Effects of Precursor Solution Aging and Other Parameters on

Article pubs.acs.org/JPCC

Effects of Precursor Solution Aging and Other Parameters on Synthesis of Ordered Mesoporous Titania Powders Jessica Veliscek-Carolan,† Robert Knott,† and Tracey Hanley*,† †

Australian Nuclear Science and Technology Organisation, Locked Bag 2001, Kirrawee DC, NSW 2232, Australia S Supporting Information *

ABSTRACT: Evaporation-induced self-assembly (EISA) of ordered mesoporous titania powders using block copolymer templates Brij 58 and F127 has been studied as a function of the precursor solution composition and age as well as the evaporation conditions. Small-angle X-ray scattering was used to monitor the degree of order in the mesoporous structure of materials synthesized under these varying conditions. Also, for the first time, the time-dependent formation of Ti structures in precursor solutions and the effect of those structures on the creation of mesostructural order have been demonstrated. The interactions of the Ti precursor with Brij 58 and F127 were investigated and showed that the different templates caused formation of Ti oligomers of unique sizes and structures. Precursor solution composition and evaporation conditions were also shown to affect the order and stability of the mesoporous titania produced. Overall, this systematic study has provided fundamental insights into the synthesis conditions that maximize the degree of order and thermal stability of the final materials. These “optimal” conditions are highly dependent on the choice of template. As a result of this improved understanding, the synthesis of ordered mesoporous titania powders using the block copolymer F127 as a template has been achieved without the use of stabilizing agents for the first time.

1. INTRODUCTION Although synthesis of ordered mesoporous titania thin films has been achievable for some time,1−3 synthesis of ordered mesoporous titania powders in large quantities via evaporation induced self-assembly (EISA) remains challenging. Mesoporous titania has long been a synthetic target due to its utility in applications such as catalysis, sorption, nuclear separations, optics and photovoltaics, solar cells, and hydrogen production.4−11 The EISA method of synthesis is a common and simple soft-templating method for producing mesoporous titania and is represented schematically in Figure 1. During EISA, a mixture of inorganic precursors, surfactant, water, and alcohol are evaporated under controlled conditions such that the surfactant molecules self-assemble into liquid crystal structures that act as templates for the inorganic precursors which are simultaneously hydrolyzed and condensed by an ingress of water from the atmosphere.12 For photoinduced applications such as catalysis, the template should then be completely removed from the titania surface.13 Thermal treatment is often used to remove the templating polymer, but this can cause structural collapse due to growth of crystallites in the pore walls.2 To prevent this collapse, additives such as ethylene diamine,14,15 ammonia,16 acetyl acetone,17,18 or sulfuric acid19 are usually required to stabilize the mesoporous titania structure. However, such modifiers can adversely impact the performance of titania materials in their final application.20 To the best of our knowledge, this paper reports the first synthesis of a substantial quantity of ordered mesoporous titania powder using commercially available block © 2015 American Chemical Society

Figure 1. Schematic representation of evaporation induced selfassembly (EISA) process.

copolymer template F127 without the use of any stabilizing additives to give a clean final product. As mentioned, the use of stabilizing additives during synthesis is undesirable because it introduces chemical Received: December 23, 2014 Revised: March 10, 2015 Published: March 11, 2015 7172

DOI: 10.1021/jp5127927 J. Phys. Chem. C 2015, 119, 7172−7183

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The Journal of Physical Chemistry C complexity that increases the time and cost of production. In addition, great care must be taken that postsynthesis removal of these additives is complete. This may require, for example, thermal treatment at substantially higher temperatures than are required to remove the template.15 Synthesis of a substantial quantity of thermally stable ordered mesoporous titania powder without the use of stabilizing additives has been achieved with only a limited number of structure directing agents: the triblock copolymers P123 (EO20PO70EO20, EO = ethylene oxide and PO = propylene oxide)21,22 and F108 (EO130PO60EO130)22 or noncommercially available surfactants such as poly(ethylene oxide)-block-polystyrene (PEO-b-PS)1,17,23 and polyoxoethylene fluoroalkyl ether.16 The time and cost advantages of using a less expensive, commercially available structure directing agent over one that must be synthesized, are compelling. Since the commercially available triblock copolymers P123 and F108 are structurally similar (EOxPOyEOx), it was of interest to investigate EISA synthesis using a structurally different diblock copolymer Brij 58 (C16H33EO20). A similar triblock copolymer with an EO/PO ratio intermediate between P123 and F108 was also investigated, namely F127 (EO100PO70EO100). This study will thus investigate the effects of varying both block type and length in block copolymer templates on the final materials produced, which is highly relevant when scaling the production for an application. To achieve formation of ordered mesoporous titania powders without the use of stabilizing additives via EISA, strict control over the rate of hydrolysis and condensation is required. Previous work on synthesis of mesoporous titania in the form of thin films or larger scale powders via EISA has shown that the structures formed are highly dependent upon the precursor solution composition24 and age25 as well as the temperature, relative humidity (RH),18,26 and length of time over which evaporation is performed.2 The effect of all these parameters was explored in this work; however, the investigation of the formation of Ti structures in precursor solutions during aging at room temperature prior to evaporation was of particular interest, as it has not been previously explored. Variation of all the conditions of synthesis was observed to affect both the degree of order and the thermal stability of the resulting materials. Mesoporous titania powders were synthesized under the conditions found to optimize the formation of order and thermal stability using Brij 58 or F127 templates. Following template removal, these materials were fully characterized using SAXS, XRD, TEM, CHN analysis, and nitrogen porosimetry, to allow comparison of the different block copolymer templates.

