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CARBON-BASED NANOMATERIALSAND NANOCOMPOSITES FORGASSENSING
CARBON-BASED NANOMATERIALSAND NANOCOMPOSITES FORGASSENSING
Editedby
NAVINCHANDRAGOPALSHIMPI
DepartmentofChemistry,UniversityofMumbai,Santacruz(East), Mumbai,Maharashtra,India
SHILPAJAIN
DepartmentofChemistry,JaiHindCollege,Churchgate, Mumbai,Maharashtra,India
Elsevier
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Listofcontributors
ShilpaJain DepartmentofChemistry,JaiHindCollege, Mumbai,Maharashtra,India
MohammadKhalid GrapheneandAdvanced2DMaterials ResearchGroup,SchoolofEngineeringandTechnology,Sunway University,JalanUniversity,SubangJaya,Malaysia
JolinaRodrigues DepartmentofChemistry,Universityof Mumbai,Santacruz(East),Mumbai,Maharashtra,India
TanushreeSen DepartmentofChemistry,Universityof Mumbai,Santacruz(East),Mumbai,Maharashtra,India
AksharaPareshShah DepartmentofChemistry,Universityof Mumbai,Santacruz(East),Mumbai,Maharashtra,India
NavinchandraGopalShimpi DepartmentofChemistry, UniversityofMumbai,Santacruz(East),Mumbai,Maharashtra, India
GolnoushZamiri CentreofAdvancedMaterials,Mechanical Engineering,FacultyofEngineering,UniversityofMalaya,Kuala Lumpur,Malaysia
Listofcontributors.................................................xi
Abouttheeditors.................................................xiii
Preface...................................................................xv
Part1Introductiontocarbonbased nanomaterials.......................................1
1Carbon-basedsmartnanomaterials.......................3
ShilpaJainandNavinchandraGopalShimpi
1.1Introductiontocarbon-based nanomaterials.................................................3
1.2Typesofcarbonnanomaterials.....................6
1.2.1Carbonnanotubes.................................6
1.2.2Fullerenes...............................................8
1.2.3Graphene...............................................9
1.2.4Carbonnanofibers...............................11
1.3Synthesismethodologiesandvariations......12
1.4Gassensorsandtheircomparison.............16 References...........................................................19
2Carbonnanomaterials-basedgassensors.........25
ShilpaJain,AksharaPareshShahand NavinchandraGopalShimpi
2.1Typesofgassensorsbasedon carbon-basednanomaterials.......................26
2.1.1Electrochemicalsensors.....................27
2.1.2Electrical/chemiresistivesensors.......27
2.1.3Mass-sensitivegassensors................29
2.1.4Thermometric(calorimetric) gassensors..........................................30
2.2Parametersofgassensor............................31
2.2.1Sensitivity............................................31
2.2.2Selectivity.............................................32
3.6.1Carbonnanotubesandmetal
3.7.1Grapheneandmetalormetaloxide nanocompositeforgassensor...........68
3.7.2Grapheneandpolymers nanocompositesforgassensor.........72
3.8Conclusion....................................................73
Acknowledgment................................................74 References...........................................................74
Part2Applicationofcarbon
4Carbonnanotube-basedgassensors..................83
TanushreeSenand
NavinchandraGopalShimpi
4.1Introduction..................................................83
4.2Sensingmechanism.....................................85
4.3Carbonnanotube/metal nanocomposite-basedgassensors............87
4.4Carbonnanotube/semiconducting metaloxidenanocomposite-based gassensors...................................................89
4.5Carbonnanotube/conductingpolymer nanocompositesforgassensors................93
4.6Functionalizedcarbonnanotubesas gassensors...................................................97
4.7Conclusionsandoutlook.............................98
References...........................................................99
5Carbonnanofiber-basedgassensors................105
JolinaRodrigues,ShilpaJain, NavinchandraGopalShimpiand AksharaPareshShah
5.1Introduction.................................................105
5.2Methodsofcarbonnanofiber preparation..................................................106
5.2.1Electrospinning..................................106
5.2.2Catalyticthermalchemicalvapor depositiongrowth.............................107
5.2.3Substratemethod..............................109
5.3Fabrication/constructionofcarbon nanofibers....................................................110
5.3.1Carbonnanofibersmodified withmetaloxides..............................110
5.4Carbonnanofibersasgassensors.............111
5.4.1ZnO/CNFs.........................................111
5.4.2Sn SnO2/CNFs................................111
5.4.3CNFs/polystyrene............................113
5.4.4SnO2/CNFs.......................................114
5.4.5V2O5/CNFs........................................114
5.4.6Au-Pt/CNFs.......................................115
