Tuning Cooperative Assembly with Bottlebrush Block Co-polymers for

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Article Cite This: Langmuir 2019, 35, 9572−9583

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Tuning Cooperative Assembly with Bottlebrush Block Co-polymers for Porous Metal Oxide Films Using Solvent Mixtures Xuhui Xia,† Garrett Bass,‡ Matthew L. Becker,‡ and Bryan D. Vogt*,† †

Department of Polymer Engineering and ‡Department of Polymer Science, University of Akron, Akron, Ohio 44325, United States

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ABSTRACT: Block copolymer templating enables the generation of well-defined pore sizes and geometries in a wide variety of frameworks, typically through evaporationinduced self-assembly (EISA). Here, we systematically modulate the solvent quality with mixtures of tetrahydrofuran−ethanol (THF−EtOH) to manipulate the unimer/ micelle ratio in the precursor solution to explore how the associated solution structure influences the final pore morphology. A bottlebrush block copolymer (BBCP) with poly(ethylene oxide) and poly(t-butyl acrylate) side chains was used as the template for pore formation. Irrespective of the solvent composition, a bimodal pore size distribution was obtained with mesopores templated by small aggregates of the BBCP unimers (potentially low aggregation number micelles) and macropores templated by large self-assembled BBCP micelles. The morphology and pore characteristics of the metal oxide films were dependent on the THF−EtOH composition. Interestingly, an intermediate solvent composition where the volume of micelles is approximately half the volume of unimers (in the precursor solution) leads to the best ordering of micelle-templated pores and also the maximum porosity in the films. The micelle/unimer ratios in the precursor solutions do not correspond directly to the bimodal pore distribution in the metal oxide films, which we attribute to kinetically trapped assembly of the BBCP at a low THF content. The increased critical micelle concentration at high THF composition leads to changes in the unimer/micelle ratio during solvent evaporation. These results appear to be universal for a number of metal oxides (cobalt, magnesium, and zinc) with the porosity maximized at a THF/EtOH ratio of 3:1. These results suggest the potential for enhancements in the porosity of block copolymer-templated films by EISA methods through judicious solvent selection.

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INTRODUCTION Since the initial reports of the mobil composition of matter family of ordered mesoporous silicates in 1992,1,2 significant efforts have been made to extend templated synthesis to a variety of frameworks, including carbon,3 metals,4 sulfides,5 transition-metal oxides.6 This diverse set of materials provides opportunities that exploit features, such as high surface area, well-defined pore architectures, and tunable pore sizes, in a variety of applications including catalysis,7,8 drug delivery,9 separations,10 energy generation,11 and energy storage.12,13 The synthetic strategies for ordered mesoporous materials can be broadly divided into hard14 and soft templating.15 The soft templating strategy uses organic molecules, such as surfactants or block copolymers (BCPs), to direct the porous structure via self-assembly.16 The pores are generated by the removal of the template through degradation or extraction.17 Conversely, the hard templating route begins with an inorganic porous template that is subsequently filled with the precursor of interest, processed to generate the desired framework chemistry, and then the template is selectively removed.14 Hard templating generally provides more control over the structure especially for crystalline frameworks18,19 but requires increased time and costs associated with the template synthesis and its subsequent removal. Soft templating is commonly thought to be more efficient, but there are issues associated © 2019 American Chemical Society

with assembly with reactive species based on the relative size of the precursor particles to the template.20 In soft templating, the size of the pores can be controlled by the selection of the template.21 Structure directing agents for zeolites generate well-defined micropores (200 °C from TGA in the air (Figure S5), which is greater than the onset temperature for the conversion from metal nitrate−citrate precursor to carbonate phase for the metal centers examined (Co: 130 °C, Zn: 175 °C, and Mg: 140 °C). FTIR spectroscopy confirms the conversion of the cobalt nitrate/ citric acid complex to cobalt carbonate at 200 °C (Figure 2)

Figure 2. FTIR spectra of (a) PEO-PtBA bottlebrush polymer, (b) ascast film, (c) cobalt carbonate film heated at 200 °C, and (d) Co3O4 films calcined at 325 °C. The FTIR spectra show the chemistry evolution of the PEO-PtBA templated films at different temperatures. After heating at 325 °C, the cobalt oxide forms, and the organic template can be fully removed.

