METALLIC NANOPARTICLES

DRAGOMIRMIRELAUNIVERSITYOFNOVAGORICA

DOCTORALSTUDY,PROGRAMMEPHYSICS1

INTRODUCTIONTOTHENANOWORLD

WHATISNANO?

Theprefixnano isvariouslysaidtoderivefromtheGreekwordo ortheLatinwordnannus,bothmeaningdwarf.ItwasadoptedasanofficialSIprefix,meaning109 ofanSIbaseunit,atthe11th

ConfrenceGnraledesPoidsetMesures(CGPM)in1960(althoughithadinformalstatusbeforethat).

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Nanoscience thestudyofthephenomenaat1100nmNanomaterials thosewhichhavestructuredcomponentswithatleastonedimensionlessthan100nm

ZeroDimensionalNanostructuresNanoparticlesOneDimensionalNanostructuresNanowiresandNanorodsTwoDimensionalNanostructuresThinFilms

Nanoparticles arenanosized structuresinwhichatleastoneofitsphaseshasoneormoredimensions(length,widthorthickness)inthenanometersizerange(1to100nm)asdepictedinfigure1.

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Figure1.Depictionofthesizeregimeofnanoparticles relatedtocommonnano scaleobjects;Aprokaryoticcell,Bultravioletwave,Cvirus,Denzyme.Bluespheresrepresenta50nmmetal

nanoparticle [RobinJ.WhiteChem.Soc.Rev,2009,38].

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2.METALLICNANOPARTICLES Thetermmetalnanoparticle isusedtodescribednanosized metalswith

dimensions(length,widthorthickness)withinthesizerange1100nm.

Theexistenceofmetallicnanoparticles insolutionwasfirstrecognizedbyFaradayin1857andaquantitativeexplanationoftheircolour wasgivenbyMiein1908.

Figure2. Thegrowthofnumberofpublicationsdealingwithmetal nanoparticles [JuanM.CampeloChemSusChem2009,2]

Figure3.Interestinspecificelementsinthepreparationofnanoparticles overthepast17years[J RobinJ.WhiteChem.Soc.Rev,2009,38]

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ThemaincharacteristicsofMNPslargesurfaceareatovolumeratioascomparedtothebulkequivalents;

largesurfaceenergies

thetransitionbetweenmolecularandmetallicstatesprovidingspecificelectronicstructure(localdensityofstatesLDOS);

plasmonexcitation;

quantumconfinement;

shortrangeordering;

increasednumberofkinks;

alargenumberoflowcoordinationsitessuchascornersandedges,havingalargenumberofdangling bondsandconsequentlyspecificandchemicalpropertiesandtheabilitytostoreexcesselectrons. 6

DensityofStatesFigure4.Relationshipbetweennanoparticle size,energyandtheprincipleofenergy

ofstates[RobinJ.WhiteChem.Soc.Rev,2009,38].

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SurfacePlasmonResonance

Figure5.Oscillationofametallicnanoparticles electroncloud(red)relativetothemetalcore(blue)inresponsetotheelectromagneticfield;thebasisforthesurfaceplasmon resonanceeffectobservedinnanoparticles [RobinJ.WhiteChem.Soc.Rev,2009,38].

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Localfield factor TheSPRcan be simply formalized,inafirstapproach,bysolving Laplaces

equation inthecaseofasingleconducting sphere surrounded byahomogeneous transparentmedium,with theappropiate continuity relationsatthemetaldielectric interfaceandassuming that thesphere radiusis muchlower than thewavelength (quasistatic approximation).Thehomogeneouslocalelectric field inside theparticle,El :

Eq.1:

where: E0 is theapplied field m =dielectric function ofmetal d =dielectric function ofthehostmedium

Thelocalfield factoris defined astheratioofthelocalfield totheapplied one:

Eq.2: f =El/E0

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SYNTHESISMETHODS

A. CHEMICALMETHODS

A.1.ChemicalreductionofmetalsaltsA.1.1.Thealcoholreductionprocess

A.1.2.Thepolyolprocess

A.2.Microemulsions

A.3.Thermaldecompositionofmetalsalts

A.4.Electrochemicalsynthesis

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B.PHYSICALMETHODS

B.1.ExplodingwiretechniqueB.2.PlasmaB.3.ChemicalvapourdepositionB.4.MicrowaveirradiationB.5.PulsedlaserablationB.6.SupercriticalfluidsB.6.SonochemicalreductionB.7.Gammaradiation

