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Investigation of the structural, optical, and photoelectrochemical properties of α-Fe2O3 nanorods synthesized via a facile chemical bath deposition
Optik - International Journal for Light and Electron Optics 200 (2020) 163454
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Original research article
Investigation of the structural, optical, and photoelectrochemical properties of α-Fe2O3 nanorods synthesized via a facile chemical bath deposition
T
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Gul Rahmana, , Zainab Najafa, Anwar ul Haq Ali Shaha, Shabeer Ahmad Mianb a b
Institute of Chemical Sciences, University of Peshawar, 25120, Peshawar, Pakistan Department of Physics, University of Peshawar, 25120, Peshawar, Pakistan
A R T IC LE I N F O
ABS TRA CT
Keywords: Chemical bath deposition Optical properties Water splitting Growth temperature Photoelectrode Photoelectrochemical cell
In this study, a simple and cost-effective chemical bath deposition was employed to deposit nansized hematite (α-Fe2O3) directly on fluorine-doped tin oxide (FTO) substrates. The structure, morphology, and optical properties of α-Fe2O3 were manipulated by adjusting the experimental parameters, particularly the deposition temperature and precursor concentration. With increase in Fe precursor concentration from 0.1 M to 0.3 M, the surface morphology was changed from granular-shape condensed particles to vertically aligned rod-like structures. XRD spectra indicated signals from FTO substrate and pure α-Fe2O3 phase which corresponds to the rhombohedral system. The XPS analysis revealed the presence of α-Fe2O3 in the film and no tin (Sn) impurities from the FTO substrate. By changing the deposition temperature from 60 ℃ to 90 ℃, the size of particles increased; whereas, the surface morphology was not altered. The resultant films were examined as photoanode for photoelectrochemical (PEC) water splitting. The optimized electrode exhibited maximum water splitting photocurrent density of 700 μA.cm−2 at 1.6 V versus reversible hydrogen electrode (RHE) under simulated solar illumination (AM 1.5, 100 mW.cm−2) in 1 M NaOH. This could be attributed to the large electrode electrolyte interfacial area and low charge transfer resistance offered by nanorods of α-Fe2O3 of high surface-tovolume ratio.
1. Introduction The current global economy critically relies on the energy production from the fossil fuels which are non-renewable sources of energy. Hence, it is becoming significant to turn energy systems towards sustainability to meet the increasing energy demand due to rapid population growth and depletion of fossil fuel sources [1]. Solar energy-driven decomposition of water by photoelectrochemical (PEC) reaction is an attractive technology for the production of clean and sustainable hydrogen [2]. Water splitting using TiO2 semiconductor photoelectrode irradiated under ultraviolet light was first reported in 1972 by Honda and Fujishima [3]. However, because of the wide bandgap of TiO2, these kinds of materials can only be excited by ultraviolet radiations: a small fraction of solar light. Various semiconducting materials such as TiO2 [4,5], ZnO [6,7], WO3 [8], BiVO4 [9], and α-Fe2O3 [10,11] have been considered for water oxidation. Recently, hematite (α-Fe2O3) has got much attention as an ideal material for water oxidation due to a suitable band gap of 2.1 eV, stability, cost-effectiveness and non-toxicity [12,13]. Conversely, its practical use is inadequate due to its
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Corresponding author. E-mail address: [email protected] (G. Rahman).
https://doi.org/10.1016/j.ijleo.2019.163454 Received 19 June 2019; Accepted 18 September 2019 0030-4026/ © 2019 Elsevier GmbH. All rights reserved.
Optik - International Journal for Light and Electron Optics 200 (2020) 163454
G. Rahman, et al.
poor electron mobility (˜10−2 cm2 V-1 s-1), short hole diffusion length (about 2–4 nm), short life span of charge carriers (˜10 ps) [14], sluggish water oxidation kinetics, and poor light absorption properties. To overcome these limitations, various strategies have been adapted, including modifications in the morphology for the improvement in absorption of light [15], doping to enhance the conductivity [16], and using water oxidation catalysts to boost the water oxidation kinetics [17]. Different types of unique morphologies such as porous films [18], nanorods [19], nanotubes [20], nanowires [21] and the dendritic nanostructures [22] have been synthesized. Among them, the nanostructuring has been found favorable approach in enhancing the light absorption and charge transfer properties in α-Fe2O3 for water oxidation [14]. To synthesize nanostructured α-Fe2O3, different methods such as atomic layer deposition [23], low-pressure metal organic chemical vapor deposition (MOCVD) [24], electrodeposition [25], spray pyrolysis [26], ultrasonic spray pyrolysis [27], and hydrothermal synthesis [2] have been tried. However, only few fabrication techniques have control on the deposition parameters, chemical composition and size of nanoparticles in the film [14]. In addition, some of the methods are complicated and use toxic solvents/chemicals for the synthesis of nano-sized α-Fe2O3. Manipulating the morphological and structural properties of α-Fe2O3 films using simple synthesis route is still a major challenge. In recent years, chemical bath deposition (CBD) has been frequently used for the synthesis of nanoarchitectured metal oxides for different applications [28–31]. This method is safe, simple, and cost-effective. Patil et al. [32] used “bottom-up” CBD technique to grow nanostructured flower-like TiO2 on stainless steel and glass substrates at room temperature. They also studied the relation of deposition time with the thickness of the NiO thin film grown via CBD technique and the results indicated that the thickness increased with deposition time to some extent and then decreased due to precipitation [33]. Sultana et al. [30] explored that the reaction time affected the growth of the CuO thin films deposited on n- Silicon substrate via CBD technique. It was observed that lower reaction time led to the formation of discontinuous film which was