SOFT X-RAY EMISSION SPECTROSCOPY STUDY OF A CATHODE
MATERIAL FOR LI-ION BATTERIES IN COMBINATION WITH
FULL-MULTIPLET CALCULATION
Daisuke Asakura1,2, Yusuke Nanba1,#, Eiji Hosono1,2, Hideharu Niwa3,4,†, Hisao Kiuchi5,$, Jun
Miyawaki3,4, and Yoshihisa Harada2,3,4
1

Research Institute for Energy Conservation, National Institute of Advanced Industrial Science andTechnology (AIST)2

AIST-UTokyo Advanced Operando-Measurement Technology Open Innovation Laboratory, AIST

3

Synchrotron Radiation Laboratory, The Institute for Solid State Physics, The University of Tokyo
4

Synchrotron Radiation Research Organization, The University of Tokyo
5

Department of Applied Chemistry, The University of Tokyo

Present affiliations: #Shinshu University, †University of Tsukuba, $Kyoto University

Enhancement of the energy-density and power-density of electrode materials is one of theimportant subjects in the research field of Li-ion batteries (LIBs). To improve theperformances of electrode materials, the redox mechanisms due to Li-ion extraction/insertion
should be clarified. In this study, high-energy-resolution soft x-ray emission spectroscopy(XES) was applied to understand changes in the electronic structure of LiMn2O4 cathode [1]by Li-ion extraction/insertion[2]. The Mn 2p-3d-2p resonant XES spectra were analyzed by
the configuration-interaction full-multiplet (CIFM) calculation, which reproduced both dd andcharge-transfer (CT) excitations.
The LiMn2O4 sample was fabricated by a sol-gel method. For electrochemical experiments,
LiMn2O4 (75 wt%), acetylene black (20 wt%), and polytetrafluoroethylene (5 wt%) wereground into a paste. Lithium metal was used as the counter and reference electrodes, and 1M
ethylene carbonate (EC)/diethyl carbonate (DEC) solutions of LiClO4 were used aselectrolytes for a three-electrode beaker cell. We prepared three samples for initial (before
charging), charged, and discharged states by constant-current charge-discharge experimentswith a current density of 10 mA/g. The corresponding voltages for the three states were 3.2,
4.3 and 3.2 V vs. Li/Li+, respectively. Mn 2p-3d-2p resonant XES measurements for theLiMn2O4 samples were carried out at BL07LSU in SPring-8 using a high resolution XES
spectrometer, HORNET[3]. The total energy resolution for XES was set to E/E = 3200 at
the Mn L3 edge. All the XES measurements were carried out at room temperature in a highvacuum. The CIFM calculation for the electronic-structure analysis of Mn atoms in LiMn2O4
was carried out by assuming an octahedral MnO6 cluster in which the Mn 3d and O 2p
orbitals and the charge-transfer effect were taken into account[4].
The resonant XES spectra taken with an excitation energy of 642.0 eV corresponding to a
Mn L3-edge absorption peak showed drastic changes between the initial and charged states(Fig. 1). The initial state should be attributed to a mixed valence state of Mn3+/Mn4+. For the
initial state, all the Mn3+ should be oxidized to Mn4+. Fo
r the discharged state, the Mn ions should partially be reduced to Mn3+. Between the initialand discharged state, the small changes of the ratio of Mn3+/Mn4+ was detected by the use ofresonant XES.
To confirm the changes and separately understand the Mn3+ and Mn4+states, we analyzed
the XES spectra by using the CIFM calculation (Fig. 2), while the Mn4+ state was regarded as
unchanged during charge-discharge. Both the oxidation states have strong ligand-to-metal CT
effect. In particular for the Mn4+ (charged) state, the CT energy  was highly negative,
resulting in the dominant d4L and d5L2 charge transfer configurations. The present results

Fig. 1. Mn 2p-3d-2p resonant XES spectra
for LiMn2O4 at the initial, charged anddischarged states[2].

Fig. 2. CIFM calculation for (a) Mn4+ withthe charged state and (b) Mn3+ states with adifference spectrum between the initial andcharged states[2].

indicate possible reasons for the capacity fading in LiMn2O4: instability of thecrystal-structure during charge-discharge and Jahn-Teller distortion at the Mn3+ site.
In summary, we measured Mn 2p-3d-2p resonant XES for LiMn2O4. Detailed informationof the electronic-structure change such as the strong CT effect was obtained. In the near future,
we will perform O 1s-2p-1s XES measurement to further understand the redox reaction.

REFERENCES[1] For example, M. M. Thackeray, Prog. Solid State Chem. 25, 1 (1997).
[2] D. Asakura et al., Phys. Chem. Chem. Phys. 21, 18363 (2019).
[3] Y. Harada et al., Rev. Sci. Instrum. 83, 013116 (2012).
[4] For example, D. Asakura et al., J. Phys. Chem. Lett. 5, 4008 (2014).

DEVELOPMENT OF ALL-SOLID-STATE LI-ION-BATTERY CELLFOR OPERANDO SOFT X-RAY EMISSION SPECTROCOSPY
Daisuke Asakura1,2, Eiji Hosono1,2, Kosuke Yamazoe3,4, Jun Miyawaki3,4, and Yoshihisa
Harada2,3,4
1

Research Institute for Energy Conservation, National Institute of Advanced Industrial Science andTechnology (AIST)2

AIST-UTokyo Advanced Operando-Measurement Technology Open Innovation Laboratory, AIST

3

Synchrotron Radiation Laboratory, The Institute for Solid State Physics, The University of Tokyo
4

Synchrotron Radiation Research Organization, The University of Tokyo

Development of high-performance lithium-ion battery (LIB) for electric vehicles is a one of
the important research subjects to realize a sustainable low-carbon society. However, thetrade-off relationships among the performances such as energy density, power performance,
cycle performance, and safety property make the development difficult. Moreover, thecombination of the materials among the cathode, anode, and electrolyte is crucial for the
improvement of total performances of LIB. Thus, comprehensive researches anddevelopments are highly important. In addition to the materials design, cutting-edge analyses
are inevitable because understanding of the redox reactions and structural/electronic
properties for the materials in LIBs is necessary to design/create innovative materials for
LIBs.
Synchrotron radiation X-ray spectroscopy is a powerful technique to understand the crystal
structure and electronic structure in LIB materials. We have been studying thecharge-discharge mechanisms in LIBs based on the electronic-structure analysis by using soft
X-ray spectroscopy at HORNET, BL07LSU. Especially, the redox reactions of electrodematerials have been investigated by operando soft x-ray emission spectroscopy under the
charge-discharge operation [1,2]. The operando cell consists of special electrode chip directlygrown on Si3N4 thin-film window, counter electrode and electrolyte solution. On the other
hand, all-solid-state LIB with solid-state electrolyte is of particular importance. We havealready developed an all-solid-state-type operando cell for operando photoemission
spectromicroscopy at 3DnanoESCA, BL07LSU [3,4].
In this study, we tried to install the all-solid-state-type operando cell for the HORNET
chamber to perform operando soft x-ray emission spectroscopy for the all-solid-state LIBs. A
special transfer vessel was developed to transfer the operando cell from a glove box filledwith Ar gas to vacuum chamber without air exposure, because the operando cell includes a
Li-metal counter electrode. The transfer vessel was fixed to the HORNET chamber and theoperando cell was moved to the beam position (Figs. 1(a) and 1(b)).
The working electrode of the operando cell was LiCoO2, a cathode material, which was
single crystalline particles. The particles were mixed with a binder as a slurry and fixed on acarbon micro grid with an Au mesh (Fig. 1(d)). Then, the working electrode was stacked withthe solid-state electrolyte sheets and Li-metal counter electrode as shown in Fig. 1(d).
Figure 1(c) shows the Co L3-edge soft X-ray emission spectra for different 4 positions on theworking electrode. For all the points, the intensity was too small, indicating that there were
few LiCoO2 particles. Because the size of the particles (m order) is comparable with thebeam size, it was difficult to find the positions of the small number of the particles. The
density of the particles on the Au mesh should be much higher. On the other hand, we
succeeded in the electrochemical charge-discharge using the operando cell and detecting thesignals of Co L3-edge soft X-ray absorption by the total-electron yield detection mode.
In summary, we installed an all-solid-state-type operando cell for the HORNET chamber.
The intensity of the Co L3-edge soft X-ray emission spectra for the LiCoO2 working electrodewas too weak due to the small number of the m-order particles. In the near future, we will
increase the number of the particles by modifying the cell structure in order to perform soft
X-ray emission measurement for the all-solid-state-type operando cell.

Fig.1. (a) A picture of the transfer vessel fixed with the HORNET chamber. (b) A picture of
the inserted all-solid-state-type operando cell into the main chamber taken via a viewport. (c)Co L3-edge soft X-ray emission spectra for different 4 positions on the working electrode. (d)A schematic picture of the all-solid-state-type operando cell [3,4].

REFERENCES[1] D. Asakura et al., Electrochem. Commun. 50, 93 (2015).
[2] D. Asakura et al., Phys. Chem. Chem. Phys. 21, 26351 (2019).
[3] K. Akada et al., J. Electron Spectrosc. Relat. Phenom. 233, 64 (2019).
[4] K. Akada et al., Sci. Rep. 9, 12452 (2019).

VERIFICATION OF TWO-CARRIER STATE
IN THE CUPRATE SUPERCONDUCTOR Pr1.4-xLa0.6CexCuO4
1

Kenji Ishii1, Shun Asano2, Masaki Fujita2,
Kohei Yamagami3, Jun Miyawaki3, Yoshihisa Harada3

Synchrotron Radiation Research Center, National Institutes for Quantum and Radiological Scienceand Technology
2
Intsitute for Materials Research, Tohoku University
3
Synchrotron Radiation Laboratory, The Institute for Solid State Physics, The University of Tokyo

High-transition-temperature superconductivity in the cuprates occurs when either holes orelectrons are doped into parent antiferromagnetic Mott insulators. Dope holes occupy theoxygen 2p orbitals predominantly. In contrast, electrons are doped into the Cu 3d orbital but
doping evolution of the electronic states of the “nominally” electron-doped cuprates is not sosimple. Most electron-doped cuprate superconductors have the chemical formula RE2 −xCexCuO4 (RE = La, Pr, Nd, Sm, Eu) and electrons are doped via partial substitution of trivalent
RE with tetravalent Ce. At low Ce concentration (x), Hall and Seebeck coefficients, which areoften used to determine the sign and number of carriers, are negative as expected. However, thecoefficients change the sign in the temperature dependence in the intermediate concentrationand the sign becomes positive in entire measured temperature at high concentration (x > 0.155
for RE = Pr and x > 0.135 for RE = La) [1-3]. Furthermore, reconstruction of the Fermi surface
with increasing the Ce concentration is observed in ARPES [4] and quantum oscillation [5].
Recently, it was reported in a transport study [6] that superconductivity in the nominallyelectron-doped cuprates emerges under the coexistence of holes and electrons and a relation
between the superfluid density of holes and the superconducting transition temperature agrees
with that of hole-doped cuprates. This result indicates that the superconductivity in thenominally electron-doped cuprates is driven by the holes rather than the electrons and a unified
mechanism of the superconductivity exists irrespective of the type of carries.
In this study, we aim to verify the two-carrier states in the nominally electron-doped cuprates,
especially to prove the existence of the holes by means of x-ray absorption spectroscopy (XAS)
and resonant inelastic x-ray scattering (RIXS) at the oxygen K-edge. In the hole-doped cuprates,
O K-edge XAS gave a direct evidence of the holes in the O 2p orbitals [7]; a peak at the preedge is ascribed to the hole state. In addition, we recently demonstrated that O K-edge RIXS is
sensitive to the charge excitations of the doped holes [8].
The experiment was performed using the HORNET spectrometer at BL07LSU of SPring-8.
Total energy resolution of RIXS was 150 meV. For the present study, we selected Pr1.4xLa0.6CexCuO4 (PLCCO), in which superconductivity is observed in wide Ce concentration (x).
We prepared single crystals of superconducting x = 0.16 and non-superconducting x = 0.08.
The crystals of were cleaved in the air just before the measurement and s-polarized x-rays were
irradiated on the ab-plane of the crystal. The ac-plane was parallel to the horizontal scattering
plane and momentum transfer in the CuO2 plane (q) was scanned by rotating the crystal along
the vertical b-axis. Scattering angle (2θ) was kept at 135˚ and temperature of the crystals was
about 15 K.
The inset of Fig. 1 shows an XAS spectrum of PLCCO x = 0.16. The peak at 528.8 eVoriginates from the absorption to the Cu 3d upper Hubbard band (UHB) hybridized with the O
2p states. In the hole doped cuprates, absorption to the O 2p hole states appears at lower energyof the UHB peak but we cannot find any trace of the hole states in PLCCO. The main panel ofFig. 1 shows incident energy (Ei) dependence across the UHB peak in XAS as indicated byvertical bars in the inset. A Raman feature at 0.4 eV is observed at Ei ≤ 528.7 eV and it graduallycharges to fluorescence at higher Ei.

Figure 2 shows momentum dependence of RIXS
measured at Ei = 528.5 eV. In the four spectra fromthe bottom, lines in light color present raw data andelastic scattering is subtracted for the lines in deepcolor. The subtracted spectra are overlaid at the top.
Most of the spectral weight below 1eV isindependent of the momentum and two-magnon
excitation is a possible origin. In stark contrast,
spectral weight in O K-edge RIXS exhibits large
momentum dependence in hole-doped cuprates [8].
Though it is tiny, a momentum-dependent
component is discernible in Fig. 2. Because thecomponent follows the momentum dependence ofthe charge excitations observed in Cu K- and L3edge RIXS [9,10], it is reasonably ascribed to thesame charge origin through the hybridization
between Cu 3d and O 2p orbitals. Regarding thedoping dependence, the momentum-independent
spectral weight is similarly observed, intensity ofthe momentum-dependent component is weaker in
x = 0.08.
In summary, present XAS and RIXS results of
PLCCO are comprehensible without assuming theexistence of the holes in the O 2p orbitals. Even if
the holes exist in the nominally electron-doped
cuprates, their orbital character is qualitatively
different from that of the O 2p holes in hole-doped
cuprates.

FIG. 1: Incident energy dependence of O Kedge RIXS in PLCCO x = 0.16. Inset shows
XAS at the edge.

REFERENCES[1] Y. Dagan et al., Phys. Rev. Lett. 92, 167001 (2004).
[2] P. Li et al., Phys. Rev. B 75, 020506 (2007).
[3] R. L. Greene et al, Annu. Rev. Condens. Matter Phys.
11, 213 (2020).
[4] N. P. Armitage et al., Phys. Rev. Lett. 88, 257001(2002).
[5] T. Helm et al., Phys. Rev. B 92, 094501 (2015).
[6] Y. Li et al., Sci. Adv. 5, eaap7349 (2019).
[7] C. T. Chen et al., Phys. Rev. Lett. 66 ,104 (1991).
[8] K. Ishii et al., Phys. Rev. B 96 ,115148 (2017).
[9] K. Ishii et al., Phys. Rev. Lett. 94 ,207003 (2005).
[10] K. Ishii et al., Nat. Commun. 5, 3714 (2014).

FIG. 2: Momentum dependence of O K-edge
RIXS in PLCCO x = 0.16.

SPIN-POLARIZED ELECTRONIC STATE OF SINGLE CRYSTAL
Co2MnSi HEUSLER ALLOY PROBED BY RESONANT INELASTIC
SOFT X-RAY SCATTERING (SX-RIXS) IN MAGNETIC FIELD
Rie Y. Umetsu1, Hidenori Fujiwara2, Jun Miyawaki3,4, Kohei Nishimoto2, Toshiyuki
Kashiuchi2, Akira Sekiyama2, Akinori Irizawa5 Yoshihisa Harada3,4 and Shigemasa Suga5
1

Institute for Materials Research (IMR), Tohoku University
Graduate School of Engineering Science, Osaka University, Osaka, Japan
3
The Institute for Solid State Physics (ISSP), The University of Tokyo, Japan
4
Synchrotron Radiation Research Organization, The University of Tokyo, Japan
5
Institute of Scientific & Industrial Research, Osaka University, Osaka, Japan
2

Introduction
It has been predicted from theoretical calculations that some of Mn- and Co-based Heusler
alloys are half-metallic ferro- or ferri-magnets with the spin polarization of 100 % aroundthe Fermi energy (EF) [1-3]. If the electrons are completely polarized around EF, it is very
efficient as a magnetic electrode in various spin dependent devices in the field of spintronics.
In the aspect of the fundamental investigations for half-metallic materials, it has been
considered how to show the real evidence of the characteristic electronic structure. Attempts
have been made worldwide to observe the detailed electronic state of Heusler alloys byphotoelectron spectroscopy. Although a magnetic field must be applied in order to make asample into a single domain state, the photoelectron orbit is greatly affected by the magnetic
field. In our previous work, we have first studied the detailed electronic structure andmagnetic properties of a single crystal Mn2VAl by SX-RIXS measurements in magnetic field[4]. The RIXS is a bulk sensitive photon-in and photon-out spectroscopy, and very powerful
to investigate such as d-d excitations for open shell 3d orbitals and magnetic excitations forspin systems as well as 2p-3d transitions in element- and symmetry-specific ways. In thisstudy, we performed the RIXS experiments for single crystal of Co2MnSi Heusler alloy,
which was predicted to be a half-metallic ferromagnet.
Experimentsa) Sample preparation
A single crystal of Co2MnSi was prepared by the Bridgman method after preparing amaster alloy by arc melting in Ar gas atmosphere, and subjected to homogenizing heat
treatment at 1373 K. The sample composition identified by an electron probe microanalyzer is
Co:50.0, Mn:25.9, Si:24.1 at.%. Crystal orientation was checked by the Laue method and thespecimens were cut out in the stripe form with the length of 6 mm along the <100> with about
1 × 1 mm2 cross section. Magnetic properties were investigated with SQUID magnetometerand the spontaneous magnetization at 5 K was 142.2 emu/g (= 5.1 μB/f.u.), being comparable
to the literature [5].
b) RIXS experiment

RIXS experiments were performed using a high-resolution soft X-ray emission
spectrometer installed at the end of the BL07LSU of SPring-8. The specimen with the (001)plane was fractured in a glove box with Ar gas atmosphere, and then transferred into theanalyzer chamber without exposure to the air. The RIXS spectra for Co and Mn 2p core
excitation were measured with use of the right and left helicity circularly polarized light atroom temperature. A permanent magnet with the field of 0.25 T was installed in the chamber
and arranged as the X-ray incidence direction and the magnetic field direction become parallel[6].

Results
Figures indicate RIXS spectra obtained in parallel (μ+) and antiparallel (μ−) configurations
between the photon helicity and the magnetic field at incoming photon energy, hvin of 779.0
eV (left figure) and 640.5 eV (right figure) for the Co and Mn-2p3/2 edges, respectively. These
photon energies correspond to the peak positions of the Co and Mn-2p3/2 absorption spectra
for the Co2MnSi. Intensity peaks without any energy loss are the elastic component, and theother peaks in the larger energy loss region are the so-called fluorescence peaks. As shown in
the figures, clear fluorescence components were observed and their circular polarization
dependence (MCD = μ+ − μ−) was also confirmed. This RIXS-MCD signals originate fromthe spin-selective dipole-transition from mj = ± 3/2 which is separated by the Zeeman splitting
due to the effective magnetic field of 3d electron states [4]. It was found that the positive andnegative MCD fluorescence components in both Co and Mn have a large contribution offluorescence from mj = +3/2 and −3/2, respectively, suggesting that the valence band
electronic structures of Co2MnSi is highly spin-polarized.

References[1] R.A. de Groot, F.M. Mueller, P.G.van Engen, K.H.J. Buschow, Phys. Rev. Lett. 50 (1983)
2024-2027.
[2] J.J. Kübler, A.R. Williams, C.B. Sommers, Phys. Rev. B 28 (1983) 1745-1755.
[3] S. Ishida, S. Akazawa, Y. Kubo, J. Ishida, J. Phys. F 12 (1982) 1111-1122.
[4] R.Y. Umetsu, H. Fujiwara, K. Nagai, Y. Nakatani, M. Kawada, A. Sekiyama, F. Kuroda, H. Fujii,
T. Oguchi, Y. Harada, J. Miyawaki, S. Suga, Phys. Rev. B 99 (2019) 134414.
[5] R.Y. Umetsu, K. Kobayashi, A. Fujita, R. Kainuma, K. Ishida, Scripta Mater. 58 (2008) 723-726[6] J. Miyawaki, S. Suga, H. Fujiwara, H. Niwa, H. Kiuchi, Y. Harada, J. Synchro. Rad. 24 (2017)449-455.

Figs. RIXS spectra recorded for parallel (μ+) and antiparallel (μ−) configurations of thephoton helicity at Co-2p3/2 with incident photon energy, hvin of 779.0 eV (left figure)and at Mn-2p3/2 of 640.5 eV (right figure). Black lines indicate MCD (=μ+ − μ−).

