Publications by authors named "Frank M F de Groot"

92 Publications

Femtosecond Charge Density Modulations in Photoexcited CuWO.

J Phys Chem C Nanomater Interfaces 2021 Apr 26;125(13):7329-7336. Epub 2021 Mar 26.

Inorganic Chemistry and Catalysis, Debye Institute for Nanomaterials Science, Utrecht University, Universiteitslaan 99, 3584 CG Utrecht, The Netherlands.

Copper tungstate (CuWO) is an important semiconductor with a sophisticated and debatable electronic structure that has a direct impact on its chemistry. Using the PAL-XFEL source, we study the electronic dynamics of photoexcited CuWO. The Cu L X-ray absorption spectrum shifts to lower energy upon photoexcitation, which implies that the photoexcitation process from the oxygen valence band to the tungsten conduction band effectively increases the charge density on the Cu atoms. The decay time of this spectral change is 400 fs indicating that the increased charge density exists only for a very short time and relaxes electronically. The initial increased charge density gives rise to a structural change on a time scale longer than 200 ps.
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http://dx.doi.org/10.1021/acs.jpcc.0c10525DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8040018PMC
April 2021

Rational strain engineering of single-atom ruthenium on nanoporous MoS for highly efficient hydrogen evolution.

Nat Commun 2021 Mar 16;12(1):1687. Epub 2021 Mar 16.

College of Materials Science and Engineering, State Key Laboratory of Advanced Design and Manufacturing for Vehicle Body, Hunan University, Changsha, Hunan 410082, China.

Maximizing the catalytic activity of single-atom catalysts is vital for the application of single-atom catalysts in industrial water-alkali electrolyzers, yet the modulation of the catalytic properties of single-atom catalysts remains challenging. Here, we construct strain-tunable sulphur vacancies around single-atom Ru sites for accelerating the alkaline hydrogen evolution reaction of single-atom Ru sites based on a nanoporous MoS-based Ru single-atom catalyst. By altering the strain of this system, the synergistic effect between sulphur vacancies and Ru sites is amplified, thus changing the catalytic behavior of active sites, namely, the increased reactant density in strained sulphur vacancies and the accelerated hydrogen evolution reaction process on Ru sites. The resulting catalyst delivers an overpotential of 30 mV at a current density of 10 mA cm, a Tafel slope of 31 mV dec, and a long catalytic lifetime. This work provides an effective strategy to improve the activities of single-atom modified transition metal dichalcogenides catalysts by precise strain engineering.
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http://dx.doi.org/10.1038/s41467-021-21956-0DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7966786PMC
March 2021

Spectroscopic characterization of electronic structures of ultra-thin single crystal LaSrMnO.

Sci Rep 2021 Mar 4;11(1):5250. Epub 2021 Mar 4.

Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA, 94720, USA.

We have successfully fabricated high quality single crystalline LaSrMnO (LSMO) film in the freestanding form that can be transferred onto silicon wafer and copper mesh support. Using soft x-ray absorption (XAS) and resonant inelastic x-ray scattering (RIXS) spectroscopy in transmission and reflection geometries, we demonstrate that the x-ray emission from Mn 3s-2p core-to-core transition (3sPFY) seen in the RIXS maps can represent the bulk-like absorption signal with minimal self-absorption effect around the Mn L-edge. Similar measurements were also performed on a reference LSMO film grown on the SrTiO substrate and the agreement between measurements substantiates the claim that the bulk electronic structures can be preserved even after the freestanding treatment process. The 3sPFY spectrum obtained from analyzing the RIXS maps offers a powerful way to probe the bulk electronic structures in thin films and heterostructures when recording the XAS spectra in the transmission mode is not available.
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http://dx.doi.org/10.1038/s41598-021-84598-8DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7933230PMC
March 2021

Self-Activated Catalytic Sites on Nanoporous Dilute Alloy for High-Efficiency Electrochemical Hydrogen Evolution.

ACS Nano 2021 Mar 3;15(3):5333-5340. Epub 2021 Mar 3.

College of Materials Science and Engineering, State Key Laboratory of Advanced Design and Manufacturing for Vehicle Body, Hunan University, Changsha, Hunan 410082, China.

Design and synthesis of effective electrocatalysts for hydrogen evolution reaction (HER) in wide pH environments are critical to reduce energy losses in water electrolyzers. Here, by using a self-activation strategy, we construct an atomic nickel (Ni) decorated nanoporous iridium (Ir) catalyst, which can create the reaction-favorable chemical environment and maximize the electrochemical active surface area (ECSA), enabling efficient HER over a wide pH range. By using X-ray absorption spectroscopy and theoretical calculations, the atomic Ni sites are identified as the synergistic sites, which not only accelerate the water dissociation under operation conditions but also activate the surface Ir sites thus leading to the efficient H generation. This work highlights the significance of atomic-level decorating strategy which can optimize the activity of surface Ir atoms with negligible sacrifice of the ECSA.
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http://dx.doi.org/10.1021/acsnano.0c10885DOI Listing
March 2021

Saturation and self-absorption effects in the angle-dependent 2p3d resonant inelastic X-ray scattering spectra of Co.

