Effect of Annealing Temperature on ECD Grown Hexagonal-Plane Zinc Oxide

14 Pages·2017·2.2 MB·English

materials
Article
Effect of Annealing Temperature on ECD Grown
Hexagonal-Plane Zinc Oxide
SukritSucharitakul‡,RangsanPanyathip‡ andSupabChoopun*,†,‡ ID
DepartmentofPhysicsandMaterialsScience,FacultyofScience,ChiangMaiUniversity,239HuayKeawRoad,
MuangChiangMai50200,Thailand;[email protected](S.S.);[email protected](R.P.)
* Correspondence:[email protected];Tel.:[phone]
† Currentaddress:ThailandCenterofExcellenceinPhysicsP.O.Box70ChiangMaiUniversity,
ChiangMai50202,Thailand.
‡ Theseauthorscontributedequallytothiswork.
(cid:1)(cid:2)(cid:3)(cid:1)(cid:4)(cid:5)(cid:6)(cid:7)(cid:8)(cid:1)
(cid:1)(cid:2)(cid:3)(cid:4)(cid:5)(cid:6)(cid:7)
Received:1June2018;Accepted:31July2018;Published:6August2018
Abstract:Zincoxide(ZnO)offersagreatpotentialinseveralapplicationsfromsensorstoPhotovoltaic
cells thanks to the material’s dependency, to its optical and electrical properties and crystalline
structurearchitypes. Typically,ZnOpowdertendstobegrownintheformofawurtzitestructure
allowing versatility in the phase of material growths; albeit, whereas in this work we introduce
an alternative in scalable yet relatively simple 2D hexagonal planed ZnO nanoflakes via the
electrochemicaldepositionofcommerciallypurchasedZn(NO ) andKClsaltsinanelectrochemical
3 2
process. Theresultinggrownmaterialswereanalyzedandcharacterizedviaaseriesoftechniques
prior to thermal annealing to increase the grain size and improve the crystal quality. Through
observationviascanningelectronmicroscope(SEM)images,wehaveanalyzedthestatisticsofthe
grownflakes’hexagonalplane’ssizeshowinganon-monotonalstrongdependencyoftheaverage
flake’shexagonalflakes’ontheannealingtemperature,whereasat300◦Cannealingtemperature,
averageflakesizewasfoundtobeintheorderof300µm2. Theflakeswerefurtheranalyzedvia
transmissionelectronmicroscopy(TEM)toconfirmitshexagonalplanesandspectroscopytechniques,
suchasRamanSpectroscopyandphotoluminescencewereappliedtoanalyzeandconfirmtheZnO
crystalsignatures. The grown materialsalsounderwent furthercharacterizationto gaininsights
onthematerial,electrical,andopticalpropertiesand,hence,verifythequalityofthematerialfor
Photovoltaiccells’electroncollectionlayerapplication.TheroleofKClinaidingthegrowthoftheless
preferable(0001)ZnOisalsoinvestigatedviavariousprospectsdiscussedinourwork. Ourmethod
offersarelativelysimpleandmass-produciblemethodforsynthesizingahighquality2Dformof
ZnOthatis,otherwise,technicallydifficulttogroworcontrol.
Keywords: ZnO;hexagonal;electrochemicaldeposition
1. Introduction
ZnO has been widely studied as a platform for a broad spectrum of applications. Thanks to
thematerial’swidelyvariableelectricalandopticalproperties,basedonitsnanostructuretypesor
effectiveplanesalign,thematerialcanbeastrongcandidateforbiosensing[1],ultravioletlasing[2],
varistor[3],catalysts[4],optoelectronicdevices[5]and,moreimportantly,photovoltaiccells[6–13].
ZnO has attracted attention from the research community. For the past decade, there have been
manyadvancementsonthematerial’sgrowthandsynthesistechniques,mostlyfocusedonZnO’s
nano particle forms; namely, ZnO nanowire, which is the 1-dimensional case. Techniques have
been developed to improve the yield and control over the grown material’s morphology, ranging
fromchemical-vapordeposition[14–16],pulselaserdeposition[17,18],hydrothermalgrowth[19,20],
Materials2018,11,1360;doi:10.3390/ma11081360 www.mdpi.com/journal/materials
Materials2018,11,1360 2of14
vapor–liquid–solidreaction[21–23],microwaveheating[24],precipitationmethod[25],sonochemical
Materials 2018, 11, x FOR PEER REVIEW 2 of 13
method[26,27],andelectrochemicaldeposition[28–31].
Asmevnatpioorn–leiqdu,idd–essopliidte trreeamctieonn do[2u1s–2a3d], vamnicceromweanvets ohfeaZtinnOg g[r2o4]w, thpriencipbiutaltkio,nn anmoedthootds ,a[2n5d], nanowire
forms throsoungohchevmairciaol umsettheocdh [n26i,q27u]e, asn,dt heleect2ro-dchiemmeicnasl idoenpoaslitfioonr m[28a–t3s1]o. f ZnO have remained relatively
As mentioned, despite tremendous advancements of ZnO growth in bulk, nanodots, and
unexplored.NotuntiltherecentdiscoverybyNovoselovetal.of2-dimensionalexfoliablematerials[32],
nanowire forms through various techniques, the 2-dimensional formats of ZnO have remained
havethecommunity’sinterestsstartedshiftingtowardstheregimeof2-dimensionalmaterials.Thishas
relatively unexplored. Not until the recent discovery by Novoselov et al. of 2-dimensional exfoliable
inspiredthmeacteormialms [3u2n],i htyavteo thfoe ccuomsmitusnaittyte’sn intitoernesstst ostafurtretdh sehriftiinnvg etoswtigaradtse thZen rOegiimnet ohfe 2-2d-idmiemnseionnsailo nalform,
bothexpermimateenriatalsl.l Tyh[i3s 0h,a3s3 in–s3p8ir]eadn thde tchoemomreutniictay ltloy fo[3cu9s, 4it0s] a.ttentions to further investigate ZnO in the 2-
dimensional form, both experimentally [30,33–38] and theoretically [39,40].
