u niversita degli s tudi di p adova corso cfma. ls-simat1 chimica fisica dei materiali avanzati part...
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Corso CFMA. LS-SIMat 1
UUNIVERSITA’ DEGLI NIVERSITA’ DEGLI SSTUDI DI TUDI DI PPADOVAADOVAUUNIVERSITA’ 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
Corso CFMA. LS-SIMat 2
UUNIVERSITA’ DEGLI NIVERSITA’ DEGLI SSTUDI DI TUDI DI PPADOVAADOVAUUNIVERSITA’ DEGLI NIVERSITA’ DEGLI SSTUDI DI TUDI DI PPADOVAADOVA
Nanoscale MaterialsNanoscale Materials
d
1nm<d<100nm
Increasing size
nanoparticle bulk materialmolecule
Milan - Duomo Florence - S. CroceSingle Electron TransistorAndres et al., Science, 1323, 1996
Corso CFMA. LS-SIMat 3
UUNIVERSITA’ DEGLI NIVERSITA’ DEGLI SSTUDI DI TUDI DI PPADOVAADOVAUUNIVERSITA’ DEGLI NIVERSITA’ DEGLI SSTUDI DI TUDI DI PPADOVAADOVA
What are they?What are they?
Nanoparticles Nanorods
0 dimensional nanomaterials:unique properties due to
quantum confinementand very high surface/volume ratio
0 dimensional nanomaterials:unique properties due to
quantum confinementand very high surface/volume ratio
1 dimensional nanomaterials:extremely efficient classical properties
1 dimensional nanomaterials:extremely efficient classical properties
NanowiresNanotubes
Corso CFMA. LS-SIMat 4
UUNIVERSITA’ DEGLI NIVERSITA’ DEGLI SSTUDI DI TUDI DI PPADOVAADOVAUUNIVERSITA’ DEGLI NIVERSITA’ DEGLI SSTUDI DI TUDI DI PPADOVAADOVA
Properties of Metal NanoparticlesProperties of Metal NanoparticlesOptical Properties
Nanoscale Materials have
different properties when
compared to their bulk
counterparts!
Nanoscale Materials have
different properties when
compared to their bulk
counterparts!
Electronic Properties
Melting point of Au vs. nanoparticle radius (Å)
Corso CFMA. LS-SIMat 5
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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
Corso CFMA. LS-SIMat 6
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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)
Corso CFMA. LS-SIMat 7
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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)
Corso CFMA. LS-SIMat 8
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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
Corso CFMA. LS-SIMat 9
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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)
Corso CFMA. LS-SIMat 10
UUNIVERSITA’ DEGLI NIVERSITA’ DEGLI SSTUDI DI TUDI DI PPADOVAADOVAUUNIVERSITA’ DEGLI NIVERSITA’ DEGLI SSTUDI DI TUDI DI PPADOVAADOVA
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+
Corso CFMA. LS-SIMat 11
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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
Mo
bili
ty +
-
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.
Corso CFMA. LS-SIMat 12
UUNIVERSITA’ DEGLI NIVERSITA’ DEGLI SSTUDI DI TUDI DI PPADOVAADOVAUUNIVERSITA’ DEGLI NIVERSITA’ DEGLI SSTUDI DI TUDI DI PPADOVAADOVA
Electrostatic stabilization of metal colloidsElectrostatic stabilization of metal colloids
Corso CFMA. LS-SIMat 13
UUNIVERSITA’ DEGLI NIVERSITA’ DEGLI SSTUDI DI TUDI DI PPADOVAADOVAUUNIVERSITA’ DEGLI NIVERSITA’ DEGLI SSTUDI DI TUDI DI PPADOVAADOVA
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.
