Facile synthesis of metal-organic cobalt hydroxide nanorods exhibiting a reversible structural transition

Article abstract of DOI:10.1039/c1cc14384k

Metal-organic cobalt hydroxide nanorods with controllable dimensions using salicylates as bridging ligands have been synthesized in water under mild conditions in a simple process. The nanorods exhibit a reversible structural transition on dehydration and rehydration with a concomitant color change and may find potential applications in nanodevices.

Full text of DOI:10.1039/c1cc14384k

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COMMUNICATION
Facile synthesis of metal–organic cobalt hydroxide nanorods exhibiting a
reversible structural transitionw
Lianying Wang,a Sen Lu,a Yuexi Zhou,b Xiaodi Guo,a Yanluo Lu,a Jing He*a and
David G. Evansa
Received 20th July 2011, Accepted 30th August 2011
DOI: 10.1039/c1cc14384k
Metal–organic cobalt hydroxide nanorods with controllable
dimensions using salicylates as bridging ligands have been
synthesized in water under mild conditions in a simple process.
The nanorods exhibit a reversible structural transition on
dehydration and rehydration with a concomitant color change
Scheme 1
and may find potential applications in nanodevices.
which requires neither sophisticated techniques nor surfactants
or modulators, and show that the material undergoes a novel
reversible structural transition and color change on dehydration
and rehydration. To the best of our knowledge, this is the first
time that one-dimensional (1D) nanostructured MOFs have
been shown to undergo a reversible crystal-to-crystal dehydration/
rehydration process and as a result represents a significant
breakthrough in this area of chemistry.
Metal–organic frameworks (MOFs, also known as coordination
polymers) have attracted increasing attention due to their
ability to give functional materials with promising potential
applications in catalysis, sensors, optics, gas storage, and
magnetism.1 In particular their reversible crystal-to-crystal
structural transformations under the influence of external
stimuli such as heat or guest species,2 which can lead to
changes in physical properties—for example, color—have
important implications for their use in devices. However,
reported examples of this type of material are quite rare and
limited to macrosize particles. For many applications in
optical, electrical, magnetic and sensing devices and in medi-
cine, however, it is necessary to limit the crystallite size of
MOFs to the nanoscale.3
We use a simple commercially available salicylate as the
organic linking block, based on its strong coordination ability
with metal ions.10 The approach is very simple and involves
directly mixing solutions of cobalt nitrate hexahydrate
and sodium salicylate (NaHsal, where Hsal denotes
o-HOC6H4COOÀ) in water under pH-controlled conditions
(Scheme 1). Typically, an aqueous solution (100 ml) of
Co(NO3)2Á6H2O (0.02 mol) was added to an aqueous solution
(150 ml) of NaHsal (0.04 mol) with continuous stirring. The
pH of the reaction mixture was adjusted to 7 with 0.5 M
NaOH solution, which ensures that the phenol proton of
salicylate (pKa2 13.5)11 remains non-ionized. The mixture
was aged at 95 1C for 48 h. The resulting pink product was
separated by centrifugation, washed repeatedly with distilled
water, and dried at 40 1C in air. Elemental analysis of the
nanorods was consistent with the chemical composition
Co(OH)(Hsal)ÁH2O (Anal. Calcd: Co 25.54, C 36.36, O,
34.63, H 3.46%; found: Co 25.35, C 36.38, O, 34.71, H
3.54%).
Over the past decade, although a variety of procedures—
such as microfluidics,4 reverse microemulsions,5 solvothermal
approaches,6 sonochemistry,7 and microwave-assisted8 and
coordination modulation methods9—have been employed
in attempts to prepare nanosized MOFs, however, these
procedures typically involve expensive reagents (organic
solvent, organometallic precursor, surfactant and modulator)
or complex manipulations and equipment. And moreover, it
still remains a great challenge to control the morphology, size,
composition, stoichiometry, and/or crystal structure. In this
communication, we report a facile, economical, and scalable
synthesis of metal–organic cobalt hydroxide nanorods with
uniform and tunable size in water under mild conditions,
The morphology of the synthesized nanorods was examined
by SEM. As shown in Fig. 1A and B, the product exhibits a
consistent rod-like morphology with an average length of
about 7 mm and diameter of 130 nm. The diameter of the
nanorods is uniform throughout their length and most of
the nanorods tend to form bundles of aligned nanorods. The
sample uniformity and narrow diameter distribution are also
confirmed by the TEM image (Fig. 1C) of a sample of
nanorods cast from a colloidal suspension onto a substrate.
a State Key Laboratory of Chemical Resource Engineering, Beijing
University of Chemical Technology, Beijing, 100029, P. R. China.
E-mail: jinghe@263.net.com; Tel: +86-10-64425280
b Department of Water Pollution Control Technology, Chinese
Research Academy of Environmental Sciences, Beijing, 100012,
P. R. China
w Electronic supplementary information (ESI) available: Instrumenta-
