MgGd4Si3O13:Ce3+, Mn2+: A Dual-Excitation Temperature Sensor.

A novel apatite-based phosphor MgGd4Si3O13[Mg2Gd8(SiO4)6O2]:Ce3+, Mn2+ was designed and successfully synthesized by a solid-state reaction. Based on the different luminescence properties under 298 and 340 nm excitations, its potential application as a dual-excitation luminescent ratiometric thermometer was studied in detail. Under the excitations of 298 and 340 nm, the fluorescent intensity ratio of Ce3+ and Mn2+ is linearly correlated in the temperature range of 303-473 K. The sensitivity showed an opposite trend with the increase of temperature, and the maximum value was 0.95% K-1. These results indicated that MgGd4Si3O13: Ce3+, Mn2+ can be used as an ideal dual-excitation luminescent ratiometric thermometer.

Introduction

Temperature is one of the important parameters for many physical and chemical reactions. Therefore, it is very significant for many scientific and industrial fields to measure temperature through accurate and effective methods. The fluorescence temperature measurement technology developed based on rare-earth ion-doped inorganic–organic luminescent materials has received wide attention from researchers due to its unique performance, such as realizing noncontact temperature detection, and can work in special environments, such as biological tissues and strong magnetic fields.[1−5] In the early research, the temperature measurement process is mainly based on the change of emission intensity with temperature. However, because the emission intensity is easily affected by external factors such as PH value and luminescence material aggregation, the accuracy of the fluorescence-based temperature sensors based on this principle will also be adversely affected. For the accuracy of temperature, the fluorescence-intensity-ratio-based thermometers which use the emission intensity ratio of two peaks changing with temperature as parameters are developed.[6,7]

Recently, the materials that can be used as temperature sensors have been mainly upconversion nanoparticles (UPNPs). There are also some rare-earth ion-doped complexes and MOF ⊃ dye composites. Compared with UPNPs, MOF ⊃ dye composites often require a complex synthesis process.[8−10] The temperature sensing range of UPNPs is usually excellent; however, the dissatisfactory sensitivity (CaWO3:Ho3+/Yb3+: 3.3–923 K, 0.5% K–1) restricted their applications.[11,12] Downconversion luminescent materials containing two or more luminescent centers have been widely used as LED phosphors. Because of the energy transfer between the luminous centers, the emission intensity of these luminous centers may have different trends with temperature. Based on this performance, some multiple-emission-downconversion luminescent materials show good application potential as temperature sensors.[13−15]

In recent years, many compounds with an apatite structure and same chemical formula of A10(XO4)6Z2 have been selected as the host of the luminescent materials because of the superiority of this structure in designing the new materials.[16,17] In our early work, the luminescence properties of Ce3+–Tb3+-doped MgGd4Si3O13[Mg2Gd8(SiO4)6O2] have been studied in detail.[18] Because the different coordination environments of two cation sites in the crystal, MgGd4Si3O13:Ce3+ exhibits an obvious difference emission band under different excitation wavelengths (blue emission excited at 298 nm and yellow emission excited at 340 nm). In this work, Ce3+–Mn2+-doped MgGd4Si3O13 was synthesized to realize the dual-excitation temperature sensors with high sensitivity. The structure and luminescence properties were discussed in detail. Under the excitation of 340 nm, over a wide temperature range (303–473 K), the intensity ratio of Ce3+–Mn2+ shows satisfactory linear correlation with the temperature and exhibits a good sensitivity of 0.95% K–1.

Results and Discussion

Phase Structure of Mn2+ Singly and Ce3+, Mn2+ Codoped MgGd4Si3O13

The X-ray diffraction (XRD) patterns of the synthesized MgGd4–Si3O13:xMn2+ (0.04 ≤ x ≤ 0.2) and MgGd4–Si3O13:0.08Ce3+, xMn2+ (0 ≤ x ≤ 0.16) samples are provided in Figure a,b. All the patterns match well with the structural refinement result of MgGd4Si3O13 and no obvious impurity peak was detected. For both series of samples, the XRD patterns shifted slightly to the large angle direction with the increase in doping concentration, which is due to the change of lattice parameters caused by the different ion radii of Mn2+ (0.67 Å, CN = 6; 0.9 Å, CN = 7) and Mg2+ (0.72 Å, CN = 6)/Gd3+ (1.07 Å, CN = 7).

Figure 1

XRD patterns of (a) MgGd4–Si3O13:xMn2+ (0.04 ≤ x ≤ 0.2) and (b) MgGd4–Si3O13:0.08Ce3+, xMn2+ (0 ≤ x ≤ 0.16) samples.

