Understandingthestructuresofg-C3N4
Theterm, “graphiticcarbonnitride”,isusedtorefertoalarge groupofmaterialsthathasvariousstructuralandchemical properties.Muchoftherecentpublicationshavereferredto thesefamilyofmaterialsas “g-C3N4” compounds,however almostnoneofthematerialsinvolvedhavetheratioof3to4 forCtoN.Commonly,theyinsteadcontainconsiderable concentrationsofhydrogenandotherelements,thatinclude oxygen.Thelayersofgraphitearealsopossiblyincomplete andthesheet-likeregionsarelesslikelytobeplanar[56]. Thereareothercategoriesofgraphiticcarbonnitridematerialsthathavebeendiscoveredandthefirstgroupisrelatedto graphiticcarbon,graphene-likecarbonsorcarbonnanotubes (CNTs).Theyarethosethataresynthesizedbyinvolvingnitrogentoadegreethatreachesuptoafewatomicpercent [57,58]andhavemetalliccharacteristics[58,59].Thisgroupis bettertermedasN-dopedcarboninsteadofgraphiticcarbon nitridesandtheirimplementationassupportmaterialsfor electrocatalystsstartedinthe1980s[60,61].Thecarbonaceous materialswerepyrolyzedalongwithprecursorsthatcontain nitrogen.Sincethebeginningoftheiruse,differentproductiontechniqueshavebeendevelopedandthroughtheseapproachessupportmaterialswereproducedthatcould potentiallyenhancethecatalyticperformancebycombining bothtransitionmetalandsupport[38,62 65].
Compoundswitharangeofpolymericorgraphiticarrangementsmakeupthelargestcategoryoftruecarbon nitridecompounds.Thesecompoundsaregeneratedfrom condensationreactionsofeasilyavailableprecursors,that includemelamine,urea,ordicyandiamide,thattakesplace afterthermaltreatmentsinthetemperaturescaleof 500 700 C[66].Theycouldpreciselybecalledascompounds ofgCNHwhosestructuresaregenerallybuiltfromlinkedunits ofheptazine(tri-s-triazine,orC6N7).Whenlowtemperature rangesareusedforsynthesis,thesepolyheptazine(PHs) becomeimperfectlyassembledribbon-likeshapes,similarto thoseestablishedinLiebig’smelon[67].Theyareended laterallybygroupsof NH2-and-NH-(Fig.1a)[68].Withrising synthesistemperatures,thecondensationextentincreases withadropinammonia-containingcomponentsandunits withsheet-likeappearancestarttodevelopgraphiticarrangements.Theprocedure,intheory,canbeextendedtothe graphiticcarbonnitride(g-C3N4)composition(Fig.1b)[68], althoughthishasnotbeenperformed[56,66].Instead,the limitingstoichiometriesobtaineduntilnowamidstgCNH materialhavebeenneartotheconstitutionofC2N3H.Another categoryofcarbonnitridesystemtonoteisconstructedfrom independentringsoftriazine(C3N3)thatareconnectedby groupsof-NH-or N ¼ .Theselayersofpolytriazineimide (PTI)offeranalternativewayofgeneratinginfinitesheetsof graphitewiththeC3N4 compositions(Fig.1candd)[68]. Initiallytheyweresynthesizedintheformofnanocrystalvia chemicalvapourdeposition(CVD)method[69,70].Afterwards, graphiticcarbonnitridebasedontriazineofcrystallineform (TGCN, Fig.1c)wasfoundduringthesynthesisofmoltensalt thatwasdepositedonthesurfaceofthemeltphaseorthe reactionvesselwalls[68,71].Itwasalsoobservedthatthe layersinthismaterialcontainC6N6 voidsthatarestacked
withapatternofABorABC[68,71].Inaddition,itsbandgap wasdeterminedtobe2.7eV.Thepolytriazineimidegroupof crystallinegraphiticcarbonnitridematerialsgrowbythe involvementofheteroatomsthatincludeLi,H,ClandBr withinthesystem,thatareinsertedduringproductionprocessinamoltensalt(e.g.,LiCl:KBr)orviahightemperaturehighpressureapproaches[32,72 75].Larger(C12N12)ring voidsarepresentinthesematerialsinsidethelayers,with bromideorchlorideionsoccupyingthespacesbetweenthe layersorthevoidsites.-NH-groupslinktheunitsoftriazine thatdecoratethevoidsites’interior,andthehydrogenscanbe partiallyorcompletelysubstitutedbycationsofLiþ.MoreLiþ speciescanalsobepresentbetweenthelayers(Fig.1dande) [68,74,76].
Propertiesofg-C3N4
Chemicalstability
Likegraphite,thepresenceofstackingwithvanderWaals bondsthatareoptimizedinthemiddleofcarbonnitridesingle layersrendersitinsolubleinmanysolvents.Therewasalso nonoticeablereactivityorsolubilityofcarbonnitrideinregularsolventssuchasalcohols,H2O,THF,DMF,toluene,and diethylether[77].Thestabilityaswellasthedurabilityof graphiticcarbonnitrideinorganicsolventswaspreviously analysedbydispersingapowderofcarbonnitrideinacetone, H2O,C2H5OH,acetonitrile,pyridine,CH2Cl2,glacialaceticacid, DMF,andanaqueoussolutionofsodiumhydroxideof0.1M concentrationforadurationof30days.Thedispersedcarbon nitridesampleswerenextdriedatatemperatureof80 Cfor 10handthenmeasurementoftheirIRspectrawastakenfor comparisonwiththefreshmaterial.ResultsofIRspectraof thesamplesthatweresoakedwereobservedtobeunaltered. Therewere,however,twoexceptionsandoneofthemwas thatthetreatmentofg-C3N4 inmoltenalkalimetalhydroxides causedthehydrolysisofthestructure.Theotherobservation madewasthattreatmentinconcentratedacidsresultedina sheet-likedissolutiontogenerateacolloidaldistribution[78], thatisneverthelesscompletelyreversible.
Thermalstability
Performingthermalgravimetricanalysis,orTGA,ongraphitic carbonnitridehasindicatedthatthissubstanceisremarkably robustandremainednon-volatileuptoatemperatureof 600 C,evenduringexposuretoair.Astrongendothermalpeak alsoappearedatatemperatureof630 Catwhichtherewas alsosuccessivecompletelossofweight.Thisrevealsthatthe fragments’thermaldecompositionandvaporizationbeganat thetemperaturevaluepreviouslymentioned.InastudyperformedbyGillan,carbonnitridewassealedinevacuatedsilica ampoulesandwasthenpositionedwithinatemperature gradient[77].Observationsfromthestudyshowedthatavery slowratesublimationofcarbonnitrideoccurredat450 Cand escalatedgreatlyat650 C.Carbonnitrideswerethen completelydecomposedat750 Candleftnoresidue.Fromthis analysis,thethermalstabilityobservedisamongstthehighest foranorganicmaterial(greaterthantheordinaryhigh-
Fig.1 Structuralmotifsobserveding-C3N4 (a)Liebig’smelonthatconsistsofchainswithzig-zagpatternofunitsof heptazine(tri-s-triazine)connectedbylinkingnitrogensandadornedontheiredgesbygroupsofN-H,(b)C3N4 layerthatis fullycondensedbasedonunitsofheptazine,(c)C3N4 layerthatistotallycondensedbuiltfromunitsoftriazine,(d) FoundationofPTIbasedonunitsoftriazinerings,connectedbybridgesofN-Hand(e)SideelevationofPTI.Liclthatisfully occupied[68].
temperaturepolymers,polyimides,andaromaticpolyamides).
Itisalsoimportanttoconsiderthatcarbonnitride’sstability ratherdiffersfromonepreparationtechniquetoanother,as foundintheliterature[72,73,77,79,80],andthiscouldbedueto differentdegreesofpolymerization.
Opticalandphoto-electrochemicalproperties
UV/Visabsorptionandphotoluminescenceexperimentshave previouslybeenusedtoinvestigatetheopticalcharacteristics ofcarbonnitride.Basedoncalculationsperformedtheoretically,carbonnitrideofpolymericformisanordinarysemiconductorthathasabandgapreaching5eV,whichdepends onthestructuralvariantsoradatoms[81].Infact,carbon nitridecommonlydemonstratesapatternofabsorptionofan organicsemiconductorthathasabandgapadsorptionthat appearsstronglyatapproximately420nm.Thisisinlinewith thepaleyellowcolouritpresentsthathasbeenreported earlierbyvariousauthors[82 84].
Animportantpointtonoteisthatsynthesisapproach, precursorsutilized,andthecondensationtemperature appliedcouldhaveaslightimpactoncarbonnitride’sabsorptionedge.Thismaybebecauseofthedistinctlocal structure,defects,andpackingthatareformedthroughout theproductionoralterationprocess[83,85].Differentalterationsofcarbonnitride,forinstance,maycauseablue-shift (byeitherprotonation[78]ordopingwithsulfur[86])orredshift(bycopolymerizationwithbarbituricacid[87]andby boronandfluorinedoping[88])oftheadsorptionedge.
Reportshavealsobeenmadeonseveralphotoluminescent speciesandsomewereabletodemonstrateblueregion emission.Itappearsthatthereisahighdependenceofphotoluminescencespectrumtowardsthecondensationdegree aswellasthepackinginthemiddleoflayers[79,84,89,90]. Conventionalpolymericcarbonnitridehas,inmanycases, exhibitedintensebluephotoluminescenceatambienttemperature.Itwasestablishedthattheluminescencespansover awiderangeof430 550nmandshowedamaximalvalueat around470nm.
Withanelectronicbandstructurethatissatisfactory, carbonnitridehasbecomeapotentialmaterialforusein systemsthatharvestandconvertsolarenergy(e.g.,photoelectrochemicalcells).Itisalsoknownthatphotocurrentwas determinedevenbybulkgraphiticcarbonnitridewhenradiatedwithvisiblelight(wavelengthofgreaterthan420nm) [87].Carbonnitride’shighlevelofchemicalandthermalstabilityimprovesthestabilityofphotoelectrochemicalcellsin presenceofoxygen.Inaddition,itispossibletotunegraphitic carbonnitride’selectronicbandstructurebynanomorphology alterationorbydopingandthishelpsintheenhancementof photocurrent.Forinstance,carbonnitridewithmesoporous structure(mpg-C3N4)canusuallyassistinboostingthecapacitytoharvestlightduetoitsmultiplescatteringeffectand largesurfacearea,andhencedemonstratedariseinphotocurrent[91].Othertypesofmodificationthatcanincreasethe photocurrentincludeprotonationanddoping[87].
Eventhoughcarbonnitridemodificationcanpartially enhancethephotocurrent,itisstillpresentlyconsideredtobe
ratherlow.Thisissupposedtobetheresultofthepresenceof grainboundarydefectsaswellasthelackofabilitytogenerate biggerdomainsizeswiththepresentsynthesisstrategy.
