Article pubs.acs.org/cm
Thermolytic Grafting of Polystyrene to Porous Silicon Joanna Wang,† Jinmyoung Joo,‡ Rhiannon M. Kennard,‡ Sang-Wha Lee,*,§ and Michael J. Sailor*,†,‡ †
Materials Science and Engineering and ‡Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, California 92093, United States § Department of Chemical and Biochemical Engineering, Gachon University, Seongnam-si, Gyeonggi-do 461-701, South Korea S Supporting Information *
ABSTRACT: Inert-atmosphere thermolysis of polystyrene, preloaded into a porous silicon (pSi) template, generates a composite in which styrenic fragments are chemically grafted via Si−C bonds to the surface of the pore walls. The quantity of styrenic material in the pores, and thus the final porosity of the composites, is controlled by the amount of polystyrene initially loaded into the pSi host and the time and temperature of thermolysis. For a host template with a porosity of 64 ± 1%, the porosity of the resulting composite can be varied from 10 to 50%. The composites are significantly more hydrophobic than bulk polystyrene, displaying water contact angles ranging from 110 to 138° compared to a value of 89° for a pure polystyrene film. The contact angle follows the Cassie rule for porosity values up to 40%, increasing with increasing porosity. For composite porosity values >40%, the contact angle is observed to decrease, and this correlates with increasing silicon oxide content and a decrease in hydrophobicity. The stability of the grafted composite material in aqueous base (>pH 12) is enhanced with increasing styrenic content.
■
acetylene26−28 or other carbon precursors.29 These reactions also suffer from an incomplete reaction of surface Si−H species, although the latter thermal carbonization (or “hydrocarbonization”) reactions tend to form good physical barriers of a relatively thick layer of glassy carbon, hydrogenated carbon, or a mixed silicon−carbon phase on the pSi surface.26−28 Barrier layers synthesized either by chemical grafting or by introduction of inert fillers can be prepared without complete filling of the pores of the pSi template, which is critically important for chemical sensor, biosensor, or drug delivery applications.26,29−35 Inspired by the well-known ability of polystyrene to be thermally depolymerized to styrene monomers, oligomers, and polymeric radicals,36−38 we reasoned that thermolysis of a composite of polystyrene and pSi could simultaneously unzip the polymer and graft the resulting reactive products to surface Si−H species, generating a stable, grafted surface that would also serve as a physical barrier layer. In this work we find the thermolytic grafting approach to be successful, and we perform a systematic study to identify the optimal conditions to maximize surface stability while retaining open porosity. Like the polymer infiltration methods discussed above, the approach provides a functional surface layer without extensive preparation or complicated air-sensitive handling methods. We find good chemical stability in caustic solutions, show that the resulting material is notably hydrophobic, and demonstrate
INTRODUCTION Over the past two decades much effort has been expended on stabilizing porous silicon (pSi) in order to improve its suitability for sensor, biomaterials, and electronics applications.1−5 The chemical and mechanical stability of pSi has been improved through three main approaches: thermal oxidation,6 introduction of inert fillers,2,7−10 and chemical grafting.11−23 Thermal oxidation converts the chemically reactive silicon skeleton to silicon oxide, while the introduction of inert fillers forms a physical barrier that protects the reactive skeleton from attack by hydrolytic or corrosive agents. Chemical grafting reactions tend to provide both chemical and physical barriers to degradationthey lower the overall reactivity of the pSi surface while excluding aggressive chemicals from the near surface. Chemical grafting is often accomplished by thermal hydrosilylation of alkenes with surface silicon-hydride groups. The resulting low polarity silicon−carbon (Si−C) bonds provide resistance to degradation in aqueous media, particularly against nucleophilic attack by strong bases.11 However, the reaction does not proceed to completion due to steric limitations,12−15 and it is sensitive to air and water impurities.13 Typical hydrosilylation conditions require excess purified alkene (or alkyne) and vacuum or Schlenk techniques.16 In addition, the hydrosilyation reaction sometimes suffers from high variability in overall efficiency, yielding anywhere from 20 to 80% conversion of the surface Si−H species to Si−C bonds.17−21 This variability can lead to inconsistent performance of the modified materials in aqueous media.13 Other methods to form stable Si−C bonds include radical coupling,15,22,23 electrochemical alkylation,17,24,25 and thermal carbonization of © XXXX American Chemical Society
Received: August 19, 2015 Revised: December 22, 2015
A
DOI: 10.1021/acs.chemmater.5b03221 Chem. Mater. XXXX, XXX, XXX−XXX
Article
Chemistry of Materials
voltage of 15 kV. Energy dispersive X-ray spectroscopy was performed on cross-sectional samples using an FEI XL30 field emission microscope. Raman spectroscopy was performed using a Renishaw InVia Raman microscope equipped with green (532 nm) and red (785 nm) lasers. Alkaline Stability Tests. The composite (or oxidized) pSi samples were mounted in a Teflon cell fitted with an optical window that allowed real-time acquisition of reflectance spectra. For the experiments performed at pH 14, the cell was filled with 3 mL of a solution (90% water, 10% ethanol, by volume) containing 1.0 M NaOH. The optical reflectance spectrum acquired immediately after introduction of the caustic solution (after liquid had infiltrated the porous film) served as the starting point (t = 0) for the measurements. For the experiments performed at pH 12, the cell was prefilled with 3 mL of a solution (90% water, 10% ethanol, by volume) containing no base, and an aliquot (61 μL) of 1.0 M NaOH was injected into the cell, resulting in a final pH of 12. The optical reflectance spectrum acquired immediately after introduction of the alkaline solution served as the starting point (t = 0) for the measurements. Tests were performed in a static mode (without solution flow), and the optical reflectance spectra were monitored at specified intervals for a period of 120 min.
