白光LED 用球形鉬酸鈣粉體的控制合成及特性

李 兆，趙西成，江元汝，趙亞娟

（西安建筑科技大學(xué)材料與礦資學(xué)院，西安 710055）

1 Introduction

White light-emitting diodes（LEDs）are versatile semiconductor devices composed of individual diode light source and phosphors excited by LED，emitting white light at room temperature.［1--6］As a novel solid-state semiconductor light source，the white LED has the advantages of energy saving，environmental friendliness and protection，long life，and easy maintenanceas the fourth generation of light source or green light.At present，the white LED mainly consists of blue LED chip and cesium doped yttrium aluminum garnet（YAG∶Ce3＋）yellow phosphor excited by blue light.Part of blue light from the blue LED is absorbed by phosphor to emit yellow light.As a result，the residual blue light and yellow light combine together to give a bright white light source.The major flaw of white LED composed of blue light and YAG∶Ce3＋is the poor color rendering property，with the value of only 85.［7--10］Some investigations indicated that the problem mentioned above is mainly caused by low luminosity of the phosphors in the red light wavelength region.However，among the existing phosphors with high light conversion efficiency and thermal stability，the effective red phosphors that can be excited efficiently by blue light and near ultraviolet（UV）light are unavailable.Trivalent rare earth ions（RE3＋）doped molybdate phosphors were extensively investigated，which have applications for white LEDs.Molybdate ions（Mo）in calcium molybdate（CaMoO4）possess special properties，and they can be effectively excited by the UV and emission light of blue LED and the energy can be transferred to those active RE ions.Moions in CaMoO4has tetrahedral configuration，and the central Mo6＋ion is coordinated with four oxygen atoms.However，Ca2＋ion is coordinated with eight oxygen atoms to form a distorted cube.Guan et al.reported that the samples of CaMoO4∶Eu3＋powder could be excited to emit a red photoluminescence with a wavelength of 616 nm under UV（395nm）and blue light（464nm）irradiation.Since molybdate has superior chemical and thermal stability，and exhibits a good compatibility with near UV and blue LED chip，it is thus a promising material in red phosphors for white LED.［11--16］The molybdate phosphors prepared via the conventional high-temperature solid-state reactions show great crystallinity and luminescent efficiency.However，the preparation method has some inherent disadvantages such as high energy consumption，serious powder agglomeration，and irregular particle morphology.The particle morphology of the phosphors prepared by microemulsion and sol-gel method can be controlled，but the luminescent efficiency is restricted due to the low crystallinity.Fortunately，the hydrothermal method has the advantages of the methods mentioned above.Therefore，the hydrothermal method is an advanced approach to prepare luminescent materials with the regular morphology，uniform distribution and intact crystallinity.The superior molybdate phosphors should be phosphors with spherical morphology，uniform distribution，and a high stacking density，which can reduce the light scattering and yield high luminescent efficiency.Moreover，spherical phosphors can be coated on the devices readily，and the phosphors disperse in a medium，thus lowering the lifting surface area and minimizing the irregular luminescent layer.Therefore，the phosphors have a longer luminescent lifetime.Raju，et al.［17］prepared spherical CaMoO4∶Eu3＋red phosphors and investigated their luminescent property.However，liitle work has been reported on the preparation of Sm3＋and Pr3＋doped CaMoO4phosphors by the hydrothermal method under different pH values.

In this paper，rare-earth ions such as Eu3＋，Sm3＋，and Pr3＋doped CaMoO4red phosphors with the spherical morphology were prepared by a hydrothermal method，and the luminescent properties of Eu3＋，Sm3＋，and Pr3＋doped CaMoO4red phosphors were investigated.In addition，the effect of alkali ions，i.e.，Li＋，on the luminescence intensity of CaMoO4∶Re3＋（Re=Eu3＋，Sm3＋，and Pr3＋）was also analyzed.

