An improved laboratory-based x-ray absorption fine structure and x-ray emission spectrometer for analytical applications in materials chemistry research

X-ray absorption fine structure (XAFS) and x-ray emission spectroscopy (XES) are advanced x-ray spectroscopies that impact a wide range of disciplines. However, unlike the majority of other spectroscopic methods, XAFS and XES are accompanied by an unusual access model, wherein the dominant use of the technique is for premier research studies at world-class facilities, i.e., synchrotron x-ray light sources. In this paper, we report the design and performance of an improved XAFS and XES spectrometer based on the general conceptual design of Seidler et al. [Rev. Sci. Instrum. 85, 113906 (2014)]. New developments include reduced mechanical degrees of freedom, much-increased flux, and a wider Bragg angle range to enable extended x-ray absorption fine structure (EXAFS) measurement and analysis for the first time with this type of modern laboratory XAFS configuration. This instrument enables a new class of routine applications that are incompatible with the mission and access model of the synchrotron light sources. To illustrate this, we provide numerous examples of x-ray absorption near edge structure (XANES), EXAFS, and XES results for a variety of problems and energy ranges. Highlights include XAFS and XES measurements of battery electrode materials, EXAFS of Ni with full modeling of results to validate monochromator performance, valence-to-core XES for 3d transition metal compounds, and uranium XANES and XES for different oxidation states. Taken en masse, these results further support the growing perspective that modern laboratory-based XAFS and XES have the potential to develop a new branch of analytical chemistry

Developing Laboratory-Based X-ray Spectroscopies for Energy and Materials Research spectroscopy

Thesis (Ph.D.)--University of Washington, 2019Advanced x-ray spectroscopies interrogate a material’s electronic structure in an element-specific manner. Traditionally, X-ray absorption fine structure (XAFS) and X-ray emission spectroscopy (XES) studies are performed at synchrotron X-ray light sources. These facilities serve to push the forefront of science and, thus, operate under an access model which necessarily excludes projects requiring routine analytical characterization, rapid feedback for prototyping, or regular access. In response to this deficit, my dissertation presents a laboratory-based XAFS and XES spectrometer of high energy resolution, reproducibility, and efficiency, along with other improvements in instrumentation, especially as pertains to the utilized crystal analyzer. A range of basic and applied materials problems were addressed with this and similar instrumentation. Select applied research studies include operando XAFS analysis of a prototype lithium-ion battery’s state-of-charge and state-of-health and an XES-based method for the quantification of hexavalent chromium in manufactured plastics that is being developed into a standard test method. Basic research spanned a study of photoexcitation dynamics in Ni metal and time-dependent density functional theory interpretations of valence-to-core XES spectra collected from a series of vanadium oxide and vanadyl phosphate energy storage materials candidates. This thesis provides strong evidence that laboratory-based X-ray spectroscopy instrumentation can serve as a powerful tool for increasing productivity and understanding in the fields of chemistry and materials science

Determination of Hexavalent Chromium Fractions in Plastics Using Laboratory-Based, High-Resolution X‑ray Emission Spectroscopy

Cr­(VI) is a well-known human carcinogen with many water-soluble moieties. Its presence in both natural and man-made substances poses a risk to public health, especially when contamination of groundwater is possible. This has led the European Union and other jurisdictions to include Cr­(VI) in restriction of hazardous substances regulations. However, for several important industrial and commercial purposes, analytical capability to characterize Cr­(VI) is known to be insufficient for regulatory purposes. For example, advanced X-ray spectroscopies, particularly synchrotron-based X-ray absorption fine structure (XAFS) studies, have shown that species interconversion and under-extraction can be difficult to prevent in many existing liquid extraction protocols when applied to plastics, mining ores and tailings, and paint sludges. Here, we report that wavelength dispersive X-ray fluorescence spectroscopy taken at energy resolutions close to the theoretical limit imposed by the core-hole lifetime, generally called X-ray emission spectroscopy (XES) in the synchrotron community, can be used in the laboratory setting for noninvasive, analytical characterization of the Cr­(VI)/Cr ratio in plastics. The selected samples have been part of ongoing efforts by standards development organizations to create improved Cr­(VI) testing protocols, and the present work provides a direct proof-of-principle for the use of such extremely high-resolution laboratory WDXRF as an alternative to liquid extraction methods for regulatory compliance testing of Cr­(VI) content. As a practical application of this work, we report a value for the Cr­(VI) mass fraction of the new NIST Standard Reference Material 2859 Restricted Elements in Polyvinyl Chloride