Effects of cathodic electrodeposition conditions on morphology and photoelectrochemical response of α-Fe2O3 photoanode

Accepted Manuscript Effects of cathodic electrodeposition conditions on morphology and photoelectrochemical response of α-Fe2O3 photoanode

Penghua Liang, Longzhu Li, Changhai Liu, Wenchang Wang, Honglei Zhang, Naotoshi Mitsuzaki, Zhidong Chen PII: DOI: Reference:

S0040-6090(18)30633-3 doi:10.1016/j.tsf.2018.09.034 TSF 36896

To appear in:

Thin Solid Films

Received date: Revised date: Accepted date:

4 January 2018 12 September 2018 13 September 2018

Please cite this article as: Penghua Liang, Longzhu Li, Changhai Liu, Wenchang Wang, Honglei Zhang, Naotoshi Mitsuzaki, Zhidong Chen , Effects of cathodic electrodeposition conditions on morphology and photoelectrochemical response of α-Fe2O3 photoanode. Tsf (2018), doi:10.1016/j.tsf.2018.09.034

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ACCEPTED MANUSCRIPT Effects of cathodic electrodeposition conditions on morphology and photoelectrochemical response of α-Fe2O3 photoanode Penghua Liang a, Longzhu Li b,c, Changhai Liu b, Wenchang Wang a, Honglei Zhang a, Naotoshi Mitsuzaki d, Zhidong Chen a,* School of Petrochemical Engineering, Changzhou University, Changzhou, 213164, Jiangsu, China

b

School of M aterials Science and Engineering, Changzhou University, Changzhou, 213164, Jiangsu, China.

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Department of Chemical and M aterials Engineering, Changzhou Vocational Institute of Engineering, Changzhou,

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a

213164, Jiangsu, China.

Qualtec Co., Ltd, Osaka 590-0906, Japan

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ABSTRACT:

α-Fe2 O3 photoanodes were prepared on Fluorine-doped tin oxide glass by annealing

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of electrodeposited Fe films. By controlling the synthesis parameters, the α-Fe2 O3 film prepared from the Fe film electrodeposited in 0.05 M ferrous sulfate solution

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(pH=2.1) for 50 s and then annealed at 600 ℃ for 3 h has the optimal photocurrent density around 257.08 μA cm-2 at 1.23 V vs. RHE (Reversible Hydrogen Electrode).

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In this paper, the effects of different synthesis parameters on α-Fe2 O 3 photoanodes were studied systematically. The morphology and properties of the sample were characterized

with

scanning

electron

microscopy,

UV-vis

spectra,

X-ray

diffractometry and photoelectrical measurements. Keywords: Water oxidation; Hematite; alpha phase ferric oxide; Photoanode; Cathodic electrodeposition; Morphology *

Corresponding author.

Tel.: +86

0519-86330239; Fax.: +86

[email protected](Zhidong Chen) 1

0519-86330239.

E-mail addresses:

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ACCEPTED MANUSCRIPT 1. Introduction With the development of society, the rising world’s energy demand and corresponding environmental crisis become so urgent that producing clean and renewable fuels is of great significance [1–7]. Among various possible fuel candidates, hydrogen fuel is an

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alternative energy carrier which is the most promising in the next generation because

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it is carbon- neutral, high in energy density, and easy to carry [8–10]. In this regard,

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the photoelectrochemical (PEC) devices that use semiconductors to absorb solar light to produce hydrogen from water is of particular interest, which makes the recycling of

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hydrogen fuel more clean and efficient [11–16].

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Among a range of efforts involved in improving a PEC system, the suitable selection of photoelectrode materials is crucial in that their advantage s, such as chemical

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stability and optical absorption characteristics, determine the performance of the

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system [17]. Most of researches are concentrated on simple metal oxides (ZnO [18], WO3 [19,20], BiVO 4 [21–23] and Fe2 O3 [24,25]) due to the relative stability for water oxidation reactions. Among them, hematite (α-Fe2 O3 ) as a promising photoanode, is

