Nanoprecursors have the benefits of both molecular and bulk crystallineprecursors.First,becausenanoprecursorscanbemade intheformsofboth powderanddispersion, theycanbeeasily molded into th
Trang 1Synthesis of delafossite CuAlO 2 p-type semiconductor with a nanoparticle-based
Tran V Thua, Pham D Thanha, Koichiro Suekunia, Nguyen H Haib, Derrick Motta, Mikio Koyanoa, Shinya Maenosonoa,*
a
School of Materials Science, Japan Advanced Institute of Science and Technology (JAIST), 1-1 Asahidai, Nomi, Ishikawa 923-1292, Japan
b
Center for Materials Science, College of Science, Vietnam National University (VNU), 334 Nguyen Trai, Hanoi, Viet Nam
1 Introduction
Transparentconductingoxides(TCOs)areveryimportantfor
variouskindsofdevicesandarecommonly usedin transparent
electronics.Thecombinationoflowelectricalresistivityandhigh
transparencyinthevisiblelightrangemakesTCOsfascinatingin
various practical applications including liquid crystal displays,
touchscreens, photovoltaicdevices,light emitting diodes, solar
cells,etc.[1–4].Proverbially,mostTCOsaren-type
semiconduc-tors.Forexample,Sn-dopedIn2O3(ITO)[5],F-dopedSnO2(FTO)[6]
andAl-dopedZnO(AZO)[7]arealln-typematerials.Thelackof
reliable preparation methods for p-type TCOs has prevented
furtherdevelopmentsoftransparentelectronics,whichbasically
requirep–njunctions.Cuprousaluminatedelafossite(CuAlO2)has
beenknownasap-typesemiconductorsince1984[8],andasa
promisingp-typeTCOsinceitsdiscoverybyHosonoandcoworkers
in 1997 [9] CuAlO2 belongs to the familyof delafossite TCOs
[10,11].Thisremarkablepropertyisthoughttobecausedbyits
crystal structure, which is identified by alternating layers of
dumbbellO–Cu–Oandparallelplanesofedge-sharedoctahedral
AlO6(Fig.1a)[9].CuAlO2thinfilmshavetypicallybeenpreparedby
high-vacuumphysicalvapourdepositiontechniquessuchaslaser
ablation[9,12],sputtering[13,14],andelectronbeamevaporation
[15].Meanwhile,someothertechniqueshavealsobeenemployed
tofabricateCuAlO2 thin films,suchasspray pyrolysis[16]and chemicalvapourdeposition[17,18].Ifonecandirectlyfabricate CuAlO2patternedthinfilmsusingwetprocesses,suchasink-jet printing,screenprinting,spincoating,dipcoating,spraycoating, roll-to-roll coating, the potential for dynamic development of transparentelectronicsdrasticallyincreases
Towardthisend,chemicalsyntheticroutesofCuAlO2havebeen extensively investigated.For example, a powdercontainingthe CuAlO2phasehasbeenpreparedfroma-LiAlO2byionexchange with CuCl at temperatures below5008C[19].CuAlO2 powders werealsopreparedusingCu2O/CuOandAl2O3powdersinmolten NaOHat3608C[20].ThepolycrystallineCuAlO2waspreparedby heatingthestoichiometricmixtureofhighpurityAl2O3andCu2O
at11008Cfor fourdays inargonatmosphere,pelletizingit and reheatingitat11008Cfortwodays[21].CuAlO2thinfilmswere preparedbythermal decompositionof athin filmcomposed of aluminum isopropoxide [(CH3)2CHO3Al] and copper nitrate [Cu(NO3)25H2O] followed by crystallization at 1000–11008C [21].CuAlO2thinfilmswerealsopreparedbythermal decomposi-tion of a thin film composed of copper acetate monohydrate [Cu(OAc)2H2O] and aluminum nitrate nonahydrate [Al(NO3)39H2O] followed by crystallization at 1000–12508C [22] Moreover, a CuAlO2 thin film was prepared by sol–gel processingusingCu(OAc)2H2Oandalumatrane[Al(OCH2CH2)3N]
asprecursorsandsubsequentthermaltreatmentat9208Cinair [23].Interestingly,CuAlO2nanoparticlescanbesynthesizedwith
A R T I C L E I N F O
Article history:
Received 11 May 2011
Received in revised form 15 June 2011
Accepted 29 July 2011
