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Structural Study of Pt-Fe Nanoparticles: New Insights Into Pt Bimetallic Nanoparticle Formation with Oxidized Fe Species Rosemary Easterday, Olivia Sanchez-Felix, Barry D. Stein, David Gene Morgan, Maren Pink, Yaroslav B. Losovyj, and Lyudmila M. Bronstein J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp507870h • Publication Date (Web): 29 Sep 2014 Downloaded from http://pubs.acs.org on September 30, 2014

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The Journal of Physical Chemistry

Structural Study of Pt-Fe Nanoparticles: New Insights Into Pt Bimetallic Nanoparticle Formation with Oxidized Fe Species Rosemary Easterday1, Olivia Sanchez-Felix1, Barry D. Stein2, David Gene Morgan1, Maren Pink,1 Yaroslav Losovyj1*, Lyudmila Bronstein1,3* 1

Indiana University, Department of Chemistry, Bloomington, IN 47405, USA 2

3

Indiana University, Department of Biology, Bloomington, IN 47405, USA

King Abdulaziz University, Faculty of Science, Physics Department, Jeddah, Saudi Arabia

ABSTRACT. A combination of physicochemical methods allowed us to assess a structure of comparatively monodisperse 3-4 nm Pt-FexOy nanoparticles (NPs) synthesized by thermal decomposition of Pt acetylacetonate in the presence of iron oxide NPs as an iron source. Unlike traditional PtFe alloys composed of zerovalent Pt and Fe species with the surface enriched by Pt atoms, the NPs discussed in this work contain Pt(0) and oxidized Fe species (most probably, Fe3+ or Fe2+ ) as is proven by X-ray photoelectron spectroscopy (XPS). Angular dependence XPS measurements demonstrated the absence of the core-shell structure although a minor enrichment of the NP surface with Fe species was observed. High resolution transmission electron microscopy and X-ray powder diffraction (XRD) reveal that these Pt-FexOy NPs are not alloys, 1

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but consist of different domains, i.e., have “cluster-in-cluster” morphology. A comparison of the XPS and XRD data allowed us to conclude that the NPs also include amorphous iron oxide. These results allow better understanding of the mechanism of such NP formation and possible predictions of their catalytic performance.

KEYWORDS Platinum, iron oxide, nanoparticles, x-ray photoelectron spectroscopy

Introduction PtFe alloy nanoparticles (NPs) where both Pt and Fe are in a zerovalent state received considerable attention because of their magnetic and catalytic properties.1-6 Numerous synthetic protocols have been developed for preparation of such PtFe alloy NPs. Monodisperse PtFe NPs were synthesized by a polyol route from Pt(acac)2 and Fe(CO)5,1 from Pt and Fe acetylacetonates,7 or from Fe(III) ethoxide and Pt(II) acetylacetonates,8 to name a few synthetic procedures. Recently we reported a novel method to form monodisperse PtFe NPs using Pt(acac)2 as a Pt source, maghemite (γ-Fe2O3) NPs as an iron source, and 1,2-hexadecanediol as a reducing agent.9 We demonstrated the influence of the surface of the γ-Fe2O3 NPs of different sizes and the amount of surfactants (oleic acid, OA, and oleylamine, OAm) on the growth of PtFe NPs. The study of the catalytic properties of Pt3Fe NPs as well as the mixtures of Pt5Fe and maghemite NPs formed in situ demonstrated exceptionally high catalytic activity in hydrogenation of methyl-3-buten-2-ol. This makes them promising for catalytic applications, in particular, in microreactors.9 Based on X-ray powder diffraction (XRD) and numerous literature data on PtFe (or FePt NPs), it was presumed that these NPs are alloys of Pt(0) and Fe(0) with a Pt-rich shell. This assumption was based on literature data,10 showing that even for NPs with the Pt:Fe ratio of 1:1, the surface 2