5.4.7Multifunctionalcarbon nanofibers........................................116
5.4.8Mesoporouscarbonnanofibers.....117
5.4.9WO3/CNFs........................................117
5.4.10Ni/CNFs............................................118
5.4.11CNFs/PPy..........................................119
5.4.12WS2/CNFs.........................................119
5.4.13Ni-CNF..............................................119
5.4.14Graphiticcarbonnanofibers...........121
5.4.15Graphitic-carbonnanofibers/ polyacrylate.....................................122
5.4.16PAN/(PAN-b-PMMA)........................122
5.4.175,6;11,12-di-o-phenlyenetetracene/ carbonnanofibers...........................123 References..........................................................125
6Graphene-basedgassensors..............................127
AksharaPareshShah,ShilpaJainand NavinchandraGopalShimpi
6.1Gassensormechanism..............................129
6.2Grapheneanditsderivative/metalbasedgassensor.........................................131
6.3Grapheneanditsderivative/metal oxide-basedgassensor..............................134
6.4Grapheneanditsderivative/polymer basedgassensor.........................................140 References..........................................................142
73DHierarchicalcarbon-basedgassensors....149
JolinaRodrigues,ShilpaJainand
NavinchandraGopalShimpi
7.1Introduction.................................................149
7.2Importanceof3Dnanomaterial.................150
7.3Construction/fabricationof3D architectures................................................151
7.43-Dmetaloxide/graphene nanocompositeasgassensors..................153
7.53-Dfunctionalizedgraphene nanocompositeasgassensors..................163
7.63-Dmetaldopedgraphene nanocompositeasgassensors..................166
7.73-Dmetaloxide/carbonnanotubeand metaloxide/grapheneoxide/carbon nanotubenanocomposite asgassensors.............................................167
7.7.1Sensingmechanismsof3DTiO2/ graphene-carbonnanotubes gassensors........................................169
7.83Dmetaloxide/carbonnanocomposite asgassensors.............................................171
7.93Dgraphene-basedgassensors................174 References..........................................................178
8Conductingpolymer-basedgassensors...........181
JolinaRodrigues,ShilpaJainand
NavinchandraGopalShimpi
8.1Introduction.................................................181
8.2Conductingpolymers-basedgassensors...182
8.3Polyanilineasagassensingmaterial.......183
8.4Polypyrroleasgassensingmaterial..........190
8.5Polythiopheneasgassensingmaterial.....207 References..........................................................228
9Futureprospects:carbon-based nanomaterialsandnanocomposites..................233
ShilpaJain,NavinchandraGopalShimpi andAksharaPareshShah
References..........................................................237
Abouttheeditors
Dr.NavinchandraGopalShimpi hasbeenworkingasan ProfessorattheDepartmentofChemistry,UniversityofMumbai, Mumbai,sinceApril2014.Previously,hewasassociatedwiththe UniversityInstituteofChemicalTechnology,Jalgaon.HecompletedPhDfromNorthMaharashtraUniversity,Jalgaon,in2006. HewastherecipientofYoungScientistAwardfromAsianPolymer Associationin2014andDnyanjotiPuraskarin2008fromShirsathe Foundation,Jalgaon.Sofar,hehaspublishedmorethan100 papersininternationaljournalsofgoodimpactfactoranddeliveredmorethan40lecturesasaninvitedspeaker.Hehasgenerated Rs1.65croresforoutstandingresearchfromvariousfundingagencies.HeisatpresenthandlingoneresearchprojectfromUGC, NewDelhi,andoneconsultancyprojectfromIndofilChemicals Ltd,Thane.Sofar,fourteenstudentshavecompletedtheirPhD, andeightaredoingtheirPhDunderhisguidance.Heishaving twograntedpatents,andfourareunderexamination.Moreover, hehasguided15studentsfortheirMTechdissertation.Besides this,hehasorganizedfivenational andinternationalconferences withfivestaffdevelopmentprogramsandfourprofessionalcertificatecourses.Heisanassociateeditorofthe InternationalJournal ofChemicalStudies andworkedasaleadguesteditorfor AdvancementinPolymericNanomaterialsandNanocomposites,a specialissueof InternationalJournalofPolymerScience
Dr.ShilpaJain hasbeenworkingasanassistantprofessorat theDepartmentofChemistry,JaiHindCollege,Universityof Mumbai,Mumbai,sinceNovember2016.Shehasdoneher PhD.undertheguidanceofDr.NavinchandraShimpifromthe DepartmentofChemistry,UniversityofMumbai.HerPhD. topicwas“Techniquedevelopmentinsynthesisofsmartnanomaterialsandtheirapplicationinsensing.”Herresearchareas includenanomaterials,nanotechnology,gassensors,andpolymernanocomposites.Shehasworkedonhybridnanomaterials andnanocompositesandtheirapplicationinhighlyefficient gassensors.Herotherareasofresearchinterestare morphology-dependentsensing,carbon-basednanostructures, andnanocatalysts.Shehaspublished10researchpapersin variousinternationaljournalswithhighimpactfactorhaving morethan200citations.
