Fabrication of Composite Polymer Foam Films at ... - ACS Publications

Feb 12, 2014 - The film formation is a result of emulsion droplet-templated assembly and ... Physical Chemistry Chemical Physics 2016 18 (3), 1945-195...
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Fabrication of Composite Polymer Foam Films at the Liquid/Liquid Interface through Emulsion-Directed Assembly and Adsorption Processes Yuanyuan Geng,† Mei Liu,† Kun Tong,† Jian Xu,† Yong-Ill Lee,‡ Jingcheng Hao,† and Hong-Guo Liu*,† †

Key Laboratory for Colloid and Interface Chemistry of Education Ministry, Shandong University, Jinan 250100, P. R. China Anastro Laboratory, Department of Chemistry, Changwon National University, Changwon 641-773, Korea

‡

S Supporting Information *

ABSTRACT: The foam films of polystyrene-b-poly(acrylic acid)-bpolystyrene (PS-b-PAA-b-PS) doped with Cd(II) or Pb(II) species were fabricated at the planar liquid/liquid interfaces between a DMF/chloroform (v/v: 1/1) solution of the polymer and aqueous solutions containing cadmium acetate or lead acetate at ambient temperature. Optical microscopic observation shows the thin film is uniform on a larger length scale. Transmission electron microscopic (TEM) investigations reveal that the foam films are made up of microcapsules with the size of several hundreds of nanometers to micrometers. The walls of the microcapsules have a layered structure decorating with nanofibers and hollow nanospheres, where numerous inorganic fine nanoparticles are dispersed homogeneously. The film formation is a result of emulsion droplet-templated assembly and adsorption of the formed microcapsules at the planar liquid/liquid interface. Because of the miscibility of DMF with chloroform and water, DMF migrates to the aqueous phase while water migrates to the organic phase across the interface, resulting in the formation of a W/O emulsion, as revealed by optical microscopic observation, freeze fracture transmission electron microscopic (FF-TEM) observation, and dynamic laser scattering (DLS) investigation. The triblock copolymer molecules and the inorganic species adsorb and selfassemble around the emulsion drops, leading to the formation of the composite microcapsules. X-ray photoelectron spectroscopic (XPS) and FTIR spectroscopic results indicate that two kinds of Cd(II) or Pb(II) species, metal oxide or hydroxide, resulting from the hydrolysis of the metal ions and the coordinated metal ions to the carboxyl groups coexist in the formed thin films, which transform to metal sulfide completely after treating with hydrogen sulfide to get metal sulfide nanoparticle-doped polymer thin films.

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process.13−24 This method has some advantages over others. First of all, this method can assemble amphiphilic polymer molecules and hydrophilic inorganic species such as metal ions, complex ions, and colloidal particles into a composite structure. Second, the planar liquid/liquid interface plays an important role in directing the formation of low-dimensional micro- and nanostructures. In addition, the fabrication can be carried out under ambient atmosphere. There are several reasonable approaches which have emerged since 2011. For example, fiber-like nanocomposites composed of CdS quantum dots and a PVK derivative,13 Ag nanoparticle-doped poly(4-vinylpyridine) porous thin films,14 and composite nanotubes of a conjugated polymer with Cu2+ ions15 have been fabricated through an adsorption and self-assembly process at the liquid/ liquid interface. Thus, there is little doubt that the planar

INTRODUCTION Functional nanoparticles including noble metal nanoparticles and quantum dots are usually immobilized on an inert support to prevent them from aggregation. Polymers are often used as such matrix due to their chemical and thermal stabilities and easy processing ability.1,2 Various inorganic nanoparticle/ polymer composite micro- and nanostructures have been fabricated by several approaches including mixing,3 layer-bylayer assembly,4 electrospan/adsorption,5 template-induced polymerization,6 and self-assembly, etc. Among these approaches, self-assembly of polymers, especially block copolymers, with inorganic species has attracted much attention in recent years as a result of their peculiar and fascinating assembly behaviors superior to their homopolymers.7−10 The planar liquid/liquid interface has been utilized to synthesize nanoparticles and to fabricate nanostructures in recent years.11,12 It has also been found that the liquid/liquid interface can be used to fabricate inorganic species/polymer composite structures through adsorption and self-assembly © 2014 American Chemical Society

Received: August 27, 2013 Revised: February 7, 2014 Published: February 12, 2014 2178

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wish this approach can be utilized as a general one to fabricate more and more composite thin films.

