Ethanol-driven Room-temperature Synthesis of Potential Green-emitting Phosphors:a Case Study of Tb3+-doped CaHPO4①

YANG Liu-Sai PENG Si-Yan YU Le-Shu LUO Wei SUN Yu-Jie

(School of Chemistry and Environmental Science, Jiangxi Province Key Laboratory of Polymer Preparation and Processing, Shangrao Normal University, Shangrao, Jiangxi 334001, China)

1 INTRODUCTION

Phosphates are a type of promising luminescent host materials for their easy-synthesis, high stabilities, superior chemical and optical properties,and have widespread applications in displays, LEDs,and fluorescent markers in biomedicine[1-9].Among them, alkali- and alkaline-earth metal phosphate phosphors with the general formula ABPO4have been investigated as efficient host materials for rare earth doped phosphors recently, where A is a monovalent cation (Li+, Na+, K+) and B is a divalent cation (Ca2+, Sr2+, Ba2+)[10-14].In this case,Eu2+-activated LiBPO4, NaBPO4, and KBPO4phosphors (B = Ca, Sr, Ba) that emit blue or green light under UV light irradiation have received great attention as used in phosphor-converted light-emitting diodes because these kinds of phosphors would not only strongly emit visible light under blue light irradiation, but also have small thermal quenching and high quantum eきciency[15-17].However, compared with alkali- and alkaline-earth metal phosphate phosphors, alkaline-earth metal hydrogen phosphate phosphors BHPO4(B = Ca, Sr, Ba) have rarely been researched.A very recent result was reported by Wang et al., which demonstrated the preparation of rare earth ions (Eu3+, Ce3+, Tb3+) doped β-SrHPO4nanocrystals by a hydrothermal method at relative high temperature[18].

Herein, it is desirable to develop a simple and economical method to prepare rare-earth doped BHPO4(B = Ca, Sr, Ba) with excellent properties.With regard to CaHPO4, it is very special among alkaline-earth metal hydrogen phosphate families which are capable of being used in powder form in some toothpastes, chewing gums and in food processing industry[19,20].However, to the best of our knowledge, luminescent properties of rare-earth doped CaHPO4as a potential luminescent material has not been revealed.

In this study, we report a novel green calcium hydrogen phosphate phosphor CaHPO4:Tb3+prepared by a room-temperature co-precipitation method driven by ethanol solvent.The crystal structure,morphology and luminescence properties of CaHPO4:Tb3+powders were investigated to be an attractive green luminescent material for display devices.

2 EXPERIMENTAL

2.1 Synthesis of Ca1-xHPO4:Tb3+x

Ca1-xHPO4:Tb3+xsamples (x = 0～0.15) were directly prepared at room temperature using a precipitation method[20].In a typical synthesis, a mixture of 4 mmol Ca(NO3)2·4H2O mmol and Tb(NO3)3·6H2O with different molar ratios was dissolved in 40 mL ethyl alcohol solution, and 4 mmol (NH4)2HPO4was dissolved together.Then,both solutions were fully mixed under 3 h constant stirring.A white homogenous precipitate was obtained, then isolated by filtration, washed with ethyl alcohol and deionized water, and then dried at 80oC for 6 h.

2.2 Sample characterization

Powder X-ray diffraction (XRD) patterns of the samples were collected using a Rigaku Miniflex II apparatus equipped with Cu-Kα radiation (λ =0.15418 nm).The average particle sizes were estimated by Scherrer formula: D = 0.9λ/βcosθ,where λ is the X-ray wavelength, θ is the diffraction angle of the diffraction peak (200), and β is defined as the full width half maximum (FWHM) of the diffraction peaks after subtracting the instrumental broadening.The morphologies and energy dispersive X-ray spectroscopy (EDS) analysis of the sample were determined by Field Emission Scanning Electron Microscope (Hitachi SU-8010).Photoluminescent spectra were carried out by a Varian Cary Eclipse Fluorescence Spectrometer.For comparison,the testing parameters like slit width, intensity of light source, and specimen mass were kept the same.

3 RESULTS AND DISCUSSION

3.1 Structural and morphological investigation

Doping effects of Tb3+on the structure of CaHPO4were examined by XRD.Fig.1(a) shows the XRD patterns of the samples prepared at different molar ratios of Tb3+ions ranging from 0 to 0.05.It is seen that the diffraction peaks matched well with the standard diffraction data for triclinic phase CaHPO4(JCPDS, No.71-1760).No traces of other impurities(such as Tb4O7, TbPO4) were observed, which indicates the formation of a pure-phase.Therefore,Tb3+could have been incorporated into CaHPO4through the room-temperature reaction.The structure of triclinic CaHPO4can be described by space group P1 with the following lattice parameters[20]; a =6.910, b = 6.627, c = 6.998 ?, α = 96.34°, β =103.82°, and γ = 88.33°.Based on the full width at half maximum (FWHM) of the intense diffraction peak (200) after subtracting the instrumental broadening, the average grain size of the undoped CaHPO4is estimated to be 29.5 nm.Fig.1(b)exhibits the partial enlarged drawing of XRD patterns with the diffraction angle (2θ) ranging from 23oto 30o.Compared to the undoped CaHPO4, the full width at half maximum (FWHM) of the intense diffraction line (200) became clearly broadened after Tb3+doping, which implied the average grain size of CaHPO4:Tb3+decreased.The average particle sizes of Ca1-xHPO4:Tb3+xnanoparticles were 19.6, 18.6,17.7 and 16.2 nm for x = 0.01, 0.02, 0.03 and 0.05,respectively.Moreover, the (200) diffraction peak intensities decreased with the increase of doping amount of Tb3+.This indicates that an increase in doping concentration deteriorates the crystallinity of Ca1-xHPO4:Tb3+xnanoparticles, which may be due to the lattice distortion of CaHPO4by the inequitable ionic valence state between the dopant Tb3+and Ca2+,thus resulting in suppressed grain growth.The morphologies of CaHPO4and CaHPO4:Tb3+samples were characterized by SEM, a typical SEM image of Ca0.95HPO4:Tb0.05nanocrystals shown in Fig.1(c).It is also noted that all samples, including Tb3+-doped and un-doped, showed similar morphology and mostly consisted of nanparticles.As shown in Fig.2,the EDS spectrum taken for Tb3+-doped CaHPO4nanocrystals shows the presence of Ca, P, O, and Tb signals.The atomic ratio of Ca and Tb is approximately 18.05:0.46, close to the stoichiometric ratio of Ca0.97HPO4:Tb3+0.03(0.97:0.03).These results indicate that Tb3+ions could be efficiently incorporated into CaHPO4nanocrystals.

