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Photoelectrochemical water splitting performance of vertically aligned hematite nanoflakes deposited on FTO by a hydrothermal method
Journal of Alloys and Compounds 608 (2014) 207–212
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Letter
Photoelectrochemical water splitting performance of vertically aligned hematite nanoflakes deposited on FTO by a hydrothermal method Ramesh Rajendran a,⇑, Zahira Yaakob a,b,⇑, Manoj Pudukudy a, Muhammad Syukri Abd Rahaman a, Kamaruzzaman Sopian b a b
Department of Chemical and Process Engineering, Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, UKM Bangi 43600, Selangor, Malaysia Solar Energy Research Institute (SERI), Universiti Kebangsaan Malaysia, UKM Bangi 43600, Selangor, Malaysia
a r t i c l e
i n f o
Article history: Received 26 March 2014 Received in revised form 15 April 2014 Accepted 16 April 2014 Available online 24 April 2014 Keywords: Nanostructured materials Chemical synthesis X-ray diffraction Photoelectrochemical measurement
a b s t r a c t In this report, vertically aligned hematite nanoflakes were deposited on fluorine-doped tin oxide (FTO) substrate by facile one step hydrothermal method for efficient photoelectrochemical water splitting. Further, the annealing effect on the PEC performance of a-Fe2O3 nanoflakes was studied. The a-Fe2O3 nanoflakes annealed at 300 °C yielded a high charge carrier density, lower flat band potential and enhanced photocurrent density of 2 mA/cm2 at 2.2 V vs RHE, which is higher than that of as-deposited and annealed at 600 °C hematite nanoflakes. The enhancement in photocurrent density is mainly attributed to the improved crystalline quality, high surface roughness and existence oxygen vacancy upon annealing process. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Global energy demand continuously has risen inexorably in the last ten decades connected with population growth and infinite industrial development [1]. Renewable solar energy is expected to be the fastest growing and a potential solution to the energy sustainability [2]. In this race solar driven water splitting has been much attracted because the natural abundance of solar and water resources [3]. Photoelectrochemical (PEC) process is a promising approach to convert solar energy to chemical energy through the splitting of water [4]. First solar driven water splitting performance demonstrated by Fujishima and Honda in 1972 through PEC approach using semiconductor based titanium dioxide (TiO2) as photoanode [5]. In the past decades, various metal oxides (TiO2, WO3, ZnO, etc.,) based nanostructured materials have been developed as a photoanode for PEC water splitting [6–12]. However, these materials can be operated in the UV light and account for only 4% of the incoming solar radiation [13]. Materials for PEC application must possess the certain characteristics such as suitable band gap energy to observe the sunlight with wavelength shorter than 560 nm, flat band potential and stability towards ⇑ Corresponding authors. Address: Department of Chemical and Process Engineering, Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, UKM Bangi 43600, Selangor, Malaysia (Z. Yaakob). Tel.: +60 173610697. E-mail addresses: [email protected] (R. Rajendran), zahirayaakob@gmail. com (Z. Yaakob). http://dx.doi.org/10.1016/j.jallcom.2014.04.105 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
photocorrosion [14]. Hematite (a-Fe2O3) is a potential material in PEC water splitting application due to its favorable optical band gap of 2.2 eV, excellent chemical stability in aqueous environments, ample abundance and hematite has a high theoretical solar-to-hydrogen conversion efficiency of 14–17% [15]. Although, Hematite possesses unfavorable optoelectronic parameters such as conduction band edge at an energy level below the reversible hydrogen potential so that external electrical bias is required to provide a hydrogen evolution at the cathode. In addition, hematite has a very short excited state lifetime (on the order of 10 ps) and poor minority charge carrier mobility (0.2 cm2 V1 s1) which results in a small hole diffusion length (2–4 nm) as compared to the light penetration depth (a1 = 118 nm at a wavelength of k=550 nm), poor kinetics for water oxidation and poor electrical conductivity, limits the use of hematite nanostructures in PEC water splitting [16,17]. Significant efforts have been directed to overcome these limitations such as metal doping, manipulation of morphology, loading of oxygen evolving catalysts and surface modification [18–20]. More importantly, the dimension control nanostructures have played an important role in enhancing the PEC water splitting process. Since the dimensionality will lead to significant influence on PEC performance, recently much effort focused on the development of 1D and 3D nanostructures such as nanowire, nanotubes and nanotube/nanorod arrays [21–25]. Such dimensions offer a short diffusion distance to reduce the recombination process of the photogenerated charge carriers, long optical path to increase light absorption coefficient and large
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surface area for reducing the carrier-scattering rate. However, there are few reports on the PEC water splitting performance of 2D hematite nanostructures such as nanoflakes and nanosheet [26]. Moreover, the PEC water splitting performance of hematite nanoflakes and nanosheet can be enhanced by controlling the thickness in atomic scale. Various approaches have been developed to synthesize a-Fe2O3 nanostructures on FTO substrate such as hydrothermal methods, spray pyrolysis, electrochemical deposition, atmospheric pressure chemical vapor deposition (APCVD), flame oxidation of iron foils and atomic layer deposition (ALD) [27–30]. Among these approaches hydrothermal method is relatively simple, low cost, requirement of non-toxic precursors, precise control of dimension and the potential of co-depositing dopant elements. In the present work vertically aligned ultra thin hematite flakes grown on an FTO by low cost solution based hydrothermal procedure with enhanced PEC performance. Further, the effect of annealing temperature on PEC performance of hematite nanoflakes was performed.
