Chinese Chemical Letters  2018, Vol. 29 Issue (2): 321-324   PDF    
Tunable white-light emission of lanthanide(Ⅲ) hybrid material based on hectorite
Jiahui Ma, Tianren Wang, Caixia Liu, Huanrong Li    
Hebei Provincial Key Lab of Green Chemical Technology and High Efficient Energy Saving, School of Chemical Engineering and Technology, Hebei University of Technology, Tianjin 300130, China
Abstract: In this work, we present novel trivalent lanthanide ions (Ln3+)-based luminescent hybrid materials, in which the organic ligands are covalently grafted on the hectorite templates and the Ln3+ ions can be well immobilized by the ligands through coordination bond. The hybrid materials exhibit tunable emission colors by varying the molar ratio of Eu3+ to Tb3+, and the one with Eu3+:Tb3+=1:1 exhibits excellent coordinate of (0.327, 0.328) located in the "white region" of the CIE 1931 chromaticity diagram (under 300 nm UV illumination). These properties make the hybrid composites suitable for fabricating optoelectronic devices such as full-color displays and white LED.
Key words: Hectorite     Lanthanide     Covalent bond     Luminescence     White light    

Trivalent lanthanide ions (Ln3+)-based luminescent hybrid materials have been of great interest in versatile practical applications such as display, lighting, optical devices and medical technology [1-10] in decades. Generally, such hybrid materials are prepared by doping Ln3+ complexes into various matrices like zeolites [11-13], titania [14, 15], and clay materials [7, 16], which afford the complexes promoted optical behaviors and improved thermo-/photo-stability, as well as good solvent processability [7, 17-22]. Recently, researchers have realized that the hybrid materials can also be prepared by covalently grafting the organic composites onto the matrices, which can guarantee molecular distribution of Ln3+ complexes in the materials, and the agglomeration of the active centers can be effectively avoided.

Amongst numerous organic components, the organically modified alkoxysilanes are well suited for this purpose and have frequently been used to provide covalent bonding between inorganic matrix and organic group to improve the mechanical properties of the hybrid materials [23, 24]. For instance, the TMC-Si (see Scheme 1 and Fig. S1 in Supporting information), a blueemitting bifunctional organic silane, is capable of both covalently bonding with inorganic matrix and coordinating with Ln3+ to form luminescent centers. Hectorite (we denoted here as LaponiteRD or Lap: Na0.7[Si8Mg5.5Li0.3]O20(OH)4) is a smectite-like synthetic clay that composed of stacked platelets, with an average diameter 25 nm and 0.92 nm in height [24-29]. The hectorite has a tendency to form partially delaminated and disordered aggregates through edge-to-face and edge-to-edge interactions, and this is beneficial for the reaction between the organic silane and hectorite [30]. Hence, in this manuscript, we utilize hectorite, TMC-Si and Ln3+ (Ln = Eu, Tb or Eu and Tb in different molar ratio) to prepare novel luminescent hybrid materials (denoted as Lap-TMC-Si-Ln) through a simple, environmentally friendly and less-time consuming method. Tunable emission colors and even white light emission can be observed by varying the molar ratio between Eu3+ and Tb3+ upon excitation at 300 nm. The detailed experimental parts were listed as follow:

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Scheme 1. Schematic representation of the hybrid material consisting of hectorite, organosilane and Ln3+ as well as digital photos of Lap-TMC-Si-Eu (red) and LapTMC-Si-Tb (green) (under 302 nm UV light illumination).

The nanoclay hectorite (denoted as LaponiteRD or Lap), 3-aminopropyltriethoxysilane, 1, 3, 5-benzenetricarbonyl trichloride were used as received without further purification. EuCl3·6H2O and TbCl3·6H2O were obtained by dissolving Tb4O7 and Eu2O3 in hydrochloric, respectively.

Synthesis of the amino silane-functionalized organics TMC-Si: 3-Aminopropyltriethoxysilane (725 μL, 3.1 mmol) and Et3N (485 μL, 3.4 mmol) were dissolved in benzene to form a solution, then 1, 3, 5-benzenetricarbonyl trichloride (0.265 g, 1 mmol) benzene solution was added dropwise with stirring and cooling at 0 ℃ under nitrogen atmosphere and white precipitate was formed, which was stirred at room temperature for 48 h. Afterwards, the crude solution was centrifuged at 10, 000 rpm for 10 minutes, and the precipitate (triethylammonium chloride) was discarded and the clear supernatant was collected. After removing the solvent by vacuum distillation, the oily solid was obtained and dried in a vacuum oven at 35 ℃ for 4 h, which was then dissolved in dichloromethane and was separated by means of silica gel column chromatography (ethyl acetate/dichloromethane = 2.5/1) and then dried in vacuum to give a white solid. 1H NMR: (400 MHz, CDCl3): δ 0.71 (m, 6H, SiCH2), 1.22 (dt, 27H, J = 9.1, 7.0 Hz, CH3), 1.76 (m, 6H, CH2), 3.50 (dd, 6H, J = 13.0, 6.8 Hz, NCH2), 3.84 (m, 18H, OCH2), 6.75 (t, 3H, J = 5.8 Hz, NH), 8.36 (s, 3H, ArH). 13C NMR (101 MHz, CDCl3): δ 7.941 (s, CH2), 18.296 (d, J = 14.9 Hz, CH3), 23.000 (s, CH2), 42.596 (s, N-CH2), 58.443 (d, J = 10.4 Hz, CH2), 127.996 (s, ring-CH), 135.414 (s, ring-C), 165.676 (s, O=C). MS (MALDI, m/z) calcd. for C36H69N3O12Si3 819.42 [2 M + Na+]; found: 819.80.

