Synthesis and formation mechanism of urchin-like nano/micro-hybrid α-MnO2

Article abstract of DOI:10.1016/j.jallcom.2009.10.004

Urchin-like nano/micro-hybrid α-MnO₂ balls were prepared through the hydrothermal reaction of H₂SO₄ and KMnO₄ without the use of templates, surfactants or other additives. The products were characterized by scan

Full text of DOI:10.1016/j.jallcom.2009.10.004

Journal of Alloys and Compounds 490 (2010) 331–335
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Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Synthesis and formation mechanism of urchin-like nano/micro-hybrid ␣-MnO2
Yong Chena,b,∗, Yuzhen Honga,b, Yanping Maa,b, Jianbao Lia,b
a Ministry of Education Key Laboratory of Application Technology of Hainan Superior Resources Chemical Materials, Haikou 570228, PR China
b Hainan Provincial Key Laboratory of Research on Utilization of Si-Zr-Ti Resources, Hainan University, Haikou 570228, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 11 June 2009
Received in revised form 1 October 2009
Accepted 1 October 2009
Available online 6 October 2009
Urchin-like nano/micro-hybrid ␣-MnO2 balls were prepared through the hydrothermal reaction of H2SO4
and KMnO4 without the use of templates, surfactants or other additives. The products were character-
ized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive
spectroscopy and X-ray diffraction (XRD). It indicates that the urchin-like ␣-MnO2 is composed of sin-
gle crystalline ␣-MnO2 nanorods. The formation mechanism of urchin-shaped ␣-MnO2 was proposed
and explained in detail on the basis of the time-dependent designed experiments combined with XRD
analysis and TEM observations of the intermediates during the formation process.
Keywords:
X-ray diffraction
Chemical synthesis
Nanostructures
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
ods. Most 1D structural MnO2 are prepared by the hydrothermal
method via deposition–precipitation at high reaction temperature
As a kind of important metal oxide, manganese dioxide is one
of the most attractive inorganic materials because of its phys-
ical and chemical properties and wide applications in catalysis
[1], molecular-sieves [2], ion exchange, biosensor, and especially
energy storage, such as electrode materials in Li/MnO2 batteries
[3,4] and supercapacitors [5,6] due to its energy compatibility with
a reversible lithium electrochemical system, environmental friend-
liness and low cost. It is well-known that manganese dioxide can
exist in different crystal structures, including ␣-, ␤-, ␥-, ␧-, ␩-, ␦-,
and ␭-MnO2, etc., when the basic structural unit [MnO6] is linked
in different manners. According to different [MnO6] octahedron
links, the MnO2 structures can be divided into three categories: the
chain-like tunnel structure such as ␣-, ␤-, and ␥-crystalline form;
the sheet or layered structure such as ␦-type; and the third cat-
egory that is composed of three-dimensional structures such as
␭-MnO2. The different crystalline structures of manganese diox-
ide exhibit different properties and life cycles [7]. In addition to
the crystal structure, the size and morphologies of MnO2 parti-
cles also play a key role in determining the properties for practical
applications. In this regard, many efforts have been made to pre-
pare nanocrystalline MnO2 with different structures and shapes.
Up to now, various nanostructures of MnO2, such as nanoparticles,
nanorods, belts, wires, tubes, fibers, urchins/orchids, mesoporous
and branched structures, have been synthesized by different meth-
and pressure [8–11]. In particular, MnO2 with the new shape has
attracted interest from researchers owing to its potential appli-
cations and novel physical and chemical properties. Generally,
urchin-like ␣-MnO2 is synthesized with the addition of a tem-
plate, surfactant, or any other additives [12–15]. In previous work,
single-crystal ␣-MnO2 nanorods were prepared in H2SO4 solution
by adding KMnO4 at the temperatures of 70–95 ◦C [16]. Interest-
ingly, it was found that ␣-MnO2 nanorods can aggregate together,
forming a ball-like shape. However, knowledge on its growth mech-
anism is still quite limited. In fact, how these nanorods aggregated
without the templates or other additives remains to be examined.
In this work, we introduced a simple approach to synthesize an
urchin-like structure ␣-MnO2 which is composed of a single-crystal
␣-MnO2 nanorod using a hydrothermal reaction at low tempera-
ture without expensive raw materials or equipments. Compared to
other methods, the current one did not use any templates, surfac-
tants, or additives. In addition, the short reaction time of about 0.5 h
and the low reaction temperature are its distinct advantages. Since
␣-MnO2 has wider application in Li-ion batteries and supercapac-
itors, the urchin-like ␣-MnO2 particles developed by the current
method may be useful for energy storage [11]. This is one relevant
topic that can be explored in the future. In addition, the urchin-like
␣-MnO2 particle formed may provide new opportunities for fun-
damental studies to elucidate the growth mechanisms of the new
shape nanomaterials without use of templates.
∗
Corresponding author at: Ministry of Education Key Laboratory of Application
2. Experimental
Technology of Hainan Superior Resources Chemical Materials, Renming Road 58,
Haikou 570228, PR China. Tel.: +86 898 66279122.
All the chemicals used in this experiment were of analytical grade and were used
without further purification. In a typical procedure, KMnO4 powder was added to
E-mail address: ychen2002@163.com (Y. Chen).
0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2009.10.004

