Dramatically Enhanced Luminescence of Layered Terbium

Article pubs.acs.org/JPCC

Dramatically Enhanced Luminescence of Layered Terbium Hydroxides as Induced by the Synergistic Effect of Gd3+ and Organic Sensitizers Liangliang Liu, Qin Wang, Cunji Gao, Hao Chen, Weisheng Liu, and Yu Tang* Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu Province, State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, P. R. China S Supporting Information *

ABSTRACT: Host−guest chemistry allows the engineering of new functional materials with tunable properties. This study focuses on layered terbium hydroxides (NO3-LTbH) codoped with Gd3+ (NO3-LTbH:Gd), which were prepared using the hydrothermal method and successfully modified using sensitizers (L). Luminescence studies showed that compared with the NO3-LTbH precursor the sensitizer-modified NO3-LTbH:Gd exhibited high luminescence intensity and high luminescence quantum efficiency (Φ = 33%). This performance results from the synergistic effect of codoped Gd3+ and intercalated sensitizers in the organic−inorganic hybrid materials, which led to stronger luminescence properties, and synergistic effect on the enhancement of Tb3+ luminescence was investigated by the spectroscopic characteristics, UV−vis absorption spectrum, lowtemperature phosphorescence, and crystal structures of the layered rare-earth hydroxides. Studies demonstrate that the mechanism for synergistic effect of significant enhancement of Tb3+ luminescence is mainly caused by Gd3+ inducing a cascaded energy transfer from the host to Tb3+ via organic sensitizers. The very interesting thing is that LTbH:Gd has multiple roles, including energy-transfer bridges that connect the sensitizers and Tb3+ (in host) to enhance the characteristic emission of Tb3+, significantly enhancing phosphorescence of sensitizers, and acting as host matrices for sensitizers. Second, the effects of intercalation conditions on luminescence were also investigated. Furthermore, a novel transparency luminescent composite film device [PMMA/L-LTbH:Gd] that exhibits strong luminescence property was also fabricated using the solvent-casting method.

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INTRODUCTION The self-assembly of molecular rare-earth complexes or the incorporation of organic sensitizers into inorganic materials such as zeolites,1−5 silica,6 and a variety of layered materials7−9 is a promising method of developing functional hybrid rareearth luminescent materials.10,11 Layered inorganic materials have interesting physical properties (e.g., tunable interlayer space and composition) and can be readily functionalized via intercalation to obtain specific properties.12 Layered rare-earth hydroxides (LRHs), which have the general composition RE2(OH)5X·nH2O (where RE = rare-earth elements and X = anion), are a new group of important inorganic layered matrices. The general structure of LRHs is similar to that of classical layered double hydroxides (LDHs), which consists of alternating positively charged hydroxocation layers and chargecompensating anion layers.13,14 Consequently, these materials © 2014 American Chemical Society

occupy a distinct position because of their potential applications in ion exchange,15 photoluminescence (PL),16 catalysis,17 and biomedical devices.18,19 However, the practical application of LRHs in optical devices is largely limited by their low luminescence emissions because of the direct coordination of water molecules and hydroxyl groups to the rare-earth metal centers; this phenomenon imposes a significant quenching effect on the emissions.20 By contrast, the extinction coefficients of rare earths are intrinsically low because of Laporte-forbidden f−f transitions. As a result, direct excitation of the metal ion center is generally difficult and inefficient.21,22 Received: March 5, 2014 Revised: June 5, 2014 Published: June 9, 2014 14511

dx.doi.org/10.1021/jp502281m | J. Phys. Chem. C 2014, 118, 14511−14520

The Journal of Physical Chemistry C

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our knowledge, this study is the first to investigate the “synergistic effect” of inert ions Gd3+ and the sensitizer that dramatically enhances the luminescent properties of the layered terbium hydroxide and such luminescent composite films.

