Temperature Dependence of the Sensitivities of Metal ‐ TiO2 Junctions to Various Reducing Gases

Abstract

The current through a Pd-TiO2 diode is sensitive to hydrogen at room temperature and becomes sensitive to other reducing gases, such as carbon monoxide, ethanol, and propylene, at high temperature. Experiments were carried out extensively for the similar junctions of TiOu with Pt, Au, A1, Ni, Cu, Mg, or Zn. The Pt-TiO2 and Au-TiO2 diodes are also sensitive to hydrogen, the sensitivity being in the order of Pd > Pt > Au. The sensitivity of the diode depends on the catalytic property of the metal to the reaction between oxygen adsorbed on the metal surface and reducing gases, resulting in a lowering of the potential barrier height at the metal-TiO2 interface.It has been shown recently that the current-potential (I-U) curve of a Pd-TiO2 diode is very sensitive to hydrogen in ambient air and its possible utilization as a hydrogen sensor has been suggested (1). The sensitivity presumably arises from catalytic reactions occurring at the metal surface. In air, oxygen is chemisorbed in the form of anions on the Pd surface, increasing the work function of the metal (2). When hydrogen exists in air, the oxygen is desorbed by reaction with it, resulting in a lowering of the potential barrier height at the Pd-TiO2 interface.Many authors have also investigated hydrogen-sensitive diodes made of Pd and semiconductors such as Si (3), CdS (4), and ZnO (5), and explained the results by the change in electrical properties arising either from the adsorption of hydrogen at the Pdsemiconductor interface (3, 5) or the formation of palladium hydride (4). Such an explanation is inconsistent with our own, as discussed in our previous paper (1).In this paper the experimental results are reported on the sensitivity of a Pd-TiO2 diode to various reducing gases such as carbon monoxide, ethanol, propylene, and isobutane together with similar experimental results on the junctions of TiO~ with other metals such as Pt, Au, Ni, A1, Cu, Mg, and Zn. ExperimentalMetal-TiO2 Schottky diodes were fabricated by depositing thin metal film, ca. 200A thick, on n-type TiOe single crystal wafers, the detail of the procedure was described in our previous paper (1). A schematic diagram of the metal-TiO2 diode is shown in Fig. 1. The current-potential (I-U) curve was measured by use of a Hokuto denko HA 101 potentiostat. Reducing gases were introduced into the measuring chamber by a syringe and stirred well with a small fan. Carbon monoxide, propylene and isobutane, guaranteed to be more than 99.9% pure, were obtained from Seite.tsu Kagaku Company. Ethanol was evaporated from 99% pure liquid.Surface potential of palladium was measured by a vibrating capacitor method using a Cu reference electrode (6). Results and DiscussionThe Pd-TiOz d~ode.--The Pd-TiO2 diode at room temperature is very sensitive to hydrogen, but insensitive to any other reducing gases studied. It, however, becomes sensitive to CO, ethanol, and propylene at higher temperature. Figure 2 shows, as an example, a change in I-U curve of the P d-TiO2 diode by introduc-* Electrochemical Society Active...

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