Fabrication of Hollow and Yolk–Shell Structured η-Fe2O3

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Fabrication of Hollow and Yolk−Shell Structured η‑Fe2O3 Nanoparticles with Versatile Configurations Junyu Zhong,†,‡ Chuanbao Cao,*,‡ Hui Liu,† Yulong Ding,§ and Jun Yang*,† †

State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China ‡ Research Center of Materials Science, Beijing Institute of Technology, Beijing 100081, China § Institute of Particle Science & Engineering, University of Leeds, Leeds LS2 9JT, United Kingdom ABSTRACT: A solution-based approach has been developed for the synthesis of hollow or yolk−shell structured η-Fe2O3 nanoparticles with versatile configurations via the Ostwald ripening process. By simply controlling the amount of PVP and reaction time in solution, hollow or yolk−shell structured η-Fe2O3 nanoparticles with spherical, egg-like, olivary, elliptical, and shuttle-like configurations can be easily fabricated. The interior structures and morphologies of the particles are found to be effective ways to affect both the specific saturation magnetization and coercivity of the η-Fe2O3 nanoparticles.

1. INTRODUCTION Nanomaterials with hollow interiors have stimulated great research interest in many emerging fields owing to their potential applications in catalysis, drug delivery, sensing, nanoreactors, and so on.1−4 Over the past 30 years, various methods based on sacrificial templates of polymer and inorganic spheres,5−16 liquid droplets,17 vesicles,18−20 and microemulsion droplets,21−25 have been developed for the synthesis of hollow structured nanomaterials, and the reports were nicely reviewed by Lou et al.26 The morphologies of the products prepared this way are usually confined to the original shape of the templates.27−29 Zeng and co-workers took a specific approach based on the Ostwald ripening process to synthesize semiconductor ZnO with a hollow interior.30 The particles thus obtained were mainly spheres. It is therefore still a challenge to develop effective ways for the fabrication of hollow structured nanomaterials with versatile configurations. Herein, we report for the first time a facile solution-based route to produce hollow and yolk−shell structured η-Fe2O3 nanoparticles with versatile shapes via the Ostwald ripening process. By simply controlling the addition of multidentate ligand poly(vinyl-pyrrolidone) (PVP) and reaction time, versatile ηFe2O3 nanoparticles, including spherical, egg-like, olivary, elliptical, or shuttle-shaped configurations with hollow or yolk−shell structures, were fabricated successfully. The interior structure was found to exhibit a strong influence on the magnetic property of the η-Fe2O3 nanoparticles, raising an effective strategy to tune the properties of nanomaterials for some specific applications.

FeSO4 and 0.5 g of PVP (MW = 30000) were dissolved in 60 mL of DMF. The solution was heated to 40 °C, and 0.8 mL of hydrazine hydrate was then added dropwise, followed by heating and maintaining at 160 °C for 2, 2.5, and 3 h with constant magnetic stirring. Then the solution was cooled to room temperature, the precipitates, which were washed several times with ethanol and distilled water, were calcined in a muffle furnace for 3 h at 500 °C. The loose red products were then collected and characterized. The yolk−shell structured η-Fe2O3 nanoparticles with other configurations can be generated by varying the amount of PVP in solution while keeping the reaction time in solution for 3 h, followed by the calcination treatment. Powder X-ray diffraction (XRD) patterns were recorded on a X’Pert Pro MPD diffractometer, using CuKα radiation (λ = 0.15418 nm). The morphology and structures of the η-Fe2O3 samples were examined using a Hitachi TM-1000 and S-4800 scanning electron microscopy (SEM), a Hitachi H-800 transmission electron microscopy (TEM) with a tungsten filament, and an accelerating voltage of 200 kV. For SEM and TEM measurements, the η-Fe2O3 nanoparticles were dispersed into ethanol by ultrasonic and then a drop of the nanoparticle solution was dispensed onto a 3 mm carbon-coated copper grid. Excess solution was removed by absorbent paper, and the sample was dried under vacuum at room temperature. The magnetic characterization was conducted on a Lake Shore 7300 vibrating sample magnetometer (VSM) at room temperature. Five milligrams of the powder was wrapped with Teflon tape and measured.

