Optical Temperature Sensing of YbNbO₄:Er³⁺ Phosphors Synthesized by Hydrothermal Method

Abstract

The novel YbNbO₄:Er³⁺ phosphors were firstly synthesized through the hydrothermal method by adding LiOH·H₂O as flux in the H₂O/EG system. YbNbO₄:Er³⁺ phosphors showed the agglomerated irregular polygons coexisting with some tiny grains. XRD and Raman spectra were measured to understand the phase structure and the crystal growth mechanism of YbNbO₄:Er³⁺ phosphors. The upconversion (UC) emission spectra, the pump power dependency and UC mechanism were studied under 980 nm excitation. Based on the fluorescence intensity ratio technique, YbNbO₄:Er³⁺ exhibited the maximum sensor sensitivity of 0.00712 K⁻¹ at 220 K, providing a promising application in optical low-temperature sensors.

1. Introduction

Non-contact optical thermometry based on fluorescence intensity ratio (FIR) technique is playing a key role in applications for harsh environments, such as high-intensity electromagnetic fields, fire detection and electrical power stations due to its excellent properties of fast response and anti-interference, high-spatial resolution and superior accuracy [1,2]. The FIR technique, which utilizes the temperature-dependent emission intensity from the two thermally coupled energy levels (TCLs) of rare-earth (RE³⁺) ions, is independent of spectrum losses and fluctuations of exciting intensity [3,4]. Er³⁺ ion is an ideal candidate for optical temperature sensor because its energy gap (ΔE) between ²H_11/2 and ⁴S_3/2 states is about 700–800 cm⁻¹, satisfying the requirement for locating in the range of 200 ≤ ΔE ≤ 2000 cm⁻¹ [5,6]. To our knowledge, much attention has been focused on the non-contact optical thermometry in the high-temperature circumstance [7,8,9]. Ye reported the maximum absolute sensitivity value of 0.0552 K⁻¹ and the relative sensitivity value of 1.49% K⁻¹ recorded from 294 to 573 K in Ba₃Y₄O₉:Ho³⁺/Tm³⁺/Yb³⁺ phosphor [7]. It has been reported by Chen that β-NaYF₄:Yb³⁺/Er³⁺ microcrystal exhibited the maximum sensitivity of 0.0037 K⁻¹ at 508 K in the temperature region of 298–653 K [8]. YPO₄:Yb³⁺/Ho³⁺/Tm³⁺ submicro-plates synthesized by Lu possessed a high temperature sensitivity of 0.0885 K⁻¹ at 563 K according to the thermo-responsive FIR of Ho³⁺ (⁵F₅ → ⁵I₈) to Tm³⁺ (³F₃ → ³H₆) emissions [9]. However, there are few reports on optical temperature sensors in the cryogenic region below 298 K. Kaczmarek and Deun showed that LaF₃:Yb³⁺/Er³⁺ nanoparticles displayed a remarkably high relative sensitivity of 0.6092% K⁻¹ at 15 K [10].

RE³⁺ ions doped lanthanide niobates (LnNbO₄, Ln = La, Gd, Tb and Y, et al.) have garnered a tremendous amount of attention since LnNbO₄ has the advanced physical properties, such as the good thermal and chemical stability, a wide transparency range, electro-optical, the high dielectric constants and nonlinear optics [11,12,13]. Zhou reported the colorful visible emissions in LaNbO₄:Yb³⁺/Er³⁺/Ho³⁺ materials under 980 nm excitation due to the low symmetry of LaNbO₄ [14]. It has been reported by Carmo that adjusting the concentrations of Tm³⁺ and Yb³⁺ ions was responsible for producing the white upconversion (UC) emission in YNbO₄:Er³⁺/Tm³⁺/Yb³⁺ phosphor [15]. YNbO₄:Eu³⁺/Er³⁺ phosphors synthesized by Yin displayed a relative sensitivity of 0.0088 K⁻¹ at 303 K [16]. However, there was only one report for YbNbO₄ thin film prepared by sol-gel method [17]. Therefore, it is desirable to discuss the phase evolution and optical low-temperature sensing properties of RE³⁺ ions in YbNbO₄ phosphors. Furthermore, the YbNbO₄:Er³⁺ phosphors will provide an opportunity to synthesize the flexible coatings for temperature sensing and photothermal conversion. For example, Gonçalves and Ferreira reported that the GeO₂-Ta₂O₅:Er³⁺/Yb³⁺ particles were dispersed in poly(methyl methacrylate) (PMMA) to form the plastic free-standing films. The high-quantum-yield UC Er³⁺/Yb³⁺-organic-inorganic hybrid coatings exhibited a relative thermal sensitivity of similar to 1.1% K⁻¹ at 300 K [18].

