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A white-light hydrotalcite-like compound (Eu/Tb/Zn-HTlc) fabricated from Eu, Tb, and Zn complexes co-doped into magnesium-aluminum hydrotalcite (MgAl-LDH) was prepared by a simple one-pot chemical method. The structural and luminescent properties of Eu/Tb/Zn-HTlc were investigated. Excited by 370 nm UV light, the simultaneous emission bands of this single-matrix material are located in the red, green, and blue regions, combined to give a pure white emission (CIE x = 0.346 and y = 0.3339). This research represents a new strategy for designing layered organic-inorganic hybrid materials for LED-pumped white light.
©2012 Optical Society of America
1. Introduction
White-light-emitting diodes (white LEDs), the next-generation of solid-state lighting, have broad applications in illuminating light sources and various displays due to their excellent properties such as brightness, high reliability, high energy efficiency, long lifetime and environment friendliness [1]. At present, a commercial white LED is realized by combining a 465nm blue-emitting InGaN chip with a broad-band yellow-emitting phosphor (YAG:Ce3+). However, such a white LED shows low color-rendering index, low color reproducibility and low luminous efficiency because of deficiency of red emission light [2]. Another approach for obtaining white light is to combine a near-UV LED (around 370-410 nm) with tri-color materials. The white light consists of the red, green, blue light emitted from the tri-color materials while excited by the near-UV LEDs. The most commercially popular white-light-emitting devices usually contain three-phased phosphors, which have the following drawbacks: for example, they are rather complicated and expensive; white-emitting color changes with input power; and poor stability [3]. Thus, one of the current academic interests is in pursuing white-light emitting single-matrix materials pumped by high-efficiency UV LEDs owing to their high efficiency in converting electrical energy to light, long duration and lifetime, good reliability and safety, easy maintenance, and low energy consumption advantages over currently employed multiphosphors or multi-LEDs [4,5].
Lanthanide β-diketonate complexes have triggered numerous studies due to their attractive emission features, which are characterized by narrow band emission spectra, high quantum efficiency and long luminescence lifetimes. However, they have so far been excluded from practical applications as luminescent devices, essentially due to their poor photo and thermal stability and mechanical properties. Therefore, in recent years, many organic-inorganic hybrid materials have been synthesized to combine the mutual advantages of both emission properties of lanthanide β-diketonate complexes and the heat, ageing-resistance of inorganic networks [6–8]. Despite the high interest in doping inorganic matrices with lanthanide β-diketonate complexes, research about the organic-inorganic white-light emitting hybrid materials is rarely reported.
Layered double hydroxides (LDHs), also called hydrotalcite-like compounds (HTlcs), are a large family of inorganic lamellar materials, which are constructed by positively charged brucite-like layers with interlamellar exchangeable anions located, as water molecules, in the interlayer space. LDHs have the general formula, [M(II)1-xM(III)x(OH)2]x+(An−x/n)·mH2O, where M(II) and M(III) are the divalent and trivalent metal cations, respectively. A is the charge compensating anion or gallery anion, m is the number of moles of co-intercalated water per formula weight of compounds, and x is the number of moles of M(III) per formula weight of the compounds [9]. With many great advantages, LDHs are widely applied in various fields, such as catalysis, adsorption, medical materials, etc. [10–12]. The optical properties of LDH have attracted much attention recently due to their potential applications for photochemical and photophysical devices [13–15]. The conventional methods to obtain the photoluminescent LDH composites are to exchange the interlayer anions with some fluorophores by intercalating, or to adsorb fluorophores on the surfaces of the LDHs layers [16].
