Abstract:
Nanomaterials may be defined as the materials with, at least, one structural dimension in
the range of 1 - 100 nm. Nanocomposites are a special class of nanomaterials and are of interest
because they exhibit interesting mechanical, electrical, optical and magnetic properties in
addition to high catalytic activity.
Although nanomaterials can be synthesized by many methods but wet synthesis
methods, often offers better control over shape, composition and structure. Wet synthesis
include thermal decomposition, pyrolysis, polyol process, hydrothermal/solvothermal, sol-gel,
electrochemical, chemical / borohydride reduction and, co-precipitation, etc. However, sol-gel is
one of the methods, which offers better control over chemistry and composition. Consequently,
sol-gel is a technique most widely used for the manufacturing and synthesis of metal/inert
ceramic nanocomposites.
Generally, it is difficult to prepare metallic nanoparticles in ceramic matrix, directly
through sol-gel method employed for the preparation of nanocomposites, and a subsequent
pyrolysis and / or hydrogen reduction treatment becomes almost essential. The metallic ions may
also be reduced chemically, but is usually often accompanied by difficulty in controlling reaction
conditions and composition and further demonstrated suitable for surface deposition only.
Metallic species in the sol-gel ceramic could be reduced by radiations also, but this method is
accompanied by inherited safety issues, and found more effective in thin films or solid sections
only. Electrolysis is another very simple and often room temperature technique, that can be
efficiently applied to reduce the metallic ionic species present in solution phase to their
corresponding metallic state. However to get electrodepositable gel, researchers in the past opted
for either long duration for gelation and /or high temperature treatments to get aged gels. In some
cases gelation time was in weeks, in other case temperatures employed were high such as;
>500 o C. Often the technique has been limited to thin gel films for ease in soaking and shorter
electrolytic conducting paths. If these limitations are overcome, this combination may possibly
the simplest, most versatile, fast enough and cost-effective for the formation of metallic
nanoparticles in the oxide matrices.
Presently emphasis has been laid on the development of a synthesis technique based on
sol-gel and electrodeposition by overcoming all the above observed problems. A new technique
based on electrolysis of alcogels has been employed for the synthesis of various metals (Ni, Co
& Fe) and alloys (Ni-Fe, Ni-Co, Fe-Co, Fe-Zn and Ni-Zn) nanoparticles in the pores of silica gel.
Chloride(s) of respective metal(s) were used as metal precursor and introduced into the alcogel
during sol formation step. The as–synthesized alcogels without subsequent heat treatments were
immediately subjected to electrochemical reduction, consequently forming metal and alloy
nanoparticles into the pores of silica alcogel. Electrolysis of as generated alcogels (i.e., without
any subsequent treatment) resulted in the formation of nickel and alloy nanoparticles within
reasonable depth of the gel. The method employed, does not require high temperatures or long
durations to form electrodepositable gel. This technique is simple and cost effective. Further it
can produce nanomaterials in bulk and in a single go. The nanoparticles were characterized by
XRD, TEM, surface area, Resistance measurements, BET, AC-Susceptibility, SQUID, VSM,
Mössbauer and M-TGA measurements etc.From XRD analysis size of FCC Ni, Ni(Fe), Ni(Co), Ni(Zn) nanoparticles ware around
17-20 nm, 8-15 nm, 11-16 nm and 9-14 nm respectively. The FCC phase in most case was also
accompanied by surface oxide; tetragonal nickel. The sample with only iron chloride in alcogel
does not revealed presence of any significant amount of BCC phase, this may probably due to
oxidation of iron; as a consequent of small particle size. The spinel iron oxide phase had size
around 8 nm. Addition of even small quantity of cobalt or zinc along with iron, resulted in the
formation of BCC phase. The BCC Fe(Co) particles were around 9-12 nm, while BCC Fe(Zn)
nanoparticles were around 6-11 nm. The particle size appeared to decrease with the increase in
the concentration of alloying elements. However in case of Fe(Co) alloys size seems independent
of alloying element concentration. In gels containing only cobalt chloride, about18 nm cobalt
nanoparticles were formed. The formation of small size of nanoparticles was further confirmed
from TEM studies. Resistance measurement was carried to further understand the structure of
samples. Composites having more metal-oxide content such as; in samples with high iron, cobalt
or zinc as alloying element, resulted in increased resistance such as; up to order of MΩ at a load
of 100 kg. This is due to the formation of higher quantity of oxides between the interconnected
necks of nanoparticles. However, complete metallic contact at low load was observed in FCC Ni
and FCC nickel alloys, having low alloying concentrations of iron or cobalt. Besides XRD, the
formation of spinel iron oxide in iron containing samples was confirmed from the presence of
superparamagnetic doublet appearing in Mössbauer spectra. This corresponds to iron in high spin
Fe 3+ state. The formation of Ni(Fe) and Fe(Co) was also confirmed by Mössbauer analysis,
showing presence of ferromagnetic sextets, having hyperfine field of the order of 260kOe and
340kOe respectively. The VSM of composites indicated formation of soft magnetic metal and
alloy nanoparticles. The coercivity measured for nickel samples comes out around 100 Oe.
While for Ni(Fe) it lies between 50 to 100Oe, with low being associated to more iron alloying.
Coercivity of Ni(Co) samples lied in the range of 150 to 250Oe with higher being associated to
higher concentration of cobalt in the gel. However coercivity of Fe(Co) samples decreased
slightly with the cobalt addition from around 160Oe to 120Oe but resulted in increased
magnetization.
M-TGA studies were also performed to magnetically characterize samples. Presence of exchange
coupling was observed in the samples due to ferromagnetic–antiferromagnetic interaction at the
surface of nanoparticles. Consequently ferromagnetic nanoparticles remained blocked up to
Curie temperature of FCC nickel in case of nickel containing samples and up to Curie
temperature of spinel ferrite in case of Fe(Co) samples. The formation of alloy was further
confirmed by the change in Curie transition of various samples. The Curie temperature of nickel
increased from 620 K to 630 K by iron addition, and it increased to ~ 900 K in case of cobalt
addition. In Fe(Co) samples, Curie transition associated with metallic phase was only observed
but in samples with higher concentration of cobalt. This probably is due to oxidation of
nanoparticles during M-TGA studies. From XRD and M-TGA quantity of alloying can be
estimated, such as; up to 20 % Fe in Ni(Fe), up to 30% Co in Ni(Co) and up to ~30-50% Co in
case of Fe(Co) samples was estimated. The present technique has proven its versatility by
depositing variety of nanoparticles, and having soft magnetic properties, with high resistance.
Therefore, if further characterized, these materials could stand potential candidates for high
frequency applications. Since surface area of most of the samples was ~100m 2 /gm, besides high
well dispersed metallic load (e.g.; 55% Ni in Ni/Silica samples), therefore this technique can
produce potential catalytic composites too.