Wear and mechanical properties of Al6061/SiC/B4C hybrid composites produced with powder metallurgy

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w w w . j m r t . c o m . b r

Availableonlineatwww.sciencedirect.com

Original

Article

Highly

efﬁcient

photocatalytic

performance

of

Cu

2

O@TiO

2

nanocomposite:

inﬂuence

of

various

inorganic

oxidants

and

inorganic

anions

Wagih

A. Sadik

a

_,

_{Abdel-Ghaffar}

_M.

_El-Demerdash

a

_,

_Adel

_W.

_Nashed

a

_,

Amr

A. Mostafa

a

_,

_Hesham

_A.

_Hamad

b,∗

a_Materials_Science_Department,_Institute_of_Graduate_Studies_and_Research_(IGSR),_Alexandria_University,_Alexandria,_Egypt

b_Fabrication_Technology_Department,_Advanced_Technology_and_New_Materials_Research_Institute_(ATNMRI),_City_of_Scientiﬁc_Research

andTechnologicalApplications(SRTA-City),NewBorgEl-ArabCity,21934,Alexandria,Egypt

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received15August2019 Accepted3September2019 Availableonline26September2019

Keywords:

Photocatalyticoxidation Cu2O@TiO2nanocomposite

Inorganicoxidants AR8dyedegradation Electricalenergyperorder.

a

b

s

t

r

a

c

t

ThisworkoffersthefacilesynthesisofCu2O@TiO2nanocompositesbysolidstatereaction

towardstheeliminationofAcid-Red8(AR8)dyeasatargetpollutantmodelthroughthe photocatalyticoxidationsystem.Anothergoalistoassessthecapacityofvariousinorganic oxidantsthatactasalternativeelectronacceptorforCu2O@TiO2mediatedphotocatalytic

oxidation.Thepreparednanocompositewasanalyzedbyvariousanalysisinstruments,and theirphotocatalyticperformancewassystemicallyassessedwithrespecttotheremovalof AR8dye.Uponirradiation,comparedtopureCu2OandpureTiO2alone,theCu2O@TiO2has

exhibitedahigherphotocatalyticperformancewhichshowthatthecombiningTiO2(n-type)

withCu2O(p-type)hasbeenimprovedtheelectronmobilityandsubsequentlydecreasing

therateofelectron-holerecombination.Akineticstudyisconﬁrmedthatthedegradation ofAR8hasbeenobeyedthepseudo-ﬁrst-ordermodel.Theperfectionofthephotocatalytic activityisachievedbyusingvariousinorganicoxidantssuchas;H2O2,Na2S2O8andNaIO4

soastoproduceanelectronscavenger.Also,theapparentrateconstant(kapp)and

appar-entquantumyield(Qapp)arehigherforalloxidantsthanwithoutoxidants,whileislower

intermsofelectricalenergyperorder(EEO)andhalf-lifetime(t0.5).Periodateionhas

con-sideredthemostefﬁcientoxidantwhencomparedwithotheroxidantsforenhancingthe photocatalyticactivityviatheformationofvariousreactiveoxygenspecies.Thedegradation efﬁciencyofthesecatalyticsystemscouldbearrangedinanascendingorder:Cu2O@TiO2/

NaIO4>Cu2O@TiO2/Na2S2O8>Cu2O@TiO2/H2O2>Cu2O>TiO2.

∗ _{Corresponding}_author.

E-mail:heshamaterials@hotmail.com(H.A.Hamad).

1. Introduction

Recently, the shortage of water is becoming more severe problemduetoanenvironmentalpollution,rapidindustrial growth,uncontrolledgroundwaterprogressandreducingthe water resources. Hence, the ﬁxing of these problems and

https://doi.org/10.1016/j.jmrt.2019.09.007

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theprogressivegrowthofthewaterpollution.Amongthem, heterogeneousphotocatalysisisesteemedamotivating tech-niquewithtoday’schallengingdemandsespeciallyforclean watertechnology[11,12].

Nanostructured TiO2 is considered as a well-known

photocatalystforthehighlyefﬁcientperformancefor decom-positionoftextilepollutantsinwater[13].Thedestructionof organicpollutantsbytitaniainthepresenceofsuitablelight isasaresultofproductionofhydroxylradical,verystrong oxidant,thatarethekeyofusingthephotocatalystfor oxi-dationoforganic molecules.Also,there weremany efforts forimprovingthephotocatalyticactivitybycombinationwith otheroxidants[14].Greatchallengeshavebeenmadeinorder toimprovetheperformanceoftitania,mainlyjointwithother semiconductor,regardlesstheexcellentprospectofefﬁcient photocatalyticmineralizationoftoxicdyesinwater[15].

