Toward High-Performance Hematite Nanotube Photoanodes: Charge

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Toward High Performance Hematite Nanotube Photoanodes: Charge Transfer Engineering at Heterointerfaces Do Hong Kim, Dinsefa Mensur Andoshe, Young-Seok Shim, Cheon Woo Moon, Woonbae Sohn, Seokhoon Choi, Taemin Ludvic Kim, Migyoung Lee, Hoon Kee Park, Koo Tak Hong, Ki Chang Kwon, Jun Min Suh, Jin-Sang Kim, Jong-Heun Lee, and Ho Won Jang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b05366 • Publication Date (Web): 23 Aug 2016 Downloaded from http://pubs.acs.org on August 24, 2016

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ACS Applied Materials & Interfaces

Toward High Performance Hematite Nanotube Photoanodes: Charge Transfer Engineering at Heterointerfaces Do Hong Kim, †, ‡ Dinsefa M. Andoshe,† Young-Seok Shim,§ Cheon-Woo Moon,† Woonbae Sohn,† Seokhoon Choi,† Taemin Ludvic Kim,† Migyoung Lee,† Hoonkee Park,† Kootak Hong,† Ki Chang Kwon,† Jun Min Suh,† Jin-Sang Kim,§ Jong-Heun Lee,*,‡ and Ho Won Jang*,†

†Department

of Materials Science Engineering, Research Institute of Advanced Materials, Seoul

National University, Seoul 151-742, Korea ‡Department §Electronic

of Materials Science Engineering, Korea University, Seoul 136-713, Korea

Materials Research Center, Korea Institute of Science and Technology, Seoul 136-

791, Korea

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ABSTRACT Vertically ordered hematite nanotubes are considered to be promising photoactive materials for high performance water splitting photoanodes. However, the synthesis of hematite nanotubes directly on conducting substrates such as fluorine-doped tin oxide (FTO)/glass is difficult to be achieved because of the poor adhesion between hematite nanotubes and FTO/glass. Here, we report the synthesis of hematite nanotubes directly on FTO/glass substrate and high performance photoelectrochemical properties of the nanotubes with NiFe co-catalysts. The hematite nanotubes are synthesized by a simple electrochemical anodization method. The adhesion of the hematite nanotubes to the FTO/glass substrate is drastically improved by dipping them in nonpolar cyclohexane prior to post-annealing. Bare hematite nanotubes show a photocurrent density of 1.3 mA/cm2 at 1.23 V vs. a reversible hydrogen electrode, while hematite nanotubes with electrodeposited NiFe co-catalysts exhibit 2.1 mA/cm2 at 1.23 V which is the highest photocurrent density reported for hematite nanotubes based photoanodes for solar water splitting. Our work provides an efficient platform to obtain a high performance water splitting photoanodes utilizing earth-abundant hematite and noble-metal-free co-catalysts.

KEYWORDS: water splitting photoanode, hematite, nanotube, NiFe co-catalysts, earth abundant

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Introduction Hydrogen, as a clean and renewable energy source, is considered as one of the most promising candidates to replace fossil fuels. One of the efficient ways to produce hydrogen is solar water splitting using photoelectrochemical (PEC) cells in which sunlight-illuminated photoelectrodes generate electrical current to transform water into useful hydrogen.1 In the past decades, various metal oxide semiconductors such as SnO2, WO3, and TiO2 have been investigated as photoelectrodes for solar water splitting.2-4 Among various metal oxide photoelectrode materials, hematite (α-Fe2O3) is one of the most attractive photoanode material owing to the highest theoretical solar energy to hydrogen conversion efficiency, 15.3%, which have 2.2 eV energy band gap and more to the point, the cheapest and most abundant elements on the earth.5 Furthermore, it possesses noticeable properties such as non-toxicity, and excellent stability in alkaline condition.6-7 However, the PEC properties of hematite as a photoanode are restricted by important factors including poor electrical conductivity with the short hole diffusion length of 2−4 nm and short excited-state lifetime.8 Therefore, most of hematite photoanodes have been reported to show poor PEC properties due to the immoderate recombination in the photoelectrodes.8-9 To overcome these problems and to ameliorate the solar-to-hydrogen conversion efficiency, numerous studies have focused on designing hematite nanostructures and controlling of their electrical conductivity using doping material.10-13 Hematite nanostructures with large specific surface area and short minority carrier diffusion length are anticipated to be highly efficient in charge carrier generation and collection.14 Recently, one-dimensional (1D) hematite nanostructures such as nanorods, nanowires, and nanotubes, have been extensively studied for photoanode materials.15-17 However, a few problems still remain unsolved, such as production in large scale and low cost process of 3