washed with acetone, ethanol and then deionized (DI) water using ultrasonic bath. In the typical experiments, ferric chloride (FeCl36H2O) and urea were dissolved in a mixture of distilled water and glycerol (3:1) solution with resistivity greater than 15 MX cm. This solution was poured into Teflon lined stainless steel autoclave containing a polycrystalline FTO glass substrate vertically placed and partially immerse into the solution. Then the autoclave was placed in a hot air oven at constant temperature 200 °C for 12 h in air. At the end of the reaction, the autoclave was left to cool to room temperature. Finally a-Fe2O3 deposited FTO substrate was taken out from the autoclave and washed in DI water in an ultrasonic bath to remove impurities and then dried in air at room temperature. Furthermore, the film was subjected to the annealing process for two different temperatures (300 and 600 °C) in a muffle furnace for 45 min. 2.2. Characterization The phases and crystalline quality of hematite films were identified by XRD (Rigaku DMax 200, using Cu Ka radiation) in the 2h range from 10° to 80° with a step scan of 0.02°/12 s. Optical absorbance spectra were measured using PerkinElmer UV WinLab 6.0.4.0738/Lambda35 1.27 spectrophotometer and all the date were corrected by deducting the baseline of the FTO substrate. Morphology and chemical composition of the samples were investigated by a field-emission scanning electron microscope (FE-SEM, S4800, Hitachi) equipped with an energy dispersive X-ray (EDX) analyzer. The surface morphology and roughness of the film were examined by the tapping mode NT-MDT atomic force microscopy.
2. Experimental procedure
2.3. Photoelectrochemical measurements
2.1. Deposition of vertically aligned a-Fe2O3 nanoflakes on FTO substrate
The steady sate current density (j–v) and electrochemical impedance spectroscopy (EIS) measurements of vertically aligned hematite thin films were investigated in a three-electrode configuration cell using an FRA equipped PGSTAT-30 from Metrohm AutoLab. Synthesized hematite thin film was used as a working electrode, an Ag/AgCl (3 M KCl) and Pt wire were used as a reference and counter electrode
A monodispersed vertically aligned a-Fe2O3 nanoflakes were deposited onto a fluorine-doped tin oxide glass (FTO, Nippon Sheet Glass Co Ltd, 15 O) substrate by a hydrothermal method. Before the deposition, the FTO glass substrate was
Fig. 1. Low and high magnified FE-SEM images of the (a) as-deposited, (b) annealed at 300 °C and (c) annealed at 600 °C samples.
R. Rajendran et al. / Journal of Alloys and Compounds 608 (2014) 207–212 respectively. The measurements were performed in 1.0 M NaOH (pH 13.6) electrolyte solution. A xenon lamp (150 W, XENON POWER SUPPLY XPS-150TM, PA, USA) was used as a solar simulator to irradiate quartz photoelectrochemical cell. The light intensity at place of working electrode was calibrated to be 100 mW/cm2 by using solar power meter (TES-1333, ZhongXuan Electronic Corp. Ltd. Shanghai). PEC measurements were performed on 0.5 cm2 area of working electrode with and without illumination of sunlight.
