J. Phys. Chem. C 2008, 112, 1901-1907
1901
Synthesis of LaPO4:Eu Nanostructures Using the Sol-Gel Template Method Mary J. Fisher, Wehzhong Wang, Peter K. Dorhout, and Ellen R. Fisher* Department of Chemistry, Colorado State UniVersity, Fort Collins, Colorado 80523-1872 ReceiVed: August 8, 2007; In Final Form: NoVember 1, 2007
The sol-gel template synthesis method was used to investigate size-induced changes in luminescence properties of Eu3+-doped LaPO4 materials. Both untemplated bulk powders as well as templated nanotubes ranging in size from 20 to 200 nm were synthesized. These materials were dried at 70 °C or dried and sintered at 650 °C. Powder X-ray diffraction data indicate that the untemplated dried bulk powder is a mix of hexagonal and monazite phases, whereas the dried and sintered bulk powder is a pure monazite phase. When the LaPO4: Eu material is templated, nanotubes are formed that reflect the dimensions of the template membrane. Luminescence data indicate the Eu3+ ion substitutes for the La3+ ion in the LaPO4 host lattice for both the bulk powders and the nanotubes. Notably, as the diameter of the tube decreases, the spectra broaden and the background signal increases significantly. This is likely the result of increased defects in smaller particles that have a larger surface-to-volume ratio.
Introduction Luminescent materials have been studied extensively in recent years because of their potential applications in devices that produce artificial light.1,2 Inorganic luminescent materials are currently used in devices such as cathode ray tubes and fluorescent lamps because they have intense electromagnetic peaks.2 Oxide materials represent a class of inorganic materials with technologically relevant properties that could be used in the production of displays, lamps, and sensors. Recently, various types of oxide materials in the form of films, nanoparticles, and doped nanoparticles have been examined to explore their viability in a range of devices. Sol-gel chemistry is one route to many oxide materials, including those of interest for display applications. The solgel method involves the preparation of a colloidal suspension in a liquid, the sol, which is then aged to produce the gel, a colloid.3 Most sol-gel preparations rely on hydrolysis of the precursor material to form the hydroxide or oxide compound. The gel is typically heated to remove any volatiles and to crystallize the material. The use of colloidal solutions prevents the precipitation of particles, which allows dopants to be added. For industrial processes, low-temperature reaction techniques such as the sol-gel method are preferred for the production of phosphor powders with small (99.5%) in 5 mL of deionized H2O. This solution was added to the clear solution containing the chelate of La(EDTA)- with vigorous stirring. The pH of the final solution was adjusted to 8.5 as necessary using the acid or base solutions described above. When scaling down the reaction to yield 1 g, the only changes from the 8 g synthesis involved constant heating of the Na2H2EDTA solution, and upon the addition of the phosphate solution, the mixture was heated for 5 min to complete the reaction. The resulting solution was allowed to gel at room temperature without heat treatment. We term materials produced in this manner “untreated bulk powders”. Preparation of phosphate nanostructures was accomplished by the dropwise addition of the sol onto Whatman porous alumina membrane templates having a thickness of 50 µm and average pore diameter of either 200, 100, or 20 nm. The sol was allowed to permeate the membrane for 1 min, and then the excess was wiped from the membrane surface. Membranes containing the sol were allowed to age, presumably allowing a gel to form within the template, and then were dried at room temperature overnight in air. One control in our process involved evaluating the dried gels within the membrane templates before sintering, samples we refer to as “dried templated”. The term “dried bulk powder” describes the powder formed by heating the untemplated sol overnight at 70 °C. Untemplated gels that were dried and then sintered using a heat treatment which entails a ramp (50 °C/h) to 650 °C, heating for 3 h in air, followed by a cooling cycle within a deenergized furnace are termed “dried and sintered bulk powder”. To dry and sinter the gels that formed upon aging of the sols within the templates, the membranes were heated following the procedures described above for the dried and sintered bulk powder. To isolate free-standing nanostructures, the residual phosphate film that formed on the membrane surface was polished clean using fine grit sand paper, as needed, and the samples were immersed in 6 M NaOH for 24-48 h to remove the aluminum oxide membrane template. After carefully washing the remaining solids by rinsing with deionized H2O and centrifuging the mixtures, the phosphate nanostructures were dried at 70 °C overnight in air. Sintering the template/gel composite above the vitrification point of the aluminum oxide membrane effectively destroyed the nanostructures; therefore, the filled membranes were not heated above 650 °C. The nanomaterials formed by drying and sintering in the templates are termed simply “nanostructures” or “nanotubes”. PXRD data were collected on a Phillips PW1830 diffractometer with Cu KR radiation (1.5406 Å). SEM images and energy-dispersive spectroscopy (EDS) data were acquired on a JEOL JSM-6500F field emission scanning electron microscope. The SEM samples were prepared as follows: after sintering, the membranes were adhered to a piece of paper towel by epoxy (Torr-Seal epoxy, Varian). After setting, the membrane-epoxy composites were polished by sandpaper as needed. The alumina membrane template was dissolved using 6 M NaOH and allowing the membrane to soak overnight. Following the dissolution of the templates, samples were washed with deionized water several times and dried at room temperature. The process yielded bundles of nanostructures seated at one end within the epoxy matrix. The dried nanostructure/epoxy matrixes were attached to SEM stubs by Cu tape or carbon tape and coated by 5-15 nm Au by an Anatech sputter coater. One of
Sol-Gel Template Synthesis of LaPO4:Eu
Figure 1. Electron microscopy images of LaPO4:Eu bulk powders: (a) TEM image of LaPO4:Eu bulk powder dried at 70 °C, showing rodlike morphology and (b) field emission SEM image of the dried and sintered bulk powder (×19 000).
