A method to construct perfect 3D polymer/graphene oxide core–shell microspheres via electrostatic self-assembly

Abstract

In this study, a method to construct perfect three-dimensional (3D) polymer/graphene oxide (GO) core–shell microspheres was proposed via electrostatic self-assembly. 2D GO nanosheets were successfully wrapped onto polymer microspheres to form a perfect 3D core–shell structure with uniform shell thickness under the action of an electrostatic attraction force. The GO nanosheets, with a thickness of 1.5–2 nm and an area over 2 × 1 μm², were firstly prepared from graphite, and then cationic polystyrene (PS) microspheres with 0.246% and 0.715% surface concentrations of –N(CH₃)₃⁺ were successfully synthesized. After that, PS/GO core–shell microspheres were constructed from the GO nanosheets and the cationic PS microspheres. It was found that different cationic PS microspheres led to different assembly speeds. SEM and TEM images of rippled silk waves on the surface of the PS/GO core–shell microspheres not only indicated the perfect polymer/GO core–shell structure, but also demonstrated the strong binding between the two materials. It was also revealed that the thicknesses of the shells of the PS/GO core–shell microspheres were under good control, and the thicknesses of shells from different cationic PS microspheres were 9–13 nm and 80–100 nm. The method proposed here has proved to be a valuable tool for the assembly of 3D microstructures from polymers and graphene oxide (or graphene).

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1. Introduction

Graphene,¹ a single carbon sheet with a 2D honeycomb crystal lattice structure made up of hexagonal carbon rings, is supposed to be the nanomaterial with the most potential, with excellent conductivity,^2–4 mechanical properties,⁵ thermal conductivity⁶ and electrochemical properties.⁷ Thus, its potential applications have been widely developed in the fields of polymer composites,^8,9 biomedical sciences,¹⁰ sensors,¹¹ batteries,¹² supercapacitors,¹³ and photocatalysis.¹⁴ Graphene has been used as a perfect 2D material in most of the recent applications. Low-dimensional materials such as 2D graphene can achieve good strength in part because of the lack of surface defects that often initiate fractures in 3D materials.¹⁵ Nevertheless, graphene and graphene oxide could be useful for the construction of three-dimensional (3D) macrostructures for use in real-life devices. Therefore, developing a method to transform the nanosheets into three-dimensional macrostructures with a uniform thickness is of great significance.

The well-established strategies such as spin-coating,¹⁶ filtration,^17,18 layer-by-layer (LbL) assembly,^19,20 and the Langmuir–Blodgett technique^21,22 can transform nanosheets into thin or ultra-thin film structures. On the contrary, constructing a thicker structure is very difficult. It is well known that because of the high specific surface area, the original 2D lamellar structure is easily distorted and aggregated into other uncontrolled shapes, resulting in a sharp decline in performance. Therefore, an important issue is how to construct a 3D macrostructure with a uniform thickness, avoiding agglomeration during the process. Several studies have investigated the 3D nanostructures of graphene in recent reports,^23–31 of which, the most commonly used method was a hydrothermal method for graphene oxide (GO) suspensions. Xie³² reported the preparation of PS particles using GO nanosheets as the stabilizer, and an intricate method which was sensitive to the initiator and a slight change of pH was provided. Yang³³ studied a self-assembly method to prepare a PMMA/graphene composite, and it was reported that PMMA/GO complex particles were formed due to the interaction between the GO nanosheets and the PMMA particles. Huang et al.³⁴ introduced a method of surface modification to form almost perfect SiO₂/GO hybrids. Li et al.³⁵ reported a facile method to fabricate polystyrene/graphene core–shell microspheres through electrostatic interaction. The polystyrene microspheres were prepared by emulsion homopolymerization using 2,2′-azobis(2-methylpropionamidine) dihydrochloride (AIBA) as a cationic initiator. In their work, the cationic groups in the PS microspheres were the residues of AIBA (initiator residues). In general, the amount of initiator used in emulsion polymerization is very low. The positive charge is derived from positively charged cationic groups. Thus, in their study, the positive charge on the surface of cationic PS microspheres was very small. This will lead to a thin shell thickness of core–shell microspheres, and the shell thickness certainly cannot be controlled.

