Communications
Growth and Characterization of Single-Crystal Y2O3:Eu Nanobelts Prepared with
a Simple Technique
Xia Li,?Qiang Li,*,?Zhiguo Xia,?Lin Wang,?Wenxun Yan,?Jiyang Wang,?and
R.I.Boughton§
Department of Chemistry,Tsinghua Uni V ersity,Beijing100084,P.R.China,The State Key Laboratory on
Crystal Materials,Shandong Uni V ersity,Jinan250100,China,and Department of Physics and Astronomy,
Bowling Green State Uni V ersity,Bowling Green,Ohio43403
Recei V ed January21,2006;Re V ised Manuscript Recei V ed July31,2006
ABSTRACT:In this paper,we report on the synthesis of single-crystalline Y2O3:Eu nanobelts via a simple,rapid,and efficient one-step technique without using any templates or catalysts.The prepared products were characterized by X-ray powder diffraction(XRD),transmission electron microscopy(TEM),high-resolution transmission microscopy(HRTEM),and selected area electron diffraction(SAED).The effects of pH,reaction temperature,and process time on the phase structure and morphology of the product are studied.The possible growth mechanism of Y2O3nanobelts is discussed on the basis of the crystal structure of the materials.Photoluminescence results indicate that the Y2O3:Eu nanobelts have a strong red5D0f7F2transition.
One-dimensional nanomaterials of different shapes,such as wire, rodlike,and tubular forms,have attracted much attention since the discovery of carbon nanotubes in1991.1The reason is that they may be suitable for various applications such as electronic devices, sensors,and energy-storage media.2One-dimensional rare earth oxides have been widely used as high-performance magnets,lumin-escence devices,catalysts,and other functional materials because of their electronic,optical,and chemical properties resulting from the4f shell of the ions.3-7These properties depend strongly on the materials’composition and structure.If rare earth oxides were obtainable in a nanostructure form,they could hold promise as high-ly functionalized materials as a result of both shape-specific and quantum-size effects.Indeed,Wakefield8has reported on some 1D nanomaterials in which the luminescence properties of the rare earth ions are modified in comparison to traditional micrometer-sized powders.
Many methods have been used for the preparation of one-dimensional rare earth nanostructures,including solution combustion (propellant)synthesis,9,10the homogeneous precipitation method,11-13 and the hydrothermal method,14-16among others.17,18Among these methods,catalyst or template-based techniques have been widely used to prepare1D nanobelts.In recent work,12Yada’s group described the synthesis of rare earth oxide nanotubes templated by dodecyl sulfate assemblies,realized by homogeneous precipitation with urea.Conversion into a hollow nanotube with an inner diameter of3nm was accomplished after anionic exchange of a surfactant with the acetate ions.However,the selection of suitable catalysts or templates for the reaction system can be a complicated process, and their addition may result in the presence of impurities in the final product.Developing the synthesis of nanorods and nanowires that involves a template-less and noncatalyst process presents a tremendous challenge.Recently,Wang et al.14prepared a series of rare earth compound nanotubes,including hydroxides,oxides,oxy-sulfides,and hydroxyfluorides.These materials have been success-fully synthesized on the basis of a simple hydrothermal method followed by subsequent dehydration,sulfuration,or fluoridation processes(700°C for2h).
Eu-doped Y2O3phosphor is a well-known red phosphor that is used widely in fluorescent lamps and cathode-ray tubes(CRT).For this reason,studies on the luminescence properties of nanocrystalline Y2O3:Eu have attracted extensive interest during the past few years. Wu et al.also reported the preparation of1DY2O3:Eu NTs by the surfactant assembly method.19
In this paper,we present a simple,rapid,and efficient one-step technique to synthesize very thin,single-crystalline Y2O3nanobelts and nanorods,using yttrium nitrate in a mixed solvent system at about200°C for3h.The nanobelts typically have an average thickness of ca.10nm,width of40-100nm,width-to-thickness ratio of4-10,and a length of up to several micrometers.The present work suggests that it is possible to grow rare earth nanobelts using an aqueous,solution-based chemical technique under con-trolled conditions without any catalysts or templates.The growth mechanism of the nanobelts is also discussed on the basis of the crystal structure of the materials.
