Synthesis of γ-Fe₂O₃@SiO₂@polypyrrole core/shell/shell nanospheres with flexible controllability of electromagnetic properties

Abstract

A new anchoring method by –SO₃H groups was proposed to prepare γ-Fe₂O₃@SiO₂@polypyrrole (γ-Fe₂O₃@SiO₂@PPy) nanospheres with core/shell/shell structure. In the reaction process involved, the sulfonic-functionalized γ-Fe₂O₃@SiO₂ (γ-Fe₂O₃@SiO₂–SO₃H) acts as the core for the polymerization of pyrrole. And PPy can be anchored to form on the surface of γ-Fe₂O₃@SiO₂ to form γ-Fe₂O₃@SiO₂@PPy nanospheres with the help of the sulfonic groups on the surface of γ-Fe₂O₃@SiO₂. Moreover, flexible controllability of the conductivity and saturation magnetization of the resulting γ-Fe₂O₃@SiO₂@PPy nanospheres can be realized through this method. The resulting electromagnetic γ-Fe₂O₃@SiO₂@PPy nanospheres show a maximum conductivity of 53 S cm⁻¹ and saturation magnetization of 8.17 A m² kg⁻¹. The method reported here may provide an efficient way to realize the best impedance matching between complex permeability and complex permittivity.

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Introduction

Electromagnetically functionalized micro/nanostructures of conducting polymers have triggered growing interest due to their specific applications in electromagnetic interference shielding and microwave absorption.^1–3 It has been reported that microwave absorbing properties can be enhanced via the complementation of dielectric loss and magnetic loss. The electromagnetically functionalized composites composed of magnetic nanoparticles (MNPs) and conducting polymers not only have dielectric loss and magnetic loss, but also a synergistic effect of the dielectric loss and magnetic loss.^4,5 So it is very interesting to obtain electromagnetically functionalized conducting polymers. Early studies on preparing electromagnetically functionalized micro/nanostructures of conducting polymers mainly focussed on a two-step method (TSM) including preparation of polyaniline (PANI) nanotubes containing Fe₃O₄ NPs⁶ and coaxial PANI/γ-Fe₂O₃ nanofibers⁷ by an in situ chemical oxidation polymerization in the presence of Fe₃O₄ NPs or γ-Fe₂O₃ nanoneedles. A remaining problem on this aspect is the aggregation of MNPs, leading to unhomogeneous distribution in conducting polymer system. By addressing this issue, we have developed a simple chemical one-step method (COSM) in our previous study to prepare PANI/γ-Fe₂O₃ nanofibers.⁸ This COSM has successfully realized the one-step synthesis of electromagnetically functionalized composite nanofibers, in which FeCl₃ can act as the oxidant for the aniline polymerization reaction and as a source of Fe^III for the precipitation of γ-Fe₂O₃ magnets, allowing for a simultaneous preparation of the conducting polymer and magnetic γ-Fe₂O₃ as well as homogeneous distribution of magnetic γ-Fe₂O₃ NPs in the hybrids. More recently, an improved COSM (ICOSM) has also been created to prepare uniform electromagnetic PPy/γ-Fe₂O₃ nanospheres having high electrical conductivity without compromising the magnetic properties.⁹ However, it is hard to regulate the electrical conductivity and saturation magnetization independently for this ICOSM.

It is noted that the use of core/shell structure has been an efficient way for improving the microwave absorption properties in recent years.^10–12 Especially, the MNPs/conducting polymer core/shell structure has also been reported.^1,13 Moreover, it is possible to control the magnetism and conductivity independently by regulating the dosage of MNPs cores and the polymerization condition of conducting polymer shell, respectively. However, the emulsifiers or/and stabilizers were introduced in order to conducting polymer coated well on the magnetic nanoparticles. The weak interactions between the magnetic nanoparticles and conducting polymers make it hard to ensure the magnetic nanoparticles to be coated well.