Here, we suggest a new method to obtain fluorescent metal complex-containing hydrotalcite-like compound. It is considered that if we modify the internal region of MgAl-LDH by intercalation of stearate anions, disperse proper ligands into the anions in the interlayer region, and at the same time, doped metal cations in the brucite-like layers of a LDH matrix. The ligands in the interlayer galleries may coordinate with metal cations in the layers, and will exhibit characteristic bright light. Firstly, the MgAl-LDH contains interlayer chloridions, which are known to be readily displaced by other anions. Stearate anions possess amphiphilic structure, which can be easily intercalated and used to organically modify the interlayer galleries of LDHs. The hydrophobic alkyl chains of stearate anions have high affinity for ligands that allow them to encapsulate in the lamellar hydrotalcite host. The driving force of coordination of ligands also makes them favor co-intercalation of the stearate anions. Secondly, the homogeneous distribution of irradiant center cations in the solid matrix can avoid metal complex aggregation and fluorescence quenching effectively. Thirdly, the particular coordination structure may increase the asymmetry of the metal complex. The asymmetric microenvironment causes the polarization of the irradiant center under the influence of the electric field of the surrounding ligands, which increases the probability for the electric dipole transition [17] and leads to an increase in the fluorescence intensity. Finally, the presence of the inorganic LDH host matrix may improve the thermal and optical stability of the incorporated metal complex.
In this report, Eu and Tb complexes functionalized by β-diketone and 1,10-phenanthroline were selected as the red and green emitters, respectively. Zinc salicylate [Zn(Sal)2] was selected as the blue-light-emitting complex due to its strong blue luminescence under UV light excitation [18]. The interlayer space of a magnesium-aluminum hydrotalcite (MgAl-LDH) was modified by intercalation of stearate anions, and ligands (thenoyltrifluoroacetone, acetylacetone, 1,10-phenanthroline, and salicylic acid) were intercalated in the interlayer space. At the same time, the irradiant center cations (Eu3+, Tb3+, Zn2+) were doped in the octahedral lattice of the brucite-like layer by partially substituted aluminum (III) and magnesium (II). The ligands in the interlayer galleries coordinated with irradiant center cations in the layers and enabled simultaneous red-, green-, and blue-light, combined to give a pure white emission. To the best of our knowledge, this is the first report of a solely white light emitting hydrotalcite-like compound.
2. Experimental
The Eu3+, Tb3+, Zn2+ doped MgAl-LDHs intercalated with thenoyltrifluoroacetone (TTA), acetylacetone (acac), 1,10-phenanthroline (o-phen), and salicylic acid were synthesized by a one-pot coprecipitation method [19]. 0.003g of Eu2O3 and 0.03g of Tb4O7 were dissolved independently in 10ml (3M) hydrochloric acid solution for the synthesis of EuCl3 and TbCl3, and added to 20 mL of an aqueous solution containing 0.18g MgCl2, 0.02g NaAlO2 and 0.2g ZnCl2. This mixed metal salt solution was solution A; 0.012g thenoyltrifluoroacetone, 0.03g 1,10-phenanthroline, 0.05g acetylacetone and 0.15g salicylic acid, were dissolved independently in absolute ethanol and added to 100 ml of a sodium stearate solution (0.1M), this was solution B. Then, solution A was added dropwise to solution B with vigorous stirring, the pH was adjusted to 10 using alkaline solution. The resulting slurry was aged at 343K for 48h. At the end of aging, the precipitate was centrifuged and washed thoroughly with deionized water until reaching a pH of 7 followed by drying in vacuo at 373K for 5h and abbreviated here as Eu/Tb/Zn-HTlc.
For comparison purposes, the pristine MgAl-LDHs; Eu3+, Tb3+, Zn2+ doped MgAl-LDHs intercalated with chloridions (MgAlEuTbZn-LDHs); Eu3+, Tb3+, Zn2+ doped MgAl-LDHs intercalated with stearate anions (MgAlEuTbZn-stearate anions-LDHs) were also prepared by using the same procedure above except in the absence of some reagents. For pristine MgAl- LDHs, solution A was an aqueous solution (30ml) containing MgCl2, NaAlO2. Solution B was 100ml distilled water; For MgAlEuTbZn-LDHs, solution A was an aqueous solution (30ml) containing EuCl3, TbCl3, ZnCl2, MgCl2, and NaAlO2. Solution B was 100ml distilled water; For MgAlEuTbZn-stearate anions-LDHs, solution A was an aqueous solution (30ml) containing EuCl3, TbCl3, ZnCl2, MgCl2, and NaAlO2. Solution B was 100 ml of a sodium stearate solution (0.1M).