3. Results and discussion The a-Fe2O3 nanostructures was prepared by deposition of hematite nanostructures on conducting FTO substrate using facile hydrothermal methods and surface morphology was observed by using FE-SEM measurement. Fig. 1 shows the high and low magnified FE-SEM images of as-deposited and annealed (300 and 600 °C) a-Fe2O3 nanostructures. Nearly uniform sized vertically aligned flake-like top morphology was observed with typical thickness around10 nm and length around 150 nm for as-deposited sample. The diameter and length of the nanoflakes were not significantly changed during the thermal treatment at 300 °C while increasing the temperature to 600 °C the nanoflakes were agglomerated to from tightly packed bundles like structure with close boundaries (Fig. 1(b)). Moreover, it is observed that compared to as deposited a-Fe2O3 film the grain size was increased for the film
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annealed at 600 °C (Fig. 1(c)). The well separated grain boundaries of oxide nanostructured thin film enhance their physical properties. For example, Straumal et al. have reported that the improvement of physical properties of semiconducting ZnO thin film by developing the well separated grains and doping impurities [31,32]. The surface roughness also can also be significantly affected the PEC performance of the hematite nanoflakes. Surface roughness and microstructure of the a-Fe2O3 nanoflakes can be accurately determined using the atomic force microscopy (AFM). The three dimensional AFM images and surface roughness of asdeposited and of 300, 600 °C annealed a-Fe2O3 nanoflakes are shown in Fig. 2. The AFM analyses clearly demonstrate that the annealing alter the morphology of the film deposited on FTO. As shown in Fig. 2(a), (b) and (c), vertically aligned a-Fe2O3 nanoflakes shows a maximum grain size for annealed sample at 600 °C than other samples such as-deposited and annealed at 300 °C, which is in good agreement with the FE-SEM images. Moreover, surface roughness of all a-Fe2O3 nanoflakes was analyzed from topography AFM images using IA-P9 program presented in Fig. 2 along with corresponding three dimensional AFM images. High temperature annealing (600 °C) shows the small roughness than as-deposited and sample annealed at 300 °C (Fig. 2(c)). The XRD measurements were performed to determine the structural properties and crystalline quality of the hematite
Fig. 2. AFM images and corresponding roughens of the (a) as-deposited, (b) annealed at 300 °C and (c) annealed at 600 °C samples.
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a-Fe2O3 nanoflakes. Fig. 3(a), (b) and (c) shows the XRD pattern of as-deposited and annealed (at 300 and 600 °C) a-Fe2O3 nanoflakes. The a-Fe2O3 nanoflakes exhibited a rhombohedral structure of hematite phase consistent with the standards JCPDS No. 890598. All the diffraction patterns of hematite showed the strong (1 1 0) diffraction peak at 35.4° indicated the synthesized nanoflakes are highly oriented in the [1 1 0] direction on the FTO substrate. Moreover, the strong (1 1 0) diffraction peak intensity progressively increased with increasing annealing temperature that may be significantly enhanced the PEC performance of hematite nanostructures [33]. Optical properties of as-deposited and annealed a-Fe2O3 nanoflakes were studied by using UV–vis. Spectroscopy. The absorbance spectra (Fig. 4(a)) shows the absorption edge around 585 nm for as-deposited a-Fe2O3 nanoflakes, which is consistent with an absorption edge of the previously reported hematite based aFe2O3 nanosteructures. The shoulder peaks around 540 and 430 nm were originated from indirect Fe3+ d to d and direct O2 p to Fe3+ d transitions of hematite [17]. Moreover, the absorbance intensity is significantly increased with increasing annealing temperature could be attributed to increases in the crystalline size and change in the morphology [34]. The indirect band gap transition of as-deposited and annealed (300 °C and 600 °C) measured as 2.1 eV, 2.07 eV and 1.96 eV using Tauc plot derived from UV–vis absorbance spectra (Fig. 4(b)). The band gap was slightly decreased with increasing annealing temperature due to increase in crystalline order and a larger particle size. The effect of annealing temperature on PEC performance of vertically aligned hematite nanoflakes were studied in an electrochemical cell of three electrode system using AM 1.5 100mW/ cm2 simulated solar illumination. Applied potentials values were converted to a reversible hydrogen electrode (RHE) scale using the Nernst relationship
Fig. 4. (a) UV–vis absorbance spectra and (b) Tauc-plots for as deposited (i), annealed at 300 °C (ii) and annealed at 600 °C (iii) samples. Black used to find the optical band gap as an intercept.
ERHE ¼ EAg=AgCl þ 0:0591 pH þ E0Ag=AgCl The current density of the as synthesized and annealed
a-Fe2O3 nanoflakes were measured and shown in Fig. 5. It was found that as-deposited a -Fe2 O 3 nanoflakes showed the
Fig. 5. J–V curve of as-deposited, annealed at 300 °C and 600 °C samples during solar light illuminations and dark.
Fig. 3. XRD patterns of (a) as-deposited (b) annealed at 300 °C and (c) annealed at 600 °C samples.
photocurrent density about 1.62 mA/cm2 at 2.19 V vs RHE. When the a-Fe2O3 nanoflakes subjected to the annealing treatment at 300 °C, a photocurrent density was increased up to 2 mA/cm2 at 2.19 V vs RHE. Moreover, a photocurrent onset potential is decreased from 1.35 to 1.15 V vs RHE for a-Fe2O3 nanoflakes annealed at 300 °C. However, the photocurrent density was significantly decreased from 2 mA/cm2 to 1.85 mA/cm2 at 2.19 V vs RHE while increasing the annealing temperature up to 600 °C. The decreasing in the photocurrent density can be related to changes in the morphologies of nanostructures during annealing treatment on the FTO substrate. When a-Fe2O3 nanoflakes treated at 600 °C, vertically aligned nanoflakes onto the FTO
R. Rajendran et al. / Journal of Alloys and Compounds 608 (2014) 207–212
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Fig. 6. (a) Mott–Schottky plots under dark conditions at a frequency of 1 kHz for as-deposited, annealed at 300 °C, and 600 °C samples and (b) Nyquist plots under the dark and under solar light illumination at 1.6 V vs RHE of annealed sample at 300 °C.
