Shell and Alloy Nanoparticles: Colloidal

Article pubs.acs.org/cm

Pt/Sn Intermetallic, Core/Shell and Alloy Nanoparticles: Colloidal Synthesis and Structural Control Xiaodong Wang,*,†,∥ Lena Altmann,‡ Jörg Stöver,§ Volkmar Zielasek,‡ Marcus Baü mer,‡ Katharina Al-Shamery,§ Holger Borchert,† Jürgen Parisi,† and Joanna Kolny-Olesiak† †

Energy and Semiconductor Research Laboratory, Department of Physics, University of Oldenburg, Carl-von-Ossietzky-Str. 9-11, 26129 Oldenburg, Germany ‡ Institute of Applied and Physical Chemistry, University of Bremen, Leobener Str. NW 2, 28359 Bremen, Germany § Institute of Pure and Applied Chemistry, University of Oldenburg, Carl-von-Ossietzky-Str. 9-11, 26129 Oldenburg, Germany S Supporting Information *

ABSTRACT: For the first time, shape-controlled Pt3Sn, PtSn, and PtSn2 intermetallic nanocrystals were synthesized in octadecene (ODE) by a versatile hot-injection method with 1,2-hexadecanediol (HDD) as the reducing agent. Transmission electron microscopy (TEM) measurements reveal that the metal composition has an influence on the particle morphology: with the increase in the Sn content, the Pt/Sn nanoparticles obtained by the hot-injection synthesis show flower-like, irregular faceted, cubic/tetrahedral, hexagonal, and spherical/nanowire structures. A facile phase-transfer preparative procedure for the synthesis of Pt/Sn core/shell nanoparticles was also developed, in which ligand-free Pt nanoparticles were used as precursors. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) measurements confirm a Pt-core/Snshell structure. The surface characteristic of the Pt/Sn core/shell nanoparticles was also investigated by IR spectroscopy of CO adsorption experiments (i.e., with a highly surface sensitive technique). These experiments reveal a few Pt atoms to be left on the surface as adsorption sites for CO. However, the intensity of the corresponding infrared (IR) bands is almost negligible. Furthermore, Pt/Sn random-alloy nanoparticles with different metal compositions and particle sizes were synthesized in this work by heating-up methods. Energy dispersive X-ray (EDX) and XRD analyses show different alloying extent of Sn with Pt. KEYWORDS: platinum/tin, bimetallic nanoparticles, intermetallic, core/shell, alloy, shape control

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tion,15 desulphurization and denitrogenation,16 as well as hydrogenation reactions,17−19 because of their superior catalytic activities and selectivities. In addition, as one of the best catalytic materials for reforming reactions and electrochemical reactions, Pt/Sn bimetallics have also been widely used in the petroleum industry, for reforming paraffins to olefins or aromatics,20−22 and in the fuel cell technology, for electroreduction of oxygen23 and electro-oxidation of gaseous24,25 or liquid fuels.26−29 In these works, Pt/Sn bimetallic nanoparticles were mostly prepared directly on catalyst supports or substrates by coimpregnation,30 sequential impregnation,31 electrocodeposition,32 coreduction,33 the sol−gel method,34 the hydrothermal approach,35 the carbonyl route,36 or surface organometallic chemistry.21 On the contrary, reports on the colloidal synthesis of unsupported Pt/Sn bimetallic nanoparticles with advanced structural control are very limited.37,38

INTRODUCTION Bimetallic nanoparticles are of great importance from a scientific and technological point of view for the improvement of the catalytic properties of monometallic nanoparticles for industrial production of chemicals,1,2 for energy conversion (e.g., in full cells3) and for energy storage (e.g., in batteries4), etc. The improvements derived from the combination of two metal elements into bimetallic particles can arise from an ensemble effect, a modified electronic structure, or the formation of new catalytic sites.5,6 Bimetallic nanoparticles composed of a noble metal and a non-noble metal have attracted increasing interest among researchers because of the high possibility of tailoring the electronic and geometric structures, which in turn can enhance the catalytic activity and selectivity.7−9 Besides, reducing the consumption of precious metals such as Pt by bimetallization with a low-cost metal is a popular approach to accelerate the practical application of noble metal-based catalysts in new areas of energy technology, such as fuel cells.10,11 Pt/Sn bimetallic nanoparticles have important applications in a variety of heterogeneous catalytic processes, including oxidation,12 reduction,13 transformation,14 hydro-deoxygena© 2012 American Chemical Society

