Exploiting the dynamic Sn diffusion from deformation

Solar Energy Materials & Solar Cells 141 (2015) 71–79

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Exploiting the dynamic Sn diffusion from deformation of FTO to boost the photocurrent performance of hematite photoanodes Pravin S. Shinde a,b, Alagappan Annamalai a, Ju Hun Kim b, Sun Hee Choi c, Jae Sung Lee b,n, Jum Suk Jang a,nn a Division of Biotechnology, Advanced Institute of Environmental and Bioscience, College of Environmental and Bioresource Sciences, Chonbuk National University, Iksan 570-752, Republic of Korea b School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), 50 UNIST-gil, Ulsan 689-798, Republic of Korea c Pohang Accelerator Laboratory (PAL), Pohang University of Science and Technology (POSTECH), Pohang 790-784, Republic of Korea

art ic l e i nf o

a b s t r a c t

Article history: Received 16 March 2015 Received in revised form 18 April 2015 Accepted 12 May 2015

Herein, we report on exploiting effective Sn diffusion from the underlying FTO (F:SnO2) substrate during post-growth annealing treatment to boost the photocurrent performance of α-Fe2O3 photoanodes. Conventionally, hematite photoanodes are activated at high temperature (800 °C 4T 4650 °C) after initial low temperature annealing. The purpose of such activation is to utilize the diffusion of Sn ions leached from FTO deformation for improving the electronic conductivity and hence the photocurrent response of hematite photoanodes. However, the pre-formed oxide layer (in the first annealing) on the surface of FTO creates obstacle for diffusion of Sn ions during FTO deformation. To overcome this difficulty and to exploit Sn diffusion, we employed a direct single-step annealing of as-grown iron-coated FTO electrodes at 800 °C for short duration followed by quenching in air. Such activation greatly improved the water oxidation photocurrent response of hematite by 37% on account of increased Sn diffusion which was confirmed from X-ray photoelectron spectroscopy. Moreover, the onset of photocurrent is shifted cathodically meaning that the water oxidation reaction proceeds at lower applied bias. Higher Sn content increased the electron donor concentration and hence improved the charge transfer kinetics of hematite as studied from EIS and Mott–Schottky measurements. Interestingly, despite higher Sn diffusion, the loss of FTO conductivity was minimal and the structural ordering was highest in onestep-activated hematite compared to conventionally activated hematite photoanodes as studied from XAFS analysis. & 2015 Elsevier B.V. All rights reserved.

Keywords: Hematite Onset potential Sn diffusion doping FTO conductivity Photoelectrochemical water oxidation

1. Introduction Hematite (α-Fe2O3) is widely studied for photoelectrochemical (PEC) water splitting due to its favorable band gap (Eg 2.1 eV), maximum theoretical efficiency (12.9%), good electrochemical stability, abundance, non-toxicity, and cost-effectiveness [1,2]. However, its PEC activity is limited by ultra-fast charge recombination (τ 10 ps), small hole diffusion length (2–4 nm), and poor carrier mobility (0.2 cm2 V 1 s 1) [3,4]. The PEC activity can be improved by increasing the charge carrier density via intentional or unintentional doping [1,4,5], by increasing the hematite–electrolyte interface area via engineering nanostructure morphology [2,6,7], and by reducing the over-potential via surface treatments n

Corresponding author. Tel.: þ 82 52 217 2544. Corresponding author. Tel.: þ 82 63 850 0846; fax: þ 82 63 850 0834. E-mail addresses: [email protected] (J.S. Lee), [email protected] (J.S. Jang).

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http://dx.doi.org/10.1016/j.solmat.2015.05.020 0927-0248/& 2015 Elsevier B.V. All rights reserved.

[7–9]. Annealing is one of the influential parameters that decide the physico-chemical properties of hematite. The photoactivity of hematite fabricated on FTO (F:SnO2) substrate at low temperature is very poor and requires a thermal activation at high temperatures (650–800 °C) to induce Sn doping from FTO for improvement [3,10]. Conventionally, hematite is activated for efficient PEC performance via two-step heat treatment that involves initial lowtemperature heat treatment at 500–550 °C for 2–10 h and natural cooling until room temperature, followed by high-temperature annealing at 800 °C for 10–20 min [3,7,11,12]. The purpose of lowtemperature heat treatment for synthesis of hematite electrodes by different methods such as colloidal, doctor-blade, hydrothermal and sputtering was reported to be for removing the organics, bringing about the nanostructured growth and particle connectivity. The improvement in photoactivity due to high-temperature annealing was mainly attributed to the n-type doping of Sn cations diffused from the deformed FTO substrate. Such cationic

