Characterization and control of adsorbates of nitrogen mono-oxide on tin dioxide surface

The adsorption of NO on the surface of SnO2 was investigated by combining the measurement of electrical resistance with a temperature programmed desorption technique. Four types of adsorbates of NO were recognized from desorption temperatures, i.e., L (30 degrees C), alpha (200 degrees C), beta (280 degrees C), and gamma (380 degrees C). These were estimated to be cationic dimer of NO (L), anionic nitrosyl (Sn-NO- (alpha)), cationic nitrosyl (Sn-NO+ (beta)), and anionic nitrito (Sn-O-NO- (gamma)). It was found that the formation or distribution of the latter three adsorbates drastically changed depending on the redox state of SnO2 surface, the presence of additives and the coexistence of oxygen gas. Because of the competition of three adsorbates, a sensor element using SnO2 showed complex behavior upon exposure to NO in air; The addition of an alkali metal oxide such as K2O to SnO2, however, could improve the NO sensing properties significantly This is attributed to the modification of adsorption behavior of NO. It seems that these additives modify the SnO2 surface to be more favorable to beta type adsorption.