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Role of amphiphilic block copolymer composition on pore characteristics of micelle-templated mesoporous cobalt oxide films Siyang Wang, Pattarasai Tangvijitsakul, Zhe Qiang, Sarang M Bhaway, Kehua Lin, Kevin A. Cavicchi, Mark Deland Soucek, and Bryan D. Vogt Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b01026 • Publication Date (Web): 03 Apr 2016 Downloaded from http://pubs.acs.org on April 6, 2016

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Role of amphiphilic block copolymer composition on pore characteristics of micelle-templated mesoporous cobalt oxide films Siyang Wang, Pattarasai Tangvijitsakul, Zhe Qiang, Sarang M. Bhaway, Kehua Lin, Kevin A. Cavicchi, Mark D. Soucek, and Bryan D. Vogt* Department of Polymer Engineering, University of Akron, Akron, Ohio 44325-0301, United States KEYWORDS: Soft templating, SBA-16, nanoporous film

Block copolymer templating is a versatile approach for the generation of well-defined porosity in a wide variety of framework chemistries. Here, we systematically investigate how the composition of a poly(methoxy poly[ethylene glycol] methacrylate)-block-poly(butyl acrylate) (PMPEG-PBA) template impacts the pore characteristics of mesoporous cobalt oxide films. Three templates with a constant PMPEG segment length and three different hydrophilic block volume fractions of 17%, 51% and, 68% for the PMPEG-PBA are cooperatively assembled with cobalt nitrate hexahydrate and citric acid. Irrespective of template composition, a spherical nanostructure is templated and elliptical mesostructures are obtained on calcination due to

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uniaxial contraction of the film. The average pore size increases from 11.4 ± 2.8 nm to 48.5 ± 4.3 nm as the length of the PBA segment increases as determined from AFM. For all three templates examined, a maximum in porosity (~35 % in all cases) and surface area is obtained when the precursor solids contain 35-45 wt % PMPEG-PBA. This invariance suggests that the total polymer content drives the structure through interfacial assembly. The composition for maximizing porosity and surface area with the micelle templating approach results from a general decrease in porosity with increasing cobalt nitrate hexahydrate content and the increasing mechanical integrity of the framework to resist collapse during template removal / crystallization as the cobalt nitrate hexahydrate content increases. Unlike typical evaporation induced selfassembly with sol gel chemistry, the hydrophilic/hydrophobic composition of the block copolymer template is not a critical component to the mesostructure developed with micelletemplating using metal nitrate-citric acid as the precursor.

Introduction Since the report of the MCM family of ordered mesoporous silicates by Mobil researchers in 1992,1,

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there has been significant interest in advancing these materials to other material

frameworks, such as carbon,3 metals,4 sulfides,5 and transition metal oxides,6 and subsequently exploring the suitability of these materials for a diverse set of applications7 ranging from drug delivery8 to catalysis9 to separations10 to energy generation and storage.11 Two dominant routes have been explored for the synthesis of these ordered mesoporous materials: hard12 and soft13 templating. Soft templating utilizes organic molecules, such as surfactants and block copolymers, (BCPs) to direct the ordered porous structure,13 while these inorganic porous materials are commonly utilized to control the nanostructure in hard templating.12 Soft templating provides efficiency advantages and generalized synthesis strategies have been developed for a variety of


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framework chemistries,14, 15, 16 but the soft templating method can be limited by the template6 due to its limited thermal stability and balancing assembly with sol gel reactions. Stucky and coworkers developed a non-hydrolytic route using metal salts as the precursor to fabricate mesoporous materials with a wide variety of metal frameworks including mixed oxides,17 but the synthesis is quite slow. More recently, we have demonstrated that soft templated mixed oxides can be rapidly fabricated using a carbonate intermediate,18 following the metal nitrate-citric acid complex precursor route developed by Kraehnert and coworkers.15, 19 This metal nitrate-citric acid complex route for fabrication of mesoporous materials provides several advantages in comparison to the more traditionally examined sol-gel method. For example, the ordering of mesosphase occurs first, then the precursors are thermally reacted to the carbonate.20 The separation of these processes avoids challenges with balancing self-assembly and reaction to generate the desired highly ordered mesostructure of the metal oxide. However, little is known about how the template selection impacts the mesostructure, except that the hydrophobic contrast for commercially available Pluronics is insufficient to generate an ordered mesostructure.19 With recent advances in controlled radical polymerizations,21 novel BCPs have been increasingly examined as a route to template large pore materials.18, 22, 23, 24, 25, 26 In general, the phase diagram of the BCP provides guidance to the expected mesostructure based on the relative hydrophilic content, which includes the precursors.27 However with high molecular weight BCPs, the mesostructure can be kinetically trapped, especially in cases of high glass transition temperature (Tg) segments or strong interactions between the block copolymer and precursors.28 In the design of BCP templates, the favorable, but weak interaction between most hydrophilic precursors and poly(ethylene oxide) (PEO) has led to PEO being a preferred hydrophilic segment.29 However, the sequential anionic polymerization of PEO-containing block


