Synthesis of ruthenium dioxide titanium dioxide aerogels: Redistribution of electrical properties on the nanoscale

High-surface area RuO2-TiO2 is an important material for catalytic and power source technologies. To create this mixed oxide as a nanoscale, high-surface-area material, we have synthesized electrically conductive aerogels of RunTi1-nOx via sol-gel chemistry and supercritical drying of the resulting gel. The structural and electrical properties of these aerogels are characterized after annealing using X-ray diffraction, X-ray photoelectron spectroscopy, surface-area measurements, and impedance spectroscopy. Their properties are affected by the nature of the RuCl3 precursor and the initial sol-gel chemistry. Whereas bulk 32 mol % RuO2.TiO2 is an electronic conductor, a dominant ionic response is measured in the impedance of Ru0.32Ti0.68Ox aerogels. This impedance is ascribed to a proton conduction mechanism associated primarily with the hydrous surface of the RuOx component. The electrical conductivity of these Ru0.32Ti0.68Ox aerogels at 25 degrees C increases from 10(-4) S cm(-1) in ambient air to 10(-2) S cm(-1) under an increased partial pressure of water, which is characteristic of a protonic conductor. The electronic conductivity of these aerogels improves with exposure to dry oxygen, which is consistent with the oxidation of Ru3+ surface defects. Synthesizing RuO2-TiO2 as an aerogel-i.e., as high-surface-area, networked interface-accentuates the surface properties of this material. The electrical (electronic + protonic) transport properties of bulk RuO2-TiO2 are redistributed when synthesized as an aerogel. Whereas electronic transport dominates the characteristics of the dense form, the protonic transport of the hydrous oxide surfaces governs the electrical properties of the aerogel. Aerogels provide a useful means to isolate, study, and control the surface of metal oxides.