Fabrication of porous thin films of block copolymer at the liquid/liquid interface and construction of composite films doped with noble metal nanoparticles

Abstract

Two approaches have been utilized to fabricate thin composite films of amphiphilic polyisoprene-block-poly(2-vinylpyridine) (PI-b-P2VP) doped with silver and gold nanoparticles. Honeycomb and foam structures were obtained at the interfaces between a chloroform solution of the polymer and aqueous solutions of either AgNO₃ or HAuCl₄ through the first route. For the second route, porous thin films were obtained at the interfaces between a chloroform solution of the crosslinked polymer by S₂Cl₂ and pure water, immersed in aqueous solutions of either AgNO₃ or HAuCl₄ to adsorb metal precursors and then treated with KBH₄ aqueous solution to form composite films. The catalytic properties of these films were evaluated by assessing the reduction of 4-nitrophenol by KBH₄ in aqueous solutions. It was found that the metal nanoparticles are unstable enough and fused in the film prepared through the first route while the composite film obtained through the second route exhibited good catalytic performance and was more stable during the catalytic reaction. In addition, the porous polymer film is also expected to adsorb other species to form various functional composites.

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1. Introduction

Polymer-based composites doped with inorganic nanoparticles have aroused great interest in recent years.¹ These composites not only have desirable properties, such as good processing ability, and mechanical and chemical stability, but also exhibit new optical and electronic properties resulting from cooperative interactions between the polymer molecules and nanoparticles.² Various approaches have been proposed to prepare these composites, including blending,^3,4 layer-by-layer assembly,^5,6 emulsion polymerization,^7,8 template-induced polymerization/adsorption,^9–11 and self-assembly^5,12 methods. Recently, the self-assembly of polymers, especially block copolymers, has attracted considerable attention due to the combination of polymer blocks containing different and unique properties. As a kind of special block copolymer, amphiphilic diblock copolymers containing separate hydrophobic and hydrophilic moieties exhibit prominent self-assembly behavior. Various micro- and nanostructures have been observed in solutions,^13–15 thin films,^15,16 and at the air/water interfaces^17,18 due to the micellization and microphase separation of the polymer molecules, and these include micelles, reverse micelles, vesicles, parallel and perpendicular cylinders, lamellar structures, ribbons, and 2D networks.

Liquid/liquid interfaces between two immiscible liquids have microenvironments that differ from the solutions and those between air and water, and have been widely used to synthesize nanoparticles and to fabricate nanostructures through interfacial reactions in recent years.^19,20 The liquid/liquid interface has also been utilized to fabricate coordination polymers^21,22 and composite materials of polymers with inorganic species, such as colloidal particles²³ and metal ions,²⁴ through adsorption and self-assembly processes. Self-assembly at liquid/liquid interfaces offers some advantages compared to other methods. First of all, the liquid/liquid interface is vital for directing the formation of the nanostructures. In addition, the fabrication can be carried out at ambient atmosphere and temperature, and the experimental process is very simple.

In recent years, we have systematically investigated the fabrication of polymer-based composites at the liquid/liquid interfaces through adsorption and subsequent self-assembly processes. Both amphiphilic homopolymers and block copolymers have been used. The polymers were dissolved in the organic phase and the noble metal ions were dispersed in the aqueous phase. Formation of the interface between the two phases results in the adsorption of both the polymer molecules and inorganic species, enabling them to interact with each other, after which they self-assemble into composite structures at the interface. In our previous work, we successfully fabricated various catalytically active composite thin films with different morphologies doped with noble metal nanoparticles by using homopolymers^25,26 and block copolymers.^27–29 The formation of these composite films was regarded as a result of a specific adsorption process in which the special interaction between the polymer molecules and the inorganic species played a crucial role. It is difficult to form stable thin films at the liquid/liquid interface without inorganic species.

