Direct Colloidal Route for Pt-Covered AuPt Bimetallic Nanoparticles

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Direct Colloidal Route for Pt-Covered AuPt Bimetallic Nanoparticles Zhichuan Xu,† Christopher E. Carlton,† Lawrence F. Allard,§ Yang Shao-Horn,*,† and Kimberly Hamad-Schifferli*,†,‡ †

Department of Mechanical Engineering, and ‡Department of Biological Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, and §Materials Science & Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831

ABSTRACT Pt-covered AuPt bimetallic nanoparticles were synthesized in a polar organic environment using HAuCl4 and H2PtCl6 as precursors. Electrochemical and UV-vis measurements showed that these AuPt nanoparticles have Pt-rich surfaces and their effective electrochemical Pt coverage is dependent on the Pt nominal composition. By mixing the two precursors in one pot, the interaction of two precursors lowered the temperature for Pt reduction and further favored the formation of a smooth Pt shell on the particle surface. The synthesis is a unique approach for controlling the surface electrochemical properties of bimetallic nanoparticles via varying particle nominal compositions. SECTION Nanoparticles and Nanostructures

t-based bimetallic nanoparticles (NPs) have gained great interest in catalysis due to their potential to improve overall efficiency.1 Recently, significant progress has been made in the synthesis of various Pt-based alloy/bimetallic NPs in terms of control over particle shape,2 size,3 composition,4 and structure.5-10 In order to reduce Pt mass loading while maintaining an active Pt surface, a coreshell structured NP with a Pt skin is highly desirable. Coreshell NPs are expected to possess unique electronic properties and reactivities which can be influenced by the core material.11 Consequently, synthesis of core-shell particles has been attempted by using different core NPs, such as Pd, Co, Au, and others.6-10 Compared to PdPt and CoPt, AuPt coreshell NPs are difficult to make. The typical colloidal route is the reduction of the shell precursor in the presence of core NPs (seeds). Unlike these other alloys, Au metal is not miscible with Pt metal in the bulk phase, making the synthesis of AuPt particles a significant challenge. Pt coatings on Au often face challenges from crystal lattice mismatch, incompatible core surface chemistry, and homogeneous nucleation. Consequently, most reports on Pt-coated Au NPs to date have dendritic Pt shells or Pt islands on the core particles.7,12 While certain applications of bimetallic particles may be enhanced by dendritic Pt coatings because of the higher Pt surface area, dendrites prevent systematic tuning of the particle surface composition as well as shell thickness. Other approaches such as the underpotential deposition (UPD)10 involve electrochemical deposition of a Cu layer onto the surface of core metal Au particles on a substrate, followed by replacement of Cu by Pt ions. While UPD is successful in making a Pt skin over NPs of various metals including Au, it does not have the benefits of wet chemical methods such as tunability of composition, particle size, and shape.

P

r 2010 American Chemical Society

In this Letter, we demonstrate a simple synthesis of bimetallic AuPt NPs with a smooth Pt skin as well as controllable Pt surface coverage. The synthesis employs HAuCl4 and H2PtCl6 as precursors and oleylamine as the solvent, stabilizer, and reducing reagent.13,14 Since oleylamine is amphiphilic, it provides a polarized solvent environment and thus further allows the interaction of the two metal precursors to lower the thermal energy (temperature) of Pt reduction for a smooth Pt coating. Such an interaction-favored metal reduction has not been found in any other one-pot synthesis of core-shell NPs.15 In a typical synthesis, HAuCl4 and H2PtCl6 in a total amount of 0.5 mmol were dissolved in 20 mL of oleylamine at 40 °C under Ar and then heated to 160 °C at a rate of 2 °C/min. The solution color changed from reddish yellow to pale yellow at 50-70 °C and then changed to dark purple at 80-90 °C. Further increasing the temperature resulted in a dark brown solution. The reaction temperature was maintained at 160 °C for 2 h to give final products. The Au/Pt composition in the NPs was controlled by varying the precursor ratio. TEM imaging (representative Figure 1, additional images are in Figure S1, Supporting Information) revealed that assynthesized AuPt NPs are monodisperse with a size of ∼7 nm. ICP of the particles purified from the unreacted reagents showed that the AuPt NP composition is directly associated with the precursor feeding ratio, further confirmed by energy dispersive X-ray spectroscopy (EDX) analysis on individual particles (Figure 1B). XRD patterns of Au3Pt1, Au2Pt1, Au1Pt1, Au1Pt2, and Au1Pt3 NPs show typical fcc crystal phases with Received Date: July 1, 2010 Accepted Date: July 29, 2010 Published on Web Date: August 06, 2010

