Structural and Photoluminescent Properties of Zn2SiO4:Mn2+ Nanoparticles Prepared by a Protected Annealing Process

11 Pages·2011·3.244 MB·English

by  Caroline Bertail, Sebastien Maron, Valerie Bruissette, Thierry Le Mercier, Thierry Gacoin, Jean-Pierre Boilot

ARTICLE
pubs.acs.org/cm
Structural and Photoluminescent Properties of Zn SiO :Mn2þ
2 4
Nanoparticles Prepared by a Protected Annealing Process
Caroline Bertail,†,‡ S(cid:1)ebastien Maron,† Val(cid:1)erie Buissette,‡ Thierry Le Mercier,‡ Thierry Gacoin,*,†
and Jean-Pierre Boilot†
†GroupedeChimieduSolide,LaboratoiredePhysiquedelaMati(cid:3)ereCondens(cid:1)ee,CNRSUMR7643,E(cid:1)colePolytechnique,
91128,Palaiseau,France
‡CentredeRecherchesd’Aubervilliers,Rhodia,52,ruedelaHaieCoq,93308,Aubervilliers,France
b
S SupportingInformation
ABSTRACT: This works presents a new method for the production of colloidal
suspensionsofZn SiO :Mn(ZSM)luminescentnanoparticles.Theprocessconsistsin
2 4
achieving a high temperature solid state reaction among silica nanoparticles homoge-
nouslydispersedwithinaZnOmatrixinlargeexcess.Highlycrystalline,welldispersed
nanoparticlesarerecoveredasacolloidalsuspensionafterdissolutionoftheZnOmatrix
inslightlyacidicwater.Yellowemittingβ-Zn SiO :MnorgreenemittingR-Zn SiO :Mn
2 4 2 4
areobtaineddependingonthereactiontemperature(either800or900(cid:1)C)andtheMn
content.Zn2(cid:2)2xSiO4:Mnxparticleswithxupto3%canbeobtainedwithamaximum
relativeemissionefficiencyofabout16%ascomparedtoabulkZSMphosphorreference.
The principle of this new synthetic route opens a new field of investigation for the
productionofcolloidaldispersionofrefractorycompoundswithahighcrystallinity.
KEYWORDS:willemite,luminescence,greenphosphor,nanomaterial,colloid,nanoparticles
’INTRODUCTION organometallic precursor routes.13 All these methods lead to
ZSMpowderswithsubmicrometricgrainsizes.
Willemite(Zn SiO )isawell-knownhostmaterialforappli-
cationsinthefield2 ofp4hosphorsforlightemittingdevices.1Red, Theultimatechallengefornanocrystallinematerialssynthesis
blue,andgreencolorsmaybeobtainedthroughdopingwithEu3þ, is the elaboration of individualized particles with a controlled
size.Thiswouldallowthefurtherelaborationofmaterialswitha
Ce3þ,andMn2þions,respectively.2Mn(II)-dopedZn SiO green
2 4 perfectly controlled microstructure, and opens the way toward
phosphor,oftenreferredtoasZSM,isofparticularinterestbecause
neworiginalapplicationssuchasbiologicallabelsortransparent
of its high luminescence efficiency, its high photostability
light emitting devices. Many examples such as YVO :Eu,14
(especially under VUV excitation), and the absence of moisture 4
LaPO :Ce,Tb16 or YAG:Ce17 are now well-known for the
sensitivity.Allthismakeitcommonlyusedinfluorescentlamps, 4
elaborationofwelldispersedoxidephosphornanoparticles,most
and in various display devices (field emission displays (FED),
of them using colloid chemistry routes. Concerning ZSM,
vacuum fluorescent displays (VFD), and plasma display panels
hydrothermal18(cid:2)20 and solvothermal21(cid:2)24 synthesis have been
(PDP)3(cid:2)5).
reported,leadingtomoreorlessdispersednanoparticles.
Asformanyphosphormaterials,synthesisofZSMisusually
Our group has been involved since many years on the
achieved through conventional solid state reaction at high
elaborationofcolloidaldispersionsofphosphormaterials,espe-
temperature.4 The optimized product in term of chemical
cially rare-earth doped LaPO and YVO obtained through
composition, oxidation state of the dopant, and crystallinity is 4 4
simple reactions of precipitation in water.14,15 As reported by
commerciallyavailableforindustrialapplications.Improvement
manyothergroupsondifferentphosphorcompounds,investiga-
oftheZSMsynthesisnowconcernsabettercontrolofthegrains'
tionoftheemissionpropertiesoftheobtainedparticlesusually
morphology and agglomeration state. These parameters are
showalteredefficienciesascomparedtorelatedbulkreference.
