Luminescent Properties of Tm 3+ -doped BaY 2 ZnO 5 Phosphors Prepared by Sol–Gel Method with Different Urea Concentrations

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Introduction
Sensors have been widely used in various fields, such as machinery, electronics, and medical, optical, and chemical industries.The important requirements of sensors are stability, sensitivity, and selectivity, which are termed 3 'S'. (1)Owing to environmental changes and abnormal weather, the use of temperature, gas, and environmental monitoring sensors has gradually become widespread, and the need for stable sensing materials has been increasing.(4)(5) Photoluminescence-based nanomaterials can be used for thermometry sensors, and lanthanide (La)-doped photoluminescent materials have advantages such as high chemical stability, narrow emission peaks, long emission decay lifetimes, temperature sensitivity, and low toxicity, which are promising for a new generation of potential applications. (5)7)(8) Rare-earth-ion-doped materials have attracted considerable attention because of their excellent photoluminescence properties. (9)Rare-earth ions in phosphors act as activators with f-f transitions that confine emission to the visible range for high efficiency and lumen-equivalent properties.Various Tm 3+ ion-doped oxide host materials such as YVO 4 :Tm 3+ , GdVO 4 :Tm 3+ , and YInGe 2 O 5 :Tm 3+ phosphors producing a blue emission with excellent color coordinates have been extensively studied because they are stable with appropriate lifetimes and color-rendering properties under high-density excitation. (9,10)aY 2 ZnO 5 has a stable orthorhombic crystal structure and high thermal stability, and its space group is Pbnm, which is an excellent photoluminescent host material. (11,12)The unit cell of BaY 2 ZnO 5 is composed of BaO 11 , YO 7 , and ZnO 5 polyhedra.
A given activator center in different host lattices exhibits various optical properties, which are attributable to activator center surroundings changed by a Tm 3+ ion-doped phosphor.In this study, BaY 2 ZnO 5 phosphors with various concentrations of Tm 3+ ions and urea were synthesized at 1200 ℃ for 8 h in air by the sol-gel method.The crystal structure and photoluminescence properties were then investigated.

Materials and Methods
Tm 3+ ion doping in BaY 2 ZnO 5 , formulated as Ba(Y 1-x Tm x ) 2 ZnO 5 , with x equal to 0-0.05 was performed by the sol-gel method.The raw materials used with purities of 99.99% were barium nitrate (BaNO 3 ), zinc acetate (C 4 H 10 O 6 Zn), yttrium acetate [Y(OOCCH 3 ) 3 ], thulium (III) nitrate [Tm(NO 3 ) 3 ], and urea (CN 2 H 4 O), which were purchased from Aldrich and Alfa Aesar Company.The doses of all starting materials were weighed stoichiometrically, and all the materials were dissolved separately in deionized water and mixed in a beaker with continuous stirring and dissolution at 50 o C for 5 h.The precursor was dissolved completely in the solution and then put in a furnace to dry at 50 o C for 4 days.After drying, the powders of the precursor were calcined at 1200 o C for 8 h using a programmable furnace.
The phases of the powders were identified using an X-ray powder diffractometer (Bruker AXS GmbH, D8 Advance, Germany), which uses Cu Ka radiation with a source power of 30 kV and a current of 20 mA.The surface morphologies of the phosphors were observed with a field emission scanning electron microscope (Hitachi S4800-I, Japan).The absorption spectra of the phosphors were measured using a Hitachi U-3010 UV-visible spectrophotometer.The photoluminescence properties of the phosphors, including excitation and emission behaviors, were measured with a Hitachi F-7000 fluorescence spectrophotometer that uses a 150 W xenon arc lamp as the excitation source.The absorption and photoluminescence properties were measured at room temperature.