J Synchrotron Radiat 2020 Jul 9;27(Pt 4):979-987. Epub 2020 Jun 9.

Debye Institute for Nanomaterials Science, Utrecht University, Universiteitsweg 99, 3584 CG Utrecht, The Netherlands.

Angle-dependent 2p3d resonant inelastic X-ray scattering spectra of a LaCoO single crystal and a 55 nm LaCoO film on a SrTiO substrate are presented. Theoretical calculation shows that, with ∼20 meV resolved Co 2p3d resonant inelastic X-ray scattering (RIXS), the excited states of the isotropic A(O) ground state are split by 3d spin-orbit coupling, which can be distinguished via their angular dependence. However, strong self-absorption and saturation effects distort the spectra of the LaCoO single crystal and limit the observation of small angular dependence. In contrast, the RIXS on 55 nm LaCoO shows less self-absorption effects and preserves the angular dependence of the excited states.
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http://dx.doi.org/10.1107/S1600577520005123DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7336173PMC
July 2020

Tensor description of X-ray magnetic dichroism at the Fe L-edges of FeO.

J Synchrotron Radiat 2021 Jan 1;28(Pt 1):247-258. Epub 2021 Jan 1.

Debye Institute for Nanomaterials Science, Utrecht University, 99 Universiteitsweg, Utrecht 3584 CG, The Netherlands.

A procedure to build the optical conductivity tensor that describes the full magneto-optical response of the system from experimental measurements is presented. Applied to the Fe L-edge of a 38.85 nm FeO/SrTiO (001) thin-film, it is shown that the computed polarization dependence using the conductivity tensor is in excellent agreement with that experimentally measured. Furthermore, the magnetic field angular dependence is discussed using a set of fundamental spectra expanded on spherical harmonics. It is shown that the convergence of this expansion depends on the details of the ground state of the system in question and in particular on the valence-state spin-orbit coupling. While a cubic expansion up to the third order explains the angular-dependent X-ray magnetic linear dichroism of Fe well, higher-order terms are required for Fe when the orbital moment is not quenched.
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http://dx.doi.org/10.1107/S1600577520015027DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7842224PMC
January 2021

Dynamic active-site generation of atomic iridium stabilized on nanoporous metal phosphides for water oxidation.

Nat Commun 2020 Jun 1;11(1):2701. Epub 2020 Jun 1.

College of Materials Science and Engineering, Hunan University, Changsha, Hunan, 410082, China.

Designing efficient single-atom catalysts (SACs) for oxygen evolution reaction (OER) is critical for water-splitting. However, the self-reconstruction of isolated active sites during OER not only influences the catalytic activity, but also limits the understanding of structure-property relationships. Here, we utilize a self-reconstruction strategy to prepare a SAC with isolated iridium anchored on oxyhydroxides, which exhibits high catalytic OER performance with low overpotential and small Tafel slope, superior to the IrO. Operando X-ray absorption spectroscopy studies in combination with theory calculations indicate that the isolated iridium sites undergo a deprotonation process to form the multiple active sites during OER, promoting the O-O coupling. The isolated iridium sites are revealed to remain dispersed due to the support effect during OER. This work not only affords the rational design strategy of OER SACs at the atomic scale, but also provides the fundamental insights of the operando OER mechanism for highly active OER SACs.
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http://dx.doi.org/10.1038/s41467-020-16558-1DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7264278PMC
June 2020

Oxygen K-edge X-ray Absorption Spectra.

Chem Rev 2020 05 10;120(9):4056-4110. Epub 2020 Apr 10.

Inorganic chemistry and catalysis, Debye Institute for Nanomaterials Science, Utrecht University, 3584CG Utrecht, The Netherlands.

We review oxygen K-edge X-ray absorption spectra of both molecules and solids. We start with an overview of the main experimental aspects of oxygen K-edge X-ray absorption measurements including X-ray sources, monochromators, and detection schemes. Many recent oxygen K-edge studies combine X-ray absorption with time and spatially resolved measurements and/or operando conditions. The main theoretical and conceptual approximations for the simulation of oxygen K-edges are discussed in the Theory section. We subsequently discuss oxygen atoms and ions, binary molecules, water, and larger molecules containing oxygen, including biomolecular systems. The largest part of the review deals with the experimental results for solid oxides, starting from s- and p-electron oxides. Examples of theoretical simulations for these oxides are introduced in order to show how accurate a DFT description can be in the case of s and p electron overlap. We discuss the general analysis of the 3d transition metal oxides including discussions of the crystal field effect and the effects and trends in oxidation state and covalency. In addition to the general concepts, we give a systematic overview of the oxygen K-edges element by element, for the s-, p-, d-, and f-electron systems.
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http://dx.doi.org/10.1021/acs.chemrev.9b00439DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7227067PMC
May 2020

Electronic parameters in cobalt-based perovskite-type oxides as descriptors for chemocatalytic reactions.