2. EXPERIMENTAL SECTION In our experiments, all chemical reagents, including FeSO4, poly(vinyl-pyrrolidone) (PVP), N,N-dimethylformamide (DMF), and hydrazine hydrate, were purchased from Beijing Chemical Co. Ltd. The chemicals were of analytical grade and used without further purification. In a typical synthesis for the hollow or yolk−shell structured η-Fe2O3 nanoparticles with spherical configuration, 0.21 g of

3. RESULTS AND DISCUSSION Synthesis of η-Fe2O3 nanoparticles with hollow interiors was carried out through a solution-based process followed by

© 2012 American Chemical Society

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September 28, 2012 December 2, 2012 December 21, 2012 December 21, 2012 dx.doi.org/10.1021/ie302652b | Ind. Eng. Chem. Res. 2013, 52, 1303−1308

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calcination treatment under ambient pressure. By controlling the amount of PVP in solution and the reaction time for the growth of particles, η-Fe2O3 samples with different morphologies and inner structures can be fabricated. Typically, 0.21 g of FeSO4·H2O and 0.5 g of PVP (MW = 30000) were dissolved in 60 mL of N,N-dimethylformamide (DMF). The solution was heated to 40 °C, and then 0.8 mL of hydrazine hydrate was added dropwise. The solution was then heated and maintained at 160 °C in a refluxed system for different times under constant magnetic stirring. When the solution was cooled to room temperature, the precipitates, which were washed several times with ethanol and distilled water, were calcined in a muffle furnace for 3 h at 500 °C. Figure 1 shows the X-ray diffraction

Figure 2. (a,b) SEM and TEM images of solid η-Fe2O3 nanoparticles obtained by controlling the amount of PVP at 0.5 g and the reaction time in solution for 2 h. (c) TEM image of η-Fe2O3 nanoparticles with vacancies in the central region obtained by controlling the amount of PVP at 0.5 g and the reaction time in solution for 2.5 h. (d) TEM image of η-Fe2O3 nanoparticles with hollow interiors obtained by controlling the amount of PVP at 0.5 g and the reaction time in solution fo 3 h. (e,f) TEM images of η-Fe2O3 nanoparticles with yolk− shell structures obtained by controlling the amount of PVP at 0.7 g and the reaction time in solution for 3 h. These images illustrate that the η-Fe2O3 nanoparticles display spherical shapes when the amount of PVP in solution is controlled at 0.5 g.

Figure 1. XRD patterns of Fe2O3 nanoparticles obtained by controlling reaction time in solution for 2 h (a), 2.5 h (b), and 3 h (c), respectively, followed by the calcination treatment. The amount of PVP in solution for all samples was set at 0.5 g. Diffraction peaks of all samples are indexed to rhombohedral Fe2O3. The XRD reference labeled as JCPDS file 21-0920 is also shown (d).

to Cu2+,31 Ni2+,31,32 Co2+,33 the Fe2+ in the solution first reacted with hydrazine to form complex [Fe(N2H4)3]2+. Then at the boiling point of DMF (∼153 °C), the excessive hydrazine acted as a reducing agent and converted [Fe(N2H4)3]2+ to Fe through homogeneous nucleation. The formation of Fe nanoparticles is likely to involve the following chemical reactions

