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 o f 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 w ater 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 wa s 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 resu lts suggest that water has a complex and multiple role in the sensing process. (i) It has an effect on the ad- and coadsorption processes, f or example there is competition between the adsorption of the examined gases and the formation of surface OH groups. (ii) It has an effect o n the sensor material itself: adsorption of molecular water - it resul ts in a resistance decrease; scission of the adsorbed water and format ion 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 inc reased resistance in the presence of H-2 and 02, while for CO the resi stance was diminished. The catalytic activity of certain centres may b e affected by their physical or chemical interaction with water, but s ome water induced surface structural changes also have to be considere d.