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Synthesis of metal, oxide, and semiconductor nanoparticles, homogeneous and heterogeneous nucleation.
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III. 0-dimensional nanostructures
Small size
Monosized
Identical shape or morphology
Identical chemical composition and crystal
structureNo agglomeration
Required features of nanoparticles:
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Bottom up approaches preferred:
1.Generation of supersaturation
Liquid
Vapor
Solid2.Nucleation
Homogeneous nucleation
Heterogeneous nucleation
3.Subsequent growth Confined space (micelle, microemulsion)
Many methods have been developed.
Synthesis of nanoparticles
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1. Nanoparticles through homogeneous nucleation
Generation of supersaturation is a prerequisite:
Reduction of T of an equilibrium mixtureIn situ chemical reaction by converting
highly soluble chemical into less soluble
chemicals.
1.1 fundamental of homogeneous nucleation
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Reduction of Gibbs free energy is the driving
force for both nucleation and growth.
Two contributions to total Gibbs energy:
1. Phase transformation: supersaturated
solution has high Gibbs free energy. It will be
reduced by segregating solute from the solution.The change of Gibbs free energy per unit volume
of the solid phase
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2. Increase in liquid/solid interface surface
energy that will be created when solutes aresegregated.
Assuming a spherical nucleus with a radius of r,
is the surface energy per unit area.
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the newly formed nucleus is stable only when its
radius exceeds a critical size, r*.
Total:
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Energy barrier that a nucleation process must overcome.
Minimum size of a stable spherical nucleus3
It also works for supersaturated vapor and a
supercooled gas or liquid
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To reduce the critical size
Increase gibbs free energy
Increasing the supersaturation
Decrease surface energy
use of different solvent
additive in solution
incorporation of impurities into solid phase
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Thermodynamics: Is nucleation possible ?
(energy minimization)
how small can you prepare?
Kinetics: How fast does it happen ?(nucleation rate)
how small can you prepare in reality?
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The rate of nucleation ,
is proportional to(i) the probability
(ii) the number of growth species per unit volume,
n, which can be used as nucleation centers (inhomogeneous nucleation, it equals to the initial
concentration, Co)
(iii) the successful jump frequency ofgrowth species, from one site to another
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high initial concentration or supersaturation
(so,a large number ofnucleation sites)
low viscositylow critical energy barrier
To form a large number of nuclei
For a given concentration of solute, a largernumber of nuclei mean smaller sized nuclei.
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No nucleation in region I
(even above equilibrium C)
Nucleation whensupersaturation/concentration
reaches certain value (to
overcome certain energy
barrier to from nuclei)Decrease of supersaturation
level.
When the concentration decreases below this specific
concentration, no more nuclei would form. Instead,
particle growth proceed until C falls below equilibrium C
or solubility
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For the synthesis of nanoparticles with uniform size:
All nuclei should be formed at a short period of time.
- All nuclei are likely to have similar size and willhave the same subsequent growth.
In practice:
to achieve a sharp nucleation, the concentrationof the growth species is increased abruptly to a
very high supersaturation and then quickly
brought below the minimum concentration for
nucleation.
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1.2 subsequent growth of nuclei
This determines the size distribution.
Two processes:
diffusion
it includes the generation, diffusion and
adsorption of growth species onto the growthsurface.
Surface growth
incorporation of growth species adsorbed on the
growth surface into solid structure.
Different controlling step will lead to different size
distribution.
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1.2.1 growth controlled by diffusion
If controlled by the diffusion of growth species
from the bulk to the particle surface
Growth rate:
ris the radius of nucleus.Dis the diffusion coefficient of the growth species
Vmis the molar volume of the nuclei
or
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Differentiate with respect to r0
or
For two particles with initial radius difference ,
the radius difference ,decreases as time
increases or particles grow bigger.
The diffusion controlled growth promotes
the formation of uniformly sized particles.
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1.2.2 Growth controlled by surface process
Fast diffusion, no C gradient
Two mechanisms for surface growth:
Mononuclear growth (growth layer by layer).
Polynuclear growth (surface process is so fastthat second layer growth proceeds before the
first layer growth is complete).
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Mononuclear growth
Growth rate:
This growth mechanism does not favor monosize
synthesis.