It is well known that silica layer formed on the surface of MNPs can screen the magnetic dipolar attraction between MNPs, favoring the dispersion of MNPs in liquid media and protects them from etching in an acidic environment.¹⁴ Furthermore, the silica surface is active to be grafted by specific functional groups such as amidogen,^15,16 sulfonic acid group,^17–19 or be modified by chitosan²⁰ or poly(N-vinylpyrrolidone)²¹ to anchor the polymerization reaction. Moreover, it is noted that silica is one of the most popular wave-transmitting materials.^22,23 The superior characteristics of silica inspire us to design a sandwiched γ-Fe₂O₃@SiO₂@PPy core/shell/shell structure. In details, the silica-coated γ-Fe₂O₃ (γ-Fe₂O₃@SiO₂) core/shell composite spheres is prepared and then modified with sulfonic acid groups (–SO₃H), which are subsequently employed to guide the oxidation polymerization of pyrrole monomers on the γ-Fe₂O₃@SiO₂ surface. Through this, the γ-Fe₂O₃@SiO₂@PPy core/shell/shell nanospheres were prepared successfully and the weak interfacial interactions between the inorganic γ-Fe₂O₃ MNPs and organic PPy, as well as the poor wave-transmissivity may be improved efficiently at the same time.

Experimental

Materials

The chemicals used in this work, such as FeCl₃·6H₂O, FeSO₄·7H₂O, tetraethoxysilane (TEOS), p-toluenesulfonic acid (p-TSA), 3-mercaptopropyltrimethoxysilane (MPTS) and pyrrole, were all of analytical purity provided by the Shanghai Sinopharm Chemical Reagent Co., and were used as received without further purification. Deionized water was used in all experiments.

Preparation of functional Fe₃O₄ NPs

Fe₃O₄ NPs were prepared by chemical precipitation method according to previous literature.¹⁴ In details: 54.06 g of FeCl₃·6H₂O (0.2 mol) and 27.80 g of FeSO₄·7H₂O (0.1 mol) were dissolved in 440 mL of deionized water. The obtained transparent solution was degassed with nitrogen flow and then 60 mL of NaOH (16.67 M) was dropped into the above solution within 1 h under vigorous agitation. After reaction at 1 h at 20 °C and subsequent heating at 90 °C for 1 h, the resulting black precipitates were collected and thoroughly rinsed with deionized water and ethanol and dried at 50 °C in vacuum for 12 h.

The as-synthesized Fe₃O₄ NPs were modified by citric acid to get highly dispersed and chemically stable aqueous magnetic fluids (MFs).²⁴ Briefly, 1 g of Fe₃O₄ NPs and 1.7 g of citric acid were dispersed in 200 mL of deionized water under supersonic agitation for 1 h. The pH value of the reagent was adjusted to 4.8 with hydrochloric acid, and the reaction was allowed to proceed with stirring for 12 h at room temperature. Subsequently, the modified Fe₃O₄ NPs were collected and thoroughly rinsed with deionized water/acetone mixture to remove the residual citric acid, the volume of the mixture was 100 mL and the volume ratio of water to acetone was 1 : 4. Finally, the modified Fe₃O₄ NPs were dispersed in 100 mL of deionized water for further use.

Preparation of γ-Fe₂O₃@SiO₂ (FS) core/shell nanospheres

20 mL of Fe₃O₄ MFs (10 mg mL⁻¹) was mixed with 80 mL of ethanol and 2.4 mL of 28 wt% aqueous ammonia. After ultrasonic agitation for 1 h, 2 mL of TEOS was added dropwise to the above dispersion and the reaction was proceeded under supersonic stirring for 2 h. The obtained product was collected by magnetic separation and rinsed with concentrated HCl, deionized water and ethanol three times, respectively, and then dried at 50 °C for 4 h. Finally, the γ-Fe₂O₃@SiO₂ nanospheres were heated at 300 °C for 5 h in air. The obtained γ-Fe₂O₃@SiO₂ was designated as FS.

