3.1 Crystal and phase structure
Three kinds of structure for YTaO4, including monoclinic M', monoclinic M and tetraclase T, have been reported. Among which, the two monoclinic phases are demonstrated to be a thermodynamically stable phase.
As can be seen from Fig. 1, the diffraction peak of prepared YTaO4 material is consistent with standard card of PDF#24-1425, indicating that pure M'-YTaO4 is successfully synthesized by chemical co-precipitation method, and its unit cell parameters are a = 5.29 Å, b = 5.45 Å, c = 5.11 Å and β = 96.45°, respectively [19]. The main diffraction peaks at 16.3°, 28.2°, 29.9°, 30.6°, 32.8° and 35.4° are the corresponding crystal planes (0 1 0), (-1 1 1), (1 1 1), (1 1 1), (0 2 0) and (0 0 2). The diffraction peaks at 28.2° and 30.6° correspond to the (-1 1 1) and (1 1 1) crystal planes of M'-YTaO4, when doped by Zr4+, the diffraction peaks at 28.2° and 30.6° reduces gradually, while a new diffraction peak locating at 29.9° enhance progressively as the Zr4+ content increases. By comparing with the standard card of PDF#77-2112, it can be confirmed that the diffraction peak of 29.9° is the (1 1 1) crystal plane of YSZ. Therefore, the prepared (ZrO2)x-(YTaO4)1−x ceramic powder is compound of M'-YTaO4 and YSZ(Y: 7% − 8%).
To further study the composition of (YSZ)x-(YTaO4)1-x, the prepared ceramic powder was analyzed by XPS subsequently. As listed in Fig. 2, the O 1s spectra of (YSZ)x-(YTaO4)1-x ceramic powder is decomposed into Y-O bond and Ta-O bond, which locates at 528.88 eV and 530.48 eV [20], respectively. Due to the little difference between Zr-O bond (530.2 eV) and Ta-O bond (530.4 eV), Zr-O bond is not successfully distinguished in O 1s spectra. However, it is clearly observed in the energy spectrum of Zr 3d, locating at 180.98 eV and 183.18 eV. Meanwhile, Y-O bond (156.08 eV, 158.18 eV) is also found in Y 3d high resolution energy spectrum. Therefore, it can be deduced that the prepared (YSZ)x-(YTaO4)1-x ceramic powder materials contain YTaO4 and YSZ, which is consistent with the results of XRD.
3.3 Fluorescent properties
The excitation spectrum of the material (YSZ)x-(YTaO4)1−x at room temperature, shown in Fig. 5(a), was obtained by monitoring the emission at 500 nm. The spectrum band between 350 nm and 420 nm can be attributed to the charge-transfer state (CTS) of TaO43−, which reaches the maximum intensity at 385 nm. By monitoring the excitation wavelength at 385 nm, the emission spectra of (YSZ)x-(YTaO4)1−x (x = 0-0.5) ceramic powder at room temperature were detected and listed in Fig. 4(b). The emission spectra are mainly composed of a broadband emission range from 450 nm to 550 nm. The maximum emission peak locates at 499 nm, corresponding to the charge transfer transition of the TaO43− group [21], which increases firstly and then decreases gradually as the Zr4+ concentration increasing. While the content of Zr4+ is 0.3, emission intensity obtains the maximum value. The maximum emission intensity of (YSZ)0.1-(YTaO4)0.9 is much lower than that of pure YTaO4, possibly attributing that the formation of YSZ cause the reduce of YTaO4. The experimental results show that the characteristic emission of TaO43− is caused by the transition of TaO43− from the ground state to the excited energy level under the excitation of ultraviolet light, and the emission intensity of TaO43− varies with the concentration of Zr4+, indicating that TaO43− and Zr4+ have a process of energy transfer.
Figure 6 shows the CIE color coordinate of (YSZ)x-(YTaO4)1−x (x = 0-0.5) powder. It can be concluded that all the prepared (YSZ)x-(YTaO4)1−x (x = 0-0.5) ceramic powder excited at 385 nm emit bluish green light, and the luminous color gradually moves from bluish green to green as the doping contents of Zr4+ increasing from 0 to 0.3, and then moves back to bluish green. The color coordinates are listed in Table 1. The color temperature of prepared (YSZ)x-(YTaO4)1−x powder is higher than 10000 K, which mainly concentrated in the bluish green light area.
Table 1
Color coordinates of (YSZ)x-(YTaO4)1−x (x = 0-0.5) powder excited at 385 nm. (ZYT-1, ZYT-2, ZYT-3, ZYT-4 and ZYT-5 represent YTaO4 powder doped with Zr4+ at concentrations of 0.1, 0.2, 0.3, 0.4 and 0.5, respectively)
Materials
|
YTaO4
(X,Y)
|
ZYT-1
(X,Y)
|
ZYT-2
(X,Y)
|
ZYT-3
(X,Y)
|
ZYT-4
(X,Y)
|
ZYT-5
(X,Y)
|
385 nm
|
0.09839,
0.35032
|
0.12950, 0.34503
|
0.17146, 0.36149
|
0.17996, 0.36064
|
0.14949, 0.37379
|
0.14982, 0.39250
|
To investigate the luminescence kinetics, fluorescence decay curves of (YSZ)x-(YTaO4)1−x (x = 0-0.5) powder was measured at room temperature. As shown in Fig. 7, all the prepared (YSZ)x-(YTaO4)1−x (x = 0-0.5) powder decays as a bi-exponential function, whose fluorescence lifetime can be fitted by the following Eq. (1) [22, 23]. The corresponding fitting values are listed in Table 2.
$${I}={{I}}_{1}\ast \mathbf{e}\mathbf{x}\mathbf{p}\left(-\frac{{t}}{{{\tau }}_{1}}\right)+{{I}}_{2}\ast \mathbf{e}\mathbf{x}\mathbf{p}\left(-\frac{{t}}{{{\tau }}_{2}}\right)+\mathbf{B}$$
1
where \({{I}}_{1}\) and \({{I}}_{2}\) are intensities at two different time, a.u.; \({{\tau }}_{1}\) and \({{\tau }}_{2}\) are the luminescence lifetime, ms; t is time, ms; B is background.
