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Formation Mechanism via a Heterocoagulation Approach of FePt Nanoparticles Using the Modified Polyol Process Watson Beck, Jr.,† Caio G. S. Souza,† Tiago L. Silva,† Miguel Jafelicci, Jr.,‡ and Laudemir C. Varanda*,† †
Instituto de Química de S~ao Carlos, Universidade de S~ao Paulo—USP, Colloidal Materials Group, CP 780, 13566-590, S~ao Carlos—SP, Brazil ‡ Instituto de Química de Araraquara, Depto de Físico-Química, Universidade Estadual Paulista—UNESP, CP 355, 14801-970, Araraquara—SP, Brazil
bS Supporting Information ABSTRACT: Herein, we report a new approach of an FePt nanoparticle formation mechanism studying the evolution of particle size and composition during the synthesis using the modified polyol process. One of the factors limiting their application in ultra-highdensity magnetic storage media is the particle-to-particle composition, which affects the A1-to-L10 transformation as well as their magnetic properties. There are many controversies in the literature concerning the mechanism of the FePt formation, which seems to be the key to understanding the compositional chemical distribution. Our results convincingly show that, initially, Pt nuclei are formed due to reduction of Pt(acac)2 by the diol, followed by heterocoagulation of Fe cluster species formed from Fe(acac)3 thermal decomposition onto the Pt nuclei. Complete reduction of heterocoagulated iron species seems to involve a CO-spillover process, in which the Pt nuclei surface acts as a heterogeneous catalyst, leading to the improvement of the single-particle composition control and allowing a much narrower compositional distribution. Our results show significant decreases in the particle-to-particle composition range, improving the A1-to-L10 phase transformation and, consequently, the magnetic properties when compared with other reported methods.
1. INTRODUCTION Magnetic nanoparticles have recently drawn much attention because of their potential applications in ultra-high-density magnetic recording devices; biomedical applications in diagnostics, therapy, and sensing; and biotechnological uses in bioseparation.1 New materials and the enhancement of their magnetic properties for applications in magnetic recording media have been driving the magnetic materials development in the last decades, mainly after the metallic and metal evaporated nanoparticle uses.2 Moreover, the synthesis of magnetic colloids has been studied for more than one century, but yet the number of preparations yielding a size series of monodisperse particles, mainly metal nanocrystals, is surprisingly small.1d,3 In this direction, one of the very important studies was reported by Sun4 and workers at the IBM group of the self-assembly and magnetic recording performance of FePt nanoparticles synthesized in an organic liquid phase using iron pentacarbonyl [Fe(CO)5] and platinum(II) acetylacetonate [Pt(acac)2] as precursors.5 After that, considerable work has been done to develop different methods of synthesizing FePt nanoparticles. In general, reported work in the literature aims at substituting Fe(CO)5 with both less toxic and volatile chemicals in order to achieve good control of the nanoparticles' composition and r 2011 American Chemical Society
size.1d,6,7 To realize the potential applications of FePt nanoparticles, uniformity in size and composition are essential, with the latter being more critical. In this context, there still remain key questions on particle-to-particle compositional variation. The established wet chemical methods used to produce FePt nanoparticles involve nucleation and growth in the reaction medium generally described by the Lamer’s model.8 One of the factors that may affect the formation mechanism is the choice of metallic precursors for Fe and Pt, for example. There are few reports in the literature that specifically correlate the average composition of nanoparticle dispersion to the composition variability on a particle-to-particle basis, mainly about the Fe(CO)5 uses.6 Yu and co-workers showed that the method proposed by Sun and others studies using metal carbonyls involving the reduction of Pt(acac)2 and the thermal decomposition of Fe(CO)5 leads to particle-to-particle composition variation from 21 to 70 at. % Fe.9 The chemical composition control and the structural homogeneity of the nanoparticles are difficult due to the high volatility of the Fe(CO)5, which remains in the vapor mixture during the Received: February 24, 2011 Revised: April 20, 2011 Published: May 06, 2011 10475
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The Journal of Physical Chemistry C synthesis time. To understand the origin of both size and compositional distribution, an insight into the formation mechanism of the FePt nanoparticles is essential. More specifically, analysis of the formation mechanism by the choice of metallic precursors is required to determine (i) if there is a specific wet chemistry method that minimizes individual particle compositional variation and (ii) the sequence of the nucleation and growth of the nanoparticle and how that contributes to the compositional variation.6 It is widely believed that, during the synthesis of FePt nanoparticles, Pt nuclei form initially, followed by growth of Fe on the Pt nuclei.4,6,10 However, to our knowledge, there are no reports that conclusively prove this belief. The use of the iron(III) acetylacetonate [Fe(acac)3] instead of the Fe(CO)5 as an iron source was recently reported in the literature by our group and is referred to as a modified polyol process, improving the individual nanoparticle compositional control.1d In the present work, we provide experimental data that show the size and compositional evolution of these nanoparticles during the modified polyol process. Results convincingly show that, initially, Pt nuclei are formed, followed by heterocoagulation of Fe cluster species formed by Fe(acac)3 decomposition onto the Pt nuclei, resulting from reduction of Pt(acac)2 by the diol. Complete reduction of heterocoagulated iron species seems to involve a CO-spillover process, in which the Pt nuclei surface acts as a heterogeneous catalyst, leading to improved particle-toparticle compositional variation control.
