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Nano Res
1
Self-powered electrochemical anodic oxidation: A new
method for preparation of mesoporous Al2O3 without
applying electricity
Huarui Zhu1, Ying Xu1, Yu Han1, Shuwen Chen1, Tao Zhou1, Magnus Willander1, Xia Cao1,2 (), and
Zhonglin Wang1,3 ()
Nano Res., Just Accepted Manuscript • DOI 10.1007/s12274-015-0860-5
http://www.thenanoresearch.com on July 13, 2015
© Tsinghua University Press 2015
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Nano Research
DOI 10.1007/s12274-015-0860-5
Self-powered electrochemical anodic oxidation: a new
method for preparation of mesoporous Al2O3 without
applying electricity
Huarui Zhu, 1 Ying Xu, 1 Yu Han, 1 Shuwen Chen, 1 Tao
Zhou, 1 Magnus Willander, 1 Xia Cao*, 1, 2 and Zhonglin
Wang*, 1, 3
1 Beijing Institute of Nanoenergy and Nanosystems,
Chinese Academy of Sciences, Beijing 100083, China
2 School of Chemistry and Biological Engineering,
University of Science & Technology Beijing, Beijing,
100083, China
3 School of Material Science and Engineering, Georgia
Institute of Technology, Atlanta, Georgia 30332-0245,
USA
The first self-powered electrochemical anodic oxidization system was
developed which is capable of synthesis of mesoporous Al2O3 driven
by TENG arrays without applying electricity.
Corresponding Authors:
*E-mail: [email protected].
*E-mail: [email protected].
Self-powered electrochemical anodic oxidation: a new
method for preparation of mesoporous Al2O3 without
applying electricity
Huarui Zhu, 1 Ying Xu, 1 Yu Han, 1 Shuwen Chen, 1 Tao Zhou, 1 Magnus Willander, 1 Xia Cao, 1, 2 () and
Zhonglin Wang, 1, 3 ()
Received: day month year
Revised: day month year
Accepted: day month year
(automatically inserted by
the publisher)
© Tsinghua University Press
and Springer-Verlag Berlin
Heidelberg 2014
KEYWORDS
triboelectric
nanogenerator,
self-powered
electrochemical anodic
oxidization, mesoporous
materials
ABSTRACT
Anodic oxidization (AO) is one of the most important methods to fabricate
mesoporous Al2O3, which can be conducted at either high potential or low
potential, but an external electricity power source is indispensible. In this work,
a novel self-powered electrochemical anodic oxidization (SPAO) system is
introduced for preparing mesoporous Al2O3 by using the newly invented
triboelectric nanogenerator (TENG) arrays driven by natural wind. Owing to
the controllable voltage output of TENG arrays, the SPAO system can regulate
the pore’s depth and size of mesoporous Al2O3. Distinguished from traditional
AO system, our technique takes the advantages of high output voltage of TENG
arrays without additional energy cost. In addition, the SPAO system can be
used for the preparation of other mesoporous materials.
1 Introduction
Mesoporous aluminum oxides (Al2O3) often display
novel physical, chemical, and mechanical properties
due to their high surface-to-volume ratio and low
densities, such as increased chemical activity and
high specific strength, which could be used as
catalyst beds, molecular sieves and hosts for
inclusion compounds [1-4]. In the synthesis of
mesoporous Al2O3, anodic oxidization is one of the
most important methods because it is relatively
simple and can be conducted at either high potential
or low potential, although the detailed mechanisms
are still under discussion [5, 6]. However, the
Nano Research
DOI (automatically inserted by the publisher)
Address correspondence to [email protected]; [email protected].
Research Article
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2 Nano Res.
application of anodic oxidation for preparation of
mesoporous Al2O3 has one major disadvantages, that
is an applied potential is required as the driving force
of the oxidation system [7-11].
Currently, triboelectric nanogenerator, as an
innovative invention, have been developed to convert
mechanical energy from irregular mechanical
vibrations to electricity, such as impacts [12, 13],
sliding [14, 15], and rotations [16, 17]. TENG exhibits
the remarkable characteristics of easy fabrication,
low cost, and high efficiency. Moreover, the output
potential of TENG is adjustable, that is the output
potential can vary from several volt to a few hundred
volt based on the contact electrification effect of
TENG [18]. Previously, we have proposed the
self-powered electrochemistry for applications of
such as water splitting, environmental particular
filtering, water purification and pollution cleaning
[19-21].
