Elsevier

Solid State Ionics

Volume 177, Issues 33–34, 15 November 2006, Pages 2939-2944
Solid State Ionics

An XAS study of the defect structure of Ti-doped α-Cr2O3

https://doi.org/10.1016/j.ssi.2006.08.028Get rights and content

Abstract

The bulk defect structure in Cr2−xTixO3 (x = 0.05, 0.20 and 0.30) has been studied by X-ray absorption spectroscopy measurements at the Cr and Ti K-edges. The results show that the Ti is predominantly present in the IV oxidation state and resides on the normal Cr host lattice site. The dopant is charge compensated by Cr3+ vacancies and there is evidence for the formation of defect clusters; however, the detailed structure of these clusters could not be deduced.

Introduction

Ti-doped α-chromium oxide (CTO or Cr2−xTixO3) is currently being used as a material for sensors to detect trace quantities of reducing gases in air (such as CO or ethanol vapour) by changing their resistance at temperatures in the range 300–500 °C [1]. The sensing mechanism involves the trapping of electrons by chemisorption of oxygen from air on the surface of a semiconducting oxide. If the bulk oxide is n-type this reduces the bulk concentration of electrons in the conduction band, whereas if the oxide is p-type it increases the concentration of holes in the valence band. In the presence of small quantities of reducing gas in air, some of the chemisorbed oxygen is removed and the trapped electrons are returned to the bulk. Thus for an n-type oxide the conductance increases (resistance decreases) in the presence of a reducing gas, whereas for a p-type oxide the conductance decreases (resistance increases). The sensor response of CTO is typical of a p-type semiconducting oxide [2], [3]. For this mechanism to generate a useful sensitivity for sensing trace quantities of reducing gases in air, the bulk concentration of carriers must be low so that the trapping of electrons by chemisorption makes a significant difference to the bulk carrier concentration.

The p-type behaviour is somewhat unexpected as earlier work would suggest that pure Cr2O3 is a narrow band semiconductor [4] and that doping with a higher valency cation like Ti4+ would result in an increased conductivity with oxygen activity dependence characteristic of n-type donor doping (i.e. increasing with decreasing pO2) [5], [6]. However, Holt and Kofstad [7] found that although Cr2O3 doped with 2 mol% TiO2 (x = 0.04) did show n-type behaviour at 1000 °C at low pO2, there was an apparent n to p transition at pO2 = 10 atm and the conductivity in air was slightly lower than pure Cr2O3. The solubility of TiO2 is very large (sensors typically have x = 0.2) and the p-type behaviour of sensors is surprising. Two possible modes of solution of TiO2 in Cr2O3 can be considered [8] and these are represented by the equations3TiO2→3TiCr + VCr + 6OOxand2TiO2→2TiCr + 2e′ + 3OOx + (1/2)O2Eq. (1) represents the compensation of Ti4+ by Cr vacancies. In contrast, Eq. (2) represents compensation by electrons and can also be regarded as dissolution of Ti3+ depending on the degree of ionisation of the neutral defectTiCrx→TiCr + e′

Recent studies [8], using a combination of X-ray diffraction, density and electrical conductivity measurements supported by computer modelling, suggested that Eq. (1) offers the best explanation of the electrical behaviour. However, more direct information on the bulk defect structure of CTO is required.

X-ray absorption spectroscopy (XAS) is the ideal experimental method to provide direct information on local structure around the Ti dopant in Cr2O3 and hence resolve the nature of the defect structure. The technique is well established and involves the study of the absorption coefficient of X-rays across the energy range for the photoemission of a core electron (K or L shell) of the target atom [9], [10], [11], [12]. The resulting spectrum can be divided into two regions. Firstly the X-ray absorption near edge structure (XANES) which primarily provides information on the oxidation state of the atom and the local geometry. Secondly the extended X-ray absorption fine structure (EXAFS) which yields quantitative information on the atom coordination to a distance of ∼ 5 Å. In this paper we report the results of Ti K-edge XAS measurements in CTO and the ensuing conclusions on the bulk defect structure.

