Tables for
Volume I
X-ray absorption spectroscopy and related techniques
Edited by C. T. Chantler, F. Boscherini and B. Bunker

International Tables for Crystallography (2023). Vol. I. Early view chapter

Micro- and nano-XAFS: spatially resolved XAFS

Gema Martinez-Criadoa,b*

aESRF – The European Synchrotron, 71 Avenue des Martyrs, 38043 Grenoble, France, and bInstituto de Ciencia de Materiales de Madrid, Consejo Superior de Investigaciones Científicas (CSIC), Calle Sor Juana Inés de la Cruz 3, Cantoblanco, 28049 Madrid, Spain
Correspondence e-mail:

Micro- and nano-XAFS is an emerging tool with important applications across multiple fields. Here, a description is given of how spatially resolved XAFS methods are accomplished and also how they are currently exploited for the study of heterogeneous materials. The key role of the instrumentation in reliable spatially resolved 2D and 3D data acquisitions with micrometre and nanometre spatial resolutions is discussed. Selected examples illustrate the ability of spatially resolved XAFS modalities to provide local information on chemical changes in electrodes during battery operation and iron speciation, as well as on catalyts under relevant reaction conditions.

Keywords: nanobeams; XAFS; synchrotrons.

1. Introduction

Although the X-ray absorption fine-structure (XAFS) technique has contributed enormously to our understanding of materials and phenomena, the ability to spatially resolve and identify local atomic and electronic structure on the micrometre and/or nanometre length scales with high sensitivity (for example at a sub-parts per billion level) is crucial to unravel macroscopic responses in heterogeneous materials. Magnetic domains, extremely diluted microstructures, nanoparticles and/or embedded agglomerates in amorphous systems are just a few examples in which ultrasmall features mediate changes, modifying their average properties and macroscopic behaviours. Accordingly, the addition of spatial resolving power to XAFS-related methods represents a key step to study local structural transformations influenced by heterogeneities, surfaces, grain boundaries, interfaces etc. in a wide range of disciplines from biomedicine to nanotechnology, chemistry and surface and environmental sciences.

Superior brilliant and low-emittance synchrotron sources (diffraction-limited storage rings; Eriksson et al., 2014[link]), advanced nanofocusing optics (Ice et al., 2011[link]; Sakdinawat & Attwood, 2010[link]) and enhanced detection schemes (Ryan et al., 2010[link]), as well as new imaging and improved microscopy approaches (Ade & Stoll, 2009[link]), are key factors which have made it possible for spatially resolved XAFS to become an established technique. The combination of large penetration depths, high throughput and excellent tunability in terms of the probe (energy, spot size, photon flux, time structure, polarization etc.) with in situ or operando exploration of short-range ordering has also been an unprecedented asset at micrometre and nanometre lengths (Smit et al., 2008[link]). Deeply buried polymeric microstructures (Takao et al., 2015[link]), nano­scale magnetic phenomena (Nolting et al., 2000[link]; Wang et al., 2015[link]) and dynamics of catalytic solids and related nano­materials (Andrews & Weckhuysen, 2013[link]) under industrially and environmentally relevant conditions are just a few scientific examples (Buurmans & Weckhuysen, 2012[link]).

Complementing spatially averaged tools and electron-microscopy methods (for example electron energy loss spectroscopy and extended electron energy loss fine structure) that operate under ultrahigh vacuum with ultrathin sample thickness and greater beam damage, micro/nano-XAFS offers unique advantages for 2D/3D imaging of local atomic environments with lower detection limits (especially for Z > 20). It also produces maps of mixed-valence states or depth chemical analysis (Lüh et al., 2012[link]) without sectioning large samples that might even be subject to realistic environments (Li, Meyer et al., 2015[link]). However, in developing approaches for spatially resolved XAFS it has proven to be difficult to technically combine a fixed focal spot position, chromaticity of the focusing lens, angular stability of the double-crystal monochromator (DCM), fast energy scanning speeds, incident beam intensity monitoring, vibration-free operation, large k-space information and low radiation dose (Ziegler et al., 2008[link]; Bertsch & Hunter, 2001[link]). In addition, data collection may be affected locally by the highly focused X-ray beam due to sample instability (for example photoreduction) or local environment changes (for example temperature). Finally, the data analysis of spatially resolved XAFS may also become more complex because of mixing of crystallographic phases or structural disorder, large surface-to-volume ratios, nanoscale effects etc. (Kuzmin & Chaboy, 2014[link]). Advanced simulation techniques are sometimes necessary to describe, for example, the structure and dynamics of nanoparticles, as well as to take into account the influence of nanosized effects (charge-transfer, vacancies etc.). The use of complementary techniques [for example transmission electron microscopy (TEM) and X-ray diffraction (XRD)] to extract reliable structural information might be required to properly process the data.