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Polynuclear growth
The absolute radius difference remains constant
regardless of the growth time and the absoluteparticle size.
This growth mechanism also favors the synthesis
of monosized particles.
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the radius difference as functions of particle size
and growth time for all three mechanisms of
subsequent growth.
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Some discussions on growth
Usually, the growth of nanoparticles involve all
three mechanisms.
monolayer growth
poly-nuclear growth
Diffusion limited growth
Different growth mechanisms can become
predominant when favorable growth conditions
are established.e.g. when the supply of growth species is very slow,
predominantly by the diffusion-controlled process.
small nuclei
Large particles
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to achieve diffusion-limited growth for monosize
synthesis:
low concentration of growth species (dilution after
nucleation stage).
Increase solution viscosity.
Introduction of diffusion barrier such as monolayer onparticle surface.
Controlled supply of growth species by reaction
control.
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1.3 Synthesis of metallic nanoparticles
Reduction of metal complex in dilute solution
Advantages: easiness of
Stabilization of nanoparticles from agglomeration
Extraction of nanoparticles from solvent
Surface modification
Processing control
Mass production
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Reduction of metal complex in dilute solution
Various precursors
Reducing agents (KBH4, alcohol, glycol, hydrogen,
ascorbic acid, sodium citrate)
Other chemicals (polymer, surfactant, pH adjusting)
Energy providing (heat, microwave, radiolysis, UV
illumination, sonication)
Objective: to promote/control reduction reactions,so that it has fast initial nucleation and
subsequent diffusion controlled growth.
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Au nanoparticles
Heat HAuCl4 aqueous solution to boiling
+
Sodium Citrate (reducing agent and diffusion barrier)
Color change
Excellent stability and uniform size.
A typical approach:
Turkevich method
(>50 years ago)
HOC (COONa) (CH2COONa)2
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Concentration effect
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Pt nanoparticles
Mix H2PtCl6+CH3OH+PVA
Reflux at 90C, pH adjustment
Color change
H2PtCl6+2CH3OH
->Pt+2HCHO+6HCl
a). Polymer
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Mix H2PtCl6+CH3OH+SB12
Reflux at 90C, pH adjustment
Color change
b). surfactant
Other reducing agents: Hydrogen, KBH4, NH2OH
ascorbic acid
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-
-
-
-
-
-
-
-
Colloid particle
SO3-
N
CH3
CH3
+
SO3
-N
CH3
CH3
+
SO3-
NCH3
CH3
+
SO3-
NCH3 CH3
+
CH3
SO3-
N
CH3
+
N
+
CH3SO3
-
CH3
SO3-
N
CH3
CH3
+
SO3-
NCH3
CH3
+
*Schematic of a Surfactant-stabilized
Colloidal Catalyst Particle
SB12
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c). Ethylene glycol
Serving as reducing agent and stabilizer.
No need for polymer and surfactant!
Reflux at 140C, pH adjustment
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Ag nanoparticles
a). Untrasonication of an aqueous AgNO3 at
10C in Ar/H2.
The ultrasound resulted in decomposition of water
into hydrogen and hydroxyl radicals. Hydrogenradicals would reduce silver ions into silver atoms,
which subsequently nucleate and grow to silver
nanoclusters.
Some hydroxyl radicals would combine to form
an oxidant, H2O2. Use H2 to remove it.
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b). UV illumination of aqueous solution of AgClO4,
acetone, 2-propanol and polymer stabilizer.
Generate ketyl radicals
protolytic dissociation
reduction by radicals
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Influence of reducing agents
a strong reduction reaction
favors the formation of more nuclei, therefore,
smaller nanoparticles.
leads to big size, in growth period.
a slow reaction may
result in wide size distribution, if it leads to
continuous formation of nuclei.
lead to diffusion limited growth and favors narrow
size distribution, if no further nucleation.
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35abrupt surge of concentration. More nuclei
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Influence of polymer stabilizer
A strong adsorption would occupy growth sites.
A full coverage would reduce diffusion of growth
species.
