Cooperative Assembly of Metal Nitrate and Citric Acid with Block

May 5, 2015 - Mark D. Soucek, and Bryan D. Vogt*. Department of Polymer Engineering, University of Akron, Akron, Ohio 44325, United States. •S Suppo...
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Cooperative Assembly of Metal Nitrate and Citric Acid with Block Copolymers: Role of Carbonate Conversion Temperature on the Mesostructure of Ordered Porous Oxides Michael C. Burroughs,†,‡ Sarang M. Bhaway,‡ Pattarasai Tangvijitsakul, Kevin A. Cavicchi, Mark D. Soucek, and Bryan D. Vogt* Department of Polymer Engineering, University of Akron, Akron, Ohio 44325, United States S Supporting Information *

ABSTRACT: The conversion of cooperatively assembled metal nitrate, citric acid, and an amphiphilic block copolymer, poly(methoxypoly[ethylene glycol methacrylate])-block-poly(butyl acrylate), films to their associated carbonate is investigated using Fourier transform infrared spectroscopy (FTIR) and spectroscopic ellipsometry for both cobalt and copper. The processing conditions associated with the formation of the carbonate significantly impact the mesostructure generated. Ex situ FTIR measurements tracked the carbonate formation and consumption of citric acid to elucidate the kinetics of the reactions and were compared to the evolution in the film thickness and refractive index by in situ spectroscopic ellipsometry. From ellipsometry, the initial rate of thickness change appears to follow an Arrhenius temperature dependence with the apparent activation energy for Co (43 kJ/mol) approximately double that for Cu (23 kJ/mol). These data elucidating the reaction kinetics enable optimization of the temperature and reaction time for improved properties and decreased fabrication time. The temperature utilized to form the carbonate impacts the mesostructure that develops and the porosity in the resultant oxide film. The optimum temperature to maximize the porosity of the oxide films is an intermediate carbonate formation temperature where the rate of conversion is not too fast to disrupt the nanostructure, but the final conversion is sufficiently high to provide thermal resilience to the framework through calcination. This knowledge enables fabrication of ordered mesoporous oxides with porosities in excess of 60%.

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INTRODUCTION Templated mesoporous materials1−4 hold tremendous promise for a wide range of applications including separations,5 solar cells,6 batteries,7 supercapacitors,8 and drug delivery.9 The fabrication of well-defined ordered mesoporous materials generally involves either hard or soft templating.3 Soft templating, pioneered by Mobil researchers,1 involves the cooperative assembly of a self-assembling organic component (surfactant or block copolymer) with the framework precursor, and calcination to remove the organic template.10 This route provides flexibility to produce a wide variety of mesoporous materials with controlled pore size,11 pore geometry,2 and framework chemistry.12 However, the fabrication details13,14 can adversely impact the quality of the mesoporous materials if not appropriately controlled. For example, the use of inorganic salts15 in place of sol gel precursors can overcome difficulties associated with the time scales for self-assembly and condensation. Similarly, new precursors have been designed to produce nonoxide frameworks with soft templating.6,12,16−18 Pioneered by Kraehnert and co-workers, the soft templated synthesis of metal carbonates using metal nitrates and citric acid as the precursors with an amphiphilic block copolymer as the template provides a simple, inexpensive route to yield ordered © 2015 American Chemical Society

mesoporous magnesium, zinc, cobalt, and aluminum carbonates.18,19 Some transition metal carbonates are promising materials for insertion batteries,20−23 so this templating approach could prove powerful to provide a facile route to generate nanomaterials of these carbonates suitable for electrodes. Moreover, heating at elevated temperatures transforms these mesoporous metal carbonates into their respective mesoporous transition metal oxides; some of these oxides can be difficult to fabricate by conventional soft templating methods. These soft templated mesoporous carbonates/oxides have only been fabricated as thin films.18,19 For other soft templating protocols, the thin films geometry can add complications to their syntheses as the mesostructure that develops can be different from that obtained for analogous bulk powders.24,25 For example, the mesostructure in thin films formed by sol−gel processing through evaporation-induced self-assembly (EISA)26 can be tuned for a short time after casting, known as the modulable steady state (MSS).27 Rankin and co-workers recently used detailed in situ FTIR measureReceived: March 5, 2015 Revised: May 5, 2015 Published: May 5, 2015 12138