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nontoxic, low-cost, abundant on the earth, and excellent chemical stability under most of the aqueous solution. In addition, for solar water splitting, α-Fe2 O 3 photoanode owing to its favorable optical bandgap (Eg≈2.1 eV) [26] that can utilize about 40 % of the incident solar spectra [27], which is better than WO 3 (Eg≈2.7 eV) and TiO 2 (Eg≈3.0 eV), and its theoretical solar-to-hydrogen efficiency is up to 12.9 % [28]. However, the photoelectrochemical activity of α-Fe2 O3 is still hindered by several drawbacks such as low conductivity, short carrier diffusion length (2-4 nm) and slow 3

ACCEPTED MANUSCRIPT water oxidation kinetics [29–31]. In order to achieve a low onset potential and high photocurrent in hematite-based PEC device, a large number of methods have been investigated to resolve these inherent drawbacks, such as morphology controlling, elemental doping and surface treatment [32].

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Various synthesis methods have been investigated for preparing the α-Fe2 O3

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photoanode, including colloidal [33], hydrothermal [34], spray pyrolysis [35], atomic

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layer deposition [36] and electrodeposition method [37,38]. Among these, the electrodeposition is a bright method because of its simplicity o f operation, ambient

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temperature and pressure processing conditions and the ability to control the

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crystallinity, phase composition as well as other physicochemical properties [37]. The reported electrodeposition methods for preparing α-Fe2 O3 films include anodic

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electrodeposition and cathodic electrodeposition. Santamaria et al. [38] obtained

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Fe2 O3 by anodizing sputterdeposited iron and annealed under air, which can effectively regulate the morphology of Fe 2 O3 . In addition, cathodic electrochemical method has the advantages of simplicity, inexpensive and ease of controlling the

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electrodeposition process.

Based on the above background, the α-Fe2 O3 photoanodes were prepared directly from Fe films followed by annealing treatment. As this method has not been studied systematically, we provide an indepth understanding of the effects of different experimental conditions such as annealing temperature, pH, ferrous sulfate concentration, and electrodeposition time on morphology, photoelectrochemical response of the α-Fe2 O3 photoanode. Furthermore, we provide the basis for the next 4

ACCEPTED MANUSCRIPT research. 2. Experimental procedure 2.1 Materials Ferrous sulfate heptahydrate (FeSO 4 ·7H2 O), sodium sulfate (Na2 SO4 ), sulfuric acid

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and ethylene glycol (EG) were purchased from Sinopharm Chemical Reagent Co.,Ltd

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(China). All these chemicals were used without further treatment. Fluorine-doped tin

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oxide (FTO) glass electrodes (14 Ω cm-1 ) were sliced into 1 × 4 cm2 pieces and cleaned in an ultrasonic bath with detergent, deionized water, acetone, and ethanol for

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2.2 Fabrication of α-Fe 2 O3 photoanodes

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20 min in sequence, and finally dried with nitrogen gas flow.

Cathodic electrodeposition method was used to synthesize Fe films onto the surface of

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FTO working electrodes followed by annealing treatment as Fig. 1 [39]. Initially, 25

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mL aqueous electrodeposition solution was prepared as 0.01-0.15 M FeSO4 ·7H2 O and 0.1 M Na2 SO4 , in which the solvent is the mixture of EG and H2 O with the volume ratio as 1/8. The pH of solution was adjusted to 1.9-3.0 by adding 1.0 M H2 SO4 . Fe

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films was deposited onto FTO at -1.5 V for given time in a three-electrode equipment with FTO glass as the working electrode, the platinum foil as the counter electrode, and Ag/AgCl electrode as the reference electrode at room temperature (25 ℃). The as-deposited Fe films were rinsed with deionized water for several times, followed by annealing treatment at 500 ℃, 600 ℃, 700 ℃ under air for 3 h. 2.3 Structural characterization The structure of iron oxide films were characterized by X-ray diffraction patterns 5

ACCEPTED MANUSCRIPT (XRD, Rigaku, max 2500 PC) with CuK α radiation (λ=0.154059 nm) at 40 kV and 150 mA. The UV- vis absorption spectroscopy (Shimadzu, UV 2450) was used to explore the optical properties. Surface morphology was characterized by scanning electron microscopy (SEM, JEOL, JSM-6360LA, 20 kV).