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synthesizing one-dimensional hematite photoanodes. Vertically ordered hematite nanotubes obtained by electrochemical anodization are considered as a promising photoanode nanoarchitectures due to their large surface areas, easy charge transport for majority carriers, short diffusion length for minority carriers, and sufficient lengths to absorb incident light.18 In addition to nanostructuring, the use of co-catalysts can ameliorate the PEC performance of hematite. The PEC performances of hematite nanostructures could be improved by noble metal decoration such as Ru and Pt, since noble metal nanoparticles enhance oxygen evolution reaction (OER) at the surface and improve the electrical conductivity of hematite.19-20 Guo et al.21 reported a high performance photoanode using Ru-doped hematite nanorods and Kim et al.22 showed that Pt additives significantly enhance PEC properties of hematite nanorods. However, terribly high cost of the noble metal restricts large-scale application of hematite for solar water splitting photoanodes. Meanwhile, noble-metal-free co-catalysts have been reported to enhance PEC performance of photoanodes.23 NiFe, for example, has attracted attention owing to the high density of catalytically active sites for OER and its earth-abundance.24-25 In this paper, we report the synthesis and PEC properties of NiFe coated hematite nanotubes for high performance solar water splitting. A unique nanotubular structure on FTO/glass substrate is produced by electrochemical anodization and annealing in two-step at 550 oC and 800 oC in air. The delamination of hematite nanotubes from the FTO/glass due to poor adhesion is prevented by surface treatment using a nonpolar solvent. Bare hematite nanotubes show an excellent photocurrent density of 1.3 mA/cm2 at 1.23 V vs. a reversible hydrogen electrode (RHE). Furthermore, after electrodeposition of NiFe co-catalysts on the surface, hematite nanotubes exhibit the photocurrent density of 2.1 mA/cm2 at 1.23 V vs. RHE, corresponding to a Photon-tocurrent Conversion Efficiency (IPCE) of 14.8 % at wavelenght of 400 nm. 4


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EXPERIMENTAL DETAILS Synthesis Fe thin films used in this study were deposited on FTO/glass substrates at 400 oC by electron beam evaporator (Rocky Mountain Vacuum Tech). The base pressure, e-beam voltage, and current were 10-6 Torr, 7.3kV and 70 mA respectively, and deposition rate was about 1.0 Å/s. The thicknesses of the Fe film were 0.5, 1, 2 and 2.5 μm, respectively. Electrochemical anodization was performed in system with two electrodes of working and counter electrode at room temperature. The electrolyte for anodization was ethylene glycol (99.5 % purity, Junsei) solution containing 0.3 wt % of NH4F (99.5 % purity, Junsei) and 3 vol % water. Applied potential was 200 V, and anodization time was 1.5−10 min, resulting in doubled thickness compared with the the original Fe film thickness. After anodization, synthesized Fe2O3 nanotubes were rinsed using deionized water and cyclohexane, and then the samples were sintered at 550 oC in air atmosphere. After sintering, the films were converted to hematite phase of Fe2O3. Subsequently, the samples were sintered at 800 oC using rapid thermal annealing system. The details of experimental procedure about anodization method may be referred to our previous study.26 Electrodeposition was performed in a standard three electrode system with a Pt mesh as counter electrode, a Ag/AgCl/saturated NaCl as reference electrode, and hematite nanotubes on FTO/glass substrate as the working electrode. The electrolytes for electrodepostion were 0.1 M FeSO4 (99.5 % purity, Daejung) and 0.1 M NiSO4 (99.5 % purity, Daejung) solution. The potential was cycled between -0.5 and 0.5 V.