Preface
Carbon-basedNanomaterialsandNanocompositesforGas Sensing mainlydealswiththeapplicationofcarbon-based nanomaterials(CBNs)suchascarbonnanotubes,fullerenes, graphene,andcarbonnanofibersandtheirnanocompositesin gassensing.Variousnanomaterialssuchasmetaloxides,semiconductingmetaloxides,polymers,nanocomposites,andCBNs havebeenstudiedextensivelyforthedetectionofvariousgases. CBNsareconsideredasasubstituteofexpensiveelectronic gradesemiconductorsbecauseoftheirextraordinaryproperties andeasymanufacturing.Theyareconsideredasoneofthe primemembersofsmartnanomaterialsandadvancednanotechnology.CBNswithhighaspectratio,highcarriermobility, uniquestructure,andpropertiesareexcellentchemicalandbiologicalsensors.Carbonnanostructureswithinherentproperties becomeanidealsensingmaterialforthenextgenerationof sensortechnology.Gassensorshavebecomeessentialcomponentinseveralindustries,processcontrolunit,andenvironmentalmonitoring.Thisbookbeginswiththedescriptionof CBNs,theirtypes,synthesismethodologies,properties,and theirapplicationingassensing.Inaddition,itprovidesanoverviewofvariousCBNsandtheirapplication,challenges,and opportunitiesforhighlyefficientgassensors.Thisbookfocuses ontheuniquecharacteristicoftheseCBNs,whichenhancesthe selectivitytowardaspecificgasandcanbeeasilytailoredwith functionalizationanddoping.Italsodescribesvarioustypesof gassensorsbasedonCBNsandtheirfabricationandsensing mechanism.Itfurtherdiscussesthemodificationsinmicrostructure,doping,functionalization,andhybridnanocompositeswithconductingpolymerstoattainselectivityand sensitivitytowardparticulargaswithmaximumgasresponse.
ThisbookgivesabroadideaaboutvarioustypesofCBNs, theirproperties,synthesismethodology,andapplicationingas sensing.Itintroducesthereaderswiththerecentdevelopments, technology,andimportanceofCBNsinhighlyefficientand smartgassensors.Also,itcoversseveralaspectsintermsof societal,academic,industrial,andresearchbenefits.Thisbook willbeofgreatimportancenotonlytothelearnersbutalsoto themoreexperiencedresearchers,researchscholars,andstudentsofpostgraduateandgraduatelevels.Theintentionbehind
editingthisbookistointroducethereaderstothefascinating worldofcarbon-basednanostructuresandtheirapplicationin gassensing.Basedonthetheme,thisbookisdividedintotwo parts: Partone:IntroductiontoCarbonbasedNanomaterials, whichcontainsthreechapters andParttwo:ApplicationofcarbonNanomaterialsingassensing,whichcontainsfivechapters.
Chapter1presentsabriefintroductiononCBNsandtheir types,properties,andsynthesistechniques.Itgivesabriefcomparisononvarioustypesofgassensors.
Chapter2presentsindetailtheparametersofgassensors, varioustypesofgassensors,theirsensingmechanism,fabrication,andfunctionalizationofCBNs.
Chapter3presentsinbriefthedetectionmechanism,carbon nanomaterials,andnanocompositesforsensingsuchascarbon black,carbonnanofibers,carbonnanotubes,andgraphene.The nanocompositeswithpolymersarediscussedforgassensing.
Chapter4dealswithcarbonnanotube-basedgassensorsin detail.Theirsensingmechanism,synthesismethodologies, characterization,andfunctionalizationarediscussedthoroughly.Inaddition,thischapterfocusesonCNT-basednanocompositeswithmetals,semiconductingmetaloxides,and conductingpolymerssuchaspolyanilineandpolypyrrole.All theseCBNsandnanocompositesarediscussedforbeingused ashighlyefficientgassensors.
