Eu³⁺ doped Sr₂CeO₄ phosphors for thermometry: single-color or two-color fluorescence based temperature characterization

Abstract

Sr₂CeO₄:Eu³⁺ phosphors containing various activator contents have been systemically investigated by temperature-dependent fluorescent method. The temperature-dependent photoluminescence spectra and the integrated emission intensity of Sr₂CeO₄:Eu³⁺ at different temperatures indicate that these samples can exhibit distinct low-temperature sensing characteristics. The sensitive intensity/temperature responses of these materials, depending on tuning of the concentration of Eu³⁺, provide a sound foundation for their potential application in single-color or two-color fluorescence thermometry techniques.

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1. Introduction

The characterization of temperature and thermal properties plays a fundamental role in all fields of science, hence the development of methods for measuring this characteristic remains in vogue.^1–5 In this vast field, fluorescence thermometry appears as a tool for non-contact and non-invasive surface temperature measurements,^6–8 which offers a number of advantages over conventional test methods (infrared pyrometry, thermocouples, thermistors), e.g., high accuracy, remote detection, high signal yield, cost-effective, and quantitative global temperature/heating information.⁹ As two models of this technique, both the single-color fluorescence thermometry method and the two-color fluorescence thermometry technique are based on a thermophosphor whose fluorescence intensity is temperature dependent.^7,10

Materials based on Ceria (CeO₂) have been extensively investigated owing to their broad application in the fields of catalysis and optics,^11–13e.g., they are employed as promoters in Three Way Catalysts, as electrolyte or electrode promoters, active supports or cocatalysts, as well as energy transfer medium in phosphors to sensitize the luminescence of Pr³⁺. Added with 2Sr(II), CeO₂ can form a novel type of solid solution Sr₂CeO₄, which can emit efficient blue light when irradiated by UV light, cathode rays or X-rays.¹⁴ Its luminescence was generally considered to originate from a ligand-to-metal O²⁻→Ce⁴⁺ charge transfer state (CTS).¹⁵ Since it is an active center with 100% concentration, intensive studies on this phosphor have been focused on its synthesis,¹⁶ structure,¹⁷ emission mechanism¹⁸ and its potential applications, such as the employment in field emission displays.¹⁹ Besides, Sr₂CeO₄ as a prominent host material for the incorporation of many trivalent luminescence centers, such as Eu³⁺, Sm³⁺, Dy³⁺, Ho³⁺, Er³⁺, Tm³⁺, etc., has continued to attract a lot of attention from researchers.^20–22 Effective energy transfer from Ce⁴⁺–O²⁻ CTS to these activation centers was proposed to be responsible for their luminescence in Sr₂CeO₄.²³ Among the many rare earth ions, Eu³⁺ with 4f⁶ electronic configuration can exhibit abundant emission colors, such as blue from ⁵D₂, green from ⁵D₁, red from ⁵D₀, which have been playing important roles in modern lighting and display fields. These characteristics of Eu³⁺ ions motivate researchers to explore the luminescence properties of Eu³⁺-activated Sr₂CeO₄ most. When Eu³⁺ concentration becomes higher, emission from the Ce⁴⁺−O²⁻ CTS will be much less until it disappears and then the emission owing to the Eu³⁺ transitions dominates finally. Correspondingly, emission colors of Sr₂CeO₄ added with different Eu³⁺ content changes from blue to red, in favor of their potential applications in low pressure mercury vapor (lpmv), high pressure mercury vapor (hpmv) lamps and TV tubes.²⁴

In this research, we have investigated the temperature-dependent photoluminescence (excitation and emission) properties of Sr₂CeO₄ doped with different concentrations of Eu³⁺ ions. And then we have proposed the idea that Sr₂CeO₄:Eu³⁺ might be employed as novel thermographic phosphors to measure temperature using the single-color or two-color fluorescence-based thermometry methods, on the basis of their various emission colors and temperature-sensitive luminescence properties.

