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Synthesis of FeS2 (pyrite) nanotube through sulfuration of Fe2O3 nanotube
Materials Letters 141 (2015) 104–106
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Synthesis of FeS2 (pyrite) nanotube through sulfuration of Fe2O3 nanotube Xiaoguo Shi a, Ang Tian a, Xiangxin Xue a,n, He Yang a, Quan Xu b a b
Liaoning Provincial Key Laboratory of Metallurgical Resources Circulation Science, Northeastern University, Shenyang 110819, China Institute of New Energy, State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, China
art ic l e i nf o
a b s t r a c t
Article history: Received 7 October 2014 Accepted 16 November 2014 Available online 26 November 2014
Pyrite (FeS2) has been widely used in the photovoltaic field due to its various advantages. In this study, FeS2 nanotube arrays were successfully synthesized by sulfuration of the precursor Fe2O3 nanotube arrays. The microstructure, crystalline phase and optical characteristics of FeS2 nanotube arrays were investigated. It was found that the precursor with tubular nano-topography could effectively accelerate the diffusion rate of sulfur atoms and make the sulfuration more complete. The structure of FeS2 nanotube could enlarge the optical absorption area and enhance the scattering of incident light which would extend the travel path of photon in the nanotube coating and ultimately promoted the optical absorption. The bandgap of FeS2 nanotube arrays was 1.24 eV. & 2014 Elsevier B.V. All rights reserved.
Keywords: FeS2 nanotube Optical property Nanocrystalline Microstructure Band gap
1. Introduction Since the photoelectric characteristics of the natural pyrite were discovered, lots of research works about preparation, crystal structure, morphology and photoelectric property of FeS2 were reported [1,2]. The optical absorption capacity of pyrite (α4105 cm 1) is two orders of magnitude higher than that of the monocrystalline Si [3] and pyrite has appropriate band gap of 0.95 eV. With these excellent properties, pyrite has been widely used as the photoanode [4], sensitizer for the energy conversion [5] and solar cells [6]. However, the S/Fe ratio in natural pyrite does not fully comply with the stoichiometric ratio. Therefore, various preparation methods have been applied for the preparation of FeS2, such as, chemical vapor deposition (CVD) [7], flash evaporation [8], magnetron sputtering and ion sputtering [9], sulfuration of Fe, Fe2O3 or Fe3O4 [10,11], etc. Among these methods, sulfuration of the precursor Fe2O3 can effectively avoid the formation intermediate phases (e.g. FeS, Fe1 xS). It is known that TiO2 nanotube arrays could be used as the photoanode of the solar cell [12,13] with larger special surface area to absorb dye molecule and highly ordered structure which could accelerate transmission of photoelectronics. The optical absorption property of nanotube arrays could be tuned by geometry size of nanotube such as inner radii, wall thickness and the length of nanotube [14,15]. For example, the increase of the wall thickness
n
Corresponding author. E-mail address: [email protected] (X. Xue).
http://dx.doi.org/10.1016/j.matlet.2014.11.084 0167-577X/& 2014 Elsevier B.V. All rights reserved.
of TiO2 nanotube would lead to the narrower forbidden band gap which could be attributed to the quantum confinement [14]. On the basis of the works mentioned above, we successfully prepared FeS2 (pyrite) nanotube array by sulfuration of the precursor Fe2O3 nanotube arrays. The microstructure, optical absorption property and bandgap of FeS2 nanotube coating were also investigated.
2. Experimental Preparation of precursor Fe3O4 and TiO2 nanotube coating: The commercial Fe foils (1 mm thick, purity 99.5%) were anodized at 70 V in the electrolyte solution containing ethylene glycol, 0.9 wt% NH4F and 3 vol% H2O for 20 min at 10 1C. Subsequently, the amorphous samples were annealed at 400 1C for 2 h in air to obtain the precursor Fe3O4 nanotube. In order to explore the effect of nanotube's morphology on the optical absorption, TiO2 nanotube array with similar morphology was prepared as the control group. The cleaned Ti foil was anodized at 40 V for 6 h in the electrolyte solution containing ethylene glycol, 5 wt% NH4F and 2 vol% hydrofluoric acid. Then the TiO2 nanotube sample was annealed at 450 1C for 1 h in air. Sulfuration of precursor Fe3O4 nanotube coating: The precursors were sealed in the quartz tube (vacuum tube furnace) with the sulfur pressure of 80 kPa. Filling and evacuating the quartz tube with nitrogen for 5 times to ensure the residual gas pressure was less than 1 10 2 Pa before annealing at 400 1C for 5 h. The control sample of FeS2 micro-particles coating (FeS2 sheet) was prepared at 600 1C for 10 h with the Fe substrate.