Sensitizationofn-typesuchas;TiO2withp-typesuchas;

Cu2OisaneffectivepathforfabricationTiO2based

photocata-lystwithextendedabsorptionandsubsequentlyimprovedthe photocatalyticactivity.ThisisduetothelowbandgapofCu2O

(E0=1.9–2.2eV)andtheformationofheterostructurebetween

TiO2andCu2Othathelpstheseparationandtransportation

ofthephoto inducedchargecarriersinCu2O@TiO2

hetero-junctionnanocomposite,andhencedeclinestheopportunity ofrecombinationandimprovesthephotocatalyticproperties

[16].Also,thesynergisticeffectbetweenTiO2andCu2Oisvery

importantforimprovingthechargeseparationandleadsto muchhigherphotocatalyticactivitythanpureTiO2andCu2O

[17,18].

Manyresearchgroupsconcerned withthefabricationof Cu2O@TiO2inordertomakeaprogressinthedestructionof

organiccontaminantsfromwater.Thephotocatalyticactivity ofTiO2/zeolite(TZ)wasimprovedthephotocatalytic

degrada-tionofbisphenolA(BPA)byaddingCu2O[19].Mixedoxidesof

Cu2O-TiO2wassynthesizedonacoppersubstratefor

reduc-tionof2,2,4,4-tetrabromodiphenylether(BDE47) [20]. Also, themorphologyissigniﬁcantfactorforenhancingthe pho-tocatalytic activity. The Cu2O/TiO2 heterostructure hollow

spheres(Cu2O/TiO2HS)forremovalofRhodamineBasaresult

ofdesigntheheterojunctionbetweenTiO2andCu2O[21].The

degradationofreactiveblue49(RB49)dyeisobservedbyusing 4wt.%Cu2O-CuO/TiO2catalyst[22].

Todate,thereisnostudies concerningthevarious inor-ganic oxidants using Cu2O@TiO2 as a photocatalyst for

destructionoftoxicdyesinanaqueoussolution.Herein,itisin

Germany. CuCl2.2H2O, NaOH and glucose were purchased

fromAldrichchemicals,Germany.AcidRed8(AR8)(anionic dye) with molecular formula C18H14N2Na2O7S2, molecular

weight of 480.42g/mol, and ␭ max 508nm was purchased

from Fluka, Germany. All oxidants such as sodium perio-date(NaIO4),hydrogenperoxide(H2O2,30%w/v),andsodium

persulphate (Na2S2O8)wereobtainedfromFisherScientiﬁc,

Germany.

2.2. SynthesisofCu2OandCu2O@TiO2

NanoCu2Owasfabricatedbyco-precipitationmethod.Cu2O

@TiO2nanocompositewaspreparedbytwosteps;oneby

syn-thesisofCu2Onanoparticles,followedbyaddingit(10wt.%of

Cu2O)toDegussaP-25(90wt.%ofTiO2)forsynthesisofCu2O

@TiO2bysolidstatereaction.Thepowdershavebeenmixed

uniformlyandformedﬁnepowderbygrinding2h.The resul-tantpowdershavebeencalcinedat500◦Cfor3hinamufﬂe furnace.Theoverallprocedureforsynthesisofnanocomposite ispresentedinFig.1.

2.3. Physicochemicalcharacterization

The physicochemical properties of the synthesized Cu2O

@TiO2 nanocomposite was observed by scanning electron

microscope (SEM,JEOL,ModelJSM-6360 LA,Japan)[Priorto the investigation,the sampleswere coatedwithgoldusing sputteringcoater(model:S150B,EdwardsHighVacuumLtd., England)], and joint it with EnergyDispersive X-ray (EDX), X–raydiffraction(XRD,Bruker,D8ADVACE,Germany)[Cu(K␣) radiations of wavelength (␭=1.5406A◦) generated at 40Kv, 40mA], and Fourier Transform Infrared spectrophotometer (FTIR,Shimadzu-8400,Japan)[IRspectraweretakenfromthe testsamples,pressedintoKBr-supporteddiscsandscanned inthewavelengthrangefrom400to4000cm−1.

