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Abstract
Owing to their ultra-high temperature sensitivity, luminescence thermometers based on the intervalence charge transfer state of Pr3+ have drawn more and more attention. But the inner mechanism of Pr3+ luminescence thermometers, especially which parameters could affect the value of relative sensitivity, is still not fully revealed. In this paper, the luminescence thermometer properties of Ba1-xCaxTiO3:1%Pr3+ (x=0-0.7) with variation temperature have been systematically studied. The process of electrons in the 3P0 state passed through the intervalence charge-transfer state to the 1D2 state has been found to be sensitive to temperature variation. Through the intervalence charge transfer state engineering, we successfully realized the effective regulation of the intervalence charge transfer state energy position by replacing Ba2+ with Ca2+ and find that the effective energy gap between the 3P0 state and the intervalence charge transfer state is the key parameter that affects the relative sensitivity of luminescence thermometers. Moreover, the maximum relative sensitivity of Pr3+-doped BaTiO3, evaluated to be 2.26% K−1 at 413K under 450 nm excitation, is the one of the best performances among the Pr3+-based luminescence thermometers. This research could lead to the novel development of an efficient and high-performance optical thermometer.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
High-sensitivity temperature sensing exerts an indispensable part in the control of the reaction procedure, commodity performance, as well as product safety [1–3]. It is less satisfying that physical exploration like thermometers and thermocouples merely exert restricted utility on the noncontact locations, small scales, and high spatial resolutions during the operation under inferior conditions or the dynamic systems [4,5]. Or else, optical thermometry contributes the approaches with feasibility which benefits by rapid-response fluorescence spectroscopy techniques [6–8]. For suiting the demands from differential technical applications, surveys in recent days, a variety of lanthanide-doped thermometric substances are drawing a great deal of interest. The Fluorescence Intensity Ratio (FIR), greatly associated with thermally coupled energy levels (TCELs) of lanthanide ions, is a prominent approach for optical thermometry and more advantageous than other elements [9]. The FIR approach has several advantages, such as independent of doping concentration or excitation power, unnoticed signal drift, self-reference, and so on. Nevertheless, TCELs observations are constrained by inherent limitations such as maximum temperature, relative sensitivity, long-term stability, imposing evident susceptibility on the minor relative temperature sensitivity (Sr) as well as the absolute temperature sensitivity (Sa) [10]. Additionally, because of the slight energy difference between TCELs, two measured emission peaks overlapped, which brings about less qualified signal discriminability. Thereby, the FIR, on the basis of non-TCELs luminescence ions, has been evolved to be the focus which indicates promising optical thermometry materials [11].
Pr3+ is a well-known emitter of greenish-blue and redness by 3P0-3H4,5,6 and 1D2-3H4 transitions. However, in some materials, the 3P0 related emissions are completely quenched, leaving red emission as the only option [12]. It is widely known that the 3P0 related emissions of Pr3+ quenched either by cross-relaxation due to high Pr3+ doping concentration or large phonon energy in host material. Recently, a new model “intervalence charge-transfer (IVCT) state” has been proposed by Boutinaud et al. for interpretation of 3P0 emission quenching [13]. The IVCT state could provide an extra channel for electrons transfer from 3P0 to 1D2 level, which results in a redistribution of electrons [14–16]. In 2016, our group first demonstrated this redistribution process is