10 nano10 madeira-lecture-11.10.10-synthesis and characterization_s

Institute of Inorganic Chemistry

Synthesis and characterization ofSynthesis and characterization of inorganic nanoparticlesinorganic nanoparticles

Matthias Epple

Inorganic Chemistry and Center for Nanointegration Duisburg-Essen (CeNIDE)

University of Duisburg-Essen

Definition of nanotechnology

Classical: "The size is below 100 nm in at least one dimension (nanolayers, nanowires, nanoparticles)."(nanolayers, nanowires, nanoparticles).

Revised: "The size of the objects determines their physical, j p y ,chemical or biological properties."

The limit of "100 nm" is arbitrarily set and may be exceeded in some cases. However, it gives a good hint about the typical dimensions of nanosystems.

Some typical examples of nanosystems:• Colloidal gold for drug delivery or as pigment• Semiconductor nanoparticles (quantum dots) for imaging• Thin layers of semiconductors in light-emitting diodes

S ll i l f bl l l• Small particles of noble metals as catalysts

Examples of nanostructuresp

a) Late Roman Lycurgus cup, b) baroque ruby glass, and c) colloidal Au nanoparticles

H. Goesmann, C. Feldmann, Angew. Chem. Int. Ed. 2010, 49, 1362–1395

Suspensions with SiO2 nanoparticles of various sizes and partial blueSuspensions with SiO2 nanoparticles of various sizes and partial blue tint from Rayleigh scattering


CdSe semiconductor quantum dots as nanoscale luminophors: q pa) Surface color of suspensions in toluene in visible light. b) Schematic diagram of band gap and emission color as a function of particle size. ) Li ht i i f i i t l h it d ith UV li htc) Light emission of suspensions in toluene when excited with UV light.


Light emitting diodes(LEDs)(LEDs)

(a) Bandgap energies of InN, GaN, AlN, and the ternary alloys InxGa1-xN and AlyGa1-yN. The lasery yenergies for reading and writing compact discs, digital video discs (DVDs), and Blu-ray discs are alsoshown, together with a rainbow depiction of the visible spectrum. (b) Main materials that wereknown to emit light prior to the advent of GaN (solid circles denote direct-bandgap materials, and

i l d i di b d i l lid li d di b d ll d d h d

MRS Bulletin, April 2008, Volume 33, p. 459

open circles denote indirect-bandgap materials; solid lines denote direct-bandgap alloys, and dashedlines denote indirect-bandgap alloys).

Light emitting diodes(LEDs)(LEDs)

High-resolution transmission electroni (0002) l tti f i imicroscope (0002) lattice fringe image

of three InGaN quantum wells separatedby GaN barriers (courtesy of T. M. y ( ySmeeton, University of Cambridge,Cambridge, UK).


LED traffic lightsLED traffic lights


Structural colour of Bragg stacks with different composition


and layer thickness; scale bars 1 µm

Some properties of nanoparticles• Nanosystems have a high specific surface area due to the small• Nanosystems have a high specific surface area due to the small

characteristic dimension.• Surface effects play an increasing role sometimes a decisive• Surface effects play an increasing role, sometimes a decisive

role.• Surface tension effects may become more important than bulkSurface tension effects may become more important than bulk

effects.• For a given sample weight more surface atoms are accessible inFor a given sample weight, more surface atoms are accessible in

a nanodisperse sample than in a conventional microcrystalline sample.sample.

• The energy of surface atoms is higher than that of atoms in the bulk of a sample, therefore the chemical reactivity is different p , y(and usually higher).

• The electronic structure changes in the nano-scale, leading to g gcolors and fluorescence which are different to bulk materials.

• Biological systems like cells react differently to nanoparticles than to bulk materials.

Synthesis of nanoparticlesy p

Synthesis of nanoparticles (diameter about 1-100 nm)

Two major concepts:

- Bottom-up: Assembly of nanoparticles from l l t Th " h i l" hmolecules or atoms: The "chemical" approach.

- Top-down: Decomposition of larger particles into nanoparticles, e.g. by grinding.p g y g g

Synthesis of inorganic nanoparticles by wet-chemical methodsmethods

• A chemical reaction that leads to a solid phase is carried out by rapidly mixing suitable reagents, e.g.