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Information Figure S1) and subsequently the block copolymer template. The PMPEGMA macroinitiator for the block copolymer was prepared by mixing the monomer MPEGMA 475 g mol−1 (21.00 g, 4.42 × 10−2 mol), the RAFT agent CPADB (0.49 g, 1.75 × 10−3 mol), the initiator AIBN (58 mg, 3.53 × 10−4 mol), and DMF (10.00 g) in a three-neck round-bottom reaction flask. The solution was gently stirred with a magnetic stir bar and sparged with nitrogen for 30 min to remove ambient oxygen. The polymerization proceeded in an oil bath at 65 °C for 20 h. Polymerization was terminated by exposure to air and quenching in an ice bath. After termination, the PMPEGMA was purified by precipitation in diethyl ether three times to remove unreacted MPEGMA, CPADB, and AIBN. Finally, the polymer was dried overnight in a vacuum oven at room temperature (% yield ∼75% by gravimetric method). The number-average molecular mass (Mn) for the PMPEGMA is 2.50 × 104 g mol−1 with a molecular weight dispersity (Đ) of 1.3, based on absolute molecular weight determined with a light scattering detector by size exclusion chromatography (SEC). This PMPEGMA was used as a macroinitiator to synthesize the desired PMPEGMA-b-PBA using a mixture of PMPEGMA (5.00 g, 2.00 × 10−4 mol), butyl acrylate (BA) (9.31 g, 7.27 × 10−2 mol), AIBN (0.03 g, 1.86 × 10−4 mol), and DMF (6.00 g) in a three-neck round-bottom reaction flask. This mixture was sparged with nitrogen for 30 min under gentle stirring. Subsequently, the reaction flask was immersed in an oil bath at 65 °C for 20 h to synthesize the copolymer. The reaction was terminated by quenching in an ice bath (0 °C) and then exposing the mixture to air. The DMF was removed using a rotary evaporator. To purify the crude solid product, it was dissolved in 5 mL of THF and precipitated in 100 mL of hexane three times to remove residual monomer and AIBN. Finally, the precipitate was dried overnight in a vacuum oven at room temperature (% yield ∼70% by gravimetric method). The number-average molecular mass (Mn) for PMPEGMA-b-PBA is 5.9 × 104 g mol−1 with a molecular weight dispersity (Đ) of 1.3, based on absolute molecular weight determined with a light scattering detector by SEC. The mole fraction of hydrophilic component ( f PMPEGMA) in the copolymer is 0.1 as calculated from 1H NMR. Film Preparation. Single-side polished (SSP) silicon wafers (600 μm thick, 1−10 Ω cm, Silicon, Inc.) and double-side polished (DSP) silicon wafers (575−625 μm thick, 1−10 Ω cm, Silicon, Inc.) were used as substrates. Transmission Fourier transform infrared (FTIR) spectroscopy measurements were performed using DSP wafers to avoid backside scattering. All wafers were cleaned using Piranha solution (H2SO4:H2O2 = 3:1 v/v) at 90 °C for 45 min. The wafers were subsequently rinsed three times in deionized water and dried with N2 prior to casting films. The single-side polished wafers were used for ellipsometry measurements to obtain the film thickness changes during the conversion to carbonate or oxide. For fabricating films, the typical casting solution contains 1.40 g of cobalt(II) nitrate hexahydrate and 0.47 g of citric acid (corresponding to a 2:1 molar ratio) or 1.08 g of copper(II) nitrate trihydrate and 0.86 g of citric acid (corresponding to a 1:1 molar ratio) dissolved in 2.25 g of ethanol. The higher citric acid concentration is necessary to readily obtain the carbonate when using copper. The solution homogeneity was ensured by moderately shaking with a vortex mixer for 1 h. In a separate vial, 0.26 g of PMPEGMA-b-PBA was dissolved in 6.75 g of THF. The metal nitrate−citric acid solution was then added

ments to elucidate routes to achieve the desired structure via EISA.28 The mesostructure from sol gel processing is dependent on other postfilm fabrication processing as illustrated by Hillhouse and co-workers with controlled humidity exposures to fabricate ordered mesoporous tin oxide.29 These prior efforts examining the synthesis of mesoporous films by sol gel processes have illustrated the importance of careful control of process parameters in their fabrication, including environmental factors like humidity,14 to obtain the desired mesostructure. Unlike sol gel processing, the precursors for soft templated carbonates (metal nitrate and citric acid) do not react until after the film is generated, which should limit the environmental sensitivity. However, the influence of other processing parameters on the properties of these mesoporous transition metal carbonate/oxide films is not well understood. Here, we examine the reaction kinetics associated with the conversion of metal nitrates (cobalt(II) nitrate hexahydrate and copper(II) nitrate trihydrate) and citric acid to metal carbonates in block copolymer templated films and the relationship between the conditions used to form the carbonate and the mesostructure in the mesoporous oxide. The temperature dependencies of the reactions are indirectly determined using in situ ellipsometry to track the film thickness changes due to the reaction-induced volumetric contraction. These data illustrate an Arrhenius-like temperature dependence on the initial film thickness change. FTIR provides a more direct probe following the consumption of citric acid and evolution of the carbonate as the films are heated. Subsequent calcination of these films after formation of the carbonate to their analogous mesoporous metal oxides reveals that there is an optimum temperature for the conversion to the carbonate to maximize the porosity of these films. Additionally, the pore size distribution of the mesoporous oxide is impacted by the conversion temperature to carbonate, even though the temperature for generating oxide is unchanged. These results illustrate the interplay between the conversion to carbonate and the ultimate mesostructure of these films.

