Highly ordered MnO2 nanowire array thin films on Ti/Si substrate as an electrode for electrochemical capacitor

Article abstract of DOI:10.1016/j.jssc.2006.01.058

AAO/Ti/Si substrate was successfully synthesized by a two-step electrochemical anodization of the aluminum film on the Ti/Si substrate and then used as template to grow nanowire arrays. The ordered MnO₂ nanowire arrays with about 40 nm diameters had been directly fabricated on AAO/Ti/Si substrate by direct current (DC) electrodeposition. The microstructure of the nanowire arrays was investigated by field-emission scanning electron microscopy, transmission electron microscopy and X-ray diffraction. Their electrochemical characterization was performed using cyclic voltammetry in 0.5 M Na₂SO₄ aqueous solution. The synthesized MnO₂ nanowires had amorphous nature until 400 °C. The deal capacitive behavior was obtained when the as-prepared sample was heat-treated at 200 °C. The specific capacitance of the electrode was about 254 F/g.

Full text of DOI:10.1016/j.jssc.2006.01.058

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Journal of Solid State Chemistry 179 (2006) 1351–1355
www.elsevier.com/locate/jssc
Highly ordered MnO2 nanowire array thin films on Ti/Si substrate as an
electrode for electrochemical capacitor
Ã
Cai-Ling Xua, Shu-Juan Baoa, Ling-Bin Kongb, Hua Lia, Hu-Lin Lia,
aCollege of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, PR China
bState Key Laboratory of Gansu Advanced Non-Ferrous Materials, Lanzhou University of Technology, Lanzhou 730050, PR China
Received 12 December 2005; received in revised form 18 January 2006; accepted 22 January 2006
Available online 14 March 2006
Abstract
AAO/Ti/Si substrate was successfully synthesized by a two-step electrochemical anodization of the aluminum film on the Ti/Si
substrate and then used as template to grow nanowire arrays. The ordered MnO2 nanowire arrays with about 40 nm diameters had been
directly fabricated on AAO/Ti/Si substrate by direct current (DC) electrodeposition. The microstructure of the nanowire arrays was
investigated by field-emission scanning electron microscopy, transmission electron microscopy and X-ray diffraction. Their
electrochemical characterization was performed using cyclic voltammetry in 0.5 M Na2SO4 aqueous solution. The synthesized MnO2
nanowires had amorphous nature until 400 1C. The deal capacitive behavior was obtained when the as-prepared sample was heat-treated
at 200 1C. The specific capacitance of the electrode was about 254 F/g.
r 2006 Elsevier Inc. All rights reserved.
Keywords: MnO2 nanowire arrays; AAO/Ti/Si substrate; Electrochemical capacitor
1. Introduction
wire array electrodes of SnO2 and V2O5 have significantly
improved rate capability compared with thin films of the
same material [28–30]. Anodic aluminum oxide (AAO)
template offers a promising route to synthesize a high
surface area, ordered nanowire electrodes because of its
advantages. Recently, amorphous manganese oxide nano-
wire arrays for high energy and power density electrodes
were prepared by the AAO template method [31]. But this
method is difficult to be used in practical purpose due to
the fragility of the AAO template.
In this work, AAO films are successfully grown on Ti/Si
substrate and this is first time it is used as a template to
synthesize high surface area and ordered MnO2 nanowire
array electrode for electrochemical capacitors. This kind of
template has unique electrodeposition properties and can
bond well with the deposited materials. The electrochemi-
cal characterization of ordered MnO2 nanowire arrays on
Ti/Si substrate is performed by cyclic voltammetry in 0.5 M
Na2SO4 aqueous solution using the nanowire array thin
films as the working electrode. The nanowire array
electrode consists of only electrochemically active material
(MnO2) and is fairly stable. The results of cyclic
Electrochemical capacitors are currently receiving con-
siderable amount of attention because of their high power
energy storage ability. It is well known that the most widely
used active electrode materials are carbon [1,2], conducting
polymers [3,4] and both noble [5–7] and transition metal
oxides [8–26]. Among these candidates, MnO2 has been
considered to be the most attractive electrochemical
capacitor materials in terms of its low cost, abundance
and more environmental friendliness than other transition
metal oxide systems [27].
Researches indicate that electrochemical characteristics
of electrode materials are highly dependent on the grain
size, texture, surface area, and morphology. The ordered,
high surface area structure of electrode materials can
enhance electrochemical characteristics. Martin and co-
workers have demonstrated that high surface area nano-
Ã
Corresponding author. Fax: +86 931 891 2582.
E-mail addresses: xucl01@163.com (C.-L. Xu), lihl@lzu.edu.cn
(H.-L. Li).
0022-4596/$ - see front matter r 2006 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2006.01.058

