Near Surface Phase Transition of Solute Derived Pt Monolayers

Abstract

As the loadings of precious metals in surfacechemical systems continue to decrease for photo-and electro-catalysts for energy and environmental applications, the study of near-surface electronic and atomic structure in functional materials becomes critically important. Extremely small quantities of active elements, whether grown as clusters or ultrathin films, exhibit changes in catalytic activity that arise from both size effects and electron-transfer effects. These size and transfer effects can be related to increased propensity for oxidation of the metallic deposit, as well as to various changes in electrochemical performance such as durability or required overpotential for a given reaction. This work establishes a minimum threshold for Pt loading beyond which bulk-type electronic behavior may be expected. By iteratively growing atomic monolayers and multilayers using selflimited electrodeposition and studying these films using core-electron spectroscopy (X-ray absorption and X-ray photoelectron spectroscopy), electrochemical methods and DFT-based computations the fundamental interactions that govern oxidation state and electron transfer near the surface of a Pt-Au bimetallic system have been explored. It has been shown that the Pt-Au system exhibits increased tendency for the Pt layer to remain cationic below a minimum threshold film thickness of two monolayers. At monodispersed levels of submonolayer coverage Pt exhibits deviated electronic structure, reactivity, and metal stability compared to films in excess of this minimum threshold thickness. At three monolayers Pt is thick enough to avoid the preference for cationicity and the resulting higher rates of metal dissolution, but thin enough to benefit from electron transfers from Au that assist in lowering the overpotentials for CO oxidation. This study shows the efficacy of a concerted method for the investigation of near-surface phenomena in multicomponent systems. By combining electrochemical and vacuum studies of solutederived samples with advanced computational techniques, a multifaceted understanding of these architectures has been achieved.

Contiguous and Atomically Thin Pt Film with Supra‐Bulk Behavior Through Graphene‐Imposed Epitaxy

“…We further observe the opposing chemical shifts in the Au and Pt 4f 7/2 peak from 2 to 3 nominal multilayers of Pt, indicating that net charge transfer between the substrate and the surface atoms (ligand effect) is dimensionally tunable. Systematic studies of the dimensional tuning of charge transfer to/from Pt monolayer films can be found elsewhere for the graphene‐less and the with‐graphene cases . These studies showed that subtle core‐level shifts are correlated to the shifts in the observed ORR potentials.…”

“…Pt and graphene architectures were prepared using chemical vapor deposition and the surface limited redox replacement (SLRR) as per procedures detailed in Supporting Information—Synthesis, while SLRR and its variant, electrochemical atomic layer deposition, have been used in the past to deposit metal on metal by Stickney et al, Brankovic et al, Fayette et al, Alamgir et al has successfully used it to produce uniform ultrathin platinum layers over various substrates, including bulk metals like gold and ruthenium, oxides such as TiO 2 and 2D materials like graphene . Extended X‐ray absortion fine structure (EXAFS) was conducted on Pt_nML/GR/Ligand samples at the National Synchrotron Light Source (NSLS Beamline X23A2) and analysis was performed to investigate the Pt atomic near‐neighbor structure ( Figure ).…”

“…including bulk metals like gold [25][26][27] and ruthenium 28 , oxides such as TiO 2 29 and 2D materials like graphene. [9][10][11] EXAFS was conducted on Pt_nML/GR/Ligand samples at the National Synchrotron Light Source (NSLS Beamline X23A2) and analysis was performed to investigate the Pt atomic nearneighbor structure (Figure 1).…”

“…Systematic studies of the dimensional tuning of charge transfer to/from Pt monolayer films can be found elsewhere for the graphene-less 25,27,28 and the with-graphene cases. 9,10 These studies showed that subtle core-level shifts are correlted to the shifts in the observed ORR potentials.…”

Synthesis, Characterization, and Computation of Catalysts at the Center for Atomic-Level Catalyst Design

“…160,182 For Pt adsorption on Au, changes in DDEC NACs as a function of Pt coverage were correlated to XPS and XAS data. 183 Although the extraction of NACs from diffraction experiments is challenging, 179 some of the most careful diffraction studies on crystals with small unit cells (e.g., formamide, MgO, and NaF) gave NACs similar to the DDEC method. 160 The DDEC method also includes a way to compute atomic spin moments (ASMs) in materials with collinear and noncollinear magnetism.…”