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n-Fe2O3 heterojunction and study of their enhanced photoelectrochemical water splitting performance
Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Materials Letters journal homepage: www.elsevier.com/locate/matlet
Preparation of nanostructured p-NiO/n-Fe2O3 heterojunction and study of their enhanced photoelectrochemical water splitting performance Ramesh Rajendran a,n, Zahira Yaakob a,b,nn, Mohd Asri Mat Teridi b, Muhammad Syukri Abd Rahaman a, Kamaruzzaman Sopian b a Department of Chemical and Process Engineering, Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, UKM Bangi 43600, Selangor, Malaysia b Solar Energy Research Institute (SERI), Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia
art ic l e i nf o
a b s t r a c t
Article history: Received 5 June 2014 Accepted 26 June 2014
The n-type α-Fe2O3 nanoflakes are deposited on the FTO substrate by a hydrothermal method and modified with p-type NiO nanoparticles to enhance the photoelectrochemical water splitting performance. X-ray diffraction, field emission scanning electron microscopy, UV–visible and impedance spectroscopy analysis are carried out to study the structural, morphological, and electrochemical characteristics. Formation of p–n heterojunction is confirmed by an impedance spectroscopy analysis and explains the transport of charge carriers. NiO/α-Fe2O3 heterojunction thin film shows the enhancement in photocurrent density (1.55 mA/cm2) compared to the α-Fe2O3 nanoflakes alone (0.08 mA/cm2) under simulated solar radiation at applied potential 1V/RHE. & 2014 Published by Elsevier B.V.
Keywords: Nanocrystalline materials X-ray diffraction Chemical synthesis Optical materials and properties Photoelectrochemical measurement
1. Introduction Hydrogen is an alternative energy source with zero carbon emission. Hydrogen can be produced by splitting of water using naturally abundant solar light through Photoelectrochemical (PEC) performance [1]. Recently, many efforts focus on the development of PEC cells, which is used to split the water molecules into molecular oxygen and hydrogen under simulated solar radiation [2]. In a typical PEC cell, semiconductor based photoelectrode is considered essential to split the water for production of hydrogen [3,4]. Hematite (α-Fe2O3) based semiconductor photocatalysts receives immense attention because of its versatile properties like chemical stability, suitable band gap to absorb the 40% of solar radiation, corrosion resistance, and nontoxicity [5]. However, it possesses some drawbacks such as short hole diffusion distance (2–4 nm) which limits their charge collection, poor minority charge carrier mobility, and low charge transfer kinetics at the electrode–electrolyte interface [6]. To overcome these limitations, many efforts explore the development of α-Fe2O3 nanostructures with various dimensions and improvement of charge transport property by doping. There are many chemical techniques have been developed for n
Corresponding author. Tel.: þ 60173610697. Corresponding author at: Department of Chemical and Process Engineering, Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, UKM Bangi 43600, Selangor, Malaysia. Tel.: þ60173610697. E-mail addresses: [email protected] (R. Rajendran), [email protected] (Z. Yaakob). nn
synthesis of α-Fe2O3 nanostructures like hydrothermal, sol–gel and Aerosol-Assisted Chemical Vapor Deposition (AACVD) [7]. Among these a hydrothermal technique is highly suitable for synthesis of nanostructures with special dimension and also can be used for effective doping metals for optical and optoelectronic applications [8–11]. In addition, the formation of heterojunction can be improved PEC performance. For example, Hernández et al. have reported that the improvement of PEC performance by the formation of the oxide based heterojunction [12]. Moreover, a few studies demonstrate the development of p–n junction using α-Fe2O3 nanostructures as an ntype semiconductor [13–15]. In this article, we report the synthesis of flakes like α-Fe2O3 nanostructures on the FTO substrate by a hydrothermal technique. Further p type NiO nanoparticles are deposited on α-Fe2O3 nanostructures to form p–n heterojunction by a solution/heat procedure. Structural, morphological, and optical properties are studied by using X-ray diffraction, field emission scanning electron microscopy (FESEM) and UV–visible spectrometer. PEC performance, charge carrier density, and charge carrier kinetics are studied in three electrode PEC cells.
2. Experimental procedures Deposition of α-Fe2O3 nanoflakes on FTO substrate: The α-Fe2O3 nanoflakes are deposited onto a fluorine-doped tin oxide (FTO, Nippon Sheet Glass Co. Ltd., 15 Ω) glass substrate by a solution-
http://dx.doi.org/10.1016/j.matlet.2014.06.157 0167-577X/& 2014 Published by Elsevier B.V.
