Physicochemical properties of metal nanoparticle by shreya modi


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Physicochemical properties of metal nanoparticle by shreya modi

  2. 2. M.Phil./Ph.D. in NANOSCIENCE AND NANOTECHNOLOGY, CENTRE FOR NANOSCIENCE, CUG. ABSTRACTThe physico-chemical properties of nanosized metal particles are different than bulk material of same metal.Physiochemical properties like Size, Structure, Surface area to volume ratio,Optical Properties, Melting point,Electron energy, Band gap, Electron arrangement, Colour, Adsorption capacity, Hardness, Mechanical strength,etc…., all have their different behavior than bulk of same metal. There are so many reasons why these properties arechanged at nanoscale because in the nanosized particles every electron of atom or molecule get equal chance toshow their activity while for the bulk metal only surface electron are activated, so because of higher surface area,smaller size, nanosized metal particles are highly reactive, highly energetic and active than bulk metal, apart fromthese nanoscale properties have given new direction to the Science, Society And Technology. In this term paper Ihave tried to explain the property change at nanoscale particle of same metal.Key Words:Physico-chemical properties, Metal nanoparticles, Surface Plasmon Resonance, Mechanical property, Magneticproperty 2
  3. 3. INTRODUCTIONWHAT IS THE PHYSICOCHEMICAL PROPERTIES?Physical properties are those that can be observed without changing the identity of the substance. The generalproperties of matter such as color, density, hardness, are examples of physical properties. Properties that describehow a substance changes into a completely different substance are called chemical properties.Flammability andcorrosion/oxidation resistance are examples of chemical properties [1].WHAT IS NANOSIZED METAL NANOPARTICLE?In nanotechnology, a particle is defined as a small object that behaves as a whole unit in terms of its transport andproperties. Particles are further classified according to size[2]: in terms of diameter, coarse particles cover a rangebetween 10,000 and 2,500 nanometers. Fine particles are sized between 2,500 and 100 nanometers. Ultrafineparticles, or nanoparticles are sized between 100 and 1 nanometers[3].Nanoparticle of Metal within the range of 1-100nm is termed as metal nanoparticle. 0AOUMshzf5Wk4aoTWtxuszOmgkYvMETAL NANOPARTICLE HISTORICAL PERSPECTIVEMetal nanoparticles show very interesting optical properties. The use of these miniscule objects for glass stainingdates back to the ancient times. “Ruby glass”, which is essentially glass containing gold nanoparticles, has been used 3
  4. 4. since antiquity until the present. A classic example of an ancient piece of art gaining its appeal from the colorproduced by metal nanoparticles is the late Roman “Lycurgus Cup”, which is exhibited in the British Museum [4].Depicting the mythological scene of Lycurgus’s entrapment by the vine-turned Ambrosia, a maenad of Dionysus,the cup shows extraordinary dichroic behavior exhibiting red color in transmission and green color in reflection (Figure 1). This beautiful effect is due to absorption and scattering of gold and silver nanoparticles which are presentin the glass from which the cup is made.[5,6]. A revolution in the use of gold metal for glass and ceramic stainingdid not take place until late 17th century when it was discovered that combining aqua regia solution of gold and tinproduces a precipitate with deep and vibrant red color.[7] Named “purple of Cassius”, after its alleged inventor, thecolorant became one of the most successful red pigments used in the production of glass and ceramics, and it is still use today[7,8], Even though Andreas Cassius had been given the credit of the discovery and the 1685 recipe ofthe “purple of Cassius” he was not the first to discover the recipe for the preparation of the famous colorant.PHYSICOCHEMICAL PROPERTIES OF THE METAL NANOPARTICLEThe physicochemical properties of the metal nanoparticle are completely different to that of the bulk material due totheir very small size and high surface to volume ratio. While the properties of bulk materials are solely composition-dependent, as the size of the particles decreases to few nm, the the electronic structure is altered from the continuousband to discrete electronic levels Thus, the properties of the nanomaterial become size-dependent [9].In addition,when nanoclusters are deposited on surfaces, their physical and chemical properties are strongly dependent not onlyon their particle size and chemical composition, but also on the structure of the surface and that of themetal/substrate interface.Some known physical properties of nanomaterials are related to different origins: for example, (i) Large fraction of surface atoms, (ii) Large surfaceenergy, (iii) Spatial confinement, and (iv) Reduced imperfections. 4