Nat Commun 2020 Jan 31;11(1):652. Epub 2020 Jan 31.

Chair of Heterogeneous Catalysis and Chemical Technology, RWTH Aachen University, Worringerweg 2, 52074, Aachen, Germany.

Perovskite-type transition metal (TM) oxides are effective catalysts in oxidation and decomposition reactions. Yet, the effect of compositional variation on catalytic efficacy is not well understood. The present analysis of electronic characteristics of B-site substituted LaCoO derivatives via in situ X-ray absorption spectroscopy (XAS) establishes correlations of electronic parameters with reaction rates: TM t and e orbital occupancy yield volcano-type or non-linear correlations with NO oxidation, CO oxidation and NO decomposition rates. Covalent O 2p-TM 3d interaction, in ultra-high vacuum, is a linear descriptor for reaction rates in NO oxidation and CO oxidation, and for NO decomposition rates in O presence. Covalency crucially determines the ability of the catalytically active sites to interact with surface species during the kinetically relevant step of the reaction. The nature of the kinetically relevant step and of surface species involved lead to the vast effect of XAS measurement conditions on the validity of correlations.
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http://dx.doi.org/10.1038/s41467-020-14305-0DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6994687PMC
January 2020

Direct observation of the electronic states of photoexcited hematite with ultrafast 2p3d X-ray absorption spectroscopy and resonant inelastic X-ray scattering.

Phys Chem Chem Phys 2020 Feb 23;22(5):2685-2692. Epub 2019 Oct 23.

Inorganic Chemistry and Catalysis, Debye Institute for Nanomaterials Science, Utrecht University, Universiteitslaan 99, 3584 CG Utrecht, The Netherlands.

Hematite, α-FeO, is an important semiconductor for photoelectrochemical water splitting. Its low charge carrier mobility and the presence of midgap states provide favourable conditions for electron-hole recombination, hence affecting the semiconductor's photoelectrochemical efficiency. The nature of the excited state and charge carrier transport in hematite is strongly debated. In order to further understand the fundamental properties of the hematite photoexcited state, we conducted femtosecond 2p (L) X-ray absorption (XAS) and 2p3d resonant inelastic scattering (RIXS) measurements on hematite thin-films at the Pohang Accelerator Laboratory X-ray Free Electron Laser (PAL-XFEL). The observed spectral changes and kinetic processes are in agreement with previous 3p XAS reports. The potential additional information that could be acquired from 2p3d RIXS experiments is also discussed.
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http://dx.doi.org/10.1039/c9cp03374bDOI Listing
February 2020

Magnetic Contrast at Spin-Flip Excitations: An Advanced X-Ray Spectroscopy Tool to Study Magnetic-Ordering.

ACS Appl Mater Interfaces 2019 Oct 20;11(39):36213-36220. Epub 2019 Sep 20.

Debye Institute for Nanomaterials Science , Utrecht University , Universiteitsweg 99 , 3584 CA Utrecht , The Netherlands.

The determination of the local orientation and magnitude of the magnetization in spin textures plays a pivotal role in understanding and harnessing magnetic properties for technological applications. Here, we show that by employing the polarization dependence of resonant inelastic X-ray scattering (RIXS), we can directly probe the spin ordering with chemical and site selectivity. Applied on the prototypical ferrimagnetic mixed-valence system, magnetite ([Fe][Fe,Fe]O), we can distinguish spin-flip excitations at the A and B antiferromagnetically coupled Fe sublattices and quantify the exchange field. Furthermore, it is possible to determine the orbital contribution to the magnetic moment from detailed angular dependence measurements. RIXS dichroism measurements performed at spin-flip excitations with nanometer spatial resolution will offer a powerful mapping contrast suitable for the characterization of magnetic ordering at interfaces and engineered spin textures.
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http://dx.doi.org/10.1021/acsami.9b10196DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6778912PMC
October 2019

Nature of cobalt species during the in situ sulfurization of Co(Ni)Mo/AlO hydrodesulfurization catalysts.

J Synchrotron Radiat 2019 May 26;26(Pt 3):811-818. Epub 2019 Apr 26.

Inorganic Chemistry and Catalysis, Debye Institute for Nanomaterials Science, Utrecht University, Universiteitsweg 99, 3584 CG Utrecht, The Netherlands.