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Goals and Problems inMetallicNanoparticlesSynthesis

Ideally, metallicnanoparticlesshouldbepreparedbyamethodwhich:

isreproducible

maycontroltheshapeoftheparticles

yieldsmonodispersemetallicnanoparticles

iseasy,cheap

uselesstoxisprecursors:inwaterormoreenvironmentallybenignsolvents(e.g.ethanol)

use theleastnumberofreagents

usea reactiontemperatureclosetoroomtemperature

withasfewsyntheticstepsaspossible(onepotreaction)

minimizingthequantitiesofgeneratedbyproductsandwaste.

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A.1. Chemical reduction of metal salts

In1857MichaelFaradayreportedasystematicstudyofthesynthesisandcolorsofcolloidalgold.

In1951,J.Turkevichreproducedstandardprotocolsforthepreparationofmetalcolloids(wasrefinedin1970byG.Frens)

Figure 6.The formation ofmetalcolloidsbythesaltreductionmethod [Helmut Boennemann Eur.J.Inorg.Chem, 2001].

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1. Nucleation takesplacebecausethesupersaturatedsolutionisthermodynamicallyunstable.Forthenucleationprocesstooccur,thesolutionmustbesupersaturatedinordertogenerateanextremelysmallsizesolparticle.

NucleationandGrowth(LaMer)

Figure 7. The concept of monodisperse colloid growth of la Merr model (A) and typically synthetic apparatus (B) [JohnA.Blackman Metallicnanoparticles,Elsevier,2009].

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NucleationandGrowth(LaMer)

2.Growth Afterthenucleiareformedfromthesolution,theygrowviadepositionofthesolublespeciesontothesolidsurface(molecularaddition).

Therelativeratesofgrowthofsmallandlargeparticlesaredifferentwhenthereactantsaredepletedduetoparticlegrowth.

Secondary growth thegrowthofparticlesbyaggregation

isfasterthanthatbymolecularaddition

itoccursbystableparticlescombiningwithsmallerunstablenuclei

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StabilizingMetallicNanoparticles

by placing them in an inert environment an inorganic matrix or

inorganic capping materials figure 8

polymer

Figure 9 . Steric stabilization of nanostructured metal colloids

[Helmut Boennemann Eur.J.Inorg.Chem.2001]

Figure 8. Electrostatic stabilization of metal colloid particles. Attractive van der Waals forces are outweighed by repulsive electrostatic forces between adsorbed ions and associated counterions at moderate interparticle separation[GunterSchmid ClustersandColloids,VCH,1994]

by adding surface-protecting reagents organic ligands figure 9

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A.1.Chemicalreductionofmetalsalts

Table1.Summaryofprecursors,reductionagentsandpolymeratabilizers[GuozongChaoNANOSTRUCTURESANDNANOMATERIALS,ImperialCollegePress,2004]

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Influencesofreducingreagents

Table2.ComparisonofaveragesizesofAunanoparticles synthesizedusingvarious reduction[GuozongChaoNANOSTRUCTURESANDNANOMATERIALS,ImperialCollegePress,2004]

Aunanoparticles

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Figure10.SEMmicrographsofgoldnanoparticles preparedwithsodiumcitrate(a)and citricacid(b)asreductionreagents,respectively,underotherwisesimilarsynthesisconditions.[W.O.Miligan andR.H.Morriss,J.Am.Chem.SOC.86,3461(1964).]

