Binary nickel and iron oxide modified Ti-doped hematite

photoanode for enhanced photoelectrochemical water splitting

Dongyu Xua, Yichuan Ruib, Vernon Tebong Mbaha, Yaogang Lic, Qinghong Zhangac,*

and Hongzhi Wanga,*

aState Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, Shanghai 201620, People’s Republic of ChinabCollege of Chemistry and Chemical Engineering, Shanghai University of Engineering Science, Shanghai 201620, People’s Republic of ChinacEngineering Research Center of Advanced Glasses Manufacturing Technology, MOE, Donghua University, Shanghai 201620, People’s Republic of China

Abstract

In this article, a novel and facile strategy to load low-cost and earth-abundant oxygen

evolution catalyst NiFeOx by spin-coating on Ti-doped α-Fe2O3 films was reported.

The NiFeOx modified hematite photoanode was prepared by a two-step process,

which consists of a hydrothermal method and a subsequent NiFeOx loading step.

Scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-

ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Electrochemical

impedance spectroscopy (EIS) were used to characterized with the resulting

photoanode. The highest photocurrent increment and the lowest onset potential were

observed with 30 mM NiFe precursor treatment. Increasing Among the loading

** Corresponding author. Tel: +86-21-67792943; fax: +86-21-67792855. E-mail address: zhangqh@dhu.edu.cn (Q. H. Zhang), wanghz@dhu.edu.cn (H. Z. Wang).

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content in a range from 10 to 90 mM, the photocurrent density increases from 0.998

for the pristine α-Fe2O3 to 1.126 mA /cm2 at 1.23 V vs RHE (i.e., 12.7% increment)

with by 30 mM NiFe precursor treatment. Concomitant with this improvement was a

cathode shift in the onset potential by nearly 55 mV and improvements in incident-

photon-to-current efficiencies. Hematite photoanodes after NiFeOx deposition showed

better performance than pristine samples, because of a lower overpotential for water

oxidation resulted from NiFeOx modifying.

1. Introduction

Since Boddy and Fujishima’s pioneering work on n-type TiO2 for water splitting

[1, 2]., photoelectrochemical (PEC) water splitting is envisioned as a promising

strategy for utilizing the solar energy and storing chemical fuel as hydrogen and

oxygen [3]. However, the bandgap of widely used TiO2 is too large, and can only be

excited by ultraviolet light which is just a small fraction of solar light [4-6].

Therefore, many other kinds of semiconductor materials owning appropriate bandgaps

have been utilized to drive this reaction, such as Fe2O3 [7-9], WO3 [10, 11], BiVO4 [12,

13], Ta3N5 [14], TaON [15] and LaTaON2 [16].

Among above-mentioned materials, hematite is especially attractive as a

photoanode candidate for efficient solar water splitting due to its favorable optical

bandgap (2.0 eV), valence band edge position, extraordinary chemical stability in

oxidative environment, abundance, and low cost [7, 17]. It has been theoretically

predicted that hematite could achieve a water splitting efficiency of 12.9 % with this

bandgap [18]. But the factual research is still much less than the theoretical value.

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Many efforts have been devoted to improve the performance of hematite photoanode,

such as morphology control [7, 19-21], elemental doping [22-25] and surface

composition modification [26-28]. However, because of the short hole-electron pair

lifetime and poor minority carrier, a high-rate recombination of photo-generated

carriers will happen in the bulk [29, 30]. Additionally, because the low kinetics of

water oxidation results in holes accumulation at the surface, surface recombination

occurs until sufficiently positive potentials are achieved for appreciable charge

transfer across the interface, which shows up a large overpotential [28, 31]. Due to

these reasons, the performance of hematite has been severely limited as a photoanode

for water oxidation.

To overcome the limitation of very slow oxygen evolving reaction (OER) kinetics

of photoanodes, different electrocatalyst were utilized to accelerate this process on the

surface of hematite. Grätzel et al. introduced IrO2 on hematite to achieve a

photocurrent over 3 mA /cm2 [32]. Wang et al. utilized earth-abundant NiFeOx OER

catalyst to decorate hematite by photochemical metal-organic deposition method,

which induced a significant cathodic shift in hematite-based photoelectrochemical

water splitting reactions [33]. To avoid utilization of noble metal and expensive

organic-ligands, a low-cost and earth-abundant method is urgent and necessary for

reducing the over-potential of α-Fe2O3 for water oxidation.

