Preparation and characterization of highly activated MnO2 nanostructure

Article abstract of DOI:10.1111/j.1551-2916.2009.03567.x

Highly activated MnO₂ nanostructure was prepared from potassium permanganate and manganese acetate, via a simple alternate drop-feeding method. This highly activated MnO₂ could combust in the atmosphere of ethanol vapors to yield nanocrystalline Mn₃O₄ at 114°C. Powder X-ray diffraction, scanning electron microscopy, transmission electron microscopy, particle size distribution, and electrochemical tests were used to characterize the as-obtained products. The results show that the as-prepared MnO₂ nanostructure shows good electrochemical properties.

Full text of DOI:10.1111/j.1551-2916.2009.03567.x

J. Am. Ceram. Soc., 93 [4] 1183–1186 (2010)
DOI: 10.1111/j.1551-2916.2009.03567.x
r 2010 The American Ceramic Society
ournal
J
Preparation and Characterization of Highly Activated MnO Nanostructure
2
Xiaoyan Han, Feng Zhang, Qingfei Meng, and Jutang Sunw
Department of Chemistry, Wuhan University, Wuhan 430072, China
Highly activated MnO2 nanostructure was prepared from
potassium permanganate and manganese acetate, via a simple
alternate drop-feeding method. This highly activated MnO2
could combust in the atmosphere of ethanol vapors to yield
nanocrystalline Mn O at 1141C. Powder X-ray diffraction,
scanning electron microscopy, transmission electron micros-
copy, particle size distribution, and electrochemical tests were
used to characterize the as-obtained products. The results show
II. Experimental Procedure
(
1) Material Preparation and Characterization
All the starting materials were purchased from commercial
sources and used without further purification.
The alternate drop-feeding process was based on ethanol
3
4
aqueous solutions (1:1) as matrix solutions. The KMnO4
(
0.25 mol/L, 200 mL) and Mn(CH CO ) (0.375 mol/L, 200 mL)
3 2 2
solutions were dripped alternately into ethanol aqueous solu-
tions with stirring to yield a black precipitate. Then, after being
filtered, washed, dehydrated with anhydrous ethanol, and dried
that the as-prepared MnO nanostructure shows good electro-
2
chemical properties.
in air for 10 h at 1001C, a black product (MnO
The black product combusted in the atmosphere of ethanol
vapors to yield a brown product (Mn ) at 1141C or above.
MnO2 with bigger particle size was obtained with KMnO4
0.5 mol/L, 200 mL) and Mn(CH CO (0.75 mol/L, 200 mL)
solutions in the same alternate drop-feeding method.
The X-ray powder diffraction (XRD) patterns were collected
using an XRD-6000 X-ray diffractometer (Shimadzu, Kanagawa,
2
) was obtained.
3
O
4
I. Introduction
(
3
2 2
)
ANGANESE OXIDES are currently under extensive investiga-
tions because of their physical and chemical properties
and wide technological applications as catalysts, electrode ma-
terials, sensor, ion-exchanging materials, energy transformation,
molecular adsorption, magnetic materials, electrochromic ma-
M
˚
Japan) with a Ni filter and CuKa1 radiation (l 5 1.54056 A) in
ꢀ1
the range of 10ꢀ901 (2y) with a scanning rate of 41 min . The
2
terials, etc. Among these, amorphous MnO has been consid-
refined unit-cell parameters were calculated by the JADE5.EXE
procedure. The particle size distribution (PSD) was obtained
with Mastersizer-2000 (Malvern instruments, Malvern, Worces-
tershire, U.K.) using a laser granulometer. The particle mor-
phology was observed using a scanning electron microscope
ered to be the most attractive candidate electrode material for
