- Email: [email protected]
Electrochemical behavior of chemically synthesized selenium thin film
Accepted Manuscript Short Communication Electrochemical behavior of chemically synthesized selenium thin film A.M. Patil, V.S. Kumbhar, N.R. Chodankar, A.C. Lokhande, C.D. Lokhande PII: DOI: Reference:
S0021-9797(16)30110-2 http://dx.doi.org/10.1016/j.jcis.2016.02.030 YJCIS 21092
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
11 December 2015 7 February 2016 9 February 2016
Please cite this article as: A.M. Patil, V.S. Kumbhar, N.R. Chodankar, A.C. Lokhande, C.D. Lokhande, Electrochemical behavior of chemically synthesized selenium thin film, Journal of Colloid and Interface Science (2016), doi: http://dx.doi.org/10.1016/j.jcis.2016.02.030
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Electrochemical behavior of chemically synthesized selenium thin film A. M. Patila, V. S. Kumbhara, N. R. Chodankara, A. C. Lokhandeb, C. D. Lokhandea,* a
Thin Film Physics Laboratory, Department of Physics, Shivaji University, Kolhapur, 416 004 (M.S), India. b
Department of Materials Science and Engineering, Chonnam National University, South Korea 500-757
*CORRESPONDING AUTHOR Prof. C.D. Lokhande Tel: +91 231 2609225, Fax: +91 231 2609233 E-mail:- [email protected]
1
Abstract The facile and low cost simple chemical bath deposition (CBD) method is employed to synthesize red colored selenium thin films. These selenium films are characterized for structural, morphological, topographical and wettability studies. The X-ray diffraction (XRD) pattern showed the crystalline nature of selenium thin film with hexagonal crystal structure. The scanning electron microscopy (SEM) study displays selenium nanoparticles ranging from 20 to 475 nm. A specific surface area of 30.5 m2g-1 is observed for selenium nanoparticles. The selenium nanoparticles hold mesopores in the range of 1.39 nm, taking benefits of the good physicochemical stability and excellent porosity. Subsequently, the electrochemical properties of selenium thin films are deliberated by cyclic voltammetry (CV), galvanostatic charge-discharge and electrochemical impedance spectroscopy (EIS) techniques. The selenium thin film shows specific capacitance (Cs) of 21.98 Fg-1 with 91 % electrochemical stability. Keywords: Selenium, Thin film, Chemical bath deposition, Electrochemical properties 1. Introduction During past few years, a significant interest in selenium has increased appreciably owing to number of applications such as modern electronics, optoelectronics, new generation sensors, photovoltaic cells etc. [1, 2]. The unique photoelectric and semiconducting properties of selenium find applications in rectifiers, photocells, switching electronic devices, memory devices and X-ray photoconductors [3]. Elemental selenium is almost present in environmental and biological materials. Selenium has several oxidation states such as Se2-, Se0, Se4+ and Se6+ and exhibits different structures namely amorphous, trigonal (hexagonal), α-monoclinic and β-monoclinic. The structure of selenium consists of chains (Sen) and rings (Se8) [4]. Now days, sulfur is replaced by selenium in Li-S batteries, because of the insulating nature of sulfur and dissolution of reductive polysulfide in organic electrolytes during cycling which moderately affect the performance of Li-S batteries [5]. More importantly, the higher electrical conductivity (~20 times greater than sulfur) is desired for lithium ion batteries [6]. The selenium is also used as a dopant in lead acid batteries, where it increases the corrosion resistance and discharge capacities [7]. Even all these features, no reports are available on the electrochemical supercapacitive features of selenium. In the past, selenium thin films have been prepared by various physical as well as chemical deposition methods [8-11]. In present work, selenium thin films are synthesized by chemical bath deposition (CBD) method, which is binder-free, surfactant-less, low cost and simple as compared to other physical and chemical deposition methods. Subsequently, the electrochemical properties of selenium thin films are studied.