Please cite this article as: Rajendran R, et al. Preparation of nanostructured p-NiO/n-Fe2O3 heterojunction and study of their enhanced photoelectrochemical water splitting performance. Mater Lett (2014), http://dx.doi.org/10.1016/j.matlet.2014.06.157i
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R. Rajendran et al. / Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎
based hydrothermal method. In the typical procedure, 2 mmol of ferric chloride (FeCl3 6H2O) and 2 mmol of urea are subsequently dissolved in a 40 ml mixture of distilled water and glycerol (3:1) solution for 10 min under magnetic stirring at room temperature. The 35 ml solution is poured into a 50 ml of Teflon-lined stainless steel autoclave containing an FTO glass substrate vertically placed and partially immersed into the solution. The autoclave is then placed in a hot air oven at a constant temperature of 200 1C for 12 h. At the end of the reaction, the autoclave is left to cool to room temperature. Finally α-Fe2O3 deposited FTO substrate is taken out from the autoclave and washed in DI water in an ultrasonic bath to remove impurities, then dried in air at room temperature. Formation of p–n heterojunction using NiO and α-Fe2O3: NiO nanoparticles are deposited on the vertically aligned α-Fe2O3
Fig. 1. XRD patterns of the α-Fe2O3 and NiO/α-Fe2O3 heterojunction thin film.
nanoflakes by a simple solution/heat procedure. In the typical experiment, nickel nitride (0.14 g) is dissolved in absolute ethanol (30 ml) at room temperature to form a light green solution. Subsequently, FTO substrate contains vertically aligned α-Fe2O3 nanoflakes immersed into the solution and quickly taken out of the solution. The FTO is dried at room temperature in air and then subjected to the annealing process at 300 1C. Finally, the substrate is washed with ethanol and DI water several times to remove the unreacted impurities. Characterizations: The phase formation of α-Fe2O3 and NiO/α-Fe2O3 heterojunction thin films are studied by XRD (Rigaku DMax 200, using Cu Kα radiation) in the 2θ range from 20 degree to 70 degree with a step scan of 0.02 degree/12 s. Optical absorbance spectra is measured using PerkinElmer UV WinLab 6.0.4.0738/Lambda 35 1.27 spectrophotometer and all the dates are corrected by deducting the baseline of the FTO substrate. Morphology and chemical composition of the samples are investigated by a field-emission scanning electron microscope (FE-SEM, S4800, Hitachi) equipped with an Energy Dispersive Xray (EDX) analyser. The steady sate current density (j–v) and electrochemical impedance spectroscopy measurements of α-Fe2O3 film and NiO/α-Fe2O3 heterojunction are investigated in a three-electrode configuration cell using an FRA equipped PGSTAT-30 from Metrohm AutoLab. Synthesised α-Fe2O3 films and NiO/α-Fe2O3 heterojunction thin film is used as a working electrode, an Ag/AgCl (3 M KCl) and Pt wire are used as a reference and counter electrode, respectively. The measurements are performed in 1.0 M KOH (pH 13.6) electrolyte solution. A xenon lamp (100 W, XENON POWER SUPPLY XPS-150™, PA, USA) is used as a solar simulator to irradiate the quartz photoelectrochemical cell. The light intensity at place of working electrodes is calibrated at 100 mW/cm2 by using a solar power meter (TES-1333, ZhongXuan Electronic Corp. Ltd. Shanghai). PEC measurements are performed on 0.5 cm2 area of working electrodes with and without illumination of simulated sunlight.
Fig. 2. FE-SEM images of (a) α-Fe2O3 and (b) NiO/α-Fe2O3 heterojunction thin film. EDS spectra of (c) α-Fe2O3 and (d) NiO/α-Fe2O3 heterojunction thin film.
Please cite this article as: Rajendran R, et al. Preparation of nanostructured p-NiO/n-Fe2O3 heterojunction and study of their enhanced photoelectrochemical water splitting performance. Mater Lett (2014), http://dx.doi.org/10.1016/j.matlet.2014.06.157i
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3. Results and discussion The phase purity α-Fe2O3 and formation of NiO/α-Fe2O3 heterojunction thin film are investigated by X-ray diffraction. Fig. 1 shows the XRD patterns of the α-Fe2O3 and NiO/α-Fe2O3 heterojunction thin film. The Bragg's diffraction peaks of the thin film can be indexed as a rhombohedral crystal structure of α-Fe2O3 (JCPDS 898103) along with FTO background (Fig. 1). The NiO/α-Fe2O3 heterojunction thin film shows the peaks at 42.9 degree and 62.3 degree, which corresponds to the (2 0 0) and (2 2 0) diffraction plane of the face-centred cubic NiO, confirming the formation of heterojunction. The morphology of the α-Fe2O3 and NiO/α-Fe2O3 heterojunction thin film is examined using a FE-SEM. Fig. 2 shows FE-SEM images of the α-Fe2O3 and NiO/α-Fe2O3 heterojunction deposited on FTO. As shown in Fig. 2a, the α-Fe2O3 nanoflakes are vertically oriented to the surface of the substrate and have a diameter of around 10 nm. Fig. 2b demonstrates that the NiO nanoparticles are uniformly covered on vertically oriented α-Fe2O3 nanoflakes with an average size of 20 nm. The composition Fe, O and Ni are detected from energy dispersive X-ray spectroscopy (EDS) analysis confirming the formation of NiO/α-Fe2O3 heterojunction (Fig. 2c and d). The UV– visible spectrometer that is used to collect the optical absorbance spectra of pure α-Fe2O3 and NiO/α-Fe2O3 heterojunction thin film is presented in Fig. 3. Absorbance spectrum shows an absorbance band around 540 nm and 400 nm originating from indirect band transition of Fe3-d to d and direct O2-p to Fe3-d [17]. In addition, a broad absorbance band is observed in the region between 516 and
Fig. 3. UV–visible absorbance spectra of the α-Fe2O3 and NiO/α-Fe2O3 heterojunction thin film.