In this work, we report a facile method of loading earth-abundant NiFeOx

electrocatalyst on Ti-doped hematite photoanode via a simple spin-coating in aqueous

solution for efficient water oxidation. The presence of mixed metal-oxide layer

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provides a relative faster rate for water oxidation, which is beneficial to collect photo-

generated holes rather than accumulation on the interface between semiconductor and

liquid. Both the cathodic shift of the onset potential and the enhancement of PEC

efficiency came from the OER catalyst loading.

2. Experimental

2.1 Preparation of Ti-doped hematite films

Films of Ti-doped hematite were prepared on fluorine-doped tin oxide (FTO)

coated glass substrates (TEC-15, 15 Ω/□, Pilkington) through two steps. Firstly, the

FeOOH was grown on FTO substrate by a modified hydrothermal method [26, 34].

Then the as-synthesized FeOOH was annealed at a high temperature in air to convert

it to hematite. Briefly, aqueous solution containing 0.15 M FeCl3 (≥ 99 %, Sigma-

Aldrich) and 0.01 M TiCl3 (20 wt. % in 3% hydrochloric acid, Alfa) was adjusted to a

pH of 1.5 by 6 M HCl. 70 mL of precursor solution was transferred into an a Teflon-

lined stainless steel autoclave with a capacity of 80 mL and a piece of FTO substrate

was declining placed at an angle facing on the wall of the autoclave. The autoclave

was sealed and heated at 120 °C for 5 h. After the autoclave was cooled down

naturally, the as-synthesized FeOOH films were taken out and washed with deionized

water (18 MΩ, Millipore). The FeOOH films were dried with compressed nitrogen at

room temperature and then annealed in air at 550 °C for 2 h to transform into α-Fe2O3.

2.2 Loading NiFeOx electrocatalyst on hematite

NiFeOx electrocatalyst was loaded on Ti-doped hematite by a modified spin

coating method [35]. Aqueous solution containing Ni2+ and Fe3+ was utilized to treat

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hematite films. 30 µl of precursor aqueous solution containing 1 wt% Triton X-100,

NiSO4 and Fe2(SO4)3 (Ni:Fe molar ratio = 1:1) was freshly prepared and dropped on

the above Ti-doped hematite film with an area of 10 × 10 mm2, then spun at 5000 rpm

for 90 s. The films were annealed at 300 °C for 30 min for converting them to metal

oxide electrocatalyst. Ni2+ and Fe3+ concentration were ranged from 0 to 90 mM.

Precursor solutions were freshly prepared before each film deposition.

2.3 PEC characterization

Hematite electrodes were masked with a 60 μm Surlyn film (Solaronix) with a

0.25 cm2 square hole to explore the active areas. Surlyn films were adhered to the

electrodes after heating to 115 °C. The PEC measurements were performed using

Zennium workstation (Zarhner, Inc.) in a home-built three-electrode electrochemical

system with 1 M KOH (pH=13.6) electrolyte. Platinum wire and standard Ag/AgCl

were employed as counter electrode (CE) and reference electrode (RE), respectively.

The light source was the simulated sunlight from a 300 W Xenon solar simulator

(67005, Newport Corp.) through a solar filter. The illumination intensity was

calibrated to standard AM1.5 sunlight (100 mW /cm2) by a standard silicon cell. 1 M

KOH electrolyte was degassed by purging nitrogen for 10 min.

All potentials reported here were normalized to the reversible hydrogen potential

(RHE) according to Formula I. Current density-voltage (J-V) curves were measured

under light and dark in 1.0 M KOH, with a scanning rate of 5 mV/s between 0.6 and

1.6 V. Electrochemical impedance spectroscopy (EIS) was measured in the 1.0 M

KOH solution with an amplitude of 5 mV and frequencies varying between 0.01 and

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100,000 Hz at a potential of 1.23 V.

ERHE = E AgCl + 0.059pH + E°AgCl with E°AgCl = 0.1976 V at 25 °C (1)

The incident-photon-to-current conversion efficiency (IPCE) spectra were

measured as a function of wavelength from 350 to 600 nm using a specially designed

IPCE system (Newport Co., USA) according to Equation 2.

IPCE ( λ )=1240 × J p( λ)

P × λ×100 % (2)

Where P and λ are the intensity (mW /cm2) and the wavelength (nm) of incident light,

respectively, Jp(λ) is the photocurrent density (μA /cm2) under the irradiation of single

wavelength λ.

