Particle Synthesis in Condensed PhasesHeinrich Hofmann

Swiss Federal Institute of Technology,EPFLLausanne, Switerland

heinrich.hofmann@epfl.ch

“Nanochemistry and Nanophysics”

Nanochemistry can be described as a special discipline of inorganic or solid state chemistry. It focuses on the synthesis of nanoparticulate systems. The nanochemist can be considered to work towards this goal from the atom „up“, whereas the nanophysicist tends to operate from the bulk „down“:

G. A. Ozin Adv. Mater. 4/10 (1992) 612ff.

Goals, Problems, and Methods in nanoparticle synthesis

The goal is to elaborate a method of synthesis, whicha) is reproduceableb) yields monodisperse nanoparticlesc) produces „perfect“ particlesd) may control the shape of the particlese) is easy, cheap

The chemical methods are either based on the kinetic control of nucleation and growth of the particles, on electrostatic stabilization in (aqueous) suspension, or on the introducion of spatial constraints. The latter include particleformation within or at the interface of micelles, vesicles, or bilayer lipidmembranes, surface monolayers and Langmuir Blodgett films, within thechannels of zeolites, in interlayers of clay, in peptides, or in biological cells.

2D. Myers Surfaces and Interfaces, VCH Publishers Inc., New York, 1991.3J. P. Spatz, A. Roescher, M. Möller Adv. Mater. 8/4 (1996)

Nucleation and Growth (La Mer)

The stages of nucleation and growth for the preparationof monodisperse NCs in the framework of the La Mer model. As NCs grow with time,a size series of NCs may be isolated by periodically removing aliquots from the reactionvessel.C. B. Murray and C. R. Kagan and M. G. Bawendi Annu. Rev. Mater. Sci. 2000. 30:545–610

Kinetics:• Most of the time, the reactions are so fast, that they can’t be controlled easily.

In some cases, better results can be obtained using a precipitation method, which is called „precipitation from homogeneous solution“:

Example: Synthesis of ZnS nanoparticles:

Zn2+ + S2-⇔ ZnS↓

A „regular“ method of synthesis for zincsulfide particles involves the reaction of Zn2+-ions with a sulfide (S2-) source i.e. H2S (Hydrogensulfide) or Na2S (Sodiumsulfide). ZnS forms instantenously at a certain pH value. The obtainedZnS powder consists of particles with a very large shape and size distribution,

ZnS-regularly precipitated particles

The equilibrium is shifted by a change in temperature, concentration, and pressure. Only when the available sulfide ions are no longer present in thereaction solution, (which means that they have reacted with Zn2+ to form zincsulfide,) new sulfide ions are released by the thioacetamide.

ZnS-precipitation from homogeneous solution

Precipitation from homogeneous solution:

Thiacetamide is used as a sulfide source. It hydrolyses according to:

CH3C

S

NH2

2 H2O H2S+ + CH3COO- + NH4+

• Sometimes the size as well as the morphology can beinfluenced by the counter-ions.

3040800Size in nm (TEM)

SphericalSphericalSpherical

Morphology

Zinc-acetylacetonate

Zinc-acetate

Zinc-tfMS

The influence of the counter-ion on the particle size is again a kinetic one. TheZn2+-ion forms complexes with the above mentioned acetate, acetylacetonate, ortrifluoromethane-sulfonate anions. Depending on the complexation, less „free“cations are available for reaction, the crystallite growth is suppressed.

Influence of Counter-ions

Lit.: R. Vacassy, S. M. Scholz, J. Dutta, H. Hofmann et al., J. Am. Chem. Soc. 81/10 (1998) 2669ff.

Quantum-dotsSize- and material-dependent emission spectra ofseveral surfactant -coated semiconductor nanocrystals in a variety of sizes (A). Blue series: different sizes of CdSe (Diameter : 2.1, 2.4, 3.1, 3.6, 4.6 nm)Green series: InP nanocrystals (Diameter: 3.0, 3.5, and4.6 nm)Red series: InAs nanocrystals (Diameter:

2.8, 3.6, 4.6, 6.0 nm)(B) A true-color image of a series of silica-coated core(CdSe)-shell (ZnS or CdS) nanocrystal probes in aqueous buffer, all illuminated simultaneously with a handheld ultraviolet lamp

Synthesis: There are many wet chemical methods of synthesis for semiconductor nanoparticles, a organic and an inorganic method arepresented here:

Cd2+ + Se2-

Stabilizer Ln

CdSeLn

Optimized synthesis parameters:9<pH<12.5Surfactant: Thioalcohols/ThioacidsAtmosphere: inert gas

[Me2Cd] + [(TMS)2Se]TOP/TOPO

CdSe

Optimized synthesisparameters:230 < T < 260°CSurfactant: TOP/TOPOAtmosphere: inert gasTransmission electron micrograph

of CdSeA. P. Alivisatos J.Phys.Chem. 100/31 (1996) 13226ff.

