WO2011149912A1

WO2011149912A1 - Core/shell nanoparticle synthesis and catalytic method

Info

Publication number: WO2011149912A1
Application number: PCT/US2011/037705
Authority: WO
Inventors: Shouheng Sun; Vismadeb Mazumder
Original assignee: Brown University
Priority date: 2010-05-24
Filing date: 2011-05-24
Publication date: 2011-12-01

Abstract

This invention comprises a method of preparing stabilized core/shell structured nanoparticles comprising core material being at least one member a first metal group, said first metal group consisting of Pd, Au, NiPd or CoPd and shell material comprising Pt and at least one member of a second metal group, said second metal group consisting of Fe, Ni, Cu comprising (a) preparing a monodispersion of said core material wherein said monodispersion is in a size range of from about 3nm to about about 20nm; (b) forming core/shell structured nanoparticles by adding shell material to monodispersion; and,(c) stabilizing said structured nanoparticles, as well as stabilized core/shell structured nanoparticles per se. Further included is a method of preparing NiPd and CoPd nanoparticles.

Description

Core/Shell Nanoparticle Synthesis and Catalytic Method

Government Rights

This invention was made with Government support awarded by the U.S.

Department of Energy, Office of Energy Efficiency and Renewable Energy, Fuel Cell Technologies Program. The Government may have certain rights in the invention.

Field of the Invention

Disclosed herein is a synthesis of core/shell Pd/FePt nanoparticles (NPs) and their catalysis, particularly for oxygen reduction reaction (ORR). Usefully a uniform FePt shell is formed by controlled nucleation of Fe(CO)₅ in the presence of Pt salt and Pd NPs at designated reaction temperatures. Without being bound by any particular theory, it is believed that the Pd/FePt NPs exhibit FePt shell-dependent catalytic properties and those with about a 1 nm FePt shell exhibit an increase in durability and activity (15 times more active with a 140 mV gain in onset potential compared to those with the 3 nm coating). These Pd/FePt NPs and their analogues Pd/Au NPs, Pd+Au/FePt and M/FePt (M = NiPd, CoPd ) are useful catalysts for fuel cell applications.

Background of the Invention

Synthesis of active and durable catalysts for oxygen reduction reaction (ORR) is one of the urgent needs in developing fuel cell devices for practical applications.

Reported work has been directed towards the optimization of the existing platinum (Pt) nanoparticle (NP) catalysts, and to the design of new catalysts with lesser or no usage of Pt. In addition to NP morphology control, recent research also focused on fabrication of multi-component metal catalysts to modify Pt electronic structure and to improve Pt catalytic efficiency. Multi-component Platinum based systems have been reported: Pt monolayers supported on Pd (111 ) film surfaces, MPt alloy films (M = Fe, Co, or Ni), and FePt NPs. Pd (111 ) is a crystal phase of Pd typically present in a film. Without being bound by any particular theory, it is believed that these catalysts are more active for ORR than pure Pt counterparts. Without being bound by any particular theory, it is believed that their enhanced activity is attributable to strong interactions between Pt and other metals present as nanoparticles. Such catalysts, however, lack durability in the strong acidic medium of either HCIO₄ or H₂SO₄ for ORR.

An example of fuel cell technology is noted in US Pub 20100081034 to Pak et al, "Supported Catalyst And Method Of Preparing The Same," the teachings of which are incorporated herein by reference in their entirety.

The following references are noted:

1. Lee, H.; Habas, S. E.; Kweskin, S.; Butcher, D.; Somorjai, G. A.; Yang, P. Angew. Chem. Int. Ed., 2006, 45, 7824-7826. (b) Ren, J.; Tilley, R. D. J. Am. Chem. Soc. 2007, 129, 3287-3290. (c) Wang, C; Daimon, H.; Onodera, T.; Koda, T.; Sun, S. Angew. Chem. Int. Ed. 2008, 47, 3588-3591.

2. Lim, B.; Jiang, M.; Camargo, P. H.; Cho, E. C; Tao, J.; Lu, X.; Zhu, Y.; Xia, Y. Science 2009, 324, 1302-1305. (b) Peng, Z.; Yang, H. J. Am. Chem. Soc. 2009, 131, 7542-7543. (c) Lefe^'vre, M.; Proietti, E.; Jaouen, F.; Dodelet, J. Science 2009, 324, 71-75.

3. Zhang, J.; Vukmirovic, M. B.; Xu, Y.; Mavrikakis, M.; Adzic, R. R. Angew. Chem.

Int. Ed. 2005, 44, 2132-2135. (b) Zhou, W.; Yang, X.; Vukmirovic, M. B.; Koel, B. E.;

Jiao, J.; Peng, G.; Mavrikakis, M.; Adzic, R. R. J. Am. Chem. Soc. 2009, 131 , 12755-

12762. (c) Wang, J. X.; Inada, H.; Wu, L; Zhu, Y.; Choi, Y.; Liu, P.; Zhou, W.-P.;

Adzic, R. R. J. Am. Chem. Soc. 2009, 141, 17298-17302.

4. Stamenkovic, V. R.; Mun, B. S.; Arenz, M.; Mayrhofer, K J. J.; Lucas, C. A.;

Wang, G.; Ross, P. N.; Markovic, N. M. Nat. Mater. 2007, 6, 241-247.

