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Applied Surface Science
j o ur na l ho me pa g e :w w w . e l s e v i e r . c o m / l o c a t e / a p s u s c
Full Length Article
Fabrication and photoelectrochemical properties of silicon nanowires/g-C 3 N 4 core/shell arrays
Zhen Chen
a,c, Ge Ma
a, Zhihong Chen
b,∗, Yongguang Zhang
d, Zhe Zhang
a, Jinwei Gao
c, Qingguo Meng
b, Mingzhe Yuan
b, Xin Wang
a,∗, Jun-ming Liu
c, Guofu Zhou
aaInstituteofElectronicPaperDisplays,SouthChinaAcademyofAdvancedOptoelectronics,SouthChinaNormalUniversity,Guangzhou,Guangdong Province,China
bShenyangInstituteofAutomation,Guangzhou,ChineseAcademyofSciences,Guangzhou511458,China
cInstituteofAdvancedMaterials,SouthChinaAcademyofAdvancedOptoelectronics,SouthChinaNormalUniversity,Guangzhou,GuangdongProvince, China
dResearchInstituteforEnergyEquipmentMaterials,TianjinKeyLaboratoryofMaterialsLaminatingFabricationandInterfaceControlTechnology,Hebei UniversityofTechnology,Tianjin300130,China
a r t i c l e i n f o
Articlehistory:
Received7September2016
Receivedinrevisedform27October2016 Accepted31October2016
Availableonline1November2016
Keywords:
PEC
Siliconnanowire/g-C3N4core/shellarrays Watersplitting
a b s t r a c t
Aphotoelectrochemical(PEC)cellmadeofmetal-freecarbonnitride(g-C3N4)@siliconnanowire(SiNW) arrays(denotedasSiNWs/g-C3N4)ispresentedinthiswork.Theas-preparedphotoelectrodeswithdif- ferentmasscontentsofg-C3N4havebeensynthesizedviaametal-catalyzedelectrolessetching(MCEE), liquidatomiclayerdeposition(LALD)andannealingmethods.Theamountofg-C3N4ontheSiNWarrays canbecontrolledbytuningtheconcentrationofthecyanamidesolutionusedintheLALDprocedure.The denseandverticallyalignedSiNWs/g-C3N4core/shellnanostructureswerecharacterizedbyscanning electronmicroscopy(SEM),transmissionelectronmicroscopy(TEM),andX-raydiffraction(XRD).Incom- parisonwithFTO/g-C3N4andSiNWsamples,theSiNWs/g-C3N4samplesshowedsignificantlyenhanced photocurrentsovertheentirepotentialsweeprange.Electrochemicalimpedancespectroscopy(EIS)was conductedtoinvestigatethepropertiesofthechargetransferprocess,andtheresultsindicatedthat theenhancedPECperformancemaybeduetotheincreasedphoto-generatedinterfacialchargetrans- ferbetweentheSiNWsandg-C3N4.Thephotocurrentdensityreached45A/cm2under100mW/cm2 (AM1.5G)illuminationat0V(vs.Pt)inneutralNa2SO4solution(pH∼7.62).Finally,asystematicalPEC mechanismoftheSiNWs/g-C3N4wasproposed.
©2016PublishedbyElsevierB.V.
1. Introduction
Energy harvested directly from sunlight offers a promising approach to satisfy theincreasing global energy demands and to resolve environmental issues caused by the overuse of fos- silfuels.Oneofthemostattractiveoptionsistheconversionof solarenergy into chemical bonds,suchasH2,through a water splittingprocesswiththehelpofsemiconductor-basedphotocat- alysts[1–4].In the pastdecades, thetraditional TiO2 hasbeen extensivelyinvestigatedinthefieldofhydrogenproductionsince FujishimaandHondafirstdemonstratedthatwatercouldbesplit intoH2 and O2 ona TiO2 electrode under light irradiation[5].
