Investigating the Luminescence Behaviors and Temperature Sensing

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Investigating the Luminescence Behaviors and Temperature Sensing Properties of Rare-Earth-Doped Ba2In2O5 Phosphors Zhiying Wang,† Huan Jiao,*,† and Zuoling Fu*,‡ †

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Key Laboratory of Macromolecular Science of Shaanxi Province, Shaanxi Key Laboratory for Advanced Energy Devices, Shaanxi Engineering Laboratory for Advanced Energy Technology, School of Chemistry & Chemical Engineering, Shaanxi Normal University, Xi’an 710062, Shaanxi, People’s Republic of China ‡ Coherent Light and Atomic and Molecular Spectroscopy Laboratory, Key Laboratory of Physics and Technology for Advanced Batteries, College of Physics, Jilin University, Changchun 130012, People’s Republic of China ABSTRACT: We present a strategy for selecting an optimal material in a particular temperature range by investigating the relationship between the absolute sensitivity (Sa) and energy gap (ΔE), as well as the relationship between Sa and temperature on the basis of Yb3+/Ln3+ (Ln = Er3+, Ho3+)-codoped Ba2In2O5 phosphors. Through an investigation of optical performance, the phosphors exhibit near-infrared (NIR) downshifting and visible upconversion (UC) emissions under 980 nm excitation. The NIR spectral range from 700 to 1800 nm is referred to as the “biological window”. The NIR emission peaks of Er3+ and Ho3+ are located at 1550 nm of the third biological window and 1192 nm of the second biological window, respectively. The temperature sensing behaviors based on the UC luminescence in Yb3+/Ln3+-codoped Ba2In2O5 phosphors are recorded by the fluorescence intensity ratio (FIR) technique in the temperature range from 303 to 573 K. The Ba2In2O5:Er3+/Yb3+ sample is usable at temperatures above 350 K, and the Ho3+/Yb3+-codoped Ba2In2O5 phosphor is suitable at temperatures below 350 K in our experimental region. The above results show that the Ba2In2O5:Ln3+/Yb3+ phosphors could be promising candidates for optical temperature sensors and applications in the biological imaging field.

1. INTRODUCTION Temperature is an essential parameter of thermodynamics which is indispensable in many fields, such as scientific research, industrial manufacturing, and scientific fields. Accurate temperature measurement is very significant in practical applications. It is well-known that the commonly used contact temperature sensors need direct thermal transmission and the subsequent heat balance between the sensor and the measured object, which usually take a long time in the measurement process and are likely to change the practical temperature of the sample, particularly when the sample’s size is very small or quite similar to that of the sensor head.1 Therefore, the temperature can be directly detected by noncontact temperature sensors using optical parameters, for instance, single fluorescence intensity, fluorescence intensity ratio (FIR), peak wavelength, emission bandwidth, fluorescence lifetime, and so on.2 However, owing to the change of the exogenous factors, the peak wavelength, the emission bandwidth, and the single fluorescence intensity will be strongly altered: for instance, pressure, light source, atmosphere, etc. In contrast, the FIR technique is realized through the varied luminescent intensities of two thermally coupled levels (TCLs) at ambient temperature, which is unaffected by spectrum losses and fluctuations of pumping intensity.3 The © XXXX American Chemical Society

TCLs are two adjacent energy levels with a very small energy gap (ΔE); ΔE is about 200−2000 cm−1.4 Accordingly, measuring temperature with high spatial resolution is extremely significant. In recent years, due to potential applications in noncontact temperature sensors, some efforts have been concentrated on the research of temperature-dependent upconversion (UC) luminescence of rare-earth ions.5 This creates optical temperature sensors that are extremely attractive for their potential applications in biological environments, electrical power stations, oil refineries, and coal mines.6 As the center of luminescence, it is particularly attractive that trivalent rareearth ions have rich energy levels, which are located in the wide wavelength range from ultraviolet to infrared.7 Currently, high quantum efficiency for UC luminescence is primarily acquired in fluoride phosphors.8 However, fluorides are usually poisonous and harmful to the environment, which could limit their applications.9 Accordingly, oxides generally exhibit superior chemical stability and eco-friendly features. Hence, a growing number of oxide-based UC phosphors have been investigated systematically in the past few years.10 It is wellReceived: March 20, 2018

