Photochemistry and Photobiology, 2014, 90: 503–510
Photophysical Properties of Metal Ion Functionalized NaY Zeolite Tian-Wei Duan1 and Bing Yan*2 1 2
Department of Chemistry, Tongji University, Shanghai, China State Key Laboratory of Pollution Control and Resource Reuse, Tongji University, Shanghai, China
Received 13 November 2013, accepted 30 December 2013, DOI: 10.1111/php.12235
ABSTRACT
coordination environments (7). Recently, researchers focus on the potential use of optically functionalized zeolite as white light and near-IR luminescent materials, optical switching, data storage, or as uniform matrix for metal ion clusters (8–12). To get some novel property materials, it is advantageous to introduce substance of interest by ion exchange, vapor impregnation, solid state diffusion or ship-in-bottle synthesis (13–18). Our group synthesized three kinds of novel macrocylic calix[4]arene derivatives functionalized SBA-15 type of mesoporous hybrids (CalixS15, Calix-NO2-S15 and Calix-NH2-S15) by cocondensation of tetraethoxysilane (TEOS) and modified organic ligand (Calix-Si, Calix-NO2-Si and Calix-NH2-Si) in the presence of Pluronic P123 surfactant as a template. Quantum efficiencies indicate that hybrid materials functionalized different calix[4]arene derivative bridges present different luminescence behavior (19). Sun and coworkers synthesize bismuth-embedded dehydrated zeolite Y and reveal that the substructures of Bi+ residing in the sodalite cages contribute to the observed ultrabroad and tunable near-IR photoluminescence (20). Kim and coworkers report that zeoliteencapsulated PbS QDs show very high third-order nonlinear optical activities (3NLO), which can be exploited for engineering optical switches, waveguides, optical limiters and many others (21). Ion exchange method should be the conventional process to modify zeolite. Because zeolite has many cages and channels, cations are free to migrate in and out zeolite structures. The luminescence properties of rare-earth exchanged zeolite are exploited to probe microenvironment of zeolite (22–25). Rare earth has large ion size, so a hydrated ion cannot migrate from a supercage to a small sodalite to replace the residing Na+ ions. The ion exchange saturated level of La-NaY is 0.69 0.01 at 25°C. The rare earth can only replace Na+ ions in the supercage unless the temperature of exchange is raised much more higher. Higher temperature of treatment facilitates rare-earth ions going into the sodalite cages and double hexagonal prisms (26,27). Luminescence emission spectra are usually used to track the transfer of energy on solid surface from absorber to an emitter, and lifetime experiments are used to obtain the number of coordination water molecules (28). Photoluminescence phenomenon of transition metal exchanged zeolites also raises the attention of scientist because photoluminescence analysis enables the precise investigation of the local structure as well as the dispersion of metal ions as active sites (29–31). Inspired by the various luminescence phenomenon of ion exchange zeolite, we have synthesized a great diversity of ion exchanged zeolite. The obtained materials can be classified into three series including monometal ion (Eu3+, Tb3+, Ce3+, Y3+, Zn2+, Cd2+, Cu2+) exchanged zeolite, rare-earth ion (Eu3+, Tb3+,
A series of luminescent ion exchanged zeolite are synthesized by introducing various ions into NaY zeolite. Monometal ion (Eu3+, Tb3+, Ce3+, Y3+, Zn2+, Cd2+, Cu2+) exchanged zeolite, rare-earth ion (Eu3+, Tb3+, Ce3+) exchanged zeolite modified with Y3+ and rare-earth ion (Eu3+, Tb3+, Ce3+) exchanged zeolite modified with Zn2+ are discussed here. The resulting materials are characterized by Fourier transform infrared spectrum radiometer (FTIR), XRD, scanning electronic microscope (SEM), PLE, PL and luminescence lifetime measurements. The photoluminescence spectrum of NaY indicates that emission band of host matrix exhibits a blueshift of about 70 nm after monometal ion exchange process. The results show that transition metal ion exchanged zeolites possess a similar emission band due to dominant host luminescence. A variety of luminescence phenomenon of rare-earth ion broadens the application of zeolite as a luminescent host. The Eu3+ ion exchanged zeolite shows white light luminescence with a great application value and Ce3+ exchanged zeolite steadily exhibits its characteristic luminescence in ultraviolet region no matter in monometal ion exchanged zeolite or bimetal ions exchanged zeolite.
INTRODUCTION Zeolites are natural or synthetic aluminum silicate whose pores are molecular dimensions. Zeolites have a wide spread application in areas such as catalysis, gas absorption, water filtration, etc. The structures of zeolite comprise a three-directional network of [AlO4]5 and [SiO4]4 tetrahedron linked via bridging oxygen atoms (1–4). This generates different kinds of holes of negative charge density due to the presence of Al3+, and the net negative charge can be balanced by metallic cations, protons or cationic complex. Cages and channels of discrete size in the zeolite are normally occupied by water molecules. Zeolite Y shows the FAU (faujasite) structure, whose pore diameter is about 7.4 Å as the aperture is defined by a 12-member oxygen ring resulting a larger cavity diameter of 12 Å. Zeolite Y is made of secondary building units S4R, S6R and D6R (5,6). Zeolites are endowed with uniform cavities and channels, which has opened new possibilities for numerous application fields, not only in catalysis and absorption but also for the utility of host materials for photoluminescence center. One of the earliest applications of zeolite photoluminescence was to probe ions *Corresponding author email: [email protected] (Bing Yan) © 2014 The American Society of Photobiology
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Ce3+) exchanged zeolite modified with Y3+ and rare-earth ion (Eu3+, Tb3+, Ce3+) exchanged zeolite modified with Zn2+. The key to create luminescence from zeolite is to perform activation scheme, which is doping the zeolite matrix with rare-earth ions by means of aqueous ion exchange. By this means, the rare-earth ions are introduced into the cage of zeolite and replace Na+ ions, which locate in the void of zeolite as a charge compensating function. Thus, zeolite Y was chosen to be loaded with the activator species for its high ion-exchange capability and numerous hospitable cavities and sites. Rare-earth double doping of zeolite has become a very interesting topic in both experiment and theoretical luminescence phenomenon studies, especially investigation on Ce3+–Tb3+ doping and Eu3+–Tb3+ doping (8,32,33). As for zeolite modified with Y3+ or Zn2+, doping monometal ion and bimetal ions were carried out to enrich photoluminescence theoretical knowledge of ion exchanged zeolite Y. In addition, we employed XRD and IR analysis method to characterize the structure of ion exchanged zeolites. To the best of our knowledge, this is the first time that photoluminescence of various ions exchanged zeolite Y has been studied systematically.