  5. 5. SizeFor a science that is all about size, one of the most interesting aspects of nanoscience is that properties ofnanoparticles change with size. It has been shown that many fundamental properties are size dependent on thenanoscale.For example, the most stable crystalline phase of a material is size dependent. From thermodynamic considerations,the total free energy is a sum of the free energy of the bulk and the surface of the nanoparticle.Gnanoparticle = Gsurface + Gbulk (1)Figure- Microscopic and macroscopic behaviors of nanoparticles depend on a number of a number of importantcharacteristics and properties (e.g., shape, concentration (dose/response), surface composition, and aggregation forpassive and active (changing) nanostructures.(Modified and adapted from Tinke et al., Am. Pharmaceut. ReV., 2006, September/October, 1.)[10]For nanoparticles, Gsurface is no longer a minor component but in fact becomes a large component of the total freeenergy.Surface free energies and surface stress are important components to the overall phase stability of nanoparticles.[11-15].Titanium dioxide is interesting as anatase becomes more stable than rutile for a particle size below 14 nm.3However, it has been recently shown that the stability of rutile nanoparticlesincreases relative to anatase and brookite at low pH due to surface charges.[13]SURFACE ARE TO VOLUME RATIOReactions take place at the surface of a chemical or material; the greater the surface for the same volume, thegreater the reactivity. 5
  6. 6. One prime example of surface area to volume ratio at the nanoscale is gold as a nanoparticle. At the macroscale,gold is an inert element, meaning it does not react with many chemicals, whereas at the nanoscale, goldnanoparticles become extremely reactive and can be used as catalysts to speed up reactions.[16]Surface area to volume ratio in nanoparticles have a significant effect on the nanoparticles properties. Firstly,nanoparticles have a relative larger surface area when compared to the same volume of the material. For example,let us consider a sphere of radius r:The surface area of the sphere will be 4πr2The volume of the sphere = 4/3(πr3)Therefore the surface area to the volume ratio will be 4πr2/{4/3(πr3)} = 3/r 6
  7. 7. It means that the surface area to volume ration increases with the decrease in radius of the sphere and vice versa. Itcan also be conclude here that when given volume is divided into smaller piece, the surface area increases.Therefore as particle size decreases, a greater portion of the atoms are found at the surface compared to those inside.For example, a particle of size 3 nm has 50% of its atoms on its surface, at 10 nm 20% of its atoms and at 30 nm has5% of its atoms on its surface. Therefore nanoparticles have a much greater surface area per unit volume comparedwith the larger particles. It leads to nanoparticles more chemically reactive. As growth and catalytic chemicalreaction occurs at surfaces, therefore a given mass of nanomaterial will be much more reactive than the same massof material made up of large particles. It is also found that materials which are inert in their bulk form are reactivewhen produced in their nanoscale form. It can improve their properties.[17]ELECTRON DYNAMICS IN METAL NANOPARTICLESOne of the most appealing features of nanometer-sized clusters (aka nanoparticles) is that their electronic propertiesare midway between those of small molecular systems and those of bulk condensed matter. This intermediate natureof clusters makes most of their properties depend on size. In our particular case, we are interested in the way inwhich size effects appear in the dynamics of electronic excitations.The recent development of experimental techniques based on femtosecond lasers has made it possible to study thedynamics of electronic excitations in a wide variety of systems, including nanoparticles. Experimental techniquesbased on the interaction of charged particles with the target provide an additional source of information on theelectronic properties of metal nanoparticles. There are two main effects that drastically modify the dynamics ofelectronic excitations in clusters with respect to the bulk analogous situation. First, the discretization of levels in theelectronic structure of