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P.S. Shinde et al. / Solar Energy Materials & Solar Cells 141 (2015) 71–79

incorporation dramatically decreases the resistivity of the film as a result of enhanced donor density, thus positively affecting the photoactivity of the photoanodes. However there are two decisive factors that may limit the photoactivity of hematite. One is behavior of FTO conductivity during two-step annealing and another is formation of oxide layer during the first low-temperature annealing. As FTO's role is to efficiently drive the photo-generated electrons to the back contact, maintaining its conductivity to optimum level is crucial during high-temperature annealing. We noticed that the loss of conductivity in FTO is severe for the hematite photoanodes fabricated by conventional annealing approach as a result of more FTO deformation due to longer hours of processing time. The pre-formed oxide layer at the interface of FTO may also pose interference for the dynamic diffusion of Sn ions during re-crystallization of hematite lattice at high-temperature annealing. Importantly, the Sn ions leached during FTO deformation have to diffuse into the hematite lattice in a dynamic way so that the structural ordering of hematite is unperturbed. Therefore, there is a need to maintain a trade-off between the amount of Sn ions leached from FTO and minimum loss in conductivity of FTO. To overcome this difficulty, we used a direct onestep activation of hematite using as-grown iron-coated FTO electrodes at high temperature for optimum duration followed by a mild quenching in air. Such heat treatment not only enables the cost-effective synthesis of hematite but also yields improved PEC performance. In this work, it is demonstrated that different post-growth annealing conditions can significantly influence the photocurrent response of hematite photoanodes. We report on efficient utilization of Sn diffusion from the underlying FTO (F:SnO2) substrate during hematite activation from FTO deformation at high temperature (HT) to boost the photocurrent of α-Fe2O3 photoanodes. We used two different states of electrodes for hematite activation: As-grown Fe/FTO (one-step activation at HT) and Fe2O3/FTO (twostep activation at 550 °C þHT). The annealing temperature and the time for HT activation of hematite were optimized systematically by means of photocurrent performance. The state of material to be annealed is found to affect the dynamics of Sn diffusion, electron transport and the PEC performance of photoanodes. The effect of Sn diffusion into hematite matrix on the changes in donor density of hematite, shift in onset potential of water oxidation, deterioration in FTO conductivity, and structural ordering are discussed. The possible mechanism during hematite activation is discussed to understand annealing phenomena.

2. Experimental 2.1. Synthesis of

α-Fe2O3 photoanodes

The iron films were grown on fluorine doped tin oxide (FTO, F:SnO2, 10–15 sq. cm 1)-coated glass substrates by a facile pulse reverse electrodeposition (PRED) method according to our previous study [13]. The process parameters of PRED such as amplitude of square wave pulse, duty cycle, pulse period, and deposition time were [10 V ( 6/ þ4 V)] 20%, 10 ms, and 45 s, respectively. The as-grown samples were rinsed in deionized water and dried immediately using nitrogen stream. For one-step annealing, the as-grown samples were inserted into the box furnace (whose temperature was already attained to high temperature, viz. 800 °C), soaked for desired duration, air-quenched for few seconds and then transferred to the oven at 100 °C for 10 min. After keeping in oven for 10 min, the samples were taken out from the oven to ambient air for cooling down completely. By following such mild air-quenching process, we avoid the breaking of hightemperature-annealed red-hot samples as well as maintain the

crystalline synthesis of hematite photoanodes. Several as-grown films under identical synthesis conditions were prepared by PRED to investigate the effects of annealing temperature and annealing time. The as-grown films were reflecting and dark greyish in color. Initially, by keeping the annealing time fixed for 15 min, the elevated temperature for one-step annealing was varied from 700 to 825 °C. In the next step, by choosing the optimized annealing temperature, the annealing time was varied from 10 to 17.5 min to decide on the optimum growth time for improving the photocatalytic activity of iron oxide. The annealing conditions of samples (iron oxide thin films) in both cases were optimized by means of their PEC performance. Later, to compare the PEC performance of optimized one-step-annealed photoanodes, the representative two-step-annealed hematite photoanodes were also prepared under identical conditions of elevated annealing temperature after the initial low temperature annealing treatment at 550 °C for 4 h followed by natural cooling to room temperature. All annealed films were reddish brown in color. Despite having little wrinkled surface, one-step-annealed films were well-adherent and showed no signs of any degradation. Alternatively, FTO substrates were also annealed along with hematite electrodes by low temperature, one-step and two-step annealing methods at representative annealing conditions. 2.2. Characterization of


