New Photocatalysts Of Doped Titania Nanotubes: Preparation And Study Of Their Photocatalytic Activities

Photocatalysis has been widely used in wastewater treatment, air cleaning, sterilization, antifouling and self-cleaning materials, dye-sensitized solar cell, cosmetic, air sensor, etc. Most of the photocatalysts are semiconductors, such as TiO2,ZnO,CdS,WO3,SnO2 and Fe2O3. Among them, TiO2 has become the most important one because of its cheap, harmless, good stability and high photocatalytic activity. Comparing with TiO2 nanoparticles, TiO2 nanotubes have higher surface area, higher photocatalytic activity and stronger adsorption ability. Preparing and developing TiO2 nanotubes has become a new focus all over the world. However, TiO2 has some drawbacks. Due to the wide bandgap (3.2 eV for anatase), TiO2 can only absorb UV light. However, the UV light only accounts for 4% of solar energy, thus TiO2 can only utilize a very small part of sunlight. It has been a long-term issue for researchers how to absorb and utilize solar energy more efficiently. Furthermore, it is difficult to recycle nano TiO2, which restricts its application in many fields because of the high cost.This paper is to overcome the drawbacks of TiO2 mentioned above and prepare new efficient photocatalysts of doped TiO2 nanotubes and degrade persistent organic pollutants. New photocatalysts of Gd, N-codoped trititanate nanotubes were prepared by hydrothermal method and ion-exchanging, as well as Zr, N-codoped TiO2 nanotube arrays by two-step electrochemical method. It has mainly the following aspects:(1) Gd-doped, N-doped and Gd, N-codoped trititanate nanotubes were prepared by hydrothermal method and ion-exchanging, and their photocatalytic activities were investigated with Rhodamine B as the model pollutant. The results showed that crystallinity was the key factor for phtotcatalytic activity under UV light irradiation and the efficiency of Gd-doped trititanate nanotubes at 600℃was 1.22 times than that of the non-doped ones prepared under like conditions. However, all the factors such as crystallinity, particle size, surface area and nanotube morphology, etc. played their roles under visible light irradiation. The efficiency of Gd-doped trititanate nanotubes at 400℃was 2.78 times than that of non-doped ones under like conditions; N-doped trititanate nanotubes were prepared by ion-exchanging with NH4+ ion, which evoked a new absorption band in visible light regions; no obvious difference was found between the photocatalytic activities of Gd, N-codoped and Gd-doped nanotubes in UV regions. However, in visible light regions, synergetic action accured between Gd and N codoping, which resulted in a significant enhancement of photocatalytic activity than that with Gd-doping or N-doping nanotubes. In two hours, 78.3% Rhodamine B was degraded with Gd, N-codoped trititanate nanotubes, while 61.6% and 57.8% were degraded with Gd-doped and N-doped nanotubes, respectively.(2) TiO2 nanotube arrays were prepared on pure titanium sheet by anodization and then doped with Zr and N to improve their photocatalytic activities and to absorb solar lights, their photocatalytic activities were investigated with Rhodamine B as model pollutant. The results showed that Zr-doping enhanced the photocatalytic activity of TiO2 nanotube arrays in UV regions. Samples calcined at 600℃exhibited higher photocatalytic activities. The efficiency of Zr-doped TiO2 nanotube arrays at 600℃was 1.55 times than that of non-doped ones. However, Zr-doped TiO2 nanotube arrays couldn't absorb visible lights. Synergetic reaction occurred between Zr and N codoping, which resulted in an enhancement of photocatalytic activity in UV regions and visible light absorption as well. The codoped nanotube arrays improved the photocatalytic efficiency by 42.6% and 62.0% in UV and visible regions, respectively. This is a simple, convenient, and less time-consuming path for preparing N-doped and N, metal-codoped TiO2 nanotube arrays. Furthermore, it opens a way to preparing new materials for photocatalysis, electrodes and sensors.(3) 4, 4'-dibromobiphenyl was degraded by TiO2,Zr/TiO2 and Zr, N/TiO2 nanotube arrays and its mechanism was investigated. The results showed that 4, 4'-dibromobiphenyl was not degraded in electrochemical process and 42.4% was photodegraded under UV light irradiation in two hours. 55.6% and 88.7% was degraded with Zr, N/TiO2 nanotube arrays in photocatalytic and photoelectrocatalysis process (under bias potential of 1 V), respectively. The degradation mechanism of 4, 4'-dibromobiphenyl was to debrominate step by step and further to mineralize in the end. In conclusion, photoelectrocatalysis process was far more efficient than photocatalysis or electrochemical process alone, which provided a new path for degradation of other persistent organic pollutants.