Study Of The Interface Enhancement And Doping Modification Of Anodic TiO₂ Nanotubes For Supercapacitors

One-dimensional TiO₂ nanotubes(called anodic TiO₂ nanotubes)grown in situ on a titanium metal by anodization have high specific surface area and unique electron transmission paths,making them ideal electrode materials for supercapacitors.However,the poor interfacial adhesion between the nanotube layer with high aspect ratio and the Ti substrate and the limited conductivity of TiO₂ greatly hinder their further application.Therefore,interface enhancement and various doping modification techniques are applied to anodic TiO₂ nanotubes to enhance their interfacial adhesion and supercapacitive properties.Firstly,the anodic TiO₂ nanotubes are prepared by anodization method.The interfacial adhesion strength is shown to decrease with increasing thickness of the anodic TiO₂ nanotube layer.Then the effects of various anodization posttreatment processes on the interfacial adhesion between the anodic TiO₂ nanotube layer and the Ti substrate and the interface enhancement mechanism are studied.When the anodization posttreatment processes are carried out at 1 m A cm^–2 or at 10 V for 5 min,significant improvement of the interfacial adhesion can be achieved.Compared with the untreated sample,their interfacial adhesions are both increased by a factor of approximately 1.5.Further,the as-treated samples also show enhanced capacitance and reduced contact resistance.A large residual stress would be created at the interface of the nanotube layer and the underlying metal during anodization,due to the remarkable volume expansion when Ti is converted to TiO₂ and the bad plasticity of Ti metal.These samples exhibit the enhanced adhesion by means of these posttreatment processes,because of the decrease or release of the residual stress.In particular,both methods are found to work well for the thin Ti sheet(?18?m).Secondly,through the liquid phase method,the anodic TiO₂ nanotubes are soaked at room temperature or hydrothermally treated in different metal salt solutions.As a result,it is found that the anodic TiO₂ nanotubes cannot be doped with metal ions by annealing after soaking in metal salt solutions at room temperature.Although the areal capacitance of the anodic TiO₂ nanotubes hydrothermally treated in 0.5 m M Fe(NO₃)₃ solution is approximately2.54 times higher than that of pristine TiO₂ nanotubes,the hydrothermal treatment destroys the integrity of the anodic TiO₂ nanotubes and the strong adhesion with the substrate,which affects their practical application.Thirdly,by vapor heat treatment,low-boiling Al Cl₃ is used as a dopant source to prepare Al-doped TiO₂ nanotubes.The Al³⁺ions are stably incorporated into the TiO₂ lattice in the form of substitution of Ti⁴⁺by Al³⁺,and oxygen vacancies are introduced.The as-prepared Al-doped TiO₂ nanotubes electrode delivers an average capacitance of 12.48 m F cm^-2 at a scan rate of 100 m V s^-1,which is?12.7 times higher than that of untreated TiO₂ nanotubes.Through vapor heat treatment,low-boiling sulfur is used as a dopant source to prepare S-doped TiO₂ nanotubes.The as-prepared S-doped TiO₂ nanotubes electrode delivers an average capacitance of 14.98 m F cm^-2 at a scan rate of 100 m V s^-1,which is?15.3 times higher than that of untreated TiO₂ nanotubes.Unlike the liquid phase method(hydrothermal treatment),the vapor heat treatment can achieve adherent TiO₂ nanotube layer on Ti substrate.Finally,the anodic TiO₂ nanotubes are doped by facile electrochemical doping method to study the effect of electrochemical doping in different pH solutions on the cycling stability.The results show that the electrochemically doped samples in KOH solution have the best cycling stability.In order to study the mechanism of improving cycling stability,the anodic TiO₂ nanotubes are electrochemically doped in KOH solution,followed by soaking them in H₂O₂ solution for a short time.The supercapacitor made by the as-prepared TiO₂ nanotube electrode exhibits an excellent cycling stability with a capacitance retention of 94.1%after10000 cycles,which is markedly higher than that of the electrochemically doped TiO₂materials reported to date.This is can be attributed to the formation of a core-shell structure with an amorphous surface layer of 2-3 nm thickness on the as-prepared TiO₂ nanotubes.