Table 1. Precursor Solution Compositions molar ratio solution name

TiCl4

EtOH

PS-28 PS-11 PS-5 PS-10

1 1 1 1

40 40 40 40

surfactant 0.028 0.011 0.005 0.010

Brij 58 Brij 58 F127 F127

H2O 10 10 10 10

chamber was internally lined with low density polyethylene (LDPE) film to prevent contamination as well as adverse reactions due to expulsion of HCl during evaporation. The evaporation chamber was also externally insulated. The temperature in the evaporation chamber was controlled by flowing water through the walls of the chamber on a closed loop and a Polyscience temperature control unit was used to set the temperature. The relative humidity and flow of air through the chamber were controlled using an IQI Instruquest HumiSys HF (High Flow) Relative Humidity Generator with a Watlow temperature control unit. Temperature and relative humidity were measured periodically during evaporation using a calibrated Rotronic HygroPalm humidity probe. As-made samples were thermally treated using a ramp rate of 0.5 °C/ min and were held at 200 °C for 2 h before higher temperature treatment to complete the template removal. After template removal, approximately 150 mg mesoporous titania powder was typically produced (photograph provided in the Supporting Information). It would be possible to obtain approximately 3 g of sample if the entire surface area of the evaporation chamber (900 cm2) were utilized instead of 80 mm diameter Petri dishes. The system chosen consisted of standard process engineering methodologies which enable up-scaling to produce larger material quantities. Small-angle X-ray scattering (SAXS) measurements were performed using a NanoSTAR SAXS camera (Bruker-AXS, Karlsruhe), with 3 pinhole collimation for point focus geometry. The instrument source was a copper rotating anode (0.3 mm filament) operating at 45 kV and 110 mA, fitted with Montel mirrors, producing Cu Kα radiation of wavelength 1.54 Å. The SAXS camera was fitted with a Vantec2000 2D detector. The selected sample-to-detector distance was 730 mm, which provided a Q-range of 0.01 to 0.39 Å−1 (Q = (4π sin θ)/λ, where 2θ is the scattering angle and λ is the wavelength of the incident X-rays). Due to the maximum resolution of the instrument, d-spacings are reported with an error value of 10 Å. The background has been subtracted from all presented SAXS data. In addition, SAXS data of precursor solutions have been placed on an absolute scale using the absolute intensity of water reported by Orthaber et al.27 For mesoporous titania powder samples, it was not possible to place the data on an absolute scale as the path length could not be quantitatively determined; however, the amount of sample in the beam was adjusted so that transmission values were comparable between samples. Thus, the degree of order could only be treated as relative variations. Relative intensities of correlation peaks in the SAXS data were calculated based on the number of counts at the peak position divided by background counts at that position. Guinier fitting of SAXS data was performed, using PRIMUS software,28 to afford values for the radius of gyration (Rg) of scattering structures. Rg is the root-mean-square distance of an object’s parts from its center of mass and as such gives a shape-independent indication of the size and density of an object. Fitting of

2. MATERIALS AND METHODS Precursor solutions were prepared by addition of titanium tetrachloride (TiCl4) to an ethanolic solution of block copolymer (Brij 58 or F127) in a glovebox to exclude oxygen and water. This was followed by addition of water in the appropriate ratios to produce a clear, colorless solution which was sealed in a Pyrex Schott bottle. Precursor solutions of interest are listed in Table 1 and are designated PS-XX, where XX was the molar ratio of surfactant multiplied by 1000. Precursor solutions were aged statically in sealed vessels at room temperature to prevent changes in sol solvent composition. Also, overall volumes were monitored to ensure consistency. After aging, 4.0 mL volumes were syringed into 80 mm diameter Petri dishes. These were placed in a custom-made 16 L evaporation chamber with controlled air flow, temperature, and humidity to undergo EISA. The evaporation 7173


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The Journal of Physical Chemistry C SAXS data using power law, Debye, unified fit and various peak models was performed using SasView 2.2.0. Details of the fitting models used are provided in the Supporting Information. Viscosity measurements were performed using a Brookfield LVDV-II+PX viscometer and an Ubbelohde size 1 Ostwald glass viscometer. Measurements using the Brookfield LVDV-II +PX viscometer were performed in triplicate at 25 °C using three different shear rates. Measurements using the Ostwald glass viscometer were performed once at 24 °C as the relatively long experiment times resulted in evaporation of the solutions, increasing the viscosity between measurements. X-ray diffraction (XRD) patterns were measured with a PANalytical X’Pert Pro diffractometer (Almelo, The Netherlands) using Cu Kα radiation (λ = 1.54 Å) at 40 kV and 30 mA. The data were recorded from 5°−95° with a step size of 0.02°. Transmission electron microscopy (TEM) samples consisted of crushed specimen dispersed in ethanol then dispensed onto holey carbon film supported on a TEM copper mesh grid. A JEOL 2010F operated at 200 keV was used to characterize the samples via bright field imaging and selected area electron diffraction. Elemental microanalyses of C, H, and N content for thermally treated mesoporous titania were performed using a model PE2400 CHNS/O elemental analyzer, PE Datamanager 2400 for Windows and a PerkinElmer AD-6 Ultra Micro Balance (CHNS/O Microanalysis Service at Macquarie University). The samples were combusted in oxidizable metal containers, and the combustion products, after scrubbing and oxidation, were quantitatively measured using frontal gas chromatography. Nitrogen adsorption−desorption isotherms were obtained by multipoint nitrogen gas sorption experiments at 77 K after degassing at 150 °C on a Micromeritics ASAP 2020 adsorption analyzer (Norcross, GA). Surface areas were estimated according to the Brunauer−Emmett−Teller (BET) method, while pore volume and pore size distributions were calculated using the Barrett−Joyner−Halenda (BJH) method based on the desorption branch.

Figure 2. SAXS patterns of samples prepared using 4 mL volumes of 1 TiCl4: 40 EtOH: xBrij 58: yH2O precursor solutions aged in a sealed vessel for 6 days then evaporated at 30 °C and approximately 65% RH.

mesoporous titania-zirconia materials with F127.29,30 SAXS parameters of the resulting materials are shown in Table 2. PS10 produced a sample with a similar d-spacing and width but higher relative intensity than PS-5, indicating a higher degree of order. The sample produced using PS-10 also demonstrated higher thermal stability. Table 2. d-Spacings and Relative Intensities of Peaks in SAXS Patterns of Samples Prepared Using 4 mL Volumes of Precursor Solution Aged in a Sealed Vessel for 6 days and Evaporated at 37 °C and Approximately 50% RH for 14 Days 100 oC

350 oC

precursor solution

d-spacing (Å)

Irel

d-spacing (Å)

Irel

PS-5 PS-10

132 ± 10 146 ± 10

9 10

part collapse, 102 ± 10 102 ± 10

3 7

3.1.2. Precursor Solution Aging. In this study, aging of precursor solutions containing TiCl4, ethanol, Brij 58, and water in a sealed vessel between 1 and 36 days was investigated. SAXS was used to directly observe the formation of nanosized objects in these solutions over time, as well as to determine the degree of ordering in mesoporous titania powders synthesized via EISA using precursor solutions of different ages. Initially, the aging of PS-28 was investigated by SAXS, and the results are shown in Figure 3. The scattering pattern from the Brij 58 polymer, in the absence of the titanium precursor, was measured in a solution of 4:1 ethanol:water in a sealed capillary with a Brij 58 concentration of approximately 1.5%. Using the Guinier approximation, a Rg of 8.9 ± 0.9 Å was calculated for the solvated Brij 58 scatterers in this solution. The Guinier plot is shown in Figure S1 of the Supporting Information. A Debye model was also used to fit the Brij 58 block copolymer solution scattering data and resulted in Rg = 10.5 ± 0.5 Å and scale = 0.016 ± 0.0004, with the goodness of fit parameter χ2/Npts = 0.6, indicating a good fit to the experimental data. The Rg values from the Guinier and Debye models are hence consistent with each other and suggest that the Brij 58 was a polymeric structure with Rg ∼10 Å when in solution with 4:1 ethanol:water. Figure 3 also displays the scattering profiles of PS-28 aged between 7 h and 36 days in a sealed vessel. The scattering was

