CO optical sensing properties of nanocrystalline ZnO-Au films: Effect of doping with transition metal ions

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Sensors and actuators B: Chemical, 161, 1, pp. 675-683, 2012-01-03

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CO optical sensing properties of nanocrystalline ZnO-Au films: Effect

of doping with transition metal ions

Della Gaspera, E.; Guglielmi, M.; Perotto, G.; Agnoli, S.; Granozzi, G.; Post,

M. L.; Martucci, A.

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SensorsandActuatorsB161 (2012) 675–683

ContentslistsavailableatSciVerseScienceDirect

Sensors

and

Actuators

B:

Chemical

j o u r n al hom 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 / s n b

CO

optical

sensing

properties

of

nanocrystalline

ZnO–Au

films:

Effect

of

doping

with

transition

metal

ions

E. Della

Gaspera

a,∗

_,

_M.

_Guglielmi

a

_,

_G.

_Perotto

b

_,

_S.

_Agnoli

c

_,

_G.

_Granozzi

c

_,

_M.L.

_Post

d

_,

_A.

_Martucci

a

a_INSTM_and_Dipartimento_di_Ingegneria_Meccanica_Settore_Materiali,_Università_di_Padova,_Via_Marzolo_9,₃₅₁₃₁_Padova,_Italy b_Dipartimento_di_Fisica,_Università_di_Padova,_Via_Marzolo_8,₃₅₁₃₁_Padova,_Italy

c_Dipartimento_di_Scienze_Chimiche_and_INSTM_Research_Unit,_Università_di_Padova,_Via_Marzolo_1,₃₅₁₃₁_Padova,_Italy

d_Institute_for_Chemical_Process_and_{Environmental}_Technology,_National_Research_Council_of_Canada,1200_Montreal_Road,_Ottawa,_Ontario_K1A_0R6,_Canada

a

r

t

i

c

l

e

i

n

f

o

Articlehistory: Received14June2011

Receivedinrevisedform27October2011 Accepted4November2011

Available online 12 November 2011 Keywords: Nanoparticles Nanocomposite Thinfilms Sensors

a

b

s

t

r

a

c

t

Zincoxidenanocrystals,pureanddopedwithtransitionmetalions,havebeensynthesizedusingcolloidal techniques;afterpurificationandconcentrationprotocols,theZnOsolutionsaremixedwith monodis-perseAucolloidalsuspensionsandusedforthinfilmdepositions.Theeffectofthedopantionsonthe structural,morphologicalandopticalpropertiesoftheas-synthesizedcolloidsaswellasthe nanocom-positethinfilmshasbeenanalyzedanddiscussed.ThedopantpresencehasbeenfoundtoaffecttheCO opticalsensingpropertiesofthenanocompositeZnO–Aufilms:comparedtopureZnO,anincreasein sensitivityupto80%and55%hasbeendetectedforCo-dopedandMn-dopedZnOrespectively,while Ni-dopedZnOfilmsshowonlyminorimprovements.Thisobservationhasbeenascribedtothemultiple oxidationstatesofcobaltandmanganeseionsthatcanfacilitateelectrontransferbetweenthetarget gasandsemiconductiveoxidematrix,andalsotothelowersurfaceconcentrationofNiionsinsideZnO crystals,ascomparedtoCoandMn.AfastandreversibleresponseafterrepeatedCOexposureshas beendetectedforalltestedsamples,andalinearresponseintensitywiththeorderofmagnitudeofCO concentrationhasbeenobservedinthe10–10,000ppmrange,withalowerdetectionlimitof1–2ppm.

1. Introduction

Theneedforhighqualityandlowtemperatureactivematerials

hasincreasedsubstantiallyinrecentyears,especiallyforprintable

electronicdevices[1],andforsensingapplicationsthatrequire

del-icateandtemperature-sensitivesubstrates,e.g.surfaceplasmon

resonance(SPR)substrateswheretheAuorAglayerisdeposited

overapolymericdiffractiongrating[2].Wetchemicalsyntheses

of inorganic colloidal nanoparticles(NPs) of the active

materi-alsrepresentausefultooltoobtainlow-temperature,crystalline

materialsdispersedinanappropriatesolventthatcanbedirectly

depositedwithavarietyoftechniques(spinning,dipping,casting,

spraying,andink-jetprinting)onthedesiredsubstrates.Sincethe

as-depositedmaterialiscrystalline,thethermaltreatmentofthe

filmcanbededicatedtoobtainthedesiredfinal properties,for

exampletopromotesintering,ortoremove organictemplating

agentsincreasingtheporosity.Moreover,thecolloidalapproach

allowsanaccuratecontrolonthesize,theshapeandother

physi-calandchemicalpropertiesofthesynthesizednanocrystals,allkey

parametersformaterialstobeusedinsensingandcatalysis.

∗ Correspondingauthor.Tel.:+390498275634;fax:+390498275505. E-mailaddress:enrico.dellagaspera@unipd.it(E.DellaGaspera).

Zincoxideisoneofthemostinvestigatedmaterials:itswide

directbandgap(3.36eVat25◦_C_[3]₎_and_its_high_{transmittance}

in thevisibleand nearinfraredrange make ZnO aninteresting

materialforawiderangeofapplications,especiallyin

optoelec-tronicdeviceslikesensors,transistors,solarcells,lightemitting

diodes(LEDs),transparentconductingelectrodesandsmart

win-dowscoatings[4–9].

