Green- and Blue-Emitting Tb3+-Activated Linde Type A Zeolite-Derived Boro-Aluminosilicate Glass for Deep UV Detection/Imaging

Tb3+-activated LTA zeolite-derived boro-aluminosilicate glass samples with a composition of xTb2O3-68(Na2O-Al2O3-SiO2)–32B2O3 (x = 0.2, 1.0 and 10 extra wt%) were prepared using the melt-quenching method. The emission spectra recorded upon ultraviolet (UV) excitation with two different wavelengths of 193 and 378 nm showed blue light (5D3 to 7FJ=6,5,4 and 5D4 to 7F6 transitions of Tb3+) and green light (5D4 to 7F5 transition of Tb3+) emissions with comparable intensities up to a Tb3+ concentration of 10 extra wt%. Of note, the mean decay times of the green luminescence of the glass samples were relatively fast (<20 μs). The synthesized glass has potential in applications concerning UV imaging, UV detection, and plasma display panels.

Different host lattices have been used for Tb 3+ -activated glasses, e.g., aluminates [14,19,21,26,30], phosphates [16,18,22,23], and germanates [24,25,31].Linde Type A (LTA) zeolites are synthetic aluminosilicates with a porous and periodic structure with the composition of Na 12 (AlSi) 12 O 48 •27H 2 O, which can adopt an amorphous structure via thermal treatment at high temperatures [32].To our knowledge, only a few studies have examined the luminescence properties of rare earth-activated LTA zeolite-derived ceramics/glasses, which have included ions of Tb 3+ [20], Dy 3+ (with or without Ag + ) [33], and Eu 2+ [20,34].In the study that used Tb 3+ as the dopant [20], the sample was prepared by ion exchanging Tb 3+ for Na + in the LTA zeolite in deionized water, followed by applying the melt quenching method.The sample was green-emitting and showed a green luminescence decay time of 2.398 ms.Thus, in the present paper, we report on the preparation, physical characterization, and luminescence study of green-and blue-emitting Tb 3+ -doped LTA zeolite-derived boro-aluminosilicate glasses that show fast green luminescence decay times on the order of µs.

Sample Preparation
The Tb 3+ -doped glass samples were prepared via the melt quenching method using LTA zeolites (4A, A. R, Tosoh, Tokyo, Japan; Na 12 (AlSi) 12 O 48 •27H 2 O), boron oxide (B 2 O 3 , 98%, Shanghai Aladdin Biochemical Technology Co. Ltd., Shanghai, China), and Tb 4 O 7 (Shanghai Diyang Chemicals Co., Ltd., Shanghai, China).The amounts of LTA zeolite and boron oxide were fixed at 34 g (68 wt%) and 16 g (32 wt%), respectively.Thus, to 50 g mixtures of LTA zeolite and boron oxide, Tb 4 O 7 was added in amounts of 0.1 g (extra 0.2 wt%; sample #1), 0.5 g (extra 1.0 wt%; sample #2), and 5 g (extra 10 wt%; sample #3).The powder mixtures were placed in an agate mortar and mixed homogeneously.Heating the mixture to 1200 • C using a high-temperature muffle furnace [35], the Tb 3+ ions diffuse and evenly distribute within the LTA molecular sieve.Subsequently, the thoroughly melted liquid is quenched by pouring it into water, thus obtaining colorless transparent glass with the nominal composition xTb 2 O 3 -68(Na 2 O-Al 2 O 3 -SiO 2 )-32B 2 O 3 (x = 0.2, 1.0 and 10 extra wt%).We have also illustrated a brief description of the preparation process [36][37][38][39] of LTA:Tb 3+ , as shown in Figure 1.treatment at high temperatures [32].To our knowledge, only a few studies have examined the luminescence properties of rare earth-activated LTA zeolite-derived ceramics/glasses, which have included ions of Tb 3+ [20], Dy 3+ (with or without Ag + ) [33], and Eu 2+ [20,34].In the study that used Tb 3+ as the dopant [20], the sample was prepared by ion exchanging Tb 3+ for Na + in the LTA zeolite in deionized water, followed by applying the melt quenching method.The sample was green-emitting and showed a green luminescence decay time of 2.398 ms.Thus, in the present paper, we report on the preparation, physical characterization, and luminescence study of green-and blue-emitting Tb 3+ -doped LTA zeolite-derived boro-aluminosilicate glasses that show fast green luminescence decay times on the order of μs.

