Adsorptive properties of semiconducting thin NiO and TiO2 films combined with an oppositely polarized ferroelectric support

Abstract

Semiconducting oxides of NiO and TiO₂ have been deposited as 30–50 nm thin films on either the positive (+) or negative (–) polar LiNbO₃ ferroelectric surface, and the polarization field effects of the support upon the adsorptive properties of the combined oxides have been studied. Thermal desorption spectra show that the activation energy for desorption, E_d, varies as a function of the direction of polarization depending on the ferroelectric support, the effects of which are reversed for p- and n-type semiconducting oxides; E_d for O₂ desorption was 102 kJ mol^–1 for NiO/(+) and 157 kJ mol^–1 for NiO/(–), whereas E_d for H₂ desorption was 51 kJ mol^–1 for TiO₂/(+) and 37 kJ mol^–1 for TiO₂/(–). When hydrogen was adsorbed on TiO₂/LiNbO₃ surfaces, the changes in the surface electrical conductivity were larger by a factor of 40 for the positive than for the negative support, which was opposite to the polarization-direction effects observed previously for O₂ adsorption on NiO/LiNbO₃ surfaces. The different adsorptive properties are accounted for in terms of the effects of band-bending induced in thin semiconducting oxides in contact with the polar ferroelectric surfaces, and it is suggested that the use of ferroelectrics as a support is effective in modifying the adsorptive properties of semiconducting oxide surfaces.