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Fabrication of nanocrystalline cobalt oxide via sol–gel coating
Materials Science and Engineering B 144 (2007) 69–72
Fabrication of nanocrystalline cobalt oxide via sol–gel coating Ahalapitiya H. Jayatissa a,∗ , Kun Guo a , Ambalangodage C. Jayasuriya b , Tarun Gupta c b
a MIME Department, The University of Toledo, OH 43560, USA Orthopaedic Department, The University of Toledo, OH 43614, USA c IME Department, Western Michigan University, MI 49008, USA
Abstract Nanocrystalline cobalt oxide thin films were fabricated by sol–gel processing method. The effect of cobalt salt amount in sol–gel mixture on film properties was investigated by means of Raman spectroscopy and atomic force microscopy. Micropatterns of these sol–gel mixtures were fabricated by spin coating followed by a lift-off method. It is generally known that sol–gel processed films are difficult to etch or remove by chemical and plasma techniques. In this novel process, cobalt oxide microstructures could be fabricated by transformation of pre-fabricated nanocrystalline cobalt oxide particles in a sol–gel process. Also, the growth of carbon nanotubes on fabricated cobalt oxide films was investigated. It was found that different size multiwall carbon nanotubes could be grown on the cobalt oxide films depending on the concentration of cobalt salt in sol–gel mixtures. © 2007 Elsevier B.V. All rights reserved. Keywords: Sol–gel processing; Cobalt oxides; Nanocrystalline thin films; Microfabrication; Carbon nanotubes
1. Introduction Fabrication of metal oxide thin films by sol–gel processing has attracted much attention because this technique allows transfer of pre-fabricated nanostructures on to a substrate. Also, sol–gel process is cost effective, scalable and reproducible. However, fabrication of micropatterns using sol–gel coated films is difficult because these films are resistance to etch using chemical or plasma methods. Therefore, application of sol–gel coated films in electronic devices and micro-electro-mechanical systems (MEMS) is limited at present [1]. The growth of carbon nanotubes has been investigated on metal catalyst coated surfaces at elevated temperatures. Cobalt, iron and nickel have been widely used as the catalysts for the growth of carbon nanotubes [2,3]. In typical nanotube growth process, metal oxide nanoparticles have been used because metal oxide nanoparticles can be pre-synthesized before coating on the substrates. Furthermore, oxide particles are more stable than thin metal films in a heating process. When compared with thin metal films, the metal oxide crystals cannot be agglomerate and grow large crystals at moderate temperature (400–600 ◦ C) where
∗
Corresponding author. Tel.: +1 419 530 8245; fax: +1 419 530 8206. E-mail address: [email protected] (A.H. Jayatissa).
0921-5107/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2007.07.078
reduction of metal oxides is taken place. Thus, sol–gel processing is an ideal way to fabricate nanostructure oxide catalyst materials for coating of nanotubes. In this paper, coating of nanocrystalline cobalt oxide thin films by a sol–gel method was investigated. Primary focus of this research is to obtain patterned nanocrystalline cobalt oxide films coated by sol–gel processing. The processing of cobalt oxide films and their surface and microstructure were investigated. Also, the application of these films as the substrate for growing carbon nanotubes is reported. The patterns produced by this process could be successfully used to grow multiwall carbon nanotubes. 2. Experimental Aluminum films were coated on glass and silicon substrates by physical vapor deposition. The thickness of aluminum layer was in the range of 200–500 nm, depending on the application. Desired microstructure was transferred to the aluminum film by optical lithography. The aluminum films were etched using a mixture of nitric acid and phosphoric acid (1:3) at 60 ◦ C. After photoresists were removed from the aluminum patterns, sol–gel mixture was coated as follows. Known amount of Co(NO)3 was dissolved in 20 ml of isopropyl alcohol. This solution was added to diethanolamine and 1% triton (x-100, Alfa Aeser). The mix-
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A.H. Jayatissa et al. / Materials Science and Engineering B 144 (2007) 69–72
Table 1 Cobalt nitrate concentration (isopropyl alcohol:diethanolamine = 1:1. Also, 1% triton X-100 and 1 ml of NH4 OH was added. The total volume of solution was 6 ml) Sample #
Co(NO3 )2 (mg/ml)
Thickness (nm/run)
1 2 3 4
0.5 2.5 5.0 25.0
∼10 17.0 22.5 78.0
Thickness per run (1 min at 4000 rpm) is also given.