MATERIAL AND METHODS Materials. NaY zeolite is synthesis by using NaAlO2, Ludox HS-40 and NaOH. Y(NO3)36H2O, Eu(NO3)36H2O,Tb(NO3)36H2O and Ce (NO3)36H2O were prepared by dissolving their corresponding oxides in concentrated nitric acid followed by evaporation. Zn(NO3)3 (99%), Cu (NO3)3 (99%) and Cd(NO3)3 (99%) were purchased from reagent company. Preparation of NaY zeolite. Synthesis of NaY zeolite was carried out according to a procedure reported in literature (34). The procedure of synthesis was as follows. 2.337 g NaAlO2 (0.029 mol) was added into 8 mL 1.25 mol L 1 NaOH and stirred for an hour. 9.845 g Ludox HS-40 was dissolved in 3.62 mL distilled water and was stirred for an hour. While stirring, silica solution was slowly added into the first solution and continued stirring for 5 h. At this time the solution was sealed in bombs and left at room temperature for 48 h and then at 373 K for 12 h. Obtained NaY zeolite was washed till pH = 7 with distilled water and was dried in the air thoroughly at 373 K overnight. Preparation of monometal ion exchanged zeolite MNaY (M = Eu3+, Tb3+, Ce3+, Y3+, Zn2+, Cd2+, Cu2+). Zeolite with different monometal ions was prepared by adding 0.5 g synthesis zeolite to 50 mL 0.5 mM different types of nitrate aqueous solution (Eu(NO3)36H2O, Tb (NO3)36H2O, Ce(NO3)36H2O, Y(NO3)36H2O, Zn(NO3)2, Cd(NO3)2, Cu(NO3)2). The ion-exchange process was carried out by stirring at 373 K for 24 h. Obtained monometal ion exchanged zeolite MNaY (M = Eu3+, Tb3+, Ce3+, Y3+, Zn2+, Cd2+, Cu2+) was dried in the air thoroughly at 373 K overnight. By this ion exchange, ions were introduced into zeolite pores. Hereafter, we denote the zeolite with the different types of ions as EuNaY, TbNaY, CeNaY, YNaY (the first Y means yttrium and the second Y means the type of zeolite), ZnNaY, CdNaY and CuNaY. Preparation of YNaY series zeolite. Zeolite loaded with Y3+ which is doped with rare earth was prepared by adding 0.5 g synthesized zeolite to 50 mL certain nitrite aqueous solution with component shown in Table 1. The ion exchange process was carried out by stirring at 373 K Table 1. The component of YNaY/ZnNaY* series. Materials YNaY-Eu YNaY-Tb YNaY-Ce YNaY-Eu/Tb YNaY-Ce/Tb
Doping Ion Mole Ratio Y:Eu = 25:1 Y:Tb = 25:1 Y:Ce = 25:1 Y:Eu:Tb = 25:1:1 Y:Ce:Tb = 25:1:1
*The component of ZnNaY series is as the same ion molar ratio as YNaY series.
for 24 h. Obtained YNaY series zeolites were washed till pH = 7 with distilled water and were dried in the air thoroughly at 373 K overnight. Hereafter, we denote the YNaY zeolite with the different types of rareearth ions as YNaY-Eu, YNaY-Tb, YNaY-Ce, YNaY-Eu/Tb and YNaYCe/Tb. Preparation of ZnNaY series zeolite. Zeolite loaded with Zn2+ which is doped with rare earth was prepared as follows: the zeolite powder was suspended in 50 mL of water containing the desired nitrite with component shown in Table 1. The ion exchange process was carried out by stirring at 373 K for 24 h. Obtained ZnNaY series zeolites were washed till pH = 7 with distilled water and were dried in the air thoroughly at 373 K overnight. Hereafter, we denote the ZnNaY zeolite with the different types of rare-earth ions as ZnNaY-Eu, ZnNaY-Tb, ZnNaY-Ce, ZnNaY-Eu/Tb and ZnNaY-Ce/Tb. Physical characterization. Infrared spectra were measured within KBr pellets from 4000 to 400 cm 1 using a Nexus 912 AO446 Fourier transform infrared spectrum radiometer (FTIR). X-ray powder diffraction patterns (XRD) were acquired on Rigaku D/max-Rb diffractometer equipped with Cu anode; the data were collected within the 2h range of 5–65°. Energy Dispersive Analysis by X-rays (EDAX) and scanning electronic microscope (SEM) images were obtained with a Philips XL-30. Thermogravimetric analysis (TG) was measure using a Netzsch STA 449C system at a heating rate of 5°C/min under the nitrogen protection. The diffuse reflectance UV diffusion reflection spectra of the powdered samples were recorded by a B&WTEK BWS003 spectrophotometer. Luminescence excitation spectra and emission spectra were measured on an Edinburgh FLS 920 fluorescence spectrometer. The lifetime measurements were measured on an Edinburgh Instruments FLS 920 fluorescence spectrometer.
RESULTS AND DISCUSSION The metal ions exchanged zeolites are conveniently prepared through suspending zeolite into certain nitrate aqueous solution, which is outlined in Scheme 1. The concentration of metal nitrate solution is relatively low since the limitation of saturate adsorption capacity of metal ions. In studies of the location of rare-earth exchanged NaY zeolite (35,36), it has been reported that the lanthanide ions are initially introduced at the more accessible SIII type, but during calcination at high temperatures a considerable fraction of these lanthanides migrates to SI and SII sites. And then, by the exchange process between the metal ions and synthesized NaY zeolite, we succeeded to exchange various ions into zeolite. This has been confirmed by the XRD and luminescence analysis of materials. In the scanning electron microscope, octahedral crystal shapes were found as shown in Figure 1. It can be seen that most of the crystals are octahedral NaY zeolite nanocrystals with a small amount of gmelinite as impurity. Meanwhile, NaY zeolite presents regular and uniform structure with the diameter of about 250 nm. The component of synthesized zeolite has also been confirmed by XRD patterns (Supporting Information Fig. S1). The morphology of zeolite NaY crystals and their size is largely dependent on the preparation conditions such as temperature, time and the composition and molar ratio of the starting gels. The time-dependent experiment is carried out to study the morphology transformation of zeolite (Supporting Information Fig. S1). If we prolong synthesis time, the morphology of zeolite transforms from zeolite NaY to gmelinite. Figure 2 shows the FTIR spectra of MNaY (M = Eu3+, Ce3+, Tb3+). The 729 cm 1 band in the IR spectra is due to the T-O (T = Si, Al) symmetric stretching vibration, whereas the strong band at 1034 cm 1 and medium band at 1124 cm 1 can be assigned to T-O asymmetric stretching. As depicted in Figure 2, peaks centered at 1653 and 3470 cm 1 can be assigned to bending vibration of water molecules located inside zeolite framework
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Scheme 1. Synthesis of the rare-earth ion exchanged zeolite: ion exchange process to introduce rare-earth ions into NaY zeolite under the condition of 373 K. The rare-earth ions are mainly on the SIII site because of relatively low heating treatment temperature.
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Figure 1. Selected scanning electronic microscope (SEM) images of synthesis NaY zeolite.