The evolution in local structure and electronic properties of cobalt was investigated during in situ sulfurization. Using a combination of 1s X-ray absorption (XAS) and 1s3p resonant inelastic X-ray scattering (RIXS), the valence, coordination and symmetry of cobalt ions were tracked in two cobalt-promoted molybdenum oxide precursors of the hydrodesulfurization catalyst system, namely Co-Mo/AlO and Co-Ni-Mo/AlO. Extended X-ray absorption fine structure shows that the Co-O bonds were replaced with Co-S bonds as a function of reaction temperature. The cobalt K pre-edge intensity shows that the symmetry of cobalt was modified from Co O and Co O to a Co ion where the inversion symmetry is broken, in agreement with a square-pyramidal site. The 1s3p RIXS data revealed the presence of an intermediate cobalt oxy-sulfide species. This species was not detected from XAS and was determined from the increased information obtained from the 1s3p RIXS data. The cobalt XAS and RIXS data show that nickel has a significant influence on the formation of the cobalt oxy-sulfide intermediate species prior to achieving the fully sulfided state at T > 400°C.
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http://dx.doi.org/10.1107/S1600577519002546DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6510205PMC
May 2019

X-Ray Absorption Near Edge Structure Spectroscopy of a Solid Catalyst using a Laboratory-Based Set-up.

ChemCatChem 2019 Feb 11;11(3):1039-1044. Epub 2019 Jan 11.

Inorganic Chemistry and Catalysis Debye Institute for Nanomaterials Science Utrecht University Universiteitsweg 99 Utrecht 3584 CG The Netherlands.

An laboratory-based X-ray Absorption Near Edge Structure (XANES) Spectroscopy set-up is presented, which allows performing long-term experiments on a solid catalyst at relevant reaction conditions of temperature and pressure. Complementary to research performed at synchrotron radiation facilities the approach is showcased for a Co/TiO Fischer-Tropsch Synthesis (FTS) catalyst. Supported cobalt metal nanoparticles next to a (very small) fraction of cobalt(II) titanate, which is an inactive phase for FTS, were detected, with no signs of re-oxidation of the supported cobalt metal nanoparticles during FTS at 523 K, 5 bar and 200 h, indicating that cobalt metal is maintained as the main active phase during FTS.
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http://dx.doi.org/10.1002/cctc.201801822DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6471006PMC
February 2019

Single platinum atoms embedded in nanoporous cobalt selenide as electrocatalyst for accelerating hydrogen evolution reaction.

Nat Commun 2019 04 15;10(1):1743. Epub 2019 Apr 15.

College of Materials Science and Engineering, Hunan University, 410082, Changsha, Hunan, China.

Designing efficient electrocatalysts for hydrogen evolution reaction is significant for renewable and sustainable energy conversion. Here, we report single-atom platinum decorated nanoporous CoSe (Pt/np-CoSe) as efficient electrocatalysts for hydrogen evolution. The achieved Pt/np-CoSe shows high catalytic performance with a near-zero onset overpotential, a low Tafel slope of 35 mV dec, and a high turnover frequency of 3.93 s at -100 mV in neutral media, outperforming commercial Pt/C catalyst and other reported transition-metal-based compounds. Operando X-ray absorption spectroscopy studies combined with density functional theory calculations indicate that single-atom platinum in Pt/np-CoSe not only can optimize surface states of CoSe active centers under realistic working conditions, but also can significantly reduce energy barriers of water dissociation and improve adsorption/desorption behavior of hydrogen, which synergistically promote thermodynamics and kinetics. This work opens up further opportunities for local electronic structures tuning of electrocatalysts to effectively manipulate its catalytic properties by an atomic-level engineering strategy.
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http://dx.doi.org/10.1038/s41467-019-09765-yDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6465355PMC
April 2019

Insights into the Synthesis Mechanism of Ag Nanoclusters.

J Phys Chem C Nanomater Interfaces 2018 Dec 19;122(49):28351-28361. Epub 2018 Nov 19.

Inorganic Chemistry and Catalysis, Debye Institute for Nanomaterials Science, Utrecht University, Universiteitslaan 99, 3584 CG Utrecht, The Netherlands.

The current understanding of the synthesis mechanisms of noble metal clusters is limited, in particular for Ag clusters. Here, we present a detailed investigation into the synthesis process of atomically monodisperse Ag clusters, prepared via reduction of AgNO in the presence of dithiolate ligands. Using optical spectroscopy, mass spectrometry, and X-ray spectroscopy, it was determined that the synthesis involves a rapid nucleation and growth to species with up to a few hundred Ag atoms. From these larger species, Ag clusters are formed and their concentration increases steadily over time. Oxygen plays an important role in the etching of large particles to Ag. No other stable Ag cluster species are observed at any point during the synthesis.
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http://dx.doi.org/10.1021/acs.jpcc.8b09360DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6369667PMC
December 2018

Single Au Atom Doping of Silver Nanoclusters.

