Dr Robertson Burgess

The dispersion of small amounts of Ce4+ ions in the bulk of ZrO2 leads to a photoactive material sensitive to visible light. This is shown by monitoring with EPR the formation and the reactivity of photogenerated (¿ > 420 nm) charge carriers. The effect, as confirmed by DFT calculations, is due to the presence in the solid of empty 4f Ce states at the mid gap, which act as intermediate levels in a double excitation mechanism. This solid can be considered an example of a third-generation photoactive material. © 2014 American Chemical Society.

Time-dependent density functional theory (TDDFT) was used to calculate the optical absorption spectra of gold clusters of 20-171 atoms. The spectra for the smallest clusters agree with previous results, and the spectra for the largest clusters show features consistent with classical Mie theory. The systematic exploration of particles of sizes within these two extremes has allowed the trends linking optical absorption spectra and particle size and symmetry to be identified. A transition from molecular-like spectra to a more classical response is observed. © 2014 American Chemical Society.

The paramagnetic defects present in pristine zirconium dioxide (ZrO 2) and those formed upon reductive treatments (either annealing or UV irradiation in H2) are described and rationalized by the joint use of electron paramagnetic resonance (EPR) and DFT supercell calculations. Three types of Zr3+ reduced sites have been examined both in the bulk of the solid (one center) and at the surface (two centers). Trapping electron centers different from reduced Zr ions are also present, whose concentration increases upon annealing. A fraction of these sites are paramagnetic showing a symmetric signal at g = 2.0023, but the majority of them are EPR silent and are revealed by analysis of electron transfer from the reduced solid to oxygen. The presence of classic F-type centers (electrons in bulk oxygen vacancies) is disregarded on the basis of the g-tensor symmetry. This is expected, on the basis of theoretical calculations, to be anisotropic and thus incompatible with the observed signal. In general terms, ZrO2 has some properties similar to typical reducible oxides such as TiO2 and CeO2 (excess electrons stabilized at cationic sites), but it is much more resistant to reduction than this class of materials. While point defects in doped (Y 3+, Ca2+) ZrO2 materials have been widely investigated for their role as ionic conductors, the defectivity of pristine ZrO2 is much less known; this paper presents a thorough analysis of this phenomenon. © 2013 American Chemical Society.