2.4. Evaluationofthephotocatalyticactivity

ThephotocatalyticperformanceofthepreparedCu2O@TiO2

nanocompositewasusedtoevaluatetheremovalefﬁciency ofAR8 dyeinan aqueoussolution usingbatchslurry pho-toreactor.Thecontents(TiO2/dye,Cu2O/dye,Cu2O@TiO2/dye

andCu2O@TiO2/dye/oxidant)oftheglasscontainerwere

agi-tated bya magneticstirrer and kept purged with air(rate 3000mlmin−1).Thedyesolutionwasagitatedwith

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photocat-Fig.1–SchematicdiagramforsynthesisofCu2O@TiO2nanocomposites.

alystforadsorptionat30minbeforeirradiationwithUVto obtainequilibriumadsorption.

Irradiation was done with a tubular low-pressure mer-curylampwithpower43Wandwavelength254nm(Voltarc Tubes Inc.,USA). The total intensity reaching the solution wasestimated4mWcm−2.ThecolordisappearingofAR8dye wasanalyzedspectrophotometricallyatitsmaximum absorp-tionwavelengthof508nm,usingShimadzu model1601PC doublebeamspectrophotometer,Japan.Samplescontaining Cu2O@TiO2weretakenperiodicallyfromthephotoreactorand

measuredafterﬁltrationusing0.2␮mpolyethersulfone mem-brane. The efﬁciency of the photocatalytic activity can be estimatedinEq.(1).

%Degradation=[Co–C]/Co×100%=[Ao–A]/Ao×100% (1) Where Co=initialconcentration of dyesolution (mg/L), C=concentrationofdyesolutionafterphotoirradiation(mg/L) attimet(minute).Aoisthevalueofabsorbanceofdye aque-oussolutionafteradsorptioninthedark,andAisthevalueof absorbanceofdyeaqueoussolutionafterreaction.

3. Results

and

discussion

3.1. FormationpathwayofCu2O@TiO2nanocomposite

In this work, Cu2O@TiO2 nanocomposite was prepared by

two steps. Cu2O nanoparticles were ﬁrstly prepared using

co-precipitationmethod,andthenCu2Onanoparticleswere

depositedontheinsideandoutsidesurfaceofTiO2by

solid-statereaction,yieldingtheCu2O@TiO2nanocomposite(Fig.1).

Firstly,Cu2+ _ions_and _solvent_were_ﬁrstly_mixed_to_form

bluesolution.Then,theadditionofNaOHandglucosethat supportsthereductionofCu2+_to_Cu+ _ions._The_addition_of

glucoseduringthesynthesisofCu2Oassociatestoreducethe

rateofagglomerationandalsoimprovethehomogeneityand thedistributionofthisparticleswithsmallgrainsize[23].After increasethetemperaturefromroomtemperatureto70◦C,a largenumberofCu2Owereformedatshorttime.Secondly,

Cu2OwasaddedtoDegussaP-25,theionsofCu+maybe

dif-fusedandadsorbedonthesurfaceofTiO2aswellasabsorbed

intotheinterstitialspaceinTiO2athighcalcination

tempera-ture,yieldingCu2O@TiO2nanocompositebecauseTiO2isused

asasubstratewithassociatedoflowamountofCu2O(10wt.%)

[19,24].

3.2. CharacterizationofCu2O@TiO2nanocomposite

Fig.2(a)showstheSEMofthepreparedCu2O@TiO2

nanocom-positewithhighhomogeneityanduniformityofaggregated semi-sphericalshape.Theaveragediameterofgrainsizeof 56nm.TheEDXofthepreparedCu2O@TiO2nanocomposite

statedthattheexistenceofconsiderableamountsofCu,Ti, andOwhichconﬁrmedthatthesuccessfulfabricationofhigh purityofCu2O@TiO2nanocompositewerenoticedbythe

exis-tenceofthedistinctiveenergypeaksforTiandCuwithoutany impurities(Fig.2b).ThespectrumshowsthestrongTisignal atabout4.5eVandaweaksignalatabout0.45eVwithatomic percentageof39.25%.Also,aweaksignalatabout0.45eVfor oxygenwasdetectedwithatomicpercentageof48.52%.The twoweaksignalsatabout0.94and 8.04eVforcopperwere detectedwithatomicpercentageof12.23%.

