Editorial pubs.acs.org/cm
Application of Modern X‑ray Spectroscopy in ChemistryBeyond Studying the Oxidation State
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in sulfur-containing catalysts. Moreover, a study by Kowalska et al. has combined S and Fe K-edge XAS and Fe K-Beta XES to obtain greater insight into the changes, which occur in a series of diferrous, mixed-valent and diferric iron−sulfur dimers.7 This study shows the dangers of utilizing conventional oxidation state “fingerprinting” approaches when using XAS or XES to assign metal oxidation states in highly covalent complexes. However, with quantitative correlation to calculations, the authors demonstrate that the Fe K-edge XAS can serve as a marker for the extent of localization or delocalization in iron dimers. Finally, Haldrup et al. utilize a combination of timeresolved XES and X-ray scattering to obtain insights into the changes in solvation dynamics which occur upon photoexcitation of [Fe(2,2′- Bipyridine)3]2+ in aqueous solution.8 This approach has great promise for monitoring the solvation dynamics in chemical reactions on a pico- to femtosecond time scale. Nowadays, RIXS became an important method for studying the structure of complex nanomaterials. Notable recent examples include hard X-ray 1s2p and 1s3p RIXS studies at cobalt, which provide L-edge- and M-edge-like information, respectively; as well as soft X-ray 2p3d and 3p3d RIXS studies, which provide d-to-d excitations. In general, XAS is a bulk technique, and as such it “sees” the average oxidation state of the probed photoabsorber. Hadt et al. demonstrate that 1s3p RIXS can overcome this limitation by providing a means to selectively isolate the Co(IV) signal from a strong Co(III) background. In this particular example, it also allows surface selectivity to be achieved with the use of a bulk probe.9 Furthermore, Cui et al. studied the size dependent change of the electronic structure of for cobalt nanoparticles using Co Mand L-edge XAS, combined with 3p3d and 2p3d RIXS.10 The RIXS measurements revealed the significant dependence of d− d excitations on particle size and demonstrated that the crystalfield splitting energy changes significantly when the particle size is reduced from 10 to 4 nm. The Co 2p3d RIXS simulations with atomic multiplet code further confirmed that the covalency of Co−O bond decreases with an increase of Co nanoparticle size. Moreover, Co K-edge XAS and 1s3p RIXS studies on cobalt oxides and sulfides further underline the role of ligand (O, S) mediated interactions in modulating the electronic properties of cobalt compounds. al Samarai et al. showed that the cobalt ions in CoS2, which are generally assumed to have a Co2+S22− oxidation state distribution, exhibit a much stronger sulfurmediated cobalt−cobalt overlap as compared with high spin CoS and are more similar to the case of the low-spin trivalent oxides (Co3O4).11 In general, 1s3p RIXS is a valuable method for determining the valence and covalence and estimating the spin state of cobalt oxides and sulfides, with the exception that for sulfides the large covalency does not allow for a distinction
n recent years, the sophistication of both soft and hard X-ray spectroscopy beamlines have evolved greatly, allowing for a wide variety of experiments that combine both X-ray absorption (XAS) and X-ray emission (XES) in order to obtain more detailed electronic structural information from a given photoabsorber. By combining the experimental results with theory, more quantitative insights are obtained, and X-ray spectroscopy has thus advanced beyond the mere fingerprinting of oxidation states that was once commonplace. This has resulted in both new experimental approaches as well as new theoretical interpretation tools that allow advanced X-ray spectroscopic approaches to be utilized for ever more complex questions, which are at the forefront of materials chemistry and catalysis research.1−3 Of particular importance have been developments for in situ and operand studies of functional systems, with time-resolved studies providing further insights into fundamental processes of bond making and breaking. In this Virtual Issue, we highlight 3 reviews and 18 original contributions published in the last 18 months in eight ACS journals demonstrating the diversity of applications of X-ray spectroscopy. The first set of publications describes experimental and theoretical efforts to establish X-ray absorption (XAS) and Xray emission spectroscopy (XES) as versatile spectroscopic tools in chemistry. Emphasis is placed on examples from (bio)inorganic and nanomaterials chemistry, which provide the nonspecialist with a sense