简介概要

Preparation, morphology and luminescence properties of Gd₂O₂S:Tb with different Gd₂O₃ raw materials

来源期刊：Rare Metals2019年第3期

论文作者：Yu-Jie Ding Peng-De Han Li-Xi Wang Qi-Tu Zhang

文章页码：221 - 226

摘要：The sulfide fusion method was used to synthesize Gd₂ O₂ S:Tb phosphors using commercial Gd₂ O₃ and freshly prepared Gd₂ O₃, respectively. The freshly prepared Gd₂ O₃ was synthesized from Gd₂ O（GO₃）₂·H₂ O precursor prepared by homogeneous precipitation method.The structure and morphology of the composites were characterized by X-ray diffraction（XRD）, scanning electron microscopy（SEM） and energy dispersive spectroscopy（EDS）. The result shows that the Gd₂ O₂ S:Tb phosphor prepared by commercial Gd2 O3（GOST-A） presents agglomerated particles with average particle size of2.1 μm; however, Gd₂ O₂ S:Tb produced from as-prepared Gd₂ O₃（GOST-B） tends to form regular hexagon particles with the average particle size of 1 μm. Furthermore, Gd3+and Tb3+ contents in GOST-B are higher than that in GOST-A. In addition, fluorescent properties were analyzed by fluorescent spectrophotometer. It is indicated that similar excitation and emission spectra can be obtained from the two phosphors, but the luminescence intensity of GOST-B is higher than that of GOST-A.

稀有金属(英文版) 2019,38(03),221-226

Preparation, morphology and luminescence properties of Gd₂O₂S:Tb with different Gd₂O₃ raw materials

Yu-Jie Ding Peng-De Han Li-Xi Wang Qi-Tu Zhang

College of Materials Science and Engineering,Nanjing Tech University

School of Materials Engineering,Yancheng Institute of Technology

作者简介：*Qi-Tu Zhang,e-mail:njzqt@126.com;

收稿日期：27 July 2014

基金：financially supported by the National Natural Science Foundation of China (No. 51202111);the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD);

Preparation, morphology and luminescence properties of Gd₂O₂S:Tb with different Gd₂O₃ raw materials

Yu-Jie Ding Peng-De Han Li-Xi Wang Qi-Tu Zhang

College of Materials Science and Engineering,Nanjing Tech University

School of Materials Engineering,Yancheng Institute of Technology

Abstract：

The sulfide fusion method was used to synthesize Gd₂ O₂ S:Tb phosphors using commercial Gd₂ O₃ and freshly prepared Gd₂ O₃, respectively. The freshly prepared Gd₂ O₃ was synthesized from Gd₂ O(GO₃)₂·H₂ O precursor prepared by homogeneous precipitation method.The structure and morphology of the composites were characterized by X-ray diffraction(XRD), scanning electron microscopy(SEM) and energy dispersive spectroscopy(EDS). The result shows that the Gd₂ O₂ S:Tb phosphor prepared by commercial Gd2 O3(GOST-A) presents agglomerated particles with average particle size of2.1 μm; however, Gd₂ O₂ S:Tb produced from as-prepared Gd₂ O₃(GOST-B) tends to form regular hexagon particles with the average particle size of 1 μm. Furthermore, Gd3+and Tb3+ contents in GOST-B are higher than that in GOST-A. In addition, fluorescent properties were analyzed by fluorescent spectrophotometer. It is indicated that similar excitation and emission spectra can be obtained from the two phosphors, but the luminescence intensity of GOST-B is higher than that of GOST-A.

Keyword：

Precursor; Phosphor; Luminescence; Homogeneous precipitation; Sulfide fusion;

Received： 27 July 2014

1 Introduction

Owing to potential applications in phosphors,optoelectronics and many leading fields,gadolinium compounds have attracted considerable research efforts in recent years.The gadolinium oxysulfide (Gd₂O₂S) is a kind of well-designed rare earth host matrix for phosphors fabrication [ 1] .Also,the most investigated rare earth ions are Eu³⁺and Tb³⁺because of their intrinsic electronic spectroscopic properties which have visible light emission under ultraviolet (UV)excitation [ 2] .Terbium-activated sulfur gadolinium oxide phosphors (Gd₂O₂S:Tb) are considered as one of the most effective phosphors in UV imaging systems [ 3, 4, 5] .

