Controlled Growth And Electrochemical Properties Of Nanocrystal Nobel Metals In Vertically Aligned TiO₂ Nanotubes

The electrochemical properties of an electrode not only depend on its macro size, but also rely on its micro scale and structures. Highly ordered and vertically aligned TiO₂ nanotubes possess large surface areas, favorable electrochemical activity and excellent photocatalysis performance. The steady chemical and physical properties of TiO₂ nanotubes make them good carrier that are suitable for the immobilization and decoration of biomolecules or catalysts. Study on the electrocatalysis of the modified nanotube electrodes with different size and microstructures by controllable synthesis may be of great importance in theory and application. In this paper, vertically aligned TiO₂ nanotube matrix was constructed by anodic oxidation, and served as substrate material. Various electrodeposition methods were applied the assistance of other conditions such as ultrasonic field, and noble metals like Au, Pt, Pd were deposited on the nanotubes. The affection of electrochemical and ultrasonic conditions on the growth and structures of noble metal crystals was studied, and the mechanism of controlled growth process was discussed. The resulted noble metal-semiconductor nanotube complex was applied to bioelectrochemistry, photoelectric catalysis and direct electrocatalytic oxidation of methanol, which offered a primary model and theoretic fundament for the design and fabrication of new type biosensors, photoelectric conversion devices and fuel cell. Research results were as follow.A novel nanostructured noble metal-TiO₂ nanotubes complex electrode has been constructed for effective immobilization of biomolecule and successful realization of its direct electrochemistry and electrocatalysis. Vertically aligned TiO₂ nanotubes (NTs) is in situ growing on titanium substrate by anodic oxidation of Ti, possessing large surface areas and high aspect ratio of the NTs. Au nanocrystal is further introduced into the TiO₂ NTs by one-stepped electrodeposition to form nanocrystal Au-TiO₂ NTs hybrid, aiming at enhancing the electrochemical performance and biocompatibility. The formation mechanism of the nanocrystal Au was elucidated from field-emission scanning electron microscopy (SEM) and X-ray diffraction (XRD) character. Nanocrystal Au can grow to different nanostructures in shape such as polyhedral nanorods with high-index facets, and honey-cave like nanocages, etc, due to the tubular confine of the TiO₂NTs. Furthermore, the multi-shape of nanocrystal Au results in better electrocatalysis for cytochrome c (Cyt c) at the nanocrystal Au-TiO₂ NTs complex electrode either immobilized on the electrode or dissolved in the solutions. Experimental results also indicated that this novel nanostructured electrode could be served as a biosensor for the reduction of H₂O₂. The linear range is 2×10^-6-3.49×10^-4mol/L, with a detection limit of 1.21×10^-6 mol/L. Moreover, it can be adapted to different pH circumstance, ranging from 3 to 8, with good sensitivity and resolution. It makes the nanocrystal AU-TiO₂ NTs complex electrode a promising platform for fabricating the third-generation biosensors.TiO₂ nanotube electrode was used as the substrate. Various electrodeposition methods were used under the controlled affection of ultrasonic and reverse pulse. Nanocrystal platinum and palladium with different microstructures and sizes were deposited on the nanotubes to form noble metal-TiO₂ nanotube complex electrode. The electrochemical behaviors and photoelectric catalysis performances of different microstructured Pt-TiO₂ nanotube electrode and Pd-TiO₂ nanotube electrode fabricated by controllable deposition were investigated. The influence of electrodeposition conditions and nanostructures on photoelectric catalysis performance was also taken into account. Experiment results indicated that ultrasonic field and pulse potential and current played remarkable roles in the nucleation and growth of nanocrystal noble metals. Moreover, due to the diversity of microstuctures caused by the effect of ultrasonic and reverse pulse, different nanocrystal noble metal-TiO₂ nanotube electrodes exhibited different photoelectric catalysis. Doping of nanocrystal noble metals could enhance the absorbance and utilization of visible light on nanotube electrode. Moreover, the structure and catalysis diversities between Pt and Pd further affected the photoelectric conversion.Base on the controlled growth conditions of noble metals, further study focused on the co-deposition behavior and controlled growth of bimetal on TiO₂ nanotubes. Platinum and palladium with high catalytic activities were chosen as primary metals, and were combined with gold or copper to form bimetallic hybrid nanocrystals. The fabricated bimetallic nanocrystal-TiO₂ nanotube electrodes were applied for the direct electrocatalytic oxidation of methanol. The diversity of the electrocatalysis for methanol oxidation affected by the compositions and microstructures of bimetallic systems was investigated. Experiment results demonstrated that microstructures and compositions of bimetal had great impact on the electrocatalysis of these catalyst systems. Both Pt and Pd exhibited favorable catalytic performance for methanol oxidation. And the cooperation and coordination of Pt and Pd further improved the catalytic activity and efficiency of bimetallic system. Cu, which served as the assistant metal, could enhance the catalytic activity of Pd-Cu system. As for Au, it showed some inhibit effect on bimetallic catalyst system. Researches on the comparison of Pt-M and Pd-M bimetallic catalytic system on TiO₂ nanotube electrode may provide relevant theoretic foundation for the design and fabrication of catalyst systems in direct methanol fuel cell.