Selective CO gas detection of SnO2–Zn2SnO4 composite gas sensor
Introduction
The n-type semiconductor gas sensors based on SnO2 and ZnO have been studied for the detection of inflammable or toxic gases such as H2, CH4, C2H5OH or CO [1]. However, the lack of selectivity, especially to CO gas, is one of their disadvantages. In order to modify the gas sensing properties, the use of metal additives such as Ag, Pt, and Pd as catalysts was widely studied [2]. In2O3 [3], [4] and SnO2 [5] gas sensors showed the selective detection of CO gas when they were mixed or coated with small amount of oxide.
Spinel zinc stannate (Zn2SnO4) has often been studied as a sensor for i-C4H10, NO, NO2 and C2H5OH gases [6], [7], [8], [9]. In our previous report, Zn2SnO4 also showed the selectivity of CO gas against H2 gas [10]. However, the Zn2SnO4 showed the current-limiting behavior at Pt (electrode)/Zn2SnO4 interface, especially in reducing atmosphere, thus restricting its application as a gas sensor. The layered-type sensors using ZnO (or SnO2) and Zn2SnO4 have been fabricated to examine the interfacial effect in detail. It was found that the conductive ZnO and SnO2 could be ohmic electrodes for Zn2SnO4 while Pt electrode was responsible for the non-ohmic current–voltage (I–V) characteristics in reducing atmosphere [10]. Since the layered-type sensors are difficult to fabricate [10], [11], the composite-type sensors were suggested since they are easy to fabricate and still contain many hetero-contacts between two phases [12], [13]. For example, ZnO(n)–CuO(p) composite sensor showed the higher sensitivity to CO gas than pure ZnO [12]. ZnO(n)–SnO2(n) composite sensor also showed the enhanced sensitivity due to the ZnO/SnO2 interface [13].
In our previous report, the addition of CuO markedly changed the CO gas sensing characteristics in addition to the electrical conductivity of SnO2 [14]. While SnO2 showed the maximum sensitivity at ∼350°C, the addition of CuO into SnO2 moved the sensitive curve to the lower temperature (150–200°C). We have also reported that the temperature showing the maximum sensitivity to CO gas for ZnO sensor also decreased from ∼400 to 200°C with the CuO addition [12].
In this study, for the selective detection of CO gas against H2 gas, CuO-doped SnO2–Zn2SnO4 composite type sensors were fabricated in the form of pellet by sintering at 1000°C for 3 h. The I–V characteristics were examined to see the influence of Zn2SnO4 content on the sensitivity. The use of SnO2 as the matrix composition is expected to provide the ohmic I–V behavior to the composite. The effect of Zn2SnO4 addition and CuO-doping was discussed in connection with the electrical conductivity, the sensitivity to reducing gases, and the selective detection of CO gas. A small amount of CuO-doping and Zn2SnO4 addition for the SnO2-based sensor was expected to provide the sensitivity to CO gas and, thus, the selectivity for CO against H2 gas at relatively low temperature.
Section snippets
Experimental procedure
Appropriate amount of tin oxide powder (99.9%, Aldrich, USA) and zinc oxide powder (99.9%, Aldrich, USA) were ball-milled with zirconia ball in ethyl alcohol for 12 h. The slurry was filtered, dried on hot plate, and crushed with Teflon-coated magnetic stirring bar in a glass vial. The powder was uniaxially pressed into pellets and subsequently cold-isostatically pressed at 2 tonnes/cm2. The pellets were sintered at 1000°C for 3 h in air.
For CuO-coating, the porous, sintered samples were dipped
Phase and microstructure
Fig. 1 shows XRD patterns of SnO2, ZTO33, and ZTO75 samples, sintered at 1000°C for 3 h. It was known that SnO2 and ZnO form the intermediate compounds ZnSnO4 and Zn2SnO4 [17], [18]. The Zn2SnO4 formation was found when ZnO and SnO2 mixtures were sintered at or above 1000°C [10]. As the Zn2SnO4 content increased, the stronger Zn2SnO4 peaks were found in this system. Since the solubility of ZnO into SnO2 is ∼1 mol%, all the samples containing ZTO in front of their sample notation are, to be
Conclusion
The I–V characteristics and the sensing properties to reducing gases of SnO2–Zn2SnO4 composites, coated or doped with CuO, were examined. Since all samples have the similar microstructure, the composition of composites was the major factor determining the gas sensitivity. All samples examined in this study showed the nearly linear I–V characteristics due possibly to the ohmic SnO2-phase in the composites. The addition or coating of CuO to SnO2 lowered the temperature showing the maximum
Acknowledgements
The authors wish to acknowledge the financial support of Korea Research Foundation made in the program year of 1998.
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