J. Phys. Chem. B 2005, 109, 18781-18785
18781
Synthesis of TiO2 Photocatalysts in Supercritical CO2 via a Non-hydrolytic Route Guangqing Guo,† James K. Whitesell,*,‡ and Marye Anne Fox*,‡ Department of Chemistry, North Carolina State UniVersity, Raleigh, North Carolina 27606, and Department of Chemistry, UniVersity of California, San Diego, La Jolla, California 92093 ReceiVed: June 6, 2005; In Final Form: August 5, 2005
Nanoscaled TiO2 powders with narrow size dispersion were prepared in supercritical carbon dioxide via nonhydrolytic acylation/deacylation of titanium alkoxide precursors with or without tris-fluorination. The microstructures of these powders were characterized by spectroscopic (FTIR, TGA, and XRD), microscopic (SEM or TEM), and surface area (BET) measurements. Photocatalytic oxidation of 1-octanol on these calcined TiO2 powders and on commercial T805 TiO2 suspended in aerated supercritical carbon dioxide revealed relative reactivity controlled by the powder microstructures. Calcined TiO2 prepared from titanium(IV) isopropoxide and trifluoroacetic anhydride was effectively dispersed in aerated supercritical carbon dioxide under stirring and exhibited high photocatalytic oxidation activity.
Introduction Supercritical carbon dioxide (scCO2) has attracted increasing attention as a medium for the preparation of nanoparticles as a consequence of its readily accessible critical point, low toxicity, nonflammability, low cost, and high compressibility.1-3 Titanium dioxide is also an extensively studied semiconductor photocatalyst, and is used widely as well as an optical brightener in paper and pigments, in electrooptical devices. It is also used as a thermal catalyst support for noble metals and transition metal oxides because of its unique physical and chemical properties. As a photocatalyst, TiO2 has been used primarily in a large number of synthetic oxidation pathways and in the degradation of toxic organic materials.4,5 Although the synthesis of TiO2 powders in scCO2 by hydrolysis of titanium(IV) isopropoxide (TIP) has been reported,6 this pathway is problematic because CO2 is highly hydrophobic and water is only slightly soluble in scCO2.7 Although stable water-in-CO2 dispersions can be formed in the presence of fluorinated anionic surfactants,8,9 the reaction conditions that produce TiO2 particles via TIP hydrolysis in H2O/CO2 microemulsions must be strictly controlled.10 Otherwise, amorphous TiO2 particles with a wide size dispersal rapidly precipitate when TIP is added to the mixture as a consequence of high hydrolysis rates.11 On the other hand, several non-hydrolytic sol-gel routes have been developed in the past decade to prepare SiO2, TiO2, and other metal oxide gels or powders.12-21 One route is the “alkyl halide elimination” from thermal condensation of metal halides with metal alkoxides, eq 1.16-20
MCln + M(OR)n f 2MOn/2 + nRCl
(1)
where M represents Si, Ti, Zr, and so forth, and R denotes alkyl. Metal alkoxides can be generated in-situ by the reaction of alcohol or dialkyl ether with metal halides, eqs 2 and 3.19-21 * Authors to whom correspondence should be addressed. Tel: 858-5345870. Fax: 858-534-0969. E-mail:
[email protected] (Prof. Marye Anne Fox);
[email protected] (Prof. James K. Whitesell). † North Carolina State University. ‡ University of California, San Diego.
