Block Copolymer Templated Synthesis of CoreâShell PtAu Bimetallic

Subscriber access provided by UNIV OF OTTAWA

Article

Block Copolymer Templated Synthesis of Core-Shell PtAu Bimetallic Nanocatalysts for the Methanol Oxidation Reaction Kyle Mikkelsen, Blake Cassidy, Nicole Hofstetter, Leah Bergquist, Audrey Taylor, and David A. Rider Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm5026798 • Publication Date (Web): 10 Nov 2014 Downloaded from http://pubs.acs.org on November 23, 2014

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Chemistry of Materials is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Chemistry of Materials

Block Copolymer Templated Synthesis of Core-Shell PtAu Bimetallic Nanocatalysts for the Methanol Oxidation Reaction Kyle Mikkelsen, † Blake Cassidy, ‡ Nicole Hofstetter, Leah Bergquist,‡ Audrey Taylor‡ and David A. Rider‡,†* †

Department of Chemistry, Western Washington University, 516 High St., Bellingham WA 98225 ‡ Department of Engineering and Design, Western Washington University, 516 High St., Bellingham WA 98225

* To whom correspondence should be addressed: [email protected]

RECEIVED DATE (to be automatically inserted after your manuscript is accepted if required according to the journal that you are submitting your paper to) KEYWORDS. Self-Assembly, block copolymer, PS-b-P4VP, fuel cell, methanol oxidation, templated synthesis, platinum-gold, core-shell, bimetallic, nanocatalyst, nanoparticle, electrocatalysis. ABSTRACT Direct methanol fuel cells (DMFCs) are an attractive portable energy technology due to their low operating temperatures, high-energy conversion efficiency and lower pollutant production. For over a half-century, the default electrocatalyst for DMFCs has been platinum (Pt). The barriers to widespread deployment of DMFCs however are largely linked to the cost of this precious metal and its propensity to become poisoned in its role as the anode catalyst for the oxidation of methanol. Bimetallic platinumgold (PtAu) catalysts however offer superior activity for the oxidation of methanol and can operate with increased electrocatalytic stability and resistance to poisoning. Here, we demonstrate a block copolymer template strategy for the preparation of arrays of clusters of PtAu nanocatalysts with tailored 1 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 31

composition, particle density and electrochemical activity. A detailed characterization by XPS, TEM, EDX and electrochemistry was used to assign a core-shell nanostructure to the 3 nm PtAu nanocatalysts that constitute the clusters. The activity Pt-rich core-shell PtAu nanocatalysts for the electrocatalytic oxidation methanol was approximately 2-4 fold that of a current Pt benchmark catalyst (ETEK), only 28% less than that of the PtRu bimetallic benchmark catalyst (XC-72R), and, in comparison to these same catalysts, exhibited a 2- to 3-fold increase in its metric for tolerance to carbonaceous poisoning (If/Ib ratio). INTRODUCTION Direct methanol fuel cells (DMFCs) are a highly promising and well-developed energy technology that may substitute for conventional batteries in portable electronics due to their low operating temperatures, high-energy conversion efficiency and lower pollutant production.1-3 DMFCs typically consist of two electrodes installed around a polyelectrolytic membrane (PEM) and produce electricity by passing oxygen (O2) through the cathode and methanol through the anode. Ideally, anode catalysts completely oxidize methanol to produce protons (H+), electrons (e-) and carbon dioxide (CO2). Hydrogen ions pass into the PEM whereas e- pass across an electrical load for work and back to the cathode catalysts where they combine with O2 and H+ from the PEM to form water. For nearly 65 years, a default monometallic material for executing the electrocatalytic steps in DMFCs has been platinum (Pt).4-6 The barriers to widespread commercialization of DMFCs have been largely linked to the limitations of Pt which include its cost, its relatively slow rate for conducting the oxygen reduction reaction (ORR) and its susceptibility to poisoning.7 A major strategy for addressing these barriers and to improve the performance of DMFCs is to install nanoscale Pt and multimetallic Pt-containing catalysts at the electrodes.8,9 The rationale for Pt-based nanoparticles (NPs) stems from the high surface area-to-volume ratio which maximizes the utility of Pt atoms but also accesses a quantum regime where catalytic activity may exceed that predicted based on miniaturization of bulk properties.10-13 The strategy for modifying Pt with complementary metals has received steadily increasing attention and has identified several bimetallic Pt NP catalysts with increased catalytic activity and stability.14-19 The current a default bimetallic system for executing the electrocatalytic steps in DMFCs is NP catalysts of PtRu.20 The enhanced catalytic properties for bimetallic Pt NP catalysts generally stem from an altered geometric and/or electronic structure of the exterior atoms of the catalyst particles. Consequently, controlled syntheses that specify the stoichiometry and structure in the NP catalyst are of great importance. Bimetallic platinum-gold (PtAu) catalysts in particular have demonstrated superior 2 ACS Paragon Plus Environment

Page 3 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


activity for the oxidation of methanol,21-25 formic acid,26-29 the reduction of oxygen,30-35 and can operate with increased electrocatalytic stability and resistance to poisoning.33,36-40 Currently, the most common routes for creating bimetallic PtAu particles employ a solvothermal method that relies on a simultaneous reduction of the respective metal salt precursors in a thermostatically controlled media that is often charged with surfactants.27,41,42 The application of polymer thin films as templates for the synthesis of bimetallic nanoparticles is less-well explored.43-45 Block copolymers, macromolecules comprised of two distinct polymer chains joined at a common chain end, have long been used to pattern and structure metals on the nanoscale.46-51 Block copolymer lithography in particular is a versatile, solution based technique that provides a high degree of control over the size and spacing of nanoparticles.52 The block copolymer template-synthesis of Pt NPs has therefore been demonstrated by many groups.53-60 The extension of this approach for the synthesis of bimetallic NPs has not yet been fully explored and represents a considerable opportunity for discovering highly active NP catalysts.45 To this end, a block copolymer route has been used to prepare improved anodes for direct formic acid fuel cells consisting of PtPb NPs fixed to nanoporous composites.61 The block copolymer approach has also been shown to be compatible with a sol-gel route capable of the preparation highly conductive palladium-carbon-silica composite electrodes that avoid carbon-based corrosion in fuel cells.62 Currently however, the application of a block copolymer method for the preparation and electrochemical testing of uniform clusters of Pt-based bimetallic catalysts has not yet been reported. In this work, we therefore investigate the applicability of block copolymer templated-synthesis for the preparation of arrays of clusters of PtAu NP catalysts. We demonstrate that the method can produce catalysts ~3 nm in diameter and permits the tuning of the catalyst composition and consequently electrocatalytic activity. Specifically, we present herein: (i) the details of block copolymer templated synthesis of arrays of clusters of PtAu NP catalysts from polystyrene-blockpoly(4-vinylpyridine) thin films, (ii) a detailed characterization of the structure and composition of the PtAu NP catalysts, and (iii) a study of the catalytic activity and stability of the PtAu NP catalysts for the methanol oxidation reaction.

EXPERIMENTAL Materials and General Instrumentation. Unless otherwise noted, all the experiments were performed under

ambient

laboratory

conditions.

Potassium

tetrachloroaurate

(KAuCl4),

potassium

hexachloroplatinate (K2PtCl6) and concentrated HF were used as received from Fisher Scientific, Inc (caution HF is highly toxic and extreme care has to be exercised while handling it). Polystyrene-block3 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 31

poly(4-vinylpyridine) (PS-b-P4VP) block copolymers of block ratio ~3:1 were selected as precursors for the templates in this study. Namely PS1392-b-P4VP471 (PDI =1.07), PS720-b-P4VP238 (PDI =1.15) and PS552-b-P4VP174 (PDI =1.14) were used as received from Polymer Source, Inc. Polystyrene homopolymer (average Mw ~280,000 Da) was used as received from Sigma Aldrich. Silicon wafer (with a native oxide or a or a 250 nm thick thermal oxide) and indium tin oxide glass substrates (8-12 Ω/☐) were acquired from Nova Electronic Materials and Delta Technologies, respectively. The substrates were cleaned by sequential rinsing in dichloromethane, ultrapure water (18 MΩ) and isopropyl alcohol, and were further cleaned by exposure to a 15 min argon plasma at ~ 0.1 torr (Harrick Plasma, PDC 32G, 18W) immediately prior to use. Similarly, polymer-based thin film coatings supported on these substrates were reactive ion etched using this equipment and procedure. Block copolymer films were prepared from filtered (Millipore syringe filters at 0.45 µm pore size) micellar toluene solutions (4 mg/ml) of PS-b-P4VP. Films were cast onto cleaned silicon wafer and indium tin oxide glass substrates by spin coating for 60 sec at 3000 rpm (WS-400B-6NPP Lite Laurell Spin Coater). Scanning force microscopy (SFM) was conducted using a Digital Instrument Nanoscope IIIa multimode instrument operated in tapping mode and equipped with conical tapping mode silicon probes (Nanoscience Instruments) with resonant frequencies close to 300 kHz. Transmission electron microscopy (TEM) was performed with a FEI Tecnai Osiris TEM system (accelerating voltage of 200 keV) equipped with and Analytical TWIN (A-TWIN) objective lens and an integrated Super-X EDX detection system. X-ray photoelectron spectroscopy (XPS) was carried out using a Sage 100, SPECS system operated at a pressure of < 10-5 Pa. All samples were analyzed using an unmonochromated Mg Kα X-ray source operated at a power of 300 W at a take-o

angle of 90°, resulting in a probe depth of 5-10 nm.