ACS Nano 2018 Dec 3;12(12):12751-12760. Epub 2018 Dec 3.

Inorganic Chemistry and Catalysis, Debye Institute for Nanomaterials Science , Utrecht University , Universiteitslaan 99 , 3584 CG Utrecht , The Netherlands.

Ag nanoclusters capped with lipoic acid (LA) can be doped with Au. The doped clusters show enhanced stability and increased luminescence efficiency. We attribute the higher quantum yield to an increase in the rate of radiative decay. With mass spectrometry, the Au-doped clusters were found to consist predominantly of AuAg(LA). The clusters were characterized using X-ray absorption spectroscopy at the Au L-edge. Both the extended absorption fine structure (EXAFS) and the near edge structure (XANES) in combination with electronic structure calculations confirm that the Au dopant is preferentially located in the center of the cluster. A useful XANES spectrum can be recorded for lower concentrations, or in shorter time, than the more commonly used EXAFS. This makes XANES a valuable tool for structural characterization.
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http://dx.doi.org/10.1021/acsnano.8b07807DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6328285PMC
December 2018

Electronic structure of PrMnNiO from x-ray photoemission, absorption and density functional theory.

J Phys Condens Matter 2018 Oct 14;30(43):435603. Epub 2018 Sep 14.

Department of Physics, Indian Institute of technology, Roorkee-247667, Uttarakhand, India. Institute of Physics, Bhubaneshwar-750012, India.

The electronic structure of double perovskite PrMnNiO was studied using core x-ray photoelectron spectroscopy and x-ray absorption spectroscopy. The 2p x-ray absorption spectra show that Mn and Ni are in 4+  and 2+  states respectively. Based on charge transfer multiplet analysis of the 2p XPS spectra of both ions, we find charge transfer energies [Formula: see text] of 3.5 and 2.5 eV for Ni and Mn respectively. The ground state of Ni and Mn ions reveal a higher d electron count of 8.21 and 3.38 respectively as compared to the ionic values. The partial density of states clearly show a charge transfer character of the system for U  -  J [Formula: see text] 2 eV. The O 1s edge absorption spectra reveal a band gap of 0.9 eV, which is close to the value estimated from analysis of Ni and Mn 2p photoemission and absorption spectra. The combined analysis of nature of spectroscopic data and first principles calculations reveal that the material is a p  -  d type charge transfer insulator with an intermediate covalent character according to the Zannen-Sawatzy-Allen phase diagram.
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http://dx.doi.org/10.1088/1361-648X/aae168DOI Listing
October 2018

Capturing the Genesis of an Active Fischer-Tropsch Synthesis Catalyst with Operando X-ray Nanospectroscopy.

Angew Chem Int Ed Engl 2018 Sep 17;57(37):11957-11962. Epub 2018 Aug 17.

Inorganic Chemistry and Catalysis group, Debye Institute for Nanomaterials Science, Utrecht University, Universiteitsweg 99, 3584, CG, Utrecht, The Netherlands.

A state-of-the-art operando spectroscopic technique is applied to Co/TiO catalysts, which account for nearly half of the world's transportation fuels produced by Fischer-Tropsch catalysis. This allows determination of, at a spatial resolution of approximately 50 nm, the interdependence of formed hydrocarbon species in the inorganic catalyst. Observed trends show intra- and interparticular heterogeneities previously believed not to occur in particles under 200 μm. These heterogeneities are strongly dependent on changes in H /CO ratio, but also on changes thereby induced on the Co and Ti valence states. We have captured the genesis of an active FTS particle over its propagation to steady-state operation, in which microgradients lead to the gradual saturation of the Co/TiO catalyst surface with long chain hydrocarbons (i.e., organic film formation).
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http://dx.doi.org/10.1002/anie.201806354DOI Listing
September 2018

Quanty4RIXS: a program for crystal field multiplet calculations of RIXS and RIXS-MCD spectra using Quanty.

J Synchrotron Radiat 2018 May 1;25(Pt 3):899-905. Epub 2018 May 1.

Debye Institute of Nanomaterial Science, Utrecht University, 3584 CA Utrecht, The Netherlands.

Some initial instructions for the Quanty4RIXS program written in MATLAB are provided. The program assists in the calculation of 1s 2p RIXS and 1s 2p RIXS-MCD spectra using Quanty. Furthermore, 1s XAS and 2p 3d RIXS calculations in different symmetries can also be performed. It includes the Hartree-Fock values for the Slater integrals and spin-orbit interactions for several 3d transition metal ions that are required to create the .lua scripts containing all necessary parameters and quantum mechanical definitions for the calculations. The program can be used free of charge and is designed to allow for further adjustments of the scripts.
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http://dx.doi.org/10.1107/S1600577518004058DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5929360PMC
May 2018

Direct Observation of Cr 3d States in Ruby: Toward Experimental Mechanistic Evidence of Metal Chemistry.

