Synthesis And X-ray Absorption Spectroscopy Study Of Nano Functional Materials

Nano functional materials of different size and shape are of great interest in many technological areas such as electronics, magnetic devices, optics, catalysts etc, because of their potential applications. Due to their unique properties, the development of simple synthetic methods including morphology-controlled processes to synthesize nanocrystals deserves a great attention.As a novel approach, synchrotron radiation-based X-ray absorption spectroscopy (XAS) is a unique element-selective tool capable to probe both the local lattice and the electronic structure of almost any materials. In this thesis, we used different processes to synthesize Sm(OH)3nanocrystals and bimetallic nanoparticles with different morphologies. Their local lattice and electronic structure were studied by X-ray absorption spectroscopy.Results open the way to create new multifunctional compounds for many future applications. Indeed, samarium hydroxide (Sm(OH)3), an important Rare Earth compound, offers many interesting technological applications.We present here a facile hydrothermal method, assisted by oleic acid surface modifications, that allows synthesis of Sm(OH)3nanocrystals with different nanoscale morphologies. The as-synthesized Sm nanocrystals were characterized by X-ray diffraction (XRD),electron microscopy, X-ray absorption spectroscopy and Superconducting Quantum Interference Device measurements. Data show that both crystallization and grain size of nanocrystals can be tuned by the content of the oleic acid (OA) surfactant while rod-like Sm(OH)3single crystals have been synthesized with a process not mediated by the oleic acid. Addition of oleic acid, reduce the length of nanorods that display a sheet-like morphology. At oleic acid concentrations greater than1ml, the morphology of the Sm(OH)3nanosheet abruptly changes in a disc-like polycrystalline structure. Data from paramagnetic resonance and magnetic measurements also show that Sm(OH)3nanorods are non-magnetic, while Sm(OH)3nanodiscs exhibit a weak ferromagnetism. In agreement with the X-ray absorption near edge structure (XANES) analysis, we assigned the origin of the ferromagnetic behaviour of these nanostructures to the localization of electrons into Sm5d orbitals following the removal of the hydroxyl groups. Also Pt-based alloysrecently triggered a lot of attentions due to interesting industrial applications. In particular they provide great opportunities for the development of low-cost and high-performance fuel-cell catalysts. Many studies already pointed out the excellent physico-chemical properties of Pt-based alloys, intimately related to their internal structure. Great efforts have been spent to characterize shape, homogeneity, dispersion, alloying extent and kinetic growth of Pt-based nano-particles. Here, we present properties of Cu-Pt bimetallic nano-particles synthesized by the thermal decomposition method under oleylamine and OA coordination. High-resolution transmission electron microscopy(HRTEM) images show that the size of Cu-Pt nanostructures capped by the surfactant with OA is about1.2nm, a dimension containing about35atoms. Accurate structural information of this system has been obtained by XRD and XAS. We found that Pt atoms occupy Cu sites and Pt-Cu bonds are formed in these Cu-Pt nanoparticles. A clear charge transfer mechanism from Cu to Pt has also been identified. The structural and electronic characterizationsprovided useful information to understand properties of such small nanoparticles. Furthermore, in order to study temperature induced internal structuraltransition,weperformedtemperature-dependent in situ XAS investigation on the correlation of atomic and electronic structures for Cu-Pt nanoparticles. Homogeneous Cu-Pt core-shell nanoparticles with the averaged size of8nm have been synthesized by chemical methods and a Cu atoms diffusion process was observed by temperature-dependent XAS. Data show that increasing temperature, in these nanoparticles Cu diffused gradually from the Cu core to the Pt shell. We also found that surface O ligands bonded Pt at the room temperature and aregradually removed by heating the sample. The analysis of the diffusion process in these bimetallic nanoparticles provide important guidelines for further nanoparticles design with tuned characteristic properties.