Variations in the ⁵D₀ → ⁷F_0–4 transitions of Eu³⁺ and white light emissions in Ag–Eu exchanged zeolite-Y

Abstract

The variations in the ⁵D₀ → ⁷F_J(J=0–2) transitions of Eu³⁺ in the 580–630 nm range can reveal the exchanged-ion distribution within the zeolite pores for Ag–Eu exchanged zeolite-Y. A laser could excite more Eu³⁺ ions at symmetric sites (SI, SU) in the cages deep inside the zeolite besides activating those at the shallow surface. Significantly, the ⁵D₀ → ⁷F₄ transition of Eu³⁺ at 696 nm can be selectively modulated and enhanced by laser beam compared to xenon light according to the intensity ratio and decay-lifetime ratio of ⁵D₀ → ⁷F₄ to ⁵D₀ → ⁷F₂ transitions, which could be ascribed to the dynamic coupling effect on Eu³⁺ for the polarization of Eu³⁺-coordinated chemical environment induced by laser. Both transitions are determined by the odd-rank static electric field parameters of Eu³⁺ site according to the point charge model, but the ⁵D₀ → ⁷F₄ transition is further influenced by the polarizability of chemical environment around Eu³⁺ following the covalent bond model. Such strengthened effect may be partially relevant to the electronegativity of the zeolite framework. White light emission is achieved in these composites for excitation of both the 266 nm xenon lamp and laser, while the latter shows better tuning behaviors. The Ag–Eu codoped zeolite composites have potential applications in white light illumination.

---

1. Introduction

Zeolites, a type of crystalline microporous alumosilicates, can be considered as SiO₂ with partial Si⁴⁺ substitution by Al³⁺, with some cations like K⁺, Na⁺, NH₄⁺ inside the cage to neutralize the electronegative framework. They have widespread and significant applications in the chemical and petrochemical industry.¹ Increasing interest in the study of guest–host composite materials² has heightened the research in these well-defined periodic microporous structures because of their low-frequency vibrational framework, almost ultraviolet (UV) transparency and inexpensive price.^3–5 Among of these, zeolites are utilized as excellent hosts for luminescence because of their strong ability of separating the luminescent centers, thus, reducing the concentration quenching.^6,7 They can offer superb sites for luminescent centers like rare earth ions,^8–10 quantum dots,^11–15 Ag^16–18 and so on. These porous composite materials could be used in white-light illumination,⁴ random laser,¹⁹ optical encoding,²⁰ etc. Zeolites exhibit no electric conductivity and behave as insulators, but impedance spectroscopy indicates that ionic conduction via the cations undergoing intracage as well as longer-range intercage motions is possible.^21,22 Hence, luminescent centers may be introduced into the cages of zeolites by ion exchange, vapor impregnation, solid state diffusion, or direct synthesis within the cavities or channels of zeolites.² The luminescence of such zeolite composite luminescent materials largely depends on the distribution and state of the luminescent centers (ions) in the microporous cages. And the electronegativity of the zeolite framework may have some influence on luminescence that can not be ignored. Furthermore, different excitation lights, such as xenon lamp light and laser light, may result in luminescence variation for the microporous zeolite composite luminescent materials, owing to the different penetration depth. However, such research work is barely found to our knowledge.

Eu³⁺ is a notable rare earth ion with red light emitting in the family of commercial phosphors. It is commonly used as a probe because its magnetic dipole transition ⁵D₀ → ⁷F₁ and electric dipole transition ⁵D₀ → ⁷F₂ are hypersensitive to the coordinated chemical environments.^23–25 Additionally, the ⁵D₀ → ⁷F₄ transition is largely influenced by the dynamic polarization of Eu³⁺-coordinated environment induced by laser.^26,27 Thus, Eu³⁺ is adopted to evaluate its distribution in the zeolite in this research work. On the other side, white light emission from one single-phased phosphor is desired for lighting.^6,28 Sensitizer is normally needed in Eu³⁺ doped red-emitting systems to gain white light emission. Optical properties of small metal clusters are very intriguing for luminescent materials. The small metal clusters (M_n^m+ with n < 100 and m < n) have discrete electronic energy states compared to bulk metals, and thus behave like molecules with respect to electronic transitions. As for noble metals such as gold and silver, bright emission upon excitation of the UV-visible light has been reported.^29–35 Meanwhile, the use of noble metals to improve optical properties of rare-earth-doped materials has received great interest. Enhanced emissions of rare-earth ions such as Sm³⁺, Tb³⁺, and Dy³⁺, have been reported through codoping silver metal ions.^36–38 Silver doped zeolites have been extensively studied since the 1970s because of their promising catalytic properties.^39–41 Thus, metallic Ag could be incorporated in the Eu³⁺-doped zeolite to realize white light emission and to perturb the ⁵D₀ → ⁷F_0–4 transitions of Eu³⁺. Commercial available and commonly used zeolite-Y (ZY) is adopted in this research, which has a SiO₂/Al₂O₃ ratio of 5.1 and porous diameters of 2.6–7.4 Å.⁴² Luminescent behaviours of Ag–Eu codoped ZY composites upon the excitation of lamp light or laser light are investigated in detail.

