About this Journal Submit a Manuscript Table of Contents
Journal of Nanomaterials
Volume 2013 (2013), Article ID 621929, 4 pages
http://dx.doi.org/10.1155/2013/621929
Research Article

Facile Synthesis of Fe-Doped Titanate Nanotubes with Enhanced Photocatalytic Activity for Castor Oil Oxidation

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China

Received 13 December 2012; Accepted 31 December 2012

Academic Editor: Shao-Wen Cao

Copyright © 2013 Guozhu Fu et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Iron-doped titanate nanotubes were synthesized by hydrothermal method, and the photocatalytic activity was greatly enhanced by iron doping.


Followed by the discovery of carbon nanotubes, synthesis of one-dimensional (1D) nanomaterials has attracted great interest because of their exceptional electrical and mechanical properties [14]. Some inorganic 1D nanomaterials including ZnO, VOx, and TiO2 have been synthesized in recent years [58]. Among these materials, titanic compound nanotubes have stimulated particular interest. Titanic nanocrystals have been extensively studied in photocatalytic or photoelectrochemical systems and so forth [912], and fabrication of tubular structures offers an effective approach to adjust their properties, which are crucial in practical applications. For example, the photocatalytic activity of TiO2 could be enhanced by the tubular structures because of their large specific surface, which leads to a higher potential of applications in environmental purification and generation of hydrogen gas and so forth [13].

Recently, particular interest is devoted to obtain H2Ti3O7-type nanotubes synthesized by hydrothermal method [1416], and these nanotubes show excellent ion-exchange ability and photocatalytic activities and may be applied to photocatalysis, photoluminescence, and dye-sensitized solar cells [3]. However, their structures are still not well understood. The photocatalytic property is originated from the charge carriers produced by the excitation process on the particle surface, and the photocatalytic efficiency is determined by the transfer rate and recombination rate of carriers [17]. However, the carriers are usually unstable and easy to recombine. To improve the photocatalytic efficiency, the transfer rate must be enhanced and recombination rate should be reduced. Introducing other elements especially the transition metal ions into the matrix has been proved to be an effective method to improve the photocatalytic efficiency, and many reports on the doped TiO2 and their properties have been published [1719]. More recently, followed by the development of researches on one-dimensional materials, fabrication of doped nanorods and nanowires with transition metal ions has stimulated much interest due to their exclusive properties and potential applications [2022]. However, doped tubelike nanostructures have never been reported. Since the tube channel has potential advantage to provide direct conduction paths for the electrons [23], it is reasonable to believe that doped tubelike structures will induce some excellent properties for their applications.

In this communication we reported that the Fe-doped titanated nanotubes (TiNTs) can be easily synthesized via a hydrothermal process. Commercial anatase TiO2 powders (2 g) and FeCl2·4H2O (0.05 g, 0.15 g, and 0.25 g) were dispersed into an aqueous solution of NaOH (10 mol dm−3) and then moved into a Teflon-lined autoclave. The autoclave was heated in an oven at 130°C for 72 h. The precipitate was filtrated and washed with diluted HCl until pH = 7. Final products were obtained by air-drying.

The structure analysis of obtained products was investigated by an X-ray diffraction (Philips X’Pert PRO MPD) operated at 40 kV and 30 mA using Cu radiation through the 2 -range from 5 to 70 degrees. The typical XRD pattern of products with various levels of iron doping is presented in Figure 1. In all cases, several weak and broad diffraction peaks exist (positioned at 2θ = 9.9, 24, 28, and 48°), which could be assigned to the diffraction peaks of titanates such as H2Ti3O7 structure (monoclinic unit cell with , ,  nm, and °) [13]. No crystalline anatase TiO2 or ferric oxide was detected in the pattern. In addition, the height of diffraction peaks is increased by the iron doping, which illustrates that the iron doping gives rise to the increase of the crystallization. Wang et al. [17] have reported that iron doping promotes the formation of rutile phase in TiO2 particles, while the influence of iron on the crystallization of titanate has never been reported.

621929.fig.001
Figure 1: XRD pattern of synthesized sample.

