Enhanced Performance of Photoelectrochemical Water Splitting with

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Enhanced Performance of Photoelectrochemical Water Splitting with ITO@#-Fe2O3 Core-shell Nanowire Array as Photoanode Jie Yang, Chunxiong Bao, Tao Yu, Yingfei Hu, Wenjun Luo, Weidong Zhu, Gao Fu, Zhaosheng Li, Hao Gao, Faming Li, and Zhigang Zou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b07470 • Publication Date (Web): 13 Nov 2015 Downloaded from http://pubs.acs.org on November 18, 2015

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ACS Applied Materials & Interfaces

Enhanced Performance of Photoelectrochemical Water Splitting with ITO@α-Fe2O3 Core-shell Nanowire Array as Photoanode Jie Yang, †‡ Chunxiong Bao, †‡ Tao Yu,†ǁ* Yingfei Hu, § Wenjun Luo,⊥ Weidong Zhu, † Gao Fu,† Zhaosheng Li,§ Hao Gao,† Faming Li,† and Zhigang Zou†ǁ †

National Laboratory of Solid State Microstructures, Eco-Materials and Renewable Energy

Research Center (ERERC), Department of Physics, Nanjing University, Nanjing 210093, China ‖

Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing

210093, China. ⊥

Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM),

Jiangsu National Synergistic Innovation Center for Advanced Materials(SICAM), Nanjing Tech University (NanjingTech), 211816, Nanjing, China. §

‡

Collage of Engineering and Applied Science, Nanjing University, Nanjing 210093, China These authors contributed equally.

* Address correspondence to [email protected] KEYWORDS: nanowire array; ITO; α-Fe2O3; core-shell; photoanode; water splitting

ACS Paragon Plus Environment

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ABSTRACT Hematite (α-Fe2O3) is one of the most promising candidates for photoelectrodes in photoelectrochemical water splitting system. However, the low visible light absorption coefficient and short hole diffusion length of pure α-Fe2O3 limits the performance of α-Fe2O3 photoelectrodes in water splitting. Herein, in order to overcome these drawbacks, singlecrystalline tin-doped indium oxide (ITO) nanowire core and α-Fe2O3 nanocrystal shell (ITO@αFe2O3) electrodes were fabricated by covering the chemical vapor deposited ITO nanowire array with compact thin α-Fe2O3 nanocrystal film using chemical bath deposition (CBD) method. The J-V curves and IPCE of ITO@α-Fe2O3 core-shell nanowire array electrode showed nearly twice as high performance as those of the α-Fe2O3 on planar Pt coated silicon wafers (Pt/Si) and on planar ITO substrates, which was considered to be attributed to more efficient hole collection and more loading of α-Fe2O3 nanocrystals in the core-shell structure than planar structure. Electrochemical impedance spectra (EIS) characterization demonstrated a low interface resistance between α-Fe2O3 and ITO nanowire arrays, which benefits from the well contact between the core and shell. The stability test indicated that the prepared ITO@α-Fe2O3 core-shell nanowire array electrode was stable under AM1.5 illumination during the test period of 40000 s.

Introduction

Since the advent of the petroleum crisis in the 1970s, worldwide research has been focused on the conversion and storage of sustainable solar energy.1 Photoelectrochemical (PEC) water splitting has been considered as a promising technology to convert solar energy directly into chemical energy in the form of hydrogen and oxygen molecular bonds. 2,3 Among various photoanode materials, hematite (α-Fe2O3) is one of the most attractive candidates in water splitting for its chemical stability and suitable band gap of 2.0-2.2 eV as well