5. Kim, J.; Lee, Y.; Sun, S. J. Am. Chem. Soc. 2010, 132, 4996-4997.

6. Mazumder, V.; Sun, S. J. Am. Chem. Soc. 2009, 131, 4588 - 4589.

7. Supporting Information

8. Kirkland, E. J.; Loane, R. F.; Silcox, J. Ultramicroscopy 1987 ^', 23, 77-96. (b)

Nellist, P. D.; Pennycock, S. J. Ultramicroscopy 1999, 78, 111-124.

9. Paz-Borbon, L. O.; Mortimer-Jones, T. V.; Johnston, R. L.; Posada-Amarillas, A.; Barcaro, G.; Fortunelli, A. Phys. Chem. Chem. Phys. 2007, 9, 5202-5208.

10. Handbook of Fuel Cells, Technology and Applications, Edited by W. Vielstich, H.A. Gasteiger, A. Lamm, Vol. 2, John Wiley & Sons, Ltd. (2003) The foregoing and all references cited herein are incorporated by reference in their entirety.

Summary of the Invention

Disclosed herein is a general synthesis of core/shell nanoparticles of

Pd/FePt, Pd+Au/FePt and M/FePt. M shall mean NiPd and CoPd. The

combination of NiPd and CoPd may be either intemingled or alloyed. In particular embodiments core dimensions are from about 5nm to about 10nm in diameter

(particularly from about 7nm to about 8nm), and shell thicknesses is from about

0.5nm to about 3nm (and particularly from about 0.1 nm to 2.5nm).

These measurement are usefully performed by .high resolution transmission electron microscopy.

In one embodiment, the formation of a generally uniform shell is achieved by maintaining monodispersion amd addressing reaction temperature as well as the weight ratio of Pt salt and Pd core nanoparitcles. These core/shell nanoparticles are activated for catalysis by an acetic acid wash.

Reference is made of NiPd and CoPd in ratios of 99:1 to 1 :99 for Ni as to

Pd and Co as to Pd; with particular reference to about 30:70 to 70:30; and more particularly about 40:60 to 60:40 (w/w).

The disclosed core/shell nanoparticles are active and durable catalysts for oxygen reduction reaction. Such activity is useful in low temperature fuel cells. Low temperature as applied to fuel cells shall mean less than about 80°C. In addition, these nanoparticles acts as catalysts for other reactions including carbon monoxide oxidation, reducing the carbon-carbon double bond and alcohol oxidation. Such reactions are typical in low temperature fuel cells and cathode reactions. They are also useful in hydrogen sensing and storage by increasing capacity.

This invention includes a method of preparing stabilized

core/shell structured nanoparticles comprising core material

being at least one member a first metal group, said first metal

group consisting of Pd, Au, NiPd or CoPd and shell material comprising Pt and at least one member of a second metal group, said second metal group consisting of Fe, Ni, Cu comprising the steps of

(a) preparing a monodispersion of said core material wherein said monodispersion is in a size range (diameter) of from about 3nm to about about 20nm;

(b) forming core/shell structured nanoparticles by adding shell material to monodispersion; and,

(c) stabilizing said structured nanoparticles. Note that the term monodispersion references or a sample said to be monodispersed is when it is size consistent by at least about 90% within the mean size. In our case, all samples are within 94% to 98% of the mean size, i.e. in Example 4.A.1 5nm Pd NPs sample has all its constituent nanoparticles between 4.6nm and 5.4nm. These sizes have been measured via High Resolution Transmission Electron Microscopy technique. (This control over size is significant, especially in the shell thickness case: in Example 4.A.2 5/1 nm Pd/FePt NPs, the FePt shell is always within 0.9 to 1.1 nm - a precision that finally controls its catalytic activity in the oxygen reduction reaction.

A sample is also said to be monodispersed when it is composition consistent along with the size consistency mentioned above in a multi-component situation, like a bimetallic alloy. In Example 4.A.8 and 4.8.9, the

monodispersion of NiPd and CoPd NPs, consists of NPs all having a composition of Ni₆oPd o or Co6oPd₄o or any other composition thereof. In particular embodiments of this method the shell material comprises a thickness of from about 0.5nm to about 3nm of the core/shell structured nanoparticles.

Note that references to size range are specific to a given size ± 10% by population. Thus a population of nanoparticles stated to be 3nm in diameter is intended to reference a population substantially with a diameter of 3nm ± 10%. i.e. the population substantially falls between about 2.3 and 3.3nm.

In particular embodiments this method comprises core/shell structured nanoparticles is in a ratio of from about 1.8:1 to about 3:1 (w/w), and particularly about 2:1.

In the present method, particular mention is made of metal the metal the first metal group being NiPd in a ratio of from about 30:70 to about 70:30 (w/w) or CoPd in a ratio of from about 30:70 to about 70:30 (w/w), and particularly as to each in a ration of about 40:60 (w/w)

In specific embodiments the method includes the step of stabilizing said structured nanoparticles comprises exposing said structured nanoparticles to surfactant in a dispersing amount. Noted surfactants are tert-butylamine, oleic acid trioctylphoshpine, oleylamine and octadecene or combinations thereof at about 75°C or higher. In addition, certain

embodiments of the method include heating wherein heat is increased at a rate of at least about 5°C/min to final reaction temperature of at least about 180°C.

In the practice of this method with NiPd or CoPd, it is useful to have begin with NiPd or CoPd, in the form of nanoparticles. The instant invention further encompasses stabilized

core/shell structured nanoparticles comprising core material

being at least one member of a first metal group, said first

metal group consisting of Pd, Au, NiPd or CoPd and shell

material comprising Pt and at least one member of a second

metal group, said second metal group consisting of Fe, Ni, Cu

comprising wherein said nanoparticle is in a size range of about

3 to about 20nm and said shell shell material comprises a

thickness of from about 0.5nm to about 3nm of said core/shell

structured nanoparticles.

