The role of hydration-dehydration effects on oxygen adsorption on SnO2

The Schottky barrier-controlled conduction of polycrystalline tin dioxide i s strongly influenced by the presence of the residual humidity in the ambie nt, at atmospheric pressure. Molecularly adsorbed water and surface hydroxy l groups build up an over-layer impeding oxygen adsorption up to 320 degree s C-340 degrees C. Extensive surface oxidation occurs only above 340 degree s C, being coupled with dehydroxylation. The thermal cycling up to this tem perature in humid ambient increases the extent of water retention, as is pr oved by TGA data in humid argon and in humid oxygen flow. Thus, depending o n the temperature range of thermal cycling and on the "quality" of the ambi ent, different surface coverages with water/hydroxyl groups will dictate th e differentiated responses of tin dioxide-based sensors or catalysts with r espect to oxygen or other adsorbing gases. The involvement of protonic cond uction in the conduction mechanism is also suggested.