Improvement of SO 2 sensing properties of WO 3 by noble metal loading YasuhiroShimizu * , Naoki Matsunaga, Takeo Hyodo, Makoto Egashira Department of Materials Science and Engineering, Faculty of Engineering, Nagasaki University, 1-14 Bunkyo-machi, Nagasaki 852-8521, Japan Abstract SO 2 sensingpropertiesofseveralsemiconductormetaloxideshavebeeninvestigatedinthetemperaturerangeof100±8008C.Eachoxide exhibitedcomplextemperature-andtime-dependentSO 2 response.Amongtheoxidestested,WO 3 exhibitedthehighestSO 2 sensitivityat 4008C,accompaniedbyaresistanceincrease,butitsresistancedecreasedinSO 2 attemperatureshigherthan5008C.ToimprovetheSO 2 sensitivityofWO 3 ,effectofametaladditionwasalsotested.Amongthemetalstested,theadditionof1.0wt.%Agwasmosteffectivefor improving the sensitivity at 4508C. In the case of the 1.0Ag/WO 3 , however, the sensor resistance decreased in SO 2 over the whole temperaturerangestudied.Thus,theadditionofAgledtochangesinboththesurfacestatesofSO 2 -relatedadsorbatesandtheelectronic interactionsbetweentheadsorbatesandWO 3 .TheresistanceincreaseofWO 3 uponexposuretoSO 2 at4008Cwassuggestedtoarisefrom theformationofSO 2  atsitesdifferentfromthoseforoxygenadsorbates,whiletheresistanceincreaseof1.0Ag/WO 3 at4508Cfromthe formationofSO 4 2 ontheAgloadedduetothereactionofagaseousSO 2 moleculeandtwoO 2 ad ionsontheAg.InterferencefromNO 2 to theSO 2 sensitivitywasfoundtobemoresigni®cantinthecaseof1.0Ag/WO 3 . # 2001ElsevierScienceB.V.Allrightsreserved. Keywords: SO 2 ;Semiconductorgassensor;WO 3 ;Ag/WO 3 1. Introduction Due to growing concern on environmental issues, much attentionhasbeenfocusedoncontinuouson-sitemonitoring ofairpollutants.Sulfurdioxideisoneoftypicalairpollu- tants to be monitored and then to be reduced from the environment by appropriate methods. For this purpose numerous efforts have been directed to developing SO 2 gas sensors, by mainly employing solid electrolytes such as alkali metal sulfates [1,2], Ag-b 00 -alumina [3], Na-b- alumina [4], NASICON Na 2 Zr 2 Si 3 PO 12 ) [5] and MgO- stabilized ZrO 2 [6,7],thelatterthreebeingcombinedwith anappropriateauxiliaryphase.Owingtoextensiveinvesti- gations, the properties of solid electrolyte SO 2 sensors are reaching the levels for practical applications [8]. On the other hand, electronic interactions between SO 2 and SnO 2 have ®rst studied by Lalauze et al. [9] in 1984. Thereafter SO 2 sensing properties of semiconductor metal oxides, such as CeO 2 [10] and SnO 2 [11,12], have been studied. However, little was known about SO 2 sensing properties of other semiconductor metal oxides. Theaimofthepresentstudywas®rsttoinvestigatethe potential of other semiconductor metal oxides as a SO 2 sensor.Inaddition,attemptsweremadetoimprovetheSO 2 sensingpropertiesofWO 3 bytheadditionofasmallamount ofnoblemetals.Cross-sensitivitytoNOandNO 2 wasalso tested. 2. Experimental 2.1. Preparation of sensor materials WO 3 powder was prepared by pyrolyzing NH 4 ) 10 W 12 O 41 5H 2 O at 10008C for 5h in air. SnO 2 powder was preparedfromanaqueoussolutionofSnCl 2 2H 2 Obyadding equimolar COOH) 2 2H 2 O. The resulting precipitate, SnC 2 O 4 , was washed with deionized water and dried at 908C for 10h. The powder was milled and then was decomposed at 4008Cfor20min.Thepowderwasmilled again and calcined at 10008C for 5h in air. ZnO, TiO 2 , In 2 O 3 and In 2 O 3 doped with 5.0mol% MgO In 2 O 3 - 5.0mol% MgO) powders were prepared from hydrolysis products of ZnNO 3 ) 2 6H 2 O, TiCl 4 , InNO 3 ) 3 3H 2 O, and InNO 3 ) 3 3H 2 O plus MgNO 3 ) 2 6H 2 O, respectively, by using an aqueous ammonia solution. Each precipitate was washedwithdeionizedwateranddriedat908Cfor10h.The resulting powder was milled, then was decomposed and calcined at 10008Cfor5hinair. Addition of a metal, typically 1.0wt.%, to the WO 3 powder was carried out by a conventional solution-based SensorsandActuatorsB772001)35±40 * Correspondingauthor.Tel.: 81-95-848-9648;fax: 81-95-848-9648. E-mail address:shimizu@net.nagasaki-u.ac.jpY.Shimizu). 0925-4005/01/$ ± see front matter # 2001ElsevierScienceB.V.Allrightsreserved. PII:S0925-400501)00669-4