Tin oxide SnO2 is used to detect pollutant gas due to its properties of being twice in configuration Sn4+ and Sn2+. But what is amazing is that it is more active when we add additives to it such as Sb ,Pt ,Cu, Ag, etc. The band structure changes and the gap diminishes but what exactly happens chemically to reach all these properties?
Must take into account the catalytic role of metals, usually nobles, which are deposited on the surface of metal oxides such as SnO2, WO3 and others. The process can be complex. You can have a first reaction between nanograins and gas molecules, which can be divided. some species can leave as a gas and other reacting with the surface of the sensitive material.
Infact, the gas and oxide surface interaction with metal assisted oxide (like SnO2) depends on the electronic structure of metal clusters they can act as a donor or acceptor. The enhanced sensing properties of the sensors with metal additive in SnO2 layer can be explained by three processes : catalytic, Fermi energy control and spillover. Catalytic process: the oxygen adsorption on SnO2 process can also be controlled by suitable metal additive which can act as a catalyst, which helps in dissociating the analyzing gas and adsorption/desorption reactions. The Fermi energy control process of Pt-SnO2 interface can be explanined with keeping in mind the Schottky barrier, where the alignment of Fermi level due to Pt-SnO2 interface has been formed. This alignment can influence the space charge region and band bending in SnO2 layers (oxygen reacts with Pt by taken electron from it and consequently Pt takes electrons form SnO2 to compensate the loss of electrons), both are controlled by oxidation and reduction of Pt at the surface. The metal clusters facilitate the reaction when the gas reacts directly with adsorbed oxygen from atmosphere. In case of H2 gas reacting with Pt assisted SnO2 thin film, H2 molecules are dissociated on metal Pt clusters and spilled over the sensing layer resulted in accelerated reaction and hence improved response and recovery time is observed (spillover effect).
There is mainly two models proposed two explain the role of additive in the detection mechanism. The first is called the spiller over effect when the additive of a simple catalyst which help only the dissociation of the polluting gases without entering in direct interaction with tin oxide,
The second model known under the name of electronic model explain the effect of additive by direct electronic interaction between metallic aggregate and SnO2 surface. Under air the metallic aggregates are presented in oxidised state, the aggregartes pass to metallic state after reaction with reducing gases (such as PtO2 + CO------Pt+CO2). In the metallic state Pt release electrons to Tin oxide and increase in consequence the its electrical conductivity. We have experimentally demonstrated that the electronic is true for Pt and Pd additives. In situ measurement under gas and under temperature using combined measurement of XAS and electrical measurement shows direct correlation between the aggregate behavior and the enhancement observed for gas detection. For more detail you can take a look to our paper published in sensors and actuators B and Journal of electrochemical society (gaidi et al).
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