2. Experimental section

The Ag and Eu doped zeolites were prepared by an ion exchange process. The NH₄ form of faujasite-type zeolite (zeolite-Y, SiO₂/Al₂O₃ = 5.1) was purchased from Alfa Aesar. The Ag doped zeolite was prepared by stirring the zeolites in the aqueous solution of AgNO₃ at 80 °C for 12 h, and the Ag–Eu codoped zeolites were made by stirring the zeolites in the aqueous solution mixed with AgNO₃ and Eu(NO₃)₃ at 80 °C for 12 h. The products were filtered, washed with deionized water, and dried in air at 80 °C. Then all the products were annealed in air at 800 °C for 1 h.

The concentrations of Ag and Eu in zeolites were determined by Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES). The samples were kept in air. The phases of the samples were characterised by a Rigaku D/max-IIIA X-ray diffractometer with Cu-Kα radiation (λ = 1.5418 Å). Morphology characterization was carried out on a field-emission scanning electron microscopy (FESEM, FEI Nova Nano SEM 430). Raman spectra were recorded at room temperature with a Renishaw invia-58P056 laser Raman spectrometer under the excitation of 785 nm laser. The emission and excitation spectra as well as decay curves were recorded by a fluorescence spectrometer (FLS 920, Edinburgh Instruments) with a red-sensitive photomultiplier tube (PMT, R928). The emission spectra activated by laser were recorded on a iHR320 fluorescence spectro-fluorometer (Horiba Jobin-Yvon Co.) equipped with an R928 PMT using a pulsing wave excitation from a 266 nm pulsed DPSS (Diode Pumped Solid State) laser (Shenzhen Leo-photoelectric Co., LTD). The emission spectra and decay curves excited by 393 nm pulsed laser were determined with a photon counting detector combined with the multichannel scaler/averager SR430 by using an output at a central wavelength of 393 nm with pulse duration of 120 fs and a variable repetition rate from 1 to 1000 Hz as the excitation source, which is generated from an optical parametric amplifier (TOPAS-Prime) pumped by a mode-locked Ti:sapphire oscillator seeded regenerative amplifier (SpectraPhysics Spitfire Ace).

3. Results and discussion

3.1 Phases, morphology and compositions of the samples

Fig. 1 is the X-ray powder diffraction (XRD) patterns of the zeolite-Y and the samples sintered at 800 °C for 1 h, and the standard card (JCPDS 43-0168) is included for comparison. It can be seen that all the samples are in agreement with JCPDS 43-0168, which clearly suggests that the structure of ZY is maintained even after thermal treatment at 800 °C. It is also confirmed by Raman spectra in Fig. S1.† However, it is worth noticing that the peak intensities for (220) and (311) at 10–15° decreased and even disappeared with the incorporation of Ag and Eu³⁺. Similar behavior was reported in La³⁺ doped ZY,^43,44 which was attributed to that La³⁺ is easier moved to sodalite cage and coordinates with framework O3. Since the La³⁺ ions are similar to Eu³⁺, it can be inferred that some of the Eu³⁺ ions have entered into sodalite cages. The corresponding field emission scanning electron microscopy (FE-SEM) images of the samples in Fig. 2 revealed that the morphology and monodispersity of the obtained particles remain almost unchanged.

Fig. 1 XRD patterns of (a) ZY; (b) ZY:0.02Ag, (c) ZY:0.05Eu, (d) ZY:0.02Ag–0.01Eu, (e) ZY:0.02Ag–0.05Eu, (f) ZY:0.02Ag–0.1Eu, (g) ZY:0.02Ag–0.15Eu and JCPDS card no. 43-0168 is included for comparison.

Fig. 2 SEM images of (a) ZY:0.02Ag, (b) ZY:0.05Eu, (c) ZY:0.02Ag–0.01Eu, (d) ZY:0.02Ag–0.05Eu, (e) ZY:0.02Ag–0.1Eu, (f) ZY:0.02Ag–0.15Eu.

Table 1 gives the nominal formulae with different concentration of Ag and Eu. Fig. 3(a) shows the real concentration of Ag and Eu in zeolites tested by Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES). The solid dark histogram of the sample without Eu shows the highest concentration of Ag, and it lowers even with 0.01 mol L⁻¹ of Eu and declines further with higher concentration of Eu (0.05, 0.10 and 0.15 mol L⁻¹). These results manifest the competition between Ag and Eu during the ion-exchange process. Meanwhile, the concentration of Eu increases as depicted by the slash red histogram, this is coincident with the experimental design. Fig. 3(b) gives the molar summation of Ag and Eu, it varies for all the samples but almost equal for the Ag or Eu singly doped samples, suggesting that the available sites in the ZY are the same for both Ag and Eu and there is competition for these two ions to be incorporated.

Table 1 Solution concentration of the samples designed

Number	Sample	Ag (mol L^−1)	Eu (mol L^−1)
1	ZY:0.02Ag	0.02	0
2	ZY:0.02Ag–0.01Eu	0.02	0.01
3	ZY:0.02Ag–0.05Eu	0.02	0.05
4	ZY:0.02Ag–0.1Eu	0.02	0.10
5	ZY:0.02Ag–0.15Eu	0.02	0.15
6	ZY:0.05Eu	0	0.05

---

Fig. 3 The ICP-OES results of Ag–Eu doped ZY composites.