Characterization
The structures of all of the obtained samples were analyzed with X-ray diffraction (XRD) (Rigaku, Model Mini Flex 600, Tokyo, Japan), using Cu Kα irradiation (λ = 1.5418Å) operated at 40 kV, 15 mA.The morphology of the fracture surfaces of the samples was observed using a scanning electron microscope (SEM), while energy dispersive X-ray spectroscopy (EDX) was performed in the SEM (Verios 5 UC, Eindhoven, The Netherlands).The optical transmission spectra were recorded with an ultraviolet-visible-NIR spectrophotometer (PerkinElmer, LAMBDA 1050, Waltham, MA, USA).The steady photoluminescence (PL) and photoluminescence excitation (PLE) spectra and the dynamic emission decay curves were recorded using a fluorescence spectrophotometer (Edinburgh Instruments, FLS-1000, Livingston, UK).

Characterization
The structures of all of the obtained samples were analyzed with X-ray diffraction (XRD) (Rigaku, Model Mini Flex 600, Tokyo, Japan), using Cu K α irradiation (λ = 1.5418Å) operated at 40 kV, 15 mA.The morphology of the fracture surfaces of the samples was observed using a scanning electron microscope (SEM), while energy dispersive X-ray spectroscopy (EDX) was performed in the SEM (Verios 5 UC, Eindhoven, The Netherlands).The optical transmission spectra were recorded with an ultraviolet-visible-NIR spectrophotometer (PerkinElmer, LAMBDA 1050, Waltham, MA, USA).The steady photoluminescence (PL) and photoluminescence excitation (PLE) spectra and the dynamic emission decay curves were recorded using a fluorescence spectrophotometer (Edinburgh Instruments, FLS-1000, Livingston, UK).