2. Experimental

Strontium cerium oxide doped with different concentrations of europium, Sr_2−xEu_xCeO₄ (x = 0, 0.005, 0.01, 0.05, 0.1, 0.3), were prepared by the solid-state reaction method at high temperature. The synthetic routes to these materials were similar to those reported in the literature.²⁰ High purity SrCO₃ (A.R.), CeO₂ (99.99%), Eu₂O₃ (99.99%) employed as starting materials were blended and ground in an agate mortar. Then the mixture was heated at 1323 K in air in an alumina crucible for 10 h. After cooling to room temperature, each sample was re-ground again and the same procedure described above was adopted for them. In order to certify that the as-prepared materials belong to Sr₂CeO₄ structure, X-ray power diffraction spectra were collected using a Rigaku D/max-IIB X-Ray Diffractometer with Cu Kα (λ = 1.5405 Å) radiation. The photoluminescence excitation (PLE) spectra of samples, between 273 and 413 K, were measured by a FLS920-Combined Fluorescence Lifetime and Steady State Spectrometer (Edinburgh Instruments). The photoluminescence emission (PL) spectra of samples were obtained at elevated temperature ranging from 303 to 473 K, by a system which consists of a computer controlled CCD detector and a heater. During the measurements, a UV lamp with maximum wavelength of 365 nm was adopted as the excitation source.

3. Results and discussion

In order to characterize the phase purity and identify the structure type, X-ray powder diffraction (XRD) measurements are performed for the as-synthesized samples at room temperature. These XRD patterns of Sr₂CeO₄:Eu³⁺ particles at different doping concentrations are plotted in Fig. 1, in good accordance with the reported powder pattern in JCPDS standard card numbered 50-0115 [Sr₂CeO₄], confirming the formation of single-phase crystalline products. Sr₂CeO₄ is indexed to an orthorhombic cell in space groupPbam.¹⁴ The structure consists of linear chains of edge-sharing CeO₆ octahedra, and the terminal Ce–O distance is about 10 pm shorter than the equatorial distance.¹⁴ Therefore, both the Ce⁴⁺-site and the Sr²⁺-site are six-coordinated by oxygen ions in the Sr₂CeO₄ structure.

Fig. 1 XRD patterns of Eu³⁺-activated Sr₂CeO₄ phosphors.

As we know, when coordination number amounts to six, the ionic radii of cations Ce⁴⁺, Eu³⁺, Sr²⁺ are 87, 94.7 and 118 pm, respectively.²⁵ Therefore, we believe that Eu³⁺ ions prefer to occupy both the Sr²⁺ sites and Ce⁴⁺ sites because of their similar radius and charge. Based on the similarity of ionic charges and radii between the doping ion (Eu³⁺) and host cations (Sr²⁺, Ce⁴⁺), we consider that the trivalent Eu³⁺ ions may non-equivalently replace divalent strontium ions or tetravalent cerium ions, acting as substitutional species, as they enter the Sr₂CeO₄ crystal. Furthermore, in order to keep the charge balance, four Eu³⁺ ions will substitute six Sr²⁺ ions or three Ce⁴⁺ ions (the total charge of four Eu³⁺ ions is equal to that of six Sr²⁺ ions or three Ce⁴⁺ ions).

Hence, there might exist two chemical reactions: (i) if Eu³⁺ ions replaced Sr²⁺ ions, one vacancy defect of V_Sr′′ with two negative charges and two positive defects of Eu^•_Sr would be produced by each substitution of every two Eu³⁺ ions in the compound: 2Eu³⁺ + 3Sr²⁺ → V_Sr′′ + 2Eu^•_Sr. (ii) If Eu³⁺ ions substituted Ce⁴⁺ ions, two vacancy defects of V^••_O with two positive charges and four negative defects of Eu_Ce′ would be created by each substitution of every four Eu³⁺ ions in the compound: 4Eu³⁺ + 3Ce⁴⁺ → 2V^••_o + 4 Eu_Ce′.