Interaction with solute/catalyst/solvent, thereby
contributing to the reaction.shape
Influence ofpolymer/Pt ion ratio
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Influence of other factors (concentration, T)
Ions that will affect reaction rate.
e.g. for the synthesis of Pt nanoparticles using an
aqueous methanol reduction of H2PtCl6, a high
concentration of chloride ions present in the
reaction mixture promoted monodispersity andnear-spherical shape
PtCl62-+ CH3OH -> PtCl4
2-+ HCHO + 2H++ 2Cl-
Slow supply of Pt atoms favors diffusion
controlled growth.
PtCl42-+ CH3OH -> Pt + HCHO + 2H++ 4Cl-
Control of pH is also very critical for many reactions
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Sequence of adding reagent
e.g. Au nanoparticles formation:
HAuCl4+ ascorbic acid+ PDDA
adding AA firstly and followed by
adding HAuCl4 into PDDAsolution
adding HAuCl4 firstly and thenadding AA into PDDA solution
AuCl4-
+PDDA will form ion pairs
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Seeding nucleation
CoxNi100-xand Fez[Cox-Ni100-x]1-zwere
synthesized by reduction and precipitation from
metallic precursors dissolved in 1,2-propanediol
with an optimized amount of sodium hydroxide
e.g.
The particle formation is initiated by adding
a small amount of solution of K2PtCl4, or
AgNO3 as nucleating agent.
Increased C reduced mean particle size
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1.4 Synthesis of semiconductor nanoparticles
Non-oxide semiconductor nanoparticles (CaSe,CdS, InP) are commonly synthesized by
pyrolysis of organometallic precursor(s)
dissolved in anhydrate solventsat elevated temperatures
in an airless environment
in the presence of polymer stabilizer or capping
material.
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1). Temporally discrete nucleation is attained by
a rapid increase in the reagent concentrations
upon injection, resulting in an abrupt
supersaturation.
2). Ostwald ripening during aging at increasedtemperatures promotes the growth of large
particles at the expense of small ones,
narrowing the size distribution.
3). Size selective precipitation is applied tofurther enhance the size uniformity.
To form monodispersed semiconductor particle:
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e.g. synthesis of InP nanoparticle.
Reaction of InCl and P(Si(CH3)3)3 in
trioctylphosphine oxide (TOPO) with
dodecylamine as capping material at elevatedtemperatures in dry box (Ar).
Size selective precipitation can be an effective
way to narrow size distribution
Initial product contains wide size distribution as
it is a slow process in which nucleation andgrowth occur simultaneously over long time
scales.
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InP nanocrystals capped with dodecylamine are
soluble in toluene and insoluble in methanol.
Methanol is added stepwise. The solution is
filtered after each addition, isolating anarrowed size distribution of nanocrystals,
which become successively smaller throughout
the precipitation series.
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1.5 Synthesis of oxide nanoparticles
Sol-gel processing
Sol: A stable suspension of colloidal solid particles within a liquid.
Gel: A colloidal suspension of a solid in a liquid, forming a
jellylike material that keeps its shape in a more solid formthan a sol.
Sol-gel processing is a wet chemical route for the
synthesis of a colloidal suspension of solid particles or
clusters in a liquid, and subsequently for the formation of
a dual-phase material having a solid skeleton filled with a
solvent through sol-gel transition.
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After evacuating the solvent, thin films and coatings,
powders, fibers and membranes can be obtained from the
gels.The sol-gel process involves the evolution of networks
through the formation of a colloidal suspension (sol) and
gelation of the sol to form a network in a continuous liquid
phase (gel).Sol-gel is a useful self-assembly process for
nanomaterials synthesis. (particularly oxide nanoparticles)
Advantages: low processing temperature andmolecular level homogeneity
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Precursor: It includes inorganic salts and organic compounds.
Al(NO3)3, Al(OC4H9)3,Si(OCH3)4, Si(OC2H5)4,Ti(OC2H5)4,
Ti(OC3H7)4, Ti(OC4H9)4
Metal alkoxides and alkoxysilanes are most popular
precursors because they react readily with water.
The most widely used alkoxysilanes are tetramethyloxysilane
(TMOS) and tetraethoxysilane (TEOS), which form silica gels.
Alkoxides such as aluminates, titanates, and borates are also
commonly used in the sol-gel process, often mixed with TMOS
and TEOS.
