XUE Y-Li ZHAO Dn SHI Chng-Ling LIU Bo-Zhong ZHAO Ji LI Fei-Fei

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Energy Transfer and Green Emitting Properties of Solid SolutionPbGd1-xTbB7O13(=0～1)①

XUE Ya-Lia, bZHAO Dana, b②SHI Chang-Lianga②LIU Bao-ZhongaZHAO Jia, bLI Fei-Feia

a(454000)b(350002)

A series of Tb-doped solid solutions PbGd1-xTbB7O13(= 0～1) were synthesized by high-temperature solid state reaction method. The luminescence properties were investigated under UV (274 nm) and near-UV (372 nm) excitation. The emission spectrum by 274 nm exciting reveals a charge-transfer between Gd3+and Tb3+ions. Under near-UV light (372 nm) excitation, PbGd1-xTbB7O13:Tb3+exhibits intense green emission centered at 543 nm due to the54→75transition of Tb3+activator. The optimum doping concentrations were found to be= 0.8 with the quantum efficiency of 35%. One may expect that PbGd1-xTbB7O13has the potential to be used as a green phosphor activated by near near-UV light.

phosphor, photoluminescence, rare-earth, borate;

1 INTRODUCTION

Nowadays, much attention has been attracted by the exploration of rare-earth doped functional materials since they can be used as highly efficient phosphors, laser materials, sensors and catalysts by virtue of their unique chemical, optical and electronic properties[1-3]. The selection of a suitable host matrix is one of the most import factors to design an efficient phosphor. Among various host materials, rare earth borates have been paid intense attention for a wide range of applications due to their significant advantages, such as low sintering temperature, low cost, broad band gap, high lumi- nous efficiency, and high chemical stability[4, 5]. The boron atoms can coordinate to three and four oxygen atoms forming a BO3triangle and a BO4tetrahedron, respectively, and these units can further polymerize to complicated BOarchitectures.

On the other hand, Tb3+ions can serve as efficient activators due to the intense54→75green emission under near-ultraviolet (UV) light excitation. The past 10 years have seen a large number of new efficient Tb3+activated borate phosphors, which have potential applications in solid state lighting, such as NaSrB5O9:Tb3+[6], CdB4O7:Tb3+[7], YA13(BO3)4:Tb3+[8], Sr3B2O6:Tb3+[9],. Our previous study on PbO?Ln2O3?B2O3(Ln = rare- earth metals) system has succefully provied two new isotypic polyborate compound PbGdB7O13[10]and PbTbB7O13[11]. In this work, wereport the synthesis and luminescent properties of their solid solutions PbGd1-xTbB7O13(= 0～1).

2 EXPERIMENTAL

2. 1 Materials and instruments

The raw analyticalreagents, Pb3O4(≥99.0%), Gd2O3(≥99.9%), Tb4O7(≥99.9%) and B2O3(≥99.5%), were purchased from Ji'nan Heng Chemical Co., Ltd. X-ray powder diffraction (XRD) analysis was performed by Rigaku DMax2500 diffractometer with graphite-monochromated Cucharacteristic radiation and the 2range of 5～70° (0.02 °/step). The morphology of the samples was examined by scanning electron microscopy (SEM) images taken using a ZEISS Merlin Compact scanning electron microscope. Photoluminescence (PL) spectra were carried out by a FLS920 Edinburgh Analytical Instrument apparatus. The steady-state measure- ments were performed using a standard 450 W continuous-wave xenon lamp as the excitation source. The step width 1 nm and integration time 0.2were used for the PL excitation and emission spectra measurement. The lifetime measurement was fulfilled by using a standard microsecond flash lampF920H with the time-correlated single-photon counting technique. The flash lamp was operated at 50 Hz pulse frequency with a pulse width of 2s. The external quantum efficiency was determined on the same instrument equipped with a barium sulfate coated integration sphere as the reflectance standard.

2. 2 Syntheses

The solid solution samples of PbGd1-xTbB7O13(= 0, 0.01, 0.05, 0.10, 0.20, 0.40, 0.60, 0.80, 1) were prepared by conventional solid-state reactions. The stoichiometric starting reagents of Pb3O4, Gd2O3, Tb4O7and B2O3werepreheated at 550 ℃ for 3 h. The temperature was raised to 780 ℃ and held for 24 h with several intermittent grindings. The purity of powder samples can be confirmed by XRD analysis.

3 RESULTS AND DISCUSSION

3. 1 Structure analysis and morphology

As our previous reports revealed that PbLnB7O13(Ln = Gd, Tb)[10, 11]are isostructures and crystallize in the monoclinic21space group. As shown in Fig. 1, the structure features a 2layer structure consisting of a [B7O13]∞layer, [Ln]∞layer, and Pb2O8dimers that are encapsulated in a [B7O13]∞layer. The solid solutions PbGd1-xTbB7O13(= 0～1) were prepared by solid state reaction method at 780 ℃. The XRD patterns of PbGd1-xTbB7O13and pure PbLnB7O13(Ln = Gd, Tb) are presented in Fig. 2. It can be clearly observed that all peaks can be indexed to the pure PbLnB7O13(Ln = Gd, Tb) phase and no impurity phase was observed in samples PbGd1-xTbB7O13forvalue of 0～1. This proved the formation of the solid solution. It should be pointed out that above 830 ℃, the samples will partially melt, leading to the formation of some unknown impurity phases.