Fig. 2 (c) shows the XRD pattern of the fabricated Cu2O@TiO2 nanocomposite which displays the diffraction

peaksat2␪=25.23,27.31,35.42,36.02,37.82,38.66,47.97,48.73, 54.04,55.05and62.61,whichmatchtoanataseandrutile tita-nia(JCPDScardNo021-1272and021–1276),respectivelyand also purecubic Cu2O (JCPDS99-0041)[25,26]. Comparedto

DegussaP-25TiO2,Cu2O@TiO2nanocompositehasthree

addi-tionalXRDpeaksthatlocatedat36.02,62.61,and73.54which ﬁtstothe(111),(220)and(311)andascribedtocupritephaseas wellasstatedthatCu2Oarereallyfoundontothesurfaceof

theP25TiO2[16].ItisindicatedthatthecombinationofCu2O

inthecrystalstructureofTiO2 whichhasnotanyeffecton

thestructureofDegussaP-25andthisstructureispreserved. Theseﬁndingsaredemonstratedthatatwo-phase composi-tionofTiO2andCu2OexistintheCu2O@TiO2heterostructure.

TheFTIRspectrumofCu2O@TiO2nanocomposite(Fig.2d)

showsabandat3471cm−1,whichisdistinctiveofthe non-hydrogen bonded surface hydroxyl groups. Also, the peak observedat1638cm1_is_ascribed_to_the_bending_mode_of_the

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Fig.2–(a)SEM,(b)EDX,(c)XRD,and(d)FT-IRofCu2O@TiO2nanocomposite.[A:anatase,R:rutile,andC:cuprite].

inorganicCu Ohasbeendetectedat378cm−1whereasthe representativebandoftheinorganicTi Ohasbeenobserved at587cm−1[16].

3.3. Photocatalysisofacidred8dye 3.3.1. Degradationkinetics

Thedecomposition kineticsofAR8 dyebyTiO2, Cu2O,and

its composite Cu2O@TiO2 were assessed by following the

Langmuir-Hinshelwoodmodel[11].Whenthechemical con-centrationCoismillimolarsolutiontheintegratedformofEq.

(2)beanapparentﬁrstorderequation.

ln(Co/C)=kappt (2)

Where,kapprepresentstheapparentﬁrstorderrateconstant,

CoandCareconcentrationbeforeandphotocatalyticreaction,

respectively.Thehalf-lifetime(t0.5)oftheﬁrstorderreaction isthetimerequiredforthereactantstobedegradedtothehalf oftheirCo.Therelationshipbetweent0.5andkappisgivenby

Eq.(3).

t0.5=0.693/kapp (3)

3.3.2. ComparisonbetweenTiO2,Cu2Oanditscomposite Cu2O@TiO2

The decomposition ofAR8 dye asan organic contaminant modelhasbeenachievedbyphotocatalyticperformanceusing TiO2P-25,Cu2OanditsurfacemodiﬁedCu2O@TiO2

nanocom-positeasphotocatalystsatroomtemperature.Thedestruction ofAR8dyewasassessedbyreducingtheCoat␭max508nmdue

totheazobond(N N).TheAR8dyewascompletelydegraded ataround90minwithobeyedthepseudo-ﬁrst-orderkinetic models (Figure not shown). In Table 1, the decomposition rate followsthe orderCu2O@TiO2>Cu2O>TiO2.Therateof

removalbypureCu2OwashigherthanthatofpureTiO2which

maybeduetothelowbandgapofCu2O.Themosteffective

catalyticsystemisobtainedbyusingcombined Cu2O@TiO2

systemwhichcanbeascribedtothemoreefﬁcientseparation ofphotoinducedelectron-hole(e−/h+₎_pairs,_and_also_improve

thesurface-activesitesincomposite[17,20,29].

3.3.3. Inﬂuenceofvariousoxidationprocessesonthe degradationbyadditionofelectronacceptors

One of the signiﬁcant drawbacks of the photocatalysis is the electron-hole recombination which signiﬁes the major energy-wasting step and subsequently leads to the reduc-tionofthequantumyield.So,thedeferringofelectron-hole recombinationbyadditionofirreversibleelectronacceptorsin

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Table1–Collectivedataofapparentrateconstants,half-lifetimes,andreactionordersfordegradationofAR8dye[conc. ofcatalyst0.1g/l].