However,therelativelyhighbandgapofTiO2 (ca.3.2eV) leads
∗Correspondingauthors.
E-mailaddresses:chenzhihong1227@sina.com(Z.Chen),wangxin@scnu.edu.cn (X.Wang).
toinsufficientuseofthesolarspectrum(itcanabsorblessthan 4%oftheavailablesolarenergy),resultinginthelowvisible-light photocatalytic activity of TiO2 [6]. Therefore, developing novel visible-light-responsivephotocatalystshasbecomeahottopicin thephotocatalysisfield.
In recent years, special attentionhas beenpaid tographitic carbonnitride(g-C3N4), duetotherelativelylowband-gap(ca.
2.8eV)andtheappropriatepositionsoftheconductionband(CB) andvalenceband(VB)forwaterreductionandoxidation,respec- tively [7–10]. Furthermore, g-C3N4 can be synthesized by the simplethermalcondensationoflow-costnitrogen-richprecursors, suchasurea, cyanamide,dicyandiamide,andmelamine, among other precursors [11–14]. However, the photocatalytic activity oftheas-obtainedg-C3N4 is usuallyrestrictedby therelatively fastrecombinationof photo-generatedelectron–hole pairs[15].
Variousstrategies, such asmorphology modification,elemental doping,andcopolymerizationaresubsequentlyappliedtoimprove
http://dx.doi.org/10.1016/j.apsusc.2016.10.203 0169-4332/©2016PublishedbyElsevierB.V.
610 Z.Chenetal./AppliedSurfaceScience396(2017)609–615 thephotocatalyticperformanceofg-C3N4[16–18].Morerecently,
combiningg-C3N4withothersemiconductors,suchasZnO,TiO2, CuInS2,Bi5O7I,InVO4,toformaphotoelectrochemical(PEC)het- erogeneouselectrodeforwatersplittinghasbeenconsideredan effectivewaytoimproveitsphotocatalyticactivity[19–25].How- ever,thesephotoelectrodesalsosufferfrominsufficientabsorption ofsolarenergy and poorsurfacearea, which mayrestricttheir practicalapplications.
SiliconisapromisingphotoelectrodematerialforPECapplica- tionsbecauseofitsnarrowbandgap,enablingeasilycontrollable electricalconductivity[26–28].ComparedtothebulkSistructure, SiNWsexhibitsignificantlyenhancedlightabsorptionandenergy conversionefficiencydue totheirfaster charge transport path- ways,moreefficientcharge collection,higher contactareawith theelectrolyteandlowerreflectivity[29–31].Hence,constructing aheterojunctionbycouplingg-C3N4andSiNWsmayprovidean alternativenovelpathwaytoaddresstheintrinsicdrawbacksof g-C3N4-basedphotoelectrodesforPECapplications.Tothebestof ourknowledge,therearenoreportedstudiesaboutSiNWs/g-C3N4 heterojunctionPECelectrodes.
Inthecurrentwork,wereportanovelmetal-freeSiNWs/g-C3N4
core/shellPECphotoanodeforwater splittingpreparedbysim- plemetal-assistedetching,liquidatomiclayerdeposition(LALD) andannealingmethodsandcharacterizedbyTEM,SEM,XPSS,DRS, XRD.Undervisiblelightirradiation,theSiNWs/g-C3N4photoelec- trodesexhibitednoticeablyenhancedPECperformancecompared toFTO/g-C3N4andSiNWphotoelectrodes,whichisattributedto thefastphoto-generatedchargetransferbetweentheSinanowires andg-C3N4. Asystematic investigationof theeffects ofg-C3N4 thicknessonphotocurrentandthePECmechanismwasconducted.
Thisworkmayprovideacommonandsimpleroutealongwith an insight into designing and understanding an efficient wide spectral-responsePECphotoelectrodeforrenewableenergyappli- cations.
