Chemistry and structure of colloids obtained by hydrolysis of Fe(III) in the presence of SiO4 ligands

Fe(III)–Si systems at various Si/Fe molar ratios and pH values were examined at the local and semi-local scale. The growth of Fe species is strongly dependent on the Si concentration: at Si/Fe<1, the presence of edge and corner Fe–Fe linkages indicates a three dimensional growth regime, whereas for Si/Fe>1, the predominance of edge linkages corresponds to a two dimensional growth. Si–O–Fe and Si–O–Si bonds are formed simultaneously from the lowest pH and the lowest Si/Fe. At low pH, silica domains of approximately 2 nm are detected within the samples having the highest Si/Fe ratios. The polymerization level of Si strongly decreases at high pH, whereas Fe polymerization is only moderately sensitive to pH variations. The high fractal dimension of these amorphous phases indicates dense aggregation. The evolution of the fractal dimension with pH is correlated with modifications of the Fe speciation.