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copolymers requires a demanding synthetic protocol, especially when compared to the preparation of hydrophilic block copolymers through controlled free radical polymerization. End functionalization of commercially available PEO provides a route to synthesize block copolymer templates through controlled free radical polymerization for a variety of frameworks.30, 31, 32, 33 In this case, the molecular weight of monofunctional PEO that is commercial available is quite limited. In this work, we systematically examine how the role of the relative length of the hydrophobic segment of a poly(methoxy poly[ethylene glycol] methacrylate)-block-poly(butyl acrylate) (PMPEG-PBA) template impacts the morphology and pore characteristics of mesoporous cobalt oxide films fabricated by co-assembly with cobalt nitrate hexahydrate and citric acid. The monomer selection for the template offers several advantages: first, PEO-like interactions are obtained with PMPEG, but this is a commercially available, liquid monomer, capable of undergoing free radical polymerization, which simplifies the polymerization protocol. Second, poly(butyl acrylate) is quite hydrophobic and its glass transition temperature (Tg) is well below ambient temperature to minimize the non-equilibrium effects due to the dynamics of the BCP template. Here, the hydrophilic volume fraction of the PMPEG-PBA is varied from 17 % to 68 %. Despite this large variance, the best-ordered and highest porosity mesoporous cobalt oxide films are obtained at an invariant blend composition (PMPEG-PBA, cobalt nitrate hexahydrate, and citric acid) irrespective of the BCP template selected. Moreover, the total porosity is nearly independent of the BCP template. These results indicate that the template composition does not significantly alter the ordering or porosity of these materials.

Experimental Section


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Materials: Cobalt (II) nitrate hexahydrate (98%), Citric acid (≥99.5%), tetrahydrofuran (THF, anhydrous, 250 ppm BHT inhibitor, ≥99.9%), methoxy poly (ethylene glycol) methacrylate (MPEGMA, 475 g mol-1), n-butyl acrylate (>99%), N, N-dimethylformamide (DMF) (anhydrous, 99.8%), ethanol (≥99.5%), sulfuric acid (95.0%-98.0 wt%), hexane (anhydrous, 95%), and toluene (anhydrous, 99.8%) were purchased from Sigma-Aldrich. Ethanol (200 proof) was purchased from Decon Labs. Hydrogen peroxide (30 wt% aqueous) was purchased from EMD Chemicals Inc. Deuterated chloroform (CDCl3, D, 99.96%) was purchased from Cambridge Isotope Laboratories, Inc. The initiator, 2,2’-Azobis(isobutyronitrile) (AIBN) (Aldrich, 98%), was purified by recrystallization from methanol and the reversible addition–fragmentation chain-transfer (RAFT) agent, 4-cyanopentanoic acid dithiobenzoate (CPADB), was synthesized following established methods.34 Amphiphilic block copolymers (BCPs) of poly(methoxy poly[ethylene glycol] methacrylate)-block-poly(butyl acrylate) (PMPEG-PBA) were synthesized by reversible addition fragmentation chain transfer (RAFT) polymerization following a previously reported protocol.20 Briefly, PMPEG, which subsequently acts as a macromolecular-RAFT agent was synthesized using MPEGMA as the macromonomer, CPADB as the RAFT agent, and AIBN as an initiator in DMF at 65 °C. Characteristics of this macromolecular-RAFT agent were determined by size exclusion chromatography (SEC) using an instrument with a refractive index and a light scattering detector: number-average molecular mass (Mn) = 2.50 × 104 g mol–1 and molecular weight dispersity (Đ) = 1.3. This PMPEGMA was then used to synthesize three BCPs with varying block lengths of PBA at 65 °C in DMF. The nomenclature used throughout this manuscript for these BCPs is PMPEG-PBA(x-y) where x and y are the average Mn in kg/mol for


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the PMPEGMA and PBA segments, respectively. The SEC chromatographs for the BCPs are shown in Figure S1. The volume fraction of PMPEGMA, fPMPEGMA, is estimated using the molar composition determined from 1H NMR (Figure S2) and assuming densities of 1.08 g/cm3 and 1.087 g/cm3 for PMPEGMA and PBA, respectively.35

