nanoparticles and qunatum dots via solution based...

Nanoparticles and qunatum dots

via

solution based chemistry

Helmer Fjellvåg and Anja Olafsen Sjåstad

Lectures at CUTN spring 2016

Nanostructures

Nanostructure:

Structures so small that chemical and physical properties become

observably different from the “normal” properties of the material.

The dimension at which this transformation become apparent depends

on the phenomena investigated.

Normally in the rage of 1 - 100 nm

Finite size effect: electronic bands are gradiually converted to molecular

orbitals – electrons as ‘particle-in-box’

Surface and interface effects: relative fraction of atoms located on the

surface/interface are significant – affects

reactivity

(1 nm = 109 m)

Size alter electronic structure

Linear combination of n atomic orbitals (AO) to form n molecular orbitals (MO)

n energy bands are formed

LUMO

CdSe quantum dots

Semiconductors as NP

Size decides - let us look into CdSe

Radiated with light

Fluorescence:

Short wavelength (blue) – small particles

Long wavelength (red) – bigger particles

Quantum dots – bandgap increasing with

decrease in particle size

5

The energy gap between the valence band and the conduction band

(corresponding to the HOMO-LUMO gap in molecular compounds) widens as

the particle size decreases. The corresponding absorption band is blue-shifted

and becomes sharper. The size distribution must be very narrow in order to obtain

good optical properties.

The optical properties may also be influenced by

interaction with e.g. solvent molecules or ligands,

which may perturb the energy levels of the surface

atoms.

Semiconductor nanoparticles in a transparent

matrix exhibits photoluminescence and third order

non-linear optical behavior (refractive index

depends in the intensity of the incident light.

Size decides – CdS (text book)

Size decides - Appliction Quantum dots

Gold

Lycurgus cup, 4. century a.c.

Size matters for metallic particles

Why does the gold look red or blue?

Colour of Au NPs are conferred by the interaction of light

with electrons on the gold nanoparticle surface.

Electrons mean free path is 5-60 nm. When particle enter this size range, electrons are

scattered at the inner surface (Mie scattering) giving rise to surface plasmons.

At a specific wavelength (frequency) of light, collective oscillation of electrons on the

gold nanoparticle surface cause a phenomenon called surface plasmon resonance

resulting in strong absorption and scattering. Frequency is related to gold nanoparticle

size, shape, surface and agglomeration state.

9

Smaller particles;

the absorption is blue shifted,

i.e. towards higher energy.

Why does the gold look red or blue or…?

Why does the gold look reddish – brown to orange?

Quantum size effect corresonding to those we have in quantum dots

10

SIMIT: Size-induced metal-insulator transition, SIMIT effect

Transition from metallic to non-metallic behavior below a few nanometers.

The energy bands gradually change toward molecular orbitals. Bulk metals follow

Ohms law (collective movement). The corresponding NPs are insulators.

In nanosized (metal) particles the

electrostatic contribution must be included

(describing the energy associated with

adding or removing an electron), Eel =

e2/2C,

C is the effective capacitance.

Add or remove single electron into

nanoparticle = single electron transfer, SET

(Possible application: Quantum computers)

Size matters for metallic particles – SIMIT effect

11

Melting point: When the mean thermal displacement (d) of the atoms becomes

larger than some fraction of the interatomic distance, the material melts.

Surface atoms have lower coordination and a higher displacement compared to bulk.

When the surface/volume ratio increases, the mean d increase, i.e. the melting point

decreases. This is more pronounced for metals. (Why?)

The melting point is only constant as long as surface effect can be neglected!

Size decides – melting point

12

Ferromagnetic materials (Fe, Co, Ni, Gd, CrO2…) spin alignment below TC,

Magnetic domains are present, separated by Block walls.

When the size of the magnetic particle is in the same range as the domain size,

only one domain is present in one particle.

Critical sizes: Fe: 14 nm, Co: 70 nm, Ni: 15 nm; Fe3O4: 128 nm.

