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A non-thermal chemical synthesis of hydrophilic and amorphous cobalt oxide films for supercapacitor application
Applied Surface Science 253 (2007) 3952–3956 www.elsevier.com/locate/apsusc
A non-thermal chemical synthesis of hydrophilic and amorphous cobalt oxide films for supercapacitor application Sunil G. Kandalkar a, C.D. Lokhande a,b,*, R.S. Mane b, Sung-Hwan Han b,* b
a Thin Film Physics Laboratory, Department of Physics, Shivaji University, Kolhapur 416 004, India Hanyang University, Department of Chemistry, Sungdong-Ku, Haengdang-dong 17, Seoul 133791, Republic of Korea
Received 16 July 2006; received in revised form 21 August 2006; accepted 21 August 2006 Available online 25 September 2006
Abstract Present work explored a room temperature, simple and low cost chemical route for the preparation of hydrophilic cobalt oxide films from alkaline cobalt chloride (CoCl2:6H2O) and double distilled water precursor solutions. As-deposited cobalt oxide films showed amorphous nature, which is one of the prime requirements for supercapacitor, as confirmed from X-ray diffraction studies. Changes in direct band gap energy and electrical resistivity of as-deposited cobalt oxide films were confirmed after annealing. Spherical grains of about 40–50 nm diameters were uniformly distributed over the substrate surface. Surface wettability studied in contact with liquid interface, showed hydrophilic nature as water contact angle was <908. Finally, presence of cobalt–oxygen covalent bond was observed from Raman shift experiment. # 2006 Elsevier B.V. All rights reserved. Keywords: Cobalt oxide; Non-thermal process; Raman shift; Electrical resistivity
1. Introduction In general, magnetic oxides containing transition elements such as cobalt, manganese or ruthenium exhibit most fascinating magnetic properties. Among these transition metal oxides, the cobalt oxide is one of the most versatile ceramic materials, since it is a p-type anti-ferromagnetic oxide semiconductor with the highest Curie temperature, Tc = 1396 K. Although, this oxide is customarily identified with its chemical formula Co3O4, in fact it is a nonstoichiometric [1,2]. Cobalt has less affinity for oxygen than iron but more than nickel [3]. It has three well-known polymorphs; the monoxide or cobaltous oxide (CoO), the cobaltic oxide (Co2O3) and the cobaltosic oxide or cobalt cobaltite (Co3O4). Cobaltous oxide (CoO) is the final product formed, when the cobalt compound or other oxides are calcinated to a sufficiently high temperature (1173 K). The pure CoO is difficult to obtain, since it readily takes up oxygen even at room temperature to reform to a higher oxide. Cobaltic oxide (Co2O3) could be formed, when cobalt compounds are heated at * Corresponding authors. E-mail addresses: [email protected] (C.D. Lokhande), [email protected] (S.-H. Han). 0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.08.026
a low temperature in presence of an excess air. Cobalt forms two stable oxides: CoO and Co3O4. At low temperatures, the Co3O4 is a stable normal spinel structure of AB2O4 type, where Co2+ ions occupy the tetrahedral 8a sites and Co3+ occupy the octahedral 16d sites [4]. Solar selective absorber, catalyst in the hydrocracking process of crude fuels, pigment for glasses and ceramics [5] and catalyst for oxygen evolution and oxygen reduction reaction [6] are some of the well-known applications of cobalt oxide, in addition to the glass industry for colored glasses [7]. These commonly known catalytic properties could be due to reduction of activation energies for chemisorption of gas molecules and are very useful to promote high gas sensitivities or lower operation temperatures. It is also widely used as an electrochromic material [8], electrochemical anodes [9] and newly in supercapacitors [10]. An emerging application of Co3O4 as an electrode material in electrochemical supercapacitors can prove itself as a promising alternate to expensive RuO2, which has been used extensively as an electrode material. These kinds of films are generally used in electrochemical capacitors for high power devices. There are many reports found in literature for the synthesis of cobalt oxide films from different chemical methods such as spray pyrolysis [10], sputtering [11], chemical vapor deposition
S.G. Kandalkar et al. / Applied Surface Science 253 (2007) 3952–3956
(CVD) [12], pulsed laser deposition [13], sol–gel process [14], electrodeposition [15], etc., on a variety of substrates. Each deposition method offers different advantages, for example electrodeposition is an effective, fast and controllable process for depositing various thin film layers on curved or cylindrical shaped substrates. On the other hand, the CVD provides uniform deposition over large areas, good coverage and selective deposition. Moreover, the pulsed-injection metal organic chemical vapor deposition (MOCVD) technique has the possibility to produce and controlling the film composition, microstructure and morphology, through a suitable choice of the substrate, precursor and reactant, as well as the deposition conditions [16]. Accordingly, the main goal of this investigation is to deposit cobalt oxide films, to study their structural, morphological and optical properties, as well as to demonstrate the potential of a non-thermal chemical method as an alternative to traditional high temperature ceramic film deposition methods. The present chemical method is a simple and convenient method for the large area deposition. The initially required precursor materials are cheap and easily available. Looking into applications, in this paper, we presented chemical synthesis of cobalt oxide thin films onto glass substrates and characterized for structural, optical and electrical properties along with surface wettability and Raman studies. 