Preparation of polyaniline (PANI)-coated Fe₃O₄ microsphere chains and PANI chain-like hollow spheres without using surfactants

Abstract

We successfully coated polyaniline (PANI) onto amino-Fe₃O₄ microsphere chains to form PANI-coated Fe₃O₄ microsphere chain (PFMC) composites without using any surfactants. Chaining the amino-Fe₃O₄ microspheres as templates was realized via a magnetic-field-induced (MFI) assembly process. The hydrogen bonding formed between amino-Fe₃O₄ microspheres and aniline molecules was the driving force of aniline polymerization on the surface of the microspheres rather than in solution. After the Fe₃O₄ microspheres cores were removed, PANI chain-like hollow spheres (PCHM) were obtained. The length and PANI shell thickness of PFMC composites and corresponding PCHM could be effectively tuned by employing different dosages of aniline. It was found that the PANI shell thickness d₁, average interparticle separation d₂, and PANI loading yield were linearly increased with increasing aniline dosage in a certain range. This effective method not only supports a simple approach to the PFMC composites and PCHM, but also demonstrates that a PANI coating shell can be easily formed via solely electrostatic interactions without the aid of surfactants.

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Introduction

The term conducting polymers, of which polyaniline (PANI) is perhaps the most prominent example, conjure up images of materials that combine the electrical properties of metals with good thermal and environmental stabilities. PANI has been the focus of many studies since MacDiarmid synthesized it by chemical oxidative polymerization in the 1980s.¹ Conducting polymer-coated latex particles are of great interest as they combine the properties of organic conductors and colloidal materials. There are numerous reports on the preparation of polymer latex particles coated with PANI and other conducting polymers such as polypyrrole (PPy) and poly(3,4-ethylenedioxythiophene) (PEDOT).^2–7 Being efficient and inexpensive, colloidal particles as templates are widely researched for the fabrication of capsular and hollow spherical structures, the size of which can simultaneously be regulated and controlled by colloidal particles.⁸ PANI capsules and hollow microspheres attract tremendous attention in last few years, and their unique physicochemical and solution-processable properties find widespread applications in catalysis, actuator, energy storage devices, corrosion protection of metals, microcavity resonance, and delivery and controlled releases, etc.^9–14 In addition, because of high electrical conductivity and multiple redox states (fully reduced leucoemeraldine, the fully oxidized pernigraniline, and the half-oxidized emeraldine), PANI capsules synthesized through using the interface of miniemulsion droplets as a template have recently been considered as an ideal candidate for delivery and controlled releases of self-healing chemicals by a redox stimulus upon electro- or chemical stimuli. The capsules exhibited delayed release under oxidation and rapid release under reduction.¹⁵

The challenge for the preparation of PANI-coated particles and PANI capsules and hollow microspheres is how to generate the PANI shell completely and uniformly on the surface of a sacrificial template cores. The key issue lies in directing and controlling the aniline polymerization on the periphery of template cores rather than in the solution. Until now, intense worldwide research has been making on the fabrication of PANI-coated particles and PANI capsules and hollow microspheres. Using core–gel-shell (polystyrene-sulfonated polystyrene (PS)) template particles, Yang's group obtained monodisperse PANI–silica composite capsules and hollow spheres. Aniline monomers were preferentially adsorbed in the gel shell and polymerized therein, resulting in the formation of PANI–gel capsules. PANI hollow spheres were derived after the polystyrene cores were dissolved.¹⁶ Uniform PANI thin shells and hollow capsules obtained utilized polyelectrolyte-coated microspheres as templates which were formed via the layer-by-layer (LBL) self-assembly procedure.¹⁷ A double-surfactant-layer-assisted polymerization method was designed to obtain well-controlled core/shell-metal/PANI nanocomposites (c/s-CuO, -Fe₂O₃, -In₂O₃, -Fe₂O₃/SiO₂/PANI). By etching the metal cores, hollow PANI capsules maintaining the original template shape were obtained.¹⁸ Well-defined SiO₂-g-P(VAn-g-PANI) core–shell structures were produced by oxidative graft copolymerization of aniline on the initiator (4-vinylaniline)-immobilized SiO₂ nanoparticles by surface-initiated atom transfer radical polymerization (ATRP). Removal of the silica cores by HF etching got the well-defined P(VAn-g-PANI) hollow nanospheres with shell of about 15–40 nm in thickness and core void of about 25 nm in diameter.¹⁹ By means of the “swelling–diffusion–interfacial polymerization method” (SDIPM), monodisperse PS/PANI composites particles using positively charged PS particles as model substrate were prepared, and their morphology could be well controlled by simply changing weight ratio of aniline/PS.²⁰ In the presence of H₃PO₄ as the dopant, aggregated, hollow octahedral PANI micro/nanostructures were synthesized by using octahedral C₂O crystals as a template, and during the polymerization these C₂O template reacted with oxidant ammonium persulfate (APS) to form a soluble Cu²⁺ salt.²¹ An in situ sacrificial oxidative template route was developed for the bulk synthesis of PANI flat hollow capsules. In this reaction system, VOPO₄·2H₂O nanoplates served as both oxidant and sacrificial template for the chemical oxidative polymerization of aniline.²² In a separate study, a unique capability in partially oxidizing the oligoaniline shell on gold nanoparticles to PANI to form yolk–shell nanostructures was demonstrated. These nanostructures could probe the rate of ionic diffusion.²³