The atomic concentrations were estimated by determining the integral peak intensities for a specific element on a Shirley background. The approximate error is 5-10%. High-resolution analysis was performed on the carbon 1s (C 1s), platinum and gold 4d (Pt 4d and Au 4d) regions as well as the platinum and gold 4f range (Pt 4f and Au 4f). The spectra were fit using 100% Gaussian components and a Shirley background subtraction. All binding energies were corrected to the C 1s peak for the background hydrocarbon component (C–C/C–Hx) at 284.9 eV.63 Synthesis of Platinum-Gold Bimetallic Nanoparticles. The synthesis of PtAu NPs follows a modified protocol as outlined by Aizawa et al.64 In brief, 0.1M H2SO4 aqueous immersion baths were formulated with an overall 10 mM metal ion concentration by changing the relative concentration of the gold or 4 ACS Paragon Plus Environment

Page 5 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


platinum reagents. PS-b-P4VP coated substrates were first reconstructed to a torroidal form by soaking in methanol and then introduced into the metal ion loading bath that had been tailored to the desired platinum:gold ratio. Following metal ion loading, the substrates were rinsed with a standardized amount of ultrapure water (18 MΩ) and dried with prefiltered air. The metal ions that were loaded into the PS-bP4VP films were then reduced by exposure to a 15 min argon plasma at ~ 0.1 torr (Harrick Plasma, PDC 32G, 18W) which also simultaneously removed the block copolymer. Arrays of PtAu NPs were transferred to a TEM grid by a polymer overcasting method. The PtAu NPs arrays were created on silicon wafer bearing a 250 nm thick thermal oxide layer using the steps outlined above. The arrays were then embedded in a transferring medium by spin coating (1.5krpm, 60 sec) a toluene solution of ~2% /wt. polystyrene homopolymer (~Mw = 280,000 Da). The substrate was then immersed in concentrated HF for ~ 1 min which led to the etching of the thermal oxide layer of the substrate which released the polystyrene layer that contained the embedded PtAu NPs arrays. The polystyrene layer was then diluted with ~10 fold amounts of ultrapure water and then a TEM grid was applied in contact to transfer the floating layer. The TEM grid was further rinsed with ultrapure water and then dried and stored for later use in high-resolution TEM and EDX analysis. AFM and XPS analysis confirmed that samples prepared by this method were similar in cluster definition and composition to those prepared directly on ITO electrodes (see supporting information Figure S16-S17).

Electrochemical Analysis. Cyclic voltammetry (CV) was carried out either using a Parstat 2273 potentiostat or a Pine WaveDriver 20 bipotentiostat employing a standard three-electrode electrochemical cell. All potentials are reported relative to a ferricyanide-corrected Ag/AgCl reference electrode recorded at a scan rate of 100 mV/s. The counter electrode was platinum wire and the working electrode consisted of an ITO electrode bearing the film of interest. CV characterization employed a degassed 0.1M H2SO4 electrolyte whereas methanol (MeOH) oxidation was conducted in degassed 2M MeOH and 0.1M H2SO4 electrolyte. RESULTS AND DISCUSSION

Diblock copolymer templates. The block copolymer template approach for synthesizing PtAu bimetallic catalysts employs polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) diblock copolymers. The general procedure to fabricate arrays of PtAu bimetallic catalysts can be summarized three steps: (1) the self-assembly and thin film processing PS-b-P4VP diblock copolymer micelles, (2) the simultaneous 5 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 31

absorption of platinate and aurate ions into the PS-b-P4VP film by immersion into an aqueous solution, (3) a plasma reactive ion etch/reduction step. A similar block copolymer thin film template strategy for creating crystalline monometallic Au NPs has been described by Mirkin et al.65 The entire process for creating PtAu bimetallic catalysts is easily carried out with standard laboratory apparatus (laboratory ambient, benchtop wet processing, glassware plasma sterilizer) and does not require sophisticated photolithography or metal evaporation equipment or other clean-room-based fabrication techniques. In order to understand how PS-b-P4VP specifies PtAu catalyst composition and size, three molecular weight-varied PS-b-P4VP diblock copolymers were selected. The selected block copolymers were PS1392-b-P4VP471, PS720-b-P4VP238 and PS552-b-P4VP174 where the subscripts denote the average number of repeat units in each block as determined from the number average molecular weight. This selection of copolymers was defined such that the PS:P4VP block mass ratio was fixed at ~3:1 which primarily favors a spherical micellar structure in toluene solvent.66 The ion loading bath used for loading Pt and Au precursors into PS-b-P4VP consist of aqueous 0.1 M H2SO4 with codissolved KAuCl4 and K2PtCl6 in proportion to create 10 mM total metal ion concentration. These high oxidation state salts were selected in order to avoid spontaneous reduction in the immersion bath. In order to distinguish each ion loading bath, the mole ratio of codissolved Pt:Au ions will be used and are reported in Table 1.

6 ACS Paragon Plus Environment

Page 7 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Scheme 1. General Procedure for the synthesis of PtAu bimetallic catalysts using thin films of PS-bP4VP micelles. The diagrams atop and below track the evolution of a single PS-b-P4VP micelle and an array of micelles, respectively. (i) Solution state self-assembly of PS-b-P4VP into spherical micelles in toluene; (ii) spin coating micelles onto an electrode substrate followed by thin film reconstruction into an array of open-micelles by soaking in MeOH; (iii) simultaneous loading of AuCl4- and PtCl62- ions into micellar array by immersion into a stoichiometrically tuned aqueous solution of the respective metal ions; (iv) reductive etching of PS-b-P4VP template by Ar plasma etching. x and y indicate the amount of AuCl4- and PtCl62- of metal ions incorporated into a single PS-b-P4VP micelle domain while z refers to possible sulfate ions from the 0.1 M H2SO4 loading bath.

In the first step (Scheme 1, step i), the self-assembly of PS-b-P4VP is triggered by dissolution of the copolymer in the block selective solvent, toluene, and afford micelles with a corona of solventswollen polystyrene and a core of solvent-incompatible P4VP. Quasihexagonal arrays of PS-b-P4VP micelles are created by casting toluene solutions of PS-b-P4VP onto flat substrates (Scheme 1, step ii). For this work, indium tin oxide (ITO) or silicon wafers (terminated with either a native oxide or a 250 nm thick thermal oxide) are used as substrates. Shown in Figure 1a-c are the height-mode scanning force microscopy (SFM) images of the arrays of PS-b-P4VP micelles created by spin casting (3 krpm) toluene solutions (4 mg/ml) of PS1392-b-P4VP471, PS720-b-P4VP238 and PS552-b-P4VP174 onto ITO, respectively.

Each image confirms a thin film morphology consisting of periodic P4VP domains

embedded in a PS matrix. Table 1 (as cast entry) summarizes the respective SFM-determined average periodicity values in the quasihexagonal arrays created from PS1392-b-P4VP471, PS720-b-P4VP238 and PS552-b-P4VP174. The periodicity dimensions are known to be primarily set by the processing 7 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 31

conditions.67 While possibly influenced by SFM tip convolution effects, the average height and fullwidth-at-half-maximum values are also reported for comparison purposes. The loading of the micellar array with Pt and Au ions consists of exposing the template films to aqueous environment. The polystyrene matrix that results from spin casting PS-b-P4VP micelles from toluene initially acts as a barrier between the pyridinyl sites in the P4VP core and any dissolved metal ions. Previous research has demonstrated that PS-b-P4VP micelles undergo reconstruction or micelle inversion when exposed to orthogonal solvents like water, acid or alcohol.68-71 During the inversion process, P4VP chains become highly swollen in solvent and increase the P4VP domain size by severalfold.72,73 The swelling forces of the solvent-swollen P4VP domains allow the P4VP chains to penetrate across any PS overlayer and better contact the aqueous solution. P4VP domains however remain constrained by the rigid substrate and the surrounding vitrified and solvophobic PS matrix and force any reconstruction to be focused at the thin film/solvent interface. An empirical screening of a variety of synthetic conditions has confirmed that PS-b-P4VP templates load metal ions reproducibly if a methanol-based micelle inversion step is included prior to a metal ion loading step (Scheme 1, step iii). Accordingly, spin cast PS-b-P4VP micelles are primed for loading by immersion into a methanol bath (10 min) and afford a thin film array of toroidal PS-b-P4VP. Shown in Figure 1d-f are the respective height-mode SFM images of the inverted arrays of micelles created by immersing ITO supported samples of PS1392-b-P4VP471, PS720-b-P4VP238 and PS552-b-P4VP174 in methanol (10 min). Each image suggests that this step has repositioned the P4VP chains such that metal-ion loading can be conducted.