filled at greater deposition time. Marcia et al. [34] concluded that growth temperature strongly affected the morphology of the BiVO4 nanostructures synthesized by CBD. At higher temperatures, it was found that the dimensions of the nanostructures increased. Moreover, Morrish et al. [35] showed that high annealing temperature improved the photo-activity of α-Fe2O3 nanorod arrays synthesized by CBD for PEC water splitting. It is well accepted that CBD is the desired method for the synthesis of materials in commercial applications due to its prospects; cost effectiveness and easy to handle, the precise control on the size and shape of deposited material; however, it requires the use of structure directing agents, especially in the synthesis of nanostructures [36,37]. Herein, we report on the fabrication of α-Fe2O3 nanorods on fluorine-doped tin oxide (FTO) via CBD technique without using any additives. The influence of various experimental conditions such as precursor concentration, deposition time and temperature, and post-synthesis heat treatment on the structural and morphological properties of α-Fe2O3 was investigated. The precursor concentration was found to have drastic effect on the morphology of α-Fe2O3, while deposition temperature influenced the size of particles and film thickness. Post synthesis annealing of the α-Fe2O3 resulted in improving the crystallinity. These parameters ultimately affected the PEC performance of the film. Various spectroscopic and electrochemical measurements were performed to characterize Fe2O3 as a photoanode for PEC water oxidation. 2. Experimental section 2.1. Growth of α-Fe2O3 FTO (TEC 8, Pilkington glass) glass was used as substrate for the deposition of α-Fe2O3. The substrate was washed with detergent prior to deposition, followed by ultrasonic cleaning with ethanol (10 min), acetone (10 min), and distilled water (10 min). For the synthesis of α-Fe2O3, an aqueous solution of iron chloride hexahydrate (FeCl3.6H2O Sigma-Aldrich) was prepared in distilled water. A small quantity of the freshly prepared precursor solution was transferred to a flask and a piece of the FTO substrate (2.5 cm2) was immersed with its conductive side facing up. The reaction was carried out in a digital constant temperature water tank at temperatures ranging from 60 to 95 ℃ for various reaction times. The as-deposited yellow film was rinsed carefully with distilled water, followed by annealing at high temperature to eliminate the impurities and get crystalline α-Fe2O3. The formation of α-Fe2O3 from iron chloride precursor can be achieved by the following reactions:
FeCl3. 6H2 O → Fe (H2 O )36 + + 3Cl−
(1)
Fe (H2 O )36 + → FeOOH + 3H+ + 4H2 O
(2)
Heat
(3)
2FeOOH → α − Fe2 O3 + H2 O 3+
Due to the amphoteric character of water, the electropositive cation (Fe ) induces the H2O ligand and forms a hexa aquo complex [Fe (H2O)63+] as shown in Eq. (1). The complex, via deprotonation, gives rise to FeOH (H2O)52+, Fe(OH)2(H2O)4+, and finally an oxyhydroxide product of iron (FeOOH) is formed [38,39] (Eq. 2). This oxyhydroxide is converted to α-Fe2O3 by annealing at high temperature in air. 2.2. Characterization X-ray diffraction (XRD) data was recorded using X-ray diffractometer (XRD-6000, Shimadzu, Japan) for the identification of the crystalline phases. The surface morphology of the α-Fe2O3 was investigated using Field emission scanning electron microscope (FE2
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Fig. 1. SEM micrographs of thin films of α-Fe2O3 using various concentrations of the precursor solution (A) 0.1 M, (B) 0.15 M, (C) 0.2 M, (D) 0.25 M, (E) 0.3 M and (F) cross-section SEM image of film (E).
SEM, HITACHI S-4100 model). The UV–vis spectra were obtained by measuring the transmittance and reflectance by UV–vis spectrophotometer (Cary-5000, VARIAN) to examine the optical properties of the grown thin films. The X-ray photoelectron spectra of the film was measured using PHI 5000 Versa Probe (Ulvac-PHI) under the conditions of high vacuum (6.8 × 10-8 pa) with a monochromatic Al Ka radiation source of 1486.6 eV. The 1 s carbon peak (284.6 eV) was used for the internal adjustment. The PEC performance α-Fe2O3 thin films was studied using IviumStat potentiostat with three-electrode cell setup. An Ag/AgCl (3 M NaCl) was used as reference electrode, platinum-wire as counter, and α-Fe2O3 photoanode as working electrode, respectively. During testing, the bare part of substrate (FTO) was masked with epoxy resin to prevent contact with the electrolyte. For PEC measurements, 1 M NaOH electrolyte (pH 13.5) was used which was continuously purged with nitrogen gas during and before experiments. The electrode surface was illuminated by a solar simulator (Sun 2000 solar simulator, ABET Technologies, USA), keeping standard 1 sun illumination conditions. Electrochemical impedance spectroscopy (EIS) was performed using the same threeelectrode setup and potentiostat. The electrodes were scanned from 200 mV to 600 mV against Ag/AgCl within the frequency range of 1 Hz to 100 kHz. The measurement was done in the dark and the data was acquired in the form of Nyquist plots. 3. Results and discussion 3.1. Concentration effect To investigate the influence of Fe precursor concentration, α-Fe2O3 films were prepared at concentrations varying from 0.1 M to 0.3 M. The SEM micrographs of the films are shown in sections A–E of Fig. 1. Fig. 1 A revealed granular shaped condensed particles for 0.1 M salt concentration. The particle size was estimated to be 60 nm. The surface morphology of film (B) (0.15 M precursor concentration) was similar to that of (A); however, a slight increase in particle size as well as particle agglomeration were observed. Fig. 1 (C) and (D) showed that the particles transformed into nanorods when the concentration is increased from 0.15 to 0.30 M. At 0.3 M (Fig. 1 E), well-aligned rod-like structures were obtained. The diameter and length of the rods were in nanometric scale. 3
Optik - International Journal for Light and Electron Optics 200 (2020) 163454
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Fig. 2. (A) XRD profiles of α-Fe2O3 thin films using various precursor concentrations (B) XRD pattern of the film fabricated using 0.3 M precursor concentration.
