- Email: [email protected]
Carbon nanotube synthesis, characteristics, and microbattery applications
Materials Science and Engineering B 116 (2005) 363–368
Carbon nanotube synthesis, characteristics, and microbattery applications Zhijing Zhanga,b , Christina Dewana , Saumya Kotharic , Saibal Mitrab , Dale Teetersa,∗ b
a Department of Chemistry and Biochemistry, The University of Tulsa, 600 South College Avenue, Tulsa, OK 74104-3189, USA Department of Physics and Engineering Physics, The University of Tulsa, 600 South College Avenue, Tulsa, OK 74104-3189, USA c Department of Chemical Engineering, The University of Tulsa, 600 South College Avenue, Tulsa, OK 74104-3189, USA
Received 13 November 2003; received in revised form 12 May 2004; accepted 14 May 2004
Abstract The stabilities and efficiencies of cathodes of microbatteries are important for their superior performance. The performance of cathode of microbattery systems can be improved by applying new materials or combining components with unique properties into cathode materials. Individual carbon nanotubes exhibit extraordinary mechanical, thermal and electrical properties. Composite materials, using carbon nanotubes as fillers are, expected to show similar superior properties. This study reports synthesis of carbon nanotubes by microwave chemical vapor deposition (MACVD) and applying them in the cathode of microbatteries to improve battery performance. Carbon nanotubes were grown on Ni deposited porous alumina substrates in a flowing gas mixture of methane and hydrogen. Characterization of the carbon nanotubes was carried out using scanning electronic microscopy, transmission electron microscopy, and Raman spectroscopy. It was found that the carbon nanotubes were randomly oriented, spaghetti-like, and with a high aspect ratio. The diameters of CNTs, as determined by transmission electron microscopy (TEM) were between 10 and 100 nm. TEM also showed that the nanotubes had a central hollow with amorphous carbons covering outside wall. The CNTs were nanocoated with a V2 O5 sol–gel, which upon curing formed a xeorgel about the carbon nanotubes. This formed a molecular composite cathode. Microbatteries/microcapacitors were made and subjected to charge/discharge cycles with a consistent maximum charge of 4.60 V. © 2004 Elsevier B.V. All rights reserved. Keywords: Carbon nanotubes; Microwave plasma; Chemical vapor deposition; Microbattery; Vanadium xerogel cathode.
1. Introduction Microelectromechanical systems (MEMS) and nanoelectromechanical systems (NEMS) need self-contained power sources so that they may be commercially exploited. Many investigations on microbatteries have been reported in the literature, with thin-film rechargeable batteries with active layers of 1–10 m being of interest, since 1980s. The majority of papers on actual functioning microbatteries either describe systems where very thin films of electrolyte materials were used to construct the battery or describe the potential for these films to be used in batteries [1–14]. Although our previous work was in the area of micro and nanobattery construction, development and characterization, in this work we describe ∗
Corresponding author. Tel.: +1 918 631 3147; fax: +1 918 631 3404. E-mail address: [email protected] (D. Teeters).
0921-5107/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2004.05.049
the construction and study of individual nanobatteries with m and nm scale anodes and/or cathodes [15–21]. Recently, there has been considerable effort to design new cathode materials and to improve the performance of cathode systems, since the stabilities and efficiencies of these electrodes are critical for enhanced battery performance. Carbon nanotubes (CNT) hold great promise in this area. Individual carbon nanotubes exhibit unique mechanical, thermal and electrical properties [22]. They are attractive candidates for electrode materials of microbattery systems or microcapacitors due to their superb characteristics of chemical stability, low mass density, low resistivity, and large surface area. Composite materials with carbon nanotubes as fillers are expected to show similar superior properties. Recently, Bachas and co-workers [23] incorporated carbon nanotubes to enhance electrical properties of sol–gel materials. Windle and co-workers [24] investigated the electrochemical capacitance
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of nanocomposite films formed by coating aligned arrays of carbon nanotubes with polypyrrole. They attributed the increased electrical and ionic conductivity of the nanocomposites relative to their component materials to the combination of electrolyte accessibility, reduced diffusion distance, and increased conductivity in the redox pseudo-capacitive composite structure. In our previous work we have shown that the incorporation of bundles carbon nanotubes in V2 O5 sol–gel used in microbattery improved battery performance [15]. In this work we describe the synthesis of CNTs. These CNTs were then mixed with the V2 O5 sol–gel to make a composite cathode. The CNTs were grown on the nanoporous alumina membranes that are typically used to make the microbatteries and then applying V2 O5 sol–gel to the membrane to make the cathode. It was hoped that the addition of better dispersed, individual carbon nanotubes in the sol–gel would further improve microbattery performance. In addition to enhanced mechanical properties, interconnected networks of carbon nanotubes are envisioned to provide direct thermal and electrical conductivity paths within the V2 O5 sol–gel of the composite cathode. Furthermore, the high surface area of carbon nanotubes provides the better access to the electrolyte. Improved capacities and battery response times to charging and discharging are expected.
