Metastable one-dimensional AgCl1-xIx solid-solution wurzite tunnel crystals formed within single-walled carbon nanotubes

Article abstract of DOI:10.1021/ja0173270

High-resolution transmission electron microscopy and spatially resolved electron loss spectroscopy have revealed that a eutectic mixture of AgCl and AgI crystallizes within single walled carbon nanotubes (SWNTs) as metastable AgCl_1-xI_x 1D solid solution crystals. The incorporated halide crystals form wurzite tunnel structures with locally varying Cl:I ratios and reduced Ag coordination. Copyright

Full text of DOI:10.1021/ja0173270

Published on Web 02/16/2002
Metastable One-Dimensional AgCl1-xIx Solid-Solution Wurzite “Tunnel”
Crystals Formed within Single-Walled Carbon Nanotubes
Jeremy Sloan,*,†,§ Mauricio Terrones,¶,‡ Stefan Nufer,¶ Steffi Friedrichs,† Sam R. Bailey,†
Hee-Gweon Woo,| Manfred Ru¨hle,¶ John L. Hutchison,§ and Malcolm L. H. Green†
Inorganic Chemistry Laboratory, South Parks Road, Oxford, OX1 3QR, United Kingdom, Department of Materials,
South Parks Road, Oxford, OX1 3PH, United Kingdom, Max Planck Institut fu¨r Metallforschung, Seestrasse 92,
D-70174 Stuttgart, Germany, and Department of Chemistry, Chonnam National UniVersity, 300 Yongbong-dong,
Puk-ku, Kwangju, 500-757, Korea
Received October 18, 2001
Single-walled carbon nanotubes (SWNTs)1 are 1-2 nm diameter
sp2 carbon cylinders capable of hosting a variety of species,
including 1D crystals of metals,2 metal salts2 and oxides;3 helical
iodine chains;4 and chains of fullerene5a or endofullerene molecules.5b,c
The synthesis of such composites has been driven by a desire to
study such materials when confined to a low-dimensional environ-
ment. For example, it has been possible to image individual
fullerenes5 and bulk crystalline structures confined to the molecular
scale.2-4 Using high-resolution transmission electron microscopy
(HRTEM) image restoration, it has been possible to study the latter
on an atom-by-atom basis2f and also to determine simultaneously
the chirality of the host SWNTs.3b Despite this level of character-
ization, it has proved difficult to study inclusions containing more
than two elements. While we previously inserted AgBr-AgCl and
KCl-UCl4 mixtures into SWNTs,2b detailed information concerning
local crystallinity or composition was unavailable. Here we describe
the application of HRTEM and spatially resolved electron energy
loss spectroscopy (EELS) to the systematic characterization of a
eutectic AgCl-AgI mixture formed inside SWNTs. This composi-
tion contains one strongly scattering halogen (i.e.: I, Z ) 53 vs Cl,
Z ) 17), which should facilitate preferential imaging via HRTEM
and scanning transmission electron microscopy (STEM). We also
show how the eutectic forms metastable 1D “tunnel” structures
within SWNTs.
Samples of SWNTs were prepared by catalytic arc synthesis6
and a premelted molten 53:47 mol % mixture conforming to the
AgCl-AgI eutectic7 was introduced via a published method.2b The
specimen was examined in a 300 kV JEOL 3000F HRTEM (0.16
nm point resolution). Energy-dispersive X-ray spectra (EDX) were
obtained with a 0.5 nm probe (LINK “ISIS” system). EELS line
scans were obtained with an energy dispersion of 0.1 eV/channel
and a 1.0 nA/1.0 nm2 probe in a VG HB501UX STEM with a Gatan
Digi-PEELS 766 detector. Pre- and post-scan high-angle annular
dark field (HAADF) images were obtained from the specimen.
HRTEM showed that ca. 50% of the observed SWNTs contained
filling that was predominantly crystalline, although ca. 30% was
glassy. The crystalline filling consisted of the following: (i) metallic
Ag filling (Figure 1a-c), presumed to originate from dissociation
of the halide mixture (cf. ref 2b), and (ii) crystalline halide filling
(Figure 1d-i). The latter is assumed to be metastable as solid
eutectic AgCl-AgI should be polycrystalline. EDX spectra obtained
from (ii) indicated that it contained Ag, Cl, and I.
Figure 1a shows a ca. 2 nm diameter SWNT filled with a fcc
type Ag metal for which 010 is parallel to the SWNT growth
axis and 001 is parallel to the electron beam. Measurements from
the image and corresponding fast Fourier transform (Figure 1b)
produced an average lattice spacing orthogonal to the SWNT axis
of ca. 0.21 nm, corresponding to d020 for bulk Ag.8 The SWNT
walls are visible as dark lines either side of the Ag nanocrystal,
which is visualized as eight {200} layers arranged parallel to the
SWNT axis (Figure 1c).
Figure 1d-f shows a HRTEM image, enlargement, and corre-
sponding FFT of crystalline halide filling formed in a 1.4 nm
diameter SWNT. An average periodicity of ca. 0.4 nm was
established along the SWNT axis, which was characteristic of all
the crystalline halide filling. Figure 1e reveals that the microstructure
consists of an ordered array of distorted dark spots. The 0.4 nm
spacing and spot configuration suggest a 1D “tunnel” structure
derived from wurzite AgI9 (i.e. Figure 1g-i). Wurzite AgI consists
of stacked hexagonal double layers of Ag and I separated by ∼0.37
nm (i.e. d0029). According to our model, each dark spot corresponds
to a staggered X-Ag-X or Ag-X-Ag column (Figure 1g, bottom).
Inspection of Figure 1e reveals that the dark spots vary unsystem-
atically in contrast, an effect that we attribute to random distribution
of weaker scattering Cl and stronger scattering I over the X sites
(Figure 1g,h; see also Supporting Information). It is noteworthy
that the formation of such a structure would result in reduction in
coordination for Ag from tetrahedral to trigonal, which may explain
the small increase in d002 (cf. 4:4 KI in SWNTs2e).
EELS line scans were used to probe the local compositions of
SWNT bundles such as the example given by the HAADF image
in Figure 2a. Figure 2b shows an end-on view of a 3D plot of 26
spectra obtained along I-II. The height of the dominant C-K
absorption edge gives an indication of the thickness of the bundle
with respect to probe position. A representative EELS spectrum
with resolved Cl-L2,3, Ag-M4,5, and I-M4,5 edges is shown in Figure
2c. The I and Cl edges were always observed together whenever
halide was detected, indicating that the AgCl-AgI mixture melted
congruently into the SWNTs. The perspective view of the 3D plot
(Figure 2d) shows the evolution of the EELS spectra along I-II.
The Cl, Ag, and I edges are absent from the group of peaks
corresponding to the SWNT nearest to I, indicating that this tube
is empty according to the configuration inset in Figure 2d. Cl:I
ratios determined from counts obtained from integrated Cl and I
absorption edges varied from ca. 10:1 to 1:10, thus supporting the
observation above that local concentrations of Cl and I vary on an
unsystematic basis. Additional EELS line scans confirmed the
† Inorganic Chemistry Laboratory.
§ Department of Materials.
¶ Max Planck Institute.
‡ Present address: IPICyT, Me´xico.
| Chonnam National University.
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2116 VOL. 124, NO. 10, 2002 J. AM. CHEM. SOC.
10.1021/ja0173270 CCC: $22.00 © 2002 American Chemical Society

