Fabrication And Characterization Of Carbon Nanowall-based Semiconductor Composites And Their Photocatalytic Ability

Carbon nanostructures, such as zero-dimensional fullerenes and one-dimensional carbon nanotubes, have attracted great interests because they are often superior to the conventional bulk carbon materials. However, as compared with the intensive research of fullerenes and carbon nanotubes, only a few studies have been carried out on two-dimensional carbon nanowalls (CNWs). Actually, the CNWs have unique field-emission and electron transport properties. In particular, the CNWs are the good candicates for catalyst supporting because they can form self-supported graphitic carbon network structure with an open boundary. TiO₂ and WO₃ have been regarded as the most attractive semiconductor materials with high Photocatalytic ability and long life stability. However, enhancing the Photocatalytic efficiency of photocatalyst to meet the practical application requirement is still a challenge, mostly because of low quantum yield caused by the rapid combination of photogenerated electrons and holes. Meanwhile, the Photocatalytic oxidation technology always suffers from the difficulties of separating suspended photocatalyst particles from aqueous solution as well as the surface area of supported photocatalyst exposed to the solution is lower than that of suspended photocatalyst in solution. In order to increase the Photocatalytic efficiency, attempt has been made to cover the TiO₂ and WO₃ on CNWs for increasing the surface area and the forming of a heterojunction which could provide a potential driving force for the separation of photogenerated charge carriers. Therefore, in present work, the TiO₂/CNW and the WO₃/CNW composite materials were prepared by chemical vapor deposition, and the Photocatalytic activity of these photocatalysis was evaluated. In this dissertation, the following several parts of work have been done:(1) A plasma enhanced hot filament chemical vapor deposition (PE-HFCVD) system was self-designed and self-made. The heat power, substrate bias voltage, and deposition area of this system is 3 kW, 300 V and 20 cm^-2, respectively. This system was employed to prepare the CNWs. The substrate is a Ti sheet, and the hydrogen and methane is used as source gases. The gas flow rates of the H₂ and CH₄ are controlled at 6 and 18 sccm, respectively. The reactor is evacuated using a rotary vacuum pump, and the pressure of the system is kept at approximately 3000 Pa during the whole experimental process. When the substrate temperature is estimated by a thermocouple to be about 600℃, a negative substrate bias of 100 V is initiated between the hot filament (anode) and the substrate holder (cathode). The typical deposition time lasted 30 min. The CNWs appeare to distribute uniformly over the whole Ti sheet surface and each nanowall stood perpendicularly on the substrate, they can form self-supported graphitic carbon network structure with an open boundary. These self-aligned CNWs grow up to nearly 2μm and the length is in the range of 500 nm to 1μm. The thickness of the CNW is approximately 10 nm. Trassion electron micrography and Raman spectrum indicate that the CNWs are graphite with sp² hybird. The growth of CNWs depends on the electric field occurred by a DC source, and the concentration of carbon particals and hydrogen are the key factors.(2) The deposition of WO₃ on CNWs is carried out in a HFCVD system using tungsten filament as tungsten source. The WO₃ could be covered on CNWs uniformly by controlling the deposition duration. The formation of WO₃/CNWS is benefit from the form the self-supported graphitic CNW structure with an open boundary. Raman spectroscopy and X-ray diffraction indicate that the crystal phase of the WO₃ coating is monoclinic. The UV-vis diffusion reflection spectrum reveals that the WO₃/CNWS have Photocatalytic ability under visable light. The photocurrent density and the photocatalystic degradation rate of p-nitrophenol are higher for WO₃/CNW electrode than WO₃ nanobelt array electrode.(3) The deposition of TiO₂ on CNWs is carried out in a metal-organic chemical vapor deposition system (MOCVD) using titanium isopropoxide as titanium source. The CNW substrate is placed in a tubular-furnace quartz reactor. The argon is used as the carrier gas with a constant flow rate of 500 sccm. When the substrate temperature reach 320℃, the solution of the titanium isopropoxide is fed continuously into the tubular quartz reactor through a capillary at a rate of 0.05 mL min^-1. After the deposition process, the argon flow is stopped and the film is annealed in the air at 430°C for 1 h with a heating rate of 2°C min^-1. The excellent uniformity of TiO₂ has been obtained on the entire CNWs to form a TiO₂/CNW composite material by controlling the deposition duration, and the thinkness of TiO₂/CNWS increase from several ten nanometers to nearly 200 nm with deposition duration increasing. Raman spectroscopy and X-ray diffraction indicate that the crystal phase of the TiO₂ coating is anatase. The asymmetry of the current-voltage plot for the material reveals that a heterojunction is formed between the TiO₂ and the carbon nanowall. As a result of this heterojunction, enhanced separation of photogenerated electrons and holes is confirmed by surface photovoltage and photocurrent measurements. The investigation of Photocatalytic ability shows that the TiO₂/CNW electrode has a higher Photocatalytic activity than TiO₂ nanotube electrode for the degradation of phenol.