Full text of DOI:10.1557/JMR.2000.0018

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Influence of reduction mechanism on the morphology of
cobalt nanoparticles in a silica-gel matrix
A. Basumallick
Department of Metallurgical Engineering, Bengal Engineering College (D.U.),
Howrah-711 103, India
G.C. Das and S. Mukherjee
Department of Metallurgical Engineering, Jadavpur University, Calcutta-700 032, India
(Received 16 June 1999; accepted 7 October 1999)
Cobalt-chloride- and dextrose-containing silica gels were reduced in situ under nitrogen
atmosphere in the temperature range of 600 to 950 °C. Analysis of kinetic data on the
in situ reduction shows that, in the temperature range of 600 to 750 °C, the contracting
geometry type and, in the temperature range of 800 to 950 °C, the nucleation and
growth type of mechanisms remain operative. The shape and size of the reduced cobalt
nanoparticles in the silica matrix was studied by examining the transmission electron
micrographs of the reduced Co/SiO2 samples. The morphology of the reduced metallic
particles was found to be influenced by the change in reduction mechanism.
I. INTRODUCTION
and CoCl2 in ethyl alcohol. The volume ratio of
C2H5OH:TEOS was maintained at 4:1 for all the gel
samples. The detailed preparation technique has been de-
scribed elsewhere.9 The resulting gels were then sub-
jected to isothermal reduction treatment in the temperature
range 600 to 950 °C at an interval of 50 °C under N2
atmosphere. The N2 flow rate was maintained at 5 cc/s
throughout the experiment. The reduction treatments
were carried out in an electrical heating furnace. The
desired temperature was maintained by a PID controller
with an accuracy of ±1 °C. The heat treatment led to the
decomposition of dextrose:
During the last few years preparation of glass–metal
nanocomposites via the sol-gel route has gained consid-
erable importance.1–5 This new class of materials exhib-
its remarkable electrical2,3,6 and magnetic7 properties,
which has raised the possibility of their commercial ex-
ploitation as substrates for semiconduction technology.8
The shape and size of the nanoparticles present in the
host matrix influence the physical properties to a great
extent. However, it has been reported that the shape and
size of the metallic islands in the host matrix vary within
wide limits.9 Such wide deviation in shape and size im-
parts a great deal of inconsistency in the results of
physical characterization of these novel materials. There-
fore, we feel strongly that, unless the physical properties
are tailored to be uniform through precise control of
shape and size, the possibility of commercial exploita-
tion of these materials appears to be eclipsed. Control
over the morphology of the particles from the fundamen-
tal viewpoint can be achieved by controlling the kinetic
parameters and thereby by the mechanism(s) of conver-
sion of gels to nanocomposites. Considering the above
views in the backdrop, the present paper reports the ef-
fect of kinetic parameters, e.g., temperature and time,
and conversion mechanism(s) on the morphology of
CoCl2-containing SiO2 gels reduced to Co–SiO2
nanocomposites.
C6H12O6
which subsequently generated H2 in situ:
6C + 6H2O 6CO + 6H2
6C + 6H2O ,
.
The H2 so generated in situ reduced CoCl2 at the sites.
CoCl2 + H2 ס Co + 2HCl
The HCl vapor generated during the course of the heat
treatment is absorbed in a known volume of double-
distilled water. The pH values of the resulting HCl so-
lution were recorded as a function of heat-treatment time.
The pH values of the solution yield [H+] in the solution
from which the fraction of CoCl2 reduced was calculated.
The gels on complete reduction would yield 5 wt% Co/
SiO2 nanocomposite.
III. RESULTS AND DISCUSSION
II. EXPERIMENTAL
The effect of time and temperature on the in situ iso-
thermal reduction of CoCl2-containing gel samples in the
temperature ranges 600 to 750 °C and 800 to 950 °C are
SiO2 gels containing CoCl2 and a stoichiometric
amount of dextrose as reductant were prepared by us-
ing a sol containing tetraethyl orthosilicate (TEOS)
102
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A. Basumallick et al.: Influence of reduction mechanism on the morphology of cobalt nanoparticles in a silica-gel matrix
shown separately in Fig. 1 and Fig. 2, respectively, since
the reduction characteristics and mechanisms are con-
spicuously different from each other in the above men-
tioned temperature ranges.
The appropriate reaction mechanism(s) which remain
operative during the course of reduction has been iden-
tified by analyzing experimental kinetic data on time-
dependent fractional conversion through reduced time
analysis. The analysis is as follows. The general equa-
tion10 for a particular mechanism is written as
find out the mechanism which remains actually opera-
tive. The list of relevant mechanisms along with the form
of g(␣) and symbols is shown in Table I.
