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# Fabrication of Fe 2 O 3 nanowire arrays based on oxidation-assisted stress-induced atomic-diffusion and their photovoltaic properties for solar water splitting

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In this research, we propose a new simple method to fabricate high-density Fe2O3 nanowire arrays for solar water splitting, based on oxidation-assisted stress-induced atomic-diffusion. In the presence of water vapor, surface oxidation was promoted during the heating process. The driving force induced by the stress gradient was enhanced due to the expansion of the oxidation layer. Therefore, Fe2O3 nanowire arrays were fabricated at a relative low temperature (350 °C) with a high density (8.66 wire per μm²). Using the nanowire array as the photoanode, a photocurrent density of 0.65 mA cm⁻² at 1.23 V vs. RHE was achieved in a three-electrode system.
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Fabrication of Fe
2
O
3
nanowire arrays based on
oxidation-assisted stress-induced atomic-diusion
and their photovoltaic properties for solar water
splitting
Yiyuan Xie, Yang Ju,*Yuhki Toku and Yasuyuki Morita
In this research, we propose a new simple method to fabricate high-density Fe
2
O
3
nanowire arrays for solar
water splitting, based on oxidation-assisted stress-induced atomic-diusion. In the presence of water
vapor, surface oxidation was promoted during the heating process. The driving force induced by the
stress gradient was enhanced due to the expansion of the oxidation layer. Therefore, Fe
2
O
3
nanowire
arrays were fabricated at a relative low temperature (350 C) with a high density (8.66 wire per mm
2
).
Using the nanowire array as the photoanode, a photocurrent density of 0.65 mA cm
2
at 1.23 V vs. RHE
was achieved in a three-electrode system.
1. Introduction
Nowadays, most of the energy consumed by human beings
comes from fossil fuel. Fossil fuel is not an ideal energy
resource for the future due to several disadvantages, such as
the limited amount, which cannot satisfy the energy demands
in the future; moreover, the combustion of fossil fuel
produces CO
2
, one of the main greenhouse gases. As a
sustainable clean energy source, hydrogen is an ideal choice
for the future. Photoelectrochemical solar fuel production,
especially solar water splitting, has been attracting increasing
interest, motivated by the recent advances in nanostructured
materials and by concerns over the environmental impact of
fossil fuels.
1
Solar water splitting uses only water and solar energy and
a catalyst to produce hydrogen. Some semiconductors have
shown great potential for this application, such as BiVO
4
,
TiO
2
,andFe
2
O
3
with the theoretical maximum solar to
hydrogen (STH) eciencies of 9.2%, 2.0%, and 15%, respec-
tively.
2
Fe
2
O
3
is the most promising of these materials due to
the small bandgap and the related visible light absorption,
natural abundance, low cost, and stability under deleterious
chemical conditions. Recently, several reports on doped
nanostructure Fe
2
O
3
usedforsolarwatersplittinghavebeen
published. For example, in 2006, Cesar et al. fabricated the
silicon-doped thin hematite lm, with the solar to hydrogen
conversion eciency of 2.1%.
3
In 2008, Hu et al. reported
aplatinum-dopedthinhematitelm with a photocurrent
density of 1.43 mA cm
2
,at0.4Vvs. Ag/AgCl.
4
In 1999,
a nanocrystalline n-Fe
2
O
3
thin-lm was synthesized by Khan
et al. with a photocurrent density of 3.7 mA cm
2
at 0.7 V vs.
saturated calomel electrode (SCE).
5
In 2009, Mohapatra et al.
used a sono-electrochemical anodization method to grow
Fe
2
O
3
nanotube arrays on an Fe plate with a photocurrent
density of 1.41 mA cm
2
at 0.5 V vs. Ag/AgCl.
6
On the other hand, Fe
2
O
3
nanowire arrays can be fabricated
by stress-induced method, as reported previously.