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Four stages:
Hydrolysis
Condensation and polymerization of
monomers to form nanoparticlesGrowth of particles
Agglomeration of particles followed by
formation of networks that extend throughout
the liquid medium resulting in thickening, which
forms a gel
Sol-Gel Formation
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Condensation results in the formation of nanoscale clusters of
metal oxide or hydroxide, often with organic group embedded
The size of the nanoscale clusters, along with the morphology and
microstructure of the final product, can be tailored by controlling the
hydrolysis and condensation reactions.
H d l i d d ti f ili lk id
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Hydrolysis and condensation of silica alkoxides are
relatively slow without addition of an external
catalyst. Therefore, acids (HCl, HNO3, HAc, etc.)
and bases (NH4OH, KOH, etc.) are commonly usedto speed up these processes.
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Summary of acid/base sol-gel conditions
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e.g. Stober approach for Silica nanoparticles
First, alcohol solvent,
ammonia, and a desired
amount of water were mixed,
and then silicon alkoxideprecursor was added under
vigorous stirring. The
formation of colloids became
noticeable just in a fewminutes.
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Various silicon alkoxides with different alkyl
ligand sizes were used as precursors, and various
alcohols were used as solvents.
The reaction rate and particle size were strongly
dependent on solvents, precursors, amount of
water and ammonia.Reaction rate: Methanol>n-butanol,
Final particle size: Methanol
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Different precursors have different chemical
reactivities.important for multi-component
colloids synthesis
a). The reactivity of a metal atom is dependent
largely on the extent of charge transfer and the
ability to increase its coordination number.
Reactivity
increases
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b). For a given metal atom, large or more complex
organic ligand would result in a less reactive
precursor.
electrostatic
stabilization.
Size control? (low concentration, or controlled release, time)
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Forced hydrolysis
The simplest method for the generation of
uniformly sized colloidal metal oxides
-- rapid and forced hydrolysis gives an abruptsupersaturation.
e.g. Stober approach for Silica nanoparticles(heat the solution before adding TEOS)
Increase T to increase hydrolysis rate.
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1.6 Solid state phase segregation
Preparation of nanoparticles of metals andsemiconductors in glass matrix.
1. Precursors + liquid glass melt at high T.
2. Rapidly quenched.3. Upon reheating, metallic ions are reduced to metallic
atoms by certain reduction agents and diffuse through
glass to form nuclei.
4. Nuclei grow further to form nanoparticles.
Metallic atom is not soluble in glass and gains
limited diffusivity with increased T> diffusion
limited growth > monosized particles
2 N ti l th h h t l ti
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2.Nanoparticles through heterogeneous nucleation
consider a heterogeneous nucleation
process on a planar solid substrate:growth species in the vapor phase impinge on
the substrate surface, these growth species
diffuse and aggregate to form a nucleus with a
cap shape
: surface energy
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Change of Gibbs free energy
Contact angle defined by Youngs equation
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Area=2Rh
h=R(1-cos)
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2
Substitute the geometric constants
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Compare homogeneous case
Substitute the geometric constants
wetting factor
= 180, no wetting, homogeneous case.
= 0, no energy barrier, the deposit is the same as
substrate.
heterogeneous is easier
3
3
Nanoparticles by heterogeneous nucleation
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Nanoparticles by heterogeneous nucleation
Surface defects are active nucleation centers due to
high energy state.To create surface defects on substrate:
thermal oxidation
Sputtering and thermal oxidationAr plasma and ulterior thermal oxidation
edge
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Example: core-shell structure
seed-mediated growth method for Au-Pt catalyst synthesis
Synthesis of core Au
nanoparticles
Deposition of Platinum shell
on Au core
Citrate stabilized Au nanoparticles were
prepared from the reduction of
HAuCl4.3H2O with NaBH4
H2PtCl6 was mixed with aqueous
NH2OH.HCl and heated to 60
o
C, thenthe Au hydrosol was added to start the
seed-mediated growth reaction
A ti l i ( d d
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Average particle size (measured and
calculated)
3
2
3 3 3[ ] and [ / ]
6000
-Pt Au
final
final core core
d nm gr cmd
d d d
Shouldform 1completeshell
Pt/Au
molar
ratio
measured
particle
size
(nm)
calculated
particle
size
(nm)
calculated
shell
thickness(nm)
specific Pt
surface
area
(m2/gr)
0 4.8 - - -
0.5 5.4 5.4 0.3 166
1 6 5.9 0.6 100
2 7 6.7 1.0 65
3 7.5 7.4 1.3 52
4 8.2 8.0 1.6 45
Pt atom diameter: 0.276nm
Cylic Voltammetry (CV) of Pt(shell)-Au(core)/C
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-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
-0.4
-0.3
-0.2
-0.1
0.0
0.1
0.2
current(mA)
E (V vs SCE)
A
B
CV of (A) Au/C and (B) PtAu/C 4:1 in 0.5 M H2SO4 at 50mV/s
Au/C: typical features of
the Au electrode were
observed.