In this paper we report the synthesis of colloidal ZnO NPs,

pureanddopedwithtransitionmetalions,andtheiruseas

start-ingmaterialstodepositnanocrystalline ZnO–Authin films.The

effectoftheiondopinghasbeenevaluatedwithstructural,

opti-calandelectroniccharacterization,andit hasbeenexploitedto

increase thesensing performances for opticalCO detection. An

enhancementintheopticalresponse,togetherwithanalmostideal

step-likedynamicbehaviorhasbeenobserved.Thedeveloped

pro-cedurecouldbeusedfortherealizationofnanocrystallineinksfor

highqualityprintablesensingmaterialsonavarietyofdifferent

substrates.

2. Materialandmethods

The syntheticprocedureemployed herefor thesynthesis of

ZnO and doped-ZnO NPs is adapted from the work presented

byGamelin and coworkers[10]:briefly, 500mg ofzinc acetate

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676 E.DellaGasperaetal./SensorsandActuatorsB161 (2012) 675–683

dihydratearedissolvedin22.5mLdimethylsulfoxide;separately,

750mgtetramethylammoniumhydroxide (TMAH)aredissolved

in7.5mLethanol.TheTMAHsolutionisaddeddropwise (about

2mL/min) tothe zinc solutionunder vigorous stirring atroom

temperature;after10minthesolutionisheatedat50◦_C_for₁_h

topromoteOstwaldripeningoftheparticles.ToprepareZnONPs

dopedwith2.5%metalions,itissufficienttosubstitute12.5mg

oftheZnprecursorwiththerespectiveamountofthedopant

pre-cursor,keepingthetotalZn+dopantmolarityconstant.TheZnO

dopingwasperformedwithmanganese(frommanganeseacetate

tetrahydrate),nickel(fromnickelacetatetetrahydrate)andcobalt

(fromcobaltacetatetetrahydrate).After1hat50◦_C,_the_solutions

werecooled downto roomtemperature, precipitatedwiththe

minimumamount(about3:1involume)ofmethylethylketone,

centrifugedat1500rpmfor5minandredispersedinethanoltoa

finalnominalconcentrationof0.8MinmolarzincorZn+dopant.

GoldcolloidswerepreparedaccordingtotheTurkevichmethod

[11,12]byreducingHAuCl4withtrisodiumcitrateinwater.Briefly,

12mLof 1%trisodiumcitrate aqueoussolutionwasaddedto a

200mLboilingsolutionof0.5mMHAuCl4.Afterthesolutionturned

a red-wine colour, it was stirred at boiling point for an

addi-tional15minand thenwascooleddowntoroomtemperature;

10,000g/molaveragemolecularweightpoly(N-vinylpyrrolidone)

(PVP)wasdissolvedinwatertoyielda50g/Lconcentration,and

thenthis solutionwasmixed withaqueousgold colloidsunder

constantstirringaccordingtotheratiogPVP/molAu=1000.After

2hthesolutionwasconcentratedinarotaryevaporatorandAu

NPswereprecipitatedwithacetone,centrifugedat4000rpmfor

5minandre-dispersedinethanolleadingtoa30mMconcentrated

solution.

Thesolutionsforthefilmdepositionswerepreparedby

mix-ingtheethanolicsuspensionofZnONPsandPVP-cappedAuNPs

leadingtoafinalAu:Znmolarratioofabout0.06.Aufreesamples

werealsopreparedforcomparisonpurposes,bysubstitutingtheAu

colloidalsolutionamountwithpureethanol.Allthesampleswere

depositedbyspincoatingwithamultilayerprocedure:solutions

werespunat2500rpmfor30soneitherSiO2orSisubstrates,and

thefilmswerestabilizedat400◦_C_for₅_min_on_a_hot_plate._This

pro-cedurewasrepeated4times,andthelastannealingwasperformed

at500◦_C_for₁_h_in_a_muffle_furnace.

Thecrystallinephasesofthethinfilmswerecharacterizedby

X-raydiffraction(XRD) byusinga PhilipsPW1710

diffractome-terequipped withgrazing incidence X-rayoptics. Theanalyses

wereperformedat0.5◦ _incidence,_using_CuK␣_Ni_filtered

radia-tionat30kVand40mA.Themeancrystallitediameterhasbeen

evaluatedusing the Scherrer equationperforming a Lorentzian

fit of themostintense diffraction peaks.Transmission electron

microscopy(TEM)analysisofnanoparticlesdepositedfrom

col-loidal solutions deposited oncarbon-coated coppergrids were

performedwithaPhilipsCM20STEMsystemoperatingat200kV.

UV–visabsorptionspectraofcolloidalsolutionsandthinfilmswere

takenusingaJASCOV-570standard spectrometer.Ellipsometry

measurementswerecarriedoutonaJ.A.WoollamV-VASE

Spec-troscopicEllipsometerinverticalconfiguration,attwodifferent

anglesofincidence(60◦_and₇₀◦_)._The_surface_composition_of_thin

filmswas analyzed by X-ray photoelectron spectroscopy (XPS)

usingamodified VGESCALABMKII(Vacuumgenerators,

Hast-ings,England)whereatwin(Mg/Al)anodeX-raysource,asputter

gun,andahemisphericalelectrostaticanalyzerwithafivechannel

detectoraremounted.TheXPSdatareportedwereobtainedusing

Mg-K␣radiation(1253.6eV)astheexcitationsource.Thecharging

effecthasbeencompensatedbyreferencingallthebinding

ener-gies(BEs)totheC1speakat284.8eV.Photoemissionspectrahave

beenobtainedatroomtemperatureusingnormalemission

geom-etry.BeforetakingXPSmeasurements,thesamplesweredegassed

overnightunderUHV(pressurelowerthan10−8_mbar)._In_order_to

Fig.1. XRDpatternsofZnOanddoped-ZnONPs(dopinglevelis2.5%)heatedat50◦_C for1h;thetheoreticaldiffractionlinesforwurtzitecrystallinephaseareindicated atthebottom.