MULTIORBITAL BOND FORMATION FOR STABLEOXYGEN-REDOX REACTION IN BATTERY ELECTRODES
Masashi Okubo
Department of Chemical System Engineering, School of Engineering, The University of Tokyo

The development of advanced electrochemical energy storage devices has been an active
research field because it offers sustainability prospects, for instance, reduction of fossil fuelreliance by realizing an electric vehicle range of 500 km per charge or integrating renewableenergy sources to an electrical grid. Market-leading lithium-ion batteries (LIBs) efficiently
store energy by lithium-ion (de)intercalation associated with redox reactions. However, theirenergy density approaches the theoretical limit, in part owing to the small capacity of thepositive electrode materials, which severely obstructs the wide deployment of LIBs.
Therefore, it is important to find an alternative battery chemistry that can exceed the existing
positive-electrode capacity limits.
The use of the extra redox reactions of oxygen in addition to conventional transition-metal
redox reactions is an attractive way to increase the capacity of transition-metal oxides.7 In
general, most oxygen 2p orbitals in conventional electrode materials LiMO2 (M: transition
metal) form σ-type bonds with axial M 3d orbitals (eg orbitals in an Oh symmetry). As theatomic energy level of oxygen 2p orbitals is usually lower than that of M eg orbitals, theσ-type bonding orbitals are predominantly from oxygen 2p orbitals. Therefore, it is difficult tooxidize oxygen 2p orbitals that have a σ-type bonding character. However, research groups
led by Bruce and Ceder postulated that oxygen in Li-excess transition-metal oxides
Li1+xM1−xO2 (Fig. 1a) intrinsically have localized (‘orphaned’) 2p orbital along the Li−O−Li
axis without the σ-type bonding character, which can contribute to an additional
oxygen-redox capacity. Indeed, large capacities exceeding the capacity limit of M redox
reactions have been reported for many Li-excess transition-metal oxides including
Li2MnO3-LiMO2 solid solution, layered honeycomb Li2IrO3, random rock salt
Li1.2Ti0.4Mn0.4O2, three-dimensional β-Li2IrO3, and anti-fluorite Li5FeO4. However, despite
the theoretical adequacy of the proposed ‘orphaned’ oxygen 2p orbital hypothesis, theexistence of the localized oxygen 2p orbitals in Li-excess transition-metal oxides has not beenverified
experimentally.
Furthermore,
although localized,
the oxygen 2p
orbital along the
Li−O−Li axis stillhasa
π-typeinteraction
withthe M t2g orbital,
which should play
an essential role inoxygen-redox
reactions. It is
importanttoFigure 1. Resonant inelastic X-ray scattering (RIXS) spectra for delocalized and localizedemphasize that the oxygen 2p orbitals. (a) Crystal structure of Li-excess layered transition-metal oxides
rigid-band model, Li[LixM1−x]O2 (M: transition metal). (b) π-type localized occupied/unoccupied states consisting*which
was of oxygen 2p and M t2g orbitals (b1/b1 bands in a C2v-symmetry OM2Li4 cluster). (c) σ-type
delocalized occupied/unoccupied states consisting of oxygen 2p and M eg orbitals (3a1/3a1* andfrequently used for 3b2/3b2* bands in a C2v-symmetry OM2Li4 cluster).

consideration of oxygen-redox electrodes, cannot qualitatively discuss M−O interaction (e.g.,
covalency), because it only considers a bandwidth and a band center energy of each band.
Therefore, for a thorough understanding of the reversible oxygen-redox reactions occurring in
Li-excess transition-metal oxides, it is necessary to clarify the nature of the π-type bonds
during the oxygen-redox reactions from the molecular orbital viewpoint. In this work, we usedoxygen K-edge resonant inelastic X-ray scattering (RIXS) spectroscopy combined with abinitio computations to demonstrate (σ + π) multiorbital bond formation as a firm criterion forstable oxygen-redox reactions.
Oxygen K-edge RIXS spectroscopy was used to examine the existence of π-type localized
oxygen 2p orbitals in Li2MnO3, a standard Li-excess layered transition-metal oxide. Oxygen
K-edge RIXS spectroscopy records the inelastic scattering during core excitation–relaxation(1s→2p→1s), in which the energy loss corresponds to valence excitation between occupied
and unoccupied 2p bands. RIXS spectroscopy has a probing depth of approximately 100 nm(bulk sensitive). Importantly, when occupied/unoccupied oxygen 2p bands have a localized
character with weak orbital hybridization, the inelastic scattering becomes Raman-like andhas constant energy loss regardless of the incident photon energy (Fig. 1b). Conversely,
delocalized occupied/unoccupied oxygen 2p bands with a large orbital hybridization gives
fluorescence-like inelastic scattering with variable energy loss depending on the incident
photon energy (Fig. 1c). Therefore, oxygen K-edge RIXS spectroscopy allows us to diagnose
localized or delocalized oxygen 2p bands.
The ab initio calculations using HSE06 hybrid functional for Li2MnO3 indicates that a
localized oxygen 2p orbital exists along the Li−O−Li axis with an orbital energy ofapproximately −0.9 to 0.0 eV versus the Fermi energy (Fig. 2a). The RIXS spectra and thecorresponding second differential RIXS map (Fig. 2b and 2c) exhibit two broad
fluorescence-like peaks, which could be related to the valence excitation between the σ-typewide oxygen 2p bands strongly hybridized with axial Mn eg orbitals (Fig. 1c). Based on theC2v symmetry of an OMn2Li4 cluster, these bands are labelled as 3a1/3b2 bands. More
importantly, the RIXS spectra show an intense Raman-like peak with constant energy loss ofapproximately 2.5 eV. This Raman-like peak clearly indicates the existence of the π-type
narrow oxygen 2p bands weakly hybridized with Mn t2g orbitals (b1/b1* bands based on the
C2v-symmetry labelling, Fig. 1b). It is important to note that the Raman-like feature is not thesimple projection of the d-d excitation of Mn L-edge, because O K-edge XAS and RIXS
involve charge-transfer processes through O-Mn interactions. In contrast, for a non-Li-excess
layered oxide LiNi1/3Co1/3Mn1/3O2, the RIXS spectra show only fluorescence-like peaks withvariable energy loss dependingon the incident photon energy
because most oxygen 2p orbitals
form σ-type wide bands withaxial M 3d orbitals. Therefore,
this is the first experimental
verification of the ‘orphaned’
oxygen 2p orbital hypothesis for
Li-excesstransition-metaloxides, and oxygen K-edge
RIXS spectroscopy is definitely
an effective diagnostic tool to Figure 2. Direct observation of orphaned oxygen 2p orbitals in Li MnO .
2
3
detect electrochemically active (a) Calculated partial density of states for oxygen and manganese in
Li2MnO3. The inset shows the spatial electron density at the energy range
oxygen states.
from −0.9 to 0 eV versus the Fermi energy. (b) Oxygen K-edge RIXS
spectra, and (c) second differential RIXS map for Li2MnO3 with excitation
photon energy from 527 to 533.75 eV.

MULTIORBITAL BOND FORMATION FOR STABLEOXYGEN-REDOX REACTION IN BATTERY ELECTRODES
Masashi Okubo
Department of Chemical System Engineering, School of Engineering, The University of Tokyo

High energy density batteries are a long-standing target for sustainability efforts, but theenergy density of the state-of-the-art lithium-ion batteries is limited in part owing to the small
capacity of positive electrode materials. Although the additional oxygen-redox reaction of
Li-excess transition-metal oxides is an attractive approach to increase the capacity, an
atomic-level understanding of the reaction mechanism has not been established. Here, usingbulk-sensitive resonant inelastic X-ray scattering (RIXS) spectroscopy combined with abinitio computations, we prove the existence of an orphaned oxygen 2p orbital that wastheoretically predicted to play a key role in oxygen-redox reactions.
As a target Li-excess layered transition metal oxide, Li1.2Ni0.13Co0.13Mn0.54O2 was used asan oxygen-redox electrode material. Before measuring the RIXS spectra, the ab initio
calculations using the HSE06 hybrid functional were carried out forLi1.2−xNi0.13Co0.13Mn0.54O2. The calculated electronic structures show that the averaged Badercharges of Ni and Co decrease only during the early stage of Li+ extraction (x < 0.4), whereasthat of oxygen continuously decreases during the whole delithiation process, suggesting theoccurrence of the additional oxygen-redox reactions for x > 0.4. Indeed,
Li1.2Ni0.13Co0.13Mn0.54O2 delivers a large capacity of approximately 250 mAh/gexceeding thetheoretical capacity (123 mAh/g) of the Ni4+/Ni2+ and Co4+/Co3+ redox reactions.
The oxygen K-edge X-ray absorption spectrum (XAS) in a bulk sensitive partial
fluorescence yield mode for the pristine Li1.2Ni0.13Co0.13Mn0.54O2 shows two absorption peaks(528.5 and 531 eV) of 1s core-electron excitation to unoccupied σ-type bands (oxygen 2p−Mneg↓/eg↑, 3a1*/3b2* in C2v) and an unoccupied π-type band (oxygen 2p−Mn t2g↓, b1* in C2v). Thisband picture agrees with the calculated partial density of states (pDOS) for Mn. Uponcharging to 4.4 V vs. Li/Li+, a new XAS shoulder emerged at 527 eV owing to hole
generation in σ-type (oxygen 2p−Ni eg) and π-type (oxygen 2p−Co t2g) bands. After oxygen
oxidation, a new absorption emerged at a relatively high absorption-energy region of
approximately 530.5 eV. Similar spectral changes were also observed during the second cycle.
The XAS signal at 530.5 eV can be ascribed to an unoccupied π-type band (oxygen 2p−Mnt2g↑) resulting from oxygen oxidation. The emergence of the absorption at a relatively highenergy region is reasonable because oxygen oxidation increases the effective nuclear charge,
which lowers the initial energy level of an oxygen 1s core and hence raises the excitationenergy18. Similar changes in the XAS signals by oxidation (the emergence of new absorption
at a high energy region) are often observed for transition-metal K-edge absorption spectra.
The RIXS spectra and the second differential RIXS map of pristine
Li1.2Ni0.13Co0.13Mn0.54O2 (Fig. 1) exhibit a Raman-like peak with constant energy loss
corresponding to a π-type narrow oxygen 2p band (π→π*), in addition to the fluorescence-likepeaks (σ→σ*). The ab initio calculations for pristine Li1.2Ni0.13Co0.13Mn0.54O2 supports theexistence of the localized oxygen 2p orbitals along the Li−O−Li axes with an orbital energy
near the Fermi level. The π→π* RIXS peak intensified after charging to 4.4 V vs. Li/Li+(transition-metal oxidation, Fig. 1b), which indicates accumulation of the localized oxygen 2p
states to the Fermi level. The initial Li+ extraction associated with Co and Ni oxidation gives
an oxygen coordination environment with a −O−Li+ or −O− axis (: Li+ vacancy),
which electrostatically raises the energy level of the localized oxygen 2p orbitals. Indeed, thefact that the energy loss of the Raman-like RIXS peak decreased from 2.5 to 2.0 eV supports
the rise in the energy level of the localized oxygen 2p orbitals. We presume that theaccumulation of the localized oxygen 2p states to the Fermi level should be an essential

preceding process to trigger the oxygen-redox reactions. For example, it is well known thatthe electrochemical properties of Li2MnO3 is poor even though oxygen has a Li+−O−Li+ axis
and hence a localized oxygen 2p orbital. Most likely, as Li2MnO3 cannot exhibit the initial Li+
extraction owing to the electrochemically inactive Mn4+, it is difficult to generate the
−O−Li+ and −O− axes, and the energy level of the localized oxygen 2p orbital remains
too low to undergo oxidation within the electrolyte stability window. Conversely, whenredox-active transition metals are present, such as in Li2MnO3-LiMO2 solid solution, the
−O−Li+ and −O− axes are easily formed by the initial Li+ extraction to raise the energy
level of the localized oxygen 2p orbital, unlocking the redox activity of oxygen. Once theoxygen-redox reaction is triggered, the −O−Li+ and −O− axes are continuously
generated to achieve a large oxygen-redox capacity.
After charging to 4.8 V vs. Li/Li+ (oxygen oxidation, Fig. 1c), the Raman-like π→π* RIXS
peaks disappear, and all the RIXS peaks become fluorescence-like. Importantly, a new intense
fluorescence-like peak emerges at an energy loss of approximately 7.0 eV with an incident
photon energy of 530.5 eV. Recalling that an incident photon of 530.5 eV excited 1s coreelectron to an unoccupied π-type band (oxygen 2p−Mn t2g↑), the inelastic scattering resonancewith the 530.5 eV incident photon includes the valence excitation between the π-type bands(π→π*). Therefore, a large energy loss of approximately 7 eV for this RIXS peak corresponds
to π-type splitting. This large π-type splitting should be induced by the hole generation in the
antibonding π-type band, and makes the Mn−O bonds more bonding (more delocalized).
Therefore, the RIXS peak is observed as fluorescence-like. The enhanced π-type interaction
stabilizes the oxidized oxygen, leading to reversible oxygen-redox reactions. Indeed, theRaman-like RIXS peak of the localized oxygen 2p orbitals was recovered after full discharge(Fig. 1d), and the changes in the RIXS spectra were reversible even during the subsequent
cycle.
To
summarize
thereaction mechanism, the
Li+−O−Li+ coordination in
the pristine state forms
localized oxygen 2p orbital
weakly hybridized with theMn t2g orbital. When theinitial Li+ extraction occurs
in association with M redox
reactions, the −O−Li+ and−O− coordination axes
are generated, which raises
the energy level of thelocalized oxygen 2p statesto the Fermi level. After thisaccumulation,oxygenoxidation
occurstodelocalize the hole within a
Mn−O−Mn
bond.
The
resultinglarge
b1/b1*
splittingstabilizes
the1. Monitoring oxygen 2p orbitals during oxygen-redox reaction ofoxidized oxygen, enabling a Figure
Li1.2Ni0.13Co0.13Mn0.54O2. Oxygen K-edge second differential RIXS map for thereversible
oxygen-redox samples (a) before charge, (b) after transition-metal oxidation, (c) after oxygen
oxidation, and (d) after discharge, with excitation photon energy from 527 toreaction.
533.75 eV. The schematic partial density of states (pDOS) for Mn 3d and oxygen
2p orbitals are shown for comparison.

UNDERSTANDING OF SOLVATION AND DESOLVATION STATE OFACTIVE MATERIALS FOR REDOX-FLOW BATTERIESDERIVED FROM HYDROGEN-BONDING ANALYSIS BY
SOFT X-RAY SPECTROSCOPY
Akihiro Ohira1, Eiji Hosono1,2,3, Daisuke Asakura1,3, Takashi Funaki1, Kosuke Yamazoe4, Jun
Miyawaki4,5, Yoshihisa Harada4,5
1

Research Institute for Energy Conservation, National Institute of Advanced Industrial Science andTechnology (AIST), 2Department of Energy and Environment, Global Zero Emission Research Center,
AIST, 3AIST-UTokyo Advanced Operando-Measurement Technology Open Innovation Laboratory(OPERANDO-OIL), 4Synchrotron Radiation Laboratory, The Institute for Solid State Physics, The
University of Tokyo 5Synchrotron Radiation Research Organization, The University of Tokyo

Power generation systems that use natural energy, such as sunlight and wind, and that do
not generate greenhouse gases have been actively introduced globally. Although they do not
generate carbon dioxide, compared to power sources like thermal and hydroelectric power,
these renewable energy sources provide an unstable power supply that is affected by various
situations. In order to use renewable energy efficiently and smoothly, the generated electricity
must be stored and discharged as required; thus, a storage battery (secondary battery) that canadjust the constantly fluctuating output over short and long cycles is significantly desired.
Large scale storage batteries are famous for lithium ion batteries, sodium-sulfur batteries, leadstorage batteries and redox flow batteries.[1-3] Lead storage batteries have been used asbatteries for automobiles and motorcycles for a long time and have abundant achievements as
small storage batteries. Lithium ion batteries also have many achievements as small batteries.
Sodium-sulfur batteries have an extremely high energy density and are widely used in load
leveling applications in substations due to their high charge and discharge efficiency.
Redox flow batteries (RFBs) have been successfully developed by NASA around 1974.[4]At the beginning of development, the Fe/Cr system was mainstream,[5] but it was not put into
practical use due to the disadvantage that mixing of the positive/negative electrolyte throughthe membrane leads to a decrease in capacity of the battery. In around 1984, Kazacos et al.
has developed a vanadium-based RFB (VRFB).[6] Since that time, the awareness of RFBs
have increased and research and development has been accelerating.
The significant issue in studying RFBs is to correctly evaluate the reactivity of electrodes
and active materials in addition to newly active material search and clarify material properties
necessary for improving battery performance.
In addition to the search for new active materials, it is important to precisely understand thereactivity of the electrodes and active materials. In particular, the state of solvation anddesolvation associated with the electrochemical reaction between active material and solvent
has not been well-understood at all, and a detailed understanding of the dynamics ofsolvation/desolvation at the interface between the active material and the electrode is required.
This will lead to a dramatic increase in solubility and redox activity, which will lead to therealization of high-performance RFBs. In this study, to obtain a guideline for the design of theinterface between the active material and the electrode, which is indispensable for therealization of a high-performance RFB, the hydration state of water-soluble active materials
was investigated by X-ray absorption and emission spectroscopy.
A dendrimer molecule with a viologen unit [7] dissolved in methanol was cast on the SiN
window and after drying the membrane, the cell was assembled and mounted in a chamber.
Water vapor was introduced while bubbling with nitrogen gas to control the relative humidity
in the cell. We observed changes in the O1s soft x-ray emission spectra of oxygen upon
hydration and dehydration of the dendrimer molecules.

Intensity (arb. units)

Figure 1 shows the change in
the O1s XES spectrum of water
XES hv=550eV
35% RHin dendrimer molecules during
1b1’ 1b1’’
25% RHthe hydration and dehydration
12%
RH
H
processes. With increasing
2O
Re 35% RH
humidity, emission spectra
Re 25% RH
similar to those of bulk water
Re 13% RH
were obtained. In addition, inthe spectrum of the dehydration
process (the change from wet todry state indicated by the signRe), 1b' is relatively small, andthe change is not reversible
when comparing the each
spectra of the hydration anddehydration
processes.
To
irreversible change in the
spectra, optical microscopy
observations of the cast films
before
and
after
the
measurements were performed.
515
520
525
530
Since the film disappeared in
Photon energy (eV)the area irradiated by X-rays, it
is inferred that the film was Figure 1 Change in the O1s XES spectrum of water
dissolved during the hydration in dendrimer molecules under humidity control.
process.
In the future, we plan to immobilize the molecules on the substrate to check the changes inhydration and dehydration.

REFERENCES[1] B. Zakeri. S. Syri, Renewable and Sustainable Energy Reviews, 42, 569 (2015).
[2] F. Shi, Reactor and Process Design in Sustainable Energy Technology, Elsevier
Amsterdam (2016).
[3] B. Dunn, H. Kamath, J.-M. Tarascon, Science, 334, 928 (2011).
[4] L. H. Theller, NASA TM X-71540, 1 (1974).
[5] R. F. Gahn, N. Hagedorn, NASA Tech Briefs February, 42 (1987).
[6] M. S. Kazacos, M. Rychcik, R. G. Robin, A. G. Fane, J. Electrochem. Soc., 133, 1057
(1986).
[7] A. Ohira, T. Funaki et al. ACS Appl. Energy Mater., 3, 4377 (2020)

PHOTOEXCITED CARRIER DYNAMICS IN LEAD SULFIDEQUANTUM DOT PHOTOVOLTAIC CELL
Kenichi Ozawa a,b, Susumu Yamamoto c, Iwao Matsuda da

Department of Chemistry, Tokyo Institute of Technology, b Institute of Materials Structure Science,High Energy Accelerator Research Organization, c Institute of Multidisciplinary Research for
Advanced Materials, Tohoku University, d The Institute for Solid State Physics, The University ofTokyo

INTRODUCTION
For realizing a sustainable society, uses of solar energy are essential. Photovoltaic (PV)devices are one of solutions to utilize the solar energy. It has been recognized that there is the
Shockley-Queisser limit, which is the theoretical maximum light-electricity conversion
efficiency of a PV cell using a single p/n junction. The limit is 34% at a band gap of 1.34 eV[1]. One of techniques to exceed this limit is tandemaization of multiple p/n junctions
composed of different materials to cover several photon energies. Another solution is the useof quantum dots (QDs) in a light-receiving layer. The QDs allow to absorb lights of various
wavelengths owing to the formation of quantum-well states. One of advantages of the QD PV
cells is that the absorption band can be controlled simply by changing the QD size so that the
production cost can be kept low in comparison with the multijunction PV cells.
Preceding studies on the QD PV cells have mainly focused on synthesis methods of various
kinds of QDs as well as their optical properties. On the other hand, accumulation ofknowledge of photoexcited carrier dynamics in the QD PV cells is still insufficient. In thepresent study, therefore, we shed the light on the dynamics of carriers generated in the QDs
by using time-resolved soft X-ray photoelectron spectroscopy (TRXPS). TRXPS has been
proved to be a powerful tool to clarify the carrier dynamics in model organic PV cells [2, 3].

RESULTS AND DISCUSSION
Fig. 1 shows S 2p and Pb 4f spectra of
the 3-nm PbS QD film. Four peaks areobserved in the S 2p spectrum; two ofthem originate from the spin-orbit splitting
S 2p levels of the S atoms in the PbS QD

Intensity (arb. units)

EXPERIMENTAL
The QDs employed in this work were lead sulfide (PbS) nanoparticles with average sizes of
3 and 9 nm (Quantum Solutions). PbS QD films were prepared under ambient conditions by
spin-coating the PbS QD solutions on 0.05 and 0.5 wt% Nb-doped rutile TiO2(110) substrates.
Since PbS and TiO2 are p-type and n-type semiconductors, respectively, a p/n junction isrealized at the PbS/TiO2 interface. The prepared QD films were treated with
3-mercaptopropionic acid (3-MPA) by dropping a 3-MPA solution onto the films and then byremoving the solution. This procedure allowed to replace a capping layer of the PbS QD fromoriginal oleic acid to 3-MPA.
The TRXPS measurements were carried out at BL07LSU of SPring-8 in the H operation
mode [4]. Visible (807 nm) and ultraviolet (404 nm) lasers with a repetition rate of 208 kHz
were used to generate the photoexcited
carriers in the PbS QD films, and the
S 2p
PbS
Pb 4f
excited states were monitored throughsynchrotron-radiation excited Pb 4f and S
2p peaks of PbS and Ti 2p peak of TiO2.
3−MPA
165

160

Binding Energy (eV)

145

140

135

Binding Energy (eV)FIGURE 1. S 2p and Pb 4f core-level peaks of the
3-nm PbS QD film on TiO2(110). The photon energy
was 370 eV.

