Hymavathib a Department of Physics, GITAM Institute of Technology, GITAM University, Visakhapatnam 530045, AP, India b Department of Physics, Anil Neerukonda Institute of Technology and
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j o u r n a l h o m e p a g e :w w w e l s e v i e r c o m / l o c a t e / j a s c e r
Full Length Article
B Rajesh Kumara,∗, B Hymavathib
a Department of Physics, GITAM Institute of Technology, GITAM University, Visakhapatnam 530045, AP, India
b Department of Physics, Anil Neerukonda Institute of Technology and Sciences (Autonomous), Sangivalasa, Visakhapatnam 531162, AP, India
Article history:
Received 19 November 2016
Received in revised form 31 January 2017
Accepted 5 February 2017
Available online xxx
Keywords:
Solid-state reaction
XRD
Scanning electron microscopy
ZnOdopedwithdifferentconcentrationsofAl2O3(2,4,6,8and10wt%)ispreparedbyconventional solid-statereactionmethod.X-raydiffractionresultsrevealedthatthesampleswerecrystallinewitha hexagonalwurtzitephase.Astheconcentrationofalumina(Al2O3)increasesinZnO,theX-raydiffraction peaksshiftstowardshigherangle.Thisshiftinginpeakpositionanddecreaseinintensityreflectthat
AlissuccessfullyreplacedZninZnOmatrix.X-raypeakbroadeninganalysiswasusedtoevaluatethe crystallitesizeandlatticestrainbytheWilliamson–Hall(W–H)methodandsize-strainplot(SSP)method Thephysicalparameterssuchasstrain,stress,andenergydensityvalueswerealsocalculatedusing W–Hmethodwithdifferentmodelsnamelyuniformdeformationmodel,uniformstressdeformation modelanduniformdeformationenergydensitymodel.Thesurfacemorphologyandelementalanalysis
ofthepreparedsampleswerecharacterizedbyfieldemissionscanningelectronmicroscopyandenergy dispersivespectra
©2017TheCeramicSocietyofJapanandtheKoreanCeramicSociety.Productionandhostingby ElsevierB.V.ThisisanopenaccessarticleundertheCCBY-NC-NDlicense(http://creativecommons.org/
licenses/by-nc-nd/4.0/)
1 Introduction
ZincOxideis awellknownimportantsemiconductordueto
itsfundamentaland technologicalimportance.It isa wideband
gap semiconductor (3.37eV) with highexciton binding energy
of60meV,excellentchemicalstability,abundanceinnatureand
non-toxic It had a wide range of applications such as solar
cells,luminescentdevices,laserdiodes,chemicalsensors,surface
acousticwave devicesetc, whoseconductivitycanbetunedby
controllingthedeviationfromstoichiometryandbydoping[1–4]
Al2O3isann-typesemiconductingmaterialwithawidebandgapof
8.8eV,differentcrystallineformandgoodthermalstability.Itcan
beactasagoodcatalystwithsemiconductorwhenthegasessuch
asH2SandEthanolcomeincontactwithit.GroupIIIAelements
(Al,Ga,In)havebeenusedtoimprovetheelectricalconductivity
andthermalstabilityofZnOmaterial[5–7].Inthiswork,alumina
dopedZnOceramicswerepreparedbytheconventionalsolid-state
reactionmethodusingZnOandAl2O3asthestartingmaterials.In
theconventionaloxide-mixingtechnique,thepowdersproduced
aremorehomogeneousafterthesinteringprocess[8]
∗ Corresponding author Fax +91 8554 255710.
E-mail address: rajphyind@gmail.com (B Rajesh Kumar).