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is strongly enriched by Pt atoms. However, in our case, it was not verified. Considering the promising catalytic properties of these PtFe NPs and a novel method of their syntheses when the γ-Fe2O3 NPs serve as an iron source and in some cases, as a magnetic component of the catalyst,9 a more detailed study of their structure was merited. Here we report an X-ray photoelectron spectroscopy (XPS) study combined with transmission electron microscopy (TEM), high resolution TEM (HRTEM), energy dispersive spectroscopy (EDS), and X-ray powder diffraction (XRD) of the Pt-Fe NPs prepared in the conditions when the initial γ-Fe2O3 NPs were destabilized and precipitated during syntheses9 to allow the analysis of solely Pt-Fe NPs. We studied three samples, the syntheses of which differed by the amount of surfactants (OA and OAm) used. As was demonstrated in our preceding paper,9 at the low surfactant loading Pt nuclei recruit more iron species from the γ-Fe2O3 NP surface along with OA and destabilize the host γ-Fe2O3 NPs. In this work, Pt2Fe, Pt0.8Fe, and Pt0.1Fe NPs were formed. It was revealed that these Pt-Fe NPs are not alloys, but include Pt(0) and iron oxide species, creating “cluster-in-cluster” morphology,11, 12 i.e., NPs consist of Pt(0) and iron oxide clusters (domains) bridged by amorphous iron oxide. To the best of our knowledge, this is the first report of monodisperse bimetallic Pt-FexOy NPs with oxidized iron species included in the NP body (not only surface).

Experimental part Materials FeCl3.6H2O (98%), octadecane (99%), dioctyl ether (99%), 1,2-hexadecane diol (90%), oleylamine (OAm, 70%), oleic acid (OA, 90%), and platinum(II) acetylacetonate (97%, Pt(acac)2) were purchased from Sigma-Aldrich and used as received. Hexanes (98.5%) and 3



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ethanol (95%) were purchased from EMD Chemicals and used as received. Acetone (99.5%) and chloroform (99.8%) were purchased from MACRON Chemicals and also used without purification.

Syntheses of Pt-Fe NPs Iron oleate was synthesized according to a published procedure.13 Iron oxide NPs were synthesized via the thermal decomposition of iron oleate using procedures published elsewhere.14,

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The NPs were synthesized in octadecane, as a solvent, which was chosen to

control the reaction temperature based on the desired NP size. The resultant NPs were oxidized by heating at 200 °C for 2 hours according to a previous study which determined that the NPs before oxidation (as-synthesized) consist of a mixture of FeO and Fe3O4 with the former phase prevailing, while after oxidation the NPs consist of γ-Fe2O3.15 The NPs were precipitated in acetone and washed in mixtures of acetone and hexane and then dissolved in chloroform. The γFe2O3 NP diameter was 12.7 ± 1.1 nm. According to thermal gravimetric analysis (TGA), the NPs contained 45.1 wt.% of OA. The Pt-Fe NPs were synthesized according to a previously published procedure.9 In a typical synthesis, chloroform was evaporated in a vacuum from a solution containing 15 mg of the γFe2O3 NPs. The NPs were dissolved in 7 mL of dioctyl ether and sonicated for 20 minutes. A three-neck round bottom flask was charged with 0.05 g of 1,2-hexadecane diol, 10 µL OAm, 10 µL OA, and the NP solution. The flask was equipped with a reflux condenser, temperature probe, argon inlet, and a stir bar. Argon was slowly bubbled into the solution via a needle inserted into a septum for 15 minutes. The solution was then heated at 10 °C/min to 285 °C. Once the solution reached 285 °C, a suspension of 0.025g of Pt(acac)2 in 0.25mL dioctyl ether was injected into the

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reaction solution and was maintained at 285 °C for 45 minutes. The solution was then slowly cooled and transferred to a vial. Table 1. Conditions of Pt-Fe NP formation and structural characteristics.

