Synthesis and Characterization of Nano-Aerogels
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- 5.2.2. Experimental Setup
- 5.2.4. Characterization
- 5.3. Results and Discussion
- 5.3.1. N 2 Adsorption/Desorption
- 5.3.4. Thermal Analysis
- 5.3.6. Spheres or Fibers
- 5.3.7. CO 2 ’s Effect on the formation of TiO 2 Microstructures
5.2. Experimental Details5.2.1. MaterialsReagent grade 97% titanium (IV) butoxide (TBO), 97% titanium (IV) isopropoxide (TIP), 99.7% acetic acid, from the Aldrich Chemical Company, were used without further purification. Instrument grade carbon dioxide (99.99%) was obtained from BOC Canada. 5.2.2. Experimental SetupThe synthesis of titania nanomaterials was carried out in the 25 mL or 10 mL view cell, or in the 100 mL autoclave equipped with in situ FTIR, as described in Chapter 3. 5.2.3. Preparation of TiO2 NanoparticlesIn a typical experiment, titanium alkoxide (TA) was quickly placed in the view cell, followed by addition of acetic acid and CO2 up to the desired temperature and pressure. A magnetic stirrer system was used for mixing the reaction mixture. An amber transparent homogeneous phase was formed at the specified temperature and pressure (Figure 5.2a). The fluid in the view cell became semi-transparent and turned to dark red (Figure 5.2b) after stirring the reaction mixture from several hours to several days, just before gelation into a white gel. After at least 72 hours of aging, a few venting droplets were placed in a test tube, followed by addition of water and nitric acid. If there was no precipitate formed, this indicated the complete polymerization of TA. In order to remove the unreacted acetic acid and condensation byproducts, i.e. alcohol, ester and water from the gel formed in the view cell, a CO2 wash was conducted. To prevent collapse of the gel network, both the CO2 wash and CO2 venting were conducted carefully in a controlled manner by means of the pneumatic control valve that was fine tuned by the LabView virtual instrument. During the CO2 wash, the syringe pump was operated in a constant flow manner at a rate of 0.5 ml/min until the pump was empty. The pressure of the view cell was maintained at ≈ 6500 psig. When the pump was empty, the pneumatic valve would shut off automatically. The pump was refilled and pressurized to 6500 psig and operated with constant-flow mode again. After the supercritical drying process was finished, CO2 was released gradually by decreasing the pressure in a controlled manner. 
Figure 5.27. Photographs following the reaction in the view cell: (a) start of the reaction; (b) the moment directly proceeding gel-formation. The prepared TiO2 aerogel was further dried at 120 °C under a vacuum of 508 mmHg for 24 hours, then calcined in air at the desired temperature for 2 hours with a ceramic crucible in a programmable furnace (Thermolyne 1500). The heating rate for each calcination temperature was 10 °C per minute to the set point. The holding time was 2 hours, and the cooling rate to room temperature was 0.5 °C per minute. 5.2.4. CharacterizationFTIR. In situ FTIR monitoring of solution concentration in the stirred autoclave was performed using a high-pressure diamond immersion probe (Sentinel-ASI Applied Systems). The probe is attached to an ATR-FTIR spectrometer (ASI Applied System ReactIR 4000), connected to a computer, supported by ReactIR software (ASI). The spectra were collected automatically within the reaction time with specified intervals. Solid TiO2 aerogel powder was characterized by means of a Bruker Vector 22 FTIR instrument using a MIRacle Single Reflection HATR (Pike Technologies). The samples were finely ground before FTIR examination. Thermal analysis. DSC and TGA analysis were performed on a Mettler Toledo DSC822e and TGA/SDTA851e, respectively, at a heating rate of 10 °C/min in nitrogen. XRD. Powder XRD was performed either on Bruker D8 Discover Diffractometer with GADDS employing CuKα radiation or on Rigaku-Geigerflex CN2029 employing CuKα1 + Kα2 radiation with a power of 40kV×35mA for the crystalline analysis. The samples were finely ground and spread on a glass substrate. N2 physisorption. Brunauer-Emmett-Teller (BET) surface area, pore size and distribution were obtained on Micromeritics ASAP 2010 at 77 K. Prior to the N2 physisorption, a sample was crushed into small pieces (without grinding) and degassed at 200 °C under vacuum. SEM. SEM micrographs were recorded using either a LEO 1530 or Hitachi S-4500 FE. The samples were crushed (without grinding) and spread on a carbon substrate without gold coating unless specified. TEM. TEM images and electron diffraction patterns were measured using either a JEOL 2010 operated at 200 kV or Philips CM 10 at 80 kV. The specimens were previously finely ground and then dispersed in distilled water with the help of ultrasonic vibration. A drop of water with dispersed TiO2 powder was placed on a copper grid covered with Formvar® or carbon and dried in the air. 5.3. Results and DiscussionEither a white opaque monolith with low density (Figure 5.3a) or cracked translucent monoliths with higher density (Figure 5.3b) were formed in the view cell after CO2 venting. The SEM showed that the low-density monolith was composed of nanofiber clusters and the high-density monoliths were composed of nano-spherical particles as described in the SEM result section. Both the low-density monolith and the high-density monolith were easy to be ground into separate fine particles in micron size. 