2 Experimental

2.1 Phosphors preparation

The CaMoO4∶Re3＋（Re=Eu，Sm，Pr）were prepared by a hydrothermal method in ethanol at 180℃.In the preparation，Ca（NO3）3·4H2O（AR）and Eu2O3（AR），Sm2O3（AR）and Pr2O3（AR）were dissolved in HNO3under vigorous stirring.Then ethanol and deionized water were mixed and added into the solution above.Afterwards，appropriate amounts of urea and Na2MoO4·2H2O（AR）were dissolved into the solutions.Finally，the transparent solutions were transferred into 50 mL autoclaves to fill 70%of the total volume and heated at 180 ℃for 24h.After the autoclaves were cooled to room temperature，the products were separated by centrifugation，washed with deionized water and ethanol for five times，then dried in air at 70℃and kept for the coming characterization.The charge compensation experiment was carried out with an appropriate amount of C2H3LiO2according to the stoichiometric ratio of the target product，Ca0.98MoO4∶（Re=Eu3＋，Sm3＋，and Pr3＋）.

2.2 Sample characterization

The crystalline structures of CaMoO4∶Re3＋（Re=Eu，Sm，Pr）phosphors were determined by a Rigaku D／max 2550X-ray diffractometer equipped with graphite monochromatic Cu Kαradiation source，under the tube voltage of 40kV，scanning step of 0.02°，step-scan time of 0.6s，in the measuring range of 3°--90°.The morphology of the as-synthesized products was observed directly by a scanning electron microscope（SEM，model QUANTA 600F，F(xiàn)EI，USA）.Prior to the characterization，the sample was dispersed in alcohol for 10min in a ultrasonic bath and then dried，followed by spraying of gold for 30s.The excitation and emission spectra were obtained by a model PELS55fluorescence spectrophotometer.

3 Results and discussion

3.1 Morphology of CaMoO4

Figure 1shows the influence of hydrothermal reac-tion time（i.e.，6，12and 24h）on the morphology of CaMoO4powders.It is seen that the spherical particles of CaMoO4are not obtained under hydrothermal treatment for 6h.However，the morphology of particles in the powder gradually transforms into spherical shape，and agglomerates with smaller particles covering the surface when the hydrothermal time increases to 12h.The further increase of the hydrothermal time to 24h results in the formation of CaMoO4powder with the spherical particles with the better dispersibility and the smooth surface.

Fig.1 SEM photographs of CaMoO4powders prepared at different hydrothermal reaction time

Figure 2 shows the SEM photographs of CaMoO4powders at different pH values.Clearly，the pH value significantly affects the morphology of CaMoO4.The preparation at pH value 4does not lead to the crystallization.However，the morphology changes drastically as the pH value increases to 7，leading to the formation of the spherical particles with the average diameter of 2μm.Moreover，the rupture and agglomeration of the spherical particles is observed at pH 10.Finally，the agglomeration phenomenon becomes more prominent at a higher pH value of 14.It is thus indicated that the pH value has an effect on the morphology of the as-prepared phosphors.At pH value 7，the solution exhibits an electrical neutrality，the spherical nanoparticles of CaMoO4were composed of the massive small grain crystals due to the minimum surface energy of sphere stacking.

Figure 3 shows the SEM photographs of CaMoO4powders prepared using different ratios of ethanol to deionized water.CaMoO4powders with extreme agglomeration and irregular shapes are obtained when deionized water is used as a solvent.However，CaMoO4powders composed of fragments are observed when ethanol is used as solution solvent（see Fig.3（d））.At the ethanol to deionized water ratio of 1∶3，the spherical particles of CaMoO4with the better dispersibility are obtained，as shown in Fig.3（b）.Therefore，the morphology of CaMoO4particles can be controlled by the adjustment of the ratio of ethanol to deionized water to obtain the spherical particles of CaMoO4with a certain size，which can be used as a potential candidate for white LED.

