Tb-codoped inorganic apatite Ca5(PO4)3F luminescent thermometer

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CERAMICS INTERNATIONAL

Ceramics International 41 (2015) 7010–7016 www.elsevier.com/locate/ceramint

An Eu/Tb-codoped inorganic apatite Ca5(PO4)3F luminescent thermometer Linlin Fua, Zuoling Fua,n, Yingning Yub, Zhijian Wub, Jung Hyun Jeongc,1 b

a State Key Laboratory of Superhard Materials, College of Physics, Jilin University, Changchun 130012, China State Key Laboratory of Rare Earth Resources Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China c Department of Physics, Pukyong National University, Busan 608-737, South Korea

Received 27 January 2015; received in revised form 1 February 2015; accepted 2 February 2015 Available online 7 February 2015

Abstract A new Eu/Tb-codoped inorganic apatite Ca5(PO4)3F luminescent thermometer has been targeted. Under ultraviolet irradiation, the Ca5(PO4)3F: Tb3 þ /Eu3 þ samples exhibit a blue-light emission of the host matrix which might originate from the CO2 radical-related defect produced by Cit3  groups, as well as the typical green emission band of the Tb3 þ ions, and a red-light emission of Eu3 þ . These lanthanide-based thermometers (Ca5(PO4)3F:Tb3 þ /Eu3 þ ) exhibit different sensitivities ranging from different temperatures based on two emissions of Tb3 þ at 548 nm and Eu3 þ at 621 nm. The temperature-dependent luminescent intensity ratios ITb/IEu can be optimized by the controlled relatively doping of different amounts of Tb3 þ and Eu3 þ ions. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: Luminescent thermometer; Inorganic apatite; Lanthanide ions

1. Introduction Temperature plays crucial roles. Almost all physical, chemical and biological processes are temperature dependent, making accurate temperature knowledge is essential for their control and understanding [1,2]. There are many areas of industry where temperature measurements are essential, such as metallurgical, glass manufacturing, material modeling as well as food manipulation and testing [3]. In most of these applications contact-based temperature measurements (using thermocouples, thermistors or resistance thermometers) are not applicable. A fluorescent sensor that makes use of temperature-dependent fluorescence properties from luminescence material can over-come the interference of strong electromagnetic noise, hazardous sparks, and corrosive environment that are inaccessible to traditional temperaturemeasurement methods [4]. In this vast field, fluorescence thermometry appears as a tool for non-contact and non-invasive surface temperature measurements, which offers a number of advantages n

Corresponding author. Tel.: þ 86 431 85167966; fax: þ 86 431 85167966. E-mail addresses: [email protected] (Z. Fu), [email protected] (J.H. Jeong). 1 Tel.: þ82 51 6295564; fax: þ 82 51 6295549. http://dx.doi.org/10.1016/j.ceramint.2015.02.004 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

over conventional test methods (infrared pyrometry, thermocouples, and thermistors), e.g., high accuracy, remote detection, high signal yield, cost-effective, and quantitative global temperature/ heating information [5]. 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 [6]. Most luminescence-based thermometry mainly relies on the temperature-dependent fluorescence intensity of one transition, the accuracy of which is susceptible to errors introduced by optical occlusion, concentration inhomogeneities, excitation power fluctuations, or environment-induced nonradiative relaxation [7–10]. Ratiometric detection based on the intensity ratio of two independent emissions of the same phosphor can circumvent these complications and give rise to a more accurate self-referencing thermal sensing, thus gaining popularity [11]. Thermally stable coordination polymers and metal–organic frameworks have been widely studied [6]. The recently developed M'Ln-MOF methodology for luminescent thermometers is based on the energy transfer from the Tb3 þ to Eu3 þ ions within the framework solids which host materials with suitable state energy in the range of 22,000– 27,000 cm  1 as the prerequisite to sensitize both Eu3 þ and Tb3 þ emissions [12–16]. Most of the inorganic temperature sensing

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material doped with Yb3 þ and Er3 þ ions that in combination with the high absorption cross-section at the pumping wavelength (980 nm) results in a strong up-conversion visible emission obtained under infrared excitation [17,18]. Herein, a new Eu/Tbcodoped inorganic apatite Ca5(PO4)3F as a luminescent thermometer has been targeted. In these inorganic apatite materials, it has an intense broad bluish emission which might originate from the CO2 radical-related defect produced by Cit3  groups [19]. These characteristics make the composites suitable for luminescent thermometer because it has the suitable state energy (22,000oΔEo27,000 cm  1) which can sensitize both Eu3 þ and Tb3 þ emissions. These lanthanide-based thermometers (Ca5(PO4)3F:Tb3 þ /Eu3 þ ) exhibit two main narrow emission bands (Tb3 þ at 548 nm and Eu3 þ at 621 nm), each of which has its own temperature dependence. The luminescence intensity ratio of the two emission bands ITb/IEu affords a self-referenced signal used for more accurate probing for temperature.