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EXPERIMENTAL SECTION Materials. Cobalt(II) nitrate hexahydrate (reagent grade 98%), copper(II) nitrate trihydrate (reagent grade 98%), citric acid (ACS reagent, ≥99.5%), N,N-dimethylformamide (99.8%, anhydrous), tetrahydrofuran (THF, ACS reagent, ≥99.0%), ethanol (ACS reagent, ≥99.5%), and hexane (anhydrous, 95%) were purchased from Sigma-Aldrich and used as received. Sulfuric acid (H2SO4, 95−98%, J.T. Baker), hydrogen peroxide (H2O2, 30%, Fisher Scientific), and deuterated chloroform (CDCl3) (Cambridge Isotope Laboratories, Inc.) were used as received without further purification. 2,2′-Azo(isobutyronitrile) (AIBN, 98%, Aldrich) was purified by recrystallization from methanol. Methoxypoly(ethylene glycol) methacrylate (PEGMA 475 g mol−1) and butyl acrylate (>99%) were purchased from Sigma-Aldrich and purified to remove free radical inhibitor. 4-Cyanopentanoic acid dithiobenzoate (CPADB, Supporting Information Figure S1) was synthesized according to a prior literature report.30 Synthesis of Block Copolymer Template. Poly(methoxypoly[ethylene glycol methacrylate])-block-poly(butyl acrylate), PMPEGMA-b-PBA, was synthesized using reversible addition−fragmentation chain-transfer (RAFT) polymerization.31,32 This synthesis involved two distinct synthesis steps to first produce the PMPEGMA macroinitiator (Supporting 12139

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Figure 1. Temporal evolution of the film thickness of the micelle templated films as determined by in situ ellipsometry for (A) cobalt-containing and (B) copper-containing films at (blue ●) 140 °C, (green ■) 160 °C, (◆) 180 °C, (pink □) 200 °C, (purple ▲) 220 °C, and (red ▶) 240 °C. An effective first-order rate constant is obtained from the initial decrease in film thickness, and this rate appears to be effectively Arrhenius for both (C) cobalt-containing and (D) copper-containing films.

chromatograph was collected using a refractive index and light scattering detector. The data were interpreted using Omnisec software v.4.7.0.406. The GPC elution curves for PMPEGMA and PMPEGMA-b-PBA are shown in the Supporting Information (Figure S2). The compositions of the RAFT PMPEGMA and PMPEGMA-b-PBA template were determined by 1H NMR (Varian NMRS-500 nuclear magnetic resonance instrument) operating at 500 MHz using deuterated chloroform (CDCl3) as a solvent (Supporting Information Figures S3 and S4, respectively). TGA (Thermal Analysis Q50) was used to determine the onset of metal carbonate and metal oxide formation. Multiple heating rates (1, 5, 10, and 20 °C min−1) were used to extrapolate the zero-rate onset temperatures. The film thickness was determined in situ during reaction using a variable angle spectroscopic (UV−vis−NIR: 350−1100 nm) ellipsometer (VASE M-2000, J.A. Woollam Co.) at a fixed incident angle of 70° and the heat stage (HTC-100, J.A. Woollam Co.) for isothermal reactions. The in situ film thickness profiles were determined from fitting the ellipsometric angles in the wavelength range of 450−1550 nm. The optical properties of the film were adequately described by a Cauchy layer with Urbach absorption. The porosity and pore size distribution of mesoporous cobalt oxide and copper oxide films were estimated from ellipsometric porosimetry (EP)33 using ethanol as the probe solvent. To determine the pore size distribution in the porous thin films, the Kelvin equation was applied to the adsorption isotherm determined from the change in refractive index of the films due to capillary condensation.34 The porosity of the film was calculated by two independent methods. First from the EP data, the change in refractive index near saturation is assumed to contain only ethanol filled voids and the metal oxide framework to calculate a lower limit for the porosity. The refractive index of the dry film is used to provide an upper limit for the porosity using the Bruggeman effective medium

dropwise to the PMPEGMA-b-PBA solution under gentle stirring at 350 rpm. Subsequently, the combined solution was stirred at 350 rpm for an additional 12−14 h to ensure micelle formation of the PMPEGMA-b-PBA. Block copolymer micelletemplated thin films were prepared by dip-coating from the aforementioned solution at 30 mm/min (withdrawal rate) and relative humidity of 45−55%. The cast films were then dried at ambient conditions for 10 min prior to FTIR or ellipsometric measurements. Isothermal heating on an ellipsometric stage (HTC-100, J.A. Woollam, Co.) at 140−240 °C converts the nitrate−citrate complex to the corresponding metal carbonates and enabled in situ ellipsometric measurements. To further convert the carbonates into mesoporous metal oxides, the carbonate films were calcined in a preheated muffle furnace (Ney Vulcan 3-130) at 300 °C for 30 min. After 30 min, the films were immediately quenched to room temperature on a metal heat sink. Samples for thermogravimetric analysis (TGA) were prepared by dissolving cobalt(II) nitrate and citric acid (2:1 molar ratio, 45 wt % solids) or copper(II) nitrate and citric acid (1:1 molar ratio, 45 wt % solids) in ethanol. The solutions were subsequently poured into a Petri dish and placed in a hood for 2 h to allow the ethanol to evaporate, and then placed in a vacuum oven at 50 °C for 12−14 h to remove excess ethanol. TGA traces were recorded by heating these dried and crushed powders in air. Characterization. The molecular weight and molecular weight distribution of the macroinitiator and PMPEGMA-bPBA block copolymer were determined by gel permeation chromatography (GPC, Waters) with HR4, HT2, HR1, HR0.5 styragel, and 500 Å Ultrastyragel columns connected in series. GPC analyses in distilled tetrahydrofuran (THF) were performed at 35 °C using 0.1% (w/v) polymer solutions. Solutions were filtered (0.45 μm), and 200 μL was injected into the column at an effluent flow rate of 1.0 mL min−1. The 12140