Influencesofreducingreagents

Aunanoparticles

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Influencesofreducingreagents

InthecaseofPd

Figure 11.TheparticlesizeofPdcolloidsasafunctionofpeakpotentialsof reduction reagent,carboxylates,inwhichsmallerpeakpotentialsmean strongerreductionreagents.[M.T.Reetz andM.Maase,Adv.Muter.11,773(1999)]

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Figure 12.Transmissionelectronmicrographsofgoldnanoparticles obtainedbycitratereductionatthefollowingpHvalues:(a)4.0,(b)4.5,(c)5.0,(d)5.5,(e)6.0,and (f)6.5

[W.Patungwasa,J.H.Hodak ,MaterialsChemistryandPhysics108(2008)]

InfluencesofpHAunanoparticles

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lnfluencesofpolymerstabilizerPt nanoparticles

Figure13.Ptnanoparticles synthesizedincolloidalsolutionandhavingdifferentshapes (11nmcubesontheleftand~7nmtetrahedronsontheright)[T.S.Ahmadi,Z.L.Wang,T.C.Green,A.Henglein,M.A.ElSayed,Science272,1924(1996).]

InfluenceofsodiumpolyacrylateontheshapeofPtNPs

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A.1.1.Thealcoholreductionprocess

Figure 14. Preparation of polymer-capped metal nanoparticle by alcohol reduction [Marcel Dekker, DekkerEncyclopediaofNanoscience andNanotechnology,NewYork,2004]

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A.2.Microemulsions

Figure15.Reversemicelleandnormalmicellestructures[Burdaetal.,Chem.Rev, 2005,105].

Theformationofreversemicelleswasconfirmedtobeaninterestingandenvironmentallyfriendlyalternativetothepreparationofmetalnanoparticles.

NormalmicellesReversemicelles

Also,byturningthereactionparameters,differentnanoparticleshapescanbeproduced

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A.2.Microemulsions

Figure 16.TEMmicrographsoftheAgpowderssynthesizedbyareversemicelleprocess withdifferentwater/surfactantratio: (a)R=4,(b)R=6,and(c)R=8

[DongSikBaeetal.,MetalsandMaterialsInt.,11,4,2005]

Nanosized Agparticleswithauniformsizedistribution havebeenpreparedusinga reversemicelleprocess

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A.3.Thermaldecompositionofmetalsalts

Figure 17. Apparatus used to make metal nanoparticles by thermally decomposing solidsconsisting of metal cations and molecular anions, or metal organic solids

[Charles P. Poole, Jr. Frank J. Owens, Introduction to nanotechnology, John Wiley & Sons, Inc., 2003].

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A.4.ElectrochemicalsynthesisTheoverallprocessofelectrochemicalsynthesis(equation)canbedividedintosixelementarysteps:

1.OxidativedissolutionofthesacrificialMbulkanode.2.MigrationofMn+ ionstothecathode.3.Reductiveformationofzerovalentmetalatoms

atthecathode.4.Formationofmetalparticlesbynucleationand

growth.5.Arrestofthegrowthprocessandstabilizationof

theparticlesbycolloidalprotectiveagents(e.g.tetraalkylammoniumions).6.Precipitationofthenanostructuredmetal

colloids.

Figure 18. Electrochemical formation of NR4+Cl- stabilized nanometal[Helmut Boennemann Eur.J.Inorg.Chem.2001] 27

B.PHYSICALMETHODSB.2.Plasma

Thistechniqueisaverypromisingandstraightforwardwaytopreparemetalnanoparticlesasitis:anenvironmentallyfriendly,fastandsimplemethodologyandalsoapromisingalternativetohydrogen

reductionathightemperatures.

However,thespecialisedequipmentneededmakesdifficultitswidespreaduse.

Anovelplasmareductionmethodatroomtemperaturehasbeenusedtopreparesupportedmetalnanoparticles.

Legrandetal.employedadihydrogenmicrowaveplasmatoreducevariousmetalsolutions(Au,PtandPtAu)onzeolites.