MechanismofmethanoloxidationreactionongC3N4 catalysts
Themechanismdescribedinthissectionisbasedonthat proposedbyZhuetal.inwhichtheyutilizedanultrathin2dimensionalg-C3N4 nanosheetasasupportforultrasmall platinumnanoclusters[54](Fig.2).Itwasobservedthatthe electrodemodifiedwiththeas-preparedPt/g-C3N4 demonstratedbetterelectrocatalyticperformanceformethanol oxidationreactionincomparisontobareplatinumnanoparticles.Muchhigherperformancewasalsoobservedwhen themodifiedelectrodewasirradiatedwithvisiblelight.
Inatraditionalelectrocatalyticmethanoloxidationprocess involvingplatinumnanoparticles,theparticleswouldserveas activesitesonwhichmethanolmoleculeswouldbeelectrooxidizedtocarbondioxide.Withtheintroductionofultrathing-C3N4 asasupport,theaggregationofplatinumduring thesynthesisprocesscouldbeavoidedtohelpinforming smallplatinumnanoparticles.Thisalsoimprovesthe adsorptionoftargetmoleculesduetothe2-dimensional structures.Furthermore,uponvisiblelightirradiation (>400nm),thegraphiticcarbonnitridecouldbeexcitedto generateelectrons(eCB)intheconductionband(CB)andholes (hVB þ )inthevalence(VB)[92 95].Therein,theholespossessan oxidativeabilityandcouldreactwithOH /H2Oadsorbedon thesurfacetoproducestrongoxidativehydroxylradicals (∙OHs)[96 99].Themethanolmoleculesbeingadsorbedon thecatalystsurfacecouldalsobeoxidizedinpresenceofthe hydroxylradicalsasinphotocatalyticmethanoloxidation reaction[96 99].Generally,thepairsofholeandelectronwill quicklyrecombineandonlysomeportionoftheholescould beutilized.Duringtheinterfacialchargetransfersbetween photoexcitedg-C3N4 nanosheetsandplatinumnanoclusters, thephotoexcitedelectronswillfirsttransfertoplatinumand thenflowtothecircuitunderexternalelectricfield.Thisis whatpreventsthechargesfromrecombining.
Fig.2 Representationofthesynergisticphotocatalytic (PC)andelectrocatalytic(EC)influenceofthePt/g-C3N4 modifiedelectrodeformethanoloxidationreaction(inthe presenceofvisiblelight)[54].
ThegroupalsoexaminedtheEISofthesynthesizedPt/gC3N4 nanosheetsmodifiedelectrodeindifferentrangeofpotentialsinboththepresenceandabsenceoflight.Theexaminationfurtherprovidedaproofofthesuggested synergisticeffectandthephoto-catalysisaspectofthe methanoloxidationreaction.Itwasestablishedthatfroma potentialof 0.60Vto 0.40Vthediametersoftheimpedance arcs(DIA)significantlyreducedwithincreasingpotential valueindarkness(Fig.3a).Thissignifiesthatatalowerpotentialtherearemoreactivesitesavailableformethanol oxidationreactionasaresultofCOintermediatespecies removalbyoxidation(COisgeneratedfrommethanoldehydrogenation[100,101]).Withincreasingpotentialvalue,from 0.40Vto 0.20V,theDIAalsoincreasedbecauseofcatalyst poisoningandoxidationatrelativelyhighervalueofpotentials(Fig.3b).However,thereisasuddenreversetrend observedinthearctothesecondquadrantatapotentialof 0.15V.ThispatternisduetoCOoxidativeremovalfromthe surfaceofthecatalystandregenerationoftheactivesites. Furtherincreaseinthepotentialfrom0Vto 0.15Vcaused thearcstoadjusttonormalpositivebehaviourgraduallywith alargeDIA(Fig.3c).Theobservedbehaviourisduetothe carbonmonoxideintermediatespeciesbeingabsentwhilethe platinummightbecoveredbyplatinumoxidesthatinhibits methanoloxidation[100].
UponlightilluminationonthePt/g-C3N4 electrode,smaller DIAsweregeneratedatthesamepotentialvalues(Fig.3d). Thisisanindicationthatthemethanoloxidationreaction charge-transferresistance(Rct)duringthephoto-assisted processisrelativelysmaller.Thearcreversingtothesecond quadrantpreviouslyobservedalsooccurredat 0.20V,a comparativelylowervaluethanthatobservedinabsenceof light,astheappliedpotentialiscontinuouslyincreased (Fig.3e).Amorenegativeonsetpotentialofnegativeimpedanceseenduringvisiblelightilluminationsuggestedthat removalofCOspeciesbyoxidationonthesurfaceofthe modifiedelectrodeismucheasier[102].Inaddition,froma potentialof0Vto 0.15V(Fig.3f),thearcsadjustedtonormal positivebehaviourgraduallywithalargeDIA,whilethetendencyofthearctoturnbacktowardsthefirstquadrantat 0.10Vwasmoreapparent.Furthermore,basedonthe Nyquistplotsobtainedbytheresearchteam,theDIAsatthe entirerangeofpotentialweresmallerthanthoseseeninthe absenceoflight.Thisindicatesthatinpresenceoflightthere islessercharge-transferresistance,duetotheeffective interfacialchargetransferonPt/g-C3N4,whichfurtherleadsto improvedphoto-electrocatalyticperformancetowardsmethanoloxidationreaction.
Variousg-C3N4 preparationandmodification techniques
Carbonnitridesarecommonlysynthesizedbydirect condensationorganicprecursorsthatcontainnitrogen.Examplesoftheseprecursorsincludethiourea,urea,dicyandiamide,melamine,cyanamide,andguanidine hydrochloride.Theyarebulkmaterialsthatarecharacterized bysmallsurfaceareasthatareoftenlessthan10m2 g 1 Providingwell-controlledporosityofnanoscalelevelinthe
Pleasecitethisarticleas:YudaA,KumarA,Areviewofg-C3N4 basedcatalystsfordirectmethanolfuelcells,InternationalJournalof HydrogenEnergy,https://doi.org/10.1016/j.ijhydene.2021.01.080
Fig.3 NyquistplotsoftheelectrodemodifiedwithPt/g-C3N4 inasolutionof1Mmethanoland1MKOHatelectrode potentialvaluesrangingfrom 0.6Vto0V(a c)intheabsenceoflightand(d f)undervisiblelightillumination[54].
bulkcarbonnitrideisnecessaryforpracticalapplicationsin areasofasinglecatalystorasupportsubstrateofco-catalysts (e.g.,heterojunctions)toimproveitsusetoalargescale.The creationofmesoporoussystemandtheenhancementof specificsurfaceareaarevitaltoproperlyadjustthephysicochemicalpropertiesandupgradethephotocatalyticperformance[103].Reportsfromanalysisinthepastindicatethat themesoporousg-C3N4 wasfirstattainedbytheperformance ofduplicationornanocastingonmatricesofmesoporoussilicathatarenotableforproducingthecorrespondingnanostructuresofcarbon[104 106].Severalexperimentswere performedaftergaininginspirationfromthishardtemplate methodtodiscovernewapproachesforg-C3N4 alteration. Thesenewtechniquesincludetheultrasonicdispersion technique[48],thesofttemplatemethod[107 109],the chemicalfunctionalizationtechnique[80,86,87,110],andthe acidicsolutionimpregnationmethod[78,111].Thestrategies mentionedarealsousefulinmodifyingtheelectronicpropertiesaswellasthetextureandsurfacechemicalproperties.
Nanoporousg-C3N4 synthesismethods
Materialswithporouscharacteristicsareparticularlyattractiveforuseasheterogeneouscatalystsorasacomponentfor catalyticreactionsthatconvertenergybecauseoftheirvast surfaceareasandaccessibleporosity[112 117].Graphitic carbonnitridewithmesoporousfeatures(meso-g-C3N4)isa potentialmaterialforproducingmetal-freecatalysts.Itis becauseitdisplaysdistinctivesemiconductorpropertiesin additiontolargesurfaceareaandavailablecrystallinepore wallstoassistwiththetransferofmass.Relativetothebulk graphiticcarbonnitride,thematerialcoulddemonstrate greaterporositythatcouldreach1.25cm3 g 1,largerspecific surfaceareathatcouldreachavalueof830m2 g 1,larger quantitiesofactivesitesaswellashighersize-orshapeselectivitythatoverallimprovethecatalyticperformance. Approachesthatarevitaltosynthesizegraphiticcarbon
nitridewithmesoporousarrangementincludethetemplating strategywhichcaneitherbethehard-templatingmethods(of nanocasting)orthesoft-templatingmethods(orselfassembly).Throughthesetechniques,acontinuousmesoporousg-C3N4 structurepossessingsphericalorcylindrical mesoporesandtheirinverseduplicatescouldbeachieved.
Mesoporousg-C3N4 soft-templatingsynthesismethod Formationofmesoporousarrayusingthistechniqueis attainablethroughcooperativeconstructionofmoleculesof amphiphilicsurfactantandguestspeciesthataredirectedby thelikelihoodofdecreasinginterfacialenergy.Thecomponentsoftheorganictemplates,aswellastheirpropertiesare essentialtoproducemesoporousstructures.Hence,theyare alsoidentifiedasstructure-directingagents(SDAs).This methodisusuallyperformedinhydrothermalenvironment thatisachievablethroughevaporationinducedself-assembly (EISA).
Tobeprecise,ionicliquids(ILS)orsurfactants(e.g.,P123, TritonX-100,F127,Brij76,Brij58,andBrij30)havealsobeen utilizedassofttemplatesforsynthesizingmeso-g-C3N4 by dicyandiamideself-polymerizationreaction[118]toobtain variousspecificsurfaceareasandporestructures.Itis important,however,tonotethatonlycertainpyrolysisdesign andtemplatepathwayscouldbeusedtoproducethedesired mesoporousarrangementsasthetemplatecomponentscould decomposebeforegraphiticcarbonnitridecouldbeformed. Moreover,somecarbonresiduecouldstillbeleftoverinthe finalproductsfromthetemplatingpolymersthatcouldseriouslyminimizethenitrogencontentoftheresultingmaterials furtheraffectingtheircatalyticperformance.Astudyreported byShenandteamdescribedthegenerationofbimodalmesoporousg-C3N4 thatpossessesporesof3.8nmandadiameter ofbetween10and40nmwhenTritonX-100templateand precursorsofglutaraldehydeandmelaminewereused[119]. Theresultingmeso-g-C3N4 surfacehasapolarnaturethat couldenhancetheimmobilizationof Candidarugosa lipase,
Pleasecitethisarticleas:YudaA,KumarA,Areviewofg-C3N4 basedcatalystsfordirectmethanolfuelcells,InternationalJournalof HydrogenEnergy,https://doi.org/10.1016/j.ijhydene.2021.01.080
andfurtherassistinmaintainingitsthermalstabilityand catalyticactivity.