liquid/liquid interface technique has been arousing an increasing research interest due to its simplicity, convenience, and usefulness. At the same time, we have also been doing some works on the fabrication of inorganic species/polymer composite microand nanostructures at the liquid/liquid interface.16−24 We found that poly(2-vinylpyridine) (P2VP) formed foam films at the interface with inorganic species including HAuCl4, H2PtCl6, and AgNO3, and the molecular weight of P2VP has a great influence on the film properties: free-standing films were obtained when P2VP with high molecular weight was used. A diblock copolymer, polystyrene-b-poly(2-vinylpyridine), selfassembled into a large area of honeycomb-like microstructures doped with AuCl4− and Ag+ ions at the interfaces, while polystyrene-b-poly(4-vinylpyridine) self-assembled into a lamellar structure and a foam film when combining with AuCl4− and Ag+ ions, respectively. This indicates that molecular structures and inorganic species have influences on the selfassembly behaviors of polymer molecules and the final microand nanostructures. After further treatment with KBH4 aqueous solution, the inorganic species embedded in the composite structures were transformed to noble metal nanoparticles that exhibited high and durable catalytic properties. The fabrication of these composite structures mentioned above took place at the liquid/liquid interface formed by two kinds of immiscible solutions. The amphiphilic polymer molecules adsorbed at the interface interacted with inorganic species and then self-assembled into composite structures. Here we describe a novel approach, the emulsion-directed assembly and adsorption process at the interface. As we know, some amphiphilic polymers including PS-b-PAA-b-PS contain functional acid groups that can combine with various transition metal ions to form composite structures. However, it can be difficult to dissolve in general organic solvents due to its stronger hydrophilicity; it is also difficult to dissolve in water to form aqueous solutions. Hence, PS-b-PAA-b-PS was dissolved in high polar solvents, such as N,N-dimethylformamide (DMF), at first, and then chloroform was added to form a mixed solution. Because DMF is miscible with both chloroform and water, the interface between the mixed solution and an aqueous solution containing metal ions becomes unclear and the organic phase becomes milky gradually due to the exchange of DMF and water between the two phases. The organic phase becomes clear again after a certain time and a thin layer appears at the planar liquid/liquid interface. The position of the interface shifts to the organic phase with the formation of a thin foam film composed of microcapsules. The film formation should be attributed to the emulsion droplet-directed self-assembly of polymer molecules with inorganic species, following the adsorption and accumulation of the fabricated emulsion droplets at the planar liquid/liquid interface. This fabrication process is unique and different from that occurred at the interface between two kinds of immiscible solutions which has been reported.16−24 Undoubtedly, this method enriches the interfacial adsorption and self-assembly approaches and will be a new way to fabricate composites based on some amphiphilic polymer molecules with stronger hydrophilicity and various inorganic species. In this paper, the fabrication of Pb2+ and Cd2+ ion-doped PS-b-PAA-b-PS composite films at the liquid/liquid interfaces was described as an example of the emulsion-directed assembly and adsorption method, and the incorporated metal ions were transformed to semiconductor nanoparticles, which is of great importance for developing functional materials. We

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EXPERIMENTAL SECTION

Chemicals. PS-b-PAA-b-PS with the Mn values of the three blocks of 3000, 8000, and 3000 g mol−1 (Mw/Mn = 1.35) was purchased from Polymer Source (Canada) and used as received. Lead(II) acetate (Pb(CH3COO)2·3H2O, ≥99.5%), cadmium acetate dihydrate (Cd(CH3COO)2·2H2O, ≥98.5%), and DMF (≥99.5%) were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China), and used as received. Chloroform (analytical reagent) containing 0.3−1.0% ethanol as a stabilizer was obtained from Tianjin Guangcheng Chem. Co. The water used was highly purified using a UP water purification system (UPHW-IV-90T, Chengdu China) with a resistivity ≥18.0 MΩ cm. Formation of Composite Films at the Liquid/Liquid Interfaces. A certain amount of PS-b-PAA-b-PS was dissolved in DMF first under ultrasonication, and then a certain amount of chloroform was added to form a mixed solution. The volume ratios of DMF to chloroform in the mixed solutions are 1:3, 1:1, and 3:1, respectively, and the concentration of the polymer is 0.20 mg mL−1. Aqueous solutions of lead acetate of 1 × 10−3 mol L−1 and cadmium acetate of 1 × 10−2 mol L−1 were prepared by dissolving the corresponding salts into pure water. About 5 mL of the polymer solution was poured in a clean and dry bottle. Then equal volume of the aqueous solution was added slowly along the bottom wall by using a pipet to cover the organic solution at last. A clear planar liquid/liquid interface was formed. The organic and the aqueous phases were called lower and upper phase, respectively. Mass transfer across the interface took place immediately as soon as the formation of the interface. The interface became unclear, and the lower phase became milky gradually. A thin film appeared at the interface when the lower phase became clear again. The thin film was deposited onto solid substrates such as carbon-coated copper grids and quartz slides after the upper phase was removed with a dropper. Generation of Metal Sulfide Nanoparticles. The transferred films were placed in a vessel with a small beaker containing a certain amount of Na2S aqueous solution. The vessel was sealed after adding some sulfuric acid into the Na2S aqueous solution. The transformation process for the coordinated metal ions to metal sulfide was monitored using UV−vis spectroscopy (HP 8453E). After 60 min the transformation was complete. General Characterization. The deposited films from the liquid/ liquid interfaces and the further treated films with H2S gas were characterized using various techniques. The morphology and structure of the untreated and treated thin films deposited on copper grids were investigated using a high-resolution transmittance electron microscope (HRTEM, JEOL-2010) with an accelerating voltage of 200 kV. The element analysis was carried out using an energy-dispersive spectroscope (EDS; Oxford INCAx-sight) equipped on the HRTEM. The formation of PbS nanoparticles in the treated thin film was further confirmed using X-ray diffractometry (XRD, Rigaku D/Max 2200PC). The compositions of these samples were probed by using X-ray photoelectron spectroscopy (XPS, ESCALAB MKII) with an Mg Kα exciting source at a pressure of 1.0 × 10−6 Pa and a resolution of 1.00 eV. In order to clarify the interaction between the polymer molecules and the metal ions, the thin film formed at the liquid/liquid interface was investigated by using FTIR spectroscopy (VERTEX-70). The thin films were deposited on glass slides first and then scraped from the slides after drying. These pieces of the films were collected, and the FTIR spectra were obtained with KBr pellet pressing. DLS Measurement. The lower phase changed from a clear organic solution to a cloudy dispersion and then to a clear solution again during the assembly process. In order to clarify this change, the lower phase as well as the upper phase was monitored using the DLS technique with a BI-200SM research goniometer and laser light scattering system (Brookhaven Instruments Corporation). An argon laser (λ = 532 nm) with variable intensity was used to cover the size 2179

