Fabrication of nickel oxide-embedded titania nanotube array for redox capacitance application

Titania nanotube array with an enlarged tube diameter of 110 nm and length of 700 nm was grown on titanium metal by a potentiostatic anodization in hydrofluoric acid-phosphoric acid-ethenyl glycol electrolyte. Nickel hydroxide was introduced into this titania nanotubes by either an electrodeposition-oxidation or hydrothermal process. Nickel oxide-titania composite was finally formed by heating treatment at 300 °C. Such a well-defined nanocomposite supported on titanium substrate was designed as a functional nanotube array electrode for the redox capacitance application. The morphology, microstructure and electrochemical properties of the nanocomposites were investigated by field emission scanning electron microscope, X-ray diffraction, energy dispersive X-ray diffraction and cyclic voltammetry measurements. It was found that nickel oxide could be embedded in titania nanotubes and extend from inner wall to top layer with an open pore mouth. The entire tube lengths were approx. 770 nm and 710 nm, meanwhile nickel-to-titanium atom ratios were determined as 9.6 at% and 36.4 at% for nickel oxide-embedded titania by an electrochemical and hydrothermal synthesis, respectively. The corresponding specific redox capacitance was also increased from 26 mF cm-2to 85 mF cm-2with highly reversible charge-discharge stability. Such an improvement was mostly ascribed to more accessible reaction interface of electroactive nickel oxide through its higher loading and a uniform dispersion on titania nanotubes. The capacitance was further increased up to 128 mF cm-2for 36.4 at% nickel-containing nickel oxide-titania/titanium electrode when a porous graphite carbon instead of a platinum sheet was used as a cathode.