X-raypeakprofileanalysis(XPPA)isusedtoestimatethe micro-structuralquantitiesandcorrelatethemwiththeobservedmaterial properties.XPPAisanaveragingmethodandapowerfultoolto esti-matethecrystallitesizeandlatticestrain[9].Thecrystallitesize andlatticestrainaffecttheBraggpeaktoincreasethepeak inten-sity,peakwidthandashiftinthe2peakposition.Themethods reportedintheliteraturetoestimatethecrystallitesizeand lat-ticestrainaretheWarren–Averbachanalysis,Rietveldrefinement andpseudo-Voigtfunction[10–12].However,theWilliamson–Hall (W–H)analysisisasimplifiedintegralbreadthmethodusedfor estimatingcrystallitesizeandlatticestrain,consideringthepeak widthasafunctionof2[13,14].Inthepresentwork,XPPAis car-riedouttoevaluatethecrystallitesize(D),latticestrain(ε),lattice stress()andlatticestrainenergydensity(u)ofAl2O3dopedZnO ceramicsbasedonmodifiedW–Hplotsusinguniformdeformation model(UDM),uniformstressdeformationmodel(USDM),uniform deformationenergydensitymodel(UDEDM)andsize-strainplot (SSP).Literaturesurveyreportsthatadetailedstudyusingthese modelsonsolid-statesinteredZnOdopedwithdifferent concen-trationsofAl2O3isnotyetreported
2 Experimental details
ZnO dopedwith2,4,6, 8and 10%of alumina(Al2O3)were synthesized by conventional solid-state reaction method The http://dx.doi.org/10.1016/j.jascer.2017.02.001
2187-0764/© 2017 The Ceramic Society of Japan and the Korean Ceramic Society Production and hosting by Elsevier B.V This is an open access article under the CC BY-NC-ND license ( http://creativecommons.org/licenses/by-nc-nd/4.0/ ).
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Fig 1. X-ray diffraction patterns of ZnO doped with (a) 2% Al 2 O 3 ; (b) 4% Al 2 O 3 ; (c) 6% Al 2 O 3 ; (d) 8% Al 2 O 3 and (e) 10% Al 2 O 3
Table 1
Structural parameters of ZnO doped with different concentrations of alumina.
Alumina concentration (%) Lattice parameters, Volume, V (Å 3 ) No of unit cells in the particle, n
Table 2
Rietveld refinement results for ZnO doped with different concentrations of alumina.
Alumina concentration (%) Lattice parameters (Å) Volume (Å 3 ) R p (%) R wp (%) R exp (%) GOF R Bragg (%) 2 R F (%)
2 a = b = 3.2722(4) 48.57(4) 4.42 8.84 5.96 1.48 2.76 2.20 3.88
c = 5.2382(5)
4 a = b = 3.2662(6) 48.30(5) 4.32 7.96 6.47 1.23 3.12 1.23 3.71
c = 5.2286(3)
6 a = b = 3.2618(3) 48.13(4) 5.21 9.88 6.29 1.57 3.64 1.57 4.27
c = 5.2238(2)
8 a = b = 3.2513(2) 47.72(4) 5.13 8.22 7.05 1.17 2.84 1.17 5.50
c = 5.2136(2)
10 a = b = 3.2378(4) 47.28(5) 6.17 8.42 6.89 1.22 2.92 1.22 4.13
c = 5.2082(5)
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Fig 2. Rietveld refinement analysis of XRD patterns of ZnO doped with (a) 2% Al 2 O 3 ; (b) 4% Al 2 O 3 ; (c) 6% Al 2 O 3 ; (d) 8% Al 2 O 3 and (e) 10% Al 2 O 3
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Fig 3. W–H analysis of ZnO doped with (a) 2% Al 2 O 3 ; (b) 4% Al 2 O 3 ; (c) 6% Al 2 O 3 ; (d) 8% Al 2 O 3 and (e) 10% Al 2 O 3 assuming UDM.