MCln + n/2ROH f MCln/2(OR)n/2 + n/2HCl
(2)
MCln + n/2ROR f MCln/2(OR)n/2 + n/2RCl
(3)
where M represents Si, Ti, Zr, and so forth, and R denotes alkyl. Another route is the acylation/deacylation of anhydrides with metal alkoxides, eq 4.13,20
M(OR)n + n/2(R′CdO)2O f MOn/2 + nR′COOR (4) where M represents Si, Ti, Zr, and so forth, and R and R′ denote alkyl. Because organic anhydride and titanium alkoxides are soluble in scCO2, the route shown in eq 4 is likely to be suitable to prepare TiO2 powders in scCO2, as the reaction can be carried out in a homogeneous phase. Moreover, the observed reaction rate at room temperature of thermal acylation/deacylation is lower than that of a hydrolytic route,20 allowing for more uniform particle sizes that are attained by a surface-assisted hydrolysis. In this paper, we report our study of the preparation of TiO2 powders via non-hydrolytic acylation/deacylation in scCO2 from TIP and acetic anhydride or trifluoroacetic anhydride, followed by calcination. The relative photocatalytic activities of these TiO2 powders were studied in aerated scCO2, with the photocatalytic oxidation of 1-octanol as a model reaction. The observed photocatalytic reactivity was correlated with nanoparticle structure. Experimental Section Materials. Titanium(IV) isopropoxide (TIP, Acros, 98+%), titanium(IV) tetrachloride (Aldrich, 99.9+%), acetic anhydride (Aldrich, 99.5+%), trifluoroacetic anhydride (Aldrich, 99+%), and 1-octanol (Aldrich, 99+%), all CAS grade, as well as T805 TiO2 powder (Degussa) were used as received. CO2 (National, minimum purity 99.99%) and O2 (National, minimum purity 99.5%, H2O < 5 ppm) gases were passed through a moisture trap (Agilent Tech, model MT-4-SS) before being transferred to the reaction vessel.
10.1021/jp0530101 CCC: $30.25 © 2005 American Chemical Society Published on Web 09/15/2005
18782 J. Phys. Chem. B, Vol. 109, No. 40, 2005 High-Pressure Vessels. Two high-pressure A316 stainless steel vessels (inner volume: 15 mL) were used in our experiments. A steel cylinder with no windows was used for the preparation of TiO2 powders in scCO2. A cubic reaction vessel equipped with four sapphire windows was used in the photocatalysis experiments. The latter vessel was also used for observations of the solubilities of various titanium(IV) precursors in scCO2. Details of construction and operation of this highpressure vessel have been described previously.22,23 Characterization. Fourier transform infrared (FTIR) spectra were measured using a Nicolet 510P FTIR spectrometer for samples dispersed in a KBr pellet. The morphology of each sample was determined via a JEOL 6400F field emission scanning electron microscope (FESEM) or a Hitachi 2000 scanning transmission electron microscope (STEM). Thermogravimetric analysis (TGA) was performed using a model 2950 TGA HR instrument. The crystal phases of the obtained powders before and after calcination were examined by large-angle powder X-ray diffraction (XRD) measurements recorded on a Rigaku D-MAXA diffractometer using Cu KR radiation. A Micromeritics Floesorb 2100 instrument was used for singlepoint Brunauer-Emmett-Teller (BET) surface area measurements. The progress of the photocatalysis was monitored by gas chromatography (GC, Hewlett-Packard, model 5890A) equipped with a HP-5 capillary column (0.25 mm × 25 m) with helium as the carrier gas. Synthesis of TiO2 in scCO2. From TIP and Acetic Anhydride (TiO2-Ac). TIP (200 µL, 0.70 mmol), acetic anhydride (130 µL, 1.4 mmol), and titanium(IV) tetrachloride (15 µL) were added by syringe to the high-pressure reaction cylinder. Liquid CO2 (about 5-8 mL) was then pumped into the vessel via a manual high-pressure generator (High-Pressure Equipment Co., 87-65-5, 60 cm3/stroke). The mixture was stirred for about 30 min at room temperature, then heated to 110 °C as the pressure was increased to 17 MPa with a high-pressure generator, while the mixture was stirred for 20 h. After the vessel had cooled to room temperature, CO2 was depressurized, inducing precipitation of a white powder. To the reaction vessel was rapidly added 5-10 mL of anhydrous hexane, and the resulting solid powders were collected by centrifugation. The powders were washed with hexane and acetone several times and dried in an oven at 60 °C overnight. Upon calcination at 500 °C for 5 h in air, TiO2 powder was obtained. From TIP and Trifluoroacetic Anhydride (TiO2-Fac). TIP (1.2 mL, 4.1 mmol), trifluoroacetic anhydride (1.2 mL, 8.1 mmol), and titanium(IV) tetrachloride (15 µL) were added to the high-pressure reaction cylinder. The same procedures as described above were performed except that reaction time was extended to 36 h, producing a white precipitate. Calcination at 500 °C for 5 h gave a white TiO2 powder. Photocatalysis of 1-Octanol in scCO2. The newly calcined TiO2 powder (30 mg) and 1-octanol (25 µL) were added to the high-pressure vessel equipped with four sapphire windows. O2 was introduced to the vessel (0.6 MPa), and liquid CO2 was pumped into the vessel at room temperature. The reaction temperature was controlled to 36 °C by circulating a thermostated ethylene glycol-ethanol mixture, and the reaction pressure was adjusted to 10 MPa with a manual high-pressure generator. The reaction mixture was stirred with a magnetic stirrer for about 30 min before irradiation. The light source was a 100-W Hg-Xe lamp (ORIEL CO., model 7340). Incident wavelengths shorter than 300 nm were removed by a cutoff filter. The excitation light was condensed by a quartz focusing lens to the center of the cell through a sapphire window.
Guo et al. SCHEME 1: Synthesis of TiO2 by Non-hydrolytic Acylation/Deacylation of TIP by Anhydrides
Reaction progress was monitored by on-line GC at 1 h intervals after the start of irradiation. After each sampling, CO2 was added to the reaction vessel to maintain the desired reaction pressure, and the GC line was flashed with compressed air or nitrogen to remove any residue of the reactant or products in the sampling system. Further experimental details about such photocatalysis experiments have been described previously.22,23 Results and Discussion Synthesis of TiO2 in scCO2. The synthesis of TiO2 powders in scCO2 via non-hydrolytic acylation/deacylation of TIP by anhydrides is shown in Scheme 1. TIP reacted with acetic anhydride or trifluoroacetic anhydride, respectively, affording TiO2 powders with organic groups on their surfaces, together with the formation of isopropyl acetate or trifluoroacetate. A small amount of titanium tetrachloride was added as a Lewis acid catalyst to promote the condensation.20 The reaction of TIP with 2 equivalents of acetic anhydride at room temperature is known to yield liquid Ti(OAc)2(OPri)2,24 which was the actual precursor of the condensation. The solubilities of the starting reactants and precursor were directly monitored through a sapphire window. When 600 µL (2.1 mmol) of TIP and 400 µL (4.2 mmol) of acetic anhydride were added, phase separation was observed. However, in a solution containing 200 µL (0.7 mmol) of TIP and 130 µL (1.4 mmol) of acetic anhydride, all the reactants and precursor were soluble in scCO2. The synthesis thus was carried out in a homogeneous solution. At 45 °C and 17 MPa, no solid was observed after 20 h. However, at 110 °C and 17 MPa, a white precipitate dispersed in a liquid was generated after the same reaction time. The liquid is likely a mixture of the byproduct (CH3COOPri), unconverted reactants, and/or a very low-molecular-weight titanate oligomer. Anhydrous hexane was rapidly added in order to prevent hydrolysis of unconverted reactants or precursors in moist air. Because fluorinated moieties were reported to be CO2philic,8,9 trifluoroacetic anhydride was used to increase the solubility of the precursor. Ti(OPri)2(OCOCF3)2, the expected product from the reaction of TIP with 2 equivalents of trifluoroacetic anhydride, was highly soluble in scCO2, and no phase separation was observed even at high concentrations. However, when the reaction for preparing TiO2-FAc was conducted in the same conditions as for preparing TiO2-Ac, only a very small amount of TiO2-FAc solid was generated. But when more reactants (4.1 mmol of TIP and 8.2 mmol of trifluoroacetic anhydride) were added and the reaction time was extended to 36 h at 110 °C, a white solid was obtained. The difference of reaction rate is probably related to the facility of the elimination from the diacylated intermediate, but details of the operative mechanism of the deacylation have not been unambiguously defined.25 Characterization. FTIR Spectroscopy. Figure 1a shows the FTIR spectra of TiO2 prepared from TIP and acetic anhydride in scCO2 before and after calcination. Several absorbance bands
Synthesis of TiO2 Photocatalysts in scCO2
J. Phys. Chem. B, Vol. 109, No. 40, 2005 18783
Figure 2. TGA profiles of (a) TiO2-Ac and (b) TiO2-FAc under N2. Heating rate: 20 °C/min.