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range involved. Measurements were carried out with a scattering angle of 90° at room temperature. Optical Microscopic Observation. The films formed at the liquid/liquid interface were deposited on glass slides and observed using an optical microscope (LWT300LPFT, China). In order to confirm the formation of emulsion in the lower phase, a drop of the emulsion was placed on a glass slide and observed using the optical microscope, too. Freeze Fracture Transmission Electron Microscope (FF-TEM) Observations. A small amount of emulsion in the lower phase was placed on a 0.1 mm thick copper disk covered with a second copper disk. Then the copper sandwich with the sample was plunged into liquid nitrogen. Fracturing and replication were carried out on a Balzers BAF-400D equipment at −150 °C. Pt/C was deposited at an angle of 45°. The replicas were examined on a JEOL-2010 HRTEM with an accelerating voltage of 200 kV.

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RESULTS AND DISCUSSION Morphology and Composition. The morphologies and structures of the thin films formed at the liquid/liquid interface were investigated using TEM, as shown in Figure 1.

Figure 2. XP spectra of the thin films of PS-b-PAA-b-PS/Pb2+ (a) and PS-b-PAA-b-PS/Cd2+ (b) formed at the liquid/liquid interfaces.

existence of more than one species. As shown in Figure 2a, the band for Pb (4f7/2) was decomposed into two peaks with the binding energies (BEs) of 139.2 and 140.0 eV, which are assigned to Pb(II) coordinating with the carboxyl groups25 and PbO,26,27 respectively. The BE of 140.0 eV is slightly higher than the literature values of 138.6 and 138.9 eV of PbO,26,27 which should be ascribed to the smaller size of PbO nanoparticles in the composite film. Similarly, the band for Cd (3d5/2) in Figure 2b was decomposed into two peaks at 405.9 and 407.5 eV, which are assigned to coordinated Cd(II)28 and CdO/Cd(OH)2,29 respectively. Because of the size effect, the BE of 407.5 eV is much greater than 404.8 eV, the literature value for Cd(OH)2.29 Formation Process. Figure 3 shows typical pictures of the assembly system at different stages to illustrate the formation process of the composite thin film. A clear liquid/liquid interface forms at the initial stage (Figure 3a). The interface becomes blurry with time, the interface shifts downward (Figure 3b), and the organic phase becomes milky gradually (Figure 3c). The organic phase becomes clear again after a long time, and a thin film is generated at the liquid/liquid interface. The position of the interface has been lowered with respect to the initial position obviously (Figure 3d). This process was monitored using DLS. Figure 4a presents the DLS spectra of the lower phase (the organic phase) during the assembly process. At the initial stage, the organic phase is clear, and only a monomodal distribution peak appears with relatively narrow size distribution, indicating that only one kind of aggregate exists. The mean hydrodynamic diameter was found to be 27 nm, which is within the size of micelles,

Figure 1. TEM micrographs of PS-b-PAA-b-PS/Pb2+ (a, b) and PS-bPAA-b-PS/Cd2+ (c, d) composite films formed at the liquid/liquid interfaces.

Microcapsules with the size of several hundreds of nanometers conglutinate to a foam structure for either polymer/Pb2+ or polymer/Cd2+ composite system. From the high-magnification images, one can see some nanofibers with layered structure and hollow nanospheres distributed on the walls (Figure 1b). The walls of the microcapsules have layered structure (Figure 1d). It can be also observed that a large numbers of nanoparticles dispersed on the walls of the microcapsules, on the walls of the nanospheres, and between the layers of the nanofibers. In fact, the film covers the entire interface. Figure S1 in the Supporting Information gives the optical microscopy images of the thin films deposited on glass slides. It can be seen that the films are uniform on a large length scale. The compositions of the thin films were analyzed based on the XP spectra illustrated in Figure 2. The bands corresponding to Pb2+ and Cd2+ appear in the spectra, indicating these metal ions are combined in the thin films. It can be noticed clearly that these bands are broad and asymmetric, hinting the 2180

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with time, and most of the aggregates are micelles when the lower phase becomes clear again (curve 5). The emulsion formed in the lower phase was investigated by using an optical microscope; the image is shown in Figure 5a. It

Figure 3. Formation process of the composite thin film at the liquid/ liquid interface: (a) initial stage; (b) after 30 min; (c) after 2 h; (d) after 1 day.

Figure 5. Optical microscopy image (a), TEM micrographs (b, c), and FF-TEM micrographs (d, e) of the milky lower phase in the polymer/ Pb2+ system.

can be seen that numerous uniform droplets with the size less than 1 μm appear, indicating the formation of the emulsion. This result is consistent with the DLS observations. In addition, a milky drop in the lower phase was deposited on a copper grid and investigated using TEM after drying. As can be seen in Figure 5b,c, hollow nanospheres with the diameter ranging from 20 to 50 nm are accumulated to constitute a porous film. It is apparent that the hollow nanospheres come from the micelles. The nanoparticles dispersed in the film should be metal oxide. We could not find microcapsules from TEM observation. It is possible that the assembled microcapsules around the mixed droplets are not stable enough; the mechanical strength is not stronger. The microcapsules may break up during the solvent evaporation process. But even so, these results confirm the formation and existence of micelles in the lower phase during the assembly process. In order to further confirm the formation of emulsion and the existence of micelles in the emulsion, the milky lower phase was investigated by using FF-TEM, as shown in Figure 5d,e.

