Synthesis of metallicnanoparticles

The most diffused method for the synthesis of metal colloidaldispersions or metallic nanoparticles is the reduction of metalcomplexes in dilute solutions.

The formation of monosized metallic nanoparticles is achieved in mostcases by a combination of a low concentration of solute andpolymeric monolayer adhered onto the growth surfaces. Bothaspects hinder the diffusion of growth species from the surroundingsolution to the growth surfaces, so that growth of initial nuclei isreduced resulting in the formation of uniformly sized nanoparticles.

In the synthesis of metallic nanoparticles various types of precursors,reduction reagents, other chemicals, and methods were used topromote or control the reduction reactions, the initial nucleation andthe subsequent growth of initial nuclei.

Precursors, reductionreagents and polymericstabilizers commonlyused in the productionof metallic colloidaldispersions.

Colloidal goldOne of the most common method is based on sodium citratereduction of chloroauric acid at 100 °C.In particular:

a) Chlorauric acid dissolves into water to make 20 ml very dilutesolution of ~2.5 X 10-4Mb) 1 ml 0.5% sodium citrate is added into the boiling solutionc) the mixture is kept at 100°C till color changes while maintainingthe overall volume of the solution by adding water.

Such prepared colloidal sol has excellent stability and uniformparticle size of ~ 20 nm in diameter.

The concentration of precursors (and also of the reduction agents)has a strong effect on the size and size distribution of goldnanoparticles and the nucleation rates.

Video Gold Nanoparticle Synthesis.

A large number of initial nuclei formed in the nucleation stage resultsin a larger number of nanoparticles with smaller size and narrowersize distribution.

Effect of concentration

Elettrochemical methodMetallic nanoparticles can also be prepared by an electrochemicaldeposition method.

This synthesis employs a simple electrochemical cell containing onlya metal anode and a metal or glassy carbon cathode. The electrolyteconsists of organic solutions (of tetraalkylammonium halogenides),which also serve as stabilizers for the produced metal nanoparticles.Upon application of an electric field, the anode undergoes oxidativedissolution forming metal ions, which would migrate toward thecathode. The reduction of metal ions by ammonium ions (NH4

+ ) leadsto the nucleation and subsequent growth of metallic nanoparticlesin the solution.

Nanoparticles of Pd, Ni and Co withdiameters ranging from 1.4 to 4.8 nm wereproduced. Furthermore, it was found thatthe current density has an appreciableinfluence on the size of metallic particles;increasing the current density resultsin a reduced particle size.

Influence of reduction reagentsThe size and size distribution of metallic colloids vary significantly withthe types of reduction reagents used in the synthesis.

In general, a strong reduction reaction promotes a fast reaction rateand favors the formation of smaller nanoparticles.

A weak reduction (low reaction) may result in either wider ornarrower size distribution depending on the rate of nuclei formation.

Using the same reduction reagent, nanoparticle size can be variedby changing the synthesis conditions.

Gold particles with spherical shape were obtained using sodium citrateor hydrogen peroxide as reduction reagents, whereas faceted goldparticles were formed when hydroxylamine hydrochloride (cubical with{100} facets) and citric acid (trigons or very thin platelets of trigonalsymmetry with {111} facets) were used as reduction reagents.

Concentration of the reduction reagents and pH value of thereagents have noticeable influences on the morphology of the growngold nanoparticles. For example, lowering the pH value causes the{111} facets to develop at the expense of the {l00} facets.

Morphology of Nanoparticles

In general, the size of metallic colloids is strongly dependent onhow strong a reduction reagent is, and stronger reducingreagents lead to smaller nanoparticles.

Stronger reduction reagent can generate an abrupt increase ofthe concentration of growth species, resulting in a very highsupersaturation and consequently, a large number of initialnuclei are produced. The final result is the production of smallersize nanoparticles.

Influence of other factorsIn the synthesis of Pt nanoparticles using an aqueous methanolreduction of H2PtCl6, a high concentration of chloride ions in thereaction mixture promotes monodispersity and near-sphericalparticle shape of the metallic colloids, favoring smoother and roundersurfaces, at the otherwise similar conditions. The high Cl ionsconcentration favors slow reaction rates since the diffusion-limitedgrowth of initial Pt nuclei is favored.In the case of silver nanoparticles synthesized by reduction of silvernitrate (AgNO3) using formaldehyde (HCOH) in aqueous solution animportant role is played by the pH of the solution.When only NaOH (strong base) isused, a higher pH favors a higherreduction rate, and result in theformation of large silverprecipitates.When sodium carbonate Na2CO3(weak base) was added to partiallysubstitute NaOH, stable silvercolloidal dispersions wereobtained.