Chapter5focusesoncarbonnanofiber-basedgassensors. Methodsofnanofiberpreparationsuchaselectrospinning, vapordepositionandsubstratemethod,characterization,and functionalizationarediscussed.Nanocompositesofcarbon nanofiberswithmetals,semiconductingmetaloxides,andconductingpolymersarediscussedwiththeirgassensingcapabilities.Thischapteralsocoversmesoporousandgraphiticcarbon nanofibersandtheircompositeswithpolymers.
Chapter6focusesongraphene-basedgassensors.Sensing mechanism,fabrication,characterization,andextraordinary sensingcapabilitiesofgrapheneanditsderivativesareindicated.Nanocompositesofgrapheneanditsderivativeswith metals,metaloxides,andconductingpolymersarediscussed withleadstohighlyselectiveandsensitivegassensorswith maximumresponse.
Chapter7dealswith3Dhierarchicalcarbon-basedgassensors.Theirimportance,synthesismethodologies,andfabricationof3Darchitecturesarediscussed.Furthermore, nanocompositeswithmetals,semiconductingmetaloxides,graphene,anditsderivativesarediscussedaseffectivegassensing materials.3DhierarchicalcarbonwithCNT,metal-doped
graphene,metaloxide/grapheneoxide,metaloxide/CNTare discussedinthischapter.Suchfunctionalizednanocomposites arepromisingcandidatesforsmartsensors.
Chapter8coversconductingpolymersandtheirapplication ingassensing.Conductingpolymerssuchaspolyaniline,polypyrrole,andpolythiophenearestudiedindetail.Theirsynthesis technique,thinfilms,characterization,doping,andfunctionalizationarediscussed.Theiradvantageinroomtemperature sensingisexploredwithhighlyselectiveandsensitivegassensorwithmaximumresponse.
Chapter9presentstheadvantages,challenges,andopportunitiesassociatedwiththeuseofcarbon-basednanostructures andtheirapplicationingassensing.Itdealswiththefuture prospectsofcarbon-basednanostructuresassmartgassensors.
NavinchandraGopalShimpi ShilpaJain
Carbon-basedsmart nanomaterials
ShilpaJain1 andNavinchandraGopalShimpi2
1DepartmentofChemistry,JaiHindCollege,Mumbai,Maharashtra,India
2DepartmentofChemistry,UniversityofMumbai,Santacruz(East),Mumbai, Maharashtra,India
Nanoscienceandnanotechnologyaidincontrollingmatterand buildingitsunitsatombyatom,layerbylayerintomoreadvanced systems.Itinvolvessubsequentfindingsofpropertiesandphenomenaatnanoscale(10 9 m)andmanipulationofmaterialsat nanometersizes.Nanotechnologyenablesworkingatmolecular levelsandcreatingthelargerstructurewithfundamentallynew molecularorganizationandproperties [1 4].Rapidlygrowing technologynotonlyrequiresminiaturizationofdevicesbutalsoan ultimateperformancewithspecificfunctionalityandselectivity. Therecenttrendintechnologyrequiressmartnanomaterialsand systemswithspecificfunctionalityandthoroughunderstandingof theirproperties.Recentadvancementinanalysistechniqueshas enabledexaminationandprobingofatomsandmoleculeswith greatprecision,leadingtocomprehensiveunderstanding,expansion,anddevelopmentofnanoscienceandnanotechnology. Withnanodimensions,materialsurfacetovolumeratioincreases manifold,leadingtoanemergenceofquantumsizeeffectsand thesurfaceatomeffects.Thiscanchangeorenhancechemical reactivity,electronic,optical,mechanical,magneticandtransport characteristicsofnanomaterialascomparedtotheirbulkanalogs [5,6].Owingtotheiradvancedpropertiesandspecificfunctionalities,nanomaterialshaveshownvastuseinseveralfields rangingfromenergystoragedevicestobiomedicalfields [7 9].
1.1Introductiontocarbon-basednanomaterials
Currently,carbon-basednanomaterials(CBNs)arevastly studiedandexploredbecauseoftheirexceptionalelectronic, optical,thermal,andmechanicalproperties.TheseCBNshave shownvastapplicationsinmaterialscience,energystorage
1
[16].