2. Methodology

Several spatially resolved XAFS layouts are currently in operation (Holt et al., 2013[link]; Karim et al., 2015[link]). They can be classified as a function of the energy of the X-ray beam (soft or hard X-rays), the specific setup (surface- or bulk-sensitive), the nature of the sample (magnetic materials, organic or biological samples) and/or the signal that is detected (photoelectrons, characteristic X-rays or transmitted beam). For example, there are multiple schemes: scanning transmission X-ray microscopes (Watts & Ade, 2012[link]), scanning X-ray fluorescence microscopes (Martínez-Criado et al., 2012[link]), near-edge X-ray absorption fine-structure microscopes (Ade & Stoll, 2009[link]), photoemission electron microscopes (He et al., 2011[link]; Gu et al., 2014[link]; Karim et al., 2015[link]) and full-field transmission X-ray microscopes (Streubel et al., 2015[link]; Meirer et al., 2011[link]). However, the most common methods can be grouped into three classes: (i) full-field transmission X-ray microscopy (TXM), (ii) energy-dispersive EXAFS microscopy (EDXAM) and (iii) scanning X-ray microscopy (SXM).

TXM produces a chemical mapping by the rapid acquisition of a set of images recorded by scanning across an absorption edge. Usually, a large and incoherent X-ray beam is focused onto the sample with a condenser lens, which defines the field of view. According to the X-ray energy and focusing optics, the spot size can vary from tens of micrometres to a few micrometres in diameter. A Fresnel zone plate often collects the X-rays that are scattered and transmitted through the sample, which must have an appropriate thickness and uniformity. Thus, the objective lens, which determines the spatial resolution, forms a real-space intensity image on a CCD camera. When properly aligned, the image stack has an XAFS spectrum for each pixel.

EDXAM, on the other hand, is based on the diffraction of polychromatic X-rays from a large horizontal divergence source by a polychromator crystal (Katayama et al., 2015[link]). With a large energy bandwidth, the beam is focused on the sample and then diverges towards a position-sensitive detector, which correlates position with energy. Thus, EDXAM acquires a full X-ray absorption fine-structure (EXAFS) transmission spectrum per pixel in a single shot, which makes the technique especially useful for time-resolved studies (Pascarelli et al., 2006[link]). Although still not fast enough to obtain large fields of view (dwell time of >1 s per pixel), the focused beam (with a spot size of typically a few micrometres) is very stable because no movement of optical elements is required to collect the spectral data.

Finally, SXM acquires images on a pixel-by-pixel basis. The beam is focused to a very small spot size by the nanofocusing optics (preferably Kirkpatrick–Baez mirrors in a grazing-incidence configuration). This spot size ranges from hundreds to tens of nanometres and usually determines the resolution of the microscope. The sample is raster-scanned through the X-ray beam, while the intensity of the characteristic X-rays, photoelectrons and/or transmitted light is recorded at each sample position. When the transmitted X-ray intensity is collected using a point detector, this instrument is called a scanning transmission X-ray microscope. Based on the images generated by a contrast mechanism (for example X-ray fluorescence; XRF), spatially resolved XAFS spectra can be collected across the sample at individual points to extract information on valence states, symmetry, electronic states, coordination number, interatomic distances etc. (Segura-Ruiz et al., 2011[link]). Alternatively, by acquiring images at different X-ray photon energies, full spectral information collected at every pixel can be used to image and distinguish between specific chemical species (Martínez-Criado et al., 2005[link]). Micro-spectroscopy and spectro-microscopy are also terminologies that are commonly used to describe spatially resolved XAFS analysis. In micro-spectroscopy, a spectrum is collected with a micro/nanobeam for a specific volume element or region of interest defined by an optical, phase or chemical contrast image or an elemental map generated via XRF. In spectro-microscopy, on the other hand, full spectra are collected at each pixel and are then used to produce an image based on spatially variable chemical differences in the sample.

Although the acquisition time per image is considerably slower compared with TXM, SXM requires a lower radiation dose and has more flexibility in terms of field of view, sample environment (electrochemical or humidity-controlled cells, magnetic and electric fields, cryostats, micro-heaters, diamond anvil cells) and detection schemes (photoelectrons, XRF, visible light, transmitted X-rays, linear and circular magnetic dichroism). It also has the potential for ultrahigh-resolution (∼1 nm) imaging with ptychography, although this imaging technique is still under considerable development (Shapiro et al., 2014[link]; Wise et al., 2016[link]).

