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A route to obtain Gd2O3:Nd3+ with different particle size

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A route to obtain Gd2O3:Nd3+ with different particle size
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  Materials Chemistry and Physics 127 (2011) 40–44 Contents lists available at ScienceDirect MaterialsChemistryandPhysics  journal homepage: www.elsevier.com/locate/matchemphys A route to obtain Gd 2 O 3 :Nd 3+ with different particle size  J.L. Ferrari a , b , ∗ , R.L.T. Parreira b , A.M. Pires c , S.A.M. Lima a , c , M.R. Davolos a a Instituto de Química, UNESP, P.O. Box 355, 14800-970 Araraquara, SP, Brazil b Departamento de Química da Faculdade de Filosofia Ciências e Letras de Ribeirão Preto – Universidade de São Paulo – USP Brazil c Faculdade de Ciência e Tecnologia, Departamento de Física, Química e Biologia, UNESP, 19060-900 Presidente Prudente, SP, Brazil a r t i c l e i n f o  Article history: Received 15 July 2010Received in revised form23 November 2010Accepted 29 November 2010 Keywords: A. Optical materialB. EtchingC. Inorganic compounds a b s t r a c t Thisworkreportsthechemicaletchingeffectofethylenediaminetetraaceticacid(H 4 EDTA)ontheparticlesize of Gd 2 O 3 :Nd 3+ phosphor obtained from the thermal decomposition of Gd 2 (OH) 2 (CO 3 ) · n H 2 O:Nd 3+ .Thermogravimetric behavior of the precursor Gd 2 (OH) 2 (CO 3 ) · n H 2 O:Nd 3+ (2at%) is presented and dis-cussed.StructuralandspectroscopicinvestigationoftheGd 2 O 3 :Nd 3+ powdersrevealedthatthisphosphordisplays a band emission in the infrared region, which is an important feature for photonic applications.The treatment with different EDTA concentrations results in different Gd 2 O 3 :Nd 3+ particle size. ThroughSEM analysis, we have demonstrated that the particle size of Gd 2 O 3 :Nd 3+ decreases with increasingamount of EDTA. Hereby, we report a chemical etching method as a route to obtain particle size controlwhich is an important requirement to improve phosphor properties and its applicability. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Many non-emissive rare earth (RE) oxides have been reportedin the literature as excellent host lattices for trivalent RE activatorions, for instance Y  2 O 3 :Eu 3+ [1–3], Gd 2 O 3 :Eu 3+ [4], Gd 2 O 3 :Er 3+ [5],Gd 2 O 3 :Pr 3+ [6] and Gd 2 O 3 :Nd 3+ [7,8], among others. These oxidesare excellent host matrices for activator ions due to their optical,mechanical,andchemicalpropertiesaswellas,lowphononenergy.Besides,thesimilaritiesconcerningradii,coordinationnumberandoxidation state among the RE ions allow the activator ions to sub-stitute the host ions with small changes in the lattice parameters.The energy value of non-radiactive losses for Y  2 O 3  and Gd 2 O 3  are409.5cm − 1 (24,420nm) and 419.9cm − 1 (23,815nm), respectively[7]; therefore, these oxides have been shown to be suitable as REhost lattice that emit in the infrared region. In particular, Gd 2 O 3 is a versatile material with high application potential in severaltechnological fields (e.g., photonics), due to its transparency fromvisible to near-infrared region. When doped with active RE ions,gadolinium oxide displays good luminescent properties. Conse-quently, this oxide has received considerable attention for laserapplications [7,9].Optical features associated with particle size can contribute todifferent desirable properties [10,11]. Therefore, several ways to ∗ Correspondingauthorat:SãoPauloUniversity–USP,DepartmentofChemistry,Faculdade de Filosofia, Ciências e Letras de Ribeirão Preto, Av. Bandeirantes, 3900,14040-901 Ribeirão Preto, SP, Brazil. Tel.: +55 16 36024850. E-mail addresses:  jeffersonferrari@gmail.com,ferrari@pg.ffclrp.usp.br (J.L. Ferrari). controlparticlesizeinpowderoxidematerialshavebeenreportedintheliteratureinordertoachievetheirbestperformance[12–17].Theabilitytocontroltheparticlesizeisveryimportantforapplica-tionssuchascatalystandoptical,mechanical,orelectronicdevices.Among all the