Synthesis and characterization of the nanometric Pr-doped ceria

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Abstract

In this paper nanometric powders of solid solution of the host compound ceria (CeO2) with Pr dopant in the lattice were synthesized by self-propagating room temperature (SPRT) synthesis with composition (Ce0.9Pr0.1O2−δ). Powder properties such as specific surface area crystallite and particle size and lattice parameters have been studied. BET, TEM, X-ray diffraction (XRD) analysis and Raman scattering measurements were used to characterize the samples at room temperature. Obtained solid solution exhibits a fluorite-type crystal structure. The average crystallite size is about 3–4 nm. Williamson–Hall plots were used to separate the effect of the size and strain in the nanocrystals. It is noticed the redshift and asymmetric broadening of the Raman F2g mode which is explained with nanocrystalline nature of powders.

Color characters of solid solution depending on calcinations temperature and their position in the chromaticity diagram were studied by UV–vis spectrophotometry (diffuse reflectance).

Introduction

The study on production of red ceramic pigments with high temperature stability is of great importance to the ceramic industry [1]. At present, the following classical red pigments used on a large scale are: iron oxide (Fe2O3) encapsulated in zircon (ZrSiO4) matrix and lead oxide (Pb3O4) in tin oxide (SnO2) matrix give pale red or pink colors [2]. The red–orange pigments in the Cd(SxSe1−x)–ZrSiO4 system, and sodium urinate are toxic and unstable above 900 °C [3]. Another possibility for obtaining red pigments is to use ceria (CeO2) doped with praseodymium (Pr4+) ions that leads to a stable dye [4], [5], [6].

Today, production in the ceramic pigments is focused on high surface area powders, since this feature influences the color intensities. Thus, there is a great interest on development of nanosized pigments. Among the processes available for synthesizing nanometric ceramic powders [7], [8], [9], [10], [11], one of the methods that are cost and time effective is the self-propagating room temperature (SPRT) synthesis. SPRT procedure is based on the self-propagating room temperature reaction between metal nitrates and sodium hydroxide. This reaction is spontaneous and after being initiated it terminates extremely fast [12], [13].

In this paper we present a study on the synthesis of nanometric Pr-doped ceria by SPRT method as well as its structural and optical properties from room temperature to 600 °C.

Apart from the interest to get more fundamental knowledge on the characteristics of solid solutions under study, it is also of interest to find out if the applicability of the Pr-doped ceria in pigments will be of practical interest. It is interesting to note that the influence of particle size on color hue was not studied in detail till now.

Section snippets

Experimental

Starting reactants used in the experiments were cerium nitrate hexahydrate (Ce(NO3)3·6H2O, Merck), praseodymium nitrate hexahydrate (Pr(NO3)3·6H2O, John Mathey) and sodium hydroxide (NaOH, Zorka Sabac). Amounts of nitrates and NaOH were calculated according to the nominal composition of the Ce0.9Pr0.1O2 solid solution and reaction:2|(1–0.9)Ce(NO3)3·6H2O + 0.9Pr(NO3)3·6H2O| + 6NaOH + (½−y)O2  2Ce0.9Pr0.1O2−y + 6NaNO3 + 15H2O + 3/2O2

The chemicals were hand-mixed in alumina mortar for 5 min. After being

Results and discussion

Typical X-ray diffraction patterns for the Pr-doped ceria are shown in Fig. 1. All of them are single phase, which are independent of dopant concentration in the range investigated. Peaks related to isolated Pr-phases are not observed and all of solid solution powders exhibit the fluorite crystal structure. This high solubility may be attributed to nanometric nature of powders. XRD analysis reveals that all peaks for each sample were significantly broadened indicating small crystallite size

Conclusion

In summary, Ce1−xPrxO2−δ solid solution (x = 0.10) was prepared by SPRT method that is easy to handle and low cost. It was found that the ceria powders with Pr dopant up to 10% are solid solution with fluorite structure. Particle size lies in the nanometric range. They exhibited larger lattice parameter than polycrystalline material due to presence of Ce3+, Pr3+ ions and O2− vacancies. However, increasing temperature results in crystal growth, which led to increasing absorption and shifting the

Acknowledgements

The authors are grateful to the Ministry of Science and Technological Development of Serbia, and A. von Humboldt Foundation, Germany, for the support.

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