substrate were agglomerated into the nano-blocks with less porosity and surface roughness. This agglomeration could compromise semiconductor/electrolyte interface reaction availability and hindering the transport of holes from the semiconductor to the electrolyte which increases the electron–hole recombination loss [35]. As a consequence, their photo-current density was significantly lower for water oxidation kinetics. Furthermore, the photocurrent density also decreased due increase in the resistance of the FTO substrate upon heating at 600 °C and decrease in surface roughness of the nanoflakes on FTO substrate [34]. The a-Fe2O3 nanoflakes with the high surface roughness has a higher surface area exposed and it is expected that more photoelectrochemical reaction due to the existence of more reaction sites for the photo generated charge carriers [36]. In order to study the role of annealing temperature on intrinsic electronic properties of a-Fe2O3 nanoflakes, the capacitance (C) was measured at the semiconductor/electrolyte junction for different electrode potentials (Vapp) at a constant frequency of 1 kHz. Mott-Schottky curves (1/C2 vs Vapp) of as-deposited and annealed a-Fe2O3 nanoflakes showed the positive slope, indicating that they are n-type semiconductors (Fig. 6(a)). Moreover, the slope of photoanode annealed at 600 °C is higher than that of annealed at 300 °C due to the decreased surface roughness and oxygen vacancy [37] and resulting in increase the PEC performance as shown in Fig. 6(a). The charge carrier density was estimated from slope determined from Mott-Schottky using equation as follows 1
at 300 °C shows a flat-band potential at 0.38 V vs. RHE and the a-Fe2O3 nanoflakes annealed at 600 °C shows a flat-band potential of 0.49 V vs RHE (Fig. 6(a)). Although, the flat band potential of the a-Fe2O3 nanoflakes annealed at 300 °C was shifted towards the cathodic side with respect to that of as-deposited and annealed at 600 °C due to the significant increase in donor density resulted from decrease of space charge region width at the semiconductor/ electrolyte [40]. The electrochemical impedance spectroscopy measurements were performed to study the kinetics of the oxidation process at the surface of a-Fe2O3 nanoflakesunder dark and solar light illumination. Fig. 6(b) shows the Nyquist plots of a-Fe2O3 nanoflakes annealed at 300 °C under dark and solar light illumination at an applied potential of 1.23 V vs RHE. The a-Fe2O3 nanoflakes exhibits one capacitance arc under dark condition attributed to the Faradaic charge transfer was the limiting the oxidation process at the a-Fe2O3 nanosteructures surface [33]. When the illumination of the solar light Nyquist plot shows the two clear capacitive arcs, one from higher (left) frequency side and another from lower (right) frequency side originated from the charge transfer resistance and mass transfer limitation respectively. Moreover, the capacitive arc shows much smaller radii due to the existence of smaller charge transfer resistance under illumination than those in the dark attributed to the photoexcited carriers increase the conductivity of the a-Fe2O3 film [33].
4. Conclusion
ND ¼ ð2=e0 e0 eÞ½dð1=C 2 ÞdV
where e0 is the electron charge, e the dielectric constant of a-Fe2O3, e0 the permittivity of a vacuum, ND the donor density, and V the potential applied at the electrode. With a dielectric constant (e) of 80 for hematite, ND was calculated as 1.42 1020, 1.8 1021 and 8.2 1020 cm3 for as-deposited a-Fe2O3 nanoflakes, annealed at 300 °C and 600 °C respectively. As compared to as-deposited sample, a donor density of sample annealed at 300 °C was increased from 1.42 1020 to 1.8 1021 cm3. This is attributed to increase in the concentration of oxygen vacancies, Fe2+ centers and hydrogen impurities during the thermal treatment, which is responsible to enhance the overall PEC performance of hematite based a-Fe2O3 nanoflakes [38]. Furthermore, the flat band potential was estimated at hematite/electrolyte interface from the extrapolation of Mott– Schottky plots. The 0.58 V vs RHE flatband potential was observed from as-deposited a-Fe2O3 nanoflakes, which is consistent with previously reported value [39]. The a-Fe2O3 nanoflakes annealed
In this letter, We have demonstrated the facile hydrothermal synthesis of vertically aligned a-Fe2O3 nanoflacks and their PEC performance were studied for different annealing temperature such as 300 °C and 600 °C. The sample annealed at 300 °C showed an increased carrier density and decreased flat-band potential with respect to their as-deposited nanoflakes and annealed at 600 °C results in the reduction of oxygen evolving potential (1.15 V vs RHE) and enhance the PEC performance. The enhancement of PEC performance of annealed at 300 °C is attributed due to increase in the crystallinity, roughness and oxygen vacancy which increase the water oxidation kinetics.