Overall, the morphology of the α-Fe2O3 thin films was well controlled by variation in the precursor concentration. Moreover, the thickness of films was observed to increase with Fe concentration (not shown here). A representative cross-sectional SEM image of the film prepared from 0.30 M Fe concentration is shown in Fig. 1 (F). The thickness of the films was approximately 400 nm. Being an indirect bandgap material of small absorption coefficient, α-Fe2O3 with a thickness of 400 nm is expected to absorb sufficient photons of solar spectrum for photoelectrochemical applications. Fig. 2 (a) depicts the XRD profiles of α-Fe2O3 deposited at various precursor concentrations. The peaks at (104), (110), (202), (116) and (300) were noticed in all the samples which confirmed the presence of pure α-Fe2O3 phase (belonging to rhombohedral system (JCPDS card: 089-8104)). The intensity of these peaks were more pronounced as the concentration of Fe solution increased. This is associated to the high crystallinity of well-defined α-Fe2O3 nanostructures in the film, as observed in SEM analysis (Fig. 1E). A close inspection of the XRD spectrum of film prepared from 0.3 M Fe solution (Fig. 2 (b)) revealed that (110) peak intensity is greater than (104) peak. The relative higher intensity of (110) peak compared to (104) is indicative of the preferential growth along [110] direction which dictates that most conductive plane (001) of hematite is aligned vertically to the substrate [40,41]. This supports the growth of rod-like structures vertically aligned on FTO (see cross-section SEM image).The films with such orientations are reported to have high photoactivity for water splitting [42]. To understand the activity of a material for solar energy conversion, the optical band gap and light capturing properties are of great importance [43]. To examine it, UV–vis spectrophotometer was set to record the % transmittance and % reflectance data of the samples. These measurements were used to calculate the absorbance by using the relationship: A=100- (T+R)
(4)
where A and T corresponds to the reflectance and transmittance, respectively. The UV–vis optical absorption spectra of α-Fe2O3 films (Fig. 3 (a)) showed a clear absorption onset at approximately 600 nm, which is consistent with the bandgap of the α-Fe2O3 [12]. The absorbance was increased for the α-Fe2O3 prepared at 0.1 M and 0.15 M. Two distinctive absorption peaks associated to hematite have been identified; one at ˜ 420 nm and the second broad one is at ˜ 530 nm. The peak at 420 nm is ascribed to ligand field transition of Fe3+ (6A1→4E1, 4A1 (4G)) [44]. The double exciton transitions of Fe3+ eFe3+ cations pair produced a peak at ˜ 530 nm which imparted a typical red color to hematite. Besides higher absorption of light, a slight shift to longer wavelengths was also found in the wavelength range of 600 to 420 nm. This is associated to the thickness of α-Fe2O3 that is increased with Fe concentration [45]. From the absorption data, the bandgap of the α-Fe2O3 was determined according to the following equation: 4
Optik - International Journal for Light and Electron Optics 200 (2020) 163454
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Fig. 3. (a) UV–vis optical absorption spectra of the α-Fe2O3 photoanodes prepared at various concentrations. (b) Tauc plots of the films synthesized at various concentrations of the precursor solution.
ahv = A0 (hv − Eg )n
(5)
e2 ⎤ A0 = ⎡ (2mr )3/2 ⎢ nch2me* ⎥ ⎣ ⎦
(6)
Where “a” is the absorption coefficient of the film, “hv” is photon energy, Eg is the bandgap, “e” is electronic charge, “c” is the velocity of light, “h” is Plank’s constant, “mr” is the reduced mass, and “me” is the effective mass. “n” is constant and its value depends on the nature of electronic transitions. Fig. 3 (b) illustrates the Tauc plots of α-Fe2O3 films deposited at different Fe concentrations. The bandgap values calculated were in the range of 2.01–2.09 eV. These values are similar to those mentioned in the literature [46]. It was observed that the bandgap slightly decreased at higher concentrations. This is associated to the thickness of the film. For thicker films, the overlapping of energy bands occurs which tend to decrease the bandgap [47]. To obtain chemical information of α-Fe2O3, X-ray photoelectron spectroscopy (XPS) of the representative film was performed. Fig. 4 (a) illustrates the XPS survey spectra of α-Fe2O3 film. The signals indicated the presence of iron (Fe), oxygen (O), carbon (C); whereas the carbon peak may be due to the impurities introduced during the sample preparation. The XPS Fe2p spectrum (Fig. 4 b) consisted of two sub-peaks. The peak at binding energy of 723.1 eV is attributed to Fe (2p1/2), while the peak at 709.4 eV stands for Fe (2p3/2). These peaks along with a peak shoulder at ˜718.3 eV confirmed the +3 oxidation state of Fe in α-Fe2O3 film [27,48,49]. The XPS 1 s spectrum of oxygen is presented in Fig. 4 c. A very intense and sharp peak can also be seen at the binding energy of 529.4 eV which is assigned to the oxygen atom directly attached to the metal ion (Fe+3) in α-Fe2O3. The shoulder of main peak near high binding energy corresponded to the adsorbed oxygen (physi- and chemi-sorbed water at the surface). No Sn signal was detected in the XPS spectra which dictated that the substrate FTO glass was stable under the prevailing experimental conditions. 3.2. Effect of growth temperature To investigate the influence of growth temperature, α-Fe2O3 thin films were deposited at different temperatures, ranging from 60 to 90 ℃. The SEM micrographs of the films are shown in Fig. 5. The film deposited at 60 ℃ (Fig. 5 A) exhibited nanorod arrays arranged in groups. The surface of the substrate was completely covered with densely-packed nanorods. At 70℃ (Fig. 5 B), the 5
Optik - International Journal for Light and Electron Optics 200 (2020) 163454
G. Rahman, et al.
Fig. 4. (a) XPS survey spectrum for the α-Fe2O3 thin film (b) Fe2p core-level spectra of the film and (c) O1 s spectra. The film was prepared at 60 ℃ with a precursor concentration of 0.1 M and reaction time of 5 h.
diameter as well as the length of nanorods were increased. Further increase in temperature to 80 and 90℃ resulted in an irregular surface of the film with agglomerated nanostructures (Fig. 5C and D). To see the effect of growth temperature on optical properties, UV–vis spectra of α-Fe2O3 films prepared at different temperatures were measured (shown in Fig. 6 a). Film prepared at 90 ℃ showed maximum absorbance in the wavelength ranging from 300 nm to 650 nm as compared to other films. The optical band gap of α-Fe2O3 films were also calculated from Tauc plots (Fig. 6b). At 60 ℃ grown film, the estimated bandgap was 2.1 eV which is reduced to 2.03 eV for the α-Fe2O3 film prepared at 90 ℃. This could be due related to the difference in dimension of rods in films prepared at different growth temperature. 3.3. PEC water splitting performance Hematite (α-Fe2O3) has been widely studied as photoanode material for PEC water oxidation due to its rich semiconducting properties. The PEC water oxidation activity of α-Fe2O3 films was studied in 1 M NaOH electrolyte under standard 1 sun illumination conditions. The photocurrent density versus applied potential curves were measured against Ag/AgCl reference electrode. The measured potentials were converted to the reversible hydrogen electrode (RHE) scale according to the correlation: 6
Optik - International Journal for Light and Electron Optics 200 (2020) 163454
G. Rahman, et al.
Fig. 5. SEM images of the surface morphology of thin films at various growth temperatures: (a) 60℃, (b) 70℃, (c) 80℃, and (d) 90℃. The Fe solution concentration was 0.1 M. All other synthesis conditions are kept constant.
