Journal of Analytical and Applied Pyrolysis 104 (2013) 461469

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Journal of Analytical and Applied Pyrolysis

journa l h om epage: www.elsev ier .co

Synthe rolto appl

Rosaria D ErneEmanuela ENEA-UTAPRAb ENEA-UTTMAc NILPRP, Laser

a r t i c l

Article history:Received 20 MAccepted 28 MAvailable onlin

Keywords:CO2 laser pyroSilicon carbideSilica nanoparTitania nanoparticlesNanoparticles synthesisNanomaterials applications

s of mpyrolexibtechn

size ction2 na

1. Introdu

Nanoparsion that is was found ties (opticaentirely diffopment of new nanowtal buildingpoint of sevrials used, ocatalysis totion to therconsidered to their verywithin cellsdelivery to c

Nowadaphases can duction of desired siz

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0165-2370/$ http://dx.doi.oction

ticles (NPs) are particles having at least one dimen-less than 100 nm in size. In the 90s, scientic evidencethat materials on the nanoscale may possess proper-l/electronic/magnetic) that are dramatically and evenerent from the bulk [1]. After this discovery, the devel-nanoscience and nanotechnology has created a wholeorld. NPs can be regarded as one of the fundamen-

blocks of nanotechnology since they are the startingeral bottom up approaches for fabricating nanomate-r being evaluated for use, in many elds ranging from

photonics and opto-electronics, from energy produc-mal management [2]. Moreover, biological systems areas the ideal playground for NPs applications. In fact, due

small size, NPs are able to gain access and even operate. Promising applications in bio-medicine span from drugancer therapy by hyperthermia and to bio-imaging [3].ys, NPs of a wide range of chemical compositions andbe prepared by a variety of methods; however the pro-large amounts of pure, non-agglomerated NPs, withe and narrow size distribution, still results to be an

ding author. Tel.: +39 069 400 5469; fax: +39 069 400 5312.ress: rosaria.damato@enea.it (R. DAmato).

extremely difcult task [4]. To this respect, the technique of CO2laser pyrolysis of gas- and vapour-phase reactants appears as ascalable synthesis route for preparing NPs with controllable mor-phology and in quantities sufcient to be tested for structural andfunctional applications [5]. This technique has been applied tothe synthesis of a large variety of oxide and non-oxide nanopow-ders. Si-based nanoparticles (SiC, Si3N4, Si/C/N, Si/Ti/C) were therst to be produced by laser pyrolysis for the development ofnanocomposite ceramic materials for high temperature structuralapplications [59]. Even if the results were interesting, it was foundthat the presence of critical defects in the structural materials basedon nanophase constituents [9,10] severely affects the nal per-formance and, consequently, the attention was shifted towardsthe development of nanocomposites for applications where thesensitivity to defect population is not very critical (like abrasionresistance [11], polymer reinforcement by addition of nanoparticlesetc.) or specic functions are required (optical [1214], magnetic[15,16], thermal [17] etc.).

A great effort was made for the development and exploitation ofthe optical and electronic properties of Si and Si-based nanopow-ders prepared by laser pyrolysis for applications in photonicsand optoelectronics [12,18,19]. In this framework, it was demon-strated that Si-based nanopowders can be effectively excited atall the photon energies above the band gap and subsequentlytransfer their energy to nearby rare-earth ions which decay emit-ting photons at their characteristic transition energies [18]. This

see front matter 2013 Elsevier B.V. All rights reserved.rg/10.1016/j.jaap.2013.05.026sis of ceramic nanoparticles by laser pyications

Amatoa,, Mauro Falconierib, Serena Gagliardib, e Serrab, Gaetano Terranovaa, Elisabetta Borsellaa

D, C.R. Frascati, via E. Fermi 45, 00044 Frascati, Roma, ItalyT, C.R. Casaccia, via Anguillarese 301, 00123 Roma, Italys Department, 409 Atomistilor, 077125 Magurele-IF, Bucharest, Romania

e i n f o

arch 2013ay 2013e 3 June 2013

lysis nanoparticlesticles

a b s t r a c t

Nanoparticles are the building blockdevices. The technique of CO2 laser nanoparticles has proven to be a very challenges in different sectors of nanosize in the range 560 nm and a narrowof structural materials and/or for funon the synthesis of SiC, TiO2 and SiOpreservation.m/locate / jaap

ysis: From research

st Popovici c,

any approaches for realizing nanostructured materials andysis of gas- or vapour-phase precursors for the synthesis ofle and versatile technique which permits to cope with severalology. Different kinds of pyrolytic nanopowders with averagedistribution, have been produced and tested for the fabricational applications in various elds. Here we report new resultsnoparticles for energetic applications and cultural heritage

2013 Elsevier B.V. All rights reserved.

462 R. DAmato et al. / Journal of Analytical and Applied Pyrolysis 104 (2013) 461469

functionality was tested on pyrolytic Si-based nanopowders and Erco-doped solgel silica glasses for the realization of optical ampli-ers emitting at 1.54 m (i.e. the telecommunication wavelength)[18] and, afterwards, for the realization of uorescent bio-markersby exploitaamino grouSi nanopart

A great optical and[3]. In this coptical propthe visualizthe laser pof magnetifor magneting with naincluding thSi nanoparpyrolysis hfor the realjet printing

These rlaser pyrolious applicboth the syticles for qresults on prompted bapplicationtion.

