HUANG Bo－tong，MA Yong－qing，ZOU Dan，ZHENG Gan－h(huán)ong，DAI Zhen－xiang

（Anhui Key Laboratory of Information Materials and Devices，School of Physics and Materials Science，Anhui University，Hefei 230039，China）

0 Introduction

So far，nano－sized YBO3：Eu3＋phosphors have been widely investigated because their morphology can be well controlled and the color purity can be significantly improved in nanocrystals［1－2］.However，post－annealing at a high temperature was usually needed to obtain single－phase YBO3：Eu3＋，and the morphology of samples were usually controlled by surfactants.

Recently，the carbon－nanotube（CNT）was used as a template to synthesize Y2O3：Yb／Er phosphors which still need a subsequent heat treatment process［3］.As reported before［4－5］，the CNT in the acidic solution（such as HNO3）was able to be oxidated and such a reaction involves the initial rapid formation of carbonyl groups（C＝O），which are then transformed into phenol or carboxylic groups（—COOH）.Oxygenated protonated functionalities such as—OH groups are of particular interest，since they can behave as coordination sites for metal cations，thus paving the way for the production of highly dispersed metal particles on CNT surfaces.So we can image that the pH value of the solution will affect the groups on the CNT surface，and consequently influence the morphology and luminescent properties of phosphors.

In this work，we develop a new and simple method to control the morphology of YBO3：Eu3＋.Specifically，the CNTs were added in the precursor solution.Then the pH value of the precursor solution was changed by NH3·H2O solution from 5to 10.Using such a precursor solution，the single－phase YBO3：Eu3＋was able to be synthesized via the hydrothermal method at 260℃ which did not need a post－annealing at high temperature.The morphology of YBO3：Eu3＋was simply and significantly altered by the pH value which did not need any surfactants.Furthermore，the color purity of YBO3：Eu3＋was also able to be improved by the pH value.The related mechanism was discussed.

1 Experiment

1.1 Synthesis

All the raw materials included yttrium nitrate hexahydrate（Y（NO3）3·6H2O），Eu2O3（99.99%），carboxy－functionalized multi－walled carbon nanotubes（hereafter also referred to as CNTs），sodium dodecyl benzene sulfonate（SDBS），nitric acid（HNO3），ammonium water（NH3·H2O），ethanol（C2H5OH）and deionized water.Tributyl borate（C12H27BO3）was used as B3＋sources for preparing YBO3：Eu3＋.

Stoichiometric Eu2O3was dissolved in HNO3，Y（NO3）3·6H2O and C12H27BO3were dissolved in deionized water to form solution，respectively.With stirring for 10minutes，the Eu（NO3）3，Y（NO3）3solution and C12H27BO3solution were mixed with continuous stirring.Next，appropriate amount of MWNT－COOH and SDBS were suspended in deionized water and ultrasonicly treated until the homogeneous suspension was obtained.Then，MWNT－COOH solution was added to the former mixture solution.Keeping the final solution of C2H5OH and H2O with the volume ratio of H2O and C2H5OH being 2∶1.NH3·H2O was added to adjust pH value to 5，6，7，8，9and 10.Finally，the mixture was placed in a 1 000mL hastelloy autoclave（Parr 4577）and reacted at 260℃for 6hwith stirring speedVs＝250r·min－1.After the autoclave was cooled to room temperature naturally，the products were separated by centrifugation，and washed with ethanol and deionized water several times and dried in a vacuum oven at 80℃to obtain the final samples

1.2 Characterization

The crystal structure of the samples was characterized by X－ray diffraction using an X－ray diffractometer（XRD；DX－2000SSC）with CuKαirradiation（λ＝1.540 6?）in the scanning range 10°to 90°with a step of 0.02°.The size and morphologies were observed by using a scanning electron microscope（SEM，S－4800，Hitachi）.（High resolution）transmission electron microscopy（（HR）TEM，JEOL JEM－2100）was used to observe lattice fringes and to obtain selected area electron diffraction（SAED）patterns.The activation and emission spectra were measured on a FL fluorescence spectrophotometer（F－4500）.All the measurements were carried out at room temperature.

