Electrochemical fabrication of lanthanum-doped TiO2 nanotube array electrode and investigation of its photoelectrochemical capability
Introduction
In recent years, titanium dioxide (TiO2) has been extensively studied and applied as a photocatalyst because of its environmentally friendly, low cost, non-toxicity, and high photocatalytic (PC) activity [1]. Accordingly, self-organized and vertically oriented TiO2 nanotube arrays formed by electrochemical anodization, first synthesized by Grimes and co-workers in 2001 [2], have been regarded as the most promising materials for PC oxidation technology due to their unique properties such as highly ordered array structure, high specific surface area, outstanding mechanical, chemical stability and eminent charge-transport properties [3]. As a result, TiO2 nanotubes have attracted much attention in fields such as photocatalysis [4], water photoelectrolysis [5], gas sensors [6], environmental purification [7], and dye-sensitized solar cells [8]. Unfortunately, the applications of TiO2 nanotubes have found that the high recombination rate of photogenerated electron–hole pairs and wide band gap (3.2 eV for anatase and 3.0 eV for rutile) were the main disadvantages that limited the PC activity and visible light response of TiO2 nanotubes [9], [10]. To improve PC activity of TiO2 under UV irradiation and extend the photo response of TiO2 further into visible light region, many attempts have been reported, including doping with non-metals such as B, N, C, F [9], [10], [11], [12] or metal ions such as Fe, Ag, Pt [13], [14], [15], or coupling it with other narrow band gap semiconductors [16], [17], [18], [19]. In addition, the development of PEC oxidation process with an externally applied anodic bias has also been receiving attention in recent years, when a bias potential is introduced into the PC process, the recombination of electron–hole pairs is reduced. Some literatures had proved that La doping could efficiently stabilize the mesostructure of TiO2 with crystallized walls and improve its photoactivity [20], [21], [22]. Moreover, incorporation of lanthanide ions into TiO2 matrix could also promote the chemical adsorption of the organic substrates on the photocatalyst surface, which also benefited the improvement of its PC efficiency [20], [21], [22]. Most researches focused on the La-doped TiO2 powders, however, the energy conversion efficiency was relatively low and it was troublesome to separate and recycle the composite powders from the reaction system. The La-doped TiO2 nanotube for its good mechanical adhesion strength, electronic conductivity and great surface area for photoelectrochemical action, may provide a better system to overcome the above problems. In this work, we have successfully prepared La-doped TiO2 nanotube arrays by electrochemical process with improved PEC activity. To the best of our knowledge, however, no report has been found so far regarding the preparation of La doping TiO2 nanotube arrays using an electrochemical approach and the investigation of its PEC characteristic.
The purpose of this study was to investigate the structural characteristics and surface morphology of the La-doped TiO2 nanotube array electrode and to measure its PEC capability. Although as a hazardous waste and typical toxic organic pollution, PNP is considered as one of the priority toxic pollutants by the United States Environmental Protection Agency [23], it has been used extensively as raw materials in the manufacture of fungicides, pesticides, dyes and pharmaceuticals [24]. At present, there have been some methods for degradation of PNP including biodegradation [25], PC destruction [26], microwave assisted oxidation [27], electrocatalytic oxidation [28] and PEC degradation [29]. Thus, we chose PNP as the probe substance to verify the PC and PEC behavior of the La-doped TiO2 nanotube array electrode. The doping nanotube arrays showed a significant enhancement in their photoelectrochemical and PEC properties compared with undoped nanotubes fabricated and tested under the same conditions. Compared with biological reactions and strong oxidants processes such as H2O2 [30] and Photo-Fenton regents [31], our electrodes were relatively high effective, cost-effective and environmentally benign.
Section snippets
Preparation of TiO2 nanotube arrays
Titanium foils (100 μm thick, 99.6% purity, from Sigma–Aldrich) were ultrasonically cleaned in acetone, isopropanol and double-distilled water for 5 min in turn prior to anodization. These Ti foils were anodized at a constant potential of 60 V for 2 h in a two-electrode electrochemical cell in an ethylene glycol solution of 0.3 wt% ammonium fluoride and 2 vol% water using graphite as the counter electrode at room temperature. The distance between the two electrodes was maintained at 3 cm. After
Morphology of La-doped TiO2 nanotube arrays
Fig. 1A and B shows the top-views of the TiO2 and La-doped TiO2 nanotube arrays, respectively. Both of the two nanotube arrays show highly ordered and vertically oriented morphologies with inner diameter of approximately 85 nm and the wall thickness of around 20 nm, indicating that the process of La doping do not destroy the structure of the nanotubes. The inset gives the cross-section image of La-doped TiO2 nanotube arrays, and the resulting nanotube array length is about 6 μm.
XPS analysis
Fig. 2A shows XPS
Conclusions
Highly ordered vertically oriented La-doped TiO2 nanotube arrays were successfully prepared by a facile electrochemical approach. The photoelectrochemical properties of La-doped TiO2 nanotube array electrode were studied and the photocurrent was dramatically enhanced. A remarkable and maximum photoconversion efficiency of 0.598% was achieved. It was conclusively shown that the PEC degradation efficiency of PNP on La-doped TiO2 nanotube arrays electrode was higher than the undoped one. According
Acknowledgments
This work was supported by the National Natural Science Foundation of China (Nos. 20927007, 21175094) and Youth Foundation of Sichuan University (No. 2010SCU11048).
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