The temperature-dependent luminescent excitation spectra of Sr_2−xEu_xCeO₄ (x = 0, 0.005, 0.3) are recorded in the temperature range T = 273 K to T = 413 K. The corresponding experimental results are shown in parts (a), (b) and (c) of Fig. 2. For x = 0, i.e. the host material, two major excitation bands are found in the excitation spectra when monitored for the Ce–O CT emission near 480 nm, as shown in Fig. 2(a). These two excitation bands are generally considered to result from the t_1g-f (high-energy peak) and t_1u-f (low-energy peak) charge transfer states of Ce⁴⁺, respectively.²¹ It is evident that Sr₂CeO₄ shows not only a dramatic decrease in its excitation intensity but also a significant spectral red shift in its excitation bands with the increase of temperature. At 273 K, the peak positions of the two luminescence excitation bands for Sr₂CeO₄ lie at about 286 and 334 nm, respectively. As temperature rises from 273 to 373 K, the first peak shifts from 286 to 305 nm, a difference of 19 nm; the second peak moves from 334 to 347 nm, difference = 13 nm.

Fig. 2 Temperature-dependent PLE spectra of Sr₂CeO₄:Eu³⁺ taken at elevated temperature showing charge-transfer bands shift. (a) x = 0, λ_em = 480 nm; (b) x = 0.005, λ_em = 480 nm; (c) x = 0.3, λ_em = 616 nm.

For x = 0.005, the charge transfer between Ce⁴⁺ and O²⁻ also results in two broad excitation bands, when monitored at 480 nm (the Ce–O CT luminescence emission), as displayed in Fig. 2(b). Furthermore, each excitation band decreases in intensity and distinguishably moves to longer wavelengths, behaving in a manner similar to that of x = 0. One band maximum varies from 297 to 302 nm, the other band maximum changes from 336 to 342 nm, as temperature is increased from 273 to 413 K.

Besides, by comparing the Ce–O charge transfer bands at 273 K and 373 K of x = 0.005 with those of x = 0, we note that (i) the doping of Eu³⁺ ions makes the two bands move to the red region of the spectrum at 273 K but shift to the blue region at 373 K, and (ii) the intensity ratio of the t_1u-f CT band (low-energy peak) to t_1g-f CT band (high-energy peak) increased as Eu³⁺ ions were added into the lattice. Perceptibly, the reasons for these changes are complicated and here we have tried to ascribe these findings to the changes in the environment around the O²⁻ ligand, which influences the distribution of the electrons in the corresponding orbitals and finally effects the process of charge transfer from O²⁻ to Ce⁴⁺.²⁶

For x = 0.3, the temperature-dependent excitation spectra of the strongest red-emission line at 616 nm of Eu³⁺ are given in Fig. 2(c). The weak sharp lines at about 395, 415 and 466 nm are ascribed to the transitions between the ⁷F₀ and the ⁵L₆, ⁵D₃, ⁵D₂ levels of Eu³⁺ ions. When the sample temperature rises to 293 K (close to RT), the excitation spectrum of Sr_1.7Eu_0.3CeO₄ can be considered as a strong broad band with a maximum at 300 nm and two shoulders at 289 and 338nm. For this broad excitation band, some authors have attributed it to the Ce⁴⁺–O²⁻ CT band,²⁷ however, some other authors have considered that the CT band of Ce⁴⁺ inhibits the CT band of Eu³⁺ in this lattice.²⁴ In the present case, we consider this broad band as the overlap of the Ce⁴⁺–O²⁻ CT band and the Eu³⁺–O²⁻ CT band. Because the Eu³⁺–O²⁻ CT band in oxides can be generally considered to be typically 0.6–0.8 eV wide and a strong band at energies between 4 and 6 eV,²⁸ it is reasonable that we suppose the Eu³⁺–O²⁻ CT band to be the middle part of this broad band, whose peak is located at 300 nm. This assumption can simplify the question: how does the sample temperature influence the Eu³⁺–O²⁻ CT excitation band in our work? Based on this assumption, it is clear that there is a red-shift for the Eu³⁺–O²⁻ CT band from 293 to 304 nm (difference = 11 nm), with increasing temperature from 273 to 413 K.