and O–H stretching vibration. Because of the clearly unique framework vibration peaks, it is suggested that the structure of zeolite is undamaged after ion exchange. Moreover, the peaks in XRD (Fig. 3) provide a positive evidence for the crystalline structure of NaY zeolite. Figure 3 depicts the XRD patterns of ion exchanged zeolite MNaY (M = Ce3+, Eu3+, Tb3+, Cd2+, Cu2+, Y3+, Zn2+).When ion exchanged is performed at low temperature and low pressure, the exchanged ions are located in the b supercage of NaY zeolite, which includes (1,1,1) crystal plane, since it is the maximum ion exchange window of NaY zeolite. The intensity of (1,1,1) crystal plane of rare-earth ion exchanged zeolite is relatively low than that of transition metal ion exchanged zeolite, mainly
because the ion radius of rare earth is much larger than that of transition metal ion, which leads to partly inevitable damage on (1,1,1) plane. However, the structure of NaY zeolite still exists with other peaks clearly as shown in XRD patterns in Figure 3. As for YNaY and ZnNaY series exchanged zeolite, the structure of exchanged zeolite is still in crystalline state and it is strongly supported by XRD characterize (Supporting Information Fig. S2). NaY Zeolite exhibits an emission broad peak centered at 550 nm when it is excited by the wavelength of 369 nm (Supporting Information Fig. S3). It is probably because defect in crystal lattice yields the contribution to the emission of zeolite matrix. In the excited of [AlO4]5 and [SiO4]4 , the hole (on oxygen) and the electron may remain together generating luminescence. When Na+ ions located in supercage are exchanged by rare-earth ions, the emission spectrum is largely influenced by rare-earth electrons transition among various energy levels. Figure 4A shows the excitation and emission spectra of EuNaY zeolite. The excitation spectrum obtained by monitoring the characteristic emission of europium ion at 614 nm shows that a broad band in the low wavelength of ultraviolet (240–300 nm) is
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mainly attribute to charge transfer state transition (CTS). It means the electrons transit from 2p shell of O2 to 4f shell of Eu3+. The peaks located at 300–500 nm are due to 4f–4f transition of Eu3+. The europium emission in EuNaY following laser excitation at 394 nm contains the 5D0 ? 7FJ (J = 0, 1, 2, 3, 4) transition at 579, 590, 614, 653 and 698 nm and also includes 5 D5 ? 7FJ transition (J = 0, 1, 2) at 524, 535 and 555 nm. The broad band around 478 nm due to the emission of excited zeolite structure is also observed in the EuNaY zeolite emission spectrum. As we know, the intensity ratio of 5D0 ? 7F2 to 5D0 ? 7 F1 is sensitive to the symmetry around Eu3+ ion and gives valuable information about the chemical microenvironment change of anions coordinating the Eu3+ ion. Among these emission peaks of the EuNaY, the most striking red fluorescence (5D0 ? 7F2) of the electric dipole transition at about 614 nm is little stronger than the orange emission intensities of magnetic dipole transition of 5D0 ? 7F1 at about 589 nm, which indicates that the Eu3+ site is situated in an environment with higher symmetry. The CeNaY zeolite excitation spectrum was obtained by monitoring the characteristic emission of cerium trivalent ion at 350 nm and is presented on the left of the Figure 4B. The obtained excitation spectrum shows that the asymmetry broad band at 293 nm is mainly attribute to the transition energy absorbance of 2F5/2 ground state to 2DJ excited levels. The cerium emission in CeNaY following excitation at 293 nm contains a broad band at 350 nm due to the transition of 5d ? 2F5/2 and 5d ? 2F7/2 ground states, which cannot be distinguished directly. Achieved by monitoring the characteristic emission of terbium ion at 545 nm, the TbNaY zeolite excitation spectrum is presented on the left of the Figure 4C. The obtained excitation spectrum shows that the broad band at 240–300 nm is mainly attribute to CTS, which means the peaks at 300–500 nm are due to 4f–4f transition of Tb3+. The terbium emission in TbNaY following excitation at 368 nm contains the 0D4 ? 7FJ (J = 3, 4, 5, 6) transition at about 621, 583, 545 and 489 nm. The broad band around 437 nm due to the emission of excited zeolite structure is also observed in the TbNaY zeolite emission spectrum. The emission spectrum of rare-earth ion exchanged zeolites reveals that the characteristic rare-earth emission peaks are generated by its nature 4f (or 5d) energy level transition, but not by the
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Figure 3. XRD of MNaY (M = Eu3+, Tb3+, Ce3+, Y3+, Zn2+, Cd2+, Cu2+) zeolite and NaY zeolite.
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Wavelength(nm) Figure 4. Excitation and emission spectrum of rare-earth ion (Eu3+, Ce3+, Tb3+) exchanged NaY. (A) EuNaY zeolite excitation spectrum is obtained by monitoring the emission of Eu3+ at 614 nm, and the excitation wavelength for the emission spectrum is 394 nm. (B) CeNaY zeolite excitation spectrum is obtained by monitoring the emission of Ce3+ at 350 nm, and the excitation wavelength for the emission spectrum is 293 nm. (C) TbNaY zeolite excitation spectrum is obtained by monitoring the emission of Tb3+ at 545 nm, and the excitation wavelength for the emission spectrum is 368 nm.
Photochemistry and Photobiology, 2014, 90
Figure 5. CIE x–y chromaticity diagram of rare-earth ion (Eu3+, Ce3+, Tb3+) exchanged NaY zeolite: (A) EuNaY; (B) CeNaY; (C) TbNaY.
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excitation band of zeolite. It is likely because the transition energy level of rare earth and defect energy level of zeolite is not match. We can get various photoluminescence colors, as illustrated in the CIE x–y chromaticity diagram of the multicolored photoluminescence of the samples (Fig. 5). It is remarkable that EuNaY exhibits white light which shows strong applied value in luminescent devices. The emission spectra of MNaY (M = Y3+, Cd2+, Cu2+, Zn2+) zeolite vary in different ions (Supporting Information Fig. S4). Because ion M (M = Y3+, Cd2+, Cu2+, Zn2+) is not conventional photoluminescence ion, the emission spectra of MNaY show the luminescence of matrices with broad peak unlike rare-earth ion photoluminescence with sharp peaks. Compared to the maximum emission wavelength of NaY zeolite around 550 nm, the maximum emission wavelength of MNaY (M = Y3+, Cd2+, Cu2+, Zn2+) zeolite is about 475 nm. The blueshift is likely due to the exchanged ion located in b supercage of NaY zeolite stabilized silicon–oxygen tetrahedron and aluminum–oxygen tetrahedron causing higher excitation energy of MNaY (M = Y3+, Cd2+, Cu2+, Zn2+) zeolite. The intensity of MNaY (M = Y3+, Cd2+, Cu2+, Zn2+) zeolite at 475 nm is YNaY > CdNaY > CuNaY > ZnNaY > NaY when the slit of fluorescence spectrometer is fixed. The increasing of emission peak intensity is probably because exchanged ions cause the lattice distortion increase the degree of defect enhancing the luminescence. The excitation and emission spectra for the obtained YNaYEu are given in Figure 6A. The excitation spectrum of YNaY-Eu exhibits a similar pattern as EuNaY. As shown in Figure 6A, a band ranging from 250 to 300 nm and narrow peak centered at 395 nm are observed, and the former band is attributed to CTS and the latter narrow peak is ascribe to the absorption of 4f–4f electron transition of Eu3+. The emission spectrum of YNaY-Eu contains a broad band ranging centered at 473 nm. Lines of YNaY-Eu are distributed mainly in the 575–725 nm range, which are assigned to 5D0 ? 7FJ (J = 1, 2, 4) transition at about 591, 614 and 598 nm, respectively. YNaY-Eu zeolite shows
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Wavelength(nm) Figure 6. Excitation and emission spectra of rare-earth ion (Eu3+, Ce3+) exchanged NaY modified with Y3+. (A) YNaY-Eu zeolite excitation spectrum is obtained by monitoring the emission of Eu3+ at 614 nm, and the excitation wavelength for the emission spectrum is 394 nm. (B) YNaY-Ce zeolite excitation spectrum is obtained by monitoring the emission of Ce3+ at 330 nm, and the excitation wavelength for the emission spectrum is 270 nm.
relatively weak Eu3+ ion emission due to low doping rate, so the emission spectrum depicts mainly the emission of ion exchanged zeolite matrix with the blueshift phenomenon compared to pure NaY zeolite emission. On account of the intensity ratios I (5D0?7F2)/I(5D0?7F1) (I02/I01) can be seen as an indicator for the local environment of ions, we concluded that the chemical environment around the europium ions is in high symmetry according to the intensity ratio listed in Table 2. The excitation spectrum of ZnNaY-Eu possesses similar emission spectrum as YNaY-Eu (Supporting Information Fig. S5A). Figure 6B depicts the excitation and emission spectra of the YNaY-Ce prepared by monitoring the characteristic emission of cerium trivalent ion at 330 nm. Excitation spectrum of YNaY-Ce reveals a broad band with doublet peaks located in 240–320 nm wavelength range, which is generated by 2F5/2 ? 2DJ transition. The emission spectrum of YNaY-Ce reveals the typical light emission of Ce3+ ions. The light emission of Ce3+ ions is result from two probable electron decay transition due to a split ground state (2F5/2, 2F7/2). In the same way, the excitation spectrum of ZnNaY-Ce is shown in
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Table 2. The luminescent efficiencies and lifetime data for ion exchanged zeolites.
