Transition metal oxides (MxOy, M = Ti, W, Zn) prepared by electrochemical anodization: synthesis, modifications, and applications

Abstract

In the past few decades, transition metal oxides (TMO) show various unique properties and wide applications due to their electronic configuration and multiple possible structures. Among different TMO, titanium dioxide (TiO2), tungsten trioxide(WO3), and zinc oxide (ZnO) are most commonly oxides used as catalyst, sensors and electric devices due to their unique physical and chemical properties. Electrochemical anodization is a facile and cost-effective way to produce 1D self-assembly metal oxides,which integrates the advantage of larger surface area, short diffusion path, low quantum confinement, high charge transfer efficiency and tunable electronic structures. Post-annealing is a possible way to rearrange the disordered atomic arrangement of the anodic oxides and tune their electronic structure. In addition, the decoration of the anodic oxides with metal nanoparticles is another efficient way to modify the as-synthesized anodic oxides, which helps supress the recombination of the photogenerated charge carriers and prolong their life time. The as-prepared and modified anodic metal oxides exhibit various applications, including the photocatalyst, electrochemical catalyst and SERS substrates.The pre-treatment of the Ti substrate and electrolyte were conducted before formal anodization. Their influence on the morphology and mechanical properties of the anodic TiO2 were discussed. The results indicated that the regular TiO2 NTs with the hierarchical shape was produced on the patterned Ti substrate prepared by two-step anodization. Although the regularity of TiO2 NTs improved a lot with the increase of aging time, the pore size decreased as well as the surface homogeneity. The nanohardness and reduced modulus showed the obvious increase with the aging time and highest adhesion between TiO2 and Ti substrate was achieved by anodizing in the 25 h aging electrolyte. Anodic TiO2 initially has an amorphous structure and crystallizes into anatase after thermal annealing. High resolution transmission electron microscopy and in situ synchrotron X-ray diffraction were employed to study the dynamic phase transformation process and the effect of annealing parameters on the atomic structure. At temperatures above 330°C the crystallization process began immediately and ceased within ~500 s. The material was not fully crystallized (crystallinity only ~70 wt.%), even when the annealing time was prolonged to 7000 s at an elevated temperature. The incomplete crystallization could be ascribed to the effect of grain boundaries, oxygen vacancies, and fluorine ions. Besides the phase structure change of anodic TiO2 after annealing, the electric resistivity and wettability of the TiO2 were found to have a close relationship with the post heat treatment. The as-synthesized and annealed TiO2 were used as the electroplating substrate for fabrication Ni-TiO2 nanocomposite. The results reveal that the deposited Ni grows inside the nanotube on annealed TiO2 substrate, while it only forms a compact layer on the top of as-anodized. The resultant Ni-TiO2 nanocomposite on annealed TiO2 substrates also exhibited better oxygen evolution performance than on amorphous substrates in terms of low overpotential at a current density of 10 mA/cm2 and small Tafel slope. The modification of TiO2 can also be achieved by a novel technique that combines magnetron sputtering and thermal dewetting. The obtained Cu-TiO2 nanocomposite catalyst exhibited 4-fold increase in the photodegradation rate of methylene blue aqueous solution under solar light irradiation than anatase TiO2 prepared with sameanodization conditions. The enhanced photocatalytic activity was attributed to thesynergistic effect of Schottky barrier and surface plasmon resonance.Similar to anodic TiO2, regular self-organized nanoporous WO3 was prepared through anodization in the electrolyte containing ethylene glycol, ammonium fluoride and DI water, which was subsequently used as a template for deposition of Ag nanoparticles by magnetron sputtering and thermal dewetting. The synthesized Ag-WO3 nanocomosites showed large SERS enhancement factor of ~2.1× 107 and a low detection limit of ~1× 10-6 M Rhodamine B. The visible light responding behaviour of WO3, synergistic interaction between Ag nanoparticles and the WO3 substrate, and the plasmonic behaviour of Ag collectively contribute to the enhanced Raman scattering. Although TiO2 nanotube and WO3 nanoporous structure can be successfully prepared in the ethylene glycol electrolyte containing the NH4F, it did not work for anodic ZnO. A new type of self-assembled Zinc oxide (ZnO) nanostructure has been prepared by electrochemical anodization of Zinc foil in NaOH aqueous electrolyte. The experimental results indicate that the primary factor affecting anodic nanostructure is the applied voltage, while anodization time and electrolyte concentration also play significant roles in tailoring nanorod morphology.