2. Experimental
2.1. SynthesisofSiNWnanoarrays
N-typeverticallyaligned SiNW arrays weresynthesizedvia metal-catalyzed electroless etching (MCEE) of the n-Si wafer substrate. A 2cm×2cm phosphorous-doped n-type Si wafers piece(2–10/cm)wascleanedbywaterandacetone,andthen immersedinanoxidantsolutioncontainingH2SO4(98%)andH2O2
(40%)(v/v=3/1)for60mintoremoveorganicsentirely.Thecleaned siliconwaferwasthenimmediatelyimmersedinanHF–AgNO3 solutioninasealedvesseltoetchfor45minandthenimmersed intheoxidantsolutionforoveranhourtoremovetheresidual Agdendriticcrystalonthewafer.Thesolutionconcentrationsof HFandAgNO3 werekeptat49%(massfraction)and0.25mol/L, respectively.
2.2. SynthesisofSiNWs/g-C3N4core/shellnanoarrays
InatypicalsynthesisoftheSiNWs/g-C3N4nanoarrays,asshown in Scheme1, Si NWswere used asthe core,and a cyanamide solutionwithacertainmassconcentrationwasusedastheshell precursor.TheSiNWs/g-C3N4samplesmentionedlaterwerefabri- catedwithcyanamidesolutionswithamassconcentrationof0.25, exceptwherenoted.Then,apieceoftheSiNWnanoarraysample wasimmersedinthecyanamideaqueoussolutionandleftstand- ingfor 30min.Afterbeingtakenout,thetreatedSiNWsample wasplacedonplanarglassandexposedtoairfor12h.Then,theSi NWs/cyanamidewaferwastransferredtoavacuumtubefurnace andmaintainedat550◦Cfor3h.
2.3. SynthesisofFTO/g-C3N4photoelectrode
To confirm the superiority of the core-shell nanowires, a FTO/g-C3N4 photoelectrode was prepared.Twenty milliliters of cyanamideaqueoussolutionwastransferredtoacruciblewitha lid.Then, thecruciblewiththecyanamide solutionwasheated to550◦C ataheatingrateof10◦C/mininamuffle furnaceand thenmaintainedatthistemperaturefor3h.Intotal,0.2gofthe as-preparedsamplewassuspendedin5mLofethanol,whichwas thengroundfor 4htoformaslurry.Theslurrywasdip-coated ona20mm×20mmfluorine–tinoxide(FTO)glasselectrode.The electrodewasthenannealedat150◦Cfor2hataheatingrateof 5◦C/min.
2.4. Characterization
The crystalline structure of the Si NWs/g-C3N4 sample was characterizedusingaShimadzu7000SX-raydiffractometer(XRD) withCuK␣ radiation.The morphologiesand nanowiresizesof theas-preparedsamplesandhigh-resolutionimageswerestudied withafield-emission(JEM2100F)transmissionelectron micro- scope (TEM). EDX mapping analysis was carried out with the sameTEMin STEM mode.Scanning electronmicroscopy (SEM) wasperformedusinga ZEISSULTRA55field-emissionscanning electronmicroscope.TheUV–vis diffusereflectionspectrawere measuredbyaShimadzuU-3010Ultraviolet–visspectrophotome- terequippedwithanintegratingsphereusingbariumsulfateas thereference.Electrochemicalimpedancespectroscopy(EIS),lin- earsweepvoltammetry(LSV)andamperometricI–Tcurve(AIT) measurementswereconductedwithanaccurateelectrochemical workstation(CHI660E)inatwo-electrodesystemusingplatinum wireasthecounterelectrode.X-rayphotoelectronspectroscopy (XPS)datawereobtainedfromaThermoScientific,Escalab250Xi X-rayphotoelectronspectrometer.
