Journal of Colloid and Interface Science 386 (2012) 28–33

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Journal of Colloid and Interface Science

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Anomalous dependence of particle size on supersaturation in the preparationof iron nanoparticles from iron pentacarbonyl

Maija Huuppola a, Zhen Zhu b,1, Leena-Sisko Johansson c, Kyösti Kontturi a, Kari Laasonen a,Christoffer Johans a,⇑a Department of Chemistry, Aalto University, FI-00076 Aalto, Finlandb Department of Applied Physics, Aalto University, FI-00076 Aalto, Finlandc Department of Forest Products Technology, Aalto University, FI-00076 Aalto, Finland

a r t i c l e i n f o

Article history:Received 2 March 2012Accepted 16 June 2012Available online 31 July 2012

Keywords:NanoparticleIronSynthesisCarbon monoxide pressureSupersaturationAggregation

0021-9797/$ - see front matter � 2012 Elsevier Inc. Ahttp://dx.doi.org/10.1016/j.jcis.2012.06.041

⇑ Corresponding author. Fax: +358 9 47022580.E-mail address: cjohans@cc.hut.fi (C. Johans).

1 Present address: Beneq Oy, P.O. Box 262, FI-01511

a b s t r a c t

Iron nanoparticles were prepared by decomposing iron pentacarbonyl (Fe(CO)5) at 170–220 �C in thepresence of amine surfactant and alkane solvent and under 1–12 bar carbon monoxide (CO) pressure.It was found that the amine not only acted as a stabilizer for the growing particles but also had a criticalrole as a promotor in the decomposition reaction. Relatively small changes in the CO pressure had anom-alous effects on the particle size distribution. Typically, monodisperse particles were obtained at 1 bar,while pressures in the 2–6 bar range led to wider and even bimodal size distributions due to an emer-gence of smaller particles. At still higher pressures, the larger particle size disappeared leaving the distri-bution monodisperse again. The CO pressure, at which the bimodal transition took place, increased withthe reaction temperature. Polycrystalline particles were formed at lower pressures and monocrystallineparticles at higher pressures. This indicates that increased CO pressure inhibits aggregation.

� 2012 Elsevier Inc. All rights reserved.

1. Introduction such as amines [4,10,11], cobalt chloride [16], and iron particles

While synthesis of cobalt nanoparticles from dicobalt octacar-bonyl has been very successful, preparation of metal nanoparticlesfrom many other transition metal carbonyls is more difficult,mainly due to their low boiling points and the high temperaturerequired for decomposition of mononuclear metal carbonyls. Forexample, while the boiling point of Fe(CO)5 is 103 �C, temperaturesof approximately 200 �C are needed for rapid decomposition. De-spite their relative ease of occurrence, decomposition reactions ofmetal carbonyls are rather complicated and very sensitive to reac-tion conditions [1–3]. The amount of CO in the synthesis solutionaffects both decomposition rate [4–6] and particle size [5,6]. Metalcarbides [7] and metal oxides [8] may form from the decomposi-tion of CO on the surface of iron. Moreover, metal carbonylcompounds can efficiently catalyze carbonylation of both doublebonds [9] and amines [9–12], thus complicating their use assurfactants. Nonetheless, Fe(CO)5 in surfactant–solvent systemshas been widely and successfully used to prepare iron nanoparti-cles [13–15].

It is generally considered that removal of the first carbonylligand is the rate determining step of Fe(CO)5 decomposition[1,2]. It can be catalyzed or promoted by a number of compounds,

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[4], as well as by thermal or photolytic means [2]. Intermediateiron carbonyls play a significant role in Fe(CO)5 decomposition,e.g. Fe(CO)4 may react with Fe(CO)5 to form Fe2(CO)9, which in turnmay react with another Fe(CO)4 or Fe(CO)5 to form CO andFe3(CO)12 [3].

Nitrogen nucleophiles promote decomposition of Fe(CO)5

[4,10,11]. Edgell et al. [10,11] have studied the reaction betweenamines and Fe(CO)5. For the case of secondary amines, such aspiperidine and pyrrolidine, nitrogen attacks, either directly or indi-rectly via the iron atom, one of the carbon atoms of Fe(CO)5, form-ing an intermediate iron carbonyl compound with one of the axialcarbonyl groups replaced by an amide group. The formation of theintermediate is reversible and promoted by polar solvents. Theproton on the incoming amine may either remain on the aminegroup or bind to another amine (R2NHþ2 ), the latter configurationbeing more probable. Subsequently, the intermediate loses onecarbonyl group to form iron tetracarbonyl with the amine nitrogenbound to the fifth coordination site. However, in the presence ofwater, the intermediate may alternatively decompose to aminehydrogen tetracarbonyl ferrate and amine carbamate.