Corso CFMA. LS-SIMat 14
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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 x
i
ondistributi Boltzmann
exp kTez xiixi
02
2
xiix ez
dx
d
Corso CFMA. LS-SIMat 15
UUNIVERSITA’ DEGLI NIVERSITA’ DEGLI SSTUDI DI TUDI DI PPADOVAADOVAUUNIVERSITA’ DEGLI NIVERSITA’ DEGLI SSTUDI DI TUDI DI PPADOVAADOVA
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
kT
ezezez
dx
d xiiixiix
exp
002
2
sxi
xxii Edxddxdxddxez 000
0 0
220
0
sE
kTE
kTdx
d
kT isix
isii0
220
2
0
00 222
Corso CFMA. LS-SIMat 16
UUNIVERSITA’ DEGLI NIVERSITA’ DEGLI SSTUDI DI TUDI DI PPADOVAADOVAUUNIVERSITA’ DEGLI NIVERSITA’ DEGLI SSTUDI DI TUDI DI PPADOVAADOVA
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
Corso CFMA. LS-SIMat 17
UUNIVERSITA’ DEGLI NIVERSITA’ DEGLI SSTUDI DI TUDI DI PPADOVAADOVAUUNIVERSITA’ DEGLI NIVERSITA’ DEGLI SSTUDI DI TUDI DI PPADOVAADOVA
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
Corso CFMA. LS-SIMat 18
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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
2
0
x
x
ii
ixi dx
d
kT
xx
x
x ee
kT
e
e
e
kT
4
1
1log
2
kTe 4tanh 0 1
Gouy-Chapman theory
xx e 0
Debye-Hükel equation
Corso CFMA. LS-SIMat 19
UUNIVERSITA’ DEGLI NIVERSITA’ DEGLI SSTUDI DI TUDI DI PPADOVAADOVAUUNIVERSITA’ DEGLI NIVERSITA’ DEGLI SSTUDI DI TUDI DI PPADOVAADOVA
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
Corso CFMA. LS-SIMat 20
UUNIVERSITA’ DEGLI NIVERSITA’ DEGLI SSTUDI DI TUDI DI PPADOVAADOVAUUNIVERSITA’ DEGLI NIVERSITA’ DEGLI SSTUDI DI TUDI DI PPADOVAADOVA
DLVO theoryDLVO theory
Consider equal sized spheres and low potentials.
The total interaction must also include the VdW attraction
RADLVO WWW
D
ARWA 12
022
200
2
2
D
DR
eR
eRW
Derjaguin & Landau (1937); Verwey & Overbeek (1944)
Corso CFMA. LS-SIMat 21
UUNIVERSITA’ DEGLI NIVERSITA’ DEGLI SSTUDI DI TUDI DI PPADOVAADOVAUUNIVERSITA’ DEGLI NIVERSITA’ DEGLI SSTUDI DI TUDI DI PPADOVAADOVA
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.
Corso CFMA. LS-SIMat 22
UUNIVERSITA’ DEGLI NIVERSITA’ DEGLI SSTUDI DI TUDI DI PPADOVAADOVAUUNIVERSITA’ DEGLI NIVERSITA’ DEGLI SSTUDI DI TUDI DI PPADOVAADOVA
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 E
nerg
y/ k
T
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
Corso CFMA. LS-SIMat 23
UUNIVERSITA’ DEGLI NIVERSITA’ DEGLI SSTUDI DI TUDI DI PPADOVAADOVAUUNIVERSITA’ DEGLI NIVERSITA’ DEGLI SSTUDI DI TUDI DI PPADOVAADOVA
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
En
erg
y /
kT
Surface Separation /Å
Debye Length = 30Å-1
Radius = 100Å
A = 2.5x10-19 J
200mV
150mV
100mV
50mV
25mV
-100
-50
0
50
100
150
200
250
300
0 50 100 150 200
pzcpHpH 0592.00
Corso CFMA. LS-SIMat 24
UUNIVERSITA’ DEGLI NIVERSITA’ DEGLI SSTUDI DI TUDI DI PPADOVAADOVAUUNIVERSITA’ DEGLI NIVERSITA’ DEGLI SSTUDI DI TUDI DI PPADOVAADOVA
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
Corso CFMA. LS-SIMat 25
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Place Exchange ReactionsPlace Exchange Reactions
Corso CFMA. LS-SIMat 26
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Ligands on NanoparticlesLigands on Nanoparticles
Corso CFMA. LS-SIMat 27
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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.
Corso CFMA. LS-SIMat 28
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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
Corso CFMA. LS-SIMat 29
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Order/Disorder TransitionOrder/Disorder Transition
10nm@1220C @200C
Deinterdigitated
Interdigitated
Corso CFMA. LS-SIMat 30
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Characterizing Metal NanoparticlesCharacterizing Metal Nanoparticles
2.7 nm
3 nm
TEM shows atoms in the core STM shows ligands in the shell