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Figure S5B by monitoring the characteristic emission of Ce3+ ions at 345 nm. Excitation spectrum of ZnNaY-Ce consists of a broad band located in 240–320 nm wavelength range, which is generated by 2F5/2 ? 2DJ transition. The emission spectrum of ZnNaY-Ce reveals the typical light emission of Ce3+ ions at about 345 nm. Tb3+ exchanged YNaY zeolite cannot exhibit the characteristic peak of Tb3+ by using the excitation wavelength of 368 nm, so does Tb3+ exchanged ZnNaY zeolite. However, by using the CTS band wavelength of 280 nm as the excitation wavelength, we can get the luminescence of Tb3+ in YNaY and ZnNaY (Supporting Information Fig. S6). The excitation and emission spectra of YNaY zeolite with double doped rare earth (Eu3+ and Tb3+) are depicted in Figure 7A. It is shown that the excitation spectra of Eu3+ and Tb3+ cover the same CTS band (250–330 nm) by monitoring the 614 and 545 nm. The emission spectrum exhibits not only the luminescence of Eu3+ but also Tb3+ under the condition of using the excitation wavelength at 318 nm. The emission lines are assigned to 5D0 ? 7FJ (J = 5, 6) and 0D5 ? 7FJ (J = 1, 2) transition at about 545, 489 nm and 591, 614 nm, respectively. In the same way, the excitation spectrum of YNaY-Ce/Tb is shown in Figure 7B by monitoring the characteristic emission of cerium trivalent ion at 350 nm. Excitation spectrum of YNaY-Ce consists of a broad band centered at 279 nm, which is generated by 2F5/2 ? 2DJ transition. The emission spectrum of YNaY-Ce/Tb mainly reveals the typical light emission of Ce3+ ions at about 348 nm, and the weak luminescence of Tb3+ at 548 nm and 545 nm because the CTS of Tb3+ is overlapped with the excitation spectrum of Ce3+. There exists energy transfer between Ce3+ and Tb3+ intra YNaY because the lifetime of Ce3+ in YNaY-Ce/Tb is extremely low compared to YNaY-Ce (Table 2). It is concluded that Tb3+ ions absorb light and transfer to Ce3+ ions. As for double rare-earth ions exchanged zeolite of ZnNaY series, it possesses the similar spectra as YNaY series (Supporting Information Fig. S7). The energy transfer is also observed between Ce3+ and Tb3+ in ZnNaY (Table 2). Figure S7 illustrates excitation and emission spectra of ZnNaY series. The maximum excitation wavelength of Ce′NaY zeolite (amount as Ce equivalent weight in ZnNaY-Ce′ or
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Wavelength(nm) Figure 7. Excitation and emission spectra of YNaY-Eu/Tb and YNaYCe/Tb (A) The excitation and emission spectrum of YNaY-Eu/Tb (a) the excitation spectrum is obtained by monitoring the emission of Tb3+ at 545 nm (b) the excitation spectrum is obtained by monitoring the emission of Eu3+ at 614 nm (c) the emission spectrum is obtained by using excitation wavelength at 318 nm. (B) YNaY-Ce/Tb zeolite excitation spectrum is obtained by monitoring the emission of Ce3+ at 350 nm, and the excitation wavelength for the emission spectrum is 280 nm.
ZnNaY-Ce/RE (RE = Eu3+, Tb3+, Y3+)) is about 273 nm and the maximum emission wavelength is about 342 nm. When ZnNaY is doped with Ce, the emission intensity of ZnNaY-Ce′ is higher than Ce′NaY. ZnNaY is double doped with Ce and other rareearth ion; however, the emission intensity is much higher than CeNaY. The lifetime measure indicated that there is energy transfer between Ce and Eu or Ce and Tb intra ZnNaY zeolite, which can be preliminary conformed by comparing the decay time among ZnNaY-Ce′, ZnNaY-Eu/Tb and ZnNaY-Ce/Tb. It is likely responding to Eu3+ or Tb3+ acting as sensitizer and Ce3+ acting as activator. For further investigation of the photoluminescence properties, we measure the luminescence lifetime decay curves of the ion exchanged zeolite at room temperature, and lifetimes are given in Tables 2 and S1.
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CONCLUSIONS We have designed and synthesized a series of ion exchanged zeolite by introducing various ions into zeolite. The photoluminescence measurements indicate that pure NaY zeolite with the emission wavelength centered at 550 nm. After ion exchange process, it exhibits a blueshift of about 70 nm by excitation wavelength of 394 nm. Rare-earth (Eu3+, Tb3+, Ce3+) exchanged zeolite exhibits its characteristic luminescence generated by its nature 4f (or 5d) energy level transition. Cerium exchanged zeolite always exhibits its characteristic luminescence no matter in monometal ion exchanged zeolite or bimetal ion exchanged zeolite. There exists energy transfer between Ce3+ and Tb3+ in zeolite modified with Y3+ or Zn2+. The finding principle may be applied to synthesis other luminescence material based on ion exchanged zeolite. Further efforts may concern on studying ions distribution and energy transfer in bimetal ions exchanged zeolite. Acknowledgements—This work was supported by the National Natural Science Foundation of China (20971100, 91122003) and Program for New Century Excellent Talents in University (NCET-08-0398).
SUPPORTING INFORMATION Additional Supporting Information may be found in the online version of this article: Figure S1. XRD patterns of zeolite at different synthesis times. Figure S2. XRD patterns of metal ion exchanged ZnNaY series and metal ion exchanged YNaY series. Figure S3. Emission spectra of MNaY (M = Y3+, Cd2+, 2+ Cu , Zn2+) zeolite. The excitation wavelength for the emission spectra is 394 nm. Figure S4. Excitation and emission spectra of ZnNaY-Eu and ZnNaY-Ce. (A) ZnNaY-Eu zeolite excitation spectra are obtained by monitoring the emission of Eu3+ at 614 nm, and the excitation wavelength for the emission spectra is 394 nm. (B) ZnNaYCe zeolite excitation spectra are obtained by monitoring the emission of Ce3+ at 345 nm, and the excitation wavelength for the emission spectra is 273 nm. Figure S5. Excitation and emission spectra of YNaY- Tb and ZnNaY-Tb. (A) YNaY-Tb zeolite excitation spectra are obtained by monitoring the emission of Tb3+ at 545 nm, and the excitation wavelength for the emission spectra is 320 nm. (B) ZnNaYTb zeolite excitation spectra are obtained by monitoring the emission of Tb3+ at 545 nm, and the excitation wavelength for the emission spectra is 320 nm. Figure S6. Excitation and emission spectra of ZnNaY-Eu/Tb and ZnNaY-Ce/Tb. (A) The excitation and emission spectra of ZnNaY-Eu/Tb; (a) the excitation spectra are obtained by monitoring the emission of Tb3+ at 545 nm, (b) the excitation spectra are obtained by monitoring the emission of Eu3+ at 614 nm, (c) the emission spectra are obtained by using excitation wavelength at 318 nm. (B) ZnNaY-Ce/Tb zeolite excitation spectra are obtained by monitoring the emission of Ce3+ at 350 nm, and the excitation wavelength for the emission spectra is 280 nm. Figure S7. Excitation and emission spectra of ZnNaY series. All the excitation spectra are obtained by monitoring the emission of Ce3+ at 350 nm, and the excitation wavelength for the emission spectra is 274 nm. Ce3+ doped in Ce′NaY is the similar
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amount as in ZnNaY-Ce, ZnNaY-Ce/Y, ZnNaY-Ce/Eu, ZnNaYCe/Tb. Table S1. The luminescent efficiencies and lifetimes for comparing Ce3+ luminescent phenomenon in ZnNaY series exchanged zeolite.
