Characterization and optical properties of nano-ceria synthesized by surfactant-mediated precipitation technique in mixed solvent system
Introduction
Rare earth compounds have wide applications because of their special electronic configuration. However, nanostructural rare earth compounds, because of high surface to volume ratio and quantum size effect [1] show specific physical (optical, mechanical, magnetic), chemical (reaction activity, catalytic) and biomedical (curing, delivery, release) properties. The cerium oxides have been extensively used as glass polishing material [2], oxygen ion conductor in solid oxide fuel cells (SOFCs) [3], gas sensor [4], UV absorbent [5], catalytic support or promoter for automotive exhaust gas conversion reaction [6]. In recent years, there has been a great scientific interest in the study of cerium oxide nanoparticles [7], [8] due to many distinctive characteristics and novel options for a wide range of applications. The fine particle of cerium oxide with very small size exhibits unique UV absorbing ability, high stability at high temperatures, high hardness and reactivity as catalyst [9], [10].
Preparation of cerium oxide powders is by no means a new research subject; however, practical methods are still needed for synthesizing high-quality ultrafine powders with required characteristics in terms of their size, uniformity, morphology, specific surface area and crystallinity. Numerous techniques have been proposed to synthesize nano-sized CeO2 particles with promising control of properties [11], [12], [13]. The large-scale production of powders needs to be economically feasible and should not be more complex. The use of mixed solvent system in the precipitation process is a new approach; the addition of low-dielectric medium to the aqueous solution can alter the thermodynamics of the reaction system and nucleation kinetics [14], [15]. By using this approach Huey-Ing Chen and Hung-Yichang synthesized CeO2 nanoparticles by homogeneous precipitation in alcohol/water mixed solvents [16]. In the present work we have used acetone as low dielectric medium and mixed with aqueous medium. The ultrafine cerium oxide nanoparticles were prepared by surfactant-mediated precipitation of ceria precursor in acetone/water mixed solvent system and studied the effect of temperature on textural properties of cerium oxide.
Section snippets
Experimental
The chemicals used in this work were: cerium nitrate hexahydrate (Ce(NO3)3·6H2O (S.D. Fine-Chem Ltd., 99%), ammonium hydroxide (NH4OH, 25 vol.%; Merck), acetone (Merck), Tween 80 (polyoxyethylene sorbitan monooleate; Merck) as starting materials. The Tween 80 was first dissolved in acetone and to this, required amount 0.5 M cerium nitrate solution was added slowly under vigorous stirring (volume ratio of 93:6.2:3.8). The resulting solution was stirred for 30 min and an excess amount (1.65 times
CeO2 formation and thermal analysis
The formation of CeO2 involves several complicated reactions [17]. As the ammonium hydroxide was added into the precursor, Ce(OH)3 precipitate was formed immediately due to extreme low-solubility constant (7 × 1021) [18]. Here, high-alkaline environment favored the oxidation of Ce(OH)3 to hydrated Ce(IV). Oxidation of Ce3+ to Ce4+ in solution takes place at high pH, i.e. Ce3+ + H2O → Ce(OH)3+ + H+ + e− with subsequent hydrolysis to Ce(OH)4 and precipitation. Hydroxyl ions play an important role in this
Conclusions
Nano-crystalline CeO2 particles with narrow size distribution were prepared by surfactant-mediated precipitation technique in acetone/water mixed solvent system. The method is found to be simple and economical for the bulk preparation of cerium oxide ultrafine particles with required characteristics. The XRD and HRTEM studies suggest that the average particle size was found to be 4.5 nm, with cubic fluoride structure The CeO2 particles showed good thermal stability, after calcination at 800 °C
Acknowledgements
The authors are thankful to Director, Prof. B.K. Mishra, Institute of Minerals and Materials Technology, Bhubaneswar, for his kind permission to publish this paper. The help provided by Dr. K.S. Rama Rao and Dr. K.V.K. Chari scientists of Indian Institute of Chemical Technology, Hyderabad, for FTIR and BET measurements is thankfully acknowledged.
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