J. Phys. Chem. C 2008, 112, 17115–17120
17115
(K1.5Eu0.5)Ta3O10: A Far-Red Luminescent Nanosheet Phosphor with the Double Perovskite Structure Tadashi C. Ozawa,*,† Katsutoshi Fukuda,† Kosho Akatsuka,†,‡ Yasuo Ebina,† Takayoshi Sasaki,†,‡,§ Keiji Kurashima,| and Kosuke Kosuda| Nanoscale Materials Center, International Center for Materials Nanoarchitectonics, and Materials Analysis Station, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan, and Graduate School of Pure and Applied Sciences, UniVersity of Tsukuba, 1-1-1 Tennoudai, Tsukuba, Ibaraki 305-8571, Japan ReceiVed: June 23, 2008; ReVised Manuscript ReceiVed: August 21, 2008
Eu3+-activated double perovskite-type nanosheet phosphor (K1.5Eu0.5)Ta3O10 has been prepared by the soft chemical exfoliation reaction of K(K1.5Eu0.5)Ta3O10 bulk precursor. The lateral size of the nanosheet product ranges from 0.1 to a few micrometers, and the thickness is uniformly 2.4(2) nm. The lattice parameter along the sheet direction obtained by the in-plane X-ray diffraction is 0.3933(3) nm, corresponding to the typical O-Ta-O distances. The photoluminescence emission can be obtained either by host or by direct photoactivator excitation, but the host excitation yields much higher emission intensity than does the direct photoactivator excitation. The emission spectra show relatively sharp peaks from the 5D0 f 7FJ manifold transitions of Eu3+. The highest emission intensity is observed around 704 nm (far-red) from the 5D0 f 7F4 transition of Eu3+. Introduction Ln (lanthanide)-activated oxide phosphors have been under intense scrutiny because of their well-defined emission from the intra-4f transitions of Ln ions and the chemical and physical stability of the oxide hosts with respect to those of other types of hosts such as chalcogenides, halides, and organic molecules. Among the oxide hosts, those with nanosheet morphology have great advantages over the bulk hosts.1-15 First, the large surface area of nanosheets is quite suitable to efficiently absorb excitation energy. Second, emission colors for various applications could finely be adjusted by sequential depositions of nanosheet phosphors with different emission colors while avoiding the cross-relaxation interactions of photoactivators in different nanosheet hosts. Third, relatively high concentrations of photoactivators can be incorporated into a two-dimensional host without quenching the luminescence as predicted by the percolation model.16-20 Thus, high emission intensity can be realized by high concentration photoactivator doping into a nanosheet host.10 Finally, two-dimensional morphology of the nanosheet is suitable for the fabrication of devices consisting of stacked layers of various functional materials21 because densely packed and well-oriented films of nanosheets can readily be formed by methods such as the LB (Langmuir-Blodgett) method9,10,22 and layer-by-layer depositions of nanosheets utilizing the electrically charged polyions such as PEI (polyethylenimine) as electrostatic glue.23 The majority of previously reported nanosheet-based phosphors consist of Ln photoactivators or Ln-containing complexes inserted between oxide nanosheets.2-8,11,13,14 The * Corresponding author. Phone: +81-29-860-4722. Fax: +81-29-8549061. E-mail:
[email protected]. † Nanoscale Materials Center, National Institute for Materials Science. ‡ International Center for Materials Nanoarchitectonics, National Institute for Materials Science. § University of Tsukuba. | Materials Analysis Station, National Institute for Materials Science.