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Figure 2. Evolution of the FTIR absorption spectra associated with the formation of (A) cobalt carbonate (1585 cm−1) at 160 °C and (B) copper carbonate (1575 cm−1) at 140 °C. Simultaneously, citric acid (1300 cm−1) is consumed during reaction at (C) 160 °C for cobalt-containing films and (D) 140 °C for copper-containing films. There is a clear progression from the (−) as-cast film to (red − − −) after 5 s of reaction to (blue - - -) after 3840 s.

spectra, the initial drop is consistent with carbonate formation at short times, but further heating at 220 and 240 °C leads to both cobalt carbonate and cobalt oxide (Supporting Information Figure S9). Thus, the carbonate reaction appears to occur very rapidly for these cobalt nitrate−citric acid containing films, irrespective of the reaction temperature. However, examination of the final thickness after 2 h of heating shows significant variance at higher temperatures with a minimum in the thickness at 220 °C. As thickness is likely correlated with porosity, this suggests that the carbonate reaction temperature may be an important factor in determining the ultimate properties of the mesoporous oxide films, especially as there is a mixture of carbonate and oxide formed at both 220 and 240 °C and the thicknesses for these films differ by nearly a factor of 2. Figure 1B illustrates the thickness evolution for coppercontaining films on heating. At low temperatures (140 and 160 °C), there is again a rapid drop in the film thickness within 2 min, and then the thickness is nearly constant at longer times, similar to the low temperature behavior for the cobaltcontaining films. At 180, 200, and 220 °C, there is a second decrease in thickness that occurs at progressively shorter times as the temperature is increased. From FTIR analysis, this second step is associated with the formation of oxide as discussed previously. However, in this case, the thickness of the film after 2 h decreases monotonically with increasing temperature. To begin to understand the effect of processing temperature on the kinetics of metal carbonate formation, effective reaction rates (k) are calculated on the basis of the initial rate of change in film thickness (Supporting Information Figures S10 and S11). For each processing temperature (T) considered, this effective reaction rate is less for cobalt carbonate (Figure 1C) formation than for copper carbonate (Figure 1D). These effective rates appear to behave Arrhenius-like for both

approximation (BEMA) and assuming a crystalline framework.35 The chemical composition of the micelle-templated films was quantified ex situ through FTIR (Thermo Scientific Nicolet iS50 FTIR spectrometer) analysis using 512 scans at a resolution of 8 cm−1 in transmission mode. For quantitative analysis, the FTIR spectra were baseline corrected using OriginPro (Origin Lab). An atomic force microscope (Dimension ICON, Veeco) was used to elucidate the surface topography of the films after each processing stage (carbonate and oxide formation). The AFM was operated in tapping mode using ACT-50 (AppNano) tips. The crystal structure of the mesoporous oxide films was determined by X-ray diffraction (Rigaku Ultima IV diffractometer) using Cu Kα radiation (1.54 Å). The average crystal size was estimated from the Scherrer equation36 using the most intense diffraction peak and assuming β = 0.9.

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RESULTS AND DISCUSSION Heating the metal nitrate−citrate complex to form the carbonate evolves H2O, CO2, CO, and NOx,37,38 which leads to a decrease in the film thickness during processing as shown in Figure 1. The temperatures are selected on the basis of the metal carbonate onset temperature (198 °C for cobalt carbonate and 138 °C for copper carbonate) estimated from TGA traces as shown in Figures S5 and S6 in the Supporting Information. The thickness profiles for cobalt-containing films undergo a stepwise decrease in thickness on heating at 220 and 240 °C as shown in Figure 1A. There is an initial period of approximately 3 min where the thickness decreases precipitously for the cobalt-containing films. This time period appears to be almost temperature independent in comparison to the second drop in thickness that is only observed for the higher processing temperatures (220 and 240 °C). From FTIR 12141

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Figure 3. Conversion as determined by FTIR from the citric acid peak for the fabrication of (A) cobalt carbonate and (B) copper carbonate nanocomposite films at (blue ●) 140 °C, (green ■) 160 °C, (◆) 180 °C, (pink □) 200 °C, (purple ▲) 220 °C, and (red ▶) 240 °C.

