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LiSrCa(SiO):Eu : A Potential Temperature Sensor with Unique Optical Thermometric Properties Rui Shi, Lixin Ning, Yan Huang, Ye Tao, Lirong Zheng, Zhibing Li, and Hongbin Liang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b22754 • Publication Date (Web): 28 Feb 2019 Downloaded from http://pubs.acs.org on March 1, 2019
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ACS Applied Materials & Interfaces
Li4SrCa(SiO4)2:Eu2+: A Potential Temperature Sensor with Unique Optical Thermometric Properties Rui Shi†, Lixin Ning‡,*, Yan Huang§, Ye Tao§, Lirong Zheng§, Zhibing Li†, Hongbin Liang†,* † MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, KLGHEI of Environment and Energy Chemistry, State Key Laboratory of Optoelectronic Materials and Technologies, School of Physics, School of Chemistry, Sun Yat-sen University, Guangzhou 510275, China. ‡ Anhui Key Laboratory of Optoelectric Materials Science and Technology, Key Laboratory of Functional Molecular Solids, Ministry of Education, Department of Physics, Anhui Normal University, Wuhu, Anhui 241000, China. § Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100039, China. Keywords: temperature sensor; optical thermometry; Eu2+; luminescence evolution; electron transfer; high sensitivity ABSTRACT: Investigations on luminescence properties of lanthanide-activated phosphors are not only essential to understand the fundamental structure-property relationship but also important to advance the development and application of relevant research techniques. We report herein a promising optical thermometric material Eu2+-doped Li4SrCa(SiO4)2 utilizing the different sensitivities of EuSr2+ and EuCa2+ emission intensities to temperature. A unique evolution of Eu2+ luminescence in the as-prepared sample is identified under the simultaneous action of UV illumination and thermal treatment. The maximum relative sensitivities are 2.87 % K-1 (at 440 K) and 1.51% K-1 (at 460 K) for the asprepared and illuminated samples, respectively. These temperature sensing features reflect a great potential of Eu2+-doped Li4SrCa(SiO4)2 for applications in the optical thermometry field.
High-precision temperature sensors have recently attracted extensive interests for their applications in the systems with controlled manufacture, micro-zone environment monitoring and safety in production.1,2 Conventional physical temperature probes such as thermocouples and thermometers have limitations in noncontact and dynamic systems. Therefore, much effort has been expended to develop advanced thermometric strategies not only from detection equipment arrangement but also from exploration of novel sensing materials.3 Among them, optical thermometry serves as an alternative and lanthanide-doped phosphors have been regarded as promising optical thermometric materials.4,5 Currently, the sensing functions of lanthanide-doped optical thermometric materials are mainly realized by exploiting the different temperature-dependent luminescence responses of “thermally coupled” excited states within 4fN configuration of dopant lanthanide ions. For example, Yin et al. reported that Er3+-doped NaYb2F7 glass-ceramics offered the potential for optical thermometry because the luminescence from Er3+ (4f11)2H11/2 and (4f11)4S3/2 states displayed different temperature dependences.6 Benayas et al. mentioned that Nd3+-doped Y3Al5O12 nanoparticles exhibited a remarkable thermometric property due to the different temperature-dependent luminescence of Nd3+ (4f3)4F5/2 and (4f3)4F3/2 states.7 However, the sensing
precision of the materials developed through the abovementioned strategy can hardly be further improved as a result of the nearly constant energy difference between the two “thermally coupled” states in different materials due to the insensitivity of 4fN energy levels to the local environment. Moreover, the actual application of the optical thermometric materials developed through this strategy requires strongly a spectrometer with high wavelength resolution, which also restricts their applicability. As an alternative, Eu2+-doped phosphors could be utilized as a feasible next-generation temperature sensor. The dopant Eu2+ may exhibit bright luminescence corresponding to 4f65d14f7(8S7/2) (5d4f) parity-allowed transitions.8-10 Because of the strong interaction of Eu2+ 5d electron with the local crystal field, the temperaturedependence of 5d4f luminescence is largely determined by the coordination environment.11,12 When substituted into the different sites of a material, Eu2+ may exhibit different temperature-sensitive luminescence properties.13 Along this line, the development of advanced Eu2+activated optical thermometric materials can be anticipated. Li4SrCa(SiO4)2 (abbreviated as LSCSO hereafter), with the outstanding chemical stability, may serves as a potential candidate host to accomplish the above purpose.14 Two different kinds of Eu2+ substitutional sites