It is well-known that the one-dimensional (1D) structures have flourished because of the proven merits in many applications that bring superior performance characteristic compared to those of their currently used bulk counterparts.²⁴ Although there are a variety of synthetic approaches for fabricating PANI capsules and hollow microspheres, the controlled synthesis of 1D PANI chain-like capsules and hollow microspheres using a facile method still remains scientifically challenging. Besides, current synthetic methods to PANI capsules and hollow spheres usually need surfactants to control aniline polymerization and stabilize PANI on the surface of templates. However, it is difficult to remove these surfactants from the resulting product. Accordingly, to find an effective route to get rid of the limit of surfactant is also significant.

Herein, 1D PANI-coated Fe₃O₄ microspheres chains (PFMC) composites were prepared by coating PANI onto the surface of amino-Fe₃O₄ microspheres chains without the help of surfactants. The PANI shell thickness was readily tunable by changing the dosage of aniline. By etching the cores of the PFMC composites in a high concentration of HCl solution, PANI chain-like capsules and hollow microspheres (PCHM) keeping the original template shape and size were got.

Experimental

Materials

Aniline (Hongyan Chemical Reagent Co., Ltd.) was purified by distilling it at a reduced pressure and stored in a refrigerator. Ferric chloride hexahydrate (FeCl₃·6H₂O; Hongyan Chemical Reagent Co., Ltd.), ethylene glycol (EG; Jinshan Chemical Reagent Co., Ltd.), sodium acetate (NaAc; Jinhua Chemical Reagent Co., Ltd.), polyethylene glycol 4000 (PEG4000; Kemiou Chemical Reagent Co., Ltd.), absolute ethanol (Fuyu Chemical Reagent Co., Ltd.), 3-aminopropyltriethoxysilane (APTES; Chenguang Chemical Reagent Co., Ltd.), ammonium hydroxide (NH₃·H₂O; Sanpu Chemical Reagent Co., Ltd.), hydrochloric acid (HCl; Beijing Chemical Reagent Co., Ltd.), and ammonium persulfate (APS; Hongyan Chemical Reagent Co., Ltd.) were of analytical grade and used as received. Deionized water was used through all the synthetic processes.

Synthesis of Fe₃O₄ microspheres

Fe₃O₄ microspheres were prepared through a solvothermal reaction.²⁵ Briefly, FeCl₃·6H₂O (2.7 g) was dissolved in EG (80 mL) to form a clear solution, followed by the addition of NaAc (7.2 g) and PEG4000 (2.0 g). The mixture sealed into a 100 mL Teflon-lined stainless-steel autoclave after the solution was homogeneous yellow under magnetic stirred for half an hour. The autoclave was heated to and maintained at 200 °C for 8 h, then cooled to room temperature. The black Fe₃O₄ microspheres were washed several times with ethanol and deionized water, then dried at 60 °C for 12 h.