Ag+ + reducing agent Ag0

2 2Cd2+ + Se2- Cd Se

• Nucleation occurs i e the formation of the first small solid particles (range ofNucleation occurs, i.e. the formation of the first small solid particles (range of 1-2 nm or less). The growth of small crystals to larger crystals is thermo-dynamically favoured due to decrease in the specific surface energy during crystal growth.

• The growth of crystals must be prevented by suitable capping agents (charged molecules surfactants or polymers) These components may also preventmolecules, surfactants, or polymers). These components may also prevent agglomeration.

• The colloidal stability can be due to electrostatic repulsion (as measurable by the zeta potential), due to steric stabilization (by a polymer shell), or due to a combination of both (electrosteric stabilization by a polyelectrolyte).

Growth and stabilization of nanoparticles


Preparation of gold nanoparticles

gold salt stabilizing agent reducing agent

colloids

Gold salt: Sodium tetrachloroaurate: Na[Au+IIICl4]

Reducing agent: sodium citrate (Turkevitch method)

Typical Stabilizers: sodium citrate, phosphine ligand, poly(N-vinylpyrrolidone)

++ reduction nucleation growth

+

+ ++

+g

Au0Au3+ gold nanoparticles

Preparation of phosphine-stabilized spherical gold nanoparticlesnanoparticles

• Tetrachlorogold acid is reduced by sodium citrate in aqueous solutionTetrachlorogold acid is reduced by sodium citrate in aqueous solution according to the Turkevich method.

• Citrate acts as reducing and stabilizing agent. Citrate-functionalized gold nanoparticles are only electrostatically stabilized and not stable for longer times or in the presence of salts.

• Citrate is exchanged by phosphine according to the Schmid methodCitrate is exchanged by phosphine according to the Schmid method.• Phosphine-stabilized particles are negatively charged and more stable due to

the strong interaction between gold and phosphorus.

O

O

ONadiNaO

O

ONaOH

sodiumcitrate phosphine

("TPPTS")O

Scanning electron and transmission electron microscopy of TPPTS-stabilized gold nanoparticlesmicroscopy of TPPTS-stabilized gold nanoparticles

20 nm140 nm

Transmission electron microscopyScanning electron microscopy

Synthesis of larger phosphine-stabilized gold nanoparticles

• Gold nanoparticles were prepared by the citrate method• A reduction of the citrate concentration from 0.17 mM to 0.017 mM leads educt o o t e c t ate co ce t at o o 0. 7 to 0.0 7 eads

to an increase of the size from 16 nm to 84 nm (measured by dynamic light scattering)Th h f h ld i l i h i l• The shape of the gold nanoparticles is not spherical anymore

• Citrate is the exchanged byphosphine (TPPTS)phosphine (TPPTS)

• The phosphine-stabilized particlesare negatively charged

250 nm

Synthetic methods to prepare silver nanoparticles

1. The polyol method: Reduction with ethylene glycol in the presence of the polymerpoly(vinylpyrrolidone), PVP:

N ti l ith d fi d i d hNanoparticles with defined size and shape, 30-200 nm characteristic dimension.

ON PVP

2. The citrate method: Reduction and stabilization with trisodium citrate: O

n

Spherical nanoparticles with a diameter of about 20 nm.

O

OONa

ONa

Na

3 Red ction ith gl cose i th f PVPHO

OOH

ONa

Trisodium citrate

3. Reduction with glucose in the presence of PVP:Spherical nanoparticles with a diameter of about 70-90 nm. OH

OHH

H

H OH

H OH

CH OHGlucose

CH2OH

Characterization of silver nanoparticlesT i i l t i h f PVP d it t f ti li d ilTransmission electron micrographs of PVP- and citrate-functionalized silvernanoparticles. The particles were purified by ultra-centrifugation to remove allcounter-ions and side-products of the preparation.cou te o s a d s de p oducts o t e p epa at o .

PVP-functionalizedCitrate-functionalized PVP-functionalizedCitrate functionalized

10 nm20 nm

Colloid-chemical characterization of silver nanoparticlesDi t if ti d d i li ht tt i (DLS) d t t ti lDisc centrifugation and dynamic light scattering (DLS) and zeta potential

of PVP-functionalized silver nanoparticles, prepared by reduction with glucose.