simple colorimetric chemical sensing and molecular size discrimination using a suitably modified porosity-modulated optical rugate filter.
■
EXPERIMENTAL SECTION
Materials. Highly boron-doped p-type silicon wafers (∼1 mΩ-cm resistivity, polished on the (100) face) were obtained from Virginia Semiconductor, Inc. Hydrofluoric acid (48% aqueous, ACS grade) was obtained from Fisher Scientific. Absolute ethanol (200 proof), sodium hydroxide, toluene (anhydrous, 99.8%), and polystyrene (Mw = 20 kDa, PDI: ∼ 1.06, GPC standard, Catalog# 81407) were obtained from Sigma-Aldrich Chemicals. Deionized (18 mΩ) water was used for all aqueous dilutions. Preparation of Porous Silicon. Porous silicon (pSi) samples were prepared by electrochemical etching of silicon wafers in an electrolyte consisting of 3:1 (v:v) of 48% aqueous HF:ethanol (CAUTION: HF is highly toxic and proper care should be exerted to avoid contact with skin or lungs). A silicon working electrode with an exposed area of 8.6 cm2 was contacted on the back side with aluminum foil and mounted in a Teflon cell. The silicon wafer was then anodized by applying a constant current density of 100 mA/cm2 for 150 s in a two-electrode configuration with a platinum counter electrode. After etching, the porous silicon layer was rinsed three times with ethanol and then dried in a stream of nitrogen gas. Optical rugate filters of pSi were prepared similarly, but the applied current density consisted of a sinusoidal waveform varying between 10 and 100 mA/cm2, with a period of 6.9 s and 35 cycles. Fabrication of pSi Composites. The freshly etched pSi samples were treated with a solution consisting of 1:7 (v:v) of 48% aqueous HF:ethanol for 10 min to clean the surface and remove any native oxide layer. The samples were then rinsed with ethanol, dried under a stream of nitrogen, and placed on a hot plate at 70 °C for 5 min, and an aliquot (5−12 μL) of polystyrene (PS) solution (5−15% by mass, in toluene) was dropped onto the preheated pSi chip. The polystyrene solution spread as a circle, and the temperature was maintained at 70 °C for 5 min. The temperature of the sample was increased to 110 °C for an additional 10 min to facilitate infiltration of polystyrene and evaporation of toluene. The sample was then transferred to a tube furnace and placed under N2 flow (2.0 L/min) for 30 min. The furnace was then ramped to the intended temperature (between 380 and 420 °C) at a rate of 10 °C/min. After a predetermined time (0.5−2 h), the furnace was allowed to cool to room temperature, while a continuous flow of N2 was maintained. The samples were removed from the furnace, soaked in pure toluene for 2 h, and then rinsed with ethanol to remove residual unreacted polystyrene. For some samples residual silicon oxides were removed by treatment with various aqueous solutions containing between 0.8 and 40% (by mass) HF and 10% (by volume) ethanol. Sample Characterization. Reflectance spectra were acquired using a CCD spectrometer (Ocean Optics USB-4000) fitted to a bifurcated fiber optic cable as previously described.39 One arm of the fiber cable was connected to the spectrometer, while the other arm was connected to a tungsten light source (Ocean Optics LS-1). The distal end of the combined fiber was attached to a microscope objective lens to allow acquisition of 180° reflectance spectra from the sample surface, with a spot size of approximately 1−2 mm2. The porosity of pSi chips (freshly etched or chemically modified) was characterized using the spectroscopic liquid infiltration method (SLIM).39 Refractive index of methanol used in the work was measured with a MettlerToledo Refracto 30GS refractometer. Fourier-transform infrared (FTIR) spectra were obtained using a Nicolet 6700 spectrometer (Thermo Scientific) equipped with an attenuated total reflectance (ATR) attachment. The ATR-FTIR spectral resolution was 4 cm−1, and 128 interferograms were averaged per spectrum. Water contact angle measurements (Theta Optical Tensiometer, KSV, Finland) were made on the pSi samples in triplicate, averaging values from different locations on the sample. Plan-view and cross-sectional scanning electron microscope (SEM) images were obtained using a field emission instrument (Hitachi S-4700) operating at an accelerating
■
RESULTS AND DISCUSSION Infiltration and Thermolysis of Polystyrene in Porous Si Templates. The synthetic strategy for thermolytic grafting of styrene to pSi is summarized in Scheme 1. The approach Scheme 1. Synthetic Procedure Used To Prepare StyrenicGrafted Compositesa
a
The freshly etched pSi matrix is infiltrated with polystyrene (MW = 20 kDa) from a toluene solution. Thermolysis under N2 depolymerizes the polymer, and the released styrenic and oligomeric fragments undergo a hydrosilylation reaction with the hydrogen-terminated pSi surface, grafting a significant quantity of polymer to the inner pore walls.