Modification of Fe₃O₄ microspheres

The amino-Fe₃O₄ microspheres were prepared through the hydrolysis with APTES. Typically, Fe₃O₄ (100 mg) and APTES (4.0 mL) were dispersed in a mixed solution of ethanol (40 mL) and deionized water (40 mL). NH₃·H₂O (1.0 mL) as catalyst was added into the solution. Then the solution was refluxed for 24 h at 70 °C. The amino-Fe₃O₄ microspheres were obtained.

Synthesis of PFMC composites

In a representative procedure, the amino-Fe₃O₄ microspheres (10 mg) and purified aniline (0.10 mL) were dissolved in a 90 mL HCl solution (0.010 M) to form a homogeneous solution by using ultrasound for half an hour. The solution was poured into a flask and a plane magnet (0.50 T) was placed to the side of the vessel with a distance of about 4 cm. 10 mL HCl solution (0.010 M) containing APS (54.7 mg) was quickly injected into the above solution to initiate the polymerization. The polymerization was left undisturbed at 20 °C for 10 h. The resulting sediment was washed with ethanol and deionized water several times and then obtained through magnetic separation.

A range of experiments getting PFMC composites with various PANI shell thickness were proceeded. All procedures were the same as the aforementioned preparation process except changing the dosages of aniline and APS: (1) aniline: 0.050 mL, APS: 27.4 mg; (2) aniline: 0.015 mL, APS: 82.1 mg; (3) aniline: 0.20 mL, APS: 109.5 mg; (4) aniline: 0.25 mL, APS: 136.9 mg; (5) aniline: 0.30 mL, APS: 164.2 mg, (6) aniline: 0.50 mL, APS: 274.0 mg.

A comparative experiment to investigate the effect of hydrogen bonding

A comparative experiment was carried out. All procedures were the same as the preparation process of PFMC composites (aniline: 0.10 mL), but the Fe₃O₄ microspheres used were not modified with APTES beforehand.

Synthesis of PANI chains-like capsules and hollow spheres

PANI chain-like capsule and hollow microspheres with different shell thickness were obtained by etching and dissolving the Fe₃O₄ microspheres chains of respective PFMC composites in HCl solution. Specific steps are as follows: the PFMC composites were immersed into 50 mL 2.0 M HCl solution for 48 h and then washed several times with water by centrifugation at 4000 rpm until the upper solution was colorless. The resulting PCHM were obtained by drying the sediment at 60 °C for 12 h.

Characterization

Field emission scanning electron microscopy (FE-SEM) images were generated with JEOL JSM-6700F microscope and ZEISS MERLIN microscope. Samples dispersed in ethanol were deposited onto silicon wafers and sputtered with platinum by a JFC-1600 auto fine coater at a current of 20 mA for 300 s before observation. Transmission electron microscopy (TEM) images were taken on a JEOL JEM-3010 microscope with Oxford 794-CCD camera at an accelerating voltage of 200 kV. Samples suspended in ethanol were dropped onto copper grids coated with carbon support film. Magnetic property of samples was acquired at room temperature by using a Quantum Design VersaLab vibrating sample magnetometer (VSM). Fourier transform infrared (FTIR) spectra were obtained from 2000 to 400 cm⁻¹ on a Bruker TENSOR 27 spectrometer. The samples needed to be pressed into pellets with KBr before measurement. X-ray photoelectron spectrum (XPS) was gained through a Kratos Axis Ultra DDL spectroscopy. Ultraviolet and visible (UV-vis) spectrum for PANI chain-like hollow spheres (PCHM) was measured from 200 to 900 nm in a LabTech UV Power spectrophotometer. The PANI content of samples was determined through HENVEN thermogravimetric analysis (TGA) in the temperature range from 20 °C to 800 °C with a heating rate of 10 °C min⁻¹.