with the reduction of the peak at ∼1640 cm−1 associated with N−O antisymmetric stretch and the formation of a broad band between 1600 and 1320 cm−1 attributed to inorganic carbonates. Calcination at 325 °C leads to CO2 evolution to form cobalt oxide with the loss of the carbonate band between 1600 and 1320 cm−1 and the appearance of the sharp peaks at 661 and 656 cm−1, which is consistent with Co3O4.59,71 During calcination at 325 °C, the BBCP template is completely removed with loss of peaks associated with the CO vibration and stretching vibrations of C−H bonds of the BBCP. The impact of these heat-induced chemical changes on the porous structure of the film prepared from EtOH is shown in Figure S6. In this case, a spherical structure is developed, displaying a broad distribution. This broad distribution is attributed to the binary nature of the template, where either unimers or the self-assembled micelles may provide structural control. This spherical structure is maintained upon heating to

Figure 3. AFM images of Co3O4 films prepared from precursor solutions containing 35 wt % (relative to solids) BBCP using (a) neat ethanol or THF/EtOH mixtures (w/w) of (b) 1/1, (c) 3/1, (d) 5.5/1, and (e) 8/1. The insets show the histogram associated with pore diameter distribution determined from three AFM micrographs, with fitted Gaussian peaks shown in Figure S7. 9576

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Langmuir films cast from ethanolis shown in the inset in Figure 3 a. Example individual histograms from individual AFM micrographs are shown in Figure S7. The histograms for the pore sizes can be fit well by two Gaussian distributions in all cases. Figure S8 illustrates the full distribution with these two Gaussian functions. The average sizes from these Gaussian fits of the pore size distribution are 50 ± 14 and 86 ± 31 nm for the histogram shown in Figure 3 a. This relatively broad distribution of sizes suggests that the poor solvent quality of EtOH for the PtBA segments leads to kinetically trapped morphologies in the casting solution. It should be noted that kinetically trapped solution structures have been widely reported for block co-polymer assembly.38 To improve the dynamics of the PtBA segments in the solution to better assemble the micelles, THF, which is a good solvent for both the PEO and PtBA segments, was added as a co-solvent to the casting solution. Using 1:1 (w/w) THF/ EtOH, the pore size distribution narrows (Figure 3b) in comparison to that obtained from the neat EtOH (Figure 3a). However, the ordering of these pores obtained from 1:1 THF/ EtOH does not improve from visual inspection, despite a narrower size distribution. The addition of THF in this case slightly decreases the average pore sizes to 45 ± 15 and 80 ± 26 nm, with the fraction of smaller pores significantly increased. This suggests that the larger pores obtained from neat EtOH could be larger than the equilibrium micelle size for the BBCP. It is to be noted that the equilibrium size of BBCP micelles corresponds to the minimum in free energy, which is dependent on the conditions such as concentration, temperature, and solvents. By further increasing the THF/EtOH ratio in the precursor solution, the self-assembly of BBCP appears to change appreciably. At 3:1 THF/EtOH, larger pores on average are obtained, appearing more regularly distributed from visual inspection (Figure 3c) but still lacking any appreciable order as determined by the fast Fourier transform of the AFM micrograph (Figure S9). The distribution, in this case, is clearly bimodal and can be fit well by two Gaussian functions that yield average pore sizes of 60 ± 14 and 139 ± 56 nm. At 3:1 THF/EtOH, the small pore size is similar to that obtained with the 1:1 THF/EtOH and neat EtOH. However, the larger pores now dominate the pore size distribution for the 3:1 THF/EtOH solution in comparison to its minor contribution to the pore size at lower THF contents in the precursor solution. Further increasing the THF content to 5:1 THF/EtOH still yields a bimodal pore size distribution (Figure 3d), but the relative fraction of large pores has decreased. The fraction of large pores is further reduced when the THF/EtOH ratio is increased to 8:1 (Figure 3e). These surface pore distributions can all be fit with two Gaussian functions, but the bimodal nature of the distribution is not as obvious from the raw histogram. The sizes of the large (∼130 nm) and small (∼50 nm) pores appear to be common, which suggests that similar sizes of large micelles and unimer aggregates (or low aggregation number micelles) are formed in this range of solvent compositions during film formation. This bimodal distribution is likely associated with the solution structure of the BBCP. To probe this solution structure, DLS measurements were used to determine the relative concentration of unimers and self-assembled micelles in the solution as a function of the solvent composition. From these DLS measurements, the volume-averaged hydrodynamic diameter distribution of the BBCP was elucidated as a function of the THF/EtOH ratio, as shown in Figure 4. There are two