Characterization.

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The morphological and structural characterization of the nanotubes were conducted by Field Emission Scanning Electron Microscope (FESEM, ZEISS, MERLIN Compact) using an acceleration voltage of 15 kV. The X-ray Diffration (XRD, BRUKER MILLER Co.) and Transmission Electron Microscope (TEM, JEOL JEM 3000F) analysis were conducted for the crystallinity and phase characterization of the nanotubes. Chemical composition of the nanotubes was investigated using X-ray Photoelectron Spectroscopy (XPS, KRATOS AXIS-His), with a monochromatic Al-Kα (hν = 1486.58 eV) radiation, to obtain high quality core-level spectra. The high resolution specta were collected using pass energy of 100 eV with step energy of 50 meV, respectively. The peaks of the nanotubes were evaluated using XPS Peak processing (ver. 2.3.16), along with a Shirley background subtraction.

Photoelectrochemical Measurements The water splitting measurements were carried out in 1M NaOH electrolyte with a threeelectrode electrochemical system composed of the hematite nanotubes as the working electrode, Ag/AgCl/saturated NaCl as reference electrode, and a Pt mesh as the counter electrode. The photoanodes were illuminated from the front side using a solar simulator with AM 1.5 G filter and light source of solar simulator was calibrated to 1 sun (100 mV/cm2). The polarization curves were measured in the dark and under illumination with a scan rate of 10 mV/s. These measurements were conducted using a potentiostat (Ivium Technologies, Nstat). The IPCE measurements were measured with an irradiation source and monochromator (MonoRa150). The Electrochemical Impedance Spectroscopy (EIS) spectra were collected in frequency range of 100 kHz to 0.1 Hz with amplitude of 10 mV. The curves were fitted using the ZsimpWin program. Rs stands for series resistance, Cct for the Constant Phase Element (CPE) for between the electrolyte 6


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and electrode interface, and Rct for the charge transfer resistance at the interface of between electrode and electrolyte. Equivalent circuit parameter 1/Rct was also calculated from fitted curves EIS data. The potential was applied from 1.23 V to 1.53 V vs. RHE with an interval of 10 mV. The potentials vs. Ag/AgCl were converted to the RHE according to the Nernst equation:27

ERHE = E Ag/AgCl +EoAg/AgCl + 0.059*pH ----- (1)

Where ERHE is the converted potential vs. RHE, E°Ag/AgCl = 0.1976 V, and EAg/AgCl is the measured potentials against RHE and Ag/AgCl reference.

RESULTS AND DISCUSSION Figure 1a shows an illustration of our anodization method to form hematite nanotubes on FTO/glass used in this study. The hematite nanotubes were formed by anodization of Fe thin film on FTO/glass substrate in ethylene glycol containing 0.3 wt% NH4F solution and 2 vol% water at 200 V.26 The Fe film on FTO/glass substrate was changed to slightly reddish nanotubular structure after anodization as shown in Figure 1b and c. The synthesis of hematite nanotubes directly on FTO/glass substrate is hard pressed to accomplish due to the poor interfacial adhesion between nanotubes and FTO/glass substrate during annealing. It is well known that iron is converted to iron hydrides (Fe-H), after washed with deionized water due to the hydrogen ions dissolved in the iron. This process can reduce the toughness of hematite. Therefore, the iron hydrides in the bottom of the nanotubes would contribute to sufficiently enhanced brittleness of