Available online 5 August 2011
Keywords:
A Electronic materials
A Oxides
B Chemical synthesis
C X-ray diffraction
D Crystal structure
A B S T R A C T
DelafossiteCuAlO2p-typenanostructuredsemiconductorwassynthesizedusingboehmite(g-AlOOH) nanorodsloadedwithcopper(I)acetate[Cu(OAc)]asaprecursor(nanoprecursor).Because Cu(OAc)-loadedg-AlOOHnanorodsarehighlyanisotropic,theytendtoforminherentbunchesconsistingof severalnanorodsduringthecourseofdryingthenanoprecursordispersiondropletonasolidsubstrate
Byannealingthenanoprecursor at11508Cinair,adelafossiteCuAlO2polycrystalwassuccessfully obtained as the dominant phase The CuAlO2 polycrystal is found to exhibit the (110) crystal orientation.ThecrystallineanisotropyofCuAlO2,whichisnotusuallyattainableusingconventional molecularprecursors,ispresumablyoriginatedintheanisotropicmorphologyofthenanoprecursor.The Seebeckcoefficient,resistivityandthermalconductivityoftheCuAlO2polycrystalat300Kwerefoundto
be+560mVK1,1.3Vmand19.4WK1m1,respectively,confirmingthep-typenatureoftheCuAlO2 polycrystal
ß2011ElsevierLtd.Allrightsreserved
* Corresponding author Tel.: +81 761 51 1611; fax: +81 761 51 1625.
E-mail address: shinya@jaist.ac.jp (S Maenosono).
ContentslistsavailableatScienceDirect
j our na l ho me pa g e : w ww e l se v i e r com / l oca t e / m a tr e sbu
0025-5408/$ – see front matter ß 2011 Elsevier Ltd All rights reserved.
Trang 2Cu(NO3)23H2O and Al(NO3)39H2O precursors using the novel
alkalotolerant and thermophilic fungus as a biocatalyst [24]
Amongthem,themostsuccessfulsyntheticstrategymaybethe
hydrothermalroute[25–28].Shannonandcoworkerssynthesized
CuAlO2 atrelativelylow temperatureof500–7008Cunderhigh
pressure[25–27] CuAlO2 waspolycrystalline andcould notbe
isolatedasapurephase.Shahriarietal.employedaTeflonpouch
techniquetosynthesizeCuAlO2fromamixtureofCuO,Cu2O,Al,
andAl2O3.Afteratwo-stepthermaltreatment(1508Cfor5hand
2108C for 48h), the gray-black CuAlO2 microcrystals were
obtained[28].Despitetheseefforts,itis stilla bigchallengeto
obtainpurephasep-typeCuAlO2viaachemicalsyntheticroute
ThedifficultyofsynthesizingpurephaseCuAlO2ismainlybecause
itisamixedoxideandCu+tendstobeeasilyoxidizedtoCu2+.To
obtain pure phase CuAlO2, an accurate control of reactant
compositionbecomesessential.Inaddition,Cu+isunstable,and
thus, is easily oxidized or reduced to Cu2+ or metallic Cu0
dependingon reaction conditions.Therefore, a strategy for the
design of reactionis quite important tosynthesize pure phase
CuAlO2readilyandreproducibly
Thenanocrystallineprecursor(nanoprecursor)hasanumberof
advantages over other molecular precursors When one uses
molecularprecursorstofabricateCuAlO2thinfilms,thenucleation
andgraingrowthwouldbelefttonatureeventhoughtheycanbe
controlledtoacertaindegreebyvaryingannealingconditions.On
theotherhand,ifonecanmixbulksinglecrystalsofcuprousoxide
(Cu2O)andg-alumina(g-Al2O3)togetherpreservingcrystallinity,
itwillbepossibletoobtainpurephasemonocrystallineCuAlO2
Obviously,however,suchprocessisquitechallengingtorealize
Nanoprecursors have the benefits of both molecular and bulk
crystallineprecursors.First,becausenanoprecursorscanbemade
intheformsofboth powderanddispersion, theycanbeeasily
molded into the forms of bulk or thin film depending on
applications.Second,a relatively uniform CuAlO2 polycrystal in
terms of composition and grain size can be obtained using
nanoprecursorsbecauseofthenano-scalesizeand
monodisper-sity.Third,ifnanoprecursorshaveananisotropicshape,i.e.,
one-dimensionalnanorods,two-dimensionalnanodiscs,etc.,theycan
