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Studies on electrochromism of spray pyrolyzed cobalt oxide thin films
Solar Energy Materials and Solar Cells 53 (1998) 229—234
Studies on electrochromism of spray pyrolyzed cobalt oxide thin films P.S. Patil*, L.D. Kadam, C.D. Lokhande Thin Film Physics Laboratory, Department of Physics, Shivaji University, Kolhapur-416 004, India
Abstract Cobalt oxide thin films were prepared by spray pyrolysis technique on to fluorine doped tin oxide (F.T.O.) coated glass substrates. The electrochromic cell was formed by using these films as working electrodes and the electrochromic characteristics were determined by using cyclic voltammetry and chronoamperometry. The films exhibited anodic electrochromism; changing colour from grey to pale yellow. ( 1998 Elsevier Science B.V. All rights reserved. Keywords: Spray pyrolysis; Electrochromism; Cobalt oxide
1. Introduction Electrochromic (EC) materials are characterised by a reversible and/or persistent change of optical properties under the action of an applied electric field. They have interesting potential application to energy efficient ‘smart windows’ with controllable throughput of radient energy, mirrors with variable reflectance [1,2] and high contrast non-emisive information display [3]. Potential advantage offered by electrochromic materials includes a long open circuit memory and a low power requirement. Among the numerous inorganic and organic electrochromic materials, the transition metal oxides are well suited. The most widely used electrochromic inorganic materials are WO [4,5], MoO [6], V O [7] and Nb O [7] etc. 3 3 2 5 2 5
* Corresponding address. Bereich Physikalische chemie, Hahn-Meitner-Institute, Glienicker str.-100, D-14109, Berlin, Germany, E-mail: [email protected] 0927-0248/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved. PII S 0 9 2 7 - 0 2 4 8 ( 9 7 ) 0 0 0 0 6 - 3
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P.S. Patil et al. /Solar Energy Materials and Solar Cells 53 (1998) 229—234
Cobalt oxide is one of the promising transition metal oxide materials which has many industrial applications, such as solar selective absorber [8], catalyst in hydrocracking process of crude fuel and pigment for glasses and ceramics [9]. However, its electrochromic properties have not been studied extensively. Few investigations have been performed on the electrochromic properties of cobalt oxide films prepared by anodically grown and chemical vapour deposited cobalt oxide thin films [10,11]. In the present investigation, cobalt oxide thin films have been prepared by a simple and inexpensive spray pyrolysis technique onto FTO coated glass substrates and preliminary results on electrochromic properties of such thin films are reported.
2. Experimental Spray pyrolysis involves the applications of a fine mist of very small droplets containing the reactants onto a hot substrates. During spray pyrolysis, the solution is atomized onto preheated glass substrates. The droplets undergo evaporation, solute condensation and thermal decomposition which results into the film formation. In the present case, cobalt oxide thin films were prepared by spraying 0.05 M, 7 ml cobalt chloride solution in doubly distilled water on to transparent (90%) FTO coated glass (sheet resistance 10 ) cm2) substrates maintained at 300°C. The time required for the film formation was 7 min. The electrochromic cell was fabricated by using three electrode system. The cobalt oxide thin film deposited onto FTO coated glass was used as a working electrode. The counter electrode was a FTO coated glass and the reference electrode was a saturated calomel electrode (SCE). The electrolyte used was 0.1 M KOH. The cyclic voltammetry was carried out using a potentiostat EG&G model PAR362 and X-½(t) recorder. All the potential values are reported versus SCE. The area of working electrode was 1 cm2. Optical transmission of the films was measured in the wavelength range 350 to 1150 nm, using spectrophotometer; Hitachi-330. For this measurement, the samples were removed from the electolyte after bleaching, rinsed in distilled water and dried. For the studies on changes in structural properties of cobalt oxide films after anodic colouration, the X-ray diffraction patterns were recorded with Phillips PW-1710 diffractometer using CuK line. a 3. Results and discussion Cobalt oxide films were prepared using spray pyrolysis technique. In this aqueous cobalt chloride solution was sprayed onto the preheated FTO coated glass substrates, which undergoes pyrolytic decomposition thereby resulting into the formation of cobalt oxide films. Detailed experimental procedure and mechanism of film formation is reported elsewhere [12].