Notations

OAm OA (µL) (µL)

Total amount of surfactants (µL)

Elemental Resultant analysis by XRF, Fe/Pt Pt-Fe NP atomic wt.% diameter ratioa) (nm) Fe Pt

Standard deviation of NP sizes (%)

Pt-Fe1

12

12

31.6

9.15

55.92

0.57

2.9

10.6

Pt-Fe2

10

10

27.6

16.30

47.71

1.19

3.9

15.0

Pt-Fe3

6

6

19.6

42.36

14.60

10.1

3.2

9.3

a)

From elemental analysis

Characterization Electron-transparent NP specimens for TEM were prepared by placing a drop of a diluted solution onto a carbon-coated Cu grid. Images were acquired at an accelerating voltage of 80 kV on a JEOL JEM1010 transmission electron microscope. Images were analyzed with the National Institute of Health developed image-processing package ImageJ to estimate NP diameters. Between 150 and 300 NPs were used for this analysis. High resolution TEM (HRTEM) images and energy dispersive X-ray spectra (EDS) were acquired at accelerating voltage 300 kV on a JEOL 3200FS transmission electron microscope equipped with an Oxford Instruments INCA EDS system. The same TEM grids were used for both analyses. TGA was performed on TGAQ5000 IR manufactured by TA Instruments. The NP solution was evaporated into a 100 µL platinum pan and dried. The experiments were carried out by heating to 700 °C at the rate of 10.0 oC/min.

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X-ray fluorescence (XRF) measurements to determine the Pt and Fe contents were performed with a Zeiss Jena VRA-30 spectrometer (Mo anode, LiF crystal analyzer and SZ detector). Analyses were based on the Co Kα line and a series of standards prepared by mixing 1 g of polystyrene with 10–20 mg of standard compounds. The time of data acquisition was constant at 10 sec. X-ray powder diffraction (XRD) patterns were collected on an Empyrean from PANalytical. X-rays were generated from a copper target with a scattering wavelength of 1.54 Å. The stepsize of the experiment was 0.02. XPS experiments were performed using PHI Versa Probe II instrument equipped with monochromatic Al K(alpha) source. The X-ray power of 25 W at 15 kV was used for 100 micron beam size. The instrument work function was calibrated to give a binding energy (BE) of 84.0 eV for Au 4f7/2 line for metallic gold and the spectrometer dispersion was adjusted to give BEs of 284.8 eV, 932.7 eV and of 368.3 eV for the C 1s line of adventitious (aliphatic) carbon presented on the non-sputtered samples, Cu 2p3/2 and Ag 3d5/2 photoemission lines, respectively. The PHI double charge compensation system was used on all samples. The ultimate Versa Probe II instrumental resolution was determined to be better than 0.125 eV using the Fermi edge of the valence band for metallic silver. XPS spectra with the energy step of 0.1 eV were recorded using software SmartSoft–XPS v2.0 and processed using PHI MultiPack v9.0 and/or CasaXPS v.2.3.14 at the pass energies of 93.9 eV, 23.5 eV, and 11.75 eV for Fe 2p, both C 1s and Pt 4f, and O 1s regions, respectively. Peaks were fitted using GL line shapes, i.e., a combination of Gaussians and Lorentzians with 30-50% of Lorentzian contents. Shirley background was used for curvefitting.

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Results and Discussion Morphology and composition by TEM and XPS The synthesis conditions of Pt-Fe NPs (Fig. 1) and their characteristics are summarized in Table 1.

Figure 1. TEM images of Pt-Fe3 (a), Pt-Fe2 (b), and Pt-Fe1 (c). Red arrows indicate remaining iron oxide NPs. For the Pt-Fe3 and Pt-Fe2 samples (Fig. 1 a,b), only small Pt-Fe NPs are observed, while in the case of the Pt-Fe1 sample (Fig. 1c) prepared with the largest amount of surfactants, a few iron oxide NPs are present in the sample confirming the conclusions of the preceding work9 that there is a threshold amount of surfactants for destabilizing initial iron oxide NPs. Figure 1a shows aggregation of NPs due to insufficient surfactant loading. For the XPS studies, the NP samples were drop cast deposited on the native Si(111) wafer surface as well as on the TEM grids for the sake of imaging after the surface treatment (Figs. S1 and S2, the Supporting Information, SI). In all the samples, Pt is metallic with a binding energy (BE) of 71.2 eV (see Fig. 2c and Table S1, SI).16 In the XPS spectrum of the initial iron oxide NPs the iron BE ranges from 710.7 eV to 711.2 eV with very weak satellite structure,17-19 7