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2.4 Photoelectrochemical measurement

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The photoelectrochemical properties of the fabricated samples were carried out using

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a standard three-electrode electrochemical workstation (CHI 760E). In which the fabricated samples were used as working electrode (photoanode), a platinum foil was

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used as the counter electrode and an Ag/AgCl (sat. KCl, E=0.1976 V vs. RHE

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(Reversible Hydrogen Electrode)) was used as the reference electrode. The 1 M NaOH (pH=13.6) aqueous solution was used as electrolyte, and the simulated solar

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light irradiation was AM 1.5 global and 1 sun (100 mW cm-2 ). The photocurrent was recorded by linear sweep voltammetry from 0.4 V to 1.7 V vs. RHE with a scan rate

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of 0.03 V s-1 . The Mott-Schottky (MS), open circuit potential (OCP) and electrochemical impedance spectroscopy (EIS) were measured by VersaSTAT 3

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potentiostat. The MS plots were measured in the dark at 1 kHz, the OCP transient tested was carried out under chopped light, and the EIS measurements were performed at 1.23 V vs. RHE with the frequency range being adjusted between 100 kHz to 0.1 Hz at amplitude frequency of 10 mV under light irradiation. The program Zsimpwin was used to fit the obtained data to the corresponding equivalent circuit model. 3. Results and discussion 6

ACCEPTED MANUSCRIPT 3.1. Morphology characterization of α-Fe2 O3 photoanodes In order to probe into the influence of process parameters such as annealing temperature, pH, ferrous sulfate concentration and electrodeposition time on the α-Fe2 O3 photoanodes morphology, SEM of the fabricated samples were carried out.

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The surface morphology of hematite thin films annealed at temperatures of 500 ℃,

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600 ℃ and 700 ℃ for 3 h from the as-deposited Fe films were shown in Fig. 2, in

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which the Fe films were electrodeposited in 0.05 M ferrous sulfate solution (pH=2.5) for 50 s. It can be seen that the fabricated samples had the similar morphology as

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nanoparticle. The average size of the nanoparticles increased from 80-90 nm (at

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500 ℃) to 200-210 nm (at 700 ℃) as the annealing temperature increased. Just as previously reported [40,41], the higher temperature accelerated the migration of

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surface atoms, which helped the incorporation of iron and oxygen atoms into the

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lattice sites, and thus the grains size increased. For investigating the influence of the pH value, the Fe films were electrodeposited in the 0.05 M FeSO 4 solution with pH=1.9-3.0 for 50 s. As shown in Fig. 3, the α-Fe2 O3 (annealed at 600 ℃) particles

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became larger and arranged loosely with the increase of pH value. The main reason may be that the lack of H+ caused by the consumption of the acid result in the formation of the Fe(OH)2 /Fe(OH)3 precipitate in the film, especially in the electrolyte system with high pH value. As shown in Fig. 4a-c, the morphology of the films electrodeposited with different solution concentration of ferrous sulfate with the pH value of 2.1, the annealing temperature of 600 ℃ and the deposition time of 50 s. When the concentration of ferrous sulfate increased from 0.03 M to 0.05 M, the 7

ACCEPTED MANUSCRIPT nanoparticles became smaller but when the concentration increased to 0.1 M further, the particles became larger. The main reason maybe that the poor conductivity and higher resistance of the lower concentration solution made the particles larger, and the formed Fe film would be larger and looser as the solution concentration increased

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further. Fig. 4d-f demonstrated that the α-Fe2 O3 films obtained with the different

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deposition time had similar nanostructure. According to the cross-sectional view, the

in consonance with the result of Zhou et al. [42].

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thickness of the film was linearly dependent on the electrodeposition time which were

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XRD patterns of the fabricated films can be indexed to the characteristic peaks of

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α-Fe2 O3 (JCPDS NO:33-0664). As seen in Fig. 5, all the samples demonstrated the obvious diffraction peak of (110), which had the conduction up to four orders of

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magnitude greater than the perpendicular directions [43]. As showed in Fig. 5a and

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Fig. 6a, the α-Fe2 O3 annealed at 600 ℃ showed the highest peak intensity ratio of (110)/(104) as 0.953. From Fig. 5b and Fig. 6b, the intensity of (110) enhanced with the increased of pH value. When the concentration was 0.01 M, the film was quite