Interestingly,ithasbeenshownthatZnOinthe2-dimensionalformof(0001)orhexagonalplane
Interestingly, it has been shown that ZnO in the 2-dimensional form of (0001) or hexagonal plane
(referred to as h-ZnO for the rest of this work) can be more effective in certain aspects than all its
(referred to as h-ZnO for the rest of this work) can be more effective in certain aspects than all its
other formosth(eer .fgo.r,mqsu (ea.ng.t, uqmuandtuomts ,dontas,n noawnoiwreirse,s,a anndd bbuulklk) )thtahnakns ktos tthoe pthlaenep bleainnge mbeorine gpomlaroizreed,p olarized,
whichallowwhsicthh aellopwlas nthee tpolabnee tsoi gbne isfiigcnaifnictalnytlmy moroere sseennssiittivive etot pooplaor lmarolemcuoleles caundle lsigahnt dintleirgahcttioinnst, eractions,
and thanks to the higher ratio of the materials in such a plane. Furthermore, the theoretical results
andthankstothehigherratioofthematerialsinsuchaplane. Furthermore,thetheoreticalresults
suggested that charge carriers have a higher mobility and lower combination rate in such planes,
suggested that charge carriers have a higher mobility and lower combination rate in such planes,
making h-ZnO not only an interesting platform for scientific studies but also a very strong candidate
makingh-Zfonr OPhontoovtooltnalicy (PaVn) ianptpelriceasttioinngs. platformforscientificstudiesbutalsoaverystrongcandidate
forPhotovoltaIinc g(ProVw)inagp hp-lZicnaOt,i othnesre. have been reports regarding the success of the growths via various
methods. The list includes pulse laser deposition (PLD) [35], commercial Simonkolleite annealing [36],
In growing h-ZnO, there have been reports regarding the success of the growths via various
hydrothermal method [37], and electrochemical depositions (ECD) [9,28,30,31,33,34,41]. However, in
methods. Thelistincludespulselaserdeposition(PLD)[35],commercialSimonkolleiteannealing[36],
the report listed so far, there have been no methods that can grow atomically thin ZnO with a high
hydrothermaspaelctm raettioh oond a[ h3e7x]a,gaonnadl pellaencet wroitchh secmalaibclae lgdroewptoh steitcihonniqsu(eEs. CInD th)is[ 9w,2o8rk,3, h0-,Z3n1O,3 3cr,y3s4t,a4ls1 a].re However,
intherepogrrtolwisnt evdia sEoCDfa, rf,oltlhowereed huapv bey btheeernmanl oanmneeatlhinogd tos mthaaxtimciazne tghreo gwrowattho manidc aimllpyrothvein thZe nasOpewct ithahigh
ratio prior to further optoelectrical investigations. The method proposed in this work combines the
aspectratioonahexagonalplanewithscalablegrowthtechniques. Inthiswork,h-ZnOcrystalsare
strength of a relatively simple electrochemical deposition, which allows mass production of flake
grownviaECD,followedupbythermalannealingtomaximizethegrowthandimprovetheaspect
nucleation in highly controlled direction, with the quality and grain size offered by the followed-up
ratioprioratnonefaulirntgh, ewrhoicpht woeilll ebcet driisccaulssinedv feusrttihgear tinio thnes .laTtehr epamrt eotf hthoisd ppaproerp. osedinthisworkcombinesthe
strength of a relatively simple electrochemical deposition, which allows mass production of flake
2. Sample Preparation and Methodology
nucleationinhighlycontrolleddirection,withthequalityandgrainsizeofferedbythefollowed-up
In this work, the authors propose the procedures for growing h-ZnO via electrochemical
annealing,whichwillbediscussedfurtherinthelaterpartofthispaper.
deposition prior to annealing at a higher temperature for the promotion of growth in (0001) plain as
shown if Figure 1a. The procedure starts with pre-cleaned cut Zn plates sonicated in Acetone for 10
2. SamplePreparationandMethodology
min prior to rinsing in Ethanol, DI water then blown dry prior to being used as electrodes.
In this work, the authors propose the procedures for growing h-ZnO via electrochemical
2.1. Materials
deposition priorto annealingat ahigher temperature forthe promotion ofgrowthin (0001)plain
In mixing electrolyte solution for the growth, commercially purchased Sigma-Aldrich (St. Louis,
asshownifFigure1a. Theprocedurestartswithpre-cleanedcutZnplatessonicatedinAcetonefor
MO, USA) 99.99% grade Zn(NO3)2 and KCl powder were mixed with DI water in a standard 100 mL
10minprioberatkoerr sienastiendg onin hoEt tphlaatne.o l,DIwaterthenblowndrypriortobeingusedaselectrodes.

Figure 1. Illustration of (a) crystal structure of ZnO highlighting the (0001) plane; and
Figure1.Illustrationof(a)crystalstructureofZnOhighlightingthe(0001)plane;and(b)electrochemical
(b) electrochemical setup used in this work.
setupu sedinthiswork.
2.1. Materials
Inmixingelectrolytesolutionforthegrowth,commerciallypurchasedSigma-Aldrich(St. Louis,
MO,USA)99.99%gradeZn(NO ) andKClpowderweremixedwithDIwaterinastandard100mL
3 2
beakerseatedonhotplate.