Fabrication of templated mesoporous cobalt oxide films: Double-side polished silicon wafers (500 ± 25 µm thick, 1-10 Ω-cm, Silicon, Inc.) were cleaned using Piranha solution (H2SO4: H2O2=3:1 v/v) at 90 °C for 45 min. Subsequently, the silicon wafers were rinsed three times with deionized (DI) water and dried with nitrogen gas before casting the films. For the casting solution, cobalt (II) nitrate hexahydrate and citric acid were dissolved in a molar ratio of 2:1 nitrate:acid (compositions listed in Table S1) in 1.125 g ethanol using a vortex mixer for 30 min. A template solution was prepared by dissolving 0.26 g PMPEGMA-b-PBA in 3.375 g THF. The ethanoic solution was added dropwise into the PMPEGMA-b-PBA THF solution under gentle stirring. The composition of casting solution was controlled by the amount of cobalt (II) nitrate hexahydrate and citric acid solution added to the PMPEGMA-b-PBA. The PMPEGMA-b-PBA micelle-templated thin films were fabricated by flow coating this solution at a constant velocity of 28 mm/s.36 The cobalt nitrate-citric acid complex was converted to cobalt carbonate by calcination at 200 °C for 1 h in a preheated muffle furnace (Ney Vulcan 3-130). The film was cooled down to room temperature before removing from the muffle furnace. Calcination at elevated temperatures led to conversion to cobalt oxide and complete removal of the block copolymer template: 325 °C for 45 min for PMPEG-PBA(2512) and PMPEG-PBA(25-34) templates, while a higher temperature (375 °C) for 45 min was


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required to fully remove the MPEG-PBA(25-126) template in the muffle furnace. The film was subsequently quenched rapidly to room temperature using a metal heat sink.

Characterization: The decomposition temperature of the three block copolymer templates was determined using thermogravimetric analysis (TGA, TA Instruments, TGA-Q50) up to 700 °C at a heating rate of 10 °C/min using air as the environment. The chemistry of the films was assessed using Fourier Transform Infrared spectroscopy (FTIR, Thermos Scientific Nicolet iS50 FITR Spectrometer) with the Deuterated TriGlycine Sulfate (DTGS) detector using a resolution of 4 cm-1 and averaging over 512 scans in transmission through the silicon wafer. A variable angle spectroscopic ellipsometer (UV-Vis-NIR: 250-1700 nm, VASE M-2000, J.A. Woollam Co.) was used to determine the film thickness by fitting the ellipsometric angles (∆ and ᴪ) to a simple stack consisting (from the bottom) of silicon, a native oxide layer consisting of 10 Å interfacial oxide layer and 8 Å SiO2 layer,37 and an appropriate optical model for the film of interest. For the as-cast composite and cobalt carbonate films, the optical properties (n and k) of the films are adequately described by the Cauchy model. For the cobalt oxide film, the absorbance in the UV is modeled with Gaussian and Lorenzian oscillators within the constructs of the general oscillator (GenOsc) model (WVase softwave, J. A. Woollam). Ellipsometric porosimetry (EP)38 was used to determine the pore size distribution and surface area for these mesoporous films. Both adsorption and desorption isotherms were measured using ethanol as the probe solvent with the vapor pressure (P) controlled by two mass flow controllers (MSK-146C-FF000-1) with near saturated solvent vapor (P0) and dry air streams with a constant total fluid flow of 800 mL/min for the EP measurements. The relative volume fraction of


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absorbed ethanol in the films is calculated by Lorentz-Lorenz effective medium approximation (EMA) as:39 V = V

n − 1

− 1 −

+2 n + 2 n −1 n + 2

where n is the refractive index at each P, no is the refractive index of the neat mesoporous film prior to any ethanol absorption, and nethanol (1.36) is the refractive index of condensed ethanol. The refractive indices at 632 nm were used in all calculations. The pore size distributions were estimated using a modified Kelvin equation39 that accounts for ellipsoidal anisotropy, p (= a/b, where a and b are the length of the major and minor axes of the ellipse). The anisotropy, p, is estimated based on uniaxial pore contraction40 using the ratio of the thickness of the mesoporous cobalt oxide to that of the as cast film. The pore size distribution was estimated as: !