For luminescent materials,the fluorescent properties are greatly influenced by the luminous efficiency,grain sizes,particle uniformity and morphology [ 6, 7] .Phosphors with smaller particle size and narrow size distribution are required for electronic devices especially in the display device [ 8, 9] .The improvement of the luminous intensity relies a lot on decreasing impurity contents and increasing crystallization degree of samples,and Gd₂O₃ is a crucial staring material to prepare Gd₂O₂S:Tb phosphors.However,more impurities and serious reunion are shown in commercial Gd₂O₃ raw materials,which decrease the properties of phosphors.In order to obtain fine phosphor with high luminescence intensity,some papers reported that using precursors to prepare phosphors was a good method [ 10] .Nowadays,there have been an increasing number of reports on the precise design and synthesis of gadolinium precursors,including gadolinium hydroxide,carbonate,phosphates,fluorides,oxalates and alkali carbonates [ 11] .

More and more attention is paid to gadolinium carbonates because they have following advantages:low reaction temperature,low toxicity and simple preparation process [ 6, 12, 13] .The carbonates of most metals are prepared by two common procedures:by alkali carbonates and by passing carbon solution containing the metal salt.

So in this paper,Gd₂0(CO₃)2·H₂O was chosen as the precursor and synthesized by homogeneous precipitation method,and Na₂CO₃ was used as the precipitation agent.On the basis of the fusion method [ 14] ,Tb-doped Gd₂0₂S phosphor was prepared by two different starting materials:commercial Gd₂O₃ and as-prepared Gd₂O₃ from Gd₂0(-CO₃)₂·H₂0 precursor,respectively.The aim is to prepare fine phosphors with narrow size distribution,smaller size and higher luminescence intensity.

2 Experimental

The starting materials were as follows:Gd₂O₃ (99.99%purity,Liyang Luodiya Rare Earth New Materials Co.,Ltd),Tb₄0₇ (99.99%Liyang Luodiya Rare Earth New Materials Co.,Ltd),nitric acid (analytical reagent,Shanghai Lingfeng Chemical Regent Co.,Ltd),sublimed sulfur (chemical pure,Sinopharm Chemical Reagent Co.,Ltd) and sodium carbonate anhydrous (chemical pure,Sinopharm Chemical Reagent Co.,Ltd).All reagents were used without further purification.

2.1 Synthesis of Gd20(CO3)2.H2O and Gd2O3 powders

First,HN0₃ was used to dissolve Gd₂O₃ thoroughly,and then,the mixed Gd(N0₃)3 solution was evaporated four times to decrease nitric acid concentration.In a typical procedure,the Gd(N0₃)₃ solution was adjusted to 0.2 mol·L^-1.Next,Na₂CO₃ solution was injected into mixed solution to adjust the pH to 10-11,and the stirrer bar was kept working during the whole process.Subsequently,the solution was enclosed in100℃thermostat water bath and kept stirring for 1 h.After that,the resulting solution was slowly cooled down to room temperature.The white precipitate was collected by pumping filtration,washed with deionized water and dried at 80℃.At last,the Gd₂0(CO₃)2·H₂0 powders were ground and calcined at 1000℃for 1 h,and the final Gd₂O₃ powders were obtained after washing with deionized water and dried at 100℃.

2.2 Synthesis of Gd2O2S:Tb phosphors

Commercial Gd₂O₃ or as-prepared Gd₂O₃,Tb₄O₇,S and Na₂CO₃ were mixed with mole ratio of 1.00:0.02:4.00:1.50,and the mixtures were kept in ball milling for 9 h.Then the compacted bulk was loaded in the double-jacketed alumina crucible with an aluminum powder layer and activated charcoal outer layer.The synthesis bulk was calcined at 1 100with 5℃.min^-1 rate for 3 h.The product obtained was washed with deionized water to eliminate extra salts.The phosphor powders were finally obtained after drying at100℃for 10 h.The Gd₂0₂S:Tb phosphor powders prepared by commercial Gd₂O₃ and from as-prepared Gd₂O₃were simplified as GOST-A and GOST-B,respectively.