Mesoporousg-C3N4 hard-templatingsynthesismethod Withthistechniquenanostructuredgraphiticcarbonnitride withnanowiresornanospheresfeaturescanbeduplicatedby makinguseofprimarynanoporesinthetemplateofsilicato synthesizeareplicaarrangementthatisstable.ThefinalgC3N4 ofnanoporousform,thatisaninverseduplicateofthe templateofsilica,isextractedafterextractingthetemplate withaqueousHForNH4HF2.However,thismethodisnot environmentallyfriendlyandisunsafe.WhenSBA-15was usedasahardtemplateandcarbontetrachloride(CCl4)and ethylenediamine((CH2-NH2)2)wereusedasprecursorsina nanocastingmethod,thegeneratedfinalproductwasanorderedmesoporouscarbonnitride(meso-CN)[120,121].Featuresoftheproductincluded140m2 g 1 specificsurfacearea, 2-dimensionalhexagonalorderedmesostructureswith approximately2.9nmpores,and0.16cm3 g 1 porevolume. Furthermore,theratioofcarbontonitrogencouldbemodified fromavalueof4.5to3.5bysteppinguptheweightratioof (CH2-NH2)2 toCCl4 from0.3to0.9.Thesametechniquecould alsobeappliedtosynthesize3-dimensionalcubicmesocarbonnitridestructureswhenahardtemplateofSBA-16 silicaisused[122].Duplicationusingahardtemplateof mesoporoussilicaIBN-4couldhelpinproducingmeso-carbon nitridenanoparticlesofabout150nm.TheproductsynthesizedinthiswaycoulddemonstrategreaterC/Natomicratio of2.3thanthosesynthesizedwithKIT-6andSBA-15templates.ThereasoncouldbeduetotheIBN-4template magnifiedporesizethatassistsitsimpregnationwithethylenediamine[123].
ExamplesofotherC-andN-richorganiccompoundsthat couldbeutilizedtoproducehighlycondensedmeso-g-C3N4 includemelem(C6N7(NH2)3),melamine(C3N3(NH2)3),dicyandiamide((CN-NH2)2),cyanamide(CN-NH2),andammonium dicyanamide(NH4(N(CN)2))[82,106,124 126].Antoniettietal. fabricatedadisorderedformofmeso-g-C3N4 usingahard templateofsilicananospheresandaprecursorofcyanamide. Thefinalproduct’scharacteristicswereporesof12nmand surfaceareasofbetweenvaluesof86and439m2 g 1 , dependingontheweightratioofprecursortosilicautilized. Regardingthecarbontonitrogenatomicratioameanvalueof 0.71wasestablishedandthisisnearto0.75,whichisthe theoreticalvalue.ImprovementofthisapproachwasperformedbyJunandco-workersbyimplementingahardtemplateoforderedmesoporoussilicaSBA-15togeneratean orderedmesoporousgraphiticcarbonnitride[127].Themesog-C3N4 producedwerewith0.59cm3 g 1 porevolume,4.89nm meanporesize,and239m2 g 1 surfacearea.Parkand researchteamutilizedthesameprecursorandperformeda faciletechniquetoformnitrogenrichmesoporousg-C3N4 withvarious2-and3-dimensionalstructuresthroughthe incipientwetnessmethodintheabsenceofsolvents[128]. Replicaswithnitrogentocarbonratioof1.13wassynthesized aftercarbonizationfor3hat550 C.The2-and3-dimensional mesoporousgraphiticcarbonnitrideswerecharacterizedby bigporevolumesof0.50and0.67cm3 g 1 andhighsurface areasof361and343m2 g 1
Thetemplateofchoicetosynthesizemeso-g-C3N4 couldbe mesoporoussilicaorcolloidalsilicaspheres.Usingatemplate ofsilicananoparticlesof12nmsize,forexample,couldcause theproductionofmesoporousg-C3N4 replicathatpossesses 12nmsphericalporesand450m2 g 1 surfaceareaorganized inanirregularmanner[129].Withhardtemplatesofcolloidal crystalsofsilicawithbiggerparticles(20 80nm),meso-or macroporousgraphiticcarbonnitridecouldbesuccessfully fabricated[130].Otherthansilicaspheres,highlyordered meso-g-C3N4 couldbesynthesizedwith2-dimensionalhexagonalSBA-15and3-dimensionalcubicKIT-6mesoporous silicathatcanfurtheractasareactivetemplateforproducing metalnitrideofmesoporousform[72,104,126,131].
Chemicalmodificationofcarbonnitride
Modifyingthroughchemicalapproachisaneffectivewayof alteringthephysicochemicalpropertiesoftheprecursorsso thattheirrangeofapplicationscanbeextended[132].This couldbeachievedbyelementaldopingofsolidmaterials.Itis astrategythatisoftenappliedtotailorthetextureandsurface chemicalcharacteristics,inadditiontotheelectronicpropertiesofmaterialssuchasTiO2 [133],carbon[134,135],and silicon[136].Applicationsofthisheteroatomdopingmethod havepreviouslybeenusedtomodifycarbonnitridematerials, andstudiesonthedopingeffectswithfluorine[88],boron [110],sulfur[86],andotherelementshavealsobeenreported.
Post-treatmentmodification
Thistechnique,alsoknownaspost-functionalization,involves introducingfunctionalgroupsontocarbonnitrides’surface.It hasbeenusedinthepasttomodifyfullerenes[137]andcarbon nanotubes[138]thatenabledthemtobesuitablefordifferent applications.Duetothenitrogen-richfeatureofcarbonnitride materials,directprotonationduringcounterionadditionisan appropriatemodificationapproach.Infact,itispossibleto reversiblyprotonatecarbonnitridebydissolvingin37%hydrochloricacidatroomtemperaturefor3h[78].XRDpatterns obtainedafterprotonationhaveshownalmostunchanged characteristics(2characteristicdiffractionpeaksatapproximately27 and13 )andthisdemonstratesthattheoriginal carbontonitrogenmatrixhasbeenpreserved(Fig.4a d).
Throughprotonation,theelectronicbandgapsofcarbon nitridecanbetunedtoimprovetheionicconductivityofthe material.Theadditionofprotonsalsoassistedindispersing thematerialinaqueoussolutionsthateasedbothcharacterizationandprocessing.Inaddition,furthermodificationofthe techniquecouldbeattainedbyasimplecounteranion exchange.
Chengetal.performedthepost-functionalizationtechniquebydopingcarbonnitridewithsulfur[86].Theresulting material,g-C3N4-xSx,wasattainedbyexposingpristineg-C3N4 toagaseoushydrogensulfideatmosphereat450 C.EnergyfilteredTEMimageshowedahomogeneoussulfurdoping displayingastrongScontrast(Fig.4e).AnalysiswithXPSand XANESspectroscopyrevealedthestructuraldetailsofdoping withsulfurintothecarbon-nitrogenframeworkandtheresultsreportedthatC-Sbondsgenerateding-C3N4-sSx through substitutionofsulfurbylatticenitrogen(Fig.4f).Inasimilar waytoprotonation,dopingwithsulfurcouldalsoalterthe
Fig.4 (a)Imagesofas-preparedg-C3N4 and(b)regeneratedg-C3N4 obtainedusingscanningelectronmicroscope(with 200nmscalebars).(c)Thephotocurrent-appliedpotentialdependenceatregeneratedC3N4/ITOelectrodeinasolutionofKCl of0.1Mconcentrationinthepresenceofvisiblelight(insetimageisthephotocurrentbiasedat 0.2V).(d)Theplotsof Nyquistimpedance(asscatter)ofg-C3N4-HþCl andg-C3N4 andsimulation(aslines).Therangeoffrequencyisfromavalue of106 to103 Hz,andtheperturbationsignalisofthevalueof100mV.Theinsetimageshowsthedottedareainlarge magnificationandtheequivalentcircuitmold.Thevalueofthecalculatedresistances(Rb)pre-andpost-protonationwere approximately28and1.5MU,respectively.(e)Imagesofenergy-filteredtransmissionelectronmicroscopyofcarbon, nitrogen,andsulfurinC3N4 (toprow)andC3N4-xSx (bottomrow).(f)Themodelofatomicstructureofaperfectg-C3N4 sheet thatcomprisesofmelemunits,and2melemunitswithasubstitutionalNatomatvariousperiodicsitesbySatom.Carbon, nitrogen,andsulfurinsites1and2aredisplayedbyspheresinyellow,red,green,andbluecolour[78,86].(For interpretationofthereferencestocolourinthisfigurelegend,thereaderisreferredtotheWebversionofthisarticle.)
Pleasecitethisarticleas:YudaA,KumarA,Areviewofg-C3N4 basedcatalystsfordirectmethanolfuelcells,InternationalJournalof HydrogenEnergy,https://doi.org/10.1016/j.ijhydene.2021.01.080
morphologyandsurfaceareaofgraphiticcarbonnitrideto makeitsuitableforcatalyticapplications.
Modifiedcarbonnitride in-situ synthesisstrategy Throughthismethodadditiveisaddedinsitubeforetheformationoflattice,andheteroatomsofboron,fluorineorothers areintroducedintothematrix.Anexamplethatimplements thistechniqueisthefabricationofmesoporouscarbonnitride polymersthatcontainsfluorineandboron[139].Themethod involvesionicliquidswithananionofBF4 fromarangeof variousionicliquids.BF4 anionisintroducedintheC-N condensationprocessduringorganicprecursor(e.g.,dicyandiamide)self-condensation.1-butyl-3-methyl-imidazolium hexafluorophosphate,orBmimPF6,isanionicliquidthatwas usedinanotherstudyasamildsourceofphosphoroustodope carbonnitride.Theimprovementachievedthroughphosphorousdopingofferedbetterphotocurrentproductionbya factorofupto5inadditiontoenhancedelectricalconductivitythatreachedfourordersofmagnitude.Thismethod utilizingionicliquidcouldbefurthermodifiedtoincludeother typesofheteroatomsbyreplacingtheionicliquids’cationor anionintomaterialsbasedoncarbonandnitrogenforother specificapplications.
Anotherexperimentdemonstratedthedopingofcarbon nitridematerialswithfluorineandboronseparatelyinvolved co-monomersofammoniumfluorideandaminoborane.Fluorinationwasperformedonthenewlythermallyinduced condensationofdicyandiamide[88].Analysisusingsolid-state MASNMRspectroscopyandX-rayphotoelectronspectroscopy showedfluorineatomsbeingintroducedintothematrixof carbonnitrideasC-Fbondsthatcausedthesp2 carbonto convertpartiallytosp3 carboninthematrixofcarbonnitride thatmayproducealowerplaneorderofthematerials.Relative tog-C3N4 thathasnotbeenmodified,XRDrevealedthatafter fluorinationhasoccurredthegraphiticstackingismuch weakened.Carbonatomsreplacementinthesystemwith boronproducesaminoborane-modifiedg-C3N4 thatformsa planarlayeredconfigurationinasimilarmannertomelon.The planarheterocyclicmacrocyclearrangementstillexistsinthe material,whiletheboronsitespresentonthesurfacemight behaveassitesofstrongLewisacid,thatthereforecomplementsthebasicsitesofnitrogeninbifunctionalcatalysis[110]. Informationofthestructureduetoboroninclusionintothe carbontonitrogenmatrixwereachievedthroughXPSand 11B solid-stateMASNMRanalysis.The 11BNMRspectrumdisplayedtwopeaksthatdesignatestwodistinctpositionsinthe structure(forbayboronandcornerboron).
