Intermetallic compounds are highly investigated, as they combine or enhance the properties of their constituting elements or even bring forth new properties. Especially at the nanoscale, these features can be exploited in heterogeneous catalysis.[1] Various methods have been developed so far to synthesize intermetallic particles, each with its own benefits and drawbacks. A facile procedure is the polyol process, which was first introduced by the group of FIÉVET and FIGLARZ in 1989. Here, the polyol serves a threefold purpose. First, it serves as the primary solvent. With respect to metal salts, it displays a solvation behavior similar to water due to the chelating properties. Second, the polyol can act as a surface-capping agent, which prevents agglomeration, resulting in finely dispersed particles. Third and last, the main attribute of the polyol is its reductive property, which increases with temperature, enabling the reduction of multiple metal cations. Compared to other synthetic routes, the polyol process can be performed with cheap starting materials, such as metal salts or oxides. The utilization of a laboratory microwave can further improve the process. A homogeneous heat distribution and contactless heating diminish side reactions. Additionally, the extreme heating rates foster homogeneous nucleation, resulting in a uniform product. Furthermore, the precise control over the temperature profile enables good reproducibility, making this setup ideal for efficient syntheses as well as investigations of reaction pathways. In this PhD thesis, formation mechanisms of Bi-M particles (M = Ni, Ir, Rh; ) were elucidated, revealing different mechanisms depending on the metal combination as well as various intermediates. Additionally, the influence of reaction parameters, such as metal precursor, anion, and pH value was investigated. In the case of BiNi particle formation, a successive reduction of bismuth and nickel cations was observed. Bismuth cations are reduced first producing bismuth particles, which act as nucleation sites for the subsequent nickel reduction. The particles grow on the surface of the bismuth core resulting in a core-shell structure. Diffusion of nickel results in Bi3Ni and eventually BiNi after full depletion of the nickel shell. The choice of nickel precursor substantially influences the required reaction time. Nickel acetate requires the shortest reaction time, whereas nickel nitrate necessitates drastically longer reaction times due to a decreased reductivity. Nickel chloride is not reducible in neat ethylene glycol due to the formation of a stable dinuclear nickel complex in solution. The overall formation kinetics are substantially promoted by increasing the pH value or temperature, leading to a higher reduction strength or improved diffusion dynamics, respectively. The study of Bi2Ir particle formation revealed a two-stage scenario. First, the starting materials are partially co-reduced to the new intermetallic suboxide Bi4Ir2O. In a second step, at higher temperatures, the suboxide is fully reduced to Bi2Ir. So far, only the combination of bismuth nitrate and iridium acetate results in the suboxide, whereas the introduction of chloride ions, i.e., iridium chloride or potassium hexachloroiridate, merely results in BiOCl and elemental iridium. A structural model of Bi4Ir2O was established based on diffraction data and quantum chemical calculations. Edge-sharing [IrBi6] octahedra form corrugated layers stacked along the c-axis, which are separated by oxide ions. The calculated band structure and DOS suggest metallic behavior within the layers, whereas a band gap was found along the stacking order, thus, making the compound a pseudo 2D metal.The formation of Bi2Rh particles follows a two-step mechanism. For rhodium acetate, the process starts with a direct co-reduction of rhodium and bismuth cations resulting in the formation of BiRh. Increasing the temperature further leads to a gradual transition into Bi2Rh via reduction of residual bismuth cations in solution followed by diffusion. In the case of rhodium nitrate, a bismuth-glycolato complex precipitates, which undergoes a reaction with rhodium at high temperatures. The addition of a base promotes the reactions by lowering the necessary reduction temperatures and preventing the precipitation of the bismuth glycolate. Rhodium chloride does not yield intermetallic phases in the desired purity and yield. These results allowed for a comparison and assessment of reactions in the synthesis of γ-BiPd particles. Similar to the above discussed reactions, chlorides resulted in the formation of BiOCl. An increased pH value was beneficial by preventing precipitation of intermediates, i.e., BiOCl or bismuth glycolates, and improved reduction strength.

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.