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DOI: 10.1021/jz100896p |J. Phys. Chem. Lett. 2010, 1, 2514–2518

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Figure 1. (A) A representative TEM image of as-synthesized Au1Pt1 NPs and (B) the particle composition determined by ICP (red circles) and single-particle EDX (green triangles) as the precursor feeding ratio was varied as 3:1, 2:1, 1:1, 1:2, and 1:3 (Au/Pt). The plot shows that NP composition is directly associated with the precursor feeding ratio.

Figure 2. (A) XRD patterns and (B) UV-vis spectra of AuPt NPs with different nominal compositions.

diffraction peak positions between Au (green) and Pt (red) peaks (Figure 2A). It is known that bulk Au and Pt are immiscible below 500 °C,16 making the synthesis of AuPt bimetallic NPs challenging. However, recent studies indicate that AuPt may form a well-mixed solid solution at a size range of 2-3 nm,17,18 consistent with theoretical predictions that find that the formation energy of well-mixed AuPt crystals below 5.8 nm is negative.16 The XRD peaks of AuPt NPs with different compositions shift with atomic Au/Pt ratio, indicative of a Pt and Au solid solution, which is consistent with what has been observed in the XRD analysis on AuPt alloy NPs.17,18 The UV-vis absorption spectra of NPs with atomic Au/Pt ratios e 2:1 lacked a surface plasma resonance (SPR) of Au (Figure 2B), where only Au3Pt NPs showed a very weak SPR (purple). These results suggest that the NP surfaces are enriched in Pt, which shields the Au SPR.19,20 Since cyclic voltammetry (CV) is a powerful tool for monitoring surface properties,10,21,22 surface Pt enrichment of the AuPt NPs was further confirmed by CV measurements. AuPt NPs were loaded onto carbon (Vulcan CX-72) and then washed with isopropanol to remove the oleylamine surfactant. CV curves were recorded in Ar-saturated 0.1 M HClO4 from 0.04 to 1.7 V (vs RHE). AuPt NPs with nominal atomic Au/Pt ratios e2:1 showed a predominantly Pt surface, with features associated with hydrogen adsorption/desorption (0.04-0.4 V) and reduction of Pt oxides (0.5-0.8 V). Reduction of Au oxides at 1.1-1.5 V were barely visible for Au1Pt1 (Figure 3C, dashed) but became apparent for Au3Pt1 (Figure 3A), which can be attributed to reduction of oxides of surface and sublayer


Figure 3. Cyclic voltammograms of AuPt NPs with different nominal compositions, (A) Au3Pt1, (B) Au2Pt1, (C) Au1Pt1, (D) Au1Pt2, and (E) Au1Pt3. (F) Pt surface coverage versus Pt nominal atomic composition based on CV.

Au upon scanning to 1.7 V.21,22 Since the electrochemical quantification method of the surface area of Au and Pt has been well-established on bulk surfaces,21,22 the effective electrochemical surface areas of Au and Pt on the AuPt NPs were obtained by integrating the charge-associated area with the reduction of Au oxides and the hydrogen adsorption/desorption on Pt, obtaining the surface fraction of Pt. The Pt surface atomic fraction was plotted against the NP nominal composition (Figure 3F), which shows that the effective electrochemical coverage can reach up to ∼90% as long as the nominal composition is >33% Pt. Electrochemical and UV-vis results show that AuPt NPs are covered with Pt and that Pt coverage can be fine-tuned by varying the NP nominal composition. Estimated Pt shell thicknesses are ∼0.33-1.16 nm for 3:1 to 1:3 Au/Pt (Supporting Information), which is supported by EDX chemical analysis across individual particles in the TEM (Figure S3, Supporting Information). To understand the formation process of the Pt-covered AuPt NPs, a set of intermediate products at different temperature stages in the synthesis of Au1Pt1 was collected during the reaction. UV-vis spectra of the intermediates were collected at 80, 100, 120, 140, 160, and 180 °C, which are at increasing time points in the reaction (Figure 4A). The SPR could be seen at 80, 100, and 120 °C. As the reaction proceeded with increasing temperature, the SPR decreased and