ERHE = EAg/AgCl + E̊Ag/AgCl + 0.0591pH
(7)
where ERHE refers to the calculated potential versus RHE, EAg/AgCl is the measured potential, and E̊Ag/AgCl is equal to 0.209 V at 25 ℃. Fig. 7 (A) shows the potential-current (I–V) curves of α-Fe2O3 photoanodes prepared at different Fe concentrations. The film deposited in 0.10 M and 0.15 M precursor concentration produced very low water oxidation photocurrent density. While that obtained from 0.2 M solution exhibited a maximum value of 700 μA.cm−2 at 1.6 V under standard illumination conditions. Further increase in precursor concentration (0.30 M film) resulted in film with lower water oxidation photocurrent density (˜215 μA.cm−2 at 1.6 V). This could be due to the low surface area of large-sized rods in the films obtained from concentrated solution [116,117]. When the film thickness is large, the electron-hole recombination becomes progressively significant as the short hole-diffusion length (2–4 nm) of α-Fe2O3 limits the ability to remove charges from the semiconductor and consequently undergo charge-carrier recombination. Fig. 7 (B) depicts the effect of growth temperature on the water splitting photocurrent density of α-Fe2O3 photoanodes. The films grown at 90 and 80 ℃ showed negligible photocurrent density. A small photocurrent of 132.2 μA/ cm2 at 1.6 V was obtained on film deposited at 70℃ while a maximum photocurrent density of 240.6 μA/ cm2 was measured for the film deposited at 60℃. As revealed by the SEM analysis (Fig. 5 A), α-Fe2O3 film grown at 60 ℃ exhibited the smallest feature sizes compared to the other films. Consequently, the electrode/electrolyte interfacial area is enhanced for water oxidation reaction. The transient water oxidation photocurrent of α-Fe2O3 electrodes was measured at constant potential of 1.6 V under light on and off conditions. Sharp anodic current spikes were observed when the light was turned-on which was due to the accumulation of holes at semiconductor liquid junction because of slow water oxidation kinetics or trapping of holes by surface states. The spikes disappeared when equilibrium was established between water oxidation and charge carrier recombination [50]. When the light was turned off, a cathodic transient spike was observed which was due to the flow of electrons from external circuit to semiconductorelectrolyte junction and subsequent recombination with holes [51]. EIS measurements were made to study the charge transfer processes at α-Fe2O3-electrolyte interface. Nyquist plots (Z′ vs. Z″) of α-Fe2O3 photoanodes deposited at different temperatures are shown in Fig. 7 (D). From the diameter of semicircle of Nyquist plots, the charge transfer resistance can be estimated. As shown in figure, the film obtained at 60 ℃ exhibited the lowest charge transfer resistance in comparison to others. This means the water oxidation reaction was facilitated by the electrode, as revealed by high water oxidation photocurrent density (Fig. 7 B). This is consistent to the SEM results since the film grown at 60 °C showed smaller feature sizes compared to the films prepared at other temperatures (Fig. 5A). Owing to the small hole-diffusion length of α-Fe2O3 (2–4 nm), majority of the holes generated in small-sized particles reached the surface to take part in water oxidation, thus showing high water splitting photocurrent density. To examine further, Mott–Schottky equation was used to determine the charge carrier density (ND) and the flat band potential (Vfb) of the fabricated photoelectrodes, as under [52]:
7
Optik - International Journal for Light and Electron Optics 200 (2020) 163454
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Fig. 6. (a) Absorption spectra of the α-Fe2O3 thin films at different growth temperatures, (b) Tauc plots generated from absorbance data.
1/C2 = (2/ƐƐoeND) (V-Vfb-KBT/e)
(8)
Here, ε shows the semiconductor dielectric constant (Ɛ = 80 for α-Fe2O3) [53], Ɛo is the electric permittivity of free space, C represents the capacitance of the space charge layer of the semiconductor, e is the charge of electron, Vfb is the flat band potential, ND is the density of charge carriers (donor), KB is the Boltzmann constant, T is the temperature, and. V is the applied potential. The values of ND are calculated from the slope while the value of Vfb was found from the intercept of the axis with potential value. The Fig. 8 shows the Mott–Schottky plot of representative α-Fe2O3 photoanode. The positive slope of the plot is due to the n-type nature of αFe2O3. From the slope of the plot, a donor density of 1.32 × 1017 cm−3 was calculated while from the intercept, Vfb was estimated as 0.73 V vs. RHE. These values are in agreement with values reported for un-doped α-Fe2O3 photoanodes [54,55]. The value of ND can be increased by doping of α-Fe2O3 with various metals such as Ti, Si, Sn etc. to improve its overall water oxidation performance.
4. Conclusions In summary, a facile chemical bath deposition method was employed to grow α-Fe2O3 on FTO-coated glass substrate for photoelectrochemical water splitting application. By controlling the experimental conditions such as concentration, deposition time, temperature, annealing temperature and time, we were able to control size and orientation of deposited material, film thickness, and surface morphology which ultimately affected the water splitting efficiency. Changing the deposition time and the concentration of the precursor solution resulted a significant increase in the thickness of α-Fe2O3 films. It was also observed that an increase in precursor concentration changes the surface morphology from granular shape condensed particles to nanorods. The growth temperature was found to have a considerable effect on the size/diameter of the rods. The optimized α-Fe2O3 film exhibited considerable activity as photoanode for photoelectrochemical water splitting. Besides water splitting, our synthesized α-Fe2O3 can be used for other applications such dye degradation and gas sensing.
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Fig. 7. (A) Concentration dependent IV curves of α-Fe2O3 photoanodes, (B) Temperature dependent current-potential curves, (C) Photocurrent density versus time of α-Fe2O3 photoanode, and (D) Nyquist plots α-Fe2O3 deposited at different growth temperatures. The Nyquist plots were obtained in 1 M NaOH in dark at a frequency of 1000 Hz.
