1

Sonochemical Synthesis of Metal

Oxide Nanoparticles

Jiri Pinkas

Department of Chemistry

Masaryk University

Brno

Czech Republic

2

OutlineIntroduction

Ultrasound and Sonochemistry

Sonochemical Synthesis of Nanoparticles

Iron Oxides - Fe2O3 - maghemite and hematite

- Fe3O4 - magnetite

Mixed Y/Fe oxides - YFeO3 and Y3Fe5O12

Ferrites CoFe2O4

Conclusions

3

UltrasoundUtrasound frequencies from 20 kHz to 50 MHz

frequency, Hz

4

Ultrasound in Chemistry

Sound = pressure waves = periodic compression/expansion

cycles traveling through a medium possessing elastic

properties (gas, liqud, solid)

frequency 20 kHz

Acoustic Cavitation = creation, growth, and implosive collapse of bubbles in a liquid

Bubble size

5

Acoustic Pressure

cIPA 2

PA = driving pressure amplitude [Pa]

I = irradiation intensity [W m2]

(500 W system - 1.3 105 W m2)

ρ = liquid density [kg m3]

c = sound velocity in liquid [m s1]

(Water 1482 m s1)

Compression and rarefaction

(expansion) regions

PA = 620 700 Pa = 6.2 bar

6

Bubble formation = breakage of liquid during expansion,

overcoming tensile strength (pure water 1500 bar, only 6.2 bar

available)

Weak spots needed = dissolved gas molecules, solid particles,

trapped gases

Bubble growth (300 µs), energy absorption, size oscillations

critical size (170-300 µm) = most efficient energy absorption, rapid

growth, inefficient energy absorption, collapse

Acoustic Cavitation

Cavitation effects = creation, growth, and implosive collapse of

bubbles in a liquid

7

Bubbles collapse = spherically symmetrical implosion,

shear forces, adiabatic compression, life time 1-2 µs

Hot spot = end of the collapse

temperature of the gas inside bubble

5 000 – 20 000 C (for 1 ns)

surrounding liquid layer 2000 C

pressure 500 – 1500 bar

Extreme cooling rates 1010 K s1

red hot steel poured into water 2500 K s1

Acoustic Cavitation

8

Cavitation effects = creation, growth, and implosive collapse of

bubbles (1-2 µs) in a liquid = implosion HOT SPOT (1 ns)

Acoustic Cavitation

stable cavitation - bubbles

oscillate for many cycles

transient cavitation -

transient cavities expand

rapidly

collapse violently

9

Sonochemical Reactions

• Chemical changes/reactions induced by ultrasound

• No direct interaction of ultrasound field with molecules (in

contrast to photons, heat, electro-magmetic field,…)

• Liquid phase reactions – chemical reactions driven by cavitation

effects – radical generation, excited species formation,

mechanochemical bond scission, …

• Solid state reactions – introduction of defects = speeding up

diffusion

10

Homogeneous Sonochemistry

Two-Site MechanismCavity interiorFilled with gases and vapors

temperatures 5 000 – 20 000 C

pressure 500 – 1500 bar

Surrounding liquid layertemperatures 2000 C

Bulk liquidShock waves, shear forces

11

• Apparent blackbody temperature

• Ar emission

• SO and O2+ emission

How to Measure the Temperature

inside a Bubble ?Sonoluminescence - Light generated during the implosive

collapse of bubbles in liquids irradiated with ultrasound

95% H2SO4(aq.)

under Ar

20 kHz (14 W/cm2)