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Figure 1. (a) A sketch illustrating our anodization method to form hematite nanotubes on FTO/glass substrate. (b) Photograph of Fe film on FTO/glass substrate before anodization. (c) Photograph of hematite nanotubes on FTO/glass substrate after anodization. (d) Photograph of hematite nanotubes on FTO/glass substrate after annealing (washed by deionized water). (e) Photograph of hematite nanotubes on FTO/glass substrate after annealing (washed by cyclohexane). (f) Cross-sectional SEM image of the hematite nanotubes (washed by deionized water). Inset: plain-view SEM image of hematite nanotubes on FTO/glass substrates. (g) High magnification SEM image of hematite nanotubes. (h) Cross-sectional SEM image of the hematite nanotubes (washed by cyclohexane). Inset: plain-view SEM image of hematite nanotubes on FTO/glass substrates. (i) High magnification SEM image of hematite nanotubes. the materials.28 Hydrogen ions are well known to be produced in the nanotubular structure during anodization. Therefore, the abundant hydrogen ions near the nanotubes can pass through the bottom of the nanotubes by the fluid pressure of electrolyte, and then permeate into the substrate. After washed with deionized water, hydrogen ions at the bottom of nanotubes can transform into iron hydrides, which results in enhancement of the brittleness of the hematite nanotubes. To overcome the severe brittleness of the nanotubes, Wang et al. used a solvent with a high solubitiy of H2 when treating nanotube films. Solvent with high solubility of H2 can enhance the interfacial adhesive force by removing H2 at the interface between nanotubes and the substrates, thereby reducing the peeling off of the nanotubes.29 The dipping of the synthesized hematite 8


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nanotubes into a nonpolar solvent retained the hydraulic pressure, which could not only keep the hematite nanotubes stable but also expedite the liberation of H2 into the solvent. Thus, cohesion of the hematite nanotubes can be retained after annealing with appropriate temperature by reducing H2 molecules at the interface. Therefore, we immersed in a solvent with low polarity, cyclohexane, and improved the adhesive force between hematite nanotubes and FTO/glass. Figure 1d shows the photograph of partially eliminated region of hematite when they are washed by deionized water and annealed at 550 °C. However, when washed by cyclohexane and annealed at 550 oC, they show clean surface of hematite nanotubes without any detached region as shown in Figure 1e. In addition, the color of the hematite nanotubes washed with deionized water becomes cloudy reddish due to poor interfacial adhesion between hematite nanotubes and FTO/glass after annealing. In contrast, the color of the hematite nanotubes washed with cyclohexane becomes dark-reddish color with good adhesion. The cross-sectional SEM images of hematite nanotubes washed by deionized water and their poor adhesion to the FTO/glass are presented in Figure 1f-g. Likewise, SEM image of hematite nanotubes treated by cyclohexane is depicted in figure 1h. The highly magnified SEM image of hematite nanotubes confirmed that Fe film is completely converted to nanotubes without peeling off. The synthesized hematite nanotubes were vertically ordered with consistent dimeters of ∼70 nm (Figure 1i). Also, we confirm that the morphology of the nanotubes is composed of open-end structure (Figure S3), and it is a indisputable proof for the transformation of Fe film to nanotubular structures. These images exhibit direct experimental evidence to assist that cyclohexane treatment enhanced the adhesion of hematite nanotube layer on FTO/glass substrate. It should be noted that the same annealing condition (550 oC) was used for both cases and there was already significant difference in adhesion before 800 oC annealing. Therefore, improvement in adhesion mainly stemmed from 9



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Figure 2 (a) Glancing angle X-ray diffraction patterns of a FTO/glass substrate, and Fe/FTO/glass substrate, and of a nanotubes on the substrate, and of a hematite nanotubes annealed at 550 oC for 2 hours. (b) Cross-sectional TEM image of hematite nanotubes for amorphous and (c) annealed at 550 oC. (d) Selected area diffraction pattern of hematite nanotubes. nonpolar solvent, cyclohexane, treatment. The XRD data of the Fe thin film and Fe2O3 nanotube films before and after sintering at 550 oC are shown in Figure 2a. The Fe peak was not found from synthesized nanotubes, which proves the perfect transformation of the Fe film to Fe2O3 nanotubes. After sintering, the nanotubes were converted to crystalline hematite phase without secondary phases. The cross-sectional TEM image in Figure 2b reveals that the hematite nanotubes are highly uniform with certain diameters. After sintering at 550 oC, the hematite 10