2 Results and discussion

2.1 Structure analyses of all samples

Fig.1 shows the XRD patterns of all samples.The diffraction peak positions and relative intensities are well matched with those of standard JCPDS card（No.16－0277）of YBO3，which has a hexagonal vaterite－type structure with the space group of P63／m（176）；no detectable secondary phases are observed.The crystallite size（D）of all samples is calculated by MDI Jade 5.0software and theDvalues are also given in Fig.1 .With increasing pH value，theDvalue first increases to 58.9nm at pH＝6，and then gradually decreases monotonously.The sample with pH＝10has the smallestDvalue of 17.7nm.

2.2 Morphology and micro－structural analyses of all samples

Fig.2 shows SEM images of samples with pH values of 5，6，7，8，9，10；TEM，SAED and HRTEM images photographed from the area marked with square in Fig.2 g for the sample with pH＝8.The inset of Fig.2 ashows the magnified plot of a single drum－like particle.For the sample with pH＝5as shown in Fig.2 a，it consists of drum－like particles which conglomerate to large particles.The drum－like particle has the smooth surface，as shown in the inset of Fig.2 a.The upper and lower surfaces exhibit a regular round shape with a diameter of about 5μm.The height of a drum is about 4－5μm.Such a morphology characteristic has not been reported before as far as we know.The sample with pH＝6exhibits a flake－like morphology，with the thickness of about 500nm.As pH＝7，the particles in Fig.2 cexhibit an hexahedron－like shape with a size range from 0.5to 1 μm.The particles with pH＝8show a hexagonal flake－like structure with the thickness of about 500nm，which has also not been observed before as far as we know.The morphology of the sample with pH＝9is the same as that with pH＝6，and such a flake－like morphology was also observed in YBO3synthesized via oxides－h(huán)ydrothermal route but the thickness of our samples is about two times larger which maybe results from the additive of CNTs in the precursor solution.Finally the sample with pH＝10exhibits a flower－like morphology which has been widely observed before in YBO3：Eu［2，6－8］，YBO3：Tb［9］and Gd2O3：Eu［10］.

Fig.2 g shows the TEM image of the sample with pH＝8，which exhibits the hexagonal shape，consistent with the SEM image of Fig.2 d.The SAED pattern in Fig.2 hshows the distinct diffraction spots of a hexagonal YBO3：Eu3＋，which is characteristic of single crystal.Furthermore，the HRTEM image in Fig.2 ishows the clear lattice fringes with interfringe distance of 0.44nm，corresponding to the（002）crystalline plane of YBO3：Eu3＋.High crystallinity will be beneficial to luminescence of the phosphor because high crystallinity generally means fewer traps［11］，which will be further confirmed below.

Based on the above results we can clearly notice that both the additive of CNTs and the pH value of the precursor solution greatly influence the morphology of the samples，and the possible reasons are suggested as follows：1）as reported before［4］，the oxidation treatment of CNTs in HNO3introduces the main functional groups on CNT surfaces in large amounts such as phenolic，lactonic，quinonic and carboxylic groups and some of them such as—OH groups can behave as coordination sites for metal cations.For our sample with pH＝5，we suggest that the—OH groups on the surface of CNT should be increased in the acidic precursor solution and then the metal cations coordinate with—OH groups；Therefore YBO3：Eu grows layer－by－layer in the perpendicular direction of CNTs and forms the drumlike particle.2）Tributyl borate can rapidly decompose into boric acid（HBO3）and alcohols.HBO3exists as a hydrolyzed form，B（OH）3at a low pH value while it exists as polyborate anions at an intermediate pH（achieved by adding NH4OH）［12］，and such a different existing form of HBO3in the precursor solution with different pH values will result in the morphology variation of the YBO3particles.3）In the strongly alkaline solution such as pH＝10，the metal cations are precipitated to form the insoluble alkaline salts rather than coordinated with CNTs，and the final sample usually exhibits flower－like structure which is schematically illustrated in Ref.［2］.Additionally the flowerlike YBO3：Eu3＋particles are usually obtained from the precursor solution with pH value around 8［2，6］，whereas they are obtained until pH＝10in this work.This indicates that the additive of CNTs inhibits the formation of flower－like YBO3：Eu3＋particles or in other word it facilitates the formation of thicker flake－like or drum－like particles.

