The competing effect of ammonia in the synthesis of iron oxide/silica nanoparticles in microemulsion/sol–gel system

Combined microemulsion/sol–gel processes may be used to prepare composite nanoparticles. In this work, iron oxide/silica nanoparticles were synthesized in such a combined system, having ammonium hydroxide acting in both processes: as precipitating agent in the synthesis of iron oxide and as catalyst in the sol–gel process of silica. The nanoparticles were synthesized in microemulsions containing tetraethoxysilane (TEOS) and composed by Triton X-100/hexyl alcohol/cyclohexane/aqueous solution, with different amounts of ammonium hydroxide. Powder materials were obtained after centrifugation, washing and drying, and they were analyzed as synthesized and after heating at 350, 500 and 1000 °C. The influence of base quantity was evaluated in relation to the size of the particles and the iron oxide phase synthesized. TEM analysis showed that nanometric iron oxide particles were formed and that they were percolated by a net of amorphous silica. The higher amount of ammonia seems to have induced TEOS condensation process instead of iron oxide particles’ growth, indicating that the quantity of base influenced both the microemulsion system and the sol–gel process, but with minor influence over the iron oxide composition, as evidenced by FT-IR and DRX.

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► Synthesis of iron oxide–silica nanoparticles in microemulsion system.
► Ammonia acts as precipitant agent in coprecipitation and as catalyst in sol–gel.
► Higher amount of ammonia results in smaller iron oxide particles.
► Nanometric iron oxide particles are percolated by a net of amorphous silica.
► Silica showed little inhibition over iron oxide transition phase during heating.