J Phys Chem A 2018 May 26;122(18):4399-4413. Epub 2018 Apr 26.

Inorganic Chemistry and Catalysis, Debye Institute for Nanomaterials Science , Utrecht University , 3584CG Utrecht , The Netherlands.

The role of transition metals in chemical reactions is often derived from probing the metal 3d states. However, the relation between metal site geometry and 3d electronic states, arising from multielectronic effects, makes the spectral data interpretation and modeling of these optical excited states a challenge. Here we show, using the well-known case of red ruby, that unique insights into the density of transition metal 3d excited states can be gained with 2p3d resonant inelastic X-ray scattering (RIXS). We compare the experimental determination of the 3d excited states of Cr impurities in AlO with 190 meV resolution 2p3d RIXS to optical absorption spectroscopy and to simulations. Using the crystal field multiplet theory, we calculate jointly for the first time the Cr multielectronic states, RIXS, and optical spectra based on a unique set of parameters. We demonstrate that (i) anisotropic 3d multielectronic interactions causes different scaling of Slater integrals, and (ii) a previously not observed doublet excited state exists around 3.35 eV. These results allow to discuss the influence of interferences in the RIXS intermediate state, of core-hole lifetime broadenings, and of selection rules on the RIXS intensities. Finally, our results demonstrate that using an intermediate excitation energy between L and L edges allows measurement of the density of 3d excited states as a fingerprint of the metal local structure. This opens up a new direction to pump-before-destroy investigations of transition metal complex structures and reaction mechanisms.
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http://dx.doi.org/10.1021/acs.jpca.8b00984DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6023264PMC
May 2018

Cobalt-to-vanadium charge transfer in polyoxometalate water oxidation catalysts revealed by 2p3d resonant inelastic X-ray scattering.

Phys Chem Chem Phys 2018 Feb;20(6):4554-4562

Inorganic Chemistry & Catalysis, Debye Institute for Nanomaterials Science, Utrecht University, Universiteitsweg 99, 3584 CG, The Netherlands.

Two isostructural cobalt containing polyoxometalate water oxidation catalysts, [Co(HO)(α-PWO)] (Co4P2) and [Co(HO)(α-VWO)] (Co4V2), exhibit large differences in their catalytic performance. The substitution of phosphorus centers in Co4P2 with redox-active vanadium centers in Co4V2 leads to electronic structure modifications. Evidence for the significance of the vanadium centers to catalysis, predicted by theory, was found from soft X-ray absorption (XAS) and resonant inelastic X-ray scattering (RIXS). The XAS and RIXS spectra determine the electronic structure of the cobalt and vanadium sites in the pre-reaction state of both Co4V2 and Co4P2. High-energy resolution RIXS results reveal that Co4V2 possesses a smaller ligand field within the tetra-cobalt core and a cobalt-to-vanadium charge transfer band. The differences in electronic structures offer insights into the enhanced catalysis of Co4V2.
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http://dx.doi.org/10.1039/c7cp06786kDOI Listing
February 2018

Charge-Transfer Analysis of 2p3d Resonant Inelastic X-ray Scattering of Cobalt Sulfide and Halides.

J Phys Chem C Nanomater Interfaces 2017 Nov 27;121(45):24919-24928. Epub 2017 Oct 27.

Inorganic Chemistry & Catalysis, Debye Institute for Nanomaterials Science, Utrecht University, Universiteitsweg 99, 3584 CG Utrecht, The Netherlands.

We show that with 2p3d resonant inelastic X-ray scattering (RIXS) we can accurately determine the charge-transfer parameters of CoF, CoCl, CoBr, and CoS. The 160 meV resolution RIXS results are compared with charge-transfer multiplet calculations. The improved resolution and the direct observation of the crystal field and charge-transfer excitations allow the determination of more accurate parameters than could be derived from X-ray absorption and X-ray photoemission, both limited in resolution by their lifetime broadening. We derive the crystal field and charge-transfer parameters of the Co ions, which provides the nature of the ground state of the Co ions with respect to symmetry and hybridization. In addition, the increased spectral resolution allows the more accurate determination of the atomic Slater integrals. The results show that the crystal field energy decreases with increasing ligand covalency. The L edge RIXS spectra show that the intensity of the (Coster-Kronig induced) nonresonant X-ray emission is a measure of ligand covalency.
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http://dx.doi.org/10.1021/acs.jpcc.7b06882DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5694969PMC
November 2017

Local vs Nonlocal States in FeTiO Probed with 1s2pRIXS: Implications for Photochemistry.

Inorg Chem 2017 Sep 5;56(18):10882-10892. Epub 2017 Sep 5.