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thin and the signal of hematite peak was not strong (Fig. 5c). By increasing the concentration (Fig. 6c) from 0.03 M to 0.15 M, it can be found that the peak intensity ratio of (110)/(104) increased as the concentration increased. At the same time, the high concentration increased the particle size and nonuniformity of iron oxide, so the effect of increasing concentration was relative. As the deposition time increased (F ig. 6d), the intensity ratio of (110)/(104) decreased. UV-vis spectra of α-Fe2 O3 photoanodes were shown in Fig. 7. The UV-vis absorbance 8

ACCEPTED MANUSCRIPT of the α-Fe2 O3 photoanodes had a clear edge around 600 nm, which corresponded to the band gap of the electrodes as 2.1 eV. With annealing temperature increased from 500 ℃ to 700 ℃, the absorption intensity gradually increased. Apparently, the notable higher absorbance was found when the electrodeposition time was 50 s, the ferrous

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sulfate concentration was 0.05 M, the pH value of the electrolyte was 2.1 and the

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annealing temperature was 600 ℃ (Fig.7). This result indicated that the light

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absorption intensity of α-Fe2 O3 film was related to the uniformity, crystallinity and lattice orientation of the film [42].

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3.2. Photoelectrochemical properties of α-Fe2 O3 photoanodes

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The photoelectrochemical performance of the α-Fe2 O3 photoanodes prepared from different experimental conditions were shown in Fig. 8. In this paper, the photocurrent

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density is read off at 1.23 V vs. RHE which is the standard reversible potential for water oxidation. From Fig. 8a, it was observed that the nanostructured hematite thin

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film annealed at 600 ℃ had a maximum photocurrent density of 76.68 μA cm-2 . With the improvement of annealing temperature, photocurrent density increased due to the

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growth of α-Fe2 O3 nanocrystals, in which the crystallinity of nanostructured hematite thin films enhanced and the voids reduced [44]. According to the SEM images, the average hematite nanocrystallites size increased with improved annealing temperature, which lowered the voids, and thus achieving non-obstructive, lower grain boundaries resistance and higher photocurrent flows at the molecular level. Glasscock et al. [45] explained that the grain boundaries decreased with the increased size of average hematite nanocrystallites, which suppressed the recombination rate of electron-hole 9

ACCEPTED MANUSCRIPT pairs. As a consequence, the photocurrent of the nanostructured hematite thin films will be improved since it reduces the recombination rate of the electron-hole pairs. However, the α-Fe2 O3 film annealed at 700 ℃ showed lower photocurrent density than that of the hematite annealed at 600 ℃, which will be discussed later in the EIS

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section. As shown in Fig. 8b, the α-Fe2 O3 photoanode synthesized with the pH of

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solution equal to 2.1 showed the highest photocurrent density of 257.08 μA cm-2 . The

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reason may be due to the electrolysis where there were two reactions existing on the working electrode surface, i.e., 2H+ + 2e = H2 (1) and Fe2+ + 2e = Fe (2), which made

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the process complicated. When the pH was higher, the precipitate was formed in the

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solution due to the reaction between Fe2+/Fe3+ and OH-. On the contrary, when the pH was too low, the H+ was plentiful enough to compete with Fe2+ to be reduced on the

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cathode, and H2 evolution was the main reaction instead of the iron deposition [46,47].

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Then with appropriate pH as 2.1, the fabricated electrode had the minimum particle size (Fig. 3) and maximum light absorption (Fig. 7b) which were beneficial to photocurrent. The influence of ferrous sulfate concentration on the photocurrent

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density can be seen in Fig. 8c, it can be seen that the α-Fe2 O3 film electrodeposited with 0.05 M concentration had the best photocurrent density as 257.08 μA cm-2 . The α-Fe2 O3 film electrodeposited in 0.10 M solution showed the low photocurrent density of 142.95 μA cm-2 , possibly because of non-uniformity of the electrodeposited Fe film (Fig. 7c) resulting in extremely low absorption intensities. Fig. 8d showed the photocurrent densities-potential curves of the α-Fe2 O3 films for different electrodeposition time (annealed at 600 ℃ for 3 h). Clearly, the α-Fe2 O3 film with a 10

ACCEPTED MANUSCRIPT electrodeposition time of 50 s showed the highest photocurrent. α-Fe2 O3 is an indirect band gap semiconductor with a short carrier diffusion length (2-4 nm) and a very short lifetime of photogenerated charge carriers (<10 ps) [48]. Although, as the electrodepositon time increased, the increased average thickness of the α-Fe2 O3 from

