InternationalJournalofNanoelectronicsandMaterialsVolume11,No.2,Apr2018[233‐240]

SynthesisofCopperOxideNano‐RodsbyMicrowave‐AssistedCombustionRouteandtheirCharacterizationStudies

YathishaROaandY.ArthobaNayakaa*.

aDepartmentofChemistry,SchoolofChemicalSciences,KuvempuUniversity,Jnanasahyadri,

Shankaraghatta‐577451,Karnataka,India.

Received4September2017;Revised13October2017;Accepted3November2017

ABSTRACTThepresentstudydemonstrateshowpetal‐shapedcopperoxide(CuO)nano‐rodstructureswerefabricatedusingthemicrowave‐assistedcombustionmethod.Thepreparedmaterialwasannealedatatemperatureof500°Candcharacterizedbymeansofopticalabsorptionspectroscopy (UV–Vis), X‐ray diffraction (XRD), Field emission scanning electronmicroscopy(FE‐SEM),andPhoto‐conductivitytechnique(I‐Vcharacteristics).Theopticalenergy gap of CuO nano‐rods was analysed through the UV–Vis NIR spectroscopictechniqueand the result illustrates that theenergygapof synthesisedCuOnano‐rods isfoundtobe1.44eV.TheXRDofthesynthesisedsampleconfirmsthatcopperoxidenano‐rodsareinpuremonoclinicphasewithlatticeparametersx=4.675Å,y=3.343Å,z=5.183Å.EnergydispersiveanalysisusingX‐raysconfirmstheexistenceofcopperandoxygenatomsinsynthesisedCuOnanocrystals.FE‐SEMmicrographsspecifythatthesynthesisedmaterialcontainsnanostructurednano‐rodswithasmallamountofagglomerationintheproduct.The electricalpropertiesofCuO filmwere studied through I‐V characterizationand theresults depict that the conductance of CuO film is found to be superior underUV‐ lightcomparedtointhedark.Keywords:Crystallinesize,Energygap,Morphology,Petalshapedrods,Tenorite.

1. INTRODUCTIONSynthesis ofmetal‐oxide and transitionmetal doped semiconducting nanomaterials provideopportunities for improved applications in different areas of science and technology due totheiruniquephysicalandchemicalpropertiescausedbytheirnano‐sizeddimensionandlargesurface/volume ratio [1]. One‐dimensional nanomaterial such as nano‐ribbons, nano‐belts,nano‐cubes, and nano‐prisms have become the focus of intensive research due to theirdistinctivepropertiesandtheirsignificanceforfabricationintohigh‐densitynanoscaledevices[2]. The transitionmetal oxides such as copper sulphide (CuS), cerium oxide (CeO2), nickeloxide(NiO),ferrousoxide(Fe2O3),andTitania(TiO2)areanimportantclassofsemiconductors.Among thesemetal‐oxides, Copper (I) oxide (Cu2O) and copper (II) oxide (CuO) have beengainingmorepopularityinrecentyearsduetopropertiessuchasexcellentthermalstability,easily available reactant materials, non‐toxic, and exhibits good optical and electricalproperties[3].Duetotheseproperties,recentlymostresearchworkismainlyfocusedonthesynthesis of different CuOnanostructures, such as nano‐flowers, nano‐flakes, nano‐pyramid,nano‐plates,andnano‐bats[4].

*CorrespondingAuthor:drarthoba@yahoo.co.in

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Therefore,anumberofCuOnanostructureshavebeenpreparedindifferentmorphologiesbyavariety of methods such as precipitation [5], solvothermal [6], hydrothermal [7], thermalevaporation [8], electrochemical [9] and combustion [10], and solid‐state [11] methods.Generally, theabovemethodsfacecertaindrawbacks:(i)needmorecomplicatedequipment,(ii)requiremoreadditionalsolvent(iii)requireveryhightemperatureandpressure(iv)takemore time for the reaction, and (v) more steps are required for the synthesis of CuOnanoparticles [11]. Hence to resolve the above problems, a simple and low‐cost method isnecessaryforthepreparationofCuOnanocrystals.Outofthevariousmethods,themicrowaveheating process has recently become a popular andmore usual process compared to othercombustion methods. In this route, the microwave has got more penetrating power and itpenetratesintothematerialsatthemolecularlevel.Thisleadstotheconversionofmicrowaveenergyintoheatthroughoutthematerialsbyeitherionicconductionordipolerotation,whichresults in reaction rate acceleration, yield of a homogeneous productwithin a fewminutes,goodsizedistributionandexcellentcontrolofstoichiometry[12].Usingthismethod,anumberoffunctionalmaterialsandcompositeswithnovelstructurescanbeobtainedsuchasα‐Fe2O3nano‐rings[13],NiS2nano‐cubes[14],CuSnano‐prisms[15],androse‐likeZnO[16]withtheirexcellentproperties.CuOisknownasapotentialP‐typesemiconductormaterialwithanarrowopticalenergygapvarying between 1.2‐ 1.4 eV [17]. When comparing CuO nanoparticles with commerciallyavailableCuOpowder,theCuOnanoparticleshaveattractedalotofpotentialapplicationsanduses due to their nanocrystals having advanced selectivity and photo catalytic activity [18].Duetotheirunconventionalbandstructure,CuOnanoparticleshaveawiderangeofpotentialapplications in the fieldsofsuperconductors [19], lithium‐ionbatteries [20],electrochemicalgas sensors [21], dye‐sensitized solar cells (DSSC) [22], and electro‐chemiluminescence.Therefore,itisoffundamentalinteresttostudythestructureandcharacterizetheopticalandelectricalpropertiesofhigh‐purity,singlephaseCuOnanocrystals[23].

