Zirconia Aerogels with High Surface Area Derived from Sols Prepared

Nov 29, 2007 - ... Microstructure of Zirconia Monolithic Aerogel. Xiaolei Li , Yuna Jiao , Huiming Ji , Xiaohong Sun. Integrated Ferroelectrics 2013 1...
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J. Phys. Chem. C 2007, 111, 18738-18743

Zirconia Aerogels with High Surface Area Derived from Sols Prepared by Electrolyzing Zirconium Oxychloride Solution: Comparison of Aerogels Prepared by Freeze-Drying and Supercritical CO2(l) Extraction Zhongqiang Zhao, Dairong Chen,* and Xiuling Jiao* School of Chemistry and Chemical Engineering, Shandong UniVersity, Jinan 250100, People’s Republic of China ReceiVed: July 3, 2007; In Final Form: August 23, 2007

Zirconia aerogels with high surface area of ca. 640 m2/g were prepared from sols made by electrolyzing zirconium oxychloride solutions at room temperature. To obtain the aerogels, supercritical CO2(l) extraction or freeze-drying was applied to remove the solvents from the wet gels. The microstructures and properties of aerogels produced by the two processes were characterized and compared. The aerogel produced by the supercritical CO2(l) drying process (S-aerogel) had a mesoporous structure (pore size, 9.7 nm) and was a transparent monolith, whereas that prepared by the freeze-drying process (F-aerogel) had a microporous structure (pore size, 0.59 nm) and was an opaque white powder, which transformed from amorphous to crystalline solids upon calcination. Moreover, the effects of the yittria on the microstucture and phase structure of the prepared aerogels were investigated.

1. Introduction Oxide aerogels exhibit high surface area/porosity and low density/dielectric constant/thermal conductivity, so they have many potential applications.1 In particular, these aerogels have attracted significant attention because of their excellent heatinsulation properties.2 The conventional approach to producing the aerogels is to first prepare the wet gels through the hydrolysis and condensation of alkoxide precursors and then dry the gels to remove the solvents.3 During this process, not only is control of hydrolysis complicated but also the required precursor is expensive. Thus, it would be desirable to develop a cost-effective method utilizing simple inorganic salts as raw materials. For this purpose, an “epoxide addition” sol-gel technique for preparing metal oxide aerogels has been developed,4 in which hydrated inorganic salts are used as raw materials and water or other polar protic liquids are used as the solvents. The method was also shown to be useful in the synthesis of a series of binary metal oxides. In a typical synthesis, Gash and co-workers prepared zirconia aerogels using ZrCl4 as the starting material.5 In addition, Deng et al. prepared zirconia aerogels through the hydrolysis and condensation of zirconyl nitrate under heating;6 however, in the precipitation reaction of ZrOCl2 or ZrOSO4 with ammonia, only supporting aerogels or aerogel powders but not monolith could be obtained.7 For large-scale preparation of aerogels with high surface areas, the inorganic salts based solgel process still remains a challenge.8 Herein, we introduce an electrolysis route to convert a metal chloride solution to a wet gel, followed by removing the solvents from the gels by extraction with supercritical CO2(l) or freeze-drying. Electrolysis has been applied in the preparation of various materials such as p-matals from fused salts, galvanic layers, and some reduced oxides; one famous example is the industrial production of NaOH by electrolyzing NaCl solution. In our earlier report, this route was also used to prepare sols for the fabrication of zirconia * Corresponding author. Telephone: +86-0531-88364280. Fax: +860531-88364281. E-mail: [email protected].