Table1

CrystallitesizeofZnOanddoped-ZnONPs(dopinglevelis2.5%)heatedat50◦_C_for 1hevaluatedwiththeScherrerequationusingtheFWHMofthefivediffraction peaksdetectedforeachsample(seeFig.1).

Sample Crystallitessize(nm)

ZnO 7.6±0.5

ZnO:Co 5.8±0.7

ZnO:Ni 5.7±0.4

ZnO:Mn 5.0±0.5

derivethechemicalcompositionofthefilms,thetheoretical pho-toemissioncrosssectionsbyYehandLindauhavebeenused[13],

whiletheelectroninelasticmeanfreepathhasbeencalculatedwith

theTPP2algorithm[14].Atomicforcemicroscopy(AFM)height

profileswererecordedusingaNT-MDTSolverProinstrumentin

tappingmode.Thesurfacestructureofthenanocompositefilms

hasbeeninvestigatedwithanxTNovaNanoLabscanningelectron

microscopy(SEM).Opticalgassensingtestswereperformedby

makingopticalabsorbance/transmittancemeasurementsoverthe

wavelengthrange400nm<<800nmwithsamplefilmsmounted

onaheaterinsideacustom-builtgasflowcellcoupledwithaVarian

Cary1Espectrophotometer.Thinfilmswereexposedtofour

differ-entconcentrations(10ppm,100ppm,1000ppmand10,000ppm,

i.e.1%v/v)ofCO,allbalancedindryair,ataflowrateofabout

0.4L/min.Testswereperformedatanoperatingtemperature(OT)

of300◦_C._The_substrate_size_was_{approximately}₁_cm_×₂_cm_and_the

incidentspectrophotometerbeamwasnormaltothefilmsurface

andcoveringa6mm×1.5mmsectionarea.

3. Resultsanddiscussion

Zinc oxide NPs synthesized with this method present the

wurtzitecrystallinestructure,withameancrystallitedimension

rangingfrom3to10nmdependingonthesyntheticparameters.

SincethefreshlypreparedNPsarefoundtocontinuouslygrow

forseveralhoursatroomtemperaturedue toOstwaldripening,

it wasdecided toperforma mild heating at 50◦_C _for ₁_h_after

theTMAHadditioninordertoacceleratethisprocessandobtain

stable colloidalsolutions. XRD patternsof ZnO and doped-ZnO

NPsaftertheannealingprocedurearereportedinFig.1.Typical

diffractionpeaksforwurtziteZnO(JCPDSno.36-1451)areclearly

detectedinallsamples,butadifferenceinpeakbroadeningcanbe

observed,withthecrystallitesizeofdopedNPsbeingsignificantly

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161 (2012) 675–683 677

Fig.2. TEMimagesof(a)pureZnOcolloidsand(b)Aucolloids.

nanocrystalshavethelowercrystallitesize,andthisismaintained

alsointhethinfilmsdepositedfromthesesolutionsandheatedat

500◦_C_(see_below).

Thiseffecthasbeenascribedtothedifferenceinsizebetween

Zn2+_and_dopant_ions:_since_these_dopants_are_{substitutional}_to_zinc

intheZnOlatticestructure[10,15,16],thedopedcrystallitesgrow

mechanicallystressed.Thence,extendedgrowthofthecrystalsis

expectedtobeenergeticallyunfavorablesincethevariationin

lat-ticeenergyisnotbalancedbythereductioninthesurface/volume

ratio.

Fig.2showsTEMimagesofas-synthesizedZnO andAuNPs:

almostsphericalZnOnanocrystalswithanaveragediameter

rang-ingfrom4to7nmcanbeseeninFig.2a,ingoodagreementwith

theXRDevaluation,suggestingthattheNPsaremainly

monocrys-talline;despitethelowcontrastofZnO,somelatticeplanescan

beseen,confirmingthecrystallinewurtzitephase.Monodisperse

(standarddeviationabout7%)sphericalAucolloidsof13nmin

diametercanbeeasilyseeninFig.2b:somefacetingand some

triangularshapescanbedetected,quitecommonphenomenafor

AuNPssynthesizedwiththecitratereductionmethod.

FromUV–visabsorptionspectraoftheZnOcolloidalsolutions

reportedinFig.3,it ispossibletoseethesharpUVabsorption

onsetofZnOnanocrystalsatabout350–360nm.FordopedZnO

Fig.3.UV–visabsorptionspectraofethanolicsuspensionofZnOcolloids.Theinsets show(a)theeffectofthedopantsontheUVabsorptiononsetofZnOand(b)the distinctiveCo2+_ion_absorption_bands.

theonsetisblueshifted,ashighlightedintheinset(a)ofFig.3.

Thisisduetothedopanteffectinreducingtheparticlesizethat

inturnleadstohigherenergiesassociatedtotheopticalbandgap,

duetothequantumconfinementeffect.Infactithasbeen

con-firmedthatthesizedependenceofZnOelectronicpropertiescan

bepresentforcrystalssizeupto7nm[17],inagreementwiththat

evaluatedbothfromXRDandTEM.Thethreedistinctive

absorp-tionbandsinthegreen–redrangethatcanbeseenintheinset

(b)ofFig.3areascribedtothed–dadsorptionlevelsofdivalent

Co2+_ions_in_tetrahedral_coordination_[18]_,_in_this_particular_case_in

thetetraoxocationcoordinationenvironmentofwurtzite[19,20],

consistentwithaCo2+_{substitutional}_site.