Figure 4. (a) Hydrodynamic size distribution from 0.5 wt. % BBCP in different THF/ethanol solutions from DLS. (b) The integrated peak area ratio of micelle to bottlebrush unimer as a function of the THF content in the solvent at a constant BBCP concentration of 0.5 wt %.

peaks in the DLS data associated with the BBCP unimers (≈30 nm) and BBCP micelles (≈440 nm). The smaller size that we attribute to the unimer is consistent with the size (25 ± 6 nm) determined from TEM micrographs (Figure 1c). Irrespective of the solvent composition, the size distribution of the BBCP in the solution appears to be bimodal (or even trimodal at low THF concentration) for all THF/EtOH compositions examined here. It is worth noting that the pore size of the fabricated cobalt oxide films differs from the size of BBCP unimers and BBCP micelles obtained from DLS measurements. The average size of the larger pores in the film templated from BBCP micelles is ≈130 nm, which is much smaller than the micelle size of the BBCP micelles in the solution. It is not surprising that the size of the larger pores in the films templated from BBCP micelles (approximately 130− 150 nm) is smaller than the size of the BBCP micelles, as a result of the framework shrinkage. It should be noted that a smaller size can also be associated with the ineffective templating of the corona for mesoporous materials using sol−gel chemistry,77 but both the corona and core contribute to the pore size in the metal nitrate−citric acid complex route as the pore size scales with the total degree of polymerization of BCP template rather than the degree of polymerization of the hydrophobic segment alone.59 However, the size of the smaller pores in the films (approximately 45−60 nm) is larger than the size of BBCP unimers. To explain this, we propose that the size of the smaller pores in the films is templated from an aggregation of several BBCP unimers or low aggregation number micelles, instead of a single BCP unimer. During the solvent evaporation process though the formation of large micelles of the BBCP is suspected to be suppressed as BBCP unimers are kinetically trapped, there is still some tendency for the aggregation of the BBCP to minimize the contact of the PtBA segments with the solvent, which can be possibly considered as low aggregation number micelles. As a result, aggregates containing several BBCP molecules will likely form during the solvent evaporation, serving as the template for the smaller pores in the cobalt oxide films. In neat ethanol, despite ethanol being considered a poor solvent for the PtBA, there is an appreciable fraction of unimers in the solution with approximately three micelles for each unimer on v/v basis. This high content of unimers can be partially attributed to the low BBCP concentration used for the DLS measurements. As shown in Figure S10, the CMC of the BBCP in ethanol is 0.02 wt %. However, intrinsic limitations in the Stokes−Einstein assumption associated with the determination of particles size from DLS prevent accurate size determination at the 4.5 wt % BBCP concentration used for templating films. The CMC can be used to estimate the expected ratio of micelle to unimers at higher concentrations. 9577

