Fabrication Of Nanostructured Anodic TiO₂ Films For Supercapacitor Applications

Recently,supercapacitors have been widely studied as a new energy storage device.TiO₂ nanotubes fabricated by anodization,feature a large surface area,direct pathway for charge transport,wide potential window,making them a candidate material for supercapacitors.However,TiO₂ is a kind of semiconductor material with the limited conductivity,and the synthesis of TiO₂ nanotubes with regular and controllable microscopic morphologies is still under development.In this work,the influences of anodizing parameters on morphology of TiO₂ films was studied,and the electrochemical properties of TiO₂ nanotubes were improved by increasing surface areas of nanotubes and doping.Firstly,the rapid growth of TiO₂ nanotubes was achieved by adding lactic acid?LA?and polyvinyl alcohol?PVA?to the electrolyte.The growth rate of TiO₂ nanotubes was up to 1.9?m min-1,and the complete and nantubular structure was maintained.The TiO₂ nanosponge was fabricated by anodization of Ti in high-water-content electrolytes,which led to increased specific surface areas.And the influences of anodizing voltage?current density?,temperature and multiple anodization on the morphology of TiO₂ films were studied.The regularity of TiO₂ nanotubes was improved by two-step anodization and increasing the anodizing voltage.Secondly,using hydrothermal solid-liquid route?HSLR?or hydrothermal solid-gas method?HSGM?,TiO₂ nanoparticles/nanotubes hybrid structure was fabricated,and the influences of temperature,vapor pressure,processing time on morphology of hybrid structures were investigated.The results suggested that the morphology transformation of TiO₂ nanotubes follows the dissolution-recrystallization mechanism.The TiO₂ films became inhomogeneous and fractured significantly after HSLR treatment,whereas the same films treated by HSGM still maintained homogeneous and defect-free.The hydrothermal vapor pressure was the key factor for controlling microscopic morphologies of hybrid structures.The surface area of the resultant samples treated by 180 ?L of water?HSGM-180?was about 70.8 m2 g-1,which was?3.16 times larger than that of the TiO₂ nanotubes without HSGM treatment.Under the saturated vapor pressure,the obtained sample treated for 20 min?HSGM-20min?could achieve the ideal hybrid structures.Thirdly,the electrochemical properties of TiO₂ nanoparticles/nanotubes hybrid structures were improved by annealing in Ar or NH3 atmosphere.Electrochemical measurements showed that though NH3 annealing would improve the areal capacitance,the rate capability and cycling stability were very poor possibly due to the unsteadily chemical state.In contrast,annealing in Ar could generate oxygen vacancies,which led to the reduction of electrode resistance.HSGM-20min annealed in Ar exhibited a significantly improved capacitance of 76.12 mF cm^-2,?5.49 times larger than that of the counterparts annealed in air without HSGM treatment,and an excellent cycling stability with 87.7%of initial capacitance retained after 2000 cycles.Finally,the modification of TiO₂ nanotubes was achieved by potentiostatically or galvanostatically electrochemical doping in metallic ion-containing solutions.The results demonstrated that Al³⁺ can insert to the crystal structure of TiO₂ by electrochemical doping in a non-aqueous solution containing Al³⁺.Compared to the H-doped TiO₂ nanotubes,the concentration of oxygen vacancies was increased and Al³⁺-doping could overcome the disadvantage of the poor cycling stability.Nevertheless,Fe³⁺ and Cu²⁺ were hard to insert to the crystal structure of TiO₂.The areal specific capacitance of the Al³⁺-doped TiO₂ nanotubes,obtained by the galvanostatic method(at-3 mA cm^-2),was 8.41 mF cm^-2,which was?3.77 times larger than that of the counterparts without doping,and it still retained 93.4%of initial capacitance at the end of 2000 cycles.