MoS₂ armored polystyrene particles with a narrow size distribution via membrane-assisted Pickering emulsions for monolayer-shelled liquid marbles

Abstract

We report an effective method to fabricate MoS₂ armored polystyrene particles with a narrow size distribution based on membrane-assisted Pickering emulsions. These prepared hybrid particles can serve as stabilizers in the formation of monolayer-shelled liquid marbles by rolling water droplets on a hybrid particle powder bed followed by solvent vapor treatment.

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One interesting class of nanocomposite (NC) polymer latexes is those with morphologies that are armored or multilayered in nature.^1–3 They are widely applied in various fields such as biomedicine, catalysis, and optics, and are also important for fundamental studies in colloid and interface science.^4–6 The traditional path to produce these complex NC colloids is a layer-by-layer approach, which involves first preparing a core, and subsequently depositing multi-shells.^7–9 The disadvantages of this approach are that it is time-consuming, it requires special design for the coupling shell materials, intermediate purification or separation, and large-scale production is limited.^4–9

Recently, the self-assembly of solid particles at the liquid–liquid interface to stabilize so-called Pickering emulsions has been well documented and offers a straightforward pathway for the production of organized nanostructures.^10–14 The solid particles first self-assemble at the liquid–liquid interface and act as effective stabilizers during the polymerization process. By polymerizing the inner monomer phase, the particle stabilizers are captured on the surface of the resultant polymer beads, resulting in a final product with a polymer core with nanoparticles armoring the core surface.^15–18 Several successful syntheses were conducted using Pickering emulsion polymerization to produce armored polymer latexes: Armes et al. described the synthesis of poly(methyl methacrylate)-silica NC particles in aqueous alcoholic media using silica nanoparticles as stabilizer;¹⁹ Bon et al. used clay discs as a stabilizer to produce a variety of armored “soft” polymer latexes.²⁰ However, only particles with suitable surface wettability can serve as Pickering emulsion stabilizers.^21–23 Particles that are too hydrophobic or hydrophilic should preferably be modified to demonstrate suitable wettability.^24–28 Moreover, as the conventional preparation of Pickering emulsions is based on a droplet breakup method by disrupting the large droplets of the pre-emulsion into smaller ones, the obtained emulsion droplets exhibit high polydispersity which limits their practical applications.^14–22

MoS₂, as a type of transition metal dichalcogenide, has drawn much attention due to its unusual physical properties, especially its complementary electronic properties compared to graphene.^29,30 For example, a monolayer of MoS₂ is a semiconductor and shows an intrinsic band gap and high mobility, making it useful for potential applications in sensors, optoelectronics, and electroluminescent devices.^31–33 MoS₂/organic nanocomposites have recently received extensive attention,^34,35 while there are few reports about research on the fabrication of MoS₂-armored polymer latexes. In this work, MoS₂ was first introduced as a Pickering stabilizer to stabilize oil-in-water (O/W) emulsions. Under a membrane emulsification process and the polymerization of the inner phase, MoS₂ armored polymeric particles with a narrow size distribution were obtained. Liquid marbles were then achieved by rolling water droplets on a powder bed of the dried obtained particles. Moreover, MoS₂ hybrid polymeric capsules were prepared from the liquid marbles via solvent vapor treatment.

Monolayer MoS₂ (mo-MoS₂) was produced by a chemical exploitation method, which involved intercalating Li into the bulk MoS₂, followed by exposing the Li_xMoS₂ to water. AFM measurements revealed a mean longest diagonal of 0.7 μm and a thickness of about 0.8 nm, which was consistent with the reported values for monolayer chemically exploited MoS₂ (Fig. S1†).³⁶ The obtained mo-MoS₂ could be homogeneously dispersed in water as it had a zeta potential value of −31.4 mV (Fig. S2†). However, such a large zeta potential value was not beneficial to stabilize oil-in-water emulsion droplets because a too large mutual repulsion of nanosheets led to an insufficient nanosheet coating on the droplet surface, and it could not provide significant prevention from coalescence to the droplets. To our best knowledge, an electrolyte of BaCl₂ was applied for the first time to the MoS₂ aqueous suspension to screen part of the charges to enhance its emulsifying ability.