Fig. 8. Mott-Schottky plot for the film prepared at 60 °C. The measurement was carried out in dark in 1 M NaOH solution at a frequency of 1000 Hz.
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Original research article
Investigation of the structural, optical, and photoelectrochemical properties of α-Fe2O3 nanorods synthesized via a facile chemical bath deposition
T
⁎
Gul Rahmana, , Zainab Najafa, Anwar ul Haq Ali Shaha, Shabeer Ahmad Mianb a b
Institute of Chemical Sciences, University of Peshawar, 25120, Peshawar, Pakistan Department of Physics, University of Peshawar, 25120, Peshawar, Pakistan
A R T IC LE I N F O
ABS TRA CT
Keywords: Chemical bath deposition Optical properties Water splitting Growth temperature Photoelectrode Photoelectrochemical cell
In this study, a simple and cost-effective chemical bath deposition was employed to deposit nansized hematite (α-Fe2O3) directly on fluorine-doped tin oxide (FTO) substrates. The structure, morphology, and optical properties of α-Fe2O3 were manipulated by adjusting the experimental parameters, particularly the deposition temperature and precursor concentration. With increase in Fe precursor concentration from 0.1 M to 0.3 M, the surface morphology was changed from granular-shape condensed particles to vertically aligned rod-like structures. XRD spectra indicated signals from FTO substrate and pure α-Fe2O3 phase which corresponds to the rhombohedral system. The XPS analysis revealed the presence of α-Fe2O3 in the film and no tin (Sn) impurities from the FTO substrate. By changing the deposition temperature from 60 ℃ to 90 ℃, the size of particles increased; whereas, the surface morphology was not altered. The resultant films were examined as photoanode for photoelectrochemical (PEC) water splitting. The optimized electrode exhibited maximum water splitting photocurrent density of 700 μA.cm−2 at 1.6 V versus reversible hydrogen electrode (RHE) under simulated solar illumination (AM 1.5, 100 mW.cm−2) in 1 M NaOH. This could be attributed to the large electrode electrolyte interfacial area and low charge transfer resistance offered by nanorods of α-Fe2O3 of high surface-tovolume ratio.
1. Introduction The current global economy critically relies on the energy production from the fossil fuels which are non-renewable sources of energy. Hence, it is becoming significant to turn energy systems towards sustainability to meet the increasing energy demand due to rapid population growth and depletion of fossil fuel sources [1]. Solar energy-driven decomposition of water by photoelectrochemical (PEC) reaction is an attractive technology for the production of clean and sustainable hydrogen [2]. Water splitting using TiO2 semiconductor photoelectrode irradiated under ultraviolet light was first reported in 1972 by Honda and Fujishima [3]. However, because of the wide bandgap of TiO2, these kinds of materials can only be excited by ultraviolet radiations: a small fraction of solar light. Various semiconducting materials such as TiO2 [4,5], ZnO [6,7], WO3 [8], BiVO4 [9], and α-Fe2O3 [10,11] have been considered for water oxidation. Recently, hematite (α-Fe2O3) has got much attention as an ideal material for water oxidation due to a suitable band gap of 2.1 eV, stability, cost-effectiveness and non-toxicity [12,13]. Conversely, its practical use is inadequate due to its
⁎
Corresponding author. E-mail address: [email protected] (G. Rahman).
https://doi.org/10.1016/j.ijleo.2019.163454 Received 19 June 2019; Accepted 18 September 2019 0030-4026/ © 2019 Elsevier GmbH. All rights reserved.
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poor electron mobility (˜10−2 cm2 V-1 s-1), short hole diffusion length (about 2–4 nm), short life span of charge carriers (˜10 ps) [14], sluggish water oxidation kinetics, and poor light absorption properties. To overcome these limitations, various strategies have been adapted, including modifications in the morphology for the improvement in absorption of light [15], doping to enhance the conductivity [16], and using water oxidation catalysts to boost the water oxidation kinetics [17]. Different types of unique morphologies such as porous films [18], nanorods [19], nanotubes [20], nanowires [21] and the dendritic nanostructures [22] have been synthesized. Among them, the nanostructuring has been found favorable approach in enhancing the light absorption and charge transfer properties in α-Fe2O3 for water oxidation [14]. To synthesize nanostructured α-Fe2O3, different methods such as atomic layer deposition [23], low-pressure metal organic chemical vapor deposition (MOCVD) [24], electrodeposition [25], spray pyrolysis [26], ultrasonic spray pyrolysis [27], and hydrothermal synthesis [2] have been tried. However, only few fabrication techniques have control on the deposition parameters, chemical composition and size of nanoparticles in the film [14]. In addition, some of the methods are complicated and use toxic solvents/chemicals for the synthesis of nano-sized α-Fe2O3. Manipulating the morphological and structural properties of α-Fe2O3 films using simple synthesis route is still a major challenge. In recent years, chemical bath deposition (CBD) has been frequently used for the synthesis of nanoarchitectured metal oxides for different applications [28–31]. This method is safe, simple, and cost-effective. Patil et al. [32] used “bottom-up” CBD technique to grow nanostructured flower-like TiO2 on stainless steel and glass substrates at room temperature. They also studied the relation of deposition time with the thickness of the NiO thin film grown via CBD technique and the results indicated that the thickness increased with deposition time to some extent and then decreased due to precipitation [33]. Sultana et al. [30] explored that the reaction time affected the growth of the CuO thin films deposited on n- Silicon substrate via CBD technique. It was observed that lower reaction time led to the formation of discontinuous film which was filled at greater deposition time. Marcia et al. [34] concluded that growth temperature strongly