P.S. Patil et al./Solar Energy Materials and Solar Cells 53 (1998) 229—234
231
Fig. 1. Cyclic voltammogram of the EC cell formed with cobalt oxide film. Electrolyte was 0.1 M KOH, scan rate was 20 mV/s.
Cobalt oxide films were found to be uniform, pin hole free and strongly adherent to the substrates. The as prepared films were grey in colour. The film thickness was measured using a Dektak-3030 profilometer and was 0.4 micrometer. The electrochromic cell was formed with the following configuaration; FTO/Cobalt oxide film/0.1 M KOH/FTO. The potential was cycled from !1 to #1 V versus SCE at a potential sweep rate 20 mV/s and cyclic—voltammetric study was carried out. Fig. 1 presents the cyclic—voltammetric result of a cobalt oxide thin film. The cyclic—voltammetric curve exhibits two clearly resolved (A and A ) and one ill resolved (A ) anodic current 1 3 2 peaks. The peak potentials for three anodic current peaks are about; A "#0.37 V, 1 A "#0.4 V and A "#0.8 V. The anodic current peaks A and A may corres2 3 1 2 pond to Co O QCoOOH and Co(OH) QCoOOH transitions respectively [10,13]. 3 4 2 A third anodic current peak A corresponds to the transition CoOOHQCoO 3 2
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P.S. Patil et al. /Solar Energy Materials and Solar Cells 53 (1998) 229—234
[10,13—15]. After the reversal of the potential scan, three cathode current peaks are observed with peak potentials at C "#0.3 V, C "!0.1 V and C "!0.5 V. The 1 2 3 colour of the electrode was changed to pale yellow at most of the cathodic potentials, while during anodic scan, at about 0.7 V (SCE) the electrode becomes grey. The general features of this cyclic—voltammogram are similar to the one obtained for cobalt oxide thin films grown by anodic electroprecipitation [16]. In the present cyclic-voltammogram, there are two or three maxima or minima in the redox current, indicating that the film has energetically different intercalation sites. These different intercalation sights may be due to different oxidation states of cobalt oxide. In order to check the electrochemical stability of the films under voltammetric cycling, potentials were cycled for 103 times, and cyclic—voltammograms were recorded at the scan rate of 20 mV/s. It was observed that the peak current densities increase up to about 10% for first five cycles and then no change has been observed up to 103 cycles. Similar type of result has been observed for evaporated tungsten oxide films which has been attributed to water incorporation in the films [13]. The time required for fully bleaching from fully coloured state (or vice-versa) is defined as the response time. Fig. 2 shows variation of current density with time. The response time was found to be 4 s for colouration and 2 s for bleaching. This time is
Fig. 2. Chronoamperometric curve of the electrochromic cell formed with cobalt oxide thin film.
P.S. Patil et al./Solar Energy Materials and Solar Cells 53 (1998) 229—234
233
Fig. 3. Optical transmittances for (A) as deposited and (B) bleached cobalt oxide films on FTO coated glass substrates.
smaller as compared with anodically grown cobalt oxide thin films. For anodically grown thin films colouration and bleaching response times were 8 and 4.5 s, respectively. Fig. 3A and B shows spectral transmittance of the as deposited film and the film which was bleached at !0.4 V(SCE) for about 20 s. The transmittance of the reduced (bleached) film was higher than that of the as-deposited cobalt oxide thin film. After bleaching the transmittance is increased throughout the spectral range with the largest change in the region 750—1050 nm. The X-ray diffraction patterns of the cobalt oxide film on FTO coated glass after colouration and bleaching were studied. Cobalt oxide film was polycrystalline and consisted of Co(III) oxide phase. No significant changes in crystal structure were observed after anodic colouration as well as bleaching of the film. 4. Conclusion The cobalt oxide thin films were prepared by using spray pyrolysis technique. The films exhibited anodic electrochromism changing originally from grey to pale yellow colour. The colouration and bleaching times were 4 and 2 s, respectively, which were smaller than that of anodically grown cobalt oxide thin films. The change in optical transmittance for bleached film was maximum in the wavelength region 750—1050 nm. No significant changes in crystal structure was evidenced after colouration and bleaching of the film, from XRD studies. Acknowledgements One of the authors (LDK) whishes to thank University Grants Commission, New Delhi, India, for the award of Teacher Fellowship. The author would like to thank the Hahn-Meitner-Institute, Berlin, for meeting the publication and printing costs.