photoluminescence properties of these internanosheet site activated phosphors tend to be quite susceptible to the amount of cointercalated species such as H2O and hydronium ions, which act as mediators of excitation energy transfer from the nanosheet hosts to the intercalated photoactivators or the complexes. Thus, these are not suitable for practical applications. In addition, more efficient energy transfer from the nanosheet host to photoactivators is expected if the activators are incorporated in intrananosheet sites rather than in internanosheet sites because of the closer interaction between the nanosheet hosts and photoactivators. We have recently prepared two types of intrananosheet site Eu3+-activated phosphors, (La0.90Eu0.05)Nb2O7 and Eu0.56Ta2O7.9,10 The structures of these nanosheets are the single perovskite layer type. Both of them exhibit visible red emission from the 5D0 f 7FJ manifold transitions of Eu3+ photoactivators either by the nanosheet host excitation or by the direct Eu3+ excitation. However, the host excitation yields much higher emission intensity than does the direct Eu3+ excitation. The host excitation dominated photoluminescence property seems to be a general behavior of the intrananosheet site activated phosphors. However, further studies of photoluminescence properties of many other intrananosheet site activated phosphors are required to corroborate this trend. Therefore, it would be quite intriguing to investigate the photoluminescence properties of a new type of intrananosheet site photoactivated nanosheets. In addition, it is also important to expand the nanosheet phosphor libraries for their utilization in diverse practical applications. In this Article, we report the preparation and photoluminescence properties of the new Eu3+-activated double perovskite-type nanosheet phosphor (K1.5Eu0.5)Ta3O10, whose structure consists of triple corner shared Ta-O octahedral layers and K+ and Eu3+ in the sites surrounded by the faces of the octahedra.
10.1021/jp805545u CCC: $40.75 2008 American Chemical Society Published on Web 10/11/2008
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Ozawa et al.
Experimental Section Materials. (K1.5Eu0.5)Ta3O10 nanosheets were prepared by three-step reactions. First, the layered double perovskite bulk precursor K(K1.5Eu0.5)Ta3O10, which is a n ) 3 member of the DJ (Dion-Jacobson) phases A[A′n-1BnO3n+1],24,25 was prepared by the solid state reaction of 5:1:3 K2CO3:Eu2O3:Ta2O5. The large excess of K2CO3 and Eu2O3 was necessary to minimize the coproduction of impurity phases. These materials are mixed thoroughly in an agate mortar, placed in a Pt crucible, which is further enclosed in a capped alumina crucible, and heated at 800 °C for 6 h and then at 1225 °C for 15 h. Second, 1.26 g of the ground K(K1.5Eu0.5)Ta3O10 powder was reacted with 126 mL of 2 M HNO3 for 3 days at room temperature under vigorous shaking to exchange K+ with H+. This protonation of the precursor was necessary to activate it for the following exfoliation reaction. After the reaction, the protonated precursor was separated from the acid by centrifugation, washed with water, and dried at room temperature. Finally, the protonated precursor was reacted with a TBAOH (tetrabutylammonium hydroxide) aqueous solution. The concentration of the TBAOH solution was adjusted so that the amount of TBA+ ions in the solution was approximately 6 times that of H+ in the protonated bulk precursor. After 1 week of vigorous shaking, a colloidal nanosheet suspension was obtained. This nanosheet suspension was centrifuged at 2000 rpm for 15 min to separate the unexfoliated residue from the nanosheet suspension. The concentrated form of the nanosheets was also prepared by the additional centrifugation of the nanosheet suspension at 20000 rpm for 30 min for its characterizations. Characterizations. The powder XRD (X-ray diffraction) patterns of the bulk precursors were obtained using Cu KR radiation on a Rigaku RINT2200V/PC diffractometer. The diffraction peaks were indexed, and the lattice parameters were refined using APPLEMAN software.26 The structural data of the previously reported analogous phase KCa2Ta3O10 were used as starting parameters for these processes.27,28 The elemental composition of the concentrated and dried nanosheets was analyzed by EPMA on JEOL JXA-8500F using the acceleration voltage of 15 kV and the beam current of 4 × 10-5 mA. The in-plane XRD pattern of (K1.5Eu0.5)Ta3O10 nanosheets, which were deposited on a Si substrate by the LB method,22 was obtained using the synchrotron radiation (λ ) 0.11975(4) nm) of the Photon Factory BL-6C at High Energy Accelerator Research Organization (KEK). TEM and SAED (selected-area electron diffraction) were performed on a JEOL JEM-1010 transmission electron microscope at an acceleration voltage of 100 kV. The specimen for this characterization was prepared by dropping and drying the diluted nanosheet suspension on a carbon microgrid. The morphology analysis of the nanosheets was performed by AFM using a Seiko Instruments SPA-400 AFM system with a Si tip cantilever (20 N m-1) in the DFM mode. The nanosheet sample for this analysis was deposited on a Si substrate using PEI as electrostatic glue.23,29 The XRD pattern of the concentrated nanosheets was obtained using Cu KR radiation on a Rigaku RINT2100 diffractometer. The increase in nanosheet concentration was necessary to minimize the halo pattern of the water medium in the diffraction profile. The diffraction data were acquired under a relative humidity of 95% to minimize the restacking of the nanosheets by drying. Finally, photoluminescence excitation and emission spectra of (K1.5Eu0.5)Ta3O10 nanosheets were obtained using a HITACHI F-7000 fluorescence spectrometer at room temperature. The excitation spectrum was corrected for the spectral distribution of the lamp intensity by the Rhodamine B method, and the
Figure 1. Powder XRD profiles of (a) as-prepared and (b) protonated K(K1.5Eu0.5)Ta3O10.