Figure 5.28. Photographs: (a) the low-density monolith, and (b) the cracked high-density monoliths. A conversion of 98% was obtained when the reaction was complete based on the amount of starting TA and the weight of calcined TiO2 at 500 °C. Determined by the monolith volume and weight, the typical aerogel apparent densities were approximately 0.13 and 0.20 g/cm3 for the ‘low-density’ monolith and ‘high-density’ monolith, respectively. There were two factors that affected the density. First, a higher initial concentration of the alkoxide was used for synthesizing the high-density monolith than that used for the low-density one. Second, upon scCO2 extraction and drying, the low-density monolith did not shrink, while the high-density monolith shrank noticeably. This can be explained by the microstructure of the aerogels. As described later, the low-density monoliths were composed of randomly-oriented nanofibers and bundles, which were hard to shrink upon drying; while the high-density monoliths were comprised of nanospherical particles, which were easier to shrink by decreasing the interstitial space when the gel was dried. Both the low-density and higher-density monoliths were fragile and easy to be crushed into a fine white powder. 5.3.1. N2 Adsorption/DesorptionThe N2 adsorption/desorption technique was employed to study the surface area and the pore structure. The average BET surface area, the single point adsorption total pore volume per gram, and the adsorption average pore diameter (4V/A by BET) of the aerogels formed under different synthesis conditions are summarized in Table 5.2. Table 5.5. Results of the reaction of TBO with acetic acid in CO2.
The as-prepared aerogels (TiO2-1 to –15), which were amorphous as described later, exhibited a high BET surface area of 228 ~ 642 m2/g. After calcination at 380 C for two hours in air, the surface area of the aerogels decreased to 101 ~ 281 m2/g, except TiO2-8 and –10 which are 39 and 68 m2/g, respectively. The surface areas of TiO2-1, -11 and –12 dropped into 74 ~ 83 m2/g after calcination at 500 C and into 61 ~ 65 m2/g after calcination at 600 C. The quick decrease of the surface area is due to the fusion of the solid face and losing of the micropores during heat treatment, as shown later in the isotherm curves. The pore diameters of the as-prepared materials were in the range of 2.0 ~ 9.6 nm, and increased to 4.5 ~20.9 nm after heat treatment at 380 C. The pore volumes of the as-prepared materials were in the range of 0.12 ~ 1.22 cm3/g, and dropped to 0.044 ~ 0.8 cm3/g after calcination at 380 C. The resulting TiO2 exhibited a Type IV adsorption isotherm (see Figure 5.4), indicating the existence of mesopores.164 The high-density monoliths exhibited H3 hysteresis loops, indicating the existence of agglomerated spherical particles.164 Figure 5.4 shows the isotherms of the TiO2-1 samples without and with calcination at different temperatures. The hysteresis loop (a) from 0.45 to 0.80 P/P0 is likely due to the mesopores inside the particles, and the loop from 0.8 to 1.00 P/P0 is assigned to the interstitial porosity due to the agglomeration of the nanoparticles.164 From the as-prepared isotherm, it seems that the mesopores inside the particles made a significant contribution. When curve (a) and (b) are compared, it is observed that the hysteresis loop between 0.4 and 0.7 disappeared after heat treatment at 380 C. This indicates that the hysteresis loop contributed by the mesopores disappeared after calcination at 500 and 600 C, the loop less than 0.80 P/P0 disappeared, indicating further loss of the mesopores inside the nanoparticles. The isotherm plots of the TiO2 aerogels, before and after calcination at different temperatures, showed that the H3 hysteresis loop moved to a higher relative pressure with increased calcination temperature, indicating a larger average pore size was generated.