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P.S. Patil et al. /Solar Energy Materials and Solar Cells 53 (1998) 229—234
References [1] J.S.E.M. Svesson, C.G. Granqvist, Solar Energy Mater. 12 (1985) 391. [2] C.G. Granqvist, Solid State Ionics. 53 (1992) 479. [3] C.M. Lampert, C.G. Granqvist (Eds.), Large area Chromogenic Materials and devices for transmittance control, SPIE Optical Engg., Press Bellingham, 1990. [4] S.I. Cordoba de Torresi, A. Gorenstein, R. Torresi, M.V. Vasquez, J. Electroanal. Chem. 318 (1991) 131. [5] C.G. Granqvist (Ed.), Materials Science for Solar Energy Conversion System, Pergamon Press, Oxford, 1991, p. 106. [6] T.C. Arnoldussen, J. Electrochem. Soc. 123 (1976) 527. [7] C.M. Lampert, Solar Energy Mater. 11 (1984) 1. [8] G. McDonald, Thin Solid Films 72 (1980) 83. [9] K. Othmer, Encyclopedia of Chemical Tachnology, vol. 6, 3rd Ed., Wiley, London, 1979, p. 481. [10] C.N. Poloda Fonseca, Marco-A De Paoli, A. Gorenstein, Solar Energy Mater, Solar Cells 33 (1994) 73. [11] T. Maruyama, S. Arai, J. Electrochem. Soc. 143 (1996) 1383. [12] P.S. Patil, L.D. Kadam, C.D. Lokhande, Thin Solid Films 272 (1996) 29. [13] W.K. Behl, E.J. Toni, J. Electroanal. Chem. 31 (1971) 63. [14] N. Sato, T. Olsuka, J. Electrochem. Soc. 125 (1978) 1735. [15] H.G. Meier, J.R. Vilche, A.J. Arvia, J. Electroanal. Chem. 138 (1982) 367. [16] C.G. Granqvist, in: Handbook of Inorganic Electrochromic Materials, Elsevier, Amsterdam, 1995, p. 95.
Studies on electrochromism of spray pyrolyzed cobalt oxide thin films P.S. Patil*, L.D. Kadam, C.D. Lokhande Thin Film Physics Laboratory, Department of Physics, Shivaji University, Kolhapur-416 004, India
Abstract Cobalt oxide thin films were prepared by spray pyrolysis technique on to fluorine doped tin oxide (F.T.O.) coated glass substrates. The electrochromic cell was formed by using these films as working electrodes and the electrochromic characteristics were determined by using cyclic voltammetry and chronoamperometry. The films exhibited anodic electrochromism; changing colour from grey to pale yellow. ( 1998 Elsevier Science B.V. All rights reserved. Keywords: Spray pyrolysis; Electrochromism; Cobalt oxide
1. Introduction Electrochromic (EC) materials are characterised by a reversible and/or persistent change of optical properties under the action of an applied electric field. They have interesting potential application to energy efficient ‘smart windows’ with controllable throughput of radient energy, mirrors with variable reflectance [1,2] and high contrast non-emisive information display [3]. Potential advantage offered by electrochromic materials includes a long open circuit memory and a low power requirement. Among the numerous inorganic and organic electrochromic materials, the transition metal oxides are well suited. The most widely used electrochromic inorganic materials are WO [4,5], MoO [6], V O [7] and Nb O [7] etc. 3 3 2 5 2 5
* Corresponding address. Bereich Physikalische chemie, Hahn-Meitner-Institute, Glienicker str.-100, D-14109, Berlin, Germany, E-mail: [email protected] 0927-0248/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved. PII S 0 9 2 7 - 0 2 4 8 ( 9 7 ) 0 0 0 0 6 - 3
230
P.S. Patil et al. /Solar Energy Materials and Solar Cells 53 (1998) 229—234
Cobalt oxide is one of the promising transition metal oxide materials which has many industrial applications, such as solar selective absorber [8], catalyst in hydrocracking process of crude fuel and pigment for glasses and ceramics [9]. However, its electrochromic properties have not been studied extensively. Few investigations have been performed on the electrochromic properties of cobalt oxide films prepared by anodically grown and chemical vapour deposited cobalt oxide thin films [10,11]. In the present investigation, cobalt oxide thin films have been prepared by a simple and inexpensive spray pyrolysis technique onto FTO coated glass substrates and preliminary results on electrochromic properties of such thin films are reported.
