Structure of SnO2 alcosols and films prepared by sol-gel dip coating

To obtain SnO2 films to be used for surface protection of fluoride glasses, a non-aqueous sol-gel route for the preparation was developed. An ethanoli c SnO2 colloidal suspension was prepared by thermohydrolysis of SnCl4 solut ion at 70 degreesC. By using this procedure, redispersable powders with nan ometer sized particles were obtained. Films were obtained by dip coating on glass and mica substrates. The structures of the ethanolic precursor suspe nsion and films were compared to those of similar samples prepared by the c lassical aqueous sol-gel route. Comparative analyses performed by photon co rrelation spectroscopy demonstrated that the powders obtained by freeze-dry ing are fully redispersable either in aqueous or in alcoholic solutions at pH greater than or equal to 8. As prepared sols and redispersed colloidal s uspensions have hydrodynamic radius distribution (2-14 nm) with an average size close to 7 nm. The variations in film structures with firing temperatu re were investigated by small-angle X-ray scattering and X-ray reflectometr y. The experimental results show that the films have a two level porous str ucture composed of agglomerates of primary colloidal particles. The sinteri ng of the primary particles leads to the densification of agglomerates and to the formation of inter-agglomerate spatially correlated pores. The volum e fraction of intra-agglomerate pores is reduced from approximate to 50% to approximate to 30% by the precipitation of precursor salts partially hydro lyzed in ethanolic solution. (C) 2001 Elsevier Science B.V. All rights rese rved.