Functionalized Nanoporous Thin Films from Metallo-Supramolecular

Article pubs.acs.org/Langmuir

Functionalized Nanoporous Thin Films from Metallo-Supramolecular Diblock Copolymers Clément Mugemana, Jean-François Gohy,* and Charles-André Fustin* Institute of Condensed Matter and Nanosciences (IMCN), Bio- and Soft Matter (BSMA), Université catholique de Louvain, Place Pasteur 1, 1348 Louvain-la-Neuve, Belgium S Supporting Information *

ABSTRACT: A polystyrene-[Ni2+]-poly(ethylene oxide) metallo-supramolecular block copolymer (PS-[Ni2+]-PEO), where -[ is a terpyridine, is used to create nanoporous thin films with free terpyridine ligands homogenously distributed on the pore walls. The PS-[Ni2+]-PEO block copolymer is synthesized by a two step assembly process, and is then self-assembled into a thin film in order to obtain PEO cylinders oriented perpendicularly to the film surface. The supramolecular junction is opened by exposing the film to an excess of a competing ligand, and the free PEO block is then rinsed away by a selective solvent. The presence of the terpyridines on the pore walls is evidenced by fluorescence spectroscopy after formation of a fluorescent complex with an europium salt.

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INTRODUCTION Block copolymers are powerful tools for preparing nanostructured materials due to their ability to self-assemble into well-ordered structures at the nanometer scale.1−5 Spheres, cylinders, lamellae, or more complex structures such as double gyroid or hexagonally perforated layers can be obtained from diblock copolymers. Among these, the cylindrical morphology is of particular interest since it can be transformed into an array of nanopores after elimination of the minor component. These materials exhibit the pore topology and the pore size of their parent structures, and can be further used as separation membranes, lithographic masks, or templates for the preparation of other nanomaterials.1−10 Several strategies have been reported to create nanopores from a self-assembled block copolymer.6−10 The most common one consists in selectively degrading the minor block using processes adapted to the chemical nature of the block to be removed such as ozonolysis of polydienes,11 UV etching of poly(methylmethacrylate),12 hydrolysis of polyesters,13 or HF etching of polysiloxanes.14 In recent years, an alternative route has emerged which involves the use of block copolymers bearing a cleavable junction between the two blocks. The few reported examples have exploited a metal−ligand complex,15 a tritylether linkage,16 a disulfide bond,17 or a photocleavable group as junction to link the blocks.18,19 This strategy is highly interesting since the cleavage is usually performed under milder conditions than for etching, and moreover it is independent of the nature of the polymer blocks so that almost any kind of polymer sequence of interest can be used. Metallo-supramolecular block copolymers, i.e., copolymers where the two blocks are linked by a metal−ligand complex, have attracted a lot of attention over the past years because they offer several advantages compared to their covalent counterparts.20−23 Indeed, the coordination bond is highly directional, a wide range of ligands is available, the interaction strength can © 2012 American Chemical Society

be fine-tuned by choosing the appropriate metal ions, and the reversibility of the supramolecular bond allows an improved control over the material properties and the construction of “smart” materials. Metallo-supramolecular block copolymers have already shown their potential for different applications such as, e.g., gels with self-healing properties,24 functionalized nanocages,25 and stimuli responsive micelles.20b,26−28 Previously, we reported on the preparation of nanoporous thin films using a polystyrene-[RuII]-poly(ethylene oxide) metallo-supramolecular block copolymer, where -[ is a terpyridine ligand, as precursor.15 After self-assembling the copolymer into a thin film, the nanopores were created by opening the metal−ligand complexes and selectively rinsing away the PEO blocks. However, because of the high stability of the terpyridineruthenium bis-complexes, harsh conditions had to be used to open them and the presence of free terpyridines on the pore walls could not be demonstrated. In this paper we report on an approach to obtain nanoporous thin films where the pore walls are bearing free terpyridine ligands. To this aim, we have synthesized a PS-[Ni2+]-PEO metallo-supramolecular block copolymer as precursor. Nickelterpyridine bis-complexes are indeed stable enough to keep the integrity of the block copolymer, i.e., to avoid spontaneous opening of the complex which would lead to a rearrangement of the blocks to yield unwanted PS-[Ni2+]-PS and PEO-[Ni2+]PEO polymers, while being easily opened upon application of an adequate stimulus. The PS-[Ni2+]-PEO block copolymer was then self-assembled into a thin film in order to obtain an array of PEO cylinders in a PS matrix. Exposing the film to a strong competing ligand induced the opening of the nickelterpyridine complexes, which allowed the release of the PEO Received: December 14, 2011 Revised: January 9, 2012 Published: January 11, 2012 3018