2.5. PECmeasurements
The PECproperties of the as-prepared sampleswere evalu- atedbyathree-electrodeelectrochemicalworkstation(CHI660E, China).Forthetwo-electrodesystem,weusedtheas-preparedSi NWs/g-C3N4nanoarraysastheworkingelectrodeandaplatinum wireasthecounterelectrodeandreferenceelectrode[32,33].An aqueoussolutioncontaining0.5MNa2SO4wasusedastheelec- trolyte.ThepHvalueof0.5MNa2SO4isapproximately7.62,which wasmeasuredbya pHmeter(HzaollPS-3C,witha precisionof 0.01).A300-Wxenonlampwithalightintensityof100mW/cm2 (AM1.5G)actedasthelightsource.LSVwascarriedoutatascan rateof50mV/sfrom−1.5to1.5Vvs.Pt.Electrochemicalimpedance spectra(EIS)weremeasuredusingthesameworkstationunderan open-circuitconditionwiththefrequencyrangingfrom0.05Hzto 100kHz.
3. Resultsanddiscussion
3.1. Characterizationofcompositionandstructure
The structures and morphologies of Si NWsand Si NWs/g- C3N4core/shellarraysareobservedbySEMandTEM.Asshown inFig.1(a),thesmoothSiNWarrayswithdiametersrangingfrom 70to300nmareevenlyandverticallyarrangedontheSisubstrate.
Moreover,fromtheviewofthecross-sectionimageoftheSiNWs, thelengthofthesingleSiNWsisapproximately10m,controlled byadjustingtheetchingtime.ThesurfaceoftheSiNWsbecomes roughaftercoatingitwithg-C3N4 bytheLALDmethod,butthe lengthoftheSiNWsdidnotchange,asindicatedbythecompari- sonofFig.1(a)and(b).Theg-C3N4iscoatedontothesurfaceofthe
Scheme1.SchematicIllustrationforthePreparationofSiNWs/g-C3N4.
Fig.1.SEMandTEMimages:(a)cross-sectionalSEMimageofpristineSiNWsand(b)SiNWs/g-C3N4;(c)TEMimagesofasingleSiNWs/g-C3N4core/shellstructure;(d) HRTEMimagesofSiNWs/g-C3N4.
SiNWsuniformlytoformtheSiNWs/g-C3N4core/shellstructure, confirmingtheintimateinterfacialcontactbetweenSiNWsandg- C3N4,asshowninFig.1(c).AnHRTEMimageofSiNW/g-C3N4is showninFig.1(d),whichindicatesthatthethicknessoftheg-C3N4 shellisapproximately15nm.
The elemental composition and chemical state of the as- preparedSi NWs/g-C3N4 sample aredetectedby XPSmeasure- ments,and theresultsare shownin Fig.2.The SiNWs/g-C3N4 compositesarecomposedofC, N,SiandO,whichconfirmsthe
coexistenceofSiNWsandg-C3N4intheSiNWs/g-C3N4compos- ite.ThetypicalhighresolutionXPSC1sandN1sspectraindifferent samplesarealsogiveninFig.2(b–c).TheC1shigh-resolutionspec- trumshowstwodifferentpeakswithbindingenergiesof284.8and 289.4eV.Thesharppeaklocatedat284.8eVisattributedtosp2 hybridizedcarbons(C C)andthepeaklocatedat289.4eVcorre- spondstoC (N)3bonds[34,35].TheN1shigh-resolutionspectrum isdecomposedintothreepeaks.Thepeakcenteredat399.8eVis attributabletothesp2Ninvolvedinthebridgingnitrogenatoms
612 Z.Chenetal./AppliedSurfaceScience396(2017)609–615
Fig.2.XPSspectra:(a)surveyspectra,high-resolutionXPSspectraof(b)C1s,(c)N1sand(d)XRDpatternsforthepureg-C3N4,SiNWs/g-C3N4andSiNWs(asshowninthe inset).
