COADSORPTION AND CROSS-SENSITIVITY ON HIGH-TEMPERATURE SEMICONDUCTING METAL-OXIDES - WATER EFFECT ON THE COADSORPTION PROCESS

The operation of high temperature oxide semiconductor gas sensors is strongly influenced by their surface properties and the processes taking place in the topmost atomic layers such as adsorption, coadsorption, gas-gas interactions and the catalytic processes on them. From our experimental and theoretical results a resistance limit was calculated; owing to the measured resistance it is possible to estimate when the surface phenomena dominate. According to our results the effect of water on the resistance is the result of at least two processes. The adsorption of molecular water is fast and results in a donor effect while the parallel formation of surface OH groups is relatively slow and results in an acceptor effect. When the temperature is increased the OH formation becomes the dominant process and at a definite temperature its influence passes a maximum. Our results prove that on beta-Ga2O3 and n-SrTiO3 the adsorption and coadsorption processes are strongly influenced by the presence of water. For both substances the effects are very similar. We suggest that similar mechanisms take place on every oxide semiconductor.