For the purpose of further studying the occupancy of the doped ions Ce3+ and Mn2+, the structural refinement of MgGd4Si3O13:0.16Mn2+ and MgGd4Si3O13:0.08Ce3+, 0.16Mn2+ is carried out by the Materials Studio program, as shown in Figure a,b. The crystal structure parameters of MgGd4Si3O13:0.16Mn2+ and MgGd4Si3O13:0.08Ce3+, 0.16Mn2+ are given in Table S1, and their atomic coordinates are given in Tables S2 and S3. It can be seen that MgGd4Si3O13: 0.16Mn2+ has a slightly smaller cell volume than MgGd4Si3O13: 0.08Ce3+, 0.16Mn2+. This is because the larger-sized Ce3+ (1.07 Å, CN = 7; 1.196 Å, CN = 9) replaces the smaller-sized Gd3+ (1.00 Å, CN = 7; 1.107 Å, CN = 9). In addition, the refinement results indicate that Mn2+ occupies both Mg/Gd1 and Gd2 sites in the lattice.

Figure 2

XRD refinement results of (a) MgGd4Si3O13:0.16Mn2+ and (b) MgGd4Si3O13:0.08Ce3+, 0.16Mn2+.

PL Properties

Figure a shows the photoluminescence excitation (PLE) spectra of MgGd4–Si3O13:xMn2+ (0.04 ≤ x ≤ 0.2) samples monitored at 602 nm. Several sharp peaks near 312, 273, and 253 nm belong to the 8S7/2–6P, 8S7/2–6I, and 8S7/2–6D transitions of Gd3+. In our previous work, the transitions of Gd3+ also appeared in the PLE spectrum of MgGd4Si3O13: Tb3+.[18] This phenomenon is caused by the obvious energy transfer mechanism from Gd3+ to the doped ions. Except for the transition of Gd3+, other five distinct peaks located at about 343, 367, 409, 414, and 467 nm existing in the excitation spectra are credited to the absorption transitions of Mn2+, respectively. The emission spectra of Mn2+-doped MgGd4Si3O13 are displayed in Figure b, which show a broad red emission around 600 nm under 409 nm excitation. When x increases to 0.16, the emission intensity of the sample reaches the maximum.

Figure 3

(a) PLE spectra of MgGd4Si3O13:xMn2+ (0.04 ≤ x ≤ 0.20) monitored at 602 nm; (b) the PL spectra of MgGd4Si3O13: xMn2+ (0.04 ≤ x ≤ 0.20) excited at 409 nm.

In our previous work, the special luminescence properties of Ce3+ were discussed. Because Ce3+ occupies different coordination environments in MgGd4Si3O13 (Mg/Gd1 site with nine-coordination and Gd2 site with seven-coordination), under different excitations, Ce3+ exhibits distinct emission spectra.[18] The PLE spectra of MgGd4Si3O13:0.16Mn2+ and MgGd4Si3O13:0.08Ce3+, 0.16Mn2+ and the PL spectra of MgGd4Si3O13:0.08Ce3+ are displayed in Figure , respectively. The emission band peaked at 410 nm is attributed to the emission of Ce3+ occupying the Mg/Gd1 site (Ce1), and the emission band peaked at 505 nm is attributed to the emission of Ce3+ occupying the Gd2 site (Ce1). Because the PL spectrum of MgGd4Si3O13:0.08Ce3+ overlaps significantly with the PLE spectrum of MgGd4Si3O13:0.16Mn2+, the energy transfer from the Ce3+ to Mn2+ ions can be expected in the Ce3+ and Mn2+ codoped host. In addition, the 4f–5d transition of Ce3+ is observed in the excitation spectrum of MgGd4Si3O13:0.08Ce3+, 0.16Mn2+ (the green line in Figure ) monitoring at 602 nm emission (the emission of Mn2+), which demonstrates that there is an effective energy transfer effect between Ce3+ and Mn2+.

Figure 4

PLE spectra of MgGd4Si3O13:0.16Mn2+ (black) and MgGd4Si3O13: 0.08Ce3+, 0.16Mn2+ (green) monitored at 602 nm. The PL spectra of MgGd4Si3O13:0.08Ce3+ excited at 298 (red) and 340 (blue) nm.

