Synthesis and Characterization of Nano-Aerogels

6.2. Experimental Details

Materials. Reagent grade 70% Zirconium (IV) Propoxide (ZPO) in 1-propanol, 80% Zirconium (IV) Butoxide (ZBO) in 1-butanol, 99.5% 1-butanol, and 99.7% acetic acid (HOAc), from the Aldrich Chemical Company, were used without further purification. Instrument grade carbon dioxide (99.99%) was obtained from BOC Canada.

For synthesis, a 10 ml stainless steel view cell was connected to a syringe pump (ISCO 100 DM) for pumping CO₂. Temperature and pressure in the view cell were measured and controlled by means of a temperature controller (Fuji), a pressure transducer (Omega) and a control valve (Badger), which communicated with a computer through a network interface (National Instruments). The details of the experimental setup were previously provided in Chapter 3.

Preparation of ZrO₂. In a typical experiment, zirconium alkoxide (ZPO or ZBO) was placed in the view-cell reactor, followed by addition of acetic acid and CO₂ up to a temperature of 40 ºC and a pressure of 6000 psig, which is above the supercritical condition of CO₂ (T_c =31 ºC, P_c =1070 psig). The amount of zirconium alkoxide and acetic acid added to the view-cell ranged from 5.47 to 11.3 mmol and from 17.6 to 35.1 mmol, respectively (see Table 5.1). A magnetic stirrer was used for mixing the reaction mixture to reach a homogeneous transparent phase. To ensure complete condensation of the precursor, a few drops of the reaction mixture were vented into water, where a white precipitate indicated that further reaction time was required. Normally, several days of aging was required for complete reaction, which is further studied in Chapter 7. In order to remove the unreacted acetic acid and condensate, i.e. alcohol and ester from the gel formed in the view cell, a scCO₂ washing step was conducted. After aging, the formed gel was washed continuously using scCO₂ at a rate of approximately 0.5 ml/min, followed by controlled venting at 0.5 ml/min to prevent collapse of the solid network. The transparent gel gradually became translucent during the scCO₂ washing and venting steps. The as-prepared ZrO₂ was then calcined using a heating rate for each calcination temperature of 10C per minute, to set-point. The holding time was 2 hours, and the cooling rate to room temperature was 0.5C per minute.

Characterization. In situ FTIR was performed on the Sentinel-ASI Applied Systems. Solid ZrO₂ aerogel powder was characterized by means of a Bruker Vector 22 ATR-FTIR instrument. DSC and TGA analysis were performed using a Mettler Toledo DSC822^e and TGA/SDTA851^e, respectively, at a heating rate of 10 C/min under a nitrogen atmosphere. XRD was conducted utilizing Rigaku-Geigerflex CN2029 employing CuKα1 + Kα2 radiation with a power of 40kVx35mA for the crystalline analysis. BET surface area, and pore size and distribution were obtained using Micromeritics ASAP 2010 at 77 K. Prior to the N₂ physisorption, the sample was degassed at 200 C under vacuum. SEM micrographs were recorded using a LEO 1530 electron microscope without gold coating. TEM images and electron diffraction patterns were obtained using a JEOL 2010f transmission electron microscope. The specimens were ground, and placed on a copper grid covered with holey carbon film.

6.3. Results and Discussion.

In this direct sol-gel route in CO₂, when mixing the Zr alkoxide and acetic acid, a transparent homogenous phase was initially formed which was either clear or yellow, depending on the starting concentrations of the precursor and acetic acid (Figure 6.1a). After stirring the transparent reaction mixture for several hours, the viscosity of the fluid became large enough to stop the stirrer whirling, and either a white opaque phase or a light yellow transparent homogeneous phase was formed (Figure 6.1b) which through ageing turned into a transparent gel (Figure 6.1c). High conversions were typically obtained using this direct sol-gel synthesis method in CO₂, approaching 98% based on the weight of starting zirconium alkoxide and calcined ZrO₂ product at 500 °C.



Figure 6.52. (a) The homogeneous transparent phase at the initial stage of the reaction; (b) the formation of the opaque white phase; (c) the formation of the transparent gel in the view cell.