Preparation of Lap-TMC-Si: TMC-Si (0.667 mmol, 0.546 g) was dissolved in toluene, then hectorite (1.00 g) was added and the mixture was stirred for 4 days without heating. The grafted hectorite were filtered under vacuum, washed with toluene for three times in order to remove the excessive silane, and dried overnight in a vacuum oven at 35 ℃ to obtain the product as a white solid, which was denoted as Lap-TMC-Si.

Preparation of the luminescent hybrid material Lap-TMC-Si-Ln: Lap-TMC-Si-Ln (Lap-TMC-Si-Eu, Lap-TMC-Si-Eu10Tb3, Lap-TMC-SiEu10Tb6, Lap-TMC-Si-Eu10Tb10, Lap-TMC-Si-Eu10Tb15, Lap-TMC-SiEu10Tb20 and Lap-TMC-Si-Tb) was obtained by dispersing LapTMC-Si (0.05 g) and 0.1 mol/L LnCl3·6H2O ethanol solution (Ln = Eu, Tb or Eu and Tb in different molar ratio) in 4 mL of absolute ethanol with the molar ratio of Ln:TMC-Si = 1:1, and the reaction mixture was treated by ultrasonic wave for 30 minutes. The as-obtained precipitate was filtered under vacuum, washed with absolute ethanol, and dried at 35 ℃ in a vacuum oven.

The 1H NMR spectra was recorded on a Bruker Biospin AG AVANCE400 spectrometer, using deuterated chloroform as the solvent. The FTIR spectra were detected by a Bruker Vector 22 using KBr pellets for solid sample from 400 cm-1 to 4000 cm-1 at a resolution of 4 cm-1 (16 scans collected). The X-ray diffraction (XRD) measurements were carried out on powdered samples by using a Bruker D8 diffractometer (40 mA, 40 kV) with monochromated Cu-Kα1 radiation (λ = 0.15405 nm). SEM images were obtained from an FE-SEM (Hitachi S-4300) at an acceleration voltage of 10 kV. The steady-state luminescence spectra was measured on an Edinburgh Instruments FS920P spectrometer, with a 450 W xenon lamp as the steady-state excitation source, a double excitation monochromator (1800 lines mm-1), an emission monochromator (600 lines mm-1), a semiconductor cooled Hamamatsu RMP928 photomultiplier tube.

The grafted amount (expressed in mmoles of grafted silane per g of bare hectorite) was calculated from the difference ΔC (wt %) of carbon content after and before grafting. The formulas as follows [24]: grafted amount (mmol/g) = 103ΔC/(1200Nc-ΔC × (M-1)), where Nc and M (g/mol) represent the number of carbon atoms and the molecular weight of the grafted silane molecule, respectively (Nc = 18 and M = 567 for TMC-Si). The grafted amount was determined using the formula from the weight loss, W between 120 ℃ and 700 ℃ corresponding to silane degradation [17]: grafted amount (mmol/g) = 103W120-700/((100-W120-700) × M), where M (g/mol) is the molecular weight of the grafted silane molecules. The grafting yield, which corresponds to the percentage of silane molecules which effectively participated in the coupling reaction, was calculated as follows: grafting yield (%) = graft density × 100/ [silane], where [silane] (mmol/g) designates the initial silane concentration.