2
2. Experimental For deposition of selenium thin film, the stock solution of 0.0125 M Na2SeSO3 was prepared by dissolving selenium powder in aqueous solution of 0.0125 M Na2SO3 with pH∼10.2 (± 0.1). The 0.0125 M Na2SeSO3 solution was acidified at room temperature (300K) by adding acetic acid and pH was maintained at 4.4 (± 0.1). The well cleaned stainless steel (SS) substrate was dipped in acidified solution of Na 2SeSO3. After 5 h, red colored selenium thin film was deposited on SS substrate. The substrate was taken out from the bath, washed several times in double distilled water (DDW) and dried at room temperature (300K). The selenium thin film structure was characterized by XRD technique using a Bruker AXS D8 advanced Model with copper radiation (Kα of wavelength = 1.54 Å). The phase confirmation was accomplished by Raman spectrometer (Bruker MultiRAM, Germany). Surface morphological investigation was done by scanning electron microscope (SEM), (JEOL-JSM 6360) and atomic force microscopy (AFM) Model-INNOVA 1B3BE units. The energy dispersive X-ray analysis (EDAX) was collected by JEOL-JSM 6360 unit. The contact angle measurement was carried out by Rame-Hart equipment with CCD camera. The specific surface area of a selenium material was calculated by Brunauer, Emmett and Teller (BET) model Quantachrome Instruments v11.02. The thickness of selenium thin film was measured using XP-1 Stylus surface profiler with laser calibration.
Electrochemical
properties of selenium thin films were executed by automatic battery cycler unit (WBCS3000) and EIS analysis was carried out using electrochemical workstation (ZIVE SP5) in 1 M Na2SO4 electrolyte. 3. Results and discussion The CBD route involves several steps for film formation such as nucleation, aggregation, coalescence and growth of particles. The nucleation conducts heterogeneous reaction on the film surface. The cluster of molecule inaugurated undergoes rapid decomposition with combination of particles to grow film up to certain thickness [12]. With addition of acetic acid in precursor solution (0.0125 M Na2SeSO3), the precipitate starts and solution becomes saturated. The proposed reaction mechanism is as follows, 2
2
3
2
3
3
(1)
3
3
2
2
Fig. 1 (A) shows XRD pattern of selenium thin film. The diffraction planes corresponding to the (100), (101) and (110) planes of hexagonal structure for selenium having lattice parameters of a= 4.3639 Å and c= 4.9595 Å [JCPDS card No. 65-1876] are observed. The peaks of SS substrate are denoted by asterisk (*). The crystallite size
3
of selenium is calculated as 29.8 nm by full width at half maxima (FWHM) for high intense peak (101) using Scherrer formula [13]. The vibrational, rotational and other low-frequency modes in a selenium material were detected by Raman spectroscopy technique. The high intense peaks observed at frequencies of 236 and 254 cm-1 are in well agreement with formation of selenium (Se8 rings) [Fig. 1 (B)]. The resonance peak observed at 236 cm-1 is a characteristic stretching mode (A1), which represents chain-like structure existing in hexagonal selenium [14]. Inset figure confirms that selenium surface is a hydrophilic, as water contact angle is 47°. Fig. 1 (C, D) shows SEM images of selenium thin film at magnifications of 2,000 X and 5,000 X. At low magnification, selenium surface is covered with nanoparticle like grains. At higher magnification, interconnection of nanoparticles distributed throughout film surface is observed. Fig. 1 (E) depicts EDAX spectrum of selenium thin film. High intense spectrum of selenium confirms selenium deposition on SS substrate. The inset of figure shows atomic (96.64 %) and weight (99.38 %) percentages of selenium. Fig. 1 (F) shows 3-D AFM image of selenium film surface with elliptical shaped nanoparticles at scan range of 10 µm, which supports the SEM analysis. The average particle size and roughness are calculated to be 120 nm and 20.7 nm, respectively. The nanoparticles consist of cluster of atoms having different size ranging from 100 to 500 nm. The thickness of selenium thin film on SS substrate is about 585.3 nm for deposition time of 5 h. The thickness of selenium thin film on SS substrate can be adjusted by varying deposition time. Also, the mass of selenium material on SS substrate of area 1 × 1 cm2 is about 0.33 m.gm. Such a small amount of selenium contribute in electrochemical reaction of electrode-electrolyte. The surface cross sectional resistance of selenium thin film on SS substrate