Synthesis and characterization of nanocrystalline cerium oxide powders by two-stage non-isothermal precipitation

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Abstract

Nanocrystalline CeO2 particles were firstly prepared by two-stage non-isothermal precipitation, i.e. precipitating at 70 °C and aging at another temperature. Experimental results showed that the intermediates at the end of precipitation stage were needle-like mixtures of Ce3+–Ce4+ compounds. The subsequent aging temperature played an important role on the shape and size of final products. As the aging temperature suddenly reduced to 0 °C, the resultant particles retained their original needle-like structure via the topotactic mechanism, which cannot be obtained by isothermal precipitation. As raising the aging temperature above 50 °C, the products were hexagonal and grown up with increasing temperature via the dissolution–recrystallization mechanism. Moreover, all products were cubic-fluorite structured CeO2 with negligible Ce3+ content. As compared to the nanohexagons (aged at 90 °C), the nanoneedles (aged at 0 °C) exhibited an unordinary red shift in the UV absorption and possessed a smaller bandgap energy.

Introduction

Cerium oxide (CeO2) is a material with fairly wide bandgaps and specific chemical reactivity, which can provide excellent performances in the fields of catalysts [1], chemical–mechanical polishing (CMP) [2], oxygen gas sensors [3], solid oxide fuel cells [4], UV absorbents [5] and so on. As reported, various techniques based on the chemical wetness routes have been extensively used to prepare CeO2 nanoparticles, such as hydrothermal [6], [7], reverse micelles [8], sonochemical [9], pyrolysis [10] and homogeneous precipitation [11], [12], [13], [14], [15], [16]. Among them, the homogeneous precipitation demonstrates promising development in fabricating CeO2 nanoparticles due to advantages of low cost, mild synthesis condition and easy scale-up.

Recently, Zhou et al. [11] produced 4 nm CeO2 particles from Ce(NO3)3 and ammonia water. Matijević and Hsu [12] acquired sub-micron Ce2O(CO3)2·H2O particles by reacting Ce(NO3)3 with urea. Chen and Chen [13] prepared CeO2 particles from Ce(NO3)3 and hexamethylenetetramine. Li et al. [14] obtained nanosized CeO2 powders from ammonia carbonate by using diethylamine as precipitant. Uekawa et al. [15] obtained 7–9 nm CeO2 fine powders from Ce(NO3)3 in the polyethylene glycol solution. Besides, Yamashita et al. [16] produced CeO2 powders by reacting CeCl3 with NaOH in the presence of hydrogen peroxide and found that the resulting particle shape changed significantly depending on the pH value. However, to the best of our knowledge, most of those products were sphere-like and obtained isothermally. Very few studies have been conducted on the synthesis of needle-shape nanoparticles. Herein, a novel non-isothermal precipitation method was employed to synthesize CeO2 nanoparticles with different shapes and sizes. The morphology, crystalline structure, Ce3+ percentage and optical properties of final products were characterized by TEM, HRTEM, XRD, XPS and UV absorption spectroscopic techniques.

Section snippets

Experimental

The CeO2 particles were synthesized by the non-isothermal precipitation starting from cerium(III) nitrate (Ce(NO3)3·6H2O, Strem, 99.9%) solution with ammonia (from Riedel-de Haën Co., 25%). The preparation process includes two successive stages: precipitation and aging. Firstly, at precipitation stage, 50 mL of 0.2 M cerium(III) nitrate solution was served as the precursor. The reaction solution was kept at 70 °C with a stirring rate of 500 rpm. As soon as the ammonia solution (3 M, 25 mL) was added,

Results and discussion

Fig. 1(a) shows XRD pattern for the sample taken at 5 min (end of precipitation stage). The characteristic peaks occurred at 2θ=28.5, 33.1, 47.5, 56.2 and 59.0° are confirmed as the cubic fluorite structured CeO2. In addition, the peaks of Ce(OH)3 at 2θ=15.8, 27.5, 39.8 and 49.1° can also be observed. The corresponding TEM image is shown in Fig. 2(a). Interestingly, the predominant shape of the sample is needle-like, which is very similar to the morphology of Ce(OH)3 [16]. Zhou et al. [11] and

Conclusion

Non-isothermal precipitation method has been firstly proposed to prepare cubic-fluorite structured CeO2 nanoparticles with different shapes and sizes. Two stages of synthesis were conducted, i.e. precipitating at 70 °C and following by aging at another temperature. At the end of precipitation stage (about 5 min), the resultant needle-like intermediates are the mixture of Ce3+–Ce4+ compounds. As the aging temperature is suddenly reduced to 0 °C, the resultant particles retain their needle-like

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