At present, the assembly of inorganic−organic hybrid phosphors with ordered nanostructures has attracted considerable attention.11 These composites are potential multifunctional materials that offer a wide variety of physical properties that depend on the inorganic and organic components as well as on the interaction between the two phases. Furthermore, the organic component can be readily modified to tune the global properties of the final material.22 The distinct and highly promising optical properties of LRHs are due to their large capacity, high affinity for anion-exchange reactions with a wide range of organic sensitizers in the interlayer space, and the advantages of rare-earth elements.23 To date, a small number of published studies have focused on the incorporation of sensitizers into LRHs to develop new photofunctional materials that have high potential in diverse applications.24−26 Byeon et al. reported the rose-like hierarchical superstructure and drastically enhanced luminescence of LEuH/polyoxomolybdate hybrid materials.24 A more recent study involved the intercalation of two organic sensitizers, namely, 4-biphenylcarboxylate and terephthalate anions, into LEuH and investigated the photoluminescence property.27 The interactions between hosts and guests indicate that the structural features of organic sensitizers in the interlayer spacing of the NO3-LTbH host can affect the luminescence of hybrid materials.28 Moreover, inert ions with stable electronic configuration (4f shells are empty, half filled, and full), such as La3+, Y3+, and Gd3+ doped crystallites, have attracted considerable research interest because they can enhance the luminescence of active rareearth ions, such as Tb3+ and Eu3+.29,30 To fabricate a new type of efficient luminescent organic− inorganic LRH hybrid phosphor and investigate the “synergistic effect” of the inert ions and organic sensitizers on the optical performance of the hybrid materials, herein, we present the preparation of Tb3+, Gd3+-codoped, and sensitizers intercalated LRH (sensitizer-LTbH:Gd, abbreviated as L-LTbH:Gd) powder samples and the discussion of Tb3+-related luminescence behavior as well as the energy transfer from host via sensitizer to Tb3+ according to the spectroscopic characteristics, crystal structures of the LRHs, UV−vis absorption spectrum, and low-temperature phosphorescence (Scheme 1). As an integral part of this work, the luminescent properties of an organic−inorganic LRH-hybrid phosphor-doped poly(methyl methacrylate) (PMMA)31 luminescent composite film fabricated using the solvent-casting method are also reported. To

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RESULTS AND DISCUSSION Intercalation Assembly and Structural Analysis of the L-LTbH Hybrid Materials. The hydrothermal synthesis of NO3-LTbH:Gd, particularly the two rare-earth ion-containing LRHs, was sensitive to pH conditions. At higher pH, RE3+ is transformed into RE(OH)3, which is considered an impurity; however, a low percentage yield of the product is obtained at a lower pH range, and not all RE3+ forms NO3-LTbH:Gd. Therefore, the pH should be slightly higher to obtain a higher NO3-LTbH:Gd yield.15,33 On the basis of CHN elemental analysis and inductively coupled plasma analysis, the compositions of the synthesized hydroxide precursors are as follows: Tb2(OH)4.64(NO3)0.89(CO3)0.25·1.43H2O [found (%): Tb 64.41, H 1.53, N 2.50, C 0.65)] and (Tb0.54Gd0.46)2(OH)4.52(NO3)0.86(CO3)0.32·1.12H2O [found (%): Tb 35.66, Gd 29.85, H 1.414, N 2.49, C 0.78]. Structural determination of the hydrothermal products was performed using powder X-ray diffraction (PXRD) (Figure 1a).

Figure 1. (a) PXRD patterns of NO3-LTbH, NO3-LTbH:Gd, LLTbH, and L-LTbH:Gd. The asterisks indicate RE(OH)3. (b) TEM images of the L-LTbH:Gd.

Scheme 1. Schematic Illustration of the Fabricated Hybrid Phosphors L-LTbH:Gd and Synergistic Effect

Both hydrothermal hydroxide precursors NO3-LTbH and NO3-LTbH:Gd exhibited strong (00l) layer reflections at a low 2θ angle attributed to diffractions by the (002) and (004) planes. A series of strong (00l) reflections and a number of general (hkl) reflections were also observed. These reflections are characteristic of a layered and ordered phase, as previously reported for LRHs.34,35 Hydrothermal ion exchange is an efficient method of fabricating L-LTbH hybrid materials. The chemical structure of the carboxylic acid sensitizers was illustrated in Figure S1 (Supporting Information). This method is generally viewed as a simple self-assembly of LRHs and sensitizers (Scheme 2a).36 The hydrothermal synthesis conditions are vital to obtaining certain ideal compounds. Moreover, the molar ratio of LRHs to 14512



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Scheme 2. Mechanism of Formation of L-LTbH

Figure 2. Fourier transform infrared (FT-IR) measurements for (a) NO3-LTbH, (b) pure Na2L, and (c) L-LTbH.