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140 nm (with 30 s) to 310 nm (with 70 s) would enhanced the light absorption and

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generated more photocharges, which benefited to photoelectrochemical performance,

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but meanwhile the recombination rate of photogenerated electron-hole pairs increased and the efficiency of charge transfer lowered. In other words, the benefit of improved

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light absorption would be offset by a rapidly increasing recombination rate, especially

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in very thick films. Hence, a moderate thickness of α-Fe2 O3 film is vital to light absorption and charge transfer in PEC applications [49]. In this paper, the suitable

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sample was electrodeposited for 50 s with a thickness of 250 nm (Fig. 4e).

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To clarify the origin of the different photocurrent response and electron transport mechanism of the fabricated photoanodes, MS plots were shown in Fig. 9. The flat-band potential and donor density of the photoanodes were deduced from

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Mott-Schottky equation:

1 𝐶2

=

2 𝜀𝜀 0𝑞 𝑁𝐷 𝐴2

(𝐸 − 𝐸𝑓𝑏 − 𝑘𝐵 𝑇⁄𝑞 )

(1)

where C is the capacitance of the space charge layer, ε is the dielectric constant of the semiconductor, ε0 is the vacuum permittivity, q is the elementary charge, ND is the donor density, A is the actual area of the electrode exposed to the electrolyte, E is the applied potential, Efb is the flat-band potential, k B is the Boltzmann constant, and T is the temperature. The positive slope of MS plot indicates that all of the fabricated 11

ACCEPTED MANUSCRIPT photoanodes are n-type semiconductor with electrons as the majority carriers. As shown in Fig. 9a, the carrier density calculated from the slopes of the MS plots were 1.98×1019 , 4.30×1018 and 1.47×1018 cm-3 for 500 ℃, 600 ℃ and 700 ℃, respectively. It can be seen that the decreased electron conductivity was caused by higher annealing

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temperature. From Fig. 9b, the carrier density increased from 2.55×10 18 cm-3 to

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7.41×1018 cm-3 when pH value decreased from 3.0 to 2.1. Hence, the α-Fe2 O3

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prepared at pH 2.1 solution demonstrated higher donor density, which accorded with relatively higher photocurrent in comparison with the other samples. From the result

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shown in Fig. 9c, increasing the concentration, carrier density decreased and then

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increased. The carrier density of the sample under 0.03 and 0.05 M were slightly lower than that of the other samples, which may due to a thicker film. Similarly, the

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carrier density value with deposition time of 50 s (7.41×1018 cm-3 ) was higher than

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that with deposition time of 70 s (2.06×1018 cm-3 ). (Fig. 9d) The longer deposition time led to lower electron conductivity, which was consistent with the photocurrent result (Fig. 8d).

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To further probe the quality of the photoanode/electrolyte junction, the transient open circuit potential (OCP) (equal to the difference of open circuit voltage upon light illumination (OCVlight ) and in dark (OCVdark )) of the fabricated photoanodes were carried out. From Fig. 10 and Table 1, it can be seen that the fabricated electrodes showed a negative increase in voltage under light irradiation, indicating that the photogenerated electrons were injected from the electrodes into the FTO substrate, and the photoanode acted as an n-type semiconductor material which was in a good 12

ACCEPTED MANUSCRIPT agreement with the MS result [50]. It should be noted that the α-Fe2 O 3 samples with higher generated photovoltage showed more remarkable photoelectric conversion ability. The OCP increased as the increase of annealing temperature (Fig. 10a). The more cathodic OCVlight values indicated the flattened energy band of the photoanodes

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(in light quasi-equilibrium with the electrolyte) by the photoexcited carriers. OCVlight

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was mostly determined by the negatively Fermi level shifts in the photoanode

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materials under illumination [51]. From Fig. 10b, the α-Fe2 O3 photoanode synthesized with the pH value of solution equal to 2.1 showed the highest OCP of 0.268 V,

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indicating that the α-Fe2 O3 photoanode possesses preferable photoelectric conversion

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ability. From the result shown in Fig. 10c, increasing the concentration of ferrous sulfate can effectively improve the OCP value. Besides, the OCVdark reflected the

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upward band bending nature of the photoanodes in dark equilibrium with the electrolyte. More positive OCVdark values were obtained as the concentration