Morphology of synthesized products was characterized by both field-emission scanning electron microscope (SEM, JSM-6700F) and transmission electron microscope (TEM, Hitachi H-800). Figure 2 gives the typical images of titanates with %. From the SEM micrograph shown in Figure 2(a) we can see that needle-shaped products with uniform morphology are obtained. The typical diameter and length are about 20 and 300 nm, respectively, and no particles or layered structures were observed from the image. The TEM image shown in Figure 2(b) reveals the nanotubular structures of synthesized products. A large quantities of tubular nanoparticles with uniform diameter about 20 nm were synthesized, and the length of obtained particles is about several hundred nanometers.

fig2
Figure 2: (a) SEM and (b) TEM images of synthesized products.

The energy dispersive X-ray spectroscopy (EDX) analysis recorded from the synthesized nanotubes illustrates that the characteristic peaks of Fe, Ti, and O were detected, indicating that Fe ions were successfully doped into the lattice of titanate.

Raman spectrum of synthesized nanotubes with different iron levels is shown in Figure 3. They are almost identical except for the intensity of peaks. The Raman features of synthesized nanotubes could be roughly regarded as a reflection of the six-coordinated layered titanate although the iron was doped into the titanate [3], but the exact assignment of the Raman spectra to specific active modes in layered titanates is still not well understood. In addition, these peaks are broadened and strengthened with increasing the iron levels, which reflects the split of lattice vibration modes caused by the decrease of symmetry. Raman spectroscopy is widely used to investigate the near-surface defect structure because of its surface sensitiveness, accordingly, the increase of peak height could be attributed to the increase of oxygen vacancies in the titanates. The defects promote charge transfer and efficiently separate the electrons and holes by shallow trapped electrons, which might give rise to remarkable increase of photocatalytic activity.

621929.fig.003
Figure 3: Raman spectra of synthesized sample.

Photocatalytic activities of synthesized nanotubes were evaluated by the conductometric determination method (CDM) which uses castor oil as probe reactant [24, 25]. It has been confirmed that this method is effective for estimating the catalytic activity of inorganic pigments in oil cosmetics. In the test, 0.2 g of synthesized TiNTs was mixed with 20 mL of castor oil and moved into a quartz tube with UV-light irradiation for 150 min. Air gas was bubbled in and then flowed out and induced into the deionized water placed in an electric conductivity measurement cell. Volatile molecules produced by the oxidation of the castor oil were trapped in the water by the effluent gas leading to the increase of conductivity. The degree of photocatalytic activity was estimated by the extent of conductivity change.

Figure 4 shows the photocatalytic activities of synthesized nanotubes, and we can conclude from the results that the photocatalytic activities are greatly enhanced by the iron doping. The change curve of electric conductivity for undoped TiNTs is very flat while the curve is relatively sharp for the doped ones. After irradiated for 150 min, the electric conductivity for 3% doped TiNTs is 52 μS/cm, which is about 2-fold higher than that of undoped TiNTs. These results obviously indicate that doping with iron ions is very effective in increasing the photocatalytic activity of TiNTs.

621929.fig.004
Figure 4: Photocatalytic activities of synthesized nanotubes.

The photocatalytic activity originates from the production of excited electron in the conduction band, along with corresponding positive holes in the valence band through the absorption of suitable illumination [17]. By introducing the Fe3+ in the matrix, Fe atoms replace the Ti in the crystal lattice, and the oxygen vacancies and defects increase to maintain charge equilibrium. During the process of photocatalytic reaction, oxygen vacancies and defects could become the centers to capture photoinduced electrons so that the recombination of photoinduced electrons and holes was effectively inhibited [26]. Thus, oxygen vacancies and defects were in favor of photocatalytic activity. The oxygen vacancies and defects were increased by the iron doping; consequently, the photocatalytic activity is enhanced by the level of iron doping as shown in Figure 4.

In conclusion, we have firstly reported the synthesis of the iron-doped TiNTs, and this method may be applied to synthesize transition metal ions doped TiNTs and other nanostructures (such as nanobelt). This work provides a facile route to improve the photocatalytic efficiency of materials, and other properties such as magnetic property may also be changed. In addition, existence of Ti-OH on the surface [27] makes the decoration of TiNTs possible, and further assembly may also be achieved. These works are still under research.