The water decomposition efficiency can be measured by applied bias photon-to-

current efficiency (ABPE), which was calculated from the linear sweep voltammetry

data based on the following equation [12].

ABPE=J (mA /c m2)×(1.23−V )

P0(100 mW /c m2)× 100 % (3)

Where J is the photocurrent density at the potential of V under simulated light, P0 is

the power of the incident illumination power density (AM 1.5G, 100 mW/cm2).

2.4 Materials characterization

Film morphology was analysized by a field emission scanning electron

microscopy (FESEM, Hitachi S-4800) and transmission electron microscopy (TEM)

(JEOL-2010) operated at 200 kV. X-ray diffraction (XRD) analysis were carried out

on a Rigaku D/max-2550 PC using Cu Kα radiation. The X-ray photoelectron

spectroscopy (XPS) measurements were carried out in a Kratos Axis Ultra DLD X-ray

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photoelectron spectrometer using a source of Al Ka radiation with the energy of

1486.6 eV.

3. Results and discussion

3.1 Phase analysis

The compositions of the pristine sample and NiFe solution treated sample were

studied with XRD. After hydrothermal reaction and subsequent annealing at 550°C

for 2 h, the FeOOH converted to hematite. As shown in Fig. S1, the XRD patterns of

FTO substrate and hematite are designated respectively. Strong (110) diffraction peak

at 2θ=35.8° indicates that the hematite nanoparticle growth orientation is along to

[110], which has a good conductivity (4 orders than orthogonal plane) [17, 36]. The

average crystallite size was calculated to be 25.9 nm, according to the Scherrer

Formula.

To elucidate the existence of NiFeOx loading on the Ti-doped hematite, XPS

spectroscopy was employed to investigate the pristine Ti-doped hematite and NiFeOx

decorated Ti-doped hematite. For both of the two samples, Ti 2p and Fe 2p signal can

be clearly observed in the survey scan of XPS spectrum in Fig. 1a. Fig. 1b shows the

existence of doublet Fe 2p3/2 and Fe 2p1/2 with binding energies of 710.2 and 724.0 eV

respectively. The satellite peak at 718.5 eV is characteristic of α-Fe2O3 [36]. The Ti 2p

detailed spectrum is shown in Fig. 1c. The binding energies of Ti 2p in both samples

are 463.4 eV and 457.4 eV, respectively, which are slightly lower than the binding

energy of Ti 2p in TiO2 (458.5 eV). It is ascribed to Ti4+ induced into hematite lattice

[36-38]. These results indisputably prove the presence of Fe3+ and Ti4+ from the two

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samples, indicating that the successful synthesis of Ti-doped hematite film. The Ni 2p

detailed spectrum is shown in Fig. 1d. Ni is observed by the increasing intensity of Ni

2p3/2 at 853.6, 854.6 and 860.4 eV in the treated NiFe solution treated sample, which

was fitted using relative peak positions for both Ni2+ and Ni3+ oxides, which and did

not have unambiguous identification of the oxidation state [39]. NiOx may consist of

the NiO Ni(OH)2 and NiOOH mixture. The spectra are nominally identical for all

samples, however, which they are consistent with NiO5/2 peaks (centered at 485.2 eV

in survey scan) [40]. In contrast, there is no signal of Ni 2p in the pristine sample.

This It indicates that the NiFeOx layer is induced into the hematite film.

3.2 Morphology and PEC studies

Fig. 2a shows photocurrent density-potential (J-V) curves of the pristine hematite

and hematite treated with different concentrations of NiFe solutions under

illumination. It was found that loading NiFeOx electrocatalyst could apparently give

influence on the PEC efficiency. The photocurrent density of pristine sample is 0.998

mA/ cm2 at 1.23 V (vs RHE). In addition, the NiFeOx decoration also increased the

photocurrent densities to 1.056 and 1.126 mA/cm2 at 1.23 V (vs RHE), respectively,

corresponding to 5.7% and 12.7% improvement compared to bare hematite at the

same potential. However, with the further increase of NiFe concentration, the

photocurrent densities gave a dramatic decline to 0.567 and 0.402 mA/cm2 at 1.23 V

(vs RHE), respectively. In addition, when hematite is treated with certain amounts (15

mM and 30 mM) of precursors, a slightly varying degree of cathode shifts occurs (20

mV and 55 mV). That is also observed by Kleiman-Shwarsctein et al [41]. Fig 2b

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shows the J-V curves in the dark to investigate the water oxidation kinetics.

