| dc.description.abstract | The confluence of intriguing size and morphology dependent optical and chemical properties with versatile application in various fields, such as energetic and magnetic makes monometallic nonmaterial of high fundamental scientific interest. However, the challenge that needs to be addressed is to achieve their synthesis with a rational control on their dimensions, morphology and dispersion for the widespread applications of these materials. In addition to synthesis, achieving long-lasting stability of nonmaterial becomes imperative in order to realize their potential applications. Miniaturization in size of particles results in an increased surface to volume ratio, conducing especially reactive metal nanoparticals prone to oxidation. This thesis describes the synthesis of nearly monodiperse colloids of metallic and intermetallic nanoparticles using solvated metal atom dispersion method and digestive ripening facilitated interatomic diffusion process. Our aim is to understand the combinatiorial effects of nanosizing and stability on the functional properties of these nanomaterials. Towards this Direction, we investigated Co, A1 and Mg monometallic, and Au/Ag-In and Au-Sn intermetallic nanoparticle systems.
Chapter 2
Describes the synthesis, detailed characterizations and magnetic properties of nearly monodisperse cobolt nanoparticles(<5nm) synthesized using a hydride synthetic protocol, solvated metal atom diserion method. The as-prepared cobalt nanoparticles in this size range exhibit intrinsic instability towards Oxidations. After 30 day of exposure to air, magnetic measurements showed drastic degration in saturation magnetization and complete conversion to antiferromagnetic cobalt oxide was confirmed. In order to achieve their stability, a heat treatment was applied to decompose the organic solvent and capping agent, resulting in carbonization of solvent/ligand around the surface of cobolt nano particles. Controlled and optimized annealing at different temperatures resulted in the formation of hexagonal closed packed (hcp) and fape-centered cubic (fcc) phases of metallic cobalt. Remarkably, the corresponding heat treated samples retained their rich magnetic behavior even after exposure to air for a duration of one year. Compared to un-annealed samples, magnetization values increased two-fold and the corecivity of nanoparticles exhibited strong dependence on the phase transformation of cobolt.
Chapter 3
Deal with an exploratory study of the synthesis, characterization, and stabilization of nanometer-sized enegetic material, aluminum. Highly monodisperse colloidal aluminum nanoparticles (3.1‡ 0.6 mm) were prepared by using hexadecy amine (HAD) as the capping agent tetrahydrofurma as a coordinating solvent in the SMAD method. Since such small particles are highly prone to oxidation, a support materials is required for their stabilization. Stability has been achived by carbonization of the capping agent on the surface of A1 nanoparticles by carrying out thermal treatment of A1-HAD nanoparticles at a modest temperature. Presence of corbon was confirmed using Raman spectroscopy and TEM measurements evidencing that annealed A1 nanoparticles are encapsulated in a corbon matrix. The exhibition of robust stability was established using thermal analysis (TGA/DTA) wherein, oxidation of aluminum in air did not occur upto 500 0C. Indirectly, the successful passivation was further exploited in the synthesis and characterization of small sized monodisperse magnesium nanoparticles. The resulting samples were hybrided and nanosized MgH2 released hydrogen at much lower temperature than that of the bulk MgH2 (573 K). The observed hydrogen release was only partially reversible. This partial reversibility could be attributed to the coalescence of small sized Mg nanoparticles upon subsequent charging/discharging hydrogen cycles.
In the next step, we exploed the intermetallic systes which are composed of more than one metallic species. Chapter 4 describes the synthesis and characterization of small sized, monodisperse (<10 nm) colloidal AuIn2 and Ag3In intermetallic nanoparticles. The formation of intermetallic nanoparticles could be explained by invoking digestive ripening facilitated atomic diffusion of Au/Ag and In nanoparticles followed simultaneously by their growth in te solution. The course of the reaction was followed using optical spectroscopy where the changes in UV-visible absorption band were correlated to the formation of AuIn/Ag3In intermetallic. Structural characterization, Performed using powder X-ray diffraction, brought out the formation of phase pure AuIn2 and Ag3In intermetallic compounds. Digestive ripening effects were clearly observed using transmission electron microscopy which showed the transformation of polydisperse physical mixture colloid of nanometallic species to uniform sized intermetallic nanoparticles. By invoking the phenomenon of interatomic diffusion at nanoscale favored by feasible thermodynamics ( G being negative) we were successful inrealizing the formation of these intermetallic nanoparticles. Optimization of temperature at which digestive ripening was performed, turned out to be a crucial factor in the successful synthesis of phase pure intermetallic nanoparticles.
These promising results inspired us to study further the preparation of Au-Sn intermetallic system which is described in Chapter 5. The potential of such an unprecedented approach has been exploited in the synthesis of homogeneous intermetallic nanaocrystals of Au5Sn and AuSn. The two monometallic collids (Au and Sn), mixed in a stoichiometric amount were subjected to digestive ripening process. 1:1 stichiometry always led to the formation of eutectic mixture (Au5Sn and AUSn), The stoichiometry of monometallic nanocrystals. Therefore, by taking an extra equivalent of Au and Sn in two different experiments, phase pure Au5Sn and AuSn intermatillic nanocrsytals were obtained, respectively. This is the first observation that has been reported regarding the phase pure synthesis if Au5Sn intermetallic nanocrystals using solution based approach. Formation of different phases was established by structural characterization which elicited srystalline nature of the samples. A combination of TEM, HRTEM, and STEM-EDS mapping techniques employed here, brought and tailored phase. In conclusion, the careful selection of solvent, stoichiometry and growth directing agents is an important prerequisite for realizing distinct phases of Au-Sn system with a controlled morphology. | en_US |
