Synthesis and Characterization of Au@Pt Nanoparticles with Ultrathin

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Synthesis and Characterization of Au@Pt Nanoparticles with Ultrathin Platinum Overlayers Ipshita Banerjee, V Kumaran, and Venugopal Santhanam J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 02 Mar 2015 Downloaded from http://pubs.acs.org on March 3, 2015

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The Journal of Physical Chemistry

Synthesis and Characterization of Au@Pt Nanoparticles with Ultrathin Platinum Overlayers Ipshita Banerjee†, V Kumaran* and Venugopal Santhanam* Department of Chemical Engineering, Indian Institute of Science, Bangalore, India-560012

ACS Paragon Plus Environment

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KEYWORDS Au@Pt nanoparticles, Cyclic voltammetry, Platinum overlayers, Room-temperature synthesis, Self-Assembly. ABSTRACT Gold-core platinum-shell (Au@Pt) nanoparticles with ultrathin platinum overlayers, ranging from sub-monolayer to two monolayers of platinum atoms, were prepared at room-temperature using a scalable, wet-chemical synthesis route. The synthesis involved the reduction of chloroauric acid with tannic acid to form 5 nm (nominal dia.) gold nanoparticles followed by addition of desired amount of chloroplatinic acid and hydrazine to form platinum overlayers with bulk Pt/Au atomic ratios (Pt surface coverages) corresponding to 0.19 (half monolayer), 0.39 (monolayer), 0.58 (1.5 monolayer) and 0.88 (2 monolayers). The colloidal particles were coated with octadecanethiol and phase-transferred into chlroform-hexane mixture to facilitate sample preparation for structural characterization. The structure of the resultant nanoparticles were determined to be Au@Pt using HRTEM, SAED, XPS, UV-Vis and confirmed by Cyclic Voltammetry (CV) studies. Monolayers of octadecanethiol coated Au@Pt nanoparticles were self-assembled at an air-water interface and transfer printed twice onto a gold substrate to form bilayer films for electrochemical characterization. Electrochemical activity on such films was observed only after the removal of the octadecanethiol ligand coating the nanoparticles, using a RF plasma etching process. The electrochemical activity (HOR, MOR studies) of Au@Pt nanoparticles was found to be highest for particles having a two atom thick platinum overlayer. These nanoparticles can significantly enhance platinum utilization in electrocatalytic applications as their platinum content based activity was three times higher than pure platinum nanoparticles.


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INTRODUCTION Nanoparticles with reduced platinum loadings are sought after for electrocatalytic applications in varied fields related to energy and environment1. For instance, polymer electrolyte (or proton exchange) membrane fuel cells (PEMFC) and direct methanol fuel cells (DMFC) are efficient, high power density, electrochemical energy devices, wherein platinum nanoparticles are used to catalyze the oxygen reduction reaction (ORR) occurring at cathode, and hydrogen oxidation reaction (HOR)/ methanol oxidation reaction (MOR) occurring at anode in PEMFC/ DMFC fuel cells, respectively. Recently, bimetallic nanoparticles have attracted interest not only as vehicles for maximizing precious metal utilization2,3,4,5, but also because of enhanced electrocatalytic activity, which is attributed to structural and electronic interactions of the constituent metals6,7,8,9. Typically, bimetallic nanoparticles are comprised of a precious, catalytically-active metal like platinum, palladium and a lower-cost, stable/noble metal like gold, ruthenium, cobalt, iron etc. Bimetallic nanoparticles can be broadly classified as alloy and core-shell nanoparticles. The performance of alloy nanoparticles decays over time, due to loss of electroactive surface area caused by particle agglomeration or by leaching of the alloying non-precious metal to the surrounding electrolyte10. Core-shell gold-platinum (Au@Pt) nanoparticles are stable over long time periods11, and are typically synthesized using lab-scale, wet-chemical methods4,12,13,14,15. The electrocatalytic activity for methanol oxidation of Au@Pt nanoparticles is higher than au-pt alloy nanoparticles in acidic media8,16,17. The most commonly reported morphology of Au@Pt nanoparticles are that of platinum particles/clusters decorating the surface of gold nanoparticles2,18,19,20. Recently, some groups have reported the electrocatalytic properties of Au@Pt nanoparticles with smooth film-like morphology5,15,21,22. However, there still exists a need for developing scalable room-temperature processes for synthesizing monodisperse Au@Pt


