Mapping metals incorporation of a whole single catalyst particle using element specific X-ray nanotomography.

Full-field transmission X-ray microscopy has been used to determine the 3D structure of a whole individual fluid catalytic cracking (FCC) particle at high spatial resolution and in a fast, noninvasive manner, maintaining the full integrity of the particle. Using X-ray absorption mosaic imaging to combine multiple fields of view, computed tomography was performed to visualize the macropore structure of the catalyst and its availability for mass transport. We mapped the relative spatial distributions of Ni and Fe using multiple-energy tomography at the respective X-ray absorption K-edges and correlated these distributions with porosity and permeability of an equilibrated catalyst (E-cat) particle. Both metals were found to accumulate in outer layers of the particle, effectively decreasing porosity by clogging of pores and eventually restricting access into the FCC particle.

Registration of the rotation axis in X-ray tomography.

There is high demand for efficient, robust and automated routines for tomographic data reduction, particularly for synchrotron data. Registration of the rotation axis in data processing is a critical step affecting the quality of the reconstruction and is not easily implemented with automation. Existing methods for calculating the center of rotation have been reviewed and an improved algorithm to register the rotation axis in tomographic data is presented. The performance of the proposed method is evaluated using synchrotron-based microtomography data on geological samples with and without artificial reduction of the signal-to-noise ratio. The proposed method improves the reconstruction quality by correcting both the tilting error and the translational offset of the rotation axis. The limitation of this promising method is also discussed.

Nanoscale morphological and chemical changes of high voltage lithium-manganese rich NMC composite cathodes with cycling.

Understanding the evolution of chemical composition and morphology of battery materials during electrochemical cycling is fundamental to extending battery cycle life and ensuring safety. This is particularly true for the much debated high energy density (high voltage) lithium-manganese rich cathode material of composition Li(1 + x)M(1 - x)O2 (M = Mn, Co, Ni). In this study we combine full-field transmission X-ray microscopy (TXM) with X-ray absorption near edge structure (XANES) to spatially resolve changes in chemical phase, oxidation state, and morphology within a high voltage cathode having nominal composition Li1.2Mn0.525Ni0.175Co0.1O2. Nanoscale microscopy with chemical/elemental sensitivity provides direct quantitative visualization of the cathode, and insights into failure. Single-pixel (∼ 30 nm) TXM XANES revealed changes in Mn chemistry with cycling, possibly to a spinel conformation and likely including some Mn(II), starting at the particle surface and proceeding inward. Morphological analysis of the particles revealed, with high resolution and statistical sampling, that the majority of particles adopted nonspherical shapes after 200 cycles. Multiple-energy tomography showed a more homogeneous association of transition metals in the pristine particle, which segregate significantly with cycling. Depletion of transition metals at the cathode surface occurs after just one cycle, likely driven by electrochemical reactions at the surface.

Hard X-ray spectroscopic nano-imaging of hierarchical functional materials at work.

Heterogeneous catalysts often consist of an active metal (oxide) in close contact with a support material and various promoter elements. Although macroscopic properties, such as activity, selectivity and stability, can be assessed with catalyst performance testing, the development of relevant, preferably quantitative structure-performance relationships require the use of advanced characterisation methods. Spectroscopic imaging in the hard X-ray region with nanometer-scale resolution has very recently emerged as a powerful approach to elucidate the hierarchical structure and related chemistry of catalytic solids in action under realistic reaction conditions. This X-ray-based chemical imaging method benefits from the combination of high resolution (∼30 nm) with large X-ray penetration and depth of focus, and the possibility for probing large areas with mosaic imaging. These capabilities make it possible to obtain spatial and temporal information on chemical changes in catalytic solids as well as a wide variety of other functional materials, such as fuel cells and batteries, in their full complexity and integrity. In this concept article we provide details on the method and setup of full-field hard X-ray spectroscopic imaging, illustrate its potential for spatiotemporal chemical imaging by making use of recent showcases, outline the pros and cons of this experimental approach and discuss some future directions for hierarchical functional materials research.

3D nanoscale chemical imaging of the distribution of aluminum coordination environments in zeolites with soft X-ray microscopy.

Which side are you on? Scanning transmission X-ray microscopy is used for the first time to elucidate the coordination and distribution of aluminum in industrial-relevant zeolites at the single-particle level. Extended regions of a few hundred nanometers, rich in higher aluminum coordination environments, are heterogeneously embedded within the zeolite particle, before and after a steaming post-treatment.

Mercury localization and speciation in plants grown hydroponically or in a natural environment.