highly susceptible to temperature by Pr3+ doped K0.5Na0.5NbO3, proposed the application of Pr3+ in the field of optical temperature sensing, and the maximum of Sr (Sr-max) of Pr3+ doped K0.5Na0.5NbO3 is almost twice as much as that of traditional TCELs sensor [17]. As can be observed, the IVCT state is not only valuably critical in the de-excitation process, but also could provide a lot more applications in optical thermometry. Successively, in 2017, Liang et. conducted a thorough study on the temperature-dependent spectroscopic properties featured by Pr3+ doped La2MgTiO6, temperature sensing by IVCT state had been further demonstrated, which Sr-max is 1.28%K−1 at 350 K [18]. In 2018, through combining the fluorescence intensity ratio with luminescence lifetime of Pr3+:Gd2ZnTiO6, Wang et. broadening the temperature measurement scope of 293 K to 593 K, the Sr-max presented the value of 1.67% K−1 at 433 K [19]. In 2019, Ye et. possesses a Sr-max of 2.49%·K−1 at 390 K for CaSc2O4: Pr3+, which further confirms the high performance of Pr3+ as a temperature detector [20]. The year 2020 witnessed the design by Eugeniusz et. on luminescence thermometers with two modes based on Lu2(Gex,Si1-x)O5:Pr3+. This study further improved the relative sensitivity of Sr-max up to 3.54% K−1 at 17 K [21]. In 2021, Mu et. designed a new dual-mode optical thermometer La2Mo3O12:Yb3+, Pr3+ based on the IVCT, upon 450 nm excitation, the Sr reached 2.0% K−1 at 648 K, under excitation at 980 nm, the Sr reached 4.3% K−1 at 598 K [22]. The literature above demonstrates that Pr3+ offers superior performance in the field of temperature sensor. However, until now, the vast majority of studies highlighted Pr3+ doping with differential host materials to improve Sr value or broaden the temperature range. Few works focus on the temperature sensing mechanism, especially the parameters which could effectively affect the value of Sr.
Consequently, a serial optical thermometric measurement on Pr3+-doped BaxCa1-xTiO3 perovskite microcrystals was reported in this paper. The cross-relaxation (CR), multi-phonon relaxation (MPR), and IVCT state were studied in the system. PL and decay measurement results showed that Pr3+-Ti4+ IVCT energy is the key determinant of the luminescence properties and finally applied to luminescence temperature measurement. Temperature-dependent luminescence properties, together with the discussions on the mechanisms to interpret the experimental results, are revealed. Notably, the Sr value depends on the effective energy difference (ΔE) ranging from the 3P0 lowest-vibronic state and the crossover-point for the IVCT state, that is, the energetic position occupied by the IVCT state. Additionally, measured Sr-max of BaTiO3:1%Pr3+ reached 2.26% K−1 at 413 K, highly ranked within those oriented with the inorganic optical thermometric. It reveals that Pr3+ doped Ba1-xCaxTiO3 phosphors also deserve to be a good element of temperature sensing materials with high performance.
2. Experimental
Ba1-xCaxTiO3:1%Pr3+ (x = 0, 0.1, 0.3, 0.4, 0.6, 0.7) samples have been given preparation by convention solid-state reaction. During the preparation, Pr6O11, BaCO3, CaCO3, and TiO2 were employed for a materials starter amidst the appropriate stoichiometric ratio. Each component is impregnated and sufficiently ground in ethanol. These compositions were then transferred into unsealed alumina crucibles and calcined at 1350 °C for 6 hours. Under ambient temperature, the samples were crushed to fine powders in an agate mortar in preparation for the following test.
X-ray diffraction (XRD) using Cu Kα radiation (Rigaku D/MAX-2600) was used to analyze the crystallization of as-prepared samples, and scanning electron microscopy (SEM) was used to evaluate the morphology (FEI NovaNano450). A spectrometer was used to measure the photoluminescence excitation (PLE) spectra (HORIBA, Fluoromax-4) and the photoluminescence (PL). The decay curves were measured utilizing FLS980.
