Article pubs.acs.org/JPCC
UV-Imprint Resists Generated from Polymerizable Ionic Liquids and Titania Nanoparticles Aswin Gopakumar,†,‡ Zhaofu Fei,† Emilia Păunescu,† Vaida Auzelyte,§ Juergen Brugger,*,§ and Paul J. Dyson*,† †
Institut des Sciences et Ingénierie Chimiques, Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland Electrochemical Materials Research Laboratory, Centre for Nanoscience and Technology, Pondicherry University, Puducherry 605 014, India § Microsystems Laboratory, Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland ‡
ABSTRACT: Ionic liquids incorporating a polymerizable anion were prepared in a facile manner and used to fabricate stable titania nanoparticle films. The nanotitania was directly patterned by UV imprinting using micrometer size test structures in a polydimethylsiloxane mold. The resulting resist microstructures are resistant to cracking with only slight shrinkage observed for the system made using a hydrophobic ionic liquid. Because ionic liquids with a plethora of different physical and chemical properties can be made (designed) in a facile manner their application in this facile process instead of organic solvents could lead to many new types of functional materials.
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INTRODUCTION Micro- and nanopatterning of metallic or ceramic nanoparticles (NPs) provides opportunities for developing nanomaterialbased electronics, energy devices, sensors, and other types of devices.1−12 Indeed, micro- and nanofabrication and structuring of titanium dioxide NPs has been achieved using various methods including electron-beam lithography,13 nanoimprint lithography,14−17 electro-hydrodynamic patterning,18 two-photon lithography,19 direct-write assembly,20 ion-beam lithography,21 and atomic force microscopy (AFM) nanolithography.22 Direct imprint of NPs could provide a facile approach for the fabrication of nanopatterned, NP-related materials, and devices and the sol−gel route is the most widely used method employed to fabricate titania films. The process involves polymerization of monomers, usually induced by photolysis, into a colloidal solution (sol) to form an integrated network (or gel) that stabilizes the titanium oxide NPs. Monomers such as methacrylate23 and styrene24−29 are frequently used for this purpose, but the resulting materials processed using volatile monomers such as those mentioned above are prone to high shrinkage and cracking.30,31 Ionic liquids (ILs) are salts with comparatively low melting points, arbitrarily defined as 100 °C or less, that can be modified in a facile way for specific applications.32 ILs have a number of interesting features including low vapor pressure, wide liquid temperature range, good solvation ability, high thermal stability, high ionic conductivity, wide electrochemical windows, and so forth.33 IL-mediated synthesis of Titania NPs has been reported previously.34−38 Moreover, the problems associated with solvent mediated imprinting appear to be reduced or even suppressed when organic solvents are replaced by ILs.39,40 © XXXX American Chemical Society
Figure 1. Structures of the 1-ethyl-3-methylimidazolium, [emim]+, and trihexyl(tetradecyl)phosphonium, [P66614]+, cations and the 2-acrylamido-2-methylpropane 1-sulfonate, [AMPS]−, anion.
Our strategy is to replace widely used monomers emloyed in sol−gel routes with polymerizable room temperature ILs (Figure 1) for the preparation of TiO2 NPs and, following spin coating, explore the titania NPs in direct imprint using a polydimethylsiloxane (PDMS) patterned mold with micrometer-sized features (this type of mold compensates for the surface energies of the sol−gel resist41−46). Subsequent UV exposure of the TiO2 NPs is expected to result in polymerization of the IL to afford a solid polymeric ionic liquid (PIL) and yielding a rigid structure containing stabilized TiO2 NPs. To the best of our knowledge this lithographic approach, Special Issue: Michael Grätzel Festschrift Received: December 28, 2013 Revised: April 15, 2014
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dx.doi.org/10.1021/jp412722y | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
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Figure 2. Schematic showing the overall process used to fabricate the micrometer patterned IL-resist. (a) Drop casting of IL on glass substrate and spin coating to form a uniform thin film, (b) pressing PDMS mold into the IL-resist, (c) exposure with UV light, and (d) removing PDMS mold.
Figure 3. Representative SEM micrograph of microsized features in PDMS mold (top left), the [Emim][AMPS]-resist (top right), magnified image of the imprinted patterns formed in the [Emim][AMPS] (bottom left), and TEM images of the TiO2 NPs generated in [Emim][AMPS] (bottom right).
methyl-1-propanesulfonic acid, H[AMPS] (8.95 g, 43.2 mmol) and ethanol (50 mL) protected from light. The mixture was stirred vigorously for 1 h at room temperature and then 1-ethyl3-methylimidazolium bromide (8.25 g, 43.2 mmol) was added. The mixture was heated at 50 °C for 1 h. A yellow precipitate formed that was centrifuged and removed by filtration, and the filtrate was collected and evaporated under vacuum to give a waxy colorless solid. Yield 13.0 g, 95%. 1H NMR (D2O): 8.60 (s, 1H, NCHN), 7.39 (s, 1H, NCHCHN, s), 7.33 (s, 1H, NCHCHN, s), 6.1 (m, 2H, HCC−, CCH), 5.5 (d, 1H, H−CC), 4.15 (q, 2H, NCH2CH3), 3.80 (s, 3H, NCH3), 3.30 (s, 2H, −CH2SO3−), 1.43(s, 6H, NH(CH3)2), 1.41 (t, 3H, NCH2CH3). IR (cm−1): 3250, 3050, 2995, 1600, 1280. Elemental analysis for C13H23N3O4S (%) calcd.: C 49.19, H 7.30, N 13.24. Found: C 49.14, H 7.28, N 13.08. ESI-MS (in MeOH) m/z: positive 111.0 for [(cation)]+; m/z: negative 206.0 for [(anion)]−.
depicted in Figure 2, employing polymerizable ILs has not been reported.