tion, FT-IR, PXRD, TGA, UV-visible diffuse reflectance and EDX
spectra, and SEM image. See DOI: 10.1039/c1cc14384k
c
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Fig. 2 (A) TGA curve of Co(OH)(Hsal)ÁH2O nanorods. (B) PXRD
patterns of Co(OH)(Hsal)ÁH2O nanorods heating at different temperatures
in N2 for 4 h and after subsequent treatment with water. (C) Photographs
of (a) original Co(OH)(Hsal)ÁH2O nanorods, (b) after heating at 300 1C,
and (c) the nanorods in (b) after treatment with water. (D) UV-visible
diffuse reflectance spectra of Co(OH)(Hsal)ÁH2O nanorods before (a) and
after heating at 300 1C (b).
Fig. 1 (A) Low-resolution SEM image, (B) high-resolution SEM
image, (C) TEM image, and (D) EDX spectrum of Co(OH)(Hsal)Á
H2O nanorods prepared at 95 1C. SEM images of Co(OH)(Hsal)Á
H2O nanorods prepared at (E) 70 1C and (F) 30 1C.
layered cobalt hydroxide intercalated with terephthalate
ligands, in which the CoII ions are octahedrally coordinated
by m3-OH and m3-Z1,Z2-carboxylate groups.13 The FT-IR
spectra (Fig. S4, ESIw) confirm that the carboxylate groups
are coordinated to the CoII ions, as evidenced by the multiple
characteristic asymmetric and symmetric COOÀ bands
centered at 1570/1538 and 1453/1401 cmÀ1, respectively,14
whereas the corresponding bands of the uncoordinated NaHSal
occur at 1586 and 1387 cmÀ1, respectively. The presence of the
salicylate n(C–O) band at 1237 cmÀ1 confirms that the phenol
group has not been deprotonated.15 A broad band centered
around 3390 cmÀ1 indicates the presence of water molecules
and extensive hydrogen bonding. The intense and sharp peak
at 3584 cmÀ1 is attributed to the O–H stretching mode of the
Co–OH groups, similar to that observed for layered cobalt
hydroxides.14,16
The sample consists of exclusively nanorods, consistent with
the SEM images. The EDX spectrum of the nanorods
(Fig. 1D) shows the presence of Co, C, and O with a Co/C
À
ratio of 1 : 7 and no N signal from NO3 detected, which
agrees well with the elemental analysis results. Additional Pt
signals arise from the Pt coating which was applied on the
sample to avoid a charging effect.
Further experiments revealed that the dimensions of the
nanorods can be easily reduced by simply decreasing the
reaction temperature, with all other conditions identical to
those described above. The nanorods prepared at 70 1C were
found to have a length of about 1 mm and diameter of 70 nm
(Fig. 1E). When the temperature was further decreased to 30 1C,
nanorods 400 nm in length and 33 nm in diameter were
obtained (Fig. 1F). EDX results show that the nanorods
prepared at different temperatures have the same chemical
composition (Fig. S1 and S2 in the ESIw). We should stress
here that the dimensions of the nanorods can be tailored by
simply varying the reaction temperature in the absence of any
surfactant or coordination modulator, and this may provide a
new way to control the size of other nanoscale MOFs. The
significance of these findings can be understood in terms of the
fact that the existing techniques, which are successful in
controlling the size of nanostructured MOFs, rely on the
surfactant or coordination modulator.5,9
Good thermal stability of MOFs is an important prerequisite
for many practical applications. The thermal stability of the
nanorods synthesized at 95 1C was investigated by TGA in N2
(Fig. 2A). The two weight loss steps from 30 to 200 1C
correspond to the removal of the water of crystallization and
a second water molecule formed by protonation of the
coordinated hydroxide ligand by the hydroxyl group of the
salicylate moiety. The observed mass loss of ca. 16.1% agrees
well with what is expected for the loss of two water molecules
from the formula Co(OH)(Hsal)ÁH2O (15.6%). The weight
remained constant from 200 to 400 1C and after that the
framework started to decompose. The resulting product at 650 1C
consists of, according to XRD (Fig. S5, ESIw), cobalt/carbon
nanocomposites.17
PXRD studies (Fig. S3, ESIw) show that the nanorods
prepared at different temperatures are crystalline and have
the same structure, which closely match the bulk phase of
cobalt salicylate.12 They show a feature typical of layered
structure with strong peaks at 6.86 (d = 12.87 A), 13.76
(6.43 A), and 27.82 (3.20 A), which can be assigned to the
basal reflection and its second and fourth-order harmonics of
a layered structure. Based on the following detailed character-
ization, the structure of the nanorods may be similar to that of
It is especially interesting to note that dehydration followed
by rehydration of the nanorods induces a reversible structure
transformation. As shown in Fig. 2B, the PXRD pattern of the
sample after heating in N2 for 4 h at 150 1C does not indicate
much change in crystallinity. A phase transition occurs at 200 1C,
characterized by a decrease in the intensity of diffraction peaks
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This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 11002–11004 11003

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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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11004 Chem. Commun., 2011, 47, 11002–11004
This journal is The Royal Society of Chemistry 2011