of3.5nm.The compositionalanalysisofZnO dopedwithAl2O3
wascarriedoutusingEnergyDispersiveX-raySpectroscopy(EDS,
Model:Horiba,EMAX,137eV)
3 Results and discussion
3.1 Structuralproperties
X-raydiffraction patterns of ZnO doped with different
con-centrations of alumina (Al2O3)are shown in Fig 1 The X-ray
diffractionpeaksatanangle2areindexedbytheirmillerindices
(100),(002),(101),(102),(110),(103),(200),(112)and(201)
correspondingtoZnO (spacegroupp63mc,JCPDSno.36–1451)
indicatingthatthephaseofthesamplewaswurtzitestructure.The
XRDpeaksshiftstowardsthehigherangleswiththeincreaseof
dopingconcentrationofAl2O3inZnO.Themajordiffractionpeak
intensitiesincreasedwiththeincreaseofaluminacontentupto6%
andthenathigherdopingconcentrations(8and10%)ofalumina
thepeakintensitydecreased,whichindicatesAl-dopingresultedin
adecreaseinthecrystallinequality.AlargeamountofAldopants
resultedinlatticedisorder,whichisassociatedwiththestress
gen-erated.Besidesthestressproblem, thegrains grewmoreeasily
whenAldopantswereincorporatedwithZnO.Thelattice
param-eters‘a’and‘c’ofthepreparedsamplesarecalculatedfromX-ray
diffractionmeasureddatausingtheequationsasreportedinthe
previousliterature[15].Thevolumeoftheunitcellandnumber
ofunit cells intheparticlearedeterminedby theequationsas reportedinthepreviouswork[16].The volume(V) oftheunit cellandnumberofunitcellsintheparticle(n)decreaseswiththe increaseofaluminaconcentrationinZnO.Thestructural parame-tersofdifferentconcentrationsofAl2O3dopedZnOestimatedfrom X-raydiffractiondataaregiveninTable1
XRDRietveldrefinementswereperformedwithFULLPROF soft-ware program The wurtzite ZnO structure (space group 186, P63mc)andAl(spacegroup225,Fm-3m)wereselectedasthe start-ingmodelstructures.Theaccurateunitcellparametersofvarious compositionsofaluminadopedZnOweredeterminedbyRietveld refinementoftheobservedX-raydiffractionprofile[17] Pseudo-Voigtpeakprofilefunctionwasadoptedandthebackgroundwas approximatedbylinearinterpolationbetweenasetofbackground pointswithrefinableheights.TherefinementresultsofXRD pat-ternsofaluminadopedZnOsamplesareshowninFig.2.Thered and black linesrepresent theobserved and fitted data, respec-tively.Thebluelinerepresentsthedifferencebetweentheobserved andfitteddata.TherefinedXRDpatternsareinverygood agree-mentwiththemeasureddata.Nosecondphaseisobservedwhich revealedthefirstindicationthatthedopantatomsareincorporated
inthewurtzitestructure.Thelatticeparameters(aandc)decreases withtheincreaseofdopingconcentrationsofAl2O3inZnO.This behaviourhasbeenattributedtothefactthattheZnatomsare replacedbytheAlatoms.BecausetheionicradiiofZn2+andAl3+
are0.074nmand0.053nm,respectivelywithincreasing
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Fig 4. W–H plots of ZnO doped with (a) 2% Al 2 O 3 ; (b) 4% Al 2 O 3 ; (c) 6% Al 2 O 3 ; (d) 8% Al 2 O 3 and (e) 10% Al 2 O 3 using USDM.
tionofAlforZnthelatticeparameters‘a’and‘c’shoulddecrease
monotonicallyduetothesmallerradiusofAlcomparedwiththat
ofZn,reflectingthelatticecontraction.Theobtainedreliability
fac-torsRp,Rwp,Goodness-of-fitting(GOF),RBragg,2,RFand lattice
parameterscalculatedfromtheRietveldrefinementaretabulated
inTable2
3.2 Williamson–Hall(W–H)methods
ThebroadeningofXRDpatternisattributedtothecrystallite
size-inducedorstraininducedbroadening.Thesignificanceofthe
broadeningof peaksevidencesgrainrefinementalong withthe
largestrainassociatedwiththepowder.Theinstrumental
broad-ening(ˇhkl)wascorrected,correspondingtoeachdiffractionpeak
usingtherelation
Theaveragecrystallitesize(D)wascalculatedusingScherrer’s
formula[18]
where D is crystallite size, K (=0.94) is shape factor and
(=0.154nm)isthewavelengthofCuk␣radiation.Itisfoundthatthe
averagecrystallitesizedecreasesfrom56to25nmwithincreasing
aluminaconcentrationinZnO.Itmaybeduetotheenhancement
insurface-to-volumeratio
Thestraininducedinpowdersduetocrystalimperfectionand distortionwascalculatedusingtheformula[19]
Assumingthattheparticlesizeandstraincontributionstoline broadeningareindependenttoeachotherandbothhavea Cauchy-likeprofile,theobservedlinebreadthissimplythesumofEqs.(2) and(3)
Byrearrangingtheaboveequation
TheaboveEqs.(4)and(5)areWilliamson–Hallequations.To makeWilliamson–Hallanalysis,aplotisdrawnwith4sinalong thex-axisandˇhklcosalongthey-axisforallorientationpeaks
ofAl2O3 dopedZnOceramicsisshowninFig.3.Thecrystalline sizewasestimatedfromthey-interceptandthestrain(ε)fromthe slopeofthelinearfittothedata.InUDMthestrainisassumedto
beuniforminallcrystallographicdirections,thusconsideringthe isotropicnatureofthecrystal,wherethematerialpropertiesare independentofthedirectionalongwhichtheyaremeasured.Itis clearthatthereisadecreaseinthelatticestrainwithincreasing dopingconcentrationsofAl2O3inZnO.Theuniformdeformation modelforAl2O3dopedZnOceramicsareshowninFig.3
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Fig 5.W–H analysis of ZnO doped with (a) 2% Al 2 O 3 ; (b) 4% Al 2 O 3 ; (c) 6% Al 2 O 3 ; (d) 8% Al 2 O 3 and (e) 10% Al 2 O 3 assuming UDEDM.