Dynamic light scattering25

10

15

20

mbe

r/ %

120000

Analytical disc centrifugation

0

5

0.1 1 10 100 1000 10000

Num

Di t /

80000

100000

sity

Z i l

Diameter / nm

20000

40000

60000

Inte

n

Zeta potential

150000

2000000,01 0,10

diameter / µm

50000

100000

Tota

l cou

nts

0-200 -100 0 100 200

Zeta potential / mV

Synthesis of silver nanoparticles with different shapeThe polyol method:The polyol method:• Silver ions are reduced in-situ by the solvent ethylene glycol. The size and

shape of resulting silver nanoparticles are controlled by the additive poly(vinylpyrrolidone) (PVP).

• The synthesis of nanoparticles of different shapes and sizes is possible by changing the reaction conditions such as reagent ratio and temperature:changing the reaction conditions such as reagent ratio and temperature:

500 nm 1 µm 300 nm

PVP A NO / M T / °C / hPVP AgNO3 / mM T / °C t / h

Cubes (200 nm) 180 mM (3 mL) 120 mM (3 mL) 160 1

Rods (100 nm · 5000 nm) 300 mM (3 mL) 100 mM (3 mL) 160 1

Spherical particles(20-50 nm by SEM, 80-100 nm by DLS)

25 mM (3 mL) 2.5 mM (3 mL) 90 1

UV spectroscopy on silver nanoparticles

Calculated UV-visible extinction (black) absorption (red) and(black), absorption (red), and scattering spectra (blue) of Ag nanocrystals, illustrating the effect of shape on spectraleffect of shape on spectral characteristics:a) sphere,b) bb) cube,c) tetrahedron,d) octahedron,e) triangular plate, andf) circular plate

Xia, Angew. Chem. Int. Ed. 2009, 48, 60-103

Problem: The synthesis of nanoparticles with a defined shape is difficult and often

d iblnot very reproducible

Nucleation and crystal growth

Reaction pathways that lead to fccmetal nanocrystals having y gdifferent shapes. First, a precursor is reduced or decomposed to form the nuclei (small clusters). Once the nuclei have grown past a certain size, they become seeds with a single crystal, singly twinned, or multiply twinned structure. If stacking faults are introduced, then plate-like seeds

ill b f d Thwill be formed. The green, orange, and purple colors represent the {100}, {111}, and {110} facets,

ti l T i lrespectively. Twin planes are delineated in the drawing with redlines. The parameter R is defined as the ratio between thedefined as the ratio between the growth rates along the (100) and (111) directions.


Nucleation and crystal growth

Overgrowth process of Ag nanocrystals, in which Ag atoms are continuously deposited


onto the {100} facets of a Ag nanocube to eventually result in an octahedron enclosed by{111} facets.

El i i f i l

Nucleation and crystal growthElectron microscopy images of single crystal Ag nanocrystals:a) cuboctahedrons prepared in ethylene

glycol with PVP as a capping agent ; b) nanocubes prepared in ethylene

glycol with PVP as a capping agent ;g yco w V s c pp g ge ;c) truncated octahedrons prepared in

1,5-pentanediol in the presence of PVP and Cu2+ ions;PVP and Cu ions;

d) octahedrons prepared in 1,5-pentanediol in the presence of PVP

d C 2+ iand Cu2+ ions ;e) nanocubes prepared by a modified

silver mirror reaction in the presence of Br- with glucose as a reducing agent ;

f) nanobars prepared in ethylene glycol ) p p y g yin the presence of PVP and Br‐.


Electron microscopy images of Ag

Nucleation and crystal growthElectron microscopy images of Ag nanocrystals with twin defects:a) singly twinned right bipyramids

d i h l l l iprepared in ethylene glycol in the presence of PVP and Br–;

b) singly twinned nanobeamsprepared in ethylene glycol in the presence of PVP and Br– ;

c) five-fold twinned nanorods)prepared in ethylene glycol with PVP as a capping agent;

d) nanoplates prepared by light-d) nanoplates prepared by light-induced conversion of Ag nanospheres;

) l t d i te) nanoplates prepared in water with PVP as a reductant ;

f) nanobelts formed by refluxing an aqueous dispersion of Ag colloids.