Ti horn directly immersed

T = 298 K

Kenneth S. Suslick

University of Illinois

8 000 – 15 000 K

12

Sonochemical Reactor

Piezoelectric Ultrasound Generator

Ultrasound Processor VCX 500 W

Piezoelectric

transducer

PZT ceramics

Titanium horn

Reaction

vessel

Argon

inlet

AC generator

Frequency 20 kHz

Cooling

Piezoelectric

transducer

13

Project Goals

Sonochemical synthesis of metal oxide nanoparticles

for magnetic, catalytic, biomedical applications

Metal acac complexes as precursors, organic solvents

Influence of reaction parameters, added water

Control of phase composition, particle size,

distribution, shape, crystallinity, metal stoichiometry in

mixed-metal oxides

14

Product Control in Sonochemical

Synthesis of Metal Oxides

• Polymorphic modification control

Iron Oxides Fe2O3 - maghemite and hematite

• Oxidation state control

Reduction to Fe3O4 magnetite and Fe

• Phase control

Y-Fe oxides – YFeO3 and Y3Fe5O12

• Composition control - CoxFe2xO4 ferrite

15

Iron Oxide-Based Materials

Magnetic nanoparticles

• Hyperthermia

• Magnetic drug delivery

• MRI contrast agents

Iron Gate

Samaria Gorge, Crete, Greece

16

OO

M

H

CH3H3C

OO

M

CH3

- C3H4

200 °C

- CH3COCH3

300 °CMCO3 MxOy

765 °C

- CO2

M(acac)n Precursors

M = Fe, Y, Co, Mn

MO

O

O

O

O

O

Me

Me

Me

Me

Me

Me

Ismail, H. M. J. Anal. Appl. Pyrolysis 1991, 21, 315326.

Thermal decompositon pathway

• Volatile

• Organics soluble

• Nontoxic

• Well studied class of compounds

• Precursors in CVD

• Easily chemically modified

17

• Solution of M(acac)3 (0.03 M)

• 50 cm3 of tetraglyme (b.p. 549.0 K)

• Water addition (0, 1, 3 cm3)

• Ar flow rate 60 cm3 min1

• Sonication 8 hrs

• Frequency 20 kHz

• Pulsed 2s / 2s

• Power 22 W cm2

• Colloidal soln. precipitated by hexane

• Centrifugation, washing with iPrOH

• Drying in air

Reaction Setup

VCX 500 W

18

Iron Oxide-Based Materials

Fe2O3

amorphous

Fe2O3

maghemite

Fe2O3

hematite

300 oC

340 oC

He, Nitrogen

bulk

dynam/isothermal, air

Fe(acac)3

)))))

tetraglyme

TGLsolution

thin layer

Fe3O4

magnetite

200 oC

Fe3O4

magnetite

linear, air

340 oC

Fe

-iron

MO

O

O

O

O

O

Me

Me

Me

Me

Me

MeDefect spinel

Corundum

Spinel

19

Sonochemical Synthesis

Amorphous Iron Oxides

Fe2O3

amorphous

)))))Fe(acac)3

Colloidal solution of iron oxide

Particle size by Dynamic Light Scattering

(Zetasizer)

Mean D = 9.2 nm (by volume)

Pinkas, J.; Reichlova, V.; Zboril, R.; Moravec, Z.; Bezdicka, P.; Matejkova, J. Ultrasonics Sonochem. 2008, 15, 257–264.

Particle size 20 - 30 nm

Spherical shape

Uniform size distribution

Specific surface area

BET 188 m2/g

20

TEMproves amorphous character

of sono-Fe2O3

Electron diffraction

21

Time under TEM beam

Maghemite or Magnetite

Crystallization of Amorphous Fe2O3

under TEM Beam

Electron diffraction

22

Amorphous

Fe2O3

Crystallization induced

by heating (300 C)

HR-TEM

Fe2O3 calcined at 300 C

23

Low blocking

temperature

25 K

Amorphous

Fe2O3

High-spin Fe3+ octahedral

superparamagnetic nanoparticles

size less than 10 nm

57Fe Mössbauer Spectra

of Sono-Fe2O3

Zero-Field

5T-Field

24

Decomposition of Acac Ligands

OO

C

HC

C

H3C CH3

Fe

H2O

O

C

CH3H3C

OO

Fe

C

CH3

OO

Fe

C

CH3

OO

Fe

C

CH3

Fe

OO

C

HC

C

H3C CH3

FeH3C C C H

Speculation about the nature of residual organic groups

WET

DRYBinding modes of acetate groups

25

3 5 0 0 3 0 0 0 2 5 0 0 2 0 0 0 1 5 0 0 1 0 0 0 5 0 0

0

0 . 2 5

0 . 5

0 . 7 5

F e J P 6

IR and MS Evidence for Acetate

in Sono-Fe2O3

as(COO)

1566 cm-1

s(COO)

1432 cm-1

Acetate stretchingDiketonate vibrations absent

Mass Spectrometry

Direct insertion probe, 70 eV

T = 50 – 300 C

m/z 60 / 45 / 43 = CH3COOH+

26

Synthesis of Fe2O3 from

Fe(acac)3Water

%vol.

