Synthesis and formation mechanism of nanoneedles and nanorods of manganese oxide octahedral molecular sieve using an ionic liquid

Article abstract of DOI:10.1021/jp0569739

Single-crystalline cryptomelane-type manganese oxide octahedral molecular sieve (OMS-2) nanoneedles and nanorods were prepared by a solution-phase approach in the presence of an ionic liquid 1-n-butyl-3-methylimidazolium tetrafluoroborate ([BMIM]BF₄). [BMIM]BF₄ can act as a cosolvent, structure-directing agent, and reducing reagent in the reaction system. Based on the redox reaction of MnCl₂ and KMnO₄ in the mixed solvents of water and [BMIM]BF₄, the formation of OMS-2 nanoneedles followed the rolling mechanism with lamellae as an intermediate. However, the direct reaction of KMnO₄ with [BMIM]BF₄ resulted in the formation of OMS-2 nanorods with diameters as small as 3-6 nm. The formation mechanism of OMS-2 nanostructures was discussed.

Full text of DOI:10.1021/jp0569739

J. Phys. Chem. B 2006, 110, 6609-6614
6609
Synthesis and Formation Mechanism of Nanoneedles and Nanorods of Manganese Oxide
Octahedral Molecular Sieve Using an Ionic Liquid
Li-Xia Yang,†,§ Ying-Jie Zhu,* Wei-Wei Wang, Hua Tong,†,§ and Mei-Ling Ruan‡
,†
†,§
State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Analysis and
Testing Center for Inorganic Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences,
Shanghai 200050, and Graduate School of the Chinese Academy of Sciences, People’s Republic of China
ReceiVed: December 1, 2005; In Final Form: February 15, 2006
Single-crystalline cryptomelane-type manganese oxide octahedral molecular sieve (OMS-2) nanoneedles and
nanorods were prepared by a solution-phase approach in the presence of an ionic liquid 1-n-butyl-3-
methylimidazolium tetrafluoroborate ([BMIM]BF
agent, and reducing reagent in the reaction system. Based on the redox reaction of MnCl
mixed solvents of water and [BMIM]BF , the formation of OMS-2 nanoneedles followed the rolling mechanism
with lamellae as an intermediate. However, the direct reaction of KMnO with [BMIM]BF resulted in the
4
). [BMIM]BF
4
can act as a cosolvent, structure-directing
2
and KMnO in the
4
4
4
4
formation of OMS-2 nanorods with diameters as small as 3-6 nm. The formation mechanism of OMS-2
nanostructures was discussed.
1
. Introduction
a variety of preparation methods have enriched the recognition
to the formation mechanisms and in hence expanded a new
regime in physical and chemical applications. One basic crystal
growth mechanism in solution systems is the well-known
In view of today’s environmental concerns, room-temperature
ionic liquids (RTILs), novel green recyclable solvents, have
gained a great deal of attention as a new class of compounds
for replacement of conventional organic solvents due to their
high fluidity, low melting temperature and extended temperature
range in the liquid state, air and water stability, low toxicity,
nonflammability, high ionic conductivity, ability to dissolve a
variety of materials, and importantly no measurable vapor
“
Ostwald ripening process”, in which the larger particles will
2
0
grow at the expense of the smaller ones. Recently, the
oriented attachment” mechanism was reported by Penn and
“
2
1
Banfield. While other mechanisms have also been proposed,
for example, the rolling mechanism,
mechanism,
recrystallization mechanism, the understanding of the forma-
tion mechanism is the key point for the realization of controlled
synthesis of one-dimensional nanomaterials.