As shown in Fig. 1a, styrene was rapidly emulsified in a mo-MoS₂/BaCl₂ aqueous mixture to produce an O/W Pickering emulsion by shear agitation, with the nanosheets self-assembling at the liquid–liquid interface. The obtained droplets appeared as a typical Pickering emulsion with a spherical shape, and an average size of about 47 μm. With increasing MoS₂ concentration, the size of the emulsion droplets decreased slightly (Fig. S3†). By confocal microscope, both markers were imaged in the emulsion droplets (Fig. 1b). Firstly, the styrene phase was labeled with Nile red, appearing as red, and the water phase appeared as black; then, the water phase was stained with rhodamine B, appearing as green, and the styrene phase appeared as black. This revealed that the water phase acted as the continuous phase with styrene being the dispersed phase, demonstrating an O/W Pickering emulsion system.

Fig. 1 (a) Schematic of the formation of MoS₂ nanosheet-stabilized styrene (St)-in-water Pickering emulsions. (b) Confocal laser scanning microscopy images of the obtained emulsion droplets: Nile red-marked oil phase on the left, and Rhodamine B-stained water phase on the right. (c) Schematic illustration of the membrane-extrusion emulsification processing. The optical microscopy images of the MoS₂ nanosheet-stabilized styrene-in-water emulsion (d) before and (e) after membrane extrusion with 100 passes. The styrene to water ratio is 1 : 3, and the MoS₂ concentration is 4 mg mL⁻¹.

For practical applications, MoS₂ armored particles with different core components are needed under varied situations. Therefore, besides styrene, various kinds of organic solvents, including liquid paraffin, toluene, n-hexane, and hexadecane, were applied as oil phases to examine the generalizability of the MoS₂ nanosheet-stabilized O/W Pickering emulsion system (Fig. S4†). It was revealed that most of the water-insoluble liquids could be successfully emulsified by this surfactant-free system. The obtained emulsions exhibited as traditional Pickering types, whilst their sizes and size distributions varied with their different densities, viscosities, and surface tensions (Fig. S5†). Moreover, increasing the nanosheet concentration decreased the mean droplet diameter, until a plateau at ca. 50 μm was reached. Higher nanosheet concentrations did not lead to any further reduction in droplet size, but merely to an excess of the latex nanosheets in solution. This observation was consistent with those reported previously.^14–17

Even though the sizes of the obtained emulsion droplets could be tuned by adjusting the nanosheet concentrations, they exhibited fairly high polydispersity. This was ascribed to the conventional pre-emulsion/droplet breakup method which could not easily control the mean droplet sizes.^21–24 In order to obtain particles with a narrow size distribution to satisfy practical requirements, many microengineering techniques, such as microchannel emulsification based microfluidic devices and membrane emulsification by Shirasu porous glass membranes,^37–39 have been developed to further treat Pickering emulsions. However, the requirement for complex devices raises barriers to application in practical production. Recently, Zhu et al. utilized a membrane-extrusion emulsification-assisted approach to generate microcapsules with uniform sizes.⁴⁰ Inspired by that, the MoS₂ nanosheet-stabilized O/W Pickering pre-emulsions were re-emulsified by a hand-driven mini-extruder containing two syringes (Fig. 1c). The poly(vinylidene fluoride) filtering membrane used in this study consisted of pore openings of 7.5 μm in diameter. As shown in Fig. 1d, the sizes of emulsion droplets fabricated by shear agitation are large and polydisperse, ranging from 20 to 120 μm with an average diameter of 50 μm. It was clear to see that the droplet sizes were significantly reduced and the size distribution became much narrower after the extrusion process (Fig. 1e and S6†).

Moreover, the droplet sizes and size distribution were found to strongly depend on the number of passes through the membrane. As shown in Fig. 2a and b, with an increase in the number of passes, the average size of the obtained droplets decreased from 47 to 25 μm. More importantly, the polydispersity index (PDI) significantly decreased from 0.199 to 0.078 over 100 passes, resulting in relatively uniform emulsion droplets (Fig. 2b). This is almost comparable to the values for the uniform particles.⁴¹

Fig. 2 (a) Size distribution curves of MoS₂ nanosheet-stabilized styrene-in-water emulsions after different numbers of membrane extrusion passes. (b) The average size and PDI of the emulsion droplets from (a). SEM images of the MoS₂ armored PS particles fabricated based on emulsions (c) before and (d) after membrane extrusion with 100 passes. The insets of (c) and (d) are the corresponding amplified images of the particles.