Compared with bare hematite sample, catalytic activities are evident between 1.45

and 1.6 V for the Ti-doped hematite/NiFeOx photoanode. Furthermore, the significant

Ni(II)/Ni(III) oxidation peak (near 1.3 V vs RHE) does not exist, which is due to the

as-deposited film. This is in accord with previous research [35, 39] . These results

confirmed that Spin-coated grown NiFeOx films exhibited catalytic activities toward

water oxidation.

The FE-SEM image of bare Ti-doped hematite film is shown on Fig. 3a. The

diameter of hematite nanoparticles is about 50~10 nm, and many of them aggregate

together. Some cracks were formed during synthesis. After treatment with 15 mM

Ni(II)Fe(III) solution, the width of cracks were reduced as observed by FE-SEM, but

the original smooth hematite particles turn into became rough (Fig. 3b), which is

easily distinguishable from the pristine simple. Due to the low concentration of the

precursor, the morphology did not change much. When the concentration increased to

30 mM, the hematite films were uniformly covered by the NiFeOx nanoparticles (Fig.

3c). Meanwhile all cracks were disappeared. With further increase of loading, the

morphology of hematite nanoparticles is did not changed further, but which is was

covered by irregularly shaped NiFeOx nanoparticles and with some cracks (Fig 3d).

That is due to the shrinkage of NiFeOx coating during the annealing step. The TEM

image of hematite treated by 30 mM Ni(II)Fe(III) solution is shown on Fig. 1e, where

the formation of a thin interfacial layer of amorphous NiFeOx in a thickness of 4~8

nm is easily observed. The HRTEM image is shown on Fig. 1f, where the lattice

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fringes of hematite (104) and the amorphous coating of NiFeOx are clearly presented.

The correlation between the PEC efficiency and the morphology is closely

relevant. When treated with appropriate amount of solution (30 mM), the photoanode

shows the best PEC efficiency. The morphology also shows the modest change after

the catalyst loading (Fig 3c). When the catalyst loading amount is too high, serious

aggregation is induced the (Fig 3d). The corresponding photocurrent also declines by

72 %. From these results, we can infer that the conformal catalyst is superior to the

agglomerates. That is due to the formation of amorphous NiFeOx layer on the surface

of hematite film. Thick amorphous NiFeOx layer is beneficial to improving water

oxidation kinetics but it is harmful to transfer the proton from semiconductor interface

to liquid [42, 43]. Because of low conductivity of the NiFeOx, the thickness should

have an optimum value to balance the OER catalysis and proton transfer. It can be

confirmed that the optimum concentration is 30 mM.

The water decomposition efficiency can be measured by applied bias photon-to-

current efficiency (ABPE). Fig S1 shows that the bare Ti-doped hematite photoanode

processes possesses the a relative low ABPE value (0.68 % at 1.11 V). After NiFeOx

loading on the Ti-doped hematite, the ABPE value of Ti-Fe2O3/NiFeOx increased by

33 % (0.10 % at 1.07 V).

Fig. 4a shows the current response to turning on the light at a constant potential

of 1.23 V vs. RHE. For the bare hematite photoanode, there is a short spike of

photocurrent that can be attributed to the trapping of photogenerated holes in surfaces

states, which quickly decays to a low steady state photocurrent density. For NiFeOx-

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modified hematite, the value of decay is relative low, which indicates the partial

suppression of surface recombination [44, 45]. That phenomenon is also consistent

with previous results of Co-Pi on Fe2O3 or BiVO4 and Ni(OH)2 on TiO2 or Fe2O3

electrodes [40, 45-47]. The stability test is shown in Fig. 4b, which demonstrates that

the NiFeOx-modified hematite photoanode can be operated steadily for more than 18

h (3 cycles), and the steady photocurrent is not caused by the corrosion by

illumination. That is due to the corrosion resistance of NiFeOx in alkaline solution

[39].

Fig 5 shows the IPCE analysis of pristine electrode and NiFeOx-decorated

electrode in 1M KOH at 1.23 V RHE, which indicates the an improvement when in

the presence NiFeOx is present. Bare hematite reached a maximum of 8.1% at 380

nm, while the hematite/NiFeOx film achieved 8.9% at same wavelength. The

amorphous NiFeOx layer is too thin (4~8 nm) to act as a light absorber and

photocatalyst. We infer that the NiFeOx layer of composite acts as proton transferrer,

which facilitates the water oxidation.