2. Laser py

The CO2gerty and cclassied asNPs. In thiswhen a sufucts is reacfast particlethan furthecle coalesceare formedpartially sierates of psmall spherof supersatof nuclei anremoving tkinetics.

In the pucts result point of thephase precuinto the syleast one otional mod10 m [9]. vapour pha(SF6) or amers due to relatively hSF6, 3.91 eVthe sequenmolecule, fo

chems.

incrion-ty thecule

ecom with

pared with other vapour-phase synthesis methods8], it is fairly evident that laser pyrolysis permits highly

ed and fast heating (leading to rapid nucleation) in ae that can be limited to a few hundred mm3, followed byenching of the particle growth (in a few ms). As a result,ith average size ranging from 5 to 60 nm and narrow sizeution are formed in the hot region. The mean NP size canied by acting on the process parameters like the residencef the particles in the ame (tres), the total pressure in then chamber and the reaction temperature. This explains theotentiality of laser pyrolysis for the preparation of ad-hoc

owders for different applications. However, as in other-phase synthesis methods, one of the major obstacles todespread use of pyrolytic NPs is the formation of chainederates of small particles. Agglomeration occurs when the

ave the high temperature region and coalescence becomesslower than coagulation. High-power ultrasonic treatment

ball milling are effective in reducing the nanoaggregatesize by disaggregation of soft-agglomerates, whereas par-intered aggregates can only be eliminated by centrifugationation, with signicant material loss. Stabilization of the NPssed in liquids is usually achieved by use of surfactants, that is

active agents adsorbed on the surface of the NPs. Repulsivearticle forces are required to overcome the attractive Vanaals forces between the particles. The two most commonds used are based either on electrostatic repulsion or onorces [1].tion of the aptitude of Yb ions to form complexes withps which are easily linkable to the surface of pyrolyticicles [20].attention was indeed devoted to the development of

magnetic probes for biomedical imaging applicationsontext, several procedures were followed to exploit theerties of Si nanoparticles prepared by laser pyrolysis foration of medically relevant targets [2023]. Moreover,yrolysis technique was employed for the productionc Fe2O3 nanoparticles to be tested as contrast agentsic resonance imaging [15,16,24]. Multimodal bioimag-noparticles has also been addressed by several routes,e synthesis of a nano-complex incorporating pyrolytic

ticles and gadolinium ions [25]. More recently, laseras been applied to the synthesis of TiO2 nanoparticlesization of UV protection products [26] and inks for ink

techniques [27].esults prove the great exibility of the method ofysis for preparing prototypal nanoparticles for var-ations and leave room for further developments innthesis process and the use of the produced nanopar-uite different functions. Here we will report newthe synthesis of SiC, TiO2 and SiO2 nanopowders,y a surge of renovated interest for these materials fors in the energetic sector and cultural heritage preserva-

rolysis for nanopowder production

laser pyrolysis technique was rst developed by Hag-o-workers at the beginning of 80s [6] and it is usually

a vapour-phase synthesis process for the production of class of synthesis routes, NP formation starts abruptlycient degree of supersaturation of condensable prod-hed in the vapour phase [28]. Once nucleation occurs,

growth takes place by coalescence/coagulation ratherr nucleation. At sufciently high temperatures, parti-nce is faster than coagulation and spherical particles. At lower temperatures, coalescence slows down andntered, non-spherical particles and/or loose agglom-articles are formed [28]. It follows that, to prepareical particles, it is necessary to create a high degreeuration for inducing the formation of a high densityd then quickly quench the particle growth either by

he source of supersaturation or by slowing down the

rocess of CO2 laser pyrolysis, the condensable prod-from laser induced chemical reactions at the crossing

laser beam with the molecular ow of gas- or vapour-rsors (see Fig. 1). The pre-requisite for energy couplingstem, leading to molecular decomposition, is that atf the precursors absorbs through a resonant vibra-e the infrared (IR) CO2 laser radiation tuned at aboutAlternatively, an inert photo-sensitizer is added to these mixture. Ethylene gas (C2H4), sulphur hexauoridemonia (NH3) are the most widely employed sensitiz-their resonant absorption at about 10 m and to theigh dissociation energy (7.2 eV for C2H4, 3.95 eV for

for NH3). The high power of the CO2 laser inducestial absorption of several IR photons in the samellowed by collision assisted energy pooling, leading to

Fig. 1. Sreactant

a rapid(vibratnied bIf moleular doccursucts.