As described in the experimental parts, the amino silanefunctionalized organics TMC-Si was synthesized through a simple one-step method (Fig. S1) and was well characterized by 1H NMR spectroscopy, 13C NMR spectroscopy and ESI-MS (Figs. S2 and S3 in Supporting information). The luminescent Ln3+ hybrid material was synthesized through a two-step strategy, and the interactions among the components of the material were demonstrated by FTIR spectra, as shown in Fig. 1. The spectrum of hectorite exhibits two bands at about 3450 cm-1 and 1640 cm-1, assigned to the presence of the absorbed water molecules, while the absorption band at 790 cm-1 (Fig. 1, curves b and c) can be ascribed to the absence of νC-H of benzene, and characteristic vibrations of the aliphatic CH2 and CH3 groups (νCH, 2887 cm-1, 2934 cm-1, and 2980 cm-1) of the TMC-Si molecules were also shown. In addition, the absorption band at 1655 cm-1 corresponding to the stretching vibration of the C=O and the stretching vibration of the N—H bond at 1295 cm-1 can be observed, representing the existence of the amide bond in hybrid material Lap-TMC-Si. These data clearly reveal that the organic ligands were successfully grafted on the hectorite templates. However, a weak absorption band of SiOCH2CH3 moieties [31] at 1166 cm-1 shown in Fig. 1, curve b almost disappeared in Fig. 1, curve c, which might be attributed to the hydrolysis during the sonication procedure for preparing LapTMC-Si, probably due to trace amounts of water in the solvents and the ethanol solution of LnCl3·6H2O. By comparing the spectrum of Eu3+-containing hybrid materials (Lap-TMC-Si-Eu) to that of LapTMC-Si, a downshift of the C=O variation from 1655 cm-1 to 1645 cm-1 upon the addition of EuCl3 (Fig. 1, curve c) was observed, indicating that coordination bond between Eu3+ ions and C=O groups was formed [32, 33]. The coordination between TMC-Si and Tb3+ can also be confirmed by the FT-IR spectrum shown in Fig. S4 (Supporting information).

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Fig. 1. FT-IR spectra of a) hectorite, b) Lap-TMC-Si and c) Lap-TMC-Si-Eu.

Generally, the silylating reaction of hectorite with the trifunctional silanes is time-dependent [24]. A maximum grafted amount was reached for the trifunctional silane after 4 days and remained nearly constant even after a long period of time, and the grafted amount increases with increasing the silane concentration in the reaction system. To learn more about the grafting amount of TMCSi on hectorite, carbon content microanalysis and thermogravimetric (TG) analysis was carried out. Fig. 2 shows the TG curves before and after grafting of the silane molecules. The region between 120 ℃ and 700 ℃ corresponds to the decomposition of the organic molecules. The grafted amount determined by TG analysis is in good agreement with the silane content determined by carbon microanalysis, as shown in Table S1 (Supporting information). We also changed the initial silane concentration and maintained the reaction time constant, and the corresponding data shown in Table S2 (Supporting information) reveal that the grafted amount increases rarely with increasing silane concentration in the treating solution, while the grafting yield significantly decreased. It is worth noting that the plateau value is determined by the concentration of silane at about 0.667 mmol/g.

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Fig. 2. TG curves of a) hectorite and b) Lap-TMC-Si.

The formation of Ln3+ complexes in the interlayers was confirmed by the powder X-ray diffraction (XRD) patterns (Fig. 3). The broad peak at approximately 5.9° is attributed to the (001) crystal plane or the basal spacing of the clay, indicating low crystallinity and small particle size of hectorite [34, 35]. There are significant differences in the basal spacing of the hectorite after modification with the amino silane-functionalized organics TMCSi, and this implies that the organics are mainly intercalated in the interlayer spaces of hectorite. Ln3+ complexes intercalated in the plates of hectorite cause the decrease of 2θ (2θ = 5.9° for the bare hectorite, 2θ = 5.1° for Lap-TMC-Si), resulting in the expansion of interlayer spaces (d spacing) of hectorite according to the Bragg equation (2dsinθ = nλ or d = nλ/2sinθ, d is inversely proportional to θ and the d spacing for the bare hectorite is 15.0 Å, and that for LapTMC-Si is 17.2 Å) [36], suggesting the formation of the polycondensates in the interlaminate space, which is similar to that of the previous report [37]. Nevertheless, the location of guest complexes on external adsorption sites on hectorite could not be excluded. It is worth concerning that Lap-TMC-Si-Ln has smaller interlayer spaces (d = 15.1 Å for the Lap-TMC-Si-Eu and d = 15.0 Å for the Lap-TMC-Si-Tb) than that of Lap-TMC-Si, indicating that the ligand can be well coordinated with Ln3+ ions. The morphology of the Lap-TMC-Si was characterized by SEM images, as shown in Fig. S5 (Supporting information). A number of uniform size nanoparticles with an average diameter of 30–50 nm were observed. After reaction with Ln3+, the particle size of the material became smaller.

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Fig. 3. XRD patterns of (a) bare hectorite, (b) Lap-TMC-Si, (c) Lap-TMC-Si-Eu and (d) Lap-TMC-Si-Tb.