affected the morphology of the BiVO4 nanostructures synthesized by CBD. At higher temperatures, it was found that the dimensions of the nanostructures increased. Moreover, Morrish et al. [35] showed that high annealing temperature improved the photo-activity of α-Fe2O3 nanorod arrays synthesized by CBD for PEC water splitting. It is well accepted that CBD is the desired method for the synthesis of materials in commercial applications due to its prospects; cost effectiveness and easy to handle, the precise control on the size and shape of deposited material; however, it requires the use of structure directing agents, especially in the synthesis of nanostructures [36,37]. Herein, we report on the fabrication of α-Fe2O3 nanorods on fluorine-doped tin oxide (FTO) via CBD technique without using any additives. The influence of various experimental conditions such as precursor concentration, deposition time and temperature, and post-synthesis heat treatment on the structural and morphological properties of α-Fe2O3 was investigated. The precursor concentration was found to have drastic effect on the morphology of α-Fe2O3, while deposition temperature influenced the size of particles and film thickness. Post synthesis annealing of the α-Fe2O3 resulted in improving the crystallinity. These parameters ultimately affected the PEC performance of the film. Various spectroscopic and electrochemical measurements were performed to characterize Fe2O3 as a photoanode for PEC water oxidation. 2. Experimental section 2.1. Growth of α-Fe2O3 FTO (TEC 8, Pilkington glass) glass was used as substrate for the deposition of α-Fe2O3. The substrate was washed with detergent prior to deposition, followed by ultrasonic cleaning with ethanol (10 min), acetone (10 min), and distilled water (10 min). For the synthesis of α-Fe2O3, an aqueous solution of iron chloride hexahydrate (FeCl3.6H2O Sigma-Aldrich) was prepared in distilled water. A small quantity of the freshly prepared precursor solution was transferred to a flask and a piece of the FTO substrate (2.5 cm2) was immersed with its conductive side facing up. The reaction was carried out in a digital constant temperature water tank at temperatures ranging from 60 to 95 ℃ for various reaction times. The as-deposited yellow film was rinsed carefully with distilled water, followed by annealing at high temperature to eliminate the impurities and get crystalline α-Fe2O3. The formation of α-Fe2O3 from iron chloride precursor can be achieved by the following reactions:
FeCl3. 6H2 O → Fe (H2 O )36 + + 3Cl−
(1)
Fe (H2 O )36 + → FeOOH + 3H+ + 4H2 O
(2)
Heat
(3)
2FeOOH → α − Fe2 O3 + H2 O 3+
Due to the amphoteric character of water, the electropositive cation (Fe ) induces the H2O ligand and forms a hexa aquo complex [Fe (H2O)63+] as shown in Eq. (1). The complex, via deprotonation, gives rise to FeOH (H2O)52+, Fe(OH)2(H2O)4+, and finally an oxyhydroxide product of iron (FeOOH) is formed [38,39] (Eq. 2). This oxyhydroxide is converted to α-Fe2O3 by annealing at high temperature in air. 2.2. Characterization X-ray diffraction (XRD) data was recorded using X-ray diffractometer (XRD-6000, Shimadzu, Japan) for the identification of the crystalline phases. The surface morphology of the α-Fe2O3 was investigated using Field emission scanning electron microscope (FE2
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Fig. 1. SEM micrographs of thin films of α-Fe2O3 using various concentrations of the precursor solution (A) 0.1 M, (B) 0.15 M, (C) 0.2 M, (D) 0.25 M, (E) 0.3 M and (F) cross-section SEM image of film (E).
SEM, HITACHI S-4100 model). The UV–vis spectra were obtained by measuring the transmittance and reflectance by UV–vis spectrophotometer (Cary-5000, VARIAN) to examine the optical properties of the grown thin films. The X-ray photoelectron spectra of the film was measured using PHI 5000 Versa Probe (Ulvac-PHI) under the conditions of high vacuum (6.8 × 10-8 pa) with a monochromatic Al Ka radiation source of 1486.6 eV. The 1 s carbon peak (284.6 eV) was used for the internal adjustment. The PEC performance α-Fe2O3 thin films was studied using IviumStat potentiostat with three-electrode cell setup. An Ag/AgCl (3 M NaCl) was used as reference electrode, platinum-wire as counter, and α-Fe2O3 photoanode as working electrode, respectively. During testing, the bare part of substrate (FTO) was masked with epoxy resin to prevent contact with the electrolyte. For PEC measurements, 1 M NaOH electrolyte (pH 13.5) was used which was continuously purged with nitrogen gas during and before experiments. The electrode surface was illuminated by a solar simulator (Sun 2000 solar simulator, ABET Technologies, USA), keeping standard 1 sun illumination conditions. Electrochemical impedance spectroscopy (EIS) was performed using the same threeelectrode setup and potentiostat. The electrodes were scanned from 200 mV to 600 mV against Ag/AgCl within the frequency range of 1 Hz to 100 kHz. The measurement was done in the dark and the data was acquired in the form of Nyquist plots. 3. Results and discussion 3.1. Concentration effect To investigate the influence of Fe precursor concentration, α-Fe2O3 films were prepared at concentrations varying from 0.1 M to 0.3 M. The SEM micrographs of the films are shown in sections A–E of Fig. 1. Fig. 1 A revealed granular shaped condensed particles for 0.1 M salt concentration. The particle size was estimated to be 60 nm. The surface morphology of film (B) (0.15 M precursor concentration) was similar to that of (A); however, a slight increase in particle size as well as particle agglomeration were observed. Fig. 1 (C) and (D) showed that the particles transformed into nanorods when the concentration is increased from 0.15 to 0.30 M. At 0.3 M (Fig. 1 E), well-aligned rod-like structures were obtained. The diameter and length of the rods were in nanometric scale. 3
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Fig. 2. (A) XRD profiles of α-Fe2O3 thin films using various precursor concentrations (B) XRD pattern of the film fabricated using 0.3 M precursor concentration.