Results and Discussion
The X-ray diffraction (XRD) results of Ba(Y 1-x Tm x ) 2 ZnO 5 (x = 0 -0.05) and Ba(Y 0.98 Tm 0.02 ) 2 ZnO 5 phosphors added with different urea concentrations and calcined at 1200 o C for 8 h in air are shown in Figs.1(a) and 1(b), respectively.These results indicate that most of the diffraction peaks of the samples doped with different concentrations of Tm 3+ ions and added with various urea concentrations were consistent with the orthorhombic structure of BaY 2 ZnO 5 (JCPDS No:89-5856).The properties of yttrium and other rare-earth ions are similar; thus, rareearth ions can easily replace yttrium ions. (13)Kaduk et al. found that the crystal structures of BaY 2 ZnO 5 and BaTm 2 ZnO 5 , which were calculated by the Rietveld refinement method, (14) have the same space group as Pbnm.These results suggest that Tm 3+ ions can be introduced to completely replace Y 3+ ions in the BaY 2 ZnO 5 lattice and form solid solutions. (15)This is because Tm 3+ (0.88 Å) and Y 3+ (0.90 Å) ions have similar ionic radii.Both ions have the same valence, and in the Ba(Y 1-x Tm x ) 2 ZnO 5 system, when the Y 3+ ion is replaced by the Tm 3+ ion, the charge compensation problem does not occur. (16)he SEM images of Ba(Y 1-x Tm x ) 2 ZnO 5 (x = 0.005-0.05)phosphors are shown in Fig. 2. Figure 3 shows the SEM images of Ba(Y 0.98 Tm 0.02 ) 2 ZnO 5 phosphors added with different concentrations of urea and calcined at 1200 o C for 8 h.The SEM images show particle sizes of approximately 40-50 nm.Regardless of whether only Tm 3+ ions were doped or different urea concentrations were added, the morphology of the particles was irregular and particle agglomeration occurred in the host system.The agglomeration is due to the small particles and the strong cohesion of phosphors prepared by the sol-gel method.The SEM images in Figs. 2 and 3 showed no significant differences; thus, the doping of Tm 3+ ions or the addition of urea at different concentrations had no effect on the surface morphology of the BaY 2 ZnO 5 host.The absorption spectra of BaY 2 ZnO 5 doped with different concentrations of Tm 3+ ions and Ba(Y 0.98 Tm 0.02 ) 2 ZnO 5 added with different concentrations of urea ions, then calcined at 1200 °C for 8 h in air, are shown in Figs.4(a) and 4(b), respectively.The results show a strong absorption appearance in the region from 200 to 250 nm, which is attributed to the band-to-band transitions of the BaY 2 ZnO 5 host. (13)The region from 270 to 400 nm has a weak broad peak that is attributed to the tightly bound Frankel excitons, which are usually observed near the band gap with a large band gap in crystals. (17)When BaY 2 ZnO 5 is doped with Tm 3+ ions or added with different concentrations of urea ions, a group of sharp peaks appear at wavelengths of 362, 464, and 694 nm, which are assigned to the typical 4f n →4f n intra-configuration forbidden transition for Tm 3+ ions. (18)With increasing urea ion concentration, the absorption band between 350 and 750 nm is  , respectively.The excitation spectra were detected using the emission wavelength of 457 nm.The excitation spectrum wavelength range between 200 and 400 nm can be observed.The center of the broadband at 244 nm is classified as the band-to-band transition for the BaY 2 ZnO 5 host. (19)In addition, the two peaks that appear between 250 and 300 nm correspond to the Tm 3+ ion 4f n →4f n intra-configuration transition for 3 H 6 → 3 P 0 and 3 H 6 → 3 P 2 , and the strongest peak at 365 nm corresponds to the 3 H 6 → 1 D 2 transition. (20)When the doping concentration of the Tm 3+ ion is 2 mol%, the excitation energy band in this region is at the maximum, and then it decreases as the doping concentration increases.It can be observed from Fig. 5(b) that the excitation peak is not shifted by adding urea ions.When the concentration of urea ions is 1 wt%, the excitation energy band at 365 nm is at the maximum, and then, it decreases as the urea ion concentration increases.Therefore, from the overall results, if Tm 3+ ions are doped at 2 mol% and 1 wt% urea ions are added, the best excitation peak intensity will be obtained.
