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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

DOI:10.1039/C7RA03298F
Authors:
Yiyuan Xie at Micron Technology, Inc.
Yiyuan Xie
Yang Ju at Nagoya University
Yang Ju
Yuhki Toku at Nagoya University
Yuhki Toku
Yasuyuki Morita
Yasuyuki Morita
Yasuyuki Morita
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Abstract and Figures

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-diffusion
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-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, 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) efficiencies 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 efficiency 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
rsc.li/rsc-advances
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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
diffraction (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 different heating temperatures under the conditions shown
in Table 1. It can be inferred from the SEM images that the
morphologies of the nanowires are different under different
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 affects the morphology
of the nanowire.
The density of the nanowires is a key factor affecting the
efficiency of the solar-hydrogen energy cycle. A comparison of
the density of the nanowire arrays fabricated at different
temperatures is shown in Fig. 4. The largest density of 14.3 wire
Table 1 Experimental conditions: different 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: different 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: different 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 different 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 different
temperatures. Diameters of the nanowires are also considered
as an important factor affecting the efficiency 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 efficiency. 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 effect 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 different
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
different 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 different
temperatures.
Fig. 5 Length statistic of the Fe
2
O
3
nanowires obtained at different
temperatures.
Fig. 6 Diameter statistic of the nanowires obtained at different
temperatures.
Fig. 7 SEM micrographs of the Fe
2
O
3
nanowire arrays for samples
heated at 450 C with different 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 diffused Fe atoms and their oxidation. The average
diameter of the nanowires shown in Fig. 10 is 144 nm,
approximately.
3.2 X-ray diffraction and discussions
Fig. 11 shows the XRD patterns of the nanowire arrays obtained
for different heating temperatures under the water vapor
condition of 0.2 L h
1
and heating time of 90 min. From data
obtained from different 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
diffusion 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 diffuse 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 efficiency (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 different 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 different
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/offfor 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 offthe 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 diffusion 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 diffusion. These diffusion atoms serve as
a continuous source for the formation of Fe
2
O
3
nanowires.
Aer the Fe atoms diffuse 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 diffuse 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 suffered 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 diffusion, 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 different from that induced by the thermal
Fig. 12 Photocurrents from the nanowire array anodes obtained at
different 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 J–tcurve 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 coefficients of Fe
2
O
3
and Fe make it difficult 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 affect 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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One-dimensional nanostructured metal oxides present great potential for both research and practical applications. Due to their catalytic properties, high chemical and thermal stability, pronounced surface chemistry and biocompatibility, such nanostructures have attracted increasing research interest. Much attention is currently devoted to the development of reliable methods for producing such nanomaterials, which require specific growth conditions, including those based on alternative processes that exploit novel physical effects. Nanostructures based on zinc oxide are currently used in sensing applications and present interest as functional electrical contact materials. Pulsed-periodic laser irradiation is promising for the creation of zinc oxide-based nanomaterials.
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... Meanwhile, Fe 2 O 3 can alter the optical and electrical properties when it is mixed with a nanocomposite material [5]. In addition to that, Fe 2 O 3 was found to be stable in poisonous chemical environments reported in [6]. Moreover, graphene oxide (GO) is known to enhance the adsorption properties by reducing the internal resistances and promoting facile electron transport [7]. ...
Influence of Fe2O3 in ZnO/GO-based dye-sensitized solar cell
Article
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This work aims to study the influence of Fe2O3 in ZnO/GO-based DSSC incorporating PAN-based gel electrolyte. ZnO–Fe2O3/GO thin films and gel electrolyte were prepared using the sol–gel technique via spin-coating and polymerization of polyacrylonitrile (PAN) methods, respectively. The insertion of Fe2O3 in ZnO/GO improved the open-circuit voltage and fill factor significantly. However, large amount of Fe2O3 (0.3%) inhibited the electron transport with high electron recombination rate (keff = 3044.62 s−1). The main reason for the low efficiency in ZnO–Fe2O3(0.3%)/GO is due to the energy band misalignment with the failure of the excited electron from the LUMO of dye into the conduction band of ZnO–Fe2O3(0.3%)/GO. The study found that the optimum concentration of Fe2O3 is 0.2% for an efficient DSSC. ZnO–Fe2O3(0.2%)/GO-based DSSC exhibited slow electron recombination of 0.751 s−1. Moreover, the fine nanoparticles of ZnO–Fe2O3(0.2%)/GO observed through field emission electron microscopy show a more porous structure that improved the short-circuit current density in DSSC.
