Published: November 23, 2010

r 2010 American Chemical Society 5140 dx.doi.org/10.1021/jp109371m | J. Phys. Chem. C 2011, 115, 5140–5146

ARTICLE

Magnetic Nanoparticles of Iron Carbide, Iron Oxide, Iron@Iron Oxide,and Metal Iron Synthesized by Laser Ablation in Organic SolventsVincenzo Amendola,*,‡ Pietro Riello,§ and Moreno Meneghetti‡

‡Department of Chemical Sciences, University of Padova, Via Marzolo 1, I-35131 Padova, Italy§Department of Physical Chemistry, University of Venezia, Via Torino 155/b, I-30172 Venezia, Italy

ABSTRACT: Iron-based nanoparticles can have useful magnetic and catalytic proper-ties. We investigated the synthesis of iron-based nanostructures by laser ablation ofbulk iron with 1064 nm nanosecond pulses in the following organic solvents:tetrahydrofuran, acetonitrile, dimethylformamide, dimethylsulfoxide, toluene, andethanol. Structural analysis carried out by transmission electron microscopy andX-ray diffraction revealed that the solvent has a dramatic influence on both thecomposition and the nanostructure of nanoparticles. Various magnetic nanoparticleslike iron carbide (Fe3C), magnetic iron oxide (magnetite/maghemite), metal iron(R-Fe), and iron@iron oxide are obtained by varying the solvent and keepingunchanged all the other experimental conditions. These results are the consequencesof the different reactivity of solvent molecules exposed to the plasma plume generatedduring the ablation process.

’ INTRODUCTION

Iron-based nanostructures attract great interest in nanotech-nology for their physical and chemical properties.1 Magneticnanoparticles composed of metal (Fe), carbide (Fe3C), andoxides (Fe3O4, magnetite, or γ-Fe2O3, maghemite) are studiedfor a wide range of applications like data storage, ferrofluids, ormagnetic microdevices.1,2 Furthermore, the biocompatibility ofsome iron-based materials makes them especially promising forapplications in nanomedicine like magnetic resonance imaging,selective drug delivery, and magnetic hyperthermia.2-4 Nanos-tructured iron oxides are investigated also for applications incatalysis, as active substrates for water splitting, or for the removalof toxic metals from aqueous solutions.5-7

Variousmethods have been proposed for the synthesis of iron-based nanoparticles. The chemical approaches are particularlyuseful for obtaining colloidal solutions of the metal and theoxides with well-defined shape and size distribution,1,8,9 while thephysical methods like spray or flame pyrolysis are useful forobtaining nanopowders of oxides and carbides.10-12 However,wet chemistry synthesis usually requires a controlled atmosphereand expensive and/or toxic chemicals and usually producespollutant waste, while pyrolysis synthesis usually gives agglom-erated nanoparticles.

To date, there are relatively few studies about the laser ablationsynthesis in solution (LASiS) of iron-based or other magneticnanostructures. LASiS consists of the ablation of a bulk targetplaced at the bottom of a cell containing the liquid solution by afocused laser pulse, and it is a versatile technique for the synthesisof a variety of nanomaterials in a variety of solvents.13,14 TheLASiS is a low-cost “green” technique because it does not requireexpensive chemicals and does not usually produce pollutantwaste as in wet chemistry methods.13,14 LASiS has been success-fully applied to metals, oxides, and semiconductor nanoparticles,

and its scalability to large-scale production was recently shown.15

The syntheses by nanosecond pulses of magnetic nanostructuresnot based on iron, like Co and Co oxide nanoparticles in water, inhexane or in toluene have been reported by various groups.16-18

Recently, femtosecond laser ablation in cyclopentanone of twobinary magnetic alloys (Sm-Co and Ni-Fe) was also investi-gated.19 A more sophisticated experiment was carried out by theMeunier group that reported the synthesis by femtosecond laserablation and by postirradiation of a Au-Co nanoalloy with acore@shell structure.20 Regarding iron-containing materials,Zeng et al. reported that the laser ablation of an iron target inwater solution of polyvinylpyrrolidone yielded wustite (FeO),which is an antiferromagnetic phase stable at high temperature.21

In this paper, we focus on the laser ablation of an iron target indifferent organic solvents. Previously, we showed that the solventcan affect the average size of noble metal nanoparticles and candetermine if isolated nanoparticles, metal@graphite nanostructures,or nanoparticles embedded in organic matrix are obtained.22-24