An excitation wavelength of 365 nm was used to excite BaY 2 ZnO 5 doped with different concentrations of Tm 3+ ions and Ba(Y 0.98 Tm 0.02 ) 2 ZnO 5 added with various concentrations of urea ions, and their emission spectra are shown in Fig. 6.The main emission peaks at 457 nm are due to the 1 D 2 3 F 4 transitions of the Tm 3+ ions. (20)There are also some very weak peaks at 480, 517, 666, and 735 nm, which correspond to the 1 G 4 → 3 H 6 , 1 D 2 → 3 H 5 , 1 G 4 → 3 F 4 , and 1 G 4 → 3 F 5 transitions, respectively.According to the literature, the rare-earth Tm 3+ ion has three emission levels: 3 P 0 (35,000 cm −1 ), 1 D 2 (27,770 cm −1 ), and 1 G 4 (21,200 cm −1 ). (21)In BaY 2 ZnO 5 :Tm phosphors, it is enough to populate only the 1 D 2 (27,770 cm −1 ) or 1 G 4 (21,200 cm −1 ) emission level because the excited energy level is from the 3 H 6 → 1 D 2 transition of the Tm 3+ ion itself. (20,22)t can be observed from Fig. 6(a) that the emission intensity increases and then decreases with increasing Tm 3+ ion concentration, with the maximum intensity being obtained at a Tm 3+ ion concentration of 2 mol%.(23)(24) Figure 6(b) shows that when the optimal doping concentration of Tm 3+ ions is 2 mol%, the addition of low-concentration urea ions can enhance the main emission peak and the optimal value is 1 wt%.Then, the emission intensity is expected to decrease as the urea ion concentration increases.Therefore, from the overall results, if Tm 3+ ions are doped at 2 mol% and 1 wt% urea ions are added, the best emission peak intensity will be obtained.concentrations of Tm 3+ ions or adding different concentrations of urea ions has no effect on the shape of the emission curve, but the emission spectrum intensity is different.It can be seen in Fig. 7(a) that as the Tm 3+ ion doping concentration increases, the CIE chromaticity coordinates change from the blue-green light region to the blue light region.The best Tm 3+ concentration located in the blue light region is 2 mol%, and the chromaticity coordinates are (x = 0.164, y = 0.07).Figure 7(b) shows that when the optimal doping concentration of Tm 3+ ions is 2 mol%, the addition of 1 wt% urea ions can enhance the luminous intensity of blue light.Its chromaticity coordinates are (x = 0.162, y = 0.067).The results show that the BaY 2 ZnO 5 phosphor doped with Tm 3+ ions and added with a low concentration of urea ions emits a vivid blue light.
The PL properties of Tm 3+ are strongly affected by not only its surroundings but also the blue emission intensity ratio and the energy transfer behavior between the Tm 3+ concentrationdependent PL intensity and the chromaticity coordinates. (25)This result is also shown in Fig. 7.

Conclusions
Lanthanide-doped BaY 2 ZnO 5 phosphors are promising candidates for luminescence temperature thermometry applications.In this study, the BaY 2 ZnO 5 phosphor doped with Tm 3+ and urea ions was successfully synthesized by the sol-gel method, and its luminescence region was blue light.The results of XRD analysis showed that all the samples doped with different concentrations of Tm 3+ and urea ions were consistent with the orthorhombic structure of BaY 2 ZnO 5 .Regardless of whether only Tm 3+ ions were doped or different concentrations of urea ions were added, the morphology of the particles was irregular and particle agglomeration occurred in the host system.The agglomeration occurred because the phosphor powder prepared by the sol-gel method has strong cohesion.An excitation wavelength of 365 nm was used to excite BaY 2 ZnO 5 , and the main emission bands obtained at 457 nm are due to the 1 D 2 → 3 F 4 transitions of the Tm 3+ ions.There are also some very weak peaks at 480, 517, 666, and 735 nm, which correspond to the 1 G 4 → 3 H 6 , 1 D 2 → 3 H 5 , 1 G 4 → 3 F 4 , and 1 G 4 → 3 F 5 transitions, respectively.When the doping concentration of Tm 3+ ions is 2 mol% and that of urea ions is 1 wt%, the CIE chromaticity coordinates are (0.162, 0.067) in the vivid blue region.