3.2 Luminescence and energy transfer in Ag–Eu doped zeolite-Y

Excitation (λ_em = 530 nm) and emission spectra (λ_ex = 278, 310 nm) of the Ag singly doped ZY sample are shown in Fig. 4(a). It was reported that the excitation peak at 245 nm belongs to 4d¹⁰(¹S₀) → 4d⁹5s(¹D₂) transition of Ag⁺ in some glass system^45,46 and SiO₂ three-dimensionally ordered macroporous materials,⁴⁷ while the peak at 310 nm was ascribed to Ag₂ in ZY,⁴⁸ and Ag₃ and Ag cluster were also considered to be the origin,^18,49 therefore, it is primarily owed to the Ag active centers (ACs) in this work. The emission spectrum excited by 278 nm xenon light shows redshift compared to that excited by 310 nm, which may originate from the contribution of different Ag ACs.

Fig. 4 Excitation and emission spectra of (a) ZY:0.02Ag, (b) ZY:0.05Eu, (c) ZY:0.02Ag–0.05Eu, (d) ZY:0.02Ag–0.05Eu.

Fig. 4(b) (blue line) presents the excitation spectrum of ZY:0.05Eu obtained by monitoring the ⁵D₀ → ⁷F₂ emission at λ_em = 617 nm. The spectrum consists of broad excitation band at 250–375 nm and sharp f–f transition peaks of the Eu³⁺ ion at the range between 350 and 600 nm. The broad band corresponds to the O²⁻–Eu³⁺ charge transfer band. The sharp peaks are assigned to the ⁷F₀ → ⁵D_J (J = 0, 1, 2, 3) transitions of Eu³⁺, including ⁷F₀ → ⁵D₄ (363 nm), ⁷F₀ → ⁵G₂ (383 nm), ⁷F₀ → ⁵L₆ (393 nm), ⁷F₀ → ⁵D₃ (417 nm) and ⁷F₀ → ⁵D₂ (466 nm). The red line in Fig. 4(b) is the emission spectrum of ZY:0.05Eu under excitation of 393 nm xenon light. It consists of some sharp emission peaks, which are typical transitions of Eu³⁺, i.e. ⁵D₀ → ⁷F₀ (576 nm), ⁵D₀ → ⁷F₁ (587 nm), ⁵D₀ → ⁷F₂ (617 nm), ⁵D₀ → ⁷F₃ (648 nm) and ⁵D₀ → ⁷F₄ (695 nm), respectively.

Fig. 4(c) shows the excitation (blue line, λ_em = 530 nm) and emission spectra of ZY:0.02Ag–0.05Eu. The excitation spectrum is analogous to that of ZY:0.02Ag in Fig. 4(a), indicating they have the same origin of Ag ACs. The emission peaks of Eu³⁺ were observed besides the emission of Ag ACs at ∼530 nm, and the former is more apparent when excited by 278 nm light compared to 310 nm light. This may be due to that the 278 nm light locates at the range of O²⁻–Eu³⁺ charge transfer band. Fig. 4(d) is the excitation (blue line, λ_em = 617 nm) and emission (red line, λ_ex = 393 nm) spectra of ZY:0.02Ag–0.05Eu. The emission spectrum is the same as that of ZY:0.05Eu. Whereas, the excitation spectrum shows a ridgy peak at 310 nm compared to that of ZY:0.05Eu, which is contributed by the adsorption of Ag ACs as shown in Fig. 4(a) (blue line) and indicates that there is energy transfer from Ag ACs to Eu³⁺.

The decay curves and lifetime of the samples were present in Fig. 5. The decay curves appear to be straight lines (the longitudinal coordinates are logarithmic values) for ZY:0.02Ag under excitation of 278 or 310 nm when monitored at 530 nm, while they are the crooked lines for those codoped with Eu³⁺, suggesting that there is energy transfer process from Ag ACs to Eu³⁺. The decline of lifetimes in Fig. 5(c) and (d) also manifests the energy transfer, which is coincident with our previous discussion. Furthermore, the lifetime of Ag ACs decreases from 268 μs (ZY:0.02Ag) to 101 μs (ZY:0.02Ag–0.05Eu) under the excitation of 278 nm, as shown in Fig. 5(c), and it changes little beyond that contents of Eu³⁺. However, the lifetime of Ag ACs decreases from 255 μs (ZY:0.02Ag) to 112 μs (ZY:0.02Ag–0.15Eu) under the excitation of 310 nm in Fig. 5(d). This may be owed to the more efficient energy transfer process from Ag ACs to Eu³⁺ and the partial contribution of charge transfer absorption of O²⁻–Eu³⁺ for former (as seen in Fig. 4).

Fig. 5 Decay curves and lifetimes of the ZY:0.02Ag–xEu samples monitored at 530 nm upon excitation of 278 nm (a and c) and 310 nm (b and d) pulsed lamp.