2. Experimental 2.1. Sample preparation Carbon nanotubes were grown by the catalytic decomposition of hydrocarbon precursor gas over metal catalyst nanoparticles that were already deposited on a substrate. Metal catalysts play a crucial role in the nucleation and growth of carbon nanotubes in a chemical vapor deposition (CVD) process. In this work, a 20 nm layer of nickel was deposited using rf-magnetron sputtering on disk-shaped porous alumina substrates. The alumina disks were 13 mm in diameter and were 60 m thick and were commercially available (Whatman Anodisc alumina filtration membranes). The pores were either 20 nm or 200 nm across. 2.2. CNTs growth process The CVD system used in this work was an ASTeX 5010 reactor with a 1.5 kW microwave power generator. The flow rates and ratio of feedstock gases were controlled by a set of mass flow controllers (MFCs) and an MKS Model 247C 4Channel Readout. The pressure in the reactor was controlled by an MKS Type 250 Pressure Controller. A type-K thermocouple was used to measure the actual temperature of the substrate stage. In recent years, plasma-enhanced (PE) CVD has been routinely used to synthesize CNTs. It offers a versatility that can be used to grow CNTs over a wide range of conditions. For example, the power density of a microwave
discharge can be altered over a wide range by adjusting the reactor pressure and transmitted power. Increasing the power density increases the fraction of molecular hydrogen that dissociate into atomic hydrogen. The high concentration of active hydrogen atoms in the plasma etches the surface of the catalyst and determines the diameter and the density of the catalyst nanoparticles that act as nucleation sites for the subsequent growth of CNTs. Also, heat is transferred to the CNTs due to the exothermic recombination of atomic hydrogen on the nanotube surfaces. This unique mode of energy transport leads to very fast growth rates. In this work, all the substrates sputtered with nickel thin film were first annealed in a tube furnace at 500 ◦ C under hydrogen (H2 ) atmosphere for 2 h. Then the annealed substrate was transported in air to the microwave-assisted PECVD reactor chamber and placed on a stainless steel substrate stage with the nickel side faced down. The bell jar chamber was pumped down to a base pressure of 3.5 × 10−3 Torr using a rotary pump. Then ultrahigh purity H2 gas was introduced into the chamber to provide a reducing environment for the catalytic metal particles. The flow rate of hydrogen was set at 160 sccm. After the introduction of hydrogen, the microwave plasma was lit inside the reactor for heating the substrate and activating the nickel catalyst nanoparticles. A microwave power of 1 kW and a reactor pressure of 20 Torr were maintained for 10 min and then 40 sccm of methane (CH4 ) gas was introduced into the reactor chamber. Introduction of CH4 initiated the growth of CNT. Typically, the CNTs were grown for 15 min. The substrate temperature was estimated to be in the range of 570–640 ◦ C. 2.3. CNTs characterization In order to have a better control of the growth of CNTs on these porous alumina substrates, it is necessary to fully understand the chemistry and the mechanisms that are involved in such a process. In this work we have used a Hitachi S-2300 scanning electronic microscopes (SEM), a Hitachi H-7000 transmission electronic microscopy (TEM), and a Jobin Yvon T64000 triple Raman spectroscopy to characterize the CNT samples. The TEM specimens were prepared by detaching the CNTs from the substrate, dispersing them in acetone ultrasonically, and then putting a small drop acetone with nanotubes onto a micro grid. 2.4. Microbattery constructions As mentioned above, we have developed a technique for constructing microbatteries [15]. Our technique consisted of filling nanoporous alumina membrane with poly ethylene oxide (PEO) lithium triflate electrolyte materials. We used micro and nanoparticles of graphite as the anode on one side of the membrane, and using composite materials composed of carbon nanotubes and V2 O5 sol–gel as the cathode on the other side of the membrane [15]. In effect a microbattery array of numerous parallel nanocells was made. One
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Fig. 1. SEM micrograph of CNTs deposited on the porous alumina substrates sputtered with 20 nm Ni film.