ofN (C)3,whereasthecontributionat398.5eVcorrespondstotri- azinerings[36].Thepeakat401.1eVmaybeascribedtothe NH2 or NHgroupsoriginatingfromtheincompletecondensationof cyanamide[37].TheXRD spectraoftheas-preparedSi NWs/g- C3N4,pureg-C3N4andSiNWsamplesareshowninFig.2(d).As forthepureg-C3N4sample,thetwonoteworthydiffractionpeaks at27.5◦and13.1◦correspondtothe(002)and(100)crystalplanes [38].AllthepeaksoftheSiNWsshowcompliancewiththerutile phase(JCPDS35-1158),andthefactthatnootherimpurityphaseis detectedindicatesthegoodcrystallinityoftheSiNWs.Noobvious diffractionpeaksforg-C3N4canbeobservedintheSiNWs/g-C3N4 core/shellarrays.ComparingtheXRDpattern ofSiNWs/g-C3N4 andtheSiNWs,itcanbeconcludedthatthediffractionpeaksat 38.5◦ and 44.7◦ oftheSiNWs/g-C3N4 samplecanbeattributed tothe(111)and(200)crystalplanesofSi.Thisresultmightbe ascribedtothefactthattheamountofg-C3N4wasverysmalland welldispersedontothesurfaceoftheSiNWs[20].
3.2. Photoelectrochemicalperformance
ThephotoelectrochemicalpropertiesoftheSiNWs/g-C3N4,Si NWs, and FTO/g-C3N4 samples are evaluated via linear sweep voltammetry experiments. The corresponding current density- potentialrelationstestedinthedarkandunderilluminationare showninFig.3(a).Underirradiation,allthepreparedphotoelec- trodesexhibitananodecurrent.TheSiNWs/g-C3N4photoanode showsahigherphotocurrentofapproximately0.316mA/cm2 at 1.0V vs. Pt compared to that of Si nanowire and FTO/g-C3N4
photoanodes,whichcanbeattributedtotheformationofahetero- junctionoftheSinanowireandg-C3N4,indicatingthatN-dopingof theWO3coatingisanefficientstrategytoenhancethePECperfor- manceofSi-basedphotoanodes.Toinvestigatetheeffectofg-C3N4 thicknessonthephotocurrentdensity,cyanamidesolutionswith differentmassconcentrationswereusedtowetthesampleofSi
NWs.Thelinearsweepvoltammetry(J–V)curves oftheresult- ingsampleswithvaryingg-C3N4 shellthicknessesareshownin Fig.3(b). Whenthemassconcentrationof cyanamideincreases from0.125to0.5,thecurrentdensityincreases.Theoptimalmass concentrationis0.25,correspondingtoa15nmg-C3N4shellthick- ness.Whenthemassconcentrationofcyanamideislargerthan 0.25,thephotocurrentoftheSiNWs/g-C3N4photoanodedecreases drasticallybecausethethickerg-C3N4shellmaydecreasethelight absorptionof the Si core.The linear sweepvoltammetry (LSV) measurementoftheFTO/g-C3N4sampleswasconductedtofur- therdemonstratethepositiveeffectsofsiliconnanowires.When g-C3N4 wasdepositedonFTO,anextremelyweakphotocurrent wasobtained(asshownintheinsetofFig.3(d)),indicatingthatthe bulkg-C3N4 hasaverylowPECperformance.Wheng-C3N4 was depositedonthesiliconnanowiresstructure,thephotocurrentwas significantlyraisedbecauseoftheincreasingoflightabsorption andcontactarea.Fig.3(c)showstheLSVcurvewithchoppedAM 1.5lightof100mW/cm2intensity.Amuchhighertransientpho- tocurrentisobservedcomparedtothesteady-statephotocurrent.