In this work, we adapt polyisoprene-block-poly(2-vinylpyridine) (PI-b-P2VP) with a hydrophobic PI block that can be crosslinked by disulfur dichloride to enhance the interaction between the polymer molecules and the stability of the composite films.^30,31 Different from our previous work, in which the polymer molecules adsorbed and self-assembled into microstructures at the liquid/liquid interfaces with the aid of inorganic species that dispersed in the aqueous phases, we found that the pre-crosslinked PI-b-P2VP molecules adsorbed and self-assembled into porous microstructures at the interface between the chloroform solution containing the polymer and pure water. In other words, we fabricated a pure polymer film that does not contain inorganic species. In addition, sulfur atoms were introduced in this film. So, both nitrogen and sulfur atoms can be used as active sites to combine heavy metal ions. This kind of film can be utilized as an adsorbent to construct various composite microstructures. In this paper, AuCl₄⁻ and Ag⁺ ions were used as adsorbates to produce composite thin films doped with Au and Ag nanoparticles. Compared with the composite films formed directly at the liquid/liquid interface, these composite films exhibit good reusability and stability for heterogeneous catalytic reactions, such as the hydrogenation of 4-nitrophenol in aqueous solution.

2. Experimental section

2.1 Chemicals and materials

PI-b-P2VP with M_n values of the two blocks of 19 200/12 200 (M_w/M_n = 1.03) was purchased from Polymer Source (Canada) and used as received. AgNO₃ (99+%) and HAuCl₄·3H₂O (99.9+%) were purchased from Shanghai Chemical Plant and Aldrich, respectively. Chloroform was obtained from Tianjin Guangcheng Chem. Co. and is an analytical reagent containing 0.3–1.0% ethanol as a stabilizer. S₂Cl₂ (98%) was received from Aladdin Industrial Co. KBH₄ (≥97.0%) was obtained from Shanghai Zhanyun Chemical. Co. Ltd, and 4-nitrophenol (4-NP) (analytical reagent) and was supplied by Tianjin Guangfu Fine Chemical Research Institute. The water that was used was purified to a high degree using a UP water purification system (UPHW-IV-90T, Chengdu China) with a resistivity ≥18.0 MΩ cm.

2.2 Preparation of composites

A certain amount of PI-b-P2VP was dissolved in chloroform to form an organic solution of 0.20 mg mL⁻¹. Aqueous solutions of AgNO₃ with a concentration of 1.0 × 10⁻² mol L⁻¹ and HAuCl₄ with a concentration of 1.0 × 10⁻³ mol L⁻¹ were prepared by dissolving the salts in pure water, respectively. 10 μL of S₂Cl₂ was added to 100 mL of the PI-b-P2VP chloroform solution to form the crosslinked polymer organic solution at room temperature, i.e., at about 25 °C for 5 days.

Two routes were adopted to fabricate composite films. For route 1, about 5 mL polymer solution was poured into a clean and dry beaker, and then 5 mL of an aqueous solution of either AgNO₃ or HAuCl₄ was slowly added using a pipette to cover the polymer solution. The beaker was placed in a closed container in a dark oven, after which a thin film gradually appeared at the liquid/liquid interface. Twenty-four hours later, this film was deposited on a solid substrate, such as a carbon-coated copper grid or a quartz slide for further treatment and characterization after the upper phase was removed carefully with a dropper.

For route 2, thin films were formed at the interface between the chloroform solution of the crosslinked polymer and pure water. Then the film was deposited on the solid substrates, and immersed in an aqueous solution of AgNO₃ or HAuCl₄ for about 24 hours to adsorb metal ions. The deposited film was subsequently rinsed twice with pure water, before it was immersed in a 2 × 10⁻² mol L⁻¹ KBH₄ aqueous solution to reduce the adsorbed metal precursors.

In addition, in order to investigate the adsorption and assembly behavior of the crosslinked polymer at the interface between the chloroform solution and the aqueous solution of the inorganic species, several liquid/liquid interfaces were formed using aqueous solutions of AgNO₃, HAuCl₄, H₂PtCl₆ and Eu(NO₃)₃, respectively. The films appeared at the interfaces were deposited on solid substrates and investigated.