%

-

−γV cosθ 1 + "# $ & = RTln (- ) '

.

where γ is the surface tension of ethanol (0.0224 N m-1), VL is the molar volume of the ethanol (0.584×10-4 m3 mol-1), Ө is the contact angle of ethanol (~0°)7 on cobalt oxide, C1 directly related to the eccentricity factor, E=(8

4 # 74)# 8

456 23 ( ) 476 6

is

, rk is the Kelvin radius, R is the gas constant,

and T is the absolute temperature for the adsorption. The physical properties of ethanol were assumed to be unchanged from the bulk system for these calculations. The surface area (S) of

of t-plots (see Supporting Information) at low partial pressures, ?@ and ?A are the densities of ethanol in liquid and gas states (?@ =0.78522 g/cm3, ?A =1.59×10-3 g/cm3).


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Atomic force microscope (AFM, Dimension ICON, Veeco) using high frequency probes (Nanosensors, PPP-NCC-50) was used to assess the surface morphology of these films. The AFM was operated in tapping mode at a scan rate of 0.5 Hz. The AFM micrographs were flattened and subsequently analyzed using Fast Fourier Transforms (FFTs) of the height image. Grazing incidence small angle X-ray scattering (GISAXS) was performed at the 8-ID-E beamline at the Advanced Photon Source (APS) of Argonne National Laboratory. The films were illuminated with 14 keV radiation at incident angles between 0.1° to 0.2°. The off-specular scattering was recorded with a Pilatus 1MF pixel array-detector (pixel size = 172 µm) with a sample to detector distance of 2164.8 mm. 1D line cuts at qz = 0 were used to better illustrate the in-plane order of the films. The in-plane (x-y) domain spacing, d, was calculated as d = 2π/q*, where q* is the position of the first order diffraction peak for the ordered mesostructure.

Results and Discussion Three different PMPEG-PBA BCPs were prepared for studying their ability to template the synthesis of cobalt oxide. Their chemical structure and schematic chain architecture are shown in Figure 1A and 1B, respectively. The characteristics of these three polymers are listed in Table 1.


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Figure 1. (A) Chemical structure of the block copolymer template, PMPEG-PBA, used in this study. (B) Schematic representation of the difference in the structure between the three PMPEGPBA copolymers. The length of the hydrophilic PMPEG block (blue) is held constant while the chain length of hydrophobic PBA block (green) is varied. Table 1. Characteristics of the poly(methoxy poly[ethylene glycol] methacrylate)-blockpoly(butyl acrylate) copolymers. Mn (kg/mol)

Đ

xMPEGMA

fMPEGMA

PMPEG-PBA(25-12)

37

1.2

0.281

0.68

PMPEG-PBA(25-34)

59

1.3

0.115

0.51

PMPEG-PBA(25-126)

151

1.2

0.047

0.17

For the micelle templating of mesoporous materials, selection of the temperatures for the reactions converting the metal nitrate citric acid complex to the metal carbonate and subsequently to the metal oxide is dependent upon the metal center(s) of interest15, 18 and the


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degradation temperature of the BCP template to ensure the templated structure is maintained through the final calcination step.20 As shown in Figure S3, the degradation of the PMPEG-PBA (on the basis of the mass loss) appears to require higher temperatures as Mn increases. FTIR (Figure 2) confirms the higher thermal stability of the PMPEG-PBA(25-126) through the different stages of processing. The evolution in the structure can be determined by the loss of peaks associated with the BCP template and the precursors. For the as-cast film (Figure 2A), the broad peak near 3300 cm-1 is attributed to O-H stretch in citric acid in PMPEG-PBA. On heating to 200 °C (Figure 2B), these peaks are eliminated indicative of the consumption of the citric acid in formation of the carbonate.15 Similarly, the N-O stretch associated with cobalt nitrate at 1388 cm-1 is significantly reduced. The degradation of the polymer requires a significantly higher temperature with the peaks associated with the CH2 stretches at 2931 cm-1 and 2940 cm-1 and C=O stretch of the (meth)acrylate at 1736 cm-1 of the BCP, remaining after heating at 200 °C. Calcination at 300 °C has previously been shown to be effective to generate mesoporous cobalt oxide,15 but peaks associated with the BCP remain even after heating at 325 °C (Figure 2C) for the film templated by PMPEG-PBA(25-126). Complete identification of all FTIR peaks is included in the Supporting Information (SI, Figure S4). This temperature (325 °C) is sufficient to completely degrade the other two analogous PMPEG-PBA copolymers that only differ by the length of the BA segment (Figure S5). On calcination at 375 °C, the PMPEG-PBA(25-126) is degraded and the peaks associated with Co3O4 at 570 cm-1 and 680 cm-1 become well defined.41


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Figure 2. Evolution of the FTIR spectrum of templated film initially containing cobalt nitrate, citric acid and 38.5 wt% PMPEG-PBA(25-126) from the (A) as-cast film to (B) cobalt carbonate (200 °C) to (C) cobalt oxide (325 °C) to (D) complete removal of PMPEG-PBA(25-126) at 375 °C. Peaks associated with (a) O-H in citric acid; (b) CH2 in PMPEG-PBA(25-126); (c) O- CH2 in PEG; (d) C=O in PMPEG-PBA(25-126); (e) symmetric stretching in cobalt oxide and (f) Co-O in cobalt oxide are labeled in spectrum A.

