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2008, 112, 1312-1315 Published on Web 01/16/2008
Eu0.56Ta2O7: A New Nanosheet Phosphor with the High Intrananosheet Site Photoactivator Concentration 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 ReceiVed: December 13, 2007; In Final Form: January 4, 2008
High photoactivator concentration nanosheets Eu0.56Ta2O7 have been prepared by the soft chemical exfoliation reaction of the layered perovskite Li2-xHxEu0.56Ta2O7 with a tetrabutylammonium hydroxide aqueous solution. In-plane X-ray diffraction and TEM and AFM observation results indicate that the Eu0.56Ta2O7 nanosheet is crystalline and retains the layered perovskite structural feature of the bulk precursors. The most intense emission is observed from the 5D0 f 7F2 transition of Eu3+. The intensity of this emission by the host excitation exceeds 18 times that by the direct Eu3+ excitation.
Nanosheets are two-dimensional materials which can be obtained by soft chemical exfoliation reactions of layered compounds. Because of their bona fide two-dimensionality, nanosheet-based phosphors have been receiving increased scrutiny.1-9 Phosphors based on layered compounds exhibit the properties that are characteristic to their two-dimensional structural components; however, small but finite degrees of the interlayer interaction such as energy transfer and cross relaxation are still expected. On the contrary, nanosheets have the bona fide two-dimensional morphology and nanoscale thickness; thus, a capacity to host a large amount of photoactivators without concentration quenching, large surface areas to efficiently receive excitation energy, and possible quantum size effects are expected in nanosheet-based phosphors. In addition, their twodimensional morphology is suitable for optical applications such as EL (electroluminescence) devices.10 Among nanosheet-based phosphors, those with Ln (lanthanide) photoactivators are particularly interesting because of their high emission intensities and well-predictable emission wavelengths from the intra-4f transitions of Ln. Most of the previously reported Ln-activated nanosheet-based phosphors consist of Ln ions or Ln-containing complexes inserted between transition-metal oxide nanosheets.2-6 While their large surface areas are advantageous for effectively receiving excitation energy, the 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 energy-transfer mediators. In addition, more efficient energy transfer from the host nanosheet unit to the photoactivators is expected if the activators are incorporated into intrananosheet sites rather than in internanosheet sites. Recently, we have reported an example of the * To whom correspondence should be addressed. E-mail: OZAWA.
[email protected]. Phone: +81-29-859-2747. FAX: +81-29-854-9061. † Nanoscale Materials Center. ‡ International Center for Materials Nanoarchitectonics. § Materials Analysis Station.
10.1021/jp711699c CCC: $40.75
intrananosheet site activated phosphor La0.95Nb2O7/Eu3+ prepared from the soft chemical exfoliation of KxH1-xLa0.95Nb2O7/ Eu3+ with the maximum Eu3+ doping amount of 5%.9 As expected, this phosphor exhibits significantly more efficient emission through the host excitation than through the direct excitation of Eu3+, and this behavior is contrary to that of its bulk precursor KxH1-xLa0.90Eu0.05Nb2O7. However, the emission intensity and efficiency need to be improved further in order to utilize such materials for practical applications. One of the most effective ways to improve the photoluminescence property is to increase the photoactivator concentration up to Pc (critical concentration limit). According to the percolation model, the optimum Pc for layered perovskites such as n ) 2 DJ (Dion-Jacobson) phases A[Mn-1BnO3n+1], n ) 2 PR (Ruddlesden-Popper) phases A2[Mn-1BnO3n+1], and perovskite nanosheets exfoliated from these precursors is 50%.11-13 In the case of phosphors with the high number of nearest-neighbor activators, such high activator concentrations lead to the diminishment of emission intensity by cross relaxation among adjacent activators. However, the number of nearest-neighbor activators in the layered systems and nanosheets is four, and this small number of nearest-neighbor activators impedes the cross relaxation among the activators up to the activator concentration of 50%. As a matter of fact, Toda et al. reported that the 50% Eu3+ doping yields the highest emission intensity in the DJ-type system RbLaTa2O7/Eu3+, and the emission at that activator concentration is as bright as that of the commercial high-brightness phosphors such as Y2O2S/Eu3+.11,14 Thus, we have attempted to prepare La0.5Eu0.5Ta2O7 nanosheets by the soft chemical exfoliation reactions of A1-xHxLa0.5Eu0.5Ta2O7 (A ) K, Rb) with a TBAOH (tetrabutylammonium hydroxide) aqueous solution. However, there was no success in exfoliating A1-xHxLa0.5Eu0.5Ta2O7 into La0.5Eu0.5Ta2O7 nanosheets for any attempted reaction conditions. Another possible candidate precursor to prepare the similar target nanosheet phosphor would be the Eu analogue of the RP-type layered perovskite © 2008 American Chemical Society