One notes that the temperature-dependent red-shift behavior of the charge-transfer excitation bands in Fig. 2 are more conducive to long wavelength UV absorption. And this red shift in excitation spectra is generally ascribed to the variations in the relative position of the transition from the base state to the CTS rather than the changes in the CTS itself.²⁹ This proposed mechanism can also be used to elucidate the experimental results in Fig. 2. In the following discussion, the Ce⁴⁺–O²⁻ and Eu³⁺–O²⁻ charge transfer states will be denoted as the C-CTS and E-CTS, respectively. For the three samples (x = 0, 0.005, 0.3), upon excitation the center or system is promoted from its ground state to the C-CTS or E-CTS, and this process is called the optical absorption transition.³⁰ At low temperature T₁, the transition to the C-CTS/E-CTS starts from one vibrational level ν₁ of the ground state; at higher temperature T₂, the transition to the C-CTS/E-CTS does not actually start at ν₁ (as it did at T₁) but at the higher vibrational level ν₂ of the ground state. Namely, as temperature rises up, the optical absorption transition becomes shorter and correspondingly requires less energy. If reflected in the excitation spectrum, this behavior means the transition yields a longer wavelength.

Under excitation with 365 nm ultraviolet irradiation, the temperature dependence of the PL in the Sr_2−xEu_xCeO₄ with x = 0, 0.005, 0.01, 0.03, 0.1, 0.3 are measured and given in Fig. 3, in the temperature range 303–473 K. It is observed that the positions and shapes of emission peaks are nearly the same with an increase in temperature. However, there appears to be striking thermal quenching for the C-CTS luminescence and the ⁵D_j-⁷F_J (j = 0, 1, 2, J = 1, 2, 3) luminescence of Eu³⁺ in all samples, except for the ⁵D₀-⁷F₂ transition in the samples with low Eu concentrations.

Fig. 3 Temperature dependence of PL intensity from all samples.

The specific temperature-dependent PL characteristics of as-prepared materials are listed in the following four aspects: (i) For x = 0, namely, the host material Sr₂CeO₄, only a broad emission band at around 480 nm is observed, which can be attributed to the charge transfer transition luminescence involving Ce⁴⁺. With an increase in sample temperature, Sr₂CeO₄ exhibits evident temperature quenching effect, i.e., there is a dramatic drop in its emission intensity. (ii) For, x = 0.005 and 0.01, i.e., the samples doped with low Eu³⁺ concentration, both the emission spectra are composed of the Ce–O CT broad band and a group of lines centered at 467, 486, 510, 535, 555, 585, 616 and 654 nm corresponding to the ⁵D₂-⁷F₀, ⁵D₂-⁷F₂, ⁵D₂-⁷F₃, ⁵D₁-⁷F₁, ⁵D₁-⁷F₂, ⁵D₀-⁷F₁, ⁵D₀-⁷F₂, ⁵D₀-⁷F₃ transitions of Eu³⁺, respectively. Obviously, the former dominates the emission spectra and overlaps with the latter. When sample temperature is elevated, the emission intensity of the ⁵D₀-⁷F₂ transition originally increases and then decreases, while that of the other emission bands and peaks decays dramatically. (iii) When the Eu³⁺ content reaches x = 0.03, the Ce⁴⁺–O²⁻ CT emission band and a series of Eu³⁺ intra-4f⁶ sharp lines matches each other in the PL spectra. The emission intensity of all the bands and peaks shows a distinct trend to decrease with increasing temperature. (iv) For x = 0.1 and 0.3, the Ce⁴⁺–O²⁻ CT emission band become such weaker that it can be neglected while the transitions from Eu³⁺ dominate in the given spectral region. As a result, these phosphors show distinct red emission. As the temperature increases, the emission intensity due to the characteristic transition peaks of Eu³⁺ becomes weaker gradually.

