In this work, highly ordered TiO2 nanotube arrays (TNAs) were firstly fabricated with anodic oxidation method on Ti foils (1 cm × 1 cm), which were used as high-surface-area substrate. The Ti foil was anodized at a constant potential of 50 V for 20 h in a two-electrode system, using an aqueous electrolytic containing 0.25 wt.% NH4F and 2 vol.% H2O. Then, hydrogen electrochemical doping was performed to improve the electric double-layer capacitance of TiO2 nanotube films. TiO2 nanotube films were used as cathodes while the carbon rod as anode. The gap between two electrodes was 2.5cm, and a voltage of 5 V was applied for 30 s in a 0.5 M Na2SO4 solution. Then, nickel oxide was deposited through the whole TiO2 nanotube films in 0.04M NiCl2 through the method of differential pulse voltammetry. Finally, the hydrogenated TiO2/NiO (H-TiO2/NiO) nanotube array composite electrode was successfully fabricated.
Fig.1 (a) and (b) presents sannning electron microscope (SEM) images of the TiO2 nanotube films before and after electrochemical hydrogenation. The pristine TiO2 nanotubes have an inner diameter of ∼120 nm and a tube length of about 10um. Obviously, there is significant change of the morphology of TiO2 nanotubes after the electrochemical hydrogenation. Fig.1 (c) and (d) show the SEM images of hydrogenated TiO2 nanotubes after NiO deposition. The narrower inner diameter of TiO2 nanotube obviously indicates NiO was deposited through the inner and outer walls of TiO2 nanotubes. Fig.2 (a), (b) and (c) show the C-V curves of pristine TiO2, H-TiO2 and H-TiO2/NiO nanotube films at different scan rates. Clearly, the C-V curves of pristine TiO2 nanotubes are close to rectangular shape, which exhibits the behavior of electric double layer capacitors. However, the specific capacitance, which was calculated to be 0.6 mF cm-2 at 0.08 mA cm-2, is quite low due to the low corresponding current. The C-V curves of H-TiO2 nanotubes exhibit perfect capacitance behavior of electric double layer capacitors and much higher current response. The specific capacitance was calculated to be 71 mF/cm2 at 0.5 mA cm-2, which was significantly enhanced after hydrogen doping. The C-V curves of H-TiO2/NiO nanotubes exhibit typical pseudo capacitance behavior, and the specific capacitance was calculated to be 285mF/cm2 at 0.5 mA cm-2 after NiO deposition shown in the EDX elemental mapping in Fig.2 (d).
In summary, the electrochemical behaviors of pristine TiO2, H-TiO2 and H-TiO2/NiO nanotube films have been fabricated and characterized by SEM and electrochemical measurements. The specific capacitance of TiO2 nanotube films can be greatly enhanced from 0.6 mF cm-2 at 0.08mA cm-2 to 71 mF/cm2 and further 285mF/cm2 at 0.5mA/cm2 after electrochemical hydrogenation and NiO deposition.
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