The heating to induce these chemical changes impact the mesostructure of the film as shown in Figure 3. In this case, the mesostructure is not well developed on casting of the film. The mesostructure appears almost worm-like in Figure 3A. The exact origins for this poor mesostructural order are not clear, but this behavior is consistent with other block copolymers with associating additives42 and could be related to surface segregation.43 This mesostructure evolves into a well-defined array of circles on heating to 200 °C, which also converts the cobalt nitrate-citric acid complex to cobalt carbonate Figure 3B). This evolution in mesostructure is


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similar to the modulable steady state (MSS) in sol gel processing,44 except that the precursor crosslinking to reduce mobility is driven by the complex conversion to carbonate. In this way, this structural evolution is similar to the thermally induced self-assembly reported for the fabrication of mesoporous carbons.45 At ambient temperature, the conversion to carbonate is inhibited, but the complex mobility is low, thus the structure is kinetically trapped. On heating to 200 °C, the mobility of the composite increases, but this is countered by the faster rate of reaction to form the essentially immobile cobalt carbonate. The average surface size for the spots in the AFM micrographs for this composite is 23.2 ± 4.1 nm. On further heating to 325 °C, the block copolymer template is removal and the carbonate is converted to oxide to yield a mesoporous cobalt oxide film. The ordered mesostructure is perturbed by this calcination step as shown in Figure 3C. First, the pore size decreases to 20.5 ± 3.9 nm, associated with lateral contraction from both conversion to and crystallization of the cobalt oxide phase.20 The distribution of the pore size is broadened significantly from the mesostructure in the carbonate. To better illustrate this degradation in the ordered mesostructure, Figure 3D shows the azimuthal average of the FFT of these micrographs. The correlation peak shifts to higher q through the processing from the as-cast film to the mesoporous cobalt oxide. The width of the correlation peak broadens from the carbonate to oxide, which is indicative of the decrease in extent of ordering on conversion to the oxide.


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Figure 3. AFM micrographs illustrating the nanostructure evolution from (A) as-cast to (B) cobalt carbonate composite to (C) mesoporous cobalt oxide film. These films were templated using PMPEG-PBA (25-34) and contains 35.5 wt% of the BCP template in the as-cast film. The insets illustrate the FFT of the micrographs and (D) the azimuthal average of the FFTs better illustrates the evolution in structure through processing. The morphology of the mesoporous cobalt oxide films is impacted by the concentration of the PMPEG-PBA in the initial precursor solution as shown in Figure 4. With 27.3 wt % of PMPEGPBA(25-34) in the as-cast film, the ordered templated mesostructure is clearly present in the mesoporous cobalt oxide film (Figure 4A). However, there are larger defects in the film that appear as nanocracks with widths of 34 ± 3.5 nm that can extend to almost 190 nm in length. The average surface pore size is 19.3 ± 2.9 nm, which is smaller than the previously described film that contained a larger PMPEG-PBA(25-34) content (35.5 wt %). Additionally, the extent of ordering is less for the film templated with 27.3 wt % of PMPEG-PBA(25-34) (Figure 4A) than with 35.5 wt % of PMPEG-PBA(25-34) (Figure 4B). Further increasing the concentration of the PMPEG-PBA to 44.3 wt % slightly degrades the order of the mesostructure as evidenced by the broadening of the ring in the FFT of the micrograph (inset of Figure 4C). The local ordering is similar with an average surface pore size of 20.5 ± 3.9 nm. A more significant degradation in structure occurs at higher PMPEG-PBA(25-34) concentrations (54.0 wt%) as shown in Figure


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4D. In this case, the film appears to be composed of nanoparticles with an average diameter of 32.4 ± 3.0 nm. There are small local regions that provide some evidence of the templated mesostructure from the PMPEG-PBA(25-34), but overall the film is predominately disordered as evidenced by the FFT in the inset of Figure 4D. A similar optimal composition (ca. 35 wt %) for the best ordered mesostructure is obtained using PMPEG-PBA(25-12) as shown in Figure S6 and PMPEG-PBA (25-126) as shown in Figure S7. The composition for the best ordered mesoporous cobalt oxide film are for film fabricated with 36.4 wt % MPEG-BA(25-12), 35.5 wt % MPEGBA(25-34), and 36.4 wt % MPEG-BA(25-126).