Results and Discussion
The SEM images did not show pores in the samples.The results of the experimental determination of the wt% of the elements in the samples using EDX are presented in Table 1.The values in Table 1 are the average values of two measurements.The inconsistencies between the nominal and experimental compositions can be explained by (i) the uncertainty in the determination of wt% of the elements (18-25%) dictated by the instrumentation used, and (ii) the fact that the glass matrix consisted of light elements such as B, Al, Si, Na, and O, which could be evaporated from the surface, e.g., B could have been undetected due to being very light.The XRD analysis of samples #1, #2, and #3 did not show crystalline peaks, thus confirming the glass nature of the samples (Figure 2).The transmission spectra of samples #1, #2, and #3, presented in Figure 3, showed that the visible light transmittance was beyond 85%.Also, photos of these samples were colorless and transparent (Figure 4).The transmittance of a standard sample showed absorption peaks at 378 nm and 485 nm, which were attributed to 7 F 6 to 5 D 3 and 7 F 6 to 5 D 4 transitions of Tb 3+ , respectively [30,31].The high transparency of the fabricated LTA zeolite glass samples in the visible light range and the low doping concentration of Tb 3+ could have resulted in overshadowing or overlapping of the absorption peak of Tb 3+ by those of the LTA base material in the absorption spectra.Therefore, the transmittance spectra of the Tb 3+ :LTA material did not show a distinct absorption peak for Tb 3+ , and there was no significant correlation with the doping amount of Tb 3+ .The peaks observed and the corresponding transitions in the PLE spectra (λ em = 543 nm) were 304 nm ( 7 F 6 to 5 H 6 ), 318 nm ( 7 F 6 to 5 H 7 ), 341 nm ( 7 F 6 to 5 L 7 ), 354 nm ( 7 F 6 to 5 L 9 ), 369 nm ( 7 F 6 to 5 D 2 ), 378 nm ( 7 F 6 to 5 D 3 ), and 485 nm ( 7 F 6 to 5 D 4 ) (Figure 5) [30,31].
The SEM images did not show pores in the samples.The results of the experimental determination of the wt% of the elements in the samples using EDX are presented in Table 1.The values in Table 1 are the average values of two measurements.The inconsistencies between the nominal and experimental compositions can be explained by (i) the uncertainty in the determination of wt% of the elements (18-25%) dictated by the instrumentation used, and (ii) the fact that the glass matrix consisted of light elements such as B, Al, Si, Na, and O, which could be evaporated from the surface, e.g., B could have been undetected due to being very light.The XRD analysis of samples #1, #2, and #3 did not show crystalline peaks, thus confirming the glass nature of the samples (Figure 2).The transmission spectra of samples #1, #2, and #3, presented in Figure 3, showed that the visible light transmittance was beyond 85%.Also, photos of these samples were colorless and transparent (Figure 4).The transmittance of a standard sample showed absorption peaks at 378 nm and 485 nm, which were attributed to 7 F6 to 5 D3 and 7 F6 to 5 D4 transitions of Tb 3+ , respectively [30,31].The high transparency of the fabricated LTA zeolite glass samples in the visible light range and the low doping concentration of Tb 3+ could have resulted in overshadowing or overlapping of the absorption peak of Tb 3+ by those of the LTA base material in the absorption spectra.Therefore, the transmittance spectra of the Tb 3+ :LTA material did not show a distinct absorption peak for Tb 3+ , and there was no significant correlation with the doping amount of Tb 3+ .The peaks observed and the corresponding transitions in the PLE spectra (λem = 543 nm) were 304 nm ( 7 F6 to 5 H6), 318 nm ( 7 F6 to 5 H7), 341 nm ( 7 F6 to 5 L7), 354 nm ( 7 F6 to 5 L9), 369 nm ( 7 F6 to 5 D2), 378 nm ( 7 F6 to 5 D3), and 485 nm ( 7 F6 to 5 D4) (Figure 5) [30,31].The PL of the samples was examined by measuring their emission spectra upon excitation with two ultraviolet (UV) light excitations, one with a wavelength of λ exc = 378 nm, which lay in the ultraviolet A (UV-A)/near-ultraviolet (N-UV) regions, and another with a wavelength of λ exc = 193 nm (a laser light), which lay in the ultraviolet C (UV-C)/farultraviolet (F-UV) regions.Peaks of both 5 D 3 and 5 D 4 transitions were present in the emission spectra upon excitation with UV-A/N-UV light, which were located at 418 nm ( 5 D 3 to 7 F 5 ), 440 nm ( 5 D 3 to 7 F 4 ), 487 nm ( 5 D 4 to 7 F 6 ), 543 nm ( 5 D 4 to 7 F 5 ), 586 nm ( 5 D 4 to 7 F 4 ), and 621 nm ( 5 D 4 to 7 F 3 ) [16,30,36] (Figure 6).Upon excitation with UV-C/F-UV light, the same peaks from the 5 D 4 to 7 F J transitions were present, but emission from 5 D 4 to 7 F J was limited to one peak at 386 nm ( 5 D 4 to 7 F 6 ) [14] (Figure 7).In all of these transitions, 5 D 4 -7 F 6 (487 nm) and 5 D 4 -7 F 5 (543 nm) are magnetically dipole and parity-forbidden transitions, respectively [20].Therefore, LTA:Tb 3+ exhibits a strong emission intensity at 543 nm.In this context, we should point out that a non-radiative relaxation from the 5 D 3 to the 5 D 4 levels via cross-relaxation to a neighbor Tb 3+ is a well-known phenomenon in Tb 3+ systems [11,[14][15][16].This process occurs because due to the closely matched energy difference between the 5 D 4 and 5 D 3 levels (5800 cm −1 ) and the 7 F 6 and 7 F 0 levels (5700 cm −1 ), excitation from 7 F 6 to 7 F 0 promotes the non-radiative drain from 5 D 3 to 5 D 4 of a nearby ion (( 5 D 3 , 7 F 6 ) → ( 5 D 4 , 7 F 0 )) [11,14,15].Thus, if the dispersion of Tb 3+ in the matrix is ideal, the ratio of the intensity of the green light to the intensity of the blue light (I G /I B ) is expected to increase when Tb 3+ concentration is increased [15].In our experiments, while I G /I B increased when the Tb 3+ concentration was increased from 1 to 10 extra wt%, it decreased when the Tb 3+ concentration was increased from 0.1 to 1 extra wt% (Figure 8), possibly because Tb 3+ pair formation, clustering, and phase separation played a role in our system [15].Interestingly, for both excitation wavelengths (λ exc = 193 and 378 nm), I B and I G were comparable for the samples studied (Figure 8).The PL of the samples was examined by measuring their emission spectra upon excitation with two ultraviolet (UV) light excitations, one