Detection of silver species in an aqueous AgNO3 solution

a) Positive mode mass spectrum of aa) Positive-mode mass spectrum of a freshly prepared 1 mM aqueous AgNO3 solution. Note that Cs+

dd d i h f f C NOwas added in the form of CsNO3as a reference for concentration calibration.

b) Plots of the concentrations of different silver species versus time,

h 1 M A NOwhen a 1 mM aqueous AgNO3solution was aged in air.


Preparation of calcium phosphate nanoparticles

DNAperistaltic pump

DNAperistaltic pump

DNA solution

calcium

DNA solution

calciumcalciumphosphate

nanoparticles

reactionvessel

PO43-Ca2+

calciumphosphate

nanoparticles

reactionvessel

PO43-Ca2+

DNA/calciumphosphate dispersionCa2+/PO4

3-

reservoirs

4

DNA/calciumphosphate dispersionCa2+/PO4

3-

reservoirs

4

ese vo sese vo s

Stabilization of calcium phosphate nanoparticles as colloids with nucleic acids (DNA or RNA)

C 2+

( )

O

HC CH

ACH2

3

Ca2+

Ca2+O

PO O

CHHC

H

PO43-

Ca2+P

O

-O O

OCH2

CPO43-

Ca2+

CHHC

HC CH

O HPO4

3-

Ca2

Ca2+O H

P

O

-O O

T

4

Ca2+

O

CHHC

HC CH

CH2

PO43-

CHHC

O H

Polymeric nanocapsulesy p

Nanocapsules by the Layer-by-Layer Technique (LbL)

NH +

=

NH3+PAH n

=

SS

SO3-

PSS

n

Coating of calcium phosphate nanoparticles with alternating layers of cationic and anionic polymersalternating layers of cationic and anionic polymers

40

60 PAH PAHPAH

20

40

V

20

0

ntia

l / m

V

-40

-20

ta p

oten

-80

-60PSS PSSze

t

1 2 3 4 5-80

number of layers

Coating of calcium phosphate nanoparticles with alternating layers of cationic and anionic polymersalternating layers of cationic and anionic polymers

240

260

+PSS+PAH

220

+PSS

+PAH200 +PSS

CaPPAHer

/ nm

160

180 +PAH

diam

ete

140

160

1 2 3 4 5number of layers

Removal of the calcium phosphate core by a combination of treatment with acid and dialysisco b o o e e w c d d d ys s

100 nm100 nm

Synthesis of inorganic nanoparticles by wet-chemical methods some personal remarksmethods – some personal remarks

• The synthesis of nanoparticles is often empirically driven. There is no strict theory about nucleation and growth.N ll h hi h d i h li ll d• Not all syntheses which are reported in the literature actually do work.It i t h TEM i ith di l ti• It is easy to show a TEM image with a monodisperse population of nanoparticles. It is much more difficult to produce a few mg or e en grams of a niform pop lation of nanoparticlesor even grams of a uniform population of nanoparticles.

• Primary nanoparticles may be agglomerated to µm-sized aggregates This phenomenon cannot be detected by electronaggregates. This phenomenon cannot be detected by electron microscopy on dried samples.

Characterization of nanoparticlesp

A model experiment: Comparison of different methods to analyze silver nanoparticles gold nanoparticles and a 1:1analyze silver nanoparticles, gold nanoparticles, and a 1:1

mixture of both

Analysis of identical samples by the following methods:

• scanning electron microscopy (SEM)

• transmission electron microscopy (TEM)

d i li ht tt i (DLS)• dynamic light scattering (DLS)

• Brownian motion analysis (NanoSight®)Brownian motion analysis (NanoSight )

• analytical disc centrifugation

A model experiment: Comparison of different methods to analyze silver nanoparticles gold nanoparticles and a 1:1analyze silver nanoparticles, gold nanoparticles, and a 1:1

mixture of bothSilver nanoparticles:• Synthesis by reduction with glucose in the presence of

pol ( in lp rrolidone) (PVP) in ater at 90 °Cpoly(vinylpyrrolidone) (PVP) in water at 90 °C.• The dispersion is purified by ultracentrifugation.

Gold nanoparticles• Synthesis by reduction with sodium citrate in water at 100 °C. • The di e i i ti ed ith PVP e i ht d ified b• The dispersion is stirred with PVP overnight and purified by

ultracentrifugation.