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as its high elements abundance.4-8 However, the experimental efficiency reported using α-Fe2O3 for water splitting is relatively low. It is mainly attributed to the inherent drawbacks presented in α-Fe2O3: low visible light absorption coefficient,9,10 short hole diffusion length of about a few nanometers,11,12 low electronic conductivity,9,13 and so on. Due to the low absorption coefficient, films as thick as 400-500 nm are required for complete light absorption.14-16 However, such thick film would significantly decrease the photo-generated hole collection efficiency via water oxidation at the semiconductor/electrolyte interface. Many strategies have been employed to solve the problem, such as cation doping to enhance electronic conductivity of α-Fe2O3,17-28 controlling the morphology to improve the surface area and ability of charge seperation,29-36 introducing high-surface-area and electron-conductive substrates as supports to provide short electron transfer pathway. The last strategy is supposed to be a general method which can address the conflict not only for α-Fe2O3 but also for other photoanode materials. To actualize the last strategy, the α-Fe2O3 films on the supports have to possess two characterizations as follows: (1) The α-Fe2O3 films should be thin, so that the hole in the films can be effectively transferred to the semiconductor/electrolyte interface. (2) The α-Fe2O3 film should be compact so that the recombination between the support and the electrolyte can be avoided. In recent years, much work has been reported using porous conductive support for α-Fe2O3 deposition. Metal oxides such as WO3,6 SnO237; some elements doped metal oxides such as Nb doped SnO2,38 Sb doped SnO2,39, 40, 41 Sn doped In2O3 (ITO),42 F doped SnO2,43, 44 and other conductive compounds such as TiSi245 have been used as support of α-Fe2O3 for electron collection. However, the conductivities of the reported conductive supports were comparably low. Atom layer deposition (ALD) and chemical vapor deposition (CVD) were generally used to prepare dense and thin αFe2O3 films for this strategy. However, both methods were expensive and complex. Some


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chemical methods such as electrodeposition and spin-coating could not prepare dense and thin film but stacked between the supports or plugged on the surface of the supports. In this work, conductive ITO nanowire arrays were introduced to the photoelectrochemical system. The ITO nanowire arrays as conductive supports grown on quartz substrates exhibited outstanding electronic conductivity (~4 Ω □-1, ~500 S cm-1), which was considered to be a suitable support that enabled efficient charge separation and improved the light absorption of the photoanode. Moreover, the resistance of the ITO substrates could keep low after calcination at 800 °C in air for 2 hours. Chemical bath deposition (CBD) method, a facile chemical method, was employed to deposit α-Fe2O3 nanoparticles on the ITO nanowire arrays. The supports were observed to be covered compactly with α-Fe2O3 thin film, which broadened the methods for compact α-Fe2O3 thin film preparation. The conductive ITO nanowire arrays and the compact αFe2O3 thin film formed a core-shell structure and more α-Fe2O3 nanoparticles were covered on the nanowire arrays than planar substrates. The ITO nanowire core and α-Fe2O3 nanocrystal shell (ITO@α-Fe2O3) electrodes without any cocatalyst and intentional doping showed higher photocurrent density than that deposited on the planar substrates. Although the ITO is not stable in acidic or alkaline environment, the compact and stable α-Fe2O3 can protect it from serious corrosion. In the stability test, the prepared ITO@α-Fe2O3 core-shell nanowire array electrode was stable under AM1.5 illumination during the test period of 40000 s.

Experimental Preparation of ITO nanowire arrays and planar ITO electrodes ITO nanowire arrays were deposited by CVD on quartz substrates (Figure 1a and Figure 1b). Firstly, the quartz substrates were cleaned by sequentially sonicating in water, ethanol and


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acetone, followed by drying under N2 flow. Then the quartz substrates were sputtered with about 10 nm thick gold nanoparticles. The ITO nanowire arrays were then deposited on the Au sputtered quartz substrates with indium and tin powder 10:1 as sources. The source was kept at 800 °C under a pressure of 50 Pa and an air flow of 1.0 sccm. The quartz substrates were placed on the right and left side of the source with the distance about 0.5 cm. Planar ITO were chemical vapor deposited on the quartz without gold sputtering. The source was composed of indium and tin powder in 10:1 and kept at 800 °C under a pressure of about 90 Pa and an air flow of 2.0 sccm. The growth time for ITO nanowire array and planar ITO were 1 h. Preparation of compact α-Fe2O3 on ITO nanowire array electrode and planar ITO electrode The compact α-Fe2O3 were deposited on the ITO nanowire arrays substrates via CBD method (Figure 1c and Figure 1d)). The precursor solution were prepared by mixing 0.02 M, 0.05 M and 0.08 M Fe(NO3)3·9H2O in deionized water. The ITO nanowire arrays substrates were put in the precursor solution and kept at 90 °C for 2 h. Then the Fe(OH)3 compact thin films grown on the ITO nanowire arrays were washed with deionized water thoroughly. To obtain α-Fe2O3 with better quality, the samples were calcinated in muffle furnace at 800 °C for 2 h. In the preparation process, 0.05 M Fe(NO3)3·9H2O in deionized water was the optimized concentration of the precursor solution for preparation of the most efficient ITO@α-Fe2O3 core-shell nanowire array electrode. The planar Pt/Si and ITO substrates were chemical bath deposited with α-Fe2O3 using the same preparation process for comparison to ITO@α-Fe2O3 core-shell nanowire array electrode. The thicknesses of the α-Fe2O3 films on Pt/Si varied with the different precursor solution concentrations.