Section snippets

Materials

Three Cr2−xTixO3 samples were studied in this work with nominal compositions x = 0.05, 0.20 and 0.30. These were prepared by ultrasonic mixing powders of α-Cr2O3 (Alfa Aesar puratronic, 99.997%, 0.7 μm particle size) and TiO2 (Sigma Aldrich, 99.999%) for 5 min in an ethanol/water mixture. The product was then ball milled using zirconia balls for 24 h and then reacted in air for ≥ 70 h at 1000 °C. Further details of the preparation and the verification of the single-phase nature of the samples can

Results and discussion

Ti K-edge XAS spectra were collected for the three Cr2−xTixO3 samples with nominal compositions x = 0.05, 0.20 and 0.30, the anatase and rutile forms of TiO2, Ti2O3 and a Ti metal foil. Cr K-edge XAS spectra were collected for the Cr2−xTixO3 sample with x = 0.3 and the Cr2O3 standard. The EXAFS results will be described and discussed first as they provide information on the Ti atom environment.

The Cr K-edge EXAFS results for the Cr2O3 standard that provide a reference for the measurements, in terms

Conclusions

We stress that the XAS techniques employed here probe the average local environments of the Ti atoms and only dominant species will be observed. Likewise the technique will not detect differences between bulk and surface species. The unambiguous conclusions of this study are:

  • The Ti dissolves predominantly by substitution on the Cr site;

  • The Ti is present predominantly as Ti4+;

  • The dominant charge-compensating defects are Cr3+ vacancies;

  • The Ti4+ ions and Cr vacancies form defect clusters, but the

Acknowledgements

We would like to thank the staff at the Daresbury SRS, in particular Dr. S.G. Fiddy, for assistance with the EXAFS experiments.

References (20)

  • P.T. Moseley et al.

    Sens. Actuators, B, Chem.

    (1990)
  • A. Holt et al.

    Solid State Ionics

    (1994)
  • A. Holt et al.

    Solid State Ionics

    (1997)
  • A. Holt et al.

    Solid State Ionics

    (1999)
  • A.V. Chadwick

    Solid State Ionics

    (1993)
  • H. Sawada

    Mater. Res. Bull.

    (1994)
  • G. Antonioli et al.

    Nucl. Instrum. Methods Phys. Res., B Beam Interact. Mater. Atoms

    (1995)
  • D. Lützenkirchen-Hecht et al.

    Surf. Sci.

    (2003)
  • I. Ayub et al.

    Solid State Commun.

    (2002)
  • G.S. Henshaw et al.

    J. Mater. Chem.

    (1995)
There are more references available in the full text version of this article.

Cited by (44)

  • On the role of Ti aiding the protective barrier layer formation for γ′(L1<inf>2</inf>)-strengthened Co-30Ni-10Al-5Cr-2Ta-2/4Ti superalloys

    2022, Corrosion Science
    Citation Excerpt :

    Ti plays a role in the formation of chromium oxide (Cr2O3 or CoCr2O4) in Ni-based superalloys [43–46]. Based on multiple observations using techniques like X-ray absorption spectroscopy (XAS) and atom probe tomography, it has been concluded that Ti helps in accelerating the formation of chromium oxide [47]. The presence of Ti at the Cr sub-lattice site increases cation vacancies and hence, enhances the transport of cations across the layer.

  • Characterization of oxidation mechanisms in a family of polycrystalline chromia-forming nickel-base superalloys

    2021, Acta Materialia
    Citation Excerpt :

    Many of these studies have observed dissolution of Ti into chromia scales, combined with the formation of discrete TiO2 particles at the oxide-air and oxide-metal interfaces. These phenomena have been linked to the observed titanium-oxidation relationship via two proposed mechanisms; The first involves increasing ion transport rates through the bulk chromia lattice via cation vacancy creation [5,10,20,21], and the second mechanism involves faster short-circuit ion diffusion along dislocations or grain boundaries [6,22–25]. To investigate the viability of these mechanisms, it is critically important to precisely locate where Ti and other potentially detrimental trace elements segregate to within the oxide layers.

View all citing articles on Scopus
View full text