be spontaneously aligned to form an orientation-controlled
higher-orderstructure during evaporation ofsolvent whenone
usesa nanoprecursordispersion.Becauseanisotropic
nanoparti-cles(NPs)aregrownintoaspecificcrystaldirectionasaresultof
preferentialgrowth,iftheyalignandsinterduringcalcination,the
resulting CuAlO2 polycrystal would have unidirectional grains
This is another interesting and important point of use of
nanoprecursors
Inthisstudy,wesynthesizedapurephaseCuAlO2polycrystal using monodispersed boehmite(g-AlOOH) nanorods (NRs)and copper(I) acetate [Cu(OAc)] as precursors g-AlOOH NRs were synthesizedbyhydrolysisanddehydrationofaluminumdiacetate [Al(OH)(OAc)2] via hydrothermal reaction Then, g-AlOOH NRs weremixedwithCu(OAc)inan appropriatesolventtoformg -AlOOH/Cu(OAc)compositeNRs.Byannealingthenanoprecursorat
11508C, a pure phase CuAlO2 polycrystal was successfully synthesized By pelletizing the nanoprecursor followed by annealingat11508C,weobtainedagray-colouredCuAlO2pellet andconfirmedthatithasp-typecarriers
2 Experimental 2.1 Materials Aluminumacetate,basic[Al(OH)(OAc)2]andcopper(I)acetate [Cu(OAc)]werepurchasedfromSigma–AldrichCorp.,andsolvents werepurchasedfromKantoChemicalCorp.Allwereusedwithout furtherpurification
2.2 Synthesisofg-AlOOHnanorods
Inatypicalsynthesis,3.5mmolofAl(OH)(OAc)2wasdissolved
in70mLof distilledwater,andthen theresultingsolutionwas transferred to an autoclave The hydrothermal reaction was performed at 2008C for 12h After which the autoclave was cooleddowntoroomtemperature,thereactionmixturewastaken out, centrifuged and repeatedly washed several times with distilledwater.Theas-preparedproductwasdriedinanovenat
608Covernighttogivewhitepowdersofg-AlOOH(boehmite)NRs Thereactionyieldwasaround74%
2.3 Preparationofg-AlOOH/Cu(OAc)compositeNRs(nanoprecursor)
g-AlOOH(0.12g,ca.2mmol)wasmixedwithCu(OAc)(0.245g,
ca.2mmol)in2mLofpyridine.Pyridinewaschosenbecauseitis capable of dissolving both g-AlOOH NRs and Cu(OAc) to form uniform dispersion We call the dispersion as a nanoprecursor dispersionhereafter.Themixturewasstirredfor6hunderinert environmentwiththeassistanceofsonication.Themixturewas thenevaporatedunderlowpressure(102bar)togivealightgreen solid
2.4 Thermaltreatmentofnanoprecursor
Alightgreensolidwasobtainedbydryingthenanoprecursor dispersiononasolidsubstrate.Thenitwasthermallytreatedfor
2hatdifferenttemperatures(400,600,800,1000,and11508C) Thermal treatment of samplesin reducing atmosphere (Ar/H2) resultedin metallic Cu (asconfirmedfromXRD) Therefore, all thermaltreatmentswereconductedinair.Forthemeasurements
of the Seebeck coefficient, thermal conductivity, and electrical conductivity,thedriednanoprecursor waspelletized at40MPa followedbyannealingat11508Cfor4h.Agraypelletwith12mm diameter was uni-axially pressed once again at 40MPa and subsequentlysinteredat11508Cfor4hinairinordertoobtaina densepellet
2.5 Characterization The samples were characterized by X-ray diffraction (XRD), transmission electronmicroscopy (TEM), selected-area electron diffraction (SAED), energy-dispersive X-ray spectroscopy (EDS), field-emission scanning electron microscopy (SEM), diffuse reflectance Fourier transform infrared spectroscopy (FT-IR),
Fig 1 Crystal structures of (a) delafossite CuAlO 2 and (b) boehmite (g-AlOOH) Red,
blue and pink spheres represent O, Cu and Al atoms, respectively (For
interpretation of the references to color in this figure legend, the reader is
referred to the web version of this article.)
