Preparation Of Self-doped TiO₂ Nanotube Arrays Based Composites And Their Properties In Photo Energy Conversion And Storage

Utilization of solar energy is a promising solution to energy crisis that will emerge in the future.Currently,only a small portion of solar energy is used.The key issue that limits the utilization of solar energy is the high cost of solar energy conversion and energy storage.Simultaneous solar energy conversion and storage can improve the integration of the whole system,which will lower the cost of solar energy utilization.Photocapacitor is one of the devices widely studied that could realize photo energy conversion and storage at the same time,and the focus in recent years is mainly on design of new stuctures and the development of advanced electrode materials.Under these backgrounds,this research paper focuses on a material with excellent photoelectrochemical properties-TiO₂ nanotube arrays,and tries to utilize it as electrode material for photocapacitors.To realize this goal,the primary is to address the poor conductivity of TiO₂ nanotube arrays,which greatly hinders its energy storage capability.Specifically,this paper proposed a novel electrochemical reduction stratergy to achieve self-doping of TiO₂ nanotube arrays,after which good conductivity and great enhancement in energy storage ability of the material were attained.Finally,the self-doped TiO₂ nanotube arrays were used as electrode material for photocapacitors.The main results of this paper are summarized as follows.Firstly,an electrochemical reduction approach was employed to achieve self-doping of TiO₂ nanotube arrays.Polarized at the potential extent of-1.2～-1.8 V（vs SCE）,oxygen vacancies were successfully introduced into the lattice of TiO₂.It was found that-1.4 V was the optimal potential to fabricate self-doped TiO₂ nanotube arrays.Remarkably,5 orders of magnitude improvement on carrier density and 39 times enhancement in capacitance could be achieved upon self-doping.These enhancements were due to the great decrease of charge-transport resistance through the solid material while the surface of TiO₂ nanotubes remained no significant changes.Secondly,the effects of self-doping on the photoelectrochemical properties of TiO₂ nanotube arrays were investigated and after which the self-doped TiO₂ nanotube arrays were used as electrode material for photocapacitors.After self-doping,an enhancement in response to visible light and a slight positive shift of conduction band position were obserbed.As results,the self-doped TiO₂ nanotube array sample delivered an increased saturated photocurrent at more position potential,compared to the pristine one.This is because the self-doped TiO₂ nanotube arrays possess excellent conductivity,which leads to decreased band bending.Based on their energy storage capability,the self-doped TiO₂ nanotube arrays could be used as photocapacitor electrode.A photo energy conversion and storage capacity of 5.98 mF/cm2（3.77 mC/cm）was achieved.The third,MnO₂ species was deposited onto the self-doped Ti02 nanotube arrays to further boost the energy storage ability of the material.By using a sequential chemical bath deposition method,MnO₂ spheres were coated on the self-doped TiO₂ nanotubes,which showed a high capacitance of 175 mF/cm,1.52-fold of the one using Ti metal as the substrate.This enhancement in performance was due to the fact that using the porous TiO₂ nanotube substrate could improve the reaction area of the coated MnO₂ and reduce the ion diffusion resistance.On the other hand,by using a high current pulse electrodeposition strategy,the distinguished nucleation trend on the nanotube walls was diminished,which resulted in the homogenous coating of Mn02 along the entire TiO₂ nanotube walls forming a TiO₂@MnO₂ stucture.Benefited from ultilizing the surface area of nanotubes to the greatest extent,this material exhibited a specific capacitance as high as 425 F/g.Finally,WO₃ was coated on self-doped TiO₂ nanotube arrays and a high performance photo energy conversion and storage system was assembled by using this composite as photo energy conversion part and the TiO₂@MnO₂ material as energy storage part.The coating of WO₃ greatly enhanced the response of self-doped TiO₂ nanotube arrays towards visible light,thus it could be used to charge the TiO₂@MnO₂.When the area ratio of photo energy conversion part to energy storage part was 1:1,the working potential window of the system was 0.35V and the capacity reached 76.6 mF/cm2（26.8 mC/cm2）,most of which was stored in the Ti02@MnO₂.Increasing the area ratio of photo energy conversion part to energy storage part could promote the photocharge rate and working potential window of the system,but the capacity per area of the system would be sacrificed.