A

DOI: 10.1021/acs.inorgchem.8b00739 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry known that the phonon energies of most typical oxides (such as silicates and phosphates) are higher than those of the fluorides, and the low conversion efficiency is not adequate for UC luminescence properties. However, the phonon energy is relatively low in some heavy-metal oxides, such as BaGd2ZnO5 (∼450 cm−1).11 In comparison with the zincate, the phonon energy of the indium salt is also low (∼475 cm−1).12 Up to now, there has been no study on the photoluminescence of rare-earth-doped in Ba2In2O5 samples. Herein, we obtained pure phases of Ho3+/Yb3+- and Er3+/Yb3+-codoped Ba2In2O5 phosphors and investigated their luminescence and temperature sensing properties. Under 980 nm excitation, the emission bands of Er3+ ions correspond to 2H11/2/4S3/2−4I15/2 (green), 4F9/2−4I15/2 (red), and 4I11/2−4I15/2 (the third nearinfrared window, NIR-III), respectively. The visible-UC and NIR-downshifting emission bands of Ho3+ are attributed to 5 S2/5F4−5I8 (green), 5F5−5I8 (red), and 5I6−5I8 (the second near-infrared window, NIR-II) transitions, respectively. We also studied the temperature sensing characters of the samples between 303 and 573 K under 980 nm excitation. The larger ΔE between the TCLs will result in a higher detection temperature and vice versa, indicating that the temperature detection range is mainly determined by the value of ΔE.13 Therefore, by properly combining two pairs of TCLs with different energy gaps, a more suitable temperature detection range for the optical temperature sensor can be provided. All of the results indicate that the Ho3+/Yb3+- and Er3+/Yb3+codoped Ba2In2O5 phosphors have potential application for optical temperature sensors and biological imaging.

Figure 1. (a) Representative X-ray diffraction patterns of Ba2In2O5 codoped with 1% Er3+, 5% Yb3+ and 2% Ho3+, 5% Yb3+ in comparison with the standard Ba2In2O5 powder diffraction file (JCPDS 81-2473). The inset is an SEM image of the Ba2In2O5:1%Er3+,5%Yb3+ powders after calcination at 1400 °C. (b) Crystal structure of Ba2In2O5.

tetragonal phase is adjudged with lattice parameters of a = 6.035 Å and c = 17.059 Å, with space group I4/mcm. Figure 1b suggests that the In3+ ion is surrounded by six O2− ions to form [InO6] ionic octahedral groups. The Ba2+ ion is surrounded by four O2− ions. The SEM micrograph of Ba2In2O5:1%Er3+,5% Yb3+ phosphors annealed at 1400 °C for 6 h is displayed in the inset of Figure 1a, which reveals that the phosphors are composed of bulk grains with an average size of about 10 μm. The agglomeration phenomenon of the microparticles is due to the high sintering temperature and the long reaction time. Because the host phonon energy has a substantial effect on the UC efficiency, the FT-IR spectra of Ba2In2O5 and precursors are shown in Figure 2. Typical absorption bands are found: the peaks at 693, 865, and 1453 cm−1 are assigned as CO32− in-plane bending and out-of-plane bending and an asymmetric C−O telescopic vibration, respectively.14 The In− O−In vibrations are generally observed below the region of 800 cm−1. The occurrence of four sharp bands with peaks at 471, 540, 567, and 604 cm−1 can be ascribed to the phonon vibrations of In−O bonds, which is characteristic of cubic In2O3.15 For the Ba2In2O5 sample calcined at high temperature, the strong absorption band at 561 cm−1 is ascribed to the In−O stretching modes. The absorption bands at around 860 and 1420 cm−1 indicate that a small amount of BaCO3 may remain in the sample.16 The Ba2In2O5 compound has a narrow band of infrared activity and low vibration frequency. As shown by the spectra, the lanthanide ions can be doped in the potential host material of Ba2In2O5 oxide. 3.2. Luminescence Properties. 3.2.1. Visible UC Luminescence. Figure 3 displays the UC emission spectra of the Ba2In2O5:1%Er3+,5%Yb3+ and Ba2In2O5:2%Ho3+,5%Yb3+ samples under 980 nm laser excitation. Figure 3a reveals the visible UC emission bands including 526, 549, and 658 nm, which are attributed to 2H11/2−4I15/2, 4S3/2−4I15/2 and 4 F9/2−4I15/2 transitions of Er3+ ions, respectively. As shown in Figure 3b, the typical multiphoton emissions of Ho3+ ions in Ba2In2O5:2%Ho3+,5%Yb3+ phosphors can be seen. The green