As discussed in the characterizations part, the prepared materials are high symmetrical spherical or elliptical, thus the decay lifetimes τ can be calculated by Eq. (2) and the corresponding values are displayed as Fig. 8. It is not difficult to classify that the doping of Zr4+ enhances the fluorescence lifetime of YTaO4. And the fluorescence lifetime of (YSZ)x-(YTaO4)1−x (x = 0-0.5) ceramics increases gradually as the doping concentration of Zr4+ increases from 0 to 0.3, while it decreases as the concentration of Zr4+ ranges from 0.3 to 0.5. That is to say, the (YSZ)0.3-(YTaO4)0.7 ceramic powder obtains the longest fluorescence lifetime (2.67 ms), which is nearly 3 times of pure YTaO4.
$${\tau }=\left({{I}}_{1}{{{\tau }}_{1}}^{2}+{{I}}_{2}{{{\tau }}_{2}}^{2}\right)∕\left({{I}}_{1}{{\tau }}_{1}+{{I}}_{2}{{\tau }}_{2}\right)$$
2
where \({\tau }\) is the average fluorescence lifetime, ms.
Table 2
Datas for the fitting curves of bi-exponential decay obtained using Eq. (1)
Character
|
I1
(a.u.)
|
I2
(a.u.)
|
τ1
(ms)
|
τ2
(ms)
|
B
|
YTaO4
|
5963.26146
±
327.01402
|
2.23748
±
0.05275
|
0.90202
±
0.00575
|
4.16924
±
0.0294
|
0.00176
±
7.52381E-5
|
ZYT-1
|
0.47578
±
0.01183
|
26.56096
±
0.53261
|
6.95386
±
0.08034
|
1.4708
±
0.00963
|
8.00173E-4
±
4.8386E-5
|
ZYT-2
|
76.96467
±
4.52968
|
0.46187
±
0.04195
|
1.84718
±
0.02697
|
8.43596
±
0.33006
|
0.00545
±
3.49672E-4
|
ZYT-3
|
0.18103
±
0.00936
|
10.3984
±
0.16739
|
10.42407
±
0.30619
|
1.94954
±
0.01494
|
8.22143E-4
±
8.5356E-5
|
ZYT-4
|
80.65733
±
488505.04356
|
1.10409
±
0.06896
|
0.62503
±
280.80457
|
9.69685
±
0.39064
|
0.01992
±
0.00193
|
ZYT-5
|
26.25157
±
0.69651
|
0.50863
±
0.0122
|
1.46092
±
0.01222
|
7.60608
±
0.09099
|
9.30869E-4
±
6.47073E-5
|
To investigate the temperature indicating capability of (YSZ)x-(YTaO4)1−x powder, high temperature fluorescence decay curves are tested for ZYT-3 samples. As shown in Fig. 8, the fluorescence lifetime decays as a bi-exponential function, thus their fluorescence lifetimes could be fitted by Eq. (1) and the corresponding fitted values are summarized in Table 3. Using Eq. (2), the average fluorescence lifetime of the material was calculated and plotted vs temperature in Fig. 9. The fluorescence lifetimes of ZYT-3 decays exponentially with the increasing of temperature, which can be described as the following Eq. (3), through which the temperature of ZYT-3 material can be analyzed by measuring fluorescence lifetime.
$${\tau }={\mathbf{C}}_{1}\ast \mathbf{e}\mathbf{x}\mathbf{p}\left(-\frac{{T}}{\mathbf{S}}\right)+{\mathbf{y}}_{0}$$
3
where T is the temperature, K; C1, s and y0 are the backgrounds, which is summarized in Fig. 10.
Table 3
Fitting datas of bi-exponential decay curves obtained using Eq. (1)
Character
|
I1
(a.u.)
|
I2
(a.u.)
|
τ1
(µs)
|
τ2
(µs)
|
B
|
350 K
|
36.64946
±
2.25412
|
0.24818
±
0.03274
|
2652.0015
±
49.24481
|
15075.95201
±
0.33006
|
0.0027
±
0.00102
|
400 K
|
93.73691
±
6.53799
|
0.33917
±
0.03087
|
2075.9531
±
33.2242
|
12273.87319
±
613.55456
|
0.00334
±
6.43985E-4
|
450 K
|
126.96357
±
8.88467
|
0.63074
±
0.03979
|
1971.37616
±
29.52926
|
9810.63196
±
276.02951
|
0.00571
±
4.00869E-4
|
500 K
|
136.10959
±
9.98179
|
0.63684
±
0.03896
|
1941.66216
±
29.83539
|
9929.02502
±
275.54348
|
0.00487
±
4.16091E-4
|