Catalyticsystem Concentrationof oxidant(M)

kapp(min−1) t0.5(min) Apparent

reactionorder(n) UV/Cu2O – 124×10−4 5.6 – UV/TiO2 84.0×10−4 8.3 UV/Cu2O@TiO2 150×10−4 4.6 UV/Cu2O@TiO2/H2O2 2×10−3 182×10−4 3.8 0.4 4×10−3 ₂₇₈_×₁₀−4 _2.5 12×10−3 ₃₇₅_×₁₀−4 _1.8 14×10−2 531×10−4 1.3 28×10−2 1257×10−4 0.6 45×10−2 1834×10−4 0.4 UV/Cu2O@TiO2/S2O82− 1×10−3 302×10−4 2.3 1.2 2×10−3 455×10−4 1.5 4×10−3 983×10−4 0.7 UO 8×10−3 3651×10−4 0.2 1.2

UV/Cu2O@TiO2/IO4− 4.0×10−5 1297×10−4 0.50 0.2

7.4×10−4 2001×10−4 0.30 1.0×10−3 2846×10−4 0.20 21×10−3 3664×10−4 0.18 48×10−3 4881×10−4 0.14 11×10−2 7229×10−4 0.10

ordertoincreasetherateofphotocatalysis,especiallyinhigh concentrationoftoxicdyes.Theobserveddrastically accel-eratedthe decompositionwere attributedtotheimproving theelectronscavengingfromtheaddedinorganicoxidants. Themeritsofadditionofelectronacceptorsistoavoidthe recombinationvia (i) increasethe number oftrapped elec-trons; (ii) generation of more radicals and other oxidizing species;(iii)increasingtheoxidationrateofthe intermedi-atecompoundsand(iv)avoidingtheproblemsoflowoxygen concentration[28–30].Hence,variousconcentrationsof oxi-dants(H2O2,Na2S2O8andNaIO4)wereaddedtotheAR8dye

solutioninthepresenceofthecombinedCu2O@TiO2system

thatillustrated inFig. 3.Theintroduction ofinorganic oxi-dantsisutilizedforenhancingtherateofdegradationofAR8 dyethroughbetterscavengetheejectedelectronsofTiO2and

hencethesurvivaltimeh+ _at_Cu

2Owillbehigherand

sub-sequentlyreacts effectivelywithAR8dye.Theeffectofthe concentrationofvariousoxidantsisdescribedwellasfollow;

(a)H2O2

TheUV/Cu2O@TiO2/H2O2systemisusedtotrapthe

elec-tronsatconductionbandandformationofmore䊉OHradicals from generation for superoxide anion radicals (䊉O2−) and

homolyticcleavageofbondsoftwohydroxylgroupsduring the photolysis,which enhancing therate ofphotocatalytic reaction.H2O2isusedasanelectronacceptorthanoxygenin

othermechanismsandsubsequentlytherateof recombina-tionwilldecrease[11].Also,H2O2ismoreelectropositivethan

O2 thatresultfromtwohydrogen atomsbondedtooxygen

atoms(H O O H).

Todeepthe illustrationofintotheeffectivereactivityof H2O2/Cu2O@TiO2 systemonthedecompositionofAR8dye

was studiedatvarious concentration ofH2O2 inFig. 3 (a).

Ithasbeenstatedthattheenhancedphoto-destructionrate withincreasingthedoseofH2O2.Also,thekappwasincreased

from182×10−4to1834×10−4min−1whentheconcentration ofH2O2wasrangedfrom2×10−3to45×10−2M(Table1).The

effectofratedeterminingspeciesisexpressedbyapowerlaw relation

kapp=K[H2O2]n (4)

WherekappandKaretheapparentandthetruerate

con-stants,respectively.Thenistheorderofthephotocatalytic reaction.FromFig.3(b),thevalueofnwas0.4(Table1).The rateofAR8dyedegradationusingUV/Cu2O@TiO2/H2O2

sys-temisbetterwhencomparedtoUV/Cu2O@TiO2alone.This

maybeascribedto(i)trapofthephotogeneratedconduction bandelectronsofCu2O@TiO2byH2O2,whichismoreefﬁcient

thantrappingbyO2,accordingtothefollowingreaction[31].

H2O2+e−(cb)→−OH+䊉OH (5)

(ii) generate other oxidizing species (䊉OH), which can contributetotheoxidativedecayprocess;and(iii)after illu-mination,substantialphotolysisoftheH2O2wouldproduce

morehydroxylradicals(Eq.6)[32].