Herein, perfect 3D polymer/graphene oxide (GO) core–shell microspheres are fabricated via electrostatic self-assembly. Considering that graphene has no functional groups on its surface, the well-dispersed graphene oxide suspension becomes the best bridge to construct the core–shell microspheres. The availability of oxygen-containing functional groups on the basal plane and edge allows the GO sheets to interact with each other or to disperse in a variety of organic and inorganic materials to form the desired structure. In this work, GO nanosheets possessing negative charges overlaid and attached tightly around the cationic PS microspheres by electrostatic attraction. Different from other 3D graphene or graphene oxide microstructures, the thickness of this shell structure is relatively uniform and truly controllable. The whole process of electrostatic self-assembly is totally spontaneous, and the formed core–shell structure can withstand intense agitation and ultrasonic treatment without being destroyed. A sketch of the electrostatic self-assembly is shown in Fig. 1. Moreover, the bonding force of the assembly is very strong and can be utilized in various applications, such as to create free-standing architectures with tailored shapes or to transform 2D graphene oxide nanosheets into various 3D structures. With such useful characteristics, we believe that this technique is probably a valuable tool for the assembly of nanostructures or nanomaterials.

Fig. 1 A sketch of the process of electrostatic self-assembly.

2. Experimental

2.1. Materials

Graphene oxide was prepared via a modified Hummers method³⁶ using crystalline flake graphite as the raw material. The crystalline flake graphite was bought from Qingdao Tianheda Graphite Co., Ltd. Styrene was distillated under a vacuum to remove inhibitors. Azobisisobutyronitrile (AIBN) was refined before use by recrystallization in methanol. Styrene, AIBN, methacryloxyethyl trimethyl ammonium chloride (DMC), polyvinyl pyrrolidone (K30), and ethanol were all of analytical grade and used as provided.

2.2. Preparation of the polymer/graphene oxide core–shell microspheres

Preparation of the polymer/GO core–shell microspheres can be divided into two steps:

(1) Copolymerization with a cationic monomer to get a positively charged polystyrene (PS) microsphere;

(2) The electrostatic assembly of the negative exfoliated GO sheets with the cationic PS microspheres.

Cationic PS microspheres were prepared by dispersed emulsion polymerization. 0.8 g polyvinyl pyrrolidone was added into a three-necked flask containing 40 mL deionized water and 30 mL ethanol. A water bath was used at 70 °C, with mechanical stirring at a speed of 300 rpm, under a nitrogen atmosphere. Half an hour later, 2 g of styrene and 0.2 g of AIBN were added into the mixture. After the mixture turned a little white, a solution of 0.2 g AIBN, 6 g styrene, and DMC (2 wt%, 10 wt% of styrene dissolved in 30 mL ethanol) was added drop by drop. This adding process was completed 2 hours later, and then the system was refluxed at 70 °C for another 3 h under a nitrogen atmosphere. The solid contents of the emulsions were 8.33 wt% and 8.34 wt%, respectively. In the present study, the emulsions with 2 wt% and 10 wt% DMC were named cationic PS 002 and cationic PS 010, respectively. A pure polystyrene emulsion was also prepared according to the above procedure, only without DMC.

The PS/GO core–shell microspheres were prepared by simply mixing the cationic PS microsphere emulsion with the GO aqueous solution. The aqueous solution of GO (0.3 wt%) was prepared by high-power ultrasonication for 30 min and high-speed (>8000 rad per min) centrifugation three times. The 5 mL resultant diluted GO solution (diluted 100 times) was added dropwise into the 5 mL diluted emulsion (diluted 100 times) under agitation. After the stirring was stopped, PS/GO core–shell microspheres were precipitated at the bottom of the beaker, leaving the upper nearly transparent aqueous solution. The PS/GO core–shell microspheres were kept at room temperature for complete aggregation.

2.3. Characterizations

2.3.1. AFM. Atomic force microscopy (AFM) micrograph of GO was obtained on a Multimode Autoprobe CP/MT Scanning Probe Microscope (Veeco Instruments, Woodbury, NY). A GO solution of 0.01 mg mL⁻¹ was dropped onto a nude mica surface, dried in the dryer at room temperature until the test. The tapping mode was used and the scan rate was 1.0 Hz.