By polymerizing the styrene phase, MoS₂ armored polystyrene (PS) particles were achieved (Fig. 2c and d). Compared with the surfactant-stabilized emulsion polymerization, the nanoparticle-stabilized Pickering emulsion polymerization could be classified as a type of suspension polymerization without any stirring, saving much energy. Moreover, adhesion between the neighboring particles could be avoided as the MoS₂ nanosheets absorbed at the oil–water interfaces acted as the physical barriers. As shown in Fig. 2d, the MoS₂ armored PS particles were spherical with a monodisperse size and the surface was rough. EDS tests revealed that there was about 3 wt% of MoS₂ in the obtained hybrid particles (Table S1†). Such uniform particles consisting of a MoS₂ coating on the surface and polymeric cores could find great potential in sensors, transistors, optoelectronics, and many other fields.

Liquid marbles are liquid droplets with liquid-repellent particles irreversibly adsorbed at the liquid–air interfaces that decrease the apparent surface tension of the liquid, and maintain their spherical shape.^42,43 Because of their unique surface properties, liquid marbles move smoothly over any substrate without leakage, and have been exploited in a wide range of applications including, but not limited to, cosmetics, micropumping, water surface pollution detection, gas storage, gas–liquid reactions, and the preparation of microreactors.⁴⁴ In this work, the obtained MoS₂ armored PS particles were applied to stabilize liquid marbles, producing capsules with MoS₂/PS hybrid shells (Fig. 3a). Firstly, a water droplet was rolled on a powder bed of monodisperse particles, and the latex particles immediately coated the water droplet. These liquid marbles remained intact after being transferred onto a polypropylene mesh (Fig. 3b). However, the stability of the obtained liquid marbles remained a challenge in practical applications, and solvent vapor treatment was applied to enhance the marbles’ mechanical strength. The liquid marble-containing polypropylene mesh was placed on M-shaped rigid aluminum foil, which was placed in a closed glass bottle containing 2 mL of toluene. After 20 seconds, the liquid marbles were taken out and the MoS₂/PS hybrid walled capsules were obtained. The hybrid particles adhere to each other by swelling to form a polymer film, which could largely enhance the obtained liquid marbles’ stability. Interestingly, unlike previous liquid marbles,^45–53 the liquid marbles obtained in this work consisted of a monolayer shell, which can be identified in Fig. 3c. This could be ascribed to the fact that the liquid marble stabilizers were particles with a narrow size distribution, and these relatively uniform stabilizers resulted in two-dimensional close packing on the surface of the water droplet without any space for the other layer particles adsorbed at the water–air interfaces.

Fig. 3 (a) Fabrication of the monolayer-shelled liquid marbles via rolling and solvent vapor treatment. (b) Digital photograph of the obtained liquid marbles. The inset of (b) is an SEM image of the obtained liquid marble. (c) SEM image of the monolayer shell of the obtained liquid marbles.

In conclusion, we demonstrated, for the first time, that chemically exploited MoS₂ could serve as a solid surfactant for producing an O/W Pickering emulsion without any molecular surfactants. Following a membrane-extrusion process and suspension polymerization, MoS₂ armored PS particles with a narrow size distribution were successfully achieved. Furthermore, these hybrid particles could serve as stabilizers to form liquid marbles by rolling water droplets on a powder bed of the particles, followed by solvent vapor treatment. It was demonstrated that the obtained liquid marbles exhibited good mechanical strength and consisted of a monolayer MoS₂/PS hybrid shell, having great potential applications in microcapsulation and surface chemistry.

Acknowledgements

Financial support from the National Natural Science Foundation of China (21274046 and 21474032), National Basic Research Program of China (973 Program, 2012CB821500), and Natural Science Foundation of Guangdong Province (S20120011057) is gratefully acknowledged.

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Footnote

† Electronic supplementary information (ESI) available: Experimental procedures and characterization data. See DOI: 10.1039/c5ra11709g

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