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nanoparticles with tunable shell thicknesses and morphology. Furthermore, in conventional fuel cells the catalyst particles are physisorbed on carbon particles and embedded in the Nafion® membrane. High cathode potential and extreme acidic environment leads to corrosion of the carbon matrix23. This leads to detachment and agglomeration of catalyst nanoparticles which reduces the overall electrocatalytic surface area and catalyst durability. The use of thin films of close-packed nanoparticles has been presented as an alternative approach to carbon-free catalyst layers20,24. The added feature of such films is that by tuning the particle size, shell thickness and bimetallic nature, one can tune their catalytic activity. However, so far, such nanoparticle ensembles have not been generated using nanoparticles with ultrathin platinum overlayers in the range of sub-monoatomic layer to few atomic layer thickness, wherein platinum utilization is expected to be optimal. In this context, we adapted a scalable protocol25 and developed a process for aqueous-phase synthesis of Au@Pt nanoparticles with controllable platinum overlayer thickness in the sub-monolayer to bilayer regime, and their subsequent self-assembly to form electrocatalytically active thin films, comprising of two layers of close-packed nanoparticle arrays. This paper reports the synthesis of Au@Pt nanoparticles with platinum overlayers ranging from sub-atomic to two atom thick layers. This aqueous-phase protocol involves successive reduction of HAuCl4.3H2O and H2PtCl6.6H2O to prepare monodisperse, Au@Pt nanoparticles with controlled platinum shell thickness. The results of high resolution-transmission electron microscopy (HR-TEM), selected area electron diffraction (SAED), X-Ray photoelectron spectroscopy (XPS), UV–Visible spectroscopy (UV-Vis), and X-Ray diffraction (XRD) measurements suggest the formation of core-shell Au@Pt nanoparticles. These Au@Pt nanoparticles were phase-transferred into an organic solvent by coating the nanoparticles with


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octadecanethiol and then self-assembled at an air-water interface into a close-packed, monolayer of ligand-coated nanoparticles26. Two such monolayers were picked up, one after the other, with a PDMS stamp and transfer-printed to form a bilayer of nanoparticle arrays on a gold-coated silicon substrate27. The octadecanethiol molecules were then removed using a gentle RF powered plasma etching process28. Only after thiol removal was eletrocatalytic activity observed. Finally, hydrogen adsorption-desorption activity and methanol oxidation activity for Au@Pt bilayers in acidic media are reported, which suggest that gold seeds coated with two layers of platinum atoms exhibit higher electrocatalytic activity and platinum atom utilization than platinum nanoparticles.

EXPERIMENTAL SECTION

Materials – Hydrogen tetrachloroaurate trihydrate (HAuCl4.3H2O, purity 99.99%), tannic acid (ACS reagent), Chloroplatinic Acid Hexahydrate (H2PtCl6.6H2O, purity 99.99%) were purchased from Sigma Aldrich, India. Potassium carbonate (K2CO3), Hydrazine hydrate, 37% sulphuric acid (H2SO4), and all solvents used were of analytical grade. Deionized water from a MilliQ® system was used. Synthesis of Au@Pt nanoparticles – Monodispersed gold nanoparticles were synthesized at room temperature as described by Sivaraman et al25. Briefly, 30 mL of 0.882 mM tannic acid solution was taken in a 100 mL beaker and its pH adjusted to 7 by adding a buffering agent, K2CO3. To this solution, 20 mL of 0.635 mM chloroauric acid solution was added dropwise to form gold nanoparticles having a nominal diameter of 5 nm (see SI Fig. S1). Pt shells were formed by subsequent addition of the desired amount of platinum as chloroplatinic acid to the gold colloid, followed by reduction with hydrazine. For example, to prepare Au@Pt


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nanoparticles with Pt:Au of 0.19, 70 µL of H2PtCl6.6H2O (taken from 19.3 mM stock solution, pH=2) was added to 25 mL of the colloidal gold solution. To this mixture, 600 µL of hydrazine was then added, and the solution was stirred for a further 20 min. The pH of the final colloidal solution was alkaline. Nanoparticles with different average Pt/Au ratios of 0.19, 0.39, 0.58, and 0.88 corresponding to theoretical platinum shell thickness of half atomic layer, one atomic layer, one and half atomic layers, and two atomic layers of platinum (on gold nanoparticles having a nominal size of 5 nm) were synthesized by adjusting the amount of chloroplatinic acid added as per the desired average Pt/Au ratios (see SI Fig.S2 for corresponding particle size histograms). Pure platinum nanoparticles were also synthesized, for comparing electrocatalytic activity, by reducing chloroplatinic acid with hydrazine to form nominally 5 nm size particles (see SI Fig.S3). RF Plasma treatment – A custom-built plasma etching unit was used for the results reported here. The area of the electrodes used was 2000 cm2, and they were separated by a distance of 10 cm. A turbo molecular pump was utilized to achieve a base pressure of 1x 10-3 mbar before starting the flow of gas. The alkanethiol capped nanoparticle bilayers were exposed for 2 minutes to an argon plasma, generated using a RF power of 100 W at a pressure of 0.5 mbar, to remove the ligands27. Details of microscopic and spectroscopic characterization methods used are presented in supplementary information.