Com[1,3,4,2localizvolumfast quNPs wdistribbe vartime oreactiogreat pnanopvapourthe wiagglomNPs lemuch and/ormean tially sor ltrdispersurfaceinter-pder Wmethosteric fatic of the set-up for laser synthesis of nanoparticles from gas-phase

ease of the average temperature in the gas through V-Translation) energy transfer processes, often accompa-

appearance of a ame in the interaction volume [29].s are excited above the dissociation threshold, molec-position, eventually followed by chemical reactions,

the formation of condensable and/or volatile prod-

R. DAmato et al. / Journal of Analytical and Applied Pyrolysis 104 (2013) 461469 463

Fig. 2. Picturereactants.

3. Experim

3.1. Experim

The set-pyrolysis isis focused bcentre of thorthogonallpower is 1be varied uthrough thegas (He or conning achamber is Typical pres

In the lasgas-phase, honly choiceof view. Theorating sysprecursor isor one of tin the evapa heater; atgas mixturereaction ch

At the equenching eproduced p

a removable bag, located in a tank between the reaction chamberand the vacuum pump.

aracterization techniques

opowl combsor

00 FT areed wremenninEG-S-lenn deCentolydctur

meavon

mone-ma

elasion bve, a

amipparopows baspersetor e

disa

of the ENEA set-up for laser synthesis of nanoparticles from gas-phase

3.2. Ch

Nansevera

IR atrum1spectraequippmeasu

Scaby a Fwith inelectrotrons (their p

StruRamanJobin-YgratingA homtion ofexcitatobjectietry.

Dyn4800 aon nanysis waare dissonicapartialental

ental set-up

up for the production of nanopowders by CO2 laser shown in Fig. 2. The CW CO2 laser beam ( = 10.6 m)y a spherical lens (focal length F.L. = 12 or 19 cm) at thee reaction chamber (volume V 6.8 103 m3) where ity intersects the reactant gas ow. The maximum laser.2 kW and the power density in the focal region canp to 275 kW/cm2. Reactant gases enter the chamber

inner tube of a coaxial stainless steel nozzle. An inertAr) ows through the outer tube with the purpose ofnd cooling the particles. The pressure in the reactionkept constant and measured by a pressure control unit.sures are in the range 6.780 103 Pa.er pyrolysis process, the reactants are most often in theowever in some cases, liquid precursors are either the

or the most advantageous from an economical point use of liquid precursors is made possible by an evap-

tem installed at the laser pyrolysis set-up: the liquid extracted from the reservoir by bubbling the inert gas,he gas-phase reactants, and subsequently introducedorator unit and maintained at proper temperature by

the exit of the evaporator the vapour precursor and are carried out through a heated exible tube to the

amber.xit of the reaction region, marked by a ame, a strongffect stops the particle growth at nanometric size. The

owder are driven by the gas ow through a chimney into

4. Nanopo

Here weSiC, SiO2 angrowth mecof these nantors and cu5.

4.1. SiC-NP

Si and Sinvestigateprecursor fthe emissio(ii) the enoa multitude

Silicon csilane withmic [5,6]. Thproportion duce powdeThe mean nacting on thsity, the reaand the innreported in

Althouga function authors [5,6induced readers prepared by laser pyrolysis are characterized byplementary techniques.ption spectra are recorded on a Perkin-Elmer Spec-IR spectrophotometer, while the UV-VIS and near IR

recorded on a Perkin-Elmer 330 spectrophotometerith a 60 mm integrating sphere for diffuse reectancents.g Electron Microscopy (SEM) micrographs are collectedEM LEO 1530 (Zeiss, Obercoken-Germany) equippeds secondary electron detector, conventional secondarytector and scintillation detector for backscattered elec-aurus) to investigate the nanoparticles diameters andispersion.al investigation is performed by Raman spectroscopy.surements were performed using a modular HORIBA-

spectrometer, consisting in a bre-coupled 550 mmochromator and a liquid-nitrogen cooled CCD detector.de sampling head equipped with a notch lter for rejec-tically scattered light was used to deliver the 532 nmeam on to the sample by a 32X long-working-distancend to collect the Raman signal in backscattering geom-

c light scattering (DLS) measurements by a Malvern PCSatus allows to estimate the hard-bound aggregate sizeders samples dispersed in water or ethanol. Data anal-ed on the model-independent Zave parameter. Powdersd using an high-power ultrasonic probe (BRANSON 450quipped with 3 mm micro-tip) for 30 min to promoteggregation whenever necessary.