sensitizers (R, R = nLRHs/nL) as well as the time (t) and temperature (T) of the reaction are critical in obtaining highperformance luminescent materials. Therefore, we used NO3LTbH and the sensitizers to study the effects of synthesis conditions on material luminescence. The PXRD patterns for the hydrothermally treated intercalated samples are shown in Figure S2 (Supporting Information). Note that ion-exchange reactions are complete when the amounts of sensitizers are greater than the theoretical values, the reaction temperature exceeds 100 °C, and the reaction time is approximately 4 h. By contrast, when the experimentally determined amounts of the sensitizers are less than the theoretical ones and the reaction temperature is below 100 °C (Figure S2a and c, Supporting Information), PXRD data show that the products consist of an unreacted host lattice and an expanded phase with interlayer separations. Therefore, in this study, the ion-exchange conditions used were R = 1:1, t = 4 h, and T = 100 °C. The typical PXRD patterns of the anion-exchange derivatives of NO3-LTbH and NO3-LTbH:Gd are shown in Figure 1a. The characterization data for the materials are summarized in Table S1 (Supporting Information). All of the hydrothermal products exhibited a series of strong (00l) reflections as well as a number of general (hkl) reflections. These results represent the most striking features of an LRH-layered and ordered phase.23 As expected of intercalated materials, all PXRD patterns of the hybrid materials show systematic shifts of the (00l) reflections toward lower diffraction angles. These shifts correspond to the shift in the basal spacing of NO3− from 8.28 to ∼18 Å during the sensitizers exchange. Such systematic shifts and significant basal spacing expansion indicate that in the intercalated structure the sensitizers are inserted between the NO3-LTbH and NO3-LTbH:Gd layers under hydrothermal conditions, and TEM images revealed L-LTbH:Gd (Figure 1b) was a wellshaped layered material. The successful intercalation of the sensitizers can be further confirmed by the FT-IR spectra as shown in Figure 2. The broad band at approximately 1370 cm−1 is characteristic of an uncoordinated nitrate anion (curve a). Similar observations were made on other layered hydroxides containing interlayer nitrate groups.13 In general, the strong, broad band at approximately 3574 cm−1 corresponds to O−H stretching vibrations (ν1 and ν3), whereas the band at 1637 cm−1 is assigned to the H−O−H bending mode (ν2); these results provide evidence for the presence of water molecules in the structure.37 The bands observed in the low-frequency region of the spectrum are interpreted as lattice vibration modes and may be attributed to M−O (850−600 cm−1) and O−M−O (near 440 cm−1) vibrations.38 The absorption at 1600 cm−1 of Na2L

is assigned to the bending vibration of the −COO− group (curve b). Notice from curve c that the adsorption of the nitrate anions completely disappeared, and the emergence of absorption at 1601 cm−1 is assigned to the bending vibration of the −COO− group, which can be further confirmed by the successful intercalation of the sensitizers.39 Combined with the results of PXRD, analyses allow us to conclude that LTbH has been formed with sensitizers intercalated between the LTbH layers. FT-IR measurements for NO3-LTbH:Gd, Na2L, and LLTbH:Gd are shown in Figure S3 (Supporting Information). The thermogravimetric and differential thermal analysis (TGDTA) curves for the NO3-LTbH precursor (Figure S4a, Supporting Information) are very similar to those of the literature, and the total weight loss is 23.87%.40 A weak endothermic peak at 150 °C corresponds to the loss of interlayer water molecules,41 and two strong exothermal peaks at 371.9 and 471.9 °C show the removal of the interlayer sensitizers. The mass ratio of the capped sensitizers was determined as approximately 13.18% (Figure S4b, Supporting Information). The TG-DTA curves for the L-LTbH:Gd are very similar to L-LTbH (Figure S4c, Supporting Information). Notice from Figure S4 (Supporting Information) that the thermostability of intercalated LTbH was nearly unchanged compared with that of NO3-LTbH and can be attributed to host−guest and guest−guest interactions (including electrostatic, coordination, and hydrogen interactions) between the sensitizers and the NO3-LTbH layer.42 Effects of Synthesis Conditions on Luminescence. Figure S5 (Supporting Information) shows the corrected excitation and emission spectra of the as-obtained optical hybrid phosphor L-LTbH at different R, t, and T. As R and T increase, the luminescence intensities of the hybrid materials initially increase, reach peak values when R > 1 and T > 100 °C, and then gradually decrease (Figure S5a and c, Supporting Information). This phenomenon may be due to the difference in the extent of ion exchange. Ion exchange is insufficient when the amounts of sensitizers are less than the theoretical value and the reaction temperature is below 100 °C. As a result, some of the Tb3+ ion cannot be sensitized, and the full luminescence of the material is not achieved (Scheme 2a). Conversely, the luminescence intensity decreases when the amounts of sensitizers are greater than the theoretical values and the reaction temperature exceeds 100 °C. This is because the interval between the sensitizers in the interlayer becomes increasingly short with increasing sensitizer loadings which 14513