We found that the luminescence performance of Lap-TMC-Si-Ln can be significantly influenced by the initial addition amount of the ethanol solution of LnCl3·6H2O. The relationship between the amount of Ln3+ ions and the Lap-TMC-Si hybrid material was analyzed. The luminescence intensity of Lap-TMC-Si-Ln increased gradually when increasing the amount of the Ln3+, which reached its maximum when the molar ratio of TMC-Si to Ln3+ was 1:1 (Fig. S6 in Supporting information). Therefore, the initial amount of TMC-Si to Ln3+ was maintained at 1:1 in the following experiments.

Fig. 4 shows the excitation and emission spectra of the nanoparticles Lap-TMC-Si-Ln. The excitation spectra of Lap-TMCSi-Eu (Fig. 4a) were obtained by monitoring the Eu3+5D07F2 transition at 612 nm. The excitation spectrum of Lap-TMC-Si-Eu exhibits a broad band ranging from 250 nm to 350 nm, due to the absorption of the ligands. There are also several discrete linelike f-f transitions of Eu3+ ions at 362 (7F05D4), 380 (7F05G2), 395 (7F05L6), 415 (7F05D3) and 465 (7F05D2) nm, which represents the self-absorption of Eu3+ ions. Upon excitation at 300 nm, the emission spectrum (Fig. 4c) exhibits five sharp emission bands at 579, 592, 612, 652, and 699 nm, attributed to the 5D07FJ (J = 0-4) transitions of Eu3+, and the hypersensitive 5D07F2 transition acts as the most prominent feature, which is responsible for the bright red emission (Scheme 1) and indicates that Eu3+ sites are indeed without a center of inversion [38]. The excitation spectrum of Lap-TMC-Si-Tb (Fig. 4b) monitored by the 5D47F5 transition of Tb3+ at 544 nm consists of a broad absorption band of ligand with maximum at 300 nm, while the absorption bands of Tb3+ can hardly be observed, implying that the energy transfer from the ligand toTb3+ is more efficient than that to Eu3+. The emission spectrum (Fig. 4c), excited at 300 nm displays four typical emission bands at 488, 542, 584, and 621 nm, corresponding to transitions from 5D47FJ (J = 6-3) of Tb3+, with the most intense peak at 547 nm as the dominant feature, which leads to the relucent green emission color shown in Scheme 1.

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Fig. 4. Excitation spectra of (a) Lap-TMC-Si-Eu and (b) Lap-TMC-Si-Tb (monitored at 612 nm and 544 nm, respectively), emission spectra of (c) Lap-TMC-Si-Ln and (d) the corresponding emission colors in the CIE 1931 chromaticity diagrams excited at 300 nm.

Furthermore, the emission color of the hybrid materials is tunable by changing the molar ratio of Eu3+ and Tb3+ in Lap-TMCSi-Ln excited at 300 nm (Figs. 4c and d). The emission spectra of Lap-TMC-Si-EuxTby (x/y = 10:3, 10:6, 10:10, 10:15, 10:20) excited at 300 nm were shown in Fig. 4c. The relative emission intensity of Eu3+ and Tb3+ changes consistently [18], leading to a series of the emission colors from red to green through white region (Fig. 4d). Apparently, Lap-TMC-Si-Eu10Tb10 has an exceptional co-ordinate of (0.327, 0.328) located in the "white region" of CIE 1931 chromaticity diagram, which is very close to the ideal co-ordinate of white light (0.33, 0.33) and is better than those of the organoclay-based Ln3+ hybrid materials reported previously [18, 19]. These properties make them appealing candidates in white-light-emitting diodes devices. Compared with the formerly obtained white-light-emitting material, Lap-TMC-Si-Eu10Tb10 has purer white light emission excited at shorter excitation wavelength (λex = 300 nm).

In summary, we have successfully prepared a novel kind of multicolored luminescent hybrid materials based on hectorite, organic ligand TMC-Si and Ln3+ through a simple procedure. The TMC-Si molecules and hectorite matrices are covalently connected, which has a good effect on the immobilization of the Ln3+ ions, and the aggregation of the active centers can be effectively avoided. Emission colors of the materials can be fine tuned by varying the molar ratio of Eu3+ and Tb3+, and white light emission (CIE 1931 coordinate: (0.327, 0.328)) was obtained as well. The unique luminescence properties, together with good stability and processability, make the hybrid material potential candidate for fabrication of optical devices. Besides, our results highlight the importance of the covalent bonds between inorganic matrices and organic group, which serve as a basis for the rational design of other luminescent hybrid materials.

Acknowledgments

We gratefully acknowledged the National Natural Science Foundation of China (Nos. 21171046, 21271060, and 21236001), the Tianjin Natural Science Foundation (No. 13JCYBJC18400), the Hebei Natural Science Foundation (No. B2016202147) and Educational Committee of Hebei Province (Nos. 2011141, LJRC021) for financial support.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cclet.2017.08.010.

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