Figure 4. AFM micrographs of cobalt oxide films templated by PMPEG-PBA(25-34). The structure is dependent on concentration of copolymer template in the as-cast film: (a) 27.3 wt% (b) 35.5 wt% (c) 44.3 wt% (d) 53.9 wt%.


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Interestingly, all mesoporous cobalt oxide films fabricated with the three BCPs examined exhibit the most ordered mesostructure when using approximately 35 wt % of the PMPEG-PBA template. Figure 5A illustrates the 1D GISAXS profiles for mesoporous cobalt oxide films fabricated with each of the PMPEG-PBA templates at this composition. A clear first order reflection is present in qxy (see Figure S8 for the 2D GISAXS profile) for each of the materials. However, the location of this diffraction peak (q*) shifts to lower q as the Mn of the PMPEGPBA template increases. This corresponds to an increase in the d-spacing from 14.3 nm to 24.7 nm to 73.1 nm for PMPEG-PBA(25-12), PMPEG-PBA(25-34), and PMPEG-PBA(25-126), respectively. The lack of clear higher order reflections precludes identification of the space group from GISAXS. For the mesoporous film templated by PMPEG-PBA(25-34), there is a weak shoulder peak around q = 0.03 that corresponds to 1.45q*.46 Although this ratio is close to that expected for body center cubic packing of spherical micelles, a similar ratio for a weak second peak has been reported for disordered spheres of block copolymers,46 which appears to be consistent with the AFM micrographs for these mesoporous films. One additional feature in these scattering data is the progressive increase in the width of the primary diffraction peak as the molecular weight of the template decreases. The width of the primary diffraction peak decreases from 9.5×10-3 Å-1 to 8.9×10-3 Å-1 to 8.2×10-3 Å-1 for PMPEG-PBA(25-12), PMPEGPBA(25-34), and PMPEG-PBA(25-126), respectively. This dependence of the width suggests that the ordering decays as the molecular weight of the template decreases. As the driving force for microphase separation of a neat block copolymer is proportional to the molecular weight of the BCP,47 this observed dependence on the ordering quality is not unexpected. To more directly illustrate the differences in the mesostructure of the mesoporous cobalt oxide films, the insets in Figure 5A shows representative AFM micrographs of these surfaces. The average surface pore


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size increases from 11.4 ± 2.8 nm to 20.5 ± 3.9 nm to 48.5 ± 4.3 nm for PMPEG-PBA(25-12), PMPEG-PBA(25-34), and PMPEG-PBA(25-126), respectively. The spacing of the surface mesostructure provides a more direct route to compare the AFM micrographs with the GISAXS profiles. The d-spacing increases from 14.3 nm to 24.7 nm to 73.6 nm for PMPEG-PBA(25-12), PMPEG-PBA(25-34), and PMPEG-PBA(25-126), respectively, for the AFM micrographs, which agrees well with the GISAXS d-spacing for each. Figure 5B illustrates the molecular weight scaling with both d-spacing and pore size obtained for these mesoporous films. Both mesostructural parameters appear to scale with the total degree of polymerization (N) to the 1/2 power. This scaling is consistent with the scaling obtained generally for BCPs.47,

48, 49

This

scaling provides a simple route to predict the pore size for any MPEG-BA template.

Figure 5. (A) GISAXS profiles (qxy) for mesoporous cobalt oxide films with approximately 35 wt% BCP for the three different templates. Inset shows AFM micrographs of the surface of these mesoporous cobalt oxide films fabricated with PMPEG-PBA(25-12), PMPEG-PBA(25-34), and PMPEG-PBA(25-126). The scale bar in the AFM micrograph is 250 nm. (B) Scaling of the pore size ( from AFM) and d-spacing ( from GISAXS) with the total degree of polymerization of the PMPEG-PBA template.