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finally disappeared at 140 °C (pink). ICP (Figure 4A, inset) shows that the NPs collected at 80 and 100 °C were almost pure Au. NPs collected at higher temperatures (and later times in the reaction) were composed of both Au and Pt. From 120 to 180 °C, the Pt amount increased from atomic at 20-50%, reaching saturation at 50%. Thus, these results indicate that first, when the reaction temperature is low, Au NPs were produced through the reduction of a partial Au precursor, followed by Pt and Au deposition to form a PtAu core at higher temperatures. Increasing the temperature favored the production of the Pt shell. Separating the synthesis into two steps where Au NPs (Figure 4B, dashed) are synthesized first and then are added to a Pt precursor that is reduced in their presence does not produce a uniform Pt coating but Pt dendrites, as evidenced by the presence of the SPR (solid). Instead, Pt dendrites on Au are formed (TEM, inset), which is typically produced via the seedmediated method for coating Pt onto a metal surface.7,12 Further, by tracking the evolution of UV-vis spectra during the synthesis of Au@Pt dendrite NPs (Figure S4, Supporting Information), it was found that the surface plasmon absorption of Au NPs remained unchanged until a higher thermal energy level was reached (160 °C). This is in contrast to what has been observed in the synthesis of Pt-covered AuPt NPs, in which the surface plasmon absorption of Au was already completely blocked by the Pt shell as early as 140 °C (Figure 4A). This control experiment excluded the possibility that Au NPs catalyze the reduction of Pt at the lower temperature. HAuCl4 or H2PtCl6 was reacted with oleylamine alone to determine the temperatures at which they are reduced. HAuCl4 could be reduced at 60-70 °C, while H2PtCl6 alone could not be reduced until 220 °C. In the synthesis of Ptcovered AuPt, the temperature stages of Au and Pt reduction were found at 80 and 120 °C, respectively. These experiments indicate that there must be electron transfer between the two

precursors at low temperatures, which allows Pt reduction to occur at 120 °C instead of 220 °C, and probably is responsible for the uniform Pt shell. It is known that oleylamine can reduce Au(III)Cl4- to Au(I)Cl2-, resulting in a color change of the solution from reddish yellow to light yellow.23,24 This color change was observed at 40-50 °C in the control experiment with AuCl4- alone. A similar color change from reddish yellow to colorless was also observed in the control experiment with PtCl62- alone at ∼200 °C, implying that there is probably a similar reduction process from Pt(IV)Cl62- to Pt(II)Cl42-.25,26 Furthermore, oleylamine can also reduce Au(I)Cl2- to Au0.23,24 In the synthesis of Pt-covered AuPt NPs, the color change from reddish yellow to light yellow was observed earlier in the reaction when the temperature was in the range of 50-70 °C, indicating that Pt(IV)Cl62- was reduced to Pt(II)Cl42-. This is at a much lower temperature than for Pt(IV)Cl62- in oleylamine alone; therefore, Pt(IV) reduction was probably aided by oxidation of Au(I)Cl2- to yield Au(III)Cl4- and Pt(II)Cl42-. Au(III)Cl4- can be immediately reduced to Au(I)Cl2by oleylamine (Scheme 1). As the reaction temperature was increased to the range of 80 °C, Au(I)Cl2- was reduced to form Au0 particles first by oleylamine23 (Scheme 1, green) as Au(I) has a higher reduction potential than Pt(II) (1.83 V versus 1.19 V).27 As the reaction proceeded to higher temperature (>100 °C), Pt(II)Cl42- could be also reduced, starting to yield Pt, which probably led to a transition region with the solid solution of AuPt between the center and edge of the NPs and finally produced a predominantly Au core and a predominantly Pt shell (Scheme 1). This is supported by the elemental analysis by the ICP results (Figure 4A, inset), where the composition of the particles initially is mostly Au and then changes to AuPt. In addition, the UV-vis spectra as a function of time support formation of a Au-dominant particle initially, as indicated by the presence of the surface plasmon resonance, while the growth of a Pt-dominated shell is supported by the diminishing of the SPR as the reaction proceeds (Figure 4A). In summary, we have demonstrated a simple colloidal route for Pt-coated AuPt bimetallic NPs. The synthesis enables tuning the electrochemical surface properties of bimetallic AuPt NPs and further indicates that in the solution-based synthetic method, interaction between precursors can significantly favor the synthesis toward the desired structures. This route could enable simpler approaches to core-shell particles of other materials.

