Nanocrystalline SnO2-Based Thin Films Obtained by Sol-Gel Route: a Morphological and Structural Investigation

The thermal evolution of sol-gel SnO2-based thin films was explored by investigating their structural and morphological features. Nanostructured SnO2 and Pt-doped SnO2 layers were obtained using tetra(tert-butoxy)tin(IV) and Pt(II) acetlylacetonate as precursors. Films were prepared by spin coating from ethanol solutions with different viscosity. After drying at room temperature, they were annealed in air at 673 and 973 K. The surface morphology was analyzed by scanning electron microscopy, atomic force microscopy, and scanning nearfield optical microscopy. The structural characterization was performed by means of glancing incidence X-ray diffraction and microdiffraction. Both drying at room temperature and thermal treatment at 673 K resulted in the formation of holes on the surface and inside the films. Their distribution and average dimension were found to depend mainly on the viscosity of the sol precursor, and on the presence of Pt in the films. After annealing at 973 K, surface segregation of PtOx phases and partial filling of the surface holes occurred. The effects of morphology on the electrical transport properties are discussed on the basis of sensitivity, S, measurements (S ) Rair/RCO, where Rair and RCO stand for the resistance in air and CO/ air, respectively).