In previous reports, the cathodic shift of the onset potential on a Fe2O3

photoanode generally came from accelerating water oxidation kinetics, passivating

surface states or ions adsorption [26, 45, 48, 49]. To further study the reason of onset

potential shift and enhancement of PEC efficiency with the decorated NiFeOx layer,

the electrochemical impedance spectroscopy (EIS) measurements were carried out to

study charge transfer characteristics of hematite with and without NiFeOx coating. The

impedance data is presented in the form of Nyquist plots in Fig. 6. For the

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measurements under illumination, the Nyquist plots are shown in Fig. 6a. The

potential is 1.23 V, which is a potential for water splitting reaction. Because charge

transfer processes are normally slower at the semiconductor-electrolyte interface than

in the bulk, the low frequency response is assigned to the semiconductor- electrolyte

charge transfer [31]. The radius of low frequency response becomes smaller after

loading NiFeOx, but it becomes larger when loading excess amount, which indicates

that the thick NiFeOx may block the holes transfer to the electrolyte. When testing in

dark, 1.53 V is chosen to apply on the electrode because the depletion region of

hematite is well developed at this potential (Fig. 6b). The smaller radius of semicircle

indicates the faster water oxidation kinetics [50, 51]. It can be seen that the loading

more NiFeOx means stronger water oxidation ability. This phenomenon is different

from passivation, because passivation the latter method usually increases the

photoanode resistance in dark [48]. Through XPS analysis and TEM images, we can

confirm that the NiFeOx exists as additional amorphous layers rather than adsorbed

ions. Therefore, we can infer that the enhanced charge transfer rate comes from

improvement of water oxidation, and there is a balance between the interface

resistance and water oxidation. This result is similar from the significant reduction of

the charge transfer resistance when a Co-Pi or Co3O4 catalyst is applied on hematite

[28, 52]. This suggests that the NiFeOx layer plays a role as an electrocatalyst. The

mechanism schematic is as shown in Fig. 7. The α-Fe2O3 serves as a light absorber,

which absorbs photons to produce electron-hole pairs. The band alignment scheme is

shown in Fig. S4. Unlike the heterojunction model, the NiFeOx serves as a co-catalyst.

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The photo-generated holes are trapped in the NiFeOx layer, which makes transforms

Ni2+ into Ni3+. Due to the better water oxidation ability of Ni3+, it the slow process of

water oxidation on bare hematite can finally be accelerated [39]. Simultaneously, the

electrons migrate to the FTO back contact and pass through the circuit to the Pt

counter electrode to participate in the water reduction reaction.

4. Conclusions

In summary, equimolar nickel and iron oxide modified hematite films for enhanced

photoelectrochemical water splitting is presented. It was found that the loading of

amorphous NiFeOx on hematite could be readily achieved by a simple spin-coating

step process without substantially changing the morphology of the hematite. After

treatment with 30 mM Ni(II)Fe(III) precursors, the formed NiFeOx coating

significantly reduced the crack between α-Fe2O3 crystallites, and PEC and EIS

measurements showed improvements in water oxidation. It was found that the highest

photocurrent increase and photocurrent onset potential shift was were observed by 30

mM NiFe solution treatment. The photocurrent density increased from 0.998 to 1.126

mA /cm2 (i.e. 12.7% increment) at 1.23 V for the pristine hematite film and the 30

mM NiFe solution treated sample respectively. Concomitant with these improvements

were a shift in the photocurrent onset potential by 55 mV, improvements in IPCE and

an increase in the efficiency of oxygen evolution. The reasons for the photocurrent

improvement are attributed to surface passivation effect through the formation of a

thin NiFeOx layer, which can reduce electron-hole recombination at hematite-

electrolyte interface and increase the water oxidation efficiency

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Acknowledgement

We gratefully acknowledge the financial support by Natural Science Foundation

of China (No. 51172042), Specialized Research Fund for the Doctoral Program of

Higher Education（20110075130001），Science and Technology Commission of

Shanghai Municipality (12nm0503900, 13JC1400200), The Shanghai Natural Science

Foundation (15ZR1401200), Innovative Research Team in University（IRT1221） ,

the Program of Introducing Talents of Discipline to Universities (No.111-2-04), and

the Fundamental Research Funds for the Central Universities(2232014A3-06).