preparation techniques reported in the litera-ture,solidstatereaction[18,19],sol–gelmethods[20–23],chemical vapor deposition (CVD) [24], co-precipitation method [25], polyol method [26], are the most frequently used. Among many pow- der synthesis techniques, the chemical solution technique has theadvantageofgivingfinesizedparticleswithhomogeneityatatomiclevel by controlling pH, temperature and concentration of met-als. In particular, the homogeneous precipitation technique hasbeen widely used because one can easily control the particle sizeand readily prepare mono-dispersed powders. In this sense theaim of this work is firstly to report on the preparation and thethermogravimetric study of the Gd 2 (OH) 2 (CO 3 ) · n H 2 O:Nd 3+ (2at%)precursor. Secondly, the control of the Gd 2 O 3 :Nd 3+ (2at%) particlesize by the etching action of EDTA solution over the surface of thepowder is investigated. 2. Experimental  2.1. Synthesis process The precursor Gd 2 (OH) 2 (CO 3 ) · n H 2 O:Nd 3+ (2at%) was prepared by using thehomogeneous precipitation technique according to the methodology reported byPires et al. [14]. Therefore, in order to provide such precursor, gadolinium and neodymium commercial oxides (Aldrich – 99.99%) were used as starting materialsandtheotherchemicalswereanalyticalgradereagents.Monodispersedgadoliniumbasic carbonate doped with Nd 3+ (2at%) was produced by heating cation chloridesandureasolutions,[urea]/[REGd 3+ ]=83ratio;thepHwasadjustedupto5bydrop-wise addition of an NH 4 OH 10% solution. Then the solution was heated to 80 ◦ C0254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2010.11.055   J.L. Ferrari et al. / Materials Chemistry and Physics 127 (2011) 40–44  41 Fig. 1.  SEMmicrographofGd 2 (OH) 2 (CO 3 ) · n H 2 O:Nd 3+ (2at%)monodispersedparti-cles.under continuous stirring. After 3h of reaction, the suspension was left to rest forparticles decantation and the solid was filtered and washed with deionized watertoeliminateimpurities.Theprecursorobtainedwasdriedinadesiccatorunderlowpressure for 12h and then analyzed by thermogravimetric (TG) analysis. Based ontheTGcurves,theprecursorwasheattreatedat750 ◦ Cfor4hinordertoobtaintheGd 2 O 3 :Nd 3+ (2at%)phosphor.ThestructureoftheobtainedoxidewasevaluatedbyX-raydiffraction(XRD).Thephotoluminescence(PL)emissionattheinfraredregionwas evaluated under 514nm excitation.Finally, the Gd 2 O 3 :Nd 3+ powder was divided into seven parts and each one wassuspendedindeionizedwaterandacetatebuffersolutionatpH4.AtthispHtherareearthmetalsdonothydrolyze,andEDTAisdeprotonatedassumingtheH 2 Y  2 − form[27]forabettercomplexationwithmetal.Toeachportionanamountof0.01molL  − 1 EDTA solution, was added considering 10, 20, 30, 40, 50, 60 and 70at% as a func-tion of the RE 3+ ions present in 50mL of final volume. These solutions were keptunderstirringfor15mininordertoobtainacolloidalsuspension,andtoavoidpar-ticle agglomeration. It is important to point out that, the colloidal suspension waskept stable for many days at room temperature. The particles were isolated by cen-trifugation, the follow re-suspended in ethanol, and analyzed by scanning electronmicroscopy (SEM).  2.2. Experimental apparatus TheTGanalysiswasperformedinaTAinstrumentsmodelSDT2960thermobal-ance. The XRD pattern were obtained by using a SIEMENS D5000 diffractometer,CuK   radiation,  =1.5418 ˚A,graphitemonochromator,0.03 ◦ asincrementandinte-grationtimeof2s,15–70 ◦ 2   range.ThephotoluminescencespectrumwasrecordedonaFluorologSPEXF2121/Jobin-YvonspectrofluorometercoupledwithaGedetec-tor cooled with liquid nitrogen. The excitation source was an Innova Ar + laser with113mWoperatingat514nm.AZEISSDSM940Ascanningelectronmicroscopewasused for imaging the samples as well as to determine the average particle size andthe size distribution. 