is measured to be 0.67 Ω for 1 × 1 cm2 area. The nitrogen adsorption-desorption measurements are used to study the porosity and textural properties of selenium nanoparticles. The nitrogen adsorption-desorption isotherm is represented in Fig. 2 (A). The curve expose type IV isotherm attended by H3 type hysteresis loop in the IUPAC classification, representing the existence of mesopores with specific surface areas of 30.5 m2g-1 [15]. The type H3 hysteresis loop is related with capillary condensation happening in nanoparticle shaped mesopores, which may create from the collection of nanoparticles of selenium thin film. The pore size distribution of the fine interconnected nanoparticles was fitted via Barrett-JoynerHalenda (BJH) model (see Fig. 2 (B)). The selenium nanoparticles show higher mesoporosity and exhibits relatively high intense peak at around 1.39 nm. It can provide low-resistant trails for the ions through the porous structure, as well as a smaller diffusion path owing to the systematic mesoporous channels. The plots confirm major volume of pores in the mesopore range [16]. Pore sizes are larger than the size of the electrolyte ions used in the energy storage
4
devices and will allow fast flow of the electrolyte ions into selenium electrode. The better specific surface area and porosity enhance the intercalation/deintercalation reaction between electrode and electrolyte. The superior surface area provides more active sides in electrochemical reaction, which improves storage capacity of selenium electrode material. 4. Electrochemical properties In this exploration, three electrode system is used with selenium thin film as a working electrode, platinum as a counter electrode and saturated calomel electrode (SCE) as a reference electrode using 1 M Na 2SO4 electrolyte solution. Fig. 3 (A) shows the cyclic voltammetry (CV) curves of selenium thin film at 5, 20, 50 and 100 mVs-1 scan rates within operating potential window of 0 to + 0.8 V/SCE. The intercalation/deintercalation of electrolyte ions at lower scan rate is maximum due to more time available for charge transfer as compared to high scan rate (Fig. 3 (B)). The nanostructured porous surface morphology is useful in order to improve the utilization of selenium material by taking sufficient time for intercalation/ deintercalation of SO4- ions, results in enhanced electrochemical performance of the selenium electrode material. The spacing as well as interconnected network of nanoparticles permit an easy access for intercalation/deintercalation of electrolyte ions. The intercalation/deintercalation reaction between selenium electrode and 1 M Na2SO4 electrolyte is represented as [17], (3) The galvanostatic charge-discharge study was performed at different current densities from 0.04 to 0.1 mAcm -2 within operating potential window of 0 to + 0.8 V/SCE as shown in Fig. 3 (C). The linear part results from voltage drops across equivalent series resistance (ESR) and non-linear part, is due to faradaic reaction at electrode surface [18]. The maximum specific capacitance (Cs) of 16.66 Fg-1 is achieved at current density of 0.04 mAcm-2 with energy density (ED) of 2.04 Whkg-1. The value of Cs decreases from 16.66 to 2.96 Fg-1 as current density increases from 0.04 to 0.1 mA.cm-2 (Fig. 3 (D)). ED and power density (PD) are calculated by formulae,
where,
is discharging time and Vmax and Vmin are maximum and minimum potential during charging and
discharging cycles. The Ragone plot region lies between conventional capacitor and battery regions as shown in Fig. 4 (A). The electrochemical stability of the selenium thin film is investigated for 1,000 cycles at 100 mVs -1 scan rate
5
as shown in Fig. 4 (B, C). The loss of electroactive material during cycling influence Cs of selenium electrode, which decreases with increase in cycle number [19]. Electrochemical stability analysis demonstrates capacity retention up to 91 % after 1,000 CV cycles. Nyquist plot for selenium electrode in the frequency range of 100 kHz to 100 mHz at AC amplitude of 5 mV is shown in Fig. 5 (A) and inset figure shows equivalent circuit and magnified Nyquist plot. At higher frequency region, small semicircle is observed, which indicates the charge is transferred at electrode-electrolyte interface due to redox reactions. The equivalent series resistance (ESR) (R1) of 1.096 Ωcm-2 arises due to combination of ionic resistance of 1M Na2SO4 electrolyte, intrinsic resistance of selenium electrode and contact resistance