In this paper, instead of utilizing an external
electricity power, we designed a self-powered
electrochemical AO system for the preparations of
mesoporous Al2O3 by using 3×3 TENG arrays driven
by the energy harvested from natural wind. It is
worth noting that the TENG arrays can harvest wind
energy from all directions and the electrical output of
the TENG arrays can be adjusted according to the
frequency. In addition, the pore size and depth of the
prepared mesoporous Al2O3 could be regulated from
the vibrational frequency of TENG arrays & the
reaction times. Last but not the least; the SPAO
system is temperate, easy to be applied and
controlled, making it feasible for preparation of other
mesoporous materials.
2 Results and discussion
2.1 Fabrication and characterization of TENG arrays
In order to evaluate the feasibility of the SPAO
system, electrical characteristics of the TENG arrays
were investigated first. The TENG is based on the
contact electrification of polydimethylsiloxane
(PDMS) and ITO, where PDMS was pretreated to
form inverted pyramid structures (Figure 1d). In
order to improve the efficiency of TENG arrays, 3×3
TENG cells were used in the SPAO system (Figures
Figure 1 (a) Scheme of the designed TENG, (b) profile of single
TENG cell, (c) profile of TENG arrays consisted of 3×3 TENG
cells; (d) SEM image of PDMS’ pyramid structures; (e) Short
circuit currents and (f) open circuit voltages of single TENG cell
at a frequency of 3 Hz; (g) Schematic diagram of the AO system
driven by TENG arrays; (h) Lighted LEDs as driven by wind
blowing.
1a-c). Figure 1e and 1f show the output current and
voltage of a single TENG cell under stimulations at a
certain frequency of 3 Hz stressed by a linear
mechanical motor. As we can see, the output current
is over 0.65 mA, and the voltage reaches about 256 V.
For the AO system powered with TENG, the cathode
was Ti plate that was immersed into the electrolyte
and the anode was Al foil. The TENG arrays was
rectified and then connected to the whole system
with the external stimulation at different frequencies
(Figure 1g). Furthermore, TENG arrays can harvest
the mechanical energy of winds, and then convert
them into electricity to provide the power for the AO
system [13, 22]. As shown in Figure 1h, multiple
light-emitting diodes (LEDs, which are arranged in
letters S P A O) were powered up driven by wind
(Detail process see video S, in supporting
information), which indicated that the SPAO system
is feasible.
2.2 Morphology characterization of mesoporous
Al2O3 synthesized by SPAO system
To get intact mesoporous Al2O3, two-step anodization
under potentiostatic mode driven by TENG arrays
was employed in this work and the synthetic process
was displayed in Figure 2. During the experiment, 1.0
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3 Nano Res.
Figure 2 Schematic diagrams for the synthetic process of
mesoporous Al2O3.
mol/L sulfuric acid solutions were employed under
potentiostatic modes which were controlled by the
vibrational frequency of TENG arrays. It is worth
noting that aluminum oxide must be removed
thoroughly before the second anodization step. At
the end of the experiment, the remaining aluminum
substrate was removed in a mixed solution of
HCl/CuCl2 [23].
At the same time, the morphologies and sizes of
typical mesoporous Al2O3 with different frequencies
of TENG arrays & reaction times were characterized
by SEM, as shown in Figure 3. Figure 3a shows the
intact pore structures obtained using different
anodization potential from changing the stimulating
frequencies of TENG arrays. From the scale size of
Figure 3a, we can see that the pore size of the of the
mesoporous Al2O3 increase from 10 nm to 50 nm with
the anodization potential increasing. However, at
higher anodization potentials, the mesoporous
structures are visibly damaged in many regions
(Figure S1a in the ESM). At the same time, the
influence of reaction times on the pore size of
samples was also studied. Figure 3b shows the
changes of the pore’s depth and size of samples with
the reaction times. It is apparent that the pore’s depth
and size of mesoporous Al2O3 are both increased
with the reaction time elapsing. These images also
describe the formation of these mesoporous samples.
What’s more, the channels of these specimens grow
straight. However, if aluminum oxide can’t remove
thoroughly before the second anodization step, the
prepared mesoporous samples contain many
irregular small pores around the original pore. These
excess pores are not vertical and seen as branch pore
structures which have been reported in the literatures
(Figure S1b in the ESM) [24].
Figure 3 SEM images of the synthesized mesoporous Al2O3,
(a) the same reaction time (2 h) with different vibrational
frequencies (fa1 to fa4 is 1, 3, 5, 7 Hz, respectively) of TENG
arrays, (b) the same vibrational frequency (3 Hz) of TENG
arrays with different reaction times (tb1 to tb4 is 0, 1, 1.5, 2 h,
respectively); and (c) EDX spectra of the mesoporous Al2O3
structures.