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thus, the hydrodynamic size distribution of the BBCP in the THF/EtOH solutions can be readily assessed from DLS. The volume-averaged hydrodynamic diameter distribution of the BBCP in the presence of either magnesium or zinc ions, as shown in Figure S12, demonstrates that the micelle to unimer ratio decreases as the THF/EtOH ratio increases. However, the presence of magnesium or zinc ions suppresses the micellization of BBCP, but the general trends observed are consistent with the salt-free THF/EtOH mixtures with the BBCP. To quantitively demonstrate the impact of the solvent composition on the micellization of the BBCP and, therefore, better understand the impact of the solvent composition on the morphology of templated cobalt oxide films, the ratio of the integrated peak areas of the micelles and unimer was examined as a function of the THF/EtOH mass ratio, as shown in Figure 4b. As the THF/EtOH ratio increases from 0:1 to 8:1, there is a continual decrease in the micelle/unimer ratio from approximately 2.97 to 0.22. In the BBCP-templated cobalt oxide films, both the BBCP unimers (as well as their aggregates) and self-assembled BBCP micelles can serve as porogens to produce mesoscale pores and macroscale pores, respectively. Using neat EtOH as the solvent, though the micelle/unimer ratio is relatively high compared to higher THF/EtOH ratios, vitrification of the PtBA segment of BBCP due to the poor solvent quality of EtOH can lead to a kinetically trapped structure of both the BBCP unimer and self-assembled BBCP micelles. Kinetic trapping, associated with the micelle formation from the large BBCP unimers with a glassy hydrophobic block, provides an explanation for the poor ordering and co-existence of what is attributed to unimerinduced and micelle-induced pores. THF is a good solvent for both the PEO and PtBA segment of the BBCP, and its addition to ethanol can serve as a plasticizer of the glassy PtBA segment during the film fabrication, enabling structural rearrangement of both BBCP unimers and micelles. Herein, though the micelle/unimer ratio formed in the precursor solution decreases as the THF/EtOH ratio increases from 0:1 to 3:1, the fraction of large macropores (attributed to the micelles) in the cobalt oxide film fabricated at a THF/EtOH ratio of 3:1 is larger than in the case of neat EtOH. However, with further increase in the THF/EtOH ratio, the fraction of macropores appears to decrease. Though at high THF/EtOH ratios, the effect of kinetic trapping of the BBCP structure can be attenuated, THF is a good solvent for both the PtBA and PEO segments that acts to increase the CMC of BBCP. The higher CMC suppresses the micellization of BBCP unimers. The increased concentration requirement to form the micelles acts to provide a kinetic barrier to assembly due to the higher viscosity at high BBCP concentration. Thus, there is a tradeoff between the low mobility of the segments in the solution (PtBA) when the micelles are thermodynamically favored (ethanol) and the increased initial segmental dynamics where the CMC is increased to disfavor the micelle formation in the dilute solution (THF). An intermediate composition (3:1 THF/EtOH) appears to strike the best balance for templating. One potential route to improve on this order is though solvent vapor annealing (SVA) treatments, which has been demonstrated effective for improving the ordering of BCP and BCP nanocomposite films.79,80 SVA relies on the solvent sorption into the BCP to impart mobility for segmental rearrangement. The large inorganic content in these materials requires a sufficiently polar solvent to provide mobility to the metal

For the 4.5 wt % BBCP, this ratio is estimated to be approximately 224:1, assuming that below the CMC no micelles are present in the solution, whereas above the CMC, unimers remain at CMC and micelles are formed by the excess BBCP.78 This result is closer to expectations for the precursor solution to contain primarily micelles, but there remains an appreciable number of unimers in this solution. Additionally, the DLS results point to even larger aggregates in ethanol that appear to be comprised of aggregates of the micelles. These could be formed by PtBA−PtBA interactions that can decrease unfavorable PtBA−EtOH interactions. For the micelles to aggregate, this suggests that the minority PEO block in the BBCP cannot sterically screen PtBA−PtBA interactions in ethanol. This aggregation from PtBA−PtBA interactions to avoid contact with the EtOH could also help to further explain the relatively broad distribution of pore sizes shown in Figure 3. The addition of THF to the BBCP solution leads to a decrease in the peaks associated with the micelles and the large aggregates from the fits of the DLS data. THF is a good solvent for both PtBA and PEO segments, so the inclusion of THF will disfavor the formation of BBCP micelles and help to screen the unfavorable interactions between the PtBA segments and the solvent to decrease aggregation. At 1:1 THF/EtOH (Figure 4a), three peaks are observed in the particle size distribution at nearly identical sizes as for the neat EtOH. However, the number of micelles and large aggregates is clearly decreased as the intensity of these two peaks is suppressed relative to those in neat EtOH. At 3:1 THF/EtOH (Figure 4a), the peak associated with the large aggregates is completely suppressed, which suggests that at this composition, the enthalpic penalty associated with the solvent contact with the PtBA not screened by the PEO is less than the entropic penalty associated with the aggregation. Interestingly, the most ordered structure by visual inspection is observed at this composition, which suggests that the avoidance of aggregates larger than the equilibrium micelles in the solution may be important to ordering. Further increasing the THF/EtOH ratio leads to additional decreases in the micelle concentration relative to the unimer. During the film formation, the solvent evaporates, and the composition for the mixed solvent will evolve due to differences in their relative volatility. THF and EtOH exhibit similar volatilities, but EtOH is slightly enriched in the liquid phase during evaporation.60,61 To understand the extent of this effect on the BBCP structure, a diluted BBCP solution in 3:1 THF/EtOH (0.25 wt %) was allowed to concentrate by evaporation to half of its original volume. This concentration for the BBCP is similar to that for the DLS measurements. As shown in Figure S11, the size distribution of BBCP in this concentrated sample and that in a freshly prepared using the same 3:1 THF/EtOH solvent are similar with the ratio between the unimer and micelles being nearly constant. This result suggests that the changes in the solvent quality from evaporation should not be a dominant effect determining in the final film morphology. However, for the fabrication of the porous films, the precursor solution also contains inorganic salt and citric acid, which will impact the effective solvent quality for the BBCP. As the cobalt(II) nitrate hexahydrate solution is deep red, quantitative DLS analysis is challenged by absorption. However, solutions of the other metal nitrates, magnesium, and zinc are effectively transparent in the visible range, and, 9578