wder preparation and characterization

report on the latest developments in the synthesis ofd TiO2 nanoparticles in terms of controlling the particle-hanism and tuning the nal structure and compositionoparticles for various applications in the energetic sec-ltural heritage preservation, as described in Paragraph

s synthesis and characterization

i-based nanoparticles are probably the most widelyd for two main reasons: (i) silane (SiH4) gas, a commonor Si radical formation, has a very high absorptivity forn line of an untuned, multimodal CO2 laser (at 10.6 m);rmous importance of Si and Si-based nanoparticles for

of applications, as reviewed in Section 1.arbide is preferably formed by laser induced reactions of

acetylene, since the global process is strongly exother-e reactive gases are injected into the chamber in molar2:1 through the use of mass ow controllers, to pro-rs whose stoichiometry comes close to that of pure SiC.anoparticle size can be varied in the range 2050 nm bye process parameters, in particular the laser power den-gent ows, the total pressure in the reaction chamberer nozzle diameter. Details on process parameters are

Table 1.h changes in the SiC nanopowder nal properties asof process parameters have been studied by several,30], the details of the ignition process and of the laserctions are more difcult to characterize. To this respect,

464 R. DAmato et al. / Journal of Analytical and Applied Pyrolysis 104 (2013) 461469

Table 1Experimental parameters, productivities and mean diameters (d) of SiC-NPs.

Powder Lens F.L. (cm) Nozzle diameter (mm) SiH4 (sccm) Laser power (W) Productivity (g/h) d (nm)SiC-15 12 3 500 630 50 25 2SiC-16 630 15 30 2SiC-17 630 50 30 3SiC-20 630 55 35 13SiC-21 630 30 45 18SiC-22 630 55 40 13SiC-23 400 50 35 3SiC-24 900 50 50 18SiC-26 630 50 50 22

Fig. 3. Left sid as owame luminos

the map of burning protime) of the

In the foing geometbe analyzedconsists in with the reget this cona spherical leter on the nozzle havilaser powerin Table 1),the pressuracetylene the connetivities rang100%.

The temby the amside. The oboptical winthe inner sutemperatur(3.2 102ow. Howeame wherAr ow. Thlaser powerproductivitlaser powersociated. SEsize distribugradient at

ctane formwer r

repoon detersrderg ge12 3 300 12 4 500 19 3 500 19 4 300 19 4 500 19 4 500 19 4 500 19 6 500

e: 3D view of the interaction region between the laser beam and the molecular gity map.

ame temperatures can be used to gain insights into thecess itself and the subsequent evolution in space (and

nanoparticle growth.llowing, the effect of two different types of laser focus-ries on the nal characteristics of SiC nanopowders will

and discussed. The most common optical congurationmatching the CO2 laser beam waist in the focal regionactant ow cross-section in the interaction volume. To

the reacles arand loresultstributiparam

In ofocusindition, in our set-up the CO2 laser beam was focused byens (F.L. = 19 cm) down to a spot of about 5 mm in diam-reactant ow, a few mm below the exit of the gas inletng an internal diameter of 4 mm (see Fig. 3 left side). The

was varied in the range 400900 W (cfr SiC 222324 all the other process parameters being kept constant:e in the reaction chamber was set at 8.0 104 Pa, theow was set at 250 sccm and the Ar ow, which ensuresment of the reaction, was set at 5 slm. Typical produc-ed between 3050 g/h and reaction yields were near to

perature distribution in the reaction zone is visualizede luminosity map in the picture shown in Fig. 3 rightserved ame asymmetry is due to the Ar ow at the

dow to avoid powder deposition and accumulation onrface. It is evident that in this focusing conguration thee distribution is rather uniform in the reaction volumemm3), which is laterally limited by the conning Ar gasver, the temperature appears lower at the edge of thee the reactant molecules are diluted (and cooled) by thee maximum temperature in the ame depends on the

density that is varied from 2.0 to 4.6 kW/cm2. The highies (and reaction yields) observed in the whole range of

densities ensure that all the reactant molecules are dis-M analysis of the produced SiC particles shows a broadtion (Fig. 4) that can be explained by the temperaturethe edge of the ame. At the ame boundaries, in fact,

of the ameTo this purow was re(F.L. = 12 cmthe ame p in the focusing geometry with F.L. (focal length) = 19 cm. Right side:

ts are more strongly diluted with Ar and smaller parti-ed as a consequence of the lower radical concentration

eaction temperature T [2830]. On the other hand, therted in Table 1 conrm that the peak of the size dis-

epends on the laser power when all the other process are kept constant (cfr SiC 222324).

to narrow the particle size distribution, a differentometry was designed to change the temperature prole and minimize the particle formation at the ame edge.pose, tight focusing of the laser beam on the reactantalized by use of a shorter focal length, spherical lens) (see Fig. 5a). After SiH4 ignition in the laser focus,ropagates in the reaction volume and the temperature

Fig. 4. SEM image of SiC nanopowder sample (SiC-24).