Here we show that in the case of laser ablation of metal iron thecomposition and the structure of the resulting nanoparticles aredramatically influenced by the solvent. In particular, one cansynthesize nanoparticles with composition ranging from ironcarbide to iron oxides or to metal iron, while the nanostructurescan vary from the case of isolated nanoparticles to particles embeddedin an organic matrix or a core@shell structure composed byiron@iron-oxide or iron@graphite. The different reactivity of

Special Issue: Laser Ablation and Nanoparticle Generation inLiquids

Received: September 30, 2010Revised: November 8, 2010

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solvent molecules exposed to the plasma plume generated by theablation process determines the structural differences of the finalproducts. All the nanoparticles obtained by LASiS have magneticproperties, and these results show that laser ablation in liquidscan be a viable technique for obtaining magnetic nanostructuresin organic solutions.

’EXPERIMENTAL SECTION

Laser ablation was performed with 9 ns Nd:YAG (QuantelYG981E) laser pulses at 1064 nm focused with a 10 cm lens on a99.99% iron plate (Aldrich) placed at the bottom of a cellcontaining the solvent. We used pulses of about 5 J cm-2 at a10 Hz repetition rate. We used six HPLC grade solvents (Sigma-Aldrich, >99.8% pure): tetrahydrofuran (THF, inhibitor-free),acetonitrile (AN), dimethylformamide (DMF), dimethylsulfox-ide (DMSO), toluene (TOL), and ethanol (EtOH).

UV-vis spectra were recorded with a Varian Cary 5 spectro-meter in 2 mm optical path quartz cells. Raman measurementswere recorded with a 5� objective of a Renishaw InVia micro-Raman instrument (CCD detector with 100 μm slits), using 1.3mW of the 633 nm line of a He-Ne laser. The samples wereprepared by evaporating some drops of the nanoparticle solutionon a glass substrate. TEM analysis was performed at 300 kV witha JEOL JEM 3010 microscope equipped with a Gatan MultiscanCCD Camera model 794. The samples were prepared byevaporating some drops of the nanoparticle solution on a coppergrid covered with an amorphous carbon holey film.

A Phillips diffractometer was employed to collect the XRDpatterns. Samples for XRD analysis were dried on a quartzsubstrate. The setup is constituted by an X’Pert vertical goni-ometer with Bragg-Brentano geometry, a focusing graphitemonochromator, and a proportional counter with a pulse-heightdiscriminator, and it is connected to a highly stabilized generator.Nickel-filtered Cu KR radiation and a step-by-step technique areemployed (steps of 2θ = 0.05�), with collection times of 30 s/step. Line broadening analysis (LBA) was carried out using apreviously published method.25 The composition analysis byX-ray diffraction was performed using the Rietveld method(DBWS9600 computer program written by Sakthivel and Youngand modified by Riello et al.)26 and according to the followingPDF files: Fe3C Pdf 35-772, Fe Pdf 6-696, Fe3O4 magnetite Pdf16-629, γ-Fe2O3 maghemite Pdf 39-1346.

’RESULTS

LASiS of iron yielded yellow-brownish colloidal suspensionsin all the six solvents considered. The stability of the colloidalsolutions ranged from weeks to months. UV-visible spectra ofthe six solutions are reported in Figure 1. Usually, iron-basednanoparticles can not be easily identified by UV-visible spec-troscopy because they lack sharp absorption bands in thisspectral interval, with the sole exception of nanoscale ironcarbide Fe3C that has a broad absorption band in the300-400 nm interval.27 We observed a well-defined absorptionband in this interval only in the case of particles obtained inEtOH. An absorption peak can be detected also in the spectrumof particles obtained in THF at 275 nm, which is a too shortwavelength to be assigned to Fe3C. Instead, this absorption peakwas reported for solutions of metal iron nanoparticles and wasconfirmed by calculations performed for nanometric iron withthe Mie model.28,29 The experimental observation of the absorp-tion peak of iron nanoparticles is infrequent in the literature

because it is usually covered by strongly absorbing organicstabilizers that are not present in the case of particles obtainedby LASiS. No well-defined absorption peaks can be detected inthe spectra of the AN, DMF, DMSO, and TOL solutions. In allcases, however, an absorption edge can be observed at shorterwavelength that is compatible with the optical spectra of mostiron oxide phases as well as with the presence of amorphous orgraphitic carbon, similar to what was previously reported aboutthe synthesis of noble metal nanoparticles in the same organicsolvents.23,24