As a precipitate tended to form after mixing acetic acid with zirconium alkoxide in CO₂, it was found crucial to select a temperature, pressure and initial concentrations of starting materials to maintain a homogeneous phase before gel formation in order to obtain well-defined nanostructures. A variety of experimental conditions were carried out, and favorable conditions were found using lower temperatures (e.g. 40 C), higher pressures (e.g. 6000 psig), higher initial zirconium alkoxide concentrations, low acid/alkoxide ratio, usage of ZBO instead of ZPO, and usage of butanol as a cosolvent. The synthesis conditions for the successful and unsuccessful ZrO₂ aerogel preparation are summarized in Table 6.1.

Table 6.6. Synthesis conditions of ZrO₂ structures in CO₂ and characterization results.

Samples	Precursor	Tre ^b	Tre ^c	Co ^d	HOAc/ZA ^e	butanol	Tcalc ^f	Sbet ^g	Dpore^h	VPore^i	morphology	microstructure
		(°C)	(psig)	(mol/L)	(mol/mol)	(ml)	(°C)	(m^2/g)	(nm)	(cm^3/g)
ZrO2-1-AP^a	ZPO^j	40	6000	1.13	2.23	-	-	257	2.9	0.19	translucent monolith	mesoporous
ZrO2-1-400							400	101	4.6	0.051
ZrO2-1-500							500	51	7.8	0.045
ZrO2-2-AP	ZBO^k	40	6000	1.13	2.23	-	-	257	4.5	0.29	translucent monolith	mesoporous
ZrO2-2-500							500	52	9.8	0.15
ZrO2-3-AP	ZBO	40	6000	1.09	3.22	-	-	215	4.9	0.26	translucent monolith	mesoporous
ZrO2-3-500							500	71	7.9	0.19
ZrO2-4-AP	ZBO	40	6000	0.547	3.22	5.5	-	361	6.0	0.54	opaque monolith	nanospheres
ZrO2-4-500							500	75	12.7	0.24
ZrO2-5-AP	ZBO	40	6000	0.547	3.22	5	-	397	5.7	0.57	opaque monolith	nanospheres
ZrO2-5-500							500	76	9.7	0.14
ZrO2-6-AP	ZBO	40	6000	0.547	3.22	4	-	399	5.3	0.53	opaque monolith	nanospheres
ZrO2-6-500							500	71	8.7	0.15
ZrO2-7	ZBO	50	6000	1.13	2.23	-	400	4.9	-	-	precipitate	chunks
ZrO2-8	ZBO	40	6000	1.13	3.22	-	-	-	-	-	precipitate	-
ZrO2-9	ZBO	40	3000	1.13	2.23	-	-	-	-	-	precipitate	-
ZrO2-10	ZBO	40	6000	0.547	2.23	-	-	-	-	-	precipitate	-

Note: a: AP: as-prepared; b: reaction temperature; c: reaction pressure; d: initial concentration of zirconium alkoxide; e: molar ratio of acetic acid over zirconium alkoxide; f: calcination temperature; g: BET surface area; h: adsorption average pore diameter (4V/A by BET); i: single point adsorption total pore volume per gram; j: zirconium propoxide; k: zirconium butoxide.

Although the zirconium alkoxide, acetic acid and CO₂ were miscible at the initial stage, two layers were formed after 10~60 minutes of reaction if the synthesis parameters of ZrO₂-2 were changed as follows: when the temperature was increased from 40 to 50 °C (ZrO₂-7), or the HOAc/ZBO ratio increased from 2.23 to 3.22 (ZrO₂-8), or the pressure decreased from 6000 to 3000 psig (ZrO₂-9), or decreased initial concentration of ZBO from 1.13 to 0.547 mol/L without butanol as the cosolvent (ZrO₂-10). The ZrO₂ produced from the lower layer was examined by SEM and N₂ physisorption, which showed that the material was badly agglomerated micron-size-spheres and exhibited a low surface area of 4.9 m²/g. Hence, in most experiments, the synthesis parameters were selected at 40 °C and 6000 psig in order to maintain the homogenous phase and the supercritical state.

