The role of lattice vibration in the terahertz region for proton conduction in 2D metal-organic frameworks

概要

The role of lattice vibration in the terahertz
region for proton conduction in 2D
metal-organic frameworks
著者

journal or
publication title
volume
page range
year
URL

Tomoya Itakura, Hiroshi Matsui, Tomofumi Tada,
Susumu Kitagawa, Aude Demessence, Satoshi
Horike
Chemical Science
11
1538-1541
2020-01
http://hdl.handle.net/10097/00130751
doi: 10.1039/C9SC05757A

Chemical
Science
View Article Online

Open Access Article. Published on 16 December 2019. Downloaded on 12/2/2020 7:50:39 AM.
This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.

EDGE ARTICLE

Cite this: Chem. Sci., 2020, 11, 1538
All publication charges for this article
have been paid for by the Royal Society
of Chemistry

View Journal | View Issue

The role of lattice vibration in the terahertz region
for proton conduction in 2D metal–organic
frameworks†
Tomoya Itakura,a Hiroshi Matsui, *b Tomofumi Tada,
Aude Demessence e and Satoshi Horike *dfgh

c

Susumu Kitagawa,

d

We studied the relationship between proton conductivity and the terahertz-regime vibrations of twoReceived 13th November 2019
Accepted 15th December 2019

dimensional MOFs. The results of spectroscopy studies clarified the essential role played by the

DOI: 10.1039/c9sc05757a

collective motions in the terahertz region in 2D layers for efficient H+ conduction. Ab initio calculations
suggested the collective motion to be predominantly determined by the valence electronic structure,

rsc.li/chemical-science

depending on the identity of the metal ion.

Solid state ion conductors are in wide demand for batteries, fuel
cells, electrochemical sensors and electrochemical catalysis.
The design and elucidation of ionically conductive paths in
solid structures have, however, been signicant challenges in
relation to these applications.1 Ion conduction generally
proceeds as a result of ions hopping to neighboring coordination sites, followed by local structural relaxation. These
processes are oen predominantly determined by lattice
vibrations below 10 THz (333 cm
1).2,3 Indeed, ion conductivity
in some representative inorganic materials correlates with the
terahertz vibration. For a-AgI, for example, Ag+ behaves as
a liquid and conducts as a consequence of sub-lattice melting
with corresponding sub-lattice vibration modes of about 5–
30 cm
1.4 For the typical proton (H+) conductors CsHSO4 and
BaCeO3, the collective vibration modes of protonic species (i.e.,

a

DENSO Corporation, 1-1, Showa-cho, Kariya, Aichi 448-8661, Japan

b

Department of Physics, Graduate School of Science, Tohoku University, 6-3, Aramaki
Aza-Aoba, Aoba-ku, Sendai 980-8578, Japan

c
Materials Research Center for Element Strategy, Tokyo Institute of Technology,
Nagatsuta-cho, Midori-ku, Yokohama, Kanagawa 226-8501, Japan
d

Institute for Integrated Cell-Material Sciences, Institute for Advanced Study, Kyoto
University, Yoshida-Honmachi, Sakyo-ku, Kyoto 606-8501, Japan. E-mail: horike@
icems.kyoto-u.ac.jp

e
Universit´e Claude Bernard Lyon 1, Institut de Recherches sur la Catalyse et
l'Environnement de Lyon (IRCELYON), UMR 5256 CNRS, Villeurbanne, France
f

AIST-Kyoto University Chemical Energy Materials Open Innovation Laboratory
(ChEM-OIL), National Institute of Advanced Industrial Science and Technology
(AIST), Yoshida-Honmachi, Sakyo-ku, Kyoto 606-8501, Japan

g
Department of Synthetic Chemistry and Biological Chemistry, Graduate School of
Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan
h

Department of Materials Science and Engineering, School of Molecular Science and
Engineering, Vidyasirimedhi Institute of Science and Technology, Rayong 21210,
Thailand
† Electronic supplementary
10.1039/c9sc05757a

information

1538 | Chem. Sci., 2020, 11, 1538–1541

(ESI)

available.

See

DOI:

HSO4
and lattice OH
, respectively) have been observed at 58–
170 cm
1 and 320–378 cm
1.2
Studies of ion conductivity for molecular-based crystals have
been recently showing rapid progress.5 Of such crystals, metal–
organic frameworks (MOFs) are a promising class of materials
for obtaining different ion conductivities.6 Researchers have
shown the ability to control the ion conductivity by making use
of guests in the channels, redox activity in metal centers, and
ligand dynamics or acidity/basicity in the frameworks. Although
there are many reports on the synthesis of ion-conductive
MOFs, there are few about observations of any relationship
between ion conductivity and lattice vibration. Several reports
have described the unique lattice dynamics and functions of
MOFs in the terahertz region,7 but it is also important to
elucidate using spectroscopy the relationship between lattice
dynamics and ion conductivity in MOFs. For this purpose, we
employed three isostructural MOFs having two-dimensional
(2D) H+-conductive pathways, and characterized their lattice
vibrations in the terahertz region to describe their mechanism
of H+ conductivity. We also carried out ab initio calculations to
analyze vibrational mode characteristics and to study the
contribution of metal ions to the lattice vibration.
We prepared three 2D MOFs having different metal ions as
shown in Fig. 1. Each of the compounds may be described using
the formula [M(H2PO4)2(TrH)2], where M ¼ Zn2+, Co2+, or Mn2+,
and TrH ¼ 1,2,4-triazole, and we denote the compounds as
ZnTr, CoTr, MnTr, respectively.8 The crystal structure of ZnTr is
shown in Fig. 1A and B, and the parameters of the crystal
structures of CoTr and MnTr are summarized in Table S1.† All
three compounds crystallized in the same crystal system and
space group (orthorhombic-Pbcn). H2PO4
was observed to be
coordinated to the metal ion axially to form a hydrogen bond
network in the ab plane, with the extended H-bonds with
H2PO4
motion providing for the long-range hopping of H+