2. Experimental details The deposition was carried out onto commercially available glass micro-slides of the size 75 mm 25 mm 1 mm. The micro-slides were boiled in chromic acid for 30 min, washed with detergent, rinsed in acetone, and finally, ultrasonically cleaned with double distilled water. Cobalt oxide films were deposited at room temperature from the cation precursor 50 mL, 0.4 M CoCl2 6H2O added with 25% ammonia solution to make pH 12 and the double distilled water with few drops of H2O2 as anion solution. Double distilled water is alternately placed in between the beakers containing cationic and anionic precursor solutions. Cleaned substrate was immersed in the solution of cobalt chloride for 40 s, where cobalt ions are adsorbed on the substrate surface which was then rinsed in double distilled water for 50 s to remove loosely bound or excess cobalt ions from the substrate. The substrate was then immersed in anionic precursor (double distilled water with H2O2) solution for 20 s where, the oxygen ions reacted with pre-adsorbed cobalt ions on the glass substrate to form cobalt oxide film. By changing the concentration of cationic solution and deposition cycles, thickness of the film was controlled. Thickness of as-deposited thin films was measured using ellipsometry technique. As-deposited and annealed cobalt oxide thin films were characterized for the structural (X-ray diffractometer using Cu Ka radiation in the 2u range of 10–808 ˚ ), optical absorption and electrical of wavelength 1.5426 A resistivity measurements. Optical studies were done to determine the band gap energy, maximum absorption coefficient and effect of annealing on band gap energy. The optical absorption spectra in the range 300–900 nm were recorded using UV–vis spectrophotometer. To study the
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electrical characterization of cobalt oxide films, the electrical resistivity measurement was carried out using dc two point probe method in the temperature range 300–550 K. For surface morphological studies, the films were coated with gold palladium by a polaron sputter coating unit E-5200. The films were loaded in the sample holder of Cambridge Stereoscan 250 MK-III unit for studying micro-structural aspects of the cobalt oxide thin films. Surface wettability was evaluated by water angle contact measurement, using a commercial contact angle meter (CA-X, Kyowakaimenkagaku Co. Ltd., Saitama, Japan). The water drops (1 mL, ultra pure) put onto the film surface using vertical syringe. Raman spectrum was recorded in air with a resolution of 2 cm 1 using a Yvon Jobin Labram spectrometer with a 775 nm Ar-ion laser, run in a back-scattered confocal arrangement. 3. Results and discussion 3.1. Growth mechanism and structural studies In this method, an immersion of substrate into separately placed cationic and anionic precursors and rinsing between every immersion with ion-exchange water prevents the solutions from forming homogenous precipitation. The CoO film formation process involves the adsorption of cations from precursor solution which when interacts with anions from anion precursor, resulted into formation nucleation which grow sequentially with number of cycles as described elsewhere [17]. Table 1 shows the optimized deposition parameter conditions for cobalt oxide thin films. It was found that the optimum concentration range of the starting solution lies in a narrow range. When the concentration of the cobalt chloride solution was low (<0.4 M), the film formation was too slow to be recognized, and when the concentration was high (>0.4 M), powdery precipitate came out quickly. This means that the oxidation of Co2+ has to take place in an appropriate rate for the heterogeneous nucleation of cobalt oxide to predominate. When thin films were prepared above the room temperature, the concentration of the cobalt chloride solution had to be accordingly lowered to obtain thin films. The adherence of the films to the substrate was good when the films were prepared at room temperature. After the deposition, the films were annealed at 623 K for 4 h. The annealing process Table 1 Optimized preparative parameters used for synthesis of amorphous colbalt oxide thin films 1 2
Cation solution Anion solution
3 4 5 6 7 8 9
Complexing agent (NH3) Rinsing solution Concentration of cation solution pH of cation solution Number of cycles Immersion time in cation solution Immersion time in anion solution
50 mL of CoCl2:6H2O 45 mL double distilled water + 5mL H2O2 50 mL Double distilled water 0.4 M 12 50 40 s 20 s
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Fig. 1. The XRD patterns of as-deposited (a) and annealed cobalt oxide thin films (b). Inset of each figure is the corresponding film color.