Inorganic Chemistry and Catalysis, Debye Institute for Nanomaterial Science, Utrecht University , 3584CG Utrecht, The Netherlands.

Metal-metal charge transfer (MMCT) is expected to be the main mechanism that enables the harvesting of solar light by iron-titanium oxides for photocatalysis. We have studied FeTiO as a model compound for MMCT with 1s2pRIXS at the Fe K-edge. The high-energy resolution XANES enables distinguishing five pre-edge features. The three first well distinct RIXS features are assigned to electric quadrupole transitions to the localized Fe* 3d states, shifted to lower energy by the 1s core-hole. Crystal field multiplet calculations confirm the speciation of divalent iron. The contribution of electric dipole absorption due to local p-d mixing allowed by the trigonal distortion of the cation site is supported by DFT and CFM calculations. The two other nonlocal features are assigned to electric dipole transitions to excited Fe* 4p states mixed with the neighboring Ti 3d states. The comparison with DFT calculations demonstrates that MMCT in ilmenite is favored by the hybridization between the Fe 4p and delocalized Ti 3d orbitals via the O 2p orbitals.
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http://dx.doi.org/10.1021/acs.inorgchem.7b00938DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5636175PMC
September 2017

In-Situ 2p3d Resonant Inelastic X-ray Scattering Tracking Cobalt Nanoparticle Reduction.

J Phys Chem C Nanomater Interfaces 2017 Aug 14;121(32):17450-17456. Epub 2017 Jul 14.

Inorganic Chemistry & Catalysis, Debye Institute for Nanomaterials Science, Utrecht University, Universiteitsweg 99, Utrecht, 3584 CG, The Netherlands.

In-situ carbon-thermal reduction of cobalt oxide nanoparticles supported on carbon nanotubes was studied by cobalt 2p3d resonant inelastic X-ray scattering (RIXS). The in-situ 2p X-ray absorption spectroscopy (XAS) and RIXS measurements were performed at 500, 600, and 700 °C, where four consistent excitation energies were used for RIXS acquisitions. After 700 °C reduction, the XAS spectrum shows a cobalt metal-like shape, while the RIXS spectra reveal the minority cobalt monoxide phase. The holistic fit on both XAS and RIXS data reveals the respective contributions from metal and monoxide. We show that the relative precision to determine the monoxide content changes from ∼5.6% in XAS results to better than 0.8% in the RIXS analysis, suggesting that RIXS is a useful tool to track the oxidation state of nanoparticles under in situ conditions. We determined a relative radiative ratio () factor of approximately 5, where this factor gives the ratio between the relative strengths of the radiative decay channels compared to the nonradiative channels in CoO and Co metal.
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http://dx.doi.org/10.1021/acs.jpcc.7b04325DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5563841PMC
August 2017

Strain-Engineered Oxygen Vacancies in CaMnO Thin Films.

Nano Lett 2017 02 23;17(2):794-799. Epub 2017 Jan 23.

Department of Physics, Temple University , 1925 North 12th Street, Philadelphia, Pennsylvania 19122, United States.

We demonstrate a novel pathway to control and stabilize oxygen vacancies in complex transition-metal oxide thin films. Using atomic layer-by-layer pulsed laser deposition (PLD) from two separate targets, we synthesize high-quality single-crystalline CaMnO films with systematically varying oxygen vacancy defect formation energies as controlled by coherent tensile strain. The systematic increase of the oxygen vacancy content in CaMnO as a function of applied in-plane strain is observed and confirmed experimentally using high-resolution soft X-ray absorption spectroscopy (XAS) in conjunction with bulk-sensitive hard X-ray photoemission spectroscopy (HAXPES). The relevant defect states in the densities of states are identified and the vacancy content in the films quantified using the combination of first-principles theory and core-hole multiplet calculations with holistic fitting. Our findings open up a promising avenue for designing and controlling new ionically active properties and functionalities of complex transition-metal oxides via strain-induced oxygen-vacancy formation and ordering.
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http://dx.doi.org/10.1021/acs.nanolett.6b03986DOI Listing
February 2017

Characterisation, degradation and regeneration of luminescent Ag clusters in solution.

Nanoscale 2016 Dec;8(47):19901-19909

Condensed Matter & Interfaces, Debye Institute for Nanomaterials Science, Universiteit Utrecht, Princetonplein 5, 3584 CC Utrecht, The Netherlands.

Luminescent Ag clusters are prepared with lipoic acid (LA) as the ligand. Using a combination of mass spectrometry, optical spectroscopy and analytical ultracentrifugation, the clusters are found to be highly monodisperse with mass 5.6 kDa. We assign the chemical composition [Ag(LA)] to the clusters, where LA likely binds in a bidentate fashion. The Ag clusters show slow degradation, retaining their deep red emission for at least 18 months if stored in the dark. Purification or exposure to light results in faster degradation. No other cluster species are observed during the degradation process. Once degraded, the clusters could easily be regenerated using NaBH, which is not usually observed for thiolate-capped Ag clusters.
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http://dx.doi.org/10.1039/c6nr04958cDOI Listing
December 2016

In situ X-ray absorption spectroscopy of transition metal based water oxidation catalysts.

