Rational Growth of Various r-MnO2 Hierarchical
Structures and β-MnO2 Nanorods via a Homogeneous
Catalytic Route
Zhengquan Li,†,‡ Yue Ding,‡ Yujie Xiong,‡ and Yi Xie*,†,‡
CRYSTAL
GROWTH
& DESIGN
2005
VOL. 5, NO. 5
1953-1958
School of Chemical and Materials Engineering, Southern Yangtze University, Wuxi,
Jiangsu, 214036, People’s Republic of China, and Nanomaterials and Nanochemistry,
Hefei National Laboratory for Physical Sciences at Microscale, University of Science and
Technology of China, Hefei, Anhui 230026, People’s Republic of China
Received May 19, 2005
ABSTRACT: Heterogeneous catalytic reactions involved in the vapor-liquid-solid (VLS) or solution-liquid-solid
(SLS) growth of 1-D nanostructures have been intensively investigated in recent years. But how to control the
homogeneous catalytic route and apply it to design new structures of materials has rarely been discussed. In this
work, a homogeneous catalyst was successfully controlled via changing the catalyst feed way, and three novel R-MnO2
hierarchical structures, namely, urchin-like structures, sphere networks, and nanowire networks, were selectively
synthesized at room temperature. β-MnO2 nanorods were also prepared via this method at higher temperatures.
The present work indicates that the homogeneous catalytic route not only can produce one certain structure of
inorganic materials but also can be rationally controlled and therefore used to grow more new structures. On the
other hand, this homogeneous catalytic method is a solution-based self-assembly route, providing a simple, mild,
and template-free way to the fabrication of hierarchical structures.
Introduction
In recent years, 1-D nanostructures have been demonstrated to exhibit superior electrical, optical, mechanical, and thermal properties, showing their potential
applications as building blocks in microscaled devices.1-4
To achieve the goal of the integration of 1-D nanostructures, developing rational routes to construct 2- or 3-D
hierarchical structures is of great significance. Many
efforts have been made on the synthesis of hierarchical
1-D nanostructures, and several hierarchical structures,
such as hierarchical ZnO nanostructures, penniform
BaWO4 nanostructures, and a trigonal Se nanowire
network, have been successfully obtained.5-7 In particular, organization of 1-D nanostructures by a solution-based self-assembly route is very attractive due to
its mildness, simplicity, and large-scale production.8
Heterogeneous catalytic reactions are usually involved in the vapor-liquid-solid (VLS) or solutionliquid-solid (SLS) growth of 1-D nanostructures since
catalysts can act as energetically favorable sites for
adsorption of reactant molecules.9,10 Hitherto, a homogeneous catalytic route has been rarely used in the
formation of 1-D nanostructures. It is generally believed
that a homogeneous catalyst can reduce the potential
energy of a chemical reaction, but whether it can control
the growth of inorganic materials is rarely discussed.
Recently, we have reported that a novel R-MnO2 coreshell structure could be obtained by introducing a
homogeneous catalyst of a Ag+ solution,11 identifying
* Corresponding author. E-mail: yxielab@ustc.edu.cn. Tel: 86-5513603987. Fax: 86-551-3603987.
† School of Chemical and Materials Engineering, Southern Yangtze
University, Wuxi, Jiangsu, 214036, People’s Republic of China.
‡ Nanomaterials and Nanochemistry, Hefei National Laboratory for
Physical Sciences at Microscale, University of Science and Technology
of China, Hefei, Anhui 230026, People’s Republic of China.
the feasibility of this idea. But how to control the
homogeneous catalytic route, and then apply it to design
new strucutrues of materials, is a new challenging work
faced by us.
In this work, we first promote a homogeneous catalytic route to prepare R-MnO2 solid urchin-like structures. Then, we control the homogeneous catalytic route
by changing the catalyst feed way and successfully
obtain two new hierarchical structures (R-MnO2 sphere
networks and nanowire networks) through different
manipulations. Moreover, β-MnO2 nanorods are prepared via this method when lifting the reaction temperature. The present work indicates that the homogeneous catalytic route not only can produce one certain
structure of inorganic materials but also can be rationally controlled and used to design more new structures.
On the other hand, this homogeneous catalytic method
is a solution-based self-assembly route, having lots of
advantages such as its low cost, mildness, convenience,
and use without additional templates and apparatus.
Thus, it is very easily scaled up to industrial application
and brings us new light to the synthesis and integration
of functional materials.
