Formation Of Oxygen Vacancy Defects Research Articles

Surface-Dependence of Defect Chemistry of Nanostructured Ceria

The defect chemistry of reduced ceria nanoshapes was investigated using in situ Raman and FTIR spectroscopy. Octahedral- and rod-shaped particles behave similarly upon exposure to CO in terms of formation of anion Frenkel pair defects and oxygen vacancies. This similarity is attributed to the preferential exposure of (111) surfaces in both type of particles. Cube-shaped particles terminated with (100)-oriented surfaces exhibit very different defect behavior in CO, revealing formation of oxygen vacancy defects at the expense of existing anion Frenkel pairs. Octahedra and rods, prereduced in H2, can be further reduced with CO. In contrast, prereduced cubes can reactively adsorb CO, forming surface-bicarbonate, only via converting Frenkel-pair defects to oxygen vacancies, without any further net reduction of the ceria.

Ultraviolet photoresponse of ZnO thin-film transistor fabricated at room temperature

Transparent thin-film transistor (TFT) with ZnO film as a channel layer is fabricated at room temperature. ZnO film has a high absorption in the UV region and ZnO-TFT is sensitive to the UV illumination. We investigate the ultraviolet photoresponse of ZnO-TFT and find that the illumination with 254 nm light results in an evident photoresponse. The residual conductivity is observed in ZnO channel even the UV light was removed one week before. The UV illumination can induce the formation of oxygen vacancy defects which will act as donors in ZnO channel.

Surface Electronic Structure Transitions at High Temperature on Perovskite Oxides: The Case of Strained La0.8Sr0.2CoO3 Thin Films

In-depth probing of the surface electronic structure on solid oxide fuel cell (SOFC) cathodes, considering the effects of high temperature, oxygen pressure, and material strain state, is essential toward advancing our understanding of the oxygen reduction activity on them. Here, we report the surface structure, chemical state, and electronic structure of a model transition metal perovskite oxide system, strained La(0.8)Sr(0.2)CoO(3) (LSC) thin films, as a function of temperature up to 450 °C in oxygen partial pressure of 10(-3) mbar. Both the tensile and the compressively strained LSC film surfaces transition from a semiconducting state with an energy gap of 0.8-1.5 eV at room temperature to a metallic-like state with no energy gap at 200-300 °C, as identified by in situ scanning tunneling spectroscopy. The tensile strained LSC surface exhibits a more enhanced electronic density of states (DOS) near the Fermi level following this transition, indicating a more highly active surface for electron transfer in oxygen reduction. The transition to the metallic-like state and the relatively more enhanced DOS on the tensile strained LSC at elevated temperatures result from the formation of oxygen vacancy defects, as supported by both our X-ray photoelectron spectroscopy measurements and density functional theory calculations. The reversibility of the semiconducting-to-metallic transitions of the electronic structure discovered here, coupled to the strain state and temperature, underscores the necessity of in situ investigations on SOFC cathode material surfaces.

Effect of Radiation Defects on the Electronic Structure of Zircon by X-Ray Photoelectron Spectroscopy Data

X-Ray photoelectron spectroscopy (XPS) is used to study the electronic structure of radiation damaged samples of ZrSiO4 zircon mineral at early and middle stages of its radiation destruction. The effects of radiation induced atomic disordering are found to be most distinctly manifested in the spectra of O1s states and to a smaller extent in the spectra of Si2p states, and also in the zircon valence band. Based on the quantum chemical calculation results the conclusion is drawn that the observed changes in XPS lines are caused by the formation of oxygen vacancy defects and an increase in the covalency of interatomic bonds near oxygen vacancies. For zircon samples with a low/moderate degree of radiation damage these changes reflect the initial stage of the polymerization of the ZrSiO4 structure due to the formation of Si—O—Si chain fragments.

Synchrotron radiation stimulated evaporation and defect formation in a-SiO2

The relation between synchrotron radiation stimulated evaporation and oxygen vacancy defect formation in a-SiO2 was investigated. The behavior is shown to be quite different for the following three temperature regimes. At temperatures lower than 500°C, the surface is soon covered by a thin SiOx (x < 2) layer upon SR irradiation. The film is etched by stimulated ejection of surface atoms from the SiOx layer. Between 500–700°C, film removal is rate-limited by the thermal desorption of SiO molecules. Accordingly, a thick layer of SiOx is accumulated over the surface. Above 700°C, the evaporation of a-SiO2 proceeds at a constant rate keeping the surface stoichiometry of SiO2. This indicates that an SiOx layer, even if created, is thermally decomposed.