J. Nanosci. Nanotechnol的一篇文章,ZnOEu3+ nanocrystals and the phtoluminescence properties

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RESEARCH ARTICLE

Copyright ?2012American Scienti?c Publishers All rights reserved

Printed in the United States of America

Journal of

Nanoscience and Nanotechnology

V ol.12,2607–2611,2012

Synthesis and Luminescence Properties of ZnO:Eu 3+

Nano Crystalline via a Facile Solution Method

Shihua Zhao 1 ?,Fangjie Shu 1,Yanmin Li 1,Cuimei Liu 1,

Wenwen Shan 2,Yuting Cui 1,and Lei Yang 3

1

Department of Physics and Information Engineering,Shangqiu Normal University,

Shangqiu,Henan Province 476000,P .R.China

2

North China University of Water Conservancy and Electric Power,Zhengzhou,

Henan Province 450011,P .R.China

3

College of Materials Science and Engineering,Hunan University,Changsha,

Hunan Province 410082,P .R.China

Different ZnO:Eu 3+nanocrystalline were obtained from a facile solution method with two different precipitators.The comparison of photoluminescence property of two different ZnO:Eu 3+nanocrys-talline was performed.The XRD patterns and the PL spectra indirectly indicate that the dopant Eu 3+

ions had entered into the crystal lattices of ZnO.The study on the PL spectra of the as-prepared ZnO:Eu 3+nanocrystalline shows that with the change of dopant concentration,the ratio of rela-tive emission intensity of electric dipole transition to magnetic dipole transition changes,which fully expresses that the presence of the inversion centers is associated with the dopant concentration of Eu 3+.

Keywords:Nanocrystalline,Electron Transition,Emission Band,Photoluminescence (PL).

1.INTRODUCTION

It is known that ZnO nanostructures have been studied extensively owing to their potential uses in nanodevices and optical materials.1–5Up to now,ZnO nanowires,6nanobelts,7nanorods,8nanoparticles,9nanoleaf 10and nanotubes,5etc.,have been synthesized.Rare earth (RE)ions incorporated into ZnO nanoparticles can result in remarkable changes in the optical properties of ZnO.Among the RE,Eu 3+ions are an interesting candidate due to its red lines at about 610nm.So,there have been many preparation methods for ZnO-based nanomaterials.11–14However,those approaches mentioned above have many disadvantages in cost,facilities and complexity.Especially,the experimental conditions are hard to be controlled.So many researchers hope to ?nd a low-temperature and simple synthesis method.To our best knowledge,the per-formance of properties comparison with different precipi-tators is very little.In this work,a facile method was used for growing precursors of ZnO with two different precip-itators in solution at room temperature.Subsequently,the precursors could transform into ZnO under the annealing

?

Author to whom correspondence should be addressed.

process.This approach has advantages in cost,facili-ties,complexity and energy consumption.Especially,this approach can be put into mass production in the future.

2.EXPERIMENTAL DETAILS

In the experiment,1mol of Zn(NO 3 2·6H 2O (AR)is dis-solved in deionized water to form aqueous solution.0.01,0.05,and 0.1mol/L are three different molar concentration of Eu(NO 3 3solution,which were obtained from dissolv-ing Eu 2O 3powders (AR)in nitric acid via evaporation process and then adding to the above-mentioned aqueous solution to form 1L of neutral mixed solution,respec-tively.That is,the molar ratios of Eu 3+to Zn 2+in the homogeneous mixed solution are 1:100,5:100and 10:100,respectively.Three solutions with different molar ratio of Eu 3+to Zn 2+are obtained.Each solution is divided into two parts and labeled 1,2,3,1#,2#and 3#,respectively,on account of using different precipitator.The solutions 1,2and 3express that urea is the precipitator and the molar ratio of Eu 3+to Zn 2+in the solutions are 1%,5%and 10%,respectively.Similar to the labels,the solutions 1#,2#and 3#show that ammonia is the precipitator and the molar ratio of Eu 3+to Zn 2+in the solutions are 1%,5%