The conversion to metal carbonate requires consumption of citric acid (1300 cm−1), which provides a secondary route to track this reaction. As shown in Figure 2C and D, the peak associated with citric acid decreases as the reaction proceeds. As expected, the peak at 1300 cm−1 rapidly decreases in the first 5 s of heating. Similar to the carbonate formation behavior, the citric acid in the cobalt-containing composite film continues to decrease between 5 and 3840 s of heating, while this decrease is much less for the copper-containing composite film. For quantification, the region associated with the citric acid consumption (Figure 2C and D) is cleaner than for the carbonate formation (Figure 2A and B). The convolution of multiple peaks in the infrared absorbance spectra from 1400 to 1700 cm−1 hinders the ability to accurately resolve the metal carbonate peak. Additional FTIR spectra (800−1900 cm−1) for cobalt carbonate (Supporting Information Figure S13) and copper carbonate formation (Figure S14) can be found in the Supporting Information. The peak height associated with the absorption due to citric acid is used to quantify conversion of metal nitrate−citrate to the corresponding metal carbonate as noted in the Supporting Information. Figure 3 illustrates the calculated conversion at several temperatures near the onset of carbonate formation as a function of reaction time. The formation of cobalt carbonate increases from near zero conversion to >60% conversion after 2 h irrespective of the reaction temperature as shown in Figure 3A. The conversion appears to continue to increase until near 100% conversion is obtained. A closer inspection of these conversion data provides insight into the temperature dependence of the reaction to form the carbonate. On heating at 240 °C, 90% conversion of cobalt nitrate to cobalt carbonate is achieved after 17 min, but then the conversion appears to decrease; it should be noted that this decrease is not due to the generation of citric acid and is likely an artifact associated with the data anaylsis. This decrease corresponds well to the second drop in film thickness from in situ ellipsometric measurements at this temperature (Figure 1A). Therefore, this apparent decrease in conversion is likely associated with the appearance of cobalt oxide, which impacts the film thickness. The apparent decrease in conversion might be associated with the thickness variations, as the exact same area of the film is not probed with the ex situ FTIR measurements. Irrespective, there are some additional peculiar features in these conversion data that are reproducible and cannot be explained by thickness variations associated with the oxide formation. At long times, the lowest conversion is observed for an intermediate temperature (200 °C), while the reaction

carbonates, so we can also approximate the activation energies for cobalt and copper carbonate formation as

k = A e−Ea / RT

(1)

where A is the pre-exponential factor (s−1), Ea is the activation energy (kJ/mol), and R is the universal gas constant (8.314 J/ mol·K). From Figure 1C, the activation energy for cobalt carbonate formation is estimated as 43 kJ/mol, while the activation energy for copper carbonate formation (Figure 1D) is estimated as 23 kJ/mol. Cobalt carbonate formation by decomposition of cobalt acetate has an activation energy of 93 kJ/mol;39 the lower estimated activation energy for formation of cobalt carbonate from the cobalt nitrate−citric acid complex is consistent with the lower stability of the complex in comparison to the acetate. The effective activation energy for copper carbonate is roughly one-half that of cobalt carbonate, which would suggest that the mesostructure for copper oxide should be less sensitive to temperature than the mesostructure of cobalt oxide if there is a correlation between the carbonate formation process and the final properties of the oxide produced from the carbonate. As an aside, we can perform a similar analysis based on the second thickness drop (Supporting Information Figure S12). For copper oxide formation, the activation energy is estimated as 148 kJ/mol, while the activation energy for cobalt oxide formation was not determined due to only two temperatures (220 and 240 °C) where a second drop in thickness was observed. To more directly investigate the conversion of cobalt and copper nitrate−citrate into corresponding carbonates, Figure 2 illustrates the temporal evolution of the FTIR spectra of block copolymer templated metal nitrate−citric acid composite films after heating at the same temperatures as investigated by in situ ellipsometry. To understand the change in the chemical composition, the evolution of absorbance peaks of cobalt carbonate (1585 cm−1) or copper carbonate (1575 cm−1) and citric acid (1300 cm−1) is quantitatively examined. A rapid increase in the intensity of the metal carbonate peak (1585 or 1575 cm−1) occurs after only 5 s of heating for both cobalt (Figure 2A) and copper (Figure 2B) containing films. This is consistent with the rapid decrease in film thickness observed in Figure 1. Increasing the reaction time from 5 to 3840 s further demonstrates the enhanced reaction kinetics for the copper carbonate relative to cobalt carbonate. There is a significant increase in the peak corresponding to the carbonate formation in the cobalt containing film, but this peak evolves significantly less for copper carbonate. 12142

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Figure 4. AFM micrographs of mesoporous cobalt oxide films calcined at 300 °C after isothermal carbonate formation at (A) 160 °C, (B) 200 °C, and (C) 240 °C and mesoporous copper oxide films calcined at 300 °C after isothermal carbonate formation at (D) 140 °C, (E) 180 °C, and (F) 220 °C (scale bar = 200 nm).

appears to be fastest at 180 °C. However, this temperature dependence of conversion does not appear to match the ellipsometry data; the initial thickness decrease appears to be completed much more rapidly than the chemical conversion to carbonate. This apparent divergent behavior is a result of the difference in the mechanisms examined by the two methods. The initial nitrate−citric acid−copolymer composite likely is not particularly mechanically robust, so volume change on conversion to carbonate will be almost completely translated to the change in film thickness. Once a sufficient carbonate framework develops, the material can store some stress generated by the conversion to carbonate, which will decrease the change in the film thickness. The FTIR measurement of conversion examines the material composition at the atomic scale. Thus, the pathway for the conversion to the carbonate could depend on the local environment, so the mechanism and pathway associated with the formation of additional carbonate could vary depending on the temperature. We hypothesize that this later variation is the origin of the temperature dependencies (low conversion at long times obtained on heating at 200 °C and the more rapid/higher conversion at long times for carbonate formation at 180 °C) on the conversion to cobalt carbonate illustrated in Figure 3A. To better understand if these temperature dependencies are specific to cobalt carbonate, identical experiments were performed for the fabrication of copper carbonate (Figure 3B). The first striking feature is that the conversion is >40% within the first 5 s of heating for all temperatures investigated. This is consistent with the much more rapid evolution of the film thickness for copper than cobalt (Figure 1). Similar to the cobalt, the films heated at 180 °C appear to achieve the highest