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consists of several bands in the 250−500 nm region. The band centered at 285 nm is assigned to the lowest 4f−5d transition band of Tb3+.51 The weaker band at approximately 327 nm can be attributed to the charge-transfer transition from the 2p orbital of O2− to the 4f orbital of Tb3+. The remaining peaks are assigned to the intra-4f8 transitions between the 7F6 and the 5 L10−7, 5G6−2, and 5D2−4 levels of Tb3+. Note that the excitation spectrum of NO3-LTbH:Gd is slightly different from that of NO3-LTbH. In addition, the 8S7/2 → 6IJ intra-f−f transition of Gd3+ is clearly observed at 273 nm of the excitation spectra for the Gd3+-containing sample (inset in Figure 3).52 The excitation spectra of L-LTbH and L-LTbH:Gd both display a broad band between 240 and 425 nm, with the maximum at approximately 365 nm. This band is attributed to the 1π−π* sensitizer transition, whereas the narrow bands are assigned to the f−f transitions of Tb3+. However, these transitions are weaker than the absorption of the sensitizers and are overlapped by a broad excitation band. These results prove that luminescence sensitization via excitation of the sensitizers is considerably more efficient than direct excitation of Tb3+. Figure 4 shows that the emission spectra of NO3-LTbH, NO3-LTbH:Gd, L-LTbH, and L-LTbH:Gd upon excitation at

generates concentration quenching between sensitizer molecules (Scheme 2b).43 With ion exchange at higher temperatures, the sensitizer packing is completely disordered, and the system does not relax back to an ordered assembly upon cooling. Also, at these higher temperatures, decarboxylation of the sensitizer chains will begin to occur. Therefore, it appears that ion exchange at progressively higher temperatures introduces disorder into the system which is only partially restored on cooling, the degree of restoration decreasing at higher temperatures, which led to the carboxylate group of the sensitizers becoming involved in weak host−guest interactions with the layers.44 In addition, we suspect that the decrease in the emission intensity with increasing reaction time is a result of fewer surface defects (Scheme 2c).45,46 Synergistic Effect. To assess the efficiency of energy transfer to rare-earth ions, the singlet energy level (S1) of the sensitizers was also determined by using the UV−vis absorption edge of the Gd−L complex as reference. The S1 value was determined as 325 nm (30 769 cm−1, Figure S6a, Supporting Information). The Gd−L complex, which is widely used in elucidating triplet energy levels, was utilized to determine the triplet energy level of the sensitizes.47 The Gd−L complex exhibits sensitizer phosphorescence as a broad band centered at 425 nm after excitation at 325 nm at 77 K in a 1:1 methanol− ethanol mixture (Figure S6b, Supporting Information).48 The triplet energy level (T1) of the sensitizers was estimated by referring to the shortest wavelength emission edge (407 nm, 24 570 cm−1) in its spectrum. It is known that an efficient sensitizer-to-metal energy transfer requires a high intersystemcrossing efficiency, which is maximized when the energy difference between the singlet and triplet states is close to 5000 cm−1.49 In this study, the energy gap between S1 and T1 is 6199 cm−1 of the sensitizer, which implies that the sensitizer has a relatively high intersystem crossing. The triplet energy level of the designed sensitizers was found to be higher than that of the 5 D4 levels of Tb3+, which indicates that the sensitizers can efficiently populate Tb3+ ions.50 Figure 3 shows the typical excitation spectra of the samples obtained by monitoring the total luminescence within the 5D4 → 7F5 line (543 nm). The excitation spectrum of NO3-LTbH

Figure 4. Emission spectra of (a) NO3-LTbH, (b) NO3-LTbH:Gd, (c) L-LTbH, and (d) L-LTbH:Gd.