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C.-L. Xu et al. / Journal of Solid State Chemistry 179 (2006) 1351–1355
1352
voltammetry show that the specific capacitance of the
electrode is about 254 F/g and has promising application in
electrochemical capacitor.
2. Experimental
The highly pure Al film (99.999%, about 3.0 mm) was
deposited on the p-type silicon substrate coated with a Ti
(about 300 nm) film by radio frequency sputtering. The
anodization was carried out in 0.3 M oxalic acid solution at
room temperature, 40 V for about 40 nm pores in diameter.
The resulting alumina film was etched away in 0.4 M
H3PO4, 0.2 M H2Cr2O4 at 40 1C for 20 min, and the
remaining aluminum was re-anodized under the same
conditions until the metal film was fully oxidized. To
remove the barrier layer, the anodization was continuously
processed for 90–120 min. The details of removing the
barrier layer will be reported in our later work.
The electrolyte was an aqueous solution of 0.5 M
Mn(CH3COO)2 for preparation of MnO2 nanowire array
thin films. Electrodeposition was carried out at room
temperature, using a three-electrode potentiostatic control
and direct current (DC) electrodeposition system with a
saturated calomel electrode as reference electrode, a
1.0 cm ꢀ 1.0 cm platinum plate as a counter electrode and
the AAO/Ti/Si substrate as working electrode. The
electrolysis was carried out at 1.0 V for 2 h.
After electrodeposition, the samples were immersed in
0.5 M NaOH solution for 1 h to remove the AAO on Ti/Si
substrate. For electrochemical measurement, the nanowire
arrays on Ti/Si substrate were used as working electrode,
platinum foil with the same area of the working electrode
and saturated calomel electrodes as the counter and
reference electrodes, respectively.
Fig. 1. FESEM images of an alumina template on a Ti/Si substrate
created using a two-step anodization process in 0.3 M oxalic acid solution
at room temperature: (a) top view, (b) side view.
The morphology of the porous anodic alumina film on
Ti/Si substrate and nanowire arrays was examined by field
emission scanning electron microscope (FESEM). Trans-
mission electron microscopy (TEM) was used to character-
ize the morphology of nanowires. The crystalline structure
of the samples was determined by X-ray diffraction
(XRD). Thermo-gravimetric and different thermal analysis
(TG–DTA) was performed for studying the change of
synthesized sample with temperature. The electrochemical
measurements of the samples were performed by means of
a CHI660 electrochemical working station.
[33]. From Fig. 1b, we can see that the thickness of the Ti
adhesion layer is approximately 300 nm and not uniform
because we cannot well control the zone of fracture of
sample when it is breached for FESEM observation. The
pores partly open to the Ti layer because of the short
oxidation time. The oxide barrier layer can be completely
removed by continuously prolonging the oxidation time for
DC electrodeposition.
The FESEM image in Fig. 2 shows the surface view of
MnO2 nanowire arrays. From Fig 2, we can find that many
clusters protrude from the Ti/Si substrate which provide
high surface area electrode. The clusters could result from
the situation in which the nanowires are uncovered from
the framework of the porous anodic alumina template but
freestanding incompletely. When the porous anodic alu-
mina template was dissolved away, the nanowires em-
bedded in the template were released gradually and inclined
to agglutinate together to minimize the system free energy.
Fig. 2 also shows that the nanowires are abundant,
uniform and well ordered in the large area. From Fig. 2
it can be estimated that the pore filling rate is above 90%,
and some nanowires are lost from the electrode surface.
The diameter and the length of the nanowires are
3. Results and discussion
After a two-step anodization in 0.3 M oxalic acid
solution at room temperature, the resulting template has
parallel pores with a fairly narrow size distribution, as
shown in Fig. 1. Fig. 1a shows that the porous alumina
structure has almost arranged the pore array with the
average pore diameter about 40 nm, the interspaces about
60 nm, and pore densities about 1010 cmꢁ2. But their
arrangement has lower order than AAO on bulk Al
probably duo to small grain [32] and thin aluminum films