Fig. 1. View of the structure of PbGdB7O13

Fig. 2. XRD patterns of samples PbGd1-xTbB7O13(0≤≤1) compared to the simulated data from single-crystal data

The morphology of as-prepared solid solution PbGd0.2Tb0.8B7O13was observed by SEM. Fig. 3a shows the morphology of the sample in large scale which reveals that the sample is composed of a large quantity of agglomerated particles. A detailed study at high magnification (Fig. 3b) presents that mor- phology of the grains is flake-shaped with diameter of about 3m. No other morphologies can be detected, indicating the formation of highly uniform 3flakes. It is worth noting that some fragments and irregular broken lines are observed due to the grinding after calcinations in the agate mortar. Other samples also exhibit similar morphology and size.

Fig. 3. SEM micrographs of PbGd0.2Tb0.8B7O13in large (a) and small (b) scales

3. 2 PL and PLE spectra

The luminescent properties of pure PbGdB7O13are recorded in Fig. 4. Under 274 nm light excitation, there exists an emission at 312 nm due to the67/2→87/2transition of Gd3+ions[12]. By monitoring the emission wavelength of 312 nm, the excitation spectrum includes a sharp peak around 274 nm, corresponding to the87/2→6I(= 15/2, 13/2, 11/2, 9/2, 7/2) transition of the Gd3+ion.

Fig. 4. Excitation at= 312 nm (left) and emission at= 274 nm (right) spectra of pure PbGdB7O13

Fig. 5a exhibits the excitation spectra of samples PbGd1-xTbB7O13(= 0.01, 0.05) by monitoring the emission at 543 nm (54→75transition of Tb3+ions) in the range of 200～400 nm. The spectra for= 0 .01 and= 0.05 are very similar except relative intensity, and mainly contain four sharp peaks at 300～400 nm, which can be assigned to the intra- configurational 4→ 4characteristic transition of Gd3+or Tb3+:87/2→6Iof Gd3+at around 274 nm,87/2→67/2of Gd3+at around 312 nm,76→59of Tb3+at around 348 nm and76→56of Tb3+at 372 nm[13, 14]. This result suggests that the energy transfer Gd3+→ Tb3+occurs when they coexist in one host matrix.

For a 274 nm excitation wavelength, the emission spectra of Tb3+activated PbGdB7O13at different molar concentrations (0.01 and 0.5) are shown in Fig. 5b. The emission spectrum consists of several peaks. One at 312 nm is due to the67/2→87/2transition of the Gd3+ions present in the host material and the other seven peaks in the region from 380 to 650 nm are the characteristic emissions of the Tb3+ions:53→76(381 nm),53→75(414 nm),53→74(436 nm),54→76(483 nm),54→75(543 nm),54→74(592 nm),54→73(624 nm)[15]. Another issue is that the positions of the emission peaks have no obvious changes for different molar concentrations of the Tb3+ions; however, the emission intensity of 5 mol% Tb3+is much stronger than that of 1 mol% Tb3+. The 4→ 4characteristic transition of Tb3+can be excited by 274 nm, and the87/2→6Itransition of Gd3+ion indicates that an efficient Gd3+→Tb3+energy transfer occurs. Simultaneously, the decrease in the emission intensity of the Gd3+ion at 312 nm after the concentration rising to 5% is also caused by the efficient energy transfer of Gd3+→ Tb3+.

Fig. 5. Excitation (a) and emission (b) spectra of Tb3+doped PbGdB7O13(1 mol% and 5 mol%)

The schematic energy levels of Gd3+and Tb3+are shown in Fig. 6, which helps us understand the energy transfer process. Upon 274 nm excitation, the6Ilevel of Gd3+ion was first populated[16]. After non-radioactive relaxation, the electrons should reach the67/2state of Gd3+and then the Gd3+ions may go back to the ground state through two ways. Firstly, the Gd3+ions at the67/2state may transfer energy to the Tb3+(57) states through the resonance energy transfer process as they are almost of the same energy. Then the Tb3+ions relax non- radioactively to54or53and give emissions to the ground state (7H= 3, 4, 5, 6), which produces several 4→ 4characteristic transitions of Tb3+. Secondly, the Gd3+ions at the67/2state may also produce emis- sion at 312 nm through the67/2→87/2transitions.

