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

Abstract

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...

A Demonstration of Pt L3-Edge EXAFS Free from Au L3-Edge Using Log–Spiral Bent Crystal Laue Analyzers

“…Extended X-ray absorption fine structure (extended XAFS or EXAFS) spectroscopy is a suitable and widely-used method to investigate the local atomic structures of fuel-cell catalysts because of its atomic selectivity and applicability to nanoparticles under electrochemical environments [6]. However, in case of Pt-Au system, it is difficult to obtain a Pt L 3 -edge EXAFS sufficient for its analysis due to the interference between Pt and Au, which are only separated by~350 eV, so that Au L 3 -edge appears at~9.6 Å −1 in Pt L 3 -edge EXAFS [7,8]. Although the problem can be solved by measuring K-edge XAFS, where Pt and Au K-edges are separated by 2300 eV [9,10], the information in the long-range order is limited by the lifetime broadening, and Pt L 3 -edge EXAFS measurement is preferable.…”

Bent crystal Laue analyser combined with total reflection fluorescence X-ray absorption fine structure (BCLA + TRF-XAFS) and its application to surface studies

“…In the XAFS spectrum without the BCLA, strong Au L X-ray fluorescence appeared above 11900 eV (the Au L 3 edge). Even under the total reflection conditions, X-rays could penetrate the bulk and excite the Au edge (Yuan et al, 2017). Fig.…”

“…By combining with the polarization dependence, we can obtain the three-dimensional structure in PTRF-XAFS. In the liquid-solid interface system, PTRF-XAFS requires a less than 1 mm thick solution to reduce absorption and scattering of the solution (Tamura et al, 2000;Masuda et al, 2016;Yuan et al, 2017Yuan et al, , 2018. However, such a thin liquid layer hinders material diffusion and cannot follow the electrochemical process continuously.…”

“…X-rays can pass through the gas phase, and thus in situ PTRF-XAFS measurements can be performed in the presence of a gas phase (Tanizawa et al, 2003;Chun et al, 1997;Asakura et al, 1997). In addition, the PTRF-XAFS method can be applied to surface analysis in the liquid phase (Masuda et al, 2012;Yuan et al, 2018Yuan et al, , 2017Gullikson, 2001;Henke et al, 1993;Nelson et al, 1994). However, the elastic X-ray scattering of the liquid overlayer becomes large and causes a serious increase of background X-rays; hence the liquid overlayer should be made as thin as possible.…”

Reaction Stoichiometry and Mechanism of Pt Deposition via Surface Limited Redox Replacement of Copper UPD Layer on Au(111)

“…XAFS Measurements and Analysis. To eliminate oxygen contamination, an in situ polychloro-trifluoroethylene (PCTFE) XAFS cell 27 was assembled in the glovebox. A Mylar film with thickness of 6 μm was used as the X-ray window.…”