facile synthesis of zno micro-nanostructures with controllable morphology and

c o m / l o ca t e / a p s u s c Facile synthesis of ZnO micro-nanostructures with controllable morphology and their applications in dye-sensitized solar cells Yi Zhoua,∗, Dang Lia, Xian

jo u rn a l h om epa g e :w w w e l s e v i e r c o m / l o ca t e / a p s u s c

Facile synthesis of ZnO micro-nanostructures with controllable morphology and their applications in dye-sensitized solar cells

Yi Zhoua,∗, Dang Lia, Xiangchao Zhangb, Jianlin Chena, Shiying Zhangb

a Department of Chemistry and Biological Engineering, Changsha University of Science and Technology, Changsha 410114, China

b Department of Science and Technology, Changsha University, Changsha 410003, China

Article history:

Received 11 May 2012

Received in revised form 31 July 2012

Accepted 31 July 2012

Available online 31 August 2012

Keywords:

ZnO

Micro-nanostructures

Urchin

Dye sensitization solar cell

Photoelectric properties

Different morphologies of ZnO micro-nanostructureswere successfullyprepared byhydrothermal methodatrelativelymildconditionsusingammoniatoadjustthepHofthereactionsystem.Thesamples werecharacterizedbyX-raypowderdiffraction,scanningelectronmicroscopy,opticalreflectance spec-tra,andphotocurrent–voltagecurve.TheresultsdemonstratedthatthemorphologiesofZnOchanged from“wire”to“flower”,“urchin”and“wire”withincreaseinthepHofthereactionsystemduetothe increasedconcentrationofammonia.Thediffusedreflectancespectraillustratedthatthereflectance

ofdenserurchin-likeZnOwaslowat18%inthevisibleregion.Whentheas-synthesizedZnO micro-nanostructureswereusedastheanodeofthedyesensitizationsolarcell,thedenserurchin-likeZnO exhibitedthebestphotoelectricproperties.Theshortcircuitcurrent(Jsc),opencircuitvoltage(Voc),and conversionefficiency()were6.50mA/cm2,0.682V,and1.92%,respectively

© 2012 Elsevier B.V All rights reserved

1 Introduction

As an important low-cost semiconductor functional

mate-rial withlarge bandgap (3.37eV) and large excitation binding

energy (60meV) [1], zinc oxide (ZnO) is recognized as one of

the most promising materials for optoelectronic applications

Micro-nanostructuredZnOhasdrawnconsiderableattentiondue

to its unique electrical, mechanical, and optical properties, in

addition to its applications in numerous fields, such as solar

cells [2], gas sensors [3,4], piezoelectric materials [5],

pho-tonic crystals[6],and optoelectric devices [7] Previous studies

demonstrated that the properties of ZnO are closely related

to the size and shape of the structures For example,

tetra-pod ZnO nanostructures exhibit strong UV emission [8] and

needle-like ZnO arrays exhibit strong blue light emission [9]

Thus, studyingthemorphology ofmicro-nanostructuredZnO is

important

ZnOmicro-nanostructureshavebeensynthesizedwithvarious

methods,suchaschemicalvapourdeposition[10],template-based

method[11],laserablation[12],spraypyrolysistechnique[13],

hydrothermal method [14], and electrodeposition method[15]

Researchershave preparedmicro-nanostructuredZnO with

dif-ferentmorphologiesusingthesedifferentmethods.Forexample,

Polsongkrametal.[14]preparedZnOnanorodsbyhydrothermal

∗ Corresponding author Tel.: +86 731 85258328; fax: +86 731 85258328.

E-mail addresses: zhouyihn@163.com, zhouyihn@yahoo.com.cn (Y Zhou).

method Chen et al [16] synthesized ZnO nanotubes by a sonochemical method at low temperature Liu and Zeng [17]

fabricated ZnO dandelions by a modified Kirkendall process Jana et al [18] prepared water lily-type ZnO flowers by a simple solution method, and Elias et al [19] prepared hollow urchin-like ZnO thin filmsbyelectrochemical deposition How-ever, majority of these studies are limited to the research of one kind of morphology Few studies have reported on ZnO micro-nanostructureswithdifferentmorphologiesusingasingle method

Inthepresentstudy,severaltypesofZnOmicro-nanostructures with different morphologies and photoelectric properties were prepared by a simple hydrothermal method at relatively mild conditions The concentration of ammonia, which can adjust the pH of the reaction system, was controlled Subsequently, the influence of pH on the photoelectric properties of the ZnO micro-nanostructures was investigated by studying the photocurrent–voltage (I–V) characteristicsof thedye-sensitized solarcell(DSSC)

2 Experimental

2.1 Materials Allchemicalswereofanalyticalreagentgradeandused with-outfurtherpurification.Allaqueoussolutionswerepreparedusing doubledistilledwater

0169-4332/$ – see front matter © 2012 Elsevier B.V All rights reserved.

