Lec-3: Zero-dimensional Nanostructures

5/26/2018 Lec-3: Zero-dimensional Nanostructures

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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


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!



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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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Lec-3: Zero-dimensional Nanostructures

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