Field emission from manganese oxide nanotubes synthesized by cyclic voltammetric electrodeposition

Article abstract of DOI:10.1021/jp0404955

Manganese oxide nanotubes are electrochemically synthesized by the cyclic voltammetric electrodeposition method. A transmission electron microscopy investigation shows that these nanotubes are MnO₂ crystalline with a diameter of 20 nm. Field-emission characteristics of the synthesized manganese oxide nanotubes are measured. The turn-on field is about 8.4 V/??m at a current density of 1 ??A/cm². The emission current density and the electric field follow Fowler-Nordheim behavior. Therefore, these manganese oxide nanotubes are useful for field-emission applications because they can be easily and economically synthesized at low temperature.

Full text of DOI:10.1021/jp0404955

© Copyright 2004 by the American Chemical Society
VOLUME 108, NUMBER 42, OCTOBER 21, 2004
LETTERS
Field Emission from Manganese Oxide Nanotubes Synthesized by Cyclic Voltammetric
Electrodeposition
Mao-Sung Wu,*,† Jyh-Tsung Lee,† Yung-Yun Wang,‡ and Chi-Chao Wan‡
Materials Research Laboratories, Industrial Technology Research Institute, Hsinchu 310, Taiwan, ROC, and
Department of Chemical Engineering, National Tsing-Hua UniVersity, Hsinchu 300, Taiwan, ROC
ReceiVed: July 20, 2004
Manganese oxide nanotubes are electrochemically synthesized by the cyclic voltammetric electrodeposition
method. A transmission electron microscopy investigation shows that these nanotubes are MnO2 crystalline
with a diameter of 20 nm. Field-emission characteristics of the synthesized manganese oxide nanotubes are
measured. The turn-on field is about 8.4 V/µm at a current density of 1 µA/cm2. The emission current density
and the electric field follow Fowler-Nordheim behavior. Therefore, these manganese oxide nanotubes are
useful for field-emission applications because they can be easily and economically synthesized at low
temperature.
Recently, research on field-emission materials has focused
on carbon-based materials such as carbon nanotube,1,2 dia-
mond,3,4 diamondlike carbon,5 and amorphous carbon6,7 because
of their low work function, high aspect ratio, and high
conductivity. However, much research has also focused on the
development of an alternative material as a field-emission
emitter. Metal oxides may be a possible replacement. Field
emission from metal oxides, including In2O3 pyramids,8 SnO2
nanostructures,9 and ZnO nanowires10-14 has also been reported.
Among these nanostructured metal oxides with wide band gaps,
the zinc oxide (ZnO) nanowire, which has a wide band gap of
3.37 eV, is the most suitable. The synthesis of ZnO nanowires
by laser vaporization, arc discharge, pyrolysis, electrodeposition,
and chemical vapor deposition is attractive for larger-area flat
panel displays.11-14
metal oxide systems. Manganese oxide is an interesting material,
and its nanostructures are attractive to the electrochemical super-
capacitor.15,16 However, the synthesis and application of such a
nanostructured material, especially for nanotubes in field emis-
sion, are rarely reported. Therefore, field-emission properties
of the manganese oxide nanotubes grown on a nickel substrate
using cyclic voltammetric electrodeposition without any template
and catalyst are summarized. More importantly, the method is
an easy process for rapid mass production at low temperature.
Manganese oxide nanotubes were prepared on nickel foils
(1 × 1 cm2) by cycling the potential for 15 min between 0.4
and 0.1 V (vs SCE) (scan rate 500 mV/s) at room temperature.
The plating bath was composed of 0.1 M manganous acetate
and 0.1 M sodium sulfate.16,17 One side of the nickel substrate
was exposed, and the other side was covered with a polymer
insulator. After deposition, the samples were rinsed several times
in deionized water and dried at 300 °C for 1 h in air. All
experiments were carried out using a three-compartment cell.
A saturated calomel electrode (SCE) was used as the reference
electrode, and a platinum foil with dimensions of 2 × 2 cm2
was the counter electrode.
In this letter, manganese oxide nanotubes are proposed as a
candidate for a field-emission emitter. The advantages of
manganese oxide are the lower cost for raw materials and the
fact that manganese is more environmentally friendly than other
* To whom correspondence should be addressed. E-mail: ms_wu@
url.com.tw. Fax: 886-3-5820442.
The surface morphology of the nanotubes was examined with
a scanning electron microscope (SEM, LEO-1530). Vertical
† Industrial Technology Research Institute.
‡ National Tsing-Hua University.
10.1021/jp0404955 CCC: $27.50 © 2004 American Chemical Society
Published on Web 09/24/2004

