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ION MORJANpresenting author

R. Alexandrescu, I. Voicu

National Institute for Lasers, Plasma and RadiationPhysics, P.O. Box MG-36, Bucharest, Romania

NEW TRENDS IN THE SYNTHESIS OF

NANOMATERIALS BY LASERPYROLYSIS


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RESEARCH TRENDS IN THE LAB

OF LASER PHOTOCHEMISTRY

RESEARCH TRENDS IN THE LAB

OF LASER PHOTOCHEMISTRY

!Due to the versatility of the laser pyrolysis method, we could

develop severalresearch directions in the field of nanomaterials:

"""" iron carbides

"filamentary iron/iron oxide nanoparticles""""nanocarbons

"""" iron/carbon composites (core-shell structures)

""""gamma iron oxide nanopowders"titanium-doped gamma iron oxide

""""carbon fibres and nanotubes

""""metal-polymer nanocomposites


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International involvement

and cooperation for the FP6 3-rd priority

International involvement

and cooperation for the FP6 3-rd priority

The Laser Photochemistry Laboratory members have developedthe last-years-activity in direct connexion with EuropeanOrganizations dedicated to the spreading of nanotechnologies and

nanomaterials

""""PINCO NoE elected in the first step, 1-st FP6 Call

""""COST 523 Action (since 2000) on Nanotechnologies

-joint research and publications (more than 5) with Cost 523 members

S. Martelli, J. Pola, O. Schneeweiss

""""PHANTOMSNetwork of Excellence (since 2003)

"Network of centers of excellence (since 2003):Interfacial Effects,

Novel Properties and Technologies of Nanostructured Materials


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Synthesis approach:Synthesis approach:Synthesis approach:Synthesis approach:

CO2 laser pyrolysis

Synthesis approach:Synthesis approach:Synthesis approach:Synthesis approach:

CO2 laser pyrolysis

By designthis process uses a precisely defined chemical

reaction zone, in which gases are combined to

form simple or complex nanoscale compounds

Advantages of this method

# pure products (no contact with surface of the chamber)#### extremely fine powders (d < 50 nm)#### small distribution of sizes#### continuous synthesis#### well defined reaction zone#### variable reaction conditions (temperature, pressure, ...)#### homogenous nucleation#### control of growing rate and residence time in reaction zone

Drawbacks####gas (vapor) precursor of the material

####the use of gas sensitizers (possible shift of reaction routes)

####often small reaction yield

The composition, the diameter and the particle growthare directly dependent of the temperature which in turndepends on chosen precursors

#### pressure in the reaction chamber

#### laser power#### flow rate of the gaseous precursors


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Resonant procesess(the gas reactant absorbs the laser radiation)

Non-resonant procesessGas + energy transfer agent: C2H4, SF6

10P20 CO2 laser line (944 cm1)

Radiation absorption by resonant gasthrough the excitation of vibrational states

C2H4, SF6 vibrational excitation

Molecular collisions leading to vibrationalenergy transfer and relaxation (V-V and V-

T, R processes)heating

rates104-

105 K/sHigh excitation of all vibrational states:

hot vibrational systems

The dissociation threshold for the reactantgas is reached

Collisionalenergy transfer

V-V, V-T,R

Increased system temperatureheating

rates104-

105 K/s

Non-specificthermal

excitation

towards dissociative levels

Thermal (non-resonant) gasdecomposition

Particularly, Fe(CO)5 (D0= 1.8 eV)

Hot radicals fusion and/or radical reactions of dissociation products

Freezing/condensation process stopped at the nano-level Nanoparticles formed by fast and sudden cooling

Evolving molecular processes

by laser pyrolysis

Evolving molecular processes

by laser pyrolysis


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Schematic view of a general purposes laserpyrolysis installation

Schematic view of a general purposes laserpyrolysis installation

!By switching the liquid precursors filamentary iron nanoparticles

"""" + air (oxygen donor) titanium-doped iron oxides!!!! By switching the additive gases iron-carbon core-shell structures"""" Sensitizer and """" hydrocarbon (carbon donor) nanocarbon


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Experimental set-up forcarbon nanotubes

deposition

Experimental set-up forcarbon nanotubes

deposition

Laser driven thermal CVD


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The preparation of the following

nano-products will

be outlined

The preparation of the following

nano-products will

be outlined1.- Nanocarbons from different

hydrocarbon sensitized precursors2.-Filamentary iron/iron oxide

nanoparticles by Fe(CO)5sensitized decomposition

3.- Gamma iron oxide (maghemite)nanopowders adding oxidizer precursors

4.- Titanium-doped iron oxides adding TiCl4 vapors

5,- Iron-carbon nanocomposite

(core-shell structure) by specific experimental conditions

6.- Nanofibres/multi-wall carbon nanotubes

catalyzed formation by LCVD method


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Nanocarbon by laser pyrolysis of hydrocarbons