In the meantime, these mesoporous specimens
were also identified by energy-dispersive X-ray (EDX)
analysis. As shown in Figure 3c, the signals of O and
Al elements are observed in the EDX spectra, which
confirm that the mesoporous material is Al2O3. The
SEM images and EDX spectra demonstrate that the
intact mesoporous Al2O3 are prepared from the SPAO
system and the pore’s depth and size of the
mesoporous material could be controlled from the
frequency of TENG arrays & reaction times.
Moreover, the pore’s depth and size of mesoporous
material are both increased with the increase of
vibrational frequency & reaction time.
2.3 Principle for self-powered formation of
mesoporous Al2O3
In the self-powered electrochemical anodic
oxidization process, the formation of mesoporous
Al2O3 can be expressed in formulas (1) and (2),
respectively [25, 26].
Anode: Al → Al3+ + 3e(priority)(1-1)
4OH- → 2H2O + O2 + 4e(secondary)(1-2)
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4 Nano Res.
4Al3+ + 3O2 → 2Al2O3
Al2O3 + 6H+→ 2Al3+ + 3H2O
Cathode: 2H+ + 2e → H2(priority)(2)
To improve the efficiency of the SPAO system, the
continuous AC output from the TENG arrays was
tuned by using the conventional transformer [19]. As
shown in Figure 4a, the open circuit potential was
reduced to about 17 V at the certain frequency of 3
Hz. At the same time, the relationship between the
transformed potential of AO system and the
triggering frequency of TENG arrays was also
studied. The corresponding transformed potential
changes of the AO system at different frequency is
shown in Figure 4b, and it is obvious that the
transformed potential of the system increased from
12.14 V to 28.82 V when the vibrational frequency
increased from 1 Hz to 9 Hz. What’s more, the value
of the transformed potential charges is almost
proportional to the frequency (as shown in Figure
4c).
Figure 4 (a) Open-circuit voltage of a single TENG cell
after applying a transformer at the frequency of 3 Hz, (b) the
transformed voltage changes of single TENG cell at
different vibrational frequencies stressed by linear
mechanical motor, (c) the relationship between the
transformed voltage with different vibrational frequencies
from 1 Hz to 9 Hz; (d) Nitrogen adsorption-desorption
isotherm of the mesoporous Al2O3 (t: 2h, f: 3Hz) and its
pore size distribution (inset) calculated from the BJH model.
In the self-powered electrochemical anodic
oxidization system, when electrolyte concentration
and temperature were set to certain values at
constant potential, a critical current density (Jc) must
be present [27, 28] and the mesoporous structure
could only be formed when the current density is
below the critical density. In this initial event,
aluminum is oxidized quickly. At the same time, OH-
migrates from cathode to anode. Lots of Al3+
concentrated on the anode at the beginning, which
lead to a compact Al2O3 layer on the aluminum
surface. The formation of the compact Al2O3 layer
will increase the voltage and electric field. Then the
dissolution rate of the compact Al2O3 layer will
accelerate to lower the voltage and electric field.
Finally, the growth of mesoporous Al2O3 occurs when
Al2O3 is formed at the same rate as that it is dissolved.
Meanwhile, the increase of temperature or electrolyte
concentration can speed up the rate of vertical and
horizontal dissolution of aluminum oxide. During
the anodizing process, higher anodization potential
results in the corresponding higher current density,
which lead to the increase of pore size and depth of
mesoporous material [29, 30]. Therefore, the pore size
and depth of the mesoporous Al2O3 increase with the
TENG arrays’ vibrational frequency increasing,
which is consistent with the SEM results.
2.4 N2 adsorption-desorption study on mesoporous
Al2O3
Figure 4d shows the N2 adsorption-desorption
isotherm and the corresponding pore size
distribution of mesoporous Al2O3 (t: 2h, f: 3Hz). They
could be classified as type Ⅰ isotherm characteristic
of mesoporous materials, indicating that the sample
have a mesoporous structure [31, 32]. In the relative
pressure range of P/P0= 0.10-0.88, the adsorbed
amount gradually increases for the sample, but above
P/P0> 0.88, N2 uptake becomes saturated. From the
pore distribution curve measured by the
Barrett-Joyner-Halenda (BJH) method (inset), one can
see that the sample have an average pore diameter of
about 42 nm. For the mesoporous Al2O3, the BET
surface area and the total pore volume are 216.3 m2
g-1 and 0.2597 cm3 g-1 respectively. These results
reveal that pore sizes obtained from N2 sorption
analysis agree well with those obtained from SEM
image analysis independently.
www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research
5 Nano Res.
3 Conclusions
In summary, we developed the first self-powered
electrochemical anodic oxidization system that is
capable of synthesis of mesoporous Al2O3 driven by
TENG arrays using the energy provided by natural
wind in the environment. Systematic studies have
found that the pore depth and size of mesoporous
Al2O3 can be regulated by changing the stimulating
frequency of the TENG arrays. What’s more, BET
data further confirm that the prepared materials have
mesoporous structures. The temperate, easy
controlled self-powered electrochemical AO system
is very feasible for preparation of other mesoporous
materials.
