Kirsten M. Ø.
Jensen
*a,
Serena
DeBeer
*b and
Dorota
Koziej
*c
aDepartment of Chemistry and Nanoscience Center, University of Copenhagen, DK-2100 Copenhagen, Denmark. E-mail: kirsten@chem.ku.dk
bMax Planck Institute for Chemical Energy Conversion, D-45470 Mülheim an der Ruhr, Germany. E-mail: serena.debeer@cec.mpg.de
cCenter for Hybrid Nanostructures (CHyN), Institute of Nanostructure and Solid State Physics, University of Hamburg, 22761 Hamburg, Germany. E-mail: dkoziej@physnet.uni-hamburg.de
In chemistry, the development of X-ray and neutron scattering and X-ray absorption and emission (XAS and XES) spectroscopy techniques speaks to a need for a deeper understanding of chemical reactions. If we are to develop better batteries, catalysts or other new functional materials, we must map the reaction mechanisms that dictate how a material forms during chemical synthesis, or how it functions in advanced applications. With advanced spectroscopy and scattering methods, we are now able to probe the atomic arrangement and electronic structure in species ranging from molecules and ionic complexes in solution to solid materials under pressure or fields, giving us completely new opportunities for studies of these mechanisms. Moreover, modern large-scale facilities allow for efficient use of the whole X-ray energy range from high energies for X-ray total scattering studies to soft edges for X-ray spectroscopy studies.
In this online themed collection, several original contributions and reviews focus on new studies taking advantage of in situ synchrotron and neutron techniques to elucidate mechanisms for material formation and function, including catalysts, batteries, gas storage materials, magnetic nanomaterials and thin films, to name a few. The wide variety of methods and approaches to data analysis applied illustrates the many options synchrotron and neutron methods now provide to chemists.
Pair Distribution Function (PDF) analysis has over the last decades grown from a specialized technique only used by a few experts to become an important tool in many chemists’ toolboxes, especially for working with nanostructured and disordered materials.1 Total scattering methods with PDF analysis can provide structural information for materials with or without long range order, making it an excellent technique for studies of e.g. material formation, or structural changes during operation.2 For example, Henriksen et al. (DOI: 10.1039/D0NR01922D) used X-ray diffraction and X-ray total scattering with Pair Distribution Function analysis to study iron(III) hydroxide phosphate hydrate as a Na-battery material. Recent studies of phosphate based materials have illustrated that order–disorder transitions play a large role in Na insertion, and here, operando synchrotron experiments show that upon Na insertion, the material turns amorphous, following a simple solid solution reaction. Using PDF, the authors show that the amorphous material has a local structure similar to that of crystalline sodium iron hydroxide phosphate hydrate, but with a much smaller coherence length. The study is an excellent example of the complementarity of diffraction and total scattering techniques, and shows how knowledge of materials with only short range order is essential in the future development of materials.
In another study, Rekhtina et al. (DOI: 10.1039/D0NR01760D) used Pair Distribution Function analysis to elucidate pathways for thermal decomposition of MgO based materials. These materials are attractive CO2 sorbents, and the carbonation process can be accelerated by the presence of molten alkali metal nitrates, although the role of the molten nitrate salt is not clear. Here, PDF is used to investigate their effect, and demonstrate that the presence of molten NaNO3 influences the thermal decomposition pathway of hydrated magnesium hydroxycarbonate. The in situ PDF data were analysed using an automated multivariate curve resolution alternating least squares (MCR-ALS) method, which allows quantitative analysis of the development of the components present in the reaction. This kind of automated analysis, along with other approaches using e.g. machine learning or data-base mining,3 is likely to play a large role in PDF analysis and prediction of XAS spectra in the future.4
Synchrotron spectroscopy has played an important role in allowing the detection of very small nanoparticles in solution that are beyond the limits of conventional TEM experiments. This is highlighted in a contribution from Kuciakowski et al. (DOI: 10.1039/D0NR02866E), where a combination of X-ray magnetic circular dichroism (XMCD) and Resonant Inelastic X-ray Scattering (RIXS) is utilized to study superparamagnetic iron oxide nanoparticles in solution. By following the field dependence of the RIXS-XMCD amplitudes, the authors were able to determine the paramagnetic response of the particles and determine their size distribution profiles.
Often it is not possible to gain a comprehensive picture of a system by employing only one method, i.e. scattering or spectroscopy. Instead, spectroscopists and scatterers are joining forces, which is also reflected in this online collection. A contribution by Ramamoorthy et al. (DOI: 10.1039/D0NR03486J) is an excellent example of the application of multiple techniques to follow the role of pre-nucleation clusters (PNCs) in the non-classical growth of Au nanostructures. Remarkably, they initially observed formation of non-reduced PNCs containing both Au(III) and Au(I) species. They are almost stable in size during the induction stage, as shown by Small Angle X-ray Scattering (SAXS), and undergo a very fast shrinkage during the nucleation stage. The PDF analysis of the early stages of the reaction confirms the presence of Au–Au aurophilic bonds observed in Au(I) and Au(III) dimers and additionally Au–Au pair distances in metallic nanoparticles. Finally, the XAS analysis enables discrimination between the highly reactive Au species involved in the nucleation and growth stages, with poorly reactive species acting as a reservoir for the reactive species.
It is apparent from the studies included in this themed collection that despite having all these methods to hand, we still need to be able to probe reactions at the relevant time scales. For the more challenging experiments to come, the development of multipurpose beamlines is needed.
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