The reduced time plots corresponding to Figs. 1 and 2
are shown in Figs. 3 and 4, respectively. It is evident
from Fig. 3 that in the low-temperature range the con-
tracting geometry (CG3) type of mechanism remains op-
erative in the initial stages of reduction which changes to
a diffusion-controlled mechanism (D1, D3, or D4) at
higher fractional conversion. The change of mechanism
has been observed to occur above a value of ␣ ס 0.45.
Therefore, suitable values of ␣ ജ 0.45 were selected over
which the reduced time analysis has been done sepa-
rately. The dotted lines in the reduced time plot (Fig. 3)
g(␣) ס kt
,
(1)
where g(␣) is an appropriate function of fractional con-
version ␣, t is the time, and k is the specific rate constant
which depends on the type of mechanism(s).
For ␣ ס n,
g(n) ס ktn
,
(2)
where tn is the time required for fractional conver-
sion of n.
Dividing Eq. (1) by Eq. (2), we get
g(␣)/g(n) ס t/tn ס ␪
,
(3)
where ␪ is the reduced time. Equation (3) is independent
of k, the reaction rate constant, which is the temperature-
dependent term in kinetics. Therefore, for the same
mechanism operative over the entire range of tempera-
ture, the computed ␣ versus ␪ plot from experimental
data will coincide with the theoretically computed ␣ ver-
sus ␪ plot for the mechanism(s). By comparing the com-
puted ␣ versus ␪ plot for the mechanisms, it is possible to
FIG. 2. Fractional conversion (␣) versus time (t) plots of the isother-
mal in situ reduction of CoCl2-containing SiO2 gels in the temperature
range of 800 to 950 °C.
TABLE I. Mechanisms of gas–solid reactions.
Name
Symbol
Form of g(␣) ס kt
Diffusion-controlled type
Parabolic
Valensi Barrer
Ginstling Brounhtein
Jander
D1
D2
D3
D4
␣2 ס kt
␣ + (1 − ␣) ln(1 − ␣) ס kt
1 − 2/3␣− (1−␣)2/3 ס kt
[1 − (1 − ␣)1/3]2 ס kt
Contracting Geometry type
Linear
CG1
CG2
CG3
␣ ס kt
Cylindrical symmetry
Spherical symmetry
Nucleation and growth type
Avrami Erofeev n ס 1.5
Avrami Erofeev n ס 2.0
Avrami Erofeev n ס 3.0
1 − (1 − ␣)1/2 ס kt
1 − (1 − ␣)1/3 ס kt
NG1
NG2
NG3
[−ln(1 − ␣)]1/1.5 ס kt
[−ln(1 − ␣)]1/2.0 ס kt
[−ln(1 − ␣)]1/3.0 ס kt
FIG. 1. Fractional conversion (␣) versus time (t) plots of the isother-
mal in situ reduction of CoCl2-containing SiO2 gels in the temperature
range of 600 to 750 °C.
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A. Basumallick et al.: Influence of reduction mechanism on the morphology of cobalt nanoparticles in a silica-gel matrix
The validity of the operating mechanisms can be es-
tablished on the basis of the fact that the vapor pressure
of CoCl2 in the temperature range 600 to 700 °C is neg-
ligibly small11 and the reduction occurs only between the
in situ generated H2 and solid CoCl2 particles present in
the nanosized interconnected pores of the SiO2 gels.
Therefore, similar to other gas–solid reactions it is quite
logical that the reduction mechanism will depend signifi-
cantly on the initial geometry of the CoCl2 particles.
Initially, a thin layer of metallic Co will be formed over
the CoCl2 particles which come in contact with the in situ
generated H2. Further, reduction will occur at the Co/
CoCl2 interface by the diffusion of H2 at the reaction
interface which will move toward the center.
Figures 5(a) and 6(a) represent the transmission elec-
tron photographs of the nanocomposites reduced at 650
and 750 °C, respectively. It is clearly evident from the
photographs that the nanosized Co particles embedded in
the SiO2 matrix possess either cylindrical or spherical
shape, and the sharpness in their contours has been ob-
served to decrease appreciably with increase in tempera-
FIG. 3. Reduced time plot of the isothermal in situ reduction of
CoCl2-containing SiO2 gels in the temperature range of 600 to 750 °C.
FIG. 4. Reduced time plot of the isothermal in situ reduction of
CoCl2-containing SiO2 gels in the temperature range of 800 to 950 °C.
represent the superimposed reduced time plots for ␪ ס
t/t0.45. The analysis has been done separately due to the
following reasons. According to the reduced time analy-
sis, g(␣1)/g(␣2) ס ktt/k2tn, where ␣1 and ␣2 are the frac-
tional conversion at times t and tn, respectively. It is
worth mentioning that k1 and k2 will be equal only when
the mechanism remains isokinetic at a particular tem-
perature in the time span of t to tn. Therefore, if it does
not remain isokinetic, the related mechanism cannot be
identified accurately.