Special Issue: Synthetic and Mechanistic Advances in Nanocrystal Growth Received: July 4, 2012 Revised: August 29, 2012 Published: September 4, 2012 1400

dx.doi.org/10.1021/cm302077w | Chem. Mater. 2013, 25, 1400−1407

Chemistry of Materials

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stepwise hot-injection method. For example, in the synthesis of Pt3Sn intermetallic nanoparticles, 280 mg of DDA and 100 mg of HDD, serving as the capping ligands and the reducing agent, respectively, were dissolved in 3 mL of octadecene. The solution was heated to 300 °C in a 25-mL three-necked flask under ambient atmosphere. In another 5-mL flask, 8.5 mg of PtCl4 and 2.4 mg of SnCl2·2H2O were dispersed in 2 mL of octadecene by sonication (nominal Pt/Sn molar ratio of 70:30). The metal salt solution was stepwise injected to the hot reaction system over 10 min (at an injection speed of 0.2 mL/ min). Afterward, the mixed solution was kept at 300 °C under refluxing for another 10 min to complete the reduction. The obtained colloidal nanoparticles in octadecene were isolated by precipitating them with ethanol and using centrifugation. The product can be further purified by repeated dissolution in hexane and precipitation with ethanol. Similar intermetallic products were also obtained when oleylamine and oleic acid were used as the capping ligands instead of DDA. For the synthesis of nanocrystals with PtSn and PtSn2 intermetallic phases, Pt and Sn chloride precursors with nominal molar ratios of 40:60 and 20:80 were used for the reactions, respectively. To prepare nanoparticles with different morphologies, synthesis with nominal Pt/ Sn molar ratios of 90:10 and 80:20 was also carried out according to the above-described steps of the hot-injection method. Preparation of Pt/Sn Core/Shell Nanoparticles. Pt-core/Snshell nanoparticles were prepared by a facile phase-transfer procedure using “ligand-free” Pt nanoparticles as precursors. The Pt particle seeds were synthesized by an alkaline ethylene glycol (EG) method:60 5 mL of NaOH/EG solution (300 mM) was mixed with an EG solution of H2PtCl6·6H2O (5 mL, 20 mM) at room temperature, which was then heated to 160 °C for 3 h to produce a homogeneous black colloidal solution of Pt metal nanoparticles (10 mM, 2.0 g Pt/L). Formation of Pt-core/Sn-shell nanoparticles was achieved by subsequent reduction of Sn ions in the presence of preformed colloidal Pt particles. In a typical experiment, 4 mL of toluene and 1 mL of oleic acid were first heated to 100 °C, and 47.2 mg of DDAB and 50 mg of TBAB in 1 mL of toluene were then added. Separately, 11.6 mg of SnCl2·2H2O was added to 2.5 mL of the colloidal solution of Pt in EG. This mixture was then injected stepwise over 10 min into the hot toluene-based solution. After the injection, the reaction was further conducted at 100 °C for another 20 min to adequately reduce the Sn ions. In this synthesis, oleic acid is the capping ligand and TBAB serves as the reducing agent. DDAB is used to increase the solubility of TBAB in toluene. After the reduction, a colorless EG phase (bottom phase) and a homogeneous black toluene phase (upper phase) were obtained as products. Finally, the oleic acid-capped Pt/Sn core/shell nanoparticles (nominal molar ratio of 1:2) in toluene were washed with methanol, precipitated by centrifugation and redispersed in chloroform. Synthesis of Pt/Sn Random-Alloy Nanoparticles. Pt/Sn random-alloy nanoparticles with different metal compositions were synthesized by a heating-up method (noninjection method). Pt/Sn nanoparticles with a nominal ratio of 75:25 were prepared in a toluene-EG dual-phase system. First, an EG solution (2 mL) containing 12.8 mg of PtCl4 and 2.9 mg of SnCl2·2H2O was mixed with 4 mL of toluene, 0.25 mL of oleylamine, and 0.25 mL of oleic acid at room temperature. 47.2 mg of DDAB and 50 mg of TBAB in 1 mL of toluene were then added under stirring. Afterward, the system was heated to 100 °C under refluxing and kept at that temperature for 1 h. The resulting alloy nanoparticles in the upper toluene phase were isolated by mixing with methanol and centrifuging. For the synthesis of Pt/Sn (nominal ratio of 50:50) nanoparticles, PtCl4 (8.5 mg), SnCl2·2H2O (5.7 mg), HDD (100 mg), and DDA (280 mg) were dissolved in octadecene (5 mL) at room temperature. The solution was then heated to 250 °C for 1 h to give a homogeneous black colloidal solution. For purification of the product, the 3-fold volume of ethanol was added to precipitate the nanoparticles, which were then isolated by centrifugation and washed by repeated dissolution in hexane and precipitation with ethanol. Characterization of the Bimetallic Nanoparticles. Transmission electron microscopy (TEM) and high-resolution (HR) TEM measurements were conducted with a EM 902A microscope