368 and 365 nm consist of sharp lines within the 450−700 nm region. These lines are associated with the transitions from the excited 5D4 → 7FJ (J = 6, 5, 4, 3, 2, 1, 0) levels of Tb3+ ions. The main emission peak at 543 nm is due to the 5D4 → 7F5 transitions of Tb3+; the peak at 487 nm is attributed to 5D4 → 7 F6 transitions; and the peaks at 580 and 620 nm are due to the 5 D4 → 7F4 and 5D4 → 7F3 transitions of Tb3+.53 Compared with the emission intensity of NO3-LTbH, that of the Gd3+containing sample (NO3-LTbH:Gd) slightly increased with Gd 3+ codoping because of the reduced concentration quenching between Tb3+ ions. Gd3+ is role-playing the reduce concentration quenching in NO3-LTbH:Gd, which can be further confirmed by lifetime measurements.54−56 The decay curves of Tb3+ in NO3-LTbH and NO3-LTbH:Gd at 543 nm under a 368 nm excitation are shown in Figure S7 (Supporting Information). The corresponding luminescence lifetimes were obtained by fitting the decay profiles with a one-exponential form. The results show that the lifetimes of NO3-LTbH and NO3-LTbH:Gd are approximately 0.287 and 0.724 ms,

Figure 3. Excitation spectra of NO3-LTbH, NO3-LTbH:Gd, L-LTbH, and L-LTbH:Gd hybrid materials. The inset figures magnify the 250− 300 nm region of the NO3-LTbH and NO3-LTbH:Gd excitation spectra. 14514



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Figure 5. UV−vis absorption spectra of (a) Na2L, NO3-LGdH, L-LGdH, (b) NO3-LTbH, L-LTbH, (c) NO3-LTbH:Gd, L-LTbH:Gd, and (d) LLTbH, L-LTbH:Gd, L-LGdH.

sensitizers that can be transferred to the host would be gathered and effectively transferred to the adjacent Tb3+ because the Dexter energy transfer process is very effective in this system, which is thanks to the specific structural characteristics of the LRHs.13 From this point of view, the host can act as an energy transfer bridge that helps energy transfer from sensitizers to Tb3+ ions. On the basis of the experimental results, we believe that there are other energy transfer channels to enhance the luminescence of Tb3+ ions in addition to this energy transfer channel. To prove the effect of Gd3+ in the host on the intercalation material, we studied UV−vis absorption spectra of NO3-LGdH and L-LGdH (the PXRD and compositional analyses characterization data are shown in Figure S8 and Table S2 (Supporting Information)), as well as NO 3-LTbH, L-LTbH, NO 3LTbH:Gd, and L-LTbH:Gd. The UV−vis absorption spectrum of NO3-LGdH exhibits the broadest absorption peak from 200 to 340 nm (Figure 5a), and the intensities of absorption peaks at 220 nm that were significantly reduced or completely disappeared during Gd3+ were partly or completely replaced by Tb3+ (Figure 5b,c). It is especially important that the absorbance spectrum of the intercalation material was different from a simple superposition of the spectra of each component. Interestingly, the absorbance spectrum of the intercalation material has an obvious red-shift, especially when Gd3+ was incorporated into the host, and the absorption intensity increases with the presence of Gd3+ ions. What was beyond our expectation is that the absorption area of intercalation material L-LGdH can be extended to 400 nm, and the strongest absorption peak exhibits the broadest absorption from 330 to 360 nm (Figure 5d). This brings us to the conclusion that codoped Gd3+ increased the absorbance of the host and the interaction between host and guest. One of the main reasons is that introduction of Gd3+ can lead to LRH structure with certain imperfection owing to Gd3+ having a limiting cationic radius or existing different ionic radius with Tb3+.17,59 Consequently, sensitizers can be directly assembled onto the