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and 10%,respectively.The beakers ?lled with the 1,2and 3solutions are sealed with the adhesive tape after some urea is added into three solutions.The solutions of the 1#,2#and 3#are agitated on the magnetic force beater while the precipitator of ammonia is added into every beaker.And then,the beakers 1,2and 3are put into an oven kept at 80 C for 24h.After reaction for 24h,the precur-sors of the samples 1,2and 3were taken out and washed repeatedly using deionized water.Subsequently,a series of samples with different dopant concentration and precipita-tor were put into a tube furnace and kept at 800 C for 2h.Finally,the samples of 1,2,3,1#,2#and 3#are obtained.The as-prepared samples were detected by X-ray diffraction (XRD)measurements on a SIEMENS D-5000X diffractometer.The morphology of the as-synthesized sam-ple was obtained with scanning electron microscopy (SEM JSM-6700F).The photoluminescence was taken on a Hitachi F-2500FL spectrophotometer with a Xe lamp as the excitation light source.

3.RESULTS AND DISCUSSION Figure 1shows the XRD patterns of the as-prepared sam-ples with different dopant concentrations and precipitators

annealed at 800 C for 2h.From the XRD patterns

shown

Fig.1.XRD patterns of the as-prepared samples with different dopant concentration and precipitators.

in Figure 1,it can be seen clearly that the six samples have the same diffraction crystal planes,which can be indexed to hexagonal ZnO according to the values in the standard card (JCPDS 36-1451).According to the Scherrer formula,the diffraction peaks in Figure 1(A)are sharper than that of Figure 1(B),so the sizes of samples labeled #are larger than samples with no label.In addition to those mentioned above,it must be pointed out that compared with JCPDS date (not shown here),all the diffraction peaks are shifted slightly towards the low diffraction angle,which indicates that the lattice parameters of Eu-doped ZnO are a little larger than those of undoped ZnO.There is no peak related to Eu,so it is suggested that Eu 3+ions may occupy Zn sites or interstitial sites in the ZnO crystal lattice.

The typical SEM images of the as-prepared ZnO:Eu 3+are depicted in Figure 2.Figures 2(A)and (B)are the SEM images of the ZnO:Eu 3+prepared by the precipi-tator of urea.Figures 2(C)–(E)are the SEM images of the ZnO:Eu 3+resulted from the precipitator of ammonia.

From Figures 2(A)and (B),it can be seen that large scale particles came up and the average sizes of those parti-cles are about 100nm.Figures 3(C)and (D)show that morphologies of the samples of ZnO:Eu 3+are in column and the diameter and length are about 2 m and 6 m,respectively.To further demonstrate morphologies of the samples,a high-magni?cation SEM of the column indi-cated by the arrow in Figure 2(D)is performed,which is shown in Figure 2(E).From Figure 2(E),it can be seen clearly that columns are composed of many ?akes and the thickness of the ?akes are about 10nm.That is to say,about 600parallel layers lap over each other to form the morphology of column.In other words,ammonia as the precipitator tends to group behavior.

The reason of group behavior is possibly that OH ?ions resulted from ammonia react with Zn 2+and Eu 3+quickly to form hydroxide precipitates,which result in the higher local concentration of precipitates rather than homogeneous concentration in the https://www.360docs.net/doc/8d3882641.html,ly,OH ?ions of ammonia combines with Zn 2+and Eu 3+ions at once rather than homogeneous dispersal,which makes for the local group behavior.However,OH ?ions arisen from hydrolyzation of urea are diffused in the mixed solution,which bring about the homogeneous reaction with the Zn 2+and Eu 3+ions.In a word,the latter reactions are based on the homogeneous mixture of Zn 2+,Eu 3+and OH ?ions,which make the hydroxide precipitates come into being uniformly in whole solution rather than the local reaction.The growth of the ZnO:Eu 3+samples may be divided into two separate steps,the reaction of Zn 2+and Eu 3+with OH ?ions,and the subsequent formation of ZnO:Eu 3+samples.Those processes involve the following reactions:

NH 3·H 2O →NH +

4

+OH ?CO NH 2 2+3H 2O →CO 2+2NH 3·H 2O

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RESEARCH ARTICLE

Fig.2.Typical SEM images:(A)and (B)show the SEM images of the samples with urea as the precipitator,(C)and (D)show the SEM images of the samples with ammonia as the precipitator,(E)is a high-magni?cation SEM image of the “column”indicated by the arrow in Figure D.