the cluster reduces the number of final states to which an electronic excitation can decayenhancing its lifeti, the reduction of dynamic screening in the proximity of the cluster surface changes the interaction potentialbetween electrons and decreases the excitation lifetimes. The interplay between these two effects makes the analysisof electron dynamics in clusters both intricate and appealing. The theoretical description of electron dynamics inmetal clustersWe basically develop two lines of research in this topic: First, we use density functional theory and the self-energyformalism to study the dependence on size of the lifetime of electronic excitations created in metal nanoparticles.Furthermore, we use time-dependent density functional theory to investigate the time scales in which the electronicscreening is developed in finite systems. Time dependent density functional theory is also employed to calculate theenergy transfer processes between moving charges and metal nanoparticles.[18]The examples of the change in electrical properties in nanomaterials are: 7
  8. 8. 1. Conductivity of a bulk or large material does not depend upon dimensions like diameter or area of cross sectionand twist in the conducting wire etc. However it is found that in case of carbon nanotubes conductivity changes withchange in area of cross section.2.) It is also observed that conductivity also changes when some shear force (in simple terms twist) is given tonanotube.3.) Conductivity of a multiwalled carbon nanotube is different than that of single nanotube of same dimensions.4.) The carbon nanotubes can act as conductor or semiconductor in behaviour but we all know that large carbon(graphite) is good conductor of electricity.These are the important electrical properties of nanomaterials with their examples.[19] possess well known atomic orbitals: they define the state of the atom (distribution of its electrons).Transitions between the states of a single atom are quantized. Bulk materials are composed of a very large numberof atoms in the states of similar energies - bands. Within a band energy difference between adjacent energy states isnegligibly small. However, a significant energy gap between the bands (band gap) can exist. Depending on itsmagnitude, materials are classified as metallic (no band gap), insulators (very large - >>kT - band gap) andsemiconductors (in between). 8
  9. 9. Band gap varies with the size of material, as depicted in Fig. 3. Note also that the number of permitted energy statesof the material per unit energy (so called density of states) also decreases with decreasing size: its very large in bulkmaterials (so that transitions are basically continuous), while at small scale particles behave more like individualatoms.PLASMONIC NANOPARTICLESThe surface plasmon band (SPB) is a phenomenon observed in transmission, due to the presence of nanoparticles, insolution or in the solid phase. For a special domain of frequency, nanoparticles interact with incident light, resultingin a global scattering of it. This macroscopic feature can be explained by the collective resonance of the conductionelectrons of the nanoparticle.This electronic motion is specific of nanoparticles due to geometrical confinement effects of the free electrons.Indeeda nanoparticle can be seen as an immobile and periodical cationic network in which a cloud of conducting electronsmove. The latter are usually considered as free electrons. NPs dimensions are very small compared to thewavelength of the UV-visible light for which the phenomenon is observed and also comparable to the mean freepath of electrons. The SPB is due to the resonance of the electronic cloud with the incident wave and the mechanicsof this phenomenon can be evaluated.[20]Plasmonic nanoparticles are metal nanoparticles (typically gold and silver particles with diameters ranging from 10-150 nm) that are highly efficient at absorbing and scattering light. By changing the size, shape, and surface coating,the color of the nanoparticles can be tuned across the visible and near-infrared region of the electromagneticspectrum. Solutions of spherical gold nanoparticles are ruby red in color due to the strong scattering and absorptionin the green region of the spectrum. Solutions of silver nanoparticles are yellow due to the plasmon resonance in theblue region of the spectrum (red and green light is unaffected). - Schematic of surface plasmonic resonance where the free conduction electrons in the metal nanoparticle aredriven into oscillation due to strong coupling with incident light.The reason for the unique spectral response of silver and gold nanoparticles is that specific wavelengths of light candrive the conduction electrons in the metal to collectively oscillate, a phenomenon known as a surface plasmonresonance (SPR). When these resonances are excited, absorption and scattering intensities can be up to 40x higherthan identically sized particles that are not plasmonic. The brightness and tunability of plasmonic nanoparticleoptical properties make them highly useful in numerous applications including molecular detection, solar energymaterials, and cancer detection and treatment.[21] 9