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418 nm for NiO/α-Fe2O3 heterojunction related to NiO nanoparticles [15]. Moreover, the optical absorbance of NiO/α-Fe2O3 heterojunction is blue shifted due to the overlap of conduction band of 3d level in Fe3 þ with d-level of Ni2 þ which enables the charge transfer transitions between electrons in the conduction band of α-Fe2O3 and d-level of Ni2 þ [16]. The electrochemical impedance spectroscopy measurements are performed to study the formation of p–n heterojunction and defining the flat-band potential at the surface of nanostructures. Fig. 4a shows Mott–Schottky plots obtained from 1/C2 versus applied potential for α-Fe2O3 and NiO/α-Fe2O3 heterojunction thin film at constant frequency of 10 kHz. A positive slope is obtained from α-Fe2O3 thin film indicating the existence of electrons as the majority carriers (n-type semiconductor). The NiO/α-Fe2O3 heterojunction shows both negative and positive slopes corresponding to the p and n type conductivity, respectively, which indicates the formation of p- and n-type semiconductor-based heterojunction [12]. The flat-band potential is obtained from the intercepts of the extrapolated straight lines on the applied potential. The measured flat-band potential is 0.6 V vs. Ag/AgCl and 0.3 V vs. Ag/AgCl for α-Fe2O3 and NiO/α-Fe2O3 heterojunction, respectively. From the slope of the straight line of the Mott–Schottky plots, the electron and hole concentration can be calculated. With e¼8 for the dielectric constant of iron oxide the electron density is calculated as 1.40 1020 and 0.5 1021 cm 3 for α-Fe2O3 and NiO/ α-Fe2O3 heterojunction thin film, respectively. The increase in the charge carrier density of NiO/α-Fe2O3 heterojunction is due to the decrease in the recombination rate within the space charge layer region [18]. Potentiostatic impedance spectroscopy measurements are used to study charge transfer characteristics of photoanode under simulated solar illumination. The impedance measurement is performed at 0.6 V vs. Ag/AgCl applied potential. As shown in Supplementary Fig. 1(a and b), impedance spectroscopy measurement presented as a Nyquist spectrum. α-Fe2O3 thin film have only one capacitance arc and one semi arc, while NiO/α-Fe2O3 heterojunction thin film has two capacitance arcs one from high frequency side and another one from low frequency side. High frequency is attributed to the Schottky barrier between α-Fe2O3 and FTO whereas lower frequency arc is due the formation of p–n junction between α-Fe2O3 and NiO. Furthermore, the impedance of α-Fe2O3 thin film is much larger than NiO/α-Fe2O3 heterojunction thin film in the same scale due to the poor conductivity of αFe2O3 thin film [19]. In order to study the charge transfer characteristics of thin film, the raw data is fitted by an equivalent circuit model and presented in Supplementary Fig. 1b as an inset.
Fig. 4. (a) Mott–Schottky plots of α-Fe2O3 and NiO/α-Fe2O3 heterojunction thin film under simulated solar light (100 mW/cm2) and (b) photocurrent density under dark and simulated solar for α-Fe2O3 and NiO/α-Fe2O3 thin film.
Please cite this article as: Rajendran R, et al. Preparation of nanostructured p-NiO/n-Fe2O3 heterojunction and study of their enhanced photoelectrochemical water splitting performance. Mater Lett (2014), http://dx.doi.org/10.1016/j.matlet.2014.06.157i
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The RC circuit model contains two types of circuits R2/CPE2 can be assigned to electrode–electrolyte interface and R1/CPE1 is attributed to the electron transport inside the electrode [18]. The water oxidation takes place at the interface of the electrode–electrolyte. The α-Fe2O3 thin film showed the high charge transfer resistance and low capacitance at the electrode–electrolyte while the NiO/αFe2O3 heterojunction showed the decreased charge transfer resistances and increased capacitances [20]. This is due to increase in the charge separation at electrode–electrolyte which promote the PEC water splitting process. The PEC performance of vertically aligned α-Fe2O3 and NiO/αFe2O3 heterojunction thin film is studied in an electrochemical cell of three electrode systems using AM 1.5 100 mW/cm2 simulated solar illuminations. Applied potential values are converted to a reversible hydrogen electrode (RHE) scale using the Nernst relationship. The current density of α-Fe2O3 and NiO/α-Fe2O3 heterojunction thin film is measured as shown in Fig. 4b. No photocurrent is observed from both α-Fe2O3 and NiO/α-Fe2O3 heterojunction thin film under the dark condition. Under simulated solar light irradiation, NiO/α-Fe2O3 heterojunction exhibits the maximum photocurrent density of 1.55 mA/cm2 at 1V/RHE, which is much higher than α-Fe2O3 thin film (0.08 mA/cm2) at 1V/ RHE. The improved average photocurrent density is due to the catalytic effect of NiO nanoparticles that can reduce the barrier potential oxygen evolution. Moreover, the dynamics of the local carrier concentrations can also be contributed in increase the photocurrent density. The local charge carrier concentrations are increased through efficient separation of photoexcited electron– hole pairs of nanostructured p-NiO/n-Fe2O3 heterojunction under simulated solar radiations [21]. The onset potential also shifts towards the negative potential side for NiO/α-Fe2O3 heterojunction thin film in comparison to the α-Fe2O3 thin film. The photocurrent density of the NiO/α-Fe2O3 nanostructures further can be improved by optimizing the deposition parameter like reaction temperature, hydrothermal reaction time and precursor concentration for α-Fe2O3 nanostructures. In addition, the optimization of α-Fe2O3 thin film thickness and particle size of NiO nanoparticles can also be improved the photocurrent density of NiO/α-Fe2O3 nanostructures heterojunction. 4. Conclusions In this article, we develop NiO/α-Fe2O3 heterojunction thin film composed of p-type NiO nanoparticles deposited on the n-type αFe2O3 nanoflakes by solution/heat procedure for enhanced photoelectrochemical water splitting application. Electrochemical impedance spectroscopy measurements of NiO/α-Fe2O3 heterojunction
thin film show the formation of p–n junction and reduction of the surface recombination process compared to α-Fe2O3 alone. A remarkable change in photocurrent is observed from 0.08 to 1.55 mA/cm2, at an applied potential of 1V/RHE for NiO/α-Fe2O3 heterojunction compared to a pure α-Fe2O3 thin film.