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suggesting the presence of mainly Fe3+ species with a small (~15%) fraction of Fe2+ ions constituting Fe3O4 (see deconvolution in Figs.S3, S4, and Table S2, SI). The observed Fe3+ vs. Fe2+ ratio as well as the overall Fe 2p spectrum shape closely resemble the iron deficient Fe3-δO4 films, whose composition is between γ-Fe2O3 and Fe3O4 as was reported in ref.20 This reveals that in fact the host NPs used in the Pt-Fe NP formation have the Fe3O4/γ-Fe2O3 structure, while the previous assumption was the oxidation of as-synthesized FeO/Fe3O4 NPs leads to purely γFe2O3 NPs.9,

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Considering, however, that the fraction of the magnetite species is low, it is

reasonable to assume that the surface maghemite species participate in the reaction with Pt(0) seeds unless Fe3+ is reduced to Fe2+ by 1,2-hexadecanediol on the Pt seed surface. Exact estimation of the Fe3+/Fe2+ ratio in the Pt-Fe NPs is complicated due to overlapping Pt 5p and Fe 3p peaks as well as due to satellites in the Fe 2p region, so further iron oxide species in bimetallic NPs will be notated as FexOy or Fe3O4/γ-Fe2O3.

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Figure 2. (a) Typical survey XPS spectra (Pt-Fe1); (b) normalized high resolution XPS of the Fe 2p region for Pt-Fe3 (red) and initial iron oxide NPs (blue); (c) High resolution XPS of Pf 4f for the Pt-Fe3 sample. In (c) the experimental data are shown in blue, the Pt 4f7/2 line is brown, the Pt 4f5/2 line is green, and the simulated spectrum is red. In agreement with the data showing only Fe oxidized species (no Fe(0)) in the Pt-FexOy samples, two oxygen species were observed in the O 1s region (Fig. 3). One, at the BE of about 532.3 eV, is assigned to atmospheric oxygen routinely present at the surface of XPS samples (green component in Fig.3). The second one, at the BE between 529.9 and 530.4 eV, is related to oxygen bound to the Fe ions (blue component in Fig.3). Thus, it was unambiguously proven that 9



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iron species in the Pt-Fe NPs are iron oxide making it, to the best of our knowledge, the first example of Pt-FexOy NPs.

Figure 3. High resolution XPS spectra of O 1s region for the initial iron oxide NPs (1), Pt-Fe3 (2), Pt-Fe2 (3), and Pt-Fe1 (4). The experimental data are indicated by a black line, the simulated XPS spectra are shown by a red line, the line for atmospheric oxygen is green, and the line for oxygen bound to the Fe ions is shown in blue. Figure 4 shows the XPS valence band (VB) spectra of the Pt-FexOy samples and of the initial Fe3O4/γ-Fe2O3 NPs. The trend of the increase of the Pt 5d intensities observed in the VB region 10


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vs. those of Fe 3d in the sequence from Pt-Fe3 to Pt-Fe2 and to Pt-Fe1 (see Fig. 4a) generally correlates with the behavior of the Pt 5p vs Fe 3p intensities (Fig.4b) along with calculated atomic percentages for the Fe 2p and Pt 4f transitions. Average Fe and Pt contents along with the Fe/Pt ratios are presented in Table 2. The Pt 5p region is heavily overlapped with Fe 3p and additionally complicated due to the input of different iron species (Fe2+ and Fe3+). Nevertheless, a deconvolution into two components (Fig. 4b), Fe (positioned between 55.5 eV and 56.5 eV BE) and Pt (positioned between at about of 52.5 eV BE), allows the satisfactory fit of the experimental spectra and reproduces the intensity trend observed in the Fe 3d and Pt 5d regions as well as the Fe/Pt ratios obtained for the Fe 2p and Pt 4f transitions. These data correlate well with the ratios obtained from the elemental analysis data on Pt and Fe (Table 1).