RESULTS AND DISCUSSION Uniform deposition of platinum on gold seeds while also suppressing secondary nucleation of pure platinum nanoparticles was achieved by ensuring an adequate number density of seed


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particles (> 1013 particles/mL), maintaining the solution at an alkaline pH, and addition of platinum precursor into a mixture of the seed solution and excess stabilising agent (tannic acid)25. Table 1 shows that the measured sizes of Au@Pt nanoparticles are equivalent to their expected mean sizes based on the assumption of uniform growth of platinum atoms onto gold nanoparticles. The equivalence of computed and measured particle sizes alongwith the absence of particles with smaller sizes in electron microscopic characterization suggests that secondary nucleation events to form pure platinum nanoparticles were negligible. Table 1: Comparison of measured and computed particle sizes

(Pt/Au)bulk

Gold seed diameter, nm

Measured Au@Pt nanoparticle diameter, nm

Computed Au@Pt nanoparticle diametera, nm

0.19

5.76 ± 0.41

6.02 ± 0.51

5.98

0.39

5.35 ± 0.53

5.98 ± 0.54

5.76

0.58

5.31 ± 0.61

6.31 ± 0.48

5.90

0.88

5.77 ± 0.38

7.38 ± 0.54

6.70

a

assuming that all seed particles have the same average size as measured and are covered uniformly by the added platinum atoms (See SI for details). The succesful synthesis of Au@Pt nanoparticles with a core-shell morphology was confirmed by both microscopic (TEM, HRTEM and SAED, Fig. 1a and Fig. S4, S5) and spectroscopic (XPS, Fig. 1b & Fig. S6; UV-Vis, Fig. 1c; XRD, Fig. S7) characterizations. HRTEM images reveal lattice spacings of 0.24 nm in the core (see SI Table S2), corresponding to gold, while SAED patterns showed two sets of (311) reflections corresponding to gold and platinum respectively, in all the samples. Scaled XPS signals from platinum 4f regions showed a linearly increasing trend


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with the overall Pt/Au ratio of the respective samples (all XPS spectra shown in Fig. 1b were scaled to ensure that their Au 4f intensities are equivalent to that of the 0.19 sample, original spectra are shown in SI Fig. S6). Quantitative estimates of Pt/Au ratio using XPS spectra (See SI Table S1) are higher, by ~ 60%, than corresponding bulk Pt/Au ratios, which indicates surface segregation of Pt atoms in these nanoparticles. The higher estimates of Pt/Au atomic ratios based on XPS data also rules out the presence of pure platinum nanoparticles of the sizes observed in TEM images. If, indeed, there were independent populations of pure Pt and Au nanoparticles then the computed Pt/Au ratios would not be significantly different from bulk Pt/Au ratios, as the values of the photoionisation cross-sections and mean escape depths of photoelectrons from platinum are of similar magnitude as gold29,30,31. In contrast, some groups have reported a nonlinear increase in Pt/Au signals with shell thickness due to attenuation of gold photoelectrons by the outer platinum shell as a proof of core-shell structure. Given that typical escape depths of photoelectrons are of the order of ~1.7 nm (@ 1400 eV K.E.) in both platinum and gold29, such dampening of the gold signals in XPS spectra can only be expected for platinum shells that are thicker than a few atomic layers. Thus, the linear trend of the scaled Pt 4f XPS signals and higher Pt/Au ratios estimated using XPS further corroborate the growth of ultrathin platinum overlayers on gold seeds. XRD patterns (see SI Fig. S7) showed the presence of peaks at 2θ values associated with bulk gold, while a shoulder was visible at higher 2θ values, corresponding to Pt (111) reflections, for the samples with Pt/Au ratio of 0.58 and 0.88. The size-induced broadening of the XRD peaks and the proximity of bulk Au and Pt XRD reflections does not allow for structural analysis by Rietveld refinement. However, the presence of peaks at reflections corresponding to bulk gold, even for the sample with Pt:Au ratio of 0.88 is consistent with a core-shell morphology rather than an alloyed structure.


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Figure 1. Characterization of synthesized Au@Pt core-shell nanoparticles. a) Representative Transmission electron microscope (TEM) images, High resolution TEM (HRTEM) images and Selected area electron diffraction (SAED) patterns of nanoparticles with different platinum composition. Magnified sections (2.4x) of SAED patterns are overlaid to highlight the detection of (311) reflections corresponding to both gold (inner) and platinum (outer) domains. The dotted arcs in the SAED patterns are an aid to visualization of the (311) reflections. Scale bars within the TEM, HRTEM and SAED images correspond to 10 nm, 2 nm, and 2 (1/nm) respectively. b) XPS spectra showing both Pt and Au 4f signals of nanoparticle samples having different platinum compositions. The Au 4f intensities were used to normalize the spectra of different samples to facilitate comparison of the amount of platinum. c) UV-Vis spectra of samples with different platinum composition is shown along with a representative spectra of the gold seed solution.


