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W. Wei et al. / Electrochimica Acta 56 (2011) 1619–1628
modes is a continuous coating with a nano-scale fibrous network
[16,21,23,25,31,32,34,35,41,44,46–48]. Through varying electrode-
position parameters, such as a dilute electrolyte [50] or applying
cyclic voltammetry [23,28,34], manganese oxide electrodes with
micro- and nano-scale fibers, rod morphologies have also been
prepared. However, most studies have generally reported the for-
mation of a type of manganese oxide nanostructure prepared
under specific electrodeposition conditions and the structure’s
corresponding electrochemical properties. There is still a lack of
systematic investigations to clarify the relationship between depo-
sition conditions and the various manganese oxide nanostructures.
A working mechanism to account for the morphological evolution
from continuous coatings to free-standing fibers and rod-like mor-
phologies during anodic electrodeposition is unavailable.
ried out by X-ray photoelectron spectroscopy (XPS) using a Kratos
AXIS Ultra X-ray photoelectron spectrometer. A monochromatic
Al source, operating at 150 W with a pass energy of 160 eV and
a step size of 0.5 eV, was utilized to collect survey spectra. For
high-resolution spectra collection, a pass energy of 20 eV and a
step size of 0.1 eV were employed. All XPS spectra were calibrated
using the C 1s line at 284.6 eV. Curve fitting, background subtrac-
tion and quantitative information evaluation were accomplished
using Casa XPS Version 2.3.13 software. Sample surfaces, for both
SEM and XPS analysis, were examined directly without any addi-
tional preparation. The crystal structure and morphology of the
manganese oxides were investigated using a JEOL 2010 transmis-
sion electron microscope (TEM) equipped with an Oxford ultra-thin
window (UTW) X-ray detector. Electron transparent samples were
prepared by scraping off the manganese oxide deposits and ultra-
sonically dispersing in methanol for 10 min. One or two drops of
the suspension were then deposited on 300 mesh, carbon-coated
Cu grids and allowed to dry in air.
In this research work, an in-depth study of morphology-
controlled growth of manganese oxide nanostructures from
acetate-containing aqueous solutions was carried out. By vary-
ing the deposition parameters, including solution composition,
pH value, deposition temperature and current density, a series
of manganese oxide nanostructures with various physicochemical
features (morphology and crystallization degree) was obtained. A
growth mechanism is proposed to clarify the physicochemical fea-
ture evolution of manganese oxide nanostructures during anodic
electrodeposition. The electrochemical properties (specific capac-
itance, rate capacity and electrochemical impedance response)
of manganese oxide nanostructures are carefully examined. The
experimental results show that a manganese oxide electrode with
an interconnected nanosheet architecture exhibits superior rate
capacity performance (∼75% specific capacitance retention as the
current density increases from 1 to 50 A g−1).
2.3. Electrochemical measurements
The electrochemical behavior of as-deposited manganese oxide
electrodes was investigated with
a Solartron 1286A poten-
tiostat/galvanostat and a Solartron 1255 impedance analyzer
under cyclic voltammetry (CV), galvanostatic charge-discharge
and potentiostatic electrochemical impedance spectroscopy (EIS)
modes in an electrolyte containing 0.5 M Na2SO4. A three-electrode
cell configuration was used and consisted of a platinum mesh
counter electrode and an Ag/AgCl reference electrode. In order
to remove the dissolved air in the 0.5 M Na2SO4 solutions, high
purity Ar was introduced and bubbled for 30 min before and during
the electrochemical tests. Cyclic voltammograms were recorded
between −0.1 and 0.9 V vs. Ag/Ag+ at a scan rate of 100 mV/s. Gal-
vanostatic charge-discharge testing, with current densities ranging
from 1 to 50 A g−1, was conducted over a potential window between
−0.1 and 0.9 V to determine the specific capacitance of as-deposited
manganese oxide nanostructures. EIS measurements were con-
ducted in constant potential mode (0 V vs. Ag/Ag+) by sweeping
frequencies from 50 kHz to 0.1 Hz at an amplitude of 5 mV.
2. Experimental
2.1. Materials synthesis
Manganese oxide nanostructures were anodically electrode-
posited on Au-coated (100 nm thick) glass substrates from 0.2 M
Na acetate solutions with various Mn(NO3)2 concentrations. Before
anodic deposition, the Au-coated glass substrates were ultrason-
ically cleaned in acetone and ethanol for 10 min, respectively.
Electrochemical deposition was performed with a Solartron 1286A
potentiostat/galvanostat in a three-electrode cell configuration
under galvanostatic control with current densities ranging from
0.05 to 20 mA cm−2. The Au-coated glass served as the working
electrode with a platinum mesh counter electrode and an Ag/AgCl
reference electrode. The working electrode and counter electrode
were placed in vertical opposition to one another with an inter-
electrode gap of 20 mm. The electrolyte pH values were adjusted
using dilute HNO3 and NaOH solutions, and the deposition temper-
atures were controlled with a water bath. After electrodeposition,
the working electrodes were rinsed with deionized water, dried
at room temperature in air and then stored in a vacuum dessi-
cator. The mass loading of the manganese oxide nanostructures
was determined with a microbalance with an accuracy of 10 g
(Sartorius CPA225D).
3.1. Surface morphology
Fig. 1 illustrates the effect of applied current density on the sur-
face morphology of as-deposited manganese oxide nanostructures
prepared from a concentrated solution containing 0.1 M Mn(NO3)2
comprised of numerous equiaxed particles to form a continuous
coating. Note that there are some micro-cracks within the oxide
coating. As the deposition current density is steadily reduced, the
equiaxed features evolve into fibrous (Fig. 1b and c) and even
length of the oxide nanofibers and inter-fiber spacing increase with
reduction in applied current density from 5 to 1 mA cm−2. When
the current density is further decreased to 0.1 mA cm−2, a rough
morphology with petal-shaped features is obtained, as shown in
Fig. 1d. The petal-shaped deposit is primarily composed of oxide
thin sheets.
2.2. Materials characterization
As the concentration of the Mn(II) species was reduced from
0.1 M down to 0.01 M, 0.002 M and 0.001 M, some distinct mor-
phologies were observed. For instance, a manganese oxide coating
containing micron-scale flower-like domains was prepared from
a 0.01 M Mn(NO3)2 solution under conditions of pH 7.6 and cur-
The morphology and chemistry of as-prepared manganese oxide
electrodes were analyzed in a JEOL 6320FV field emission scanning
electron microscope (FE-SEM). Chemical state analysis was car-