Preparation of cobalt nanoparticles by hydrogen reduction of cobalt chloride in the gas phase

Article abstract of DOI:10.1016/j.materresbull.2003.09.023

Cobalt nanoparticles were produced by the hydrogen reduction of cobalt chloride vapor in a multistage tubular aerosol flow reactor. Reaction zone temperature, preheating temperature, mole fractions of CoCl₂ and H₂, and residence time were considered as key process variables for the control of particle size and size distribution. Ranging from 50 to 78 nm in average diameter, cobalt nanoparticles with narrow size distributions were synthesized throughout our experiments. All of the considered process variables affected the particle size and size distribution in the synthesis of cobalt nanoparticles. As the reaction zone temperature and the CoCl₂ mole fraction increased, the average particle diameter increased. But the average particle diameter decreased as the residence time of reactants increased.

Full text of DOI:10.1016/j.materresbull.2003.09.023

Materials Research Bulletin 39 (2004) 63–70
Preparation of cobalt nanoparticles by hydrogen reduction of
cobalt chloride in the gas phase
a,*
Hee Dong Jang , Dae Won Hwang , Dong Pyo Kim , Heon Chang Kim ,
b
b
c
d
Byung Yoon Lee , In Bum Jeong
d
a
Nano-Materials Group, Korea Institute of Geoscience and Mineral Resources, Daejeon 305-350, South Korea
Department of Fine Chemical Engineering and Chemistry, Chungnam National University, Deajeon 305-764, South Korea
b
c
Department of Chemical Engineering, Hoseo University, Asan 336-795, South Korea
d
R/D center, Chang Sung Co., Incheon 405-100, South Korea
Received 19 May 2003; received in revised form 18 August 2003; accepted 5 September 2003
Abstract
Cobalt nanoparticles were produced by the hydrogen reduction of cobalt chloride vapor in a multistage tubular
aerosol flow reactor. Reaction zone temperature, preheating temperature, mole fractions of CoCl and H , and
2
2
residence time were considered as key process variables for the control of particle size and size distribution.
Ranging from 50 to 78 nm in average diameter, cobalt nanoparticles with narrow size distributions were
synthesized throughout our experiments. All of the considered process variables affected the particle size and size
distribution in the synthesis of cobalt nanoparticles. As the reaction zone temperature and the CoCl mole fraction
2
increased, the average particle diameter increased. But the average particle diameter decreased as the residence
time of reactants increased.
#
2003 Elsevier Ltd. All rights reserved.
Keywords: A. Metals; B. Nano structures; C. Chemical synthesis
1
. Introduction
Metal particles smaller than 100 nanometers in primary particle diameter are generally considered as
nanoparticles. Such metal nanoparticles often exhibit very interesting electronic, magnetic, optical, and
chemical properties. For example, their high surface-to-volume ratios have large fractions of metal
atoms at surface available for catalysis [1,2]. In the case of cobalt (Co) nanoparticles, they are expected
to possess exceptionally high-density magnetic property, sintering reactivity, hardness levels, excellent
*
Corresponding author. Fax: þ82-42-861-9727.
E-mail address: hdjang@kigam.re.kr (H.D. Jang).
0025-5408/$ – see front matter # 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2003.09.023

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impact resistance properties, etc. Many studies on synthesis and magnetic properties of nanoscale metal
particles such as Fe, Au, Pd, and composites have been reported [3–6].
But only a few studies for the synthesis of Co particles by gas phase chemical reaction have been
reported in the literature. The preparation of metal particles by gas phase chemical reaction becomes
increasingly important mainly due to the easy control of process conditions, in turn particle size,
particle crystal structure and purity [7]. Saeki et al. [8] prepared cobalt particles from the cobalt
chloride (CoCl ) both by gas–solid reaction and by gas–gas reaction. They found that the hexagonal
2
crystal structured Co particles of several microns in diameter were produced by the gas–solid reaction
while cubic Co nanoparticles by the gas–gas reaction. However, the effects of process variables on the
control of the particle size and the crystal structure were not investigated in their study. Otsuka et al. [9]
performed series of experiments to produce Ni, Co, Fe particles by the hydrogen reduction of NiCl2,
CoCl , and FeCl , respectively. Their main focus was the preparation of Ni particles ranging from 50 to
2
2
138 nm in particle diameter with process conditions, but the detailed discussions on the characteristics
of the formation and growth of metal particles were not performed.
This paper presents a systematic experimental study on the preparation of Co nanoparticles by the gas
phase reduction of CoCl with hydrogen in a multistage tubular aerosol flow reactor. The characteristics
2
of the formation and growth of cobalt nanoparticles in the gas phase are discussed. In our experiments,
reaction zone temperature, preheating zone temperature, mole fractions of reactants, and residence time
in the aerosol reactor were considered as key process variables for the control of particle size, size
distribution, and crystal structure.
2
. Experiments
A schematic drawing of our experimental apparatus is shown in Fig. 1. The apparatus consists of
particle generation, particle collection, and off-gas treatment parts. A multistage tubular aerosol reactor,
made of 155 cm long quartz, in the particle generation part is subdivided into a CoCl evaporation zone,
2
a reactant preheating zone, and a reaction zone. A detailed explanation of characteristics of the
apparatus and experimental procedure were described in our previous paper [10].
The particle size and morphology were characterized by a transmission electron microscopy (TEM,
Philips Model CM12). The average particle diameter and geometric standard deviation of the powders
H
2
T
T
Ar
Off gas
Co Cl
2
T
Evaporation
Preheating
Reaction
Particle Generation
Particle Collection
Fig. 1. Schematic drawing of experimental apparatus for the synthesis of Co nanoparticles.

