Photon Factory Activity Report 2005 #23 Part B (2006)
Atomic and Molecular Science

7C/2004G079

Structural anomalies of hydrated Cu2+ confined in slit-shaped carbon nanospace
Yohko NOBUHARA1, Takahiro OHKUBO3, Hiroshi NOGUCHI1, Yoshiyuki HATTORI2,Takehisa KONISHI2, Hirofumi KANOH1, 2, Tanaksi FUJIKAWA1,2, Katsumi KANEKO*1, 2
1Graduate school of science, Chiba Univ., Inage, Chiba 263-8522, Japan2
Department of chemistry, Faculty of science, Chiba Univ., Inage, Chiba 263-8522, Japan
3
Research institute for Sci. & Tech., Tokyo Univ. of Sci., Noda, Chiba 278-8510, Japan
Introduction
Molecules are confined in nanospace to form anordered structure [1-3]. The structures of confinedmolecules in nanospace are of interest in variety of areas
such as electrolytic solutions and electric double layer.
Recently, Okubo et al. showed a characteristic hydration
structure of Rb ion confined in slit-shaped solid
nanospace by EXAFS analysis [4,5]. The understandingof hydration structure confined in nanospace has offeredtopics for research in nanospace science. In this work, wecharacterized the hydration structures of ionic solution ofCu in slit-shaped carbon nanospace by means of EXAFS
technique and the structural anomalies of ionic solutionsin nanospace will be discussed.
Experimental
Pitch-based activated carbon fibers (A7 and A20) wereused as a nanoporous carbon. The average width of slitshaped nanospace in A7 and A20 was evaluated to be 0.7
and 1.1 nm, respectively. The Cu ionic solution wasintroduced into slit-shaped nanospace and dried in vacuo
at 383 K for 2 h. After the dispersion of Cu ions, watervapor was adsorbed in the slit-shaped nanospace. In thisreport, we denote the hydrated Cu ions in the nanospace
as “nanosolution”. The XAFS measurements wereperformed on Cu K-edge at BL-7C beam line withtransmission mode. Samples were adjusted in a sufficientquality to give an edge jump and installed in a glass cellwith windows of Lumirror film. The program code
FEFF8 was used to calculate the theoretical phase andamplitude for curve fitting.
Results and Discussion
Figure1 shows the radial distribution function of Cu
nanosolutions and aqueous bulk solutions of 1 mol/l. The
peak centered at 0.15 nm assigned to the first hydration
shell of Cu ion. This figure shows that Cu-O distance ofCu nanosolution is almost the same as that of the bulksolution. Table1 shows the best fits data of Fourier filtered
EXAFS spectra for first hydration shell. The coordination
number around a Cu ion in the nanosolution is quit small
compared with that in the bulk solution. Also, the DebyeWaller factor of nanosolution decreased by 2.5%
compared to the bulk solution. The change in thehydration structures of nanosolutions becomesremarkable with the decrease in the pore width, indicatingthat the hydration structures in the nanospace are strongly

4Users' Reportaffected by the pore walls. Water molecules confined innanospace cannot form the hydration structure in thesame as in the bulk solution. These results suggest thatthe anomalous hydration structure was stabilized by themolecular potential field in slit-shaped carbon nanospace.

Figure1. The radial distribution functions of Cu bulk
solution and nanosolutions. NSN(0.7) and NSN(1.1)
denote nanosolutions formed in the nanospace of 0.7 and1.1 nm, respectivery.
Table1: The local structural parameters around Cu ion.
22NR/ Åσ /Å
NSN (0.7)
2.1
1.93
0.0017
NSN (1.1)
3.4
1.92
0.0028
Bulk
4.0
1.95
0.068
N: hydration number, R: Cu-O distance, σ: DebyeWaller factor

References[1] T. Iiyama et al., J. Phys. Chem., 99, 10075 (1995).
[2] T. Ohkubo at al., J. Phys. Chem. B, 103, 1859 (1999).
[3] K. Kaneko, Carbon, 28, 287 (2000).
[4] T. Ohkubo et al., J. Am. Chem. Soc., 124, 11860(2002).
[5] T. Ohkubo et al., J. Phys. Chem. B, 107, 13616 (2003).
*kaneko@pchem2.s.chiba-u.ac.jp