By utilizing different colloidal chemistry methods, we recently synthesized a series of Pt/Sn alloy nanoparticles with systematically varied structural parameters such as particle size, shape, metal composition, and organic capping agents,39 which are excellent candidates for the investigation of the structure−reactivity relationship for a number of catalytic reactions. For a given bimetallic system, the behavior as catalysts depends, in general, not only on the composition but also on the arrangement of both metals in the compound. In this sense, intermetallic nanoparticles,40−44 composed of two (or more) metals arranged in a specific crystal structure, and core/shell nanoparticles,45−51 derived from the heterogeneous nucleation and growth of two metals, can be more promising materials than random-alloy nanoparticles owing to their unique magnetic, electronic, and catalytic properties. Pt-based intermetallic and core/shell nanoparticles such as Pt/Sn are gaining particularly attention because of their remarkable performances in many important chemical and electrochemical reactions.52−55 However, studies on the structural control in the synthesis of Pt/Sn intermetallic and core/shell nanoparticles are nearly absent. To date, Cable and Schaak developed wetchemical methods for synthesizing PtSn intermetallic nanoparticles56 and solution-mediated reactions for converting PtSn to Pt3Sn or PtSn2 intermetallics,57 whereas the sizes of the asprepared nanocrystals ranged from 10 to above 100 nm with a rather poor uniformity due to significant particle agglomeration and sintering. Another work concerning the synthesis of PtSn nanoparticles and PtSn@Pt core/shell structures was published by Eichhorn’s group.53 Recently, the synthesis of Pt3Sn nanoparticles in the size range below 10 nm was also reported.58,59 In these works, the resulting particles invariably show (quasi-)spherical shapes, no success in the synthesis of shape-controlled Pt/Sn intermetallic nanocrystals has been achieved previously. It is a formidable challenge but a crucial task to develop methodologies for the effective synthesis of Pt/Sn ordered intermetallic, random-alloy and heterostructure core/shell nanoparticles with precise control on the particle size and morphology. In the present work, we established a one-step hot-injection method for the synthesis of Pt3Sn, PtSn, and PtSn2 intermetallic nanocrystals. To the best of our knowledge, this is the first time reporting a versatile synthetic strategy applicable to Pt/Sn nanoparticles of three different intermetallic phases and also the first time for the production of shapecontrolled Pt/Sn intermetallic nanocrystals. Furthermore, Ptcore/Sn-shell and Pt/Sn random-alloy nanoparticles with different particle sizes and metal compositions were also successfully synthesized in this work, by a facile phase-transfer preparative procedure and a heating-up synthesis method, respectively.