The emission spectrum of Eu/Tb/Zn-HTlc was recorded at room temperature with an Edinburgh Company FL/FS 920 TCSPC fluorescence spectrophotometer equipped with a 450 W xenon lamp as the excitation source, with excitation slit of 1.0 nm and emission slit of 0.5 nm. The structure of samples were characterized by powder X-ray diffraction (XRD) analysis with a Philips XPert MPD diffractometer using monochromatic Cu Kα radiation (λ = 1.5418 Å), operating at 40 kV and 40 mA. The XRD data for phase identification were collected in 2θ range from 5 to 80° and from 1° to 10°. Microstructures of samples were observed by transmission electron microscopy (TEM) on a Philips Technai F20 (Holland) transmission electron microscope. The Fourier transform infrared (FT-IR) spectra of samples were scanned over the wave number range 400-4000 cm–1 using the Thermo Nicolet Company’s AVATAR360, America, KBr disks. A thermogravimetry and differential scanning calorimeter study (TG-DSC) was performed on a NETZSCH STA 449C thermal analyzer, thermobalance by heating the samples from 296 to 1073K under air atmosphere at a heating rate of 10K/min.
3. Results and discussion
3.1 Fluorescence spectra analysis
The photoluminescence excitation and emission spectra of the Eu/Tb/Zn-HTlc sample were measured at room temperature and are shown in Fig. 1 . The sample shows wide and strong excitation bands in a range of 295 – 395 nm. The peak is located at 370 nm, so it matches to the emission wavelength of a 370 nm-emitting InGaN chip, indicating that the Eu/Tb/Zn-HTlc is suitable for integration with the NUV-emitting InGaN-based LED. Excited by 370 nm-light, the Eu/Tb/Zn-HTlc exhibits four narrow emission peaks centered at 614 nm (5D0→7F2), 591 nm (5D0→7F1), 545 nm (5D4→7F5), and 491 nm (T1π, π* to the ground state), which are responsible for the characteristic red, green, and blue colored emission of the Eu, Tb, and Zn complexes, respectively. The simultaneous emission bands in the red, green, and blue regions combined to give a pure white emission, for which compared with that of MgAlEuTbZn-LDH, the intensity is much stronger.
Fig. 1 (a) Excitation spectra (λem = 613nm), and (b) emission spectra (λex = 370nm) of Eu/Tb/Zn-HTlc (solid) and MgAlEuTbZn-LDH sample (dash).
Luminescence color of Eu/Tb/Zn-HTlc excited at 370 nm was characterized by Commission International de I’Eclairage (CIE) chromaticity diagram and shown in Fig. 2 . Remarkably, the CIE coordinate of Eu/Tb/Zn-HTlc (X = 0.346, Y = 0.339) is close to the standard equal energy white light illuminate (X = 0.333, Y = 0.333). The photograph shows the bright white emission from Eu/Tb/Zn-HTlc excited by the 370 nm UV lamp. This indicates that Eu/Tb/Zn-HTlc can act as a promising white-emitting material for UV LED chips.