Sulfonic-functionalization of γ-Fe₂O₃@SiO₂ (FSH) core/shell nanospheres

The FS nanospheres were functionalized with –SO₃H according to previous literature¹⁸ and the approach employed was shown in Scheme 1. The FSH nanospheres were achieved by reacting MPTS with the silanol terminal groups to introduce mercapto groups on the surface of silica followed by oxidation with H₂O₂ resulting in covalently bounded –SO₃H groups. Briefly, 1 g of FS nanospheres were dispersed in a mixture containing 30 mL of acetic acid and 4 mL of H₂O₂ (30 wt%) under supersonic agitation. Subsequently, 2 g of MPTS were added and the reaction was allowed to proceed at 80 °C for 3 h. After that, the precipitates were collected by magnetic separation and washed with ethanol three times, and then dried at 50 °C for 12 h. The obtained γ-Fe₂O₃@SiO₂–SO₃H was designated as FSH.

Scheme 1 Schematic illustration for synthesizing sulfonic-functionalization FS nanospheres.

Preparation of γ-Fe₂O₃@SiO₂@PPy (FSHP_x-y) core/shell/shell nanospheres

The typical processes for γ-Fe₂O₃@SiO₂@PPy (FSHP_x-y) were as follows: stoichiometric FSH nanospheres were dispersed in 55 mL of aqueous solution containing pyrrole and p-TSA under supersonic agitation. Under mechanical stirring at a speed of 800 rpm in an ice-water bath, 20 mL of FeCl₃ solution was added dropwise to the dispersion within 1 h for the in situ oxidative polymerization of pyrrole monomers. The reaction system was kept at 0–4 °C and stirred for 6 h. Finally, the black products were collected and rinsed with deionized water and ethanol, and dried at 50 °C for 12 h.

As a reference, γ-Fe₂O₃@SiO₂@PPy nanocomposites (assigned as FSP_x-y) using unfunctionalized FS seeds were also prepared as a comparison to investigate the effect of –SO₃H on the morphology and conductivity of the γ-Fe₂O₃@SiO₂@PPy composites. In FSHP_x-y or FSP_x-y, x and y referred to the grams of FSH (or FS) and the batch number of different pyrrole/p-TSA/FeCl₃·6H₂O molar ratios (Table 1), respectively.

Table 1 Summary of reaction conditions used in the preparation of γ-Fe₂O₃@SiO₂@PPy composites

Sample	FSH or FS (g)	Pyrrole (mmol)	Pyrrole/p-TSA/FeCl3·6H2O (molar ratio)	Water solution (mL)
FSHP0.6-1	0.6	5	1 : 0 : 4	75
FSHP0.6-2	0.6	5	1 : 1 : 4	75
FSHP0.6-3	0.6	5	1 : 2 : 4	75
FSHP0.6-4	0.6	5	1 : 1 : 2	75
FSHP0.05-2	0.05	5	1 : 1 : 4	75
FSHP0.1-2	0.1	5	1 : 1 : 4	75
FSHP0.2-2	0.2	5	1 : 1 : 4	75
FSHP1.0-2	1.0	5	1 : 1 : 4	75
FSP1.0-2	1.0	5	1 : 1 : 4	75

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Characterization

Field emitting scanning electron microscope (SEM, JSM-6700F), energy dispersive X-ray (EDX) and transmission electron microscope (TEM, JEM-200CX and FEI TECNAI G2 F20) were used to study the morphologies of the γ-Fe₂O₃@SiO₂–SO₃H and γ-Fe₂O₃@SiO₂@PPy nanocomposites. The thermogravimetric (TG) analysis was carried out on a SETSYS Evolution TGA (Setaram) from room temperature to 800 °C at a heating rate of 10 °C min⁻¹ and a dynamic dry air flow. FTIR spectra (KBr pellet, Perkin-Elmer System), X-ray photoelectron spectroscopy (XPS, PHI-5702) and X-ray diffraction (XRD, RINT2000 wide angle goniometer) were used to characterize the chemical structures of the resulting nanocomposites. The magnetization curves at different magnetic field (−800 to 800 kA m⁻¹) were measured at room temperature by an extracting sample magnetometer (Lakeshore 7404). The conductivity at room temperature was measured using four-probe method with a KEITHLEY 2400 multimeter and a KEITHLEY 2182A nanovoltmeter.