The as-grown iron or low-temperature annealed hematite and the representative one-step and two-step-annealed hematite photoanodes (those prepared with optimized conditions of elevated annealing temperature) were characterized further to understand their contribution toward enhancing the PEC performance. The structural analysis was performed using PANalytical X′ pert Pro MPD diffractometer equipped with Cu Kα radiation source (wavelength Kα1 ¼1.540598 Å and Kα2 ¼1.544426 Å) operated at 40 kV and 30 mA. The X-ray diffraction patterns were recorded in the diffraction angle range of 20–80° with step size of 0.0167° s 1 and dwell time of 50.165 s. The morphology of the α-Fe2O3 films was examined on Field Emission Scanning Electron Microscope (FESEM) (SUPRA 40VP, Carl Zeiss, Germany), equipped with X-ray energy dispersive spectrometer (EDS). The chemical state and elemental quantification in the freshly synthesized iron oxide samples was performed using X-ray photoelectron spectroscopy (XPS). The XPS analysis was performed on a Thermo Scientific XPS spectrometer equipped with a monochromatic Al Kα X-ray source (hν ¼1486.6 eV). Wide survey spectra (Binding Energy, BE: 1200–0 eV) were recorded for the samples using the X-ray spot size of 400 mm at room temperature with an analyzer pass energy of 200 eV and energy step size of 1 eV. High resolution spectra in the region of interest were acquired at the pass energy of 50 eV and step size of 0.1 eV. The calibration of the binding energy of the high resolution spectra of all the iron oxide samples was made by referring the maximum of the adventitious C 1s carbon peak at 284.8 eV. The processing of XPS data was performed with the help of an XPS Peak-fit program using Shirley background. The high-resolution XPS spectra were de-convoluted with a least squares fitting routine provided within the software. A symmetric Gaussian–Lorentzian product function was used to approximate the line shapes of the fitting components. Raw experimental data were used for analysis with no preliminary smoothing. The cross-sectional scanning electron microscopic images of one-step and two-step-annealed Fe2O3 samples were prepared with a FEI Helios 450HP NanoLab dual-beam focused ion beam (FIB) system using a gallium ion beam. The samples were partly melted due to the high energy of FIB during the sample preparation. It was difficult to obtain clear images at high magnification (150k). The elemental mapping of cross-sectional


samples was carried out. The local structures of iron oxide electrodes were investigated with X-ray absorption fine structure (XAFS). X-ray measurements were performed on 7D beamline of Pohang Accelerator Laboratory (PLS-II, 3.0 GeV). The synchrotron radiation was monochromatized using a Si(111) double crystal monochromators. At room temperature, the spectra for the Fe K-edge (E0 ¼7112 eV) was taken in a fluorescence mode. The incident beam was detuned by 30% in order to minimize the contamination of higher harmonics and its intensity was monitored using a He-filled IC SPEC ionization chamber. The fluorescence signal from the sample was measured with a passivated implanted planar silicon (PIPS) detector attached to a He-flowing sample chamber. The obtained data were analyzed with ATHENA in the IFEFFIT suite of programs [14]. The UV–vis absorption study in the wavelength range of 350–800 nm was performed using a dual beam spectrophotometer (Shimadzu, UV-2600 series). The photocurrent response of synthesized α-Fe2O3 photoanodes was measured in a PEC cell consisting of three-arm glass compartment with a circular quartz window for light illumination. The PEC cell comprised of Fe2O3/FTO photoanode as working electrode, Pt wire as counter electrode, Ag/AgCl (saturated with KCl) as reference electrode immersed in 1 M NaOH electrolyte. All the potentials mentioned in the work were measured with reference to Ag/AgCl electrode and were revised for the reversible hydrogen electrode (RHE) scale using the Nernst equation. The 1 sun (100 mW cm 2) simulated light illumination was provided using a solar simulator (Abet Technologies). The photocurrent–voltage (J–V) curves, electrochemical impedance spectroscopy (EIS), and Mott–Schottky studies were performed using a portable potentiostat (COMPACTSTAT.e, Ivium, Netherland) equipped with electrochemical interface and impedance analyzer facility. The EIS study was performed under 1 sun illumination at 1.23 V vs. RHE. The experimental EIS data was validated using the Kramers–Kronig transform test and then fitted to the suitable equivalent circuit model using the ZView (Scribner Associates Inc.) program. The fitting was done based on a model that describes the processes occurring in the PEC system. Mott–Schottky measurements were performed in a dark condition with a DC applied potential window of 0.6 to 0.7 V vs. Ag/AgCl at 0.5 kHz AC frequency. The amplitude of AC voltage was 10 mV in both EIS and Mott–Schottky measurements. Then, using the slopes determined from Mott–Schottky plot (Csc 2 vs. V), the electron donor densities are estimated using the equation for planar electrodes as [15] −1 −2 ND = (2/eεεo ) ⎡⎣d Csc /dV ⎤⎦

( )

(1)

where Csc is the space charge layer capacitance, e is the electron charge (1.6022 10 19 C), ε is the dielectric constant of α-Fe2O3 (80) [16], εo is the permittivity of vacuum (8.854 10 14 F cm 1), ND is the donor density, and V is the applied potential. The smaller the slope, the larger is the donor density.