Embodiments of nanoparticles presented herein include

the ratio of core material to shell material in said core/shell

structured nanoparticle is from about 1.8:1 to about 3:1 (w/w),

with particular reference to about 2:1. Attention is drawn to

nanoparticles wherein NiPd or CoPd is present in a ratio of

from about 30:70 to about 70:30 (w/w), and more particularly

about 40:60.

Further presented is method of preparing NiPd or CoPd

nanoparticles comprising the steps of converting Ni or Co to an acetate-like salt such as Ni (or

Co) acetate or Ni (or Co) acetylacetonate and converting Pd to

a Pd halogen salt such as fluoride, chloride, bromide or iodide); and heating said salts in the presence of surfactant at a

temperature of at least about 200°C and more particularly at

least about 245°C.

Brief Description of the Drawings

Fig. 1(A) HAADF-STEM, (B) high resolution HAADF-STEM, and (C) elemental mapping images of the 5/1 nm Pd/FePt NPs. The experiments were carried out on an aberration corrected JEOL 2200FS microscope. HAADF-STEM images were acquired with a convergence angle of 27 mrad and an inner collection angle of 100 mrad. EDS analysis was carried out with an electron beam size of ~ 2k.

Fig. 2 EDS point (1-4) analysis of a single 5/1 nm Pd/FePt NP and the corroborating EDS line scan data; the FePt (shell) is restricted to points 1 & 4 and the Pd core is restricted in points 2 & 3 of the NP structure.

Fig. 3 TEM image of the 5 nm Pd NP seeds used for core/shell NP synthesis.

Fig. 4 TEM images of the (A) 5/2 nm Pd/FePt and (B) 5/3 nm Pd/FePt NPs

Fig. 5 (A) Bright field STEM image and (B) HAADF-STEM image of the 5/1 nm Pd/FePt NPs after the acetic acid treatment.

Fig. 6 (A) Displays a polarization curves showing the ORR current for (A) three kinds of Pd/FePt and the commercially available 3.2 nm Pt NP catalyst. The current was normalized against the total mass of NPs used and the electrode rotation rate was kept at 1600 rpm. (B) Comparative ORR activities of the 5/1 nm Pd/FePt NPs before and after 10,000 potential cycles. (C) TEM image of the 5/1 nm Pd/FePt NPs/C catalyst after ORR test and 10,000 potential cycles. (D) HAADF-STEM image of a typical 5/1 nm Pd/FePt NP after 10,000 potential cycles.

Fig. 7 This figure displays the current generated from the ORR reaction catalyzed by 5/1 nm Pd/Fe₅Pt95, 7 nm Fe₂oPteo ("alloy" type NPs have 20% Iron and 80% Platinum (w/w]) and the commercial 3.2 nm Pt NPs standardized by the weight of Platinum in the sample. Table - description of mass and specific activity of the three catalysts.

Fig. 8 TEM images of (a) the 5/2 nm and (b) the 5/3 nm Pd/FePt NPs after the stability test.

Fig. 9 Representative TEM images of commercial Pt/C catalyst after the stability test.

Fig. 10 (A)Representative TEM image of 7/1 nm Ni/Pd NPs and its self assembly through a hexane dispersion (B). Elemental mapping of a singular representative 7/1 nm Ni/Pd NPs (C) Overall EDS spectra of the core/shell Ni/Pd NPs providing the elemental distribution (D)

Fig. 11 Transmission Electron Microscopy images of 8nm (A), 4nm (D) and 14nm (E) CoPd NPs stabilized by trioctylphosphine. High Resolution TEM image of 8nm CoPd NPs (B). Elemental mapping for Co and Pd within 8nm Co₆₀Pd₄o NPs (C). White Scale Bar = 25nm; Black Scale Bar = 2nm. Here, and throughout the application, subscripts such as CoggPd₄o are to ratios w/w.

Fig. 12 (A) Representative low resolution TEM images of 7/1/1 nm Ni/Pd/FePt NPs and their hexane dispersion (B) High Resolution STEM-EDS analysis of this sample with FePt restricted in points 1&4 (C). Representative TEM images of 8nm CoPd NP seeds (D) and the 8/1 nm CoPd/FePt NPs (E).

Fig. 13 Representative (A) TEM, (B) HR-TEM, image of the 5/1 nm Pd/Au NPs; (C) representative HAADF-STEM image and (D) High Resolution-STEM-EDS analysis for a single 5/1 nm Pd/Au NP with Au restricted to points 1&4 and Pd in the core points of 2&3. Note that Pd/Au references a nanoparticle core wherein a Pd element Is coated with Au, but in the practice of this invention, this will include the a shell Pt along with Fe, Ne or Cu.

Fig. 14 Representative (A) TEM and (B) HR-HAADF-STEM image of a single 5/1/2 nm Pd+Au/FePt NP; (C) high resolution line-scan EDS analysis across the NP. The analysis was obtained by scanning 150 points with a beam size of ~lk as determined by image analysis.

