Preparation of high-quality magnetic nanoparticles with a narrow size distribution, reproducible physical properties and production with short processing times are some of the key issues in nanoparticle research today. Recent studies have also focused on the development of novel synthesis techniques for the production of uniform magnetic oxide materials. Among these, ferrites, such as magnetite (Fe3O4), are of great interest because of their potential for a wide range of applications from information storage and electronic devices to medical diagnostics and drug delivery. Magnetite nanoparticles are commonly produced by the co-precipitation, hydrothermal, microemulsion or sonochemical method etc. But one of the major drawbacks of these processes are that they have had very limited success towards the synthesis of smaller (<25 nm) monodisperse nanoparticles. Here in, we report the synthesis of magnetite nanoparticles using inexpensive and non-toxic metal precursors. The advantages of this method are that it can be scaled up easily to produce nanocrystals on an ultra large scale in a single reaction and without a further size-sorting process. Secondly, the synthetic process is environmentally friendly and economical. Thirdly, the synthetic method is a generalized process that can be used to synthesize different kinds of monodisperse nanocrystals.
To synthesize iron oxide nanoparticles, iron (III) acetylacetonate and oleic acid were dissolved in 1-octadecene and heated to 100oC under vacuum and kept at that temperature 30 minutes. Then they were further heated to 320oC under nitrogen and kept at that temperature for another 30 minutes. Finally they were cooled to room temperature the synthesized nanoparticles were precipitated by ethanol. The particles were then dissolved in spectra grade toluene and put on a gold grid covered by amorphous carbon and the toluene was allowed to evaporate. The nanoparticles were subsequently characterized using the SEM (Zeiss Auriga) and FE(S) TEM (FEI Tecnai F20) of the Institute of Optics at the University of Rochester. Electron flight simulation was carried out in the SEM whereas the high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) and energy dispersive X-ray spectroscopy (EDX) were done in the TEM. Image colorizations were done in Adobe Photoshop CS2.
Results and Discussion
The following figure illustrates the SEM micrograph of the nanoscales magnetite particles. It could be seen that the particles were spherical and tended to cluster together randomly on the carbon film and some on its edges.
Figure: SEM image of the magnetite nanoparticles
The morphology of magnetite nanoparticles were further studied by TEM and the obtained bright field images are depicted in series in the figure below. The TEM images showed that the particles mostly had a diameter of 14 ± 0.4 nm but there were a few with diameters 14±1. The diameters were measured using ImageJ program. The TEM image at the bottom right corresponds to an isolated particle at the edge of the carbon film showing clearly the lattice fringes of the particle.
Figure: TEM image of the magnetite nanoparticles
For both SEM and TEM it could be seen that the particles tended to aggregrate together. This is because magnetic iron oxide nanoparticles have a large surface to volume ratio and therefore possess high surface energies. Consequently, they tend to aggregrate to minimize surface energies.
The HAADF detector was used in the TEM in order to take STEM micrographs of the sample and is shown in the figure below. The quality of the micrograph is not nearly as good here as for the previous TEM micrographs.
Figure: HAADF STEM image of the magnetite nanoparticles.
Figure: EDX spectrum (top) of the magnetite nanoparticles along with the elemental mapping of the Fe(K) (bottom left) and Fe(L) (bottom right).
Electron flight simulation was utilized in order to simulate the electron-sample interaction for the NPs in the SEM as shown in the figure below. The simulation parameters did not allow the input diameter for the NP to be less than 500 nm, even though the NPs are much smaller than that and it can be observed that the beam is relatively unperturbed as it interacts with the sample.
flight simulation of the magnetite nanoparticles.
In order to add aesthetic effect, to clarify structure add to add a realistic appearance to the sample, some of the TEM images were colored and is shown in the figure below.
Figure: Colorized TEM images.
It could be concluded that magnetite nanoparticles were synthesized with a mean diameter of 14 nm and size variation of less than 20%. The future work in this area must be focused on reducing the size variation further, to about less than 5%, possibly by carrying out the reaction with a uniform rate of heating during the nucleation and growth steps.
I would like to thank Brian McIntyre for his unparalleled lessons, guidance and help with the project. I would also like to thank Dr. Kathryn Knowles for giving me the project idea, Beckah Burke for guiding me during the synthesis and Rakan Ashour for being a great TA!
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