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. The Ba2In2O5:1%Er3+,5%Yb3+ phosphors were obtained by a high-temperature solid-state method. In2O3 (Analytical Reagent, A.R.), BaCO3 (A.R.), Er2O3 (99.99%), and Yb2O3 (99.99%) were used as precursors and were stoichiometrically weighed and ground for 30 min in an agate mortar. Then, the above powders were sintered at 1400 °C for 6 h. Finally, we obtained the final products by further grinding of the powder samples at room temperature. The Ba2In2O5:2%Ho3+,5%Yb3+ samples were prepared by the same procedures. 2.2. Characterization. The X-ray diffraction (XRD) patterns were measured with a Rigaku MiniFlex 600 X-ray instrument with a diffraction intensity versus 2θ range of 10−80°. The spectra were collected by a fluorescence spectrometer (Andor Shamrock SR-750) with a 980 nm laser. A charge coupled device (CCD) detector combined with a monochromator and a spectrometer (Andor Technology Co, Andor SR-500i, Belfast, U.K.) was used to collect signal intensity and measure the fluorescence spectra, respectively. The morphology of the samples was determined by scanning electron microscopy (SEM, Philips-FEI Quanta 200, America). The FT-IR spectra were acquired from KBr pellets by a FT-IR spectrometer (Bruker Tensor 27) in the range from 400 to 4000 cm−1. We used the Diamond software to draw the crystal structure diagram.

3. RESULTS AND DISCUSSION 3.1. XRD and Crystal Structure. Typically, the high crystallization of Ba2In2O5 is carried out via a facile hightemperature solid-state approach. Then, different Er3+/Yb3+ and Ho3+/Yb3+ molar ratios are codoped into Ba2In2O5 materials. The XRD patterns of blank Ba2In2O5 and codoped Ba2In2O5:1%Er3+,5%Yb3+ and Ba2In2O5:2%Ho3+,5%Yb3+ are displayed in Figure 1a, respectively. All of the samples exhibit diffraction patterns corresponding to pure Ba2In2O5 on the basis of JCPDS No. 81-2473. The crystal structure of the B

DOI: 10.1021/acs.inorgchem.8b00739 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