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2. Experimental sections A detailed synthetic procedure for the incorporation of rare earth ions (Tb3 þ and Eu3 þ ) into Ca5(PO4)3F microrods, as well as a microstructural characterization have been described in our previous work [19]. For luminescence experiments, the temperature-dependent luminescence spectra were recorded on an Edinburgh Instrument FLSP-920 spectrometer. Measurements at low temperature (98–300 K) were performed with a nitrogen bath cryostat (Oxford Instruments, Optistat DN) and a temperature controller (Oxford, Instruments, ITC 502S). 3. Results and discussion Fig. 1(a) shows the emission and excitation spectra of Ca5(PO4)3F:Tb3 þ . The curve on the left shows the excitation spectrum of the Ca5(PO4)3F:Tb3 þ sample by the emission at 548 nm. The broad band with a maximum at 299 nm which is assigned to the spin-allowed 4f7-4f75d transition of Tb3 þ . The curve on the right shows the emission spectrum of the Ca5(PO4)3F:Tb3 þ sample by the excitation at 299 nm. Some sharp peaks from the characteristic 5D4-7FJ transitions of Tb3 þ , and the dominant one are the 5D4-7F5 transition at 548 nm. Furthermore, we can see that the Tb3 þ doped Ca5(PO4)3F has an intense broad bluish emission centered at about 425 nm (23,529 cm  1) under the excitation of 299 nm, which might originate from the CO2 radical-related defect produced by Cit3 groups [19]. The methodology for luminescent thermometers is based on the energy transfer from the Tb3 þ to Eu3 þ ions within the host materials. Host materials with suitable state energy in the range of 22,000–27,000 cm  1 are the prerequisite to sensitize both Eu3 þ and Tb3 þ emissions. Fig. 1(b) shows the schematic representation of energy migration, emission, and processes in Eu/Tb-codoped inorganic apatites Ca5(PO4)3F luminescent thermometer. The schematic in which the solid arrows represent radiative transitions of the CO2 radical-related defect transition

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λem=548nm

λem=621nm λex=258nm

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Fig. 1. (a) The excitation (left) and emission (right) spectra of Ca5(PO4)3F: Tb3 þ microrods. (b) Schematic representation of energy migration, emission, and processes in Eu/Tb-codoped inorganic apatites Ca5(PO4)3F luminescent thermometer. Abbreviations: S¼ singlet; K¼ radiative or nonradiative transition probability. The solid arrows represent radiative transitions; dotted arrows indicate nonradiative transitions.

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Wavelength (nm) Fig. 2. Excitation spectra of Eu3 þ (blue line) and Tb3 þ (red line), and the emission spectrum (green line) of Eu/Tb co-doped Ca5(PO4)3F sample in room temperature. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)

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Fig. 4. Emission spectra of the (a)Ca5(PO4)3F:0.06Tb3 þ , xEu3 þ (x¼0.04–0.10) and (b) Ca5(PO4)3F:0.10Tb3 þ , xEu3 þ (x¼ 0.04–0.10) microcrystals.

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Temperature (K) Fig. 3. Emission spectra of (a) Ca5(PO4)3F:Tb3 þ and (b) Ca5(PO4)3F:Eu3 þ recorded between 98 and 573 K (excited at 258 nm), and (c) temperaturedependent intensity of the 5D4-7F5 transition of Ca5(PO4)3F:Tb3 þ and 5 D0-7F2 transition of Ca5(PO4)3F:Eu3 þ .

produced by Cit3 groups, the 5D0-7FJ transition of Eu3 þ and the 5D4-7FJ transition of Tb3 þ , while dotted arrows indicate nonradiative transitions. From the schematic we can see, it is necessary to have a higher state energy for sensitization as a luminescent thermometers host materials. In this work, from the emission spectrum we know that the Ca5(PO4)3F host materials match the condition that could sensitize both Eu3 þ and Tb3 þ emissions because it has the suitable sensitization state energy. So we choose the inorganic apatite Ca5(PO4)3F as the host matrix to investigate the properties of temperature sensing.