Experimental Procedures
Synthesis. Five identical homogeneous solutions were
prepared by mixing MnSO4‚H2O (0.3380 g, 2 mmol) and
(NH4)2S2O8 (0.4564 g, 2 mmol) in 50 mL of distilled water, and
each of them was used once to prepare a target nanostructure
of R- or β-MnO2. A 10 mL AgNO3 solution used as a catalyst
was prepared by dissolving solid AgNO3 (0.1052 g, 0.059 mmol)
in 10 mL of distilled water. Ag foil was also used to slowly
feed Ag+ as a catalyst without any extra treatment. The
concentration of sulfuric acid was 98%.
(a) Synthesis of r-MnO2 Urchin-Like Structures. A 1
mL AgNO3 solution was added in a 50 mL solution of MnSO4‚
H2O and (NH4)2S2O8. After the homogeneous solution stood
10.1021/cg050221m CCC: $30.25 © 2005 American Chemical Society
Published on Web 08/19/2005
1954
Crystal Growth & Design, Vol. 5, No. 5, 2005
for 2 days at room temperature, the products were filtrated
off, washed with absolute ethanol and distilled water for
several times, respectively, and then dried in a vacuum
(product 1).
(b) Synthesis of r-MnO2 Sphere Networks. A 2 cm × 2
cm Ag foil was put in the bottom of a 50 mL solution of MnSO4‚
H2O and (NH4)2S2O8 and then stood for 3 days at room
temperature. The black products were washed with absolute
ethanol and distilled water several times, respectively, and
then were collected and dried in a vacuum (product 2).
(c) Synthesis of r-MnO2 Nanowire Networks. A 2 cm
× 2 cm Ag foil was put in the bottom of a 50 mL solution of
MnSO4‚H2O and (NH4)2S2O8. When the solution stood for about
10 h at room temperature, the whole solution was ultrasonically dispersed for 1 min. The long stand (10 h) and short
ultrasonic treatment (1 min) of the solution was periodically
taken until 5 days had passed. The black products were
washed by absolute ethanol and distilled water several times
and then dried in a vacuum (product 3).
(d) Synthesis of β-MnO2 Nanorods. Two parallel hydrothermal processes were carried to synthesize β-MnO2 nanorods. One was to heat a 50 mL solution of MnSO4‚H2O and
(NH4)2S2O8 with a 2 cm × 2 cm Ag foil in the bottom of a sealed
flask at 80 °C for 1 day. When the flask cooled to room
temperature naturally, the back product was collected and
washed by absolute ethanol and distilled water several times
and then dried in a vacuum (product 4). The other experiment
was carried out similarly to the previous procedures, just
replacing the 2 cm × 2 cm Ag foil with a 1 mL AgNO3 solution
(product 5).
Characterization. X-ray powder diffraction (XRD) analysis
was performed using a Japan Rigaku D/max-γA X-ray diffractonmeter equipped with graphite monochromatized highintensity Cu KR radiation (λ ) 1.54178 Å). The accelerating
voltage was set at 50 kV, with a 100 mA flux at a scanning
rate of 0.06°/s in the 2θ range of 10-70°. Field emission
scanning electron microscopy (FESEM) images were taken on
a JEOL JSM-6700F SEM. The transmission electron microscopy (TEM) images were taken on a Hitachi Model H-800
instrument with a tungsten filament, using an accelerating
voltage of 200 kV.
Results and Discussion
XRD Patterns of the Prepared Products. The
X-ray diffraction (XRD) patterns of the prepared products are shown in Figure 1. All the peaks of the products
prepared at room temperature, including products 1-3,
were much alike. All the diffraction peaks from them
can be indexed to tetragonal symmetry with space group
of I4/m (No. 87). Lattice constants are calculated to be
a ) 9.79 Å and c ) 2.87 Å, which were in good
agreement with those reported for pure phase of R-MnO2
(JCPDS Card, No. 44-0141, a ) 9.784 Å and c ) 2.863
Å). The peaks of products 4 and 5 that were prepared
at 80 °C are apparently indexed to the pure tetragonal
phase of β-MnO2 (JCPDS Card, No. 24-0735, a ) 4.399
Å and c ) 2.874 Å). Obviously, room temperature favors
the formation of R-MnO2, while a higher temperature
favors the formation of β-MnO2 in this homogeneous
catalytic route. It is known that some counterions are
usually left in the tunnel of R-MnO2, and a further
analysis of these products is shown in the Supporting
Information (SI).