2.3 Luminescent properties of all samples

Fig.3 ashows the excitation spectra withλem＝611nm.Fig.3 bshows the emission spectra withλex＝235nm（b）for samples with different pH values in the precursor solution.The excitation spectra of all samples exhibit three bands locating at 210nm，220nm and around 247nm.The band at 210nm can be assigned to the charge transfer transition between Y3＋and O2－［6］.The band at 220nm is probably associated with an excitonic transition（ET）from O 2p valence bands to Y（4d＋5s）conduction bands［13］.The broad band around 247nm results from the charge transfer transition of Eu3＋，i.e.，the electronic transition from the 2p orbital of O2－to the 4forbital of Eu3＋.

The emission spectra of all samples（Fig.3 b）exhibit sharp lines at 593（5D0－7F1），611and 627（5D0－7F2），650and 675（5D0－7F3）nm，resulting from the emission of Eu3＋due to the5D0－7FJ（J＝1，2and 3）transitions.The red emissions（R）at 611and 627nm results from the electric dipole transition5D0－7F2with the selection ruleΔJ＝2，while the orange emission（O）at 593nm from the5D0－7F1transition is a typical magnetic－dipole transition.Predominant emissions locate at 593nm（orange），611and 627nm（red）and the emissions at 650and 675nm are much weaker，which is consistent with previous reports［8，14－15］.

The intensity of electric dipole transitions depends strongly on the site symmetry in the host crystal.Magnetic dipole f－f transitions are not greatly affected by the site symmetry，because they are parity－allowed.If the Eu3＋ion occupies a centrosymmetric site in the crystal lattice，the magnetic dipole transition5D0－7F1（orange）is the dominant transition；otherwise，the electric dipole transition5D0－7F2（red）becomes dominant.YBO3：Eu3＋possesses a hexagonal vaterite－type structure，in which the Eu3＋ions occupy the Y3＋site which has point symmetry S6，and therefore the orange emission at 593nm from the transition5D0－7F1is dominant［11］，which results in a bad color purity and is not suitable for illumination［16］.Generally，the red light at 611nm closely matches the eye sensitive curves and makes the phosphor ideal for illumination purpose［16］.Therefore the intensity ratio between red（611nm）and orange（593nm）emissions，i.e.，R／O has usually been used to judge color purity.The R／O ratios for all samples with different pH values in the precursor solution are given in the upper panel of Fig.3 .The R／O values are distinctly enhanced for the samples with pH＝9and 10；they are 1.07and 1.22，respectively，much larger than 0.81，0.78，0.74and 0.69which were found for YBO3：Eu3＋hydrothermally synthesized at temperatures of 200，220，240，and 260 ℃，respectively［17］.The possible reasons for larger R／O ratios in our samples are as follows：1）As mentioned above，the5D0－7F2transition is a hypersensitive transition，and therefore is strongly influenced by the surrounding environment of Eu3＋.When Eu3＋is located at a low－symmetry local site（without an inversion center），the emission from the5D0－7F2transition often dominates the emission spectra；this has been observed in many other host materials such as Ca9Ln（VO4）7（Ln＝Y(jié) and Gd）and Sr2V2O8［18－21］.However，the reason for the low－symmetry environment surrounding Eu3＋in our samples is somewhat different from that in previously reported samples（where the lowsymmetry environment is because of the symmetry of the crystal lattice itself）.All our samples are YBO3：Eu3＋，and they have the same crystal structure.what is the origin for low－symmetry environment surrounding Eu3＋in our samples？As given in Fig.1 ，the samples with pH＝9and 10 have the smaller crystallite sizes of 36.5and 17.7nm than the other samples，resulting in the larger area of grain boundaries.2）As shown in Fig.2 f，the sample with pH＝10consists of flower－like particles，which results in the larger surface－to－volume ratio than the other samples.In the situations of 1）and 2），many atoms at the grain boundaries or particles’surface can not be completely bound，leading to numerous defects.These defects may increase the degree of disorder in the crystal field symmetry，and lower the local symmetry of the Eu3＋ions.This then increases the probability of the5D0－7F2red－emission transition，meaning that this transition dominates the emission spectra，resulting in the higher R／O value based on the Judd－Ofelt theory［11，22－23］.Additionally，the sample with pH＝8 shows the most intense emission，which may result from the good crystallinity and suitable particle size.