1:1 mixture of the silver and gold nanoparticles• Both samples of gold and silver nanoparticles were diluted to a

t ti f 37 3 L-1 Th i t d f id ti lconcentration of 37.3 mg L-1. The mixture was prepared from identical volumes of each sample. That led to a ratio of 50 wt% gold and 50 wt% silver.

Scanning electron microscopy (SEM)Measuring principle:Measuring principle:Imaging of the sample surface by scanning with a beam of electrons in a scan pattern in high vacuum. Detection of secondary electrons and back-scattered electrons.

Advantages:Advantages: • direct imaging technique• particle detection >10 nm possible• particle composition can be determined by energy dispersive X-ray spectroscopy (EDX)

• particle morphology is visibleparticle morphology is visible

Disadvantages: • electron beam may influence the sample • artefacts may be caused by drying (agglomeration)• the analysis of agglomeration in the dispersion is not possiblethe analysis of agglomeration in the dispersion is not possible• only the solid core of a nanoparticle is visible• limited resolution in size• only a small number of particles is examined• precipitation of other dissolved compounds from the dispersion is possible.

Scanning electron microscopy (SEM)

Silver nanoparticles Gold nanoparticlesp p

Scanning electron microscopy (SEM)

1:1 mixture of silver and gold nanoparticles

Transmission electron microscopy (TEM)

Measuring principle: I i f th l b t i i d diff ti f l t b iImaging of the sample by transmission and diffraction of an electron beam inhigh vacuum.

Advantages: • direct imaging technique

ti l d t ti 1 ibl A hi h l ti th th SEM• particle detection >1 nm possible: A higher resolution than the SEM• the particle composition can be determined by energy dispersive X-ray spectroscopy (EDX)p py ( )

• particle morphology and crystallinity are visible

Di d tDisadvantages: • electron beam may influence the sample • artefacts may be caused by drying (agglomeration)y y y g ( gg )• the analysis of agglomeration in the dispersion is not possible• only the solid core of a nanoparticle is visible

l ll b f ti l i i d• only a small number of particles is examined• precipitation of other dissolved compounds from the dispersion is possible


Silver nanoparticles

200 nm

Gold nanoparticles

100 nm

Silver nanoparticles Gold nanoparticles

Particle size distribution of silver and gold nanoparticles from electron microscopy by visual inspectionfrom electron microscopy by visual inspection

50

Ag60

Au

30

40Ag

40

50Au

s

20

30

of p

artic

les

20

30

er o

f par

ticle

0

10

num

ber

0

10num

be

0.1 1 10 100 10000

size / nm0.1 1 10 100 1000

0

size / nm

determined from SEM image determined from TEM image


Ag

Au

20 nm

1:1 mixture of silver and gold nanoparticles

Dynamic light scattering (DLS)M i i i lMeasuring principle : Scattering of a laser light beam at the surface of dispersed nanoparticles and detection of the backscattered light.detect o o t e bac scatte ed g t.

Advantages: i i f di d d l d i l• examination of dispersed and agglomerated nanoparticles

• easy and fast method• determination of zeta potential ("charge of the particle") is possibledete at o o eta pote t a ( c a ge o t e pa t c e ) s poss b e• the sample is recovered after measurement• applicable to a minimum particle size of about 1 nm

d i i f h h d d i di• determination of the hydrodynamic diameter• statistical observation of the whole dispersion

Disadvantages: • limited applicability to polydisperse systems, unsuitable for unstable

di i b di idispersions because sedimentation may occur• not applicable to dispersions with different nanoparticle morphologies• larger or aggregated particles show higher scattering intensity (I islarger or aggregated particles show higher scattering intensity (I is

proportional to r6) compared to smaller particles, which may lead to misinterpretations.

Dynamic light scattering (DLS)

Brownian motion analysis (NanoSight®)

Measuring principle: Scattering of a laser light beam at the surface of dispersed nanoparticles and

i l t ki f ti l tvisual tracking of particle movement.

