Synthesis and formation mechanism of hollow carbon spheres encapsulating magnetite nanocrystals

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Hollow carbon spheres encapsulating magnetite nanocrystals were obtained in high-pressure argon at 600 °C followed by hydrolysis of Fe(NH3)2Cl2 in the hollow interiors at room temperature and heat treatment in argon at 450 °C for 2 h. The structure, morphology, and properties of the products were characterized by x-ray diffraction, field-emission scanning electron microscopy, transmission electron microscopy, thermogravimetric analysis, and vibrating sample magnetometry. The hollow carbon spheres have diameters of 1–10 ␮m and wall thicknesses of hundreds of nanometers; the wt% of magnetite nanocrystals in them is ∼13.2%. Equiaxed magnetite nanocrystals range in size from 15 to 90 nm, while acicular magnetite nanocrystals have diameters of ∼20 nm and lengths of 120–450 nm. The saturation magnetization value of the hollow carbon spheres encapsulating magnetite nanocrystals is 4.29 emu/g.

I. INTRODUCTION

Hollow carbon spheres (HCSs), exhibiting various excellent properties, such as distinct biocompatibility,1 high chemical inertness and specific surface area, low effective density, and high compressive strength,2 have scale-dependent applications in drug delivery,3 active material encapsulation,4 Li-ion battery and hydrogen storage,5 hollow sphere composites,6 damping materials,7 and surface functionalization.8 Magnetite (Fe3O4) particles, with properties of high-saturation magnetization, high magnetic susceptibility, and low toxicity,9 have also been studied for applications in biology and medicine,10 including cell labeling and magnetic separation, drug delivery, magnetic resonance imaging contrast enhancement, hyperthermia treatment of cancer, and tissue engineering. For these reasons, HCSs, serving as extremely small carriers encapsulating magnetite nanocrystals, can easily be used to enhance biocompatibility, to provide functional groups, to prevent the natural tendency of aggregation, and also to enhance the wear and corrosion resistance of the magnetic nanocrystals in their applications in biomedicine.11 In recent years, because of their improved properties over single-component counterparts and potential applications, different kinds of magnetic nanocrystals encapsulated in carbon shells have been successfully synthesized by various techniques, such as chemical vapor

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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0244 1980

http://journals.cambridge.org

J. Mater. Res., Vol. 23, No. 7, Jul 2008 Downloaded: 14 Mar 2015

deposition (CVD),12 pyrolysis of organometallic compounds,13 arc-evaporation techniques,14 and solvothermal methods.15 However, study of intact HCSs directly serving as the shells has been seldom reported, and only tin-encapsulated HCSs were successfully prepared through a sol-gel method that may suffer from disadvantages of complicated procedures and scale-up problems.16 So it is reasonable, and necessary, to develop an efficient and easy route to synthesize HCSs with a coreshell structure. In our previous work,17 HCSs were successfully prepared