of the added value of rigorously developing techniques and perspectives on future directions. They are followed by a collection of publications, featuring operando X-ray spectroscopic studies on materials and devices for energy applications. In (bio)inorganic chemistry, of particular note are recent studies by Yan et al., which have shown that the Fe L-edge XAS can be utilized to obtain information about the extent of delocalization within iron nitrosyl complexes.4 While iron nitrosyls play an essential role in biology, the electronic structure of the fragment is often difficult to understand due to the intrinsic noninnocence of NO. In this study, the authors show that through a detailed analysis of the Fe L-edge the electronic structure of highly delocalized species can be experimentally defined. In another soft X-ray study, Van Kuiken et al. show that by combing V L-edges with X-ray emission, in a 2p3d resonant inelastic X-ray scattering (RIXS) measurement, the d-to-d bands in molecular vanadium complexes can be mapped out.5 Further, these studies show that due to spin orbit coupling in the intermediate state, formally spin forbidden transitions can also be observed. These experiments thus pave the way for using 2p3d RIXS for detailed electronic structure mapping. In the “tender” X-ray region, Ochmann et al. have shown that time-resolved S K-edge XAS can be used to follow radical formation and isomerization in thiophenols.6 This work thus provides important precedence for the use of time-resolved S K-edge XAS as a means to follow the evolution of intermediates © 2017 American Chemical Society
Published: September 12, 2017 7051
DOI: 10.1021/acs.chemmater.7b03455 Chem. Mater. 2017, 29, 7051−7053
Chemistry of Materials
Editorial
For fuel and photoelectrochemical studies it is important to realistically mimic the mass-transfer and photon-absorption conditions, respectively. Siebel et al. unequivocally showed that, in contrast to previous studies in a conventional liquid electrolyte-based electrochemical cell, the palladium hydride phase is maintained during the hydrogen oxidation reaction in a fuel cell anode environment.17 Moonshiram et al. used timeresolved XAS at the Ni K-edge to follow the changes, which occur in a photocatalytic Ru/Ni system.18 Their results are able to provide insight into the rate-limiting step for photocatalytic H2 production. Another major challenge in following electrocatalytic processes is the ability to probe the changes that are occurring at the solid−liquid interface. Herein, Favaro et al. elegantly utilized a combination of ambient pressure XPS and hard X-ray XAS in order to achieve both surface and bulk sensitive probes, respectively.19 Particularly relevant to the field of catalysis is that, unlike conventional EXAFS, XES gives the possibility to distinguish between C, N, and O ligands. For example, Lomachenko et al. demonstrated the advantages of combining XAS and XES methods in tackling the origin of an outstanding performance of Cu-CHA zeolites in the NH3-assisted selective catalytic reduction of nitrogen oxides.20 In this study, they successfully differentiate between Z-[Cu(II)OH−], Z-[Cu(II)NO3−], and Cu(I) (NH3)2 complexes and correlate them with the catalyst activity of Cu-CHA in the temperature range from 150 to 400 °C. It is apparent that the understanding of how chemical bonds are made and broken is at the heart of chemical research. Therefore, the last example features a precise activation of chemical bonds with an X-ray laser. Beye et al. use timeresolved oxygen XAS and XES to follow the breaking of a Ru− O bond.21 By utilizing the complementarity of XAS and XES, they are able to probe the energetic changes, which occur in both the bonding and the antibonding molecular orbitals, thus directly assessing the strength of the bonding interactions and its evolution upon photoinduced cleavage. This virtual issue presents a window into the rapid evolution of modern X-ray spectroscopy, which provides new experimental approaches, that are changing the way chemists and material scientists can approach scientific questions. The results highlighted here have expanded the scientific communities understanding of the fundamental nature of chemical bonding and how this applies to questions at the forefront of materials and catalysis research. The curiosity of a broader chemical community has been fueled, and even though the data analysis and interpretation are seldom straightforward, the number of operando studies is steadily increasing. On the horizon there are more challenging experiments to come, with promise to bring understanding to many of chemistry’s grand challenges.