3.3 Variations of ⁵D₀ → ⁷F_0–4 transitions of Eu³⁺ and white light emissions

The variations of ⁵D₀ → ⁷F_0–4 transitions of Eu³⁺ and perturbation of Ag on them were detected and compared upon excitation of different light sources. As shown in Fig. 6(a), the emission profiles of ZY:0.02Ag are almost the same under excitation of 266 nm xenon light and 266 nm laser. But with focused laser, broadening of the peak is observed, which may be due to that more species of Ag ACs are excited by the focused laser. For the co-doped samples in Fig. 6(b)–(e) (the spectra are normalized at the emission of Ag ACs), the emission intensity of Eu³⁺ are enhanced under the excitation of 266 nm laser compared to that of 266 nm xenon light excitation (except for ZY:0.02Ag–0.05Eu), and it is further enhanced with focused laser (green lines). Two possible reasons could explain this phenomenon. One is the surface plasma resonance enhancement, but the Ag particles are not detected in these systems in Fig. 1 and the enhancement of Raman peaks is not observed either in Fig. S1.† Similar reports given by Sa chu Rong gui et al.⁵⁰ also showed that surface plasma resonance was not found. Thus, the surface plasma resonance may be excluded. Another reason may be that the laser could excite more Eu³⁺ ions located at deep inside of ZY, which will be further discussed below. Fig. 6(f) gives the spectra of ZY:0.05Eu. The spectra lines are normalized at ⁵D₀ → ⁷F₂ (617 nm) transition. It can be seen that the transition of ⁵D₀ → ⁷F₄ (696 nm) is enhanced by laser excitation compared to ⁵D₀ → ⁷F₂ (617 nm) transition, which will also be discussed below.

Fig. 6 Emission spectra of (a) ZY:0.02Ag, (b) ZY:0.02Ag–0.01Eu, (c) ZY:0.02Ag–0.05Eu, (d) ZY:0.02Ag–0.1Eu, (e) ZY:0.02Ag–0.15Eu, (f) ZY:0.05Eu upon excitation of 266 nm xenon lamp light (Xe266), 266 nm laser diode (LD266), and focused 266 nm laser diode (LDF266). The spectra of (a–e) are normalized for Ag ACs emission while that of (f) are normalized at ⁵D₀ → ⁷F₂ (617 nm) transition.

In order to compare the different influence of xenon light and laser excitation on the Ag and Eu³⁺ luminescence, the luminescence intensity ratios between the ⁵D₀ → ⁷F₂ of Eu³⁺ and Ag ACs are presented in Fig. S2.† As can be seen, the intensity ratio firstly increases and then declines as the Eu³⁺ concentration increases under excitation of 266 nm xenon lamp (black squares). However, under the excitation of 266 nm laser (red dots), it continuously go up along with the Eu³⁺ concentration. And the trend is more obvious under the excitation of focused laser beam. These can also be ascribed to that higher density of excitation light would excite more Eu³⁺ ions located at deep inside of ZY.

The Commission Internationale de I'Eclairage (CIE) chromaticity coordinates of the systems under excitation of different lights are presented in Fig. 7 and the specific coordinates and color rendering index (CRI) are listed in Tables S1 and S2.† The ZY:0.02Ag–0.1Eu and ZY:0.02Ag–0.15Eu samples show white light emission in Fig. 7(a) for 266 nm xenon lamp excitation, while ZY:0.02Ag–0.01Eu and ZY:0.02Ag–0.05Eu samples demonstrate white light emission in Fig. 7(b) for the focused 266 nm laser excitation. It can be seen that the chromaticity coordinates can be better tailored upon the 266 nm laser excitation, which is potentially applicable in illumination.

Fig. 7 The CIE chromaticity coordinates of the Ag–Eu doped ZY under excitation of 266 nm (a) xenon lamp and (b) focused laser diode.

The variations of ⁵D₀ → ⁷F_0–4 transitions of Eu³⁺ will be discussed in detail as follow. Eu³⁺ is commonly used as structural probe because its luminescence is very sensitive to the ambient environment.^23–25 According to the group theory dealing with symmetry, the integral 〈J|H|J′〉 can be used to evaluate the transition probability of Eu³⁺ (zero or nonzero), in which J and J′ are the total angular momentum of initial and final state, respectively, and H is Hamilton operator of the magnetic dipole transition or the electric dipole transition. The ⁵D_0(J=0) → ⁷F_1(J′=1) transition is the magnetic dipole transition, which is allowed when the local symmetry of Eu³⁺ site presents a center of inversion.^23,51 The ⁵D_0(J=0) → ⁷F_2(J′=2) transition is the electric dipole transition allowed when the center of inversion is absent.^23,51 Therefore, the ratio of these two transitions can roughly reveal the relative amounts of symmetric and asymmetric Eu³⁺ ions. The emission of ⁵D_0(J=0) → ⁷F_0(J′=0) transition when allowed by a low symmetry, which is normally caused by the mixing of ⁷F_2(J′=2), is always a single peak and never splitting no matter what symmetry Eu³⁺ stayed. Thus, its FWHM could give information on how low the symmetry is. While for ⁵D_0(J=0) → ⁷F_4(J′=4) transition, it is also the electric dipole transition and only allowed by a low symmetry, as the case of ⁵D_0(J=0) → ⁷F_2(J′=2) transition. In order to figure out the luminescence behaviors, the Eu³⁺ luminescent spectra are deconvoluted in Fig. S3.† The integrated intensity of the peaks and the full width at half maximum (FWHM) of ⁵D₀ → ⁷F₀ transitions are calculated. Because of the weak intensity of Eu³⁺ luminescence for the samples ZY:0.02Ag–0.01Eu and ZY:0.02Ag–0.05Eu, they could not be well deconvoluted. The case of ZY:0.02Ag–0.15Eu is analogous with that of ZY:0.02Ag–0.1Eu. Fig. 8 presents the FWHM of ⁵D₀ → ⁷F₀ transitions and different peak intensity ratios under excitation of 266 nm xenon lamp and 266 nm laser for comparison. The variation of ⁵D₀ → ⁷F₀ transition, including the FWHM and intensity, suggesting that more low-symmetric Eu³⁺ ions are excited by 266 nm xenon lamp. It is also convinced by the lower intensity ratio of ⁵D₀ → ⁷F₁ to ⁵D₀ → ⁷F₂ transitions when excited by 266 nm xenon lamp, since the former is magnetic dipole transition sensitive to symmetry centre while the latter is dielectric dipole transition sensitive to symmetry without centre.⁵¹ Whereas, the variation for the ratios of ⁷F₀/⁷F₂ and ⁷F₄/⁷F₂ is contrary in Fig. 8, which is not fully understood owing to that all of them are the dielectric dipole transitions and could be considered to be originated from the same ⁵D₀ level of the same Eu³⁺ ions.