50 51 52 53 54 55 56 57 Acknowledgements 58 59 This project is financed by Universiti Kebangsaan Malaysia 60 under grant FGS/2/2013/ST01/UKM/01/1 and GUP-2013-063. 61 62 63 Appendix A. Supporting information 64 65 Supplementary data associated with this article can be found in 66 the online version at http://dx.doi.org/10.1016/j.matlet.2014.06. 67 157. 68 69 70 References 71 72 [1] Lewis NS, Nocera DG. Proc Nat Acad Sci USA 2006;103 (15729-35). [2] Lin Y, Yuan G, Sheehan S, Zhou S, Wang D. Energy Environ Sci 2011;4 73 (4862-69). 74 [3] Hernández S, Tortello M, Sacco A, Quaglio M, Meyer T, Bianco S, et al. 75 Electrochim Acta 2014;131 (184-94). [4] Singh N, Kumari B, Sharma S, Chaudhary S, Upadhyay S, Satsangi V, et al. 76 Renewable Energy. 2014;69 (242-52). 77 [5] Sivula K, Formal FL, Grätzel M. ChemSusChem 2011;4 (432-49). 78 [6] Wang G, Ling Y, Wang H, Xihong L, Li Y. J. Photochem. Photobiol., C 2014;19 79 (35-51). [7] Wheeler DA, Wang G, Ling Y, Li Y, Zhang JZ. Energy Environ Sci 2012;5 80 (6682–702). 81 [8] Zang Z, Zang Y. Appl Opt 2012;51 (424-30). 82 [9] Zhu M, Wang Y, Meng D, Qin X, Diao G. J Phys Chem C 2012;116 (16276 85). [10] Zang Z, Zang Y. J Mod opt 2012;59 (161-65). 83 [11] Zang Z. Appl Opt 2013;52 (5701-06). 84 [12] Hernández S., Cauda V., Hidalgo D., Rivera V.F., Manfredi D., Chiodoni A., et al. J Alloys Compd Article in Press, http://dx.doi.org/10.1016/j.jallcom.2014.02.010. Q2 85 [13] Kleiman-Shwarsctein A, Hu Y, Stucky GD, McFarland EW. Electrochem Com86 mun 2009;11 (1150-53). 87 [14] Li J, Meng F, Suri S, Ding W, Huang F, Wu N. Chem Commun 2012;48 88 (8213-15). 89 [15] McDonald KJ, Choi KS. Chem Mater 2011;23 (4863-69). ́ [16] Fuertea A, Hernández-Alonsoa MD, Mairaa AJ, Martınez-Ariasa A, Fernández90 ́ M, Conesaa JC, et al. J Catal 2002;212:1–9. Garcıaa 91 [17] Al-Kuhaili MF, Saleem M, Durrani SMA. J Alloys Compd 2012;521 (178-82). 92 [18] Kumar P, Sharma P, Shrivastav R, Dass S, Satsangi VR. Int J Hydrogen Energy 2011;4 (2777-84). 93 [19] Kim ES, Nishimura N, Magesh G, Kim JY, Jang JW, Jun H, et al. J Am Chem Soc 94 2013;1313 (5375-83). 95 [20] Badia-Bou L, Mas-Marza E, Rodenas P, Barea EM, Fabregat-Santiago F, Gimenez S, et al. J Phys Chem C 2013;117 (3826-33). 96 [21] Wang G, Ling Y, Lu X, Zhai T, Qian F, Tong Y, et al. Nanoscale 2013;5 (4129-33). 97
Please cite this article as: Rajendran R, et al. Preparation of nanostructured p-NiO/n-Fe2O3 heterojunction and study of their enhanced photoelectrochemical water splitting performance. Mater Lett (2014), http://dx.doi.org/10.1016/j.matlet.2014.06.157i
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Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Preparation of nanostructured p-NiO/n-Fe2O3 heterojunction and study of their enhanced photoelectrochemical water splitting performance Ramesh Rajendran a,n, Zahira Yaakob a,b,nn, Mohd Asri Mat Teridi b, Muhammad Syukri Abd Rahaman a, Kamaruzzaman Sopian b a Department of Chemical and Process Engineering, Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, UKM Bangi 43600, Selangor, Malaysia b Solar Energy Research Institute (SERI), Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia
art ic l e i nf o
a b s t r a c t
Article history: Received 5 June 2014 Accepted 26 June 2014
The n-type α-Fe2O3 nanoflakes are deposited on the FTO substrate by a hydrothermal method and modified with p-type NiO nanoparticles to enhance the photoelectrochemical water splitting performance. X-ray diffraction, field emission scanning electron microscopy, UV–visible and impedance spectroscopy analysis are carried out to study the structural, morphological, and electrochemical characteristics. Formation of p–n heterojunction is confirmed by an impedance spectroscopy analysis and explains the transport of charge carriers. NiO/α-Fe2O3 heterojunction thin film shows the enhancement in photocurrent density (1.55 mA/cm2) compared to the α-Fe2O3 nanoflakes alone (0.08 mA/cm2) under simulated solar radiation at applied potential 1V/RHE. & 2014 Published by Elsevier B.V.