2.3.2. SEM and TEM. Scanning electron microscopy (SEM, Hitachi Limited S-3400, Japan) with an acceleration voltage of 10 kV was applied to analyze the morphology of the cationic PS microspheres and the PS/GO core–shell microspheres. The samples were diluted in a certain concentration and dried on coverslips in a dryer overnight. Transmission electron microscopy (TEM) was carried out on a Tecnai G² F20 microscope. All the samples were prepared at room temperature by using a droplet of the diluted water of the sample on a copper grid and drying it in the air.

2.3.3. XPS. The powders of GO, the PS microspheres, and the cationic PS microspheres were pulverized and detected by X-ray photoelectron spectroscopy (XPS). The oxygen content of GO and the element N content of the polymer surface can be obtained by calculations. The X-ray photoelectron spectra were recorded on a Kratos Axis Ultra-DLD system with Al Kα, 1000 meV, and 150 W.

2.3.4. Zeta potential. The zeta potential of the cationic PS emulsion and GO were determined by dynamic light scattering (DLS) using a Malvern Zetasizer Nano-ZS with a laser of 532 nm wavelength at 25 °C. The emulsions and GO dispersion were diluted 100 times with deionized water before the test. All the samples were pH neutral in the tests.

2.3.5. Particle size analysis. Particle size analysis was conducted on a Mastersizer 2000 laser particle size analyzer. The samples of the cationic microsphere emulsions were diluted with water for testing.

2.3.6. XRD. X-ray diffraction was measured using an X’Pert PRO diffractometer (PANalytical, Holland) with Cu radiation of 40 kV and 35 mA. The samples were all powders and the scanning speed and step size were 0.2° min⁻¹ and 0.03°, respectively.

3. Results and discussion

3.1. Graphene oxide

The AFM image, TEM image, and XPS spectra of the GO nanosheets are shown in Fig. 2. The sample of GO exfoliated by ultrasonic treatment was diluted by water for AFM and TEM testing. In Fig. 2, the area of the nanosheets can be measured as being over 2 × 1 μm², and the AFM height image shows that the thickness of nanosheets is 1.5–2 nm, which clearly indicates the graphite oxide is completely exfoliated via the ultrasonic treatment. In general, pure graphene has a thickness of 0.34 nm, and graphene oxide possesses a thicker size due to epoxy and hydroxyl groups randomly distributed on the basal plane and several layers stacking together. Fig. 2(c) and (d) illustrate the XPS spectra of GO. The oxygen content of GO is calculated as 29.26 wt%, indicating a high degree of oxidation. The C1s XPS spectrum of GO can be fitted into four peaks in the region of 284.6–288.5 eV, which represents the existence of four different carbon valence bonds.³⁷ From the C1s XPS spectrum of GO, it is proved that hydroxyl, epoxide, and carboxyl groups are generated after oxidation. These groups are hydrophilic, and therefore, GO can be easily made into an aqueous suspension. The zeta potential of GO is −40.3 mV, which is in accordance with the reported values.³⁸ The results above indicate the successful preparation of the GO nanosheets from graphite.

Fig. 2 Characterization of graphene oxide. (a) AFM height image of the graphene oxide nanosheets; (b) TEM image of the graphene oxide nanosheets; (c) XPS spectrum and (d) C1s XPS spectrum of GO.

3.2. Cationic polymer microspheres

In this study, the cationic PS microspheres are successfully obtained. Fig. 3 shows the SEM images of the 002 and 010 cationic PS microspheres. The zeta potentials of the 002 and 010 cationic PS microspheres are +37.1 mV and +70.3 mV, respectively. Similar results are reported in the literature.^39,40 Reversal potentials enable strong electrostatic interactions between the cationic PS microspheres and the GO nanosheets. The average particle size of the 002 cationic PS microspheres is D(0.5) = 0.351 μm, and that of the 010 cationic PS microspheres is D(0.5) = 0.336 μm. The D(0.5) is the particle size from a volume fraction of 50%, it can be regarded as the volume average particle size.

Fig. 3 SEM images of the cationic PS microspheres. (a) Cationic PS 002 microspheres; (b) cationic PS 010 microspheres. Please see section 2.2 for the meaning of 002 and 010.

XPS was carried out to prove the existence of the cationic group, which plays an important role during the process of self-assembly. Compared with Fig. 4(b), an obvious new peak located at 402.42 eV is observed in Fig. 4(c) and (d), indicating the generation of the cationic group.⁴¹ The other states of N1s from PVP and AIBN exist in the binding energy of 399–401 eV.⁴² Here, an equation to determine the surface concentration of an element from XPS is given below:

C_x = (A_x/S_x)/(∑A_i/S_i)

where C_x is the surface concentration of the desired element, and A_x is the peak area of the desired element. S_x is the sensitivity factor.