An infrared photon is absorbed by Yb3+ ions and transitions from the 2F7/2 ground state to 2F5/2 excited state. The groundstate Er3+ ions are originally excited by ground-state absorption (GSA) or ET to the excited state 4I11/2.18 In addition, through the excited-state absorption (ESA) and ET processes, the excited ions at the 4I11/2 level are pumped to the 4F7/2 level. These electrons decay to the lower excited states 2H11/2 and 4 S3/2 by nonradiative relaxation, respectively, and return to the ground state once more to generate green emission. The electrons also can further relax to the 4F9/2 level of Er3+, and the 4F9/2 level declines to the ground-state 4I15/2 level via radiative relaxation, resulting in the red emission. As shown in Figure 4b, the superior red emission in comparison to the green emission is due to the resonant cross relaxation (CR) of 4 F7/2−4F9/2 and 4I11/2−4F9/2 (Er3+). As illustrated in Figure 5a, the UC intensity I of the Ba2In2O5:2%Ho3+,5%Yb3+ phosphors is also tested as a function of the excitation power P and plotted as a logarithmic diagram. The green UC emission and red emission centers are located at 540 and 660 nm, whose slopes are 1.98 and 1.99, respectively. Therefore, both green and red UC emissions are two-photon processes. A possible UC mechanism scheme is shown in Figure 5b. The Yb3+ ions are first excited from the ground state to the excited state 2F5/2, and then the subsequent step involves an energy transfer to the 5I6 or 5I7 intermediate excited state of the Ho3+ ions due to the sensitizing effect of Yb3+. Once the two states are populated, some electrons are excited to 5F4 and 5S2 levels via a second ET from the neighboring Yb3+ ions or ESA, which generate the green emission of 5F4,5S2−5I8.19 At the same time, some electrons of the 5I6 state are relaxed to the 5I7 state and are subsequently excited to the 5F5 state through ET or ESA, generating the red emission. 3.2.2. NIR Downshifting Emission. The general requirements of phosphors to be used as temperature sensors in biological imaging are that they have superior biocompatibility and are small in size. The measurement temperature is in the physiological range of 25−45 °C.20 It is generally known that NIR light (700−1800 nm) deeply penetrates into tissue due to the resisting force of long-wavelength light to scattering from the various structures in the tissue.21 Consequently, this wavelength zone is called the “biological window”. Commonly, the NIR wavelength region from 700 to 900 nm is the first biological window. The wavelength regions over 1000 nm are divided into the second (NIR-II 1000−1350 nm) and third

Figure 2. FT-IR spectra of In2O3, BaCO3, and Ba2In2O5 sample.

emission centered at around 550 nm comes from the 5 F4(5S2)−5I8 transition of Ho3+ ions, and the red emission peak at 653 nm is assigned to the transitions of Ho3+ from 5F5 to 5I8, respectively. To investigate the dependence of the UC emission intensity on the pump power, we first determined the numbers of photons needed for the main emissions, on the basis of the equation Ι up ∝ (Ρpump)n

where n is the required pump photon number, P is the pumping power, and Iup is the UC emission intensity.17 Through changes in the incident pump power of a 980 nm laser diode, Figure 4a shows that the slopes of the three fitted straight lines in Ba2In2O5:1%Er3+,5%Yb3+ are 2.10, 1.92, and 1.79, respectively. This confirms that all the emissions from the Er3+ ions: 2H11/2−4I15/2, 4S3/2−4I15/,2 and 4F9/2−4I15/2 transitions are two-photon processes. Under 980 nm excitation, the energy transfer (ET) process and UC mechanisms of the Er3+/ Yb3+-codoped Ba2In2O5 system are also displayed in Figure 4b.

Figure 3. Under 980 nm excitation, the UC spectra of (a) Ba2In2O5:1%Er3+,5%Yb3+ and (b) Ba2In2O5:2%Ho3+,5%Yb3+ at room temperature. C

DOI: 10.1021/acs.inorgchem.8b00739 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 4. (a) Pump power dependence of the UC emissions in Ba2In2O5:1%Er3+,5%Yb3+. (b) Energy level diagram of Yb3+ and Er3+ ions and possible UC processes under 980 nm excitation.

Figure 5. (a) Pump power dependence of the UC emission in Ba2In2O5:2%Ho3+,5%Yb3+ phosphor. (b) Energy level diagram of Ho3+ and Yb3+ ions and possible UC mechanism.

Figure 6. NIR emissions of (a) Ba2In2O5:1%Er3+,5%Yb3+ and (b) Ba2In2O5:2%Ho3+,5%Yb3+ upon 980 nm excitation.

windows enables potential applications for biological imaging. Studies of the fluorescence thermometry properties of Ho3+/ Er3+ codoped in Ba2In2O5 phosphors are under way. 3.3. Temperature Sensing Properties. Figure 7a presents the emission spectra of Ba2In2O5:1%Er3+,5%Yb3+ excited at 980 nm at various temperatures. Specifically, the UC emission wavelengths are invariable with changes in temperature, whereas the UC luminescence intensity ratios of red emission at 660 nm (4F9/2−4I15/2) decreases as the temperature increases and the green emission at 526 nm (2H11/2−4I15/2) shows an increase and then decays in intensity relative to the green emission at 549 nm (4S3/2−4I15/2). Figure 7b illustrates this situation at both 526 and 549 nm in more detail. With an increase in internal temperature, enhanced lattice vibrations promote the nonradiative relaxation rate between two closely spaced energy levels, which keeps them quasi