Metal loading diblock copolymer templates. The pyridinyl sites in the P4VP core are Brönsted bases with a pKa ~ 4-574,75 and therefore the P4VP chains represent a collection of sites capable of bearing positive charge. The immersion baths containing KAuCl4 and K2PtCl6 were therefore adjusted to low pH (pH~1) with H2SO4 such that the majority of the pyridinyl groups in the primed P4VP domains would be protonated and rendered cationic. Accordingly, the negatively charged platinate and aurate complexes, [PtCl6]2− and [AuCl4]−, can coordinate to the cationic pyridinium groups primarily through electrostatic interactions.76 In an effort to understand the scope for simultaneously loading P4VP domains with both ions, a series of immersion baths were tested. The mole ratio of Pt:Au in the series of baths was set to 100:0, 98:2, 95:5, 88:12, 50:50 and 0:100 while the overall metal ion concentration was held constant (10 mM). After loading the primed templates with metal anions, a 15 min Ar plasma phase reactive ion etch is used to reduce the metal ions and remove the PS-b-P4VP template (Scheme 1, step iv). Shown in Figure 2 are the SFM height images for arrays of compositionally varied NPs created 8 ACS Paragon Plus Environment

Page 9 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


from the PS1392-b-P4VP471 template. Inspection of this series of images reveals that the PS1392-b-P4VP471 film is capable of templating a quasihexagonal array of well-defined NP from exposure to compositionally varied platinate and aurate ion loading baths. The SFM-determined average dimensions of the NPs created from PS1392-b-P4VP471, PS720-b-P4VP238 and PS552-b-P4VP174 templates are reported in Table 1. Under these conditions, there is no statistical difference in the dimensions of PtAu NPs created from exposing primed PS-b-P4VP templates to different platinate and aurate ion baths. Overall the periodicity the arrays of PtAu NPs closely match with that of the parent template, which confirms that the PS-b-P4VP has specified for the positioning of the PtAu NPs.

(a)

(d)

(b)

(c)

(e)

(f)

Figure 1. SFM height images for as cast (a) PS1392-b-P4VP471, (b) PS720-b-P4VP238, (c) PS552-b-P4VP174 and templates created by inversion of the respective films (d-f). The in-the-plane dimensions of each image are 1µm x 1µm. The height scale at right applies to all images.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 31

(a)

(b)

(c)

(d)

(e)

(f)

Figure 2. SFM height images for arrays of compositionally tuned PtAu NPs created plasma reduction of films after immersion of PS1392-b-P4VP471 templates into 0.1M H2SO4 aqueous solutions of (a) 10.0 mM K2PtCl6, b) 9.8 mM K2PtCl6 + 0.2 mM KAuCl4, c) 9.5 mM K2PtCl6 + 0.5 mM KAuCl4, d) 8.8 mM K2PtCl6 + 1.2 mM KAuCl4, e) 5.0 mM K2PtCl6 + 5.0 mM KAuCl4 and f) 10.0 mM KAuCl4. The in-theplane dimensions of each image are 2µm x 2µm. The height scale at right applies to all images.


Page 11 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1. Comparison of the diameter, height, and periodicity of the PS-b-P4VP templates created on ITO glass with those of the resulting PtAu Bimetallic NPs created from immersion into 0.1M H2SO4 / 0.1 M [KAuCl4 + K2PtCl6]. Bath Pt:Au mole ratio

None (as cast)

100:0

98:2

95:5

88:12

50:50

0:100 a

PSn-b-P4VPm Diblock copolymer templatea and AFM estimated dimensionsb in nm (Std. Dev.)

Dimension

Diameter (FWHM)

PS1392-bP4VP471 54 (5)

PS720-bP4VP238 32 (3)

Height

26 (3)

16 (3)

14 (2.5)

Periodicity

101 (8)

53 (11)

63 (8)

Diameter (FWHM)

56 (4)

26 (2)

38 (5)

Height

11 (2)

6 (1)

7 (1)

Periodicity

100 (13)

49 (6)

69 (15)

Diameter (FWHM)

54 (5.6)

24 (2)

28 (4)

Height

11 ( 2.6)

9 (1)

13 (3)

Periodicity

96 (25)

51 (6)

64 (7)

Diameter (FWHM)

50 (5)

22 (4)

27 (5)

Height

12 (2)

10 (1.5)

9 (1)

Periodicity

108 (18)

48 (10)

57 (10)

Diameter (FWHM)

48 (5)

30 (3)

31 (6)

Height

12 (2)

8 (1.5)

9 (2)

Periodicity

100 (16)

49 (10)

66 (8.5)

Diameter (FWHM)

44 (5)

29 (3)

29 (5.5)

Height

15 (3)

10 (3)

9 (1)

Periodicity

94 (16)

48 (5)

72 (13)

Diameter (FWHM)

37 (6)

25 (3)

23 (4)

Height

16 (3)

11 (1)

8 (1)

Periodicity

89 (26)

41 (9)

73 (11)

PS552-b-P4VP174 35 (5)

- n and m represent the average number of repeat units of the PS and P4VP blocks, respectively. - average of 15 particles.

b



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 31

The opportunity for a complex nanostructure to occur in the Pt- and Au-based NPs that are isolated from the PS-b-P4VP templated synthesis warrants high resolution characterization by transmission electron microscopy (TEM). The series of arrays of Pt-rich target NPs created from PS1392-b-P4VP471, PS720-b-P4VP238 and PS552-b-P4VP174 templates (entries i-iv in Table 2) were transferred from a silicon-thermal oxide substrate to a TEM-grid by a polymer-overcasting and HFetch procedure (see experimental section). In general, it was observed that the templated PtAu NPs consist of clusters of small nanoparticles (12-30 NPs per cluster) with individual NPs having diameter values in the 2-4 nm range. Shown in Figure 3 are the representative TEM images for arrays of clusters of Pt-Au NPs produced from the three PS1392-b-P4VP471, PS720-b-P4VP238 and PS552-bP4VP174 templates that were loaded using a common platinate and aurate solution (100mM H2SO4 and 9.8 mM K2PtCl6 + 0.2 mM KAuCl4). As observed by SFM, the TEM-estimated periodicity the clusters of NPs closely matches that of the parent block copolymer template, which further confirms that the selection and processing of the PS-b-P4VP has specified for the positioning of the PtAu NP clusters. Statistical quantification the average NP diameter and number of NPs per cluster for these Pt-rich series of NPs was conducted and is expressed as the representative frequency vs. diameter histograms in Figure 3d-f and further extended into the data in Table 2. In general, all NPs synthesized from bimetallic-loaded platinate and aurate solutions have diameter values that are within a standard deviation of each other suggesting that the PVP-loaded domains of the block copolymer yield NPs created by a similar reduction and growth mechanism. The high NP count per cluster suggests that rapid nucleation and slower NP growth mechanisms occur during the plasma phase reduction step. The monomodal nature of the distribution in NP diameter found for all NPs from each series is a testament to the controlled synthetic conditions NP nucleation imparted by the block copolymer template. The NP population values per cluster found for the largest polymer template (PS1392-b-P4VP471) were generally found to be statistically greater than those for the two smaller templates used in this study. This suggests that the PVP domain size in the cast PS-b-P4VP templates prescribes for the number of small nanoparticles per cluster that can be isolated from the block copolymer templated NP synthesis and plasma reduction method.


Page 13 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. (a-c) Representative TEM images for arrays of PtAu NPs created by plasma reduction of PS1392-b-P4VP471, PS720-b-P4VP238and PS552-b-P4VP174 templates that were loaded from a 0.1M H2SO4 aqueous solution of 9.8 mM K2PtCl6 + 0.2 mM KAuCl4. (d-f) Respective particle size distribution plots of Pt-Au NPs determined from (a-c).



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 31

Table 2. Dimensions, Stoichiometry and Pt-ECSA of PtxAuy Bimetallic NPs produced from PSn-bP4VPm diblock copolymer template.

