Introduction

Lanthanide ions are well known for their unique luminescent properties such as a broad spectral range (from ultraviolet to infrared region, especially and efficient narrow-width emission band in the visible region) and a long decay lifetime for various applications [1, 2]. Organic lanthanide complexes are well known to be the efficient luminescent species for the energy transfer between organic ligands and lanthanide emitters [3, 4]. However, they have so far been limited for practical application as phosphor materials or devices due to their poor stabilities under high temperature or moisture conditions and low mechanical strength. So it can be expected to investigate and develop lanthanide organic–inorganic hybrid materials [5]. In these hybrids, luminescent lanthanide species can be assembled with hosts such as silica or mesoporous silica, microporous zeolite, and polymer [6–10]. To date, typical paths are utilized to afford the special chemical linkage to construct lanthanide hybrids, which depend on the modification of organic ligands for lanthanide ions with crosslinking siloxane reagents [8–10]. The covalently grafting onto the host materials can be easily realized. On the other hand, for some hosts such as zeolites, the ion exchange is also an important path to functionalize them [6, 11]. Besides, ionic liquid compounds also can be utilized to behave as double functional linker to construct lanthanide hybrids [12, 13].

On the basis of the design and assembly of luminescent lanthanide hybrid materials, the strategy and path can also be applied to introduce other functional species besides lanthanide species, which leads to the multi-component assembly of hybrid materials [14]. Especially the photophysical properties of these hybrids can be integrated for different photofunctional units in the hybrid system. Certainly, different lanthanide species can be fabricated into one hybrid system to realize the luminescence tuning and integrating.

In this work, we put forward a so-called inside-outside double modification path of zeolite A (ZA), and two different lanthanide species (lanthanide complex and lanthanide polyoxmetallate) can be introduced the hybrid systems together with titania and ionic liquids. The photophysical tuning of these hybrids are discussed in detail.

Experimental section

Materials

Zeolite A (ZA) crystals with chemical purity and high crystallinity were synthesized according to the reported procedure [15]. Ln(NO3)3⋅6H2O (Ln = Eu, Tb, Dy) were obtained by dissolving their respective oxides in concentrated nitric acid (69.2 %). Thenoyltrifluoroacetylacetone (TAA) and acetylacetone (AA) acetylacetone, sodium hydroxide, and potassium hydroxide (Aladdin) were from Aladdin. Colloidal silica (40 %, Ludox HS-40, Sigma) and tetraisopropyltitanate(Ti(OCH(CH3)2)4) from Aladdin were used without further purification. All the other chemicals were analytically pure and purchased from China National Medicines Group and used as received.

The synthesis of ionic liquid compound 1-methyl-3-propionyloxy imidazolium bromide (IM+Br−)

Highly purified 1-methyl-3-propionyloxy imidazolium bromide (IM+Br−) was synthesized according to a previously reported procedure [16]. 3-Bromopropionic acid (100 mmol) was first dissolved in 10 mL absolute ethyl alcohol, and then an equal amount of substance 1-methyl imidazole was added to the solution stirred for 8 h at 348 K. The coarse product was washed with ether by three times and then taken down on a rotary evaporator to remove excess solvent to obtain a pale yellow ionic liquid, referred to as IM+Br−. The yield was 80 %. For IM+Br−: 1H NMR (400 MHz, DMSO-d6): δ (9.25, s, 1H), δ (7.70, s, 1H), δ (7.60, s, 1H), δ (4.44, t, 2H), δ (2.92, t, 2H), δ (3.93, s, 3H).

Synthesis of four kinds of lanthanide polyoxometalates (LnW10)

Na9LnW10O36⋅32H2O ((LnW10)) can be abbreviated to LnW10 (Ln = Eu, Tb, Dy). The synthesis of POMs was prepared with the method reported by Peacock and Weakley [17]. First, 100 mmol of Na2WO4⋅2H2O was dissolved in 10 mL deionized water, the solution was heated to 358 K, and then pH was adjusted to about 7–8 with glacial acetic acid. One-millimole aqueous solution of Ln(NO3)3⋅6H2O (0.8 mL) was added to the solution dropwise by stirring. In the process of adding, a lot of white precipitate was immediately generated, and in the end, the heating and cooling to room temperature were stopped. The product was obtained by filtration and then dried under normal atmospheric conditions. The colorless crystals are LnW10 (Ln = Eu/Tb).