between electrode and current collector. The semicircle illustrates the combination of charge transfer resistance (R2 and R3) and constant phase element (CPE1), pseudocapacitance (C1), which arises due to surface roughness, non-uniform current distribution and conductivity difference between the selenium electrode (electronic conductivity) and 1M Na2SO4 electrolyte (ionic conductivity). The total charge transfer resistance (Rct) of 2.87 Ωcm-2 is calculated for selenium electrode using R2 and R3. The line nearly at 45° in the mid frequency range relative to the diffusion of SO42- ions in the selenium electrode is denoted as Warburg constant (W). Fig. 5 (B) shows Bode plot of selenium electrode material in which phase angle increases up to -82.73º, as it goes near to -90° which indicates capacitor behavior supports to Nyquist plot results. Fig. 5 (C) depicts imaginary (C’’im) and real (C’r) parts of capacitance, in which higher frequency region shows plateau line indicative of capacitive behavior. Conclusions: In this investigation, crystalline selenium thin films are synthesized by simple chemical method. The XRD analysis confirms hexagonal crystal structure of selenium thin film. The nanoparticles of selenium are suitable for energy storage applications due to maximum contribution of electroactive material with specific surface area of 30.5 m2g-1. The specific capacitance of 21.98 Fg-1 is achieved at scan rate of 5 mVs-1 with electrochemical stability up to 91 % retention after 1,000 CV cycles. The EIS analysis shows high power implementation, low ESR values, excellent frequency and rate response. This study exploits inexpensive and simple chemical method of selenium thin film deposition for possible energy storage application. Acknowledgement: Authors appreciate DAE-BRNS, Mumbai, India for the financial support through research project no. 2012/34/67/BRNS/2911 dated 07-03-2013.
6
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B. Maranowski, M. Strawski, W. Osowiecki, M. Szklarczyk, J. Electroanal. Chem. 752 (2015) 54.
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D.S. Han, B. Batchelor, A.A. Wahab, J. Colloid Interf. Sci. 368 (2012) 496.
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V.S. Saji and C.W. Lee, RSC Adv. 3 (2013) 10058.
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X. Li, Y. Li, S. Li, W. Zhou, H. Chu, W. Chen, I. Li, Z. Tang, Cryst. Growth Des. 5 (2005) 911.
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D.P. Dubal, Z.C. Huertas, R. Holze, P.G. Romero, Electrochim. Acta, 191 (2016) 346.
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G.S. Gund, D.P. Dubal, N.R. Chodankar, J.Y. Cho, P.G. Romero, C. Park, C.D. Lokhande, Sci. Rep., 5 (2015) 12454.
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A. Abouimrane, D. Dambournet, K. Chapman, W. Wei, K. Amine, J. Am. Chem. Soc. 134 (2012) 4505.
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X. Zheng, X. Yan, Y. Sun, Y. Yu, G. Zhang, Y. Shen, Q. Liang, Q. Liao, Y. Zhang, J. Colloid Interf. Sci. 466 (2016) 291.
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S. Maiti, A. Pramanik, S. Chattopadhyay, G. De, S. Mahanty, J. Colloid Interf. Sci. 464 (2016) 73.
7
Figure captions: Fig. 1 (A) XRD pattern of selenium thin film on SS substrate, (B) Raman spectrum (inset image shows water contact angle) of selenium thin film. (C, D) The SEM images of selenium thin film surface at magnification of 2,000 X, and 5,000 X, respectively (E) EDAX spectrum of selenium thin film (Inset table shows weight and atomic percentage), and (F) 3-D AFM image of selenium thin film. Fig. 2 (A) The nitrogen adsorption/desorption isotherm of selenium, and (B) pore size distribution curves of powder selenium sample. Fig. 3 (A) The cyclic voltammetry plots, (B) the graph of specific capacitance (Cs) versus potential scan rates, (C) the galvanostatic charge-discharge curves, and (D) the plot of Cs versus current densities of selenium electrode in 1 M Na2SO4 electrolyte. Fig. 4 (A) Ragone plot of selenium thin film electrode, (B) the cyclic voltammetry (CV) graph showing electrochemical stability at constant 100 mVs-1 scan rate of selenium film, and (C) the plot of capacity retention versus CV cycle number of selenium electrode in 1 M Na2SO4. Fig. 5 (A) The Nyquist plot (Inset figure shows equivalent circuit and magnified Nyquist plot), (B) the Bode plot, and (C) difference of capacitance (C’r and C’’im) with log frequency (f) of selenium electrode in 1 M Na2SO4.