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Photophysical Properties of Metal Ion Functionalized NaY Zeolite Tian-Wei Duan1 and Bing Yan*2 1 2
Department of Chemistry, Tongji University, Shanghai, China State Key Laboratory of Pollution Control and Resource Reuse, Tongji University, Shanghai, China
Received 13 November 2013, accepted 30 December 2013, DOI: 10.1111/php.12235
ABSTRACT
coordination environments (7). Recently, researchers focus on the potential use of optically functionalized zeolite as white light and near-IR luminescent materials, optical switching, data storage, or as uniform matrix for metal ion clusters (8–12). To get some novel property materials, it is advantageous to introduce substance of interest by ion exchange, vapor impregnation, solid state diffusion or ship-in-bottle synthesis (13–18). Our group synthesized three kinds of novel macrocylic calix[4]arene derivatives functionalized SBA-15 type of mesoporous hybrids (CalixS15, Calix-NO2-S15 and Calix-NH2-S15) by cocondensation of tetraethoxysilane (TEOS) and modified organic ligand (Calix-Si, Calix-NO2-Si and Calix-NH2-Si) in the presence of Pluronic P123 surfactant as a template. Quantum efficiencies indicate that hybrid materials functionalized different calix[4]arene derivative bridges present different luminescence behavior (19). Sun and coworkers synthesize bismuth-embedded dehydrated zeolite Y and reveal that the substructures of Bi+ residing in the sodalite cages contribute to the observed ultrabroad and tunable near-IR photoluminescence (20). Kim and coworkers report that zeoliteencapsulated PbS QDs show very high third-order nonlinear optical activities (3NLO), which can be exploited for engineering optical switches, waveguides, optical limiters and many others (21). Ion exchange method should be the conventional process to modify zeolite. Because zeolite has many cages and channels, cations are free to migrate in and out zeolite structures. The luminescence properties of rare-earth exchanged zeolite are exploited to probe microenvironment of zeolite (22–25). Rare earth has large ion size, so a hydrated ion cannot migrate from a supercage to a small sodalite to replace the residing Na+ ions. The ion exchange saturated level of La-NaY is 0.69 0.01 at 25°C. The rare earth can only replace Na+ ions in the supercage unless the temperature of exchange is raised much more higher. Higher temperature of treatment facilitates rare-earth ions going into the sodalite cages and double hexagonal prisms (26,27). Luminescence emission spectra are usually used to track the transfer of energy on solid surface from absorber to an emitter, and lifetime experiments are used to obtain the number of coordination water molecules (28). Photoluminescence phenomenon of transition metal exchanged zeolites also raises the attention of scientist because photoluminescence analysis enables the precise investigation of the local structure as well as the dispersion of metal ions as active sites (29–31). Inspired by the various luminescence phenomenon of ion exchange zeolite, we have synthesized a great diversity of ion exchanged zeolite. The obtained materials can be classified into three series including monometal ion (Eu3+, Tb3+, Ce3+, Y3+, Zn2+, Cd2+, Cu2+) exchanged zeolite, rare-earth ion (Eu3+, Tb3+,
A series of luminescent ion exchanged zeolite are synthesized by introducing various ions into NaY zeolite. Monometal ion (Eu3+, Tb3+, Ce3+, Y3+, Zn2+, Cd2+, Cu2+) exchanged zeolite, rare-earth ion (Eu3+, Tb3+, Ce3+) exchanged zeolite modified with Y3+ and rare-earth ion (Eu3+, Tb3+, Ce3+) exchanged zeolite modified with Zn2+ are discussed here. The resulting materials are characterized by Fourier transform infrared spectrum radiometer (FTIR), XRD, scanning electronic microscope (SEM), PLE, PL and luminescence lifetime measurements. The photoluminescence spectrum of NaY indicates that emission band of host matrix exhibits a blueshift of about 70 nm after monometal ion exchange process. The results show that transition metal ion exchanged zeolites possess a similar emission band due to dominant host luminescence. A variety of luminescence phenomenon of rare-earth ion broadens the application of zeolite as a luminescent host. The Eu3+ ion exchanged zeolite shows white light luminescence with a great application value and Ce3+ exchanged zeolite steadily exhibits its characteristic luminescence in ultraviolet region no matter in monometal ion exchanged zeolite or bimetal ions exchanged zeolite.
INTRODUCTION Zeolites are natural or synthetic aluminum silicate whose pores are molecular dimensions. Zeolites have a wide spread application in areas such as catalysis, gas absorption, water filtration, etc. The structures of zeolite comprise a three-directional network of [AlO4]5 and [SiO4]4 tetrahedron linked via bridging oxygen atoms (1–4). This generates different kinds of holes of negative charge density due to the presence of Al3+, and the net negative charge can be balanced by metallic cations, protons or cationic complex. Cages and channels of discrete size in the zeolite are normally occupied by water molecules. Zeolite Y shows the FAU (faujasite) structure, whose pore diameter is about 7.4 Å as the aperture is defined by a 12-member oxygen ring resulting a larger cavity diameter of 12 Å. Zeolite Y is made of secondary building units S4R, S6R and D6R (5,6). Zeolites are endowed with uniform cavities and channels, which has opened new possibilities for numerous application fields, not only in catalysis and absorption but also for the utility of host materials for photoluminescence center. One of the earliest applications of zeolite photoluminescence was to probe ions *Corresponding author email: [email protected] (Bing Yan) © 2014 The American Society of Photobiology
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Ce3+) exchanged zeolite modified with Y3+ and rare-earth ion (Eu3+, Tb3+, Ce3+) exchanged zeolite modified with Zn2+. The key to create luminescence from zeolite is to perform activation scheme, which is doping the zeolite matrix with rare-earth ions by means of aqueous ion exchange. By this means, the rare-earth ions are introduced into the cage of zeolite and replace Na+ ions, which locate in the void of zeolite as a charge compensating function. Thus, zeolite Y was chosen to be loaded with the activator species for its high ion-exchange capability and numerous hospitable cavities and sites. Rare-earth double doping of zeolite has become a very interesting topic in both experiment and theoretical luminescence phenomenon studies, especially investigation on Ce3+–Tb3+ doping and Eu3+–Tb3+ doping (8,32,33). As for zeolite modified with Y3+ or Zn2+, doping monometal ion and bimetal ions were carried out to enrich photoluminescence theoretical knowledge of ion exchanged zeolite Y. In addition, we employed XRD and IR analysis method to characterize the structure of ion exchanged zeolites. To the best of our knowledge, this is the first time that photoluminescence of various ions exchanged zeolite Y has been studied systematically.
MATERIAL AND METHODS Materials. NaY zeolite is synthesis by using NaAlO2, Ludox HS-40 and NaOH. Y(NO3)36H2O, Eu(NO3)36H2O,Tb(NO3)36H2O and Ce (NO3)36H2O were prepared by dissolving their corresponding oxides in concentrated nitric acid followed by evaporation. Zn(NO3)3 (99%), Cu (NO3)3 (99%) and Cd(NO3)3 (99%) were purchased from reagent company. Preparation of NaY zeolite. Synthesis of NaY zeolite was carried out according to a procedure reported in literature (34). The procedure of synthesis was as follows. 2.337 g NaAlO2 (0.029 mol) was added into 8 mL 1.25 mol L 1 NaOH and stirred for an hour. 9.845 g Ludox HS-40 was dissolved in 3.62 mL distilled water and was stirred for an hour. While stirring, silica solution was slowly added into the first solution and continued stirring for 5 h. At this time the solution was sealed in bombs and left at room temperature for 48 h and then at 373 K for 12 h. Obtained NaY zeolite was washed till pH = 7 with distilled water and was dried in the air thoroughly at 373 K overnight. Preparation of monometal ion exchanged zeolite MNaY (M = Eu3+, Tb3+, Ce3+, Y3+, Zn2+, Cd2+, Cu2+). Zeolite with different monometal ions was prepared by adding 0.5 g synthesis zeolite to 50 mL 0.5 mM different types of nitrate aqueous solution (Eu(NO3)36H2O, Tb (NO3)36H2O, Ce(NO3)36H2O, Y(NO3)36H2O, Zn(NO3)2, Cd(NO3)2, Cu(NO3)2). The ion-exchange process was carried out by stirring at 373 K for 24 h. Obtained monometal ion exchanged zeolite MNaY (M = Eu3+, Tb3+, Ce3+, Y3+, Zn2+, Cd2+, Cu2+) was dried in the air thoroughly at 373 K overnight. By this ion exchange, ions were introduced into zeolite pores. Hereafter, we denote the zeolite with the different types of ions as EuNaY, TbNaY, CeNaY, YNaY (the first Y means yttrium and the second Y means the type of zeolite), ZnNaY, CdNaY and CuNaY. Preparation of YNaY series zeolite. Zeolite loaded with Y3+ which is doped with rare earth was prepared by adding 0.5 g synthesized zeolite to 50 mL certain nitrite aqueous solution with component shown in Table 1. The ion exchange process was carried out by stirring at 373 K Table 1. The component of YNaY/ZnNaY* series. Materials YNaY-Eu YNaY-Tb YNaY-Ce YNaY-Eu/Tb YNaY-Ce/Tb
Doping Ion Mole Ratio Y:Eu = 25:1 Y:Tb = 25:1 Y:Ce = 25:1 Y:Eu:Tb = 25:1:1 Y:Ce:Tb = 25:1:1
*The component of ZnNaY series is as the same ion molar ratio as YNaY series.