Keywords: Nanocrystalline materials X-ray diffraction Chemical synthesis Optical materials and properties Photoelectrochemical measurement
1. Introduction Hydrogen is an alternative energy source with zero carbon emission. Hydrogen can be produced by splitting of water using naturally abundant solar light through Photoelectrochemical (PEC) performance [1]. Recently, many efforts focus on the development of PEC cells, which is used to split the water molecules into molecular oxygen and hydrogen under simulated solar radiation [2]. In a typical PEC cell, semiconductor based photoelectrode is considered essential to split the water for production of hydrogen [3,4]. Hematite (α-Fe2O3) based semiconductor photocatalysts receives immense attention because of its versatile properties like chemical stability, suitable band gap to absorb the 40% of solar radiation, corrosion resistance, and nontoxicity [5]. However, it possesses some drawbacks such as short hole diffusion distance (2–4 nm) which limits their charge collection, poor minority charge carrier mobility, and low charge transfer kinetics at the electrode–electrolyte interface [6]. To overcome these limitations, many efforts explore the development of α-Fe2O3 nanostructures with various dimensions and improvement of charge transport property by doping. There are many chemical techniques have been developed for n
Corresponding author. Tel.: þ 60173610697. Corresponding author at: Department of Chemical and Process Engineering, Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, UKM Bangi 43600, Selangor, Malaysia. Tel.: þ60173610697. E-mail addresses: [email protected] (R. Rajendran), [email protected] (Z. Yaakob). nn
synthesis of α-Fe2O3 nanostructures like hydrothermal, sol–gel and Aerosol-Assisted Chemical Vapor Deposition (AACVD) [7]. Among these a hydrothermal technique is highly suitable for synthesis of nanostructures with special dimension and also can be used for effective doping metals for optical and optoelectronic applications [8–11]. In addition, the formation of heterojunction can be improved PEC performance. For example, Hernández et al. have reported that the improvement of PEC performance by the formation of the oxide based heterojunction [12]. Moreover, a few studies demonstrate the development of p–n junction using α-Fe2O3 nanostructures as an ntype semiconductor [13–15]. In this article, we report the synthesis of flakes like α-Fe2O3 nanostructures on the FTO substrate by a hydrothermal technique. Further p type NiO nanoparticles are deposited on α-Fe2O3 nanostructures to form p–n heterojunction by a solution/heat procedure. Structural, morphological, and optical properties are studied by using X-ray diffraction, field emission scanning electron microscopy (FESEM) and UV–visible spectrometer. PEC performance, charge carrier density, and charge carrier kinetics are studied in three electrode PEC cells.
2. Experimental procedures Deposition of α-Fe2O3 nanoflakes on FTO substrate: The α-Fe2O3 nanoflakes are deposited onto a fluorine-doped tin oxide (FTO, Nippon Sheet Glass Co. Ltd., 15 Ω) glass substrate by a solution-
http://dx.doi.org/10.1016/j.matlet.2014.06.157 0167-577X/& 2014 Published by Elsevier B.V.