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Figure 3. (a) J-V curves of treated hematite nanotubes prepared with different thickness. (b) J-V curves of untreated 4-μm-thick nanotubes and treated hematite nanotubes. (c) Electrochemical impedance spectroscopy (EIS) analysis of untreated 4-μm-thick nanotubes and treated 4-μmthick nanotubes. Inset graph magnifies the 0-to-400 Ω cm2 results for better understanding. nanotubes have high crystallinity with preferred orientations as shown in Figure 2c. The Selected Area Electron Diffraction (SAED) pattern (Figure 2d) shows the polycrystalline nature of the hematite nanotubes after sintering at 550 oC. To improve the PEC performance of hematite, high temperature annealing is typically needed to promote Sn doping from FTO. It is widely known that the Sn doping in the hematite film by high temperature annealing (800 oC) enhances the donor density and electrical conductivity.30 Accordingly we performed a rapid thermal annealing process at about 800 oC to promote Sn activation into the hematite nanotubes.31 As a result, the 4-μm-thick hematite nanotubes with annealing in two steps at 550 oC and 800 oC exhibited a noticeable photocurrent density of 1.3 mA/cm2 at 1.23 V vs RHE, which is much higher than that of hematite nanotubes annealed at 550 oC as shown in Figure S1. Such an excellent PEC performance could originate from high carrier density of hematite with Sn doping upon high temperature annealing, resulting that the Sn4+ dopants substitute the Fe3+ sites of hematite lattice. Figure 3a displays the photocurrent densities of cyclohexane treated hematite nanotubes with different nanotube thicknesses. The cyclohexane treated 1-μm-thick hematite nanotubes showed a relatively low photocurrent density 11



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of 0.3 mA/cm2 at 1.23 V vs RHE. The photocurrent densities are improved as function of nanotube thickness from 1-μm to 4-μm. The extreme increase of photocurrent density for nanotubes with thickness of 4-μm indicates that increasing thickness of nanotubes provides sufficiently large area for the absorption of light into hematite nanotubes. However, the 5-μmthick hematite nanotubes showed lower photocurrent density than that of 4-μm-thick hematite nanotubes since the carrier diffusion path is much longer than their minority-carrier diffusion length. Therefore, we concluded that the 4-μm is the optimum thickness for improving the PEC performance of the hematite nanotubes. This result suggests a strong correlation between the thickness and photocurrent density of hematite nanotubes. Figure 3b shows the photocurrent densities of the 4-μm-thick hematite nanotubes washed with deionized water and cyclohexane treated hematite nanotubes annealed at 800 oC. Because of poor interfacial adhesion between the hematite nanotubes and the FTO/glass substrate, the hematite nanotubes washed with deionized water exhibit lower photocurrent density than that of cyclohexane treated hematite nanotubes. The cyclohexane treated 4-μm-thick hematite nanotubes exhibit an excellent photocurrent density of 1.3 mA/cm2 at 1.23 V vs. RHE resulting in the outstanding PEC performance for photoanodes based on pure hematite under this standard condition.5 The impedance spectra of hematite nanotubes were measured by applying 1.23 V vs. RHE under simulated solar illumination. A high photoactivity can be confirmed by a small semicircle in the electrochemical impedance plots shown in Figure 3c, composed of CPE for the hematite (CPE1), the NiFe cocatalysts (CPE2), series resistance from the wiring contact and the remaining resistive loss in the system (Rs), those from hematite nanotubes to the NiFe co-catalysts (Rct_1) and from NiFe cocatalysts to the electrolyte (Rct_2). The variation in the measured series resistance was slight, suggesting efficient charge transfer from wiring with FTO substrate. Even if the series resistance 12


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Figure 4(a) A sketch illustrating our electrodeposition method to form NiFe co-catalyst on hematite nanotubes (b) Cross-sectional SEM image of the hematite nanotubes and (c) after NiFe deposition. (d) Selected area diffraction pattern with indexing. (e) High-magnification TEM image of NiFe coated hematite nanotubes. (f-g) EDS element maps of (f) Fe (g) Ni for hematite nanotubes. (h) XPS spectra for hematite nanotubes. (i) Ni 2p XPS spectra for NiFe on hematite nanotubes. of treated hematite is only slightly lower than untreated one, the charge transfer resistances of 13



