Spectroscopy and scattering for chemistry: new possibilities and challenges with large scale facilities

Kirsten M. Ø. Jensen

Kirsten M. Ø. Jensen is an associate professor in the Department of Chemistry at University of Copenhagen. She received her Ph.D. in Chemistry from Aarhus University in 2013, and following a postdoc position at Columbia University, she started her research group in Copenhagen in 2015. The research in her group concerns nanomaterials, focusing especially on the use of X-ray and neutron scattering to elucidate the relationship between the structure and formation mechanisms of nanoparticles. Pair distribution function analysis is central to her research, and the Jensen group is active in the development of new PDF modelling methods and experiments.

Serena DeBeer

Serena DeBeer is a Professor and Director at the Max Planck Institute for Chemical Energy Conversion in Mülheim an der Ruhr, Germany. She is also an Adjunct Professor in the Department of Chemistry and Chemical Biology at Cornell University, an honorary faculty member at Ruhr University in Bochum, and the group leader of the PINK Beamline at the Energy Materials In-Situ Laboratory at Helmholtz Zentrum in Berlin. She received her B.S. in Chemistry at Southwestern University in 1995 and her Ph.D. from Stanford University in 2002. From 2002 to 2009, she was a staff scientist at the Stanford Synchrotron Radiation Laboratory, before moving to her faculty position at Cornell. She is the recipient of a European Research Council Synergy Award (2019), the American Chemical Society Inorganic Chemistry Lectureship Award (2016), the Society of Biological Inorganic Chemistry Early Career Award (2015), a European Research Council Consolidator Award (2013), a Kavli Fellowship (2012), and an Alfred P. Sloan Research Fellowship (2011). Research in the DeBeer group is focused on the development and application of advanced X-ray spectroscopic tools for understanding key mechanisms in biological, homogeneous and heterogeneous catalysis.

Dorota Koziej

Dorota Koziej has been a professor of experimental physics at University of Hamburg since August 2017. She studied applied physics at Silesian University of Technology, Poland. In December 2006 Dorota received a joint doctoral degree from University of Tübingen, Germany and Silesian University of Technology, Poland. After a postdoctoral stay in the group of Prof. M. Niederberger at ETH Zurich, Switzerland, she received a Swiss National Science Foundation Fellowship for advanced researchers and moved to the group of Prof. D. Weitz at Harvard University, USA in 2010. In 2011–15 she was a team leader in the Laboratory for Multifunctional Materials, ETH Zürich. She was awarded the EurJIC Wöhler GDCh Young Investigator Prize in 2015 and an ERC Consolidator Grant in 2018. Dorota’s research focus is on studying the genesis of nanomaterials in solution with complementary X-ray spectroscopic, scattering and microscopic methods. Moreover, she aims at translating the unique properties emerging at the nanoscale to complex, functional devices.

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Over the last decades, the use of synchrotron and neutron radiation has transformed many fields in science: from being tools for niche studies in condensed matter physics to now playing a large role in geology, cultural heritage, the biosciences and chemistry. Dedicated beamlines and instruments with flexible sample environments have made spectroscopy and scattering methods accessible for scientists from a range of fields, and the continued advancement of large-scale facilities will without a doubt play an even larger role in materials development in the future.

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.

Acknowledgements

SD acknowledges the Max Planck Society for funding. DK acknowledges University of Hamburg and funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme Grant agreement No. 818941 — LINCHPIN — ERC-2018-COG.

References

1. S. J. L. Billinge and I. Levin, Science, 2007, 316, 561–565.
2. T. L. Christiansen, S. R. Cooper and K. M. Ø. Jensen, Nanoscale Adv., 2020, 2, 2234–2254.
3. S. Banerjee, C.-H. Liu, K. M. Ø. Jensen, P. Juhás, J. D. Lee, M. Tofanelli, C. J. Ackerson, C. B. Murray and S. J. L. Billinge, Acta Crystallogr., Sect. A: Found. Adv., 2020, 76, 24–31.
4. C. D Rankine, M. M. M. Madkhali and T. J Penfold, A Deep Neural Network for the Rapid Prediction of X-ray Absorption Spectra, J. Phys. Chem. A, 2020, 124(21), 4263–4270.

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