Fig. 4 XPS spectra. (a) Powders of the pure PS microspheres; (b) N1s spectra of the pure PS microspheres; (c) N1s spectra of the cationic PS 002 microspheres; (d) N1s spectra of the cationic PS 010 microspheres.

The actual concentration of –N(CH₃)₃⁺ was calculated according to the equation above. The surface charge density depends on the concentration of –N(CH₃)₃⁺, therefore, the surface charge density can be calculated from the –N(CH₃)₃⁺ group. Then, the issue becomes to calculate the state of –N(CH₃)₃⁺ among all the N-containing groups. For the cationic PS 002 microspheres, the measured surface concentration of N element in –N(CH₃)₃⁺ is 0.246%. However, the theoretical concentration of N element in –N(CH₃)₃⁺ is 0.106%. The actual concentration is more than twice the theoretical value. We think these results can be interpreted in terms of the strong hydrophilic nature of the cationic monomer. The hydrophilic –N(CH₃)₃⁺ will certainly migrate toward the surface of the cationic PS microspheres in solution, resulting in the enhancement of the actual surface concentration of N element in –N(CH₃)₃⁺. Similarly, the cationic PS 010 microspheres gain a 0.715% surface concentration of N element in –N(CH₃)₃⁺, which is also higher than the theoretical value (0.506%). The migration of –N(CH₃)₃⁺ to the surface of the cationic PS microspheres is a good phenomenon for our work, because the surface concentration of –N(CH₃)₃⁺ is proportional to the surface charge density, which plays an important role in electrostatic self-assembly.

3.3. Electrostatic self-assembly

The results of electrostatic self-assembly after 240 min between cationic PS and GO with different content are shown in Fig. 5. In the first row, 1-a is the digital image of the cationic PS emulsion, and 1-b is the pure PS emulsion mixed with the GO aqueous suspension, which produces no sediment. 1-c is the digital image of 3 wt% cationic PS 002 mixed with 1 wt% GO, and 1-d is 3 wt% cationic PS 010 mixed with 5 wt% GO. 1-e is the GO aqueous suspension. In the second row, 2-a, 2-b, 2-c, 2-d, and 2-e are 3 wt% cationic PS 002 emulsions mixed with 1, 2, 3, 4 and 5 wt% GO aqueous suspensions, respectively. In the third row, 3-a, 3-b, 3-c, 3-d, and 3-e are 3 wt% cationic PS 010 emulsions mixed with 1, 2, 3, 4 and 5 wt% GO aqueous suspensions, respectively.

Fig. 5 Digital images of electrostatic self-assembly after 240 min. In the first row: (1-a) 3 wt% cationic PS emulsion; (1-b) 3 wt% pure PS emulsion mixed with 3 wt% GO aqueous suspension, which produces no sediment; (1-c) 3 wt% cationic PS 002 mixed with 1 wt% GO; (1-d) 3 wt% cationic PS 010 mixed with 5 wt% GO; (1-e) 3 wt% GO aqueous suspension. In the second row, 2-a, 2-b, 2-c, 2-d, and 2-e are 3 wt% cationic PS 002 emulsions mixed with 1, 2, 3, 4 and 5 wt% GO aqueous suspensions, respectively. In the third row, 3-a, 3-b, 3-c, 3-d, and 3-e are 3 wt% cationic PS 010 emulsions mixed with 1, 2, 3, 4 and 5 wt% GO aqueous suspensions, respectively.

Table 1 shows the total element contents of each sample calculated according to the XPS spectra in Fig. 2, 4, and 6. Elemental carbon, oxygen and nitrogen are contained in each sample. The total contents of element O in cationic PS 002 and cationic PS 010 are 3.1% and 4.0%, respectively. The total contents of element O in cationic PS 002 with GO and cationic PS 010 with GO are 7.2% and 8.8%, respectively. The increase of oxygen content, along with the decrease of nitrogen and carbon contents, clearly demonstrates that GO has already been overlaid onto the cationic PS microspheres. A careful study of the nitrogen content reveals more details. The N content of PS 002 with a GO coating is 2.0%, which is close to the 2.9% of pure cationic PS 002. However, 1.1% of PS 010 with a GO coating is closer to the 0.6% of GO. This phenomenon indicates that cationic PS 010 mixed with GO generates a thicker core–shell structure. The thick shell of GO makes XPS unable to detect the interior structure. A detailed analysis of O1s in Fig. 6 gives a similar result.