The low-density monoliths also exhibited H3 hysteresis loops. Figure 5.5 shows the isotherms of TiO2-13 samples without and with calcination at different temperatures. Again, the H3 hysteresis loop from 0.2 to 0.8 P/P0 is due to the mesopores inside the particles (fibers in this case), and the loop from 0.8 to 1 P/P0 is assigned to the interstitial porosity due to the agglomeration of the nanofibers. Like Figure 5.4, after heat treatment at 380 and 500 C, the hysteresis loop became narrow; after calcination at 600 C, the loop at less than 0.80 P/P0 disappeared, indicating loss of the mesopores inside the nanoparticles. 
Figure 5.30. Isotherm: TiO2-13 low-density monolith. (a) as-prepared and calcined at (b) 380 °C, (c) 500 °C, and (d) 600 °C. 5.3.2. SEMThe microstructure of the TiO2 aerogels was carefully examined by means of SEM. The morphology and the size of the nanoparticles were summarized in Table 5.2. The high-density monoliths were found to be composed of nanospherical particles with a diameter of 20 ~ 30 nm, while the low-density monolith was composed of nanofibers with a diameter of 10 ~ 80 nm and a length in the micron range. While the ratio of HOAc/alkoxide has a significant effect on the morphology and size of the aerogel particles, the reaction temperature and pressure show a rather insignificant effect. The reaction temperature’s effect on the morphology of the aerogel was studied by varying the temperature from 40 to 70 C, while maintaining the pressure at 6000 psig, the initial concentration of TBO at 1.5 mol/L, and HOAc/alkoxide ratio at 4.2, as shown in Table 5.2 TiO2-1~4. In this temperature range, high-density monoliths composed of spherical particles with a diameter of ca. 20 nm were obtained. The results show that temperature has no obvious effect on the morphology and the size of the aerogel. This is also true when using TIP as the precursor. TiO2-12~14 were prepared at the temperature range of 40 ~ 70 C, while the other synthesis parameters remained constant, and low-density monoliths composed of fibers with a diameter of 10 nm were obtained. The reaction pressure’s effect on the morphology of the aerogel was studied by varying the pressure from 4000 to 7000 psig, while keeping the temperature at 60 C, the initial concentration of TBO at 1.5 mol/L, and HOAc/alkoxide ratio as 4.2, as shown in Table 5.2 TiO2-5~7. In this pressure range, the high-density monoliths composed of spherical particles with a diameter of ca. 20 nm were obtained. When using TIP as the precursor, the pressure was varied from 4000 to 6000 psig while the other synthesis conditions remained constant, no change of the morphology and size was observed, as shown by TiO2-18~19 in Table 5.2. The ratio of acetic acid/titanium alkoxide has a significant effect on the morphology of the resulting aerogels. Keeping the initial TBO concentration at 1.1 mol/L, the temperature at 40 C and the pressure at 6000 psig, and varying the HOAc/alkoxide ratio at 4.0, 4.5, 5.0 and 5.5 (mol/mol) resulted in formation of 20 nm spheres, 20 nm spheres, 1 m rods and 80 nm fibers, respectively (see TiO2-8 ~11 in Table 5.2). When using TIP as the precursor, and the HOAc/alkoxide ratio was varied from 3.0 to 5.5 while the other synthesis conditions remained constant, curled 10 nm fibers and straight 40 nm fibers were obtained (see TiO2-16~18 in Table 5.2). The mechanism of nanostructure formation will be discussed in section 5.3.7. TiO2 Spheres. Both the as-prepared and the calcined high-density TiO2 monoliths were examined by SEM, and were found to be composed of agglomerated spherical particles. At low magnification, the surface of the high-density monoliths looks rough, even though it looks very smooth by eye. Figure 5.6a shows the surface of a small piece of high-density monolith under a magnification of 4.86 K. At a higher magnification of 67 K, however, it can be observed that the high-density monoliths, either the as-prepared or the calcined TiO2, were composed of fused nano spherical particles, which have an identical diameter of about 25 nm (Figure 5.6b). Here we can also observe the interstitial space between the particles. This agrees with the N2 physisorption isotherms, as described earlier. Also, the nanoparticles seem to be loosely attached, which explains why the monolith can be easily crushed and ground into a fine powder. The powder can be dispersed in anhydrous organic solvents with ultrasonic, so that separate TiO2 nano spherical particles can be obtained. 