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exist in the structure: Sr2+ sites with Cs symmetry coordinated by ten oxygens and Ca2+ sites with Ci symmetry coordinated by six oxygens. In our previous work,15 a combined experimental and computational study predicted that the temperature dependence of Eu2+ luminescence substituted at Sr2+ and Ca2+ sites (denoted as EuSr2+ and EuCa2+, respectively) could be much different from each other. This makes Eu2+-doped LSCSO (LSCSO:Eu2+) a good candidate to demonstrate the potential of Eu2+-activated phosphors with multi-site occupation in optical thermometry. In the following, sitedependent luminescence properties of Eu2+ in LSCSO are first studied in the VUV-UV-vis region at low temperatures. Temperature-dependent luminescence are then investigated, and the materials exhibit a unique evolution in luminescence properties during the cyclical “temperature heating-cooling” phases. To the best of our knowledge, it is the first time that such a luminescence evolution is reported in the case of bulk inorganic phosphors. Finally, the temperature sensing features are evaluated, and the results imply a great potential of Eu2+doped LSCSO for high-sensitive optical thermometry applications. 18
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Eu2+ centers, which are supposed to be Eu2+ situated at Sr2+ and Ca2+ sites. Furthermore, the shorter- and longerwavelength excitations can be assigned to EuSr2+ and EuCa2+ centers, respectively, in view of the fact that the larger Sr2+ (smaller Ca2+) coordination polyhedron should result in a smaller (larger) crystal field splitting of dopant Eu2+ 5d levels and thus a higher (lower) energy for its first 4f5d transition. Upon 330 and 425 nm excitations, two broad emission spectra are observed with maxima at 425 and 575 nm, respectively, which are ascribed to 5d4f transitions of EuSr2+ and EuCa2+. Note that EuCa2+ emission is absent in the emission spectrum obtained upon 330 nm excitation, and no EuSr2+ 4f5d excitation is observed in the excitation spectrum of 570 nm emission. This indicates inefficient EuSr2+EuCa2+ energy transfer in the as-prepared sample. Figure 1b displays the concentration-dependent EuSr2+ and EuCa2+ emission intensities under their characteristic 4f5d excitations at RT. The intensities of EuSr2+ emission are much stronger than those of EuCa2+ emission in all samples, showing that most dopants Eu2+ are situated at Sr2+ sites which is consistent with the closeness of their ionic sizes. With the increase of Eu concentration, both EuSr2+ and EuCa2+ emission intensities decrease as a result of concentration quenching, which is also manifested by the decrease of luminescence lifetimes as shown in Figure 1c and 1d.
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Figure 1. (a) Site-dependent VUV-UV-vis excitation/emission spectra of the as-prepared LSCSO:0.004Eu2+ at 20 K. (b) Concentration-dependent EuSr2+ and EuCa2+ emission intensities at RT. (c, d) Site-dependent luminescence decay curves of Eu2+ with different concentrations under 330 and 425 nm excitations.
A series of Eu2+-doped LSCSO was prepared using a convenient high temperature solid-state reaction route. The details of the preparation are described in the Supporting Information (SI). Figure 1a shows the sitedependent excitation/emission spectra of the as-prepared LSCSO:0.004Eu2+ at 20 K. (Note that the phrase “asprepared” means that the studied sample has never been used for spectral measurements before.) By monitoring the emission at 425 nm, the excitation spectrum is mainly in the range of 225–400 nm with maximum at 290 nm. When changing the monitoring emission wavelength to 570 nm, a broad excitation spectrum peaking at 375 nm is observed. These results suggest the presence of two distinct sets of
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Figure 2. (a) Temperature-dependent area-normalized emission spectra of LSCSO:0.004Eu2+ under 330 nm excitation and the inset denotes the variations of EuSr2+ and EuCa2+ luminescence intensities with increasing of temperature. (b) Emission spectra of the studied sample under 425 nm excitation at different temperatures.
