Study On New Carbon Nanomaterials For Electrochemical Sensors And Supercapacitors

Since the discovery of carbon nanotubes (CNTs) and graphene (GR), they have attracted much attention in the field of scientific research due to their excellent physical and chemical properties. Both CNTs and GR possess excellent electrical conductivity, large surface area, significant mechanical strength, chemical flexibility and ideal porous structure, which make them attractive materials for preparation of nanocomposites, electrochemical sensors, and nano devices. At present, there is little study on nonenzymatic biosensors and supercapacitors based on metal oxides/CNTs and metal oxides/GR. The metal oxides/CNTs nanocomposites possess the properties of individual components or even with a synergistic effect and the metal oxides/GR nanocomposites could fully utilize the advantages of GR as double-layer capacitors and metal oxides as pseudocapacitors. In addition, the fabrication of electrode is simplified by using vertical aligned MWCNTs(VACNTs) and the disadvantages of conventional painting method can be effectively avoided. Therefore, in this thesis, studies on the development of metal oxides/VACNTs and metal oxides/GR nanocomposites, and their applications for electrochemical sensors and supercapacitors were carried out. Based on that, some valuable results have been obtained. The main points of this thesis are summarized as follows:1. An electrochemical sensor for sensitive and convenient determination of salicylic acid (SA) was constructed using vertically aligned multiwalled carbon nanotubes (VACNTs) which was synthesized by chemical vapor deposition. Compared to the glassy carbon electrode, the electro-oxidation of SA was significantly enhanced at the VACNTs electrode. The VACNTs electrode shows a sensitivity of 59.25?A mmol^-1 L, a low detection limit of 0.8×10^-6 mol L^-1 and a good response linear range with SA concentration from 2.0×10^-6 to 3.0×10^-3 mol L^-1. In addition, acetylsalicylic acid was determined indirectly after hydrolysis to SA and acetic acid, which simplified the detection process. The mechanism of electrochemical oxidation of SA at the MWCNT electrode is also discussed.2. Potentiostatic method was employed for preparation of MnO₂/VACNTs nanocomposite, which was applied for electrocatalysis of H₂O₂ oxidation. Compared to the VACNTs electrode, the MnO₂/VACNTs electrode displays high electrocatalytic activity towards the oxidation of H₂O₂ in borate buffer (pH 7.8, 0.20M). At an applied potential of +0.45 V, the MnO₂/VACNTs electrode exhibits a linear dependence (R = 0.995) on the H₂O₂ concentration from 1.2×10^-6 mol L^-1 to 1.8×10^-3 mol L^-1 with high sensitivity of 1.08×10⁶?A mol^-1 L cm^-2 and a detection limit of 8.0×10^-7 mol L^-1 with signal/noise=3. Meanwhile, interference from the oxidation of common interfering species such as ascorbic acid, dopamine and uric acid is effectively avoided. The MnO₂/VACNTs nanocomposite electrode can also be used as an amperometric sensor for routine analysis of H₂O₂ in milk samples, which indicates that the sensor based on the MnO₂/VACNTs nanocomposite electrode is promising for practical application. In addition, the mechanism of electrochemical oxidation of H₂O₂ at the MnO₂/VACNTs electrode is also discussed.3. The WO₃-modified VACNTs nanocomposite was successfully prepared by magnetron sputtering deposition. In this nanocomposite, VACNTs not only serve as support, but also play as a conductor just like other metals in metal/metal oxide pH sensors. The pH sensor based on the WO₃/VACNTs electrode shows a sensitivity of about 41 mV pH^-1 at 20?and a linear working range from pH 2 to12. Although the measured sensitivity is lower than the theoretical value, the WO₃/VACNTs electrode exhibits high stability (over a month), reproducibility (RSD < 1%), fast response (<90 s) and anti-interference property. When the WO₃/VACNTs electrode was employed to examine the pH value of some real samples, satisfactory results could be obtained. All these results demonstrate that the sensor based on the WO₃/VACNTs electrode is promising for practical application. The method also provides a potential to miniaturize pH sensor.4. A novel type electrode based on RuO₂ nanoparticles-modified vertically aligned carbon nanotubes (RuO₂/VACNTs) was prepared by magnetron sputtering deposition. The morphology of the RuO₂/VACNTs nanocomposite was characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). In this nanocomposite, VACNTs not only serve as support, but also play as a conductor just like other metals in metal/metal oxide pH sensors. Compared with the WO₃/VACNTs pH sensor, the RuO₂/VACNTs electrode displays higher sensitivity (-55mV/pH) and faster response(<40 s). In pH measurement, it also demonstrates high stability (over a month), good reproducibility (RSD <5%), favorable anti-interference property. When the electrode was employed to examine the pH value of some real samples, satisfactory results could be obtained. Moreover, this method also makes it possible to miniaturize pH sensors and the determination of pH values in vitro and in vivo intracellular could be achieved. All these have demonstrated that this sensor based on the RuO₂/VACNTs electrode is promising for practical application.5. Hydrous ruthenium oxide/graphene (RuO₂/GR) nanocomposites with different loadings of RuO₂ are prepared by combining sol–gel and low-temperature annealing processes. In composite, the RuO₂ nanoparticles with diameter of about 5 nm are homogeneously deposited on the surface of GR. The supercapacitor based on RuO₂/GR nanocomposite shows good supercapacitive behaviors in 1.0 mol L^-1 H₂SO₄. When the loading level of RuO₂ reaches 38%, the RuO₂ nanoparticles in the composite achieve a specific capacitance of 546.6 F g^-1. Compared with bare RuO₂, RuO₂/GR nanocomposite also exhibits enhanced rate capability, excellent electrochemical stability ( 93% retention after 1000 cycles), and high energy density (26.5 Wh kg^-1) at low operation rate or high power density (5000 W kg? 1) at a reasonable energy density. RuO₂/GR nanocomposite could fully utilize the advantages of GR as double-layer capacitors and metal oxides as pseudocapacitors. So, it is reasonable to believe that the novel particle–sheet structured RuO₂/GR can be applied in high-performance energy-storage systems.