Preparation of TTA-Eu ⊂ ZA and AA-Tb ⊂ ZA

TTA-Eu ⊂ ZA and AA-Tb ⊂ ZA were synthesized according to the ion exchange reaction. ZA (100 mg) was stirred in 1.2 mL of a 0.05 M aqueous solution of Eu(NO3)3⋅6H2O (Tb(NO3)3⋅6H2O) for 12 h at 353 K. The product was collected by centrifugation, washed with deionized water by three times, and then dried for 5 h at 353 K under normal atmospheric conditions. The organic–inorganic hybrid materials TTA-Eu ⊂ ZA and AA-Tb ⊂ ZA were synthesized by “ship in a bottle” method. After Eu ⊂ ZA and Tb ⊂ ZA were degassed and dried at 423 K for 2 h to get rid of the solvent molecules and water molecules, it was exposed to the TTA/AA vapor at 453/393 K for 18 h. The product was washed with CH2Cl2 by three times and dried at 333 K for 4 h under vacuum.

Preparation of TTA-Eu(AA-Tb) ⊂ ZA-Ti-IM-LnW10(Ln = Eu, Tb, Dy)

IM+Br− (0.9 mmol) was first suspended in 20 mL of ethanol solution; 0.1 mmol LnW10 was then added for ion exchange and refluxed in normal atmospheric conditions at 343 K for 24 h. Ti(OCH(CH3)2)4 (0.9 mmol) and 100 mg ZA were added and refluxed at 343 K for another 6 h. Stop heating, 3.6 mmol deionized water was used to promote the hydrolysis reaction stirring at room temperature. The molar compositional ratio of the resulting gel was 1LnW10/9IM/9Ti(OCH(CH3)2)4/36H2O. The products were washed with anhydrous ethanol for three times and then dried for 10 h under vacuum.

Physical measurements

Fourier transform infrared (FTIR) spectra are measured within the 4000–400 cm−1 region on a Nexus 912 AO446 spectrophotometer with the KBr pellet technique. The X-ray powder diffraction (XRD) patterns are recorded on a Bruker D8 diffractometer (40 mA–40 kV) using monochromated Cu Ka1 radiation (k = 1.54 Å) over the 2θ range of 10°–70° and 0.6°–6°. Scanning electronic microscope (SEM) images are obtained with a Philps XL-30. Luminescence excitation and emission spectra of the solid samples are obtained on Edinburgh FLS920 spectrophotometer. The outer luminescent quantum efficiency was measured using an integrating sphere (150-mm diameter, BaSO4 coating) with the Edinburgh FLS920 phosphorimeter. The luminescence spectra were corrected for variations in the output of the excitation source and for variations in the detector response. The quantum yield was defined as the integrated intensity of the luminescence signal. Thermogravimetry (TG) data were measured on Netzsch STA 409C under nitrogen atmosphere by heating/cooling at the rate of 15 °C/min with the crucibles of Al2O3.

Results and discussion

Figure 1 shows the scheme of the composition and preparation process of multi-component hybrid system TTA-Eu(AA-Tb) ⊂ ZA-Ti-IM-LnW10. First, Eu3+(Tb3+) functionalized zeolite A (Eu/Tb ⊂ ZA) is prepared by ion exchange reaction, which is degassed and dried under vacuum. Then, it is exposed to TTA/AA gas under vacuum vapor. In this process, the coordination reaction occurs between TTA/AA and Eu3+/Tb3+ ion in ZA, resulting in the host–guest hybrid material TTA-Eu(AA-Tb) ⊂ ZA. On the other hand, LnW10 is linked to IM through the ion exchange reaction, resulting in IM-LnW10. Then, IM-LnW10 is connected to titania by the chelating reaction between propionyloxy group of IM and Ti(OCH(CH3)2)4. Finally, the final hybrids are assembled under mild conditions after hydrolysis and condensation process of Ti(OCH(CH3)2)4 and hydroxyl groups onto the surface of ZA.

