“Nanocasting”:  Using SBA-15 Silicas as Hard Templates to Obtain Ultrasmall Monodispersed γ-Fe₂O₃ Nanoparticles

Abstract

This work describes the use of mesoporous SBA-15 silicas as hard templates for the size-controlled synthesis of oxide nanoparticles, with the pores acting as nanoscale reactors. This fundamental work is mainly aimed at understanding unresolved issues concerning the occurrence and size dependence of phase transitions in oxide nanocrystals. Aqueous solutions of Fe(NO3)3*9H2O are deposited inside the pores of SBA-15 silicas with mesopore diameters of 4.3, 6.6, and 9.5 nm. By calcination, the nitrate salt transforms into FeOx oxides. The XRD peaks of nanocrystals are broad and overlapping, resulting in ambiguities attributed to a given allotropic variety of Fe2O3 (alpha, epsilon, or gamma) or Fe3O4. The association of XRD, SAED, and Raman information is necessary to solve these ambiguities. The metastable gamma-Fe2O3 variety is selectively formed at low Fe/Si atomic ratio (ca. 0.20) and when a low calcination temperature is used (773 or 873 K followed by quenching to room temperature once the targeted temperature is reached). The small size dispersion of the patterned nanoparticles, suggested on a local scale by TEM, is confirmed statistically by magnetic measurements. The nanoparticles have a superparamagnetic behavior around room temperature. Their magnetic moments (from 220 to 370 mB), their sizes (from 4.0 to 4.8 nm), and their blocking temperatures (from 36 to 58 K) increase with the silica template mesopore diameter. Their magnetic properties are compared to those of standard gamma-Fe2O3 nanoparticles of similar size, obtained by coprecipitation in water and stabilized by a citrate coating.

Facile synthesis of ordered magnetic mesoporous γ-Fe2O3/SiO2 nanocomposites with diverse mesostructures

“…3 are type IV isotherms with a H 1 -type hysteresis loop, which are characteristic of mesopores with very uniform pore size. There is no obvious information related to the pore clogging from the single step in the hysteresis loop, implying that most of the γ -Fe 2 O 3 domains were embedded in the silica matrix [23]. But the surface area and pore volume were slightly decreased and the pore size distribution was broader with iron content increased, implying that the pore uniformity decreases, especially for P MM-30.…”

“…The specific surface areas were much lower than those synthesized from iron nitrate (387.7 and 221.6 m 2 /g for P MM-10-FeCl 3 and P MM-20-FeCl 3 , respectively). The iron oxide domains dispersed in the mesopores may account for the above phenomena [23]. After calcination at 550 • C, iron oxide grew in the pores to form a large domain of α-Fe 2 O 3 and decreased surface area.…”

Magnetic properties of Fe2O3 nanoparticles embedded in hollows of periodic nanoporous silica

“…Various metal or metal oxides can be grown in the silica matrix creating nanocomposites with interesting magnetic properties and rigid structure. 4,5 The stability of electronic and high-density data storage devices made of magnetic nanocomposite is limited by their superparamagnetic transition. 3 Therefore, there is a strong interest to understand the magnetization processes in the superparamagnetic region.…”

Fabrication of mesoporous SiO2–C–Fe3O4/γ–Fe2O3 and SiO2–C–Fe magnetic composites

“…inside the pores of porous silica materials). This approach has been investigated by different researchers [16][17][18][19][20][21][22][23]. It has been observed that it is difficult to obtain magnetic mesoporous silica composites that combine a high magnetic saturation with a superparamagnetic behaviour.…”