Selected-control hydrothermal synthesis of γ-MnO2 3D nanostructures

Article abstract of DOI:10.1021/jp035567r

Highly uniform γ-MnO₂ 3D urchinlike and sisallike nanostructures have been successfully prepared by a common hydrothermal method based on the reaction between MnSO₄ and KBrO₃. Reaction temperature and the additives of the polymers play an important role in influencing the morphologies of the as-obtained products. These urchinlike and sisallike nanostructures, which own the highly specific area on the surface of the particles may provide more possibility to give an ideal host material for the insertion and extraction of lithium ions, to realize region-dependent surface reactivity, and to act as molecular sieves.

Full text of DOI:10.1021/jp035567r

J. Phys. Chem. B 2003, 107, 13583-13587
13583
Selected-Control Hydrothermal Synthesis of γ-ΜnO2 3D Nanostructures
Changzheng Wu, Yi Xie,* Dong Wang, Jun Yang, and Tanwei Li
Structure Research Laboratory, Department of Chemistry, UniVersity of Science and Technology of China,
Hefei, Anhui 230026, P. R. China
ReceiVed: June 4, 2003; In Final Form: September 25, 2003
Highly uniform γ-MnO2 3D urchinlike and sisallike nanostructures have been successfully prepared by a
common hydrothermal method based on the reaction between MnSO4 and KBrO3. Reaction temperature and
the additives of the polymers play an important role in influencing the morphologies of the as-obtained products.
These urchinlike and sisallike nanostructures, which own the highly specific area on the surface of the particles
may provide more possibility to give an ideal host material for the insertion and extraction of lithium ions,
to realize region-dependent surface reactivity, and to act as molecular sieves.
Introduction
oxidizing MnSO4 in KMnO4 and K2S2O8, respectively.6 Very
recently, our group synthesized γ-MnO2 nanowires through a
Because different morphologies and crystallographic forms
are generally believed to be responsible for their properties, how
to control the anisotropic inorganic materials at the mesoscopic
level is one of the most challenging issues presently faced by
synthetic chemists. Recently, research on nanostructures is
expanding rapidly into the assembly of nanoparticles in two-
dimensional (2D) and three-dimensional (3D) ordered super-
structures.1 Much effort has been made in the fabrication of
patterns of well-arranged nanocrystallites, especially the ar-
rangement of one-dimensional nanotubes and nanorods because
of their interesting physical properties and potential applications
in many areas.2 Due to their anisotropic structure, the oriented
growth of nanorods and nanowires is difficult and usually
requires solid templates, such as porous alumina, polymer
nanotubes, and patterned catalysts, to control the direct growth.3
However, the introduction of templates and substrates introduces
heterogeneous impurities and increases the production cost,
which may restrict the wide development of researches and
applications. Thus how to develop facile, mild, easily-controlled,
and template-free methods to create novel patterns on self-
generated homogeneous substrates is of great significance.
Many polymorphic forms of manganese dioxide, such as R-,
â-, γ-, and δ-type, have drawn considerable attention because
of their distinctive properties and now are widely used as
catalysts and electrode materials in Li/MnO2 batteries.4 More-
over, manganese dioxides with well-controlled dimensionality,
size, and crystal structure have also been regarded as critical
factors that may bring some novel and unexpected properties,
for example, isotropic or anisotropic behavior and region-
dependent surface reactivity. Therefore, development of the
morphologically controllable synthesis of MnO2 nanoparticles
is urgently important to answer the demand for exploring the
potentials of MnO2. Over the past few years, remarkable process
has been made in the synthesis of ΜnO2 with different
morphologies and different crystallographic forms. R- and
Tokorokite-type MnO2 were found to have fiber or needle
morphologies.5 Li and co-workers reported the hydrothermal
preparation of R-MnO2 nanowires and â-MnO2 nanorods by
coordination-polymer-precursor route.7 However, how to control
both the morphology and the phase of γ-MnO2 nanowires by
the direct reaction route is still unsolved,6,7 and how to
accomplish highly oriented growth of γ-MnO2 nanostructures
remains a key research challenge.
