Elsevier

Applied Surface Science

Volume 280, 1 September 2013, Pages 523-529
Applied Surface Science

Nitrogen doped TiO2 nanotube arrays with high photoelectrochemical activity for photocatalytic applications

https://doi.org/10.1016/j.apsusc.2013.05.021Get rights and content

Highlights

  • Well aligned and highly ordered TiO2 nanotube arrays (TNAs) were successfully prepared via anodization method.

  • N-doped TiO2 nanotube arrays (N-TNAs) were achieved by immersing TNAs into the ammonia aqueous solution.

  • Photocurrent and photocatalytic activity of N-TNAs annealed at 500 ̊C have the best photocatalytic activity.

  • N-TNAS show obvious response to visible light, higher photocatalytic activity and photoelectrochemical activity.

  • The photocatalytic mechanism of organic pollutants degradation (MO) was discussed based on our experiment.

Abstract

Nitrogen doped TiO2 nanotube arrays (N-TNAs) were prepared by immersing TNAs in 1 M NH3·H2O solution and then annealing in different temperatures. The morphology, structure and composition of the N-TNAs were characterized by field-emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and UV–vis spectroscopy, respectively. Effects of annealing temperatures on structure, photocatalytic properties, and the crystal structure transformation process of the N-TNAs were discussed. Photocatalytic properties of the N-TNAs were evaluated in term of the degradation of methyl orange (MO) under UV light and visible light, and the photocurrent of N-TNAs were tested by electrochemical workstation. The XPS results showed that the N-TNAs were achieved by interstitial doping and substitutional doping, and the FESEM results showed the morphology was not changed after doping process. Compared with the pure TNAs, the N-TNAs annealed at 500 ̊C for 2 h with a mixed phase of anatase and rutile exhibited higher photocatalytic degradation activity to MO. Furthermore, the photocatalytic mechanism of organic pollutants degradation (MO) was discussed based on our experiments.

Graphical abstract

Photocurrent spectra of as-prepared TiO2 nanotube arrays and N-doped TiO2 nanotube arrays. A pure TiO2 nanotube arrays without annealing, b pure TiO2 nanotube arrays annealed at 500 ̊C; N-doped TiO2 nanotube arrays annealed at different temperatures, c 300 ̊C, d 400 ̊C, e 500 ̊C, f 600 ̊C, g 700 ̊C. The inset shows the photocurrent versus annealing temperature of N-doped TiO2 nanotube arrays.

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Introduction

Nanostructured materials, especially the highly ordered nanotube materials, have attracted a great deal of attention in various fields. In recent decades, the preparation of nanomaterials has diversified with the development of science and technology. Advances in the nanoscale technology [1] facilitated the fabrication of highly ordered and multidimensional structured materials. Many efforts have focused on new synthesis methods and photoelectrochemical properties of the tubular structure, large specific surface area, oriented charge transfer channel and the other distinct properties. TiO2 nanotube arrays as nanostructure semiconductor compound have attracted increasing research interests in photocatalysis [2], [3], [4], [5], dye sensitized solar cells [6], [7], gas sensors [8], [9], biomedical applications [10] and so on. Particularly, TNAs are expected to exhibit better photocatalytic properties compared with nanoparticles or other forms of titanium dioxide [11], due to their high specific surface area, short diffusion path and high activity in the band-edge positions, which make it more suitable to be used as catalyst [12].

Consequently, the synthesis or modification of TiO2 nanotube arrays have been widely studied [13], [14], and considerable efforts of fabrication TiO2 nanotubes such as hydrothermal treatment [15], template-deposition [16], sonoelectrochemical method [17] and anodic oxidation [18], [19] have been developed. Gong and co-workers [20] pioneered the synthesis of vertically ordered TiO2 nanotube arrays up to 500 nm in length by a potentiostatic electrochemical anodization of titanium in hydrofluoric acid aqueous electrolyte. The experimental process is very convenient without any complex apparatus. Subsequently, various organic electrolytes including dimethyl sulfoxide [21], formamide [22] and ethylene glycol [23] have been adopted to fabricate TiO2 nanotube arrays with greatly extended length.

Though TNAs as photocatalysts were firstly used in environmental applications [24], many challenges still remain such as the TNAs could not absorb visible light (λ > 387 nm) of the solar spectrum efficiently because of their large band gap (3.2 eV) as well as the recombination of photogenerated electrons and holes. In order to overcome these disadvantages, considerable efforts have been made to modify TNAs in order to reduce the band gap. In the present case, many transition metal ions [25], [26] and nonmetal ions [27], [28], [29], [30] have been studied to increase the visible light absorption or suppress the recombination of photogenerated electron–holes. Asahi et al. [31] investigated a visible-light photocatalysis in nitrogen-doped titanium oxides by sputtering the TiO2 target in a N2/Ar gas mixture. Tokudome and co-workers [32] reported nitrogen-doped TiO2 nanotubes by a wet process. In this paper, nitrogen doped TiO2 nanotube arrays (N-TNAs) were fabricated by immersing TNAs in ammonia aqueous solution following with annealing in air atmosphere. Effects of annealing temperature on the photocatalytic performance of N-TNAs were investigated.

Section snippets

Preparation of nitrogen doped TiO2 nanotube arrays

Highly ordered TiO2 nanotube arrays were fabricated by anodization method. Titanium foil was anodized in ethylene glycol electrolytes containing 0.3 M ammonium fluoride and 2 vol% water with potential of 60 V for 6 h. The as-prepared TNAs samples were immersed in 1 M NH3·H2O solution for 10 h and then annealed in a tube furnace for 2 h at different temperatures with heating and cooling rates of 2 ̊C/min.

Characterization of N-TNAs

The surface morphologies of samples were observed using the field-emission scanning electron

The morphology of TNAs and N-TNAs

SEM morphologies of typical TNAs are shown in Fig. 1. The TNAs are well-aligned with average diameter of 140 nm, wall thickness of 10 nm, and length of 30 μm. The morphologies of N-TNAs are shown in Fig. 2. N-TNAs annealed at 500 ̊C are shown in Fig. 3. Both undoped and the N-doped TiO2 nanotube arrays annealed at 500 ̊C show similar morphologies with the as-fabricated TNAs. These results indicate no significant effect of annealing on surface morphology and microstructure of the TNAs. However, the

Conclusions

In this study, TiO2 nanotube arrays were successfully prepared by anodization. N-doped TiO2 nanotube arrays were then synthesized by immersing the TNAs into the ammonia aqueous solution. The XPS characterization results showed that the N-TNAs were mainly achieved through substitutional doping and interstitial doping. And the morphology was not changed after annealing and doping process. Photoelectrochemical experiment showed that photocurrent and photocatalytic activity of N-TNAs strongly

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (Nos. 91023030, 51272062, 51128201 and 51202052), Natural Science Foundation of Anhui Province (No. 1308085QE74), the Specialized Research Fund for the Doctoral Program of Higher Education (No. 20100111110012) and the International Scientific and Technological Cooperation Project of Anhui Province (No. 10080703017).

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