2.2 PreparationofZnOfilm

ZnO micro-nanostructures were synthesized using a

hydro-thermalmethod.Theprocedurewasasfollows:1.18gzincnitrate

hexahydrate(Zn(NO3)2·6H2O)and0.56ghexamethylenetetramine

(C6H12N4)wereaddedto40mLdoubledistilledwaterunderstrong

magneticstirringatroomtemperaturetoobtainatransparentand

homogeneoussolution.NH3(25%)wasthendroppedintothe

solu-tionat60drops/mintochangethepHfrom7to11.Thesolution

waskeptatroomtemperaturefor0.5hundervigorousstirringto

obtaintheprecursor

TheZnOmicro-nanostructuresusedinthisworkweregrownon

FTO-coatedglasssubstrates.First,theFTO-coatedglasssubstrates

were successively cleaned in an ultrasonic bath with acetone,

ethanol,anddoubledistilledwaterfor15mintoremovedustand

preventsurfacecontamination.The FTO-coatedglass substrates

werethendippedin0.5mol/LZnCl2 aqueoussolutionfor5min

atroomtemperature.Finally,thesubstrateswerepulledupward

byahoistwithconstantspeed,andthendriedintheairtoobtain

functionalizedFTO-coatedglasssubstrates[20]

Theprecursorsolutionandthefunctionalizedsubstrateswere

transferredtoaTeflon-sealedautoclave.Thenthereactionwaskept

at90◦Cfor9htosynthesizetheZnOmicro-nanostructures.After

deposition,thesampleswerecleanedseveraltimeswithdouble

distilledwaterandthendriedintheair

2.3 Constructionofdye-sensitizedsolarcell

The construction of DSSC has been reported in a previous

research[21]

2.4 Characterization

The samples were characterized using scanning electron

microscopy(SEM,JEOLJSM-6700F)andX-raydiffractionpatterns

wererecordedonanX-raydiffractionsystem(SIEMENSD5000)

ThediffusedreflectancespectraweremeasuredbyanIPCEtester

(SolarCellScan100,BeijingZhuoLiHanGuang).TheI–V

charac-teristicsweremeasuredusingacomputer-controlleddigitalsource

meter(Keithley,Model2400)undertheilluminationofaNewport

solarsimulator(AM1.5,100mW/cm2)

3 Results and discussion

3.1 Morphologyandstructuralanalyses

Fig.1 depicts theSEM imagesof micro-nanostructured ZnO

grownunderdifferentpH,namely,7,8,9,10and11.Thegrowth

temperatureandtimewere90◦Cand9h,respectively.Asshownin

Fig.1,themorphologyoftheas-grownmicro-nanostructuredZnO

wascloselyrelatedtothepHoftheprecursorsolution

Fig.1aindicatesthatZnOnanowiresformedonthesubstrate

when the applied pH was 7 The dense ZnO nanowires with

hexagonalstructure were vertically well-alignedand uniformly

distributed onthesubstrate.Theaveragediameters of theZnO

nanowires wereapproximately 30–50nm; thelength–diameter

ratioswereapproximately6–10.ThesamplepreparedwithpH=8

resultedintheformationofflower-likeZnO whosepetalswere

approximately500–700nmin lengthand300–400nminwidth

(Fig.1b).WhenthepHwasincreasedto9,urchin-likeZnOwere

formed.Fig.1crevealsthat theurchin-likeZnO wascomprised

ofnanorods,whichhadsimilarcentersandwereapproximately

5–6␮minlengthand300–500nminwidth.Notably,urchin-like

ZnOalsoformedwhentheappliedpHwascontrolledat10(Fig.1d)

However,thisurchin-likeZnOwascomprisedofneedle-likeZnO

nanowires,andthesizesandamountsofZnOnanowireswerealso

differentfromthoseinFig.1c.WhentheappliedpHwasincreased

to11,theurchin-likemorphologiesdisappearedandchangedto ZnOnanowireswithpoororientations.Theaveragediametersof these ZnO nanowires were approximately 50–80nm, and their averagelengthswereapproximately500–600nm

Fig.1fandgisthelowermagnificationSEMimagesofFig.1cand

d,respectively.Theurchinstructureswerelinedbyasinglelayeron thesubstrate,andalltheZnOnanowireswererelatively homoge-neous.Fig.1hrevealsthesideviewimagesofmicro-nanostructured ZnOatpH=10.AsshowninFig.1h,thefilmthicknesseswereabout