of the major tasks of making the nanobattery system was the construction of the cathode. The carbon nanotubes were nanocoated with a V2 O5 sol–gel that upon curing formed a xerogel about the carbon nanotubes. This formed a molecular composite cathode. The resulting cathode on the membrane was silver pasted onto a 0.125-mm thick nickel foil, which served as a current collector. The microbatteries were charged and discharged proving that these small batteries could function. Detail information about the construction and characterization of the microbattery can be found in our previous work [15].
3. Results and discussion 3.1. CNTs morphology and structure Fig. 1 shows an SEM micrograph of as-deposited CNTs on the porous alumina substrate. It can be seen that the synthesized CNTs are spaghetti-like, long, and curved. Nickel nanoparticle caps are observed on the tips of some CNTs. This supports the generally accepted tip-growth mechanism of carbon nanotubes [25–27]. The carbonaceous gas first decomposes on the surface of the catalyst particle, then the carbon dissolves in the catalyst, diffuses through it under an activity gradient, and finally the carbon precipitates out on the opposite side of the catalytic particle to form the nanotubes. The substrate, which was placed with nickel side facing down, effectively created a sustained condition for subsequent growth of carbon nanotubes. The growth of CNTs will not slow down and stop as it would otherwise. Because, the nickel nanoparticles were protected by the alumina substrate from being etched away by plasma, CNTs with a high aspect ratio were grown as showed in Fig. 1. The diameters of the grown CNTs in Fig. 1 are the same as those of nickel nanoparticles and are between 10 and 100 nm. TEM analysis was performed on the carbon nanotubes to determine the wall structure and is shown in Fig. 2.
Fig. 2. TEM image of as-deposited CNTs.
This and other TEM micrographs indicated that nanotubes were mostly multi-walled. The outer surface of the nanotube walls were often covered with an amorphous carbon layer. Plasma-enhanced CVD methods to date are only capable of producing multi-walled carbon nanotubes and the growth of single-walled carbon nanotubes using plasmaenhanced CVD has not been reported [28]. According to Kanzow and Ding [29], it is not favorable for single-walled carbon nanotubes to grow in view of the fact that the high concentration of carbon-bearing radicals in plasma-enhanced processes. Raman spectroscopy is also a very effective method for characterization of carbon nanotubes. It gives information on the average properties of the material in a sample volume within the diameter of the laser beam. Several vibration modes are commonly observed in the visible Raman spectrum of various forms of carbon. The positions and relative intensities of the G mode (for “graphite”) and D mode (for “disorder-induced”) give much information about the structure and domain size of a carbon material. Fig. 3 shows a first-order Raman spectrum of the as-deposited CNTs. The excitation source was 514.50 nm Ar-ion laser and the ex-
Fig. 3. Raman spectrum of as-deposited CNTs displayed in Fig. 1.
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posure time used was 60 s. We observed two characteristic peaks centered at 1595 cm−1 (G band) assigned for the tangential stretching mode of highly ordered graphite and centered at 1351 cm−1 (D band) ascribed to defects in the graphite crystals and by the finite sizes of microcrystalline graphite but not in single-crystal graphite. The D band and the G band correspond to sp2 and sp3 carbon stretching modes, respectively. The strength of the D band relative to the G band is a measure of the amount of disorder in the carbon nanotubes. The Raman spectrum shown in Fig. 3 is similar to the spectra of CNTs previously reported in the literature [30–32]. The positions of the D band and the G band are similar. The derivation of the D band and the G band could be due to the disorder in the carbon nanotubes. The two wide, distinctive peaks shown in Fig. 3 indicate the presence of crystalline graphitic carbon and microcrystalline graphite as well as amorphous carbon in the carbon nanotube layer. Overall, the Raman spectrum shows a lower crystalline quality. 3.2. Effects of sample annealing In order to grow the carbon nanotubes by CVD, the metal catalyst has to be transformed from a flat thin film to isolated nano-sized particles because these nanoparticles acted as nucleation seed and template of tube structure. These nanosized particles have to be in an activated state. To realize this transformation, sample pretreatments, such as annealing, or plasma etching are effective steps to fragment the catalyst layer for CNTs growth. Fig. 4 shows the effect of annealing on a continuous thin film of nickel on porous alumina substrate. It clearly shows that after annealing the continuous nickel thin film on the surface of the porous alumina substrate was broken into uniformly distributed nanoparticles due to surface tension and the compressive stress caused by the mismatch of the thermal expansion coefficients of Al2 O3 and Ni [25].
