Preparation: Hot injection method

Supersaturation 1. Injection of reagentsinto hot solvent => rapid nucleation and cooling below the solubility product

2. Slow heating ”alongthe solubility curve”to grow the nucleiuniformly

The solution is cooled: i) cold liquid is injectedii) nucleation is usually endothermic

Preparation: Hot injection method

Preparation: Hot injection method• Separated nucleation and growth stages

– Instantaneous nucleation– Narrow size distribution– Good control of particle size– High temperature => Good crystal structure

• Disadvantages: – Expensive and poisonous chemicals– Difficult to scale-up for commercial production

Hot injection- CdSe semiconductor particlesCdMe2+Se-TOP

~300°CTOPOStabilizers

TOPO = Trioctylphosphine oxide”strongly coordinatingsolvent”TOP=Trioctylphosphine

www.mrsec.wisc.edu/edetc/SlideShow/shows/quantumdot.html

Hot injection - Cobalt particlesCo2(CO)8

~200°CDichlorobenzeneStabilizers

Co2(CO)8 = Complexwhere the oxidation stateof cobalt is 0

∆T

½ nCo2(CO)8 ==> Con + 4nCO

Preparation: Hot injection –shape control

• Differences in adsorption of surfactants on different crystal faces

• Surfactant mixtures => The particle grows anisotropically

V. F. Puntes, K. M. Krishnan and A. P. Alivisatos, Science, 2001,291, 2115.

Hot injection

• Celso de Mello Doneg, Peter Liljeroth, and Daniel Vanmaekelbergh, Physicochemical Evaluation of the Hot-Injection Method, a Synthesis Route for Monodisperse Nanocrystals, Small 2005, 1, No. 12, 1152 – 1162.

Preparation: Brust-Schiffrin method• Simple method for preparing relatively

monodisperse gold particles by reduction with NaBH4

• Does not require special equipment –can be carried out in virtually any laboratory

• Can be adapted to copper and silver

Preparation: Brust-Schiffrin method

HAuCl4 in water(yellow)

tetraoctylammoniumbromide in tolueneTOA+Br-

1. 2. 3. 4.

phase transfer:Br-(o) → Br-(aq)AuCl4-(aq) → AuCl4-(o) (red)

addition of alkanethiol into toluene forms a polymeric gold-thiol complex (colourless)

adding NaBH4(aq) and separating the toluene phase with the particles,

⇒ formation of alkanethiol-protected gold nanoparticles

M. Brust et al., Chem. Commun. (1994) 801.

Preparation: Reverse micelles (microemulsions)

• In reverse micelles small water droplets in an organic solvent are stabilized by surfactants

• The size of the micellesis controlled by the water/surfactant ratio

• The droplets work as ”microreactors” whereonly a limited amount of rectants are available

[ ][ ]Sa

OHVR aq

W23

=

Preparation: Synthesis in templates• Forming the nanoparticles in pores of a membrane • Dissolving the membrane

Example: Electrodeposition of Cu-Co rods in a polycarbonate membrane

ElectrodeMembrane

Solution with metal ions

Post-synthetic size control

• Most methods give too large size distributions for advanced applications

Post: Size fractionation• A non-solvent is added to a good solvent with particles (e.g.

Ethanol-toluene for alkanethiol stabilized gold particles)• Small particles are more soluble than big• The precipitate (big particles) is separated from the solution

(small particles)• The procedure is repeated until sufficiently narrow distribution

have been obtained• Method is very slow...• ...but fractions with very narrow size can be obtained• The physical chemistry can be approached, e.g. scaled particle

theory• Suitable only for particles that can be redispersed

Post: Size fractionation

Example: Supercritical-CO2 – Hexane system

*Semi-continuous operation

Roberts et al.

Post: Teranishi method• Nanoparticle precipitate is heated• Small particles sinter to bigger particles• Bigger particles remain unchanged• => Narrow size distribution• Higher temperature => bigger particles• The size dependent melting point of the particles has

been suggested as an explanation for the process

Teranishi et al.

Post: Digestive ripening• Reflux of a colloid

in presence of excess surfactants

• Big particles are etched

• Small particles grow

• Difficult to explain

Post: Digestive ripening

Klabundeet al.

Ligand excessReflux 110ºC

Post: Digestive ripening

•Digestive ripening is reversible!

Klabunde,

Digestive ripening1. Mix a gold colloid and a

copper colloid2. Reflux the solution at

198°C

⇒Gold-copper alloyparticles

Nanoparticles are extremely reactivecompared to bulk materials

Ligands play a key role in atom transfer reactions

Crystal structure and shape

Properties : Core size and Shape of MPCs

The number of atoms per core tendstoward closed shell structures (”magicnumbers”) with an equilibriumtruncated octahedral shape.

A.C. Templeton, W.P. Wuelfing, R.W. Murray, Acc. Chem. Res. 33 (2000) 27.