Trang 3surface area analyzer XRD data were obtained using a Rigaku
RINT2500diffractometerwithCuKaradiation(l=1.542A˚;40kV,
100mA).MorphologyofsampleswascharacterizedusingTEMand
SEM.TEMsampleswerepreparedbycastingseveralmicrolitresof
NRdispersionontoacarbon-coatedMogridfollowedbydryingin
air.TEMimageswereobtainedusingHitachiH-7100andH-7650
transmission electron microscopes both operated at 100kV
Elemental analyses were carried out using a Hitachi H-7650
transmissionelectronmicroscopeequippedwithanEDSdetector
SAEDpatternswererecordedonanHitachiH-9000NAR
transmis-sionelectronmicroscopeoperatedat300kV SEMimages were
obtainedusinganHitachiS4100scanningelectronmicroscope
FT-IRspectrawererecordedonaPerkinElmerSpectrum100.TG/DTG
datawereobtainedinairusingaSeikoTG/DTA6200(heatingrate
108Cmin1).Thenitrogenadsorptionanddesorptionisothermof
g-AlOOHNRsweremeasuredat77KusingaBELSORP-maxsurface
areaanalyzer(BELJapan,Inc.).BeforetheBETmeasurement,the
samplesweredegassedanddehydratedinvacuumat2008Cfor
2h.TheSeebeckcoefficient,thermalconductivity,andelectrical
conductivityoftheCuAlO2 pellet weremeasured ona physical
propertymeasurementsystem(QuantumDesign,PPMS)usingthe
thermaltransportoption(TTO)package
3 Resultsanddiscussion
3.1 Synthesisandcharacterizationofg-AlOOHNRs
Fig.2aandbshowshigh-andlow-magnificationTEMimagesof
g-AlOOHNRs,respectively.AsseeninFig.2aandb,theNRshavea
diameterofabout10–30nmandalengthofabout60–400nm Theg-AlOOHNRsarereadilydispersed inwaterorotherpolar solvents athighconcentrationwithexcellentcolloidalstability TheinsetofFig.2ashowsaphotographofaNRdispersiontaken2 weeksafterthepreparation.Noprecipitationoraggregationwas observedby visualinspection.Evenafterseveral months,there wasnosignificantchangeintheappearance.Theexcellentstability
of NRsoccurspresumablybecausethesurfacesof NRsarewell hydrated.Fig.2cshowsanSEMimageofg-AlOOHNRs.Theytend
toforminherentbunchesconsistingofseveralNRs.Thealignment
ofthesehighlyanisotropicNRsispresumablyachievedduringthe courseofdryingthenanoprecursordispersiondropletonasolid substrate Fig 2 shows the XRD pattern of g-AlOOH NRs indicating that the NRs have an orthorhombic g-AlOOH single phase(Fig.1b).TheprimarypeakintheXRDpatterncorresponds the(200)planeasindicatedbytheg-AlOOHreferencepattern Thisindicatesthattheg-AlOOHNRsgrewalongthea-axis.This structural feature did not change even when the reaction conditions were varied In general, chemically synthesized g -AlOOH NPs tend to have one-dimensional morphologies, e.g nanowires[29],nanofibers[30,31]andnanorods[32–42],whichis similartoourresult.Thechemicalequationsofthereactioncanbe expressedas:
Dehydrationreaction(Eq.(2))mightbeinitiatedwithincreasein temperatureordecreaseinpH[43,44].Theas-producedCH3COOH might act as a shape directing agent adsorbing onto specific
Fig 2 High- (a) and low-magnification (b) TEM, and (c) SEM images ofg-AlOOH NRs The inset of panel a shows a photograph of an aqueous dispersion ofg-AlOOH NRs taken
Trang 4-AlOOH,andthus,g-AlOOHNRswereselectivelyformed.Finally,
the surface area of g-AlOOH NRs was determined by the BET
(Brunauer–Emmett–Teller)surfaceareaanalyzer.Asaresult,the
specificBETsurface area wasestimatedtobe38m2g1.Ifone
assumesthatag-AlOOHNRhasacylindricalshapewithdiameter
of30nmandlengthof400nm,thetheoreticalvalueiscalculated
tobe45.7m2g1,whichiscomparabletotheBETsurfacearea.This