2␮m

Urchin-likeZnO micro-nanostructureswere formedon FTO-coatedglasssubstratesbyahydrothermalmethod.Theformation processcanbeexpressedasfollows[22]:

(CH)6N4+6H2O →6HCHO+NH3 (1)

NH3+H2O →NH4++OH− (2)

Zn2++NH3→Zn(NH3)4 + (3)

Zn2++4OH−→ Zn(OH)4 − (4) Zn(NH3)4 ++2OH−→ ZnO+4NH3+H2O (5) Zn(OH)4 −→ ZnO+H2O+2OH− (6) BasedonthegrowthhabitsofZnO crystalsinaqueous solu-tions,urchin-likeZnOmicro-nanostructurescanbeobtainedonly whenthepHofthebulksolutionarecontrolledatcertainvalues WhenthepHislow,theconcentrationsofOH−andNH3 inthe precursorsolutionarecorrespondinglylow,leadingtothesmall amountofZn(OH)4 −and Zn(NH3)42+,whichareinsufficientto formthenuclei.Therefore,ZnOnanowiresformonthesubstrate whentheappliedpHiscontrolledat7(Fig.1a).Theamountof Zn(OH)4 −andZn(NH3)42+increaseswiththepH.WhenthepH

isabove8,Zn(OH)4 −andZn(NH3)42+willgatheranddecompose

toZnOnucleiatthebeginningofthereaction.Thegrowthunits

of Zn(OH)4 − and Zn(NH3)42+ are then adsorbed onthenuclei due tointermolecularabsorptionforces,suchasvanderWaals interactions, and finally growto nanowires in all directions to formthree-dimensionalurchin-likeZnO.TheSEMmicrographsin

Fig.1revealthatthemorphologiesofthree-dimensionalZnOare differentunderdifferentconditions.AsthepHincreases,the mor-phologies ofZnO changefrom“flower”(Fig.1b),tosparse“sea urchin”(Fig.1c)and denser“sea urchin”(Fig.1d) The pHhas

an important functionduring the formationof the ZnO micro-nanostructures Thiscanbeexplainedasfollows.Ammonia can easilyseparatefromthesolutionwhenthepHishigh,whichresults

inanincreaseintheairpressureoftheTeflon-sealedautoclave ThiswillinfluencethegrowthofZnOmicro-nanostructures, lead-ingtochangeinthemorphologyofZnO.ThepHofthereaction solutionincreaseswiththeadditionofammonia.Atthesametime, thegrowthunitsaremorelikelytocomeincontactwiththenuclei ThisphenomenoncanbepropitiousfortheformationofZn(OH)4 − and Zn(NH3)42+,finallyleadingtoanincrease intheamountof nanowires onthenuclei.However,when thepHincreasestoa certaindegree,theairpressureintheTeflon-sealedautoclavewill increasetoagreaterdegree;thus,Zn(OH)4 −andZn(NH3)42+are unabletogathertoformtheinitialZnOnuclei.Therefore,the pre-cursorsolutionisgeneratedforthesingleandindependentZnO nanowires,hindering themfromformingflower- orurchin-like ZnO.Theconcentrationofammonia,thepH,andtheairpressure

oftheTeflon-sealedautoclavehasanimportantfunctionin the abovetransformation processesofZnO morphologies.However, thespecificreactionmechanismrequiresfurtherresearch

Fig 1.SEM images of the micro-nanostructured ZnO under different pH: (a) 7; (b) 8; (c and f) 9; (d and g) 10; and (e) 11 Lower magnification SEM images (f) and (g) Side view images of micro-nanostructured ZnO at pH = 10 (h).