2. Experimental Spray pyrolysis involves the applications of a fine mist of very small droplets containing the reactants onto a hot substrates. During spray pyrolysis, the solution is atomized onto preheated glass substrates. The droplets undergo evaporation, solute condensation and thermal decomposition which results into the film formation. In the present case, cobalt oxide thin films were prepared by spraying 0.05 M, 7 ml cobalt chloride solution in doubly distilled water on to transparent (90%) FTO coated glass (sheet resistance 10 ) cm2) substrates maintained at 300°C. The time required for the film formation was 7 min. The electrochromic cell was fabricated by using three electrode system. The cobalt oxide thin film deposited onto FTO coated glass was used as a working electrode. The counter electrode was a FTO coated glass and the reference electrode was a saturated calomel electrode (SCE). The electrolyte used was 0.1 M KOH. The cyclic voltammetry was carried out using a potentiostat EG&G model PAR362 and X-½(t) recorder. All the potential values are reported versus SCE. The area of working electrode was 1 cm2. Optical transmission of the films was measured in the wavelength range 350 to 1150 nm, using spectrophotometer; Hitachi-330. For this measurement, the samples were removed from the electolyte after bleaching, rinsed in distilled water and dried. For the studies on changes in structural properties of cobalt oxide films after anodic colouration, the X-ray diffraction patterns were recorded with Phillips PW-1710 diffractometer using CuK line. a 3. Results and discussion Cobalt oxide films were prepared using spray pyrolysis technique. In this aqueous cobalt chloride solution was sprayed onto the preheated FTO coated glass substrates, which undergoes pyrolytic decomposition thereby resulting into the formation of cobalt oxide films. Detailed experimental procedure and mechanism of film formation is reported elsewhere [12].
P.S. Patil et al./Solar Energy Materials and Solar Cells 53 (1998) 229—234
231
Fig. 1. Cyclic voltammogram of the EC cell formed with cobalt oxide film. Electrolyte was 0.1 M KOH, scan rate was 20 mV/s.
Cobalt oxide films were found to be uniform, pin hole free and strongly adherent to the substrates. The as prepared films were grey in colour. The film thickness was measured using a Dektak-3030 profilometer and was 0.4 micrometer. The electrochromic cell was formed with the following configuaration; FTO/Cobalt oxide film/0.1 M KOH/FTO. The potential was cycled from !1 to #1 V versus SCE at a potential sweep rate 20 mV/s and cyclic—voltammetric study was carried out. Fig. 1 presents the cyclic—voltammetric result of a cobalt oxide thin film. The cyclic—voltammetric curve exhibits two clearly resolved (A and A ) and one ill resolved (A ) anodic current 1 3 2 peaks. The peak potentials for three anodic current peaks are about; A "#0.37 V, 1 A "#0.4 V and A "#0.8 V. The anodic current peaks A and A may corres2 3 1 2 pond to Co O QCoOOH and Co(OH) QCoOOH transitions respectively [10,13]. 3 4 2 A third anodic current peak A corresponds to the transition CoOOHQCoO 3 2
232
P.S. Patil et al. /Solar Energy Materials and Solar Cells 53 (1998) 229—234
[10,13—15]. After the reversal of the potential scan, three cathode current peaks are observed with peak potentials at C "#0.3 V, C "!0.1 V and C "!0.5 V. The 1 2 3 colour of the electrode was changed to pale yellow at most of the cathodic potentials, while during anodic scan, at about 0.7 V (SCE) the electrode becomes grey. The general features of this cyclic—voltammogram are similar to the one obtained for cobalt oxide thin films grown by anodic electroprecipitation [16]. In the present cyclic-voltammogram, there are two or three maxima or minima in the redox current, indicating that the film has energetically different intercalation sites. These different intercalation sights may be due to different oxidation states of cobalt oxide. In order to check the electrochemical stability of the films under voltammetric cycling, potentials were cycled for 103 times, and cyclic—voltammograms were recorded at the scan rate of 20 mV/s. It was observed that the peak current densities increase up to about 10% for first five cycles and then no change has been observed up to 103 cycles. Similar type of result has been observed for evaporated tungsten oxide films which has been attributed to water incorporation in the films [13]. The time required for fully bleaching from fully coloured state (or vice-versa) is defined as the response time. Fig. 2 shows variation of current density with time. The response time was found to be 4 s for colouration and 2 s for bleaching. This time is
Fig. 2. Chronoamperometric curve of the electrochromic cell formed with cobalt oxide thin film.