Fig. 8 Full width at half maximum (FWHM) of ⁵D₀ → ⁷F₀ transition and integrated intensity ratios of ⁵D₀ → ⁷F₀, ⁵D₀ → ⁷F₁, ⁵D₀ → ⁷F₄ to ⁵D₀ → ⁷F₂ transitions upon excitation of 266 nm xenon lamp and 266 nm laser diode for (a) ZY:0.05Eu and (b) ZY:0.02Ag–0.1Eu.

In order to further understand the difference of ⁵D₀ → ⁷F₄ and ⁵D₀ → ⁷F₂ transition, direct excitation by 393 nm xenon lamp and pulsed laser are carried out, which corresponds to the strongest adsorption of ⁷F₀ → ⁵L₆ transition. The emission spectra are shown in Fig. 9 (the spectra are normalized at 617 nm). It can be seen that the peaks at 696 nm are obviously enhanced under 393 nm pulsed laser excitation compared to that excited by 393 nm xenon lamp. The decay curves and lifetimes for ⁵D₀ → ⁷F₄ and ⁵D₀ → ⁷F₂ transition are also investigated in detail and shown in Fig. 10. The lifetime ratio of ⁵D₀ → ⁷F₄ to ⁵D₀ → ⁷F₂ shortens under 393 nm pulsed laser excitation compared to that excited by pulsed xenon lamp, especially for the sample with Ag. It should be noticed that excitation by these lights would not directly populate the ⁵D₀ level but higher level ⁵L₆. No matter which higher levels are directly populated, they would inevitably relax and finally populate the ⁵D₀ level (with evidence of the presence of only ⁵D₀ → ⁷F_J(J=0–4) transitions for the Eu³⁺-concentrated zeolite samples in Fig. 4, 6 and 9). Both the electric dipole transitions of ⁵D₀ → ⁷F₂ and ⁵D₀ → ⁷F₄ are from the same ⁵D₀ level of the same coordination-asymmetric Eu³⁺. Therefore, the ratio difference of population distribution upon 393 nm laser excitation compared to that upon 393 nm xenon excitation would not be significantly different.

Fig. 9 Luminescence of Eu³⁺ under excitation of 393 nm xenon lamp (Xe) and 393 nm pulsed laser (PL) for (a) ZY:0.05Eu and (b) ZY:0.02Ag–0.1Eu.

Fig. 10 Decay curves of Eu³⁺ under excitation of 393 nm pulsed xenon lamp and 393 nm pulsed laser for (a) ZY:0.05Eu (λ_em = 617 nm), (b) ZY:0.05Eu (λ_em = 696 nm), (c) ZY:0.02Ag–0.1Eu (λ_em = 617 nm), (d) ZY:0.02Ag–0.1Eu (λ_em = 696 nm); lifetime ratios of ⁵D₀ → ⁷F₄ to ⁵D₀ → ⁷F₂ for (e) ZY:0.05Eu and (f) ZY:0.02Ag–0.1Eu.

It can be rationalized as follows. The integrated coefficient of spontaneous emission of a transition between two manifolds J and J′ in standard theory is given by^23,52,53

where ω is the angular frequency of the transition, e is the electronic charge, c is the velocity of light, ħ is Planck's constant over 2π and n is the refractive index of the medium. S_ed and S_md, which are the electric and magnetic dipole strengths, respectively, are given bywhere the quantities Ω_λ are the so-called Judd–Ofelt intensity parameters,^52,53 U^(λ) is the unit tensor operator; m is the electron mass. The intensity parameters Ω_λ are theoretically given by²³in whichwhere ΔE is the energy difference between the barycenters of the excited 4f^N−15d and ground 4f^N configurations, 〈r^x〉 is a radial expectation value, θ(t, λ) is a numerical factor, (1 − σ_λ), 〈f||C^(λ)||f〉 and δ_{t, λ+1} are shielding factor, one-electron reduced matrix element and Kronecker delta function, respectively. The first term in the right-hand-side of eqn (5) corresponds to contribution of the static forced electric dipole mechanism and the second term corresponds to contribution of the ligand polarizability-dependent dynamic coupling mechanism.²³ γ_p^t, the so-called odd-rank ligand field parameters, and Γ_p^t (t = 1, 3, 5, 7) contain the dependence on the coordination geometry and the nature of chemical environment around lanthanide ion, which can be expressed by²³in eqn (6) where α_j is the isotropic polarizability of the jth ligand atom, or groups of atom, at position R_j and Y_p^t is the spherical harmonic of rank t. In eqn (7) where ρ_j is the magnitude of the total overlap between 4f and ligand wavefunctions and β_j = 1/(1 + ρ_j), g_j is the charge factors. The eqn (7) can be interpreted as a ligand field parameter produced by effective charges-ρ_jg_je located at the mid-points of the lanthanide–ligand chemical bonds.²³