the TEM instruments used was a JEOL 2000 EXII (JEOL, U.S.A.) microscope, located at Colorado State University. TEM was also performed at the Colorado School of Mines, on a Philips CM200 (FEI microscope). Thermal analysis of isolated nanostructures removed from the epoxy was performed on a TA Instruments thermal analysis and rheology DSC 2010. Room-temperature luminescence spectra were recorded using a Fluorolog-3 spectrofluorometer with a Tau-3 lifetime system (Instruments S.A. Inc.). The excitation wavelength, λex, was either 266 or 396 nm. Results The morphology and size of the synthesized phosphate nanomaterials were observed directly using SEM and TEM. The TEM image of the dried bulk powders shown in Figure 1a reveals the particles are similar in size and shape. From this image, it is clear that most of the dried bulk powder is composed of short rods. The diameter of the rods ranged from 3 nm to 1 µm, and the length ranged from 10 nm to 8 µm. The preferential formation of rods may be related to the presence of the chelate, EDTA4-, which has been shown to influence the morphology of nanomaterials such as LaPO4:Eu, CdS, and Bi2Te3.9,23,24 For the LaPO4:Eu materials, increasing the EDTA/La3+ ratio favored formation of spherical particles over spindle-like particles. Thus, our results are consistent with formation of more rodlike materials as our EDTA/La3+ ratio was lower than that used by Fujishiro et al.9 Work by Haase and co-workers demonstrated that the morphology of LaPO4:Eu materials formed via wet chemical synthesis methods (i.e., without EDTA) is highly dependent on pH and degree of doping.1,19 At high pH nanoparticles are formed, whereas fibers are formed under acidic conditions. Doped LaPO4 nanoparticles with an undoped LaPO4 shell also demonstrate elongated forms. The morphology of the dried and sintered bulk powder, Figure 1b, shows submicrometer particles are formed upon heating to 650 °C. These particles appear to be more spherical and could be the result of elimination of any of the remaining EDTA upon heating. SEM images of templated materials reveal the formation of ∼50 µm long nanostructures with 200, 100, or 20 nm diameters, Figure 2, depending on the pore size in the original template. Bundles of 200 nm phosphate nanostructures isolated from the template are shown in Figure 2a. This side view of the nanostructure array shows straight and conformal 200 nm diameter tubes. The dimensions of the nanostructures are in accordance with the size and shape of the pores of the membrane templates. The open end of the nanostructures, depicting the hollow structures, can be more clearly seen in the SEM image of the top view, Figure 2b, further supporting the hypothesis
J. Phys. Chem. C, Vol. 112, No. 6, 2008 1903 that nanostructures were produced. Figure 2c shows LaPO4:Eu tubes formed from a template with 100 nm pores. In this image, the 100 nm tubes are lying on their sides and the tube diameter is 100 nm ((1 nm) at one end of the tubes, increasing to 200 nm ((1 nm) at the other end of the tubes. Figure 2d shows a side view of the tubes formed using a 20 nm diameter template membrane. Here, the tube diameter range is 35-36 nm at one end of the tube, increasing to 130-131 nm at the other end of the tubes. Note the pores for the commercial 100 and 20 nm diameter template membranes are not uniform across the membrane. The pore diameter is approximately 150-200 nm on one side and narrows to the defining diameter (either 100 or 20 nm, respectively) on the other side. These data indicate that the tubes are conformal to the pore size and that at the defining diameter end of the tubes the variation in size is approximately (1 nm at any given point along the nanotube array. Figure 3a shows a TEM image of an isolated nanostructure formed using a 200 nm pore diameter template; the outer diameter of the single tube is ∼200 nm. The walls and hollow nature of the structure are clearly shown in this image. Parts b and c of Figure 3 show TEM images of dried templated nanostructures made using the 20 nm pore diameter template. Again the hollow nature of the 20 nm diameter particles indicates that nanotubes were formed. Phase identification and composition of our reaction products were performed using PXRD, EDS, and X-ray photoelectron spectroscopy (XPS). PXRD evaluation