Results and discussion

The whole procedure to prepare γ-Fe₂O₃@SiO₂@PPy nanospheres is illustrated in Scheme 2. First, the Fe₃O₄ NPs were prepared by chemical precipitation method and modified by citric acid to get MFs. Then, SiO₂ shell was coated on the Fe₃O₄ NPs surface through a sol–gel process. Subsequently, the achieved Fe₃O₄@SiO₂ nanospheres were transformed into γ-Fe₂O₃@SiO₂ by heating in air. After reacting MPTS with the γ-Fe₂O₃@SiO₂ and further oxidation with H₂O₂, the –(CH₂)₃SO₃H groups were successfully grafted onto SiO₂ surface. Finally, pyrrole monomer polymerized on the surface of γ-Fe₂O₃@SiO₂–SO₃H nanospheres by in situ oxidation polymerization, generating core/shell/shell γ-Fe₂O₃@SiO₂@PPy nanospheres.

Scheme 2 A schematic diagram for the fabrication process of γ-Fe₂O₃@SiO₂@PPy nanospheres.

Morphologies and formation mechanism

Typical SEM and TEM images of γ-Fe₂O₃@SiO₂–SO₃H, γ-Fe₂O₃@SiO₂@PPy were employed to investigate the morphologies of the as-prepared samples. SEM image (Fig. 1a) indicates that the resultant γ-Fe₂O₃@SiO₂–SO₃H indeed have a nanosphere in shape, and TEM image (Fig. 1a′) confirms the γ-Fe₂O₃@SiO₂–SO₃H nanospheres possess a well-defined core/shell structure. Compared with γ-Fe₂O₃@SiO₂–SO₃H nanospheres, well-defined γ-Fe₂O₃@SiO₂@PPy core/shell/shell three-layer spherical structure can be observed clearly by SEM and TEM images (Fig. 1b and b′), which indicate the polymerization of pyrrole monomers has occurred on the surface of SiO₂ shell.

Fig. 1 SEM and TEM images of FSH (a and a′); FSHP_1.0-2 (b and b′); FSP_1.0-2 (c and c′). The blue arrow represents the bare FS two-layer structure and the red arrow represents the freely deposited PPy particles.

It is noteworthy to mention that the surface of SiO₂ shell is hydrophilic, while pyrrole monomer is hydrophobic. In this fashion, the combination of SiO₂ with PPy is a challenge by in situ polymerizing PPy in SiO₂ solution. We can facilely realize the homogeneous coverage of PPy on γ-Fe₂O₃@SiO₂ by anchoring PPy with modified γ-Fe₂O₃@SiO₂–SO₃H. As a reference, the unmodified γ-Fe₂O₃@SiO₂ has also been utilized as the core to prepare γ-Fe₂O₃@SiO₂@PPy nanospheres. As illustrated in TEM image of Fig. 1c′ the resulting γ-Fe₂O₃@SiO₂@PPy composite contains both bare γ-Fe₂O₃@SiO₂ two-layer structure (blue arrow) and freely deposited PPy particles (red arrow). In other words, the polymerization process isn't favoured on the unmodified silica surface. Therefore, –SO₃H group is the key factor to influence the core/shell/shell morphology.

Energy dispersive X-ray spectroscopy (EDX) elemental mapping of the nanospheres also clearly displays the expected core/shell/shell structure.²⁵ The different colour images shown in Fig. 2c–f indicate Fe-, Si-, C-, and N-enriched areas of the FSHP_1.0-2 sample, respectively, which confirm the presence of both SiO₂ and PPy serving as the shell of γ-Fe₂O₃ core. The images also confirm that the coating of SiO₂ and PPy is uniform, and after the coating of PPy, the nanospheres agglutinate. The line profiles further confirm the three-layer structure (Fig. 2g) and the 38 nm thickness of the SiO₂ shell and 46 nm thickness of the PPy shell were calculated accordingly, which in correspondence with the HAADF STEM image (Fig. 2b).