3.2 XRD analysis of CaMoO4∶Re3＋（Re=Eu，Sm，Pr）

Figure 4shows the X-ray diffraction（XRD）patterns of CaMoO4∶Re3＋（Re=Eu，Sm，Pr）phosphors.All the diffraction peaks are in agreement with a cubic phase structure of CaMoO4（JCPDS 29--0351），without any impurity peaks being detected.The results indicate that CaMoO4∶Re3＋（Re=Eu，Sm，Pr）phosphors could be prepared by the hydrothermal method.Since the radii of Eu，Sm，and Pr ions are smaller than that of Ca2＋ions，Eu，Sm，and Pr ions are capable of substituting Ca2＋ions.CaMoO4has a tetragonal system（in pace group：I41／a，a=0.539 4nm，c=1.201 8nm）with only one Ca lattice site in CaMoO4.Thus，one Ca2＋ion can coordinate with eight O2-to form a distorted cube.Mo6＋coordinates with four O2-ions to form a coordinated tetrahedron，and Mo6＋is in the center of the oxygen tetrahedron，thus forming the MoO2-4anionic complex.

Fig.2 SEM photographs of CaMoO4powders prepared at different pH values

Fig.3 SEM photographs of CaMoO4powders prepared under different ratios of ethanol to deionized water

3.3 Excitation and emission spectra of CaMoO4∶Re3＋（Re=Eu，Sm，Pr）

Fig.4 XRD patterns of phosphors CaMoO4∶Re3＋（Re=Eu，Sm，Pr）

Figure 5 （a）shows the excitation spectrum of CaMoO4∶Eu3＋under the monitoring wavelength of 616nm，showing two excitation peaks at 395and 465 nm corresponding to the high energy level transition of 4f--4f in Eu3＋ion.The emission curve in the range of 350--500nm is attributed to the high energy level transition of 4f--4fin activator Eu3＋ion，with the most intensive emission peaks at 395nm（7F0→5D1）and 465nm（7F0→5D2）.This result indicates that CaMoO4∶Eu3＋phosphors can be excited by UV irradiation at 395nm and blue light at 465nm，which are in agreement with the excitation of UV LED chip and blue LED chip.

Figure 5（b）shows the excitation spectrum of CaMoO4∶Sm3＋.The atomic number of Sm is 61，and its electron configuration is［Xe］4f66s2.In the formation of Sm3＋ion，two 6Selectrons and one 4felectron are lost，and the electron configuration changes into［Xe］4f5.Sm3＋ions emit red light in many host materials，which are similar to Eu3＋ions.The radius of Sm3＋ions is similar to that of some alkali metal ions；therefore，Sm3＋ions can easily substitute alkali metal ions.As shown in Fig.5（b），the excitation peaks of CaMoO4∶Sm3＋phosphors under the monitoring wavelength of 649nm are attributed to the transition of f--fin Sm3＋ions，and the emission peaks are located at 364，377，406，445and 481nm，which correspond to the transitions of6H5／2→4D15／2，6H5／2→4L17／2，6H5／2→4K11／2，6H5／2→4M19／2and6H5／2→4I13／2，respectively.The peaks at 406nm and 480nm are the main excitation peaks，indicating that Sm3＋doped molybdate phosphors can be used for the excitation by UV LED and blue LED.Thus，CaMoO4∶Sm3＋phosphors match well with yellow-green phosphors to have a high efficiency luminescence of white LED.

Figure 5（c）shows the excitation spectrum of CaMoO4∶Pr3＋under the monitoring wavelength of 650nm，exhibiting three excitation peaks of Pr3＋ions at 453，475and 490nm corresponding to energy level transitions of3H4→3P2，3H4→3P1and3H4→3P0，respectively.The results indicate that CaMoO4∶Pr3＋phosphors can be excited by blue light at 450--500nm，thus matching well with the excitation of blue LED chip.

Fig.5 Excitation spectra of phosphors CaMoO4∶Re3＋（Re=Eu，Sm，Pr）

Figure 6 （a）shows the emission spectrum of CaMoO4∶Eu3＋under the excitation wavelength of 395nm.Clearly，there are two emission peaks at 591and 618nm corresponding to5D0→7F1magnetic dipole transition and5D0→7F2electric dipole transition of Eu3＋ions，respectively.According to the transferring mechanism between different 4f configurations in Eu3＋ions，when the Eu3＋ions are located at the inversion center，only magnetic dipole transition is permitted.However，at the lattice site without inversion center，electric dipole transition also occurs.The intensity of5D0→7F2in the emission spectrum of CaMoO4∶Eu3＋is greater than that of5D0→7F1，indicating that the Eu3＋ions are not located at the center of symmetry sites.

