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Effects of annealing temperature on the physicochemical, optical and photoelectrochemical properties of nanostructured hematite thin films prepared via electrodeposition method
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Effects of annealing temperature on the physicochemical, optical and photoelectrochemical properties of nanostructured hematite thin films prepared via electrodeposition method Yi Wen Phuan a , Meng Nan Chong a,b, * , Tao Zhu a , Siek-Ting Yong a , Eng Seng Chan a,b a
School of Engineering, Chemical Engineering Discipline, Monash University Malaysia, Jalan Lagoon Selatan, Bandar Sunway 46150 Selangor DE, Malaysia Sustainable Water Alliance, Advanced Engineering Platform, Monash University Malaysia, Jalan Lagoon Selatan, Bandar Sunway 46150 Selangor DE, Malaysia b
A R T I C L E I N F O
A B S T R A C T
Article history: Available online xxx
Hematite (a-Fe2O3) is a promising photoanode material for hydrogen production from photoelectrochemical (PEC) water splitting due to its wide abundance, narrow band-gap energy, efficient light absorption and high chemical stability under aqueous environment. The key challenge to the wider utilisation of nanostructured hematite-based photoanode in PEC water splitting, however, is limited by its low photo-assisted water oxidation caused by large overpotential in the nominal range of 0.5–0.6 V. The main aim of this study was to enhance the performance of hematite for photo-assisted water oxidation by optimising the annealing temperature used during the synthesis of nanostructured hematite thin films on fluorine-doped tin oxide (FTO)-based photoanodes prepared via the cathodic electrodeposition method. The resultant nanostructured hematite thin films were characterised using field emission-scanning electron microscopy (FE-SEM) coupled with energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), UV-visible spectroscopy and Fourier transform infrared spectroscopy (FTIR) for their elemental composition, average nanocrystallites size and morphology; phase and crystallinity; UV-absorptivity and band gap energy; and the functional groups, respectively. Results showed that the nanostructured hematite thin films possess good ordered nanocrystallites array and high crystallinity after annealing treatment at 400–600 C. FE-SEM images illustrated an increase in the average hematite nanocrystallites size from 65 nm to 95 nm when the annealing temperature was varied from 400 C to 600 C. As the crystallites size increases, the grain boundaries reduce and this suppresses the recombination rate of electron–hole pairs on the nanostructured hematite thin films. As a result, the measured photocurrent densities of nanostructured hematite thin films also increased. The highest measured photocurrent density of 1.6 mA/cm2 at 0.6 V vs Ag/AgCl in 1 M NaOH electrolyte was achieved for the nanostructured hematite thin film annealed at 600 C. This study had confirmed that strong interdependencies exist between the average hematite nanocrystallites size and grain boundaries with annealing temperature on the eventual PEC water splitting performance of nanostructured hematite thin films. The annealed hematite thin films at a higher temperature will enhance the nanocrystals growth and thus, suppressing the electron–hole pairs recombination rate, lowering the grain boundary resistance and enabling higher photocurrent flow at the molecular level. As a result, the photocurrent density and thus, the overall PEC water splitting performance of the nanostructured hematite thin films are significantly enhanced. ã 2014 Elsevier Ltd. All rights reserved.
Keywords: Thin films X-ray diffraction Optical properties Electrochemical properties Surface properties
1. Introduction
* Corresponding author at: School of Engineering, Chemical Engineering Discipline, Monash University Malaysia, Jalan Lagoon Selatan, Bandar Sunway 46150 Selangor DE, Malaysia. Tel.: +60 3 5516 1840; fax: +60 3 5514 6207. E-mail address: [email protected] (M.N. Chong).
Recently, the photoelectrochemical (PEC) water splitting process has received increasing attention as an effective means to convert solar energy into chemical energy to resolve the potential energy crisis caused by rapid fossil fuels depletion. Hydrogen produced via the PEC approach is highly attractive owing to its environmental benign with zero carbon footprint, in addition
http://dx.doi.org/10.1016/j.materresbull.2014.12.059 0025-5408/ ã 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Y.W. Phuan, et al., Effects of annealing temperature on the physicochemical, optical and photoelectrochemical properties of nanostructured hematite thin films prepared via electrodeposition method, Mater. Res. Bull. (2014), http://dx.doi.org/10.1016/j.materresbull.2014.12.059
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to being an efficient chemical energy carrier with high energy density [1,2]. In a typical PEC cell, it consists of a semiconductorbased photoanode that absorbs light photons to induce the formation of photogenerated electron–hole pairs. The photogenerated electron–hole pairs will further diffuse to the external photoanode surface to enable: (1) valence band holes react with water molecules to produce protons and oxygen gas and (2) conduction band electrons transfer to cathode through an external circuit and eventually reduce protons at cathode to produce hydrogen gas [3]. At present, however, the PEC process still exhibits low photo-assisted water oxidation efficiency for hydrogen production due to the high band gap energy requirement of semiconductors [4]. In order to find a suitable photoanode material, various semiconductor metal oxide photocatalysts such as titanium dioxide (TiO2) [5,6], zinc oxide (ZnO) [7], tungsten trioxide (WO3) [8], bismuth vanadate (BiVO4) [9], copper(I) oxide (Cu2O) [10] and iron(III) oxide (Fe2O3) [11] have been widely investigated for PEC water splitting application. Among the various metal oxide semiconductors, hematite (a-Fe2O3) appears to be a potential photoanode material for PEC water splitting owing to its narrower band gap energy level that can absorb up to 40% of solar irradiation. In addition, hematite is abundant, low-cost, highly