Gas sensors for air quality monitoring: realisation and characterisation of undoped and noble metal-doped SnO2 thin sensing films deposited by the pulsed laser ablation

Undoped and noble metal-doped very thin SnO2 sensing layers deposited by th e pulsed laser ablation (PLA) technique upon micromachined Si substrate hea ter elements have been extensively characterised. The main objective behind the effort carried out in this work has been the challenging perspective t o define a sensing layer technology and a sensor operating mode (continuous (DC) vs. pulsed temperature (PT) mode) which allows the long term measurin g and the easy discrimination of very low concentration of benzene by an in terfering species like carbon monoxide, both present in the outdoor air. Th e experimental results reported in this work have shown that combining the sensor technology with a suitable catalytic element (in the present case, g old) and a proper sensor operating mode, very low concentration of benzene can be revealed with a sufficient selectivity towards CO. One of the major problems encountered during the long term test of the sensors has been the loss of the sensitivity to benzene with the time. An attempt has been made to explain the reason of this experimental result. We firmly believe that m any factors contribute to it. Among them, the annealing of point defects as sociated with the oxygen vacancies as well as the loss of the catalytic eff iciency by the added Au layer as effect of a possible coalescence of the me tal clusters at the sensor working temperature (T congruent to 400 degrees C). To partially reduce the cluster mobility and to preserve the benzene se nsitivity, the last assumption would suggest to operate the sensors, no mat ter if in DC or in PT mode, at a temperature lower than that used in this w ork. (C) 1999 Elsevier Science S.A. All rights reserved.