devices,aerospace,optoelectronics,catalysis,lightemitters,biotechnologyandsensorsetc. [10 15] Fig.1.1 illustratestheapplicationofCBNsinvariousfieldsbasedontheirproperties [16]
Owingtotheirexceptionallyhighmechanicalstrength,opticalproperties,electrical,magneticandthermalconductivity, CBNsareconsideredasoneoftheprimemembersofsmart nanomaterialsandadvancednanodevices.Carbonitselfis aninimitableuniqueelementwiththerelativeabundanceof 180 270ppm [17].Carbonisthemostsignificantelement(after oxygen)inthehumanbody [18],earthcrust(17thinrelative abundance),andtheentireuniverse(6thmostcommon) [19]. Carbonhasauniquepropertyofcatenationanditcanform severalmetastablephasesatambientconditions.Conventionally, carbonhastwoallotropesknownasgraphiteanddiamond(crystallineform),whichareentirelydifferentfromeachotherin crystalstructureandproperties.Diamondisanelectricallyinsulatingandhardestnaturalsubstanceknown,whilegraphiteis softandconducting [20 22].Thevariousnanocrystallineforms ofcarbonwerediscoveredattheendof20thcenturywith
Figure1.1 Propertiesandapplicationsofcarbon-basednanomaterialsinvariousfields
progressinnanotechnology.Fullerenes(C60)werediscovered byHaroldKroto,RichardSmalleyandRobertCurlin1985ina sootyresidueofvaporizedgraphiteintheheliumatmosphere. Fullerenescanbedescribedascagelikemoleculeswith60carbonatoms(heldbysingleanddoublebonds).Fullerenescan beimaginedashollowsphereresemblingfootballwith12pentagonaland20hexagonalfaces.Consideringtheproperties andapplicationinvariousfields,thetriowasawardedwiththe “NobelprizeinChemistry”in1996fortheirdiscoveryandsynthesisoffullerenes.Theirdiscoveryfueledintenseresearchof CBNsandfurtherin1991,aJapanesephysicistSumioIijima discoveredcarbonnanotubes(CNTs)usinganarcdischarge methodandstartedaneweraofcarbon-basedsmartnanomaterials.DiscoveryofCNTanditsextraordinarypropertiesacceleratedthegrowthofCBNs.Furtherin2004,AndreGeimand KonstaninNovoselovextractedsingleatomthickcarbontermed as“graphene”fromgraphiteusingmicromechanicalcleavageor scotch-tapemethodattheUniversityofManchester.Theywere awardedknighthoodand2010Nobelprizeinphysicsfortheir discoveryofthewondermaterialgraphenes.
Thevariousnanocrystallineformsofcarbonconsistofcarbonnanotubes,fullerenes,carbonfibers,nondiamond,andgraphene,whichareillustratedin Fig.1.2.
Figure1.2 Differenttypesofcarbonnanostructures.
Table1.1Comparisonofsomepropertiesofcarbon-basednanomaterials [30 32].
1.2Typesofcarbonnanomaterials
CBNscanbebroadlyclassifiedasCNTs,fullerenes [23,24], carbonnanofibers(CNFs),graphene [25,26],anditsderivatives suchasgrapheneoxide,reducedgrapheneoxide,andquantum dots [27 29].PropertiesofCBNscomparedwithgraphiteare shownin Table1.1.
1.2.1Carbonnanotubes
Carbonnanotubesaredefinedashollowcylindersofcarbon withthehexagonallatticeofsinglegraphiticsheetsrolledup withinthediameterrangeoffewnanometers.Theyarefurther classifiedassinglewalledcarbonnanotube(SWCNT,diameter , 1nm)ormultiwalledcarbonnanotube(MWCNT,diameter . 100nm).ThediscoveryofCNTiscreditedtoProf.Sumio Iijimawitharevolutionarypaper“Helicalmicrotubesofgraphiticcarbon”reportingMWCNTsfromarcdischargemethod [33].Carbonsourcewasco-vaporizedinthepresenceoftransitionmetalcatalystironinanAr/CH4 atmosphereandtheCNTs werefoundinthedepositedsoot.Furtherin1993,Bethune etal.synthesizedCNTbyvaporizingcarbonmonoxideand graphiteunderheliumatmosphereandacceleratedresearchon CNTs [34].MWCNTscontainsmanytubesofgraphenewhich arerolledmultipletimeswithaninterlayerspacingof3.4A ˚ . Twomodels“RussianDollmodelandSwissrollmodel”are usedtodescribeMWCNTs [35 37] asillustratedin Fig.1.3.
Figure1.3 (A)Swissrollmodel,(B)Russiandollmodelofcarbonnanotubes. [Ref-nanotechweb.org, nanotech-now.com].
Figure1.4 Graphenesheetlatticewiththelatticevector“a”and“b”andtheangles θ and ϕ,whichdeterminethe typeofnanotube.
AccordingtotheSwissrollmodel,asinglesheetofgraphiteis rolledinarounditselfresemblingascrollofparchmentora rollednewspaper.IntheRussianDollmodel,sheetsofgraphite arearrangedinconcentriccylinderslikesmallerdiameter SWCNTwithinalargerdiameterSWCNT [38 40].