Peak Shift (meV)Intensity (arb. units)

film, and the other two arise from 3-MPA.
(a)
(b)
150
The lower and higher binding-energy
S 2p
Pb 4f7/2
Pb 4f7/2
components are associated with PbS and100
3-MPA, respectively. This assignment isrationalized because (1) the PbS/3-MPA
50intensity ratio is higher in the film
pump off
pump on (800 nm)
composed of the 9-nm QDs than the 3-nm
0
164
162
160 139 138 137
1 10 100 1000
Binding Energy (eV) Binding Energy (eV)Delay Time (ns)QDs and (2) the low binding-energy
component is transiently shifted upon laser
pulse irradiation (this will be described FIGURE 2. (a) Comparison of the S 2p and Pb 4f7/2
peaks of 3-nm PbS QD with and without 800-nm
below). The Pb 4f7/2 and 4f5/2 peaks have a laser pulse irradiation. The photon energy was 370
tailing towards higher binding energies eV. The spectra with laser irradiation were acquired(Fig. 1). This suggest an existence of at a delay time of 1 ns. (b) Change in the magnitude
oxidation species such as PbO. Although of the Pb 4f7/2 peak shift as a function of a delay time.
the PbS QD films were prepared andtreated under ambient conditions, oxidation of the PbS QD is limited because of a relatively
low intensity of the tail structure.
As the PbS QD films are irradiated by either visible or ultraviolet laser, the PbS-originatedpeaks move to the higher binding energies. Fig. 2a compares the S 2p and Pb 4f7/2 spectra of
the 3-nm PbS QD film acquired with and without the visible laser (807 nm = 1.54 eV). Except
for the S 2p component of 3-MPA, all peaks move by ~100 meV upon laser irradiation. The
direction of the peak shift can be interpreted by a loss of an excited electron in the PbS QD by
charge transfer from PbS to TiO2.
Fig. 2b shows that the magnitude of the laser-induced shift of the PbS peaks diminisheswith a delay time. This suggests that electron-hole recombination should proceed with time if
the magnitude of the shift is proportional to the number of the PbS cations. It is apparent thatthe decay curve can be reproduced neither by a biexponential function nor by a triexponentialfunction. This is in contrast to decay curves obtained from model organic PV cells [2], where
the curves are reproduced by a biexponential function. A gradual decrease of the peak shift in
the present case implies that there should be several PbS cations with different lifetimes. A
certain distribution of the QD size, though the average size is 3 nm in the case of Fig. 2b, may
be responsible for a wide range of the lifetime.
A point to be noted is that the peak shift remains even after 4.8 μs, the longest delay time
examined in the present study. A long lifetime of the excited state of PbS, i.e., the cationicstate of PbS, is one of advantages for the efficient light-electricity conversion because there isan enough time for the free electrons and holes to be transported to the electrodes before they
are quenched via electron-hole recombination. Thus, the PbS QD/TiO2 system can be a goodcandidate for a PV cell with high efficiency.
ACKNOWLEDGEMENTThe TRXPS study was conducted using the facilities of the Synchrotron Radiation Research
Organization, The University of Tokyo (Proposal No. 2019B7460). A financial support from
MEXT, Japan (KAKENHI 16H06027) is appreciated.
REFERENCES[1] S. Rühle, Sol. Energy 130, 139−147 (2016).
[2] K. Ozawa et al., J. Phys. Chem. C 123, 4388−4395 (2019).
[3] F. Roth et al., Phys. Rev. B 99, 020303(R) (2019).
[4] S. Yamamoto et al., J. Synchrotron Radiat. 21, 352−365 (2014).

Photo-induced valence transition dynamics of SmS studied by time-resolved
x-ray absorption spectroscopy
H. WatanabeA, T. NakamuraA, Y. ShibataA, K. YamagamiB, Y. HirataC, K. IkedaB, Y. ZhangD,
H. WadatiD, K. ImuraE, H. S. SuzukiB, N. K. SatoE, S. KimuraAA
Osaka University, BISSP. The University of Tokyo, CNational Defense Academy,
D
University of Hyogo, ENagoya University
Samarium monosulfide (SmS) is known to show a phase transition (namely BG transition)from a black insulator phase to a valence-fluctuating golden metal phase by applying pressure
and/or elemental substitution (yttrium doping) [1]. Although the research has a long history forabout 50 years, the origin of the BG transition has not been revealed yet. One possibility of theorigin is due to an instability of excitonic states, namely excitonic instability [2]. If so, whenthe exciton states are produced by photo-irradiation, a phase transition like the BG transition is
supposed to occur. So far, we have reported the existence of a metastable state of yttrium-doped
SmS accompanied by high carrier density by photo-irradiation [3]. However, it has not been
revealed that the metastable state is related to the valence transition. In this study, to clarify thevalence change of Sm ions by photo-irradiation, we measured the temporal structure of the Sm
3d5/2 XAS after the photo-irradiation.
The time-resolved pump-probe XAS experiment was performed at BL07LSU of SPring-8
using the H mode (11/29-filling + 1-bunch). The Sm 3d5/2 core level absorption peak at about1080 eV was used to evaluate the ratio between Sm2+ and Sm3+ ions. A Ti:sapphire laser (pulse
width: 50 fs, photon energy: 1.55 eV, reputation rate: 1 kHz) synchronized with the synchrotron
radiation x-ray (pulse width: 50 ps) was used as the pump light.
Figure 1 shows the time evolution of the signal intensity of 1077 eV (Sm2+ 3d5/2 core level)
before and after the one-laser-pulse irradiation at 0 ps, but no significant change was observed
after the photo-irradiation. The same result was obtained at the Sm3+ 3d5/2 peak at 1080 eV.
These suggest that the spectral change due to the one-pulse irradiation was less than theexperimental accuracy of about 1 % estimated from the variation in strength before the pumppulse irradiation (t < 0 ps) as shown in Fig. 1. According to our time-resolved reflectancemeasurements, a 20-% change of the reflectivity was observed just after the photo-irradiation,
but the change was relaxed to less than 1 % within 1 ps. Therefore, no spectral change in theFig. 1. Time development of the peak intensity at
1077 eV before and after photo-irradiation at thetime of 0 ps.

Fig. 2. XAS spectra of the Sm 3d5/2 core level
before (red), just after (green) and 10 minutes
after (blue) the photo-irradiation (PI). The spectra
were normalized by the peak intensity at 1077 eV.

XAS result by one laser pulse isconsistent with our previous timeresolved reflectance measurement.
Next, the time evolution of the Sm
3d5/2 XAS spectrum after many lightpulses irradiation was measured. Figure
2 shows the Sm 3d5/2 core absorption
spectra before (red), immediately after(green) and 10 minutes after (blue) thephoto-irradiation for 1 hour at atemperature of 58 K. The absorption
shape obviously changes by timesuggesting the temporal change of themean valence. Also, combined with notemporal change by one pulse excitation, Fig. 3. Time evolution of the mean valence before andthe valence change by one pulse is too after one-hour laser pulse irradiation (PI). The meansmall to be observed and it is pilled up by valence is obtained by the ratio between the 1077-eV andmany pulses because it does not relax 1080-eV peaks.
completely before the next pulse after 1
ms.
Before the photo-irradiation, not only the peak at 1077 eV (mainly originating from a Sm2+
component) but also the peak at 1080 eV (mainly from Sm3+) was observed suggesting theintermediate valence of Sm ions. From Figure 2, combined with core level calculations, themean valence can be roughly evaluated from the ratio of peak intensity at 1077 eV and 1080eV as shown in Fig. 3. The mean valence before the photo-irradiation was evaluated as 2.40.
Just after the photo-irradiation shown by blue circles, the intensity of the Sm3+ peak increased
compared to that of Sm2+ suggesting the mean valence of Sm increased to 2.51 by many laser
pulses. After 10 minutes, the mean valence of Sm decreased to 2.47, but the mean valence does
not come back to the original value even after 1 hour. The origin of the remained higher valencecannot be explained by a very long relaxation time because surface oxidation also makes avalence change from Sm2+ to Sm3+. However, since the decrease of the mean valence after 10
min is the opposite change to the oxidation, it suggests the relaxation from Sm3+ to Sm2+ without
photo-irradiation. This implies that the photo-excited Sm3+ intermediate state decays to theSm2+ ground state by a very long relaxation time for several 10 min. The detailed relaxation
dynamics is reported in a separate report [4].
REFERENCES[1] K. Matsubayashi et al., J. Phys. Soc. Jpn. 76, 064601 (2007).
[2] T. Mizuno et al., J. Phys. Soc. Jpn. 77, 113704 (2008).
[3] R. Ikeda et al., J. Phys.: Conf. Ser. 1220, 012005 (2019).
[4] H. Watanabe et al., ACTIVITY REPORT OF SYNCHROTRON RADIATION LABORATORY 2019,
ISSP, The University of Tokyo.

Relaxation dynamics of Photo-induced valence transition of SmS
studied by time-resolved x-ray absorption spectroscopy
H. WatanabeA, T. NakamuraA, Y. ShibataA, K. YamagamiB, Y. HirataC, K. IkedaB, Y. ZhangD,
H. WadatiD, K. ImuraE, H. S. SuzukiB, N. K. SatoE, S. KimuraAA
Osaka University, BISSP. The University of Tokyo, CNational Defense Academy,
D
University of Hyogo, ENagoya University
Samarium monosulfide (SmS) is known to show a phase transition (BG transition) from a
black insulator phase to a valence-fluctuating golden metal phase by applying pressure and/or
elemental substitution (yttrium doping) [1]. We have reported the existence of a metastable stateof yttrium-doped SmS accompanied with high carrier density by photo-irradiation [2] and thevalence transition from Sm2+ to Sm3+ could be occurred by photo-irradiation at 58 K by thetime-resolved x-ray absorption spectroscopy (trXAS) [3]. In the trXAS, the valence change of
Sm could not be observed by one-pulse excitation, but after the many pulse irradiation, thevalence finally changes from 2.40 to 2.51. This suggests that the photo-excited metastable state
has a very longer life time than the interval of the pump pulse (1 ms) and it is piled up by themulti-pulse excitation, and finally, the photo-excited state could be observed. The origin of themetastable state is related to the pressure-induced BG transition and recently observed nonlinear conduction phenomena of black-SmS [4].
The origin of the metastable state, as well as the valence transition by the photo-irradiation,
has not been clarified yet. One possibility is the BCS-to-BEC transition of the photo-excited
exciton state, namely “excitonium” [5]. The excitonium is expected to have carriers, so theother experiment to detect the time-dependence of carrier density, band structure, and lattice
constants must be combined with trXAS. In this study, to evaluate the lifetime of the photoexcited state, we measured the relaxation dynamics after multi-pulse excitation.
The time-resolved pump-probe XAS experiment was performed at BL07LSU of SPring-8
using the H mode (11/29-filling + 1-bunch). The Sm 3d5/2 core level absorption peak at about1080 eV was used to evaluate the ratio between Sm2+ and Sm3+ ions. A Ti:sapphire laser (pulse
width: 50 fs, photon energy: 1.55
eV, reputation rate: 1 kHz)synchronized with the synchrotron
radiation x-ray (pulse width: 50 ps)
was used as the pump light.
Figure 1 shows the temporal
structure of the 1080-eV peak after
the 5-min (about 300,000 laser
pulse) laser irradiation. The valenceof the photo-excited state has been
evaluated as about 2.5 [3], which issmaller than that of the golden
phase (2.6-2.7) induced by pressure[6]. So, the consistency between thephoto-excited state and the golden
phase is still unclear but it isobviously different from that of the Fig. 1. Temporal structure of the 1080-eV peak at 52.7 K.
divalent black phase. The photo- The time dependence can be fitted by an exponential decay
excited state might be a new curve with a lifetime of 1400 s.

intermediate state. The decay curve can be fitted by an exponential relaxation with the a lifetimeof 1400 s (about 23 min), i.e., the photo-excited intermediate state has a very long time. To
clarify the origin of the photo-excited state, a further trXAS and other time-resolved
experiments are needed.
REFERENCES[1] K. Matsubayashi et al., J. Phys. Soc. Jpn. 76, 064601 (2007).
[2] R. Ikeda et al., J. Phys.: Conf. Ser. 1220, 012005 (2019).
[3] H.Watanabe et al., ACTIVITY REPORT OF SYNCHROTRON RADIATION LABORATORY 2019,
ISSP, The University of Tokyo.
[4] H. Ando et al., JPS Conf. Proc. 30, 011132 (2020).
[5] B. I. Halperin and T. M. Rice, Rev. Mod. Phys. 40, 755 (1968).
[6] K. Imura et al., JPS Conf. Proc. 30, 011131 (2020).

LOW-TEMPERATURE METHANE OXIDATIVE COUPLING ON
MODIFIED PD MODEL CATALYSTS STUDIED BY OPERANDOAMBIENT-PRESSURE XPS
Takanori Koitaya,† Susumu Yamamoto,‡ Iwao Matsuda,‡ Jun Yoshinobu‡ and Toshihiko
Yokoyama††

Department of Materials Molecular Science, Institute for Molecular Science‡

The Institute for Solid State Physics, The University of Tokyo

Direct observation of heterogeneous catalysts under operation condition is important forunderstanding reaction mechanism and nature of active sites on the catalysts. Various operando
methods have been developed to investigate surface chemistry on heterogeneous catalysts. We
have investigated reactions of inert molecules, such as carbon dioxide and methane, using an
ambient-pressure XPS (AP-XPS) system at SPring-8 soft X-ray beam line BL07LSU [1, 2].
The XPS measurements can be currently performed in the presence of reactant gases up to 2000
Pa using synchrotron radiation or a conventional X-ray source. In the gas environment, thesample can be heated up to around 900 K to investigate thermal reaction on the surface of
catalysts.
In this study, oxidation of methane on the Pd(110) surface was studied by AP-XPS and QMS.
Fig. 1 shows QMS spectra of reaction products and sample temperature of the Pd(110) surface
in the presence of 2.0 mbar oxygen and 5.6 mbar methane. As shown in Fig. 1, reactivity of thePd(110) sample was very low, when the sample was heating from room temperature to 615 K
in the presence of the feed gases. However, the amount of the products increased significantly
after heating to 780 K.
Chemical states of the surface during the reaction were investigated by operando AP-XPS.
Fig. 2 shows AP-XPS spectra measured at sample temperature of 615 K. Before heating to 780
K, the surface of the sample was oxidized, and PdO thin film was formed as indicated by Pd 3d
and O 1s spectra. In addition, peaks of adsorbed species were observed in O 1s and C 1s, which
can be attributed to oxidized carbon species (COx). Since the surface before heating to 780 K
is less reactive, the observed adsorbate is not an important reaction intermediate, but a spectatoror poisoning species. After the activation process, Pd 3d and O 1s spectra shows that the sample
was still (partially) oxidized. In O 1s and C 1s spectra, gas-phase peaks of the produced species(water and CO2) were observed, indicating that this surface is much more reactive. Note thatthe adsorbed COx was not observed after the activation process. This indicates that coverageof the intermediates is negligible during the methane oxidation reaction. Therefore, the ratelimiting step for the methane oxidation is dissociative adsorption of methane. Once theformation of CH3 species occurs, following reactions, including desorption of products, proceed
at much faster rates compared to the initial methane dissociation.

(II)

(I)

Figure 1. QMS spectra of oxidation products (water, CO, CO2) on the Pd(110) surface in thepresence of 2.0 mbar oxygen and 5.6 mbar methane. Sample temperature is also shown in the
figures.

(I)

(II)

Figure 2. AP-XPS spectra measured at region (I) and (II) indicated in Fig. 1. During themeasurement, the Pd(110) sample was kept at 615 K in the presence of 2.0 mbar oxygen and
5.6 mbar methane gases.

REFERENCES[1] Koitaya, T. et al., Top. Catal. 59, 526-531 (2016).
[2] Koitaya, T. et al., e-J. Surf. Sci. Nanotech. 17, 169-178 (2019).

X-ray Spectral Imaging Analysis of Interface States at p+-WSe2/n-MoS2
Hetero Junctions in TFET Device Structure
Naoka Nagamura1,2, Keigo Nakamura3, Hayami Kawamoto3, Naoki Higashitarumi3,
Keishi Akada4, and Kosuke Nagashio3
1

National Institute for Materials Science (NIMS), Tsukuba, Ibaraki, Japan.
PRESTO, Japan Science and Technology Agency, Honcho, Saitama, Japan.
3
Department of Materials Engineering, The University of Tokyo, Hongo, Tokyo, Japan.
4
Institute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba, Japan.
2

Reducing power consumption in semiconductor devices is an imminent challenge, but thepresent field effect transistors (FETs) require at least 60 mV of gate voltage to increase thecurrent by one order of magnitude at room temperature. Tunnel FETs can overcome thislimitation because of the band-to-band tunnelling (BTBT).
Two-dimensional materials are promising for TFETs. Recently, using p+-WSe2 source
doped by charge transfer from a WOx surface oxide layer, we succeeded in observing a clear
BTBT in the stable TFET system of p+-WSe2/n-MoS2[1].
In this study, we systematically study the local bandalignment in the p+-WSe2/MoS2 TFET device structure asshown in Fig. 1 by scanning photoelectron emission
microscopic(SPEM)
measurementsusingathree-dimensional nanoscale electron- spectroscopy
chemical analysis (3D nano-ESCA) system installed at
BL07LSU of SPring-8.
2D materials used in the sample are fabricated by
Fig. 1 Schematic image of a
exfoliation of bulk crystals. WOx side of WSe2 film is p+-WSe2/MoS2 TFET device
transferred onto h-BN on a SiO2/n+-Si substrate after structure
forming WSe2/WOx by O3 annealing. Then, thep+-WSe2/MoS2 heterostructures were fabricated via dry transfer method using PDMS under
the alignment system. Ni/Au was deposited as source/drain electrodes by EB lithography.
Figure 2a shows an intensity
mapping image for the Mo 3d 5/2
peak for the 3L-n-MoS2/p+-WSe2
heterostructure on h-BN. The
position of the heterostructure isclearly identified. Figure 2b
shows the pinpoint core-level
spectra for W 4f 5/2 and 4f 7/2
peaks recorded at points of (i)p+-WSe2/h-BN
and
(ii)
+
3L-n-MoS2/p -WSe2/h-BN. W 4f
peaks for (ii) p+-WSe2 in contact Fig. 2 (a) Photoelectron intensity mapping image for the Mo 3dwith 3L-n-MoS2 shifts to the 5/2 peak for the 3L-n-MoS2/p+-WSe2 heterostructure on h-BN.
higher binding energy side than (b) Pinpoint core-level spectra for W 4f 5/2 and 4f 7/2 peaks
that for (i) p+-WSe2 directly on recorded at points (i) and (ii).
h-BN. When p+-WSe2 is dopedwith electron, EF in p+-WSe2 increases, resulting in the higher binding energy of W 4f peaks.
Therefore, this data supports the p-doping reduction in WSe2[2]. The mapping of the EF
modulation in p+-WSe2 by the top gate is obtained by high-throughput peak fitting analysis
using machine learning techniques[3].

We also worked on chemical characterization of monolayer tin monosulfide (SnS), a
two-dimensional group IV monochalcogenides[4]. The special distribution of different phases,
SnS, Sn2S3, and SnS2, around the SnS single crystal nanosheet grown by the physical vapor
deposition on a mica substrate has been clarified[5].
REFERENCES[1] J. He et al. Adv. Electron. Mater. 4, 1800207 (2018).
[2] K. Nakamura et al., submitted.
[3] T. Matsumura et al. Sci. Technol. Adv. Mater. 20, 733 (2019).
[4] N. Higashitarumi et al. Nature Commun. 11, 2428 (2020).
[5] H. Kawamoto et al., submitted.

SOFT X-RAY ADSORPTION AND EMISSION SPECTROSCOPY OF
THE INTERMEDIATE WATER IN BIOCOMPATIBLE POLLYMERS
Shin-nosuke Nishimura1, Keishi Akada3, Daiki Murakami1,2, Kosuke Yamazoe3, Tomoya
Ueda2, Jun Miyawaki3, Yoshihisa Harada3, Masaru Tanaka1,2
1Institute of Materials Chemistry and Engineering, Kyushu University,
2
Graduate School of Engineering, Kyushu University,
3
Institute for Solid State Physics, The University of TokyoINTRODUCTION
Recently, significant efforts have been applied toward the design of polymer biomaterials
which have central roles in regenerative medicine and tissue engineering application. Water
definitely exists on an interface between material and biotissue, of which hydration stateessentially affects for biocompatibility [1]. Hydrated water around polymer, as represented
by poly(2-methoxyethyl acrylate) (PMEA), is classified into three types, non-freezing water(NFW), intermediate water (IW), free water (FW) in ascending order of the intensity ofinteraction with polymer chain [2]. Among them, the IW has a key role on the affinity ofpolymer with an organism. However, it has not been clarified what it is in a chemical
structure of polymer that works on formation of the IW. As the first step to systematize
relationship between the IW and polymer structure, we evaluated a hydration process on the
PMEA brush depending on humidity and succeeded to detect the IW by soft x-ray
absorption/emission spectroscopy (XAS/XES), whereas origin of the IW is still unclear [3, 4].
In this study, we designed and prepared the cyclic PMEA and linear poly(ethylene glycol)
monomethyl ether (mPEG) brushes to examine effect of a polymer end and polymer species,
respectively (Figure 1). The hydration process on these polymer brushes was evaluated by
XAS/XES measurement using the same method as our previous reports.
EXPERIMENTAL
We prepared the cyclic PMEA andlinear mPEG brushes on gold-coated
SiC substrate by grafting-to method
using sulfur-gold interaction (Figure 1).
Thiol-terminated PMEA at both end
wassynthesized
via
reversible
addition-fragmentation chain transfer(RAFT) polymerization and subsequentaminolysis.
The obtained polymer
was treated with iron(III) chloride
under high-diluted condition, resulting Figure 1. Chemical structure of cyclic PMEA and linear
in an objective cyclic PMEA. The PEG brushes used in this study. These polymer brushes
number average molecular weight (Mn) were prepared by grafting-to method.
and polydispersity index (Ð) of cyclic PMEA were estimated to be 76000 g mol-1 and 1.06
from size exclusion chromatography measurement, respectively. A commercially available
product was used for thiol-terminated mPEG (Mn = 10000 g mol-1, Ð = 1.13). The
gold-cated SiC substrates were soaked in methanol solutions of these polymers. The
substrates were washed with methanol and dried in vacuo to give objective polymer brushes.
Grafting density of the cyclic PMEA and linear was determined to be 0.048 and 0.11 chains
nm-2 by quartz crystal microbalance measurement, respectively.
XAS/XES measurement was performed at BL07LSU HORNET station in SPring-8. The
humid air with controlled humidity was flowed on the samples to change the water contents inthe brush samples

RESULTS AND DISCUSSION
In the XES spectrum of bulk liquid water, two significant peaks are basically observed at ca.
525.6 eV and 526.8 eV corresponding to ordered hydrogen bonds (tetrahedral coordination)and distorted hydrogen bonds, respectively [5]. Figure 2 shows XES spectra of the cyclic
PMEA and linear mPEG brushes under the humidity control. In the low humidity range
below a relative humidity of 65%, broad profiles peaked at the tetrahedrally-coordinatedcomponent dominates along with a sharp peak at ca. 527.1 eV. Above a relative humidity of
75%, increase of the peak at 526.5 eV was observed from the both spectra. We reported that
IW was formed on the linear PMEA brush at a relative humidity of 75% [4]. The hydration
behaviors obtained from this experiment were similar to that of our previous study, indicatingthat the IW existed on the cyclic PMEA and linear PEG brushes. Interestingly, formation of
the IW on the cyclic PMEA brush significantly increased compared with that on the linear
PMEA brush even though these polymer brushes had the same monomer unit and chemicalcomponent. This result suggested that the cyclic PMEA brush formed nano-scaled space
based on its loop structure and promoted adsorption of interfacial water by confinement effect.
There is a possibility that the cyclic PMEA brush shows higher biocompatibility than thelinear PMEA.
In this study, we successfully carried out to trace hydration process of the interfacial water
including IW onto the cyclic PMEA and linear mPEG brushes by XAS/XES measurementwith humidity control. State of polymer chain on the outermost of surface might has a great
effect on structure of the interfacial water. Although this challenging experiment make it
increasingly clear that the origin of IW and the relationship of chemical structure with IW
formation, the knowledge still is not enough to accomplish the ultimate design of biomaterial.
Further investigation will be performed in our future work in order to clarify the essential
feature of IW and the role on biocompatibility.