Othermodificationtechniquesincludenitrogenprecursor (e.g.,dicyandiamide)copolymerizationwithotherorganic additives(e.g.,barbituricacid(BA))[87].Whencomparedto theinorganicmodificationapproaches,thecarbonnitride obtainedfromtheorganiccopolymerization-basedmethod displayedsignificantopticalabsorptionredshiftfromavalue of470nmto750nmwithescalatingamountofBA.Thiswould enablephotochemicalapplicationofwavelengthsthatcould leadtoamaximumsolarphotonflux.
Itisalsopossibletodissolvemetalsaltsintothecavitiesof nitrogeninthematrixofgraphiticcarbonnitride.Such strategywasreportedbyKawaguchietal.in1995tofabricate
metal-modifiedcarbonnitride[80].Theteamreportedthatthe proposedcarbonnitridestructureconsistsofaholebeing boundedbythreeradicalsofaminoinaunitcell.Thehole mentionedwasexpectedtopermittheintroductionofspecific speciesofchemicalsthatcouldbemetalatoms.Whenthe powderof[(C3N3)2(NH)3]n washeatedwithmetalchloride(e.g., ZnCl2 orAlCl3)foranhourat500 Cayellowishmaterialwas produced.ThefinalproducthadanIRspectrumthatwasakin tothatoftheinitialcarbonnitridematerial,whichfurther provedtheconservationoftheoriginalframework.Even thoughthewideningandweakeningof(002)diffractionwas detectedinthefinalmaterial,itspositionremainedunchangedandthisindicatedagraphiticstackingarrangement likethatofthehostmaterial.Thenitrogen “pots” contained largequantitiesofmetalthatcouldbepartiallyincluded(upto 59wt%).Furthermore,theXRDpatternsshowednometal(Zn orAl)ormetalsalt(ZnCl2 orAlCl3)diffractionthatsignifiedthe homogeneousincorporationofthemetalinthehostmaterial matrix.Besides,onlyminutequantitiesofcounterionswere foundinthematrixandthisindicatesthattheCNframework musthavetakenupthenegativecharges.
Similarly,theironionsinclusionintothematrixofcarbon nitridehavealsobeenobservedwithoutdamagingthehost’s graphiticsystem[140].Opticalabsorptionspectrastudiesand XRDpatternssuggestedthatironinsertionintothecarbon nitridematrixcouldstronglyalterthematerial’selectronic propertiesandgeneratesystemswithnovelcapabilities.
Typesofelectrocatalystthatusesg-C3N4 forMOR
Electrocatalystsofgraphiticcarbonnitridewithnon-metals
Graphiticcarbonnitrideitselfisastoichiometricmaterial whichconsistssitesofLewisacidandbase(terminaland bridginggroupofNH-andlonepairsofnitrogeninringsof triazine/heptazine,respectively)thatcouldpotentiallyactas platinumanchoringsitesandCOadsorptionsites.Itsfirst knownapplicationasasupportmaterialforacatalystwas performedbyYuandco-workersin2007foruseinDMFC[141]. Fromtheirstudy,itwasobservedthatanelectrocatalystof PtRubeingsupportedbygraphiticcarbonnitridecould demonstrate78 83%greaterpowerdensitythanwhen VulcanXC-72wasusedasthesupport.
AresearchstudyperformedbyMeenuetal.implementedgC3N4 todevelop,forthefirsttime,morphologicallyengineered metal-freeg-C3N4 fortheoxidationofmethanol(Fig.5)[142].gC3N4 of2-dimensionalnanosheets,1-dimensionalnanorods, and0-dimensionalquantumdotsweresuccessfullysynthesizedfrombulkbyaprocessthatinvolvedthermo-chemical etching.TheteamstudiedthebehaviourofthedifferentgC3N4 materialsforMORinbasicmediumusingcyclicvoltammetry(CV)andfoundthatthematerialinquantumdotsform showedgreatermethanoloxidationactivityincomparisonto theothertypesofgraphiticcarbonnitridedeveloped.Thisis becauseoftheexistenceofabundantedgesinnanomorphologyandmaximumactivesiteatomicpercentagesof pyridinicnitrogen.However,thequantumdotsg-C3N4 itself hasunstableelectrocatalyticactivityasitexhibitedamarked
Fig.5 (a)CVresultsobtainedforCNQDandCNQD-PANIinasolutionofNaOHwith0.5Mconcentration(with50mV/sscan speed).(b)CyclicvoltammogramtracesofmethanoloxidationreactionofglassycarbonelectrodemodifiedwithCNQDand CNQD-PANIinasolutionofmethanolandNaOH,eachwithaconcentrationof0.5M.(c)CurvesofCVof1st(linedinblack) and150th(linedinred)cycleinasolutionofmethanolandNaOH,eachwithaconcentrationof0.5M.(d)Curvesofcyclic voltammogramofmethanoloxidationreactionofCNQD-PANIindifferentconcentrationsofmethanolsolutions[142].(For interpretationofthereferencestocolourinthisfigurelegend,thereaderisreferredtotheWebversionofthisarticle.)
reductioninthefirstcyclefrom13Ag 1 to10.8Ag 1 inthe hundredthcycle.Suchphenomenoncouldbebecauseofthe slowcatalystdissolutioninbasicsolutionandmaterial’spoor electricalconductivity.Theproblemwassubjugatedwhenthe 0-dimensionalg-C3N4 wassupportedonconductingpolyaniline(CNQD-PANI).Theresultingmaterialdidnotonlydisplay bettermethanoloxidationelectrocatalyticactivitythanthePt/ Ccommerciallyavailable,butalsobettercarbonmonoxide resistivity.Thenanocompositewasabletodisplayhighcurrent densityinthefirstcyclewithavalueof28.4Ag 1 andmanaged tomaintainupto92%ofthevalueevenafter1000electrocatalyticcycles.Thereasonforthisdevelopmentcouldbe assignedtotheelectrostaticinteractionthatexistsbetween conductingpolyaniline(PANI)and0-dimensionalg-C3N4, whichcouldfurtherenhancethemethanoladsorptionand electricalconductivityofCNQD-PANIandoxidationofadsorbedCOtominimizeCOpoisoning.
g-C3N4 asasupportformetalnanoparticles
Containingnoblemetals
Platinum:Withregardstonoblemetals,platinumisapopular catalystmaterialforuseinmethanoloxidationreaction [143 146].TheelectrooxidationproducesCO-basedby-productsthatreducestheelectrocatalyst’sperformance.Theissue ofCOpoisoningandslowoxidationkineticsarethemain disadvantagesfacedwithusingplatinumcatalyst.Furthermore,platinumisalsoanexpensivematerialthatmakesit
economicallyunsuitableforfuelcellscommercialization. Hence,properutilizationofplatinumisnecessaryfortheir implementationinfuelcells.Platinumnanoparticlesoffew nanometersdimensionandnarrowsizedistributioncould offerappropriateuseofnoblemetalsbyenhancingtheelectrocatalyticactivity.Anumberofresearchteamsreported thatthesizeandshapeofplatinumnanoparticlescouldinfluencetherateofelectrooxidation[147].Thereisalsothe problemoflong-termdurabilityofanodeplatinumcatalysts beingsupportedbycarbonformethanoloxidation.ThecarbonsupportthatisoftenusedcouldalsobeeffortlesslyelectrochemicallyoxidizedtoCO2 duringfuelcelloperation, whichcouldresultinstructuraldegradationofthesupport. Platinumnanoparticlescould,inaddition,beelectricallydisengagedfromthesupportingmaterial.Platinumnanoparticles’migration,aggregation,andOstwaldripening[148] coulddegradetheelectrochemicalcharacteristics.Thereare twostrategiesoftenimplementedtoupgradethedurability andcatalyticactivityofplatinumelectrocatalysts.Thefirstis bysynthesizingaplatinumalloywithothermetalsandthe secondisbyutilizinggoodsupportmaterials.
ThedesignandfabricationofacatalystcomprisingofAgPtbimetalwasperformedbyLiuandco-workersthatdisplayedenhancedperformancetowardsmethanoloxidation byminimizingCOchemisorptionaffinity[149].Theinclusion ofrutheniumintoelectrocatalystcontainingplatinumcould assistintheoxidationofcarbonmonoxidebeingproduced duringmethanolelectrooxidation.Inthiswaytheissueof
carbonmonoxidepoisoningcouldbeavoidedduringthe oxidationofmethanol[150].However,theissuebeingfaced withutilizingbimetalliccatalystisinitssynthesisbecause maintainingspecificratiooftwometalsisvitalinadditionto itsshapetoacquirethedesiredeffectivecatalytic performance.
A2-dimensionalsupportmaterialknownasgraphenehas beendeterminedtobeasuitablematerialtoactasasupport fornanoparticledispersiondueitsphysicalandchemical properties[151]thatincludehighelectricalconductivity,surfacearea,andthermalstability[152 154].Nonetheless,graphenealsohasitslimitationforelectrocatalyticpurposes suchashighhydrophobicityandlowpolarity.Grapheneaggregationinaqueoussolutionandthechallengeinachieving metalNPsuniformloadingongraphenesurfacelimitsfurther enhancementintheelectrocatalyticactivities[155].Furthermore,theuseofacidicmediacanleadtocarbonsupport corrosionthatgenerallytakesplaceattheinterfacewiththe novelmetalNPsthatfurtherdegradesupportedcatalysts’ performance.Toupgradeboththestabilityanddispersionof theNPsonthesurfaceofgraphene,sheetsofgrapheneneed tobefunctionalized.ThisstrategyhelpsenhancethesolubilityandtheNPsdispersionabilityonthegraphenesurface. Grapheneattachmentwithsurfactantmoleculesorpolymer, andgrapheneoxidationtogenerategrapheneoxide(GO) reducegrapheneaggregationandimprovemetalNPsdispersionresultinginanenhancementoftheelectrocatalyticactivities[156,157].
Ontheotherhand,graphenedopingwithheteroatom(e.g., nitrogen,boron,etc.)canbeperformedtoaltergraphene’s characteristics.Dopingwithnitrogen,forinstance,can improvetheelectrocatalyticactivityinadditiontobetter durabilitybecauseofthestronginteractionthatexistsbetweenthecatalystandthesupport[158,159].Acarbon-based materialofg-C3N4,similarinstructurewithgraphene,could alsobeutilizedasasupportbecauseoftheexistenceofsitesof Lewisacid/baseintothemoiety[160 166].Metal/semiconductorNPsbeingsupportedbyg-C3N4 havebeenproven tobeaneffectivecompositefororgano-catalysis[167],photocatalysis[168],biosensors[169],andelectrochemicalapplications[170].Recently,variousresearchstudieshavealsobeen performedtosynthesizemonometallicplatinumcatalysts thataresupportedbyporousgraphenecomposites[171 173] andbynitrogen-dopedgraphene[159]toelectrochemically oxidizemethanol.