Detailed Description of the Invention

This invention will be better understood with reference to the following definitions:

A. "core/shell" in reference to a nanoparticle shall mean core and shell of different compositions in contact. By way of non-limiting example, reference is made to, inorganic/organic, inorganic/inorganic, organic/organic, or inorganic/biological compositions; where the core material is completely and uniformly covered by the shell materials. This specific morphology differs it from any other form where two different materials are in close contact, e.g. alloys. Note that MPt alloy films (M = Fe, Co, or Ni) are not nanoparticles," 7 nm Fe₂oPt8o ("alloy" type NPs have 20% Iron and 80%

Platinum) are present as nanoparticles.

B. "Nanoparticle" shall mean an object behaving like a single unit in terms of both physical and chemical properties within the size range of about 1-100nm. For catalytic applications, the more useful size range is from about 1 to about 10nm, and as to the coating, the 3 nm and 1 nm. Useful particle size in total is about 5-15nm.

C. Measurement of FePt shell stated to be about 1 nm or less shall be understood to be about 1 nm or less as measured by a series of microscopy

experiments (TEM, STEM) on about 40 single nanoparticles amd taking the mean of their diameters. Measurements are usefully corroborated by High Resolution EDS line scans and elemental mapping experiments.

D. "Activity" shall mean the increase in current produced from the oxygen reduction reaction in an acidic electrolyte at 0.9V (vs NHE). The power produced from a fuel cell is the product of potential and current. Activity further implies the maximum current produced at the highest potential, leading to more power being produced by a fuel cell. As applied to the oxygen reduction reaction means the onset potential of the reduction portion of the cyclic voltammetry curve - the earlier the onset potential the faster the reaction occurs. This is generally viewed as "better" catalytically.

E. "Durability" shall mean the non-agglomeration of catalyst nanoparticles after potential cycling between 0.6V and 1.1 V (Standard Hydrogen Electrode) at 35°C for 10,000 Cyclic Voltammetry cycles. A non-durable catalyst nanoparticle will agglomerate leading to a drop in active surface area and the consequent drop in activity, i.e. a drop in onset potential.

Herein we present a useful synthesis of the sub-10 nm Pd/Au NPs having a gold shell with a controlled thickness of 1-2 nm. These Pd/Au NPs can also serve as NP seeds for FePt shell coating to give quaternary Pd+Au/FePt NPs with Au and FePt forming two layers over the Pd NP core. Compared to the Au NP catalyst of similar size for oxygen reduction in 0.5M KOH, the core/shell Pd/Au NPs are much more active, and their activities are dependent on the thickness of the gold shell, with the thinnest (1 nm) shell exhibiting the higher activity.

Disclosed in one embodiment is core/shell structured Pd/FePt NPs with about a 5 nm Pd core and a FePt shell with its thickness tunable from about 1 to about 3 nm. The present invention also presents a method of "tuning" shell thickness. To tune shell thickness is advantageous for this type of core/shell structure - the thinner FePt shell displays higher activity in the order: 1nm>2nm>3nm; with 1nm shell displaying the highest activity. In the case of Pd/Au core/shell NPs the observed activity series with respect to the Au shell is : 0.5nm>1 nm>2nm. The thinnest dimension can be better understood with respect to the measurement of a single atomic layer being in the order of ~0.3nm. It is possible to make shell thickness in any thickness between about 0.5 nm and 5 nm by our reported synthesis.

Without being bound by any particular theory, the ORR catalysis of Pd/FePt is believed to be FePt thickness dependent as well as employing an FePt shell of less than about 0.5nm to about 1 nm. Such catalyst is both active and durable for ORR in

0.1 M HCIO₄ solution. These Pd/FePt NPs are useful for fuel cell applications.

Example 1

Core/Shell Pd/FePt NP Synthesis

Core/shell Pd/FePt NPs were made by the following process. 5 nm Pd NPs were prepared by the reduction of Pd(acac)₂ (acac = aceylacetonate) in the presence of oleylamine and borane f-butylamine at 75°C. The Pd NPs were coated with FePt. 5/1 nm Pd/FePt NPs, Pt(acac)2 (0.15 mmol), oleylamine (6 mmol) and oleic acid (3 mmol) were mixed in 1-octadecene solvent (8 ml_) and heated to 110 °C. The notation "5/1" means that the core diameter is about 5 nm and the shell thickness is about 1 nm.

The Pd seeding NPs (30 mg) in hexane (2 ml_) were then added into the reaction mixture. Iron pentacarbonyl (Fe(CO)s, 0.1 ml_, 0.15 mmol) was injected into the reaction solution that was further heated to 180° C. The reaction mixture was cooled down to room temperature, about 20°C. The product, Pd/FePt NPs, was separated and re-dispersed in 5 ml_ hexane. The Pd/FePt NPs were characterized by transmission electron microscopy (TEM) and aberration-corrected high angle annular dark field scanning transmission electron microscopy (HAADF-STEM). TEM provides the overall size and structural information of various core/shell NPs. Contrast variations in HAADF-STEM images are proportional to the square of the element's atomic number, and provide information regarding the elemental distribution within nanostructures at sub-A resolution. Fig. 1A&B are the HAADF-STEM images of the 5/1 nm Pd/FePt NPs. The high resolution HAADF-STEM image shows the darker contrast in the core and brighter contrast in the shell, indicating that the core contains the lighter elements (Pd) than the shell (Pt). The core sizes and shell thicknesses were measured to be 4.9 ± 0.8 nm/0.9 ± 0.1 nm based on an average measurement from 40 NPs. The compositional architecture of these core/shell NPs was further supported by high resolution energy dispersive spectroscopy (EDS) with an electron probe size of ~2 A at an A-level resolution (Fig.4). A minimum of 18 individual NPs were compositionally analyzed and the compositional distribution within each NP from the same sample was found to be consistent. Figure 1C displays the elemental mapping of a typical 5/1 nm Pd/FePt NP. We can see that the NP contains Pt (green) and Fe (red) in the shell and Pd (blue) in the core. The FePt composition was further quantified by inductively coupled plasma atomic emission spectroscopy (ICP-AES). The FePt shell in the 5/1 nm Pd/FePt NPs has a composition of Pd/Fe₄gPt₅i.