Figure 4. (a) Valence band XPS spectra: Pt-Fe3 (1), Pt-Fe2 (2), Pt-Fe1 (3), and Fe3O4/γ-Fe2O3 NPs (4). Bands between 10 eV and 20 eV originated from the carbon and nitrogen 2s and 2p 11



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levels, respectively. (b) Fe 3p (green lines) and Pt 5p (red lines) transition region for Pt-Fe3 (1), Pt-Fe2 (2), Pt-Fe1 (3), and Fe3O4/ γ-Fe2O3 NPs (4). For the curve (1) on the panel (b) the intensity is scaled up by a factor of 10.

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Table 2. The XPS and XRD parameters of the Pt-FexOy NPs Samples Recording on the conditions Si(111) substrate

Fe, at.%

Pt, at.%

Fe/Pt ratioa)

Fe3O4-γFe2O3, wt.%b)

Pt(0), wt.%b)

Molar phase Atomic Crystallite size, ratio iron Fe/Pt nmc) b) b) oxide/Pt ratio

Pt-Fe1

at 90°, bulk

33.9

66.1

0.51

77

23

4.36

1.52

2.7

at 15°, surface

40.3

59.7

0.68

at 90°, bulk

55.6

44.4

1.25

43

57

0.92

0.32

2.0

at 15°, surface

52.8

47.2

1.12

at 90°, bulk

90.5

9.5

9.53

22

78

0.34

0.12

2.3

at 15°, surface

94.5

5.5

17.2

Pt-Fe2

Pt-Fe3

a)

from XPS; b)from XRD, accounts only for crystalline phases. For recalculation of weight percentage of iron oxide crystalline phase to molar percentage, maghemite phase was used; c)from Scherrer equation using reflection at 40.4° two theta.

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The Pt-FexOy samples were found to exhibit very different Fe and Pt photoemission signals (see Table 2 for corresponding atomic concentrations) depending on the amounts of surfactants used in the NP syntheses. The Pt-Fe3 sample prepared at the lowest surfactant loading (Table 1) contains the highest fraction of Fe oxide, supporting the hypothesis that at the low surfactant loading Pt seeds reside on the iron oxide NP surface for a longer time and recruit a higher fraction of the iron oxide species than in the case of a higher surfactant loading.9 This is due to insufficient stabilization of Pt seeds when the surfactant loading is low. The Pt-Fe2 sample prepared at a medium surfactant loading contains much less Fe which is consistent with the above reasoning. Finally, in the Pt-Fe1 sample the iron content drops even further although this sample contains a few host iron oxide NPs, indicating that less iron species are included in the Pt-FexOy NPs. EDS carried out for Pt-Fe3 (Fig. S5, SI), confirms the high Fe content in this sample.

“Cluster-in-cluster” structure by XRD, HRTEM, and XPS The XRD profiles presented in Figure S6 (SI) are in agreement with the XPS data. They show two sets of reflections: extremely broad reflections whose positions are consistent with those of Pt(0) (or PtFe alloy) crystallites,9 and narrow reflections whose positions correlate with those of spinel nanocrystals (either Fe3O4 or γ-Fe2O3).21 Because the Fe(0) species were not observed by XPS, the PtFe alloys can be excluded from consideration. This allows us to conclude that the broad reflections come mainly from Pt(0). The semi-quantitative calculation of the phase composition gives weight percentage of the crystalline Fe3O4/γ-Fe2O3 and Pt(0) phases (Table 2). Amorphous phases are excluded from the calculation. The data presented in Table 2 indicate that for all the samples, in addition to crystalline phases containing Fe and Pt, there are amorphous phases. This conclusion is based on the comparison of the Fe/Pt ratios obtained from XPS and 14


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XRD. For Pt-Fe1, the Fe/Pt ratio from crystalline phases is higher by a factor of 3 than that from XPS, revealing a higher fraction of crystalline iron oxide. This is likely due to a few remaining initial iron oxide NPs present in the sample (Fig. 1c). For Pt-Fe2 and Pt-Fe3, the Fe/Pt ratio from XRD is much lower than that from XPS (especially, for Pt-Fe3), indicating that iron oxide species in Pt-FexOy NPs are largely amorphous. The HRTEM image of Pt-Fe3 shown in Figure 5 reveals a polycrystalline nature of these NPs and their “cluster-in-cluster” morphology i.e., NPs consist of Pt(0) and iron oxide clusters (domains) probably connected by amorphous iron oxide.

