Experimental and theoretical studies on the receptor and transducer functions of SnO2 gas sensors

Rutile-structure SnO₂ (cassiterite) is a wide-gap, n-type semiconductor with oxygen vacancies as bulk donors. It has many technical applications, e.g., in the fields of oxidation catalysis and chemical sensing where its surface properties play a key role. Its ability to transduce different gas-surface interactions to conductivity changes makes it useful in gas-sensor applications. The gas-surface interactions, such as adsorption/desorption, removal of lattice oxygen and other chemical reactions with a solid surface product (e.g., a possible ion exchange in the case of volatile sulphides) form the basis for the receptor function. The removal of lattice oxygen from the SnO₂(110) surface is considered here as an example of the defect mechanism for the receptor function. The electronic and atomic structures of the surface change with changes in the lattice oxygen content. This leads to a surface relaxation and changes in the dipole layer at the ionic surface in addition to changes in the Schottky barrier which is a result of the charge accumulation onto the surface from the bulk Bloch states of the n-type semiconductor. Changes in the dipole layer and the Schottky barrier change the work function of the semiconductor oxide and may reflect in its electrical conductivity. Grain contacts together with electrode-ceramic interfaces play the key role for the transducer function in the structure of a porous ceramic gas sensor. A transistor model is described for the transducer function in the case of the adsorption/desorption mechanism and the role of tin(II) ions at the reduced SnO₂(110) surface is discussed in the case of the surface defect mechanism.