7
Nanowire
array structure was considered having two advantages used for
solar water splitting. The rst is that due to the high surface to
volume ratio of nanowire structure, it could provide large
electrode/electrolyte interface area to enhance the chemical
reaction, thereby improving the water splitting performance
eventually. The second is that nanowire array could absorb
more light energy than the other structures such as thin lm or
nanoparticulates because the nanowire array is in a 3D struc-
ture which can absorb not only the incident light but also the
reected one. Fe
2
O
3
nanowire arrays can be obtained by
heating a high-purity iron substrate under ambient condi-
tions, which is a simple and low-cost method. However,
because the density of these nanowire arrays is not high
enough, they are unfavorable for solar water splitting. In this
work, a new method is proposed to synthesize high-density
Fe
2
O
3
nanowire arrays on an iron plate, under low-
temperature conditions used for solar water splitting. In the
presence of water vapor, surface oxidation was promoted
during the heating process, thereby enhancing the driving
force induced by stress gradient due to the expansion of the
oxidation layer. Consequently, it is possible to fabricate high-
density Fe
2
O
3
nanowire arrays at a relatively low temperature
(350 C) compared to that used in the traditional method (500
800 C).
8,9
Department of Mechanical Science and Engineering, Graduate School of Engineering,
Nagoya University, Nagoya 464-8603, Japan. E-mail: ju@mech.nagoya-u.ac.jp
Cite this: RSC Adv.,2017,7,30548
Received 21st March 2017
Accepted 6th June 2017
DOI: 10.1039/c7ra03298f
30548 |RSC Adv.,2017,7,3054830553 This journal is © The Royal Society of Chemistry 2017
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2. Experimental
2.1 Nanowire fabrication
Commercial iron plate with the purity of 99.95% was used as
the substrate for the nanowire fabrication. The thickness
of the iron plate is 0.1 mm and the size of each substrate is
10 mm 10 mm.
The iron plate was heated by a ceramic heater in an atmo-
sphere of water vapor. In order to nd the best conditions for
the nanowire array fabrication, some key parameters are
investigated, which include the heating temperature, heating
time, water vapor volume, and the duration of heating. Heating
temperature was set between 250 and 700 C, as shown in Table
1. A humidier was used to provide the water vapor condition,
with a gas ow rate ranging from 0.2 L h
1
to 1.25 L h
1
,as
shown in Table 2. Heating time of the iron plate on the ceramic
heater was set to 30, 60, and 90 min, respectively, as shown in
Table 3. Aer the fabrication, all the samples were analyzed by
scanning electron microscopy (SEM, JSM-7000FK) and X-ray
diraction (XRD).
2.2 Photocurrent measurements
Photocurrent measurement was carried out using a three-
electrode system, as shown in Fig. 1. The fabricated Fe
2
O
3
nanowire array is used as the photoanode, the cathode is a Pt
wire with a diameter of 0.05 mm, and Ag/AgCl is used as the
reference electrode. These three electrodes were placed in a 1 mol L
1
NaOH solution. The light source is a quartz halogen
ber optic illuminator (Fiber-Lite PL800), the spectrum of the
light source was measured as shown in Fig. 2, and the optical
power density was measured to be 154 mW cm
2
by a power
meter (COHERENT LM-10).
IPCE measurements were performed using a Xe lamp with
the single-wavelength lters from 400 nm to 650 nm. The light
energy of the incident light from the lamp was measured with
a power meter (COHERENT LM-10). All IPCE measurements
were carried out with the applied bias of 0.234 V versus Ag/AgCl
reference electrode (1.23 V vs. RHE).
3. Results and discussion
3.1 Experimental results and discussions
Fig. 3 shows the SEM images of the nanowire arrays fabricated
at dierent heating temperatures under the conditions shown
in Table 1. It can be inferred from the SEM images that the
morphologies of the nanowires are dierent under dierent
temperatures, besides the density, length, and diameters of the
nanowires. The nanowires heated at 350, 450, and 500 C
(Fig. 3(b), (c) and (d), respectively) are cone-shaped and those
heated at 600 C (Fig. 3(e)) and 700 C (Fig. 3(f)) are wire-shaped.
Similar morphologies are observed for a given temperature,
indicating that the heating temperature aects the morphology
of the nanowire.
The density of the nanowires is a key factor aecting the
eciency of the solar-hydrogen energy cycle. A comparison of
the density of the nanowire arrays fabricated at dierent
temperatures is shown in Fig. 4. The largest density of 14.3 wire
Table 1 Experimental conditions: dierent heating temperatures
No.
Heating time
(min)
Temperature
(C)
Water vapor volume
(L h
1
)
1 90 250 0.2
2 350
3 450
4 500
5 600
6 700
Table 2 Experimental conditions: dierent water vapor volumes
No.
Heating time
(min)
Heating temperature
(C)
Water vapor volume
(L h
1
)
7 90 450 0.2
81
9 1.25
Table 3 Experimental conditions: dierent heating times
No.
Heating time
(min)
Heating temperature
(C)
Water vapor volume
(L h
1
)
10 30 450 0.2
11 60
12 90
Fig. 1 Schematic of the three-electrode measurement system.