While amines tend to substitute one of the carbonyl groups ofFe(CO)5, acids oxidize the iron [17–19]. Adding Fe(CO)5 to a solu-tion that contains excess of oleic acid at 100 �C leads to an iron ole-ate complex [17], which can be further decomposed to Fe, c-Fe2O3

[17] or Fe3O4 [20,21].

M. Huuppola et al. / Journal of Colloid and Interface Science 386 (2012) 28–33 29

Iron particles can promote Fe(CO)5 decomposition. It has beensuggested that Fe(CO)5 decomposes on the surfaces of the nucle-ated particles in the growth stage, resulting in zero order depen-dence of the evolution of CO on the concentration of Fe(CO)5 [4].However, the decomposition is inhibited by the adsorption of COon the particle surface [4,22].

Iron carbide nanoparticles have been prepared by using Fe(CO)5

[23,24] and iron oxide nanopowder [25] as iron sources. Typicalcarbon sources are CO and hydrocarbons [24,25], but also othercompounds, such as methyl methacrylate [23], have been used.Synthesis methods include a gas-phase laser pyrolysis approach[23] and a solid-state approach, where Fe2O3 nanopowder washeat-treated at 400 �C in CO atmosphere [25]. A typical chemicalsynthesis route, where Fe(CO)5 was decomposed in diphenyl etherat 200–257 �C under either argon, methane or acetylene atmo-sphere [24], has also been used. In addition, small amounts of car-bon (11 at.%) have been found in amorphous iron nanoparticlesprepared at less than 200 �C [7]. It is suggested that carbon atomsenter the particles by chemisorption and disintegration of CO onthe surface of the particles, similar to the carburization of iron cat-alysts in Fischer–Tropsch synthesis [7].

In this work, we have studied the decomposition of Fe(CO)5 toiron nanoparticles in alkane–alkyl amine solutions. In the experi-ments, Fe(CO)5 was first heated under high CO pressure to a170–220 �C temperature range. The decomposition reaction wasthen initiated by a rapid pressure drop to a desired decompositionpressure, which induced a supersaturated state in the solution andfurther nucleation and growth of particles [5,6,26]. Rather smallchanges in the reaction temperature, decomposition pressure, oramine to iron ratio caused marked changes in the particle size dis-tribution. In principle, a small decomposition pressure should re-sult in a high supersaturation yielding small particle size, andvice versa. Here, we observed an anomalous dependence of theparticle size on the decomposition pressure, in which an increasein pressure resulted in a bimodal transition phase in the size distri-bution with the appearance of drastically smaller particles. The ef-fects of reaction temperature, CO pressure, and reactantconcentrations were studied.

2. Experimental part

The chemicals used were iron pentacarbonyl (Fe(CO)5, Aldrich),dodecyl amine (DA, Aldrich, 98%), oleyl amine (OA, Fluka, 70%),tridodecyl amine (TDA, Fluka, 85%), dodecane (Sigma–Aldrich,99%), tetradecane (Aldrich, 99.0%), liquid paraffin (Yliopiston Apt-eekki), ethanol (Altia, ETAX Aa, 99.5%), toluene (Merck, 99.9%),polystyrene (Dow Plastics, STYRON 678E), carbon monoxide (CO,AGA, 99%), and nitrogen (AGA, 99.999%). All chemicals were usedas received.

Iron nanoparticles were prepared in an autoclave (Parr 4560,100 ml) following a previously published method [5,6,26], seeFig. S1 (Supplementary material) for details. In the synthesis proce-dure, Fe(CO)5 (in most cases 1 ml) was added to a mixture of aminesurfactant and solvent (in most cases 30 ml of dodecane for tem-peratures up to 200 �C and tetradecane for temperatures higherthan 200 �C) that had been purged with CO. The molar ratio ofamine to iron was one, unless otherwise stated. The CO pressurewas increased to 50 bar in order to prevent Fe(CO)5 from decom-posing during heating, and the system was heated up at a constantrate of 3 �C/min. When the desired temperature was reached, theCO pressure was rapidly dropped to the desired decompositionpressure by opening a gas release valve for 10 s. The desireddecomposition pressure was set by adjusting the pressure of anexternal vessel connected to the gas outlet of the reactor. Thepressure drop induced the decomposition of the Fe(CO)5 and thus

initiated the nucleation and growth of nanoparticles. The evolutionof gas from the decomposition reaction was monitored with a pres-sure sensor. The conversion of Fe(CO)5 was calculated from thepressure rise in the reactor after the pressure drop, under theassumption that the ideal gas law applies and that the pressuredependence of the CO solubility in the solvent is negligible. Thereaction considered here was Fe(CO)5 ? Fe0 + 5 CO (g).

