Preparation of stable magnetic nanofluids containing Fe3O4@PPy nanoparticles by a novel one-pot route

Stable magnetic nanofluids containing Fe3O4@Polypyrrole (PPy) nanoparticles (NPs) were prepared by using a facile and novel method, in which one-pot route was used. FeCl3·6H2O was applied as the iron source, and the oxidizing agent to produce PPy. Trisodium citrate (Na3cit) was used as the reducing reagent to form Fe3O4 NPs. The as-prepared nanofluid can keep long-term stability. The Fe3O4@PPy NPs can still keep dispersing well after the nanofluid has been standing for 1 month and no sedimentation is found. The polymerization reaction of the pyrrole monomers took place with Fe3+ ions as the initiator, in which these Fe3+ ions remained in the solution adsorbed on the surface of the Fe3O4 NPs. Thus, the core-shell NPs of Fe3O4@PPy were obtained. The particle size of the as-prepared Fe3O4@PPy can be easily controlled from 7 to 30 nm by the polymerization reaction of the pyrrole monomers. The steric stabilization and weight of the NPs affect the stability of the nanofluids. The as-prepared Fe3O4@PPy NPs exhibit superparamagnetic behavior.


Introduction
Nanofluids, which contain nanoparticles dispersed in base fluids, have been proposed as a new kind of heat transfer medium because they can improve the heat transport and energy efficiency and may have potential applications in the field of heat transfer enhancement [1]. Magnetic nanofluids, suspension containing magnetic nanoparticles, such as magnetite (Fe 3 O 4 ), iron (Fe), nickel (Ni), and cobalt (Co), show both magnetic and fluid properties and have important applications in industries [2,3]. Fe 3 O 4 nanoparticles are always used to form magnetic nanofluids because of their broad potential and practical technological applications and fundamental scientific significance [3][4][5][6][7][8][9][10]. Recently, heat transfer enhancements were reported for γ-Fe 2 O 3 magnetic nanofluids [3]. However, long-term stability of nanofluids is a major concern for the engineering applications [11][12][13]. Nanoparticles tend naturally to aggregate and sediment in the base fluid. Also, stable solution with large volume concentration is not easy to obtain. Therefore, technique breakthroughs are needed to produce well-dispersed and long-term stable nanofluids. Use of functionalized nanoparticles is a promising approach to achieve long-term stability of nanofluid [2].
Organic-inorganic nanocomposites with an ordered structure show new functional hybrids of organic and inorganic materials. Incorporation of nanosized particles in organic polymeric materials has been extensively studied because they combine the advantages of the inorganic materials and the organic polymers. Moreover, new properties of these hybrids can also show up because of synergetic effects, which can be scarcely obtained from the individual components. Combination of various conducting polymers with magnetic ironoxides is more and more intensively investigated, because materials possessing both high magnetic susceptibility and high conductivity can be used in different applications, such as nonlinear optics, electrical, and magnetic shielding, magnetic electrocatalysis, and as microwave absorbers [14].
This study reports on the synthesis of stable nanofluids with a novel one-pot hydrothermal route, in which Fe 3 O 4 coated by PPy NPs were contained. FeCl 3 ·6H 2 O was used as the iron source and the oxidant to polymerize pyrrole monomers. Trisodium citrate was used as the reducing reagent. The procedure was carried out at low temperature and without any protection atmosphere. To our knowledge, this is the first report to direct synthesis of Fe 3 O 4 @PPy nanofluids. The mechanism of suspension was proposed.

