Multi-Scale X-ray Scattering to Address Fundamental Research Questions
Imagine you are kayaking in the middle of your favorite lake on a fine summer day and you decide to touch the water with your hand. You see water ripples on the surface of the water moving in all directions. We can observe the same phenomenon by just dipping a plastic ball in the water. It becomes interesting when we use two plastic balls, because we start to see the interaction of the ripples from both sources. A similar phenomenon occurs when light waves emitted by a point source (analogous to the plastic ball) pass through slits. This interference phenomenon could be called constructive or destructive depending upon how the waves (or water ripples) interact. When constructively interfering, the waves form a pattern, which can be observed on a screen placed in the path of the interacting light waves.
The same principle governs the powerful X-ray scattering technique, where the X-rays, similar to light waves, emitted by a point source interact with the materials of interest such as metals, minerals, or soft materials. Upon interacting, these X-rays are scattered in multiple directions and the constructively interfering scattered waves produce patterns. These patterns can help provide details about the structural arrangement of atoms in different materials, and the size and shape of particles. The type of information obtained from scattering X-rays off a material depends upon the angle at which the waves are collected by the detector. Typically, X-rays scattered at angles less than 10° provide information about the size, shape, and arrangement of particles in the system of interest, while those at angles larger than 10° are enriched with information related to the internal structure of crystalline materials. Based on the scattering angles, the former is termed small-angle X-ray scattering (SAXS) and the latter is wide-angle X-ray scattering (WAXS) or X-ray diffraction, alternatively. Both SAXS and WAXS are sometimes used in conjunction to obtain both types of information by probing a wide range of length scales from angstroms (1 Å = 10-10 m) to a few microns (1 µm = 10-6 m).
Because of the robustness of these techniques and the variety of information that can be obtained by using them, researchers at different Energy Frontier Research Centers (EFRCs) are using these powerful instrumental capabilities to answer fundamental research questions in the fields of material science, chemistry, environmental engineering, and energy. Another aspect that makes these techniques unique is the ability to study the materials of interest dynamically by understanding the properties and performance of those materials upon external perturbations. This understanding is made possible by the advancement of synchrotron light sources, which enable experiments to be performed under different in situ and in operando conditions and data to be collected within a few minutes. This means that the measurements are taken while the material is subjected to the real conditions. These real-time insights obtained from such dynamic measurements are helping scientists across EFRCs to not only answer the research questions but also develop a synergy between experimental and theoretical methods. Examples of such efforts are reflected by some of the recently published work by the Multi-Scale Fluid-Solid Interactions in Architected and Natural Materials Center (MUSE), where researchers are looking at structural and morphological changes in different materials of interest in response to various chemical and thermal perturbations.
The SAXS technique was employed by investigators at MUSE to understand the evolution of pore-networks in silica nanoparticles during thermal treatment at temperatures as high as 1050°C—temperature comparable to the melting point of gold (106°C). In a similar effort, SAXS was used in conjunction with infrared spectroscopy to understand the development of pore channels in mesoporous silica (pore size = 2–50 nm [1 nm = 10-9 m]) and how the variation in solution chemistry affects the size and shape of pores and particles. Because one of the aims of MUSE is to understand the transport and interfacial properties of fluids under confinement, the insights obtained from these research efforts could facilitate the rational design of materials with a better control over confined geometry.
Another interesting aspect of the dynamic probing of material systems is using the insights obtained to develop models to support computational efforts. In recent work, a WAXS-ReaxFF (reactive forcefield molecular dynamics) unified map was presented, which summarizes different structural transformation regions during the heat treatment of a naturally occurring clay. The findings from experimental WAXS measurements were used to develop and validate the computational model. This synergy between the two approaches could open new avenues for developing models that provide reliable predictability of material properties and performance under extreme environments and pave a path for advancement in basic sciences in different EFRCs.
H. Asgar, V. Semeykina, M. Hunt, S. Mohammed, I. Kuzmenko, I. Zharov, G. Gadikota, Thermally-Induced Morphological Evolution of Spherical Silica Nanoparticles Using in-operando X-ray Scattering Measurements, Colloids and Surfaces A: Physicochemical and Engineering Aspects, 586, 2020, 124260.
H. Asgar, S. Seifert, I. Kuzmenko, M. H. Bartl, G. Gadikota, Mechanistic Insights into the Colloidal Assembly of Mesoporous Silica Using In-Operando Cross-Scale X-Ray Scattering and Spectroscopic Measurements, Materialia, 12, 2020, 100764.
M. G. Muraleedharan, H. Asgar, S. Mohammed, G. Gadikota, A. C. T. van Duin, Elucidating Thermally Induced Structural and Chemical Transformations in Kaolinite Using Reactive Molecular Dynamics Simulations and X-ray Scattering Measurements, Chemistry of Materials, 32 (2), 2020, 651-662.
The above mentioned works were supported as part of the EFRC-MUSE, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award #DE-SC0019285.