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We demonstrate highly enhanced ORR activity and electrochemical stability of Pt/Ir(111) model core-shell catalysts prepared by molecular beam epitaxy (MBE) in ultra-high vacuum (UHV). Reflection high-energy electron diffraction patterns for the surfaces show that Pt grew epitaxially on the clean Ir(111) substrate and the corresponding scanning tunneling microscope images collected in UHV reveal atomically flat terraces with 50–80 nm widths at a substrate temperature of 673 K. In contrast, the corresponding surfaces prepared at a substrate temperature of 303 K show island-like topmost surface structures. The two-monolayer (ML)-thick Pt grown on Ir(111) (Pt2ML/Ir(111)) surfaces, prepared at substrate temperatures of 303 K and 673 K, show ca. 6 and 24 times higher ORR activities than clean Pt(111), respectively. The anomalous activity enhancement for the latter surface prepared at 673 K is probably caused by homogeneous surface strain acting on the Pt shells that is derived from the 2.2% lattice mismatch between the Pt and Ir. The preparation-temperature–dependent ORR activity suggests that the activity can be dominated by the topmost surface and interface structures of the Pt shell–Ir(111) bimetallic system. Furthermore, while the initial ORR activity of pristine surfaces decreases with increasing Pt shell thickness, the stability during room temperature potential cycling between 0.6 and 1.0 V in a 0.1 M HClO4 solution was greatly enhanced above three ML thickness; the Pt4ML/Ir(111) surface prepared at 673 K retained 6.5 times higher ORR activity than Pt(111), even after 5000 potential cycles. The ORR activity and electrochemical stabilities for the Pt/Ir(111) bimetallic surfaces are the highest among the MBE-prepared Pt/M(111) (M = Ir, Pd, Au) systems reported to date. The results obtained in this study show that Pt/Ir core–shell nanostructures are potential candidates for highly active and durable ORR catalysts.

We investigated oxygen reduction reaction (ORR) activity and electrochemical stability of Pt/Pd(111) model electrocatalysts having well-defined topmost and sub-surface structures. The Pt/Pd(111) bimetallic surfaces were prepared by vacuum-deposition of Pt on clean Pd(111) at substrate temperatures of x ( x -Pt y nm /Pd(111), where x = 573 K or 673 K, y = 0.6 or 1.2) in ultra-high vacuum (UHV). Reflection high-energy electron diffraction patterns and UHV scanning tunneling microscopy images revealed that Pt grew epitaxially on the Pd(111) substrate under the aforementioned Pt deposition conditions. High substrate temperatures resulted in thermal diffusion of deposited Pt with substrate Pd atoms. The Pt atomic ratio at the topmost surface of 573K-Pt 0.6nm /Pd(111) is estimated to be 95% using He-ion scattering spectroscopy (ISS), which is 8% greater than that of 673K-Pt 0.6nm /Pd(111). ORR activities of the 573K-Pt 0.6nm /Pd(111) and 673K-Pt 0.6nm /Pd(111) surfaces were 6.3 and 3.6 times higher than that of clean Pt(111), respectively, indicating that the activity is sensitive to the topmost surface Pt atomic ratios. Moreover, electrochemical stability of 573K-Pt 0.6nm /Pd(111) evaluated under potential cycle loadings (0.6 V–1.0 V) is better than that of 673K-Pt 0.6nm /Pd(111). Depth profiles of the surfaces judged by corresponding ISS spectra suggest that the stability stems not only from the topmost Pt atomic ratios but also effective Pt-shell thickness determined by thermal diffusion of Pt and Pd. Furthermore, an increase in Pt-thickness from 0.6 nm to 1.2 nm improved the electrochemical stability: 573K-Pt 1.2nm /Pd(111) retained 5 times more activity vs. clean Pt(111) even after 2000 potential cycles, at which the activity of 573K-Pt 0.6nm /Pd(111) was the same as that for initial activity of clean Pt(111). The results obtained in this study demonstrate that atomic-level structures Pt-Pd bimetallic alloy surfaces determine the ORR activity and electrochemical stability of practical Pd@Pt core–shell catalysts.

