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

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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.

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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

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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

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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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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.

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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.

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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.

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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

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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.