generally makes the film free from the defects with improved crystallinity and electrical conductivity. Fig. 1(a and b) shows the XRD profiles of as-deposited and that of annealed cobalt oxide thin films deposited onto the glass substrate. The asdeposited films did not show any distinct diffraction peak, which probably means that the film consisted of hydrous Co3O4 colloidal particles with low crystallinity. After annealing, the film did not show clearly an improvement in crystallinity. However, here we found considerable change in film color i.e. the as-deposited films were brown in color, which became black after annealing [insets of Fig. 1(a and b)]. 3.2. Optical and electrical resistivity studies The variation of absorbance (at) of as-deposited and annealed cobalt oxide films of thickness 0.91 mm measured using ellipsometry is shown in Fig. 2(a). Observed spectra reveals that the cobalt oxide film has absorbance in the visible region, which increases towards higher wavelength after annealing. The theory of optical absorption gives the relationship between the absorption coefficient, a and the photon energy, hn for direct allowed transition. Fig. 2(b) shows the plots of (ahn)2 versus hn for as-deposited and annealed cobalt oxide films. The estimated band gap energy for asdeposited cobalt oxide film is 2.5 eV which decreased to 2.20 eV after annealing, which is in consistent with the band
gap energy of cobalt oxide films reported earlier for spray pyrolysis method [10]. The electrical resistivity of as-deposited and annealed cobalt oxide films was measured in the range of 300–500 K to demonstrate semiconducting behavior and to know magnitude of resistivity at room temperature. The electrical resistivity under reversible measurements showed no significant difference. Fig. 3 shows the variation of dark electrical resistivity (log r) with temperature (1000/T) for both the films. The asdeposited cobalt oxide film shows the semiconductor behavior with two different transport mechanisms. After annealing, the film showed similar trend with no significant change in room temperature resistivity magnitude (104 V cm). Resistivity in lower temperature region is almost same to that of as-deposited and lowered in high temperature region due to removal of defect levels/staking faults if any. The activation energy calculated from the slopes of graphs represents the location of trap levels below the conduction band. The activation energies calculated in low temperature region were almost the same (0.08 eV) and in high temperature region, were 0.67 and 0.8 eV, respectively for as-deposited and annealed cobalt oxide films. 3.3. Surface morphology and wettability studies The adherence of the films to the substrates was so good that the films were not peeled or scraped off with stainless
Fig. 2. (a) The variation of optical absorbance of as-deposited and annealed cobalt oxide films as a function of incident radiation wavelength and the measurement of band gap energies by extrapolating straight lines to zero absorption coefficient for the same films (b).