indeed poorly controlled in conventional solid state synthesis
Werecentlyreportedthat,inthecaseofYVO :Euparticlesinthe
becausethecalcinationtemperaturesrequiredtohaveaperfect 4
size range of about 30(cid:2)40 nm, the main limitation of the
homogeneityofthechemicalcompositionleadtotheconcomitant
emissiondoesnotarisefromsurfacequenchingbutfromdefects
growthandagglomerationofthephosphorgrainsthroughsinter-
inthevolumeoftheparticlesorstructuraldisorderresultingfrom
ing.Moststudiesrelyontheintimatemixingoftheprecursorsprior thelowtemperatureofsynthesis.14Thisresultwasdemonstrated
to their calcinations to decrease the required temperature of
calcination.Thiscanbeachievedstartingfrommolecularprecur- Received: February25,2011
sors or salts using spray pyrolysis,6 sol(cid:2)gel process,7(cid:2)9 meso- Revised: April16,2011
porous template routes,10,11 solution combustion method,12 or Published: May17,2011
r2011AmericanChemicalSociety 2961 dx.doi.org/10.1021/cm2005902|Chem.Mater.2011,23,2961–2967
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Figure1. Generalschemeofthesyntheticrouteinvestigatedinthiswork.(a)Solutioncontainingthezincsaltandsilicaparticles,(b)precipitationof
Zn(OH) byadditionofabase,(c)washingandadditionoftheMn2þsalt,(d)dryingandrecoveryasapowder,(e)calcinationleadingtotheformation
2
oftheZSMnanoparticles,and(f)selectivedissolutionoftheZnOmatrix,washing,andrecoveryoftheparticlesasacolloidaldispersion.
consideringthedrasticimprovementofemissionyieldobserved by sonication and washed with distilled water by centrifugation
by achieving a so-called “protected annealing” treatment at (11363g(cid:2)1h).Thewhiteprecipitatewasthendispersedin150mL
1000 (cid:1)C.25 This annealing process was achieved on particles of an aqueous solution of MnCl234H2O (98þ% ACS, Aldrich) and
dispersed in a silica matrix to prevent particles growth and homogenizedbysonification.Awhitepowderwasfinallyobtainedby
sintering, and allowing their recovery as a colloidal suspension evaporationofwaterundermildconditions(40(cid:1)C,30Torr).
afterdissolutionofthematrix. Thermal Treatment for Solid State Reaction (Step d of
The mainidea ofthepresentworkistoextend thisstrategy Figure 1). The powder was put into a tubular furnace and heated
and develop a new synthetic procedure for the preparation of between810and915(cid:1)Cfor4hunderargonatmosphere.
highly crystalline Zn SiO :Mn2þ colloidal nanoparticles ob- MatrixDissolutionandZSMColloidalSuspensionRecov-
2 4
tainedafterathermaltreatmentathightemperature.Wefocused ery(StepeofFigure2).Thecalcinedpowder,amixtureofZnOand
ourattentiononthedevelopmentofaprocessthatreliesonthe Zn2SiO4,waswashedwitha5mol3L(cid:2)1solutionofaceticacid(glacial,
insitugenerationofnanoparticleswithinaZnOmatrixthrough CarloErba)indimethylformamidecontaining5%volH2O.Theamount
solidstatereactionbetweenprecursorsathightemperature.The ofaceticacidusedwastakenas12molequivascomparedtoZnO.After
generalprocessisshownonFigure1. 10minunderstirringat60(cid:1)C,thesuspensionwassonicatedfor2min
TheZnOmatrixaimstolimitparticlessizeandtopreservethe andcentrifuged(11363g(cid:2)1h).Theprecipitatewasthenpurifiedby
nanoparticle dispersion during calcination. After its selective washingtwicewithDMF.Thewholeprocessisrepeatedasecondtime.
dissolution, non aggregated zinc silicate nanoparticles are ex- At theend of this step, the ZnO matrix had been dissolved and zinc
silicateparticleswererecoveredasasuspensionindimethylformamide.
pectedtoberecoveredanddispersedinanappropriatesolventto
Finally, ZSM nanoparticles were transferred into ethyleneglycol
get the colloidal suspension. ZnO was also chosen considering
(99.8þ%, Aldrich) by centrifugation/dispersion using sonication. A
thatitisaprecursoroftheZSMphaseandthatitcanbeeasily
dissolved using slightly acidic water. Moreover, the ZnO/SiO fewdropletsofanaqueoussodiumpoly(acrylicacid)solution(Mw=
2 1200, 45 wt % in water, Aldrich) were then added, and the pH was
phase diagram only has ZSM as a definite compound so that
adjusted to 9 with an aqueous solution of tetramethylammonium
stoichiometry in the presence of ZnO in large excess must be
hydroxide(1mol L(cid:2)1).
preserved.Smallsilicacolloidalparticleswereusedasprecursor 3
Characterizations. X-ray diffraction (XRD) studies were per-
ofthesilicatecomponent.
formed on nanocrystals powders using a Philip X-Pert diffractometer
Inthispaper,wepresentourresultsontheinvestigationofthe
with Cu KR radiation (λ = 1.54 Å). Dynamic light scattering experi-
proposedstrategyandontheeffectofseveralparametersamong
mentswerecarriedoutwithZetasizerNanoZSequipmentsuppliedby
which are the temperature of calcination and the Mn content.