Zn 2++2OH ?→Zn OH 2Eu 3++3OH ?→Eu OH 3Zn OH 2→ZnO +H 2O 2Eu OH 3→Eu 2O 3+3H 2O

To further demonstrate the dependence of different pre-cipitators and dopant concentration on the photolumines-cence properties,the PL spectra of the ZnO:Eu 3+samples are performed,which are shown in Figures 3and 4.

The PL excitation spectra monitored by 611nm is shown in Figure 3.Figures 3(A)and (B)express excitation spectra of different samples with different dopant concen-tration and precipitators.From Figure 3,it can be seen clearly that 392nm is the most effective to the emission band of 611nm.According to the reports of Ref.15,there is a main peak at about 395nm in the excitation spectra of pure ZnO or Eu 2O 3,so using 392nm as the excitation wavelength can possibly excite ZnO and Eu 3+ions,simul-taneously.In addition to those mentioned above,it must be pointed out that the two shoulder peaks of 405nm and 408nm are possibly resulted from the energy level split of 395nm.

Figure 4shows PL emission spectra of the samples with different dopant concentration and precipitators,mea-sured with an excitation of the 392nm lines of a Xe lamp.From Figure 4,it can be seen that the as-prepared ZnO:Eu 3+crystal can emit salmon pink lines (592nm)

(A)

(B)

Fig.3.Excitation spectra of the as-prepared samples with different dopant concentration and precipitators.

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(A)

(B)

Fig.4.Emission spectra of the as-prepared samples with different dopant concentration and precipitators.and red lines (611nm).It is well-known that the peaks at

595and 612nm derive mainly from the allowed magnetic

dipole transition (MDT 5D 0→7F 1 and electric dipole transition (EDT 5D 0→7F 2 of Eu 3+ions,respectively.

According to the reported paper 16,the peaks near 595nm derived mainly from the allowed MDT,which required the inversion centers.However,the EDT did not require the inversion centers.So,from the comparison of the PL inten-sity of the MDT and EDT,it can be presumedly conjec-tured that whether the dominant Eu 3+

ions have located the inversion centers or not.It should be pointed out that with the increase of dopant concentration,the number of inver-sion centers will change,which can be demonstrated by the Figure 4.It can also been seen that with the change of dopant concentration,the ratio of relative emission inten-sity of EDT to MDT change,which fully expresses that the presence of the inversion centers is associated with the dopant concentration of Eu 3+

.In addition to those men-tioned above,it should be emphasized that there is no 510nm peak in Figure 4,which shows the possibility of energy transfer between ZnO and Eu 3+

.If the pho-ton energies are absorbed by the defects on the surface states of ZnO,there will be a weak visible emission around 510nm.

17

Fig.5.A schematic drawing for possible mechanisms of electron tran-sitions in the excitation and emission processes.

Based on the work,a schematic drawing for possi-ble mechanisms of electron transitions in the excitation and emission processes is proposed,which is shown in

Figure 5.When the 392nm lines (3.17eV)excite the as-prepared samples,the electrons of Eu 3+absorb the pho-tons to transfer from the 7F 0energy level to 5L 6and 5D 2energy level,so there are two absorption peaks at 392nm

and 466nm in Figure 3.After electrons are jumped to the higher energy lever,the deexcitation of the electrons would transfer back to the lower energy lever,which results in the luminescence due to the 5D 0→7F 1 2transitions.In the ZnO system,the electrons excitated to the higher energy

level would possibly return to the 5D 0level of Eu 3+ions,rather than to the ground state,which could be demon-strated by the absence of the 510nm visible emission band

in Figure 4,which is also the evidence to express that there is possibly an energy transfer between ZnO and Eu 3+.4.CONCLUSIONS

In summary,the ZnO:Eu 3+nanocrystallines have been prepared by a facile solution method.This approach has advantages in cost,facilities,complexity and energy con-sumption.From the XRD patterns,it is suggested that

Eu 3+ions may occupy Zn sites or interstitial in the ZnO crystal https://www.360docs.net/doc/8d3882641.html,pared with the SEM images of two different samples,it can be concluded that ammonia as the

precipitator tends to group behavior.The study on the PL spectra of those ZnO:Eu 3+crystallines at room tempera-ture shows that there is the possibility of energy transfer between ZnO and Eu 3+.A schematic drawing for possible mechanisms of electron transitions in the excitation and

emission processes is also proposed.