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Scheme 1. Schematic Representation of the Preparation of Functionalized Nanoporous Thin Films from a PS-[Ni]-PEO Block Copolymer

Scheme 2. Synthetic Strategy toward PS300-[Ni2+]-PEO230 Diblock Copolymers

Thin Film Preparation. The silicon wafers were first immersed in a piranha solution (H2SO4 98%/H2O2 30% 70/30) for 15 min and thoroughly rinsed with ultrapure water. The substrates were dried by the spin-coater at a velocity of 4000 rpm for 40 s. The copolymer solution was filtered through a 0.2 μm PTFE filter and further spincoated onto the wafer at 2000 rpm for 40 s. The thickness of the resulting thin film depends on the concentration of the solution and was determined by ellipsometry. The thin films were annealed in solvent vapors by placing them under a glass case with two beakers containing THF and water. This setup was then placed in an oven to keep the temperature constant at 30 °C. Before extraction of the PEO blocks, the PS matrix was slightly cross-linked by UV light (1.8 J cm−2) emitted by three Rayonet

blocks and their removal by a selective solvent (Scheme 1). The presence of free terpyridine ligands on the pore walls was evidenced by forming fluorescent complexes with an europium salt.

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EXPERIMENTAL SECTION

Terpyridine End-Functionalized Polymers. Terpyridine functionalized poly(ethylene oxide) was prepared by reacting 4chloroterpyridine with monohydroxy PEO following a reported procedure.29 Polystyrene was prepared by nitroxide mediated controlled radical polymerization (NMP) of styrene using a terpyridine functional initiator as previously reported.30 3019