Materials 2018, 11, x FOR PEER REVIEW 3 of 13
Materials2018,11,1360 3of14
2.2. Preparation of Hexagonal-Plane Zinc Oxide Via Electrochemical Deposition
2.2. PTrheep agrraotiwontho fcHonexdaigtioonnasl- aPrlean seeZt itnoc bOex siidmeVilaiar Etole cwtroorckh ecmoincadluDcteepdos bityio nPradhan et al.; initially, the
bath’sT ehleecgtrroowlytthe ciosn fdiristtio mnsixaerde swetittho 0b.e04s iMmi loafr Zton(wNoOrk3)c2 oannddu 0c.t1e dMb yofP KraCdlh iann deetiaoln.;iizneidti awllay,tethr e(DbaI)t ha’ts
room temperature, then the bath is preheated until the temperature is at an equilibrium at 45 °C as
electrolyteisfirstmixedwith0.04MofZn(NO ) and0.1MofKClindeionizedwater(DI)atroom
3 2
rteeamdp beyra tthuer eD,tSh1e8nb2th0 ewbaattehrpisroporef htheaetremdaul nsetinlstohre. tAemgipleenrat tEu3r6e3i3sAat paonweeqru siluibprpiulym isa tth4e5n◦ sCeta sinr edaidrebcty
current (DC) voltage bias mode with 1.1 V potential difference between the Cathode and the Anode
theDS18b20waterproofthermalsensor.AgilentE3633Apowersupplyisthensetindirectcurrent(DC)
as demonstrated in the figure as the growth commences. Commercial grade Zn plates were cut into
voltagebiasmodewith1.1VpotentialdifferencebetweentheCathodeandtheAnodeasdemonstrated
1 cm × 5 cm pieces via metal cutter and used as electrodes for the growth process. Grown crystals are
inthefigureasthegrowthcommences.CommercialgradeZnplateswerecutinto1cm×5cmpieces
then extracted from the solution for further investigation and annealed at various temperatures on a
via metal cutter and used as electrodes for the growth process. Grown crystals are then extracted
hot plate in an ambient environment with calibrated equilibrium temperature. The process of
fromthesolutionforfurtherinvestigationandannealedatvarioustemperaturesonahotplateinan
material preparation was systematically studied through Cyclic voltammetry over ranges of
ambientenvironmentwithcalibratedequilibriumtemperature.Theprocessofmaterialpreparationwas
electrolyte concentration of 0.1 M–0.5 M before 0.1 M was picked as the growth condition (the reason
systematicallystudiedthroughCyclicvoltammetryoverrangesofelectrolyteconcentrationof0.1M–0.5M
is discussed later in the article) while the growth period is a controlled 30 min, followed by annealing
before0.1Mwaspickedasthegrowthcondition(thereasonisdiscussedlaterinthearticle)whilethe
at different temperatures ranging from 100 to 300 °C for the final product. The annealing was done
growthperiodisacontrolled30min,followedbyannealingatdifferenttemperaturesrangingfrom100to
a3t0 0am◦Cbifeonrtt hgeafsin palrepsrsoudruec ta.nTdh ecaonmnepaolisnitgiowna stod opnreoavtidaem bthieen togxaysgperne sfsourr emanadtecrioaml’sp ocsoitmiopnotsoitpioronv iadse
discussed later in the article. The appropriate analysis and characterizations are separated into
theoxygenformaterial’scompositionasdiscussedlaterinthearticle. Theappropriateanalysisand
different stages of growth to gain insights on the grown material’s properties in each stage in order
characterizationsareseparatedintodifferentstagesofgrowthtogaininsightsonthegrownmaterial’s
to demonstrate insights on the growth process.
propertiesineachstageinordertodemonstrateinsightsonthegrowthprocess.
22..33.. CChhaarraacctteerriizzaattiioonn aanndd AAnnaallyyssiiss ooff tthhee OObbttaaiinneedd MMaatteerriiaallss
In this work, in order to quantify and qualify the materials, electron microscopic and
Inthiswork,inordertoquantifyandqualifythematerials,electronmicroscopicandspectroscopic
spectroscopic techniques were used. For scanning electron microscope (SEM) and electron diffraction
techniqueswereused. Forscanningelectronmicroscope(SEM)andelectrondiffractionspectroscopy
spectroscopy (EDS) techniques, the results were collected using a FE-Scanning Electron Microscope:
(EDS)techniques,theresultswerecollectedusingaFE-ScanningElectronMicroscope: JSM6335F
JSM 6335 F while all information obtained using tunneling electron microscopy, the JEOL JEM-2010
whileallinformationobtainedusingtunnelingelectronmicroscopy,theJEOLJEM-2010TEMmodel
TEM model (MA, USA) was utilized on synthesized samples collected on standard Cu grids.
(MA,USA)wasutilizedonsynthesizedsamplescollectedonstandardCugrids.
Immediately after the growth was finished, the h-ZnO crystals were extracted and investigated
Immediatelyafterthegrowthwasfinished,theh-ZnOcrystalswereextractedandinvestigated
via SEM and EDS to gain insight on the immediate composition after the growth and prior to
viaSEMandEDStogaininsightontheimmediatecompositionafterthegrowthandpriortoannealing.
annealing. Results are as demonstrated in Figure 2d,e.
ResultsareasdemonstratedinFigure2d,e.

Figure 2. Scanning electron microscope (SEM) images of the extracted crystal grown for (a) 5; (b) 10
Figure2.Scanningelectronmicroscope(SEM)imagesoftheextractedcrystalgrownfor(a)5;(b)10
and (c) 30 min and electron diffraction spectroscopy (EDS) results of ZnO crystals grown (with K
and(c)30minandelectrondiffractionspectroscopy(EDS)resultsofZnOcrystalsgrown(withK
element as weighing factor in pink rectangle) for (d) 5 and (e) 30 min).
elementasweighingfactorinpinkrectangle)for(d)5and(e)30min).
Materials2018,11,1360 4of14
AsillustratedinFigure2a–c,growthstartsinitiallywithK+depositionincompetitionwithZn2+
ontotheCathode. K+depositsbetterthanZn2+thankstoitshighermobilitygivingthestructurerather
complicatedshapeinitiallyasshownin5min. Thecrystalsstartgrowingmoreuniformlyandshowing
more rigid domains of vertical h-ZnO growth as time goes on, forming clusters on the substrates.
Thisisincompatiblewithpreviouswork[33],wheregrownflakesareintheorderofafewmicrons.