takes advantage of two chemical reactions: the ability of polystyrene to undergo thermal depolymerization to styrene monomers, oligomers, and polymeric radicals,36−38 and the ability of such species to undergo a thermal hydrosilylation reaction with the Si−H species at the pSi surface.40−42 The pSi templates were prepared by electrochemical etch of single crystal Si substrates in an electrolyte containing HF. The freshly prepared pSi surface is terminated with Si−H species in the form of SiH, SiH2, and SiH3 moieties, whose characteristic vibrational modes were confirmed in the FTIR spectrum: ν(Si− Hx) stretching vibrations at 2110 cm−1, δ(Si−H2) scissors B
DOI: 10.1021/acs.chemmater.5b03221 Chem. Mater. XXXX, XXX, XXX−XXX
Article
Chemistry of Materials mode at 910 cm−1, and ν(Si−Hx) deformation mode at 663 cm−1 (Figure 1A).39 These hydrides, as well as the underlying
this higher viscosity polymer. No obvious difference in properties of the grafted composites was observed for 10 kDa polystyrene relative to 20 kDa polystyrene. Characterization of Thermolytic Grafting by Infrared and Raman Spectroscopy. A pSi sample of 63% porosity was preloaded with polystyrene and thermolyzed at 400 °C for 1 h under a dry N2 atmosphere. The resulting composite was soaked in excess toluene for 2 h to remove nongrafted polystyrene residues, rinsed thoroughly with ethanol, and dried. The FTIR spectrum at this point (Figure 1B) showed several bands attributed to the styrenic aromatics, a loss of the bands associated with Si−H surface species (by 55−60%), and a substantial Si−O stretching band at 1028 cm−1.43 Thus, even though the sample was heated under a nitrogen atmosphere, adventitious water or oxygen was apparently present in sufficient quantities to oxidize the pSi sample. FTIR measurements of a pSi sample performed immediately after polystyrene casting showed an increase in the Si−O stretching band, and a control experiment involving thermolysis of a pSi sample in the absence of polystyrene resulted in an Si−O stretching band of similar amplitude. We conclude that each step in the procedure leads to some additional oxidation of the porous Si substrate. The presence of crystalline Si in the porous Si skeleton of the composite was confirmed by Raman spectroscopy (Si−Si lattice mode at 516 cm−1, Figure S2 in the Supporting Information). In order to confirm that the styrenic fragments generated in the thermolysis reaction were grafted to the pSi surface via Si− C bonds, the composite was treated successively with aqueous ethanolic solutions of 0.8% HF (for 10 min) and 3.3% HF (35 min). This type of HF treatment has been used previously as a test for grafted Si−C species, because HF selectively attacks and dissolves silicon oxides while leaving Si−C-grafted species intact.39,42 After HF treatment, the Si−O band in the FTIR spectrum was quite weak, indicating substantial removal of silicon oxide. The bands associated with styrenic vibrations became more apparent after HF treatment (Figure 1C and Figure S3 in the Supporting Information). The most distinctive bands at 698 and 757 cm−1 are attributed to the C−C and C− H out of plane bending modes, respectively.44 In addition, three characteristic C−C skeletal in-plane phenyl vibrational bands appear at 1600 cm−1, 1492 cm−1, and 1452 cm−1,44 and strong absorption bands assigned to asymmetric and symmetric C−H stretching vibrations of aliphatic −CH2− appear at 2923 and 2851 cm−1, respectively. The CC stretching band of the terminal alkene (RHCCH2) of (unreacted) styrene is expected to appear with medium intensity at 1630 cm−1. No significant band was observed in this region (Figure S3 in the Supporting Information), indicating that thermolysis and workup leaves no unreacted styrene monomer in the sample.45 The HF-treated samples also displayed bands characteristic of residual surface Si−H species. Mechanism of Thermolytic Grafting. The grafting reaction of Scheme 1 depicts addition of a styrenic radical fragment, implying a radical mechanism. Polystyrene is an amorphous and linear polymer formed via a free radical mechanism, and thermal degradation of polystyrene produces polymeric radicals along with styrene monomers and oligomers through depolymerization, chain scission, intermolecular transfer, and radical−radical interactions.36−38 Phenylic radical intermediates are relatively stable due to resonance delocalization of the unpaired electrons in the aromatic rings,46 which imparts antioxidant properties to many phenylic compounds.47 The phenylic radicals generated by thermal degradation of
Figure 1. FTIR spectra of the composite at selected stages of preparation: (A) freshly etched pSi before polystyrene infiltration; (B) after polystyrene infiltration, thermolysis (400 °C for 1 h under N2), and soaking in toluene for 2 h to remove excess unreacted polymer; (C) after treatment with 0.8% HF(aq) in 10% ethanol (10 min) and 3.3% HF(aq) in 10% ethanol (35 min) to remove adventitious silicon oxides. For the pSi sample represented by these measurements, the porosity reduces from an initial value of 63% to 42% for the composite (after thermolytic grafting and removal of oxide).