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photochemical reactor UV lamps at 254 nm. The thin film was then immersed in a MeOH/H2O (9:1) solution mixture (20 mL) containing 5 × 10−3 M of KCN for 24 h, thoroughly rinsed with a MeOH/H2O (1:1) mixture, and finally dried under vacuum. Characterization Methods. Scanning force microscopy was performed on a Digital Instruments Nanoscope V in tapping mode using NCL cantilevers (Si, 48 N/m, 330 kHz, Nanosensors). Transmission electron microscopy images were recorded on a LEO 922 microscope, operating at 200 kV accelerating voltage in bright field mode. A thin film was spin-coated onto a silicon wafer covered with a 400 nm thick silicon oxide layer. Afterward, the film was annealed for 2 h under THF/H2O vapors at 30 °C. The film was removed from the substrate by floating it at the surface of a dilute hydrofluoric acid solution (5 wt %), transferred in water, and then picked up with a TEM grid (copper, mesh 200). The sample was then dried overnight in vacuum at room temperature. The sample was stained under RuO4 vapors for 2 h. The emission spectra were recorded on a SPEX Fluorolog 1681 spectrofluorometer for the thin film samples and on a Varian Cary Eclipse spectrofluoremeter for the solutions. Synthesis of the PS300-[NiII]-PEO230 Metallo-Supramolecular Block Copolymer. NiCl2.4H2O (0.5 × 10−3 g, 1.05 equiv, 2.1 × 10−6 mol) was dissolved in 1 mL of dimethylformamide (DMF) to give the NiCl2·6DMF precursor complex. On the other hand, PS300-[ block (0.060 g, 1 equiv, 2 × 10−6 mol) and PEO230-[ (0.020 g, 1 equiv, 2 × 10−6 mol) blocks were, respectively, dissolved in 5 and 1.5 mL of DMF. In the case of PS300-[, pyridine (1.9 × 10−3 g, 6 equiv, 1.2 × 10−5 mol) was added to the solution. The PS300-[Ni2+(py)3 mono-complex was prepared by a dropwise addition of the PS300-[ block to the precursor complex. The subsequent addition of the second PEO230-[ block resulted in the PS300-[NiII]-PEO230 heteroleptic complex formation. The mixture was reacted three hours at room temperature. Finally, ammonium hexafluorophosphate (3.2 × 10−3 g, 10 equiv, 2 × 10−5 mol) was added and allowed to react for one hour to substitute chloride counterions by hexafluorophosphate ones. Toluene was added at the end of the reaction to form an azeotropic mixture, easily removed under reduced pressure. The solid residue was dried and dissolved in dichloromethane to remove the excess of the insoluble ammonium hexafluorophosphate salts. The purification process to remove homoleptic complexes consisted of successive extractions over four days in diethyl ether (containing 5% v/w of toluene) and four hours in water−methanol (9:1) mixture to remove, respectively, PS and PEO homopolymers. The reaction yield was estimated to be 15%. 1 H NMR (CDCl 3 , 500 MHz): δ 7.20−6.28 (m, 1500H; HPS backbone aromatics), 3.90−3.40 (m, 960H, HPEO backbone), 2.20−1.20 (m, 900H; HPS backbone aliphatics). SEC (PS calibration): Mn 44 200; Mw 57 900; PDI 1.30.

PEO230 heteroleptic complex. Finally, ammonium hexafluorophosphate was added to exchange the initial Cl− counterions by PF6− ones to improve the solubility of the copolymer in organic solvents. The purification process, to remove residual homopolymers, was performed by successive extractions in selective solvents. The PS300-[Ni2+]-PEO230 block copolymer has a PEO volume fraction of 0.24 and should thus self-assemble in a morphology made of PEO cylinders in a PS matrix. In addition to the morphology, it is important to be able to control the orientation of the cylinders considering the targeted application. We reported previously, for a PS-[Ru2+]-PEO metallo-supramolecular block copolymer, that the presence of charged metal−ligand complexes at the junction between the blocks has a positive impact on the self-assembly of the copolymer and favored the perpendicular orientation of the PEO cylinders with respect to the film surface.32 The same effect is therefore expected for the PS300-[Ni2+]-PEO230 block copolymer since only the metal ion forming the complex is different. The PS300-[Ni2+]-PEO230 block copolymer was dissolved in THF and further spin-coated onto a silicon wafer, leading to 68 nm thick film. We indeed showed previously that THF is the solvent giving the best results for preparing thin films from metallo-supramolecular copolymers,32 as opposed to benzene for “classical” covalent PS-b-PEO copolymers.33 This difference is due to the better solubility of the charged metal−ligand complexes in THF compared to benzene. Figure 1a shows the phase contrast SFM image recorded on a thin film of this copolymer. An array of nanoscopic cylinders of PEO is observed at the surface of the film, suggesting their normal orientation with respect to the substrate. The average center-to-center distance of the cylindrical microdomains, λC−C, is 41 nm. This result confirms that PS-[Ni2+]-PEO metallo-supramolecular block copolymers readily self-assemble in thin films with a perpendicular orientation of the cylinders. In order to improve the lateral ordering of the PEO cylinders, an annealing in solvent vapors was performed. Practically, the thin film was annealed two hours under THF/ water vapors at a constant temperature of 30 °C. Figure 1b shows the SFM phase image of the thin film after annealing where short-range ordering of the PEO microdomains was reached. Longer annealing times did not improve further the lateral ordering, and even started to induce defects in the film, as opposed to PS-b-PEO34 and PS-[Ru2+]-PEO block copolymers.32 This observation can be explained by the greater lability of the nickel-terpyridine complexes, compared to ruthenium-based complexes, and, obviously, to covalent bonds. Indeed, for long annealing times nickel-terpyridine complexes have enough mobility and time to open themselves and recombine, which can lead to the formation of small amounts of PS-[Ni2+]-PS and PEO-[Ni2+]-PEO homopolymers. The presence of those homopolymers can in turn affect the ordering process during the annealing, as previously reported for mixtures of block copolymers and homopolymers.35 Nevertheless, this is not a strong limitation since many applications do not require long-range lateral order of the cylindrical microdomains. After the annealing, the average center-to-center distance of the cylindrical microdomains λC−C slightly increased to 44 nm. This increase is due to the selective swelling of the PEO microdomains by water, which evaporates more slowly than THF during the drying step at the end of the annealing.

