Chimica Fisica dei Materiali Avanzati Part 5 – Colloids and nanoparticles

Corso CFMA. LS-SIMat 1

UUNIVERSITA’ DEGLI NIVERSITA’ DEGLI SSTUDI DI TUDI DI PPADOVAADOVA

Chimica Fisica dei Materiali AvanzatiChimica Fisica dei Materiali Avanzati

Part 5 – Colloids and nanoparticlesPart 5 – Colloids and nanoparticles

Laurea specialistica in Scienza e Ingegneria dei MaterialiCurriculum Scienza dei Materiali



Nanoscale MaterialsNanoscale Materials

d

1nm<d<100nm

Increasing size

nanoparticle bulk materialmolecule

Milan - Duomo Florence - S. CroceSingle Electron TransistorAndres et al., Science, 1323, 1996



What are they?What are they?

Nanoparticles Nanorods

0 dimensional nanomaterials:unique properties due to

quantum confinementand very high surface/volume ratio

1 dimensional nanomaterials:extremely efficient classical properties

NanowiresNanotubes



Properties of Metal NanoparticlesProperties of Metal NanoparticlesOptical Properties

Nanoscale Materials have different properties when compared to their bulk counterparts!

Electronic Properties

Melting point of Au vs. nanoparticle radius (Å)



A brief historical backgroundA brief historical background

it is probable that “soluble” gold appeared around the 5th or 4th century B.C. in Egypt and China.

the Lycurgus Cup that was manufactured in the 5th to 4th century B.C. It is ruby red in transmitted light and green in reflected light, due to the presence of gold colloids.

The reputation of soluble gold until the Middle Ages was to disclose fabulous curative powers for various diseases, such as heart and venereal problems, dysentery, epilepsy, and tumors, and for diagnosis of syphilis.

the first book on colloidal gold, published by the philosopher and medical doctor Francisci Antonii in 1618. This book includes considerable information on the formation of colloidal gold sols and their medical uses, including successful practical cases.

In 1676, the German chemist Johann Kunckels published another book, whose chapter 7 concerned “drinkable gold that contains metallic gold in a neutral, slightly pink solution that exert curative properties for several diseases”. He concluded that “gold must be present in such a degree of communition that it is not visible to the human eye”.

Marie-Christine Daniel and Didier Astruc, Chem. Rev. 2004, 104, 293-346



A colorant in glasses, “Purple of Cassius”, is a colloid resulting from the heterocoagulation of gold particles and tin dioxide, and it was popular in the 17th century.

A complete treatise on colloidal gold was published in 1718 by Hans Heinrich Helcher. In this treatise, this philosopher and doctor stated that the use of boiled starch in its drinkable gold preparation noticeably enhanced its stability.

These ideas were common in the 18th century, as indicated in a French dictionary, dated 1769, under the heading “or potable”, where it was said that “drinkable gold contained gold in its elementary form but under extreme sub-division suspended in a liquid”.

In 1794, Mrs. Fuhlame reported in a book that she had dyed silk with colloidal gold.

In 1818, Jeremias Benjamin Richters suggested an explanation for the differences in color shown by various preparation of drinkable gold: pink or purple solutions contain gold in the finest degree of subdivision, whereas yellow solutions are found when the fine particles have aggregated.

A brief historical background (cont’d)A brief historical background (cont’d)



In 1857, Faraday reported the formation of deep red solutions of colloidal gold by reduction of an aqueous solution of chloroaurate (AuCl4

-) using phosphorus in CS2 (a two-phase system) in a well known work.

He investigated the optical properties of thin films prepared from dried colloidal solutions and observed reversible color changes of the films upon mechanical compression (from bluish-purple to green upon pressurizing).

The term “colloid” (from the French, colle) was coined shortly thereafter by Graham.

A brief historical background (cont’d)A brief historical background (cont’d)



Synthesis of Metal NanoparticlesSynthesis of Metal NanoparticlesThe citrate method (J. Turkecitch et al., 1951)

It is easy It requires only water

It is used to produce modestly monodisperse spherical gold nanoparticles suspended in water of around 10–20 nm in diameter.

•Take 5.0×10−6 mol of HAuCl4, dissolve it in 19 ml of deionized water (should be a faint yellowish solution) •Heat it until it boils •While stirring vigorously, add 1 ml of 0.5% sodium citrate solution; keep stirring for the next 30 minutes•The colour of the solution will gradually change from faint yellowish to clear to grey to purple to deep purple, until settling on wine-red. •Add water to the solution to bring the volume back up to 20 ml (to account for evaporation).