Letters Li2La2/3Ta2O7.15 We initially intended to prepare the precursor with the nominal composition of Li2Eu2/3Ta2O7, but the EPMA analysis results of the precursor product indicated the composition of 0.56:2 Eu/Ta, which is close to the ideal 50% Eu3+ content. In this letter, we report the preparation and photoluminescence properties of Eu0.56Ta2O7 nanosheets prepared by the soft chemical exfoliation of the RP-type layered Ln perovskite with the extra Ln-site vacancies Li2Eu0.56Ta2O7. Eu0.56Ta2O7 nanosheets were prepared in three-step reactions. The first precursor Li2Eu0.56Ta2O7 was synthesized by the solidstate reaction of Li2CO3, Eu2O3, and Ta2O5 at 1600 °C, similar to that reported for its La analogue Li2La2/3Ta2O7, with some modifications.15 Li2Eu0.56Ta2O7 was then ground and reacted with 2 M HNO3 for 3 days at room temperature under vigorous shaking to exchange its Li+ with H+.15-19 This process was necessary in order to activate the bulk precursor for the following exfoliation reaction. The elemental compositions of the first and protonated precursors analyzed by EPMA are 0.56:2 Eu/Ta for both precursors, corroborating no reduction of the Eu content from the first precursor during the protonation reaction. Finally, the protonated precursor Li2-xHxEu0.56Ta2O7 was reacted with an approximately 3-fold excess of TBAOH aqueous solution. After 1 week of vigorous shaking, a translucent white colloidal nanosheet suspension was obtained. The more detailed sample preparation procedures are provided in Supporting Information. The powder XRD (X-ray diffraction) peaks of both Li2Eu0.56Ta2O7 and Li2-xHxEu0.56Ta2O7 can be indexed to tetragonal unit cells like those of the analogous phases Li2La2/3Ta2O7 and Li2SrTa2O7, except for some low-intensity peaks from a trace amount of impurity phases such as Li3TaO4.15,20 The refined lattice parameters are abulk ) 0.5430(2) nm and cbulk ) 1.8447(7) nm for Li2Eu0.56Ta2O7 and abulk ) 0.5458(3) nm and cbulk ) 1.809(1) nm for Li2-xHxEu0.56Ta2O7. For these tetragonal cells, abulk/x2 corresponds to the Ta-O-Ta distance along the intralayer direction. The lattice parameters abulk of Li2Eu0.56Ta2O7 and Li2-xHxEu0.56Ta2O7 are slightly smaller than that of the La analogue Li2La2/3Ta2O7 (0.5564 nm).15 This is likely a reflection of the smaller ionic radius of Eu3+ than that of La3+ and the additional Ln-site vacancies in Li2Eu0.56Ta2O7 and Li2-xHxEu0.56Ta2O7. On the other hand, the lattice parameter cbulk of Li2Eu0.56Ta2O7 is slightly larger than that of the La analogue (1.8134 nm).15 Le Berre et al. reported that the lattice parameter cbulk of the layered Ta-O perovskite is highly dependent upon the distortion of Ta-O octahedral units.21 Therefore, the larger lattice parameter cbulk of Li2Eu0.56Ta2O7 with respect to that of the La analogue is likely a reflection of the difference in the Ta-O octahedra distortions between these two phases. In addition, hydration of layered perovskites is a frequently observed phenomenon; thus, the larger lattice parameter cbulk of Li2Eu0.56Ta2O7 might also originate from the interlayer H2O.15,18,19,22 On the other hand, the lattice parameter cbulk of Li2-xHxEu0.56Ta2O7 is significantly smaller than that of the first precursor Li2Eu0.56Ta2O7, reflecting the replacement of larger Li+ with smaller H+, as commonly observed in the protonation of layered oxides.19 Phase homogeneity and crystallinity of Eu0.56Ta2O7 nanosheets were characterized by TEM and SAED (selected-area electron diffraction), and the results are shown in Figure 1. All of the Eu0.56Ta2O7 nanosheet products have the sheet morphology, and no scroll morphology was observed, unlike the structurally related SrTa2O7 nanosheet products.23 The monotonic contrast of the nanosheet images indicates their homogeneous thickness. The SAED pattern of a single nanosheet exhibits intense
J. Phys. Chem. C, Vol. 112, No. 5, 2008 1313
Figure 1. (a) TEM image and (b) SAED pattern of Eu0.56Ta2O7 nanosheets.