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RESULTS AND DISCUSSION The PS300-[Ni2+]-PEO230 metallo-supramolecular block copolymer used in this study has been synthesized following a sequential two-step process (see Scheme 2). The first step consisted in forming a mono-complex between the metal ion and one of the terpyridine-functionalized polymer blocks. The second block was then subsequently added in situ to the intermediate mono-complex to form the heteroleptic PS300[Ni2+]-PEO230 complex. We reported previously a procedure toward such block copolymers,31 but we describe here an improved methodology. The PS300-[ block was first dissolved in DMF in the presence of pyridine (6 equiv with respect to the terpyridine end-groups). This mixture was added to a NiCl2·xDMF precursor complex to yield the PS300-[Ni2+(py)3 mono-complex. The role of pyridine, which is a stronger ligand than DMF, is to stabilize the mono-complex and thus minimize the formation of homoleptic PS300-[Ni2+]-PS300 bis-complex during this first step. The PEO230-[ block was then added in situ to displace the pyridines and yielded the PS300-[Ni2+]3020

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Figure 2. SFM phase image of a thin film of the PS300-[Ni2+]-PEO230 block copolymer that was floated and flipped to show the bottom of the original film. The image is 1.5 × 1.5 μm2.

containing 0.005 M of KCN for 24 h, and then thoroughly rinsed using a MeOH/H2O (1:1) mixture. The cleavage and removal of the PEO blocks were investigated by FTIR analysis (see Supporting Information) and imaging techniques such as SFM and TEM. SFM was used to probe the top surface of the thin film after the extraction of the PEO blocks. Figure 3

Figure 1. SFM phase images of a thin film of the PS300-[Ni2+]-PEO230 block copolymer (a) after spin-coating and (b) after annealing 2 h in THF/water vapors. The images are 2 × 2 μm2.

Before addressing the cleavage of the complexes, we should first ensure that the PEO cylinders span across the entire film thickness. To this end, the bottom of the thin film was checked by SFM. In this respect, a PS300-[Ni2+]-PEO230 thin film was spin-coated onto a silicon wafer covered with a 400 nm thick silicon oxide layer. Afterward, the film was annealed for 2 h under THF/H2O vapors at 30 °C. The film was removed from the substrate by floating it at the surface of a dilute hydrofluoric acid solution (5 wt %) to etch the silica oxide, then transferred to a water bath, and picked up with a clean silicon wafer. Finally, the film was flipped over another silicon wafer in such way that the bottom of the original film was now on the top. The SFM image (Figure 2) evidenced an array of cylinders perpendicularly oriented to the substrate at the bottom of the film, suggesting that the microdomain orientation propagates through the entire film. The next step was to open the terpyridine-nickel biscomplexes to release the PEO blocks and allow their removal to create the nanopores. This was achieved by exposing the film to an excess of cyanide, a strong competing ligand, to form a more stable complex of K2[Ni(CN)4]. The PS300-[Ni2+]-PEO230 sample was thus immersed in a MeOH/H2O (9:1) solution