Si−Si lattice, are quite air sensitive, and under atmospheric conditions they slowly oxidize.39 In the first step of the procedure, this reactive surface is coated with a thin layer of polystyrene, by casting a toluene solution of the polymer into the pores and heating it slightly above the glass transition temperature to aid infiltration into the micro/mesoporous Si nanostructure. At this point the surface is expected to still be reactive toward oxygen or water, but it should be protected from degradation somewhat by the presence of the polymer barrier. The degradation of polystyrene occurs at temperatures >350 °C, and the reaction is controlled by temperature and heating time. Levin et al. reported that at 350 °C, 18−19% of the total mass volatilized is styrene, whereas at 450 °C styrene constitutes 50−51% of the total mass volatilized.37 At lower temperatures (350 °C, with complete loss of mass observed by 430 °C (10 °C/min temperature ramp, Figure S1 in the Supporting Information). The reaction in the present study was optimized over a temperature range of 380−420 °C and heating times of 0.5−2 h as a means of adjusting the quantity of polymer remaining in the pSi layer. We also prepared polystyrene-grafted composites using polystyrene of different molecular weights (10, 20, and 35 kDa) to optimize the process. The higher Mw polymer (35 kDa) gave less consistent coverage, presumably due to less effective pore infiltration of C
DOI: 10.1021/acs.chemmater.5b03221 Chem. Mater. XXXX, XXX, XXX−XXX
Article
Chemistry of Materials
pores (methanol in the present case), and the pSi skeleton as inputs. In this work the index of air was taken as 1.00, and the index of methanol was measured to be 1.329. The refractive index of the pSi skeleton can vary between 3.8 and 1.55, depending on its extent of oxidation, surface chemistry, and other factors. In addition, the polymer grafted to the pore walls adds a second refractive index component. The refractive index of polystyrene is in the range 1.57−1.61. Based on optical measurements on a number of samples and comparison with thickness measured by SEM (Figure S5 in the Supporting Information), a value of 2.3 was chosen for the average refractive index of the skeleton. While this number is only an approximation, it provided an internally consistent means to compare samples with differing quantities of polymer grafted to the skeleton. Thus, the values of residual porosity quoted in this work should be considered as semiquantitative measures of the relative amount of polymer loaded in each sample. Effect of Thermolysis Temperature on Polymer Content. The initial porosity of the pSi templates used in this portion of the study was 64 ± 1%. Each sample was infiltrated with a 10% by mass solution of polystyrene in toluene. We found that 10% polystyrene gave the most consistent results; a 15% solution was more viscous and did not always fully infiltrate the pores, and a 5% solution yielded incomplete and heterogeneous infiltration. The toluene was allowed to evaporate, and the samples were subjected to thermolysis for various temperatures and times, as indicated in Figure 2 and Table S1 in the Supporting Information. After removal of the soluble residues with toluene, the final porosity was then measured by SLIM. As expected the porosity of the composites was smaller than the 64% porosity of the original template, ranging from 10 to 60%, depending on thermolysis conditions. This smaller final porosity indicates that some residue of the polymer remained in the template after thermolysis. In general, the final porosity of the pSi composite increased with increasing thermolysis temperature or with increasing thermolysis time for a fixed temperature (380 °C). This is consistent with the mechanism of degradation of the polymer mentioned above and verified by TGA measurements (Figure S1 in the Supporting Information); at higher temperatures or longer times, more of the polystyrene in the template is degraded and volatilized. Concomitant with this degradation process, a portion of the reactive polymer fragments become grafted to the pSi framework and immobilized. The porosity data cannot be explained by adsorption of a simple monolayer of styrene, which would not decrease the porosity to the extent seen. Thus, large quantities of insoluble polymeric or oligomeric species must be deposited within the pores, particularly at the lower thermolysis temperatures. Although the high temperature thermolysis is important to provide significant coverage of grafted material, the grafting reaction appears to proceed to some extent even at lower temperatures. A control experiment was performed to see if the 110 °C temperature used during the polystyrene infiltration process was sufficient. After infiltration and heating at 110 °C for 10 min, followed by extensive (48 h) rinsing in toluene, FTIR measurements detected residual styrenic fragments on the pSi surface. These fragments were not removed with an ethanolic HF rinse, strongly suggesting that some material became grafted to the pSi surface even at 110 °C. The coverage was low for this material, as the porosity of the infiltrated sample only decreased from 64% (before infiltration) to 61%.
