Modification Of Carbon Nanotubes By Semiconductive Metal Oxides And Their Electrocatalysis And Photoelectrocatalysis

Since their discovery in 1991, carbon nanotubes (CNTs) have been the focus of intensive study due to their excellent physical and chemical properties. The excellent electrical conductivity, large surface area, significant mechanical strength, chemical flexibility and ideal porous structure make CNTs attractive materials for preparation of nanocomposites, electrochemical sensors, and nano devices. Especially, there is little study on nonenzymatic sensor and photoelectrocatalysis based on CNTs and metal oxides. The metal oxides/CNTs nanocomposites possess the properties of individual components or even with a synergistic effect. In addition, the fabrication of electrode is simplified by using MWCNTs arrays and the disadvantages by the conventional painting method can be effectively avoided. Therefore, in this thesis, studies on the development of metal oxides/CNTs nanocomposites and their application for nonenzymatic senors and photoelectrocatalysis were carried out, and some valuable results were obtained. The main points of this thesis are summarized as follows:1. Dopamine sensor was fabricated by using RuO₂/MWCNTs nanocomposite which was synthesized by magnetron sputtering deposition. The RuO₂-modified carbon nanotube electrode showed higher electrocatalytic activity towards the oxidation of dopamine than the MWCNTs electrode in 0.10 M phosphate buffer solution. At an applied potential of +0.4 V, the RuO₂/MWCNTs electrode exhibited a wide detection range up to 3.6×10^-3 M with detection limit of 6.0×10^-8 M (signal/noise=3) for dopamine determination. Meanwhile, the optimized sensor for dopamine displayed a sensitivity of 83.8μA mM-1 and response time of 5 s with addition of 0.20 mM dopamine. In addition, DPV experiment revealed that interfering species such as ascorbic acid and uric acid could be effectively avoided. The RuO₂/MWCNTs electrode presents stable, highly sensitive, favorable selectivity and fast amperometric response of dopamine.2. Potentiostatic method was employed for preparation of TiO₂/MWCNTs nanocomposite, which was applied for electrocatalysis of H₂O₂ oxidation. Compared to the MWCNTs electrode, the TiO₂/MWCNTs electrode diplays high electrocatalytic activity towards the oxidation of H₂O₂ in 0.1 M phosphate buffer solution. At an applied potential of +0.4 V, the TiO₂/MWCNTs electrode exhibits a linear dependence (R=0.998) in the H₂O₂ concentration up to 15.0×10^-3 M with a sensitivity of 13.4μA mM^-1 and detection limit of 4.0×10^-7 M with signal/noise=3. The optimal response time is less than 5 s with addition of 1 mM H₂O₂. In addition, interference from the oxidation of common interfering species such as ascorbic acid, dopamine and uric acid is effectively avoided. The TiO₂/MWCNTs electrode allows highly sensitive, low-potential, stable and fast amperometric sensing of H₂O₂, which is promising for the development of nonenzymatic H₂O₂ sensor.3. CuO modified MWCNTs nanocomposite was successfully prepared by magnetron sputtering deposition. The morphology of the CuO/MWCNTs nanocomposite was characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The MWCNTs electrode showed much lower electrocatalytic activity towards oxidation of glucose. In contrast, a substantial decrease in the overvoltage of the glucose oxidation was observed at the CuO/MWCNTs electrode due to excellent catalytic activity of the CuO nanoparticles to the glucose oxidation and high surface area and good charge transport characteristic of MWCNTs. At an applied potential of +0.40 V, the CuO/MWCNTs electrode presented a high sensitivity to glucose with 2596μA mM-1 cm-2. In addition, linear range was obtained over a concentration up to 1.2 mM with a detection limit of 0.2μM (signal/noise=3). The response time is about 1 s with addition of 0.10 mM glucose. More importantly, the CuO/MWCNTs electrode is also highly resistant against poisoning by chloride ion, and the interference from the oxidation of common interfering species such as ascorbic acid, dopamine, uric acid and carbohydrate compounds is effectively avoided. In addition, the CuO/MWCNTs electrode was also used to analyze glucose concentration in human serum samples. The CuO/MWCNTs electrode exhibits an enhanced electrocatalytic property, low working potential, high sensitivity, excellent selectivity, good stability, and fast amperometric sensing towards oxidation of glucose, thus is promising for the future development of nonenzymatic glucose sensors.4. ZnO-NWs modified MWCNTs nanocomposite was successfully prepared by a hydrothermal process. The morphology of the ZnO-NWs/MWCNTs nanocomposite was characterized by SEM and TEM. Compared with pure ZnO-NWs on tantalum substrate, the charge transfer rate of the ZnO-NWs/MWCNTs nanocomposite was remarkably increased because of the MWCNTs. Mott-Schottky plot displayed a high donor density of 3.9×1019 cm-3, a flat band potential of -0.8 V and a space charge layer of 7 nm. In addition, the ZnO-NWs/MWCNTs nanocomposite yielded higher photocurrent than pure ZnO-NWs. The decay constant of the ZnO-NWs/MWCNTs nanocomposite was also lower than that of its pure counterpart. The recombination of photoinduced electron-hole pairs in the ZnO-NWs/MWCNTs heterojunction was hindered, thus enhanced the photoelectrical conversion efficiency. The heterojunction of ZnO-NWs/MWCNTs provides potential applications in the field of photocatalysis and photoelectrical devices. 5. The MWCNTs arrays on tantalum foils were successfully coated with TiO₂ nanoparticles by a hydrothermal process. The prepared TiO₂/MWCNTs nanocomposite was characterized by SEM and TEM. The charge transfer properties and photocatalytic degradation of rhodamine B with and without bias potential under UV irradiation were investigated. The MWCNTs promoted the separation of photoinduced carriers in the TiO₂, thus enhanced photocatalytic activity. Applying bias potential on the photoanode further enhanced its catalytic activity. Hydroxide ion, which can react with photogenerated holes to inhibit the recombination of photogenerated carriers, is also favorable for improving the photocurrent and photocatalytic activity. Modification of TiO₂/MWCNTs with polyaniline (PANI) endows the resulting nanocomposite with visible light activity. The PANI/TiO₂/MWCNTs nanocomposite showed remarkable photocatalytic activity for the degradation of RhB under visible light irradiation. The enhanced photocatalytic activity of the PANI/TiO₂/MWCNTs nanocomposite originated from the effective charge transfer properties of the heterojunction. The efficient charge transportation and high photoelectrocatalytic activity towards degradation of rhodamine B made these novel composite materials promising for photocatalysts and for the development of photoelectrical devices. Photoelectrocatalysis; Heterojunction...