22-24
solution-liquid-solid
25,26
1-4
and nucleation-dissolution-anisotropic growth-
pressure. Therefore, RTILs have found applications in various
fields such as organic chemical reactions, catalysis, extractions,
2
7
and electrochemistry.3-7 Recently, RTILs have aroused much
interest as new reaction media in the inorganic nanomaterials
synthetic field because there are many kinds of RTILs and their
solubility with water can be varied from complete miscibility
to immiscibility, which can be used to prepare different
morphologies of inorganic materials. Several nanomaterials
have been successfully synthesized through employing RTILs
Octahedral molecular sieves (OMS) of mixed-valent man-
2
8-31
ganese oxide have widespread applications in catalysis,
ion-
3
1
32
exchange, and cathode in batteries. Cryptomelane-type
manganese oxide (OMS-2) is one group of the OMS family,
containing 2 × 2 tunnel structure with a pore size of 4.6 Å. In
recent years, increasing research interest has been aroused with
respect to OMS-2 nanomaterials. A number of synthetic methods
for OMS-2 nanomaterials have been reported, for example,
oxidation of Mn by KMnO or H O by refluxing in acidic
hydrothermal treatment of birnessites, and sol-
gel-assisted solid-state reaction using cross-linking reagents.
8
,9
8
,9
reaction systems, for example, TiO2 nanocrystals,
gold
nanoparticles and nanosheets,1
0,11
palladium nanoparticles,
12
13
nanorods and nanowires of tellurium, Bi2S and Sb2S nano-
rods,1 and CoPt nanorods. However, this new kind of reaction
media still needs to be expanded in other inorganic systems.
The exploration of facile preparation methods for nanostruc-
tures and systematic control over their shape and properties are
the main aims that material scientists are pursuing. Particularly,
control over morphology and hence over dimensionality is of
great interest. One-dimensional nanostructured materials such
as nanotubes, nanorods/nanowires, and nanobelts/nanoribbons
are receiving more attention with respect to their potential
applications in nanodevices as compared to their bulk or
4
15
2+
4
2
2
33,34
35
conditions,
36
The reflux route is commonly used to prepare OMS-2 nano-
materials. However, these methods usually need relatively high
temperatures or long reaction time and sometimes complex
preparation procedures. Fast and simple methods for the
synthesis of OMS-2 nanomaterials with specific morphology
are desirable for their applications.
Herein, we report a facile solution-phase approach for the
synthesis of OMS-2 nanoneedles and nanorods using an ionic
liquid [BMIM]BF4 for the first time. [BMIM]BF4 was used as
a cosolvent, structure-directing agent, and reducing reagent in
the formation of OMS-2. The multifunctional role of [BMIM]-
BF4 simplifies the preparation procedure to the synthesis of
OMS-2 one-dimensional nanomaterials at a relatively low
temperature for a short period of time.
16-19
spherical nanoparticles of the same composition.
Up to now,
*
Corresponding author. Fax: +86-21-52413122. E-mail: y.j.zhu@
mail.sic.ac.cn.
†
State Key Laboratory of High Performance Ceramics and Superfine
Microstructure, Shanghai Institute of Ceramics.
‡
Analysis and Testing Center for Inorganic Materials, Shanghai Institute
of Ceramics.
§
Graduate School of the Chinese Academy of Sciences.
1
0.1021/jp0569739 CCC: $33.50 © 2006 American Chemical Society
Published on Web 03/11/2006

6
2
610 J. Phys. Chem. B, Vol. 110, No. 13, 2006
Yang et al.
. Experimental Section
All chemicals used were of analytical grade and were used
as received without further purification. The ionic liquid used
was 1-n-butyl-3-methylimidazolium tetrafluoroborate ([BMIM]-
BF4), and its chemical structure is shown schematically below:
Synthesis of Cryptomelane Nanoneedles. In a typical
procedure, MnCl2‚4H2O (36 mg) was dissolved in [BMIM]BF4
(0.8 mL) in a 10-mL tube by magnetic stirring at room
Figure 1. XRD pattern of sample 3 prepared at 90 °C for 60 min.