8
(C)
10 µm (D)
(F)
47°
5 µm
Fig. 1
Fig. 2 9
Fig. 3
Fig. 4
10
Fig. 5
11
(a)
(b)
Graphical abstract: (a) Image shows surface morphology of selenium thin film with unequal nanoparticles. Inset AFM figure supports SEM analysis. The inside Ragone plot displays supercapacitive behavior of selenium electrode material, and (b) The formation of selenium thin film on stainless steel substrate as nucleation, aggregation, coalescence and grouth of particles.
S0021-9797(16)30110-2 http://dx.doi.org/10.1016/j.jcis.2016.02.030 YJCIS 21092
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
11 December 2015 7 February 2016 9 February 2016
Please cite this article as: A.M. Patil, V.S. Kumbhar, N.R. Chodankar, A.C. Lokhande, C.D. Lokhande, Electrochemical behavior of chemically synthesized selenium thin film, Journal of Colloid and Interface Science (2016), doi: http://dx.doi.org/10.1016/j.jcis.2016.02.030
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Electrochemical behavior of chemically synthesized selenium thin film A. M. Patila, V. S. Kumbhara, N. R. Chodankara, A. C. Lokhandeb, C. D. Lokhandea,* a
Thin Film Physics Laboratory, Department of Physics, Shivaji University, Kolhapur, 416 004 (M.S), India. b
Department of Materials Science and Engineering, Chonnam National University, South Korea 500-757
*CORRESPONDING AUTHOR Prof. C.D. Lokhande Tel: +91 231 2609225, Fax: +91 231 2609233 E-mail:- [email protected]
1
Abstract The facile and low cost simple chemical bath deposition (CBD) method is employed to synthesize red colored selenium thin films. These selenium films are characterized for structural, morphological, topographical and wettability studies. The X-ray diffraction (XRD) pattern showed the crystalline nature of selenium thin film with hexagonal crystal structure. The scanning electron microscopy (SEM) study displays selenium nanoparticles ranging from 20 to 475 nm. A specific surface area of 30.5 m2g-1 is observed for selenium nanoparticles. The selenium nanoparticles hold mesopores in the range of 1.39 nm, taking benefits of the good physicochemical stability and excellent porosity. Subsequently, the electrochemical properties of selenium thin films are deliberated by cyclic voltammetry (CV), galvanostatic charge-discharge and electrochemical impedance spectroscopy (EIS) techniques. The selenium thin film shows specific capacitance (Cs) of 21.98 Fg-1 with 91 % electrochemical stability. Keywords: Selenium, Thin film, Chemical bath deposition, Electrochemical properties 1. Introduction During past few years, a significant interest in selenium has increased appreciably owing to number of applications such as modern electronics, optoelectronics, new generation sensors, photovoltaic cells etc. [1, 2]. The unique photoelectric and semiconducting properties of selenium find applications in rectifiers, photocells, switching electronic devices, memory devices and X-ray photoconductors [3]. Elemental selenium is almost present in environmental and biological materials. Selenium has several oxidation states such as Se2-, Se0, Se4+ and Se6+ and exhibits different structures namely amorphous, trigonal (hexagonal), α-monoclinic and β-monoclinic. The structure of selenium consists of chains (Sen) and rings (Se8) [4]. Now days, sulfur is replaced by selenium in Li-S batteries, because of the insulating nature of sulfur and dissolution of reductive polysulfide in organic electrolytes during cycling which moderately affect the performance of Li-S batteries [5]. More importantly, the higher electrical conductivity (~20 times greater than sulfur) is desired for lithium ion batteries [6]. The selenium is also used as a dopant in lead acid batteries, where it increases the corrosion resistance and discharge capacities [7]. Even all these features, no reports are available on the electrochemical supercapacitive features of selenium. In the past, selenium thin films have been prepared by various physical as well as chemical deposition methods [8-11]. In present work, selenium thin films are synthesized by chemical bath deposition (CBD) method, which is binder-free, surfactant-less, low cost and simple as compared to other physical and chemical deposition methods. Subsequently, the electrochemical properties of selenium thin films are studied.
