Chemical and Morphological Inhomogeneity of Aluminum Metal and Oxides from Soft X-ray Spectromicroscopy

Abstract

ABSTRACT. Oxygen and aluminum K-edge X-ray absorption spectroscopy (XAS), imaging from a scanning transmission X-ray microscope (STXM), and first principles calculations were used to probe the composition and morphology of bulk aluminum metal, α-and γ-Al2O3, and several types of aluminum nanoparticles. The imaging results agreed with earlier transmission electron microscopy studies that showed a 2 to 5 nm thick layer of Al2O3 on all the Al surfaces. Spectral interpretations were guided by examination of the calculated transition energies, which agreed well with the spectroscopic measurements. Features observed in the experimental O and Al K-edge XAS were used to determine the chemical structure and phase of the Al2O3 on the aluminum surfaces. For unprotected 18 and 100 nm Al nanoparticles, this analysis revealed an oxide layer that was similar to γ-Al2O3 and comprised of both tetrahedral and octahedral Al coordination sites. For oleic-acid protected Al nanoparticles, only tetrahedral Al oxide coordination sites were observed.The results were correlated to trends in the reactivity of the different materials, which suggests that the structures of different Al2O3 layers have an important role in the accessibility of the underlying Al metal towards further oxidation.Combined, the Al K-edge XAS and STXM results provided detailed chemical information that was not obtained from powder X-ray diffraction or imaging from a transmission electron microscope.

Structural properties of ultra-small thorium and uranium dioxide nanoparticles embedded in a covalent organic framework

“…18,[72][73][74][75][76] Unfortunately, the use of surfactant molecules also complicates comparisons to analogous bulk materials. For example, surface ligands have been shown to directly alter nanoparticle surface charge and electronic structure, [77][78][79] as well as shape and surface reconstruction, 80 while also impacting their functionality in catalysis [81][82][83] and luminescence properties. [84][85][86] Hence, it has remained difficult to identify changes in physical behavior that are a direct consequence of particle size, particularly in ultrasmall nanoparticles where surface speciation is a major and sometimes dominant component of the overall composition.…”

4D Imaging of ZnO-Coated Nanoporous Al2O3 Aerogels by Chemically Sensitive Ptychographic Tomography: Implications for Designer Catalysts

“…The characteristic ZnO L 3 spectral features are a “triplet” at 1022.5, 1024.1, and 1030.2 eV and an additional peak at 1033 eV, all of which arise from excitations of Zn 2p electrons to unoccupied Zn 4s and Zn 4d states. The Al K-edge spectra of α-Al 2 O 3 (corundum), other Al 2 O 3 allotropes, and alumina silicates have been presented previously. ,,− In this case there is a large variability in Al K-edge spectra, depending on the crystal structure. Several authors have noted a significant correlation between Al K-edge spectral shape and position and the relative proportion of octahedral ([6]-Al) and tetrahedral ([4]-Al) sites. , In general, the main Al 1s → 3p transition intensity in Al­(III) oxidation state species in either the pure oxides, aluminosilicate minerals, or zeolites occurs around 1566.5 eV for tetrahedral ([4]-Al) sites and about 2 eV higher, around 1568.5 eV, in octahedral ([6]-Al) sites.…”

“…The energy scales were set by using values reported in the literature. The first feature in the Zn L 23 spectrum was calibrated to 1024.1 eV, − and the second peak in the Al K spectrum was calibrated to 1568.0 eV. − Quantitative OD1 spectra (response for 1 nm of pure ZnO or Al 2 O 3 ) were derived by matching spectral shapes and intensities below and far above each edge to the response predicted from the elemental compositions from standard mass absorption coefficient tabulations and the density of bulk ZnO (5.61 g cm –3 ) and Al 2 O 3 (3.95 g cm –3 ). These OD1 spectra were then used for quantitative analysis.…”

Gauging aromatic conjugation and charge delocalization in the aryl silanes PhnSiH4−n (n = 0–4), with silicon K-edge XAS and TDDFT

“…STXM methodology was similar to that discussed previously. 31,43 Single-energy images and silicon K-edge XAS spectra were acquired using the STXM instruments at the at the Canadian Light Source (CLS) spectromicroscopy beamline 10ID-1 and at the Advanced Light Source-Molecular Environmental Science (ALS-MES) beamline 11.0.2. The CLS operated in decay mode (250 to 150 mA) while the ALS operated in topoff mode (500 mA).…”