stable under most of the aqueous environment and has a high theoretical solarto-hydrogen (STH) efficiency of 12.9% [12]. Despite the various advantages of hematite, the feasible application of hematite as the ideal photoanode material is still hindered by its low conductivity, short hole-diffusion length, fast electron–hole pairs recombination rate [13]. However, recent studies indicated that the nanostructured morphology is a solution to overcome these limitations and improve the overall PEC performance [14,15]. In this instance, nanostructured metal oxide photocatalysts offer a large semiconductor liquid junction wherein the redox reactions can occur to enable charge separation, in addition to minimise the diffusion length of minority carriers [16]. To date, a number of synthesis methods have been investigated for synthesising nanostructured hematite thin films such as colloidal method [17,18], hydrothermal method [19,20], spray pyrolysis method [21–24], atomic layer deposition (ALD) method [15,25], atmospheric pressure chemical vapor deposition (APCVD) method [26–28] and electrodeposition method [29–31]. Although, it is well-accepted from the open literatures that high performance nanostructured hematite thin films can be fabricated with almost all of the reported synthesis methods, most of them however are not suitable for large-scale fabrication due to the high production cost. Among all, the electrodeposition method is a promising and alternative method for the synthesis of nanostructured hematite thin films due to its simplicity, low-cost, ambient temperature and pressure processing conditions and the ability to control the crystallinity, phase composition as well as other physicochemical properties [30,32]. This study was concentrated on the fundamental understanding of nanostructured hematite thin films prepared by using the electrodeposition method. Previously, Tamboli et al. [33] found that the physical, chemical and PEC properties of nanostructured hematite thin films can be manipulated by simply tuning the electrodeposition synthesis condition. Thus, the aim of this work was to investigate the effects of annealing temperature on the physiochemical and PEC properties of nanostructured hematite thin films under visible light irradiation. The nanostructured hematite thin films were synthesised via cathodic electrodeposition approach followed by varying the annealing treatment in the range of 400–600 C. The resultant nanostructured hematite thin films were characterised using field emission-scanning electron microscopy (FE-SEM) coupled with energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), UV-visible
spectroscopy and Fourier transform infrared spectroscopy (FTIR). This work constitutes a preliminary study on the more fundamental approach toward nanostructured hematite-based photoanode in improving the overall PEC water splitting performance on hydrogen production. 2. Experimental 2.1. Materials Iron(III) chloride (FeCl3), potassium chloride (KCl), sodium fluoride (NaF) and hydrogen peroxide 30 wt% (H2O2) were purchased from HmbG Chemicals (USA). All of these chemicals were used without further purification. Fluorine-doped tin oxide (FTO) glass slides were supplied by ChemSoln (USA). The FTO glass slides were further cut into smaller 10 mm 25 mm pieces. The cut smaller FTO glass slides were cleaned with acetone and ethanol, followed by rinsing with deionised water for subsequent synthesis of nanostructured hematite thin films. 2.2. Synthesis of nanostructured hematite thin films The cathodic electrodeposition method was employed to synthesise nanostructured hematite thin films onto the surface of FTO glass slides. The precursor solution was made up of 5.0 mM FeCl3, 5.0 mM NaF, 0.1 M KCl and 1.0 M H2O2. Electrodeposition synthesis was performed using a conventional three-electrode electrochemical cell containing platinum (Pt) rod, Ag/AgCl saturated by 3 M KCl and FTO substrate as the counter, reference and working electrodes, respectively. The electrochemical cell was connected to an Autolab potentiostat that was used for regulating the electrodeposition synthesis conditions of nanostructured hematite thin films, as well as for other electrochemical-related measurements. The nanostructured hematite thin films were prepared by cyclic voltammetry process at a potential sweep rate of 0.1 V/s, from 0.5 V to 0 V for 100 cycles. During the electrodeposition process, the temperature of precursor solution was set constant at 21 C (room temperature). After the electrodeposition synthesis, the as-deposited amorphous iron oxy-hydroxide (FeOOH) thin films were washed with deionised water for several times, followed by annealing treatment at 400 C, 500 C and 600 C with a dwell time of 4 h. 2.3. Structural characterisations The polycrystalline structures of nanostructured hematite thin films samples were analysed by using XRD analysis with an X-ray powder diffractometer (Bruker D8 Discover) employing Cu Ka radiation with 40 kV and 100 mA at 0.02 /s scan rate. FE-SEM coupling with EDX (SU8010 model, Hitachi) was used to estimate the average hematite nanocrystallites size, as well as to examine the surface morphologies and chemical elemental compositions of the nanostructured hematite thin films, respectively. The FTIR-spectra of nanostructured hematite thin films samples were recorded using the Thermo Scientific Nicolet iS10 spectrophotometer. The optical properties of nanostructured hematite thin films were characterised by using the UV-visible spectrophotometer (Agilent Technologies Cary Series 100). 2.4. Photoelectrochemical characterisations The PEC characterisations of nanostructured hematite thin films on FTO (working electrode) were performed by using the same Autolab potentiostat setup with three-electrode electrochemical cell containing Pt rod and Ag/AgCl saturated by 3 M KCl as the counter and reference electrodes, respectively. The active
Please cite this article in press as: Y.W. Phuan, et al., Effects of annealing temperature on the physicochemical, optical and photoelectrochemical properties of nanostructured hematite thin films prepared via electrodeposition method, Mater. Res. Bull. (2014), http://dx.doi.org/10.1016/j.materresbull.2014.12.059