More hydroxyl ions

Influence of polymerstabilizer

Polymer stabilizers are introduced primarily to form a monolayeron the surface of nanoparticles so as to prevent agglomeration ofnanoparticles.

The presence of polymer stabilizers during the formation ofnanoparticles can have various influences on the growth process ofnanoparticles. Interaction between the surface of a solid particleand polymer stabilizer may vary significantly depending on thesurface chemistry of solid, the polymer, solvent and temperature.

A strong adsorption of polymer stabilizers would occupy thegrowth sites and thus reduce the growth rate of nanoparticles.

Polymer stabilizers may also interact with solute, catalyst, orsolvent, and thus directly contribute to the reaction.

In Platinum nanoparticles synthesis, under the sameexperimental conditions and using the same polymer stabilizer,changing the ratio of the concentration of the cappingmaterial to that of Pt ions from 1:1 to 5:1 produce differentshapes of Pt nanoparticles, with cubic particles correspondingto a ratio of 1:1 and tetrahedral particles to a 5:1 ratio.

Capping material concentration effect

Metal nanoparticles can be also prepared without using any polymerstabilizer. The dispersion is likely to be stabilized by electrostaticstabilization mechanism. The particle size is sensitively dependenton the synthesis temperature.

Higher synthesistemperaturesproduce biggernanoparticles.

Temperature effect

Synthesis of metal nanoparticles with magnetic properties isimportant for many applications.In particular, nanomaterials (metals, metal alloys, oxides, oxide core-shel structures) with high magnetic moment are important for theirpotential biomedical applications such as bioseparation,biosensing, magnetic imaging, drug delivery and magnetic fluidhyperthermia.

Cobalt has a high magnetic moment and can be magnetized by weakmagnetic field. Stabilization of Co nanoparticles can be attained byAu, Pt, CdSe, SiO2 surface coating.

Alloys of FeCo are also exploited, but they can be easily oxidized andtherefore coating with graphite is a possible and efficient method toincrease solubility and stability in water and to allow biologicalapplications.

Fe based nanoparticles are also exploited but they are quite unstablesince they are easily oxidized. Also for this material, alloys or coreshell procedure are adopted to increase stability and increase thepossible applications.

Magnetic nanoparticles

Time evolution and temperature effect

Synthesis of semiconductornanoparticles

Nonoxide semiconductor nanoparticles are commonly synthesized bypyrolysis of organometallic precursor(s) dissolved in anhydratesolvents at elevated temperatures in an airless environment in thepresence of polymer stabilizer or capping material. Cappingmaterials are linked to the surface of nanocrystallites via either covalentbonds or other bonds such as dative bonds.

Formation of monodispersed semiconductor nanocrystallites is generallyachieved by the following approaches:

o Discrete nucleation is attained by a rapid increase in the reagentconcentrations upon injection, resulting in an abruptsupersaturation.

o Ostwald ripening during aging at increased temperatures promotesthe growth of large particles at the expense of small ones, narrowingthe size distribution.

o Size selective precipitation is applied to further enhance the sizeuniformity.

Organic molecules are used to stabilize the colloidal dispersionsimilarly to metallic colloidal dispersions, but the organic monolayerson the surfaces of semiconductor nanoparticles play a relatively lesssignificant role as a diffusion barrier during the subsequent growthof initial nuclei. This is simply because there is a less extent ornegligible subsequent growth of initial nuclei due to the depletion ofgrowth species and the drop of temperature at the nucleation stage.

A general procedure of synthesis of CdE (E= S, Se, Te) semiconductornanocrystallites is reported as an example. Dimethylcadmium (Me2Cd)was used as the Cd source and bis(trimethylsilyl) sulfide ((TMS)2S),trioctylphosphine selenide (TOPSe), and trioctylphosphine telluride(TOPTe) were used as S, Se and Te precursors, respectively. Mixed tri-n-octylphosphine (TOP) and tri-n-octylphosphine oxide (TOPO)solutions were used as solvents and capping materials.

The procedure is a high temperature (300 C°) process starting fromTOP/TOPO solutions to which are added the reagent mixture. Injectionof reagents into the hot reaction vessel results in a short burst ofhomogeneous nucleation due to an abrupt supersaturation andsimultaneously a sharp drop in temperature associated with theintroduction of room temperature precursor solution.

Colloidal dispersion is purified by several rinses and centrifugation inorder to select specific sizes and to remove the by-products andunreacted solvents.