Morphologies of the Prepared Products. The
morphologies of R-MnO2 prepared in different ways are
observed by the field emission scanning electron microscope (FESEM) and transmission electron microscopy
(TEM), shown in Figures 2-4, respectively. Figure 2A
is a panoramic FESEM image of product 1, indicating
Li et al.
Figure 1. XRD patterns of the prepared products. (A) R-MnO2
urchin-like structures; (B) R-MnO2 sphere networks; (C)
R-MnO2 nanowire networks; (D) β-MnO2 nanorods prepared
with Ag foil; and (E) β-MnO2 nanorods prepared with AgNO3
solution.
that the product consisted of spherical structures with
high quantities. The magnified FESEM image (Figure
2B) shows that the surfaces of these spherical structures
are fixed with many nanorods and take on an urchinlike appearance. The diameters of these urchin-like
structures are about 1.6 to ∼2.0 µm. More careful
observation of a typical urchin-like structure is shown
in Figure 2C, indicating that these nanorods with
uniform diameters of 30 to ∼40 nm are spherically and
densely aligned. The R-MnO2 urchin-like structures are
also studied by TEM (SI), which confirms the structural
characteristics of the products observed by the FESEM.
The TEM images also show that the inner parts of the
products are solid.
The panoramic FESEM image of product 2 is shown
in Figure 3A, revealing that many regular spheres are
stacked together. The magnified FESEM image (Figure
3B) shows that these spheres are interconnected by
many nanowires, forming a big sphere network. The
diameters of the nanowires connecting these spheres are
about 30 to ∼40 nm, while their lengths are determined
by the distance between two adjacent spheres. A typical
single sphere in the sphere networks is shown in Figure
3C, indicating that the surface of these spheres is
composed of many ends of flexible 1-D nanowires with
diameters of 20 to ∼30 nm. Although the inner structures of the sphere cannot be observed directly, it is
believed that the spheres are composed of tightly
aggregated nanowires, judging from the morphologies
of their intermediates (SI). When the reaction time of
this product prolongs to 5 days, many nanowires can
be directly observed on the surface of these spheres (SI).
The fact indicates that the epitaxial growth of R-MnO2
nanowires is continuous when Ag foil is used to feed
the catalyst Ag+ at room temperature.
Figure 4A is the panoramic FESEM image of product
3, showing that the product consists of loosely inter-
Homogeneous Catalytic Growth of MnO2
Crystal Growth & Design, Vol. 5, No. 5, 2005 1955
Figure 2. (A), (B) and (C) were the FESEM images of the R-MnO2 urchin-like structures with different magnification. The
products were prepared by standing the solution for 2 days with AgNO3 solution.
Figure 3. Panels A-C were the FESEM images of the R-MnO2 sphere networks with different magnification. The products were
prepared by standing the solution for 3 days with Ag foil.
Figure 4. FESEM images of the R-MnO2 nanowire networks. (A) Panoramic image and (B) magnified image. The products were
prepared by periodical ultrasonic treatment and by standing the solution for 5 days with Ag foil.
twisting nanowires, and many of them form irregular
spherical structures. But, these spherical structures
have no distinctive region between each other, and most
of them are connected together. The magnified image
(Figure 4B) shows that these nanowires are flexible with
diameters of 30 to ∼50 nm, and their lengths are up to
several micrometers.
Products 4 and 5 prepared at 80 °C were studied by
TEM. Figure 5A,C is the panoramic image of products
4 and 5, respectively, and Figure 5B,D is their corresponding magnified TEM image. From these images,
one can see that both products are β-MnO2 nanorods
with high quantities. More careful observation shows
that the diameters of the nanorods prepared with Ag
foils (Figure 5B) are about 30 to ∼40 nm and that their
lengths range from 100 to 800 nm, while the diameters
of the nanorods prepared with Ag+ solution (Figure 5D)
are about 30 to ∼80 nm and their lengths are about 100
to ∼500 nm. Obviously, catalytic growth of β-MnO2
nanorods with Ag foil favors the formation of thinner
and uniform products. The β-MnO2 nanorods prepared
with different catalytic resources are also observed by
FESEM, which indicates that the nanorods obtained in
both products are on a large scale (SI).
