Abstract
The influence of the water content in the initial composition on the size of silica particles produced using the Stöber process is well known. We have shown that there are three morphological regimes defined by compositional boundaries. At low water levels (below stoichiometric ratio of water:tetraethoxysilane), very high surface area and aggregated structures are formed; at high water content (>40 wt%) similar structures are also seen. Between these two boundary conditions, discrete particles are formed whose size are dictated by the water content. Within the compositional regime that enables the classical Stöber silica, the structural evolution shows a more rapid attainment of final particle size than the rate of formation of silica supporting the monomer addition hypothesis. The clearer understanding of the role of the initial composition on the output of this synthesis method will be of considerable use for the establishment of reliable reproducible silica production for future industrial adoption.
Highlights
The use of sol-gel methods allows the fabrication of silica nanoparticles that is alternative to the conventional pyrogenic or precipitation/ion-exchange methods
As with most sol-gel synthesis routines the practical undertaking appears straightforward, but the resultant materials are dependent on a raft of process parameters
We postulate an upper boundary condition that exists between 40 wt% and 48 wt% of water, but whose influence may appear at even lower water content
Summary
The use of sol-gel methods allows the fabrication of silica nanoparticles that is alternative to the conventional pyrogenic or precipitation/ion-exchange methods. The transformation of silicon alkoxides into silica progresses through a series of chemical reactions that have subtle interdependencies This was described clearly by Assink and Kay [8] who postulated that each of the 15 potential intermediates have slightly different hydrolysis and condensation reaction rates, where the intermediates are described by the general formula Si(OC2H5)x(OH)y(OSi)z, where x + y + z = 4. The subtleties emerge as a function of the number of hydrolysis reactions that can take place on each silicon-containing moiety; each moiety effectively becomes a new intermediate capable of further reactions. Such reactions include, potentially, further hydrolysis and polymerisation via condensation
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