Herein, we demonstrate a facile selected-control hydrothermal
method to synthesize highly pure uniform urchinlike 3D γ-MnO2
nanostructures (in these structures, nanorods or nanowires grow
radially from the core particles) via the direct reaction between
only MnSO4 and KBrO3. When the polymers (such as poly-
(ethylene glycol)-20000 (PEG-20000), poly(ethylene glycol)-
400 (PEG-400), and poly(vinylpyrrolidone)-K30 (PVP-K30,
average MW 58 000)) were introduced into this system, the
sisallike 3D nanostructures (in these structures, broad leaves
grow radially from the core particles) can be obtained. As we
all know, up to now, there has been no report on selectively
controlled 3D morphologies (urchinlike nanostructures, sisallike
nanostructures) for γ-MnO2 under mild conditions. And the
urchinlike and sisallike nanostructures, which own the highly
specific area on the surface of the particles, may provide more
possibility to give an ideal host material for the insertion and
extraction of lithium ions, to realize region-dependent surface
reactivity, and to act as molecular sieves.
Experimental Section
The chemical reaction that we employed for synthesis of the
γ-MnO2 3D nanostructures can be formulated as
5MnSO4 + 2KBrO3 + 4H2O f
5γ-MnO2 + Br2 + K2SO4 + 4H2O
To prepare urchinlike nanostructures, a 50 mL aqueous solution
containing 2 mmol of MnSO4 and 1 mmol of KBrO3 was put
in a conical flask and was stirred with a magnetic stirrer until
a transparent solution was obtained. The solution was then
transferred into a 60 mL Teflon-lined stainless steel autoclave,
sealed, and was slowly heated to 110 or 130 °C at 1 °C/min.
And then the autoclave was maintained at 130 °C for 16 h.
To prepare γ-MnO2 sisallike nanostructures, 0.4 g of PEG
(or 0.6 g of PVP) was introduced, and the autoclave was heated
to 110 °C (or 90 °C for PVP) and maintained for 16 h.
* Corresponding author. Tel: 86-551-3603987. Fax: 86-551-3603987.
E-mail: yxielab@ustc.edu.cn.
10.1021/jp035567r CCC: $25.00 © 2003 American Chemical Society
Published on Web 11/14/2003

13584 J. Phys. Chem. B, Vol. 107, No. 49, 2003
Wu et al.
The panoramic morphology of the product was obtained by the
field emission scanning electron microscopy (FE-SEM), in
which the solid sample was mounted on a copper mesh without
any dispersion treatment, indicating that all samples (Figure 2A)
are uniform urchinlike nanostructures with diameter of 4-10
µm, and the proportion of the urchinlike nanostructures in the
whole sample is above 85%. Careful observation (Figure 2B,C)
can find that the urchinlike nanostructures consist of uniform
nanorods with the diameter of 40-100 nm and length up to 1
µm. With a raised temperature of 130 °C, we obtained nanowires
with high aspect ratio grown radially from the core particles.
As shown in Figure 3A, these nanowires have uniform diameter
ranging from 20 to 50 nm with length up to 10 µm. More details
about the structure of as-obtained nanowires were investigated
by the electron diffraction (ED) patterns (inset image in Figure
3B) and high-resolution transmission electron microscopy
(HRTEM) (Figure 3B), which indicate that they are structurally
uniform monocrystalline grown along [002] direction.
Figure 1. XRD pattern of the as-obtained γ-MnO2 3D nanostructures
obtained by the direct reaction between MnSO4 and KBrO3 at 110 °C.
After the reaction completed, the resulting black solid product
was filtered, washed with distilled water and absolute ethanol
to remove byproducts, and then dried at 60 °C in air. The
obtained black powders were collected for the following
characterization.