3. Results and discussions Fig. 1 shows the SEM image of the precursorGd 2 (OH) 2 (CO 3 ) · n H 2 O:Nd 3+ , from where one can observe itsuniformity with an average diameter of about 150nm. This behav-ior illustrates the fact that from this route it is possible to obtainparticles with controlled size. The particles morphology dependson a number of parameters such as the pH, the temperature, theatomic ratio between urea and metals, and also the aging time.Fig. 2 shows the TG analysis of the precursor. Three decom-position steps are observed. From the study on the thermaldecomposition of lanthanides basic carbonates conducted by Fig. 2.  TG analysis of Gd 2 (OH) 2 (CO 3 ) · n H 2 O:Nd 3+ (2at%). D’Assunc¸ão et al. [28], we suggest the following reaction steps: Gd 2 (OH) 2 (CO 3 ) · H 2 O : Nd 3 + 2 at% (s)  = 240  ◦ C −→ Gd 2 O 2 CO 3  : Nd 3 + 2 at% (s) + CO 2  (g) + 3H 2 O (g) (1)Gd 2 O 2 CO 3  : Nd 3 + 2 at% (s)  = 388  ◦ C −→ Gd 2 O (2 −  x ) CO 3(1 −  x )  : Nd 3 + 2 at% (s) +  x CO 2  (g) (2)Gd 2 O (2 −  x ) CO 3(1 −  x )  : Nd 3 + 2 at% (s)  = 581  ◦ C −→ Gd 2 O 3  : Nd 3 + 2 at% (s) + (1 −  x )CO 2  (g) (3)The observed mass loss is about 33% for the thermal decom-positionofGd 2 (OH) 2 (CO 3 ) · n H 2 O:Nd 3+ (2at%)intotheGd 2 O 3 :Nd 3+ (2at%) product. The oxide formation is a consequence of the highthermodynamic affinity of rare earth elements for oxygen and thestability of their trivalent oxidation state. It is important to pointoutthatat750 ◦ Callcarbonateiseliminated.Becauseofthis,750 ◦ Cwas the temperature chosen for heat treating the precursor.In order to confirm the conversion from Nd 3+ -doped basic car-bonate to Nd 3+ -oxide, the product was analyzed by XRD after heattreatmentat750 ◦ Cfor4h.Fig.3showsthediffractionpatternthat Fig. 3.  X-ray diffraction pattern of Gd 2 O 3 :Nd 3+ (2at%) obtained from heat treat-ment of the Gd 2 (OH) 2 (CO 3 ) · n H 2 O:Nd 3+ (2at%) precursor at 750 ◦ C for 4h in staticair atmosphere.  42  J.L. Ferrari et al. / Materials Chemistry and Physics 127 (2011) 40–44 Fig.4.  PhotoluminescenceemissionspectraofGd 2 O 3 :Nd 3+ (2at%)underexcitationat 514nm using Ar + laser. Fig. 5.  Nd 3+ ion energy levels. Fig. 6.  SEM micrographs of Gd 2 O 3 :Nd 3+ (2at%) after chemical etching with (A) 10, (B) 20, (C) 30, (D) 40, (E) 50, (F) 60 and (G) 70at% of EDTA.   J.L. Ferrari et al. / Materials Chemistry and Physics 127 (2011) 40–44  43 Fig. 7.  Histograms of Gd 2 O 3 :Nd 3+ (2at%) after chemical etching with (A) 10, (B) 20, (C) 30, (D) 40, (E) 50, (F) 60 and (G) 70at% of EDTA. corresponds to the Gd 2 O 3  cubic phase, space group Ia3, in accor-dance to JCPDS No. 01-086-2477 [29]. The four most intense peaks relatedtoGd 2 O 3  phaseat2   :28.54 ◦ ,33.10 ◦ ,47.51 ◦ ,and56.33 ◦ areassignedtothe(222),(211),(400)and(321)planes,respectively.Any other reflection peak different from the cubic gadoliniumoxide pattern was not detected in the diffractogram. Consideringthe limit of the technique, this fact is a strong indication that nootherphasewasformed.Therefore,weconcludethattheNd 3+ ionsare homogeneously distributed into the Gd 2 O 3  host matrix with-outdetectablephasesegregation.Thiswasexpectedsincetheionicradiioftheseionsaresimilar(Gd 3+ =1.20 ˚AandNd 3+ =1.26 ˚A)[30],and they have the same oxidation state (RE 3+ ) and coordinationnumber(eightand/orsix).Itcanalsobeseenthatthediffractogramdisplaysintenseandnarrowpeaksthatawell-crystallizedmaterial.Ferrari et al. [3] have reported that the Y  2 O 3  host lattice accom-modates up to 7at% of Eu 3+ ions in the cubic structure, withoutthe detection of any other phase; the luminescence quenching isobserved when the percentage of Eu 3+ ions is higher than 5at%.Fig. 4 depicts the emission spectrum of the phosphorGd 2 O 3 :Nd 3+ (2at%) before EDTA treatment. The emission detectedbetween 1300 and 1500nm is assigned to internal levels of Nd 3+ ion located in the Gd 2 O 3  host lattice that corresponds to the 4 F 3/2  → 4 I 13/2  and  4 F 7/2  → 4 I 15/2  transitions. The energy levels dia-gram of Nd 3+ ion is presented in Fig. 5. Although it was not attempted in this work, the broadening of the emission band maybedirectlyassociatedwiththeparticlesize.Theluminescencespec-tra of Gd 2 O 3 :Nd 3+ (2at%) after different EDTA treatments are stillinprogresstostudytheparticlesizeeffectonthespectrashapeandintensity.The phosphor powder was then divided into seven portionsand each one was treated with different concentration of EDTA.The amount of EDTA ranged from 10 to 70at% in ratio to the total Fig. 8.  