Synthesis and characterization of Gd2O2 S: Tb3+ phosphor powder for X-ray imaging detectors

INTRODUCTION

The rare earth oxysulfide phosphors have gained considerable attraction in recent years due to their advantages such as high luminescence efficiency [1]. These materials are widely used as luminescent host materials for X-ray application because of their high conversion efficiency (12–25%) of the exciting radiation [2-5]. Terbium activated gadolinium oxysulfide (Gd₂O₂S:Tb), one of the rare earth oxysulfide group of phosphors, is known to be an efficient phosphor and shows bright green luminescence and high efficiency under UV, cathode-ray and X-ray excitations [6].

Terbium activated gadolinium oxysulfide phosphors were prepared by different methods such as flux method [7], combustion [8] and solvothermal [9]. However, in most of these methods to achieve the desired size, the particles must be milled, which causes defects in the surface and non-radiative recombination of electrons and holes with a drop in the luminescence efficiency. Among these methods, the homogeneous precipitation method [1] is more appropriate than others due to its simplicity, rapidity and economy.

Herein, gadolinium oxysulfide phosphor doped with trivalent terbium was prepared via urea homogenous precipitation and followed by sulfurization at 800 °C under argon atmosphere.

MATERILAS AND METHODS

Gadolinium oxide and terbium activated oxysulfide were prepared through homogenous precipitation method [1].

For preparing of gadolinium oxide, typically, 0.54 g Gd(NO₃)₃ . 6H₂O was dissolved in 25 mL water (0.5 M), then diluted to 500 mL with deionized water and heated for 30 min at 90 °C. After that, urea solution (30 g urea dissolved in 120 mL water) was added to it and stirred for 30 min at 90^°C. The mixture solution was aged for a period when visible bluish tint occurred. After aging, the precipitates generated were observed on the bottom of the beaker. The resulting precipitate was separated by centrifugation (4000 rpm), washed two times with deionized water and once with ethanol, then was dried at 100℃ for 24 h. The dried precipitate was calcined at 800℃ for 1 h to obtain the white Gd₂O₃ powder.

Terbium activated gadolinium oxysulfide were also prepared through homogenous precipitation method via two steps. In the first step, typically, 25 mL Gd(NO₃)₃ . 6H₂O (0.5 M) and the proper amount of Tb(NO₃)₃. 5H₂O (0.01 M) stock solutions were weighed out, mixed, then diluted to 500 mL with deionized water and heated for 30 min at 90 °C. After that, urea solution (30 g dissolved in 120 mL water) was added to it and stirred for 30 min at 90 ^°C. The obtained solution was aged overnight. After that, urea solution (30 g urea dissolved in 120 mL water) was added to it and stirred for 30 min at 90 ^°C. The mixture solution was aged for a period when visible bluish tint occurred. After aging, the precipitate generated was observed on the bottom of the beaker. The resulting precipitate was separated by centrifugation (4000 rpm), washed two times with deionized water and once with ethanol, and was dried at 100℃ for 24 h. The dried precipitate was calcined at 800℃ for 1 h to obtain the yellowish Gd₂O_3:Tb powder.

In the second step, the sulfurization of oxide was carried out by the solid-gas reaction. A mix of sulfur and the Gd₂O₃:Tb³⁺ powders (ratio of S to Gd₂O₃:Tb³⁺ was 1 to 1.5) was placed into a quartz tube and the sulfurization reaction was performed at 900℃ under argon atmosphere. The heating temperature of the sulfur powder was 400℃, and the sulfur vapor flow was controlled by adjusting the intensity of argon gas flow. After the reaction was kept for 1 h, the sulfurization was stopped and only the argon gas flow was supplied until the sample was cooled to the room temperature. Finally, the white Gd₂O₂S:Tb³⁺ powder was obtained.