Experimental
Pyrrole monomer, iron (III) chloride hexahydrate (FeCl 3 ·6H 2 O), sodium hydroxide (NaOH), trisodium citrate (Na 3 cit), and sodium dodecyl sulfate (SDS) were purchased from Sinopharm Chemical Reagent Company. Pyrrole monomer was distilled under reduced pressure, and other reagents were of analytical grade and used as received without further treatment. All solutions were prepared with twice-distilled water.
In a typical experiment, NaOH 0.0800 g (2.0 mmol) and 0.0230 g SDS (0.008 M) were dissolved into a 9 mL aqueous solution containing 0.1 M Na 3 cit (C 6 H 5 Na 3 O 7 ·2H 2 O) under constant stirring. 1.0 mL of FeCl 3 solution (1.0 M) and 0.3 mL pyrrole monomer were added into the solution. The whole mixture was stirred vigorously for 5 min to give a homogeneous solution. Subsequently, the solution was transferred into a 50 mL Teflon-lined stainless steel autoclave, and maintained at 160°C for 24 h. Afterward, the autoclave was allowed to cool down to room temperature naturally, and the resulting black Fe 3 O 4 @PPy nanofluid was produced. The deposit was obtained by magnetic separation method, and washed with distilled water and absolute ethanol for several times. The as-prepared black precipitate was dried under vacuum at 50°C for 24 h.
X-ray powder diffraction (XRD) measurements were performed on a Bruker D8 Advance X-ray Diffractometer with Cu Kα radiation (λ = 1.5418 Ǻ). The 2θ range used in the measurement was from 20°to 70°. Standard transmission electron microscopy (TEM) measurements were performed on a JEOL-2010 TEM at an acceleration voltage of 200 kV. Samples were first ultrasonically dispersed in deioned water and drop-cast onto copper grids. Magnetic characterization was carried out at room temperature using a vibrating sample magnetometer (VSM, Lakeshore 7307, USA). Infrared spectrum (FT-IR) measurements were performed on a Nicolet Aexus 470, with scanning from 4000 to 400 cm -1 by using KBr pellets under ambient temperature.

Results and discussion
Stability of the as-synthesized Fe 3

O 4 @PPy nanofluids
Well-dispersed nanofluid can be prepared as shown in Figure 1A and no sedimentation was observed as shown in Figure  1D(b), in which the nanoparticles were synthesized with 0.3 mL pyrrole monomer. This kind of nanofluid can be stable for more than 1 month. In order to investigate the effect of amount of pyrrole monomer on stability of the nanofluid, various amounts of pyrrole were added into the reaction system. Figure 1a,c shows the nanofluids with 0.1 and 0.5 mL pyrrole, respectively. Sedimentations can be clearly found after the nanofluids have been standing for 1 and 4 h as shown in Figure 1B (a, c) and 1C(a, c), respectively. When no pyrrole was added into the reaction solution, no stable was prepared (the figure is not shown here). These results proved that the most stable nanofluid was produced with 0.3 mL pyrrole.
Effects of different amounts of pyrrole monomer on the structure and size of the as-prepared nanoparticles  [15]. Nevertheless, as magnetite (Fe 3 O 4 ) and maghemite (γ-Fe 2 O 3 ) havethe inverse spinel structure and have very similar XRD patterns, precise studies on the phase composition of the as-synthesized products were required. Using the formula 2d*sinθ = n*λ to calculate d, and compare to the standard numerical value [16], these as-prepared products are all magnetite (Fe 3 O 4 ). These data are listed in Table 1. Figure 3 shows the FT-IR spectra of the as-prepared Fe 3 O 4 @PPy samples with different amounts of pyrrole monomer. The absorption band at 583 cm -1 is attributed to Fe-O bond, the N-H stretching band of a pyrrole appears at about 3379 cm -1 , a weak band at 2928 cm -1 in Figure 3b,c is due to the C-H stretching vibration absorption. Other spectra at 1650-1029 cm -1 also show the characteristic polypyrrole absorption [17]. A weak absorption at about 1620 cm -1 is assigned to the C = C ring stretching of pyrrole. The C-N ring stretching band of pyrrole occurs at 1044 cm -1 . These results demonstrated that polypyrrole was synthesized. The size and shape of the precipitates were determined by TEM. Figure 4 presents the typical TEM images of the precipitates produced under various amounts of pyrrole monomer. All of these precipitates present uniform sizes and morphologies. Figure 4a shows the Fe 3 O 4 @PPy NPs prepared with 0.1 mL pyrrole monomer with an average diameter of 15 nm. When the amount of pyrrole monomer is 0.3 mL, the mean diameter of the NPs is about 7 nm as shown in Figure 4b. When the amount of pyrrole is increased to 0.5 mL, the average diameter of the NPs is about 70 nm as shown in Figure 4c. Core-shell structures can be found in Figure 4a, especially in Figure 4c. This kind structure was not clearly found in Figure 4b because of small particles produced in the experimental conditions. In order to study the structure of the sample shown in Figure 4b, a magnified figure was applied. Figure 5 shows the magnified image of the sample synthesized with 0.3 mL pyrrole, in which core-shell structure was clearly found (as arrows' directions). The corresponding SAED pattern as the inset in Figure 5 shows amorphous   Figure 4. When the amount of pyrrole monomer is small, such as 0.1 mL pyrrole added into the reaction system, the formation rate of the PPy is slower and the shell of PPy is thinner than those produced with higher amounts of the pyrrole, such as 0.3 and 0.5 mL pyrrole. The Fe 3 O 4 NPs grow quickly with small amounts of pyrrole. When the amount of pyrrole monomer is too high, such as 0.5 mL pyrrole added into the reaction system, the formation rate of the PPy is faster and the shell of PPy is thicker than those produced with smaller amounts of the pyrrole. The particle size becomes bigger because of the formation of the thick PPy shell. This kind of shell can be clearly found in Figure 4c. When the amount of the pyrrole was 0.3 mL, the size of the as-produced particle is the smallest in the present experimental conditions.