Entry

i

ii

iii

iv

v

vi

Bath Pt:Au mole ratioa

Characteristic

Dimensions, Stoichiometry and Pt-ECSA of PtxAuy Bimetallic NPs produced from PSn-b-P4VPm diblock copolymer templateb

PS1392-bPS720-bPS552-bP4VP471 P4VP238 P4VP174 NP Diameter (std. dev.) 2.0 nm (1.0) 2.2 nm (0.7) 1.8 nm (0.5) NPs / Cluster (std. dev.) 25 (3) 13 (2) 30 (3) 100:0 XPS-Estimated Composition Pt1.00Au0.00 Pt1.00Au0.00 Pt1.00Au0.00 2 Pt-Specific ECSA (m /g) 104 68 60 NP Diameter (std. dev.) 3.0 nm (0.5) 2.8 nm (0.5) 3.0 nm (0.6) NPs / Cluster (std. dev.) 16 (2) 14 (2) 17 (3) 98:2 XPS-Estimated Composition Pt0.73Au0.27 Pt0.72Au0.28 Pt0.81Au0.19 2 Pt-Specific ECSA (m /g) 75 151 161 NP Diameter (std. dev.) 3.0 nm (0.6) 3.1 nm (0.6) 2.9 nm (0.6) NPs / Cluster (std. dev.) 18 (1) 15 (3) 15 (2) 95:5 XPS-Estimated Composition Pt0.57Au0.43 Pt0.61Au0.39 Pt0.68Au0.32 2 Pt-Specific ECSA (m /g) 58 121 83 NP Diameter (std. dev.) 3.0 nm (0.6) 3.1 nm (0.6) 2.9 nm (0.6) NPs / Cluster (std. dev.) 18 (1) 15 (3) 15 (2) 88:12 XPS-Estimated Composition Pt0.49Au0.51 Pt0.56Au0.44 Pt0.61Au0.39 2 Pt-Specific ECSA (m /g) 49 48 47 NP Diameter (std. dev.) 3.0 nm (0.6) 3.1 nm (0.6) 2.9 nm (0.6) 50:50 NPs / Cluster (std. dev.) 18 (1) 15 (3) 15 (2) XPS-Estimated Composition Pt0.38Au0.62 Pt0.41Au0.59 Pt0.37Au0.63 2 Pt-Specific ECSA (m /g) N/A N/A N/A NP Diameter (std. dev.) 2.9 nm (0.8) 3.2 nm (0.7) 3.8 nm (0.7) NPs / Cluster (std. dev.) 23 (3) 15 (2) 13 (2) 0:100 XPS-Estimated Composition Pt0.00Au1.00 Pt0.00Au1.00 Pt0.00Au1.00 2 Pt-Specific ECSA (m /g) N/A N/A N/A a –The ratio of Pt:Au in the immersion bath that is used for loading platinate and aurate ions into the block copolymer templates is reported as the mole:mole ratio for K2PtCl6(aq) to KAuCl4(aq). The immersion bath also consists of 0.1M H2SO4(aq) and is 10mM in total metal ion. b - n and m represent the average number of repeat units in the polystyrene and poly(4vinylpyridine) blocks, respectively.


Page 15 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Further compositional and structural information of the block copolymer templated PtAu NPs is gleaned from energy dispersive X-Ray (EDX) analysis and lattice quantification of high-resolution TEM images. EDX elemental mapping confirmed that Pt and Au atoms were coincident in their distribution throughout the clusters of NPs produced from the block copolymer template method (see supporting information). Figure 4a-b depict the distribution of the Pt and Au components by TEM-EDX line-profile analysis and the corresponding high-angle annular dark field (HAADF) TEM image for the Pt-Au NPs that were synthesized from the PS720-b-P4VP238 template. These isolated PtAu NPs were created from the platinate- and aurate-loaded template using a loading solution of Pt:Au molar ratio of 88:12 (100mM H2SO4 and 8.8 mM K2PtCl6 + 1.2 mM KAuCl4) and, as designated by the line in the plot, the NPs were ~4 nm in diameter. The profile analysis indicates that the Pt component is continuously distributed over the nanoparticle. The Au intensity however is more concentrated to the interior of the nanoparticle. This evidence indicates the formation of a core(PtAu)–shell(Pt) structure similar to that shown in Figure 4c. Analogous EDX profiles were obtained for PtAu NPs that were isolated from the other platinate- and aurate-loaded templates. Further magnification of the sample in Figure 4b permitted characterization of the lattice fringes in the PtAu bimetallic particle. Figure 4d depicts a PtAu NP where two distinct lattices are in view within the same particle. This is observation is more clearly identified by a Fast-Fourier Transform (FFT) depiction of this image, presented as Figure 4e, which shows that four pronounced intensities occur at the reciprocal radii identified as (i), (ii), (iii) and (iv). The platinum rich shell is identified from intensities (ii) and (iv) which closely match that of Pt (111) and Pt (200), respectively. The lattice fringe highlighted in Figure 4d is therefore assigned as Pt-rich {111} planes with the interplanar distance of ~2.3 Å.77,78 The PtAu core is identified by intensity (i) and (iii) which correspond to the Au-rich (111) and Au-rich (200), respectively. A face centered cubic bimetallic PtAu solid– solution alloy structure is therefore assigned for the core lattice – a conclusion that agrees with previous research on bulk and nanoscaled PtAu materials.79,80 For a series where the polymer template is coincident and the selection of the immersion bath is varied in the synthesis of PtAu NPs, similar coreshell nanostructures resulted (see supporting information Figures S5 and S6). Similarly, for a series where the immersion bath is coincident and the selection of the polymer template is varied in the synthesis of PtAu NPs, the analogous core-shell nanostructures is found (see supporting information Figures S7-S9). As shown by previous groups, the locating of a Pt shell over a larger-lattice Pt-Au alloy core results in an expansion of the Pt-shell lattice which affects the surface reactivity for the formation of surface oxides – a characteristic that is observed in the electrochemical characterization described 15 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 31

herein (vide infra). Overall, the block copolymer templated synthesis of Pt-Au nanocatalysts favors PtAu NPs with a PtAu alloy core and a Pt-rich shell nanostructure.

Figure 4. (a) TEM-EDX line-profile analysis Pt and Au for NPs that were synthesized using the PS720b-P4VP238 template and a platinate- and aurate loading bath of Pt:Au molar ratio = 88:12 (100mM H2SO4 and 8.8 mM K2PtCl6 + 1.2 mM KAuCl4). (c) Depiction of a core(PtAu)–shell(Pt) NP structure. (d) High-resolution TEM image of Pt and Au for NPs that were synthesized as indicated above. (e) FastFourier Transform (FFT) of image in (d).

The generation of a Pt-enriched shell surrounding a PtAu alloy core is proposed to be a consequence of (i) the differences in the rate of reduction of the aurate and platinate ions and (ii) the high mobility of Au atoms and aggregates. Indeed, previous studies on mixed AuCl4- and PtCl62- ions have shown the aurate ions reduce more easily than platinate ions.81 Similarly, when these same ions are co-loaded into a polyvinylpyrrolidinone matrix, AuCl4- were also found to reduce in the initial stages (initially leading to Au-rich NPs) followed by a platinum reduction stage which slowly forms a Pt-rich shell on the newly formed gold-based core NPs.82 The offset rates of reduction and NP growth ultimately lead to a gold rich core that incorporates Pt to form an alloy phase. In a relevant recent study, the borohydride reduction of AuCl4- and PtCl62- co-loaded poly(2-aminoethylmethacrylate) produced PtAu alloy NPs by a similar mechanism.83 This proposed NP mechanism is consistent with the slower mobility of the surface platinum (surface diffusion coefficient, Ds,Pt = 1x10-18 cm2/s) in relation to the surface mobility of gold (surface diffusion coefficient, Ds,Au = 1x10-13 cm2/s).84 The differences in the 16 ACS Paragon Plus Environment

Page 17 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


rate of reduction are a consequence of the standard reduction potentials of the aurate and platinate ions where, the reduction of the latter is the more energetically demanding step. The standard reduction potentials of PtCl62- and AuCl4- are described85 by equations 1 and 2 (standard hydrogen electrode, SHE): AuCl4- + 3e- Au + 4Cl-

Eo = 0.994V

(1)

PtCl62- + 4e- Pt + 6Cl-

Eo = 1.47V

(2)

In this study, the composition of the block copolymer templated Pt1-xAux NPs was controlled by varying the ratio of KAuCl4 to K2PtCl6 in the immersion baths. XPS characterization was used to determine the oxidation state and the relative content of platinum and gold elements in the Pt1-xAux NPs. The high-resolution XPS spectra for the 4d core electron region were qualitatively similar for all Pt1xAux

NPs synthesized from each of the three block copolymer templates (see supporting information).