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surface area of nanostructured hematite thin films photoanodes was physically measured before the photocurrent measurements. During the photocurrent measurements, the electrodes were immersed in 1 M NaOH electrolyte solution. The nanostructured hematite thin films photoanodes were scanned from 0.5 V to 0.8 V vs Ag/AgCl at a scan rate of 0.1 V/s. The nanostructured hematite thin films samples were illuminated from backside using a 100 W halogen lamp (Philips) and at a lamp-to-sample distance of 10 cm. For the dark photocurrent measurements, the whole setup was set in a black box to ensure that no light reached the surface of nanostructured hematite thin films photoanodes. 3. Results and discussion 3.1. Structural analysis In order to transform the as-deposited amorphous FeOOH thin films into nanostructured hematite thin films, annealing treatment at elevated temperature was performed and optimised to yield the favourable physicochemical and PEC properties. In this study, the as-deposited amorphous FeOOH thin films were thermally annealed in the range of 400–600 C with a dwell time of 4 h. During the annealing treatment, it can be observed that the colour of thin films changes from yellowish-brown to complete-brown owing to the complete conversion of FeOOH to hematite. Fig. 1 shows the surface morphology of nanostructured hematite thin films synthesised at different annealing temperatures where after the annealing treatment, densely packed hexagonal-shaped hematite nanocrystallites were observed. As the annealing temperature increased, the average hematite nanocrystallites size increased from the initial 65 nm (at 400 C) to the final 95 nm (600 C). This is due to the migration of surface atoms that helps the incorporation of iron and oxygen atoms into the lattice sites and thus, the grains size increases [34]. Subsequently, the EDX analysis was performed in order to validate the elemental chemical composition in nanostructured hematite thin films, as well as for other potential chemical impurities and the over-exposure of underlying FTO surfaces. Based on the EDX analysis, it was observed that the nanostructured hematite thin films containing both the iron (Fe) and (O) elements. The theoretical atomic ratio of Fe:O based on the atomic percentage of Fe should be 0.2:0.3, but the atomic percentage of O element was significantly higher as compared to Fe element. This is because other metal oxide such as tin oxide (SnO2) from the FTO glass slide has contributed to the excess measurement of O element during the EDX analysis. Some other impurity elements such as sodium (Na) and carbon (C) were found alongside where, they were believed to be contributed by the contamination of the thin film samples. The effects of annealing temperature on the crystal structure of nanostructured hematite thin films were also analysed by using XRD, as shown in Fig. 2. The XRD patterns showed a weak hematite
3
diffraction pattern superimposed on a strong FTO background. In addition, the XRD analysis has proven that the nanostructured hematite thin films samples display the same diffraction peaks of which only hematite crystalline phase (2u = 24.14 (0 1 2), 33.15 (1 0 4), 35.61 (11 0), 39.28 (0 0 6), 40.86 (11 3), 43.52 (2 0 2), 49.48 (0 2 4), 54.09 (11 6), 56.15 (2 11), 57.59 (0 1 8), 62.45 (2 1 4), 63.99 (3 0 0) and 66.10 (1 2 8)) was present. It was also observed that the peak intensity of hematite increases when the annealing temperature was increased. Besides, the crystallinity of nanostructured hematite thin films increased from 71.9% to 79.3% with increasing annealing temperature from 400 C to 600 C. This was due to the increase in average hematite nanocrystallites size with increasing annealing temperature. The crystallinity of nanostructured hematite thin films will affect the amount of light being absorbed. [35]. Nanostructured hematite thin film with higher crystallinity will have better visible light absorption. Thus, in general, increasing the annealing temperature will enhance the crystallinity and improve the PEC performance of the nanostructured hematite thin films. This was followed by using the UV-visible spectrophotometry to investigate the optical properties of nanostructured hematite thin films. Fig. 3 shows the UV-visible spectra for the nanostructured hematite thin films that were thermally annealed at different temperatures. The absorption peaks at 340 nm, 400 nm and 490 nm observed in the nanostructured hematite thin films are consistent with the transitions reported in a previous study by Marusak et al. [36]. They attributed the absorption peaks by 6tlu#! 2t2g#, 3eg"! 3eg# and 2t2g"! 2t2g# ligand field transitions for interlevel transitions for lower energy peaks, as well as high energy absorption. The small differences in the observed optical densities of nanostructured hematite thin films may be due to the variation in the annealing temperatures used. In this instance, the onset wavelength was measured around 600 nm, which is in accordance with the band gap of hematite. It can be seen from the UV-visible spectra that the nanostructured hematite thin films showed an obvious red-shift in the band gap transition. Subsequently, the UV-visible spectra were used to estimate for the optical band gap (Eg) of the nanostructured hematite thin films. The relationship between the absorption coefficient (a) and the incident photon energy (hn) of semiconductor photocatalysts is given by the following equation: ðahnÞn ¼ Aðhn Eg Þ
(1)
where a is the absorption of the nanostructured hematite thin film, hn is the photon energy, n is a constant of either 0.5 or 2 for indirect or direct transition semiconductor, respectively, A is the absorption coefficient and Eg is the separation between the bottom of the conduction band and the top of the valance band. Fig. 4 shows the Tauc plot for direct band gap transition against band gap energy. When the annealing temperature was increased, the reduction in band gap energy from the initial 2.17 eV (at 400 C) to final 2.13 eV (at 600 C) was observed. The optical band gap measurements of
Fig. 1. FESEM images of nanostructured hematite thin films annealed at different temperatures of (a) 400 C, (b) 500 C and (c) 600 C.