Figure 2. XES spectra of cyclic PMEA and linear mPEG brushes under the humidity control.

REFERENCES[1] T. Tsuruta, J. Biomater. Sci. Polym. Ed., 2010, 21, 1831-1848.
[2] M. Tanaka et al., Polym. Int., 2000, 49, 1709-1713.
[3] D. Murakami et al., Activity Report of Synchrotron Radiation Laboratory 2018, 2019, 68-69.
[4] D. Murakami et al., Activity Report of Synchrotron Radiation Laboratory 2018, 2019, 70-71.
[5] T. Tokushima et al., Chem. Phys. Lett., 2008, 460, 387-400.

DEVELOPMENT OF TIME-RESOLVED X-RAY PHOTOELECTRONDIFFRACTION FOR THE OBSERVATION OF THE STRUCTURAL
DYNAMICS OF EPITAXIAL SILICENE
Kouichi Hayashi1,2, Artoni Kevin R. Ang1, Koji Kimura1, Yuichiro Fukatsu1,
Susumu Yamamoto3, Iwao Matsuda3, Takahiro Yonezawa4, Antoine Fleurence4,
Yukiko Yamada-Takamura4
1

Department of Physical Science and Engineering, Nagoya Institute of Technology, Nagoya, Japan
Frontier Research Institute for Materials Science, Nagoya Institute of Technology, Nagoya, Japan
3
Institute of Solid State Physics, The University of Tokyo, Kashiwanoha, Chiba, Japan
4
School of Materials Science, Japan Advanced Institute of Science and Technology, Ishikawa, Japan
2

Introduction
Over the past half-century, photoelectron spectroscopy (PES) has become one of the mostimportant experiments in physics, chemistry and material science. Among the related techniques,
X-ray photoelectron diffraction (XPD) has grown into a powerful technique in studying surface
structures. Since the first reported XPD experiments by Siegbahn and Fadley in the 1970’s, XPD has
been used to investigate the structures of surface reconstructions, adsorbates, interfaces and thin films.
Through the years, PES has evolved significantly alongside the rapid developments in electron
spectrometers, synchrotron radiation facilities, laser light sources, and other related technologies.
Typical PES experiments can now resolve nm-resolution spatial information, time-resolved and evenspin-resolved information. Taking advantage of these developments, new dimensions of space-, timeand spin- can incorporated into XPD experiments.
In this work, we demonstrate, for the first time,
time-resolved X-ray photoelectron diffraction (trXPD) of an
epitaxial silicene layer using a time-resolved two-dimensional
angle-resolved time-of-flight (2DARTOF) system. The
evolution of the diffraction rings (DR) and the forward
focusing peaks (FFP) after laser irradiation can be used to
extract atomic resolution structural information about thesurface structure (Fig. 1). The 2DARTOF system in BL07LSU
is well suited for this experiment because of its high energy
and angular resolutions, and most importantly, its
Fig. 1. Schematic diagram of thesimultaneously recording of the photoemission intensity as atime-resolved XPD experiment.
function of θx, θy, and E.
Experiment and data processing
The experiments were performed using an epitaxial silicene sample grown on a ZrB2 thin film [1].
It has been shown that this epitaxial silicene layer has minimal electronic interactions with thesubstrate, and that the silicene layer buckled easily, resulting in a mixed sp2/sp3 hybridization that canbe tuned to control its electronic properties [2,3]. XPS studies have shown that the Si atoms occupy atleast 3 chemically different sites, and that the surface structure consists of a planar surface structurewith an adatom [3,4]. This sample was chosen because of its relatively simple surface structure, andthe current interest in silicene for next generation electronic device applications.
The sample was prepared in situ by the spontaneous segregation during the annealing of theZrB2/Si(111) substrate at 800°C [1]. LEED and the Si 2p core-level spectra confirms the formation ofa well-defined silicene layer.
The photoemission experiments were done using a photon energy of hν = 300 eV. The ARTOF
detector was set to simultaneously measure the photoemission intensity with an acceptance angle of±13°, within the energy range 97-103 eV. To obtain a larger section of the hologram pattern, thesample’s orientation was scanned along the [1120] direction. For the time-resolved experiments, a 1.5
eV, 208 kHz laser was used.

Compared to other electron detector
systems used in XPD, the 2DARTOF
system has a major advantage in that it cansimultaneously measure I(θx, θy, E) (Fig.
2a). To extract the angle-integrated Si 2p
spectra, the raw data is integrated along
both the θx and θy directions. To get the
XPD spectra, the raw data is integrated
within a small energy region centredaround the Si 2p binding energy (Fig. 2c).
However, this will contain XPD signal
from Si atoms at all sites. To separate the
XPD patterns from the Si atoms occupying
different sites, peak fitting of the spectra ateach (θx, θy) pixel is done [5].
The extracted XPD patterns are thennormalized by a background image Fig. 2. a) Schematic diagram of the raw data obtained by theobtained by integrating the raw 3D data 2DARTOF system, b) a typical Si 2p spectra from a (θx, θy)within a narrow energy region in the low pixel, and c) a raw 2D cut taken at the Si 2p binding energy.
binding energy side.
Results
Figure 3a shows the “planar” structuralmodel of epitaxial silicene showing the 3
distinct sites occupied by Si atoms. From thepeak fitting of the angle integrated Si 2p
spectra (Fig. 3b), the 2 main peak components
corresponding to the Si atom at the SiA and SiB
sites were resolved. The simulated andexperimental XPD patterns extracted for thesepeak components are shown in Fig. 3c) and3d), where the DR and FFPs in the simulation
are well resolved in the experiment.
Epitaxial silicene consists mostly of
in-plane scatterers, and thus the XPD patterns
are dominated by these DRs. In XPD patternof SiB, a strong FFP at θ~48° is observed,
which corresponds to the direction of the SiC
atom from the SiB emitter. This confirms thatthe 2DARTOF system can be a viable system
for XPD measurements.
Fig. 3. a) “Planar” model for epitaxial silicene, b) the Si
Conclusion
2p core level spectra measured from the sample, andIn this work, we have demonstrated for simulated and experimental XPD patterns from the c) Si A
the first time that the 2DARTOF system is and d) SiB component.
well-suited in XPD experiments. The
simultaneous measurement of I(θx, θy, E), makes acquisition of XPD patterns faster, opening up the
possibility of time-resolved XPD measurements. In our future work, we will obtain the trXPD patternsof epitaxial silicene in the pump-probe scheme.
REFERENCES[1] A. Fleurence, et al., Phys. Rev. Lett. 108, 245501 (2012).
[2] R. Friedlein and Y. Yamada-Takamura. J. Phys. Condens. Matter 27, 203201 (2015).
[3] C.C. Lee, et al., Phys. Rev. B, 90, 075422 (2014).
[4] R. Friedlein, et al., J. Chem. Phys. 140, 184704 (2014).
[5] K. Tsuitsui, et al., Nano Lett. 17, 7533 (2017).

TIME-RESOLVED X-RAY PHOTOELECTRON DIFFRACTIONSTUDY OF EPITAXIAL SILICENE
Kouichi Hayashi1,2, Artoni Kevin R. Ang1, Koji Kimura1, Yuichiro Fukatsu1,
Susumu Yamamoto3, Iwao Matsuda3, Takahiro Yonezawa4, Antoine Fleurence4,
Yukiko Yamada-Takamura4
1

Department of Physical Science and Engineering, Nagoya Institute of Technology, Nagoya, Japan
Frontier Research Institute for Materials Science, Nagoya Institute of Technology, Nagoya, Japan
3
Institute of Solid State Physics, The University of Tokyo, Kashiwanoha, Chiba, Japan
4
School of Materials Science, Japan Advanced Institute of Science and Technology, Ishikawa, Japan
2

Introduction
Silicene is a monolayer of Si atoms arranged in a honeycomb lattice that exhibits massless Diracfermions, similar to its carbon based counterpart in graphene. The development of silicene based
devices has seen significant interest recently because of the expected remarkable electronic properties,
stronger spin-orbit coupling of Si, and the ease of adapting such technologies into the already Si-based
manufacturing infrastructure. Until recently, silicene could only be synthesized on conductive
substrates, resulting in electronic interactions with the substrate. It was recently demonstrated thatsilicene with minimal substrate interactions can be grown on a ZrB2/Si(111) substrate through thesurface segregation of Si atoms[1]. This silicene layer was found to buckle easily, resulting in sp 2/sp3
hybridization that can tailored to tune its electronic properties [1,2]. Photoemission studies haveshown that the Si atoms occupy at least 3 chemically different sites on the silicene layer (Fig. 1a) [1,3].
In this study, we investigate the local structures of epitaxial silicene and the structural dynamics
induced by laser irradiation.
Experiment
The sample was prepared in situ by the spontaneous segregation during the annealing of theZrB2/Si(111) substrate at 800°C [1]. The ZrB2/Si(111) substrate was prepared elsewhere, and degassedin the prior to sample preparation. The silicene surface was confirmed by LEED and the core-level
spectra.
The photoemission experiments were done using a photon energy of hν = 300 eV. The ARTOF
detector was set to simultaneously measure the photoemission intensity with an acceptance angle of±13°, within the energy range 97-103 eV. To
obtain a larger section of the hologram
pattern, the sample’s orientation was scannedalong the [1120] direction. For thetime-resolved experiments, a 1.5 eV, 208 kHz
laser was used.
Results
The LEED and Si 2p spectra confirmed
the formation of a well-defined silicene layer.
From the peak fitting of the Si 2p spectra (Fig.
1b), the 2 main peak components of silicene
were resolved. These 2 peaks correspond to
the Si atoms occupying the SiA and SiB sites
in Fig. 1a [2,3]. The XPD patterns of these 2
peak components were extracted by peakfitting [5], and are shown in the righthemispheres of Fig. 1c and 1d. The
corresponding simulations are shown in theleft hemispheres. Some diffraction rings (DR)and forward focusing peaks (FFP) in the Fig. 1. a) “Planar” model for epitaxial silicene, b) the Si
simulations are well reproduced in the 2p core level spectra measured from the sample, andsimulated and experimental XPD patterns from the c) Si A
experiment.
and d) SiB component.

In this sample, most of the neighboring
scatterers are in-plane, thus the XPD patterns
are dominated by these DRs. Meanwhile, theXPD pattern of SiB shows a strong FFP at
θ~48°, which corresponds to the direction of
the Si atom at the SiC site from the emitter at
SiB.
In the time-resolved experiments, we
focused on this FFP at θ~48° to observe anychanges in the surface structure. At -100 ns,
the FFP is observed as a strong peak (marked
with the black circle in Fig. 2a). At later delay
times (Figs. 2b, 2c, 2d), the FFP is first seen
decreasing in peak intensity and shifting tolower θ (0.1 and 0.5 ns), before shifting tohigher θ and recovering its intensity (1.0 ns).
At 1.0 ns, the shift to higher polar angles
corresponds to an in-plane expansion of thesilicene layer, which could be explained bylaser-induced thermal expansion.
Meanwhile, at 0.1 and 0.5 ns, the decreasein FFP peak intensity suggests an intermediate
structure that occurs before the onset ofthermal equilibrium between the photoexcitedelectrons and phonons. This could be due tobond softening due to the increased
anti-bonding character of the excited charge
distribution. At higher laser fluence, it has
been reported that this can even result in bondbreaking and the formation of disordered state,a non-thermal melting of the surface [6,7].
Fig. 2. The time-resolved XPD patterns of the SiB
While the details of the structural changes
component taken at various delay times: a) -100ns, b) 0.1
observed in silicene is still not clear, this workns, c) 0.5 ns and d) 1.0 ns, and d) the line profiles taken
has demonstrated that time-resolved XPD
across the FFP.
experiments are now feasible and can be used
to probe the structural dynamics of surfaces. Analysis of the evolution of the diffraction rings andforward focusing peaks in the XPD patterns can provide element sensitive, atomic resolution structure
information about surfaces. Further experiments and first principles calculations are needed tocompletely understand the evolution of the epitaxial silicene structure after laser-induced heating.
REFERENCES[1] A. Fleurence, et al., Phys. Rev. Lett. 108, 245501 (2012).
[2] R. Friedlein and Y. Yamada-Takamura. J. Phys. Condens. Matter 27, 203201 (2015).
[3] C.C. Lee, et al., Phys. Rev. B, 90, 075422 (2014).
[4] R. Friedlein, et al., J. Chem. Phys. 140, 184704 (2014).
[5] K. Tsuitsui, et al., Nano Lett. 17, 7533 (2017).
[6] K.J. Gaffney, et al., Phys. Rev. Lett. 95, 125701 (2005).
[7] M. Harb et al. Phys. Rev. Lett., 100, 155504 (2008).

ELECTRIC STRUCTURE OF WATER IN THE VICINITY OF MODELORGANIC SURFACES BY X-RAY EMISSION SPECTROSCOPY
Tomohiro Hayashi
Department of Materials Science and Engineering, School of Materials and Chemical Technology
Tokyo Institute of Technology
JST-PRESTO

Introduction
Self-assembled
monolayers (SAMs)
have been employed
as model organic
surfaces since the1980s (Figure 1). Justby changing theterminal groups of themolecules, we caneasily control thephysicochemical
properties of thesurfaces. In a field of Figure 1 illustration of self-assembled monolayers (SAMs) ofbiomaterial
and alkanethiols on gold.
biosensing, SAMs areoften employed as scaffolding layers for protein and cells or passivating layer preventing nonspecific adsorption of non-target molecules. 1 Recently, water-mediated force has been foundto play an important role to govern the interaction of materials with protein molecules and cells.
2-5
However, the properties of the interfacial water, in particular, hydrogen bonding state andthe mechanism underlying the water-mediated force are still matters of intense debate. 1 In thiswork, we attempt to answer these questions by using X-ray emission spectroscopy (XES),
which provides a detailed picture of the hydrogen bonding state of water.

Experimental
In this work, we employed a measurement
setup shown in Fig. 2. SAMs were formed on
the backside of the window. In this work, weused 8 types of SAMs. They are hydrophobic(metyl-terminated:
C8),hydrophilic(hydroxy-terminated: OH), positively charged(trimethylamine-terminated:
TMA),
negatively charged (sulfuric acid-terminated:SA), and non-fouling [oligoethylene glycol:
EG and sulfobetaine: SB]. The humidity ischanged to vary the amount of water
condensed on the SAMs. The energy of theincident X-ray was fixed at 550 eV.

Figure 2 schematic illustration of themeasurement system田先生と の共同研究)
吸着水の厚さ : 8 n m
Results and discussion
We confirmed that the thickness of thewater strongly depended on the humidity
in the chamber. From the quartz crystal
microbalance (QCM), the thickness ofwater at relative humidity at 28 degree in
Celcius is about 0.5 to 1 nm.
Figure 3 shows XE spectra of water
adsorbed on the SAMs. For watermolecules in the vicinity of OH- and C8SAMs, the tetrahedral-coordinated water(ice-like water) is concentrated only near
the interface (with a film thickness of lessthan 0.5 nm) and steeply changes to thestructure of bulk water. In contrast, it was
found that water molecules with hydrogenbonding in the distorted tetrahedral
coordination was dominant for nonfouling SAMs (Fig. 3). Therefore, thedistorted tetrahedral-coordinated waterlayer is key to the anti-adhesion properties,
in agreement with our previous findings.

T. To k u sh im a , e t a l. C h em Ph ysREFERENCES
Lett 2 0 0 8 , 4 6 0 , 3 8 7 .

(1) Hayashi, T.; Hara, M., Nonfouling selfassembled monolayers: mechanisms
underlying protein and cell resistance.
Current Physical Chemistry 2011, 1, 90Figure 3 X-ray emission spectra of water
98.
(2) Chang, R.; Asatyas, S.; Lkhamsuren, adsorbed on SAMs.
G.; Hirohara, M.; Mondarte, E. A. Q.;
Suthiwanich, K.; Sekine, T.; Hayashi, T., Water near bioinert self-assembled monolayers. Polym.
J. (Tokyo, Jpn.) 2018, 50, 563-571.
(3) Kanayama, N.; Sekine, T.; Ozasa, K.; Kishi, S.; Nyu, T.; Hayashi, T.; Maeda, M., TerminalSpecific Interaction between Double-Stranded DNA Layers: Colloidal Dispersion Behavior andSurface Force. Langmuir 2016, 32, 13296-13304.
(4) Sekine, T.; Tanaka, Y.; Sato, C.; Tanaka, M.; Hayashi, T., Evaluation of Factors To
Determine Platelet Compatibility by Using Self-Assembled Monolayers with a Chemical
Gradient. Langmuir 2015, 31, 7100-5.
(5) Tanaka, M.; Hayashi, T.; Morita, S., The roles of water molecules at the biointerface of
medical polymers. Polym. J. (Tokyo, Jpn.) 2013, 45, 701-710.

基に大き く 依存する
面体水素結合状態から ひずんだ状態)の割合が低い。

ま っ て おり 、 よ り 詳細な スペク ト ルの門家と 連携し たい。

MEASUREMENT OF THE THICKNESS DEPENDENCE OF THE
MAGNET-OPTICAL CONSTANTS OF METAL FE USING FAST
POLARIZATION SWITCHING
Yasuyuki Hirata1, Keisuke Ikeda1, Kohei Yamagami1, Yujun Zhang2, Hiroki Wadati2,
Souliman El Moussaoui1, Iwao Matsuda1
1

Synchrotron Radiation Laboratory, Institute for Solid State Physics, University of Tokyo
2

Graduate School of Material Science, University of Hyogo

Magneto-optical effect is quite important not only for its use in devices such as
magneto-optical storage devices, but also as a probe for unraveling the spin and orbital statesof electrons in magnetic materials, therefore researchers of magnetism have measured
magneto-optical spectrum at various wavelengths. In particular, the measurement of resonant
X-ray magnetic circular dichroism (R-XMCD) and resonant X-ray magneto-optical Kerr
effect (R-XMOKE) using soft X-ray are powerful measurement methods that can selectively
observe the magnetic behavior of each 3d-transition-metal element, which is a main player ofmagnetism. However, despite the fact that the fine structure of the spectrum is important foranalysis, R-XMCD for materials with a small magnetic signal has a difficulty on improving
accuracy because it requires subtraction between two absorption/reflection spectra for theincident light with different polarizations. R-MOKE measurement is free from this problem,
but it has an experimental limitation that it takes a long time for measurement because it
requires an ellipsometry scan for each measurement point.
Taking advantage of the characteristics of the BL07LSU light source where high-speed
polarization switching is available [1,2], from the 2016A period we have performed the
R-MOKE measurement using the optical delay modulation method [3], and measured themagneto-optical constant of the Fe thin film (30 nm thickness) [4]. With this method, the realpart (corresponding to Kerr rotation angle) and the imaginary part (corresponding toellipticity) of the off-diagonal component of the permittivity can be simultaneously measured
independently in a short time, without Kramers-Kronig transformation analysis. Although theobtained permittivity of Fe thin film was generally in good agreement with the result of thefirst-principles calculation, a dip structure at Fe L3 absorption edge in the measured spectrum
could not be reproduced from the calculation. It is very important to distinguishexperimentally whether the dip structure is intrinsic in Fe metal, comes from interference withthe thin film [5], or has another origin. In this study, we performed an R-MOKE measurement
on Fe thin films with different film thickness by the optical delay modulation method using
polarization switching, and clarified the film thickness dependence of the magneto-optical
spectrum. The dip structure was hardly reproduced, which indicates it is not intrinsic in themagneto-optical spectrum of Fe metal. No systematic film thickness dependence wasobserved in the permittivity spectrum.
We prepared two Fe thin films with different thickness (30 nm thick and 15 nm thick) on
MgO substrate, and capped with Ru layer (5 nm thick) to prevent surface oxidization. The
reflection spectroscopy measurement with grazing geometry (incident angle of 80 degree) was
performed at Fe L3 and L2 edge under the in-plane magnetic field of 0.3 T applied bysuperconducting coil magnet. The polarization of the incident light was continuously
modulated between left circular polarization and right circular polarization with the frequencyof 13 Hz. The reflected light was analyzed with ellipsometer at both p-polarization ands-polarization geometry. The 13 Hz component of analyzed signal corresponded to ellipticity,and the 26 Hz component of analyzed signal corresponded to Kerr rotation angle.