of the dried nanomaterials confirmed the amorphous nature of the solids. The PXRD pattern for the 200 nm rare-earth phosphate nanotubes, Figure 4a, was identified as the high-temperature monoclinic monazite phase. For comparison, PXRD patterns of the dried bulk powder and the dried and sintered bulk powders from the same batch used for preparing the nanostructures are also displayed in Figure 4. The drying and sintering conditions for the bulk powders were the same as for the templated nanostructures. The diffraction pattern of the dried bulk powder, Figure 4b, could be indexed as a mix of the low-temperature hexagonal phase and the high-temperature monoclinic monazite phase of lanthanum phosphate.25,26 After drying and sintering, the powder was identical to monoclinic monazite lanthanum phosphate, Figure 4c.25 Note that we were unable to acquire reliable PXRD data for the 20 nm templated nanostructures because of the extremely small amounts of material. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) data for the dried bulk powders were collected and analyzed. These data demonstrate an exothermic peak around 207 °C, significantly lower than the reported transition at 335 °C, which was attributed to loss of adhered EDTA.9 After sintering, however, no phase change for the untemplated materials was observed. Analysis of the dried and sintered bulk powder by EDS, Figure 5, confirms the presence of La, P, Eu, and O. Similar spectra were collected for the nanotubes, although the Eu peaks at ∼1.1 eV (which overlaps with the Na peak) and 5.8 eV were often very small. The oxygen peak lies at the edge of our detector range and could be attributed to either the remains of the template or the phosphate material. Spectra acquired for the dried bulk powder as well as 200, 100, and 20 nm nanotubes are very similar to those shown in Figure 5 for the dried and sintered bulk powder. Elemental mapping data did not provided any definitive evidence for the location of the Eu dopant (i.e., we could not determine if the Eu was located primarily at grain boundaries) as signal from these peaks were relatively uniformly distributed spatially. Overall, the EDS data suggest there are no appreciable differences in the elemental composition of all
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Fisher et al.
Figure 2. Field emission SEM images of 200 nm nanotubes: (a) side view, showing bundle formation after the removal of the AAO template (×15 000), and (b) top view, showing the tubular structure (×18 000). The tubes have an outer diameter of 200 nm and are ∼50 µm long. Field emission SEM images of dried and sintered LaPO4:Eu nanotubes formed using AAO templates with (c) 100 nm (×18 000) and (d) 20 nm diameter pores (×40 000).
Figure 3. TEM images of LaPO4:Eu nanotubes: (a) A single dried and sintered 200 nm tube. Images shown in (b) and (c) are for as-prepared 20 nm nanotubes at two different magnifications.
of these materials. The compositional information from the EDS data were confirmed with XPS data, which reveal both the bulk powders and the nanotubes have a La/P atomic ratio of 1:1, consistent with formation of a LaPO4 material. Eu was not always observed in the XPS spectra, which may be because of the relatively low doping levels. Alternatively, it could also be because of the inconsistencies in the surface doping levels leading to Eu not being at the extreme surface of the nanoparticles.19 Emission spectra were acquired for the bulk powder to verify the presence of Eu3+. Figure 6a shows the emission spectrum for dried bulk powder using λex ) 396 nm. The peaks observed in this spectrum correspond to the 5D0-7F1-4 transitions for Eu3+ in the LaPO4 matrix.1,2,20,27,28 Figure 6b displays the emission spectrum for the dried and sintered bulk powder, with λex ) 396 nm. This spectrum shows the same four peaks attributable to 5D0-7F1-4 transitions of Eu3+. The emission spectra in Figure 6 demonstrate how the optical properties of the bulk powder change as a function of heat treatment. First, the peaks at 590, 615, and 700 nm in the spectrum for the dried
Figure 4. PXRD patterns of LaPO4:Eu materials: (a) 200 nm nanotubes, (b) dried bulk powder, and (c) dried and sintered bulk powder. Data shown in (d) are reference data for the monazite phase from the JCPDS no. 32-493.
