Valence Evaluation of Cerium in Nanocrystalline CeO₂Films Electrodeposited on Si Substrates

Abstract

The oxidation state of the cerium in ceria (CeO 2 ) nanocrystalline films electrodeposited on Si(001) substrate was probed by three different complementary techniques: X-ray photoelectron spectroscopy (XPS), X-ray absorption near-edge structure (XANES) and electron energy loss spectroscopy (EELS). CeO 2 films were prepared under different oxygenation conditions, and subsequently thermal annealed as well as submitted to 30 and 350 keV Ne + irradiation with fluences ranging from 2 × 10 14 to 2 × 10 16 ions/cm 2 . Trivalent cerium concentrations higher than 50% were reached by using 30 keV Ne + irradiation with high fluencies. Strong valence deviations in the film volume with specimen regions practically fully-reduced to CeO 1.5 were obtained. These findings points out the potential for applications of low-cost nanocrystalline ceria thin films in many kinds of devices.Ceria (CeO 2 ) is an oxide with important applications in areas like catalysis, electrochemistry, photochemistry, and materials science 1-4 due to its ability to undergo a facile conversion between 4+ and 3+ formal oxidation states. CeO 2 is a material having fluorite-type Fm3m crystal structure even with some Ce 4+ ions replaced by Ce 3+ ions. In these cases, the charge is balanced by some of the Ce 3+ going to interstitial sites and, more importantly, by formation of oxygen vacancies. 5 The electrons left behind by released oxygen are transferred to cerium ions. 5, 6 As a result of lower coulomb repulsion due to the reduction of the cerium cation from 4+ to 3+ state, the lattice constant increases. Nevertheless, CeO 2-δ preserves a fluorite-type structure even under strong oxygen deficiency. [5][6][7] The oxygen atoms in the fluorite structure are all in the same plane, allowing high diffusion rates either by release or uptake of oxygen, depending on the number of oxygen vacancies. 5, 6 For nanocrystalline CeO 2-δ , relatively high diffusion rates are observed since an increase of the number of oxygen defects occurs as crystallite size decreases. High ambipolar oxygen diffusivity, high surface reaction rate, and crystallographic stability, even with a large concentration of oxygen vacancies are desirable characteristics in catalytic converters. Because of this, ceria is commonly used to reduce the emissions of CO, NO x , and hydrocarbons from automobile exhaust, 2 or is used as a base material of electrolytes and electrodes in solid oxide fuel cells. 3 Ceria-supported noble metal catalysts are capable of storing oxygen under oxidizing conditions and releasing oxygen under reducing conditions through a transformation between Ce 4+ and Ce 3+ oxidation states. 4 CeO 2-δ associated with other catalysts is also used in thermo-chemical water splitting process for hydrogen production 8, 9 and in the walls of self-cleaning ovens as a hydrocarbon catalyst during high-temperature cleaning process. Besides, ceria integrated on Si can be used as gate dielectric for a Si metal-oxide semiconductor field-effect transistor or as an alternative to ferroelectric mater...

Effect of annealing temperature on oxygen vacancy concentrations of nanocrystalline CeO2 film

“…It was reported that Ce 4+ can be reduced to Ce 3+ by means of ion and x-ray irradiation [15,16]. However, there are few reports focused on controlling oxygen vacancies by annealing treatment in nanocrystalline CeO 2 films.…”

Structural and Chemical Characterization of Cerium Oxide Thin Layers Grown on Silicon Substrate

“…Investigations by high resolution transmission electron microscopy (HRTEM) and electron energy loss spectroscopy (EELS) have already been performed on ceria systems [7][8][9] but mainly for ceria nanopowders [10][11][12][13][14][15][16][17] and not for thin films. The combination of the two techniques allows the correlation of spectroscopic information with detailed structural knowledge of a material [15][16][17].…”

Growth, characterization and interfacial reaction of magnetron sputtered Pt/CeO2 thin films on Si and Si3N4 substrates

“…In Fig. a, Ce3d spectrum of Pt/CeO 2 /Si film, three doublet peaks labeled as v–u (882.5 and 900.9 eV), v′′–u′′ (888.5 and 907.0 eV) and v′′′–u′′′ (897.9 and 916.4 eV) are assigned for the Ce 4+ species and peaks labeled as v o –u o (881.7 and 899.1 eV) and v′–u′ (885.3 and 903.4 eV) are associated with Ce 3+ species . The u′′′ peak is relatively well separated from the rest of the spectrum and is the characteristic of the presence of tetravalent Ce (Ce 4+ ) in Ce compounds.…”

“…Significant works on the growth, structure and interfacial reaction between CeO 2 and different substrates have been in the literature . Recently, we have investigated interfacial reactions between CeO 2 and substrates like Si, Al, Ti–6Al–4 V alloy, Si 3 N 4 and glass using XPS .…”