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EXPERIMENTAL SECTION Materials and Characterization. Chemicals were obtained from commercial sources and used without further purification. 1H NMR spectra were recorded at 20 °C with a Bruker DMX 400 instrument using SiMe4 as an external standard; the chemical shifts are reported in parts per million (ppm). IR spectra were recorded on a PerkinElmer FT-IR 2000 instrument. Electrospray ionization mass spectra (ESI-MS) in methanol were recorded on a ThermoFinnigan LCQ Deca XP Plus quadrupole ion trap instrument. Elemental analysis was performed on a Thermo Scientific Flash 2000 Organic Elemental Analyzer. Synthesis of 1-Ethyl-3-methylimidazolium 2-acrylamido 2-methyl 1-propanesulfonate, [Emim][AMPS]. Silver oxide (5.02 g, 21.6 mmol) was added to a solution of 2-acrylamido-2B
dx.doi.org/10.1021/jp412722y | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
Article
Synthesis of Trihexyl(tetradecyl)phosphonium 2-acrylamido-2-methylpropane 1-sulfonate, [P66614][AMPS]. Trihexyl(tetradecyl)phosphonium chloride (23.45 g, 45.1 mmol) was added to a solution of H[AMPS] (8.95 g, 43.2 mmol) in water (50 mL). To this mixture a solution containing NaOH (1.80 g, 45 mmol) in water (30 mL) was added dropwise at room temperature. The resulting reaction mixture forms two phases and was stirred continuously for 24 h. The two phases were separated and the IL phase was washed with water (5 × 20 mL) and dried under vacuum to give a yellowish liquid. Yield 24.7 g, 83%. 1H NMR (D2O): 8.70 (s, 1H, −CONH), 6.1 (m, 2H, HCC−, CCH), 5.5 (d, 1H, H−CC), 2.75 (s, 2H, −CH2SO3−), 2.10(overlapping signals, 8H, 4× −P−CH2), 1.53 (s, 6H, −NH(CH3)2), 1.50−1.15 (m, overlapping signals). 0.90−0.94 (m, 12H, overlapping signals, 4× −CH2CH3). IR (cm−1): 3255, 3045, 2990, 1610, 1285. Elemental analysis for C39H80NO4PS (%) calcd.: C 67.88, H 11.68, N 2.03. Found: C 67.76, H 11.57, N 2.01. ESI-MS (in MeOH) m/z: positive 483.0 for [(cation)]+; m/z: negative 206.0 for [(anion)]−. Preparation of the IL Titania NP Resists. Titanium isopropoxide (0.24 mL) was added to a solution of ionic liquid, [Emim][AMPS] or [P66614][AMPS] (1.5 mL) in absolute ethanol (8 mL). To the resulting homogeneous viscous solution, acetic acid (0.19 mL) was added dropwise to afford a turbid solution. IRGACURE-907 (0.05 g, photoinitiator) was added and was ultrasonicated for 5 min and the resulting viscous solution was employed for UV-imprinting procedure. Hereafter, the resist formulations will be referred to as IL-resists. UV-Imprint Lithography. Continuous and uniform thin films of each IL-resist were obtained by spin coating at a rotation speed of 2500 rpm. In the case of the [P66614][AMPS]resist, the high hydrophobicity of the IL makes adherence to the glass surface problematic and a brief surface treatment of the glass substrate with trichloromethylsilane was required to increase the hydrophobicity of the glass substrate, as described elsewhere.47−57 A thickness of ca. 1 μm was achieved. The films were imprinted with a PDMS mold and then irradiated using a UV lamp (Vakuum UV Belichtungsgerät 4) for 5.5 min. The exposure dose was optimized by systematically varying the irradiation time and a time of 5.5 min was found to be optimal. The ILs were efficiently polymerized in the presence of the initiator following treatment with UV irradiation as the samples become solid. A schematic of the overall procedure is shown in Figure 2. Characterization of the IL-Resists and Imprints. The IL-resists (IL-embedded TiO2 NPs) were analyzed by TEM (Philips FEI CM12 model) to determine their morphology and particle size. The features made after imprinting were analyzed using AFM, scanning electron microscopy (SEM), and energy dispersive X-ray (EDX) in order to determine the surface topology. The crystallinity of the samples was determined using powder X-ray diffraction (XRD) analysis on a Bruker D8 four circle XRD with a Cu−K alpha source.