The 4d core electron region was selected as the signal from adventitious carbon could be included in each scan in order to ensure proper assignments of any oxidation states (see Supporting information for full range). Figure 5a depicts the high-resolution XPS spectra for the 4d core electron region for the nanoparticles created from the PS1392-b-P4VP471 block copolymer template. From the peaks found in the topmost curve (curve i), monometallic Pt NPs exhibit only two energy bands at 314.5 eV and 331.4 eV corresponding to Pt 4d5/2 and Pt 4d3/2 core electrons, respectively. The position of these peaks, and the doublet splitting binding energy (∆E = 16.9 eV), strongly indicate metallic Pt(0).86-88 From the peaks found in the bottommost curve (curve vi), monometallic Au NPs exhibit two energy bands at 334.2 eV and 352.4 eV corresponding to Au 4d5/2 and Au 4d3/2 core electrons, respectively. Similarly, the position of these peaks, and the doublet splitting binding energy (∆E = 18.2 eV), also strongly indicate metallic Au(0).89,90 For the spectra of bimetallic Pt1-xAux nanoparticles isolated from PS1392-bP4VP471, the energy values of Pt 4d5/2 and Au 4d3/2 core electrons were easily resolved at ~314 eV and ~352 eV, respectively (curves ii-v). The corresponding peaks for Pt 4d3/2 and Au 4d5/2 core electrons were found to overlap in these spectra and required curve fitting for further examination. After curve fitting, the set of peaks for core 4d electrons for Au and Pt were found to closely align with that of the parent monometallic Pt and Au curves indicating that the oxidation states of the metals in the bimetallic nanoparticles were also zero. Figure 5b depicts the high-resolution XPS spectra for the 4f core electron region for the NPs created from the PS1392-b-P4VP471 block copolymer template. This region was selected for complementary analysis to the 4d data above as shifts in the binding energy of 4f electrons are often 17 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 31

easily identified and useful for elucidation of alloy phases.91,92 From the peaks found in the topmost curve (curve i), monometallic Pt NPs exhibit only two energy bands at 74.5 eV and 71.2 eV corresponding to Pt 4f5/2 and Pt 4f7/2 core electrons, respectively. The position of these peaks, and the doublet splitting binding energy (∆E = 4.4 eV) further indicate metallic Pt(0).86,87,93 From the peaks found in the bottommost curve (curve vi), monometallic Au NPs exhibit two energy bands at 87.7 eV and 84.0 eV corresponding to Au 4f5/2 and Au 4f7/2 core electrons, respectively. Similarly, the position of these peaks, and the doublet splitting binding energy (∆E = 3.7 eV), also reconfirms Au(0).89,90 In this case, the doublets for the 4f core electrons in all high resolution XPS spectra were well resolved from each other permitting an important characterization. The Pt 4f7/2 peaks were observed to shift to lower binding energy values as the complement metal increased in content in the bimetallic NPs. Shown in Figure 5c is a data for position of curves that were fit to the traces in Figure 5b where the peak position for the Pt 4f7/2 and 4f5/2 components decreased with increasing Au content in the Pt1-xAux NPs. The significant decrease of the binding energy of these 4f electrons to values results from the charge transfer between highly mixed Au and Pt and confirms that an alloy phase is present in the Pt1-xAux NPs synthesized by this approach.91,92 The intensity associated with the 4d core electrons detected by XPS was used to estimate the content of Pt and Au in the nanoparticles. The estimate of the Pt1-xAux composition from the intensity associated with the 4f core electrons were also in agreement with those determined from the 4d signal. From curves ii-v in Figure 3a, the compositions of Pt1-xAux NPs created from the PS1392-b-P4VP471 block copolymer template were determined to be Pt0.62Au0.38, Pt0.59Au0.41, Pt0.47Au0.53 and Pt0.23Au0.77 and are reported in Table 2. Table 2 also reports the Pt1-xAux composition for all NPs created from the three block copolymer templates. From this data, it is evident that the P4VP domains are preferentially loaded with AuCl4- ions during the immersion procedure. An interesting observation is that as larger PS-bP4VP block copolymers are applied to the same immersion bath for metal-ion loading, a subtle decrease in the Pt content results. The composition data reported in Table 2 suggests that the tailoring the composition of the immersion baths provides access the entire compositional range for the PtAu bimetallic system.


Page 19 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(a)

(b)

(c)

Figure 5. Offset high resolution XPS plots for (a) 4d and (b) 4f range for Pt-Au bimetallic NPs created from PS1392-b-P4VP471 (see Table 2 entries i-vi). Spectra correspond to samples Pt1.00Au0.00, Pt0.73Au0.27, Pt0.57Au0.43, Pt0.49Au0.51, Pt0.38Au0.62 and Pt0.00Au1.00 (top to bottom respectively). (c) Plots for the binding energy of Pt 4f

7/2

and Pt 4f

5/2

electrons as a function of the Au content in the PtAu NPs

described in (a) and (b). The dotted lines are intended to guide the eye. 19 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 31

Electrochemical Characterization. The cyclic voltammetry (CV) plots for the series of Pt1-xAux NPs synthesized from each of the three block copolymer templates were qualitatively similar (see supporting information). Shown in Figure 6 are the Ag/AgCl-referenced CV plots studied in 0.1 M H2SO4 solution for the Pt1-xAux bimetallic nanocatalysts produced from the PS1392-b-P4VP471 template on an ITO electrode. Overall, the plots exhibit the typical hydrogen adsorption/desorption region (for Pt rich samples), a charging region for the formation of an electrochemical double-layer and a region for the oxidation/reduction of the metal surface. The CV for the pure Pt NPs exhibits diagnostic Hads/des peaks at ~ -0.17 V and -0.26 V which corresponds to (i) (110)-type Pt sites and (ii) a combination of those from (100) step sites on (111) terraces and the sites close to the steps on (100) terraces.94 The broad anodic peak found at 0.94 V corresponds to the formation of surface oxides (Pt-OxHy) and is complemented by a subsequent cathodic peak for reduction of the surface oxide at 0.30 V. In the case of the purely Au NPs, the electrochemical formation of surface oxides (Au-OxHy) is observed as a broad peak centered at a potential of 1.25 V while the corresponding reduction peak for this surface oxide is observed at ∼0.79V. As found in other published work,95,96 the CV data of bimetallic Pt1-xAux NPs created from the PS1392-b-P4VP471 template appear as a combination of the features found for both pure metals. Upon increasing the Au content in the PtAu NPs, a decrease in the intensities of the peaks associated with the reduction of Pt-OxHy and the Hads/des is observed. Resolving the potential value of each Hads anodic peak becomes increasingly difficult in Au-rich NPs. As the Au content increases, the intensity of the peak associated with the formation of Au-OxHy increases. An important trend in the potential for the reduction peak of Pt-OxHy is observed, where that for pure Pt NPs is found at E = 0.30 V and those for Pt1-xAux NPs appear at lower values. In the case of Pt49Au51 NPs, this Pt-OxHy reduction peak can be observed at ~ E = 0.17 V. The increased resistance to reduction of surface oxides in Au-rich alloys indicates a stronger Pt−O binding energy as compared with Pt-rich alloy electrodes. As the Au content increases in PtAu alloys, the increase in the lattice parameter value increases the stabilization of oxygentype adsorbates (such as O and OH species) at the surface of expanded Pt lattice.97,98 This characteristic further supports the identification of the Pt-Au alloy structure identified in the TEM and XPS studies. The characteristics of the CV data from Pt1-xAux NPs created from PS720-b-P4VP238 and PS552-b-P4VP174 templates are largely similar to that indicated above and therefore support the conclusion that the PtAu bimetallic NPs that are isolated from this block copolymer template route consist of a nanostructure with an alloyed core. 20 ACS Paragon Plus Environment

Page 21 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6. Steady-state cyclic voltammetry (vs. Ag/AgCl) for Pt-Au bimetallic NPs created from PS1392b-P4VP471 (see Table 2 entries i-vi). The plots were acquired in 0.1M H2SO4 at 100 mV/sec. The plots characterize Pt1.00Au0.00, Pt0.73Au0.27, Pt0.57Au0.43, Pt0.49Au0.51, Pt0.38Au0.62 and Pt0.00Au1.00 NPs (top to bottom respectively). The dotted lines are intended to guide the eye.

The electrochemical surface area (ECSA) per mass unit of Pt (termed Pt-ECSA herein) for all bimetallic Pt1-xAux NPs was calculated by determining the charge associated with the Hads peaks found in the potential range of ~ -0.05 to -0.18 V from the steady state CV curves in Figure 6 and Figure S13-S15 (supporting information). For the calculation, a charging density of 210 µC/cm2 was used to estimate the charge associated with the adsorption of a hydrogen monolayer on Pt.65,72 Further, the Pt-loading on the electrode was determined by inductively coupled plasma mass spectroscopy (ICP-MS; see supporting information). Table 2 reports the specific ECSA for the PtAu bimetallic NPs created from the three tested PS-b-P4VP templates. The specific ECSA of the Pt/C catalyst (E-TEK) has been previously determined to be 43.3 m2/g whereas that for PtRu/C (Hi-SPEC / Johnson Matthey) is 126.6 m2/g.63,73,99 The block copolymer templated synthesis of bimetallic PtAu alloy core Pt-shell NPs therefore produces nanocatalysts with a very high density of electrochemically active surface Pt sites. Furthermore, it is observed that as the Pt-content in bimetallic Pt1-xAux NPs decreases within a block copolymer series, the 21 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 31

surface Pt-specific ECSA decreases (approaching that of E-TEK Pt/C catalyst). This observation suggests as the Pt-content decreases in a NP, the shell-based Pt is either deactivated or preferentially decreased in population relative to core-associated Pt. The overall discrepancy in Pt-ECSA for monometallic Pt NPs from smaller block copolymer templates and similar Pt1-xAux NPs produced from differing block copolymer templates remains as a subject of current research. A proposed hypothesis for the discrepancy is that the arrangement and/or overlap of NPs within a cluster is responsible for the reduction of the surface-accessible Pt in some cases.

