To understand the properties of many useful materials, the chemical structures that form within them from elements of low relative atomic mass must be determined. A new X-ray imaging technique does just that.
One of the most striking properties of X-rays is their ability to penetrate matter. In fact, X-rays are partially absorbed as they pass through an object, and cast a 'shadow' of the structures inside — an effect that underpins medical X-ray imaging. As objects become bigger, harder (more energetic) X-rays are needed to penetrate them. The problem is that the harder the X-rays, the smaller the imaging contrast for light elements — those that have low relative atomic masses, such as carbon, nitrogen and oxygen. Reporting in Nature Materials, Huotari et al.1 now describe a way of using hard X-rays to make three-dimensional images of objects that also detects light elements with high sensitivity. The technique even distinguishes between different bonding modes of those elements, and should enable studies of the interiors of diverse objects, such as biological systems, working catalytic reactors and fuel cells.
When a crimson rose is illuminated by sunlight, it appears red to us even though the light contains all the colours of the rainbow. This is because the different colours in the light are absorbed and scattered differently by molecules in the petals, which effectively reflect only the red part of the spectrum. Modern spectroscopic techniques make use of this effect to determine the chemical composition and structure of an object. In photoabsorption spectroscopy, visible or ultraviolet light, or X-rays of a given energy, are shone onto an object, and the amount of light absorbed is measured as the radiation passes through2. When the energy (colour) of the radiation is varied, the transmittance — a measure of the amount of light that passes through a sample — changes in a way that is characteristic of the atoms in the sample and their chemical bonds to neighbouring atoms. In this way, detailed information about the chemical environments in an object can be obtained.
Carbon, nitrogen and oxygen are particularly abundant in both natural and technologically relevant materials. In order to understand the properties of these materials, it is important to determine the diverse chemical structures that form within them. But to investigate these structures using photoabsorption spectroscopy, for example, soft X-ray light must be used. Most materials are rather opaque to this part of the electromagnetic spectrum — even in water, soft X-rays travel the distance of less than a hair's breadth. It is therefore difficult to use this technique to investigate thicker materials such as biological tissues, or the chemistry in the vicinity of a battery electrode. Analogous electron-based spectroscopic methods could in principle also be used, but electrons penetrate even less deeply into materials than do soft X-rays.
Huotari et al.1 have addressed these difficulties by using hard X-rays, which easily penetrate large sample volumes and a variety of specialized sample environments. In particular, they have used hard X-rays in a technique called inelastic X-ray scattering spectroscopy3. In their system, a sample is partly illuminated by a sheet-like, hard-X-ray beam of fixed energy (Fig. 1a). The X-rays are scattered by the atomic structures in the illuminated slice of the sample. The scattered X-ray light is then reflected by a spherically curved crystal, which focuses a two-dimensional image of the slice onto a detector. Three-dimensional images are obtained by stepping a sample through the beam and assembling the two-dimensional images that are acquired at each step.
In addition to focusing images of samples onto the detector, the curved crystal has a second vital function: because its reflectivity is strongly selective, it reflects and images only scattered light of a certain energy onto the detector. For example, by setting up the system so that only light that has the same energy as the incident beam is reflected, an image can be formed using elastically scattered X-rays — those that are scattered without transferring part of their energy to the sample. Such an image depicts the static (time-averaged) structure of the sample. Similarly, the energy reflected by the crystal can be set to a given energy transfer (the difference in energy of incident and scattered X-rays). This allows imaging of inelastically scattered X-rays — those that transfer part of their energy to the sample, for example by exciting vibrations, valence electrons or collective motions of electrons.
Crucially for Huotari and colleagues, hard X-rays can also transfer energy to atoms of light elements in excitations similar to those that occur in photoabsorption spectroscopy. The resulting inelastic-scattering spectrum thus reveals the local chemical structure of those atoms in a material. So, by tuning the crystal in their experimental set-up to reflect scattered X-rays at transfer energies corresponding to such excitations, the authors could obtain images that reveal details of the chemical bonds of a given light element in samples.
To illustrate the strength of this approach, Huotari et al. analysed a test sample consisting of diamond enclosed by graphite — two allotropes of carbon in which the chemical bonding between atoms is different. Using their technique, the authors were able to map out clearly the positions of the different allotropes in the sample, effectively locating the diamond and graphite components on the basis of their bond structures (Fig. 1b). No other technique could do this, although the diamond could perhaps have been detected using classical absorption imaging because diamond is denser than graphite. But the classical approach would provide no information about the chemical structures of the allotropes.
Because inelastically scattered X-ray signals are typically weak, the authors' method requires the brightest available X-ray sources. This currently restricts the use of the technique to researchers with access to modern synchrotron radiation sources. Another limitation is the spatial resolution of the technique — the authors studied millimetre-sized samples, but it would be desirable to use the method to study microscopic samples.
Nevertheless, the ability to characterize the chemical bonding of light elements inside thick samples, and of samples in specialized environments (such as in chemical reactors or pressurized cells) opens up many applications. For example, one could use the authors' method to visualize local bond structure in composite materials, or to identify different phases in polymer blends. It could also be used to study the reactions of carbon and oxygen in industrially useful catalytic reactions, such as the catalytic partial oxidation of methane4 — one of the steps involved in the conversion of natural gas into liquid fuels.
Huotari, S., Pylkkänen, T., Verbeni, R., Monaco, G. & Hämäläinen, K. Nature Mater. 10, 489–493 (2011).
Koningsberger, D. C. & Prins, R. (eds) X-Ray Absorption (Wiley, 1988).
Schülke, W. Electron Dynamics by Inelastic X-Ray Scattering (Oxford Univ. Press, 2007).
Grunwaldt, J.-D. & Schroer, C. G. Chem. Soc. Rev. 39, 4741–4753 (2010).
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Medical Physics (2018)