Fig. 4. SEM surface morphology of porous alumina substrates sputtered with 20 nm Ni film. (a) As sputtered smooth surface before annealing. (b) After annealing at 500 ◦ C under H2 for 2 h.
3.3. Applications in microbatteries
mately 4.6 V then discharging at 50 nA to a potential of 0.5 V. Fig. 5 shows several charge–discharge cycles of the microbatteries with a consistent maximum charge of 4.60 V. This large voltage may be due to overcharging of the system and some capacitance behavior; however, compared to the microbatteries that we previously made without carbon nanotubes
Having successfully grown CNTs on porous membranes, we used these CNT covered membranes to make microbatteries which were then characterized. Recently, Bachas and co-workers [23] incorporated carbon nanotubes to enhance electrical properties of sol–gel materials and we have incorporated them in the battery cathodes where enhanced performance was observed. The addition of carbon nanotubes to the V2 O5 sol–gel studied here should increase the electrical conductivity in the resulting xerogel composites. Similar to our previous experimental technique [15], the charging studies were done under ambient conditions using a Digital Instruments Nanoscope IIIa AFM where the cantilever tip was brought into contact with a selected graphite particle in ∼100 nm diameter, the anode of the microbatteries. The microbattery was subjected to charge–discharge cycles which consisted of charging at 50 nA to the peak voltage of approxi-
Fig. 5. Charge–discharge data for the microbattery having a V2 O5 xerogel and carbon nanotubes composite cathode.
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bon nanotubes as electrodes and fabricating true nanobattery systems with nanotube electrical connections.
Acknowledgements The authors would like to acknowledge the financial support for this research from the NASA EPSCoR, the National Science Foundation EPSCoR, and the Office of Naval Research.
References
Fig. 6. Capacity data for two microbatteries.
in the vanadium xerogel cathode, the addition of the carbon nanotubes to the V2 O5 sol–gel making a composite cathode significantly improved battery performance [15]. This can be seen in Fig. 6 where potential versus capacity data for a microbattery fabricated in this work with the individual CNTs at the porous membrane interface is compared with capacity data for a microbattery made in our previous work [15] where CNT bundles are distributed in the bulk composite cathode. It is observed that at any capacity the battery with the individual carbon nanotubes has a voltage, that is, approximately twice that of the battery made in our previous work. This enhanced performance is most likely due to the superior electrolyte access in the composite cathode through its interconnected networks and a high surface area of carbon nanotubes.