result suggests that the surfaces of g-AlOOH NRs are smooth
withoutmesopores
3.2 Phasetransformationbehaviourofg-AlOOHNRs
Because g-AlOOH NRs will be used as a precursor and be
calcinedtogetherwithcopperprecursortoformCuAlO2inthenext
step, thethermal behaviourof g-AlOOH NRs was analyzed by
thermogravimetry(TG/DTG).Fig.3showsa TG/DTGresultwith
respecttotheg-AlOOHNRs.Theslightmasslossobservedbelow
2008C(4%)isattributedtodesorptionofphysisorbedwater.The
masslossobservedbetween3008Cand5008Cisassignedtothe
phase transformation from g-AlOOH to Al2O3 The observed
amount of the mass loss was 16% which agrees well to the
theoreticalmasslossof15%calculatedaccordingtoEq.(3)
In order to examine this further, we carried out structural
characterizationsforthesampleobtainedbythermaltreatmentof
g-AlOOHNRsat6008Cfor1hinair.Afterthethermaltreatment,a
whitesolidmaterialwasobtained.Fig.4aandbshowshigh-and
low-magnification TEM images of the product,respectively As
seeninFig.4aandb,theNRshaveadiameterofabout10–20nm andalengthofabout40–300nm.Comparingthesedimensions with those of g-AlOOH NRs, the NRs have almost the same diameterasg-AlOOHNRs,buthaveashorterlengththang-AlOOH NRs.Fig.4cshowstheSAEDpatternoftheNRs,whichcorresponds
totheg-Al2O3phase.TheXRDpatternoftheNRs(Fig.4d)agrees wellwithcubicg-Al2O3.Thethermaldecompositionofg-AlOOH startswiththeremovalofthestructuralwatermoleculesfollowed
by thecrystallizationof g-Al2O3.The primarypeak in theXRD patterncorrespondsthe(400)planeasindicatedbytheg-Al2O3
referencepattern These results clearly indicate that thephase transformationfromg-AlOOHNRstog-Al2O3NRswascompleted withoutasignificantarchitecturalchange,whichisconsistentwith previous reports [45,46].Surprisingly, theg-Al2O3 NRs arestill
Fig 3 TG and DTG curves ofg-AlOOH NRs.
Fig 4 High- (a) and low-magnification (b) TEM images, and (c) SAED pattern ofg-Al 2 O 3 NRs (d) XRD pattern ofg-Al 2 O 3 NRs Bars indicate reference peaks ofg-Al 2 O 3 (JCPDS
Trang 5readily-dispersible in water or other polar solvents at high
concentrationwithexcellentcolloidalstability
Tofurtherconfirmthephasetransformationfromg-AlOOHto
g-Al2O3,andtoinspectthepurity,weconductedFT-IR
measure-mentsforg-AlOOHandg-Al2O3NRs.Fig.5showstheFT-IRspectra
ofbothNRs.Inthecaseofg-AlOOHNRs,theintensebandsat3300
and 3112cm1 correspond to asymmetric nas(Al)O–H and
symmetric ns(Al)O–H stretching vibrations, respectively These
two strong and well-separated bands are indicative of a good
crystallinity of g-AlOOH NRs [38] The peaks at 1556 and
1651cm1 represent bending modes of adsorbed water Two
peaksat1147and1062cm1areascribedtoasymmetricdasAl–O–
HandsymmetricdsAl–O–Hbendingvibrationinthecrystallattice
Thebroadpeakat620cm1correspondstothevibrationmodeof
AlO6octahedra.Inthecaseofg-Al2O3NRs,twopeaksat3468cm1
(broad) and 1647cm1 (sharp) are observed corresponding to
stretchingvibrationsof–OHgrouponthesurfaceofg-Al2O3NRs
andbendingmodeofadsorbedwater,respectively.Thebroadpeak
at576cm1correspondstothevibrationmodeofAlO6octahedra
Inconsequence,itisverifiedthattheas-synthesizedg-AlOOHNRs
andg-Al2O3NRsarepurephasewithoutimpurities
3.3 Preparationandcharacterizationofg-AlOOH/Cu(OAc)composite
nanoprecursor
To prepare the nanoprecursor for the CuAlO2 synthesis, g
-AlOOH NRs and Cu(OAc) were simultaneously dissolved in
pyridine at a Cu/Al atomic ratio of 1:1 (Scheme 1 TEM and