OurinitialgrowthsdemonstratetheroleofCl−asacappingion,distinguishingandallowingthe(0001)
planetogrowmoreenergeticallyandfavorably,inaccordancewithargumentsanddemonstrations
proposedinpreviousworks[33,42]. FromFigure2d,e,theEDSresultsfurtherdemonstratetheroleof
K+ andCl− inthegrowthprocess. Firstly,K+ initiallydepositedonCathodewithahigherrelative
ratio compared to Zn2+ thanks to the ions’ higher mobility in the solution. From our observation,
thisisgenerallyreducedasthegrowthtimeincreases(forindepthdiscussionsregardingthistopic,
pleaserefertoourSupplementaryMaterial). WhileCl− ionsfacilitatethemobilityofZn2+ andthe
remainingCl− ionsinthesolutionareexpectedtodepositonthefinalproductafterthesolventpartof
thesupernatantisevaporated. ItisnoteworthythatalthoughCl− ionsarefunctioningasintended
as a facilitator of Zn2+ deposition and capping of the (0001) plane on ZnO growths, they are also
consideredunwanteddefectswhentheh-ZnOflakesareusedforPVapplication,asitreducesmobility
andincreasestherecombinationrateofcarriers. Thus,weproposetoannealtheproductsafterthe
growth,whichwillbediscussedlaterinthearticle.
3. GrowthMechanismDiscussionsandFurtherAnnealing
In order to obtain h-ZnO structures with high yield, Cl− ion had been attributed as the
“cappingions”,whichbecomesstronglyattractedtothe(0001)planewhosepolarizationisstronger
thantheother2DplanesofZnO,whilebeinglessenergeticallyfavorabletogrowths[33,42]. Although
themechanismforsuchgrowthshasundergoneextensivedebate,sofarnoconclusiveevidencehas
beenunearthed. ForfurtherqualitativeunderstandingontheroleofKClinh-ZnOgrowth,theauthors
have performed cyclic voltammetry (CV) utilizing the ER461 EChem Startup System on the KCl
solutioninthesetupusedinthisworktodeterminetheroleoftheK+ andCl− ionsinthegrowth
contributioninthegrowthsetup. Itisnoteworthythatintheresultstheanodicoxidationandcathodic
reductionpeakswerebarelyobservedduringthescanrange, allofwhichfallswithinthe1.1Volt,
whichisthegrowthvoltage[43].
3.1. ThePrimaryZnOGrowth
The nucleation process of the electrochemical method takes place prior to more complicated
secondary processes. The rate of this growth process depends on the reaction rate (oxidation and
reductionrate)onthesurfaceelectrode(Zn)intheelectrochemicaldepositedmethod[44]. Thegrowth
mechanismofZnOintheprimaryprocesshasthereactionprocesswithEquation(1),whichcanbe
separatedintothereactionsasthecathodeprocess,suchasintheequationlistedbelow(2–8)whilethe
anodewasextracting,Equation(9)definestheaquaioninreaction. Theoverallreactionfornucleation
was dependent on the deposition process to the cathode surface. Hence, considering the primary
growthreactionofZnOviaECDwithZn(NO ) (Zn(NO ) 6H O)andKClasreagents,theycanbe
3 2 3 2 2
brokendowninthechemicalreactionslistedbelow[45–48]:
Zn(NO ) + 2KCl (cid:11) ZnCl + 2KNO (1)
3 2 2 3
Cathode:
Zn(NO ) →Zn2++2NO− (2)
3 2 3
KCl→K++Cl− (3)
NO−+H O+2e− →NO−+2OH− (4)
3 2 2
Zn2++2OH− → Zn(OH) (5)
2
Materials2018,11,1360 5of14
Zn(OH) →ZnO+H O (6)
2 2
Zn2++2Cl− → Zn(Cl) (7)
2
K++NO− →KNO (8)
3 3
Anode:
1
H O→ O +2H++2e− (9)
2 2
2
Primary growth of ZnO is the heterogeneous reaction between the bulk solution (electrolyte)
and electrode surface cations. The anions in the electrolytes are separated by electrical activation
formingreactionsontheelectrodeasshownintheequations. Thisnucleationmechanismisstrongly
influencedbyelectrontransfersneartheelectrodesurfaceandelectrolyte. Viatheobservationofthe
ratioI /I ,furtherinsightsinthenucleationmechanismcanbeobtained[49],wheretheredox
P(re) P(ox)
reactioninanelectrochemicalprocesscanbedeterminedwiththemaximumcurrent(I )quantified
P
fromtheCVplotandtheRandles–SevcikEquation(10)[49]:
I =0.446nFAC(nFDv/RT)1/2 (10)
p
where I is current maximum in amps, n is number of electrons transferred in the redox event,
p
A is electrode area in cm2, F is Faraday constant in C mol−1, D is diffusion coefficient in cm2/s,
Cisconcentrationinmol/cm3,vυisscanrateinV/sandRisgasconstantinJK−1mol−1 andTis
temperatureinK.Furtherdiscussionaboutenergeticandkineticinformationofthereactioncanbe
foundinourSupplementaryMaterialSectionS1(c.f. FigureS1).
Aspointedoutinthemainarticlethecyclicvoltammetricdatarepresentsthereversibleprocess
andfastelectrontransferbehavior[49]throughthecyclicvoltammograminhexagonalZnOshown
inFigure3a,bofthemainarticle. Thehysteresisoffersevidencethattheredoxprocessisstrongly
dependentontheratechosenforthemeasurement. Theareaofvoltammogramwasseparatedasthe
anodiccurrentoxidationprocessandcathodiccurrentreductionprocess,bothofwhicharecomponents
usedtodiscussthesynthesizedhexagonalZnO,therefore,theelectrochemicalmechanisminFigure3a
started at 0.64 V, as indicated by the slope of the growth up to 1.0 V, reaching maximum cathodic
current I for 6.47 mA at 0.93 V, then the reversible process began as the voltage was ramped
P(re)
to 1.0 V and reached the oxidation zone at 0.64 V, which observed the maximum anodic current
(I )of5.41mAat0.66V.Aspreviouslydiscussedinanearlierstudy[49],therateofnucleation
P(ox)
ontheelectrodesurfacecanbeinfluencedcompletelydifferentlybasedonhowfarthenucleation
actuallytakesplacefromthesurface, whichcanbeseparatedinto3zones: thebulksolutionzone
(electrolytezone),diffusionlayerzone(~1nmto500µmformelectrodesurface),andcompactlayer
zone,whichisdirectlyonthesurfaceuptoapproximately0.1nmthick. Thephysicsthatseparates
thecompactlayer(a.k.a. InnerHelmholtz)fromthediffusionlayer(a.k.a. Gouy–Chapman)isthe
saturationofthediffusionrangewherethegrowthmechanicsinthefirstaredeterminedbysolvent’s
dipolemomentasthatofthelatterisstronglyaffectedbytheelectrontransferanddiffusionrate[49].