2.3 Dynamic light scattering (DLS) measurements

The crosslinking process of PI-b-P2VP in the mixed chloroform solution was monitored using a DLS technique with a BI-200SM research goniometer and laser light scattering system (Brookhaven Instruments Corporation, USA). An argon laser (λ = 532 nm) with variable intensity was used to cover the size range involved. Measurements were carried out at a scattering angle of 90° at room temperature.

2.4 General characterization

The morphology and structure of the deposited films were investigated using a high-resolution transmittance electron microscope (HRTEM, JEOL-2010) with an accelerating voltage of 200 kV and a field emission scanning electron microscope (FESEM, Model JSM-7600F, JEOL Ltd, Tokyo, Japan). Element analysis was carried out using an energy-dispersive spectroscope (EDS; Oxford INCAx-sight) attached to the HRTEM. Film compositions were probed by X-ray photoelectron spectroscopy (XPS, ESCALAB MKII) with an Mg Kα excitation source at a pressure of 1.0 × 10⁻⁶ Pa and a resolution of 1.00 eV. Confirmation that the PI block was really cross-linked by S₂Cl₂ was obtained by investigating the dry films and the pure polymer by FTIR spectroscopy (VERTEX-70) by using pressed KBr pellets.

2.5 Catalytic reaction

The catalytic activities of these thin composite films were evaluated by assessing the reduction of 4-NP in aqueous solutions. Thus, 0.5 mL of an aqueous solution of 4-NP with a concentration of 1 × 10⁻⁴ mol L⁻¹ was poured into a 1 cm quartz cuvette, to which 1.0 mL aqueous solution of KBH₄ with a concentration of 2 × 10⁻² mol L⁻¹ was added. The final concentrations of 4-NP and KBH₄ in the mixture were 3.33 × 10⁻⁵ and 1.33 × 10⁻² mol L⁻¹, respectively. The thin composite film deposited on a quartz slide and treated with KBH₄ aqueous solution was immersed in the reaction system to catalyze the reduction of 4-NP. The progress of the reaction was monitored by using UV-vis spectroscopy (HP8453). The reaction temperature was maintained at 25 °C with a thermostat.

3. Results and discussion

The TEM micrographs of the composite film formed at the liquid/liquid interface through route 1 are shown in Fig. 1. In the composite film of PI-b-P2VP/Ag, a honeycomb-like structure composed of polygons, such as hexagons and pentagons, formed, as shown in Fig. 1(a). This structure was similar to that of the composite film of PS-b-P2VP/Ag prepared in our previous work,²⁸ indicating that PI-b-P2VP adopted similar steps to construct this kind of honeycomb structure. As seen in Fig. 1(b), the nanoparticles are distributed homogeneously on both the spindle-like sides and the bottom faces of the honeycomb holes. The PI-b-P2VP/Au composite film was a foam film composed of different sized microcapsules (Fig. 1(c) and (d)), and the nanoparticles are also distributed homogeneously on the film (Fig. 1(e) and (f)). As revealed in our previous work,^27,29 the formed microstructures depended on experimental conditions, including the polymer structure, the inorganic species, and the concentrations. Hence, the PI-b-P2VP/Ag and PI-b-P2VP/Au composite films exhibited different morphologies. The formation of Ag and Au nanoparticles was attributed to the reduction of the corresponding metal ions by the small amount of alcohol during the adsorption and assembly process. According to the formation mechanism of the honeycomb structure proposed in the literature,²⁸ the sides and the bottom faces of the honeycomb structure of PI-b-P2VP/Ag were made up of P2VP and PI blocks, respectively. The homogeneous distribution of Ag nanoparticles across the bottom faces indicated that not only the P2VP block, but also the PI block, can combine with metal nanoparticles or precursors. It is known that the π–electrons of the benzene rings can interact weakly with the metal nanoparticles. For example, the benzene rings in the PS block can interact with metal nanoparticles to embed them in the layers with the aid of the weak π–electron–metal interactions.^28,32,33 In the same way, the PI block can also interact with metal nanoparticles through the π–electrons of the conjugated carbon–carbon double bond.