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increased, suggesting the effective passivation of the α-Fe2 O3 surface states for the reduced surface Fermi leveling pinning effect. The influence of electrodeposition time

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on the OCP can be seen in Fig. 10d where the OCP with deposition time of 70 s (0.197 V) was lower than that with 50 s (0.268 V), presumably because of the increased electron-hole recombination in the thick Fe2 O3 film. The results demonstrated that samples with large generated photovoltage tend to exhibit higher photocurrent values. To explore the charge dynamics of the photoanode, EIS was performed at 0.23 V (vs. Ag/AgCl) for all the samples as shown in Fig. 11 and Table 2. The Nyquist impedance 13

ACCEPTED MANUSCRIPT of the fabricated photoanodes analyzed in a frequency range of 0.1-105 Hz and the corresponding equivalent circuit model fitted from the obtained EIS data by program Zsimpwin, in which the dots in the plots represent the experimental data and the solid lines represent the result of fitting. Di Franco et al. [52] suggested that the

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contributions on C for an iron oxide based electrode are different and important, so it

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is particularly important to choose a suitable model to interpret the impedance results.

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The EIS equivalent circuit model and fitted impedance parameter values were shown in Fig. 11e, including a space charge capacitance of the bulk hematite Cbulk , a surface

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state capacitance C ss, a resistance represents the trapping of holes in the surface states

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Rtrap , a charge transfer resistance from the surface states to solution Rct,trap, and a resistance Rs represents total series resistances resulting from external circuit, solution

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etc [53]. As seen in Fig. 11a, the α-Fe2 O 3 sample annealed at 600 ℃ had the lowest

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Rct,trap , which was responsible for the water oxidation reaction due to a favorable morphology, and allowed the easy penetration of the electrolyte. A decreased Rct,trap means faster charge transfer from surface states which should produce a lower steady

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state concentration of trapped holes. The Css controls the presence of surface or defect states that are formed due to oxygen vacancies [54]. The surface states with or without coupling with protons play pivotal role by determining the thermodynamics of surface hole-trapping in the first step, followed by controlling the hematite water oxidation activity. The sample annealed at 600 ℃ has good crystallinity and few surface defects, which contribute to lower Rct,trap and higher C ss, having the highest photocurrent. When the temperature was too high (700 ℃), the increased Rs caused by 14

ACCEPTED MANUSCRIPT the increased resistance of FTO, Rtrap and Rct,trap resulted in the weaken photoelectrochemical response. Comparing the samples fabricated in the different pH solutions (Fig. 11b), the fitting results indicated that the photoanodes electrodeposited in the pH 2.1 solution exhibited lowest Rct,trap . This suggested that the losing of

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recombination due to surface states were lower in α-Fe2 O3 photoanodes prepared with

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lower pH, resulting in improved photoelectrochemical performance. In addition, the

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Cbulk of Fe2 O3 photoanode (pH 2.1) was increased significantly. The comprehensive analysis showed that the reduced of the pH enhanced the absorbance of visible- light

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and increased the reactive sites at the surface, which benefited the separation of

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photogenerated charge carriers. And the sample had a smaller Rtrap and Rct,trap , so that it had excellent photoelectrochemical performance. Similar results were observed for

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the α-Fe2 O3 photoanodes prepared at different concentration of ferrous sulfate as shown in Fig. 11c. The α-Fe2 O3 photoanodes prepared at 0.05 M had the lowest Rct,trap .

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This means that hole transport from the α-Fe2 O3 (0.05 M) surface to the electrolyte was much faster than from other samples. At the same time, the sample had the

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highest Css at 0.05 M. Zandi et al. [55] consider a larger C ss indicates larger number of holes that can be trapped and thus participate in the water oxidation reaction. While, the sample had the higher Rtrap at 0.05 M may due to a thicker film, which made the e--h+ recombination. With the comprehensive analysis of UV-vis, I-V, etc., when the concentration was 0.05 M, the α-Fe2 O3 had uniform film, big specific surface area, high absorbance and low surface recombination which are beneficial to the high photocurrent. In addition, the α-Fe2 O3 films with longer deposition time had much 15