Acknowledgment

Financial support from the National High Technology Research and Development Program of China (863 Program) (Grants 2009AA03Z802 and 2009AA03Z803) is gratefully acknowledged.

References

  1. P. X. Gao, C. S. Lao, Y. Ding, Z. L. Wang, and Z. L., “Metal/semiconductor core/shell nanodisks and nanotubes,” Advanced Functional Materials, vol. 16, pp. 53–62, 2006. View at Publisher · View at Google Scholar · View at Zentralblatt MATH
  2. P. Hu, F. Yuan, L. Bai, J. Li, and Y. Chen, “Plasma synthesis of large quantities of zinc oxide nanorods,” Journal of Physical Chemistry C, vol. 111, no. 1, pp. 194–200, 2007. View at Publisher · View at Google Scholar · View at Scopus
  3. P. Hu, L. Y. Bai, L. J. Yu, J. L. Li, F. L. Yuan, and Y. F. Chen, “Shape-controlled synthesis of ZnS nanostructures: a simple and rapid method for one-dimensional materials by plasma,” Nanoscale Research Letters, vol. 4, pp. 1047–1053, 2009. View at Publisher · View at Google Scholar
  4. Z. Zhang, C. Shao, P. Zou et al., “In situ assembly of well-dispersed gold nanoparticles on electrospun silica nanotubes for catalytic reduction of 4-nitrophenol,” Chemical Communications, vol. 47, no. 13, pp. 3906–3908, 2011. View at Publisher · View at Google Scholar · View at Scopus
  5. P. Hu, N. Han, X. Zhang et al., “Fabrication of ZnO nanorod-assembled multishelled hollow spheres and enhanced performance in gas sensor,” Journal of Materials Chemistry, vol. 21, no. 37, pp. 14277–14284, 2011. View at Publisher · View at Google Scholar
  6. H. Yu, Z. Zhang, M. Han, X. Hao, and F. Zhu, “A general low-temperature route for large-scale fabrication of highly oriented ZnO nanorod/nanotube arrays,” Journal of the American Chemical Society, vol. 127, no. 8, pp. 2378–2379, 2005. View at Publisher · View at Google Scholar · View at Scopus
  7. P. Hu, X. Zhang, N. Han, W. Xiang, Y. Cao, and F. Yuan, “Solution-controlled self-assembly of ZnO nanorods into hollow microspheres,” Crystal Growth & Design, vol. 11, no. 5, pp. 1520–1526, 2011. View at Publisher · View at Google Scholar · View at Scopus
  8. C. C. Tsai and H. Teng, “Structural features of nanotubes synthesized from naoh treatment on TiO2 with different post-treatments,” Chemistry of Materials, vol. 18, pp. 367–373, 2006. View at Publisher · View at Google Scholar
  9. D. Li and Y. Xia, “Fabrication of titania nanofibers by electrospinning,” Nano Letters, vol. 3, no. 4, pp. 555–560, 2003. View at Publisher · View at Google Scholar · View at Scopus
  10. M. Zhang, Y. Bando, and K. Wada, “Synthesis of coaxial nanotubes: titanium oxide sheathed with silicon oxide,” Journal of Materials Research, vol. 16, no. 5, pp. 1408–1412, 2001. View at Publisher · View at Google Scholar · View at Scopus
  11. A. Hagfeldt and M. Gratzel, “Light-induced redox reactions in nanocrystalline systems,” Chemical Reviews, vol. 95, no. 1, pp. 49–68, 1995. View at Publisher · View at Google Scholar
  12. S. U. M. Khan, M. Al-Shahry, and W. B. Ingler, “Efficient photochemical water splitting by a chemically modified n-TiO2,” Science, vol. 297, no. 5590, pp. 2243–2245, 2002. View at Publisher · View at Google Scholar · View at Scopus
  13. Q. Chen, W. Z. Zhou, G. H. Du, and L. M. Peng, “Trititanate nanotubes made via a single alkali treatment,” Advanced Materials, vol. 14, no. 17, pp. 1208–1211, 2002. View at Publisher · View at Google Scholar