Please cite this article in press as: Y.W. Phuan, et al., Effects of annealing temperature on the physicochemical, optical and photoelectrochemical properties of nanostructured hematite thin films prepared via electrodeposition method, Mater. Res. Bull. (2014), http://dx.doi.org/10.1016/j.materresbull.2014.12.059
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Fig. 2. XRD patterns of hematite films annealed at different temperatures where (a) bare FTO substrate, (b) 400 C, (c) 500 C and (d) 600 C. Peaks from FTO substrate are indicated by *.
Fig. 3. UV-visible spectra of nanostructured hematite thin films annealed at different temperatures.
0.01
400 °C 500 °C
(αhv)2
0.008
600 °C
0.006 0.004 0.002 0
1.8
1.9
2
2.1
2.2 hv (eV)
2.3
2.4
2.5
2.6
Fig. 4. Tauc-plots of nanostructured hematite thin films annealed at different temperatures.
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Fig. 5. FTIR spectra of nanostructured hematite thin films annealed at different temperatures.
nanostructured hematite thin films annealed at 400–600 C were consistent with the literature values of 1.9–2.2 eV [11]. To further investigate the chemical characteristics of nanostructured hematite thin films, FTIR spectra were obtained for different annealing temperatures as shown in Fig. 5. From the FTIR spectra, two distinct peaks between 700 cm1 and 900 cm1 region were observed. These peaks are ascribed to the stretching vibration of the Fe—O bond in hematite according to Ruan et al. [37]. As the annealing temperature increased, the peaks gradually augmented which imply the enhancement of crystallisation. The FTIR peaks at 1420 cm1 and 1650 cm1 correspond to the surface absorbed water [38], while the peak at 2249 cm1 corresponds to the Sn—O bond. All these peaks gradually diminished as the annealing temperature increased. The FTIR results obtained in this study coincide well with the XRD analysis where, both analyses showed the presence of hematite in the synthesised thin films. 3.2. Photoelectrochemical performance Fig. 6 shows the effects of annealing temperature on the PEC performance of nanostructured hematite thin films by measuring the photocurrent density outputs. Table 1 shows the measured photocurrent density outputs at constant potentials of 0.4 V, 0.5 V and 0.6 V against the Ag/AgCl reference electrode. Results showed that the measured photocurrent density output of nanostructured hematite thin film annealed at 400 C was the lowest at 0.0102 mA/ cm2 at 0.5 V vs Ag/AgCl. However, the PEC performance of
nanostructured hematite thin films exhibits an upward trend with the increase in annealing temperatures. Higher measured photocurrent density with the increase in annealing temperature was observed. The enhancement of photocurrent density at elevated annealing temperature was linked to the growth of hematite nanocrystals whereby the simultaneous crystallinity enhancement and the reduction of voids within the nanostructured hematite thin films were evidenced [39]. As evidenced from the FESEM images, the average hematite nanocrystallites size increases with annealing temperature that reduce the voids and thus, enabling non-obstructive, lower grain boundaries resistance and higher photocurrent flows at the molecular level. Glasscock et al. [40] explained that the grain boundaries will reduce when the average hematite nanocrystallites size increases and this will suppress the recombination rate of electron–hole pairs. As a result, the measured photocurrent densities of nanostructured hematite
Table 1 The photocurrent density values of nanostructured hematite thin films synthesized at different annealing temperatures with reference to the standard Ag/AgCl electrode. Annealing temperature ( C)
Photocurrent (mA/cm2) At 0.4 V
At 0.5 V
At 0.6 V
400 500 600
0.0015 0.3583 0.1613
0.0102 0.9016 0.9102
0.0637 0.9086 1.6331
Fig. 6. Photocurrent density vs potential of the nanostructured hematite thin films synthesized at different annealing temperatures.
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thin films will be improved since it is interrelated to the recombination rate of the electron–hole pairs. In this instance, the highest measured photocurrent density of 0.91 mA/cm2 at 0.5 V vs Ag/AgCl and reaching 1.6 mA/cm2 before the dark current flow starts at 0.6 V vs Ag/AgCl were achieved at the annealing temperature of 600 C with an onset potential of approximately 0.39 V vs Ag/AgCl.
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4. Conclusions Nanostructured hematite thin films were successfully synthesised via the cathodic electrodeposition method. It was observed that the variation in annealing temperature plays a critical role in determining the physicochemical, optical and PEC properties of the nanostructured hematite thin films. Fine tuning of the electrodeposition synthesis process parameters is a dominant route in enhancing the PEC performance of the nanostructured hematite thin films. The nanostructured hematite thin films annealed at 600 C shows the highest photocurrent densities of 0.91 mA/cm2 at 0.5 V vs Ag/AgCl and reaching 1.6 mA/cm2 before the dark current flow starts at 0.6 V vs Ag/AgCl with an onset potential of 0.39 V. Increasing the annealing temperature will increase the average hematite nanocrystallites size and reduce the voids in the nanostructured thin film formed. Thus, this will suppress the electron–hole pairs recombination rate, allow lower resistance and higher photocurrent flow at the molecular level. The synthesised nanostructured hematite thin films provide a potential application in PEC water splitting process for sustainably producing hydrogen as the next-generation renewable energy resource.