Results and discussion

Fabrication of PFMC composites and PCHM

The schematic illustration of preparation of PFMC composites and PCHM is displayed in Fig. 1. SEM and TEM images (Fig. 1(a)) exhibit the Fe₃O₄ microspheres prepared by solvothermal method,²⁵ which have smooth surface and about 400 nm diameter. All detected diffraction peaks ((220) (311) (400) (422) (511) (440)) in XRD pattern (ESI Fig. S1†) of these microspheres are indexed to face centered cubic Fe₃O₄ microspheres (JCPDS card no. 75-1609). After being cleaned, Fe₃O₄ microspheres in an aqueous dispersion were hydrolytically treated with APTES to immobilize amino groups onto their surface to improve their affinity to aniline monomers. These modified Fe₃O₄ microspheres were sequentially mixed with aniline monomers (0.10 mL) and HCl solution. Then these modified Fe₃O₄ microspheres absorbed a layer of aniline monomers onto their surface, and assembled into magnetic dipole chains via magnetic-field-induced (MFI) assembly process as applying an external magnetic field.^26–28 Upon oxidation and polymerization, a layer of PANI deposited on the surface of the magnetic dipole chains, thus gradually acquiring the PFMC composites. As shown in SEM image (Fig. 1(b)), the resulting PFMC composites have chain-like morphology whose length ranges from several microns to tens of microns. The core–shell structure of them can be better observed in the TEM image. In Fig. 1(c), the periodically arrayed Fe₃O₄ microspheres (black color) assemble into chains by head-to-tail interactions in a continuous PANI shell (gray color). The thickness of the PANI shell is estimated to be 50–130 nm. Note that the Fe₃O₄ microspheres are linearly arranged, indicating the pre-organization of Fe₃O₄ microspheres into chains through MFI self-assembly process before the polymerization of aniline.

Fig. 1 Schematic illustration of preparation of PFMC composites and PCHM using 0.10 mL aniline in 0.010 M HCl solution. Inset electron microscope images: (a) SEM and inset TEM images of Fe₃O₄ microspheres, (b) SEM and (c) TEM images of PFMC composites, (d) SEM and (e) TEM images of PCHM, scale bar of inset SEM image in (d): 1 μm.

After etching and dissolving Fe₃O₄ microspheres cores in a strong HCl solution (2.0 M), the resulting PCHM were acquired. In SEM image (Fig. 1(d)), it can be seen that although the cores have been removed, the PCHM still maintain the same 1D chain-like structure as the PFMC composites. In inset SEM image, a magnified view shows the PCHM have rough surface. The TEM image (Fig. 1(e)) exhibits the PFMC have the chain-like capsular and hollow spherical structure, and their diameter and shell thickness are about 400 nm and 50–130 nm, in consistence with the original PFMC composites.

The magnetic properties of Fe₃O₄ microspheres and PFMC composites were examined by VSM, as exhibited in Fig. 2(a). Their saturation magnetization value measured at 300 K is 78.01 and 33.18 emu g⁻¹, respectively. The PFMC composites have much smaller saturation magnetization value than Fe₃O₄ microspheres, which is rationalized on the decrease of magnetic content due to the introduction of PANI. Based on the remarkable distinction of two saturation magnetization values, it is powerful evidence that PANI shell is generated. Meanwhile, zero coercivity in magnetization curves shows both samples have paramagnetic property, suggesting that they can be readily manipulated by external magnetic field.^29,30 As illustrated in inset optical pictures, the separation-redispersion behavior of Fe₃O₄ microspheres (left) and PFMC composites (right) in water manifests both samples possessing excellent magnetic responsivity. During the experimental process, this excellent property can shorten the post-treatment time, effectively simplifying the experimental procedures.

Fig. 2 (a) VSM curves measured at 300 K and (b) FTIR spectra of Fe₃O₄ microspheres and PFMC composites, inset optical photographs in (a): separation-redispersion behavior in water of Fe₃O₄ microspheres (left) and PFMC composites (right) under an magnetic field (0.50 T). (c) XPS (inset: N1s core-level) spectra of PFMC composites. (d) UV spectrum of PCHM in water.