ture was thoroughly stirred for 10 min. After this process, 1 ml of 0.1 M ammonium hydroxide was added to the solution and stirred for 8 h. The resulting solution was coated on aluminum micro patterns. Table 1 lists the concentration of Co(NO)3 in the solutions and coating thickness per run. The spin coating was carried out at 4000 rpm for 1 min followed by heating at 250 ◦ C on a hot plate. After coating process was completed, the films were heated at 500 ◦ C for 6 h in the ambient. Fig. 1 indicates the processing sequence of cobalt oxide micropatterns. Cobalt oxide coated aluminum patterns were etched in nitric acid and phosphoric acid solution at 60 ◦ C. After this etching process, cobalt patterns were obtained. Carbon nanotubes were grown on these nanocrystalline cobalt oxide films using a chemical vapor deposition (CVD) system. Cobalt oxide coated substrates were placed in a quartz tube furnace and heated at 450 ◦ C in hydrogen ambient for 2 h. In this process,
Fig. 1. Fabrication sequence of sol–gel based cobalt oxide micropatterns.
the surface layer of cobalt oxide reduces to cobalt layer. After this process, the furnace temperature was increased to 600 ◦ C. The growth of nanotubes was carried out with methane at a flow rate of 100 ccm for 1 h. Cobalt oxide thin films and carbon nanotubes were characterized using atomic force microscopy (AFM), Raman
Fig. 2. AFM image of cobalt oxide films coated with different cobalt salt concentrations as given in Table 1 (area of scan = 1 m2 ).
A.H. Jayatissa et al. / Materials Science and Engineering B 144 (2007) 69–72
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indicate the crystal size of thin films, AFM images clearly indicates that the surface morphology has a direct relationship with the cobalt concentration in the solution. Larger grains appeared for higher concentration of cobalt salt. Large area AFM scanning (15 m × 15 m) indicated that quite uniform cobalt oxide layers could be fabricated using all concentrations of sol–gels.
Fig. 3. Raman spectra of nanostructure cobalt thin films. Raman intensity of sol sample-2 was multiplied by two times. Spectrum of sample-3 was shifted for clarity (see Table 1 for sample description).
(excitation 532 nm) and scanning electron microscopy (SEM). 3. Results and discussion In order to investigate the dependence of crystallinity and surface morphology on cobalt content in sol–gel mixture, four different cobalt concentrations were used for the coating of cobalt oxide thin films (Table 1). The resulting films were measured with AFM as shown in Fig. 2. The scanning area of these images was 1 m2 . All these patterns indicated that the film surfaces have rough surface and grain-like morphology. When the cobalt concentration was reduced, the films became smoother and the size of these grains was reduced. Though AFM does not
Fig. 4. SEM image of array fabricated using sample-4 (see Table 1) on glass substrate. Diameter of array is 5 m. (a) Large area view and (b) small area view. Inset is the high-resolution image of one of the patterns.
Fig. 5. Carbon nanotubes grown using cobalt oxide film array fabricated by sol–gel processing. Nanotubes coated on sample-1 (insert picture), sample-2 (scale bar 100 nm) and sample-4 (scale bars are 500 nm) are shown.