Main component parts:

!!!! Continuous wave CO2 laser

(max. 1 kW at 10P20 emission line)

!!!! stainless steel reactor

!!!! Gas control system (mass flow controllers)

!!!! Downstream pressure control system

!!!! Powder recovery system

Experimental parameters

Ar as carrier gas7508001.20.8SF6330C6H6/SF6/N2O/Ar

Ar as carrier gas750800--C2H4250C6H6/C2H4/Ar

Ar as carrier gas750800-0.8SF6200C6H6/SF6/Ar

750800-0.53SF6300C2H2/SF6

600950400900--C2H4300C2H2/C2H4

450950500900--C2H4100300C2H4

Obs.Pressure[mbar]

Laser power[W]

(C/O)at

(C/F)at

Absorbtion gas(at =10.6 m )

Total flow(sccm)

Typesof mixtures


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Morphology investigation

revealed

amorphous carbon, partial crystalline carbon with small graphitic sheets, fullerene-like structures

Curved structures are a general feature of the morphology of carbon particles

The soot is made of very fine particles, which are rather spherical andmay be dispersed or may coalesce in bigger particle, forming chains


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A g g l o m e r a t i o n

Different number of soot nuclei suggest

that agglomeration occurs already in the

flame, during the formation of the sootnuclei and throughout the process of

surface growth

Rather diffuse images of diffraction rings

(Selected Area Electron Diffraction analysis)suggest the absence of a long-range

crystalline order


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The controlled presence of oxygen in the reactive gas mixture couldproduce a major change in soot morphology

C6H6/SF6 C6H6/SF6/N2O

Fullerene-like carbon powder

better chemical activity

(interesting effects in applications)

turbostratic structure graphene sheets spaced at 4

particle diameter: ~ 35 nm

specific surface: ~ 134 m2/g

fullerenic structure (curved carbon layers,

onion-like formations and fullerenes)

particle diameter: ~ 20 nm

C/O = 1.2; C/F = 0/8

specific surface: 245 m2/g

!!!! The oxidizing character of fluorine released by SF6 decomposition

(could produce some alteration of the classic turbostratic structure even without oxygen-)


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Iron/iron oxide compositesIron/iron oxide composites

Filamentary iron oxide covered iron nanoparticles,produced like a spider-web inside the reaction chamber

-Laser pyrolysis of Fe(CO)5 (vapors)

Fe(CO)5 Fe + (5-x)CO, x = 0-4

Ethylene flow (400 sccm) as carrier gas for Fe(CO)5

HREM image of an iron nanoparticle surrounded

by a thick Fe3O4 layer(lower part: the corresponding diffractograms)


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Iron/iron oxides morphologies andparticle size distributions

Iron/iron oxides morphologies andparticle size distributions

Medium-resolution TEM micrographs exhibiting

chain-like agglomeration of iron nanoparticles

at two different scales

Particle size distribution of iron nanoparticles

obtained by laser synthesis of Fe(CO)5/ C2H4.

The given diameter pertains to the total diameter

including the oxide shell. The ordinate value refer to

the number of particle evaluated (N = 342)


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Gamma - iron oxideGamma - iron oxide

Filamentary iron oxide covered iron nanoparticles,produced like a spider-web inside the reaction chamber

Detail of experimental set-up:Collection chamber for - Fe2O3

Some representative experimentalparameters for the synthesis of iron oxides

Run Carriergas

Carrier

sccm

Air

l/min

Ptorr

V SF6 8 0.5 150

VI C2H4 250 0.5 150

* The total argon flow (for gas confinement and windowscleaning) was maintained at 1.3 l/min. In all runs the CO2laser power was about 100 watt


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X ray diffraction analysisX ray diffraction analysis IR-SpectroscopyIR-Spectroscopy

IR transmission spectra of samples V(curve a) and VI (curve b), showing

the characteristic gamma oxideabsorption band around 600 cm-1

X-ray diffraction pattern of sample VIshowing the characteristic diffraction planes

for - Fe2O3


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Titanium-doped gamma iron oxidesTitanium-doped gamma iron oxides

!!!! Basic pyrolysed precursors: TiCl4/Fe(CO)5/air/C2H4

Experimental parameters for some representative runs:

70800150150080350250SF 22

6040015015001009068SFT 1

50500150120016540024SF 16

5050015012004540018SF 14

7550015012004540010SF 10

PL(watt)

P

(mbar)

ArW(sccm)

ArC(sccm)

Air

(sccm)

C2H4 flow through Fe(CO)5(sccm)

C2H4 flow through TiCl4(sccm)

Run

The most relevant runs show different degrees of titanium incorporation, mainly by simplepenetrating the iron oxide network-forming different extents of gamma titanium maghemite

(with similar diffraction peaks)

