Electrical Properties of Rutile Type Oxides: Doped TiO2 and RuO2

Conducting oxides have long been used in electrochemical and photoelectrochemical devices because of their stability and catalytic properties. Among them, the rutile structure oxides constitute a family of compounds with a wide spectrum of electronic properties. For example pure TiO2 is insulating but becomes a good semiconductor when doped with donors such as Nobium, Tantalum or Lithium, or when non-stoichiometric (oxygen vacancies and interstitial donor Ti3+ ions). Pure RuO2 by contrast shows metallic conductivity of around 105 Ω-1 cm-1 at room temperature. In spite of the fact that these two oxides have the same crystal structure and similar lattice parameters, the solid solubility limits are only around 5% of each oxide in the other. The behaviour of Ruthenium as an impurity in TiO2 is surprising. Although in RuO2 the four Ru 4d electrons are delocalised in the conduction band, the Ru impurity in TiO2 retains its electrons and is present as the Ru4+ ion.