Preparation of α-MnO2 nanorods and Co3O 4 submicrooctahedrons by molten salt route

Article abstract of DOI:10.1246/cl.2007.688

α-MnO₂ nanorods and Co₃O₄ submicroctahedrons have been obtained by simple molten salt preparation route using cheap reagents. Particle size and morphology of the powders were studied by transmission electron microscopy (TE

Full text of DOI:10.1246/cl.2007.688

688
Chemistry Letters Vol.36, No.5 (2007)
Preparation of ꢀ-MnO2 Nanorods and Co3O4 Submicrooctahedrons by Molten Salt Route
Songlin Ran and Lian Gaoꢀ
State Key Laboratory of High Performance Ceramics and Superfine Microstructure,
Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, P. R. China
(Received February 5, 2007; CL-070136; E-mail: liangaoc@online.sh.cn)
ꢀ-MnO2 nanorods and Co3O4 submicroctahedrons have
NaNO3, and the reactive temperature was 350 ꢁC.
been obtained by simple molten salt preparation route using
cheap reagents. Particle size and morphology of the powders
were studied by transmission electron microscopy (TEM) and
field emission scanning electron microscopy (FESEM).
The phases of the products were identified by X-ray powder
diffractometry (XRD, D/MAX-2550V, Rigaku, Tokyo, Japan).
High-resolution transmission electron microscopy (HRTEM)
images and selected area electron diffraction patterns (SAED)
were taken with transmission electron microscopy (TEM,
JEM-2100F, JEOL, Tokyo, Japan). Scanning electron micro-
graph was obtained by using field emission scanning electron
microscopy (FESEM, JSM-6700F, JEOL, Tokyo, Japan).
Figure 1 shows the X-ray diffraction patterns of the samples.
The diffraction peaks of Figure 1a and Figure 1b can be indexed
to tetragonal ꢀ-MnO2 phase (JCPDS 44-0141) and cubic
Co3O4 phase (JCPDS 43-1003), respectively. The XRD patterns
indicate that pure ꢀ-MnO2 and Co3O4 can be obtained under the
current synthetic conditions.
Figure 2a shows the TEM image of the prepared ꢀ-MnO2
powders. The ꢀ-MnO2 powders show nanorod morphology.
Figure 2b exhibits a single ꢀ-MnO2 nanorod with a diameter
of ꢂ6:3 nm. Figure 2c shows the HRTEM image of the single
nanorod. The space between the adjacent lattices planes parallel
to the growing direction is ꢂ0:484 nm, which consistent with the
(200) lattice spacing of ꢀ-MnO2.
Controlling the architecture and morphology of materials
at all dimensions from the nanoscale to macroscopic scale has
recently attracted much attention because these materials not
only have some morphology-induced novel properties but also
have potential applications in the design of devices.1
Manganese dioxide and tricobalt tetraoxide are important
inorganic materials for their superior physical and chemical
properties. MnO2 and Co3O4 both show potential applications
in the fields of catalysts, sensors, magnetic materials, and partic-
ularly in energy storage.2–6 Recently, MnO2 and Co3O4 powders
have been prepared with different morphologies, such as rods,7,8
wires,9,10 cubes,11 hollow spheres,12 sea urchin shapes,6 etc.,
using hydrothermal, solvothermal, catalytic, surface-assisted,
and electrochemical methods. However, these techniques relied
on high pressure, long reaction time, surfactant, organometallic
precursor, or catalysts and were not suitable for large-scale
production. Therefore, seeking a one-step, convenient, low-cost,
mass-production route for the preparation of MnO2 and Co3O4
powders at atmospheric pressure is highly desired.
The morphology of the as-prepared Co3O4 powders is
characterized by FESEM and TEM. Figure 3 and Figure 4a
reveal that the products obtained are octahedrons. Figure 4b
and Figure 4c show the TEM images and corresponding electron
Molten salt synthesis (MSS) method is one of the simplest
techniques to prepare solid inorganic materials.13 Up to now,
materials with various morphologies have been prepared
by the MSS method, including ꢀ-Al2O3 platelets,14 BaTiO3
nanocubes, CoFe2O4 nanobelts,15 BaCeO3 nanoflowers, and
BaMnO3 nanorods.16 Although both the preparation of
(a)
(b)
17
18
ꢀ-MnO2 and Co3O4 by MSS has been already published,
the processes were complex owing to the using of electrolysis
and surfactants. In the present work, we report the preparation
of ꢀ-MnO2 nanorods and Co3O4 submicrooctahedrons by
simple molten salt route at normal atmospheric pressure, in the
absence of any templates, surfactants or catalysts.
10
20
30
40
50
60
70
80
2θ / degree
The preparation of the ꢀ-MnO2 follows the following steps.
MnSO4 xH2O of analytic reagent grade was calcined at 500 C
Figure 1. XRD patterns of the prepared (a) ꢀ-MnO2 and (b)
Co3O4 powders.
ꢁ
.
for 2 h to remove the water of crystallization. 0.01 mol of
anhydrous MnSO4 was mixed with 20 g of KNO3 by grinding
the mixture for 30 min. The above mixture was placed in an
alumina crucible and heated at 400 ꢁC for 30 min. After the
crucible was cooled to room temperature, deionized water was
added to the solid product. The product was filtered, washed
by deionized water and absolute ethanol for several times to
remove ions possibly remaining in the final products, and finally
dried in vacuum at 60 ꢁC for characterization. For the prepara-
tion of Co3O4, all of the procedures were the same as those stated
Figure 2. (a) TEM image of prepared ꢀ-MnO2 powders; (b)
Typical TEM image of a single nanorod and (c) its correspond-
ing HRTEM image.
.
above except that the raw materials were CoCl2 6H2O and
Copyright Ó 2007 The Chemical Society of Japan