Figure 5.31. SEM: (a) TiO2-1 monolith at low magnification, and (b) at high magnification. The sample was calcined at 500 °C. The arrows indicate where the mesopores are present. TiO2 Nanofibers. Contrasting to the microstructure of the high-density monolith, the low-density monolith is composed of nanofiber clusters. Figure 5.7 reveals the microstructures of the as-prepared TiO2-11 by increased magnification. At low magnification, the monolith looks very porous (Figure 5.7a). At high magnification, the nanofibers with a diameter less than 100 nm are loosely contacted with each other. If the synthesis process was well controlled, especially with caution taken during the CO2 extraction and venting steps, the low-density monoliths exhibited a well-defined nanofiber structure under the SEM. 
Figure 5.32. SEM of as-prepared TiO2-11 monolith at increasing magnification: (a) 50 times; (b) 100 times; (c) 1,000 times and (d) 10,000 times. The arrow indicates where the macropores are present. SEM images showed that the nanofibers did not break, even after heat treatment at 800 °C for 2 hours. After heat treatment in the temperature range from 300 to 800 °C, no obvious morphology change can be observed by using SEM at low magnification. Under high magnification, however, the calcined TiO2 nanofibers were composed of connected beads, which is different from the relatively smooth surface of the as-prepared TiO2 nanofibers (Figure 5.8a and b). It should be noted that the nanofibers described above were synthesized from TBO. By using TIP instead of TBO as the polymerization precursor, TiO2 nanofibers with a smaller diameter were obtained. Under a low magnification, the aerogel looks like coral (Figure 5.8c). Under a higher magnification, very fine fibers can be observed (Figure 5.8d). The fine fibers form clusters and the clusters are connected with each other to form the monolith. The mechanism of the morphology change will be discussed in section 5.3.6. 
Figure 5.33. SEM: (a) as-prepared TiO2-11; (b) TiO2-11 calcined at 380 °C (the sample was coated with gold to prevent charging); (c) TiO2-13 nanofiber clusters under a low magnification; and (d) TiO2-13 nanofiber clusters under a high magnification. It was found that the HOAc/TIP ratio has an effect on the morphology and diameter of the nanofibers. An increasing HOAc/TIP ratio resulted in formation of straight fibers with a larger diameter of 40 nm (Figure 5.9a). A decreasing HOAc/TIP ratio resulted in formation of curled fibers with a smaller diameter (Figure 5.9b). 
Figure 5.34. SEM: (a) TiO2-18 nanofibers with a diameter of 40 nm, calcined at 380 °C; (b) TiO2-17 curled nanofibers, calcined at 380 °C. TiO2 Rods. The aerogel derived from TBO (TiO2-10) was composed of rods with a diameter of about 1 μm (Figure 5.10a). The surface of the rods is revealed under a higher magnification (Figure 5.10b). It appears that the nanospheres were fused together forming the rods. 
Figure 5.35. SEM. The TiO2-10 rods calcined at 380 °C: (a) at a low magnification, and (b) at a high magnification. As described earlier in the experimental section, complete condensation, so-called aging, is required to achieve desired microstructure. If the condensation is not complete, unreacted titanium alkoxide will be removed while venting CO2, and a mixture of fiber-like bundles and chunks will be produced (Figure 5.11). 
Figure 5.36. SEM. The result of scCO2 drying before the complete condensation. In order to study the effect of CO2 on the formation of the microstructures, the synthesis was also carried out under similar conditions using isopropanol as solvent, instead of CO2, and a sheet-like structure was obtained without scCO2 drying (Figure 5.12). This result shows that CO2 takes a critical role in the formation of nanofibers. 