Intheuniformstressdeformationmodel(USDM)thereisalinear
proportionalitybetweenstressandstraingivenby=Yhklwhich
isknownastheHook’slawwithintheelasticlimit.Inthis
rela-tion,isthestressofthecrystalandYistheYoung’smodulus
Hook’slawisareasonableapproximationtoestimatethelattice
stress.Inthisapproach,theWilliamson–Hallequationismodified
bysubstitutingthevalueofεinthesecondtermofEq.(5)yields
ˇhklcos=(K/D)+(4sin/Yhkl) (6)
YhklistheYoung’smodulusinthedirectionperpendiculartothe
setofcrystallatticeplane(hkl).Theuniformstresscanbe
calcu-latedfromtheslopelineplottedbetween4sin/Yhklandˇhklcos
andthecrystallitesize(D)fromthey-interceptasshowninFig.4
ThestraincanbemeasuredifYhklofhexagonalZnO
nanoparti-clesisknown.Forsampleswithahexagonalcrystalphase,Young’s
modulusYhklisrelatedtotheirelasticcompliancesSijas[20]
Yhkl= [h2+(h+2k)2/3)+(al/c)2]2/[S11(h2+(h+2k)2/3)2
+S33(al/c)4+(2S13+S44)(h+2k)2/3)(al/c)2] (7)
whereS11, S13,S33,and S44 aretheelasticcompliances ofZnO
andtheirvaluesare7.858×10−12,2.206×10−12,6.940×10−12and
23.57×10−12m2N−1,respectively[21,22]
Theuniformdeformation energy density model (UDEDM)is
used to determine the energy density of a crystal In Eq (4),
weconsideredthatthecrystalsareassumedtohavea
homoge-neous,isotropicnature.Inmanycases,theassumptionofisotropy and homogeneity is not justified Moreover, the constants of proportionalityassociatedwiththestress-strainrelationarenot independent when the strain energy density ‘u’ is considered AccordingtoHooke’slaw,theenergydensity uasafunctionof strainisu=ε2Yhkl/2.Therefore,Eq.(6)canbemodifiedtotheform
ˇhklcos=(K/D)+(4sin(2u/Yhkl)1/2) (8) Plotofˇhklcosand4sin(2/Yhkl)1/2isshowninFig.5andthe valueofenergydensity‘u’wascalculatedfromtheslopeofthe lin-earfitandthecrystallitesizeisestimatedfromthey-intercept From Eqs.(6) and (8), thedeformation stress and deformation energydensityarerelatedasu=2/Yhkl.Thelatticestraincanbe calculatedbyknowingtheYhklvaluesofthesample
3.3 Size-strainplot(SSP)method The corresponding W–H plots described that line broaden-ingwasessentiallyisotropic.Thishighlights thatthediffracting domains wereisotropic due tothe contributionof microstrain
In case of isotropic linebroadening, a betterevaluation of the size-strainparameterscanbeobtainedbyconsideringanaverage
“size-strainplot”(SSP).Thismethodhadanadvantagethatless importanceisgiventodatafromreflectionsathighangles,where theprecisionisusuallymuchlower.Inthismethod,itisassumed thatthecrystallitesizeprofileisdescribedbyaLorentzianfunction
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Fig 6.Plot of (d hkl ˇ hkl cos hkl ) 2 versus d hkl2ˇ hkl cos hkl.