We performed transmission electron microscopy (TEM) andX-ray diffraction (XRD) analysis to have an accurate identifica-tion of the composition and the nanostructure of iron-basednanoparticles. In the case of laser ablation of iron in tetrahy-drofuran, nanoparticles with core@shell structure and an averagesize of 20 ( 7 nm were observed by TEM analysis (Figure 2a).The core shell structure was present in 79% of the ca. 300nanoparticles examined. High-resolution TEM (HRTEM) anal-ysis shows that both the core and the shell of the particles arecrystalline; however, the interplanar distance measured on thecores is of 0.20 nm, while the shells have interplanar distances of0.25 or 0.48 nm (Figure 2b). The fraction of non core@shellparticles shows only interplanar distances of 0.25 or 0.48 nm.Within the experimental error, the interplanar distance of 0.20nm observed by HRTEM is compatible with metal iron (cubicFe), while a distance of 0.21 nm is compatible with iron carbide(Fe3C cementite) nanocrystals. Interplanar distances of 0.25 or0.48 nm are characteristic of the spinel structure of magnetite(Fe3O4) or maghemite (γ-Fe2O3). Moreover, particle cores aredarker than shells in the TEM images; therefore, they have ahigher electronic density. From the TEM images, one canobserve also that the oxide shell surrounding the metal corehas a rather uniform thickness independent of particle size: theaverage shell thickness is 4.2 nm( 9%, while the average particlesize is 20 nm( 35%. These observations suggest that metal ironnanoparticles are formed during LASiS in THF and that apassivating oxide layer of about 4 nm appears soon after theprocess due to the presence of oxygen in solution. XRD analysiswas performed on the same samples to confirm the presence ofmagnetite/maghemite and ofmetal iron compositions. The XRDspectrum (Figure 2c) shows reflections that can be assigned onlyto magnetite/maghemite and to metal iron, while the typicalsignatures of iron carbides were not detected. The discriminationbetweenmagnetite andmaghemite is not possible in our spectrumbecause diffraction peaks are large and the difference between the

Figure 1. UV-visible absorption spectra of the solutions obtained byLASiS in tetrahydrofuran (black line), acetonitrile (red line), dimethyl-formamide (green line), dimethylsulfoxide (blue line), toluene (cyanline), and ethanol (magenta line).

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reflections of the two phases is minimal.30 These results agreealso with what was observed by UV-visible spectroscopy on thesame solution, where no spectral signatures of carbides werepresent, while a peak compatible with metal iron nanoparticles at275 nm was detected.

In the case of laser ablation of iron in acetonitrile anddimethylformamide, nanoparticles with a well-defined sphericalshape and an average size of 29( 10 nm for AN and of 30( 15nm for DMF were observed by TEM analysis (Figures 3a,b).High-resolution TEM analysis evidenced that particles are poly-crystalline, and only the interplanar distances of 0.25 or 0.48 nmwere measured, which are compatible with magnetite/maghe-mite compositions (Figure 3c). In the case of DMF, TEM imagesshow the presence of a fraction of amorphous material probablyascribable to organic byproduct due to solvent degradation(Figure 3b). The reflections and their relative intensities mea-sured by the selected area electron diffraction (SAED) on varioussingle nanoparticles from both the AN and DMF samples con-firmed the presence of magnetite or maghemite (Figure 3d). Theresolution of the SAED pattern does not allow the discriminationbetween magnetite and maghemite.30 However, Raman spec-troscopy can be useful for the identification of the iron oxidephase.31 In the Raman spectrum collected on the sample obtainedin DMF (Figure 3e), we found a band at 670 cm-1 compatiblewith magnetite, plus two more intense bands at 1350 and 1600cm-1 that are due to the presence of amorphous carbon,32 aspreviously suggested by TEM images. The magnetic response ofnanoparticles obtained by LASiS in DMF is qualitatively con-firmed by their migration in a few minutes toward a NdFeBmagnet placed in the proximity of the solution (Figure 3f). The

yellow color of the solution after the migration of nanoparticlescan be ascribed to the presence of residual nonmagnetic amor-phous carbon in solution.

