INFLUENCE OF WATER ON THE COADSORPTION OF OXIDIZING AND REDUCING GASES ON THE BETA-GA2O3 SURFACE

This work deals with the effect of water on the coadsorption process of oxidizing and reducing gases on beta-Ga2O3. The results obtained by resistance measurements and by SIMS are discussed on the basis of the complex chemical and structural changes of the surface caused by the water vapour. The water effect on adsorption and coadsorption is shown by comparing the R-p characteristics measured in the presence of water to those taken in 'dry' conditions. The studied temperature interval was 823-1103 K, the water content of the atmospheric pressure gases was 15 or 30 mbar while the partial pressure of CO, H-2 and 02 reactant gases ranged from 0.1 to 10 mbar. N2 was used as carrier gas. Our results suggest that water has a complex and multiple role in the sensing process. (i) It has an effect on the ad- and coadsorption processes, for example there is competition between the adsorption of the examined gases and the formation of surface OH groups. (ii) It has an effect on the sensor material itself: adsorption of molecular water - it results in a resistance decrease; scission of the adsorbed water and formation of surface OH groups - it results in a resistance increase; water also affects the activity of certain catalytically active centres. The formation of surface OH groups, in N2 carrier gas, resulted in an increased resistance in the presence of H-2 and 02, while for CO the resistance was diminished. The catalytic activity of certain centres may be affected by their physical or chemical interaction with water, but some water induced surface structural changes also have to be considered.