Nanocrystalline Semiconductors: Synthesis, Properties,and Perspectives (review) Tito Trindade et al. Chem. Mater. 2001, 13, 3843-3858

Brus and co-workers suggested that sulfurvacancies, located at the surface of the material, might be important in mediating low-energy emissions. There are severalreasons for this, one of which is the considerable size of such shallow traps. Moreover, as the size of these trapsapproaches that of the nanoparticle, the wave functions of the trap and excited state overlap. Transfer to these levels, in the form of a separate event, should subsequentlybe minimized, and the possibility of electron-hole recombination, with emission close to the absorption peak of the bound exciton, can become the predominantevent.

Arrested Precipitation in Solution

Controlled precipitation reactions can yield dilute suspensions of quasi monodispersed particles. This synthetic method sometimes involves the use of seeds of very small particles for the subsequent growth of larger ones. The stability of the initially small crystallites formed is influenced by the dynamic equilibrium illustrated in

Small crystallites are less stable than larger ones and tend to dissolve into their respective ions. Subsequently, the dissolved ions can recrystallize on larger crystallites, which are thermodynamically more stable (Ostwald ripening). The use of acetonitrile, as a solvent, or the addition of styrene/maleic anhydride copolymer allowed the preparation of stable CdS nanoparticles, with anaverage size of 34 and 43 Å, respectively.Cubic ZnS and CdS nanocrystallites were synthesized in aqueous and methanolic solutions without organic surfactant (capping agent).

Nanocrystalline Semiconductors: Synthesis, Properties,and Perspectives (review) Tito Trindade et al. Chem. Mater. 2001, 13, 3843-3858

Preparation of semiconductor nanocrystallites:Solutions of (CH3)2Cd and tri-n-octylphosphine selenide (TOPSe) are injected into hot tri-n-octylphosphine oxide (TOPO) in the temperature range 120-300 °C. This produced TOPO capped nanocrystallites of CdSe.

Example CdSe

C. B. Murray and C. R. Kagan and M. G. Bawendi Annu. Rev. Mater. Sci. 2000. 30:545–610

ZnS:Mn (Me) PhotoluminescenceZnSO4 + Na2S + (MnCl2)

L-cysteineZnS(:Mn2+) + 2 Na+ + SO4

2- + (Cl-)

Absorption Spectrum

00.20.40.60.8

11.21.41.61.8

260.0 280.0 300.0 320.0 340.0wavelength (nm)

30 min1h1h 30 min2h

0.0

20.0

40.0

60.0

80.0

100.0

120.0

400.01400.02400.03400.0wavenumber (cm-1)

CysteineCysteine on particles

2559 cm-1

2090 cm-1

1581 cm-1 1531 cm-1

2559 cm-1: S-H stretch band• 2090 cm-1: NH3

+ stretch band• 1581 cm-1: COO- stretch band• 1531 cm-1: NH3

+ deformation band

Attachment of cysteine on the ZnS surface occurs via mercapto-group. Free carboxylic groups at the particle surface

SO

NH2

O

S

O NH2

O

S

O NH2

O

S

O

NH2 O

S

O

NH2

O

ZnS:Mn (Me) Photoluminescence

SiO2

SEM micrographs of SiO2 nanoparticles formed by the hydrolysisof TEOS in ethanol containing ammonia after 24 h reaction time (SiO2 100/0).

Silica powders were prepared by precipitation in ethanol according to the St¨ober et al. method, which is a precipitationtechnique based on controlled hydrolysis of a silicon alkoxide(tetraethylorthosilicate, TEOS) in a mixture of ethanol, aqueous ammonia (21%), and water. The porogen used, 3-aminopropyltriethoxysilane (APTES) can be mixed to the TEOS prior to adding to the solvent mixture. Five TEOS/APTES ratios were investigated (100/0, 90/10, 80/20, 50/50, and 20/80), the total alkoxide concentration being kept constant and equal to 0.2 M and the water concentration being maintained at 3.2 M. The use of glycerol (Aldrich, reagent ACS, 99.5%) as a porogen was investigated since this organic compound is known to adsorb very well to cations and oxide surfaces under basic conditions.