Sadhukhanetal.utilizedg-C3N4 toproduceacompositeof Pt/CNx byinvolvingsodiumborohydridereductionthat involveultrasoundtechnique(Fig.6)[174].Theresulting compositeofPt/CNx possessedalargeelectrochemicalsurface area(ECSA)duetobetterplatinumnanoparticlesdistribution onsheetsofCNx.AnalysisusingXPSandFT-IRperformedby theteamhavereportedtheexistenceofgreatinteractions betweenthemetalandsupportmaterialinthecomposite.To observetheelectrocatalyticperformanceofthematerialson methanoloxidationtheteamperformedtheexperimentsin anaqueousH2SO4 solutionof1Mconcentrationconsistingof 1Mmethanolusingcyclicvoltammogram(CV).Comparison wasalsomadewithaPt/Cmodifiedglassycarbon(GC)electrodeusingCVatascanrateof50mV/sinanacidicsolution. TheCVcurvesgeneratedbyboththesamplesdisplayedtwo
peaks(intheforwardscanandthebackwardscan).Theforwardscanpeakisthepeakcharacterizingmethanoloxidation whilethepeakinthebackwardscanindicatestheefficiencyto removecarbonaceousspecies.ThePt/CNx (310mA/mgPt)displayedaforwardpeakcurrentdensitythatwas2.7times greaterthanthatofPt/Ccatalystthatdisplayed114mA/mgPt TheonsetpotentialsofPt/CNx andPt/Ctowardsmethanol oxidationare0.193Vand0.28V,respectively.Thelowonset potentialandthehighmassactivitysignifythatPt/CNx isa superiorcatalystforMORrelativetocommercialPt/C.The forwardtothebackwardcurrentdensityratio(If/Ib),anindicatorofthecatalyst’spoisonresistancetowardscarbonaceousspecies,wasalsodetermined.Theteamfoundoutthat theIf toIb ratioofPt/CNx was2.68andthisisgreaterthanPt/C commerciallyavailable(If/Ib ¼ 1.9)by1.41times.Thisshows thatMORtakingplaceonthePt/CNx catalystsurfaceismuch easierintheforwardscantoformsmallerquantitiesof carbonaceouscomponents.Inaddition,thePt/CNx catalyst coulddemonstrateanexcellentlong-termstability.This abilitywasanalysedthroughchronoamperometric(CA)experimentsataconstant0.7Vpotentialinasimilarsolution saturatedwithnitrogen.Duringthestudy,Pt/CNx continuouslydisplayedgreatercurrentdensitythanthatofPt/C duringthewholescanduration.When5000sofamperometric scanhadpassed,thecurrentdensityofPt/CNx (57.3mA/mgPt) was17timeslargerthanthevalueobtainedforPt/C(3.3mA/ mgPt).
Mansoretal.developedthreedistinctmaterialsof graphiticcarbonnitrideintheformofpolymericcarbon nitride(gCNM),poly(triazine)imidecarbonnitride(PTI/ LiþCl ),andboron-dopedgraphiticcarbonnitride(B-gCNM) [175].Whentheresearchgroupperformedtheaccelerated corrosionexperiment,allthedevelopedmaterialswere observedtobeelectrochemicallystableincomparisonto conventionalcarbonblack(CB)withboron-dopedgraphitic carbonnitridedisplayingthesuperiorstability.Analysisof performancewasalsoconductedwithplatinumnanoparticles depositiononeachmaterialanditwasdeterminedthatPt/ PTI-LiþCl electrocatalystexhibitedgreaterdurabilitywith ECSAlossofonly19%relativeto36%obtainedforPt/Vulcan after2000scans.
Huangandco-workersdevelopedanefficienttechniqueto produceplatinum-decorated3-dimensionalarchitectures constructedfromgraphiticcarbonnitridenanosheetsand graphene(Pt/G-CN)asananodeelectrocatalystsforthe oxidationofMeOH[173].Thefinalproductpossessedlarge, accessible,multi-sizeporesthatcouldenablereactantstobe swiftlytransferredtotheelectroactivesitesandfavourable conditionstopreservethecatalyst’selectricalconductivity.In comparisontoPt-VulcanXC-72andPt-graphenehybrids(Pt/C andPt/G,respectively),the3-dimensionalPt/G-CNelectrocatalystcoulddemonstratebetterpropertiessuchasgood activity,satisfactorystability,andunusualpoisoningspecies toleranceduringMOR.TheresearchteamalsovariedtheG/CN ratiosoftheelectrocatalyst’sarchitectureandinvestigationof itsactivityusingCVinH2SO4 solutionof1Mconcentration showedthatPt/G3-(CN)7 possessthegreatestelectrochemicallyactivesurfaceareavalueof69m2 g 1.Whencomparedto Pt/C,Pt/G,andPt/CN,theydisplayedlowerECSAvaluesofless than40.8m2 g 1.Methanoloxidationmeasurements
Fig.6 (a)CVofMORofthecatalystsPt/CandPt/CNx inasolutionofMeOHandH2SO4,eachwithaconcentrationof1M(at 50mV/sscanrate).(b)Pt/CNx andPt/CcatalystsonsetpotentialsbeingcomparedforMOR.(c)Massactivityandspecific activitycomparisonbetweenthecatalystsofPt/CandPt/CNx atapotentialof0.65VinforwardscanforMOR.(d)Pt/CNx cyclicvoltammogramcurvesinasolutionofmethanolandH2SO4,eachwithaconcentrationof1M,atvariousscanrates (from10mV/sto200mV/s).(e)Aplotofforwardpeakcurrentdensitiesagainstscanratessquareroot.(f)Curvesobtained fromchronoamperometryanalysisofthecatalystsPt/CandPt/CNx forMORinasolutionofmethanolandH2SO4,eachwith aconcentrationof1M(at0.7Vconstantpotential).(g)Thevariationofforwardpeakcurrentdensitywithnumbercyclefor MORofthePt/CandPt/CNx catalystsinasolutionofmethanolandH2SO4,eachwithaconcentrationof1M[174].
conductedinH2SO4 andmethanolsolutionof1Mand2M concentration,respectively,showedthattheforwardanodic peakcurrentdensityofthecatalystsofPt/G-CNdeclinesinthe followingpattern:Pt/G3-(CN)7 (15.7mAcm 2) > Pt/G5-(CN)5 (12.2mAcm 2) > Pt/G7-(CN)3 (10.8mAcm 2) > Pt/G1-(CN)9 (1.3mAcm 2).Aninterestingdiscoveryreportedbythegroup wasthatwhenthecontentofnanosheetsofg-C3N4 increase from30%to90%,thecatalystpoisonresistanceimproved.The If/Ib ratioofPt/G3-(CN)7 is1.82and1.3timesgreaterthanthe ratioobtainedforPt/CandPt/Gcatalysts,respectively.In addition,Pt/G3-(CN)7 hasamassactivityvalueof 612.8mAmg 1 anditsresistivitytowardspoisoningspecies arebetterthanthoseoftherecentlydiscoverednanostructuresthatarebasedonplatinumthatincludePt/graphene[176,177],Pt/CNTs[178],Pt/N-dopedcarbons[159,179], Pt/porouscarbons[180],andPt-basedbimetalliccatalysts [37,181].
Zhangetal.previouslypublishedtheirworkonthe developmentof3-dimensioalhierarchicallyporouscarbon nanocompositesthatconsistedofg-C3N4 thatwaschemically combinedwithrGOthroughC-NcovalentbondstosupportPt NPs(3DPt-g-C3N4-rGO)[182].Thesynthesizednanocomposite possessedhighnitrogencontent,largesurfacearea,fine electricalconductivity,andinterconnectedporousnetworks. TowardsMOR,thecatalystcoulddisplayhighactivity,good
poisonresistivity,andexcellentstabilityinthelongrun.It alsopossessedhighvalueofelectrochemicallyactivespecific surfaceareaof80.3m2 g 1 thatisfargreaterthanotherrecent state-of-the-artplatinum-basedelectrocatalysts.The enhancedfeaturescouldbeduetotheexistenceofgood conductivityandlargespecificsurfaceareaofg-C3N4-rGO, superlativestructuralstabilityasaresultofcovalentinteractionsthatexistbetweenrGOandgraphiticcarbon nitride,aswellasthelargetriple-phaseboundariesgenerated bythePtNPsthatarefinelydistributed.
Zhuandresearchteampreparedasupportforultrasmall platinumnanoclustersintheformofultrathin2-dimensional g-C3N4 nanosheet[54].Theplatinumnanoclustershadamean sizeofapproximately3.2nm.Inadditiontodemonstrating betterelectrocatalyticabilitytowardsMORrelativetobarePt NPs,duringtheirradiationofvisiblelighthigherMORperformancewasdiscoveredrelativetotraditionalambient electrocatalyticoxidation.Thereasoncouldbeduetothe synergisticinfluencesofbothphoto-andelectro-catalytic oxidationofMeOHalongwithefficienttransferofinterfacial chargeinPt/g-C3N4.Linearsweepvoltammetric(LSV)behavioursofPt/g-C3N4 andpureplatinumNPswereperformed underlightanddarkilluminationinasolutionofCH3OHand KOH,eachhavingaconcentrationof1M.ThePt/g-C3N4 electrodewhenilluminatedbylightshowedavalueofonset
potentialthatwasmorenegativethanintheabsenceoflight. Inaddition,Pt/g-C3N4 electrodedisplayedavalue 132mAmg 1 asthecurrentdensityundervisiblelightirradiationandthisisapproximately2.4and3.1timeshigherthan thevalueobtainedforPt/g-C3N4 withoutlightirradiation (56mAmg 1)andofpureplatinumNPselectrode (42.3mAmg 1),respectively.
Lietal.supportedanalloyofplatinumandruthenium (PtRu)onag-C3N4 nanosheetcoveredVulcanXC-72carbon black(C@g-C3N4 NS)toproduceanelectrocatalystthatis synthesizedthroughamicrowave-assistedpolyolprocess (MAPP)[183].AnexcellentactivitywasobservedduringelectrochemicalmeasurementsofPtRu/C@g-C3N4 NSbecauseof evendistributionandminutesizeofNPsofPtRu,andgreater stabilityowingtorobustinteractionbetweenthecomposite supportandthePtRuNPs.AnalysisusingX-raydiffraction (XRD),X-rayphotoelectronspectroscopy(XPS),andtransmissionelectronmicroscopy(TEM)determinedthattheshell ofthebulkgraphiticcarbonnitrideontheexteriorofthe producedbulkgraphiticcarbonnitridecoatedVulcanXC-72 carbonblack(C@bulkg-C3N4)infactexfoliatedtog-C3N4 of layerednanosheetsandresultedinacompositematerialof VulcanXC-72coatedwithnanosheetsofg-C3N4.Furthermore, themasscatalyticactivityofsynthesizedcatalystsignificantly improvedby2.1timesgreaterthanthemasscatalyticactivity observedforPtRu/Ccatalystproducedbysimilarmethod. Acceleratedpotentialcyclingtests(APCTs)showedthatthe catalystcandisplay14%greaterstabilityinadditiontohigher poisonresistivitywhencomparedtotheas-synthesizedPtRu/ C.Theenhancementinperformanceobservedcouldbedueto afewreasons.Thefirstonecouldbeduetothesupport’s upgradedelectronconductivitybygeneratingC@g-C3N4 NS core-shellstructure.Secondly,thereisalsothepresenceof excellentmechanicalresistanceandstabilityofthenanosheetsofg-C3N4 inoxidativeandacidicconditions.Thirdly,a strongerinteractionispresentbetweenthemetalandsupport (SMSI)thatexistsbetweenthecompositesupportandthe metalnanoparticles.