Fig. 1
figure 1

The scheme for synthesis process and composition for the hybrid materials TTA-Eu(AA-Tb) ZA-Ti-IM-LnW10 (Ln = Eu, Tb, Dy)

Full size image

The morphology of ZA is the typical truncated cubes with an average dimension of 2 μm, whose structure of ZA material is visible in the scanning electron microscopy (SEM) picture in Supplementary Fig. S1 (top). After the preparation of ZA, all individual channels can be loaded with fitted dye molecules, leading to samples with obvious optical anisotropic properties on a macroscopic scale. The X-ray diffraction (XRD) pattern of ZA material is shown in Supplementary Fig. S1 (bottom), and the 2θ range of 5–70° for this measurement. Supplementary Fig. S2 shows the selected SEM image of the cylindrical structure of AA-Tb ⊂ ZA-Ti-IM-EuW10TTA-Eu ⊂ ZA-Ti-IM-EuW10 (top) and AA-Tb ⊂ ZA-Al-IM-EuW10 (bottom). The crystal framework belonging to ZA still can be observed except for it becomes irregular as pure ZA for the introduction of other functionalized components in the hybrid system.

Figure 2 exhibits the selected FTIR spectra of hybrid materials (a) TTA-Eu ⊂ ZA-Ti-IM-EuW10, (b) TTA-Eu ⊂ ZA-Ti-IM-TbW10, (c) AA-Tb ⊂ ZA-Al-IM-EuW10, and (d) AA-Tb ⊂ ZA-Al-IM-TbW10. They are dominated by the intense anti-symmetric V as (M-O, M = Si, Al, or Ti) stretching vibrations at about 1095 cm−1. The bands for the stretching and bending vibrations at 1200–400 cm−1 are corresponded to the ZA framework modes. The water molecules present in the materials are characterized by the stretching and the bending vibrations at about 3430 cm−1. Furthermore, the C = O group of organic molecules present in the sample are characterized by the stretching vibration at 1655 cm−1 and can be easily recognized.

Fig. 2
figure 2

The selected FTIR spectra of hybrid materials (a) TTA-Eu⊂ZA-Ti-IM-EuW10, (b) TTA-Eu⊂ZA-Ti-IM-TbW10, (c) AA-Tb⊂ZA-Ti-IM-EuW10, and (d) AA-Tb⊂ZA-Ti-IM-TbW10

