Epitaxial Growth of Ag on Au(111) by Galvanic Displacement of Pb and Tl Monolayers

We studied the initial stage of a Pt monolayer produced by surface-limited redox replacement (SLRR) using polarizationdependent total reflection fluorescence X-ray absorption fine structure (PTRF-XAFS). Different from the widely accepted understanding that metallic monolayer islands are formed, our XAFS showed that the Pt monolayer, initially present on the Au(111) substrate, was mainly in the form of a planar [PtCl 4 ] 2¹ complex with its molecular plane parallel to Au(111). This result provides a new insight into the mechanism of SLRR.Keywords: X-ray absorption fine structure (XAFS) | Surface-limited redox replacement reaction | Pt monolayerPolymer electrolyte fuel cells (PEFCs), which emit no greenhouse gases, have attracted much attention because of the necessity for the modern society to shift from fossil fuels to clean energy. However, several issues concerning electrocatalysts, including their cost and durability, have greatly inhibited the practical use of PEFCs.1 To reduce the cost of the electrocatalyst, we need to increase the surface area of Pt, which is the main component of the fuel cell catalyst currently. Considerable effort has been devoted to investigating the electrodeposition of Pt. 24Owing to the high surface energy and low wettability of Pt, it is difficult to obtain monolayer deposition on a flat surface with conventional electrodeposition methods.2,4 Brankovic et al. developed a method for Pt monolayer deposition by galvanic displacement of a layer of sacrificial underpotential deposition (UPD) metal, which is called surface-limited redox replacement (SLRR).5 Several sacrificial materials, like Cu, 68 Pb,7,9 and H,10 have been studied for the deposition of Pt monolayers. In addition to Pt, the SLRR was extended to the monolayer deposition of other metals, such as Pd, 11 Ru, 12 and Ag. 5,13 Furthermore, bimetallic monolayer depositions were also achieved by the SLRR.14,15 However, there are only limited numbers of studies regarding the mechanism of the deposition process on atomic levels. In situ scanning tunneling microscopy (STM) was used to study the structure of Pt submonolayers.8 Brankovic et al. systematically studied the reaction mechanism of SLRR to replace the UPD Cu with Pt. They proposed a kinetic model, 16 based on the stoichiometry of SLRR, 6 and also proposed the nucleation mechanism of the Pt monolayer clusters, 17 and the UPD procedure on the Pt monolayer modified surface. 9,15 However, some aspects remain unclear. For example, what is the chemical state and structure of the Pt submonolayers and Pt submonolayerAu interaction, which are very important parameters since they are related to the catalytic properties? To examine the Pt initial structure created by the SLRR of the Cu UPD layer, we applied polarization-dependent total reflection fluorescence-X-ray absorption fine structure (PTRF-XAFS) techniques. 1820PTRF-XAFS is a powerful way of characterizing the surface structure and electronic state of less than a monolayer of adsorbate on atomically flat surfaces even und...

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confidence: 99%

“…In addition to Pt, the SLRR was extended to the monolayer deposition of other metals, such as Pd, 11 Ru, 12 and Ag. 5,13 Furthermore, bimetallic monolayer depositions were also achieved by the SLRR. 14,15 However, there are only limited numbers of studies regarding the mechanism of the deposition process on atomic levels.…”

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confidence: 99%

Polarization-dependent Total Reflection Fluorescence X-ray Absorption Fine Structure (PTRF-XAFS) Studies on the Structure of a Pt Monolayer on Au(111) Prepared by the Surface-limited Redox Replacement Reaction

et al. 2017

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“…The importance of minimizing dissolved oxygen in the e-ALD electrolytes must be emphasized here. This is a known issue in many other electrochemical ALD processes 30,36 which also suffer from extreme sensitivity to dissolved oxygen content of the electrolytes.…”

Section: Methodsmentioning

confidence: 99%

Electrochemical Atomic Layer Deposition of Cobalt Enabled by the Surface-Limited Redox Replacement of Underpotentially Deposited Zinc

Venkatraman

Dordi

Akolkar

2017

J. Electrochem. Soc.

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Electrochemical atomic layer deposition (e-ALD) process for fabricating cobalt (Co) nano-films is reported. The e-ALD process employs a two-step approach in which underpotential deposition (UPD) is first used to form a sacrificial adlayer of zinc (Zn) on a ruthenium (Ru) substrate. The sacrificial Zn adlayer then undergoes spontaneous surface-limited redox replacement (SLRR) by nobler Co. This provides an atomic layer of Co on the substrate surface. The two-step process is repeated cyclically to build multilayers of Co. The unique feature of the e-ALD approach presented herein is that it utilizes Zn as the sacrificial adlayer instead of Pb or Cu used conventionally in the e-ALD sequence of noble metals. The use of sacrificial Zn uniquely renders its redox replacement by Co to be thermodynamically favorable, thereby enabling Co e-ALD. In the present report, we discuss the electrochemical characteristics of the UPD and SLRR process steps, the e-ALD deposition rate and the deposit surface roughness. The Co deposits formed via e-ALD do not exhibit roughness amplification during the first 10 cycles of e-ALD, which is indicative of atomic-scale layer-by-layer growth of Co on the underlying Ru substrate. High-performance integrated circuits utilize nano-scale, currentcarrying copper (Cu) interconnects. In accordance with the Moore's law, 1 miniaturized interconnects are required in modern devices to obtain superior device performance. The continued shrinkage in the size (cross-sectional area) of each interconnect leads to an increase in its electrical resistance. This detrimentally impacts the electrical performance of the integrated circuit.2 Furthermore, aggressive interconnect size scaling below the 10 nm node poses challenges to void-free interconnect fabrication. State-of-the-art interconnects are fabricated of Cu metal due to its low electrical resistivity and superior electromigration resistance.3 The interconnect signal delay (τ) is given by τ = RC, where R is the interconnect resistance and C is the capacitance of the surrounding inter-layer dielectric. The interconnect resistance R increases dramatically for Cu interconnect dimensions below 40 nm due to the substantial increase in the electrical resistivity of Cu at such narrow dimensions. The resistivity increase is typically attributed to electron scattering processes at grain boundaries and at interfaces within the Cu interconnect structure. Such scattering processes become dominant when the interconnect dimension approaches the electron mean free path (EMFP = 39 nm at room temperature) in Cu.4,5 As a consequence, the interconnect signal delay increases. To overcome this critical issue, an interconnect material which exhibits lower electrical resistivity at narrow (sub 10 nm) dimensions is required. Cobalt (Co) is considered as a promising alternative interconnect material to replace the conventionally used Cu.6,7 At narrow dimensions, i.e., below 10 nm, Co exhibits comparable or lower electrical resistivity compared to Cu, largely attributed to the lower ...