bulk powder have some splitting, although it is not very pronounced. These same transitions in the emission spectrum
Sol-Gel Template Synthesis of LaPO4:Eu
Figure 5. EDS spectrum of dried and sintered bulk powder with peaks from La, P, and O. The atomic % ratio for La/P/O is 1.0:0.80:4.7, whereas the weight % of La/P/O is 1.0:0.17:0.54. This spectrum is representative of the dried bulk powder and the nanotubes and does not show the presence of Eu.
Figure 6. Room-temperature luminescence spectra (λex ) 396 nm) for the synthesis of LaPO4:Eu (a) dried bulk powder and (b) dried and sintered bulk powder (multiplied by a factor of 2). The four major peaks correspond to the 5D0-7F1-4 transitions in Eu3+.
of the dried and sintered bulk powder show significantly more splitting. Second, the electric-dipole transition, 5D0-7F2, is the most intense for the dried bulk powder; however, when heated to 650 °C, the magnetic-dipole transition, 5D0-7F1, is the most intense peak. Emission spectra for comparable amounts of 200 nm and the 20 nm nanotubes (λex ) 396 nm) are shown in Figure 7. Both spectra contain peaks corresponding to the 5D0-7F1-4 transitions of Eu3+ in a LaPO4 matrix, similar to those observed for the powders. The peak shape changes from narrow and more welldefined peaks for the 200 nm nanotubes to broader, poorly defined peaks with a broader background for the 20 nm nanotubes. This suggests the crystal environment of the Eu3+ ion in the matrix may be influenced by particle size. Figure 8 shows the emission spectra for the untreated bulk powder, dried bulk powder, and dried and sintered bulk powder using λex ) 266 nm. These spectra demonstrate that when the dried powder is heated, the intensity of the luminescence peaks in the 580750 nm range decrease. In addition, the peak at 530 nm, attributed to the 5D1-7F0,1 transition,22 changes shape with different heat treatments. This spectral feature narrows significantly with elevated temperatures, and the splitting observed in the spectrum for the untreated bulk powder is not present in the spectra of materials treated at either 70 or 650 °C.
J. Phys. Chem. C, Vol. 112, No. 6, 2008 1905
Figure 7. Room-temperature luminescence spectra (λex ) 396 nm) for sol-gel template synthesized LaPO4:Eu nanoparticles: (a) 200 nm nanotubes and (b) 20 nm nanotubes. The four major peaks correspond to the 5D0-7F1-4 transitions in Eu3+.
Figure 8. Room-temperature luminescence spectra (λex ) 266 nm) of the LaPO4:Eu powders: (a) untreated bulk powders (dashed line), (b) dried bulk powder (solid line), and (c) dried and sintered LaPO4:Eu (dash-dot line).
Discussion We have integrated sol-gel chemistry with the template synthesis method to produce nanoscale LaPO4 materials doped with Eu3+ while also controlling the morphology and size of the nanoparticles produced. Untemplated bulk powders of LaPO4:Eu were easily produced using sol-gel chemistry. The sol-gel template synthesis method successfully resulted in nanotubes with diameters ranging from 20 to 200 nm. Indeed, TEM images of the nanotubes indicate that regardless of the heat treatment or pore size, the template method can control the shape and size of the particles formed within the channel. Comparison between the bulk powders and the nanoparticles reveals potential particle size-based changes in the properties of the LaPO4 materials. XPS and EDS data indicate the presence of LaPO4 in the bulk powders, implying the EDTA4- chelates to the La3+ ion and prevents the reaction to form lanthanide oxide, allowing the LaPO4 to form preferentially, in part because the La(EDTA)complex is stabilized at [EDTA]/[PO43-] ratios of 1 or higher.9 These data also indicate that the composition of the material produced using the sol-gel method does not vary significantly with different heat treatments. TEM, SEM, and PXRD data, however, clearly indicate the morphology and phase of our bulk powders change with different heat treatments. This is somewhat
1906 J. Phys. Chem. C, Vol. 112, No. 6, 2008 consistent with the results of Fang et al., who found that although the composition and morphology of their LaPO4 nanowires and nanorods did not change after a 900 °C heat treatment, the phase of the materials did.18 The change in morphology we observe is likely the result of using a different synthetic technique. As noted in the Introduction, the synthetic technique and pathway can significantly affect the morphology of the materials produced.8 For example, Deng et al. found that using different chelating species in the solvothermal synthesis of Bi2Te3 materials resulted in formation of either rods, sheet-rods, nanosheets, or rag particles.23 With LaPO4 materials, the solution pH as well as the synthetic route can severely