Adsorptive properties of semiconducting thin NiO and TiO2 films combined with an oppositely polarized ferroelectric support
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, Ed, 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; Ed for O2 desorption was 102 kJ mol–1 for NiO/(+) and 157 kJ mol–1 for NiO/(–), whereas Ed 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.