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Laurea specialistica in Scienza e Ingegneria dei Materiali
Curriculum Scienza dei Materiali
Chimica Fisica dei Materiali Avanzati
Part 5 – Colloids and nanoparticles
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Nanoscale Materials
1nm<d<100nm
d
Increasing size
molecule
Milan - Duomo
nanoparticle
SingleCorso
Electron
Transistor
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Andres et al., Science, 1323, 1996
bulk material
Florence - S. Croce
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What are they?
0 dimensional nanomaterials:
unique properties due to
quantum confinement
and very high surface/volume ratio
Nanoparticles
Nanorods
1 dimensional nanomaterials:
extremely efficient
classical properties
Nanowires
Nanotubes
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Properties of Metal Nanoparticles
Nanoscale Materials have
different properties when
compared to their bulk
counterparts!
Optical Properties
Melting point of Au vs.
nanoparticle radius (Å)
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Electronic Properties
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A brief historical background
Marie-Christine Daniel and Didier Astruc, Chem. Rev. 2004, 104, 293-346
 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”.
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A brief historical background (cont’d)
 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.
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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.
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Synthesis of Metal Nanoparticles
The citrate method (J. Turkecitch et al., 1951)
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).
 It is easy
 It requires only water
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
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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)
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Colloids 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+
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Point of Zero Charge
Mobility
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.
amphoteric
+
pH
3
5
7
9
pAg
-
COOH
Only surface
The pzc is sensitive to adsorbed
ions and molecules, crystallinity
of the particles and ionic strength
Typically a particular material
has a characteristic pH at which
it is neutral. This is called the
point of zero charge (pzc)
Mine ral
Fe2O3, FeOOH
TiO2
SiO2
MnO2
Al2O3, AlOOH
PbS, CdS
proteins
Latex particles (COOH)
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pzc
6-8
4-6
2-3
2-4
8-10
2-3
6-8
4-6
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Electrostatic stabilization of metal colloids
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The 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.
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Basic relations determining the ionic distribution

The chemical potential of an ion, of
valency zi and number density i
x at
a distance x from the surface, is
constant
i  i0  zi e x   kT log i x 

In the bulk, where the ionic
concentration is
i , the potential is
  x       0

Therefore, at the surface (x = 0)
 xi  i exp zi e x kT 
Boltzmann distributi on

The two unknown quantities are
related by the Poisson equation
d 2 x
ezi  xi


dx 2
 0
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Poisson-Boltzmann equation
d 2 x
zi e xi
zi ei
 zi e x 




exp


dx 2
 0
 0
kT 

 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 s. Thus


s     zi e xi dx   0  d 2 x dx 2 dx   0  d x dx 0    0 Es
0
whence
i
0
Es 
s
 0
 A further general relation can be worked out
 0  d x 
s2
0i   si  i 
E  i 

  i 
2kT  dx 0
2kT
2 0 kT
2
 0
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2
s
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Charged 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  x    xi and the net charge
i
density is  zi e xi .
i
 The total concentration of ions at an isolated surface of charge
density s is given by
2




s
2 0 kT
 0i  i
i
(in number per m3 )
i
this is known as Grahame equation. Once we replace on the
LHS 0i  i exp zi e 0 kT  , it provides a link between
potential and surface charge density.
s   0 0
 For low potentials it simplifies to
12
where


    i e 2 zi2  0 kT 
 i

[m-1 ]
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The Debye length
 The diffuse electric double layer near a charged surface has a
characteristic length or ‘thickness’ known as the Debye length
1 .
 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
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Potential and ionic concentrations
 Worked out from the general relation

i
xi
  i 
i
0  d x 
2


2kT  dx  x
(cf. slide 15)
 For 1:1 electrolytes (e.g., NaCl)
1  ex  4kT x
2kT
x 
log 

e
x 
e
e
1  e 
Gouy-Chapman theory
 x   0ex
Debye-Hükel equation
where   tanhe 0 4kT  . For high potentials   1 .
For low potentials
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Interaction 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
W D  2 0 02eD  2s 2eD 0
 For two spheres of radius R
W D, R   2R 0 02eD  2Rs 2eD  2 0
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DLVO theory
Derjaguin & Landau (1937); Verwey & Overbeek (1944)
 Consider equal sized spheres
and low potentials.
 The total interaction must also
include the VdW attraction
WDLVO  WA  WR
WA  
AR
12 D
WR  2R 0 02e D
 2Rs 2e D  2 0
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DLVO theory, colloid stability and coagulation

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.
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Effect of Hamaker Constant
20
A = 2.5x10
-20
 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.
J
Interaction Energy/ kT
15
A = 10
10
-20
J
5
0
A = 10
-19
J
-5
A = 2.5x10
-10
-19
J
Radius = 100Å
Surface Potential = 0.05V
-15
Debye Length = 30Å
-20
0
50
100
150
200
-1
250
Surface Separation / Å
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Effect of Surface Potential
300
200mV
Interaction Energy / kT
250
Debye Length = 30Å
Radius = 100Å
200
A = 2.5x10
150
-19
 When the particle charge is
low enough the van der Waals
attraction will lead to
coagulation.
 For a metal oxide particle, we
can approximate:
-1
J
150mV
100
 0  0.0592 pH  pH pzc 
100mV
50
0
50mV
-50
25mV
-100
0
50
100
150
Surface Separation /Å
200
 When salt is added the
repulsive curve decays more
quickly, and the maximum
repulsion decreases.
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Metal Nanoparticles Synthesis
Metal Salt (AuHCl4) +
HS
+
Reducing Agent (NaBH4)
HS
Direct mixed
ligands reaction**
F. Stellacci, et al. Adv. Mat. 2002, 14, 194
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Ligand exchange
reaction*
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A. C. Templeton, M. P. Wuelfing and R. W. Murray, Accounts Chem. Res. 2000, 33, 27
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Place Exchange Reactions
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Ligands on Nanoparticles
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Solubility Issue
liquid state
Solid interdigitated state
Solid de-interdigitated state
DHde-int
DHsol
4
CAB1-cycle 1
CAB1-cycle 2
2
Differential Scanning
Calorimetry
< ENDO
0
liquid
-2
-4
decomposition
DH
deinterdigitation
-6
DSC -
Pradeep et al, Phys. Rev. B, 2(62), R739,2000.
Badia et. al. Chem. Europ. J., 2(3), 359,1996.
-8
0
50
100
150
temperature C
200
250
300
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Control 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
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Order/Disorder Transition
@1220C
10nm
@200C
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Deinterdigitated
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Interdigitated
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Characterizing Metal Nanoparticles
3 nm
TEM shows atoms in the core
STM shows ligands in the shell
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2.7 nm
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