3. Results and discussion 3.1. Photoelectrochemical study of


Fig. S1a and b shows the current density–voltage (J–V) curves and photocurrent density (Jph) variation measured at 1.23 V vs. RHE (VRHE) for hematite photoanodes prepared with different onestep annealing temperatures. The one-step-annealed hematite prepared at 800 °C for 15 min showed Jph of 561 mA cm 2 at 1.23 VRHE, which is 11% higher than that of the two-step-annealed hematite prepared at the same high temperature condition. The onset potential (Vonset) of the water oxidation photocurrent is in the range of 0.8–0.85 VRHE, except for the 825 °C one-step-

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Fig. 1. (a) Jph variation of one-step-annealed hematite photoanodes measured at 1.23 VRHE in 1 M NaOH for different annealing times. For comparison, Jph value at 1.23 VRHE of two-step-annealed α-Fe2O3 photoanode prepared by similar higher temperature annealing condition is shown. (b) J–V curves of one- and two-stepannealed hematite photoanodes with similar high temperature annealing at 800 °C for 13.5 min. The error bars represent the standard deviations in PEC measurements of the independently prepared series of hematite photoanodes. Light illumination, 1 sun; scan rate, 50 mV s 1.

annealed hematite. An anodic shift of Vonset (0.1 V) for the 825 °C annealed hematite indicates inferior water oxidation kinetics. The shift could be due to surface trapping sites as a result of severe distortion of the structure as well as excess Sn diffusion (causing more loss of Sn from underlying FTO) at annealing temperature4 800 °C [11]. Annealing time is another critical factor that governs the growth and the electronic properties of hematite. Fig. 1a shows the Jph variation of hematite photoanodes at 1.23 VRHE for the one-step annealing time. The one-step-annealed hematite (at 800 °C for 13.5 min) showed a prominent Jph of 650 mA cm 2, with an increment of 37% at 1.23 VRHE relative to that of the two-step-annealed hematite (with similar high temperature annealing condition), and reached a maximum of 900 mA cm 2 at 1.6 VRHE (Fig. 1b). As discussed below, the increased Jph was due to the improved electronic properties as a result of the facile diffusion of Sn ions from FTO during the onestep annealing at 800 °C while the crystalline transformation of hematite proceeds [10]. The diffusion of Sn4 þ ions at high temperature leads to instability in the electronic structure of hematite (Fe3 þ ). Hence, to balance the charge, Sn4 þ ions induce partial reduction of Fe from Fe3 þ to Fe2 þ on the surface of the hematite, which can provide extra 3d electrons. Such charge-neutrality increases the electron donor concentration and thus the photocurrent [17,18]. A relatively low Vonset (0.71 VRHE) was observed for one-step-annealed hematite showing a cathodic shift of 0.1 V

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from typical values (0.8–1.0 VRHE) [3,5]. Ling et al. noticed similar shift for Sn-doped hematite electrodes and it was ascribed to be due to the nature of synthesis method [4]. The much desired cathodic shift in the onset potential observed for the one-step annealed hematite is advantageous as the water oxidation reactions can proceed at a lower applied bias. This is a significant development in the context that several researchers utilized cocatalysts such as RuOx or cobalt-phosphate [5,19] to shift the onset potential of the water oxidation reaction. We achieve such shift with the help of modified annealing approach from underlying FTO substrate without using any co-catalysts. Use of such co-catalysts led to decrease in onset potential at the expense of decrease in photocurrent response. The onset potential and photocurrent of one-step activated hematite have the positive effects. This suggests that interfacial hole transfer is improved by reducing the deleterious carrier recombination processes. Fig. S2 shows the J–V curves of one-step-annealed hematite photoanodes for all annealing temperatures and annealing times. Next, the one- and two-stepannealed hematite photoanodes formed under similar high-temperature conditions (800 °C for 13.5 min) were further investigated to understand their structural, optical and electrochemical properties for photocurrent contribution. 3.2. XRD and FESEM studies As shown in Fig. 2a, the as-grown film was in the form of metallic iron. Upon annealing at 800 °C (4550 °C) for 13.5 min, the iron film was transformed to a polycrystalline hematite (α-Fe2O3) phase having preferred growth along the (104) and (110) planes. The crystallite sizes determined from the high-intensity diffraction peaks of two- and one-step-annealed hematite are ca. 62 nm and 76 nm, respectively. Fig. 2b reveals the FESEM image of as-grown iron film ( 200 nm thick) deposited by PRED. The nanostructured growth and particle inter-connectivity during lowtemperature annealing at 550 °C has been reported previously [13] (Supporting information). In brief, the as-grown film consists of agglomerated morphology with nanocrystalline clusters of