Figure 5.37. TiO2 synthesized in isopropanol. The synthesis condition: the initial concentration of TIP = 1.1 mol/L, HOAc/alkoxide molar ratio = 5.5, 60 C. Scale bar: 200 nm. 5.3.3. TEMDifferent from SEM images that “scan” the specimen and only provide surface information of the nanomaterials, TEM images can provide more details about the microstructure underneath the surface, and as important, the crystal information. Figure 5.13 shows the TEM image of TiO2-1 calcined at 380 °C at a low magnification. Both the crystalline and amorphous phases can be observed. HRTEM of the sample provides the reflection pattern of the crystalline phase (Figure 5.20). The d-spacing of 0.35 nm indicates that the (101) lattice plane of anatase is present.235 
Figure 5.38. TiO2-1 nano spherical particles calcined at 380 °C: (a) TEM and (b) HRTEM. Figure 5.14a shows the TiO2 nanofibers with a diameter of 80 nm, which were calcined at 380 °C. The connection between beads can be clearly observed. Figure 5.14b shows the TEM of TiO2 nanofibers with a diameter of 10 nm, in which, connected beads can also be observed. Figure 5.14c shows the HRTEM of the TiO2 nanofibers with a diameter of 10 nm, in which the reflection pattern of anatase can again be observed. Figure 5.14d shows the TEM image of the straight fiber with a diameter of 40 nm. Different from Figure 5.21 ~ 5.23, the fibers are straight and they are not composed of beads. Furthermore, the white spots in the fiber show the presence of mesopores. 
Figure 5.39. TEM and HRTEM images: (a) TiO2-11 nanofibers calcined at 380 °C; (b) TEM of TiO2-13 nanofibers calcined at 380 °C; (c) HRTEM of TiO2-13 nanofibers calcined at 380 °C; and (d) TEM of TiO2-18 nanofibers calcined at 380 °C. 5.3.4. Thermal AnalysisDSC. The DSC analysis on the TiO2 synthesized under different reaction temperatures showed that the reaction temperature had an effect on the aerogel thermal properties. The TiO2 synthesized under a temperature range of 40 ~70 ºC, while other synthesis parameters remained constant, were characterized with DSC. Figure 5.15 shows the thermal behavior of fresh titania aerogels of TiO2-1, -2, -3 and –4, prepared with the initial TBO concentration = 1.5 mol/L, HOAc/alkoxide = 4.2 (mol/mol) at 6000 psig. The endothermal peaks at 100 and 350 °C are due to the evaporation of the organic liquid trapped in the pores and decomposition of the organic groups, respectively. The exothermal peaks are due to the formation of the crystalline phases. The DSC curves at the temperature range of 300 ~ 370 °C are quite complicated due to the formation of the crystalline phase and the quick decomposition at the temperature range, as described later. 
Figure 5.40. DSC of TiO2-1, -2, -3, -4 aerogels synthesized at 6000 psig and different temperature (a) 40 °C; (b) 50 °C; (c) 60 °C; (d) 70 °C. The initial concentration of TBO: 1.48 mol/L, acetic acid/HOAc: 4.15. From this figure, it is observed that, at 300 °C, the aerogels synthesized at 70 °C (curve d) released more energy compared with those synthesized at lower temperatures, suggesting the formation of more crystallites. The powder XRD results support the DSC results. While the TiO2-1, -2 and –3 remained amorphous after calcination at 300 °C, TiO2-4 exhibited the anatase phase after heat treatment at 300 °C. This result suggests that a lower calcination temperature could be used to obtain the TiO2 crystalline phase, by means of using a higher reaction temperature. As described by Jensen, research was carried out to synthesize TiO2 crystals at low temperature, which simplified the synthesis process and required less energy.142 Although calcination is still necessary for crystallization of the TiO2 spheres and fibers, it is encouraging to see that the calcination temperature could be decreased significantly when the reaction temperature was raised. TGA. When the temperature increases, the weight loss of the as-prepared titania aerogel provides information about evaporation of the moisture and decomposition of the aerogel that contains organic groups. A typical TGA result is shown in Figure 5.16. The TGA curve can be divided into 5 sections. From 25 to 90 °C, the curve was flat, indicating no significant organic solvents in the sample. In the temperature region of 90 ~190 °C, a small slope of weight loss is shown, probably due to evaporation of the entrapped water and acetic acid, and decomposition of some organic groups such as the ending alkoxide groups that are easy to be removed. The slope becomes larger in the temperature range of 190 ~ 310 °C and reached its maximum at 310 ~ 370 °C, probably due to the decomposition of acetate bidentate groups. It becomes flat again after the temperature is over 400 °C. The TGA result agrees with the DSC observations. 
Figure 5.41. TGA of as-prepared TiO2-11. 5.3.5. XRDThe wide-angle powder XRD results showed that the crystalline phase in the TiO2 is particle-size dependent. A larger nanoparticle size facilitated the formation of rutile nanocrystallites, while a smaller nanoparticle size favored the formation of anatase phase. Figure 5.17 shows the XRD patterns of the TiO2 nano spherical particles (with a diameter of 20 ~30 nm) that were synthesized at 60 or 70 °C and calcined at temperature of 300, 380, 500 and 600 °C. 