polystyrene can therefore be expected to persist sufficiently to react with the Si−H or free radicals on the hydrogenterminated pSi surface.48−50 Silicon-based radicals are relatively easy to generate on silicon surfacesfor example mechanical scribing of a silicon wafer generates surface radicals that participate in hydrosilylation reactions with alkenes.14,15,51,52 In addition, alkenes (such as the styrene monomer) are known to undergo thermal hydrosilylation reactions on pSi surfaces, adding across the Si−H bond at relatively low temperatures.16 Thus, either thermal or radical mechanisms might be responsible for grafting of phenylic fragments to the pSi surface under the thermolytic reaction conditions used. The result in the present case is that the grafting reaction attaches oligomers and polymers, in addition to possibly simple styrenic monomers, to the surface. Optical Porosity Measurements. The hypothesis that the grafting reaction attaches higher order oligomers to the pSi surface is supported by porosity measurements, which were made as a function of thermolysis temperature and time (Figure 2). Porosity was quantified by spectral reflectance measure-
Figure 2. Final measured porosity of pSi composites as a function of thermolysis temperature for the thermolytic grafting of infiltrated polystyrene (10 wt %). The initial porosity of all the pSi templates (prior to polymer infiltration) was 64 ± 1%. Thermolysis was carried out under flowing dry N2. Black squares, red circles, and blue triangles indicate different thermolysis times, as indicated. Error bars represent average (and standard deviation) of three independent samples.
ments using a technique known as the spectroscopic liquid infiltration method (SLIM), described in detail elsewhere.39 In brief, SLIM provides a nondestructive means to quantify porosity and thickness of a porous film based on two optical reflectance measurements: one with the sample in air and one with the sample immersed in a fluid of known refractive index. In the present case the analysis was performed as follows: the Fabry-Pérot interference fringes in the reflectance spectrum of the composite layer was recorded (Figure S4a in the Supporting Information). Fourier transformation of the reflectance spectrum (inverse wavelength vs intensity) directly yielded the effective optical thickness, 2nL, of the pSi layer, where n is the average refractive index and L is the physical thickness of the layer. From the values of nL determined with the sample pores filled with air and with methanol (Figure S4b in the Supporting Information), the open porosity and physical thickness of the layer were calculated using a least-squares fit to the Bruggeman effective medium model. The SLIM model requires values of the refractive index of air, the liquid filling the D
DOI: 10.1021/acs.chemmater.5b03221 Chem. Mater. XXXX, XXX, XXX−XXX
Article
Chemistry of Materials
Silicon Oxide Content in Composites and Selective Removal with HF(aq). As discussed above, the FTIR analysis indicated that a substantial quantity of silicon oxide is incorporated into the pSi composite during thermolysis, and the oxide could be selectively removed by treatment with aqueous ethanolic HF solutions. Figure 4 presents FTIR spectra for a single composite made from a pSi template with initial porosity of 63% and a final porosity (after styrenic grafting) of 41%. The sample was treated with aqueous ethanolic solutions containing successively increasing concentrations of HF (ranging from 0.8 to 40%) and/or treatment times. The most obvious trend observed was a substantial reduction in the magnitude of the band associated with Si−O at 1028 cm−1. Relative to the Si−O band, the styrenic bands decreased much more gradually with progressive HF treatments. In between treatments and prior to each spectral acquisition, the sample was rinsed thoroughly with ethanol and toluene, so the styrenic residue observed by FTIR must have become either highly insoluble (cross-linked) or chemically grafted to the pSi surface. A control experiment performed with polystyrene on flat (nonporous) silicon showed that the styrenic residue is readily soluble in toluene under these conditions. Identical experiments performed with a composite containing more styrenic residue (final porosity after styrenic grafting 21%) exhibited similar changes in the FTIR spectra, although relatively less oxide and styrenic species were removed by the treatment (Figure S7 in the Supporting Information). Stability of Composites in Alkaline Media. Porous Si readily dissolves in caustic solutions, but it shows a marked increase in stability when modified with organic species via hydrosilylation or other grafting reactions.13,18,42,55 This enhanced stability is also seen with the thermolysis-modified samples of the present work. Alkaline stability was tested in aqueous ethanolic solutions containing 0.02 or 1.0 M NaOH. Degradation of each sample was monitored in real time by RIFTS as described above, and the optical thickness nL was calculated from the optical reflectance spectrum. Figure 5 compares the fractional change in optical thickness of three composites of varying polymer content as a function of exposure time to a pH 12 solution. Here the fractional change in optical thickness is defined as56