Figure 1. FTIR spectra of (a) as-prepared and calcined TiO2-Ac; (b) as-prepared and calcined TiO2-FAc.
suggest the presence of organic groups on the surfaces of asprepared TiO2-Ac particles. Two strong bands at 1529 and 1451 cm-1 were attributed to bidentate (chelating or bridging) acetate ligands.26 A small band at 1710 cm-1 was also observed, indicating a small amount of unidentate acetate bound to the powder surfaces. Hydroxyl groups were evident (3370 cm-1), likely because of partial hydrolysis of the appended organic groups in moist air. After the as-prepared powders were calcined at 500 °C for 5 h, the absorbance bands assigned to acetate and isopropoxide groups disappeared, and TiO2 with a fully hydroxylated surface was obtained. Figure 1b shows the FTIR spectra of TiO2 prepared from TIP and trifluoroacetic anhydride in scCO2. Absorbance bands at 1646, 1469, 1208, and 1155 cm-1 confirmed that trifluoroacetate and isopropoxide groups were present on the surfaces of the as-prepared TiO2-FAc powders. However, it is difficult to say whether trifluoroacetate groups were bound to the TiO2 surface by a bidentate or a unidentate mode. After heat treatment of as-synthesized TiO2-FAc at 500 °C for 5 h in air, a very small band at 1643 cm-1 was observed, indicating the presence of a few residual trifluoroacetate groups at the surface of the calcined TiO2-FAc. That is, most of the organic groups had been removed by the pyrolytic calcination. TGA Analysis. Figure 2, parts a and b, show TGA profiles of TiO2-Ac and TiO2-FAc under N2. After initial surface
dehydration to remove physisorbed water, the loss of surfacebound organic groups could be observed. For TiO2-Ac, thermal weight loss (about 32%) from desorption of acetate and isopropoxide groups mainly took place between 190 and 400 °C, indicating that most of these organic groups on the TiO2Ac surface desorbed over this temperature range. Above 500 °C, the mass only slightly decreased until 1000 °C. For TiO2FAc, weight loss (about 24%) due to the desorption of trifluoroacetate and isopropoxide groups mainly took place between 220 and 400 °C. Above 500 °C, no further mass loss could be observed. These results indicate that there were fewer organic groups on the surface of as-prepared TiO2-FAc than on that of TiO2-Ac. Presumably, oligomeric TiO2-FAc was more soluble in scCO2 than the corresponding TiO2-Ac analogue, and the reaction time for producing TiO2-FAc was longer than that for for TiO2-Ac. Thus, the final degree of condensation derived from Ti(OPri)2(OCOCF3)2 was higher than that from Ti(OPri)2(OAc)2. Powder XRD Characterization. Both as-prepared TiO2-Ac and TiO2-FAc were amorphous powders, as shown by XRD analyses (Figure 3). After TiO2-Ac was calcined at 500 °C for 5 h, a clear peak at 2θ ) 25 and several small peaks at 2θ ) 48, 54, 64, and 76 were detected (Figure 3a), indicating the presence of a poorly crystalline anatase phase. On the other hand, several sharp and clear peaks at 2θ ) 25, 38, 48, 54, 64, and 76 in Figure 3b suggested that a more highly crystalline form of anatase was formed after the same heat treatment of TiO2-FAc. Although the reason for poor crystallinity of calcined TiO2-Ac is not very clear, the different behavior probably resulted from a higher level of inorganic impurities
18784 J. Phys. Chem. B, Vol. 109, No. 40, 2005
Guo et al.