Theoretically, the ⁵D₀ → ⁷F₂ transition is governed by the effective operator Ω₂U⁽²⁾ while ⁵D₀ → ⁷F₄ transition is governed by Ω₄U⁽⁴⁾ in the electric dipole transition intensity calculation with Judd–Ofelt theory.^23,52,53 The lower odd-rank γ_p^t and Γ_p^t (t = 3, mostly contribute to Ω₂ for ⁵D₀ → ⁷F₂) are more sensitive to change in symmetry, and the contribution of dynamic coupling effects is larger to the higher odd-rank γ_p^t and Γ_p^t (t = 5, mostly contribute to Ω₄ for ⁵D₀ → ⁷F₄), which may interpret the variation of ⁵D₀ → ⁷F₂ transition and ⁵D₀ → ⁷F₄ transition. In brief, we could roughly draw the picture that both the ⁵D₀ → ⁷F₂ and ⁵D₀ → ⁷F₄ transitions are determined by the odd-rank static electric field parameters of Eu³⁺ site according to the point charge model, while the latter is further influenced by the polarizability of chemical environment around Eu³⁺ following the covalent bond model. It is also coincident with the previous work.^26,27,54,55 That is, dynamic coupling of polarization effect of Eu³⁺-coordinated environment induced by laser has strong influence on the ⁵D₀ → ⁷F₄ transition. Such strengthened effect may be relevant to the electronegativity of the zeolite framework.

However, it can not fully rule out the different population of the energy levels when excited by the high density of laser beam. As stated above, no matter which higher levels are directly populated or indirectly populated via piling up two or more photons (upconversion), they would inevitably relax and finally populate the ⁵D₀ level. But it may change the population distribution in ⁷F_J(J=0–4) upon excitation of laser via cross relaxation with ⁵D_J(J=0–3), very likely resulting in variation of the branching ratios for ⁵D₀ → ⁷F_J(J=0–4). Further investigation is needed, such as two-beam excitation.

3.4 Schematic summary

Thus, the luminescence variations upon different excitation sources could be correlated to the chemical environment that Eu³⁺ stayed. For an ideal ZY structure⁷ in Fig. 11(a), SI is located in the center of the hexagonal prism which is inaccessible to the detrimental –OH bonds or the water molecules. SI′ is located in sodalite cage close to the center of the prism-sodalite shared 6-member ring. SII′ is located in the sodalite cage close to the center of the sodalite self-own 6-member ring. SII is in the super cage, next to SII′ and in line with SII and the center of the sodalite self-own 6-ring. SII* is located deeper in the super cage and also in line with SII′ and SII. SU is in the center of the sodalite cage. It is worth noticing that some of the ions may cling to the surfaces of the composite materials, these sites are asymmetric. It is more sophisticated for a real case, factors like temperature, exotic molecular/ions can change the symmetry of the sites within or outside the ZY. Overall, some of the Eu³⁺ ions would occupy the symmetrical sites.

Fig. 11 Schematic diagrams of (a) cation sites in the ideal ZY framework, the black skeleton stands for the ZY framework, red circles stand for the cation sites; (b) luminescence of Eu³⁺ in ZY, the blue balls stand for Eu³⁺ ions on the shallow surface of ZY, the red balls stand for Eu³⁺ ions deep inside the ZY pores; (c) luminescence processes of Ag/Eu³⁺ in ZY.

Laser, which is collimated light beam with high density, may penetrate deeper into the ZY materials. Under the xenon lamp excitation, it's more likely to excite the Eu³⁺ ions on the shallow surface of the composite materials, which are asymmetric Eu³⁺ ions in majority. While under the excitation of laser, it is more likely to reach Eu³⁺ ions located at the more symmetric sites of SI and SU deep in the ZY composites besides the shallow Eu³⁺, as depicted in Fig. 11(b). Thus, the FWHM of ⁵D₀ → ⁷F₀ transitions is smaller and intensity ratio of ⁵D₀ → ⁷F₁ to ⁵D₀ → ⁷F₂ is larger for the laser excitation. Furthermore, the collimated light beam of laser may cause dynamic polarization of Eu³⁺-coordinated environment, resulting in the significant variation of the ⁵D₀ → ⁷F₄ transition.

The energy transfer and luminescence process are schematically presented in Fig. 11(c). For the luminescence of Eu³⁺ in singly doped sample, O²⁻–Eu³⁺ charge transfer is responsible for 278 nm or 266 nm excitation, followed by the energy transfer to Eu³⁺ ion and nonradiative relaxation to its ⁵D₀ level, then it gives ⁵D₀ → ⁷F₀, ⁵D₀ → ⁷F₁, ⁵D₀ → ⁷F₂, ⁵D₀ → ⁷F₃ and ⁵D₀ → ⁷F₄ transitions, respectively, corresponding to 576, 587, 617, 648, 696 nm luminescence. For the Ag–Eu codoped samples, Ag ACs absorb 278 and 310 nm photon energy and give 490 to 540 nm emission. Then energy transfer from Ag ACs to Eu³⁺ could take place, resulting in the luminescence of Eu³⁺.

