Facile synthesis and characterization of magnetic MnFe2O4/CNT nanocomposites
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In this paper, the effects of carbon nanotubes (CNTs) were studied as supports for the synthesis of MnFe2O4 nanocomposite. The synthesis of nanocomposite powder MnFe2O4/CNTs was performed by direct precipitation method in aqueous solution. The prepared samples were analyzed by X-ray diffraction, scanning electron microscopy and Fourier transform infrared spectroscopy. The results represent the considerable change in the MnFe2O4 nanoparticle size and also the morphology of MnFe2O4/CNT nanocomposite powder from agglomerative into nanorod in shape.
KeywordsCarbon nanotube Nanoparticles Nanocomposite Morphology Direct precipitation
In recent years, there has been wide research in the field of nanostructures, especially magnetic nanoparticles [1, 2, 3]. Due to their unique characteristics, magnetic nanoparticles have found a special status in various sciences and multiple applications such as biological materials, medicine and industry [3, 4, 5, 6, 7, 8]. Among magnetic nanoparticles, MnFe2O4 nanoparticles have attracted most attention among researchers due to their high saturation, low toxicity, good biocompatibility, strong super magnetic properties, drug delivery, audio filters, magnetic heads and magnetic memories and so on [9, 10, 11, 12, 13, 14].
Several parameters such as nanoparticle size, purity, morphology, crystallinity and their distribution are important factors influencing the physical properties of nanoparticles [15, 16, 17, 18, 19, 20, 21, 22]. These parameters can be affected by their production methods. Among the different methods such as sol–gel , hydrothermal [19, 23], reverse micelle  and microwave irradiation , direct co-precipitation is the most appropriate method to produce MnFe2O4 nanoparticles due to its low cost, easy control of the synthesis conditions and high efficiency .
On the other hand, carbon nanotubes have been widely used in various fields because of their unique properties. Due to the high aspect ratio of the carbon nanotubes, they prevent the agglomerating of the particles and increase their applicability considerably [25, 26].
In the present paper, we intend to study the effect of applying carbon nanotube support on size and morphology of MnFe2O4 nanoparticles prepared by direct precipitation.
Materials and characterizations
To provide the samples, we used manganese chloride tetrahydrate (MnCl2·4H2O Merk, purity >99 %), iron nitrate hexahydrate [Fe(NO3)2·6H2O, Merk, purity >99 %], ammonium hydroxide solution (NH4OH, Merck, 25 % of ammonia), carbon nanotubes (CNTs, Neutrino China, 20 nm < d < 30 nm, purity >95 %), sulfuric acid (H2SO4) and nitric acid (HNO3).
The crystal structure of prepared samples was investigated by X-ray diffraction (XRD, Philips, Cu (Kα) Spectra, λ = l.54 Å, Pw 1,800). A scanning electron microscope (SEM, Philips, 15 kV, 60 kx) was used to check the morphology of the MnFe2O4/CNT nanocomposites. Fourier transform infrared spectroscopy (FT-IR Spectra, Shimadzu, 8,400 s) was used to study the functional groups on the surface of CNTs. In addition, the sizes of MnFe2O4 nanoparticles were determined by Scherrer equation.
Functionalization process of carbon nanotubes
To form strong covalent bonds between atomic ions of Fe3+ and Mn2+ with carbon nanotubes, the nanotube surface must have functional groups of carboxyl or hydroxyl bonds. To remove carbon contamination, thermal oxidation was carried out for 1.5 h in a horizontal electric furnace at 500 °C under air flow condition (the increasing rate of furnace temperature was 10 °C/min). To functionalize the carbon nanotubes, they were sonicated in 100 cc of acidic solution including a combination of H2SO4: HNO3 (6 M) for 30 min; then the solution was stirred for 3 h in 50 °C. Then, the obtained solution was passed through a filter paper and washed with distilled water until it reached pH = 7. At the end, the prepared samples were dried in an oven at 120 °C.
Synthesis of MnFe2O4 nanoparticles
To prepare MnFe2O4 nanoparticles, first the required amount of Fe(NO3)3·6H2O and MnCl2·4H2O is dissolved in 60 cc distilled water at a ratio of (Fe3+)/(Mn2+) = 2:1 to achieve the stoichiometry of the spinel ferrite. While stirring the mixed Fe3+ and Mn2+ ions, 6.5 cc of NH4OH was overflown into it. Then the obtained solution was stirred up to 30 min at room temperature. Finally, a brown precipitate was formed at the bottom of the container. Following the completion of the process, the resulting brown precipitate was then washed in distilled water and then dried in an oven at 120 °C for 4 h. Then, the obtained dry powders were calcined in a horizontal electrical furnace at various temperatures of 400, 600 and 800 °C.
Synthesis of MnFe2O4 nanoparticles decorating carbon nanotubes
To prepare MnFe2O4/CNTs nanocomposite powder, first 0.2 g of functionalized CNTs was poured into 60 cc distilled water and sonicated for 10 min. While stirring the resulting solution, a solution containing the required amount of (Fe3+)/(Mn2+) = 2:1 was added. For precipitation, ammonium hydroxide reducer was added instantaneously and the solution was stirred for 30 min. Finally, a dark precipitate was formed at the bottom of the container. Following the completion of the process, it was then washed in distilled water and then dried in an oven at 110 °C for 4 h. Then, the obtained dry powders were calcined in a horizontal electrical furnace at various temperatures of 400, 600 and 800 °C under argon flow condition.
Results and discussion
FT-IR spectrum resulting from CNTs
Analysis of X-ray diffraction spectra
Figure 1 presents the spectra resulting from X-ray diffraction of the prepared samples. Existing peaks in (XRD) spectra within the range of 2θ = 5–80° suggest the formation of a two-phase structure from MnFe2O4 and α-MnO2 nanoparticles. The peaks in 2θ = 28.97, 34.06, 43.6, 53.59, 57.21, 64.5 and 75.2° (shown by circles) are related to MnFe2O4 nanoparticles, which are related to the reflection of crystal plates of (220), (311), (400), (422), (511), (440) and (553), respectively [13, 15, 23]. On the other hand, existing peaks located at 2θ = 47.5, 50.59, 53.59, 60.39, 66.5, 68, 70.52 and 73.79° (shown by squares) are associated with α-MnO2 crystal structure which are related to reflection of crystal plates of (510), (411), (440), (531), (002), (202), (541) and (312), respectively . Moreover, the peaks being sharp represent the crystalline order and high crystalline structure of MnFe2O4 nanoparticles. There were no additional peaks, due to the existing impurity in the prepared powder.
After comparing the two XRD spectra in Figs. 3 and 4, for calcined samples at temperature of 400 °C and regarding the result of Scherrer equation, it is observed that the presence of carbon nanotubes’ support in the synthesis process increases the mean size of MnFe2O4 nanoparticles. The increase in the size of nanoparticles may be due to existing CNTs in solution and the decreased required space for nucleation of MnFe2O4 nanoparticles. By increasing the calcination temperature, CNTs prevent the formation of larger crystallite nanoparticles compared to the state of pure MnFe2O4 (Fig. 3b, c).
In this study, MnFe2O4 nanoparticles and MnFe2O4/CNTs nanocomposite powder were prepared using the direct precipitation method. Noticeable results were obtained in terms of MnFe2O4 nanoparticle size. SEM images suggest that nanotubes as nanoparticles growth support will reduce the agglomerated level of nanoparticles and change the powder morphology from the mass state to the string state.
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