NiFe2O4@SiO2 Nanoparticles Stabilized by Porous Silica Shells

Abstract

NiFe 2 O 4 nanoparticles stabilized by porous silica shells (NiFe 2 O 4 @SiO 2 ) were prepared using a onepot synthesis and characterized for their physical and chemical stability in severe environments, representative of those encountered in industrial catalytic reactors. The SiO 2 shell is porous, allowing transport of gases to and from the metal core. The shell also stabilizes NiFe 2 O 4 at the nanoparticle surface: NiFe 2 O 4 @SiO 2 annealed at temperatures through 973 K displays evidence of surface Ni, as verified by H 2 TPD analyses. At 1,173 K, hematite forms at the surface of the metallic cores of the NiFe 2 O 4 @SiO 2 nanoparticles and surface Ni is no longer observed. Without the silica shell, however, even mild reduction (at 773 K) can draw Fe to the surface and eliminate surface Ni sites.

Tuning Physical Properties of NiFe2O4 and NiFe2O4@SiO2 Nanoferrites by Thermal Treatment

“…Nevertheless, we cannot exclude that some part of SiO 2 burned during the annealing process, and the iron concentration in this sample increased, and, consequently, the drastic increase of M S value is observed. Similar enhancement (i.e., after annealing process) of M S was noticed, e.g., by Chaudhuri et al, [36] Wang et al [37] and Mitra et al, [39] but the morphology of their samples is significantly different, so it is hard to compare them to our annealed NFO@SiO 2 sample. At 100 K and 300 K, the M(H) dependences are almost linear with zero coercivity as for paramagnetic materials, which confirms the decomposition of some nanoparticles to paramagnetic fractions.…”

“…Similarly, the refined value of crystallite size equals d crys = 57.7 ± 3.1 nm and d crys = 90.7 ± 4.4 nm, respectively. Such secondary phase was already noted for ferrites, e.g., by Huo et al, [4] Shukla et al [37] and Pozo Lopez et al [42,43] as a result of synthesis and further processing.…”

“…As reported, a sol-gel method was predominantly applied in preparing such nanocomposites based on, e.g., NiFe 2 O 4 , CoFe 2 O 4 , CuFe 2 O 4 , ZnFe 2 O 4 , (NiZn)Fe 2 O 4 and SiO 2 matrix. [35][36][37][38][39][40][41][42][43][44][45] One of the positive features of the sol-gel method is the homogeneous dispersion of NPs. Beyond preventing particle aggregation, the silica matrix is used in active catalytic applications, whereas a porous shell allows the transport of gases from and to the ferrite core.…”

“…Over the past few years, several studies have been done on NiFe 2 O 4 @SiO 2 nanoparticles synthesized mainly by sol-gel or ball-milling. [35][36][37][38][39][40][41][42][43][44][45] Mitra et al [39] for NiFe 2 O 4 NPs showed that the duration of heat treatment at different temperatures determines the particle size ≤ 25 nm. However, the superparamagnetic behavior dependent on the NPs size, in this case, was evidenced only at room temperature for particles < 10 nm.…”

“…In addition, the diamagnetic SiO 2 shell has a noticeable impact on magnetic properties, causing the reduction of magnetization. Shukla et al [37] revealed that the shape of NiFe 2 O 4 @SiO 2 nanoparticles, e.g., nanosphere and nanorod, depends on the applied synthesis route; thus, it affects the shape anisotropy.…”