Figure 4. (a) TEM image of TiO2-Ac particles after being calcined at 500 °C for 5 h; (b) SEM image of TiO2-FAc particles after being calcined at 500 °C for 5 h.
Figure 3. XRD patterns of (a) TiO2-Ac and (b) TiO2-FAc before and after calcination at 500 °C for 5 h.
in calcined TiO2-Ac, because calcined TiO2-Ac was a light yellow solid, whereas calcined TiO2-FAc was pure white. SEM and TEM Characterizations. A TEM image of a calcined powder of TiO2-Ac (Figure 4a) revealed mostly spherical particles with smaller particles (80-200 nm, average 140 nm) sticking to larger ones. The calcined particles derived from TiO2-FAc, in contrast, existed in both a spherical and irregular shapes (Figure 4b), and these particles exhibited a size range between 100 and 500 nm with an average size of 270 nm. Aggregates derived from TiO2-FAc were not as evident as that derived from TiO2-Ac particles. The size dispersions of both TiO2-Ac and TiO2-FAc were narrower than those of TiO2 prepared from hydrolysis of TIP in scCO2,11 but they were wider than those of TiO2 obtained by TIP hydrolysis in the presence of reverse micelles formed in CO2 under strictly controlled reaction conditions.10 The final TiO2-FAc particles were bigger than those of TiO2-Ac, because the solubility of oligomeric TiO2-Ac in scCO2 was low, and small spherical particles precipitated. On the other hand, the solubility of the oligomer of TiO2-FAc was higher than that of the corresponding TiO2-Ac, permitting the growth of bigger and more irregular-shaped particles. BET Surface Area Measurements. Single-point N2 BET measurements showed that both calcined TiO2-Ac and TiO2FAc had specific surface areas of 79 m2/g. This value was
somewhat higher than that of TiO2 prepared by hydrolysis of isopropoxide in scCO2 (65 m2/g after calcination).11 Presumably, the thermal removal of organic groups from the surfaces or interior of the as-prepared TiO2-Ac and TiO2-FAc particles resulted in voids in their calcined structures. BET measurements were made after degassing the samples for 2 h at 125 °C and subsequently for 2 h at 235 °C. Calcined TiO2-FAc particles were much larger than TiO2-Ac particles, but they had almost identical BET surface areas. This difference likely resulted from relatively higher porosity of calcined TiO2-FAc, because a rough surface of calcined TiO2-FAc was observed in SEM image. Photocatalysis of 1-Octanol in scCO2. Previous study indicated that moisture in the reaction vessel may result in aggregates and precipitates of hydrophilic TiO2 from scCO2,22 so the reactions were carried out under very dry conditions. The reaction cell was dried in an oven at 60 °C overnight before being used. CO2 and O2 were passed through a moisture trap before entering the reaction vessel. Photocatalytic oxidations of 1-octanol in aerated scCO2 on calcined TiO2-Ac, calcined TiO2-Fac, and commercial T805 TiO2 produced the oxidation products (1-octanal and octanoic acid) previously reported.22,23 For comparison, all photocatalytic experiments were carried out under completely identical conditions. Figure 5 shows the dependence of the photocatalytic oxidative conversion of 1-octanol on reaction time upon band gap irradiation of each photocatalyst suspended in aerated scCO2. The photocatalytic oxidations here followed zeroth-order kinetics, unlike previous reports of pseudo-first-order kinetics.23 Because the reaction rate is greatly influenced by the adsorbed concentration of substrates, kinetic studies are usually conducted in dilute solution, that is,
Synthesis of TiO2 Photocatalysts in scCO2
J. Phys. Chem. B, Vol. 109, No. 40, 2005 18785 Conclusion TiO2 nanopowders with narrow size dispersal were prepared via a non-hydrolytic acylation/deacylation route in scCO2 from titanium alkoxide precursors. The as-prepared TiO2 nanopowders were amorphous, but they could be readily converted to anatase by heat treatment at 500 °C for 5 h. The microstructures of these powders were characterized by FTIR, TGA, XRD, SEM (or TEM), and surface area (BET) measurements. TiO2 prepared from titanium(IV) isopropoxide and trifluoroacetic anhydride could be effectively dispersed in scCO2, and its photocatalytic activity in scCO2 was higher than that of TiO2 prepared from titanium(IV) isopropoxide and acetic anhydride, or of commercial T805 TiO2.