Magnetic property of the as-prepared samples
Magnetic properties of the as-prepared products were investigated by a VSM at room temperature in the applied magnetic field from -8 to 8 KOe. The hysteresis loops of the Fe 3 O 4 @PPy NPs are shown in Figure  6. The products showed typical superparamagnetic behavior with negligible coercivity and remanence. Superparamagnetic behavior is often observed at room temperature with Fe 3 O 4 NPs smaller than 20 nm [18,19]. In order to study the effect of the PPy shell on magnetic property of the samples, the hysteresis loop of the Fe 3 O 4 NPs is shown as Figure 6A, in which the Fe 3 O 4 NPs were prepared without any pyrrole and other experimental conditions kept the same. The saturation magnetizations of the as-prepared products were determined to be 23. 7, 22.4, 20.7, and 17.2 emu/g for the samples fabricated with 0, 0.1, 0.3, and 0.5 mL pyrrole, respectively. These data shown that the saturation magnetization became smaller with increase of pyrrole added into the reaction system, for which the shell of the as-prepared shell-core samples became thicker with increase of pyrrole. These results demonstrated that Fe 3 O 4 @PPy NPs were produced.

Effects of trisodium citrate (Na 3 cit) on the structure of the sample
In order to study the effects of Na 3 cit on the structure of the product, different amounts of Na 3 cit were used and other experimental conditions kept the same in the reaction systems. Figure 7 shows XRD patterns of the as-prepared samples under 9 mL 0.06 M Na 3 cit aqueous solution and 9 mL water instead of the 9 mL 0.1 M Na 3 cit aqueous solution as described in the "Experimental" section, respectively. When the amount of the Na 3 cit was decreased from 9 mL 0.1 M to 9 mL 0.06 M Na 3 cit aqueous solution, pure Fe 3 O 4 was also obtained as shown in Figure 7a. When no Na 3 cit was added into the solution, almost pure α-Fe 2 O 3 was produced as shown in Figure 7b. These results demonstrated that the Na 3 cit was used as the reducing reagent to form Fe 3 O 4 .
The size and shape of the product prepared in 9 mL 0.06 M Na 3 cit aqueous solution were determined by TEM as shown in Figure 8, in which the mean diameter of the particles is about 30 nm. Compared with the product prepared in 9 mL 0.1 M Na 3 cit aqueous solution as shown in Figure 4b, the size of the sample increased from 7 to 30 nm with the decrease of the amount of Na 3 cit in the solution. These results were in agreement with references [20,21], in which Na 3 cit was used as a dispersant.