R. Vacassy, R. J. Flatt, H. Hofmann, K. S. Choi, and R. K. Singh ,Journal of Colloid and Interface Science 227, 302–315 (2000)

Evolution of the particle size and particle size distribution of silica during the hydrolysis precipitation of TEOS (SiO2 100/0). The results were determined using PCS, and error bars indicate the spread of the particle size distribution.

SiO2 nanoparticles formed by the co-hydrolysis of TEOS in ethanol containing ammonia at an intermediate stage of the synthesis (4 h reaction time, SiO2 100/0).

Total particle interaction energy (potential barrier) after 24 h reaction as a function of center to centerseparation for two particles of the same size (a) and of different sizes (b)

The total pair interaction energy V for particles of masses i and j and for the center-to-center separation R is the sum of Van der Waals,VA, electrostatic, VE, and solvation, VS, interactions.

R-ri-rj (nm)

Evolution of the maximum of the total particle interaction energy(potential barrier) during the early stage of the silica nanoparticle synthesis,considering the interaction between primary and growing particles. Dashed lines present linear corrections due to particle density variations.

Side Reactions

Very often, syntheses, which seem straight forward are in fact very complicatedand result in various reaction products. A very good example is the well known

iron oxide Fe2O3.2Fe3+ + 6OH- ⇔ Fe2O3 + 3H2O

In detail:[Fe(H2O)6]3+ [Fe(OH)(H2O)5]2+ + H+ pH: 0-2

2 [Fe(OH)(H2O)5]2+ [(H2O)4 Fe(OH)2Fe(H2O)4]4+ + 2H2O pH: 2-3

[(H2O)4 Fe(OH)2Fe(H2O)4]4+ Isopolyoxo-cations pH: 3-5

Fe2O3·xH2O

Overall reaction:

Jean-Pierre Jolivet et al. C h e m . C o m m u n . , 2 0 0 4 , 4 8 1 – 4 8 7

Nanosized particles in a biological environment are complex systems

1 nm 10 nm 102 nm 103 nm 104 nm 105 nm

Nanoparticle Inorganic or Organicbead with nanoparticles

Parts of DNA, Proteins Virus Cells

Nanoparticles Beads

Bacteria

Biotin

Avidin

CH2 CH O (CH2)3 O CH2 CH CH2O

OH OH

NH

PVA, Silica

Typical Functionalisation and Derivatisation

Iron oxide-PVA-Linker-Transferrin

Spacer

8 nm 10 nm 2 – 30 nm

BiocompatibleCoating

Core Fuctionalisation Derivatisation

Mn:ZnSFe2O3

CarboxylAminoThiol

Drug, Proteine,..

Physics Chemistry BiologyColloidal chemistry

Base

Bareparticles

Fe2+

Fe3+

Sedimentation

γ-Fe2O3

Fe3O4Oxidation/Redispersion

PRODUCT

Synthesis ofMaghemite

Wet chemical co-precipitation

+ Polymer

2 nm

HRTEM

30 nm

Silica beads showing well separated iron oxide particles

Bare particle with double layer + PVA PVA coated particle

NH2

NH2

PVA coated & functionalized PARTICLE & BEAD

++

NH2

NH2

Synthesis in Templates

TEM micrograph of a Cobalt-Al2O3·2SiO2 composite prepared by infiltrating a poroushost matrix with a cobalt-nitrate precursorsolution, and a thermal treatment at 900

°C under H2 atmosphere.

Example for nanostructure tailoring by precursor entrapping:

The high porosity of the gels/xero gels enables the substitution of the water logged in thepores by a designed liquid precurser. The densification of the the host (xero) gel matrix will entrap the precursor which will be transformed. The low temperature densifiction preventsin most cases an uncontrolled reaction between the matrix and the entrapped particles.

R. Nayak, J. Galsworthy, P. Dobson, J. Hutchison J. Mater. Res. 3/4 (1998) 905ff.

Gold particles in micelles

Synthesis: A-B diblock copolymer is used for micelle formationPolymer: Poly(styrene-block-2-vinyl-pyridine)Idea: An inorganic compound such as HAuCl4 is bound selectively to the

Polyvinylpyridine block of the polymer and thus solubilized within thecore of the micelle. Afterwards, the compound is transformed bychemical reaction to the metal.