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THF content increases, fewer micelles are initially present. However, as the solvent evaporates and the solution concentrates, the equilibrium structure for the BBCP is driven toward micelles, which corresponds to the original EISA approach described by Brinker.30 THF is a good solvent for both the PtBA and PEO segments to provide mobility of the PtBA to rearrange toward an equilibrium structure, but THF also increases the CMC of the BBCP to suppress the micellization of BBCP unimers. Thus, there is a tradeoff between the thermodynamically favored micellization and kinetic trapping that is induced by low mobility of the PtBA segments, which results in the best balance for templating at an intermediate composition (THF/EtOH of 3:1). As AFM only probes the surface morphology, cross-sectional SEM micrographs were obtained for the calcined thin films to determine the impact of the solvent composition on the structure through the thickness of the films (Figure 5). Based on these micrographs, the pores appear to be ellipsoidal, which is attributed to anisotropic film shrinkage during the template removal and conversion to the cobalt oxide. The films are initially ≈1 μm thick after casting, but after calcination the thickness decreases to ≈200 nm. The asymmetric pore geometry is commonly observed in thin films when the framework contracts due to the constraint of the substrate.84,85 The long-axis dimension of the ellipsoidal pores is consistent with the surface structure obtained from AFM and the expected original size of the BBCP micelle. The ratio of the long to short axis dimensions of the pores is approximately 5:1, consistent with the relative decrease in the thickness of the film on calcination. These data suggest that surface pore sizes obtained from AFM micrographs are representative of the initial dimensions of the BBCP template in the film prior to their removal. Figure S13 shows the GIXD patterns of the templated cobalt oxide film prepared with a 3:1 THF/ethanol mass ratio calcined at 325 °C. All peaks presented in the pattern can be indexed to Co3O4 with a cubic crystal system (PDF card number: 01-078-1969). No characteristic peaks of other crystalline structures are observed. As the diffraction patterns are rings with intensities consistent with the standard powder diffraction, the cubic crystalline Co3O4 films are polycrystalline with no preferred orientation in the films. As the crystal grain size can provide microporosity and also influences performance, the average grain size of the Co3O4 crystallites was estimated from the Scherrer Equation using a shape factor value of 0.9, to be approximately 5.6 nm. This is consistent with the size of the Co3O4 crystals obtained via similar processing using much smaller linear BCP templates.56,59 This indicates that the template selection does not influence the crystallization behavior of the metal oxide in these porous materials. To further demonstrate the role of the THF/EtOH solvent composition on the structure developed in the porous cobalt oxide films, the porosity of the films was estimated from the optical properties obtained from spectroscopic ellipsometry using the Bruggeman effective medium approximation. This approximation has been shown to be effective in determining the porosity of metal oxide films.86 Figure 6 shows how the composition of the solvent used in the precursor solution impacts the porosity of the final porous cobalt oxide films as determined from the refractive index of the films. Although gas adsorption and desorption is an ideal method to quantify pore properties,87 these films contain primarily macropores, which