Fig. 1: Kerr rotation angle, ellipticity, and off-diagonal permittivity  of Fe film with 30
nm thick (left panel) and Fe film with 15 nm thick (right panel). Solid line (dashed line)
is obtained from p-polarization (s-polarization) reflection measurement, respectively.

Figure 1 shows the measured spectra of Kerr rotation angle, ellipticity, and off-diagonal
permittivity of Fe film (30 nm thick) and Fe film (15 nm thick). Off-diagonal permittivity wasderived from Kerr rotation angle, ellipticity, and refraction index of Fe [6]. Under bothp-polarization and s-polarization reflection geometry, obtained off-diagonal permittivity
spectra show good agreement with theoretical calculation. We cannot find any systematic
dependency on thickness in the spectra of the 30 nm thick film and the 15 nm thick film, anda large dip structure observed in previous experiment has almost disappeared. Our resultclearly shows the dip structure is neither real magnetic signal of Fe nor originating from an
optical interference within the film. The capping layers of the samples are different between
previous measurement (Cu 2 nm thick and Ta 2 nm thick) and ours (Ru 5 nm thick), but it isless likely that the dip structure comes from the capping layer because Cu and Ta do not haveany optical structure at the energy around Fe L-edge. One possible origin of the dip structure
is surface oxidization of the sample which may be caused by x-ray radiation.
In conclusion, we have observed the thickness dependence of off-diagonal permittivity of Fe
thin film at Fe L-edge by the optical delay modulation method. No systematic dependency onthickness was found out, and the dip structure in spectrum observed in previous experiment
was shown not to be intrinsic.
REFERENCES[1] S. Yamamoto et al., J. Synchrotron Radiat. 21, 352 (2014).
[2] I. Matsuda et al., Nucl. Instrum. Methods Phys. Res., 767, 296 (2014).
[3] K. Sato, Jpn. J. Appl. Phys. 20, 2403 (1981).
[4] Y. Kubota et al., Phys. Rev. B 96, 214417 (2017).
[5] S. Valencia et al., Physica B 345, 189 (2004).
[6] H.-C. Mertins et al., J. Magn. Magn. Mater. 240, 451 (2002).

Creation of multi-dimensional operando photoelectron spectroscopy (I)Selection of industry-government-academia collaborative model sample
Hirokazu Fukidome1, Takamasa Kamogawa1, Naoka Nagamura2,3, Masaharu Oshima4,
Masato Kotsugi5, and Issei Watanabe6
1Research Institute of Electrical Communication, Tohoku University, Sendai, Miyagi, Japan.
2
National Institute for Materials Science (NIMS), Tsukuba, Ibaraki, Japan.
3
PRESTO, Japan Science and Technology Agency, Honcho, Saitama, Japan.
4
Institute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba, Japan.
5Tokyo University of Science, Katsushika, Tokyo, Japan.
6
National Institute of Information and Communications Technology (NICT), Koganei, Tokyo, Japan.

Virus and environmental issues
threaten the sustainability of mankind.
To balance the suppression of virus
infection and active socio-economic
activities,
weshoulddevelopinformation technologies enabling ahigh telepresence communications. For
thispurpose,thewireless
communication using THz frequencies
is needed, indicating 100 times
speeding up, compared to the current Fig. 1 A schematic view of the high telepresence
wireless communication, so-called 5G.
communication
Two-dimensional (2D) materials suchas graphene and ultrathin semiconductor devices
using AlGaN/GaN [1] and InGaAs heterointerfaces
is promsing for THz devices for high speed wirelesscommunication, 6G and more. This due to excellent
electronic properties, such as giant carrier mobility
and ultrahigh saturation velocity, and suppression of
so-called short-channel effects arising from theultrathinness of the channles. With the advance ofminiaturization and integration of devices, theimportance of interfaces further increases. Especially,
2D materials are sensitive to interface environment,so many environmental parameters drive theelectronic and chemical properties of 2D materials.
Moreover, better spatial and time resolution isrequiredfortheadvanceddevice analysis. Then, we attempt to realize
multi-dimensional
operando
photoelectron
spectroscopy using machine learning analysis
Fig. 2 Angle resolved Photoelectron
techniques in the viewpoint of Measurement Informatic.
At first, we performed depth profiling of III-V spectroscopy of Ga 3d peak for thecompound semiconductor heteroepitaxial layers by GaAs heteroepitaxial layers.
ARPES measurements. Maximum Entropy Method isapplied to enable sub nm order analysis (Fig. 2)[2].
In-plane 2D spectral mapping is also carried out for InP high electron mobility transistor(HEMT) device structures. Present in-plane spatial resolution of 3D nano-ESCA is ~70 nm,

which is deteriorated bymechanicalvibrations,
thermal drift, charging
effect, and so on. To
overcome the limit of spatial
resolution and make itpossible to observe objects
in the size of several nms,
we adopt sparse modelling
approach for the spectral
intensity mapping datasets.
Fig. 3(a) is a low-resolution Fig. 3 (a) Photoelectron intensity mapping image for the Au 4f peak forimage taken by coarse steps the InP-HEMT device structure. Low-resolution image by downin short measurement time. sampling. (b) Super-resolution image using sparce modeling.
Using
this
kind
of
down-sampling mappings, super-resolution image processing by sparse modelling [3] wasperformed as shown in Fig. 2(b). Effective spatial resolution seems to be improved by thismachine learning approach.
Our final goal is to achieve below 10 nm-resolution with the aid of machine learning, suchas super-resolution. Then, we will plan to execute industry-government-academia
collaboration that utilize operando 3D nano-ESCA to develop novel next-generation
communication devices necessary for balancing the suppression of virus infection and active
socio-economic activities.

REFERENCES[1] H. Fukidome et al. Sci. Rep.8, 13268 (2018).
[2] K. Horiba et al. Rev. Sci. Instrum. 82, 113701 (2011).
[3] T. Kato et al. Neurocomputing 240, 115 (2017).

OPERAND MEASUREMENT OF ALL SOLID-STATE LI-ION
BATTERYS BY PHOTOELECTRON MICROSPECTROSCOPY
Eiji Hosonoa,b,c, Wenxiong Zhangd, Daisuke Asakurab,c, Naoka Nagamurae,
Masaharu Oshimaf, Yoshihisa Haradac,d, faNational Institute of Advanced Industrial Science and Technology (AIST), Global Zero
Emission Research Center, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japanb
National Institute of Advanced Industrial Science and Technology (AIST), Research Institutefor Energy Conservation, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japanc
AIST-UTokyo Advanced Operando-Measurement Technology Open Innovation Laboratory,
5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8565, Japand
Institute for Solid State Physics (ISSP), The University of Tokyo,
5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8581, Japane
National Institute for Materials Science, Research Center for Advanced Measurement andCharacterization, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japanf
Synchrotron Radiation Research Organization, The University of Tokyo, 7-3-1
Hongo, Bunkyo-ku, Tokyo, 113-8656, Japan

Development of high-performance lithium-ion battery (LIB) for stationary type and vehicle isan important research topic. The total development of LIB is very difficult, because
performances such as energy density, power performance, cycle performance, and safety
property are in the trade-off relationships. Thus, comprehensive researches and developments
are highly important to development of LIB. Detailed understanding of the redox reactions by
Li-insertion/extraction, structural change, electronic state change for the materials in LIBs is
important to develop innovative materials for LIBs. In these days, cutting-edge analyses
methods are necessary in addition to the innovative materials development.

Figure 1 XPS images of LiCoO2

To truly understand LIBs, synchrotron radiation soft X-ray spectroscopy is a usefultechnique with elemental selectivity, valence selectivity and orbital selectivity. We have been
studying the charge-discharge mechanisms of LIBs based on the electronic-structure analysis
by soft X-ray spectroscopy. In addition, operando measurement is very important because
LIB is a high voltage device. Even deconstruction of LIB in the glovebox filled by inert Argas, the potential control of active material is difficult. Operando measurement system in soft
X-ray beamline is our core-technique. We reported the operando soft x-ray emission
spectroscopy under the charge-discharge operation [1,2].
Furthermore, microspectroscopy is another important point because the redox reaction bycharge-discharge reaction influenced by the position like a facet of an electrode material andthe interfaces between the electrode materials and electrolytes. Spectroscopy with spatial
resolution is of particular importance.
We reported operando photoemission spectromicroscopy of single crystalline Li4Ti5O12 (ananode material) [2] and LiCoO2 (an cathode materials) [3] by 3DnanoESCA [4] withall-solid-state LIB. The soft X-ray photoemission spectromicroscopy experiments have been
conducted at BL07LSU of SPring-8.
Here, we measured single crystalline LiCoO2 cathode material with clear facet. Figure 1
shows images of XPS mapping image of single crystalline LiCoO2. From the clear image ofeach facet, resonant XPS was conducted.
ACKNOWLEDGEMENT
This work was conducted under the international joint research program for innovative energy
technology by METI, Japan.
REFERENCES[1] D. Asakura, E. Hosono, H. Niwa, H. Kiuchi, J. Miyawaki, Y. Nanba, M. Okubo, H.
Matsuda, H.S. Zhou, M. Oshima, and Y. Harada, “Operando Soft X-ray Emission
Spectroscopy of LiMn2O4 Thin Film Involving Li-ion Extraction/insertion Reaction”,
Electrochem. Commun., 50, 93 (2015).
[2] D. Asakura, Y. Nanba, M. Okubo, H. Niwa, H. Kiuchi, J.Miyawaki, M. Oshima, E.
Hosono, Y. Harada, “Operando soft X-ray emission spectroscopy of the Fe2O3 anode toobserve the conversion reaction”, Physical Chemistry Chemical Physics, 21, 26351 (2019).
[3] K. Akada, T. Sudayama, D. Asakura, H. Kitaura, N. Nagamura, K. Horiba, M. Oshima, E.
Hosono, Y. Harada, “Operando measurement of single crystalline Li4Ti5O12 withoctahedral-like morphology by microscopic X-ray photoelectron spectroscopy”, J. Electron
Spectrosc. Relat. Phenom., 233, 64 (2019).
[4] K. Akada, T. Sudayama, D. Asakura, H. Kitaura, N. Nagamura, K. Horiba, M. Oshima, E.
Hosono, Y. Harada, “Microscopic photoelectron analysis of single crystalline LiCoO2
particles during the charge-discharge in an all solid-state lithium ion battery”, Scientific
Reports, 9, 12452 (2019).
[5] K. Horiba, Y. Nakamura, N. Nagamura, S. Toyoda, H. Kumigashira, M. Oshima, K.
Amemiya, Y. Senba, H. Ohashi, Scanning photoelectron microscope for nanoscale
three-dimensional spatial-resolved electron spectroscopy for chemical analysis. Rev. Sci.
Instrum. 82, 113701 (2011).

MAPPING MEASURMENT OF SINGLE CRYSALLINE LITHIUMMETAL OXIDE BY ALL SOLID-STATE LI-ION BATTERYS
Eiji Hosonoa,b,c, Wenxiong Zhangd, Daisuke Asakurab,c, Naoka Nagamurae,
Masaharu Oshimaf, Yoshihisa Haradac,d, faNational Institute of Advanced Industrial Science and Technology (AIST), Global Zero
Emission Research Center, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japanb
National Institute of Advanced Industrial Science and Technology (AIST), Research Institutefor Energy Conservation, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japanc
AIST-UTokyo Advanced Operando-Measurement Technology Open Innovation Laboratory,
5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8565, Japand
Institute for Solid State Physics (ISSP), The University of Tokyo,
5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8581, Japane
National Institute for Materials Science, Research Center for Advanced Measurement andCharacterization, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japanf
Synchrotron Radiation Research Organization, The University of Tokyo, 7-3-1
Hongo, Bunkyo-ku, Tokyo, 113-8656, Japan

Many researchers have studied development of clean energy devices for the realization of alow-carbon society. There is a great need to develop innovative clean energy devices. In
particular, high performance LIBs are important not only for electric vehicles, but also for
power source of next-generation semiconductor devices for "Beyond 5G", which will enablehigh-speed, high-capacity data communication in the IoT society.
In response to these high societal needs, LIBs with higher performance than the current state
are required. In recent years, research and development of all-solid-state LIBs have been
focused for innovative improvement of the performances of LIBs. It is necessary to establish
design guidelines based on fundamental science because a lot of technical issues remainunresolved in the all-solid-state LIBs. Thus, understanding the charge-discharge mechanismof LIBs is important to improve their properties.
In addition, operando measurement under the control of potential is important for detail andprecise understanding because LIBs are high voltage device. Even deconstruction of LIB in
the glovebox filled by inert Ar gas, the potential maintaining of active material is difficult.
Recently Akada and Hosono et al. have successfully introduced operando soft X-ray
photoelectron spectroscopy as a novel method to understand the LIBs during
Li-intercalation/deintercalation process in LIB cathode and anode materials [1, 2]. These areresults of photoelectron microscopy with spatially resolved information by 3D nanoESCA
station at BL07LSU of SPring-8 [3]. Microspectroscopy is another important point in addition
to operando measurement because the redox reaction by charge-discharge reaction affected by
the position like a facet of an electrode material and the interfaces between the electrodematerials and solid-state electrolytes. Spectroscopy with spatial resolution is important
techniques.
Figure 1 shows the intensity mapping of Ni 3p and Mn 3s of LixNi0.5Mn1.5O4. Here, lithium
ratio is controlled by the cell using liquid electrolyte. We obtained intensity mapping. The
next step is operando measurement by all-solid-state LIBs. We will create oprerando
measurement system and conduct XPS mapping measurement.

Ni3p

Ni3p

Mn3s

Mn3s

Figure 1 Intensity mapping of Ni 3p and Mn 3s of LixNi0.5Mn1.5O4.

ACKNOWLEDGEMENT
This work is partially conducted on the basis of the International joint research program forinnovative energy technology by Ministry of Economy, Trade and Industry, Japan.
REFERENCES[1] K. Akada, T. Sudayama, D. Asakura, H. Kitaura, N. Nagamura, K. Horiba, M. Oshima, E.
Hosono, Y. Harada, “Operando measurement of single crystalline Li4Ti5O12 withoctahedral-like morphology by microscopic X-ray photoelectron spectroscopy”, J. Electron
Spectrosc. Relat. Phenom., 233, 64 (2019).
[2] K. Akada, T. Sudayama, D. Asakura, H. Kitaura, N. Nagamura, K. Horiba, M. Oshima, E.
Hosono, Y. Harada, “Microscopic photoelectron analysis of single crystalline LiCoO2
particles during the charge-discharge in an all solid-state lithium ion battery”, Scientific
Reports, 9, 12452 (2019).
[3] K. Horiba, Y. Nakamura, N. Nagamura, S. Toyoda, H. Kumigashira, M. Oshima, K.
Amemiya, Y. Senba, H. Ohashi, Scanning photoelectron microscope for nanoscale
three-dimensional spatial-resolved electron spectroscopy for chemical analysis. Rev. Sci.
Instrum. 82, 113701 (2011).

Resonant inelastic soft x-ray diffraction of nanoparticles
J. Miyawaki1, T. Yachi2, S. Maki2, K. Kanie2 and Y. Harada1
1

2

Institute for Solid State Physics, The University of Tokyo
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University

The excitation energy dependence of soft X-ray emission spectra (XES) was first measured
for TiN in 1988 [1]. Four years later, Ma et al. proposed that the excitation energy
dependence could be interpreted by the theory of resonant inelastic X-ray scattering (RIXS),
and band dispersions of electronic state were observed [2]. This indicates that the absorptionand emission-processes are described by a single scattering theory, and the momentum isconserved during that process. In other words, when a core electron with momentum k is
excited to the conduction band and an electron at the valence band is decayed to the core hole,the emissions from the different cores could interfere with each other, and Bragg-like
diffraction results in the emissions of the same momentum k. These facts suggest that a moregeneral Bragg’s law, qin = qout + kh − ke + G is valid, in addition to the energy conservation
law in RIXS, hνin = hνout + Eh – Ee, where hνin (qin) and hνout (qout) are the energy (momentum)of the incident and scattered X-ray, Eh (kh) and Ee (ke) are the energy (momentum) of the holein the valence band and the electron in the conduction band at the final state, and G is thereciprocal vector of the crystal. When the excitation and scattered energies are the same, themomentum conservation law gives qin = qout + G, which is widely accepted as Brass’s law.
The more general Bragg’s law qin = qout + kh − ke + G (the momentum conservation law in
RIXS) has been validated by the RIXS studies of the band dispersions in SiC [3], graphite,diamond [4], Si [5], and LiBC [6]. Recent observations of the dispersions of orbitons,
magnons, and phonons also confirmed the validity of the principle [7].
In the above discussion, the Bragg diffraction and RIXS were treated as independent
processes. However, if the Bragg diffraction and RIXS occur simultaneously, the intensity of
RIXS is expected to increase assisted by the Bragg diffraction. We named this method
resonant inelastic X-ray diffraction (RIXD). In the case of nanoparticles, the momentum
dispersion of the elementary excitations is small, and the diffraction from the nanoparticles,that is, small angle X-ray scattering (SAXS), is expected to increase the intensity of RIXS.
Therefore, we measured soft X-ray SAXS of FePt nanoparticles by a RIXS spectrometer anddiscussed the feasibility of RIXD by comparing the SAXS profile obtained by the elastic
scattering and the momentum dependence of RIXS.
The measurement of SAXS and RIXD were conducted at HORNET end-station ofSPring-8 BL07LUS using θ-2θ scan system. In this scan system, RIXS spectra can bemeasured at an arbitrary scattering angle 2θ in the range between 45° and 135°, and SAXS
and RIXD profiles can obtained from momentum dependence of the intensity of the elastic
and inelastic peaks, respectively. The measured sample was FePt nanoparticles. The average
particle size and standard deviation were 2.7 nm and 0.37 nm, respectively.
Figure 1 shows the RIXS spectra of the FePt nanoparticle measured at 708.1 eV,
which is the peak top of Fe L3-edge X-ray absorption spectra. A SAXS profile was obtainedby integrating the intensity of the elastic peak (Fig. 2). A peak was observed in the SAXS
profile at around θ=31°. In the hard X-ray SAXS, the SAXS profile is observed at small
angles (typically 0.1–10°), but in the case of soft X-ray SAXS, SAXS profile is observed atlarger angle, since the wave length of the soft X-ray is longer than that of hard X-ray. The
obtained profile is well reproduced by a simulation with the particle size of 2.73 nm andstandard deviation of 0.37 nm, and thus the obtained q-dependence of the elastic peak was
confirmed to be SAXS. Furthermore, the momentum dependence of the inelastic scattering
indicated by a1, a2, a3, and a123 in Fig. 1 was calculated, and the increase of the intensity ofa3 peak was found at the same scattering angle of the SAXS peak, indicating the observationof RIXD. In the future, we will measure the particle size and incident energy dependences ofvarious samples to further establish RIXD and elucidate its mechanism.

Fig. 1 RIXS spectra of FePt nanoparticle measured at 708.1 eV and 2θ=66°.

Fig. 2 The momentum dependence of the elastic peak intensity in RIXS spectra of FePt nanoparticle.
Red circle and blue line correspond to the experimental result and simulation, respectively.

REFERENCES[1] J. E. Rubensson, et al., Phys. Rev. Lett. 60, 1759 (1988).
[2] Y. Ma, et al., Phys. Rev. Lett. 69, 2598 (1992).
[3] J. Luning, et al., Phys. Rev. B 56, 13147 (1997).
[4] A. V. Sokolov, et al., J. Phys. Condes. Matter 15, 2081 (2003).
[5] A. V. Sokolov, et al., J. Electron Spectrosc. 137, 591 (2004).
[6] P. F. Karimov, et al., J. Phys. Condens. Matter 16, 5137 (2004).
[7] L. J. P. Ament, et al., Rev. Mod. Phys. 83, 705 (2011).

Development of X-ray Polarization Precession Spectroscopy
J. Miyawaki, Y. Hirata†, K. Yamagami‡, I. Matsuda, and Y. Harada
Institute for Solid State Physics, The University of Tokyo
Magnonics is a next-generation technology, that utilizes a collective excitation of electron
spin (magnon or spin wave), and is an alternative technology to electronics, which utilizes
electrons for information transfer and computation. Compared with electron- and spin-based
electronics and spintronics, magnonics does not suffer from Joule heat. Magnon has longer
mean free path, which is advantageous for long-distance information transfer. Magnon has awave nature, which offers a possibility of quantum computation. Thus, researchers are trying
to use high-frequency terahertz magnons to speed up magnonic devices and observe magnons
injected to magnonics materials to understand the operating mechanism of devices [1–6].
For the magnon observation, ferromagnetic resonance (FMR), Brillouin lightscattering (BLS), inelastic neutron scattering (INS), spin-polarized electron energy loss
spectroscopy (SPEELS), and time-resolved spectroscopy have mainly been used. FMR andBLS observe magnons only at q≈0 nm−1 and cannot observe propagating magnons at
q≠0 nm−1. INS and SPEELS can observe at larger q, but INS cannot observe higher energy
magnons and require large sample volume, and SPEELS is surface sensitive. Therefore, INS
and SPEELS is not suitable for operando experiments. Thus, a new technique to observe
propagating magnons in operando condition has been explored. In this study, we developed
X-ray polarization precession spectroscopy (XPPS), which is a new method to observe
dispersion of magnons by X-ray with high energy resolution to understand the properties ofmagnon in devices.
XPPS was proposed as a new technique to measure the frequency of spin waves byobserving a rotational/angular Doppler effect in the polarization of scattered X-ray induced byspin waves in the sample via magnetic linear dichroism [7]. Since magnetic linear dichroismis employed in XPPS, resonant condition is required to measure XPPS signal. The
measurement principle and experimental setup are shown in Fig. 1(a). The scattered X-ray isirradiated to a multilayer polarizer, the intensity of the reflected X-ray is spatially resolved by
CCD, and the period of the rotation of the polarization can be detected. The energy resolutionof XPPS reaches 50 GHz (0.2 meV), which is very high energy resolution for X-ray based
spectroscopy to obtain the energy of magnons. Thus, XPPS has a promising technique for
operando experiment of magnonic devices.
XPPS experiments were conducted at free-port end-station of SPring-8 BL07LSU
using superconducting magnet chamber equipped with a polarization analyser (Fig. 2). The
linear vertical polarization was used for the incident X-ray, and the multilayer polarizer of thepolarization analyser was positioned orthogonal to the electronic vector of the incident X-ray.
The position dependence of the reflected X-ray intensity was measured by CCD.
Figure 3 shows a XPPS result of Y3Fe5O12 (YIG) and Pt. 705.0 eV and 709.6 eV
correspond pre-edge and the peak-top of Fe L3-edge X-ray absorption spectra of YIG. YIG
showed the difference between pre- and post-edge of Fe L3-edge, while Pt, which is
non-magnetic, showed no difference. In YIG, the difference spectra showed the intensity
modulation with a period of ~100 μm. This period corresponds the magnon energy of~5.5 meV, which matches the energy obtained by inelastic neutron scatterings [8]. Thus, we
succeeded in detecting XPPS signal and extracting the energy of magnon.
†Present Address: Department of Applied Physics, National Defense Academy‡Present

Address: Quantum Materials Science Unit, Okinawa Institute of Science and Technology
Graduate University

(b)

(a)

FIG. 1. (a) The measurement principle and (b)experimental setup of XPPS. Linearly polarized
X-ray of a resonant energy is irradiated to asample that possesses a spin wave of frequency
Ω, and the precessional motion of frequency 2Ω
is induced to the polarization vector of thescattered X-ray. The multilayer polarizer reflects
the vertically polarized component of thescattered X-ray, and thus spatially resolved
reflection intensity gives the intensity modulation
due to the precession in the scattered X-ray.