Figure 2a and 2b give the temperature-dependent emission spectra of the as-prepared LSCSO:0.004Eu2+. Upon 330 nm excitation, the intensity of EuSr2+ emission decreases dramatically while that of EuCa2+ emission increases simultaneously with increasing temperature from 300 to 500 K (Figure 2a and the inset). At each temperature, after 330 nm excited emission measurement, EuCa2+ emission under 425 nm excitation were also measured. Surprisingly, an incredible enhancement of EuCa2+ emission intensity with the increase of temperature
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ACS Applied Materials & Interfaces was detected; the intensity at 500 K is 9.3 times larger than that at 300 K (Figure 2b). This observation is also displayed in PHASE 1 of Figure 3a, where the whole temperaturecycling measurement process is divided into five phases. This indicates that the number of Eu2+ ions at Ca2+ sites increases significantly upon simultaneous action of 330 nm illumination and thermal treatment, at the expense of the number of Eu2+ ions at Sr2+ sites. At 500 K, EuSr2+ emission intensity is almost stable under 330 nm excitation, while EuCa2+ emission intensity under 425 nm excitation still increases somewhat (PHASE 2). Figure 3b gives the CIE chromaticity coordinate variation of the as-prepared sample in PHASE 1-2. Under 330 nm excitation, the emission color changes from blue to yellowish-white and finally becomes bright yellow, and the corresponding luminescence photographs are exhibited on the right of the figure. We note that the EuSr2+ and EuCa2+ luminescence evolution in PHASE 1-2 is a result of combined action of 330 nm illumination and thermal treatment of the asprepared sample, and this point has also been confirmed, as described in Figure S6.
studied sample under 330 nm excitation in PHASE 4. As temperature rises, EuCa2+ yellow luminescence becomes predominant while EuSr2+ blue luminescence becomes weakened stepwise, resulting in an emission color tuning in a large color gamut as shown in Figure 3d. To understand the above luminescence evolution, Eu L3edge X-ray absorption near-edge structure (XANES) spectral measurements of LSCSO:xEu2+ as-prepared samples were performed first. The results show that in the samples most europium ions exist as Eu3+, with only a small amount of Eu2+ (Figure S7). Next, the analysis of luminescence properties of Eu3+ in LSCSO (Figure S8 and S9) reveals that Eu3+ prefers to be at the smaller Ca2+ site (denoted as EuCa3+) rather than at the larger Sr2+ site (denoted as EuSr3+) that are favorable for Eu2+ occupation. Finally, our recent work showed that the energy levels of the emitting EuSr2+(5d) and EuCa2+(5d) excited states were close to the host conduction band (CB),15 enabling thermal ionization of the 5d electron into the host conduction band. With these in mind, we propose the following simplified scenario to explain the luminescence evolution. EuSr2+(4f) + h (330 nm) EuSr2+(5d) EuSr3+(4f) + eCB (1) and EuCa3+(4f) + eCB EuCa2+(5d) EuCa2+(4f) + h (575 nm) (2)
Figure 3. (a) EuSr2+ and EuCa2+ luminescence intensities of LSCSO:0.004Eu2+ under 330 and 425 nm excitations in different temperature-variation phases. (b) CIE chromaticity coordinate changes of the as-prepared sample under 330 nm illumination in PHASE 1-2 and the corresponding luminescence photographs. (c) 3D color-filled contour of areanormalized emission spectra of the sample under 330 nm excitation in PHASE 4. (d) CIE chromaticity coordinate changes of the sample under 330 nm illumination in PHASE 35 and the corresponding luminescence photographs.