In the Pd/FePt synthesis, the FePt composition was controlled by the molar ratio of Fe(CO)5 and Pt(acac)2 and the FePt shell thickness was tuned by controlling the weight ratio of the Fe-, Pt-precursors and Pd NP seeds. In the condition similar to the synthesis of 5/1 nm Pd/FePt NPs, 25 mg and 20 mg of Pd NP seeds resulted in

Pd/FePt NPs with 2 nm Fe₄₀Pt₆o shell (Fig. 5A) and 3 nm Fe₃₃Pt₆7 shell (Fig. 5B), respectively.

The presence of Fe(CO)s is a significant factor in the uniformity of FePt coating on Pd NPs. The presence of Fe(CO)₅ is significant in nucleation of Pt on the Pd NP seeds. Pt NPs are formed separately and precipitated out from the reaction solution.

Without being bound by any particular theory, it is believed that that under the disclosed synthesis condition, the decomposition of Fe(CO)₅ initiates nucleation and then facilitates the growth of FePt on the Pd NP surfaces. To control FePt growth on Pd, the reaction temperature is usefully kept below 200°C and large amounts (about 0.2ml_) of Fe(CO)₅ are best avoided. Again, without being bound by any particular theory, it is believed that higher temperatures or adding more Fe(CO)s (for example, > 0.5 mmol) promotes the separate nucleation of Fe components. The amount of Pd NP seeds also affects the FePt shell formation. In the presence of 20 mg to 30 mg Pd NPs, FePt nucleates and grows uniformly on the Pd seeds. More Pd NPs (for example 35 mg) would lead to multiple nucleations and the formation of both core/shell Pd/FePt and Pt NPs. In some instances, this is avoided by using more (for example 70 mg) Pt precursor. In the disclosed synthetic condition, the weight ratios of Pt(acac)2 over Pd NPs in the range of about 3:1 to about 2:1 are significant in the formation of continuous FePt shells on Pd NPs.

The controlled FePt coating over the Pd core offers Pd/FePt NPs structural and compositional tunability for ORR catalysis. Example 2

Catalysis Testing

Catalytic was tested by depositing the Pd/FePt NPs on the Ketjen carbon support via sonication of equal amounts of the two constituents in 5 ml_ each of hexane and acetone. The surfactant surrounding each NP was removed by an acetic acid (99%) wash at 70 ^°C. During the acid wash, a significant portion of the Fe was lost, leaving only -5% Fe within the FePt shell in each of the Pd/FePt NPs with 1 , 2, and 3 nm FePt coating. However, Fe leaching did not significantly affect the core/shell morphology of the Pd/Fe₅Pt₉₅ NPs, as confirmed by HAADF-STEM image (Fig. 6). After this acid wash three different kinds of Pd/Fe₅Ptg5 NP catalysts are present with the variation among them being the FePt shell thicknesses.

The Pd/Fe₅Pt₉₅/C samples were re-dispersed in de-ionized water (2 mg/mL) and 20 μΙ_ of this dispersion was deposited on the glassy carbon surface of a rotating disk electrode (RDE) for ORR studies in O₂-saturated 0.1 M HCIO₄ at 308 K.⁷ The current generated from ORR was normalized by dividing the measured raw electrode currents with the mass of NPs (20pg).⁷ Fig. 2A summarizes the observed current of the ORR for the three different kinds of Pd/FePt NPs and the commercial 3.2 nm Pt catalyst (BASF, New Jersey, USA). At the half-wave potential (-0.7 V), the current density generated from the 5/1 nm Pd/FePt NPs is -12 times higher than that from the commercial Pt catalyst (BASF Pt/C fuel cell catalyst). The Pd/FePt NPs with the 1 nm FePt coating are 15 times more active than those with the 3 nm coating and possess a -140 mV more positive ORR onset potential.

Without being bound by any particular theory it is believed that the ORR activity of the Pd/FePt NPs is FePt shell thickness-dependent with thinner FePt shell having higher activity.

Example 3

Durability Testing

The 5/1 nm Pd/FePt NPs were stable in the ORR condition. Durability tests were performed by cycling the potential between 0.4 V and 0.9 V (vs. Ag/AgCI) in the O2- saturated 0.1 M HCIO₄. Fig. 2B shows the ORR activities for the 5/1 nm Pd/FePt NPs before and after 10,000 potential cycles. The overlapped l-V curves indicate that the NPs have no obvious loss in surface area or activity after these potential cycles. TEM and HAADF-STEM analyses further support the view that the 5/1 nm Pd/FePt NPs have no noticeable change in morphology after ORR test and 10,000 potential cycles, and the core/shell structure is maintained (Fig 2C&D).