X. Shi et al. / Materials Letters 141 (2015) 104–106
Characterization: The morphology and crystallinity of the samples were evaluated by field emission scanning electron microscopy (FE-SEM, ZEISS ULTRA) and X-ray diffraction (XRD, Rigaku D/ Max 2500). The transmission electron microscopy (TEM, JEM2100) was used to characterize the single nanotube's morphology and crystallinity. Optical absorption spectra of the samples at ambient temperature were recorded at wavelength between 400 and 1100 nm with UV-2550 spectrophotometer which spectral resolution is 0.1 nm.
3. Result and discussion The morphologies of different samples were shown in Fig. 1a. The vertically-aligned FeS2 nanotubes were highly ordered arranged and the caliber, length and wall thickness were 90710 nm, 2.570.2 μm and 1572 nm respectively. The XRD pattern of FeS2 nanotube was index to represent the pyrite phase (PDF Card 99-0087) (Fig. 1d). The diffraction peak of Fe also appeared in the pattern because of the Fe substrate. The TiO2 nanotube had the caliber, length and wall thickness of 100710 nm, 370.1 μm and 2072 nm, respectively (Fig. 1b). The
105
crystalline phase of the TiO2 nanotube was anatase mixed with rutile (Fig. 1e). Fig. 1c shows the morphology of FeS2 micro-particles with the least diameter of 200 nm. The corresponding XRD pattern showed that the intermediate phase (FeS and Fe1 xS) also exist with pyrite (Fig. 1f). The tubular structure of FeS2 nanotube could be clearly observed in Fig. 2a. The diffraction spots' lattice spacing of 0.164 nm, 0.189 nm and 0.315 nm corresponded to the d value of (311), (220) and (111) crystal planes of FeS2 (pyrite), respectively (Fig. 2b). It meant that the nanotube had the cubic crystal and single crystal characterization. As shown in Fig. 3a, the absorption curve could be divided into three stages: weak absorption region in the range below 1.25 eV; abrupt absorption region in the range of 1.25–2.0 eV and the stable absorption region in the range above 2.0 eV. Among the three samples, the stable optical absorption coefficient of FeS2 microparticles is 1.8 105 cm 1 and FeS2 nanotube has the maximum absorption coefficient of 4.6 105 cm 1. The pattern of the abrupt absorption region indicated that all of the samples possessed classical semiconductor properties [16]. The excellent absorption performance of the FeS2 nanotube could be attributed to the morphology of nanotube. The nanotube array has larger surface area which could effectively increase the diffusion rate of sulfur
Fig. 1. SEM and XRD of different samples: morphology of (a) FeS2 nanotube (b) TiO2 nanotube (c) FeS2 microparticles; XRD pattern of (d) FeS2 nanotube (e) TiO2 nanotube (f) FeS2 microparticles.
Fig. 2. TEM of single FeS2 nanotube: (a) morphology (b) diffraction pattern.
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X. Shi et al. / Materials Letters 141 (2015) 104–106
Fig. 3. Optical absorption property of different samples: (a) photo absorption coefficient α. (b) Plots of (αhν)1/2 vs. hν. Inset: zoom magnified picture of TiO2 nanotubes' absorption coefficient.
