Electrophoresis Coating of Titanium Dioxide on Aligned Carbon Nanotubes for Controlled Syntheses of Photoelectronic Nanomaterials

Owing to its excellent semiconducting and photoelectronic properties, TiO2 has recently attracted a great deal of interest for a large variety of applications. Examples include the use of TiO2 nanoparticles/films as photocatalysts for environment protection, photoelectron mediators for sensors, photosensitizers in light-emitting devices, and solar cells. With the recent development in nanoscience and nanotechnology, there is now a pressing need to integrate multicomponent nanoscale entities into multifunctional materials and devices. In this regard, TiO2 nanoparticles have been deposited onto various catalytic supports to improve their photocatalytic activities. In particular, activated carbon fibers and spheres were used as a class of chemically stable mesoporous catalytic supports to provide multiple active sites and to allow effective diffusion of reactants for the photocatalytic reactions. Because of their unique one-dimensional electronic structure, large surface area, good chemical and thermal stability, and excellent mechanical properties, carbon nanotubes (CNTs) may work better than other carbon forms to support TiO2 for a wide range of applications, especially in photocatalytic and optoelectronic systems. As a consequence, some recent attempts have been made to coat nonaligned multiwalled CNTs with TiO2 thin films, [10] whilst several synthetic routes were devised to prepare nonaligned TiO2 nanotubes, [11] nanowires, or nanomembranes. It will be a significant advantage if we can coat vertically aligned CNTs (VACNTs) with TiO2 as the coaxial structure should allow the nanotube framework to provide a good mechanical stability, high thermal/electrical conductivity, and large surface/interface area necessary for efficient optoelectronic and sensing devices. The alignment structure will also facilitate surface modification for adding novel surface/interfacial characteristics to the TiO2 and VACNT hybrids, and allow the constituent nanotube devices to be collectively addressed through a common substrate/electrode. We previously prepared various vertically aligned conducting polymer–CNT coaxial nanowires by electrochemically depositing a concentric layer of an appropriate conducting polymer onto the individual aligned CNTs for advanced biosensing applications. In this Communication, we report the use of VACNTs not only as the support for electrophoretic coating with TiO2 but also as the template for producing aligned TiO2 nanotubes and nanomembranes. The resultant aligned TiO2–VACNT coaxial nanowires, TiO2 nanotubes, and TiO2 nanomembranes were demonstrated to possess novel photocurrent responses and photoinduced electron-transfer properties. In a typical experiment, we prepared VACNTs by pyrolyzing iron(II) phthalocyanine (FePc) on a Si substrate according to our published procedures. For electrophoresis, a Si-supported VACNT film thus produced was used as the cathode and a graphite rod as the anode. Both electrodes were immersed in a sol–gel solution approximately 2 cm apart from each other. The sol–gel solution was prepared by dissolving titanium (IV) isopropoxide (10 mL) in 30 mL of ethanol containing glacial acetic acid (12 wt %), followed by the addition of 0.6 mL of HCl in deionized water (pH 2) under magnetic stirring for 1 h. A potential of 1 V was then applied between the two electrodes for several minutes to electrophoretically deposit the positively charged, protonated TiO2 clusters onto the VACNT cathode. After the electrophoretic deposition, the samples were dried at 80 °C for l h. The chemical stoichiometry, crystal structure, and thickness of the resultant TiO2 coating can be regulated by controlling the electrophoresis conditions (e.g. applied voltage, deposition time) and a post-electrophoresis heat treatment for crystallization and densification (typically, 500 °C in air for 20 min). Figure 1a shows a typical scanning electron microscopy image of ca. 7 lm long VACNTs. The corresponding SEM image under higher magnification in Figure 1b shows an average diameter of ca. 50 nm for the as-synthesized CNTs. Upon electrophoresis, TiO2 clusters were deposited on sidewalls of the VACNTs as nanoparticles (Fig. 1c). Further electrophoresis deposition up to ca. 30 min caused additional TiO2 clusters to fill up the spaces between the pre-deposited TiO2 nanoparticles, leading to the growth of a continuous TiO2 coating along C O M M U N IC A IO N