Synthesis and characterization of transition-metal nanocrystals and their application in catalysis

Extended X-ray absorption fine structure (EXAFS) spectroscopy and transmission electron microscopy (TEM) have been used to investigate Ni/Co bimetallic nanoparticles (bi-MNPs) produced via laser vaporization of a carbon-metal mixture. Results show that the structures of these nanoparticles are face-center-cubic, and each of the nanoparticles examined contains 50% of Ni and Co. A detailed analysis of the EXAFS data using Fourier filtering and single scattering theory has revealed that the surface of the nanoparticles is predominantly occupied by Ni atoms.; In order to further study the application of the transition-metal nanocrystals in catalysis, different sizes of Co nanocrystals are synthesized using thermo-decomposition. Extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) are used as tools, theoretically and experimentally, for studying the structures and dynamics of these Co nanocrystals. These Co nanocrystals are used as catalysts to grow self-aligned one-dimensional CoSi2 nanostructures on Si substrates by chemical vapor deposition (CVD). Scanning electron microscopy (SEM), atomic force microscopy (AFM), and transmission electron microscopy (TEM) are used to characterize as-prepared nanostructures. The effects of reaction temperature, Si substrates, flow rates of the feeding gases and nanocrystal size on the growth of CoSi2 have been studied systematically. The nanostructure so produced are of high quality and are well arranged, making them available to many applications. We also expand the Co nanocrystals synthesis to Ni nanocrystals and Ni/Co bimetallic nanocrystals. Transmission electron microscope (TEM) and X-ray diffraction (XRD) and energy dispersive x-ray spectroscopy (EDX) have been used to characterize as-prepared nanocrystals.; A high repetition rate, compact terawatt Ti: Sapphire laser system is presented. The oscillator produces an 82-MHz pulse train consisting of broad bandwidth pulses of 0.5 nJ/pulse and 9-fs pulse duration. The laser system is used to generate x-ray pulses, and can also generate a lower peak power, dual-pulse output that can excite, simultaneously and coherently, Raman modes within an adjustable bandwidth (up to 700-cm-1) and at a tunable central vibrational frequency. The potential application of the laser system in "machining" the metallic nanocrystals is discussed.