Organically Hybridized In 2 O 3 Thin Film Gas Sensors

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Organically Hybridized In2O3 Thin Film Gas Sensors Ichiro Matsubara, Norimitsu Murayama, Woosuck Shin, and Noriya Izu National Institute of Advanced Industrial Science and Technology, Shimo-Shidami, Moriyama-ku, Nagoya 463-8560, Japan ABSTRACT Organically hybridized In2O3 thin films have been prepared. The surface of In2O3 thin films was hybridized by organic components with various kinds of functional groups. Upon exposure to CO gas, the electrical resistance of the hybrid sensor with amino group in the organic components increased (R-increasing response), whereas H2 gas caused the decreasing in the sensor resistance. No response was obtained to CH4 gas. For the n-type metal oxide semiconductors, the R-increasing response cannot be explained by the ordinary combustion mechanism. The response of R-increasing or R-decreasing to CO was controlled by functional groups of organic components as in the case of SnO2-based hybrid thin films. The approach of organic-inorganic hybridization is effective to realize the selective detection of reducing gas molecules.

INTRODUCTION Polycrystalline metal-oxide semiconductors are the most widely used materials for gas sensors [1, 2]. Among them, n-type semiconductors such as tin oxide (SnO2) and indium oxide (In2O3) have been reported to show pronounced sensing properties to H2, CO and other gases [3-6].The metal-oxide sensors operated on the ordinary combustion mechanism [1] can detect any kinds of reducing gases in principle. The lack of gas selectivity, e.g. between CO and H2 gases has been regarded as a problem to be solved. A lot of efforts have been paid to improve the gas selectivity by adding small amount of noble metals and/or other oxides [7-11]. Matsubara et al. have proposed a new concept of organic-inorganic hybrid sensors for the improvement in the gas selectivity [12, 13]. The most important point of the concept is that the roles of molecular recognition and transducer are shared by the organic and inorganic components, respectively. Based on this concept, organically hybridized SnO2 thin films have been prepared by using 3-aminopropyltrimethoxysilan [12, 13]. Among reducing gases (CO, H2 and CH4), the hybridized SnO2 sample exhibited resistance-increasing (R-increasing) response only to CO gas, whereas electrical resistance decreased to other gases. The R-increasing response cannot be explained by the ordinary combustion mechanism. The appearance of such the anomalous R-increasing response to CO gas depends on the functional groups of the organic components. Matsubara et al. have proposed a possible mechanism for the R-increasing response based on hydrogen bonding between the organic components and SnO2 surface (Figure 1). The electric charge is expected to be supplied from the amino groups of the organic components to the SnO2 surface through the hydrogen bonding, resulting in a decrease in the depth of the space-charge region. When this type of sensor is exposed to gas molecules having stronger interactions with the amino groups of the organic components,

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