J. P. Spatz, A. Roescher, M. Möller Adv. Mater. 8/4 (1996)

Synthesis in a Structured MediumA number of matrices have been used for the preparation of semiconductor nanoparticles including: zeolites, layered solids, molecular sieves, micelles/microemulsions, gels, polymers, and glasses. These matrices can be viewed as nano-chambers which limit the size to which crystallites can grow. The properties of the nanocrystallites are determined, not only by the confinements of the host material but also by the properties of the system, which include the internal/external surface properties of the zeolite and the lability of micelles.

Nanocrystalline Semiconductors: Synthesis, Properties, and Perspectives (review) Tito Trindade et al. Chem. Mater. 2001, 13, 3843-3858

Segmented Flow Tubular Reactor

Reactant 1

Reactant 2

Immiscible Fluid

Tubular section

Film on tube wall limits fouling

Well mixedreactants Immiscible

Fluid

Parobolic flow Quasi - Plug Flow

Mixer-segmenter

Segmentation – plug flow not parabolic

30 m longSegmentingFluidDodecane

Perfect Segmentation –no fouling

10 cm

In

Out

• Temperatures 5 - 95°C

• Flow rates 1.4 L/hr• Residence times 1-60 mins

• pH 1-14

Previous Results- Narrower size distributions

Copper Oxalate 25°C – self assembled nanocyrstals

geometrical standard deviation σg

log(particle size)

Freq

uenc

y

σg = 4.3

Batch

log(particle size)

Freq

uenc

y

σg = 1.7

SFTR

Continuous Production - 25 hrs - CaCO3

Conditions: - Ca0 = 0.02 M, - PAA = 0.01 %- C/Ca = 1.01- S = 46

0

5

10

15

0.01 0.1 1 10

SFTR 1hSFTR 9hSFTR 25hMini-batch

Freq

vol

%

d [µm]

dv50 = 0.39 µmspan = 1.06

Crystallographic Control Seeding(25°C)- CaCO3

Seed

Calcite

Vaterite

Aragonite

Powder

Calcite

Vaterite

Calcite

BaTiO3 synthesis – Batch vs SFTR

• Batch 6 litre reactor• 85°C• Nitrogen atmosphere • 5hrs ageing• Washing• Freeze drying

Ba(OH)2 + TiCl4 + 4NaOH→ BaTiO3 + 4NaCl + 3H2O

• SFTR

• 95°C

• Nitrogen segmenting fluid

• 4mm φ tube PTFE• Residence time 10 mins• Washing• Freeze Drying

Low Temperature Aqueous Synthesis (LTAS) developed at GenoaReactants 0.6 M pH 12-14

A.Testino, M.Viviani, M.T.Buscaglia, V.Buscaglia, P.NanniInstitute for Physical Chemistry of Materials - CRN, Genoa, ItalyChemical and Process Engineering Department - University of Genoa, Italy

Powder Characterisation (1)

• Stoichiometry well controlled –batch and SFTR

• Secondary phases – lower BaCO3 with SFTR

• SFTR Finer

Powder Ba/Ti (±1%)nominal experim.

BaCO3%

SFTR 1.12 1.11 0.5%

Batch 1.025 1.01 1-3%

Powder Characterisation (2) Granulometry

• SFTR powder• Finer, • High surface area,

Powder SSAm2/g

dBETnm

PSD (nm)dv16 dv50 dv84

Fag

(dv50 / dBET)

SFTR 50.3 23.7 49.7 67.5 111 2.8

Batch 37.6 30.9 54.1 86.3 328 2.8

0

20

40

60

80

100

0 0.05 0.1 0.15 0.2 0.25 0.3

Batch

SFTR

% V

olum

eDiameter (µm)

Sintering of Nanometer BaTiO3

SFTR- 90% < 100 nm- density – 97 %- grain size 80 nm

Batch - 50% < 100 nm- density - 96% - grain size 150 nm

• Initial powdersprimary particles22-40 nm

•Sintering SPS- 50 MPa, - vacuum – N2- 800-1000°C

Dr.Zhao Zhe,Prof. Mats Nygren, Dr. Zhijian ShenDept. of Inorg. Chem., Arrhenius Lab., Stockholm Univ. S106 91, SwedenB10 Paper 659 Tuesday 14.50, Dolmabahce C