EXPERIMENTAL SECTION Figure 4. (A) UV-vis spectra of samples collected at different temperature stages. The inset is the corresponding compositions. (B) The UV-vis spectra and TEM images (inset) of Au NPs and Au@Pt dendrites. The scale bar is 10 nm.

Synthesis of Pt-Covered AuPt Bimetallic NPs. The Pt-covered AuPt NPs were synthesized by reducing HAuCl4 and H2PtCl6 in oleylamine. In a typical synthesis of Au1Pt1, 0.25 mmol of

Scheme 1. Proposed Formation Process of AuPt NPs


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HAuCl4 and 0.25 mmol of H2PtCl6 were dissolved in 20 mL of oleylamine at 40 °C under an Ar blanket. The yellow clear solution was then slowly heated up to 160 °C with a heating rate of 2 K/min. The solution color changed from yellow to colorless at 50-70 °C and then changed from colorless to dark purple at 80-90 °C. Further increasing of the temperature resulted in a dark brown solution. The reaction temperature was maintained at 160 °C for 2 h and then cooled down to room temperature. The NPs were collected by adding 100 mL of ethanol, followed by centrifugation. The as-prepared NPs were dispersed well in a nonpolar solvent such as hexane and toluene. Electrochemical Measurements. The AuPt NPs were loaded onto carbon Vulcan CX-72 with a metal loading of 40 wt % and then washed with isopropanol three times to remove excess surfactant. So-prepared catalysts were dispersed in deionized water to give a catalyst ink solution of 1 mg/mL. The 10 μL ink solution was dropped onto a glass carbon electrode (GCE, 5 mm in diameter) and then dried slowly in a water vapor to form a flat thin film fully covering the surface of GCE. The electrochemical measurements were performed on a Pine Instrument. A Pt wire and a saturated calomel electrode (SCE, Analytical Sensor, Inc.) were used as the counter electrode and the reference electrode, respectively. The cyclic voltammograms of the AuPt NPs were recorded in Ar-saturated 0.1 M perchloric acid (HClO4) with a scan rate of 50 mV/s at the potential window of ∼0.04-1.7 V (vs RHE). Other Characterizations. The basic TEM imaging was carried out on a JEOL 200CX transmission electron microscope at 200 kV. STEM-EDS analysis on individual particles was carried out on a JEOL 2010F microscope. High resolution images and X-ray elemental distribution maps were acquired using a probe-corrected JEOL 2200FS TEM/STEM, available through the Oak Ridge National Laboratory's High Temperature Materials Laboratory user program. This microscope was chosen for its ability to generate a 0.2 nm probe with very high current, allowing high resolution EDS maps to be generated. The UV-vis absorption spectra were recorded by a Cary 100 UV-vis spectrometer. The X-ray diffraction pattern was collected on a PANalytical X'pert Pro with Cu KR radiation (λ =1.5418 Å). The NPs and the intermediate products were collected and washed with hexane and ethanol three times by centrifugation to separate them from unreacted reagents. The powders were then dissolved in aqua regia under boiling conditions. The resulting solutions were diluted with deionized water for ICP analysis.

ACKNOWLEDGMENT This work was supported by the MRSEC Program of the National Science Foundation under Award Number DMR-0819762. The authors thank S. Chen for TEM/EDS, W. Sheng and J. Kim for fruitful discussion, H. A. Gasteiger and J. Suntivich for establishing the electrochemical method to estimate surface Pt coverage of AuPt NPs. This research at the Oak Ridge National Laboratory's High Temperature Materials Laboratory was sponsored by the U. S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Vehicle Technologies Program.

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SUPPORTING INFORMATION AVAILABLE

TEM images and EDX spectra of AuPt bimetallic NPs. The methods for electrochemical surface area calculation and shell thickness estimation. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION Corresponding Author:

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*To whom correspondence should be addressed. Email: shaohorn@ mit.edu (Y.S.-H.); [email protected] (K.H-S.).


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Direct Colloidal Route for Pt-Covered AuPt Bimetallic Nanoparticles

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