Formation and Characterization of Composite Nanostructures from Photocatalytic, Dielectric and Ferroelectric Materials

In this study, titania (TiO2) nanotube arrays of different structures and morphologies were fabricated by anodic oxidation of titanium foils in aqueous ammonium sulfate or ethylene glycol electrolytes. After the anodizing process, the nanotube arrays were adopted as templates to fabricate barium titanate (BaTiO3) and TiO2 nanostructure composites using the hydrothermal method with barium hydroxide precursors at 160-180 °C. Based on the SEM, XRD and XPS analyses, the nanostructure composites can be classified into four types according to their forms: BaTiO3 thin film, flower-like BaTiO3/TiO2 composite, BaTiO3/TiO2 double-layer composite and porous BaTiO3/TiO2 composite. This study also reports the use of atomic layer deposition (ALD) to fabricate hafnium dioxide (HfO2) nanotubular capacitors in the prepared TiO2 nanotube arrays or porous BaTiO3/TiO2 composites. In other words, the TiO2 nanotube arrays or BaTiO3/TiO2 composites were used as templates with their pores filled with HfO2 by ALD. After using the impedance analyzer and equivalent circuits to investigate the electrical properties of the HfO2 nanotubular capacitors, it is found that the capacitors with specific TiO2 nanotube (template) dimensions performed the best – 800-900 nFcm-2 was achieved with 570-nm-long TiO2 nanotubes and 1.7 μFcm-2 with 4-μm-long TiO2 nanotubes. These capacitance values are significantly higher than those of HfO2 film capacitors of the same dielectric layer thickness. It is believed that the HfO2 nanotubular capacitors formed within the TiO2 nanotube arrays behave like capacitors in parallel connection; hence, the significant increase in capacitance. Another topic of this study was to examine the influence of the polarization direction of a ferroelectric substrate on the photocatalytic performance of TiO2. A two-step hydrothermal treatment was adopted to grow vertically aligned, ferroelectric BaTiO3 nanorod arrays on conductive FTO substrates. This involved growing rutile phase TiO2 nanorods first and then converting them into BaTiO3 nanorods. The gaps and pores between the ferroelectric BaTiO3 nanorods were then filled with TiO2 by ALD, forming film-like BaTiO3/TiO2 composites. The TiO2 component within the composites could provide both photoinduced charge carriers and a conduction path for the charge carriers. It is found that when the polarization of the BaTiO3 nanorods was poled downward toward the FTO substrate, the photoinduced electrons from the TiO2 were encouraged to flow to the FTO substrate, resulting in a significant increase in the photocurrent density.