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B.5.PulsedlaserablationThelaserapproachinvolves:

thevaporisationofmetalsemployingapulsedlaser(e.g.NdYAG)and subsequentcontrolleddepositiononthesurfaceofthesupportunderwelldefinedconditionsoftemperatureandpressureorpulsedlaserablationoftargetsinliquidenvironment(insolution)

AdvantagesofPLAinsolution:

alesserneedtoaddsurfactantforcappingofcolloidalparticlessomorepureparticlescanbeachieved

nanoparticlescanbeproducedinarbitrarysolution changingthenatureoftheliquidenvironmentcancontrolthesizedistributionandstabilityofmetallicNPs

Someparametersthatinfluenceablation,nucleation,growthandaggregationmechanismsare:

Laserwavelength:thediameterofNPsdecreasedwithanincreaseofphotonenergyorlaserlight

Pulseenergy:thediameterofNPsdecreasedwithanincreaseofpulsenumber

Pulseduration,

Repetitionrate

Liquidenvironment 29

Figure19.TypicalTEMimageofsilvernanoparticles inethanolanditssizedistribution[Tilaki etal.Appl.Phys.A,84,2006]

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Figure 20. TypicalTEMimageofsilvernanoparticles inwateranditssizedistribution [Tilaki etal.Appl.Phys.A,84,2006]

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Characterization

Figure21.CommonmethodsavailableforthecharacterizationofmatallicNPs[J.D.AikenIII,JournalofMolecularCatalysisA:Chemical145(1999)]

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PROPERTIES

Physicalproperties

Effectofsizeonmeltingtemperature

Figure 22. Size dependence of the melting temperature of gold [J.C.Bertolini,NanomaterialsandNanochemistry,Springer,Berlin,2008]

Themanysurfaceatoms,beinglessrestrictedintheirthermalmotions,willfluctuatemoreeasilyspatially,therebyloweringthemeltingtemperature

ThemeltingtemperaturedecreaseswiththedimensionsofmetallicNPs

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Opticalproperties

Figure 23.Schematicdrawingoftheinteractionofanelectromagneticradiationwithametalnanosphere.Adipoleisinduced,whichoscillatesinphasewiththeelectricfieldoftheincominglight.[LuisM.LizMarzn Nanometals:formationandcolor,Elsevier,2004]

Figure24.Left:TransmissionelectronmicrographsofAunanospheresandnanorods (a,b)andAgnanoprisms (c,mostlytruncatedtriangles)formedusingcitratereduction,seededgrowth,and DMFreduction,respectively.Right:PhotographsofcolloidaldispersionsofAuAg alloynanoparticleswithincreasingAuconcentration(d),Aunanorods ofincreasingaspectratio(e),andAgnanoprisms withincreasinglateralsize(f)[LuisM.LizMarzn Nanometals:formationandcolor,Elsevier,2004]

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ApplicationsInCatalysis:

CatalystsbasedonmetalNPsare:

9 Highlyactive9 Selective9 Exhibitlonglifetimeforseveralkindofreactions

Figure25.Schematicillustrationofthestructuresof(a)conventionalmetalcatalystand(b)metalnanoparticlescappedbypolymers[Marcel Dekker, DekkerEncyclopediaofNanoscience andNanotechnology,NewYork,2004]

conventionalmetalcatalyst

metalnanoparticlescappedbypolymers

heterogenouscatalysts immobilizedoninnorganicsupports(figure25 a)

homogenouscatalysts metalNPssurroundedwithstabilizers(figure25 b)

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Table3. ApplicationsofvarioussupportedAumetalnanoparticlesincatalysis[JuanM.Campelo,ChemSusChem2009,2]

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Applicationsofhomogenouscatalysts: Olefinhydrogenation Nitrilehydrogenation Photoinducedelectrontransfer

Applicationsofheterogeneouscatalysts: Oxidationreactions Hydrogenation Hydrodechlorination SynthesisofH2O2 Water gasshift

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ApplicationinFuelCellCatalysts Thefuellcelltechnology allowsthedirectconversionof

chemicalenergyintoelectricity

HydrogenfuelcellcatalystsrelyonpurePt Ptalloyelectrocatalystsareemployedfortheconversionofreformergas

intoelectricity

Examples ColloidalPt/Rucatalystsareusedin

directmethanolfuelcells DMFCs InPEMFCs

PotentialApplicationsinMaterialsScience Planararraysofuniformmetalnanoparticleswouldalowthedesignofnewsupercomputershavingasuperiordatastoragecapacity NickelNPsareusedasmagneticrecordingmedium,electricalconductivepastes,batterymaterials,etc. 38