14

ReferencesUncategorized References

[1] Boddy P. Oxygen evolution on semiconducting TiO2. Journal of The Electrochemical Society. 1968;115:199-203.[2] Fujishima A. Electrochemical photolysis of water at a semiconductor electrode. Nature. 1972;238:37-8.[3] Grätzel M. Photoelectrochemical cells. Nature. 2001;414:338-44.[4] Wang G, Wang H, Ling Y, Tang Y, Yang X, Fitzmorris RC, et al. Hydrogen-treated TiO2 nanowire arrays for photoelectrochemical water splitting. Nano letters. 2011;11:3026-33.[5] Liu J, Yu X, Liu Q, Liu R, Shang X, Zhang S, et al. Surface-phase junctions of branched TiO2 nanorod arrays for efficient photoelectrochemical water splitting. Applied Catalysis B: Environmental. 2014;158-159:296-300.[6] Li J, Cushing SK, Zheng P, Senty T, Meng F, Bristow AD, et al. Solar hydrogen generation by a CdS-Au-TiO2 sandwich nanorod array enhanced with Au nanoparticle as electron relay and plasmonic photosensitizer. Journal of the American Chemical Society. 2014;136:8438-49.[7] Kay A, Cesar I, Grätzel M. New benchmark for water photooxidation by nanostructured α-Fe2O3 films. Journal of the American Chemical Society. 2006;128:15714-21.[8] Ling Y, Wang G, Wheeler DA, Zhang JZ, Li Y. Sn-doped hematite nanostructures for photoelectrochemical water splitting. Nano letters. 2011;11:2119-25.[9] Li S, Zhang P, Song X, Gao L. Ultrathin Ti-doped hematite photoanode by pyrolysis of ferrocene. International Journal of Hydrogen Energy. 2014;39:14596-603.[10] Marsen B, Miller EL, Paluselli D, Rocheleau RE. Progress in sputtered tungsten trioxide for photoelectrode applications. International Journal of Hydrogen Energy. 2007;32:3110-5.[11] Vidyarthi VS, Hofmann M, Savan A, Sliozberg K, König D, Beranek R, et al. Enhanced photoelectrochemical properties of WO 3 thin films fabricated by reactive magnetron sputtering. international journal of hydrogen energy. 2011;36:4724-31.[12] Kim TW, Choi KS. Nanoporous BiVO4 photoanodes with dual-layer oxygen evolution catalysts for solar water splitting. Science. 2014;343:990-4.[13] Li M, Zhao L, Guo L. Preparation and photoelectrochemical study of BiVO 4 thin films deposited by ultrasonic spray pyrolysis. international journal of hydrogen energy. 2010;35:7127-33.[14] Li Y, Takata T, Cha D, Takanabe K, Minegishi T, Kubota J, et al. Vertically Aligned Ta3N5 Nanorod Arrays for Solar Driven Photoelectrochemical Water Splitting. Advanced materials. 2013;25:125-31.‐[15] Higashi M, Domen K, Abe R. Fabrication of efficient TaON and Ta3N5 photoanodes for water splitting under visible light irradiation. Energy & Environmental Science. 2011;4:4138-47.[16] Zhang L, Song Y, Feng J, Fang T, Zhong Y, Li Z, et al. Photoelectrochemical water oxidation of LaTaON 2 under visible-light irradiation. International Journal of Hydrogen Energy. 2014;39:7697-704.[17] Sivula K, Le Formal F, Grätzel M. Solar water splitting: progress using hematite (α Fe2O3)‐ photoelectrodes. ChemSusChem. 2011;4:432-49.[18] Murphy A, Barnes P, Randeniya L, Plumb I, Grey I, Horne M, et al. Efficiency of solar water splitting using semiconductor electrodes. International journal of hydrogen energy. 2006;31:1999-2017.[19] Brillet J, Gratzel M, Sivula K. Decoupling feature size and functionality in solution-processed, porous hematite electrodes for solar water splitting. Nano letters. 2010;10:4155-60.[20] Satsangi VR, Kumari S, Singh AP, Shrivastav R, Dass S. Nanostructured hematite for photoelectrochemical generation of hydrogen. International Journal of Hydrogen Energy. 2008;33:312-