Figure 6（b）shows the emission spectrum of CaMoO4∶Sm3＋under the excitation of 405nm，with scanning scope between 500 and 750 nm.Four emission peaks corresponding to Sm3＋ions appear at 568，602，610and 649nm under the irradiation of violet laser with a wavelength of 406nm.The emission peak at 649nm shows the maximum intensity.The peak at 568nm corresponds to4G5／2→6H5／2transition in the range of 550--570nm，the peaks at 602and 610nm correspond to4G5／2→6H7／2transition in the range of 580--610nm，and the peak at 649nm is attributed to4G5／2→6H9／2transition at 630--650nm.All the four emission peaks originate fromf--ftransitions of 4felectrons in Sm3＋ions.

Figure 6（c）shows the emission spectrum of CaMoO4：Pr3＋under the excitation of 453nm.The emission spectrum displays the characteristic red emissions corresponding to the 4f--4ftransitions in Pr3＋ions，among which the red emission peak at 655nm corresponds to3P0→3F2electric dipole transition of Pr3＋ions，and red emission peaks at 608and 624nm correspond to1D2→3H4and3P0→3H6transitions，respectively.The CaMoO4∶Pr3＋red phosphors can be excited by blue LED to emit red light，thus acting as a superior red light compensation powder.

Fig.6 Emission spectra of phosphors CaMoO4∶Re3＋（Re=Eu，Sm，Pr）

3.4 Effect of charge compensation on CaMoO4∶Re3＋（Re=Eu，Sm，Pr）

Figure 7 shows the XRD patterns of the CaMoO4∶Re3＋（Re=Eu，Sm，and Pr）phosphors after Li＋charge compensation，in which the diffraction peaks of the CaMoO4∶Re3＋（Re=Eu，Sm，and Pr）phosphors are identical before and after the Li＋charge compensation.The XRD patterns of the CaMoO4∶Re3＋（Re=Eu，Sm，and Pr）phosphors are consistent with the standard spectrum of cubic CaMoO4（JCPDS 29--0351）.

Fig.7 XRD patterns of the CaMoO4∶Re3＋（Re=Eu，Sm，and Pr），Li＋

Figures 8，9and 10show the emission spectra of CaMoO4∶Re3＋（Re=Eu，Sm，and Pr）phosphors before and after the Li＋charge compensation.Clearly，the Li＋doping slightly affects the peak shapes and positions，but significantly affects the peak intensities.The intensities of the three phosphors were enhanced after the Li＋charge compensation.The alkaline-earth metal ions have an effect on the photoluminescence properties of alkalinerare earth doped alkaline-earth metal molybdates phosphors.The rare earth ions，in general，show＋3valence state.However，alkaline-earth metal ions always exhibit＋2valence state.When the alkaline-earth metal ions with ＋2valence state are substituted by rare earth ions，the charge difference can produce the vacancies of positive charge.Conversely，when the alkaline-earth metal ions are substituted by earth ions with ＋1valence state，the charge difference can produce the vacancies of negative charge.Thus，the charge imbalance and lattice stress can be reduced.Therefore，the lattice structure is more intact，which can effectively reduce the non-radiative transition and change the luminescent properties.

4 Conclusion

Fig.8 Emission spectra of CaMoO4∶Eu3＋before and after the Li＋charge compensation

Fig.9 Emission spectra of CaMoO4∶Sm3＋before and after the Li＋charge compensation

Fig.10 Emission spectra of CaMoO4∶Pr3＋before and after the Li＋charge compensation

The spherical particles of CaMoO4were prepared by the hydrothermal method.The parameters for the preparation（i.e.，hydrothermal time，pH value，and ratio of ethanol to deionized water）affected the preparation of the CaMoO4particles.Eu3＋，Sm3＋，and Pr3＋ions doped molybdate red phosphors were prepared using the CaMoO4particles as host materials，and their main excitation peaks were located at 395，406and 453nm，respectively，indicating that the phosphors could be excited from UV to visible lights.Their main emission peaks were located at 618，649and 655nm，corresponding to red lights.The introduction of alkalis ion Li＋as a charge compensator could improve the lu-minous intensity of CaMoO4∶RE3＋（RE=Eu，Sm，Pr）.Therefore，this phosphor could be used either as a trichromatic phosphor for white LED or as a red light compensation powder for“blue chip and yellow phosphors”white LED.