The FTIR measurements indicate that the packing density of hydrocarbon chains increased with increasing quantity of grafted styrenic species in the pores. The band position of the methylene symmetric and asymmetric C−H stretches has been shown to provide an indication of the conformational order (or packing density) of alkyl chains.53 For instance, liquid hydrocarbons exhibit higher frequency methylene stretching vibrations than crystalline hydrocarbons with conformationally ordered alkyl chains.54 In the present work, the asymmetric methylene stretching bands from the samples exhibited values of 2923, 2920, 2919, and 2917 cm−1, for sample porosity values of 48%, 36%, 28%, and 21%, respectively (Figure S6 in the Supporting Information). This red shift with decreasing porosity indicates that the conformational order (or packing density) of polymer chains increased with increasing styrenic content. Characterization of Composite Morphology by Scanning Electron Microscopy. Plan-view and cross-sectional SEM images support the hypothesis that the thermolytic process coats the inner pore walls with substantial quantities of polymer. Figure 3 shows SEM images of several composite
Figure 3. Plan-view and cross-sectional SEM images of pSi host matrix before (A) after (B−D) infiltration and thermolytic grafting of various quantities of polystyrene: (A) pSi template (porosity = 63%); (B−D) samples similar to (A) but with increasing polymer content. For these samples, porosity is reduced to 42% (B), 35% (C), and 17% (D). All scale bars represent the same length of 300 nm. Insets show magnified views, revealing the gradual decrease in pore size with increasing polymer deposit.
(nLt − nL0) ΔnL = nL0 nL0
(1)
where nLt is the value of nL measured at time t after the introduction of the alkaline solution, and nL0 is the value of nL measured with the sample immediately after introduction. The optical thickness measurement provides a means of assessing the relative stability of a composite because the amount of silicon or silicon oxide in the porous skeleton is related to the value of nL.39 The refractive index of silicon or silicon oxide is larger than the refractive index of the alkaline solution used in these dissolution experiments, so as the porous skeleton dissolves and becomes replaced with solution, the average value of the refractive index of the film is expected to decrease, and it follows that the measured optical thickness value nL will also decrease. As might be expected, the stability of the composites toward attack by these caustic aqueous solutions increased substantially with increasing polymer content. Thus, a composite with residual porosity of 44% displayed a decrease in nL of 1.0% within 60 min in pH 12 solution, while a composite with residual porosity of 39%
samples prepared with different degrees of styrenic content. The pore size of the pristine pSi template (prior to polymer infiltration, porosity 63%) was approximately 30 nm (Figure 3A). There was no obvious difference in pore appearance for a composite prepared with low polymer content (sample porosity 42%, Figure 3B) relative to the original pSi template. A sample with higher polymer content (porosity 35%, Figure 3C) showed clear signs of a reduction in average pore diameter, and this became more apparent with increasing polymer content (porosity 17%, Figure 3D). Energy dispersive X-ray (EDX) maps of the cross-sectional SEM images verified the presence of a uniform distribution of carbon throughout the porous layer (Figure S5 in the Supporting Information). Consistent with the FTIR analysis, the EDX maps indicated substantial increases in carbon and oxygen content after thermolytic grafting of the pSi samples. E
DOI: 10.1021/acs.chemmater.5b03221 Chem. Mater. XXXX, XXX, XXX−XXX
Article
Chemistry of Materials
Figure 4. (A) FTIR spectra of a composite (template porosity 63%, composite porosity 41%) exposed to progressively more aggressive HF treatments. Sample was removed from the indicated solution, rinsed with ethanol and toluene, and dried prior to spectral acquisition. (B) Expanded view of the silicon oxide stretching region of the FTIR spectrum for the same series. The concentration of polystyrene in the solution used to load the template was 15%, and the thermolytic conditions were 380 °C for 1 h under dry N2 (flow rate 2 LPM).