FTIR spectra (Fig. 2(b)) reveal the obvious differences between Fe₃O₄ microspheres and PFMC composites. There is only one peak at 580 nm corresponding to Fe–O vibration observed for Fe₃O₄ microspheres.³¹ But for PFMC composites, the peaks at 1643, 1618 cm⁻¹ are attributed to C=C stretching vibration of quinoid rings and benzene rings.³² There is a band due to an aromatic vibration at 1553 cm⁻¹.³³ Bands at 1416 and 1102 cm⁻¹ assigns to C–N and C=N stretching vibrations.³² The peaks at 1024 and 810 cm⁻¹ is ascribed to aromatic C–H bending in the plane and out of the plane for para disubstituted rings.³⁴ The characteristic peak presents at 630 cm⁻¹ is related to Fe–O vibration.³¹ These characteristic peaks of PFMC composites indicate that PANI forms on the surface of Fe₃O₄ microspheres chains.

XPS was employed to investigate the composition of the resulting PFMC composites. In Fig. 2(c), it can be viewed that these PFMC composites are mainly composed of C, N, O, and Fe elements. Nevertheless, the binding energy for Fe2p is difficult to observe, which further supports the Fe₃O₄ microspheres in the composite are encased within a shell of PANI, in accordance with the TEM image (Fig. 1(c)). In addition, the imine nitrogen atoms of PANI are protonated in part or in whole to produce a series of oxidation states. Accordingly, the oxidation states of PANI can be quantitatively differentiated by scrutinizing the binding energy of nitrogen atoms.³⁵ Inset N1s core-level spectrum is deconvoluted into three major components with binding energy at 403.5, 404.5, and 405.6 V, which are attributed to the quinonoid imine (=N–), benzenoid amine (–NH–), and nitrogen (–N⁺–), respectively. The XPS results demonstrate that the PANI shell is in emeraldine salt, in agreement with the previous result.³⁶ Furthermore, the atomic percent of three kinds of nitrogen atoms is 17.18 (=N–), 63.58 (–NH–), and 19.24% (–N⁺–), so the doping level of the PANI shell is only 19.24%. This insufficient doping can be effortlessly affirmed by the brown color of PFMC composites (inset optical pictures (right) of Fig. 2(a)), while sufficient doping of PANI (emeraldine salt) usually possesses green or dark green color.³⁷

Fig. 2(d) shows the UV-vis spectrum of PCHM which are gained by etching the PFMC composites in 2.0 M HCl solution and re-dispersing in water. A peak at 338 nm is ascribed to π–π* transition of the benzene ring, showing that the PCHM is in emeraldine salt presumably as a result of PANI re-doping during the etching process in 2.0 M HCl solution.³⁸ The peak for the π-to-polar band transition around 600 nm is not found. The specific reason about the disappearance of this peak is still unclear, although we speculate the electrostatic interaction between residual Fe³⁺ ions and PANI molecules eliminates the π-to-polar band transition.

Effect of hydrogen bonding

The PFMC composites and PCHM have been successfully produced. During the preparation process, the key is to induce the chaining of the amino-Fe₃O₄ microspheres by exposing to external magnetic field during the PANI coating process so that the microspheres stay temporarily connected, allowing additional PANI deposition to fix the chains into mechanically robust 1D chains. There are two difficulties should be overcome. One is inducing chaining of the Fe₃O₄ microspheres, which has been easily solved by using MFI self-assembly process.^39–41 The other is getting Fe₃O₄–PANI core–shell structure. The mechanistic rationale for the synthesis of core–shell structure of PANI-coated colloidal particles composites have been roughly explained by many previous results, in most of which surfactant layer surrounding on the surface of colloidal particles adsorbs aniline molecules through intermolecular interactions, followed by forming PANI shell. However, in our case the underlying formation mechanism is quite different, since there are no surfactants but amino-Fe₃O₄ microspheres used in the reaction process. To elucidate the special formation mechanism of PFMC composites, the indispensability of APTES modification to Fe₃O₄ microspheres is discussed in the following part.

A comparative experiment in the presence of external magnetic field but without using APTES to modify Fe₃O₄ microspheres was carried out, as shown in Fig. 3. The SEM image (Fig. 3(a)) only exhibits the irregular PANI (gray color) in micron size and scattered Fe₃O₄ microspheres (white color), but no PANI-coated Fe₃O₄ microspheres chain-like morphology. In TEM image (Fig. 3(b)), one can easily find scattered Fe₃O₄ microspheres (black color) and short PANI nanofibers (gray color). The direct results from electron microscope images imply that aniline molecules polymerize in the solution rather than on the surface of Fe₃O₄ microspheres. Hence, it is proved that once the Fe₃O₄ microspheres are not modified with amino groups on their periphery, the PFMC composites cannot be obtained.