In both the SXM and TXM approaches the choice of the X-ray energy range plays a key role in the resolution limits, depth of foci, sample thicknesses and available sample environments (Winarski et al., 2012[link]). The instruments are often grouped into hard (energy > 4 keV) and soft (energy < 4 keV) X-ray instruments. With depths of focus of tens of micrometres and lateral resolutions down to 25–50 nm (for example Kirk­patrick–Baez mirrors), the former can operate in air with focal lengths of centimetres, accommodating multiple sample environments (Martínez-Criado et al., 2013[link]). On the other hand, the latter operate in vacuum or helium conditions and feature depths of focus below 10 µm, better lateral resolutions of about 12–40 nm and focal lengths in the millimetre range. Several comprehensive reviews have described the principles, capabilities and applications of these micro/nano-XAFS-related techniques (Suzuki & Terada, 2016[link]; Fischer et al., 2010[link]; Ade, 2002[link]). A collection of papers also discuss progress in the field in areas such as focusing optics, data-collection modes, evolution and impacts on various research topics (Bertsch & Hunter, 2001[link]; Hitchcock et al., 2005[link]; Grunwaldt et al., 2005[link]; Pascarelli et al., 2006[link]; Sakdinawat & Attwood, 2010[link]; Ice et al., 2011[link]). Generally, X-ray spot sizes from micrometres down to tens of nanometres are produced by three types of focusing devices: refractive, reflective and diffractive optics. The choice depends on the X-ray wavelength range, the experimental setup (TXM or SXM) and the source characteristics. For details and up-to-date references on focusing methods and diffraction-limited optics for spectroscopy, the reader is referred to Simionovici et al. (2022[link]). Finally, the EDXAM approach, in addition to the other spatially resolved XAFS schemes, was introduced by Newton and Dent in the book In-situ Characterization of Heterogeneous Catalysts (Newton & Dent, 2013[link]).

3. Instrumentation

To date, little information has been published concerning the experimental difficulties of conducting spatially resolved XAFS. Some major considerations should be carefully taken into account to obtain a highly stable operation and vibration-free design (Tucoulou et al., 2008[link]; Kaznatcheev et al., 2013[link]; Sung et al., 2015[link]).

Concerning the X-ray source, combining the requirements for high spatial resolution, high photon flux and a large energy range calls for an undulator source (De Andrade et al., 2011[link]). Since the focus size is roughly proportional to the square root of the emittance, operation of a micro/nano-XAFS beamline with a highly stable (less than microradians) and small beam is crucial in addition to full tunability over the chosen energy range (Weckert, 2015[link]). To fulfil the conditions for EDXAM, on the other hand, the requirement to produce a sufficiently large energy bandpass after diffraction by the polychromator also demands an insertion device coupled with a horizontally focusing mirror to enhance the divergence to the required value. An incoming horizontal divergence of about 1 mrad is necessary at the level of the polychromator (Pascarelli et al., 2006[link]).

The high heat load produced by these insertion devices must be effectively minimized on the optical elements. As is usual, unwanted high-energy photons at low energies can be suppressed by a mirror, whereas unwanted low-energy photons at high energies can be suppressed by absorption filters. Optimization of the positions, cooling systems and quality of all optical components (for example slope errors) is the key technical challenge (Siewert et al., 2014[link]). When a long-length beamline design (100–200 m) is adopted to obtain large source-size demagnifications (Schroer et al., 2010[link]; Somogyi et al., 2015[link]; Martínez-Criado et al., 2016[link]), extreme stability of the fixed-exit DCM becomes imperative. Thus, for routine nano-XAFS operations the DCM needs cutting-edge capabilities not only in terms of fixed exit (angular stability ≪ 1 µrad) but also of repeatability, thermal stability, speed and long-term stability (Yoder et al., 2011[link]; Kristiansen et al., 2015[link]). Microradian angular fluctuations can induce movements of the effective point source, resulting in deviations of the focused beam (Tucoulou et al., 2008[link]). Furthermore, the limited beam acceptance of common nanofocusing optics also implies that the angular fluctuations can cause movements of the incident beam that turn into intensity fluctuations in the focused beam. Thus, an overillumination approach of the focusing optics can help to avoid large intensity changes. To further optimize the intensity variation of the incoming beam, the change in energy (i.e. the DCM) should be followed by the undulator gap in the scanned energy range, which can become very complicated due to speed-synchronization issues. Moreover, monitoring of the intensity before the sample at the same time as the transmitted beam or XRF signal also involves serious difficulties for the normalization of absorption spectra. Different strategies might be adopted: sequential data acquisition, collection of the XRF intensity from an element homogeneously distributed in the sample, scanning a slit over the beam fan after the polychromator etc.

