Influences of Al, Pd and Pt additives on the conduction mechanism as well as the surface and bulk properties of SnO2 based polycrystalline thick film gas sensors

The reasons of the effect of Pd, Pt and Al additives on the sensing and conduction mechanism of SnO2 based thick film porous gas sensing layers are studied by a combination of DC-resistance, work function changes and catalytic conversion measurements. This is done by analyzing the dependence of the DC resistance on the corresponding band bending changes over a large range and the use of previously reported conduction models. The gained information deals with the surface band bending in the absence of ambient atmosphere oxygen, the position of the Fermi level, the concentration of free charge carriers, the Debye length and the width of the surface charge layer in various ambient conditions. Very interestingly, we found that in all cases the “doping” had an impact on both surface and bulk properties even if the additives and the technology of “doping” were targeted towards surface activation, in the case of Pt and Pd, and bulk compensation of donors, in the case of Al. Besides that, the catalytic conversion experiments indicated that the presence of Pt is associated with the reduction of the material in the absence of ambient atmosphere oxygen.