The morphology of the particles was examined with a Tecnai 12transmission electron microscope (TEM) operating at a 120 kVacceleration voltage and a few samples were further characterizedwith a JEOL 2200FS double aberration corrected TEM, denoted hereas high resolution TEM (HRTEM), operating at an 80 kV accelera-tion voltage. Typically, a drop of a freshly prepared nanoparticlesolution was placed on a formvar/carbon or carbon coated coppergrid (Electron Microscopy Sciences) and the solvent was evapo-rated in air. To get rid of possible impurities in the case of theHRTEM imaging, particles were first precipitated and washed withethanol in the presence of a strong magnet and then transferredwith either ethanol or toluene on the grid. Size analysis was per-formed with Gatan Digital Micrograph 3.8.2 software by measuringthe diameter of at least 1000 particles for each sample.

X-ray diffraction (XRD) measurements were carried out usinga Philips MPD 1880 instrument with 0.154 nm Cu Ka radiation.Since the measurements were carried out under ambient condi-tions, the particles were embedded in polystyrene to minimizeoxidation. Polystyrene (2 g) was added to a freshly preparednanoparticle solution in the autoclave while purged with nitro-gen. The reactor was closed, and the solvent was evaporated un-der vacuum for 1.25 h at 220 �C. After cooling, the polymer platewas removed from the reactor and XRD measurements were car-ried out. The maximum exposure to air prior to the measure-ments was 1 h, and the XRD experiment lasted for 12 h. TheXRD spectrum of a polystyrene plate prepared by the aboveprocedure in the absence of Fe(CO)5 was used as a blank and sub-tracted from the spectra.

Surface compositions of the iron particles were analyzed withan AXIS 165 X-ray photoelectron spectrometer (XPS, Kratos Analyt-ical). The measurements were performed on the polystyreneembedded particles described above, using monochromated AlKa irradiation at 100 W, a mg lens configuration, and a charge neu-tralizer. Wide scans and trace measurements of the N 1s regionwere carried out using 80 eV pass energy and 1 eV step, whilethe C 1s, O 1s, and Fe 2p regions were recorded in high resolutionmode (20 eV pass energy and 0.1 eV step). The analysis area wasless than 1 mm2 and the analysis depth was less than 5 nm. Severaldata points were recorded for each sample. Binding energies of theregional spectra were corrected using the C 1s signal at 285 eV. Thepolymer plates were stored under ambient conditions before theexperiments and the air-exposed surface was measured, sincescraping led to loss of iron.

Infrared spectroscopy was carried out for the reagent mixtureprior to heating and after heating under high CO pressure, as wellas, for samples taken during syntheses. Here we have assumed thatthe composition of the sample is representative of the synthesisconditions, despite reactions that may occur during rapid coolingand handling. The samples were diluted with CO purged solventand analyzed within a few minutes with a Fourier transform infra-red (FTIR) spectrometer (Perkin–Elmer Spectrum One).

3. Results and discussion

Iron nanoparticles were prepared by a pressure drop method,where Fe(CO)5, amine surfactant, and alkane solvent were heatedunder high CO pressure and then, at the desired temperature, thepressure was rapidly dropped to induce decomposition of the ironcarbonyl. When heated, Fe(CO)5 ideally decomposes according to

Fig. 1. Temperature sweeps at a rate of 1 �C/min for Fe(CO)5 in DA–dodecane(solid), OA–tetradecane (dash), and TDA–tetradecane (dash-dot-dot) mixtures with1 bar initial CO pressure.

30 M. Huuppola et al. / Journal of Colloid and Interface Science 386 (2012) 28–33

the overall reaction Fe(CO)5 ? Fe0 + 5 CO (g), where the CO gas iseasily removed from the solution leaving only pure iron behind.The decomposition can be more complicated under less ideal con-ditions, resulting in iron carbides [7] or iron oxides [8]. Here wehave used the CO pressure to prevent decomposition of Fe(CO)5

by controlling the balance between the reactant and products inthe above reaction. The effect of CO pressure on the above reactionwas first verified by heating Fe(CO)5 in liquid paraffin from roomtemperature to 270 �C with initial CO pressures of 1 and 32 bar.Only the experiment performed at 1 bar showed gas evolution.