EthanolicsuspensionsofAuandZnONPsaremixedtogether

anddirectlyusedforfilmdeposition,asdescribedinSection2.All

thecharacterizationspresentedinthefollowing,ifnotdifferently

specified,refertoAu-containingfilmsannealedat500◦_C,_since_no

significantdifferenceinstructural,morphologicalandoptical

prop-ertiesofZnOfilmsisdetectedwhenAuNPsareembeddedinside

theoxidematrix(excepttheexpectedpresenceoftheAudiffraction

andplasmonpeaks).

TheXRDpatternsofthesynthesizedfilmsareshowninFig.4:

bothAu(JCPDSno.04-0784)andZnO(JCPDSno.36-1451)

diffrac-tionpeaksareclearlyevident,andabigdifferenceintheFWHM

of ZnO peakscompared totheas synthesizedNPs canbeseen

Fig.4.XRDpatternsofZnOanddoped-ZnOthinfilmscontainingAuNPsannealedat 500◦_C;_the_theoretical_diffraction_lines_for_wurtzite_(straight_lines)_and_Au_(dashed lines)crystallinephasesareindicatedatthebottom.

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678 E.DellaGasperaetal./SensorsandActuatorsB161 (2012) 675–683 Table2

CrystallitesizeofZnOanddoped-ZnOthinfilmscontainingAuNPsannealedat 500◦_C_for₁_h_evaluated_with_the_Scherrer_equation_using_the_FWHM_of_the_five_ZnO diffractionpeaksdetectedforeachsample(seeFig.4).

Sample ZnOcrystallitesize(nm)

ZnO 19.5±0.7

ZnO:Ni 19.0±0.6 ZnO:Co 15.8±0.6 ZnO:Mn 10.7±0.4

Table3

SurfacestoichiometriesoftheZnOanddoped-ZnOthinfilmscontainingAuNPs annealedat500◦_C_derived_from_the_XPS_data_reported_in_Figs.₅_and_6.

Atomiccomposition ZnO ZnO:Ni ZnO:Co ZnO:Mn

Zn 48.5 47.0 47.6 42.8

O 49.0 50.6 49.0 51.4

Au 2.5 2.0 2.4 2.9

M2+_dopant _– _0.4 _1.0 _2.9

(comparewithFig.1).Thisisduetocrystalgrowthpromotedby

hightemperatureannealing.TheaveragediameteroftheZnO

crys-tallitesisbetween11and20nmaccordingtothetypeofdopant,

asshowninTable2.

Again,theinfluenceofthedopantistoreduceoralmostprevent

thetemperature-drivenZnOcrystallitegrowth,evenifthiseffect

ismuchmoreevidentforMncomparedtotheotherions.

Besidesthealreadydiscusseddifferenceindopantsize,a

pos-sibleexplanationofthisbehaviorcanbeascribedalsotodifferent

oxidationstatesofthecations:whileZnandNiionsexistmainlyin

the+2oxidationstate,CoandMnpresentmultipleoxidationstates

(+2and+3themorecommon,butMncanbealsofoundin

com-poundswithhigheroxidationstates).Thepresenceofanionwitha

differentoxidationstate,higherthanZn2+_,_would_lead_to_a

substan-tialamountofnegativelychargedZnvacancies,inducingahigher

extentofmechanical stressinthelattice,inhibitingthe

crystal-litegrowth.ConsideringthatsamplesdopedwithCoandMnshow

thesmallestcrystallitesizes,whiletheeffectofNiinreducingthe

crystallitesizeisnotsopronouncedafterthe500◦_C_annealing,_the

differentoxidationstateofthedopantionscouldbeanexplanation

ofthedifferenceincrystalgrowth.EvenifCo2+_absorption_bands_are

onlydetectedinthecorrespondingopticalspectra,aminor

contri-butionfromCo3+_species_cannot_be_excluded._Actually,_ZnO_crystals

dopedwithseveralions,includingFeandCo,havebeenprepared

andanalyzedbyXANES[16]:thepresenceofbothFe2+_and_Fe3+

specieshasbeendemonstrated,whileinthecaseofCothe

major-ityoftheionsaredivalent,evenifabroadeningatshorterlengthsof

peakprofilesoftheradialdistributionfunctionshasbeenascribed

totheeffectofCo3+_species.

Asexpected,Audiffractionpeaksarenotevidentlyaffectedby

thetypeofZnONPs,andtheydonotundergoanyrelevantchange

inshape, indicatinggood stabilityof AuNPsinsidetheseoxide

matrixes,asobservedalsointhepastforTiO2-based

nanocompos-ites[21].

Inordertoevaluatethesuccessfulincorporationofdopantions

insidetheZnOfilmsandtoevaluatetheiroxidationstate,XPS

anal-ysiswascarriedout.AsfarastheZnOmatrixandtheAuNPsare

concerned,theXPSresultsarequitehomogeneousinallthe

exam-inedsamples:thephotoemissionspectraoftheZn2pandAu4f

corelevelsdonotshowsignificantchangesinallthesamples(see

Fig.5)andarecompatiblewiththepresenceofstoichiometriczinc

oxideandmetalAuNPs,respectively.InTable3theelemental

sur-facestoichiometriesofthedifferentfilmsarereported:theamount

ofAufoundinthedifferentsamplesisonlyslightlyvaryingranging

from2.9%intheMndopedsampleto2.0inthecaseoftheNidoped

one.However,itcanbeseenthatlargedifferencescanbeobserved

Table4

ThicknessandsurfaceroughnessforthefoursamplescontainingAuannealedat 500◦_C_evaluated_from_AFM_images.