The sodium citrate first acts as a reducing agent, and later the negative citrate ions are adsorbed onto the gold nanoparticles and introduce the surface charge that repels the particles and prevents them from aggregating. To produce bigger particles, less sodium citrate should be added (possibly down to 0.05%, after which there simply would not be enough to reduce all the gold). The reduction in the amount of sodium citrate will reduce the amount of the citrate ions available for stabilizing the particles, and this will cause the small particles to aggregate into bigger ones (until the total surface area of all particles becomes small enough to be covered by the existing citrate ions).

It requires skills It has reproducibility issues



What is a Colloid?What is a Colloid?Definition: A colloid is a dispersion of small particles

in a medium. The particles may be solid, liquid or gas, and the medium is normally liquid but may be a gas.

Types of Colloid Sol: Dispersion of very small solid particles in solution.

Example: Faraday gold sol (15nm) Aerosol: Dispersion of droplets or particles in air.

(Example: steam) Emulsion: Dispersion of oil in water or water in oil.

(Example: salad dressing)



Colloids and nanoparticlesColloids and nanoparticles If only VdW forces were operating, we might expect all dissolved

particles to stick together (coagulate) immediately and precipitate out of solution as a mass of solid material.

Fortunately, there are repulsive forces that prevent coalescence, like electrostatic, solvation and steric forces.

Particles suspended in water or any other liquid of high dielectric constant are usually charged

Charge can arise from several chemical processes: Dissolution of the lattice

E.g., AgI(s) Ag+(aq) + I-(aq) Adsorption of ions, polymers & detergents

E.g., humic acid (a poly-carboxylic acid) onto soil particles Surface Acid Base reactions

E.g., Ti-OH(s) TiO- + H+

T.W. Healy and L.R. White Adv. Coll. Interface Sci., 9, 303 (1978). Electron Transfer

E.g., (CH3)2COH + Agn (CH3)2CO + Agn- + H+



Point of Zero ChargePoint of Zero Charge

Typically a particular material has a characteristic pH at which it is neutral. This is called the point of zero charge (pzc)

Mineral pzcFe2O3, FeOOHTiO2SiO2MnO2Al2O3, AlOOH

6-84-62-32-48-10

PbS, CdSproteins

2-36-8

Latex particles (COOH) 4-6

pAg

Mob

ility +

-

3 5 7 9

pH

amphoteric

COOHOnly surface

The pzc is sensitive to adsorbedions and molecules, crystallinityof the particles and ionic strength

A surface charge is at its point of zero charge (pzc) when the surface charge density is zero. It is a value of the negative logarithm of the activity in the bulk of the charge-determining ions.



Electrostatic stabilization of metal colloidsElectrostatic stabilization of metal colloids



The electric double layerThe electric double layer

Independent of the charging mechanism, the surface charge is balanced by an equal but oppositely charged region of counterions. Some of them are transiently bound to the surface and form the Stern or Helmoltz layer.Other form a diffuse layer in rapid thermal motion: the electric double layer.



Basic relations determining the ionic distributionBasic relations determining the ionic distribution The chemical potential of an ion, of

valency zi and number density at a distance x from the surface, is constant

In the bulk, where the ionic concentration is , the potential is

Therefore, at the surface (x = 0)

The two unknown quantities are related by the Poisson equation

xkTxez iiii log0

xi

0 xi

ondistributi Boltzmann

exp kTez xiixi

02

2

xiix ez

dxd



Poisson-Boltzmann equationPoisson-Boltzmann equation

It is a non-linear second-order differential equation Subjected to the boundary condition of overall neutrality of

the system, i.e., the total charge of the counterions must counterbalance the charge on the surface . Thus

whence

A further general relation can be worked out

kTezezez

dxd xiiixiix

exp

002

2

sxi

xxii Edxddxdxddxez 0000 0

220

0sE

kTE

kTdxd

kT isix

isii0

220

2

0

00 222



Charged surfaces in electrolyte solutionsCharged surfaces in electrolyte solutions If, as in most practical cases, the charged surfaces interact

across a solution that already contains electrolyte ions, the total ionic concentration is and the net charge density is .

The total concentration of ions at an isolated surface of charge density is given by

this is known as Grahame equation. Once we replace on the LHS , it provides a link between potential and surface charge density.

For low potentials it simplifies to

where

i

xix

ixiiez

)mper number (in 2 30

20

ii

ii kT

exp 00 kTeziii

00

][m 121

022 -

iii kTze



The Debye lengthThe Debye length The diffuse electric double layer near a charged surface has a

characteristic length or ‘thickness’ known as the Debye length .