The temperature-dependent luminescence of Eu³⁺ has been explored in many systems in the past several decades. Fonger and Struck observed the thermal quenching phenomenon of Eu³⁺ luminescence in LaOCl, Y₂O₂S, and La₂O₂S.^31,32 They interpreted the phenomenon taking into account the Franck–Condon shift between the Eu³⁺–O²⁻ CT band and the 4f states and the resultant resonance crossovers between these states. That is, with increasing temperature, the excited electrons in ⁵D states go back to the ground state nonradiatively via the crossovers. In the present work, we think that the thermal quenching behaviors of the luminescence of Sr₂CeO₄ and Sr₂CeO₄:Eu³⁺ can be attributed to the similar mechanism.

Based on the above discussion, two configuration coordinate (CC) models for the mechanism of the present case are given in Fig. 4. As shown in Fig. 4(a), for the Ce⁴⁺←O²⁻ charge transfer state (C-CTS), it intersects with the ground state of Ce⁴⁺ at a point. With an increase in sample temperature, the electrons in the excited C-CTS are thermally churned up to the intersection point and then nonradiatively go back to the ground state, leading to the thermal quenching of the Ce–O CT luminescence. As shown in Fig. 4(b), for the Eu³⁺←O²⁻ charge transfer state (E-CTS), it crosses not only with the ground state of Eu³⁺ but also with the three excited states ⁵D₂, ⁵D₁, ⁵D₀. When the sample temperature increases, the electrons in the ⁵D₀ excited state are thermally agitated to the crossover point and then nonradiatively go back to the ground state via the E-CTS, leading to the thermal quenching of luminescence from the ⁵D₀ state. Nevertheless, for the electrons in the ⁵D_1,2 excited states, parts of them might go through the same path as those in the ⁵D₀ state, while parts of them might originally cascade to the lowest ⁵D₀ state via the two paths “⁵D₂→E-CTS→⁵D₀” or “⁵D₁→E-CTS→⁵D₀” and then go through the same path as those in the ⁵D₀ state. This not only describes the thermal quenching processes happening to Eu³⁺, but also can be considered as the main reason for the abnormal thermal behavior of the ⁵D₀-⁷F₂ emission in the samples (x = 0.005, 0.01) where the emission from the ⁵D_1,2 excited states and the Ce–O CTS is much stronger in the emission spectra. Another possible reason for the abnormal thermal behavior of the ⁵D₀-⁷F₂ emission at least in the case of low Eu³⁺ concentration is that the excitation spectrum shows a small red-shift at higher temperature.

Fig. 4 Two configuration coordinate models for the thermal quenching mechanism of Sr₂CeO₄:Eu³⁺. C-CTS and E-CTS represent the Ce⁴⁺–O²⁻ and Eu³⁺–O²⁻ charge transfer states, respectively. ΔE, ΔE (⁵D₀), ΔE (⁵D₁) and ΔE (⁵D₂) refer the thermal activation energy of emissions from the Ce–O CT and the ⁵D₀, ⁵D₁, ⁵D₂ excited states of Eu³⁺, respectively. G. S. indicates the ground state of Ce⁴⁺ or Eu³⁺.

In order to further illustrate temperature response characteristics of PL intensity for these materials, their integrated emission intensity at different temperature is shown in Fig. 5. Here, the integrated intensity of all samples is defined as the area under their PL curves calculated by integrating from 440 to 680 nm. If the thermal quenching temperature (T_q) is defined as the temperature at which the integrated luminescence intensity is 50% of its original value (in this case, the original value refers to the emission intensity at around RT),^33,34 the T_q value can be deduced from the intensity of emission peaks at different temperature in Fig. 3. Note that the thermal quenching temperature of samples changes with different Eu³⁺ doping content. For x = 0, 0.005, 0.01, they have the same T_q value, equal to about 358 K. This observation indicates that a small amount of Eu³⁺ ions hardly influence the thermal quenching temperature. However, for x = 0.03, 0.1, 0.3, the T_q values of them amount to 362, 399, and 446 K, respectively, which are obviously higher than those of samples with low Eu content.