H2O2+h␯→2䊉OH (6)

Insummary,theincreasingtheH2O2concentrationsleads

to an increase in both electron scavenging action, and hydroxylradicals,whichoxidizethedyeandleadtoahigher rateofdecomposition.

(b)PeroxydisulphateionsS2O82−

Fig.3(c)displaysthe inﬂuenceofaddingS2O82− to

com-bined Cu2O@TiO2 system onthe destruction ofAR8dye.It

wasfoundthattherateofremovalwasenhancedwiththe addition of a low concentration of the S2O82− (1×10−3M)

to UV/Cu2O@TiO2 system. Accordingto aﬁrst-order model

for dye destruction, it has been found that the enhanced photodegradationrateisresultfromtheincreasingthe con-centration of S2O82−. Also, the kapp was increased from

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Fig.3–(a,c,e)Changeofln(C/Co)withtimeatdifferentconcentrationsofH2O2,S2O82−,andIO4−,respectively,and(b,d,f)

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Table2–Collectivedataofapparentrateconstants,electricalenergyperorderandapparentquantumyieldfor degradationofacidred8dye[conc.ofcatalyst0.1g/l].

Catalyticsystem Concentrationof oxidant(M)

kapp(min−1) EEO(kWh/m3) Qapp%

(mol/Einstein) UV/Cu2O – 124×10−4 526.3 1.07 UV/TiO2 84.0×10−4 769.2 0.73 UV/Cu2O@TiO2 150×10−4 333.3 1.3 UV/Cu2O@TiO2/H2O2 2×10−3 182×10−4 357.1 1.58 4×10−3 ₂₇₈_×₁₀−4 _238.1 _2.41 12×10−3 ₃₇₅_×₁₀−4 _175.4 _3.25 14×10−2 531×10−4 125 4.60 28×10−2 1257×10−4 52.6 11.0 45×10−2 1834×10−4 36 16.0 UV/Cu2O@TiO2/S2O82− 1×10−3 302×10−4 217.4 2.62 2×10−3 455×10−4 145 4.0 4×10−3 983×10−4 67.1 8.52 UV/CuO@TiO/SO 8×10−3 3651×10−4 18.1 31.6 UV/Cu2O@TiO2/IO4− 4.0×10−6 1297×10−4 51 11.2

7.4×10−4 2001×10−4 33 17.3 1.0×10−3 2846×10−4 23.2 24.7 21×10−3 3664×10−4 18.1 31.8 48×10−3 4881×10−4 13.5 42.3 11×10−2 7229×10−4 9.1 62.7

S2O82−wasrangedfrom1×10−3to8×10−3M(Table1).The

completedecompositionofthedyeinashorttime(20min)at conc.8×10−3MofS2O82−.Therateorder ofphotocatalytic

reaction withrespect toS2O82−, which was obtainedfrom

Fig.3(d),wasfoundtobe1.2(Table1).

TheadditionofS2O82−ionscouldhaveincreasedthe

pho-tocatalyticperformanceduetotheinteractionwithlightand its interaction with Cu2O@TiO2. The photolysis of

persul-fateleadstotheformationofsulphateradicalswhoseredox potential 2.5–3.1V/NHE is higher than that of S2O82− ions

(1.96V/NHE) [32]. TheS2O82− anionscan traptheelectrons

thatformedfromtheconduction bandofCu2O@TiO2 more

thantheelectronsfromO2,andthus formedotheroxidant

SO4䊉−(Eq.7)[33]

S2O82−+e−(cb)→SO42−+SO4䊉− (7)

Uponirradiation,theSO4•−isalsogeneratedandcan

con-tributeinthephotocatalyticreactionwithwaterforformation of•OH,accordingtoEqs.(8and9).

S2O82−+h␯→2SO4䊉− (8)

SO4䊉−+H2O→䊉OH+SO42−+H+ (9)

So, the increased concentrations of the S2O82− have

increasedthetrappingoftheelectronsfromtheconduction bandsofCu2O@TiO2andtheproductionofSO4䊉− and䊉OH.

Thesetwofactorsareresponsibleforhigherrateof photode-compositionofAR8dye.