We also measured the emission spectra under excitation wavelength of Hg ray，such as 254nm，and the results are shown in Fig.4 .

As shown in Fig.4 ，the peak positions and origin are the same as in the emission spectra withλex＝235nm.The R／O value is 1.11，1.11，1.13，1.10，1.25and 1.29for the sample with pH＝5，6，7，8，9，and 10，respectively.The sample with pH＝10has the highest R／O ratio and the sample with pH＝8shows the most intense emission，which has the same mechanism as discussed above.These results indicate that the nano－sized YBO3：Eu3＋phosphors have potential application in lighting because of better color purity.

3 Conclusion

Multi－walled carbon nano－tubes were added to the precursor solution，and the pH value of precursor solution was adjusted to 5，6，7，8，9and 10by ammonia aqueous solution.Using such precursor solutions with different pH values，a series of YBO3：Eu3＋samples were synthesized by the hydrothermal method at 260 ℃.The single phase YBO3：Eu3＋can be directly obtained at 260 ℃without experiencing a subsequent post－annealing at high temperature.Rich morphologies are observed due to the variation of pH value，which include drum－，flake－，hexahedron－，hexagonal－and flower－like particles.The possible reasons for the different morphologies are suggested as follows：functional groups are grafted to the surface of carbon nano－tubes which behave as the coordination sites for metal cations，leading to the crystal layer－by－layer growth；different forms of HBO3in the precursor solution with different pH values also affect the morphology variation of the YBO3particles；in the strongly alkaline solution such as pH＝10，the metal cations are precipitated to form the insoluble alkaline salts rather than coordinated with CNTs，resulting in the flower－like structure which has been widely observed and discussed.

All samples exhibit the characteristic emissions of Eu3＋due to the5D0－7FJ（J＝1，2and 3）transitions.The R／O ratios are enhanced for the samples with pH＝9and 10which are 1.07and 1.22，respectively，indicating better color purity.The underlying origins are attributed to the lower local symmetry of the Eu3＋ions at the grain boundaries or particles’surface.

［1］Wei Z G，Sun L D，Liao C S，et al.Fluorescence intensity and color purity improvement in nanosized YBO3：Eu［J］.Appl Phys Lett，2002，80：1447－1449.

［2］Xu Y F，Ma D K，Chen X A，et al.Bisurfactant－controlled synthesis of three－dimensional YBO3／Eu3＋architectures with tunable wettability［J］.Langmuir，2009，25（12）：7103－7108.

［3］Huang W S，Shen J F，Wan L，et al.Y2O3：Yb／Er nanotubes：layer－by－layer assembly on carbon－nanotube templates and their upconversion luminescence properties［J］.Materials Research Bulletin，2012，47：3875－3880.

［4］Gerber I，Oubenali M，Bacsa R，et al.Heoretical and experimental studies on the carbon－nanotube surface oxidation by nitric acid：Interplay between functionalization and vacancy enlargement［J］.Chem Eur J，2011，17：11467－11477.

［5］Chen J，Hamon M A，Hu H，et al.Solution properties of single－walled carbon nanotubes［J］.Science，1998，282：95－98.