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Fig. 3 indicates the Raman spectra of cobalt oxide films coated with samples 2, 3 and 4 described in Table 1. Here, asrecorded spectra are presented without any processing. It can be seen that all films have peaks at 483, 522, 621 and 694 cm−1 corresponding, respectively, to Eg , E1 2g , F2 2g and A1g modes of crystalline Co3 O4 [4]. Slight differences of Raman bands can be associated with residual stress of the films. No detectable signal in Raman spectra was observed for the film coated with sol–gel sample-1 perhaps due to the small crystallites or ultrathin layer on the surfaces. If the sol mixture was dried to make a thicker film, we could observe the Raman bands for this sol concentration (not shown). The Raman spectra indicated the bands corresponding to Co3 O4 only [4,5]. The background of the spectrum is slightly increased for smaller concentrations and this kind of phenomenon in Raman spectroscopy can be associated with scattering and luminescence phenomenon. However, we do not know the exact reason for this phenomenon at present. Fig. 4 shows SEM image of cobalt oxide micropatterns deposited with sample # 2. In this preliminary investigation, we have fabricated such array patterns only in 1 cm × 1 cm. The diameter and the distance between centers of patterns were 5 and 55 m, respectively. The thickness of cobalt layer estimated to be 85 ± 5 nm (5 runs). The fabrication was successful for high concentrations as shown in Fig. 4(a). But for the lowest concentration (0.5 mg/ml), the fabrication was not very successful. At low concentrations, disconnectivity among crystallites and thermal stress may result in delaminating of micropatterns. At higher concentration of cobalt nitrate, uniform and dense cobalt oxide thin films were deposited as shown in Fig. 2. Carbon nanotubes were grown on cobalt oxide films using chemical vapor deposition of a methane hydrogen gas mixture. Detail description of this growth system has been reported elsewhere [6]. Nanotubes grown on cobalt oxide films made using three different cobalt nitrate concentrations are shown in Fig. 5. The nanotube grown using films made by samples 2 and 3 were very close. However, these SEM images indicate that size of nanotubes strongly depends on the cobalt nitrate concentration in sol–gel solution. According to SEM based estimations, the diameter of nanotubes for sol–gel samples 1, 2 and 4 were 5, 16 and 78 nm, respectively. These results clearly indicate that the size of nanotubes can be controlled by change of the concentration of cobalt in sol–gel precursors. Low concentrations made small crystallites in cobalt oxide thin films and therefore, it can lead to grow small multiwall carbon nanotubes.
In this investigation, the cobalt oxide thin films with different size nanoparticles were fabricated employing a sol–gel process. For instance, one can coat crystal cobalt oxide films directly by sputtering or evaporation followed by annealing at high temperature. However, it is difficult to control the size of crystallites to a few nanometers using vapor deposition and annealing methods. In sol–gel processing technique, pre-fabricated nanoparticles can be coated on different substrates and these crystals are stable in preheating temperature near by 600 ◦ C. Nevertheless, films produced by sol–gel methods are difficult to etch and remove unwanted parts in a microfabrication process. In the present technique, a lift off method was employed to fabricate simple micropatterns that can be used to grow carbon nanotubes. 4. Conclusions Processing of cobalt oxide thin films by sol–gel coating technique was developed. The process could be applied to generate micropatterns of cobalt oxides employing a lift-off method. By changing the concentration of cobalt in sol–gel mixture, surface morphology, crystallinity and film thickness per coating could be controlled. Such films could be used to grow different size carbon nanotubes depending on the cobalt oxide particle size. Authors believe that this technique can be used to incorporate multiwall carbon nanotubes on MEMS and field emission display devices. Acknowledgements This research was supported by the National Science Foundation (NSF) of USA with grant number 0401690. Authors acknowledge Dr. G. Sumanasekera at the University of Louisville for allowing using Raman spectrophotometer. References [1] T.G. Cooney, L.F. Francis, J. Micromech. Microeng. 6 (1996) 291. [2] S.P. Chai, Z.S.H. Sharif, M.A. Rahman, Chem. Phys. Lett. 426 (2006) 345. [3] D. Nishide, H. Kataura, S. Suzuki, S. Okubo, Y. Achiba, Chem. Phys. Lett. 392 (2004) 309. [4] V.G. Hadjiev, M.N. Iliev, I.V. Vergilov, J. Phys. C: Solid State Phys. 21 (1988) L199. [5] H.C. Choi, Y.M. Jung, I. Noda, S.B. Kim, J. Phys. Chem. B 107 (2003) 5806. [6] Guo, A.H. Jayatissa, Proceedings of the ASME–IMECE Conference, Chicago, USA, November 5–10, 2006.