nitrate−citric acid complexes, while effectively plasticizing the PtBA in the BBCP. The citric acid and metal nitrate react slowly even at room temperature, which may form a network to inhibit the large-scale rearrangements of the BBCPs necessary for improving the order. We have found that the dry films can be swollen by solvent vapors, but no appreciable change in the morphology was observed. The large size of the BBCP micelles will challenge their organization in a concentrated solution to form well-ordered structures, similar to kinetic trapping of colloids in colloidal crystallization.50 To estimate the effect of kinetic trapping on the evolution of BBCP structure during solvent evaporation, the Peclet number (Pe) approach described by Routh and Russel81 is used Pe =

EH D0

(3)

where E is the rate of evaporation, which was estimated by the mass loss of solutions at room temperature, H is the initial thickness of the film, and D is the diffusion coefficient of the BBCP in the solution as estimated from Stokes−Einstein from the DLS data. Kinetic trapping of the colloidal structure is prevalent if Pe ≫ 1.82,83 The Peclet number was estimated for both pure EtOH and high THF content (THF/EtOH = 8/1) to be approximately 41.5 and 15.4, respectively. These large values suggest that kinetic trapping of the micelles is applicable to understand the BBCP assembly behavior. The fabrication process using BBCP as the template is an EISA process with different initial structures in the precursor solution, as illustrated in Scheme 1. A fraction of the BBCP micelles with a micellization extent dependent on the solvent composition is present in the precursor, but a majority of the BBCP in the precursor solution remains as unimers. As the Scheme 1. Schematic Illustrating the Film Formation Process at Different Solvent Compositions (THF/EtOH)a

a

At low THF/EtOH, the PtBA is not well solvated and effective micellization is inhibited by the aggregation of PtBA segments to minimize contact with the solvent. At high THF/EtOH, the CMC for the BBCP is high, so the assembly of the unimers to micelles and the ordering of the micelles are then hindered at these higher concentrations. The best-ordered structure is formed at an intermediate solvent composition that balances these effects. 9579

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Figure 5. Cross-section SEM micrographs of Co3O4 films prepared from precursor solutions containing 35 wt % BBCP (relative to solids) using THF/ethanol (w/w) ratios of (a) 0, (b) 1/1, (c) 3/1, and (d) 8/1. The black dash lines indicate the border of the top surface and the cross-section. Scale bars: 500 nm.

the porosity for the film fabricated with the THF/EtOH mass ratio of 3:1. These porous films are thermally stable on heating to 600 °C with the structure from AFM (Figure S16) and refractive index from ellipsometry unchanged. Using the same templating strategy, films based on magnesium oxide or zinc oxide were fabricated. As shown in Figure 6, the porosity of the cobalt, zinc, and magnesium oxide films fabricated using the citric acid-mediated BBCP templating method are maximized at the THF/EtOH ratio of 3/1. However, the overall porosity of magnesium oxide films is lower than the porosity of zinc oxide or cobalt oxide films, especially when deviating from the 3:1 THF/EtOH. Figure 7

Figure 6. Impact of casting solvent composition on the porosity of the (box solid) Co3O4, (red circle solid) ZnO, and (blue triangle up solid) MgO films as determined from spectroscopic ellipsometry. The porosity is maximized with THF/ethanol at 3/1 (w/w).

are difficult to measure using gas adsorption techniques as the pressure required for condensation in the pores is not significantly altered from the bulk fluid. As shown in Figure S14, for the film prepared with 3:1 THF/EtOH, ellipsometry porosimetry (EP) can identify some micro- and mesoporosity in the films. The porosity of the films determined from EP (Vad/Vfilm) was approximately 11% based on the change in refractive index at the highest solvent vapor pressure examined. As the refractive index was still increasing with increasing partial pressure (Figure S14), it is reasonable to assume that not all of the pores were filled under these conditions. This porosity from EP is significantly less than expected based on the refractive index, which is attributed to the macroporosity in these films that cannot be assessed by EP. From the refractive index of the films, the porosity is maximized at the THF/EtOH mass ratio of 3:1, which coincides with the largest fraction of macropores in the cobalt oxide films for the various solvent compositions examined. This maximum in porosity cannot be attributed to templating by water31 or other small molecule components as EP measurements show similar or greater micro-/mesoporosity in films fabricated with other solvent compositions (Figure S15). All of the films exhibit a similar pore size distribution from the EP measurements. These results suggest that the porosity from the macropores is better conserved through the calcination to produce a maximum in