2. EXPERIMENTAL SECTION Synthesis of FePt Nanoparticles. The synthesis of Fe55Pt45 was carried out using commercially available reagents purchased from Sigma-Aldrich Co. used as received without further purification according to the modified polyol process reported early.1d,2d In a three-neck round-bottom flask under a nitrogen atmosphere, Pt(acac)2 (0.2 mmol), Fe(acac)3 (0.24 mmol), and 1,2-hexadecanediol (0.6 mmol) in 20 mL of dioctylether were mixed and heated to 120 °C for 20 min, resulting in the complete dissolution of the salts. Oleic acid (0.5 mmol) and oleylamine (0.5 mmol) were then added via a syringe in the hot solution. The resulting mixture was heated to reflux (∼298 °C) and allowed to reflux for 30 min, giving a black dispersion. The heat source was removed and the dispersion cooled to room temperature. Ethanol was then added to separate the nanoparticles, and the particles were isolated by centrifuging. The particles were redispersed in hexane, precipitated with ethanol, and isolated by centrifuging several times for purification. For the formation mechanism study, eight additional samples following the same procedure using a final temperature at 130, 140, 160, 170, 180, 190, 230, and 260 °C were carried out, in which reaction mixtures remained for 5 min in these temperatures. In all cases, after this time, the reaction was quenched by using an ice bath, allowed to cool to room temperature, and purified. Additionally, at temperatures of 130, 190, 230, and 298 °C, the vapor phase present in the reaction flask was collected using an appropriated preevacuated ampule, quenched in an ice bath, and cooled to room temperature. Both the gaseous phase and precipitated nanoparticles were analyzed by infrared spectroscopy in order to determine the presence of either free CO2 and CO or metaladsorbed CO species. Characterization. Morphology, particle size, and size distributions were investigated by transmission electron microscopy (TEM) using a Philips CM200 microscope operating at 200 kV.
