Structural and photocatalytic properties of iron- and europium-doped TiO2 nanoparticles obtained under hydrothermal conditions

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Abstract

Iron- and europium-doped (≤1 at.%) TiO2 nanoparticles powders have been synthesized by a hydrothermal route at 200 °C, starting with TiCl4, FeCl3·6H2O and EuCl3·6H2O. The structure, morphology and optical peculiarities were investigated by means of X-ray diffraction (XRD), transmission electron microscopy (TEM), extended X-ray absorption fine structure (EXAFS), Mössbauer spectroscopy and UV–vis measurements. The photocatalytic performance was analysed in the photodegradation reaction of phenol. Rietveld refinements of XRD patterns reveal that the as-prepared samples consist in iron- and europium-doped TiO2 in the tetragonal anatase structural shape, with particle size as low as 15 nm. By means of Mössbauer spectroscopy on both 57Fe and 151Eu isotopes as well as by EXAFS analyses, the presence of Fe3+ and/or Eu3+ ions in the nanosized powders has been evidenced. It was found that iron and europium ions can substitute for titanium in the anatase structure. From the UV–vis reflection spectra, by using the transformed Kubelka–Munk functions, the band gap energy (Eg) of the hydrothermal samples has been determined in comparison with that of Degussa P-25 photocatalyst. A decrease of Eg from 2.9 eV found for Degussa photocatalyst to 2.8 eV for the titania doped with 1 at.% Fe has been evidenced, indicating a valuable absorption shift (∼20 nm) towards visible light region. However, the best photocatalytic activity in the photodegradation reaction of phenol was evidenced for the hydrothermal sample, TiO2: 1 at.% Fe, 0.5 at.% Eu, in both UV and visible light regions. The photocatalytic activities of iron-doped and iron–europium-codoped samples are high and practically the same only in visible light. The photocatalytic properties in correlation with the structural and optical peculiarities of the hydrothermal samples are discussed.

Introduction

Titanium dioxide (titania) is a cheap, nontoxic and highly efficient photocatalyst being extensively applied for the degradation of organic pollutants, air purification, water splitting, and reduction of nitrogen to ammonia [1], [2], [3], [4], [5], [6], [7], [8], [9], [10]. However, only a small UV fraction of solar light (<5%) can be utilised because of large band gap (∼3 eV) of titanium dioxide semiconductor structure. Recently, a considerable number of studies were devoted to the development of efficient visible light sensitive photocatalysts and to photocatalytic properties improvement [11], [12], [13], [14], [15]. Transition metal selective doping is one of the common approaches to extend the spectral response of titania to the visible light region. In particular the iron doping was found to increase the photocatalytic activity up to 2.5 times [12]. Investigation on chloroform photodegradation revealed a significant photocatalytic reactivity increase over nanocrystalline TiO2 codoped with Fe3+ and Eu3+ by sol–gel method, as compared with undoped or monodoped TiO2 nanoparticles [16].

It is already established that material properties depend strongly on precursors and synthesis methods in correlation with the thermodynamic process parameters. For the synthesis of nanoparticle systems the hydrothermal method was intensively utilised in the last decade [17], [18], [19], [20], [21]. However, no reports on the hydrothermal synthesis of iron- and europium-codoped TiO2 materials have been published, by our knowledge.

It is the aim of this work to present the hydrothermal synthesis of iron- and europium-doped and -codoped TiO2 nanoparticle systems, their microstructure, morphology and catalytic properties in the photodegradation of phenol, in both UV and visible light region.

Section snippets

Hydrothermal synthesis

Fe3+- and Eu3+-doped and -codoped nanocrystalline titania samples have been synthesized by a hydrothermal route, starting with titanium (IV), iron (III) and europium (III) chlorides in solution. Titanium tetrachloride has been obtained by air oxidation under vigorous stirring (30 h) from a 15% titanium trichloride in hydrochloric acid solution. Europium trichloride has been prepared by dissolving the corresponding europium oxide amount in 2N hydrochloric acid. A 25% ammonium hydroxide solution

X-ray diffraction

Primary structural information was given by XRD patterns of the hydrothermal samples (Fig. 1a–d).

All spectra display the characteristic patterns of TiO2 in tetragonal anatase phase (the anatase characteristic lines are indexed in Fig. 1a). No relevant differentiation can be observed by changing the doping element (Fig. 1b–d) except for small line intensity variations and a slight increase of diffraction line widths from undoped (Fig. 1a) to codoped (Fig. 1d) samples. Rietveld refinement of XRD

Conclusions

Iron- and europium-doped TiO2 nanoparticles were obtained by a hydrothermal route, at mild temperature and pressure (∼200 °C and ∼15 atm, for 1 h). Rietveld refinements of the XRD patterns reveal the exclusive presence of iron- and europium-doped anatase phase in hydrothermally synthesized samples; the particle mean size was less than 15 nm and the morphology was found to depend on doping element. EXAFS analysis strongly support that both Fe3+ and Eu3+ ions enter the TiO2 lattice, by substituting

Acknowledgements

This work was supported by the Romanian Ministry of Education and Research through Contract CEEX-Matnantech no. 23/2005. We gratefully acknowledge the valuable assistance of Dr. Edmund Welter and Dr. Dariusz Zajac (HASYLAB) during the EXAFS experiments, as well as the whole scientific support of Prof. Eberhardt Burkel and Dr. Radu Nicula (University of Rostock).

References (30)

  • J. Arana et al.

    Appl. Catal. B: Environ.

    (2001)
  • J. Bandara et al.

    Appl. Catal. B: Environ.

    (2001)
  • A. Di Paola et al.

    Catal. Today

    (2002)
  • P. Fernandez et al.

    Catal. Today

    (2005)
  • S. Sakthivel et al.

    Appl. Catal. B: Environ.

    (2006)
  • W.J. Zhang et al.

    Chem. Phys. Lett.

    (2003)
  • R.S. Sonawane et al.

    Mater. Chem. Phys.

    (2004)
  • M. Kitano et al.

    Appl. Catal. A: Gen.

    (2007)
  • P. Yang et al.

    Mat. Lett.

    (2002)
  • Y.V. Kolen’ko et al.

    Appl. Catal. B: Environ.

    (2004)
  • L. Diamandescu et al.

    Appl. Catal. A: Gen.

    (2007)
  • W.G. Zhang et al.

    Mater. Chem. Phys.

    (2007)
  • J. Yu et al.

    Appl. Catal. B: Environ.

    (2007)
  • S. Zhu et al.

    Phys. B

    (2007)
  • A. Sobczynski et al.

    J. Mol. Catal. A: Chem.

    (2004)
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