Influence of Fe3+-doping on optical properties of CeO2−y nanopowders

doi:10.1016/j.ceramint.2012.11.087

Ceramics International

Volume 39, Issue 5, July 2013, Pages 4929-4936

https://doi.org/10.1016/j.ceramint.2012.11.087 Get rights and content

Abstract

Ce_1−xFe_xO_2−y (0≤x≤0.05) nanopowders were synthesized using hydrothermal method at low calcination temperature and low doping regime. Structural and morphological characterization has been carried out by the X-ray diffraction method and non-contact atomic force microscopy. Vibrational properties were investigated by Raman spectroscopy. It was observed that the content of oxygen vacancies increased significantly with Fe doping up to 3 mol%. For higher dopant concentration, phase separation was detected. The optical properties of pure and Fe³⁺-doped CeO_2−y samples were investigated by spectroscopic ellipsometry. Several analytical models were applied to analyze the optical absorption onset of ceria defective structure. It was found that, Cody–Lorentz model most suitably described the sub-band gap region of CeO_2−y nanopowders and consequently gave more accurate band gap values, which are closer to the direct band gap transitions than to the indirect ones. The increased content of localized defect states in the ceria gap and corresponding shift of the optical absorption edge towards visible range in Fe-doped samples can significantly improve the optical activity of nanocrystalline ceria.

Introduction

Ceramic materials based on cerium oxide (ceria, CeO₂) have been a very active research field in the area of photocatalytic technologies such as production of oxygen trough water decomposition [1] and photodegradation of toluene in gas phase [2], methylene blue [3], acidic black 10b [4] and acid orange 7 [5] industrial dyes. The recent study of Ji et al. [5] have revealed that nanophased CeO₂ is more efficient photocatalyst in the visible region for dye wastewater treatment than the commercial TiO₂ [5]. Doping with transition metals, such as Fe, enhances the oxygen mobility and oxygen storage capacity of ceria lattice [6] which can improve photocatalytic properties of CeO₂ nanomaterials [3]. Also Fe-doped ceria nanoparticles were used for pigmented ultraviolet filter applications [7]. The optimization of photocatalytic technologies which utilize solar radiation is directed towards the development of ceria materials with precisely controlled position of the absorption edge and tunable electronic band structure.

Even though extensive research has been performed on the optical properties of ceria based nanomaterials, still a lot of controversy exists in the literature regarding the nature of the optical band gap transition (E_g) which corresponds to the O 2p→Ce 4f transition. Guo et al. [8] reported the optical band gap values of 3.6 eV and 3.3 eV for direct and indirect transitions respectively, in a case of bulk (crystalline film) ceria sample. A certain number of authors [9], [10], [11], [12] have calculated the optical band gap of various ceria based materials, treating it as a direct transition. Contrary to them, a group of other authors [13], [14], [15], [16], [17] ascribed the optical band gap of CeO₂ to the indirect transition. Independent measurements performed on oxygen deficient ceria structures using non optical techniques, gave the values of 3.3 eV [18], and 3.4–3.6 eV [19] without defining the type of the transition. The present uncertainty in the band gap calculations of nanostructured CeO_2−y increases a need for better explanation of its electronic properties using sophisticated analytical models which incorporate nanosized ceria defective structure.

The decrease of the particle size to nanodimensions leads to the increase of the band gap energy in majority of materials due to the quantum confinement effects [20]. In a case of nanoceria an anomalous behavior in the band gap energy was observed for the grain size less than 20 nm [9]. The decrease of the band gap energy with grain size decrease can be explained by a transformation from the crystalline into the amorphous state when crystals become of nanodimension [9] and increase in the number of defects within the band gap [16]. All these nano-size related effects are reflected in the spectral dependence of the complex dielectric function which is, through its imaginary part (ε₂(E)), related to the energy-band structure. Being a nondestructive and very sensitive technique, spectroscopic ellipsometry (SE) represents a very convenient method for precise and direct determination of the complex dielectric function of nanostructured materials from which important information about materials electronic structure can be obtained.

The aim of this work was to investigate the influence of Fe³⁺-doping on the optical properties and band gap behavior of CeO_2−y nanopowders. We applied different ellipsometric models in order to deduce which model describes best the electronic properties of nonstoichiometric pure and Fe³⁺-doped CeO_2−y.