Table 1 Total element contents of each sample calculated according to the XPS spectra in Fig. 2, 4, and 6

Sample	Total element contents
	C	O	N
Cationic PS 002	94.0%	3.1%	2.9%
Cationic PS 010	93.5%	4.0%	2.5%
GO	69.9%	29.5%	0.6%
3 wt% cationic PS 002 + 1 wt% GO	90.8%	7.2%	2.0%
3 wt% cationic PS 010 + 2 wt% GO	90.1%	8.8%	1.1%

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Fig. 6 XPS spectra of the PS/GO core–shell microspheres. (a) Pure cationic PS 002. (b) O1s spectra of the cationic PS 002 microspheres mixed with a 1 wt% GO suspension. (c) O1s spectra of the cationic PS 010 microspheres mixed with a 2 wt% GO suspension.

In Fig. 5, the images of the first row clearly illustrate that the specific electrostatic attraction only exists between the cationic PS and GO, rather than for other mixed systems. The second and the third rows show the results of electrostatic self-assembly with different contents of the GO aqueous suspension, from 1 wt% to 5 wt%. It can be observed that PS/GO assembly is quickly precipitated at the bottom of the bottles in the case of 1 wt% GO with cationic PS 002 (2-a) and 3–5 wt% GO with cationic PS 010 (3-c, 3-d, 3-e), leaving a transparent aqueous solution at the top of the bottles. This also indicates that the used concentration of the cationic PS and GO nanosheets in 2-a, 3-c, 3-d and 3-e are exactly matched, which leads to the charge neutralization between the positive charge in the cationic PS and the negative charge in the GO nanosheets, resulting in a fast precipitation. Actually, all the samples in the second and third rows of Fig. 5 produce a precipitate. However, except for 2-a, 3-c, 3-d, 3-e, all the other samples need 5–7 days. It is noted that both the PS emulsion and the GO aqueous suspension are of near neutral pH in our study. The samples of 1-a, 1-b, 2-a, 2-c, and 3-c were chosen for further SEM and TEM analysis.

Fig. 6 shows the O1s spectra of the core–shell microspheres. Fig. 6(b) is the O1s spectra of the PS/GO core–shell microspheres assembled by 3 wt% cationic PS 002 and 1 wt% GO, and Fig. 6(c) is the O1s spectra of the core–shell microspheres generated from 3 wt% cationic PS 010 and 2 wt% GO. The peaks at 532.82 eV and 531.78 eV are the characteristic peaks of O=C–O, C=O and C–O in GO in Fig. 6(b) and (c).⁴³ The peak at 533.72 eV is assigned to O=C–O–C of cationic monomer DMC. It can be observed that the peak intensity at 533.72 eV of the PS 010/GO core–shell microspheres is obviously weaker than that of PS 002/GO. The calculated oxygen contents from O=C–O–C in Fig. 6(b) and (c) are 1.0% and 0.5%, respectively. This phenomenon is similar to the total oxygen contents listed in Table 1. The decrease of the oxygen content of O=C–O–C indicates a thicker GO layer in the PS 010/GO microspheres, which agrees with the discussion for Table 1.

The morphology of the electrostatic assembly is observed by SEM, and is shown in Fig. 7. Clearly, the PS/GO core–shell microspheres are formed via electrostatic assembly. Fig. 7(a) and (b) show the images of the typical core–shell microsphere assembly of 3 wt% cationic PS 002 and 1 wt% GO (2-a in Fig. 5). Fig. 7(c) is a SEM image with both smooth and wrinkled microspheres, which is just a simple mixture of pure PS emulsion and 3 wt% GO suspension (1-b in Fig. 5). The GO nanosheets only cover parts of the PS microspheres. It can be seen that all the polymer microspheres are successfully encapsulated by flexible and ultra-thin GO nanosheets in Fig. 7(a) and (b). In addition, rippled silk waves formed by redundant GO nanosheets can be clearly observed between each PS/GO core–shell microsphere, as marked by the arrow and the circle in Fig. 7(b). The reason behind this phenomenon is probably that the GO nanosheets with a large area will not perfectly match with a single particle, resulting in the formation of a ribbon structure between the microspheres.