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Photoelectrochemical property measurements The photoelectrochemical properties of the samples were tested in a three-electrode cell using an electrochemical analyzer (CHI-630D, Shanghai Chenhua) under simulated AM1.5 solar illumination. The electrolyte was 1 M NaOH aqueous solution (pH 13.6). The under tested αFe2O3 photoanode was used as a working electrode. A Pt wire and a saturated calomel electrode (SCE) were used as a counter and a reference electrode, respectively. A RHE potential was calculated following the formula: VRHE=VSCE+0.059pH+ESCE=VSCE+1.04. The film area exposed to the light was 0.28 cm2. The photocurrent of the samples was measured from the front side (electrolyte/α-Fe2O3 interface).

Figure 1. Scheme for preparation of ITO@α-Fe2O3 core-shell nanowire array electrodes. (a) Sputtering Au catalysts on quartz substrates. (b) Synthesis of ITO nanowire arrays by CVD method. (c) Coating Fe(OH)3 thin films on the ITO nanowire arrays via CBD method. (d) Calcinating the ITO@ Fe(OH)3 core-shell nanowire array to ITO@α-Fe2O3 core-shell nanowire array. Results and discussion


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Figure 2. (a) SEM image and (b) XRD spectrum of ITO nanowire arrays on quartz substrate. (c) EDX spectrum of ITO nanowire arrays. (d) Sheet resistance of ITO nanowire arrays after calcinated in air with different temperature for 2 hours. The morphology of the ITO nanowire arrays was investigated using scanning electron microscopy (SEM). A typical low magnification SEM image of the ITO nanowire arrays was presented in Figure 2a and a higher magnification SEM image was shown in the inset. It can be seen that the diameters of these nanowires are around 200 nm. The faceted shape and smooth side surfaces indicate the nanowires have good crystallinity, which also can be shown in the sharp peaks of the typical X-ray diffraction (XRD) spectrum (Figure 2b). High intensity of diffraction peak corresponding to (400) crystallographic planes indicates high orientation of the


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ITO nanowires. Energy dispersive X-ray spectrometry (EDX) reveals that the ITO nanowire arrays are mainly composed of In and Sn elements. As shown in the table inserted in Figure 2c, the average weight percentage of In and Sn is 94.37:5.63 and the atomic percentage is 94.54:5.46. High conductivity and high temperature tolerance of the ITO nanowire arrays is important when used as the electron collection layer in PEC. The relationship between the sheet resistances and calcination temperatures were plotted in Figure 2d. The as-prepared ITO nanowire array substrates possess low sheet resistance of about 4 Ω □-1, which is lower than that of commercial ITO substrates. Meanwhile, the sheet resistance can maintain low (~5.3 Ω □-1) after calcinated in air at 800 °C for 2 hours, which condition is significant for the high quality αFe2O3 fabrication.


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Figure 3. Full XPS spectrum (A) and fine XPS spectra of Fe 2p scan (b) and O 1s scan (c) for ITO@α-Fe2O3 core-shell nanowire array electrode. (d) Light absorption of pure ITO nanowire arrays electrode and ITO@α-Fe2O3 core-shell nanowire array electrode. After CBD treatment in Fe3+ aqueous solution and calcination at 800 °C for 2 hours, the ITO nanowire arrays were coated with a compact layer. The composition of the compact layer was characterized by x-ray photoelectron spectroscopy (XPS).46-48 According to the full XPS spectrum of the as-prepared electrode in Figure 3a, Sn 3d, In 3d, Fe 2p and O 1s were detected in the film. The Sn 3d and In 3d were not sure from the incorporation of dopant ions from the substrates or not, because the signal of the Sn 3d and In 3d were probably from the ITO core. The Fe 2p spectrum was shown in Figure 3b. The peak at 710.1 eV and 724 eV are corresponding to Fe 2p3/2 and Fe 2p1/2 respectively. A satellite peak at 719.5 eV indicates the presence of Fe3+, which is consistent with the value reported for α-Fe2O3. The peak at 529.2 eV in Figure 3c is assigned to the O2- ions in Fe2O3. The light absorption for the ITO@α-Fe2O3 core-shell nanowire array electrode and pure ITO substrate were characterized by UV- Vis spectrometry to study the absorption properties of the pristine ITO and the ITO@α-Fe2O3 core-shell nanowire arrays. As illustrated in the Figure 3d, the absorption edge of the pure ITO nanowire arrays is in the wavelength of about 350 nm. Whereas the ITO@α-Fe2O3 core-shell nanowire array electrode shows wider light absorption with sharp absorption edge at about 600 nm and significant absorption enhancement from 350 nm to 600 nm compared to pure ITO nanowire arrays. The absorption edge of the ITO@α-Fe2O3 core-shell nanowire array electrode is in accordance to the absorption edge of α-Fe2O3 reported in the literature.10