SEM and TEM. After calcinations, samples ZrO₂-1 to ZrO₂-3 were found to produce relatively hard translucent monoliths while ZrO₂-2 to ZrO₂-4 produced fragile opaque monoliths. The microstructure of these materials was examined by means of electron microscopy. The SEM results showed that the translucent monolith samples exhibited a relatively smooth surface (Figure 6.2a), while the TEM results revealed wormlike mesopores on the thin flakes of the specimen (Figure 6.2b). The SEM (Figure 6.2c) of the fragile opaque monolith showed that it was composed of loosely compacted nanospherical particles with diameters of approximately 20 nm. The HRTEM of this sample showed the size of the crystallites and reflection pattern of the polycrystalline phases (Figure 6.2d). The d-spacings of 0.37 nm and 0.32 nm correspond (011) and (-1 11) planes of monoclinic ZrO₂, respectively.^258 The crystallite size of ZrO₂-4-500 determined by HRTEM was measured to be 10 nm, which is in agreement with the XRD results estimated using Scherrer’s equation (see below). Figure 6.3 shows the lumber-like precipitate of ZrO₂-7.



Figure 6.53. SEM and TEM images. The translucent monolith of ZrO₂-2-400: (a) SEM and (b) TEM. The translucent monolith of ZrO₂-2-400: (a) SEM and (b) HRTEM.



Figure 6.54. SEM. Precipitate chunks of ZrO₂-7.

N₂ Physisorption. The N₂ physisorption (77K) was employed to study the surface area and pore structure of the aerogel-like materials. For the various synthesis conditions of this study for as-prepared and calcined ZrO₂, the average BET surface area, the single point adsorption total pore volume per gram, and the adsorption average pore diameter (4V/A by BET), are summarized in Table 6.1. The surface areas of the as-prepared translucent monoliths (ZrO₂-1 to 3) ranged from 215 to 257 m²/g, while when calcined at 500 °C, decreased to 51 ~ 71 m²/g, likely due to the fusion of the solid network. The surface areas of the opaque monoliths (ZrO₂-4 to 6) were as high as 361 ~ 399 m²/g as-prepared, while when calcined at 500 °C also declined into the 71 ~ 75 m²/g range. Several samples were also heat treated in an inert atmosphere using the technique of Suh and Park,^249 however, the surface areas were not found to be noticeably higher. Comparing with the literature, the surface areas of the calcined ZrO₂ (Table 6.1) were higher than those obtained using the conventional sol-gel process (48-56 m²/g),^259 and lower than those obtained by careful control of sol-gel parameters, i.e., between 69 and 134 m²/g by Ward et al.,^260 and between 96 and 145 m²/g by Suh et al..^249

All the resulting aerogel materials in this study (samples 1-6) exhibited a Type IV isotherm, indicating the existence of mesopores.^261 The translucent monoliths (samples 1-3) exhibited H2 hysteresis loops (Figure 6.4a), while the opaque fragile monoliths with nanostructure (samples 4-6) exhibited H3 loops (Figure 6.4b). After heat treatment for both samples, the isotherms: a) showed lower volume of N₂ gas adsorbed per relative pressure indicating lower surface areas, and b) moved to a higher relative pressure region, indicating that larger pore sizes were produced. These isotherms were found to be typical for both sets of samples.



Figure 6.55. Isotherms of the ZrO₂ as-prepared and calcined samples at 500 °C: (a) ZiO₂-2; (b) ZrO₂-4. See Table 5.1 for the synthesis conditions for ZrO₂-2 and -4.

The pore sizes of the experimental samples were in the range of 2.9 ~ 6.0 nm before calcination, and increased to 4.6 ~ 12.7 nm after calcination (Table 6.1) due to the evolution of gas (i.e., CO₂ and water vapor) during heat treatment.^262 At the same time, the pore volume was 0.19 ~ 0.57 cm³/g before calcination, and was reduced to 0.051 ~ 0.24 cm³/g after calcination, indicating the formation of denser materials. By comparing Figures 6.5a and Figure 6.5b, which show the BJH desorption pore-size distribution, we can see that ZrO₂-2 exhibited a smaller pore size and pore-size distribution than ZrO₂-4. Careful examination and measurement of the TEM image of ZrO₂-2 (Figure 6.2b) shows relatively uniform pores with an approximate accessible diameter of 4.0 nm. This TEM result is similar to the N₂ physisorption result (4.8 nm). In the case of ZrO₂-4, the pores are formed by the interstitial space between the loose-compact nanoparticles inside the monolith. These pore volumes are larger and more polydisperse than the translucent monolith samples (Figure 6.5b), and can be visualized by electron microscopy in Figure 6.2c.