fibers,9 which indicates that the sol precursor has an important effect on the microstructures and properties of the product. Thus, to prepare the ZrO2 sol precursor for the preparation of the aerogels by a new way might give a new and deep understanding on the ZrO2 aerogels. On the other hand, the electrolysis method was a green process and suitable for large-scale preparation of the sols and the byproducts (H2 and Cl2) could be reclaimed. In the present work, the method is extended to prepare wet gels for the fabrication of zirconia aerogels with high surface area, and the microstructures and properties of the aerogels were also studied and compared. Furthermore, the crystallization and properties of the aerogels after calcination were also studied. As a polymorphic oxide, zirconia exhibits a monoclinic phase at room temperature, and tetragonal or cubic phases at higher temperatures. With the substitution of Zr4+ cations by rare earth or alkali earth cations such as Y3+, Ca2+, and Mg2+, stable tetragonal or cubic phases at room temperature are obtained, depending on the doping contents. Zirconia materials involving stabilized zirconia have extensive applications in many fields such as fire-resistant or thermal barriers, sensors, special glasses, catalyst/catalyst carriers, piezoelectric ceramics, and pigments, etc.10 It is expected that the aerogels will have extensive applications because of their high surface area, high porosity, and low density. The objective of this research is to find an effective route to the zirconia aerogels with high surface area derived from inorganic salts instead of alkoxide precursors and to enrich the basic theory and applications of inorganic salts on the basis of the sol-gel process. 2. Experimental Section (2.1) Synthesis. All reagents were analytical grade and used without further purification. (2.1.1) Preparation of Wet Gel. In a typical experiment, 30.0 mL of 0.3 M ZrOCl2‚8H2O was electrolyzed in an electrolytic cell (Figure 1) at 25.0 °C, while the electrode voltage was kept at 5.0 V. After the electrolysis reaction was conducted

10.1021/jp075150b CCC: $37.00 © 2007 American Chemical Society Published on Web 11/29/2007

High-Surface-Area Zirconia Aerogels from Sols

J. Phys. Chem. C, Vol. 111, No. 50, 2007 18739 Kawazoe) theory. The Fourier transform infrared (FT-IR) spectra were recorded on a Nicolet 5DX-FT-IR spectrometer using the KBr pellet method in the range of 400-4000 cm-1. Element analyses were conducted by using the Inductively Coupled Plasma (ICP, ICP-6000, Thermo Electro Corp.). 3. Results and Discussion

Figure 1. Diagram of the electrolytic cell. The positive electrode is a platinum filament with a diameter of 1.0 mm and length of 1.2 cm, and the negative electrode is a platinum sheet with length of 3.0 cm, width of 1.0 cm, and thickness of 0.5 mm. The distance between the two electrodes is 4.0 cm.

for 120.0 h and the concentration of Cl- anions decreased to ca.0.01 M on the basis of chemical analysis, the solution gradually transformed to a transparent sol. Then 30.0 mL of isopropyl alcohol was added under stirring, and a wet gel formed after the sol was aged for 24.0 h. Finally, several drops of 1,2epoxypropane were added into the gel to deplete the remaining Cl- anions, and then the gel was aged in a closed container for an additional 24.0 h. (2.1.2) Formation of Aerogels. (2.1.2.1) S-Aerogels. The wet gel was immersed in absolute ethanol for 120.0 h to exchange the solvent (mainly water) in the network. The ethanol was renewed five times during the exchange period. Then, the ethanol was extracted with liquid CO2 in a supercritical extractor (HL-0.5L, Huali) for 72.0 h to remove solvent from the gel; the extraction temperature and pressure were kept at 45.0 °C and 10.0 MPa. When the autoclave was depressurized slowly, a lump of transparent supercritical aerogel, which is called S-aerogel, was obtained. (2.1.2.2) F-Aerogels. The wet gel without exchanging with ethanol was put into a flask and quickly frozen with liquid N2 and then freeze-dried (ALPHA 1-2 LD, CHRIST) for 12.0 h at -50.0 to -60.0 °C and a pressure less than 10.0 Pa to give the freeze-dried aerogel, which is called F-aerogel. To investigate the effects of yttria on the zirconia aerogels, 30.0 mL of 0.3 M ZrOCl2‚8H2O + YCl3‚6H2O solution, in which the molar ratio of Y2O3 to ZrO2 was adjusted to 3:97, 6:94, and 8:92, was electrolyzed under the same conditions as the typical process. The aerogels with molar ratio Y2O3:ZrO2 of 0, 3:97, 6:94, and 8:92 are marked as S-aerogel-0, -3, -6, -8 and F-aerogel-0, -3, -6, -8, in which S and F denote supercritical extraction and freeze-dried, respectively. C-S-aerogel and C-Faerogel denote the calcinated samples. (2.2) Characterization. A transmission electron microscope (TEM, JEM 100-CXII) with an accelerating voltage of 80.0 kV and a high-resolution TEM (HR-TEM, JEOL-2010) were used to observe the morphology and microstructure of the products. X-ray diffraction (XRD) patterns collected on a Rigaku D/Max 2200PC diffractometer with a graphite monochromator (Cu KR radiation, λ ) 0.154 18 nm) were applied to analyze the crystal structures of the products. Thermogravimetric (TG) analyses were conducted using a SDTQ600 thermal analyzer (TA Instruments, Ltd.) in the temperature range of 50-800 °C under a flow of air with a heating rate of 10.0 °C/min. N2 adsorptiondesorption data were measured on a QuadraSorb SI apparatus at liquid N2 temperature (T ) -196 °C) after pretreatment at 100.0 °C for 10.0 h at devaporized condition, surface areas were determined by the BET (Brunauer-Emmett-Teller) method. The size distribution and volume of the mesopores were calculated by the BJH (Barrett-Joyner-Halenda) theory, and those of the micropores were calculated by the HK (Horvath-