atoms and complete the sulfuration, eventually leading to the better optical property than the FeS2 micro-particles mixed with FeS or Fe1 xS [17]. In addition, compared with micro-particles, the nanotubes with hollow tubular structure could enable more lights to be irradiated inside the cavity and promote the light refraction [18] as well as the subsurface scattering [19], therefore, the absorption was increased. Similarly, the weak optical absorption of TiO2 nanotube (1.5 104 cm 1) in visible region might be attributed to the photonic band generated from nanotube topography and multiple scattering of light [20]. The indirect band gap of FeS2 could be concluded from the curve of (αhν)1/2 vs. photoelectron energy in Fig. 3b. The optical absorption of indirect semiconductor was caused by the transitions of electrons from the valence band to the conductive band as well as assisting phonons [21]. The band gap of FeS2 microparticles was 0.88 eV and the bandgap of FeS2 nanotube arrays was about 1.24 eV which is 0.29 eV higher compared with the bulk pyrite of 0.95 eV. The narrower band gap of FeS2 micro-particles would be attributed to the lower barrier height of grain boundary [3] and interfusion of intermediate phase – FeS with small bandgap of 0.1 eV and Fe1 xS [22]. In addition, the Fe1 xS phase leaded to the generation of FeS clusters, the FeS clusters with different spin constructions would change the molecular energy level [23], and cause the shift of the band gap. On the other hand, the band gap shift of FeS2 nanotube could be ascribed to the quantum size effect initiated by the small grain size and thickness of nanotube wall [24]. In this study, the thin thickness of pyrite nanotube wall (15 72 nm) would limit the growth of the crystal, and enlarge the forbidden band gap via the quantum confinement [14,15]. The effects of size dependence of the nanotube and the spin construction of FeS cluster on the band gap would be investigated in our further studies. 4. Conclusions FeS2 nanotube array coating was successfully fabricated by sulfuration of precursor Fe3O4 nanotube. The structure of nanotube which had large specific surface area was beneficial for the diffusion rate of sulfur atoms and completion of sulfuration. Meanwhile, the
FeS2 nanotube coating showed better optical property than FeS2 micro-particles which could attribute to the morphology effect that the nanotube enhanced the scatter of light and promoted the optical absorption.
Acknowledgments This work is supported by the National Natural Science Foundation of China (U1261120, 51002027), Post Doctoral Foundation of China (2013M530930), Scientific Foundation of Educational Department of Liaoning Province (L2012084), Project of Ministry of Education of Basic Scientific Research (N130402001).
References [1] Jaegermann W, Tributsch H. J Appl Electrochem 1983;13:743–50. [2] Xia J, Jiao JQ, Dai BL. RSC Adv 2013;3:6132–40. [3] Ennaoui A, Fiechter S, Pettenkofer C, Alonso-Vante N, Buker K, Bronold M, Hoopfner C. Sol Energy Mater Sol Cells 1993;29:289–370. [4] Takada K, Lwamoto K, Kondo S. Sol State Ion 1999;117:273–6. [5] Song XM, Wu JM, Meng L. J Am Ceram Soc 2010;93:2068–73. [6] Bucher E. Appl Phys 1978;17:1–26. [7] Oertel J, Ellmer K, et al. J Cryst Growth 1999;198:1205–10. [8] de las Heras C, Sanchez C. Thin Solid Films 1991;199:259–67. [9] Bronold M, Kubala S, et al. Thin Solid Films 1997;304:178–82. [10] Smestad G, Ennaoui A, Fiechter S, Tributsch H, Hofmann WK, Birkholz M, Kautek W. Sol Energy Mater 1990;20:149–65. [11] Meng L, Liu YH, Tian L. J Cryst Growth 2003;253:530–8. [12] Liu CW, Tian A, Yang H, Xu Q, Xue XX. Appl Surf Sci 2013;287:218–22. [13] Liu CW, Tian A, Yang H, Xue XX. Mater Lett 2013;106:1–4. [14] Cummings FR, Le Roux LJ, Mathe MK, et al. Mater Chem Phys 2010;124:234–42. [15] Lai YK, Sun L, Chen C, et al. Appl Surf Sci 2005;252:1101–6. [16] Dong YZ, Zheng YF, Duan H, Sun YF, Chen YH. Mater Lett 2005;59:2398–402. [17] Wang MD, Xing CC, Cao K. J Mater Chem A 2014;2:9496–505. [18] Liu YL, Chen YQ, Chen KQ, et al. Electrochim Acta 2014;146:838–44 http://dx. doi.org/10.1016/j.electacta.2014.09.09. [19] Mohamed A, Jeffrey S, Lu WT, Thomas C, Latika M, Christian R. J Appl Phys 2014;115:014306. [20] Guo M, Yong ZH, Xie KY. ACS Appl Mater Interfaces 2013;5:13022–8. [21] Abass AK, Ahmed ZA, Tahir RE. J Appl Phys 1987;61:2339–41. [22] Xu Y, Schoonen AAM. Am Mineral 2000;85:543–56. [23] Matusiewicz M, Czerwiński M, Kasperczyk J, et al. J Chem Phys 1999;111:6446–55. [24] Lai YK, Sun L, Chen YC, Zhuang HF, Lin CJ, Chin JW. J Electrochem Soc 2006;153: D123–D127.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Synthesis of FeS2 (pyrite) nanotube through sulfuration of Fe2O3 nanotube Xiaoguo Shi a, Ang Tian a, Xiangxin Xue a,n, He Yang a, Quan Xu b a b