H.D. Jang et al. / Materials Research Bulletin 39 (2004) 63–70
65
were determined by counting more than 300 particles from TEM pictures [11]. X-ray diffractometer
XRD, Rigaku Co. Model RTP 300 RC) was used to obtain X-ray diffraction patterns of the powders.
(
3
. Results and discussion
3
.1. Effect of temperature
The overall reaction of CoCl and H in gas phase can be described as follows.
2
2
CoCl ðgÞ þ H ðgÞ ! CoðsÞ þ 2HClðgÞ
2
2
Since the changes of Gibb’s free energy for this reaction become negative at temperature above
500 8C [12], the reaction zone temperature was varied in the range of 800–950 8C. The effects of the
reaction zone temperature on the conversion as well as on the particle diameter were investigated
while fixing all other conditions (preheating temperature: 900 8C, CoCl mole fraction: 0.0168%,
2
total gas flowrate: 4 l/min). As the reaction zone temperature increased from 850 to 950 8C, the
conversion monotonically increased from 72.5 to 99.9% and the average particle diameter also
increased from 51 to 60 nm, as shown in Fig. 2. To explain the effects of the reaction zone
temperature on the particle growth in the tubular aerosol reactor, two competing aspects need to be
considered. Firstly, the rapidly increased nucleation rate at higher temperature produces much larger
number of nuclei and sintering rate becomes high, then greatly enhancing the opportunity to grow
into larger particles by high coagulation and sintering rate among nuclei [13]. Secondly, the
residence time of the reactants in the reaction zone decreases at higher temperature due to volume
expansion, providing the generated nuclei less chance to grow into larger particles by coagulation
and sintering [14]. Since the particle diameter and the conversion monotonically increased with the
reaction zone temperature, the particle growth mechanism in our experimental condition should be
dominated by the first aspect.
1
1
0
0
0
0
.1
.0
.9
.8
.7
.6
750
800
850
900
950
1000
o
Temperature ( C)
Fig. 2. Degree of conversion of CoCl
2
into Co increased as the temperature of reaction zone in the tubular aerosol reactor
increased (preheating temperature: 900 8C, total gas flowrate: 4 l/min, CoCl mole fraction: 0.0168%).
2

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The preheating zone temperature was previously reported as one of the effective variables to control
the particle diameter and size distribution. Jang and Jeong [14] reported that the smaller particle with
narrower size distribution of titania powder could be synthesized by heating reactants up to the reaction
zone temperature prior to entering the reaction zone. Here the effects of preheating temperature on the
particle diameter and sized distribution were also investigated for cobalt nanoparticles. As the
preheating zone temperature decreased from 900 to 800 8C by 50 8C while fixing the reaction zone
temperature at 900 8C, the average particle diameter slightly decreased from 53 to 50 nm. However the
geometric standard deviation of the powder increased from 1.34 to 1.45. When the preheating zone
temperature is lower than the reaction temperature, the conversion can decrease and a temperature
gradient can develop in the reaction zone. As a consequence, the particle diameter is expected to
decrease due to lower conversion, and nuclei generated at the nozzle surface may act as seeds for
heterogeneous nucleation resulting in wider size distribution of the powder. From these results the
preheating zone temperature was found to be effective also for the synthesis of Co particles having
smaller diameter and narrower size distribution.
3
.2. Effects of reactant concentrations
As the concentration of aerosol precursor increases at the fixed reaction temperature and gas flow
rate, the growth rate of nuclei by the coagulation and surface reaction becomes faster than the
nucleation rate during the gas phase reaction. Thus, larger primary particles are generated at the higher
precursor concentrations [15]. Nanometer-sized primary particles form aggregates in the gas phase
because of interactive forces between particles. Such aggregates change into large particles when the
temperature in the gas phase is high enough to sinter the aggregates [16,17].
Fig. 3 shows the effects of the CoCl mole fraction on the particle size and size distribution at the
2
constant conditions (preheating temperature: 900 8C, reaction zone temperature: 950 8C, total gas
flowrate: 4 l/min). As CoCl mole fraction increased from 0.0084 to 0.0862%, the average particle size
2
100
2.0
1
1
1
1
.8
.6
.4
.2
8
6
4
2
0
0
0
0
1.0
1
0-3
10-2
10-1
100
Mole fraction of CoCl (%)
2
Fig. 3. As the mole fraction of CoCl increased, average particle diameter increased but the geometric standard deviation of
2
powder was not changed in the tubular aerosol reactor (preheating temperature: 900 8C, reaction temperature: 950 8C, total gas
flowrate: 4 l/min).

