure on CuO NWs.
Second, comparing to HAc, PA and PA + XL result in wider branching of the NP-chain because
higher moles of gas are generated with greater expansion due to larger heat of combustion (Table 1).
The average Co3O4 particle size decreases from 60 nm to 47 nm when the solvent is changed from
HAc to PA (Fig. 2d). This can be a combined effect of the increased gas and heat production with
PA during the solvent combustion process, compared to HAc as shown in Table 2. Third, we try to
decouple the contribution of gas and heat production on the morphology of Co3O4 by comparing PA
and mixture of PA and Xylenes (PA:XL=1:1 (v/v)). As listed in Table 1, the heat of combustion of
PA+XL is 30.45 kJ/mol that is about 44% higher than that of PA (21.18 kJ/mol). The amount of gas
generation by PA+XL is 0.0772 mol/mL that is about 96% of PA (0.0802 mol/mL) and the
difference is very small. The mean particle size and size distribution are very close for PA and
4
PA+XL (Fig. 2b-d and Table 2), so the amount of heat generation appears to have only a minor
influence on the chain morphology. Thus, the amount of gas production during solvent combustion
is believed to be the controlling factor of the formation of Co3O4 NP chains and the size of Co3O4
NPs. Higher amount of gas generation from the solvent combustion process leads to smaller
nanoparticle diameter with higher degree of branching.
3.2 Effects of cobalt salt precursor on the morphology of Co3O4 on CuO NWs
So far, we have focused on the effect of solvent on the morphology of the Co3O4 NPs in the
nanowire heterostructure. However, the chemical composition of the cobalt salt precursor is another
important parameter to consider because it affects the solubility, precipitation, and crystallization
behaviors. Here, the cobalt salt is changed from Co(OAc)2 to Co(NO3)2 to illustrate the effect of
cobalt salt on the Co3O4 morphology. The solvent used is acetic acid (HAc) and the sample is air
dried 0.4 h at 25 oC after dip-coating to leave larger amount of solvent. The SEM image in Fig. 3a
clearly shows that when Co(NO3)2 dissolved in HAc is used as the cobalt precursor solution, a shell
instead of NP-chain is formed on CuO NWs. This shell is about 9 nm thick (Fig. 3b) that is coated
on the surface of CuO NWs. The TEM-EDS analysis (Fig. 3c) shows the presence of both Cu and
Co peaks along with the O peak. Further high resolution TEM (HRTEM) characterization (Fig. 3d)
indicates that the CuO NW is a single crystal with a [111] growth direction and the thin film shell is
polycrystalline with interplanar spacing of 0.25 nm, which corresponds to the spacing of (311)
planes of Co3O4. The polycrystalline nature of the Co3O4 layer suggests that the thin shell layer is
formed by sintering of nanoparticles during the flame treatment process.
For the formation of the polycrystalline Co3O4 shell, we believe that the solubility of the cobalt salt
in the acetic acid solvent plays a critical role. Co(NO3)2 has higher solubility in acetic acid than that
of Co(OAc)2 and more soluble salt leads to the formation of shell. A schematic growth mechanism
for Co3O4 is illustrated in Fig. 4. First, CuO NWs are dip-coated with the cobalt precursor solution
containing both solvent and cobalt salt. After the air drying process, we assume that the thickness of
the dip-coated cobalt salt solution layer at the CuO NWs surface is about the same when using the
same solvent. During the flame annealing step, solvent is evaporated, decomposed and combusted
continuously, and the concentration of the cobalt salt in the remaining solvent increases
simultaneously. Co(OAc)2 has lower solubility in acetic acid, so it starts to precipitate out as the
solvent is evaporating and combusting. The gaseous product from the solvent combustion process
blows the precipitated Co(OAc)2 particles radially away from the CuO NW surface. Co(OAc)2
decomposes by the gas heating at the same time, leading to the formation of Co3O4 NP-chain
morphology [19]. Co(NO3)2, in comparison to Co(OAc)2, has higher solubility in acetic acid solution,
due to the common ion effect in that the solubility of a soluble salt is reduced in a solution that
contains an ion in common with that salt [21]. As a result, Co(NO3)2 only starts to precipitate out
when larger amount of the acetic acid has evaporated and combusted. Less solvent leads to the
formation of a shell of Co3O4 composed of sintered nanoparticles, which is very similar to the case
of Fig. 1f where solvent is intentionally evaporated by high temperature drying process before flame
annealing.