ACCEPTED MANUSCRIPT higher resistance (Rtrap ) (Fig. 11d). The α-Fe2 O3 obtained at long time deposition, was too long for electrons transferring from the top to the bottom because the long electron pathway could enhance the probability of e --h+ recombination. In total, the initial increase of PEC performance (50 s) was mainly due to the increased amount of

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photoactive α-Fe2 O3 nanoparticles, which improved the light absorption as well as the

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number of photoexcited charge carriers. When the deposition time extended, the

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thickness of this film further increased and the recombination rate of the electrons and holes became the major factor in the water splitting process. It’s well known that the

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carrier may need more time to reach the films’ surface in compact and thick film,

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leading to the reduction in separation efficiency of the photogenerated carriers and the photocurrent. In summary, the pH and concentration have a great influence on the

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photoelectrochemical properties of α-Fe2 O3 . These combined results suggest that the

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synergistic effect of capacitance and resistance is important to the enhanced photocurrent response.

The stability of the photoanode in PEC water splitting is crucial in both fundamental

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and practical studies. Fig. 12a showed the photocurrent curve of optimized α-Fe2 O3 photoanode measured at 1.23 V vs. RHE in 1 M NaOH electrolyte under AM 1.5 illumination for 3500 s. The optimized α-Fe2 O3 film, obtained from a Fe film in 0.05 M ferrous sulfate solution (pH=2.1) for 50 s and then annealed at 600 ℃ for 3 h. Fig. 12b showed the photocurrent curve of optimized α-Fe2 O3 photoanode measured after soaking in aqueous solutions at different times. The stability tests indicate that the fabricated samples exhibited excellent stability. 16

ACCEPTED MANUSCRIPT 4. Conclusions In this study, cathodic electrodeposition method was used for synthesizing α-Fe2 O3 photoanode by annealing of electrodeposited Fe film. The effects of annealing temperature, pH, ferrous sulfate concentration and electrodeposition time on the

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preparation of α-Fe2 O3 were studied. The optimized α-Fe2 O 3 film, obtained from a Fe

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film in 0.05 M ferrous sulfate solution (pH=2.1) for 50 s and then annealed at 600 ℃

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for 3 h, showed the highest photocurrent around 257.08 μA cm-2 at 1.23 V vs. RHE. The results show that the higher annealing temperature, lower pH value and suitable

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ferrous sulfate concentration and electrodeposition time are favorable to the maximum

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photoelectrochemical properties. Among them, the concentration and the pH value of the solution are critical to the preparation of α-Fe2 O3 film with a high

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photoelectrochemical response. The enhanced photoelectrochemical response is

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mainly attributed to the strong light absorption ability as well as the reduced resistance of the charge transfer inside the electrode and across interface of electrode and electrolyte. This work provides a useful insight into the preparation of the

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α-Fe2 O3 films with high photoelectrochemical performance for water splitting. Acknowledgments

The authors greatly acknowledge financial support from the National Natural Science Foundation of China (No. 51702025, 51574047), Natural Science Foundation of Jiangsu Province (No. BK20160277), Natural Science Foundation of the Jiangsu Higher Education Institutions of China (Grant No. 16KJA430004). References 17

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electrodes: resurrection of the dead layer, Energy Environ. Sci. 6 (2013) 634–642.

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OCVdark (V)

OCVlight (V)

OCP (V)

500 ℃

-0.177

-0.297

0.120

600 ℃

-0.210

-0.344

0.134

700 ℃

-0.128

-0.383

0.255

3.0

-0.201

-0.377

2.5

-0.210

-0.344

0.134

2.3

-0.222

-0.384

0.162

2.1

-0.165

-0.433

0.268

1.9

-0.236

-0.434

0.198

0.01 M

-0.253

-0.406

0.153

0.03 M

-0.239

-0.409

0.170

-0.165

-0.433

0.268

-0.121

-0.423

0.302

-0.114

-0.414

0.300

30 s

-0.175

-0.429

0.254

50 s

-0.165

-0.433

0.268

70 s

-0.218

-0.415

0.197

0.10 M

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0.05 M

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Samples

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Table 1 The parameters of the α-Fe2 O3 photoanodes obtained from the OCP analysis.