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Fig. 3. The logarithmic dark electrical resistivity (log r) variation with reciprocal of temperature (1/T) for annealed cobalt oxide film.
steel knives. Fig. 4 shows SEM image of annealed cobalt oxide thin film due to the nearly same surface morphology for as-deposited. The film surface looks smooth and composed of very fine elongated particles smaller than 80 nm in length connected by two-three spherical grains of about 40–45 nm in diameters. From SEM image, overgrowth of clusters is clearly seen. Initially grown nanograins may have increased their size by further deposition and come closer to each other. The cobalt oxide film surface is well covered without any pinholes and cracks. Such surface morphology may offer increased surface area, feasible for supercapacitor application [18]. The annealed cobalt oxide films were employed in water contact angle measurement. The wetting of solid with water, where air is the surrounding medium, is dependent on the relation between the interfacial tensions (water/air, water/ solid and solid/air). The ratio between these tensions determines the contact angle ‘u’ between a water droplet on a given surface. A contact angle of 08 means complete wetting, and a contact angle of 1808 corresponds to complete nonwetting. Both super-hydrophilic and super-hydrophobic surfaces are important for practical applications. Prior to contact angle measurements or treatment, film was rinsed with
Fig. 4. The SEM image of annealed cobalt oxide film showing smooth surface area with well defined grain boundaries.
Fig. 5. Measurement of water contact angle on the surface of annealed cobalt oxide film.
acetone and de-ionized water. The average contact angle was obtained by measuring for at least five separate drops on each sample surface by delivering de-ionized water with a microsyringe. Measurement of surface water contact angle is inversely proportional to the wettability and can be determined by Young’s relation. Fig. 5 shows the water contact measurements for cobalt oxide thin films. Interestingly, cobalt oxide exhibits hydrophilic behavior (60.738), as water contact angle is <908. Generally, low water contact angle increases the electrochemical performance, where interfacial contact at electrolyte-electrode is important [19]. 3.4. Raman studies The Raman imaging was performed via the point illumination method. The surface of the sample had to be smooth, and
Fig. 6. Raman spectra of glass and annealed cobalt oxide film deposited onto the glass substrate.
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the specimens were therefore pressed with microslide. The Raman scattered light was collected to form a file containing a Raman spectrum for each individual sample point. The computer was programmed to make a map of the surface. This map showed the intensity of a given Raman peak at each point. The Raman spectrum was measured between the limit of 260–780 cm 1 for the annealed cobalt oxide films as shown in Fig. 6. The spectra for as-deposited and glass were the same and hence only for glass is shown. The peaks marked by arrow at 480 and 682 cm 1 [Fig. 6] are the same for cobalt oxide [20]. 4. Conclusions The cobalt oxide thin films were prepared onto glass substrate by chemical method from an aqueous bath and annealed at 623 K for 4 h, where amorphous structure was noticed except change in color responsible for change in band gap. The observed hydrophilic surface nature from water contact angle measurement indicates that cobalt oxide may be a candidate for supercapacitor application. Raman spectrum showed the presence of cobalt oxide peaks at two different positions, confirming the formation of cobalt oxide. Acknowledgements This work is supported by Hanyang University Next Generation Development Program. RSM wish to thank KOSEF for awarding Brain Pool fellowship in 2006–2007.
References [1] C.R. Gravaglia, C.M. Mari, S. Trasatti, C. Asmundis, Surf. Technol. 19 (1983) 197. [2] G.A. Kokarev, V.A. Kolensnikov, A.F. Gubin, A.A. Korobanov, Electrokhimia 18 (1982) 466. [3] H. Reny, Treatise on Inorganic Chemistry, vol. II, Elsevier Publ. Co., Amsterdam, 1956. [4] R.S. Young (Ed.), Cobalt, Its Chemistry Mutallurgy and Users, Reinhold Publ. Corp., New York, 1960. [5] T. Seike, J. Nagai, Sol. Energy Mater. 22 (1991) 107. [6] N. Bahlawane, Appl. Catal. B 67 (2006) 168. [7] J. Wollenstein, M. Burgmair, G. Plescher, T. Sulima, J. Hildenbrand, H. Bottner, I. Eisele, Sens. Actuators B 93 (2003) 442. [8] P.S. Patil, L.D. Kadam, C.D. Lokhande, Sol. Energy Mater. Sol. Cells 53 (1998) 229. [9] C. Cantalini, M. Post, D. Buso, M. Guglielmi, A. Martucci, Sens. Actuators B 108 (2005) 184. [10] V.R. Shinde, S.B. Mahadik, T.P. Gujar, C.D. Lokhande, Appl. Surf. Sci. 252 (2006) 7487. [11] C.L. Liao, M.T. Wu, J.H. Yen, I.C. Leu, K.Z. Fung, J. Alloys Compd. 