Photon Factory Activity Report 2019 #37 (2020)

BL-13A/2018G597

Partial density of states in functional zeolite by soft x-ray emission spectroscopy
Shinya HOSOKAWA,1,* Hitoshi SATO,2 and Yasuhisa TEZUKA3
Department of Physics, Kumamoto University, 2-39-1 Kurokami, Chuo-ku, Kumamoto 860-8555,
Japan
2Hiroshima Synchrotron Radiation Center, Hiroshima University, 2-313 Kagamiyama, HigashiHiroshima 739-0046, Japan3
Department of Advanced Physics, Hirosaki University, 3 Bunkyo, Hirosaki 036-8561, Japan

1 Introduction
Zeolites are functional materials that the nature ismanufactured by self-organization. They are mostly
composed of aliminosilicate networks with a considerable
number of holes with a nanometer size [1], and one gramof zeolite possesses a surface of holes with an area wider
than a tennis court. Zeolites have many applications, suchas classical ones of molecular sieves and adsorbents formoistures, radioactive Cs ions etc., or recently high durable
exhaust gas purification devices by P-including ones,
petrochemical catalysts by hydrophobic ones, antibacterialagents by Ag in their holes, and so on. More than two
million tons per year are manufactured all over the world,and thus, human lives do not work without zeolites.
To consider the functions of zeolites, three parts should
be carefully discussed, i.e., 1) frameworks made of mainly
Si, Al, and O atoms, 2) Cations located at the hole surface
like Na+ compensating the fourfold Al- ions, and 3)
molecules or nano-scaled atomic groups adsorbing into theholes. Although the applications of zeolites are widely
spread in the human lives, basic knowledges on thesematerials are not so rich, and thus, higher advancements
are not improved on the basis of the fundamental
understandings of basic scientific properties. This was
because in addition to complex structures of zeolites, mostof active sites for the functions, such as Al- ions in theframeworks, cations at the hole surfaces, and adsorbed
atomic groups, have no long-range periodicity, whichmakes difficult to determine atomic positions by usual
diffraction methods. As regards the electronic structures, it
is difficult to measure photoemission spectroscopy since
they are insulators.
To investigate electronic density of states (DOS) forinsulating materials, soft x-ray emission spectroscopy(SXES) and soft x-ray absorption spectroscopy (SXAS)
are available with element- and angular momentum
quantum number-selective [2]. By using these methods, wepreviously measured O 2p DOS of SiO2 [3] and B2O3 [4]glasses. In this article, we report results of SXES andSXAS spectra on one of the typical zeolites, mordenite(Na2O·Al2O3·20SiO2), and discuss the O 2p DOS.
2 Experiment
Synthetic mordenite powder with an one-dimensionalholes of about 0.66 nm and a sample size of about 1 µm
was supplied by Toyo Soda Manufacturing Co., Ltd. (No.
TSZ 640NAA). The mordenite powder was washed bydistilled water. The powder sample was pressed to make apellet with a diameter of 10 mm and a thickness of about 1mm. It was dehydrated by heating it at 150°C undervacuum in the sample introduction device.
SXES and SXAS measurements were carried out at
BL13A of the PF-KEK. The SXES spectra were measured
by using a Rowland-type monochromator with a sphericalgrating with a radius of 5 m and a line density of 1200
line/mm and a photon detector with a CsI-coatedmultichannel plate. The energy resolution of SXES isabout 0.3 eV. Details of the experimental setups are given
elsewhere [5].
The SXES spectra of O 2p valence-band partial DOS
were obtained with incident photon energies of 535-550 eV,
beyond the corresponding 1s absorption edge. The SXAS
spectra of O 2p conduction-band partial DOS were
measured around the O 1s absorption edge in total electron
yield mode. The energy resolution of SXAS was higher
than 0.1 eV. All the SXES and SXAS experiments werecarried out at room temperature under ultrahigh vacuum
condition of less than 10-7 Pa.
3 Results and Discussion
Figure 1 shows the SXAS spectra of mordenite near theO K absorption edge, corresponding to the O 2p
conduction-band partial DOS. At a glance, the spectralfeatures are very similar to those of crystalline and glassy
SiO2 [3]. However, the pre-shoulder structures are very
prominent at 533-535 eV, which may be related to thecovalent-ionic bonding to the Al+ ions. Besides, there areseveral characteristic structures as indicated by arrows,XAS intensity (Arb. units)

1Mordenite
530

535

540
545
Energy (eV)550

Fig. 1: The SXAS spectra of mordenite near the O K
absorption edge.

Photon Factory Activity Report 2019 #37 (2020)

which were used for the incident photon energies, E0, forthe subsequent SXES measurement.
Figure 2 shows the SXES spectra of mordenite,
corresponding to the O 2p valence-band partial DOS, atseveral E0 values from 533.7 to 550 eV. The SXES spectra
exhibit four characteristic peaks and shoulders as indicatedby dashed lines. Most of the structures do not change withvarying E0.
4.5

Reduced XES intensity (Arb. units)

4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
510

Mordenite

E0 (eV) =
550.0
543.5
540.0
537.5
536.7
535.2
533.7
515

520

525

Energy (eV)

530

535

Fig. 2: SXES spectra of mordenite at several E0 values.
At the lower E0 values of 533.7 and 535.2 eV, however,the prominent peak at about 526 eV shifts towards thelower energies, and the shoulder at about 524 eV changes
the spectral shapes. These anomalies would be related tothe origin of the pre-shoulder in Fig. 1, or in other words,
the dispersion relations of O 2p electronic bands near theband gap, reflecting the bond natures of O atoms with Aland Si atoms.
An ab initio molecular dynamics simulation is now inprogress for calculating the electronic DOS and forclarifying the role of Al- ions for the electronic propertiesof mordenite. The most serious problem for simulating it
in a first principle sense is a large size of unit cell
containing one hundred atoms, which will besophisticatedly solved very soon.
References[1] D. W Breck, Zeolite Molecular Sieves: Structure,Chemistry, and Use (Wiley, New York, 1974).
[2] A. Kotani and S. Shin, Rev. Mod. Phys. 73, 203 (2001).
[3] S. Hosokawa, H. Sato, K. Mimura, Y. Tezuka, D.
Fukunaga, Y. Matsuzaki, and F. Shimojo, J. Phys. Soc.
Jpn. 84, 024605 (2015).
[4] S. Hosokawa, H. Sato, K. Mimura, Y. Tezuka, D.
Fukunaga, K. Shimamura, and F. Shimojo, J. Phys. Soc.
Jpn. 83, 114601 (2014).

[5] S. Shin, A. Agui, M. Fujisawa, Y. Tezuka, T. Ishii, andN. Hirai, Rev. Sci. Instrum. 66, 1584 (1995).
* shhosokawa@kumamoto-u.ac.jp