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EXPERIMENTAL SECTION

Chemicals. Platinum tetrachloride (PtCl4, 99%) and dodecylamine (DDA, 98%) were obtained from Acros Organics. Hexachloroplatinic acid hexahydrate (H2PtCl6·6H2O, ≥ 37.5% Pt basis), tin dichloride dihydrate (SnCl2·2H2O), 1,2-hexadecandiol (HDD, 90%), didodecyldimethylammonium bromide (DDAB, 98%), and tetrabutylammonium borohydride (TBAB, 98%) were purchased from Aldrich. Sodium hydroxide (NaOH) was supplied by Carl Roth. All organic solvents and other chemicals were of analytical grade and used as received. Preparation of Pt/Sn Intermetallic Nanoparticles. Pt/Sn intermetallic nanoparticles were synthesized in 1-octadecene by a 1401



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(Zeiss) operated at 80 kV, and a Tecnai F20 S-TWIN microscope (FEI) and a Jeol JEM-2100F, both operated at 200 kV. Samples for TEM and HRTEM measurements were prepared by placing a drop of the Pt/Sn colloidal solution on a carbon-coated copper grid and then evaporating the solvent at ambient conditions overnight. Energy dispersive X-ray (EDX) analysis was carried out by using a Helios Nanolab 600i focused ion beam (FEI) equipped with an Apollox EDAX detector (Ametek). EDX samples were prepared by coating an Al sample holder with the Pt/Sn colloidal solution and subsequently drying the sample at elevated temperatures. X-ray diffraction (XRD) patterns of the nanoparticles were obtained on a X’PertPro MPD diffractometer (PANalytical), where the samples were prepared by dropping the colloidal solution on a single crystal Si substrate followed by evaporation of the solvent. In some cases, Rietveld analysis was used to evaluate the diffraction patterns in detail. Rietveld analysis was performed with the help of the program MAUD,61 as described in detail in ref 39. X-ray photoelectron spectroscopy (XPS) was performed with a Leybold-Heraeus surface analysis system equipped with a hemispherical EA-10 analyzer and an Al Kα (1486.6 eV) X-ray source. Samples were prepared by depositing a small amount (∼10 μL) of the colloidal solution on a Si(111) wafer with native oxide layer and letting the solvent evaporate. The Sn 3d and Pt 4f signals were fitted using the software Spectral Data Processor (SDP v4.5).62 After subtraction of a Shirley-type background function, the signals were fitted with pseudo-Voigt line shapes. For both peaks of a corresponding doublet, the same full width at half-maximum (fwhm) and Gauss−Lorentz ratio were used. The areas of the Pt 4f and the Sn 3d5/2 signals, divided by empirical relative sensitivity factors from the literature,63 were used for calculating the Pt/Sn ratio after correction for the transmission function of the analyzer. In situ infrared (IR) spectroscopy of CO adsorption on the nanoparticles (supported on γAl2O3) was conducted at room temperature in diffuse-reflectance geometry (DRIFTs) with a Varian-670 FT-IR spectrometer. The supported particle samples were pressed into pellets and investigated in a reaction cell equipped with a controlled CO supply system for in situ studies. In a typical experiment, CO was preadsorbed on the particle surface by purging the IR cell with a gas mixture containing 1 vol % CO in He for 15 min to ensure saturation coverage at 30 °C. Before taking a spectrum, the IR cell was purged with pure He another 15 min to remove the CO from the gas phase, since the gas phase vibration signal would otherwise overlay weaker vibration bands of adsorbed CO. All spectra were recorded with a resolution of 2 cm−1 in the adsorption mode.

cubic symmetry is in agreement with the fact that they possess a face-centered cubic (fcc) crystallographic structure. The selectivities of the sample to cubic and tetrahedral shapes are 85% and 15%, respectively. The side length of cubic nanoparticles ranges from 6.0 to 12.0 nm with an average value around 8.7 nm, whereas the tetrahedral particles have a side length of 14.0 ± 1.3 nm. The HRTEM images (Figure 1b and c) reveal that the cubic and tetrahedral particles are highly crystalline. An EDX elemental mapping (Figure 1d) confirms that the nanoparticles are bimetallic in nature with a homogeneous distribution of Pt and Sn. Figure 2 shows the

Figure 2. XRD pattern of Pt3Sn intermetallic nanoparticles. The drop lines are the reference data for Pt3Sn (ICDD 01-072-2977).