414 (2006) 302. [12] N. Bahlawane, E.F. Rivera, K. Kohse-Ho¨inghaus, A. Brechling, U. Kleineberg, Appl. Catal. B: Environ. 53 (2004) 245. [13] Y. Makimura, A. Rougier, J.-M. Tarascon, Appl. Surf. Sci. 252 (2006) 4593. [14] E. Barrera-Calva, J.C. Martı´nez-Flores, L. Huerta, A. Avila, M. OrtegaLo´pez, Solar Energy Mater. Solar Cells 90 (2006) 2523. [15] I.G. Casella, M. Gatta, J. Electroanal. Chem. 534 (2002) 31. [16] A.U. Mane, S.A. Shivashankar, J. Cryst. Growth 254 (2003) 368. [17] H.M. Pathan, C.D. Lokhande, Bull. Mater. Sci. 27 (2004) 85. [18] T.P. Gujar, V.R. Shinde, C.D. Lokhande, R.S. Mane, S.-H. Han, Appl. Surf. Sci. 250 (2005) 161. [19] E. Hedborg, F. Winquist, I. Lundstrom, Thin Solid Films 240 (1994) 147. [20] O. Bockman, T. Ostvold, G.A. Voyiatzis, G.N. Papatheodorou, Hydrometallurgy 55 (2000) 93.
A non-thermal chemical synthesis of hydrophilic and amorphous cobalt oxide films for supercapacitor application Sunil G. Kandalkar a, C.D. Lokhande a,b,*, R.S. Mane b, Sung-Hwan Han b,* b
a Thin Film Physics Laboratory, Department of Physics, Shivaji University, Kolhapur 416 004, India Hanyang University, Department of Chemistry, Sungdong-Ku, Haengdang-dong 17, Seoul 133791, Republic of Korea
Received 16 July 2006; received in revised form 21 August 2006; accepted 21 August 2006 Available online 25 September 2006
Abstract Present work explored a room temperature, simple and low cost chemical route for the preparation of hydrophilic cobalt oxide films from alkaline cobalt chloride (CoCl2:6H2O) and double distilled water precursor solutions. As-deposited cobalt oxide films showed amorphous nature, which is one of the prime requirements for supercapacitor, as confirmed from X-ray diffraction studies. Changes in direct band gap energy and electrical resistivity of as-deposited cobalt oxide films were confirmed after annealing. Spherical grains of about 40–50 nm diameters were uniformly distributed over the substrate surface. Surface wettability studied in contact with liquid interface, showed hydrophilic nature as water contact angle was <908. Finally, presence of cobalt–oxygen covalent bond was observed from Raman shift experiment. # 2006 Elsevier B.V. All rights reserved. Keywords: Cobalt oxide; Non-thermal process; Raman shift; Electrical resistivity
1. Introduction In general, magnetic oxides containing transition elements such as cobalt, manganese or ruthenium exhibit most fascinating magnetic properties. Among these transition metal oxides, the cobalt oxide is one of the most versatile ceramic materials, since it is a p-type anti-ferromagnetic oxide semiconductor with the highest Curie temperature, Tc = 1396 K. Although, this oxide is customarily identified with its chemical formula Co3O4, in fact it is a nonstoichiometric [1,2]. Cobalt has less affinity for oxygen than iron but more than nickel [3]. It has three well-known polymorphs; the monoxide or cobaltous oxide (CoO), the cobaltic oxide (Co2O3) and the cobaltosic oxide or cobalt cobaltite (Co3O4). Cobaltous oxide (CoO) is the final product formed, when the cobalt compound or other oxides are calcinated to a sufficiently high temperature (1173 K). The pure CoO is difficult to obtain, since it readily takes up oxygen even at room temperature to reform to a higher oxide. Cobaltic oxide (Co2O3) could be formed, when cobalt compounds are heated at * Corresponding authors. E-mail addresses: [email protected] (C.D. Lokhande), [email protected] (S.-H. Han). 0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.08.026
a low temperature in presence of an excess air. Cobalt forms two stable oxides: CoO and Co3O4. At low temperatures, the Co3O4 is a stable normal spinel structure of AB2O4 type, where Co2+ ions occupy the tetrahedral 8a sites and Co3+ occupy the octahedral 16d sites [4]. Solar selective absorber, catalyst in the hydrocracking process of crude fuels, pigment for glasses and ceramics [5] and catalyst for oxygen evolution and oxygen reduction reaction [6] are some of the well-known applications of cobalt oxide, in addition to the glass industry for colored glasses [7]. These commonly known catalytic properties could be due to reduction of activation energies for chemisorption of gas molecules and are very useful to promote high gas sensitivities or lower operation temperatures. It is also widely used as an electrochromic material [8], electrochemical anodes [9] and newly in supercapacitors [10]. An emerging application of Co3O4 as an electrode material in electrochemical supercapacitors can prove itself as a promising alternate to expensive RuO2, which has been used extensively as an electrode material. These kinds of films are generally used in electrochemical capacitors for high power devices. There are many reports found in literature for the synthesis of cobalt oxide films from different chemical methods such as spray pyrolysis [10], sputtering [11], chemical vapor deposition