electrochemical capacitors due to its abundant, environmentally
friendly, low cost, and favorable pseudocapacitive characteris-
tics than noble metal oxides and other transition metal oxide
1
–4
systems. In addition, MnO and Mn O nanosized or nano-
2
3
4
(SEM, Hitachi 400, Hitachi, Tokyo, Japan) and a transmission
electron microscope (TEM, JEM-2010 FEF, Tokyo, Japan).
structured materials with high specific surface and internal tun-
nel structures may become more ideal host materials for the
insertion and desertion of cations in electrode materials for re-
5
,6
chargeable lithium batteries and chemical reactions. It is well
known that the functional properties of materials highly depend
on their dimensionality, morphologies, particle size and crystal-
line structure, and bulk density, and many efforts have been
made to control these parameters for synthesizing the manga-
(2) Electrochemical Measurements
2
The electrochemical performance of MnO was examined on a
Neware battery test system at room temperature. The dis-
charge–charge tests were carried out using the coin cells (size:
2
016), which consisted of a working electrode and a lithium foil
counter electrode separated by a Celgard-2300 membrane
Celgard Inc., Charlotte, NC). The working electrode consisted
7
–10
nese oxides with unique properties.
Synthetic processes of amorphous MnO
oped including thermal oxidation of Mn(II) salts,
2
have been devel-
(
3
,11
reducing
of active material along with acetylene black and polytetrafluo-
ethylene binder in a weight ratio of 80:15:5. A stainless-steel
1
2
13
14
Mn(VII) salts, chemical coprecipitation, electrodeposition,
15
16
templated-assisted, and sol–gel method. Some of these meth-
ods require high-temperature hydrothermal, complex apparatus
or sophisticated techniques, metal catalyst, organic reducing
agent, and/or templates, etc.
mesh acted as the current collector. A 1 mol/L solution of LiPF
dissolved in ethylene carbonate and dimethyl carbonate solution
1:1 volume ratio) was used as the electrolyte. The cells were
assembled in an argon-filled glove box (Mikrouna, Super 1220/
50, Shanghai, China). The cells were discharged and charged
6
(
2
In this study, highly activated MnO with polyporous
7
1
sponge-like structure was prepared using a simple alternate
drop-feeding method. It was found that the as-prepared MnO2
could combust in the atmosphere of ethanol vapors to yield
between 1.5 and 4.0 V versus Li/Li at a constant current den-
sity of 100 mA/g.
3 4
nanocrystalline Mn O at 1141C. The morphology and micro-
structure of the products were characterized, and the electro-
chemical performance of the as-prepared MnO2 was
investigated.
III. Results
(
1) Powder XRD Analysis
Figure 1(a) shows the XRD patterns of the as-prepared MnO
2
powder. A lack of sharp and strong diffraction peaks except for
weak and broad peaks shows the mixture of amorphous and
nanocrystalline nature of the sample. The two weak and broad
diffraction peaks can be indexed to (210), (011), and (412) of
D. Johnson—contributing editor
Manuscript No. 26475. Received July 2, 2009; approved November 23, 2009.
This work was supported by the National Natural Science Foundation of China (No.
orthorhombic phase MnO
2
with a space group of Pnma (62)
˚
JCPDS card no. 39-0375, a 5 9.27 A, b 5 2.866 A, and
˚
(
c 5 4.533 A). The main peaks of the as-prepared MnO cannot
2
0771087).
wAuthor to whom correspondence should be addressed. e-mail: jtsun@whu.edu.cn
˚
2
1
183