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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. ...
Exploring (11¯\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\overline{1}$$\end{document}2)-related ordered structure in oxidation-synthesized α-Fe2O3 nanowhiskers
Article
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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.
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... 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 . ...
Innovative multifunctional hybrid photoelectrode design based on a ternary heterojunction with super-enhanced efficiency for artificial photosynthesis
Article
Full-text available
  • Jun 2020
Electrochemical cells for direct conversion of solar energy to electricity (or hydrogen) are one of the most sustainable solutions to meet the increasing worldwide energy demands. In this report, a novel and highly-efficient ternary heterojunction-structured Bi4O7/Bi3.33(VO4)2O2/Bi46V8O89 photoelectrode is presented. It is demonstrated that the combination of an inversion layer, induced by holes (or electrons) at the interface of the semiconducting Bi3.33(VO4)2O2 and Bi46V8O89 components, and the rectifying contact between the Bi4O7 and Bi3.33(VO4)2O2 phases acting afterward as a conventional p–n junction, creates an adjustable virtual p–n–p or n–p–n junction due to self-polarization in the ion-conducting Bi46V8O89 constituent. This design approach led to anodic and cathodic photocurrent densities of + 38.41 mA cm–2 (+ 0.76 VRHE) and– 2.48 mA cm–2 (0 VRHE), respectively. Accordingly, first, this heterojunction can be used either as photoanode or as photocathode with great performance for artificial photosynthesis, noting, second, that the anodic response reveals exceptionally high: more than 300% superior to excellent values previously reported in the literature.
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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] . ...
Fabrication of γ-Fe2O3 Nanowires from Abundant and Low-cost Fe Plate for Highly Effective Electrocatalytic Water Splitting
Article
Full-text available
  • Mar 2020
Water splitting is thermodynamically uphill reaction, hence it cannot occur easily, and also highly complicated and challenging reaction in chemistry. In electrocatalytic water splitting, the combination of oxygen and hydrogen evolution reactions produces highly clean and sustainable hydrogen energy and which attracts research communities. Also, fabrication of highly active and low cost materials for water splitting is a major challenge. Therefore, in the present study, γ-Fe2O3 nanowires were fabricated from highly available and cost-effective iron plate without any chemical modifications/doping onto the surface of the working electrode with high current density. The fabricated nanowires achieved the current density of 10 mA/cm² at 1.88 V vs. RHE with the scan rate of 50 mV/sec. Stability measurements of the fabricated Fe2O3 nanowires were monitored up to 3275 sec with the current density of 9.6 mA/cm² at a constant potential of 1.7 V vs. RHE and scan rate of 50 mV/sec.
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... Fe 2 O 3 nanostructures removes the performance limitation imposed by the inherently short hole diffusion length of α-Fe 2 O 3 and can improve the electron collection efficiency [1]. Large-area synthesis of arrayed α-Fe 2 O 3 nanowhiskers (NWs) can be simply and economically realized through thermal oxidation of an iron-based substrate [2][3][4]. In the situation the oxidation temperature is below 570°C, an iron oxide scale composed of a magnetite (Fe 3 O 4 ) layer and an α-Fe 2 O 3 layer is expected to initially form on the iron-based substrate. ...
Understanding ordered structure in hematite nanowhiskers synthesized via thermal oxidation of iron-based substrates
Article
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Hematite (α-Fe2O3) nanowhiskers (NWs) with (001) basal faces synthesized via thermal oxidation of iron-based substrates are known to contain an ordered structure. The ordered structure has been identified to be related to oxygen vacancy ordering. However, the cause of its formation remains a mystery. In this study, with a high-resolution transmission electron microscopy (HR-TEM) investigation based on negative-Cs imaging (NCSI) and atomic-column position analysis, we observed tensile strain in the above-mentioned α-Fe2O3 NWs and revealed that the ordered structure was actually periodic interplanar gap expansions induced by oxygen vacancy accumulations. These findings were further confirmed in a monochromated electron energy loss spectroscopy (EELS) analysis of the α-Fe2O3 NWs. The EELS data indicated that, in comparison to pristine α-Fe2O3, the α-Fe2O3 NWs possessed expanded average FeO and OO interatomic distances and were oxygen-deficient. Clarifying oxygen deficiency in the α-Fe2O3 NWs was not attributed to an insufficient oxygen supply during the NW growth, we concluded the ordered structure formed to accommodate tensile strain in the α-Fe2O3 NWs. This work demonstrates the applicability of integrating NCSI and monochromated EELS for the examination of strain-induced microstructural and microchemical variations in lightly strained metal oxides.
