There have some studies on the absorption of a flexible polymer
chains, such as PAA [29]. Its groups can be immobilized on the surface
of magnetic supports and result in conjugates for the absorption
of materials. Based on the above analysis, a possible procedure
for the deposition of titania on the silica surface is proposed and
schematically shown in Fig. 5 to show the role of PAA treatment.
The molecular structure of PAA is shown in Fig. 5a and its abbreviated
form is given in Fig. 5b. During PAA treatment process, the
Fe3O4/SiO2 sample was suspended in the dimethylformamide solution
of PAA and the PAA macromolecules would form a thin coating
on the surface of silica. Hydrogen bridge type reaction is responsible
for the adsorption process of PAA macromolecules on the surface
of SiO2 [30,31]. Thus, the surface of silica after PAA treatment is full of carboxyl group which improves the dispersibility in the
reaction medium, as shown in Fig. 5c. When TBOT was hydrolyzed
with water and the resulting amorphous titania would form.
Adsorption of titania clusters could take place by the hydrogen
bridges and chemical interaction between titania surface and the
carboxyl group on the support [32], as shown in Fig. 5d. Through
PAA treatment, formation of titania nanoparticles on the surface
of silica is favorable. During subsequent heat treatment, the
bonded amorphous titania converted into crystalline anatase particles
and the PAA molecules decomposed completely. The covalent
bonds between silica and titania formed as the result.