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treated and untreated hematite show significant difference. This result may stem from the relatively poor interfacial adhesion of untreated hematite nanotubes. Because the electron transfer from the surface of untreated hematite is hindered, it leads to electron-hole recombination and consequently results in the increase of Rct. The measured charge transfer resistances are listed in Table 1. The co-catalysts have been employed to enhance the PEC performance of metal oxide nanostructure photoanodes. In genernal, drop coating and physical vapor deposition (PVD) methods can be used to deposit a catalyst on surface of nanostructures. However, the results were not so impressive because these methods led to the coating of co-catalysts on only a part of the surface of the nanostructures.32 In this work, we coated noble-metal-free NiFe co-catalyst on the hematite nanotubes by using electrodeposition method, with similar procedure reported in the works of Talbot et al.33 The Figure 4a shows the schematic illustration of our electrodeposition method to form NiFe co-catalysts on hematite nanotubes. The electrodeposition was conducted in electrolyte containing 0.1 M NiSO4 and 0.1 M FeSO4, resulting in NiFe coated hematite nanotubes (Figure 4b-c). After electrodeposition, the NiFe coated hematite nanotubes were damaged because of the harsh chemical condition of low pH, but they still retained original structure of bare nanotubes. The NiFe co-catalysts were coated on hematite nanotubes with specific direction of NiOOH [110], [101], [121], Fe064Ni0.36 [200] as shown in SAED patterns (Figure 4d). According to TEM-EDS mapping in Figure 4e-g, NiFe co-catalysts is uniformly distributed over the hematite nanotubes. The film composition and quality of the NiFe coated hematite nanotubes were analyzed using XPS which is shown in Figure 4h. High resolution XPS spectra of the Fe 2p, Ni 2p core level were observed. The spectra reveal existence of NiFe catalysts on the synthesized nanotubes. (Figure 4i) 14


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Figure 5. (a) J-V curves of NiFe/hematite nanotubes prepared with different thickness. (b) J-V curves of 4-μm-thick nanotubes and NiFe/hematite nanotubes. (c) IPCE measurements of 4-μmthick nanotubes and NiFe coated hematite nanotubes (d) EIS analysis of 4-μm-thick nanotubes and NiFe coated hematite nanotubes. Inset graph magnifies the 0-to-100 Ω cm2 results for better understanding The J-V curves of NiFe coated hematite nanotubes as function of nanotube thicknesses are shown in Figure 5a. The 4-μm-thick hematite nanotubes with NiFe co-catalysts exhibit a excellent photocurrent density of 2.1 mA/cm2 at 1.23 V vs RHE, which is much higher than that of previously suggested photoanodes based on hematite nanostructures such as nanoporous hematite, nanoflakes, and nanorods (Table 2).16, 31, 34-39 The resulting excellent PEC performance 15



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Table 1. Fitted charge transfer resistance.

Photoanode

Rs(Ω cm2)

Rct_1 (Ω cm2)

Rct_2 (Ω cm2)

Untreated hematite NTs (4 μm)

14.33

14178.15

-

Treated hematite NTs (4 μm)

10.83

3250.38

-

NiFe/treated hematite NTs (4 μm )

