As part of our surface science studies, we are interested in characterizing the response mechanisms of SnO2 -based gas sensors. The above figure shows some of the various surface and interfacial effects which influence operational behavior. The gas molecule to be detected comes in contact with a SnO2 surface which may be modified by the addition of a noble metal catalyst such as Pt or Pd. The catalyst is often deposited in the monolayer regime and exists as a dispersed film with an island structure.
The electrical conductance of the catalyst modified SnO2 surface layers is affected by adsorption or surface reaction of gas molecules in the sensing environment. The degree of conductance modulation possible is dependent on the initial conductance of the semiconducting oxide surface and the interactions that occur between the gas and the sensing oxide and/or the gas and the metal catalyst.
Crystalline specimens are often used in model studies of SnO2 based gas sensors as a means of isolating various chemical, structural and additive effects and more clearly evaluating their importance in sensing.
Shown above from Cox et al. 1988 is a comparison between variations in the composition of the topmost surface layer, as measured by ion scattering spectroscopy (ISS), and the changes in the 300K electrical conductivity for heating an oxidized surface in vacuum. The change in conductivity is small for annealing temperatures up to 700K. As can be seen from the ISS measurements, this temperature range is associated with the removal of the top layer of bridging oxygens. The higher temperature anneal treatments caused a much larger increase in conductivity. Because surface oxygen vacancies are known to act as n-type donors, the conductivity increase is assigned to the fromation of a second type of oxygen vacancy, most likely an in-plane oxygen vacancy.
Interpretation of the electrical conductance changes caused by gas adsorption is complex. By dosing single gases in an ultrahigh vacuum system, it is possible to get a better picture of the mechanisms for sensing. UHV conductance measurements by Fryberger et al. 1989 have shown that the response of a SnO2 (110) surface to hydrogen at 400 K is dramatically enhanced by the addition of 3 ML Pd. Shown above from Fryberger et al. 1990 is the conductance vs. time for sequential hydrogen-pump-oxygen-pump cycles of the 3 ML Pd/SnO2 (110) surface at 400 K. Steps 1-3 show the response on the freshly annealed, Pd-dosed surface, prior to any oxygen exposure. The exposures to hydrogen (Step 2) and oxygen (Step 4) produce conductance changes that are not reversible upon pumping. Spillover mechanisms are thought to produce chemisorbed hydrogen atoms and oxygen atoms following hydrogen and oxygen exposures respectively. For Steps 4 and beyond, the exposure to hydrogen or oxygen produces two effects: reaction with the previously chemisorbed species to produce water, and production of additional chemisorbed species of the dosed gas.
Ion scattering spectroscopy supports this interpretation. Above is a set of spectra showing the results of exposure of a vacuum annealed 3 ML Pd/SnO2 (110) surface to 10-3 Pa 18O2, and then 10-3 Pa H2, each treatment at 400K. ISS is particularly useful here because of its top layer sensitivity, allowing a qualitative determination to be made of the atomic composition at the sample surface. It is seen that approximately one third of the total signal from the labeled-oxygen treated surface is due to 18O. Hydrogen exposure preferentially removes 18O. The most likely explanation is that 18O is present as a chemisorbed rather than a lattice oxygen species, and the the reaction 2Hads + Oads 
H2Ogas is removing the labeled oxygen.
References
1. Cox, D.F., Fryberger, T.B., and Semancik, S., "Oxygen vacancies and defect electronic states on the SnO2(110) 1 x 1 surface," Phys. Rev., B38, 2072-2083 (1988).
3. Fryberger, T.B. and Semancik, S., "Conductance response of Pd/SnO2(110) model gas sensors to H2 and O2," Sensors and Actuators, B2, 305-309 (1990).
Contact:
Steve Semancik
Last updated November 18, 1997