Morphology Control And Field Emission Properties Of Multi-Wall Carbon Nanotubes

In this stduy, multi-wall carbon nanotubes (MWNTs) were prepared with chemical vapor deposition (CVD) on three substrates: quartz, nano-SiO₂ and nanoSiO₂/ Graphitized Polyaniline (GPANI), respectively. To some extent, the micro-structure of resulted carbon nanotubes were controlled, i.e.: 1) the array of carbon nanotubes was formed on quartz, and then multi-layer carbon nanotubes further grew on this array; 2) the narrow distribution carbon nanotubes grew rapidly by using nano-SiO₂ to control Fe catalyst space; 3) by changing experiment factors(such as the concentration of the promoter thiophene, the inject rate of carbon source) and with the aid of nano-SiO₂, the bamboo-like carbon nanotubes was synthesized with the control of bamboo joint carbon layer intensity; 4) the insertion state nano-SiO₂ on the composite substrate of nano-SiO₂/ GPANI made bamboo-like nanotubes film grow much faster. Therefore, the intensity of bamboo-like carbon nanotubes was controlled by the distribution (intensity) of nano-SiO₂ at the surface.The micro-structure of above carbon nanotubes were analyzed with scanning electron microscope (SEM) and transmission electron microscopy (TEM). With the combination of X-ray photoelectron spectroscopy analysis (XPS) and X-ray microanalysis (EDS), the growing mechanism of bamboo-like carbon nanotubes was investigated tentatively.The thermal gravimetric analysis (TGA) and Raman spectrum indicated there were many defects on the bamboo-like carbon nanotubes.The effects of purifying methods, air oxidation and mixed-acid oxidation, on the structure of carbon nanotubes were analyzed. The moderate method of the two-step treatment was proposed. The resulted carbon nanotubes was rinsed in HF and oxidized in the air at 430℃for 30 min. The high purified carbon nanotubes was obtained in the expense of slight damage of the structure of the carbon nanotubes. And then oil of vitriol and dense nitric acid (volume ratio for 3:1) oxidated CNTs at 60℃, the result of SEM and TEM analyses showed that the CNTs were cut into short,and the shell of the tubes produced carboxyl and hydroxy with oxidization time. The carbon nanotubes/low melting point glass composite film was formed tightly on the surface of the ITO glass. The study of the carbon nanotube field emission performance with the aid of screen printing paste indicated that, due to the defect of resulted carbon nanotubes, the field emission performance of bamboo-like carbon nanotubes was higher that of common carbon nanotubes.The short carbon nanotubes film was prepared with laser sculpture technology. The result of SEM and TEM analyses showed that the CNTs were truncated and the tube became rough. Raman spectra showed that the carbon layer crystal lattice structure was contorted and disordered. The film possessed field emission properties and electronic emission stability.A new field emission material, GPANI, was prepared at different reaction conditions. The results of field emission tests indicated that the optimum reaction conditions were: pressure under 60-100MPa,then treated with hydrogen under 900℃for 2h. XRD, SEM, TEM, Raman spectra and XPS results indicated that GPANI was a kind of N-doped amorphous carbon material, its field emission performance resulted from that the surface containing convex structure and the sp²C structure containing sp³C,and the appropriate N-doped structure would decrease the surface potential barrier which improved the field emission performance.The preparation of field emission cathode material for in situ growing the carbon nanotube on the GPANI substrate was also studied. The rapid preparation of Fe/SiO2 catalyst was realized with rotation evaporation and high temperature reduction. Once this catalyst was deposited in the gap of the surface of GPANI substrate, the carbon nanotubes would grow by low pressure chemical vapor deposition (LPCVD). The results indicated that the gap was closed by partial carbon atoms during the deposition when the carbon nanotubes grew, which made the root of the carbon nanotubes fixed firmly on the substrate. Therefore, this material possessed perfect field emission stability. Finally, the study of growing the bamboo-like carbon nanotubess film on composite substrate of nano-SiO2/GPANI was carried out, then the resulted film was use as field emission cathode material. The result showed that this material had low turn-on filed strength and threshold field strength, and good field emission properties.