Better understanding of mercury (Hg) accumulation, distribution, and speciation in plants is required to evaluate potential risks for the environment and to optimize phytostabilization strategies for Hg-contaminated soils. The behavior of Hg in alfalfa (Medicago sativa) plants grown under controlled conditions in a hydroponic system (30 µM HgCl2) was compared with that of naturally occurring Horehound (Marrubium vulgare) plants collected from a mining soil polluted with Hg (Almadenejos, Spain) to characterize common mechanisms of tolerance. Synchrotron X-ray Fluorescence microprobe (µ-SXRF) showed that Hg accumulated at the root apex of alfalfa and was distributed through the vascular system to the leaves. Transmission electron microscopy (TEM) implied association of Hg with cell walls, accompanied by their structural changes, in alfalfa roots. Extended X-ray absorption fine structure (EXAFS) determined that Hg was principally bound to biothiols and/or proteins in M. sativa roots, stems, and leaves. However, the major fraction of Hg detected in M. vulgare plants consisted of mineral species, possibly associated with soil components. Interestingly, the fraction of Hg bound to biothiols/proteins (i.e., metabolically processed Hg) in leaves of both plants (alfalfa and M. vulgare) was similar, in spite of the big difference in Hg accumulation in roots, suggesting that some tolerance mechanisms might be shared.

In Operando X-ray diffraction and transmission X-ray microscopy of lithium sulfur batteries.

Rechargeable lithium-sulfur (Li-S) batteries hold great potential for high-performance energy storage systems because they have a high theoretical specific energy, low cost, and are eco-friendly. However, the structural and morphological changes during electrochemical reactions are still not well understood. In this Article, these changes in Li-S batteries are studied in operando by X-ray diffraction and transmission X-ray microscopy. We show recrystallization of sulfur by the end of the charge cycle is dependent on the preparation technique of the sulfur cathode. On the other hand, it was found that crystalline Li(2)S does not form at the end of discharge for all sulfur cathodes studied. Furthermore, during cycling the bulk of soluble polysulfides remains trapped within the cathode matrix. Our results differ from previous ex situ results. This highlights the importance of in operando studies and suggests possible strategies to improve cycle life.

TXM-Wizard: a program for advanced data collection and evaluation in full-field transmission X-ray microscopy.

Transmission X-ray microscopy (TXM) has been well recognized as a powerful tool for non-destructive investigation of the three-dimensional inner structure of a sample with spatial resolution down to a few tens of nanometers, especially when combined with synchrotron radiation sources. Recent developments of this technique have presented a need for new tools for both system control and data analysis. Here a software package developed in MATLAB for script command generation and analysis of TXM data is presented. The first toolkit, the script generator, allows automating complex experimental tasks which involve up to several thousand motor movements. The second package was designed to accomplish computationally intense tasks such as data processing of mosaic and mosaic tomography datasets; dual-energy contrast imaging, where data are recorded above and below a specific X-ray absorption edge; and TXM X-ray absorption near-edge structure imaging datasets. Furthermore, analytical and iterative tomography reconstruction algorithms were implemented. The compiled software package is freely available.

Extended depth of focus for transmission x-ray microscope.

A fast discrete curvelet transform based focus-stacking algorithm for extending the depth of focus of a transmission x-ray microscope (TXM) is presented. By analyzing an image stack of a sample taken in a Z-scan, a fully in-focus image can be generated by the proposed scheme. With the extended depth of focus, it is possible to obtain 3D structural information over a large volume at nanometer resolution. The focus-stacking method has been demonstrated using a dataset taken with a laboratory x-ray source based TXM system. The possibility and limitations of generalizing this method to a synchrotron based TXM are also discussed. We expect the proposed method to be of important impact in 3D x-ray microscopy.

Imaging translocation and transformation of bioavailable selenium by Stanleya pinnata with X-ray microscopy.

Selenium hyperaccumulator Stanleya pinnata, Colorado ecotype, was supplied with water-soluble and biologically available selenate or selenite. Selenium distribution and tissue speciation were established using X-ray microscopy (micro-X-ray fluorescence and transmission X-ray microscopy) in two dimensions and three dimensions. The results indicate that S. pinnata tolerates, accumulates, and volatilizes significant concentrations of selenium when the inorganic form supplied is selenite and may possess novel metabolic capacity to differentiate, metabolize, and detoxify selenite concentrations surpassing field concentrations. The results also indicate that S. pinnata is a feasible candidate to detoxify selenium-polluted soil sites, especially locations with topsoil polluted with soluble and biologically available selenite.

3D elemental sensitive imaging using transmission X-ray microscopy.