4. Conclusions Carbon nanotubes have been successfully synthesized by microwave plasma assisted CVD on Ni-sputtered porous alumina substrates. The grown CNTs are randomly oriented, spaghetti-like, and with a high aspect ratio. Nickel nanoparticle tips appeared on the ends of some CNTs. The diameters of the carbon nanotubes are as the same as those of the nickel nanoparticles. The TEM image proved that nanotubes instead of nanofibers were produced. The Raman spectrum of the CNTs confirms the presence of crystalline graphitic carbon and microcrystalline graphite in the carbon nanotube layer and point outs the presence of carbon impurities in the CNTs. The carbon nanotubes were used to construct a V2 O5 xerogel and carbon nanotubes composite cathode, which resulted in a microbattery with a voltage consistently of 4.60 V. For applications in micro batteries and nanoscale devices, controlled growth of vertically aligned nanotubes is important. Future work will involve improving the synthesis techniques of carbon nanotubes for a better control over alignment, density, purification, nanotube diameter, wall thickness and improving our prototype batteries by applying car-
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Carbon nanotube synthesis, characteristics, and microbattery applications Zhijing Zhanga,b , Christina Dewana , Saumya Kotharic , Saibal Mitrab , Dale Teetersa,∗ b
a Department of Chemistry and Biochemistry, The University of Tulsa, 600 South College Avenue, Tulsa, OK 74104-3189, USA Department of Physics and Engineering Physics, The University of Tulsa, 600 South College Avenue, Tulsa, OK 74104-3189, USA c Department of Chemical Engineering, The University of Tulsa, 600 South College Avenue, Tulsa, OK 74104-3189, USA
Received 13 November 2003; received in revised form 12 May 2004; accepted 14 May 2004
Abstract The stabilities and efficiencies of cathodes of microbatteries are important for their superior performance. The performance of cathode of microbattery systems can be improved by applying new materials or combining components with unique properties into cathode materials. Individual carbon nanotubes exhibit extraordinary mechanical, thermal and electrical properties. Composite materials, using carbon nanotubes as fillers are, expected to show similar superior properties. This study reports synthesis of carbon nanotubes by microwave chemical vapor deposition (MACVD) and applying them in the cathode of microbatteries to improve battery performance. Carbon nanotubes were grown on Ni deposited porous alumina substrates in a flowing gas mixture of methane and hydrogen. Characterization of the carbon nanotubes was carried out using scanning electronic microscopy, transmission electron microscopy, and Raman spectroscopy. It was found that the carbon nanotubes were randomly oriented, spaghetti-like, and with a high aspect ratio. The diameters of CNTs, as determined by transmission electron microscopy (TEM) were between 10 and 100 nm. TEM also showed that the nanotubes had a central hollow with amorphous carbons covering outside wall. The CNTs were nanocoated with a V2 O5 sol–gel, which upon curing formed a xeorgel about the carbon nanotubes. This formed a molecular composite cathode. Microbatteries/microcapacitors were made and subjected to charge/discharge cycles with a consistent maximum charge of 4.60 V. © 2004 Elsevier B.V. All rights reserved. Keywords: Carbon nanotubes; Microwave plasma; Chemical vapor deposition; Microbattery; Vanadium xerogel cathode.
1. Introduction Microelectromechanical systems (MEMS) and nanoelectromechanical systems (NEMS) need self-contained power sources so that they may be commercially exploited. Many investigations on microbatteries have been reported in the literature, with thin-film rechargeable batteries with active layers of 1–10 m being of interest, since 1980s. The majority of papers on actual functioning microbatteries either describe systems where very thin films of electrolyte materials were used to construct the battery or describe the potential for these films to be used in batteries [1–14]. Although our previous work was in the area of micro and nanobattery construction, development and characterization, in this work we describe ∗
Corresponding author. Tel.: +1 918 631 3147; fax: +1 918 631 3404. E-mail address: [email protected] (D. Teeters).
0921-5107/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2004.05.049
the construction and study of individual nanobatteries with m and nm scale anodes and/or cathodes [15–21]. Recently, there has been considerable effort to design new cathode materials and to improve the performance of cathode systems, since the stabilities and efficiencies of these electrodes are critical for enhanced battery performance. Carbon nanotubes (CNT) hold great promise in this area. Individual carbon nanotubes exhibit unique mechanical, thermal and electrical properties [22]. They are attractive candidates for electrode materials of microbattery systems or microcapacitors due to their superb characteristics of chemical stability, low mass density, low resistivity, and large surface area. Composite materials with carbon nanotubes as fillers are expected to show similar superior properties. Recently, Bachas and co-workers [23] incorporated carbon nanotubes to enhance electrical properties of sol–gel materials. Windle and co-workers [24] investigated the electrochemical capacitance
364
Z. Zhang et al. / Materials Science and Engineering B 116 (2005) 363–368
of nanocomposite films formed by coating aligned arrays of carbon nanotubes with polypyrrole. They attributed the increased electrical and ionic conductivity of the nanocomposites relative to their component materials to the combination of electrolyte accessibility, reduced diffusion distance, and increased conductivity in the redox pseudo-capacitive composite structure. In our previous work we have shown that the incorporation of bundles carbon nanotubes in V2 O5 sol–gel used in microbattery improved battery performance [15]. In this work we describe the synthesis of CNTs. These CNTs were then mixed with the V2 O5 sol–gel to make a composite cathode. The CNTs were grown on the nanoporous alumina membranes that are typically used to make the microbatteries and then applying V2 O5 sol–gel to the membrane to make the cathode. It was hoped that the addition of better dispersed, individual carbon nanotubes in the sol–gel would further improve microbattery performance. In addition to enhanced mechanical properties, interconnected networks of carbon nanotubes are envisioned to provide direct thermal and electrical conductivity paths within the V2 O5 sol–gel of the composite cathode. Furthermore, the high surface area of carbon nanotubes provides the better access to the electrolyte. Improved capacities and battery response times to charging and discharging are expected.
