Metal Oxide Films on Ferroelectric Substrates for Switchable Catalysis

Polar surfaces result whenever crystals of compound materials are oriented such that a dipole moment can be associated with each bulk repeat unit perpendicular to the surface. For crystals of these materials, very large electrostatic potentials build up with increasing thickness of the material. Without compensating charges at the surfaces, the electric fields are divergent, and the surfaces are unstable. Because opposite sides of the crystal are oppositely charged, the charge compensation mechanism, which may include electronic and chemical reconstructions, will be different for each surface. Ferroelectric materials are materials where each unit cell possesses a net dipole, and the direction of the dipole can be switched by applying an electric field. Considering the different charge compensation mechanisms on oppositely poled surfaces, one can envision the intriguing possibility of a material with “switchable” surface properties. A step further is the idea of using ferroelectric films as substrates for tunable catalysts by depositing catalytically active metals or metal oxides onto them. This talk focuses on recent work with two model film/ferroelectric substrate systems currently being investigated in our lab: Cr 2O3 on LiNbO3 and RuO2 on Pb(Ti,Zr)O3. For RuO2 on Pb(Ti,Zr)O3, theoretical results and results from temperature programmed desorption measurements for the activity of this system toward NO are presented and compared to results for RuO2 layers on non-ferroelectric substrates. Using the RuO2/Pb(Ti,Zr)O3 system as an example, it is shown that by adding an extra degree of freedom (i.e. the change in polarization direction of the substrate) to the catalyst system, it may be possible to achieve novel behavior, such as the direct decomposition of NOx.