3. RESULTS AND DISCUSSION 3.1. Precursor Solution. 3.1.1. Effect of Precursor Solution Composition. To determine the optimal precursor solution composition using Brij 58 as a template, mesoporous titania was synthesized using TiCl4, ethanol, Brij 58 and water, with a molar ratio of 1:40:x:y, varying the amounts of Brij 58 and water. The results of the SAXS measurements on the resulting materials are shown in Figure 2. The SAXS patterns of all the as-made samples contained correlation peaks, indicating the presence of mesoporous order in the samples. The dspacing of the peaks indicated a 63−69 Å repeating unit size. As such, the d-spacing did not vary substantially for the different precursor compositions. The widths and relative intensities of the peaks in the SAXS patterns provide an indication of the degree of order in the samples. Narrower, more intense peaks indicate a greater degree of ordering. As such, it is clear that the sample prepared from precursor solution with composition 1 TiCl4:40 EtOH: 0.028 Brij 58:10 H2O (designated PS-28) was the most ordered. Synthesis of ordered mesoporous titania using F127 as a template was performed using precursor solution compositions of 1 TiCl4:40 EtOH:0.005 F127:10 H2O (PS-5) or 1 TiCl4:40 EtOH:0.01 F127:10 H2O (PS-10), as these compositions were previously shown to produce ordered, thermally stable 7174


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1.1), confirming the polymeric nature of the scatterers. The Rg from the Debye fit of 29.6 ± 0.2 Å was also similar to the Guinier and unified fit models. The SAXS data of PS-28 aged for 36 days in a sealed vessel showed a substantial change in shape and an increase in scattering intensity relative to the solutions aged for shorter periods of time (Figure 3). At low Q, the scattering curve had a weak dependence on Q, so a Guinier analysis was performed (Figure S1 of the Supporting Information). This indicated that Rg had again increased to 41.4 ± 0.3 Å. A Q−2.1 dependence was observed in the mid to high Q range for this scattering data, suggesting the presence of polymeric structures in solution. However, the Debye model did not provide a good fit to the scattering data (χ2/Npts = 13), suggesting that the polymer chain did not display Gaussian statistics (that is, it was not fully flexible). As a result, a unified fit was applied, since this model is able to fit SAXS data from polymeric mass fractals with mass fractal dimensions that deviate from 2.0.33 The unified fit delivered Rg = 41.4 ± 0.1 Å and power law = −2.2 ± 0.01 (χ2/ Npts =1.6 (other parameters are given in the Supporting Information). This Rg was consistent with that determined via Guinier analysis, and the polymeric constraint was obeyed. The deviation of the power law from 2.0 indicated the presence of a polymeric mass fractal. The fact that the fitted power law of 2.2 was greater than 2.0 suggests that there was more mass inside the Rg of the polymeric structure than in a Gaussian chain. This increased density of the polymer may be a result of branching or closer packing of the polymer chains due to the initiation of self-assembly. The pair distance distribution function P(r)34 was also calculated for this system and provided evidence that the polymeric mass fractals had an elongated structure, with Rg = 43 Å and the maximum dimension of the structure, Dmax = 160 Å (Figure S4 of the Supporting Information). To investigate how the aging of precursor solutions was affected by composition, specifically the amount of surfactant present, SAXS data of a precursor solution containing a lower concentration of Brij 58 (1 TiCl4:40 EtOH:0.011 Brij 58:10 H2O, PS-11) was collected over time (Figure 4). SAXS data of the Brij 58 polymer in the absence of Ti is also shown in Figure 4, again in a solution of 4:1 ethanol:water in a sealed vessel and, in this case, with a concentration of approximately 0.5%. Using the Guinier approximation, an Rg of 11.6 ± 0.9 Å was calculated

Figure 3. SAXS patterns of PS-28 aged between 7 h and 36 days in a sealed vessel. Data is on an absolute scale, but traces have been offset for clarity.

dominated by the structure of the Ti-complexes in solution because Ti has a much higher atomic number than the C, H, and O in the Brij 58. The SAXS data of PS-28 aged between 7 h and 6 days had a Q−x dependence where x = 0.01−0.4. As a consequence, only Guinier analysis31 was performed, and the results in Table 3 indicate that PS-28 contained Ti-based structures that slowly increased in size over 6 days of aging. Guinier plots are shown in Figure S1 of the Supporting Information. Viscosity measurements of PS-28 immediately after preparation and 6 days after preparation showed that the viscosity of the aged sol was unchanged (see data in the Supporting Information). Table 3. Growth of Ti Structures in Precursor Solutions over Time. nd = Not Determined. PS-28

PS-11

age (days)

Rg (Å)

Rg (Å)

0.3 2 3 4 5 6 17 36

2.5 ± 0.3 2.4 ± 0.3 4.1 ± 0.3 5.8 ± 0.3 8.6 ± 0.4 11.8 ± 0.6 25−30 41−43

1.5 ± 0.3 1.5 ± 0.4 2.4 ± 0.3 2.8 ± 0.3 nd 3.8 ± 0.2 15.9 ± 0.5 35−36

When PS-28 had been aged for 17 days, more intense scattering at low Q was observed (Figure 3). Guinier analysis indicated that the Rg had increased to 25.0 ± 0.8 Å (Figure S1 of the Supporting Information). Beyond the Guinier region, the scattering decreased with a Q−1.5 dependence, suggesting scattering behavior intermediate between a rod and a Gaussian chain. A unified fit model (level 1)32 was used to successfully fit the scattering data (χ2/Npts = 1.1) and the fitting parameters, which are given in full in the Supporting Information, included Rg = 27.9 ± 0.2 Å and power law = −2.0 ± 0.1. As such, the Rg values determined by the Guinier and unified fit models were consistent. The power law of −2.0 combined with the unified fit obeying the polymeric constraint33 indicated that the scatterers were polymeric in nature, so a Debye fit was performed. This also provided a good fit to the data (χ2/Npts =

Figure 4. SAXS patterns of PS-11 aged between 7 h and 36 days in a sealed vessel. Data is on an absolute scale, but traces have been offset for clarity. 7175