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Figure 4. SEM images of the ITO@α-Fe2O3 core-shell nanowire arrays prepared with different precursor concentrations (C1: 0.02 M (a), C2: 0.05 M (b), C3: 0.08 M (c)) for chemical bath deposition of α-Fe2O3. SEM images of the ITO@α-Fe2O3 core-shell nanowire array electrodes fabricated with different precursor solution concentrations were displayed in Figure 4. The precursor solution concentration for α-Fe2O3 deposition were varied as C1 (0.02 M), C2 (0.05 M) and C3 (0.08 M), respectively and the SEM images of the corresponding ITO@α-Fe2O3 core-shell nanowire array electrodes were shown from Figure 4a to Figure 4c. As illustrated from Figure 4a to Figure 4c, the amount of the α-Fe2O3 nanoparticles deposited on the ITO nanowire arrays increase with the increase of the precursor concentration. When the precursor concentration was 0.02 M, few αFe2O3 nanoparticles were deposited on the ITO nanowire arrays and many parts of the ITO nanowire was exposed to the outside, which would increase the possibility of charge recombination and decrease the light absorption of the α-Fe2O3 film. When the precursor concentration was 0.05 M, α-Fe2O3 nanoparticles were compactly covered on the ITO nanowire arrays. When the precursor concentration increased to 0.08 M, many α-Fe2O3 nanoparticles piled up and the thickness of the α-Fe2O3 film increased, which was not benefit for the hole collection from α-Fe2O3 to the electrolyte.


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Figure 5. SEM and TEM images of the synthesized ITO@α-Fe2O3 core-shell nanowire array electrode. (a) The SEM image of the ITO@α-Fe2O3 core-shell nanowire array in the top-down view with the higher-resolution image in the inset. (b) The SEM image of ITO@α-Fe2O3 coreshell nanowire array electrode in the cross-section view with the nanowire about 4.4 µm. (c) The TEM image of compact α-Fe2O3 nanoparticles deposited ITO nanowire arrays with the α-Fe2O3 film 34 nm thick. (d) The crystalline lattice of the ITO@α-Fe2O3 with the ITO single crystalline nature and α-Fe2O3 multi-crystalline nature. Figure 5 shows the SEM and TEM images of the optimized and compact α-Fe2O3 coated ITO nanowire arrays. Figure 5a presents the low and high (inset) resolution images of the ITO@αFe2O3 core-shell nanowire array electrode. From Figure 5a, we can see that the α-Fe2O3 nanoparticles deposited on the ITO nanowire arrays are compact and no α-Fe2O3 nanoparticles are stacked between the ITO nanowires. The cross-section SEM image in Figure 5b shows that


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the ITO nanowire is about 4.4 µm in length and possesses good orientation. Figure 5c is the TEM image of the ITO@α-Fe2O3 core-shell nanowire array electrode. From the TEM image, we can see that the α-Fe2O3 film is about 34 nm thick, which is approaching to the charge diffusion length, permitting effective electron collection and further reducing the recombination of the electron-hole pairs. Figure 5d presents a typical high-resolution TEM image of α-Fe2O3 coated ITO nanowire arrays. Clear crystal lattice fringes in both core and shell of nanowire indicate that they are well crystallized. The distance of lattice fringes in shell part is about 0.22 nm which is consistent with the interplanar spacing of (113) planes of α-Fe2O3, while the orientation of ITO nanowire according to the selected area electron diffraction (SEAD) patterns inset in the Figure 5d is along the (400) planes of In2O3. It also can be seen that even though the lattice of α-Fe2O3 nanocrystals displays mismatch with the ITO lattice, with the lattice mismatch of 12.7%, they linked with each other smoothly with no obvious impurity or amorphous layer, which indicates the α-Fe2O3 nanocrystals contact well with the ITO core. This should benefit the transfer of electrons from α-Fe2O3 to ITO. Figure 6a shows the J-V curves of the ITO@α-Fe2O3 core-shell nanowire array electrodes prepared with the precursor concentration C1, C2 and C3. As we can see in Figure 6a, the ITO@α-Fe2O3 core-shell nanowire array electrodes prepared with precursor concentration C2 showed the best performance with the current density at 1.23V vs. reversible hydrogen electrode (RHE) of about 1.1 mA cm-2. To study the performance of α-Fe2O3 as a function of the thicknesses, we choose comparably stable Pt/Si as planar substrates excluding the factor of substrates difference. J-V curves of α-Fe2O3 with different thickness on the Pt/Si were shown in Figure 6b, and the relationship between the thickness and the current density of the α-Fe2O3 on Pt/Si at 1.23V vs. RHE were displayed in Figure 6c. The thicknesses of the α-Fe2O3 films on