conversion at fixed reaction time, and the reaction kinetics seems to be more hindered at 200 °C. These data for both copper and cobalt carbonate formation appear to indicate that there is something intriguing about the carbonate formation at 180 °C that leads to the most rapid and highest conversion. The carbonate formation temperature impacts the final templated mesostructure of metal oxide films after calcination at 300 °C for 30 min as illustrated in Figure 4. At the lowest temperatures examined for cobalt carbonate formation (160 °C), the mesoporous film obtained on template removal and decomposition of carbonate to cobalt oxide exhibits a poorly ordered porous structure (Figure 4A). This film is comprised of cobalt oxide nanoparticles with diameter of 15 ± 3 nm. The block copolymer templated structure is not clearly ascertained from the micrographs, but the nanoparticles that comprise the film appear to be relatively uniform in size. On the basis of TEM micrographs (Supporting Information Figure S17), the average cobalt oxide nanoparticle size is 13 ± 4 nm, which is consistent with the AFM micrographs. At 200 °C (Figure 4B), the ordered morphology templated by the block copolymer is readily apparent with uniform spherical pores that are 15 ± 2 nm wide. Further increasing the carbonate formation temperature to 240 °C (Figure 4C) results in partial retention of the ordered structure, but many nanocracks appear in the film. The occurrence of such cracks can generally be attributed to inorganic framework shrinkage on calcination,19 but the oxide calcination conditions are identical for all of the micrographs shown in Figure 4. Thus, the stresses that develop through the carbonate formation appear to significantly impact the structure that forms on calcination to generate the mesoporous oxide. 12143

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Figure 5. X-ray diffraction profiles for (A) mesoporous cobalt oxide and (B) mesoporous copper oxide calcined at 300 °C after isothermal carbonate formation at the temperatures listed.

Figure 6. Adsorption−desorption isotherms of mesoporous (A) cobalt oxide and (B) copper oxide determined by ellipsometric porosimetry using ethanol. The influence of carbonate reaction temperature on the pore size distribution is determined from the adsorption isotherm for (C) mesoporous cobalt oxide with the carbonate formed at 160 °C (−), 200 °C (red − − −), and 240 °C (blue − - −) and (D) mesoporous copper oxide with the carbonate formed at 140 °C (−), 180 °C (red − − −), and 220 °C (blue − - −).

significantly higher temperatures (220 °C) as shown in Figure 4F. Additionally, the pore size becomes nonuniform; these asymmetric mesopores are likely formed by crack propagation similar to what is found for cobalt oxide (Figure 4C), but the cracks do not propagate as far in copper oxide. AFM micrographs of cobalt and copper oxides formed at other carbonate formation temperatures are included in the Supporting Information (Figure S16). To further investigate the structure of porous oxides, X-ray diffraction profiles were obtained (Figure 5). The diffraction peaks for mesoporous cobalt oxide (Figure 5A) can be indexed to (220), (311), (222), (400), and (511) crystal planes of cubic Co3O4 (PDF card no. 01-078-1969). A cubic Co3O4 crystal phase is found for all of the mesoporous cobalt oxide,

Similar conclusions can be drawn from the copper-containing films. Copper carbonate formation at the lowest temperature examined (140 °C) leads to a mesoporous copper oxide film (Figure 4D) that consists of copper oxide nanoparticles (diameter: 16 ± 3 nm). These are slightly larger than the nanoparticles of copper oxide determined from TEM micrographs (9 ± 4 nm). For the copper oxide films, some larger mesopores appear that are not obtained for the cobalt oxide films (Figure 4A). Fabricating the copper carbonate at 180 °C leads to relatively uniform mesopores with a diameter of 15 ± 6 nm after calcination (Figure 4E). Local near hexagonal packing of the mesopores is consistent with the templating by the block copolymer. The ordered mesostructure in copper oxide degrades significantly if the copper carbonate forms at 12144

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Figure 7. Influence of initial carbonate reaction temperature on the porosity of mesoporous (A) cobalt oxide and (B) copper oxide films as determined by ellipsometric porosity (EP) measurements (blue ●) and from the refractive index of the films using the Bruggeman effective medium approximation (red ▲).