Terbium activated gadolinium oxysulfide phosphor layers were fabricated using by sedimentation method [10]. In this method, the coating solution was first prepared by dissolving 15 g polyvinyl alcohol (PVA) into 500 mL deionized water with proper agitation. After the PVA was fully dissolved, the solution poured into a, flat-bottomed Plexiglas vessel for mixing with phosphor particles. The solution in the vessel was stirred, then a small amount of the additives was introduced followed by gradual addition of weighted phosphor particles. The overall mixing process must be subject to agitation until the phosphor particles were able to fully disperse into the polymer matrix. Here, the rate of agitation played a critical role. It must yield enough centrifugal force to keep the phosphor particles suspended in solution for dispersion. Agitation was terminated when the particles were fully dispersed and then a homogeneous coating system was achieved. The scintillator layer thickness was varied by varying the amount of used Gd₂O₂S:Tb³⁺ powder: 0.9, 1.9 and 2.9 g. The amount of used PVA as a binder was constant (15 g). After that, the glass substrates introduced to solutions and put settled for different times, as shown in Table. 1. When all of the particles settled down completely, the phosphor-coated substrates were removed and calcined at 60 °C in an oven for 1 hr. Fig. 1(a) and (b) show Plexiglas vessel containing PVA solution and phosphor coated layers, respectively.

The crystal structures were identified by a powder X-ray diffractometer (XRD, Inel model Equnox-3000) employing Cu Ka radiation (k=1.5418 A°). The XRD Patterns of phosphors were confirmed by comparing with the JCPDS (Joint Committee on Powder Diffraction Standards) data. The morphology of the synthesized phosphors was imaged by scanning electron microscopy (SEM, Tescan model MIRA3 XMU). Chemical composition of the synthesized phosphors was determined by Energy dispersive x-ray spectroscopy (EDS, Samx) and particle induced x-ray emission (PIXE). The PIXE analysis was carried out using conventional RBS-PIXE reaction chamber at the facilities of Van de Graff Lab in Tehran [11].  A 2000 keV proton beam of about 1 mm diameter was applied for analysis. The X-ray spectra were collected by a Si(Li) X-ray detector placed at a scattering angle of 135°.  FT-IR spectra (4000–400 cm^-1) in KBr were recorded using a Bruker-vertex 70 spectrometer. Emission spectra of Gd₂O₂S:Tb³⁺ were measured by a Varian Carry Eclipse fluorescent spectrometer at room temperature with a xenon flash lamp as excitation source. An X-ray tube model Baltospot Ceram35 was used for irradiation of prepared phosphor layers.

RESULTS AND DISCUSSION

Terbium activated gadolinium oxysulfide was synthesized through two steps. In the first step, spherical hydroxyl carbonate precursor powder was prepared using the urea homogeneous precipitation method at over 90˚C [12]. The homogeneous precipitation technique is based on the slow hydrolysis of Gd³⁺ ion for the preparation of Gd(OH)CO₃. The whole process can be simplified as the release of CO₂ and NH₃ by urea decomposition, followed by the sequential addition of the ligands OH^-1 and CO₃^2- to the Gd³⁺ ion until the concentration of reactants reaches critical supersaturation and then precipitation occurs [13]. The chemical reactions to obtain precursors are given below:

H₂NCONH₂ → NH₄⁺ + OCN^-

OCN^- + 2H⁺ + 2H₂O → H₂CO₃ + NH₄⁺

[GdOH(H₂O)n]²⁺ + H₂CO₃ → Gd(OH)CO₃. H₂O + (n-1) H₂O

In the second step, obtained precipitate was converted to the gadolinium oxysulfide through sulfurization treatment under argon atmosphere.

The Fig. 2 shows the XRD patterns of different synthesized samples. It is obvious from the XRD patterns (curve (a)), the diffraction peaks at 2𝜃= 28.6, 33.1, 47.5, and 56.4^∘ are for (222), (400), (440) and (622) of cubic Gd₂O₃ [14], in good agreement with reported data (JCPDS Card number 43-1014). The high intensity of the diffraction peaks in (curve (b)) indicates good crystallinity of the Gd₂O₃:Tb³⁺. This also means that the Tb³⁺ions have been effectively built into the host lattice of Gd₂O₃ [15]. After the Gd₂O₃:Tb³⁺ powders were sulfurized at 800℃ for 1 h under nitrogen atmosphere containing sulfur vapor (curve (c)), the XRD lines of sample are matched with the data of JCPDS Card No.26-1422, which shows pure hexagonal phase of Gd₂O₂S:Tb³⁺ powder.