(Y0.95Eu0.05)2O3nanobelts were prepared by a mixed solvothermal process.In the preparation,stoichiometric amounts of yttrium oxide (Y2O399.99%)and europium oxide(Eu2O399.99%)were dissolved in diluted nitric acid(A.R.)under vigorous stirring.Then,15% NaOH(or KOH)solution was added to adjust the system to a pH of～8-14.Next,100mL of precursor precipitate was placed in an autoclave container with a volume of500mL.A quantity of 300mL of ethanol was added to it,and the resulting suspension was finally heated to the desired temperature for2h and allowed to cool to room temperature naturally.The gray precipitate was
*Corresponding author.Tel:86-10-62797871.E-mail:lix@ https://www.doczj.com/doc/4d1568942.html,.
?Tsinghua University.
?Shandong University.
§Bowling Green State University.
CRYSTAL GROWTH &DESIGN
2006 VOL.6,NO.10 2193-2196
10.1021/cg0600400CCC:$33.50?2006American Chemical Society
Published on Web08/29/2006
filtered using suction filtration and washed several times each with distilled water and absolute ethanol.
Results on the purity and phase structure of the products were obtained by X-ray diffraction in a D max-γA model (Japan Rigaku)X-ray with Ni-filtered CuK R radiation (λ)0.1541784nm).The morphology of the samples was observed by transmission electron microscopy (TEM)measurements,which were performed on a Hitachi Model H-800transmission electron microscope using an accelerating voltage of 200kV with a tungsten filament.The microstructure of the Y 2O 3:Eu nanorods was analyzed by high-resolution transmission electron microscopy (HRTEM),which was performed with a Philips Tecnai 20v-Twin transmission electron microscope using an accelerating voltage of 200kV.Samples for the TEM and HRTEM were prepared by ultrasonically dispersing the as-synthesized products into absolute ethanol,placing a drop of this suspension onto a copper grid coated with an amorphous carbon film,and drying it in air.The room-temperature emission spectra were obtained on a spectrophotometer (model Edinburgh FLS920).
Figure 1shows a typical XRD pattern of the Y 2O 3:Eu samples at different temperatures.All of the peaks can be indexed to the pure cubic phase of Y 2O 3with a measured lattice constant of a 0)1.0602nm (JCPDS 88-1040).No purity assessment can be determined from the XRD analysis in view of the technique’s detection limit.According to X-ray diffraction data,there are no differences in the peak positions observed between the two samples except for the latter having higher intensity.This indicates that the crystallite size of the Y 2O 3increases at higher reaction temperatures.Yttrium oxide,as well as the entire rare earth sesquioxide series,presents polymorphic forms,denoted as A,B,and C,and classified as being hexagonal,monoclinic,and cubic,respectively.The C-form structure is of the cubic bixbyite type,related to a doubled-edge fluorite in a regular way.For Y 2O 3,it is the low-temperature form at ordinary pressures.20Therefore,a pure Y 2O 3phase was apparently obtained using this technique.Figure 2is the IR spectrum of the prepared product.The absorption band centered around 3443cm -1can be attributed to moisture absorbed on the surface of the sample.The peaks centered about 1539,1450,and 1396cm -1can be attributed to CO 32-in the bond-stretching mode.The strong metal -oxygen vibrations centered at 460and 560cm -1,in accord with earlier reports,are characteristic of the Y -O stretching frequencies.This result is in agreement with the above XRD result.