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Samples were made by dropping the dilute particle dispersion onto a carbon-coated copper grid, and the solvent slowly evaporated at room temperature. The average particle size diameter and standard deviation (d ( σ) were statistically determined by counting around 200 nanoparticles. Nanoparticle structure and phases were determined by X-ray powder diffraction (XRD) on a Rigaku RINT2000 diffractometer using CuKR radiation (λ = 1.5406 Å). The samples with a thickness of ∼0.5 μm were deposited onto a microslide substrate (1 1 cm). Compositional microanalyses of both single FePt nanoparticles and bulk FePt nanoparticles dispersions were also performed on a 200 kV FEG-FEI Tecnai F20 TEM microscope equipped with the energy-dispersive X-ray spectroscopy (EDS) probe (OXFORD) with the sample deposited on the carbon-coated copper grid and using the scanning transmission electron microscopy mode (STEM). In the case of the individual nanoparticles, the EDS were carried out in the highresolution (HR-EDS) and in a Z-contrast imaging mode using a high-angle annular dark-field detector in STEM using the drift corrections with time intervals of 10 s. The data were collected in nanoprobe mode with an acceleration voltage of 200 kV and a collection time of 100 s for each particle, and individual particles well separated from any other nanoparticles by at least 2025 nm to remove any potentially unwanted EDS signals from neighboring particles were analyzed. The spot size was adjusted in order to obtain a typical EDS spatial resolution of 12 nm, and the background-subtracted integrated intensity of the peaks corresponding to FeK and PtL lines in the spectra were used for the sample quantifications using the Tecnai Imaging and Analysis software. In addition, the average composition of the FePt nanoparticles was also determined by inductively coupled plasmaatomic emission (ICP) in a Plasma 40 PerkinElmer spectrometer previously calibrated with element standard solutions. For this analysis, dried powder was first dissolved with concentrated HCl and then diluted with ultrapure water. The resulting samples from different stages of the particle formation were analyzed by Fourier transform infrared (FTIR) spectroscopy using an FTIR Spectrum 2000 PerkinElmer spectrometer with a spectral resolution of 1 cm1. After the purification process using ethanol and centrifugation, the collected solid samples were dried in a vacuum oven for 5 h at 70 °C and measured using the pellets prepared by mixing their solids with KBr previously dried in a vacuum oven at 120 °C for 24 h. The mass ratio of the sample/KBr has been fixed at 1 mg/100 mg, and the spectra were acquired in the spectral range of 4000250 cm1. Free CO and CO2 present in the vapor phase during the reaction time were also analyzed by FTIR spectroscopy. The vapor phase present in the samples synthesized at final temperatures of 190, 230, and 298 °C were collected in an appropriated pre-evacuated flask with a gas valve. During the final temperature of the reaction, the pre-evacuated flask was connected to the reaction vessel and the collected vapor phase was quenched by an ice bath. The quenching process allows any liquid phases condensation present in the vapor. The gas phase remaining in the flask was then transferred to the spectrometer gaseous sample holder previously deaired with nitrogen gas and vacuum. Spectra were acquired in the specific expected adsorption region around 2350 and 2145 cm1 for free CO2 and CO species, respectively. In addition, the presence of the adsorbed CO on metallic Fe or Pt in the nanoparticles' surface was also evaluated by FTIR by analysis of the characteristic adsorption bands in the 20002100 cm1 spectral region. For that, after reaching the room temperature, a nanoparticle suspension aliquot was mixed with the KBr pellets and analyzed. 10476
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3. RESULTS AND DISCUSSION FePt nanoparticles with very narrow particle-to-particle compositional variation were successfully synthesized by the modified polyol process using Fe(acac)3 as an iron source. The final nanoparticles obtained after refluxing for 30 min at 298 °C show the average particle size of 4.2 ( 0.1 nm, determined by TEM analysis with the average composition evaluated by EDS and confirmed by ICP analyses of Fe55Pt45, as expected according to the previously reported work.1d Both these results and other experimental data are presented and discussed in the context of the nanoparticle formation mechanism based on the new heterocoagulation approach. The formation mechanism hypothesis using the polyol process and the Fe(CO)5 was first presented by Sun et. al,11 suggesting a binary nucleation process, which remains plausible when metal carbonyl was used as an iron source, though the global and particle-to-particle composition control are not achieved due to carbonyl volatility. After that, few studies in the literature investigate the formation mechanism of the FePt nanoparticles using either Fe(CO)5 (polyol process) or FeCl2 (superhydride