with a wavelength of λexc = 378 nm, which lay in the ultraviolet A (UV-A)/near-ultraviolet (N-UV) regions, and another with a wavelength of λexc = 193 nm (a laser light), which lay in the ultraviolet C (UV-C)/farultraviolet (F-UV) regions.Peaks of both 5 D3 and 5 D4 transitions were present in the emission spectra upon excitation with UV-A/N-UV light, which were located at 418 nm ( 5 D3 to 7 F5), 440 nm ( 5 D3 to 7 F4), 487 nm ( 5 D4 to 7 F6), 543 nm ( 5 D4 to 7 F5), 586 nm ( 5 D4 to 7 F4), and 621 nm ( 5 D4 to 7 F3) [16,30,36] (Figure 6).Upon excitation with UV-C/F-UV light, the same peaks from the 5 D4 to 7 FJ transitions were present, but emission from 5 D4 to 7 FJ was limited to one peak at 386 nm ( 5 D4 to 7 F6) [14] (Figure 7).In all of these transitions, 5 D4-7 F6 (487 nm) and 5 D4-7 F5 (543 nm) are magnetically dipole and parity-forbidden transitions, respectively [20].Therefore, LTA:Tb 3+ exhibits a strong emission intensity at 543 nm.In this context, we should point out that a non-radiative relaxation from the 5 D3 to the 5 D4 levels via crossrelaxation to a neighbor Tb 3+ is a well-known phenomenon in Tb 3+ systems [11,[14][15][16].This process occurs because due to the closely matched energy difference between the 5 D4 and 5 D3 levels (5800 cm −1 ) and the 7 F6 and 7 F0 levels (5700 cm −1 ), excitation from 7 F6 to 7 F0 promotes the non-radiative drain from 5 D3 to 5 D4 of a nearby ion (( 5 D3, 7 F6) → ( 5 D4, 7 F0)) [11,14,15].Thus, if the dispersion of Tb 3+ in the matrix is ideal, the ratio of the intensity of the green light to the intensity of the blue light (IG/IB) is expected to increase when Tb 3+ concentration is increased [15].In our experiments, while IG/IB increased when the Tb 3+ concentration was increased from 1 to 10 extra wt%, it decreased when the Tb 3+ concentration was increased from 0.1 to 1 extra wt% (Figure 8), possibly because Tb 3+ pair formation, clustering, and phase separation played a role in our system [15].Interestingly, for both excitation wavelengths (λexc = 193 and 378 nm), IB and IG were comparable for the samples studied (Figure 8).A graph of the green PL decay curves is shown in Figure 9 (λexc = 378 nm; λem = 543 nm).The curves did not follow single exponential decays, indicating the presence of a radiationless process due to the energy transfer among active Tb 3+ ions, caused by either cross-relaxation or a cooperative energy transfer to upper levels [21].We employed two methods for obtaining the luminescence decay times.In the first method, the following   A graph of the green PL decay curves is shown in Figure 9 (λexc = 378 nm; λem = 543 nm).The curves did not follow single exponential decays, indicating the presence of a radiationless process due to the energy transfer among active Tb 3+ ions, caused by either cross-relaxation or a cooperative energy transfer to upper levels [21].We employed two methods for obtaining the luminescence decay times.In the first method, the following A graph of the green PL decay curves is shown in Figure 9 (λ exc = 378 nm; λ em = 543 nm).The curves did not follow single exponential decays, indicating the presence of a radiationless process due to the energy transfer among active Tb 3+ ions, caused by either crossrelaxation or a cooperative energy transfer to upper levels [21].We employed two methods for obtaining the luminescence decay times.In the first method, the following theoretical intensity curve with two exponential decay terms was fitted to the experimental data (Figure 9): Materials 2024, 17, x FOR PEER REVIEW 7 of 10 theoretical intensity curve with two exponential decay terms was fitted to the experimental data (Figure 9): The values for the luminescence decay times (τ1 and τ2) obtained using this method were similar for the samples and included τ1 = 1.21, 1.21, and 1.20 μs and τ2 = 11.80,12.28, and 12.00 μs for samples #1, #2, and #3, respectively.In the second method, the mean luminescence decay time (τm) was calculated from the following equation [17,22,30,36] The values obtained for τm from this method were 18, 14, and 17 μs for samples #1, #2, and #3, respectively.A decay time on the order of μs is potentially suitable for applications concerning static imaging, UV detection [27], and plasma display panels [28][29][30].Importantly, these decay times are substantially shorter than those typically obtained for Tb 3+ -doped glass materials, which are on the order of ms (Table 2).The Tb 3+ -doped glass materials listed in Table 2 were based upon calcium aluminosilicate [14,19,21], fluorophosphate [16], fluoroborate [17], zinc phosphate [16,20], LTA zeolite-derived aluminosilicate [20], zinc phosphate [20], zinc fluorophosphate [21], borogermanate [22,29], lead germanate [25], strontium aluminoborate [26], and strontium fluoroaluminate [30] glasses.Of note, in the only other example of LTA zeolite-derived Tb 3+ -doped glass we are aware of [20], the sample was green-emitting and had a decay time of 2.398 ms.The values for the luminescence decay times (τ 1 and τ 2 ) obtained using this method were similar for the samples and included τ 1 = 1.21, 1.21, and 1.20 µs and τ 2 = 11.80,12.28, and 12.00 µs for samples #1, #2, and #3, respectively.In the second method, the mean luminescence decay time (τ m ) was calculated from the following equation [17,22,30,36] The values obtained for τ m from this method were 18, 14, and 17 µs for samples #1, #2, and #3, respectively.A decay time on the order of µs is potentially suitable for applications concerning static imaging, UV detection [27], and plasma display panels [28][29][30].Importantly, these decay times are substantially shorter than those typically obtained for Tb 3+ -doped glass materials, which are on the order of ms (Table 2).The Tb 3+ -doped glass materials listed in Table 2 were based upon calcium aluminosilicate [14,19,21], fluorophosphate [16], fluoroborate [17], zinc phosphate [16,20], LTA zeolite-derived aluminosilicate [20], zinc phosphate [20], zinc fluorophosphate [21], borogermanate [22,29], lead germanate [25], strontium aluminoborate [26], and strontium fluoroaluminate [30] glasses.Of note, in the only other example of LTA zeolite-derived Tb 3+ -doped glass we are aware of [20], the sample was green-emitting and had a decay time of 2.398 ms.