Figure 3. SFM phase image of a thin film of the PS300-[Ni2+]-PEO230 block copolymer after opening of the -[Ni2+]- complexes and rinsing to remove the PEO blocks. The image is 1.6 × 1.6 μm2.

presents the SFM image of the film after the cleavage of the complex and rinsing of the film. The inversion of the phase contrast compared to Figure 1 is clearly seen, suggesting the removal of the PEO blocks.15 As additional proof, transmission electron microscopy (TEM) was performed to demonstrate the formation of nanoporous thin films. To this end, the thin films were floated onto the surface of a dilute hydrofluoric acid solution (5 wt %), transferred in water, and then picked up with a TEM grid. Figures 4a,b show the films (stained with RuO4 vapors for 2 h) 3021

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Figure 5. Fluorescence spectra recorded on a nanoporous thin film before (dotted line) and after (red solid line) immersion in a Eu(NO3)3 solution.

after immersion of the nanoporous thin film in the Eu(NO3)3 solution, two bands appear at 617 and 592 nm. These bands have an intensity ratio of 2.5 and are characteristic of the emission of Eu(III)-terpyridine complexes.37 Moreover, this fluorescence spectrum is very similar to the one of Eu(III)terpyridine complexes prepared in solution (see Supporting Information). These results clearly show that free terpyridines are present on the pore walls and are accessible for further complexation with the desired ions.

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CONCLUSIONS

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ASSOCIATED CONTENT

In this paper we have demonstrated that nanoporous thin films bearing free terpyridine ligands on the pore walls can be successfully obtained using a PS300-[Ni2+]-PEO230 metallosupramolecular block copolymer as precursor. Our strategy involves the self-assembly of the PS300-[Ni2+]-PEO230 block copolymer into a thin film exhibiting a cylindrical morphology, followed by the selective cleavage of the supramolecular junction by exposing it to an excess of a strong competing ligand and extraction of the minor phase. SFM and TEM characterizations demonstrated the creation of cylindrical nanopores. We provided evidence for the presence and accessibility of free terpyridine ligands on the pore walls by forming fluorescent complexes with a europium salt. Thanks to the ability of terpyridines to form complexes with a large variety of metallic cations, the nanoporous materials reported in this paper could find application in various domains such as selective membranes, sensors, and catalysis.

Figure 4. TEM images of (a) a thin film of the PS300-[Ni2+]-PEO230 block copolymer stained with RuO4 vapors and (b) a nanoporous thin film stained with RuO4 vapors.

before and after PEO extraction, respectively. A significant difference between both films is clearly seen. Before extraction no contrast is visible, indicating a homogeneous staining of both phases (PS + PEO) of the film (Figure 4a). On the contrary, for the film after extraction (Figure 4b), small white dots are observed indicating that only the PS matrix has been stained, and, therefore, that PEO has been extracted. After removal of the PEO microdomains, the final step consisted in proving the presence of free terpyridine ligands lining the pore walls. To this aim, we selected lanthanide ions, well-known for their photoluminescent complexes with terpyridine or other ligands.36 Indeed, lanthanide-ion-based complexes constitute a class of efficient luminescent agents since they emit a long living, narrow band, of practically monochromatic radiation via ligand-to-metal ion energy transfer.36 We selected Eu(NO3)3 to form complexes with the terpyridines on the pore walls. A nanoporous thin film was immersed in a MeOH solution containing 0.001 M of Eu(NO3)3 for 2 h and was then thoroughly rinsed with MeOH. The emission spectra were measured at λexc of 333 nm on a spectrofluorometer adapted for solid samples. The fluorescence spectra presented in Figure 5 clearly show that,

S Supporting Information *

Additional characterizations of the PS300-[Ni2+]-PEO230 block copolymer and of the thin films. 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] (C.-A.F.); [email protected] (J.-F.G.). 3022

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ACKNOWLEDGMENTS C.-A.F. is Research Associate of the FRS-FNRS. C.M. thanks FRIA for financial support. J.-F.G. thanks EU for financial support in the frame of the SELFMEM project (No. 228652).

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