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Self-supporting nanoporous Ni/metallic glass composites with hierarchically porous structure for efficient hydrogen evolution reaction
Article
Searching for free-standing and cost-efficient hydrogen evolution reaction (HER) electrocatalysts with high efficiency and excellent durability remains a great challenge for the hydrogen-based energy industry. Here, we report fabrication of a unique hierarchically porous structure, i.e., nanoporous Ni (NPN)/metallic glass (MG) composite, through surface dealloying of the specially designed Ni40Zr40Ti20 MG wire. This porous composite is composed of micrometer slits staggered with nanometer pores, which not only enlarges effective surface areas for the catalytic reaction, but also facilitates the release of H2 gas. As a result, the NPN/MG hybrid electrode exhibited the prominent HER performance with a low overpotential of 78 mV at 10 mA cm⁻² and Tafel slope of 42.4 mV dec⁻¹, along with outstanding stability in alkaline solutions. Outstanding catalytic properties, combining with their free-standing capability and cost efficiency, make the current composite electrode viable for HER applications.
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Bandgap Engineering in the NaBiO 3 ·2H 2 O/NaBiO 3 · x H 2 O Heterostructures for High Photoelectronic Response
Article
  • Jul 2020
High photoelectronic response with broad spectral range in photoelectric materials is of the great importance for the photovoltaics and photocatalysis applications. However, the existing photoelectric materials, such as TiO2 and α-Fe2O3, exhabit only high photoelectronic response or only broad spectral response because of wide bandgap limitation of light absorbance or low photogenerated charge separation efficiency. Here, we report NaBiO3•2H2O annealed at given temperature to form NaBiO3•2H2O/NaBiO3•xH2O heterostructures, which efficiently drives the photogenerated charge separation in a broad spectral range. The best performance of wide photoelectronic response and high surface photovoltage was obtained in the sample annealed under 130 oC. The high surface photovoltage with wide spectral range is attributed to the bandgap engineering of NaBiO3•2H2O/NaBiO3•xH2O heterostructures for the efficient photogenerated charge separation. These findings regarding the use of optimized NaBiO3•2H2O/NaBiO3•xH2O heterostructures suggest that fine-tuning heterostructure of the photoelectric materials is an effective approach for improving the photoelectrical performance in optoelectronic applications.
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Magnetization reversal switching in nanocylinders using dynamic techniques
Article
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The high aspect ratio Ni nanocylinders (nanowires and nanotubes) were fabricated by means of potentiostatic electrochemical deposition in porous anodic alumina templates with pore diameter of 200 nm and a thickness of 60 µm. The growth of nanocylinders was found to depend upon deposition parameters. The structural, morphological and magnetic analysis revealed the face-centred cubic texture, uniformity and magnetic properties of nanowires and nanotubes respectively. The dynamic properties of one-dimensional uniform arrays of Ni nanowires (NWs) and nanotubes (NTs) were studied by ferromagnetic resonance technique at various frequencies ranging from 6 to 25 GHz. In particular, we identify the onset of magnetization reversal from the angular variation studies of resonance fields [Hr(θH)] and FMR linewidths [ΔHr(θH)]. Parameters such as gyromagnetic ratio (γ), damping constant (α) and effective field (Heff) were derived from theoretical fits to Hr and ΔHr data. We found that Hr and ΔHr both depend sensitively on the angle between the magnetization direction and wire-axis. For a NT geometry, the fitting of ΔHr(θH) data divulges that the magnetization reversal mechanism dominant by coherent rotation and thereafter beyond transition angle (∼80°) curling mode contributes. In case of nanowires, the curling reversal mode is the dominant magnetization reversal process.
