Polarity reversal of resistance response to trace H2 gas in the air between asymmetrically shaped electrodes on rutile-TiO2 single crystal

We investigated the resistance response to trace hydrogen gas in the air between the asymmetrically shaped point contact- (Pt tip) and plane contact- (Al thin-film) electrodes formed in-plane on a rutile TiO2 single crystal at 673 K. It was found that the polarity of the resistance response, that is, the increase or the decrease of the resistance by exposing to hydrogen, reverses depending on the bias voltage polarity. This reversal mechanism of the resistance response was analyzed from the electrical conduction properties and the depth profile of the oxygen tracer (18O) in the electric field-induced diffusion. The central mechanism is proposed to be the oxygen transfer reaction at the Pt/TiO2 interface, accompanied by the modulation of the positively charged-oxygen vacancy concentration, thus the resistance between the two terminals. It is also proposed that the proton hinders the transfer reaction at the interface; and hence, the resistance increases by exposing to hydrogen with biasing the positive voltage to the Pt tip electrode. The application of this reversal of the resistance response for separating hydrogen and ethanol in the trace gas sensing is discussed, which is quite limited for a conventional semiconductor gas sensor.