  10. 10. MECHANICAL STRENGTHMechanical properties of materials increase with a decreasing size. A lot of work in this regard has been done onwhiskers; and it has been found that increase in mechanical strength starts at the micrometer scale, which isnoticeably different from other size dependent properties.Two possible explanations have been proposed to explain enhanced strength of nanowires and nanorods (withdiameter less than 10 microns). These are: increased strength due to high internal perfection in nanowires; secondone being perfection of side faces resulting in less surface defects.Copper nanoparticles smaller than 50 nm are considered super hard materials that do not exhibit the samemalleability and ductility as bulk copper.Experimentally, it has been found that nanostructured metals can have higher as well as lower hardness and strengthcompared to coarse grained materials, depending on the method used to vary grain size. Although significant workhas been done on silver, copper, palladium, gold, iron and nickel, actual role of grain size on mechanical propertiesis still not clear. Other properties such as Youngs modulus, creep and superplasticity have also been studied,however, a conclusion is yet to be relating these properties to size dependence of particles.[15]The mechanical properties of materials increase with a decreasing size. Many studies have beenfocused on the mechanical properties of onedimensional structure; particularly a lot of work hasbeen done on whiskers. It was found that a whisker can have a mechanical strengthOPTICAL PROPERTIES 10
  11. 11. The reduction of materials dimension and size have pronounced effects on the optical properties. Thesize dependence can be generally classified into two groups. One is due to the increased energy levelspacing as the system becomes more confined, and the other is related to surface plasmon resonance1) The properties like color and transparency are considered as optical properties. These properties are observed tochange at nano scale level. For example bulk gold appears yellow in colour while in nanosize gold appear red incolour.2) Bulk silicon appears grey in colour while nanosized silicon appears red in colour.3) Zinc oxide, which at bulk scale blocks ultraviolet light and scatters visible light and gives white appearance.While nanoscale zinc oxide is very small in particle size compared with wavelength of visible light and it does notscatters it. Thus it appears transparent.Reason for change in optical properties in nanoscale1.) The main reason for change in optical properties at nanoscale level is that nanoparticles are so small thatelectrons in them are not as much free to move as in case of bulk material. Due to this restricted movement ofelectrons, nanoparticles react differently with light as compared to bulk material.[23]Nanoshells are a recent product from the field of nanotechnology. A dielectric core is coated with metal, and aplasmon resonance mechanism creates color, the wavelength depending on the ratio of coating thickness to coresize. For gold, a purple color gives way to greens and blues as the coating shell is made thinner. In the future,jewelry applications may include other precious metals, such as platinum. diameter of gold nanoparticles determines the wavelengths of light absorbed. The colors in this diagramillustrate this effect.[24] 11
  12. 12. of the different sized arrangement of gold atomsabsorbs and reflects light differently based on its energylevels,which are determined by size and bondingarrangement. This is true for many materials when theparticles have a sizethat is less than 100 nanometers in at least one dimention.ADSORPTION PROPERTYSmaller nanoparticles are predicted to show enhanced adsorption due to an increase in interfacial tension and surfacefree energy with decreasing particle size.[25]Metal nanoparticle have advanced property of adsorption capacity than bulk of same metal,for example, a cube ofbulk charcoal can not adsorb heavy metals as compared to activated nanoparticle charcoal made up from same bulkmetal. Apart from this,Carbon nanotubes (CNTs), a member in carbon family, are relatively new adsorbents thathave been proven to possess great potential for removing many kinds of pollutants such as chlorobenzenes [26],herbicides [27,28] as well as lead [29] and cadmium ions [30]. The hexagonal arrays of carbon atoms in graphitesheets of CNTs surface have strong interactions with other molecules or atoms. While the carbon metal in bulkform cannot give such results 12