(NIR-III 1550−1850 nm) biological windows, which are generally used for in vivo fluorescence bioimaging.22 In this work, Figure 6a displays the NIR emission of Ba2In2O5:1%Er3+,5%Yb3+ upon 980 nm excitation; the NIR emission band centered at 1550 nm in the third (NIR-III) biological window region is attributed to 4 I 11/2 − 4 I 15/2 transitions of Er3+ ions. The NIR emission at 1192 nm comes from the 5I6−5I8 transition of Ho3+ ions, which belongs to the NIR-II biological window region, as displayed in Figure 6b. Currently, the majority of fluorescence thermometry strategies use the visible emission of fluorescent probes, which are excited by ultraviolet or short-wavelength visible light. Ultraviolet and visible lights exhibit a limited penetration depth; they cannot easily be used for deep-tissue imaging in biological samples. Consequently, the integration of the NIR-II (1192 nm, Ho3+) and NIR-III (1550 nm, Er3+) biological D

DOI: 10.1021/acs.inorgchem.8b00739 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 7. (a) Temperature evolution of the Ba2In2O5:1%Er3+,5%Yb3+ UC emission spectra excited by a 980 nm laser from 303 to 573 K. (b) Histogram of the corresponding integrated intensities of 526 and 549 nm emissions versus temperature. (c) Log plot of the FIR as a function of inverse absolute temperature. (d) FIR between the green emissions as a function of temperature in the range of 303−573 K. (e) Sensor sensitivity as a function of temperature.

Figure 8. (a) UC emission spectra of Ba2In2O5:2%Ho3+,5%Yb3+ under the excitation of a 980 nm laser at different temperatures. (b) Rectangular illustration of the corresponding integrated intensities of 653 and 661 nm emissions versus temperature. (c) Log plot of the FIR as a function of inverse absolute temperature. (d) Plot of FIR relative to the temperature. (e) Plot of sensor sensitivity under 980 nm excitation.

and I1 and I2 are the integrated intensities of 4S3/2−4I15/2 (integrated from 539 to 592 nm) and 2H11/2−4I15/2 (integrated from 508 to 539 nm) transitions, while N1 and N2 are the corresponding numbers of ions, respectively. ω, g, and σ define the angular frequency, degeneracy, and emission cross section of corresponding transitions, respectively. KB = 0.695 K−1 cm−1 is the Boltzmann constant, ΔE is the energy gap between the TCLs (4S3/2 and 2H11/2), and T is the absolute temperature.24 If the natural logarithm of both sides of eq 1 is taken, we can obtain the natural logarithm of FIR as a function of inverse absolute temperature as follows:

thermally balanced, suggesting that both 2H11/2 and 4S3/2 are thermally coupled. According to the Boltzmann-type distribution law, the corresponding relative population for two thermally coupled electronic states can be mathematically expressed as23 ÄÅ É ÅÅ −ΔE ÑÑÑ g2σ2ω2 I2 N2 Å ÑÑ = = FIR = expÅÅÅ Ñ ÅÅÇ ΚΒT ÑÑÑÖ I1 N1 g1σ1ω1 ÄÅ É ÅÅ −ΔE ÑÑÑ Å ÑÑ = C expÅÅÅ Ñ ÅÅÇ ΚΒT ÑÑÑÖ

(1)

ln FIR = ln C −

where C=

ΔE ΚΒT

(2)