Onthebasisofstructure,CNTscanbefurtherclassifiedas armchair,zigzag,andchiraldependingontherollinggraphitic sheetsasshownin Fig.1.4.Thenanotubesaregenerally describedas(n,m),where“n”indicatesthecarbonatoms aroundthetube,and“m”determinestheoffsetofNTswrappingaround.DuringtheformationofcylindricalpartofCNT,
Table1.2Comparisonofsomepropertiesofcarbonnanotubes [41 43].
endsofchiralvectormeeteachotherduringrollingofthegraphenesheet.Thischiralvectoranddifferentvaluesofnandm determinesthechiralityor“twist”oftheCNT.Itispossibleto recognizezigzag(n or m 5 0, θ 5 0degrees),armchair(n 5 m, θ 5 30degrees),andchiralCNTs(0degrees , θ , 30degrees) bytheircross-sectionalstructureandpatternacrossdiameter (Fig.1.4).
Dependingonlength,diameter,chirality,andfunctionalization,CNTscanbemetallicorsemiconductiveandpossess intrinsicsuperconductivity,highthermalconductivity,andfield emissionproperties [41,42].Owingtohighaspectratioand ballisticone-dimensionalelectronictransport,CNTsareconductingwithminimalheatingloss(currentdensitiesupto109 to 1010 A/cm2) [43].Furthermore,reactionswithinCNTsinterwall canredistributecurrentnon-uniformlyovertheindividual tubes.Owingtoexceptionallyhighelectronicandthermalproperties,SWCNTsareusedinsmartminiaturizedelectronics. CNTshaveextraordinarymechanicalpropertiesandtheirmeasuredtensilestrength,rigidityandelasticityaremorethan someindustrialhigh-strengthmaterialssuchaskevlar,hightensilesteelandcarbonfibers.SeveralcompositesofCNTsarealso studiedwithmetaloxides,ceramicsandclaywhichenhanceits multifunctionalnature.Attributabletoitsuniquemechanical, thermal,opticalandelectricalproperties,CNTsshowinteresting potentialinthefieldofelectronics,optics,fieldemission,flat paneldisplays,andreinforcingmaterials [42,44]. Table1.2 shows variousphysicalandelectricalpropertiesofCNTs.
1.2.2Fullerenes
Fullerenesaretheman-madethirdallotropeofcarbonwith structuresimilartofootballandnamedafterarchitectRichard BuckminsterFuller.Theyarecagedmoleculeswiththesheet
Differenttypesoffullerenes.
likehexagonalandpentagonal(orsometimesheptagonal)rings, whichmakesitnon-planarandimpartclosedshellstructure [45].Thestructureoffullerenescomprisesoftruncatedicosahedronbutwithouthexagonalpacking.Thestructureoffullerenes comprisesofhexagons,pentagons(orevenheptagons)which bendsthesheetintospheres,ellipsesorcylinders.Fullerenes consistofabout20hexagonaland B12pentagonalrings(icosahedralsymmetry)withclosedcagestructureandthisstructural andelectronicbondingleadstohighstability [46].Depending onthedifferentnumberofpentagonalandhexagonalrings,an infinitenumberoffullerenescanexist. Fig.1.5 showsthestructuresofvariousfullereneswithsizevariation.Thefullerenes showhighaffinityfororganicmoleculessuchasC60 with60 π electronsareusedasgoodadsorbent.Although,itcan’tadsorb metalions,anionsandpolarmoleculeswhichimpartsselectivityandthereforefullerenescanbeusedaselectrochemicalsensors.Fullerene-basednanostructuresarefoundtobepotential sensingmaterialforthesurfaceacousticwave-andquartz microbalances-basedgassensors.