Here, we synthesised high purity CuO nano‐rods using the microwave irradiation methodwhere a solution of copper sulphate, sodiumhydroxide, and ureawas used. The diffractionpattern of the CuO was recorded on an X‐ray diffractometer. The band gap was calculatedusingUV‐Visabsorptionstudies.I‐VcharacteristicsweremeasuredonaKeithleysourcemeter.Therod‐likestructureofCuOwasinvestigatedthroughFE‐SEM.

2. EXPERIMENTALDETAILS

2.1 ChemicalsUsed

Copper sulphate pentahydrate (Cu(SO)4·5H2O), sodium hydroxide (NaOH), and Urea(NH2CONH2)wereprocuredfromHimediaLaboratoriesPvt.Ltd.,andacetone(CH3COCH3)waspurchased from Merck. All chemicals were of analytical grade and were used as receivedwithoutfurtherpurification.Doubledistilledwaterwasusedforthepreparationofsolutions.The synthesisofnanosizedCuO rodswas carriedout in adomesticmicrowaveoven (Onidapowersolo17D)operatingat100%powerof700W.

2.2 SynthesisofCuONano‐Rods

ToprepareCuOnanoparticles,100mLof0.4Msodiumhydroxidewasaddeddrop‐wiseintoabeakercontaining100mLof0.8Mcoppersulphateand0.8Mureasolutionunderconstantstirring.Whentheblue‐colouredhomogenousreactionmixtureturnedintopartialgreen,theresultingsolutionwaskeptinamicrowaveovenandthereactionmixturewasirradiatedwith60% power (175°C) of microwave energy for 15 minutes. Then, the obtained dark greenprecipitateof copperhydroxidewascentrifuged,washedseveral timesusingdistilledwater,

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alcohol,andacetonesuccessivelyandthendriedat80°Cinanovenfor5hours.Theobtainedsampleswerecalcinated(500°C)intheairfor1hourtogetthedesireddarkbrowncolouredCuOnanocrystal.2.3 CharacterizationDetails

ThecrystallinityanalysisofthenanoparticleswascarriedoutonanX‐RaydiffractometerusingCu‐Kαradiation( =0.15406nm).Thepatternswererecordedintherangeof10ºto80º(2 ).ThecrystallitesizeandstructureofthesampleswerestudiedusingaFieldemissionscanningelectron microscope (FESEM) fitted with an Energy‐dispersive X‐ray spectroscopy (EDAX)[Model:FE‐SEMCarlZeissAG‐ULTRA55].TheabsorbanceofCuOnanomaterialwasmeasuredusingaUV‐Vis spectrophotometer (Model:USB4000,OceanOptics,USA).PhotoconductivitypropertieswereanalysedusingaKeithleylowvoltagesourcemeterwiththemeasuredvoltagebetween0‐20V[Model:Keithley2401].3. RESULTSANDDISCUSSION3.1 XRDAnalysis

Figure1showstheXRDpatternofCuOnanoparticlespreparedusingthemicrowaveheatingmethod.Ascanbeseen inFigure1,alldiffractionpeaksareassignedwell to themonoclinicstructureof a tenorite system.Themajorpeaksareobserved at2θ valuesof32.55o, 35.50o,38.69o,48.70o,53.53o,58.57o,61.67o,66.24o,68.04o,72.24o,75.10o[24].Inaddition,,nootherimpuritypeaksofcopperhydroxidewereobservedintheXRDpattern,showingasinglephaseCuOformation,whichisinagreementwithJCPDScardNo:03‐0884.Themeasuredvaluesofd‐spacingandFWHMofthesynthesisedCuOnanocrystalsarewell‐matchedwiththeJCPDScardno. In addition, the peak of the XRD is broader due to the obtainedmaterials containing ananorange of particles. The crystalline size of the synthesised CuO nanoparticles wascalculatedusingScherrer’sformula[25];

Averagecrystallinesize(D)=

cos

9.0 (1)

where isthewavelengthofCuKαradiation, isFWHM,and isBragg’sreflectionangle.TheaveragecrystallinesizeofCuOwasfoundtobeintherangeof~21–54nm.