TABLE 1: Lattice Parameters of (K1.5Eu0.5)Ta3O10 Nanosheet and Related Phases K(K1.5Eu0.5)Ta3O10 as-prepared K(K1.5Eu0.5)Ta3O10 dried at 200 °C K(K1.5Eu0.5)Ta3O10 acid treated (K1.5Eu0.5)Ta3O10 nanosheet (in-plane XRD) (K1.5Eu0.5)Ta3O10 nanosheet (SAED) (K1.5Eu0.5)Ta3O10 nanosheet (concentrated suspension) KCa2Ta3O10a a
a (nm)
b (nm)
c (nm)
0.39949(9)
3.3553(8)
0.39825(9)
0.4069(3)
2.992(1)
0.3952(1)
0.4030(2)
2.754(1)
0.3897(2)
0.3933(3)
)a
0.40(1)
)a
0.3921(1)
)a
0.38657(3)
2.9777(2)
0.38520(3)
See ref 27.
emission spectrum was corrected for the spectral response of the instrument using a substandard light source. The relative quantum yield of the (K1.5Eu0.5)Ta3O10 nanosheet suspension was estimated using quinine in aqueous 0.5 M H2SO4 (quantum yield: 55%) as the reference.30 Results and Discussion The powder XRD pattern of the as-prepared bulk precursor is shown in Figure 1a. The diffraction pattern can be indexed to the orthorhombic cell similarly to that of the analogous phase KCa2Ta3O10 consisting of Ca-Ta-O double perovskite layers interspersed with K+.27 The refined lattice parameters of the as-prepared bulk precursor are a ) 0.39949(9), b ) 3.3553(8), and c ) 0.39825(9) nm, where a and c are parallel to the layers and b is perpendicular to the layers. The lattice parameters a and c are consistent with the typical Ta-O-Ta lengths in perovskite-type compounds. On the other hand, b is larger than that (2.9777(2) nm) of KCa2Ta3O10.27,28 It is well-known that many of the layered perovskite compounds can easily be hydrated, and the lattice parameter along the interlayer direction can be expanded as much as several tenths of a nanometer.31,32 Thus, the large lattice parameter b of the as-prepared bulk precursor is considered to be due to the existence of the interlayer water. As a matter of fact, when this precursor was heated at 200 °C, the lattice parameter b decreased to 2.992(1) nm, while no significant changes in a and c were observed as shown in Table 1 and Figure S1 in the Supporting Information. A couple of diffraction peaks of the as-prepared bulk precursor overlap with those of Eu3TaO7 (PDF #00-038-1411). In addition,
(K1.5Eu0.5)Ta3O10 a few peaks that cannot be indexed on the basis of the layered double perovskite model were observed. The origin of these low intensity peaks is considered to be due to a trace amount of impurity phases other than Eu3TaO7. The complete structure of the as-prepared bulk precursor could not be solved by a technique such as the Rietveld method because of the significant overlap of diffraction peaks from secondary phases. However, we expect that the as-prepared precursor consists of the double perovskite-type layers interspersed with K+, similar to the structure of KCa2Ta3O10, and the chemical formula would be K(K1.5Eu0.5)Ta3O10 based on the following results and facts. First, the double perovskite-type nanosheet without the interlayer alkali metal ions prepared by the exfoliation of this precursor has the elemental composition of K:Eu:Ta ) 1.5:0.5:3 as described later. Second, the exfoliation reaction of layered oxides into nanosheets is topotactic, and elemental composition in the layer units is generally retained in the nanosheets.9,10,33-37 The inclusion of the large size alkali metal ion such as K+ in the A′-site of layered perovskites is rare. However, the occupation of the alkali metal ions in the A′-site has been reported for some layered perovskites.25,38-41 In addition, a well-known perovskite such as KTaO3 contains a large size cation in A-site.42 Therefore, it is rational to expect that the A′-site of the as-prepared bulk precursor is partially occupied by K+ ions. The stoichiometric ratio of the starting materials described in the Experimental Section might potentially yield K2Eu2/ 3Ta2O7, which is a n ) 2 member of the Ruddlesden-Popper phases A2[A′n-1BnO3n+1].39,43 However, no trace of the diffraction peaks from such phase was observed. We have attempted the various ratios of reaction components to prepare K(K1.5Eu0.5)Ta3O10 including the reaction stoichiometry corresponding to the composition stoichiometry of K(K1.5Eu0.5)Ta3O10. Among those reaction conditions, the product with the least amount of impurity phases was obtained for the ratio of the starting components described in the Experimental Section. Figure 1b shows the diffraction pattern