Liandteamalsoimplementeda p-p stackingtechniqueto synthesizeasupportforultrafinePtRunanoparticlesinthe formofgraphene/ultrathing-C3N4 nanosheetcompositematerialformethanolelectrooxidation[160].Graphiticcarbon nitrideelectricalconductivityisduetotheelectron-hole puddlethatdevelopedonthesheetinterfacesbetweengrapheneandg-C3N4.Duetotheremarkabletexturalproperties thatincludelargesurfacearea,layeredstructure,highnitrogencontent,homogeneousultrafinePtRuNPsdispersion,and thestronginteractionbetweenthenanosheetofg-C3N4 and metalnanoparticles,thefinalmaterialcouldperformwellas anelectrocatalyst.Theenhancedpropertiesincludegreater stability,betterpoisontolerance,andexcellentelectrocatalyticactivity.Theexistenceof25%g-C3N4 contentinthe mixedsupportenabledthecatalyst(PtRu/G75-(CN)25)to performwithbestactivityandstability.Theforwardpeak currentdensitiesof0.91AmgPt1 wasdeterminedforthis catalyst,whichis1.4timesgreaterthanthevalueachievedby PtRu/rGO.Increasingthecontentofg-C3N4 wouldleadto graphiticcarbonnitridenanosheetagglomerationontherGO surfacethatcouldreducethecatalyticactivitybecauseofpoor conductivityofelectron.However,increasingtheg-C3N4 NS
contentfrom0%to30%causedthecatalysts’poisonresistanceabilitytobecomemuchbetter.Hence,PtRu/G70-(CN)30 hashigherIf/Ib ration(4.06)thanthatofPtRu/G75-(CN)25 (2.34). Suchphenomenoncouldbeduetothemarginalnitrogen compositioninthesupportsthatwouldactivateahuge quantityofneighbouringatomsofcarbonandfurtherspeed upthegenerationofOHbyH2Odissociation.Therefore,promotionoftheoxidativeremovaloftheintermediatepoisoning speciesthatwereabsorbedtakesplace.
Wangetal.depositedatraditionalelectrocatalystofPt andAuona2-dimensionalvisible-light-activatednanosheetsofgraphiticcarbonnitridethroughafacileone-pot hydrothermaltechnique[184].Alteringthequantitiesof goldandplatinumallowedtheconstructionof3dimensionalplatinumisland-on-goldarchitectureson graphiticcarbonnitridenanosheetssurface(Pt-Au/CN).The performanceoftheresultingPt-Au/CNcompositetowards MORwasanalysedbytheresearchteam.Incomparisontoa graphiticcarbonnitride-supportedplatinum(Pt/CN)modifiedelectrode,thepreparedcatalystcoulddemonstratean enhancedelectrocatalyticactivityforMORby13.8times. Visiblelightilluminationalsocontinuouslyenhancedthe currentdensityandstabilityofthecomposite.Factorsthat contributetotheimprovedstabilityandelectrocatalyticactivityincludethebimetallicelectroniceffectsofplatinum andgold,3-dimensionalPtislands-on-Auarchitectures,2dimensionalsupportnanosheetofgraphiticcarbonnitride, andthesynergisticinfluenceofelectro-andphoto-catalytic processes.
Palladium:Manyresearchershavestudiedtheinclusionof palladiumasanoblemetaltosubstituteplatinumtoproduce non-Ptbasedanodecatalysts.ForexampleQianetal.reported thedevelopmentofhigh-performancePd/g-C3N4/carbonblack fortheelectrooxidationofbothMeOHandformicacid[185]. Theproductionwasachievedbyafacile2-steptechniquethat involvedg-C3N4 coatingonthecarbonblacksurfacebyheating treatmentatlowtemperaturefollowedbyuniformdeposition ofpalladiumNPsthroughwetchemistryapproach.Promoted bytheindividualcomponents’synergisticinfluences,the resultingcatalystcouldexhibitexcellentforwardpeakcurrent densitiesofthevalue1720mAmg 1 Pd forMORinalkalinesolutions.Thisperformanceissignificantlybetterthanthat shownbythecommercialPd-Ccatalyst.
InanotherstudyperformedbyFangandco-workers,gC3N4 andrGOwascombinedtofabricateahybridsupportto anchorultrafinePdNPsthroughasimpletechniqueinvolving aone-stepelectrodeposition[186].Characterizationstudyof themorphologyandstructurebyimplementingXRD,scanning/transmissionelectronmicroscopy(SEM/TEM),XPS,and RamanspectroscopyconfirmedtheuniformdispersionofPd NPsong-C3N4@rGOsupportwithparticlesof5.87nmmean size,originatingfromtheNingraphiticcarbonnitridethat contributetotheelectrontransporthighwayonthereduced grapheneoxidenanosheetlayersurface.Inaddition,reports fromelectrochemicalexperimentsindicatedthatthesynthesizedcatalystdisplayedahighefficiencyofelectrocatalytic activityforMORwithavalueofcurrentdensityof 0.131mAcm 2 Zhangandteamdevelopedacovalentlycoupledg-C3N4 andrGOhybridtoloadPdNPsusinganinsituchemical
synthesistechnique[33].PdNPswitha3.83nmmeandiameterweredepositedevenlyontheg-C3N4-rGOsurface.When comparedtocatalystsofcommercialPd-activatedcarbon(PdAC)andPd-rGO,thenanocompositeofternaryPd-g-C3N4-rGO possessesgreatelectrocatalyticcharacteristicsfortheoxidationofbothformicacidandMeOH.Theseremarkablepropertiesincludenotablyhighforwardpeakcurrentdensities, largeelectrochemicallyactivesurfacearea(ECSA),andfine long-termstability.Reasonsfortheimprovedactivitycouldbe duetothespecificpropertiesofthecatalyst’sspecialnanostructureandthesynergisticinfluenceoftheindividualspecies.Theseincludethelargespecificsurfaceareaofthe mesoporousstructure,finerrGOconductivity,well-dispersed PdNPsduetotheinfluenceofplanargroupsintroducedby g-C3N4,andsatisfactorystabilityofthestructureduetothe covalentinteractionsthatexistsbetweenrGOandgraphitic carbonnitride.
Zhangetal.alsodevelopedanovelsupportmaterialrich innitrogen,consistingofrGOandg-C3N4 intheformof nanoflakelets(CNNF)toloadPdNPs[35].TheCNNFwas formedbysplittingdecompositionofthepolymerofgraphitic carbonnitrideonreducedgrapheneoxideathighertemperaturevalues.TheCNNFproducedwasalsointimatelycombinedwiththereducedgrapheneoxidenanosheets.Itwas observedthattheCNNFcouldhelpinofferingedgesitesthat aremoreexposedandspeciescontainingactivenitrogenfor thegreatdistributionofpalladiumNPs.Itwasalsodeterminedthatthenanoparticlesofpalladiumhadamean diameterofthevalue3.92nmthatwereevenlydistributedon sheetsofCNNF-G.DFTcomputationsreportedthattheCNNF couldtrapthepalladiumadatom,andthereforebehaveasa nucleationsiteforpalladiumatwhichatomsofpalladium aremorelikelytogathertogeneratepalladiumclusters.The excellentpropertiespossessedbythefinalPd-CNNF-G nanocatalystweresatisfactoryforbothMORandformic acidoxidationandtheyincludelargeECSAvalues (91.2m2 g 1),finestabilityanddurability,andremarkably highforwardpeakcurrentdensities.Theseproperties enabledPd-CNNF-GnanocatalysttosurpassPd-graphene, commercialpalladiumcatalystsupportedbyactivatedcarbonorPd-carbonnanotubes.Pd-CNNF-Ghadtheabilityto displayapeakcurrentdensityasgreatas1770mAmg 1 for MORinNaOHandmethanolsolution,eachwithaconcentrationof1M.Thisvalueismuchgreaterthanthevalue foundforPd-g-C3N4-rGO(1550mAmg 1),Pd-rGO (835mAmg 1),Pd-CNT(725mAmg 1),Pd-AC (535mAmg 1),Pd-g-C3N4 (67mAmg 1).Inaddition,the recordedvalueisevengreaterthanthoserecentlyreported fornanostructuresbasedonpalladiumthatincludePd-SnO2/ MWNTs(778.8mAmg 1)[187],Pd/graphenehydrogelona foamofnickel(788mAmg 1)[188],Pd1Ag1 nanoparticles/ graphene(630mAmg 1)[189],Pd-Cubimetallicnanoparticles/graphene(392.6mAmg 1)[190],Pd1Pt3/nanoplateletsofgraphite(385.22mAmg 1)[191],Pd/CuO-TiO2 (381mAmg 1)[192],Pd-Ni-PNPs/carbonblack (~360mAmg 1)[193],andporousPt-Pdnanospheres/graphene(180mAmg 1)[194].
Eswaranandteampreviouslyreportedasimpleone-step approachofproducinganewg-C3N4/polyaniline/palladium NPs(g-C3N4/PANI/PdNPs)basednanohybridcompositealtered
screen-printedelectrode(SPE)toachieveefficientMOR[195]. Thestructureofthenanohybridwasattainedbyasingle-step simultaneouselectro-depositiontechniquethatinvolvedan electrolytesolutionofaniline,palladiumchloride,andnanosheetsofgraphiticcarbonnitride.Thefinalproductalso featuredneedle-shapedpolyanilineandsphericalnanoparticlesofpalladiumbeingpositionedbetweennanomatrix ofg-C3N4.Analysisoftheas-preparedcompositealteredSPE usingCVandCAhaveshownthatagreatelectro-oxidation andexcellentcatalyticperformanceforMORcouldbe exhibitedbythematerialincomparisontocommercial10%Pd depositedcarbonblackandothertypeofcatalystspreviously studied.