Size decides – cooperative magnetism

13

Size decides – cooperative magnetism

-60000 -40000 -20000 0 20000 40000 60000

-0.010

-0.005

0.000

0.005

0.010 Cat-A

M (

em

u)

H (Oe)

Ms

-2000 -1000 0 1000 2000

-1.0

-0.5

0.0

0.5

1.0 Cat-A

Cat-B

M/M

s

H(Oe)

Ferromagnetic

Size – surface – crystallographic planes

Can act as a catalyst and transform carbon monoxide (CO) to carbon dioxide (CO2),

catalyst used in the removal of odours and toxins or to clean automotive exhaust gases.

Size of gold particles is more than 10 nm.

The particles are almost spherical and the perimeter attached

to the support (active sites) is short. Inactive as a catalyst

Size of gold particles is less than 5 nm.

The particles are almost hemispherical and the perimeter

attached to the support (active sites) is long. Active as a

catalyst

Size of gold particles is less than 2 nm (no. of atoms < 300).

Catalytic activity is abruptly changed by the number of atoms

and steric structure of particles.

Catalytic selectivity is abruptly changed by the

crystallographic form of supports.

Gold catalysts

Silver catalysts

The shape of a catalyst may be as important as its size.

Shape Morphology

Size control Chemical composition

• Average composition

• “Phase” segregation

• Compositional gradients

(core shell vs. solid solution)

• Contaminations

Atomic arrangement

18

How to perform the synthesis?

Main ingrediensens: Metal precursor(s) – surfactant(s) – solvent

Hot injection – case studies Co NPs and CdSe NPs

Microwave assisted synthesis

Reversed micelles

Hot injection for burst nucleation

Case studies – Co NPs and CdSe NPs

19

Extra papers related to this topic:

- Zacharaki et al. 2016 (Co NPs)

- Murray et al. 1993 (CdSe NPs)

A nanoparticle of 5 nm core diameter with different hydrophobic ligand molecules.

Left to right: trioctylphosphine oxide (TOPO), triphenylphosphine (TPP), dodecanethiol

(DDT), tetraoctylammonium bromide (TOAB) and oleic acid (OA).

Some selected surfactants for NPs

R. A. Sperling, W. J. Parak (2010)

DOI: 10.1098/rsta.2009.0273

Burst nucleation via hot injection – case study Co NPs - I

Solvent : 1,2 dichlorobenzene

C6H4Cl2

Tb = 180.5 °C

Cobalt source: Dicobalt octacarbonyl

Co2(CO)8

Tdecomp: 52 oC

Burst nucleation refers to the formation of a large number of nuclei in a short

period of time, followed by growth without additional nucleation (La Mer theory).

Surfactant: Oleic acid (OA)

CH3(CH2)7CH=CH(CH2)7COOH

Tb = 360 oC

Reflux

Burst nucleation via hot injection Case study – Co NPs –II

Sun et al. 2012

Co2(CO)8 2Co(solv) + 8CO(g)

Burst nucleation via hot injection – case study – Co NPs –III; LaMer theory

How can we trig – alter supersaturation?

- Solubility increases normally with temperature – higher injection temperature smaller

supersaturation (and larger particles)

- Solubility may decrease with increasing temperature (retrograde solubility) – higher

injection tempertaure larger supersaturation (and smaller particles)

Characterization – stable suspensions and particles

2. Suspension stability (DLS)

- Hydrodynamic diameter 13 – 25 nm

- Polydispersivity index (PDI) 0.06 0.02

- Suspensions stable for more than 1 month

1. Yield synthesis (after 4 washing cycles): 75 %

3. Phase purity and atomic arrangement

- -Co (not fcc or hcp)

- Phase pure – occasionally some CoO

- Crystallite size

4. Particle size and morphology

- TEM

- SEM in STEM mode

12 16 20 24 28 32

Inte

nsity (

a.u

.)