4. Conclusions
To summarize, we have investigated the fundamental aspects of morphology control of decorating
CuO NWs with Co3O4 heterostructures by the sol-flame method. Both the solvent and the cobalt
precursor salt greatly impact the final morphology of Co3O4. First of all, the gaseous product
5
generated by solvent combustion is responsible for the formation of Co3O4 NP chains. With higher
amount of gas generation, the degree of Co3O4 NP branching increases and the average diameter of
the NP decreases. Moreover, cobalt salts with high solubility in the solvent prefer the formation of
the Co3O4 shell morphology rather than a nanoparticle chain. Finally, we believe that this new
understanding will facilitate the use of the sol-flame method for the synthesis of nanowire
heterostructures with tailored morphologies to satisfy the needs of diverse applications such as
catalysis, sensors, solar cells, Li-ion batteries and photosynthesis.
Acknowledgements
This research was funded by the ONR/PECASE program and Army Research Office under the grant
W911NF-10-1-0106.
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7
Fig 1. Effects of solvent on the morphology of Co3O4 on CuO NWs. (a) Schematic drawing of the
sol-flame procedure, for which bare CuO NWs (b) are dip-coated with a cobalt precursor containing
cobalt salt and solvent and air dried (c), followed by a rapid flame annealing process to form Co3O4
@CuO NW heterostructure. SEM image of Co3O4 @CuO prepared by the sol-flame method with
different air drying conditions: (d) 25 oC for 0.4 h, (e) 25
oC for 22 h, (f) 130
oC for 1.5 h and (g)
first dried at 130 oC for 1.5 h, then reapplied acetic acid and dried at 25
oC for 0.4 h. Removal of
solvent by extensive drying reduces the formation of the Co3O4 NP-chain morphology.
8
Fig 2. Effects of solvent on the degree of branching and size distribution of Co3O4 NPs. SEM
images of Co3O4 NPs @CuO NW synthesized by the sol-flame method using different solvents: (a)
acetic acid, (b) propionic acid, and (c) propionic acid : xylenes = 1:1 (v/v). (d) Histogram of Co3O4
NPs size distribution for different solvents. Higher amount of gas generation from the solvent
combustion process leads to smaller nanoparticle diameter with higher degree of branching.
9
Fig 3. Effects of cobalt salt precursor on the morphology of Co3O4 on CuO NWs. A thin film of
Co3O4 is formed when cobalt nitrate is used as the metal salt precursor. (a) SEM image of the
CuO/Co3O4 core/shell NWs. (b) TEM image, (c) TEM-EDS spectrum and (d) HRTEM of the
core/shell NW edge, indicating that the shell is a 9nm thick Co3O4 polycrystalline film.
10
Fig 4. Schematic illustration of the effects of metal salt solubility on the morphology of Co3O4
on CuO NWs. (Top) Metal salt with low solubility in the solvent results in the formation of NP-
chain. (Bottom) Metal salt with high solubility results in formation of thin film morphology.
11
Table 1. Comparison of gas production and heat of combustion of different solvents
upon combustion.
# Solvent Combustion reaction Gas Production Heat of Combustion
mol / mL kJ/mL
1 Acetic acid C3H4O2 + 2 O2 --> 2 CO2 + 2 H2O 0.0699 13.74
2 Propionic acid C3H6O2 +
O2 --> 3 CO2 + 3 H2O 0.0802 21.18
3 Xylene C8H10 +
O2 --> 4 CO2 + 5 H2O 0.0734 35.32
4 Propionic acid:
Xylene =1:1 (v/v) 0.0772 30.45
12
Table 2. Statistics of Co3O4 nanoparticle size distribution with different solvents
# Solvent Particle size (nm) Gas
increase° Heat
increase° Mean STD§
1 Acetic acid 60 15 0 % 0 %
2 Propionic acid 47 12 15 % 54 %
3 Propionic acid : Xylene
=1:1 (v/v) 50 10 10 % 122 %
o The percentage increase is calculated relative to the indicated reference solvent: acetic acid; §
STD=standard deviation of the sample.