Fig. 2 Spectrum of the halogen light used in photocurrent density
measurement.
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per mm
2
is achieved for the sample heated at 450 C. When the
iron plate was heated at 250 C, only a small quantity of the
nanowires could be observed on the sample surface. With the
increase in the heating temperature, the density of the nanowire
array increased up to 450 C. However, it decreased for
temperatures above 450 C. The density is only 1 wire per mm
2
at
700 C.
The length and diameter statistics of the nanowires obtained
at dierent temperatures are shown in Fig. 5 and 6, respectively.
With the increase in the heating temperature, the average
length of the nanowires increased, and the longest nanowires of
9.98 mm average lengths were obtained at 700 C. Fig. 6 shows
the diameter statistic of the nanowires fabricated at dierent
temperatures. Diameters of the nanowires are also considered
as an important factor aecting the eciency of solar to
hydrogen energy conversion; nanowires with larger diameters
could absorb more light than those with small diameters, which
could eventually improve the conversion eciency. Unlike the
variation in the average length, the average diameter of the
nanowires decreases with the increase in heating temperature.
The largest average diameter of 300 nm was obtained for
nanowires fabricated at 250 C. The eect of the water vapor
volume on the nanowire growth was also investigated in this
study. The volume of the water vapor was set to be 0.2, 1, and
1.25 L h
1
, respectively, as shown in Table 2. From the SEM
images shown in Fig. 7, it can be easily observed that the density
of the nanowires decreased with an increase in the water vapor
volume.
Fig. 8 shows the results of the iron samples heated for 30, 60,
and 90 min, respectively, under the conditions listed in Table 3.
When the sample was heated for a very short duration, some
weak spots were generated on the iron plate surface, without
any nanowire growth (Fig. 8(a)). In the sample heated for 60 min
(Fig. 8(b)), nanowires were formed, but with very dierent
lengths and the density was lower than that of the sample
heated for 90 min, as shown in Fig. 8(c). The nanowires had the
highest density when the sample was heated for 90 min. The
experiments were also carried out with longer heating times,
120 and 150 min, but this did not increase the density of the
nanowire array.
Fig. 3 SEM micrographs of the Fe
2
O
3
nanowire arrays obtained at
dierent heating temperatures: (a) 250; (b) 350; (c) 450; (d) 500; (e)
600; and (f) 700 C.
Fig. 4 Density statistic of the Fe
2
O
3
nanowires obtained at dierent
temperatures.
Fig. 5 Length statistic of the Fe
2
O
3
nanowires obtained at dierent
temperatures.
Fig. 6 Diameter statistic of the nanowires obtained at dierent
temperatures.
Fig. 7 SEM micrographs of the Fe
2
O
3
nanowire arrays for samples
heated at 450 C with dierent water vapor volumes: (a) 0.2; (b) 1; and
(c) 1.25 L h
1
.
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The cross section of the fabricated sample has also observed
by using the FESEM, as shown in Fig. 9. Three layers can be
easily observed from the SEM image, which include the nano-
wire layer, the oxide layer and the iron layer. The morphology of
the nanowires fabricated at 450 C was shown in Fig. 10. The
shape of nanowires looks like grass, which indicated that
nanowires grew from the top of themselves with the precipita-
tion of diused Fe atoms and their oxidation. The average
diameter of the nanowires shown in Fig. 10 is 144 nm,
approximately.
3.2 X-ray diraction and discussions
Fig. 11 shows the XRD patterns of the nanowire arrays obtained
for dierent heating temperatures under the water vapor
condition of 0.2 L h
1
and heating time of 90 min. From data
obtained from dierent samples, it can be inferred that when
the heating temperature is higher than 450 C, the formed
Fe
2
O
3
layer on Fe substrate is thicker than that formed at
450 C. By comparing the densities of the nanowire arrays, it is
considered that although the heating temperature of over
500 C could provide a larger driving force to increase the
diusion of the Fe atoms, the formed thicker oxidation layer
will hinder the growth of the nanowires, due to the decrease of
numbers of weak spots in Fe
2
O
3
layer. Therefore, low density
nanowire arrays were obtained at relative high temperatures. In
the case of the sample heated at 450 C, the oxidization rate of
the iron plate surface is optimal, generating more weak spots in
Fe
2
O
3
layer, and the driving force is also large enough to make
the Fe atoms diuse from the inner part to the Fe/Fe
2
O
3
interface.