Advantages: g• examination of dispersed and agglomerated nanoparticles possible• fast method

d t i ti f t t ti l i i i l ibl• determination of zeta potential in principal possible• sample is reusable after measurement• applicable to a minimum particle size of about 10 nmpp p• determination of the hydrodynamic diameter

Di d tDisadvantages: • limited applicability to polydisperse systems, unsuitable for unstable

dispersionsp• not applicable for dispersions with different nanoparticle morphologies• examination of only a small part of the dispersion is possible, dilution may

bbe necessary• to obtain reliable results, an experienced operator is required


3.0

3.5

4.0 A m

L-1

Ag0.5

0.6

0.7 BAu

mL-1

1.5

2.0

2.5

cent

ratio

n / 1

06

0.3

0.4

entra

tion

/ 106 m

0.0

0.5

1.0

parti

cle

conc

0 0

0.1

0.2

parti

cle

conc

e

0.1 1 10 100 10000.0

size / nm0.1 1 10 100 1000

0.0

size / nm

2 5

3.0 CAu+Ag

1 5

2.0

2.5g

on /

106 m

L-1

0.5

1.0

1.5

le c

once

ntra

tio

0.1 1 10 100 10000.0pa

rticl

size / nm


Measuring principle: Sedimentation of dispersed nanoparticles due to centrifugal force on a rotating disc (anal tic centrif gation) in a gradient s gar sol tion (higher densit anddisc (analytic centrifugation) in a gradient sugar solution (higher density and viscosity)

Advantages : • examination of polydisperse and agglomerated systems possible• ti le h l be t ke i t t• particle morphology can be taken into account• particle detection down to 5 nm possible

Disadvantages: • possibly, there are interactions between the sugar solution used and th ti lthe nanoparticles

• some properties of the particles must be known (density, morphology)


140000Ag Au

120000Ag Au

Ag+Au

80000

100000

60000

80000

tens

ity

40000

int

20000

0.1 1 10 100 10000

i /size / nm

Summary of the results

th ddiameter Ag Au 1:1 mixture of Ag and Au

methodas to… nanoparticles nanoparticles nanoparticles

72-97 nm (Ag; 5 %, by number)SEM 70 nm 13 nm

( g; , y )

12 nm (Au; 95 %, by number)

TEM 70 1543-112 nm (Ag; 7 %, by number)

TEM 70 nm 15 nm13 nm (Au; 93 %, by number)

DLS intensity 124 nm 52 nm 121 nm (average)

particle volume 94 nm 30 nm 86 nm (average)

particle number 63 nm 22 nm 55 nm (average)

z average 102 nm 42 nm 121 nm (average)z-average 102 nm 42 nm 121 nm (average)

zeta potential -36 mV -57 mV -34 mV (average)

NanoSight® 95 nm 46 nm 85 nm (average)NanoSight 95 nm 46 nm 85 nm (average)

disc centri-

fugation42 nm 11 nm

34 nm (Ag; 66%, by number)

13 nm (Au; 34%, by number)g ( y )

Results of the model experiment

• The application of different methods led to variable results due to the physical principles that form the basis of the measurement methods.

• Gold nanoparticles (very uniform regarding size and morphology) showed aGold nanoparticles (very uniform regarding size and morphology) showed a size conformity in scanning electron microscopy, transmission electron microscopy and disc centrifugation; dynamic light scattering showed only an agreement by particle number analysis (not in the 1:1-mixture).

• The best choice for the examined dispersions was transmission electronThe best choice for the examined dispersions was transmission electron microscopy because this was the only method which could differentiate quantitatively between gold and silver nanoparticles. However, it could not give information about the agglomeration state of the particles in dispersion.

• Disc centrifugation also had the ability to differentiate qualitatively betweenDisc centrifugation also had the ability to differentiate qualitatively between silver and gold nanoparticles in the 1: 1 mixture.

Synthesis and characterization of nanoparticles:Some conclusionsSome conclusions

Typically, high-end analytical methods like transmission electron microscopy or atomic force microscopy are necessary to study nanosystems.

Colloid chemical methods are very important – remember that ti l d t d " ll id " til th "i ti " fnanoparticles were denoted as "colloids" until the "invention" of

nanotechnology.

Not every method is applicable to every kind of nanoparticle system therefore it is advisable to use more than one method forsystem, therefore it is advisable to use more than one method for the characterization of nanoparticles.

The progress of nanosciences is driven by the development of synthetic methods as well as by the development of new analyticalsynthetic methods as well as by the development of new analytical techniques (like AFM).

10 nano10 madeira-lecture-11.10.10-synthesis and characterization_s

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