Determination of the heterogeneous distribution of metals in alloy/battery/catalyst and biological materials is critical to fully characterize and/or evaluate the functionality of the materials. Using synchrotron-based transmission x-ray microscopy (TXM), it is now feasible to perform nanoscale-resolution imaging over a wide X-ray energy range covering the absorption edges of many elements; combining elemental sensitive imaging with determination of sample morphology. We present an efficient and reliable methodology to perform 3D elemental sensitive imaging with excellent sample penetration (tens of microns) using hard X-ray TXM. A sample of an Al-Si piston alloy is used to demonstrate the capability of the proposed method.

Three-dimensional imaging of chemical phase transformations at the nanoscale with full-field transmission X-ray microscopy.

The ability to probe morphology and phase distribution in complex systems at multiple length scales unravels the interplay of nano- and micrometer-scale factors at the origin of macroscopic behavior. While different electron- and X-ray-based imaging techniques can be combined with spectroscopy at high resolutions, owing to experimental time limitations the resulting fields of view are too small to be representative of a composite sample. Here a new X-ray imaging set-up is proposed, combining full-field transmission X-ray microscopy (TXM) with X-ray absorption near-edge structure (XANES) spectroscopy to follow two-dimensional and three-dimensional morphological and chemical changes in large volumes at high resolution (tens of nanometers). TXM XANES imaging offers chemical speciation at the nanoscale in thick samples (>20 µm) with minimal preparation requirements. Further, its high throughput allows the analysis of large areas (up to millimeters) in minutes to a few hours. Proof of concept is provided using battery electrodes, although its versatility will lead to impact in a number of diverse research fields.

Complexation of Hg with phytochelatins is important for plant Hg tolerance.

Three-week-old alfalfa (Medicago sativa), barley (Hordeum vulgare) and maize (Zea mays) were exposed for 7 d to 30 µm of mercury (HgCl(2) ) to characterize the Hg speciation in root, with no symptoms of being poisoned. The largest pool (99%) was associated with the particulate fraction, whereas the soluble fraction (SF) accounted for a minor proportion (<1%). Liquid chromatography coupled with electro-spray/time of flight mass spectrometry showed that Hg was bound to an array of phytochelatins (PCs) in root SF, which was particularly varied in alfalfa (eight ligands and five stoichiometries), a species that also accumulated homophytochelatins. Spatial localization of Hg in alfalfa roots by microprobe synchrotron X-ray fluorescence spectroscopy showed that most of the Hg co-localized with sulphur in the vascular cylinder. Extended X-ray Absorption Fine Structure (EXAFS) fingerprint fitting revealed that Hg was bound in vivo to organic-S compounds, i.e. biomolecules containing cysteine. Albeit a minor proportion of total Hg, Hg-PCs complexes in the SF might be important for tolerance to Hg, as was found with Arabidopsis thaliana mutants cad2-1 (with low glutathione content) and cad1-3 (unable to synthesize PCs) in comparison with wild type plants. Interestingly, high-performance liquid chromatography-electrospray ionization-time of flight analysis showed that none of these mutants accumulated Hg-biothiol complexes.

Phase retrieval using polychromatic illumination for transmission X-ray microscopy.

An alternative method for quantitative phase retrieval in a transmission X-ray microscope system at sub-50-nm resolution is presented. As an alternative to moving the sample in the beam direction in order to analyze the propagation-introduced phase effect, we have illuminated the TXM using X-rays of different energy without any motor movement in the TXM system. Both theoretical analysis and experimental studies have confirmed the feasibility and the advantage of our method, because energy tuning can be performed with very high energy resolution using a double crystal monochromator at a synchrotron beam line, and there is zero motor error in TXM system in our approach. High-spatial-resolution phase retrieval is accomplished using the proposed method.

Arsenic localization, speciation, and co-occurrence with iron on rice (Oryza sativa L.) roots having variable Fe coatings.

Arsenic contamination of rice is widespread, but the rhizosphere processes influencing arsenic attenuation remain unresolved. In particular, the formation of Fe plaque around rice roots is thought to be an important barrier to As uptake, but the relative importance of this mechanism is not well characterized. Here we elucidate the colocalization of As species and Fe on rice roots with variable Fe coatings; we used a combination of techniques--X-ray fluorescence imaging, µXANES, transmission X-ray microscopy, and tomography--for this purpose. Two dominant As species were observed in fine roots-inorganic As(V) and As(III) -with minor amounts of dimethylarsinic acid (DMA) and arsenic trisglutathione (AsGlu(3)). Our investigation shows that variable Fe plaque formation affects As entry into rice roots. In roots with Fe plaque, As and Fe were strongly colocated around the root; however, maximal As and Fe were dissociated and did not encapsulate roots that had minimal Fe plaque. Moreover, As was not exclusively associated with Fe plaque in the rice root system; Fe plaque does not coat many of the young roots or the younger portion of mature roots. Young, fine roots, important for solute uptake, have little to no iron plaque. Thus, Fe plaque does not directly intercept (and hence restrict) As supply to and uptake by rice roots but rather serves as a bulk scavenger of As predominantly near the root base.

