2p3d Resonant X-ray emission spectroscopy of cobalt compounds

Abstract

This manuscript demonstrates that 2p3d resonant X-ray emission spectroscopy (RXES) yields unique information on the chemically relevant valence electrons of transition metal atoms or ions. Experimental data on cobalt compounds and several theories were used hand-in-hand. In chapter 1 2p3d RXES was situated within the scientific disciplines of chemistry, catalysis and spectroscopy. It was emphasized that the technique measures many-electron effects in great detail and that it allows investigating their role in the chemical bond. Prior to any RXES measurements, a novel, general organometallic route was developed in chapter 2 to obtain CoxNi1-x and CoxFe1-x alloy metal nanoparticles with a fully tunable composition and a size of 4 to 10 nm. A combined ultraviolet-visible (UV/Vis), 2p X-ray absorption spectroscopy (XAS) and 2p3d RXES study of 3d-orbitals in cobalt carboxylates was presented in chapter 3. Using cobalt(II) carboxylates, it was shown that 2p3d RXES reveals more and more intense d-d excitations than UV/Vis spectroscopy and that 2p XAS cannot discriminate the compounds. Ligand field multiplet (LFM) theory was applied to explain why the technique is highly sensitivity to the metal ion ligand field. In chapter 4 a total experimental 2p3d RXES resolution better than 100 meV full-width-at-half-maximum was exploited to investigate CoO nanocrystals with an average diameter of 4.2 nm and a CoO single crystal. The RXES data allowed the first X-ray observation of the CoO 4T1g(4F) manifold that occurs within the first 120 meV above the ground state. LFM modeling was performed to assess the tetragonal crystal field splitting, spin-orbit and superexchange parameters. Chapter 5 reports 2p3d RXES measurements on 8.4 and 5.0 nm cobalt and 3.6 nm cobalt-nickel nanoparticles coated with oleate molecules that exhibit low energy resonant Raman features at 0.3 and 0.75 eV. In combination with time-dependent density functional theory (TD-DFT) and LFM calculations these are ascribed to d-d excitations of cobalt ions in two different, but both possible, chemical environments. A consequence of the first discussed model would be that 2p3d RXES is a unique tool for the selective identification of surface metal-adsorbate interactions. In chapter 6 both 2p XAS and 2p3d RXES spectra of cobalt nanoparticle batches with average diameters of 4.0, 4.2, 5.0, 8.4 and 15.2 nm are reported. Most 2p XAS data cannot conclusively distinguish whether the nanoparticles are oxidized or metallic. In contrast, the high-energy resolution 2p3d RXES spectra clearly reveal particle oxidation. Chapters 7a, 7b and 8 are not dealing with RXES measurements, but use XAS and electron energy loss spectroscopy (EELS) in combination with scanning transmission microscopes to demonstrate synergetic possibilities resulting from X-ray and electron spectromicroscopes. Two dimensional and tomographic scanning transmission X-ray microscopy (STXM) is shown to be an attractive and complementary imaging tool to probe the interior of colloidal, photonic crystals. High angle annular dark field scanning transmission electron microscopy (STEM) combined with EELS is used to directly visualize and quantify the organic and inorganic components of lipid-coated silica nanoparticles that contain smaller semiconductor quantum dots; particles that are of interest as multimodal medical contrast agents