Fabrication And Mechanism Study Of Anodic TiO₂ Nanotubes For Supercapacitor Application

Titanium dioxide?TiO2?is one kind of wide bandgap functional semiconductor materials.It has been used in various industrial fields.In recent years,TiO2 nanomaterials have become one of hotspots in academic research due to the thrust of nanotechnology.Due to its large specific surface area and outstanding stability,TiO2 nanotubes have been considered as a promising material for photocatalysis and supercapacitor applications.Although some breakthroughs have been made in the research of TiO2 nanotubes,most of them were focused on the efficiency improvement of TiO2nanotubes as photocatalytic material.Comparatively,there is not enough research focusing on TiO2nanotubes as electrode material for supercapacitors.Anodization is a facile method for fabricating TiO2 nanotubes.The TiO2 nanotubes obtained from anodization are connected with Ti substrate directly and are highly ordered.Therefore,anodization is a suitable method for fabricating TiO2 nanotube electrode material.However,the relation between capacitive properties of the anodized TiO2 nanotubes and the thermal treatments and doping processes has not been broadly investigated and deeply understood.In the present work,the fabrication of anodic TiO2 nanotubes have been modified from the aspects of thermal treatments and doping process.The capacitive properties of the TiO2 nanotube electrode materials have been measured using various electrochemical measurements.The relation between the capacitive properties of TiO2 nanotube electrode materials and the processing parameters have been systematically studied and summarized.By combining electrochemical measurement results and microstructural analyses,the fundamental reasons for the variations of capacitive properties of TiO2 nanotube electrode materials caused by process modification have been explored.The conclusions obtained in the present work have a significance of understanding the relation between TiO2 nanotubes fabrication process and the capacitive properties of the obtained TiO2nanotubes,and will help broaden the application of TiO2 nanotube electrode materials in the supercapacitor field.First,the effects of thermal treatments on the capacitive properties of TiO2 nanotube electrode materials have been systematically studied.It is found that the heating rate has a huge influence on the capacitive properties of TiO2 nanotube electrode.The capacitance of TiO2 nanotube electrode can be significantly enhanced by using a rapid heat-up treatment.When TiO2 nanotubes were heated at a rate of 50?/min to 400?and held for 1 h,the best overall capacitive properties can be obtained.The capacitance of TiO2 nanotubes reaches 52.4 mF/cm²,with a rate capability of 65.9%and the capacitance retention of 95.2%after 5000 cycles of CV measurements.Moreover,the capacitive properties enhancement brought by rapid heat-up process can be obtained in other thermal treatment conditions including different temperature and holding durations.Therefore,rapid heat-up process can be seen as an effective way to improve the capacitive properties of TiO2 nanotube supercapacitors.It is found that increasing the heating rate has two effects on the TiO2 nanotubes.On one hand,the chemical state of TiO2 surface is changed.The content of Ti³⁺and oxygen vacancy has increased,while the content of the hydroxyl groups on the surface of TiO2 has also increased.On the other hand,the rapid heat treatment has an effect on the phase transformation of TiO2 from amorphous to anatase,raising the initial transition temperature,lowering the number of nuclei during the transition,and resulting in larger crystallite at the end of the thermal treatment.Both of these effects contribute to the conductivity enhancement and the capacitive properties of TiO2 nanotube supercapacitors.By controlling the composition of the anodization electrolyte,the anodized TiO2 nanotubes are successfully doped during the anodization process.It is found that the pH value has a dominant effect on whether the TiO2 nanotube can be successfully doped.When the pH value is near 7,TiO2 nanotube cannot be doped during the anodization process.However,when the pH value is about 2,the doping can be realized during the anodization.The doped TiO2 nanotubes can obtain a capacitance much higher than the undoped TiO2 nanotubes,with an enhancement of 94.7%.The doped TiO2 nanotubes were systematically analyzed by XPS.Several elements,including N,F and Fe were detected in the doped TiO2 nanotubes.As the doping level increases,the amount of N and Fe incorporated in the TiO2 lattice increases.The F element was absorbed on the surface in the F^-formation.The capacitive property enhancement obtained by the doped TiO2 nanotube originates from the pseudocapacitance brought by these doping elements.To broaden the application of TiO2 nanotubes,a TiO2-SiNW composite micro-supercapacitor was successfully fabricated by spin-coating TiO2 onto the SiNW.Compared to SiNW micro-supercapacitor,the composite material electrode possesses higher cyclic stability because of the TiO2coating.The TiO2 coating protects SiNW from the oxidation when it is used as an electrode in the aqueous electrolyte.It is found that there is an optimal coating amount,with which the capacitance of TiO2-SiNW is much higher than that of pure SiNW electrode.At the optimized condition,the TiO2-SiNW composite electrode exhibits higher capacitance,higher rate capability and higher stability than the SiNW electrode.It is found by the EIS measurement that coating TiO2 onto SiNW lowers the charge transfer resistance of the electrode.Furthermore,the resistance through the TiO2 coating layer is small.These will contribute to the capacitance enhancement of the electrode.However,the accessible area of the electrode is reduced by coating TiO2 layer onto SiNW,which will reduce the capacitance of electrode.Therefore,there is a trade-off between these two factors.When the coating amount is optimized,which in this study is 3 times of spin coating procedure,the best capacitive properties of the electrode materials is obtained.The calculation of the energy band diagram at various junctions confirms the reduction of charge transfer resistance when SiNW is coated with TiO2.The barrier between TiO2-SiNW interface restricts the transport of carriers within the TiO2 layer,resulting the electrochemical active area changes from SiNW to TiO2 coating.