BiologicalApplications AuNPs1.4nmwerefoundtoarrangethemselvesintoalinearrowwhenattachedtosinglestrandedDNAolinucleotides

Magneticnanoparticles(withinorganicshells)havebeenstudiedforbilogicalapplicationsuchas:

ForbindingBSAFordrugdeliveryForbiosensingForbioseparations

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Sensors

1.Electrochemicalsensors theintroductionofmetalNPs(mostlysupported)ontheelectrodecan:

decreasetheoverpotentialsofmanyelectrochemicalreactions

turnintoreversiblesomeredoxreactionsthatarenormallyirreversibleinconventionalunmodifiedelectrodes

Examples:sensitiveNO,H2O2 andsugarandaminosacidsensors

2.Biosensors: canenhancetheelectrontransferbetweenbiomolecules AgNPssupportedonglass usedasselectivebiosensorsfor

thebiotinstreptavidinsystem

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Silvernanoparticles

AgNPs exhibitsthehighestefficiencyofplasmon excitation

isbecominganincreasinglyimportantmaterialinmanytechnologies

is the only material whose plasmon resonance can be tuned to any wavelength in the visible spectrum

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SYNTHESISMETHODS

1.Traditional methods

2.Nontraditional methods

AgNPs

Aqueous solutionreduction

Laser ablationofametaltargetintoasuspendingliquid

Hightemperaturereductioninporoussolidmatrices

Vaporphasecondensationofametaltargetontoasolidsupport

Microemulsion techniques

Gamma radiationinducedmethods Photoreduction ofAgions

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Goals andproblems regardingthesynthesisofAgNPs

Withthetraditionalmethods,themajorproblemisoftenalimitedflexibilityinthesizeofparticlesthatcanbeproducedandsuchmethodsareusuallysoldontheirabilitytomake

OpticalpropertiesofAgNPs

AgNPsshowremarkablephysicalandchemicalpropertieszhataresizeandshapedependent.

ThemainfactorsthatcancontroletheopticalpropertiesofAgNpsare:

nanoparticlemorphology sizeandshape

therefractiveindexofthesurroundingmedium

adsorptionphenomenaatthesolidsolutioninterface

thedistancebetweentheparticles

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InfluenceoftheparticlemorphologyontheSPRofAgNPS

inthecaseofasphere,asinglepeakintheopticalabsorptionspectrawasfound for20and100nmnanospheres,SPRpeaksareatabout370and600nm,respectively whenananoparticlebecomestruncated,themainresonanceisblueshifted theresonancesvanishasthenumberoffacesincreases,orwhenthesymmetryofthe

nanoparticleislarger

Figure26.DeviationsfromsphericalgeometrystronglyaffecttheopticalpropertiesofAg NPs[JuanM.Campelo ChemSusChem2009,2]

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InfluenceofthesurroundingmediumThelocationofthesurfaceplasmonresonancesissensitivetothedielectricenvironment,andastherefractionindexincreases,theSPABisshiftedtolongerwavelengths

Figure27.(A)ResonantRayleighscatteringspectraofanindividualAgNPinvariousdielectricenvironments(N2,methanol,1propanol,chloroform,andbenzene).(B)AplotdepictingthelinearrelationshipbetweenthesolventrefactiveindexandtheNPsmax.[ChristyL.Haynes,NanoparticleswithtunablelocalizedSPR,TopicsinFluorescenceSpectroscopy,Volume8:Radiative DecayEngineering,NewYork,2005] 46

ApplicationsofAgNPs1.Catalysis

SupportedAgareimportantin: theselectiveoxidationofalcohols,alkanesandalkenes forthesynthesisofindustriallyinterestingproductsincludingepoxides andaldehydes

2.Photocatalysis: watersplitting degradationoforganicpollutants

3.BiologyandMedicine: Agisahighlyantimicrobialmaterialusedin: waterpurification woundcare medicaldevices drogdelivery

4.Optics:Optoelectronicdevices activewaveguidesinopticaldevices(amplifiers)

5.Electronics:electronicallyconductiveadhesives(ECAs)

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CONCLUSIONSThe21stcenturyhasbroughtagreatinterestand

expansionofthenanomaterialsduetouniquespropertiesthatexistatthenanometricscales.