H.D. Jang et al. / Materials Research Bulletin 39 (2004) 63–70
67
Fig. 4. TEM image of Co nanoparticles prepared at the different mole fraction of CoCl . Primary particles looked like single
2
spherical crystals of nearly uniform size and the directional linkage of particles was observed.
of Co powders increased from 55 to 78 nm. However, the geometric standard deviation remained nearly
constant around 1.35. Fig. 4 shows the TEM images of Co powders produced at the condition of Fig. 4.
The morphology of Co particles was spherical in shape, primary particles seemed to be single crystals of
nearly uniform size, and the directional linkage of particles due to magnetic interaction between particles
was observed. By the XRD analysis as shown in Fig. 5, the crystal structure of the cobalt powders was
cubic. The production rate of Co nanoparticles was 1 g/h at 0.0862% of CoCl mole fraction.
2
As can be seen in Fig. 4, the increase in the number of larger particles with CoCl concentration
2
appears to indicate that they are produced by the coagulation, surface reaction, and sintering. However,
smaller particles that seemed to be grown by the coagulation and surface reaction are observed at the
low CoCl concentration. If the sintering is dominant in the particle growth, it is expected that there
2
should be many large particles at the low CoCl concentration because the primary particle size is
2
comparatively small at the low CoCl concentration. Therefore, it is suggested that particle growth by
2
the sintering is not dominant, and is only effective at the higher CoCl concentrations in the present
2
experimental condition.
The effects of the H mole fraction on the particle size were also investigated while fixing all other
2
conditions (CoCl mole fraction: 0.0168%, total gas flowrate: 4 l/min, preheating temperature: 900 8C,
2
reaction zone temperature: 900 8C). As the H mole fraction increased from 12.49 to 49.99% by
2

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H.D. Jang et al. / Materials Research Bulletin 39 (2004) 63–70
500
400
300
200
100
0
:
Co (cubic)
20
30
40
50
60
70
2θ (degrees)
Fig. 5. X-ray diffraction pattern of product particles showed pure cobalt crystal having cubic structure.
100
2.0
1
1
1
1
.8
.6
.4
.2
80
60
40
20
1.0
5
10
100
Mole fraction of hydrogen (%)
Fig. 6. Change of hydrogen flowrate at the fixed total flowrate was not effective on the average particle diameter and
geometric standard deviation in the tubular aerosol reactor (reaction temperature: 900 8C, preheating temperature: 900 8C,
total gas flowrate: 4 l/min, CoCl mole fraction: 0.0168%).
2
varying the hydrogen flow rate from 0.5 to 2 l/min at the fixed total flowrate, the average particle
diameter slightly decreased from 53 to 48 nm, as shown in Fig. 6. The alternation of the H mole
2
fraction, in the range of large excess H , gave small effect on particle size and distribution.
2
3
.3. Effects of residence time
The effect of the residence time in the reaction zone was also investigated by varying the total gas
flow rate from 3 to 5 l/min while fixing all other conditions (preheating temperature: 800 8C,
ꢁ
5
temperature of reaction zone: 900 8C, CoCl feed rate: 3 ꢀ 10 mol/min). As the residence time
2
increased from 0.64 to 1.07 s, the particle size increased from 50 to 59 nm but the uniform size

H.D. Jang et al. / Materials Research Bulletin 39 (2004) 63–70
69
100
2.0
1
1
1
1
.8
.6
.4
.2
8
6
4
2
0
0
0
0
1.0
1.25
0
.50
0.75
1.00
Residence time (sec)
Fig. 7. As residence time of reactants in the reaction zone decreased, average particle diameter decreased but geometric
standard deviation was nearly constant in the tubular aerosol reactor (reaction temperature: 900 8C, preheating temperature:
feed rate: 3 ꢀ 10ꢁ mol/min).
5
9
00 8C, CoCl
2
distribution was maintained, as shown in Fig. 7. This result is due to the reduction of possibility for the
particles to grow by coagulation among small particles as the residence time decreases.
4
. Conclusions
The synthesis of Co nanoparticles from CoCl vapor by hydrogen reduction in the gas phase was
2
investigated with a multistage aerosol reactor. Following important characteristics on the growth of
cobalt nanoparticles were obtained.
1
. As the reaction zone temperature increased, the particle diameter and the conversion of CoCl2
monotonically increased. The increase of particle diameter with the reaction zone temperature was
dominated by the high coagulation and sintering rate at higher temperature. The preheating of
reactants was effective for the synthesis of Co nanoparticles having smaller diameter and narrower
size distribution.
2
3
4
. As the CoCl mole fraction increased, the average particle diameter increased. The particle growth
2
was not dominated by sintering.
. As the residence time increased, the particle size increased. This is due to the reduction of possibility
for the particles to grow by coagulation among small particles.
. Throughout present experiments, spherical cobalt nanoparticles showing nearly uniform size were
produced and the crystal structure was cubic.
Acknowledgements
This study was supported by the Korea Ministry of Commerce, Industry and Energy.

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