The detailed preparation procedure is described in the Experimental
Section.
temperature. Next, 19 mg of KMnO4 was dissolved in 0.8 mL
of deionized water. The MnCl2 solution was microwave-heated
to 90 °C, and then the KMnO4 aqueous solution was added
dropwise. The mixed solution was maintained at 90 °C for 30
s (sample 1), 10 min (sample 2), and 60 min (sample 3),
respectively, and then microwave heating was terminated and
the solution was cooled to room temperature. The microwave
oven used was a focused single-mode microwave synthesis
system (Discover, CEM). The products were separated by
centrifugation, washed with deionized water and ethanol, and
dried at 60 °C in air.
Synthesis of γ-MnO2 Nanosheets. The same procedure as
for the preparation of cryptomelane nanoneedles of sample 2
was used, except that deionized water was used instead of
[BMIM]BF4 in equal volume and that the quantity of KMnO4
was decreased to 10 mg. This sample was denoted as sample
4.
Synthesis of Cryptomelane Nanorods. Typically, 0.8 mL
of [BMIM]BF4 in a 10-mL tube was microwave-heated to 90
C, and 0.8 mL of KMnO4 (containing 19 mg of KMnO4)
°
aqueous solution was added dropwise. The solution was
maintained at 90 °C for 10 min (sample 5) and 60 min (sample
6
), respectively.
Characterization. X-ray powder diffraction (XRD) was
performed on a Rigaku D/max 2550 V X-ray diffractometer
with high-intensity Cu KR radiation (λ ) 1.54178 Å) and a
graphite monochromator. Elemental analysis was carried out
on an electron probe microanalyzer (EPMA, JEOL JXA-8100)
and an energy dispersive spectrometer (EDS, INCA Energy).
Scanning electron microscopy was performed with a JEOL JSM-
Figure 2. (a-c) TEM micrographs and (d) HRTEM micrograph of
sample 3; the inset of (b) is the corresponding SAED pattern of a single
OMS-2 nanoneedle.
transmission electron microscopy (HRTEM) micrographs of the
OMS-2 nanoneedles. Figure 2a shows that sample 3 consisted
of nanoneedles with diameters in the middle varying from 8 to
50 nm and lengths of 100-800 nm. A few nanoneedles have
lengths below 100 nm. A typical morphology was presented in
Figure 2b with clear steps, which were indicated by arrows.
The SAED pattern (inset of Figure 2b) taken from a randomly
chosen nanoneedle by focusing the electron beam along the
[111] direction indicates the single-crystalline structure of the
OMS-2 nanoneedle. The reflections can be indexed as (1-10)
and (2-1-1) of a tetragonal OMS-2, which is consistent with
the XRD result (Figure 1). Figure 2c shows the OMS-2
nanoneedle structure consisting of a number of nanorods with
diameters as small as several nanometers. The HRTEM image
of a typical nanoneedle with a step is shown in Figure 2d, from
which one can see that the lattice spacings in two sides of the
steps were the same (0.694 nm), corresponding to the (110)
plane. The distance between lattice planes is consistent with
that from XRD and SAED.
6
700F field emission scanning electron microscopy (FESEM),
and the sample was coated with platinum. Transmission electron
microscopy (TEM) and selected-area electron diffraction (SAED)
were obtained using a JEOL JEM-2100F field emission
transmission electron microscope.
3
. Results and Discussion
Figure 1 shows the XRD pattern of sample 3 prepared at 90
°
C for 60 min using MnCl2, KMnO4 in the mixed solution of
deionized water and [BMIM]BF4. The product was a single
phase of OMS-2 with a tetragonal structure, which indicates
that OMS-2 crystals could be obtained by microwave heating
at 90 °C for 60 min in the presence of [BMIM]BF4. Elemental
analysis of sample 3 shows that three elements K, Mn, and O
were detected. Assuming that it follows the stoichiometric
composition of MnO2, the structure formula calculated was K0.10-
MnO2‚xH2O.