3.3 Photocurrent measurements
The photovoltaic properties of the nanowires have been inves-
tigated using a three-electrode system (Fig. 1), and the results
are shown in Fig. 12. The Fe
2
O
3
nanowire photoanode fabri-
cated at 350 C showed the largest photocurrent density among
all the photoanodes, 0.65 mA cm
2
at 1.23 V vs. a reversible
hydrogen electrode (RHE). Although the nanowire photoanode
fabricated at 450 C has the largest density of nanowires, the
photocurrent density is lower at 0.47 mA cm
2
, due to the
smaller average diameter of the nanowires (127 nm) compared
to that of the nanowire photoanode fabricated at 350 C (161
nm). The samples heated at 250, 500, 600, and 700 C show very
small photocurrent values, possibly owing to the poor nanowire
density.
The incident-photon-to-current eciency (IPCE) of the
nanowire photoanode fabricated at 450 C was measured to
conrm the performance of water splitting, as shown in Fig. 13.
The IPCE decreased with the increase of wavelength, and the
maximum value is 5.54% at 400 nm. This value is relative high
than that of other pure Fe
2
O
3
photoanodes without any func-
tional modication, reported by the literatures, such as the
Fig. 8 SEM micrographs of the Fe
2
O
3
nanowire arrays fabricated at
450 C for dierent heating durations: (a) 30; (b) 60; and (c) 90 min.
Fig. 9 SEM cross section observation of the Fe
2
O
3
nanowire sample.
Fig. 10 SEM image of the Fe
2
O
3
morphology.
Fig. 11 XRD patterns of the Fe
2
O
3
nanowire arrays at dierent
temperatures.
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Fe
2
O
3
lm with the IPCE of 2% at 400 nm,
10
and Fe
2
O
3
nanorods
with the IPCE of 1.3% at 400 nm.
11
It should be mentioned that
the IPCE value could be remarkably improved by functional
modication of the Fe
2
O
3
nanowire array. It has been reported
that the Pt-doped Fe
2
O
3
nanorods can reach the IPCE up to 55%
at 400 nm,
12
Pt-doped polycrystalline thin-lm electrodes of
Fe
2
O
3
exhibit an IPCE of 25% at 400 nm,
4
and Fe
2
O
3
thin lms
modied with a catalytic cobalt layer has the IPCE of 46% at
370 nm.
13
The stability of photocurrent was measured at 1.23 V vs. RHE
by a chopped illumination with 10 s on/ofor 120 seconds, for
aFe
2
O
3
nanowire array photoanode fabricated at 450 C, as
shown in Fig. 14. The photocurrent density is very stable and
increased and decreased quickly with on and othe light which
shows the good photoresponse properties of the Fe
2
O
3
nano-
wire array photoanode.
3.4 Mechanism
When the iron plates are heated, the thermodynamically stable
oxide layer, Fe
2
O
3
topmost layer are formed. Because the molar
volumes of Fe
2
O
3
(30.39 cm
3
mol
1
)
14
is great larger than that of
Fe (7.09 cm
3
mol
1
),
14
tensile stress is generated in the iron
plate due to the volume expansion of Fe
2
O
3
layer.
15
Thus,
a stress gradient is generated from the center of the Fe plate to
the Fe/Fe
2
O
3
interface. The gradient of stress can serve as the
driving force for the atomic diusion and the atomic ux
propagates from the low tensile area to high tensile area.
Therefore, with the formation of the Fe
2
O
3
layer, the Fe atoms
move from the center of Fe plate to Fe/Fe
2
O
3
interface due to the
stress-induced atomic diusion. These diusion atoms serve as
a continuous source for the formation of Fe
2
O
3
nanowires.
Aer the Fe atoms diuse along the stress gradient to the Fe/
Fe
2
O
3
interface, they cumulate at the interface and then nd the
weak spots of Fe
2
O
3
layer and penetrate them to form nanowires
accompanying the oxidation of the Fe atoms. Aer the nano-
wires are formed, Fe atoms continue to diuse along the
nanowires due to the high driving force (see Fig. 15), which
explains the formation of longer nanowires with the increase in
the heating time. Under the water vapor condition, greater
amounts of iron can be oxidized into Fe
2
O
3
, which could
increase the thickness of the Fe
2
O
3
layer on the Fe substrate.
Therefore, the tensile stress that the Fe layer suered from the
Fe
2
O
3
layer is much larger in the presence of water vapor than
that created under an atmosphere condition. This increase the
stress gradient and the driving force for atom diusion, thereby
resulting in an increase in the density of the nanowires. It
should be noted that the driving force induced by the stress
gradient is due to the volume expansion of the Fe
2
O
3
oxidation
layer, which is dierent from that induced by the thermal
Fig. 12 Photocurrents from the nanowire array anodes obtained at
dierent heating temperatures.