The high activity of the 5/1 nm Pd/FePt NPs may result from the electronic structure change of Pt upon its alloying with Fe.⁴ We note that the NPs contain only 5% of Fe and pure FePt NPs do not show significant activity enhancement in ORR over the commercial Pt catalyst (Fig. 7). Without being bound by any particular theory it is believed that interfacial interactions between the thin FePt shell and the Pd core dominate the ORR activity increase. The enhanced ORR stability of the 5/1 nm

Pd/Fe₅Pt₉₅ NPs may also arise from the thin FePt coating, as a thin metallic shell can offer thermodynamic stability for a metallic core/shell structure.⁹ As a comparison, under the similar test conditions, the Pd/FePt NPs with thicker FePt coatings, or the commercial Pt catalyst, showed lesser stability with various NP aggregations (Figs. 8/9). Attention is drawn to the uniform FePt shell formed by controlled nucleation of Fe(CO)5 in the presence of Pt salt and Pd NPs at designated reaction temperatures. The Pd/FePt NPs show the FePt shell-dependent catalytic properties and those with the 1 nm coating exhibit increases in durability and activity (15 times more active with a 140 mV gain in onset potential compared to those with the 3 nm coating). The multimetallic core/shell NPs reported here have several distinct advantages over any of the previously reported NP systems: the catalytically active shell is deposited uniformly on the core surface and is readily activated; the shell thickness is controlled and core/shell interactions are tuned to optimize catalytic performance; the synthesis is versatile and allows different metals to be incorporated into either the core or the shell structure, offering a rich variety of core/shell NPs for catalytic applications.

Example 4

Nanoparticle Synthesis Examples

A synthesis was carried out using standard airless procedures and commercially available reagents. Oleylamine (>70%), oleic acid, platinum(ll) acetylacetonate, palladium(ll) acetylacetonate, iron pentacarbonyl, acetic acid, borane i-butylamine complex and Nafion solution were all purchased from Sigma Aldrich.

A.1. Synthesis of 5 nm Pd NPs

0.25 mmol Pd(acac)₂ (acac = acetylacetonate) was mixed with 20 ml_ of oleylamine and heated to 60 °C under a constant nitrogen flow. Upon observation of complete dissolution of the Pd precursor, the solution of 3 mmol of borane i-butylamine in 2 ml_ of oleylamine was injected into the Pd(acac)₂ solution and the reaction mixture was heated to 75°C under a nitrogen blanket. After 1 hour, the reaction mixture was cooled down to room temperature and 40 mL of ethanol was added to precipitate out the product. The product was separated by centrifugation at 8000 rpm for 8 minutes. The 5 nm Pd NPs were dispersed in hexane.

A.2. Synthesis of 5/1 nm Pd/FePt NPs Pt(acac)₂ (0.15 mmol), oleylamine (6 mmol) and oleic acid (3 mmol) were first mixed in 1 -octadecene solvent ( 8 mL) and heated to 110 °C under a constant nitrogen flow to ensure the dissolution of Pt(acac)₂ and the removal of the moisture/air trapped in the reaction system. The Pd seeding NPs (30 mg) in hexane (2 mL) were then added into the reaction mixture. The low boiling hexane was evaporated at 1 10 °C. Under a nitrogen blanket, iron pentacarbonyl (Fe(CO)5, (0.1 mL, 0.15 mmol) was injected into the reaction solution that was further heated to 180 °C and kept at this temperature for 20 minutes. The reaction mixture was cooled down to room temperature (20°C) and isoproponol (40 mL) was added. The suspension was centrifuged at 6000 rpm and the solid product was separated. The product was dispersed in hexane (30 mL) and precipitated out by adding 40 mL ethanol. The final product, 5/1 nm Pd/FePt NPs, was dispersed in 5 mL hexane for further use.

A.3. Synthesis of 5/3 nm Pd/FePt NPs

60 mg of Pt(acac)₂ was mixed with 8 mL of 1-octadecene, 2 mL of oleylamine, 1 mL of oleic acid. This solution was heated to 1 10 °C at a rate of 6-7 °C/min, and 20 mg of Pd nanoparticles dispersed in hexane was injected into the reaction mixture. The reaction mixture was heated to 125 °C and 0.01 mL of Fe(CO)s was added under a nitrogen blanket. The mixture was heated at the same rate to 180°C and was kept at this temperature for 20 minutes. The mixture was cooled down to room temperature (20°C). Isopropanol (40 mL) was added to precipitate out the product. The NPs were separated as described in A.2 and dispersed in hexane.

In the same reaction condition, increasing the Pd seed amount to 25 mg gave 5 nm/2 nm Pd/FePt and further to 35 mg resulted in 5 nm/1 nm Pd/FePt NPs.

A.4. Synthesis of 7 nm FePt NPs

Pt(acac)₂ (0.5 mmol), oleic acid (4 mmol), and oleylamine (4 mmol) were mixed with 10 mL of 1 -octadecene in an argon atmosphere. The mixture was heated to 120 °C at a heating rate of 6-7 °C/min. The flask was maintained at this temperature for 10 minutes to ensure the dissolution of Pt(acac)₂. Under a blanket of argon gas, 0.20 mL of Fe(CO)₅ was added. The solution was then heated to 240°C at a heating rate of 5 °C/min, and kept at this temperature for 1 h. The heating source was then removed, and the solution was cooled to room temperature (20°C). A black product was precipitated by adding 40 mL of isoproponol, and separated as described in A.3 and dispersed in hexane.

ICP analysis revealed the composition of the as-synthesized NPs to be FesoPtso-

Alloy composition was changeable to Fe₄₀Pt6o and Fe₅₈Pt42 by using the 0.14 mL and 0.28 mL of Fe(CO)₅ respectively.