Investigations on size dependent phase stability and transformations in isolated nanoparticles have gained momentum in recent times. Size dependent phase stability generates size specific particle microstructure which consequently yields size specific functionality. One important prerequisite for conducting studies on nanoparticles is their synthesis. A substantial amount of research effort has therefore been focused on devising methodologies for synthesizing nanoparticles with controlled shapes and sizes. The present thesis deals with both these two aspects: (a) synthesis of nanoparticles and (b) phase transformations in nanoparticles. The system chosen in this study is AuCu intermetallic nanoparticles. The choice of AuCu nanoparticle was due to the fact that the literature contains abundance of structural and thermodynamic data on Au–Cu system which makes it a model system for investigating size dependence of phase transformations. With respect to synthesis, the present thesis provides methodologies for synthesizing alloyed Au–Cu nanoparticles of different sizes, Au–Cu nano-chain network structures and uniform Au–Cu2S hybrid nanoparticles. For every type, results are obtained from a detailed investigation of their formation mechanisms which are also presented in the thesis. With respect to phase transformation, the thesis presents results on the size dependence of fcc to L10 transformation onset in Au–Cu nanoparticles under isothermal annealing conditions. The present thesis is divided into eight chapters. A summary of results and key conclusions of work presented in each chapter are as follows. The ‘introduction’ chapter (chapter I) describes the organization of the thesis. Chapter II (literature study) presents a review of the research work reported in the literature on the various methodologies used for synthesizing Au–Cu based nanoparticles of different shapes and sizes and on ordering transformation in AuCu nanoparticles. The chapter also presents a brief discussion on the reaction variables that control the process of nucleation and growth of the nanoparticles in solution. Chapter III titled ‘experimental details and instrumentation’ describes the synthesis procedures that were used for producing various nanoparticles in the present work. The chapter also briefly describes the various characterization techniques that were used to investigate the nanoparticles. The fourth chapter titled ‘synthesis and mechanistic study of different sizes of AuCu nanoparticles’ provides two different methodologies for synthesis, referred as ‘two-stage process’ and ‘two-step process’ that have been used for producing alloyed AuCu nanoparticles of different sizes (5, 7, 10, 14, 17, 25 nm). The ‘two-stage’ process involved sequential reduction of Au and Cu precursors in a one pot synthesis process. Whereas, the ‘two-step’ process involved a two-pot synthesis in which separately synthesized Au nanoparticles were coated with Cu to generate alloyed AuCu nanoparticles. In the two-stage synthesis process it was observed that by changing the total surfactant-to-metal precursor molar ratio, sizes of the alloyed AuCu nanoparticles can be varied. ‘Total surfactants’ here include equal molar amounts of oleic acid and oleylamine surfactants. Interestingly, it was observed that there exists a limitation with respect to the minimum nanoparticle size that can be achieved by using the two-stage process. The minimum AuCu nanoparticle size achieved using the two-stage synthesis process was 14 nm. Mechanism of formation of AuCu nanoparticles in the two-stage synthesis process was investigated to find out the reason for this size limitation and also to determine how the synthesis process can be engineered to synthesize alloyed AuCu nanoparticles with smaller (<14nm) sizes. Studies to evaluate mechanism of synthesis were conducted by investigating phase and size of nanoparticles present in the reaction mixture extracted at various stages of the synthesis process. Their studies revealed that (a) the nanoparticle formation mechanism in the two-stage synthesis process involves initial formation of Au nanoparticles followed by a heterogeneous nucleation and diffusion of Cu atoms into these Au rich seeds to form Au–Cu intermetallic nanoparticles and (b) by increasing the relative molar amount of the oleylamine surfactant, size of the initial Au seed nanoparticles can be further reduced from the minimum size that can be achieved in the case when equal molar amounts of oleylamine and oleic acid surfactants are used. The information obtained from the mechanistic study was then utilized to design the two-step synthesis process. In the two-step process, Au nanoparticles were synthesized in a reaction mixture containing only the oleylamine surfactant. Use of only oleylamine resulted in production of pure Au nanoparticles with sizes that were well below 10 nm. These Au nanoparticles were washed and dispersed in a