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This work reports the effect of oxygen activity on surface segregation for TiO2 co-doped with two cations, indium and niobium (0.076 at.% In + 0.103 at.% Nb). In this work, we studied the effect of annealing at 1273 K in the gas phase of controlled oxygen activity on surface segregation of both ions. The applied oxygen activity included pure oxygen, p(O2) = 100 kPa, and argon, p(O2) = 10 Pa. The segregation-induced concentration gradients were determined using both secondary ion mass spectrometry and X-ray photoelectron spectroscopy. The obtained results indicate that annealing of the studied TiO2 specimens in argon results in cooperative segregation of both ions leading to the formation of a surface structure involving comparative concentrations of both cations. However, annealing in oxygen results in preferential segregation of indium leading to the formation of a In2TiO5-type surface structure. The obtained results are considered in terms of the effect of multicomponent segregation on processing of the surface layer with controlled properties that are desired for specific applications. The present work indicates that oxygen activity may be used as the parameter in surface engineering of the solid solution. [ABSTRACT FROM AUTHOR]

Segregation in crystals results in surface layer enrichment of selected lattice elements. The present work considers the phenomenon of segregation in solid solutions of TiO2 doped with donor-type elements, such as niobium and tantalum. The focus is on the effect of oxygen activity on segregation at elevated temperatures. It is shown that the effect of oxygen activity on the segregation-induced enrichment may be used for engineering of oxide materials in general and TiO2-based materials in particular with controlled surface composition that is required to achieve the desired reactivity and photoreactivity for energy conversion in general and solar energy conversion in particular. The development of the related surface engineering procedures requires correct determination of oxygen activity during processing and the segregation-induced concentration gradients within the surface layer. [ABSTRACT FROM AUTHOR]

Abstract: The present work considers the reaction kinetics between titanium dioxide, TiO2 (rutile) single crystal and a thin layer of indium oxide, In2O3 deposited on its surface. The reported experimental data are reflective of the chemical reaction involving the diffusive transport of In3+ in the rutile phase. The reaction progress at 1173K is monitored using the secondary ion mass spectrometry (SIMS) technique. It is shown that the SIMS depth profiles may be considered in terms of two distinctly different components, related to the surface layer of In2O3, and the rutile single crystal phase beneath. The depth profile of the rutile phase involves the region related to bulk diffusion of indium as well as the background composition. The bulk diffusion coefficient of indium, In3+ in single crystal TiO2 (rutile) at 1173K and p(O2)=21kPa was determined to be 4.4(±0.2)×10−18 m2 s−1. [Copyright &y& Elsevier]

The increasing demand for sustainable energy results in the development of new technologies of energy generation. The key objective of hydrogen economy is the introduction of hydrogen as main energy carrier, along with electricity, on a global scale. The key goal is the development of hydrogen-related technologies needed for hydrogen generation, hydrogen storage, hydrogen transportation and hydrogen distribution as well as hydrogen safety systems. It is commonly believed that hydrogen is environmentally clean since its combustion results in the formation of water. However, the technology currently employed for the generation of hydrogen from natural gas, does in fact lead to the emission of greenhouse gases and climate change. Therefore, the key issues in the introduction of hydrogen economy involve the development of environmentally clean hydrogen production technology as well as storage and transport. The clean options available for hydrogen generation using nuclear energy; such as advanced nuclear fission and, ultimately, nuclear fusion, are discussed. The latter, which is environmentally clean, is expected to be the primary approach in the production of hydrogen fuel at the global scale. The present work considers the effect of hydrogen on properties of TiO 2 and its solid solutions in the contexts of photocatalytic energy conversion and the effect of tritium on advanced tritium breeders. [ABSTRACT FROM AUTHOR]

Photocatalytic activity of oxide semiconductors is commonly considered in terms of the effect of the band gap on the light-induced performance. The present work considers a combined effect of several key performance-related properties (KPPs) on photocatalytic activity of TiO2 (rutile), including the chemical potential of electrons (Fermi level), the concentration of surface active sites, and charge transport, in addition to the band gap. The KPPs have been modified using defect engineering. This approach led to imposition of different defect disorders and the associated KPPs, which are defect-related. This work shows, for the first time, a competitive influence of different KPPs on photocatalytic activity that was tested using oxidation of methylene blue (MB). It is shown that the increase of oxygen activity in the TiO2 lattice from 10-12 Pa to 105 Pa results in (i) increase in the band gap from 2.42 to 2.91 eV (direct transitions) or 2.88 to 3 eV (indirect transitions), (ii) increase in the population of surface active sites, (iii) decrease of the Fermi level, and (iv) decrease of the charge transport. It is shown that the observed changes in the photocatalytic activity are determined by two dominant KPPs: the concentration of active surface sites and the Fermi level, while the band gap and charge transport have a minor effect on the photocatalytic performance. The effect of the defect-related properties on photoreactivity of TiO2 with water is considered in terms of a theoretical model offering molecular-level insight into the process. [ABSTRACT FROM AUTHOR]