Liaoning Provincial Key Laboratory of Metallurgical Resources Circulation Science, Northeastern University, Shenyang 110819, China Institute of New Energy, State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, China
art ic l e i nf o
a b s t r a c t
Article history: Received 7 October 2014 Accepted 16 November 2014 Available online 26 November 2014
Pyrite (FeS2) has been widely used in the photovoltaic field due to its various advantages. In this study, FeS2 nanotube arrays were successfully synthesized by sulfuration of the precursor Fe2O3 nanotube arrays. The microstructure, crystalline phase and optical characteristics of FeS2 nanotube arrays were investigated. It was found that the precursor with tubular nano-topography could effectively accelerate the diffusion rate of sulfur atoms and make the sulfuration more complete. The structure of FeS2 nanotube could enlarge the optical absorption area and enhance the scattering of incident light which would extend the travel path of photon in the nanotube coating and ultimately promoted the optical absorption. The bandgap of FeS2 nanotube arrays was 1.24 eV. & 2014 Elsevier B.V. All rights reserved.
Keywords: FeS2 nanotube Optical property Nanocrystalline Microstructure Band gap
1. Introduction Since the photoelectric characteristics of the natural pyrite were discovered, lots of research works about preparation, crystal structure, morphology and photoelectric property of FeS2 were reported [1,2]. The optical absorption capacity of pyrite (α4105 cm 1) is two orders of magnitude higher than that of the monocrystalline Si [3] and pyrite has appropriate band gap of 0.95 eV. With these excellent properties, pyrite has been widely used as the photoanode [4], sensitizer for the energy conversion [5] and solar cells [6]. However, the S/Fe ratio in natural pyrite does not fully comply with the stoichiometric ratio. Therefore, various preparation methods have been applied for the preparation of FeS2, such as, chemical vapor deposition (CVD) [7], flash evaporation [8], magnetron sputtering and ion sputtering [9], sulfuration of Fe, Fe2O3 or Fe3O4 [10,11], etc. Among these methods, sulfuration of the precursor Fe2O3 can effectively avoid the formation intermediate phases (e.g. FeS, Fe1 xS). It is known that TiO2 nanotube arrays could be used as the photoanode of the solar cell [12,13] with larger special surface area to absorb dye molecule and highly ordered structure which could accelerate transmission of photoelectronics. The optical absorption property of nanotube arrays could be tuned by geometry size of nanotube such as inner radii, wall thickness and the length of nanotube [14,15]. For example, the increase of the wall thickness
n
Corresponding author. E-mail address: [email protected] (X. Xue).
http://dx.doi.org/10.1016/j.matlet.2014.11.084 0167-577X/& 2014 Elsevier B.V. All rights reserved.
of TiO2 nanotube would lead to the narrower forbidden band gap which could be attributed to the quantum confinement [14]. On the basis of the works mentioned above, we successfully prepared FeS2 (pyrite) nanotube array by sulfuration of the precursor Fe2O3 nanotube arrays. The microstructure, optical absorption property and bandgap of FeS2 nanotube coating were also investigated.
2. Experimental Preparation of precursor Fe3O4 and TiO2 nanotube coating: The commercial Fe foils (1 mm thick, purity 99.5%) were anodized at 70 V in the electrolyte solution containing ethylene glycol, 0.9 wt% NH4F and 3 vol% H2O for 20 min at 10 1C. Subsequently, the amorphous samples were annealed at 400 1C for 2 h in air to obtain the precursor Fe3O4 nanotube. In order to explore the effect of nanotube's morphology on the optical absorption, TiO2 nanotube array with similar morphology was prepared as the control group. The cleaned Ti foil was anodized at 40 V for 6 h in the electrolyte solution containing ethylene glycol, 5 wt% NH4F and 2 vol% hydrofluoric acid. Then the TiO2 nanotube sample was annealed at 450 1C for 1 h in air. Sulfuration of precursor Fe3O4 nanotube coating: The precursors were sealed in the quartz tube (vacuum tube furnace) with the sulfur pressure of 80 kPa. Filling and evacuating the quartz tube with nitrogen for 5 times to ensure the residual gas pressure was less than 1 10 2 Pa before annealing at 400 1C for 5 h. The control sample of FeS2 micro-particles coating (FeS2 sheet) was prepared at 600 1C for 10 h with the Fe substrate.