Figure 4. DLS spectra of the lower phase at different time after the formation of the interface (a) and the upper phase (b).

implying the formation of polymer micelles in the DMF/ chloroform (v/v: 1/1) mixed solution. This phase is getting milkier with time after several tens of minutes. Curve 2 in Figure 4a corresponds to the DLS result when it just turns to be milky. A distribution peak with a mean hydrodynamic diameter of 120 nm appears, suggesting the formation of emulsion droplets. The organic phase becomes milkier and milkier with time. The corresponding size distribution at this stage is bimodal, as illustrated in curves 3 and 4 in Figure 4a. The mean hydrodynamic diameters were obtained to be 29 and 350 nm from curve 3 and 34 and 380 nm from curve 4, which correspond to micelles and emulsion droplets, respectively. It can be seen that the relative amount of the droplets decreases 2181

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Scheme 1. Schematic Presentation of the Composite Film Formation Process at the Liquid/Liquid Interface: (A) Formation of Emulsion in the Lower Phase Due to the Migration of the Aqueous Solution, and the DMF Droplets Formed in the Upper Phase Disappear Rapidly; (B) Polymer Molecules and Micelles Adsorb, Combine with Metal Ions, and Self-Assemble into Microcapsules around the Emulsion Droplets; (C) Microcapsules Adsorb at the Planar Liquid/Liquid Interface and Organize into a Foam Film

droplet, and organize into a thin film with lamellar structure around the droplet. The nanofibers with lamellar structure and the walls shown in TEM micrographs should be attributed to microphase separation of the block copolymer molecules during the assembly process. These polymer-shell-capped droplets are captured by the planar liquid/liquid interface gradually. More and more droplets adsorb at the interface. They accumulate and fuse eventually to form a thin foam film. The formation process is schematically illustrated in Scheme 1. It is apparent that the water or water/DMF droplets act as templates and inorganic species suppliers for the fabrication of the composite polymer microcapsules. On the other hand, the polymer molecules and metal ions stabilize the water droplets. A comparative experiment supports this deduction. When pure water and chloroform/DMF mixed solution without polymer formed a liquid/liquid interface, we found that the organic phase became milky rapidly within 2 min and turned to be clear in about 20 min. It should be pointed out that DMF in the organic phase is necessary for the formation of the emulsion. Without DMF, the organic phase will keep clear during the adsorption and assembly process, as revealed in our previous works.16−24 Besides the migration of water to the organic phase, DMF also migrates to aqueous phase. The upper phase was also monitored using DLS (Figure 4b). It was observed that bigger particles with the size of several hundreds even several thousands of nanometers formed and disappeared rapidly with time. These particles should be the DMF droplets. Because the stronger interaction between DMF and water,

Not only the images corresponding to the emulsion droplets appear in the FF-TEM micrographs, but the small round images with the size of about 25 nm appear. This size is coincident with the DLS and TEM results and locates in the size range of micelles, further confirming the formation of micelles in the clear organic phase and existence of the micelles in the lower phase emulsion. In general, there is a dynamic equilibrium between micelles and free molecules. The free polymer molecules and the micelles should coexist in the system. The formation of the emulsion in the lower phase should be ascribed to the exchange of DMF and water between the two liquid phases. One can see mass transfer clearly between the upper and lower phases through the interface with the naked eye. Because of the affinity characteristics of two liquids (water and DMF), water in the upper phase migrates to the lower phase; meanwhile DMF migrates to the upper phase. Water droplets or water/DMF mixed droplets are generated in the lower phase when sufficient amounts of water and the metal ions enter the lower phase because water and chloroform are immiscible, leading to the formation of an emulsion. At the initial stage of the emulsion formation, the size of the discrete phase droplets was measured to be 120 nm and became larger with time. The droplet is not stable enough because of the larger surface area and higher surface free energy. The mean diameter of the droplets increases to 350−400 nm at last and the emulsion keeps stable for several hours. At this stage, free polymer molecules and micelles in the continuous phase should adsorb, interact with metal ions in the 2182

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Figure 6. HRTEM micrographs (a−d), size distribution histogram (e), XRD pattern (f), and EDS (g) of the polymer/Pb2+ composite films after H2S gas treatment for 60 min.

vibrations of COO−.34,37,38 Meanwhile, the vibration peaks of carboxyl and hydroxyl groups disappear. This confirms the dissociation of the carboxyl groups and the coordination between the carboxylate anions and Cd2+ ions. The peak at 1650 cm−1 belongs to the H−O−H deformation mode of physically adsorbed molecular water.39,40 These results verify the interaction between the polymer molecules and the metal ions, which is the driving force for the composite microcapsule fabrication. Generation of Metal Sulfide Nanoparticles. PbS and CdS nanoparticles are generated by exposing the as-prepared composite films to H2S gas. Figure S3 shows the successive UV−vis spectra of the composite films during the H2S treating process. It can be seen that the absorption increases with increasing the exposure time and does not change any longer after 60 min. This suggests that the coordinated metal ions and the oxides/hydroxides are transformed to metal sulfide completely. It is known that PbS is a direct band gap semiconductor with a rather small bulk band gap of 0.41 eV at 300 K and a relatively large exciton Bohr radius of 18 nm.41 The absorption of PbS nanoparticles can be tuned from visible to infrared regions within the size of less than 18 nm.42 After the H2S treatment for 60 min, the generated PbS absorbs the light lower than 600 nm, indicating the formation of smaller nanoparticles. The edge absorption of CdS is around 400 nm, implying the formation of smaller particles. The morphology, composition, and structure of the H2Streated samples were investigated. Figure 6a−d shows the HRTEM images of the H2S treated polymer/Pb(II) sample. The foam structure was preserved. As can be seen in Figure 6b, the lamellar structure of the walls was not affected by the treatment, and the hollow spheres coming from the micelles still existed on the walls. From Figure 6c, one can see that the hollow spheres have the size of less than 50 nm with