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Pt/Si were determined using a DEKTAK 150 Profilometer. SEM images of the α-Fe2O3 on Pt/Si with different film thicknesses were shown in Figure S2. According to Figure 6c, α-Fe2O3 on Pt/Si have a maximum current density of 0.56 mA cm-2 (at 1.23 V vs. RHE) when the thickness reach to around 250 nm. The photocurrent increases with the thickness of the α-Fe2O3 films from near 0 to around 250 nm, for the key impact factor of the photocurrent is the light absorption, and a thicker film can absorb the incident light more completely. When the thickness

Figure 6. (a) J-V curves of the ITO@α-Fe2O3 core-shell nanowire arrays prepared with different precursor concentrations (C1, C2, C3) for chemical bath deposition of α-Fe2O3. (b) J-V curves of the α-Fe2O3 on Pt/Si with different film thicknesses. (c) Current density of various α-Fe2O3 films on Pt/Si with different thickness. (d) Current-voltage curves of ITO@α-Fe2O3 core-shell nanowire array electrode, α-Fe2O3 on Pt/Si, α-Fe2O3 on planar ITO and pure ITO nanowire arrays.


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further increases, the current density reduces, because the key impact factor of the photocurrent for the thicker film is the hole collection efficiency. A thicker film has a lower hole collection efficiency. The optimized thickness of about 250 nm for α-Fe2O3 on Pt/Si at 1.23 V vs. RHE could just reach 0.56 mA cm-2, which demonstrated the limit of planar structure for α-Fe2O3 in water splitting. To overcome the drawbacks presented in α-Fe2O3, ITO@α-Fe2O3 core-shell nanowire array electrode were prepared as photoanodes for water splitting. The optimized ITO@α-Fe2O3 coreshell nanowire array electrode was used to be compared with the optimized α-Fe2O3 on Pt/Si and pure ITO nanowire arrays. From Figure 6d, we can see the pure ITO nanowire arrays show a photocurrent approximately of zero. However, with compact α-Fe2O3 nanoparticles decoration, the photocurrent significantly increases to about 1.1 mA cm-2 at 1.23 V vs. RHE. It is worth noting that the ITO@α-Fe2O3 core-shell nanowire array electrode was decorated without any cocatalyst and intentional doping. In order to further confirm that the enhancement of current density for ITO@α-Fe2O3 core-shell nanowire array electrode was attributed to the core-shell structure, current density of α-Fe2O3 on planar ITO was tested. The current density of the optimized α-Fe2O3 on planar ITO at 1.23 V vs. RHE reached 0.51 mA cm-2, which was approximately one half of that of ITO@α-Fe2O3 core-shell nanowire array electrode and a little smaller than that of α-Fe2O3 on Pt/Si. The small difference in current density of two planar αFe2O3 electrodes was probably attributed to the reabsorption of incident light due to the reflection of Pt/Si substrate. The ITO@α-Fe2O3 nanowire array electrode compared to planar αFe2O3 could address the trade-off between sufficient light harvesting and efficient hole colletion, therefore the optimized ITO@α-Fe2O3 nanowire array electrode showed better performance than planar α-Fe2O3. The absorption spectra of ITO@α-Fe2O3 core-shell nanowire arrays with