Sample Thickness(nm) Surfaceroughness(nm)

ZnO 92.3±2.8 3.4

ZnO:Co 67.9±2.5 2.3

ZnO:Ni 78.1±5.9 2.4

ZnO:Mn 67.6±3.3 2.7

intheconcentrationofthedopantitself,whichvariesfrom0.4% inthecaseofNito2.9%inthecaseofMn(i.e.almostoneorderof magnitude).

One possible explanation for this trend is surface

segrega-tion [22] of the larger dopant ions (the ionic radii scale as

Mn2+_>_Co2+_>_Ni2+₎_induced_by_the_strain,_considering_that_the

prob-ingdepthofXPSmeasurementsisratherlow(3–5nm).

Itisinterestingtonotethatthereisalsoadirectrelationbetween

thedopantsurfaceconcentration,asdeterminedby

photoemis-sion,andthecrystallitesize,asdeterminedbyXRD,whichisabulk

sensitivetechnique:themorethedopantconcentrationatthe

sur-face,thesmalleristhecrystallitesize(asshowninTables2and3).

Thereforeitseemsreasonabletointerpretbothcompositionaland

structuraldatawitha uniqueframework that istheamountof

dopinginducedstrain.

Fortheidentificationofthechemicalstatesofdopingcations,

highresolutionspectraofthecorresponding2plevelshavebeen

measured(see Fig.6).A clear and definitiveassessment ofthe

chemicalstatesisratherdifficultsincethesemetalsarepresent

onlyintraceamountsand theexpectedchemicalshiftbetween

differentoxidationstatesisquitesmall.Moreusefulinthisrespect

istoanalyzethesatellitefingerprintofthephotoemissionlines,

which,inthecaseof2ptransitionmetalsisverydistinctiveofthe

oxidationstate[23].Thusbycomparingthepositionsandrelative

intensitiesofthemainphotoemissionpeakandsatellites[24–28]

itcanbeconcludedthatallthedopingionsarepresent

predom-inantlyasdivalentcations.Howeverthelowsignaltonoiseratio

preventsexcludingthepossibilityofthepresenceofsmall

contri-butionsfromminorityoxidationstates.

Atomic force microscopy measurements were conducted to

analyzesample morphologyandevaluatethickness: Fig.7a and

Table4showtheresultsoftheAFManalysis.Ascanbeseen,the

sur-faceofthesamplesisquitestructured,duetoZnOcrystallinegrains;

neverthelessthesurfaceroughnessisnothigh,beingbelow4nm

forallthesamples,providinggoodopticalqualityandabsenceof

scatteringphenomena.OnlyoneAFMscanhasbeenpresentedhere

becausethedifferentsamplesarerather similar,andjustminor

differencesinsurfaceroughnesshavebeendetected.Thethickness

evaluatedbyscratchingthesamplesandusingtheAFMtiptoscan

theedgeofthelines,isintherange70–90nmforallthesamples,

butatrendinthethicknesscanbeseen,withthepureZnOsample

beingthethickest,andtheCo-dopedandMn-dopedsamplesthe

thinnest.

AconfirmationofthesurfacemorphologycomesalsofromSEM:

atypicalmicrographofaZnO–AufilmisreportedinFig.7b.Ascan

beseenthesurfaceroughnessaswellasthehighporosityextent

canbeappreciatedfromthisimage.

Opticalabsorptionspectra ofthe Au-containingsamplesare

showninFig.8:theUVabsorptiononsetofZnOisstillclearly

evi-dent,anditisslightlyredshiftedcomparedtoas-synthesizedNPs;

moreoverthedifferencebetweenthe4samplesisnotaslargeas

theonefoundforthecolloidalsolutions.Thisbehaviorisduetothe

temperature-drivencrystalgrowththatcausestheparticlenotto

besubjectedanymore(oronlyweakly)tothequantum

confine-mentregime,reachingtheZnObulkbandgapvalue.AlsotheAuSPR

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161 (2012) 675–683 679

Fig.5. XPSAu4fandZn2pdataofZnOanddoped-ZnOthinfilmscontainingAuNPsannealedat500◦_C.

Fig.6. XPSCo2p,Mn2pandNi2pdataofdoped-ZnOthinfilmscontainingAuNPsannealedat500◦_C.

shiftedcomparedtotheAuSPRpeakfortheas-synthesized

col-loidsinethanolrecordedat523nm.Thisisduetothedifference

inrefractiveindexbetweenethanolandthenanocrystallinezinc

oxidefilms,thelatterbeingmuchhigherthantheformer.

Infact,therefractiveindexvaluesoftheZnOfilmsarefound

tobeinthe1.8–1.85rangeat600nm,asmeasuredwith

spectro-scopicellipsometry,substantiallyhighercomparedtotherefractive

indexofethanolatthesamewavelength(1.36[29]).Byusingthe

effectivemediumapproximation(EMA)models,theporosityofthe

filmscanbeestimatedfromthecomparisonbetweenthe

experi-mentalrefractiveindex,andthetheoreticalvalueforfullydensified

material(2.01at600nmforwurtziteZnO[30,31]):the

nanocom-positefilmisthusmodeledasaneffectivemediumcomposedof

denseZnOandpores.AmongthedifferentEMAmodelsthatcan

beused,theBruggemannfunction[32]isuniversallyacceptedand

widelyemployedforporosityevaluation [33,34].Table5shows

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Fig.8. OpticalabsorptionspectraofZnOanddoped-ZnOthinfilmscontainingAu NPsannealedat500◦_C.

Table5

Experimentalrefractive indexat600nm andestimatedporosityforthe sam-pleswithoutAuannealedat500◦_C,_evaluated_from_{spectroscopic}_ellipsometry measurements.