Thisphenomenonmaysimplybeascribedtocarrierrecombination atthesurfaceofg-C3N4.Wealsoinvestigatedtheamperometric I–TcurveofSiNWs/g-C3N4,SiNWsandFTO/g-C3N4at0Vvs.Pt underAM1.5light,asshowninFig.3(d).TheSiNWs/g-C3N4sample achievesastablephotocurrentdensityof45A/cm2,whichis2.5 timesthatoftheSiNWsampleand562.5timesthatofFTO/g-C3N4. Thispromotionofphotocurrentdensityindicatesthatconstructing aSiNWs/g-C3N4core/shellheterojunctionisaneffectiverouteto addresstheintrinsicdrawbacksofg-C3N4-basedphotoanodesfor PECapplications.
3.3. Opticalabsorptionandchargetransferproperties
TheopticalabsorptionoftheSiNWs,FTO/g-C3N4andSiNWs/g- C3N4sampleswereinvestigatedbyUV–visspectrophotometry,and
Fig.3.(a-c)LinearsweepJ–Vmeasurementsunder100mWcm2ofsimulatedsunlightillumination:(a)FTO/g-C3N4,SiNWsandSiNWs/g-C3N4samples;(b)photocur- rentdensityoverg-C3N4-modifiedSiNWswithdifferentcyanamideconcentrations;(c)SiNWs/g-C3N4arrayelectrodesilluminatedwithchoppedwhitelight;and(d) amperometricI–tcurvesofFTO/g-C3N4,SiNWsandSiNWs/g-C3N4samplesat0Vvs.Ptwith25slighton/offcycles.
Fig.4. UV–visabsorptionspectraoftheFTO/g-C3N4,SiNWsandSiNWs/g-C3N4
samples.
theresultsareshowninFig.4.BothSiNWs/g-C3N4andSiNWsam- plesshowstronglightabsorptioninthespectralregionfrom350to 900nm,whiletheFTO/g-C3N4justshowsabsorptionintherangeof wavelengthsshorterthan450nm,indicatingthattheformationof SiNWs/g-C3N4isanefficientroutetoenhancethelightabsorbing abilityofg-C3N4PECelectrodes.
ThechargetransferpropertiesofSi NWs,FTO/g-C3N4 andSi NWs/g-C3N4samplesweredetectedbyelectrochemicalimpedance spectroscopy(EIS),andtheresultsareshowninFig.5.Asmallerarc wasfoundintheNyquistcurveoftheSiNWs/g-C3N4photoanode comparedtothearcsobservedforbareSiNWsandFTO/g-C3N4, indicatingthattheresistancebetweentheSiNWs/g-C3N4photoan- odeinterfaceandelectrolyteismuchsmallerthanthatoftheSi NWsandFTO/g-C3N4.TheformationtheSiNWs/g-C3N4core/shell nanostructure can significantly improve the rate of charge
transferbecauseofthelowerchargetransferresistancebetween theSiNWs/g-C3N4 photoanodeinterfaceand electrolyte.Under lightirradiation,photo-generatedelectronswillflowintothecon- ductionbandandreactattheelectrode/electrolyteinterface.The lifetimeofelectronsforrecombinationwithatimeconstant(n)is correlatedwithcharacteristicmaximumfrequency(notedasfmax), whichcouldbeacquiredfromtheBodephaseplots[39].Fig.5(b) shows theBode phase plots of theSi NWs/g-C3N4,Si NW and FTO/g-C3N4photoanodes.Thereisnodifferencebetweenthetwo photoanodesintermsofcharacteristicmaximumfrequencypeaks.
SinceSiNWs/g-C3N4andSiNWshavethesamecharacteristicmax- imumfrequencypeaks,theyexhibitsimilarcarrierrecombination kinetics. However,arelative highfmaxmeasuredinFTO/g-C3N4
electrodeunderthesameconditionsasSiNWs/g-C3N4 indicates therelativelyshortnofelectronsgeneratedinFTO/g-C3N4.Thus, wecandeclarethatthecombinationofSiNWsandg-C3N4 pro- motescarrierseparation.