R. DAmato et al. / Journal of Analytical and Applied Pyrolysis 104 (2013) 461469 465

Fig. 5. Left side: 3D view of the interaction region between the laser beam and the molecular gas ow in the focusing geometry with F.L. (focal length) = 12 cm. Right side:ame image and luminosity map.

distribution in the gas mixture was established as visualized by theame luminosity map in the Fig. 5b. It is evident that in this cong-uration the reaction volume at the highest temperature is smaller(1.8 102 mm3) and more conned reactions are expected tooccur in the central part of the ame, with a lower effect of the dilu-tion (and conarrower siindeed obseSiC-17 with

In selectnanoparticlthe nanopavelocity of tby decreasidiameter. TSiC nanopaprepared bthe comparand SiC-22.particles (cSiC-26).

All the nat 830 cm1

(Fig. 7).

4.2. SiO2-N

Silica NPthat reacts

Amorphous, nanosized Si/C/O powders were synthesized usingalkoxysilane as liquid precursor [31]. In order to obtain pure silicananoparticles, we chose a different precursor, i.e. tetraethoxysilane(TEOS) Si(OC2H5)4, a liquid at room temperature, with a vapourpressure of about 130 Pa, in which Si atoms are already oxidized.

s not a strong absorber of laser radiation at 10 m, thus eth-gas (C2H4) was added as a reaction sensitizer to increasepling of the laser energy in the system, due to its resonanttion . The

carrator.0 C

t for the ow e real par

acco as-stionne denecefortu

owde for mecreaoling) of the reactants at the outer edge of the ame. Aze distribution of the produced SiC nanoparticles wasrved in this conguration (cfr. SiC-15 with SiC-20 and

SiC-22), as shown by the SEM pictures in Fig. 6.ed laser focusing conditions, the nal mean size of thees can also be varied by acting on the residence time ofrticles in the reaction volume, that is by changing thehe reactants. The speed of the reagents can be decreasedng the molecular ow or by changing the inner nozzlehe effect of a change in these parameters on the nalrticle size is shown in Table 1. Bigger particles can bey decreasing the molecular ow as it is evidenced byison between SiC-15 and SiC-16 and between SiC-21

Also the use of a larger nozzle diameter leads to largerompare SiC-15 with SiC17 and SiC-20 with SiC-22 and

anopowders present the typical infrared feature of SiCand the X-ray diffraction patterns of crystalline -SiC

Ps synthesis and characterization

s formation needs a precursor different from silane, violently with oxygen and can cause an explosion.

TEOS iylene the couabsorpenergytle andevaporTev = 14diluenbeforemass and thmentavaried

Thetaminaethyleit was 6 h. Unnanopsirablewas dFig. 6. SEM image of SiC nanopowder sample (SiC-17).Fig. 7. TypicalSiC nanopowdat about 10 m and to the relatively high dissociation aerosol was produced by bubbling Ar into the TEOS bot-ied to the reaction chamber by Ar ow through a liquid

The temperature of the evaporator was settled at either, 160 C, 170 C or 180 C. Ar gas can also be used as athe reagents. C2H4 and TEOS aerosol were mixed justexit of the inlet nozzle (6 mm in diameter). The TEOSresulted to be of about 50 g/h (aerosol ow = 400 slm)ctor pressure was xed at 5.3 104 Pa. Other experi-ameters such as ethylene ow and laser power wererding to data reported in Table 2.ynthesized powders generally appeared dark for con-

with free-carbon coming from TEOS and/or unwantedcomposition. In order to remove these contaminantsssary to perform a thermal treatment at T = 600 C fornately, the DLS measurements on thermally treatedrs revealed a high degree of aggregation, that is unde-ost applications. Consequently, carbon contaminationsed to less than 6%, w/w (as shown by thermal IR spectrum and X-ray diffraction pattern (inset) of laser synthesizeders.

466 R. DAmato et al. / Journal of Analytical and Applied Pyrolysis 104 (2013) 461469

Table 2Effect of synthesis parameters: experimental parameters, carbon contamination, mean diameters (d, as obtained by SEM analysis) and Zave (as obtained by DLS) of SiO2-NPs.

Powder Tev (C) C2H4 (sccm) Laser (W) %C (w/w) d (nm) Zave (DLS)TEOS-1 140 400 800 10 12 221TEOS-2 140 0 1200 10 15 248TEOS-3 140 400 1200 20 20 241TEOS-4 140 600 1200 18 18 235TEOS-8 160 400 800 8 28 150TEOS-9 160 400 1000 10 25 158TEOS-7 160 400 1200 13 22 190TEOS-14 170 0 700 15 18 204TEOS-12 170 200 700 9 15 180TEOS-10 170 400 700 6 35 157TEOS-15 170 600 700 6 25 186TEOS-11 170 400 900 9 35 183TEOS-16 180 600 700 5 28 195TEOS-17 180 400 800 9 10 204

gravimetric analysis) by a ne tuning of the process parameters (seeTable 2) such as evaporator temperature, laser power and ethyleneow.