X di i


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X ray diffraction analysisX ray diffraction analysis X ray energy dispersive

analysis

X ray energy dispersive

analysis

Sample SF 22 high increase of titanium

component

-SF 16 with identified -Fe2O3 diffraction peaks

- showing the presence of-FeO(OH), in Cl

2atmosphere (TiCl4 decomposition)

HRTEM images of high Ti doped iron oxide sample

HRTEM images of high Ti-doped iron oxide sample


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HRTEM images of high Ti-doped iron oxide sample(SFT 1)

HRTEM images of high Ti doped iron oxide sample(SFT 1)

Image showing mainly large grains withinterplanar distances corresponding to -FeOOH

[ICSD 29-129]

Image with region of lower size particles showinginterplanar distance 3.0 corresponding either to

-Fe2O3 [ICSD 87-119] or -titanium maghemite[84-1595]

C b l t d i ti l


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Carbon encapsulated iron particlesCarbon encapsulated iron particles

$one-step procedure$the carbon embedding degree depends on the control of experimental parameters and radiation geometry$narrow size distribution, particle mean diameter 4 6.5 nm

""""C2H4 (sensitizer for Fe(CO)5 fast decomposition) - through the inner nozzle""""C2H2+C2H4 (hydrocarbon mixture) through the middle tube

Experimental parameters(pressure: 650-700 mbar; Ar flow confinement: 1100 sccm)

50371020CF7

50731920CF6

85703050CF5

952955100CF4

PLWatt

Acetylene inmixture

sccm

Ethylene in mixture

sccm

Ethylene (Fe(CO)5carrier)

sccm

Sample

TEM image of the as prepared sample CF5 SAED


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TEM image of the as-prepared sample CF5, SAEDpattern and particle size distribution (inserts)

TEM image of the as-prepared sample CF5, SAEDpattern and particle size distribution (inserts)

$even dispersion of Fe nanoparticlesembedded in a carbon matrix

$$$$almost each particle present a core(dark contrast) surrounded by a shell

$$$$the particle distributionmean size diameter of 7nm

$$$$electron diffraction pattern (SAED) the presence of-Fe (110) and (211) (hkl)

planes and possibly -Fe2O3 partialoxidation in the ambient)

HRTEM i f F C i d l

HRTEM images of Fe C nanocomposite as prepared sample


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Sample CF 5 Sample CF 6

HRTEM images of Fe-C nanocomposite as-prepared sampleHRTEM images of Fe-C nanocomposite as-prepared sample

$$$$the structure: rather spherical core(identified as -Fe) and outer carbon layers (3-4)surrounding the iron particles

$$$$partial coalescent features between particles

$$$$increased number of carbon layers (about 7)showing a peculiar feature: they seem to be

partially shared by neighbor iron covered particles

C b fib / t b d d b d

Carbon fibers/nanotubes grown on seeded carbon-covered


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Carbon fibers/nanotubes grown on seeded carbon-coverediron nanoparticles

Carbon fibers/nanotubes grown on seeded carbon-coverediron nanoparticles

Different nanofibre/MWNT structures catalytically grown by acetylene decomposition on dispersed

Fe-C nanocomposite, using the laser induced CVD method.

!!!!the embedding of metal-based nanoparticles in carbon layers confers inertness and resistance to externaldetrimental conditions!could be used as catalysts for the growth of carbon nanotubes!preliminary CO2 laser-induced CVD experiments: %%%% using a flowing gas mixture containing acetylenesensitized by a low flow of SF6! the C-Fe nanocomposite was dispersed on silicon (single crystal) substrates

CONCLUSIONS

CONCLUSIONS


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CONCLUSIONSCONCLUSIONSThe laser pyrolysis method was employed in the gas phase for the synthesis of

different nanopowders and composites

$$$$Soot containing different carbon nanoparticles were obtained by the laser pyrolysis ofdifferent hydrocarbons

$$$$Nanometric size -Fe2O3 particles were obtained from gas-phase reactants (iron

pentacarbonyl (vapors) and air as oxygen donor)

$$$$Different titanium-based iron oxide nanocomposite powder prepared from TiCl4/

Fe(CO)5/ air/ C2H4 precursors show different degrees of titanium incorporation, mainly

by simply penetrating the iron oxide network (mean sized between 1.5 and 8 nm)

$$$$Filamentary iron nanostructures were obtained from laser-induced pyrolysis of ironpentacarbonyl and ethylene mixtures

$$$$Single-step experiment was leading to the synthesis of Fe-C nanocomposite formed of

iron nanoparticles (4.5-6 nm mean diameters) with a low degree of agglomeration,

which are covered by carbon layers

$$$$ Preliminary CVD experiments demonstrate the catalytic properties for growing

fibers/nanotubes of the as-prepared Fe-C nanocomposite

The presented results demonstrate that the laser pyrolysis technique mayopen new opportunities for the fabrication of nanomaterials and

com osites

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