Fig. 2 (a) Schematic of γ-Fe₂O₃@SiO₂@PPy nanospheres. (b) Typical HAADF STEM image of FSHP_1.0-2. (c–f) Elemental mapping of three agglutinate γ-Fe₂O₃@SiO₂@PPy nanospheres in FSHP_1.0-2 with Fe, Si, C and N signals. (g) EDS line scanning profiles of FSHP_1.0-2 across the map. The red line in (b) corresponds to the EDS line scanning region.

The synthesis conditions, as summarized in Table 1, have significant impact on the morphologies of γ-Fe₂O₃@SiO₂@PPy.

Fig. 3 shows the morphology evolution of γ-Fe₂O₃@SiO₂@PPy with FSH dosage ranging from 19.2 to 83.1 wt% (mass fraction calculated by TGA data, ESI Table S1†). Apparently, enhancement of FSH dosage is beneficial to homogeneous distribution of γ-Fe₂O₃@SiO₂@PPy nanospheres and to the reduction of γ-Fe₂O₃@SiO₂@PPy diameter. However, we can't find apparent deviations by tuning molar ratio of pyrrole/p-TSA/FeCl₃ from 1 : 0 : 4 to 1 : 1 : 2 while fixing FSH dosage of 0.6 g, yielding spherical morphology in all aspects, as shown in Fig. 4. Notably, the p-TSA dopant does not affect the morphologies of γ-Fe₂O₃@SiO₂@PPy composites.

Fig. 3 SEM images of (a) FSHP_0.05-2, (b) FSHP_0.1-2, (c) FSHP_0.2-2, (d) FSHP_0.6-2 and (e) FSHP_1.0-2.

Fig. 4 SEM images of (a) FSHP_0.6-1, (b) FSHP_0.6-2, (c) FSHP_0.6-3 and (d) FSHP_0.6-4.

A possible formation mechanism behind the aforementioned results for γ-Fe₂O₃@SiO₂@PPy nanospheres is illustrated in Scheme 3. It has been widely investigated that the morphologies of PPy composites highly depend on seed templates' shape in the polymerization system of pyrrole monomers, such as particulates, nano/microspheres and nano-needles.^20,26,27 As has demonstrated in previous work,²⁶ the micelles consisting of p-TSA and pyrrole can be formed in the reaction solution resulting from the hydrophobic pyrrole adhering to the oil-wet side of p-TSA and the presence of hydrophilic –SO₃H group in the p-TSA, thus the pyrrole/p-TSA supermolecules serve as the “soft-templates” in the formation of PPy-p-TSA nanostructures. Herein, the outer –SO₃H groups are hydrophilic and the inner γ-Fe₂O₃@SiO₂–(CH₂)₃– structure is hydrophobic (Scheme 2), therefore the γ-Fe₂O₃@SiO₂–SO₃H nanosphere has a micelle-like structure (micelle-like A). In the absence of p-TSA, pristine pyrrole diffuses into the inner oil-wet side of micelle-like A to form micelle-like B. In the presence of p-TSA, micelle-like C occurs when the oil-wet side of p-TSA adheres to micelle-like A, the subsequently added pyrrole diffuses into micelle-like C to form micelle-like D. B and D are believed as the templates to form γ-Fe₂O₃@SiO₂@PPy nanospheres. Due to fact that the solubilized pyrrole molecules are oxidized by FeCl₃, the polymerization reaction is believed to proceed in the interface of micelles and water.²⁸ In our experiments, it is obvious that the micelle-like B and D mainly act as the template judged from the morphology of the as-prepared γ-Fe₂O₃@SiO₂@PPy. In case of the decided amount of pyrrole monomers and polymerization condition used in the reaction systems, the amount of the pyrrole monomers distributed to every γ-Fe₂O₃@SiO₂–SO₃H core decrease when the amount of γ-Fe₂O₃@SiO₂–SO₃H cores increase. As a result, the thickness of PPy shell decreases and thus leads to smaller diameter and more sphere-like of the γ-Fe₂O₃@SiO₂@PPy nanospheres (Fig. 3a–e). On the other hand, when the amount of Fe₂O₃@SiO₂–SO₃H cores is decided, there's no apparent deviation on the morphologies (Fig. 4a–d).