3. Results and discussion
3.1 Structural properties
Figure 1(a) presents the XRD of 1%Pr3+ doped Ba1-xCaxTiO3 (x = 0, 0.1, 0.3, 0.4, 0.6 and 0.7). All the patterns show a polycrystalline perovskite nature. The pure BaTiO3 and Ba0.9Ca0.1TiO3 ceramics show a tetragonal crystal structure (space group P4 mm, PDF#75-2118). When x≥0.3, a biphasic structure was produced, tetragonal and orthorhombic phases were coexisted. The additional phase to be orthorhombic CaTiO3 (space group Pnma, PDF#72-1192) phase, which marked ‘‘#’’ in Fig. 1(a). To study the effects of the Ca2+ doping on crystal structure, the fine scanning of XRD spectra in the 2θ range of 31–33° was performed, which are shown in Fig. 1(b). With increasing Ca2+ concentration, positions of diffraction peaks slightly shift to higher angles, which is ascribed to the size mismatch between the small Ca2+ (0.112 nm, 8 coordinate) ions and the large Ba2+ (0.142 nm, 8 coordinate). The TOPAS program was employed for implementing the process of Rietveld Structure Refining by the high-qualified XRD information. Fig. S1 (a)-(f) stands for XRD figures (•) from the experimental achievement, the obtained Bragg positions (|), the diversification (blue line) and the refined outcome (red line) associated with Ba1-xCaxTiO3:1%Pr3+. Unit cell characteristics and bond lengths are interpreted in Table S1. Fig. S2 show the variation in the serial samples positioned with the closest Ba2+/Ca2+–Ti4+ distance. The nearest Ba2+/Ca2+–Ti4+ distance is monotonically decreasing as Ca2+ concentrations rise because of the difference in ionic radius between Ba2+ and Ca2+.
Fig. 1. (a) The XRD of Ba1-xCaxTiO3:1%Pr3+ (x = 0, 0.1, 0.3, 0.4, 0.6, 0.7). (b) Partially enlarged XRD patterns, range from 31° to 33°.
3.2 Luminescence of Ba1-xCaxTiO3:1%Pr3+
Figure 2(a) demonstrates the normalized PLE and PL spectra of BaTiO3:1% Pr3+ under ambient temperature. The excitation spectra got respectively monitored at 490 and 602 nm. Each of the spectra is made up of the lines within the visible region of 420−450 nm (3H4-3P2 transition of Pr3+) and the broadband in UV region. The broadband contains host absorption and Pr3+−Ti4+ IVCT, and the IVCT state provides an extra channel for the electrons on 3P0 level transfer to 1D2 level [18]. The right part of Fig. 2(a) shows the PL spectra under 360 and 450 nm. Both PL spectra are mainly consist of 3P0-3F2 (650 nm), 3P0-3H6 (617 nm), 3P0-3H4 (490 nm) and 1D2-3H4 (602 nm) transitions. But the emission intensity of the 1D2–3H4 transition shows a significant rise under bandgap excitation at 360 nm. This occurrence is consistent with the findings of the excitation spectra, implying that the IVCT state efficiently de-excitation of electrons in the 3P0 level [23]. Fig. 2(b) and 2(c) show the decay kinetics of the emissions at 490 and 602 nm in BaTiO3:1%Pr3+ at 303K. The decay profiles present excellent fittings with two exponential functions: $I(t)\textrm{ = }\Sigma _{i = 1}^n{A_i}\exp ( - t/{\tau _i}),(n = 2)$, with average lifetime ($\bar{\tau}$) conducted according to $\mathop \tau \limits^\_ = \frac{{\Sigma _{i = 1}^na{\tau _i}^2}}{{\Sigma _{i = 1}^na{\tau _i}}}(n = 2)$. Figure 2(b) and (c) suggest that the average decay time attained for 3P0 (∼11 µs) presents shorter time compared to the time from 1D2 (∼60 µs), due to the spin-allowed 3P0-3H4 transition and the spin-forbidden 1D2-3H4 transition. The decay curves observed for 450 and 360 nm excitation vary significantly. The longer lifetime for 360 nm excitation is thought to be due to charge carrier recombination processes, which can only occur at higher energies. Figure 3 indicates that a schematic diagram illustrating the recombination and excitation processes in Ba1-xCaxTiO3:1%Pr3+ is provided based on the foregoing findings. When Ba1-xCaxTiO3:1%Pr3+ samples under 450 nm excitation, the electrons in ground state transfer into the 3P2 excited state, and rapidly relaxes to 3P0 and 1D2 levels, resulting in a broad emission spectrum. Under 360 nm excitation, electrons in the 3H4 ground-state are transported to the conduction band. These electrons then relax to both the IVCT and 3P0 states with different nonradiative rates. Then the electrons in the IVCT state swiftly transition to the 3P0 and 1D2 levels by absorbing the thermal energy. Owing to a larger energy barrier between IVCT state and 3P0 level than the IVCT state and 1D2 level, the electron preferentially populates the 1D2 level, leading to a considerable rise in absolute 1D2-3H4 emission intensity under 360 nm excitation, which is exhibited in Fig. 2(a). As a result, under 360 nm excitation, the relative intensity of 1D2 emission is greater than under 450 nm excitation. So, in order to avoid the complex electron distribution in the first stage of Variable temperature experiment, 450 nm is used as the excitation wavelength in the following sections.