Electrocatalytic Oxidation of Methanol. In order to assess the relevance of Pt-Au bimetallic NPs created from the PS-b-P4VP templates for DMFC catalysis, methanol oxidation was investigated using acidic aqueous media and a cyclic voltammetry technique.69 Figure 7a depicts the steady-state cyclic voltammograms from 50 consecutive scans of methanol oxidation catalyzed by ITO-supported bimetallic nanocatalysts produced from the PS1392-b-P4VP471 template (-0.1 to 0.9 V; 100 mV/sec; 2M MeOH and 0.1M H2SO4). For Pt-rich catalysts, a well-defined symmetric anodic peak was found at ~0.60-0.65 V in the forward scan which corresponds to the oxidation of methanol. In the reverse scan, a complementary anodic peak was found at ~0.30-0.35V which is assigned as the oxidation of surface absorbed intermediates created from previous incomplete oxidation pathways.70 In the case of Au-rich catalysts, no methanol oxidation peaks were discernable from background charging currents (Figure 7a, v and vi) confirming that a critical amount of Pt is necessary to catalyze this reaction.

The CV for

methanol catalysis from the NPs produced from PS720-b-P4VP238 and PS552-b-P4VP174 templates exhibited large similarities to that indicated above. Several proposed mechanisms have been described for the oxidation of methanol on Pt and PtAu catalyst surfaces.71,86,87,100,101 A recent spectroscopic investigation coupled to a potentiodynamic assessment of methanol oxidation proposed that the primary step consists of adsorption of methanol molecules on the Pt surface followed by secondary disassembly steps to produce CO2 via surface adsorbed intermediates such as Pt(CHO)ads and Pt(CO)ads.20,46,102 The presence of Au assists in the activation of water to create Pt(OH)ads groups which react with the carbonaceous intermediates to create CO2. In the CV-experiment for methanol oxidation the forward scan anodic peak consists of both primary and secondary steps while the reverse scan emphasizes the oxidation current for surface adsorbed carbonaceous intermediates which leads to the second anodic peak. Since the Pt(CO)ads is considered to be the most prevalent surface intermediate, any remaining unreacted groups of this sort indicate a poisoned catalyst site. A criterion that is frequently used to assess the electrocatalytic activity 22 ACS Paragon Plus Environment

Page 23 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


and tolerance to CO-poisoning during methanol oxidation in Pt-based catalysts is the ratio between the anodic peak current of the forward anodic peak (If) to that of the anodic peak current of the reverse anodic peak (Ib).89,90,93,103 By this metric, a high the If/Ib indicates that the methanol oxidation to CO2 is dominant and that any remnant surface intermediates, including Pt(CO)ads, are negligible which indicates that the tested catalyst is tolerant to carbonaceous poisoning. Shown in Figure 7b-d are summary charts for the If, Ib and the If/Ib ratio for all the active catalysts produced by this block copolymer template approach. The peak If and Ib are reported in mass activity units (A/g) which is estimated from the highest peak current found during the 50 cycle methanol oxidation CV experiment, the Pt-loading and the Pt-specific ECSA. The block copolymer template approach described herein produces three different arrangements of pure Pt. As the size of the block copolymer is decreased, overall peak If mass activity of the resulting monometallic Pt nanocatalyst decreases from 189 A/g to 39 A/g. Similarly, the If/Ib metric decreases from 2.0 to 1.5 with decreasing block copolymer size. The peak If mass activity for methanol oxidation catalysis by a commercial PtRu NP catalyst (PtRu/XC-72R from Johnson Matthey Catalyst and Chemicals Division; PtRu on C) has recently been reported 590 A/g.104 The If/Ib metric for tolerance to poisoning for this catalyst is ~1.11. The block copolymer template approach described here therefore produces relatively high activity Pt NP catalysts with improved tolerance to poisoning. The evolution in the peak If and Ib mass activities within a series of catalysts produced from a common block copolymer are largely similar. Within each series, as the Pt-content is reduced, the peak If and Ib generally decrease on an exponential-type decline. This decrease in mass activity is likely a result of the decreased amount of electroactive Pt in the shell of the bimetallic PtAu alloy core Pt-shell NPs as identified in the ECSA characterization. Interestingly, as the block copolymer is decreased in overall size, the extent of the declined electroactivity decreases. In the smallest block copolymer, a very highly active catalyst is identified. The highest activity bimetallic PtAu catalyst was the Pt0.81Au0.19 alloy core Pt-shell NPs which exhibited a catalytic activity for methanol oxidation (426 A/g) that far exceeded that of the pure Pt NPs. The If/Ib ratio for this catalyst was also very close to that of the pure Pt NPs produced by the block copolymer template approach. As previously reported, the introduction of Au into the templated Pt NP catalysts generally assists in the activation of water for the oxidation of methanol oxidation byproducts and increases the peak If while decreasing peak Ib – or an overall increase in the If/Ib ratio for Au-containing NP catalysts. In particular, the bimetallic PtAu alloy core Pt-shell NPs with Pt:Au ratio of ~0.7:0.3 produced from PS1392-b-P4VP471 and PS720-b-P4VP238 had very high If/Ib ratios at ~2.5 in value. Interestingly, the discrepancies in the overall trends lie in the NPs produced from the 23 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 31

smallest block copolymer template, namely PS552-b-P4VP174. When benchmarked against the commercial PtRu NP catalyst, the block copolymer template approach described here therefore produces relatively higher activity bimetallic PtAu catalysts with improved tolerance to poisoning. The overall discrepancy in the trends in catalyst parameters for the bimetallic catalysts produced from the smallest block copolymer template remains as a subject of current research - a proposed hypothesis for the discrepancies is that the subtle balance in Pt-content, size and arrangement of catalysts particles within clusters from smaller block copolymers impacts the accessibility and lattice of the surface Pt in these NPs.


Page 25 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(a)

(b)

(c)

(d)

Figure 7. (a) Steady-state cyclic voltammetry (vs. Ag/AgCl) plots acquired in 0.1M H2SO4 for PtAu NP arrays created from PS1392-b-P4VP471 templates. (b-d) Summary electrocatalytic activity charts for peak mass activity (If and Ib) and If/Ib ratio observed during 50 cycles of MeOH oxidation for all PtAu NPs created from PS1392-b-P4VP471 (b), PS720-b-P4VP238 (c) and PS552-b-P4VP174 (d). CONCLUSIONS This article outlines a block copolymer templated method for the preparation of clusters of PtAu nanocatalysts. The composition of the NP is specified by the tailoring the Pt:Au ion ratio in an immersion bath that is used to simultaneously load pyridinyl-sites in the PS-b-P4VP templates with platinate and aurate ions. Upon plasma phase reduction, arrays of PtAu NP are produced where the arrangement of PVP domains in the metal ion-loaded templates prescribes the dimensions and pattern of the array. The NPs located within each cluster were found to have similar diameter values at ~2-4 nm 25 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 31

which confirms that the block copolymer templated method uniformly distributes the NPs over the entirety of the substrate. Further, the selection of the PS-b-P4VP and its processing prescribes the number of low-diameter NPs per cluster. Importantly, this template synthesis yields a PtAu nanostructure comprised of a Pt-rich shell is located on a PtAu alloy core. The PtAu core-shell bimetallic NPs were found to have a very high density of electrochemically active Pt surface sites. Accordingly, the activity of Pt-rich core-shell PtAu nanocatalysts for the electrocatalytic oxidation methanol was approximately 2-4 fold that of a monometallic Pt benchmark catalyst (ETEK) and only 28% less than that of the PtRu bimetallic benchmark catalyst (XC-72R). The Au that is present in the PtAu NPs was beneficial to Pt-rich electrocatalysts as the carbonaceous poisoning If/Ib metric was greatly enhanced relative to both benchmark materials (~ 2-3 fold increase). We anticipate that this block copolymer template method preparing clusters of low-diameter, core-shell NPs will have a positive impact on several emerging areas in nanoscience such as optoelectronics, bionanotechnology, and heterogeneous catalysis.