Figure 5.42. XRD: amorphous, anatase (a) and rutile (r) phases of the TiO2 mono spherical particles calcined at different temperatures. (A). TiO2-4 synthesized at 70 °C calcined at 300 °C; (B) TiO2-3 synthesized at 60 °C and calcined at 300 °C (B), 380 °C (C), 500 °C (D) and 600 °C (E). The initial concentration of TBO: 1.5 mol/L, acetic acid/HOAc: 4.12, pressure: 6000 psig. A pure anatase phase could be observed from the TiO2 calcined at 380 and 500 °C. From the sample calcined at 600 °C (Figure 5.17), besides 93.8 % of anatase, 6.2 % of rutile could be observed. The percentage of the crystalline phase was based on Zhang and Banfield’s correlation:157  (5.18)
where The results show that anatase was the main crystalline phase at a relatively low calcination temperature (380-600 °C), even though rutile is more thermodynamically stable under ambient pressure compared with anatase and brookite. This might be explained by Zhang et al.’s study that anatase becomes more stable when the nanoparticles are smaller than 14 nm.236, 237 No obvious peaks could be observed from the TiO2-3 synthesized at 60 °C and calcined at 300 °C (Figure 5.17 curve B). However, anatase phase is observed from the sample TiO2-4 synthesized at 70 °C and calcined at 300 °C (Figure 5.17, curve A). This agrees with the DSC results that higher reaction temperatures resulted in lower calcination temperatures for crystallite formation. The XRD patterns of the as-prepared and calcined TiO2-11 nanofibers with an average diameter of 80 nm are presented in Figure 5.18. No crystalline phase could be observed from the as-prepared TiO2 aerogel. Rutile phase was obtained after calcination at 380 °C. Polycrystalline phases, rutile and anatase with a ratio of 5.3: 94.7, were obtained after calcination at 500 C. The only crystalline phase observed was anatase after calcination at 600 C. However, the anatase phase transferred back into the rutile phase after calcination at 800 C. The formation of rutile at 380 C is due both to the larger size of the nanofibers (80 nm) and the fact that the rutile phase is more thermodynamically stable.
The TiO2-13 nanofibers with a diameter of around 10 nm were also examined by XRD (Figure 5.19). Similar to the nano spherical particles of 20-30 nm, lower calcination temperature, i.e., 380 C, resulted in anatase. When the calcination temperature was 600 C, rutile was formed with a relative percentage of 56 %. 
Figure 5.44. XRD: nanocrystalline TiO2-13, r: rutile, a: anatase (A) calcined at 380 °C, (B) 500 °C (C) 600 °C. 5.3.6. Spheres or Fibers?ATR-FTIR Measurements Powder ATR-FTIR spectra of the resulting nanospherical powder (TIO2-1) in Figure 5.20 shows the IR spectra of both the as-prepared (spectrum a) and calcined TiO2 nano spherical particles of TiO2-1 (b-d). The peaks at 1546, 1447 and 1410 cm-1 in Figure 20a are due to a Titanium-acetate complex.238 The small peak at 1343 cm-1 is contributed by the CH3 group.232 Besides the titanium-acetate absorbance, there are also small peaks at 1132, 1117 and 1022 cm-1 corresponding to Ti-O-C, and the ending and bridging butoxyl groups, respectively. The oxo bonds can be observed by the presence of wide bands below 800 cm-1. With increasing calcination temperature (Figure 20a to d), the bidentate acetates, and the ending and bridging butoxyl groups were gradually removed, similar to the TGA observation. At 400 °C (Figure 20d), only the oxo bands can be observed. In addition, with increased calcination temperature, the oxo bands shifted to a smaller wave number, as a possible result of the removal of adjacent organic groups from the oxo-bond network. 