Acknowledgments:This work was supported by the

Youth Scienti?c Research Foundation of Shangqiu Normal

University,China (Grant No.2011QN14),the Founda-tion and High-Tech Project of Henan Province (Grant

No.112300410258),the Henan Provincial Science and

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Technology Research Projects (Grant No.092102210343),the Henan Provincial Science and Technology research projects (Grant No.112300410269),the Natural Science Foundation of Henan Province (Grant No.2009A140009),the National Natural Science Foundation of China (Grant No.11104066),the National Natural Science Foundation of China (Grant No.11074160).

References and Notes

1.Z.J.Wang,P.L.Zhou,J.Liu,F.J.Wang,Y .J.Peng,and X.B.Yu,J.Nanosci.Nanotechnol.11,6772(2011).

2.S.H.Zhao,H.F.Ma,L.L.Wang,L.Yang,and Y .T.Cui,Surf.Rev.Lett.17,425(2010).

3.P.Soman,M.Darnell,M.D.Feldman,and S.Chen,J.Nanosci.Nanotechnol.11,6880(2011).

4.R.K.Kumar,M.Husain,and Z.A.Ansari,J.Nanosci.Nanotechnol.

11,6940(2011).

5.S.H.Zhao,L.L.Wang,L.Yang,and Z.Y .Wang,Physica B

405,3200(2010).

6.S.Hindley,A.C.Jones,S.Ashraf,J.Bacsa,A.Steiner,P.R.

Chalker,P.Beahan,P.A.Williams,and R.Odedra,J.Nanosci.

Nanotechnol.11,8294(2011).7.Y .Q.Chen,Y .Shao,X.H.Zhang,C.Jia,Y .Su,Q.Li,L.Z.Liu,and T.B.Guo,J.Nanosci.Nanotechnol.11,1205(2011).

8.J.Kim,G.J.Lee,I.Park,and Y .Lee,J.Nanosci.Nanotechnol.11,7238(2011).

9. A.Susarrey-Arce,V .Petranovskii,M. A.Hernández-Espinosa,R.Portillo,and W.D.L.Cruz,J.Nanosci.Nanotechnol.11,5574(2011).

10.J.J.Qiu,X.M.Li,X.D.Gao,X.Y .Gan,W.Z.He,H.K.Kim,

and Y .H.Hwang,J.Nanosci.Nanotechnol.11,2175(2011).

11. D.W.Kim,J.H.Kang,Y .S.Lim,M.H.Lee,W.S.Seo,H.H.Park,K.H.Seo,and M.G.Park,J.Nanosci.Nanotechnol.11,1617

(2011).12.P. A.Tanner and L.X.Yu,J.Nanosci.Nanotechnol.8,1307(2008).13.Y .P.Du,Y .W.Zhang,L.D.Sun,and C.H.Yan,J.Phys.Chem.C 112,12234(2008).14.L.Armelao,F.Heigl,A.Jurgensen,R.I.Blyth,T.Regier,X.T.

Zhou,and T.K.Sham,J.Phys.Chem.C 111,10194(2007).

15.S.H.Zhao,L.Yang,L.L.Wang,B.H.Yu,Y .X.Chen,and Y .T.Cui,Func.Mater.Lett.3,285(2010).16.G.L.Song,K.X.Sun,and Y .T.Yang,J.Chin.Spectrosc.Spec.Anal.27,639(2007).17.G.L.Song and H.Liang,J.Chin.Syn.Cryst.35,1032(2006).Received:6August 2011.Accepted:8November 2011.