A.5. Synthesis of 5/1 nm Pd/Au NPs

HAuCI₄-3H₂O (0.2 mmol) was dissolved in 1 -octadecene (8 mL) and

oleylamine(2 mL). The resultant orange solution was slowly heated to 808C, and the Pd NP seeds (20 mg) dispersed in hexanes (2 mL) were injected into the solution. The reaction mixture was kept at the same temperature for 2 h and then cooled down to room temperature. Isopropanol (40 mL) was added and the suspension was centrifuged at 8000 rpm to separate the solid product. The product was dispersed in hexane (30 mL) and precipitated out by adding ethanol (40 mL). The final product, 5/1 nm Pd/Au NPs, was dispersed in hexanes (5 mL) for further use. By tuning the amount of Au precursor used, Pd/Au NPs with different Au shell thickness were synthesized: the 1.5 nm Au shell was obtained from 0.25 mmol of Au precursor, and the 2 nm shell was formed from 0.3 mmol of Au precursor.

A.6. Synthesis of 5/1/2 nm Pd+Au/FePt NPs

Pt(acac)2 (0.233 mmol) was mixed with 1 -octadecene (8 mL), oleylamine (2 mL), and oleic acid (1 mL). This solution was heated to 120°C at a rate of 6-7 °C/min, and 5/1 nm Pd/Au NPs (20 mg) dispersed in hexane (2 mL) was injected into the reaction mixture. The reaction mixture was heated to125°C and iron pentacarbonyl [Fe(CO)5] (0.07 mL) was added under a nitrogen blanket. The mixture was heated at 5°C/min to 2008C and was kept at this temperature for 20 min. The mixture was cooled down to room temperature. Isopropanol (40 mL) was added to precipitate out the product. The NPs were separated as described in A.5. and dispersed in hexanes. A.7. Synthesis of 9nm NiPd NPs

0.2 mmoi each of Ni(ac)₂ (ac = acetate) and PdBr₂ were mixed with oleyamine (18ml_) and trioctylphosphine (0.5mL). This solution was heated to 245°C at 5°C/min under nitrogen flow and kept at that temperature for 1 hr under a nitrogen blanket. The mixture was cooled down to room temperature (20°C). Isopropanol (40 mL) was added to precipitate out the product. The NPs were separated as described in A.5 and dispersed in hexane. ICP analysis revealed the composition of the as-synthesized NPs to be Ni₆₀Pd o.

A.8. Synthesis of 8nm CoPd NPs

0.4 mmol of Co(acac)2 (acac = acetylacetonate) and 0.3 mmol of PdBr₂ were mixed with oleyamine (18mL) and trioctylphosphine (0.5ml_). This solution was heated to 260°C at 5°C/min under nitrogen flow under vigorous magnetic stirring. The reaction mixture was kept at that temperature for 1 hr under a constant nitrogen blanket. The mixture was cooled down to room temperature (20°C). Isopropanol (40 mL) was added to precipitate out the product. The NPs were separated as described in A.5 and dispersed in hexane. ICP analysis revealed the composition of the as-synthesized NPs to be Co₆oPd₄o.

A.9. Synthesis of 9/1 nm NiPd/FePt and 8/1 nm CoPd/FePt NPs

The protocol of A.6 yields the core/shell NPs wherein 9/1 nm NiPd/FePt and 8/1 nm CoPd are substituted for 5/1/2 nm Pd+Au/FePt NPs. Importantly, these particles a in the cataclyic size range being about 10nm.

Example 5

Preparations of Core/Shell NPs for Electrocatalysis

B.1. Surfactant Removal from the Core/Shell NPs

An equal amount (10 mg each) of Pd/FePt NPs and Ketjen carbon support were mixed in 10 mL of hexane/acetone (v/v 1/1 ) and sonicated with a Fischer Scientific FS 110 for 60 minutes. The colorless solvent was decanted and 20 mL of acetic acid was added. The suspension was heated to 70 C for 10 hours. The NPs on the carbon support were separated by centrifugation and re-dispersed in de-ionized water to form a 2 mg/mL suspension. ICP analysis showed that the NPs had -5% Fe remaining for all the core/shell Pd/FePt NPs treated in the same way. Moreover, the Pd/FePt NPs had 30% (1 nm FePt), 34% (2 nm FePt) and 38% (3 nm FePt) Pt content (by weight). A 20 μΐ_ of this dispersion was used for catalytic studies.

The acetic acid treatment described above was also carried out for the FePt NPs.

After the treatment, three FePt NPs synthesized in A.4 became Fe₂5Pt75, Fe₂2Pt78 and Fe2oPtao-

B.2. Deposition of NP Catalyst on the Working Electrode

20 pL of the 50% NPs/C (40 Cg) in water prepared in B.1 was dropped on a rotation disk electrode (RDE) with a glassy carbon surface (5 mm in diameter from Hokuto Denko Corp., Japan). After water evaporation, 10 μΙ of 0.1 wt% Nafion® solution was dropped onto the NPs to ensure the tight binding of the catalyst to the electrode surface.

B.3. Electrochemical Measurements

The electrochemical measurements were performed on a Pine Electrochemical Analyzer, Model AFCBP1 , by typical cyclic voltammetry (CV) technique. Ag/AgCI and Pt wire were used as reference and counter electrodes respectively.

