High quality CeO2 nanocrystals stabilized by a double hydrophilic block copolymer

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Abstract

High quality monodisperse cerium(IV) oxide (CeO2) particles with a particle size less than 2 nm can be synthesized easily in large scale by a hydrothermal hydrolysis reaction at 120 °C using (NH4)2Ce(NO3)6 as precursor in the presence of a double hydrophilic block copolymer. The nanoparticles show a strong blue emission with an intense peak at 425 nm. With dilution of the solution, an obvious violet emission was observed at room temperature with concentration-dependent blue-shift characteristics.

Introduction

CeO2 has found important applications as catalyst, electrolyte material of solid oxide fuel cells (SOFC), or as a material of high refractive index, and as an insulating layer on silicon substrates [1], [2], [3], [4]. The previous synthesis strategies of cerium oxide mainly involved in using high temperature approaches. Ultrafine CeO2 powder has been prepared and used to decrease the sintering temperature from 1500 °C to 1200 °C [5]. Direct mixing of equal volumes of a Ce(NO3)3 solutions and hexamethylenetetramine at room temperature led to the formation of CeO2 nanoparticles within a size range of 3–12 nm [6]. A solvothermal process has been employed to synthesize colloidal solutions of ultrafine ceria particles, which were directly obtained by the reaction of cerium metal in 2-methoxyethanol at 200–300 °C [7]. A hydrothermal process has also been used for producing CeO2 nanoparticles [8], [9], [10]. In addition, homogeneous solution precipitation in the presence of hexamethylenetetramine [5] or urea was also used to produce CeO2 nanoparticles [11]. Masui et al. [12] reported the synthesis of cerium oxide nanoparticles using reverse micelles. Recently, sonochemical synthesis of CeO2 nanoparticles was reported using cerium nitrate and azodicarbonamide as starting materials with addition of ethylenediamine or tetraalkylammonium hydroxide as additives [13], or in the presence of poly(ethyiene glycol) and NaAc [14]. A semi-batch reactor method was applied for producing CeO2 nanoparticles within a size range of 3–5 nm [15]. Even though there are many intensive reports on the synthesis of CeO2 nanoparticles, it is still difficult to obtain a high quality and stabilized homogeneous CeO2 nanoparticle dispersion.

Herein, we present a facile way to produce high quality cerium oxide nanoparticles in large scale by a hydrothermal hydrolysis of (NH4)2Ce(NO3)6 in the presence of a double hydrophilic block copolymer, poly(ethylene glycol)-block-poly(methacrylic acid) (PEG-b-PMAA), as a stabilizer. These polymers are highly efficient additives for the crystallization control of inorganic particles as reviewed in [16]. A concentration-dependent blue shift towards the violet region for as-prepared CeO2 nanoparticles was observed.

Section snippets

Synthesis

A commercial block copolymer poly(ethylene glycol)-block-poly(methacrylic acid) (PEG-b-PMAA, PEG = 3000 g mol−1, 68 monomer units, PMAA = 700 g mol−1, 6 monomer units) was obtained from Th. Goldschmidt AG, Essen, Germany. The synthesis was conducted in a Telfon innered autoclave. 0.01 mol (NH4)2Ce(NO3)6 was dissolved in 25 ml 1 g L−1 PEG-b-PMAA block copolymer solution under vigorous stirring for 15 min. Then, the clear yellow solution was poured into a Teflon-linned autoclave. The autoclave was kept at

Results and discussion

The XRD pattern was shown in Fig. 1, in which the four typical peaks 1 1 1, 2 0 0, 2 2 0, 3 1 1 can be indexed as cubic fluorite phase of CeO2 (JCPDS card 34-0394). The broadened baseline in the range of 20–45° is due to the presence of amorphous polymer existed in the sample. The composition of the nanoparticles was examined by X-ray photoelectron spectroscopy. The Ce3d spectrum in Fig. 2 clearly shows two strong peak at around 917 eV and 899 eV, which were assigned to Ce3d5/2 for the Ce4+ state.

Conclusion

In summary, we have shown that extremely small CeO2 nanoparticles stabilized by the block copolymer can be produced by a hydrothermal hydrolysis reaction. The nanoparticles shows a strong blue emission with a peak at 425  nm. With dilution of the solution, an obvious violet emission was observed at room temperature with concentration-dependent blue-shift characteristics. The produced high quality dispersion can be readily used for the deposition of thin films on self-assembled monolayers (SAMs)

Acknowledgements

S.H. Yu thanks for the special funding support, the Centurial Program of Chinese Academy of Sciences, the Distinguished Youth Fund, the Distinguished Team of the National Science Foundation of China (NSFC, No. 20325104, No. 20321101), and NSFC No. 50372065 for financial support. H.C. thanks the Max-Planck-Society for financial support. Roland Hauert (EMPA Dübendorf) is gratefully acknowledged for XPS measurements.

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