Sample Refractiveindex Porosity%

ZnO 1.84 16

ZnO:Co 1.84 16

ZnO:Mn 1.80 20

ZnO:Ni 1.82 18

refractiveindexvaluesat600nmandcorrespondingporesvolume fractionsforthefoursamples:ascanbeseen,porosityvaluesarein the16–20%rangeforallsamples.Theellipsometricmeasurements wereperformedonAu-freesamplesinordertoobtainaCauchy dispersionofthe refractiveindex in thevisible range,because, asaconsequenceof thestrongSPR absorptionpeakof AuNPs, ZnO–Aufilmspresentaperturbationoftherefractiveindex dis-persioncurve.Nevertheless,intheNIRregion,therefractiveindex valuesofAu-freeandAu-containingsamplesarealmostidentical, suggestingthattheporosityamountisonlyweaklyaffectedbyAu NPspresence.

Besidesthealready mentionedred shiftof AuNPsSPR peak whenthemetalisembeddedinside aZnO matrix comparedto ethanol,alsoaclearbroadeningandasmallredshiftoftheAuSPR peakisdetectedwhenAuNPsareembeddedinsideadoped-ZnO matrixwithrespecttopureZnO.Thiseffectmightberelatedtoa differenceintheelectronicpropertiesofthedoped-ZnOcrystals comparedtotheundopedones,thatcaninteractwiththesurface freeelectronsofAuNPs,affectingtheSPRfrequencies,asdescribed bytheMietheory[35].

FromtheintensityoftheZnOexcitonpeakitispossiblehave

someinsightonthethicknessofthefilms,asreportedinthe

lit-erature[36]: theundopedZnO filmis thethickest, followed in

orderbyNi-doped,Co-dopedandMn-doped.Thesedataareingood

agreementwiththethicknessestimatedfromAFMmeasurements.

Sincetheprecipitation–redispersionprotocolforallthedifferent

ZnOcolloidalsolutionswasthesame,andsowerethedeposition

parameters,thisdifferenceinthicknesscanbeascribedtoa

dif-ferentprecipitationthresholdoftheZnOcolloidsaccordingtothe

typeofdopant,ortodifferentsolubilityoftheseparticlesinthe

sol-vent/nonsolventmixture(beingalsothesizeoftheparticlesslightly

different).Thisleadstoadifferenceinnanocrystalsconcentration

andalsotheviscosityof thefinal solutionusedfordepositions.

Moreover,alsoexperimentalvariabilityofthespinningprocedure

cannotbeexcludedasacauseofsuchthicknessdifferences,aswell

asalimitedsinteringofZnOcrystalsduringtheannealingofthe

filmsduetothepresenceofthedopants.Theactualthicknessof

thesamplehasthustobetakenintoconsiderationwhencomparing

thegassensingresults.

Thesematerialsweretestedasopticalsensorsfor CO

detec-tionwheredifferentoperatingtemperatures(OTs)wereused,in

orderto identifythebestoperative conditions.Allthesamples

gavelowresponsebetween25◦_C_and₂₀₀◦_C_OTs,_with_quite_long

transienttimes,whileinthe250–350◦_C_range_the_response_was

easilydetected,andallthetestsreportedinthefollowinghavebeen

carriedoutat300◦_C_OT._Au-free_samples_did_not_give_any

appre-ciableopticalresponse,asalreadyobservedinthepast[37],while

Au-containingsamplesshowedthetypicalwavelengthdependent

behaviorofAu-dopedtransitionmetaloxides[38,39].Allsamples

presentablueshiftoftheAuSPRpeakwhenexposedtothe

reduc-inggas,consistentwiththeoxidationofCOandthesubsequent

electroninjectioninsidethen-typeZnOmatrix,leadingtotheshift

tohigherfrequencies oftheplasmonband. Itisreasonablethat

otherreducing gasses likeH2 willreactsimilarly withtheZnO

matrix,whileoxidizinggasseslikeNO2 willcausea redshiftof

theSPRpeak.Thesensingperformancesweremonitoredusingthe

opticalabsorptionchangeparameter,definedasthedifferencein

absorbanceduringtarget gasexposureand duringair exposure

(OAC=ACO−Aair).Fig.9ashowstheOACcurvesforthe4prepared

samplesexposedto1%COat300◦_C_OT.

Thedistinctiveshapeoftheresponseversuswavelengthcurve

can be seen, resulting in wavelengths were the response is

maximized (about 570nm and 670nm, positive and negative,

respectively)andwavelengthsinwhich thesensing responseis

extremelylowornull(below450nm,∼625nmandover800nm),

allowingtotunethematerialsensitivityandpossiblyselectivityby

usingwavelengthmodulation.

Adifferenceintheintensityoftheresponsecanbeseen

compar-ingthefoursampleswithundopedZnOshowingthelessintense

OACmaximumandminimum,despitehavingthehighest

thick-ness,whiletheCo-dopedsampleisthemostsensitivetoCO,even

thoughthethicknessismuchlowerthanwiththepureZnOsample.

Thisdifferenceinresponse becomes clearerwhen

normaliz-ingthesevaluestothesamplethicknessexpressedinmicrons,as

showninFig.9b.Thenormalizationhasbeenperformedby

cal-culatingtheaverageresponseofeachsampleand summingthe

absoluteOACvaluebothatthepositiveandnegativemaximum

wavelengths,andthendividingbythesamplethicknessas

mea-suredwithAFM (thustheunitsareexpressedin␮m−1).Cobalt

andmanganesedopedsamplesshowevidentlythebestsensing

response,whileundopedZnOhasthelowerresponse,confirming

thepositiveeffectofdopingZnOcrystalswithtransitionmetalions.