Oxygen reduction reaction (ORR) catalyzed by the NP catalysts was evaluated in the O₂-saturated 0.1 M HCIO₄ at 308 K. The RDE rotation speed was controlled from 1225 - 2500 rpm and the scan rate was at10 mV/s in the 1.0V to -0.2 V region scanned. The mass current (mA/mg) originating from ORR was standardized by dividing the measured electrode currents with the mass of the total metal content of the 50% NP/C catalyst (20pg).

Example 6

Sample Analysis

Samples for TEM analysis were prepared by depositing a single drop of diluted NP dispersion in hexane on amorphous carbon coated copper grids. Images were obtained by a Philips EM 420 (120 kV). HRTEM image was obtained on a JEOL 2010 TEM (200 kV). XRD patterns were obtained on a Bruker AXS D8-Advanced

diffractometer with Cu Ka radiation (λ = .54 8 A). The ICP measurements were carried on a JY2000 Ultrace ICP Atomic Emission Spectrometer equipped with a JY AS 421 autosampler and 2400g/mm holographic grating.

High resolution STEM and EDS analysis were carried out on an aberration corrected JEOL 2200FS microscope. HAADF-STEM images were acquired with a convergence angle of 27 mrad an inner collection angle of 100 mrad. Bright field STEM (BF-STEM) images were recorded simultaneously with HAADF-STEM images to get complete information of the microstructures. EDS analysis was carried out with an electron beam size of ~lk. Before conducting any measurements, the NP samples were tested against possible electron beam induced damage, and no damage was observed under the STEM operational conditions.

Claims

1. A method of preparing stabilized core/shell structured nanoparticles comprising core material being at least one member a first metal group, said first metal group consisting of Pd, Au, NiPd or CoPd and shell material comprising Pt and at least one member of a second metal group, said second metal group consisting of Fe, Ni, Cu comprising the steps of

(a) preparing a monodispersion of said core material wherin said monodispersion is in a size range of from

about 3nm to about about 20nm;

(c) stabilizing said structured nanoparticles.

2. The method of Claim 1 wherein said shell material comprises a thickness of from about 0.5nm to about 3nm of said core/shell structured nanoparticles.

3. The method of Claim 1 wherein the ratio of core material to shell material in said core/shell structured nanoparticles is in a ratio of from about 1.8:1 to about 3:1 (w/w).

4. The method of Claim 3 wherein said ratio is about 2:1.

5. The method of Claim 1 wherein said metal of said first metal group is Pd.

6. The method of Claim 1 wherein said metal of said first metal group is Au.

7. The method of Claim 1 wherein said metal of said first metal group is NiPd.

8. The method of Claim 7 wherein said NiPd is in a ratio of from about 30:70 to about 70:30 (w/w).

9. The method of Claim 1 wherein said metal of said first metal group is CoPd.

10. The method of Claim 7 wherein said CoPd is in a ratio of from about 30:70 to about 70:30 (w/w)

11. The method of Claim 1 wherein the step of stabilizing said structured nanoparticles comprises exposing said structured nanoparticles to surfactant, tert- butylamine at about 75°C or higher.

12. The method of Claim 11 wherein said surfactant oleylamine present at a dispersing amount; said tert-butylamine is present at a reducing amount; and, said heating is increased at a rate of at least about

5°C/min to final reaction temperature of at least about 180°C.

13. The Method of Claim 12 wherein said surfactant further comprises a co-surfactant selected from the group comprising oleic acid trioctylphoshpine, oleylamine and octadecene.

14. The method of claim 1 wherein said NiPd or CoPd is in the form of nanoparticles.

15. A stabilized core/shell structured nanoparticle

comprising core material being at least one member of a first metal group, said first metal group consisting of Pd, Au, NiPd CoPd and shell material comprising Pt and at least one member of a second metal group, said second metal group consisting of Fe, Ni, Cu comprising wherein said nanoparticle is in a size range of about 3 to about 20nm and said shell shell material comprises a thickness of from about 0.5nm to about 3nm of said core/shell structured nanoparticles.

16. The nanoparticle of Claim 15 wherein the ratio of core material to shell material in said core/shell structured

nanoparticle is from about 1.8:1 to about 3:1 (w/w).

17. The nanoparticle of Claim 16 wherein said ratio is about 2:1.

18. The nanoparticle of Claim 15 wherein said metal of said first metal group is Pd.

19. The nanoparticle of Claim 15 wherein said metal of said first metal group is Au.

20. The nanoparticle of Claim 15 wherein said metal of said first metal group is NiPd.

21. The nanoparticle of Claim 20 wherein said NiPd is in a ratio of from about 30:70 to about 70:30 (w/w).

22. The nanoparticle of Claim 15 wherein said metal of said first metal group is CoPd.

23. The nanoparticle of Claim 22 wherein said CoPd is in a ratio of from about 30:70 to about 70:30 (w/w)

24. A method of preparing NiPd nanoparticles comprising the steps of converting Ni to an acetate-like salt and converting Pd to a Pd halogen salt; and heating said salts in the presence of surfactant at a temperature of at least about 245°C.

25. The method of Claim 24 wherein said Ni acetate-like salt is Ni acetate or Ni acetylacetonate and said Pd halogen salt is Pd bromide.

26. A method of preparing CoPd nanoparticles comprising the steps of converting Co to an acetate-like salt and converting Pd to a Pd halogen salt; and heating said salts in the presence of surfactant at a temperature of at least about 245°C.

27. The method of Claim 26 wherein said Co acetate-like salt is Co acetate or Co acetylacetonate and said Pd halogen salt is

Pd bromide.