Concerning the nanofocusing optics, the influence of thermal distortions, roughness (for example superpolished, <1 Å), fabrication slope errors (∼0.1 µrad r.m.s.) and figure errors (∼1 nm peak to valley) on the focusing properties and energy resolution is critical (Ziegler et al., 2008[link]; Barrett et al., 2011[link]). Efficient and high-quality achromatic 2D focusing lenses are essential to achieve large numerical aperture optics that simultaneously provide a large flux and a fixed and small focus (Matsuyama et al., 2006[link]). The use of achromatic Kirkpatrick–Baez mirrors, for example, offers the possibility of measuring spatially resolved XAFS spectra of several elements in the very same sample spot without realignment of the experimental setup (Segura-Ruiz et al., 2011[link]).

The nanopositioning required for 2D/3D chemical imaging also demands sample (piezo)stages with very high thermal/vibration stability, resolution, repeatability, low angular errors and ultrahigh precision (Zhu et al., 2015[link]; Schroer & Falkenberg, 2014[link]; Nazaretski et al., 2013[link]). For tomographic acquisitions, in addition, a high-accuracy spindle (angular resolution of ∼10−3, eccentricity of less than micrometres, wobble of less than microradians) must be anticipated. Online metrology can also be crucial to reach ultrahigh-precision positioning (error compensation using laser interferometers/capacitive sensors to measure online angular errors and introduce online compensation, vibration-control techniques etc.). To speed up data collection and to reduce possible beam-induced sample damage, the development of fast-scanning strategies (degrees per second) is highly desirable, such as, as mentioned, quick energy scans involving continuous movement of the DCM with parallel tuning of the undulator gap (Müller et al., 2016[link]).

The detectors are also one of the keystones of the micro/nano-XAFS experiment itself (Heald, 2015[link]). Depending on the detection energy range, several options are commercially available for XRF analysis (Letard et al., 2006[link]). Multi-element silicon drift detectors (SDDs) are commonly used to quantitatively measure XRF signals from samples at energies from 2 keV to ∼25 keV, at which the detector efficiency falls to ∼20% due to the limited silicon X-ray absorption; high-energy measurements are preferentially obtained with a germanium diode array. These systems reach spectral energy resolutions down to about 140 eV FWHM at the Mn Kα line (5.9 keV) but featured a limited count rate (up to 4 × 106 counts per second), involving fairly long dwell times per point/pixel or point/energy (i.e. 0.1–1 s). Faster performance energy-resolving detectors for XRF-based XAS acquisitions with a larger solid angle, a higher count rate and higher efficiency are under development. The Maia-384 massively parallel detector is a good example (Ryan et al., 2010[link]). The system consists of an annular array of 384 silicon diode detectors positioned in a back-scattering geometry with respect to the incident X-ray micro/nanobeam; this geometry ensures that a large solid angle is subtended (approximately 1.3 sr). An approach based on an event-mode data collection with real-time processing capabilities allows the recording of maps with a minimum dwell time down to ∼0.05 ms per pixel and a total count-rate capacity greater than 106 s−1 while avoiding readout overheads. A real-time full spectra elemental deconvolution can be obtained through an integrated algorithm based on a matrix-transform method called dynamic analysis (Ryan et al., 2010[link]). However, the Maia detectors suffer from higher scatter and backgrounds, difficulty in positioning visible optics, poor low-energy performance and an energy resolution that is not comparable to SDDs, as well as working-distance limitations.

EDXAM acquisitions in transmission mode, on the other hand, are presently based on a 2D CCD camera exploited in a 1D fashion (Labiche et al., 2007[link]; Salvini et al., 2005[link]). The general requirements include a pixel size of <50 µm for energy-resolution purposes, a pixel array of >2048 to span the required energy range, a large dynamic range (104–105), a fast readout time (<1 µs) and very low noise. For time-resolved experiments on the millisecond time scale, a strip detector based on germanium technology is a good choice. An XSTRIP for high energies has been developed specifically for EDXAM on third-generation synchrotrons (Headspith et al., 2007[link]). The system collects data at high energies with good efficiency (>90% absorption at 40 keV), spatial resolution and radiation-damage tolerance. A key element of this system is the 1024-element germanium microstrip sensor, fabricated using amorphous germanium (a-Ge) contact technology, featuring a strip pitch of 50 µm. This sensor coupled to the STFC-designed X2CHIP ASIC readout electronics provides an excellent linearity (better than 0.1%) and ultrafast readout performance (100 kHz frame rate).