respectively. The increased lifetime of NO3-LTbH:Gd is due to a decrease in the concentration quenching because of Tb3+ ion dilution. Furthermore, replacing Tb3+ with Gd3+ reduces the hydration of the LRH structure and thus partially shifts the Ln3+ coordination from [Ln(OH)8H2O] (C4v) to [Ln(OH)7H2O] (C1), thereby reducing the vibration of the layer hydroxyl groups which induces the nonradiative relaxation of the rare-earth ions in the excited states.57−59 Meanwhile, the emission spectra of L-LTbH (Figure 4) indicate an efficient energy transfer from the excited state of the intercalated sensitizers to the Tb3+ centers in the host layers, which improves the luminescence intensity of NO3-LTbH. We observed the occurrence of an interesting phenomenon when the luminescence spectrum of NO3-LTbH was compared with that of L-LTbH:Gd. The high emission intensity of the hybrid material L-LTbH:Gd is 24.4-fold higher than that of NO3LTbH which does not contain Gd3+ and sensitizers. Moreover, the luminescence quantum efficiency (Φ) of L-LTbH:Gd is 33.0% (the Φ data of other samples are listed in Table S3, Supporting Information). By contrast, the emission intensity of NO3-LTbH exhibited a 1.5- or 4.7-fold increase upon the addition of Gd3+ or of sensitizers, respectively. We believe that the dramatically enhanced luminescence of L-LTbH:Gd compared with NO3-LTbH is mostly due to the synergistic effect between Gd3+ and the sensitizers and thus cannot be achieved by simple mixing. We propose that the cause of the synergistic effect apart from Gd3+ ions has reduced the concentration quenching effect between the Tb3+ ions, and starting from the viewpoint of energy transfer is also very important. According to the references30 and experimental results, the excited levels of Gd3+ which situate above the excited triplet level of sensitizers are not accessible by 365 nm excitation. Nonetheless, if the intercalated sensitizers nearby with Gd3+ cannot intensify adjacent Tb3+, the presence of Gd3+ will not affect the luminescent intensity of the Tb3+ band, which is not in accordance with the experimental result. Therefore, we may be sure that the excited energy at the triplet state of 14515



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Figure 6. Low-temperature (77 K) phosphorescence spectra of (a) Na2L and (b) phosphorescence of sensitizer L dramatically enhanced by NO3LGdH. (c) When Tb3+ and Gd3+ ions exist simultaneously in the host, the phosphorescence emission from sensitizer L was quenched, and characteristic peaks of Tb were observed by a cascaded energy transfer from host (NO3-LTbH:Gd) to Tb via sensitizers. (d) Phosphorescence spectra of L and L-LTbH, indicating the energy transfer from sensitizer L to Tb3+. Spectra were obtained with excitation at 330 nm for Na2L, 375 nm for L-LGdH, 365 nm for L-LTbH:Gd, and 358 nm for L-LTbH.

dramatic increase of the luminescence lifetime (τ) of L in the intercalated sample L-LGdH (0.639 ms) compared with Na2L (5.5 ns) (Figure S11, Supporting Information) can also support this conclusion. A decrease of phosphorescence of sensitizers and a remarkable increase of Tb3+ photoluminescence are shown in Figure 6c, demonstrating energy transfer from sensitizer L to Tb3+. A comparison between the phosphorescence spectra of L-LTbH (Figure 6d) and L-LTbH:Gd (Figure 6c) intercalation material clearly shows the synergistic effect of LTbH:Gd and sensitizers on the increased luminescence of Tb3+ ions. In this respect, LTbH:Gd can increase luminescence of sensitizers not only as a host matrix for sensitizers, and sensitizers act as energy-transfer bridges that connect the LTbH:Gd and Tb3+ to intensify characteristic emission of Tb3+ by a cascaded energy transfer.60,61 These results are very similar to those previously reported by Park et al.62 Consequently, we propose that the mechanism for the synergistic effect of significant enhancement of Tb 3+ luminescence is mainly caused by a Gd3+-induced increased absorption cross-section, enhanced interaction of host and guest, and a cascaded energy transfer from host to Tb3+ via sensitizers. Analyzing the layered structure of LRHs, the direct UV absorption of an organic sensitizer intercalated through ion exchange in the interlayer space is possibly difficult or is limited by the shielding effect of the layered structure (Figure 2b). Therefore, in the L-LTbH:Gd hybrid material, the vast majority of UV absorption is initially performed by the LTbH:Gd rather than directly by the sensitizers, which is more advantageous to a cascaded energy transfer from host → sensitizers → Tb3+ (in host). On the basis of all of the above results, the reason why the synergistic effect to create remarkable luminescence property is caused by the energy-transfer pathway involves the following steps: (i) UV absorption by the LTbH:Gd host,