C O MMU N I C A T I O N S
Figure 1. (a) HRTEM image of Ag crystal within a SWNT (scale bar ) 1 nm). (b) FFT from the boxed region in part a. Arrow indicates scattering from
SWNT. (c) Structure model. (d) HRTEM image of crystalline AgCl1-xIx filling (scale bar ) 2 nm). (e) Detail from part d (scale bar ) 0.2 nm). (f) FFT from
part d. (g) Side-on and end-on representations of proposed “tunnel” structure. (h) Staggered ball-and-stick model showing Ag coordination. (i) Derivation
of tunnel structure from the AgI wurzite structure.
1D AgCl1-xIx crystals represent the lowest dimensionality recorded
for any solid-solution crystal system.
Acknowledgment. We acknowledge the Petroleum Research
Fund, administered by the ACS (Grant No. 33765-AC5), and the
EPSRC (Grant Nos. GR/L59238 and GR/L22324). J.S. is indebted
to the Royal Society, M.T. to the A. v. Humboldt Stiftung, S.N. to
the Deutsche Forschungsgemeinshaft (Grant No. Ru 342/11-2), S.F.
to BMBF and Fonds der Chemischen Industrie, and H.G.W. to the
RSC Journals Grants 2000 and KRF grant (2000-D00165).
Supporting Information Available: AgCl-AgI phase diagram;
derivation of the 1D wurzite tunnel structure and HRTEM image
simulation; additional integrated EELS profiles (PDF). This material
is available free of charge via the Internet at http://pubs.acs.org.
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