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Acknowledgements The authors are grateful to the financial support provided by the Fundamental Research Grant Scheme (FRGS) (Project Reference Code: FRGS/1/2014/SG06/MUSM/02/1) from Ministry of Education (MOE), Malaysia. Similar gratitude also goes to the Advanced Engineering Platform, Monash University Malaysia.
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Effects of annealing temperature on the physicochemical, optical and photoelectrochemical properties of nanostructured hematite thin films prepared via electrodeposition method Yi Wen Phuan a , Meng Nan Chong a,b, * , Tao Zhu a , Siek-Ting Yong a , Eng Seng Chan a,b a
School of Engineering, Chemical Engineering Discipline, Monash University Malaysia, Jalan Lagoon Selatan, Bandar Sunway 46150 Selangor DE, Malaysia Sustainable Water Alliance, Advanced Engineering Platform, Monash University Malaysia, Jalan Lagoon Selatan, Bandar Sunway 46150 Selangor DE, Malaysia b
A R T I C L E I N F O
A B S T R A C T
Article history: Available online xxx
Hematite (a-Fe2O3) is a promising photoanode material for hydrogen production from photoelectrochemical (PEC) water splitting due to its wide abundance, narrow band-gap energy, efficient light absorption and high chemical stability under aqueous environment. The key challenge to the wider utilisation of nanostructured hematite-based photoanode in PEC water splitting, however, is limited by its low photo-assisted water oxidation caused by large overpotential in the nominal range of 0.5–0.6 V. The main aim of this study was to enhance the performance of hematite for photo-assisted water oxidation by optimising the annealing temperature used during the synthesis of nanostructured hematite thin films on fluorine-doped tin oxide (FTO)-based photoanodes prepared via the cathodic electrodeposition method. The resultant nanostructured hematite thin films were characterised using field emission-scanning electron microscopy (FE-SEM) coupled with energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), UV-visible spectroscopy and Fourier transform infrared spectroscopy (FTIR) for their elemental composition, average nanocrystallites size and morphology; phase and crystallinity; UV-absorptivity and band gap energy; and the functional groups, respectively. Results showed that the nanostructured hematite thin films possess good ordered nanocrystallites array and high crystallinity after annealing treatment at 400–600 C. FE-SEM images illustrated an increase in the average hematite nanocrystallites size from 65 nm to 95 nm when the annealing temperature was varied from 400 C to 600 C. As the crystallites size increases, the grain boundaries reduce and this suppresses the recombination rate of electron–hole pairs on the nanostructured hematite thin films. As a result, the measured photocurrent densities of nanostructured hematite thin films also increased. The highest measured photocurrent density of 1.6 mA/cm2 at 0.6 V vs Ag/AgCl in 1 M NaOH electrolyte was achieved for the nanostructured hematite thin film annealed at 600 C. This study had confirmed that strong interdependencies exist between the average hematite nanocrystallites size and grain boundaries with annealing temperature on the eventual PEC water splitting performance of nanostructured hematite thin films. The annealed hematite thin films at a higher temperature will enhance the nanocrystals growth and thus, suppressing the electron–hole pairs recombination rate, lowering the grain boundary resistance and enabling higher photocurrent flow at the molecular level. As a result, the photocurrent density and thus, the overall PEC water splitting performance of the nanostructured hematite thin films are significantly enhanced. ã 2014 Elsevier Ltd. All rights reserved.
Keywords: Thin films X-ray diffraction Optical properties Electrochemical properties Surface properties
1. Introduction
* Corresponding author at: School of Engineering, Chemical Engineering Discipline, Monash University Malaysia, Jalan Lagoon Selatan, Bandar Sunway 46150 Selangor DE, Malaysia. Tel.: +60 3 5516 1840; fax: +60 3 5514 6207. E-mail address: [email protected] (M.N. Chong).
Recently, the photoelectrochemical (PEC) water splitting process has received increasing attention as an effective means to convert solar energy into chemical energy to resolve the potential energy crisis caused by rapid fossil fuels depletion. Hydrogen produced via the PEC approach is highly attractive owing to its environmental benign with zero carbon footprint, in addition
http://dx.doi.org/10.1016/j.materresbull.2014.12.059 0025-5408/ ã 2014 Elsevier Ltd. All rights reserved.