Malvern.ThesynthesizedproductswerecharacterizedusingaHitachi
The obtained particles are fully characterized in terms of size,
FEGS4800scanningelectronmicroscope(SEM)andaPhilipsCM30
crystalline structure, and dispersion state. We show that the
transmission electron microscope (TEM) operating at 300 kV. The
temperature of calcination allows governing not only the size of
samplesforTEMwerepreparedbyfirstdepositingadropletofpolymer
theparticlesbutalsotheircrystallinephaseleadingtoparticleswith
(polyethyleneimine) and then a droplet of colloidal solution onto a
either green or yellow emission properties. All these results are
TEM grid. A Netzsch thermoanalyzer STA409 was used for simulta-
discussedinrelationwithamechanismfortheparticlesformation
neousthermalanalysiscombiningthermogravimetry(TG)anddiffer-
correspondingtotheinitialstrategythatwasproposed. ential thermal analysis(DTA) with aheating rateof10 (cid:1)Cmin(cid:2)1 in
argon.29SiMASNMRcharacterizationsofpowderswereachievedona
TecMagApollospectrometerwitha8.46Twide-boreOxfordmagnet.
’EXPERIMENTALSECTION
TheMASexperimentswereperformedwithaBr€ukerprobeequipped
Synthesis of Zn SiO :Mn2þ colloidal nanoparticles was achieved with 4mmZrO2rotors ataspinningrateof14kHz.The29SiMAS
2 4
followingtheprocessschematicallydescribedonFigure1. experiments were runwitharecycle timeof500s,90(cid:1)pulselengths
MixtureofPrecursorstotheSolidState(Stepsatodof (2.3ms),adwelltimeat20μs,adeadtimeat25μs,and680scansin
Figure1).A0.896mol L(cid:2)1solutionofZn(OAc) .2H O(98þ%ACS, eachexperiment.The29Sichemicalshiftswerereferencedtotetraethox-
3 2 2
Aldrich)indimethylformamide(DMF,99.9þ%,Aldrich)(10mL,8.96 ysilane(δ(Si)=(cid:2)82ppm).AllNMRexperimentswereconductedwith
mmol)wasmixedwith83μLofacolloidalsilicasol(NISSANMAST, non doped zinc silicate to avoid the effects of paramagnetic ion.
pH=2,diameter=15nm,S=263m2/g,[SiO2]=323g/Linmethanol) Luminescencespectraandlifetimemeasurementswererecordedona
undermagneticstirring.Theclearandcolorlessresultingmixturewas Hitachi F-4500 spectrofluorometer. Relative quantum yields were
subsequentlyslowlyaddedintoabasicaqueoussolutionoftetramethy- determined by comparing the integrated emission of as-synthesized
lammonium hydroxide (25 wt % in water, Sigma Aldrich) (25 mL, zincsilicatenanoparticleswiththeemissionunderthesameexcitation
0.5mol L(cid:2)1)undervigorousstirring,leadingtoawhiteprecipitate.ThepH wavelength ofamicrometer sizedzincsilicate obtainedbysolidstate
3
attheendoftheadditionisabout7.3.Theprecipitatewashomogenized reactionwithoutZnOinexcess.
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Figure2. SEMimageofZSMcolloidalparticlesobtainedintheabsence(a)orinthepresence(b)oftheZnOmatrixevidencingitsefficiencytolimitthe
sizeandthesinteringoftheparticles.TEMimageofthesameparticlesasinb(c).
Zincandmanganeseatomiccontentsinnanoparticleswerededuced wascircumventedbyachievingthedissolutionusingaceticacid
fromchemicalanalysisperformedbyinductivelycoupledplasmaemis- inDMFwith5%water(thiswaterwasfoundnecessarytoallow
sionspectrometry(ICP-ES)afterdissolutionofnanoparticlesinacidic thedissolutionoftheformed Znsalt). Thetotal reaction yield
solution. was found to be 45(cid:2)50% for 810 (cid:1)C (but residual SiO was
2
detectedinthesolid)and60(cid:2)65%at915(cid:1)C.
