Gas Sensing Interactions at Hydrogenated Diamond Surfaces

Hydrogenated diamond (HD) samples exhibit a p-type surface conductivity (SC) which is caused by transfer doping to an adsorbed liquid electrolyte layer. We report on gas sensing experiments showing that such samples selectively respond to NO₂ and NH₃ gases at room temperature. Successive substitution of H-terminated surface sites by O-termination ones causes an increase in both the sensor baseline resistance and the gas-induced resistance changes. Thermal desorption of the surface electrolyte layer, on the other hand, causes the sensor baseline resistance to increase and the gas sensing effect to disappear. Readsorption of the surface electrolyte reestablishes both the sensor baseline resistance and the gas sensing effect. Our results indicate that the gas sensing effect is caused by local pH-changes due to acid/base reactions of the adsorbed gas molecules in the surface electrolyte layer. It is argued that this dissociative gas sensing mechanism represents a valuable complement with regard to the established surface combustion mechanism that is operative on heated metal oxide surfaces.