LRH surface through the stronger coordination interactions not just based on electrostatic interactions which exist in the interlayer gallery. On the other hand, highly polarizable hydroxocation layers possess stronger interaction with sensitizers in the interlayer gallery when Gd3+ doped.15,37 Returning to the luminescence studies, it seemed reasonable that excitation spectra of the Na2L and intercalation material showed a similar effect compared with UV−vis absorption spectra (Figure S9, Supporting Information). We studied whether a better emission will exist, if the L-LTbH:Gd intercalation material was excited with different excitation wavelength, especially for 273 nm which is the energy level of the Gd3+ excited states. As can be seen from Figure S10 (Supporting Information), better results have therefore not followed use of 273 nm as the excitation wavelength, which indicates that Gd3+-induced host sensitization of sensitizers occurs in L-LTbH:Gd rather than itself. This result can be further strongly supported by experimental evidence discussed below. To further investigate the mechanism for dramatically enhanced luminescence of layered terbium hydroxides as induced by the synergistic effect of Gd3+ and sensitizers, we studied the function of Gd3+ and traced a path of energy transfer by low-temperature (77 K) phosphorescence spectra (Figure 6). As can be seen from Figure 6a and 6b, the phosphorescence of sensitizers was significantly enhanced when sensitizers intercalated into NO3-LGdH. The main reason to choose NO3-LGdH as the host matrix for sensitizers is to clearly observe significantly enhanced phosphorescence of L, and this kind of phenomenon cannot be observed in LLTbH:Gd because of the efficient energy transfer from sensitizer L to Tb3+ ions. Thus, this brings us to the conclusion that Gd3+ played a very important role in helping the host absorb energy even though it cannot be excited by 365 nm. The 14516



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other rare-earth luminescent materials with unique structures and unusual functionalities, which have potential applications in various technological fields.

followed a cascaded energy transfer by sensitizers which serve as energy-transfer bridges that connect the host and Tb3+ to enhance the characteristic emission of Tb3+; (ii) intersystem crossing of the sensitizers from the singlet to the triplet state and then intermolecular energy transfer from the sensitizer (very close to Tb3+) triplet state to the excited 4f states of Tb3+; (iii) the excited energy at the triplet state of sensitizers (adjacent with Tb3+), which can be transferred to the host acting as an energy transfer bridge, would be gathered and transferred to the adjacent Tb3+. All three of the energy-transfer pathways give rise to strong radiative transition from the Tb3+ ion-emission states to lower-energy states, which results in the characteristic Tb3+ green emission.63,64 A schematic diagram of the L-LTbH:Gd energy-transfer pathways is shown in Scheme 3.

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EXPERIMENTAL SECTION Materials. Terbium nitrates (Tb(NO3) 3 ·6H 2O) and gadolinium nitrates (Gd(NO3)3·6H2O) were obtained by dissolving Tb4O7 and Gd2O3 (99.99%, Shanghai Yuelong) in nitric acid followed by successive fuming to remove excess acid. Other chemicals were obtained from commercial sources and used without further purification. Synthesis of Layered Rare-Earth Hydroxides. NO3LTbH, NO3-LTbH:Gd, and NO3-LGdH powders were synthesized via a hydrothermal route.13,15,16,36,37 A mixed solution was prepared by dissolving Tb(NO3)3·6H2O (0.5 mmol), Gd(NO3)3·6H2O (0.5 mmol), and KNO3 (8 mmol) in deionized water, and an aqueous KOH solution was added dropwise until the pH of the solution was adjusted to ca. 6.9. Then the mixed solution was treated hydrothermally at 150 °C for 48 h in a Teflon autoclave. The obtained products were filtered, washed with deionized water, and then vacuum-dried at 60 °C for 24 h. Synthesis of Organic Sensitizers H2L. The carboxylic acid sensitizers (H2L, listed in Figure S1, , Supporting Information) were prepared according to the literature procedure,32 and chemical structures of the carboxylic acid sensitizers H2L and NMR data are available in the Supporting Information. Synthesis of the Gd(III) Complex Gd−L. H2L (3.0 mmol) and Gd(NO3)3·6H2O (2.0 mmol) were dissolved in a suitable volume of methanol at room temperature (RT). The pH value of the solution was adjusted by adding ammonia solution until the sediment was observed. The mixture was continually stirred for 5 h at RT. The reactive solution was filtered, and the precipitate was washed with methanol and dried in vacuum. Elemental analysis (%) calcd (found) for C60H84N4O28N2Gd2: C, 38.72 (38.85); H, 5.94 (5.87); N, 3.87 (9.957). Intercalation Assembly of the Hybrid Materials LLRHs. LRHs (100 mg) were dispersed in deionized water (15 mL) and ultrasonicated for 15 min. The sensitizers were dissolved in methanol solution and deprotonated beforehand by adding NaOH aqueous solution. Then the mixture of LRHs and sensitizers was sealed in a 25 mL Teflon-lined autoclave and heated at 100 °C for 4 h. The autoclave was cooled to RT, and the precipitates obtained were filtered, washed with a mixed solvent of acetone and water three times, and then dried at 60 °C for overnight. Preparation of Composite Film (PMMA/L-LTbH:Gd). PMMA powder (500 mg) was dissolved in DMF (4 mL), was heated at 60 °C for 30 min, followed by the addition of LLTbH:Gd (25 mg) which was dispersed in DMF (1 mL), and ultrasonicated for 1 h, followed by continuous stirring for 5 h at RT. Subsequently, the hybrid material was casted onto clean quartz glasses, and the composite film was obtained after the total evaporation of the solvent at RT. Measurements. NMR spectra were recorded on a Bruker 400 MHz spectrometer. Chemical shifts (δ) are given in parts per million (ppm). The compositions of all samples were determined by CHN elemental analyses on an Elementar Vario EL analyzer. Mass spectra were recorded on a Bruker UHRTOF maXis 4G mass spectrometer. FT-IR spectra of the materials were conducted within the 4000−400 cm−1 wave-

