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.
J. Mater. Res., Vol. 15, No. 1, Jan 2000
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