SA

m2 g1

H Pore vol.

cm3 g1

Fe

%

C

%

H

%

Mass loss %

at 500 ºC

dry 48 H3 0.304 22.0 30.5 4.52 63.1

2 105 H1 0.611 35.6 22.6 3.48 46.0

6 147 H3 0.455 43.6 15.3 2.75 35.2

2 + wet Arc 260 H2 0.291 42.0 15.5 2.73 37.3

Increasing water concentration

= larger surface area

= lower organics content

= higher metal content

27

Nitrogen Adsorption-Desorption

Isotherms

0

0

0 11

)1(p

p

CV

C

CV

p

pV

p

p

mm

BET

28

Specific Surface Area

BET surface area

of the Fe2O3 heated

to different

temperatures

during 12h

outgassing periods

120

140

160

180

200

220

0 50 100 150 200 250 300 350 400

Temperature, ºC

SA

, m

2/g

The oxide surface area increases as the acetate groups are

removed, then the particle size increases because of sintering

Surface area 48 to

260 m2 g-1 (BET)

depending on H2O

content

29

BET Constant C

RT

qqC L1exp

q1 is the adsorption heat of the first layer

qL is the liquefaction heat

C is an indication of the affinity of the adsorbed molecule for the solid :

if this affinity is high, q1 >> qL and C is high (for example 100)

if the affinity is low, q1 ≥ qL and C may be as low as a few units.

For BET validity C = 20 – 200

0

0

0 11

)1(p

p

CV

C

CV

p

pV

p

p

mm

30

BET Constant C

C constant (BET)

10,00

30,00

50,00

70,00

90,00

0 50 100 150 200 250 300 350 400

Temperature, oC

C

The strenght of interaction of nitrogen with oxide surface increases

as the acetate groups are removed

31

Composite Particles of Sono-Fe2O3

HR-TEM (5 nm bar)

TEM (20 nm bar)

after heating to 250 °C

Iron oxide particle size 2 - 3 nm

Embedded in organic matrix

32

Thermal Transformations of

Amorphous Iron Oxides

Fe2O3

amorphous Fe2O3

hematite

dynam/isothermal, air

Fe(acac)3

)))))

tetraglyme

340 oC

thin layer

Heating in on a hot-stage of an XRD diffractometer

Sample in a thin layer on a Pt holder

Ramp 2.5 °C min-1, 2 min data collection, 25 °C steps

Polymorphic modification control

33

HT-PXRD of Sono-Fe2O3

Amorphous Hematite

Crystallization

325 - 350 C

34

Hematite Particle Size

30

40

50

60

70

80

350 400 450 500 550 600

Temperature, °C

D,

nm

D/nm, 104

Coherence length

D (nm)

By Scherrer

equation

Dependence of the coherence length, D (nm) of -Fe2O3 on

the crystallization temperature under dynamic-isothermal

conditions of the HT-XRD measurement

35

Crystallization to hematite induced by heating (above 300 C)

BET Surface area decreased from 188 to 120 m2/g

HR-TEM

Fe2O3 calcined at 300 CFe2O3 calcined at 600 C

TEM

36

Thermal Transformations of

Amorphous Iron Oxides

Fe2O3

amorphous Fe2O3

maghemite

linear, air

Fe(acac)3

)))))

tetraglyme

300 oC

thick layer

Heating in a DSC crucible

Sample in a thick layer

Ramp 2.5 °C min1

360 °C250 °C

300 °C

Polymorphic modification control

37

20 40 60 80 100

2[deg]

2000

3000

4000

cou

nts

2000

4000

6000

8000

16000

24000

maghemite

250 oC

300 oC

360 oC

PXRD

of amorphous Fe2O3

heated dynamically in air

up to 250, 300, and 360 °C

250 °C

300 °C

360 °C

Maghemite

38

In-field 57Fe Mössbauer Spectrum of

Amorphous Fe2O3 Heated to 300 °C

subspectra for the tetrahedral and octahedral positions of

Fe3+ in the defect-spinel structure of maghemite resolved

some content (cca 21%) of amorphous phase is still present

20 K / 6 T

Maghemite

39

Solution Thermolysis Amorphous Iron Oxide

Red-brown colloidal solution after sonolysis

Particle size by DLS (Zetasizer)

Mean D = 9.2 nm (by volume)

Black suspension, magneticParticle size by DLS (Zetasizer)

Mean D = 11.3 nm (by volume)

Surface area 103 m2 g1 BET

Heating to 250 C

3 hrs in air

Fe2O3

amorphous Fe3O4

magnetite

TGL solution

Fe(acac)3

)))))

tetraglyme

250 oC

Oxidation state control

40

HR-TEM

Fe2O3 thermolysed at 250 C

TEM

SAED

41

XRD of Fe2O3 Thermolysed at 250 C

Coherence domain size

by Scherrer equation

8 - 9 nm

Magnetite Fe3O4

42

57Fe Mössbauer Spectrum of Fe2O3

Thermolysed at 250 C

T

(K) 0.01

(mm/s)