In order to study the energy transfer from Ce3+ to Mn2+, a series of MgGd4Si3O13:0.08Ce3+, xMn2+ (0 ≤ x ≤ 0.16) samples have been prepared. Figure a,b displays the PL spectra excited at 298 and 340 nm, respectively. It can be seen that with the increase of Mn2+ concentration, the emission intensity of Ce3+ decreases monotonically, which can be used as the evidence of energy transfer between Ce3+ and Mn2+. Therefore, according to the different emission positions of Ce3+ and Mn2+, the color can be adjusted from blue to red (as shown in Figure S1). Figure S2 displays the decay curves of Ce3+, and based on the Dexter’s and Reisfeld’s theories and decay curves, the dominant energy transfer mechanism from Ce3+ to Mn2+ was identified as a dipole–quadrupole interaction (as shown in Figure S3).[19−22]

Figure 5

PL spectra of MgGd4Si3O13:0.08Ce3+, xMn2+ (0 ≤ x ≤ 0.16) excited at (a) 298 and (b) 340 nm.

Thermal Stability and Ratiometric Temperature Sensing

The temperature-dependent PL spectra of MgGd4Si3O13:0.08Ce3+excited at 298 and 340 nm are shown in Figure a,b, respectively. The luminescence intensities of Ce3+decrease obviously with the increase of temperature. Moreover, when the temperature rises to 200 °C, the emission band at 505 nm disappears, while the band at 410 nm occurs. All these changes have been shown in the inset of Figure b, which indicates that the thermal stability of Ce1 and Ce2 is obviously different. Generally, the Ce3+ located at the rigid host lattice is expected to have good thermal stability with a narrow emission band.[23−25] By comparing the emission spectra of Ce1 and Ce2, it is found that the half-height width of Ce1 is significantly smaller than that of Ce2, which indicates that the Mg/Gd1 site shows better structural rigidity than the Gd2 site. Therefore, Ce1 has better thermal stability than Ce2.The obvious difference in thermal stability between Ce1 and Ce2 is attributed to the different structural rigidities of Mg/Gd1 and Gd2 sites. Due to the stronger rigidity of the Mg/Gd1 site, Ce1 exhibits a relatively narrower emission band and better thermal stability. The results of normalized spectra (shown in Figure c,d) also indicate that with the increase of temperature, the excitation and emission spectra of Ce2 appear more obvious broadening, indicating that Ce2 is more obviously affected by phonons. The spectral broadening results show that after the electrons of Ce2 are excited to the 5d level, they are more likely to return to the 4f level in a radiation-free way through cross-relaxation, resulting in a more obvious emission intensity reduction.

Figure 6

Temperature-dependent PL spectra of MgGd4Si3O13:0.08Ce3+ excited at (a) 298 and (b) 340 nm, respectively. The normalized PLE and PL spectra of MgGd4Si3O13:0.08Ce3+ excited at (c) 298 and (d) 340 nm with the temperature increased from 25 to 125 °C, and the insets show the enlarged view of the spectral overlap region.

The temperature-dependent PL spectra of MgGd4Si3O13:0.08Ce3+, 0.16 Mn2+ excited at 298 and 340 nm are shown in Figure a,b, respectively. The luminescence intensities of Ce3+ and Mn2+ decrease obviously with the increase of temperature. To further investigate the thermal quenching phenomenon of the MgGd4Si3O13:0.08Ce3+, 0.16 Mn2+ sample, a configurational coordinate diagram based on the spectra is shown in Figure .[26−28] When MgGd4Si3O13: Ce3+, Mn2+ was excited under 340 nm excitation, some of the excited electrons jumped to the ground state and produced the yellow light emission, and some of them back to the ground state with nonradiation relaxation by passing through the intersection of the ground state and the excited state, causing the loss of luminescence intensity (① and ②). At the same time, some excited electrons transfer energy to the electrons of Mn2+, making the electrons of Mn2+ transition to the excited state (③). Similarly, some of these electrons transition back to the ground state to produce red light emission, and some of them relax to the ground state without radiation through the intersection of the ground state and the excited state (④ and ⑤). When the material is excited at 298 nm, the excited electrons of Ce1 will not only produce the blue emission and nonradiation relaxation (⑥ and ⑦) but also transfer energy to Ce2 and Mn2+ (⑧ and ⑨). Therefore, under 298 nm excitation, the process ① to ⑤ and the emission of Ce2 and Mn2+ will also occur.

Figure 7

Temperature-dependent PL spectra of MgGd4Si3O13:0.08Ce3+, 0.16 Mn2+ excited at (a) 298 and (b) 340 nm, respectively. The inset is the enlarged view of the spectra in the range of 360–520 nm.

Figure 8

Configurational coordinate diagram of the ground and excited state of Ce1, Ce2, and Mn2+.

