Uniform magnetic hollow nanospheres (GdNi2, Co5Gd) coated with Gd2O3 have been successfully prepared on a large scale via a urea-based homogeneous precipitation method using silica (SiO2) spheres as sacrificed templates, followed by subsequent heat treatment. Nitrogen sorption measurements and scanning electron microscope reveal that these hollow-structured magnetic nanospheres have the mesoporous shells that are composed of a large amount of uniform nanoparticles. After reduction treatment, these nanoparticles exhibit superparamagnetism that might have potential applications in medicine. Furthermore, the developed synthesis route may provide an important guidance for the preparation of other multifunctional hollow spherical materials.
Hollow nanospheres with tailored structures have attracted tremendous interest due to their higher specific surface area, better permeation, lower density than their bulk counterparts, and their extensive potential applications in magnetic, catalysis, chemical reactors, drug delivery, and optical materials [
As a form of nanocomposites, the particles combine magnetic materials and gadolinium oxide to integrate the functions of magnetic and optical responses of the individual components. In particular, shell/shell nanocomposites of magnetic materials and Gd2O3 combine the advantages of individual shells to enhance space utilization of the capsules and greatly extend the application. However, few studies have been involved in preparation of such shell/shell nanocomposites. Recently, template-directed synthesis, with soft templates and hard templates, has been demonstrated to be an effective approach to prepare inorganic hollow spheres. Up to now, using carboxylate silica particles [
In this work, we have successfully prepared monodisperse shell/shell nanocomposites comprising magnetic hollow spheres and Gd2O3 shells. The main steps of the preparation involve synthesis of monodisperse SiO2 templates, installation of inorganic seeds, and growth and formation of inner or outer inorganic shells via a layer-by-layer deposition process [
Schematic illustration of the preparation of M@Gd2O3 (M = GdNi2, Co5Gd, Fe3O4) hollow nanospheres.
Nickel nitrate hexahydrate, cobalt nitrate hexahydrate, and gadolinium (III) nitrate hexahydrate were from Aladdin Chemistry Co. Ltd. Iron (III) nitrate nonahydrate was from Sinopharm Chemical Reagent Co. Ltd. Other reagents were used without further purification.
The synthesis procedure of silica/Ni3Si2O5(OH)4 [
The concentrations of gadolinium nitrate, urea, and nickel-silica monodisperse spheres were 0.005 M, 0.05 M, and 12 g/L, respectively, which were prepared by following the same procedure. By immersing the products in 200 mL of 0.5 M NaOH, the silica cores were dissolved by alkaline and the remainder shells were collected by centrifugation. Then, these powders were obtained through heat treatment at 800°C for 2 h in air at a heating rate of 10°C min−1. Finally, the particles were reduced in H2 gas (40 cm3/min) at 450°C (at a rate of 10°C/min) for 2 h in the fixed-bed reactor. After the completion of the reduction process, the sample was protected at room temperature in the flowing-inert gas for 12 h. The core-shell structured M@Gd2O3 (M = Fe3O4, and Co5Gd) are prepared by the same method (Table
SiO2 |
|
Gd(NO3)3·6H2O |
---|---|---|
1.2 g | 0.21 g Fe(NO3)3·9H2O | 0.23 g |
1.2 g | 0.15 g Co(NO3)2·6H2O | 0.23 g |
1.2 g | 0.15 g Ni(NO3)2·6H2O | 0.23 g |
The structures of the samples were characterized by X-ray diffractometer (XRD) (Bruker D8 ADVANCE) using Cu-K
The layer-by-layer (LBL) route was adapted to fabricate the core-shells sphere using SiO2 as the sacrificed template, followed by deposition of desired coating chemicals or nanoparticles with opposite surface charges [
SEM images of (a) SiO2 spheres, ((b) and (d)) silica/Ni3Si2O5(OH)4 core-shell structures, Ni3Si2O5(OH)4 hollow spheres with an average size of 580 nm. ((c) and (e)) SiO2@Ni3Si2O5(OH)4@Gd2O3 core-shell structures, Ni3Si2O5(OH)4@Gd2O3 hollow spheres with an average size of 600 nm. (f) Ni3Si2O5(OH)4@Gd2O3 hollow spheres (200 nm).
SEM image of (a) iron-silica composite nanospheres. (b) Composite hollow spheres (400 ± 5 nm). (c) Iron-gadolinium hollow nanospheres (485 ± 5 nm). (d) After reduction. (e) Cobalt-silica composite nanospheres and (f) removing silica cores (510 ± 5 nm). (g) Precursor nanospheres and (h) removing silica cores after reduction (530 ± 5 nm).
To confirm the composition of products, EDS mapping was devoted for elaborate analysis of the hollow spheres, as shown in Figure
EDS surface analysis of nickel-gadolinium, iron-gadolinium, and cobalt-gadolinium composite two-layer hollow spheres: ((a), (e), and (i)) dark field image, ((b), (f), and (j)) silicon map, (c) nickel map, (g) iron map, (k) cobalt map, and ((d), (h), and (l)) gadolinium map.
These powders were reduced in H2 current (40 cm3/min) at 450°C (at a rate of 10°C/min) for 2 h in the fixed-bed reactor. After the completion of the reduction process, the sample was protected at room temperature in the flowing-inert gas for 12 h. These multilayer hollow spheres were also characterized by XRD as shown Figure
The X-ray powder diffraction patterns of GdNi2@Gd2O3, Fe3O4@Gd2O3, Co5Gd@Gd2O3 hollow spheres after reduction in H2 current at 450°C for 2 h.
The magnetic properties of the heterodimers of Gd2O3 and GdNi2 nanoshells are measured using a vibrating sample magnetometer at room temperature. As shown in Figure
Room-temperature (300 K) magnetic hysteresis loops of GdNi2@Gd2O3, Fe3O4@Gd2O3, and Co5Gd @Gd2O3 hollow spheres, respectively. The upper left inset of (a) shows GdNi2@Gd2O3 hollow spheres suspended in water and separated from solution under an external magnetic field and the lower right shows close-up of the central region of the magnetization curves. The inset of (b) shows the highlight of low field region.
The specific surface area and porosity of the as-prepared M@Gd2O3 (M = GdNi2, Co5Gd, Fe3O4) hollow spheres were determined by nitrogen sorption measurements. Figure
The pore diameter distribution and the inset of N2 adsorption/desorption isotherm of the hollow spheres. (a) GdNi2@Gd2O3; (b) Fe3O4@Gd2O3; (c) Co5Gd@Gd2O3.
In summary, we have demonstrated a feasible route to prepare uniform hollow shell-shell microspheres of composite materials (M@Gd2O3, M = GdNi2, Co5Gd, Fe3O4) by a template–directed method with silica microspheres as templates. Moreover, the nanoshells thickness can be easily tuned by the times of coating and the diameter of microsphere can also be altered by changing the size of the core. The as-prepared composite materials exhibit excellent adsorption performance for N2 and superparamagnetism at room temperature and are expected to be useful in water treatment, drug delivery, and many other applications. Thus, the facile synthesis method can readily be extended to the preparation of other magnetic layers @RE2O3 functional materials with shell-shell structure.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work was supported by the National Natural Science Foundation of China (11174165), the Natural Science Foundation of Ningbo (2012A610051), and the K. C. Wong Magna Foundation.