2
2. Experimental For deposition of selenium thin film, the stock solution of 0.0125 M Na2SeSO3 was prepared by dissolving selenium powder in aqueous solution of 0.0125 M Na2SO3 with pH∼10.2 (± 0.1). The 0.0125 M Na2SeSO3 solution was acidified at room temperature (300K) by adding acetic acid and pH was maintained at 4.4 (± 0.1). The well cleaned stainless steel (SS) substrate was dipped in acidified solution of Na 2SeSO3. After 5 h, red colored selenium thin film was deposited on SS substrate. The substrate was taken out from the bath, washed several times in double distilled water (DDW) and dried at room temperature (300K). The selenium thin film structure was characterized by XRD technique using a Bruker AXS D8 advanced Model with copper radiation (Kα of wavelength = 1.54 Å). The phase confirmation was accomplished by Raman spectrometer (Bruker MultiRAM, Germany). Surface morphological investigation was done by scanning electron microscope (SEM), (JEOL-JSM 6360) and atomic force microscopy (AFM) Model-INNOVA 1B3BE units. The energy dispersive X-ray analysis (EDAX) was collected by JEOL-JSM 6360 unit. The contact angle measurement was carried out by Rame-Hart equipment with CCD camera. The specific surface area of a selenium material was calculated by Brunauer, Emmett and Teller (BET) model Quantachrome Instruments v11.02. The thickness of selenium thin film was measured using XP-1 Stylus surface profiler with laser calibration.
Electrochemical
properties of selenium thin films were executed by automatic battery cycler unit (WBCS3000) and EIS analysis was carried out using electrochemical workstation (ZIVE SP5) in 1 M Na2SO4 electrolyte. 3. Results and discussion The CBD route involves several steps for film formation such as nucleation, aggregation, coalescence and growth of particles. The nucleation conducts heterogeneous reaction on the film surface. The cluster of molecule inaugurated undergoes rapid decomposition with combination of particles to grow film up to certain thickness [12]. With addition of acetic acid in precursor solution (0.0125 M Na2SeSO3), the precipitate starts and solution becomes saturated. The proposed reaction mechanism is as follows, 2
2
3
2
3
3
(1)
3
3
2
2
Fig. 1 (A) shows XRD pattern of selenium thin film. The diffraction planes corresponding to the (100), (101) and (110) planes of hexagonal structure for selenium having lattice parameters of a= 4.3639 Å and c= 4.9595 Å [JCPDS card No. 65-1876] are observed. The peaks of SS substrate are denoted by asterisk (*). The crystallite size
3
of selenium is calculated as 29.8 nm by full width at half maxima (FWHM) for high intense peak (101) using Scherrer formula [13]. The vibrational, rotational and other low-frequency modes in a selenium material were detected by Raman spectroscopy technique. The high intense peaks observed at frequencies of 236 and 254 cm-1 are in well agreement with formation of selenium (Se8 rings) [Fig. 1 (B)]. The resonance peak observed at 236 cm-1 is a characteristic stretching mode (A1), which represents chain-like structure existing in hexagonal selenium [14]. Inset figure confirms that selenium surface is a hydrophilic, as water contact angle is 47°. Fig. 1 (C, D) shows SEM images of selenium thin film at magnifications of 2,000 X and 5,000 X. At low magnification, selenium surface is covered with nanoparticle like grains. At higher magnification, interconnection of nanoparticles distributed throughout film surface is observed. Fig. 1 (E) depicts EDAX spectrum of selenium thin film. High intense spectrum of selenium confirms selenium deposition on SS substrate. The inset of figure shows atomic (96.64 %) and weight (99.38 %) percentages of selenium. Fig. 1 (F) shows 3-D AFM image of selenium film surface with elliptical shaped nanoparticles at scan range of 10 µm, which supports the SEM analysis. The average particle size and roughness are calculated to be 120 nm and 20.7 nm, respectively. The nanoparticles consist of cluster of atoms having different size ranging from 100 to 500 nm. The thickness of selenium thin film on SS substrate is about 585.3 nm for deposition time of 5 h. The thickness of selenium thin film on SS substrate can be adjusted by varying deposition time. Also, the mass of selenium material on SS