The morphology of the Y 2O 3nanobelt was characterized by transmission electron microscopy (TEM).As can be seen from Figure 3a,the product consists of a uniform nanobelt (over 90vol %),in addition to a small number of nanorods.The TEM images can provide more geometric detail on the Y 2O 3.As shown in panels b and c of Figure 3,most of the nanobelts are very thin,and each nanobelt has a uniform width and thickness along its entire length.The nanobelts have thicknesses of 10-20nm and typical lengths of several micrometers.The width-to-thickness ratio of the nanobelts is estimated to lie in the range of 4-10.It can be seen from the enlarged selected area image that most of the nanobelts have rectangularly shaped tips that are easily viewed at the open end.The tips of the nanobelts are a regular rectangular shape,different from the faceted tip morphology of other oxide nanobelts and nitride nanobelts.21,22
Detailed microstructure information of individual Y 2O 3nanobelts was further examined by HRTEM and nanobeam ED analysis.Figure 4a is a TEM image of a segment of such an individual nanobelt.As shown in Figure 4b,the ED pattern recorded with the incident electron beam perpendicular to the wide surface of an individual nanobelt can be indexed to the diffraction pattern of the [110]zone axis of cubic single-crystal yttrium.The selected-area electron diffraction (SAED)pattern (Figure 4b,c)reveals that the as-synthesized Y 2O 3nanobelts are structurally uniform,single crystalline,free of detectable defects and dislocations,and have a growth direction of [001].Diffraction patterns taken from different regions along the nanobelt axis show the same features,indicating the same crystal orientation along the entire length of the nanobelts.The high temperature and high-pressure field produced during this method provides a favorable environment for the
anisotropic
Figure 1.XRD patterns of the samples prepared by the mixed solvothermal method at different temperatures for 2h:(a)200,(b)220°
C.
Figure 2.IR spectrum of the as-prepared Y 2O 3
nanobelt.
Figure 3.(a)TEM image of the as-synthesized Y 2O 3product.(b)TEM images and magnification of a single Y 2O 3nanobelt.(c)Tip of a single
nanobelt.
Figure 4.(a)TEM image of a single nanobelt (b)SAED pattern reveals the [001]growth direction of the Y 2O 3nanobelt.(c)HRTEM image of the belt’s top surface,exhibiting marked interplanar d -spacing (1.03nm)corresponding to the {001}lattice planes of cubic Y 2O 3.
2194Crystal Growth &Design,Vol.6,No.10,2006Communications
growth of nanocrystals.For a better understanding of the synthesis of Y 2O 3nanobelts,we carried out a series of experiments employing different reaction conditions.We found that the pH of the starting reaction system plays a crucial role in the formation of Y 2O 3nanobelts and related nanostructures under ambient solvothermal conditions.Other research groups 23have found that the same phenomenon,i.e,pH of the mixture solution,affects the morphology of the synthesized product.The uniform Y 2O 3nanobelts with a large aspect ratio can be obtained only when the reaction system is adjusted so that the pH lies in the range 12-13.With decreasing pH,the tendency to grow along a certain direction is weakened to some extent,and the product obtained exhibits a lower aspect ratio and less uniform morphology,as shown in Figure 5.
A possible explanation of this result is provided as follows .In the case for which the pH was around 8-10in the precursor solution,the Y source was primarily in the form of [Y(C 2H 5O)6]3-,whereas the remaining Y source existed in the form of Y(OH)3precipitates.During the reaction process,large quantities of Y 2O 3nuclei were first formed because of the decomposition of part of the Y(OH)3precipitates.On the other hand,certain Y(OH)3precipitates transformed into the growth units of [Y(OH)6]3-under alkaline conditions.It is well-known that the polar growth of Y 2O 3crystal along the [001]direction proceeds through the adsorption of growth units of [Y(OH)6]3-onto (001)plane.However,in the present case,the negatively charged [Y(C 2H 5O)6]3-complexes preferably adsorbed on the (001)plane,thus repelling the growth units of [Y(OH)6]3-.Accordingly,the intrinsically anisotropic growth of Y 2O 3along the [001]direction was substantially suppressed.As a consequence,the flakelike Y 2O 3crystals were formed.