Thismodelcanbeusedtoexplainthenucleation-growthmechanismintheZnOsingle-layer
crystal. FromtheCVresultsinFigure3ainthemainarticleandasdiscussedinthesupplemental
materialsectionSI,theroleoftheKClconcentrationontheelectrontransferratecanbeinferredvia
(I ,I )sinceelectrontransferisstronglyinfluencedbytheKClconcentration,allowingusto
P(re) P(ox)
somewhat control the electron transfer rate by the variation of (KCl) while observing the ratio of
I /I . BasedonourresultsinthemainarticledemonstratedinFigure3b,at(KCl)rangingfrom
P(re) P(ox)
0.3Mto0.5Mdemonstrateshighelectrontransferphenomena. Thereasonofthisprocesswasthe
inducediondiffusiontoelectrodesurfacewitharelativelyhighdiffusionrateasaresultofthehigher
concentrationoftheions,resultinginawiderdiffusionrangecomparedtothe0.1Mand0.2Mascan
beimpliedfromthepeakcurrentandtheratioofI /I demonstratedinFigure3. However,
P(re) P(ox)
Materials2018,11,1360 6of14
suchaneffectisnotfavorableforcontinuousgrowthinthesecondaryprocessastheuniformitycanbe
reducedwithahighergrowthrate.
MateriaTlso 20d18e,m 11o, xn FsOtrRa tPeEEtRh eREpVrIeEvWio usly explained growth mechanism, SEM images of ZnO c6r yofs 1ta3 l
grown under our standard condition with (KCl) as 0.1 M were taken at different growth times
demonstrated in Figure S2 (c.f. Supplementary Material). It can be inferred from Figure S2a that the
and demonstrated in Figure S2 (c.f. Supplementary Material). It can be inferred from Figure S2a
secondary growths already took place after 30 s into the growth, the vertical crystal seeded from the
thatthesecondarygrowthsalreadytookplaceafter30sintothegrowth,theverticalcrystalseeded
compact layer grew correspondingly with time, as can be seen in Figure S2b. After 300 s of growth,
fromthecompactlayergrewcorrespondinglywithtime,ascanbeseeninFigureS2b. After300sof
the flakes had a dense uniformity to a bulk layers as Figure S2c [41] and Figure S2d where the
growth,theflakeshadadenseuniformitytoabulklayersasFigureS2c[41]andFigureS2dwherethe
saturation of ZnO crystal growth in forms of bulk flakes [50], which can improve the crystal sizes
saturationofZnOcrystalgrowthinformsofbulkflakes[50],whichcanimprovethecrystalsizeswith
with annealing temperature in the next process.
annealingtemperatureinthenextprocess.

Figure 3. Cyclic voltammetry (CV) measurement of (a) ZnO with 0.1 M of KCl; and (b) ZnO growth
Figure3.Cyclicvoltammetry(CV)measurementof(a)ZnOwith0.1MofKCl;and(b)ZnOgrowth
dependence of 0.1 M–0.5 M for KCl concentration. The effect of KCl concentration in cyclic
dependenceof0.1M–0.5MforKClconcentration.TheeffectofKClconcentrationincyclicvoltammetry
voltammetry to growth ZnO; (c) peak oxidation and reduction in each KCl concentration; (d) ratio of
togrowthZnO;(c)peakoxidationandreductionineachKClconcentration;(d)ratioofredoxreaction
redox reaction vs KCl concentration ranging from 0.1 M to 0.5 M.
vsKClconcentrationrangingfrom0.1Mto0.5M.
As demonstrated in Figure 3a, the maximum current ratio of cathodic and anodic
As demonstrated in Figure 3a, the maximum current ratio of cathodic and anodic
(I / I ~1) in the electrochemical process can be extracted and utilized to reveal the fast electron
P(re) P(ox)
(I /I ∼1) in the electrochemical process can be extracted and utilized to reveal the fast
traPn(srfee)r mP(eocxh)anism during the reversible reaction of reduction and oxidation redox respectively.
electron transfer mechanism during the reversible reaction of reduction and oxidation redox
Moreover, the mechanism can be used to explain the role of KCI in the electrochemical process by
respectively. Moreover,themechanismcanbeusedtoexplaintheroleofKCIintheelectrochemical
testing the reaction with a KCl concentration of 0.1 M–0.5 M during the synthesis of h-ZnO. It is
processbytestingthereactionwithaKClconcentrationof0.1M–0.5Mduringthesynthesisofh-ZnO.