Figure 4. EDX spectra of (top) the [Emim][AMPS]-resist patterned material and (middle) the [P66614][AMPS]-resist patterned material and (bottom) stacked powder XRD diffractograms for the colloidal TiO2 thin films obtained from [Emim][AMPS] (red, above) and [P66614][AMPS] (black, below).
methylimidazolium cation to the [AMPS]− and Br− anions is similar and use of the silver salt ensures the removal of the halide, that is, a AgBr precipitate forms that is removed by filtration to yield [Emim][AMPS] as waxy solid after removal of the solvent. For the synthesis of [P66614][AMPS], the sodium salt Na[AMPS] was prepared prior to anion exchange with the phosphonium chloride, [P66614]Cl in water. The bulky [P66614]+ cation and the [AMPS]− anion combine to form a hydrophobic IL that is insoluble in aqueous solution. The two ILs were characterized by 1H NMR and IR spectroscopy and ESI mass spectroscopy (see Experimental Section). The imidazolium-based IL, [emim][AMPS], is highly soluble in water and sparingly soluble in dichloromethane whereas [P66614][AMPS] is practically insoluble in water and highly soluble in dichloromethane. Both ILs are soluble in ethanol. Consequently, titanium isopropoxide, [Ti(OiPr)4] was dispersed in a mixture of each IL with ethanol as a co-solvent and acetic acid was added to generate to titanium dioxide NPs (IL-resist). The photoinitiator, IRGACURE-907, was added
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RESULTS AND DISCUSSION Polymerizable ILs derived from 2-acrylamido-2-methylpropane 1-sulfonic acid, H[AMPS] (Figure 1), allow facile control of the polymer properties via the IL cation employed. The usual route to these IL-based monomers involves the conversion of H[AMPS] into alkali metal or silver salts and subsequent anion exchange with the appropriate halides58 or hydroxides.59 We used the silver salt of 2-acrylamido-2-methylpropane 1-sulfonic acid, that is, Ag[AMPS], as the precursor for ion exchange with 1-ethyl-3-methylimidazolium bromide, [emim]Br, as the anion affinity of the 1-ethyl-3C
dx.doi.org/10.1021/jp412722y | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
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Figure 5. Representative AFM images of the micrometer patterned [Emim][AMPS]-resist; (top, left) AFM 3D image, (top, right) top view, and (bottom) AFM cross-section analysis along the white line shown marked in the top view showing the depth and width of the patterned features.
to the NP suspension for direct spin coating and subsequent imprinting. Transmission electron microscopy (TEM) of the IL-resists revealed that the titania NPs have an average particle size of ca. 50 nm for both ILs. Micrometer patterned titania structures were fabricated in IL-resists according to the procedure illustrated in Figure 2. Representative SEM micrographs (Figure 3) show the replicated pattern formed from the [Emim][AMPS]-resist. The replicated feature dimensions in both IL-resists correspond well to the dimensions in on the mold without any noticeable shrinkage of the features. The magnified SEM image of the imprinted features on the IL-resist indicates that the titania NPs are well encased within the PIL. This feature appears to be substantiated by transmission electron microscopy (TEM) of the of the titania NPs (Figure 3) with the IL presumably coating the TiO2 NPs causing them to aggregate. The materials were analyzed by energy-dispersive X-ray spectroscopy to determine their elemental compositions confirming the presence of the expected elements in each sample, notably phosphorus is present in the material derived from the [P66614][AMPS]-resist and absent in the [Emim][AMPS]-resist material (Figure 4). The presence of chlorine in the [P66614][AMPS]-resist and the large signal for silicon are probably due to the treatment of the glass substrate with trichloromethylsilane. The structure of the TiO2 NPs in the spin coated thin films obtained from [Emim][AMPS] and [P66614][AMPS] was studied by powder XRD analysis (Figure 4), which did not show any notable peaks suggesting that the TiO2 NPs formed in both ILs are amorphous. The surface of UV-direct imprinted patterns of both ILresists was further studied by AFM after baking at 120 °C (to
Table 1. Relative Shrinkage in the IL-Resists lateral gap between stamp features
ionic liquid
SEM value, before baking (μm)
AFM value, postbaking (μm)
[Emim][AMPS] [P66614][AMPS]
2.5 2.5
2.0 2.3
linear difference shrinkage (μm) (%) 0.5 0.2
20% 8%
remove any residual volatile components). The approximate lateral width between the TiO2 walls in the patterned material are ca. 2.0 μm for one derived from the [Emim][AMPS]-resist and ca. 2.3 μm for the [P66614][AMPS]-resist (Figure 5). Relative linear shrinkages for the structures (compared to values from the SEM data) are listed in Table 1. The shrinkage percentages were far less than previous reports based on sol−gel resists prepared using organic solvents.16 The shrinkage phenomenon has been attributed to the evaporation of volatile sol−gel components and trapped air pockets, which are presumably reduced with the ILresist, especially the hydrophobic [P66614]+-based system. Moreover, no visible cracks were observed, a significant achievement for imprinted structures. Titania walls of ca. 150 nm in height were obtained with the [Emim][AMPS]-resist (Figure 5) and