According to eq 2, ln(I526/I549) is inversely proportional to absolute temperature, as shown in Figure 7c,d. Through a linear fitting of the experimental data, the slope and intercept

g2σ2ω2 g1σ1ω1 E

DOI: 10.1021/acs.inorgchem.8b00739 Inorg. Chem. XXXX, XXX, XXX−XXX

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exhibit relatively better sensitivities. In addition to the temperature sensitivity, an important parameter for evaluating a thermometer is temperature resolution (δT). The δT value is estimated from eq 426

can be obtained and are equal to 1097.8 and 2.50, respectively. Ultimately, the constant C and energy gap ΔE are also estimated to be 12.21 and 763 cm−1, respectively. Considering the practical applications, the important factor for temperature detection is the absolute sensitivity (Sa), which stands for the absolute variation of FIR in unit temperature, as given by the equation25 ÄÅ É ÅÅ ΔΕ ÑÑÑ d(FIR) ΔE Å ÑÑ = FIR ΔE expÅÅÅ Sa = =C Ñ 2 ÅÅÇ ΚΒT ÑÑÑÖ dT ΚΒT ΚΒT (3)

δT =

When ΔE = KBT, we can obtain dSa/dΔE = 0 according to eq 5. This indicates that we could select a better material for an object temperature range. We select T1 = 313 K and T2 = 773 K, and so ΔE = 217.54 and 537.24 cm−1, respectively. The fitted curve of the relationship between Sa and ΔE is displayed in Figure 10. The ΔE values of Ba2In2O5:1%Er3+,5%Yb3+ and Ba2In2O5:2%Ho3+,5%Yb3+ phosphors are represented by the light cyan point and light magenta point on the black curve, using FIR(I526/I549) and FIR(I653/I661), respectively. Figure 10a shows that the ΔE value of Ba2In2O5:2%Ho3+,5%Yb3+ phosphor using FIR(I653/I661) is closer to the ΔE value of 217.54 cm−1 than that of Ba2In2O5:1%Er3+,5%Yb3+ phosphor at low temperature. Meanwhile, the ΔE value for Yb3+/Er3+codoped Ba2In2O5 phosphor using FIR(I526/I549) is closer to the value of 537.24 cm−1 than that of Yb3+/Ho3+-codoped Ba2In2O5 phosphor at high temperature. That is to say, when the material is fixed, the Er 3+ /Yb 3+ -codoped Ba2 In 2 O 5 phosphor is more suitable for temperature measurement in the high-temperature region. In the low-temperature region, the Ho3+/Yb3+-codoped Ba2In2O5 phosphor is more suitable. On account of the above results, it can be found that the energy gap ΔE has a significant effect on the temperature detection range of the sensor. In addition, as can be deduced from eq 3, the value of the temperature Tmax with the maximum sensitivity can be determined by the equation42 ÑÉ ÅÄÅ ÑÉÑ ÑÉÑÄÅÅ ÅÄÅ dSa ÅÅ ΔE Ñ ÑÅ CΔE ÑÑÑ Å ÑÑ expÅÅÅ − ΔE ÑÑÑ = ÅÅÅ − 2ÑÑÑÑÅÅÅÅ ÅÅ Κ T ÑÑ ÅÅÇ ΚΒT ÑÑÖÅÅÇ ΚΒT 3 ÑÑÑÖ dΤ ÅÇ (6) Β Ñ Ö

Table 1. Maximum Sensitivity Values of Different Yb3+/Ln3+ (Ln = Er3+, Ho3+) Codoped Phosphors and Temperature Ranges temp range (K)

Sr(max) (K−1) (temp (K))

Er3+, Er3+, Er3+, Er3+, Er3+, Er3+, Er3+, Er3+,

Yb3+ Yb3+ Yb3+ Yb3+ Yb3+ Yb3+ Yb3+ Yb3+

α-NaYF4 β-NaLuF4 Y2O3 Al2O3 BaMoO4 SrWO4 LiNbO3 Na0.5Bi0.5TiO3 ceramics Bi7Ti4NbO21 TeO2−WO3 glass Ba2In2O5

298−318 303−523 93−613 295−973 303−523 300−518 285−453 173−553

0.0030 (515) 0.0052 (303) 0.0044 (427) 0.0051 (495) 0.0206 (463) 0.01498 (403) 0.0075 (310) 0.0035 (493)