Overall, the morphology of the α-Fe2O3 thin films was well controlled by variation in the precursor concentration. Moreover, the thickness of films was observed to increase with Fe concentration (not shown here). A representative cross-sectional SEM image of the film prepared from 0.30 M Fe concentration is shown in Fig. 1 (F). The thickness of the films was approximately 400 nm. Being an indirect bandgap material of small absorption coefficient, α-Fe2O3 with a thickness of 400 nm is expected to absorb sufficient photons of solar spectrum for photoelectrochemical applications. Fig. 2 (a) depicts the XRD profiles of α-Fe2O3 deposited at various precursor concentrations. The peaks at (104), (110), (202), (116) and (300) were noticed in all the samples which confirmed the presence of pure α-Fe2O3 phase (belonging to rhombohedral system (JCPDS card: 089-8104)). The intensity of these peaks were more pronounced as the concentration of Fe solution increased. This is associated to the high crystallinity of well-defined α-Fe2O3 nanostructures in the film, as observed in SEM analysis (Fig. 1E). A close inspection of the XRD spectrum of film prepared from 0.3 M Fe solution (Fig. 2 (b)) revealed that (110) peak intensity is greater than (104) peak. The relative higher intensity of (110) peak compared to (104) is indicative of the preferential growth along [110] direction which dictates that most conductive plane (001) of hematite is aligned vertically to the substrate [40,41]. This supports the growth of rod-like structures vertically aligned on FTO (see cross-section SEM image).The films with such orientations are reported to have high photoactivity for water splitting [42]. To understand the activity of a material for solar energy conversion, the optical band gap and light capturing properties are of great importance [43]. To examine it, UV–vis spectrophotometer was set to record the % transmittance and % reflectance data of the samples. These measurements were used to calculate the absorbance by using the relationship: A=100- (T+R)
(4)
where A and T corresponds to the reflectance and transmittance, respectively. The UV–vis optical absorption spectra of α-Fe2O3 films (Fig. 3 (a)) showed a clear absorption onset at approximately 600 nm, which is consistent with the bandgap of the α-Fe2O3 [12]. The absorbance was increased for the α-Fe2O3 prepared at 0.1 M and 0.15 M. Two distinctive absorption peaks associated to hematite have been identified; one at ˜ 420 nm and the second broad one is at ˜ 530 nm. The peak at 420 nm is ascribed to ligand field transition of Fe3+ (6A1→4E1, 4A1 (4G)) [44]. The double exciton transitions of Fe3+ eFe3+ cations pair produced a peak at ˜ 530 nm which imparted a typical red color to hematite. Besides higher absorption of light, a slight shift to longer wavelengths was also found in the wavelength range of 600 to 420 nm. This is associated to the thickness of α-Fe2O3 that is increased with Fe concentration [45]. From the absorption data, the bandgap of the α-Fe2O3 was determined according to the following equation: 4
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Fig. 3. (a) UV–vis optical absorption spectra of the α-Fe2O3 photoanodes prepared at various concentrations. (b) Tauc plots of the films synthesized at various concentrations of the precursor solution.
ahv = A0 (hv − Eg )n
(5)
e2 ⎤ A0 = ⎡ (2mr )3/2 ⎢ nch2me* ⎥ ⎣ ⎦
(6)
Where “a” is the absorption coefficient of the film, “hv” is photon energy, Eg is the bandgap, “e” is electronic charge, “c” is the velocity of light, “h” is Plank’s constant, “mr” is the reduced mass, and “me” is the effective mass. “n” is constant and its value depends on the nature of electronic transitions. Fig. 3 (b) illustrates the Tauc plots of α-Fe2O3 films deposited at different Fe concentrations. The bandgap values calculated were in the range of 2.01–2.09 eV. These values are similar to those mentioned in the literature [46]. It was observed that the bandgap slightly decreased at higher concentrations. This is associated to the thickness of the film. For thicker films, the overlapping of energy bands occurs which tend to decrease the bandgap [47]. To obtain chemical information of α-Fe2O3, X-ray photoelectron spectroscopy (XPS) of the representative film was performed. Fig. 4 (a) illustrates the XPS survey spectra of α-Fe2O3 film. The signals indicated the presence of iron (Fe), oxygen (O), carbon (C); whereas the carbon peak may be due to the impurities introduced during the sample preparation. The XPS Fe2p spectrum (Fig. 4 b) consisted of two sub-peaks. The peak at binding energy of 723.1 eV is attributed to Fe (2p1/2), while the peak at 709.4 eV stands for Fe (2p3/2). These peaks along with a peak shoulder at ˜718.3 eV confirmed the +3 oxidation state of Fe in α-Fe2O3 film [27,48,49]. The XPS 1 s spectrum of oxygen is presented in Fig. 4 c. A very intense and sharp peak can also be seen at the binding energy of 529.4 eV which is assigned to the oxygen atom directly attached to the metal ion (Fe+3) in α-Fe2O3. The shoulder of main peak near high binding energy corresponded to the adsorbed oxygen (physi- and chemi-sorbed water at the surface). No Sn signal was detected in the XPS spectra which dictated that the substrate FTO glass was stable under the prevailing experimental conditions. 3.2. Effect of growth temperature To investigate the influence of growth temperature, α-Fe2O3 thin films were deposited at different temperatures, ranging from 60 to 90 ℃. The SEM micrographs of the films are shown in Fig. 5. The film deposited at 60 ℃ (Fig. 5 A) exhibited nanorod arrays arranged in groups. The surface of the substrate was completely covered with densely-packed nanorods. At 70℃ (Fig. 5 B), the 5
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Fig. 4. (a) XPS survey spectrum for the α-Fe2O3 thin film (b) Fe2p core-level spectra of the film and (c) O1 s spectra. The film was prepared at 60 ℃ with a precursor concentration of 0.1 M and reaction time of 5 h.
diameter as well as the length of nanorods were increased. Further increase in temperature to 80 and 90℃ resulted in an irregular surface of the film with agglomerated nanostructures (Fig. 5C and D). To see the effect of growth temperature on optical properties, UV–vis spectra of α-Fe2O3 films prepared at different temperatures were measured (shown in Fig. 6 a). Film prepared at 90 ℃ showed maximum absorbance in the wavelength ranging from 300 nm to 650 nm as compared to other films. The optical band gap of α-Fe2O3 films were also calculated from Tauc plots (Fig. 6b). At 60 ℃ grown film, the estimated bandgap was 2.1 eV which is reduced to 2.03 eV for the α-Fe2O3 film prepared at 90 ℃. This could be due related to the difference in dimension of rods in films prepared at different growth temperature. 3.3. PEC water splitting performance Hematite (α-Fe2O3) has been widely studied as photoanode material for PEC water oxidation due to its rich semiconducting properties. The PEC water oxidation activity of α-Fe2O3 films was studied in 1 M NaOH electrolyte under standard 1 sun illumination conditions. The photocurrent density versus applied potential curves were measured against Ag/AgCl reference electrode. The measured potentials were converted to the reversible hydrogen electrode (RHE) scale according to the correlation: 6
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Fig. 5. SEM images of the surface morphology of thin films at various growth temperatures: (a) 60℃, (b) 70℃, (c) 80℃, and (d) 90℃. The Fe solution concentration was 0.1 M. All other synthesis conditions are kept constant.
