at 2y 4 101, which becomes more marked on raising the
temperature further to 350 1C. The position of the first
diffraction peak corresponding to the basal reflection of the
layered structure is unchanged even after 4 h of heating at 350 1C,
although there is some decrease in its intensity. Accompanying
this structural change is a dramatic color change from pink to
dark blue (Fig. 2C). The UV-visible diffuse reflectance spectra
(Fig. 2D) were measured in order to determine the absorption
features responsible for the color change. The spectrum of the
original nanorods is characteristic of CoII ions in an octahe-
dral geometry with a broad major peak at 550 nm and two
shoulders around 470 and 500 nm.16 In contrast, the spectrum
of the dehydrated nanorods displays a pair of transitions at 638
and 580 nm arising from CoII in tetrahedral coordination,18
and a band at 470 nm which can be attributed to octahedral
CoII with a higher ligand field splitting than in the original
material.
framework is perhaps due to the presence of the phenol group
in the Hsal ligands, which can interact with the CoII center, as
seen by FT-IR, stabilizing the high temperature state, but
allowing a return to the original structure on rehydration.20
The other properties and potential applications of the material
deserve further investigation. For example, the presence of
unsaturated four-coordinate CoII sites in the high temperature
form suggests it should have a high ability to adsorb guest
molecules.1b,2b,2f
This work was supported by the National Natural Science
Foundation of China, the Major Projects on Control and
Rectification of Water Body Pollution (No. 2008ZX07207-
004) and the 973 Program (No. 2011CBA00504).
Notes and references
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When the dehydrated nanorods were immersed in water or
exposed to air for several days at room temperature, they
completely reverted to their initial color (Fig. 2C). The original
structure was also recovered, as shown by comparing the
PXRD pattern recorded after treating the nanorods dehy-
drated at 300 1C with water (Fig. 2B, top) with the original
powder pattern (Fig. 2B, bottom). Accordingly, the UV-visible
diffuse reflectance spectrum and TGA curve of the rehydrated
nanorods, as shown in Fig. S6 and S7 (ESIw), respectively, are
identical to those of the original material. It is noteworthy that
the intrinsic shape of the nanorods is also retained during the
dehydration/rehydration cycle (Fig. S8, ESIw).
We also investigated the structural transformation of the
nanorods using FT-IR spectroscopy (Fig. S9, ESIw). The FT-
IR spectrum of the dehydrated nanorods confirms the removal
of the water molecule and hydroxide group by virtue of the
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.
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c
11004 Chem. Commun., 2011, 47, 11002–11004
This journal is The Royal Society of Chemistry 2011