studies,33,57,58 pSiO2 displayed slow dissolution in the pH 12 solution (Figure 5), while the “bare” pSi template dissolved immediately (not shown). The data demonstrate that composites with a higher degree of styrenic content (P < 35%) are substantially more stable than thermally oxidized pSiO2 samples. Sample stability was also assessed in a more aggressive alkaline solution, at pH 14 (Figure S8 in the Supporting Information). The composites were less stable in this more highly alkaline solution. Only the composite with the highest styrenic content (P = 17%) was more stable than the thermally oxidized pSi sample. Hydrophobicity of the Composites. The wettability of the various composites was evaluated by measuring the water contact angle (CA). Figure 6A shows the measured CA as a function of sample residual porosity, which is inversely related to styrenic content as discussed above. The composites exhibited contact angles in the range of 110135°, substantially larger than that of the pure polystyrene starting material (89°). This behavior is consistent with the Cassie and Baxter relationship,59 which predicts that the apparent contact angle (CA) of water on a rough or porous surface with low surface energy increases with decreasing solid surface fraction or increasing air surface fraction.60 The composite samples of residual porosity 10 to 40% followed the Cassie rule, displaying a water CA that increased linearly with increasing porosity (Figure 6A). This indicates that the hydrophobicity of these samples can be accounted for by the increasing surface fraction of air at the porous surface. For samples with porosity >40%, the water CA decreased with increasing porosity, and optical reflectance measurements showed an increase in optical thickness when the porous layer was submerged in water, suggesting a transition from Cassie to Wenzel behavior.61,62 Wenzel behavior corresponds to infiltration of water into the pores, which is favored as pore size increases and/or the pore walls become more hydrophilic. Indeed, thermal treatment is expected to increase the hydrophilicity of the pSi composite for two reasons. First, the longer thermal treatment and higher temperatures used to prepare the higher porosity composites (samples of porosity >40%) are expected to increase the fraction of SiOx in the PS matrix, and second, the thermal
Figure 5. Comparison of alkaline stability of pSi composites of different porosities, measured as percent relative change in optical thickness as a function of time exposed to pH 12 aqueous solution (containing 10% ethanol). Each porosity (P) value indicated in the figure corresponds to the measured porosity of the as-prepared composite; to prepare composites with different final porosities, samples were subjected to different thermolysis temperature−time conditions as described in the text. All samples were prepared from pSi templates of porosity 64 ± 1%. Thus, a lower final porosity corresponds to a sample with higher styrenic content. Sample denoted as “Thermally oxidized pSi” was prepared by air-oxidation of a pSi template (containing no polystyrene) at 800 °C for 1 h. Prior to introduction of the basic solutions, each sample was first wetted with deionized water containing 10% ethanol.
exhibited only a 0.25% decrease, and a composite with residual porosity of 35% exhibited less than a 0.01% decrease (Note that a lower composite residual porosity value corresponds to a higher polymer content. All these samples were prepared from pSi templates of nominally the same original porosity.). For comparison, a thermally oxidized pSiO2 film and an empty pSi template were subjected to the same degradation conditions. The empty pSi template was prepared using the same etching protocol and thermolytic conditions (400 °C for 1 h under dry N2) as the polystyrene-filled composites, though with no added polymer. The pSiO2 sample was prepared by air oxidation of a pSi template at 800 °C for 1 h. Consistent with prior F
DOI: 10.1021/acs.chemmater.5b03221 Chem. Mater. XXXX, XXX, XXX−XXX
Article
Chemistry of Materials
Figure 6. (A) Water contact angle as a function of styrenic content in pSi template, measured as residual porosity of the pSi-polystyrene composite. Each sample was prepared by loading polystyrene in a pSi template and then subjecting the sample to thermolytic grafting as described in the text. The quantity of styrenic material remaining in the pSi template is inversely related to the residual porosity. Residual porosity of a given sample was controlled by thermolysis temperature (range 380−420 °C), time (range 0.5−2 h), and polystyrene concentration used in the loading solution (range 5−10 wt %). For this series of samples, the porosity of the empty pSi template was 63−65%. The inset shows the water CA measured on a flat polystyrene sample (polystyrene spin-coated on a polished silicon substrate). (B) FTIR spectra in the Si−O stretching region (1028 cm−1 band assigned to Si oxide) for five representative composites. Sample residual porosity as indicated.
consistent with the quantity of styrenic residue in each film, as the increasing average refractive index is expected to increase the stop band of the rugate filter.7,73,75 The apparent colors of the pSi rugate filter composites were quantified by spectral reflectance measurements (Figure 7B). The pristine pSi rugate filter template displayed a strong stop band centered at 560 nm, and the stop band of a control rugate filter, which had been subjected to thermolytic conditions but did not contain polystyrene, blue-shifted to 530 nm. The blue shift of the control pSi template is caused by a decrease of the effective refractive index of the pSi layer due to Si oxide formed under the thermolytic conditions (420 °C, 30 min).74 Thermolytic grafting of polystyrene resulted in a distinct red shift of the stop band and an increase in the width of the stop band. The residual porosity of the rugate composites was estimated (by SLIM) as 34%, 45%, and 54% for the red, orange, and green samples, respectively. The porosity of a pristine pSi rugate template was 65%, and the measured porosity of the control pSi rugate filter was 66%. The water CAs were 130° ± 3°, 138 o ± 2°, and 132 o ± 3° for