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To further demonstrate the role of BCP content on the mesostructure that develops in the mesoporous cobalt oxide films, the pore texture was assessed using elllipsometric porosimetry (EP)38, 39 where modulation of the refractive index of the film by the condensation of ethanol, allows the porosity to be analyzed for the absorption/desorption isotherms and the pore size distribution to be modelled from ellipsometry data. Figure 6 illustrates how the PMPEGPBA(25-34) content impacts the sorption isotherms. For all the isotherms, the hysteresis between the adsorption and desorption isotherms is generally negligible. At low BCP content, the sorption follows a pseudo type-IV isotherm (Figure 6A). There is an initial increase in sorption at P/P0≈0.4, with a plateau between 0.6 < P/P0 < 0.75. Subsequently, the sorption increases further. This isotherm is peculiar for a templated mesoporous film, where typically well-defined hysteresis loops exist.50 The hysteresis loop that is present in most mesoporous materials is attributed to pore blocking through the inkbottle effect.51 In these porous cobalt oxide films (Figure 6), the hysteresis loop is very small and suggests that the mesopores are interconnected by rather large mesopores to prevent blockage that leads to the more defined hysteresis loop. This behavior can be rationalized when examining the deformation of the nanostructure during the conversion to cobalt oxide. During this process, the templated mesostructure partial coalesce and collapse to generate more slit-like pores that are interconnected by defects. Near the best composition for the ordered mesostructure, the total sorption increases significantly as shown in Figure 6B. The sorption increases sharply at P/P0 ≈ 0.5, indicative of slightly larger pores than for the mesoporous cobalt oxide templated with only 15.4 wt%, which is consistent with the improved mesostructural integrity using ca. 35 % BCP. The adsorption increases with increasing P/P0 up to near saturation. On desorption, there is a slight hysteresis down to


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P/P0≈0.95 prior to any emptying of the pores. This suggests that the large pores that do not fill completely are connected by slightly smaller mesopores. Further increasing the concentration of BCP used in the fabrication results in a significant decrease in sorption with a more traditional type-IV isotherm obtained, but again without a significant hysteresis loop (Figure 6C). Adsorption and desorption isotherms for mesoporous cobalt oxide films templated with PMPEGPBA(25-12) and PMPEG-PBA(25-126) are shown in Figure S9 and S10, respectively. These isotherms are similar to those shown in Figure 6.

Figure 6. Adsorption () and desorption () isotherms for mesoporous cobalt oxide films templated by PMPEG-PBA(25-34) from EP with ethanol as the adsorbate. The porosity and pore size is strongly dependent on the composition of the as-cast film with BCP wt%, (A) 15.4% (B) 35.5% (C) 53.9%. The connection lines are the fitted isotherm curves.

From these sorption isotherms, the porosity of the films can be estimated from the maximum sorption (Vab/Vfilm at P/P0 ≈ 1). This provides a lower limit for the porosity as not all of the pores are filled by ethanol in this case with the adsorption capacity continuing to increase as saturation is approached. From Figure 6, the porosity is significantly enhanced for the sample with the best ordering. Figure 7 shows how the BCP concentration during fabrication impacts the porosity of


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the final mesoporous cobalt oxide film. Irrespective of the composition of the BCP template examined, the porosity is maximized near ca. 35 wt % of the PMPEG-PBA. This maximum in porosity is lowest for PMPEG-PBA(25-12) (≈ 30%), which is attributed to the smaller micelle size and more significant contraction of the ordered mesopore size on calcination. The estimated maximum porosities for the PMPEG-PBA(25-34) and PMPEG-PBA(25-126) are around 35%, despite the greater than 4x difference in pore size (Figure 5). In these cases, the adsorption isotherm is still increasing as saturation pressure is approached, so this is a lower estimate for the porosity. This independence of the relative hydrophobicity of the BCP is unexpected for cooperative assembly templating. Typically, the phase diagram for the BCP template can be utilized as a reference to predict the phase and morphology of the resultant mesoporous film.27 However in this case, the large difference in composition of the BCP would suggest that different BCP loadings would be necessary, but from Figure 7 this is not the case. We attribute this difference to templating by preformed micelles and this compositional invariance suggests that the inorganic phase may be completely segregated from the BCP template. As the carbonate is more hydrophobic than the precursors, this may drive the segregation, similar to the crosslinking of phenolic resin leads to a loss of hydroxyl groups to generate a more hydrophobic environment that partially segregates from PEO segments of the initial template.52


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Figure 7. Impact of composition on the porosity of mesoporous cobalt oxide films templated by () PMPEG-PBA(25-12), () PMPEG-PBA(25-34), and () PMPEG-PBA(25-126). The porosity is determined from the EP measurements. Dashed lines are the best Gaussian fits for the porosity. Error bars correspond to one standard deviation.


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Figure 8. Surface area for adsorption of ethanol on cobalt oxide films using () PMPEGPBA(25-12), () PMPEG-PBA(25-34), and () PMPEG-PBA(25-126) BCP templates. The surface area is dependent on concentration of BCP template used in the synthesis. The larger BCP leads to smaller specific surface area. Error bars correspond to one standard deviation.

