A four-point probe correlation of oxygen sensitivity to changes in surface resistivity of TiO2(001) and Pd-modified TiO2(001)

The objective of the current study was to investigate changes in the surface electrical resistivity of single crystal TiO2(0 0 1) under ultra-high vacuum conditions for gas sensor studies. Using a linear four-point probe (FPP), we investigated the effects of surface stoichiometry and of exposure to oxygen on the surface resistivity. We also investigated the effect of palladium (Pd) particles of varying sizes on the surface resistivity using both FPP and scanning tunneling microscopy. Our results showed that there was a strong dependence of the resistivity on the average Ti oxidation state of the clean TiO2 (0 0 1) surface. A decrease in the surface resistivity of TiO2 (0 0 1) was observed after the surface was reduced by Ar+ ion sputtering, whereas an increase in the resistivity of the sputtered-TiO2 surface was detected after reacting with gas-phase oxygen. In addition, we found that the presence of Pd particles enhanced the sensing performance to oxygen, and that the degree of enhancement depended strongly on the size of the Pd nanoparticles. (C) 2003 Published by Elsevier B.V.