Elsevier

Thin Solid Films

Volume 520, Issue 4, 1 December 2011, Pages 1348-1352
Thin Solid Films

X-ray absorption fine structure investigations on heat-treated Cr-doped titania thin films

https://doi.org/10.1016/j.tsf.2011.04.124Get rights and content

Abstract

Chromium-doped titanium oxide thin films were investigated in the as-deposited state and after thermal treatment (723 K for 3 h in air). X-ray diffraction data revealed an improvement in film crystallinity induced by the thermal treatment. Extended X-ray absorption fine structure data revealed similar atomic neighboring around Cr atoms in both as-deposited and annealed samples. A lattice contraction of ~ 2% is observed in the annealed samples. The 67% enhancement of the amplitude of the Cr 1 s X-ray absorption fine structure pre-edge peak after thermal treatment, which is a sign of “dipole-forbidden” 1 s  3 d transitions, suggests strong alteration in the number of Cr 3 d vacancies, in spite of similar Cr local environment in the two kinds of investigated samples. We discuss here the Cr+  Cr4+ and Cr2+  Cr6+ changes induced by thermal treatment, and/or the evolution in local structures without inversion center.

Refractive index dispersion spectra in the visible wavelength domain allowed us to compute the values of the dispersion energy, the single-oscillator energy and the coordination number of Ti atoms in both as-deposited and annealed samples.

Introduction

Transition metal-doped titania is nowadays a class of materials with promising applications in photocatalysis, gas sensing, diluted magnetic semiconductors etc. [1], [2], [3], [4], [5], [6]. The technological interest has been the main stimulating factor for the basic research of these materials. Important information on the local environment around a low-concentration element in a matrix can be derived, among others, from the non-destructive Extended X-ray Absorption Fine Structure (EXAFS) technique [7]. The X-ray absorption near-edge structure (XANES) and EXAFS techniques are based on the changes in the absorption spectrum of the material obtained by gradually increasing the incident X-ray photon energy in a synchrotron facility.

After complete absorption of a photon (whose energy range is chosen to include the absorption edge of the investigated element in the material), an inner photoelectron is ejected from the absorbing atom, thus leading to occurrence of an inner hole in the atom. As a result, a sharp increase in the absorbance of the studied material (the edge) is observed. The interaction between the photoelectron and the surrounding atoms results in interference between the propagated and reflected waves. The desired information on the coordination environment of the central excited atom could be derived from the oscillating part of the absorption spectrum, following the edge.

The aim of this paper is to understand the structural modifications in amorphous Cr-doped titania thin films, induced by thermal treatment. A study on the alteration of the optical constants induced by annealing was also performed, and the coordination number of titanium atoms was evaluated by applying the single-oscillator model.

Section snippets

Experimental details

Chromium-doped titania thin films were obtained on glass substrates (microscope slides) by DC reactive sputtering, in an O2/Ar mixture, by using three sputtering targets (two Ti discs and one Cr disc, 50 mm in diameter each). The samples were prepared without additional substrate heating (during sputter deposition, the substrate temperature reached about 100 °C).The total pressure has been set at 0.35 N/m2, the partial pressure of the oxygen being of 0.10 Pa. The film thickness (240 nm) was measured

Results and discussion

From the XRD patterns (Fig. 1), very weak signals corresponding to anatase A(101) and rutile R(110) peaks for an as-deposited sample can be noticed. The heat treatment led to a visible increase in the degree of crystallinity of chromium-doped sample, favoring the development of larger anatase crystallites (11.33 ± 0.27 nm) in comparison with the rutile ones (5.76 ± 0.51 nm). Crystallite sizes were calculated by using Eq. (1), where the maxima in the XRD plots were deconvoluted with two Lorentzian

Conclusions

Chromium-doped titanium oxide thin films, deposited by DC reactive sputtering, on unheated glass substrates, present an amorphous structure. After heat treatment, a mixed anatase/rutile structure developed (crystallites size under 12 nm), a large amount of the material still remaining in the amorphous state. XANES and EXAFS techniques, used to derive information on the local structure around chromium atoms present in trace concentration (2 at.%) in the as-deposited and annealed sputtered TiO2

Acknowledgements

One of the authors (D. Mardare) is grateful to Professor F. Levy from the Institute of Applied Physics of the Polytechnic Federal School of Lausanne, Switzerland for providing the necessary laboratory facilities to carry out a part of this investigation. We acknowledge qualified support from Dr. Edmund Welter and Dr. Dariusz Zajac from the Hasylab staff.

This work was supported by CNCSIS Contract PCCE-ID_76/2010 and by grant POSDRU/89/1.5/S/49944.

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