Please cite this article as: Rajendran R, et al. Preparation of nanostructured p-NiO/n-Fe2O3 heterojunction and study of their enhanced photoelectrochemical water splitting performance. Mater Lett (2014), http://dx.doi.org/10.1016/j.matlet.2014.06.157i
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R. Rajendran et al. / Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎
based hydrothermal method. In the typical procedure, 2 mmol of ferric chloride (FeCl3 6H2O) and 2 mmol of urea are subsequently dissolved in a 40 ml mixture of distilled water and glycerol (3:1) solution for 10 min under magnetic stirring at room temperature. The 35 ml solution is poured into a 50 ml of Teflon-lined stainless steel autoclave containing an FTO glass substrate vertically placed and partially immersed into the solution. The autoclave is then placed in a hot air oven at a constant temperature of 200 1C for 12 h. At the end of the reaction, the autoclave is left to cool to room temperature. Finally α-Fe2O3 deposited FTO substrate is taken out from the autoclave and washed in DI water in an ultrasonic bath to remove impurities, then dried in air at room temperature. Formation of p–n heterojunction using NiO and α-Fe2O3: NiO nanoparticles are deposited on the vertically aligned α-Fe2O3
Fig. 1. XRD patterns of the α-Fe2O3 and NiO/α-Fe2O3 heterojunction thin film.
nanoflakes by a simple solution/heat procedure. In the typical experiment, nickel nitride (0.14 g) is dissolved in absolute ethanol (30 ml) at room temperature to form a light green solution. Subsequently, FTO substrate contains vertically aligned α-Fe2O3 nanoflakes immersed into the solution and quickly taken out of the solution. The FTO is dried at room temperature in air and then subjected to the annealing process at 300 1C. Finally, the substrate is washed with ethanol and DI water several times to remove the unreacted impurities. Characterizations: The phase formation of α-Fe2O3 and NiO/α-Fe2O3 heterojunction thin films are studied by XRD (Rigaku DMax 200, using Cu Kα radiation) in the 2θ range from 20 degree to 70 degree with a step scan of 0.02 degree/12 s. Optical absorbance spectra is measured using PerkinElmer UV WinLab 6.0.4.0738/Lambda 35 1.27 spectrophotometer and all the dates are corrected by deducting the baseline of the FTO substrate. Morphology and chemical composition of the samples are investigated by a field-emission scanning electron microscope (FE-SEM, S4800, Hitachi) equipped with an Energy Dispersive Xray (EDX) analyser. The steady sate current density (j–v) and electrochemical impedance spectroscopy measurements of α-Fe2O3 film and NiO/α-Fe2O3 heterojunction are investigated in a three-electrode configuration cell using an FRA equipped PGSTAT-30 from Metrohm AutoLab. Synthesised α-Fe2O3 films and NiO/α-Fe2O3 heterojunction thin film is used as a working electrode, an Ag/AgCl (3 M KCl) and Pt wire are used as a reference and counter electrode, respectively. The measurements are performed in 1.0 M KOH (pH 13.6) electrolyte solution. A xenon lamp (100 W, XENON POWER SUPPLY XPS-150™, PA, USA) is used as a solar simulator to irradiate the quartz photoelectrochemical cell. The light intensity at place of working electrodes is calibrated at 100 mW/cm2 by using a solar power meter (TES-1333, ZhongXuan Electronic Corp. Ltd. Shanghai). PEC measurements are performed on 0.5 cm2 area of working electrodes with and without illumination of simulated sunlight.
Fig. 2. FE-SEM images of (a) α-Fe2O3 and (b) NiO/α-Fe2O3 heterojunction thin film. EDS spectra of (c) α-Fe2O3 and (d) NiO/α-Fe2O3 heterojunction thin film.
Please cite this article as: Rajendran R, et al. Preparation of nanostructured p-NiO/n-Fe2O3 heterojunction and study of their enhanced photoelectrochemical water splitting performance. Mater Lett (2014), http://dx.doi.org/10.1016/j.matlet.2014.06.157i
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3. Results and discussion The phase purity α-Fe2O3 and formation of NiO/α-Fe2O3 heterojunction thin film are investigated by X-ray diffraction. Fig. 1 shows the XRD patterns of the α-Fe2O3 and NiO/α-Fe2O3 heterojunction thin film. The Bragg's diffraction peaks of the thin film can be indexed as a rhombohedral crystal structure of α-Fe2O3 (JCPDS 898103) along with FTO background (Fig. 1). The NiO/α-Fe2O3 heterojunction thin film shows the peaks at 42.9 degree and 62.3 degree, which corresponds to the (2 0 0) and (2 2 0) diffraction plane of the face-centred cubic NiO, confirming the formation of heterojunction. The morphology of the α-Fe2O3 and NiO/α-Fe2O3 heterojunction thin film is examined using a FE-SEM. Fig. 2 shows FE-SEM images of the α-Fe2O3 and NiO/α-Fe2O3 heterojunction deposited on FTO. As shown in Fig. 2a, the α-Fe2O3 nanoflakes are vertically oriented to the surface of the substrate and have a diameter of around 10 nm. Fig. 2b demonstrates that the NiO nanoparticles are uniformly covered on vertically oriented α-Fe2O3 nanoflakes with an average size of 20 nm. The composition Fe, O and Ni are detected from energy dispersive X-ray spectroscopy (EDS) analysis confirming the formation of NiO/α-Fe2O3 heterojunction (Fig. 2c and d). The UV– visible spectrometer that is used to collect the optical absorbance spectra of pure α-Fe2O3 and NiO/α-Fe2O3 heterojunction thin film is presented in Fig. 3. Absorbance spectrum shows an absorbance band around 540 nm and 400 nm originating from indirect band transition of Fe3-d to d and direct O2-p to Fe3-d [17]. In addition, a broad absorbance band is observed in the region between 516 and
Fig. 3. UV–visible absorbance spectra of the α-Fe2O3 and NiO/α-Fe2O3 heterojunction thin film.