XRD profile of the as-synthesized Pt3Sn nanoparticles. The diffraction pattern is very well consistent with reference data for fcc Pt3Sn. EDX analysis indicates a particle composition of 75:25 (molar ratio), which is exactly in line with the stoichiometric ratio of Pt to Sn for the Pt3Sn intermetallic phase. PtSn intermetallic nanoparticles were produced by a synthesis with the nominal Pt/Sn molar ratio of 40:60. Figure 3a shows the TEM image of the resulting PtSn nanoparticles. Different from the Pt3Sn sample, the PtSn nanoparticles were found to have a less defined shape. Nevertheless, faceted particles with characteristic lengths in the range from 4.5 to 12.0 nm with an average value of 7.1 nm were observed. The recorded XRD pattern matches precisely with the reference pattern of the hexagonal PtSn phase (Figure 3b). This is in good agreement with the EDX analysis, where the composition of the particles was determined to be 50:50, indicating a full alloying of the Pt and Sn to the intermetallic PtSn phase. In the synthesis with DDA as the stabilizing ligands, when further increasing the relative content of Sn (Pt/Sn molar ratio of 20:80), large faceted particles of 30−120 nm and aggregates of several hundred nanometers (Figure S1a and b, Supporting Information) were obtained as the main products instead of small uniform colloidal particles. The formation of large particles and aggregates can be attributed to the insufficient stabilizing ability of DDA to the elevated amount of Sn. This sample, having a metal composition of 33:67 as measured by EDX, was confirmed to have a PtSn2 intermetallic structure (Figure S1c, Supporting Information). Colloidal synthesis of the PtSn2 intermetallic phase has been considered as a formidable task. Schaak and co-workers prepared PtSn2

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RESULTS AND DISCUSSION Synthesis of Pt/Sn Nanoparticles with Different Intermetallic Phases. By using a facile hot-injection colloidal chemistry method, for the first time, we synthesized Pt/Sn intermetallic nanoparticles with well-controlled morphologies. Figure 1a displays a TEM image of the as-prepared Pt3Sn intermetallic nanoparticles, which show well-defined cubic and tetrahedral shapes. The formation of these crystallites with

Figure 1. (a) TEM image, (b and c) HRTEM images, and (d) HRTEM-EDX elemental mapping of Pt3Sn intermetallic nanoparticles. 1402



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Figure 4. TEM images of Pt/Sn nanostructures with different Sn contents: (a) 10 mol %, prepared by using DDA as capping ligands; (b) 20 mol %, capped by DDA; (c) 60−76 mol %, synthesized with a high amount of oleylamine and oleic acid as stabilizing agents (0.25 mL + 0.25 mL); (d) 56−74 mol %, prepared with a small amount of oleylamine and oleic acid as capping ligands (0.01 mL + 0.01 mL). The scale bars are 5 nm in the insets of (a) and (b), and 2 nm in that of (d). Figure 3. (a) TEM image and (b) XRD pattern of PtSn intermetallic nanoparticles. The scale bar in the inset of (a) is 5 nm. The drop lines in (b) are the reference pattern of PtSn (COD 99-000-8332).

nanoparticles. In this sample, several different morphologies can be found, such as cubic, tetrahedral, cross-, star-, or heart-like structures. These irregular shapes can be deemed as a cubic or tetrahedral structure losing a part in the middle of facet(s). The recorded XRD pattern of the Pt/Sn (80:20) sample is similar to that of the cubic/tetrahedral nanocrystals (Figure 1), suggesting that it still has a Pt3Sn intermetallic structure (Figure S2b, Supporting Information), although the molar ratio of Pt/Sn as determined by EDX (80:20) differs slightly from the ideal value of 75:25. HAADF imaging coupled with EDX analysis/mapping demonstrates homogeneous distribution of Pt and Sn for both the flower-like and irregular cubic/tetrahedral particles (Figure S3, Supporting Information). An investigation on the shape evolution of the Pt/Sn (90:10) and (80:20) samples by TEM reveals that the flower-like and irregular cubic/tetrahedral particles already formed within 1 min after the hot-injection of metal salts, no obvious change in the particle structure occurred with prolonged synthesis time. For the synthesis of uniform colloidal particles with Sn content of higher than 50 mol %, oleylamine and oleic acid were employed as capping ligands (for stabilizing Pt and Sn, respectively) instead of DDA. It was found that using a high amount of ligands (0.25 mL of oleylamine and 0.25 mL of oleic acid) facilitated the production of spherical Pt/Sn nanoparticles with average size of 3.8 nm (Figure 4c). However, decreasing the amount of capping ligands (0.01 mL of oleylamine and 0.01 mL of oleic acid) resulted in the formation of nanowire structures (Figure 4d). The diameter of the nanowires is comparable to that of the spherical nanoparticles (Figure 4c). It is reasonable to deduct that the nanowires are formed via linear aggregation of spherical particles as a consequence of low surface ligand coverage.64 XRD measurements on a nanowire sample (Pt/Sn composition of 44:56 according to EDX) show