SEMimagesofthesample(Fig.6aandb)confirmedthatCu(OAc)
werehomogeneously deposited onto all thesurfaces of theg
-AlOOH NRs Fig 6c shows a photograph of the nanoprecursor
dispersion The well-dispersedstable dispersion is a promising
materialforwetprocessing.IfonetakesacloselookatFig.6aandb, Cu(OAc) is found to be homogeneously attached on the NR surfaces.ThehomogeneousattachmentofCu(OAc)on g-AlOOH NRs is thoughtto bea good sign for thesynthesis of uniform CuAlO2polycrystal.Inaddition,thehydrophobicinteraction[47] between Cu(OAc) may facilitate a self-assembly of NRs This structureisquitebeneficialforthesubsequentsolidstatereaction becauseofthehugenumberdensityofreactivespots,wherethe reactiontakesplace,andthequite-shortdiffusionlengththanksto thenano-scaledimension
Tofurther confirmtheformationofg-AlOOH/Cu(OAc) nano-composite,theelementalcompositionofasmallareacontaining just afewNRs wasanalyzed usingTEM-EDS.Fig.6 showsthe TEM-EDSspectrum confirmingthepresenceofCu(26.6at%),Al (26.5at%)andO(46.9at%)inthenanocompositeevidencingthe homogeneousdistributionofCu(OAc)intheNRframework.Note thatMoandCpeaksseenintheEDSspectrumarefromthe carbon-coatedMogrid
3.4 Phasetransformationbehaviourofg-AlOOH/Cu(OAc)composite nanoprecursor
Fig 7 shows TEM and SEM images of g-AlOOH/Cu(OAc) nanocompositeannealedfor 2hatdifferenttemperatures:400,
600,1000,and11508C.AsseeninFig.7a,themorphologyofNRs didnotchangemuchafterannealingat4008C.However,smallNPs (blackspotsintheTEMimage)seemtohaveaggregated.Fig.8a shows an XRD pattern of g-AlOOH/Cu(OAc) nanocomposite annealed at 4008C TheXRD pattern clearlyshows coexistence
of g-AlOOH and CuO phases This indicates that Cu(OAc) is oxidizedtoformCuOaccordingtothefollowingreaction:
Ontheotherhand,thephasetransformationfromg-AlOOHtog
-Al2O3hadnotproceededmuch,andthus,nog-Al2O3wasobserved Whentheannealingtemperaturewasincreasedto6008C,NPs becomesinteredtoformlargerNPsasshowninFig.7b.However, themorphologyoftheNRsremainsunchanged.Fig.8 showsan XRD pattern of g-AlOOH/Cu(OAc) nanocomposite annealed at
6008C.TheXRDpatternshowsthedisappearanceoftheg-AlOOH phaseandacoexistenceofg-Al2O3andCuOphases.Thisresultis reasonablebecausethephasetransformationfromg-AlOOHtog
-Al2O3wasfoundtobecompletedwhenitwasannealedat6008C for1hinair(seeFig.4
Whentheg-AlOOH/Cu(OAc)nanocompositewasannealedat
10008C,bothNRsandNPsbecomesinteredtogethertoformbigger crystalsasshowninFig.7c.Fig.8 showsanXRDpatternofthe sample Itis evidenced that CuO and copper aluminate spinel (CuAl2O4)phasesdominantlycoexist.Thisphase transformation reactioncanbeexpressedas
Atthistemperature,thereactionbetweenCuONPsandg-Al2O3 NRseventuallytakesplace.Todeterminethethreshold tempera-ture forthe solid statereaction between CuOand g-Al2O3, we annealedtheg-AlOOH/Cu(OAc)nanocompositeat8008C.Fig.8c showsanXRDpatternofthesampleannealedat8008C.Inthis case,thecoexistenceofCuOandCuAl2O4phasesisalsoobserved, while the relative intensity of peakscorresponding toCuAl2O4
phase ismuchlowerthanthoseofFig.8d Thismeansthatthe thresholdtemperatureforthesolidstatereactionbetweenCuO andg-Al2O3mightbeinbetween600and8008C
Finallytheannealingtemperaturewasfurtherincreasedupto
11508C As shownin Fig 7d, all nanoparticulatemorphologies disappearedandlargecrystalsemerged.However,thelargecrystal
Fig 5 Diffuse reflectance IR spectra ofg-AlOOH (bottom) andg-Al 2 O 3 NRs (top).