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to being an efficient chemical energy carrier with high energy density [1,2]. In a typical PEC cell, it consists of a semiconductorbased photoanode that absorbs light photons to induce the formation of photogenerated electron–hole pairs. The photogenerated electron–hole pairs will further diffuse to the external photoanode surface to enable: (1) valence band holes react with water molecules to produce protons and oxygen gas and (2) conduction band electrons transfer to cathode through an external circuit and eventually reduce protons at cathode to produce hydrogen gas [3]. At present, however, the PEC process still exhibits low photo-assisted water oxidation efficiency for hydrogen production due to the high band gap energy requirement of semiconductors [4]. In order to find a suitable photoanode material, various semiconductor metal oxide photocatalysts such as titanium dioxide (TiO2) [5,6], zinc oxide (ZnO) [7], tungsten trioxide (WO3) [8], bismuth vanadate (BiVO4) [9], copper(I) oxide (Cu2O) [10] and iron(III) oxide (Fe2O3) [11] have been widely investigated for PEC water splitting application. Among the various metal oxide semiconductors, hematite (a-Fe2O3) appears to be a potential photoanode material for PEC water splitting owing to its narrower band gap energy level that can absorb up to 40% of solar irradiation. In addition, hematite is abundant, low-cost, highly stable under most of the aqueous environment and has a high theoretical solarto-hydrogen (STH) efficiency of 12.9% [12]. Despite the various advantages of hematite, the feasible application of hematite as the ideal photoanode material is still hindered by its low conductivity, short hole-diffusion length, fast electron–hole pairs recombination rate [13]. However, recent studies indicated that the nanostructured morphology is a solution to overcome these limitations and improve the overall PEC performance [14,15]. In this instance, nanostructured metal oxide photocatalysts offer a large semiconductor liquid junction wherein the redox reactions can occur to enable charge separation, in addition to minimise the diffusion length of minority carriers [16]. To date, a number of synthesis methods have been investigated for synthesising nanostructured hematite thin films such as colloidal method [17,18], hydrothermal method [19,20], spray pyrolysis method [21–24], atomic layer deposition (ALD) method [15,25], atmospheric pressure chemical vapor deposition (APCVD) method [26–28] and electrodeposition method [29–31]. Although, it is well-accepted from the open literatures that high performance nanostructured hematite thin films can be fabricated with almost all of the reported synthesis methods, most of them however are not suitable for large-scale fabrication due to the high production cost. Among all, the electrodeposition method is a promising and alternative method for the synthesis of nanostructured hematite thin films due to its simplicity, low-cost, ambient temperature and pressure processing conditions and the ability to control the crystallinity, phase composition as well as other physicochemical properties [30,32]. This study was concentrated on the fundamental understanding of nanostructured hematite thin films prepared by using the electrodeposition method. Previously, Tamboli et al. [33] found that the physical, chemical and PEC properties of nanostructured hematite thin films can be manipulated by simply tuning the electrodeposition synthesis condition. Thus, the aim of this work was to investigate the effects of annealing temperature on the physiochemical and PEC properties of nanostructured hematite thin films under visible light irradiation. The nanostructured hematite thin films were synthesised via cathodic electrodeposition approach followed by varying the annealing treatment in the range of 400–600 C. The resultant nanostructured hematite thin films were characterised using field emission-scanning electron microscopy (FE-SEM) coupled with energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), UV-visible
spectroscopy and Fourier transform infrared spectroscopy (FTIR). This work constitutes a preliminary study on the more fundamental approach toward nanostructured hematite-based photoanode in improving the overall PEC water splitting performance on hydrogen production. 2. Experimental 2.1. Materials Iron(III) chloride (FeCl3), potassium chloride (KCl), sodium fluoride (NaF) and hydrogen peroxide 30 wt% (H2O2) were purchased from HmbG Chemicals (USA). All of these chemicals were used without further purification. Fluorine-doped tin oxide (FTO) glass slides were supplied by ChemSoln (USA). The FTO glass slides were further cut into smaller 10 mm 25 mm pieces. The cut smaller FTO glass slides were cleaned with acetone and ethanol, followed by rinsing with deionised water for subsequent synthesis of nanostructured hematite thin films. 2.2. Synthesis of nanostructured hematite thin films The cathodic electrodeposition method was employed to synthesise nanostructured hematite thin films onto the surface of FTO glass slides. The precursor solution was made up of 5.0 mM FeCl3, 5.0 mM NaF, 0.1 M KCl and 1.0 M H2O2. Electrodeposition synthesis was performed using a conventional three-electrode electrochemical cell containing platinum (Pt) rod, Ag/AgCl saturated by 3 M KCl and FTO substrate as the counter, reference and working electrodes, respectively. The electrochemical cell was connected to an Autolab potentiostat that was used for regulating the electrodeposition synthesis conditions of nanostructured hematite thin films, as well as for other electrochemical-related measurements. The nanostructured hematite thin films were prepared by cyclic voltammetry process at a potential sweep rate of 0.1 V/s, from 0.5 V to 0 V for 100 cycles. During the electrodeposition process, the temperature of precursor solution was set constant at 21 C (room temperature). After the electrodeposition synthesis, the as-deposited amorphous iron oxy-hydroxide (FeOOH) thin films were washed with deionised water for several times, followed by annealing treatment at 400 C, 500 C and 600 C with a dwell time of 4 h. 2.3. Structural characterisations The polycrystalline structures of nanostructured hematite thin films samples were analysed by using XRD analysis with an X-ray powder diffractometer (Bruker D8 Discover) employing Cu Ka radiation with 40 kV and 100 mA at 0.02 /s scan rate. FE-SEM coupling with EDX (SU8010 model, Hitachi) was used to estimate the average hematite nanocrystallites size, as well as to examine the surface morphologies and chemical elemental compositions of the nanostructured hematite thin films, respectively. The FTIR-spectra of nanostructured hematite thin films samples were recorded using the Thermo Scientific Nicolet iS10 spectrophotometer. The optical properties of nanostructured hematite thin films were characterised by using the UV-visible spectrophotometer (Agilent Technologies Cary Series 100). 2.4. Photoelectrochemical characterisations The PEC characterisations of nanostructured hematite thin films on FTO (working electrode) were performed by using the same Autolab potentiostat setup with three-electrode electrochemical cell containing Pt rod and Ag/AgCl saturated by 3 M KCl as the counter and reference electrodes, respectively. The active