Supporting Information Available: Calibration and statistical data on TEM images for all samples are included in the supporting information file. Additionally, selected EDX elemental maps, FFT analyses are also included. ICP-MS data and interpretation as well as high resolution XPS data (4d and 4f region) are also found therein. Electrochemical characterization and methanol oxidation curves are included supporting information file as well as AFM and XPS characterization that compare ITO-supported catalysts to those created by the polystyrene transfer method. This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgments. This work was supported by a faculty start-up grant from the Advanced Materials Science and Engineering Center at Western Washington University (AMSEC WWU) and by external grants from the American Chemical Society Petroleum Research Fund (ACS-PRF UNI # 51559-UNI10 and ACS-PRF UR # 54780-UR10). NH is grateful to the National Science Foundation Research Experiences for Undergraduates program (NSF REU # 1062722) for a research stipend for 2013. KM, BC and LB are grateful for research grants from the Vice Provost for Research at WWU. Brandy Pilapil, Dr. Xin Zhang and Prof. Byron Gates at 4D labs - SFU are gratefully acknowledged for electron microscopy and helpful discussions. 26 ACS Paragon Plus Environment

Page 27 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table of Contents Graphic:



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 31

REFERENCES

(1) McGrath, K. M.; Prakash, G. K. S.; Olah, G. A. J. Ind. Eng. Chem. 2004, 10, 1063-1080. (2) Baldauf, M.; Preidel, W. J. Power Sources 1999, 84, 161-166. (3) Wee, J. H. Journal of Power Sources 2006, 161, 1-10. (4) Wasmus, S.; Kuver, A. J. Electroanal. Chem. 1999, 461, 14–31. (5) Cameron, D. S.; Hards, G. A.; Harrison, B.; Potter, R. J. Platinum Metals Rev. 1987, 31, 173-181 (6) Kordesch, K.; Marko, A. Oesterr. Chem. Ztg. 1951, 52, 125. (7) Yasuda, K.; Mityata, S. Durability Targets for Stationary and Automotive Applications in Japan; Springer Science and Business Media LLC: New York, 2009; Vol. 489. (8) Wang, L.; Yamauchi, Y. Chem. Mater. 2011, 23, 2457–2465 (9) Wang, R.; Wang, C.; Cai, W. B.; Ding, Y. Adv. Mater. 2010, 22, 1845–1848. (10) Adlhart, C.; Uggerud, E. Chem. Commun. 2006, 2581-2582. (11) Sun, Y.; Zhuang, L.; Lu, J.; Hong, X.; Liu, P. J. Am. Chem. Soc. 2007, 129, 1546515467. (12) Yamamoto, K.; Imaoka, T.; Chun, W. J.; Enoki, O.; Katoh, H.; Takenaga, M.; Sonoi, A. Nat. Chem. 2009, 1, 397-402. (13) Xu, C.; Wang, L.; Wang, R.; Wang, K.; Zhang, Y.; Tian, F.; Ding, Y. Adv. Mater. 2009, 21, 2165–2169. (14) Hasché, F. O., M.; Strasser, P., J. Electrochem. Soc. 2012, 159, B24-B34. (15) Strasser, P. O., M.; Hasché, F.; Koh, S.; Yu, C.; Srivastava, R., Chem. Ingen. Tech. 2008, 80, 1267. (16) Sao-Joao, S. G., S.; Penisson, J. M.; Chapon, C.; Bourgeois, S.;; Henry, C. J. Phys. Chem. B 2005, 109, 342-347. (17) Soal, N. S. E., U.; Mohwald, H.; Giersig, M. J. Phys. Chem. B 2003, 107, 7351-7454. (18) Giorgio, S. H., C. R. Eur. Phys. J. Ap. 2002, 20, 23-27. (19) Zhang, J. L., F. H. B.; Shao, M. H.; Sasaki, K.; Wang, J. X.; Hanson,; J.; Adzic, R. R. J. Phys. Chem. B 2005, 109, 22701-22704. (20) Liu, Z. L.; Ling, X. Y.; Su, X. D.; Lee, J. Y. J. Phys. Chem. B 2004, 108, 8234–8240. (21) Choi, J. H.; Park, K. W.; Park, I. S.; Kim, K.; Lee, J. S.; Sung, Y. E. J. Electrochem. Soc. 2006, 153, A1812-A1817. (22) Yang, L.; Chen, J. H.; Zhong, X. X.; Cui, K. Z.; Xu, Y.; Kuang, Y. F. Coll. Surf. A Physicochem. Eng. Aspects 2007, 295, 21-26. (23) Hernandez-Fernandez, P.; Rojas, S.; Ocon, P.; de Frutos, A.; Figueroa, J. M.; Terreros, P.; Pena, M. A.; Fierro, J. L. G. J. Power Sources 2008, 177, 9-16. (24) Yang, L. X.; Yang, W. Y.; Cai, Q. Y. J. Phys. Chem. C 2007, 111, 16613-16617. (25) Zhao, D.; Xu, B. Q. Phys. Chem. Chem. Phys. 2006, 8, 5106-5114. (26) Xu, J. B.; Zhao, T. S.; Liang, Z. X. J. Power Sources 2008, 185, 857-861. (27) Xu, J. B.; Zhao, T. S.; Liang, Z. X.; Zhu, L. D. Chem. Mater. 2008, 20, 1688-1690. (28) Wang, J. P.; Thomas, D. F.; Chen, A. C. Chem. Commun. 2008, 5010-5012. (29) Wang, S. Y.; Kristian, N.; Jiang, S. P.; Wang, X. Electrochem. Commun. 2008, 10, 961964. 28 ACS Paragon Plus Environment

Page 29 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(30) Luo, J.; Njoki, P. N.; Lin, Y.; Wang, L. Y.; Zhong, C. J. Electrochem. Commun. 2006, 8, 581-587. (31) Hernandez-Fernandez, P.; Rojas, S.; Ocon, P.; de la Fuente, J. L. G.; Fabian, J. S.; Sanza, J.; Pena, M. A.; Garcia-Garcia, F. J.; Terreros, P.; Fierro, J. L. G. J. Phys. Chem. C 2007, 111, 29132923. (32) El Roustom, B.; Sine, G.; Foti, G.; Comninellis, C. J. Appl. Electrochem. 2007, 37, 12271236. (33) Zhang, J.; Sasaki, K.; Sutter, E.; Adzic, R. R. Science 2007, 315, 220-222. (34) Brown, B.; Wolter, S. D.; Stoner, B. R.; Glass, J. T. J. Electrochem. Soc. 2008, 155, B852-B859. (35) Kumar, S. S.; Phani, K. L. N. J. Power Sources 2009, 187, 19-24. (36) Liang, Z. X.; Zhao, T. S.; Xu, J. B. J. Power Sources 2008, 185, 166-170. (37) Moller, H.; Pistorius, P. C. J. Electrochem. Soc. 2004, 570, 243-255. (38) Habrioux, A.; Vogel, W.; Guinel, M.; Guetaz, L.; Servat, K.; Kokoh, B.; Alonso-Vante, N. Phys. Chem. Chem. Phys. 2009 11, 3573–3579. (39) Yamauchi, Y.; Tonegawa, A.; Komatsu, M.; Wang, H.; Wang, L.; Nemoto, Y.; Suzuki, N.; Kuroda, K. J. Am. Chem. Soc. 2012, 134, 5100–5109. (40) Ge, X. B.; Wang, R. Y.; Liu, P. P.; Ding, Y. Chem. Mater. 2007, 19, 5827–5829. (41) Schrinner, M.; Proch, S.; Mei, Y.; Kempe, R.; Miyajima, N.; Ballauff, M. Adv. Mater. 2008, 20, 1928-+. (42) Zhou, S. G.; McIlwrath, K.; Jackson, G.; Eichhorn, B. J. Am. Chem. Soc. 2006, 128, 1780-1781. (43) Croy, J. R.; Mostafa, S.; Hickman, L.; Heinrich, H.; Cuenya, B. R. Appl. Catal. A-Gen. 2008, 350, 207-216. (44) Ono, L. K.; Cuenya, B. R. Catal. Lett. 2007, 113, 86-92. (45) Boyd, D. A.; Hao, Y.; Li, C.; Goodwin, D. G.; Haile, S. M. ACS Nano 2013, 7, 49194923. (46) She, M.-S.; Lo, T.-Y.; Hsueh, H.-Y.; Ho, R.-M. NPG Asia Mater 2013, 5, e42. (47) Park, S.; Lee, D. H.; Xu, J.; Kim, B.; Hong, S. W.; Jeong, U.; Xu, T.; Russell, T. P. Science 2009, 323, 1030-1033. (48) Park, M.; Harrison, C.; Chaikin, P. M.; Register, R. A.; Adamson, D. H. Science 1997, 276, 1401-1404. (49) Tang, C.; Lennon, E. M.; Fredrickson, G. H.; Kramer, E. J.; Hawker, C. J. Science 2008, 322, 429-432. (50) Jung, Y. S.; Chang, J. B.; Verploegen, E.; Berggren, K. K.; Ross, C. A. Nano Lett. 2010, 10, 1000-1005. (51) Bates, F. S. Science 1991, 251, 898-905. (52) Spatz, J. P.; Mössmer, S.; Hartmann, C.; Möller, M.; Herzog, T.; Krieger, M.; Boyen, H. G.; Ziemann, P.; Kabius, B. Langmuir 2000, 16, 407-415. (53) Sohn, B.-H.; Choi, J.-M.; Yoo, S. I.; Yun, S.-H.; Zin, W.-C.; Jung, J. C.; Kanehara, M.; Hirata, T.; Teranishi, T. J. Am. Chem. Soc. 2003, 125, 6368-6369. (54) Sidorov, S. N.; Bronstein, L. M.; Valetsky, P. M.; Hartmann, J.; Colfen, H.; Schnablegger, H.; Antonietti, M. J. Colloid Interface Sci. 1999, 212, 197-211. (55) Mizuno, H.; Buriak, J. M. ACS Appl. Mater. Interfaces 2009, 1, 2711-2720. (56) Warren, S. C.; Messina, L. C.; Slaughter, L. S.; Kamperman, M.; Zhou, Q.; Gruner, S. M.; DiSalvo, F. J.; Wiesner, U. Science 2008, 320, 1748-1752. 29 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 31