3.2 XRDpatterns

Fig.2showstheXRDspectraofthemicro-nanostructuredZnO

underdifferentpH.Alldiffractionpeakscanbeindexedtoa

hexag-onalwurtzitephaseofZnO,inagreementwiththestandardcard

(JCPDS78-2486).Nocharacteristicpeaksofanyimpurities,except

polycrystallineSnO2 (fromtheFTOsubstrate),weredetectedin

thepattern,confirmingthattheobtainedproductsarepureZnO

Thecharacteristicpeakswerehighinintensityandnarrow,which

indicatedthatZnOmicro-nanostructurehadhighcrystallinity.The

intensitiesofthediffractionpeaksofmicro-nanostructuredZnO

wereobviouslydifferent,indicatingthatthepHhadeffectonthe

crystallinityofgrownZnOmicro-nanostructure

3.3 Opticalreflectionspectraanalyses

Fig 3 shows the optical reflection spectra of the

micro-nanostructured ZnO under different pH Fig 3 indicates that

the light scattering of the ZnO micro-nanostructure is closely

related to its morphology The surface areas of denser

urchin-likestructure,sparseurchin-likestructure,verticallywell-aligned

nanowirestructure,disorderlynanowirestructureandflower-like

structurewere350,290,223,147,96m2/g,respectively.Anddue

tothedifferentsurfaceareasofthedifferentmorphologies,the order ofintensityoftheZnO micro-nanostructurelight scatter-ingisasfollows:denserurchin-likestructure<sparseurchin-like structure<verticallywell-alignednanowirestructure<disorderly nanowire structure<flower-like structure Among these ZnO micro-nanostructures, the reflectance of the denserurchin-like ZnO is the lowest at approximately 18% in the visible region Thereflectanceoftheflowerstructureisthehighest.Thiscould attributetothesize[23]andthemorphology[24]ofZnO,which playimportantrolesforcontrollingthelightscattering.Inaddition, thegraphindicatesthattheentireultravioletabsorptionspectrum edge is approximately380nm, which isin agreementwiththe directwidebandgap(3.37eV)ofZnO

3.4 ApplicationofZnOmicro-nanostructureinDSSC

Fig.4comparestheI–VcharacteristicsoftheDSSCbasedonZnO with different micro-nanostructures The corresponding values are summarized in Table 1, which demonstrates photoelectro-chemical characteristics,suchascurrentdensityatshortcircuit (J ), voltageat opencircuit(V ), fill factor (FF),and efficiency

Fig 2. XRD spectra of the micro-nanostructured ZnO under different pH: (a) 7; (b)

11; (c) 8; (d) 9; and (e) 10.

Fig 3.Optical reflection spectra of the micro-nanostructured ZnO under different

pH: (a) 8; (b) 11; (c) 7; (d) 9; and (e) 10.

Fig 4.Photocurrent–voltage characteristics of ZnO with different micro-nanostructure-based DSSCs (a) Denser urchin-like ZnO; (b) sparse urchin-like ZnO; (c) orderly ZnO nanowire; (d) flower-like ZnO; and (e) disorderly ZnO nanowire.

Table 1

Photovoltaic parameters of micro-nanostructured ZnO with different micro-nanostructures.

thatthephotoelectrochemicalcharacteristicsoftheDSSC-based denserurchin-like ZnO arehigh, reachinga maximumvalueof 1.92%.Notably,thedenserurchin-likestructureisbeneficialforthe transferofelectrolytes,andthisspecificstructurecanincreasethe productionofcarriersandphotoelectricactivityduetoitslarger surfaceareaand increasedactivitycentersforabsorbingdye.In theurchin-likestructure,theneedle-likeZnOnanowireshavethe samecenterextendedtothesurroundings.Thisspecialstructure can effectively improve the efficiency of electron transmission, decreasesthetransmissionpathofchargeintheelectrode materi-alsandtherecombinationofcarriers.Asaresult,thephotoelectric activityofZnOincreases

Table1furthershowsthattheZnOnanowireswereformedon thesubstratewhen theappliedpHwascontrolledat7 and11, butthephotoelectricparametersofthesetwodifferentnanowires varied enormously Due totheir excellent orientation,the ZnO nanowirespreparedatpH=7canprovideaneffective transmis-sionpathforelectrons,reducetherecombinationofcarriers,and increasethephotoelectricactivityofZnO

4 Conclusions

SeveralkindsofZnOmicro-nanostructureswithdifferent mor-phologiesandphotoelectricpropertieshavebeenpreparedbya simple hydrothermal methodat relatively mild conditions.The

pH is essential in the growth of ZnO, and resultsin the mor-phology ofZnO changing from“wire”to“flower”,“urchin”and

“wire”withtheadditionofdifferentamountsofammonia Due

to thedifferent surface areas of the various morphologies, the orderof intensityoftheZnOmicro-nanostructure light scatter-ingisasfollows:denserurchin-likestructure<sparseurchin-like

nanowirestructure<flower-likestructure.Amongtheforegoing,

thereflectivityofthedenserurchin-likestructurewasthelowest

at18%.WhentheobtainedZnOmicro-nanostructureswereusedas

theanodeoftheDSSC,thephotoelectrochemicalcharacteristicsof

theDSSCbasedonZnOwithdifferentmicro-nanostructuresvary

Thedenserurchin-likeZnOmicro-nanostructuresdisplayexcellent

photoelectricpropertiesduetotheirlargersurfacearea,increased

activitycenters,andmoreeffectivetransmissionpaths

Acknowledgments

This work was supported by the National Natural Science

FoundationofChina(grantno.21171027) Theauthorsarealso

gratefultotheaidprovidedbytheScienceandTechnology

Inno-vativeResearchTeaminHigherEducationalInstitutionsofHunan

Province

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