ΔEQ 0.01

(mm/s)

Bhf 0.03

(mm/s)

0.01

(mm/s)

A

()

Sextet 293 K 0.42 -0.03 26.89 0.61 84.3

Dublet 0.35 0.81 ---- 0.47 11.3

Singlet 0.34 --- --- 0.47 4.5

high-spin Fe3+ octahedral

superparamagnetic nanoparticles

size less than 10 nm

43

Thermal Transformations of

Amorphous Iron Oxides

Fe2O3

amorphous Fe3O4

magnetite

Helium, Nitrogen

Fe(acac)3

)))))

tetraglyme

250 oC

Fe

-iron

Heating in on a hot-stage of an XRD diffractometer

Flowing He gas

Sample in a thin layer on a Pt holder

Ramp 5 °C min-1, 20 min data collection, 50 °C steps

TG/DSC under flowing nitrogen

Sample in a thick layer in a Pt crucible

Ramp 5 °C min-1

Oxidation state control

44

100 200 300 400 500 600 700 800 900Temperature /°C

40

50

60

70

80

90

100

TG /%

Mass Change: -43.90 %

Mass Change: -15.48 %

[1]

300 °C

250 °C

700 °C

**

* = Pt holder

Magnetite Fe3O4PXRDof amorphous Fe2O3

heated dynamically in He

Magnetite Fe3O4

Wustite FeO

Iron Fe

TG/DSC in N2

45

Sonochemical Synthesis

of Ternary Y/Fe Oxides

amorphous

)))))

)))))

Y(acac)3

YxFeyOz

Fe(acac)3

crystalline

Y3Fe5O12

YFeO3

crystalline

Y/Fe = 3/5

Y/Fe = 1/1

TetraglymeCalcination

One-step synthesis

Perovskite (YIP)

Garnet (YIG)SEM

Phase Control

Pinkas, J.; Reichlova, V.; Serafimidisova, A.; Moravec, Z.; Zboril, R.; Jancik, D.; Bezdicka, P.

J. Phys. Chem. C 2010, 114, 13557-13564

46

SEM and TEM of Sono-YFeO3

As-prepared sono-YFeO3 is amorphous

Particle size 30-50 nm

Uniform particle size distribution

BET surface area 85 m2 g1

47

57Fe Mössbauer Spectra

of Sono-Y-Fe-Oxide

Temperature

KComponent

IS

mm s-1

EQ

mm s-1

Q

mm s-1B / T A / % Assignment

RT D 0.34 0.79 – – 100.0Fe3+ HS

octahedral

25 D 0.46 0.84 – – 100.0

Fe3+ HS

octahedral

amorphous

Quadrupole doublet

High-spin Fe3+

Octahedral coordination

Superparamagnetic nanoparticles

Size less than 10 nm

Amorphous

YFeO3

Same parameters for amorphous Fe2O3, YFeO3, Y3Fe5O12

48

4000 3500 3000 2500 2000 1500 1000 500

Absorbance

-1

-0.8

-0.6

-0.4

-0.2

Acac to Acetate Transformation

as(COO)

1587 cm-1

s(COO)

1386 cm-1

Acetate stretchingDiketonate vibr. absent

OO

C

HC

C

H3C CH3

Fe

H2O

O

C

CH3H3C

OO

Fe

C

CH3

OO

Fe

C

CH3

OO

Fe

C

CH3

Fe

OO

C

HC

C

H3C CH3

FeH3C C C H

Mass spectrometry

Direct insertion probe, 70 eV

T = 50 – 300 C

m/z 60 / 45 / 43 = CH3COOH+

49

Crystallization of Sono-YFeO3

Perovskite YFeO3

700 °C

800 °C

780 °C

760 °C

740 °C

720 °C

Ramp 1 °C min-1, 1 min equilb.,

30 min data collect., 20 °C steps

HT-XRD25-1000 °C

Size 60 nm

50

Growth of Particle Size in YFeO3

Dependence of the coherence length, D (nm) of YFeO3 on

the crystallization temperature under dynamic-isothermal

conditions of the HT-XRD measurement under air

coherence

length

D (nm)750 °C

70 nm

51

Calcination of Sono-YFeO3 in Air

200 400 600 800 1000Temperature /°C

-3

-2

-1

0

1

2

3

DSC /(mW/mg)