Figure 6.56. BJH desorption pore-size-distributions of the as-prepared and calcined at 500 °C: (a) ZrO₂-2; (b) ZrO₂-4. See Table 5.1 for the synthesis conditions for ZrO₂-2 and -4.

DSC. The as-prepared translucent and opaque monoliths were examined by means of DSC, which are shown in Figure 6.6a and b for samples ZrO₂-2 and ZrO₂-4, respectively. The endothermal peaks in Figure 6.5a for ZrO₂-2 are at 132 (T1) and 332 °C (T2), while the ZrO₂-4 sample shows a weak peak around 100 °C, and a strong peak at 328 °C (T5). These peaks correspond to the evaporation of the organic solvent trapped in the pores, and removal of the organic groups from the ZrO₂ network, which is also shown by the FTIR and TGA results (see below). The exothermal peaks at 464 (T3, tetragonal) and 520 °C (T4, monoclinic) for ZrO₂-2, and at 467 °C (T6, monoclinic) for ZrO₂-4, indicate the formation or transformation of the crystalline phases, which were recognized by powder XRD analysis as described later. For ZrO₂-2, the slope from 400 to 464 °C indicates the development of crystalline phases, and the inflection point at 445 °C suggests the formation of more than one crystalline phase. In contrast to ZrO₂-2, ZrO₂-4 is flat near 400 °C, indicating there is no significant formation of crystal structure at this temperature. Interestingly, ZrO₂-4 exhibited a very sharp exothermal peak (T6) contrasting to ZrO₂-2, suggesting a rapid crystallization of nanoparticles at the specified temperature.

The TGA result (Figure 6.6 inset) shows two obvious regions when the temperature was increased: the first region started from 80 °C and ended at 260 °C with a relatively small slope; while the second region started at 320 °C and ended at 490 °C with a large slope indicating more weight loss in this region. This agrees with the DSC results.

XRD. The wide-angle powder XRD patterns of translucent ZrO₂-2 calcined at 400 and 500 °C are displayed in Figure 6.7a. Mainly tetragonal phase was found present in the sample calcined at 400 °C while the monoclinic baddeleyite became the main phase after calcination at 500 °C. The tetragonal crystallite size of the ZrO₂ calcined at 400 °C is 4.1 ± 0.9 nm, while the monoclinic crystallite size of the ZrO₂ calcined at 500 °C is 8.2 ± 1.7 nm. The crystallite size was estimated by using Scherrer’s equation:

D_scher = 0.90 λ/(βcos θ) (6.19)

where, D_scher is the crystallite size, λ the X-ray wave length, θ half the angle of diffraction, and β is the full width at half-height of the diffraction peak, as described in Chapter 3.^263 The opaque ZrO₂-4 sample consisting of nanospheres, exhibited the similar crystalline phases as the translucent ZrO₂-2 when calcined at 500 °C (Figure 6.7b), with a crystallite size of 10.1 ± 2 nm. The main phase in the ZrO₂ calcined at 400 °C was amorphous, with a very small amount of tetragonal phase. The XRD results are consistent with the DSC observations. The samples were also examined in the small angle XRD region between 0 and 20 2-theta. No peaks were observed, indicating the mesophases were not in a regular pattern that could be detected using this technique.