In the case of laser ablation of iron in dimethylsulfoxide, weobserved nanoparticles with an average size of 4.7 ( 2.3 nm byTEM analysis, which is a much smaller size than in previous cases(Figure 4a). We found that these particles are embedded in athick amorphous carbon matrix. One can deduce that the matrixis quickly formed during the laser ablation process with the effectof preventing the growth and coalescence of nanoparticles. High-resolution TEM images showed particles with a highly defectiveand polycrystalline structure and with interplanar distance of0.20 nm (Figure 4b), while the reflections and the relativeintensity contained in the SAED pattern match well with thecubic iron composition. The identification of metal iron is com-patible with the presence of a dense matrix surrounding theparticles that is effective in preventing air oxidation of thenanoparticles. The UV-visible spectrum of particles obtainedin DMSO solution reported in Figure 1 is very similar to thespectrum previously reported for silver nanoparticles embeddedin an amorphous carbon matrix obtained by LASiS in DMSO.23

In both cases, no absorption bands of embedded particles aredetectable due to the thick matrix.

In the case of laser ablation of iron in toluene, TEM imagesshow nanoparticles with an average size of 14( 9 nm (Figure 5a).HRTEM analysis evidenced that these nanoparticles have a nano-structure much different from the previous cases (Figure 5b)because a graphitic shell with thickness of 4-6 nm surrounds acore that is completely amorphous. The interplanar distance of0.35 nm in the graphitic shell is typical of bulk graphite along the

Figure 2. (a) Representative TEM image of nanoparticles obtained in THF and the relative size distribution. (b) HRTEM image showing onecore@shell particle with interplanar distance of 0.20 nm in the core and of 0.25 and 0.48 nm in the shell. (c) Powder XRD analysis showing the peaks ofcubic R-Fe (*) and of magnetite/maghemite (þ) crystalline structures.

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c-axis direction. The UV-visible spectrum of the colloid intoluene, reported in Figure 1, is in agreement with the presenceof a large amount of graphitc or amorphous carbon and is very

similar to that previously reported for AuNP obtained by LASiSin toluene.24 The SAED patterns collected on single particles aswell as onmultiple nanoparticles do not show any peak ascribable

Figure 3. Representative TEM images of nanoparticles obtained in AN (a) and DMF (b) and the relative size distributions. HRTEM images showingthe polycrystalline structure with interplanar distances of 0.25 and 0.48 nm in one particle from the AN sample (c). SAEDpatterns of single nanoparticlesobtained in DMF (d) showing the reflections typical of magnetite/maghemite. Raman spectrum recorded on the sample obtained in DMF showing thepresence of magnetite and of amorphous carbon (e). Nanoparticles obtained in DMF readily migrate toward a NdFeB magnet within a few minutes (f).

Figure 4. Representative TEM image of nanoparticles obtained in DMSO (a). HRTEM image showing the polycrystalline structure of these particleswith interplanar distances of 0.20 nm (b). (c) SAED patterns of the nanoparticles showing the reflections typical of cubic iron (R-Fe).

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to ordered crystalline compositions. However, due to the higherelectronic contrast of the core with respect to the graphitic shellobserved in TEM images, the core is clearly composed byelements heavier than carbon, like iron. This is confirmed bythe X-ray fluorescence energy dispersion spectroscopy (EDS)analysis performed on a single nanoparticle by TEM (Figure 5c),which evidenced the presence of the iron peaks. On the basis ofour data, we were unable to determine if the particles arecomposed by a core of amorphous iron or of amorphous ironcarbide. However, the synthesis of similar nanoparticles has beenrecently reported where amorphous-cobalt@graphite nanopar-ticles were obtained by nanosecond laser ablation of a cobalttarget in toluene.17We qualitatively tested the magnetic behaviorof our nanoparticles obtained in toluene by applying a NdFeBmagnet to the solution, and we observed the limited accumula-tion of nanoparticles to the magnet side only 5 days after itspositioning (Figure 5d). This result can be an effect of eitherhigher solubility or of a low magnetization of the particles insolution.