method) as an iron source, suggesting an initial Pt nucleation, followed by Fe growth on Pt nuclei surfaces processes.6 However, there is no conclusive evidence of this mechanism. Moreover, the particleto-particle compositional variation in both reported methodologies was associated with posterior Fe incorporation onto the nanoparticle. The iron pentacarbonyl route produces particles that have a markedly wider compositional distribution ranging from 10 to 100 at. % Pt, whereas particles produced by the superhydride (lithium triethylborohydride, Li(C2H5)3BH) method are within 3070 at. % Pt. To investigate the formation mechanism of the FePt nanoparticles synthesized by the modified polyol process using the Fe(acac)3 as an iron source, samples at different final temperatures were prepared instead of aliquot extracting during the reaction course. By using the aliquot extracting strategy, when the portions are removed from the dispersion medium and quenched at different stages of the reaction and temperatures, the concentration and proportions of reagent in the remaining reaction medium were continuously changed and the initial conditions could not be warranted. Thus, several samples were prepared at the same conditions, only varying the final temperature, promoting a similar environment expected to be found at different reaction stages, but without changes in medium proportions. After synthesis, all samples were exhaustively washed with 1:8 mixtures of hexane and ethanol, followed by centrifugation. In this process, the surfactants and soluble reactants were discarded and the main products, intermediates, and unreacted reagents were evaluated. These cleaning steps were adopted because neither ethanol nor a mixture of hexane and ethanol dissolved Fe(acac)3, Pt(acac)2, and 1,2-hexadecanediol. Moreover, as described in the Experimental Section, the temperature of the synthesis was first kept at 120 °C for 20 min, which was found to be high enough to promote the complete reagent dissolutions. Quenched samples synthesized at final temperatures of 130 and 140 °C present a yellow-reddish precipitate at room temperature. After the washing procedure, infrared spectroscopy analysis was performed in both samples that only shows the presence of the metallic precursors and 1,2-hexadecanediol mixture (available in the Supporting Information, Figure S1 with band assignments described in Table S1). The presence of the
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Figure 1. XRD patterns of FePt nanoparticles synthesized at different final temperatures of (a) 170, (b) 190, and (c) 298 °C (reflux for 30 min) and (d) Fe3O4 synthesized in the absence of a Pt precursor. PseudoVoigt fits (lines) of the experimental data (symbols) and reflection shifts (inset) are shown. Standard reflection (JCPDS) is also presented.
precursors and the absence of either secondary or intermediary compounds at these temperatures infers that no reaction has occurred in this stage and that, at these temperatures, only the precursor dissolution took place. In the next stage, the reaction medium changed from transparent and reddish to a slightly opaque and quite black solution in the temperature range from 160 to 170 °C. According to XRD patterns of the quenched and washed sample synthesized at 170 °C, metallic platinum in the face-centered cubic (fcc) structure is the only present phase in the sample (Figure 1, pattern a). The (111) sample reflection centered in 39.56° in 2θ is in good agreement with the 39.75° observed for standard pure metallic platinum, as shown in the inset of Figure 1. TEM results (Figure 2a) of this sample indicate the presence of nanoparticles with sizes ranging from 1.5 to 2.0 nm, and the pure platinum phase was confirmed by the electron diffraction patterns. Typical diffraction halos corresponding to the nanosized materials are observed in the electron diffraction patterns, but the same nonordered diffracted spots can also be observed, indicating low sample crystallinity associated with the chemically disordered fcc structure. Atom dislocations are generally observed in nanoparticles smaller than 2.0 nm.12 Moreover, the diffracted spots' distance corresponds to the fcc (111) direction, in agreement with the XRD that presents a very large diffraction pattern for this reflection. According to the XRD patterns (Figure 1, pattern a), the fcc (111) reflection is very intense and the reflection corresponding to fcc (200) is not observed, which suggests that particles are essentially formed from (111) planes,1d,13 in agreement with the magic number used in the nanosized materials theory for particles smaller than 2.0 nm.5a,14 Borchert et al. demonstrated that, for highly monodispersed nanoparticles, particle sizes estimated from the line broadening 10477
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Figure 2. TEM images of the FePt nanoparticles synthesized at final temperatures of (a) 170, (b) 190, (c) 230, and (d) 298 °C (reflux for 30 min). Insets: (a) electron diffraction (above) and HRTEM (below) and (b) high magnification of the three image regions.
