Photon Factory Activity Report 2018 #36 (2019)

AR-NW10A/2018G076

Adsorbed Structure of Iodine Species of CsI Restricted in Cylindrical Pore ofSingle-Walled Carbon Nanotube
Yuri Hirano1, Hiroki Nakayasu1, and Takahiro Ohkubo1,*
Department of Chemistry, Graduate School of Natural Science and Technology,
Okayama University,
3-1-1 Tsushimanaka, Kita-ku, Okayama 700-8530, Japan

1 Introduction
To understand properties of hydrated ions under nanoconfined conditions is a subject of great importance. We
have, therefore, engaged in research works to unveil
specific structure of hydrated metal ions confined incarbon nanospaces. Through our investigations, we foundsurplus adsorption phenomena of anions in carbon
micropores whose pore wall is constructed by -electrons.
Such specific adsorption of anion species was firstly
investigated by the adsorption of aqueous RbBr solution
on single-walled carbon nanotube (SWCNT) where theadsorbed amount of Br− was 10-100 times larger than Rb+[1]. In addition, the solution pH, which is typically around
6 at the initial state, increased up to around 10 after theadsorption of ions indicating the strong adsorption ofproton on SWCNT even though the concentration ofproton in the aqueous solution is much smaller than that ofboth Br− or Rb+. Herein, we briefly summarize specificstructure formed around an iodine species of cesium iodide(CsI) confined in the nanospace of SWCNT to study
dependence of anionic species to the surplus adsorption
phenomena.
2 ExperimentCsI was impregnated into the cylindrical micropore ofSWCNT (purchased from Meijo Nano Carbon Co., Ltd.;
average pore width = 1.2 nm) by using CsI aqueoussolution (0.5 M). Here, all CsI species can be successfully
impregnated inside the cylindrical pore because theadsorbed amounts of iodine species on close- and openended SWCNT are 0.02 and 0.94 mmol g-1, respectively
meaning that no CsI species can be adsorbed outside thepore. Such CsI-deposited SWCNT (denoted as SWCNTCsI) was obtained after drying in a desiccator. XAFS
measurements were performed at KEK-PFAR (NW10A)
around the I K-edge region.
3 Results and Discussion
Fig. 1 shows Fourier-transformed spectra of I K-edge
EXAFS spectra of SWCNT-CsI, aqueous CsI solution (CsIaq.), and CsI crystal, respectively. The spectum ofSWCNT-CsI is obviously different from both CsI aq. andCsI crystal. Especially, the largest band around 0.27 nm for
SWCNT-CsI is not observed in any other spectra,
indicating an existence of unique structure initiated by theconfinement of SWCNT pores. We carefully analyzed theband observed in SWCNT-CsI and were able to obtain an
incredible result; the major species of iodine adsorbed on
SWCNT was not I− but I3−. We reached such an argument

because the distance between nearest I-I (0.289 nm) for
SWCNT-CsI was similar to the literature value of I3- (0.290nm) [2]. Generally, the oxidation reaction from I− to I3− inaqueous solution needs three special conditions, i.e. (i) ahighly concentrated I− condition (ii) in an acidic solutionand (iii) UV-light irradiation. However, I3− species wasspontaneously synthesized through only a simple
adsorption process in the present study. This is because ahighly concentrated condition of iodine species wasachieved by the confinement effect of SWCNT andSWCNT could also adsorb H+ species from aqueous
solution to balance between cation and anion species.
Actually, the pH value of aqueous CsI solution increased
from 6.6 to 10.1 by the adsorption of CsI. In addition, we
already reported that a suitable nano-confinement
condition can initiate a unique photoreaction [3], thus, thereaction to I3− must be launched under any room-lightirradiated condition. Therefore, the three conditions
mentioned above might be satisfied to produce I3− from I−
species inside the pore of SWCNT.

|FT|

100.1

0.20.3r / nm

0.40.5

Fig. 1: Fourier-transformed spectra of I K-edge EXAFS
spectra of SWCNT-CsI (red), aqueous CsI (blue), and CsI
crystal (black).
Acknowledgement
This work was partially supported by the financial
support from The Society of Iodine Science (SIS).
References[1] M. Nishi et al., J. Colloid Interface Sci. 508, 415 (2017).
[2] K. E. Aasmundtveit et al., Synth. Met. 101, 363 (1999).
[3] T. Ohkubo et al., J. Colloid Interface Sci. 421, 165(2014).
* ohkubo@okayama-u.ac.jp