of the protonated precursor. The diffraction peaks of the protonated precursor can be indexed to an orthorhombic unit cell similar to that of the as-prepared precursor, indicating the topotactic nature of the protonation reaction retaining the layered double perovskite structure. The refined lattice parameters are a ) 0.4030(2), b ) 2.754(1), and c ) 0.3897(2) nm. The lattice parameters a and c of the protonated precursor are slightly different from those of the as-prepared precursor (Table 1). However, these lattice parameters are still in the typical range of the Ta-O-Ta lengths in the perovskite-type compounds. In addition, the lattice parameter b along the interlayer direction of the protonated precursor is smaller than that of the as-prepared precursor dehydrated at 200 °C. This indicates that the interlayer K+ of the bulk precursor has successfully been exchanged with smaller H+ by the acid treatment. Furthermore, TGA (thermogravimetric analysis) of the protonated precursor revealed that there was weight loss between 115 and 520 °C corresponding to the deprotonation of the precursor. This result indicates that the bulk precursor was evidently protonated by the acid treatment.9,10 A more detailed result of TGA is provided in the Supporting Information. The protonated precursor was successfully exfoliated into a translucent white colloidal nanosheet suspension by reacting it with a TBAOH aqueous solution (Figure 2). The Tyndall effect was demonstrated by the scattered LASER beam in the colloidal nanosheet suspension. The result of EPMA indicates that the elemental composition of the nanosheets is 1.5:0.5:3.0 K:Eu: Ta. This result, the XRD results of the bulk precursors, and the
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Figure 2. Photograph of a colloidal suspension of (K1.5Eu0.5)Ta3O10 nanosheets.
Figure 3. In-plane XRD profile of (K1.5Eu0.5)Ta3O10 nanosheets deposited on a Si substrate.
results of the following nanosheet characterizations indicate that the nanosheet is the (K1.5Eu0.5)Ta3O10 double perovskite type. This nanosheet is expected to be negatively charged like other types of oxide nanosheets, and its charge is balanced with the cations such as TBA+ and H+ in the suspension.29,33-37,44 In fact, this negatively charged nanosheet can be self-assembled onto a substrate that is coated with positively charged polyions such as PEI as shown in a later section for the AFM observation. Figure 3 shows the in-plane XRD profile of the nanosheet product deposited on a Si substrate. All of the reflections can be indexed to the tetragonal cell similarly to those of the previously reported perovskite-type nanosheets.9,10 The a- and c-axes are designated as the in-plane axes, and b as the axis perpendicular to the plane to be consistent with the crystallographic axes of the bulk precursors. Contrary to the bulk precursors, whose unit cells are orthorhombic, no difference in two of the orthogonal in-plane lattice parameters a and c was observed. The lack of coordination by proton or K+ interlayer species in the nanosheets might be the possible reason for this slight symmetry difference between the bulk precursors and the exfoliated nanosheets. The refined in-plane lattice parameter is a ) 0.3933(3) nm, and it is quite close to the in-layer lattice parameters of the bulk precursors. This result suggests that the nanosheet product is single phase and topotactically retains the double perovskite structural feature of the bulk precursors. Figure 4a and b shows the TEM image and SAED pattern of (K1.5Eu0.5)Ta3O10 nanosheets, respectively. The nanosheet product has the lateral size ranging from 0.1 to a few micrometers. The monotonic contrast of the sheet images reflects the uniform thickness of the nanosheets. The SAED pattern of a single nanosheet exhibits intense diffraction spots, indicating the single crystalline nature of the nanosheet. Similarly to the in-plane XRD pattern of the nanosheets, the diffraction spots of the single nanosheet were indexed to the tetragonal type where no difference in two of the orthogonal lattice parameters along the in-sheet directions (a- and c-axes) was observed within the experimental resolution limit. The refined lattice parameter of
17118 J. Phys. Chem. C, Vol. 112, No. 44, 2008
Figure 4. (a) TEM image and (b) SAED pattern of (K1.5Eu0.5)Ta3O10 nanosheets.