(XRD) pattern of the nanoparticles obtained by controlling the reaction time in solution for 2, 2.5, and 3 h, respectively. The diffraction peaks can be indexed to rhombohedral Fe2O3 (JCPDS file 21-0920). Information on the morphologies and inner structures of ηFe2O3 nanoparticles are revealed by the typical scanning electron microscopy (SEM) and transmission electron microcopy (TEM) images from Figure 2. As revealed by Figure 2a and b, when the amount of PVP was kept at 0.5 g and the solution was maintained at 160 °C for 2 h, solid η-Fe2O3 products with spherical shape were obtained. The η-Fe2O3 products consisting of numerous smaller particles have an average diameter of ∼800 nm. As the reaction time in solution was prolonged for 2.5 h, vacancy was observed in each η-Fe2O3 nanoparticle, while the size and shape of the particle were kept unchanged, as shown in Figure 2c. After the reaction time was further prolonged for 3 h, hollow η-Fe2O3 nanospheres with enlarged void space were obtained, where the hollow interior and outer shell can be differentiated by the brightness contrast, as shown in Figure 2d. The void space was then kept unchanged even as the reaction time in solution was further increased. The shell thickness of the final hollow η-Fe2O3 nanospheres is ∼250 nm, while the shape and size are the same as those of the solid η-Fe2O3 products. Regarding the mechanism for the formation of η-Fe2O3 nanoparticles with hollow interiors, we believe that the Ostwald ripening process plays an important role. To start with, similar

Fe2 + + N2H4 → [Fe(N2H4)3 ]2 +

(1)

[Fe(N2H4)3 ]2 + + N2H4 → Fe ↓ + 4NH3 ↑ +2N2 ↑ +H 2 ↑ +2H+

(2)

As illustrated in Figure 3, the obtained small Fe nanoparticles were protected by PVP, and they then diffused and aggregated to form larger nanoparticles due to the magnetic attraction among the small Fe nanoparticles. Most probably, PVP serves both as stabilizing agents and soft templates to direct the assembly of small Fe nanoparticles along the radial orientation with the size increased from the center to the surface for minimizing the steric hindrance. As time involved, Ostwald ripening, defined as “the growth of larger crystals from those of smaller size which have a higher solubility than the larger ones”,34,35 took place in the aggregated Fe nanoparticles, and the solid evacuation started at the center of the particle, finally resulting in the formation of Fe nanoparticles with hollow interiors, which were stable as there is no size gradient along the radial orientation (path (a) in Figure 3). Then the 1304

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Figure 3. Schematic for the formation of η-Fe2O3 nanoparticles with different interior structures: (1) Fe2+ ions. (2) Small Fe nanoparticles synthesized by N2H4 reduction of [Fe(N2H4)3]2+ in the presence of stabilizing agent PVP. (3) Aggregates of small Fe nanoparticles by magnetic attraction; circles with different contrasts indicate particles of different sizes, where organizations in particle aggregates are greatly affected by the amount of PVP in solution. (4) η-Fe2O3 nanoparticles with different interior structures fabricated by the Ostwald ripening process and calcination treatment.

calcination in a muffle furnace at 500 °C converted the hollow Fe particles into η-Fe2O3 nanoparticles with hollow interiors. The mechanism we proposed above might be only one of the rational approaches for the formation of Fe2O3 nanostructures. In addition, it should be mentioned that because Fe particles are easily oxidized in aqueous phase, the aggregated particles before calcination in a muffle furnace might have been oxidized into an intermediate product consisting of complex Fe oxides. Interestingly, when the amount of PVP was increased to 0.7 g and the reaction time in solution was still kept for 3 h or longer, spherical η-Fe2O3 nanoparticles with yolk−shell structures were fabricated as dominant products (Figure 2e and f). The diameter of the whole sphere and core are ∼800 and ∼500 nm, respectively, and the thickness of the shell is ∼150 nm. It is concluded that at high concentration of PVP in solution, the solid evacuation via Ostwald ripening started at a particular region underneath the immediate surface layer, which divided the pristine solid sphere into two discrete regions and formed a homogeneous yolk−shell structure (path (b) in Figure 3). Moreover, as displayed in Figure 2e and f, the cores of the ηFe2O3 nanoparticles are entirely detached from the shell, which endow the yolk−shell structured nanoparticles with a welcome feature to be applied as nanoreactors. In addition to being used as stabilizer to prevent nanoparticles from aggregation in chemical preparation of metal nanoparticles, including gold,36 silver,37,38 palladium,39 and platinum,40 PVP is usually used to facilitate shape control and highly shape-selective synthesis of nanomaterials. A large number of nanomaterials of different shapes can be achieved by simply varying the concentration of PVP in solution.41−47 In our work, the influence of PVP on the morphology of η-Fe2O3 nanoparticles has been systematically investigated. Keeping the reaction time in solution for 3 h before the calcination treatment, when the amount of PVP in solution was increased to 1.2, 1.7, 2.2, and 2.8 g, yolk−shell structured η-Fe2O3 nanoparticles with egg-like (Figure 4a and b), olivary (Figure 4c and d), elliptical (Figure 4e and f), and shuttle-like shapes (Figure 4g and h) were successfully fabricated, respectively. The morphologies of η-Fe2O3 nanoparticles obtained under different experimental conditions are summarized in Table 1. In particular, when the PVP in solution was controlled at 0.6 and 1.1 g, respectively, spherical and egg-like η-Fe2O3 nanoparticles with double-shelled hollow structures were also dominantly observed (Figure 5a and b), which enriched the design and