4. Applications

4.1. 3D chemical imaging of batteries using TXM

Electric vehicles are becoming more and more popular owing to the reduction in oil dependence and greenhouse gas production. However, one major barrier must be overcome: their battery technology (Dunn et al., 2011[link]). Further improvements to lithium-ion batteries are still necessary to meet energy density, cost, life-cycle and safety goals. New strategies for the design of next-generation high energy-density devices require the monitoring of chemical changes in electrodes during battery operation (i.e. insertion/extraction of lithium ions). In this context, chemical X-ray absorption near-edge structure (XANES) microscopy plays a key role in the diagnostics of lithium-ion battery electrodes. Lim and coworkers, for example, tracked the reaction dynamics of LiFePO4 (the electrode material) by recording the relative fractions of iron(II) and iron(III) contained within it (Lim et al., 2016[link]). In the same way, Li and coworkers reported the effects of particle size, electronic connectivity and incoherent nanoscale domains on the sequence of lithiation in LiFePO4 porous electrodes by STXM (Li, Meyeret al., 2015[link]). Two other studies have provided 2D and 3D chemical information about the changes that take place in the electrodes, revealing the location of nickel and nickel oxide (Meirer et al., 2011[link]; Wang et al., 2011[link]). In close correlation with changes in the morphology and porosity of the cathode, these studies provide a new perspective on lithium-ion battery electrodes that could direct new design concepts for the next generation of batteries.

The Xradia TXM instrument located on beamline 6-2 of the Stanford Synchrotron Radiation Lightsource at the SLAC National Accelerator Laboratory, USA has been used to study lithium-ion battery electrodes (Liu et al., 2011[link]). Single-pixel XANES spectra were collected with a depth of focus of about 50 µm and a field of view of 30 µm, which was extendable to the millimetre range through mosaic mapping. Fitting of the XANES results in a chemical phase map at 30 nm resolution (Meirer et al., 2011[link]). The potential impact of this technique is illustrated in Fig. 1[link] with the study of the changes that take place in NiO when cycled in a lithium battery. NiO is considered to be an alternative anode material because of its very high charge-storage capability. 2D XANES images were collected from 8250 to 8600 eV in 154 steps across the Ni K edge at 8333 eV. The zone plate was adjusted to maintain focus. The two chemical phases present, NiO and nickel, were fitted to XANES spectra of pure NiO and pure nickel metal, respectively. Tomography acquisitions were acquired at 13 distinct energy points identified from the 2D XANES. 3D chemical imaging was performed from −67° to 67° in 1° steps within 17.5 h. In brief, the use of TXM to analyze lithium-ion NiO battery electrodes in different charge states results in a set of images in which the presence of NiO and nickel, the phase produced upon reduction, was resolved and correlated with changes in morphology and porosity.

[Figure 1]

Figure 1

Principles of data processing for 3D XANES microscopy. (1) One image is acquired in absorption contrast at each energy in the XANES scan. (2) XANES is constructed from each pixel, plotting normalized absorption versus energy. (3) XANES from each pixel is fitted to create a chemical phase map. (4) One phase map is generated at each angle in the tomographic scan. (5) The set of phase maps is used for tomographic reconstruction to retrieve 3D chemical speciation. Adapted with permission from Meirer et al. (2011[link]).

These findings provide detailed insight into the conversion of NiO to nickel during the discharge process. While the smallest particles are homogeneously converted, the transformation of the larger particles is concentrated around the edges. 3D chemical imaging revealed that while conversion to nickel is extensive on the surface of the larger agglomerates, it can also take place through cracks that go through the interior of the grains. It is likely that the fine grains of NiO observed in the smallest particles are already the end result of electrochemical grinding. This observation also implies that the pores between and within particles/agglomerates are large enough for the electrolyte to penetrate and provides direct evidence of the critical role of open spaces within the composite architecture of a lithium-ion battery electrode.

Wang and coworkers also reported a similar study on in situ tracking of chemical phase transformation, mapping and composition information of a battery with CuO as the anode at sub-30 nm resolution with a 40 × 40 µm field of view using TXM combined with spectroscopy (Wang et al., 2011[link]). A size-dependent and core–shell lithiation–delithiation mechanism was suggested for the electrochemical reaction. Likewise, operando hard X-ray spectro-imaging enabled the first visualization of the electrochemical reaction in high-capacity FeF3 cathodes on the nanoscale (Li, Chen-Wiegart et al., 2015[link]). In summary, 3D XANES microscopy is a unique technique that combines unprecedented spatial and energy resolution with large fields of view and fast acquisition, the capabilities and high throughput of which may have an overarching impact in several research fields.