2. Experimental 2.1. Sample preparation Carbon nanotubes were grown by the catalytic decomposition of hydrocarbon precursor gas over metal catalyst nanoparticles that were already deposited on a substrate. Metal catalysts play a crucial role in the nucleation and growth of carbon nanotubes in a chemical vapor deposition (CVD) process. In this work, a 20 nm layer of nickel was deposited using rf-magnetron sputtering on disk-shaped porous alumina substrates. The alumina disks were 13 mm in diameter and were 60 m thick and were commercially available (Whatman Anodisc alumina filtration membranes). The pores were either 20 nm or 200 nm across. 2.2. CNTs growth process The CVD system used in this work was an ASTeX 5010 reactor with a 1.5 kW microwave power generator. The flow rates and ratio of feedstock gases were controlled by a set of mass flow controllers (MFCs) and an MKS Model 247C 4Channel Readout. The pressure in the reactor was controlled by an MKS Type 250 Pressure Controller. A type-K thermocouple was used to measure the actual temperature of the substrate stage. In recent years, plasma-enhanced (PE) CVD has been routinely used to synthesize CNTs. It offers a versatility that can be used to grow CNTs over a wide range of conditions. For example, the power density of a microwave
discharge can be altered over a wide range by adjusting the reactor pressure and transmitted power. Increasing the power density increases the fraction of molecular hydrogen that dissociate into atomic hydrogen. The high concentration of active hydrogen atoms in the plasma etches the surface of the catalyst and determines the diameter and the density of the catalyst nanoparticles that act as nucleation sites for the subsequent growth of CNTs. Also, heat is transferred to the CNTs due to the exothermic recombination of atomic hydrogen on the nanotube surfaces. This unique mode of energy transport leads to very fast growth rates. In this work, all the substrates sputtered with nickel thin film were first annealed in a tube furnace at 500 ◦ C under hydrogen (H2 ) atmosphere for 2 h. Then the annealed substrate was transported in air to the microwave-assisted PECVD reactor chamber and placed on a stainless steel substrate stage with the nickel side faced down. The bell jar chamber was pumped down to a base pressure of 3.5 × 10−3 Torr using a rotary pump. Then ultrahigh purity H2 gas was introduced into the chamber to provide a reducing environment for the catalytic metal particles. The flow rate of hydrogen was set at 160 sccm. After the introduction of hydrogen, the microwave plasma was lit inside the reactor for heating the substrate and activating the nickel catalyst nanoparticles. A microwave power of 1 kW and a reactor pressure of 20 Torr were maintained for 10 min and then 40 sccm of methane (CH4 ) gas was introduced into the reactor chamber. Introduction of CH4 initiated the growth of CNT. Typically, the CNTs were grown for 15 min. The substrate temperature was estimated to be in the range of 570–640 ◦ C. 2.3. CNTs characterization In order to have a better control of the growth of CNTs on these porous alumina substrates, it is necessary to fully understand the chemistry and the mechanisms that are involved in such a process. In this work we have used a Hitachi S-2300 scanning electronic microscopes (SEM), a Hitachi H-7000 transmission electronic microscopy (TEM), and a Jobin Yvon T64000 triple Raman spectroscopy to characterize the CNT samples. The TEM specimens were prepared by detaching the CNTs from the substrate, dispersing them in acetone ultrasonically, and then putting a small drop acetone with nanotubes onto a micro grid. 2.4. Microbattery constructions As mentioned above, we have developed a technique for constructing microbatteries [15]. Our technique consisted of filling nanoporous alumina membrane with poly ethylene oxide (PEO) lithium triflate electrolyte materials. We used micro and nanoparticles of graphite as the anode on one side of the membrane, and using composite materials composed of carbon nanotubes and V2 O5 sol–gel as the cathode on the other side of the membrane [15]. In effect a microbattery array of numerous parallel nanocells was made. One
Z. Zhang et al. / Materials Science and Engineering B 116 (2005) 363–368
365
Fig. 1. SEM micrograph of CNTs deposited on the porous alumina substrates sputtered with 20 nm Ni film.