［6］Zou D，Ma Y Q，Qian S B，et al.Improved luminescent properties of novel nanostructured Eu3＋doped yttrium borate synthesized with carbon nanotube templates［J］.Journal of Alloys and Compounds，2014，584：471－476.

［7］Zhang X W，Marathe A，Sohal S，et al.Synthesis and photoluminescence properties of hierarchical architectures of YBO3：Eu3＋［J］.J Mater Chem，2012，22：6485－6490.

［8］Choi S，Park B Y，Seo J H，et al.Emissive transparent luminescent layer using shape controlled YBO3：Eu3＋nanophosphors prepared by solvothermal reactions［J］.Electrochemical and Solid－State Letters，2012，15（5）：J19－J23.

［9］Yang L，Zhou L Q，Chen X G，et al.Hydrothermal synthesis of YBO3：Tb3＋microflowers and their luminescence properties［J］.Journal of Alloys and Compounds，2011，509：3866－3871.

［10］Pavitra E，Yu S J.A facile large－scale synthesis and luminescence properties of Gd2O3：Eu3＋nanoflowers［J］.Materials Letters，2013，90：134－137.

［11］Li Z H，Zeng J H，Li Y D.Solvothermal route to synthesize well－dispersed YBO3：Eu nanocrystals［J］.Small，2007，3：438－443.

［12］Jung K Y，Kim E J，Kang Y C.Morphology control and optimization of luminescent property of YBO3：Tb phosphor particles prepared by spray pyrolysis［J］.Journal of The Electrochemical Society，2004，151（3）：H69－H73.

［13］Wei Z G，Sun L D，Liao C S，et al.Size dependence of luminescent properties for hexagonal YBO3：Eu nanocrystals in the vacuum ultraviolet region［J］.Journal of Applied Physics，2003，93：9783－9788.

［14］Boyer D，Bertrand－Chadeyron G，Mahiou R，et al.Synthesis and characterization of sol－gel derived Y3BO6：Eu3＋powders and films［J］.Opt Mater，2003，24：35－41.

［15］Jin D L，Yang J J，Miao X，et al.Highly enhanced photoluminescence of YBO3：Eu3＋micro－spheres by coadding Li ion and alkaline－earth metal ions［J］.Mater Lett，2012，79：225－228.

［16］Gundiah G，Shimomura Y，Kijima N，et al.Novel red phosphors based on vanadate garnets for solid state lighting applications［J］.Chem Phys Lett，2008，455：279－283.

［17］Jiang X C，Yan C H，Sun L D，et al.Hydrothermal homogeneous urea precipitation of hexagonal YBO3：Eu3＋nanocrystals with improved luminescent properties［J］.J Solid State Chem，2003，175：245－251.

［18］Qian S B，Ma Y Q，Dai Z X，et al.Photoluminescence properties of Na＋－free and Na＋co－doped Ca9Gd（VO4）7：Eu3＋，Dy3＋［J］.Mater Res Bull，2013，48：521－525.

［19］Ma Q，Ma Y Q，Qian S B，et al.Photoluminescence properties and effect of Bi3＋dopant of Eu3＋and Dy3＋codoped Sr3V2O8［J］.J Rare Earths，2012，30：426－430.

［20］Zhu W L，Ma Y Q，Cao S，et al.Photoluminescence properties of Sr3（VO4）2，Sr2Y2／3－yEuy（VO4）2and Sr2Y2／3－zSmz（VO4）2［J］.Opt Mater，2011，33：1162－1166.

［21］Cao S，Ma Y Q，Quan C M，et al.Photoluminescence properties of Ca9Y（VO4）7and Ca9Y0.95Ln0.05（VO4）7（Ln3＋＝Eu3＋，Sm3＋，Pr3＋）［J］.J Alloys Compd，2009，487：346－350.

［22］Yadav R S，Dutta R K，Kumar M，et al.Improved colorpurity in nano－size Eu3＋－doped YBO3redphosphor［J］.J Lumin，2009，129：1078－1082.

［23］Judd B R.Preparation and structure analysis of optical materials［J］.Phys Rev，1962，127：750－753.