Scheme 3. Schematic Representation of the Energy-Transfer Mechanism for L-LTbH:Gd

PMMA exhibits excellent mechanical and optical properties and has been used in the fabrication of optical devices.31 To determine the potential of these hybrid materials in various applications, PMMA/L-LTbH:Gd composite film devices were fabricated using the solvent-casting method65−67 and showed the high transparency and excellent luminescent properties (see the Supporting Information, Figures S12−15).

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CONCLUSIONS In conclusion, we have successfully fabricated novel organic− inorganic LRH hybrid materials by a simple hydrothermal method. We demonstrated that the synergistic effects of an inert ion (Gd3+) and sensitizers in NO3-LTbH result in significantly enhanced luminescent properties and high quantum efficiency (Φ = 33%). On the basis of the study of luminescence spectroscopic characteristics, crystal structures of the LRHs, UV−vis absorption spectrum, and low-temperature phosphorescence, we speculate that the mechanism for synergistic effect is caused by the multiple energy-transfer channels, especially a cascaded energy transfer. The very interesting thing is that LTbH:Gd can enhance phosphorescence of sensitizers and not only act as host matrices for sensitizers but also act as energy-transfer bridges that connect the sensitizers and Tb3+ (in the host) to enhance the characteristic emission of Tb3+. This study provides additional insight into the design of hybrid phosphors based on LRHs by integrating rare-earth coordination chemistry and the aforementioned synergistic effect. The results of this work may have high theoretical and practical significance for the fabrication of 14517



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number range using a Nicolet 360 FT-IR spectrometer with the KBr pellet technique. PXRD was determined with a RigakuDmax 2400 diffractometer using Cu Kα radiation over the 2θ range of 3−65°. TG-DTA were performed on a PerkinElmer thermal analyzer up to 800 °C at a heating rate of 10 °C·min−1 under air. The phosphorescence spectra of samples were measured at 77 K in a methanol−ethanol mixture (v:v = 1:1) on a Hitachi F-4500 spectrophotometer. The steady-state corrected luminescence spectra and the lifetime measurements were measured on an Edinburgh Instruments FSL920 fluorescence spectrometer, with a 450 W Xe arc lamp as the steady-state excitation source and a Nd-pumped OPOlette laser as the excitation source for lifetime measurements. The overall quantum yields of the samples were determined by an absolute method using an integrating sphere (150 mm diameter, BaSO4 coating) on an Edinburgh Instrument FLS920.68−70 Three parallel measurements were carried out for each sample, so that the presented value corresponds to the arithmetic mean value. The errors in the quantum yield values associated with this technique were estimated to be within 10%.

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ASSOCIATED CONTENT

* Supporting Information S

The data of 1H NMR and 13C NMR of H2L, PXRD patterns, the FT-IR spectra, TG-DTA curves, excitation and emission spectra, phosphorescence spectra, absorption spectrum, luminescence decay curves, characterization data for samples, and PMMA/L-LTbH:Gd composite film section. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 86 931 8912552. Fax: 86 931 8912582. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Project 20931003, 21071068). REFERENCES

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Dramatically Enhanced Luminescence of Layered Terbium

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