Fig. 3 (a) SEM and (b) TEM images of a comparative sample obtained in the presence of magnetic field but without using APTES to modify Fe₃O₄ microspheres, the other reaction conditions are same as the preparation of PFMC composites.

During the polymerization process, hydrogen bonding formed spontaneously between amino-Fe₃O₄ microspheres and aniline molecules is deemed to be the fundamental driving force for the formation of Fe₃O₄–PANI core–shell structures. As shown in Fig. 1, the amino-Fe₃O₄ microspheres and aniline monomers are fully dispersed in HCl solution (0.010 M). Afterwards, aniline molecules are adsorbed on the surface of amino-Fe₃O₄ microspheres through the interaction of hydrogen bonding. This adsorption results in great increase of the local concentration of aniline molecules near the microspheres surface, which is favorable for the initial aniline polymerization in a low monomer concentration.¹⁸ Under the external magnetic field, these Fe₃O₄–aniline structure assembles into 1D temporary chains. Once PANI nucleation is generated on the surface of amino-Fe₃O₄ microspheres chains, the polymerization takes place preferentially and continuously in the close proximity of already existing PANI owing to the low nucleation energy.⁴² Therefore, the aniline polymerization is successfully initiated, propagated, and terminated on the surface of amino-Fe₃O₄ microspheres chains instead of in the solution. Finally, a continuous, homogeneous, and uniform PANI shell is generated and Fe₃O₄–PANI core–shell structure appears. Moreover, the PCHM are got through the subsequent removal of the Fe₃O₄ microspheres cores in a strong HCl solution (2.0 M).

Different dosages of aniline

Furthermore, the shell thickness of PANI in PFMC composites (FTIR spectra shown in ESI Fig. S2,† the measurement process of coating shell shown in ESI Fig. S3†) and PCHM can be readily tuned by taking advantage of different dosages of aniline, as exhibited in Fig. 4. In SEM image (Fig. 4(a1)), when 0.050 mL aniline is used, very short PFMC composites with 2–6 μm in length are viewed. Extremely thin PANI shell estimated to be 8–23 nm is displayed in TEM image (Fig. 4(a2)). In inset TEM image, smooth PANI shell tightly wraps Fe₃O₄ microspheres, and no PANI protuberances are discovered. It is explicit that the appearance of these short chains is a consequence of the weak immobilization of thin PANI shell. However, as aniline dosage is increased to 0.15 mL, the PFMC composites with length of tens of microns are typically obtained (SEM image of Fig. 4(b1)). From TEM image (Fig. 4(b2)), it is easily found the thickness of PANI shell is 20–59 nm. In inset TEM image, there are PANI protuberances concomitantly generated. This phenomenon is correlated with the disordered accumulation of rigid PANI molecules. Similar outcome is observed in the reaction system in which 0.20 mL aniline is used (SEM image of Fig. 4(c1)), except that the PANI shell increases to 40–130 nm (TEM image of Fig. 4(c2)). Interestingly, a distinguishing consequence emerges once the dosage of aniline is up to 0.25 mL (SEM image of Fig. 4(d1)). Aside from the PFMC composites produced, a few short PANI nanofibers grow on their surface. In TEM image (Fig. 4(d2)), a short PANI nanofiber is clearly seen (highlighted by arrow). At the same time, the PANI shell increases to 81–167 nm, and there are more and bigger PANI protuberances appeared. With the dosage of aniline increasing to 0.30 mL (SEM image of Fig. 4(e1)), as expected, PANI nanofibers begin to dominate the morphology. The reason is that there are enough aniline monomers to support the continually growing of short nanofibers. At a closer look, SEM image (Fig. 4(e2)) exhibits the magnified PANI nanofibers with the diameter of 100–195 nm and it is better viewed in TEM image (Fig. 4(e3), highlighted by arrows). When the dosage of aniline is 0.50 mL, more PANI nanofibers with diameter of 65–200 nm and PANI granules on their surface (SEM image of ESI Fig. S4†) are found, suggesting that secondary growth of PANI happens in the case of adding excess aniline.^43,44