Fig. 1 TEM micrographs of composite films of PI-b-P2VP/Ag (a and b) and PI-b-P2VP/Au (c–f) formed at the liquid/liquid interface through route 1.

We evaluated the catalytic activities of these composite films by investigating the reduction of 4-NP to 4-aminophenol (4-AP) by KBH₄ in aqueous solution.³⁴ The progress of the reaction was recorded by monitoring the absorption peak of 4-nitrophenolate ions at 400 nm using UV-vis spectroscopy. This reaction is usually regarded as a pseudo-first order one because the concentration of KBH₄ in the reaction system is considerably larger than that of 4-NP and considered to be constant during the reaction process. Therefore, the apparent rate constant k_app can be obtained. Fig. 2 shows the obtained k_app values. When the PI-b-P2VP/Ag composite film was used as catalyst, k_app increased gradually and obviously as the number of reaction cycles increased until 18 cycles were completed. At this point the k_app decreased sharply and became nearly constant after the 20^th cycle, as shown in Fig. 2(a). From Fig. 2(b) one can see that k_app behaved similarly when the PI-b-P2VP/Au composite film was used. The surprising and unusual characteristic of k_app suggests that these two composite films were unstable, and that the structure and composition underwent changes during these reaction cycles. The increase in k_app means that the metal nanoparticles take part in the catalytic reaction to an increasing extent, which may be attributed to the outward diffusion of the metal nanoparticles. The decrease in k_app points to a reduction in the number of metal nanoparticles, which may be attributed to part leaching of the metal nanoparticles or fusing of the small nanoparticles to form larger particles. The question therefore is: why does this partial outward diffusion of Ag or Au away from the films occur? As mentioned above, two kinds of metal nanoparticles existed in the composite films. One of them combined with the pyridine groups, and the other was embedded in the PI blocks. The π–electrons in the carbon–carbon double bonds of the PI blocks were insufficiently electronegative compared to the benzene rings in the PS-b-P2VP films,²⁸ and the steric hindrance of the methyl group also prevented the interaction between the metal nanoparticles and the PI block from becoming sufficiently strong. Thus, the metal nanoparticles embedded in the PI blocks diffused outwardly and were removed from the films with an increasing number of reaction cycles. After complete leaching of these nanoparticles or complete fusion to form stable larger particles, the k_app value stabilized.

Fig. 2 The relationship between the reaction rate constants and the cycle times using composite films of PI-b-P2VP/Ag (a) and PI-b-P2VP/Au (b) formed by route 1 as catalyst. The reaction temperature is 298 K.

In order to clarify why the k_app decreases after a certain reaction cycles, the thin films deposited on quartz slides were monitored using UV-vis spectroscopy during the catalytic reaction and the thin films deposited on carbon-coated copper grids were investigated using TEM after 30 cycles. Fig. S1(a) in ESI† shows the UV-vis spectra of the PI-b-P2VP/Ag system. The absorption intensity of the as-deposited PI-b-P2VP/Ag film increases steadily from 800 to 500 nm, and a broad band corresponding to the surface plasmon resonance (SPR) of Ag nanoparticles appears at 430 nm, suggesting the co-existence of tiny Ag nanoclusters and nanoparticles with the size greater than 3 nm.^35–37 After using as a catalyst for several cycles, the absorption intensities from 800 to 490 nm and from 390 to 300 nm decrease. Simultaneously, the SPR band shifts slightly to 433 nm, the intensity of the band increases. However, the overall absorption does not change obviously. This implies that the number of the tiny nanoclusters decreases, some larger nanoparticles form, and the Ag species is almost not lost during the catalytic reaction. It was reported that the presence of NaBH₄ in the reaction medium led to the fusion of metal nanoparticles.^38,39 It is possible that the Ag nanoclusters aggregate and fuse into larger nanoparticles during the catalytic reaction, resulting in the decrease of k_app after a certain cycles. The UV-vis spectra of the PI-b-P2VP/Au film (Fig. S1(b)†) exhibit similar feature.