［1］SHI H，ZHU C，HUANG J，et al.Luminescence properties of YAG∶Ce，Gd phosphors synthesized under vacuum condition and their white LED performances［J］.Opt Mater Expr，2014，4（4）：649-655.

［2］HUANG C H，LAI Y T，CHAN T S，et al.A novel greenemitting SrCaSiAl2O7∶Eu2＋phosphor for white LEDs［J］.RSC Adv，2014，4（15）：7811-7817.

［3］ZHANG S，LI C，PANG R，et al.Long-lasting phosphorescence study on Y3Al5O12doped with different concentrations of Ce3＋［J］.J Rare Earths，2011，29（5）：426-430.

［4］SUN H，ZHANG X，BAI Z.Synthesis and characterization of nano-sized YAG∶Ce，Sm spherical phosphors［J］.J Rare Earths，2013，31（3）：231-234.

［5］YANG Z P，LI X，LI X M，et al.Synthesis of red-emitting phosphors Ca1-xSrxS∶Eu2＋used in white light emitting diodes by combustions［J］.J Chin Ceram Soc，2006，34（8）：966-969.

［6］HAO M R，LI G F，HE W W.Preparation and luminescence properties of CaWO4∶Eu3＋red emitting phosphor for white LEDs［J］.J Chin Ceram Soc，2013，41（12）：1730-1734.

［7］LIU Y，DING Y，PENG Z.Novel full color greenish-whiteemitting Sr3Bi（PO4）3∶Eu2＋phosphor for white light-emitting diodes［J］.Opt Eng，2014，53（1）：010501（1-3）.

［8］HEESUN Y，DONG K L，YONG S K.Spectral variantions of nano sized Y3Al5O12∶Ce phosphors viacodoping substitution and their white LED characteristics［J］.Mater ChemPhys，2009，114：665-669.

［9］ZHANG Y F，LI L，ZHANG X S，et al.Temperature effects on photo luminescence of YAG∶Ce3＋phosphor and performance in white lightemitting diodes ［J］.J Rare Earths，2008，26（3）：446-449.

［10］ZHANG J，YIN J，LIU P，et al.Preparation and luminescent properties of MgTiO3∶Eu3＋phosphor for white LEDs［J］.J Rare Earths，2012，30（10）：1009-1012.

［11］YAN S，ZHANG J，ZHANG X，et al.Enhanced red emission in CaMoO4∶Bi3＋，Eu3＋［J］.J Physl Chem C，2007，111（35）：13256-13260.

［12］CAVILLI E，BOVERO E，BELLETTI A.Optical spectroscopy of CaMoO4∶Dy3＋single crystals［J］.J Phys：Condens Matter，2002，14（20）：5221.

［13］PARCHUR A K，NINGTHOUJAM R S.Preparation and structure refinement of Eu3＋doped CaMoO4nanoparticles［J］.Dalton Trans，2011，40（29）：7590-7594.

［14］LI G，WANG Z，QUAN Z，et al.Growth of highly crystalline CaMoO4∶Tb3＋phosphor layers on spherical SiO2particles viasol-gel process：Structural characterization and luminescent properties［J］.Cryst Growth Design，2007，7（9）：1797-1802.

［15］GUAN L，WEI W，LIU C，et al.Preparation and luminescent properties of LiGd（MoO4）2∶Sm3＋red phosphor［J］.J Chin Ceram Soc，2013，41（1）：62-65.

［16］LIN Y，GAO S K.Preparation and photoluminescence of a phosphor SrMoO4∶Dy3＋［J］.J Chin Ceram Soc，2012，40（12）：1755-1759.

［17］RAJU G S R，PAVITRA E，KO Y H，et al.A facile and efficient strategy for the preparation of stable CaMoO4spherulites using ammonium molybdate as a molybdenum source and their excitation induced tunable luminescent properties for optical applications［J］.J Mater Chem，2012，22（31）：15562-15569.