The emission and excitation spectra measured at room temperature for the powder samples of the Tb3 þ and Eu3 þ codoped Ca5(PO4)3F are displayed in Fig. 2. The curve on the left shows the excitation spectra of the sample by the emission at 621 nm (blue line) and 548 nm (red line). By monitoring the 548 nm emission of Tb3 þ and the 621 nm emission of Eu3 þ , excitation spectrum of Ca5(PO4)3F:Tb3 þ , Eu3 þ displays strong broad band and sharper peaks, which corresponds to the spinallowed 4f7-4f75d transition of Tb3 þ , the Eu3 þ –O2 charge transfer band, defect-related luminescence as well as the 4f–4f transitions of Tb3 þ and Eu3 þ [19]. However, under the excitation at 258 nm, the emission spectrum (green line) simultaneously contains defect-related luminescence, 621 nm (5D0-7F2) of Eu3 þ and the 548 nm (5D4-7F5) transition of Tb3 þ in Ca5(PO4)3F microrods. So we finally choose the 258 nm as the excitation wavelength to discuss the other temperature-dependent spectra. The temperature-dependent photoluminescent (PL) properties of the Ca5(PO4)3F:Tb3 þ and Ca5(PO4)3F:Eu3 þ were investigated in terms of intensity in order to establish their potentials as luminescent thermometers. The temperature-dependent emission

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10% Tb 6% Eu

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Fig. 5. Emission spectra of (a) Ca5(PO4)3F:0.06Tb3 þ , 0.08Eu3 þ and (b) Ca5(PO4)3F:0.10Tb3 þ , 0.06Eu3 þ recorded between 98 and 573 K (excited at 258 nm), and temperature-dependent intensity of the 5D4-7F5 and 5D0-7F2 transition for (c) Ca5(PO4)3F:0.06Tb3 þ , 0.08Eu3 þ and (d) Ca5(PO4)3F:0.10Tb3 þ , 0.06Eu3 þ .

spectra of Ca5(PO4)3F:Tb3 þ and Ca5(PO4)3F:Eu3 þ from 98 to 573 K are illustrated in Fig. 3(a) and (b) while temperaturedependent intensity of the 5D4-7F5 (Tb3 þ , 548 nm) and 5 D0-7F2 (Eu3 þ , 621 nm) transitions is shown in Fig. 3(c). As expected, the luminescent intensities of both Tb3 þ and Eu3 þ in Ca5(PO4)3F:Tb3 þ and Ca5(PO4)3F:Eu3 þ decrease gradually as the temperature increases, which is normally due to the thermal activation of nonradiative-decay pathways [20]. In order to better clarify the properties of temperature sensing, we straightforwardly synthesize a series of rare earth-doped Ca5(PO4)3F microrods in which variable molar ratios of Tb3 þ and Eu3þ ions can be systematically incorporated into the host. Tb3þ ions with the same concentration and the Eu3þ ions with different concentrations were doped into Ca5(PO4)3F. Fig. 4(a) and (b) displays the emission spectra of the Ca5(PO4)3F:0.06Tb3þ , xEu3 þ and Ca5(PO4)3F: 0.10Tb3þ , xEu3þ microcrystals with different Eu3þ concentrations respectively. The luminescent intensity of Tb3þ and Eu3þ varies with simply adjusting the relative doping concentrations of the Tb3þ and Eu3þ ions in the Ca5(PO4)3F. We select a better luminous one of these isostructural samples which are that Ca5(PO4)3F:0.06Tb3þ , 0.08Eu3þ and Ca5(PO4)3F:0.10Tb3 þ , 0.06Eu3þ to investigate the Eu/Tb-codoped inorganic apatites Ca5(PO4)3F luminescent temperature sensing property.

To examine the thermal effect on the PL behavior, the emission intensity of the Ca5(PO4)3F:6% Tb3 þ , 8% Eu3 þ and the Ca5(PO4)3F:10% Tb3 þ , 6% Eu3 þ was recorded at temperatures ranging from 98 to 573 K under the excitation at 258 nm (Fig. 5 (a) and (b)). The emission intensities of both Tb3 þ and Eu3 þ ions in Ca5(PO4)3F:6% Tb3 þ , 8% Eu3 þ show an obvious decrease with the rise in sample temperature. However, it is interesting that the intensity of Tb3 þ (548 nm) exhibits a significantly different temperature-dependent luminescence behavior as compared to that of Eu3 þ (621 nm) in Ca5(PO4)3F:6% Tb3 þ , 8% Eu3 þ (Fig. 5(c)). The emission intensity of 548 nm (Tb3 þ ) shows a linear temperature dependence whereas the 621 nm (Eu3 þ ) emission is much less affected by the change of temperature and even a little bit increase in the 150–250 K temperature range. And it is also the same tendency of Ca5(PO4)3F:10% Tb3 þ , 6% Eu3 þ sample (Fig. 5(d)). The different temperature-dependent luminescent emissions of 5D4-7F5 (Tb3 þ , 548 nm) and 5D0-7F2 (Eu3 þ , 621 nm) in the same host materials have enabled them to be excellent candidates for self-referencing luminescent thermometers [21]. Therefore, in this work, the emission intensity ratio of the 5 D4-7F5 (Tb3 þ at 548 nm) to 5D0-7F2 (Eu3 þ at 621 nm) transition (denoted as ITb/IEu) is used as a self-calibrated reference for the sample temperature, such that the well-known drawbacks