DMF molecules diffuse into water rapidly to form a homogeneous solution. The position of the interface lowers obviously due to the migration of DMF from the organic phase into the aqueous phase, as shown in Figure 3d. The generation of the metal oxide or hydroxide nanoparticles dispersed in the microcapsule walls should be related to the hydrolysis of the Pb2+ and Cd2+ ions. The interaction between water and DMF molecules in the mixed droplets leads to the release of hydroxyl groups because DMF is a kind of organic base, resulting in the formation of hydroxides. Lead hydroxide is easy to decompose into lead oxide at ambient temperature.30 So it is reasonable to suppose that the nanoparticles in the composite films are lead oxide and cadmium hydroxide, respectively. In order to demonstrate the interaction between metal ions and the polymer molecules, FTIR spectra of the polymer/Cd2+ composite film as well as the pure polymer were obtained, as shown in Figure S2. The spectrum of the pure polymer shows a broad CO stretching vibration band at 1710 cm−1, indicating that the polymer was in an associated state.31 The peaks appear at 2550 and 1946 cm−1 are satellite bands for the stretching vibrations of hydrogen-bonded hydroxyl group.31,32 This is evident that the existence of hydrogen bonding between the PAA groups. In addition, the broad band at 1252 cm−1 is related to C−O stretching coupled with O−H in-plane bending.33 The characteristic peaks of PS appear at 1452, 1493, and 1600 cm−1 due to the stretching vibrations of aromatic C−C and 697 cm−1 due to the out-of-plane deformation of C−H.34−36 The peaks corresponding to the PS blocks appear in the spectrum of the composite film. However, the peaks associated with the PAA block were altered obviously. The CO stretching vibration is shifted to 1552 and 1412/1388 cm−1, which are assigned to asymmetric and symmetric stretching 2183

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Figure 7. HRTEM micrographs (a−c), EDS (d), and size distribution histogram (e) of the polymer/Cd2+ composite film after 60 min treatment in H2S gas.

nanoparticles dispersing on their walls. Figure 6d gives a HRTEM image of a smooth part of the microcapsule wall where numerous nanoparticles are distributed homogeneously. Form the size distribution histogram shown in Figure 6e, the mean diameter of the nanoparticles was 1.39 ± 0.49 nm. The formation of PbS nanoparticles was further confirmed by XRD, as shown in Figure 6f. Three characteristic diffraction peaks of rock-salt PbS nanocrystals corresponding to (111), (200), and (220) facets appear, evidently proving the generation of PbS nanoparticles. The EDS spectrum shown in Figure 6g demonstrates the existence of the elements of lead and sulfur in the composite film. Figure 7 gives the HRTEM images and EDS spectrum of the H2S treated PS-b-PAA-b-PS/Cd(II) sample. The foam structure with the lamellar walls and hollow nanospheres on the walls kept after the treatment, and numerous fine nanoparticles were dispersed homogeneously on the microcapsule walls. The mean diameter of the nanoparticles was found to be 0.71 ± 0.24 nm. The EDS spectrum gives a clear proof on the existence of cadmium and sulfur elements in the treated films. The transformation to metal sulfide nanoparticles was also confirmed using XPS. Comparing with the XP spectra of the composite films obtained from the liquid/liquid interfaces (Figure 2), the XP spectra of the H2S gas treated thin films give narrower and symmetric peaks, as shown in Figure 8. It indicates the existence of only one species in each sample. The BEs were found to be 139.0 and 405.7 eV for 4f7/2 of Pb(II) and 3d5/2 of Cd(II), respectively. These values are higher than those of PbS and CdS in the reported literature,25,28,43 suggesting the smaller size of the formed nanoparticles.44 Effects of the Volume Ratios of DMF/CHCl3. The effects of the volume ratios of DMF to chloroform in organic phase on morphology and structure of the formed films were

Figure 8. XP spectra of the H2S gas treated composite films.