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1353
determined from Fig. 3 to be about 40 nm and 1mm,
respectively. These nanowire diameters are identical to the
pore diameters of the AAO template on Ti/Si substrate.
From the diameter, filling rate and the length of the
nanowires it is easy to show that the nanowire array
electrode have 11 cm2 of MnO2 area per cm2 of substrate
electrode area.
Fig. 4 illustrates the XRD patterns of the samples heat-
treated at different temperatures up to 400 1C. The main
peaks of the a-MnO2 [34] can be roughly identified in
Fig. 4. However, the amorphous nature of the samples is
confirmed by the XRD spectrum, which shows broad and
low intense peaks related to a poorly crystallized com-
pound. The amorphous nature is still retained for the
material heat-treated at 400 1C.
Fig. 5 shows the TG–DTA curves of the as-prepared
sample. The first weight loss at 35–104 1C results from the
removal of physically absorbed water, the second step
weight loss at 104–252 1C is the removal of chemically
bound water in the samples, and the third step weight loss
are due to the structure transformation. DTA displays two
obvious exothermic peaks at 206 and 513 1C, which
correspond to the removal of chemically bound water
and the transition of MnO2 to Mn2O3 as determined by
Reddy [35], respectively.
Fig. 2. FESEM image of MnO2 nanowire arrays grown on AAO/Ti/Si
substrate.
Typical cyclic voltammograms of the samples heat-
treated at different temperatures for 2 h measured in 0.5 M
Na2SO4 at 5 mV/s scan rate are shown as curves (a), (b)
and (c) in Fig. 6. Within the potential range ꢁ0.1 to 0.9 vs.
SCE, the capacitive-like and symmetric I=E responses of
the CV curves indicate that the oxides through different
temperature treatments exhibit electrochemical character-
istics suitable for pseudocapacitor application. But only the
sample obtained at 200 1C has the ideal regular rectangle,
as shown in curve (b). It is ascribed to the variation of
crystalline water content as depicted in the literature [24].
(c)
(b)
(a)
0
10 20 30 40 50 60 70 80 90 100 110
2θ/ degree
Fig. 4. XRD patterns of the samples heat-treated at different tempera-
tures for 2 h: (a) as-prepared, (b) heat-treated at 200 1C, (c) heat-treated at
400 1C.
Fig. 3. TEM images of MnO2 nanowires: (a) truss nanowires, (b) single
nanowires.

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1354
2
0.5
100%
0.4
0.3
0.2
(a)
80%
60%
40%
20%
0%
(b)
(c)
0.1
0
0
-0.1
-0.2
-0.3
-2
-0.4
23
100
200
300
400
500
600
-0.5
Temperature (°C)
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
Fig. 5. TG and DTA curves for as-prepared sample. Heating rate: 10 1C/
min.
E/V (vs.SCE)
Fig. 7. CV curves of the samples heat-treated at 200 1C at different scan
rates: (a) V ¼ 20 mV/s, (b) V ¼ 10 mV/s, (c) V ¼ 5 mV/s.
0.3
where Ccv is the specific capacitance (in F/g), Q is the
charge (in C), DE is the potential window (in V), and m is
the mass of active material (in g). The above results
demonstrate that the prepared MnO2 nanowire array thin
films on Ti/Si substrate in 0.5 M Na2SO4 aqueous
electrolyte can be as an excellent electrode for electro-
chemical capacitor.
0.2
0.1
0
(a)
(b)
(c)
-0.1
-0.2
-0.3
4. Conclusion
In this work, MnO2 nanowire arrays were electrodepos-
ited in AAO template on Ti/Si substrate. The AAO/Ti/Si
substrate had unique electrodeposition properties and
could bond well with the deposited materials. XRD
analysis indicated that the synthesized sample had amor-
phous nature until 400 1C. The MnO2 nanowire array
electrode heat-treated at 200 1C showed ideal capacitive
behavior and the specific capacitance can be up to 254 F/g.
So it had promising application in electrochemical capa-
citor.
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
E/V (vs.SCE)
Fig. 6. CV curves of the samples heat-treated at different temperatures for
2 h at 5 mV/s scan rate: (a) as-prepared, (b) heat-treated at 200 1C, (c) heat-
treated at 300 1C.
The results indicate that the sample heat-treated at 200 1C
has the ideal capacitive behaviors for electrochemical
capacitors.
Acknowledgments
Fig. 7 shows the CV curves of the sample heat-treated at
200 1C at different scan rates. Note that the shape of these
voltammetric curves is not significantly influenced by the
change in scan rate of CV. In addition, voltammetric
currents are directly proportional to the scan rate of CV.
These results indicate ideal capacitive behavior of the
nanowire arrays heat-treated at 200 1C. Because the
prepared nanowire arrays possess high surface area and
high aspect ratio, the interface between the active material
and electrolyte increase. The specific capacitance of
the sample heat-treated at 200 1C at 10 mV/s scan rate is
254 F/g, which can be estimated from the cyclic voltammo-
gram according to the following equation [34]:
This work is supported by the National Natural Science
Foundation of China (NNSFC60471014).
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