Z^eta potential measurements were achieved to study the
’RESULTSANDDISCUSSION
redispersionoftheparticlesobtainedaftertheZnOdissolution
step(seeSupportingInformation).OverthefullrangeofpH,the
Process.Preliminaryexperimentswereachievedtodetermine
surfacechargeofzincsilicateparticleswasfoundtobenegative,
thebestprecursorstobechosentofollowtheproposedsynthesis
with a value lower than (cid:2)30 mV (assumed to be required to
protocol.Toavoidthepresenceofresidualelementsthatcould
interfere with the ZSM phase formation, alkaline counterions ensure correct dispersion) for pH between 10 and 12. Never-
wereprohibited.Zincacetatewaschosensincetheacetatewould theless, dispersion under these conditions of pH did not give
beremovedduringcalcination,andneutralcolloidalsilicainstead satisfactory results, presumably because of surface reactivity of
of silicates. A critical step of the process is to achieve the silanols or dissolution/precipitation problems. Particles were
appropriatehomogeneousdispersionofthesilicaparticleswithin preferably dispersed into ethyleneglycol, which is known to be
theZnOhostmatrix,thatis,toavoidtheirsegregationleadingto agooddispersingsolventforoxideswhilebeingmorechemically
uncontrolled dispersion and growth of the obtained ZSM neutral than water. This solvent was also chosen regarding the
particles.Thisisallthemoredifficultconsideringthehighionic further studies on luminescence that requires a non absorbing
strengthofthesolutionresultingfromthepresenceofzincsaltat solventunderUVexcitation.Slightimprovementofthecolloid
highconcentration.WefoundthatusingDMFasthesolventand dispersion was also observed by adding poly(acrylic acid) ,
colloidal silica in methanol allows to mix Zn and Si precursors leadingtoaz^etapotentialof(cid:2)55mV.
withoutanyprecipitation,thuspreservingtheirintimatemixing. Characterization of the colloids by dynamic light scattering
Startingfromthishomogeneousdispersion,precipitationofthe experiments leads to a typical size of about 180 nm in term of
Zn(OH) matrix was then achieved by slow addition of tetra- volumicfractionanalysis(seeSupportingInformation).Electron
2
methylammonium hydroxyde. Complete precipitation of the microscopy images are shown on Figure 2. Well individualized
ionswasmonitoredbycontrollingthepHincreaseuptoabout particles are observed, evidencing the role of the ZnO matrix
7.Atthisstep,Mn2þionsareaddedafteraroughwashingofthe when comparing with the image of a stoichiometric sample
precipitatetoremovepartofthecounterions.NotethatMnwas obtained without ZnO in excess (Figure 2a). The size of the
addedinasecondstepandnotmixedinitiallywiththeZnions particlesaretypicallyof40(cid:2)70nmforparticlesheatedat810(cid:1)C
becauseoftheadditionofastrongbasethatleadstoprecipitation and of 80(cid:2)100 nm for particles heated at 915 (cid:1)C. This is
ofMnoxo/hydroxide(seenthroughtheappearanceofabrown compatible with the coherence length as measured from X-ray
color). The neutral pH at the time of Mn addition to the diffraction,attestingtothegoodcrystallinityoftheparticles.
precipitateallowspreservingthe2þoxidationstate. Structural Characterization of the Particles. Willemite
Afterdryingoftheprecipitate,therecoveredpowdercontains Zn SiO isanaturalorthosilicatewithaphenacitelikestructure
2 4
the silica particles and the Mn ions dispersed within the ZnO (space group R3). In this structure, Zn2þ ions occupy two
matrix.Solidstatereactionisthenachievedbyhightemperature different sites both having four oxygen atoms as their nearest
treatmentunderargontoavoidMnoxidation.Theformationof neighbors inaslightly distorted tetrahedral(T )configuration.
d
the ZSM phase (which is the only compound that can be In a previous work, electron paramagnetic resonance (EPR)
obtained as referring from the ZnO/SiO phase diagram) was studies showed that Mn could be incorporated on both Zn
2
found to start at about 750 (cid:1)C as checked by X-ray diffraction sites,26sincethetwoionshaveidenticalvalencyandveryclose
(Figure2). ionicradii(0.60ÅforZn2þand0.66ÅforMn2þ(CN=4)27).
Thenextstepwastherecoveryoftheparticlesasacolloidal TwopolymorphsoftheR-Zn SiO phase(theonlystableform)
2 4
solution through dissolution of the ZnO matrix under acidic exist and show different luminescent properties: the β-form,
conditions. Preliminary studies have shown that the ZSM exhibiting a yellow luminescence, and the γ-form, with a red
particles maydissolveinacidicaqueous solution. Theproblem luminescence.28(cid:2)30β-Zn SiO hasbeenobservedinthecaseof
2 4
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Figure3. (a)X-raypatternsofzincsilicatenanoparticles:nanoparticlesheatedat785(cid:1)Cwiththeβ-Zn SiO monoclinicstructure(JCPDS19-1479)
2 4
andNanoparticlesheatedat915(cid:1)CwiththeR-Zn SiO phenacitestructure(JCPDS37-1485).(b)Evolutionofcoherencelengthasafunctionofthe
2 4
temperatureofcalcinationwith(ratioZn/Si=20,emptydots)orwithoutmatrix(ratioZn/Si=2,plaindots).
very small grain size, obtained after rapid cooling of SiO / temperature required to complete the formation of the ZSM
2
ZnO mixtures thermally treated at high temperature (about phase. Further conclusion would probably require a detailed
1500(cid:1)C).29Itwasalsoobtainedfromthethermaldehydration characterization of the SiO dispersion within the ZnO matrix
2
ofhemimorphite(Zn Si O (OH) H O)30orthroughconfined priortocalcination,forexample,usingelectronmicroscopy.