4 Experimental section
4.1 Fabrication of the self-powered AO system
The basic structure of the SPAO system is composed
of TENG arrays device and an electrochemical AO
system, as schematically illustrated in Figure 1. The
TENG arrays consist of 3×3 TENG cells. Each one is
based on the contact electrification of PDMS and ITO
(Figure 1a). The fabrication of TENG has been
described in detail previously [21, 33]. In a typical
procedure, PDMS film was transferred onto an
ITO-coated polyester (PET) film. Then, another clean
ITO-coated PET film was placed onto the prepared
PDMS-ITO-PET film and sealed at the two ends,
leaving a small gap of 3 mm between the two contact
surfaces by forming an arched structure. The
effective size of TENG cell is 9 cm×9 cm, and the total
thickness is about 1 mm. The output voltage and
current were measured by the Keithley 6514 System
Electrometer and SR570 low noise current amplifier
from Stanford Research Systems. In the self-powered
electrochemical AO system, the anode was aluminum
foils that were immersed into the electrolyte and the
cathode was Ti plates.
4.2 Preparation of mesoporous Al2O3
High purity aluminum foils (99.99%,
15mm×10mm×0.2mm) were used as working
electrodes. In order to get the uniform mesoporous
materials, two-step anodization was employed for
the process [34, 35]. Prior to anodization, the
aluminum foils were degreased by ultrasonic in
ethanol for 10 min and then electro-polished in a
mixture solution of HClO4 and ethanol
(VHClO4/Vethanol=1/4) at 20 V for 5min. Then 1.0 mol/L
sulfuric acid solutions were employed under
potentiostatic modes which were controlled by the
frequency of TENG arrays. Following the first
anodization step, the specimens were immersed in 6
wt % phosphoric acid for 0.5 h to remove aluminum
oxide then proceeded second anodization in
conditions as same as the first step. In the whole
process of oxidation, a powerful ice-water bath
system was used to maintain the low temperature
needed for the high-field anodization. Subsequently,
the as-anodized sample was immersed into a mixture
solution of 0.2 mol/L CuCl2 and 6.1 mol/L HCl to
remove the underlying Al substrate [23, 36].
4.3 Characterization
Scanning electron microscopy (SEM) images
equipped with an EDX spectrometer were collected
to analyze the surface morphology and
microstructure of mesoporous Al2O3. The 1 kV
accelerating voltage was used to avoid damaging the
mesoporous microstructure. The porosity and
specific surface areas of the yielded mesoporous
Al2O3 was substantiated by adsorption-desorption of
ultrapure N2 on a Micromeritics ASAP 2020 unit at
313 K.
Acknowledgements
We thank the financial support from the National
Natural Science Foundation of China (NSFC No.
21173017, 51272011 and 21275102), the Program for
New Century Excellent Talents in University
(NCET-12-0610), the science and technology research
projects from education ministry (213002A), National
“Twelfth Five-Year” Plan for Science & Technology
Support (No.2013BAK12B06), the “thousands
talents” program for pioneer researcher and his
innovation team, China, National Natural Science
Foundation of China (Grant No. 51432005;
No.Y4YR011001), Beijing City Committee of science
and technology (Z131100006013004,
Z131100006013005).
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6 Nano Res.
Electronic Supplementary Material: Supplementary
material (SEM images of mesoporous Al2O3 without
growing straight, included Figure S1a, S1b; Video of
multiple light-emitting diodes (LEDs) powered up
driven by wind.) is available in the online version of
this article at
http://dx.doi.org/10.1007/s12274-***-****-*
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Nano Res.
Electronic Supplementary Material
Self-powered electrochemical anodic oxidation: a new
method for preparation of mesoporous Al2O3 without
applying electricity
Huarui Zhu, 1 Ying Xu, 1 Yu Han, 1 Shuwen Chen, 1 Tao Zhou, 1 Magnus Willander, 1 Xia Cao, 1, 2 () and
Zhonglin Wang, 1, 3 ()
Supporting information to DOI 10.1007/s12274-****-****-*
Figure S1 SEM images of the synthesized mesoporous Al2O3 (a) at higher anodization potentials, (b) the
aluminum oxide without removing thoroughly after the first anodization step.
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