Qianetal.have,inthepast,utilizedbimetallicNPsofPt-Pd tofabricateacompositecatalystwithg-C3N4 modifiedcarbon blackthroughafacile2-stepapproach[196].Anextraordinarilyfinecatalyticactivityaswellasstabilityinthelongtermcouldbedemonstratedbytheresultingmaterialsfor oxidizingMeOH,C2H5OH,glycerolandethyleneglycolinbasic solutions.Theenhancedpropertiescouldbeduetotheplanar aminogroupofgraphiticcarbonnitridepromotingatemplate effectforthedistributeddecorationofplatinum-palladium nanoparticles,withcomplementaryrolesofplatinum(site fordehydrogenation)andpalladium(siteforremovalofCOlikespecies).Otherfactorscouldincludethepresenceofcarbonblackhavinglargespecificsurfaceareatopromoteafast diffusionofelectrolyte,quickeliminationofcarbonaceous species,andthesupport’sstructuralstabilityestablishedon thecovalentlinkthatexistsbetweenCBandg-C3N4 forpreservingthecatalyticsystemdurability.Thegroupstudiedthe behaviouroftheas-preparedcatalysttowardsMORinalkaline mediumusingCV.ItwasdeterminedthatPt-Pd-gCN-CBdisplayedthegreatestvalueofanodicpeakcurrentdensityof 4420mAmg 1metal.Thisvalueisthenfollowedbytheresultingcurrentdensitiesinthefollowingorder:Pt-Pd-CB(2017mA mg 1metal) > Pt-gCN-CB(2000mAmg 1Pt) > Pd-gCN-CB (955mAmg 1Pd).ThevalueobtainedforPt-Pd-gCN-CBiseven remarkablyhigherthanthevalueattainedforPt/Ccommerciallyavailable(1410mAmg 1Pt)[197],Pt-Pd/graphene (636mAmg 1metal)[198],Pt@Pd/RGO(588mAmg 1metal) [199],andcommercialPd/C(459mAmg 1Pd)[185].
Containingnon-noblemetals
JunlanLvandco-workersutilizedalow-costtransitionmetal ofironandg-C3N4 tofabricateanoveltypeofanodecatalyst [200].Ironwaschosenbecausethed-orbitalofitsatomsis likelytoobtainorloseelectrons,andtheresultingcatalyst couldpossessstrongredoxproperties.Catalystscontaining ironhavealsobeenshowntohavegreatperformancetowards methanoldecompositionandoxidation[201,202].Adsorption resultshavealsoreportedthatmethanolmoleculescouldget adsorbedontheironatomsthroughtheOatoms,thatthey couldeasilydissociateintoHandCH3O,andthenC-Hbond couldbegraduallybrokentogeneratethefinalproducts. Usingg-C3N4 asacatalystsubstratethatpossessessix-fold cavitieshashigherpotentialofcapturingmetalatoms.The energyrequiredforO-Hbondcleavageinmethanolis0.99eV, whichsignifiesthatthereactioncouldproceedeasily. Furthermore,theadsorptionenergyofCO2 ( 3.44eV)is smallerinvaluethanthatofCO( 4.65eV),suitablefor
Fig.7 CurvesobtainedfromCVanalysisofglassycarbonelectrodesmodifiedwith(a)Ni/CN,(b)(Ni-Cu)/CN,andCu/CNina solutionof1Msodiumhydroxideanddifferentconcentrationsofmethanol(at50mV/spotentialscanrate).(d)Curves attainedfromCVstudyofglassycarbonelectrodemodifiedwithcatalystsofCu/CN(inblack),Cu-Ni/CN(inred),andNi/CN (inblue)inasolutionof1Msodiumhydroxideand3MMeOH(at50mV/sscanrate).Insetimagedisplaysthemagnified versionoftheCVcurvestoshowtheonsetofMOR.(e)Aplotofcurrentdensitiesagainsttimeforaglassycarbonelectrode modifiedwithfilmsofNi/CN(inblue),Ni-Cu/CN(inred),andCu/CN(inblack)catalystsinasolutionof1Msodiumhydroxide and3MMeOH(atE ¼ 0.5VvsAg/AgCl).(f)AgraphofcurrentdensitiesofelectrodesmodifiedwithNi/CN,Ni-Cu/CN,andCu/ CNinthepresenceandabsenceofvisiblelight[203].(Forinterpretationofthereferencestocolourinthisfigurelegend,the readerisreferredtotheWebversionofthisarticle.)
minimizingthetoxicityofCOtoironandenhancingthe efficiency.
Pietaetal.synthesizednanostructuresofCu,Ni,andCu-Ni thatwerehomogeneouslyembeddedonultrathin2dimensionalg-C3N4 (Fig.7)[203].Theresearchteamdropcastedthenovelhierarchicalhetero-structuresonglassy carbonanodes.Thebehaviourofthenovelelectrocatalyst towardMORwasalsoexaminedunderbasicconditions. Nanosizednickelparticles,finelydispersedong-C3N4,were veryactivetowardsMORandexhibitedavalueof0.35Vasthe onsetpotentialandachargetransferresistanceof0.12kU ThemodifiedglassycarbonelectrodestabilitywasalsostudiedusingCAandaconsistentcurrentdensityofaround 36Ag 1 (12Acm2)wasattainedduringthedurationofthe entireexperiment(upto160min)when4wt%ofnickeloxide wasloadedontheelectrode.Inaddition,illuminationbyUV lightwithwavelengthof~400nmassistedinenhancingthe currentdensityofallthecatalystsstudied.Itwasdetermined that4wt%Ni/CNcatalystexhibitedthehighestcurrentdensityvalueof127Ag 1 (22Acm2).Theinclusionofcopperinto thehybridmaterialhelpedreducetheactivitylossesdueto Cuþ undergoingirreversiblereduction/oxidationtoCu0 and Cu2þ,segregationofcopperoxide,andaffectingtheprocessof electrontransferthatleadtoincreasedredoxpotential.
Table1 presentsalistofselectedC3N4 basedelectrocatalysts alongwithkeyexperimentalobservationsforaquick reference.
Keychallengesandfutureprospects
Withthemanystudiesperformedonelectrocatalystsconsistingofgraphiticcarbonnitride,significantprogresscouldbe observedinthisfield.However,furthersystematicexaminationsarerequiredasthestudiesarestillintheearlystages. Implementingrationaldesignofcomplexheterostructures couldenablethesimultaneousachievementofefficientcarrier formation,separation,transfer,andutilizationinadditionto efficientopticalabsorptionthatisfundamentaltothedevelopmentofnewgenerationofexcellentperformingphotocatalysts.Challengesstillexistinthedevelopmentofsuch complexstructurethatneedstobeconsideredasfollows:
-Itisnecessarytounderstandthebasicsofchargetransport processtakingplaceinphotocatalystswithmultiheterostructuretooptimizethechargecascadingprocess toensurethemaximumusageofphotogeneratedcharge carrierstoachievethesatisfactoryreduction-oxidation chemistry.Althoughelectron-holeseparationandtransportationina2-componentheterostructurecaneasilybe understood,theunderstandingbecomesmuchmore complexinmulti-componentheterostructurestocontrol chargetransport,especiallyinthescaleofnanometers. Thephotocatalystsinterfacepropertiesdeterminethe resultingefficiencyofthephotocatalyst.Furthermore,itis
Table1 Listofvariouselectrocatalystscontaininggraphiticcarbonnitrideandtheirelectrocatalyticpropertiestoward methanoloxidationreaction.
CatalystsMethodMass/Specificactivity/ Currentdensity
CNQD-PANIThermo-chemicaletching28.4Ag 1 (massspec.peakforward) 3.580.5MMeOH þ 0.5MNaOH[142]
StellatedPtNPs þ AgcoreOne-potapproach72mAcm 2 (peakcurrent density forward) 1.1251MMeOH þ 0.1MHClO4 [149]
Pt-CNx Ultrasoundmediated sodiumborohydride reductionofH2PtCl6 (CNx nanosheetspresent) 310mAmgPt1 (massactivity forwardscan)
Pt/gCNMThermolysisand condensationreactions, ethyleneglycolreduction
Pt/PTI-LiþCl
Pt/B-gCNM 209mAcm 2 ECSA
[174]
[175]
[175]
þ 0.1MHClO4 [175]
Pt/G3-(CN)7 ModifiedHummersmethod, liquid-phaseexfoliation 15.7mAcm 2 (peakcurrent density forward) 1.642MMeOH þ 1MH2SO4 [173]
3DPt-g-C3N4-rGO
1 (peakcurrent density forward)
þ 0.5MH2SO4 [182]
Pt/g-C3N4 nanosheetsSimplerefluxmethod158.9mAmg 1 (in darkness), 520.4mAmg 1 (visiblelight irradiation) 1MMeOH þ 1MKOH[54]
PtRu/C@g-C3N4 NSMicrowave-assistedpolyol process 1.14AmgPt1 (peakcurrent density forward) 1.730.5MMeOH þ 0.5MH2SO4 [183]
PtRu/G75-(CN)25 Microwave-assistedpolyol process 0.91AmgPt1 (peakcurrent density forward) 2.340.5MMeOH þ 0.5MH2SO4 [160]
Pt10-Au1/CNOne-pothydrothermal approach 1.52mAcm 2 (indarkness), 2.58mAcm 2 (visiblelight irradiation)(peakcurrent density forward)
1MMeOH þ 1MKOH[184]
Pd/g-C3N4/carbonblack-30Facileultrasonicprocess, wetchemistrymethod 1.66mAcmPd2 (peakcurrent density forward) 1MMeOH þ 1MNaOH[185]
Pd/g-C3N4@rGOOne-stepelectrodeposition technique 0.131Acm 2 (peakcurrent density forward)
TernaryPd-g-C3N4-rGO-2 nanocomposite Insitu chemicalsynthesis method,modified Hummers’method
1550mAmg 1 (peakcurrent density forward)
Pd-CNNF-GModifiedHummers’method1770mAmg 1 (peakcurrent density forward)
ASPE-g-C3N4/PANI/PdNPs
Pt-Pd-gCN-CBSurfactant-freesoft chemistryprocess
3.42mAcm 2 (peakcurrent density forward)
4420mAmgmetal 1 (peak currentdensity forward)
Ni/CNOne-pothydrothermal synthesis 57Ag 1 (indarkness), 127Ag 1 (visiblelight irradiation)
criticaltohaveagoodunderstandingofthenanoscaleinterfacesofchargegeneration,separation,andtransportation.Studiesperformedsofarhavemostlyfocusedon thephotocatalyticsystem’soverallapparentefficiency.In ordertofurtheradvanceinthefield,detailedstudyonthe mechanismofchargetransfershouldbeperformedfor photocatalyststhatconsiststhreeormorecomponents.
-Alessaddressedpropertyofg-C3N4-basednanocomposites isthestability,whichcouldbecomethemainobstaclein thedevelopmentofphotocatalysts.Evenifaphotocatalyst withveryhighefficiencyisdevelopeditsapplicationcould belimitedifitslifetimeisshort.Inmanycases,aphotocatalyst’schemicalcorrosionand/orphoto-degradation
1MMeOH þ 1MKOH[186]
1MMeOH þ 1MNaOH[33]
1MMeOH þ 1MNaOH[35]
2.7361MMeOH þ 0.5MKOH[195]
1MMeOH þ 1MNaOH[196]
3MMeOH þ 1MNaOH[203]
cannotbeprevented.Sofar,satisfactorystabilityhas onlybeenachievedbyfewphotocatalystswhichwas possiblethroughverysophisticatedsynthesistechniques. Oneofthefocusoffutureresearchcouldbetodevelopa stablephotocatalystpossessinggreatefficiencyatarelativelycheapercostthatcouldbesuitableforpracticalapplicationsandcommercialization.