Chem Soc Rev 2017 Jan;46(1):102-125

Department of Chemistry, University of California - Berkeley, 419 Latimer Hall, Berkeley, CA 94720, USA.

X-ray absorption studies of the geometric and electronic structure of primarily heterogeneous Co, Ni, and Mn based water oxidation catalysts are reviewed. The X-ray absorption near edge and extended X-ray absorption fine structure studies of the metal K-edge, characterize the metal oxidation state, metal-oxygen bond distance, metal-metal distance, and degree of disorder of the catalysts. These properties guide the coordination environment of the transition metal oxide radical that localizes surface holes and is required to oxidize water. The catalysts are investigated both as-prepared, in their native state, and under reaction conditions, while transition metal oxide radicals are generated. The findings of many experiments are summarized in tables. The advantages of future X-ray experiments on water oxidation catalysts, which include the limited data available of the oxygen K-edge, metal L-edge, and resonant inelastic X-ray scattering, are discussed.
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http://dx.doi.org/10.1039/c6cs00230gDOI Listing
January 2017

Robust Ferromagnetism of Chromium Nanoparticles Formed in Superfluid Helium.

Adv Mater 2017 Jan 27;29(1). Epub 2016 Oct 27.

Department of Chemistry, Utrecht University, Universiteitsweg 99, 3584CG, Utrecht, The Netherlands.

Chromium nanoparticles are formed using superfluid helium droplets as the nanoreactors, which are strongly ferromagnetic. The transition from antiferromagentism to ferromagnetism is attributed to atomic-scale disorder in chromium nanoparticles, leading to abundant unbalanced surface spins. Theoretical modeling confirms a frustrated aggregation process in superfluid helium due to the antiferromagnetic nature of chromium.
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http://dx.doi.org/10.1002/adma.201604277DOI Listing
January 2017

Distorted Tetrahedral Co in KH[CoWO]·xHO Probed by 2p3d Resonant Inelastic X-ray Scattering.

Inorg Chem 2016 Oct 27;55(20):10152-10160. Epub 2016 Sep 27.

Inorganic Chemistry & Catalysis, Debye Institute for Nanomaterials Science, Utrecht University , Universiteitsweg 99, 3584 CG Utrecht, The Netherlands.

The Co 2p X-ray absorption spectroscopy and high-energy-resolution (∼0.09 eV fwhm) 2p3d resonant inelastic X-ray scattering (RIXS) spectra of the single-cobalt-centered polyoxometalate KH[CoWO]·xHO were measured. The low-energy dd transition features at 0.55 eV, unmeasurable with ultraviolet-visible (UV/vis) spectroscopy, were experimentally revealed in 2p3d RIXS spectra. RIXS simulations based on ligand-field multiplet theory were performed to assess the potential cobalt tetragonal symmetry distortion, which is described with the ligand-field parameters 10D (-0.54 eV), D (-0.08 eV), and D (0.005 eV). Because 2p3d RIXS probes not only the optical spin-allowed transitions but also the spin-forbidden transitions, we show that the current 2p3d RIXS simulation enables a series of dd feature assignments with higher accuracy than those from previous optical data. Furthermore, by wave-function decomposition analyses, we demonstrate the more realistic and detailed origins of a few lowest dd transitions using both one-electron-orbital and term-symbol descriptions.
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http://dx.doi.org/10.1021/acs.inorgchem.6b01228DOI Listing
October 2016

CTM4DOC: electronic structure analysis from X-ray spectroscopy.

J Synchrotron Radiat 2016 09 23;23(Pt 5):1264-71. Epub 2016 Aug 23.

Inorganic Chemistry and Catalysis Group, Debye Institute for Nanomaterials Science, Utrecht University, Universiteitsweg 99, Utrecht 3584 CG, The Netherlands.

Two electronic structure descriptions, one based on orbitals and the other based on term symbols, have been implemented in a new Matlab-based program, CTM4DOC. The program includes a graphical user interface that allows the user to explore the dependence of details of electronic structure in transition metal systems, both in the ground and core-hole excited states, on intra-atomic electron-electron, crystal-field and charge-transfer interactions. The program can also track the evolution of electronic structure features as the crystal-field parameters are systematically varied, generating Tanabe-Sugano-type diagrams. Examples on first-row transition metal systems are presented and the implications on the interpretation of X-ray spectra and on the understanding of low-spin, high-spin and mixed-spin systems are discussed.
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http://dx.doi.org/10.1107/S1600577516012443DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5006656PMC
September 2016
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