(3.1) Formation of the Wet-Gel. The hydrolysis of ZrOCl2 solution follows eqs 1 and 2.11a As the electrolysis proceeding, the pH value of the solution gradually increased along with a decrease in the concentration of Cl- anions, which tended to shift eqs 1 and 2 toward products. When the pH was higher than 2.0, the [Zr4(OH)8‚(H2O)16]8+ species could further decompose to release H+ cations as shown in eq 3.11b

4ZrOCl2 + 12H2O h [Zr4(OH)8(H2O)16Cl6]2+ + 2Cl- h [Zr4(OH)8(H2O)16]8+ + 8Cl- (1) 2[Zr4(OH)8(H2O)16]8+ h [Zr8(OH)20(H2O)24]12+ + 4H+ + 4H2O (2) [Zr4(OH)8 (H2O)16]8+ h [Zr4(OH)8+4xH2O16-4x](8-4x)+ + 4xH+ (3) During the electrolysis, the Cl- anions were oxidized to form Cl2 gas on the anode (E° ) 1.358 V), and the H+ cations were reduced to form H2 gas on the cathode. In addition, there may be another reaction at the anode:

2H2O f O2 + 4H+ + 4e-

(4)

whose standard electropotential is 1.229 V. From the standard electro potentials, it should be O2 gas, not Cl2 gas eliminated from the anode. However, the overpotential value of O2 gas at the Pt electrode greatly increases as the electric current density increases, but Cl2 gas at the Pt electrode only exhibits a small overpotential. If the current density of the Pt anode is large enough, the electropotential of O2 becomes larger than that of Cl2, and then Cl2 is released from the anode. To avoid production of O2 by reaction 4, a platinum filament with diameter of 1.0 mm and length of 1.2 cm was used as the anode, whose current density was between 260 and 5200 A/m2 during the electrolysis, and the cathode was a platinum sheet with a larger area to decrease the current density. During the electrolysis, the concentration of OH- gradually increased, allowing hydroxyls coordination to Zr4+ to produce cluster species such as [Zr3(OH)4]8+, [Zr3(OH)6Cl3]3+, and [Zr4(OH)8]8+, which may further condense to form inorganic polymers. When the concentration of the hydrolyzed complex increased to a critical value, the polynuclear species called sol particles gradually transformed to a stable gel through olation and oxolation reactions. The addition of isopropyl alcohol resulted in a rapid decrease of the critical concentration and promoted formation of the gel,12 which was due to the lower solubility of zirconia sol in the mixture of water and isopropyl alcohol than in water resulted from the decreasing of the dielectric constant of the solution when isopropyl alcohol was added.13 The experimental results reveal that the concentration of the metal ions should be less than 0.4 M; otherwise precipitation or gelation easily occurred during the electrolysis process. However, it was difficult to form the gels if the metal ion concentration was less than 0.2 M. In addition, the electrode voltage also had a great effect on the formation of the sol.

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Figure 3. N2 adsorption-desorption isotherms of the S-aerogel-0 (a) and F-aerogel-0 (b). The insets are the corresponding pore size distribution plots.

Figure 2. TEM and HRTEM images of S-aerogel-0 (a, c) and F-aerogel-0 (b, d).