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Y. Chen et al. / Journal of Alloys and Compounds 490 (2010) 331–335
Fig. 1. SEM images of the urchin-like ␣-MnO2 (a) and their enlarged view (b).
the sulphuric acid aqueous solution at 55 ◦C with strong magnetic stirring in a digital
water bath then it was heated up to 85 ◦C. Precipitates were immediately produced
after the addition of KMnO4 powder, and the solution color changed during the
reaction. The reaction course was monitored through the color change from purple
to brown. The whole reaction lasted for about 0.5 h. After reaction the system was
cooled to room temperature. The obtained products were washed thoroughly with
deionized water in order to remove the possible remaining ions, and then were dried
at 80 ◦C for 48 h.
The chemical composition of the as-synthesized products was analyzed by
energy dispersive spectroscopy (EDS, Oxford unit) at an acceleration voltage of 20 kV
in transmission electron microscopy (TEM, JEOL 2010), and their crystal structures
were analyzed by X-ray diffraction (XRD, Bruker D8 Advance) with CuK␣ radiation.
The morphologies of the samples obtained were observed with TEM, high-resolution
transmission electron microscopy (HRTEM) at an acceleration voltage of 200 kV, and
scanning electron microscopy (SEM, JSM-6301F). TEM specimens were prepared by
dispersing the as-prepared samples in alcohol, then dipping a TEM sample grid into
the dispersion. The specific surface area was measured by nitrogen cryo-adsorption
apparatus (Micromeritics, ASAP 2010) at 77 K. Powder products were outgassed at
200 ◦C under 10−4 Pa for 9 h in vacuum before the measurement.
Fig. 1a). The products are composed of tiny fibers (shown in Fig. 1b).
In addition, the shapes are spherical or elliptical.
To further investigate their microstructures, the spherical par-
ticles were examined by TEM. Uniform ␣-MnO2 nanorods having
sharp tips with a mean diameter of 10 nm and a length of 200 nm
were aggregated to an urchin-like ball as shown in Fig. 2a. The
enlarged view can be seen in the inset of Fig. 2a. It shows the typi-
cal HRTEM image of a single nanorod as shown in Fig. 2b. The clear
lattice fringes illustrate that the nanorod is a single crystal, and
the inter-layer space is about 0.7 nm, corresponding to the [1 1 0]
plane of ␣-MnO2. Likewise, a typical tip of a nanorod can be clearly
seen to be gradually sharpened (Fig. 2c). This is probably because
of the reduced concentration of the KMnO4 towards the end of the
reaction.
The chemical compositions of the as-prepared product were
examined by using EDS in TEM. The result (Fig. 3) shows that the
products are composed of elements Mn, O, and a small amount of
K. The peak of element Cu was caused by the Cu micro-grid which
was used for supporting the samples. The atomic ratio of the com-
ponent elements is listed in Table 1. As shown, the atomic ratio of
element O to Mn is 2.14, a little bigger than 2. The surface Cu of the
3. Results and discussion
It is interesting to find that all the products observed by SEM
are uniformly spherical with a mean diameter of 1 ␮m (as shown in
Fig. 2. TEM images of the ␣-MnO2 nanorods with a diameter of 10 nm (a) and their enlarged view (inset), their HRTEM image corresponding to [1 1 0] (b) and the tip of the
nanorod (c).