aggregates by a solution-mediated conversion process with PtSn colloidal particles as precursor. Herein, we succeed for the first time in the direct one-pot synthesis of PtSn2 intermetallic nanostructures by a colloidal chemistry method. It is worthwhile to note that the high injection temperature (300 °C) plays a crucial role in the formation of shapecontrolled Pt/Sn intermetallic nanocrystals. It was found in our work that when the hot-injection temperature was lower than 250 °C, no intermetallic particles with aforementioned different shapes were obtained. The observations suggest that the polyhedral Pt/Sn intermetallic nanocrystals require high activation energies during growth. This is probably the reason why in previously reported work, only (quasi-)spherical Pt3Sn or PtSn intermetallic nanoparticles were synthesized.52−59 Shape Control of Pt/Sn Nanoparticles. In this work, it is a surprise to find that for the nanoparticles synthesized by the hot-injection method described above, the Sn content has an influence on the particle morphology. To systematically investigate this effect, Pt and Sn chloride precursors with different molar ratios were used for the synthesis reactions. As shown in Figure 4a, the as-synthesized nanoparticles with composition of 90:10 (determined by EDX) display interesting flower-like shapes. Their size ranges from 8.4 to 19.2 nm with an average value of 14.6 nm. It is worth noting that the variation in the molar ratio between Pt and Sn caused also a change in the crystallographic structure. XRD measurement reveals that these nanoparticles consist of a random-alloy phase instead of an intermetallic phase (Figure S2a, Supporting Information). Figure 4b shows a TEM image of Pt/Sn (80:20) 1403



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that the diffraction peaks are consistent with those of the hexagonal PtSn phase (Figure 5), which confirms the intermetallic nature of the nanowires. The synthesis of PtSn nanowire materials with intermetallic structure is an unprecedented work.

Figure 5. XRD pattern of Pt/Sn nanowires. The drop lines are the reference pattern of intermetallic PtSn (COD 99-000-8332).

Preparation of Pt/Sn Core/Shell Structured Nanoparticles. Pt-core/Sn-shell nanoparticles were prepared in a toluene−EG dual-phase system via a facile phase-transfer procedure. The “ligand-free” Pt nanoparticles (synthesized in the absence of the usual protective ligands, stabilized only with small solvent molecules and simple anions)60 were used as the precursors. The subsequent reduction of Sn ions in the Pt/EG colloidal solution is intended to deposit the formed Sn atoms on the surface of the Pt nanoparticles. The surface Sn atoms are susceptible to coordination with the oleic acid ligands, which would then transfer the Pt-core/Sn-shell nanoparticles from the EG phase to the toluene phase. A blank experiment shows that by the similar steps but without addition of Sn ions, monometallic Pt nanoparticles cannot be transferred to the toluene phase. This means that oleic acid can obviously not bind efficiently to the surface of the pure Pt nanocrystals; by consequence, the monometallic Pt particles stay in the EG phase. These results demonstrate that the nanoparticles obtained in the toluene phase are bimetallic in nature. Moreover, as a side aspect, the results provide clear evidence that oleic acid can bind to Sn but is not a suitable ligand to bind to Pt surface sites. Figure 6a shows the TEM image of the as-synthesized Pt/Sn nanoparticles. They have a quasi-spherical shape with an average size of 2.3 nm (standard deviation of 0.4 nm). The HRTEM image (inset in Figure 6a) reveals that the nanoparticles have high crystallinity. In XRD measurements, the recorded pattern of the Pt/Sn bimetallic nanoparticles (Sn content of 54 mol % according to EDX) did not show any shift of the Pt diffraction peaks to lower 2θ value when compared with the pattern of monometallic Pt nanoparticles having similar size and shape (Figure 6b). This suggests that in the bimetallic sample no Sn was incorporated into the cubic Pt lattice. By consequence, the Sn responsible for transferring the particles into the toluene phase must be located at the particle surface as a shell surrounding the Pt core.