Figure 7. AFM images of (a) zinc oxide and (b) magnesium oxide films prepared from THF/ethanol = 3/1. The insets show the histogram associated with pore diameter distribution determined from three AFM micrographs.

illustrates the surface morphology of the zinc oxide and magnesium oxide films fabricated from a THF/EtOH ratio of 3:1. The bimodal distribution in pore sizes is observed for these metal oxides as well as shown in the inset in Figure 7a,b, but the fraction of the larger pores is less than that for the cobalt oxide films. One potential reason for the differences in the morphology is the requirements associated with the processing of the films. The temperature for metal carbonate and metal oxide formation depends on the metal nitrate selected, which entails different heating conditions for syntheses of zinc and magnesium oxide films, as shown by 9580

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Langmuir Author Contributions

FTIR analysis (Figures S17 and S18). The templated Co3O4 and MgO films both exhibit enhanced porosity at the 3:1 solvent composition (Figure 6). The large variation in the refractive index for ZnO films prohibits making any strong conclusion about the effect of solvent composition on the film porosity, but the highest average porosity is also obtained with 3:1 solvent composition. This common composition suggests that the key factor being changed is the conformation of the BBCP template through the solvent composition.

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Science Foundation under Grant No. CBET-1510612. This research used the Complex Materials Scattering (CMS/11-BM) beamline of the National Synchrotron Light Source II, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory under Contract No. DE-SC0012704. The authors express thanks to Dr. Min Gao at Kent State University for assistance with the SEM and TEM measurements. The SEM and TEM data were obtained at the Liquid Crystal Institute Characterization Facility, Kent State University, supported by the Ohio Research Scholars Program Research Cluster on Surfaces in Advanced Materials.

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CONCLUSIONS Porous metal oxide films were fabricated through the metal nitrate−citric acid route using a BBCP as the template for the pores. The larger size of the BBCP than that for typical templates enables pores templated by both the micelles and unimers to be resolved. This allows new insights into the influence of the solvent composition on the morphology and pore characteristics of the porous metal oxide films that are difficult to be detected using linear BCPs as the template. A continuum in solvent quality was examined using mixtures of ethanol and THF, which are highly selective and near neutral solvents for the BBCP segments, respectively. Due to the presence of both unimers and micelles in the solution, these porous templated metal oxide films exhibit a bimodal pore size distribution with mesopores templated by small aggregates of the BBCP unimers and macropores templated by the BBCP micelles. The morphology and pore characteristics are highly dependent on the composition of the solvent used in the precursor solution. Specifically, a 3:1 THF/EtOH ratio leads to the highest fraction of macropores and the largest porosity. This is inconsistent with the micelle/unimer ratios in the precursor solution. We hypothesize that the high porosity and higher fraction of larger pores at the intermediate composition of 3:1 are a result of kinetically trapped structures in BBCP at low THF composition and the difficulty in forming the micelles when the precursor is still sufficiently dilute for good mobility of the BBCP due to the increased CMC at high THF composition. This interplay in kinetics and thermodynamics that is controllable through the solvent composition provides a handle to manipulate the structure in templated porous materials that could be especially powerful in the development of novel templates. The extendability of these concepts to other novel templates remains to be determined.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.9b01363.

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REFERENCES

Additional information including experimental synthetic details, NMR spectra, TGA curves, FTIR spectra, AFM images, DLS size distribution, GIXD patterns, and EP isotherms (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Matthew L. Becker: 0000-0003-4089-6916 Bryan D. Vogt: 0000-0003-1916-7145 9581

DOI: 10.1021/acs.langmuir.9b01363 Langmuir 2019, 35, 9572−9583

Article

Langmuir

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