It is well-known that using a polymer-assisted reaction to
control the nucleation and growth is a simple but effective way,
and different environments for nucleation and growth of
products will be created and will affect the morphology of the
precipitations by using different polymers with various molec-
ular weights. For the case of γ-MnO2, sisallike nanostructures
can be synthesized with the addition of polymer. Figure 4A-C
shows the as-obtained sisallike nanostructures synthesized with
the addition of PEG-20000 at 110 °C. The panoramic morphol-
ogy (Figure 4A) of the obtained product shows that the
as-obtained products consist of highly uniform spherical ag-
gregates with a diameter of 5-6 µm that consist of acicular
crystallites radiating from the center (Figure 4B) and these
uniform acicular crystallites are approximately 3 µm long by
200-400 nm wide (Figure 4C). However, when low molecular
weight polymers (PEG-400) were introduced into this system,
the coexistence of the broad leaves and high aspect ratio
nanowires grown from the particles were obtained as shown in
Figure 4D and its magnified image Figure 4E. The use of
another high molecular weight polymer (PVP-K30, average MW
58 000) led to the uniform broad leaves (Figure 4E,F) similar
to those obtained by PEG-20000 (Figure 4A-C), implying that
molecular weight plays an important role in the formation of
broad-leave structures.
The samples were characterized by X-ray powder diffraction
(XRD) pattern with a Japan Rigaku D/max rA X-ray diffrac-
tometer equipped with graphite monochromatized high-intensity
Cu KR radiation (λ ) 1.541 78 Å). The transmission electron
microscopy (TEM) images were performed with a Hitachi model
H-800 instrument with a tungsten filament, using an accelerating
voltage of 200 kV. The field emission scanning electron
microscopy (FE-SEM) images were taken on a JEOL JSM-
6700F SEM. The electron diffraction (ED) patterns and high-
resolution transmission electron microscopy (HRTEM) images
were carried out on a JEOL-2010 TEM at an acceleration
voltage of 200 kV. IR absorption spectra were performed with
a Nicolet FT-IR-170SX spectrometer in the range of 250 ( 450
cm-1 at room temperature with the sample in a KBr disk.
Results and Discussion
XRD Patterns of the Products. X-ray powder diffraction
(XRD) patterns reveal the phase and purity of as-obtained
products. Figure 1 shows a typical XRD pattern of γ-MnO2
urchinlike nanostructures obtained by the direct reaction between
MnSO4 and KBrO3 at 110 °C. All of the reflection peaks can
be readily indexed to pure orthorhombic γ-MnO2 (JCPDS card
14-644, a ) 6.36 Å, b ) 10.15 Å, c ) 4.09 Å). Similar XRD
patterns were obtained from the sisallike nanostructures and
other product of the comparison experiment, all of which
exhibited orthorhombic γ-MnO2. No characteristic peaks are
observed for other impurities such as R-, â-, and δ-MnO2.
The Morphology of the Obtained Product. The urchinlike
nanostructures were obtained through the direct reaction between
MnSO4 and KBrO3 at 110 and 130 °C. Shown in Figure 2 is
the as-obtained urchinlike nanostructures synthesized at 110 °C.
Formation Mechanism of the As-Obtained Sisallike Nano-
structures. To study the influence of the polymer, PEG-20000
was taken as an example. Both IR absorption spectrum (Figure
5) and TEM image (Figure 6A,B) indicate that there is a strong
interaction between the surfaces of the MnO2 nanoparticles and
PEG-20000, although the exact bonding geometry and the nature
of selectivity between different crystallographic planes are still
unclear. To investigate the formation process of the γ-MnO2
sisallike nanostructures, the IR absorption spectrum of the pure
PEG-20000 was measured (Figure 5A). In the IR region, the
Figure 2. FE-SEM images of obtained urchinlike nanostructures at 110 °C: the panoramic morphologies (A), low magnification (B), and high
magnification images (C).