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“…The surface area of NPG was obtained simply by dividing the average charge by 300 μC/cm 2 , which is the charge density of a Pb UPD layer on a flat polycrystalline Au electrode, prepared by a procedure described elsewhere. 40 Adhesion assessment.-NPG fabricated on modified GC was subjected to rotation using a Pine MRS Rotator System which was operated by a Pine MRS Motor Controller. The rotation was administered at open circuit potential for two hours at 1600 rpm in a solution of 0.1 M HClO 4 .…”

Section: Methodsmentioning

confidence: 99%

Enhanced Adhesion of Ultrathin Nanoporous Au Deposits by Electrochemical Oxidation of Glassy Carbon

Xia

Rooney

Ambrozik

et al. 2015

J. Electrochem. Soc.

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Electrochemical (EO) and Thermochemical (TO) oxidation have been considered as means for improved adhesion of metals to glassy carbon (GC) substrates. This paper is aimed at studying the effect of EO on the all-electrochemical processing of nanoporous gold (NPG) catalysts with emphasis on their adhesion to a GC substrate. It has been shown by two independent assessment approaches that EO improves twice the amount of retained NPG on GC helping to keep about 90% originally existing material after a two-hour rotation disk electrode test at 1600 rpm. This work also demonstrates that EO modifies the GC in a way making it more passive in the alloy deposition and subsequent dealloying processes. This effect results in a progressive nucleation density decrease with the depth of oxidation which in turn leads to a lack of continuity of the NPG layer. This trend is also accompanied by a decrease in deposition efficiency as a substantial part of the cathodic current is spent on the reduction of the pre-oxidized GC surface. Our study identifies the best outcome of the trade-off scenario between nucleation density, deposition efficiency and adhesion strength to be associated with depth of oxidation in the range 0.10 to 0.25 μm. NPG based catalyst on glassy carbon (GC).-Nanoporous Au (NPG) is a nano-material with three dimensional (3D) continuous structure featuring Au with interconnected nano-sized pores and ligaments.1 NPG and other nanoporous metals have been intensively investigated in recent years. This conductive material has an open, 3D porous framework with surface areas that even in an ultrathin NPG layer configuration could be an order of magnitude larger than the area of planar Au. Besides its continuous crystal lattice throughout the porous network, it has a pore size that is adjustable via simple thermal post-processing, which makes it potentially applicable in various fields such as electro-and gas-phase catalysis, optics, sensors and actuators. [2][3][4][5] It is known that nanoparticulate gold possesses remarkable catalytic activity toward oxidation reactions, should the particles be well supported and smaller than 5 nmin size. 6 Recently, ultrathin films of NPG have been shown to serve as catalysts for fuel cell applications as an alternative to their nanoparticle (NP) counterparts.7-9 These films are generated electrochemically under precise potential/current control, which minimizes material loss and surface contamination over the multi-step NP synthetic routines. 7,10 Compared to nanoparticle synthesis which often lacks control due to the large number of the processing steps, ultra thin films of NPG are cost effective, simple to fabricate, and show better reproducibility in terms of quality and reactivity. Another advantage of NPG films is that the edge effect that lowers the catalytic activity characteristic for NP based catalysts does not exist in NPG films due to its continuous nature. 11In our previous work, we established an all-electrochemical synthetic method for a NPG-based catalyst that maximized t...

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Epitaxial Growth of Ag on Au(111) by Galvanic Displacement of Pb and Tl Monolayers

Cited by 64 publications

References 37 publications

Polarization-dependent Total Reflection Fluorescence X-ray Absorption Fine Structure (PTRF-XAFS) Studies on the Structure of a Pt Monolayer on Au(111) Prepared by the Surface-limited Redox Replacement Reaction

Polarization-dependent Total Reflection Fluorescence X-ray Absorption Fine Structure (PTRF-XAFS) Studies on the Structure of a Pt Monolayer on Au(111) Prepared by the Surface-limited Redox Replacement Reaction

Electrochemical Atomic Layer Deposition of Cobalt Enabled by the Surface-Limited Redox Replacement of Underpotentially Deposited Zinc

Enhanced Adhesion of Ultrathin Nanoporous Au Deposits by Electrochemical Oxidation of Glassy Carbon

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