spherical grains (30–50 nm). Upon low-temperature annealing at 500 °C or 550 °C, the agglomerated clusters transform into a morphology that consists of compactly arranged nano-crystalline grains with good inter-connectivity as seen from Fig. S3. Additionally, the nano-rods as long as 400–700 nm and 50 nm in diameter also appear. The high-temperature annealing collapses the nanostructures and brings about further grain growth with different size distributions as shown in Fig. 2c and d. The quality and surface morphology of electrodes is almost similar for two-step and one-step annealed hematite samples except the grain size. In general, two-step annealing produced smaller grains, which was consistent with the XRD study. Increased grain size (190 nm) in one-step annealing thus reduces the undesired grain boundary sites. Such a compact and interconnected morphology provides easy and uninterrupted pathways for charge carriers to move to the back contact, which in turn improves the photo-response. 3.3. XPS study Fig. S4 shows the XPS survey spectra of low temperature (550 °C), two-step and one-step-annealed iron oxide samples. The binding energy (BE) peaks centered at 711, 530, 486, 285, 94, 56 and 23 eV were assigned to Fe 2p, O 1s, C 1s, Sn 3d, Fe 3s, Fe 3p, and O 2s photo-electrons, respectively. The broad peaks centered at 974 eV and 894, 847, and 785 eV were assigned to the OKLL, FeLML and Auger transitions, respectively. Fig. 3a–c shows the highresolution XPS spectra of the Fe2p, O1s, and Sn3d regions for all annealed iron oxide samples (curves 1–3). Elemental quantification of these peaks is shown in Table 1. In the Fe 2p region (Fig. 3a), an Fe 2p3/2 peak was observed at a binding energy (BE) of 710.6 70.1 eV, and its absence at 709.3 eV suggests the oxidation state of Fe to be þ3, confirming the formation of phase-pure α-Fe2O3. The broadness of the peaks was attributed to electrostatic interactions between the photo-ionized Fe 2p core hole and unpaired Fe3d electrons, spin-orbit coupling, and crystal field interactions [20]. The presence of two additional satellite peaks (ascribed to charge transfer and/or shake-up processes) around

Fig. 2. (a) XRD patterns of as-grown iron film and two-step and one-step annealed hematite photoanodes. FESEM images of (b) as-grown iron film, (c) two-step annealed hematite, and (d) one-step annealed hematite. Insets in Fig.4b and c show the corresponding cross-sectional images.


Fig. 3. High resolution XPS spectra of (a) Fe 2p, (b) O 1s, and (c) Sn 3d regions for different iron (oxide) samples prepared on FTO substrate. Samples: (curve 1) low temperature annealing at 550 °C for 4 h, (curve 2) two-step annealing at 550 °C followed by 800 °C, and (curve 3) one-step annealing at 800 °C. Table 1 The elemental ID and quantification of iron oxide films synthesized by PRED and annealed via low temperature (550 °C), two-step and one-step methods. Peak elementsa

Fe 2p O 1s Sn 3d5

550 °C annealed iron oxide

Two-step-annealed iron oxide

One-step-annealed iron oxide

Peak BE

at%

Peak BE

at%

Peak BE

at%

710.6 529.50 –

24.67 75.33 –

710.60 529.60 486.10

23.78 74.97 1.24

710.70 529.70 486.10

23.74 73.81 2.46

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530 eV was assigned to oxygen atoms in the iron oxide lattice, O 1s (Fe–O), and the peak around 531 eV to the lattice hydroxyl groups, O 1s(Fe–OH lattice) [20,22]. The O 1s(Fe–O) peak for the one-stepannealed sample is close to the ideal value (529.9 eV). However, a slight shift in its position toward lower BE for other samples could be due to a slightly disordered electronic structure of the Fe2O3. The shapes of the Fe 2p and O 1s spectra are similar to those reported for α-Fe2O3 [23]. In Fig. 3c, the Sn 3d5/2 peak was absent at 486.10 eV for the 550 °C annealed sample, but appeared for both two- and one-step-annealed samples. This suggests that, at elevated annealing temperatures, leaching and diffusion of tin (from the underlying FTO substrate) into the Fe2O3 matrix occurs, causing electronic modification and changes in charge carrier density. Sn was reported to exist in hematite with a 4þ oxidation state. However, the appearance of an Sn 3d5/2 peak at slightly lower BE (486.1 eV) in the present study than that for SnO2 ( 486.3 70.1 eV) could be related to oxygen deficiency. The increased Sn 3d5 peak intensity for the one-step-annealed sample indicates a higher amount of diffused Sn content (ca. 2.46 at%) than that for the 2-step-annealed sample (ca. 1.24 at%). Hence, it is evident that the higher amount of diffused Sn in the one-stepannealed hematite improved its photocurrent. Such unintentional Sn doping in Fe2O3 is advantageous for improving photoactivity [24]. 3.4. FIB study and schematic illustration

a

Peak values were calibrated with reference to adventitious carbon at 284.8 eV.