In the case of laser ablation of iron in ethanol, nanoparticleswith a well-defined spherical shape and an average size of 19( 11nm were observed (Figure 6a). High-resolution TEM analysisshows that nanoparticles are prevalently monocrystalline with aninterplanar distance of 0.21 nm (Figure 6b). Such an interplanardistance observed byHRTEM is compatible with bothmetal ironor iron carbide nanocrystals. We also detected the presence ofnanocrystals having size of ca. 1 nm and interplanar distancesof 0.25 nm, evidenced by the circles in Figure 6b, which ischaracteristic of iron oxides like magnetite or maghemite, asrecalled above. We also performed a powder XRD analysis on thesame sample (Figure 6c) that clearly indicates the presence of theFe3C phase. These results are in agreement with the absorptionband at 330 nmobserved byUV-visible spectroscopy (Figure 1)

characteristics of nanometric iron carbides. Two broad bandscentered at 35� and 60� are also present in the XRD pattern.These peaks are compatible with the presence of very smallcrystals of magnetite (Fe3O4) or maghemite (γ-Fe2O3). Anotherless intense peak at 45� can be ascribed to traces of cubic Fe. Theaverage crystal size estimated by the Debye-Scherrer formula isof the order of 1 nm for the oxide phase, in close agreement withwhat is observed by HRTEM.We recorded the Raman spectrumto obtain more indications about the presence of magnetite ormaghemite (Figure 6d), and we found one band at 670 cm-1 thatis in agreement with the presence of magnetite and two broadbands typical of amorphous carbon at 1350 and 1600 cm-1.Unfortunately, iron carbide has no Raman active vibrationalmodes.33 Therefore, LASiS of iron in ethanol yields two differentpopulations of nanoparticles, the first one composed by Fe3Cwith a size of tens of nanometers and the second one composedby magnetite with sizes of the order of 1 nm. The iron carbidecomposition is very interesting because the bulk saturationmagnetization of Fe3C is 140 emu/g, higher than bulk magnetiteor maghemite (<90 emu/g) although lower than metal iron(<220 emu/g).11,12 Contrary to iron oxides and metal iron,however, iron carbide is stable in air up to 200 �C, and it is highlyresistant to acidic dissolution; therefore, it can be easily purifiedfrom the other compounds by acidic treatment. The goodmagnetic response of nanoparticles obtained by LASiS in EtOHis qualitatively confirmed by their migration in a few minutestoward a NdFeB magnet placed in proximity of the solution(Figure 6e).

’DISCUSSION

Laser ablation produces a plasma plume at high temperatureand pressure containing highly ionized and excited species from

Figure 5. Representative TEM image of nanoparticles obtained in TOL and the relative size distribution (a). HRTEM image showing the amorphousstructure of the cores and the surrounding graphitic shell with interplanar distances of 0.35 nm (b). (c) EDS spectrum recorded showing the signals ofiron at 6.6 and 7.1 eV. (d) The limited magnetic migration of nanoparticles toward a NdFeB magnet takes place only some days after its positioning.

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both the target and the solvent. When the solvents can react withthe ablated material, nanoparticles with composition differentfrom the target can form during the nucleation and growthprocesses. For instance, LASiS of gold in CH2Cl2 yielded goldchloride nanoparticles.34 When the solvent does not react withthe ablated material, it still has an important role in thedetermination of the average size of nanoparticles or can forma matrix embedding nanoparticles, as shown for gold and silvernanoparticles obtained by LASiS in various organic solvents.22-24

Both circumstances take place during the LASiS of iron in the sixorganic solvents considered here (Table 1).

The formation of Fe3C nanoparticles in EtOH is an example ofa solvent that is chemically active with the target atoms in theplasma plume. The reactivity of ethanol with iron atoms at hightemperature to form carbides was reported also for electric

plasma discharge in liquids and is a consequence of the chemicalspecies formed during thermal decomposition of ethanol mole-cules.35,36

The formation of a graphitic shell surrounding nanoparticlesobtained by LASiS in toluene was formerly reported for AuNPsand, more recently, for CoNPs and explained with the solventpyrolysis promoted by the high temperature reached during theablation process.17,24 The formation of the amorphous corescontaining iron is compatible with the fast cooling of the plasmaplume in the liquid.14 However, the reaction of solvent moleculeswith iron atoms in the high-temperature and -pressure conditionsgenerated by the laser ablation cannot be excluded as anotherpossible cause of the formation of an amorphous core composedby iron carbide. In the past, these extreme conditions allowed theformation of various phases not thermodynamically stable instandard conditions like a silver-nickel alloy, diamond nano-particles, or cubic Ge nanoparticles.13,14