Figure 5.45. Powder ATR-FTIR spectra of TiO2-1 nanospherical particles. (A) as-prepared, (B) calcined at 200 °C, (C) calcined at 300 °C, (D) calcined at 400 °C of TiO2-1. In order to investigate the polycondensation reaction process of the two precursors (TIP and TBO) in CO2, in situ FTIR was also utilized. Formation of the Ti-acetate complex, and polycondensation products along with consuming of acetic acid and titanium alkoxide, were observed. Selected IR spectra during the sol-gel process using TIP as the metal alkoxide in CO2, with experimental conditions of TiO2-19, are presented in Figure 5.21. Straight TiO2 fibers with a diameter of 40 nm were produced afterwards. The curve (a) was the spectrum of TIP, and spectra (b) to (e) were taken at a reaction time from 10 to 4320 minutes. Acetic acid consumption can be conveniently observed from the decreasing peak at 1715 cm-1, while the consumption of TIP alkoxide precursor can only be observed from the peak at 860 cm-1, as the other strong peaks of TIP from 950 to 1120 cm-1 are in the fingerprint region of acetic acid, isopropanol and propyl acetate, hence being obscured. At the reaction time of 10 minutes, which is at the initial stage of the polycondensation reaction (spectrum b), the presence of peaks at 1596, 1557 and 1447 cm-1 provided evidence for the formation of the hexaprismane shape Ti6O6(OPri)6(OAc)6 complex 2, (see Table 5.1). After the reaction time of 4320 minutes, the peaks from the complex shifted to lower wave numbers at 1549, 1445 and 1397 cm-1, likely due to the OCO angle change of the bridging acetate, during further condensation of the hexamer. The gradual increasing of the peaks in the region below 800 cm-1 in spectra (b) ~ (e) indicates the formation of oxo bonds. 
Figure 5.46. (a) IR spectrum of TIP. (b) ~ (e) in situ FTIR spectra of polymerization of TIP with acetic acid, at 60 °C and 4500 psig, initial concentration: TIP=1.1 mol/L, HOAc/TIP=5.5 (mol/mol). Reaction time: (b) 10 min; (c) 230 min; (d) 250 min; (e) 4320 min. To further examine the role of hexamer formation in fiber growth, Figure 5.22 compares the powder FTIR spectrum of complex 2 previously synthesized in CO2 (molar ratio of HOAc/alkoxide = 1.33:1 and isolated as a crystal) to the in situ FTIR spectrum at 10 minutes polycondensation time using TIP alkoxide with a HOAc/TIP ratio of 3.5. Comparing the peaks in Figure 5.32 at 1604, 1543 and 1458 of the Ti6O6(OPri)6(OAc)6 complex, the peaks at 1603 1543 and 1448 cm-1 of the self-assembling fiber are very close, providing further evidence for the formation of this hexamer in the early stages of the self-assembly process. Curled TiO2 fibers with a diameter of 10 nm were produced under these lower HOAc/TIP ratio conditions. 
Figure 5.47. (a) FTIR spectrum of complex 2 synthesized in CO2. The infrared spectrum was recorded on a Bruker Vector 22 FTIR instrument. The crystals were dispersed in a KBr tablet. (b). In situ FTIR spectrum of polymerization of TIP with acetic acid, at 60 °C and 4500 psig, initial concentration: TIP=1.1 mol/L, HOAc/TIP=3.5 (mol/mol). The polymerization of TBO by acetic acid in CO2 under conditions leading to sphere formation was also examined by in situ FTIR (Figure 5.23). At the reaction time of 10 minutes, the peaks at 1580~1600, 1560 and 1449 cm-1 were observed. These absorption peaks are similar to those of the previously reported rutilane shape complex Ti6O4(OBun)8(OAc)8 (1 in Table 5.2). After the reaction time of 100 minutes, the peaks from the complex moved to 1547, 1457 and 1420 cm-1 due to further condensation. A large amount of butyl acetate was also produced according to the presence of the peaks at 1243 cm-1, 1368 and 1742 cm-1, due to the higher initial TBO concentration. Figure 5.24 shows the in situ FTIR during the polymerization of TBO by acetic acid in CO2 under conditions leading to fiber formation. At the reaction time of 10 minutes, there were three Ti-acetate bidentate peaks at 1565, 1447 and 1418 cm-1 (Figure 5.24a), which is different from Figure 5.23b, indicating another type of complex being produced. Variation of acid ratio to alkoxide is known to give different hexamer structures.231 
Figure 5.48. (a) IR spectrum of TBO. (b) to (e) in situ FTIR spectra of polymerization of TBO with acetic acid, at 60 °C and 4500 psig, initial concentration: TBO=1.5 mol/L, HOAc/TBO=5.0 (mol/mol). Reaction time: (b) 10 min; (c) 100 min; (d) 1280 min; (e) 5840 min. 