Figure 4. (a,b) SEM and TEM images of yolk−shell structured ηFe2O3 nanoparticles with egg-like shapes obtained by controlling the amount of PVP at 1.2 g. (c,d) SEM and TEM images of yolk−shell structured η-Fe2O3 nanoparticles with olivary shapes obtained by controlling the amount of PVP at 1.7 g. (e,f) SEM and TEM images of yolk−shell structured η-Fe2O3 nanoparticles with elliptical shapes obtained by controlling the amount of PVP at 2.2 g. (g,h) SEM and TEM images of yolk−shell structured η-Fe2O3 nanoparticles with shuttle-like shapes obtained by controlling the amount of PVP at 2.8 g. For all samples, the reaction time in solution was controlled for 3 h.

Table 1. Morphologies of η-Fe2O3 Nanoparticles under Different Experimental Conditions samples

amount of PVP (g)

reaction time in solution (h)

1 2 3 4

0.5 0.5 0.5 0.6

2 2.5 3 3

5

0.7

3

6

1.1

3

7

1.2

3

8

1.7

3

9

2.2

3

10

2.8

3

morphology solid spheres spheres with vacancies inside spheres with enlarged void spaces spheres with double-shelled hollow structures spheres with yolk−shell structures egg-like particles with doubleshelled hollow structures egg-like particles with yolk−shell structures olivary shapes with yolk−shell structures elliptical shapes with yolk−shell structures shuttle-like shapes with yolk− shell structures

construction of η-Fe2O3 nanoparticles fabricated via the Ostwald ripening process. It is obvious that the amount of 1305

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Figure 5. (a) TEM image of spherical η-Fe2O3 nanoparticles with double-shelled hollow structures obtained by controlling the amount of PVP at 0.6 g. (b) TEM image of egg-like η-Fe2O3 nanoparticles with double-shelled hollow structures obtained by controlling the amount of PVP at 1.1g. (c,d) SEM images of two cracked yolk−shell structured η-Fe2O3 nanoparticles obtained by controlling the amount of PVP at 1.2 g. The reaction time in solution was 4 h for (a) and 3 h for (b, c, and d). Figure 6. (a) Room-temperature magnetic hysteresis loops of spherical η-Fe2O3 nanoparticles with solid, hollow, and yolk−shell structures. (b) Room-temperature magnetic hysteresis loops of yolk− shell structured η-Fe2O3 nanoparticles with spherical and elliptical morphologies.