2.3 Measurement

The composition of phosphor samples was characterized by X-ray diffractometer (XRD,Rigaku,D/Max2500) with Cu Kαradiation.The microstructure of powders was characterized by scanning electron microscopy (SEM,JSM-5610LV)and field electron scanning electron microscopy (FE-SEM,Hitachi SU8010).The component element types and contents of phosphors were detected by energy-dispersive spectrometer (EDS,NS7).In addition,structures of the precursor were also investigated by Fourier transform infrared spectroscopy(FT-IR,Nicolet 5700).The photoluminescence (PL) properties were analyzed by fluorescent spectrophotometer (Lumina) at 500 V,1 nm/1 nm slit.Specific surface area (S_BET)of particles was measured by the Brunauer-Emmett-Teller(BET) method.The pore size was computed using the B arrettJoyner-Halenda (BJH) method.The data of and pore size were obtained by physisorption analyzer (ASAP 2020,Micromeritics) under the condition of-196.15℃.

3 Results and discussion

3.1 Structural characteristics of GOST-A and GOST-B

XRD measurement was performed to identify the resulting particles,and apparent changes of crystal structures from Gd₂0(CO₃)₂·H₂0 to Gd₂O₃ to Gd₂O₂S:Tb are shown in Fig.1.All peaks of precursors (Curve (1) in Fig.1) well correspond to Gd₂O(CO₃)2.H₂0 (reference to PDF 43-0604).Following the hydration process,Gd₂0(CO₃)₂·H₂O could be obtained with the reaction of [ 11, 13] :

In the chemical reaction above,Gd³⁺is provided by Gd(NO₃)₃ solution and is provided by Na₂CO₃solution.After the 1000℃calcination process,the product obtained is in good accordance with the cubic structure of Gd₂O₃ (reference to PDF 12-0797).Gd₂O₃ is obtained by the following two steps [ 11, 15] :

When Na2CO3,Tb407 and S are added,there is an obvious change of crystal structure from cubic Gd₂O₃ to Gd₂O₂S treated by flux fusion method.All sharp peaks existing in the final phosphors (Curves (3) and (4) in Fig.1) agree well with hexagonal Gd₂O₂S (reference to PDF 26-1422).That is to say,sublimed sulfur successfully enters into the Gd₂O₃ crystal lattice and changes the crystalline phase.It also indicates that Tb and Na₂CO₃ do not change the crystal structure.The process of formation is as follows [ 16] :

Fig.1 XRD patterns of (1) precursor:Gd₂O(CO₃)₂·H₂O,(2) Gd₂O₃calcined from Gd₂O(C0₃)₂·H₂O at 1000℃.(3) GOST-A and (4)GOST-B

FT-IR analysis (Fig.2) was conducted for both Gd₂0(CO₃)₂·H₂O and GOST-B phosphor.Absorption mode of water molecule in Gd₂O(CO₃)₂·H₂0 is shown in3484 cm^-1,while the three double-peak characteristic bands of V_as O-C-O (1516 and 1426 cm^-1),δ (854and 822 cm^-1) andπ (724 and 699 cm^-1) are attributed to the presence of carbonate [ 11] ,which also confirms the formation of Gd₂O(CO₃)₂.However,no obvious peaks are shown for GOST-B in Fig.2,meaning no residual stretching and O-H stretching existing.

3.2 Morphologies of GOST-A and GOST-B

From the FE-SEM image of commercial Gd₂O₃ (Fig.3a),it is shown that small grains pack tightly into irregular flakes.During the heating process,CO₂ gas escapes from Gd₂0(CO₃)₂·H₂O,causing many holes among powders,which makes an effect on blocking the reunion among particles [ 13] .Compared with raw commercial material Gd₂O₃,the as-prepared Gd₂O₃ (Fig.3b) presents better dispersion properties with submicron particles.Interestingly,both Fig.3c,d belong to hexagonal Gd₂O₂S,but two different shapes and sizes are shown.GOST-A shows the nearly spherical morphology,yet GOST-B tends to be regular hexagon particles.From XRD patterns of Curves(3) and (4) in Fig.1,the intensities of (100),(101),(102)and (110) in GPST-B are higher than those in GOST-A.So,more growth in (100),(101),(102) and (110) may contribute to the formation of regular hexagon.

Fig.2 FT-IR spectra of Gd₂0(C0₃)₂·H₂O precursor and GOST-B

Fig.3 FE-SEM images of a commercial Gd₂O₃ and b as-prepared Gd₂O₃ powders;SEM images of c GOST-A and d GOST-B

Figure 4 shows the size distributions of GOST-A and GOST-B.Obviously,the particle size range of GOST-A(from 0.5 to 4.5μm) is wider than that of GOST-B (from0.5 to 2.0μm).It is calculated that the average size of GOST-A is 2.1μm,yet the average size of GOST-B is1μm.

