Preparation of titania nanotubes doped with cerium and their photocatalytic activity for glyphosate
Introduction
Heterogeneous photocatalysis based on semiconductor oxides especially TiO2 has been proven to be a promising technology for the destruction of toxic organic contaminants in air and water. However, titania can only operate under UV light irradiation because of its wide bandgap (3.2 eV for anatase). Moreover, the recombination of the photogenerated electron–hole pairs takes place quickly on a time scale of 10−9 to 10−12 s. In order to improve the photocatalytic activity and utilize visible light, the development of highly efficient and visible light responsive photocatalyst is demanded [1].
In recent years, photocatalyst modification techniques have been extensively investigated, including noble metal loading [2], transition metal ion doping [3], non-metal doping [4], and dye sensitization [5], metal and non-metal codoping [6], etc. and some of them have been proved to be useful. Enhanced photocatalytic activities and redshift of photo-response have been observed for rare earth metal doped titania [7], [8]. Moreover, TiO2 nanotubes have attracted a lot of interest due to their stability and large specific surface area. Compared with any other form of TiO2, many recent studies have reported that TiO2 nanotubes improved properties for various applications in photocatalysis [9], sensing [10] and photovoltaics [11].
Although various noble metals [9], [12], transition metal ions [13], and non-metal [14], [15] doped TiO2 nanotubes have been successfully prepared, there have been a few reports on synthesizing rare-earth metal doped TiO2 nanotubes. According to our knowledge, there are reports only about rare-earth metal europium [16], lanthanum [17] and neodymium [18] doped TiO2 nanotubes. Furthermore, many attempts have been made to the preparation and characterization of doped TiO2 nanotubes. However, only a few cases, the photocatalytic property of doped TiO2 nanotubes was also investigated [18]. Rare earth oxides are found to have polymorph, good thermal stability, etc. due to their f electron and multi-electron configuration. Many studies have shown that Ce doping enhanced the photocatalytic activity of TiO2 [19], [20], so we tried to improve the photocatalytic activity of TiO2 nanotubes by doping cerium. For the reasons given above, TiO2 nanotubes were first fabricated through hydrothermal treatment method, and then a series of Ce–TiO2 nanotubes were prepared by impregnation method. In order to understand the mechanism, Ce–TiO2 nanotubes were characterized with TEM, XRD, DRS, FT-IR, BET and XPS. Moreover, the photocatalytic activity of Ce–TiO2 nanotubes for the degradation of broad-spectrum herbicide glyphosate, which is the most widely used in the world [21], was presented in this paper.
Section snippets
Preparation of titanic acid nanotubes
The hydrothermal treatment of rutile-phase TiO2 powder, which was obtained through annealing P25 TiO2 at 800 °C for 1 h, with 10 mol l−1 NaOH aqueous solution in a Teflon vessel at 130 °C was carried out for 24 h, and then cooled to room temperature in air. The as-prepared precipitates were neutralized, and subsequently dispersed in a 0.1 mol l−1 HNO3 aqueous solution. After ultrasonication for 30 min in the above acid solution, the precipitate was washed continually with distilled water to pH 7, and
XRD analysis
Fig. 1 shows XRD patterns of TiO2 hydrothermally treated and then calcined at 400 °C and 0.15% Ce–TiO2 nanotubes prepared at different calcination temperatures. The X-ray diffraction peaks at two-theta angle of 24.4°, 28.3° and 48.5° are ascribed to the characteristic peak of titanic acid (H2Ti2O5·H2O), so the sample prepared with hydrothermal treatment is titanic acid nanotubes [22]. For TiO2 nanotubes annealed at 400 °C (Fig. 1a), the sharp peak at 25.3° and the small peak at 27.5° correspond
Conclusions
0.15% Ce–TiO2 nanotubes calcined at 400 °C are 5–8 nm diameter with the lengths in the range of 50–200 nm. For 0.15% Ce–TiO2 nanotubes, the amount of OH− groups and the average pore size increased, and SBET decreased with increasing calcination temperature. Cerium doping in TiO2 nanotubes caused a redshift in the absorption spectra, and the light absorption increased with increasing cerium content in the range of visible light. Both Ce3+ and Ce4+ coexisted in Ce/TiO2 nanotubes, and cerium species
Acknowledgements
The authors would like to thank Guangdong Science & Technology Development Foundation (2008B030303027) and the Key Academic Program of the 3rd phase “211 Project” of South China Agricultural University (2009B010100001).
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