Scheme 1 From preparation ofg-AlOOH/Cu(OAc) composite nanoprecursor to the
synthesis of CuAlO polycrystal.
Trang 6still retains some remnants of NR morphology in its internal
structure.Fig.8eshowsanXRDpatternofthesampleannealedat
11508C.Inthiscase,CuAlO2isthedominantphasewithsomevery
minorpeakscorrespondingtotheCuAl2O4phase.Reflectingthe
largegrainsize,theXRDpeaksaremuchsharper thanthoseof
samplesannealedatlowertemperatures.Thisphase
transforma-tionreactioncanbeexpressedas
Wealsofoundthat,iftheCu/Alatomicratiowaslessthan1,the
relativeintensityofCuAl2O4peaksincreasedatthistemperature
WhentheCu/Al atomicratio was0.5, thedominantphase was
switched toCuAl2O4 The average grain size of CuAl2O4 phase
estimatedbytheScherrerformulausingthe(311)primarypeak
alsodramaticallyincreasedwithdecreasingtheCu/Alatomicratio
Thisphenomenoncanbeelucidatedbytwodifferentmechanisms
First,CuOphaseisconsumedfasterthanAl2O3phaseaccordingto
Eq.(5)preventingthereactionexpressedbyEq.(6),becauseofthe
Al-rich composition Second, CuAlO2 phase once formed might
returntoCuAl2O4phaseaccordingtoEq.(7)
AsseeninFig.8e,therelative(110)peakintensityoftheCuAlO2
polycrystal synthesized using a nanoprecursor followed by
annealingat 11508Cis significantlyenhanced compared tothe
standardpatternforCuAlO2bulkcrystal.Thisresultindicatesthat
ourCuAlO2polycrystalhasanorientationalongthe(110)lattice
planes.Thecrystal structureof CuAlO2can bedescribedas the
alternatestackingofedgesharedAlO6octahedrallayersandCu+
ionlayersperpendiculartothec-axis.EachoftheCu+ionlayersis
linearlycoordinatedbytwoO2anions.TheenergylevelsofCu+
andO2poverlap,givingrisetoincreasedmobilityofholes,which makes them suitable for p-type conduction Because of this structural anisotropy,electrical conductivityalongtheab plane wasreportedtobeover25-foldhigherthanthatalongthecaxis [48], indicating that the main conduction path of the CuAlO2
polycrystalisclosed-packedCu+layers(seeFig.1a).Forthisreason, the(110)-orientedCuAlO2polycrystalsynthesizedinthepresent study is quite promising, because such a structure is usually unattainableusingsolidstatechemicalsynthesis
3.5 Assessmentoftheelectrical/thermalpropertiesofCuAlO2
synthesizedusingg-AlOOH/Cu(OAc)nanoprecursor For the measurements of the Seebeck coefficient, thermal conductivity,andelectricalconductivity,thedriednanoprecursor waspelletizedat40MPafollowedbyannealingat11508Cfor4h
A gray pellet of 12mm diameterwas uni-axiallypressed once againat40MPaandsubsequentlysinteredat11508Cfor4hinair
inordertoobtainadensepellet.Fig.9showsaphotographofa pieceofthepellet(massof33.04mg,areaof3.6mm3.4mmand thicknessof0.6mm).Thedensityofthepelletwas4.4gcm3,87%
ofthetheoreticalvalue.Then,twosidesurfacesofthepelletwere coatedwithgoldpastetocontactwiththeelectrodesofthesample holder.AsseeninFig.9,thecolouroftheCuAlO2pelletisblue-gray
orgray-black.Similarresultshavebeenreportedintheliterature Forexample,thecolourofCuAlO2fibrousmatsappearedtobegray [49],whereasCuAlO2samplessynthesizedbysolidstatereaction hydrothermal processing were correspondingly blue-gray and gray-black[50].InthecaseofCuAlO2synthesizedviaionexchange [19],alargeamountofdeepstatesintheforbiddenbandgapof
Fig 6 (a) TEM and (b) SEM images ofg-AlOOH/Cu(OAc) composite nanoprecursor (c) A photograph of a pyridine dispersion of the nanoprecursor (d) TEM-EDS spectrum of
g-AlOOH/Cu(OAc) composite nanoprecursor.