Fig. 13 IPCE of Fe
2
O
3
nanowire array photoanode at 0.234 V vs. Ag/
AgCl (1.23 V vs. RHE).
Fig. 14 Jtcurve of Fe
2
O
3
nanowire array photoanode under chop-
ped illumination at a bias of 0.234 V vs. Ag/AgCl (1.23 V vs. RHE).
Fig. 15 Schematic of the mechanism of the nanowire growth.
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expansion mismatch generated in Al/Si
16
or Cu/Si
17
structured
samples. The similar thermal expansion coecients of Fe
2
O
3
and Fe make it dicult to create a stress based driving force
based on thermal expansion mismatch.
4. Conclusions
In summary, a new oxidation-assisted stress-induce method to
fabricate high-density semiconductor nanowire array has been
demonstrated. Large area Fe
2
O
3
nanowire arrays with high
density were fabricated successfully at low temperatures under
the water vapor condition. The best growth condition for the
Fe
2
O
3
nanowire arrays is heating for 90 min at 350 C, and
a water vapor volume of 0.25 L h
1
. Both the density and
diameter of the nanowires aect the photocurrent density of the
nanowire photoanode, which reached 0.65 mA cm
2
for the
nanowire array with the density and diameter of 8.66 wire per
mm
2
and 161 nm, respectively. The photocurrent measurements
indicate the good potential of the Fe
2
O
3
nanowire array pho-
toanodes for solar water splitting.
Acknowledgements
This work was supported by the Japan Society for the promotion
of science with Grants-in-Aid for Science Research (A) 26249001.
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... Large-scale facile synthesis of aligned a-Fe 2 O 3 nanowhiskers (NWs) can be achieved through oxidation of an iron-based substrate under a suitable environmental condition [4][5][6][7][8]. a-Fe 2 O 3 NWs synthesized via such a means normally exhibit tapered quasi-1D geometries with geometrically basal faces. ...
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Hematite (α-Fe 2 O 3 ) nanowhiskers (NWs) synthesized via oxidation of iron-based substrates are a promising photoanode material for photoelectrochemical water splitting. Such synthesized α-Fe 2 O 3 NWs have been found to contain ordered axial structures. Herein, we reveal that the known (1 $$\overline{1}$$ 1 ¯ 2)-related ordered structure actually exists in bicrystalline α-Fe 2 O 3 NWs instead of single-crystalline α-Fe 2 O 3 NWs and that it is associated with another known (3 $$\overline{3}$$ 3 ¯ 0)-related ordered structure. Through a spherical aberration (C S )-corrected high-resolution transmission electron microscopy (HR-TEM) investigation, the microstructural characteristic of the (1 $$\overline{1}$$ 1 ¯ 2)-related ordered structure is verified to be periodic atomic column displacements serving as tensile strain accommodation. The HR-TEM observation are also supported by a monochromated O K-edge EELS analysis, which indicates that α-Fe 2 O 3 NWs hosting the (1 $$\overline{1}$$ 1 ¯ 2)-related ordered structure are indeed associated with lattice expansion. In sum, our microstructural study elucidates the root cause of the long-asserted relationship between the (1 $$\overline{1}$$ 1 ¯ 2)-related ordered structure and oxygen vacancy ordering.
... The conductivity characteristics of the heterojunction-structured BVO material were partly elucidated by the Mott-Schottky electrochemical characterization, and interpreted through the mathematical model described by the equation below: from which a linear behavior between the inverse of the square capacitance (Cp -2 ) of the space-charge layer and the applied potential (V) is predicted. In this expression, N D is the number of donor ions, k B the Boltzmann's constant, ε the dielectric constant of the semiconductor, A the area of the electrode, ε 0 the dielectric permittivity of free space, T the absolute temperature, e the electronic charge and V fb the flat-band potential 47,48 . ...
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... with scratched surface was compared with unscratched Fe plate using FESEM analysis (Fig. S1, Supporting Information). From the images it is observed that the scratched surface ( Fig. S1(a)) showed substantial growth of nanowires than unscratched surface ( Fig. S1(b)) and the corresponding mechanisms ( Fig. S2) for nanowire formation has drawn on the basis of experimental results and previous reports 11,12 . A reducing environment or straining the surface by applying external force has already been employed to prepare γ-Fe 2 O 3 [28][29][30] . ...
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