In this work, YbNbO₄:Er³⁺ phosphors are synthesized by hydrothermal method in H₂O/EG solution system for the first time. The phase evolution, optical characteristics and optical low-temperature sensing of Er³⁺ ions in YbNbO₄ phosphors are discussed.

2. Materials and Methods

In a typical hydrothermal method for the synthesis of YbNbO₄:x mol% Er³⁺ (x = 0.1 and 0.2), 1.5 mmol of Yb(NO₃)₃·5H₂O (99.9%), 3.6 mmol of Nb₂O₅ (99.99%), 0.1 mmol/0.2 mmol of Er(NO₃)₃·5H₂O (99.9%) and 7.0 mmol of LiOH·H₂O were dissolved in H₂O/EG mixture solution (volume ratio of 40 mL:40 mL). Here, an excess of Nb₂O₅ was used due to its poor solubility. Then, the above solution was heated at 270 °C for 12 h. After cooling naturally to ambient temperature, the white precipitates were centrifuged and washed with deionized water and ethanol three times, and dried at 70 °C. Finally, YbNbO₄:0.1 mol% Er³⁺ and YbNbO₄:0.2 mol% Er³⁺ phosphors, named as YNE-1 and YNE-2, respectively, were obtained through annealed at 900 °C for 2 h.

The powder X-ray diffraction (XRD) spectra were measured by using a powder diffractometer equipped with Cu Κα radiation source (40 kV, 30mA, λ = 1.5406 Å, Bruker AXS D8-Advance, Karlsruhe, Germany). Scanning electron microscopy (SEM, Hitachi SU8010, Tokyo, Japan) was used to observe the morphology. Raman spectra were studied by a micro co-focal Raman spectrometer (Horiba LabRAM HR Evolution, Longjumeau, France). Under a 980 nm laser excitation, the temperature-dependent UC emission spectra were measured by a fluorescence spectrometer system (Zolix, Beijing, China) equipped with a temperature controller (Lake Shore Model 336, Westerville, OH, USA).

3. Results

The XRD patterns shown in Figure 1a indicate that the main diffraction peaks of YNE-1 and YNE-2 phosphors can be indexed to monoclinic phase YbNbO₄ (JCPDS Card No. 81-1976) with an impurity phase Yb₂O₃ observed from its 2θ reflection at 29.7° (JCPDS Card No. 43-1037). An impurity phase Nb₂O₅ (JCPDS Card No. 72-1121) is observed in YNE-2.

Figure 1b shows the vibrations associated to Raman scattering for YNE-1 and YNE-2 phosphors. Raman bands around 117, 183, 303, 316, 334, 418, 444 and 816 cm⁻¹ are agreed with the literature data reported for the YbNbO₄ [19]. The Raman peaks at 418 cm⁻¹ and 334/816 cm^-1 are assigned to Nb-O anti-symmetric and symmetric modes of NbO₄ tetrahedral structure, respectively, meaning that YNE-1 and YNE-2 have a regular NbO₄ tetrahedron with no interactions and distortions [20]. Additionally, YbNbO₄:Er³⁺ phosphors here also possess the similar structure to H-Nb₂O₅ due to the appearance of Raman bands at 238, 630, 678 and 992 cm⁻¹ [21]. The Raman peaks observed at 540, 834, 901 and 935 cm⁻¹ represent the existent of the impurity phase Nb₂O₅ [22,23,24], and Raman bands at 470 cm⁻¹ is assigned to the phase Yb₂O₃ [25]. The phonon energies below 300 cm⁻¹, including 135, 155 and 276 cm^-1, are assigned to external vibrations [26]. Referring to XRD and Raman spectra, the formation evolution of YbNbO₄:Er³⁺ could be understood by the following equations [20,27]:

In the dissolution-precipitation processes:

3 Nb₂O₅ + 8 OH⁻ → Nb₆O₁₉⁸⁻ + H₂O

(1)

Nb₆O₁₉⁸⁻ + 34 OH⁻ → 6 NbO₆⁷⁻ + 17H₂O

(2)

NbO₆⁷⁻ + Li⁺ + 3 H₂O → LiNbO₃ + 6 OH⁻

(3)

LiNbO₃ + Yb³⁺ + 2 OH⁻ → YbNbO₄ + Li⁺ + H₂O

(4)

Yb³⁺ + 3 OH⁻ → Yb(OH)₃

(5)

In the calcination process:

2 Yb(OH)₃ = Yb₂O₃ + 3H₂O

(6)

In a dissolution-precipitation process, due to the poor solubility and weak acidic of Nb₂O₅, LiOH·H₂O is used as the flux. At the initial stage, Nb₂O₅ is dissolved into Nb₆O₁₉⁸⁻ ions based on a similar neutralization reaction between an acid and a base (Equation (1)). Then, Nb₆O₁₉⁸⁻ furtherly reacts with more OH⁻ to form single octahedron NbO₆⁷⁻ anions via complex transformations (Equation (2)). In a supersaturated medium (Equation (3)), the phase LiNbO₃ is occurred after producing the tiny crystalline nucleation. Finally, YbNbO₄ is generated through the exchange reaction between Yb³⁺ and Li⁺ ions based on Equation (4). It is inevitable that Yb³⁺ ions would react with OH^- in the dissolution-precipitation process (Equation (5)). After calcinating, the Yb₂O₃ phase is formed by the decomposition reaction of Yb(OH)₃ [28]. The phase Nb₂O₅ appeared in YNE-2 (Figure 1a) is caused by the suppressed exchange reaction process between Yb³⁺ and Li⁺ ions, since the enhanced Er³⁺ ions consume OH^- ions. It is speculated that the impurities Nb₂O₅ and Yb₂O₃ would produce the new defect centers in YbNbO₄ host matrix, which may decrease the UC emissions under 980 nm excitation.

SEM images shown in Figure 2 display YNE-1 and YNE-2 phosphors are composed of the agglomerated irregular polygons with an average diameter of 0.5–1.0 μm and tiny grains. It is possible that the non-uniform tiny grains may be the impurity Yb₂O₃ particles.

Figure 3a shows the UC emission spectra of YbNbO₄:Er³⁺ phosphors under 980 nm excitation. The two green UC emissions at 530/558 nm and a red UC emission at 672 nm are attributed to the ²H_11/2/⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 transitions of Er³⁺ ions, respectively [29]. It is obvious that both green and red UC emissions decrease with the increasing concentrations of Er³⁺ ions in YNE-2 phosphor. Combination of XRD, Raman spectra and UC emission spectra, there are no studies on YbNbO₄ phosphors doped with higher concentration of Er³⁺ ions. This is because the increasing Er³⁺ content leads to the appearance of the impurity of Nb₂O₅ and the reduced UC emissions.

To understand UC mechanisms, the intensity of fluorescence (I_f) is measured as a function of the pump power (P). Log–Log plots of pump power dependence in YbNbO₄:Er³⁺ phosphors are shown in Figure 3b. For an unsaturated UC process, the number of pump photons (n) required to populate the emitting state can be obtained via the formula: I_f ∝ Pⁿ [30]. As for YNE-1 and YNE-2, respectively, the slope values of the green UC emissions are fitted to be 2.25 and 2.34, and the red UC emissions yield n = 1.83 and 1.94, indicating that at least two 980 nm laser photons are needed to populate both green and red UC emissions [31].