Figure1.XRDpatternofpetal‐shapedrod‐likeCuOnanoparticles.

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3.2 StructuralandElementalAnalysis

Figure 2 shows FE‐SEM images of synthesised pure CuO nanoparticles. The FE‐SEMmicrographofsamplesshowsthattheCuOstructurespreparedat60%powerconsistofpetal‐shaped rod‐like CuO nanocrystals, while the low resolution (2 m) image show the minorflower‐shaped particles. Particle aggregation is amajor factor that controls themorphologyandstructureofthefinalproduct.Eventhoughitisnotyetunderstood,itisevidentthatNaOHplays an important role in the formation of the flower‐like structure.Microwave irradiationhas also got an influence on the surface morphology of CuO nanostructures due to itsvolumetric heating effect.Moreover, the vibrational frequencyof the atomsduringdifferentreactionconditionsplaysanimportantroleindecidingthesizeofthenanoparticles[26].EDAXspectra were used to analyse the composition of CuO particles at 500oC and are shown inFigure 3. Based on the determination of the elemental composition of CuO nanorods(Cu=59.03%andO=40.97%), theweightpercentofCu/Oratiowasfoundtobe1.45andtheEDAXspectrumobtainedfromtheCuOnanoparticlesshowspeaksthatconfirmthepresenceof Cu and O only,which confirms that the synthesised CuOwas of single phase and a highpuritymaterial.[27].

Figure2.FE‐SEMimagesofpureCuOnanoparticles(a)petalshapeofrods(b)flower‐likeparticles.

Figure3.EDAXspectraofsynthesisedpureCuOnanoparticles.

(a) (b)

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3.3 UV‐VisibleSpectralAnalysisFigure4showstheopticalabsorptionspectraofthesampleannealedat500oCtemperature.AUV‐Vis spectrumwasmeasured in the range of 300 to 900nm in the wavelength region atroomtemperature.ThemeasuredenergygapofCuOwas1.44eV,whichwascalculatedfromTauc’srelation, (αh)1/r=B(h‐Eg)whereh is theplanksconstant,B isaconstant,α is theabsorptioncoefficient,isthefrequencyoftheincidentradiation,andrisdeterminedbythetype of optical transition of a semiconductor. The energy band gap (Eg) can be obtained byplotting thegraphof (h)against (αh)1/r forvariousvaluesof rand itbecomespossible todeterminethenatureofthetransitionsinvolved.Theinterceptsofthestraightlineportionofthe respective plots (at h Eg) corresponding to zero absorption on the energy axis givesvaluesofenergygaps[28].

Figure4.UV‐Visabsorptionspectraandtauc’splotofpureCuOnanoparticles.

3.4 I‐VCharacterization

TheCurrent‐Voltage (I‐V) characteristics of CuO filmwere studied in dark aswell as underlight condition to check the nature of the contact between CuO and silver paste using twoprobemethods. The typical current‐voltage curves of CuO film in dark and under light areshowninFigure5.Theareaofusedsamplesis2x2cm2.AsseeninFigure5,theconductivityofthickCuOfilmwasextremelysmallinthedarkcondition.Itisduetotheslowdownmotionofelectrons present in the grain boundaries. These boundaries operate as diffusion sites andimpendingwalls,whichwoulddirect theelectronmovementbyreducing theresistanceandenhancingtheconductivity.Furthermore,theimprovementinconductanceundertheUV‐lightconditionmaybethereasonfortheactivationofchargecarrierssuchaselectronsandalsoforthe enhancedmobility of charge carriers [29]. Figure 5 shows themeasured current of CuOfilmsat5V,whichis0.45A(darkcondition),butthesamephoto‐currentwasalsoobservedatlowvoltage(4V)intheUV‐lightcondition.Themeasuredopticalenergygap,conductivity,andresistivityareveryvaluablefortheapplicationoffabricationofopticaldevices[30].

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Figure5.I‐VcharacteristicsofCuOnanoparticlesunderdarkaswellasUV‐lightconditions.4. CONCLUSIONSInsummary,rod‐likeCuOnanoparticleswithmonoclinicstructureweresynthesisedusingthemicrowavecombustionmethodwithureausedasfuel.XRDstudiesshowthatthesynthesisedCuOnanoparticlesareofmonoclinicstructure;italsoshowtheaveragesizeoftheparticles.AFESEMobservationdisplaysthesurfacemorphologyofrod‐shapedCuOparticlesinthenano‐meter dimension.UV‐Vis spectra show that the optical band gap of CuO is 1.45eV,which iscalculated fromTauc’s relation. I‐V indicates thephoto‐current conversion efficiency of CuOnanoparticles.ACKNOWLEDGEMENTSTheAuthorswouldliketothanktheScienceandEngineeringresearchboard(SERB),Delhiforproviding financialsupport tocarryout thisresearchwork.Theauthorsarealso thankful totheIndianInstituteofScience(IISC),BangaloreforprovidingXRDandSEMfacilities.

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