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different thickness and α-Fe2O3 on planar ITO were present in Figure S5. From Figure S5, we can see the light absorption of the ITO@α-Fe2O3 core-shell nanowire arrays increased with the increasing thickness of α-Fe2O3. The optimized ITO@α-Fe2O3 core-shell nanowire arrays possessed higher light absorption than the optimized α-Fe2O3 on planar ITO, for the higher light scattering of ITO nanowire arrays compared to planar ITO. The photocurrent density under chopped light has been given in Figure S6. The ITO@α-Fe2O3 core-shell nanowire arrays shorten the carrier collection pathway, so the recommendation of electron-hole was smaller than α-Fe2O3 on planar ITO. In order to further verify the photocurrent improvement, the incident photon-to-charge conversion efficiencies (IPCE) of the ITO@α-Fe2O3 core-shell nanowire array electrode and two planar α-Fe2O3 electrodes were measured as a function of wavelength. IPCE is expressed as IPCE = (1240·I)/ (λ·P), where I is the current density (mA cm-2), λ is the incident light wavelength (nm), and P is the power density of monochromatic light at a specific wavelength (mW cm-2). The IPCE curves at 1.23 V vs. RHE were given in Figure 7a. In comparison to two planar α-Fe2O3 electrodes, the ITO@α-Fe2O3 core-shell nanowire array electrode shows substantially enhanced IPCE values over the entire wavelength range of 300-610 nm, which is consistent with the J-V curves. Above 610 nm, the photoresponse of both the ITO@α-Fe2O3 core-shell nanowire array electrode and planar α-Fe2O3 drop to zero, which is in accordance with the band gap of α-Fe2O3. The observed IPCE enhancement of the ITO@α-Fe2O3 core-shell nanowire array electrode could be ascribed to enhanced light absorption as well as the efficient charge collection.


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Figure 7. (a) IPCE spectrum of ITO@α-Fe2O3 core-shell nanowire array electrode, α-Fe2O3 on Pt/Si and α-Fe2O3 on planar ITO at 1.23 V vs RHE. (b) Nyquist plots of the impedance measurements at different potential. The frequency varied from 105 Hz (far left) to 1 Hz (far right). Z’ and Z’’ are the real and imaginary parts of the impedance, respectively. The solid symbols plot the experimental data and the dash lines plot the fitting result with the inset equivalent circuit. (c) Different resistances from EIS data fitting plotted versus applied potential. (d) Mott-Schottky plot generated using the capacitance values derived from the EIS data fitting, from which a flatband potential of 0.34 V vs. RHE can be exacted. Because a low impedance across the ITO/α-Fe2O3 interface is critical for the performance improvement of the ITO@α-Fe2O3 core-shell structure, electrochemical impedance spectra (EIS) measurement was carried out to study the impedance of the ITO@α-Fe2O3 core-shell nanowire


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array electrode in a three-electrode configuration with the frequency range from 1 Hz to 105 Hz. The system was applied with a potential of 0.8 to 1.8 V vs. RHE in dark under steady condition. The Nyquist plots of the EIS measurement under different applied potential were shown in Figure 7b. Due to the large surface areas and high electrical conductive of the ITO nanowire arrays, the capacitance of the space charge region cannot be neglected. Therefore, an equivalent electrical circuit composed of 2 RC elements in series was used for the EIS data fitting (the inset of Figure 7b). In this model, the element RS is the series resistance, which includes the ITO resistance and the contact resistance of the test system. RSC represents the resistances of the electrical charge transfer in the α-Fe2O3 and the ITO/α-Fe2O3 interface, and RCT represents the resistance of the semiconductor-electrolyte charge transfer. The two capacitances CSC and CH represent the capacitance of the space charge in the semiconductor and the Helmholtz capacitance of the semiconductor/electrolyte interface, respectively. This equivalent electrical circuit can be fitted well to the experimental data (Figure 7b) and the fitted values of different resistances have been plotted versus applied potential in Figure 7c. RS is constant around 30 Ω in different applied potential, which confirms that RS represent the serials resistance and the equivalent circuit is reasonable. The values of RSC and RCT were fitted as around 50 and 5k Ω, respectively, which are much smaller than that of planar thin film49 and cauliflower-like Sidoped α-Fe2O3.50 The decrease of the resistances of our ITO@α-Fe2O3 core-shell nanowire array electrode can be ascribed to their high contact area with electrolyte, the ultra-thin α-Fe2O3 film and the efficient charge transfer between α-Fe2O3 and ITO. From the Figure 7c, it also can be seen that RSC and RCT drop steeply at about 1.5 to 1.7 V vs. RHE, which coincides with the onset of the water oxidation dark current (Figure 6d). The Helmholtz capacitance CH derived from the EIS data fitting decreased monotonically with increasing applied potential. We plotted the CH-2