By comparing carbon contamination values reported in Table 2,rst of all we can observe that an increase in the evaporator tem-perature improves the nanopowder purity, since it favours thecomplete dissociation of TEOS precursor and its conversion tosilica. Another important parameter is the reaction (and ame)temperaturylene ow.place with unwanted dTEOS-23is low, the ature, conscarbon contWhen the rhigh ethyleincreases agbest results16, in whichpowder colneeded.

A controwith meanFig. 8). An ow resulteconditions zone due to

The nanopowders were found to be slightly aggregated, asevidenced by Zave values reported in Table 2. The evaporator tem-perature (Tev) has an inuence on the aggregate size: at Tev = 140 C,Zave is of about 240 nm, while at higher evaporator temperaturesZave is less than 200 nm. Aggregate mean size depends also on thereaction temperature: in fact at high temperature coalescence isthe most important process and the NP mean size increases, whileat low temp

s ag infrrs ar

abso cm

ype.

O2-N

nia Nniumethyl waeters400 sre in. Th

f C col treas fo

size oe which depends on both the laser power and eth- If the laser power is high, TEOS dissociation takesa limited, or even without, addition of ethylene whoseecomposition is the main contamination source (see

4 in Table 2). On the other hand, when the laser powerpresence of ethylene increases the reaction temper-equently TEOS conversion to SiO2 is complete andamination is decreased as found in TEOS 10121415.eaction temperature is too high (high laser power andne ow, cfr TEOS 789) the carbon contaminationain, probably for side reactions of decomposition. The

were obtained in case of TEOS-10, TEOS-15 and TEOS- carbon contamination is less than 6%, the as-preparedour was almost white and no thermal treatment was

l on NPs dimensions was achieved and nanopowders size ranging from 10 to 40 nm were produced (seeincrease in the evaporator temperature and ethylened in the synthesis of bigger particles, because in these

there is a high concentration of radicals in the reaction almost complete dissociation of TEOS.

towardThe

powdestrongat 809silica t

4.3. Ti

Titaof titaeither aerosoparamow = pressuTable 3ence otherma

It wtallite Fig. 8. SEM image of the nanosilica sample TEOS-7.erature coagulation is favoured and there is a tendencygregation rather than to NP size growth.ared absorption features of the as-formed nanosilicae very similar. A typical IR spectrum is shown in Fig. 9. Arption band appears at 1100 cm1 with a weaker bands1 which correspond to Si O stretching of the opaline

Ps synthesis and characterization

Ps were synthesized by laser pyrolysis of an aerosol (IV) isopropoxide (TTIP) Ti[OCH(CH3)2]4 mixed withlene or ammonia (NH3) as reaction sensitizer. Thes produced as described above. Typical reaction

were: evaporator temperature Tev = 180 C, aerosolccm, laser power = 1100 W. The gas ows and the total

the chamber used in different runs are reported ine as-prepared powders are black or blue for the pres-ntamination that was subsequently eliminated by softatment in air at T = 500 C for 3 h.und by XRD and TEM analysis that the average crys-f sample TiO2-1, produced with ethylene as sensitizer,Fig. 9. IR spectrum of the nanosilica sample TEOS-10.

R. DAmato et al. / Journal of Analytical and Applied Pyrolysis 104 (2013) 461469 467

Table 3Experimental parameters for the synthesis of pure and N-doped TiO2-NPs.

Powder Sensitizer TTIP (g/h) sensitizer (sccm) Pressure (Pa)

TiO2-1 C2H4 50 400 5.3 104N-TiO2-2 NH3 40 600 8.7 104N-TiO2-3 NH3 40 600 4.7 104N-TiO2-4 NH3 50 600 5.7 104

was of about 10 nm, the crystalline phase being approximately 30%rutile and 70% anatase (data not shown here). When NH3 was usedas sensitizer, the average crystallite size resulted to be of about15 nm and the crystalline phase was mostly anatase. The crys-talline phase of all these samples was also determined by meansof Raman spectroscopy. Data reported in Fig. 10 show the presenceof a small percentage of rutile in sample TiO2-1, and a practicallypure anatase phase in the other samples [32]. From the structuralpoint of view, rutile is known to be the TiO2 high-temperature sta-ble phase and anatase the low-temperature one. Throughout thedifferent synthesis runs, remarkable changes in the pyrolysis amecould be seen with the naked eye. During the synthesis of samplesN-TiO2-234 the ame intensity was lower than in the presence ofethylene (TiO2-1), probably for the cooling effect of NH3. As a resultof this lower ame temperature, the rutile phase is not present inthese samples.

Moreovegen can beshift the opas evidenceafter the thwhite in casamples. A cient of theKubelkaM[33] reportis observedpresence of

It is impsamples canticular, a dein the champowders duthe reaction

Fig. 10. Ramarutile () phaseof anatase pha

Fig. 11. Absoranalysis of difthe low-intensgen doping.