FIG. 2. XPPS of Pt and YIG at Fe L3-edge.
Spatially resolved reflection intensity of thescattered X-ray from the multilayer polarizer
was measured at pre- (red) and post-edge(blue) of Fe L3-edge. Since the intensity
profiles by the pre-edge excitation has noresonant effect, the signal can be regarded as a
background affected by the surface of thesample and the polarizer.

REFERENCES[1] S. Neusser et al., Adv. Mater. 21, 2927 (2009).
[2] Y. Kajiwara et al., Nature 464, 262 (2010).
[3] V. V. Kruglyak et al., J. Phys. D: Appl. Phys. 43, 264001 (2010).
[4] A. V. Chumak et al., Nat. Phys. 11, 453 (2015).
[5] H. Yu et al., Nat. Comm. 7, 11255 (2016).
[6] C. Du et al., Science 357, 195 (2017).
[7] R. Röhlsberger, Phys. Rev. Lett. 112, 117205 (2014).
[8] J. S. Plant, J. Phys. C: solid State Phys. 10 4805 (1977).

Resonant Inelastic Soft X-ray Scattering of Y3Fe5O12
J. Miyawaki and Y. Harada
Institute for Solid State Physics, The University of Tokyo

Further development of the information society requires innovation in information technology,
and faster, compact, and low energy-consumption devices have been pursued. One of the next
generation technologies to realize this goal is magnonics, which utilizes magnons (spinwaves), that is, a collective excitation of spins instead of electrons for the transmission andprocessing of information. Since no electronic current is induced when the magnon
propagates through the magnetic material, there is no energy dissipation due to Joule heating.
There is a very wide frequency of magnons known, and if higher frequency magnons can beused, ultra-fast devices could be created. Currently, fundamental research is in progress torealize actual devices. Higher-frequency terahertz magnons have been investigated to speed
up the devices, and the magnons injected in the magnonic materials has been investigated tounderstand the behaviour of the magnons in the device and efficiently utilize magnons.
Magnons have been observed by using ferromagnetic resonance, Brillouin light scattering,inelastic neutron scattering, spin-polarized electron energy loss spectroscopy, andtime-resolved spectroscopy. However, these experimental methods are not suitable forobserving high-energy magnons at larger q, and the development of a new method to observea propagating magnons is urgently needed. In this study, we focus on X-ray spectroscopy forthe observation of the high-frequency propagating magnons and apply resonant inelastic soft
X-ray scattering (SX-RIXS) to magnonic materials to elucidate the behaviour of thehigh-energy magnon.
A Y3Fe5O12(YIG)(111) single crystal was measured, which is very promising for
magnonics due to its very low damping and long mean free path of magnon. X-ray absorption
spectra (XAS) and RIXS were measured at HORNET end-station of SPring-8 BL07LSU. All
the measurements were performed at room temperature. XAS was measured in partial
fluorescence yield mode using a silicon drift detector, and Fe L3-edge inverse partial
fluorescence yield (IPFY) XAS was obtained from O K emission. The energy resolution of
RIXS at Fe L-edge was set at 175 meV. A momentum-resolved RIXS was obtained by θ-2θscan of 2θ=45–135° and 9° step. During the θ-2θ scan, an elastic peak was measured at each2θ from Au with σ polarization. For RIXS of YIG, π polarization was used.
Fe L3-edge XAS of YIG is shown in Fig. 1(a). The spectral shape of XAS is good
agreement of YIG, having two characters of Fe3+ in octahedral (Oh) and tetrahedral (Td)symmetry [1,2]. Figure 1(b) shows the incident photon energy dependence of RIXS spectra of
YIG at θ=45° and 2θ=90°. In addition to dd excitations at ~1.3 eV and ~1.8 eV, which mainly
originate from Oh Fe3+, dd excitations around 2.5 eV were observed, which would be assigned
for Td Fe3+. The peak width at around loss energy of 0 eV is clearly larger than the energy
resolution, indicating that there is a contribution of inelastic scattering. For the
momentum-resolved RIXS, #7 (710.1 eV) was chosen since the contribution of inelastic
scattering was largest among the measured energies. Figure 2 shows RIXS spectra near theelastic peak obtained by θ-2θ scan. The peak at each 2θ was analyzed by a peak fitting of two
gaussian profiles: One is the elastic peak, ant the other is for inelastic scattering. The
experimental results (red circles) were well reproduced by the two gaussian profiles. Since theenergy resolution was set at 175 meV, it was difficult to identify low energy magnons below
100 meV. The inelastic peaks (green lines) at ~110 meV showed a weak dispersion. Theenergy scale of the inelastic peaks ranges from ~110 meV to ~130 meV. Inelastic neutron
scattering measurements and a theoretical calculation revealed that YIG possessed magnon
dispersions up to 100 meV [3,4]. The energy scale of the observed dispersion is higher thanthat of magnons of YIG, indicating that the observed inelastic scattering would be assigned tobimagnon. The identification of the bimagnon requires a complete picture of the magnetic
excitations in YIG. Further analysis will be conducted to understand RIXS spectra and thedispersion of the inelastic peak near the elastic peak.

FIG. 1. Fe L3-edge (a) XAS and (b)RIXS spectra of YIG(111) single
crystal. The numbers labeled in (a)indicate the incident photon energies
used to measure RIXS spectra in (b).

FIG. 2. Fe L3-edge RIXS spectra of
YIG measured at different θ-2θ angles.
Red circles are experimental results,
and black, green, and blue lines
correspond to the fitting results ofelastic peaks, inelastic peaks, and sumof two peaks and background. Angles
labeled on the spectra indicate 2θ
angle.

REFERENCES[1] F. M. F. de Groot et al., J. Phys. Chem. B 109, 20761 (2005).
[2] H. B. Vasili et al., Phys. Rev. B 96, 014433 (2017).
[3] J. S. Plant, J. Phys. C: Solid State Phys. 10, 4805 (1977).
[4] A. J. Princep et al., npj Quantum Materials 2, 63 (2017).

Development of Coherent Soft X-ray Diffraction Imaging Technique for
Observation of Ultra-fast Magnetic Dynamics
Yuichi Yamasaki1,2,3, Kohei Yamamoto4,5, Yuichi Yokoyama1,6, Yasuyuki Hirata4,
Hiroki Wadati4,7, and Taka-hisa Arima3,81

Research and Services Division of Materials Data and Integrated System (MaDIS),
National Institute for Materials Science (NIMS)
2PRESTO, Japan Science and Technology Agency (JST)3

4
5

RIKEN Center for Emergent Matter Science (CEMS)

The Institute for Solid State Physics, University of Tokyo

Department of Materials Molecular Science, Institute for Molecular Science
6
7
8Japan Synchrotron Radiation Institute (JASRI)

Graduate School of Material Science, University of Hyogo

Department of Advanced Materials Science, University of Tokyo

Introduction:
In recent years, topological magnetic structures, such as magnetic skyrmions, have been
actively studied. Magnetic skyrmions, nanometer-scale vortex-like magnetic structures, canbe controlled by external stimuli, such as electric field and pressure, and thus are expected to
be a promising candidate for next-generation spintronics device materials. To understand the
dynamics of magnetic skyrmions, it is important to observe them at high spatial and temporal
resolutions. Resonant soft X-ray scattering is also known to be a suitable technique forobserving such magnetic skyrmion dynamics because of its high sensitivity in detecting spinpolarization [1-3]. Therefore, in the present study, we have developed a diffractometer ofsmall-angle resonant soft x-ray magnetic scattering to observe topological magnetic texturesin the undulator beamline BL07LSU, SPring-8. The apparatus is equipped with a vacuum
chamber having a background pressure of < 1×10-5 Pa, a CCD camera (2048x2048 pixel,
Princeton Instrument), a direct beam catcher, and a sample holder. To observe transmitted soft
X-ray scattering, we prepared thin plate sample thinned by FIB method or magnetic thin films
fabricated on Si3N4 substrates.
Results:
We tried to measure resonant soft X-ray small-angle
scattering from magnetic multilayer films andmultiferroic BiFeO3 lamellas. The magnetic films
were fabricated with thickness of several tens
nano-meters and with different compositions,
Ta5[Pt3CoTa2]10, Ta5[Pt3Co1.2Ta2]10 , [Co/Pt]15, and[Co/Pt]6. The thickness of the samples were
sufficiently thin for soft X-rays to penetrate, andabsorption measurements confirmed the presence ofthe sample. A pinhole with a diameter of several
micro-meters was placed on the thin film sample toirradiate a circularly shaped soft X-ray to the sample.
Figure 1 shows resonant soft x-ray diffraction image

Figure 1: Diffraction patternfrom Co/Pt multilayer thin film.

from [Co/Pt]6 observed at the Co L-edge resonance energy. However, the scattering intensity
was weak and the diffraction pattern was not sufficiently accurate to reconstruct the real space
image from the obtained coherent diffraction pattern. This is probably due to the small
perpendicular magnetization component in the magnetic multilayer film. The other samples
were damaged during the experiment and no significant data could be obtained.
The multiferroic iron oxide BiFeO3 is known to have a ferroelectric polarization, which
gives rise a long-period cycloidal magnetic structure with a wave-length of 62 nm. The
competition between ferromagnetic interaction and the Dzyaloshinskii-Moriya interactions
between the Fe ions results in the non-collinear magnetic structure. We performed
small-angle scattering experiments at the L-absorption edge of Fe on the lamella samples toobserve the magnetic scattering derived from thecycloid magnetic structure. Figure 2 shows diffraction
patterns from the lamella sample of BiFeO3. Magnetic
scattering was observed with superimposing on theconcentric diffraction pattern from pinhole. In theprevious study, six magnetic scatters were observed
for the (110) surface sample, whereas in the present
experiment, magnetic scattering was found to bebroad in the azimuthal direction. It is assumed that themagnetic structure may be different due to the
influence of FIB processing. To confirm whether themagnetic scattering observed in the lamella sample isintrinsic or not, a follow-up experiment comparing
with other FIB processed samples is required in the
Figure 2: Diffraction pattern
future.
from BiFeO3 lamella sample.
Conclusion:
In this study, we developed the equipment for resonant soft x-ray small angle scattering andsucceeded in observing the magnetic scattering from the magnetic thin films and multiferroic BiFeO3
lamella sample. As a next step, we aim to observe temporal variations of the magnetic structure in real
space by combining the coherent soft x-ray diffraction imaging technique with time-resolved
pump-probe measurement.

References[1] “Dynamical Process of Skyrmion-Helical Magnetic Transformation in Chiral-Lattice Magnet FeGe
as Probed by Small-Angle Resonant Soft X-ray Scattering”, Y. Yamasaki, D. Morikawa, T. Honda,
H. Nakao, Y. Murakami, N. Kanazawa, M. Kawasaki, T. Arima, and Y. Tokura, Phys. Rev. B 92,
220421(R) (2015)[2] “Coherent Resonant Soft X-ray Scattering Study of Magnetic Textures in FeGe”, Victor Ukleev, Y.
Yamasaki, Daisuke Morikawa, Naoya Kanazawa, Yoshihiro Okamura, Hironori Nakao, Yoshinori
Tokura, Taka-hisa Arima, Quantum Beam Science. 2 [1] 3 (2018)[3] “Element-specific soft x-ray spectroscopy, scattering, and imaging studies of the skyrmion-hosting
compound Co8Zn8Mn4”, V. Ukleev, Y. Yamasaki, D. Morikawa, K. Karube, K. Shibata, Y.
Tokunaga, Y. Okamura, K. Amemiya, M. Valvidares, H. Nakao, Y. Taguchi, Y. Tokura, T. Arima,
Physical Review B. 99 [14] 144408 (2019)

REVERSIBLE AND IRREVERSIBLE ANIONIC REDOX FOR
HIGH-CAPACITY BATTERY MATERIALS
Naoaki Yabuuchi1 and Daisuke Asakura2
1

Department of Chemistry and Life Science, Yokohama National University
Research Institute for Energy Conservation, National Institute of Advanced Industrial Science andTechnology

2

Voltage / V

To realize the sustainable energy development, the electrification of automobiles has
progressed in the past several years. This movement reduces the dependence on fossil fuels
used for internal combustion engine technology together with accelerating the use ofrenewable energy, including solar, wind, and biomass. In the past three decades, technologyof rechargeable lithium batteries, i.e., lithium-ion batteries, has been highly sophisticated, andcurrently Ni-based layered materials dominate the market of lithium-ion batteries for electricvehicles. However, further increase in energy density of lithium-ion batteries is required toextend cruising distances of electric vehicles. In the past few years, many lithium-excess
metal oxides with layered and disordered rocksalt structures, Li2MnO3-based electrodematerials have been studied as potential high capacity electrode materials. However, Mn ionsin Li2MnO3 (Li1.33Mn0.67O2) cannot be oxidized beyond the tetravalent limit, and therefore,
anionic species, oxide ions, donate electrons on Li extraction. Nevertheless, low reversibilityof anionic redox for pure Li2MnO3 is experimentally observed, leading to insufficient
reversibility as electrode materials. To increase in the reversibility of anionic redox, thesubstitution of metal ions with higher ionic characters with oxide ions, for instance, Nb5+
(Li3NbO4) and Ti4+ (Li2TiO3), has been proposed. For the Li-excess metal oxides, the charge
compensation process on Li extraction is achieved by electron donation from oxide ions, but
detailed reaction mechanisms remain as a debatable subject, e.g., dimerization of oxygen andformation of holes in oxygen.
In this study, a binary system of Li2TiO3–LiMnO2 is examined as electrode materials for Li
storage applications, and its reaction mechanism is studied by resonant inelastic X-ray
scattering (RIXS) study. In this binary system, nanosized Li7/6Ti1/3Mn1/2O2 has been preparedby mechanical milling from a mixtureof Li2TiO3 and LiMnO2.[1] TEM
observation reveals that particle size is
reduced to 5 – 20 nm, and these
Li7/6Ti4+1/3Mn3+1/2O2
1 -5
5.0
nanosize particles are agglomerated
for each other, forming secondary
3.0
particles with diameters of 50 – 200
nm
after
milling.
Nanosize
1.0
10 mA g-1 at 25 oC
Li7/6Ti1/3Mn1/2O2
delivers
large
reversible capacities in Li cells, and an
Li7/6Mn4+1/3Mn3+1/2O2
1
5
2
5.0
initial discharge capacity reaches 295
mA h g-1 even at room temperature
3.0(Figure 1). Charge compensation
processesfornanosize
1.0
Li7/6–yTi1/3Mn1/2O2
were
further
examined by O K-edge RIXS study
0
100
200
300
400
(Figure 2a). From the data of Mn-1
Capacity / mAh g
K-edge and L-edge XAS study, Mn
oxidation from a trivalent to Fig. 1. Charge/discharge curves of nanosize
tetravalent state is evidenced on Li7/6Ti1/3Mn1/2O2 at room temperature. The data of
4+3+charge for the slope region of 3 – 4 V Li7/6Mn 1/3Mn 1/2O2 is also shown for comparison.
[1], but no oxidation of Mn ions is
st

th

th

st

nd

found at the plateau region. O K-edge RIXS spectra clearly reveals that a new peak isobserved after charge at 8 eV, which suggests that charge compensation is achieved by the
donation of electron from oxide ions at the plateau region, and the formation of holes byelectrochemical oxidation.[2]To further clarify the role of Ti ions on anionic redox reversibility, a binary system of
Li2MnO3-LiMnO2 is also prepared and tested as electrode materials. Li7/6Mn4+1/3Mn3+1/2O2
was also successfully prepared by mechanical milling from the mixture of Li 2MnO3 and
LiMnO2. This phase is metastable containing Mn4+ and Mn3+ ions and cannot be obtained byaconventional
calcination
process.
Charge/discharge
curvesofnanosize
Li7/6Mn4+1/3Mn3+1/2O2 in a Li cell are also shown in Figure 1. An observed initial discharge
capacity reaches 350 mA h g-1,
which is larger than that of
Li7/6Ti1/3Mn1/2O2. Nevertheless,(a)cyclability is insufficient aselectrode materials, and areversible capacity of ca. 100 mA
h g-1 is lost on initial five cycles.
Much better capacity retention isachieved for Li7/6Ti1/3Mn1/2O2.
Reversibility of anionic redox for
Li7/6Mn4+1/3Mn3+1/2O2 has been
also examined by RIXS study as
shown in Figure 2b. A similar(b)peak with Li7/6Ti1/3Mn1/2O2 at 8eV
isobserved
forLi7/6Mn4+1/3Mn3+1/2O2 after charge.
However, intensity of the peak ismuchhigherfortheLi7/6Ti1/3Mn1/2O2, indicating thatoxygen redox chemistry is clearlystable for the Ti-substituted
sample. This result directly
supports the role of Ti4+ ions,which are effective to stabilize Fig. 2. Comparison of changes in RIXS spectra for nanosize
anionic redox associated with the Li7/6Ti1/3Mn1/2O2
and
Li7/6Mn4+1/3Mn3+1/2O2
oncharge/discharge
processes.
high ionic character and donationof electron to oxide ions.
These findings open a new direction to design high-capacity positive electrode materials
with highly reversible electrochemistry on the basis of anionic redox. Further optimization ofchemistries, compositions, and particle sizes on these electrode materials contributes thedevelopment of advanced high-energy rechargeable Li batteries in the future.
REFERENCES[1] Y. Kobayashi, M. Sawamura, S. Kondo, M. Harada, Y. Noda, M. Nakayama, S. Kobayakawa, W.
Zhao 1, A. Nakao, A.Yasui, H. B.Rajendra, K.Yamanaka, T.Ohta, and chi, Materials Today,
in-press[2] T. Sudayama, K. Uehara, T. Mukai, D. Asakura, X.-M. Shi, A. Tsuchimoto, B. Mortemard de
Boisse, T. Shimada, E. Watanabe, Y. Harada, M. Nakayama, M. Okubo and A. Yamada, Energy &Environmental Sciences, in-press

ELECTRONIC STATE OF INTERMEDIATE WATER IN
BIOCOMPATIBLE POLYMERS INVESTIGATED BY XAS AND XES
Koichi Ozaki1, Jumpei Yahiro1, Masaru Nakada1, Masahiro Kunisu1,
Kosuke Yamazoe2, Jun Miyawaki2 and Yoshihisa Harada2
1
Toray Research Center, Inc.,
2
The Institute for Solid State Physics, The University of Tokyo
Introduction
Poly (vinyl-pyrrolidone) (PVP) and Poly (ethylene-glycol) (PEG) have been widely used forvarious biomedical applications because of their excellent biocompatibility. Biocompatibility
is one of the important factors for polymers used for biomedical applications. It is consideredthat adsorbed water on the polymer surface contributes to expression of biocompatibility.
Water molecules existing near the biocompatible polymer could be classified into three types;
free water, intermediate water and non-freezing water [1]. In particular, the intermediate water
has an important role to control the biocompatibility of polymers. However, the local
structural and dynamical properties of intermediate water are not well understood yet. It has
been reported that water molecules confined in a polyelectrolyte brush form characteristic
local hydrogen-bonded structure [2]. Based on this result, water molecules in the close
vicinity of a biocompatible polymer are expected to form a specific hydrogen-bonded
structure to express biocompatibility. In order to reveal the role of hydrogen-bonded structureof water on biocompatibility, it is necessary to obtain accurate information about thehydrogen-bonded structure of those water molecules. In this study, we examine thehydrogen-bonded structure of water molecules hydrating PVP and PEG molecules in detailusing X-ray absorption spectroscopy (XAS) and X-ray emission spectroscopy (XES).
Experimental
The PVP (Mw=4000-5000) and PEG (Mw=3350 and 20000) were purchased from
Sigma-Aldrich. PVP and PEG aqueous solutions at different concentrations were prepared asmeasurement samples. Absolutely dry PVP (dry-PVP) and PEG (dry-PEG) were alsomeasured. O 1s XAS and XES measurements were performed at the BL07LSU HORNET
station in SPring-8. The measurement of the polymer aqueous solution was performed using asolution cell under ambient pressure by circulating the aqueous solution. The dry samples
were mounted on a Cu sample plate and measured in a high vacuum condition.
Results and Discussion
Figure 1 shows the O 1s XES spectra of dry-PEG(Mw=20000) measured at various scan rates of the Cu
plate. XES measurements were performed at theexcitation energy of 537 eV. A broad peak around 527
eV was observed for the XES spectra with the scan rateof 0.16 and 0 m/s. The shape of the XES spectrachanged gradually with increasing the scan rate,
indicating the presence of X-ray induced degradation of
the polymer during the XES measurement at lower
scan rate. Since the shapes of the XES spectra above
the 5.0 m/s scan rate were almost the same, we set thescan rate of 5.0 m /s to obtain the XES spectra of thedry-PVP and dry-PEG samples.
Figure 2(a) depicts O 1s XES spectra of PVP aqueous
Figure 1. O 1s XES spectra ofsolution at the excitation energy of 550 eV. XES dry-PEG with various scan rate: 7.2,
measurements were performed with a flow rate of 5.0
5.0, 3.2, 1.6, 0.16 and 0 (m /s).
ml/s to avoid the accumulation of radiation damage.