Scheme 3 Schematic illustration of the formation mechanism of the γ-Fe₂O₃@SiO₂@PPy nanospheres.

The typical SEM and TEM images of FSHP_1.0-2 (Fig. 1b and b′) show that these nanospheres are aggregated, which has also been indicated by dynamic light scattering (DLS) data (ESI Fig. S1†). The mean hydrodynamic diameter of 858 nm and polydispersity (PDI) of 0.34 suggest the nanospheres are not monodispersed.²⁹ The aggregation of the nanospheres may result from the relatively high Hamaker constant reported for PPy³⁰ and the interaction between Fe³⁺ and –OH (from –SO₃H on γ-Fe₂O₃@SiO₂–SO₃H surface) in the reaction system. While some steric stabilizers such as poly(N-vinylpyrrolidone)³¹ and chitosan²⁰ were used in the PPy coating process, which would prevent the latex particles from aggregating, it can lead to the low and uncontrollable conductivity of the resultant composites. Herein, without the steric stabilizer, the PPy coated nanospheres have a propensity to aggregate, and on the other hand, the huge amounts of iron ions in the reaction system may lead micelle-like B or D to aggregate, and the pyrrole monomers residual in the solution are oxidized by Fe³⁺ to polymerize between the micelles, which responsible for the agglutination of the nanospheres.

Structural characterization

Fig. 5 shows the thermo gravimetric analysis (TGA) curves of FS, FSH, FSHP_1.0-2 and PPy. It's reasonable to consider that the variation of residual mass fraction at 800 °C between FSH and FS is attributed to the propylsulfonic thermal degradation in FSH (Fig. 5a and b). There are two obvious stages in the process of thermal decomposition for pure PPy (Fig. 5d). The first stage in the range of 30–180 °C results from the evaporation of some impurities such as oligomers and absorbed water, the other one at above 330 °C is caused by the degradation of PPy chains, and it has decomposed completely above 550 °C. In contrast with the pure PPy, the temperature corresponding to maximum decomposition rate of FSHP_1.0-2 is higher than that of pure PPy, which indirectly indicates that the PPy shell indeed has some interaction with the FSH cores, thus the thermal stability of FSHP composites has been improved. The contents of magnetic FSH cores in FSHP_x-2 and FSHP_0.6-y samples have also been determined by TGA data (ESI Fig. S2, Table S1†), which are responsible for the variation of magnetism of the samples.

Fig. 5 TG curves of FS (a), FSH (b), FSHP_1.0-2 (c), PPy (d).

X-photoelectron spectroscope (XPS) is utilized to determine the molecular structures. Fig. 6 shows the full XPS spectrums of the as-prepared FS and FSH. In comparison with unfunctionalized FS (Fig. 6a), the characteristic peak occurs at 169.1 eV is attributed to the sulfur atom from the silane coupling agent for the functionalized FSH (Fig. 6b). The peak centred at 168.6 eV corresponding to –SO₃H group^32,33 is detected as shown in the S 2p high-resolution spectrum for the functionalized FSH (inset of Fig. 6), while the peak near 164 eV due to the –SH group^34,35 is undetected. This result further demonstrates the mercapto groups have been oxidized completely to sulfonic ones. The S/Si atomic ratio of ∼0.4 determined by XPS data manifests the surface of silica is enriched with sulfonic groups.³² In addition, there is also a substantial increase in the relative intensity of C 1s peak compared with the untreated FS. The increase in the relative intensity of C 1s peak can be attributed to the propyl chain from MPTS grafted to silica. In the case of the untreated FS, the C 1s peak mainly results from adventitious carbon contamination. Besides, for both FS and FSH, the typical peaks at 155.1 and 103.5 eV are attributed to Si 2s, Si 2p, respectively, while the peaks corresponding to characteristic doublets of Fe 2p_3/2 and 2p_1/2 core-level spectra in iron oxide, cannot be observed. This phenomenon can be explained by the fact that the SiO₂ layer thickness (38 nm) exceeds the detecting depth of XPS measurement.