Figure 3c depicts schematically possible UC mechanisms of YbNbO₄:Er³⁺ phosphors under 980 nm excitation. Previous studies on LnNbO₄ indicated that Ln³⁺ ions acted not only as one component of host materials but also as the sensitizer to absorb laser excitation and transfer its energy to active ions. For example, The Gd³⁺ ion, a part of GdNbO₄ self-activated compound, could transfer its absorbed energy to the state ¹D₂ of Tm³⁺ and the state ⁴F₉ of Dy³⁺ ions, leading to the blue and green emissions under UV light excitation [32]. TbNbO₄ emitted the green emission at 549 nm arising from the ⁵D₄ → ⁷F₅ transition of Tb³⁺ ions [33]. Therefore, 980 nm laser excitation of Yb³⁺ ions is only considered here, since Yb³⁺ ion has a much larger absorption cross section and could efficiently transfer its absorbed energy to Er³⁺ ions [34]. As illustrated in Figure 3c, the ⁴F_7/2 state of Er³⁺ ions is populated by the energy transition (ET) processes of ET1: ⁴I_15/2 (Er³⁺) + ²F_5/2 (Yb³⁺) → ⁴I_11/2 (Er³⁺) + ²F_7/2 (Yb³⁺) and ET2: ⁴I_11/2 (Er³⁺) + ²F_5/2 (Yb³⁺) → ⁴F_7/2 (Er³⁺) + ²F_7/2 (Yb³⁺) [35]. Subsequently, the Er³⁺ ions on the ⁴F_7/2 state nonradiatively relax to the ²H_11/2/⁴S_3/2 states, which decay radiatively to the ⁴I_15/2 state, producing the green UC emissions. The Er³⁺ ions at ⁴F_9/2 state, which are populated by ET3 process of ²F_5/2 (Yb³⁺) + ⁴I_13/2 (Er³⁺) → ²F_7/2 (Yb³⁺) + ⁴F_9/2 (Er³⁺), radiatively depopulate to the ⁴I_15/2 state, generating the red UC emission. It is proposed that the energy back-transfer (EBT) process of ⁴S_3/2 (Er³⁺) + ²F_7/2 (Yb³⁺) → ⁴I_13/2 (Er³⁺) + ²F_5/2 (Yb³⁺) may occur [36]. An increase in the Er³⁺ concentrations leads to a shortened distance between Yb³⁺-Er³⁺ pairs and a fast EBT process because the rate of ET process is inversely proportional to the distance between two neighboring ions. Consequently, the reduction of green and red emissions in YNE-2 arises from more efficient EBT process (see Figure 3c). This is an indication that the optical quenching exists in YbNbO₄:Er³⁺ phosphors at high doping concentration of Er³⁺ ions.

Figure 4 shows temperature-dependent on green emissions at 530 nm/558 nm of YNE-1 phosphor in the temperature range of 120–280 K under 980 nm excitation. As the temperature increases (Figure 4a), the intensity of the green emission at 530 nm is observed to increase with respect to the emission at 558 nm, and the red UC emission at 672 nm increases. As illustrated in Figure 4b, the intensity ratio of the overall green to red emission (Ratio of green and red) is increased with increasing temperature. The international commission on illumination (CIE) chromaticity diagram used to reflect the true color of luminescences at different temperatures is shown in Figure 4c. Based on 1931 CIE chromaticity theory [37], the color coordinates (x, y) of YNE-1 phosphor are calculated to be (0.40, 0.59), (0.39, 0.60), (0.38, 0.61), (0.37, 0.62), (0.37, 0.62), (0.35, 0.63), (0.34, 0.64), (0.33, 0.65) and (0.32, 0.66), respectively, from the temperature ranging from 120 to 280 K. Therefore, the observed gradual change of color tone from yellow to green region implies that YbNbO₄:Er³⁺ may have an ability of the temperature-dependent color tuning property.