Single-nanometer-scale Pt Si fine particles (Pt Si NPs) were synthesized via the arc-plasma deposition of Si on a highly oriented pyrolytic graphite at a fixed substrate temperature of 600 °C, followed by electron-beam deposition of Pt at the temperatures between 100 and 600 °C in ultra-high vacuum. X-ray diffraction patterns of the vacuum-synthesized Pt Si NPs showed that a solid solution of Pt Si was the major component of the particles. Minor diffraction peaks, due to the intermetallic compounds (Pt3Si1 and Pt12Si5), at the Pt-deposition temperature (Tsub-Pt) of 500 °C, were also observed. Scanning tunneling microscopic images exhibited that Pt Si NPs with an average diameter less than 10 nm were dispersed in the substrate at Tsub-Pt temperatures up to 500 °C. The Pt Si NPs synthesized at a Tsub-Pt of 300 and 450 °C showed 1.7 times higher initial mass activity for oxygen reduction reaction (ORR) compared to commercial carbon-supported Pt NPs catalysts and showed better electrochemical stability than pure Pt NPs. These results demonstrate that the arc-plasma deposition of Si NPs, followed by e-beam deposition of metal elements (Pt) in ultra-high vacuum is a new class dry-synthesis for the single-nanometer-scale fine particles of Pt Si for electrocatalysis, e.g. ORR.

Pt 0.3–0.6nm /Ni 0.3–0.6nm /Pt(1 1 1) and Pt 0.3–0.6nm /Ni 0.3–0.6nm /Pt(1 1 0) atomic sandwich structures were prepared through alternating vacuum depositions of Ni followed by Pt onto clean Pt(1 1 1) and (1 1 0) substrates at room temperature under ultra-high-vacuum (UHV) conditions. After the samples were transferred from UHV to a 1-atm N 2 atmosphere, their oxygen reduction reaction (ORR) activities were evaluated in O 2 -saturated 0.1 M HClO 4 at 0.9 V vs. reversible hydrogen electrode. Pt 0.6nm /Ni 0.6nm /Pt(1 1 1) and Pt 0.6nm /Ni 0.6nm /Pt(1 1 0) were most active among the respective Pt/Ni/Pt(1 1 1) and Pt/Ni/Pt(1 1 0) sandwich series: the activities of the former and latter sandwich structures were approximately five- and threefold greater than those of the corresponding clean Pt(1 1 1) and (1 1 0) substrate surfaces. Scanning tunneling microscopy images of the as-prepared Pt 0.6nm /Ni 0.6nm /Pt(1 1 1) and Pt 0.6nm /Ni 0.6nm /Pt(1 1 0) surfaces revealed three-dimensionally grown hexagonal-shaped small domains of Pt(1 1 1) (approximately 2 nm in size) and parallelogram-shape (1 1 0) terrace islands oriented along 〈1 1 0〉, respectively. The results indicate that not only the atomic arrangements of the topmost Pt layers but also the nanoscale morphologies of Pt–Ni in the surface vicinities determine the enhancement of the ORR activity of Pt–M alloy catalysts.

Oxygen reduction reaction (ORR) activities for 0.3 nm thick Co deposited Pt(111), Pt(100), and Pt(110) surfaces prepared by molecular beam epitaxy (MBE) were investigated after the samples were transferred from ultra-high vacuum (UHV) to an electrochemical system without being exposed to air. The low-energy electron diffraction and IR reflection–absorption spectroscopic results for adsorbed carbon monoxide indicated that Co deposition on the low-index single-crystal Pt substrates at 753 K–773 K led to Pt-enrichment at the topmost surfaces through surface segregation of the substrate Pt atoms. The ORR activity was evaluated in O 2 -saturated 0.1 M HClO 4 at 0.9 V vs. reversible hydrogen electrode (RHE) for the as-prepared Pt-enriched Co 0.3 nm /Pt(111), Co 0.3 nm /(100), and Co 0.3 nm /(110) surfaces; the respective surfaces exhibited about 10, 2, and 1.5 times the ORR activity relative to their corresponding clean surfaces. When 1000 potential cycles between 0.6 V and 1.0 V were applied, the activities for the Pt-enriched Co 0.3 nm /Pt(111) and Co 0.3 nm /Pt(110) surfaces decreased and came close to that for as-cleaned Pt(110). These results show that the ORR activity enhancement for Pt–Co alloy surfaces significantly depends upon the atomic arrangement of the Pt-enriched topmost surfaces and the underlying Co atoms.