Formation mechanism of the stable fluid
In the present conditions, the Na 3 cit acted as a reductant and a dispersant. The formation processes of the The forming processes are shown in Figure 9. At the same time, these processes prevent the Fe 3 O 4 growing. Thus, this polymerizing reaction can be applied to control the size of the as-prepared sample.
A steric stabilization effect is always used to explain the stability of particles suspended in the base fluid [22]. This steric stabilization effect arises from the fact that polymers coating on the surface of NPs occupy a certain amount of space. Thus, the space becomes compressed when nanoparticles are brought too close together. An associated repulsive force makes separate nanoparticles from each other and restrains the aggregation of nanoparticles. On the other hand, the weight of the NPs affected the stability of the nanofluids as shown in Figure 1.
In the present experiments, SDS was also used as a dispersant. The size of the Fe 3 O 4 NPs became smaller with SDS (these results are not shown here). Compared with surfactants, the steric effect by polymers onto the surface of NPs is a better way to stabilize NPs suspended in the based fluid [2]. The PPy-coated Fe 3 O 4 NPs have the steric stabilization effect and achieve a better and larger solubility of NPs in water because of the solubility rule of similarity. Thus, stable nanofluids containing magnetic Fe 3 O 4 NPs can be produced with the help of PPy.
The nanofluids containing Fe 3 O 4 @PPy NPs can be used to enhance heat transfer, as magnetic nanofluids and other special applications, as PPy is one of the conducting polymers. This facile method can be used to prepare other organic-inorganic NPs.

Conclusion
A facile and novel method for preparing nanofluids containing Fe 3 O 4 @PPy was presented, in which one-pot route was used. The nanofluid can keep long-term stability and very good dispersing. Fe 3 O 4 @PPy NPs can still keep dispersing well after the nanofluid has been standing for 1 month and no sedimentation is found. The Na 3 cit acted as a reductant and a dispersant to form Fe 3 O 4 NPs. Fe 3+ ions remained in the solution adsorbed on the surface of the Fe 3 O 4 NPs. The polymerization reaction of the pyrrole monomers took place with these Fe 3+ ions as the initiator. The particle size of the as-prepared Fe 3 O 4 @PPy can be easily controlled from 7 to 30 nm by adjusting the amount of pyrrole monomer. The steric stabilization and weight of the NPs affect the stability of the nanofluids. The as-prepared Fe 3 O 4 @PPy NPs exhibit superparamagnetic behavior with various saturation magnetizations.

Additional material
Additional file 1: In a typical experiment, NaOH 0.0800 g (2.0 mmol) and 0.0230 g sodium dodecyl sulfate (0.008 M) were dissolved into a 9 mL aqueous solution containing 0.1 M Trisodium citrate (C 6 H 5 Na 3 O 7 2H 2 O) under constant stirring. 1.0 mL of FeCl 3 solution (1.0 M) was added into the solution. The whole mixture was stirred vigorously for 5 min to give a homogeneous solution. Subsequently, the solution was transferred into a 50 mL Teflon-lined stainless steel autoclave, and maintained at 160°C for 24 h. Afterward, the autoclave was allowed to cool down to room temperature naturally. The deposit was obtained by magnetic separation method, and washed with distilled water and absolute ethanol for several times. The as-prepared black precipitate was dried under vacuum at 50°C for 24 h. The crystalline structure and chemical composition of the product were determined by XRD analyses as shown in Figure 1. The positions and relative intensities of all diffraction peaks can be well indexed to the magnetic cubic structure of Fe 3 O 4 . The XPS spectrum of the product is shown in Figure 2a. The detail spectrum of the sample corresponding to the binding energies of Fe2p is shown in Figure 2b. The photoelectron peaks at 710.9 and 724.4 eV correspond to Fe 2p3/2 and Fe 2p1/2, respectively. These results are in concert to previous report [1]. The size and shape of the products were determined by TEM. Figure 3 shows the typical TEM images of the Fe 3 O 4 prepared with various concentrations of Na 3 cit.