noteworthy that without the usage of KCl into the solution, reactions (c.f. Supplementary Material
ItisnoteworthythatwithouttheusageofKClintothesolution,reactions(c.f. SupplementaryMaterial
for further details) leading to h-ZnO flakes cannot take place, this is due to the lack of catalyst to
for further details) leading to h-ZnO flakes cannot take place, this is due to the lack of catalyst to
support the ion for redox reaction with an electrode. On the other hand, after KCl is used for 0.1 M in
supporttheionforredoxreactionwithanelectrode. Ontheotherhand,afterKClisusedfor0.1Min
the electrochemical process ZnO can be produced and then by increasing the KCl concentration from
theelectrochemicalprocessZnOcanbeproducedandthenbyincreasingtheKClconcentrationfrom
0.2 M to 0.5 M revealed CV, the results of which are shown Figure 3b, in which the comparison of CV
0.2Mto0.5MrevealedCV,theresultsofwhichareshownFigure3b,inwhichthecomparisonofCV
and ZnO in each KCl concentration is demonstrated. It can be inferred that when the KCl
andZnOineachKClconcentrationisdemonstrated.ItcanbeinferredthatwhentheKClconcentration
concentration is more than 0.1 M (e.g., 0.2 M–0.5 M), the CV plot’s area increases with the
concentration, as can be seen from our results. This can be attributed to the relatively higher redox
reaction rate enabled by the higher concentration of the K+ and Cl− ions compared to the 0.1 M case,
while the latter allows a higher mass-producibility and uniformness of the h-ZnO flakes [51,52]. As
previously discussed in the EDS section, there was evidence to suggest that K+ and Zn2+ compete to
grow as crystals, which the EDS result confirmed: K+ deposited faster than Zn2+ [52], especially in the
Materials2018,11,1360 7of14
ismorethan0.1M(e.g.,0.2M–0.5M),theCVplot’sareaincreaseswiththeconcentration,ascanbe
seenfromourresults. Thiscanbeattributedtotherelativelyhigherredoxreactionrateenabledby
thehigherconcentrationoftheK+andCl− ionscomparedtothe0.1Mcase,whilethelatterallowsa
highermass-producibilityanduniformnessoftheh-ZnOflakes[51,52]. Aspreviouslydiscussedinthe
EDSsection,therewasevidencetosuggestthatK+andZn2+competetogrowascrystals,whichthe
EDSresultconfirmed: K+depositedfasterthanZn2+[52],especiallyintheearlystage. Additionally,
excessiveamountsofKClcouldpromoteothergrowthspecies,suchasKNO andZnCl while0.1Mis
3 2
proventobesufficientforthepurposeofcappingthe(0001)plane[52]. Thus,0.1Mwaschosenasthe
standardgrowthconditioninthiswork[45,53]. Throughextractionofthereductioncurrent(I )and
P(re)
oxidationcurrent(I )fromFigure3bintoFigure3c,thereisaclearenhancementtrendshownwith
P(ox)
increasingKClconcentration,whilepresentedinFigure3distheratioofI /I ,whichremains
P(ox) P(re)
relatively constant as a function of [KCl]. It is also noteworthy to point out that during the cyclic
voltammetry,twochannelsarecomparedinFigure3b;thechangeofeffectivehysteresisdifferential
conductanceisclearlyobservable. Thismayimplythattheusageof0.1M,assuggestedbyprevious
work[33],hasastronghysteresis,allowingthereverseofthedifferentialconductanceata100mV/s
scan rate while the concentration increases and the hysteresis is reduced. This demonstrates that
given0.1M,theKClconcentrationusedintheactualgrowthwouldhaveastrongtimedependence,
especiallyduringthebeginningofthegrowth,allowingnon-equilibriumdepositionofKandClto
takeplaceexplainingtheinhomogeneityofourgrowthsatlowergrowthtime,asshowninFigure2a.
Inaddition,h-ZnOCVshowedsuperiorelectrochemicalperformanceandstabilityathighcurrent
densitywithlowvoltage,thusitissuitableforsolarcellapplications[45].
3.2. TheSecondaryGrowth
This process takes place after the nucleation of the compact layer is finished. The secondary
growthcanbeattributedtotheverticalgrowthsthatareobservedinthemainarticle. Thisprocessis
similartothecoalescencebehavioroftwodropletsofwaterwhentheyareconnectedandthenform
onebiggerdropletinsteadofstayingseparately,hencethenameoftheprocess. Throughthisprocess,
thegrowthmechanismcombinesthenucleationgrainsofZnOfromprimarygrowthtotheformingof
largescaleofparticlesthatformflakesizesthroughthecoalescenceprocess[54–57].
Tofurthershedsomelightontheroleofthecoalescenceprocessandtheeffectivenessof0.1Mof
KClconcentrationinthegrowthofh-ZnOflakes,Gibsfreeenergyneedstobeconsidered. Through
Equation(11),onecancalculatetheGibsfreeenergychangeofthenucleationprocess:
4
∆G∗ =4πr∗2γ + πr∗3∆G (11)
N f 3 v
where,γ issurfaceenergyofnucleationatom,r*iseffectivecriticalradianofatomnucleation(unitless)
f
and∆G isfreeenergyofthebulkcrystal(Gibbsfreeenergyofvolumegrowth).
v
WiththeknownchangeofGibbsfreeenergychange,theprobabilityofcoalescenceprocessto
occurinsecondarygrowth(P )[58]witheachannealingprocesswhereinthecasethatthechangein
N
Gibbsfreeenergyoftheprocessisgivenby∆G∗ cansimplybecalculatedviaEquation(12)asfollows
N
PN =e(−∆GN∗/kBT) (12)
where,k istheBoltzmannconstantandTisthetemperatureofthegrowthintheprocess.
B
Thus,fromourexperimentallyobtaineddata,onecandeterminethevalueofP ofZnOnucleation
N
in0.1M(KCl)caseatvariousannealingtemperaturesfrompreviouslycalculated∆G◦ asshownin
FigureS1intheSupplementaryMaterial.
Thus, by using Equation (12) through ∆G◦ information previously obtained in Figure S1a,
thecoalescenceprobabilitycanbecalculatedviaEquation(12). Astheannealingtemperatureincreases,
onecaninferthattherecombinationrateisalsoincreased;e.g.,higherfinal∆G◦. Moreover,thegrains
Materials2018,11,1360 8of14
wereinfluencedbyhigh∆G◦ aftertheZnOflakewasgrownupwiththeannealingprocess[59]to
31.97µm2,52.01µm2and437.79µm2,respectively,asshowninFigureS1banddiscussedinsectionSI
ofourSupplementaryMaterial.
Withintherangeofourtrials,sincethe0.1Mconcentrationhasthelowestreactionrateinthe
primarygrowthprocessandhashigh∆G◦ inthesecondarygrowthprocess,wewerethusledinto
choosing 0.1 M of KCl as our default growth choice in order to enhance the hexagon ZnO flake
sizeswiththeannealingmethodforimprovingthedegreeofZnOcrystallinity[60]. Throughthis
agglomerationprocessincombinationwithsecondarynucleationontopoftheZnOseedingmolecules
formedintheprimarygrowth,ZnOcrystals,asshownasthefinalproductinFigure2c,canbeobtained.