of XRD reflections by the Scherrer equation agreed within 5% with the values obtained from TEM and SAXS (small-angle X-ray scattering) measurements, when the Scherrer equation was adapted to spherical geometry, changing the traditional value used of 0.9 for the Scherrer constant by a value that considered the shape geometric effect.15 In this case, the value of 0.94 was used according to the spherical particles crystallized in the fcc structure.16 The estimated average crystallite sizes using this correction and the (111) Pt reflection at around 40° in 2θ was 1.6 ( 0.1 nm, for the sample prepared at 170 °C. The very small average crystallite size obtained for this sample could explain the broad diffraction signal around 2θ ∼ 40° as well as the absence of other diffraction peaks expected for pure Pt. Early studies on small particle effects show that the relationship between the number of surface atoms (NS) and the number of total atoms (NT) in the metal particle becomes equal to 1 when the particle diameter is e1 nm, that is, when every atom in the metal particle becomes a surface atom. Considering particles with a cuboctahedral structure, Kinoshita has shown that the mass-averaged distribution (MAD), the distribution of surface atoms normalized to the total number of atoms in the particle, shows a maximum for the (111) crystal face for a particle size of ∼2 nm and that the distribution of atoms on the (111) face is about 10 times higher than that on the (100) face. Additionally, it has been suggested that very small particles could have thermodynamically stable structures different from those expected for the macroscopic material and that very small particles of fcc metals could have icosahedral structures consisting primarily of [111] triangular surfaces.17 The presence of platinum as the only metallic element present in the sample synthesized at a final temperature of 170 °C was also confirmed by the HR-EDS (Figure 4a), which showed that the particle-to-particle composition was related to the single-particle composition distribution obtained with measurements of 45 individual nanoparticles in the sample, as detailed in the Experimental Section. Analysis
performed on a larger set of particles showed no significant differences in the compositional average value. The most considerable change in the reaction mixture occurs in the temperature range from 180 to 200 °C. In this stage, the solution changes from a quite opaque to a black suspension, and at temperature around 190 °C, a blanket of white haze can be observed in the reaction flask, suggesting gas or vapor formation. After quenching at room temperature and purification, the sample synthesized at 190 °C presented a phase mixture assigned as FePt and Fe3O4, according to the XDR results shown in the Figure 1, pattern b. The majority phase was identified as FePt in a chemically disordered fcc structure, showing peaks at 39.98° and 46.91° in 2θ corresponding to the (111) and (200) reflections, respectively. As observed in Figure 1, the (111) reflection of the sample synthesized at 190 °C was displaced to higher 2θ values in comparison with the pure Pt phase obtained at 170 °C (from 39.56° to 39.98°), in agreement with the partial FePt phase formation and the presence of a few smaller iron atoms in the pure fcc Pt structure leading to the contraction one.1d,2d The second phase was identified as iron oxide (magnetite) with (220), (311), and (400) reflections centered at 31.2, 35.3, and 43.5° in 2θ, respectively, and broad diffraction patterns similar to the diffraction halos observed for amorphous materials. In fact, using the Scherrer equation and the (311) reflection to estimate the average crystallite size for the iron oxide phase, the calculated value was 0.9 ( 0.3 nm, which was found to be in the accurate limit of the Scherrer equation, indicating the absence of or very low crystallinity. Concerning the base of our knowledge, the simultaneous presence of the iron oxide and FePt phases is the first time that it is observed as an intermediate reaction product during the FePt nanoparticle formation by the modified polyol process. Recently, Zhao et. al, also observed the formation of oxidized species of iron (FeOx) as intermediate compounds during the FePt nanoparticle formation.18 According to the 10478
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Figure 3. EDS spectra of the sample synthesized at a final temperature of 190 °C: (a) Pt nuclei, (b) iron oxidized species, and (c) nanoparticles with a coreshell structure, corresponding to the 1, 2, and 3 regions indicated in Figure 2b, respectively.
authors, the synthesis was carried out at temperatures around 340 °C in the absence of a diol, using a large excess of stearic acid, which was related to form the precursor iron stearate besides acting as a reducing agent. Although the synthetic procedure has been quite different compared with the modified polyol process, the formation mechanism of nanoparticles discussed by the authors corroborates our results concerning the formation of oxidized iron species as intermediate species in the FePt nanoparticle formation. In our work, the result suggests that a homogeneous bimetallic nucleation mechanism is unlikely and that heterocoagulation reactions seem to be the key to the nanoparticle formation. In fact, this result is in good agreement with the recently reported discussions about the FePt mechanism concerning the compositional distribution of individual nanoparticles that present both Pt-rich and Fe-rich phases during the polyol synthesis using Fe(CO)5 or FeCl2 and superhydride as precursors.6 TEM analysis in this sample (Figure 2b) shows the presence of three different kinds of particles with distinguished sizes and contrast, which are highlighted in the Figure 2b details and identified with the numbers 13. The first kind of particle (detail 1) shows spherical nanoparticles with the strongest contrast and particle sizes around 1.82 nm, which were analyzed by HR-EDS (Figure 3a), indicating only the presence of platinum element and the absence of iron. Thus, according to the analysis performed by XRD, TEM microscopy, and EDS, these nanoparticles were assigned to metallic Pt nuclei also observed at 170 °C. The second nanoparticle set presents a large size distribution between 2 and 5 nm and a smallest contrast. HR-EDS analysis, different from that performed in the first kind of particle, infers the presence of iron element and the absence of platinum in this region (Figure 3b) due to iron oxide phase formation. Finally, the coreshell nanostructure presented in the number 3 detail consists of both Pt and Fe elements, according to the EDS analysis (Figure 3c). Thus, the coreshell nanoparticles seem to be formed by a metallic core of either Pt or a Pt-rich phase, as indicated by an intermediate shift of the (111) reflection in Figure 1, pattern b, capped with iron
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Figure 4. Single-particle composition distribution for different samples synthesized at final temperatures of (a) 170, (b) 190, (c) 230, and (d) 298 °C (reflux for 30 min). Red lines correspond to the Gaussian fits.