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Figure 9. TEM micrographs of the films formed at the liquid/liquid interfaces between aqueous solution of lead acetate and the polymer solution with different volume ratios of DMF/chloroform: (a, b) 1:3; (d, e) 3:1. Images c and f correspond to the micrographs of the deposited lower phases with the two kinds of volume ratios on copper grids.

investigated. Figure 9 shows the TEM micrographs of the composite films formed at the liquid/liquid interface when organic phases with different volume ratios were used. Differing sharply from the foam film shown in Figure 1, the pieces of thin film decorating with round dots were visualized from the TEM field of view when the organic phase with the volume ratio of 1:3 was used (Figure 9a). It was observed that the round dots with a mean diameter of 10 nm organized into a twodimensional hexagonal close-packing array, as illustrated in Figure 9b. Actually, the round dots are nanospheres which inorganic nanoparticles are embedded in. When the volume ratio of 3:1 was used, a three-dimensional network structure was formed. This structure is made up of interconnected nanotubes with the diameter of 15−20 nm and hollow nanospheres (Figure 9e). During the film forming process, these organic phases became milky and clear sequentially, too. The milky solutions were dropped on copper grids and visualized using TEM. As can be seen from Figure 9c,f, the round aggregates of smaller particles existed in the milky disperse system when the volume ratio of 1:3 was used, while nanotubes and hollow spheres existed in the system of 3:1. It is clear that these results have close relevance to the film obtained at the interfaces. It should be noticed that the nanospheres are reverse micelles formed in the mixed 1:3 system. The PAA blocks were surrounded by the hydrophobic PS block to form reverse micelles because chloroform is a poor solvent for PAA. In this case, there are little free polymer molecules in the system, and the reverse micelles are difficult to organize into stable microcapsules around the droplets. Instead, they adsorb at the planar liquid/liquid interface and assemble into 2D arrays. It is apparent that inorganic species were incorporated into the reverse micelles during the assembly process. The nanotubes should be the formed rod-like or worm-like micelles in the mixed 3:1 system. Here, DMF, the good solvent for PAA blocks, would play a leading role to control the

organization of the polymer molecules. Compared with the 1:1 system in which spherical micelles with the mean diameter of about 30 nm were formed, micelles with larger size from 30 to 1000 nm were formed in the 3:1 mixed solution. It was revealed by the DLS spectra (Figure S4) and confirmed by the FF-TEM investigation (Figure S5) of the milky lower phase. The size and size distribution of the particles in the 3:1 solution and the milky dispersion are similar, as can be seen in the DLS spectra, suggesting that the size of the formed droplets in the dispersion system is comparable to that of the aggregates. From Figure S5 one can see that spheres with the diameter of about 30 nm, belt-like structures with the width of about 30 nm, and the length of several hundreds of nanometers to several micrometers coexist in the system, confirming the formation of spherical and rod-like or worm-like micelles. One can also see round structures with the size of about 200 nm, corresponding to the formed emulsion droplets. We have not fabricated foam thin film at the liquid/liquid interface when the 3:1 organic solution of DMF/chloroform was used. A possible cause for this could be associated with why no stable microcapsule formed in the thin composite film even though there are a lot of free polymer molecules in the system. Another reason may be related to the poor stability of the droplets because there is too much DMF in the system. In brief, the 1:1 volume ratio of DMF/chloroform is suitable for fabricating the composite foam film at the liquid/liquid interface through emulsion-directed assembly and adsorption process, and the morphologies of the composite films can be tuned through changing the experimental conditions.

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CONCLUSIONS In summary, the composite thin films of polymer/transition metal ions were fabricated successfully at the liquid/liquid interface through an emulsion-directed assembly and adsorption process. The amphiphilic block copolymer molecules dispersed in mixed DMF/chloroform solutions where various 2185