Figure 5. (a) AFM image and (b) cross-sectional profile of (K1.5Eu0.5)Ta3O10 nanosheets deposited on a Si substrate.
Ozawa et al. This thickness is quite close to that of the analogous double perovskite-type nanosheet Ca2Nb3O10 (2.36 nm).35,45 These thicknesses of the nanosheets observed by AFM are larger than those expected from the sum of the ionic radii of their components (1.54 nm).46 A similar trend was also observed for many other oxide nanosheets, and the larger thicknesses of the oxide nanosheets observed by AFM with respect to those expected from the ionic radii of their components are considered to be due to the adsorption of oxonium and TBA+ ions and other uncertain factors.5,9,10,15,29,36,37,47-50 In this sense, the (K1.5Eu0.5)Ta3O10 nanosheet product is fairly likely to be homogeneously unilamellar. Figure 6 shows the XRD pattern of the concentrated gluelike nanosheets obtained by high-speed centrifugation of the colloidal nanosheet suspension. The diffraction pattern mainly consists of several broad peaks overlapped with several relatively sharp peaks reminiscent of those of the analogous Ca2Nb3O10 nanosheets.35 The sharp peaks can be attributed to the in-plane periods of the nanosheets; thus, those can be indexed to the tetragonal cell of the layered perovskite structure similarly to the in-plane XRD pattern. The refined lattice parameter obtained from the positions of these sharp peaks is a ) 0.3921(1) nm, agreeing well with the results of SAED and in-plane XRD. The broad peaks are characteristic of a particular structure type of nanosheets.34-36,44,50-52 The dotted gray line in Figure 6 represents the product of Lp (Lorentz-polarization factor) and the square of F0k0 (structure factor of the unilamellar (K1.5Eu0.5)Ta3O10 nanosheet) as in
I0k0 ) Lp|F0k0|2 )
Figure 6. XRD profile of the concentrated gluelike (K1.5Eu0.5)Ta3O10 nanosheets (red line). The gray dotted line is a calculated diffraction profile based on the unilamellar (K1.5Eu0.5)Ta3O10 layer model. The arrows indicate the positions of possible diffraction from the restacked nanosheets by drying.
Figure 7. A partial structure model of the (K1.5Eu0.5)Ta3O10 layer used in the calculation of the XRD profile of unilaminar (K1.5Eu0.5)Ta3O10 nanosheets.
the nanosheet along the in-sheet direction is a ) 0.40(1) nm, and it is quite close to those of the bulk precursors and the result of the in-plane XRD. This result also suggests that the exfoliation reaction is topotactic, and the nanosheet product retains the double perovskite layer structure of its bulk precursors. Figure 5a shows a topographic image of the (K1.5Eu0.5)Ta3O10 nanosheets deposited on a Si substrate. The observed nanosheets have the lateral size ranging from 0.1 to a few micrometers, consistent with the result of TEM. A cross-sectional height profile of the (K1.5Eu0.5)Ta3O10 nanosheet is shown in Figure 5b. The thickness of the nanosheets is uniformly 2.4(2) nm.