Precipitation or gelation also occurred easily during the electrolysis process when the voltage was higher than 6.0 V, but voltages less than 3.0 V led to a very slow electrolysis. Thus, in the present study the concentration of the metal ions was selected to be 0.3 M, and the voltage was set at 5.0 V. As the electrolysis proceeded, the concentration of Cl- anions in the solution decreased, resulting in a decreased electropotential. When the concentration of Cl- anions decreased to ca.0.01 M, the electrolysis almost stopped. In fact, at the end of the electrolysis there were few Cl- anions in the sol. A small amount of 1,2-epoxypropane was used to completely eliminate the free Cl- anions. After the gels were aged in a closed container for 24.0 h, they were dried by freeze-dring techinique directly or by the supercritical CO2(l) extraction after alcohol exchange for 5 days. Aerogels obtained by freeze-drying were called F-aerogels and those obtained by supercritical CO2(l) drying were called S-aerogels. The aim was to get rid of solvent in gels’ network for both freeze-drying and supercritical CO2(l) drying. After dring, the solvent in the gels’ network was removed and the aerogels with a lot of pores were formed. (3.2) Microstructure and Character of Aerogels. The S-aerogel-0 was transparent monolith, and the F-aerogel-0 was opaque white powder. The XRD patterns showed that all the as-prepared aerogels were amorphous (Supporting Information, Figure S1) The TEM images shown in Figure 2 indicate that the aerogels were composed of interconnected networks of spherelike particles of diameter ca.30-40 nm for the S-aerogel-0 and ca.20-30 nm for the F-aerogel-0. From the contrast difference in the HRTEM images, it can be concluded that there existed a number of pores in both S-aerogel-0 and F-aerogel-0. The HRTEM images indicate the mesoporous structure of the S-aerogel-0 with a pore size of ca. 1.5-4.0 nm, and the microporous structure of F-aerogel-0 with the pore size of ca. 0.5-1.5 nm (Figure 2c,d). The N2 adsorption-desorption curve of the S-aerogel-0 shows the type IV isotherm with a H1-type hysteresis loop (Figure 3a), confirming the mesoporous structure. The S-aerogel-0 had a surface area of 641.8 m2/g. In a previous report, the surface area of zirconia aerogels produced by supercritical CO2(l) extraction usually ranged from 350 to 676 m2/g.14 The electrolysis method provides a slow and mild process for the formation of the sols. The high surface area might result from

Figure 4. TG curves of S-aerogel-0 (a) and F-aerogel-0 (b).

the slow hydrolysis and condensation during electrolysis compared with a traditional sol-gel process such as the hydrolysis and condensation of alkoxides.8 Materials with high surface areas are suitable to be used as absorbents, catalysts, or catalyst supports. For the S-aerogel-0, the mean pore size was 9.7 nm (Figure 3a, inset). However, the N2 adsorptiondesorption curve of the F-aerogel-0 shows a type I isotherm (Figure 3b), demonstrating the microporous structure. The F-aerogel-0 had a surface area of 397.4 m2/g, which was less than that of the corresponding S-aerogel-0. For the F-aerogel0, the mean pore size was 0.59 nm (Figure 3b, inset). The difference illuminates that the drying process affects the surface area and pore size of the resulted aerogels. During the freezing process, crystallization of the solvent destroys the gel network and causes cracking of the gel, and as a result powder but not monolith was formed after the removal of the solvent.2a Also, the frozen solvent sublimated directly from solid to gas and the interface between solid and solid became a gas-solid interface under vacuum, producing stresses that resulted in the shrinking of the gel network to form small pores. However, supercritical CO2(l) drying prevents the formation of the liquidvapor meniscus and successfully annihilates the liquid surface tension and avoids the pore collapse phenomenon.1a Thus, transparent monolith was formed in the present preparation. Here, the discrepancy of the pore sizes from the HRTEM observation, and those from the N2 adsorption-desorption calculation may be due to the pores in aerogel are irregular, the TEM observation reflects one part of the aerogel and the later is based on the mode of perfect and regular pore structure. The TG curve of S-aerogel-0 (Figure 4a) exhibits three weight losses from room temperature to ∼500 °C. The first weight loss of ca.15.2% from room temperature to 178 °C is attributed to the loss of water and organics absorbed on the particle surface and those physicosorbed in the mesopores. The second one from 178 to 180 °C is ca.19.5%, which is attributed to the removal of chemisorbed water and organics in the mesopores. The removal temperature for water and ethanol is higher than their boiling points because of capillary forces in the nanopores and hydrogen bonds formed between the solvent molecules and Zr-

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J. Phys. Chem. C, Vol. 111, No. 50, 2007 18741

Figure 5. IR spectra of the S-aerogel-0 (a) and F-aerogel-0 (b).