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(7) Schacher, F. H.; Rupar, P. A.; Manners, I. Functional block copolymers: nanostructured materials with emerging applications. Angew. Chem., Int. Ed. 2012, 51, 7898−7921. (8) Zhang, H.; Liu, Y.; Yao, D.; Yang, B. Hybridization of inorganic nanoparticles and polymers to create regular and reversible selfassembly architectures. Chem. Soc. Rev. 2012, 41, 6066−6088. (9) Pavan, M. J.; Shenhar, R. Two-dimensional nanoparticle organization using block copolymer thin films as templates. J. Mater. Chem. 2011, 21, 2028−2040. (10) van Zoelen, W.; ten Brinke, G. Thin films of complexed block copolymers. Soft Matter 2009, 5, 1568−1582. (11) Rao, C. N. R.; Kulkarni, G. U.; Agrawal, V. V.; Gautam, U. K.; Ghosh, M.; Tumkurkar, U. Use of the liquid−liquid interface for generating ultrathin nanocrystalline films of metals, chalcogenides, and oxides. J. Colloid Interface Sci. 2005, 289, 305−318. (12) Hu, L.; Chen, M.; Fang, X.; Wu, L. Oil−water interfacial selfassembly: a novel strategy for nanofilm and nanodevice fabrication. Chem. Soc. Rev. 2012, 41, 1350−1362. (13) Yang, S.; Wang, C.-F.; Chen, S. Interface-directed assembly of one-dimensional ordered architecture from quantum dots guest and polymer host. J. Am. Chem. Soc. 2011, 133, 8412−8415. (14) Ren, Z. B.; Liu, J.; Chen, Y. P.; Chen, M.; Qian, D. J. Facile fabrication of porous poly(4-vinylpyridine) films at the immiscible interfaces of water-organic solvents as template for growth of silver nanoparticles. Chin. Chem. Lett. 2011, 22, 867−870. (15) Zhou, C.; Chen, N.; Yang, J.; Liu, H.; Li, Y. Ion-specific selfassembly of low-dimension aggregate structures of conjugated polymer at two-phase interface. Macromol. Rapid Commun. 2012, 33, 688−692. (16) Chen, L.-J.; Ma, H.; Chen, K.; Cha, H.-R.; Lee, Y.-I.; Qian, D.-J.; Hao, J.; Liu, H.-G. Synthesis and assembly of gold nanoparticle-doped polymer solid foam films at the liquid/liquid interface and their catalytic properties. J. Colloid Interface Sci. 2011, 362, 81−88. (17) Chen, L.-J.; Ma, H.; Chen, K.-C.; Fan, W.; Cha, H.-R.; Lee, Y.-I.; Qian, D.-J.; Hao, J.; Liu, H.-G. Synthesis and assembly of catalytically active platinum-doped polymer nanocomposites at the liquid/liquid interface. Colloids Surf., A 2011, 386, 141−150. (18) Lin, L.; Shang, K.; Xu, X.; Chu, C.; Ma, H.; Lee, Y.-I.; Hao, J.; Liu, H.-G. Formation of Ag nanoparticle-doped foam-like polymer films at the liquid-liquid interface. J. Phys. Chem. B 2011, 115, 11113− 11118. (19) Chu, C.; Yang, D.; Wang, D.; Ma, H.; Liu, H.-G. Formation of Au nanoparticle-doped PVK microcapsules and foam-like structures at the liquid/liquid interface. Mater. Chem. Phys. 2012, 132, 916−922. (20) Ma, H.; Geng, Y.; Lee, Y.-I.; Hao, J.; Liu, H.-G. Free-standing poly(2-vinylpyridine) foam films doped with silver nanoparticles formed at the planar liquid/liquid interface. J. Colloid Interface Sci. 2013, 394, 223−230. (21) Ma, H.; Geng, Y.; Lee, Y.-I.; Hao, J.; Liu, H.-G. Interfacial assembly of Pt nanoparticle-doped free-standing polymer foam films and their catalytic performance. Colloids Surf., A 2013, 419, 201−208. (22) Geng, Y.; Liu, M.; Ma, H.; Hao, J.; Liu, H.-G. Catalytic polymer/copper composite thin films formed at the liquid/liquid interface through self-assembly and hydrolysis process. Colloids Surf., A 2013, 431, 161−170. (23) Wang, D.; Ma, H.; Chu, C.; Hao, J.; Liu, H.-G. Honeycomb-like thin films of Polystyrene-block-poly(2-vinylpyridine) embedded with gold or silver nanoparticles formed at the planer liquid/liquid interface. J. Colloid Interface Sci. 2013, 402, 75−85. (24) Liu, Y.; Chen, L.; Geng, Y.; Lee, Y.-I.; Li, Y.; Hao, J.; Liu, H.-G. Fabrication of composite thin films with microstructures of honeycomb, foam, and nanosphere arrays through adsorption and selfassembly of block copolymers at the liquid/liquid interface. J. Colloid Interface Sci. 2013, 407, 225−235. (25) Guo, S.; Popovitz-Biro, R.; Weissbuch, I.; Cohen, H.; Hodes, G.; Lahav, M. Synthesis of semiconductor quantum particles in matrices of thin films and crystals of α,ω-alkanedicarboxylate salts. Adv. Mater. 1998, 10, 121−125. (26) Yang, Y.; Taggart, D. K.; Brown, M. A.; Xiang, C.; Kung, S.-C.; Yang, F.; Hemminger, J. C.; Penner, R. M. Wafer-scale patterning of

aggregates including rodlike, spherical micelles, and reverse micelles were formed. When a planar liquid/liquid interface is formed between the polymer solution with an appropriate volume ratio of DMF/chloroform and an aqueous solution of a salt, water, and DMF migrate across the interface, leading to the formation of an emulsion. The free polymer molecules and the micelles adsorb around the emulsion droplets and coordinate to metal ions to form microcapsules. Continuous adsorption and accumulation of the microcapsules make thin films at the planar interface at last. These thin films have hierarchical micro- and nanostructures including microcapsules, micelles, nanofibers, and arrays of reverse micelles depending on the volume ratios of DMF/chloroform in the polymer organic solution. The coordinated metal ions and other related metal species transformed to metal sulfide nanoparticles after treating them with hydrogen sulfide gas. This is a new way to fabricate the micro- and nanostructures of composite polymer/metal ion at the liquid/liquid interface using the different assembly mechanism from that of the fabrication at the liquid/liquid interface formed by two kinds of immiscible phases. We wish that this method can be used to fabricate more and more functional polymer-based composite materials.

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ASSOCIATED CONTENT

* Supporting Information S

Figures S1−S5. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (H.-G.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (No. 21273133, 21033005, and 20873078) and the National Basic Research Program of China (973 Program, No. 2009CB930103).

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REFERENCES

(1) Zhang, H.; Han, J.; Yang, B. Structural fabrication and functional modulation of nanoparticle−polymer composites. Adv. Funct. Mater. 2010, 20, 1533−1550. (2) Sarkar, S.; Guibal, E.; Quignard, F.; SenGupta, A. K. Polymersupported metals and metal oxide nanoparticles: synthesis, characterization, and applications. J. Nanopart. Res. 2012, 14, 715 (24 pages). (3) Jang, S. G.; Kramer, E. J.; Hawker, C. J. Controlled supramolecular assembly of micelle-like gold nanoparticles in PS-bP2VP diblock copolymers via hydrogen bonding. J. Am. Chem. Soc. 2011, 133, 16986−16996. (4) Lee, B.; Kim, Y.; Lee, S.; Kim, Y. S.; Wang, D.; Cho, J. Layer-bylayer growth of polymer/quantum dot composite multilayers by nucleophilic substitution in organic media. Angew. Chem., Int. Ed. 2010, 49, 359−363. (5) Kundu, S.; Gill, R. S.; Saraf, R. F. Electrospinning of PAH nanofiber and deposition of Au NPs for nanodevice fabrication. J. Phys. Chem. C 2011, 115, 15845−15852. (6) Zotti, G.; Vercelli, B.; Berlin, A.; Chin, P. T. K.; Giovanella, U. Self-assembled structures of semiconductor nanocrystals and polymers for photovoltaics. 1. CdSe nanocrystal-polymer multilayers. optical, electrochemical, photoelectrochemical and photoconductive properties. Chem. Mater. 2009, 21, 2258−2271. 2186