The magnitude of the Debye length depends solely on the properties of the liquid and not on the charge or potential of the surface Values

1



Potential and ionic concentrationsPotential and ionic concentrations Worked out from the general relation

For 1:1 electrolytes (e.g., NaCl)

where . For high potentials . For low potentials

15) slide (cf. 2

20

x

x

ii

ixi dx

dkT

xx

x

x eekT

ee

ekT

4

11log2

kTe 4tanh 0 1

Gouy-Chapman theory

xx e 0

Debye-Hükel equation



Interaction free energiesInteraction free energies From the above results, the interaction free energy per unit

area is estimated (at low surface potentials) For two planar surfaces at distance D

For two spheres of radius R

022

00 22 DD eeDW

0222

00 22, DD eReRRDW



DLVO theoryDLVO theory Consider equal sized spheres

and low potentials. The total interaction must also

include the VdW attraction

RADLVO WWW

DARWA 12

022

200

2

2

D

DR

eR

eRW

Derjaguin & Landau (1937); Verwey & Overbeek (1944)



DLVO theory, colloid stability and DLVO theory, colloid stability and coagulationcoagulation

Depending on the electrolyte concentration and surface charge density or potential, one of the following may occur:

(a) – For highly charged surfaces and dilute electrolytes (long Debye length): strong long range repulsion peaking at the energy barrier (some 1 – 4 nm away from the surface)

(b) – For higher electrolyte conc. A significant secondary minimum appears before the energy barrier. The potential energy minimum at constant is called primary minimum. The colloids are kinetically stable because overcoming the energy barrier is a slow process and the particles either sit in the secondary minimum or remain dispersed.

(c) – For surfaces of low charge density or potential, the energy barrier is much lower leading to slow aggregation, known as coagulation or floculation.

(d) – Above some concentration of electrolyte the energy barrier falls below zero and the particles coagulate rapidly: the colloids are now unstable.

(e) – As the surface charge approaches zero the interaction curve approaches the pure VdW curve and the two surfaces attract each other at all separations.



Effect of Hamaker ConstantEffect of Hamaker Constant

DLVO Theory was a seminal moment in solution chemistry and physics. By assuming an attractive force between particles balanced by the repulsive charge, can quantitatively predict particle coagulation, settling, adhesion.In

tera

ctio

n En

ergy

/ kT

Surface Separation / Å

Radius = 100ÅSurface Potential = 0.05V

Debye Length = 30Å -1

A = 10-20 J

A = 2.5x10-20 J

A = 10-19 J

A = 2.5x10-19 J

-20

-15

-10

-5

0

5

10

15

20

0 50 100 150 200 250



Effect of Surface PotentialEffect of Surface Potential

When the particle charge is low enough the van der Waals attraction will lead to coagulation.

For a metal oxide particle, we can approximate:

When salt is added the repulsive curve decays more quickly, and the maximum repulsion decreases.

Inte

ract

ion

Ener

gy /

kT

Surface Separation /Å

Debye Length = 30Å-1

Radius = 100ÅA = 2.5x10-19 J

200mV

150mV

100mV

50mV25mV

-100

-50

0

50

100

150

200

250

300

0 50 100 150 200

pzcpHpH 0592.00



Metal Nanoparticles SynthesisMetal Nanoparticles Synthesis

Metal Salt (AuHCl4) + HS

Reducing Agent (NaBH4)+HS

Ligand exchange reaction*

Direct mixed ligands reaction**

A. C. Templeton, M. P. Wuelfing and R. W. Murray, Accounts Chem. Res. 2000, 33, 27F. Stellacci, et al. Adv. Mat. 2002, 14, 194



Place Exchange ReactionsPlace Exchange Reactions



Ligands on NanoparticlesLigands on Nanoparticles



Solubility IssueSolubility Issue

-8

-6

-4

-2

0

2

4

0 50 100 150 200 250 300

CAB1-cycle 1CAB1-cycle 2

< E

ND

O

temperature C

liquiddecomposition

H deinterdigitation

DSC - Differential Scanning Calorimetry

Solid interdigitated stateSolid de-interdigitated state

liquid state

Hde-int Hsol

Badia et. al. Chem. Europ. J., 2(3), 359,1996.Pradeep et al, Phys. Rev. B, 2(62), R739,2000.



Control of interdigitationControl of interdigitation The shorter the ligand the smaller the interdigitation

enthalpy The larger the nanoparticle the smaller the

interdigitation enthalpy, probably because of surface curvature effect

Mixture of ligands lower the interdigitation enthalpy



Order/Disorder TransitionOrder/Disorder Transition

10nm@1220C @200C

Deinterdigitated

Interdigitated



Characterizing Metal NanoparticlesCharacterizing Metal Nanoparticles

2.7 nm

3 nm

TEM shows atoms in the core STM shows ligands in the shell

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Chimica Fisica dei Materiali Avanzati Part 5 – Colloids and nanoparticles