9.05

204.908

0.0678

may originate from three factors that facilitate and yield an excellent charge transfer and lower recombination of photogenerated minority carriers at the anode. i) the highly ordered nanotubular structure with large surface area, ii) enhanced charge transfer due to the excellent adhesion of hematite nanotubes and FTO/glass substrate, and iii) high catalytic activity of noblemetal-free NiFe co-catalysts. Figure 5b shows photocurrent densities for the cyclohexane treated 4-μm-thick hematite nanotubes and that modified with NiFe co-catalysts. As compared with the bare hematite nanotubes, the NiFe coated hematite nanotubes show higher photoactivity due to excellent charge transfer by NiFe co-catalysts.40 The IPCE spectra of the NiFe coated hematite nanotubes are displayed in Figure 5c. Compared with the bare hematite nanotubes, the NiFe coated hematite nanotubes demostrated high IPCE values over the overall wavelength of 350– 600 nm. Particularly, the IPCE value at 400 nm was enhanced from 7.3 % to 14.8 % at 1.23 V vs RHE, after the NiFe coating. On the other hand, the IPCE values significantly decrease to zero when the wavelengths are longer than 550 nm, which is a exact agreement with hematite band gap (2.2 eV). The impedance spectra of NiFe coated hematite nanotubes measured by applying 1.23 V vs RHE, show the photoactivities of the NiFe co-catalysts. Fitted small semicircle by in the Nyquist plots shows the high photoactivity as shown in Figure 5d. The measured data were 16


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Table 2. Photocurrent density of photoanode materials based on hematite, as reported in the literature and the present study.

Photoanode materials

Method

Electrolyte

Photocurrent density (mA/cm2) at 1.23 V

Ref.

NiFe/ α-Fe2O3 NTs

Anodization

NaOH

2.1

In this study

α-Fe2O3 NTs

AAO templating method

NaOH

0.5

34

Porous α-Fe2O3

Solution process

NaOH

1.1

35

Sn doped α-Fe2O3

Hydrothermal

NaOH

1.86

31

α-Fe2O3 nanorods

Hydrothermal

NaOH

0.52

16

Ti doped α-Fe2O3

Hydrothermal

NaOH

0.8

36

Pt doped α-Fe2O3

PECVD

NaOH

0.63

37

Au/α-Fe2O3 nanoflakes

Thermal annealing

KOH

1.0

38

Co3O4 decorated α-Fe2O3 nanorods

Hydrothermal

NaOH

1.2

39

analyzed by constructing equivalent circuit as shown in Figure 5d inset. The Rct value for the NiFe coated hematite nanotubes is lower than that for the bare hematite nanotubes. Since the small Rct means fast charge transfer, the above results clearly indicate that the interfacial charge transfer is dramatically accelerated upon increasing the potential (Figure S2). The obtained Rct value for NiFe coated hematite nanotubes is in exact correspondence with the resulted excellent PEC performance. The PEC stability of NiFe coated hematite nanotubes was assessed over 20,000 sec of PEC operation under an applied voltage of 1.23 V vs. RHE with the simulated AM 1.5 G solar illumination at 100 mW/cm2 (Figure S4a). Also, the electrochemical stability of the nanotubes was evaluated during a period of 20,000 sec under an applied voltage of 1.6 V vs. RHE in the dark (Figure S4b). During the test, the NiFe coated hematite nanotubes showed little degradation during long-term operation, and these results verify that the NiFe is catalytically stable and mechanically robust. 17



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CONCLUSION We have successfully synthesized vertically ordered NiFe coated hematite nanotubes on FTO/glass substrates by electrochemical anodization and electrodepostion method. We treated bare hematite nanotube with cyclohexane to improve adhesion to FTO/glass, leading to extremely high photocurrent densities due to enhanced charge transfer of photogenerated minority carriers. Furthermore, NiFe coated hematite nanotube photoanode increased the carrier density to enhance IPCE compared to bare hematite nanotubes at visible wavelength of 400 nm from 7.3 % to 14.8 % at 1.23V vs RHE. We believe that the NiFe coated hematite nanotube photoanodes are very suitable for high performance photoanode materials for solar hydrogen generation.

ACKNOWLEDGEMENTS This work was supported by Samsung Research Funding Center of Samsung Electronics.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected]

ASSOCIATED CONTENT Supporting Information Available: Detailed information about the morphologies of the hematite nanotubes, effects of two-step annealing on electrochemical properties, EIS data, and stability

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results of NiFe coated hematite nanotubes. This material is available free of charge via the Internet at http://pubs.acs.org.

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Table of Contents Graphic

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Toward High-Performance Hematite Nanotube Photoanodes: Charge

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