Fig 7.Variation of crystallite size (D) with alumina concentration obtained from Scherrer method.
andthestrainprofilebyaGaussianfunction[23].Accordingly,we
have:
(dˇhklcos)2=(K/D)(d2ˇhklcos)+(ε/2)2 (9)
whereKisaconstantthatdependsontheshapeoftheparticles;
forsphericalparticlesitisgivenas3/4.InFig.6,similarlytothe
W–Hmethods,theterm(dˇhklcos)2isplottedwithrespectto
(d2ˇhklcos)fortheallorientationpeaksofaluminadopedZnO
Inthiscase,theparticlesizeisdeterminedfromtheslopeofthe
linearlyfitteddataandtherootofthey-interceptgivestheroot
meansquare(RMS)strain
TheresultsobtainedfromtheScherrermethod,W–Hmodels
(UDM,USDMand UDEDM)and SSPmethodsareshowninFigs
7–9.ThevaluesofaveragecrystallitesizeofaluminadopedZnO
ceramicsobtainedfromthedifferentmodelsaremoreorless
sim-ilar,implyingthattheinclusionofstraininvariousformshasa
verysmallchangeintheaveragecrystallitesize.Byinspectionof
theplots,itappearsthattheresultoftheSSPmodelisfoundto
bemoreaccuratethantheUDM,USDMandUDEDM,asthedata werefittedmoreaccuratelyinthismethod,withallhigh-intensity pointstouchingthelinearfit.Itwasobservedthatthestrainand stressvaluesdecreasedwithdecreasingaveragecrystallitesizeas thedopantconcentrationwasincreased
3.4 Surfacemorphologyandelementalanalysis FESEMimagesofZnOdopedwithdifferentconcentrationsof
Al2O3 areshowninFig.10(a)–(e).Intheimages,thegreygrains are ZnO grains and the white onesare Al2O3 grains It can be observedfrom themicrographsthatthesurface morphologyof
Al2O3 dopedZnOshows porousnatureand petal-shapedgrains orientedrandomlyofsizesvariesfrom190to276nm.Itcanbe seenthatastheAl2O3contentisincreasingthesamplesare becom-ingdense;showslessporesandthedistributionofthegrainsare
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Fig 8. Variation of D, ε, and u with alumina concentration estimated from (a) UDM; (b) USDM; and (c) UDEDM.
Fig 9.Variation of crystallite size (D) and strain (ε) with alumina concentration obtained from SSP method.
uniform.TheEDSisrecordedtodeterminetheelemental
compo-sitionofZnOdopedwithdifferentconcentrationsofalumina.The
relativecompositionsobtainedfromEDSareinanatomicratioof
Zn/O/Alare46.90/51.27/1.83%,45.37/50.81/3.82%,44.88/49.28/5.84
and44.37/47.87/7.76%,42.54/48.50/8.96%forZnOdopedwith2,4,
6,8and10%ofAl2O3
4 Conclusions
ZnO doped with different concentrations of Al2O3 were synthesizedbyconventionalsolid-statereactionmethodand char-acterized by X-ray diffraction The results of Rietveld analysis indicate that thesamples belongs tothe hexagonalZnO phase (spacegroup186,P63mc)andthecubicAlphase(spacegroup225,
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Fig 10.FESEM images of ZnO doped with (a) 2% Al 2 O 3 ; (b) 4% Al 2 O 3 ; (c) 6% Al 2 O 3 ; (d) 8% Al 2 O 3 and (e) 10% Al 2 O 3
Fm-3m).Thelatticeparameters(aandc)ofthesamplesare
decreas-ingowingtothelatticecontraction,henceconfirmingthatAlhas
beensuccessfullydopedinthelattice.TheX-raypeakbroadeningof
thepreparedsamplesareduetothesmallcrystallitesizeandstrain
AmodifiedW–Hplothasbeenworkedouttodeterminethe
crystal-litesizeandstrain-inducedbroadeningduetolatticedeformation
andenergydensityvaluewithacertainapproximation.The
aver-agecrystallitesizeobtainedfromScherrer’sformula,W–Hanalysis
andSSPmethodshowsasmallvariationbecauseofthedifferencein
averagingtheparticlesizedistribution.Thestrainvaluesobtained
fromthegraphsplottedforvariousformsofW–Hanalysisi.e.,UDM,
USDM,andUDEDMwerefoundtobeaccurateandcomparable.The aboveexplainedmethodswerehelpfulindeterminingthe crystal-litesize,strain,stress,andenergydensityvalue,andamongthem SSPmethodishighlypreferabletodefinethecrystalperfection
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