H 2 -type hysteresis indicates an interconnected porous architecture in these copper oxide thin films. To better illustrate the difference in the pore texture of the films, the pore size distribution (PSD) is estimated from the absorption isotherms using the Kelvin equation. For the cobalt oxide films, the PSD varies tremendously based on the temperature used to form the carbonate (Figure 6C). At the lowest temperature (160 °C), there is a bimodal distribution: the majority of the pores exhibit Kelvin radius (rk) of approximately 4.5 nm with a minority population of rk ≈ 2.9 nm for this film. This sharp PSD is somewhat surprising as the film appears to consist of nanoparticles and is not highly ordered. However, the sharpness of the PSD is not always correlated with the extent of order in mesoporous materials.40 Similarly, carbonate formation at 200 °C also leads to a bimodal PSD. This film appears to be the best-ordered film (Figure 4B), but the PSD is broader for both the primary and the secondary pore sizes than for the lower carbonate temperature. In this case, the primary pore size is rk ≈ 3.8 nm, which is similar to the small pore size obtained at lower temperature, but the larger pore size is centered at rk = 6.4 nm. This larger pore size (rk = 6.4 nm) differs slightly from the pore radius (∼7.5 nm) determined from AFM (Figure 4B) due to the presence of preadsorbed ethanol layer33 formed at lower p/p0 during ellipsometric porosimetry measurements. The formation of cobalt carbonate at 240 °C leads to a substantial broadening of the PSD in the cobalt oxide. The mode in rk is 3.7 nm (Figure 6C), but the mesopores extend from 2 to >9 nm from the primary distribution. Additionally, a minor fraction of the pore extends to much larger sizes; we attribute these larger mesopores to the nanocracks that form across the film surface (Figure 4C). For the copper oxide, there is significantly less variance in the average rk, approximately 4.5 nm, irrespective of carbonate formation temperature. However, the PSD for the mesoporous copper oxide is significantly broader than that for the cobalt oxide. This broad distribution of pore sizes is consistent with the surface morphologies for the copper oxide films (Figure 4D−F). As the temperature for carbonate formation increases, the PSD narrows, but there is a distribution of mesopores between 2 and 10 nm in all cases. From the EP measurements, the porosity of the films can be estimated from the maximum volume fraction of ethanol adsorbed from the isotherms (Figure 6A and B). Figure 7 illustrates the porosity of cobalt and copper oxide films generated from different carbonate formation temperatures. In addition to using the EP data, the refractive index of the neat mesoporous film can be used to estimate the porosity through

irrespective of the carbonate formation temperature. The average cobalt oxide crystal size from the Scherrer equation decreases from 5.5 to 4.3 nm as the carbonate formation increases from 160 to 240 °C. The smaller oxide crystals at higher temperatures may be associated with the formation of nuclei during carbonate formation. For example, cobalt oxide begins to form concurrently with the carbonate at 240 °C after 17 min (as determined by FTIR). Carbonate formation at 160 °C does not appear to generate any oxide. Thus, in these cases, the oxide crystal size depends on relative nucleation and growth rates. The average cobalt oxide crystal size is smaller than the average nanoparticle size (∼13 nm) determined from TEM micrographs, which also indicates that these cobalt oxide nanoparticles that comprise the films are polycrystalline. For mesoporous copper oxide (Figure 5B), the diffraction peaks can be indexed to (1̅11), (002), (111), and (200) crystal planes of monoclinic C2/c CuO (PDF card no. 00-048-1548). The average copper oxide crystal size is 6.8 nm from the Scherrer equation, and independent of carbonate formation temperature. This invariance in average crystal size is similar to the mesostructured particles of copper oxide that is almost independent of carbonate processing temperature (Figure 4). The AFM micrographs clearly demonstrate a significant influence of the carbonate reaction temperature on the surface morphology, but this does not provide insight into the internal pore structure of the film. Ellipsometric porosimetry (EP) can quantify the differences in porosity and pore size for mesoporous metal oxide thin films subjected to different carbonate formation temperatures as shown in Figure 6. The refractive index of the mesoporous metal oxide films is used to determine the volume fraction of adsorbed ethanol by the Lorentz−Lorenz effective medium approximation (EMA).34 Figure 6A illustrates the type-IV isotherm with a H1-type hysteresis loop obtained for the cobalt oxide film that was processed through carbonate formation at 160 °C. Increasing the temperature to 200 °C, the mesoporous cobalt oxide film exhibits two capillary condensation steps indicative of bimodal porosity in the film. This bimodal pore size distribution is not clear from the surface morphology in the AFM micrograph (Figure 4B). For carbonate formation at 240 °C, a broad typeIV isotherm with H4-type hysteresis loop is observed in the mesoporous cobalt oxide film. This isotherm is suggestive of a broad pore size distribution, which is not unexpected based on the distribution of sizes present at the surface of the film (Figure 4C). All copper oxide films exhibit H2-type hysteresis loops, corresponding to a wide distribution of pore radii and a narrow distribution of pore length as shown in Figure 6B. The 12145

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The Journal of Physical Chemistry C the Bruggeman effective medium approximation (BEMA).35 The porosity of both metal oxides exhibits a maximum with respect to the carbonate formation temperature. For the cobalt oxide (Figure 7A), there is a significant difference in the porosity calculated from the two methods. The porosity from EP is always significantly less than that from BEMA, almost 30% lower from EP. In application of BEMA, the refractive index of the cobalt oxide framework (n = 2.16)41 assumes a fully crystalline cubic (spinel) metal oxide. As this mesoporous cobalt oxide is semicrystalline, the actual refractive index of the framework will be lower, so the porosity from BEMA will be an upper estimate. This is consistent with the large difference between the average crystal size from XRD (Figure 5A) and the average particle size from AFM (Figure 4A−C). Conversely, the porosity determined from the EP measurements will provide a lower estimate as the pores are assumed to be fully filled by ethanol. Any mesopores that are not accessible will not be filled. Comparing the AFM micrographs of the cobalt oxide films obtained at 180 °C (Supporting Information Figure S16) and 200 °C (Figure 4), the film heated after forming carbonate at 200 °C is better ordered with well-maintained porous mesostructure as confirmed by the width associated with correlation peak from the fast Fourier transform (FFT) of the AFM micrographs: the peak from the film fabrication at 180 °C (22.5 μm−1) is much broader than that at 200 °C (17.15 μm−1). However, the porosity from the film with 180 °C carbonate formation (43.3% from EP; 64.8% from BEMA) is greater than the porosity with 200 °C carbonate formation (41% from EP; 59.6% from BEMA). For block copolymer templated mesoporous alumina, the extent of order of the mesoporous structure is not always correlated with the surface area and porosity,40 so the observed changes in porosity are not unexpected. Moreover, the total surface areas of these mesoporous cobalt oxide films can be estimated from t-plots as STOT = βTOT

