Advanced Materials Research Vols. 47-50

Abstract: Recently, we have proposed some theoretical models, power laws and effect of particle shape and size, for semiconductor gas sensors. The models show that a depletion theory of semiconductor can be combined with the dynamics of adsorption and/or reactions of gases on the surface. In the case of SnO2, the relative resistance (R/R0) is proportional to PO 2 n, where n is a constant value (n=1/2) on oxygen partial pressure. In addition, carrier concentration in SnO2 influences depth of the depletion. In this study, to experimentally reveal such effects, we tried to control the carrier concentration in SnO2 by foreign doping and examined their electrical resistance and sensor response. Correlations between doping concentration, crystalline size, and partial pressures of oxygen and H2 on the electric resistance are discussed to reveal the material design for semiconductor gas sensors.

Abstract: The deposition of Pd nanoparticles onto thiol-functionalized SnO2 nanoparticles was carried out at the aqueous/organic liquid/liquid interface to prepare Pd-loaded SnO2 nanoparticles for high-sensitive sensor materials. The method is based on the self-assembly deposition of Pd onto dimercaptosucinic acid (DMSA)-functionalized SnO2 nanoparticles (mean diameter: 4 nm) prepared by a hydrothermal method. Pd nanoparticles of 2-3 nm in chloroform were prepared by thermal decomposition of a Pd complex at high temperature (170 °C). Thin film-type device using the prepared Pd-loaded SnO2 nanoparticles was fabricated and tested for its sensing properties.

Abstract: H2 sensing properties of anodic TiO2 films equipped with Pd or Pd-Pt alloy electrodes has been investigated in air and in N2 atmosphere at 250°C and room temperature. The use of a Pd-Pt alloy electrode and a Pt paste improved the magnitude of H2 response, response time, stability and pretreatment- and atmosphere-dependent response properties under humid environments.

Abstract: The organic/MoO3 hybrid sensors show a distinct response to aldehyde gases, formaldehyde and acetaldehyde, by changing the electrical resistance, whereas they show almost no response to other VOCs. The organic/MoO3 hybrids have a potential for selective VOC detection because the sensing properties of organic/MoO3 hybrids can be controlled by the kind of interlayer organic components. In this study, we have carried out the detail characterization of the interlayer of the organic/MoO3 hybrids as well as the valence of Mo and evaluation of their sensing properties.

Abstract: CO sensing properties of various transition metal oxide-loaded SnO2 sensors were investigated in a reducing atmosphere such as wet H2 gas. Among transition metal oxide-loaded SnO2 sensors used in the present study, 5wt%CuO-loaded SnO2 sensor exhibited the highest sensitivity to 1% CO in wet 50% H2 at around 150 °C, while it showed no sensitivity in dry 50% H2. From XRD analysis, CuO was reduced to metallic Cu in a reducing atmosphere at above 150 °C. H2-temperature-programmed reduction (H2-TPR) and diffuse reflectance fourier transform infrared (DRIFT-IR) spectroscopy measurements were carried out to discuss the sensor response mechanism.

Abstract: For gas sensors, not only sensitivity and selectivity but also fast response speed is very important. We fabricated a restive type gas sensor using ceria (cerium oxide) porous thick film with a particle size of about 100 nm in order to improve the response speed of the sensor. First, we investigated the sensing properties for the change of oxygen partial pressure. The sensor was able to detect the variation of oxygen partial pressure in the wide range from 10-18 Pa to 105 Pa at 873 K. The response time (t90), which is defined as the time until response changes 90% of the saturated value after a sudden change of oxygen partial pressure, was about 10 ms at 1073 K. Next, we investigated the sensing properties for carbon monoxide. We developed the new sensor having two ceria thick films with and without catalyst layer to give selectivity. The sensor was able to detect carbon monoxide over 330 ppm at 773 K, and t90 was 2 s at 723 K. By using Pt/Al2O3 catalyst, the sensor hardly responded for hydrogen, so it was implied that the sensor had a good selectivity for carbon monoxide.

Abstract: A novel composite electrode-containing gold/platinum nanoparticles on the vertically aligned multi-wall nanotubes (MWCNTs) by electron beam evaporation (e-beam evaporation) is reported herein. The gold/platinum nanoparticles were less than 10 nm, and distributed uniformly on the surface of vertically aligned MWCNTs under the control of e-beam evaporation. The composite electrodes could be applied in the field of biomedical sensor and energy storage devices. This process was straightforward and contained high specific surface areas of gold/platinum nanoparticles on the MWCNTs. It was helpful to enhance the efficiency of organic fuel decomposition as well as advance the sensor precision. In addition, the nanoparticles on the MWCNTs could also facilitate the high electron mobility and chemical stability.