Based on the above discussion, we can take the luminescence intensity ratio of Ce3+ and Mn2+ (Δ = ICe/IMn) as a temperature measurement parameter. Figure a,b shows the changes of the luminescence intensity ratio at different temperatures when MgGd4Si3O13:0.08Ce3+, 0.16 Mn2+ is excited at 298 and 340 nm, respectively. The Δ values are normalized to the Δ303K value (Δ value at room temperature). It can be seen that the normalized Δ values are linearly correlated in the range of 303–473 K. According to the linear fitting results, the relative sensitivity (Sr) can be calculated according to the following formula[29]where Δ is the intensity ratio and T is the temperature. Figure c,d exhibits the variation in the Sr values at different temperatures. Under different excitations, the sensitivity shows different trends with the temperature. Under 298 nm excitation, the Sr decreased with the increase of temperature, and the maximum value was 0.21% K–1. Under 340 nm excitation, the Sr increased with the increase of temperature, and the maximum value was 0.95% K–1. Besides, the five-time cyclic heating test of MgGd4Si3O13:0.08Ce3+, 0.16Mn2+ between 303 and 473 K is given in Figure . The MgGd4Si3O13:0.08Ce3+, 0.16Mn2+ sample showed that the recycling performance and the change of fluorescence intensity ratio are less than 5% after five cycling steps. The Sr value and temperature range of MgGd4Si3O13:0.08Ce3+, 0.16Mn2+ and some Ce3+–Mn2+ codoped thermometers are provided in Table .[14,30,31] Comparing with our previous report [NaBaSc(BO3)2: Ce3+, Mn2+], the sensitivity of MgGd4Si3O13: Ce3+, Mn2+ is not outstanding, but it still has advantages over other phosphors. These results showed that MgGd4Si3O13: Ce3+, Mn2+ phosphors could be used as a high-sensitivity optical thermometer.

Figure 9

(a,b) Temperature-dependent intensity ratio (Δ/Δ303 K) and linearly fitted curve under 298 and 340 nm excitation; (c,d) Variation of relative sensitivity with temperature under 298 and 340 nm excitation.

Figure 10

Cyclic heating test of MgGd4Si3O13:0.08Ce3+, 0.16Mn2+ between 303 and 473 K excited at (a) 298 and (b) 340 nm, respectively.

Table 1

Sr Values and Temperature Range of MgGd4Si3O13:0.08Ce3+, 0.16Mn2+ and Some Ce3+–Mn2+ Codoped Thermometers

material	Sr (% K–1)	temperature range (K)	references
MgGd4Si3O13:Ce3+, Mn2+	0.21 (298 nm)	300–473	this work
	0.95 (340 nm)	303–473
(Ca,Sr)10Li(PO4)7:Ce3+, Mn2+	0.4	293–473	(30)
NaMgBO3: Ce3+, Mn2+	0.69	293–473	(31)
NaBaSc(BO3)2: Ce3+, Mn2+	3.16	298–473	(14)

Conclusions

In summary, a novel apatite dual-excitation luminescent ratiometric thermometer MgGd4Si3O13: Ce3+, Mn2+ was successfully synthesized via the solid-state method. The energy transfer between Ce3+ and Mn2+ was discussed in detail. The luminescent properties of Mn2+ and the temperature-dependent luminescent properties of MgGd4Si3O13: Ce3+, Mn2+ have been investigated. Under the excitations of 298 and 340 nm, the fluorescent intensity ratio of Ce3+ and Mn2+ is linearly related to the temperature in the range of 303–473 K with high sensitivity of 0.95% K–1. The above results indicate that MgGd4Si3O13: Ce3+, Mn2+ can serve as a good candidate material for luminescent ratiometric thermometry.

Experimental Section

Preparation of Materials

All samples in our work were synthesized by the solid-state reaction. The starting materials were Gd2O3 (99.99%), MnCO3 (99.99%), CeO2 (99.99%), H2SiO3 (A.R.), and MgO (A.R.). These materials were used as raw materials without any further purification. After been mixed well in agate mortar, these ingredients were calcinated at 1480 °C in a reducing atmosphere of 20% H2–80% N2 for 6 h.

Characterization

The phase structures of the obtained samples were tested by a Rigaku D/Max-2400 diffractometer with Ni-filtered Cu Kα radiation. The PL properties (PL, PLE spectra, and PL decay curves) were obtained by a FLS-920T fluorescence spectrophotometer equipped with Xe and nF900 ns Flashlamp source. All the measurements were performed at room temperature. The temperature-dependent emission was tested combined with the heating apparatus (TAP-02).