Fig. 7 (a and b) PS/GO core–shell microspheres with different magnifications (2-a in Fig. 5). (c) Pure PS microspheres mixed with GO 3 wt% GO suspension (1-b in Fig. 5).

Fig. 8 shows the influence of the cationic PS with different amounts of cationic monomer on the PS/GO core–shell microspheres. Both Fig. 8(a) and (b) show rippled silk waves on the surface of each microsphere. Compared to Fig. 8(a), the microspheres in Fig. 8(b) have more attachments, presenting slight deformation on the surface. This indicates a stronger electrostatic attraction force between the cationic PS 010 microspheres and the negatively charged GO nanosheets, and it leads to more GO covers and the deformation. Here the surface density of the positive charge plays a key role in controlling the surface morphology and core–shell structure of the PS/GO microspheres.

Fig. 8 SEM images of the PS/GO core–shell microspheres from (a) 3 wt% cationic PS 002 and 3 wt% GO aqueous suspension (2-c in Fig. 5) and (b) from 3 wt% cationic PS 010 and 3 wt% GO aqueous suspension (3-c in Fig. 5).

TEM images of the PS/GO core–shell microsphere assembly from cationic PS 002 and 1 wt% GO aqueous suspension (2-a in Fig. 5) with different magnifications are shown in Fig. 9. A nearly perfect core–shell microsphere can be observed with a layer of about 9–13 nm GO adsorbed tightly on the spherical surface, showing a clear outline of the microspheres.

Fig. 9 TEM images of the PS/GO core–shell microsphere assembly from cationic PS 002 and 1 wt% GO aqueous suspension with different magnifications (2-a in Fig. 5).

TEM images of the PS 010/GO core–shell microspheres (Fig. 10) have obvious differences from the PS 002/GO core–shell microspheres (Fig. 9) both in shape and the thickness of the shell structures. In Fig. 10, the edge of the core–shell microspheres in the TEM images indicates that the GO nanosheets anchor tightly and thickly on the surface of the cationic polymer microspheres, as marked by the arrows. In addition, the margin of the core–shell microspheres in Fig. 10(a) also shows the ribbon-like GO pieces, which are similar to the ribbon materials observed in the SEM images. Particularly, in Fig. 10(b), from the edge of the PS/GO core–shell microspheres, the thicknesses of the shells can be measured within 80–100 nm, as indicated by the arrows. Here, the thickness of the shell is uniform and much thicker compared to SiO₂/GO hybrids constructed through hydrogen bonding between –NH₂ and –COOH.³⁴ In the present study, the method proposed achieves a 3D graphene oxide construction both with a uniform thickness and a strong binding force.

Fig. 10 TEM images of the PS/GO core–shell microspheres, which are from 3 wt% cationic PS 010 and 3 wt% GO aqueous suspension (3-c in Fig. 5).

Fig. 11(a)–(c) show the particle size distributions. Apparently, the particle size distribution of pure PS and the PS/GO core–shell microspheres from 3 wt% cationic PS 002 and 1 wt% GO (2-a in Fig. 5) are mainly within the scope of 0.3–0.4 μm, and that of the PS/GO core–shell microspheres from 3 wt% cationic PS 010 and 3 wt% GO (3-c in Fig. 5) is within the scope of 0.4–0.5 μm, along with a higher frequency in a larger size.

Fig. 11 Particle size distribution of the PS/GO core–shell microspheres and the XRD patterns. (a) Powders of pure PS; (b) the PS/GO core–shell microspheres from 3 wt% cationic PS 002 and 1 wt% GO (2-a in Fig. 5); (c) the PS/GO core–shell microspheres from 3 wt% cationic PS 010 and 3 wt% GO (3-c in Fig. 5); (d) the XRD patterns of GO, cationic PS 002, and the PS/GO core–shell microspheres from 3 wt% cationic PS 002 and 1 wt% GO (2-a in Fig. 5).