TheenhancingeffectofCoandMnionsmightbeagainrelatedto

theirmultipleoxidationstates,thatallowsavariationofcharge

car-rierconcentrations,therebyimprovingthematerialperformances,

asshownintheliteratureforZnOfilmsdopedwithtrivalentions

likeAl3+_or_Ga3+_[8,40,41]_,_as_a_matter_of_fact_Ni-doped_ZnO_does_not

showagreatimprovement,andaccordingtowhatwasdiscussed

earlier,thismightbeascribedtothemainoxidationstateofNi,

beingthesameasZn.Moreover,asshownabove,theNiamount

atthesurfaceofZnOcrystalsisratherlowcomparedtotheother

dopants,andsincethegassensingmechanismincludesasurface

process,thismaybeanadditionalexplanationtotheratherlow

opticalresponseoftheNi-dopedZnOnanocomposite.

Sincethereisnoconfirmationofthedifferentoxidationstateof

thedopantions,duetotheaccuracyprovidedbyXPS

measure-mentsnotallowingtodistinguishclearly possible3+ species,it

cannotbestatedunequivocallythattrivalentionsarepresent.In

anycase, evenassumingthatallthespeciesareinthe2+state,

the presence of cations that can exist with different oxidation

statesisusefulinthegassensingperformancebecausethedopants

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161 (2012) 675–683 681

Fig.9.(a)Opticalabsorptionchangeplot(OAC=ACO−AAir)forZnOanddoped-ZnOthinfilmscontainingAuNPsannealedat500◦Cfor1h.(b)NormalizedOACintensityfor thefourtestedsamples.

theatmospheretheyareincontactwith,facilitatingtheelectron

transferbetweenthegasandmatrix,improvingthesensing

perfor-mances.Forexample,itisknownthatWO3crystalswhenexposed

tohydrogen,undergotheformationofsomeW5+_species_in

oppo-sitiontothepristineW6+_ions,_changing_both_the_electrical_and_the

opticalpropertiesofthematerial[42].

All ZnO doped samples show acceptable dynamic behavior

afterrepeatedexposurestodifferentCOconcentrationsinair,as

reportedinFig.10: theresponsetimesarerelativelyfast,while

therecoverytimesarealittlelongerbutstillsatisfactory.If

con-sideringanair-1%CO–aircycle,theresponsetimes(definedasthe

timesneededtoreach90%ofthetotalresponse)forallthe

sam-plesareinthe15–20srange,whiletherecoverytimes(defined

as thetimes neededto recover90% of theinitial baseline)are

around2–3min.IfconsideringlowerCO concentrationsinstead,

responsetimesdonotchangesignificantly,whilerecoverytimes

Fig.10.Dynamicresponseof(a)ZnO;(b)ZnO:Ni;(c)ZnO:Coand(d)ZnO:Mn sam-plescontainingAuNPsatthewavelengthcorrespondingtothemaximumofthe OACcurve(Fig.9a)andat300◦_C_OT,_during_exposure_to_different_CO_{concentrations} expressedinppm.

aresubstantiallylower.Ascanbeseen,eventhoughthesesamples

arerelativelythin,theyareabletoeasilydetectdownto10ppm

CO,withalinear-logarithmicrelationshipbetweenintensityofthe

opticalvariationandtargetgasconcentration,asalreadyobserved

fornanocrystallineTiO2–Ausamplespreparedbyourgroup[21].

Comparingtheresponseintensitiesofthetime-resolvedtests,it

canbeseenagainthatsamplesdopedwithcobaltandmanganese

ionsshowthebestperformance,whiletheundopedZnOhasthe

lowerintensity:thisisaconfirmationofthenormalizedresponse

showninFig.9b.Fromthesedynamicteststhesensitivityandthe

detectionlimitscanbeevaluated.Theparameterusedforthis

eval-uationistheresponseintensity(RI),definedasthefollowing:

RI=

OAC t

wheretisthethicknessofthesamplesexpressedinmicrons.

Basi-cally,theresponse intensityisdefined astheabsolutevalueof

OACtakenatonewavelengthandnormalizedtothesample

thick-ness;theRIvaluehasbeendefinedinthiswayinordertotake

intoaccountthedifferenceinthicknessbetweenthefoursamples,

andtoobtainalwayspositivevaluesofthesensingresponse,since

consideringtheOACcurvesshowninFig.9a,thereisarangeof

fre-quenciesinwhichtheOACisnegative.MeasuringtheRIvaluesfor

twodifferenttestsforeachsample,andplottingtheaveragevalue

versustheCOconcentration(Fig.11),alinearrelationshipbetween

theresponseintensityandtheorderofmagnitudeofthetargetgas

concentrationcanbeseen,andalsoextrapolatingthedataatlower

concentrationsthedetectionlimitcanbeestimatedtobeinthe

1–2ppmrange.Thelinearfitsoftheexperimentaldatashownas

dashedlinesconfirmthelinearrelationship,andmake the

cali-brationcurvecalculationsimple.Thesensitivityishencedefined

astheslopeoftheresponsecurveversusconcentration,andthe

Co-dopedsampleshowedthehighestsensitivitytogetherwiththe

highestresponseintensityasisalsoindicatedfortheMn-doped

sample.BydefiningthesensitivitySasthefollowing:

S= RI

Log([CO])

where[CO]istheCOconcentrationexpressedinppm,theSvalue

isexactlytheslopeofthelinearfitspresentedinFig.11.

The obtained sensitivity values – taking the horizontal axis

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Fig.11. SensitivityplotsforCOdetectionforthefoursamplestestedatthe wave-lengthcorrespondingtomaximumofOACcurvesandat300◦_C_OT._Linear_fits_for thefourexperimentalsetsofdataarealsoreported.