Y. Chen et al. / Journal of Alloys and Compounds 490 (2010) 331–335
333
Fig. 3. EDS analysis of the as-prepared ␣-MnO2 products.
Table 1
EDS analysis of the as-prepared ␣-MnO2 products.
Element
Weight %
Atomic %
Fig. 5. XRD pattern of the as-prepared ␣-MnO2 sample: (a) 55 ◦C; (b) 65 ◦C; (c) 75 ◦C;
O
K
29.43
3.40
59.34
2.81
(d) 85 ◦C.
Mn
Cu
47.29
19.87
27.77
10.09
The appearance of the clear hysteresis of N2 isotherms suggests that
the urchin-like ␣-MnO2 possess some mesopores originated from
the aggregated ␣-MnO2 nanorods.
Total
100.00
100.00
To investigate the as-synthesized products’ crystal structures,
XRD was employed. The XRD pattern obtained is shown in Fig. 5d.
All the diffraction peaks can be perfectly indexed to the tetrago-
nal ␣-MnO2 single crystal structure [space group: I4/m (87)] with
lattice constants of a = 9.784 Å and c = 2.863 Å, indicating the high
purity and crystallinity of the obtained product.
The ␣-MnO2 crystal structure is composed of an [MnO6] octa-
hedron, as shown in Fig. 6. ␣-MnO2 is constructed from the double
chains of [MnO6] octahedral forming 2 × 2 tunnels. The Mn atoms
located at the octahedral center, the six oxygen atoms located in
the octahedral angle, and the [MnO6] octahedral edge were con-
nected to single-stranded or double-stranded chains. The chains
were found on top of the tunnel structure, which formed the gap. It
is known that ␣-MnO2 has a tunnel structure, generally with some
large cations (such as K+, Ba2+, Pb2+ or NH+) embedded in its tun-
nel cavity [17,18] to stabilize its structure. Thus, the small amount
of K (approximately 2.81%) detected by EDS (in Fig. 3 and Table 1)
located within the tunnel of ␣-MnO2 was possibly introduced by
KMnO4 in the reaction course.
Fig. 4. The cryo-nitrogen adsorption isotherm of the urchin-like ␣-MnO2 balls at
77 K.
From previous literatures, the specially shaped materials are
generally prepared in the presence of a template or other ions.
The urchin-like ␣-MnO2 in our experiment was formed without
any templates or surfactants. To investigate the formation pro-
cess of the urchin-shaped structures, we collected some of their
intermediates during the formation process and observed them by
SEM. As shown in Fig. 7a, the intermediates obtained under 55 ◦C
micro-grid, which was oxidized, can account for this observation.
Fig. 4 is the nitrogen adsorption and desorption isotherms of
urchin-like ␣-MnO2 balls. The samples were found to possess a high
BET specific surface area up to 85.9 m2/g. A gradual uptake of N2 is
observed at the medium P/P0, which indicates that the urchin-like
␣-MnO2 has large external surface area due to their small diameter.
Fig. 6. [MnO6] octahedral structure and ␣-MnO2 structure.