1 + cos2 2θ |F0k0|2 sin2 θ cos θ
F0k0 was calculated using the double perovskite structure model based on the atomic positions in the Ca2Ta3O10 layer of KCa2Ta3O1027 as in Figure 7 and the formula:
F0k0 )
∑ mifi cos[2π(2yi sin θ/λ)] i
where mi, fi, and yi are the site multiplicity, atomic scattering factor, and atomic position along the interlayer direction (baxis) of the ith atom, and λ is the X-ray wavelength. The calculated broad-peak profile matches well with the experimental broad-peak profile, indicating that the nanosheet product is unilamellar, and its structure is indeed the double perovskite type. In addition, the peaks on the left side shoulders of the broad peaks indicated by arrows around 19.3°, 29.1°, and 39.2° have the d-spacing of 0.459, 0.306, and 0.230 nm, respectively, and these are all integer fractions of 0.918 nm. Thus, these peaks on the shoulders can be indexed as 0 2xk 0, 0 3xk 0, and 0 4xk 0, respectively, where x is a positive integer. Suppose restacking of the nanosheets occurred during the diffraction experiment, then their peaks could be attributed to the interspacing (0.918x nm) of those nanosheets. However, not all of the peaks expected from such restacking were observed. The identification of the origin of these shoulder peaks requires further investigations. The photoluminescence excitation and emission spectra of the as-prepared (K1.5Eu0.5)Ta3O10 nanosheet suspension (5.2 × 10-4 M) and the concentrated nanosheets painted and dried on a quartz substrate at room temperature are shown in Figure 8a and b, respectively. The photoluminescence properties of these two forms of (K1.5Eu0.5)Ta3O10 nanosheets are virtually the same except for the emission intensity and the slight shift of the excitation maximum. The difference in the emission intensities between these two forms is mainly because of the difference in the amount of the nanosheet phosphors along the excitation and
(K1.5Eu0.5)Ta3O10
Figure 8. Photoluminescence (a) excitation and (b) emission spectra of the 5.2 × 10-4 M (K1.5Eu0.5)Ta3O10 nanosheet suspension (blue line) and concentrated nanosheets painted and dried on a quartz substrate (red line) at room temperature. The excitation spectra were monitored at 704 nm emission for both forms, and the emission spectra were obtained by exciting at 314 nm for the suspension form and at 322 nm for the dried form. The inset of (b) shows red luminescence of the nanosheet suspension irradiated by UV light.
emission light path. The excitation maximum of the suspension form is at 314 nm, whereas that of the dried form is at 322 nm. When the thickness or the amount of the concentrated nanosheets painted on a quartz substrate was increased, the excitation maximum shifted further to the higher wavelength. Similar redshift of excitation maxima by an increase in nanosheet phosphor concentration was also observed in another system.15 We expect that one of the reasons for the difference in the wavelengths of the excitation maxima between the suspension and dried forms of the nanosheets is due to their difference in the UV absorption. In the excitation spectra monitored at 704 nm emission, the broad host excitation band around 320 nm and the sharper peaks originated from the 7F0 f 5D4 and 7F0 f 5LJ (J ) 6, 7) transitions of the Eu3+ photoactivators were observed. The host excitation yields much higher emission intensity than does the direct photoactivator excitation. The same trend was also observed in the other types of nanosheet phosphors with the intrananosheet site Eu3+ photoactivators.9,10,15 Thus, the host excitation dominated photoluminescence is probably a general behavior of the intrananosheet site activated phosphors. The emission spectra of (K1.5Eu0.5)Ta3O10 nanosheets show relatively sharp peaks from the 5D0 f 7FJ manifold transitions of Eu3+. No splitting of the emission peak from the nondegenerate 5D0 f 7F0 transition was observed, indicating