Two 1b1-derived peaks (1b1’ and 1b1’’) were observed in the XES spectra of pure water [3].
The raw XES spectra for aqueous solutions contain X-ray emission signals from oxygens bothin water molecules and in PVP. Subtracting the polymer component, we can obtain the signal
from water molecules. In order to remove the PVP polymer contribution from the raw XES
spectra, we focused on the energy range below 522 eV. In this region, the spectral intensity
seems to well reflect the relative contributions of water and polymer. Thus each contribution
was estimated by performing least-squares fit of the XES spectrum of dry-PVP and pure waterin the energy range below 522 eV. The results after subtraction of the polymer contribution
are shown in the bottom of Figure 2(a). It was found that the 1b1’ peak is dominant in theextracted spectra. The 1b1’-dominant peak profile with a clear shoulder structure around 524.5
eV (possibly assigned as 3a1 derived peak) obtained for the 65 wt% PVP solution is quite
similar to the previously obtained spectrum for water molecules confined in a polyelectrolyte
brush [2]. Therefore water molecules in the 65 wt% PVP solution may exist almost in theform of non-freezing and intermediate water hydrating the PVP polymer. Water molecules inthe 30 wt% PVP solution consist of strong 1b1’ and weak 1b1’’ profiles, which can be
reproduced by the mixture of bulk liquid water as well as non-freezing and intermediate water
hydrating the PVP polymer. Detailed analysis of the XES fraction for the water hydrating PVP
may have rich information
about the hydrogen-bonded
structure of intermediatewater and its relation to thebiocompatibility of PVP.
Figure 2(b) shows the O 1s
XES
spectraofPEG(Mw=3350) aqueous solutions.
Broad peaks at ~526.5 eV and~528.0 eV were observed for
the XES spectra of dry-PEG.
In this case, in addition to theenergy range below 522 eV,
the energy range above 526
eV was also used for thefitting to estimate the PEG
contribution. The results after Figure 2. O 1s XES spectra of (a) PVP and (b) PEG aqueous
removal of the polymer solutions. The spectra of pure water and dried polymer powder
contribution are shown in the were also shown for comparison. Bottom: Spectra with thebottom of Figure 2(b). It polymer contribution subtracted.
should be noted that the extracted XES profiles are close to the spectrum of pure water, incontrast to the PVP results. The present study raises the possibility that a statichydrogen-bonded structure of intermediate water is different between PVP and PEG aqueous
solutions. In contrast, dynamical hydrogen-bonded structure of intermediate water hydrating
PVP and PEG aqueous solutions is similar [4, 5]. Consequently, to understand the behavior ofintermediate water, it is important to understand both the dynamical and the statichydrogen-bonded structures. Several questions remain unanswered at present yet. Futurestudies on the current topic are recommended.
References[1] T. Tsuruta, J. Biomater. Sci. 21 (2010) 1831.
[2] K. Yamazoe et al., Langmuir 33 (2017) 3954.
[3] T. Tokushima et al., Chem. Phys. Lett. 460 (2008) 387.
[4] T. Takei et al., Anal. Sci.26 (2010) 337.
[5] M. Nakada et al., Materialia 12 (2020) 100743.

Present status of the development of time-resolved ambient pressure
X-ray photoelectron spectroscopy system at SPring-8 BL07LSU

Susumu Yamamoto* and Iwao Matsuda
The Institute for Solid State Physics, The University of Tokyo* Present affiliation: Institute of Multidisciplinary Research for Advanced Materials,
Tohoku University

Introduction
Operando spectroscopy, observation of working catalysts under reaction conditions, isessential for our understanding of reaction mechanisms as well as the rational development
guidelines for novel catalysts. Ambient pressure X-ray photoelectron spectroscopy (AP-XPS)
is one of the most powerful operando spectroscopies, which permits the investigation of theelectronic and chemical states of adsorbates and catalysts under gas atmospheres atnear-ambient pressure. However, the typical temporal resolution of AP-XPS has been limited
to ms, and thus the dynamic processes faster than ms could not be studied. A pump-probe
measurement can overcome the limitation of the temporal resolution, in which a sample ispumped by ultrafast optical laser and probed by X-ray. At SPring-8 BL07LSU, we developed
time-resolved X-ray photoelectron spectroscopy (tr-XPS) in ultrahigh vacuum in a
pump-probe fashion [1, 2]. The temporal resolution of tr-XPS is ~50 ps, which is determinedby the pulse width of probe X-ray.
In this study, we aimed to develop a time-resolved ambient pressure X-ray photoelectron
spectroscopy (tr-APXPS) by combining AP-XPS with tr-XPS. tr-APXPS permits monitoring
the time evolution of photocatalytic reactions in real time and under reaction conditions; all
the players in photocatalytic reactions such as photo-excited carriers, adsorbates, substrates
can be investigated by tr-APXPS.

Experimental
The experiments were performed at the soft X-ray undulator beamline BL07LSU at
SPring-8 [3]. AP-XPS experiments were carried out using an AP-XPS apparatus thatcombines a differentially pumped electron analyzer (SPECS, PHOIBOS 150 NAP) with anambient-pressure gas cell [4]. A one-dimensional delay-line detector (DLD) is adopted as adetector of the electron analyzer.
Femtosecond (fs) laser systems were installed at SPring-8 BL07LSU. The fs laser pulses
were synchronized to synchrotron radiation X-ray pulses, and the delay time of fs laser pumppulse relative to X-ray probe pulse can be controlled. There were two different fs laser
systems; high pulse energy and low repetition rate (mJ, 1 kHz) and low pulse energy and highrepetition rate (J, 208 kHz). Optical parametric amplifier (OPA) was installed in both laser
systems, which can change the wavelength of fs laser. In the present study, high pulse energy
and low repetition rate (mJ, 1 kHz) laser system was used.

Results and discussion
In the development of tr-APXPS, we reported the installation of a soft X-ray chopper at
SPring-8 BL07LSU. The soft X-ray chopper was operated at 1 kHz, and reduced therepetition rate of probe X-ray (208 kHz in the H-mode operation of SPring-8) to 1 kHz tomatch that of pump laser. Consequently, time-resolved XPS measurements became possible
with the same experimental setup as in conventional static XPS measurements. In addition,
soft X-ray chopper blocks all the X-ray pulses that are not used in pump-probe measurements.
This decreases unnecessary burden on the sample and the detector. Thus, the photon flux canbe increased by opening slits in X-ray beamline to improve measurement efficiency.
Test tr-APXPS measurements were performed on the naturally oxidized p-type Si(111)(SiO2/p-Si(111)) surface in 1 mbar CO2. The wavelength of pump laser was changed from
800 nm to 400 nm. This was because the small pre-pulses in 800 nm can induce the surface
photovoltage effect of Si. The amplitude of the pre-pulses was much decreased by frequency
doubling to 400 nm. In addition to the energy shift of Si 2p core level due to surface
photovoltage effect, we could detect the energy shift of CO2 gas phase peak in O 1s core level.
The energy shift of gas phase peak in AP-XPS reflects the change of the sample work
function. Notably, the energy shift was different between Si 2p and O 1s core levels. The
present results demonstrate that of tr-APXPS can provide a wealth of information inphoto-excited systems and will be very suited to study various light-induced reactions
including photocatalysts and plasmonic chemical reactions.

REFERENCES
[1]
[2]
[3]
[4]

K. Ozawa, S. Yamamoto, et al., e-J. Surf. Sci. Nanotechnol. 17, 130-147 (2019).
M. Ogawa, S. Yamamoto, et al., Rev. Sci. Instrum. 83, 023109 (2012).
S. Yamamoto et al., J. Synchrotron Rad. 21, 352-365 (2014).
T. Koitaya, S. Yamamoto, et al., e-J. Surf. Sci. Nanotechnol. 17, 169-178 (2019).

Development of time-resolved X-ray absorption and photoelectron
spectroscopy system using time-of-flight electron analyzer
Susumu Yamamoto1,*, Rei Hobara2, Kenichi Ozawa3, and Iwao Matsuda1
1

The Institute for Solid State Physics, The University of Tokyo
2
3

Department of Physics, The University of Tokyo

Department of Chemistry, Tokyo Institute of Technology* Present affiliation: Institute of Multidisciplinary Research for Advanced Materials,
Tohoku University

Introduction
Understanding the dynamics of photoexcited carriers is essential for improving theefficiency of light-energy conversion materials such as photocatalysts and solar cells.
Photoexcited carriers (electrons/holes) generated by light absorption move between an
occupied state and an unoccupied state. Therefore, it is important to observe both occupied
and unoccupied states in order to get the whole picture of the photoexcited carriers dynamics.
However, there has been no experimental method that allows the observation of both
occupied and unoccupied states dynamics.
In this study, we aim to develop a picosecond time-resolved X-ray absorption/photoelectron
spectroscopy (XPS/XAS) system using a time-of-flight (TOF) electron analyzer, and track the
temporal changes of electronic structures in both occupied and unoccupied states. In addition,
the TOF electron analyzer has a unique feature that all kinetic energy electrons can bedetected and energy-resolved by the detector. In the case of a hemispherical electron analyzer,
only electrons with the fixed kinetic energy determined by electrostatic lenses are detected by
the detector. Taking advantage of this unique feature of TOF electron analyzer, we canmeasure XAS spectra with different probing depths at once. The time-resolved XAS spectrausing TOF analyzer will allow the temporal evolution of excited electronic states along thedepth direction.
Experimental
The experiments were performed using a TOF electron analyzer (VG Scienta, ARTOF 10k)at the soft X-ray undulator beamline BL07LSU at SPring-8 [1]. We developed a picosecond
time-resolved XPS system using the TOF electron analyzer, and studied the relaxation
dynamics of photoexcited carriers on various surfaces and interfaces [2]. In the present study,
thus, we aim to extend the application of the TOF electron analyzer to XAS measurements.
We developed the software program that can control both TOF electron analyzer and the soft
X-ray beamline (undulator and monochromator) to measure XAS spectra. The sample studied
in this research was the rutile TiO2(110) surface. The sample surface was cleaned by Ar ion
sputtering (1.0 kV, 1x10-4 Pa Ar, 10 min), and annealing in an oxygen atmosphere (920 K in
1x10-4 Pa O2). The surface order was confirmed by low energy electron diffraction (LEED).

Results and discussion
Figure 1 shows the Ti L-edge XAS spectrum of the clean TiO2(110) surface measured using
TOF electron analyzer. The XAS spectrum in total electron yield (TEY) was obtained byintegrating the whole TOF region indicated by the blue lines in Fig. 1(a). The obtained XAS
spectrum in Fig. 1(b) is in good agreement with the TEY-XAS spectrum measured with thesample current. This confirms the successful measurement of XAS spectra using TOF
electron analyzer. By changing the TOF window for the integration, we can change theprobing depth. For example, we could obtain the surface sensitive XAS spectra if the TOF
window is tuned to Auger electrons. Now we plan to perform XAS experiments on the sample
that has a distribution of chemical states in the depth direction (e.g., ice H2O thin films
adsorbed on the TiO2 surface).

Figure 1. Ti L-edge XAS spectrum of the clean TiO2(110) surface measured using TOF
electron analyzer. (a) TOF spectra as a function of photon energy (b) XAS spectrum obtainedby integrating the TOF region indicated by the blue lines in the panel (a).
REFERENCES[1][2]

S. Yamamoto et al., J. Synchrotron Rad. 21, 352-365 (2014).
S. Yamamoto et al., J. Phys. Soc. Jpn. 82, 021003 (2013).

SOFT X-RAY ABSORPTION SPECTROSCOPY OF Ca2RuO4
UNDER ELECTRIC FIELD
Daiki Ootsuki1, Yukie Takasuka1, Ayuko Hishikawa1,
Tatsuhiro Ishida1, Teppei Yoshida1, Kosuke Yamazoe2, Tomoyuki Tsukada2,
Jun Miyawaki2, Yoshihisa Harada2,
3Chanchal Sow , Shingo Yonezawa3, Yoshiteru Maeno3, and Fumihiko Nakamura4
1

Graduate School of Human and Environmental studies, Kyoto University,
Synchrotron Radiation Laboratory, The Institute for Solid State Physics, The University of Tokyo,
3
Department of Physics, Kyoto University,
4
Department of Education and Creation Engineering, Kurume Institute of Technology

2

The exploration of non-equilibrium phase transition and non-linear conductivity in strongly
correlated electron systems is one of the frontier of condensed matter physics. By applying
electric fields, Mott insulators exhibit the insulator to metal transition with a non-linear
conductivity [1-2], which is so called “Mott breakdown” analogous to dielectric breakdown[3]. Recently, Nakamura et al. have discovered an electric-field induced insulator to metaltransition of Ca2RuO4 with nonlinear conductivity. The threshold electric field Eth of ~40
V/cm is weakest among the Mott insulators [4]. It would be desirable to understand how theelectronic structure is changed by applying electric field and to clarify the origin of the
non-linear conductivity of Ca2RuO4.
The various stimuli such as temperature, chemical substitution, and applied pressure induce
the metal to insulator transition of Ca2RuO4 with a structural phase transition [5-7]. The
structural phase transition affects the orbital configuration of this system through RuO6
octahedra [5]. Actually, the insulating state of Ca2RuO4 is characterized by the crystal field
splitting as well as on-site Coulomb interaction. The recent polarization-dependence ofangle-resolved photoemission spectroscopy combined with dynamical mean-field theory(DMFT) revealed the coexistence of the band insulating state of dxy orbitals originating fromthe crystal-field splitting and the Mott insulating states of half-filled dxz/yz orbitals [8].

Figure 1. O 1s XAS spectra of Ca2RuO4 as a function of (a) angle  and (c) current
density J. Intensities of apical and in-plane components as a function of (b) angle  and(d) current density J.

In this study, we have investigated the electric-filed dependence of the electronic structure
for Ca2RuO4 by means of soft x-ray absorption spectroscopy (XAS) under electric field at
SPring-8 BL07LSU. High quality single crystals of Ca2RuO4 were grown by floating-zone
method. In order to apply the electric field, the gold wires were attached to the terminals bothends of the sample using silver epoxy. The electric field was applied parallel to the ab-plane
and the voltage-current characteristics were measured in situ during the XAS measurements.
The sample temperature is monitored by the directly attached Pt thermometer and controlled
by liquid helium. Although the current flow gives rise to the joule heating effect, the sample
heating kept less than 3 K during all measurements.
Figure 1 (a) shows the angle dependence of the O 1s x-ray XAS spectra take at T = 300 K.
The XAS spectrum strongly depends on the angle . We focus on the intensities at ~528.3 eV
and ~529.1 eV derived from the apical and in-plane oxygen, respectively. To quantify theintensities of the apical and in-plane components, we fitted the intensities derived from apicaland in-plane oxygen with a Gaussian as shown in Fig. 1 (b). The angle dependence of XAS
spectra is basically consistent with the previous XAS study reported by Mizokawa et al [9].
This indicates that the carrier concentration between the Ru 4dxy and 4dyz/zx orbitals is close to
1:1 at T = 300 K. Figures 1 (c) and (d) show the electric field dependence of the XAS spectra
at T = 300 K. Here, we obtained the electric-field dependence of XAS spectra at the normal
incidence. By increasing current density J, theapical intensity decreases, while the in-plane
intensity increases. As shown in Fig. 1 (d), theintensity ratio of the apical and in-planecomponents gradually increases with current
density J.
Figure 2 shows the relationship between the
crystal-field splitting , spin-orbit coupling so,and the intensity ratio of the apical and in-planecomponents of XAS spectra. The solid curve
indicate the theoretical estimation on the basis of
the simple Hamiltonian of Ref. 10. From thetheoretical curve, we obtained the crystal-field
splitting  and spin-orbit coupling so. With
increasing current density, the crystal field
splitting energy increases from the compressed Figure 2. XAS intensity ratio between
RuO6 octahedra to the elongated RuO6 octahedra. the apical and in-plane components.
These results suggest that the electric field The solid line is the model expectationinduces the elongation of the RuO6 octahedra.
of the Hamiltonian of Ref. 10.
REFERENCES[1] Y. Taguchi et al., Phys. Rev. B 62, 7015 (2000).
[2] K. Inagaki et al., J. Phys. Soc. Jpn. 73, 3364-3369 (2004).
[3] T. Oka, Phys. Rev. B 86, 075148 (2012).
[4] F. Nakamura et al., Sci. Rep. 3, 2536 (2013).
[5] O. Friedt et al., Phys. Rev. B 63, 174432 (2001).
[6] F. Nakamura et al., Phys. Rev. B 65, 220402 (2002)[7] S. Nakatsuji and Y. Maeno, Phys. Rev. Lett. 84, 2666–2669 (2000).
[8] D. Sutter et al., Nat. Commun. 8, 15176 (2017).
[9] T. Mizokawa et al., Phys. Rev. Lett. 87, 07702 (2001).
[10] G. Fatuzzo et al., Phys Rev. B 91, 155104 (2015).

OPERANDO XPS STUDY OF CO2 HYDROGENATION ON A SINGLEATOM ALLOY Pd-Cu MODEL CATALYST
J. Yoshinobu, S. Tanaka, K. Mukai, W. Osada, T. Matsuda, S. Nakano, Y. Choi,
S. Yamamoto, and I. Matsuda
The Institute for Solid State Physics, The University of Tokyo
Introduction
Single-atom alloy catalyst (SAAC) is an alloy in which a small amount of functional metal
atoms are deposited and embedded in the surface of a metal substrate. The deposited atoms givea new specific function to the substrate, where the main catalytic activity of the substrate is still
active [1]. When high-resolution XPS measurement is performed using high-intensity
synchrotron radiation, a very small amount (a few percent or less) of embedded atoms on thesubstrate surface can be detected, and their chemical states can be clearly identified.
Furthermore, it is possible to observe the changes in the electronic and chemical states of theadsorbed atoms/molecules as well as the catalyst surface in ambient conditions using NAPXPS. Operando measurments using mass spectrometry and NAP-XPS can elucidate thecatalytic function of SAAC during the reaction.
In this study, we prepared a single-atom alloy catalyst (SAAC) Pd-Cu(111) and investigatedthe hydrogenation of carbon dioxide (CO2) using NAP-XPS. CO2 is one of the greenhouseeffect gases. At present, methanol synthesis from CO2 is performed at high temperature(~500K) and under high pressure using Cu/ZnO catalysts. This reaction is an exothermic
reaction, in which the molar number decreases for the forward direction. Thus, more active andefficient catalysts which can be operated at lower temperature are highly demanded.
Recently, we found the desorptions of hydrogenated species after a large amount of hydrogen
exposure to formate species (HCOO) on a Pd-Cu(111) surface using temperature programmed
desorption. In the present study, we aimed to clarify the hydrogenation reaction process of CO2
on a single-atom alloy catalyst Pd-Cu(111) by NAP-XPS.
Experimental
Experiments were carried out using NAP-XPS system at SPring-8 BL07LSU. The cleanCu(111) was prepared in a UHV preparation chamber of NAP-XPS system. A small amount of
Pd was vapor-deposited on the surface to prepare a single atom alloy model catalyst Pd-Cu(111).
The amount of Pd deposited was estimated to be 0.02 ML (monolayer). In a NAP cell, CO2 (0.9mbar) and H2 (0.5 mbar) were introduced under isothermal conditions near room temperature.
During this process, the model catalyst surface was observed by NAP-XPS. Next, the substratetemperature was raised stepwise by heating under constant pressure of CO2 (0.9 mbar) and H2(0.5 mbar).
Results and discussion
Before the introduction of mixed gases (CO2 + H2), only H2 was gradually introduced in the
NAP cell at 300 K up to 3.0 mbar, and the model catalyst surface was investigated by NAPXPS; the core-level spectra of Cu 2p, Cu LVV Auger, Cu 3p, and Pd 3d were measured.
However, any peak shifts due to hydrogen adsorption and an appearance of new peaks were notobserved under the present conditions. Even if the dissociative adsorption of hydrogen occurs,hydrogen is thermally desorbed at room temperature, i.e., a sufficient amount of hydrogen is
not adsorbed on the surface.
CO2 (0.9 mbar) and H2 (0.5 mbar) were introduced into the NAP cell at 300 K, and Pd3d,
C1s, and O1s were observed by AP-XPS (see Fig. 1). Based on our previous study [2], major
surface species were attributed to CO3 species; a small amount of atomic oxygen were also
observed. In the case of Pd3d, a small peak was newly observed on the high binding energy
side. This may be assigned to the Pd atoms bonded to atomic oxygen species.
When heating the sample from 300K to 500K, the C1s and O1s peaks originating from CO3
species were observed up to 375K but almost disappeared above 390K. The O1s peak attributed
to atomic oxygen almost disappeared above 375K. On the other hand, the peak derived fromcarbon species (~284 eV) gradually increased with heating.
The present study suggests that the dissociation of CO2 and the formation of CO3 and O
species occur at 300 K (disproportionation). However, no experimental evidence was detected
to indicate hydrogenation of CO2 at the present experimental conditions. Therefore, in order toachieve a sufficient amount of adsorbed hydrogen atoms at the surface to induce hydrogenation,
lower temperature and/or higher H2 pressure may be needed.

-

Figure 1 A series of XPS spectra of C 1s, O 1s and Pd 3d of the Pd-Cu(111) surface
under CO2 (0.9 mbar) and H2 (0.5 mbar) at 300 K and subsequent heating.
REFERENCES[1] G. Giannakakis et al., Accounts of Chemical Research, 52, 237 (2018).
[2] T. Koitaya et al., Topics in Catalysis, 59, 526 (2016).