719 and 733 eV was consistent with the presence of α-Fe2O3 [20,21]. In O1s (Fig. 3b), the lowest BE peak centered around

To assist the XPS study and to corroborate Sn diffusion phenomenon, the cross-sectional elemental mapping of one-step and two-step-annealed α-Fe2O3 photoanodes was carried out as shown in Fig. 4a and b. It is clear from these images that more Sn is

Fig. 4. Cross-sectional scanning electron microscopic images of α-Fe2O3 photoanodes prepared by (a) one-step and (b) two-step annealing method. The textured region is represented by a dark-field scanning electron microscopy images at a magnification of 150k and corresponds to the elemental maps of Fe, Sn, and O. The samples were partly melted due to the high energy of FIB during the sample preparation. (c) Schematic illustration of growth mechanism during the one- and two-step annealing processes.

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diffused into the hematite matrix for one-step-annealed α-Fe2O3 photoanode. Fig. 4c shows the schematic illustration of the proposed growth mechanism of hematite during one- and two-step annealing. Right before the hematite activation at 800 °C from FTO deformation, the state of as-processed film is different in two annealing processes—a metallic iron for one-step annealing (Fe/ FTO) and a well-formed hematite at 550 °C for two-step annealing (Fe2O3/FTO). During second high-temperature annealing (800 °C), the pre-formed oxide layer in Fe2O3/FTO has to undergo re-crystallization and the Sn diffusion during re-crystallization of preformed oxide becomes difficult as this creates interference for dynamic migration of Sn ions. More FTO deformation is likely to occur in two-step annealed samples as seen from the increased series resistance (sheet resistance of FTO). In contrast, during hematite activation directly from Fe/FTO at 800 °C, the diffusion of Sn ions in semi-molten hot Fe mass is more favorable, resulting in Sn enrichment in hematite. Although oxide formation rate is higher during one-step annealing at high temperature, the diffusion of Sn ions occurs dynamically with minimal FTO deformation in comparison to the conventional two-step annealing approach. EIS spectra of one-and two-step annealed FTO substrates (Fig.6c) supports the above phenomena. Thus, hematite activation of asgrown Fe/FTO electrode by skipping intermediate low-temperature heat-treatment greatly enriched the Sn cations into the hematite matrix to improve the electron donor concentration while keeping the deformation of FTO to a minimal level. Such activation improved the water oxidation performance of hematite photoanodes. These findings are well-supported by XPS and FIB analysis. 3.5. XAFS study The degree of crystallinity, defined as the ordering of the structure, is an important factor determining the photoactivity of

the semiconductor because structural defects can act as recombination centers. However, the XRD and XPS did not provide ample clarification about the structural disorder in the hematite samples. Hence, to account for the structural disorder, the XANES and EXAFS studies were performed. Fig. 5a displays the Fe K-edge XANES spectra of iron oxide photoanodes prepared with lowtemperature (550 °C, LT550), one-step and two-step annealing. The spectra for all the samples were exactly the same as that of the reference α-Fe2O3, as evidenced from the observation of the preedge peak at 7115 eV denoting a quadrupole transition of 1s-3d. Moreover, the absorption rising feature and energy positions were also the same. Although the XANES analysis revealed identical electronic local structure, the geometrical structure study from EXAFS was different. In comparison with the reference α-Fe2O3, k2-weighted EXAFS function of one-step-annealed sample exhibited enhanced amplitude feature at 10–11 Å 1 (Fig. 5b). The Fourier transforms of corresponding samples are shown in Fig. 5c. The intensity of the first peak at 0.8–2.0 Å increased in a sequence of α-Fe2O3 otwo-step∼LT550 oone-step. As the Fe–O coordination number corresponding to interaction between the nearest neighbors and a central iron atom was invariant, the increase in the intensity indicated the smaller Debye–Waller factor. The Debye–Waller factor, indicator of the structural disorder, is a good structural parameter to elucidate the ordering of thin film. The second peak at 2.1–3.9 Å consists of complicated Fe–Fe and Fe–O scatterings (1 Fe–Fe at 2.90 Å, 3 Fe–Fe at 2.97 Å, 3 Fe–Fe at 3.36 Å, 3 Fe–O at 3.40 Å, 3 Fe–O at 3.60 Å, 6 Fe–Fe at 3.70 Å, and 3 Fe–O at 3.78 Å), thus revealing that each shell cannot be separated. With the enhanced EXAFS amplitude and the highest Fe–O peak intensity, it was concluded that the structural ordering was the highest for the one-step-annealed hematite sample. This finding is consistent with the XRD, FESEM and XPS results. It is important to note that despite of higher Sn leaching from FTO deformation and their migration into the hematite lattice, the structural ordering in one-step-annealed hematite was maintained. This suggests that Sn