3.2 XRD profile analysis
The XRD patterns of MgAl-LDHs (a), MgAlEuTbZn-LDHs (b), MgAlEuTbZn-stearate anions-LDHs (c), and Eu/Tb/Zn-HTlc (d) (The insert is the small-angle XRD) are given in Fig. 3 . A typical XRD pattern of the lamellar materials, with strong and symmetric reflections indexed to a hexagonal lattice with R-3m rhombohedral symmetry commonly used for the description of LDH structures, is shown in Fig. 3(a). Upon treatment with EuCl3, TbCl3 and ZnCl2 the reflections of Fig. 3(b) are essentially unshifted, the first recorded peak around 11° corresponds to basal spacing of 0.78nm. However, 110 reflections are broad and asymmetric, indicating stacking faults in matrices [20]. Such crystallographic changes were induced by the incorporation of Eu3+, and Tb3+ ions in the brucite-like layers. As the ionic radii of Eu(III) (r = 0.0947nm), Tb(III) (r = 0.0923nm) are very different from the ionic radii of Al(III) (r = 0.0535nm) and Mg(II) (r = 0.072nm), which could induce a distortion of the octahedral layer. Within each octahedron, the cation-oxygen distances must, therefore, no longer be equal. Moreover, the cation-cation distances are exclusively compatible with a corrugation of the octahedral layers, which lose their planarity. A similar phenomenon was also observed by others when Tb3+, Ce3+ cations were incorporated into LDH lattices [19,21,22]. After treatment with stearate, a new peak appeared in Fig. 3(c), and corresponds to a basal spacing of 1.52 nm (2θ = 5.8°). This result shows that stearate was intercalated between the layers. Two peaks are present at 2θ = 5.8° and 2θ = 10.9°; this indicates that stearate anions and Cl- groups were present in the interlayer galleries of MgAlEuTbZn-stearate anions-LDHs, and that the stearate anions had not displaced Cl- groups completely [23]. It can be observed from Fig. 3(d) that the interlayer spacing (d003) of Eu/Tb/Zn-HTlc increased from 1.52nm in the MgAlEuTbZn-stearate anions-LDHs to 4.42nm (2θ = 2.0°), indicating that the mixed ligands were co-intercalated in the galleries of LDH. Note that the peaks of (003) reflections were broadened and reduced in intensity. Moreover, the higher order peaks (015, 018, 116, 205) either disappear or become extremely weak, an indication of possible partial exfoliation in structure [24].
Fig. 3 XRD patterns of MgAl-LDHs(a), MgAlEuTbZn-LDHs (b), MgAlEuTbZn-stearate anions-LDHs (c), and Eu/Tb/Zn-HTlc (d). (The inset is the small-angle XRD)
3.3 Microscopic morphologic analysis
The intercalation/exfoliation structure coexistence in the Eu/Tb/Zn-HTlc was further confirmed through transmission electron microscope (TEM) images shown in Fig. 4 . The MgAl-LDHs have an average (003) spacing of approx. 0.78 nm, in line with the spacing determined using XRD. Compared with the face-to-face orientated structures of MgAl-LDHs, the Eu/Tb/Zn-HTlc forms a disordered layered structure with average spacing of approx. 4.4nm. Besides the intercalated structures, some exfoliated LDH layers are also found with a disordered morphology. The gradual expansion of the interlayer spacing may destroy the ordered crystal structure and thus lead to the exfoliated structure [24]. The mechanism of fabricated Eu/Tb/Zn-HTlc was illustrated in Fig. 5 .
3.4 FT-IR spectra analysis
The FT-IR spectra of MgAl-LDHs, MgAlEuTbZn-LDHs, and Eu/Tb/Zn-HTlc are shown in Fig. 6 . All the samples displayed a very intense and broadened band around 3450 cm−1, which are the typical stretching vibrations of hydroxyl groups in the brucite-like layer and physisorbed water [25]. The lattice vibrations of the ν (M-O) and δ (M-O-M) [26] appear at the region below 700 cm−1. The band at 1629 cm−1 is due to the HOH bend of water molecules [27]; the band at 1376 cm−1 is assigned to the skeleton vibration of Cl−1 [28]. The FT-IR spectrum of the MgAlEuTbZn-LDHs is not very different from that of the MgAl-LDHs. This phenomenon is similar with that of incorporation of Eu3+ into LDH lattices [29].
