Surface chemistry of neat tin oxide sensor for response to hydrogen gas in air

The response of a neat tin oxide device to hydrogen gas in air was analyzed in detail, shedding light on the surface chemistry involved. The response was clearly divided into two categories depending on the partial pressure of hydrogen (P_H2), one in which the response was controlled by the reaction of O^2- ions (at small P_H2) and another in which the response was by that of O^- ions (at higher P_H2). Typical of the former category, a threshold P_H2 appeared, indicating that the O^- ions had been produced transiently through the reaction of O^2- ions. Interestingly, a considerable amount of O^2- ions was shown to persist even in the stage of the latter category, leading to an increase in resistance. Mechanism of switching between the two categories, the persistent amount of O^2- ions and the meaning of the threshold P_H2 could be accounted for satisfactorily based on the diagram of partition of the transferred electrons between O^- and O^2-. The characteristic quantities of the response in the two categories were evaluated.