Samina Azad

X-ray Photoelectron Spectroscopy Studies of Oxidized and Reduced CeO2(111) Surfaces

We have studied the electronic structure of oxidized and reduced CeO2(111) surfaces using x-ray photoelectron spectroscopy (XPS). The 50 nm thick CeO2(111) film was grown on a YSZ(111) substrate using oxygen plasma assisted molecular beam epitaxy (OPA-MBE). This film has been characterized using in situ (RHEED) reflection high energy electron diffraction and ex situ x-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), and Rutherford backscattering spectroscopy (RBS). The lattice mismatch between CeO2(111) and YSZ(111) is less than 5% and yields a flat surface that is comprised of an equivalent number of Ce4+ and O2− ions. Oxidation with O2 at 773 K under UHV conditions was sufficient to fully oxidize the CeO2(111). Surface reduction was carried out by annealing in UHV at 973 K. Ceria is a technologically important metal oxide with many interesting catalytic properties. The most common use of ceria is in the treatement of automobile exhaust gases, primarily due to its oxygen s...

Distortion of the oxygen sublattice in pure cubic-ZrO2

Multilayer films of pure ZrO 2 and CeO 2 were grown using molecular beam epitaxy on a yttria-stabilized zirconia (YSZ) substrate. Distinctive forbidden diffraction spots of (odd, odd, even) type were observed on the selected-area electron-diffraction patterns of the film. Dark-field imaging clearly revealed that these forbidden diffraction spots were solely due to the ZrO 2 layers. Comparison of the electron diffraction pattern with that simulated by dynamical calculations suggest that the pure ZrO 2 layers possess a cubic structure of space with the group P 4 3 m oxygen sublattice being displaced diagonally, rather than along the c axis as suggested for YSZ. Our results further suggest that the displacement of the oxygen from the ideal (¼, ¼, ¼) position might have been introduced during the film growth process.

Adsorption and reaction of NO on oxidized and reduced SrTiO3(100) surfaces

Adsorption and reaction of NO on oxidized and reduced SrTiO3(100) surfaces have been studied using temperature programmed desorption (TPD). Major desorption peaks for NO from the fully oxidized surface are found at 140 and 260 K, along with a long tail that continues up to 500 K. The desorption features at 140 and 260 K correspond to activation energies of 36 and 66 kJ/mol, respectively, using a simple Redhead analysis. NO reacts nondissociatively on the fully oxidized surface. Reactivity of reduced SrTiO3(100) is relatively higher than that of the fully oxidized surface and is influenced by the adsorption temperature of the NO molecules on the surface. NO and N2O are the major desorption products following adsorption of NO on the reduced surface at 110 K. Desorption of N2O from significantly reduced SrTiO3(100) indicates that the oxygen atoms of the adsorbed NO molecules are preferentially extracted by the surface oxygen vacancy sites, whereas the surface oxidizes as a result of the deoxygenation of the ...