Acknowledgment This project is financed by Universiti Kebangsaan Malaysia under Grants FRGS/2/2013/ST01/UKM/01/1 and GUP-2013-063.
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Letter
Photoelectrochemical water splitting performance of vertically aligned hematite nanoflakes deposited on FTO by a hydrothermal method Ramesh Rajendran a,⇑, Zahira Yaakob a,b,⇑, Manoj Pudukudy a, Muhammad Syukri Abd Rahaman a, Kamaruzzaman Sopian b a b
Department of Chemical and Process Engineering, Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, UKM Bangi 43600, Selangor, Malaysia Solar Energy Research Institute (SERI), Universiti Kebangsaan Malaysia, UKM Bangi 43600, Selangor, Malaysia
a r t i c l e
i n f o
Article history: Received 26 March 2014 Received in revised form 15 April 2014 Accepted 16 April 2014 Available online 24 April 2014 Keywords: Nanostructured materials Chemical synthesis X-ray diffraction Photoelectrochemical measurement
a b s t r a c t In this report, vertically aligned hematite nanoflakes were deposited on fluorine-doped tin oxide (FTO) substrate by facile one step hydrothermal method for efficient photoelectrochemical water splitting. Further, the annealing effect on the PEC performance of a-Fe2O3 nanoflakes was studied. The a-Fe2O3 nanoflakes annealed at 300 °C yielded a high charge carrier density, lower flat band potential and enhanced photocurrent density of 2 mA/cm2 at 2.2 V vs RHE, which is higher than that of as-deposited and annealed at 600 °C hematite nanoflakes. The enhancement in photocurrent density is mainly attributed to the improved crystalline quality, high surface roughness and existence oxygen vacancy upon annealing process. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Global energy demand continuously has risen inexorably in the last ten decades connected with population growth and infinite industrial development [1]. Renewable solar energy is expected to be the fastest growing and a potential solution to the energy sustainability [2]. In this race solar driven water splitting has been much attracted because the natural abundance of solar and water resources [3]. Photoelectrochemical (PEC) process is a promising approach to convert solar energy to chemical energy through the splitting of water [4]. First solar driven water splitting performance demonstrated by Fujishima and Honda in 1972 through PEC approach using semiconductor based titanium dioxide (TiO2) as photoanode [5]. In the past decades, various metal oxides (TiO2, WO3, ZnO, etc.,) based nanostructured materials have been developed as a photoanode for PEC water splitting [6–12]. However, these materials can be operated in the UV light and account for only 4% of the incoming solar radiation [13]. Materials for PEC application must possess the certain characteristics such as suitable band gap energy to observe the sunlight with wavelength shorter than 560 nm, flat band potential and stability towards ⇑ Corresponding authors. Address: Department of Chemical and Process Engineering, Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, UKM Bangi 43600, Selangor, Malaysia (Z. Yaakob). Tel.: +60 173610697. E-mail addresses: [email protected] (R. Rajendran), zahirayaakob@gmail. com (Z. Yaakob). http://dx.doi.org/10.1016/j.jallcom.2014.04.105 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
photocorrosion [14]. Hematite (a-Fe2O3) is a potential material in PEC water splitting application due to its favorable optical band gap of 2.2 eV, excellent chemical stability in aqueous environments, ample abundance and hematite has a high theoretical solar-to-hydrogen conversion efficiency of 14–17% [15]. Although, Hematite possesses unfavorable optoelectronic parameters such as conduction band edge at an energy level below the reversible hydrogen potential so that external electrical bias is required to provide a hydrogen evolution at the cathode. In addition, hematite has a very short excited state lifetime (on the order of 10 ps) and poor minority charge carrier mobility (0.2 cm2 V1 s1) which results in a small hole diffusion length (2–4 nm) as compared to the light penetration depth (a1 = 118 nm at a wavelength of k=550 nm), poor kinetics for water oxidation and poor electrical conductivity, limits the use of hematite nanostructures in PEC water splitting [16,17]. Significant efforts have been directed to overcome these limitations such as metal doping, manipulation of morphology, loading of oxygen evolving catalysts and surface modification [18–20]. More importantly, the dimension control nanostructures have played an important role in enhancing the PEC water splitting process. Since the dimensionality will lead to significant influence on PEC performance, recently much effort focused on the development of 1D and 3D nanostructures such as nanowire, nanotubes and nanotube/nanorod arrays [21–25]. Such dimensions offer a short diffusion distance to reduce the recombination process of the photogenerated charge carriers, long optical path to increase light absorption coefficient and large
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surface area for reducing the carrier-scattering rate. However, there are few reports on the PEC water splitting performance of 2D hematite nanostructures such as nanoflakes and nanosheet [26]. Moreover, the PEC water splitting performance of hematite nanoflakes and nanosheet can be enhanced by controlling the thickness in atomic scale. Various approaches have been developed to synthesize a-Fe2O3 nanostructures on FTO substrate such as hydrothermal methods, spray pyrolysis, electrochemical deposition, atmospheric pressure chemical vapor deposition (APCVD), flame oxidation of iron foils and atomic layer deposition (ALD) [27–30]. Among these approaches hydrothermal method is relatively simple, low cost, requirement of non-toxic precursors, precise control of dimension and the potential of co-depositing dopant elements. In the present work vertically aligned ultra thin hematite flakes grown on an FTO by low cost solution based hydrothermal procedure with enhanced PEC performance. Further, the effect of annealing temperature on PEC performance of hematite nanoflakes was performed.