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samples with a higher degree of order. Increasing the evaporation time from 1 to 7 days for 6 day aged PS-28 caused a decrease in order as evidenced by the broadening of the correlation peak. Comparison of the SAXS data for both samples evaporated for 7 days shows a substantially less intense peak and, hence, less order was present after aging PS-28 for 14 days rather than 6 days. In order to determine the effect of aging on F127 precursor solutions, SAXS patterns of PS-10 in a sealed vessel were measured over time, and the results are shown in Figure 6. The plot on the left in Figure 6 shows the scattering profile of the sealed solution without the Ti precursor added and gives an indication of the scattering from the F127 polymer itself. Strong scattering is evident from the solution of F127 in 4:1 ethanol:water. Using the Guinier approximation, a Rg of 19.1 ± 0.3 Å was determined for the F127 structures in this solution. The Guinier plot is shown in Figure S3 of the Supporting Information. Beyond the Guinier region, the scattering decreased with a Q−2.0 dependence, which is indicative of polymeric scattering. However, the Debye model did not provide a satisfactory fit to the experimental data (χ2/Npts = 3.2), therefore the unified fit model (level 1) was employed. The unified model provided a good fit (χ2/Npts = 1.1) with Rg = 17.8 ± 0.1 Å and power law = −4.1 ± 0.3 (other parameters are given in the Supporting Information). The polymeric condition was not fulfilled. This Rg was similar to the Guinier analysis, and the power exponent of −4.1 was suggestive of Porod scattering from smooth three-dimensional objects such as spheres. The indirect Fourier transform of the SAXS data gave a Rg of 20 Å and Dmax of 70 Å (Figure S5 of the Supporting Information), suggesting elongation of the scattering structure. The SAXS data for PS-10 aged from 7 h to 36 days in a sealed vessel is also shown in Figure 6. These data show that addition of TiCl4 to the precursor solution dramatically decreased the intensity of scattering, indicating the disruption of the F127 structures previously described. As PS-10 was aged over 36 days in a sealed vessel, its SAXS pattern remained relatively unchanged. For example, the scattering decreased with a Q−x dependence, where x = 1.0−1.3 in the mid to high Q region independent of the solution age, which suggests that the scattering objects were consistently elongated or rod shaped. Guinier analysis also indicated that the Rg was similar at 7 h (15.8 ± 0.5 Å) and 36 days (17.1 ± 0.5 Å). The Guinier plots are shown in Figure S3 of the Supporting Information. The main observable difference in the data was an increase in the intensity of scattering between 6 and 17 days, suggesting an increase in the number of scattering objects present in solution. Viscosity measurements of PS-10 immediately after preparation and 6 days after preparation showed that the viscosity did not change appreciably and increased by only 1−2 cP upon aging (see data in the Supporting Information). 3.1.3. Summary of Precursor Solution Effects. In terms of precursor composition, PS-28 and PS-10 produced the most ordered mesoporous titania using Brij 58 and F127, respectively. These compositions were considered optimal and were used in further experiments to determine the evaporation conditions required to maximize the degree of order produced. In regard to precursor solution aging, it has been shown previously that the ordering of mesoporous titania films produced via EISA can depend on the age of the precursor solution used for spin-coating of the films.35 Acidic precursor

for the solvated Brij 58 scatterers in this solution. The Guinier plot is shown in Figure S2 of the Supporting Information. A Debye model was also applied and provided a good fit to the experimental data (χ2/Npts = 0.8) with Rg = 11.7 ± 1.1 Å and scale = 0.008 ± 0.0004. The SAXS data in Figure 4 for PS-11 aged between 7 h and 6 days in a sealed vessel had a Q−x dependence, where x = 0.03− 0.2, as was seen previously for PS-28. Rg values from Guinier analysis of SAXS data for these samples are given in Table 3 and indicate the presence of Ti-based structures that grew over time. Guinier plots are shown in Figure S2 of the Supporting Information. The intensity of scattering from PS-11 aged for 17 days in a sealed vessel only slightly increased relative to the “younger” solutions. Guinier analysis of this solution (Figure S2 of the Supporting Information) indicated an Rg of 15.9 ± 0.5 Å, and the scattering curve exhibited a Q−1.0 dependence in the mid to high Q range, suggesting rod type scattering structures. This Rg indicates further growth of the scattering structures with increased age. The SAXS data of PS-11 aged for 36 days in a sealed vessel showed a more substantial increase in scattering intensity relative to the solutions aged for shorter periods of time (Figure 4). At low Q, the scattering curve had a weak dependence on Q, so a Guinier analysis (Figure S2 of the Supporting Information) was performed. This indicated the Rg was equal to 34.5 ± 0.4 Å. In the mid to high Q range, the data showed a Q−2.0 dependence, indicating scattering from polymeric structures. Although a Debye model provided a reasonable fit for this data (χ2/Npts = 2.0), the unified fit model (level 1) provided a better fit (χ2/Npts = 1.1). The unified fit delivered Rg = 36.5 ± 0.2 Å and power law = −2.4 ± 0.04 (other parameters are given in the Supporting Information). This Rg is similar to that determined via Guinier analysis, and the polymeric constraint was obeyed. The power law fitted value of −2.4 suggests the presence of a branched or close-packed polymeric mass fractal. To determine the effect of the observed changes in the precursor solutions with age on the structure of the mesoporous titania produced from these solutions, SAXS measurements were performed on as-made samples synthesized using PS-28 that had been aged between 3 and 14 days in a sealed vessel. The results are given in Figure 5. It is clear from these data for aging up to 6 days, longer aging times produced

Figure 5. SAXS patterns of as-made samples prepared using 4 mL volumes of PS-28 evaporated at 29 °C and approximately 65% RH. 7176


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Figure 6. SAXS patterns of sealed solutions of F127 in 4:1 ethanol:water before Ti addition (left) and PS-10 aged between 7 h and 36 days (right). Data are on an absolute scale, but traces have been offset for clarity.