P.S. Patil et al./Solar Energy Materials and Solar Cells 53 (1998) 229—234
233
Fig. 3. Optical transmittances for (A) as deposited and (B) bleached cobalt oxide films on FTO coated glass substrates.
smaller as compared with anodically grown cobalt oxide thin films. For anodically grown thin films colouration and bleaching response times were 8 and 4.5 s, respectively. Fig. 3A and B shows spectral transmittance of the as deposited film and the film which was bleached at !0.4 V(SCE) for about 20 s. The transmittance of the reduced (bleached) film was higher than that of the as-deposited cobalt oxide thin film. After bleaching the transmittance is increased throughout the spectral range with the largest change in the region 750—1050 nm. The X-ray diffraction patterns of the cobalt oxide film on FTO coated glass after colouration and bleaching were studied. Cobalt oxide film was polycrystalline and consisted of Co(III) oxide phase. No significant changes in crystal structure were observed after anodic colouration as well as bleaching of the film. 4. Conclusion The cobalt oxide thin films were prepared by using spray pyrolysis technique. The films exhibited anodic electrochromism changing originally from grey to pale yellow colour. The colouration and bleaching times were 4 and 2 s, respectively, which were smaller than that of anodically grown cobalt oxide thin films. The change in optical transmittance for bleached film was maximum in the wavelength region 750—1050 nm. No significant changes in crystal structure was evidenced after colouration and bleaching of the film, from XRD studies. Acknowledgements One of the authors (LDK) whishes to thank University Grants Commission, New Delhi, India, for the award of Teacher Fellowship. The author would like to thank the Hahn-Meitner-Institute, Berlin, for meeting the publication and printing costs.
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P.S. Patil et al. /Solar Energy Materials and Solar Cells 53 (1998) 229—234
References [1] J.S.E.M. Svesson, C.G. Granqvist, Solar Energy Mater. 12 (1985) 391. [2] C.G. Granqvist, Solid State Ionics. 53 (1992) 479. [3] C.M. Lampert, C.G. Granqvist (Eds.), Large area Chromogenic Materials and devices for transmittance control, SPIE Optical Engg., Press Bellingham, 1990. [4] S.I. Cordoba de Torresi, A. Gorenstein, R. Torresi, M.V. Vasquez, J. Electroanal. Chem. 318 (1991) 131. [5] C.G. Granqvist (Ed.), Materials Science for Solar Energy Conversion System, Pergamon Press, Oxford, 1991, p. 106. [6] T.C. Arnoldussen, J. Electrochem. Soc. 123 (1976) 527. [7] C.M. Lampert, Solar Energy Mater. 11 (1984) 1. [8] G. McDonald, Thin Solid Films 72 (1980) 83. [9] K. Othmer, Encyclopedia of Chemical Tachnology, vol. 6, 3rd Ed., Wiley, London, 1979, p. 481. [10] C.N. Poloda Fonseca, Marco-A De Paoli, A. Gorenstein, Solar Energy Mater, Solar Cells 33 (1994) 73. [11] T. Maruyama, S. Arai, J. Electrochem. Soc. 143 (1996) 1383. [12] P.S. Patil, L.D. Kadam, C.D. Lokhande, Thin Solid Films 272 (1996) 29. [13] W.K. Behl, E.J. Toni, J. Electroanal. Chem. 31 (1971) 63. [14] N. Sato, T. Olsuka, J. Electrochem. Soc. 125 (1978) 1735. [15] H.G. Meier, J.R. Vilche, A.J. Arvia, J. Electroanal. Chem. 138 (1982) 367. [16] C.G. Granqvist, in: Handbook of Inorganic Electrochromic Materials, Elsevier, Amsterdam, 1995, p. 95.