459

314

225

140116

Laser desorption/ionization mass spectra of a dodecanethiolate MPC mixture

Magnetic properties of nanoparticles

•Superparamagnetism = susceptibilitycomparable to ferromagnetic materials, but no magnetic memory•Curie temperature decreases with particle size•One domain per particle•Magnetic nanoparticles have manyapplications

•Soft magnetic materials•Magnetic memories•Spintronics•Targeted drug delivery•Recyclable catalysts•Therapeutic hyperthermia

•Properties should be optimized for each application•Particle-particle interactions are alsoimportant

Optical properties of semiconductor nanoparticles

Fluorescence as a function of size in CdSe. Dr. D. Talapin, University of Hamburg, http://www.chemie.uni-hamburg.de/pc/Weller/

• Absorption and Fluorescence are determined by the band gap structureof the particles. As the size decreasesthe gap gets smaller, compare to particlein a box model.

Optical properties• Plasmon resonance:

– Collective motion of electrons in response to light

– Cu, Ag, Au have the strongest plasmonsabsorption in the visible range

– Sensitive to interparticle distance!! => Proximitysensors and switches

– Modeled by Mie theory

BiosensorsBiosensor s

Stabilization of particles

Interactions between Particlesin colloids

• Electrostatic = determined by the charge carried by the particle, the ionic strength and dielectric properties of the medium

• van der Waals forces between ligand-ligand, core-core and core-ligand

Stabilization:• Electrostatic determined by the charge on the particle i.e. on the

core and ions adsorbed on the particle• Steric repulsion between ligands or polymers

Distance

Ene

rgy

Stabilizingenergybarrier >kT

Deep minimumDue to high surfaceenergy of solids=>permanent aggregation

Electrostatic stability – DVLO theory

Vtot= Vrep+ Vatt

• Attractive forces are short range, pictured by the Hamakerconstant A

• Repulsive forces long range electrostatic• Steric forces not considered• Critical coagulation concentration depends on counter ion

valence z-6

( )

[ ][ ] 12/exp

12/exp

1085.9..

0

0

626

4534

+−=

×≈

kTze

kTze

zAeN

kTccc

A

ψψγ

γε

-

- -

- -

-

--

- -

- -

-

-

+

+

++

++

++

+

+

+ ++

Steric stabilization• Important from a technological point: High

concentrations which cannot be achieved with electrostatic stabilization, nonpolar solvents etc.

• Particles stabilized by organic ligands or polymers• When two particles approach each other their

protecting layers are mixed. Can be understood for example using a Flory-Huggins type interaction model between the solvent and the polymer:– ∆GR > 0 => stable colloid

– ∆GR < 0 => aggregation

• ∆GR =∆HR –T∆SR

– Consider solvent-solvent, solvent-polymer and polymer-polymer interaction for ∆HR

Steric stabilization• Protecting layer is confined to the particle surface cmp. polymer

solutions– Particle size matters => when adding a non-solvent to a colloid

large particles precipitate first

• Strong core-core interactions must also be considered, e.g.magnetic dipoles

• Aggregation can be reversible => Nanoparticles can be treatedas molecular entities!

DistanceE

nerg

y

Localminimum induced by adding a non-solvent

Permanent aggregation requires removingligands

Steric stabilization• Common ligands• Alkane chains equipped with:

– Thiols R-SH– Au, Ag, Cu, Semiconductors

– Carboxylic acids R-COOH– Ni, Co, Fe, Semiconductors

– Amines– Pt, Pd, Co, Ni, Semiconductors

– Phosphines and phosphine oxides-Semiconductors, Co

– Alcohols, silanes, nitriles, amides, other– Usually weak adsorption

Self-assembled monolayers

Surfactant monolayers on nanoparticles

• The polar group (R-SH, R-COOH, R-NH2,...) is adsorbed on the particle surface.

• The bond strengths can be comparable to covalent bonds, e.g. Au–S-R >200 kJ/mol

• Charge transfer between the ligand and the core can occur, e.g. In gold particles the gold atom to which the thiol is attached has a formal charge +1.

• HS-R + Au0 => ½H2 + RS-—Au+

• Despite the high bond strengths ligands move along the surface.• Ligand exchange also occurs if excess ligands are added to a

solution of nanoparticles.• Usually high energy sites (kinks, edges, corners) are occupied

first• The most preferential sites are in the gaps between the atoms

Monolayers on nanoparticles

NP assemblies

Binary particle assemblies

Binary particle assemblies

Particle assemblies

Oleic acid

Hexanoic acid

Properties : Effect of interparticle separation distance on the electronic structure of the QD

superlattice

Weakly interacting QD: Coulomb gapin the electronic band structure=> insulator

As distance decreases, the Coulomb gapcloses => electrical conductivity

G. Markovich, C.P. Collier, S.E. Henrichs, F. Remacle, R.D. Levine, J.R. Heath, Acc. Chem. Res. 1999, 32, 415.

Au147

Cu2+

Cu2+

Sensing properties depend on the metal ion bridge

Sensor