Field Emitters and Supercapacitors Based on Carbon Nanotube Films
This research is focused on the synthesis of CNTs using thermal chemical vapor deposition (CVD) at atmospheric pressure with Pd, Ni, and Co as catalysts, and characterization of the CNTs for field emission applications. In this work, a systematic study was performed on the catalyst processing parameters and morphologies of as-grown CNT arrays. By controlling catalyst pretreatment parameters, high-density catalyst nanoparticles with uniform size have been produced. The result indicates that Pd is a more effective catalyst than conventional catalysts such as Co and Ni, and the corresponding CNT cathodes demonstrated comparable field emission behavior (turn-on field of 2.5V/um and field enhancement factor of 7800). Secondly, synthesis of vertically aligned CNT arrays under atmospheric pressure has been achieved by thermal CVD using cobalt (Co) and nickel (Ni) as catalysts. These well-aligned nanotubes demonstrate lower turn-on field (~ 1.2V/um) than randomly oriented CNTs. Finally, an innovative approach has been developed to fabricate CNT/transition-metal-oxide (TMO) nanocomposite thin film for supercapacitor electrodes. Particular effort has been invested into manganese dioxide (MnO2) due to its excellent pseudocapacitance, low cost, non-toxicity and readily availability. This novel approach of using nano-structured CNTs architectures provides a high surface area of attachment for MnO2 nano-particles to maximize the charge efficiency and the power density and to reduce the series resistance. In this newly developed system, the charge transfer between CNTs and MnO2 nanoparticles is very efficient due to the exceptional electronic conductivity of CNTs. The direct growth of CNTs on conductive Si substrate helps to reduce contact resistance and ESR is significantly reduced and power is enhanced. A high capacitance of 30000uF (270F/g) has been observed on CNT/ MnO2 composite electrode which is over 400 times that of MnO2-free CNT sample. And a record-breaking charging/discharging current of 1.92mA, or 17.32 A/g, has been achieved.