Formation of Various R-MnO2 Hierarchical Structures with Different Catalyst Feedways. The chemical reaction in the homogeneous catalytic route to
synthesize various R-MnO2 structures at room temperature could be described as follows:
Ag+
MnSO4 + (NH4)2S2O8 + 2H2O 98
The trace amount of Ag+ was known to act as an
1956
Crystal Growth & Design, Vol. 5, No. 5, 2005
Li et al.
Figure 5. Panoramic TEM image (A) and magnified TEM
image (B) of the β-MnO2 nanorods prepared with Ag foil.
Panels C and D were the panoramic and magnified TEM
images of the β-type MnO2 nanorods prepared with AgNO3
solution.
effective catalyst in this reaction, and several steps were
involved in it.12
S2O82- f 2SO4-
(1)
2SO4- + Ag+ f 2SO42- + Ag3+
(2)
Ag3+ + Mn2+ + 2H2O f R-MnO2V + Ag+ + 4H+ (3)
Without the existence of a catalyst Ag+ solution or
Ag foil, no products were formed at room temperature.
It was known that the role of catalyst Ag+ could reduce
the potential energy of this chemical reaction so that
the reaction could proceed with the existence of catalyst
Ag+ even at room temperature. In our experiments, Ag+
was also found to play important roles in the formation
of various R-MnO2 structures besides being an effective
catalyst. Comparing the synthetic procedures of different products in the Experimental Procedures, it was
clearly revealed that the catalyst feedway plays a crucial
role to selectively synthesize different products; the
experimental manipulation and temperature also can
greatly alter the growing process.
Before the investigation of formation processes of
various R-MnO2 hierarchical structures, it was noteworthy that both R- and β-MnO2 had the growth habit
of forming 1-D nanostructures at suitable environments.13,14 Although there is still no report about the
synthesis of R-MnO2 nanorods/nanowires at room tem-
perature, considering that no any additional template
materials were added in our formation processes, the
nanorods/nanowires on the surface of various R-MnO2
hierarchical structures were believed to be epitaxially
grown when the homogeneous catalytic environment
was comparatively stable. On the basis of this consideration, the key step to build various R-MnO2 hierarchical structures was thought to result from different
intermediates, which were produced before the epitaxial
growth of nanorods/nanowires.
To investigate the formation process of the R-MnO2
urchin-like structures, we collected some intermediates
of them during the formation process and observed them
by the FESEM. Two kinds of morphologies could be
found in these intermediates. For instance, the intermediates collected in 3 h were large spheres with size
of 600 to ∼800 nm constructed by small nanoparticles,
while the intermediates collected after 6 h were even
larger spherical particles with diameters of 1.4 to ∼1.6
µm, but the surface of them was wholly covered by little
nanorods (SI). Compared the intermediates with the
final products, the nanorods on the surface of R-MnO2
urchin-like structures were believed to be epitaxially
grown from the surface nanoparticles of the big spheres.
The studies of the intermediates revealed that two
growing environments emerged during the growth of the
urchin-like structures, one favoring the growth of aggregate nanoparticles and the other favoring the growth
of 1-D nanoparticles. The formation of urchin-like
structures could be interpreted based on the process of
the homogeneous reaction. At the beginning of the
catalytic reaction, the concentration of reactants and
catalyst Ag+ was comparatively high, so the formation
rate of R-MnO2 was very fast, and nanoparticles were
quickly built and aggregated to big spheres. As the
reaction proceeded, the concentration of the reactants
was reduced. Meanwhile, the effective concentration of
catalyst Ag+ may also decrease through ways such as
confinement by the aggregate colloids or being trapped
in the large 2 × 2 tunnels of R-MnO2. As a result, the
whole system was transferred to a thermodynamically
stable environment. The newly formed MnO2 colloids
tended to nucleate heterogeneously and grow larger, so
R-MnO2 nanorods were then epitaxially grown from the
surface nanoparticles of the spheres and finally formed
the urchin-like structures.