1
184
Journal of the American Ceramic Society—Han et al.
Vol. 93, No. 4
The main population is formed by particles of 5ꢀ15 mm, with a
modal particle size of about d50 5 7.44 mm. In comparison, the
granularity distribution of MnO with bigger particle size was
also measured (shown in Fig. 2(b)), which shows a modal par-
2
ticle size of about d50 5 12.006 mm.
Figures 3a1–a3 show the SEM and high-resolution TEM
images of the as-prepared MnO . It is observed that each MnO
2
2
particle consists of an agglomeration of small spherical particles
with a polyporous sponge-like structure. Each spherical particle
consists of nanosized grains (Fig. 3a2), with an individual par-
ticle size of about 0.4 mm. Figure 3a3 indicates that the MnO
2
particle possesses amorphous properties; only some particles
have a partial crystallization phenomenon. And the inset in
Fig. 3a3 is the fast Fourier transform (FFT) diffraction pattern
of the corresponding selected area. The interplanar spacing is
˚
ꢀ
2
(
.431, 1.833, and 1.899 A, which is consistent with the ð011Þ,
302), and (311) plane of the orthorhombic MnO , respectively.
ꢀꢀ
2
Fig. 1. X-ray diffraction patterns of the as-obtained MnO
Mn (b).
2
(a) and
The zone axis is ½233ꢁ.
3 4
O
The microstructure and feature of obtained Mn
3
4
O powder
can be observed in Figs. 3b1ꢀb3. The powder is constituted of
many agglomerates with an individual particle size of about
2
mm. Each Mn O particle observed by TEM is made of several
be identified and neither of the diffraction peaks is the strongest
line, which suggest that the crystal orientation of nanocrystallite
has changed. All results indicate that the as-prepared MnO
3 4
nanocrystals with a uniform crystallite size of about 10 nm
Fig. 3b2). The HRTEM image (Fig. 3b3) shows that the nano-
crystalline Mn O possesses microporous property and the crys-
(
2
powder possess an amorphous-like character. The reactions of
KMnO with Mn(CH CO ) can be described as follows:
3
4
tal lattice is very clear. The FFT pattern (inset in Fig. 3b3) can
ꢀ
4
3
2 2
be indexed to the ½131ꢁ zone axis of tetragonal-phase Mn
3 4
O ,
ꢀ
and the diffraction spots are corresponding to the (112), ð101Þ,
2
KMnO4 þ 3MnðCH3CO2Þ þ 2H2O
2
(
1)
and (211) plane, respectively.
!
5MnO2 þ 2KCH3CO2 þ 4CH3COOH
(
3) Electrochemical Properties
Figure 4 shows the first two cycles of discharge and charge pro-
files for the MnO /Li composite electrodes at a constant current
Figure 1(b) shows that the brown product is crystalline
Mn with no other phases present. As seen from the figure,
all the diffraction peaks can be indexed to tetragonal phase
3 4
O
2
density of 100 mA/g. The initial two charge profiles of the MnO2
electrodes are almost the same, indicating the same electrochem-
ical behavior. Moreover, the discharge capacity is mainly located
at 2.25ꢀ3.0 V and the main charge capacity at 2.75ꢀ3.75 V.
Mn O with a space group of I4 /amd (no. 141). The refined
1
3
4
˚
unit-cell parameters were calculated to be a 5 5.7656(2) A and
˚
c 5 9.4774(6) A, which are in good agreement with the reported
data for the pure phase of Mn
˚
3
O
4
(JCPDS card no. 80-0382,
a 5 5.765 A and c 5 9.442 A). Thus, the so-obtained Mn O
˚
MnO electrode delivers an initial discharge and charge capac-
2
3
4
ities of 147.8 and 134.7 mAh/g, respectively. The first cycle cou-
lombic efficiency is 91.1%. The second discharge and charge
2 3 4
were of high purity. The conversion of MnO to Mn O in the
atmosphere of ethanol vapors may be formulated as
capacity of MnO
tively. The coulombic efficiency is 99.9%.
The cycle performance of the MnO electrode between 1.5
2
electrode are 133.8 and 133.6 mAh/g, respec-
3
MnO2 þ CH3CH2OH ! Mn3O4 þ CH3COOH þ H2O (2)
2
and 4.0 V is shown in Fig. 5. The capacity retention of discharge
is 65% from the second to the 50th cycle and the average ca-
pacity loss was o1 mAh/g per cycle. It is noticeable that the
(2) Particle Size Distributions and Morphological
Characterization
MnO
2
electrode maintains high coulombic efficiency (about
Figure 2 shows the granularity distribution in terms of percent-
98%) during 50 cycles.
age of total particle volume versus particle diameter of MnO
and Mn (c). The PSD of MnO is narrow and symmetrical,
with a size ranging from 1.66 to 7.58 mm. The main population
about 87%) is formed by particles of 2ꢀ5 mm, with a modal
particle size of about d50 5 3.46 mm. The PSD of Mn is wider
than that of MnO , with a size ranging from 2.18 to 17.3 mm.
2
(a)
O
3 4
2
IV. Discussion
(
The alternate drop-feeding method for preparing amorphous
MnO is easy, safe, and high yield. The morphology and particle
3 4
O
2
2
size can be controlled by changing the solution concentration,
drop-feeding speed, stirring speed, and reaction temperature.
The alternate drop-feeding fashion and continuously stirring
mode are the key techniques for performing spheroidization,
which can effectively decrease crystallization and the agglomer-
ꢀ
21
ation phenomenon of MnO
ions are alternatingly adsorbed and deposited on the surface
of the formed MnO particles. The adsorbed and deposited
2
. In this process, MnO and Mn
4
2
circulation is occurring continuously, and uniform spherical
particles are obtained, which is beneficial to high electro-
chemical capacity.
2
The XRD patterns show that the as-prepared MnO is an
amorphous-like structure. SEM and TEM results show that the
as-prepared MnO is a nanostructured material, consisting of
2
nanosized grains, which would lead to high chemical reactivity.
The combustion reaction and formation of nanocrystalline
Fig. 2. Granularity distribution of the as-obtained MnO
Mn (c).
2
(a, b) and
Mn
3 4
O indicate the high chemical activity of the as-prepared
3 4
O
MnO . Our repeated experiments prove that MnO with bigger
2
2