4.2. Iron speciation imaging using EDXAM

Iron is one of the most important transition elements in minerals and silicate melts (Sanloup et al., 2013[link]). Its redox state is fundamental in order to determine the thermodynamical conditions under which rocks and magma form. Fe3+/Fe2+ ratios can fluctuate considerably depending on the redox conditions during rock formation. As a proof of concept, a chemical imaging approach based on EDXAM was applied to iron in a chlorite mineral from a metamorphic rock from the southwest of Japan (Sambagawa belt) to validate the technique (Muñoz et al., 2006[link]). The iron speciation in this geological material characterized by a highly heterogeneous iron distribution may provide key information to understand the formation of mountain belts and the geodynamics of convergent zones on the lithospheric scale. For example, the iron oxidation state in a garnet from a subduction setting has been determined by micro-XANES (Borfecchia et al., 2012[link]).

The EDXAM experiments were performed with a focal spot of about 5 × 5 µm at the Fe K edge on the microfocused dispersive EXAFS beamline ID24 at the European Synchrotron Radiation Facility, France (Pascarelli et al., 2006[link]). Using XRF detection, a 30 µm thick sample was mapped over an area of 390 × 180 µm. Several minerals that are highly differentiated in terms of iron content were present: chlorite (∼30 wt%), phengite (∼4 wt%) and quartz (hundreds of p.p.m.). Sampling was performed every 5 µm so that around 3000 XANES spectra were collected in around 100 min. Thus, it was possible to generate maps from the raw data at different energies, which is comparable to the standard XANES mapping technique (i.e. a series of maps collected at different energies).

Fig. 2[link] presents qualitative maps obtained after normalization of the XANES spectra. The edge-jump information displays the iron distribution in the sample, whereas the edge position qualitatively indicates the iron oxidation state. Although the post-edge and edge regions of the spectra may significantly be affected by the linear polarization of the X-ray beam, the influence on the pre-edge region does not appear to be significant. For iron speciation, another image displays the normalized absorbance at 7137 eV. The results revealed that iron is mainly located in chlorite (red region), while phengite and quartz are much less concentrated (Fig. 2[link]b, blue and dark blue regions, respectively). Also, the iron oxidation state in chlorite appears highly contrasted (Muñoz et al., 2006[link]). Dark blue corresponds to more reduced regions, whereas red corresponds to more oxidized regions (Fig. 2[link]c). Moreover, the speciation image indicates a clear contrast for quartz and a relatively good correlation with the redox distribution in chlorite (Fig. 2[link]d).

[Figure 2]

Figure 2

Top: experimental setup for EDXAM. (a) Optical image showing chlorite (middle), quartz (top) and phengite (between chlorite and quartz and at the bottom), (b) iron contents (edge jump), (c) iron redox (edge position), (d) iron speciation. Averaged and single-point XANES spectra based on the different masks defined from the maps are shown to the left. Blue, chlorite–iron(II); green, phengite; orange, quartz; red, chlorite–iron(III). Masks corresponding to each region are also highlighted to deduce quantitative data. The corresponding XANES spectra are also plotted in the figure. Adapted with permission from Muñoz et al. (2006[link]). Copyright (2006) American Geophysical Union.

The findings point to trivalent iron preferentially being located in the octahedral interfoliar layer of the chlorite, which is in good agreement with a thermodynamic approach to the crystallography of chlorites (Vidal et al., 2005[link]).

This report illustrated the potential of EDXAM to study the speciation of heterogeneous samples over large areas with high sensitivity on micrometre length scales. Further applications include that of 2D chemical mapping to in situ studies, a field which is expected to expand rapidly over a variety of scientific domains (catalysis, chemistry and high pressure).

4.3. Probing catalysts at work using SXM

Catalysts are materials that speed up chemical reactions without being consumed, playing a key role in the production of many industrially relevant chemicals; for example, catalytic converters are applied to reduce toxic emissions from vehicle exhaust systems (Newton & Dent, 2013[link]). Accordingly, a clear understanding of how catalysts work and how to improve them is a great challenge owing to their heterogeneous nature on the nanoscale and their operation under gaseous atmospheres at high temperatures. Usually, the active elements consist of nanosized metal or metal oxide particles dispersed on high-surface-area supports. Therefore, multiple factors such as particle size, composition and reactions among support, reactants and products affect the resulting reactivity. Thus, the evolution of all of these elements as a function of time under realistic conditions is critical to provide insight into the mechanisms behind the chemical transformations. A recent tutorial review has demonstrated that X-ray microscopy is an exceptional technique for detecting structural changes in catalyst beds, catalyst pellets and particles in a spatially resolved fashion (Grunwaldt & Schroer, 2010[link]). Similarly, a combinatorial approach of micro-XRF, micro-XANES and micro-XRD has provided 2D/3D chemical information on commercially used fluid catalytic cracking catalyst materials at the individual particle level (Ruiz-Martínez et al., 2013[link]).

