Article pubs.acs.org/JPCC
Ceria Nanocubes: Dependence of the Electronic Structure on Synthetic and Experimental Conditions Michelle N. Revoy, Robert W. J. Scott,* and Andrew P. Grosvenor* Department of Chemistry, University of Saskatchewan, 110 Science Place, Saskatoon, Saskatchewan, Canada, S7N 5C9 S Supporting Information *
ABSTRACT: The properties of nanostructured ceria are both size- and shapedependent. Here, ∼3−10 nm ceria nanocubes having exposed (002) surfaces were hydrothermally synthesized. For the first time, the electronic properties of such nanocubes were systematically studied using X-ray photoelectron spectroscopy and X-ray absorption spectroscopy. Comparisons of the collected Ce 3d XPS spectra after a single sweep revealed that the concentration of Ce3+ present within the nanocubes is independent of the particle size and the Ce precursor used. Ce 3d spectra collected after averaging 20 sweeps demonstrated the reduction of Ce4+, and that the degree of reduction was also independent of the synthetic conditions. This observed surface reduction/damage is mainly the result of exposure to high intensity X-ray radiation. These conclusions were further confirmed through examination of Ce M4,5- and N4,5-edge XANES spectra. On the basis of these results, it is clear that the concentration of Ce3+ on the surface of ceria nanocubes is independent of the particle size. This fact will become essential when designing new and improved catalytic support systems as the size of the nanocubes will not affect the surface concentration of Ce3+, and therefore may not affect their potential catalytic activity.
1. INTRODUCTION Ceria-based materials have become of increasing interest to the scientific community owing to their fascinating properties. Ceria (CeO2) has a cubic, fluorite crystal structure which is prone to surface defects.1 It has been widely accepted that the reactivity of ceria is based on the Ce3+/Ce4+ redox couple which allows for the easy reduction/oxidation of ceria under various conditions. The low redox potential allows ceria to have many applications in the fields of catalysis,2,3 oxygen storage,4 fuel cells,5,6 gas sensors,7 and even sun-care products.8 Most notably, ceria can be used as a three-way catalyst to remove NOx, CO and other hydrocarbons from automobile exhaust,9 and to reduce soot from diesel fumes.10 One current area of interest involves the use of ceria nanoparticles (NPs), as their properties are quite different from those of the bulk material, especially since oxygen defects along with the presence of Ce3+ have been observed.11 The degree of oxygen vacancies at the surface of the NPs, i.e., the concentration of Ce3+ ([Ce3+]) on the surface, will affect how easily the NPs can reversibly absorb and release oxygen.1 There have been many reports on the synthesis and characterization of a variety of ceria NPs having different shapes and sizes, and how these differences affect the redox capabilities of ceria. It has been reported that ceria NPs can be synthesized via hydrothermal,12 solvothermal,13 precipitation,14 microemulsion,14 thermal decomposition,15 sol−gel,16 and microwave heating methods.17 Most of the literature which is focused on this area involves the study of the unit cell parameters of cubic CeO2, and how the unit cell expands or contracts with varying [Ce3+].18,19 These reports © 2013 American Chemical Society
show that smaller NPs have larger unit cell parameters, but it is unclear whether this correlation is due to an increased Ce3+ presence in the smaller particles.14 X-ray and electron characterization techniques, such as X-ray absorption near-edge spectroscopy (XANES), X-ray photoelectron spectroscopy (XPS), and electron energy loss spectroscopy (EELS), are useful methods to investigate the presence of multiple oxidation states within a material. Using EELS, it was shown that commercially available CeO2−x NPs contained a reduced Ce3+ ‘shell’ that surrounded a ‘core’ composed of mainly Ce4+.20 The thickness of the ‘shell’ was found to be dependent upon the crystallographic surface type as well as the size of the NPs. This analysis showed that smaller particles (