S.G. Kandalkar et al. / Applied Surface Science 253 (2007) 3952–3956
(CVD) [12], pulsed laser deposition [13], sol–gel process [14], electrodeposition [15], etc., on a variety of substrates. Each deposition method offers different advantages, for example electrodeposition is an effective, fast and controllable process for depositing various thin film layers on curved or cylindrical shaped substrates. On the other hand, the CVD provides uniform deposition over large areas, good coverage and selective deposition. Moreover, the pulsed-injection metal organic chemical vapor deposition (MOCVD) technique has the possibility to produce and controlling the film composition, microstructure and morphology, through a suitable choice of the substrate, precursor and reactant, as well as the deposition conditions [16]. Accordingly, the main goal of this investigation is to deposit cobalt oxide films, to study their structural, morphological and optical properties, as well as to demonstrate the potential of a non-thermal chemical method as an alternative to traditional high temperature ceramic film deposition methods. The present chemical method is a simple and convenient method for the large area deposition. The initially required precursor materials are cheap and easily available. Looking into applications, in this paper, we presented chemical synthesis of cobalt oxide thin films onto glass substrates and characterized for structural, optical and electrical properties along with surface wettability and Raman studies. 2. Experimental details The deposition was carried out onto commercially available glass micro-slides of the size 75 mm 25 mm 1 mm. The micro-slides were boiled in chromic acid for 30 min, washed with detergent, rinsed in acetone, and finally, ultrasonically cleaned with double distilled water. Cobalt oxide films were deposited at room temperature from the cation precursor 50 mL, 0.4 M CoCl2 6H2O added with 25% ammonia solution to make pH 12 and the double distilled water with few drops of H2O2 as anion solution. Double distilled water is alternately placed in between the beakers containing cationic and anionic precursor solutions. Cleaned substrate was immersed in the solution of cobalt chloride for 40 s, where cobalt ions are adsorbed on the substrate surface which was then rinsed in double distilled water for 50 s to remove loosely bound or excess cobalt ions from the substrate. The substrate was then immersed in anionic precursor (double distilled water with H2O2) solution for 20 s where, the oxygen ions reacted with pre-adsorbed cobalt ions on the glass substrate to form cobalt oxide film. By changing the concentration of cationic solution and deposition cycles, thickness of the film was controlled. Thickness of as-deposited thin films was measured using ellipsometry technique. As-deposited and annealed cobalt oxide thin films were characterized for the structural (X-ray diffractometer using Cu Ka radiation in the 2u range of 10–808 ˚ ), optical absorption and electrical of wavelength 1.5426 A resistivity measurements. Optical studies were done to determine the band gap energy, maximum absorption coefficient and effect of annealing on band gap energy. The optical absorption spectra in the range 300–900 nm were recorded using UV–vis spectrophotometer. To study the
3953
electrical characterization of cobalt oxide films, the electrical resistivity measurement was carried out using dc two point probe method in the temperature range 300–550 K. For surface morphological studies, the films were coated with gold palladium by a polaron sputter coating unit E-5200. The films were loaded in the sample holder of Cambridge Stereoscan 250 MK-III unit for studying micro-structural aspects of the cobalt oxide thin films. Surface wettability was evaluated by water angle contact measurement, using a commercial contact angle meter (CA-X, Kyowakaimenkagaku Co. Ltd., Saitama, Japan). The water drops (1 mL, ultra pure) put onto the film surface using vertical syringe. Raman spectrum was recorded in air with a resolution of 2 cm 1 using a Yvon Jobin Labram spectrometer with a 775 nm Ar-ion laser, run in a back-scattered confocal arrangement. 