Depending on the aging time CdSe nanoparticles with a series of sizesranging from ∼1.5 nm to ∼ 11.5 nm in diameter are prepared.The coordinating solvent plays a crucial role in controlling thegrowth process, stabilizing the resulting colloidal dispersion, andelectronically passivating the semiconductor surface.

X-ray powder diffraction spectra of CdSe crystallites ranging from -1.2 to 11.5 nm in diameter, and indicates CdSe crystallites have apredominantly wurtzite crystal structure with the lattice spacing ofthe bulk. Finite size broadening in all diffraction peaks isevident.

The depletion of reagents through the nucleation (when reagents areadded to the solution and temperature is decreased) prevents furthernucleation and hinders the subsequent growth of existing nuclei.Monodispersion is further achieved by gently reheating the solutionto promote slow growth of initial nuclei.

An increased temperature favors the solubility and Ostwald ripeningcan play an important role. Such a growth process can result in theproduction of highly monodispersed colloidal dispersions fromsystems that may initially be polydispersed.

Lowering the synthesis temperature results in a wider sizedistribution with an increased amount of small particles. A loweredtemperature results in an increased supersaturation favoring continuednucleation with smaller sizes. On the contrary, an increasedtemperature will promote the growth of nanoparticles with a narrowsize distribution.

Size-selective precipitation can further narrow the size distributionof the colloids. Using stepwise addition of methanol to the reactionsolution (InP nanocrystals capped with dodecylamine are soluble intoluene but no in methanol) results in the incremental size-selectiveprecipitation of the nanocrystals. If small enough volumes of methanolare used, a sufficiently careful precipitation series can resolve sizedistributions separated by as little as 0.15 nm (range 2-5 nm).

Thermal decomposition of complex precursor in a high-boiling solventis another method for the production of compound semiconductornanoparticles with a narrow size distribution

Nanoparticles of InP, GaP and GaInP, are well crystallized with bulk zincblende structure. An increase in heating duration was found toimprove the crystallinity of the nanoparticles. Different particle sizesranging from 2.0 to 6.5 nm are obtained by changing the precursorconcentration or by changing the temperature.

The size of the nanoparticles can be influenced also by the pH of thesolution.CdS nanoparticles can besynthesized by mixing Cd(ClO4),and (NaPO3)6 solutions with pHadjusted with NaOH and bubbledwith argon gas. The starting pHvalue was found to have asignificant influence on theaverage size of the particlessynthesized. The particle sizeincreases with a decreasingstarting pH value.

VIDEO on the semiconductor QDsSynthesis

V(x) = 0 per 0<x<L

V(x) = ∞ per x<0 e x>L

Possibile soluzione equazione Schroedinger:ψ(x) = A sin kx + B cos kxcon A e B costanti e k2 = 2mE/ħ2 (parabola)

Soluzioni: En = n2h2/8mL2 Energia è quantizzata e n numero quantico

Buca di Potenziale

Dentro la buca si ha:

Con le funzioni d’onda normalizzate: ψn(x) = (2/L)1/2 sin (nπx/L) dette autofunzioni

• Cosa succede se si uniscono più atomi isolati insieme?

• Principio di Pauli • Funzioni d’onda dei singoli atomi

interagiscono e quindi si ha uno splitting dei livelli di energia discreti in nuovi livelli caratteristici dell’insieme di atomi

• Nei solidi si ha la formazione di bande di energia costituite da livelli discreti vicinissimi alternati con zone proibite (gap)

• Elettroni in bande di energia non completamente occupate determinano le proprietà del solido.

• Si ha la formazione di bande di valenza e di conduzione separate da una zona di energia proibita (gap di energia)

• In un cristallo il potenziale è di tipo periodico V(x) = V(x+a) = V(x+2a) =….con a periodicità del reticolo (passo reticolare).

• Singolo elettrone che si muove lungo una linea di atomi assimilabili ad una serie di buche di potenziale (modello Kronig-Penney).

• Discontinuità tra valori di energia consentiti e proibiti si hanno per valori di vettore d’onda k = ±nπ/a con n intero.

• La zona -π/a< k < π/a si chiama prima zona di Brillouin

• Si usa spesso la rappresentazione ridotta in cui l’andamento E-k è riportato alla sola prima zona di Brillouin

In un cristallo reale la relazione E-k è chiaramente più complicata:

Forme delle bande dipendono dall’orientazione del vettore d’onda rispetto agli assi cristallografici

Si può parlare di semiconduttori a gap diretta e indirettase il massimo della banda di valenza e il minimo della banda di conduzione hanno lo stesso valore k