2(o)

a = 6.0984 0.0004 Å

( = 0.50557 Å)

Co particle size control

1) Injection time

2) Reaction time

3) Injection temperature

4) [𝐶𝑜]

[𝑆𝑢𝑟𝑓𝑎𝑐𝑡𝑎𝑛𝑡]

5) Reactant concentrations

Co2(CO)8

Dichlorobenzene (DCB)

Oleic Acid (OA)

Co particle size control; Temperature

Variable temperature and constant [𝐶𝑜]

[𝑆𝑢𝑟𝑓𝑎𝑐𝑡𝑎𝑛𝑡] = 12.9

Particle size (nm)

168 oC 179 oC

174 oC 164 oC

Particle size (nm)

Co particle size control; Temperature

162 164 166 168 170 172 174 176 178 1800

2

4

6

8

10

12

Present study (TEM) (Ratio = 12.9; Reaction time = 30 min)

Iablokov et al. 2012 (TEM) (Ratio = 6.5; Reaction time = 20 min)

Avera

ge p

art

icle

siz

e (

nm

)

Injection temperature (oC)


1) Injection time

2) Reaction time


4) [𝐶𝑜]



Co2(CO)8


Oleic Acid (OA)

Co particle size control; Variable [𝑪𝒐]

[𝑺𝒖𝒓𝒇𝒂𝒄𝒕𝒂𝒏𝒕]

0 2 4 6 8 10 12 14 16 18 200

2

4

6

8

10

12 Present study (XRD) (T = 168

oC); Reaction time = 30 min)

Ma et al. 2004 (TEM) (T = 190 oC; Reaction time = 10 min)

Avera

ge s

ize (

nm

)

[Co]/[OA]


1) Injection time

2) Reaction time


4) [𝐶𝑜]



Co2(CO)8


Oleic Acid (OA)

31

Synthesis of CdSe nanoparticles – hot injection

•Se dissolved in TOP (Se2-)

•Cold solution is injected into

hot CdMe2 in TOPO (300ºC)

•Temperature drops to approx.

170ºC

•Increase of temperature to

higher temperature (below

300ºC) for a specified time

Kinetically controlled synthesis

•Nucleation

•Growth

•Shape

•Composition

TOPO: Tri-n-octylphosphine oxide

TOP: Tri-n-octylphosphine

32


Se2-

Cd2+

Ksp =[Se2-][Cd2+]

33


34

Particle growth mechanisms

Coalescence

Oswald Ripening

Side-note: Earthworms goes to work…….

CdSe quantum dots

Element distribution in bi-metallic NPs

36

Bi-metallic NPs – element distribution

Is alloying in form of a solid solution favoured by decreasing the particle size?

Yes, it can be shown from thermodynamic considerations and molecular dynamic

simulations that solid solutions are favored when particle size goes down

(see p. 2894 in You et al. 2013)

Note – kinetics may play a role and alter what thermodynamic calculations are

predicting (Pt-Rh Example, next slides)

Bi-metallic NPs – element distribution

When can you expect to form a solid solution between to elements A and B in

bulk metallic materials?

Hume-Rothery substitutional solubility rules

1. Crystal structure of each element of the pair is the same

2. Atomic sizes of the atoms do not differ more than 15%

3. The elements do not differ greatly in electronegativity

4. Elements should have same valence

Metals – overview structures (Extra slide)

ccp

A1

bcc

A2

hcp

A3

40

Solid solution – Rh100xPtx - bulk

Immiscibility dome

Solid solution – Rh100xPtx NPs

0 20 40 60 80 100

3.80

3.85

3.90

230oC

Nanoparticles - reaction temperature 220 oC

Linear fit - bulk

a-a

xis

(Å

)

x in Rh100-x

Ptx

190oC

Metallic radii: rPt

> rRh

Observe that a-axis become longer

when:

- Increasing reaction temperature

- Increasing reaction time

Pt Rh

Rh rich core

with a Pt rich shell

5 10 15 20 25 30

a = 6.2286 0.0004 Å

( = 0.31010 Å)

Inte

nsity (

a.u

.)

2(o)

Co-Re nanoparticles taking the -Mn type structure

12 16 20 24 28 32

Inte

nsity (

a.u

.)

