SnO2 thin-films prepared by a spray–gel pyrolysis: Influence of sol properties on film morphologies

CLEMENTE LUYO; ISMAEL FÁBREGAS; L. REYES; JOSÉ L. SOLÍS; JUAN RODRÍGUEZ; WALTER ESTRADA; ROBERTO J. CANDAL

THIN SOLID FILMS

Elsevier Science

Año: 2007 vol. 516 p. 25 - 33

0040-6090

Nanostructured tin oxide films were prepared by depositing different sols using the so-called spray–gel pyrolysis process. SnO2 suspensions (sols) were obtained from tin (IV) tert-amyloxide (Sn(t-OAm)4) or tin (IV) chloride pentahydrate (SnCl4·5H2O) precursors, and stabilized with ammonia or tetraethylammonium hydroxide (TEA-OH). Xerogels from the different sols were obtained by solvent evaporation under controlled humidity.The Relative Gelling Volumes (RGV) of these sols strongly depended on the type of precursor. Xerogels obtained from inorganic salts gelled faster, while, as determined by thermal gravimetric analysis, occluding a significant amount of volatile compounds. Infrared spectroscopic analysis was performed on raw and annealed xerogels (300, 500 °C, 1 h). Annealing removed water and ammonium or alkyl ammonium chloride,increasing the number of Sn–O–Sn bonds. SnO2 films were prepared by spraying the sols for 60 min onto glass and alumina substrates at 130 °C. The films obtained from all the sols were amorphous or displayed a very small grain size, and crystallized after annealing at 400 °C or 500 °C in air for 2 h. X-ray diffraction analysis showed the presence of the cassiterite structure and line broadening indicated a polycrystalline material with a grain size in the nanometer range.Results obtained from Scanning Electron Microscopy analysis demonstrated a strong dependence of the film morphology on the RGVof the sols.Films obtained from Sn(t-OAm)4 showed a highly textured morphology based on fiber-shape bridges, whereas the films obtained from SnCl4·5H2O had a smoother surface formed by “O-ring” shaped domains.Lastly, the performance of these films as gas sensor devices was tested. The conductance (sensor) response for ethanol as a target analyte was of the same order of magnitude for the three kinds of films. However, the response of the highly textured films was more stable with shorter response times.