Fig. 6 XPS spectra of (a) FS, (b) FSH. The inset shows the expanded spectra of S 2p.

The grafting of –SO₃H groups was also confirmed by FTIR spectra. As shown in Fig. 7b, FS shows the absorption bands in the range of 3654 cm⁻¹ and 1260–1000 cm⁻¹ assigned to SiO–H and Si–O coordination, respectively. Other intense bands at 1635 cm⁻¹ (O–H bending vibration), 966 cm⁻¹ (Si–O stretching vibration), 806 cm⁻¹ (Si–O–Si bending vibration) and 450 cm⁻¹ (Si–O–Si rocking vibration) are also observed for FS.¹⁹ For FSH (Fig. 7c), the characteristic peaks of –SO₃H group attributed to O–H stretching vibration band centred at 3400 cm⁻¹ and the bands at 2940, 2880 cm⁻¹ (C–H stretching vibration) and 1460–1420 cm⁻¹ (C–H bending vibration) related to the C–H and C–C vibrational modes of the propyl group³⁶ can be observed clearly. The peaks assigned to SO₂ are overlapped by the intense band of Si–O at 1260–1000 cm⁻¹. FTIR spectra indicate that the sulfonic-functionalization of γ-Fe₂O₃@SiO₂ was well done. In the FTIR spectrum of FSHP_1.0-2 (Fig. 7d), the characteristic absorption bands at 1545, 1453, 1310, 1166, 1037, and 897 cm⁻¹ attributed to the coated of PPy are similar to those of PPy/γ-Fe₂O₃ synthesized.^11,26 The peaks at 1545 and 1453 cm⁻¹ are ascribed to the C=C and C–N stretching vibrations. The peaks at 1313 and 1186 cm⁻¹ correspond to the C–H in-plane vibration. The peaks at 1037 and 885 cm⁻¹ are attributed to the C–H in-plane bending and the ring deformation, respectively. These results indicate that the structure of the main PPy chain in the γ-Fe₂O₃@SiO₂@PPy nanospheres prepared is identical to those of the PPy/Fe₃O₄ nanostructures prepared previously.³⁷

Fig. 7 FTIR spectra of (a) γ-Fe₂O₃, (b) FS, (c) FSH and (d) FSHP_1.0-2.

Fig. 8 shows the XRD patterns of FSHP_1.0-2, FSH, γ-Fe₂O₃ synthesized and standard γ-Fe₂O₃. The sharp peaks at 2θ = 30.2°, 35.6°, 43.3°, 53.7°, 57.3°, and 62.9°are observed in the synthesized γ-Fe₂O₃ (Fig. 8c), which match well with the standard peaks of γ-Fe₂O₃ (JCPDS file no. 39-1346, Fig. 8d). Apart from the peaks attributed to γ-Fe₂O₃, a broad diffraction peak centred at 2θ = 23.5° attributed to the amorphous SiO₂ (Fig. 8b) and a broad diffraction peak centred at 2θ = 23.5°assigned to PPy (Fig. 8a) are observed in the XRD patterns of FSH and FSHP_1.0-2.^11,38

Fig. 8 XRD patterns of (a) FSHP_1.0-2, (b) FSH, (c) as-synthesized γ-Fe₂O₃, and (d) standard γ-Fe₂O₃ (JCPDS no. 39-1346).