Fig. 2. (a) PLE spectra of BaTiO3:1%Pr3+ monitored at 490 and 602 nm, PL spectra of BaTiO3:1%Pr3+ under 360 and 450 nm excitation. (b) decay kinetics of 3P0-3H4 emissions under 360 and 450 nm excitation. (c) decay kinetics of 1D2-3H4 emissions under 360 and 450 nm excitation.
Figure 4(a) presents the interdependence of the PL spectra of BaxCa1-xTiO3:1%Pr3+ phosphors on the Ca2+ doping concentrations. The fluorescent intensity ratio (FIR) of 1D2-3H4 transition versus 3P0 related emissions increases with the concentration of Ca2+, as shown in Fig. 4(b). This phenomenon is could be explained by the model of Boutinaud: as the Ca/Ba ratio rises, the IVCT state witnesses an energy drop resulting from the reduction of the Pr3+−Ti4+ shortest bonds [24]:
Fig. 4. (a) PL spectra of BaxCa1-xTiO3:1%Pr3+ under 450 nm excitation. (b) the FIR of I602/I490 with the concentration of Ca2+. (c) decay dynamics of 3P0 emissions in BaxCa1-xTiO3:1%Pr3+ with different Ca2+ contents. (b) decay dynamics of 1D2 emissions in BaxCa1-xTiO3:1%Pr3+ with different Ca2+ contents.
3.3 Temperature-dependent photoluminescence properties of (Ba1-xCax)TiO3:1%Pr3+
Figure 5(a)-(f) presents the emission spectra of BaxCa1-xTiO3:1%Pr3+ in the range of the 303−413 K under 450 nm excitation. As the temperature is increasing, the emission peaks from 3P0 and 1D2 are all quenching in a dramatic way. And, in comparison to 1D2-3H4 transition, the thermal quenching effects are being more highlighted oriented with the transitions relevant to 3P0 (3P0-3H4, 3H5, 3H6, 3F2). Generally, three likely non-radiative de-excitation pathways might undertake the responsibility of 3P0-related luminescence thermal-quenching: (i) Cross-relaxation (CR), (ii) Multi-phonon Relaxation (MPR), and (iii) crossover to IVCT state. According to the report, CR in possession of [3P0, 3H4]-[1D2, 3H6] and [3P0, 3H4]-[1G4, 1G4] constitute the favorable parts of phonon assistance, and the efficiency goes up as temperature e increases [28]. But for the reason of extremely low Pr3+ ions concentration, it is exerting less critical in-system importance. In terms of MPR from 3P0 to 1D2 level, it is routinely 4−5 phonons needed for bridging the energy gap between 3P0 and 1D2 (ΔEa ≈ 3800 cm−1). That MPR ratio (WNR) could be assessed in virtue of the exponential energy-gap law under the modification of Van Dijk and Schuurmans [29,30]:
βel = 107 s−1, α = 4.5(±1) 10−3 cm−1[31,32] and hωmax is the maximum phonon energy (under the determination of Raman spectra in Fig. S4, hωmax = 719 cm−1 in Pr3+ doped BaxCa1-xTiO3). The MPR rate which 2.42 × 102 s−1 is featured by Eq. (2). The following offers the MPR rate dependence exerted by temperature [33]: p represents the phonon number, determined by p = ΔEa/hωmax; k refers to the Boltzmann constant. The MPR rate upgrades to 3.67 × 102 s−1 at 413 K based on the calculation Eq. (3). It is a lower value compared to the representative 3P0 non-radiative rate of ∼106 s−1 in titanates [31], which is unable to make these outstanding quenching effects. Thereby, the MPR mechanism can be excluded for the strong quenching with 3P0 emission in BaxCa1-xTiO3:1%Pr3+.Fig. 5. (a)-(f) Temperature dependent PL spectra of Ba1-xCaxTiO3:1%Pr3+ (x = 0, 0.1, 0.3, 0.4, 0.6, 0.7) in the range 303-413 K. (a) x = 0, (b) x = 0.1, (c) x = 0.3, (d) x = 0.4, (e) x = 0.6 and (f) x = 0.7.