The morphology of OMS-2 samples was investigated by
TEM. Figure 2 shows the typical TEM and high-resolution
To investigate the formation mechanism of OMS-2 nanon-
eedles, the effect of reaction time was studied. Sample 1 was

Synthesis of Nanoneedles and Nanorods
J. Phys. Chem. B, Vol. 110, No. 13, 2006 6611
Figure 4. The schematic demonstration of the formation process of
OMS-2.
According to the results reported by Li et al. and Wei et al.
2+
-
who employed an aqueous system containing Mn and MnO4
Figure 3. (a,b) TEM micrographs of sample 1; the inset of (b) is the
corresponding SAED pattern. (c,d) TEM micrographs of sample 2; the
inset of (d) is the corresponding SAED pattern. (e, f) XRD patterns of
samples 1 and 2, respectively.
to prepare one-dimensional MnO2 nanostructures, the intermedi-
ate product was birnessite, a kind of manganese dioxide with
23,24
layered structure.
However, in the presence of [BMIM]BF4,
no birnessite was obtained. In this case, the formation process
of OMS-2 nanoneedles is suggested schematically in Figure 4.
At the early stage, the lamellae of a layered material with poor
prepared at 90 °C for 30 s and sample 2 for 10 min, while other
preparation conditions were the same as for sample 3. As shown
in Figure 3e, the XRD pattern of sample 1 was poorly
crystallized. EDS result shows the existence of K, Mn, and O;
however, the K/Mn molar ratio (0.01) was lower than that in
sample 3 (0.1). Sample 2 has an XRD pattern (Figure 3f) similar
to that of sample 3 and can be indexed to a single phase of
OMS-2 with a tetragonal structure, except that the relative
intensity of the (110) peak was lower than that of sample 3,
indicating that sample 3 has a more preferred orientation of
nanoneedle structures. The nanoneedles tend to lie on the (110)
planes, as revealed by HRTEM (Figure 2d). Sample 1 shows
lamellar morphology (Figure 3a and b). Most of them have
curling edges with a tendency to roll up, and the thicknesses of
the nanosheets estimated were about 1-3 nm. A few of them
have grown into tubular nanostructures, as indicated with arrows
in Figure 3a. The SAED pattern taken from the lamellae shows
a poorly crystallized structure. When the reaction time was
prolonged to 10 min (sample 2), many nanoneedles were
observed (Figure 3c). Lamellae and irregular nanoparticles were
also observed as minor morphologies. The diameters in the
middle of the nanoneedles were below 50 nm, and lengths were
up to several hundred nanometers. A typical SAED pattern (inset
of Figure 3d) shows the single-crystalline structure of the
nanoneedle.
+
crystallization formed, [BMIM] adsorbed on layers of the
precursor. FTIR of sample 1 confirmed the CdC stretch
-
1 46
vibration of the imidazolium ring at 1585 cm , indicating
+
the presence of [BMIM] in sample 1. Thermogravimetric
analysis of sample 1 showed a weight loss of ca. 10% between
100 and 220 °C, corresponding to an exothermic peak ca. 200
°C in the DSC curve, which was due to the removal of the
surface-bound ionic liquid, consistent with the FTIR result. The
+
+
adsorption of [BMIM] greatly reduced the number of K
stabilized in the interlayer. At the same time, the big size and
+
hydrophobicity of BMIM molecule made the immobilization
of this layered structure difficult, resulting in the final collapse
of layered precursor, and favoring the exfoliation process of
the layered precursor and the formation of the tunneled
cryptomelane. We suggest that [BMIM]BF4 played a role as a
structure-directing agent in the formation of OMS-2.
In the formation of cryptomelane, the morphology evolved
from lamellae to nanoneedles with an increase in the reaction
time, as shown in Figure 3a-d. Recently, the synthesis of
layered precursors and their transformation to one-dimensional
nanostructures via a rolling mechanism were reported.2
3,24,37-46
The controlled formation of lamellae was a key point in the
synthesis of one-dimensional materials. On the basis of the

6612 J. Phys. Chem. B, Vol. 110, No. 13, 2006
Yang et al.