Figure 5.49. In situ FTIR spectra of polymerization of TBO with acetic acid, at 60 °C and 4500 psig, initial concentration: TBO=1.1 mol/L, HOAc/TBO=5.5 (mol/mol). Reaction time: (a) 10 min; (b) 100 min; (c) 300 min; (d) 330 min; (e) 5800 min. Additionally, it is noticeable that lower peaks at 700 ~ 800 cm-1 in Figure 5.23e are lower than those in Figure 5.21e and 5.24e, suggesting less oxo bond being formed during formation of spherical colloidal particles than that of the fibrous equivalent. Formation Scheme of TiO2 Fibers and Spheres In order to consider how these FTIR results can shed light on the reaction pathways during polycondensation to nanostructures in CO2, we will consider both the cases where nanofiber formation is favored, and where nanosphere formation is favored. As described above for nanofiber formation using TIP alkoxide, our in situ FTIR results provided evidence for the formation of the titanium-acetate complex 2 at the initial stage of the reaction. The schematic of the skeletal arrangement of this hexamer is redrawn (omitting the bridging acetate, etc.) for clearer observation (Figure 5.25a). In the structure, all six OPri groups are vertical, with three upward and another three downward. The condensation of this structure can only take place either above or underneath the ending OPri. In other words, one-dimensional condensation is favored with this hexamer, leading to step-growth of straight polymers. In the case of spherical nanoparticles formed from TBO alkoxide, the in situ IR results provided evidence for the formation of complex 1 at the initial stage of the reaction. The schematic of the skeletal arrangement of this hexamer is shown in Figure 5.25b, in which there are six ending OBun groups, two upward, two downward, one to the left and one to the right. This structure would easily permit the formation of polycondensate chains with branches. This branching facilitates three-dimensional growth and subsequent formation of spheres.
The evolution of the macromolecules into nanofibers or nanospheres can be explained by aggregation of rigid colloidal particles as described by Brinker.105 When the straight polymers grow long enough, the solubility decreases and small spherical concentrated regions, called coacervates, are formed to decrease the interfacial energy (Figure 5.26). The arrangement of the macromolecules in the coacervates results in elliptical tactoids, in which the straight polymers are organized due to the interaction among the straight macromolecules. The macromolecules end up with a rigid fiber structure (crystalloid) as observed by electron microscopy. 
Figure 5.51. Schematic of nanostructure formation. Polycondensation of complex 2 or 1 led to formation of either straight or irregular-shape macromolecules. Coacervates and tactoids were formed to lower the energy of the macromolecules in CO2, which in turn resulted in the formation of either fibers or spheres.105 As described earlier, both the HOAc/Ti ratio and the alkoxide structure are the primary factors determining whether fibers or spheres are formed. In the case of TBO as a precursor, formation of spherical particles at low acid ratios (e.g. HOAc/TA= 4) was explained due to the skeletal structure of complex 1; however, formation of nanofibers from TBO at higher acid ratios (e.g. 5.5) is more difficult to explain due to the lack of IR and XRD crystal data. For the TIP alkoxide system, increased HOAc/TIP ratios resulted in the formation of fibers with a larger diameter and more straight morphology. This may be due to the presence of a small amount of complex 3 with 2, which was previously identified in our XRD studies. Similar to complex 1, complex 3 will facilitate 3-dimensional condensation as shown schematically in Figure 5.25. This has the potential to act as a cross-linking agent among the straight macromolecules, which could make the fiber thicker and stronger upon heat-treatment. 5.3.7. CO2’s Effect on the formation of TiO2 MicrostructuresThe zero interfacial tension of scCO2 maintained the nanostructures and a high surface area of the resulting materials. The acetate group in the complex also likely plays a role in the colloidal stabilization by enhanced solubility of the macromolecules in CO2, analogous to that observed with Beckman surfactants239 or Wallen sugars.240 As well, hydrogen bonding between individual acetic acid molecules241, 242 will slow down the condensation process, facilitating nanofiber formation and minimizing precipitate formation. Further studies on CO2’s effect on the sol-gel process will be described in Chapter 8. Yüklə 0,88 Mb. Dostları ilə paylaş:
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represents the weight fraction of rutile,
is the integrated peak intensity of rutile peak at 27.5 (2-theta), and 
is the integrated intensity of anatase at 25.3 (2-theta). 