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418 nm for NiO/α-Fe2O3 heterojunction related to NiO nanoparticles [15]. Moreover, the optical absorbance of NiO/α-Fe2O3 heterojunction is blue shifted due to the overlap of conduction band of 3d level in Fe3 þ with d-level of Ni2 þ which enables the charge transfer transitions between electrons in the conduction band of α-Fe2O3 and d-level of Ni2 þ [16]. The electrochemical impedance spectroscopy measurements are performed to study the formation of p–n heterojunction and defining the flat-band potential at the surface of nanostructures. Fig. 4a shows Mott–Schottky plots obtained from 1/C2 versus applied potential for α-Fe2O3 and NiO/α-Fe2O3 heterojunction thin film at constant frequency of 10 kHz. A positive slope is obtained from α-Fe2O3 thin film indicating the existence of electrons as the majority carriers (n-type semiconductor). The NiO/α-Fe2O3 heterojunction shows both negative and positive slopes corresponding to the p and n type conductivity, respectively, which indicates the formation of p- and n-type semiconductor-based heterojunction [12]. The flat-band potential is obtained from the intercepts of the extrapolated straight lines on the applied potential. The measured flat-band potential is 0.6 V vs. Ag/AgCl and 0.3 V vs. Ag/AgCl for α-Fe2O3 and NiO/α-Fe2O3 heterojunction, respectively. From the slope of the straight line of the Mott–Schottky plots, the electron and hole concentration can be calculated. With e¼8 for the dielectric constant of iron oxide the electron density is calculated as 1.40 1020 and 0.5 1021 cm 3 for α-Fe2O3 and NiO/ α-Fe2O3 heterojunction thin film, respectively. The increase in the charge carrier density of NiO/α-Fe2O3 heterojunction is due to the decrease in the recombination rate within the space charge layer region [18]. Potentiostatic impedance spectroscopy measurements are used to study charge transfer characteristics of photoanode under simulated solar illumination. The impedance measurement is performed at 0.6 V vs. Ag/AgCl applied potential. As shown in Supplementary Fig. 1(a and b), impedance spectroscopy measurement presented as a Nyquist spectrum. α-Fe2O3 thin film have only one capacitance arc and one semi arc, while NiO/α-Fe2O3 heterojunction thin film has two capacitance arcs one from high frequency side and another one from low frequency side. High frequency is attributed to the Schottky barrier between α-Fe2O3 and FTO whereas lower frequency arc is due the formation of p–n junction between α-Fe2O3 and NiO. Furthermore, the impedance of α-Fe2O3 thin film is much larger than NiO/α-Fe2O3 heterojunction thin film in the same scale due to the poor conductivity of αFe2O3 thin film [19]. In order to study the charge transfer characteristics of thin film, the raw data is fitted by an equivalent circuit model and presented in Supplementary Fig. 1b as an inset.
Fig. 4. (a) Mott–Schottky plots of α-Fe2O3 and NiO/α-Fe2O3 heterojunction thin film under simulated solar light (100 mW/cm2) and (b) photocurrent density under dark and simulated solar for α-Fe2O3 and NiO/α-Fe2O3 thin film.
Please cite this article as: Rajendran R, et al. Preparation of nanostructured p-NiO/n-Fe2O3 heterojunction and study of their enhanced photoelectrochemical water splitting performance. Mater Lett (2014), http://dx.doi.org/10.1016/j.matlet.2014.06.157i
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The RC circuit model contains two types of circuits R2/CPE2 can be assigned to electrode–electrolyte interface and R1/CPE1 is attributed to the electron transport inside the electrode [18]. The water oxidation takes place at the interface of the electrode–electrolyte. The α-Fe2O3 thin film showed the high charge transfer resistance and low capacitance at the electrode–electrolyte while the NiO/αFe2O3 heterojunction showed the decreased charge transfer resistances and increased capacitances [20]. This is due to increase in the charge separation at electrode–electrolyte which promote the PEC water splitting process. The PEC performance of vertically aligned α-Fe2O3 and NiO/αFe2O3 heterojunction thin film is studied in an electrochemical cell of three electrode systems using AM 1.5 100 mW/cm2 simulated solar illuminations. Applied potential values are converted to a reversible hydrogen electrode (RHE) scale using the Nernst relationship. The current density of α-Fe2O3 and NiO/α-Fe2O3 heterojunction thin film is measured as shown in Fig. 4b. No photocurrent is observed from both α-Fe2O3 and NiO/α-Fe2O3 heterojunction thin film under the dark condition. Under simulated solar light irradiation, NiO/α-Fe2O3 heterojunction exhibits the maximum photocurrent density of 1.55 mA/cm2 at 1V/RHE, which is much higher than α-Fe2O3 thin film (0.08 mA/cm2) at 1V/ RHE. The improved average photocurrent density is due to the catalytic effect of NiO nanoparticles that can reduce the barrier potential oxygen evolution. Moreover, the dynamics of the local carrier concentrations can also be contributed in increase the photocurrent density. The local charge carrier concentrations are increased through efficient separation of photoexcited electron– hole pairs of nanostructured p-NiO/n-Fe2O3 heterojunction under simulated solar radiations [21]. The onset potential also shifts towards the negative potential side for NiO/α-Fe2O3 heterojunction thin film in comparison to the α-Fe2O3 thin film. The photocurrent density of the NiO/α-Fe2O3 nanostructures further can be improved by optimizing the deposition parameter like reaction temperature, hydrothermal reaction time and precursor concentration for α-Fe2O3 nanostructures. In addition, the optimization of α-Fe2O3 thin film thickness and particle size of NiO nanoparticles can also be improved the photocurrent density of NiO/α-Fe2O3 nanostructures heterojunction. 4. Conclusions In this article, we develop NiO/α-Fe2O3 heterojunction thin film composed of p-type NiO nanoparticles deposited on the n-type αFe2O3 nanoflakes by solution/heat procedure for enhanced photoelectrochemical water splitting application. Electrochemical impedance spectroscopy measurements of NiO/α-Fe2O3 heterojunction
thin film show the formation of p–n junction and reduction of the surface recombination process compared to α-Fe2O3 alone. A remarkable change in photocurrent is observed from 0.08 to 1.55 mA/cm2, at an applied potential of 1V/RHE for NiO/α-Fe2O3 heterojunction compared to a pure α-Fe2O3 thin film.