PtAu/C: above featuresdisappear an Pt oxide
formation/reduction
observed..
Voltammetry can be viewed as a surface sensitive technique, as it reflects only the
electrochemical properties of the surface rather than the bulk electrode
Cylic Voltammetry (CV) of Pt(shell) Au(core)/C
CV of PtAu/C with different Pt/Au ratios
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-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2
-0.20
-0.15
-0.10
-0.05
0.00
0.05
0.10
0.15
curren
t(mA)
E (V vs SCE)
2:1
1:1
1:2
3:1
Pt:Au 0.7 0.8 0.9 1.0 1.1
CV of PtAu/C with different Pt/Au ratios
Not epitaxial layer growth
Complete coverage for Pt:Au=2:1 and above
Core-shell Au-Pd prepared by sonochemical technique
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Core-shell Au-Pd prepared by sonochemical technique.
a: annular dark field scanning TEM
and b: TEM of Au-Pd nanoparticles
NaAuCl42H2O and PdCl22NaCl3H2O
Stabilized by sodium dodecyl sulfate (SDS)
* T. Akita, et al, Catalysis Today, 131 (2008), 90-97.
Atomic number:Au (79)
Pd (46)
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Reversible change of core shell structure*
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Reversible change of core-shell structure
NO, O2
H2, CO
Rh-Pd system
Pd shellRh shell
In reducing(oxidizing) environment, Pd(Rh) shell forms.
The surface energy: Pd < Rh
Pd oxide > Rh oxide
* Gabor A. Somorjai, et al, Science, 322 (2008), 932.
3 Kinetically confined synthesis of nanoparticles
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3. Kinetically confined synthesis of nanoparticles
Spatially confine the growth so that the growth
stops when the limited amount of sourcematerials is consumed or the available space is
filled up.
(i) liquid droplets in liquid, such as micelle andmicro emulsion synthesis,
(ii) liquid droplets in gas phase
including aerosol synthesis and spray pyrolysis,(iii) template-based synthesis,
(iv) self-terminating synthesis.
3 1 Synthesis inside micelles or using
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3.1 Synthesis inside micelles or using
Microemulsions (soft template)
by confining the reaction
in a restricted space.
When surfactant Cexceeds CMC, form self
assemblymicelle.
Reverse-microemulsion:Dispersion of water in
organic solvent.
Molecular Packing Parameter
3.2 Growth termination
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3.2 Growth termination
Terminate the particle growth by occupying
growth sites with organic components oralien ions.
thiophenol
an increasing amount of capping moleculesrelative to sulfide precursor resulted in a
reduced particle size.
3 3 Template based synthesis (hard template)
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3.3 Template-based synthesis (hard template)
e.g.
Infiltration of precursor into porous polymer
matrix, or zeolite.
Formation of nanoparticle inside the template
by reaction.
Removal of the template
Paraformaldehye+phenol
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Suk Bon Yoon, et. al. Advanced Materials 14 (2002) 19
Paraformaldehye phenol
SEM: Porous silica TEM: Hollow carbon
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SEM: Porous silica
BET surface area:1345 m2/g.
Mesopore: 4 nm.Micropore: 0.8 nm.Micropore area: 345 m2
External area: 1000 m2
80% Pt/HC
3 4 Aerosol synthesis
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3.4 Aerosol synthesis
An aerosol is defined as a suspension of
solid or liquid particles in a gas.Aerosol processes in material synthesis
can be classified as:
Gas-to-particle conversion
Droplet-to to-particle conversion
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