Figure 5. Dependence of photocatalytic oxidation of 1-octanol on irradiation time using different catalysts: calcined TiO2-Ac at 500 °C for 5 h (filled circles); calcined TiO2-FAc at 500 °C for 5 h (filled triangles); commercial T805 TiO2 (blank circles). Photocatalysis conditions: 30 mg of TiO2 catalyst, 25 µL of 1-octanol, 36 °C, 10.3 MPa, λ > 300 nm from Hg-Xe source.
12.5 µL of 1-octanol over 50 mg of TiO2. In this study, however, the photocatalytic oxidations were conducted in more concentrated solutions over less TiO2 photocatalyst. Figure 5 thus only represents the changes of reactant concentration with irradiation time, but does not accurately represent absolute kinetics. Figure 5 shows that the observed photocatalytic activity of calcined TiO2-Ac in aerated scCO2 was lower than that of calcined TiO2-FAc. When calcined TiO2-FAc was used, more than 70% of 1-octanol was converted to the corresponding oxidative products after 9 h of irradiation, but when calcined TiO2-Ac was used, only about 43% conversion took place after the same irradiation time. As described above, after calcination of the as-prepared powders at 500 °C for 5 h, an anatase phase with higher crystallinity was obtained from TiO2-FAc than from TiO2-Ac. Because anatase TiO2 is more active in photocatalysis than the rutile or amorphous phases, the observed photocatalytic activity of calcined TiO2-FAc was expected to be greater. In addition, in scCO2, calcined TiO2-Ac, as Degussa P25 reported previously,22 tended to aggregate and to deposit onto the bottom or windows of the vessel, likely because of its hydrophilic surface. This phenomenon produced an internal filter effect and reduced photocatalytic efficiency. Calcined TiO2FAc particles, however, remained effectively dispersed in scCO2. The different degrees of dispersion of calcined TiO2-Ac and TiO2-FAc in scCO2 likely relate to the differences in sizes, shapes, porosities, densities, and surface properties of the two catalysts. Small amounts of residual trifluoroacetate groups present on the surface of calcined TiO2-FAc (Figure 1b) might also contribute to the enhanced dispersion of TiO2-FAc in scCO2, because fluorinated moieties are CO2-philic, as described above. The photocatalytic activity of calcined TiO2-FAc was slightly higher than that of hydrophobic T805 TiO2. This difference is probably related to the composition of T805 TiO2 (70% anatase and 30% rutile) and to the lower specific surface area of T805 (50 m2/g). Another possible reason for the difference is that photocatalytic oxidation of 1-octanol on T805 would be accomplished in competition with the photocatalytic oxidation of surface-bound alkyl silane groups by TiO2 photocatalysis itself, although no evidence for an induction period expected from this competition could be observed.