Trang 7observationagreeswellwiththepreviousreports.Interestingly,as
seeninFig 8 therelative (110)peakintensityoftheCuAlO2
pellet dramatically decreased compared to that of CuAlO2
polycrystalbeforepelletizing(Fig.8e),andtherelativeintensities
ofallpeaksarealmostidenticaltothestandardpatternforCuAlO2
bulkcrystal.Thisresultsuggeststhatthe(110)crystalorientation
was destroyed during pelletizing Note that the weak peaks
observed at 31.3, 36.0, 38.8, 59.6 and 66.3 degrees in Fig 8f
correspond to CuAl2O4(220), CuO( ¯111)/(002), CuO(111)/
(200),CuAl O(511)andCuO(022)/( ¯311),respectively
The Seebeck coefficient (S), resistivity (r), and thermal conductivity (k) of the CuAlO2 pellet were measured using a physicalpropertymeasurementsystem(QuantumDesign,PPMS) using the thermal transport option (TTO) package with two electrodes.Asaresult,thevaluesofS,r,andkat300Karefoundto
be 560mVK1, 1.3Vm, and 19.4WK1m1, respectively The signofSispositiveconfirmingp-typeconductivity.Themagnitude
of theSeebeckcoefficientis fairlylarge (indicative of semicon-ducting behaviour) comparedtothat ofa CuAlO2single crystal (300mVK1at300K)[51].Ontheotherhand,theresistivityisan orderofmagnitudelargerthanthatofsinglecrystal(0.15Vmat
Fig 7 TEM (left) and SEM (right) images of samples after annealing at (a) 400;(b) 600; (c) 1000; and (d) 1150 8C.
Trang 8300K)[51].Thethermalconductivity,k,ofCuAlO2isreportedtobe stronglydependingongrainsize.Forexample,inthecaseofbulk crystal, k is 20WK1m1 at 300K, while it decreases to
2WK1m1at300KinthecasesofnanocrystallineCuAlO2and disorderedCuAlO2crystal[52].Thismeansthatourpelletsample hasalmostthesamethermalconductivityasbulkCuAlO2crystal
4 Conclusions DelafossiteCuAlO2wassynthesizedusingg-AlOOHNRsloaded withCu(OAc)asaprecursor.Theg-AlOOHNRsweresynthesized
bya facilehydrothermaltechnique andtheresultingNRs were foundtogrowalongthea-axis.TheNRsweremonodispersedand readilydispersibleinwaterathighconcentration.Bymixingthe NRswithCu(OAc)inpyridine,g-AlOOH/Cu(OAc)compositeNRs (nanoprecursor) were obtained Cu(OAc) was homogeneously loaded onto all the NR surfaces The nanoprecursor was then annealed at different temperatures As a result, the phase transformation of the nanoprecursor was found to go through thefollowingpath:(1)Cu(OAc)becomeCuONPswhileg-AlOOH NRs remain unchanged at T4008C, where T denotes the annealingtemperature.(2)When T=6008C, g-AlOOHNRsstart
tobetransformedintog-Al2O3NRs.Simultaneously,CuONPsreact with g-Al2O3 NRs to form copper aluminate spinel (CuAl2O4) phase.(3)WhenT iselevatedfurtherto10008C,thefractionof CuAl2O4phaseincreasesasaresultofreactionbetweenCuOandg
-Al2O3 phases (4) When T=11508C, delafossite CuAlO2 phase appears as the dominant phase Interestingly, the CuAlO2
polycrystal exhibits the (110) orientation presumably due to the crystalline anisotropy of the nanoprecursor The Seebeck measurementfortheCuAlO2polycrystalconfirmedthatatroom temperaturetheprimarychargecarriersareholeswitharelatively largeSeebeckcoefficientof560mVK1
Acknowledgements TranV.ThuacknowledgestheVietnamesegovernmentfora
322 scholarship and financial support from the Cooperation Independent Research Foundation (JAIST) The authors wish to thank Prof Tatsuya Shimoda and Kazuhiro Fukada for their generoussupportfortheTG/DTAmeasurements
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