species, probably in the form of iron oxide, as also indicated by XRD analyses (Figure 1, pattern b). The compositional distribution of individual nanoparticles for this sample presented in Figure 4b shows a very large distribution varying from pure elements, presenting an intermediate distribution as like-equivalent atomic ratio to iron- and platinum-rich phases. To confirm the presence of iron oxide formation at 190 °C during the synthesis of FePt, the synthetic procedure was realized without Pt(acac)2 addition at the same conditions, resulting in the magnetite formation, as indicated by XRD analysis (Figure 1, pattern d) and confirmed by M€ossbauer spectroscopy (see the Supporting Information, Figure S2), which also agrees with other reported results for magnetite synthesis in the absence of Pt using the modified polyol process.19 The temperature of 190 °C is close to the Fe(acac)3 thermal decomposition temperature (∼182 °C), and the results seem to infer that the reduction potential of the used diol is not high enough to promote the complete reduction of small reducing potential metallic ions, such as Fe3þ (0.04 V), but promotes a partial reduction to Fe2þ, leading to magnetite formation. In summary, besides the metallic Pt and magnetite (or iron oxide species) nanoparticle formation, the sample realized at 190 °C also presents the coreshell nanoparticles assigned to iron oxide species heterocoagulated on the Pt or Pt-rich FePt phase nuclei. The sample synthesized at 230 °C does not show the presence of the distinguished phase in TEM analysis (Figure 2c), in which only FePt nanoparticles with a spheroidal morphology and size distribution varying from 3 to 5 nm are observed. Particle-toparticle compositional analysis in this sample (Figure 4c) shows the average composition of around 45 at. % Pt, in agreement with the average global composition. However, at this temperature, the composition variation is larger than that expected for the final product, because the composition varied from 38 to 57 at. % Pt with a standard deviation of σ = 6.3. Finally, at 298 °C after 30 min, the nanoparticles are characterized as FePt in a fcc phase, according to the XRD results (Figure 1, pattern d) with the (111) reflection displaced from 39.56° to 40.35° in 2θ, in agreement with the pure Pt and FePt in a fcc structure phase, respectively. This latter reflection position is larger than 39.98° in 2θ observed 10479
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The Journal of Physical Chemistry C for the sample synthesized at 230 °C, suggesting that more Fe atoms seem to be incorporated in the Pt structure. The TEM image in Figure 2d reveals a narrow particle size distribution with nanoparticle diameters of 4.2 ( 0.1 nm, inferring a monodisperse system. Agreeing with these results, the particle-to-particle composition analysis shown in Figure 4d also present a narrow composition distribution with an average value close to the charged amount and variation from 43 to 52 at. % Pt with σ = 2.1. This compositional variation is smaller than those best observed results for the FePt nanoparticle synthesis using FeCl2 as an iron source and superhydride as a reducing agent.6b In this case, although the global average composition is similar to that observed here, our results show a total variation of 8 at. % Pt, whereas if FeCl2 is used, this variation is around 40 at. % Pt. Thus, this result agrees with the previously reported results, showing that, when Fe(acac)3 was used as an iron source in the modified polyol process, the chemical composition of the nanoparticles
Figure 5. FTIR spectra of CO and CO2 bands' evolution in the (a) gaseous atmosphere into the reaction flask during the nanoparticle formation and (b) the region of the CO adsorption onto the nanoparticles' surfaces in the samples synthesized at different final temperatures.