3.3. AnnealingofGrownh-ZnONanoflakes
TheannealingofZnOfromSimonkolleiteinambientinordertoobtainlargeh-ZnOnanoplates
wasrealizedandbroughttolightbyZhangetal.[61]. Thankstotheprecursor,Simonkolleite,whichis
alsoahexagonalcrystallineprecursor,theprocesscanprovidehighqualityh-ZnO,however,theshape
andthecrystalaspectratiooftheresultingcrystalsarelesscontrolled,albeitwithporousfeaturesleft
behindbythechloridehydroxidemonohydratechainsdiffusingoutofthematerial,leavingbehind
porousfeaturesatahighertemperature. Inthiswork,suchamethodisappliedtothegrowncrystals,
allowing a higher aspect ratio of (0001) plane while keeping the crystallinity through the process
whilestartingfromh-ZnOasaprecursor,allowingtherelocationsanddiffusionofZnandOatomsto
becomemorefavorableandcontrolled. Additionally,webelievethatthecoalescenceprocessplayeda
strongroleindictatingthegrowncrystalsize(c.f. SectionSIofSupplementaryMaterial).
AsillustratedinFigure4a–ctheannealingtemperaturedoesplayanimportantroleinincreasing
thesizeof(0001)plane. ItisnoteworthythataccidentaldopingbybothCl− andK+canbefixedvia
annealing. Sincetheannealingisconductedinanambientenvironment,K+istreatedandremoved
byhumidity,thus,thereductionofK+ionwhileCl− effectivelycappedtheZnOonthe(0001)plane,
makingtheionsdifficulttoremove. AsdemonstratedinFigure4c,annealingathighertemperature
giveshigherthermalenergyforthealreadydepositedZn2+ andO2− todecomposeandrecompose
intonewseededh-ZnO,allowingforthecompositionoflargerfinalannealedflakes. Asdemonstrated
in the EDS results, the K+ component is either decomposed or becomes less detectable due to the
yield and size of the ZnO flakes. It is also noteworthy to point out that the h-ZnO prepared this
way has less Cl contamination than that prepared by the annealing of commercial Simonkolleite,
allowingforgrowncrystalssuperiorintermsofelectrontrappingforPVdevices,especiallyinthe
case of nano-scale flakes [62], and efficiency as a gas sensor [63]. Also, by annealing only up to
300C,thegrownflakeshavelesstendencytobecomeporous[64]and, thus, haveahighercarrier
mobilityinPVapplications,albeitsuchpropertiescouldalsobebeneficialforagassensingapplication.
Furthermore, this proposed method allows significantly larger grown flakes, as demonstrated in
Figure4f,despitethembeinglessfavorablethermodynamicallyaccordingtoourGibbsfreeenergy
analysis(c.f.SectionSIofSupplementaryMaterials).
3.4. Post-AnnealingCharacterization
Tofurtherconfirmthequalityoftheannealedsamples,aseriesoftestswereconductedtoverify
theintegrityofthefinalproductsfromourproposedprocess,TEMwasfirstusedtocharacterizethe
crystallinityofthesample. Figure5a–cillustratedTEMresultsontheannealedcrystals,whichwere
grown for 30 min prior to annealing at 300 ◦C for 1 h in an ambient environment. The d-spacing
ofthecrystalswasdeterminedtobe3.989Angstromsandthezoneanalysispatternsdemonstrates
the high crystallinity in the grown samples, which is in good agreement with previous work [65].
AsillustratedinFigure4e,f,fromtheRamanresultsontheannealedcrystals,aprominent438cm−1E
2
peakcorrespondingto(0001)planewasdetectedat300◦Cannealing. Thispeakwaslessobservedin
lowerannealingtemperature,likelyduetoadifferentorientationofthe(0001)planewhilsttheplanes
aregrownvertically. Asmallshiftof393cm−1and254cm−1peakswereobserved,thisislikelydue
Materials2018,11,1360 9of14
tothechangeinsizeconstraintsastheflakesgrowlargerwiththeannealingtemperature,relaxing
theaveragemodeofthephononsinasimilarcasetothequantumdotscasepreviouslystudiedby
Chengetal.[38]. AsshowninFigure4f,thephotoluminescencedemonstratesthesuperiorityofthe
crystalsannealedat300◦C,asacrystalpeakat389nmwavelengthisclearlydominant. Weattribute
thebroadeningofthePLpeaksonoursamplestooxygenvacanciesatlowerannealingtemperature.
Again,thisconfirmsourassumptionofreductionofdefectsofaccidentaldopingbytheelectrochemical
deposition process with higher temperature annealing in ambient and the results are also in good
agreementwithadiscussionprovidedbyZhangetal.,2017[61]regardingtheannealingtemperature
dependencetrendandadevice’sperformance.Inaddition,ourresultsalsosuggestedthatinworkdone
byZhang,thepoorperformanceofflakesannealedatlowertemperaturemayhavebeenduetotheoxide
vacancies,ascanbeobservedinouropticalresults. Regardless,suchissuescanbeavoidedthroughour
proposedmethodofusingECDgrownh-ZnOnanoplatesasaprecursorinsteadofSimonkolleite.
Materials 2018, 11, x FOR PEER REVIEW 10 of 14

Figure 4. SEM images of the grown ZnO crystals grown for 30 min prior to annealing at (a) 100; (b)
Figure 4. SEM images of the grown ZnO crystals grown for 30 min prior to annealing at (a) 100;
200; and (c) 300 °C for 1 h; and (d,e) more SEM images at different magnifications and locations for
(b)200;and(c)300◦Cfor1h;and(d,e)moreSEMimagesatdifferentmagnificationsandlocations
flakes annealed for 300 °C for 1 h for comparison; (f) statistical summary of (0001) plane flake size
forflakesannealedfor300◦Cfor1hforcomparison;(f)statisticalsummaryof(0001)planeflakesize
demonstrating statistics of flakes annealed at different temperature; and (g) flake’s statistics at
demdoinffsetrreantitn agnnsteaatliisntgic tsemofpfleraakteusrea (nrnoueanldeedda dtodwifnf etroe dnetctiemmapl peroaintutsr.e ;and(g)flake’sstatisticsatdifferent
annealingtemperature(roundeddowntodecimalpoints.