3). The inhibited growth of these Ti oxo oligomeric structures may be attributed to the greater acidity of a precursor solution using TiCl4 rather than a Ti alkoxide as the inorganic precursor, since HCl rather than alcohol is produced during hydrolysis. For PS-28, approximately 14 days of aging was required to build Ti oxo oligomers ∼25 Å in size, whereas the use of a Ti alkoxide precursor led to immediate formation of clusters of this size. The previously postulated “modulation of the hybrid interface” (MHI) mechanism, for hydrolysis and condensation of Ti precursors and PEO-based templates,38 can explain the necessity of Ti oxo oligomers in the precursor solution in order to achieve ordered mesoporous structures. According to the MHI mechanism, the presence of excess acidic water (relative to Ti) is necessary to achieve ordered mesoporosity because hydrophilic Ti oxo oligomers form in solution under these conditions, which interact weakly (via H-bonding) with the polymer template. Evidence of interaction between the Brij 58 template and the Ti oxo oligomers has been observed in the present system. This interaction allows folding and selfassembly of the template to occur during evaporation, and the Ti oxo oligomers can then co-condense around these selfassembled template structures to form the ordered mesoporous titania network. Unless excess water is present, the Ti does not form oxo oligomers. This makes the Ti more hydrophobic so it interacts more strongly with the template, causing it to unfold and leading to wormhole mesophases.38 The present work is consistent with this MHI theory. The most-ordered mesoporous titania materials were produced when PS-28 was aged for 6 days. This suggests that it is beneficial to the ordering of the final material for scattering structures with a Rg of approximately 12 Å to be present before commencing evaporation. Scattering structures smaller than 9 Å or larger than 25 Å appeared to be detrimental to the formation of ordered mesoporous titania. This suggests that the presence of Ti oxo oligomers with Rg of approximately 12 Å, which can be formed by aging the precursor solution for 6 days, are of the ideal hydrophilicity to interact appropriately with Brij 58 and encourage the formation of ordered mesoporous structures. However, there was no change in the viscosity of PS28 after aging for 6 days, which indicates that the formation of these small Ti oxo oligomers was not detectable via viscosity. Thus, SAXS can provide details of structural changes occurring in the sols before they become evident macroscopically. For the F127 system, elongated Ti structures, most likely Ti oxo oligomers, appeared to form almost immediately upon

solutions containing inorganic titanium precursors, ethanol, and block copolymers aged for 48 h at 40 °C produced less-ordered films than similar precursor solutions aged 3−24 h. This was attributed to the formation of anatase nanocrystallites in the precursor solutions; the nanocrystallites grew during aging until they were too large to fit within the walls of the mesostructure formed during EISA. The aging of similar precursor solutions at room temperature has not been investigated and is unlikely to produce crystalline material because mesoporous titania synthesized via EISA is typically amorphous prior to thermal treatment.2,3,36 It was of interest therefore to determine if aging in a sealed vessel at room temperature would similarly affect mesostructural order. The results of precursor solution aging were very different for the Brij 58 and F127 systems. For the Brij 58 system, the age of the precursor solution did affect the degree of order in mesoporous titania produced via EISA. For both PS-11 and PS28, scattering structures were present that grew in size over time. However, the scattering structures in PS-11 were consistently smaller than in PS-28, and the intensity of scattering was substantially lower for PS-11 aged for 17 and 36 days. This suggests scattering structures were smaller and less numerous in the less-concentrated Brij 58 solutions. Also, although both PS-11 and PS-28 contained branched or closepacked polymeric mass fractals after aging for 36 days, the mass fractal dimension of PS-11 was larger than for PS-28, indicating more dense packing of the scattering structures in the PS-11 solution. These differences are evidence that the template Brij 58 interacts with the Ti-based scattering structures formed in the precursor solution, since its concentration affected the growth of the Ti structures. The presence of Ti oxo oligomers in sols containing Ti alkoxide, alcohol, and aqueous acid has previously been determined to consist of Rg 20−25 Å subunits which cocondense to varying degrees, depending on the water and acid content of the sol.37 Small oligomers of mean composition Ti(X)x(OH) yO2−(x+y)/2 (X = OEt, OiPr, O nBu) which condensed into larger species over time were postulated. Changing the Ti precursor was observed to have a large impact on the composition of the oligomers formed in solution, but there were no data provided for sols containing the TiCl4 precursor that was used in this work. However, it is likely that similar Ti oxo oligomers were formed by hydrolysis of TiCl4 in the present solutions. For PS-28, smaller Ti oxo oligomer subunits of approximately 2 Å were initially formed, which increased in size up to approximately 40 Å over 36 days (Table 7177


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synthesis via EISA using Brij 58 as a template was 65%24 and using F127 as a template was 50%,30 this parameter was not varied in this work. 3.2.1. Temperature. For the Brij 58 system, PS-28 aged 6 days was evaporated at a temperature between 25 and 37 °C for 14 days. The relative humidity was maintained as 65% at each temperature so the absolute humidity was varied. The resulting SAXS parameters of as-made samples are shown in Table 4 and indicate that samples evaporated at 29 °C were more ordered than those evaporated at higher or lower temperatures.

preparation of PS-10 and underwent minimal growth during 36 days of aging. Thus, precursor solution aging is unlikely to be a critical synthesis parameter when using F127 as a template. A small increase in viscosity of only 1−2 cP was observed in PS10 upon aging for 6 days despite the SAXS data showing no change. This small increase is still well below the appreciable change in viscosity which would be required to indicate hydrolysis and gelation of a sol.8 The aging behavior of PS-10 was very different to that of the Brij 58 solutions. This provides strong evidence of the interaction between the template and Ti in the precursor solution, since the choice of template affects the growth and structure of Ti oxo oligomers in solution. F127 appears to interact more strongly with the Ti in the precursor solutions than Brij 58, as it quickly formed a stable structure with Rg ∼16 Å, whereas Brij 58 required 6 days to form comparably sized structures of Rg ∼12 Å. This stronger interaction with F127 may be hypothesized to be due to its larger hydrophilic PEO blocks (EO100 versus EO20 for Brij 58) which have been previously shown to be the part of the F127 molecule that interacts with hydrophilic titania.39 The molar ratio of Ti:EO was approximately 3:2 in PS-28 and 1:2 in PS-10, so the excess of Ti in the Brij 58 solution may have allowed for the formation of unassociated titania structures. Further work would be required to test this hypothesis, but if true, aging precursor solutions containing P123, which also has small EO20 blocks, may also improve the degree of order in mesoporous materials produced. Before the addition of TiCl4, 0.5% or 1.5% Brij 58 in 4:1 ethanol:water solution formed structures with Rg ∼10 Å. The main difference for the less concentrated solution was that the scale of the Debye fit was half that of the more concentrated solution, indicating there were fewer Brij 58 structures present. The measured Rg of ∼10 Å is substantially smaller than the Rg of 30 Å reported previously for micelles present in a water solution of similar Brij 58 concentration40 and suggests incomplete micelle formation due to the presence of ethanol. Similar phenomena upon addition of ethanol have been observed previously for the block copolymers Brij 3541 and P123.42 The F127 solution before TiCl4 addition consisted of approximately 8% F127, 83% ethanol, and 9% water. No selfassembled structures were expected to form, since it has been previously shown that in solutions that contain greater than 40% v/v ethanol, F127 does not form micelles.43 However, elongated scattering structures of Rg ∼20 Å were present. As might be expected from its higher molecular weight, these F127 structures were approximately double the size of the Brij 58 micelles (Rg ∼10 Å). Since the unified fit of the SAXS data indicated Porod scattering, it appeared that F127 did not behave as a Gaussian chain but rather as a smooth threedimensional scatterer, perhaps due to a sphere of unsolvated PPO. The elongated structure suggested from the P(r) function would be consistent with solvated PEO chains extending away from this dense PPO agglomerate. The disruption of these F127 structures upon the addition of TiCl4 is evidence for the strong interaction between F127 and the Ti oxo structures formed. This interaction has been demonstrated previously in ordered mesoporous thin films synthesized via EISA using F127, where the hydrophilic PEO blocks of the polymer template penetrate into the titania framework.39 3.2. Evaporation Conditions. Since previous work has shown that the optimal relative humidity for mesoporous titania