Fabrication of nanocrystalline cobalt oxide via sol–gel coating Ahalapitiya H. Jayatissa a,∗ , Kun Guo a , Ambalangodage C. Jayasuriya b , Tarun Gupta c b
a MIME Department, The University of Toledo, OH 43560, USA Orthopaedic Department, The University of Toledo, OH 43614, USA c IME Department, Western Michigan University, MI 49008, USA
Abstract Nanocrystalline cobalt oxide thin films were fabricated by sol–gel processing method. The effect of cobalt salt amount in sol–gel mixture on film properties was investigated by means of Raman spectroscopy and atomic force microscopy. Micropatterns of these sol–gel mixtures were fabricated by spin coating followed by a lift-off method. It is generally known that sol–gel processed films are difficult to etch or remove by chemical and plasma techniques. In this novel process, cobalt oxide microstructures could be fabricated by transformation of pre-fabricated nanocrystalline cobalt oxide particles in a sol–gel process. Also, the growth of carbon nanotubes on fabricated cobalt oxide films was investigated. It was found that different size multiwall carbon nanotubes could be grown on the cobalt oxide films depending on the concentration of cobalt salt in sol–gel mixtures. © 2007 Elsevier B.V. All rights reserved. Keywords: Sol–gel processing; Cobalt oxides; Nanocrystalline thin films; Microfabrication; Carbon nanotubes
1. Introduction Fabrication of metal oxide thin films by sol–gel processing has attracted much attention because this technique allows transfer of pre-fabricated nanostructures on to a substrate. Also, sol–gel process is cost effective, scalable and reproducible. However, fabrication of micropatterns using sol–gel coated films is difficult because these films are resistance to etch using chemical or plasma methods. Therefore, application of sol–gel coated films in electronic devices and micro-electro-mechanical systems (MEMS) is limited at present [1]. The growth of carbon nanotubes has been investigated on metal catalyst coated surfaces at elevated temperatures. Cobalt, iron and nickel have been widely used as the catalysts for the growth of carbon nanotubes [2,3]. In typical nanotube growth process, metal oxide nanoparticles have been used because metal oxide nanoparticles can be pre-synthesized before coating on the substrates. Furthermore, oxide particles are more stable than thin metal films in a heating process. When compared with thin metal films, the metal oxide crystals cannot be agglomerate and grow large crystals at moderate temperature (400–600 ◦ C) where
∗
Corresponding author. Tel.: +1 419 530 8245; fax: +1 419 530 8206. E-mail address: [email protected] (A.H. Jayatissa).
0921-5107/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2007.07.078
reduction of metal oxides is taken place. Thus, sol–gel processing is an ideal way to fabricate nanostructure oxide catalyst materials for coating of nanotubes. In this paper, coating of nanocrystalline cobalt oxide thin films by a sol–gel method was investigated. Primary focus of this research is to obtain patterned nanocrystalline cobalt oxide films coated by sol–gel processing. The processing of cobalt oxide films and their surface and microstructure were investigated. Also, the application of these films as the substrate for growing carbon nanotubes is reported. The patterns produced by this process could be successfully used to grow multiwall carbon nanotubes. 2. Experimental Aluminum films were coated on glass and silicon substrates by physical vapor deposition. The thickness of aluminum layer was in the range of 200–500 nm, depending on the application. Desired microstructure was transferred to the aluminum film by optical lithography. The aluminum films were etched using a mixture of nitric acid and phosphoric acid (1:3) at 60 ◦ C. After photoresists were removed from the aluminum patterns, sol–gel mixture was coated as follows. Known amount of Co(NO)3 was dissolved in 20 ml of isopropyl alcohol. This solution was added to diethanolamine and 1% triton (x-100, Alfa Aeser). The mix-
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A.H. Jayatissa et al. / Materials Science and Engineering B 144 (2007) 69–72
Table 1 Cobalt nitrate concentration (isopropyl alcohol:diethanolamine = 1:1. Also, 1% triton X-100 and 1 ml of NH4 OH was added. The total volume of solution was 6 ml) Sample #
Co(NO3 )2 (mg/ml)
Thickness (nm/run)
1 2 3 4
0.5 2.5 5.0 25.0
∼10 17.0 22.5 78.0
Thickness per run (1 min at 4000 rpm) is also given.