Fig. 11(d) shows the XRD patterns of GO, cationic PS 002, and the PS/GO core–shell microspheres from 3 wt% cationic PS 002 mixed with 1 wt% GO (2-a in Fig. 5). All the samples in Fig. 11(d) were dried into powders in a vacuum oven overnight. The strong diffraction peak at 2θ = 10.59° is assigned to GO (001),⁴⁴ and the corresponding d-spacing value of graphene oxide is calculated as d₍₀₀₁₎ = 0.835 nm based on the Bragg equation (λ = 2d sin θ), which is identical to the reported studies.^45–47 For the pattern of the cationic PS, a characteristic broad peak at around 2θ = 20.04° is obtained, which indicates the amorphous structure.⁴⁸ The pattern of the PS/GO core–shell microspheres from 3 wt% cationic PS 002 and 1 wt% GO is similar to that of the cationic PS, and the only difference is the appearance of a weak peak at about 10.26°, which is attributed to GO binding to the surface of the microspheres.⁴⁴

4. Conclusions

In this study, a perfect 3D polymer/graphene oxide core–shell microsphere structure was successfully prepared via electrostatic self-assembly. 2D graphene oxide nanosheets were successfully wrapped onto microspheres to form a 3D core–shell structure under the action of an electrostatic attraction force and produced shells with a uniform thickness. The SEM and TEM images of rippled silk waves on the surface of the PS/GO core–shell microspheres not only indicated the perfect polymer/GO core–shell structure, but also demonstrated the strong binding between the two materials. It was also revealed that the thicknesses of the shells of the PS/GO core–shell microspheres were under good control and the thicknesses of the shells from different cationic PS microspheres (with 0.246% and 0.715% surface concentration of –N(CH₃)₃⁺ groups) were 9–13 and 80–100 nm. We believe the method proposed here is a valuable tool for the controllable assembly of polymers and graphene oxide (or graphene). This method will probably start a new understanding of the interaction between graphene oxide and polymers.

Acknowledgements

This work was supported by the Research Center for Application of Graphene (Sichuan University-WuXi), the State Key Laboratory of Polymer Materials Engineering (Grant no. sklpme2014-3-06), and the National Natural Science Foundation of China (Grant no. 51473104).

