Nanochemistry of Ceria Abrasive Particles
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Nanochemistry of Ceria Abrasive Particles Shelley R. Gilliss, James Bentley§ and C. Barry Carter Dept. of Chemical Engineering and Materials Science, University of Minnesota 421 Washington Ave. S.E., Minneapolis MN 55455 USA § Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN ABSTRACT Surfaces of ceria (CeO2) particles have been studied by electron energy-loss spectroscopy in a field-emission gun scanning transmission electron microscope. All the ceria particles analyzed contained Ce3+ at the surface. Rare-earth impurities such as La were enriched at the surface and were observed for particles ranging from tens to hundreds of nanometers in size. The oxidation state of the cerium ion is measured from the Ce M5/M4 white-line intensity ratio. INTRODUCTION Ceria (CeO2), which has many technological applications, is the most widely used abrasive for the chemical-mechanical polishing (CMP) of silicate glasses [1-4]. CMP is a process that has been employed for centuries to form precise optics, but has been widely studied in recent decades due to its critical role in the microelectronics industry [5-8]. Ceria is also used to finish CRT screens and LCDs and is used in the ophthalmic industry for prescription lens polishing [911]. Ceria is widely used in other technologies due to the ease with which it is reversibly oxidized or reduced. For example, it is a component of automotive emission-control catalysts and also promotes the water-gas-shift reaction in precious-metal catalysis [12, 13]. Ceria-based electrolytes are attractive candidates for intermediate-temperature (< 800˚C) solid-oxide fuel cells due to their high ionic conductivity [14, 15]. Ceria is also a promising UV-ray broadspectrum blocking material for use in personal-care products due to its low refractive index and low photocatalytic activity when compared with commonly used titanium oxide and zinc oxide [16]. In spite of its wide use, the mechanism of chemical reactivity (the ‘C’ in CMP) is not well understood. To understand the mechanism of ceria reactivity in such processes as CMP and catalysis, a thorough understanding of the surface of ceria is necessary. In this study commercial ceria abrasives for CMP have been studied by electron-energy loss spectroscopy (EELS) in a field-emission-gun (scanning) transmission electron microscope (FEG (S)TEM). The advantage of using the EELS technique in a FEG (S)TEM is the ability to form a small probe with sufficient current to enable short dwell times and the ability to analyze small volumes of material while simultaneously imaging the specimen. Commercially available ceria abrasives can range in color from deep red, to pale pink to white, depending on the composition of the material. With such a diversity of appearance and composition all being sold as “ceria polishing abrasives”, it makes it difficult to distinguish the role chemistry plays in the activity of the ceria abrasives. To maximize the relevance of this study to “real world” CMP applications, commercial abrasives were chosen
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