3. Results and discussion 3.1. Growth mechanism and structural studies In this method, an immersion of substrate into separately placed cationic and anionic precursors and rinsing between every immersion with ion-exchange water prevents the solutions from forming homogenous precipitation. The CoO film formation process involves the adsorption of cations from precursor solution which when interacts with anions from anion precursor, resulted into formation nucleation which grow sequentially with number of cycles as described elsewhere [17]. Table 1 shows the optimized deposition parameter conditions for cobalt oxide thin films. It was found that the optimum concentration range of the starting solution lies in a narrow range. When the concentration of the cobalt chloride solution was low (<0.4 M), the film formation was too slow to be recognized, and when the concentration was high (>0.4 M), powdery precipitate came out quickly. This means that the oxidation of Co2+ has to take place in an appropriate rate for the heterogeneous nucleation of cobalt oxide to predominate. When thin films were prepared above the room temperature, the concentration of the cobalt chloride solution had to be accordingly lowered to obtain thin films. The adherence of the films to the substrate was good when the films were prepared at room temperature. After the deposition, the films were annealed at 623 K for 4 h. The annealing process Table 1 Optimized preparative parameters used for synthesis of amorphous colbalt oxide thin films 1 2
Cation solution Anion solution
3 4 5 6 7 8 9
Complexing agent (NH3) Rinsing solution Concentration of cation solution pH of cation solution Number of cycles Immersion time in cation solution Immersion time in anion solution
50 mL of CoCl2:6H2O 45 mL double distilled water + 5mL H2O2 50 mL Double distilled water 0.4 M 12 50 40 s 20 s
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Fig. 1. The XRD patterns of as-deposited (a) and annealed cobalt oxide thin films (b). Inset of each figure is the corresponding film color.
generally makes the film free from the defects with improved crystallinity and electrical conductivity. Fig. 1(a and b) shows the XRD profiles of as-deposited and that of annealed cobalt oxide thin films deposited onto the glass substrate. The asdeposited films did not show any distinct diffraction peak, which probably means that the film consisted of hydrous Co3O4 colloidal particles with low crystallinity. After annealing, the film did not show clearly an improvement in crystallinity. However, here we found considerable change in film color i.e. the as-deposited films were brown in color, which became black after annealing [insets of Fig. 1(a and b)]. 3.2. Optical and electrical resistivity studies The variation of absorbance (at) of as-deposited and annealed cobalt oxide films of thickness 0.91 mm measured using ellipsometry is shown in Fig. 2(a). Observed spectra reveals that the cobalt oxide film has absorbance in the visible region, which increases towards higher wavelength after annealing. The theory of optical absorption gives the relationship between the absorption coefficient, a and the photon energy, hn for direct allowed transition. Fig. 2(b) shows the plots of (ahn)2 versus hn for as-deposited and annealed cobalt oxide films. The estimated band gap energy for asdeposited cobalt oxide film is 2.5 eV which decreased to 2.20 eV after annealing, which is in consistent with the band
gap energy of cobalt oxide films reported earlier for spray pyrolysis method [10]. The electrical resistivity of as-deposited and annealed cobalt oxide films was measured in the range of 300–500 K to demonstrate semiconducting behavior and to know magnitude of resistivity at room temperature. The electrical resistivity under reversible measurements showed no significant difference. Fig. 3 shows the variation of dark electrical resistivity (log r) with temperature (1000/T) for both the films. The asdeposited cobalt oxide film shows the semiconductor behavior with two different transport mechanisms. After annealing, the film showed similar trend with no significant change in room temperature resistivity magnitude (104 V cm). Resistivity in lower temperature region is almost same to that of as-deposited and lowered in high temperature region due to removal of defect levels/staking faults if any. The activation energy calculated from the slopes of graphs represents the location of trap levels below the conduction band. The activation energies calculated in low temperature region were almost the same (0.08 eV) and in high temperature region, were 0.67 and 0.8 eV, respectively for as-deposited and annealed cobalt oxide films. 3.3. Surface morphology and wettability studies The adherence of the films to the substrates was so good that the films were not peeled or scraped off with stainless
Fig. 2. (a) The variation of optical absorbance of as-deposited and annealed cobalt oxide films as a function of incident radiation wavelength and the measurement of band gap energies by extrapolating straight lines to zero absorption coefficient for the same films (b).