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The surface condition effect of Cu2O flower/grass-like nanoarchitectures grown on Cu foil and Cu film
Article
Full-text available
  • Oct 2013
Cu2O flower/grass-like nanoarchitectures (FGLNAs) were fabricated directly on two category specimens of Cu foils and Cu film using thermal oxidation method. The FGLNAs are approximately 3.5 to 12 μm in size, and their petals are approximately 50 to 950 nm in width. The high compressive stress caused by a large oxide volume in the Cu2O layer on the specimen surface played an important role in the growth of FGLNAs. The effects of surface conditions, such as the surface stresses, grain size, and surface roughness of Cu foil and Cu film specimens, on the FGLNA growth were discussed in detail. PACS 81. Materials science; 81.07.-b Nanoscale materials and structures: fabrication and characterization; 81.16.Hc Catalytic methods
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Growth of metal and metal oxide nanowires driven by the stress-induced migration
Article
Full-text available
  • May 2012
High quality Al and CuO nanowries are fabricated by simply heating the Al and Cu samples in air. Although the experimental operations and the stress-induced migration processes are quite similar, the causes of the driving forces and the growth mechanism are completely different. For the growth of Al nanowires, the driving force is determined to be the compressive stresses caused by the thermal expansion mismatch between Al film and Si substrate, and the growth mechanism is proposed to be the extrusion of atoms from the bases of nanowires (EAFB). For the growth of CuO nanowires, the driving force is determined to be the compressive stresses caused by the formation of Cu oxide layers, and the growth mechanism is proposed to be the formation of oxide molecules on surfaces of the nanowires (FOOS). The direct experimental observations of both EAFB and FOOS are presented. It is also demonstrated that stress distribution on the macroscopic level, which is caused by thermal or mechanical manipulation, can also influence the growth of CuO nanowires, which makes it prospective to control the growth of metal oxide nanowires by designing the stress distribution within the sample from which the nanowires are generated.
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Single-crystalline, wormlike hematite photoanodes for efficient solar water splitting
Article
Full-text available
  • Sep 2013
A hematite photoanode showing a stable, record-breaking performance of 4.32 mA/cm(2) photoelectrochemical water oxidation current at 1.23 V vs. RHE under simulated 1-sun (100 mW/cm(2)) irradiation is reported. This photocurrent corresponds to ca. 34% of the maximum theoretical limit expected for hematite with a band gap of 2.1 V. The photoanode produced stoichiometric hydrogen and oxygen gases in amounts close to the expected values from the photocurrent. The hematitle has a unique single-crystalline "wormlike" morphology produced by in-situ two-step annealing at 550°C and 800°C of β-FeOOH nanorods grown directly on a transparent conducting oxide glass via an all-solution method. In addition, it is modified by platinum doping to improve the charge transfer characteristics of hematite and an oxygen-evolving co-catalyst on the surface.
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Achieving High Performance Electromagnetic Wave Attenuation: A Rational Design of Silica Coated Mesoporous Iron Microcubes
Article
  • Jul 2014
Silica coated mesoporous Fe (Fe@SiO2) microcubes were designed for high performance electromagnetic wave attenuation. Silica coating lowered the permittivity significantly over the bare Fe cubes. Most importantly, silica coating was able to keep the shape of iron particles as well as to ensure a mesoporous structure. The synthetic approach consists of three steps. α-Fe2O3 microcubes were firstly synthesized by a hydrothermal method. Then, the cubes were coated with silica. The silica coated α-Fe2O3 microcubes were finally reduced under hydrogen gas at 500 oC. The reduction of iron oxide resulted in a removal of oxygen atoms and subsequently left the empty space as pores inside of silica coated iron cubes. The silicon resin composites containing Fe@SiO2 microcubes exhibited impressive electromagnetic wave attenuation characteristics. The reflection loss value of – 54 dB could be obtained at 3.2 GHz with a thickness of 4.5 mm. In addition, the mesoporous characteristic offered a low density of Fe@SiO2 mesoporous microcubes. The microcubes enabled a RL of – 15 dB with a film thickness as thin as 3 mm. The silica coated mesoporous iron microcubes significantly reduced the usage/thickness of silicon resin composite. They are very promising as a strong-attenuation and light-weight electromagnetic wave attenuation material.