This journal is © The Royal Society of Chemistry 2020

View Article Online

Open Access Article. Published on 16 December 2019. Downloaded on 12/2/2020 7:50:39 AM.
This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.

Edge Article

Fig. 1 (A) The crystal structure of ZnTr along the bc plane. (B) The
hydrogen-bond network formed by PO4 tetrahedra in ZnTr in the ab
plane. Zn2+ and TrH are omitted for clarity. Zn: gray, P: yellow, O: red,
C: black, N: blue, H: pink. (C) Plots of the H+ conductivity of ZnTr
(circle), CoTr (triangle), and MnTr (square) under dry N2 gas from 30 to
150  C. The dashed lines represent the fittings of the linear expression
to the data. (D) Infrared spectra at 27  C, with regions I and III attributed
to intramolecular modes of TrH and PO4 tetrahedra, region II attributed to the O–H modes, band A to the O–H stretching mode, and
bands B and C to the Fermi resonance with combinations involving
O–H bending modes.

through the layer. The O(H)–O hydrogen bond distances
between PO4 groups were measured to vary by less than 0.046
˚ along the a and b directions, respectively. The X-ray
and 0.073 A
results indicated negligible differences between the crystal
structures of the three compounds.
The ratio of the amount of H2PO4
to that of metal ion was
determined using ICP-AES for each of ZnTr, CoTr, and MnTr
and found to be 1.58, 1.63, and 1.55. The lower value of the
measured relative amount of H2PO4
compared with the stoichiometric amount in the crystal structure was attributed to
H2PO4
site defects as described previously.9 These results also
indicated the quantities of structural defects for the three
compounds to be comparable. Therefore, the H+ conductivities
for these structures were attributed to the mobility levels of the
H2PO4
groups in the layers.
Fig. 1C shows the plots of anhydrous H+ conductivity under
an N2 atmosphere. The conductivity of ZnTr was found to be the
highest: the values at 150  C were measured to be 4.6  10
4
(ZnTr), 1.9  10
6 (CoTr) and 9.1  10
8 S cm
1 (MnTr). The
total activation energy of ZnTr was determined to be 0.50 eV,
about half of those of CoTr and MnTr (1.06 and 1.17 eV). The
considerable differences between the H+ conductivities of these
MOFs were found despite the near identities of their crystal
structures and compositions, implying the predominant inuence of other factors on the conductivity.
Infrared (IR) spectroscopy measurements at 27  C were taken
to investigate the characteristics of the hydrogen bonds
(Fig. 1D). Regions I and III were attributed to intramolecular

This journal is © The Royal Society of Chemistry 2020

Chemical Science
modes of TrH and PO4 tetrahedra, and region II to O–H modes.
We assigned three broad bands, known as ABC bands and
observed here from 1600 to 3100 cm
1 to the O–H modes,
specically to O–H stretching modes in Fermi resonance with
combinations involving O–H bending mode, and are typically
seen in strong hydrogen-bonding systems such as metal dihydrogenphosphates.10 Although two kinds of hydrogen bonds in
MTr were in theory identied from the crystal structure, we
could not distinguish between them due to small difference
between their O–O distances. These O–H modes were indicated
to be identical in their band shapes and frequencies, consistent
with the results of their similar O–O distances from single
crystal XRD (Table S1†). The IR spectroscopy results did not
provide information about any distinct H+ conductivity.
To investigate the lattice vibration, we performed terahertz
time-domain spectroscopy (THz-TDS) at 27  C (Fig. 2A–C). We
also computed the theoretical spectra using ab initio quantum
mechanical calculations to identify the vibration modes (dotted
lines and bars in Fig. 2A–C). The calculations were performed

Fig. 2 (A–C) THz-TDS spectra of (A) ZnTr, (B) CoTr, and (C) MnTr at
27  C. The dotted lines are spectra obtained using DFT, applied to
a 5 cm
1 FWHM Lorentzian-shaped line. The colors of the bars
represent the types of motions of the PO4 tetrahedra: R mode (red), Ta
and Tb mode (blue), and Tc mode (green). ...

この論文で使われている画像