for 24 h. Obtained YNaY series zeolites were washed till pH = 7 with distilled water and were dried in the air thoroughly at 373 K overnight. Hereafter, we denote the YNaY zeolite with the different types of rareearth ions as YNaY-Eu, YNaY-Tb, YNaY-Ce, YNaY-Eu/Tb and YNaYCe/Tb. Preparation of ZnNaY series zeolite. Zeolite loaded with Zn2+ which is doped with rare earth was prepared as follows: the zeolite powder was suspended in 50 mL of water containing the desired nitrite with component shown in Table 1. The ion exchange process was carried out by stirring at 373 K for 24 h. Obtained ZnNaY series zeolites were washed till pH = 7 with distilled water and were dried in the air thoroughly at 373 K overnight. Hereafter, we denote the ZnNaY zeolite with the different types of rare-earth ions as ZnNaY-Eu, ZnNaY-Tb, ZnNaY-Ce, ZnNaY-Eu/Tb and ZnNaY-Ce/Tb. Physical characterization. Infrared spectra were measured within KBr pellets from 4000 to 400 cm 1 using a Nexus 912 AO446 Fourier transform infrared spectrum radiometer (FTIR). X-ray powder diffraction patterns (XRD) were acquired on Rigaku D/max-Rb diffractometer equipped with Cu anode; the data were collected within the 2h range of 5–65°. Energy Dispersive Analysis by X-rays (EDAX) and scanning electronic microscope (SEM) images were obtained with a Philips XL-30. Thermogravimetric analysis (TG) was measure using a Netzsch STA 449C system at a heating rate of 5°C/min under the nitrogen protection. The diffuse reflectance UV diffusion reflection spectra of the powdered samples were recorded by a B&WTEK BWS003 spectrophotometer. Luminescence excitation spectra and emission spectra were measured on an Edinburgh FLS 920 fluorescence spectrometer. The lifetime measurements were measured on an Edinburgh Instruments FLS 920 fluorescence spectrometer.
RESULTS AND DISCUSSION The metal ions exchanged zeolites are conveniently prepared through suspending zeolite into certain nitrate aqueous solution, which is outlined in Scheme 1. The concentration of metal nitrate solution is relatively low since the limitation of saturate adsorption capacity of metal ions. In studies of the location of rare-earth exchanged NaY zeolite (35,36), it has been reported that the lanthanide ions are initially introduced at the more accessible SIII type, but during calcination at high temperatures a considerable fraction of these lanthanides migrates to SI and SII sites. And then, by the exchange process between the metal ions and synthesized NaY zeolite, we succeeded to exchange various ions into zeolite. This has been confirmed by the XRD and luminescence analysis of materials. In the scanning electron microscope, octahedral crystal shapes were found as shown in Figure 1. It can be seen that most of the crystals are octahedral NaY zeolite nanocrystals with a small amount of gmelinite as impurity. Meanwhile, NaY zeolite presents regular and uniform structure with the diameter of about 250 nm. The component of synthesized zeolite has also been confirmed by XRD patterns (Supporting Information Fig. S1). The morphology of zeolite NaY crystals and their size is largely dependent on the preparation conditions such as temperature, time and the composition and molar ratio of the starting gels. The time-dependent experiment is carried out to study the morphology transformation of zeolite (Supporting Information Fig. S1). If we prolong synthesis time, the morphology of zeolite transforms from zeolite NaY to gmelinite. Figure 2 shows the FTIR spectra of MNaY (M = Eu3+, Ce3+, Tb3+). The 729 cm 1 band in the IR spectra is due to the T-O (T = Si, Al) symmetric stretching vibration, whereas the strong band at 1034 cm 1 and medium band at 1124 cm 1 can be assigned to T-O asymmetric stretching. As depicted in Figure 2, peaks centered at 1653 and 3470 cm 1 can be assigned to bending vibration of water molecules located inside zeolite framework
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Scheme 1. Synthesis of the rare-earth ion exchanged zeolite: ion exchange process to introduce rare-earth ions into NaY zeolite under the condition of 373 K. The rare-earth ions are mainly on the SIII site because of relatively low heating treatment temperature.
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Figure 1. Selected scanning electronic microscope (SEM) images of synthesis NaY zeolite.
and O–H stretching vibration. Because of the clearly unique framework vibration peaks, it is suggested that the structure of zeolite is undamaged after ion exchange. Moreover, the peaks in XRD (Fig. 3) provide a positive evidence for the crystalline structure of NaY zeolite. Figure 3 depicts the XRD patterns of ion exchanged zeolite MNaY (M = Ce3+, Eu3+, Tb3+, Cd2+, Cu2+, Y3+, Zn2+).When ion exchanged is performed at low temperature and low pressure, the exchanged ions are located in the b supercage of NaY zeolite, which includes (1,1,1) crystal plane, since it is the maximum ion exchange window of NaY zeolite. The intensity of (1,1,1) crystal plane of rare-earth ion exchanged zeolite is relatively low than that of transition metal ion exchanged zeolite, mainly
because the ion radius of rare earth is much larger than that of transition metal ion, which leads to partly inevitable damage on (1,1,1) plane. However, the structure of NaY zeolite still exists with other peaks clearly as shown in XRD patterns in Figure 3. As for YNaY and ZnNaY series exchanged zeolite, the structure of exchanged zeolite is still in crystalline state and it is strongly supported by XRD characterize (Supporting Information Fig. S2). NaY Zeolite exhibits an emission broad peak centered at 550 nm when it is excited by the wavelength of 369 nm (Supporting Information Fig. S3). It is probably because defect in crystal lattice yields the contribution to the emission of zeolite matrix. In the excited of [AlO4]5 and [SiO4]4 , the hole (on oxygen) and the electron may remain together generating luminescence. When Na+ ions located in supercage are exchanged by rare-earth ions, the emission spectrum is largely influenced by rare-earth electrons transition among various energy levels. Figure 4A shows the excitation and emission spectra of EuNaY zeolite. The excitation spectrum obtained by monitoring the characteristic emission of europium ion at 614 nm shows that a broad band in the low wavelength of ultraviolet (240–300 nm) is
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mainly attribute to charge transfer state transition (CTS). It means the electrons transit from 2p shell of O2 to 4f shell of Eu3+. The peaks located at 300–500 nm are due to 4f–4f transition of Eu3+. The europium emission in EuNaY following laser excitation at 394 nm contains the 5D0 ? 7FJ (J = 0, 1, 2, 3, 4) transition at 579, 590, 614, 653 and 698 nm and also includes 5 D5 ? 7FJ transition (J = 0, 1, 2) at 524, 535 and 555 nm. The broad band around 478 nm due to the emission of excited zeolite structure is also observed in the EuNaY zeolite emission spectrum. As we know, the intensity ratio of 5D0 ? 7F2 to 5D0 ? 7 F1 is sensitive to the symmetry around Eu3+ ion and gives valuable information about the chemical microenvironment change of anions coordinating the Eu3+ ion. Among these emission peaks of the EuNaY, the most striking red fluorescence (5D0 ? 7F2) of the electric dipole transition at about 614 nm is little stronger than the orange emission intensities of magnetic dipole transition of 5D0 ? 7F1 at about 589 nm, which indicates that the Eu3+ site is situated in an environment with higher symmetry. The CeNaY zeolite excitation spectrum was obtained by monitoring the characteristic emission of cerium trivalent ion at 350 nm and is presented on the left of the Figure 4B. The obtained excitation spectrum shows that the asymmetry broad band at 293 nm is mainly attribute to the transition energy absorbance of 2F5/2 ground state to 2DJ excited levels. The cerium emission in CeNaY following excitation at 293 nm contains a broad band at 350 nm due to the transition of 5d ? 2F5/2 and 5d ? 2F7/2 ground states, which cannot be distinguished directly. Achieved by monitoring the characteristic emission of terbium ion at 545 nm, the TbNaY zeolite excitation spectrum is presented on the left of the Figure 4C. The obtained excitation spectrum shows that the broad band at 240–300 nm is mainly attribute to CTS, which means the peaks at 300–500 nm are due to 4f–4f transition of Tb3+. The terbium emission in TbNaY following excitation at 368 nm contains the 0D4 ? 7FJ (J = 3, 4, 5, 6) transition at about 621, 583, 545 and 489 nm. The broad band around 437 nm due to the emission of excited zeolite structure is also observed in the TbNaY zeolite emission spectrum. The emission spectrum of rare-earth ion exchanged zeolites reveals that the characteristic rare-earth emission peaks are generated by its nature 4f (or 5d) energy level transition, but not by the
500
Wavelength(nm)
o 2θ ( )
Figure 3. XRD of MNaY (M = Eu3+, Tb3+, Ce3+, Y3+, Zn2+, Cd2+, Cu2+) zeolite and NaY zeolite.