Figure 5. HRTEM images of the Pt-Fe3 sample. Three insets show zoomed-in views of individual NPs. Yellow and green arrows indicate different domains in a single particle.

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The estimation of crystallite sizes from the XRD patterns (Fig. S6, SI) using Scherrer equation (Table 1) showed that in all the samples the sizes of Pt crystallites are below the NP sizes indicating that the NPs include the Pt crystalline domains. To better understand the morphology of these Pt-FexOy NPs, we applied the angular dependent XPS using the native Si(111) wafer surface. This study allowed us to determine a surface vs bulk ratio of the Fe and Pt species across the particle.22 While in general this method was developed for the thin film analysis,23 the TEM data presented in Figure 1 allow us to assume that the spherical NPs assemble into mono/multilayer films on the high quality flat Si(111) surface. Taking into account that the Pt-FexOy particle sizes are between ~3 and 4 nm and that the mean free path of the photoelectrons emitted from the Fe 2p is below 7 Å,24,

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the considerations

widely used for thin films can be also applied in our case.23 If we assume that spherical NPs possess a core-shell structure with the Pt shell and the iron oxide core (Fig. S7, SI), the differences for bulk vs surface sensitive geometries in the XPS experiment should be observed, as the Pt input into the photoemission signal should be proportional to the effective cross-section probed by an analyzer while detecting the emitting photoelectrons at different take-off angles. However, for all the samples studied, we observed no significant differences in measured intensities at both geometries. Only a slight enrichment of the Pt-Fe1 and Pt-Fe3 NP surface with iron oxide is observed, while for Pt-Fe2, the NP surface is slightly enriched with Pt (Table 2). These observations point towards the “cluster-in-cluster” morphology with uneven distribution of domains through the individual NPs.

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XPS sputtering experiments The sputtering experiments further confirm the absence of a core-shell structure. A gentle sputtering treatment (500 eV Ar ions for 30 sec per cycle, up to four cycles) was applied to the Pt-FexOy samples.

Figure 6. XPS spectra in the VB region for Pt-Fe3 (a) and Pt-Fe2 (b) before (red) and after (blue) sputtering. Similarly to a plasma cleaning process (see “Cleaning and UPS measurements” in SI), the bands between 10 eV and 20 eV originated from the carbon and nitrogen 2s and 2p levels vanish 17



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just after the first sputtering cycle. On the other hand, the XPS intensities increase in all other regions (2p, 3p, 4f, 3d and 5d) upon applying subsequent argon sputtering cycles. This behavior is common for all the samples studied. Figure 6 shows, for example, the near Fermi level region for Pt-Fe3 and Pt-Fe2 samples as inserted and after one cycle of the Ar sputtering. The evolution of the VB region of the sample Pt-Fe3 extended to Pt 4f upon four sputtering cycles is presented in Figure.7a. Similarly, the XPS intensities increase in the Fe 2p region (Fig. 7b).

Figure 7. The VB (a), Fe 2p (b) and O 1s (c) regions of the XPS spectra of Pt-Fe3 sample upon sputtering. The number of sputtering cycles are indicated. 18