3D nanoscale imaging of the yeast, Schizosaccharomyces pombe, by full-field transmission X-ray microscopy at 5.4 keV.

Three-dimensional (3D) nanoscale structures of the fission yeast, Schizosaccharomyces pombe, can be obtained by full-field transmission hard X-ray microscopy with 30 nm resolution using synchrotron radiation sources. Sample preparation is relatively simple and the samples are portable across various imaging environments, allowing for high-throughput sample screening. The yeast cells were fixed and double-stained with Reynold's lead citrate and uranyl acetate. We performed both absorption contrast and Zernike phase contrast imaging on these cells in order to test this method. The membranes, nucleus, and subcellular organelles of the cells were clearly visualized using absorption contrast mode. The X-ray images of the cells could be used to study the spatial distributions of the organelles in the cells. These results show unique structural information, demonstrating that hard X-ray microscopy is a complementary method for imaging and analyzing biological samples.

Nanoscale X-ray microscopic imaging of mammalian mineralized tissue.

A novel hard transmission X-ray microscope (TXM) at the Stanford Synchrotron Radiation Lightsource operating from 5 to 15 keV X-ray energy with 14 to 30 microm2 field of view has been used for high-resolution (30-40 nm) imaging and density quantification of mineralized tissue. TXM is uniquely suited for imaging of internal cellular structures and networks in mammalian mineralized tissues using relatively thick (50 microm), untreated samples that preserve tissue micro- and nanostructure. To test this method we performed Zernike phase contrast and absorption contrast imaging of mouse cancellous bone prepared under different conditions of in vivo loading, fixation, and contrast agents. In addition, the three-dimensional structure was examined using tomography. Individual osteocytic lacunae were observed embedded within trabeculae in cancellous bone. Extensive canalicular networks were evident and included processes with diameters near the 30-40 nm instrument resolution that have not been reported previously. Trabecular density was quantified relative to rod-like crystalline apatite, and rod-like trabecular struts were found to have 51-54% of pure crystal density and plate-like areas had 44-53% of crystal density. The nanometer resolution of TXM enables future studies for visualization and quantification of ultrastructural changes in bone tissue resulting from osteoporosis, dental disease, and other pathologies.

Life and death of a single catalytic cracking particle.

Fluid catalytic cracking (FCC) particles account for 40 to 45% of worldwide gasoline production. The hierarchical complex particle pore structure allows access of long-chain feedstock molecules into active catalyst domains where they are cracked into smaller, more valuable hydrocarbon products (for example, gasoline). In this process, metal deposition and intrusion is a major cause for irreversible catalyst deactivation and shifts in product distribution. We used x-ray nanotomography of industrial FCC particles at differing degrees of deactivation to quantify changes in single-particle macroporosity and pore connectivity, correlated to iron and nickel deposition. Our study reveals that these metals are incorporated almost exclusively in near-surface regions, severely limiting macropore accessibility as metal concentrations increase. Because macropore channels are "highways" of the pore network, blocking them prevents feedstock molecules from reaching the catalytically active domains. Consequently, metal deposition reduces conversion with time on stream because the internal pore volume, although itself unobstructed, becomes largely inaccessible.

Nanoscale examination of microdamage in sheep cortical bone using synchrotron radiation transmission x-ray microscopy.

Microdamage occurs in bone through repeated and excessive loading. Accumulation of microdamage weakens bone, leading to a loss of strength, stiffness and energy dissipation in the tissue. Imaging techniques used to examine microdamage have typically been limited to the microscale. In the current study microdamage was examined at the nanoscale using transmission x-ray microscopy with an x-ray negative stain, lead-uranyl acetate. Microdamage was generated in notched and unnotched beams of sheep cortical bone (2×2×20 mm), with monotonic and fatigue loading. Bulk sections were removed from beams and stained with lead-uranyl acetate to identify microdamage. Samples were sectioned to 50 microns and imaged using transmission x-ray microscopy producing projection images of microdamage with nanoscale resolution. Staining indicated microdamage occurred in both the tensile and compressive regions. A comparison between monotonic and fatigue loading indicated a statistically significant greater amount of stain present in fatigue loaded sections. Microdamage occurred in three forms: staining to existing bone structures, cross hatch damage and a single crack extending from the notch tip. Comparison to microcomputed tomography demonstrated differences in damage morphology and total damage between the microscale and nanoscale. This method has future applications for understanding the underlying mechanisms for microdamage formation as well as three-dimensional nanoscale examination of microdamage.