400
5
15
7F
10
5D 4
5
200 250 300 350 400 450 500 550 600 650 700
Wavelength(nm) Figure 4. Excitation and emission spectrum of rare-earth ion (Eu3+, Ce3+, Tb3+) exchanged NaY. (A) EuNaY zeolite excitation spectrum is obtained by monitoring the emission of Eu3+ at 614 nm, and the excitation wavelength for the emission spectrum is 394 nm. (B) CeNaY zeolite excitation spectrum is obtained by monitoring the emission of Ce3+ at 350 nm, and the excitation wavelength for the emission spectrum is 293 nm. (C) TbNaY zeolite excitation spectrum is obtained by monitoring the emission of Tb3+ at 545 nm, and the excitation wavelength for the emission spectrum is 368 nm.
Photochemistry and Photobiology, 2014, 90
Figure 5. CIE x–y chromaticity diagram of rare-earth ion (Eu3+, Ce3+, Tb3+) exchanged NaY zeolite: (A) EuNaY; (B) CeNaY; (C) TbNaY.
473 nm
5
7 D0 F1 5 7 D0 F2
D2 5
F0,1 7
FJ 5 F0,1 HJ 7 F0,1 5D4 7 F0,1 5GJ
5
7
F0,1
D0 7F4
7
Intensity(a.u.)
7
F0,1
5
L6
A
5
CTS
200 250 300 350 400 450 500 550 600 650 700 750
Wavelength(nm) B 330 nm
270 nm 288 nm
Intensity(a.u.)
excitation band of zeolite. It is likely because the transition energy level of rare earth and defect energy level of zeolite is not match. We can get various photoluminescence colors, as illustrated in the CIE x–y chromaticity diagram of the multicolored photoluminescence of the samples (Fig. 5). It is remarkable that EuNaY exhibits white light which shows strong applied value in luminescent devices. The emission spectra of MNaY (M = Y3+, Cd2+, Cu2+, Zn2+) zeolite vary in different ions (Supporting Information Fig. S4). Because ion M (M = Y3+, Cd2+, Cu2+, Zn2+) is not conventional photoluminescence ion, the emission spectra of MNaY show the luminescence of matrices with broad peak unlike rare-earth ion photoluminescence with sharp peaks. Compared to the maximum emission wavelength of NaY zeolite around 550 nm, the maximum emission wavelength of MNaY (M = Y3+, Cd2+, Cu2+, Zn2+) zeolite is about 475 nm. The blueshift is likely due to the exchanged ion located in b supercage of NaY zeolite stabilized silicon–oxygen tetrahedron and aluminum–oxygen tetrahedron causing higher excitation energy of MNaY (M = Y3+, Cd2+, Cu2+, Zn2+) zeolite. The intensity of MNaY (M = Y3+, Cd2+, Cu2+, Zn2+) zeolite at 475 nm is YNaY > CdNaY > CuNaY > ZnNaY > NaY when the slit of fluorescence spectrometer is fixed. The increasing of emission peak intensity is probably because exchanged ions cause the lattice distortion increase the degree of defect enhancing the luminescence. The excitation and emission spectra for the obtained YNaYEu are given in Figure 6A. The excitation spectrum of YNaY-Eu exhibits a similar pattern as EuNaY. As shown in Figure 6A, a band ranging from 250 to 300 nm and narrow peak centered at 395 nm are observed, and the former band is attributed to CTS and the latter narrow peak is ascribe to the absorption of 4f–4f electron transition of Eu3+. The emission spectrum of YNaY-Eu contains a broad band ranging centered at 473 nm. Lines of YNaY-Eu are distributed mainly in the 575–725 nm range, which are assigned to 5D0 ? 7FJ (J = 1, 2, 4) transition at about 591, 614 and 598 nm, respectively. YNaY-Eu zeolite shows
507
200 220 240 260 280 300 320 340 360 380 400
Wavelength(nm) Figure 6. Excitation and emission spectra of rare-earth ion (Eu3+, Ce3+) exchanged NaY modified with Y3+. (A) YNaY-Eu zeolite excitation spectrum is obtained by monitoring the emission of Eu3+ at 614 nm, and the excitation wavelength for the emission spectrum is 394 nm. (B) YNaY-Ce zeolite excitation spectrum is obtained by monitoring the emission of Ce3+ at 330 nm, and the excitation wavelength for the emission spectrum is 270 nm.
relatively weak Eu3+ ion emission due to low doping rate, so the emission spectrum depicts mainly the emission of ion exchanged zeolite matrix with the blueshift phenomenon compared to pure NaY zeolite emission. On account of the intensity ratios I (5D0?7F2)/I(5D0?7F1) (I02/I01) can be seen as an indicator for the local environment of ions, we concluded that the chemical environment around the europium ions is in high symmetry according to the intensity ratio listed in Table 2. The excitation spectrum of ZnNaY-Eu possesses similar emission spectrum as YNaY-Eu (Supporting Information Fig. S5A). Figure 6B depicts the excitation and emission spectra of the YNaY-Ce prepared by monitoring the characteristic emission of cerium trivalent ion at 330 nm. Excitation spectrum of YNaY-Ce reveals a broad band with doublet peaks located in 240–320 nm wavelength range, which is generated by 2F5/2 ? 2DJ transition. The emission spectrum of YNaY-Ce reveals the typical light emission of Ce3+ ions. The light emission of Ce3+ ions is result from two probable electron decay transition due to a split ground state (2F5/2, 2F7/2). In the same way, the excitation spectrum of ZnNaY-Ce is shown in
Tian-Wei Duan and Bing Yan A 7
Table 2. The luminescent efficiencies and lifetime data for ion exchanged zeolites.
F6
508
EuNaY CeNaY TbNaY YNaY-Eu YNaY-Ce YNaY-Eu/Tb
1.15 – – 1.07 – 0.72
YNaY-Ce/Tb ZnNaY-Eu ZnNaY-Ce ZnNaY-Eu/Tb
– 0.86 – 0.63
ZnNaY-Ce/Tb
–
s (ls)†
D4
I02/I01*
7 5
D4
a
7
D0
b
F2
7
F1
CTS
5
(Tb) (Eu) (Ce)
D0
5
(Eu) (Tb) (Ce)
Intensity(a.u.)
76 51 474 221 953 18 26 18 41 566 7 14 49
F5
5
Materials
CTS
*The emission intensity ratios of the 5D0 ? 7F1 transition (I01) and the 5 D0 ? 7F2 transition (I02) for europium exchanged zeolite. †Lifetimes (s) of 5D0 energy level, 5D4 energy level and 5F7/2, 5/2 for Eu3+, Tb3+ and Ce3+ excited state, respectively.