Conclusions
Colorless, transparent, Tb 3+ -doped, LTA zeolite-derived boro-aluminosilicate glass samples were prepared using the melt quenching method.The emission spectra obtained using two different excitation wavelengths (λ exc = 193 and 378 nm) showed blue and green light emissions with comparable intensities.The green luminescence decay curves were not single-exponential.The mean decay times were 18, 14, and 17 µs for 0.2, 1.0, and 10 extra wt% of Tb 3+ , respectively, and the computed decay times from fitting a theoretical curve with two decay terms were ~1.2 and ~12 µs (irrespective of the Tb 3+ concentration).Given their relatively fast green luminescence decay times, these synthesized glass materials have potential for applications concerning static imaging, UV detection, and plasma display panels.

Figure 1 .
Figure 1.Schematic diagram of the preparation process of LTA:Tb 3+ glass samples.

Figure 1 .
Figure 1.Schematic diagram of the preparation process of LTA:Tb 3+ glass samples.

Figure 4 .
Figure 4. Photos of the Tb 3+ -activated LTA zeolite-derived boro-aluminosilicate glass samples (The bottom font is the name of our laboratory).

Figure 4 .
Figure 4. Photos of the Tb 3+ -activated LTA zeolite-derived boro-aluminosilicate glass samples (The bottom font is the name of our laboratory).

Figure 4 .
Figure 4. Photos of the Tb 3+ -activated LTA zeolite-derived boro-aluminosilicate glass samples (The bottom font is the name of our laboratory).

Figure 4 .
Figure 4. Photos of the Tb 3+ -activated LTA zeolite-derived boro-aluminosilicate glass samples (The bottom font is the name of our laboratory).

Figure 8 .
Figure 8. Variation in IG/IB with varying concentrations of Tb 3+ (IG and IB were determined by integrating the intensities of the green and blue peaks in the emission spectra, respectively).

Figure 8 .
Figure 8. Variation in IG/IB with varying concentrations of Tb 3+ (IG and IB were determined by integrating the intensities of the green and blue peaks in the emission spectra, respectively).

Figure 8 .
Figure 8. Variation in I G /I B with varying concentrations of Tb 3+ (I G and I B were determined by integrating the intensities of the green and blue peaks in the emission spectra, respectively).

Figure 9 .
Figure 9. Green luminescence decay curves of the Tb 3+ -activated LTA zeolite-derived boro-aluminosilicate glass samples (each curve was normalized according to the intensity of sample #1).The solid lines represent the results of fitting Equation (1) to the experimental data.

Figure 9 .
Figure 9. Green luminescence decay curves of the Tb 3+ -activated LTA zeolite-derived boroaluminosilicate glass samples (each curve was normalized according to the intensity of sample #1).The solid lines represent the results of fitting Equation (1) to the experimental data.

Table 1 .
Comparison of nominal and experimental compositions obtained from EDX analysis.

Table 1 .
Comparison of nominal and experimental compositions obtained from EDX analysis.

Table 2 .
Green luminescence decay times of various Tb 3+ -doped glasses under excitation with UV light.