FTIR. The as-prepared material, as well as the materials after calcination at 200 ~ 500 °C, were examined by means of ATR-FTIR (Figure 6.8). The peak in the 1500-1600 cm^-1 spectral range is due to the asymmetric stretch of acetate bidentates, while the peaks in the 1360-1480 cm^-1 spectral range are due to the symmetric stretching of acetate bidentates.^264 The small peak at 1342 cm^-1 is contributed by CH₃ group. The peaks at 1049 and 1026 cm^-1 are due to the ending and bridging butoxyl groups, respectively. The spectra show that there were still organic groups after calcination at 300 °C (Spectra c), and the absorbance drops greatly from 300 to 400 °C (Spectra d). After calcination at 500 °C, essentially no organic groups remain. The absorbance frequencies of Zr-O-Zr oxo bands (lower than 800 cm^-1) decreased noticeably with increased calcination temperature, as a possible result of the removal of adjacent organic groups from the oxo-bond network. The powder IR spectra showed a sudden absorbance decrease when the calcination temperature was increased from 300 °C to 400 °C (Spectra c and d in Figure 6.8), which agrees with the TGA observation.

In situ ATR-FTIR was used to monitor the sol-gel process under actual reaction conditions in CO₂. Figure 6.9 curve (a) shows the spectrum of ZPO (w/w 70%) dissolved in propanol. The peaks from 1381 to 1458 cm^-1 are due to the stretching and vibration of the aliphatic CH₂ and CH₃ groups, and the peaks from 1015 to 1160 cm^-1 are due to the Zr-OPr groups.^265 Spectra (b) - (e) were recorded at different reaction times in CO₂. The condensation of ZPO can be observed by the decreasing peak at 1134 cm^-1, while other Zr-OPr peaks from 1015 to 1104 cm^-1 are in the fingerprint range of n-PrOH, hence obscuring analysis. At the reaction time of 10 minutes (Spectra b), the quick formation of the peaks at 1600, 1567, 1544, 1478, and 1455 cm^-1 indicates the formation of a Zr-alkoxo-acetate coordination compounds. Gradual formation of ester during polycondensation can be observed from the peaks at 1239 and 1744 cm^-1 due to the reaction of acetic acid and 2-propanol into ester and water (Figure 6.9 b ~ e). At the same time, obvious movement of the acetate bidentate peaks (at the range 1455 ~ 1600 cm^-1) can be observed, due to the OCO bond angle and length change during condensation.^230 The chemical reactions between zirconium alkoxide and acetic acid will be discussed in Chapter 7.

In order to examine the role of CO₂ in the sol-gel process, the reaction of zirconium alkoxide with acetic acid was also conducted in butanol, instead of in CO₂, under similar synthesis conditions. No gel was formed at 40 C when ZBO = 1.13 mol/L and the HOAc/ZBO ratio = 2.23. When ZBO = 0.547 mol/L and HOAc/ZBO = 3.22, gel was formed and subsequently dried using scCO₂. However, a relatively low surface area of 35 m²/g was obtained before calcination, and SEM analysis did not show any nanostructure. In addition, increased gel shrinkage was observed providing a very dense material compared to the zirconia synthesized in CO₂. Hence, the results show that CO₂ played an important role in the nanostructure formation and mesoporous structure. The zero interfacial tension of scCO₂ maintained the nanostructures and a high surface area of the resulting materials.^113 Acetic acid is known to decrease the hydrolysis rate of metal alkoxides in water by coordination of the acetate group to the metal ions, hence slowing down the sol-gel process, and preventing precipitate formation for TiO₂ and ZrO₂ aerogels.^{227, 266} Also, according to Yamamoto et al. and Han et al., there is significant hydrogen bonding between acetic acid molecules in scCO₂,^{241, 242} which will similarly slow down the sol-gel process, hence likely facilitating the formation of uniform nanostructures. The acetate group in the complex also likely plays a role in the colloidal stabilization by enhanced solubility of the polycondensates in scCO₂, analogous to that observed with Beckman surfactants or Wallen sugars.^{239, 240} Further studies on CO₂’s effect on the sol-gel process will be described in Chapter 8.

The results in this chapter showed that acetic acid was an excellent reaction agent for producing nanoparticles and mesoporous monolith of ZrO₂ in CO₂. This research and our former synthesis of TiO₂ nanofibers and nanospheres show the promise of this technique for synthesizing nanomaterials, compared to the more challenging conventional aerogel process.