In addition,the reaction temperature,time,and pressure are important factors for the synthesis of yttrium nanobelts.When the reaction temperature is higher than 220°C with all other conditions remaining fixed,only microsized rods were obtained (as shown in Figure 6).Moreover,we found that the morphology of the as-prepared product depends strongly on the reaction time.After a
hydrothermal treatment for 1h,a typical TEM image (Figure 7a)reveals that the sample is composed of Y 2O 3nanobelts with lengths of several nm.The XRD results prove that this precipitate is a crystalline cubic structure.When the reaction time is increased to 2h,the belts grow to a diameter of 20-30nm and to lengths of up to about 2-3μm.After 3h of treatment,the widths of the nanobelts are invariable,but the lengths are increased to 5-6μm.All of the above studies show that growth kinetics affects the morphology of the yttrium product.We speculated that proper control of the reaction conditions may lead to the formation of yttrium nanobelts.
Until now,template-directed approaches based on the use of polymers,surfactants,or strong chelating ligands 24,25have proven to be particularly versatile in making confined structures and obtaining a controlled size and shape.However,there are also much simpler systems,where control over the dimensionality of the nanoparticles is achieved by the solvent.26In our previous section,it was proved that the ratio of Y(OH)3to [Y(C 2H 5O)6]3-substan-tially determines the morphology of Y 2O 3crystals.Concretely speaking,under the condition that the precursor solution contained a large quantity of Y(OH)3and a small quantity of [Y(C 2H 5O)6]3-,the anisotropic growth of Y 2O 3was favorable when the pH was greater than 10,because certain Y(OH)3precipitates transformed into the growth units of [Y(OH)6]3-under strong alkaline condi-tions.According to the “oriented attachment”view of crystal growth proposed by Penn and Banfield,27the impetus for the aggregation of nanoparticles is a group for the growth of ZnO nanorods in a solvothermal colloid system.They also found that the nanoparticles served as “seeds”for the growth of the nanobelt and that the growth is increased by higher monomer concentration,which is coincident with our observation during the growth of the Y 2O 3nanobelt.We think that in this case,the growth of the Y 2O 3nanobelt is also based on an aggregation mechanism;here,[Y(OH)6]3-is the growth unit.
As is well-known,the (111),(100),and (110)surfaces of the face-centered cubic structured particles are different,not only in their surface atom densities but also in their electronic structure,bonding,and possibly chemical reactivities.28The surface free energy of the crystallographic planes descends in the order γ(100)>γ(110)>γ(111).It may thus be inferred that the activation energy of the (100)facet should be lower than those of the (110)and (111)facets,resulting in the bonding ability and chemical reactivity of the (100)facet being greater than those of the other two.Thus preference growth of Y 2O 3crystal along the [001]direction proceeds through the adsorption of growth units of [Y(OH)6]3-onto (001)plane.The higher monomer concentration in the reaction solution leads to different growth rates for different facets.29At the same time,in the strong alkaline solution (pH 13)contained a large quantity of [Y(OH)6]3-to meet the rapid growth of the (001)direction.The limited amount of monomers maintained by diffusion is consumed by the quick growth of the predominant facet.The velocity of crystal growth should be faster because of the higher monomer concentration,and thus nanobelts with smaller diameter and higher aspect ratio can be formed.Our present understanding of the formation mechanism of yttrium nanobelts is still limited.More in-depth studies are in progress.
Figure 8.shows the emission spectrum of a (Y 0.95Eu 0.05)2O 3nanobelt measured at room temperature.Because of the
shielding
Figure 5.TEM images of the as-prepared product under various pH conditions:(a)pH 8,(b)pH
10.
Figure 6.TEM of microsized
rods.
Figure 7.TEM images of Y 2O 3nanobelts obtained after processing for (a)1,(b)2,and (b)3h.