washed with acetone, ethanol and then deionized (DI) water using ultrasonic bath. In the typical experiments, ferric chloride (FeCl36H2O) and urea were dissolved in a mixture of distilled water and glycerol (3:1) solution with resistivity greater than 15 MX cm. This solution was poured into Teflon lined stainless steel autoclave containing a polycrystalline FTO glass substrate vertically placed and partially immerse into the solution. Then the autoclave was placed in a hot air oven at constant temperature 200 °C for 12 h in air. At the end of the reaction, the autoclave was left to cool to room temperature. Finally a-Fe2O3 deposited FTO substrate was taken out from the autoclave and washed in DI water in an ultrasonic bath to remove impurities and then dried in air at room temperature. Furthermore, the film was subjected to the annealing process for two different temperatures (300 and 600 °C) in a muffle furnace for 45 min. 2.2. Characterization The phases and crystalline quality of hematite films were identified by XRD (Rigaku DMax 200, using Cu Ka radiation) in the 2h range from 10° to 80° with a step scan of 0.02°/12 s. Optical absorbance spectra were measured using PerkinElmer UV WinLab 6.0.4.0738/Lambda35 1.27 spectrophotometer and all the date were corrected by deducting the baseline of the FTO substrate. Morphology and chemical composition of the samples were investigated by a field-emission scanning electron microscope (FE-SEM, S4800, Hitachi) equipped with an energy dispersive X-ray (EDX) analyzer. The surface morphology and roughness of the film were examined by the tapping mode NT-MDT atomic force microscopy.
2. Experimental procedure
2.3. Photoelectrochemical measurements
2.1. Deposition of vertically aligned a-Fe2O3 nanoflakes on FTO substrate
The steady sate current density (j–v) and electrochemical impedance spectroscopy (EIS) measurements of vertically aligned hematite thin films were investigated in a three-electrode configuration cell using an FRA equipped PGSTAT-30 from Metrohm AutoLab. Synthesized hematite thin film was used as a working electrode, an Ag/AgCl (3 M KCl) and Pt wire were used as a reference and counter electrode
A monodispersed vertically aligned a-Fe2O3 nanoflakes were deposited onto a fluorine-doped tin oxide glass (FTO, Nippon Sheet Glass Co Ltd, 15 O) substrate by a hydrothermal method. Before the deposition, the FTO glass substrate was
Fig. 1. Low and high magnified FE-SEM images of the (a) as-deposited, (b) annealed at 300 °C and (c) annealed at 600 °C samples.
R. Rajendran et al. / Journal of Alloys and Compounds 608 (2014) 207–212 respectively. The measurements were performed in 1.0 M NaOH (pH 13.6) electrolyte solution. A xenon lamp (150 W, XENON POWER SUPPLY XPS-150TM, PA, USA) was used as a solar simulator to irradiate quartz photoelectrochemical cell. The light intensity at place of working electrode was calibrated to be 100 mW/cm2 by using solar power meter (TES-1333, ZhongXuan Electronic Corp. Ltd. Shanghai). PEC measurements were performed on 0.5 cm2 area of working electrode with and without illumination of sunlight.
3. Results and discussion The a-Fe2O3 nanostructures was prepared by deposition of hematite nanostructures on conducting FTO substrate using facile hydrothermal methods and surface morphology was observed by using FE-SEM measurement. Fig. 1 shows the high and low magnified FE-SEM images of as-deposited and annealed (300 and 600 °C) a-Fe2O3 nanostructures. Nearly uniform sized vertically aligned flake-like top morphology was observed with typical thickness around10 nm and length around 150 nm for as-deposited sample. The diameter and length of the nanoflakes were not significantly changed during the thermal treatment at 300 °C while increasing the temperature to 600 °C the nanoflakes were agglomerated to from tightly packed bundles like structure with close boundaries (Fig. 1(b)). Moreover, it is observed that compared to as deposited a-Fe2O3 film the grain size was increased for the film
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annealed at 600 °C (Fig. 1(c)). The well separated grain boundaries of oxide nanostructured thin film enhance their physical properties. For example, Straumal et al. have reported that the improvement of physical properties of semiconducting ZnO thin film by developing the well separated grains and doping impurities [31,32]. The surface roughness also can also be significantly affected the PEC performance of the hematite nanoflakes. Surface roughness and microstructure of the a-Fe2O3 nanoflakes can be accurately determined using the atomic force microscopy (AFM). The three dimensional AFM images and surface roughness of asdeposited and of 300, 600 °C annealed a-Fe2O3 nanoflakes are shown in Fig. 2. The AFM analyses clearly demonstrate that the annealing alter the morphology of the film deposited on FTO. As shown in Fig. 2(a), (b) and (c), vertically aligned a-Fe2O3 nanoflakes shows a maximum grain size for annealed sample at 600 °C than other samples such as-deposited and annealed at 300 °C, which is in good agreement with the FE-SEM images. Moreover, surface roughness of all a-Fe2O3 nanoflakes was analyzed from topography AFM images using IA-P9 program presented in Fig. 2 along with corresponding three dimensional AFM images. High temperature annealing (600 °C) shows the small roughness than as-deposited and sample annealed at 300 °C (Fig. 2(c)). The XRD measurements were performed to determine the structural properties and crystalline quality of the hematite
Fig. 2. AFM images and corresponding roughens of the (a) as-deposited, (b) annealed at 300 °C and (c) annealed at 600 °C samples.