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Y. Chen et al. / Journal of Alloys and Compounds 490 (2010) 331–335
Fig. 7. SEM images of the sample obtained at 55 ◦C (a) and their enlarged view (b).
hydrothermal conditions for 10 min, which underwent fast cool-
ing and washing by deionized water and drying at temperature of
110 ◦C, are microspheres with diameters of 1 ␮m; they consist of
small nanorods. These are shown in an enlarged SEM view in Fig. 7b
and TEM view in Fig. 8a. Due to the lower initial temperature (55 ◦C)
at the original time, the reaction rate of H2SO4 and KMnO4 was
slow, which led to the formation of a bulk product. From SEM, the
products were also shown as well-defined ball-like particles with
diameters of 1 ␮m, and a lot of tiny rods were inserted into the film.
It shows the TEM image of the intermediates (in Fig. 8b) obtained
under 65 ◦C hydrothermal conditions for 10 min, followed by fast
cooling and washing. The image indicates that many nanorods are
epitaxially grown from nanoparticles on the surface of the micro-
spheres.
nano/micro-hybrid ␣-MnO2 was formed. This suggests that the
crystal shape and size of the products obtained strongly depend
on the reaction temperature.
It is shown by further XRD characterization that the crystal
structure of the samples obtained at temperature of 55 ◦C and 65 ◦C
is that of ␦-MnO2 (Fig. 5a and b). However, its crystal structure was
transformed to that of ␣-MnO2 with an increase temperature to
75 ◦C (Fig. 5c). Likewise, the diffraction peaks are wider while their
intensities are lower, suggesting poor crystallinity. Fig. 5d shows
the diffraction peaks of the final products obtained under the 85 ◦C
hydrothermal conditions. Note that all the diffraction peaks can be
perfectly indexed to the tetragonal ␣-MnO2 single crystal structure.
This indicates that the samples obtained at 85 ◦C are well-defined
␣-MnO2 and correspond to a complete crystal structure in accor-
dance with the HRTEM observations (Fig. 2b). It was found that only
␦-MnO2 microspheres were obtained when the sulphuric acid con-
centration was below 2.5 mol/L. While above this concentration,
urchin-shaped ␣-MnO2 can be obtained, which was also affected
by the reaction temperature generally at or above 85 ◦C. The length
of nanorods was increased with the increasing reaction time.
The existence of inter-layer K+ and water molecules in the lay-
eredstructureof ␦-MnO2 is the basis forkeepingstructuralstability.
Thermo-gravimetric analysis showed that the bound water of ␦-
MnO2 was about 20%, while it was only 5% in ␣-MnO2. This further
indicates that the crystal structure of MnO2 has a close relation with
bound water. ␦-MnO2 contains more bound water and inter-layer
K+ due to its large layer interval; in contrast, the amount of bound
water in ␣-MnO2 tunnel structure was much lower than that in ␦-
Upon a further increase of the reaction temperature to 75 ◦C
under hydrothermal conditions for 10 min, the morphology of the
obtained MnO2 turned into a follower-shape and the nanorods in
the follower-shaped structure were formed and became long as
shown in Fig. 9a. In the enlarged HRTEM image of a single nanorod,
the clear lattice fringes illustrate that the nanorod is a single crys-
tal, of which inter-layer space is about 0.7 nm, corresponding to the
[1 1 0] plane of ␣-MnO2. Therefore, it is reasonable to deduce that
these rods grew longer with the increase of reaction time. Com-
pared with the intermediates, the final products obtained under
the 85 ◦C hydrothermal conditions show that the microspheres
transformed into a sea urchin-shaped ␣-MnO2. At the end of the
reaction, due to the depletion of KMnO4, the sharp tip of the rod
grew to nanorods with a length of 200 nm. Thus, the urchin-like
Fig. 8. TEM images of the sample obtained at 55 ◦C (a) and 65 ◦C (b).

Y. Chen et al. / Journal of Alloys and Compounds 490 (2010) 331–335
335
Fig. 9. TEM images of the sample obtained at 75 ◦C (a) and their enlarged view (b).
MnO2. According to the above results, the generation of ␣-MnO2
at higher sulphuric acid concentration and reaction temperature
indicates that acid has great effect on the decrease of bound water
in crystal structure. The increase of sulphuric acid concentration
weakened the bonding capability between K+ and water molecules
and demolished their bond, and the increased temperature further
promoted the demolishing capability of acid; due to the large layer
interval in the layered structure of ␦-MnO2, destroyed inter-layer
hydrated ions of K+ may lead to the layered structure to collapse,
thus forming ␣-MnO2 tunnel structure. With further increase in
temperature, the ␣-MnO2 nanorods grew out from the sphere, lead-
ing to the formation of sea urchin-like structures.
cally grown single crystalline ␣-MnO2 nanorods with diameters of
10 nm and length of 200 nm.
Acknowledgements
This work was supported by the National Natural Science Foun-
dation of China (No. 50762003), the Natural Science Foundation of
Hainan Province (No. 807009), and the Education Department of
Hainan Province (Hjkj2008-03).
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In summary, urchin-like ␣-MnO2 microspheres composed of
␣-MnO2 nanorods with small diameters have been successfully
synthesized via a simple hydrothermal process with KMnO4 in
H2SO4 solution. The experimental results showed that the crystal
structure, shape and size of the products obtained depend strongly
on both the H2SO4 concentration and reaction temperature.
Through the structural analysis and morphological observation of
the intermediate product obtained at different time during the for-
mation process, it was found that the ␦-MnO2 ball-shaped particles
were synthesized at a low temperature at the initial time. With
increasing reaction temperature, a lot of ␣-MnO2 nanorods were
formed and gradually grew out of the layered ␦-MnO2 structure,
consequently forming the urchin-like ␣-MnO2 consisted of radi-
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