solution containing Cu precursor. Introduction of a reducing agent into this reaction mixture led to the heterogeneous nucleation of Cu onto the Au seed particles and their subsequent diffusion into them to form alloyed AuCu nanoparticles with sizes of ~5, 7 and 10 nm. The study present in this chapter essentially signified that the surfactants used in the reaction mixture not only prevent nanoparticles from agglomerating in the final dispersion but also control their nucleation and growth and therefore can be used as a tool to tune nanoparticle sizes. The fifth chapter titled ‘size dependent onset of FCC-to-L10 transformations in AuCu alloy nanoparticles’ illustrates the effect of AuCu nanoparticle size on the onset of ordering under isothermal annealing conditions. Nanoparticles in this study were annealed in-situ in a transmission electron microscope. Samples were prepared by drop drying a highly dilute dispersion of as-synthesized nanoparticles onto an electron transparent TEM grid. Nanoparticles sitting on the TEM grid were well separated from each other to minimize particle sintering during the annealing operation. It was however observed that during the isothermal annealing, particle coarsening due to atomic diffusion was appreciable for 5 nm particles but negligible for 7 and 10 nm particles. Therefore for this study only 7 nm and 10 nm sized particles were considered. Onset of ordering was determined from the time when first sign of the diffraction spot, corresponding to the ordered phase, appears in the selected area electron diffraction pattern from a region containing large number of AuCu nanoparticles. Through a series of isothermal experiments it was observed that the time for onset of ordering increased with decrease in size of the nanoparticles. It is speculated that the delay in onset of ordering may be due to the fact that with a decrease in nanoparticle size the probability of a nanoparticle containing a fluctuation that shall generate a thermodynamically stable nuclei of the ordered phase decreases. A sharp interface between the ordered and the disordered phase inside the particle was also observed which suggested that the ordering transformation in as-synthesized fcc AuCu nanoparticles is a first order transformation. The sixth chapter titled ‘synthesis and characterization of Au1-xCux–Cu2S hybrid nanostructures: morphology control by reaction engineering’ provides a modified polyol method based synthesis strategy for producing uniform Au–Cu2S hybrid nanoparticles. Detailed compositional and structural characterization revealed that the hybrid nanoparticles are composed of cube shaped Au-rich, Au–Cu solid solution phase and hemispherical shaped Cu2S phase. Interestingly, the hemispherical Cu2S phase was attached to only one facet of the cube shaped phase. A study on the formation mechanism of hybrid nanoparticles was also conducted by characterizing specimens extracted from the reaction mixture at different stages of the synthesis process. The study revealed that the mechanism of formation of hybrid nanoparticles involved initial formation of isolated cube shaped pure Au nanoparticles and Cu–thiolate complex with a sheet morphology. With increase in time at 180°C, the Cu–thiolate complex decomposed and one part of the Cu atoms that were produced from the decomposition were utilized in forming the spherical Cu2S and other part diffused into the Au nanoparticles to form Au–Cu solid solution phase. The chapter also presents a study on the effect of dodecanethiol (DDT) on achieving the hemisphere-on-cube hybrid morphology. In this study it is illustrated that an optimum concentration of dodecanethiol is required both for achieving size and morphological uniformity of the participating phases and for their attachment to form a hybrid nanoparticle. The seventh chapter titled ‘synthesis of Au–Cu nano-chains network and effect of temperature on morphological evolution’ provides methodology for synthesizing fcc Au– Cu nano-chain network structures using polyvinylprrolidone (PVP) surfactant. It was observed that with increase in the molar amount of PVP in the reaction mixture, morphology of the as-synthesized product gradually changed from isolated nanoparticles to branched nano-chain like. The nano-chains contained twins which indicated an absence of continuous growth and possibility of growth by oriented attachment of initially formed Au–Cu nanoparticles. Both in-situ and ex-situ annealing of the nano-chains led to their decomposition into isolated nanoparticles of varying sizes. Annealing also caused fcc-to¬L10 phase transformation. Investigation of the wave length of perturbation leading to breaking of a nano-chain into particles indicated that the surface energy anisotropy affects the splitting of nano-chain network structure into nano-sized particles. The thesis ends with a last chapter where we have presented possible future extension of current work.