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Rs (Ω cm2 ) Cbulk (μF cm-2 ) Rtrap (Ω cm2 ) Css (μF cm-2 ) Rct,trap (Ω cm2 ) 47.01

18.22

820.1

41.19

12670

600 ℃

55.18

26.32

1420

58.76

10390

700 ℃

1098

23.58

1585

23.93

31340

3.0

61.8

10.91

1795

2.5

55.18

26.32

2.3

63.24

17.65

2.1

61.15

38.01

1.9

69.38

24.51

0.01 M

61.17

0.03 M

61.99

0.05 M

61.15

0.10 M

61.12

1420

58.76

10390

1602

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29.87

12560

1245

29.31

9384

1370

13.25

19480

31.73

533.7

13.62

38170

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51060

1112

27.17

10340

38.01

1245

29.31

9384

13.57

862.3

28.62

12960

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24.68

32.54

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500 ℃

0.15 M

62.63

15.61

1785

14.01

15510

30 s

60.45

29.95

1138

17.93

18700

61.15

38.01

1245

29.31

9384

59.85

19.03

822.4

81.61

4715

50 s 70 s

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Fig. 1. Schematic diagram of the systematic procedure for preparation of α-Fe2 O3

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photoanode.

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Fig. 2. Top-view SEM images of the α-Fe2 O3 photoanodes annealed at different

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temperatures of (a) 500 ℃, (b) 600 ℃ and (c) 700 ℃. All the Fe films were

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electrodeposited in 0.05 M ferrous sulfate solution (pH=2.5) for 50 s.

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Fig. 3. Top-view SEM images of the α-Fe2 O3 photoanodes electrodeposited at different pH of (a) FTO, (b) 3.0, (c) 2.5, (d) 2.3, (e) 2.1 and (f) 1.9. All the α-Fe2 O3 annealed at 600 ℃ and Fe films were electrodeposited in 0.05 M ferrous sulfate 29

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solution for 50 s.

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Fig. 4. (a)-(c) Top-view SEM images of the α-Fe2 O3 photoanodes electrodeposited at

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different FeSO 4 concentration of (a) 0.03 M, (b) 0.05 M, (c) 0.10 M. All the Fe films

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were electrodeposited in ferrous sulfate solution (pH=2.5) for 50 s and annealed at 600 ℃. (d)-(f) Top-view SEM and Cross-section SEM images of the α-Fe2 O3

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photoanodes electrodeposited at different time of (d) 30 s, (e) 50 s and (f) 70 s. All the Fe films were electrodeposited in 0.05 M ferrous sulfate solution (pH=2.5) and

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annealed at 600 ℃.

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Fig. 5. X-Ray diffraction patterns of the α-Fe2 O3 photoanodes under different experimental conditions. (a) annealing temperature, (b) pH, (c) ferrous sulfate concentration, (d) electrodeposition time. Peaks from FTO substrate are indicated by ♥.

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Fig. 6. Ratio between intensities of the XRD peaks (110)/(104) for the α-Fe2 O3 photoanodes under different experimental conditions. (a) annealing temperature, (b) pH, (c) ferrous sulfate concentration, (d) electrodeposition time.

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Fig. 7. UV-vis absorbance spectras of the α-Fe2 O 3 photoanodes under different experimental conditions, inset shows the digital images of the photoanodes. (a) annealing temperature, (b) pH, (c) ferrous sulfate concentration, (d) electrodeposition 34

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time.

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Fig. 8. Photocurrent densities-potential curves of the α-Fe2 O3 photoanodes (AM 1.5 global, 1 sun (100 mW cm-2 ) under different experimental conditions. (a) annealing

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Fig. 9. Mott-Schottky plots of the α-Fe2 O 3 photoanodes under different experimental

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electrodeposition time.

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Fig. 10. Open circuit potential (OCP) curves of the α-Fe2 O3 photoanodes under

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different experimental conditions with chopped light. (a) annealing temperature, (b)

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Fig. 11. (a)-(d) Nyquist plots of the α-Fe2 O 3 photoanodes under different experimental conditions. (a) annealing temperature, (b) pH, (c) ferrous sulfate concentration, (d)

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Fig. 12. (a) Photocurrent-time curve of the α-Fe2 O3 photoanode measured at 1.23 V vs.

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RHE in 1 M NaOH electrolyte. (b) The photocurrent densities-potential curve of the

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α-Fe2 O3 photoanode measured after soaking in aqueous solutions at different times.

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