X. Shi et al. / Materials Letters 141 (2015) 104–106
Characterization: The morphology and crystallinity of the samples were evaluated by field emission scanning electron microscopy (FE-SEM, ZEISS ULTRA) and X-ray diffraction (XRD, Rigaku D/ Max 2500). The transmission electron microscopy (TEM, JEM2100) was used to characterize the single nanotube's morphology and crystallinity. Optical absorption spectra of the samples at ambient temperature were recorded at wavelength between 400 and 1100 nm with UV-2550 spectrophotometer which spectral resolution is 0.1 nm.
3. Result and discussion The morphologies of different samples were shown in Fig. 1a. The vertically-aligned FeS2 nanotubes were highly ordered arranged and the caliber, length and wall thickness were 90710 nm, 2.570.2 μm and 1572 nm respectively. The XRD pattern of FeS2 nanotube was index to represent the pyrite phase (PDF Card 99-0087) (Fig. 1d). The diffraction peak of Fe also appeared in the pattern because of the Fe substrate. The TiO2 nanotube had the caliber, length and wall thickness of 100710 nm, 370.1 μm and 2072 nm, respectively (Fig. 1b). The
105
crystalline phase of the TiO2 nanotube was anatase mixed with rutile (Fig. 1e). Fig. 1c shows the morphology of FeS2 micro-particles with the least diameter of 200 nm. The corresponding XRD pattern showed that the intermediate phase (FeS and Fe1 xS) also exist with pyrite (Fig. 1f). The tubular structure of FeS2 nanotube could be clearly observed in Fig. 2a. The diffraction spots' lattice spacing of 0.164 nm, 0.189 nm and 0.315 nm corresponded to the d value of (311), (220) and (111) crystal planes of FeS2 (pyrite), respectively (Fig. 2b). It meant that the nanotube had the cubic crystal and single crystal characterization. As shown in Fig. 3a, the absorption curve could be divided into three stages: weak absorption region in the range below 1.25 eV; abrupt absorption region in the range of 1.25–2.0 eV and the stable absorption region in the range above 2.0 eV. Among the three samples, the stable optical absorption coefficient of FeS2 microparticles is 1.8 105 cm 1 and FeS2 nanotube has the maximum absorption coefficient of 4.6 105 cm 1. The pattern of the abrupt absorption region indicated that all of the samples possessed classical semiconductor properties [16]. The excellent absorption performance of the FeS2 nanotube could be attributed to the morphology of nanotube. The nanotube array has larger surface area which could effectively increase the diffusion rate of sulfur
Fig. 1. SEM and XRD of different samples: morphology of (a) FeS2 nanotube (b) TiO2 nanotube (c) FeS2 microparticles; XRD pattern of (d) FeS2 nanotube (e) TiO2 nanotube (f) FeS2 microparticles.
Fig. 2. TEM of single FeS2 nanotube: (a) morphology (b) diffraction pattern.
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X. Shi et al. / Materials Letters 141 (2015) 104–106
Fig. 3. Optical absorption property of different samples: (a) photo absorption coefficient α. (b) Plots of (αhν)1/2 vs. hν. Inset: zoom magnified picture of TiO2 nanotubes' absorption coefficient.