Table 4. d-Spacings and Relative Intensities of Peaks in SAXS Patterns of Samples Prepared using 4 mL Volumes of PS-28 Aged for 6 Days in a Sealed Vessel and Evaporated at Approximately 65% RH for 14 Days evaporation temp (oC)

d-spacing (Å)

Irel

37 29 25

68 ± 10 68 ± 10 74 ± 10

2 6 4

For the F127 system, PS-5 aged 6 days was evaporated for 14 days at a temperature of 32 or 37 °C with a relative humidity of 50%. It was also of interest to isolate the effect of temperature from that of humidity by preparing a sample in an environment with the same absolute humidity to 32 °C and 50% RH but at lower temperature, namely 26 °C and 65% RH. The resulting SAXS parameters of samples are shown in Table 5. Since the absolute humidity difference between samples evaporated at 65% RH at 26 °C or 46% RH at 32 °C was insignificant, the increased thermal stability of the sample evaporated at 50% RH and 32 °C must be attributed to its higher temperature of evaporation. Increasing the evaporation temperature from 32 to 37 °C while maintaining 50% RH created an environment with a higher amount of water molecules present. This produced asmade samples with a similar degree of order but enhanced thermal stability (Table 5). Table 5. d-Spacings and Relative Intensities of Peaks in SAXS Patterns of Samples Prepared Using 4 mL Volumes of PS-5 Aged for 6 Days in a Sealed Vessel and Evaporated for 14 Days 300 oC evaporation temp (oC)

RH (%)

d-spacing (Å)

Irel

26 32 37

65 50 50

collapsed part collapse, 105 ± 10 104 ± 10

4 11

3.2.2. Evaporation Time. Mesoporous titania templated with Brij 58 were synthesized with evaporation times between 1 and 21 days and all as-made samples showed ordered mesoporosity as indicated by peaks in their SAXS patterns. To determine the effect of increasing the evaporation time, mesoporous titania samples were prepared using PS-28 with evaporation at 29 and 37 °C for 7 and 14 days. The SAXS parameters of these samples are shown in Table 6 and show that longer evaporation times resulted in greater thermal stability in samples evaporated at lower temperatures (29 °C) but had little impact on samples prepared at higher temperatures (37 °C). Further increasing the evaporation time to 21 days for samples evaporated at low temperature had little impact on the order or thermal stability of the mesoporous titania produced. 7178


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In the F127 system, increased evaporation time improved stability even at 37 °C, which suggests that more time and more water is required for Ti condensation and cross-linking to be completed in the F127 system. 3.3. Template Removal. 3.3.1. Brij 58. SAXS data of asmade Brij 58 templated mesoporous titania synthesized under the conditions found to maximize order (4 mL PS-28 aged 6 days and evaporated at 29 °C and 65% RH for 14 days) is shown in Figure 7. These data were fitted using SASView with a power law exponent of −2.5, which is expected for any powdered system, and a Lorentzian peak with d-spacing 68 ± 10 Å, indicative of ordered mesoporosity. The fitted data is shown as a solid line in Figure 7 and provided a good fit to the experimental data (χ2/Npts = 5.8). Transmission electron microscopy (TEM) was also performed on the Brij 58 templated mesoporous titania after drying at 100 °C, and some of the images collected are shown in Figure 8. Ordered porous channels of approximately 30 Å in diameter can be seen perpendicular to the edge of the titania material. Given the dspacing of 68 Å indicated by the SAXS data, this suggests a titania wall thickness of approximately 40 Å.

Table 6. d-Spacings and Relative Intensities of Peaks in SAXS Patterns of Samples Prepared Using 4 mL Volumes of PS-28 Aged for 6 Days in a Sealed Vessel and Evaporated at Approximately 65% RH then Heated to 200 °C evaporation temp (oC)

evaporation time (days)

d-spacing (Å)

Irel

37 37 29 29

7 14 7 14

64 ± 10 66 ± 10 collapsed 62 ± 10

2 2 2

To determine the effect of evaporation time on F127templated mesoporous titania, samples were synthesized using PS-5 aged for 6 days and evaporated at approximately 50% RH and 37 °C for 7 or 14 days. Both as-made samples showed ordered mesoporosity according to SAXS, but thermal treatment at 100 °C caused partial collapse of the ordered mesoporosity in the sample evaporated for 7 days. However, the sample evaporated for 14 days did not begin collapse until temperatures of 350 °C were reached. 3.2.3. Summary of Effects of Varying Evaporation Conditions. The temperature at which evaporation is performed during EISA affects the rate at which the alcohol and HCl evaporate. Also, at the same relative humidity, the absolute amount of water present in the atmosphere is greater at higher temperatures hence the rate of water ingress into the samples and the rate of Ti hydrolysis and condensation will also increase with temperature. As a result, the temperature of evaporation can have a profound impact on the order and stability of the mesoporous titania produced. Mesoporous titania with the highest degree of order was produced with evaporation at 29 °C for Brij 58 solutions and at 37 °C for F127 solutions. Higher temperature evaporation also improved the thermal stability of the F127-templated mesoporous titania materials produced. Since higher temperatures increase the rate of evaporation, this result suggests that more ordered mesostructures were formed by F127 solutions evaporated with faster rates of titania hydrolysis and condensation. Fast titania hydrolysis may “lock in” the ordered, self-assembled F127 structure as well as ensuring complete cross-linking and condensation of the titania which is beneficial to thermal stability. It should also be noted that applying the conditions of synthesis that produced the greatest order for the Brij 58 system (evaporation at 27 °C and 65% RH) using PS-5, resulted in materials that exhibited disordered wormhole mesoporosity after heating to 100 °C. Thus, it is clear that different conditions are required to produce stable, ordered mesoporous titania with different templating polymers. In terms of evaporation time, the precursor solution aging data in Figure 5 suggest that longer evaporation times result in as-made samples with less order for PS-28 solutions. However, it is clear from the results in section 3.2.2 that longer evaporation times are nevertheless beneficial in terms of thermal stability for PS-28 solutions evaporated at 29 °C and PS-5 solutions evaporated at 37 °C. It is likely that the observed increase in thermal stability with increased evaporation time occurred due to increased cross-linking of the titania during the longer evaporation period, which strengthened the mesoporous structure. However, for PS-28 solutions evaporated at 37 °C, hydrolysis and condensation of the titania appeared to be complete within 7 days so that increasing the evaporation time to 14 days provided no stabilization benefit. This can be attributed to the increased water ingress at higher temperature.