FIG. 5. (a) TEM photograph of the CoCl2-containing SiO2 gel sample
reduced at 650 °C. (b) Corresponding SAD pattern.
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A. Basumallick et al.: Influence of reduction mechanism on the morphology of cobalt nanoparticles in a silica-gel matrix
In the temperature range of 800 to 900 °C the reduced
time plot (Fig. 4) clearly indicates that the nucleation and
growth (NG) type of mechanism remains operative as
compared to the operative contracting geometry (CG)
type of mechanism in the lower temperature range. The
validity of the nucleation and growth type of mechanism
can be physically established by taking recourse to the
fact that the equilibium vapor pressure of CoCl2 rises
sharply in the range of 800 to 950 °C.11 Therefore, it is
highly probable that the in situ generated H2 will reduce
CoCl2 in the vapor state. Since the melting point of Co is
very high (1480 °C), solid Co particles will precipitate
out from the vapor phase by a nucleation and growth
process. However, conversion of CoCl2 to metallic Co in
the gaseous state will decrease the vapor pressure of
CoCl2. Therefore, to maintain the equilibrium vapor
pressure, CoCl2 will be vaporized continuously. Since
the reduction in the gaseous phase is much faster than the
gas–solid or gas–liquid reaction, nucleation and growth
of Co from the vapor state will control the kinetics of
reduction. The amount of Co precipitation and the
growth of fine Co particles on increasing the reduction
temperature have been clearly revealed in the TEM pho-
tograph. Therefore, the nucleation and growth type of
mechanism is physically viable.
Figures 7(a) and 8(a) represent the TEM micrographs
of the reduced gel samples at 850 and 950 °C. On ex-
amination of the micrographs carefully, an agglomera-
tion of very fine spherical particles, which possess
features similar to those of the particles deposited from
the gaseous phase,12 can be observed. It is also observed
that with the increase in temperature the deposition of
FIG. 6. (a) TEM photograph of the CoCl2-containing SiO2 gel sample
reduced at 750 °C. (b) Corresponding SAD pattern.
metallic Co particles from the gaseous phase increases
and the fine particles deposited initially grow in size by
ture. This has been attributed to the insipient fusion of
unreduced CoCl2 in the Co–CoCl2 mixture. The above
observations are direct evidence of the fact that the re-
duction mechanism is dependent on the initial geometry
of the CoCl2 particles and is of contracting geometry
type. The presence of Co particles in the matrix has been
confirmed by computing the dhkl values from the corre-
sponding selected-area diffraction (SAD) pattern shown
in Figs. 5(b) and 6(b), respectively. The computed dhkl
values have been found to match reasonably well with
the ASTM dhkl values of Co.
particle coarsening. The particles deposited at the initial
stages act as nuclei for further deposition. Therefore, this
observation corroborates the fact that the nucleation and
growth type mechanism remains operative. Figures 7(b)
and 8(b) represent the corresponding SAD pattern in this
temperature range. The dhkl values computed from the
SAD pattern have been again found to match reasonably
well with the ASTM standard dhkl values of Co.
In the later stage of reduction, the adsorped water va-
por becomes exhausted and the H2 gas generated at this
stage is only through the water vapor available from the
polycondensation of the SiO2 gel. The polycondensation
reaction being sluggish in nature tends to lower H2 gas
generation and subsequently lowers the availability of H2
at the reaction interface, which in turn lowers the overall
gas–solid reaction and the rate of fractional conversion.
The above effects causes the change in mechanism from
contracting geometry to diffusion-controlled type.
IV. CONCLUSIONS
The conclusions which are drawn from the present
study are as follows.
(1) During the in situ reduction of CoCl2 in the silica-
gel matrix in the temperature range of 600 to 750 °C, the
contracting geometry type of mechanism remains opera-
tive in the initial stages which changes to the diffusion-
controlled type of mechanism at higher fractional
conversion.
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A. Basumallick et al.: Influence of reduction mechanism on the morphology of cobalt nanoparticles in a silica-gel matrix
FIG. 7. (a) TEM photograph of the CoCl2-containing SiO2 gel sample
reduced at 850 °C. (b) Corresponding SAD pattern.
FIG. 8. (a) TEM photograph of the CoCl2-containing SiO2 gel sample
reduced at 950 °C. (b) Corresponding SAD pattern.
REFERENCES
(2) The operating in situ reduction mechanism in the
temperature range of 800 to 950 °C is markedly different
from the operating mechanism at 600–750 °C and has
been found to be of nucleation and growth type.
(3) In the low-temperature range the initial geometry
of the CoCl2 particles present in the silica matrix exerts
a significant influence on the shape and size of the re-
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(4) The nanosized reduced Co particles present in the
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of precipitation from the gaseous phase have been ob-
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particles has been observed to increase with the increase
in reduction temperature.
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