During heating, the high CO pressure prevents the decomposi-tion of Fe(CO)5, which was confirmed by following the pressurein the reactor. FTIR measurements show that the reactant mixtureremains virtually unchanged during heating, except for new bandsat 3453, 1711, 1671–1672, 1565, and 1497 cm�1 (Fig. S2 and ab ini-tio calculations in Supplementary material), which may indicateformation of small amounts of amide, as has been observed previ-ously for the reaction between Fe(CO)5 and various amines [10,11].

At the desired temperature, the pressure was rapidly dropped toa desired level, referred to as decomposition pressure, by openingthe gas release valve for 10 s. The pressure drop initiated thedecomposition reaction and consequently the nucleation andgrowth processes. In a typical experiment, the pressure in the reac-tor increases gradually after the gas release valve has been closedand reaches a constant level when no more gas is evolved, indicat-ing that the reaction is complete. The pressure data serves as a con-venient means to compare the reaction rates between differentconditions. Due to the volatile nature of Fe(CO)5, the absolute con-version varies with decomposition pressure and temperature, sincesome of the Fe(CO)5 escapes during the pressure drop. WhileFe2(CO)9 and Fe3(CO)12 would avoid the problem of volatility, wehave not found suitable experimental conditions to obtain goodparticles. FTIR spectroscopy was carried out for samples taken dur-ing two syntheses performed at 220 �C with DA as the surfactantand with 1 and 9 bar decomposition pressures. The stretchingbands of the C–O bonds in Fe(CO)5 can be found at 2022 and1999 cm�1 (Fig. S2, Supplementary material). The decrease in thesebands as Fe(CO)5 is consumed is in fair agreement with that ob-tained from the change in pressure. The roles of amine, tempera-ture, and pressure are discussed in detail in the followingparagraphs.

The primary role of the amine was to stabilize the particles byadsorbing on their surfaces and to prevent aggregation. However,it turned out that the role of the amine is much more complicatedand that it is critical for the decomposition of Fe(CO)5. The decom-position rate was several orders of magnitude faster when primaryamines (DA or OA) were used instead of a tertiary amine (TDA) un-der otherwise similar conditions. The reaction rate, determinedfrom the pressure increase in the reactor during a typical synthesis,was nearly independent of the amount of amine for DA/Fe ratiosfrom 0.08 to 3.00, but several orders of magnitude faster than inthe absence of amine. This was also observed in experiments,where Fe(CO)5 was slowly (1 �C/min) heated from room tempera-ture to 270 �C with an initial CO pressure of 1 bar. The conversionscalculated from the pressure data, assuming that the ideal gas lawapplies, are shown in Fig. 1 for Fe(CO)5 in DA–dodecane, OA–tetradecane, and TDA–tetradecane mixtures. The presence of pri-mary DA or OA significantly shifted the decomposition reactionto lower temperatures by 30–60 �C compared to the tertiary TDA.The effect of surfactants on the decomposition reaction of metalcarbonyls has also been observed in a hot-injection synthesis of co-balt nanoparticles from dicobalt octacarbonyl in the presence oftrioctylphosphine oxide and oleic acid [27].

An interesting finding is that while the decomposition rate isindependent of the amine concentration, the particle size dependsstrongly on it. The impact of the DA to iron ratio on the particle size

and decomposition rate at 180 �C with 1 bar decomposition pres-sure is shown in Fig. 2. The average particle size for a DA/Fe ratioof 0.08 was 4.0 nm (SD 49%). The particle ensemble consists mainlyof small particles of a few nanometers. There are also some largerparticles of irregular shape, which indicates that the particles havebeen formed by aggregation of smaller particles. The oxide layerformed from contact with air can be clearly seen surrounding themetallic core. For a DA/Fe ratio of 0.17, the average particle sizewas 9.4 nm (SD 34%). Mainly larger particles are formed, but a sig-nificant fraction of smaller particles also exists, which is reflectedin the broad standard deviation. The fraction of small particles isstill smaller at a DA/Fe ratio of 1.00, but increases dramatically ata DA/Fe ratio of 3.00, the average sizes being 12.1 nm (SD 5%) forthe 1.00 ratio and 2.7 nm (SD 40%) for the 3.00 ratio. The fact thatthe growth rate is nearly independent of the amine concentrationwhile the particle size depends strongly on it directly implies thatthe reaction rate and the total surface area in the system are inde-pendent, which suggests that the rate limiting step of particlegrowth takes place in the bulk solution rather than at the particlesurface.