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a-Fe2O3 nanoflakes. Fig. 3(a), (b) and (c) shows the XRD pattern of as-deposited and annealed (at 300 and 600 °C) a-Fe2O3 nanoflakes. The a-Fe2O3 nanoflakes exhibited a rhombohedral structure of hematite phase consistent with the standards JCPDS No. 890598. All the diffraction patterns of hematite showed the strong (1 1 0) diffraction peak at 35.4° indicated the synthesized nanoflakes are highly oriented in the [1 1 0] direction on the FTO substrate. Moreover, the strong (1 1 0) diffraction peak intensity progressively increased with increasing annealing temperature that may be significantly enhanced the PEC performance of hematite nanostructures [33]. Optical properties of as-deposited and annealed a-Fe2O3 nanoflakes were studied by using UV–vis. Spectroscopy. The absorbance spectra (Fig. 4(a)) shows the absorption edge around 585 nm for as-deposited a-Fe2O3 nanoflakes, which is consistent with an absorption edge of the previously reported hematite based aFe2O3 nanosteructures. The shoulder peaks around 540 and 430 nm were originated from indirect Fe3+ d to d and direct O2 p to Fe3+ d transitions of hematite [17]. Moreover, the absorbance intensity is significantly increased with increasing annealing temperature could be attributed to increases in the crystalline size and change in the morphology [34]. The indirect band gap transition of as-deposited and annealed (300 °C and 600 °C) measured as 2.1 eV, 2.07 eV and 1.96 eV using Tauc plot derived from UV–vis absorbance spectra (Fig. 4(b)). The band gap was slightly decreased with increasing annealing temperature due to increase in crystalline order and a larger particle size. The effect of annealing temperature on PEC performance of vertically aligned hematite nanoflakes were studied in an electrochemical cell of three electrode system using AM 1.5 100mW/ cm2 simulated solar illumination. Applied potentials values were converted to a reversible hydrogen electrode (RHE) scale using the Nernst relationship
Fig. 4. (a) UV–vis absorbance spectra and (b) Tauc-plots for as deposited (i), annealed at 300 °C (ii) and annealed at 600 °C (iii) samples. Black used to find the optical band gap as an intercept.
ERHE ¼ EAg=AgCl þ 0:0591 pH þ E0Ag=AgCl The current density of the as synthesized and annealed
a-Fe2O3 nanoflakes were measured and shown in Fig. 5. It was found that as-deposited a -Fe2 O 3 nanoflakes showed the
Fig. 5. J–V curve of as-deposited, annealed at 300 °C and 600 °C samples during solar light illuminations and dark.
Fig. 3. XRD patterns of (a) as-deposited (b) annealed at 300 °C and (c) annealed at 600 °C samples.
photocurrent density about 1.62 mA/cm2 at 2.19 V vs RHE. When the a-Fe2O3 nanoflakes subjected to the annealing treatment at 300 °C, a photocurrent density was increased up to 2 mA/cm2 at 2.19 V vs RHE. Moreover, a photocurrent onset potential is decreased from 1.35 to 1.15 V vs RHE for a-Fe2O3 nanoflakes annealed at 300 °C. However, the photocurrent density was significantly decreased from 2 mA/cm2 to 1.85 mA/cm2 at 2.19 V vs RHE while increasing the annealing temperature up to 600 °C. The decreasing in the photocurrent density can be related to changes in the morphologies of nanostructures during annealing treatment on the FTO substrate. When a-Fe2O3 nanoflakes treated at 600 °C, vertically aligned nanoflakes onto the FTO
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Fig. 6. (a) Mott–Schottky plots under dark conditions at a frequency of 1 kHz for as-deposited, annealed at 300 °C, and 600 °C samples and (b) Nyquist plots under the dark and under solar light illumination at 1.6 V vs RHE of annealed sample at 300 °C.