affect the resulting particle morphology.1,8,27 When a metal oxide material is doped, it is useful to understand where the dopant is located in the crystal lattice as this often determines many of the physical and chemical properties of a material. Additionally, improvements to materials properties can be made if the location of dopant atoms is known. Emission spectra of the LaPO4:Eu bulk powders, Figure 6, show the Eu3+ ion replaces the La3+ ion in the matrix regardless of the heating process used. With the different heat treatments, however, the phase of the powder changes, Figure 4, resulting in an alteration of the crystal environment around the Eu3+. Because the crystal splitting depends on the coordination environment of the Eu3+ ion,5,23,29 the peaks in the emission spectrum of the dried and sintered powder have increased splitting. This is most notable with the 5D0-7F4 peak at ∼690 nm and the 5D0-7F2 peak at ∼620 nm, Figure 6. One additional feature in the emission spectra of the bulk powders deserves attention. The 5D0-7F1 and 5D0-7F2 transitions are important as the former is the magnetic-dipole transition and the latter is the electric-dipole transition of the Eu3+. The relative intensities of these two peaks change with the two different heat treatments. In the spectrum for the dried bulk powder (70 °C), the 5D0-7F2 peak is the most intense peak, whereas the 5D0-7F1 transition is the most intense for the dried and sintered bulk powder (650 °C). When Eu3+ is in a lowsymmetry site, without a center of inversion, the electric-dipole transition is the most intense peak in the luminescence spectrum.5,20,30 Typically, the magnetic-dipole transition will be most intense if there is a center of inversion. The ratios we observe are consistent with the Eu3+ ions being located at the surface of the crystal when subjected to the lower heating profile (dried only).22 At the surface, the chemical environment could change because of “oxide” or “hydroxide” on the surface of the particle. Moreover, the site symmetry at the surface may be different than in the bulk monoclinic monazite phase. The use of a different excitation wavelength can reveal emissions from different 5DJ states and can isolate emissions from Eu3+ ions in different crystallographic sites. Using 266 nm excitation, Figure 8, we found that the dominant feature in the spectra of all of the bulk powders is the 5D1-7F0,1 transition at ∼540 nm. The crystal splitting of this emission line is observed only in the untreated bulk powder and is reduced to a single peak in the spectra for materials subjected to heat treatments. Similar changes in relative intensities and splitting have been observed with other lanthanum-doped materials and are most likely the result of the Eu3+ ion residing in a different crystalline environment when the higher heat treatment is used.1,22,28 It is also possible that the number and occupation of doped sites in the LaPO4 crystal changes upon heating.28 Nonetheless, it is clear from the luminescence spectra of all of the powders that the symmetry of the dopant site changes upon heat treatment.
Fisher et al. Examination of the luminescence spectra for the nanomaterials also reveals some interesting trends. From the luminescence spectra for our nanotubes, Figure 7, the Eu3+ ion appears to have substituted in the La3+ site in the host lattice when using the sol-gel template method. These results are similar to those obtained by Yu et al. when producing LaPO4:Eu nanomaterials using hydrothermal synthesis techniques.8 As with the dried and sintered bulk powders, the electric-dipole transition (5D0-7F2) is not as intense as the magnetic-dipole transition (5D0-7F1) for both the 200 and the 20 nm tubes. This suggests that the nanomaterials produced likely contain no center of inversion. This also suggests that the drying and sintering process is the same for both nanotubes and bulk powders. Interestingly, the relative intensities of individual peaks in the luminescence spectra of the nanotubes are significantly different from those of the powders. This is likely the result of a combination of factors, including the presence of multiple europium sites, the orientation of the crystal axis relative to the polarization vector of the incident light, and potentially, energy transfer occurring between two adjacent Eu3+ sites.1,22,28 When luminescence results for two different sizes of nanoparticles are compared, Figure 7, it is clear that significant changes have occurred in the spectra. Specifically, the luminescence peaks are extremely broad in the spectrum for the 20 nm particles. There is also a comparatively broader background for the entire spectrum, but most notably for the 5D0-7F1 transition at ∼580 