Since the populations of the ²H_11/2 and ⁴S_3/2 TCLs of Er³⁺ ion obey the Boltzmann thermal equilibrium, the relationship between FIR, which is the ratio of I₅₃₀ to I₅₅₈, and the corresponding temperature is expressed as the following equation [38,39]:

$$\mathrm{FIR}=\frac{I_{530}}{I_{558}}=\mathrm{C}\exp (-\frac{-\Delta E}{{\mathrm{k}}_{\mathrm{B}}T})$$

(7)

Where I₅₃₀ and I₅₅₈ represent the intensities of green emissions around 530 and 558 nm, respectively; ΔE means an energy gap between the two TCLs; k_B, T and C are the Boltzmann constant, the absolute temperature and the constant, respectively.

Figure 5a illustrates FIR of I₅₃₀ and I₅₅₈ as a function of the temperature in the range of 120~280 K, giving the slope value of ΔE/k_B is fitted to be about −454.96 cm⁻¹*K in YNE-1. The behavior that the fitted ΔE of 318 cm⁻¹ is much smaller than the experimental ΔE of 723 cm⁻¹ determined from Figure 3a may be attributed to the nonradiative cross-relaxation channels (Channel 1: ⁴S_3/2 + ⁴I_15/2 → ⁴I_13/2 + ⁴I_9/2 and Channel 2: ⁴S_3/2 + ⁴I_15/2 → ⁴I_9/2 + ⁴I_13/2) (see Figure 3c).

The absolute sensor sensitivity S_a plays an important role in determining the practical application of optical thermal sensing, which can be obtained by [40]:

$$S_a=\frac{d FIR}{dT}=FIR\frac{\Delta E}{k_B{T}^2}$$

(8)

In Figure 5b, the sensor sensitivity S_a calculated by means of Equation (8) decreases at elevated temperature. It has been found that the maximum S_a (S_max) of 0.00712 K⁻¹ at 220 K in YNE-1 phosphor is higher than S_max of 0.0053 K⁻¹ at 350 K in Gd₂MoO₆:Yb³⁺/Er³⁺ [41], S_max of 0.0053 K⁻¹ at 93 K in BaCaTiO₃:Ho³⁺/Yb³⁺ [42], S_max of 0.0044 K⁻¹ at 637 K in NaYF₄:Er³⁺/Yb³⁺ [43] and S_max of 0.0037 K⁻¹ at 508 K in β-NaYF₄:Er³⁺/Yb³⁺ phosphor [44], and is comparable to S_max of 0.0073 K⁻¹ at a temperature of 473 K in YNbO₄:Yb³⁺/Er³⁺ phosphor [45]. Therefore, the S_max of 0.00712 K in YbNbO₄:Er³⁺ phosphor is as large as possible to provide a promising application for monitoring the low temperature.

4. Conclusions

In summary, the monoclinic YbNbO₄:Er³⁺ phosphors composed of the agglomerated irregular polygons and tiny grains are successfully synthesized for low-temperature optical sensors. Here, Yb³⁺ ions, one component of YbNbO₄ host matrix, could transfer their absorbed energy to excite Er³⁺ ions to the ²H_11/2/⁴S_3/2 and ⁴F_9/2 emitting states. The elevating temperature leads to a multicolor change from yellow to green in YNE-1. The reduced green and red UC emissions caused by the increased concentrations of Er³⁺ ions are resulted from the fast EBT process of ⁴S_3/2 (Er³⁺) + ²F_7/2 (Yb³⁺) → ⁴I_13/2 (Er³⁺) + ²F_5/2 (Yb³⁺). In the low temperature range, a high maximum sensor sensitivity of 0.00712 K⁻¹ at 220 K is achieved in YNE-1, contributing a feasible and expansible way to further survey the sensitivity of optical temperature sensor and promote its applications.