PVP can sufficiently affect the Ostwald ripening process, which takes place at the region where crystallites are small and/or less dense (path (c) in Figure 3), through determining the aggregated forms of small Fe nanoparticles generated by N2H4 reduction of Fe2+ ions. The precise control induced by PVP is yet to be explored. Nonetheless, all the schemes in Figure 3 allow us to pursue even higher levels of synthetic architecture. For instance, η-Fe2O3 nanoparticles with multipleshelled hollow structures may be created, which can provide different types of concentric space within a nanosphere. Figure 5c and d display the SEM images of two cracked yolk−shell ηFe2O3 nanoparticles fabricated with the amount of PVP at 1.2 g, from which the inner details are clearly observed. Both the shell and the core of these yolk−shell structured η-Fe2O3 nanoparticles with rough surfaces were found to be aggregated from small nanoparticles of approximate 25 nm. Different from the approaches developed for the preparation of heterogeneous nanoparticles with yolk−shell structures (core and shell regions are made of different types of materials),27,28,48−50 reports on the synthesis of homogeneous yolk−shell structured nanoparticles (core and shell regions are made of same materials) still remain scarce.30,51 To investigate the effect of interior structures on the property, magnetic hysteresis was analyzed at room temperature for the spherical η-Fe2O3 nanoparticles with solid, hollow, and yolk−shell structures. As shown in Figure 6a, the three samples have strong magnetic responses to a varying magnetic field and exhibit typical ferromagnetic behavior,52−54 and the influence of the interior structures of the particles on the magnetic properties is evident. The specific saturation magnetization and coercivity for spherical η-Fe2O3 with solid, hollow, and yolk−shell structures are 0.50 emu g−1 and 0.42 kOe, 0.07 emu g−1 and 0.19 kOe, and 0.03 emu g−1 and 0.12 kOe, respectively. The sizes of the particles forming the three structured η-Fe2O3 nanoparticles were definitely different after the Ostwald ripening process. The reduction in saturation magnetization and coercivity with the order of solid > hollow > yolk shell seemed to contradict those reported in the literature, in which nanomaterials consisting of smaller-sized particles

possess weaker saturation magnetization due to the thermal agitation and surface spin-canting effects.55−58 The void space in η-Fe2O3 nanoparticles with hollow and yolk−shell structures may be a possible reason to induce the reduction of specific saturation magnetization and coercivity. In addition, the numerous pores on the particle surfaces enable the hollow or yolk−shell structured η-Fe2O3 nanoparticles to have good substrates to host the residues of PVP and other impurities brought by the calcination treatment, which may have negative influences on the magnetic properties. Interestingly, the morphologies also have significant effects on the magnetic properties of the η-Fe2O3 nanoparticles. As displayed in Figure 6b, the specific saturation magnetization of yolk−shell structured η-Fe2O3 nanoparticles with elliptical shapes was higher than that of yolk−shell structured η-Fe2O3 nanoparticles with spherical shapes and cannot be saturated magnetically in the fields as high as 10 kOe. This is reasonable because the interior void structures as well as the numerous pores on the surface of the yolk−shell structured η-Fe2O3 nanoparticles must be different with the changes in morphologies. Both of them can affect the magnetic properties of the nanoparticles. The results of this work indicate that the magnetic properties of η-Fe2O3 nanoparticles can be tuned to some extent by simply modulating their morphologies.

4. CONCLUSION In summary, a solution-based approach has been successfully developed for the synthesis of hollow or yolk−shell structured η-Fe2O3 nanoparticles with versatile configurations via the Ostwald ripening process. By simply controlling the amount of PVP and reaction time in solution, hollow or yolk−shell structured η-Fe2O3 nanoparticles with spherical, egg-like, olivary, elliptical, and shuttle-like configurations have been easily fabricated. The interior structures and morphologies of 1306

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the particles were found to be effective ways to affect both the specific saturation magnetization and coercivity of the η-Fe2O3 nanoparticles.

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AUTHOR INFORMATION

Corresponding Author

*Tel: 86-10-68913792 (C.C.), 86-10-82544915 (J.Y.). Fax: 8610-68913792 (C.C.), 86-10-82544814 (J.Y.). E-mail: cbcao@ bit.edu.cn (C.C.), [email protected] (J.Y.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by the National Natural Science Foundation of China (Project No. 21173226), China Postdoctoral Science Foundation (Project No. 20110490595), and 100 Talents Program of the Chinese Academy of Sciences.

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dx.doi.org/10.1021/ie302652b | Ind. Eng. Chem. Res. 2013, 52, 1303−1308