When Ag foil was used instead of a Ag+ solution, the
Ag+ released from Ag foil also could act as homogeneous
catalyst in solution. But there were two significant
changes if the catalyst feedway varied. One was that
the concentration of Ag+ released from Ag foil was fairly
low. The other was that the Ag foil could continuously
provide Ag+ in the solution. In our experiments, it was
observed that MnO2 colloids were formed at a very slow
rate with Ag foil. The initial black products were
precipitated from the solution rather than from the
surface of the Ag foil, confirming that the catalytic
reaction was promoted by Ag+ in the solution released
form Ag foil. To investigate the role of Ag foil in the
formation of R-MnO2 sphere networks, we used trace
amounts of Ag+ solution (1/100 of that used for urchinlike structures) to replace Ag foil. But, the final products
were also the urchin-like structures, showing that the
continuous feed of catalyst was very important for the
Homogeneous Catalytic Growth of MnO2
formation of sphere networks. We also prolonged reaction time of the experiments with AgNO3 solution, but
there were no longer 1-D nanostructures obtained
regardless of how much AgNO3 solution was added. But,
in contrast, if prolonging the experiments with Ag foil,
namely, changing the catalyst feedway, longer nanowires continuously grew on the sphere networks (SI).
The previous results suggested that Ag foil was unique
to the formation of sphere networks and longer nanowires. As mentioned in the previous text, Ag+ decreased
as the homogeneous reaction proceeded; thus, providing
a continuous supply of Ag+ in the solution was necessary
for the continuous growth of 1-D R-MnO2 nanostructures. So, the changing of the catalyst feedway exhibited
obviously different products.
The intermediates of the sphere networks collected
at different times were also studied via FESEM (SI).
The initial formed intermediates (collected after 12 h)
were aggregate 1-D nanostructures rather than nanoparticles, confirming that Ag foil could provide a stable
homogeneous catalytic condition and favor the growth
of 1-D R-MnO2 nanostructures. The intermediates collected later, such as those collected at 24 h, were even
larger spheres composed of longer nanowires, showing
that the 1-D nanostructures were epitaxially grown
around the sphere’s surface and that the tightly aggregate nanowires still took on a spherical appearance.
The nanowires connecting two adjacent spheres in the
final product were believed to grow from one sphere and
then were enwound by other nanowires grown from
adjacent spheres.
From the growing process of the sphere networks, it
was clearly shown that Ag foil favored the growth of
long nanowires and that these nanowires were likely
to tightly aggregate together. To obtain dispersed
nanowires, it was found that periodical ultrasonic
treatment of the solution for a short time could achieve
the goal. It should be noted that long time ultrasonic
treatment or stirring of the solution could not produce
any 1-D nanostructures, and only irregular R-MnO2
particles were produced. This fact revealed that the
epitaxial growth of 1-D R-MnO2 needed a calm environment, while a chaotic environment did not favor the selfassembly process. However, the epitaxial growth of 1-D
nanostructures was not disrupted by short ultrasonic
treatment. Short and periodical ultrasonic treatment
not only could enlarge the distance of the growing
spheres but also induce outward growing of the nanowires on their surface. The intermediates of the nanowire networks prepared for 2 and 4 days were also
observed by FESEM (SI), confirming that the short
ultrasonic treatment could effectively induce R-MnO2
nanowires growing outward rather than being spherically enwound tightly like the sphere networks.
Formation of β-MnO2 Nanorods at High Temperature. Manganese dioxide has many kinds of polymorphs, such as R-, β-, δ-, and -type, when the basic
unit [MnO6] octahedron links in different ways. The
R-type is constructed from double chains of [MnO6] with
2 × 2 tunnels, while the β-type consists of a single chain
of [MnO6] with 1 × 1 tunnels.15 Although many researches on the synthesis of a certain phase of MnO2
have been reported, the development of a simple and
convenient route to selectively prepare R- and β-type
Crystal Growth & Design, Vol. 5, No. 5, 2005 1957
MnO2 with 1-D nanostructures is rarely reported, while
in our experiments, it can be clearly seen that R- and
β-type MnO2 nanorods could be selectively obtained via
a homogeneous catalytic route at different reaction
temperatures.
It is noteworthy that 1-D β-MnO2 nanostructures also
can be obtained by heating the solution of Mn2+ and
S2O82- above 120 °C without Ag+.13,14 The result indicates that the main role of catalyst Ag+ in the synthesis
of β-MnO2 nanorods is to reduce the formation temperature. Therefore, the formation of various phases at
different temperatures does not result from catalyst Ag+
but from the growth nature of MnO2 at different
temperatures. Previous research has revealed that the
formation of R-MnO2 generally needed large stabilizing
ions in their large 2 × 2 tunnels.16-20 In our experiments, it is thought that NH4+ and H3O+ may act as
stabilizing ions temporarily to induce the formation of
R-type MnO2 at room temperature. At high temperature, the diffusing speed of NH4+ and H3O+ was so fast
that they cannot act as stabilizing ions anymore.