Communications Crystal Growth &Design,Vol.6,No.10,20062195
effect of the 4f electrons by 5s and 5p electrons in the outer shells of the europium ion,narrow emission peaks are expected,consistent with the sharp peak observed.The intensity is comparable with that of ytttria-based crystalline and lamellar nanostructures syn-thesized by Pinna et al.26This spectrum comprises a series of resolved features at 586,592,598,and 610nm,which are assigned to the 5D 0f 7F J ,J )1,2,3,transitions,respectively.The emission lines reveal much about the local environment of the Eu 3+ion.The emission band at 598nm,which corresponds to the 5D 0f 7F 1transition,is a magnetic dipole transition and hardly varies with crystal field strength around Eu 3+.However,the hypersensitive transition 5D 0f 7F 2at 610nm is electronic dipole allowed.Consequently,it depends on the local electric field and,hence,local symmetry.It is well-established that there are two Y 3+sites in cubic Y 2O 3;75%of these sites are noncentrosymmetric with C 2sym-metry,and the remaining 25%are centrosymmetric,having S 6symmetry.30When the Eu 3+ion is located at a low-symmetry local site without an inversion center,this forced-electric dipole transition is often dominant in the emission spectrum.So the strongest 5D 0f 7F 2
transition (610nm)and nearly all of other features in the spectrum are due to the Eu 3+on a C 2site,except for the three other 5D 0f 7F 1(586,592,598nm)transition lines,which are expected to arise from both Eu 3+C 2and S 6sites.
In conclusion,pure cubic yttrium nanobelts were synthesized by a convenient solvothermal synthetic technique.It was demon-strated that the size and morphology of the synthesized product can be controlled by controlling the pH of the starting solution,and the phase of the nanobelts can be controlled by controlling the process temperature.The as-synthesized nanobelts typically have an average thickness of ca.10nm,width of 40-100nm,width-to-thickness ratio in the range of 4-10,and length of up to several micrometers.The photoluminescence results indicate that the Y 2O 3:Eu nanobelts have a strong red 5D 0f 7F 2transition.On the basis of this method,other kinds of rare earth compounds and composite nanosized structures show promise for easy synthesis.
References
(1)Iijima,S.Nature (London)1991,354,56.
(2)Holmes,J.D.;Zeigler,K.J.;Doty,R.C.;Pell,L.E.;Johnston,K.
P.;Korgel,B.A.J.Am.Chem.Soc.2001,123,3743.
(3)Bhargava,R.N.;Gallagher,D.;Hong,X.;Nurmikko,A.Phys.Re V .
Lett .1994,72,416.
(4)Goldburt,E.T.;Kulkarni,B.;Bhargave,R.N.;Taylor,J.;Libera,
M.J.Lumin.1997,72-74,190.
(5)Williams,D.K.;Bihari,B.;Tissue,B.M.;Mchale,J.M.J.Phys.
Chem.B 1998,102,917.
(6)Capobianco,J.A.;Vetrone,F.;Boyer,J.C.J.Phys.Chem.B 2002,
106,1181.
(7)Nelson,J.A.;Brant,E.L.;Wagner,M.J.Chem.Mater.2003,15,
688.
(8)Wakefield,G.;Holland,E.;Dobson,P.J.;Hutchison,J.L.Ad V .
Mater.2001,13,1157.
(9)Capobianco,J.A.;Vetrone,F.;Boyer,J.C.J.Phys.Chem.B 2002,
106,1181.
(10)Song,H.W.;Chen,B.;Peng,H.S.;Zhang,J.S.Appl.Phys.Lett.
2002,81(10),776.
(11)Yada,M.;Taniguchi,C.;Torikai,T.;Watari,T.;Faruta,S.;Katsuki,
H.Ad V .Mater.2004,16,1448.
(12)Yada,M.;Mihara,M.;Mouri,S.;Kuroki,M.;Kijima,T.Ad V .Mater.
2002,14(4)309.
(13)Yada,M.;Ichinose,A.;Machida,M.;Kijima,T.Angew.Chem.,Int.
Ed.1999,38,3506.