Figure 6. XRD pattern of (a) sample 5 and (b) sample 6 prepared
using [BMIM]BF and KMnO at 90 °C for 10 and 60 min, respectively.
4 4
obtained OMS-2 with a tetragonal structure (Figure 6b),
implying that [BMIM]BF4 acted as a reducing reagent to react
with KMnO4. From the chemical structure of [BMIM]BF4 (see
Experimental Section), one can see that the organic cation
+
[
BMIM] consists of a five-membered heterocyclic ring in
Figure 5. (a) FESEM micrograph, (b) XRD pattern, and (c,d) TEM
micrographs of γ-MnO (sample 4). The inset of (d) is the corresponding
2
SAED pattern.
which there are two nitrogen atoms. We deduce that the reducing
ability of [BMIM]BF4 is attributed to the nitrogen atoms of the
five-membered heterocyclic ring. The reducing property has
recently been reported for poly(vinylpyrrolidone) (PVP) and
above observation, a possible rolling mechanism of OMS-2
nanoneedles was proposed. A piece of lamella with a certain
elasticity tended to roll up into nanoneedles when the system
was supplied with adequate energy. The nanoneedles with steps
were formed by the lamellae with irregular shapes, as repre-
sented in Figure 2d. In fact, most of the lamellae were
overlapped like flowers; a single nanoneedle formed from an
individual lamella was seldom observed. Most nanoneedles
consisted of a number of assembled nanoneedles with smaller
sizes. In this process, longer time was needed to ensure the
recrystallization of the product because Ostwald ripening made
the larger nanoneedles grow at the expense of dissolving the
smaller ones.
4
8
pyrrole. There is also a five-membered heterocyclic ring
containing a nitrogen atom in the PVP or pyrrole molecule. It
was proposed that the reducing property of PVP or pyrrole
resulted from the nitrogen atom of the five-membered hetero-
4
8
cyclic ring. The XRD peaks are wider than those in Figure 1,
indicating this product consisted of smaller nanoparticles. The
EDS result was similar to that of sample 3, with a K/Mn molar
ratio close to 0.11.
TEM micrographs of samples 5 and 6 are given in Figure 7.
One can see nanoparticles and short nanorods in Figure 7a and
b. Lattice fringes are not clearly visible in the HRTEM image
of a single rod in Figure 7c, indicating a relatively poor
crystallization. However, nanoparticles almost vanished in
sample 6 (Figures 7d-f). OMS-2 nanorods of sample 6 have
diameters mostly ranging from 3 to 6 nm and lengths up to
200 nm. Each nanorod is straight and has a uniform diameter
along its entire length, and it is quite different from the
nanoneedle morphology of sample 3. The aspect ratios of most
nanorods are in the range from 1 to 20, but a few of nanowires
with aspect ratios higher than 20 are also observed. HRTEM
image in Figure 7f shows that nanorods are single crystalline
with an interplanar spacing of about 0.690 nm, corresponding
to the (110) lattice spacing of OMS-2. Lattice fringe of 0.483
nm was also observed, coincident with the (200) plane of OMS-
2.
In the methods for the synthesis of OMS-2 involving the
reaction of Mn2 and MnO4 , they were usually conducted
under refluxing or hydrothermal conditions in aqueous solution
for a long period of time to ensure the crystallization of OMS-
+
-
2
. In contrast, a relatively lower reaction temperature and a
greatly reduced reaction time were used in our method. [BMIM]-
BF4 did have some influence on this reaction system. In this
case, [BMIM]BF4 was substituted with water as the only solvent.
