Monodisperse single-crystal mesoporous magnetite nanoparticles induced by nanoscale gas bubbles
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- Dong, F., Guo, W. & Ha, C. J Nanopart Res (2012) 14: 1303. doi:10.1007/s11051-012-1303-9
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Monodisperse single-crystal mesoporous magnetite nanospheres with particle size of ~100 nm and pore size of 7.6 nm were synthesized through a solvothermal process. Transmission electron microscopy images clearly show the mesoporous structure of the products. Nitrogen adsorption–desorption data confirmed that the pore size is in the range of mesoscale. Based on the evolution experiments, a plausible mechanism was proposed including a gas bubble induced mesoporous structure formation process. The mesoporous magnetite nanospheres show high magnetization value, which resulted from the single crystalline structure, as confirmed by the high-resolution transmission electron microscopy data. By simply decreasing the concentration of ammonium acetate, magnetite hollow spheres or aggregated nanoparticles could be obtained. This work may provide new advances in the approaches to fabricate magnetite with different interior structures.
Synthesis and assembly of porous materials has attracted considerable attention for their applications in the fields of sensors, catalysis, and drug delivery (Lou et al. 2008; Zeng 2006; Trewyn et al. 2007; Tiemann 2007; Zhang et al. 2009). Most of the porous structures are based on silicon-containing materials. However, some important transition metal oxides (such as Fe3O4, Co3O4, TiO2, etc.) have also been fabricated in the past decade (Lu and Schuth 2006; Tian et al. 2004; Li et al. 2009; Liu et al. 2009a; Dong et al. 2011). To date, various interior porous architectures including simple hollow spheres, core–shell and mesoporous structures for metal oxide have already been constructed (Liu et al. 2004; Suh et al. 2006, Sun et al. 2011).
Designing nanoscale pores into magnetic materials has attracted intense interest for enhanced performance in the field of absorbent, efficient catalysis, magnetic driven drug delivery system, etc. (Caruso et al. 2001; Andersson et al. 2007; Cao et al. 2008). In particular, mesoporous Fe3O4 have been prepared by different methods in recent years. Jiao and co-workers fabricated the mesoporous iron oxide using three-dimensional mesoporous silica (KIT-6) as a hard template (Jiao et al. 2006). Xia et al. fabricated the magnetite porous materials with polymer surfactants as soft templates (Xia et al. 2009). Kwak group recently reported mesoporous magnetite using triblock copolymer (PEO100-PPO65-PEO100) micelles as pore-forming template (Yu and Kwak 2010). Guo et al. (2009) prepared mesoporous magnetite particles with a similar-surfactant structure worked as soft template. These template methods usually include some complicated processes such as repeat treatments with corrosive solution or calcinations at high temperatures to remove the template.
Gas bubble induced porous structure has been emerged as a facile and environment-friendly method to introduce pores into materials (Craig 2011). N2, Ar, or N2/CO2 have been used as the gas bubble to induce the formation of expected hollow structured from ZnSe, Fe3O4, or CaCO3 materials, respectively (Peng et al. 2003; Lynch et al. 2011; Han et al. 2009). In this study, ammonia gas produced from our reaction system was used as a template to fabricate mesoporous Fe3O4 nanospheres. By simply adjusting the concentration of the reaction materials (ammonium acetate), appropriate amount of ammonia gas could be released and worked as the templates to form the mesopores in the magnetite spheres. Compared with the conventional template method, this simple gas-bubble template approach may be more convenient. Magnetite hollow spheres or magnetite nanoparticles could also be fabricated by decreasing the ammonium acetate concentration.
Preparation of magnetite nanoparticles
Ferric chloride hexahydrate (FeCl3·6H2O, extra pure) was purchased from Junsei Chemical Co., Ltd. Ammonium acetate (NH4Ac, >98 %) and ethylene glycol (EG, >99 %) were purchased from Aldrich. All chemicals were used as received. Magnetite products were prepared through a solvothermal reaction. For a representative synthesis, 5 mmol of FeCl3·6H2O was dissolved in 60 mL of ethylene glycol with stirring to form a clear homogeneous solution, followed by the addition of 0.1 mol of NH4Ac (1.67 mmol/L). The mixture was stirred vigorously until it became homogeneous and then was sealed in a Teflon-lined stainless steel autoclave (100 mL capacity). The autoclave was heated to 200 °C and maintained 12 h, and then it was cooled to room temperature. The black precipitate was collected by magnetic separation and sequentially rinsed for five times with ethanol and five times with water under ultrasonication to remove the solvent thoroughly, and then dried in a vacuum oven at 60 °C for 12 h.
Transmission electron microscopy (TEM) images were obtained using a JEOL JEM 2010 microscope operating at 200 kV accelerating voltage. Scanning electron microscopy (SEM) images were taken by a field emission scanning electron microscope (FE-SEM; Philips XL30SFEG and Hitachi S-4800). A thin gold film was sprayed on samples before measurements. For the particle size estimation, over 100 particles on the SEM images were averaged. Nitrogen adsorption measurements were performed on an ASAP 2010 volumetric adsorption analyzer (Micromeritics, Norcross, GA, USA) at 77 K. Brunauer-Emmett-Teller (BET) method was utilized to determine the surface area. Pore size distribution was calculated using the adsorption branch of the isotherm by the Barrett–Joyner–Halenda (BJH) method. Powder X-ray diffraction (XRD) patterns were recorded using a Philips diffractometer with a Geiger counter. The X-ray tube was operated at 40 kV and 30 mA (Cu Kα radiation with Ni filter, λ = 1.5406 Å). Magnetization of nanoparticles was measured using a superconducting quantum interference device (SQUID, MPM5-XL-5). XPS spectrum was obtained using a VG Scientific ESCALAB 250 XPS spectrometer with a monochromatic Al Kα source including charge compensation at Korea Basic Science Institute (KBSI) (Busan, S. Korea).
Results and discussion
In summary, we have proposed a gas bubble induced formation mechanism for fabrication of mesoporous Fe3O4 nanoparticles. The evolution experiment demonstrated a nanocrystal aggregation and bubble-assisted recrystallization process. It is found that enough high concentration of ammonium acetate is necessary for producing ammonium bubbles to form the mesopores in magnetite nanospheres. Interestingly, by decreasing the amount of ammonium acetate, the simple hollow spheres and tiny aggregated particles could be also prepared facilely. Both mesoporous and simple hollow spheres show superparamagnetic behavior and strong magnetic response with higher saturation magnetization value compared with the materials without using ammonium acetate. Other metal oxide materials with mesoporous or hollow structure will be fabricated via this new approach involving the gas bubble induced mesopores process. The high Ms value combined with the unique interior structure would make the magnetite as ideal candidates for targeted drug delivery system, where the particles can be directed by an external magnetic field.
The work was supported by the National Research Foundation of Korea (NRF) Grant funded by the Ministry of Education, Science and Technology, Korea (MEST) (Acceleration Research Program Program (No. 2012-0000108); Pioneer Research Center Program (No. 2012-0000421/2012-0000422)), and the Brain Korea 21 Project of the MEST.