1.2.3Graphene
Grapheneknownas“Wondermaterial”issingleatomthick carbonmonolayer(sp2-bonded)withhoneycombcrystallattice andinterlayerspacingof3.4A ˚ andbondlengthof1.42A ˚ [25,26].Grapheneisstrongerthandiamondwithextremelyhigh tensilestrength(130GPa).Uniquecovalentlybondedandplanartwo-dimensionalstructurewiththevanderWaal’sforces makesitharderthandiamond,tougherthansteelyetlighter thanaluminum.Grapheneismostattractiveandstudiedmaterialofthepresenteraowingtoitsuniquestructureandextraordinaryproperties.Theedgeandbasalplanesofgrapheneshows
Figure1.5
variantelectrochemicalpropertiessuchasedgehashigherelectronmotion,greaterspecificcapacitanceandcatalyticpower [47].Graphenehassomepeculiarpropertiessuchasoptically transparentwithhighelectronmobility,impermeability,high electricalandthermalconductivityandextremelyhighsurface areaetc. [48 51].Itisexpectedthatdefectfreesinglesheet ofgrapheneshouldpossessexcellenttransportproperties andexperimentallyithasremarkablyhighelectronmobility (200,000cm2/Vs,RT),lowresistivity(10 6 Ω cm),lowerthan metallicsilverandunexpectedlyhightransparency(onlyabsorbs B2.3%ofwhitelight).Owingtotheseextraordinaryelectrical andopticalproperties,extremelyhighsurfaceareaandfree π electrons,itistermedasawondermaterialandidealcandidate forseveralapplications [26,51].Variousderivativesofgraphene suchasgrapheneoxide(GO)andreducedgrapheneoxide(rGO) havedifferentpotentialapplicationsduetothepresenceof free π π electron,aromaticringandreactivefunctionalgroups. GOhasoxygenousfunctionalgroupswithhydroxylandepoxide groupsonitsbasalplaneandcarboxylic,ketoneandaldehyde groupsonitslayeredge [52].Thereareseveralmethods purposedforthestructureofGOnamelytheHofmann,Ruess, Scholz-Boehm,Nakajima-Matsuo,Lerf-KlinowskiandSzabo models [53] asshownin Fig.1.6.Afterdetailedandcomprehensiveanalysisbysolid-stateNMRandX-raydiffraction,the“LerfKlinowski”molecularstructureiswidelyacceptedastherole modelofGO [54].
Duringanoxidationprocess,variousdefects,impurities, structuraldisorders,fragmentation,andotherstructuralattributesaregeneratedwhichinfluenceselectronic,opticaland adsorptionproperties.Further,GOcanbeconvertedintoreduced grapheneoxide(rGO,withcarbontooxygenratioof8:1 246:1) bychemicalorphysicalreductionmethods [20].Anotherderivativeofgraphenewithmicro,meso-andmicroporousstructure knownas“three-Dimensionalhierarchalgraphene”possess uniquefunctionalities.Controllablesynthesiswhichdetermines theporosity,surfacearea,highelectrontransportandsuperior electrochemicalperformancesmakesitaversatilematerial andimpartshighstructuralandmechanicalpropertiesover CNTsandgraphene.Anotherattractivenanomaterialisgraphenequantumdots(GQDs).Theycanbedescribedasazerodimensionalgraphenesheetwithsizelessthan100nminone toafewlayers(3 10) [51].TheGQDsexhibitshighsurface area,hightransparency,highphotoluminescence,excellenthole transportingproperties,andchemicalstabilityduetoquantum confinement [55,56].
Figure1.6 Structuresofpristinegraphene(A)anditsgrapheneoxidederivativesbasedonHofmann(B),Ruess (C),Scholz-Boehm(D),Nakajima-Matsuo(E),Lerf-Klinowski(F),andSzabo(G)models [54].
1.2.4Carbonnanofibers
Carbonnanofibersaretheultralong,thinstrandofcarbon (diameter B10 1000nm)withatomsbondedtogetherincrystalliteandalignedparalleltothelongeraxisofthefiber.Fibers havingdiameterlessthan100nmaregenerallyconsidered asNFs(asperNationalScienceFoundation) [57].CNFsare widelystudiedowingtotheirexceptionalphysicalandchemical propertieswhichissimilartofullerenesandCNTs [58,59]. CNFsinheritshighaspectratio,highthermalandelectricalconductivity,lowdensity,smallernumberofdefects,highspecific
modulusandstrength,whichmakesthempotentialcandidate forvariousapplications.Theuniquecrystalalignmentimparts highmechanicalstrengthtoCNFs.Asthediameterdecreases, surfacefunctionalitiessuchassurfacearea,mechanicaland thermalpropertiesenhancesmanifold [30,60].InCNFs,the graphitelayersformananglewithinnertubeaxis,whichmay beholloworsolidlikeinterior.TherangeofCNFsdiameter is B10to500nmwithlength B10 μm.Duetobasalandedge planes,CNFscanbeeasilyfunctionalizedandtheirsurfacecan bemodifiedaccordinglyformingfunctionalhybridCNF-based nanomaterials.ThesefunctionalizedCNFshavepotentialapplicationinthevariousfieldsofenergy,sensors,biomedicine, nanoelectronics,environmentalscienceandtissueengineering [31,32,61,62].