4 2 7 2 2
reactionwithinaporoushostmedia.10 The interesting point is nevertheless that we are able, by
β-Zn SiO isthermodynamicallymetastableandconvertsinto playingwithtemperature,tosynthesizeparticlesexhibitingeither
2 4
thestableR-phaseuponprolongedheating.Thephasetransition theβ-phase(around800(cid:1)C)ortheR-phase(around900(cid:1)C).
isanalogoustoordinarytopotacticchanges.Theoxygenframe- Thecoherencelengthswerefound tobeabout30to40nmfor
work tends to remain largely unchanged, and smaller highly β-phase and 80 nm in size for R-phase, respectively. This is
charged cations (Si and Zn in our case) tend to migrate most consistent with particle sizes observed on SEM pictures
readily.31Thestructureofβ-Zn SiO isnotfullycharacterized.It (Figure2),indicatingthattheparticlesaremainlymonocrystalline.
2 4
probably derives from a distorted tridymite- or cristobalite-like A first explanation why the β-phase is obtained at low
framework in which half of the silicon is replaced by zinc and temperaturewhiletheR-phaseisobtainedathighertemperature
additionalatomsofzincintroducedintosuitableinterstices(in couldbethatthemechanismofphaseformationiskineticallyin
6-foldcoordination). favor of the metastable β-phase, which further converts into
Figure3showsthepowderdiffractionpatternsofzincsilicate R-phase upon increasing temperature. This is consistent with
nanoparticlesobtainedafter4hat785and915(cid:1)Crespectively. TGA/TDAmeasurementsperformedonnanoparticlespowders
Diagram from the sample heated at 785 (cid:1)C is found to (SupportingInformation)thatexhibitanexothermicsignalnear
correspond to the β-Zn SiO structure. Some contribution in 900(cid:1)Cwhichcorrespondstothetransformationofβ-willemite
2 4
thebaselinefromanamorphousphase,probablySiO ,indicates into the R-phase.7 But another possible explanation is that
2
that thereaction isnotcompleteundertheseconditions.Con- particles exhibit the β-phase because of their small size and a
cerningthesampleheatedat915(cid:1)C,itisfoundtocorrespondto lowersurfaceenergyofthisphaseascomparedtotheR-phase,
R-Zn SiO , with still some residue of β-Zn SiO that are not thelatterhavingalowervolumeenergy.Inthatcase,theβ-to
2 4 2 4
foundwhen thereactionis achieved under stoichiometriccon- R-transformationuponincreasingfrom700to900(cid:1)Cwouldbe
dition(Zn/Si=2,i.e.,withoutaZnOmatrix). explainedjustbyanincreaseofparticlesizeandthusadecreaseof
Evolution of the coherence length as a function of the thepredominanceofsurfaceversusvolumeenergies.Thelimited
temperatureofannealingwasdeterminedusingtheSherrerlaw efficiencyoftheZnOmatrixtolimitparticlegrowthuponheating
on the (113) peak of the R phase or the ((cid:2)132) peak of the doesnotallowadefinitiveconclusion.Nevertheless,thepresence
βphaserespectively.ResultsareshownonFigure3bonasam- ofasmallamountofβ-phase,eveninsamplesheatedat1000(cid:1)C,
ple with Zn/Si equal to 20 (large excess of ZnO) and 2 playsinfavorofthesecondexplanation(lowersurfaceenergyofthe
(stoichiometric) for comparison. At 800 (cid:1)C, for which a sig- β-phase). Such a size dependent crystalline structure has been
nificantpartoftheprecursorshasreacted,thecoherencelengthis commonlyobservedinmanyothersystems(forexampleCdS32).
alreadyabout60nmandnocleardifferencesareseendepending Starting from our ability to prepare particles with the two
ontheexcessofZnO.Weconcludethatverysmallparticlesare differentstructures,sampleswerecharacterizedusingsolidstate
not obtained for a significant conversion rate of the precursor. 29Si-MAS NMR spectroscopy to study the environment and
Increasingthetemperatureupto1000(cid:1)Cleadstoanincreaseof the connectivity of SiO 4(cid:2)structural units and thus tentatively
4
thecoherencelengthinbothcases,butthesamplewithZnOin explain the difference of emission properties between the two
largeexcessexhibitsamuchlowerdomainsize.Intheidealcase,a structures. Figure 4 displays the spectra of almost pure
saturationofthecoherencelengthwouldhavebeenexpectedif β-Zn SiO and R-Zn SiO samples obtained at 785 and
2 4 2 4
the solid-state reaction was just confined at the ZnO/SiO 915(cid:1)Crespectively,andonesampleobtainedunderintermedi-
2
interface. The fact that a continuous increase is observed may ateconditions(850(cid:1)C)displayingamixtureofboththeRand
be explained either by some aggregation of the initial SiO theβ-phases.