Fig.1.(a) XRD patterns of un-doped CaHPO4 and Ca1-xHPO4:Tb3+x nanoparticles prepared at room temperature.Vertical bars represent the standard diffraction data for bulk CaHPO4.(b) Partial enlarged drawing of XRD patterns.(c) SEM image of Ca0.95HPO4:Tb3+0.05 nanoparticles

Fig.2. EDS spectrum of Ca0.97HPO4:Tb3+0.03 nanoparticles

3.2 Photoluminescence properties

Based on the above XRD and EDS analyses, the rare-earth Tb3+could have been incorporated into CaHPO4.In the following, the doping effects on the luminescent properties of Ca1-xHPO4:Tb3+xsamples were investigated in detail.All samples show similar excitation spectra by monitoring the5D4→7F5emission (544 nm).As exhibited in Fig.3 (left), a very strong broad band ranging from 250 to 300 nm is due to the4f8→4f75d1transition of Tb3+, while several sharp lines in the longer wavelength region located at 304, 318, 340, 352, 370 and 377 nm are assigned to the intrinsic transitions from the ground state,7F6, to the excitation multiplets,3H6,5D0,5L7,5L9,5G5and5G6of Tb3+ions[21,22].

Under excitation at 370 nm, the emission spectra of Ca1-xHPO4:Tb3+xwere observed except for the undoped CaHPO4(Fig.3(right)) which have similar profiles and exhibit four obvious lines centered at 488, 544, 586, and 622 nm, originating from the transitions from the5D4excited state to the7FJ(J = 6,5, 4, 3) ground states of Tb3+ion, respectively[22].Among them, the green emission5D4→7F5(544 nm)is the most prominent group[23].Under UV light excitation, Tb3+-doped CaHPO4samples both displayed a green emission, as con fi rmed by photographs upon excitation at 254 nm with a UV lamp(insets of Fig.3).Meanwhile, the samples without doping Tb3+still retained the color of UV light at 254 nm due to that there is no emission from the CaHPO4host matrix.

Fig.3. Typical excitation spectra (λem = 544 nm) and emission spectra (λex = 370 nm) of Ca1-xHPO4:Tb3+x:(a) x = 0, (b) x = 0.01, (c) x = 0.02, (d) x = 0.03, and (e) x = 0.05.Inset shows direct views of the green colour of CaHPO4:Tb3+ nonacrystals when excited using a UV lamp with a wavelength of 254 nm (1.5 W)

Moreover, different contents of Tb3+-doped CaHPO4have been prepared to study the effect of doping concentration on the luminescence behavior.No apparent changes in the emission curve of Tb3+-doped CaHPO4samples were observed, except for some peak intensities.As illustrated in Fig.4, the emission intensity of Tb3+(5D4→7F5transition at 544 nm) become strengthened gradually with increasing the content of Tb3+up to x = 0.11 and then quenched quickly as further increasing the Tb3+ions.Initially, the increase of emission intensity can be attributed to the fact that high Tb3+concentration might give rise to more active sites that promote the energy transfer process.This decrease of emission intensity beyond 11 mol% concentration in present studies may therefore be the consequence of concentration quenching phenomena[24,25], where the excited Tb3+follows non-radiative cross-relaxation with neighboring Tb3+ions.These observations show that optimum doping concentration of Tb3+ion can be 11 mol% to obtain maximum PL emission intensity by using this wet chemical synthesis.

4 CONCLUSION

Novel phosphors on a phosphate-based host matrix,CaHPO4:Tb3+powders, were prepared at room temperature using a direct precipitation.Tb3+could have been incorporated into CaHPO4through the roomtemperature reaction driven by ethanol sovent.With the increase of doping concentration, the particle-like morphologies were barely changed except for the decreased average grain size.The emission luminescence intensity reached maximum when the doping concentration of Tb3+ions in CaHPO4:Tb3+phosphor was 11 mol%, and then decreased above the co centration due to the concentration quenching effect.A novel green CaHPO4:Tb3+phosphor could be potentially suitable for LEDs and other display applications.

Fig.4. Variations of emission intensity (5D4 → 7F5 transition of Tb3+ at 544 nm) of samples Ca1-xHPO4:Tb3+x as a function of Tb3+ doping concentrations.The intensity values were normalized by taking reference of the emission for Ca1-xHPO4:Tb3+x nanocrystals at x = 0.05

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