Figure 2. In-plane X-ray diffraction profile of Eu0.56Ta2O7 nanosheets.
Figure 3. (a) AFM image and (b) cross-sectional height profile of Eu0.56Ta2O7 nanosheets.
diffraction spots, indicating its single-crystal nature. These intense diffraction spots can be indexed to a tetragonal cell with a lattice parameter of ananosheet ) 0.39 nm. This lattice parameter corresponds well to abulk/x2 of the bulk precursors, within the experimental uncertainty corroborating the topotactic exfoliation of the layered perovskite precursor into Eu0.56Ta2O7 nanosheets. The reduced lattice parameter of the nanosheet with respect to those of bulk precursors suggests that the tilted Ta-O octahedral units in the bulk precursors are aligned in the period of the TaO-Ta distance along the intralayer direction in the case of the nanosheet deposited on a carbon membrane of the TEM grid. In addition, the phase homogeneity and crystallinity of Eu0.65Ta2O7 nanosheets, which were deposited on a Si substrate by the Langmuir-Blodgett (LB) method,24 were also characterized by in-plane XRD using synchrotron radiation (λ ) 0.11973(9) nm). The result shown in Figure 2 indicates that all of the diffraction peaks of the Eu0.56Ta2O7 nanosheets can be indexed to h k 0 of the layered perovskite precursor structures, except that the lattice parameter ananosheet corresponds to abulk/x2 of the bulk precursor structures, like the case of the SAED pattern. The refined lattice parameter ananosheet of the deposited nanosheets is 0.38545(2) nm. This result also indicates that the nanosheet products are single phase and retain the layered perovskite structural feature of the precursors. The morphology analysis on Eu0.56Ta2O7 nanosheets was performed by AFM. Figure 3a shows a topographic image of Eu0.56Ta2O7 nanosheets deposited on a Si substrate using PEI (polyethylenimine) as an electrostatic glue layer.25,26 The lateral
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Letters
Figure 4. Photoluminescence (a) excitation and (b) emission spectra of Eu0.56Ta2O7 nanosheets. The excitation spectra were monitored at 616 nm emission, and the emission spectra were obtained by exciting at 276 nm. The inset shows red luminescence of Eu0.56Ta2O7 nanosheets irradiated by a UV lamp.