The thermal quenching of the 3P0 emission in BaxCa1-xTiO3:1%Pr3+ is thought to be caused by the crossover to Pr3+-Ti4+ IVCT state. A crossover of configurational parabolas might lead to thermal depopulation of the 3P0-related emissions, according to Fig. 3 [16]. As the temperature rises, the electrons of 3P0 level can surpass the energy gap (ΔE) by absorbing thermal energy and transfer to the 1D2 state through the IVCT state, resulting in a significant reduction in the luminescence intensity of 3P0-related emissions. Subsequently, the strength of 1D4-3H4 transition grows with the rising temperature in comparison to the 3P0-related transitions due to the electron feeding mechanism from 3P0. In Pr3+-doped luminous materials, the differing thermal sensitivities of the 3P0 and 1D2 multiplets are regarded as significant in temperature monitoring. The FIR of 1D2-3H4 (602 nm) and 3P0-3H4 (490 nm) in Pr3+-doped BaxCa1-xTiO3 can be used as a reliable thermometer. The temperature-dependent FIR fit equation given by a Boltzmann equation [34]:
k is the Boltzmann constant, ΔE is the effective energy gap between the 3P0 lowest vibronic state and the crossover point to the IVCT state, I490 and I602 are the emission intensities of 3P0-3H4 and 1D2-3H4, B and C are constant. The experimental results are perfectly matched with eq 4, as seen in Fig. 6(a)-(f), indicating a high relative sensitivity of 3646.7/T2, 3396.7/ T2, 2928.6/T2, 2703.5/T2, 2246.5/T2, and 1745.0/T2. These findings suggest that Pr3+-doped BaxCa1-xTiO3 can be an effective material in optical thermometry.Fig. 6. (a-f) FIR of I602 and I490 of Ba1-xCaxTiO3:1%Pr3+ (x = 0, 0.1, 0.3, 0.4, 0.6, 0.7) in the range of 303–413 K. (a) x = 0, (b) x = 0.1, (c) x = 0.3, (d) x = 0.4, (e) x = 0.6 and (f) x = 0.7.
The following expressions produce the absolute temperature sensitivity (Sa) and relative temperature sensitivity (Sr), respectively:[35]
Figure 4(S)(a)-(f) and Fig. 5(S)(a)-(f) present the temperature dependences of Sa and Sr computed from experimental results using the preceding formulae. The eq 5 and 6 reflect the observational data adequately, and the Sr-max of Ba1-xCaxTiO3:1%Pr3+ (x = 0, 0.1, 0.3, 0.4, 0.6, 0.7) are 1.79%K−1 at 399K, 1.59%K−1 at 414K, 1.38%K−1 at 394K, 1.35%K−1 at 379K, 1.31%K−1 at 345K and 1.10%K−1 at 318K, respectively. The Sa and Sr of BaTiO3:1%Pr3+ excited by 360 nm are also studied here, as shown in Fig. S7, the Sr-max is 2.26%K−1 at 413K, being larger than Sr-max under 450 nm excitation [23]. Merely the Sr value is regarded as crucial for temperature monitoring effectiveness in real-world applications. Table 1 summarizes the results for Sr in contrast to the published data. The Sr-max of BaxCa1-xTiO3:1%Pr3+ is prioritized for inorganic optical thermometric substances, and the BaxCa1-xTiO3:1%Pr3+ can be employed as a superior optical thermometric material amidst IVCT state interference.