Polyacrylonitrile(PAN)isoneofthemostcommonpolymer forthesynthesisofCNFs.PANhasseveraladvantagesover otherprecursorssuchasstability,spinnabilityandability forlargescaleproduction.PANisextensivelyexploreddueto itshighcarboncontentandabilitytotailorCNFsstructurewith variousfunctionalities,dopingandsurfacemodifications. VariousPANbasedprecursors,compositesandblendsaresynthesizedfromPANhomopolymerforfunctionalizedCNFs. NanocompositesoftheseCNFswithmetals,SMOs,polymers, CNTsetc.arestudiedforvariousapplications.Thesenanocompositeshaveextremelyhighsurfaceareaandexcellentthermal, mechanical,electricalandopticalproperties.Theyhavepotentialapplicationingassensors,biosensors,tissueengineering andenergystoragedevices.
1.3Synthesismethodologiesandvariations
ThereareseveralmethodsforthesynthesisofCBNs,but thesearebroadlyclassifiedasthetop-downandbottom-up approach.Intop-downapproach,carbonnanomaterialslike CNTs,grapheneandCNFsetc.aregeneratedfrombulkcarbon materialsuchasgraphite.Severalmethodssuchaslaser ablation,chemicalexfoliationandarc-dischargemethodetc. areusedastop-downapproach.Whilebottom-upapproach includeschemicalvapordepositionandpyrolysistechniques, whereCNTs,graphene,andCNFsetc.aresynthesizedfrom simplehydrocarbons.
Intop-downapproach,thearc-dischargemethodisoneof theprimarymethodsusedforthesynthesisofCNTsandfullerenes [33,63].Inthismethod,hightemperatureisgenerated
usinganelevatedcurrent(B20 25V)betweenthetwocarbon electrodeswhichinstigatesvaporizationofoneofthecarbon electrodes.Thesecarbonvaporsgetdepositedonthesurface oftheotherelectrode(rod-shapedcarbongrowth)leadingto CNTsandfullerenesaccordingtoreactionconditions.Thecarbonrodsinanarc-vaporizationchamberareplacedendtoend separatedbyadistanceof1mminaninertgasatmosphere atlowpressure.Fromarc-dischargetechnique,althoughCNTs areobtainedinhigheryieldbutthemixtureofcomponents areobtainedwhichentailsfurtherpurificationtechniquesand removalofresidualcatalyticmetalsfromcrudeproduct [6,64]. Anotherfamoustop-downtechnique“Laserablation”involves removalofmatterfromthesurface(evaporationorsublimation) usingalaserbeamatthelowerfluxinaclosedchamberathigh vacuumoraninertatmosphere.Normallylaserisusedincontinuouspulses,sothelaserintensityisusuallykepthighto carryoutlaserablationofmaterial.In1995Guoetal.studied laserablationtechniqueusingablockofgraphiteandfurther withcatalyticmetalinit [65,66].Differentcatalyticmetalssuch ascobalt,niobium,platinum,nickelandcopperetc.wereused forthegrowthofCNTsfromcarbonplasmastate.Inbottomup approach,chemicalvapordeposition(CVD)isanimportant techniqueforthesynthesisofvariousCBNs.InCVD,carbonin itsgaseousformisdepositedonasubstrateatambienttemperature.Severalfactorssuchasnatureofgas,catalyst,pressure andtemperatureofthereactionchamberinfluencesthegrowth ofCBNs.Hydrocarbonssuchasmethane,acetyleneand xyleneetc.areusedasprecursorgasforthesynthesisofCNTs andgraphenealongwithcarriergases [67,68].Thesegasesat hightemperaturereactsanddecomposes,whenintocontact withaheatedsubstrateresultinginsolidgrowthontothesubstrate.AlthoughthehistoryofCVDtechniquedatesbackto the19thcentury,butin1890someFrenchresearchersstudied thegrowthofcarbonfilamentsusingcyanogen(overred-hot porcelain)whichacceleratedtheinterestinCVDtechnique [69]. In1993Yacamanetal.usedCVDtechniquetogrowMWCNTs fromacetylenegasusingironparticlesascatalystat 600 C 800 C [70].SinceSWCNTshavehigherenergyofformation,thetemperatureoftheCVDchambershouldbenotably higher(900 C 1200 C)andprecursorgaseslikecarbonmonoxideormethaneareusedowingtotheirsuperiorstability(comparedtoacetylene)athighertemperatures.InCVDtechnique, propertiesofCNTssuchaslength,shape,chirality,defects, diameterandgraphitizationetc.canbetailoredandcontrolled usingseveralfactorssuchasprecursorgas,temperatureand