Figure 6. (a) TEM image and (b) XRD pattern of Pt/Sn core/shell nanoparticles. The scale bar in the inset of (a) is 1 nm. An XRD spectrum of monometallic Pt nanoparticles is used in (b) as the reference.

To further verify the core−shell structure and to clarify if the Pt core is completely covered by Sn, we performed additional characterization experiments by XPS and IR spectroscopy of CO probe molecules. In XPS measurements, the Pt/Sn core/ shell sample was compared with a Pt/Sn random-alloy sample (having a similar particle size and overall composition of ∼50:50 as measured by EDX) prepared by a method reported previously.39 The Pt/Sn ratio determined by XPS is 0.90 for the random-alloy sample. This value is below 1 because the particle surface is enriched with Sn and only a part of Sn was incorporated into the Pt lattice. In comparison, the Pt/Sn core−shell sample shows a Pt/Sn ratio of 0.65, which is significantly lower than that of the random-alloy sample, indicating a much higher fraction of Sn to be located at the particle surface. This is consistent with the Pt-core/Sn-shell structure. (Please note that for small nanoparticles of ∼2 nm diameter, XPS analysis is not highly surface-sensitive anymore; signals from the core of the particles will also give a significant contribution to the peak intensity.) Figure 7 shows the IR spectra of CO adsorption on monometallic Pt, random-alloy Pt/Sn,39 and the as-prepared core/shell Pt/Sn nanoparticles. A clear decrease in the intensity of linearly adsorbed CO on Pt sites was found from Pt > PtSn (random alloy) > Pt@Sn (core/ shell). Although it is, in general, difficult to compare intensities of IR spectra from different samples, this points to a reduction of available surface Pt atoms and an increase of Sn surface coverage. The observation of a small IR band corresponding to linear CO adsorption on Pt sites in the case of the core/shell 1404



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the particle product. Here, the difference from the previous work might be due to the utilization of both Pt- and Snstabilizing ligands (amine and carboxylic acid, respectively) for the Pt/Sn (75:25) sample and the employment of high synthesis temperature (250 °C) for the Pt/Sn (50:50) sample. XRD measurements reveal that when compared with monometallic Pt nanoparticles, the Pt/Sn bimetallic samples show a clear shift of the diffraction peaks to lower values of 2θ (Figure 9). This shift, originating from an increased lattice

Figure 7. IR spectra of CO adsorption on supported monometallic Pt, random-alloy Pt/Sn, and core/shell Pt/Sn nanoparticles.

sample means that the Sn shell is not perfect. However, in comparison to the random-alloy sample, the CO adsorption is negligible for the Pt/Sn core/shell sample, suggesting there is hardly Pt on the particle surface. Deeper insight can be gained from the C−O stretching frequencies, the CO adsorption band shifts to lower wavenumbers from 2080 cm−1 for Pt to 2077 cm−1 and 2037 cm−1 for Pt/Sn alloy and core/shell samples, respectively. This shift is a consequence of the dilution of surface Pt adsorption sites by Sn and the resulting decrease in dipole coupling. The low frequency observed for the core/shell sample implies that there are only a few more or less isolated Pt atoms left at the surface. The disappearance of the bridge adsorption of CO (1841 cm−1) in this sample can also be attributed to the Sn surface covering and the absence of neighboring Pt surface atoms. Preparation of Pt/Sn Random-Alloy Nanoparticles with Different Compositions and Sizes. Pt/Sn randomalloy nanoparticles with compositions of 75:25 and 50:50 were synthesized by a non-hot-injection method (heating-up method) in toluene and octadecene, respectively. Figure 8