Therefore, the formation of 1 × 1 tunnels is advantageous. At the same time, the fast diffusing speed at high
temperature makes the formation environment of
β-MnO2 comparatively stable, so nanorods rather than
hierarchical structures are obtained. It has been mentioned that Ag foil could provide Ag+ in the solution with
a low concentration continuously, so the formation speed
of β-MnO2 was stably slow, leading to the growth of
uniform nanorods.
Addition of Acid in Different Formation Processes. It has been reported that if acid was added in
the homogenous catalytic reaction with AgNO3 as a
catalyst resource, R-MnO2 core-shell structures could
be obtained,11 while urchin-like structures were obtained with nonacid conditions in this work. The fact
revealed that the acid could play a key role to selectively
synthesize different R-MnO2 hierarchical structures. In
our experiments, it was found that the addition of 1 to
∼3 mL sulfuric acid favored the formation of R-MnO2
core-shell structures, while a mixture of R-MnO2 coreshell structures and urchin-like structures was built
with less acid (<1 mL). It was known that when suitable
amounts of acid exist in the homogeneous catalytic
reaction, the catalytic course changed due to the formation of intermediate Mn3+ in acidic conditions.
Ag3+ + 2Mn2+ f Ag+ + 2Mn3+
(4)
2Mn3+ + 2H2O f R-MnO2V + Mn2+ + 4H+
(5)
Too much acid (>5 mL) would produce no product
since Mn3+ was very stable in high acidic conditions and
would not disproportionate to R-MnO2 and Mn2+ anymore. With the addition of 2 mL of acid, the solution
was slightly red, the characteristic color of Mn3+, while
the solution was colorless without the addition of acid.
At the same time, the formation rate of MnO2 greatly
reduced with the addition of acid. For example, black
precipitates appeared after 3 h in the formation of
urchin-like structures, while appearing 8 h later in
core-shell structures. The growing processes of the
core-shell structures have been investigated to be three
stages, in which the first and second stages were similar
to those of the urchin-like structures. The third stage
1958
Crystal Growth & Design, Vol. 5, No. 5, 2005
was the crystallization, which separated the core and
shell. We have revealed that the last crystallization step
of the core-shell structures was promoted for their poor
crystallinities of intermediates formed in acidic conditions.11 To further confirm this conclusion, the XRD
patterns of the intermediates of urchin-like structures
were also investigated, and no amorphous components
were detected. From the different reaction processes of
the urchin-like structures and core-shell structures, it
was thought that the amorphous components built in
acidic condition might result from the formation of intermediate Mn3+. But the detailed reason was not yet clear.
It was noteworthy that the addition of acid had little
effect on the final products prepared with Ag foil since
the growth rate of nanowires was slow and the whole
system constantly maintained a thermodynamically
stable environment. Although R-MnO2 was also disproportionate from Mn3+ with Ag foil and the addition of
acid, the formation rate of R-MnO2 was very slow due
to the low concentration of Ag+ released from Ag foil,
thus avoiding the rapid formation of MnO2, which likely
built amorphous components in R-MnO2. The XRD
analyses of the intermediates of products 2 and 3 were
also carried out, and no amorphous components were
detected, confirming our previous speculation. At high
temperature, no products were produced if more that 1
mL of acid was added, for Mn3+ was very stable at high
temperature even if the acid concentration was lowered.
At the same time, no obvious morphological changes
were observed in the final products if a small amount
of aid was added.
Conclusion
In summary, we promoted a homogeneous catalytic
route to prepare R-MnO2 solid urchin-like structures.
We then controlled the homogeneous catalytic route by
changing catalyst feedway and successfully obtained two
new hierarchical structures, R-MnO2 sphere networks
and nanowire networks, through different manipulations. Moreover, β-MnO2 nanorods are prepared via this
method at higher temperature. The formation process
of the hierarchical structures was also investigated and
interpreted. This low-cost, mild, template-free, and
Li et al.
solution-based self-assembly route will bring new light
to the synthesis and integration of new functional
materials.
Acknowledgment. The National Natural Science
Foundation of China, Chinese Academic of Science, and
Chinese Ministry of Education are acknowledged.
Supporting Information Available: TEM images of the
products and FESEM studies of the intermediates. This
material was available free of charge via the Internet at http://
pubs.acs.org.
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