  14. A. Thorne, A. Kruth, D. Tunstall, J. T. S. Irvine, and W. Zhou, “Formation, structure, and stability of titanate nanotubes and their proton conductivity,” Journal of Physical Chemistry B, vol. 109, no. 12, pp. 5439–5444, 2005. View at Publisher · View at Google Scholar · View at Scopus
  15. C. C. Tsai and H. Teng, “Regulation of the physical characteristics of titania nanotube aggregates synthesized from hydrothermal treatment,” Chemistry of Materials, vol. 16, no. 22, pp. 4352–4358, 2004. View at Publisher · View at Google Scholar · View at Scopus
  16. T. Kasuga, M. Hiramatsu, A. Hoson, T. Sekino, and K. Niihara, “Formation of titanium oxide nanotube,” Langmuir, vol. 14, no. 12, pp. 3160–3163, 1998. View at Publisher · View at Google Scholar · View at Scopus
  17. X. H. Wang, J. G. Li, H. Kamiyama et al., “Pyrogenic iron(III)-doped TiO2 nanopowders synthesized in RF thermal plasma: phase formation, defect structure, band gap, and magnetic properties,” Journal of the American Chemical Society, vol. 127, pp. 10982–10990, 2005. View at Publisher · View at Google Scholar
  18. R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, and Y. Taga, “Visible-light photocatalysis in nitrogen-doped titanium oxides,” Science, vol. 293, no. 5528, pp. 269–271, 2001. View at Publisher · View at Google Scholar · View at Scopus
  19. H. Irie, Y. Watanabe, and K. Hashimoto, “Nitrogen-concentration dependence on photocatalytic activity of TiO2xNx powders,” Journal of Physical Chemistry B, vol. 107, no. 23, pp. 5483–5486, 2003. View at Publisher · View at Google Scholar · View at Scopus
  20. B. C. Cheng, Y. H. Xiao, G. S. Wu, and L. D. Zhang, “Controlled growth and properties of one-dimensional ZnO nanostructures with Ce as activator/dopant,” Advanced Functional Materials, vol. 14, no. 9, pp. 913–919, 2004. View at Publisher · View at Google Scholar
  21. B. D. Yuhas, D. O. Zitoun, P. J. Pauzauskie, R. R. He, and P. D. Yang, “Transition-metal doped zinc oxide nanowires,” Angewandte Chemie International Edition, vol. 45, no. 3, pp. 420–423, 2006. View at Publisher · View at Google Scholar
  22. J. H. He, C. S. Lao, L. J. Chen, D. Davidovic, and Z. L. Wang, “Large-scale Ni-doped ZnO nanowire arrays and electrical and optical properties,” Journal of the American Chemical Society, vol. 127, no. 47, pp. 16376–16377, 2005. View at Publisher · View at Google Scholar
  23. M. A. Khan, H. T. Jung, and O. B. Yang, “Synthesis and characterization of ultrahigh crystalline TiO2 nanotubes,” Journal of Physical Chemistry B, vol. 110, no. 13, pp. 6626–6630, 2006. View at Publisher · View at Google Scholar · View at Scopus
  24. J. Frank, J. V. Geil, and R. Freaso, “Automatic determination of oxidation stability of oil and fatty products [Food quality control, vegetable and animal fats],” Food Technology, vol. 36, no. 6, pp. 71–76, 1982.
  25. M. K. Läubli and P. A. Bruttel, “Determination of the oxidative stability of fats and oils: comparison between the active oxygen method (AOCS Cd 12-57) and the rancimat method,” Journal of the American Oil Chemists' Society, vol. 63, no. 6, pp. 792–795, 1986. View at Publisher · View at Google Scholar
  26. L. Q. Jing, X. J. Sun, B. F. Xin, B. Q. Wang, W. M. Cai, and H. G. Fu, “The preparation and characterization of la doped TiO2 nanoparticles and their photocatalytic activity,” Journal of Solid State Chemistry, vol. 177, no. 10, pp. 3375–3382, 2004. View at Publisher · View at Google Scholar · View at Scopus
  27. X. T. Zhang, Y. M. Wang, C. M. Zhang et al., “Chemical modified titanate nanotubes and their stable luminescent properties,” Science in China Series B, vol. 35, pp. 1–6, 2005.