2
particles or by some significant diffusion of the silicate within Intheseexperiments,allspectraexhibitasharppeaklocatedat
theZnOmatrix(probablythroughitsporosityinthiscase)atthe (cid:2)65 ppm and (cid:2)71 ppm for the R and β-phases, respectively.
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Figure4. 29SiMASNMRspectra.(a)β-Zn SiO andR-Zn SiO sampleobtainedat915and785(cid:1)C,respectively.(b)Spectraofasampleobtainedat
2 4 2 4
anintermediatetemperature(850(cid:1)C)exhibitingadditionalcontributionsascomparedtotheβphasesample.
These peaks are characteristic of monomeric Q orthosilicate
0
unitsasexpectedfromthestructures(Q representsSiO units
n 4
that share their bonds with n other neighboring silicon
atoms).13,33Inthecaseoftheβ-sample,acontributionisclearly
detected at (cid:2)110 ppm, corresponding to Q groups from
4
amorphous SiO resulting from the incomplete reaction. The
2
sharpness of the Q peaks in both samples indicates that the
0
silicon atoms have a precise crystallographic environment, cor-
respondingtothecrystallinephaseasdetectedbyX-raydiffrac-
tion.The6ppmdifferencebetweentheQ signalsfromthetwo
0
structuressuggestsastrongerelectronicdensityonthesiliconin
theβphase.34
Figure4bdisplaysthespectrainthe(cid:2)62to(cid:2)72ppmrange
ofasampleobtainedatanintermediatetemperature(850(cid:1)C),
andexhibitingamixtureofR-andβ-phasesasobservedinthe Figure 5. Evolution of X-ray diffraction patterns of four samples
X-ray diagrams. The NMR spectra indeed shows the peaks at containinganincreasingMnrate.
(cid:2)65ppmand(cid:2)71ppmwhicharecharacteristicoftheβ-phase
and R-phase, respectively, but at least two other peaks at correspondingsampleshowsthatthiscolorappearsconcomitantly
intermediate values of chemical shift ((cid:2)66 and (cid:2)68 ppm) are withanewphasethatcoexistswiththeZSMandwasindentifiedas
also clearly observed. It thus seems that this sample not only ZnMn O phase (JCPDS 24-1133; Supporting Information)). 3
2 4
contains a mixture of R and β-phase domains, but also inter- mol%thusappearstobethemaximumMncontentthatcanbe
mediatephasesthatcouldcorrespondtointermediateenviron- incorporatedinsidetheZSMphasethroughthisprocess.
ments, resulting from the progressive topotactic transition AnothereffectofMnincoprorationinsidetheparticlesisthat
between the two phases. We note that such a progressive itwasfoundtoinfluencetheβ-phasestability.Figure5displays
transition,involvingstackingfaultsandtwinboundarieshasalso XRD patterns of samples prepared at 915 (cid:1)C with increasing
beenobservedinII(cid:2)VIsemiconductors'nanocrystals32 Mn/(ZnþMn)molarratiofrom0.7%to3.4%.
Mn2þ-Incorporation into the Nanoparticles. In our syn- Itappearsclearlythattheβ-phasefractionincreaseswithMn
theticprocess,theMn2þionsareaddedintheZnOmatrixwhich content.AfirstexplanationwouldbethattheMnionsdecrease
isinlargeexcess.Thequestionisthentodetermineitsdegreeof thesizeoftheparticles,forexample,actingasnucleationcenters,
incorporationinsidetheZSMparticles,thatis,thefinaldistribu- butnoclearevolutionofcoherencelengthandparticlesizecouldbe
tionofmanganesebetweentheZnOmatrixandtheparticles.For seen.Anotherexplanationisthatforagivenparticlesize,theMn
thisstudy,differentamountsofMnsalthavebeenaddedinthe ionshelpinstabilizingtheβ-structure.Itisallthemorepossible
initial formulation, varying between 0.1 and 1% in term of that,intheβ-structure,halftheZnareintetrahedralsitesandhalfin
Mn/(MnþZn )molarratio.ThefinalamountofMnwithin octahedralsites,whileintheR-phaseallareintetrahedralsites.As
total
the particles was determined by ICP elemental analysis octahedral coordinance fits better to the Mn2þ ionic radius, this
(Supporting Information). Whatever the initial amount of Mn couldexplainthestabilizingeffectoftheMnfortheβ-phase.
and the temperature of synthesis between 810 and 915 (cid:1)C EmissionPropertiesoftheParticles.Figure 6 displays the
leading to the R and β-phase, respectively, the concentration emissionfromcolloidaldispersionsofparticleswiththeR-and
ofMnintheparticlesisfoundtobeabout3timeshigherthanin β-phasesrespectively.Acleardifferenceofemissionisobserved
theinitialZnOcomposition.ThisshowsthattheMnionshavea fromyellow(β-)togreen(R-)color,24allthemorepronounced
somewhathigheraffinityforthesilicatestructurethanforZnO. becausetheeyespectralsensitivityisveryhigh.