From the adsorption isotherms (see Figure 6 as example), the surface area of the mesoporous films is estimated using t-plots (Figures S12-S14). As expected, the surface area is inversely correlated with the pore size of the mesoporous cobalt oxide. Moreover, the surface area is directly correlated with the porosity of the film for a given template. At the maximum, the surface area decreases from approximately from 180 m2/cm3 to 155 m2/cm3 to 95 m2/cm3 for films templated using PMPEG-PBA (25-12), PMPEG-PBA (25-34), and PMPEG-PBA (25-126), respectively. These surface areas are comparable to other mesoporous materials templated using similar large block copolymers.15

Figure 9. Schematic of the cooperative assembly driven fabrication of mesoporous cobalt oxide where the precursors are dissolved and cast, subsequent heating transforms the cobalt nitrate-


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citric acid to cobalt carbonate and subsequently cobalt oxide along with degradation of the PMPEG-PBA template. The mesostructure is tuned by the PMPEG-PBA template to cobalt complex ratio. Based on these results, a basic understanding of the underlying mechanisms that define the mesostructure and porosity of these micelle templated mesoporous films has been elucidated. The optimal composition of the BCP loading for the fabrication can be understood in terms of two competing factors as shown schematically in Figure 9. These general concepts should be independent of the BCP used, but the exact lengthscales associated with the structures will depend on the BCP selection. At high BCP content (> 60 wt%), the wall thickness is insufficient to generate a mechanically robust nanostructure and the ordered pore structure collapses on calcination. At low BCP content (< 15 wt%), the porogen (BCP template) is dispersed within the inorganic precursor to generate low porosity. At an intermediate composition, the framework wall is sufficiently thick to prevent collapse of the mesostructure, which leads to be best-ordered structure, highest porosity and largest surface area. This composition is independent of the composition of the BCP template for the three MPEG-BA copolymers examined herein. This independence of the mesostructure with respect to the BCP composition is distinct from most prior reports for evaporation induced self assembly53 associated with the sol-gel fabrication of mesoporous films.54, 55 CONCLUSIONS The influence of the composition of the BCP template on the cooperative assembly with cobalt nitrate and citric acid was systematically examined. Intriguingly, the relative hydrophobicity of the BCP template does not influence its optimal loading for the best-ordered and most porous


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mesostructure ultimately in the mesoporous cobalt oxide. The optimal loading is found to be approximately 35 wt% of BCP for all three BCPs examined with a maximum porosity of 30-35 % with a smaller porosity for the lower molecular weight template. The size of the mesopores scaled with the total degree of polymerization to the 1/2 power, consistent with the typical dspacing for block copolymers alone. This suggests that the entire BCP (both hydrophobic and hydrophilic segments) acts to template the mesopores in these materials. For the three block copolymers examined, the pore size determined from AFM was tuned from 14.3 nm to 73.6 nm based solely on the molecular weight of the BCP used for the template. Additionally, a close packed sphere morphology is obtained for all of the BCP templates examined. These results provide insight into the mechanism for BCP templating for metal nitrate-citric acid systems and provides a route to predict the pore size as well as best formulation to yield a highly porosity metal oxide.

ASSOCIATED CONTENT Supporting Information. Composition of each examined films. TGA curves of the three BCP templates. SEC and NMR spectra for the BCP templates. Evolution of the FTIR spectrum from as-cast film to oxide film templated with PMPEG-PBA(25-12) and PMPEG-PBA(25-34). AFM images illustrate the surface morphology of films templated with PMPEG-PBA(25-12) and PMPEG-PBA(25-126). GISAXS patterns for the highest porosity films with three BCP templates. Adsorption and desorption isotherms with t-plots of the films templated with PMPEGPBA(25-12) and PMPEG-PBA(25-126) are included. This material is available free of charge via the Internet at http://pubs.acs.org.


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AUTHOR INFORMATION Corresponding Author * [email protected] to whom correspondence should be addressed. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT This work was partially supported by the National Science Foundation under grant no. CBET1510612. SY thanks Dr. Rong Bai for assistance with TGA and AFM measurements. The help of Clinton Wiener and Changhuai Ye with analyses is appreciated.

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Table of Contents Graphic

Synopsis: Relative hydrophilic fraction of block copolymer (BCP) does not impact the BCP:precursor ratio required to generate ordered and highly porous cobalt oxide films


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Role of Amphiphilic Block Copolymer Composition ... - ACS Publications

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