Figure 9. XRD patterns of Pt/Sn random-alloy nanoparticles with different compositions. An XRD pattern of monometallic Pt nanoparticles is used as the reference. The data of the bimetallic samples were analyzed by Rietveld refinement. The fits are shown as solid red lines, whereas the experimental data are represented in black.

parameter of the fcc structure, provides definite evidence for the incorporation of Sn into the Pt lattice. The lattice parameters were quantitatively determined by fitting the diffraction patterns as described in ref 39. By assuming a linear increase in the lattice parameter between two reference points established by pure Pt nanoparticles (3.914 Å in our case) and a defined Pt3Sn alloy (4.002 Å according to the ICDD database), the amount of incorporated Sn (i.e., the fraction of lattice sites occupied by Sn) was determined to be 8% for the Pt/Sn (75:25) sample and 21% for the Pt/Sn (50:50) sample. The amount of Sn incorporated into the lattice is lower than the corresponding overall Sn content (detected by EDX) in both samples, suggesting that part of the Sn is segregated at the surface of the random-alloy nanocrystals. This picture is further supported by evaluating the broadening of the Bragg reflections. A Rietveld analysis of the diffraction data, following a procedure described previously,39 reveals average diameters of 1.4 and 3.6 nm for the Pt/Sn 75:25 and 50:50 samples, respectively. In both cases, the values are slightly lower than the diameters determined by the TEM measurements, which points to a highly crystalline Pt/Sn core with a more disordered Snenriched surface shell.

Figure 8. TEM images of Pt/Sn random-alloy nanoparticles with metal compositions of (a) 75:25 and (b) 50:50.

shows the TEM images of the Pt/Sn random-alloys with the different compositions. Both samples display quasi-spherical shapes. The average diameter of the Pt/Sn (75:25) nanoparticles is only 1.7 nm, with a narrow size distribution (standard deviation of 0.3 nm), whereas the Pt/Sn (50:50) sample synthesized at higher temperature have larger sizes (average value of 4.4 nm). In contrast to our previous synthesis where the Sn content of the Pt/Sn bimetallic nanoparticles was always lower than the nominal value used in the reaction,39 in this work, the full amount of Sn used for synthesis is found in

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CONCLUSIONS A versatile hot-injection method for the synthesis of Pt/Sn nanoparticles with different intermetallic phases (e.g., Pt3Sn, PtSn, and PtSn2) has been developed. For the first time, we succeeded in the colloidal preparation of Pt/Sn intermetallic nanocrystals with well-defined morphologies, such as nano1405



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cubes, tetrahedrons, nanowires, etc. It was found that the high hot-injection temperature (300 °C) plays a crucial role in the formation of shape-controlled intermetallics and the metal composition has a distinct influence on the particle shape. By using a facile phase-transfer approach with “unprotected” Pt nanoparticles as seeds (seed-mediated growth method), Pt/Sn core/shell colloidal particles with small sizes of 2.3 ± 0.4 nm were also successfully prepared in this work. XRD, XPS, and IRCO adsorption experiments confirm the Pt-core/Sn-shell structure, although IR spectroscopy reveals that a few Pt sites can still be found on the surface of these particles. In addition, Pt/Sn random-alloy nanoparticles with different compositions and sizes were synthesized by the heating-up methods. XRD and EDX measurements reveal a surface segregation of Sn on the nanoparticles. The Pt/Sn intermetallic, core/shell, and alloy nanocrystals with the well-defined morphologies have potential application in heterogeneous selective hydrogenation reactions and electrochemical reactions in full cells (e.g., direct methanol fuel cell), which will be the subject of future studies.

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ASSOCIATED CONTENT

* Supporting Information S

Additional TEM, XRD, and EDX mapping results of Pt/Sn intermetallic and alloy nanocrystals. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address ∥

Department of Chemistry, Institute of Technical Electrochemistry, Technical University of Munich, Lichtenbergstrasse 4, 85748 Garching, Germany Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support of the German Science Foundation (DFG) is gratefully acknowledged. Furthermore, this work was partly supported by the EWE AG, Oldenburg. We thank Dr. Erhard Rhiel for help with HRTEM measurements.

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REFERENCES

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Shell and Alloy Nanoparticles: Colloidal

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