  13. 13. The size dependence of adsorption properties of metal nanoparticles for CO as a probe on Pdn clusters with n = 13–116 atoms. For large particles, the values slowly decrease with cluster size from the asymptotic value for an (ideal)infinite surface. For clusters of 13–25 atoms, starting well above the asymptotic value, the adsorption energies dropquite steeply with increasing cluster size. These opposite trends meet in an intermediate size range, for clusters of30–50 atoms, yielding the lowest adsorption energies. These computational results help to resolve a controversy onthe size-dependent behavior of adsorption energies of metal nanoparticles.[31] POINT DEPRESSIONMelting-point depression is a term referring to the phenomenon of reduction of the melting point of a material withreduction of its size. This phenomenon is very prominent in nanoscale materials which melt at temperatureshundreds of degrees lower than bulk materials.At small sizes, nanoparticles may melt at temperatures significantlybelow the bulk melting point (MP) [32-36], due to increasing surface energy at small sizes [32] .This phenomenon has been studied for decades, along with the parallel rapid evaporation of such nanoparticles dueto their increased vapor pressure at high surface curvatures [37] . An example plot is shown in Figure, for threedifferent metals, illustrating that a unified theory may be possible and that significant reductions require nanoparticledimensions of ∼5 nm or less.Different electron microscopy techniques have been used to determine the MP.Sambles [38] , for example,monitored the evaporation of small particles at controlled temperatures and noted the size when the evaporation ratechanged, interpreting this as the melting point. Others have noted the transition from sharp to diffuse electrondiffraction rings [39] , or the loss of diffraction ring intensity [40] .Physics Of Melting Point DepressionThe increased surface to volume ratio means surface atoms have a much greater effect on chemical and physicalproperties of a nanoparticle. Surface atoms bind in the solid phase with less cohesive energy because they havefewer neighboring atoms in close proximity compared to atoms in the bulk of the solid. Each chemical bond an atomshares with a neighboring atom provides cohesive energy, so atoms with fewer bonds and neighboring atoms have 13
  14. 14. lower cohesive energy. The average cohesive energy per atom of a nanoparticle has been theoretically calculated asa function of particle size according to Equation 1.[41]Where: D=nanoparticle size d=atomic size Eb=cohesive energy of bulkAs Equation 1 shows, the effective cohesive energy of a nanoparticle approaches that of the bulk material as thematerial extends beyond atomic size range (D>>d).Atoms located at or near the surface of the nanoparticle have reduced cohesive energy due to a reduced number ofcohesive bonds. An atom experiences an attractive force with all nearby atoms according to the Lennard-Jonespotential. The Lennard-Jones pair-potential shown in Figure 2 models the cohesive energy between atoms as afunction of separation distance.Figure- A Lennard-Jones potential energy curve. The model shows the interactive energy between 2 atoms at anormalized distance, d/d0, where d0=atomic diameter. The interaction energy is attractive where the curve isnegative, and the magnitude of the energy represents the cohesive energy between a pair of atoms. Note that theattractive potential extends over a long range beyond the length of a chemical bond, so atoms experience cohesiveenergy with atoms further than their nearest neighbors.The cohesive energy of an atom is directly related to the thermal energy required to free the atom from the solid.According to Lindemann’s criterion, the melting temperature of a material is proportional to its cohesive energy,av (TM=Cav)[42] Since atoms near the surface have fewer bonds and reduced cohesive energy, they require lessenergy to free from the solid phase. Melting point depression of high surface to volume ratio materials results fromthis effect. For the same reason, surfaces of bulk materials can melt at lower temperatures than the bulk material[43].The theoretical size-dependent melting point of a material can be calculated through classical thermodynamicanalysis. The result is the Gibbs–Thomson equation shown in Equation [44]. 14