between high-spin and low-spin. While the three aforementioned Co studies were on solid reference model complexes, the presented approach has great promise for in situ studies of operating catalysts. A contribution by Liu et al. is an example of an ingenious application of multielement EXAFS fit analysis to follow the emergence of hybrid nanostructures.12 The proximity of the As K-edge (11 867 eV) and Au L3-edge (11 919 eV) prohibits direct tracking of the diffusion of Au in InAs nanocyrstals. Hence, the authors instead used the In−O bond as a marker. Remarkably, they observe a reversed Kirkendall effect, and Au initially present as clusters at the InAs surface diffuses into it, substituting for In host atoms. The diffusion rate of the inward diffusing Au species is faster than that of the outward diffusing species (InAs) leading to the formation of a crystalline Au core surrounded by an amorphous, oxidized InAs shell containing nanoscale voids. Further, we note that the advantages of using XAS and RIXS are not limited to d-electron systems. Recently Hirsch et al. showed that the careful analysis of HERFD-XANES spectra and RIXS maps allows for the unambiguous distinction between the temperature-induced changes of the Pr valence state from the changes related to delocalization of the f-electrons.13 Since modern highly brilliant synchrotron sources facilitate in situ studies of catalytic reactions or electrochemical devices in modus operandi, XAS has taken a firm place in the toolkit of chemists and materials scientists. This brings to light technologically relevant phenomena, which we previously could not be accounted for. For the state-of-the-art Li-ion batteries, mechanistic X-ray spectroscopic studies hitherto revolved around the question of the reversibility of redox reactions on the transition metal ions during lithiation and delithiation.3 However, recent studies have made it apparent that the “oxidation state” formalism works only in the first approximation and does not fully reflect the complexity of the electronic structure of compounds during (electro-) chemical reactions. For example, Benerjee et al. addressed the longstanding and controversial question of the solubility of Mn ions in LiPF6 electrolyte solution by combining the Mn K-edge XANES, ICP, and EPR studies.14 In contrast to the commonly accepted dogma, they observed that Mn3+ and not Mn2+ is the main soluble species in the electrolyte solution. In another study, Kim et al. applied continuous Cauchy wavelet transform to analyze the in situ EXAFS data measured at Ni, Mn, and Co K-edges during the first cycle.15 Even though XANES shows that the oxidation state of Ni, Mn, and Co are reversible, decomposition of the EXAFS signal into k- and Rspace point out differences between the local structure changes around transition ions. The local environment around the Mn hardly changes, while reversible and irreversible changes in the Co−O and Ni−O distances, respectively, are observed. Finally, Luo et al. gained deeper insight into the redox chemistry of cobalt free LiMnxNiyO2 cathodes operated at and above 4.5 V by combining the XANES measurement of the redox reaction associated with the transition metal cation at the K-edge with the measurements of oxygen redox reactions at the O K-edge.16 The soft XANES and RIXS measurements provide key information about the oxidation of O22− anions and O loss from the lattice, which was not previously accessible. Here, the localized electron−hole states at oxygen are preferentially coordinating to less covalent Mn4+ than to strongly covalent Ni4+.