Materials2018,11,1360 10of14
Materials 2018, 11, x FOR PEER REVIEW 10 of 13

Figure 5. Transmission electron microscopy (TEM) image at (a) lower; and (b) higher resolution of
Figure5. Transmissionelectronmicroscopy(TEM)imageat(a)lower;and(b)higherresolutionof
ZZnnOO flflaakkeess ggrroowwnn ffoorr 3300 mmiinn pprriioorr ttoo aannnneeaalliinngg aatt 330000 ◦°CC ffoorr 11 hh iinn aammbbiieenntt eennvviirroonnmmeenntt;; ((cc)) EElleeccttrroonn
diffraction and zone analysis for the crystals; and (d) Raman spectrum and; (e) photoluminescence
diffractionandzoneanalysisforthecrystals;and(d)Ramanspectrumand;(e)photoluminescence(PL)
(PL) results of flakes grown for 30 min then annealed at different temperatures.
resultsofflakesgrownfor30minthenannealedatdifferenttemperatures.
4. Conclusions and Outlook
4. ConclusionsandOutlook
In this work, a relatively simple and scalable process for h-ZnO crystals growth for PV
Inthiswork,arelativelysimpleandscalableprocessforh-ZnOcrystalsgrowthforPVapplication
application via ECD in combination with annealing is proposed. Through observations via SEM
viaECDincombinationwithannealingisproposed. ThroughobservationsviaSEMimages,wehave
images, we have analyzed the statistics of the grown flakes’ hexagonal plane’s size. Our results
analyzedthestatisticsofthegrownflakes’hexagonalplane’ssize.Ourresultsshowedanon-monotonal
ssthroowngedd eap nenodne-mncoynooftothneala svterroangge dfleapkeen’sdheenxcayg oonf athlefl aakveesr’asgiez efluapket’os thheexaavgeornaagle folafk3e0s0’ µsimze2 uatp3 t0o0 t◦hCe
average of 300 μm2 at 300 °C annealing temperature as shown in Figure 4f, which is a large
annealingtemperatureasshowninFigure4f,whichisalargeimprovementonthe(0001)preferredplane
gimropwrothvevmiaeants iomn itlhare m(0e0t0h1o) dp.reTfheerrreodl epslaonfeC gl−roawntdh Kvi+ai ao nsismwilearre minevthesotdig. aTthede rvoilaesC oVf mCle− aasnudr eKm+ eionntss.
were investigated via CV measurements. Through optoelectrical analysis via Raman spectroscopy,
ThroughoptoelectricalanalysisviaRamanspectroscopy,SEMandTEM,thehigh-qualityh-ZnOcrystals
SEM and TEM, the high-quality h-ZnO crystals grown were confirmed and the role of affiliating ions
grownwereconfirmedandtheroleofaffiliatingionsinelectrochemicalgrowthmechanismswere
in electrochemical growth mechanisms were further understood. Furthermore, through our I-V
furtherunderstood.Furthermore,throughourI-Vmeasurement,wehavedemonstratedthepotentialof
measurement, we have demonstrated the potential of h-ZnO flakes as a platform for PV based device
h-ZnOflakesasaplatformforPVbaseddeviceapplicationsviaalowheatpreparationprocess(c.f.our
applications via a low heat preparation process (c.f. our Supplementary Material Section SII for
SupplementaryMaterialSectionSIIforelectricaltestresults). Thisprocessnotonlyprovidedhigh
electrical test results). This process not only provided high quality h-ZnO crystals but also holds
qualityh-ZnOcrystalsbutalsoholdspromisesforamass-producibilitytransitionforindustriesthanks
promises for a mass-producibility transition for industries thanks to the method’s relative simplicity
tothemethod’srelativesimplicityandscalability. Andfinally,throughthiswork,wehaveprovided
and scalability. And finally, through this work, we have provided further insight into the growth
furtherinsightintothegrowthmechanismsofh-ZnOintheECDprocessandtheroleofassistiveions
smuecchhaasnKis+masn odf Chl-−ZninOs uinc hthme eEcChaDn ipsrmoscedsest aainledd tihne oruorle2 -osft eapssgisrotiwveth iomnos dseulc.h as K+ and Cl− in such
mechanisms detailed in our 2-step growth model.
SupplementaryMaterials:Thefollowingareavailableonlineathttp://www.mdpi.com/1996-1944/11/8/1360/
sS1u.pFpigleumreeSn1t.aPrylo Mtoaftcearlicaulsla: tTedhe( af)otlhloewreiancgt ioarner aatveaiinlaebqleu iolinblriinuem aKt Cwawnwd.tmhedvpai.rcioambl/exoxfxG/si1b. bFsigfruereee nSe1r. gPyloletv oefl
(c∆alGcu◦)lavteerds u(as)K tChel croenaccteinotnr artaioten iann edq(ubi)litbhreiuhm-Z nKOC ’asnnda nthoep lvaaterilaetblseiz oefs ’Gpilbobttse fdrewe iethne∆rGgy◦ .lFeivgeul r(e∆SG2°.) SvEeMrsuism KagCel
ofgrownh-ZnOflakesontheelectrodeat(a)30s,(b)120s,(c)300sand(d)1800sintothegrowth. FigureS3.
concentration and (b) the h-ZnO’s nanoplatelet sizes’ plotted with ∆G°. Figure S2. SEM image of grown h-ZnO
Illustrationof(a)thedevicefabricatedfordepositedZnOflakes,(b)I-VcurveoftheZnOnanoflakesdevicewith
[fKlaCkel]s =on0. 5thMe ealnecdtr(oc)dteh aetc (aal)c u30la ste, d(bc) o1n2d0u sc, t(acn) c3e00o fsZ annOd d(de)v 1ic8e00v ss [iKnCtol ]thues egdroinwtthhe. Fsyignutrhee sSi3s.. Illustration of (a)
the device fabricated for deposited ZnO flakes, (b) I-V curve of the ZnO nanoflakes device with [KCl] = 0.5 M
and (c) the calculated conductance of ZnO device vs [KCl] used in the synthesis.
Author Contributions: All the authors contributed equally in this project in different manners and prospective,
S. Choopun majorly contributes in ideas, growth discussion main project’s and technical directions, S.