Fig. 4 The shell thickness of PFMC composites and PCHM can be effectively tuned by varying the dosage of aniline: (a): 0.050 mL, (b): 0.10 mL, (c): 0.20 mL, (d): 0.25 mL, (e): 0.30 mL. (a1), (b1), (c1), (d1), (e1) SEM and (a2), (b2), (c2), (d2) TEM images of PFMC composites. (a3), (b3), (c3), (d3), (e3) SEM and (a4), (b4), (c4), (d4), (e4) TEM images of PCHM obtained by etching the corresponding PFMC composites in 2.0 M HCl solution. The inset in (a2), (a3), (b2), (c4) show a magnified view of respective samples. (e2) SEM image shows magnified PANI nanofibers.

From the above electron microscope images, the PFMC composites with controllable PANI shell are realized through using various dosages of aniline. After removing the Fe₃O₄ microspheres cores by employing a 2.0 M HCl solution, the PCHM are attained. As 0.050 mL aniline is applied (SEM image of Fig. 4(a3)), ellipsoidal PCHM with inner diameter about 400 nm are discerned. This non-spherical morphology is obviously observed in inset SEM image. Chain-like structure made up of ellipsoidal capsules is verified in TEM image (Fig. 4(a4)). PANI shell cannot retain the spherical contour and deforms into a degassed ellipsoidal morphology, because of the limited dosage of aniline. When increasing aniline dosage to 0.15 mL (SEM and TEM images of Fig. 4(b3) and (b4)), the length and shell thickness of PCHM as well as PANI protuberances are similar to those of the original PFMC composites. From this case, the chain-like structure and spherical contour of PANI can be well maintained even though the PFMC composites undergo an etching process followed by purification processes. A similar morphology but with thicker PANI shell and more PANI protuberances are generated as using 0.20 mL aniline (SEM and TEM image of Fig. 4(c3) and (c4)). As shown in SEM and TEM images (Fig. 4(d3) and (d4)), one can see short PANI nanofibers on the surface of PCHM and thick PANI shell like the pristine PFMC composites (0.25 mL). When further increasing the dosage of aniline to 0.30 mL (TEM image of Fig. 4(e4)), longer PANI nanofibers can be clearly observed (highlighted by arrows).

From the experimental results above, the PFMC composites and PCHM with adjustable length and shell thickness are realized. Besides, it is noteworthy that the upper critical value of aniline dosage for forming PANI shell is 0.25 mL. Once beyond this critical value, PANI nanofibers deriving from PANI shell begin to grow.⁴⁵ From a viewpoint of nucleation, homogeneous nucleation of PANI results in nanofibers, while heterogeneous nucleation leads to granular particulates.^46–48 In our polymerization process, PANI nucleation is firstly supposed to be heterogeneous nucleation on the surface of Fe₃O₄ microspheres chains due to the effect of hydrogen bonding, resulting in the formation of PANI shell. As the polymerization proceeds, the sufficiently high supersaturation level of PANI molecules is easily achieved, beneficially giving rise to homogeneous nucleation. Then homogenous nucleation dominates the nucleation mode, and ultimately PANI nanofibers form.

The aniline dosage plays a decisive role in controlling the thickness of PANI shell. With limited aniline, ellipsoidal PCHM were obtained, while with excess aniline monomers PANI nanofibers start to grow. Only within a certain range (aniline: 0.0–0.25 mL), the growth of PANI shell is dependent on the increase of aniline dosage. Interestingly, the increase of average interparticle separation is consistent with the change of aniline dosage as well. Fig. 5(a) and (b) summarize PANI shell thickness d₁ and average interparticle separation d₂. The application of less aniline (0.050 mL) results in a thin layer of PANI (d₁ = 15.7 nm) between neighboring Fe₃O₄ microspheres (d₂ = 8.6 nm). As using 0.10, 0.15, 0.20, 0.25 mL aniline, the d₁ is 30.6, 75.0, 107.9, 114.9 nm, and the d₂ is 9.7, 15.2, 19.7, 20.3 nm. The d₁ and d₂ respectively reveal an approximate linear relationship (d₁ = 558.7X − 17.9, d₂ = 66.8X + 4.66 (X: dosage of aniline)). We speculate more hydrogen bonding formed with increasing aniline dosage, thus making more aniline molecules absorbed on the surface of amino-Fe₃O₄ microspheres, which is the essential reason for linear increasing of d₁ and d₂.