Figure 7. SAXS pattern of optimal as-made and template removed Brij 58 ordered mesoporous titania.

After thermal treatment at 300 °C for 3 h, % CHN analysis of the thermally treated optimal sample confirmed that template removal was complete, since the % C measured was 0.3%. X-ray diffraction (XRD) of the thermally treated optimal Brij 58 templated titania showed it to be anatase (Figure S6 of the Supporting Information). The mean-ordered crystallite size was calculated according to the Sherrer equation,44 using the full width half-maximum (fwhm) of the XRD peaks, to give a value of 183 ± 31 Å. SAXS data of the optimal Brij 58 ordered mesoporous titania after template removal is also shown in Figure 7. These data were fit using SASView using a power law background, a broad peak, and a Gaussian peak model. The fitted power law exponent was −3.5, which is expected for any powdered system. The fitted Gaussian peak with d-spacing 233 ± 10 Å dominated from the SAXS data and was substantially wider than the Lorentz peak in the SAXS data of the as-made material. The fitted broad peak had d-spacing 72 ± 10 Å and was wider than the Gaussian peak. The fitted data is shown as a solid line in Figure 7 and can be seen to provide a good fit to the experimental data (χ2/Npts = 1.5). 7179


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Figure 8. TEM images of optimal, as-made Brij 58 ordered mesoporous titania.

An open but disordered wormhole structure with pores of approximately 30 Å and walls of approximately 40 Å can be visualized in the TEM images of the template removed sample (Figure S7 of the Supporting Information). Some reasonably large densely agglomerated areas, approximately 200 Å in size, can also be seen in the TEM images. These are consistent with the crystallite size from the XRD data and with the broad Gaussian peak in the SAXS data, which indicated a large size distribution of titania particles based around 233 Å. The nitrogen adsorption−desorption isotherm for the Brij 58 templated mesoporous titania material after template removal is shown in Figure 9. The isotherm is characteristic of a type IV isotherm with H2 hysteresis, indicative of capillary condensation in mesopores. The BET surface area of this mesoporous titania material was 210 m2/g, and the pore volume was 0.16 mL/g. The t-plot had a negative y-axis intercept, indicating that there were no micropores in this material. The inset to Figure 9 indicates a narrow pore size distribution centered around 30 Å. This is consistent with the pore size seen in the TEM images (Figure S7 of the Supporting Information). 3.3.2. F127. SAXS data of as-made F127-templated mesoporous titania synthesized under the conditions found to maximize order (4 mL PS-10 aged 6 days and evaporated at 37 °C and 50% RH for 14 days) is shown in Figure 10. This data was fit using SASView using a two power law background, as well as a Lorentzian peak. From Q = 0.011 to 0.018 Å−1, the power law exponent was −3.1 and from 0.018 to 0.25 Å−1, the power law exponent was −1.1. The presence of two different power law exponents at different Q ranges indicates that there were different scattering structures seen at different length scales. At low Q, the power law exponent was −3.1, which suggests a typical powdered structure on a large length scale. At high Q, the power law was −1.1, which suggests rod type structures, perhaps polymers of titania within the titania walls, were seen on a small length scale. The prominent, narrow Lorentzian peak had d-spacing 146 ± 10 Å. The fitted data is shown as a solid line in Figure 10 and provides a good fit to the experimental data (χ2/Npts = 19). After thermal treatment at 350 °C for 4 h template removal was complete, as confirmed by % CHN analysis in which the % C measured was 0.5%. X-ray diffraction (XRD) of the thermally treated optimal F127-templated titania showed it to be anatase (Figure S6 of the Supporting Information). The mean-ordered crystallite size was calculated according to the Sherrer equation,44 using the fwhm of the XRD peaks, to give a value of 174 ± 30 Å. SAXS data of the optimal F127-ordered mesoporous titania after template removal is also shown in Figure 10. The SAXS

Figure 9. Nitrogen adsorption−desorption isotherm for optimal Brij 58 templated mesoporous titania after template removal at 300 °C for 3 h. Inset: pore size distribution.

Figure 10. SAXS pattern of optimal, as-made, and template-removed F127 ordered mesoporous titania.

data were fit using SASView with a power law background of exponent of −2.5, which is consistent with a powdered system, and 3 peaks. The peak at low Q was a broad peak with dspacing of 157 ± 10 Å. This corresponds to the morphology of approximately spherical particles of this size seen in the TEM images (Figure S8 of the Supporting Information) and was consistent with the crystallite size determined from the XRD 7180


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thin film demonstrated a similar pore size of 25 Å.2 A larger quantity of Brij 58 templated ordered mesoporous titania synthesized by EISA for 24 h followed by thermal stabilization at 60−100 °C produced materials with BET surface areas of 90−160 m2/g, pore volumes of 0.09−0.21 mL/g, and 43−57 Å pore diameters.36 Thermal stabilization at 120 °C was also performed but produced materials without order. As such, it appears that the small pore size of the present materials, which is consistent with what has been observed previously in thin films, gives them greater surface area and similar pore volume to materials with shorter EISA and thermal stabilization at 60− 100 °C. Calcination of the literature samples at 360 °C for 2 h also caused the loss of the ordered structure, as was observed in the present system. F127 has been used extensively for synthesis of mesoporous titania via EISA.2,7,45,46 However, in this work, synthesis of a substantial quantity of ordered mesopous titania powder using F127 as a template has been achieved without the addition of a stabilizing agent for the first time. This is beneficial since additives can contaminate the final titania product or increase the temperature required to obtain clean titania.15 The retention of order and inferred higher thermal stability of F127 templated materials was not the only difference between the Brij 58 and F127 systems. The F127 templated material had a much larger d-spacing than the Brij 58 templated titania, as would be expected due to the higher molecular weight of the F127 polymer. The d-spacing of the F127templated mesoporous titania also showed a substantial amount of shrinkage (41 Å) upon thermal treatment, while the Brij 58 templated materials did not. Similar contraction of the mesostructure upon calcination has, however, been seen previously for F127-templated mesoporous titania films.47 The F127-templated mesoporous titania demonstrated a similar nitrogen adsorption isotherm shape and BET surface area to the Brij 58 templated mesoporous titania but with a substantially larger pore volume due to the larger pore size. This is again consistent with the larger molecular weight of the F127 block polymer relative to Brij 58. Several examples exist in the literature of F127-templated ordered mesoporous titania materials, either as thin films or additive stabilized materials synthesized on a larger scale. Most of these materials had comparable surface areas and pore sizes to the present F127-templated titania material. For example, an ordered mesoporous titania thin film thermally treated at 350 °C demonstrated a surface area of 150 m2/g and pore size of 58 Å.2 An ytterbium-stabilized titania thin film thermally treated at 400 °C had a similar pore size of ∼50 Å but a lower surface area of only 110 m2/g.48 In terms of ordered mesoporous titania materials synthesized in larger quantities, acetic acid stabilized materials have been synthesized with a surface area of 190−230 m2/g, pore volume of 0.36 mL/g and pore size of 50−63 Å after thermal treatment at 350 °C.46,49 These properties are comparable to the present material, but the acetic acid appeared to retard the growth of anatase crystallites, which were only 70 Å in the acetic acid stabilized titania,46 as opposed to ∼170 Å in the current material, despite the same thermal treatment. It was also of interest to compare the present results with ordered mesoporous titania materials synthesized in substantial quantities with commercially available templates and without the use of stabilizing additives. Using P123, ordered mesoporous titania calcined at 350 °C was produced with surface area of 220 m2/g, pore volume of 0.28 mL/g, and pore size of 41 Å.22 With a slightly higher calcination temperature of