Electrical and magnetic properties

It has been demonstrated that magnetic and electrical properties can be successfully integrated into a single COSM.⁸ Subsequently, the conductivity and magnetization can be enhanced simultaneously by controlling the amounts of FeCl₂ ([Fe^II]/[Fe^III] ratio < 0.046) for ICOSM.⁹ However, the conductivity and saturation magnetization decrease negatively simultaneously when [Fe^II]/[Fe^III] ratio is higher than 0.046. By controlling the synthesis conditions, as summarized in Table 1, an electrical conductivity ranging from 17 to 53 S cm⁻¹ (Fig. 9) and a high saturation magnetization in a range of 2.18–8.17 A m² kg⁻¹ (Fig. 10) can be realized using γ-Fe₂O₃@SiO₂–SO₃H cores. It's worth mention that the conductivities maintain a high level (50–60 S cm⁻¹) even the γ-Fe₂O₃@SiO₂–SO₃H mass fraction reaches 83.1 wt%, which can be attributed to the complete encapsulation of γ-Fe₂O₃@SiO₂–SO₃H cores by PPy shell. By contrast, the conductivity of the FSP_1.0-2 sample obtained by using unfunctionalized γ-Fe₂O₃@SiO₂ as seeds is as low as 8 S cm⁻¹, which is one magnitude lower than that of FSHP_1.0-2 (55 S cm⁻¹). Till now, we can conclude that the merits on high conductivity and high magnetization can also be integrated into a single nanosphere using the method reported here. Moreover, we can realize the independent control of electrical conductivity and saturation magnetization of the electromagnetically functionalized conducting polymer.

Fig. 9 Effect of the polymerization condition on the conductivity and saturation magnetization of FSHP_0.6-y samples.

Fig. 10 Effect of the FSH mass fraction on the conductivity and saturation magnetization of FSHP_x-2 samples.

Fig. 11 shows the dependence of magnetism (M) on applied magnetic field (H) for FSHP_0.6-y samples and FSHP_x-2 samples. All the γ-Fe₂O₃@SiO₂@PPy nanocomposites exhibit superparamagnetic behavior (i.e., no hysteretic loop). The saturation magnetization (M_s) of the γ-Fe₂O₃@SiO₂@PPy composites increases with an increase of the FSH content (Fig. 11b). And when the dosage of FSH cores was decided, the obtained γ-Fe₂O₃@SiO₂@PPy composites with different polymerization conditions have slightly different contents of FSH, and the M_s of γ-Fe₂O₃@SiO₂@PPy composites is directly proportional to the contents of FSH (Fig. 11a).

Fig. 11 Dependence of the magnetism on the applied magnetic field at room temperature: (a) FSHP_0.6-y samples with different polymerization conditions; (b) FSHP_x-2 samples with different FSH dosages. FSH mass fractions were given based on TGA data.

Conclusions

Electromagnetic functionalized γ-Fe₂O₃@SiO₂@PPy core/shell/shell nanospheres have been successfully realized by in situ polymerization of pyrrole monomers in the presence of spherical γ-Fe₂O₃@SiO₂–SO₃H templates. A prerequisite of realizing the γ-Fe₂O₃@SiO₂@PPy nanospheres is to anchor PPy shell onto SiO₂ surface by –SO₃H groups, allowing for the homogeneously spherical morphology and high conductivity of the γ-Fe₂O₃@SiO₂@PPy composites at high γ-Fe₂O₃@SiO₂–SO₃H dosage. In particularly, the conductivity as high as 17–53 S cm⁻¹ and saturation magnetization of 2.18–8.17 A m² kg⁻¹ can be independently tuned by controlling polymerization conditions and γ-Fe₂O₃@SiO₂–SO₃H dosages.

Acknowledgements

This project was supported by the National Natural Science Foundation of China (No. 41476059) and National Science and Technology Support Program (2012BAB15B02).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra25576g

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