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value vs. applied potential in Figure 7d and found that the CH-2 value increased linearly with increasing potential as described by the Mott-Schottky relation. Fitting the Mott-Schottky relation with the plot we can extract the flatband potential (Vfb) as 0.34 V vs. RHE, which can further confirm the equivalent circuit is reasonable. The Nyquist plots of the optimized ITO@αFe2O3 core-shell nanowire arrays in dark and under illumination were shown in Figure S7. From the figure, we can see the impedance decrease under illumination compared to that under dark, which origins from the carrier density increase when the semiconductor absorbs photons. The variations between the impedance under illumination and dark increase when the applied potential increase from 0.9 V to 1.4 V, which was consistent with the corresponding J-V curves in dark and under illumination. It is well known that ITO is not so stable in acidic or alkaline environment. Therefore the photocurrent stability is very important for the future practical application. Figure 8 shows the photocurrent vs. time curves of ITO@α-Fe2O3 core-shell nanowire array electrode at 1.23 V vs. RHE. The photocurrent in 40000 s showed no obvious decay in 1 M NaOH aqueous under AM1.5 illumination. This result demonstrated that the ITO@α-Fe2O3 core-shell nanowire array electrode is stable for the α-Fe2O3 nanoparticles were interfaced with ITO nanowire arrays compactly to resist corrosion of ITO in alkaline solution. O2 evolution from the optimized ITO@α-Fe2O3 core-shell nanowire array electrode under AM1.5 illumination was detected by using GC. The amount of O2 as a function of the time was studied and the curve was given in Figure S8. According to the detected O2 amount, about 75% of the faradaic efficiency was obtained.


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1.2 1.0 0.8

J/J0

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0.6 0.4 0.2 0.0 0

5000

10000 15000 20000 25000 30000 35000 40000

Time (Second)

Figure 8. Photocurrent of the ITO@α-Fe2O3 core-shell nanowire array electrode at the potential of 1.23 V vs. RHE under AM1.5 illumination for 40000 s. Conclusion In summary, conductive ITO nanowire arrays on quartz (about 4 Ω □-1) were introduced to the PEC system and the compact α-Fe2O3 thin films (about 34 nm) were coated on the ITO nanowire arrays by CBD method as shown in the SEM and TEM images. The EIS characterization demonstrated a low resistance across the interface between α-Fe2O3 and ITO nanowire arrays. The ITO@α-Fe2O3 core-shell nanowire array electrode showed better performance in photoelectrochemical water splitting than planar structure as illustrated in J-V curves and IPCE curves. The current density for ITO@α-Fe2O3 core-shell nanowire array electrode reached 1.1 mA cm-2 at 1.23 V vs. RHE without any cocatalyst, which was nearly twice higher than that of two planar α-Fe2O3 electrodes. The enhanced performance was considered to be attributed to efficient hole collection and higher light absorption in core-shell structure than planar structure. Photocurrent stability of ITO@α-Fe2O3 core-shell nanowire array electrode in 1 M NaOH


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aqueous under AM1.5 illumination was tested and showed no obvious decay during the test time of 40000 s, which indicated ITO@α-Fe2O3 core-shell nanowire array electrode a promising photoanode for future practical application in PEC water splitting. ASSOCIATED CONTENT Supporting Information. SEM image of chemical vapor deposited ITO nanowire arrays, αFe2O3 on Pt/Si with different thicknesses, planar ITO and α-Fe2O3 on planar ITO, J-V curves of the optimized ITO@α-Fe2O3 core-shell nanowire arrays, absorption spectra of ITO@α-Fe2O3 core-shell nanowire arrays, chopped light J-V curves measurements, Nyquist plots of the impedance measurements and the amount of O2 evolution per area under AM1.5 illumination for 200min and the Faradaic efficiency of O2 evolution. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Author Contributions ‡These authors contributed equally. Funding Sources This work was supported primarily by the National Natural Science Foundation of China (61377051 and 11174129), the National Basic Research Program of China (2013CB632404), the Science and Technology Research Program of Jiangsu Province (BK20130053) and the College Postgraduate Research and Innovation Project of Jiangsu Province (Grant KYZZ_0025), the


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Enhanced Performance of Photoelectrochemical Water Splitting with

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