The formation of an amorphous organic matrix around ironnanoparticles in DMSO recalls what was already observed forAgNP in the same solvent.23 In this case, the matrix is crucial forpreventing iron oxidation and for preventing further growth orcoalescence of the particles. A different case is that of AN andDMF, where the solvents do not seem to influence the finalproduct by the formation, for example, of an organic matrix. Infact, although the DMF and AN molecules are rather different,we obtained iron oxide nanoparticles with similar average size,size distribution, and composition. It is likely that iron atoms

Figure 6. (a) Representative TEM image of nanoparticles obtained in EtOH and the size distribution for the Fe3C particles. (b) HRTEM imageshowing one large particle with interplanar distance of 0.21 nm and two very small particles with interplanar distance of 0.25 nm. (c) Powder XRDanalysis showing the reflections corresponding to the Fe3C (x), magnetite (þ), and cubic Fe (*) compositions. (d) Raman spectrum recorded on thesame sample. (e) Magnetic migration of nanoparticles toward a NdFeB magnet occurs within a few minutes.

Table 1. Summary of the Type of Magnetic NanoparticlesObtained in the Different Solvents by LASiS

bare core@shell

THF metal@oxide

AN oxide

DMF oxide

DMSO metal

TOL amorphous@graphite

EtOH carbide, oxide

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were free to interact with atmospheric oxygen dissolved in thesolvents to yield iron oxides. The importance of oxygen dissolvedin the organic solvents was evidenced by some studies about theLASiS of metals in hexane, cyclopentanone, or acetone.15,16,20,37

In these cases, the formation of partially nonoxidized nanostruc-tures was possible by deaerating the solvents. As observed forTHF, AN, and DMF, also these studies have found that thechemical interaction between the solvent molecules and themetals does not play a relevant role for the final compositionof the nanoparticles. Moreover, the presence of iron@iron oxidenanoparticles in THF indicates that the 4 nm oxide layer is able toprevent the oxidation of the core also when nanoparticles areexposed to atmospheric oxygen at room temperature. From thisfinding, it follows that the formation of 100% iron oxidenanoparticles in AN and DMF takes place during the nucleationand growth process and not after it. These differences can bedue to the different chemical and physical properties of AN andDMF with respect to THF. On one hand, it is possible that theconcentration of atmospheric oxygen dissolved in THF is lowerthan in AN and DMF. On the other hand, it is possible that THFmolecules have chemical reactivity with oxygen molecules insolution different from AN and DMF. In particular, at hightemperature and low oxygen concentration, molecules withsaturated chemical bonds like THF are prone to react withoxygen, thus competing with the oxidation of Fe atoms, whilemolecules with unsaturated chemical bonds like AN and DMFare more prone to polymerize and are less competitive with theoxidation of Fe atoms.

’CONCLUSIONS

In this work, we showed that LASiS is a suitable method forobtaining a variety of magnetic nanostructures in organic liquids.By varying the solvents, nanoparticles with different composi-tions and structures were observed. We obtained iron@iron-oxide or iron oxide nanoparticles using THF or AN and DMF,respectively. Metal iron nanoparticles embedded in a denseamorphous organic matrix were obtained in DMSO, while parti-cles composed by an iron-containing amorphous core surroundedby a graphitic shell are obtained in TOL. Finally, large Fe3C andsmaller iron oxide nanoparticles can be obtained by LASiS inEtOH. All nanoparticles are composed by magnetic phases, andtherefore they can find applications in various fields, from nano-medicine to the fabrication of magnetic devices. Our experimentssuggest that the reactivity of the solvent molecules with all thespecies present in the plasma plume, during the ablation process,plays a relevant role in determining the final product. Therefore,we conclude that the solvent is an important parameter forobtaining new functional nanomaterials by LASiS.

’AUTHOR INFORMATION

Corresponding Author* E-mail: vincenzo.amendola@unipd.it.

’ACKNOWLEDGMENT

V.A. andM.M. would like to thank theUniversity of Padua andProf. S. Polizzi of the University of Venice for help with TEM andHRTEM measurements.

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