Sensors and Actuators B 126 (2007) 174–180 Gas response times of nano-scale SnO 2 gas sensors as determined by the moving gas outlet technique A. Helwig a,∗ , G. M ¨ uller a , G. Sberveglieri b , G. Faglia b a Corporate Research Centre Germany, EADS Deutschland GmbH, D-81663 M¨ unchen, Germany b C.N.R. - INFM & Universit ` a di Brescia, I-25133 Brescia, Italy Available online 18 December 2006 Abstract We report on measurements of the gas response time of pure and platinum (Pt) catalysed SnO 2 films using the moving gas outlet method. In this mode of operation changing gas flows are forced directly onto the sensor surface to avoid the formation of a stagnant layer and to reveal gas response times unaffected by the usual diffusive delays. We find that the intrinsic response and recovery time constants τ follow an exponential temperature dependence τ (T)= τ 0 exp(E a /k B T), with E a standing for the activation energies for adsorption or desorption, respectively. We find that catalytic enhancement lowers the activation energies both for adsorption and desorption, with the more pronounced changes generally taking place in the case of the adsorption time constants. We further show that catalyst-induced changes in E a are accompanied by changes in the prefactor τ 0 which partly compensate the accelerating effect of a lower E a . © 2006 Elsevier B.V. All rights reserved. Keywords: Metal oxide; Response time; Moving gas outlet; Thin film; Compensation rule 1. Introduction Gas sensitive metal oxide materials (MOX) exhibit a bell- shaped variation of the steady-state gas response S with sensor operation temperature T. This S(T) variation leads to a sensitiv- ity maximum S M at a gas-specific temperature T M . It is further recognized that small additions of noble metal can enhance the gas response of MOX materials, leading to a lowering of T M and an increase in S M [1–3]. In a recent paper [4] we have presented a rate equation model that successfully describes measured S(T) curves in terms of four kinetic parameters: the first two relating to the coverage of surface oxygen ions (O - ) in a clean-air environment and the other two relating to a reducing gas species that may interact with the surface O - ions to produce a sensor response. These latter two parameters are the energy of adsorption of the reducing analyte gas on the MOX surface (E ads ) and the activation energy (E act ) for a first-order reaction of an analyte gas molecule with a ∗ Corresponding author at: EADS Corporate Research Centre, Microsystems and Electronics, LG-ME, 81663 Munich, Germany. Tel.: +49 89 607 28197; fax: +49 89 607 24001. E-mail address: andreas.helwig@eads.net (A. Helwig). surface oxygen ion. In this previous paper we have also shown that E ads and E act are reduced when catalytically active noble metal impurities are added to nano-scale SnO 2 films. In the present paper we should like to extend on our pre- vious work, presenting measurements of the intrinsic response and recovery time constants for two oxidising (O 2 , NO 2 ) and two reducing gas species (H 2 ,C 2 H 4 ) as obtained by the moving gas outlet method. This method has first been developed by the Link¨ oping group [5] and been applied to silicon carbide exhaust gas sensors revealing millisecond response times for H 2 and hydrocarbons at sensor operation temperatures in the vicinity of 600 ◦ C [6,7]. To our knowledge the present paper is the first report on measurements of the intrinsic gas response time τ of nanoscaled SnO 2 films using the moving gas outlet method. We observe that τ follows an exponential temperature dependence τ (T)= τ 0 exp(E a /k B T) and that the activation energies for both adsorption and desorption are lowered by catalytic enhance- ment. We further observe that, upon changing the target gas or upon introducing noble metal impurities, concomitant changes in the activation energy E a and in the prefactor τ 0 are corre- lated to each other via τ 0 = τ 00 exp(-E a /k B T 00 ); τ 00 ∼ 700 s; T 00 ∼ 780 K. This means that the effect of lowering E a is partly compensated by opposing changes in τ 0 . 0925-4005/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2006.11.032