pattern. The mid Q-range peak in the SAXS data, which was the most prominent, was a narrow, intense Lorentz peak with dspacing of 105 ± 10 Å. The narrow width of this peak, as well as its intensity, indicate that there was still some ordered mesoporosity in this material. The final peak at high Q in the SAXS data was a low intensity, wide Gaussian peak with dspacing of 68 Å. This peak may be a repeat unit from the peak with d-spacing of 105 Å, as it is consistent with a higher-order peak of a hexagonal structure. The fitted data is shown as a solid line in Figure 10 and can be seen to provide a good fit to the experimental data (χ2/Npts = 780). The nitrogen adsorption−desorption isotherm for the optimal F127-templated mesoporous titania material after template removal is shown in Figure 11. The isotherm is characteristic of a type IV isotherm with H2 hysteresis, indicating capillary condensation in mesopores. The BET surface area of this mesoporous titania material was 190 m2/g, and the pore volume was 0.31 mL/g. The t-plot had a negative y-axis intercept, indicating that there were no micropores in this material. The inset to Figure 11 shows the narrow pore size distribution centered around 50 Å.

Figure 11. Nitrogen adsorption−desorption isotherm for optimal F127-templated mesoporous titania after template removal at 350 °C for 4 h. Inset: pore size distribution.

3.3.3. Comparison of Template-Removed Mesoporous Titania Materials. Synthesis of large quantities of ordered mesoporous titania using Brij 58 is known to be particularly challenging as this template produces smaller pores and thinner pore walls than pluronic template materials such as F127 and P123.2 This means that it can only accommodate small crystallites in the pore walls and, as a result, typically has low thermal stability.2,36 Several treatments such as thermal treatment under inert atmosphere, solvent extraction, hydrothermal treatment, and water-soaking of as-made samples were attempted to increase thermal stability or decrease the synthesis time for Brij 58 mesoporous titania, but all were unsuccessful (details in the Supporting Information). The broadening of the correlation peak with d-spacing of ∼70 Å in the SAXS data of the Brij 58 templated mesoporous titania upon template removal indicated a loss of order due to low thermal stability, as has been seen previously. Nevertheless, mesoporous titania with pores approximately 30 Å in diameter and BET surface area 210 m2/g has been produced. Relatively few examples of Brij 58 templated mesoporous titania materials exist for comparison. A Brij 58 templated mesoporous titania 7181


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The Journal of Physical Chemistry C 400 °C, a P123-templated, ordered mesoporous titania with a surface area of 210 m2/g and 65 Å pores was produced.21 The crystallite size of P123-templated, ordered mesoporous titania calcined at 400 °C was only 24 Å,21 which is substantially smaller than the approximately 170 Å crystallites seen in the present system despite the lower calcination temperature of 350 °C. This accounts for the higher degree of order seen in the P123-templated titania material. Ordered mesoporous titania synthesized using F108 as a template and calcined at 350 °C also demonstrated a similar surface area, of 210 m2/g, and pore size of 63 Å.22 The similar pore size values for P123, F108, and F127 suggests that the central PO block size rather than the EO block size controls this property. The smaller pore size of Brij 58 is also consistent with this hypothesis as it has a small C16H33 central block.

and a description of post-EISA treatments. This material is available free of charge via the Internet at http://pubs.acs.org.

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Corresponding Author

*E-mail: [email protected]. Tel: +61 2 9717 3300. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Robert Aughterson for provision of the TEM, Inna Karatscheva for provision of the BET, and Gordon Thorogood for provision of the XRD. This work benefited from SasView software, originally developed by the DANSE project under NSF Award DMR-0520547

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4. CONCLUSIONS Ordered mesoporous titania powders have been synthesized using the commercially available block copolymer templates Brij 58 and F127 by optimizing the conditions of synthesis. Precursor solution composition was shown to affect the degree of order in both Brij 58 and F127-templated systems and the thermal stability of F127-templated materials. Precursor solution aging at room temperature was explored for the first time and showed very different behavior for the Brij 58 and F127 systems. This is a particularly important result given that it provides the first-ever evidence of time-dependent Ti structure formation in EISA precursor solutions. Brij 58 precursor solution aging had a large impact on the order of the structures formed after EISA, with aging of 6 days producing Ti oxo oligomers of ∼12 Å in the precursor solution which were the optimal size to interact with the Brij 58 template so as to form ordered mesostructures. On the other hand, F127-templated systems were unaffected by precursor solution age. This is strong evidence that the choice of template affects the growth and structure of Ti oligomers in the precursor solution, possibly due to the size of the EO blocks in the block copolymer templates. The optimal evaporation temperature during EISA to produce ordered, stable mesoporous titania was also dependent on the choice of template. When using Brij 58, evaporation at 29 °C produced samples with the greatest order but when using F127, higher temperature evaporation at 37 °C produced more thermally stable materials. Evaporation time, on the other hand, had similar effects for Brij 58 and F127-templated materials. That is, increasing evaporation length up to 14 days improved thermal stability in both systems. Overall, a deeper understanding of the factors that control creation of ordered mesoporous titania structures and thermal stability has been achieved. As a result, synthesis of ordered mesoporous titania powders using the block copolymer F127 as a template has been achieved for the first time without the use of stabilizing agents. It has been demonstrated that the formation of ordered, thermally stable mesoporous titania is highly sensitive to the choice of template as well as to the conditions of synthesis.

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AUTHOR INFORMATION

REFERENCES

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ASSOCIATED CONTENT

* Supporting Information S

Photograph of mesoporous titania powder, description of SAXS fitting models, Guinier plots, viscosity measurements, SAXS unified fit parameters, pair distance distribution functions, X-ray diffraction patterns, transmission electron microscopy images, 7182


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