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Lastly, UV-Vis spectra show a complete dampening of the surface plasmon resonance band associated with gold seeds after addition of platinum. Although SPR dampening can occur either by core-shell formation or by alloying of gold atoms in the outer shell leading to changes in dielectric properties, the observation of lattice spacings corresponding to gold in HRTEM images of the nanoparticle cores, the absence of reflections at 2θ values in-between those corresponding to gold and platinum, expected for alloys in XRD, and the absence of peaks at smaller BE values than those of bulk gold, expected for alloying of platinum and gold, are all consistent only with a Au@Pt core-shell morphology for these samples. Also, the increased absorbance and slope of the UV-Vis absorbance spectra for samples with Pt/Au ratios of 0.58 and 0.88 is indicative of the formation of contiguous platinum atom domains32, as would be expected for nominally 5 nm sized Au@Pt nanoparticles. The Au@Pt nanoparticles were phase-transferred to an organic solvent (50:50 chloroformhexane mixture) after being coated with octadecanethiol ligands and subsequently self-assembled into close-packed monolayers at a curved air-water interface. Two such monolayers were consecutively picked up onto an elastomeric PDMS stamp pad and then transferred onto a goldcoated substrate to form pin-hole free bilayer samples for testing their electrocatalytic activity. For activity comparison, bilayers of nominally 5-nm sized platinum particles were also prepared. The octadecanethiol coated samples did not show any electrocatalytic activity in cyclic voltammograms (Fig. 2, dotted curves in HOR activity panels), indicating that the octadecanethiol coating on the nanoparticles was dense and impenetrable for the redox species, while also attesting to the pin-hole free nature of the self-assembled films formed on the gold substrates. In order to expose the nanoparticle surface, a gentle RF plasma process was used to remove the organic ligands. After two minutes of argon plasma treatment, XPS results (see SI


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Fig. S8) demonstrate the successful removal of ligands by the absence of sulphur related signal attributed to the thiol head group. Also, no significant change could be detected in XPS spectra of Pt 4f regions due to argon plasma treatment, indicating that the RF plasma treatment did not alter the electronic state of pt atoms. Furthermore, the topology of the nanoparticle bilayers was not significantly altered by the RF plasma treatment process (inset FESEM images in Fig 2, and SI Fig. S9). Finally, all the samples exhibited electrocatalytic activity after RF plasma treatment, which further confirms that the ligands covering the nanoparticle surfaces have been removed.

Figure 2. Steady state cyclic voltammograms of bilayers of Au@Pt nanoparticles obtained after two minutes of argon plasma treatment to remove octadecanethiol ligands. The top panels show hydrogen oxidation reaction (HOR) activity, while the bottom panels show corresponding methanol oxidation reaction (MOR) activity of the samples with varying Pt/Au ratios. The insets in the top panels (65 x 50 nm) show representative FESEM images of the bilayers after 2 minutes of plasma treatment. The dotted curves in the top panels correspond to CVs obtained prior to plasma treatment.


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HOR and MOR activity measurements show the onset of gold oxidation and reduction peaks at 1.1 V (vs. SHE) and 0.94 V (vs. SHE) for Au@Pt nanoparticles having Pt:Au ratios of 0.19 and 0.38, corresponding to half and complete platinum atom coverage of the surface of a nominally 5 nm sized gold nanoparticles, while these signals are absent from the samples with higher Pt:Au values. These results are in concord with our earlier conclusion on the formation of a continuous platinum shell on gold cores at higher Pt:Au ratios. The sample with Pt:Au ratio of 0.88 showed a peak in the cathodic scan for HOR at a voltage of 0.74 V (vs. SHE), which corresponds well with the range of potentials reported for platinum reduction in nanoparticles21, while there appears to be a systematic shift to lower potential values for lower Pt:Au ratios. The MOR activity of the 0.88 sample is similar to that expected for bulk platinum electrodes, with a If/Ib value of 2.24 (291/130) that is indicative of a very high tolerance to the presence of CO and facile oxidation of methanol to CO237,38, while negligible activity was observed for lower Pt:Au ratios. The much larger shifts, in HOR activity, of the platinum reduction peaks from bulk values for ultrathin platinum overlayers, as compared to many earlier studies on similar nanoparticles dispersed on porous carbon electrodes15,33,34, is attributed either to a modification due to support interactions24,35 or a result of surface reconstructions36, caused by the initial voltage cycling prior to CV measurements, and consequent structural or electronic-effects9. As such, the MOR results also suggest that the underlying gold core significantly affects the lattice spacing of the platinum overlayers in the samples with Pt:Au ratios

Synthesis and Characterization of Au@Pt Nanoparticles with Ultrathin

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