-Itisstillnecessarytounderstandthemechanismsinvolved inthephotocatalyticenhancementbysemiconductor compositesbasedongraphiticcarbonnitride.Inaddition, itisimportanttocomeupwithauniformtechniqueto evaluatethephotocatalyticperformanceascurrentlythere aremultipleassessmentmethodsavailable.
-Effortsshouldbemadetoexploretheenhancementin propertiesfromthecombinationofg-C3N4 andnovel photocatalysts.Goodunderstandingofthephotocatalytic mechanismofthecombinationinadditiontorapidnovel nanomaterialsdevelopmentcouldhelpovercomethe bottleneckthatarisesfromglobalenergyandenvironmentalissues.
Conclusionsandoutlook
Manyresearchstudieshaveutilizedg-C3N4 invariousforms andassupportstodesignandconstructsuitableelectrocatalystsformethanoloxidationreaction.Graphiticcarbon nitrideinclusionasasupportcomponenthasbeenprovento improvetheproperties,andhencetheperformanceofthe finalmaterialowingtoitsideal2-dimensionalstructure, remarkablechemicalandthermalstability.
Basedonthestudiesthathaveusedg-C3N4 asanadditional electrocatalystmaterialforMORthereareseveralfabrication techniquesthatcouldbefollowedtoensurethatasatisfactory performanceisachievedduringmethanoloxidationprocess. Incorporatingmesoporousstructuresandmodifyingthespecificsurfaceareacouldassistinobtainingappropriatephysicochemicalpropertiessuitableforbetterphoto/electrocatalyticperformance.Suchimprovementsareachievable byperformingnanocasting,ormesoporoussilicamatrices duplication.Mesoporousfeaturesongraphiticcarbonnitride allowstheenhancementofsemiconductorpropertiesand introducesmoreporewallsofcrystallineformsuitablefor enhancingmasstransfer.Utilizingsofttemplatesofsome ionicliquidsorsurfactantsformeso-g-C3N4 synthesis(via self-polymerizationreactionofdicyandiamide)isanoptionto achievevariousformsofporestructuresaswellasspecific surfacearea.Withthehard-templatingmethodthatinvolve usingprimarynanoporespresentinthesilicatemplate,a stablereplicastructurecouldbefabricated.Utilizationof ethylenediamineprecursors,carbontetrachlorideandahardtemplateofSBA-15innanocastingmethodproducesanorderedmeso-CN.TheratioofCtoNofthefinalmaterialcould alsobeadjustedfromavalueof4.5to3.5byescalatingthe weightratioofethylenediaminetoCCl4 fromavalueof0.3to 0.9.Thehardtemplatethatcouldalsobeutilizedtosynthesize ameso-carbonnitrideNPswithrelativelygreaterCtoN atomicratioof2.3isthemesoporoussilicaIBN-4.Thiscould bearesultoftheenlargedporesizeofthetemplatehelping withtheethylenediamineimpregnation.Implementinga precursorofcyanamideandahardtemplateofsilicananospherescanhelpdevelopmesoporousgraphiticcarbonnitride withanaverageatomicratioofCtoNof0.71,neartothe theoreticalvalueof0.75.
Modificationofcarbonnitridebyelementaldopingofsolid materialscanhelpinadjustingthephysicochemicalproperties.Withthisstrategy,onecanchangethetexture,surface chemicalcharacteristics,andtheelectronicpropertiesof carbon-basedmaterials.Directprotonationmethodduring counter-ionadditionisasuitablemodificationtechniqueas thecharacteristicsofthecarbonnitridecouldbemaintained
(originalC/Nmatrixisunchanged).Furthermore,protonation cantunetheelectronicbandgapstoenhancetheionicconductivityofthematerial.Protonationcouldalsohelpin achievingfinedispersionofthematerialsthatcanthenmake characterizationandprocessingsimpler.Sulfurdopinghas emergedasanalternativetoprotonationthatcouldalso modifythesurfaceareaandmorphologyofg-C3N4 allowingit tobeusedincatalyticprocesses.
Involvinganionicliquidof1-butyl-3-methyl-imidazolium hexafluorophosphate(BmimPF6)asamildphosphorous sourceforcarbonnitridedopingcouldhelpobtainenhanced generationofphotocurrentbyafactorthatcouldreachavalue offive.Improvedelectricalconductivitycouldalsobeachieveduptofourordersofmagnitude.
RelatedtothepossibilityofdissolvingmetalsaltsintogC3N4 matrix’snitrogencavities,theincorporationofironions intothecarbonnitridematrixcouldhelpavoiddamagingthe hostgraphiticstructure.Inaddition,ironintroductioncould stronglymodifythefinalproduct’selectronicproperties.
Graphiticcarbonnitrideof0-dimensionalquantumdots couldshowbetterperformancetowardsMORincomparison tothosewith1-dimensionalnanorodsor2-dimensional nanosheetsstructure.Factorsthatcontributetothe enhancementincludemaximumactivesiteatomicpercentagesofpyridinicNandabundantedgesinnano morphology.Furtherimprovementofthecarbon-based materialspropertycouldbeattainedbysupportingiton conductingpolyaniline.Inthisway,bettercatalyticperformanceintheformofbetterelectricalconductivityandCO poisoningresistivitycouldbeobservedintheresulting combinationforMORbecauseoftheelectrostaticinteraction betweenthetwocomponents.
UsingPtbyitselfasanelectrocatalystisachallengeasit hasslowoxidationkineticsanditispronetopoisoningbyCO speciesproducedduringthemethanoloxidationreactions.In addition,anodeplatinumcatalystsalsohaveanissueof maintainingstabilityinthelongrun.Nanoparticlesofplatinumcouldmigrate,aggregate,andundergoOstwald ripeningleadingtopoorelectrochemicalperformance.Approachessuchasalloyingofplatinumwithothermetalsand supportingplatinumongoodsupportmaterialswithhigh conductivity,durabilityandsuitableporestructurecould leadtoenhancedmasstransfertotheactivesites.For instance,preparingaPt/CNx compositecanhelpachieve largeelectrochemicalsurfaceareaasPtNPsarewidely dispersedonCNx sheets.Asaresult,lowonsetpotentials, highmassactivity,betterpoisontolerancetowardscarbonaceousspecies,andreliablelong-termstabilitycouldbe demonstratedbyPt/CNx incomparisontocommercially availablePt/C.Synthesizingg-C3N4 materialsintheformof poly(triazine)imidecarbonnitride(PTI/LiþCl )andusingitto supportplatinumcanenhancethedurabilitybymaintaining 81%oftheECSAduringMOR.SupportingPtong-C3N4 nanosheetsandgraphenecouldhelpbuildlargeporesofdifferent sizestopromotethereactantstransportationtotheelectroactivesitesaswellassuitableenvironmentformaintaining goodelectricalconductivity.Inaddition,Pt/G3-(CN)7 displayedthehighestforwardanodicpeakcurrentdensity
relativetootherPt/G-CNelectrocatalystwithdifferentG/CN ratiosforMOR.Increasingtheg-C3N4 nanosheetscontentto 90%alsoimprovedthecatalystabilitytoresistpoisoning species.
Alloyingplatinumwithrutheniumandsupportingthem onaVulcanXC-72carbonblackcoatedwithgraphiticcarbon nitridenanosheetcouldhelpestablishanevendispersionof smallnanoparticlesofPtRuandfabricateacatalystthatis highlystableasaresultofastronglinkthatispresentbetweenthecompositesupportandthebimetallicnanoparticles.AlloyingPtwithgold,ontheotherhand,and depositingthetwometalsonthegraphiticcarbonnitride nanosheetssurfacecoulddemonstrategreateractivityrelativetoPt/CN.Illuminatingtheelectrocatalystwithvisiblelight furtherimprovedboththecurrentdensityandstabilityofthe composite.Replacingplatinumwithpalladiumandsupportingthemetalonreducedgrapheneoxideandgraphiticcarbon nitridehybridsupportenabledtheachievementofremarkablyhighvalueofforwardpeakcurrentdensities,largeECSA, andstableMORperformanceforlongduration.Thefactors thatcontributetothesatisfactorypropertiesarebecauseof theexistenceofgoodrGOconductivity,mesoporousstructure withhighvalueofspecificsurfacearea,finepalladiumNPs dispersion,andgreatstabilitybecauseofthecovalentlinks thatarepresentbetweenreducedgrapheneoxideand graphiticcarbonnitride.Formingbimetallicnanoparticlesof platinumandpalladiumandfabricatingcarbonblackmodifiedwithg-C3N4 couldalsohelpindevelopingextraordinary catalystswithlong-termstabilityandsatisfactoryactivity. Theimprovementistheresultofacombinationofthetemplateeffect,complementaryrolesofplatinumandpalladium, highspecificsurfaceareaofcarbonblack,andthecovalent interactionbetweeng-C3N4 andcarbonblack.Thetemplate effectispromotedbythegraphiticcarbonnitrideplanar aminogroupresultinginhighlydisperseddecorationof nanoparticlesofPt-Pd.
Regardingtheuseofnon-noblemetals,supportingironon g-C3N4 allowsthemetalatomstobeeasilycapturedbythe carbon-basedsupport.Theresultisanelectrocatalystthat couldefficientlybreaktheO-HbondinCH3OHshowingagreat potentialfornon-noblemetalMORcatalysis.Thecombination alsominimizedtheadsorptionenergyofCO2 tolessthanthat forCOthatfurthersignifiestheabilitytoreduceCOtoxicityto iron.
Eventhoughsignificantadvancementshavebeenestablishedrelatedtotheutilizationofg-C3N4 withanarrayof materialsbasedonthepropertiesandimprovements mentionedabove,furtherinvestigationsarestillnecessaryto designcomplexstructuresachievingasatisfactoryperformanceformethanoloxidationreaction.Itisimportantto undertakedetailedmechanismanalysisofthechargetransfer processandthereactionpathwaystakentoprogressfurther inthefield.Asabetterunderstandingofthereactionmechanismandmolecularlevelinteractionsatthecatalystsurface aredeveloped,itwouldbepossibletoenvisionthedevelopmentofasuitablecatalystinthefuturethatisactiveand stableforMORreactionleadingtolargescalecommercial applications.
Declarationofcompetinginterest
Theauthorsdeclarethattheyhavenoknowncompeting financialinterestsorpersonalrelationshipsthatcouldhave appearedtoinfluencetheworkreportedinthispaper.
Acknowledgements
Theauthor(s)wouldliketoacknowledgethesupportfrom QatarUniversityinternalgrantQUCG-CENG-19/20-7.
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