RecentadvancesinthedesignandpreparationofmetallicNPshaveprovedthatanumerousvarietyofMNPscannowadaysbesynthesisedthroughdifferentpreparationroutes.

SynthesisofMNPsisimportantbecauseoftheirnovelelectrical,optical,magneticalandchemicalproperties.

AgNPsareofstrongresearchfocusbecauseoftheiruniquefunctionalpropertieswhichleadtovariedapplications.

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REFERENCES1. RobinJ.White Chem.Soc.Rev,38,2009,

2. JuanM.Campelo ChemSusChem,2,2009

3. JohnA.Blackman Metallicnanoparticles,Elsevier,2009

4. GunterSchmid ClustersandColloids,VCH,1994

5. HelmutBoennemann Eur.J.Inorg.Chem.,2001

6. GuozongChaoNANOSTRUCTURESANDNANOMATERIALS,ImperialCollegePress,2004

7. M.T.Reetz andM.Maase,Adv.Muter.11,773,1999

8. LuisM.LizMarzn Nanometals:formationandcolor,Elsevier,2004

9. Burdaetal.,Chem.Rev,2005,105

10. Tilaki etal.Appl.Phys.A,84,2006

11. MarcelDekker,DekkerEncyclopediaofNanoscience andNanotechnology,NewYork,2004

12. J.D.AikenIII,JournalofMolecularCatalysisA:Chemical145,1999

13. G.Chumanov,ChemPhysChem.,6,2005

14. C.Noguez,J.Phys.Chem.C2007,111,3806

15. M.K.Temgire,RadiationPhysicsandChemistry71,2004

16. A.L.Gonzalez,Phys.Stat.Sol,4,11,2007

17. DapengChen,JMaterSci44,2009

18. W.H.Qi,MaterialsChemistryandPhysics88,2004

19. P.A.Chernavskii,RussianJournalofPhysicalChemistryB,1,4,2007

20. Tabrizi,J.Nanopart.Res.,Springer,2009

21. V.N.Pustovit,Eur.Phys.JB69,2009

22. W.Wangetal.,Journal ofColloidandInterfaceScience,323,200849

THANK YOU FOR YOUR TIME!

METALLIC NANOPARTICLESINTRODUCTION TO THE NANOWORLDSlide Number 3Figure 1. Depiction of the size regime of nanoparticles related to common nano scale objects; A-prokaryotic cell, B-ultraviolet wave, C-virus, D-enzyme. Blue spheres represent a 50 nm metal nanoparticle [Robin J. White-Chem. Soc. Rev, 2009, 38].2. METALLIC NANOPARTICLESThe main characteristics of MNPsDensity of States Surface Plasmon ResonanceLocal field factorSYNTHESIS METHODSB. PHYSICAL METHODSGoals and Problems inMetallic Nanoparticles SynthesisA.1. Chemical reduction of metal saltsNucleation takes place because the supersaturated solution is thermodynamically unstable. For the nucleation process to occur, the solution must be supersaturated in order to generate an extremely small size sol particle. Nucleation and Growth (La Mer)Stabilizing Metallic NanoparticlesA.1. Chemical reduction of metal saltsInfluences of reducing reagentsInfluences of reducing reagentsInfluences of reducing reagentsSlide Number 21lnfluences of polymer stabilizerA.1.1. The alcohol reduction processA.2. MicroemulsionsA.2. MicroemulsionsA.3. Thermal decomposition of metal saltsA.4. Electrochemical synthesisB. PHYSICAL METHODSB.5. Pulsed laser ablationSlide Number 30Slide Number 31CharacterizationPROPERTIESOptical propertiesApplicationsSlide Number 36Slide Number 37 Application in Fuel Cell Catalysts Biological Applications SensorsSilver nanoparticlesSYNTHESIS METHODSGoals and problems regarding the synthesis of Ag NPsOptical properties of Ag NPsInfluence of the particle morphology on the SPR of Ag NPSInfluence of the surrounding mediumApplications of Ag NPsCONCLUSIONS REFERENCESSlide Number 50