  15. 15. Where: TMB=Bulk Melting temperature σsl=solid–liquid interface energy Hf=Bulk heat of fusion ρs=density of solid d=particle diameterA normalized Gibbs–Thomson Equation for gold nanoparticles is plotted in Figure , and the shape of the curve is ingeneral agreement with those obtained through experiment.[45]Experimental melting point depression for Au, Sn, and Pb, normalized to the bulk melting points [22] (withpermission)Experimental melting point depression for Au, Sn, and Pb, normalized to the bulk meltingpoints [22] (with permission)The two-step melting processleads to complexities in alloy systems, due to compositional phase changes [29] .Thehigh MPs of no-Pb solders lead to higher thermomechanical stresses thanfor conventional eutectic Sn–Pb solder, andmelting point depression may be onemechanism to reduce the process temperatures and thermomechanical failurerates.The MP of Sn–Ag alloy, for example, has been shown to be reduced from 222 to 193°C for 5-nm radius particles[34] .SOLUBILITYWithin the size–pressure approximation, which considers the stress induced by the surface tension and the curvatureof the particle, it was shown that the size of the particle affects the composition and temperature of a eutectic point(Fe-C ) the solubility of C in Fe and Fe:Mo Nano clusters.Reduced solubility can affect the catalytic properties ofnanoparticles. In fact it has been shown that size-induced instability of Fe-C mixtures represents the thermodynamiclimit for the thinnest nanotube that can be grown from Fe nanocatalysts. 15
  16. 16. MAGNETIC PROPERTIESFerromagnetic materials (Fe, Co, Ni, Gd, CrO2…) spin alignment below TC, the Curie temperature. Magneticdomains are present, separated by Block walls.When the size of the magnetic particle is in the same range as the domain size, only one domain is present in oneparticle.Critical sizes: Fe: 14 nm, Co: 70 nm, Fe3O4: 128 nm.The coactivity is the field needed to reverse the magnetization.This is higher for single domain particles than for bulk, because the spin has to be flipped, notrotated by moving the block wall. Smaller particles may display superparamagnetism(e.g. 7.6 nm for Co at 300K)i.e. the coercivity =0The coercivity of magnetic particles increases with decreasing size, but the decreases below a critical diameter. 16
  17. 17. Giant magneto resistance (GMR): a dramatic drop in electric resistance in amagnetic field. Observed in Nanocomposite materials (and multilayers offerromagnetic and nonmagnetic materials (Fe/Cr, Co/Cu) (and colossalmagnetoresistance, CMR, in e.g. manganite perovskites)Here below example of gold metal is given to explain property change at nanoscale.GOLD ACQUIRES NEW PROPERTIES AT THE NANOSCALE. 17
  18. 18. CONCLUSIONBy studying all the above properties of metal nanoparticles it can be definitely concluded that, the physico-chemicalproperties of nano-sized metal particles are different than bulk of same metal.Nanomaterial may have a significantly lower melting point or phase transition temperature andappreciably reduced lattice constants, due to a huge fraction of surface atoms in the total amount ofatoms.Mechanical properties of nanomaterials may reach the theoretical strength, which are one or twoorders of magnitude higher than that of single crystals in the bulk form. The enhancement inmechanical strength is simply due to the reduced probability of defects.Nanoscale metal particles areharder than bulk metal.Optical properties of nanomaterials can be significantly differentfrom bulk crystals. For example, theoptical absorption peak of asemiconductor nanoparticle shifts to a short wavelength, due toanincreased band gap. The color of metallic nanoparticles may change with their sizes due to surfaceplasmon resonance.Electrical conductivity decreases with a reduced dimension due toincreased surface scattering.However, electrical conductivity ofnanomaterials could also be enhanced appreciably, due to thebetterordering in microstructure, e.g. in polymeric fibrils.Magnetic properties of nanostructured materials are distinctly differentfrom that of bulk materials.Ferromagnetism of bulk materials disappearsand transfers to superparamagnetism in the nanometerscaledue to the huge surface energy. 18
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