Fig. 5 The integrated emission intensity of all samples as a function of temperature.

The quenching temperature of emission from luminescent centers in different host lattices has been studied by G. Blasse and A. Bril.³⁵ They have argued that there are two factors influencing on the T_q value of emission, for the luminescent centers that can be excited by charge transfer from the anion to the central cation. One is the energy of the charge transfer state (CTS); the other is Δr, the difference between the equilibrium configuration of the excited state and that of the ground state. According to their viewpoints and the CC models in Fig. 4, it can be easily summarized that the T_q value of emission exactly depends on the thermal activation energy of emission. Therefore, it can be speculated qualitatively from the T_q values that the activation energy of the Eu³⁺ emission might be higher than that of the Ce–O CT emission.

In addition, it is also observed that for the samples without the Ce–O CT emission, namely, x = 0.1 and x = 0.3, the T_q value of the latter is higher than that of the former. It can be unraveled by the following analysis: (i) in ref. 32, the authors have found that the ⁵D emissions of Eu³⁺ quenched sequentially in the simple order ⁵D₂, ⁵D₁, ⁵D₀ with increasing temperature in host materials Y₂O₂S and La₂O₂S. For the thermal quenching trend of the ⁵D emissions of Eu³⁺ in the host Sr₂CeO₄, it can be derived from the T_q values of emissions from the ⁵D₂, ⁵D₁, ⁵D₀ excited states in the sample x = 0.3. Therefore, the integrated emission intensity of luminescence from the ⁵D states of Eu³⁺ in x = 0.3 as a function of temperature is shown in Fig. 6. And then the T_q values of the ⁵D₀, ⁵D₁ and ⁵D₂ emissions are found to be 448, 388 and 348 K, respectively. This outcome suggests that the thermal quenching order of the ⁵D emissions of Eu³⁺ in Sr₂CeO₄ is the same as that in Y₂O₂S and La₂O₂S. Moreover, this order accords with the decreasing trend of ΔE (⁵D₀) > ΔE (⁵D₁) > ΔE (⁵D₂) observed from the CC models in Fig. 4. (ii) The integrated intensity ratio R of emission from the ⁵D₀ state to ⁵D₁, ⁵D₀ to ⁵D₂ and ⁵D₁ to ⁵D₂ are denoted as R_0/1, R_0/2, R_1/2, respectively. At 303 K, i.e., the initial temperature in the present research, when the Eu³⁺ molar content reaches x = 0.3 from x = 0.1, the integrated intensity ratio values vary from 9.623 to 77.84 for R_0/1, from 12.41 to 52.80 for R_0/2 and from 0.7754 to 1.474 for R_1/2, respectively. In other words, the proportion occupied by the ⁵D₀ state becomes larger among the ⁵D emissions of Eu³⁺. As a result, the T_q value of x = 0.3 are higher than that of x = 0.1.

Fig. 6 The integrated intensity of the ⁵D_j (j = 0, 1, 2) emissions of Eu³⁺ in Sr_1.7Eu_0.3CeO₄ at different temperatures. The annotation “*30” indicates 30 times the original data.

4. Conclusion

The temperature-dependent PL studies show that Eu³⁺-activated Sr₂CeO₄ with various Eu³⁺ concentrations have obvious thermal quenching effects. This observation suggests that these materials are not suitable as potential phosphors in lpmv, hpmv or TV tubes. However, they may be employed in fluorescence thermometry. On the one hand, the materials with x = 0, 0.1 and 0.3, radiating monochromatic light (blue or red), may be applied in the single-color fluorescence-based thermometry technique. On the other hand, the materials with x = 0.005, 0.01 and 0.03, which can emit dichromatic light (concentrated in the blue and red spectral regions), may be considered as candidates for the two-color thermographic phosphors.

Acknowledgements

The financial support for this work was extended by National High-tech R&D Program of China (Grant No. 2010AA03A404) and National Natural Science Foundation of China (Grant No. 20921002).

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