15

8.[21] Vincent T, Gross M, Dotan H, Rothschild A. Thermally oxidized iron oxide nanoarchitectures for hydrogen production by solar-induced water splitting. international journal of hydrogen energy. 2012;37:8102-9.[22] Kim JY, Magesh G, Youn DH, Jang JW, Kubota J, Domen K, et al. Single-crystalline, wormlike hematite photoanodes for efficient solar water splitting. Scientific reports. 2013;3:2681.[23] Cesar I, Kay A, Gonzalez Martinez JA, Grätzel M. Translucent thin film Fe2O3 photoanodes for efficient water splitting by sunlight: nanostructure-directing effect of Si-doping. Journal of the American Chemical Society. 2006;128:4582-3.[24] Kumari S, Singh AP, Deva D, Shrivastav R, Dass S, Satsangi VR. Spray pyrolytically deposited nanoporous Ti 4+ doped hematite thin films for efficient photoelectrochemical splitting of water. international journal of hydrogen energy. 2010;35:3985-90.[25] Kumar P, Sharma P, Shrivastav R, Dass S, Satsangi VR. Electrodeposited zirconium-doped α-Fe 2 O 3 thin film for photoelectrochemical water splitting. international journal of hydrogen energy. 2011;36:2777-84.[26] Cao D, Luo W, Feng J, Zhao X, Li Z, Zou Z. Cathodic shift of onset potential for water oxidation on a Ti4+ doped Fe2O3 photoanode by suppressing the back reaction. Energy & Environmental Science. 2014;7:752.[27] Xi L, Chiam SY, Mak WF, Tran PD, Barber J, Loo SCJ, et al. A novel strategy for surface treatment on hematite photoanode for efficient water oxidation. Chemical Science. 2013;4:164.[28] Zhong DK, Sun J, Inumaru H, Gamelin DR. Solar water oxidation by composite catalyst/α-Fe2O3 photoanodes. Journal of the American Chemical Society. 2009;131:6086-7.[29] Le Formal F, Tétreault N, Cornuz M, Moehl T, Grätzel M, Sivula K. Passivating surface states on water splitting hematite photoanodes with alumina overlayers. Chemical Science. 2011;2:737.[30] Lee WJ, Shinde PS, Go GH, Doh CH. Cathodic shift and improved photocurrent performance of cost-effective Fe 2 O 3 photoanodes. International Journal of Hydrogen Energy. 2014;39:5575-9.[31] Klahr B, Gimenez S, Fabregat-Santiago F, Hamann T, Bisquert J. Water oxidation at hematite photoelectrodes: the role of surface states. Journal of the American Chemical Society. 2012;134:4294-302.[32] Tilley SD, Cornuz M, Sivula K, Gratzel M. Light-induced water splitting with hematite: improved nanostructure and iridium oxide catalysis. Angewandte Chemie. 2010;49:6405-8.[33] Du C, Yang X, Mayer MT, Hoyt H, Xie J, McMahon G, et al. Hematite-based water splitting with low turn-on voltages. Angewandte Chemie. 2013;52:12692-5.[34] Vayssieres L, Beermann N, Lindquist S-E, Hagfeldt A. Controlled aqueous chemical growth of oriented three-dimensional crystalline nanorod arrays: application to iron (III) oxides. Chemistry of materials. 2001;13:233-5.[35] Trotochaud L, Ranney JK, Williams KN, Boettcher SW. Solution-cast metal oxide thin film electrocatalysts for oxygen evolution. Journal of the American Chemical Society. 2012;134:17253-61.[36] Zhang P, Kleiman-Shwarsctein A, Hu Y-S, Lefton J, Sharma S, Forman AJ, et al. Oriented Ti doped hematite thin film as active photoanodes synthesized by facile APCVD. Energy & Environmental Science. 2011;4:1020-8.[37] Zhang M, Luo W, Li Z, Yu T, Zou Z. Improved photoelectrochemical responses of Si and Ti codoped α-Fe2O3 photoanode films. Applied Physics Letters. 2010;97:042105.[38] Lian X, Yang X, Liu S, Xu Y, Jiang C, Chen J, et al. Enhanced photoelectrochemical performance of