The temperature affects both reaction rate and particle mor-phology. Faster decomposition at higher temperatures was accom-panied by a decrease in particle size as shown in Fig. 3 for the 180–220 �C range. The average diameter of the particles was 3.3 nm (SD27%) at 170 �C, 11.9 nm (SD 11%) at 180 �C, 8.2 nm (SD 20%) at190 �C, 7.4 nm (SD 5%) at 200 �C, and 6.4 nm (SD 5%) at 220 �C.At 180 and 190 �C, the average sizes were decreased and the stan-dard deviations were increased due to small particles among thelarger ones. The surfactant used in these experiments was DAand the decomposition pressure was 1 bar. The particle size reduc-tion following the temperature increase arises from relatively fas-ter nucleation at higher temperatures, which results in a largenumber of nuclei and thus small particles [28]. The synthesis car-ried out at 170 �C deviates from this trend.

The effect of the decomposition pressure on the particle sizewas studied at 220 �C with OA as the surfactant, see Fig. 4. Theaverage diameter of the particles increased from 6.3 nm (SD 9%)to 11.1 nm (SD 17%) as the decomposition pressure was increasedfrom 1.3 bar to 2.7 bar. At 4.2 bar, the particle size was 11.7 nm (SD41%), with a clearly bimodal distribution composed of small parti-cles of ca. 5 nm and large particles of ca. 15 nm. The shape of thelarge particles is irregular, suggesting an aggregation growthmechanism. At 6.1 bar, the particle size had decreased to 6.5 nm(SD 34%). The average diameter was 5.3 nm (SD 10%) at 9.0 bardecomposition pressure and 6.3 nm (SD 10%) at 11.8 bar decompo-sition pressure. Summarizing these results as a function of increas-ing decomposition pressure, the particle size and polydispersity

Fig. 2. TEM micrographs of iron particles synthesized at 180 �C with 1 bar decomposition pressure and with DA/Fe ratios of: (a) 0.08, (b) 0.17, (c) 1.00, and (d) 3.00 as well as(e) relative conversions for the syntheses and f) corresponding size distributions. The 0.08, 0.17, 1.00, and 3.00 ratios are denoted with solid, dash, dash-dot, and dash-dot-dotlines.

Fig. 3. TEM micrographs of iron particles synthesized at: (a) 170 �C, (b) 180 �C, (c) 190 �C, (d) 200 �C, and (e) 220 �C and (f) corresponding size distributions. The surfactantused was DA and the decomposition pressure was 1 bar.

M. Huuppola et al. / Journal of Colloid and Interface Science 386 (2012) 28–33 31

initially increases, then much smaller particles appear, after whichthe larger particles disappear, leaving smaller particles with a nar-row size distribution. The size of the smaller particles increaseswith the decomposition pressure. Similar observations were madefor syntheses carried out at 180 and 220 �C using DA as the surfac-tant, see Figs. S3 and S4 (Supplementary material) for details. Thegeneral trend was that higher pressures shifted the bimodal tran-sition to higher temperatures.

The relative conversions corresponding to the OA experimentsdescribed above are shown in Fig. 5a. The experiments performedat low decomposition pressures show a simple shape with an

initial rapid increase in conversion which gradually reaches a con-stant value. This is similar to that observed for growth of cobaltparticles [5,6,26] and can be quantitatively described by using sim-ple growth models assuming instantaneous nucleation and growththrough the addition of monomeric units to the particles. However,as the decomposition pressure increases, the conversion splits intotwo distinct phases separated by a shoulder, which can be readilyobserved at 6.1, 9.0, and 11.8 bar decomposition pressures. Thiscoincides with the transition from large to small particles dis-cussed earlier. Careful analysis of the temperature in the reactorshows that the pressure drop causes a significant increase in

Fig. 4. TEM micrographs of iron particles synthesized at 220 �C with OA and with: (a) 1.3 bar, (b) 2.7 bar, (c) 4.2 bar, (d) 6.1 bar, (e) 9.0 bar, and (f) 11.8 bar decompositionpressures and g) corresponding size distributions.

Fig. 5. Relative conversions calculated from the pressure data of iron particlessynthesized at 220 �C with OA and (a) with 1.3 bar (solid), 2.7 bar (long dash),4.2 bar (short dash), 6.1 bar (dash-dot), 9.0 bar (dash-dot-dot), and 11.8 bar (dot)decomposition pressures or (b) with 8.9 bar decomposition pressure (solid) andconcurrent temperature profile (dash).