Particle size as a function of EDTA percentage.  44  J.L. Ferrari et al. / Materials Chemistry and Physics 127 (2011) 40–44 Fig. 9.  Illustration of the mechanism involved in the EDTA chemical attack on theGd 2 O 3 :Nd 3+ (2at%) particles surface. amount of RE 3+ . Fig. 6 shows SEM images of the Gd 2 O 3 :Nd 3+ pow-deraftertheEDTAchemicaletchingatdifferentratios.ItisobservedthatparticlesdonotchangeshapeaftertheEDTAetching;however,in some cases, an agglomeration is observed. This particle agglom-eration suggests that an initial densification process between theparticles has started during the decomposition process from theprecursor to Gd 2 O 3 :Nd 3+ . From the analysis of SEM images, sta-tistical histograms of the nanoparticle size average diameter wereestimated and are shown in Fig. 7. Another effect observed is that the Gd 2 O 3 :Nd 3+ particle size decreases as the amount of EDTAincreases from 10at% to 60at%. By increasing the EDTA amount upto 70at%, a saturation effect is observed whereas the Gd 2 O 3 :Nd 3+ particles size remains practically constant. From these observa-tions, we believe that the chemical etching effect that the EDTAchelatingagentpromotesonthesurfaceofGd 2 O 3 :Nd 3+ changestheparticle size by dissolving the surface. The metal ions localized ontheparticlesurfacemaybecomplexedbytheEDTAligandandthenremoved from the surface with consequent particle size decreas-ing. In order to summarize the EDTA influences on the phosphorparticle size, a graphic relating EDTA percentage and Gd 2 O 3 :Nd 3+ particlesize,wasbuilt(Fig.8),basedonthemeandiametersizeval- ues estimated from the histograms displayed in Fig. 7. In general, EDTA is most likely aggressive to rare earth oxides surface. More-over, the EDTA concentration is critical and an important fact to beavoided is an undesirable degradation on the material surface.In Fig. 9 a possible mechanism that could explain particle sizedecreasing is suggested whereas it is shown the representation of EDTA chelating effect to the surface metal ions. 4. Conclusions From all results it was possible to demonstrate that the precur-sor Gd 2 (OH) 2 (CO 3 ) · n H 2 O:Nd 3+ (2at%) was successfully preparedbythehomogeneousprecipitationmethodyieldingmonodispersedparticles.Theheattreatmentat750 ◦ Cfor4hofthenanosizedpre-cursor produced the Gd 2 O 3 :Nd 3+ (2at%) phosphor powder withluminescent properties in the infrared region. The chemical etch-ing of EDTA on the Gd 2 O 3 :Nd 3+ (2at%) powder results in particlediameter size decreasing. In this sense the method reported inthis article is able to provide a particle size control, emphasizingthat the phosphor particle size is directly dependent on the EDTApercentage. Based on these promising results, a further investiga-tion on the emission band in the infrared region and some effectsto avoid the densification process need to be carried on in thefuture.  Acknowledgments The authors acknowledge FAPESP and CNPq agencies for thefinancial support and AUDAX for the help in building the EDTAattach figure. Ferrari J.L., thanks FAPESP for the scholarship(PROC. No. 03/10195-1), Jennifer Esbenshade for English revi-sions, and the Laboratory of Photonic Materials (Prof. Dr. S.J.L.Ribeiro and Prof. Dr. Y. Messaddeq) for use of laser as excitationsource. References [1] G. Blasse, B.C. Grabmaier, Luminescent Materials, Springer, Berlin, 1994.[2] T. Justel, H. Nikol, C. Ronda, Angew. Chem. Int. Ed. 37 (1998) 3085.[3] J.L. Ferrari, A.M. Pires, M.R. Davolos, J. Mater. Chem. Phys. 113 (1999) 587.[4] A.M. Pires, M.F. Santos, M.R. Davolos, E.B. Stucchi, J. Alloys Compd. 344 (2002)276.[5] H. Guo, Y. Li, D. Wang, W. Zhang, M. Yin, L. Lou, S. Xia, J. 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