dx.doi.org/10.1021/la500187d | Langmuir 2014, 30, 2178−2187

Langmuir

Article

lead telluride nanowires: structure, characterization, and electrical properties. ACS Nano 2009, 3, 4144−4154. (27) Abel, K. A.; FitzGerald, P. A.; Wang, T.-Y.; Regier, T. Z.; Raudsepp, M.; Ringer, S. P.; Warr, G. G.; van Veggel, F. C. J. M. Probing the structure of colloidal core/shell quantum dots formed by cation exchange. J. Phys. Chem. C 2012, 116, 3968−3978. (28) Guo, S.; Konopny, L.; Popovitz-Biro, R.; Cohen, H.; Porteanu, H.; Lifshitz, E.; Lahav, M. Thioalkanoates as site-directing nucleating centers for the preparation of patterns of CdS nanoparticles within 3-D crystals and LB films of Cd alkanoates. J. Am. Chem. Soc. 1999, 121, 9589−9598. (29) Gulino, A.; Castelli, F.; Dapporto, P.; Rossi, P.; Fragala, I. Synthesis and characterization of thin films of cadmium oxide. Chem. Mater. 2002, 14, 704−709. (30) Chen, K.-C.; Wang, C.-W.; Lee, Y.-I.; Liu, H.-G. Nanoplates and nanostars of b-PbO formed at the air/water interface. Colloids Surf., A 2011, 373, 124−129. (31) Hao, E.; Lian, T. Buildup of polymer/Au nanoparticle multilayer thin films based on hydrogen bonding. Chem. Mater. 2000, 12, 3392− 3396. (32) Chen, Y.; Pang, Y.; Wu, J.; Su, Y.; Liu, J.; Wang, R.; Zhu, B.; Yao, Y.; Yan, D.; Zhu, X.; Chen, Q. Controlling the particle size of interpolymer complexes through host-guest interaction for drug delivery. Langmuir 2010, 26, 9011−9016. (33) Shaikha, J. S.; Pawara, R. C.; Moholkarb, A. V.; Kimb, J. H.; Patil, P. S. CuO−PAA hybrid films: Chemical synthesis and supercapacitor behavior. Appl. Surf. Sci. 2011, 257, 4389−4397. (34) Boyes, S. G.; Akgun, B.; Brittain, W. J.; Foster, M. D. Synthesis, characterization, and properties of polyelectrolyte block copolymer brushes prepared by atom transfer radical polymerization and their use in the synthesis of metal nanoparticles. Macromolecules 2003, 36, 9539−9548. (35) Su, B.; Ma, Z.; Min, S.; She, S.; Wang, Z. Preparation of TiO2/ PS complex nanoparticles. Mater. Sci. Eng., A 2007, 458, 44−47. (36) Tseng, C.-C.; Chang, C.-P.; Sung, Y.; Ou, J.-L.; Ger, M.-D. Noble metallic nanoparticles reduced by PS-based oligomers through one-step synthesis. Colloids Surf., A 2009, 333, 138−144. (37) El Khoury, Y.; Dorlet, P.; Faller, P.; Hellwig, P. New insights into the coordination of Cu(II) by the amyloid-B 16 peptide from fourier transform IR spectroscopy and isotopic labeling. J. Phys. Chem. B 2011, 115, 14812−14821. (38) Dong, R.; Lindau, M.; Ober, C. K. Dissociation behavior of weak polyelectrolyte brushes on a planar surface. Langmuir 2009, 25, 4774− 4779. (39) Barroso-Bujans, F.; Serna, R.; Sow, E.; Fierro, J. L. G.; Veith, M. Grafting of poly(acrylic acid) onto an aluminum surface. Langmuir 2009, 25, 9094−9100. (40) Al-Abadleh, H. A.; Grassian, V. H. FT-IR study of water adsorption on aluminum oxide surfaces. Langmuir 2003, 19, 341−347. (41) Wise, F. W. Lead salt quantum dots: the limit of strong quantum confinement. Acc. Chem. Res. 2000, 33, 773−780. (42) Cademartiri, L.; Bertolotti, J.; Sapienza, R.; Wiersma, D. S.; Freymann, G.; Ozin, G. A. Multigram scale, solventless, and diffusioncontrolled route to highly monodisperse PbS nanocrystals. J. Phys. Chem. B 2006, 110, 671−673. (43) Leontidis, E.; Kyprianidou-Leodidou, T.; Caseri, W.; Robyr, P.; Krumeich, F.; Kyriacou, K. C. From colloidal aggregates to layered nanosized structures in polymer-surfactant systems. 1. Basic phenomena. J. Phys. Chem. B 2001, 105, 4133−4144. (44) Lim, W. P.; Low, H. Y.; Chin, W. S. IR-luminescent PbSpolystyrene nanocomposites prepared from random ionomers in solution. J. Phys. Chem. B 2004, 108, 13093−13099.

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