A preliminary study of the impedance data as a function of frequency (100 m
Hz-100 kKz) for pure and surface ruthenated tin oxide with and without LPG
indicates the important role of surface states in enhancing the sensitivity
of the surface functionalized tin oxide gas sensor. Different types of pro
cesses such as diffusion in the pores and electron-transfer due to surface
reactions are found to occur at different operating temperatures and the me
chanism of hydrocarbon sensing is found to drastically change with temperat
ure in the operational range. For example, the samples at 250 degrees C sho
w diffusion controlled behaviour while those at 350 degrees C predominantly
show charge transfer control. The behaviour of these systems are studied w
ith plausible equivalent circuits and a clear distinction has been observed
for the surface ruthenated samples, which could be responsible for the enh
anced sensitivity at these temperatures. (C) 1999 Elsevier Science S.A. All
rights reserved.