OH. The last weight loss from 180 to 500 °C (ca. 11.6%) is mainly attributed to the removal of residual organics and the hydroxyls from the particle surface and the inner surfaces of the nanopores. As a result, the total weight loss of the S-aerogel-0 is 46.3%. However, the F-aerogel-0 (Figure 4b) exhibits continuous weight loss of ca.62% from room temperature to 700 °C. From room temperature to ∼250 °C, the weight loss was 41.1%, which is attributed to the removal of the water and organics adsorbed on the particle surface and in the pores. Then the chemisorbed solvent molecules start to decompose as the temperature rises. Because oxygen is limited due to the small pore size, incomplete decomposition results in the formation of carbon in the pores, which is oxidized to CO2 at temperatures higher than 600 °C. The removal of the surface hydroxyls occurred at ∼400 °C, which can also be confirmed by IR analysis. The IR spectra of both S-aerogel-0 and F-aerogel-0 exhibit the same absorptions, although the intensities are different (Figure 5). As shown in Figure 5a, the IR spectrum of S-aerogel-0 exhibits -OH bending vibrations between 1450 and 1630 cm-1 besides the broad band at ∼3400 cm-1 from the hydroxyls of absorbed water and ethanol, as well as Zr-OH. The absorptions at ∼2900 and 1380 cm-1 are attributed to the C-H vibrations and those at 1000-1260 and 810-950 cm-1 are assigned to the C-O vibrations, which indicate the presence of organics. The band below 700 cm-1 reveals the presence of Zr-O bonds. After calcination at 240 °C, the absorption at ∼2900 cm-1 significantly decreases, demonstrating that most of the organics are eliminated. After calcination at 500 °C, the absorptions at 3400 cm-1 are still observed, indicating that the -OH groups still exist. The absorptions between 1450 and 1630 cm-1 are attributed to the -OH vibrations and the bidentate carbonate and bicarbonate species derived from the reaction of acidic CO2 with the basic sites of ZrO2,15 revealing that some CO2 molecules are absorbed on the aerogels. Although the aerogels were not treated with CO2, the decomposition of the organics including isopropyl alcohol and 1,2-epoxypropane in the gels produced CO2 upon calcination at 500 °C which may absorbed on the aerogels. Combined with the TG analysis, it is concluded that absorbed water and CO2 as well as a few of Zr-OH groups and organics should exist in the S-aerogel-0 calcined at 500 °C. For the F-aerogel-0, its IR spectrum (Figure 5b) is similar to that of S-aerogel-0. As the aerogel is heated to 250 °C, the peaks around ∼2900 cm-1 obviously decrease, demonstrating decomposition of the organics. After calcination at 500 °C, the absorptions between 1450 and 1630 cm-1 still exist and that at 3400 cm-1 vanishes, indicating that the organics and -OH groups are removed from the sample, and the bidentate carbonate and bicarbonate species form. (3.3) Crystallization and Microstructure of Aerogels after Calcination. The S- and F-aerogel-0 were heated to 500 °C in air with a heating rate of 2.0 °C/min and then held at that

Figure 6. XRD patterns of C-S-aerogel-0 (a) and C-F-aerogel-0 (b) after calcinations at 500 °C.