Notes and references

1. X. Huang, X. Y. Qi, F. Boey and H. Zhang, Chem. Soc. Rev., 2012, 41, 666–686.
2. H. Bi, J. Chen, W. Zhao and S. R. Sun, RSC Adv., 2013, 3, 8454–8460.
3. A. Bhardwaj, A. K. Shukla, S. R. Dhakate and D. K. Misra, RSC Adv., 2015, 5, 11058–11070.
4. J. K. Lee, C. S. Park and H. Kim, RSC Adv., 2014, 4, 62453–62456.
5. Q. Peng and S. De, RSC Adv., 2013, 3, 24337–24344.
6. C. Y. Chang, S. P. Ju, J. W. Chang, S. C. Huang and H. W. Yang, RSC Adv., 2014, 4, 26074–26080.
7. A. Pandikumar, G. T. S. How, T. P. See and F. S. Omar, et al., RSC Adv., 2014, 4, 63296–63323.
8. F. You, D. G. Wang, X. X. Li and M. J. Liu, et al., RSC Adv., 2014, 4, 8799–8807.
9. O. Jankovský, P. Šimek and D. Sedmidubský, et al., RSC Adv., 2014, 4, 7418–7424.
10. Y. Wang, Z. H. Li, J. Wang, J. H. Li and Y. H. Lin, Trends Biotechnol., 2011, 29, 205–212.
11. N. Li, Z. P. Chen, W. C. Ren, F. Li and H. M. Cheng, Proc. Natl. Acad. Sci. U. S. A., 2012, 109, 17360–17365.
12. Y. Gu and Y. Wang, RSC Adv., 2014, 4, 8582–8589.
13. Y. S. Luo, D. Z. Kong, Y. L. Jia and J. S. Luo, et al., RSC Adv., 2013, 3, 5851–5859.
14. R. Fang, X. P. Ge, M. Du, Z. Li and C. Z. Yang, et al., Colloid Polym. Sci., 2014, 292, 985–990.
15. G. H. Lee, R. C. Cooper, S. J. An and S. Lee, et al., Science, 2013, 340, 1073–1076.
16. V. C. Tung, L. M. Chen and M. J. Allen, et al., Nano Lett., 2009, 9, 1949–1955.
17. G. Eda, G. Fanchini and M. Chhowalla, et al., Nat. Nanotechnol., 2008, 3, 270–274.
18. X. W. Yang, L. Qiu and C. Cheng, et al., Angew. Chem., Int. Ed., 2011, 50, 7325–7328.
19. T. Sasaki and Y. Ebina, et al., Chem. Commun., 2000, 2163–2164.
20. P. Podsiadlo, M. Michel and J. Lee, et al., Nano Lett., 2008, 8, 1762–1770.
21. L. J. Cote, F. Kim and J. X. Huang, J. Am. Chem. Soc., 2009, 131, 1043–1049.
22. X. L. Li, G. Zhang and X. D. Bai, et al., Nat. Nanotechnol., 2008, 3, 538–542.
23. Y. X. Xu, K. X. Sheng, C. Li and G. Q. Shi, ACS Nano, 2010, 4, 4324–4330.
24. F. Liu and T. S. Seo, Adv. Funct. Mater., 2010, 20, 1930–1936.
25. Z. J. Fan, J. Yan, L. J. Zhi, Q. Zhang, T. Wei and J. Feng, et al., Adv. Mater., 2010, 22, 3723–3728.
26. J. L. Vickery, A. J. Patil and S. Mann, Adv. Mater., 2009, 21, 2180–2184.
27. Z. H. Tang, S. L. Shen, J. Zhuang and X. Wang, Angew. Chem., Int. Ed., 2010, 49, 4603–4607.
28. X. Jiang, Y. Ma, J. Li, Q. Fan and W. Huang, J. Phys. Chem. C, 2010, 114, 22462–22465.
29. H. L. Luo, G. Y. Xiong, Z. W. Yang and S. R. Raman, et al., RSC Adv., 2014, 4, 14369–14372.
30. X. C. Dong, Y. F. Cao and J. Wang, et al., RSC Adv., 2012, 2, 4364–4369.
31. Y. F. Zhang, M. Z. Ma and J. Yang, et al., RSC Adv., 2014, 4, 8466–8471.
32. P. F. Xie, X. P. Ge, B. Fang and Z. Li, et al., Colloid Polym. Sci., 2013, 291, 1631–1639.
33. J. T. Yang, X. H. Yan and M. J. Wu, et al., J. Nanopart. Res., 2012, 14, 717–725.
34. L. Huang, P. L. Zhu and G. Li, et al., J. Mater. Chem. A, 2014, 2, 18246–18255.
35. S. Y. Li, T. Qian, S. S. Wu and J. Shen, Chem. Commun., 2012, 48, 7997–7999.
36. W. S. Hummers Jr and R. E. Offeman, J. Am. Chem. Soc., 1958, 80, 1339.
37. S. Stankovich, D. A. Dikin and R. D. Piner, et al., Carbon, 2007, 45, 1558–1565.
38. S. Chandra, S. Sahu and P. Pramanik, Mater. Sci. Eng., B, 2010, 167, 133–136.
39. G. W. Zhang, W. Ao, C. Yang, J. Liu and J. J. Liu, Petrochem. Technol., 2011, 40, 1057–1062.
40. X. Z. Kong, J. Lian, X. L. Zhu, X. L. Gu and Z. G. Zhang, Acta Polym. Sin., 2008, 8, 787–802.
41. M. Fossa, S. Diplasb and E. Gulbrandsenc, Electrochim. Acta, 2010, 55, 4851–4857.
42. X. Mei, J. Y. Chen and N. Huang, J. Funct. Biomater., 2007, 38, 1819–1821.
43. A. Bagri, C. Mattevi, M. Acik and Y. J. Chabal, et al., Nat. Chem., 2010, 2, 581–587.
44. Z. S. Zhao and C. Zhang, et al., J. Nanyang Norm. Univ., 2010, 9, 42–44.
45. G. X. Zhao, L. Jiang, Y. D. He and J. X. Li, et al., Adv. Mater., 2011, 23, 3959–3963.
46. M. Song, L. L. Yu and Y. M. Wu, J. Nanomater., 2012, 135138–135143.
47. Y. H. Xue, Y. Liu, F. Lu and J. Qu, et al., J. Phys. Chem. Lett., 2012, 3, 1607–1612.
48. Y. B. Zhao, J. F. Zhou and Z. J. Zhang, Chem. Res., 1998, 9, 16–19.

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