Dorota Koziej, Guest Editor University of Hamburg
Serena DeBeer, Guest Editor
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Max Planck Institute for Chemical Energy Conversion
AUTHOR INFORMATION
Notes
Views expressed in this editorial are those of the authors and not necessarily the views of the ACS. 7052
DOI: 10.1021/acs.chemmater.7b03455 Chem. Mater. 2017, 29, 7051−7053
Chemistry of Materials
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Editorial
(16) Luo, K.; Roberts, M. R.; Guerrini, N.; Tapia-Ruiz, N.; Hao, R.; Massel, F.; Pickup, D. M.; Ramos, S.; Liu, Y. S.; Guo, J. H.; Chadwick, A. V.; Duda, L. C.; Bruce, P. G. Anion Redox Chemistry in the Cobalt Free 3d Transition Metal Oxide Intercalation Electrode Li[Li0.2Ni0.2Mn0.6]O-2. J. Am. Chem. Soc. 2016, 138 (35), 11211− 11218. (17) Siebel, A.; Durst, J.; Proux, O.; Hasché, F.; Tromp, M.; Gasteiger, H. A.; Gorlin, Y. Identification of Catalyst Structure during the Hydrogen Oxidation Reaction in an Operating PEM Fuel Cell. ACS Catal. 2016, 6 (11), 7326−7334. (18) Moonshiram, D.; Guda, A.; Kohler, L.; Picon, A.; Guda, S.; Lehmann, C. S.; Zhang, X.; Southworth, S. H.; Mulfort, K. L. Mechanistic Evaluation of a Nickel Proton Reduction Catalyst Using Time-Resolved X-ray Absorption Spectroscopy. J. Phys. Chem. C 2016, 120 (36), 20049−20057. (19) Favaro, M.; Drisdell, W. S.; Marcus, M. A.; Gregoire, J. M.; Crumlin, E. J.; Haber, J. A.; Yano, J. An Operando Investigation of (Ni−Fe−Co−Ce)Ox System as Highly Efficient Electrocatalyst for Oxygen Evolution Reaction. ACS Catal. 2017, 7 (2), 1248−1258. (20) Lomachenko, K. A.; Borfecchia, E.; Negri, C.; Berlier, G.; Lamberti, C.; Beato, P.; Falsig, H.; Bordiga, S. The Cu-CHA deNO(x) Catalyst in Action: Temperature-Dependent NH3-Assisted Selective Catalytic Reduction Monitored by Operando XAS and XES. J. Am. Chem. Soc. 2016, 138 (37), 12025−12028. (21) Beye, M.; Oberg, H.; Xin, H.; Dakovski, G. L.; Dell’Angela, M.; Fohlisch, A.; Gladh, J.; Hantschmann, M.; Hieke, F.; Kaya, S.; Kuhn, D.; LaRue, J.; Mercurio, G.; Minitti, M. P.; Mitra, A.; Moeller, S. P.; Ng, M. L.; Nilsson, A.; Nordlund, D.; Norskov, J.; Ostrom, H.; Ogasawara, H.; Persson, M.; Schlotter, W. F.; Sellberg, J. A.; Wolf, M.; Abild-Pedersen, F.; Pettersson, L. G.; Wurth, W. Chemical Bond Activation Observed with an X-ray Laser. J. Phys. Chem. Lett. 2016, 7 (18), 3647−51.
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(1) MacMillan, S. N.; Lancaster, K. M. X-ray Spectroscopic Interrogation of Transition-Metal-Mediated Homogeneous Catalysis: Primer and Case Studies. ACS Catal. 2017, 7 (3), 1776−1791. (2) Koziej, D. Revealing Complexity of Nanoparticle Synthesis in Solution by in Situ Hard X-ray Spectroscopy-Today and Beyond. Chem. Mater. 2016, 28 (8), 2478−2490. (3) Talaie, E.; Bonnick, P.; Sun, X. Q.; Pang, Q.; Liang, X.; Nazar, L. F. Methods and Protocols for Electrochemical Energy Storage Materials Research. Chem. Mater. 