Fig. 5 The dependence of (a) PANI shell thickness d₁ and (b) average interparticle separation d₂ on the dosage-point of aniline (0.050, 0.10, 0.15, 0.20, 0.25 mL). (c) TGA curves of different PFMC composites obtained with different loadings of aniline (0.0, 0.050, 0.10, 0.15, 0.20, 0.25, 0.30 mL) from 20 to 800 °C. (d) The dependence of PANI loading yield of different PFMC composites on the dosage-point of aniline.

As shown in Fig. 5(c), the mass percent of PANI in different PFMC composites are investigated by TGA. With changing aniline dosage from 0.0 to 0.30 mL, the weight loss, that is, the mass percent of PANI, is 3.7, 5.6, 13.0, 19.8, 28.2, 41.4, and 48.6%, respectively. For Fe₃O₄ microspheres (0.0 mL), the weight loss at 20–300 °C assigns to residual water and surfactant PEG4000 that is against the particle agglomeration in the formation process of Fe₃O₄ microspheres.²⁵ For other PFMC composites (0.050, 0.10, 0.15, 0.20, 0.25, 0.30 mL), a sharp weight loss from 300–600 °C is mainly attributed to the thermal degradation of PANI shell, in line with the previous results.^49,50 According to TGA data, PANI loading yield of PFMC composites is described in Fig. 5(d). It is also found PANI loading yield is proportional to the dosage of aniline (Y = 158.21X − 0.8321 (X: dosage of aniline, Y: PANI loading yield)). This case is a prominent feature in contrast with the PANI deposition onto PS particles, in which PANI loading yield gradually declines.⁵¹ Even when a myriad of PANI nanofibers appear (0.30 mL), PANI loading yield is also accordant with linear relationship. On the basis of d₁, d₂, and TGA data, it is easily known that the growth of PANI shell is in direct proportion to the aniline dosage in a certain range (0.0–0.25 mL). The possible explanation is attributable to the fact that Fe₃O₄ microspheres have enough amino-groups to capture aniline molecules to form PANI shell owing to the effect of hydrogen bonding. Although it is beyond the largest capture ability (for example: 0.30 mL), the rest of aniline molecules dispersed in the solution also polymerize and grow on the PANI shell, making PANI loading yield continually conform to the linear relationship.

Conclusions

In summary, we successfully fabricated PFMC composites with adjustable length and shell thickness using amino-Fe₃O₄ microspheres chains as a template without employing any surfactants. The corresponding PCHM were also obtained by etching PFMC composites in a strong HCl solution. In a certain range (the dosage of aniline: 0.0–2.5 mL), the growth of PANI shell was linearly increased. When excess aniline was used, PANI nanofibers began to grow from PANI shell. It should be of considerable interest that the method in which the effect of hydrogen bonding is the driving force and no surfactants are used will open a versatile way for preparing an overlayer on any template materials.

Acknowledgements

We gratefully appreciate the support of the National Natural Science Foundation of China (Grant 51173146), the National High Technology Research and Development Program of China (863 Program) (Grant 2012AA02A404), and the National Key Basic Research Program of China (Grant 2010CB635111). This research is supported by NPU Foundation for Graduate Innovation.

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Footnote

† Electronic supplementary information (ESI) available: XRD pattern of Fe₃O₄ microspheres, FTIR spectra of PFMC composites obtained with different dosages of aniline, the measurement process of PANI shell thickness, SEM images of PFMC composites and PANI nanofibers obtained at different magnifications (aniline: 0.50 mL). See DOI: 10.1039/c5ra20330a

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