50 51 52 53 54 55 56 57 Acknowledgements 58 59 This project is financed by Universiti Kebangsaan Malaysia 60 under grant FGS/2/2013/ST01/UKM/01/1 and GUP-2013-063. 61 62 63 Appendix A. Supporting information 64 65 Supplementary data associated with this article can be found in 66 the online version at http://dx.doi.org/10.1016/j.matlet.2014.06. 67 157. 68 69 70 References 71 72 [1] Lewis NS, Nocera DG. Proc Nat Acad Sci USA 2006;103 (15729-35). [2] Lin Y, Yuan G, Sheehan S, Zhou S, Wang D. Energy Environ Sci 2011;4 73 (4862-69). 74 [3] Hernández S, Tortello M, Sacco A, Quaglio M, Meyer T, Bianco S, et al. 75 Electrochim Acta 2014;131 (184-94). [4] Singh N, Kumari B, Sharma S, Chaudhary S, Upadhyay S, Satsangi V, et al. 76 Renewable Energy. 2014;69 (242-52). 77 [5] Sivula K, Formal FL, Grätzel M. ChemSusChem 2011;4 (432-49). 78 [6] Wang G, Ling Y, Wang H, Xihong L, Li Y. J. Photochem. Photobiol., C 2014;19 79 (35-51). [7] Wheeler DA, Wang G, Ling Y, Li Y, Zhang JZ. Energy Environ Sci 2012;5 80 (6682–702). 81 [8] Zang Z, Zang Y. Appl Opt 2012;51 (424-30). 82 [9] Zhu M, Wang Y, Meng D, Qin X, Diao G. J Phys Chem C 2012;116 (16276 85). [10] Zang Z, Zang Y. J Mod opt 2012;59 (161-65). 83 [11] Zang Z. Appl Opt 2013;52 (5701-06). 84 [12] Hernández S., Cauda V., Hidalgo D., Rivera V.F., Manfredi D., Chiodoni A., et al. J Alloys Compd Article in Press, http://dx.doi.org/10.1016/j.jallcom.2014.02.010. Q2 85 [13] Kleiman-Shwarsctein A, Hu Y, Stucky GD, McFarland EW. Electrochem Com86 mun 2009;11 (1150-53). 87 [14] Li J, Meng F, Suri S, Ding W, Huang F, Wu N. Chem Commun 2012;48 88 (8213-15). 89 [15] McDonald KJ, Choi KS. Chem Mater 2011;23 (4863-69). ́ [16] Fuertea A, Hernández-Alonsoa MD, Mairaa AJ, Martınez-Ariasa A, Fernández90 ́ M, Conesaa JC, et al. J Catal 2002;212:1–9. Garcıaa 91 [17] Al-Kuhaili MF, Saleem M, Durrani SMA. J Alloys Compd 2012;521 (178-82). 92 [18] Kumar P, Sharma P, Shrivastav R, Dass S, Satsangi VR. Int J Hydrogen Energy 2011;4 (2777-84). 93 [19] Kim ES, Nishimura N, Magesh G, Kim JY, Jang JW, Jun H, et al. J Am Chem Soc 94 2013;1313 (5375-83). 95 [20] Badia-Bou L, Mas-Marza E, Rodenas P, Barea EM, Fabregat-Santiago F, Gimenez S, et al. J Phys Chem C 2013;117 (3826-33). 96 [21] Wang G, Ling Y, Lu X, Zhai T, Qian F, Tong Y, et al. Nanoscale 2013;5 (4129-33). 97
Please cite this article as: Rajendran R, et al. Preparation of nanostructured p-NiO/n-Fe2O3 heterojunction and study of their enhanced photoelectrochemical water splitting performance. Mater Lett (2014), http://dx.doi.org/10.1016/j.matlet.2014.06.157i