After PHASE 2, when the temperature decreases to 300 K (PHASE 3), increases again to 500 K (PHASE 4), and the rest (e.g. PHASE 5), both EuSr2+ and EuCa2+ emission intensities exhibit a conventional temperature dependence, which is usually described by the empirical Arrhenius equation.16 Thus, the combined action of 330 nm illumination and thermal treatment in PHASE 1-2 makes the as-prepared sample undergo an irreversible conversion from a non-equilibrium to an equilibrium condition (PHASE 3-5) in terms of temperature dependence of EuSr2+ and EuCa2+ luminescence. Figure 3c shows the 3D colorfilled contour of area-normalized emission spectra of
Initially, for the as-prepared sample, most dopant Eu ions are in the form of Eu3+ and its number at Ca2+ sites is larger than that at Sr2+ sites, with only a small amount of Eu2+ mostly at Sr2+ sites. Under 330 nm illumination, the EuSr2+(4f) ground-state was excited into the EuSr2+(5d) excited state, and the 5d electron therein can be thermally ionized into the host CB with Eu2+ becoming EuSr3+(4f) (expression (1)). Since the number of EuCa3+(4f) ions is larger than that of EuSr3+(4f) ions, EuCa3+(4f) has a larger probability than EuSr3+(4f) to capture the electron (eCB) in the host CB and thus becomes EuCa2+(5d), which finally returns to EuCa2+(4f) by emitting a photon at 575 nm (expression (2)). With increasing temperature, the thermal ionization probability of EuSr2+ 5d electrons into the host CB increases, resulting in an enhancement of EuCa2+(5d) number and the emission intensity (PHASE 1-2). This process continues until an equilibrium between the 5d thermal ionization from EuSr2+(5d) and EuCa2+(5d) and the capture of eCB by EuSr3+(4f) and EuCa3+(4f) is reached, and then the EuSr2+ and EuCa2+ emission intensities come back to the conventional temperature dependence (PHASE 3-5). Note that this temperature dependence of emission intensities was stable if the temperature-cycling measurement was further continued in the illuminated sample. In view of the distinct difference between the thermalinduced luminescence properties of EuSr2+ and EuCa2+ under 330 nm excitation, Eu2+-doped LSCSO material may serve as a promising optical thermometric sensor. Generally, the ratiometric emission intensity measurement (RIM) is the frequently-used temperature read-out strategy to calibrate the thermometric characteristics of materials because of the merit of self-referencing.1,2,17 Herein, RIM of
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EuCa2+/EuSr2+ emission intensities—I(EuCa2+)/I(EuSr2+) under 330 nm excitation can be treated as a sensitive probe and the obtained RIMs in the irreversible PHASE 1 are shown
a unique evolution of Eu2+ luminescence in the as prepared sample by a combined action of UV illumination and thermal treatment. The application potential of the material as the temperature sensor were evaluated. The results demonstrate the usability of Eu2+-activated phosphors with multiple substitutional sites in optical thermometry.
ASSOCIATED CONTENT Supporting Information. Sample preparation and experimental details, XRD patterns of samples, Eu L3-edge XANES spectra and site-dependent luminescence properties of Eu2+/Eu3+ in LSCSO. This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION Corresponding Author Figure 4. (a1) RIM [I(EuCa2+)/I(EuSr2+)] at different temperatures and (a2) temperature-dependent Sr values in PHASE 1. (b) RIM [I(EuCa2+)/I(EuSr2+)] at different temperatures and the inset gives temperature-dependent Sr values in PHASE 4. (c) RIM in temperature-cycling measurements.