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Ti-doped hematite thin films prepared by the sol–gel method. Applied Surface Science. 2012;258:2307-11.[39] Louie MW, Bell AT. An investigation of thin-film Ni-Fe oxide catalysts for the electrochemical evolution of oxygen. Journal of the American Chemical Society. 2013;135:12329-37.[40] Young KM, Hamann TW. Enhanced photocatalytic water oxidation efficiency with Ni(OH)2 catalysts deposited on α-Fe 2O3 via ALD. Chemical communications. 2014;50:8727-30.[41] Kleiman-Shwarsctein A, Hu Y-S, Stucky GD, McFarland EW. NiFe-oxide electrocatalysts for the oxygen evolution reaction on Ti doped hematite photoelectrodes. Electrochemistry Communications. 2009;11:1150-3.[42] Lokhande C, Dubal D, Joo O-S. Metal oxide thin film based supercapacitors. Current Applied Physics. 2011;11:255-70.[43] Gong M, Li Y, Wang H, Liang Y, Wu JZ, Zhou J, et al. An advanced Ni–Fe layered double hydroxide electrocatalyst for water oxidation. Journal of the American Chemical Society. 2013;135:8452-5.[44] Klahr B, Gimenez S, Fabregat-Santiago F, Bisquert J, Hamann TW. Electrochemical and photoelectrochemical investigation of water oxidation with hematite electrodes. Energy & Environmental Science. 2012;5:7626-36.[45] Zhong DK, Choi S, Gamelin DR. Near-complete suppression of surface recombination in solar photoelectrolysis by "Co-Pi" catalyst-modified W:BiVO4. Journal of the American Chemical Society. 2011;133:18370-7.[46] Zhong DK, Cornuz M, Sivula K, Grätzel M, Gamelin DR. Photo-assisted electrodeposition of cobalt–phosphate (Co–Pi) catalyst on hematite photoanodes for solar water oxidation. Energy & Environmental Science. 2011;4:1759.[47] Lin F, Boettcher SW. Adaptive semiconductor/electrocatalyst junctions in water-splitting photoanodes. Nature materials. 2014;13:81-6.[48] Yang X, Liu R, Du C, Dai P, Zheng Z, Wang D. Improving hematite-based photoelectrochemical water splitting with ultrathin TiO2 by atomic layer deposition. ACS applied materials & interfaces. 2014;6:12005-11.[49] Spray RL, McDonald KJ, Choi K-S. Enhancing Photoresponse of Nanoparticulate α-Fe2O3Electrodes by Surface Composition Tuning. The Journal of Physical Chemistry C. 2011;115:3497-506.[50] Yang X, Du C, Liu R, Xie J, Wang D. Balancing photovoltage generation and charge-transfer enhancement for catalyst-decorated photoelectrochemical water splitting: A case study of the hematite/MnOx combination. Journal of Catalysis. 2013;304:86-91.[51] Lopes T, Andrade L, Ribeiro HA, Mendes A. Characterization of photoelectrochemical cells for water splitting by electrochemical impedance spectroscopy. international journal of hydrogen energy. 2010;35:11601-8.[52] Riha SC, Klahr BM, Tyo EC, Seifert Sn, Vajda S, Pellin MJ, et al. Atomic layer deposition of a submonolayer catalyst for the enhanced photoelectrochemical performance of water oxidation with hematite. ACS nano. 2013;7:2396-405.

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Figures Captions

Scheme 1. Schematic illustration of the formation of NiFeOx loaded Ti-doped

hematite film for efficient water oxidation

Fig.1 XPS spectra of survey (a), high resolution Fe 2p scans (b), Ti 2p scans (c)

and Ni 2p scans for the pristine Ti-αFe2O3 and Ti-αFe2O3/NiFeOx photoanodes.

Fig.2 J–V curves under illumination (a) and in dark (b).

Fig.3 Fe-SEM images of the pristine Ti-doped hematite (a), hematite after

treatment with different concentrations of Ni(II)Fe(III) aqueous solution: 15 (b), 30

(c), and 90 (d) mM. TEM image (e) and HRTEM image (f) of hematite after treatment

with 30 mM Ni(II)Fe(III) aqueous solution.

Fig.4 Chronoamperometry at an applied potential of 1.23 V versus RHE under

light-chopping conditions (AM 1.5 100 mW/cm2), uncorrected for Ohmic losses (a).

Current density of the NiFeOx decorated hematite device is plotted as a function of

time at 1.23 V RHE in 1 M KOH solution (b).

Fig.5 IPCE spectra of hematite photoanode with and without Ni(II)Fe(III)

treatment. IPCE measurements were carried out at an applied potential of 1.23 V RHE

in a 1 M KOH electrolyte.

Fig.6 Nyquist plots of the EIS measurements on the bare or NiFeOx decorated Ti-

doped hematite sample under illumination (a) and in dark (b), electrolyte: 1 M KOH,

potential: 1.23 V RHE.

Fig.7 Mechanism for the NiFeOx decorated hematite for enhanced

photoelectrochemical water splitting.

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Scheme 1.

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Fig. 1

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Fig. 2

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Fig. 3

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Fig. 4

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Fig. 5

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Fig. 6

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Fig. 7

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