For Mn concentration (in term of Mn/[Mn þ Zn] molar In both cases, the radiative emission is attributed to the
ratio)ofmorethan3mol%intheparticles,particleswerefound 4T -6A spinfliptransition within the d-electrons of the Mn2þ
1 1
to exhibit a brown color. X-ray diffraction analysis of the ion.3Thistransitionisforbiddeninnaturesothattheemission
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Figure6. ExcitationandemissionspectraofR-Zn SiO (left)andβ-Zn SiO (right)colloidalsuspensions.Inset:emissionofthesamplesasobserved
2 4 2 4
bytheeyesunder254nmUVillumination.
lifetimeisverylong(decaytimeat10%(τ )around30ms).4
10%
Excitationoftheemitting4T leveloccursafterabsorptionofUV
1
lightbytheMn2þinitsgroundstates,leadingtoitsoxidationinto
Mn3þ and the promotion of the electron into the conduction
band of the host silicate. The electron in the conduction band
then relax to the 4T excited state through non radiative
1
processes,andthe4T to6A radiativetransitionfollowsleading
1 1
togreenemissionpeakingat525nmfortheR-phaseandyellow
luminescencepeakingat570nmfortheβ-phase.4,10,24Lifetime
measurements(notshown)givevaluesof22msfortheR-phase
(0.7% Mn doping) and 35 ms for the β-phase (3.4% Mn
doping).4,10
The origin of the difference of spectroscopic properties
between the two phases has to be found in the difference of
MncrystalsitewithinthehostZn SiO matrix.Thevariationof
2 4
energyofthe4T and6A levelsasafunctionofthecrystalfieldin
1 1
a tetrahedral or octahedral environment is given by the corre-
Figure 7. Relative quantum yield as a function of Mn content as
sponding Tanabe(cid:2)Sugano diagram [3, Supporting Informa-
measured on powders of nanoparticles synthesized at 915 (cid:1)C with
tion].ConsideringthegreenandyellowemissionoftheRand
reference to a micronic ZSM powder. White squares correspond to
β phases, respectively, one thus expects a stronger ligand field
samples with a brownish color because of the presence of the
aroundtheMn2þionintheβ-phase.IntheR-phase,allpossible ZnMn O phase.
2 4
sites for the Mn (i.e., Zn sites) are tetrahedral, whereas in the
β-phase half are octahedral and half are tetrahedral. A first intensityhasbeeninvestigatedonpowderssynthesizedat915(cid:1)C
possibility to explain a higher ligand field in the β phase could byrelativeemissionyieldmeasurementswithreferencetoabulk
be that the Mn ions are still in the tetrahedral sites, but the ZSMmaterial.ResultsarepresentedonFigure7.
differenceofstructureleadstoahighercrystalfield.Nevertheless, Concerning bulk ZSM, the maximum emission yield is of
thisseemsnottobeconsistentwiththechemicalshiftvariation 70(cid:2)80%foranoptimumMncontentofabout5(cid:2)8%.4,24Higher
betweenthetwophasesasobservedin29SiNMR.Itwasindeed amountsleadtoadecreaseoftheemissionpropertiesasaresult
concludedthattheβ-phaseexhibitsahigherelectrondensityon ofMn(cid:2)Mninteractions.Inthecaseofourparticles,maximum
thesiliconatom,whichwouldthenleadtoalowerligandfieldfor emission intensity is found to be 16% as compared to a bulk
neighboringZnorMnions.Itthusappearsasmoreprobablethat reference. This value is reached for Mn content of about 2%,
thedifferenceofligandfields,andresultingspectroscopicproper- which is thus lower than the optimal content for the bulk
ties, originates from adifference of location of theMn ions: in materials. The difference of emission yield should be first
tetrahedralsitesfortheRphaseandoctahedralfortheβphase.A explained by the limited efficiency of Mn incorporation in our
better characterization of the β-phase would be required to nanoparticles. Nevertheless, reported value of emission yield
confirmthisassumption,butwenotethatthisisinaccordance versus Mn content show that the emission yield of the bulk
withthediscussionontheeffectofstabilizationoftheβ-phaseby compound doped with 2% Mn is already about 80% of the
theMnions,explainedbyabetterfitoftheMn2þionicradiusto maximum, which means the improvement between 2% and
octahedralsites. 5(cid:2)8%isnotsoimportant.4Wemustthenconcludethatsome
The Mn2þactivator concentration is well-known to play an alteration of the emission may still be related to surface and
importantrolefortheoptimizationoflightemissionefficiencyin residual defects within the particles. Refering to our previous
phosphors.TheinfluenceofMncontentonphotoluminescence work,14wecould neverthelessexpecttheseeffectstobeofless
2966 dx.doi.org/10.1021/cm2005902|Chem.Mater.2011,23,2961–2967
ChemistryofMaterials
ARTICLE
importanceconsideringthehightemperatureofsynthesisofthe ’ACKNOWLEDGMENT
particles(nearlythesameasthebulkmaterial)andtheirrelatively
This work was partly funded by Rhodia, CNRS and Ecole
bigsize(>50nm)sothatsurfaceeffectsareexpectedtobelimited.