of the major tasks of making the nanobattery system was the construction of the cathode. The carbon nanotubes were nanocoated with a V2 O5 sol–gel that upon curing formed a xerogel about the carbon nanotubes. This formed a molecular composite cathode. The resulting cathode on the membrane was silver pasted onto a 0.125-mm thick nickel foil, which served as a current collector. The microbatteries were charged and discharged proving that these small batteries could function. Detail information about the construction and characterization of the microbattery can be found in our previous work [15].
3. Results and discussion 3.1. CNTs morphology and structure Fig. 1 shows an SEM micrograph of as-deposited CNTs on the porous alumina substrate. It can be seen that the synthesized CNTs are spaghetti-like, long, and curved. Nickel nanoparticle caps are observed on the tips of some CNTs. This supports the generally accepted tip-growth mechanism of carbon nanotubes [25–27]. The carbonaceous gas first decomposes on the surface of the catalyst particle, then the carbon dissolves in the catalyst, diffuses through it under an activity gradient, and finally the carbon precipitates out on the opposite side of the catalytic particle to form the nanotubes. The substrate, which was placed with nickel side facing down, effectively created a sustained condition for subsequent growth of carbon nanotubes. The growth of CNTs will not slow down and stop as it would otherwise. Because, the nickel nanoparticles were protected by the alumina substrate from being etched away by plasma, CNTs with a high aspect ratio were grown as showed in Fig. 1. The diameters of the grown CNTs in Fig. 1 are the same as those of nickel nanoparticles and are between 10 and 100 nm. TEM analysis was performed on the carbon nanotubes to determine the wall structure and is shown in Fig. 2.
Fig. 2. TEM image of as-deposited CNTs.
This and other TEM micrographs indicated that nanotubes were mostly multi-walled. The outer surface of the nanotube walls were often covered with an amorphous carbon layer. Plasma-enhanced CVD methods to date are only capable of producing multi-walled carbon nanotubes and the growth of single-walled carbon nanotubes using plasmaenhanced CVD has not been reported [28]. According to Kanzow and Ding [29], it is not favorable for single-walled carbon nanotubes to grow in view of the fact that the high concentration of carbon-bearing radicals in plasma-enhanced processes. Raman spectroscopy is also a very effective method for characterization of carbon nanotubes. It gives information on the average properties of the material in a sample volume within the diameter of the laser beam. Several vibration modes are commonly observed in the visible Raman spectrum of various forms of carbon. The positions and relative intensities of the G mode (for “graphite”) and D mode (for “disorder-induced”) give much information about the structure and domain size of a carbon material. Fig. 3 shows a first-order Raman spectrum of the as-deposited CNTs. The excitation source was 514.50 nm Ar-ion laser and the ex-
Fig. 3. Raman spectrum of as-deposited CNTs displayed in Fig. 1.
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posure time used was 60 s. We observed two characteristic peaks centered at 1595 cm−1 (G band) assigned for the tangential stretching mode of highly ordered graphite and centered at 1351 cm−1 (D band) ascribed to defects in the graphite crystals and by the finite sizes of microcrystalline graphite but not in single-crystal graphite. The D band and the G band correspond to sp2 and sp3 carbon stretching modes, respectively. The strength of the D band relative to the G band is a measure of the amount of disorder in the carbon nanotubes. The Raman spectrum shown in Fig. 3 is similar to the spectra of CNTs previously reported in the literature [30–32]. The positions of the D band and the G band are similar. The derivation of the D band and the G band could be due to the disorder in the carbon nanotubes. The two wide, distinctive peaks shown in Fig. 3 indicate the presence of crystalline graphitic carbon and microcrystalline graphite as well as amorphous carbon in the carbon nanotube layer. Overall, the Raman spectrum shows a lower crystalline quality. 3.2. Effects of sample annealing In order to grow the carbon nanotubes by CVD, the metal catalyst has to be transformed from a flat thin film to isolated nano-sized particles because these nanoparticles acted as nucleation seed and template of tube structure. These nanosized particles have to be in an activated state. To realize this transformation, sample pretreatments, such as annealing, or plasma etching are effective steps to fragment the catalyst layer for CNTs growth. Fig. 4 shows the effect of annealing on a continuous thin film of nickel on porous alumina substrate. It clearly shows that after annealing the continuous nickel thin film on the surface of the porous alumina substrate was broken into uniformly distributed nanoparticles due to surface tension and the compressive stress caused by the mismatch of the thermal expansion coefficients of Al2 O3 and Ni [25].