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was controlled.1d,2d Additionally, the average crystallite size changes from 1.8 ( 0.2 and 3.1 ( 0.3 to 4.1 ( 0.2 nm for samples synthesized at 190, 230, and 298 °C, respectively. This result could be associated with the major Fe incorporation in the Pt structure, which is in a good agreement with the (111) reflection displacement observed in the XRD analysis. In fact, the absence of the (200) reflection (Figure 1, pattern a) also implies the existence of structural defects. In the solid state, the rearrangement of atoms, besides energy, depends mainly on their mobility in the lattice. Structural defects favor the mobility of the atoms in the lattice, similar to what was reported by Kang et. al,5a who synthesized Ag-doped FePt nanoparticles, in which Ag segregation during annealing generates structural defects (vacancies) and favors the mobility of platinum and iron atoms, providing decreases in the annealing temperature. The initial Pt nuclei seem to present structural defects, besides the smaller size and a great number of atoms on the nanoparticles' surface, which could contribute to Fe diffusion on the Pt lattice and FePt phase formation in a chemical disordered structure as occurs with the fcc phase. Concerning the nanoparticle formation mechanism, because the diol seems not to be able to promote complete iron oxide species reduction to metallic iron, as inferred by magnetite formation in the absence of platinum, another reduction process must be simultaneously occurring in the medium, leading to iron reducing and FePt formation. The solid iron species formation, either oxidized or incorporated in the Pt structure, was only detected in the temperature range of 180190 °C. At this temperature, the iron(III) acetylacetonate complex thermal decomposition takes place, releasing Fe3þ ions in the medium, and a blanket of white haze can also be observed in the reaction flask. At this point, it could be explained that some Fe3þ ions can be partially reduced to Fe2þ due to diol and magnetite phase formation. However, the process was carried out in a nitrogen inert atmosphere and the absence of oxygen during the thermal decomposition of the acetylacetonate ion leads to carbon monoxide (CO) formation, which is a strong reducing agent. Thus, the CO gas in the medium could be acting as an additional reducing agent. Additionally, the Pt catalytic effect is well-known, and the fresh primary particles of metallic Pt act as heterogeneous
Figure 6. Schematic representation of the FePt nanoparticle formation mechanism indicates the heterogeneous nucleation, followed by iron oxide species heterocoagulation and iron reduction CO-spillover-assisted as intermediate processes for nanoparticle formation. 10480
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The Journal of Physical Chemistry C catalysts. The iron species reaches the Pt surface, and due to CO molecules also adsorbed on Pt surface, this latter is oxidized to CO2, promoting the iron reducing by a CO-spillover-assisted process.21 The presence of either free or adsorbed CO and CO2 molecules in the medium was investigated by FTIR spectroscopy analysis, and the results are presented in Figure 5. The FTIR spectra shown in Figure 5 were collected in the wavenumber range of both free CO and CO2 molecules and adsorbed CO on the metallic nanoparticle regions. In Figure 5a, the absorption bands in the region of 2350 and 2145 cm1 were assigned to OCO asymmetric stretching and to CO stretching bands, respectively, for free CO2 and CO molecules in the gas phase.21 These absorption bands confirm the formation of the gaseous molecules and suggest that free CO molecules have been formed by incomplete thermal decomposition of acetylacetonate ions when the temperature rose up to 180 °C. Oleic acid and oleylamine used as surfactants present high thermal stability and are not expected to experience any decomposition at temperatures below 380 °C. The free CO band was not observed for the collected gas sample at a final temperature of 140 °C. The sample at 190 °C presents strong and weak intense bands in the free CO and CO2 regions, respectively. According to FTIR results, the CO band intensity decreases while the intensity band assigned to CO2 molecules increases with temperature increases too. Besides the qualitative aspects related to a band intensity comparison between samples synthesized at different temperatures, this behavior can be associated with a higher free CO amount formed due to thermal decomposition of acetylacetonate around 180 °C and their consumption by a spillover process during the synthesis, producing CO2. Additionally, the nanoparticles present in the reaction medium were also analyzed by FTIR measurement (Figure 5b). An aliquot of 500 μL of the suspension without any treatment or further purification was mixed with KBr. The mixture was then evaporated at room temperature and dried at 50 °C for 24 h before pellet preparation. Spectra were obtained in the range of 20002250 cm1 corresponding to adsorbed CO on the metal surface region.21c,22 The results shown in Figure 5b indicate the presence of a low intensity absorption band around 2080 cm1 for all samples synthesized at different final temperatures. This band is assigned to adsorbed CO stretching on the Pt surface due to the PtCO bond.22 The absorption band expected for FeCO in the region of 20002032 cm1 is absent, implying that the CO molecules preferentially adsorb on the Pt surfaces, in agreement with reported results and with the adsorbed CO on the Pt surface, which has driven the spillover process.22c Moreover, decreases in the band intensity with temperature increases are attributed to both CO consumption in the Fe ion reduction process and the high Fe atom incorporation on the Pt nuclei in order to obtain the FePt alloy nanoparticles. The FePt alloy nanoparticles were also favored by increasing the Fe diffusion on the disordered fcc Pt structure at high temperature. These results agree with the overall proposed mechanism based on Pt and Fe heterogeneous nucleation, followed by heterocoagulation, as intermediate reactions in the FePt nanoparticle formation by the modified polyol process and are schematically summarized in Figure 6.
4. CONCLUSIONS A very important limiting factor for FePt nanoparticle applications in ultra-high-density magnetic recording (UHMR) media is their large particle-to-particle compositional variation, which
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affects the A1-to-L10 transformation as well as their magnetic properties. This restriction could be solved or, at least, minimized by understanding the nanoparticle formation mechanism and adjusting the experimental procedure to improve this large particle-to-particle compositional distribution. However, there are many controversies in the literature concerning the mechanism of the FePt formation. In the present work, a procedure to study the evolution of FePt nanoparticles' composition by the modified polyol process has been described and also used to analyze the formation mechanism of the nanoparticles. Our results convincingly show that the dominating mechanism involves reduction of Pt(acac)2 by the diol acting as a reducing agent and, initially, Pt nuclei formation at a temperature of around 130 °C. At ∼190 °C, the thermal decomposition of Fe(acac)3 then leads to the formation of oxidized Fe clusters, which heterocoagulate onto fresh Pt nuclei. Complete reduction of heterocoagulated iron species seems to involve a CO-spillover process, in which the Pt nuclei surface acts as a heterogeneous catalyst. The incomplete decomposition of acetylacetonate ion under a nitrogen atmosphere used to avoid the presence of oxygen into the reaction medium was assigned to the CO formation. The spillover process takes place after simultaneous adsorption of Fe clusters and CO molecules on the Pt surface, which acts as a heterogeneous catalyst promoting the electron transfer and Fe reduction accomplished by CO-to-CO2 oxidation. The modified polyol process produces particles that have a markedly much narrower composition distribution ranging from 43 to 52 at. % Pt than particles produced by other reported methods. Furthermore, there is a larger fraction of the nanoparticles, around 92%, in the dispersion having the desired charged molar ratio composition. In summary, our data showed a strong enhancement of the particle-to-particle composition control and presented a new formation mechanism approach based on Pt and Fe heterogeneous nucleation, followed by heterocoagulation and CO spillover as intermediate reactions in the FePt nanoparticle formation by the modified polyol process.
’ ASSOCIATED CONTENT
bS
Supporting Information. M€ ossbauer spectra of the magnetite phase synthesized in the absence of Pt as well as the discussion of the M€ossbauer assignments and FTIR results and assigned bands for quenched samples synthesized at final temperatures of 130140 °C are presented. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This work was supported by Brazilian agencies Fundac-~ao de Amparo a Pesquisa do Estado de S~ao Paulo—FAPESP (Grant Nos. 2007/07919-9, 2008/07568-4, and 2008/08791-9) and Conselho Nacional de Desenvolvimento Científico e Tecnologico— CNPq (Grant No. 577166/2008-5) ’ REFERENCES (1) (a) Lu, A. H.; Salabas, E. L.; Sch€uth, F.; Abadia, A. R. Angew. Chem., Int. Ed. 2007, 46, 1222–1244. (b) Selvan, S. T.; Tan, T. T. Y.; 10481
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