32 M. Huuppola et al. / Journal of Colloid and Interface Science 386 (2012) 28–33

temperature, followed by a decrease, which coincides with theshoulders observed in the conversions (Fig. 5b). First of all, an ini-tial increase in temperature is unexpected and can only result from

an exothermic reaction. The timescale is much shorter than theheating hardware can produce. The following drop in temperatureis caused by the response of the PID controller to the initial in-crease. The two distinct steps in the conversion curves could indi-cate two nucleation stages, the first one occurring in the perioddirectly after the pressure release. The drop in temperature slowsdown nucleation, while the following increase in temperaturestarts a second nucleation wave. We discard this hypothesis oftwo distinct nucleation stages, since it cannot explain why we ob-tain monodisperse particles at 9.0 and 11.8 bar decompositionpressures and why the particle size decreases with increasing pres-sure, i.e. lowered supersaturation.

The XRD patterns of polystyrene embedded particles from twosyntheses performed at 220 �C with DA as the surfactant and with1 and 9 bar decomposition pressures are shown in Fig. S5 (Supple-mentary material). These samples were chosen for analysis sincetheir particle size is nearly equal, see Fig. S4 (Supplementary mate-rial). While the peaks are broad indicating that the crystal size israther small, both metallic iron and Fe3O4 can be identified inthe samples. The oxide was likely formed when the polystyreneembedded particles were handled under ambient conditions. Ifthe particles were precipitated instead of embedded in the polysty-rene matrix, the oxide peaks grew strongly at the expense ofmetallic iron. Interestingly, the peak half width of the 1 bar sampleis slightly broader than in the 9 bar sample. While the quality ofthe data is weak, we speculate that this is due to smaller crystallitesize in the 1 bar sample. This finding is supported by HRTEMimages (Fig. S6, Supplementary material), where the particles orig-inating from the 1 bar synthesis are polycrystalline and the parti-cles originating from the 9 bar synthesis are mainlymonocrystalline. Note that the particles have been handled in airand are oxidized and their crystal structure may not fully corre-spond to that of non-oxidized particles.

The XPS spectra of polystyrene embedded particles from twosyntheses performed at 220 �C with DA as the surfactant and with1 and 9 bar decomposition pressures are shown in Figs. S7 and S8(Supplementary material). Neither iron carbide nor amide could beobserved in the samples.

M. Huuppola et al. / Journal of Colloid and Interface Science 386 (2012) 28–33 33

Despite the unexpected behavior of the particle size distribu-tions and conversion curves, the XPS and XRD studies showed thatthe particles synthesized with different decomposition pressuresare chemically similar and the FTIR studies suggest that the syn-thesis solutions are similar. The most striking difference is the for-mation of polycrystalline particles at low decomposition pressuresand monocrystalline particles at higher decomposition pressures.This may indicate that aggregation occurs at low CO pressures,while at higher CO pressures growth takes primarily place throughaddition of monomers to the particle. This mechanism is in agree-ment with the finding that temperature and CO pressure haveopposite effects on the transition from large to small particles.

4. Conclusions

Iron nanoparticles were prepared by a pressure drop method,where Fe(CO)5 was first heated together with amine surfactantand alkane solvent under high CO pressure, that prevented itsdecomposition during heating, and then decomposed by rapidlydropping the pressure. Depending on the reaction conditions, theparticle size varied between 3 and 12 nm. While nearly monodis-perse particles were obtained in certain conditions, e.g. particleswith an average diameter of 7.4 nm (SD 5%) were obtained at200 �C with 1 bar decomposition pressure, small changes in thereaction conditions, especially changes in the decomposition pres-sure, had remarkable effects on the particle size distribution. Whenthe decomposition pressure was increased, the size distributionfirst broadened and moved towards larger sizes, then became bi-modal due to smaller particles emerging among the larger ones,and finally narrowed again due to the disappearance of the largerparticles. Higher pressures shifted the bimodal transition to highertemperatures. The chemical composition of the particles was thesame throughout the pressure range, however, polycrystalline par-ticles were obtained at low decomposition pressure and monocrys-talline particles at high decomposition pressure. These findingswere explained by inhibition of aggregation by adsorbed CO athigher pressures.

Acknowledgments

We thank Tekes (the Finnish Funding Agency for Technologyand Innovation), OMG, Outotec, Magnasense, and Academy ofFinland for financial support.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.jcis.2012.06.041.

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