In the spectrum of Eu/Tb/Zn-HTlc, the bands at 2925 cm−1 and 2852 cm−1 are attributed to the asymmetric CH3 and CH2 stretching vibration of intercalated organic anions [30]. The characteristic peaks of free o-phen ligand at 1561 (νC = C + C = N), 854 (δC-H), and 739 cm−1 (δC-H) were red shifted to 1519, 848, 723 cm−1 in Eu/Tb/Zn-HTlc, this indicates that coordination bonds have formed between metal ions and N from phen in Eu/Tb/Zn-HTlc [31]. The characteristic peak of free acetylacetone ligand at 1624 cm−1 (νC = O) was red shifted to 1573 cm−1 in Eu/Tb/Zn-HTlc, suggesting that the coordination bonds have formed between metal ions and O from acac in Eu/Tb/Zn-HTlc. The coordination of (TTA) to metal ions was indicated by the frequency shift associated with νC = O displacement (from 1642 cm−1 at free thenoyltrifluoroacetone ligand to 1600 cm−1 at the Eu/Tb/Zn-HTlc), which provides good evidence that the metal ion is coordinated through the β-diketone oxygen atoms [32]. In addition, the absorption band located at 1456 (νas COO-) and 1392 cm−1 (νs COO-) indicated the complexation reaction between metal ion and salicylic acid [33].
3.5 Thermal stability analysis
The TG-DSC curves for MgAl-LDH and Eu/Tb/Zn-HTlc are shown in Figs. 7(a) and 7(b). In the case of MgAl-LDH, there are two apparent mass losses observed in Fig. 7(a) that coincided with one broad and two sharp endothermic peaks in the DSC profiles. The first mass loss before 217°C (about 13% wt) was ascribed to the loss of physically adsorbed and interlayer water. The second mass loss between 330°C and 481°C was attributable to the removal of hydroxyl groups from the hydrotalcite-like layers and anions from the interlayer space [9]. These corresponded to two remarkable sharp DSC endothermic peaks at 371°C and 435°C. A small mass loss above 481°C was attributable to the decomposition of residual hydroxyl groups. The TG curve of Eu/Tb/Zn-HTlc in Fig. 7(b) shows a gentle mass loss (about 9.6%) before 309°C corresponding to a weak exothermic peak in the DSC at 252°C which was due to the removal of water physisorbed on the external surface of the crystallites as well as water intercalated in the interlayer galleries. The rapid mass loss (about 49.6%) in the interval temperature of 309 to 528°C occurs with the combustion of the organic chain of the interlayer anion and dehydroxylation of the layers [34,35], and the corresponding sharp exothermic peaks of DSC located at 361, 450 and 519°C, respectively. The mass loss peaks of Eu/Tb/Zn-HTlc shifted toward high temperature, indicating that Eu/Tb/Zn-HTlc exhibited greater thermal stability than MgAl-LDH, which was ascribable to the interaction between the metal coordination structure and the layer structure.
The Eu/Tb/Zn-HTlc did not decompose until 309°C, compared with that of lanthanide β-diketonate complexes (275-305°C) [36] and zinc salicylate (220-240°C) [37], and the thermal stability is obviously enhanced, which may be attributable to the protection effect of layer structure.
4. Conclusion
The paper has described a novel one-pot, cost-effective approach to producing white light-emitting single-matrix material by incorporation of Eu, Tb, and Zn complexes into layered double hydroxides. A pure white light emission with the CIE coordinates (X = 0.346, Y = 0.339) was obtained under 370 nm excitation. A broad excitation band extending from 295 to 395 nm matches the emission spectral range of UV LEDs. This research may provide a new strategy for designing novel luminescent layered organic-inorganic hybrid materials used in fabrication of UV excited LEDs, and further studies are currently underway.
Acknowledgments
This work was supported by the National Basic Research Program of China (973 Program, 2009CB930601), National Natural Science Foundation of China (60976019, 20804019 and 20774043), Program for New Century Excellent Talents in University (NCET-07-0446), Specialized Research Fund for the Doctoral Program of Higher Education (SRFDP 20093223110002), Natural Science Foundation of Jiangsu Province of China (BK2009427, BZ2008116), Natural Science Fund for Colleges and Universities in Jiangsu Province (08KJD430017), Program for Innovative Research Team in Science and Technology in Fujian Province University (IRTSTFJ), Scientific and Technological Innovation Teams of Colleges and Universities in Jiangsu Province (TJ207035), and Nanjing University of Posts and Telecommunications (NY207161).
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