that all Eu3+ photoactivators are likely to be located at the sites with the identical symmetry.53,54 This result is consistent with the nanosheet structure model in Figure 7. The hypersensitive forced electric dipole 5D0 f 7F2 transition is parity forbidden for the system with photoactivators at the site with inversion symmetry. However, the emission from this transition is higher in intensity than that from the magnetic dipole 5D0 f 7F1 transition. This result indicates that the Eu3+ photoactivators are located at the site with no inversion symmetry as expected from the double perovskite-type nanosheet host structure with the distorted Ta-O octahedra.4,9,10,55,56 Contrary to most of the Eu3+-activated phosphors whose highest emission intensities are from either 5D f 7F or 5D f 7F transition, the highest emission intensity 0 1 0 2 of (K1.5Eu0.5)Ta3O10 nanosheets is from the 5D0 f 7F4 transition observed around 704 nm (far-red).57-59 The emission intensity from the 5D0 f 7F4 transition is approximately 3 times higher than that from the 5D0 f 7F2 transition in case of the (K1.5Eu0.5)Ta3O10 nanosheets. Similar behavior has also been reported for other Eu3+-activated phosphors, and it is attributed to the distorted local symmetry of Eu3+ with high polarizability.58,60 Thus, Eu3+ in (K1.5Eu0.5)Ta3O10 nanosheets might also be located in a similar environment. The strong emission from the
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Figure 9. Photograph of the concentrated (K1.5Eu0.5)Ta3O10 nanosheet suspension painted on a quartz substrate under (a) room light and (b) UV irradiation in a darkroom. A Chinese character meaning “light” was calligraphically written on the substrate using a paint brush. 5D 0
f 7F4 transition is suitable for varieties of practical applications requiring the conversion of UV light to near 700 nm wavelength light. For example, Si-based solar cells require photon energy in the range of 700-900 nm where their maximum sensitivity resides.57 Thus, a light-energy conversion film utilizing the materials with the strong photoluminescence emission from the 5D0 f 7F4 transition of Eu3+ seems quite suitable for the enhancement of Si-based solar cell performance. The inset of Figure 8b shows the photograph of the 5.2 × 10-4 M (K1.5Eu0.5)Ta3O10 nanosheet suspension under UV irradiation. The red photoluminescence can visually be confirmed for this as-prepared nanosheet suspension even though its quantum yield is low (0.02%). The emission intensity of this nanosheet suspension can be enhanced even more by increasing its concentration and possibly by the dilution of Eu3+ with Gd3+.15,17 Furthermore, the concentrated nanosheet phosphor prepared by high-speed centrifugation is viscous enough that it can be painted on a substrate such as quartz without being shed. To demonstrate this feature, a Chinese character meaning “light” was calligraphically written on a quartz substrate using the concentrated (K1.5Eu0.5)Ta3O10 nanosheet phosphor-paint and a paint brush (Figure 9). With the amount of the nanosheetpaint applied for this writing, the painted part of the substrate remains transparent (Figure 9a) even though it turns translucent if more of the nanosheet-paint is applied. Under UV irradiation, the painted character was pronounced by the red photoluminescence of the (K1.5Eu0.5)Ta3O10 nanosheet-paint (Figure 9b). This kind of nanosheet phosphor-paint can possibly be utilized for practical applications such as the energy-conversion film for solar cells and emission layers of electroluminescence devices, and the methods like spin-coating, ink-jet, and screen printing might be suitable for the fabrication of such practical components. Summary and Conclusion The new nanosheet phosphor (K1.5Eu0.5)Ta3O10 was prepared by soft chemical exfoliation reaction of the layered double perovskite precursor K(K1.5Eu0.5)Ta3O10. The nanosheet product has a quite large aspect ratio of its lateral size to its thickness. The lateral size ranges from 0.1 to a few micrometers, whereas the thickness is uniformly 2.4(2) nm. The photoluminescence of (K1.5Eu0.5)Ta3O10 is dominated by the host excitation rather than the direct excitation of Eu3+ photoactivators. The highest emission intensity is from the 5D0 f 7F4 transition of Eu3+ around 704 nm. The dominant emission from this transition is rare in Eu3+-activated oxide phosphors despite its technological importance for the energy-conversion applications. The relative