April 2010
Preparation and Characterization of MnO Nanostructure
2
1185
Fig. 3. Scanning electron microscopy and transmission electron microscopy images of the as-obtained MnO
2
(a1, a2, a3) and Mn
O
3 4
(b1, b2, b3).
particle size without combustion reaction in the same conditions
was observed. Moreover, as seen in Eq. (2), the related reaction
sized grains with almost the same size in a regular array. This
polyporous sponge-like structure provides a more effective sur-
face to create electrochemical capacity.
is quite simple, which demonstrates that the MnO
2
nanostruc-
ture with appropriate particle size and high porosity possesses
Based on above results and discussion, reaction conditions
and the synthetic method affect the structure, morphology, and
particle size of MnO cathode material, which play an important
2
1
7
2
very high reaction activity. The conversions of MnO in
1
8–22
Mn O
3 4
have been reported previously in several papers,
but to the best of our knowledge, the conversion of amorphous
MnO to crystalline Mn O at such a low temperature has not
role in the chemical activity and electrochemical properties.
Thus, by controlling the reaction conditions and improving
2
3
4
been reported. Moreover, this may provide us another method
for the preparation of Mn
All electrochemical tests indicate a typical electrochemical
the experimental techniques, our process may produce MnO
materials with novel property and function.
2
O .
3 4
behavior of MnO electrode. The mechanism of electrochemical
2
V. Conclusions
charge and discharge of MnO
the ion diffusion theory. MnO
2
electrodes could be described by
electrode exhibits good electro-
2
Amorphous-like MnO nanostructure has been successfully pre-
2
chemical properties, which is mainly attributed to the porosity.
The insertion process of Li ions between the layers of the cath-
ode is a diffusion process. Therefore, the smaller particles or
open network of polyporous materials tend to improve the ca-
pacity of a battery by reducing the ion diffusion pathway during
pared by an alternate drop-feeding process of potassium per-
manganate and manganese acetate solutions. The as-prepared
2
MnO shows high chemical activity, it combusts in the atmo-
sphere of ethanol vapors to form nanocrystalline Mn O at
3
4
1141C. This may provide a new method to prepare nanocrys-
1
insertion and desertion processes of Li ions. Moreover, the as-
prepared MnO is a polyporous material, consisting of nano-
2
talline Mn . The nanostructured properties and combustion
3 4
O
reaction indicate that the as-prepared MnO can be expected to
2
exhibit excellent catalytic activity for chemical reaction. More-
over, as an active material for lithium ion batteries, the as-pre-
pared MnO displays an initial discharge capacity and reversible
2
capacity of 147.8 and 134.7 mAh/g in the range of 1.5 and 4.0 V,
respectively.
Fig. 4. Discharge–charge performance of the as-prepared MnO
.5ꢀ4.0 V.
2
over
2
Fig. 5. Cycle behavior of the as-prepared MnO at a constant current
of 100 mA/g.
1

1
186
Journal of the American Ceramic Society—Han et al.
Vol. 93, No. 4
12
Y. U. Jeong and A. Manthiram, ‘‘Nanocrystalline Manganese Oxides for
Electrochemical Capacitors with Neutral Electrolytes,’’ J. Electrochem. Soc., 149
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&