nm. Although it is possible that this background arises from increased scattering off of the smaller particles, it may also indicate that there are significant concentrations of defects in the smaller nanotubes relative to the 200 nm nanotubes or the bulk powders. Additional evidence for this hypothesis comes from literature results which show that as the LaPO4:Eu particle size was reduced from 40 to 5 nm a broad background appeared in the luminescence spectrum.22 This broad background was attributed to either the development of a disordered boundary region or a high defect level resulting from the high surface-to-volume ratio of the 5 nm particles. Interestingly, Wang also found a higher 5D0-7F2/5D0-7F1 ratio for the smaller nanoparticles, indicative of Eu3+ ions located closer to an inversion center. Although there is significant broadening in the spectrum of our 20 nm nanotubes, Figure 8, it is clear that the intensity of the 5D0 f 7F2 transition is not higher than that of the 5D0-7F1 transition. This suggests that no change in crystallographic site symmetry occurs when particle size is reduced for our sol-gel synthesized nanotubes. Given that we do not have reliable PXRD data for the 20 nm nanotubes, we cannot eliminate the possibility that the changes in the luminescence spectra between the 200 and the 20 nm materials may be in part the result of a crystallinity change. There are, however, several counter arguments that could be made to this. First, as discussed above, this type of change in luminescence spectra accompanying a change in particle size has been observed previously.22 Second, it has been shown that optical properties such as band gap, UV-vis absorption, and luminescence of nanoparticles vary nonlinearly with particle size.31-34 Indeed, as particle size decreases, both bonding and electronic structure change and surface effects become much more pronounced. All of these effects could lead to the observed changes in luminescence with no appreciable change in crystallinity of the nanomaterials. Third, further substantiation comes from the data of Yu et al. for microscale and nanoscale LaPO4:Eu particles and wires formed via hydrothermal synthesis discussed above. They found that their materials had very similar XRD patterns, regardless of shape
Sol-Gel Template Synthesis of LaPO4:Eu or size, but both size and shape strongly affected the luminescence features of these materials. Likewise, Cannas et al. demonstrated that phase changes in their Eu3+-doped Y2SiO5 nanoparticles did not significantly affect the luminescence spectra.35 Rather, it was the Eu3+ doping level that had the most pronounced effect on the intensity and shape of individual spectral lines. Thus, the observed changes in luminescence spectra in Figure 7 are most likely a size effect and not a crystallinity effect. Summary We have successfully employed the sol-gel template synthesis method to produce LaPO4:Eu nanotubes with sizes ranging from 20 to 200 nm. The composition, phase, and morphology of the particles was determined using XPS, EDS, PXRD, and electron microscopy. The luminescence spectra for these materials reveal that as the nanoparticle size decreases, the Eu3+ emission become broader and less well defined. This is likely a result of higher defect levels in the smaller particles. The spectra also demonstrate that the primary crystallographic site for the Eu3+ ions is the same for both 200 and 20 nm nanotubes. For bulk powders, the luminescence spectra show a considerable increase in the 5D0-7F2/5D0-7F1 intensity ratio for powders treated at lower temperatures, thereby suggesting Eu3+ resides in a more disordered LaPO4 phase. This is supported by PXRD data which indicate lower crystallinity and a phase change for the dried powders when compared to powders sintered at elevated temperatures. Acknowledgment. The authors acknowledge the financial support of the Department of Energy (DE-FG02-03ER46098 and DE-FG03-99ER45791). We also thank Professor Nancy E. Levinger for the use of the fluorimeter and Dr. John Chandler (Colorado School of Mines) for some of the TEM data. References and Notes (1) Meyssamy, H.; Riwotzki, K.; Kornowski, A.; Naused, S.; Haase, M. AdV. Mater. 1999, 11, 840. (2) Rambabu, U.; Buddhudu, S. Opt. Mater. 2001, 17, 401. (3) Sol-Gel Technology for Thin Films, Fibers, Preforms, Electronics, and Specialty Shapes; Klein, L. C., Ed.; Noyes Publications: Park Ridge, NJ, 1987; p 407. (4) Wang, M.; Wang, J.; Chen, W.; Cui, Y.; Wang, L. Mater. Chem. Phys. 2006, 97, 219. (5) Yu, M.; Lin, J.; Fu, J.; Zhang, H. J.; Han, Y. C. J. Mater. Chem. 2003, 13, 1413. (6) You, X.; Chen, F.; Zhang, J. J. Sol.-Gel Sci. Technol. 2005, 34, 181.
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