2(o)

a = 6.0984 0.0004 Å

( = 0.50557 Å)

1 0 0 n m1 0 0 n m

EDS: 10.5 2.4 at. % Re

ICP-AES:

13.9 at. % Re

Co Co85Re15

0 20 40 60 80 100

6.00

6.25

6.50

6.75

7.00 Dinega et al., 1999

Vavilova et al. 1991

This study

a-a

xis

(Å

)

x in Co100-xRex

43

Particle morphology – faceting – I

For pristine single crystals and nanoparticles surface energy plays the most important

role on the growth of the crystals.

Thermodynamics surface energies come into play

Often low-index facets {111}, {110}, {100} lowest surface energy

Construct/predict shape of NPs with basis in knowlegde on the low index factes

surface energy and Wulff-constructions (ref. Barmparis et al., 2015)

Wulff construction for Au NPs

in vacuum and 28 nm (left);

in CO(g) and 27 nm (right)

44

Particle morphology – faceting – II

Synthesis conditions affects morphology – relative rearrangment of most stable facets

and possible kinetic effects.

- Ratio between metal salt and capping agent

- Nature of solvent and surfactant/capping agent (and possible other addidatives)

octopod-cube cube truncated-cube spherical

PVP/PtCl42 = 0.007 PVP/PtCl4

2 = 0.07 PVP/PtCl42 = 0.14 PVP/PtCl4

2 = 0.28

Kalyva et al.; Submitted April 2016

45

Particle morphology – faceting – III

You et al. 2013; (See also Tables 1 and 2 same ref)

Silver catalysts

The shape of a catalyst may be as important as its size.

0 2 4 6 8 10 12 140

10

20

306.7 ± 1.0 nm

Part

icle

Popula

tion

dtotal

(nm)

Ni /Al2O3 catalysts for methanation

Pt at.60% Rh at.40%

Pt at.70% Rh at.30%

13 nm

0 2 4 6 8 10 12 140

20

40

5.5 ± 1.0 nmRSD: 18 %

Part

icle

Popula

tion

Particle Size (nm)

NiO deposited on Al2O3 NiO/Al2O3 300 oC 1% O2

0 2 4 6 8 10 12 140

10

20

30

40

50 5.1 ± 1.0 nmRSD: 20 %

Part

icle

Popula

tion

Particle Size (nm)

Ni/Al2O3 400 oC 4% H2

Microwave assisted synthesis

48

Microwave Irradiation Synthesis of NPs

Several attributes of microwave heating contribute to greener

nanosynthesis, including shorter reaction times, reduced energy

consumption, and better product yields.

dipolar polarization

ionic conduction

The heating mechanism Involves two main processes:

The dielectric

properties

of the solvents are

critical parameters for

the MWI synthesis.

Synthesis by microwave dielectric heating, is based, on the ability of a specific

material (e.g. solvent and/or reagents) to absorb microwave energy and to convert it

into heat.

Rh(acac)3, PtCl4 @100 ⁰ C

Mixture of Rh(acac)3, PtCl4 @100 ⁰ C

After MIW @ 900 W for 10 min

Pt-Rh NPs - Color of the starting materials at 100 ⁰ C

Metallic precurcors: Rh(acac)3 (97%, rhodium(III) acetylacetonate), PtCl4 (99.9+%, platinum(IV) chloride)

Liganding solvents:

OAm (tech. 70%, oleylamine) & PVP10

Polyvinylpyrrolidone average mol wt 10,000 and

(DMF, (99.8%, N,N-dimethylformamide) Dielectric constant: 36.71

52

Particle morphology – faceting – II

Synthesis conditions affects morphology – relative rearrangment of most stable facets

and possible kinetic effects.

- Ratio between metal salt and capping agent

- Nature of solvent and surfactant/capping agent (and possible other addidatives)

octopod-cube cube truncated-cube spherical

PVP/PtCl42 = 0.007 PVP/PtCl4

2 = 0.07 PVP/PtCl42 = 0.14 PVP/PtCl4

2 = 0.28

Kalyva et al.; Submitted April 2016

Reversed micelles

53

Nanoparticles using microemulsions as confined

reaction media - principles

Nanoreactor range 2-50 nm

Oil in Water

Water in Oil

Lecture notes KJM-5500 spring 2012, F. K. Hansen, UiO.