5. Recent apyrolysis

laseovens to llengecenving ve b

ing.

velo

neeely guids,e uiquidike hes ahanned ventation for a variety of applications ranging from coolingctronic components, transformer oil and heat-exchanges to the development of better performing chillers, domesticratorsfreezers and more [34].his framework, pyrolytic nanoparticles of TiO2, SiO2 and SiCsed for the preparation and testing of selected nanouids. Anement in the thermal conductivity (with respect to distilled

was observed in all these nanouids at different concen-s in water. The thermal conductivity of the nanouids wasined either with conventional methodologies [36] or with a

que based on optical, laser-induced grating, which providesssibility for in situ monitoring of coolant performances [37].ative thermal conductivity of the water based nanouids wasr, as a consequence of partial dissociation of NH3, nitro- incorporated in the NPs. N-doping has the effect totical absorption of titania towards the visible ranged by the different colour of the nanopowders; in factermal treatment, the powders colour changes fromse of pure TiO2 to yellow in case of N-doped TiO2semi-quantitative estimation of the absorption coef-

produced nanopowders (Table 3) can be obtained fromunk analysis of their diffuse reectance (DR) spectraed in Fig. 11. An absorption feature at about 430 nm

in the spectra taken on samples synthesized in the ammonia and conrms the nitrogen-doping.ortant to note that the level of N concentration in the

be varied by changing the process parameters. In par-crease of the TTIP ow and an increase of the pressureber led to increased N concentration in the producede to the higher relative concentration of N radicals in

zone.

Thehas prpermitin chaMore rand sation hafollow

5.1. De

Thehas latnanoin bassolidltages lparticlmicrocenhancto conexploitof eledevicerefrige

In twere uenhancwater)trationdetermtechnithe poThe reln spectra of TiO2 samples and attribution of peaks to anatase () and on the basis of their intensity and position, showing the prominencese.

found in sumodel [35]

Erosion conducted for direct cSiC water-bption coefcient of TiO2 nanopowders obtained from KubelkaMunkfuse reectance spectra. Note the logarithmic scale used to evidenceity absorption at wavelengths longer than 400 nm, induced by nitro-

pplications of nanopowders prepared by laser

r pyrolysis method for the synthesis of nanoparticles to be a very exible and versatile technique whichnely tune NP properties for a variety of applicationsing research elds and competitive industrial sectors.tly, applications in critical sectors like energy generationas well as in new elds like cultural heritage preserva-een started. Preliminary results are presented in the

pment of nanouids for enhanced heat exchange

d for more efcient and better performing coolantsenerated considerable interest for the development of

that are dilute and stable suspensions of nanoparticlesids [34,35]. In principle, if compared to conventional

suspensions, nanouids should offer several advan-igher specic surface areas for heat transfer betweennd uids, reduced clogging when passing throughels, better thermal properties. A rm assessment of

heat transfer coefcient in nanouids with respectional coolants would push towards their industrialbstantial agreement with the prediction of the Maxwell.tests of nanouids owing on different targets wereby use of an experimental facility designed to allowomparison with distilled water [38]. It was found thatased nanouids have a negligible effect, if compared

468 R. DAmato et al. / Journal of Analytical and Applied Pyrolysis 104 (2013) 461469

to distilled water, when impact on Al, Cu and stainless steel tar-gets. Moreover, evaluation tests of the heat transfer performanceof water-based nanouids containing either TiO2 or SiC pyrolyticnanoparticles (in different concentrations) were carried out in thesame dual tparameter, the nanoubeen attribfor the samthus allowiever, due tpressure drtypical of thues are reqwhen consipractical ap

5.2. Develo

TiO2 nanareas as phelectrodes fWhile tradiapproachesprovide advphology of

Presentlcations in pthe visible extend the range propciency gainare reportepreliminaryoptimised Dthose of a sport propertheir respecations reqgeneration,

5.3. Developreservation

In the laals have beeThe modulacoating canrial with suinanotitaniaimproved w

For thatof about 15as nanomefor protectpolymer. Tgation are nanocompofact the syndispersion method, solthe polymeprepared avery command veined both in clim

simulate real degradation processes in terms of photo-thermaleffects and mechanical damage.

The performances of the different nanocomposites were eval-uated comparatively by means of several diagnostic techniques.

sults idatil cha

the ally i

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Nano HaggeSteinfe1, pp.