Chemistry Letters Vol.36, No.5 (2007)
689
Similar chemical equations have been used to explain the
mechanism for preparation of Al2O319 and CeO220 from the re-
actions of anhydrous sulphate or chloride with molten nitrates.
The formation of ꢀ-MnO2 nanorods could be explained by
the anisotropic growth habit of ꢀ-MnO2 owing to the markedly
different lattice parameters of a0 (9.784 nm) and c0 (2.863 nm).
Three same lattice parameters of Co3O4 result in its octahedral
morphology. In the reaction process, molten salt not only
serves as reactant but also provides fluid phases which accelerate
the reaction, lower the reaction temperature, and penetrate
among the solid particles as well as prevent the agglomeration
of particles.18
Figure 3. FESEM images of the prepared Co3O4 powders.
In summary, we successfully prepared ꢀ-MnO2 nanorods
and Co3O4 submicrooctahedrons by molten salt preparation
method using inexpensive sulphate, chloride and nitrate as raw
materials. Molten salt synthesis method is a simple, low cost
and especially mass-production technique for the preparation
of inorganic materials.
This work was supported by the National Key Project
of Fundamental Research (Grant No. 2005CB6236-05) and
the Shanghai Nanotechnology Promotion Center (Grant Nos.
0552nm045 and 0652nm022), respectively.
Figure 4. (a) TEM image of prepared Co3O4 powders. (b) and
(c) Typical TEM images and corresponding SAED patterns.
diffraction patterns of the octahedrons when the electron beams
are parallel to the h111i and h013i, respectively. These typical
TEM images further demonstrate their octahedral geometries.
The mechanism for the preparation of MnO2 and Co3O4 is
suggested as follows. Acid–base-type reactions occurring in
molten oxosalts were classified as Lux–Flood (L–F) acid–base
reactions, oxide donors and acceptors were defined as L–F bases
and acids, respectively. In molten nitrates, the nitrate anion acted
as an oxygen anion donor, or an oxygen atom donor. The follow-
ing equations have been proposed to describe L–F interaction of
molten nitrates with appropriate ionic or neutral oxoacids:13
ꢃ
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NO3 ! NO2þ þ O2ꢃ
ð1Þ
ð2Þ
1
ꢃ
NO2þ þ NO3 ! 2NO2 þ O2
2
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8
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ð3Þ
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1
ꢃ
Mn2þ þ 2NO3 ! MnO þ 2NO2 þ O2
ð4Þ
ð5Þ
2
1
MnO þ O2 ! MnO2
2
For the preparation of Co3O4, the chemical reaction can be for-
mulated as
3CoCl2 þ 6NaNO3 ! Co3O4 þ 6NO2 þ 6NaCl þ O2 ð6Þ
which comprises two reaction processes
1
ꢃ
Co2þ þ 2NO3 ! CoO þ 2NO2 þ O2
ð7Þ
ð8Þ
2
1
3CoO þ O2 ! Co3O4
2
Published on the web (Advance View) May 5, 2007; doi:10.1246/cl.2007.688