In 2008 a nanoreactor originally designed for in situ TEM was used in soft SXM in order to identify the chemical species present at the nanoscale for an iron-based Fischer–Tropsch synthesis catalyst (Smit et al., 2008[link]). In this process, a gas mixture of CO and H2 (typically at 1.2 bar at 500°C) is converted through a surface polymerization reaction into liquid hydrocarbons of various forms that can then be used in the production of high-purity chemicals and transportation fuels without the need for crude oil. The iron-based catalyst consisted of an iron oxide phase dispersed on silicon oxide (SiO2), with copper oxide and potassium oxide promoters added to improve its selectivity, activity and stability. All experiments were performed on the interferometrically controlled SXM microscope on beamline 11.0.2 of the Advanced Light Source at the Lawrence Berkeley National Laboratory, USA (Kilcoyne et al., 2003[link]). As a compromise between spatial resolution and the longer working distance that was required, a 35 nm zone-plate lens was used with a spatial resolution of about 40 nm (Fig. 3[link]). The X-ray absorption spectra and images were measured with a 35 × 35 nm step size of the piezoelectric sample stage.

[Figure 3]

Figure 3

Experimental setup and data-acquisition method. (a) Diagram of the in situ STXM technique. Soft X-ray light is focused on the sample using a Fresnel-type zone-plate lens. An order-sorting aperture filters out higher order diffraction. The nanoreactor containing the sample is placed in an adaptor that holds up to two nanoreactors at the same time. The adaptor can be translated with nanometre precision by an interferometrically controlled (x, y, z) piezoelectric stage, allowing the acquisition of raster scans. (b) Close-up of the nanoreactor, showing the windows and the embedded heater spiral. The reactor dimensions are 500 × 500 × 50 µm. The platinum heater has four electronic connections for simultaneous power supply and resistive temperature measurements. The sample itself is supported on the SiNx windows. The measurements are performed in the circular areas (5.5 µm diameter) where the 1.2 µm-thick SiNx windows are etched down to a thickness of about 10 nm. (c) Diagram of a typical STXM data-acquisition method. By acquiring images at different X-ray photon energies (for example E1, E2 and E3), a 3D data cube with full spectral information at every pixel is obtained. These data can be used to image and distinguish between specific chemical species (for example species A, B and C). Adapted with permission from Smit et al. (2008[link]). Copyright (2008) Macmillan Publishers Lid.

The SXM data collection involved three stages: (i) initial observation at room temperature under helium, (ii) observation after 2 h at 350°C under reducing hydrogen gas and (iii) observation after a further 4 h at 250°C under synthesis gas. To identify the valence and coordination of the iron species, absorption spectra were recorded for each pixel at the Fe L3 and L2 edges, at the O K edge for oxygen-containing species and at the C K edge for carbon-containing species. The measurements generated chemical contour maps for the species that were present. Quantitative analysis of the spectra yielded estimations of the content of the various phases present at each point.

The results revealed that initially there was a heterogeneous distribution of predominantly α-Fe2O3 (the catalyst precursor) and SiO2 (the support) with no evidence of carbides (Fig. 4[link]). After the reducing treatment in hydrogen, there were significant changes involving the appearance of a heterogeneous distribution of Fe3O4, FeSiO4 and Fe0 species. During catalysis, the proportion of FeSiO4 grew at the expense of Fe3O4. C K-edge spectra indicate the preferential presence of carbon with Fe0, suggesting the formation of iron carbides, a finding consistent with prior knowledge that iron oxide and metallic iron (α-Fe) usually coexist during reaction, with the iron phases largely converted into iron carbide. The presence of reactant carbon species in iron-deficient areas indicates that the support might play some role in the spillover of (hydro)-carbon species from the metal to the support, thereby preventing blocking of the active sites of the catalyst (Smit et al., 2008[link]).

[Figure 4]

Figure 4

Iron-species contour maps of the catalyst material before catalysis. Maps (top row) of a 400 × 750 nm region were constructed from corresponding Fe L3- and L2-edge (bottom row) and O K-edge (not shown) spectra taken at room temperature in helium before treatment (left) and after 2 h in hydrogen at 350°C (right). The dotted lines in the L-edge spectra are fits by a linear combination of reference spectra. The bar graphs represent the calculated relative percentage contributions of the different iron phases at the sampling points. Adapted with permission from Smit et al. (2008[link]). Copyright (2008) Macmillan Publishers Ltd.

This report demonstrated that SXM can provide details of the morphology and composition of complex catalytic systems under relevant reaction conditions, opening new opportunities for nanometre-resolution imaging of a range of key chemical processes that take place on solids in gaseous or liquid environments.