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Back Electron-Hole Recombination in Hematite Photoanodes for Water Splitting
Article
The kinetic competition between electron / hole recombination and water oxidation is a key consideration for the development of efficient photoanodes for solar driven water splitting. In this study, we employed three complementary techniques, transient absorption spectroscopy (TAS), transient photocurrent spectroscopy (TPC) and electrochemical impedance spectroscopy (EIS), to address this issue for one of the most widely studied photoanode systems: nanostructured hematite thin films. For the first time, we show a quantitative agreement between all three techniques. In particular all three methods show the presence of a recombination process on the 10 ms - 1 s time scale, with the time constant and yield of this loss process being dependent upon applied bias. From comparison of data between these techniques, we are able to assign this recombination phase to recombination of bulk hematite electrons with long-lived holes accumulated at the semiconductor / electrolyte interface. The data from all three techniques are shown to be consistent with a simple kinetic model based on competition between this, bias dependent, recombination pathway and water oxidation by these long-lived holes. Contrary to most existing models, this simple model does not require the consideration of surface states located energetically inside the band gap. These data suggest two distinct roles for the space charge layer developed at the semiconductor / electrolyte interface under anodic bias. Under modest anodic bias (just anodic of flatband), this space charge layer enables the spatial separation of initially generated electrons and holes following photon absorption, generating relatively long lived holes (milliseconds) at the semiconductor surface. However under such modest bias conditions, the energetic barrier generated by the space charge layer fields is insufficient to prevent the subsequent recombination of these holes with electrons in the semiconductor bulk on a timescale faster than water oxidation. Preventing this back electron / hole recombination requires the application of stronger anodic bias, and is a key reason why the onset potential for photocurrent generation in hematite photoanodes is typically ~ 500 mV anodic of flat band and therefore needs to be accounted for in electrode design for PEC water splitting.
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Water Photooxidation by Smooth and Ultrathin ??-Fe 2 O 3 Nanotube Arrays
Article
  • Jul 2009
One-dimensional nanostructures exhibit quantum confinement that leads to unique electronic properties, making them attractive as the active elements for various applications. Iron oxide (α-Fe2O3 or hematite) nanotubes are of particular interest in catalysis, sensor devices, Li-ion battery, environmental remediation and photocatalysis. Here, we report a simple sonoelectrochemical anodization method to grow smooth and ultrathin (5−7 nm thick) Fe2O3 nanotube arrays (3−4 μm long) on Fe foil in as little as 13 min. The prepared catalyst has shown tremendous potential to split water to generate hydrogen under solar light illumination. A photocurrent density of 1.41 mA/cm2 is observed for hematite nanotube arrays with more than 50% being contributed by the visible light components of the solar spectrum.
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Pt-Doped α-Fe 2 O 3 Thin Films Active for Photoelectrochemical Water Splitting
Article
  • Jun 2008
In an attempt to improve photoelectrochemical (PEC) processes that is important in solar-to-chemical energy conversion, various materials have been proposed that have the potential for hydrogen production. For instance, hematite has many advantages for hydrogen production since it is somehow stable, has a relatively narrow bandgap, inexpensive, abundant and environmentally responsible. However, their use in PEC devices have been limited by several factors like poor conductivity and high electron-hole pair recombination rates. Strategies to overcome this limitation have been produced such as designing the hematite structure to permit more efficient transport and collection of photogenerated charge carriers. Others include the addition of surface electrocatalysts on iron oxide photoelectrodes, and doping the iron oxide with heteroatoms. Dopant species have also been introduced in the literature. However, there have been few reports on the synthesis of hematite films by electrodeposition and virtually no reports of electrodeposition of doped hematite. As such, an electrochemical route to prepare thin film photoanodes of nanocrystalline Pt-doped iron oxide has been proposed. It showed improvements in the performance of PEC when compared to pure iron oxide thin films.
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Controlled Growth of Large-Area, Uniform, Vertically Aligned Arrays of ??-Fe 2 O 3 Nanobelts and Nanowires
Article
  • Jan 2005
Vertically aligned iron oxide nanobelt and nanowire arrays have been synthesized on a large-area surface by direct thermal oxidation of iron substrates under the flow of O(2). The effects of reactive gas pressure, composition, and temperature have been systematically studied. It was found that nanobelts (width, tens of nanometers; thickness, a few nanometers) are produced in the low-temperature region (approximately 700 degrees C) whereas cylindrical nanowires tens of nanometers thick are formed at relatively higher temperatures (approximately 800 degrees C). Both nanobelts and nanowires are mostly bicrystallites with a length of tens of micrometers which grow uniquely along the [110] direction. The growth habits of the nanobelts and nanowires in the two temperature regions indicate the role of growth rate anisotropy and surface energy in dictating the ultimate nanomorphologies.
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