SYNTHESIS OF METAL AND ALLOY NANOPARTICLES BY ALCOGEL ELECTROLYSIS

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.