size of the nanosheet products ranges from 0.2 to 2 µm. A crosssectional height profile of the Eu0.56Ta2O7 nanosheet is shown in Figure 3b. The sheet thickness is uniformly 1.97(9) nm. This thickness is larger than that of the single Eu0.56Ta2O7 layer (1.07 nm) estimated from the ionic radii of its components.27 The thickness of many other kinds of oxide nanosheets observed by AFM is also larger than that estimated from the ionic radii of their components because of the absorption of oxonium and TBA+ ions and other uncertain factors.6,9,26,28-33 Especially, this thickness closely agrees with that of the structurally related unilaminar perovskite nanosheet La0.90Eu0.05Nb2O7, within the experimental uncertainty.9 In this sense, Eu0.56Ta2O7 nanosheets are also fairly likely to be homogeneously unilamellar. The photoluminescence excitation and emission spectra of the 1.0 × 10-3 M Eu0.56Ta2O7 nanosheet suspension at room temperature are shown in Figure 4. Among many allowed direct excitation transitions of Eu3+, only the very weak and broad peak, which is attributed to the convolution of the 7F0 f 5L6 and 7F0f5D3 transitions, was observed at around 402 nm. On the other hand, the host excitation peak at 276 nm is quite intense. The host excitation peak intensity is more than 18 times that of the direct excitation peak at 402 nm, indicating that the photoluminescence emission of the Eu0.56Ta2O7 nanosheets is dominated by the host excitation rather than the direct excitation of Eu3+. This behavior was also observed in another intrananosheet activated phosphor La0.90Eu0.05Nb2O7.9 Thus, the host excitation-dominated photoluminescence might be a general behavior of the intrananosheet site activated phosphors. The emission spectrum of Eu0.56Ta2O7 nanosheets exhibits relatively sharp peaks from the 5D0 f 7FJ manifold transitions of Eu3+, whereas no host luminescence was observed. The highest emission intensity was obtained from the hypersensitive forced electric dipole 5D0 f 7F2 transition at 616 nm, and it was slightly stronger than the emission intensity from the magnetic dipole transition (5D0 f 7F1). Even though the centrosymmetric environment of Eu3+ is expected from the SAED and in-plane XRD results for the nanosheet samples deposited on a carbon membrane and a Si substrate, respectively, this slight domination of the hypersensitive forced electric dipole transition of the nanosheet suspension indicates that Eu3+ in the nanosheet suspension is occupying the noncentrosymmetric site.4,9,34,35 The nanosheets have a large aspect ratio of the lateral dimension to the thickness, and they might not sustain their ideal flat plane morphology in suspension without solid supports such as a carbon membrane or a Si substrate. Thus, the noncentrosymmetric site occupancy of Eu3+ might be a reflection of the slight corrugation of the nanosheets in the suspension. As expected
from the nearly optimum Eu3+ concentration in Eu0.56Ta2O7 nanosheets, the emission intensity from this 5D0 f 7F2 transition of the Eu0.56Ta2O7 nanosheets is 6.4 times higher than that of the La0.90Eu0.05Nb2O7 nanosheet suspension with the same formula/volume concentration.9 Even though emission intensity is not necessarily linearly proportional to the photoactivator concentration, the emission performance of the Eu0.56Ta2O7 nanosheets could be improved further if the activator concentration is adjusted to the exact ideal value and the Ln-site vacancies, which might be acting as luminescence killer centers, could be eliminated by doping lower valent ions in the Ln site. Nonetheless, the red emission of Eu0.56Ta2O7 nanosheets is already intense enough that it can be confirmed visually when the nanosheets are irradiated by a UV lamp, as shown in the inset of Figure 4b, indicating their potential for the practical phosphor applications. In conclusion, we have successfully prepared the new nanosheet with the high intrananosheet site photoactivator concentration by the soft chemical exfoliation of the layered perovskite precursor Li2-xHxEu0.56Ta2O7. The Eu0.56Ta2O7 nanosheet retains the layered perovskite structural feature of its bulk precursors. The photoluminescence emission by the host excitation is dominated over the direct excitation of Eu3+, and this seems to be a general propensity of the intrananosheet site activated phosphors. The Eu3+ activator concentration in the Eu0.56Ta2O7 nanosheet is, in fact, nearly the optimum one (50%) expected from the percolation model,11-13 and its photoluminescence emission is much more intense than that of the previously reported lower Eu3+ concentration nanosheet phosphor.9 Phosphors with the high intrananosheet site activator concentration are promising candidates for optical device applications because of their morphology, efficient host to activator energy transfer, and high luminescence intensities. To the best of our knowledge, the Eu0.56Ta2O7 nanosheet is the first paradigm of such phosphors with the high intrananosheet site activator concentration. Further investigation of phosphors with the high intrananosheet site activator concentration is under way to optimize their photoluminescence performance for the “nanosheet-lighting” applications. Acknowledgment. We thank Drs. A. Watanabe and M. Osada for use of their research facilities. This research was financially supported by CREST of the Japan Science and Technology Agency. Supporting Information Available: The detailed experimental procedures for sample preparation and characterizations.
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