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Abstract
The effects of laser irradiation on γ-Fe2O3 4 ± 1 nm diameter maghemite nanocrystals synthesized by co-precipitation and dispersed into an amorphous silica matrix by sol-gel methods have been investigated as function of iron oxide mass fraction. The structural properties of γ-Fe2O3 phase were carefully examined by X-ray diffraction and transmission electron microscopy. It has been shown that γ-Fe2O3 nanocrystals are isolated from each other and uniformly dispersed in silica matrix. The phase stability of maghemite nanocrystals was examined in situ under laser irradiation by Raman spectroscopy and compared with that resulting from heat treatment by X-ray diffraction. It was concluded that ε-Fe2O3 is an intermediate phase between γ-Fe2O3 and α-Fe2O3 and a series of distinct Raman vibrational bands were identified with the ε-Fe2O3 phase. The structural transformation of γ-Fe2O3 into α-Fe2O3 occurs either directly or via ε-Fe2O3, depending on the rate of nanocrystal agglomeration, the concentration of iron oxide in the nanocomposite and the properties of silica matrix. A phase diagram is established as a function of laser power density and concentration.
Highlights
The properties of iron oxide nanomaterials have recently attracted much interest because of their applications in active catalytic, magnetic, nonlinear optics materials and photo-electrodes.[1,2,3] Magnetic nanoparticles (NPs) have received considerable attention in the last decade, in nanotechnology because NPs offer interesting magnetic and surface properties while their nontoxicity, biodegradability and biocompatibility allow applications in biomedicine and biotechnology.[4,5,6,7,8,9]Iron oxide is widely used in industry
We report the effect of both laser irradiation and thermal treatment on the structural stability and phase transitions of γ-Fe2O3 NPs homogeneously dispersed in silica matrix. γ-Fe2O3 NPs of 4 ± 1 nm diameter have been prepared by co-precipitation and sol-gel methods and randomly dispersed into an amorphous silica matrix with different mass fraction values (0.07 to 1)
The results presented in this study confirm that the maghemite concentration and the nature of the host matrix play an important role in the stability of the maghemite nanocrystals and can influence the oxidation behavior of iron oxides under either annealing process or in situ laser irradiation
Summary
The properties of iron oxide nanomaterials have recently attracted much interest because of their applications in active catalytic, magnetic, nonlinear optics materials and photo-electrodes.[1,2,3] Magnetic nanoparticles (NPs) have received considerable attention in the last decade, in nanotechnology because NPs offer interesting magnetic and surface properties while their nontoxicity, biodegradability and biocompatibility allow applications in biomedicine and biotechnology.[4,5,6,7,8,9]. The transition temperature and the mechanism of the structural transformation can be largely influenced by numerous factors, such as particle size, pressure, lattice defects, surface phenomena and functionalization.[9,10,11,12] Recently, we reported how the phase transition from γ‐Fe2O3 to α‐Fe2O3 NPs can be induced by thermal treatment and laser irradiation. In the case of nanocomposites consisting of maghemite nanocrystals dispersed in a silica matrix, the matrix stabilizes the nanocrystals, delaying their thermal transformation into α-Fe2O3.[19] In addition, previous investigations [22] on γ-Fe2O3/SiO2 nanocomposites heated at different temperatures have even evidenced the possibility to stabilize an ε-Fe2O3 phase. We report the effect of both laser irradiation and thermal treatment on the structural stability and phase transitions of γ-Fe2O3 NPs homogeneously dispersed in silica matrix. Much attention was paid to the growth and presence of ε-Fe2O3 polymorph versus oxide concentration and in addition a structural diagram will be established for the γ-Fe2O3/ SiO2 system versus laser irradiation power
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