S.G. Kandalkar et al. / Applied Surface Science 253 (2007) 3952–3956
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Fig. 3. The logarithmic dark electrical resistivity (log r) variation with reciprocal of temperature (1/T) for annealed cobalt oxide film.
steel knives. Fig. 4 shows SEM image of annealed cobalt oxide thin film due to the nearly same surface morphology for as-deposited. The film surface looks smooth and composed of very fine elongated particles smaller than 80 nm in length connected by two-three spherical grains of about 40–45 nm in diameters. From SEM image, overgrowth of clusters is clearly seen. Initially grown nanograins may have increased their size by further deposition and come closer to each other. The cobalt oxide film surface is well covered without any pinholes and cracks. Such surface morphology may offer increased surface area, feasible for supercapacitor application [18]. The annealed cobalt oxide films were employed in water contact angle measurement. The wetting of solid with water, where air is the surrounding medium, is dependent on the relation between the interfacial tensions (water/air, water/ solid and solid/air). The ratio between these tensions determines the contact angle ‘u’ between a water droplet on a given surface. A contact angle of 08 means complete wetting, and a contact angle of 1808 corresponds to complete nonwetting. Both super-hydrophilic and super-hydrophobic surfaces are important for practical applications. Prior to contact angle measurements or treatment, film was rinsed with
Fig. 4. The SEM image of annealed cobalt oxide film showing smooth surface area with well defined grain boundaries.
Fig. 5. Measurement of water contact angle on the surface of annealed cobalt oxide film.
acetone and de-ionized water. The average contact angle was obtained by measuring for at least five separate drops on each sample surface by delivering de-ionized water with a microsyringe. Measurement of surface water contact angle is inversely proportional to the wettability and can be determined by Young’s relation. Fig. 5 shows the water contact measurements for cobalt oxide thin films. Interestingly, cobalt oxide exhibits hydrophilic behavior (60.738), as water contact angle is <908. Generally, low water contact angle increases the electrochemical performance, where interfacial contact at electrolyte-electrode is important [19]. 3.4. Raman studies The Raman imaging was performed via the point illumination method. The surface of the sample had to be smooth, and
Fig. 6. Raman spectra of glass and annealed cobalt oxide film deposited onto the glass substrate.
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the specimens were therefore pressed with microslide. The Raman scattered light was collected to form a file containing a Raman spectrum for each individual sample point. The computer was programmed to make a map of the surface. This map showed the intensity of a given Raman peak at each point. The Raman spectrum was measured between the limit of 260–780 cm 1 for the annealed cobalt oxide films as shown in Fig. 6. The spectra for as-deposited and glass were the same and hence only for glass is shown. The peaks marked by arrow at 480 and 682 cm 1 [Fig. 6] are the same for cobalt oxide [20]. 4. Conclusions The cobalt oxide thin films were prepared onto glass substrate by chemical method from an aqueous bath and annealed at 623 K for 4 h, where amorphous structure was noticed except change in color responsible for change in band gap. The observed hydrophilic surface nature from water contact angle measurement indicates that cobalt oxide may be a candidate for supercapacitor application. Raman spectrum showed the presence of cobalt oxide peaks at two different positions, confirming the formation of cobalt oxide. Acknowledgements This work is supported by Hanyang University Next Generation Development Program. RSM wish to thank KOSEF for awarding Brain Pool fellowship in 2006–2007.
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