atoms and complete the sulfuration, eventually leading to the better optical property than the FeS2 micro-particles mixed with FeS or Fe1 xS [17]. In addition, compared with micro-particles, the nanotubes with hollow tubular structure could enable more lights to be irradiated inside the cavity and promote the light refraction [18] as well as the subsurface scattering [19], therefore, the absorption was increased. Similarly, the weak optical absorption of TiO2 nanotube (1.5 104 cm 1) in visible region might be attributed to the photonic band generated from nanotube topography and multiple scattering of light [20]. The indirect band gap of FeS2 could be concluded from the curve of (αhν)1/2 vs. photoelectron energy in Fig. 3b. The optical absorption of indirect semiconductor was caused by the transitions of electrons from the valence band to the conductive band as well as assisting phonons [21]. The band gap of FeS2 microparticles was 0.88 eV and the bandgap of FeS2 nanotube arrays was about 1.24 eV which is 0.29 eV higher compared with the bulk pyrite of 0.95 eV. The narrower band gap of FeS2 micro-particles would be attributed to the lower barrier height of grain boundary [3] and interfusion of intermediate phase – FeS with small bandgap of 0.1 eV and Fe1 xS [22]. In addition, the Fe1 xS phase leaded to the generation of FeS clusters, the FeS clusters with different spin constructions would change the molecular energy level [23], and cause the shift of the band gap. On the other hand, the band gap shift of FeS2 nanotube could be ascribed to the quantum size effect initiated by the small grain size and thickness of nanotube wall [24]. In this study, the thin thickness of pyrite nanotube wall (15 72 nm) would limit the growth of the crystal, and enlarge the forbidden band gap via the quantum confinement [14,15]. The effects of size dependence of the nanotube and the spin construction of FeS cluster on the band gap would be investigated in our further studies. 4. Conclusions FeS2 nanotube array coating was successfully fabricated by sulfuration of precursor Fe3O4 nanotube. The structure of nanotube which had large specific surface area was beneficial for the diffusion rate of sulfur atoms and completion of sulfuration. Meanwhile, the
FeS2 nanotube coating showed better optical property than FeS2 micro-particles which could attribute to the morphology effect that the nanotube enhanced the scatter of light and promoted the optical absorption.
Acknowledgments This work is supported by the National Natural Science Foundation of China (U1261120, 51002027), Post Doctoral Foundation of China (2013M530930), Scientific Foundation of Educational Department of Liaoning Province (L2012084), Project of Ministry of Education of Basic Scientific Research (N130402001).
References [1] Jaegermann W, Tributsch H. J Appl Electrochem 1983;13:743–50. [2] Xia J, Jiao JQ, Dai BL. RSC Adv 2013;3:6132–40. [3] Ennaoui A, Fiechter S, Pettenkofer C, Alonso-Vante N, Buker K, Bronold M, Hoopfner C. Sol Energy Mater Sol Cells 1993;29:289–370. [4] Takada K, Lwamoto K, Kondo S. Sol State Ion 1999;117:273–6. [5] Song XM, Wu JM, Meng L. J Am Ceram Soc 2010;93:2068–73. [6] Bucher E. Appl Phys 1978;17:1–26. [7] Oertel J, Ellmer K, et al. J Cryst Growth 1999;198:1205–10. [8] de las Heras C, Sanchez C. Thin Solid Films 1991;199:259–67. [9] Bronold M, Kubala S, et al. Thin Solid Films 1997;304:178–82. [10] Smestad G, Ennaoui A, Fiechter S, Tributsch H, Hofmann WK, Birkholz M, Kautek W. Sol Energy Mater 1990;20:149–65. [11] Meng L, Liu YH, Tian L. J Cryst Growth 2003;253:530–8. [12] Liu CW, Tian A, Yang H, Xu Q, Xue XX. Appl Surf Sci 2013;287:218–22. [13] Liu CW, Tian A, Yang H, Xue XX. Mater Lett 2013;106:1–4. [14] Cummings FR, Le Roux LJ, Mathe MK, et al. Mater Chem Phys 2010;124:234–42. [15] Lai YK, Sun L, Chen C, et al. Appl Surf Sci 2005;252:1101–6. [16] Dong YZ, Zheng YF, Duan H, Sun YF, Chen YH. Mater Lett 2005;59:2398–402. [17] Wang MD, Xing CC, Cao K. J Mater Chem A 2014;2:9496–505. [18] Liu YL, Chen YQ, Chen KQ, et al. Electrochim Acta 2014;146:838–44 http://dx. doi.org/10.1016/j.electacta.2014.09.09. [19] Mohamed A, Jeffrey S, Lu WT, Thomas C, Latika M, Christian R. J Appl Phys 2014;115:014306. [20] Guo M, Yong ZH, Xie KY. ACS Appl Mater Interfaces 2013;5:13022–8. [21] Abass AK, Ahmed ZA, Tahir RE. J Appl Phys 1987;61:2339–41. [22] Xu Y, Schoonen AAM. Am Mineral 2000;85:543–56. [23] Matusiewicz M, Czerwiński M, Kasperczyk J, et al. J Chem Phys 1999;111:6446–55. [24] Lai YK, Sun L, Chen YC, Zhuang HF, Lin CJ, Chin JW. J Electrochem Soc 2006;153: D123–D127.