Full size image

Figure 3 shows the luminescent excitation and emission spectra of lanthanide hybrids, for (a) TTA-Eu ⊃ ZA-Ti-IM-EuW10, (b) TTA-Eu ⊃ ZA-Ti-IM-TbW10, and (c) TTA-Eu ⊃ ZA-Ti-IM-DyW10, respectively. For TTA-Eu ⊃ ZA-Ti-IM-EuW10 hybrids in Fig. 3a, the excitation spectra of them (200∼500 nm) are obtained by monitoring the emission of Eu3+ ion at 613 nm, which shows the broad excitation band ranging from 250 to 350 nm in the ultraviolet region, which is due to the formation of charge transfer state (CTS) band of Eu-O. Another shoulder band overlapping the wide excitation is due to the π-π* electronic transitions of TTA coordinated to Eu3+. Besides, the sharp excitation peak for f-f transition of Eu3+ is observed with stronger intensity, especially the transitions 7 F0 → 5 L6 at 397 nm and 7 F0 → 5D2 at 466 nm. The corresponding emission spectral bands of the hybrids are assigned the 5D0 → 7FJ (J = 0–4) transitions at around 579, 591, 613, 622, and 700 nm under excitation at 274 nm [18]. Within the 5D0 → 7 F2 transition is the strongest emission at about 613 nm, which is different from the parent EuW10 with strong intensity of 5D0 → 7 F1 transition. The hybrid system consists of both Eu-TTA and EuW10, resulting in the different emission performance from single EuW10. But, we still can find the emission feature of EuW10, whose luminescent band is apparent for 5D0 → 7 F4 transition in Fig. 3a. Figure 3b shows the excitation and emission spectra of hybrid TTA-Eu ⊃ ZA-Ti-IM-TbW10. When the emission wavelength at 545 nm for Tb3+ is selected to measure the excitation spectrum, it presents the wide band ranging in 250∼350 nm for the CTS Eu-O. Besides, some weak excitation lines of f-f transition for Eu3+ and Tb3+ at long wavelength 350–500 nm can be observed from the excitation spectrum, among which the f-f transitions (397 nm for 7 F0 → 5 L6 and 466 nm for 7 F0 → 5D2) for Eu3+ possess the strong excitation intensity [18]. The characteristic emissions of the two lanthanide ions appear in Fig. 5. The two peaks at 490 and 544 nm are for the 5D4 → 7 F6 and 5D4 → 7 F5 transitions of Tb3+ [19]. Another two peaks at 650 and 700 nm are for 5D0 → 7 F3 and 5D0 → 7 F4 transitions of Eu3+. The remaining two emission peaks at around 590 and 618 nm are mainly from 5D0 → 7FJ (J = 0, 1, 2) transition of Eu3+, overlapped with the 5D4 → 7 F6 and 5D4 → 7 F5 transitions of Tb3+. This is clearly seen from the width and ref-shift of the emission band at maximum of 618 nm. Figure 3c shows the excitation and emission spectrum of TTA-Eu ⊃ ZA-Ti-IM-DyW10 hybrids. The excitation spectrum monitored at 615 nm shows essentially broad band centered at about 200∼450 nm. Among the wide excitation band at the range of 230–350 nm is mainly originated from the DyW10 unit and Eu-TTA to form charger transfer state (CTS) of Eu-O. Besides, some weak excitation lines of f-f transition for Eu3+ and Tb3+ at long wavelength of 350–500 nm can be observed to overlap with the excitation of host. The emission spectrum of TTA-Eu ⊃ ZA-Ti-IM-DyW10 is measured with excitation wavelength of 282 nm. The characteristic sharp bands at 485 and 575 nm, and 590, 615, 650, and 700 nm can be observed, which are ascribed to f-f transitions of Dy3+ (6H9/2–6HJ, J = 15/2, 13/2) and Eu3+ (5D0–7FJ, J = 0–4), respectively [20]. This indicates that Eu-TTA and DyW10 are both functionalized to the hybrid system.

Fig. 3
figure 3

The luminescence spectra of hybrid materials (a) TTA-Eu⊂ZA-Ti-IM-EuW10, (b) TTA-Eu⊂ZA-Ti-IM-TbW10, and (c) TTA-Eu⊂ZA-Ti-IM-DyW10