ERHE = EAg/AgCl + E̊Ag/AgCl + 0.0591pH
(7)
where ERHE refers to the calculated potential versus RHE, EAg/AgCl is the measured potential, and E̊Ag/AgCl is equal to 0.209 V at 25 ℃. Fig. 7 (A) shows the potential-current (I–V) curves of α-Fe2O3 photoanodes prepared at different Fe concentrations. The film deposited in 0.10 M and 0.15 M precursor concentration produced very low water oxidation photocurrent density. While that obtained from 0.2 M solution exhibited a maximum value of 700 μA.cm−2 at 1.6 V under standard illumination conditions. Further increase in precursor concentration (0.30 M film) resulted in film with lower water oxidation photocurrent density (˜215 μA.cm−2 at 1.6 V). This could be due to the low surface area of large-sized rods in the films obtained from concentrated solution [116,117]. When the film thickness is large, the electron-hole recombination becomes progressively significant as the short hole-diffusion length (2–4 nm) of α-Fe2O3 limits the ability to remove charges from the semiconductor and consequently undergo charge-carrier recombination. Fig. 7 (B) depicts the effect of growth temperature on the water splitting photocurrent density of α-Fe2O3 photoanodes. The films grown at 90 and 80 ℃ showed negligible photocurrent density. A small photocurrent of 132.2 μA/ cm2 at 1.6 V was obtained on film deposited at 70℃ while a maximum photocurrent density of 240.6 μA/ cm2 was measured for the film deposited at 60℃. As revealed by the SEM analysis (Fig. 5 A), α-Fe2O3 film grown at 60 ℃ exhibited the smallest feature sizes compared to the other films. Consequently, the electrode/electrolyte interfacial area is enhanced for water oxidation reaction. The transient water oxidation photocurrent of α-Fe2O3 electrodes was measured at constant potential of 1.6 V under light on and off conditions. Sharp anodic current spikes were observed when the light was turned-on which was due to the accumulation of holes at semiconductor liquid junction because of slow water oxidation kinetics or trapping of holes by surface states. The spikes disappeared when equilibrium was established between water oxidation and charge carrier recombination [50]. When the light was turned off, a cathodic transient spike was observed which was due to the flow of electrons from external circuit to semiconductorelectrolyte junction and subsequent recombination with holes [51]. EIS measurements were made to study the charge transfer processes at α-Fe2O3-electrolyte interface. Nyquist plots (Z′ vs. Z″) of α-Fe2O3 photoanodes deposited at different temperatures are shown in Fig. 7 (D). From the diameter of semicircle of Nyquist plots, the charge transfer resistance can be estimated. As shown in figure, the film obtained at 60 ℃ exhibited the lowest charge transfer resistance in comparison to others. This means the water oxidation reaction was facilitated by the electrode, as revealed by high water oxidation photocurrent density (Fig. 7 B). This is consistent to the SEM results since the film grown at 60 °C showed smaller feature sizes compared to the films prepared at other temperatures (Fig. 5A). Owing to the small hole-diffusion length of α-Fe2O3 (2–4 nm), majority of the holes generated in small-sized particles reached the surface to take part in water oxidation, thus showing high water splitting photocurrent density. To examine further, Mott–Schottky equation was used to determine the charge carrier density (ND) and the flat band potential (Vfb) of the fabricated photoelectrodes, as under [52]:
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Fig. 6. (a) Absorption spectra of the α-Fe2O3 thin films at different growth temperatures, (b) Tauc plots generated from absorbance data.
1/C2 = (2/ƐƐoeND) (V-Vfb-KBT/e)
(8)
Here, ε shows the semiconductor dielectric constant (Ɛ = 80 for α-Fe2O3) [53], Ɛo is the electric permittivity of free space, C represents the capacitance of the space charge layer of the semiconductor, e is the charge of electron, Vfb is the flat band potential, ND is the density of charge carriers (donor), KB is the Boltzmann constant, T is the temperature, and. V is the applied potential. The values of ND are calculated from the slope while the value of Vfb was found from the intercept of the axis with potential value. The Fig. 8 shows the Mott–Schottky plot of representative α-Fe2O3 photoanode. The positive slope of the plot is due to the n-type nature of αFe2O3. From the slope of the plot, a donor density of 1.32 × 1017 cm−3 was calculated while from the intercept, Vfb was estimated as 0.73 V vs. RHE. These values are in agreement with values reported for un-doped α-Fe2O3 photoanodes [54,55]. The value of ND can be increased by doping of α-Fe2O3 with various metals such as Ti, Si, Sn etc. to improve its overall water oxidation performance.
4. Conclusions In summary, a facile chemical bath deposition method was employed to grow α-Fe2O3 on FTO-coated glass substrate for photoelectrochemical water splitting application. By controlling the experimental conditions such as concentration, deposition time, temperature, annealing temperature and time, we were able to control size and orientation of deposited material, film thickness, and surface morphology which ultimately affected the water splitting efficiency. Changing the deposition time and the concentration of the precursor solution resulted a significant increase in the thickness of α-Fe2O3 films. It was also observed that an increase in precursor concentration changes the surface morphology from granular shape condensed particles to nanorods. The growth temperature was found to have a considerable effect on the size/diameter of the rods. The optimized α-Fe2O3 film exhibited considerable activity as photoanode for photoelectrochemical water splitting. Besides water splitting, our synthesized α-Fe2O3 can be used for other applications such dye degradation and gas sensing.
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Fig. 7. (A) Concentration dependent IV curves of α-Fe2O3 photoanodes, (B) Temperature dependent current-potential curves, (C) Photocurrent density versus time of α-Fe2O3 photoanode, and (D) Nyquist plots α-Fe2O3 deposited at different growth temperatures. The Nyquist plots were obtained in 1 M NaOH in dark at a frequency of 1000 Hz.
Fig. 8. Mott-Schottky plot for the film prepared at 60 °C. The measurement was carried out in dark in 1 M NaOH solution at a frequency of 1000 Hz.
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