(c)PeriodateionIO4−

Oxyhalogenshaveatomofoxygen and anotheratomof halogen as a central atom. The difference in polarization between oxygen and halogen leads to the capturing the ejectedelectron.Theeffectofoxyhalogensystemisextremely based on the differences in the electronegativity and the atomicradiusofthehalogen[34].Theelectropositive

halo-genoxidantisusedasastrongerelectronscavengerviathe generationofhVB+andformationof䊉OH.

Theincreasingoftheatomicradiusleadstothedecreasing oftheelectronegativitythatresultfromcapturingtheejected electronbyhalogen.Hence,theatomicradiusofIhashigher thanthatofClorBrandithasmorebondedwithoxygenthan ClorBr,and theabilityofelectronsoverphotocatalystwill behigher.So,theUV/Cu2O@TiO2/IO4䊉isthebestsystemfor

decompositionofAR8dyebytheoxyhalogenoxidant.

Fig. 3(e) showstheinﬂuenceofaddition ofvarious con-centration of periodate ions to UV/Cu2O@TiO2 system on

the destruction of AR8 dye.The addition ofvery low con-centrationsofIO4−(4.0×10−5M)toCu2O@TiO2hasresulted

in a higher decomposition rate than UV/Cu2O@TiO2 only.

Also,whenincreasingtheconcentrationoftheperiodateto (11×10−2M)hasresultedindyedestructionaftertheshortest time(18min).AccordingtotheIO4−,Thereactionrateorder

is0.2(Fig.3(f)andTable1).Thescavengingoftheelectronsby Cu2O@TiO2/IO4−systemleadstotheimprovementofthe

pho-tocatalyticdecompositionofAR8dye,whichismoreefﬁcient thantrappingwithO2orS2O82−asfollows[35];

IO4−+8e−(cb)+8H+→4H2O+I− (10)

Also,uponirradiation,thephotolyticdecompositionofthe IO4−,involvestheformationofanumberofhighlyreactive

radical-andnon-radicalintermediates(IO3䊉,䊉OH,O䊉− and

IO4䊉)(Eqs.11–13)asfollows[35].

IO4−+h␯→IO3䊉+O•− (11)

O䊉−+H+↔䊉OH (12)

䊉_OH ₊_IO₄−_→_OH−_+IO₄䊉 ₍₁₃₎

Theseintermediatesareresponsiblefortheenhancingthe decompositionoforganiccompounds[32].Increasingthe

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con-teredandnotabsorbedbythedyesolution.Theredoesnot usuallyexistanypossibilitytodetermineexperimentallythe amount oflightabsorbed bythe photocatalyst. Inorder to bypassthe difﬁcultyofdeterminingquantum yieldsin het-erogeneousphotocatalysis,anotherparameteroftenreported istheapparentquantumyield(Qapp)whichdeﬁnedasshown

inEq.(14)[36,37]

Apparentquantumyield(Qapp,mol/Einstein)= Rateof

disappearanceofreactantmolecules/Rateoﬁncident photonsinsidereactorcell=kappCo/I (14)

Wherekappistheapparentﬁrst-orderrateconstant,Cois

theinitialdyeconcentrationandIisthetotalintensityof inci-dentphotonsenteringthereactorcell.Itwasobservedfrom

Table2thatQappforUV/Cu2O@TiO2/IO4-systemishigherthan

those for UV/Cu2O@TiO2/H2O2 and UV/Cu2O@TiO2/S2O82−

systems. This may be due to the higher activity of perio-datewhencomparedtohydrogenperoxideandpersulfateas obtainedinthepreviousstudies[33,36].

3.3.5. Figureofmerit

Nowadays,theeconomicstudyofeachprocessisasigniﬁcant factorwhichitincludesthemajorfractionofoperatingcost. Hence,itisnecessarytostudytheelectricalenergy consump-tionoftheAOPsunderexperimentalconditions.Theelectrical energyperorder(EEO)isaninformativefactorforthe

photo-catalyticdegradationbecauseitobeystheﬁrstorderkinetic model[6].TheﬁguresofmeritEEOallowsforarapid

deter-minationoftheelectricalenergycostandtheyindicatethe totalpowerrequired.Forcomparativepurpose,thetreatment efﬁcienciesforthedifferentprocessesareevaluatedthrough the EEO values. The EEO is deﬁned as the number of kWh

ofelectricalenergyrequiredtoreducetheconcentrationof pollutantbyoneorderofmagnitude(90%)in1m3_of

contami-natedwater.Consideringﬁrst-orderdegradationkinetics,the UVdoseswerecalculatedforallAOPSusingEq.(15)[38].From theUVdoses,thesimplestformoftheestimationofEEOcan

alsobecalculatedusingEq.(16)[37].