40

50

60

70

80

90

100

TG /%

[1] FeY88air_1.dsu TG

DSC

Mass Change: -43.44 %

Mass Change: -19.39 %

Mass Change: -1.65 %

End: 347.0 °C

End: 723.6 °C

[1]

[1]

exo

Removal of weakly bound groups

Oxidation of tightly bound groups

OH removal

Crystallization

TG

DSC

52

Calcination of Sono-YFeO3 in N2

sono-YFeO3YFeO3 + Y2O3 + Fe

Reduction of Fe(III) to Fe(0) by organic residuals

Removal of weakly bound groups

Removal of tightly bound groups

Reduction

Crystallization XRD

TG

DSC886 C

Metal-Oxide Composite

53

Sonochemical Synthesis of

CoFe2O4

amorphous

)))))

)))))

Co(acac)3

CoFe2O4

Co(acac)2

)))))

Fe(acac)3

CoFe2O4

crystalline Spinel

Composition control

54

SEM/TEM of Amorphous Co-Fe Oxide

TEM of CoFe Oxide Calcined at 300 C

55

CoFe2O4 - Metal Stoichiometry

Single phase CoFe2O4

obtained for the final product stoichiometry

Co : Fe from 1 : 1.3 to 1 : 2.0

Excess Co resulted in CoO separation

Excess Fe resulted in Fe2O3 separation

Composition control

56

HT-XRD of CoFe Oxide

Direct

crystallization to

CoFe2O4

300 oC

400 oC

500 oC

600 oC

700 oC

800 oC

900 oC

1000 oC

Ramp 1 °C min-1, 1 min equilb., 30 min data collect., 20 °C steps

No hematite

Size 15 nm

57

TG-DSC of Sono-CoFe2O4 in Air

Removal of weakly bound groups

Removal of tightly bound groups

Complete removal of organics

below 300 oC

TG

DSC

58

HT-XRD of CoFe Oxide300 - 1000 C

8,3

8,32

8,34

8,36

8,38

8,4

8,42

8,44

8,46

8,48

8,5

0 200 400 600 800 1000

Temperature, oC

0

10

20

30

40

50

60

70

80

90

100

a sp

D sp

Particle Size

of CoFe2O4

Lattice parametr

of CoFe2O4

Particle size

under 20 nm

up to 600 C

59

TEM of CoFe Oxide Calcined at 300 C

CoFe2O4

nanoparticles

covered with an

amorphous layer

preventing their

growth

60

TEM of CoFe Oxide Calcined

at 400 C

61

Calcination of Sono-CoFe2O4 in N2

200 400 600 800 1000Temperature /°C

-1.5

-1.0

-0.5

0.0

DSC /(mW/mg)

40

50

60

70

80

90

100

TG /%

[1] CoFe13-1_1.dsu TG

DSC

DSC: 936.0 °C

[1]

[1]

exo

TG

DSCMp. 936 C

Complete

removal of

organics

below 600 oC

XRD Co-Fe alloy

Complete reduction of CoFe2O4 nanoparticles by carbon-containing

residues in the absence of oxygen

62

ConclusionsPolymorphic modification control

Iron Oxides Fe2O3 - maghemite and hematite

Oxidation state control

Reduction to Fe3O4 magnetite and Fe

Phase control

Y-Fe oxides – YFeO3 and Y3Fe5O12

Composition control

CoxFe2xO4 ferrite, Co : Fe ratio from 1 : 1.3 to 1 : 2.0

63

AcknowledgmentFinancial support:

Ministry of Education

MSM0021622410 and 6198959218

Grant Agency of the Czech Republic (203/08/1111,

106/09/0700)

Ph.D. students:

Zdenek Moravec

Jan Chyba

Petr Ostrizek

M.S. students:

Iva Kolhammerova

Martin Janicek

David Skoda

Ales Styskalik

B.C.

Martin Kejik

Prof. Radek Zboril, Mössbauer sp.

Prof. Zdenek Travnicek, CHN

Dr. Petr Bezdicka, HT-XRD

Dr. Mariana Klementova, TEM

Dr. Nataliya Murafa, TEM

Dr. Dalibor Jancik, TEM

Dr. Eva Vecernikova, TG-MS

Dr. Oldrich Schneewiess, Magnetic

Dr. Petr Pikal, TG-MS

Dr. Jirina Matejkova, SEM

64

Department of Chemistry

Masaryk University

Brno

Czech Republic