the templated copper oxide is crystalline and has an interconnected porous structure. This agreement is consistent with the small difference in the average crystal size from XRD and the primary particle size determined from TEM. Similar to the cobalt oxide, there is a maximum in porosity at 180 °C for the copper oxide. The total surface area determined from tplots indicates that mesoporous copper oxide obtained through carbonate formation temperature at 140 °C has a low surface area of 141 m2/cm3, while copper oxide fabricated through carbonate formation temperature at 180 and 220 °C provides a higher total surface area of 216 and 262 m2/cm3, respectively. To explain why 180 °C for carbonate formation leads to the highest porosity, the differences in the composition prior to calcination were determined from the kinetic studies of conversion by FTIR (Figure 3). At long reaction times for carbonate formation, the highest conversion of metal nitrate− citrate to metal carbonate is obtained at 180 °C for both metal nitrates. This suggests the clear correlation between metal carbonate formation and final oxide mesostructure. Thus, with careful selection of metal carbonate formation processing temperature, control over the metal oxide mesostructure and pore texture is likely possible for a wide variety of transition metal oxides.

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CONCLUSION In this work, the impact of metal carbonate formation temperature on the final structure and pore properties of micelle templated porous cobalt oxide and copper oxide is determined. The metal nitrate−citrate strategy used here to fabricate mesoporous metal oxides through a metal carbonate intermediate enabled facile control of the pore texture and mesostructure by changes in the intermediate processing parameters. Using in situ ellipsometric film thickness measurements along with FTIR spectroscopy, we demonstrated that the kinetics of copper carbonate formation is much faster than that of cobalt carbonate formation with a lower apparent activation energy for copper carbonate (23 kJ/mol) than for cobalt carbonate (43 kJ/mol). From FTIR, the conversion to carbonate is not monotonic with temperature as typically expected; we attribute this to differences in pathways for carbonate formation. The porosity of the mesoporous oxide film appears to be impacted by the extent of the conversion to carbonate prior to calcination with increased carbonate conversion leading to larger porosity. The optimum carbonate formation temperature for maximizing porosity for both cobalt and copper is 180 °C. These results indicate that the carbonate formation temperature is a powerful route to modulate the structure of soft templated, ordered mesoporous transition oxides.

ρL ρG

where ρL and ρG are the densities of ethanol adsorbate in liquid and gaseous state, respectively, and βTOT is the slope of t-plot (Supporting Information Figure S18) at low relative vapor pressure. The slope βTOT represents the change in volume of absorbed ethanol as a function of the thickness of ethanol layer. The t-plot methodology has been used previously to determine surface areas of block copolymer templated mesoporous silica films using water as an adsorbate.42 However, the surface areas determined using this method are underestimated due to uncertainty associated with ρL and ρG of ethanol. In case of mesoporous cobalt oxide obtained through carbonate formation at 160 °C, the surface area is only 33 m2/cm3. When a higher carbonate formation temperature of 200 °C is utilized, the surface area increases to 178 m2/cm3, while it decreases to 98 m2/cm3 when the carbonate formation temperature is 240 °C. The trend in surface area change matches well with the porosity data, emphasizing the need to utilize an optimum carbonate formation temperature to obtain high porosity and surface area in the mesoporous cobalt oxide thin films. For the copper oxide films, the difference in porosity between EP and BEMA calculations is much less, except at the lowest temperatures for copper carbonate formation. At temperatures greater than 180 °C, there is no statistical difference in porosities between the two techniques. This consistency in porosity between EP and BEMA indicates that

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ASSOCIATED CONTENT

S Supporting Information *

Chemical structures, GPC elution curves, 1H NMR characterization, TGA traces of PMPEGMA-b-PBA and metal nitrate− citric acid complexes, temporal evolution of refractive index of thin films at different temperatures, FTIR spectra evolution with time and temperature for cobalt nitrate−citrate and copper nitrate−citrate, determination of reaction rates, fits of conversion data to first order, second order, and Jander reaction models, additional AFM images of mesoporous cobalt and copper, TEM micrographs of scraped thin films of cobalt and copper oxide, t-plots for determining surface area in mesoporous cobalt oxide and copper oxide films, and 12146

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explanation of metal carbonate conversion calculations. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b02177.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address †

Department of Chemical and Biomolecular Engineering, North Carolina State University, Raleigh, North Carolina 27695, United States.

Author Contributions ‡

M.C.B. and S.M.B. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by the National Science Foundation (NSF) through award no. CBET-1336057. We thank Yuanzhong Zhang for his help with FTIR spectra deconvolution and analysis. M.C.B. acknowledges support from NSF REU site in Polymers at the University of Akron (grant no. 1359321).

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