Figure 7. HRTEM images of C-S-aerogel-0 (a, c) and C-F-aerogel-0 (b, d) calcined at 500 °C.

temperature for 2.0 h. The XRD patterns of the products indicate their crystalline nature (Figure 6). After calcinations, the S-aerogel-0 exhibited a mixture of tetragonal ZrO2 (t-ZrO2) and monoclinic ZrO2 (m-ZrO2), while the F-aerogel-0 showed the single t-ZrO2 phase. IR analyses demonstrated that there are a few of organics in the S-aerogel-0 calcined at 500 °C, while there was residual carbon in the calcined F-aerogel-0, which can be identified from the dark color of the C-F-aerogel-0. Thus, it is concluded that the phase structure related to the impurities in the aerogels such as the organics and the carbon derived from the incomplete decomposition of the organics. For F-aerogel0, carbon is produced from the incomplete oxidation of the organics during calcination and its presence in the micropores might accelerate the formation of t-ZrO2. On the basis of the full width at half-maximum (fwhm) of the (111) (m-ZrO2), (200) (t-ZrO2) reflections of the C-S-aerogel-0 and the (200) reflection of the C-F-aerogel-0, the particle sizes of the t- and m-ZrO2 in the C-S-aerogel-0 are 10.8 and 10.3 nm and that in the C-Faerogel-0 is 27.2 nm as calculated by the Scherrer equation. The HRTEM image in Figure 7a shows that the particles comprising the C-S-aerogel-0 are ca.10 nm, which is in good agreement with the result calculated from the XRD pattern. But Figure 7b indicates that the size of the particles comprising C-Faerogel-0 is less than 10 nm, which is much smaller than the XRD calculated result. To further probe the microstructure of the calcined aerogels, further HRTEM observations were conducted. It can be seen from Figure 7c that the C-S-aerogel-0

18742 J. Phys. Chem. C, Vol. 111, No. 50, 2007 was composed of nanocrystals with the size of 10-12 nm, which is consistent with the XRD result. Unlike the C-S-aerogel-0, the C-F-aerogel-0 exhibited continuous parallel lattice planes in the range of tens of nanometers (Figure 7d), although the particles seem to be less than 10 nm in size from Figure 7b. The continuous lattice fringes were formed during the nucleation in the calcination stage, which would result in the decreasing of the interfacial energy between the nanoparticles and a relative larger particle size from the XRD calculation, but the resulting continuous lattice fringes were not straight. The lattice spacing of 0.358 nm (Figure 7c) is indexed to the (110) plane of m-ZrO2 for the C-S-aerogel-0, and those of 0.312 and 0.299 nm with included angle 71° for the C-F-aerogel-0 (Figure 7d) are indexed to the (101) and (011) planes of t-ZrO2, indicating its tetragonal ZrO2 nature. After calcination at 500 °C, the surface area of the S-aerogel-0 remained 126.9 m2/g with the mean pore size decreasing to 7.9 nm. However, for the C-F-aerogel-0, the surface area sharply decreased to 22.5 m2/g and the pores almost disappeared. (3.4) Effect of Yttria. As discussed in a previous report, Y3+ cations are usually used as the stabilizer to adjust the structure and properties of zirconia. Here, different contents of YCl3‚ 6H2O are added into the starting solution to investigate the effect of the yttrium on the prepared aerogels. In the present system, the hydrolysis of Y3+ as described in eq 5 should occur during the electrolyzing.

YCl3 + nH2O h [Y(H2O)6-n(OH)n](3-n)+ (n ) 0, 1, 2, 3) + nH+ + 3Cl- (5) During the electrolysis, the concentration of OH- gradually increases, and the cluster species such as [Y(OH)]2+, [Y(OH)2]+, [Y2(OH)2]4+, [Y3(OH)4]5+, and [Y3(OH)5]4+ are produced, which may further condense to form inorganic polymers. The experiments indicate that the electrolyzing time is shortened with yttrium content increasing, which may be due to the different hydrolysis and condensation abilities of Y3+ and Zr4+ cations and their clusters. TEM observations of the aerogels with different contents of Y3+ did not show obvious differences of the particle size and microstructure from the pure zirconia aerogels, as well as similar IR absorptions (Supporting information, Figures S2-S4). The N2 adsorption-desorption experiments revealed that both Saerogels and F-aerogels contained different contents of yittria showing the same types of isotherms as those without yittria, but the surface area slightly increased with the addition of yittria accompanying the change of the mean pore size (Supporting information, Table S1). The result indicates that Y3+ may affect the structure of the gel network and then modify the surface area and pore size of zirconia aerogels. But the drying process is the most important determinant of the surface area and the pore size. After calcinations at 500 °C, the Y2O3 contents in the C-Faerogels should be the same as those in the starting solutions and the molar ratio of Y2O3:ZrO2 for C-S-aerogel-3, -6, and -8 are respectively 2.8:97.2, 5.8:94.2, and 8.1:91.9 by the ICP analyses. However, all C-S-aerogels and C-F-aerogels with different yittria contents exhibit a phase structure similar to the pure zirconia aerogels. That is, the C-S-aerogels show a mixture of t-ZrO2 and m-ZrO2, and the C-F-aerogels show the single t-ZrO2 phase. Y-doping appears to have little effect on the phase transformation of zirconia, and the particle sizes of the Caerogels calculated by the Scherrer formula based on the fwhm of the (111) (m-ZrO2), (200) (t-ZrO2) reflections of the C-S-