substrate of area 1 × 1 cm2 is about 0.33 m.gm. Such a small amount of selenium contribute in electrochemical reaction of electrode-electrolyte. The surface cross sectional resistance of selenium thin film on SS substrate is measured to be 0.67 Ω for 1 × 1 cm2 area. The nitrogen adsorption-desorption measurements are used to study the porosity and textural properties of selenium nanoparticles. The nitrogen adsorption-desorption isotherm is represented in Fig. 2 (A). The curve expose type IV isotherm attended by H3 type hysteresis loop in the IUPAC classification, representing the existence of mesopores with specific surface areas of 30.5 m2g-1 [15]. The type H3 hysteresis loop is related with capillary condensation happening in nanoparticle shaped mesopores, which may create from the collection of nanoparticles of selenium thin film. The pore size distribution of the fine interconnected nanoparticles was fitted via Barrett-JoynerHalenda (BJH) model (see Fig. 2 (B)). The selenium nanoparticles show higher mesoporosity and exhibits relatively high intense peak at around 1.39 nm. It can provide low-resistant trails for the ions through the porous structure, as well as a smaller diffusion path owing to the systematic mesoporous channels. The plots confirm major volume of pores in the mesopore range [16]. Pore sizes are larger than the size of the electrolyte ions used in the energy storage
4
devices and will allow fast flow of the electrolyte ions into selenium electrode. The better specific surface area and porosity enhance the intercalation/deintercalation reaction between electrode and electrolyte. The superior surface area provides more active sides in electrochemical reaction, which improves storage capacity of selenium electrode material. 4. Electrochemical properties In this exploration, three electrode system is used with selenium thin film as a working electrode, platinum as a counter electrode and saturated calomel electrode (SCE) as a reference electrode using 1 M Na 2SO4 electrolyte solution. Fig. 3 (A) shows the cyclic voltammetry (CV) curves of selenium thin film at 5, 20, 50 and 100 mVs-1 scan rates within operating potential window of 0 to + 0.8 V/SCE. The intercalation/deintercalation of electrolyte ions at lower scan rate is maximum due to more time available for charge transfer as compared to high scan rate (Fig. 3 (B)). The nanostructured porous surface morphology is useful in order to improve the utilization of selenium material by taking sufficient time for intercalation/ deintercalation of SO4- ions, results in enhanced electrochemical performance of the selenium electrode material. The spacing as well as interconnected network of nanoparticles permit an easy access for intercalation/deintercalation of electrolyte ions. The intercalation/deintercalation reaction between selenium electrode and 1 M Na2SO4 electrolyte is represented as [17], (3) The galvanostatic charge-discharge study was performed at different current densities from 0.04 to 0.1 mAcm -2 within operating potential window of 0 to + 0.8 V/SCE as shown in Fig. 3 (C). The linear part results from voltage drops across equivalent series resistance (ESR) and non-linear part, is due to faradaic reaction at electrode surface [18]. The maximum specific capacitance (Cs) of 16.66 Fg-1 is achieved at current density of 0.04 mAcm-2 with energy density (ED) of 2.04 Whkg-1. The value of Cs decreases from 16.66 to 2.96 Fg-1 as current density increases from 0.04 to 0.1 mA.cm-2 (Fig. 3 (D)). ED and power density (PD) are calculated by formulae,
where,
is discharging time and Vmax and Vmin are maximum and minimum potential during charging and
discharging cycles. The Ragone plot region lies between conventional capacitor and battery regions as shown in Fig. 4 (A). The electrochemical stability of the selenium thin film is investigated for 1,000 cycles at 100 mVs -1 scan rate
5