Table 1. The maximum relative thermal sensitivity (Sr-max), the excitation wavelength (λex), and the operating range for Pr3+-based luminescence thermometers
Further, according to Table 1, the maximum value of Sr mainly relying upon the ratio of Ba2+/Ca2+. As studied above, the change of Ba2+/Ca2+ ratio can effectively adjust the energy position of IVCT state according to the Boutinaud model, which will induce the ΔE changed. To sum up, the greater value of ΔE, the greater the maximum value of Sr. For a clear illustration, a schematic diagram was shown in Fig. 7. Case 1: the IVCT state has an extremely low position. On this occasion, thermal quenching of the 1D2 tends to be serious for the reason of facilitating electron migration range of the 1D2 level to the IVCT state and there is no thermometric performance appeared. Case 2: position of the IVCT state possesses a suboptimal position, like Ba1-xCaxTiO3:1%Pr3+ with a small value of ΔE expressed by our essay. On this occasion, the FIR thermometry based on Pr3+ luminescence gives poor performance. And because of facilitating electron transition within the range of 1D2 level to the IVCT state, Sr is to impose less value [36,40,41]. Case 3: the IVCT state has a moderate position, like BaTiO3:Pr3+ with a big value of ΔE in which our essay expressed. As for the occasion, the FIR thermometry abiding by the Pr3+ luminescence presents ideal performance, which will show a big value of Sr.
Fig. 7. Configurational coordinate diagrams showing the four cases of the relative configuration between IVCT state and 3P, 1D2 levels.
However, there is a special case to be made. Case 4 reveals the IVCT state is located in a rather high position, such as Ca0.97Pr0.02ZrO3 [36]. The electrons in 3P0 level cannot be transferred to 1D2 level through IVCT state due to the obstacles of the overlarge activate energy (ΔE), harboring negativity on the thermometry performance. In summary, we can conclude that the relative position of the Pr3+−Mn+ IVCT state and 3P0 level could effectively affect the value of Sr. These results might help navigate in-depth development of the updated temperature sensor materials.
4. Conclusions
In this paper, Ba1-xCaxTiO3:1%Pr3+ (x = 0, 0.1, 0.3, 0.4, 0.6, 0.7) was effectively synthesized using a high-temperature solid-state reaction. The applicability of the effect of IVCT engineering in luminescence thermometry could be properly explored because the structure does not change across the entire range of compositions.
It is exemplified that thermometric characterization in virtue of the FIR method is a superiorly applicable optical thermometric performance featuring a maximized relative sensitivity of 2.26% K−1 featured by 413 K in BaTiO3:1%Pr3+. The Pr3+−Ti4+ intervalence charge transfer state has proved to be the primary cause serving so well-functional thermometric characteristics. More than that, our finding is that IVCT engineering is not merely to allow controlling and tuning the luminescence properties of 4f-4f transitions of Pr3+. More importantly, it features a feasible value as a fine-tuner of a thermometer Sr. Controlling the Ba/Ca ratio enables fine customization of the IVCT state, which has an impact on the Sr of thermometers. We believe that our research will aid in the development of high-efficiency lanthanide-doped luminous materials that can be used as emitters or optical thermometers with high relative sensitivity.
Funding
Basic and Applied Basic Research Foundation of Guangdong Province (2020A1515110652).
Acknowledgments
This work is supported by Guangdong Basic and Applied Basic Research Foundation (2020A1515110652).
Disclosures
The authors declare no conflicts of interest.
Data availability
No data were generated or analyzed in the presented research.
Supplemental document
See Supplement 1 for supporting content.
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