3.4. Mechanismanalysis
The curvesbased ontheKubelka–Munk functionversus the energyoflightareshowninFig.6(a).Theestimatedbandgapof g-C3N4isapproximately2.79eV.TheVBXPSspectraofthepris- tineg-C3N4areshowninFig.6(b).TheVBofg-C3N4isobservedat
∼1.88eV,whichisconsistentwiththepreviousresult[40].TheVB edgeoftheSiNWsis∼0.73eV.
ECB= EVB−Eg (1)
TheCBofg-C3N4 andSiNWscanbecalculatedbytheabove equation,andtheCBenergiesofg-C3N4 andSiNWsweredeter- minedtobe−0.91and−0.39eV,respectively.Onthebasisofthe resultsofallthemeasurementsoftheas-preparedsamples,apossi- blemechanismfortheperformanceimprovementofphotocatalytic activityoftheSiNWs/g-C3N4core/shellstructureisproposed,as
614 Z.Chenetal./AppliedSurfaceScience396(2017)609–615
Fig.5.EISmeasurementsofFTO/g-C3N4,SiNWsandSiNWs/g-C3N4photoanodesunderillumination:(a)Nyquistand(b)Bodephaseplots.
Fig.6.(a)UV–visdiffusivereflectancecurvesofthepristineg-C3N4;(b)XPSVBspectraofthepristineg-C3N4andSiNWs.
Scheme2.TheproposedmechanismforthechargeseparationandtransferattheSiNWs/g-C3N4heterojunction.
showninScheme2.TheCBpotentials oftheSiNWsarelower thanthoseofg-C3N4,sothephoto-generatedelectronsintheCB ofg-C3N4couldtransfertotheCBoftheSiNWs.Forthejunction betweenn-Siandn-g-C3N4,thedepletionregionmostlyliesinn- g-C3N4becauseofthehigherconcentrationofelectronofthen-Si NWs(itslowresistivityisapproximately2–10cm).Thedirec- tionofthebuilt-inelectricfieldbetweenthetwosemiconductorsis fromthen-Sitotheg-C3N4.Underillumination,theelectron–hole pairsgeneratedinthedepletionregion(g-C3N4)increasethenum- berofelectronsintheconductionband(CB)ofg-C3N4aswellas
theSiNWs.Then,theelectronsintheCBofg-C3N4willdrifttothe n-SiNWs,whichisbeneficialforcarrierseparationonthesurface ofg-C3N4.ThesurvivingabundantholesintheVBofg-C3N4can oxidizewaterandproduceO2.
4. Conclusion
In conclusion, metal-free Si NWs/g-C3N4 heterojunction core/shellarrayphotoelectrodeswithdifferentg-C3N4masscon- tentshavebeenfabricated.Thesephotoelectrodesnotonlyexpand
theopticalabsorptionrangeoftheg-C3N4-basedphotoelectrodes butalsosignificantlyimprovethePECperformanceofbulkphase g-C3N4.TheSiNWarrayshavebeenproventofurthertoaccelerate themigrationofchargecarriersandimprovetheeffectivesepa- rationofthechargesgenerateding-C3N4.Conversely,thestrong opticalabsorptionperformanceoftheSiNWsalsocontributesa criticalroleintheimprovementofPECperformance.Meanwhile, theexaminedphotocatalystforneutralwatersplittingprovidesa promisingprospectforoverallsplittingofwaterusingmetal-free materials.Alltheconclusionsdiscussedabovewillprovideconve- nientguidanceforresearchersstudyingthephotoelectrochemical process.
Acknowledgments
The authors acknowledge the financial support from the National Natural Science Foundation of China (Grant Nos. 21406052, 51602111), Guangdong Province Grant Nos. 2014A030308013, 2014B090915005, 2015A030310196, 2015B050501010,14KJ13,andthePearlRiverS&TNovaProgram ofGuangzhou(201506040045),GuangdongInnovativeResearch TeamProgram(No.2011D039),PCSIRTProjectNo.IRT13064.
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