Energy-dispersive spectroscopy (EDS) was further used to determine the chemical composition of the two phosphors.As shown in Fig.5,the gadolinium content is32.65 at%in GOST-A,which is lower than that in GOST-B (39.11 at%),so it demonstrates that the content of Gd in commercial Gd₂O₃ is lower than that in as-prepared Gd₂O₃,that is to say,the preparation of precursors has the effect of purifying gadolinium oxide.From the perspective of S content,S with higher content could be introduced into the GOST-A lattice [ 17] .According to Reaction (4),the S element is important to form Gd₂O₂S.Parts of S are used for the flux agents of Na₂S_x-1 and parts are used to form Gd₂0₂S,so the increase in S content may be contributed to the formation of Gd₂O₂S hexagon crystal.

Fig.4 Bar graphs of particle size distribution in a GOST-A and b GOST-B

Fig.5 EDS spectra of a GOST-A and b GOST-B

Moreover,Gd₂0(CO₃)₂·H₂O can be transformed to mesoporous Gd₂O₃ with high surface area below 800℃;however,after heated at 1000℃,microporous Gd₂O₃ with low surface area will be formed [ 18] .For commercial Gd₂O₃,the as-prepared Gd₂O₃ has smaller specific surface area as shown in Table 1.So generally speaking,after 1000℃calcination,the generated Gd₂O₃ powders have a lower adsorption c apacity than raw commercial Gd₂O₃.Relatively,GOST-A is easier to gather more grains to form one larger particle.

3.3 PL characterization of Gd2O2S:Tb

PL properties of gadolinium oxysulfide samples prepared by two different starting materials were analyzed using254 nm excitation source (emission spectra) and monitored at 544 nm (excitation spectra).This particular excitation wavelength was used because most of the phosphors were characterized at this exciting wavelength for application [ 19] .The two phosphor samples exhibit similar excitation and emission spectra as depicted in Fig.6.

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Table 1 Textual properties of Gd₂0₃ powders

The broadband around 300 nm in the excitation spectra(left in Fig.6) is due to the absorption of host lattice,and the peak at 293 nm may be related to 4d-5f transition of Tb³⁺,and the narrow excitation peaks at 313 nm arise from the P_J transition of Gd³⁺,of which excitation energy can effectively transfer to Tb³⁺ [ 20] .The emission characteristic in the yellow-green visible region is attributed to the predominant F_J transitions (right in Fig.6) [ 21] .Consequently,Tb³⁺plays an important role in luminous emission.It is obvious that both the intensities of excitation and emission spectra of GOST-B are higher than those of GOST-A.

Fig.6 Excitation spectrum (monitored at 544 nm) and emission spectrum (254 nm excitation source) of GOST-A and GOST-B

EDS analysis also shows that the content of Tb for GOST-A is 0.8 at%(Fig.5).However,that is 1.2 at%for GOST-B.Obviously,the great decrease in Tb content directly leads to a decline in luminescence intensity.Also,the higher content of Gd in GOST-B can increase the ultraviolet absorption,which increases the intensity of emission spectrum.

Under the same test conditions,qualitative comparison of the crystallinity could be made between the two samples.The intensity of XRD pattern of Curve (4) in Fig.1 is higher than that of Curve (3) in Fig.1,so the crystallinity of GOST-B is higher than that of GOST-B.Shea and McKittrick [ 22] documented that luminous efficiency was only related to crystal size rather than particle size.For this,the luminescence intensity of GOST-B is higher.

Surface condition of phosphor is another important factor to affect luminous performance.The existence of dangling bonds,dead layers and bound electrons on surface could decrease the luminescence intensity [ 22, 23] .In addition,compared to Fig.3d,agglomerated particles in GOST-A make the surface rougher,which may increase the probability of scattering.

4 Conclusion

Compared to commercial Gd₂O₃ raw material,Gd₂O₃prepared from Gd₂0(CO₃)₂·H₂0 presents more excellent dispersion.Particle morphology in GOST-B is more regular than that in GOST-A.Also,lager particles and higher reunion are obtained in GOST-A,yet GOST-B tends to have narrow size distribution with the average size of1μm.The two phosphors share the similar PL excitation and emission spectra,and the luminescence intensity of GOST-A is weaker than that of GOST-B.