4. Conclusions

In conclusion, variations of ⁵D₀ → ⁷F_J(J=0–4) transitions of Eu³⁺ in Ag–Eu codoped ZY are investigated upon different excitation light sources. It is found that more symmetric Eu³⁺ deep in the ZY pores could be excited by laser, according to the variation of ⁵D₀ → ⁷F₀, ⁵D₀ → ⁷F₁ and ⁵D₀ → ⁷F₂ transitions of the structural probe ion Eu³⁺. Moreover, ⁵D₀ → ⁷F₄ transition of Eu³⁺ is more easily influenced by the collimated light beam of laser, owing to the dynamic coupling polarization effects of Eu³⁺-coordinated chemical environment induced by laser. Both the ⁵D₀ → ⁷F₂ and ⁵D₀ → ⁷F₄ transitions are determined by the odd-rank static electric field parameters of Eu³⁺ site according to the point charge model, while the latter is further influenced by the polarizability of chemical environment around Eu³⁺ following the covalent bond model. Such strengthened effect may be partially ascribed to the electronegativity of the zeolite framework. White light emissions can be achieved in the Ag–Eu codoped ZY samples with both 266 nm xenon lamp and laser excitation, but the latter shows better tuning behaviour. The Ag–Eu codoped ZY may find potential application in white light illumination.

Acknowledgements

This work is financially joint supported by the NSFC (Grant No. 21101065, 51472088), the Fundamental Research Funds for the Central Universities (2015PT019, SCUT), Guangdong Innovative Research Team Program of China (201101C0105067115), Outstanding Young Teacher Training Program of Guangdong provincial Institute of higher education (Yq2013011) and Guangdong Natural Science Funds for Distinguished Young Scholars (2014A030306009).

