Sparse ab initio x-ray transmission spectrotomography for nanoscopic compositional analysis of functional materials.

The performance of functional materials is either driven or limited by nanoscopic heterogeneities distributed throughout the material's volume. To better our understanding of these materials, we need characterization tools that allow us to determine the nature and distribution of these heterogeneities in their native geometry in 3D. Here, we introduce a method based on x-ray near-edge spectroscopy, ptychographic x-ray computed nanotomography, and sparsity techniques. The method allows the acquisition of quantitative multimodal tomograms of representative sample volumes at sub-30 nm half-period spatial resolution within practical acquisition times, which enables local structure refinements in complex geometries. To demonstrate the method's capabilities, we investigated the transformation of vanadium phosphorus oxide catalysts with industrial use. We observe changes from the micrometer to the atomic level and the formation of a location-specific defect so far only theorized. These results led to a reevaluation of these catalysts used in the production of plastics.

Iron-doped nickel oxide nanocrystals as highly efficient electrocatalysts for alkaline water splitting.

Efficient electrochemical water splitting to hydrogen and oxygen is considered a promising technology to overcome our dependency on fossil fuels. Searching for novel catalytic materials for electrochemical oxygen generation is essential for improving the total efficiency of water splitting processes. We report the synthesis, structural characterization, and electrochemical performance in the oxygen evolution reaction of Fe-doped NiO nanocrystals. The facile solvothermal synthesis in tert-butanol leads to the formation of ultrasmall crystalline and highly dispersible FexNi1-xO nanoparticles with dopant concentrations of up to 20%. The increase in Fe content is accompanied by a decrease in particle size, resulting in nonagglomerated nanocrystals of 1.5-3.8 nm in size. The Fe content and composition of the nanoparticles are determined by X-ray photoelectron spectroscopy and energy-dispersive X-ray spectroscopy measurements, while Mössbauer and extended X-ray absorption fine structure analyses reveal a substitutional incorporation of Fe(III) into the NiO rock salt structure. The excellent dispersibility of the nanoparticles in ethanol allows for the preparation of homogeneous ca. 8 nm thin films with a smooth surface on various substrates. The turnover frequencies (TOF) of these films could be precisely calculated using a quartz crystal microbalance. Fe0.1Ni0.9O was found to have the highest electrocatalytic water oxidation activity in basic media with a TOF of 1.9 s(-1) at the overpotential of 300 mV. The current density of 10 mA cm(-2) is reached at an overpotential of 297 mV with a Tafel slope of 37 mV dec(-1). The extremely high catalytic activity, facile preparation, and low cost of the single crystalline FexNi1-xO nanoparticles make them very promising catalysts for the oxygen evolution reaction.

Surface patterning of silver using an electron- or photon-assisted oxidation reaction.

We have investigated a recently developed method of patterning Ag surfaces. The method uses an electron beam to irradiate Ag surfaces during NO(2) dosing at 300 K and leads to sharp oxide patterns on otherwise metallic surfaces. Investigations were performed on an Ag(111) single crystal and on an Ag foil with LEEM (low-energy electron microscopy), LEED (low-energy electron diffraction), MEM (mirror electron microscopy), and XPEEM (X-ray photo-emission electron microscopy). The oxidation reaction, which is based on the electron-induced desorption of NO molecules, proceeds in steps, from a layer of O atoms adsorbed on the metallic Ag via an intermediate phase to an amorphous Ag(2)O film. Our measurements evidence a high cross section for electron-induced NO desorption with 30-40 eV electrons, indicating that only a few electrons per adsorbing NO(2) molecule are required to initiate the process. The intermediate phase, which forms a partially ordered quadratic structure, contains oxygen species in an oxide-like environment, coexisting with an adsorbate covered metallic Ag(111) surface. While the intermediate phase dissolves within hours under UHV conditions, fully developed oxide patches, consisting of several layers of thick, amorphous Ag(2)O, are kinetically stable. The oxidation method also works with 40 eV (and 700 eV) photons instead of electrons. In preliminary experiments local patterns could also be created with photons, suggesting that mask techniques can be applied for the process.