The study of iridium oxide/CNT nanocomposites

• Main Authors: Yi-Min Chen, 陳宜民
• Other Authors: Ying-Sheng Huang
• Format: Others
• Language: en_US
• Published: 2011
• Online Access: http://ndltd.ncl.edu.tw/handle/e59kjx

博士 === 國立臺灣科技大學 === 電子工程系 === 99 === Iridium oxide (IrOx) with various nanometer scale size morphologies, such as nanoparticles (NP), nanotubes (NT) and nanofoils (NF), were deposited on the surface of multiwall carbon nanotubes (CNT) via vertical-flow cold-wall metal organic chemical vapor deposition (MOCVD) and reactive radio-frequency magnetron sputtering (RFMS) techniques. The detailed characterization focusing on the morphologies, sizes, crystal structures, crystal orientations, and chemical composition of various IrOx/CNT samples have been carried out by means of field-emission scanning electron microscope (FESEM), transition electron microscope (FETEM), X-ray diffractometer (XRD), Energy-dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS) and micro-Raman scattering. The field emission properties and electrochemical characteristics of IrOx/CNT nanocomposites were also studied. The SEM image of IrO2 nanostructures, deposited on CNT by MOCVD, showed a dense coalescence of IrO2NP with uniform size distribution on the nanotube walls. EDX confirmed the growth of the IrO2 nanoparticles on CNT. TEM showed IrO2 deposited on CNT with the relationship (110)IrO2/(001)CNT and lattice mismatch related defects appeared at the interfaces. The high angle annular dark field (HAADF) image indicates that the center region of the CNT still retains a tubular structure after depositing IrO2NP. The combined effects of the geometrical structure of IrO2NP/CNT, and the natural conducting and enhanced resistance to oxidation properties of IrO2 lead to a low turn-on field of 1 V/?慆 at a current density of 0.1 ?嫀/cm2, a low threshold field of 2.7 V/?慆 at a current density of 1 mA/cm2, a high field enhancement factor of 7.4 x 103, and long-term stability for the IrO2NP/CNT nanocomposites. The results indicated that IrO2NP can be used as a protective layer on CNT, providing stable and uniform field emission applications. By increasing the growth times, the FESEM micrographs showed that the surface morphology of the as-deposited IrO2 varied from nanoparticle to nanotube. The nanotube-like structure can increase the surface-to-volume ratio which makes the IrO2NT/CNT nanocomposites as attractive candidate for supercapacitor applications. Synthesis of the hierarchical structure with open porosity has been performed by a dense deposition of IrO2 short tubes along the long wires of CNT on a substrate of stainless steel (SUS). The rutile IrO2 tube grows along the [001] direction with an opening at its tip, surrounded by very thin walls. The IrO2 addition on the CNT template increased the capacitance of the CNT thin film effectively, because of pseudocapacitance of the IrO2 surface. For this particular composite, featured with two tubular nanostructures, the specific capacitance increases from 11 F g-1 (CNT) to 69 F g-1 (IrO2NT/CNT), measured using the galvanostatic discharge experiment. Its property of fast retrieving the stored charge was assured in the impedance measurement, showing the IrO2NT/CNT nanocomposites electrode was similar to an ideal capacitor. Similar characterizations were also performed for nanoparticle like IrO2 coated on CNT by RFMS. The as-synthesized IrO2NP on CNT showed a low turn-on field of 0.7 V/?慆 at a current density of 0.1 ?嫀/cm2, a low threshold field of 2.3 V/?慆 at a current density of 1 mA/cm2, a high field enhancement factor of 1 x 104. In addition, long-term stability for the IrO2-coated CNT was also demonstrated. Large surface area IrOxNF, suitable for supercapacitor applications, were deposited on the CNT/SUS templates by RFMS. This IrOxNF/CNT/SUS electrode was featured with intriguing IrOx curved foils of 2-3 nm in thickness and 400-500 nm in height, grown on top of the vertically aligned CNT film with a tube diameter ??0 nm. These nanofoils were moderately oxidized during the process of reactive sputtering and appear translucent under the electron microscope. Detailed structural analysis showed they comprise of contiguous grains of iridium metal, iridium dioxide, and glassy iridium oxide. The attribute of nanosized iridium oxide was further manifested by the considerable Raman line broadening. Two capacitive properties of the electrode are significantly enhanced with addition of the curved IrOx foils. Firstly, the IrOxNF reduced the electrode ohmic resistance, which measures 3.5 ? cm2 for CNT/SUS and 2.5 ? cm2 for IrOxNF/CNT/SUS using impedance spectroscopy. Secondly, IrOxNF also raised the electrode capacitance from 17.7 F g-1 for CNT/SUS to 317 F g-1 for IrOxNF/CNT/SUS, measured with cyclic voltammetry. This notable increase was further confirmed with the galvanostatic charge/discharge experiment, measuring 370 F g-1 after uninterrupted 2000 cycles between -1.0 and 0.0 V (vs Ag/AgCl). We have demonstrated that the IrO2NP coated on CNT can be used as a protective layer on CNT, providing stable and uniform field emission applications. The large surface area IrOxNF deposited on the CNT/SUS templates by RFMS are suitable for supercapacitor applications.