ture was thoroughly stirred for 10 min. After this process, 1 ml of 0.1 M ammonium hydroxide was added to the solution and stirred for 8 h. The resulting solution was coated on aluminum micro patterns. Table 1 lists the concentration of Co(NO)3 in the solutions and coating thickness per run. The spin coating was carried out at 4000 rpm for 1 min followed by heating at 250 ◦ C on a hot plate. After coating process was completed, the films were heated at 500 ◦ C for 6 h in the ambient. Fig. 1 indicates the processing sequence of cobalt oxide micropatterns. Cobalt oxide coated aluminum patterns were etched in nitric acid and phosphoric acid solution at 60 ◦ C. After this etching process, cobalt patterns were obtained. Carbon nanotubes were grown on these nanocrystalline cobalt oxide films using a chemical vapor deposition (CVD) system. Cobalt oxide coated substrates were placed in a quartz tube furnace and heated at 450 ◦ C in hydrogen ambient for 2 h. In this process,
Fig. 1. Fabrication sequence of sol–gel based cobalt oxide micropatterns.
the surface layer of cobalt oxide reduces to cobalt layer. After this process, the furnace temperature was increased to 600 ◦ C. The growth of nanotubes was carried out with methane at a flow rate of 100 ccm for 1 h. Cobalt oxide thin films and carbon nanotubes were characterized using atomic force microscopy (AFM), Raman
Fig. 2. AFM image of cobalt oxide films coated with different cobalt salt concentrations as given in Table 1 (area of scan = 1 m2 ).
A.H. Jayatissa et al. / Materials Science and Engineering B 144 (2007) 69–72
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indicate the crystal size of thin films, AFM images clearly indicates that the surface morphology has a direct relationship with the cobalt concentration in the solution. Larger grains appeared for higher concentration of cobalt salt. Large area AFM scanning (15 m × 15 m) indicated that quite uniform cobalt oxide layers could be fabricated using all concentrations of sol–gels.
Fig. 3. Raman spectra of nanostructure cobalt thin films. Raman intensity of sol sample-2 was multiplied by two times. Spectrum of sample-3 was shifted for clarity (see Table 1 for sample description).
(excitation 532 nm) and scanning electron microscopy (SEM). 3. Results and discussion In order to investigate the dependence of crystallinity and surface morphology on cobalt content in sol–gel mixture, four different cobalt concentrations were used for the coating of cobalt oxide thin films (Table 1). The resulting films were measured with AFM as shown in Fig. 2. The scanning area of these images was 1 m2 . All these patterns indicated that the film surfaces have rough surface and grain-like morphology. When the cobalt concentration was reduced, the films became smoother and the size of these grains was reduced. Though AFM does not
Fig. 4. SEM image of array fabricated using sample-4 (see Table 1) on glass substrate. Diameter of array is 5 m. (a) Large area view and (b) small area view. Inset is the high-resolution image of one of the patterns.
Fig. 5. Carbon nanotubes grown using cobalt oxide film array fabricated by sol–gel processing. Nanotubes coated on sample-1 (insert picture), sample-2 (scale bar 100 nm) and sample-4 (scale bars are 500 nm) are shown.