Fig. 5. (a) Fe K-edge XANES spectra, (b) k2-weighted EXAFS functions, and (c) corresponding Fourier transforms of low temperature (LT550), one-step, and two-stepannealed iron oxide photoanodes.


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diffusion is unperturbed and dynamic. Conversely, pre-formed oxide layer in the case of two-step annealed hematite serves as a barrier for dynamic diffusion of Sn ions, and ultimately causes structural disordering of hematite. Hence, the grain growth and the improved crystallinity due to the structural ordering in onestep-annealed hematite together contributed to enhance the photoactivity.

of Sn causes as large as 2-fold enhancement of optical absorption coefficient as a result of structural distortion of the hematite lattice [4]. Fig. S5d and e reveals the band gap energies (Eg) determined from Tauc plots of two-step and one-step-annealed hematite photoanodes. The Eg values for direct and indirect optical transition are 2.13–2.14 eV and 2.11 eV, respectively, which are consistent with reported values [3,7].

3.6. UV–vis spectroscopy

3.7. EIS and MS study

The understanding of light absorption properties in hematite is an important aspect that can provide important information on the interactions of photocatalytic materials with photon energies, in order to choose the proper light to activate semiconducting materials for solar energy conversion. Fig. S5a and b shows the absorbance (measure of absorbed photons) for one-step-annealed hematite photoanodes at representative annealing temperatures and annealing times. Absorbance values at each wavelength were increased with high temperature annealing from 700 °C to 800 °C but decreased at 825 °C (Fig. S5a). The reduced absorbance could be due to excess diffusion of Sn ions into the hematite lattice altering the optical properties. No significant change was noticed in the absorbance for annealing time variation (Fig. S5b). The absorbance of one-step-annealed photoanodes was higher than that of two-step-annealed hematite at each wavelength as shown in Fig. S5c and also exhibited significant response at wavelengths greater than the band gap ( 590 nm) due to scattering caused by the large particles. Such scattering effect was lower in the twostep-annealed hematite due to lower grain size as evidenced from FESEM study. The observation of our study is in accordance with the literature in which it was demonstrated that the incorporation

The charge transfer characteristics of the hematite photoanodes was studied by electrochemical impedance spectroscopy (EIS) at 1.23 VRHE. Fig. 6a shows the Nyquist plots (Z′ vs. Z″) of one-step and two-step-annealed α-Fe2O3 photoanodes that were fitted using Randles equivalent circuit [25]. The equivalent circuit elements include a series resistance (Rs), a space-charge capacitance of the bulk hematite (Cbulk), a surface state capacitance (Css), a resistance for trapping of holes in the surface states (Rtrap), and a charge transfer resistance from the surface states to solution (Rct,trap). Recently, the role of surface states in water splitting has been of great interest. Surface states may mediate charge transfer by performing a transit shipment of the photogenerated holes for the oxidation reaction or provide recombination centers [25]. The values of Rs and Rtrap for one-step-annealed hematite are considerably lower than that for two-step-annealed hematite. No significant change in Cbulk is seen. A smaller Css means that the density of surface states responsible for recombination losses is relatively lower. Css is slightly higher for one-step annealed hematite. This validates lower onset of photocurrent in one-stepannealed hematite. Importantly, lower Rct,trap in one-step annealed hematite surpasses the recombination losses. Hence, surface states

Fig. 6. (a) Nyquist plots (under light) and (b) Mott–Schottky plot (under dark) of two- and one-step-annealed hematite photoanodes in 1 M NaOH. The inset of Fig. 6a shows the equivalent circuit and the results of fitting. Inset of Fig. 6b shows the ND and Vfb values. (c) Nyquist plots (under light) of 550 °C (4 h), one-step (15 and 13.5 min), and two-step (15 and 13.5 min)-annealed FTO electrodes. The Nyquist high frequency region is expanded in the inset. Inset shows the series resistance for all the electrodes. The light illumination was 100 mW cm 2 and the electrolyte was 1 M NaOH.