as shown in Fig. 4 (B, C). The loss of electroactive material during cycling influence Cs of selenium electrode, which decreases with increase in cycle number [19]. Electrochemical stability analysis demonstrates capacity retention up to 91 % after 1,000 CV cycles. Nyquist plot for selenium electrode in the frequency range of 100 kHz to 100 mHz at AC amplitude of 5 mV is shown in Fig. 5 (A) and inset figure shows equivalent circuit and magnified Nyquist plot. At higher frequency region, small semicircle is observed, which indicates the charge is transferred at electrode-electrolyte interface due to redox reactions. The equivalent series resistance (ESR) (R1) of 1.096 Ωcm-2 arises due to combination of ionic resistance of 1M Na2SO4 electrolyte, intrinsic resistance of selenium electrode and contact resistance between electrode and current collector. The semicircle illustrates the combination of charge transfer resistance (R2 and R3) and constant phase element (CPE1), pseudocapacitance (C1), which arises due to surface roughness, non-uniform current distribution and conductivity difference between the selenium electrode (electronic conductivity) and 1M Na2SO4 electrolyte (ionic conductivity). The total charge transfer resistance (Rct) of 2.87 Ωcm-2 is calculated for selenium electrode using R2 and R3. The line nearly at 45° in the mid frequency range relative to the diffusion of SO42- ions in the selenium electrode is denoted as Warburg constant (W). Fig. 5 (B) shows Bode plot of selenium electrode material in which phase angle increases up to -82.73º, as it goes near to -90° which indicates capacitor behavior supports to Nyquist plot results. Fig. 5 (C) depicts imaginary (C’’im) and real (C’r) parts of capacitance, in which higher frequency region shows plateau line indicative of capacitive behavior. Conclusions: In this investigation, crystalline selenium thin films are synthesized by simple chemical method. The XRD analysis confirms hexagonal crystal structure of selenium thin film. The nanoparticles of selenium are suitable for energy storage applications due to maximum contribution of electroactive material with specific surface area of 30.5 m2g-1. The specific capacitance of 21.98 Fg-1 is achieved at scan rate of 5 mVs-1 with electrochemical stability up to 91 % retention after 1,000 CV cycles. The EIS analysis shows high power implementation, low ESR values, excellent frequency and rate response. This study exploits inexpensive and simple chemical method of selenium thin film deposition for possible energy storage application. Acknowledgement: Authors appreciate DAE-BRNS, Mumbai, India for the financial support through research project no. 2012/34/67/BRNS/2911 dated 07-03-2013.
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Figure captions: Fig. 1 (A) XRD pattern of selenium thin film on SS substrate, (B) Raman spectrum (inset image shows water contact angle) of selenium thin film. (C, D) The SEM images of selenium thin film surface at magnification of 2,000 X, and 5,000 X, respectively (E) EDAX spectrum of selenium thin film (Inset table shows weight and atomic percentage), and (F) 3-D AFM image of selenium thin film. Fig. 2 (A) The nitrogen adsorption/desorption isotherm of selenium, and (B) pore size distribution curves of powder selenium sample. Fig. 3 (A) The cyclic voltammetry plots, (B) the graph of specific capacitance (Cs) versus potential scan rates, (C) the galvanostatic charge-discharge curves, and (D) the plot of Cs versus current densities of selenium electrode in 1 M Na2SO4 electrolyte. Fig. 4 (A) Ragone plot of selenium thin film electrode, (B) the cyclic voltammetry (CV) graph showing electrochemical stability at constant 100 mVs-1 scan rate of selenium film, and (C) the plot of capacity retention versus CV cycle number of selenium electrode in 1 M Na2SO4. Fig. 5 (A) The Nyquist plot (Inset figure shows equivalent circuit and magnified Nyquist plot), (B) the Bode plot, and (C) difference of capacitance (C’r and C’’im) with log frequency (f) of selenium electrode in 1 M Na2SO4.
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(C)
10 µm (D)
(F)
47°
5 µm
Fig. 1
Fig. 2 9
Fig. 3
Fig. 4
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Fig. 5
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(a)
(b)
Graphical abstract: (a) Image shows surface morphology of selenium thin film with unequal nanoparticles. Inset AFM figure supports SEM analysis. The inside Ragone plot displays supercapacitive behavior of selenium electrode material, and (b) The formation of selenium thin film on stainless steel substrate as nucleation, aggregation, coalescence and grouth of particles.