200
300
c
400
500
600
700
800
Wavelength(nm) B
D4
250
300
350
400
450
7
F6
D4
7
500
550
5
200
F5
348 nm
5
279 nm
Intensity(a.u.)
Figure S5B by monitoring the characteristic emission of Ce3+ ions at 345 nm. Excitation spectrum of ZnNaY-Ce consists of a broad band located in 240–320 nm wavelength range, which is generated by 2F5/2 ? 2DJ transition. The emission spectrum of ZnNaY-Ce reveals the typical light emission of Ce3+ ions at about 345 nm. Tb3+ exchanged YNaY zeolite cannot exhibit the characteristic peak of Tb3+ by using the excitation wavelength of 368 nm, so does Tb3+ exchanged ZnNaY zeolite. However, by using the CTS band wavelength of 280 nm as the excitation wavelength, we can get the luminescence of Tb3+ in YNaY and ZnNaY (Supporting Information Fig. S6). The excitation and emission spectra of YNaY zeolite with double doped rare earth (Eu3+ and Tb3+) are depicted in Figure 7A. It is shown that the excitation spectra of Eu3+ and Tb3+ cover the same CTS band (250–330 nm) by monitoring the 614 and 545 nm. The emission spectrum exhibits not only the luminescence of Eu3+ but also Tb3+ under the condition of using the excitation wavelength at 318 nm. The emission lines are assigned to 5D0 ? 7FJ (J = 5, 6) and 0D5 ? 7FJ (J = 1, 2) transition at about 545, 489 nm and 591, 614 nm, respectively. In the same way, the excitation spectrum of YNaY-Ce/Tb is shown in Figure 7B by monitoring the characteristic emission of cerium trivalent ion at 350 nm. Excitation spectrum of YNaY-Ce consists of a broad band centered at 279 nm, which is generated by 2F5/2 ? 2DJ transition. The emission spectrum of YNaY-Ce/Tb mainly reveals the typical light emission of Ce3+ ions at about 348 nm, and the weak luminescence of Tb3+ at 548 nm and 545 nm because the CTS of Tb3+ is overlapped with the excitation spectrum of Ce3+. There exists energy transfer between Ce3+ and Tb3+ intra YNaY because the lifetime of Ce3+ in YNaY-Ce/Tb is extremely low compared to YNaY-Ce (Table 2). It is concluded that Tb3+ ions absorb light and transfer to Ce3+ ions. As for double rare-earth ions exchanged zeolite of ZnNaY series, it possesses the similar spectra as YNaY series (Supporting Information Fig. S7). The energy transfer is also observed between Ce3+ and Tb3+ in ZnNaY (Table 2). Figure S7 illustrates excitation and emission spectra of ZnNaY series. The maximum excitation wavelength of Ce′NaY zeolite (amount as Ce equivalent weight in ZnNaY-Ce′ or
600
Wavelength(nm) Figure 7. Excitation and emission spectra of YNaY-Eu/Tb and YNaYCe/Tb (A) The excitation and emission spectrum of YNaY-Eu/Tb (a) the excitation spectrum is obtained by monitoring the emission of Tb3+ at 545 nm (b) the excitation spectrum is obtained by monitoring the emission of Eu3+ at 614 nm (c) the emission spectrum is obtained by using excitation wavelength at 318 nm. (B) YNaY-Ce/Tb zeolite excitation spectrum is obtained by monitoring the emission of Ce3+ at 350 nm, and the excitation wavelength for the emission spectrum is 280 nm.
ZnNaY-Ce/RE (RE = Eu3+, Tb3+, Y3+)) is about 273 nm and the maximum emission wavelength is about 342 nm. When ZnNaY is doped with Ce, the emission intensity of ZnNaY-Ce′ is higher than Ce′NaY. ZnNaY is double doped with Ce and other rareearth ion; however, the emission intensity is much higher than CeNaY. The lifetime measure indicated that there is energy transfer between Ce and Eu or Ce and Tb intra ZnNaY zeolite, which can be preliminary conformed by comparing the decay time among ZnNaY-Ce′, ZnNaY-Eu/Tb and ZnNaY-Ce/Tb. It is likely responding to Eu3+ or Tb3+ acting as sensitizer and Ce3+ acting as activator. For further investigation of the photoluminescence properties, we measure the luminescence lifetime decay curves of the ion exchanged zeolite at room temperature, and lifetimes are given in Tables 2 and S1.
Photochemistry and Photobiology, 2014, 90
CONCLUSIONS We have designed and synthesized a series of ion exchanged zeolite by introducing various ions into zeolite. The photoluminescence measurements indicate that pure NaY zeolite with the emission wavelength centered at 550 nm. After ion exchange process, it exhibits a blueshift of about 70 nm by excitation wavelength of 394 nm. Rare-earth (Eu3+, Tb3+, Ce3+) exchanged zeolite exhibits its characteristic luminescence generated by its nature 4f (or 5d) energy level transition. Cerium exchanged zeolite always exhibits its characteristic luminescence no matter in monometal ion exchanged zeolite or bimetal ion exchanged zeolite. There exists energy transfer between Ce3+ and Tb3+ in zeolite modified with Y3+ or Zn2+. The finding principle may be applied to synthesis other luminescence material based on ion exchanged zeolite. Further efforts may concern on studying ions distribution and energy transfer in bimetal ions exchanged zeolite. Acknowledgements—This work was supported by the National Natural Science Foundation of China (20971100, 91122003) and Program for New Century Excellent Talents in University (NCET-08-0398).
SUPPORTING INFORMATION Additional Supporting Information may be found in the online version of this article: Figure S1. XRD patterns of zeolite at different synthesis times. Figure S2. XRD patterns of metal ion exchanged ZnNaY series and metal ion exchanged YNaY series. Figure S3. Emission spectra of MNaY (M = Y3+, Cd2+, 2+ Cu , Zn2+) zeolite. The excitation wavelength for the emission spectra is 394 nm. Figure S4. Excitation and emission spectra of ZnNaY-Eu and ZnNaY-Ce. (A) ZnNaY-Eu zeolite excitation spectra are obtained by monitoring the emission of Eu3+ at 614 nm, and the excitation wavelength for the emission spectra is 394 nm. (B) ZnNaYCe zeolite excitation spectra are obtained by monitoring the emission of Ce3+ at 345 nm, and the excitation wavelength for the emission spectra is 273 nm. Figure S5. Excitation and emission spectra of YNaY- Tb and ZnNaY-Tb. (A) YNaY-Tb zeolite excitation spectra are obtained by monitoring the emission of Tb3+ at 545 nm, and the excitation wavelength for the emission spectra is 320 nm. (B) ZnNaYTb zeolite excitation spectra are obtained by monitoring the emission of Tb3+ at 545 nm, and the excitation wavelength for the emission spectra is 320 nm. Figure S6. Excitation and emission spectra of ZnNaY-Eu/Tb and ZnNaY-Ce/Tb. (A) The excitation and emission spectra of ZnNaY-Eu/Tb; (a) the excitation spectra are obtained by monitoring the emission of Tb3+ at 545 nm, (b) the excitation spectra are obtained by monitoring the emission of Eu3+ at 614 nm, (c) the emission spectra are obtained by using excitation wavelength at 318 nm. (B) ZnNaY-Ce/Tb zeolite excitation spectra are obtained by monitoring the emission of Ce3+ at 350 nm, and the excitation wavelength for the emission spectra is 280 nm. Figure S7. Excitation and emission spectra of ZnNaY series. All the excitation spectra are obtained by monitoring the emission of Ce3+ at 350 nm, and the excitation wavelength for the emission spectra is 274 nm. Ce3+ doped in Ce′NaY is the similar
509
amount as in ZnNaY-Ce, ZnNaY-Ce/Y, ZnNaY-Ce/Eu, ZnNaYCe/Tb. Table S1. The luminescent efficiencies and lifetimes for comparing Ce3+ luminescent phenomenon in ZnNaY series exchanged zeolite.
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