M. Sanchez-Dominguez et al., “New Trends on the Synthesis of Inorganic Nanoparticles Using Microemulsions as Confined Reaction Media”, 2010.

CTAB

PtRh nanoparticles in W/O microemulsions-II

Nanoreactor range 2-50 nm


PtRh nanoparticles in W/O microemulsions-I

Water phase: Rh(NO3)3 + H2PtCl6

Oil: n-heptane

Surfactant: polyethyleneglycol-dodecylether

F.J. Vidal-Iglesias et al. ; Journal of Power Sources 171 (2007) 448–456.

Water phase: NaBH4 or N2H4 (reducing agent)

Oil: n-heptane

Surfactant: polyethyleneglycol-dodecylether


Pt25Rh75

Size (TEM): D = 3.8 ± 0.9 nm

XPS: 19.9/80.1 (PtRh)

EDX: 25/75 (PtRh)

PtRh nanoparticles – how to break the microemulsion?

Pt25Rh75

58

May be produced by precipitation or by reaction in confined space.

Synthesis in confined spaces. Nanosized reactors, “Ship-in-a-bottle”. May be

pores or channels in solids or liquid droplets.

Reversed micelle (water in oil microemulsions).

Formed by adding a small amounts

of water to a surfactant in a

hydrocarbon solvent.

E.g. precipitation of CdS by adding

sulfide to a solution of a cadmium

salt.

The size is determined by the size

of the droplet (controlled by the

water/surfactant ratio).

Reversed Micelles – text book

Extra – from text book

59

60

Formation of nanoparticles from vapor.

Aerosol route – advantage: Particle size may be

controlled by droplet size, concentration etc.

Gas condensation method:

Ultra high vacuum (filled with ca. 100

Pa He).

Liquid nitrogen cold finger, a scraper,

and in-situ compactor.

Evaporation of a solid (mostly

metals) by heating.

Clusters are formed, which grow by

cluster-cluster condensation.

Convective transport towards cold

finger.

Gram quantities may be produced.

Synthesis of nanoparticles: Condensation from gas phase

61

Mechanical attrition. (abrasion, grinding)

High energy ball milling may reduce particle size to 2-20 nm. (may also

be used for synthesis or alloying)

Top – bottom synthesis

62

Thermolysis of e.g. transition metal carbonyls under an inert atmosphere may

produce metal colloids.

e.g. thermolysis of Fe(CO)5 Fe + 5CO in an organic surfactant produces

“ferrofluids” of metallic iron nanoparticles (8.5 nm).

Ni, Co, Fe and Cu nanoparticles have been prepared from the formiates:

M(OOCH)x M + xCO2 + x/2 H2 (or M + x/2 CO + x/2 CO2 + x/2 H2O)

Or from the oxalates

Photolysis, e.g. photochemical decomposition of silver halide nanocrystals.

Synthesis of nanoparticles: Thermolysis

Variant of Co NPs as discussed earlier.

Delamination Layered structures may be used to

construct new nanomaterials by

restructuring them. Typical starting

material may be layered silicates

such as clay. However, many

transition metals also form layered

structures.

These may also be curled to form

nanoscrolls.

Synthesis of nanoparticles: Delamination

Ceria/zirconia nanoparticles with

excellent thermal stability. Such

materials are used in automotive exhaust

gas treatment.

Two-nozzle flame synthesis of

Pt/Ba/Al2O3 catalysts

Flame made Pt/TiO2. Pt

particles d<3nm are indicated

by arrows.

Nanocatalyst materials can be made by spray pyrolysis. By crossing two such sprays, it

is possible to produce nanoparticles of one material on another material.

Synthesis of nanoparticles: Spray pyrolysis

Nanoparticles can be made by laser ablation of a material. They are typically collected

on a cooled substrate and most often form films. But the particles may also be used as

is in the gas phase.

Synthesis of nanoparticles: Laser ablation

nanoparticles and qunatum dots via solution based...

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