(14)Wang,X.;Sun,X.;Yu,D.;Zou,B.;Li,Y.D.Ad V .Mater.2003,15
(17),1442.
(15)Fang,Y.P.;Xu,A.W.;You,L.P.;Song,R.Q.;Yu,J.C.;Zhang,
H.X.;Li,Q.;Liu,H.Q.Ad V .Funct.Mater.2003,13(12),955.(16)Wang,X.;Li,Y.D.Angew.Chem.,Int.Ed.2002,41,4790.
(17)Nelson,J.A.;Brant,E.L.;Wagner,M.J.Chem.Mater.2003,15,
688.
(18)Kang,Y.C.;Roh,H.S.;Park,S.B.Ad V .Mater.2000,12(6),451.(19)Wu,C.;Qin,W.;Qin,G.;Zhao,D.;Zhang,J.;Huang,S.Appl.Phys.
Lett.2003,82,520.
(20)Haire,R.G.;Eyring,https://www.doczj.com/doc/4d1568942.html,parasions of Binary Oxides.In Handbook
on the Physics and Chemistry of Rare Earths ;Gschneidner,K.A.,Jr.,Eyring,L.,Choppin,G.R.,Lander,G.R.,Eds.;Elsevier Science BV:Amseterdam,The Netherlands,1994;Vol.18Lanthanides/Actinides Chemistry,p 413.
(21)Wu,Q.;Hu,Z.;Wang,X.Z.;Chen,Y.J.Phys.Chem.B 2003,107,
9726.
(22)Pan,Z.W.;Dai,Z.R.;Wang,Z.L.Science 2001,291,1947.(23)(a)Caswell,K.K.;Bender,C.W.;Murphy,C.J.Nano Lett.2003,3,
667.(b)Zhang,H.;Yang,D.;Li,D.;Ma,X.;Li,S.;Que,D.Cryst.Growth Des.2005,5,547.
(24)(a)Pinna,N.;Wild,U.;Urban,J.;Schlogl,R.Ad V .Mater .2003,15,
329.(b)Xu,T.T.;Zheng,J.G.;Nicholls,A.W.;Stankovich,S.;Piner,R.D.;Ruoff,R.S.Nano Lett.2004,4,2051.
(25)(a)Leontidis,E.;Orphanou,M.;Leodidou,T.K.;Krumeich,F.;
Caseri,W.Nano Lett.2003,3,569.(b)Xu,G.;Rong,Z.H.;Du,P.Y.;Weng,W.J.;Shen,G.;Han,G.R.Ad V .Mater.2005,17,907.(c)Lemyre,J.L.;Ritcey,A.M.Chem.Mater .2005,17,3040.(26)(a)Pinna,N.;Garnweitner,G.;Beato,P.;Niederberger,M.;Antoni-etti,M.Small 2005,1,112.(b)Pinna,N.;Neri,G.;Antonietti,M.;Niederberger,M.Angew.Chem .2004,116,4445;Angew.Chem.,Int.Ed.2004,43,4345.
(27)Banfield,J.F.;Welch,S.A.;Zhang,H.;Ebert,T.T.;Penn,R.I.
Science 2000,289,751.
(28)Wang,Z.L.J.Phys.Chem.B 2000,104,1153.
(29)(a)Peng,X.G.;Wickham,J.;Alivisatos,A.P.J.Am.Chem.Soc.
1998,120,5343.(b)Leontidis,F.;Kleitou,K.;Kypianidou-Ieodidou,T.;Bekiari,V.;Lianos,https://www.doczj.com/doc/4d1568942.html,ngmuir 2002,18,3659.
(30)(a)Silver,J.;Martunez,M.I.;Ireland,T.G.;Withnall,R.J.J.Phys.
Chem.B 2001,105,7200.(b)Maitric,M.;Antic,B.;Balanda,M.;Rodic,D.;Napijalo,M.L.J.Phys.:Condens.Matter 1997,9,4103.
CG0600400
Figure 8.Emission spectrum of Y 2O 3:Eu nanobelts.
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