Fig. 4. SEM surface morphology of porous alumina substrates sputtered with 20 nm Ni film. (a) As sputtered smooth surface before annealing. (b) After annealing at 500 ◦ C under H2 for 2 h.
3.3. Applications in microbatteries
mately 4.6 V then discharging at 50 nA to a potential of 0.5 V. Fig. 5 shows several charge–discharge cycles of the microbatteries with a consistent maximum charge of 4.60 V. This large voltage may be due to overcharging of the system and some capacitance behavior; however, compared to the microbatteries that we previously made without carbon nanotubes
Having successfully grown CNTs on porous membranes, we used these CNT covered membranes to make microbatteries which were then characterized. Recently, Bachas and co-workers [23] incorporated carbon nanotubes to enhance electrical properties of sol–gel materials and we have incorporated them in the battery cathodes where enhanced performance was observed. The addition of carbon nanotubes to the V2 O5 sol–gel studied here should increase the electrical conductivity in the resulting xerogel composites. Similar to our previous experimental technique [15], the charging studies were done under ambient conditions using a Digital Instruments Nanoscope IIIa AFM where the cantilever tip was brought into contact with a selected graphite particle in ∼100 nm diameter, the anode of the microbatteries. The microbattery was subjected to charge–discharge cycles which consisted of charging at 50 nA to the peak voltage of approxi-
Fig. 5. Charge–discharge data for the microbattery having a V2 O5 xerogel and carbon nanotubes composite cathode.
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bon nanotubes as electrodes and fabricating true nanobattery systems with nanotube electrical connections.
Acknowledgements The authors would like to acknowledge the financial support for this research from the NASA EPSCoR, the National Science Foundation EPSCoR, and the Office of Naval Research.
References
Fig. 6. Capacity data for two microbatteries.
in the vanadium xerogel cathode, the addition of the carbon nanotubes to the V2 O5 sol–gel making a composite cathode significantly improved battery performance [15]. This can be seen in Fig. 6 where potential versus capacity data for a microbattery fabricated in this work with the individual CNTs at the porous membrane interface is compared with capacity data for a microbattery made in our previous work [15] where CNT bundles are distributed in the bulk composite cathode. It is observed that at any capacity the battery with the individual carbon nanotubes has a voltage, that is, approximately twice that of the battery made in our previous work. This enhanced performance is most likely due to the superior electrolyte access in the composite cathode through its interconnected networks and a high surface area of carbon nanotubes.
4. Conclusions Carbon nanotubes have been successfully synthesized by microwave plasma assisted CVD on Ni-sputtered porous alumina substrates. The grown CNTs are randomly oriented, spaghetti-like, and with a high aspect ratio. Nickel nanoparticle tips appeared on the ends of some CNTs. The diameters of the carbon nanotubes are as the same as those of the nickel nanoparticles. The TEM image proved that nanotubes instead of nanofibers were produced. The Raman spectrum of the CNTs confirms the presence of crystalline graphitic carbon and microcrystalline graphite in the carbon nanotube layer and point outs the presence of carbon impurities in the CNTs. The carbon nanotubes were used to construct a V2 O5 xerogel and carbon nanotubes composite cathode, which resulted in a microbattery with a voltage consistently of 4.60 V. For applications in micro batteries and nanoscale devices, controlled growth of vertically aligned nanotubes is important. Future work will involve improving the synthesis techniques of carbon nanotubes for a better control over alignment, density, purification, nanotube diameter, wall thickness and improving our prototype batteries by applying car-
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