Pt-deposited Au(1 1 1) surfaces were prepared by molecular beam epitaxy (MBE). Surface structures of the samples were verified with reflection high-energy electron diffraction (RHEED), scanning tunneling microscopy (STM) and infrared reflection absorption spectroscopy (IRRAS) for adsorbed carbon monoxide in an ultra-high vacuum (UHV) condition. The UHV-results show that epitaxial growth of Pt(1 1 1) on clean Au(1 1 1). A 0.3-nm-thick Pt deposition found to almost cover the Au surface. In addition, monoatomic-height Pt islands were observed on the topmost surface. Electrochemical properties were evaluated by cyclic voltammetry (CV) and linear sweep voltammetry (LSV) using a rotating disk electrode apparatus set in an N2-purged glove box. A CV curve of the Pt0.3nm/Au(1 1 1) recorded in the potential region of 0.05–1.0 V vs. reversible hydrogen electrode (RHE) was similar to that of clean Pt(1 1 1), except for the lack of ‘butterfly’ peaks at 0.8 V. Oxygen reduction reaction (ORR) activities of the samples were evaluated by kinetic controlled current densities at 0.9 V; the activity of the Pt0.3nm/Au(1 1 1) was ca. 1.8 times higher than that of the clean Pt(1 1 1). During CV curve measurements of the Pt0.3nm/Au(1 1 1) in the region of 0.05–1.7 V, a redox feature at 0.13 V became apparent; the ORR activity evaluated after the CV measurements was higher than the as-prepared sample. An UHV-STM image collected after re-transfer from the glove box showed two to three monoatomic-height Pt mounds whose slopes were 10–20°. The results clearly showed that surface defects at the topmost Pt(1 1 1) layer such as steps contribute to the ORR enhancement of MBE-prepared Pt/Au(1 1 1).

Oxygen reduction reaction (ORR) activities were evaluated for clean Pt(111) and Ni/Pt(111) model catalysts fabricated by molecular beam epitaxy. Exposure of clean Pt(111) to 1.0 L CO at 303 K produced linear-bonded and bridge-bonded CO-Pt IR bands at 2093 and 1858 cm−1. In contrast, 0.3-nm-thick Ni deposited on Pt(111) at 573 K (573 K-Ni0.3 nm/Pt(111)) produced broad IR bands for adsorbed CO at around 2070 cm−1; the separation of reflection high-energy electron diffraction (RHEED) streaks is slightly wider for 573 K-Ni0.3 nm/Pt(111) than for the clean Pt(111). For 823 K-Ni0.3 nm/Pt(111), the separation of the RHEED streaks is the same as that for the Pt(111), and a single sharp IR band due to adsorbed CO is located at 2082 cm−1. The results suggest that for the 823 K-Ni0.3 nm/Pt(111), a Pt-enriched outermost surface (Pt-skin) was formed through surface segregation of the substrate Pt atoms. ORR activities for the 573 K- and 823 K-Ni0.3 nm/Pt(111) as determined from linear sweep voltammetry curves were five times and eight times higher than that for clean Pt(111), respectively, demonstrating that Pt-skin generation is crucial for developing highly active electrode catalysts for fuel cells. Keywords: Pt(111), Pt–Ni surface alloy, Molecular beam epitaxy, Oxygen reduction reaction, Polymer-electrolyte fuel cells

Infrared reflection absorption spectroscopy (IRRAS) was used to investigate carbon monoxide (CO) adsorption on Pt(1 0 0) surfaces deposited with Co layers with different thicknesses. Pt(1 0 0) surfaces cleaned in ultrahigh vacuum showed surface reconstruction, i.e., Pt(1 0 0)-hex: two absorption bands ascribable to adsorbed CO on the 1 × 1 surface and hex domains emerge at 2086 and 2074 cm −1 , respectively, after 1.0 L CO exposure. Deposition of a 0.3-nm-thick-Co layer on Pt(1 0 0)-hex at 333 K changes the low-energy electron diffraction (LEED) pattern from hex to p(1 × 1), indicating that the deposited Co lifts the reconstruction. The IRRAS spectrum for 1.0-L-CO-exposed Co 0.3 nm /Pt(1 0 0)-hex fabricated at 333 K yields a single absorption band at 2059 cm −1 . For Co 0.3 nm /Pt(1 0 0)-hex fabricated at 693 K, the LEED pattern shows a less-contrasted hex and the pattern remains nearly unchanged even after CO exposure of 11 L, although only 1.0 L CO exposure to Pt(1 0 0)-hex lifts the surface reconstruction. A Co 0.3 nm /Pt(1 0 0)-hex surface fabricated at 753 K exhibits an absorption band at 2077 cm −1 , which is considered to originate from CO adsorbed on the Pt-enriched surface alloy. Co 0.3 nm /Pt(1 0 0)-hex surfaces fabricated above 773 K show a clear hex-reconstructed LEED pattern, and the frequencies of the adsorbed CO bands are comparable to those of Pt(1 0 0)-hex, indicating that the deposited Co atoms are diffused near the surface region. The outermost surface of the 3.0-nm-thick-Co-deposited Pt(1 0 0)-hex is composed of Pt–Co alloy domains even at a deposition temperature of 873 K. Based on the LEED and IRRAS results, the outermost surface structures of Co x /Pt(1 0 0)-hex are discussed.