the red, orange, and green samples, respectively. These data are consistent with the CA measurements made on pSi single layer samples of Figure 5, where the CA maximizes at ∼40% residual porosity. For the rugate filter samples, the composite with residual porosity of 45% exhibited the largest CA, while the rugate filter composites with lower (34%) or higher (54%) porosity displayed smaller CA due to the interplay of air pocket fraction, pore size, and hydrophilicity as discussed above. As with the pSi single layers, FTIR measurements indicated greater silicon oxide content relative to styrenic residues in the films with 54% porosity (Figure S9 in the Supporting Information). Contact angles were also measured on the samples using formamide as a liquid. As compared to water (γL = 72.8 mJ m−2; γLD = 21.8 mJ m−2, γLP = 51 mJ m−2), liquid formamide (γL = 58.2 mJ m−2; γLD = 39.5 mJ m−2, γLP = 18.7 mJ m−2) has a relatively higher dispersion (nonpolar) component in its van der Waals attractions.76−78 Here, γLD and γLP represent the dispersive and polar components of the liquid surface tension
depolymerization process can also be expected to induce oxidation of the residual polymer.63,64 Both of these reactions should increase hydrophilicity of the composite. Consistent with this interpretation, FTIR spectroscopic measurements indicate an increase in silicon oxide content in the pore walls with increasing residual porosity of the composite (Figure 6B). When normalized to the intensity of the styrenic vibrational band at 698 cm−1, the Si oxide band at 1028 cm−1 increased with increasing composite porosity for P > 40%, while there was no significant change in the oxide band intensity for samples with residual porosity 40% can be attributed to either one or both of these factors.62,65,66 Composite Photonic Crystals. In order to assess the utility of the grafting reaction for chemical sensing applications, pSi templates were prepared in the form of rugate filter photonic crystals. The sharp stop band of these types of structures provides a characteristic structural color that changes in the presence of various analytes and can be monitored by spectrometer, by digital imaging, or by the naked eye.5,67−73 The photonic crystals were prepared by modulating the anodic current during electrochemical preparation of the pSi templates, which results in a spatially modulated porosity.74 Figure 7A shows a schematic representation of the procedure used to prepare the composites. Three composite structures are presented, which differ in the temperature−time profile used in the thermolytic grafting step. As discussed above, this results in differring styrenic content in the pores, which, in the case of the rugate filter templates, generates different colors. A photonic composite thermolyzed at 380 °C for 1 h appeared red under ambient conditions, while a composite thermolyzed at 400 °C for 1 h appeared orange, and one thermolyzed at 420 °C for 30 min appeared green. The apparent colors are G
DOI: 10.1021/acs.chemmater.5b03221 Chem. Mater. XXXX, XXX, XXX−XXX
Article
Chemistry of Materials
Figure 7. (A) Schematic illustration of thermolysis of polystyrene preloaded into a pSi template containing a spatial porosity modulation (rugate filter). Three thermolysis conditions are shown, resulting in different quantities of styrenic residues grafted to the inner pore walls as illustrated. Digital photographs of each of the three resulting sample types are shown (approximately 1.5 × 1.5 cm). At the far right are images and values representative of the water contact angle for each sample. (B) Relative intensity of reflected light spectrum of the pSi rugate template and composites as indicated. Porosity values given in the annotations are final residual porosity for each sample; template porosity for all samples was 65% prior to polymer infiltration/processing. The control rugate filter without polystyrene was prepared by thermal treatment at 420 °C for 30 min under inert atmosphere. In addition to the stop band peak characteristic of the multilayer, the spectra display well-resolved Fabry−Perot interference fringes due to thin film optical interference across the porous layer. (C) Relative change in the quantity nL, extracted from the thin film optical interference portion of the spectrum, as a function of infiltration time of liquid ethanol (n = 1.361) or isopropyl alcohol (n = 1.378), as indicated. The quantity nL0 is the optical thickness measured in air prior to introduction of the test liquid.
(γL), respectively. The CA of formamide79 on a control spincoated polystyrene film was 72°, while the rugate composites of residual porosity 34, 45, and 54% exhibited CA values of 100°, 105°, and 17°, respectively with formamide. In this case, a substantial change in CA from 105° to 17° was observed for the 45 and 54% residual porosity samples, respectively, suggestive of a sharp transition to Wenzel behavior. This is attributed to more substantial dispersive interactions between the formamide and the silicon oxide layer, relative to the water-silicon oxide interaction.79−81 Liquids with lower surface energy such as ethanol and isopropyl alcohol readily infiltrate the porous nanostructures. Wenzel behavior is clearly indicated in this case by the large increase in the refractive index of the porous matrix, and the rate of infiltration of these liquids into the rugate filter composites showed a distinctive dependence on composite porosity. Figure 7C shows the optical changes monitored as a
function of time. The rugate samples were mounted in a sealed cell equipped with an optical microscope and spectrometer focused on a ∼1.0 mm-diameter spot on the pSi surface, which allowed real-time monitoring of the spectrum during liquid infiltration. The control rugate filter (pSi template with no polymer, porosity 66%) exhibited an instantaneous increase in optical thickness (nL) when the liquid replaced air in the porous film. In the case of the rugate composites with high styrenic content, infiltration of liquids was more gradual. When ethanol was used as a test liquid, the rate of increase in nL for composite samples of residual porosity 54% and 45% was faster than the time resolution of the experiment (