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surface area of nanostructured hematite thin films photoanodes was physically measured before the photocurrent measurements. During the photocurrent measurements, the electrodes were immersed in 1 M NaOH electrolyte solution. The nanostructured hematite thin films photoanodes were scanned from 0.5 V to 0.8 V vs Ag/AgCl at a scan rate of 0.1 V/s. The nanostructured hematite thin films samples were illuminated from backside using a 100 W halogen lamp (Philips) and at a lamp-to-sample distance of 10 cm. For the dark photocurrent measurements, the whole setup was set in a black box to ensure that no light reached the surface of nanostructured hematite thin films photoanodes. 3. Results and discussion 3.1. Structural analysis In order to transform the as-deposited amorphous FeOOH thin films into nanostructured hematite thin films, annealing treatment at elevated temperature was performed and optimised to yield the favourable physicochemical and PEC properties. In this study, the as-deposited amorphous FeOOH thin films were thermally annealed in the range of 400–600 C with a dwell time of 4 h. During the annealing treatment, it can be observed that the colour of thin films changes from yellowish-brown to complete-brown owing to the complete conversion of FeOOH to hematite. Fig. 1 shows the surface morphology of nanostructured hematite thin films synthesised at different annealing temperatures where after the annealing treatment, densely packed hexagonal-shaped hematite nanocrystallites were observed. As the annealing temperature increased, the average hematite nanocrystallites size increased from the initial 65 nm (at 400 C) to the final 95 nm (600 C). This is due to the migration of surface atoms that helps the incorporation of iron and oxygen atoms into the lattice sites and thus, the grains size increases [34]. Subsequently, the EDX analysis was performed in order to validate the elemental chemical composition in nanostructured hematite thin films, as well as for other potential chemical impurities and the over-exposure of underlying FTO surfaces. Based on the EDX analysis, it was observed that the nanostructured hematite thin films containing both the iron (Fe) and (O) elements. The theoretical atomic ratio of Fe:O based on the atomic percentage of Fe should be 0.2:0.3, but the atomic percentage of O element was significantly higher as compared to Fe element. This is because other metal oxide such as tin oxide (SnO2) from the FTO glass slide has contributed to the excess measurement of O element during the EDX analysis. Some other impurity elements such as sodium (Na) and carbon (C) were found alongside where, they were believed to be contributed by the contamination of the thin film samples. The effects of annealing temperature on the crystal structure of nanostructured hematite thin films were also analysed by using XRD, as shown in Fig. 2. The XRD patterns showed a weak hematite
3
diffraction pattern superimposed on a strong FTO background. In addition, the XRD analysis has proven that the nanostructured hematite thin films samples display the same diffraction peaks of which only hematite crystalline phase (2u = 24.14 (0 1 2), 33.15 (1 0 4), 35.61 (11 0), 39.28 (0 0 6), 40.86 (11 3), 43.52 (2 0 2), 49.48 (0 2 4), 54.09 (11 6), 56.15 (2 11), 57.59 (0 1 8), 62.45 (2 1 4), 63.99 (3 0 0) and 66.10 (1 2 8)) was present. It was also observed that the peak intensity of hematite increases when the annealing temperature was increased. Besides, the crystallinity of nanostructured hematite thin films increased from 71.9% to 79.3% with increasing annealing temperature from 400 C to 600 C. This was due to the increase in average hematite nanocrystallites size with increasing annealing temperature. The crystallinity of nanostructured hematite thin films will affect the amount of light being absorbed. [35]. Nanostructured hematite thin film with higher crystallinity will have better visible light absorption. Thus, in general, increasing the annealing temperature will enhance the crystallinity and improve the PEC performance of the nanostructured hematite thin films. This was followed by using the UV-visible spectrophotometry to investigate the optical properties of nanostructured hematite thin films. Fig. 3 shows the UV-visible spectra for the nanostructured hematite thin films that were thermally annealed at different temperatures. The absorption peaks at 340 nm, 400 nm and 490 nm observed in the nanostructured hematite thin films are consistent with the transitions reported in a previous study by Marusak et al. [36]. They attributed the absorption peaks by 6tlu#! 2t2g#, 3eg"! 3eg# and 2t2g"! 2t2g# ligand field transitions for interlevel transitions for lower energy peaks, as well as high energy absorption. The small differences in the observed optical densities of nanostructured hematite thin films may be due to the variation in the annealing temperatures used. In this instance, the onset wavelength was measured around 600 nm, which is in accordance with the band gap of hematite. It can be seen from the UV-visible spectra that the nanostructured hematite thin films showed an obvious red-shift in the band gap transition. Subsequently, the UV-visible spectra were used to estimate for the optical band gap (Eg) of the nanostructured hematite thin films. The relationship between the absorption coefficient (a) and the incident photon energy (hn) of semiconductor photocatalysts is given by the following equation: ðahnÞn ¼ Aðhn Eg Þ
(1)
where a is the absorption of the nanostructured hematite thin film, hn is the photon energy, n is a constant of either 0.5 or 2 for indirect or direct transition semiconductor, respectively, A is the absorption coefficient and Eg is the separation between the bottom of the conduction band and the top of the valance band. Fig. 4 shows the Tauc plot for direct band gap transition against band gap energy. When the annealing temperature was increased, the reduction in band gap energy from the initial 2.17 eV (at 400 C) to final 2.13 eV (at 600 C) was observed. The optical band gap measurements of
Fig. 1. FESEM images of nanostructured hematite thin films annealed at different temperatures of (a) 400 C, (b) 500 C and (c) 600 C.
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Fig. 2. XRD patterns of hematite films annealed at different temperatures where (a) bare FTO substrate, (b) 400 C, (c) 500 C and (d) 600 C. Peaks from FTO substrate are indicated by *.