Fig. S2† shows the TEM micrographs of the composite films after 30 catalytic cycles. It can be seen clearly that the honeycomb and foam structures of the PI-b-P2VP/Ag and PI-b-P2VP/Au films were preserved. Besides the nanoclusters and small nanoparticles, larger particles with the size of several tens of nanometers were generated, indicating the fusion of small nanoparticles during the reaction process. This is consistent with the UV-vis spectral analysis. It was seen that the larger particles dispersed on both the sides and bottoms of the honeycomb-like structure. This should be related to the diffusion of the nanoclusters. They diffused not only longitudinally but also horizontally to the film surface and fused into larger particles. Although the particle leaching can not be excluded completely, the main reason why the k_app decreases after a certain cycles should be the fusion of the nanoclusters and small nanoparticles.

In order to improve the stability of the composite film, the polymer molecules were crosslinked in the chloroform solution by adding a certain amount of S₂Cl₂. Please note that the crosslinking would increase the interaction between the polymer molecules, because the PI blocks of some molecules were connected by covalent bonds after crosslinking other than intermolecular interactions. So we thought that a thin film might form at the interface between the crosslinked polymer chloroform solution and pure water with the help of specific adsorption of the molecules arising from the stronger interaction between the molecules.

In order to confirm this deduction, we fabricated a film at the liquid/liquid interface between the pre-crosslinked PI-b-P2VP chloroform solution and pure water. As shown in Fig. 3, the film consists of porous foam composed of microcapsules. Fig. 3(b) shows the EDS spectrum of the film. The presence of S suggests the PI-b-P2VP was cross-linked by S₂Cl₂. In addition, the dry films and the pure polymer were investigated by FTIR spectroscopy. The new broad band at 2594 cm⁻¹ in the spectrum of the thin film (Fig. 4) was assigned to S–H stretching.⁴⁰ The S–H bonds may result from the breaking of some S–S bonds.

Fig. 3 TEM micrograph (a), EDS spectrum (b) and SEM micrographs (c and d) of the composite film formed between the pre-crosslinked PI-b-P2VP solution and pure water liquid/liquid interface.

Fig. 4 FTIR spectra of the pure polymer and the film of crosslinked PI-b-P2VP.

The crosslinking process was monitored using DLS, as shown in Fig. 5. Small particles with the size of 2 nm exist in the PI-b-P2VP solution, indicating no aggregate forms. After adding S₂Cl₂, aggregates with the size of about 22 and 33 nm appeared one day and two months later, indicating the crosslinking occurred. It should be noted that the solution kept clear during the process, indicating no larger aggregates formed. In addition, no film appears at the air/chloroform solution interface, indicating that the crosslinked aggregates are dispersed in the organic medium freely and can not adsorb at interfaces without adding water.

Fig. 5 DLS spectra of the polymer chloroform solution before and after crosslinking.