in Figure 4a1. The temperature-dependent relative sensitivities (Sr) are given in Figure 4a2 and the maximum value is 2.87 % K-1 (at 440 K), which is higher than the sensing precision of most lanthanide-doped inorganic optical thermometric materials.18 After the luminescence evolution in PHASE 1-2, the illuminated sample exhibits another different but steady optical thermometric characteristics. Figure 4b gives the obtained RIMs at different temperatures and the maximum Sr value is 1.51% K-1 (at 460 K), as shown in the inset. Moreover, Figure 4c presents the temperature-cycling measurement of the material in PHASE 5 to assess its reversibility. The RIMs are almost equal with each other at the same temperature in different periods, indicating the stable temperature sensing accuracy of the illuminated Eu2+-doped Li4SrCa(SiO4)2 as a potential optical thermometric material. The temperature-sensing range of the asprepared and illuminated material covers the fire point of paper (~456 K) and the heat deflection temperature of most daily-use polymer materials (370–460 K), which makes Eu2+-doped LSCSO a promising optical thermometric material to be applied in the fields of fire alarm and safety protection. Considering that the luminescence properties of Eu2+-doped LSCSO material are strongly related to the site-occupation preference and valence state distribution of Eu ions in the system, the optical thermometric capability of the material could be further improved by optimizing the reaction atmosphere and chemical composition of the host material in the preparation process. In this contribution, the temperature dependences of Eu2+ luminescence at the Sr2+ and Ca2+ sites of LSCSO were studied in detail. In particular, we showed for the first time
* E-mail:
[email protected] (L.N.). * E-mail:
[email protected] (H.L.).
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT The present work was supported by the National Natural Science Foundation of China (Grants, 21671201, 11574003, U1632101, and U1432249), the Science and Technology Project of Guangdong Province (Grant 2017A010103034) and the China Postdoctoral Science Foundation (Grant 2018M643285).
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ACS Applied Materials & Interfaces [10] Huang, K. W.; Chen, W. T.; Chu, C. I.; Hu, S. F.; Sheu, H. S.; Cheng, B. M.; Chen, J. M.; Liu, R. S. Controlling The Activator Site To Tune Europium Valence in Oxyfluoride Phosphors. Chem. Mater. 2012, 24, 2220–2227. [11] Qiao, J.; Ning, L.; Molokeev, M. S.; Chuang, Y. C.; Liu, Q.; Xia, Z. Eu2+ Site Preferences in the Mixed Cation K2BaCa(PO4)2 and Thermally Stable Luminescence. J. Am. Chem. Soc. 2018, 140, 9730–9736. [12] Denault, K. A.; Brgoch, J.; Gaultois, M. W.; Mikhailovsky, A.; Petry, R.; Winkler, H.; DenBaars, S. P.; Seshadri, R. Consequences of Optimal Bond Valence on Structural Rigidity and Improved Luminescence Properties in SrxBa2–xSiO4:Eu2+ Orthosilicate Phosphors. Chem. Mater. 2014, 26, 2275–2282. [13] Wagatha, P.; Weiler, V. P.; Schmidt, J.; Schnick, W. Tailoring Emission Characteristics: Narrow-Band Red Luminescence from SLA to CaBa[Li2Al6N8]:Eu2+. Chem. Mater. 2018, 30, 7885–7891. [14] Akella, A.; Keszler, D. A. The New Orthosilicate Li4SrCa(SiO4)2: Structure and Eu2+ Luminescence. Inorg. Chem. 1995, 34, 1308−1310.
[15] Shi, R.; Huang, X.; Liu, T.; Lin, L.; Liu, C.; Huang, Y.; Zheng, L.; Ning, L.; Liang, H. Optical Properties of Ce-Doped Li4SrCa(SiO4)2: A Combined Experimental and Theoretical Study. Inorg. Chem. 2018, 57, 1116−1124. [16] Ueda, J.; Dorenbos, P.; Bos, A. J.; Meijerink, A.; Tanabe, S. Insight into the Thermal Quenching Mechanism for Y3Al5O12:Ce3+ through Thermoluminescence Excitation Spectroscopy. J. Phys. Chem. C 2015, 119, 25003–25008. [17] Shi, R.; Lin, L.; Dorenbos, P.; Liang, H. Development of a Potential Optical Thermometric Material through Photoluminescence of Pr3+ in La2MgTiO6. J. Mater. Chem. C 2017, 5, 10737–10745. [18] Dramićanin, M. D. Sensing Temperature via Downshifting Emissions of Lanthanide-Doped Metal Oxides and Salts. A Review. Methods Appl. Fluoresc. 2016, 4, 042001.
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