Ž . Thin Solid Films 391 2001 204᎐208 XPS, ISS and TPD study of Pd᎐Sn interactions on Pd᎐SnO x systems N. Tsud, V. Johanek, I. Stara, K. Veltruska, V. Matolın U ´ ´ ´ ´ Department of Electronics and Vacuum Physics, Charles Uni ¨ersity, V Holeso ¨ickach 2, 18000 Prague 8, Czech Republic ˇ ´ Abstract The sensitivity and selectivity of SnO based gas sensors could be improved by doping of small amount of transition me 2 this work we used X-ray photoelectron spectroscopy, ion scatteringspectroscopy, and thermaldesorption techniques to investigate Pd evaporated on SnO thin layer substrate, prepared by spray pyrolysis. The evolution of PdrSnO layer morphology x x with increasing amount of Pd deposits was studied using the XPS inelastic background shape analysis. The observations are compared to the results obtained from natural SnO crystal and metallic Sn substrates. A strong Pd᎐Sn bimetallic interact 2 observed, resulting in the formation of PdSn alloy of noble metal-like electronic structure. This feature also corresponds to the presence oftwo CO desorption states with low energy peaks at approximately 390 K. The relation ofour resultswith the operation mechanism of gas detection are discussed. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Palladium; Tin; Tin oxide; XPS, ISS, TPD 1.Introduction Semiconductor gas sensors based on tin dioxide are widely used forthe detection oftoxic and explosive gases in air.The generaloperation mechanism of gas detection by a change in surface conductivity has been w x widely described in the literature 1 ᎐ 5 . In an atmo- sphere of air containing inflammable gases, three reac- Ž . tions occur independently on the surface of SnO : i 2 gas molecules react with the surface-adsorbed O 2y and Ž. 2y release electrons;ii O are desorbed by thermal 2y y Ž. vibrations O sO q2e ; and iii oxygen molecules 2 are readsorbed in the form of O 2y and consume car- rier electrons.Differentcontributionsto the overall conductivity of SnO have been described: the grain- 2 size effect in the case of polycrystalline samples, which depends on the sample preparation and could be af- fected by a small amount of a foreign metal, influence U Corresponding author. Tel.: q42-2-2191-2323; fax: q42-2-688- 5095. Ž . E-mail address: matolin@mbox.troja.mff.cuni.cz V. Matolın . ´ of the texture ofSnO type and nature of electrical 2 contactsand namelythe presence ofadditivesand impurities into the bulk and on the surface. The bond- Ž . ing of transition metal atoms Pd, Pt,Ag at surface or bulk sites influences considerably the sensor response. The ‘doping effect’ has been studied extensively in the past,however,numerouscontradictories appeared in the literature.Two possible sensitization mechanisms wx Ž have been proposed6 : an electroniceffect direct exchange of electrons between the semiconductors and . the metal additive particles and a catalyticone Ž . metallic clusters activate the gaseous species . In the case ofPd the electronic sensitization has been sug- wx gested in Yamazoe3 . This type of mechanism was wx opposed by Kappler 7 , who ruled out the existence of metallic clusters or even atoms in the metallic state at the surface,which did notprovide an explanation of the sensoreffectsby meansof spill-overeffectsor Fermi energy control. Oxidation ofPd at the surface has been found necessary for bulk doping by in-diffu- sion of evaporated Pd, because only the small Pd 2q w x species could penetrate into SnO 7,8 .On the other 2 0040-6090r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. Ž . PII: S 0 0 4 0 - 6 0 9 0 0 1 0 0 9 8 3 - X