It should be pointed out that no film formed at the interface of the uncrosslinked PI-b-P2VP chloroform solution and pure water through route 2, and no film formed when using other polymers, such as P2VP, P4VP, PS-b-P2VP, PS-b-P4VP, P2VP-b-PS-b-P2VP, and P4VP-b-PS-b-P4VP without the presence of inorganic species in the aqueous solution. The driving force behind the adsorption of polymer molecules at the liquid/liquid interface is the reduction of interfacial tension in the absence of inorganic species, causing the adsorption and desorption to finally reach equilibrium. However, besides reducing the interfacial tension, the attraction between the polymer molecules and the inorganic species in the aqueous solution offers an additional driving force for the adsorption of these uncrosslinked polymer molecules, leading to specific adsorption of the polymer. More and more polymer molecules adsorbed at the interface, eventually self-assembling into microstructures. This is the reason why the composite films can form through route 1. The adsorption of the crosslinked PI-b-P2VP at the interface between the chloroform solution and pure water is also specific, because the interaction between the polymer molecules was enhanced, and small aggregates were formed, as illuminated in Fig. 5. That is to say, a large number of molecules adsorb at the interface in a short time, and the interaction between these molecules is strengthened by crosslinking. It is difficult for these molecules to desorb, and then eventually self-assemble into a foam film. It is clear by comparing the size of the microcapsule with that of the aggregate in the organic solution that the microcapsules were formed by the aggregation and assembly of these smaller aggregates in the chloroform solution of the crosslinked polymer. It is very interesting to prepare such pure polymer films, because various composite systems can be fabricated by adsorbing other species in their porous structure. This method has some advantages over the other methods. The process can be proceeding under ambient pressure and temperature, the film is formed rapidly, and the formed film can be deposited on various substrates. Various methods have been developed for the preparation of crosslinked porous thin polymer films, such as simultaneous crosslinking and solvent evaporation method^41,42 and in situ crosslinking breath figure formation.^43,44 The adsorption and assembly method at the liquid/liquid interface supplies another choice for the fabrication of porous polymer films.

We also prepared thin films at the liquid/liquid interface by using the crosslinked polymer solution and aqueous solutions of inorganic species, including AgNO₃, HAuCl₄, H₂PtCl₆ and Eu(NO₃)₃ to investigate the effects of the interaction between polymer and inorganic species and the interaction between the crosslinked polymer molecules on the film formation. The morphology, structure and composition of these films were investigated using TEM and EDS, as shown in Fig. S3–S6 in ESI.† It was found that foam films were formed in these cases. When using AgNO₃ and HAuCl₄ aqueous solutions, Ag and Au nanoparticles formed and adsorbed on the surface of the films, and no nanoparticle was embedded in the foam structure; when using H₂PtCl₆ and Eu(NO₃)₃ aqueous solutions, Pt and Eu elements were not detected using EDS. This indicates that the formation of the foam film is independent of the inorganic species. It is clear that the interaction between the crosslinked polymer molecules is more important than the interaction between the polymer and the inorganic species for the formation of these films. Therefore it is easy to understand the formation of the pure polymer porous films at the chloroform solution/pure water interface.

These films were immersed in AgNO₃ and HAuCl₄ aqueous solutions, respectively, to adsorb Ag⁺ and AuCl₄⁻ ions with the help of the coordination of the N atoms of the pyridine groups and the S atoms to Ag⁺ or Au³⁺ ions and electrostatic interactions between the protonated pyridine groups and AuCl₄⁻ ions. After adsorption, the films were rinsed with pure water, and then treated with an aqueous KBH₄ solution to produce Ag and Au nanoparticles. Fig. 6 exhibits the TEM micrographs of the treated films, together with the size distribution histograms of the nanoparticles in the film. It can be seen that the foam structure was preserved, and Ag and Au nanoparticles were distributed homogeneously in the films. The mean diameters of the Ag and Au nanoparticles were measured to be 2.59 and 2.17 nm, respectively.

Fig. 6 TEM micrograph of the KBH₄ aqueous solution treated composite films of PI-b-P2VP/Ag (a–c) and PI-b-P2VP/Au (e–g) formed by route 2 and the corresponding size distribution histogram (d and h) of the particles, respectively.

The composition of the films formed using route 2 was checked using XPS. In general, the S 2p spectra display 2p_3/2 and 2p_1/2 peaks with an intensity ratio of 2 : 1 and a spin–orbit splitting of 1.2 eV, as theoretically determined from the spin–orbit splitting effect.^45,46 Fig. 7(a) shows the XPS spectrum of the sulfur species in the pure polymer film. The curve was decomposed into two pairs of peaks with the binding energies located at 163.4/164.9 and 164.1/165.5 eV, which were assigned to S 2p_3/2/S 2p_1/2 of R–SH and R–S–R/R–S–S–R, respectively.^46–48 The formation of R–S–R and R–S–S–R bonding is the result of crosslinking by S₂Cl₂,⁴⁹ and the formation of R–SH is likely the result of the breaking of R–S–S–R links under certain conditions.⁵⁰ This result is consistent with the FTIR spectra shown in Fig. 4.