As a result, γ-MnO2 nanosheets were obtained (sample 4). The
XRD pattern of sample 4 was shown in Figure 5b, indicating
the formation of a single phase of γ-MnO2 with an orthorhombic
structure. It is interesting that sample 4 consisted of γ-MnO2
nanosheets with the thicknesses of 6-20 nm and lateral sizes
up to 200 nm (Figure 5a). TEM micrograph in Figure 5c
represents a typical sheetlike morphology of γ-MnO2 with some
standing nanosheets. The SAED pattern (inset of Figure 5d)
taken from an individual nanosheet shows the single-crystalline
structure. The diffraction spots can be indexed as the (131)
reflections of orthorhombic γ-MnO2. In some cases, hexagonal
nanosheets were observed as indicated in Figure 5a. Similar
morphology was reported when refluxing the aqueous solution
containing Mn(ClO4)2, (C2H5)4NOH, and CsMnO4 at 100 °C
for 11 days.47
Apparently, the formation of OMS-2 nanorods differs from
that of nanoneedles. The formation process may be explained
as follows: once KMnO4 aqueous solution was added into
[
BMIM]BF4, amorphous MnOx nuclei formed immediately,
which acted as the precursor, and then crystal growth occurred.
At the early stage, the coexistence of short nanorods and
irregular nanoparticles was observed. With the reaction going
on, the nanorods show an anisotropic growth from the central
particles (Figure 7b). The irregular nanoparticles vanished and
the longer nanorods formed, suggesting that the longer nanorods
grew at the expense of smaller particulates. The formation
process of nanorods/nanowires follows a nucleation-dissolu-
tion-anisotropic growth-recrystallization mechanism.27 The
formation of â-MnO2 nanotubes was also proposed to follow
The direct reaction of KMnO4 with [BMIM]BF4 without using
MnCl2 was also investigated. After heating at 90 °C for 10 min,
the XRD pattern shows that sample 5 was poorly crystallized
(
Figure 6a). By increasing the reaction time to 60 min, we

Synthesis of Nanoneedles and Nanorods
J. Phys. Chem. B, Vol. 110, No. 13, 2006 6613
Figure 7. (a-c) TEM and HRTEM micrographs of sample 5. (d-f) TEM and HRTEM micrographs of sample 6.
such a mechanism.49 For the reaction system of KMnO4 and
MnCl2 in the mixed solvents of [BMIM]BF4 and water, the
nanoparticles shown in Figure 3c resulted from the reaction of
KMnO4 with [BMIM]BF4, the nucleation-dissolution-aniso-
tropic growth-recrystallization mechanism cooperated with the
rolling mechanism.
nanoneedles followed the rolling mechanism with lamellae as
an intermediate. However, the direct reaction of KMnO4 with
[BMIM]BF4 resulted in the formation of OMS-2 nanorods. This
method is simple, fast, and surfactant-free. We expect that this
method may be extended to synthesize nanostructures of other
metal oxides.
Based on the above discussion, the ionic liquid [BMIM]BF4
acted as a cosolvent, structure-directing agent, and reducing
agent in the formation of OMS-2 nanoneedles/nanorods, which
played a crucial role in this reaction system. When water was
used as the only solvent to substitute [BMIM]BF4, γ-MnO2
nanosheets (sample 4) were obtained. In contrast, OMS-2
nanoneedles formed in the presence of [BMIM]BF4. In fact,
manganese dioxide has several polymorphic forms, such as R-,
â-, γ-, and δ-type. Cryptomelane can be classified as R-MnO2,
which means that cryptomelane is constructed from double
chains of [MnO6] octahedra forming 2 × 2 tunnels (R-MnO2)
Acknowledgment. Financial support from the National
Natural Science Foundation of China (50472014) and the
Chinese Academy of Sciences under the Program for Recruiting
Outstanding Overseas Chinese (the Hundred Talents Program)
is gratefully acknowledged. We also thank the Fund for
Innovation Research from the Shanghai Institute of Ceramics,
Chinese Academy of Sciences.
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