One-dimensional Arrays Of Pt, Co And TiO₂: Fabrication, Characterization And Properties

Significant research progresses of one-dimensional (1-D) systems have been achieved in the past decades. Researchers have been focusing on the following three areas. First, explore controllable synthesis approaches on 1-D materials. Second, study the fundamental physical and chemical properties on individual units. Third, assemble the 1-D nanomaterials as building blocks in low power consumption, high performance and highly integrated devices. In this study, we devote our efforts on the three different but close-related areas based on the novel structures of porous anodic aluminum and titanium oxides.A unique and robust method is developed to design and fabricate anodic aluminum oxide (AAO) membranes with serrated nanochannels in phosphoric acid solution. The synthesis can be carried out under room temperature and in a wide operation voltage (10⁸0 V). Due to high field conduction and anionic incorporation, an increase of anodizing voltage leads to the increase of the impurity levels as well as the electric field across the barrier layer. The initialization and formation of serrated channels are attributed to the evolution of oxygen gas bubbles followed by plastic deformation in the oxide film based on both experiment and simulation results. Alternating anodization in oxalic and phosphoric acids is applied to construct multilayered membranes with smooth and serrated channels, which demonstrates three-dimensional hierarchical system with controllable morphology and composition.To reveal the inside serrated channel structure, platinum is electrodeposited into the template. The interval distances, branch length, and total length of the as-synthesized serrated nanowires are about 250 nm, 250 nm, and 4μm, which are in accordance with the serrated channel spacing, length, and template thickness, respectively. X-ray diffraction (XRD) pattern and high resolution transmission electron microscope (HRTEM) image indicate that the crystal size is in the range of 5⁹ nm. The electrocatalytic activities for methanol oxidation are evaluated on serrated nanowires in a conventional three-electrode system, which exhibits superior electrocatalytic activity compared to the straight nanowires. Combined with finite-element numerical simulation, we believe that the strengthened electric field around the serrated tips contributes predominantly to the enhanced electrocatalytic activities in addition to the increased surface area per unit Pt mass.The AAO with smooth channels are used as templates to synthesize ferromagnetic cobalt nanowires and nanotubes. HRTEM and XRD results on Co nanowires (diameter⁹0 nm) show predominantly single crystal hexagonal close-packed (hcp) structure with the magnetocrystalline easy axis (c-axis) perpendicular to the wire axis. The conductivity of Co nanocrystal performed on an individual wire is around 75μ? ? cm. Hysteresis loops illustrate the dominance of shape anisotropy. Furthermore, the magnetic structures of Co nanowires are studied using magnetic force microscopy (MFM), which reveals a strong dipole at the two ends of the wire, together with a spatial magnetization modulation along the wire. Based on theoretical modeling, such intrinsic modulation is attributed to magnetization frustration due to the competition between the magnetocrystalline polarization along the easy axis and the shape anisotropy along the wire axis.Subsequently, Co nanotubes are demonstrated by direct electroplating method inside the through-hole AAO membranes. The results manifest that the nanotubes are mainly hcp single crystals with the c-axis perpendicular to the tube axis. MFM imaging shows a weak magnetic signal and SQUID measurement reveals sheared hysteresis responses for field applied along the tube axis. Combined with theoretical modeling taking into account the shape demagnetization, crystal anisotropy, magnetic exchange, and external magnetic interaction energies, our measurements confirm that the magnetization curls circumferentially around the tube in order to minimize the total magnetic energy for small external fields.Based on our successful achievement on the AAO, self-organized TiO₂ nanotube arrays are demonstrated by two-step anodization method in NH4F based electrolyte. The growth rate of TiO₂ nanotubes is ¹67 nm/min under 60 V bias voltage in fresh electrolyte. The energy dispersive X-ray analysis (EDX) on TiO₂ nanotubes indicates trace amount F element resulting from the dissolution effect of NH4F. After the annealing treatment under 500℃for 6 h, the amorphous TiO₂ is converted to dominant anatase phase and trace amount of thermally grown rutile structure. Moreover, a conductive AFM tip coated with Ti/Au is contacted on an individual nanotube for the conductance measurement which exhibits three orders of magnitude higher than that of amorphous nanotubes. In order to further increase the conductance at the nanotube bottom, reductive doping method is adopted to facilitate more conductive barrier layer.TiO₂ nanotube membranes are employed as photoanode in dye-sensitized solar cells (DSSCs). The energy conversion efficiency is achieved to be 2.94 % by optimizing the annealing parameters and TiCl4 treatment. The condensed barrier layer at nanotube bottom causes a relative high series resistance, corresponding to the low fill factor and conversion efficiency. Thus, the reductive doping of TiO₂ nanotubes is performed before the solar cell assembling. The packaged device exhibits improved performance with higher short circuit current density (8.28 mA/cm2) and energy conversion efficiency (3.95 %).The enhanced conductivity of TiO₂ nanotube bottom makes it feasible to electrochemically deposit desired materials. Due to the 1-D structure and functionality of TiO₂ itself, three-dimensional core-shell structures is promising to exhibit superior performance in photovoltaics and energy storage devices. Herein, p-type Cu₂O is successfully filled into TiO₂ nanotube channels by electrodeposition. The three-dimensional Cu₂O/TiO₂ p-n junction films exhibit an energy conversion efficiency up to 0.009 %, which is much enhanced compared to the two-dimensional Cu₂O/TiO₂ films. The improved photovoltaic performances benefit from the high junction interface as well as the 1-D carriers'pathway due to the radial hetero-junction nature. Conclusively, the presented Cu₂O/TiO₂ system would be a promising candidate of solid-state low cost solar cell and photocatalyst due to their abundant, inexpensive and environmental friendly nature once the efficiency could be further improved by optimizing the device parameters.