28 29 30 31 32 33 34 35

Er3+, Yb3+ Er3+, Yb3+ Er3+, Yb3+

163−613 300−690 313−573

0.0031 (400) 0.0029 (690) 0.0065 (498)

Y2O3

10−300

0.0097 (85)

36 37 this work 38

Ho3+, Yb3+ Ho3+, Yb3+ Ho3+, Yb3+

Ba0.77Ca0.23TiO3

93−300

0.0053 (93)

39

Ba2In2O5

313−573

0.0025 (273)

this work

host

(4)

where ΔFIR/FIR = 0.5% is the relative error when the temperature parameter is measured, which is the typical value of collection setting. 27 The calculated δT curves of Ba2In2O5:1%Er3+,5%Yb3+ and Ba2In2O5:2%Ho3+,5%Yb3+ thermometer at 303−573 K are shown in Figure 9, which demonstrate that the minimum temperature uncertainty values (δTmin) are 0.4 and 2.5 K, respectively. For optical temperature sensors, it is essential to obtain a precise theoretical relationship between sensitivity and ΔE. In addition, the relative sensitivity (Sr = 1/FIR × (d(FIR)/dT) = ΔE/KBT2) greatly depends on the ΔE value between the TCLs. Sr represents the relative variation in FIR relative to itself.40 Sr is linearly related to ΔE in the range of 200−2000 cm−1 and increases with an increase in ΔE. However, the absolute sensitivity Sa is nonlinearly dependent on ΔE, which signifies the absolute change of FIR value in unit temperature. As can be derived from eq 3, the relationship between Sa and ΔE can be expressed as41 ÄÅ É ÉÑ ÄÅ ÅÅ ÅÅ ΔE ÑÑÑ dSa ΔE ÑÑÑ C Å ÑÑ Å ÑÑ = ÅÅÅ1 − expÅÅÅ− Ñ ÅÅÇ ÅÅÇ ΚΒT ÑÑÑÖ ΚΒT ÑÑÑÖ T 2 dΔE (5)

The sensitivity of the present material increases with an increase in temperature from 303 to 573 K; the maximum sensitivity is about 0.0065 K−1 and then gradually decreases with an increase in temperature. Figure 7e plots the sensitivity curve as a function of temperature. Furthermore, to explore the possible applications of Ba2In2O5:2%Ho3+, %Yb3+ in temperature sensors, the temperature-dependence spectra of the Ba2In2O5: 2%Ho3+,5%Yb3+ sample was measured in the temperature range of 303−573 K under 980 nm laser excitation. In Figure 8a, two emission bands at around 653 and 661 nm can be observed, which are assigned to the 5F5(1)−5I8 and 5F5(2)−5I8 transitions of Ho3+ ions, respectively. The temperature-dependent UC emission intensity of the Ho3+/Yb3+-codoped Ba2In2O5 sample is displayed in Figure 8b. In accordance with eq 2, ln(I653/I661) grows in inverse proportion to the absolute temperature, as presented in Figure 8c, and the integral FIR values of the two UC emission bands suggest that there is obvious dependence on the temperature, as shown in Figure 8d. In terms of eq 3, Figure 8e exhibits that the maximum sensitivity value reaches 0.0025 K−1 at the starting temperature (273 K) and keeps dropping as the temperature is further increased to 573 K. For comparison of the sensitivity with those of other materials, we provide the maximum sensitivity values of different Yb3+/Ln3+ (Ln = Er3+, Ho3+)-codoped phosphors, as shown in Table 1. In comparison with other materials, the Yb3+/Ln3+ (Ln = Er3+, Ho3+)-codoped Ba2In2O5 phosphors

rare-earth ions

ΔFIR FIR × Sr

ref

Therefore

Tmax = F

ΔE 2ΚΒ

(7) DOI: 10.1021/acs.inorgchem.8b00739 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 9. Temperature resolution for the (a) Ba2In2O5:1%Er3+,5%Yb3+ and (b) Ba2In2O5:2%Ho3+,5%Yb3+ phosphors.

