Nb, Ta and V-doped TiO 2 aerogels and xerogels have been synthesized as possible new alternatives to carbon blacks for Proton Exchange Membrane Fuel Cells catalyst supports. A comparative study of different dopants was realized in a single study. Nb, Ta and V showed different behaviors with respect to the final material structure and morphology, composition and electronic conductivity. They are all prone to surface segregation, to different extents. V-doped TiO 2 apart, the rutile structure could only be obtained after calcination in a reducing atmosphere at 800 °C for Nb or Ta-doped TiO 2 . The electronic conductivity exhibited a maximum at 10 at.% for Nb and Ta, 5 at.% for V. Nb revealed to be the most appropriate dopant to increase the electronic conductivity of TiO 2 , followed by Ta and V.4 to 5 orders of magnitude were gained after Nb doping for xerogels conductivity to reach almost 0.1 S cm -1 . The role of point defects was discussed to account for phase transition and evolution of conductivity.

Single-crystalline Mg3N2 thin films are grown on MgO(100) substrates by plasma-assisted molecular beam epitaxy. To prevent the oxidation of the Mg3N2 films and allow further physical characterization, a polycrystalline MgO cap is deposited in-situ. The growth orientation of the Mg3N2 films can be tuned from ( 100) to ( 111) by changing the growth conditions and the associated epitaxial relationships have been determined by means of X-ray diffraction. The lattice constant of Mg3N2 films has been monitored as a function of temperature from 300 K to 900 K, determining thereby the linear thermal expansion coefficient. Transmission measurements indicate an optical bandgap of crystalline Mg3N2 around 2.9 eV at room-temperature, consistent with diffuse reflectance measurements on micrometric Mg3N2 particles. These results demonstrate the possibility of exploiting Mg3N2 as a new crystalline semiconductor in the blue-violet range.