(57) Matos, J.; Ono, L. K.; Behafarid, F.; Croy, J. R.; Mostafa, S.; DeLaRiva, A. T.; Datye, A. K.; Frenkel, A. I.; Cuenya, B. R. Phys. Chem. Chem. Phys. 2012, 14, 11457-11467. (58) Cuenya, B. R.; Croy, J. R.; Mostafa, S.; Behafarid, F.; Li, L.; Zhang, Z. F.; Yang, J. C.; Wang, Q.; Frenkel, A. I. J. Am. Chem. Soc. 2010, 132, 8747-8756. (59) Bronstein, L. M.; Sidorov, S. N.; Zhirov, V.; Zhirov, D.; Kabachii, Y. A.; Kochev, S. Y.; Valetsky, P. M.; Stein, B.; Kiseleva, O. I.; Polyakov, S. N.; Shtykova, E. V.; Nikulina, E. V.; Svergun, D. I.; Khokhlov, A. R. J. Phys. Chem. B 2005, 109, 18786-18798. (60) Croy, J.; Mostafa, S.; Heinrich, H.; Cuenya, B. Catal. Lett. 2009, 131, 21-32. (61) Shim, J.; Lee, J.; Ye, Y.; Hwang, J.; Kim, S.-K.; Lim, T.-H.; Wiesner, U.; Lee, J. ACS Nano 2012, 8, 6870-6881. (62) Warren, S. C.; Perkins, M. R.; Adams, A. M.; Kamperman, M.; Burns, A. A.; Arora, H.; Herz, E.; Suteewong, T.; Sai, H.; Li, Z.; Werner, J.; Song, J.; Werner-Zwanziger, U.; Zwanziger, J. W.; Grätzel, M.; DiSalvo, F. J.; Wiesner, U. Nat. Mater. 2012, 11, 460-467. (63) Sun, S.; Zhang, G.; Geng, D.; Chen, Y.; Li, R.; Cai, M.; Sun, X. Angew. Chem. Int. Ed. 2011, 50, 422-426. (64) Aizawa, M.; Buriak, J. M. J. Am. Chem. Soc. 2006, 128, 5877-5886. (65) Chai, J.; Huo, F.; Zheng, Z.; Giam, L. R.; Shim, W.; Mirkin, C. A. Proc. Nat. Acad. Sci. 2010, 107, 20202-20206. (66) Riess, G. Prog. Polym. Sci. 2003, 28, 1107-1170. (67) Bansmann, J.; Kielbassa, S.; Hoster, H.; Weigl, F.; Boyen, H. G.; Weidwald, U.; Ziemann, P.; Behm, R. J. Langmuir 2007, 23, 10150-10155. (68) Yoo, S.; Cho, H.; Lee, J.-P.; Kim, K. T.; Park, S. Chemistry – An Asian Journal 2012, 7, 692-695. (69) Cho, H.; Choi, S.; Kim, J. Y.; Park, S. Nanoscale 2011, 3, 5007-5012. (70) Zu, X.; Hu, X.; Lyon, L. A.; Deng, Y. Chem. Commun. 2010, 46, 7927-7929. (71) Wang, L.; Montagne, F.; Hoffmann, P.; Pugin, R. Chem. Commun. 2009, 3798-3800. (72) Loxley, A.; Vincent, B. Colloid Polym Sci 1997, 275, 1108-1114. (73) Fernández-Nieves, A.; Fernández-Barbero, A.; Vincent, B.; de las Nieves, F. J. Macromolecules 2000, 33, 2114-2118. (74) Nisit Tantavichet, M. D. P. C. M. B. J. Appl. Polym. Sci. 2001, 81, 1493– 1497. (75) Ripoll, C.; Muller, G.; Selegny, E. Eur. Polym. J. 1971, 7, 1393– 1409. (76) Bronstein, L. M.; Sidorov, S. N.; Valetsky, P. M. Langmuir 1999, 15, 6256-6262. (77) Xu, J. B.; Zhao, T. S.; Yang, W. W.; Shen, S. Y. Int. J. Hydrogen Energ. 2010, 35, 86998706. (78) Chen, J. Y.; Herricks, T.; Geissler, M.; Xia, Y. N. J. Am. Chem. Soc. 2004, 126, 1085410855. (79) Irissou, E.; Laplante, F.; Garbarino, S.; Chaker, M.; Guay, D. J. Phys. Chem. C 2010, 114, 2192-2199. (80) Okamoto, H.; Massalski, T. B. Bull. Alloy Phase Diag. 1985, 6, 46-56. (81) Chen, C. W.; Serizawa, T.; M., A. Chem. Mater. 2002, 14, 2232-2238. (82) Yonezawa, T.; Toshima, N. J. Chem. Soc. Faraday Trans. 1995, 91, 4111-4119. (83) Schrinner, M.; Ballauff, M.; Talmon, Y.; Kauffmann, Y.; Thun, J.; Möller, M.; Breu, J. Science 2009, 323, 617-619. (84) Gomez-Rodriguez, J. M. B., A. M.; Vazquez, L.; Salvarezza, R. C.; Vara, J. M.; Arvia, A. J. J. Electrochem. Soc. 1990, 137, 2161-2165. (85) Handbook of Chemistry and Physics; ; 67th ed.; CRC Press, Inc.: Boca Raton, FL, 1986. (86) Shyu, J. Z.; Otto, K. Appl. Surf. Sci. 1988, 32, 246-252. 30 ACS Paragon Plus Environment

Page 31 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(87) Engelhard, M.; Baer, D. Surf. Sci. Spect. 2000, 7, 1-68. (88) Nyholm, R.; Berndtsson, A.; Martensson, N. J. Phys. 1980, 13, L1091-L1096. (89) Turner, N. H.; Single, A. M. Surf. Interface Anal. 1990, 15, 215-222. (90) Van Attekum, P. M. T. M.; Trooster, J. M. Phys. Rev. B 1979, 20, 2335-2340. (91) Bastl, Z.; Pick, S. Surf. Sci. 2004, 832, 566-568. (92) Ge, X.; Yan, X.; Wang, R.; Tian, F.; Ding, Y. J. Phys. Chem. C 2009, 113, 7379-7384. (93) Nyholm, R.; Berndtsson, A.; Martensson, N. J. Phys. 1980, 13, L1091-____. (94) Vidal-Iglesias, F. J.; Arán-Ais, R. M.; Solla-Gullón, J.; Herrero, E.; Feliu, J. M. ACS Catalysis 2012, 2, 901-910. (95) Brown, B.; Wolter, S. D.; Stoner, B. R.; Glass, J. T. Journal of The Electrochemical Society 2008, 155, B852-B859. (96) Tang, W.; Jayaraman, S.; Jaramillo, T. F.; Stucky, G. D.; McFarland, E. W. J. Phys. Chem. C 2009, 113, 5014-5024. (97) Hyman, M. P.; Medlin, J. W. J. Phys. Chem. C 2007, 111, 17052-17060. (98) Mayrhofer, K. J. J.; Blizanac, B. B.; Arenz, M.; Stamenkovic, V. R.; Ross, P. N.; Markovic, N. M. J. Phys. Chem. B 2005, 109, 14433-14440. (99) Joghee, P.; Pylypenko, S.; Olson, T.; Dameron, A.; Corpuz, A.; Dinh, H. N.; Wood, K.; O’Neill, K.; Hurst, K.; Bender, G.; Gennett, T.; Pivovar, B.; O’Hayre, R. J. Electrochem. Soc. 2012, 159, F768-F778. (100) Zhong, W. H.; Liu, Y. X.; Zhang, D. J. J. Phys. Chem. C 2012, 116, 2994–3000. (101) Singh, A. K.; Xu, Q. Chem. Cat. Chem. 2013, 5, 652–676. (102) Mancharan, R.; Goodenough, J. B. J. Mater. Chem. 1992, 2, 875–887. (103) Ruban, A. V.; Skriver, H. L.; Norskov, J. K. Phys. Rev. B: Condens. Matter Mater. Phys. 1999, 59, 15990-16000. (104) Liao, S. J.; Holmes, K. A.; Tsaprailis, H. B., V. I. J. Am. Chem. Soc. 2006, 128, 35043510.


Block Copolymer Templated Synthesis of CoreâShell PtAu Bimetallic

Recommend Documents

Block Copolymer Templated Synthesis of CoreâShell PtAu Bimetallic