Figure 10. Fitted curves of the relationship between Sa and ΔE: (a) in the low-temperature zone (T = 313 K); (b) in the high-temperature zone (T = 773 K). The solid light cyan and light magenta balls on the curve represent the ΔE values of Ba2In2O5:Yb3+/Ho3+ and Ba2In2O5:Yb3+/Er3+, respectively.

Obviously, the value of Tmax is closely related to the ΔE value. It is implied that when the ΔE value of a temperature sensor material is given, the optimum temperature with reference to the maximum Sa can be determined. As depicted in Figure 11, the black curve and the stars on the curve

Table 2. Values of ΔΕ, Sa, and Tmax for Ba2In2O5 Materials rare-earth ions 3+

3+

Ho , Yb Er3+, Yb3+

5 2

transitions

ΔΕ (cm−1)

Sa (K−1)

Tm (K)

F5(1), F5(2) → I8 H11/2,4S3/2 → 4I15/2

124 763

0.0025 0.0065

89.42 548.92

5

5

It is obvious that a lower or higher ΔE value would extend the detection range to a lower or higher temperature region. According to Figures 10 and 11, we could come to the conclusion that the Er3+/Yb3+-codoped Ba2In2O5 phosphor is more suitable for temperature measurement in the hightemperature region and the Ho3+/Yb3+-codoped Ba2In2O5 phosphor is more suitable for the low-temperature region.

4. CONCLUSION Yb3+/Ln3+ (Ln = Er3+, Ho3+)-codoped Ba2In2O5 multifunctional phosphors have been synthesized by a facile and efficient high-temperature solid-state method. Under 980 nm excitation, the Ba2In2O5:1%Er3+,5%Yb3+ phosphors exhibit efficient visible UC and NIR downshifted emissions, including green emission centered at 528 and 547 nm and red emission centered at 660 nm, which are assigned to the 2 H11/2,4S3/2−4I15/2 and 4F9/2−4I15/2 transitions of Er3+ ions; the NIR downshifted emission centered at 1550 nm comes from the 4I13/2−4I15/2 transition of Er3+ ions. Similarly, the Ba2In2O5:2%Ho3+,5%Yb3+ samples display clear UC emissions (540 and 660 nm) and NIR emission (1192 nm) under 980 nm laser excitation. Through evaluation of the temperaturesensing behavior of two pairs of TCLs, the maximum sensitivity of Ba2In2O5:1%Er3+,5%Yb3+ is found to be 0.0065 K−1 at 498 K and the maximum sensor sensitivity of Ba2In2O5:2%Ho3+,5%Yb3+ samples is around 0.0025 K−1 at

Figure 11. Fitting curves of the relationship between Sa and T. The stars on the curve indicate the experimental values of Sa.

represent the fitting results of Sa theoretical values and the experimental values, respectively. According to the S a theoretical value, the Er3+/Yb3+-codoped Ba2In2O5 phosphor is more suitable for the high-temperature zone. In the lowtemperature region, the Yb3+/Ho3+-codoped Ba2In2O5 phosphor is more suitable, which is consistent with the above analysis of the relationship of Sa and ΔE. Related information is summarized in Table 2. G

DOI: 10.1021/acs.inorgchem.8b00739 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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273 K. We also have discussed the relationship between Sa and ΔE and the relationship between Sa and T, respectively. We could select an optimal material for a specific temperature range. All of the results indicate that the rare-earth-doped Ba2In2O5 phosphors can be regarded as promising fluorescent thermometers for deep tissue and offer a perfect potential application as optical temperature sensors with relatively high sensitivity.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for H.J.: [email protected]. *E-mail for Z.F.: [email protected]. ORCID

Zuoling Fu: 0000-0003-0176-2886 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Project of the National Natural Science Foundation of China (Nos. 21401122, 51272151, 51672167), the Natural Science Foundation of Shaanxi province (2014JZ002, 2015JQ2041), and Fundamental Research Funds for the Central Universities (GK201603050) for financial support.

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DOI: 10.1021/acs.inorgchem.8b00739 Inorg. Chem. XXXX, XXX, XXX−XXX