Section snippets

Experiment

Non-stoichiometric nanopowders of pure CeO_2−y (coefficient y denotes the oxygen deficiency and for small particles ranges from 0–0.2 [21], [22]) and CeO_2−y doped with 1 mol%, 3 mol% and 5 mol% of trivalent iron ions were synthesized by the hydrothermal method using NH₄OH solution and relatively low calcination temperature (200 °C). Detailed preparation procedure is published elsewhere [23].

The X-ray diffraction (XRD) patterns were collected using Rigaku RINT2200 powder diffractometer. Non-contact

Analytical models

From the measurements of ellipsometric angles Ψ and Δ, by using the two-phase model approximation (CeO_2−y nanoparticles/air), we have calculated the pseudo-dielectric function spectra directly from the complex reflectance ratio ρ(E)=tan Ψexp(iΔ) using the relation [24] $〈 ε (E) 〉 = \sin^{2} θ_{i} [1 + t g^{2} θ_{i} {(\frac{1 - ρ (E)}{1 + ρ (E)})}^{2}],$ where θ_i is the incidence angle. Measurement of pseudo-dielectric function enables direct determination of real and imaginary part of the complex dielectric function spectrum ε(E)=ε₁(Ε)+iε₂(Ε).

Results and discussion

In Fig. 1 are given the XRD spectra of pure and Fe³⁺-doped CeO_2−y samples. All synthesized samples have fluorite type crystal structure of cerium dioxide and no secondary phase was detected. Characteristic Miller indices are denoted for each diffraction peak.

Significant broadening of the peaks indicates that investigated powders consist of nanometric size crystallites. The crystallite size D of the samples was estimated from XRD data by taking into account strain contribution, using the

Conclusion

The hydrothermal method has been employed to synthesize ceria nanopowders doped with iron. XRD analysis showed that all investigated samples have fluorite cubic crystal structure with particle size in nanometric range, confirmed by NC-AFM results. From Raman spectroscopy measurements we have observed an increase of oxygen vacancies concentration with increasing amount of Fe up to 3 mol%. In the Raman spectrum of Ce_0.95Fe_0.05O_2−y sample, the formation of iron oxide wüstite (FeO) phase was

Acknowledgment

This work was financially supported by the Serbian Ministry of Education and Science under the Projects ON171032 and III45018, Swiss National Science Foundation through Grant IZ73Z0-128169 and Serbian-Spanish bilateral project through Grant AIB2010SE-00160.

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Influence of Fe3+-doping on optical properties of CeO2−y nanopowders

Abstract

Introduction

Section snippets

Experiment

Analytical models

Results and discussion

Conclusion

Acknowledgment

Journal of Alloys and Compounds

Materials Letters

Applied Catalysis B

Applied Catalysis B

Solid State Sciences

Journal of Physics and Chemistry of Solids

Journal of Materials Science and Engineering

Physica B

Surface Science

Surface Science

Solar Cells

Acta Metallurgica

Solid State Communications

Physica B

Journal of Physics and Chemistry of Solids

The photocatalytic oxidation of water to O2 over pure CeO2, WO3, and TiO2 using Fe3+ and Ce4+ as electron acceptors

Applied Catalysis B

Synthesis, characterization and computational study of nitrogen-doped CeO2 nanoparticles with visible-light activity

Physical Chemistry Chemical Physics

Synthesis and characterization of Fe doped CeO2 nanoparticles for pigmented ultraviolet filter applications

Journal for Nanoscience and Nanotechnology

A spectroscopic–ellipsometry study of cerium dioxide thin films grown on sapphire by rf magnetron sputtering

Journal of Applied Physics

Microstructure-property relationships in nanocrystalline oxide thin films

Ionics

Preparation of monodispersed nanocrystalline CeO2 powders by microwave irradiation

Chemical Communications

Morphology-controllable synthesis of mesoporous CeO2 nano- and microstructures

Chemistry of Materials

Influence of Fe³⁺-doping on optical properties of CeO_2−y nanopowders

The photocatalytic oxidation of water to O₂ over pure CeO₂, WO₃, and TiO₂ using Fe³⁺ and Ce⁴⁺ as electron acceptors

Synthesis, characterization and computational study of nitrogen-doped CeO₂ nanoparticles with visible-light activity

Synthesis and characterization of Fe doped CeO₂ nanoparticles for pigmented ultraviolet filter applications

Preparation of monodispersed nanocrystalline CeO₂ powders by microwave irradiation

Morphology-controllable synthesis of mesoporous CeO₂ nano- and microstructures