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Fig. 6. Temperature-dependent intensity ratio of Tb3þ (548 nm) to Eu3þ (621 nm) and the fitted curve for (A) Ca5(PO4)3F:0.06Tb3þ , 0.08Eu3þ between different temperature ranges (a1) 98–300 K, (a2) 273–373 K and (a3) 323–420 K; (B) Ca5(PO4)3F:0.10Tb3þ , 0.06Eu3þ range from (b1) 98 K to 300 K, (b2) 273 to 373 K and (b3) 323 K to 523 K.

of one transition intensity-based measurements (e.g., quantity of the luminophore, excitation power or strong electromagnetic noise) can be easily circumvented. From 98 to 300 K the following linear dependence of ITb/IEu on temperature is found for: I Tb =I Eu ¼ 1:782–0:0038T

ð1Þ

I Tb =I Eu ¼ 2:925–0:0053T

ð2Þ

Eqs. (1) and (2) are represented for the change of the intensity ratio on temperature of the Ca5(PO4)3F:6% Tb3 þ , 8% Eu3 þ and the Ca5(PO4)3F:10% Tb3 þ , 6% Eu3 þ sample respectively. As shown in Fig. 6A(a1) and B(b1), the sensitivity of this new Eu/ Tb-codoped inorganic apatites Ca5(PO4)3F luminescent thermometer is about 0.38% and 0.53% per K in the temperature range 98–300 K, respectively, which is relatively high value for inorganic materials even higher than that 0.38% per K in

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Eu0.0069Tb0.9931-DMBDC organic materials [12]. These results suggest that both Ca5(PO4)3F:6% Tb3 þ , 8% Eu3 þ and the Ca5(PO4)3F:10% Tb3 þ , 6% Eu3 þ are excellent luminescent thermometers in this temperature range. Moreover, we have also examined the sensitivity of both samples from 273 K to 373 K as the Fig. 6A(a2) and B(b2) shown, which temperature range is better applied to biology in our daily life. The sensitivity of 0.13% per K and 0.25% per K for Ca5(PO4)3F:6% Tb3 þ , 8% Eu3 þ and the Ca5(PO4)3F:10% Tb3 þ , 6% Eu3 þ is relatively higher than ever reported in this temperature range [22]. Furthermore, Fig. 6A(a3) and B(b3) gives the sensitivities of both samples in the high temperature range (320–530 K). Although its sensitivity nearly 0.064% per K of both sample is not very high compared with others that investigated above, this temperature range has been rarely reported before and may have some reference value for further investigation. In this work, we have investigated two different doping concentrations Eu/Tb-codoped inorganic apatites Ca5(PO4)3F. From the above results we can easily find out that the sensitivity of Ca5(PO4)3F:10% Tb3 þ , 6% Eu3 þ sample is a little higher than that Ca5(PO4)3F:6% Tb3 þ , 8% Eu3 þ sample in different temperature ranges. Indeed, the temperature-dependent luminescent intensity ratios ITb/IEu can be optimized by the controlled relatively doping of different amounts of Tb3 þ and Eu3 þ ions. In the following work, we will concentrate on how to obtain the most suitable inorganic luminescent thermometer nano-materials for its application in biological field. 4. Conclusions In summary, a new Eu/Tb-codoped inorganic apatite Ca5(PO4)3F as a luminescent thermometer has been targeted. Different sensitivities of Eu/Tb-codoped Ca5(PO4)3F samples between different temperature ranges have been investigated. The temperaturedependent luminescent intensity ratios ITb/IEu can be optimized by the controlled relatively doping of different amounts of Tb3 þ and Eu3þ ions. The sensitivities of Eu/Tb-codoped Ca5(PO4)3F samples are 0.37% and 0.53% per K in the temperature range 98–300 K, 0.13% per K, 0.25% per K ranging from 273 to 373 K and nearly 0.064% per K in the higher temperature range for Ca5(PO4)3F:6% Tb3þ , 8% Eu3þ and the Ca5(PO4)3F:10% Tb3þ , 6% Eu3 þ , respectively. In most of the field, this new kind of Eu/Tb-codoped inorganic apatite luminescent thermometer may be a new direction for temperature sensing technology. We expect to design and construct practically useful inorganic luminescent thermometers with high sensitivity and further investigation is ongoing. Acknowledgments This work was supported by the Science and Technology Development Planning Project of Jilin Province (20130522173JH), partially sponsored by China Postdoctoral Science Foundation (No. 2013M540241), supported by National Found for Fostering Talents of Basic Science (No. J1103202) and by Outstanding Young Teacher Cultivation Plan in Jilin University (No. 419080500300).

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