5. Conclusions

Spatially resolved XAFS is an evolving field that has unique attributes and is expanding in a number of new and exciting directions. Many full-field transmission X-ray microscopes, energy-dispersive EXAFS microscopes and scanning X-ray microscopes are available today and additional instruments are planned or under construction at third-generation synchrotron-radiation sources (Pascarelli et al., 2006[link]; Schroer et al., 2010[link]; Somogyi et al., 2015[link]; Martínez-Criado et al., 2016[link]). By pushing the spatial resolution, new research strategies exploiting spatially resolved XAFS are emerging: combined data-collection modalities (for example beam-induced current, X-ray-excited optical luminescence), novel spectroscopic concepts (for example ptychography; Shapiro et al., 2014[link]; Wise et al., 2016[link]) and new imaging and microscopy approaches (microscopes at long beamlines, cryomicroscopy, XANES tomography and resonant XRD). Ptychography, for example, combines point scanning with 2D detection of the coherently scattered X-rays. Using a coherent X-ray beam, a sample is illuminated with overlapping spots, overcoming the phase problem. Thus, extended structures can be imaged with elemental and chemical contrast mechanisms (Shapiro et al., 2014[link]). Another promising nanoscale imaging technique of buried structures with elemental specificity is resonant X-ray diffraction microscopy (Song et al., 2008[link]). Different diffraction patterns are acquired near the absorption edge of a specific element and are directly phased to obtain high-resolution chemical state maps. The ultimate resolution of the microscope is limited only by the X-ray wavelengths and can in principle reach the atomic level. Tomographic methods, as discussed, are also promising for 3D imaging of chemical phase transformation at the nanoscale (Andrews & Weckhuysen, 2013[link]; Simionovici et al., 2008[link]).

Future spatially resolved XAFS investigations can also be anticipated under extreme conditions of pressure (>200 GPa), temperature (2–4000 K) and pulse magnetic fields (30 T) by in situ static or dynamic EDXAM approaches, as well as ultrafast processes in a pump–probe scheme. Further developments will emerge in differential EDXAM for quantitative estimation of femtometre-scale atomic displacements in strain-related phenomena such as magnetostriction (Pettifer et al., 2005[link]) or chemical variations using SXM with atomic detection capabilities and/or internal 3D local information with sub-parts per billion sensitivity under controlled sample environments (pH, operando, magnetic and electric fields).

Efforts are also under way to achieve better spatial resolutions, versatile chemical contrast mechanisms and high-throughput and high-energy spatially resolved XAFS analysis. Forthcoming challenges and highly promising developments comprise combined atomic force microscopy (AFM)/scanning tunnelling microscopy (STM)/SXM methods (for example the nanoXAS instrument; Watts & Ade, 2012[link]; Cummings et al., 2012[link]; Pilet et al., 2012[link]), as well as complementary instrumentation integration within hybrid techniques (for example confocal chemical imaging, scanning electron microscopy (SEM)/TEM/SXM, nano-XAFS-SNOM; Takao et al., 2015[link]; Ade & Stoll, 2009[link]; Larcheri et al., 2008[link]). When coupled with the polarization state of the photons, X-ray magnetic circular dichroism (XMCD)–SXM also characterizes bond orientation and magnetic domain structure.

Future impact on spatially resolved XAFS can be also expected from the even higher photon flux, higher brightness and coherence of the synchrotron radiation at future diffraction-limited storage rings and free-electron lasers (FELs; McNeil & Thompson, 2010[link]; Eriksson et al., 2014[link]). Beyond synchrotron radiation, with pulses of about 200 fs in length, FELs provide a peak brilliance 1010 higher than the best synchrotron light sources and photon pulses of 1012 photons per femtosecond. Thus, a step forward would be 4D local structural monitoring capabilities (chemical imaging with space, time and energy resolutions) at future FEL facilities to access ultrafast phenomena (kinetics, photocatalysis, rapid fluctuations and chemical reactions).

However, to allow such breakthroughs key instrumental and methodological improvements need to be fostered in the following areas: (i) nanofocusing optics, and more generally optical elements, that are able to provide stable and reliable beams on the nanometre length scale, (ii) nanocharacterization tools to monitor nanodisplacements of samples and mechanical control and stability (piezo, feedback etc.), (iii) sample preparation and nanolocalization or nanomarking protocols to precisely record the areas of interest and relate them to observations on a larger length scale, (iv) low-noise, large solid angle and ultrafast detectors and electronics (data readout), (v) further cryo-techniques to reduce radiation damage and (vi) in situ measurements under monitored environmental conditions.


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