substrate were agglomerated into the nano-blocks with less porosity and surface roughness. This agglomeration could compromise semiconductor/electrolyte interface reaction availability and hindering the transport of holes from the semiconductor to the electrolyte which increases the electron–hole recombination loss [35]. As a consequence, their photo-current density was significantly lower for water oxidation kinetics. Furthermore, the photocurrent density also decreased due increase in the resistance of the FTO substrate upon heating at 600 °C and decrease in surface roughness of the nanoflakes on FTO substrate [34]. The a-Fe2O3 nanoflakes with the high surface roughness has a higher surface area exposed and it is expected that more photoelectrochemical reaction due to the existence of more reaction sites for the photo generated charge carriers [36]. In order to study the role of annealing temperature on intrinsic electronic properties of a-Fe2O3 nanoflakes, the capacitance (C) was measured at the semiconductor/electrolyte junction for different electrode potentials (Vapp) at a constant frequency of 1 kHz. Mott-Schottky curves (1/C2 vs Vapp) of as-deposited and annealed a-Fe2O3 nanoflakes showed the positive slope, indicating that they are n-type semiconductors (Fig. 6(a)). Moreover, the slope of photoanode annealed at 600 °C is higher than that of annealed at 300 °C due to the decreased surface roughness and oxygen vacancy [37] and resulting in increase the PEC performance as shown in Fig. 6(a). The charge carrier density was estimated from slope determined from Mott-Schottky using equation as follows 1
at 300 °C shows a flat-band potential at 0.38 V vs. RHE and the a-Fe2O3 nanoflakes annealed at 600 °C shows a flat-band potential of 0.49 V vs RHE (Fig. 6(a)). Although, the flat band potential of the a-Fe2O3 nanoflakes annealed at 300 °C was shifted towards the cathodic side with respect to that of as-deposited and annealed at 600 °C due to the significant increase in donor density resulted from decrease of space charge region width at the semiconductor/ electrolyte [40]. The electrochemical impedance spectroscopy measurements were performed to study the kinetics of the oxidation process at the surface of a-Fe2O3 nanoflakesunder dark and solar light illumination. Fig. 6(b) shows the Nyquist plots of a-Fe2O3 nanoflakes annealed at 300 °C under dark and solar light illumination at an applied potential of 1.23 V vs RHE. The a-Fe2O3 nanoflakes exhibits one capacitance arc under dark condition attributed to the Faradaic charge transfer was the limiting the oxidation process at the a-Fe2O3 nanosteructures surface [33]. When the illumination of the solar light Nyquist plot shows the two clear capacitive arcs, one from higher (left) frequency side and another from lower (right) frequency side originated from the charge transfer resistance and mass transfer limitation respectively. Moreover, the capacitive arc shows much smaller radii due to the existence of smaller charge transfer resistance under illumination than those in the dark attributed to the photoexcited carriers increase the conductivity of the a-Fe2O3 film [33].
4. Conclusion
ND ¼ ð2=e0 e0 eÞ½dð1=C 2 ÞdV
where e0 is the electron charge, e the dielectric constant of a-Fe2O3, e0 the permittivity of a vacuum, ND the donor density, and V the potential applied at the electrode. With a dielectric constant (e) of 80 for hematite, ND was calculated as 1.42 1020, 1.8 1021 and 8.2 1020 cm3 for as-deposited a-Fe2O3 nanoflakes, annealed at 300 °C and 600 °C respectively. As compared to as-deposited sample, a donor density of sample annealed at 300 °C was increased from 1.42 1020 to 1.8 1021 cm3. This is attributed to increase in the concentration of oxygen vacancies, Fe2+ centers and hydrogen impurities during the thermal treatment, which is responsible to enhance the overall PEC performance of hematite based a-Fe2O3 nanoflakes [38]. Furthermore, the flat band potential was estimated at hematite/electrolyte interface from the extrapolation of Mott– Schottky plots. The 0.58 V vs RHE flatband potential was observed from as-deposited a-Fe2O3 nanoflakes, which is consistent with previously reported value [39]. The a-Fe2O3 nanoflakes annealed
In this letter, We have demonstrated the facile hydrothermal synthesis of vertically aligned a-Fe2O3 nanoflacks and their PEC performance were studied for different annealing temperature such as 300 °C and 600 °C. The sample annealed at 300 °C showed an increased carrier density and decreased flat-band potential with respect to their as-deposited nanoflakes and annealed at 600 °C results in the reduction of oxygen evolving potential (1.15 V vs RHE) and enhance the PEC performance. The enhancement of PEC performance of annealed at 300 °C is attributed due to increase in the crystallinity, roughness and oxygen vacancy which increase the water oxidation kinetics.
Acknowledgment This project is financed by Universiti Kebangsaan Malaysia under Grants FRGS/2/2013/ST01/UKM/01/1 and GUP-2013-063.
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