Borselonimolaser terialsest rig [39]. Using the Reynolds number as a referenceit has been found that the heat transfer coefcient forids is higher than for the basic uid. This result hasuted to the higher viscosity of the nanouids, which,e Reynolds number, corresponds to a higher velocity,ng for a better heat transfer performance [39]. How-o the higher velocity for the same Reynolds number,ops associated with nanouids are higher than thosee basic uid. It follows that higher pumping power val-uired. These conclusions have to be taken into accountdering the perspectives for exploitation of nanouids inplications.

pment of TiO2-based porous semiconductor electrodes

opowders are considered for applications in many hototocatalysis, functional surface coating and workingor dye-sensitized solar cells (DSSC) just to name a few.tional synthesis methods like ame pyrolysis or solgel

are the most investigated, laser-assisted pyrolysis canantageous material characteristics related to the mor-aggregates, and to the possibility of efcient doping.y, we are exploring nitrogen-doped titania for appli-hotocatalysis and DSSC electrodes. In the rst case,absorption band produced by doping is expected tophotocatalytic activity of the material beyond the UVer of intrinsic titania [40], with obvious energy ef-. For DSSC applications, the N-doped working electrodesd to improve the electron collection efciency [41]. Our

results show that the conversion efciency of a non-SSC device using N-doped titania compare well with

tandard device. Work is in progress to study the trans-ties of doped and undoped electrodes, in order to assessctive electronic performances also in different appli-uiring porous semiconductor electrodes as hydrogen

sensors, and others.

pment of nanocomposites for cultural heritage

st few years, nanoparticles and nanostructured materi-n applied to restoration and conservation of artworks.tion of physical and chemical properties of a protective

be obtained by a proper blending of the coating mate-tably chosen nanoparticles. In particular, nanosilica and

were selected for their physical properties, such as theater repellence.

reason, SiO2 and TiO2 nanoparticles with mean size nm, as produced by CO2 laser pyrolysis, were addedtric llers to two commercial products, largely usedive coatings, i.e. an acrylic resin and a silicon-basedhe purity of the nanopowders and their low aggre-considered important items to obtain homogeneoussites that do not alter the colour of the artworks. Inthesis of nanocomposites to get a nal homogenous

is a tricky step, depending on the chosen preparationvent and particle concentrations. Different solutions ofrs with dispersed silica and titania nanoparticles werend applied on the surface of two different litotypes,on in outdoor cultural heritage: white marble (statuaryCarrara) and travertine [42]. Articial ageing processes,atic chamber and in solar box, were carried out to

The reconsolstantiahinderespeci

6. Con

Wenanopcope wnologyof nanotant cophotonby appresultsthe nnanomal exdopedDSSC etests scles prnanocoan emnology

Ackno

Wediscusbri andof thecharacgravimDr. F. positesfor coties. FFP5-ISCollabinvolvdevelo

Refere

[1] A.Scat

[2] M.CRes

[3] M. bio

[4] O. Ann

[5] N. nanand

[6] J.S.J.I. 198

[7] E. Dikby Mademonstrate that nanoparticles enhance the efcacy ofng and protective materials because they induce sub-nges in the surface morphology of the coating layer andphysical damage observed during articial weathering,n alkylsiloxane products [42].

ions and perspectives

e shown the capability of laser pyrolysis to producees with the composition and morphology adequate tohallenging applications in different sectors of nanotech-he past years, attention was mainly focused on the useicles in the eld of structural nanoceramics, wear resis-gs and functional nanomaterials for opto-electronics,nd bio-imaging. More recently interest was attractedions in the energetic sector. In this perspective, newe been reported here showing the possibility to tuneroperties of SiC nanoparticles for the preparation ofwith improved thermal conductivity for enhanced ther-ge. Moreover, we have addressed the development ofpure TiO2 NPs for applications in photocatalysis andodes fabrication. Finally, we have described preliminarying the promising performances of SiO2 nanoparti-ed by laser pyrolysis for the realization of protectivesite coatings for cultural heritage preservation. This isg and important application eld in which nanotech-pected to give a signicant contribution.

gments

h to thank Dr. R. Fantoni for helpful and valuable on the process of nanoparticle formation; Dr. F. Fab-

F. Rondino for their contribution in different phaseserimental activity on the synthesis, processing andation of nanoparticles; Dr. M. Carewska for thermal

analysis and thermal processing; Dr. L. Caneve anda for collaboration on the development of nanocom-

cultural heritage preservation; Dr. F. DAnnibaletion in the research activities on nanouid proper-

ng for our research came from EC funded Projects:ERGIA, FP6-Life-Science-BONSAI and FP7-Large-Scale-

ve-HENIX (NanoHex) Project. We thank all the partners these Projects for their precious contribution to thent of our activities.

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Synthesis of ceramic nanoparticles by laser pyrolysis: From research to applications1 Introduction2 Laser pyrolysis for nanopowder production3 Experimental3.1 Experimental set-up3.2 Characterization techniques

4 Nanopowder preparation and characterization4.1 SiC-NPs synthesis and characterization4.2 SiO2-NPs synthesis and characterization4.3 TiO2-NPs synthesis and characterization

5 Recent applications of nanopowders prepared by laser pyrolysis5.1 Development of nanofluids for enhanced heat exchange5.2 Development of TiO2-based porous semiconductor electrodes5.3 Development of nanocomposites for cultural heritage preservation

6 Conclusions and perspectivesAcknowledgmentsReferences