Fig. 3. UV-visible spectra of nanostructured hematite thin films annealed at different temperatures.
0.01
400 °C 500 °C
(αhv)2
0.008
600 °C
0.006 0.004 0.002 0
1.8
1.9
2
2.1
2.2 hv (eV)
2.3
2.4
2.5
2.6
Fig. 4. Tauc-plots of nanostructured hematite thin films annealed at different temperatures.
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Fig. 5. FTIR spectra of nanostructured hematite thin films annealed at different temperatures.
nanostructured hematite thin films annealed at 400–600 C were consistent with the literature values of 1.9–2.2 eV [11]. To further investigate the chemical characteristics of nanostructured hematite thin films, FTIR spectra were obtained for different annealing temperatures as shown in Fig. 5. From the FTIR spectra, two distinct peaks between 700 cm1 and 900 cm1 region were observed. These peaks are ascribed to the stretching vibration of the Fe—O bond in hematite according to Ruan et al. [37]. As the annealing temperature increased, the peaks gradually augmented which imply the enhancement of crystallisation. The FTIR peaks at 1420 cm1 and 1650 cm1 correspond to the surface absorbed water [38], while the peak at 2249 cm1 corresponds to the Sn—O bond. All these peaks gradually diminished as the annealing temperature increased. The FTIR results obtained in this study coincide well with the XRD analysis where, both analyses showed the presence of hematite in the synthesised thin films. 3.2. Photoelectrochemical performance Fig. 6 shows the effects of annealing temperature on the PEC performance of nanostructured hematite thin films by measuring the photocurrent density outputs. Table 1 shows the measured photocurrent density outputs at constant potentials of 0.4 V, 0.5 V and 0.6 V against the Ag/AgCl reference electrode. Results showed that the measured photocurrent density output of nanostructured hematite thin film annealed at 400 C was the lowest at 0.0102 mA/ cm2 at 0.5 V vs Ag/AgCl. However, the PEC performance of
nanostructured hematite thin films exhibits an upward trend with the increase in annealing temperatures. Higher measured photocurrent density with the increase in annealing temperature was observed. The enhancement of photocurrent density at elevated annealing temperature was linked to the growth of hematite nanocrystals whereby the simultaneous crystallinity enhancement and the reduction of voids within the nanostructured hematite thin films were evidenced [39]. As evidenced from the FESEM images, the average hematite nanocrystallites size increases with annealing temperature that reduce the voids and thus, enabling non-obstructive, lower grain boundaries resistance and higher photocurrent flows at the molecular level. Glasscock et al. [40] explained that the grain boundaries will reduce when the average hematite nanocrystallites size increases and this will suppress the recombination rate of electron–hole pairs. As a result, the measured photocurrent densities of nanostructured hematite
Table 1 The photocurrent density values of nanostructured hematite thin films synthesized at different annealing temperatures with reference to the standard Ag/AgCl electrode. Annealing temperature ( C)
Photocurrent (mA/cm2) At 0.4 V
At 0.5 V
At 0.6 V
400 500 600
0.0015 0.3583 0.1613
0.0102 0.9016 0.9102
0.0637 0.9086 1.6331
Fig. 6. Photocurrent density vs potential of the nanostructured hematite thin films synthesized at different annealing temperatures.
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thin films will be improved since it is interrelated to the recombination rate of the electron–hole pairs. In this instance, the highest measured photocurrent density of 0.91 mA/cm2 at 0.5 V vs Ag/AgCl and reaching 1.6 mA/cm2 before the dark current flow starts at 0.6 V vs Ag/AgCl were achieved at the annealing temperature of 600 C with an onset potential of approximately 0.39 V vs Ag/AgCl.
[10] [11] [12]
[13]
4. Conclusions Nanostructured hematite thin films were successfully synthesised via the cathodic electrodeposition method. It was observed that the variation in annealing temperature plays a critical role in determining the physicochemical, optical and PEC properties of the nanostructured hematite thin films. Fine tuning of the electrodeposition synthesis process parameters is a dominant route in enhancing the PEC performance of the nanostructured hematite thin films. The nanostructured hematite thin films annealed at 600 C shows the highest photocurrent densities of 0.91 mA/cm2 at 0.5 V vs Ag/AgCl and reaching 1.6 mA/cm2 before the dark current flow starts at 0.6 V vs Ag/AgCl with an onset potential of 0.39 V. Increasing the annealing temperature will increase the average hematite nanocrystallites size and reduce the voids in the nanostructured thin film formed. Thus, this will suppress the electron–hole pairs recombination rate, allow lower resistance and higher photocurrent flow at the molecular level. The synthesised nanostructured hematite thin films provide a potential application in PEC water splitting process for sustainably producing hydrogen as the next-generation renewable energy resource.
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Acknowledgements The authors are grateful to the financial support provided by the Fundamental Research Grant Scheme (FRGS) (Project Reference Code: FRGS/1/2014/SG06/MUSM/02/1) from Ministry of Education (MOE), Malaysia. Similar gratitude also goes to the Advanced Engineering Platform, Monash University Malaysia.
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Please cite this article in press as: Y.W. Phuan, et al., Effects of annealing temperature on the physicochemical, optical and photoelectrochemical properties of nanostructured hematite thin films prepared via electrodeposition method, Mater. Res. Bull. (2014), http://dx.doi.org/10.1016/j.materresbull.2014.12.059