Acknowledgment. The synthesis and characterization of these particles were supported by the National Science Foundation Science and Technology Center (CHE-9876684) at North Carolina State University. The relative photocatalytic activity measurements were conducted with the support of the U.S. Department of Energy, Office of Basic Energy Science, at the University of California, San Diego. References and Notes (1) Sui, R.; Rizkalla, A. S.; Charpentier, P. A. J. Phys. Chem. B 2004, 108, 11886. (2) Shah, P. S.; Husain, S.; Johnston, K. P.; Korgel, B. A. J. Phys. Chem. B 2001, 105, 9433. (3) Cooper, A. I. AdV. Mater. 2001, 13, 1111. (4) Fox, M. A.; Dulay, M. T. Chem. ReV. 1993, 93, 341. (5) Nagamine, S.; Ueda, T.; Masuda, I.; Mori, T.; Sasaoka, E.; Joko, I. Ind. Eng. Chem. Res. 2003, 42, 4748. (6) Tadros, M. E.; Adkins, C. L. J.; Russick, E. M.; Youngman, M. P. J. Supercrit. Fluids 1996, 9, 172. (7) King, M. B.; Mubarak, A.; Kim, J. D.; Bott, T. R. J. Supercrit. Fluids 1992, 5, 296. (8) Johnson, K. P.; Harrison, K. L.; Clarke, M. J.; Howdle, S. M.; Heitz, M. P.; Bright, F. V.; Carlier, C.; Randolph, T. W. Science 1996, 271, 624. (9) Ohde, H.; Ohde, M.; Bailey, F.; Kim, H.; Wai, C. M. Nano Lett. 2002, 2, 721. (10) Lim, K. T.; Hwang, H. S.; Ryoo, W.; Johnston, K. P. Langmuir 2004, 20, 2466. (11) Stallings, W. E.; Lamb, H. H. Langmuir 2003, 19, 2989. (12) Trentler, T. J.; Denler, T. E.; Bertone, J. F.; Agrawal, A.; Colvin, V. L. J. Am. Chem. Soc. 1999, 121, 1613. (13) Fujiwara, M.; Wessel, H.; Park, H. S.; Roesky, H. W. Chem. Mater. 2002, 14, 4975. (14) Parala, H.; Devi, A.; Bhakta, R.; Fischer, R. A. J. Mater. Chem. 2002, 12, 1625. (15) Guerrero, G.; Mutin, P. H.; Vioux, A. Chem. Mater. 2000, 12, 1268. (16) Crouzet, L.; Leclercq, D.; Mutin, P. H.; Vioux, A. Chem. Mater. 2003, 15, 1530. (17) Hay, J. N.; Raval, H. M. J. Mater. Chem. 1998, 8, 1233. (18) Rice, G. L.; Scott, S. L. Chem. Mater. 1998, 10, 620. (19) Arnal, P.; Corriu, R. J. P.; Leclercq, D.; Mutin, P. H.; Vioux, A. Chem. Mater. 1997, 9, 694. (20) Arnal, P.; Corriu, R. J. P.; Leclercq, D.; Mutin, P. H.; Vioux, A. J. Mater. Chem. 1996, 6, 1925. (21) Popa, A. F.; Mutin, P. H.; Vioux, A.; Delahay, G.; Coq, B. Chem. Commun. 2004, 19, 2214. (22) Resmi, M. R.; Whitesell, J. K.; Fox, M. A. Res. Chem. Intermed. 2002, 28, 711. (23) Hirakawa, T.; Whitesell, J. K.; Fox, M. A. J. Phys. Chem. B 2004, 108, 10213. (24) Pande, K. C.; Mehrotra, R. C. Z. Anorg. Allg. Chem. 1957, 290, 95. (25) Caruso, J.; Roger, C.; Schwertfeger, F.; Hampden-Smith, M. J.; Rheingold, A. L.; Yap, G. Inorg. Chem. 1995, 34, 449. (26) Doeuff, S.; Henry, M.; Sanchez, C.; Livage, J. J. Non-Cryst. Solids 1987, 89, 206.