γ-ΜnO2 3D Nanostructures
J. Phys. Chem. B, Vol. 107, No. 49, 2003 13585
the aggregated particles are stable because of the assistance of
the organic additives.10 These as-obtained lamellar structures
will grow longer and thicker during the following hydrothermal
process, and finally the sisallike nanostructures formed. How-
ever, when the low molecular weight polymer (such as PEG-
400) was introduced, the coexistence of broad leaves and some
nanowires grown from the core particles could be seen from
the products. The exact mechanism for the formation of such
morphology resulting from the additive of PEG-400 is still under
investigation; however, it appears to be related to the weaker
interaction between the surface of the aggregated particles and
the low molecular weight polymers due to the higher moving
rate and moving range of low molecular weight polymers. Thus,
some of the lamellar structures will curl and collapse into
nanowires (shown in Figure 4D,E) without the effective
influence of coordination effect. Moreover, the additive of PVP
may provide another proof to confirm the above mechanism,
in that the pure broad leaves grow radially from the core particles
as shown in Figure 4E.
Figure 3. (A) FE-SEM image of the as-obtained urchinlike nano-
structures radiated with high aspect ratio nanowires at 130 °C and (B)
HRTEM image of the γ-MnO2 nanowires covered on urchinlike
nanostructures (inset is its corresponding ED pattern).
absorption peaks at 1110 and 2896 cm-1 are the -O- and C-H
stretching vibrations in PEG-20000, respectively.8 By compari-
son, the obtained precipitates with reaction times of 2, 4, and
12 h were also determined by IR spectra. It was found that the
absorption peeks of the -O- and C-H stretching vibration in
PEG gradually disappeared. On the other hand, the absorption
peaks at 475-710 cm-1, which were assigned to the Mn-O
stretching vibration of γ-MnO2,9 appeared and their intensity
increased with increasing reaction time. These characterization
results reveal that the dissociation of the coordination polymer
and the formation of γ-MnO2 sisallike nanostructures occurred
at the same time, indicating that polymer additive might play a
certain role in the formation of the broad leaves. TEM image
clearly shows that the spherical particle was covered tightly by
the polymers, which provides a further evidence for the function
of the polymer (Figure 6B).
Detailed TEM study on the formation of sisallike nano-
strucrtures was shown in Figure 6, from which one can clearly
see that the polymer covered on the MnO2 particles was thinner
as the MnO2 particles grew larger. When the particle was so
large that the polymer cannot cover the aggregation particle
tightly, the anisotropic growth will occur and some initial broad
leaf structures will form on the edge of the particles as shown
in Figure 6E, from which one can see that there exist no curling
structures. It is worth noting that the intermediate structure as
shown in Figure 6D is similar to the aggregated particle as
shown in Figure 7A, which was obtained by the direct reaction
at 110 °C. These two similar structures lead to two different
results: the former results in the formation of sisallike nano-
structures; however, the latter one leads to urchinlike nano-
structures. We believe that this difference may result from the
influence of the polymers. The lamellar structures grown from
Formation Mechanism of the As-Obtained Urchinlike
Nanostructures. TEM study on sisallike nanostructures clearly
indicates polymers play a crucial role in the formation of broad
leaves on the aggregated particles. In fact, the formation
mechanism of the urchinlike nanostructures is similar to that
of the sisallike nanostructures in the aggregation process, and
then the nanorods and nanowires were produced on the
aggregated particle core as a result of the “rolling-broken-
growth” (RGB) process because the polymers were absent. An
intermediate sample was obtained in an experiment at 110 °C
with a short reaction time of 1 h and was investigated by TEM.
As shown in Figure 7A, one can clearly see the aggregated
particle core, just like that in sisallike nanostructures. Careful
observation of the core particle can find that there are some
thin flakes (Figure 7B) and curling structures (Figure 7C) formed
on the edge of the accumulated particle, which may serve as a
powerful proof that a “rolling-broken-growth”(RBG) happened
and lead to the initial formation of the nanorods and nanowires
in the urchinlike nanostructures. Moreover, the thin flakes
structures were grown out from the aggregated particles,
indicating that the aggregation process may occur before the
formation of the lamellar structures. At the beginning of the
reaction, the MnOx units will appear first in the solution, and
these units tend to aggregate with each other to form the
aggregated particles. Through a condensation reaction, they will
form thin flakes11 on the edge of the aggregated particles (Figure
Figure 4. FE-SEM images: (A-C) the panoramic morphologies, low magnification, and high magnification images of the obtained sisallike
nanostructures with the addition of PEG-20000, respectively; (D-E) low magnification and high magnification images of the as-obtained sisallike
nanostructures with the addition of PEG-400, showing that the coexistence of the broad leaves and some nanowires grown from the core particles;
(F) the obtained sisallike nanostructures with the addition of PVP-K30.