Zhao et al. aerogel-0 and the (200) reflection of the C-F-aerogel-0 on the XRD patterns are similar (Supporting Information, Figure S5, Table S2). Further experiments reveal that the solvents in the pores (water or ethanol) do not affect the crystalline structure of the calcined aerogels. This further indicates that in the present experiment the crystalline structure of the calcined aerogels is strongly influenced by the drying process, but the effects of the solvent in the pores and the yittia content are not obvious, which is inconsistent with the previous reports. Considering that a calcination temperature higher than 700 °C is needed to prepare Y3+ stabilized ZrO2 for a homogeneous distribution of Y3+ in the ZrO2 crystal lattice, it is assumed that this inconsistency might be due to the inhomogeneous distribution of Y3+ in the ZrO2 resulting from the lower temperature (500 °C). To confirm this point, the obtained aerogels were calcined at higher temperature (900 °C), and the homogeneous distribution of Y3+ in the ZrO2 crystal lattice may be achieved, and then the phase structure of the C-aerogels should change with the Y3+ content increasing. As expected, both of the S-aerogels and F-aerogels with the Y2O3 contents of 0, 3, 6, 8 mol % respectively are shown as m-ZrO2, t-ZrO2, c-ZrO2, and c-ZrO2 (Supporting Information, Figure S6). On the basis of the experimental results, it is concluded that the Y3+ does not enter the crystal lattice of ZrO2 homogeneously and completely at the temperature of 500 °C, and then no obvious effect of the Y2O3 content on the phase structure is observed. This inhomogeneous distribution of Y3+ in the ZrO2 lattice may be due to the different hydrolysis and condensation conditions during the formation of sol and gel and the lower calcination temperature (500 °C). 4. Conclusions Zirconia aerogels with high surface areas were successfully prepared by a combined electrolysis/sol-gel method, followed by supercritical extraction or freeze-drying. This provides a facile route to production of ZrO2 aerogels using inorganic salts as precursor and might be extended to the preparation of other metal oxide aerogels. The S-aerogel was a transparent monolith with mesoporous structure (average pore size, 9.7 nm) and surface area of ca. 640 m2/g. However, the F-aerogel had microporous structure with surface area and mean pore size of ca. 400 m2/g and ca. 0.6 nm. After calcination at 500 °C, the S-aerogel exhibited a mixture of m-ZrO2 and t-ZrO2, while the F-aerogel showed a single t-ZrO2 phase. Yittria-stabilized zirconia aerogels showed similar properties including particle size, microstructure, pore size, and surface area, as well as the phase structure of the calcined samples, as the pure zirconia aerogels. Detailed investigation indicated that the Y3+ did not enter the zirconia crystal lattice completely, so there had not been obvious effects of the Y2O3 contents on the crystalline structure of the calcined zirconia aerogels. Acknowledgment. This work was supported by the Program for New Century Excellent Talents in University, People’s Republic of China. Supporting Information Available: Photopictures and XRD patterns of F-aerogel-0 and S-aerogel-0, TEM images of S-aerogels-3, -6, -8 and F-aerogels-3, -6, -8 and surface areas and pore sizes of the S-aerogels and F-aerogels with different yttria contents, IR spectra of the S-aerogels and F-aerogels with different yttria contents, XRD patterns of the S-aerogels and F-aerogels with different yittria contents calcined at 500 °C, and XRD patterns of S-aerogels with different yittria contents

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