16332 J. Phys. Chem. B, Vol. 108, No. 42, 2004
Letters
Figure 3. X-ray photoelectron spectroscopy (XPS) of the deposited
manganese oxide nanotubes treated at 300 °C before and after field-
emission (FE) measurements.
Figure 1. SEM morphology of the manganese oxide nanotubes.
Figure 4. Typical plot of emission current density of the manganese
oxide nanotube cathode versus the applied dc electric field at different
cycle numbers and the relationship between ln(J/E2) and 1/E (inset).
nanotubes was also confirmed by X-ray photoelectron spec-
troscopy (XPS, VG Microlab MKIII), and the results are shown
in Figure 3. These peaks of binding energies, which correspond
to Mn 2p1 (653.7 eV) and Mn 2p3 (642.2 eV), indicate that
the chemical composition is MnO2. The XPS results of the
synthesized nanotubes are similar to the sol-gel-derived nano-
particles by Pang and Anderson.18
Figure 2. (a) Typical TEM image of the manganese oxide nanotubes,
(b) SAD pattern, (c) magnified view of the nanotubes, and (d) lattice
image.
nanotubes were grown densely on the nickel substrate, shown
in Figure 1. The result shows the nanotube with sharp, pointy
tips and an average diameter of approximately 20 nm. The
nanostructure of manganese oxide nanotubes was further
examined with transmission electron microscopy (TEM, JEOL-
JEM-2000EX) at an accelerating voltage of 200 kV. The TEM
specimen was prepared via the following procedure: some
nanotubes were stripped off and dispersed in anhydrous ethanol
with ultrasonic vibration for 5 min, and then a drop of the
supernatant was transferred onto a standard holey carbon-
covered copper TEM grid. TEM observation (Figure 2a) shows
that these manganese oxide nanotubes are hollow. The SAD
(selected area diffraction) pattern, shown in Figure 2b, of the
nanotubes is a ring pattern. The pattern indicates that the
manganese oxide nanotube is crystalline-structured. Figure 2c
shows that the end of the nanotube is round and closed. The
lattice image in Figure 2d is layer-structured, similar to a
multiwalled carbon nanotube, with layer spacing of about 3.5
Å. The chemical composition of the deposited manganese oxide
Field-emission experiments were carried out in a vacuum
chamber (1 × 10-6 Torr) at room temperature. The film
composed of manganese oxide nanotubes serves as a cathode,
and a platinum film serves as an anode to collect electrons from
manganese oxide nanotubes. The distance between the platinum
anode and the tips of the manganese oxide nanotubes was about
50 µm. The measured emission area was 0.0015 cm2. dc voltage
sweeping from 0 to 1000 V was applied to the sample. The
emission current was monitored with a Keithley programmable
electrometer. Figure 4 shows a typical plot of the emission
current density of the manganese oxide nanotubes cathode versus
the applied dc electric field at different cycle numbers. The turn-
on field, which is defined as the field required to detect a current
of 1 µA/cm2, is estimated to be about 8.4 V/µm. The emission
current density reaches 0.16 mA/cm2 at a bias field of 18 V/µm.
The turn-on field of manganese oxide nanotubes is comparable
to the well-aligned zinc oxide nanowires reported by Lee et
al.10 and is suitable for field-emission applications. The good

Letters
J. Phys. Chem. B, Vol. 108, No. 42, 2004 16333
References and Notes
field-emission characteristics are probably attributed to the
geometry of the nanotubes.
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â2E2
φ
Bφ3/2
âE
J ) A
exp -
(
)
(
)
where J is the current density, E is the applied electric field, φ
is the work function, A and B are constant values of 1.56 ×
10-10 (A V-2 eV) and 6.83 × 103 (V eV-3/2 µm-1), respectively,
and â is the field-enhancement factor defined as the ratio of
the induced local electric field to the applied external field.
The curves in Figure 4 are independent of the cycle number.
Obviously, there is no destruction or composition change in the
nanotubes during the continuous measurements. It also can be
found in the XPS data (Figure 3) that the binding energies of
peaks are essentially the same before and after 15 cycles of
field-emission tests, indicating that the oxidation states of Mn
in these nanotubes have not changed upon cycling.
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Acknowledgment. This work was supported by the Ministry
of Economic Affairs of Taiwan under contract no. 93-EC-17-
A-08-R7-0312.