UVDose=[Lamppower(kW)×Time(h)×1000]

/[Treatedvolume(L)] (15)

Fig.4–Changeoflog(Co/C)withUVdosefordegradation ofAR8dyeusing0.1gmofCu2O@TiO2withdifferent

concentrationsof(a)H2O2,(b)S2O82−,and(c)IO4−,

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EEO= UVDose/[Log(Co/C)] (16)

FromEq.(16),theEEOvalueswereobtainedfromtheinverse

oftheslopeofaplotoflog(Co/C)versusUVdose(Fig.4).Itwas

foundthattheﬁgure-of-meritEEOisappropriateforestimating

theelectricalenergyefﬁciency.Itisnotonlyprovingthe reduc-tionintheneededelectricitybythephotocatalyticsystem,but alsoofferingtheconsiderableinﬂuenceoftheUVdoseonthe EEOintheprocess.TheEEOvalueswerefoundtobe

depen-dentontheconcentrationofH2O2,S2O82−,andIO4-.Also,the

EEO valuesofadditionaloxidants toUV/Cu2O@TiO2 system

islowerthan thatblankUV/Cu2O@TiO2 systemwhich

indi-catethatthelowerenergyconsumptionisduetothegreater appliedpotential and generation ofhighly reactive radical speciesatthehigherpotential.TheEEOvaluesshowthatthe

additionofperiodateexhibitedalsoalowenergyconsumption comparedtoperoxideandpersulfate(Table2).Insummary, theseﬁndingscanbeappliedfordesigningphotocatalytic sys-temwithlessconsumptionofelectricalenergy,higherrate constantandloweroperationcost.

3.3.6. Comparisonbetweenvariousoxidants

This study has presented that the three oxidants, perox-ide, persulfate and periodate, have enhanced the rate of UV-induced decomposition of AR8 dye in the presence of combined Cu2O@TiO2 system. This enhancement is as a

resultofthescavengingtheactivespeciesradicals,suchas hydroxyl radicals, and holes by the inorganic ions [29]. It can be seen that the addition of very low concentrations IO4− to Cu2O@TiO2 has resulted in a higher

decompo-sition rate than UV/Cu2O@TiO2, UV/Cu2O@TiO2/H2O2 and

UV/Cu2O@TiO2/S2O82−, thus indicating the effectiveness of

IO4−overH2O2andS2O82−asaresultofhigherofkapp,shorter

t0.5_,_higher_Q

app,and lowerEEO.Thehighlyreactive radical

speciesinperiodate may directlyabstract H from AR8 dye molecules.So,theirradiatedIO4−solutioncanquicklyoxidize

theAR8molecules[39].

4. Conclusions

In summary, we revealed that the facile and proﬁcient approachfor the fabrication ofCu2O@TiO2 nanocomposite

via a solid-state approach. The formation ofthis compos-itewasrevealedbySEM-EDX,XRDandFT-IR.Thecombined Cu2O@TiO2 heterojunction system exhibited much higher

photocatalyticactivitythan thatofpristineCu2OandTiO2.

This is due to the transfer ofelectrons from Cu2O to the

conductionbandofTiO2isconcurrentlywiththe

photogen-eratedholes atthe valencebandofCu2O,whichfacilitates

theelectronandholeseparationandimprovesthe photocat-alyticactivityofCu2O@TiO2heterojunctionnanocomposite.

Itwas found that the heterogeneousphotocatalytic degra-dation, under UV illumination, using the following order: Cu2O@TiO2/NaIO4>Cu2O@TiO2/Na2S2O8>Cu2O@TiO2/H2O2

>Cu2O>TiO2.Thephotocatalyticactivityenhancementwas

attributedto the various reactive oxygen species like 䊉OH andIO4䊉.Thehigherdecolorizationandquantumefﬁciency

andlowerenergyperorderandenergyconsumptionof com-binedCu2O@TiO2systemwithoxidantsthanbareCu2O@TiO2.

ThecombinationofCu2O@TiO2nanocompositewithvarious

oxidantsmaybringanewinsightintohighlyefﬁcient photo-catalyticapplicationsinthefuture.

Conﬂicts

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interest

Theauthorsdeclarenoconﬂictsofinterest.

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