Deposition of low-density thick silica films from burning sol-gel derived alcogels

In the current study we show that the combustion of sol-gel derived alcogels with specifically tailored composition leads to the release of silica nanoparticles from the burning alcogel in a controlled manner which enables direct deposition of the released nanoparticles into low-density silica thick films. The process has some similarities to flame spray pyrolysis but requires no aerosol generator or other sophisticated instrumental setup. By the proper choice of catalysts and mixture of silicon alkoxides for the synthesis of the alcogel, preferential hydrolysis and polycondensation of one of the alkoxides is achieved. This leads to the formation of an alcogel with volatile silica precursor trapped in the gel pores. Resulting alcogels were burned to deposit uniform porous silica films with density of ~0.1 g/cm³ and primary particle size of ~10 nm. Demonstrated method yields silanol-free silica directly, without additional treatment steps and enables straightforward control over the deposition rate and coarseness of the layer by simple adjustment of the composition of the silica alcogel. The maximum layer thickness is limited only by the deposition time (in the current work up to 134 μm). Such technique of porous oxide film preparation could potentially be extended to the preparation of porous films from other oxides by using respective metal alkoxides as precursors.