AP-XPS STUDY OF ELECTRONIC STATES OF METAL NANOPARTICLES AND METAL HYDRIDE COMPLEX
J. Yoshinobu1, W. Osada1, Y. Choi1, S. Tanaka1, K. Mukai1,
T. Koitaya2, S. Yamamoto3, and I. Matsuda1
1
The Institute for Solid State Physics, The University of Tokyo
2
Institute for Molecular Science
3
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University
Introduction
When a hydrogen atom is bonded to metal, the chemical state of hydrogen can be classified
into atomic hydrogen (H), proton (H+) or hydride (H-), or an intermediate state between them;it can easily change, depending on its surrounding environment. The chemical state of metal
which is bonded to hydrogen also changes accordingly. By utilizing various hydrogen species
on a metal surface, a new reaction path can be developed for "highly selective", "verydifficult" and/or "highly efficient" processes. One might think that a metal is “reduced” when
it is bonded to hydrogen. However, when neutral hydrogen takes an electron from metal andit becomes hydride (H-), the metal is now “oxidized”. Using high-resolution X-ray
photoelectron spectroscopy (XPS), the chemical state of metal can be elucidated at each site,
based on the chemical shift of its core level peak. In the case of surface chemistry, we cantrace the chemical states of surface and bulk during the processes including hydrogen
adsorption, absorption and desorption. Although the formal charges of hydrogen and metal
have been estimated based on valency in a hydrogen-containing metal complex, one canexperimentally elucidate the chemical state of a metal atom using XPS. In this study, weplanned to perform ambient pressure (AP) XPS measurements of metal-alloy nanoparticles
and multinuclear metal complexes in the presence of hydrogen.
Experimental
In this beam time, we carried out XPS measurements of Pt nano particles and Pt75Pd25 nanoparticles synthesized by Yamauchi group at Kyushu University, and multinuclear Ti
complexes including Ti4N4 [C5Me4SiMe3]4 synthesized by Shima group at RIKEN (hereafter
abbreviated as Ti4N4 complex).
(A) In the case of Pt and Pt75Pd25 nano particles, these nano particles were deposited on a
Au foil. First, we measured Pt 4f and Pd 3d XPS in vacuum at room temperature. Next, thesample was heated to 250 °C in vacuum, and Pt 4f and Pd 3d XPS spectra were measured in
vacuum at room temperature. Finally, the sample was heated under H2 (20 mbar) at 250 °C
for 20 minutes, and XPS of Pt 4f and Pd 3d measurements were performed at room
temperature.
(B) In the case of Ti4N4 complex, a small amount of Ti4N4 complex was dissolved indegassed n-hexane in an Ar glove box and 5 μL of this solution was dropped on a PdCu alloy
substrate. After evaporating and drying the sample solvent, the sample was carefully moved
from the glove box to the load-lock chamber of the AP-XPS system using a vacuum transfer
device. First, we measured XPS spectra of Ti 2p, C 1s, O 1s and N 1s in vacuum at room
temperature. Next, H2 gas (5 mbar) was introduced and subsequently evacuated; during thisprocess AP-XPS measurement was carried out for Ti 2p, C 1s, O 1s and N 1s.
Results and discussion(A) XPS spectra of Pt nano particles and Pt75Pd25 nano particles during the above processes
were obtained. When comparing the XPS spectra of the as-installed sample and the heatedsample at 250°C, we confirmed that the O 1s intensity was decreased after heating, but aremarkable change in the spectral shapes of Pt 4f and Pd 3d was not observed. Even after
heating at 250°C under H2 pressure (20 mbar) for 20 minutes, no significant change wasobserved in Pt 4f and Pd 3d XPS in vacuum. Thus, the significant surface segregation of Pd
atoms in the alloy nano particle was not observed under the present hydrogen pressure andheating conditions.
(B) Our previous experiments suggested that the present Ti4N4 complex reacts with oxygen
and water, and it is easily oxidized. In this experiment, we carefully prepared the sample andtransferred it to the AP-XPS chamber as described above. Figure 1 shows Ti 2p, C 1s, O 1s andN 1s XPS spectra of Ti4N4 complex on a PdCu substrate. Ti 2p3/2 and N1s peaks were observed
at 456.8 eV and 397.2 eV, respectively, indicating that Ti takes a tetravalent state and N atoms
are bonded to the metal. Next, we also carried out AP-XPS measurements during the H2
exposure and pumping processes. Upon H2 exposure, we found the disappearance of N 1s peak,the increase of O1s and the peak shift of Ti 2p3/2 to 459.6eV. These results indicate that there
is no nitrogen in the complex and it was oxidized. We assume that upon H2 exposure the partialpressure of water vapor was increased in the AP cell by some reactions between hydrogen andthe inner wall of a stainless-steel AP cell, and thus produced water reacts with the complex.
For future work, we need to control the gases in the AP cell to eliminate the possibility ofoxidation of metal complex.

Figure 1 Ti 2p, C 1s, O 1s and N 1s XPS spectra of Ti4N4 complex deposited on a PdCu

substrate. XPS measurements were carried out in vacuum at room temperature (hn =680 eV).

DYNAMICS OF PHOTOCATALYTIC MATERIALS FOR WATERSPLITTING REACTION STUDIED BY TRANSIENT X-RAY
ABSORPTION SPECTROSCOPY
Yohei Uemura,1 Ahmed Ismail,1 Hebatalla Elnaggar,1 Federica Frati,1 Hiroki Wadati,2,3
Yasuyuki Hirata,2 Yujun Zhang,2 Kohei Yamagami,2 Susumu Yamamoto,2 Iwao Matsuda,2Frank de Groot1
1

Inorganic Chemistry and Catalysis, Debye Institute for Nanomaterials Science, Utrecht University,
Universiteitslaan 99, 3584 CG Utrecht, The Netherlands
2Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan
3
Graduate School of Material Science, University of Hyogo, Kamigori, Hyogo 678-1297, Japan

Introduction
From the viewpoint of building a sustainable society, developing a renewable energy
resource is an important subject in the modern science and technologies. Water splitting reaction
where water molecules (H2O) are decomposed into hydrogen (H2) and oxygen (O2) is expected
to be a key reaction to achieve a renewable energy process. Photocatalysts and photoelectrodes
play a central role in the water splitting reaction utilizing sunlight. Semiconductors such as TiO2,
-Fe2O3 is typical photocatalysts/photoelectrodes. They have been developed for several
decades and new materials are still explored to enhance its catalytic efficiency for our daily use.
Fundamental studies on the mechanisms of photocatalysts/photoelectrodes have been based ontheoretical calculations, electrochemical methods and spectroscopic studies. X-ray absorption
spectroscopy (XAS) is a new trend to study the excited states of photocatalytic materials.
XAS can address local symmetries and electronic states of a specific element in the material
under investigation and give complementary information which is not provided from optical
spectroscopies. In order to understand dynamics of excited states of photocatalysts andphotoelectrodes, we measured thin films of hematite (-Fe2O3) and copper tungstate (CuWO4).
-Fe2O3 and CuWO4 are have been studied as photoelectrodes for solar-assisted water splitting
reaction. We have already measured fast carrier dynamics of both the samples based on
femtosecond XAS measurements in PAL-XFEL. 1 We focused on how the photoexcited states
are progressed in hundreds of picoseconds. Here, we will present the result of the transient XAS
study on the photoexcited state of -Fe2O3, which has been recently published in ref. 1.
Experimental details
Pump probe XAS experiments were performed at BL07LSU, SPring-8. An illustration of
the setup employed in this study is displayed in Figure 1 (a) and the details of the setup isdescribed in ref. 2. The repetition of the excitation laser was ~1 kHz and its wavelength was~400 nm which was generated by doubling the fundamental wavelength of a Ti:Sapphire laser(~800 nm) using a BBO crystal. The samples were thin films of -Fe2O3 and CuWO4 depositedon an Indium-Tin-oxide-coated glass substrate. The thickness of the samples was ~ 50 nm.
Results
Transient XAS measurements by the sample current: In our experiments, an x-ray chopper
developed in SPring-8 was employed in order to select x-ray synchronised with an excitation
laser.3 In the previous experiments 2, the signals from the x-ray pulses synchronised with an
excitation laser were accumulated by using electric gates. Although the x-ray signals can be
efficiently by this method, a fast-response detector (response time: nanoseconds) is needed tobe employed. The x-ray chopper can isolate the x-rays synchronised with the laser and only theselected x-rays hit a sample. As the consequence, a relatively slow-responce detector (~miliseconds) could be employed. In our beamtime, we tried to measure transient XAS bymeasuring the sample current in addition to counting photoelectrons from the sample. transient
XAS of the -Fe2O3 sample was measured by the sample current.

Although XAS spectra witha good signal-to-noise ratio (S/N)
was obtained by measuring thesample current, we did not confirm
transient signals measured by the
sample current. It would because
the excitation laser affected thesample current signals, which
could be smeared out transient
signals.
However,thesynchronization of the x-ray pulses
was achieved and obtained XAS
spectra with a good S/N. It would Figure 1 (a) an illustration of the experimental setup at
be possible to measure XAS with BL07LSU, (b) the transient Fe LIII edge XAS spectra ofa transmitting geometry by hematite (c) the kinetic trace at 709.6 eV
employing the x-ray chopper,
which should extend the choice of the samples to be measured. It would be possible todistinguish transient signals from bulk and from surface of a sample when the sample current
and PEY are employed at the same time.
The transient XAS of -Fe2O3: Transient Fe LIII XAS of hematite was measured by a partial
electron yield (PEY) detection. The fluence of the excitation laser was set to 10 mJ/cm2, which
is below the damage threshold of the sample. The transient XAS at a delay time of 80 ps isshown in the central panel of Figure 1. There are three distinctive transient features foundaround 708.6, 709.6 and 710.2 eV. The transient feature around 709.6 eV was also observed by
the transient experiments in PAL-XFEL, which appeared just after the sample was excited by
the laser.1 The transient feature originates from the creation of Fe(II) species compared by thetheoretical calculations. The transient features seen around 708.6 eV and 710.2 eV were notobserved at faster delay time (< 10 ps) in PAL-XFEL. These new features could come fromlocal structural changes which happens between 10 ps and 80 ps after the photoexcitation. Small
polaron creation in a hematite thin film was observed around a delay time of 1 ps after
photoexcitation by femtosecond transient extreme ultraviolet spectroscopic experiments4.
Further local structural changes could happen in later delay time. Considering the time rangeof water splitting reaction, the excited state of hematite should be important to understand thedriving force of the water splitting reaction. We have also measured transient XUV spectra ofseveral hematite samples by a delay time of 1 ns. We would like to combine the transient Fe LIII
XAS data with the XUV data to understand more details of photoexcited states of hematite.
REFERENCES
1.
Ismail, A. S. M., et al., Direct Observation of the Electronic States of Photoexcited Hematitewith Ultrafast 2p3d X-Ray Absorption Spectroscopy and Resonant Inelastic X-Ray Scattering. Phys.
Chem. Chem. Phys. 2020, 22, 2685-2692.
2.
Takubo, K., et al., Capturing Ultrafast Magnetic Dynamics by Time-Resolved Soft X-Ray
Magnetic Circular Dichroism. Appl. Phys. Lett. 2017, 110, 162401.
3.
Osawa, H.; Ohkochi, T.; Fujisawa, M.; Kimura, S.; Kinoshita, T., Development of Optical
Choppers for Time-Resolved Measurements at Soft X-Ray Synchrotron Radiation Beamlines. Journalof Synchrotron Radiation 2017, 24, 560-565.
4.
Carneiro, L. M.; Cushing, S. K.; Liu, C.; Su, Y.; Yang, P.; Alivisatos, A. P.; Leone, S. R.,
Excitation-Wavelength-Dependent Small Polaron Trapping of Photoexcited Carriers in Α-Fe2o3. Nat.
Mater. 2017, 16, 819-825.

INVESTIGATION OF THE ELECTRONIC STRUCTURE OF COBALTOXIDE SPINEL ON ELECTRODES FOR WATER OXIDATIONCATALYSTS USING CO 2P3D RIXS
Minmin Chen,a Kosuke Yamazoe,b,c Jun Miyawaki,b,c Yoshihisa Harada,b,c Serena DeBeer,aOlaf Rüdigeraa

Max Planck Institute for Chemical Energy Conversion, Mülheim an der Ruhr, GermanybInstitute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba 277-8581, Japanc

Synchrotron Radiation Research Organization, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku,
Tokyo 113-8656, Japan

Development of new anode materials based on earth abundant transition metals for water
splitting is strategically important for industrial and large-scale applications. Water splitting
has been considered one of the most promising strategies for energy conversion and storage.1
Here, we study Co3O4 spinel on electrode surfaces by spectroelectrochemistry. The spinels aresynthesized as nanoparticles of around 10 nm that form aggregates of 100-300 nm diameter.
The materials are drop cast on the electrode surface, in this case a gold-coated SiN window.
In order to stabilize the small catalyst particles (particle size 300 nm) on the electrode surface,
Nafion is used as a binder. Unfortunately, Nafion showed extreme X-ray sensitivity during
our experiments. Being a fluorine-rich polymer, the F K-edge at 690 eV absorbed the X-rays
and damaged the polymer structure, which resulted in a simultaneous decrease of the Co
intensity.
The use of Fumion as a binder
solves this problem. Fumion
does not contain any F atoms in
its structure and showed no
detectable
photodegradation
duringthespectroelectrochemical
experiments. Nevertheless, we
did not detect any significant
changes on the 2p3d RIXS or at
the Co L-edge XAS with theapplied potential (Figure 1).
In order to understand the
2p3d RIXS of the spinel system,
we also measured spectra from
Figure 1. Co L-edge XAS (top-left) and 2p3d RIXS spectra of an
Al2CoO4, Co2ZnO4 and CoO aselectrode prepared by drop casting Co3O4 nanoparticles andpowder samples. The Al2CoO4 is
Fumion as a binder. The conditions are indicated on the figure, theelectrolyte was KOH 0.1 M.
a spinel, where the Co3+ at the
Oh sites has been substituted
3+2+with Al atoms. Similarly, on the Co2ZnO4, the Td Co has been substituted with Zn2+ ions.
CoO has a NaCl-like structure with all the Co2+ ions in a Oh geometry. The total electron yield
Co L-edge XAS shown in Figure 2 (left), correspond to what would be expected for a Co3+
HS in the case of the Co2ZnO4. The lower L3 edge position for the CoO and Al2CoO4 is also
in agreement with a lower oxidation state of the cobalt and the different fine structure may
reflect the different multiplet structure resulting from a Td vs an Oh coordination geometry.
The Al2CoO4 L3 edge shows a broad peak around 785 eV that at this point we suspect is anartefact. The RIXS spectra of the CoO show a well-defined feature at 0.9 eV and a second
feature that appears at higher excitation energies at 2 eV. The Al2CoO4 has 2 features at 0.5
and 0.9 eV. At higher incident energy an additional band appears at 2 eV. The Co2ZnO4 does

not show any well-defined bands, only a peak a 1.8 eV and at higher excitation energies(780-784 eV, not shown) a broad band was observed. The 2p3d RIXS spectra have been
compared to the powder Co3O4 measured at Sextants (Figure 2, right). The spectra have beennormalized to the elastic peak, except for the spectra at 779 where the elastic peak was quite
weak. At low excitation energies there is a band at 0.75 eV, most likely resulting from theCo2+ sites on the spinel, while as the excitation energy is increased the most prominent
features at around 2 eV could be assigned to the Co3+ sites, demonstrating the potential of
2p3d RIXS to study such materials. Multiplet calculations are planned for the near future to
analyse these results in detail.

Figure 2. Co L-edge XAS (TEY) from powder samples (left) and 2p3d RIXS (right) of the same samples atthree different incident energies. The spectra of Co3O4 powder measured at SOLEIL Sextants beamline wasadded to the figure as a comparison.

No significant changes were observed for the Co3O4 particles under different applied
electrode potentials. This could be a limitation of the experimental setup used in soft X-ray
measurements. Electrochemistry takes place at the interface with the electrolyte. If particle
films are too thick and the changes with the applied potential take place far from the electrodesurface, the X-ray beam may not penetrate enough on the film to observe any changes.
Another possibility is that the particle conductivity is too small, as a result, only a minor
fraction of the film may be under potential control. Consequently, we may be measuring all
the time the bulk of the particles, which remain unresponsive to the applied potential (seeFigure 3). We have repeated thesemeasurements at the Co K-edge. When using
hard X-rays, we probe the whole film.2
Similarly to what we have observed on thesoft X-ray regime, we could not detectsignificant changes on the spectral signatureswith the applied potential. This suggests, thatthe particles may be isolating, and that only aminor fraction is responding to the potential.
Figure 3. Schematic representation of the particles
We are currently investigating how to
on an electrode and the attenuation lengths at
producehomogeneous
Co3O4
different energies for X-ray. The arrows on the side
spinel-structured
films
with
controlledof the figure are scaled to the represented particle
size (assuming particles of around 150 nm inthicknesses on electrodes to see if we candiameter).
overcome this limitation in future
experiments.
References
1.
2.

T. Grewe, M. Meggouh and H. Tuysuz, Chem.-Asian J., 2016, 11, 22-42.
H. J. King, M. Fournier, S. A. Bonke, E. Seeman, M. Chatti, A. N. Jumabekov, B. Johannessen, P.
Kappen, A. N. Simonov and R. K. Hocking, The Journal of Physical Chemistry C, 2019, 123,
28533-28549.

INVESTIGATION OF THE ELECTRONIC STRUCTURE OF
ELECTRODEPOSITED COBALT OXIDE FILMS ON ELECTRODESFOR WATER OXIDATION CATALYSTS USING CO 2P3D RIXS
Minmin Chen,a Kosuke Yamazoe,b,c Jun Miyawaki,b,c Yoshihisa Harada,b,c Serena DeBeer,aOlaf Rüdigeraa

Max Planck Institute for Chemical Energy Conversion, Mülheim an der Ruhr, GermanybInstitute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba 277-8581, Japanc

Synchrotron Radiation Research Organization, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku,
Tokyo 113-8656, Japan

Development of new anode materials based on earth abundant transition metals for water
splitting is strategically important for industrial and large-scale applications. Water splitting
has been considered one of the most promising strategies for energy conversion and storage.1
Cobalt oxide films electrodeposited from Co2+ salts in neutral pH buffered solutions showhigh catalytic activity at low overpotential in mild environments, combined with extended
stability as a result of a self-healing mechanism.2 Although this catalyst has been studied overthe last years using different spectroscopic techniques (including operando Co K-edge XAS)
and EPR, most of the studies presented so far, did not employ ultra-pure electrolytes and it
has recently been shown that trace amounts of iron dramatically improve the catalytic
performance of such materials.3 At this point, the role of iron in catalyst activation is still not
fully understood.
We measured the Co L-edge XAS while we were electrodepositing Co oxide on theelectrode from a solution of Co(NO3)2 0.5 mM in phosphate buffer solution (PBS) at pH 7.0.
In Figure 1, one can see that the electrode did not contain any Co. Once the cell was filledwith the Co(NO3)2 solution, but kept at open circuit potential (OCP), we can already detect acobalt signal from the Co in solution. As we apply a potential of 0.9 V (vs Ag/AgCl), theintensity of the Co L-edge increases. After the second 60 s potential pulse, the intensity does
not increase any further, indicating that either the film does not grow anymore, or that the film
is too thick and the growth goes beyond the penetration limit of the X-rays. After theelectrodeposition, the electrolyte was switched to PBS without cobalt. Unfortunately, the Co
signal rapidly disappears upon changing the electrolyte. Even if the potential was kept high tostabilize higher oxidation states on the film. If the electrolyte was switched to KOH 0.1 M,
the films were more stable so that we could measure spectra at different applied potentials asa result of the lower solubility of Co at high pH.

Figure 1. Co L-edge XAS of an electrode where Co was electrodeposited. On the left, the spectra measured
during electrodeposition are shown. Measured in PBS pH 7.0 and in the presence of Co(NO3)2 0.5 mM insolution. On the right figure, the L-edges measured at different applied electrode potentials in KOH 0.1 M.

Interestingly, the Co L-edge shifts to higher energy at 0.7 V (vs Ag/AgCl), with a maximum
intensity for the L3 at 782 eV, similar to what was measured for Co2ZnO4 (see our 2019activity report on spinel Co3O4), suggesting that the cobalt has been oxidized to Co3+. On theother hand, when the potential was increased and the catalyst started doing water oxidation
the L-edge maximum shifts to lower energy (Figure 1).
The 2p3d RIXS spectra of these electrodeposited Co oxides is less structured than thatmeasured for the spinel systems. The operando measurements showed just a small change at
779 eV excitation energy, where the spectra recorded at the highest potential did not show aseparate feature at 1 eV (Figure 2).

Figure 2. Corresponding 2p3d RIXS spectra measured for the electrodeposited Co oxide film shown in figure
6 at 3 different incident energies as indicated on the figure. Measured in KOH 0.1 M.

Operando studies in the soft X-ray regime are extremely challenging. On one hand, theexperimental setup requires a vacuum environment, which makes the design of thespectroelectrochemical cell challenging. Nevertheless, we have successfully designed a cellthat operates on a three-electrode configuration. The cell did not leak during themeasurements.
The electrodeposited films showed a small response to the applied potential. We could
monitor the film growth by following the Co L-edge XAS. Under basic pH conditions, themeasured spectra suggest the formation of Co3+ at applied potentials just before the catalytic
wave starts. Under turnover conditions, the L3 edge shifts to lower energy, suggesting a Co2+
configuration. This may be because the rate-limiting step is the oxidation of Co2+, and once
this state is formed, water oxidation takes place at a fast rate, and the higher valence statescannot be observed on the time scale of the experiment. This contrasts with what we havemeasured using in situ EPR for the same catalyst, where a Co4+ signal was detected.4
Nevertheless, EPR measurements are not real operando experiments, since the measurement
requires low temperatures (10K). The freezing process may slow down the catalytic cycle andallow the detection of Co4+.
REFERENCES
1.
2.
3.
4.

T. Grewe, M. Meggouh and H. Tuysuz, Chem.-Asian J., 2016, 11, 22-42.
C. Costentin and D. G. Nocera, Proceedings of the National Academy of Sciences, 2017, 114,
13380-13384.
F. Song, L. Bai, A. Moysiadou, S. Lee, C. Hu, L. Liardet and X. Hu, J. Am. Chem. Soc., 2018, 140,
7748-7759.
Y. Kutin, N. Cox, W. Lubitz, A. Schnegg and O. Rüdiger, catalysts, 2019, 9, 926.