Infrared reflection absorption spectroscopy (IRRAS) was used to investigate carbon monoxide (CO) adsorption on 0.15 nm-thick–0.6 nm-thick Pd-deposited Pt(1 1 1) bimetallic surfaces: Pd x /Pt(1 1 1) (where x is the Pd thickness in nanometers) fabricated using molecular beam epitaxial method at substrate temperatures of 343 K, 473 K, and 673 K. Reflection high-energy electron diffraction (RHEED) measurements for Pd 0.15–0.6 nm /Pt(1 1 1) surfaces fabricated at 343 K showed that Pd grows epitaxially on a clean Pt(1 1 1), having an almost identical lattice constant of Pt(1 1 1). The 1.0 L CO exposure to the clean Pt(1 1 1) at room temperature yielded linearly bonded and bridge-bonded CO–Pt bands at 2093 and 1855 cm −1 . The CO–Pt band intensities for the CO-exposed Pd x /Pt(1 1 1) surfaces decreased with increasing Pd thickness. For Pd 0.3 nm /Pt(1 1 1) deposited at 343 K, the 1933 cm −1 band caused by bridge-bonded CO–Pd enhanced the spectral intensity. The linear-bonded CO–Pt band (2090 cm −1 ) almost disappeared and the bridge-bonded CO–Pd band dominated the spectra for Pd 0.6 nm /Pt(1 1 1). With increasing substrate temperature during the Pd depositions, the relative band intensities of the CO–Pt/CO–Pd increased. For the Pd 0.3 nm /Pt(1 1 1) deposited at 673 K, the linear-bonded CO–Pt and bridge-bonded CO–Pd bands are located respectively at 2071 and 1928 cm −1 . The temperature-programmed desorption (TPD) spectrum for the 673 K-deposited Pd 0.3 nm /Pt(1 1 1) showed that a desorption signal for the adsorbed CO on the Pt sites decreased in intensity and shifted ca. 20 K to a lower temperature than those for the clean Pt(1 1 1). We discuss the CO adsorption behavior on well-defined Pd-deposited Pt(1 1 1) bimetallic surfaces.

We investigated carbon monoxide (CO) adsorption and desorption behaviors on 0.1-nm-, 0.15-nm-, and 0.3-nm-thick-Pd-deposited Cu(1 1 0) surfaces using infrared reflection absorption (IRRAS) and temperature-programmed desorption (TPD) spectroscopic methods. CO was exposed to the 0.1-nm-thick-Pd/Cu(1 1 0) surface at the substrate temperature of 90 K. The IR band attributable to CO bonded to Cu atoms emerged at 2092 cm−1: the band was located at 2100 cm−1 at saturation coverage, with a shoulder at 2110 cm−1. In addition to these bands, weak absorptions attributable to the Pd CO bonds appeared at 2050 and 1960 cm−1. With increasing Pd thickness, the Pd related-bands became increasingly prominent. Particularly at the early stage of exposure, the band at 2115 cm−1 became visible. The band at 2117 cm−1 dominated the spectra all through the exposures for the 0.3-nm-thick-Pd surface. The TPD spectra of the surfaces showed two remarkable features at around 220–250 and 320–390 K, ascribable ,respectively, to Cu CO and Pd CO. The desorption peaks shifted to higher temperatures with increasing Pd thickness. Based on the TPD and IRRAS results, we discuss the adsorption–desorption behaviors of CO on the Pd/Cu(1 1 0) surfaces.