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Fig. 3 indicates the Raman spectra of cobalt oxide films coated with samples 2, 3 and 4 described in Table 1. Here, asrecorded spectra are presented without any processing. It can be seen that all films have peaks at 483, 522, 621 and 694 cm−1 corresponding, respectively, to Eg , E1 2g , F2 2g and A1g modes of crystalline Co3 O4 [4]. Slight differences of Raman bands can be associated with residual stress of the films. No detectable signal in Raman spectra was observed for the film coated with sol–gel sample-1 perhaps due to the small crystallites or ultrathin layer on the surfaces. If the sol mixture was dried to make a thicker film, we could observe the Raman bands for this sol concentration (not shown). The Raman spectra indicated the bands corresponding to Co3 O4 only [4,5]. The background of the spectrum is slightly increased for smaller concentrations and this kind of phenomenon in Raman spectroscopy can be associated with scattering and luminescence phenomenon. However, we do not know the exact reason for this phenomenon at present. Fig. 4 shows SEM image of cobalt oxide micropatterns deposited with sample # 2. In this preliminary investigation, we have fabricated such array patterns only in 1 cm × 1 cm. The diameter and the distance between centers of patterns were 5 and 55 m, respectively. The thickness of cobalt layer estimated to be 85 ± 5 nm (5 runs). The fabrication was successful for high concentrations as shown in Fig. 4(a). But for the lowest concentration (0.5 mg/ml), the fabrication was not very successful. At low concentrations, disconnectivity among crystallites and thermal stress may result in delaminating of micropatterns. At higher concentration of cobalt nitrate, uniform and dense cobalt oxide thin films were deposited as shown in Fig. 2. Carbon nanotubes were grown on cobalt oxide films using chemical vapor deposition of a methane hydrogen gas mixture. Detail description of this growth system has been reported elsewhere [6]. Nanotubes grown on cobalt oxide films made using three different cobalt nitrate concentrations are shown in Fig. 5. The nanotube grown using films made by samples 2 and 3 were very close. However, these SEM images indicate that size of nanotubes strongly depends on the cobalt nitrate concentration in sol–gel solution. According to SEM based estimations, the diameter of nanotubes for sol–gel samples 1, 2 and 4 were 5, 16 and 78 nm, respectively. These results clearly indicate that the size of nanotubes can be controlled by change of the concentration of cobalt in sol–gel precursors. Low concentrations made small crystallites in cobalt oxide thin films and therefore, it can lead to grow small multiwall carbon nanotubes.
In this investigation, the cobalt oxide thin films with different size nanoparticles were fabricated employing a sol–gel process. For instance, one can coat crystal cobalt oxide films directly by sputtering or evaporation followed by annealing at high temperature. However, it is difficult to control the size of crystallites to a few nanometers using vapor deposition and annealing methods. In sol–gel processing technique, pre-fabricated nanoparticles can be coated on different substrates and these crystals are stable in preheating temperature near by 600 ◦ C. Nevertheless, films produced by sol–gel methods are difficult to etch and remove unwanted parts in a microfabrication process. In the present technique, a lift off method was employed to fabricate simple micropatterns that can be used to grow carbon nanotubes. 4. Conclusions Processing of cobalt oxide thin films by sol–gel coating technique was developed. The process could be applied to generate micropatterns of cobalt oxides employing a lift-off method. By changing the concentration of cobalt in sol–gel mixture, surface morphology, crystallinity and film thickness per coating could be controlled. Such films could be used to grow different size carbon nanotubes depending on the cobalt oxide particle size. Authors believe that this technique can be used to incorporate multiwall carbon nanotubes on MEMS and field emission display devices. Acknowledgements This research was supported by the National Science Foundation (NSF) of USA with grant number 0401690. Authors acknowledge Dr. G. Sumanasekera at the University of Louisville for allowing using Raman spectrophotometer. References [1] T.G. Cooney, L.F. Francis, J. Micromech. Microeng. 6 (1996) 291. [2] S.P. Chai, Z.S.H. Sharif, M.A. Rahman, Chem. Phys. Lett. 426 (2006) 345. [3] D. Nishide, H. Kataura, S. Suzuki, S. Okubo, Y. Achiba, Chem. Phys. Lett. 392 (2004) 309. [4] V.G. Hadjiev, M.N. Iliev, I.V. Vergilov, J. Phys. C: Solid State Phys. 21 (1988) L199. [5] H.C. Choi, Y.M. Jung, I. Noda, S.B. Kim, J. Phys. Chem. B 107 (2003) 5806. [6] Guo, A.H. Jayatissa, Proceedings of the ASME–IMECE Conference, Chicago, USA, November 5–10, 2006.