Full size image

Figure 4a shows the excitation and emission spectrum of AA-Tb ⊂ ZA-Ti-IM-EuW10 hybrids. The excitation spectrum monitored at 545 nm shows essentially broad band centered at about 200∼500 nm. Among the wide excitation band at the range of 230–320 nm is mainly originated from the EuW10 unit to form charger transfer state (CTS) of Eu-O. Another shoulder band overlapping the wide excitation is due to the π-π* electronic transitions of AA coordinated to Tb3+. Besides, some weak excitation lines of f-f transition for Eu3+ and Tb3+ at long wavelength 350–500 nm can be observed from the excitation spectrum, among which the f-f transitions (397 nm for 7 F0 → 5L6 and 466 nm for 7 F0 → 5D2) for Eu3+ possess the strong excitation intensity [17]. The emission spectrum of AA-Tb ⊂ ZA-Ti-IM-EuW10 is measured with excitation wavelength of 319 nm. The characteristic sharp bands at 489 and 545 nm, and 580, 590 (595), 615 (620), 650, and 700 nm can be observed, which are ascribed to f-f transitions of Tb3+ (5D4–7FJ, J = 6, 5) and Eu3+ (5D0–7FJ, J = 0–4), respectively [20]. This indicates that Tb-AA and EuW10 are both functionalized to the hybrid system. Among the emission bands at around 590 (595) and 615 (620) nm belong to the overlap of both Eu3+ (5D0–7 F1,2) and Tb3+ (5D4–7 F4,3) transitions. It is worth pointing out that 5D0–7 F0 transition exhibits stronger intensity than 5D0–7 F1 one, which is different the character of parent EuW10 with high 5D0 - 7 F1 emission. This may be due to two reasons: One is the introduction of other units in the hybrids, resulting in the different environment from parent EuW10. The other is the overlap from the emission to the Tb3+ (5D4–7 F4,3) transitions (5D4–7 F4,3). Figure 4b shows the excitation and emission spectra of AA-Tb ⊂ ZA-Ti-IM-TbW10 hybrids. The excitation spectrum of monitored at 545 nm shows essentially broad band centered at about 250∼450 nm. Among the wide excitation band at the range of 250–350 nm is mainly due to the absorption of TbW10 host together with π-π* electronic transitions of AA, both of which behave as the energy donor for the luminescence of Tb3+. Meanwhile, some weak sharp excitation peaks for the f-f transition of Tb3+ at long wavelength 350–450 nm can be observed. The corresponding emission spectrum of AA-Tb ⊂ ZA-Ti-IM-TbW10 is measured with excitation wavelength of 313 nm. The characteristic sharp bands at 490, 545, 590, and 620 nm can be observed, which are ascribed to f-f transitions of Tb3+ (5D4–7FJ, J = 6, 5, 4, 3). It is worth pointing out that 5D4–7 F5 transition exhibits the extremely stronger intensity than other transitions, which is the feature of TbW10 unit. The final hybrids show the green color emission.

Fig. 4
figure 4

The luminescence spectra of hybrid materials (a) AA-Tb⊂ZA-Al-IM-EuW10 and (b) AA-Tb⊂ZA-Ti-IM-TbW10

Full size image

Figure 5 shows the selected digital photos of hybrid materials TTA-Eu ⊂ ZA-Ti-IM-EuW10 (top), and AA-Tb ⊂ ZA-Al-IM-EuW10 (bottom) under UV irradiation (λex = 254 nm for left; 365 nm for right). For TTA-Eu ⊂ ZA-Ti-IM-EuW10 hybrids, under the UV irradiation of short wavelength 254 nm and long wavelength 365 nm, the color of the hybrids changes from orange-red to violet-red. This is due to the different dominant europium luminescent species. More apparent for AA-Tb ⊂ ZA-Al-IM-EuW10 hybrids, its color changes from light purple-red to light blue-green. So the selectively excitation of hybrids can realize the color tuning for the two luminescent species. Besides, the luminescent quantum yields of the five hybrid materials are determined, which are 49 % for TTA-Eu ⊂ ZA-Ti-IM-EuW10, 40 % for TTA-Eu ⊂ ZA-Ti-IM-TbW10, 25 % for TTA-Eu ⊂ ZA-Ti-IM-DyW10, 43 % for AA-Tb ⊂ ZA-Al-IM-EuW10, and 33 % for AA-Tb ⊂ ZA-Ti-IM-TbW10, respectively.

Fig. 5
figure 5

The selected digital photos of hybrid materials TTA-Eu⊂ZA-Ti-IM-EuW10 (top) and AA-Tb⊂ZA-Ti-IM-EuW10 (bottom) under UV irradiation (λex = 254 nm for left; 365 nm for right)

Full size image

Conclusions

In summary, Eu/Tb complex and lanthanide (Eu, Tb, Dy) polyometallates (LnW10) are introduced to functionalize zeolite A and titania by an inside-outside double modification path. Eu/Tb complex modified zeolite A (TTA-Eu(AA-Tb) ⊂ ZA) is achieved by ionic exchange reaction and gas dispersion. LnW10 modified titania (Ti-IM-LnW10) is obtained through ionic liquid compound as linker. Then, multi-component hybrids TTA-Eu(AA-Tb) ⊂ ZA-Ti-IM-LnW10 are assembled through condensation reaction between surface hydroxyl groups onto ZA and titania. The prepared hybrids show the red and green luminescence, which provides a useful path to obtain multi-component lanthanide hybrids.