Fig. 6. Schematic energy level diagrams to present the energy transfer process of Gd3+→Tb3+

3. 3 Concentration quenching

We notice from Fig. 7 that the 372 nm light (76→56of Tb3+) is more suitable to activate Tb3+-doped PbGdB7O13phosphor. It can be seen that the emission spectrum is composed of five distinct groups of peaks in the 480 ～ 695 nm range, cor- responding to the characteristic 4→ 4transition of Tb3+activator, the same as Fig. 5b. The green emission (54→75) is the strongest one for all Tb3+concentrations, and the variation of emission inten- sity as the function of Tb3+concentration is shown in the insert of Fig. 7. The highest integrated emission intensity at the Tb3+concentration of 80% can be thought as the critical concentration. The concentra- tion quenching is mainly caused by non-radioactive energy transfer among the Tb3+ions, which gene- rally occurred due to the exchange interaction, radia- tion re-absorption or a multiple-multiple interac- tion[17]. The type of interaction mechanism can be identified by calculating the critical distance (R) between the nearest Tb3+ions. TheRof energy transfer for Tb3+ion in PbGd1-xTbB7O13phosphor can be calculated by using the Blasse[18]equation given below:

whereis the volume of one unit cell,is the number of Tb3+ions in the unit cell andXis the critical concentration of the activator ion. For PbGd1-xTbB7O13,cis 0.8,is 4, andis 0.91607 nm3, then thecwas calculated to be 0.818 nm, which is larger than the typical distance for exchange interaction (< 0.5 nm). Hence, we tentatively put forward that the electric multipolar interaction plays an important role in energy transfer between Tb3+ions in host lattice rather than exchange interaction.

Fig. 7. Emission spectra of PbGd1-xTbB7O13(= 0.01, 0.05, 0.10, 0.20, 0.40, 0.60, 0.80, 1) phosphor under 372 nm light

3. 4 Luminescent decay curve

In addition, the decay curve for the green emis- sion at 543 nm (54→75) excited by 372 nm of PbGd0.2Tb0.8B7O13phosphor is measured, as shown in Fig. 8. The curve can be well fitted with a mono-exponential function:

(t)=I+1exp(–/)

where0is the baseline correction (-offset),1is a pre-exponential factor obtained from the curve fitting andrepresents the lifetime of the excited state. The value ofis calculated to be 2.15 ms, which is comparable with other Tb3+activated phosphors[19, 20].

Fig. 8. Decay curves of Tb3+in PbGd0.2Tb0.8B7O13at excitation 372 and 543 nm emission wavelength excitation

3. 5 CIE and quantum efficiency

It is well known that three main colors recognized by the human vision system are red, green and blue. These three colors are usually referred as the 1931 color coordinates, which are the current standard for lighting specifications on the market. In general, the color of any light source in this color space can be represented as an (,) coordinate. Under the excitation at 372 nm, the calculated CIE chroma- ticity coordinates for PbGd0.2Tb0.8B7O13are calculated to be (= 0.297,= 0.583), correspon- ding to the green light, as shown in Fig. 9.

The quantum efficiency (QE) of a phosphor is an important parameter to be considered for practical applications. The QE can be measured and calcula- ted according to the following equation:

whereLrepresents the emission spectrum,Ethe excitation spectrum, andEthe background. Upon excitation at 372 nm, the corresponding QE of phosphor PbGd0.2Tb0.8B7O13was 35%. Because the significant factor, morphology of the sample, for QE was not optimized, one may expect that compound PbGd1-xB7O13:Tb3+can be used as a good green phosphor for illumination or display.

Fig. 9. Chromaticity coordinates of PbGd0.2Tb0.8B7O13phosphor with excitation at 602 nm in the CIE 1931 chromaticity diagram

4 CONCLUSION

A series of phosphors PbGd1-xTbB7O13(= 0, 0.01, 0.05, 0.10, 0.20, 0.40, 0.60, 0.80, 1) were synthesized by the high temperature solid-state reaction method. The XRD results indicate pure solid solution phase was formed. An energy transfer between Gd3+and Tb3+ions happened by 274 nm light excitation. The PL spectra of PbGd1-xTbB7O13samples exhibit green light emission under 372 nm light excitation in which the band around 543 nm due to the54→75transition of Tb3+is more dominant than the other bands. The optimum doping concentration was found to be 80% for Gd3+ion. The mono-exponential behavior of the decay curves of phosphor PbGd0.2Tb0.8B7O13reveals that the Gd3+/Tb3+activators can reside in two unique crystallographic distinct sits. The calculated CIE coordinates of phosphor PbGd0.2Tb0.8B7O13are (0.297, 0.583), corresponding to the green color. The QE of the phosphor PbGd0.2Tb0.8B7O13is 35%, which shows that the as-prepared material PbGdB7O13has promising potentials in using as host matrix for Tb3+fluorescence.

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29 December 2017;

3 May 2018

① This work was supported by the Open Foundation (No. 20160004) ofState Key Laboratory of Structural Chemistry, and the Fundamental Research Funds for the Universities of Henan Province (No. NSFRF170301)

Zhao Dan, born in 1982, Ph.D, associate professor, Tel: +86-13839155305, E-mail: iamzd1996@163.com; Shi Chang-Liang, born in 1984, Ph.D, instructor, Tel: +86-13721456053, E-mail: scl303@126.com

10.14102/j.cnki.0254-5861.2011-1938