References

1. D. E. Perea, I. Arslan, J. Liu, Z. Ristanović, L. Kovarik, B. W. Arey, J. A. Lercher, S. R. Bare and B. M. Weckhuysen, Nat. Commun., 2015, 6, 7589.
2. G. A. Ozin, Adv. Mater., 1992, 4, 612–649.
3. S. Engel, U. Kynast, K. Unger, F. Schüth, J. Weitkamp, H. Karge, H. Pfeifer and W. Hölderich, Stud. Surf. Sci. Catal., 1994, 84, 477.
4. Z. Bai, M. Fujii, K. Imakita and S. Hayashi, J. Lumin., 2014, 145, 288–291.
5. M. Lezhnina, F. Laeri, L. Benmouhadi and U. Kynast, Adv. Mater., 2006, 18, 280–283.
6. H. Li, H. Zhang, L. Wang, D. Mu, S. Qi, X. Hu, L. Zhang and J. Yuan, J. Mater. Chem., 2012, 22, 9338–9342.
7. H. Lin, S. C. R. Gui, K. Imakita and M. Fujii, J. Appl. Phys., 2014, 115, 033507.
8. H. Li, H. Zhang, L. Wang, D. Mu, S. Qi, X. Hu, L. Zhang and J. Yuan, J. Mater. Chem., 2012, 22, 9338–9342.
9. Z. Bai, M. Fujii, K. Imakita and S. Hayashi, Microporous Mesoporous Mater., 2013, 173, 43–46.
10. H. Lin, S. C. Rong-Gui, K. Imakita and M. Fujii, J. Appl. Phys., 2014, 115, 033507.
11. T.-W. Duan and B. Yan, CrystEngComm, 2014, 16, 3395.
12. W. Chen, X. Zhang and Y. Huang, Appl. Phys. Lett., 2000, 76, 2328.
13. W. Chen, R. Sammynaiken and Y. Huang, J. Appl. Phys., 2000, 88, 5188.
14. W. Chen, Z. Wang and L. Lin, J. Lumin., 1997, 71, 151–156.
15. W. Chen, A. Joly and J. Zhang, Phys. Rev. B: Condens. Matter Mater. Phys., 2001, 64, 041202.
16. H. Lin, K. Imakita and M. Fujii, Appl. Phys. Lett., 2014, 105, 211903.
17. A. Nakamura, M. Narita, S. Narita, Y. Suzuki and T. Miyanaga, J. Phys.: Conf. Ser., 2014, p012033.
18. G. De Cremer, E. Coutino-Gonzalez, M. B. Roeffaers, B. Moens, J. Ollevier, M. Van der Auweraer, R. Schoonheydt, P. A. Jacobs, F. C. De Schryver and J. Hofkens, J. Am. Chem. Soc., 2009, 131, 3049–3056.
19. H. He, E. Ma, Y. Cui, J. Yu, Y. Yang, T. Song, C.-D. Wu, X. Chen, B. Chen and G. Qian, Nat. Commun., 2016, 7, 11087.
20. G. De Cremer, B. F. Sels, J.-i. Hotta, M. B. Roeffaers, E. Bartholomeeusen, E. Coutiño-Gonzalez, V. Valtchev, D. E. De Vos, T. Vosch and J. Hofkens, Adv. Mater., 2010, 22, 957–960.
21. P. K. Dutta and M. Severance, J. Phys. Chem. Lett., 2011, 2, 467–476.
22. U. Simon and M. Franke, Microporous Mesoporous Mater., 2000, 41, 1–36.
23. G. De Sa, O. Malta, C. de Mello Donegá, A. Simas, R. Longo, P. Santa-Cruz and E. Da Silva, Coord. Chem. Rev., 2000, 196, 165–195.
24. S. Ye, C. H. Wang and X. P. Jing, J. Electrochem. Soc., 2008, 155, J148–J151.
25. S. Ye, Y. Li, D. Yu, Z. Yang and Q. Zhang, J. Appl. Phys., 2011, 110, 013517.
26. S. Ye, D. C. Yu, X. M. Wang, E. H. Song and Q. Y. Zhang, J. Mater. Chem. C, 2013, 1, 1588–1594.
27. Y. Li, T. Liu, S. Ye, T. Deng, X. Yi, Q. Zhang and X. Jing, J. Mater. Chem. C, 2015, 3, 4997–5003.
28. S. Ye, F. Xiao, Y. Pan, Y. Ma and Q. Zhang, Mater. Sci. Eng., R, 2010, 71, 1–34.
29. J. T. Petty, J. Zheng, N. V. Hud and R. M. Dickson, J. Am. Chem. Soc., 2004, 126, 5207–5212.
30. C. Felix, C. Sieber, W. Harbich, J. Buttet, I. Rabin, W. Schulze and G. Ertl, Chem. Phys. Lett., 1999, 313, 105–109.
31. M. Treguer, F. Rocco, G. Lelong, A. Le Nestour, T. Cardinal, A. Maali and B. Lounis, Solid State Sci., 2005, 7, 812–818.
32. W. Schulze, I. Rabin and G. Ertl, ChemPhysChem, 2004, 5, 403–407.
33. C. I. Richards, S. Choi, J.-C. Hsiang, Y. Antoku, T. Vosch, A. Bongiorno, Y.-L. Tzeng and R. M. Dickson, J. Am. Chem. Soc., 2008, 130, 5038–5039.
34. S. Fedrigo, W. Harbich and J. Buttet, J. Chem. Phys., 1993, 99, 5712–5717.
35. T. Vosch, Y. Antoku, J.-C. Hsiang, C. I. Richards, J. I. Gonzalez and R. M. Dickson, Proc. Natl. Acad. Sci. U. S. A., 2007, 104, 12616–12621.
36. J. Li, R. Wei, X. Liu and H. Guo, Opt. Express, 2012, 20, 10122–10127.
37. J. Jimenez, S. Lysenko, G. Zhang and H. Liu, J. Mater. Sci., 2007, 42, 1856–1863.
38. H. Guo, X. Wang, J. Chen and F. Li, Opt. Express, 2010, 18, 18900–18905.
39. P. A. Jacobs, J. B. Uytterhoeven and H. K. Beyer, J. Chem. Soc., Chem. Commun., 1977, 128–129.
40. J. Howard, T. C. Waddington and C. J. Wright, J. Chem. Soc., Chem. Commun., 1975, 775–776.
41. P. A. Jacobs, J. B. Uytterhoeven and H. K. Beyer, J. Chem. Soc., Faraday Trans., 1979, 75, 56–64.
42. H. T. Sun, A. Hosokawa, Y. Miwa, F. Shimaoka, M. Fujii, M. Mizuhata, S. Hayashi and S. Deki, Adv. Mater., 2009, 21, 3694–3698.
43. J. H. Shan, X. Q. Liu, L. B. Sun and R. Cui, Energy Fuels, 2008, 22, 3955–3959.
44. C. Deng, J. Zhang, L. Dong, M. Huang, B. Li, G. Jin, J. Gao, F. Zhang, M. Fan and L. Zhang, Sci. Rep., 2016, 6, 23382.
45. M. El Masloumi, Photoluminescence et cristallochimie des polyphosphates de formule Na_1−xAgxM(PO₃)₄·(M : La,Y) à l'état cristallisé ou vitreux, Université Sciences et Technologies-Bordeaux I, Université Cadi Ayyad, Faculté des sciences Semlalia, Marrakech, Maroc, 2008.
46. J. Jiménez, S. Lysenko and H. Liu, J. Appl. Phys., 2008, 104, 054313.
47. S. Lai, Z. Yang, J. Li, B. Shao, J. Yang, Y. Wang, J. Qiu and Z. Song, J. Mater. Chem. C, 2015, 3, 7699–7708.
48. G. A. Ozin and F. Hugues, J. Phys. Chem., 1983, 87, 94–97.
49. H. Lin, K. Imakita, S. C. R. Gui and M. Fujii, J. Appl. Phys., 2014, 116, 013509.
50. S. C. Rong-gui, K. Imakita, M. Fujii and S. Hayashi, Opt. Mater., 2014, 36, 916–920.
51. G. Blasse, A. Bril and W. Nieuwpoort, J. Phys. Chem. Solids, 1966, 27, 1587–1592.
52. B. R. Judd, Phys. Rev., 1962, 127, 750.
53. G. S. Ofelt, J. Chem. Phys., 1962, 37, 511.
54. W.-L. Chan, Z. Liu, S. Lu, P. A. Tanner and K.-L. Wong, J. Mater. Chem. C, 2015, 3, 960–963.
55. R. A. Sá Ferreira, S. S. Nobre, C. M. Granadeiro, H. I. S. Nogueira, L. D. Carlos and O. L. Malta, J. Lumin., 2006, 121, 561.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra20632h

---