Polytechnique.
We then note that dielectric effects should also be discussed
similarly to what has been evidenced in the case of YVO4:Eu
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’AUTHORINFORMATION SilicatesandZeolites;JohnWiley&Sons:Chichester,1987.
(35) Li,B.;Zhou,J.;Zong,R.;Fu,M.;Li,L.;Li,Q.J.Am.Ceram.Soc.
CorrespondingAuthor 2006,89,2308–2310.
*E-mail:[email protected]:33(1)69334656.
2967 dx.doi.org/10.1021/cm2005902|Chem.Mater.2011,23,2961–2967
SUPPORTING INFORMATION AVAILABLE
Structural and photoluminescent properties of
2+
Zn SiO :Mn nanoparticles prepared by a protected
2 4
annealing process
Caroline Bertail,†,‡ Sébastien Maron, † Valérie Buissette,‡ Thierry Le Mercier,‡ Thierry Gacoin,†,* and
Jean-Pierre Boilot†
†Groupe de Chimie du Solide, Laboratoire de Physique de la Matière Condensée, CNRS UMR 7643,
École Polytechnique, 91128, Palaiseau (France), and ‡ Centre de Recherches d’Aubervilliers, Rhodia,
52, rue de la Haie Coq, 93308, Aubervilliers (France).

[email protected]
Telephone number - 33 (1) 69 33 46 56

1
Dynamic light scattering characterization of a the colloidal dispersions (left : beta phase – 810°C,
right : alpha phase – 915°C) :

20
20
Intensity
volume analyse en intensité
number analyse en volume
analyse en nombre
15 15
Class 10 Class 10
% in % in
5 5
0 0
-5 -5
200 400 600 800 1000 200 400 600 800 1000
Size(nm) Size (nm)

Zeta potential characterization of the ZSM colloidal suspensions :
0
ZnSiO nanoparticles
2 4
-5
-10
V)
m-15
al (
nti-20
e
ot
p
a -25
êt
Z
-30
-35
-40
4 6 8 10 12
pH
Measurements were performed with a constant ionic strength (KCl à 10-1 mol/L); pH was controlled
through the addition of either tetramethylammonium hydroxide or chlorhydric acid.

Thermogravimetric and differential thermoanalysis on powders of nanoparticles. Left : with a heating
rate of 10°C.min-1 in argon until 900°C. Right : with a heating rate of 10°C.min-1 in argon until 900°C
and a landing of 4 hours.
2
110 0.6 110
exo exo 0.6
100 0.4 100
0.4
D 900°C -4h D
G (%)90 0.2TA (µV G (%) 90 0.2TA (µV
T80 0 /m T 80 0 /m
g) β(cid:198)α g)
-0.2
70 -0.2 70
-0.4
60 -0.4 60
100 200 300 400 500 600 700 800 900 0 40 80 120 160 200 240 280 320
temperature (°C) time (min)
The total weight loss is evaluated at 40% and occurs in three steps. The first one below 200°C
corresponds to the release of water molecules adsorbed on the powder surface and is associated with an
endothermic peak whose maximum is at 131.5°C. The second in the 200°C-400°C range corresponds to
the crystallization of ZnO and is associated with an exothermic broad band (maximum to 440°C). The
DTA also shows two exothermic signals at 600°C and 900°C (Figure 5 (b)). These peaks could be
assigned to the crystallization of β-willemite and its transformation to α-phase [7]. One can see that
slow weight loss continuing at higher temperature above 800°C, due to the OH removal by temperature
with samples densification.

Mn content analysis
Mn content in
nanoparticles
Mn introduced (%)
Mn / (Zn+Mn) x 100
Mn/(Mn + Zn )
total
810°C 915°C
0.1 0.3 0.3
0.2 0.6 0.7
0.4 1.2 1.2
0.6 1.9 1.9
0.8 2.3 2.6
1 3.6 3.4

3