Synthesis and characterization of nitrogen-doped CNTs and carbon-coated GaN nanostructures: sensor and solar cell applications
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Date
2019
Authors
Usman, Ibrahim Bala
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Abstract
Carbon based nanostructures have inspired extensive research due to their superior physico-chemical and electronic properties which can be tuned through doping with heteroatoms on the surface or at the ends. Besides their high surface to volume ratio and thermal stability have seen them being explored as promising candidates for sensor applications either as active layer or as templates for active layer support. In addition the ballistic transport of the charge carriers opens up applications as transparent conducting electrodes in photovoltaic technology and fast response electronic devices. The aim of this work was to design nitrogen doped carbon nanostructures based sensors for diverse applications in standard and harsh environments. The synthesis and functionalization of nitrogen doped carbon nanotubes (aligned and non-aligned) either as carbon coated GaN or metal oxide or chalcogenide based carbon nanostructures allow for application in sensors and hybrid photovoltaic devices to monitor harmful gases in the environment as well as to reduce or eliminate global warming.
Chemical vapour deposition (CVD) was used to fabricate nitrogen doped carbon nanotubes (N-CNTs) and aligned N-CNTs (A-N-CNTs). The N-CNTs acted as a support for the transition metal-VI alloys (SnO2 and CdS) based nanomaterial encapsulated on the nanotube surface. The optimized N-CNTs coated with SnO2 were tested for detection of H2, CO, NH3 and CH4 gases at room temperature and 250 C. The sensor showed no response to all the gases at room temperature. However the response to NH3, CO and H2 was observed at 250 C. The sensitivity to NH3 at 250 °C surpassed its sensitivity to any other gas studied in the temperature range investigated.
The A-N-CNTs were synthesised using an aerosol assisted CVD with a fixed concentration of ferrocene catalyst. The magnetic properties of the residual impurities in the A-N-CNTs made up of Fe-nanoparticles (NPs) left after the purification process were investigated using hyperfine interaction techniques and a SQUID magnetometer. Mössbauer spectroscopy studies revealed the presence of -Fe, -Fe, Fe2O3 and Fe3C. This was further confirmed by the magnetization curves with high coercive fields indicating the presence of size effects on the magnetization and coercivity. Nitrogen doping was used to limit the concentration, shape and aspect ratio of NPs embedded in the cavity of the carbon nanotubes through the formation of bamboo like structures.
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As a proof of concept GaN and carbon coated GaN (C-GaN) have been implemented as active layers in sensors for detection of CO, NH3, CH4 and H2 gases at room temperature and 250 °C. The sensor based on pure GaN have shown no response to all gases at these temperature ranges. However, C-GaN sensors response to all the four gases was observed and the mechanism of function identified to be driven by surface defects sites on the carbon coated GaN. The sensor (C-GaN TA) responded better compared to C-GaN CN (C-GaN TA (carbon source toluene (T) and Ar as carrier gas) and C-GaN TN (carbon source acetylene (C) and N2 as carrier gas). The ability for C-GaN to respond to CO, NH3, CH4 and H2 gases at room temperature opens up the possibility for its easy use in indoor and outdoor monitoring.
Description
A Thesis submitted to the faculty of science at the university of Witwatersrand Johannesburg, in the fulfillment for the degree of Doctor of Philosophy