2017, 29 (1), 90−105. (4) Yan, J. J.; Gonzales, M. A.; Mascharak, P. K.; Hedman, B.; Hodgson, K. O.; Solomon, E. I. L-Edge X-ray Absorption Spectroscopic Investigation of {FeNO}6: Delocalization vs Antiferromagnetic Coupling. J. Am. Chem. Soc. 2017, 139 (3), 1215−1225. (5) Van Kuiken, B. E.; Hahn, A. W.; Maganas, D.; DeBeer, S. Measuring Spin-Allowed and Spin-Forbidden d−d Excitations in Vanadium Complexes with 2p3d Resonant Inelastic X-ray Scattering. Inorg. Chem. 2016, 55 (21), 11497−11501. (6) Ochmann, M.; von Ahnen, I.; Cordones, A. A.; Hussain, A.; Lee, J. H.; Hong, K.; Adamczyk, K.; Vendrell, O.; Kim, T. K.; Schoenlein, R. W.; Huse, N. Light-Induced Radical Formation and Isomerization of an Aromatic Thiol in Solution Followed by Time-Resolved X-ray Absorption Spectroscopy at the Sulfur K-Edge. J. Am. Chem. Soc. 2017, 139 (13), 4797−4804. (7) Kowalska, J. K.; Hahn, A. W.; Albers, A.; Schiewer, C. E.; Bjornsson, R.; Lima, F. A.; Meyer, F.; DeBeer, S. X-ray Absorption and Emission Spectroscopic Studies of [L2Fe2S2](n) Model Complexes: Implications for the Experimental Evaluation of Redox States in IronSulfur Clusters. Inorg. Chem. 2016, 55 (9), 4485−4497. (8) Haldrup, K.; Gawelda, W.; Abela, R.; Alonso-Mori, R.; Bergmann, U.; Bordage, A.; Cammarata, M.; Canton, S. E.; Dohn, A. O.; van Driel, T. B.; Fritz, D. M.; Galler, A.; Glatzel, P.; Harlang, T.; Kjaer, K. S.; Lemke, H. T.; M?ller, K. B.; Nemeth, Z.; Papai, M.; Sas, N.; Uhlig, J.; Zhu, D.; Vanko, G.; Sundstrom, V.; Nielsen, M. M.; Bressler, C. Observing Solvation Dynamics with Simultaneous Femtosecond X-ray Emission Spectroscopy and X-ray Scattering. J. Phys. Chem. B 2016, 120 (6), 1158−1168. (9) Hadt, R. G.; Hayes, D.; Brodsky, C. N.; Ullman, A. M.; Casa, D. M.; Upton, M. H.; Nocera, D. G.; Chen, L. X. X-ray Spectroscopic Characterization of Co(IV) and Metal-Metal Interactions in Co4O4: Electronic Structure Contributions to the Formation of High-Valent States Relevant to the Oxygen Evolution Reaction. J. Am. Chem. Soc. 2016, 138 (34), 11017−11030. (10) Cui, Z. Z.; Xie, C. L.; Feng, X. F.; Becknell, N.; Yang, P. D.; Lu, Y. L.; Zhai, X. F.; Liu, X. S.; Yang, W. L.; Chuang, Y. D.; Guo, J. H. Revealing the Size-Dependent d-d Excitations of Cobalt Nanoparticles Using Soft X-ray Spectroscopy. J. Phys. Chem. Lett. 2017, 8 (2), 319− 325. (11) Al Samarai, M.; Delgado-Jaime, M. U.; Ishii, H.; Hiraoka, N.; Tsuei, K.-D.; Rueff, J.-P.; Lassale-Kaiser, B.; Weckhuysen, B. M.; de Groot, F. M. F. 11. 1s3p Resonant Inelastic X-ray Scattering of Cobalt Oxides and Sulfides. J. Phys. Chem. C 2016, 120 (42), 24063−24069. (12) Liu, J.; Amit, Y.; Li, Y. Y.; Plonka, A. M.; Ghose, S.; Zhang, L. H.; Stach, E. A.; Banin, U.; Frenkel, A. I. Reversed Nanoscale Kirkendall Effect in Au-InAs Hybrid Nanoparticles. Chem. Mater. 2016, 28 (21), 8032−8043. (13) Hirsch, O.; Kvashnina, K.; Willa, C.; Koziej, D. Hard X-ray Photon-in Photon-out Spectroscopy as a Probe of the TemperatureInduced Delocalization of Electrons in Nanoscale Semiconductors. Chem. Mater. 2017, 29 (4), 1461−1466. (14) Banerjee, A.; Shilina, Y.; Ziv, B.; Ziegelbauer, J. M.; Luski, S.; Aurbach, D.; Halalay, I. C. On the Oxidation State of Manganese Ions in Li-Ion Battery Electrolyte Solutions. J. Am. Chem. Soc. 2017, 139 (5), 1738−1741. (15) Kim, T.; Song, B. H.; Lunt, A. J. G.; Cibin, G.; Dent, A. J.; Lu, L.; Korsunsky, A. M. Operando X-ray Absorption Spectroscopy Study of Atomic Phase Reversibility with Wavelet Transform in the LithiumRich Manganese Based Oxide Cathode. Chem. Mater. 2016, 28 (12), 4191−4203. 7053
DOI: 10.1021/acs.chemmater.7b03455 Chem. Mater. 2017, 29, 7051−7053