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associated with one-step-annealed hematite photoanodes mediate the charge transfer. The electron donor densities (ND) were estimated using the slopes from the linear fits of Csc 2 vs. V (Fig. 6b). The intercept of linear fit to the voltage axis gives the flat band potential (Vfb) of hematite. The ND value for one-step-annealed hematite (8.812 1019 cm 3) obtained from the plot is two times higher than that of the two-step-annealed hematite (3.740 1019 cm 3). Consistent ND values in the range of 1018–1019 cm 3 were reported for high-temperature annealed hematite electrodes [4,12,26]. The enhanced ND in one-step-annealed hematite can be directly related to the relatively higher amount of diffused Sn (as evidenced by XPS) from the underlying FTO without deteriorating its conductivity. The higher ND value reduces the depletion (space charge) layer thickness of the hematite, consequently leading to more effective separation and transport of charge carriers suppressing the electron–hole recombination. The Vfb values for twoand one-step-annealed hematite were ca. 0.24 and 0.42 VRHE, respectively, suggesting that higher Sn doping in one-stepannealed hematite also promotes a significant anodic shift in Vfb compared to the two-step-annealed hematite. The higher value of Vfb may be associated with Sn enrichment in the hematite grain surface, which increases band-bending that in turn facilitates the charge separation process. The Vfb is ideally equal to the photocurrent onset potential (Vonset), but in practice it deviates from Vonset by an amount called an overpotential (to drive the watersplitting reaction). The greater shift in Vfb for two-step-annealed hematite can be related to increased surface trap resistance as evidenced from EIS, suggesting a high recombination rate and slow oxygen evolution kinetics at the hematite surface. This observation can be validated from the smaller difference (0.29 V) in Vfb and Vonset for one-step-annealed hematite compared to the two-step-annealed hematite (0.54 VRHE). To validate the hypothesis of higher ND value on account of enriched Sn diffusion, EIS study of FTO electrodes were carried out separately. Fig. 6c shows the representative Nyquist plots (real vs. imaginary impedance) of the FTO electrodes prepared by low-temperature, one-step and two-step annealing methods, measured at a bias potential of 1.23 VRHE under 1 sun illumination. The series resistance (Rs), which describes the total resistance from the contributions of FTO, electrical contacts, etc., was determined. The low-temperatureannealed (550 °C) hematite shows lowest value of Rs due to the fact that no loss of tin occurs because the Sn diffusion starts at annealing temperatures greater than 700 °C. Among the high temperature annealed FTO electrodes, one-step-annealed FTO (800 °C for 13.5 min) exhibited lowest Rs value of 40.10 Ω cm2, suggesting lower deterioration of conductivity in comparison to that annealed by the two-step method. By this analogy, we conclude that in the hematite–FTO system, the one-step annealing at 800 °C for 13.5 min, in contrast to other studied annealing conditions, maintains the required FTO conductivity despite higher amount of Sn leaching. Thus, direct one-step activation of hematite using as-grown Fe/FTO via dynamic Sn cations diffusion from FTO deformation can be an effective way to fabricate efficient photoanodes for water oxidation reactions.

4. Conclusions Hematite activation from FTO deformation at high temperature (viz. 800 °C) has been the choice of common practice to induce Sn doping in order to improve electronic conductivity and hence the PEC performance of hematite. This work introduces an effective way to exploit the dynamic diffusion of Sn ions leached during hematite activation from FTO deformation at 800 °C for 13.5 min by avoiding an intermediate low-temperature annealing step

commonly employed for conventional approach. Such activation allows dynamic diffusion of Sn cations while preserving the optimum conductivity of FTO necessary for efficient charge collection as well as maintains the highest structural ordering. In contrast, a pre-formed oxide layer at the interface of FTO during intermediate low-temperature annealing created interference for dynamic Sn diffusion. Moreover, these photoanodes suffer from the structural disordering during re-crystallization of pre-formed oxide layer and more loss of FTO conductivity possibly due to longer hours of processing time. Hence, the increased electron donor concentration due to controlled and dynamic Sn-diffusion doping in onestep activated hematite provides a plausible explanation for the enhanced photoactivity of our hematite photoanodes. In addition to improved photocurrent, much anticipated cathodic shift in onset potential of water oxidation reaction was observed without using any surface co-catalysts. This is an important finding of this study.

Acknowledgments This research was supported by BK Plus and the Basic Science Research Programs of the National Research Foundation of Korea (NRF, 2012R1A6A3A04038530). This subject is supported by Korea Ministry of Environment (2014000160001) as Public Technology Program based on Environmental Policy.

Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.solmat.2015.05. 020.

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