13586 J. Phys. Chem. B, Vol. 107, No. 49, 2003
Wu et al.
Figure 7. The TEM image (A) of the intermediate in the formation
of urchinlike nanostructures and (B,C) the images with higher
magnification of image A, from which one can find the lamellar (B)
and curling structures (C).
Figure 5. IR spectrum study of the function of PEG-20000: (A) PEG-
20000; (B-D) precipitates after reaction for 2, 4, and 12 h, respectively.
Figure 8. (A) The structure of γ-MnO2 with a random arrangement
of single and double chains of MnO6 octahedra.7 The dots represent
the positions of Mn cations. In panel B, the framework of the Mn cations
in the γ-MnO2 crystal lattice is shown, from which the chainlike
building blocks can be seen along [001] direction.
Figure 6. TEM study on the formation mechanism of the sisallike 3D
nanostructures. At the beginning (panel A), the initial nuclei were very
small, while the relatively thicker polymer was tangled seriously. Thus
the newly formed particles can hardly be seen under the heavy cover
as shown in panel A. Then the particle grew bigger (panel B), and the
polymer molecule became loose and sparse to accommodate the grown
particle. When the particle grew large enough, the polymer can hardly
cover the particle (panel C) and formed only an obscure ring in the
TEM image. In panel D, the particle was too large to be covered
completely by the polymer layer and some broad leaves grew out from
the core particle. Panel E shows the magnified image of the rectangle
part in image D.
Along the [001] direction, the chainlike building blocks can be
clearly seen. And the anisotropic nature of these building blocks
in the crystal structure plays a crucial role in the formation of
low-dimension nanomaterials, which provides a possibility for
the formation of γ-MnO2 nanowires in the soft solution
processing strategy. As for the influence of temperature, it is
generally believed that a higher temperature is preferable for
the anisotropic growth of crystal and results in the product with
high aspect ratio grown from the core particles (Figure 3A);
meanwhile, at a lower temperature of 110 °C, shorter nanorods
will be obtained as shown in Figure 2.
7B). The thin flakes tend to curl (Figure 7C) under the elevated
temperature and pressure and will collapse into nanorods without
any assistance via “rolling-broken-growth” (RGB) process.
During the following hydrothermal process, the shorter ones
may redissolve into the solution phase, and the longer ones grow
into much longer ones, similar to the formation of Se12 and Ag13
nanowires.
It is worth noting that the aspect ratios of the final products
are determined by the anisotropic growth of the corresponding
crystal structures. We believe that the formation of γ-MnO2
nanowires is actually the outward embodiment of the nature of
the initial crystal structure. Although γ-MnO2 consists of a
random arrangement of single and double chains of MnO6
octahedra14 (Figure 8A), it is found that in γ-MnO2 each Mn
cation is connected to three adjacent cations through oxygen
anions to build up distorted Mn hexagonal units in layers, which
are composed of Mn cation frameworks shown in Figure 8B.7
Conclusions
In conclusion, we have developed a facile selected-control
hydrothermal method to synthesize highly uniform urchinlike
γ-MnO2 nanostructures via the mild and the direct reaction
between MnSO4 and KBrO3. When the polymers (such as PEG
and PVP) were introduced into this system, the sisallike
nanostructures with narrow or broad leaves can be obtained via
the addition of polymers with different molecular weight,
respectively. Possible mechanism was proposed to elucidate their
formation. These urchinlike and sisallike nanostructures, which
own the highly specific area on the surface of the particles, may
bring some novel and unexpected properties, for example,
molecular sieves and catalysts, especially for the Li/MnO2

γ-ΜnO2 3D Nanostructures
J. Phys. Chem. B, Vol. 107, No. 49, 2003 13587
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Acknowledgment. This work was supported by National
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