2.9× 10−2␮m−1ppm−1forundoped,Ni-doped,Mn-dopedand

Co-dopedZnOfilms,respectively.Atrendisclearlyidentified,being

Co-doped ZnOfilm thebetter,withthesensitivity almost

dou-bledcomparedtopureZnO(theactualincreaseismorethan80%);

thehighestsensitivitytogetherwiththehighestresponseintensity

makescobaltionstheidealdopanttoenhanceZnOsensing

prop-erties.Inaddition,promisingsensingpropertieshavebeenshown

alsoforZnO:Mnsamples(sensitivityisincreasedmorethan55%),

whileonlyminorpositiveeffectshavebeendetectedforNi-doped

films(sensitivityimprovedby25%).

4. Conclusion

High quality nanocrystalline ZnO and transition metal

ions-doped ZnO films have been prepared starting from colloidal

solutionsofcrystallineNPs,andtheincorporationofmonodisperse

AuNPshasalsobeenperformed.Morphologicalandoptical

charac-terizationofbothcolloidalsolutionsandthinfilmsconfirmedthe

dopantspresenceandtheireffectonthecrystallitesizesandonthe

UVabsorptiononsetofZnO.OpticalgassensingtestsforCO

detec-tionshowedafast,reversibleandlinear-logarithmicresponsein

the10–10,000ppmrangeforallthesamplestested,andthe

pos-itiveeffectofthedopantshavebeenexploitedtoincrease both

themagnitudeoftheresponseandthesensitivityofthe

nanocom-posites.CobaltionsinsidetheZnOlatticestructurearefoundto

increasetheCOsensitivityofZnO–Aufilmsbymorethan80%.

Thesenanocrystallinecolloidalsolutionsareapromisingtool

fortherealizationoflow-temperatureactivethinfilmsformany

applicationslikesensors,LEDs,transistors,andcoatingsingeneral.

Acknowledgements

This work has been supported through Progetto Strategico

PLATFORMSofPadovaUniversity.E.D.G.thanksFondazione

CARI-PAROforfinancialsupport.ThispaperisissuedasNationalResearch

CouncilofCanada,NRCC#53007.

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Biographies

EnricoDellaGasperagraduatedinmaterialsengineeringin2007andreceivedhis PhDinmaterialsscienceandengineeringin2011attheUniversityofPadova.He iscurrentlyapost-doctoralresearchfellowatthemechanicalengineering– mate-rialsdepartmentatthesameuniversity.Hisworkisfocusedonthesynthesisof noblemetal/metaloxidenanocompositesthinfilmswithtailoredmorphologyand propertiesusingwetchemistrytechniquesforopticalsensingofreducinggases. MassimoGuglielmiisfullprofessorofmaterialsscienceandtechnologyatthe UniversityofPadova,Italy.Hewasawardedwiththe“ProfessorVittorioGottardi MemorialPrize”bytheinternationalcommissiononglassin1992.Hisscientific interestisfocusedonthesynthesis,characterizationandapplicationof nanostruc-turedmaterialsobtainedbysol–gelmethods.Heisauthororco-authorofmorethan 200papers,mostofthempublishedoninternationalscientificjournals.

GiovanniPerottograduatedinmaterialsciencein2007andgotthePhDinmaterial scienceandengineeringin2011attheUniveristyofPadova.Heiscurrentlya post-doctoralresearchfellowatthephysicsdepartmentofthesameuniversity.Hiswork isfocusedonthenanofabricationofdifferentfunctionalorderednanostructures arrangedas2Darraysonsurfacesforplasmonicapplications.

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161 (2012) 675–683 683 StefanoAgnolitookhisMScandPhDinmaterialsscienceattheUniversityofPadova

(Italy).HeworkedasresearchassociateatthechemistrydepartmentofBrookhaven NationalLaboratory(US)andnowisassistantprofessorattheUniversityofPadova. Hisresearchisfocusedonthegrowthandcharacterizationofnanostructured sur-facesandonthestudyoftheirchemicalproperties.

GaetanoGranozzigraduatedinchemistry,professorofsurfaceandsolidstate chem-istryanddirectorofthePhDschoolinmaterialscienceandengineeringatthe UniversityofPadova.Hisresearchfieldisultrathinoxidefilms,supported nanopar-ticlesandmodelcatalysts.Authorofca.250papers.

MichaelPostreceivedhisPhDinchemistryfromtheUniversityofSurrey,UK,and isaprincipalresearcherintheenvironmentalmonitoringtechnologiesprogram attheICPETinstituteofthenationalresearchcouncilofCanada,wherehehas beenanactiveresearcherinmaterialssciencesince1975.Projectshaveincluded

X-raydiffractionandstructuredetermination,intermetalliccompoundsfor hydro-genstorageandphasestudiesofhightemperaturesuperconductingceramics. Currentresearchinterestsaredirectedtowardtheinvestigationofstructuraland functionalrelationshipsofthinandthickfilmnonstoichiometriccompoundsand nanomaterialcompositesforapplicationasgassensors.

AlessandroMartuccigraduatedinphysicsattheUniversityofPadovaandin1997 hegotthePhDdegreeinmaterialsscienceandengineeringatthesame univer-sity.From1999heholdsafacultypositionatPadovaUniversityandfrom2007 heisassociateprofessorteachingmaterialsscienceandengineeringatthe fac-ultyofengineering.Hismainresearchactivityisdevotedtonanoparticlesdoped sol–gelmaterialsforphotonicsandgassensingapplications.Heisresponsibleof nationalandinternationalresearchprojectsandholdsmorethan100international publications.