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The sputtering data for Pt-Fe3 presented in Figure 6 and 7 are consistent with the removal of the surface contamination (atmospheric gases and surfactants) using plasma cleaning. Indeed, the C 1s signal related to the hydrocarbons as well as to adsorbed CO diminishes, while the Fe 2p and Pt 4f signal intensities increase (see also Fig.S7, SI). Note that the Fe 2p BE remains mainly unchanged after several sputtering cycles suggesting no significant reduction of Fe species. This and unchanged NP appearance after sputtering confirmed by TEM (Fig. S10, SI) suggest robustness of the Pt-FexOy NP morphology and structure at least in the conditions used in this study. A slight Fe 2p shift towards lower BEs could be attributed to a small fraction of metallic iron atoms at the NP surface, if the loss of surface oxygen took place due to preferential sputtering. That possibility, however, seems very unlikely as for all samples, we record a steady increase of the intensity of the iron oxide oxygen component (Fig.3) upon sputtering as is seen in Figure 7c. This picture is consistent with a simultaneous decrease of the contaminant oxygen component at the BE= 532.3 eV. Therefore, the observed shift of the Fe BE is most likely due to partial reduction of Fe3+ to Fe2+. This seems to be plausible in relation to the appearance of weak satellite structures at ~714 eV and ~729 eV BEs for the sample after four consequent sputtering cycles. Table S3 (SI) shows that for the Pt-Fe2 and Pt-Fe3 samples, the Pt and Fe contents remain largely unchanged upon sputtering. This is an additional proof of the stable “cluster-incluster” morphology vs. the core-shell one, whose structure would change upon removal of the shell during sputtering. Based on the above data we suggest the following mechanism of the NP formation (Scheme 1).

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Scheme 1. Schematic reapresentation of the formation of Pt-FexOy NPs. (a) Pt seeds are formed in solution; (b) Due to shortage of surfactants Pt seeds land on the iron oxide NP surface; (c) Pt seeds recruit iron oxide species and additional surfactants and become self-sufficient in solution; (d) Destabilized iron oxide NPs precipitate from the reaction medium while Pt-FexOy NPs remain unaffected. Please note that pane (d) is a zoomed out pane (c).

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Pt(0) seeds form in the reaction solution upon injection of the Pt(acac)2 solution into the hot reaction medium containing host iron oxide NPs followed by the Pt(acac)2 decomposition and reduction with 1,2-hexadecanediol (Scheme 1a). In the conditions of insufficient surfactant loading, the Pt seeds are poorly stabilized and precipitate on the surface of iron oxide NPs (Scheme 1b). There, the Pt seeds strip additional surfactants from the host NPs, making surface of iron oxide unprotected. This may lead to two events. First event is the removal of iron oxide species and their capture by growing Pt NPs (Scheme 1c) leading to the morphology where a single particle consists of different domains. The second event is destabilization of remaining iron oxide NPs resulting in their aggregation and precipitation from the reaction solution (Scheme 1d).

Conclusion The data for Pt-FexOy NPs obtained by a combination of XPS, electron microscopy and XRD, allowed us to prove that the Pt species are in a zerovalent state, while the iron species are oxidized mostly to the Fe3+ ions, with up to 15% of the Fe2+ species. This leads to a morphology, in which a single NP consists of iron oxide crystalline domains merged with metallic Pt clusters via amorphous iron oxide rather than traditional PtFe alloy structure. The composition of such mixed NPs could be varied from Pt2Fe to Pt0.8Fe and even to Pt0.1Fe by decreasing the amounts of surfactants in the reaction solution. The angular dependent XPS and sputtering measurements support the absence of Pt-rich shell or any core-shell structure, instead proving the “cluster-in-cluster” morphology. To the best of our knowledge, this is the first report of nearly monodisperse Pt-FexOy NPs with such morphology. Easy variation of the NP composition, presence of two phases side-by-side in a single particle, and the robustness of the NP composition and morphology make these NPs


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promising for applications in complex catalytic processes where multiple reactions are carried out in an one-pot procedure.

ASSOCIATED CONTENT AUTHOR INFORMATION Corresponding Author *[email protected]; [email protected] Author Contributions The manuscript was written through contributions of all authors. ACKNOWLEDGMENT This work has been supported in part by the IU Faculty Research Support Program and NSF grant CHE-1048613. L.B. thanks the Russian Foundation for Basic Research under grant no. 1403-00876. We also thank the IU Nanoscale Characterization Facility for access to the instrumentation. Supporting Information. XPS, EDS, and XRD data and TEM images. This material is available free of charge via the Internet at http://pubs.acs.org.

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SYNOPSIS


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Structural Study of PtâFe Nanoparticles - American Chemical Society

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