17120 J. Phys. Chem. C, Vol. 112, No. 44, 2008 emission intensities from the 5D0 f 7FJ manifold transitions are highly dependent upon the host materials. In this sense, tantalate double perovskite-type nanosheets might be suitable host candidates of Eu3+-activated phosphors for the UV to farred energy-conversion applications. Acknowledgment. We thank Dr. K. Takada for use of his research facilities. This work has been supported by World Premier International Center Initiative (WPI Initiative) on Materials Nanoarchitectonics, MEXT, Japan, and CREST of the Japan and Technology Agency (JST). The crystal structure in Figure 7 was drawn with “Balls and Sticks”, a free crystal structure visualization software.61 Supporting Information Available: Powder X-ray diffraction of the bulk precursor K(K1.5Eu0.5)Ta3O10 dried at 200 °C and thermogravimetric analysis of the protonated precursor. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Sasaki, T.; Watanabe, M. J. Phys. Chem. B 1997, 101, 10159. (2) Xin, H.; Ebina, Y.; Ma, R.; Takada, K.; Sasaki, T. J. Phys. Chem. B 2006, 110, 9863. (3) Xin, H.; Ma, R.; Wang, L.; Ebina, Y.; Takada, K.; Sasaki, T. Appl. Phys. Lett. 2004, 85, 4187. (4) Matsumoto, Y.; Unal, U.; Kimura, Y.; Ohashi, S.; Izawa, K. J. Phys. Chem. B 2005, 109, 12748. (5) Ida, S.; Araki, K.; Unal, U.; Izawa, K.; Altuntasoglu, O.; Ogata, C.; Matsumoto, Y. Chem. Commun. 2006, 3619. (6) Ida, S.; Ogata, C.; Unal, U.; Izawa, K.; Inoue, T.; Altuntasoglu, O.; Matsumoto, Y. J. Am. Chem. Soc. 2007, 129, 8956. (7) Ida, S.; Unal, U.; Izawa, K.; Altuntasoglu, O.; Ogata, C.; Inoue, T.; Shimogawa, K.; Matsumoto, Y. J. Phys. Chem. B 2006, 110, 23881. (8) Nakano, H.; Mitsuoka, T.; Harada, M.; Horibuchi, K.; Nozaki, H.; Takahashi, N.; Nonaka, T.; Seno, Y.; Nakamura, H. Angew. Chem., Int. Ed. 2006, 45, 6303. (9) Ozawa, T. C.; Fukuda, K.; Akatsuka, K.; Ebina, Y.; Sasaki, T. Chem. Mater. 2007, 19, 6575. (10) Ozawa, T. C.; Fukuda, K.; Akatsuka, K.; Ebina, Y.; Sasaki, T.; Kurashima, K.; Kosuda, K. J. Phys. Chem. C 2008, 112, 1312. (11) Ida, S.; Ogata, C.; Shiga, D.; Izawa, K.; Ikeue, K.; Matsumoto, Y. Angew. Chem., Int. Ed. 2008, 47, 2480. (12) Sasaki, T. Supramol. Sci. 1998, 5, 367. (13) Ida, S.; Unal, U.; Izawa, K.; Ogata, C.; Inoue, T.; Matsumoto, Y. Mol. Cryst. Liq. Cryst. 2007, 470, 393. (14) Ida, S.; Ogata, C.; Inoue, T.; Izawa, K. Chem. Lett. 2007, 36, 158. (15) Ida, S.; Ogata, C.; Eguchi, M.; Youngblood, W. J.; Mallouk, T. E.; Matsumoto, Y. J. Am. Chem. Soc. 2008, 130, 7052. (16) Honma, T.; Toda, K.; Ye, Z.-G.; Sato, M. J. Phys. Chem. Solids 1998, 59, 1187. (17) Toda, K.; Kameo, Y.; Ohta, M.; Sato, M. J. Alloys Compd. 1995, 218, 228. (18) Toda, K.; Honma, T.; Sato, M. J. Lumin. 1997, 71, 71. (19) Van Uitert, L. G.; Iida, S. J. Chem. Phys. 1962, 37, 986. (20) Van Uitert, L. G.; Linares, R. C.; Soden, R. R.; Ballman, A. A. J. Chem. Phys. 1962, 36, 702. (21) Frey, G. L.; Reynolds, K. J.; Friend, R. H. AdV. Mater. 2002, 14, 265. (22) Muramatsu, M.; Akatsuka, K.; Ebina, Y.; Wang, K.; Sasaki, T.; Ishida, T.; Miyake, K.; Haga, M. Langmuir 2005, 21, 6590. (23) Sasaki, T.; Ebina, Y.; Watanabe, M.; Decher, G. Chem. Commun. 2000, 2163. (24) Dion, M.; Ganne, M.; Tournoux, M. Mater. Res. Bull. 1981, 16, 1429. (25) Jacobson, A. J.; Johnson, J. W.; Lewandowski, J. T. Inorg. Chem. 1985, 24, 3727.
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