Fig. 7 XPS spectra of sulfur (a, b and d), silver (c) and gold (e) elements in the pure polymer film (a), composite films containing Ag (b and c) and Au (d and e) after treatment with KBH₄ aqueous solution.

Fig. 7(b) and (d) represent the XPS spectra of sulfur species in the composite films containing Ag and Au, respectively, which were decomposed into three pairs of peaks. The first pairs at 162.0/163.3 eV in Fig. 7(b) and 163.1/164.4 eV in Fig. 7(d) are attributed to S 2p_3/2/S 2p_1/2 of R–S–Ag and R–S–Au, respectively.^51,52 This indicates that Ag and Au nanoparticles were formed and combined with sulfur atoms. The second pairs at 163.7/164.9 eV in these two spectra is assigned to S 2p_3/2/S 2p_1/2 of R–S–R/R–S–S–R, indicating the existence of un-bounded sulfur species in the films. The third pairs at 168.6/169.9 and 168.3/169.3 eV in these two figures are related to oxidized sulfur⁴⁷ that would be generated during the adsorption process, because both Ag(I) and Au(III) possess efficient oxidizing abilities.

Two bands appeared between 366 and 376 eV in the XPS spectrum of Ag in the composite film shown in Fig. 7(c), which correspond to Ag 3d_5/2 and Ag 3d_3/2, respectively. Each band was decomposed into two peaks. For the band of Ag 3d_5/2, two peaks at 367.6 and 368.1 eV were obtained, corresponding to Ag(I) and Ag(0), respectively.^53,54 This reveals that Ag⁺ and Ag(0) coexisted in the film. Ag⁺ ions surround the nanoclusters constituted of Ag atoms and interacted with S to formed S–Ag bond, as described above.

Fig. 7(e) refers to the XPS spectrum of Au in the composite film. Two bands appeared between 82 and 92 eV, corresponding to Au 4f_7/2 and 4f_5/2 respectively. Two pairs of peaks were obtained by decomposition. The first pair at 84.1 and 87.8 eV was assigned to 4f_7/2 and 4f_5/2 of spin–orbit coupling of Au(0) and the second pair of peaks at 85.3/89.0 eV was assigned to Au(I).⁵⁵ This means that two kinds of gold species coexist in the film. The existence of Au(I) should be related to the S–Au bond.

We have also evaluated the catalytic activities of these composite films formed using route 2. As shown in Fig. 8, the k_app values are close to each other for all the cycles. Although the k_app values of these films were less than those of the films formed using route 1, the catalytic performance of these films was stable and durable. In addition, no metal nanoparticles were found to become dislodged from the films or to fuse in the films.

Fig. 8 The relationship between the reaction rate constants and the cycle times of reduction using cross-linked composite films of PI-b-P2VP/Ag (a) and PI-b-P2VP/Au (b) formed by route 2. The reaction temperature is 298 K.

4. Conclusion

A stable porous polymer thin film was fabricated at the liquid/liquid interface between an organic polymer solution and pure water for the first time. Generally, this kind of film should be formed at the liquid/liquid interface in the presence of inorganic or other species in the aqueous phase that strongly interact with the polymer molecules in the organic phase, as described in our previous work^25–29 and other literature.^23,56 The film adsorbed Ag⁺ and AuCl₄⁻ ions effectively, and the treated adsorbed film exhibited efficient catalytic activity and good reusability. The pure polymer thin film can be expected to adsorb various species and to form various functional composite materials, and to be used widely in many areas.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 21273133).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra12001b

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