Fig. 3 shows the FTIR spectra of the Gd₂O₃ (curve (a)), Gd₂O₃:Tb³⁺ (curve (b)) and Gd₂O₂S:Tb³⁺ (curve (c)). For the precursor, the broad absorption band around 3460 cm⁻¹ can

be assigned to O−H stretching vibration; the bands around 1460 and 1630 cm⁻¹ result from C−O asymmetrical stretching vibration; the peak that appears at 1042 cm⁻¹ can be assigned to C−O symmetric stretching vibration; the strong peak at 420-510 cm⁻¹ associated with the vibration of Gd-O and Gd-S [16] is observed, indicating the formation of Gd₂O₂S:Tb³⁺.

Spherical nano-particles formation of Gd₂O₂S:Tb³⁺ phosphor were confirmed by SEM images as shown in Fig. 4. According to it, the phosphor particles are well separated from each other and exhibit smooth surfaces with a spherical shape.

The chemical composition of gadolinium oxide, terbium doped gadolinium oxide and terbium doped gadolinium oxysulfide were analyzed by EDS and the results are shown in Fig. 5. The results of quantitative analysis confirm the presence of Gd and O in Gd₂O₃ (graph (a)), Gd, O and Tb in Gd₂O₃: Tb³⁺ (graph (b)) and Gd, O, S and Tb in Gd₂O₂S: Tb³⁺ (graph (c)), which are compatible with the obtained result from particle induced X-ray emission (PIXE) for c sample (Fig. 6).

The Photoluminescence emission spectra of the Gd₂O₂S:Tb³⁺ particles excited at 254 nm respectively is shown in Fig. 7. The luminescence peaks in the figure arise from the transitions of ⁵D₃ and ⁵D₄ excited state levels to ⁷F_J (J= 0–6) ground state levels, respectively, and belong to the characteristic emission of Tb³⁺. The emission lines between the 370 and 450 nm correspond to the ⁵D₃→⁷F_J (J= 0–6) transitions, and the emission lines between the 480 and 600 nm correspond to the ⁵D₄→⁷F_J (J= 3, 4, 5, 6) transitions [17].

Moreover, the photoluminescence property of oxysulfide phosphor layer was investigated by ion beam induced luminescence (IBIL). As is shown in Fig. 8, emitting of green light from phosphor layer confirm its luminescence property.

Eight of Gd₂O₂S: Tb³⁺scintillator layers with various thickness were prepared and optically coupled with a CCD image sensor for X-ray imaging performance measurement. Table. 2 shows the thickness of prepared layers.

The pixelated Gd₂O₂S: Tb³⁺ scintillator layers were tested by 300 kVp X-ray beam at 5 mA as beam current. The light outputs were measured in a dark boxby a CCD camera. The relative light output of the layers with various thickness was estimated from the pixel values of the CCD images. Fig. 9 shows the light intensity in terms of a number of pixel for phosphor layers.

As seen in Fig. 9, maximum light output ofthe Gd₂O₂S: Tb³⁺ scintillator layer was obtained for sample no.5 with 193 µm thickness.The effect of different thickness on the contrast is also presented in Table. 3. The results listed in this table shows that the contrast is increased by increasing the thickness of the phosphor layer, then is reduced.The reason may be that a thicker layer of phosphor can absorb more radiation, which increases the dispersion and so reduce the contrast.

CONCLUSION

In this study, green phosphor Gd₂O₂S:Tb³⁺ scintillator were obtained using urea homogeneous precipitation method. Hexagonal structure of Gd₂O₂S:Tb³⁺ phosphor powder was confirmed by XRD. Obtained phosphor have spherical shape. The phosphor powders were deposited on the glass substrates using poly vinyl alcohol as a paste via sedimentation method. The effect of thickness layer on light output and contrast were investigated. The results show, the optimum thickness was 193 µm. Moreover, emitting of green light from phosphor layer confirm its luminescence property.

ACKNOWLEDGEMENTS

The authors wish to thank Nuclear Science & Technology Research Institute.

CONFLICT OF INTEREST

The authors declare that there are no conflicts of interest regarding the publication of this manuscript.