Roles of structure and electron mobilization in enhanced ethanol sensing by Al doped SnO2 nanoparticles

Improvement in sensing performance by metal oxide based materials has usually been achieved by doping, morphology tuning, particle size tailoring or porosity modification. The existing models of band bending and grain boundary depletion layer that explain chemiresistive sensing address the above issues in terms of qualitative electronic behaviour. However, for thick film sensors in particular, there is a clear demarcation between the surface and the bulk. Thus the concept of gross electronic distribution needs refinement in terms of variable behavior of electrons in the surface and in the bulk so that the roles of different layers in a sensor film are distinguishable. Using the case of improved 98.2% ethanol sensing response by paramagnetic aluminium doped tin oxide mesoporous nano-systems, we have demonstrated how Al doping promotes difference in electronic behavior in the surface and in the bulk of thick film sensors, complementing the existing charge depletion model. The 4-fold improvement in sensing responses by Sn0.947Al0.144O1.881 and Sn0.869Al0.242O1.888 as compared to tin oxide have been explained in terms of shortening of the unit cell by virtue of aluminium doping, and the 2.4-fold improvement in sensing response by Sn0.869Al0.242O1.888 as compared to Sn0.947Al0.144O1.881 has been explained by surface and bulk electron mobilization. Considering the conceptual inclusiveness of correlation studied, it is expected to be applicable in explaining sensing mechanisms based on electron mobilization in other metal oxide sensor systems also.