Using “Tender” X-ray Ambient Pressure X-Ray Photoelectron Spectroscopy as A Direct Probe of Solid-Liquid Interface

Abstract

We report a new method to probe the solid-liquid interface through the use of a thin liquid layer on a solid surface. An ambient pressure XPS (AP-XPS) endstation that is capable of detecting high kinetic energy photoelectrons (7 keV) at a pressure up to 110 Torr has been constructed and commissioned. Additionally, we have deployed a “dip & pull” method to create a stable nanometers-thick aqueous electrolyte on platinum working electrode surface. Combining the newly constructed AP-XPS system, “dip & pull” approach, with a “tender” X-ray synchrotron source (2 keV–7 keV), we are able to access the interface between liquid and solid dense phases with photoelectrons and directly probe important phenomena occurring at the narrow solid-liquid interface region in an electrochemical system. Using this approach, we have performed electrochemical oxidation of the Pt electrode at an oxygen evolution reaction (OER) potential. Under this potential, we observe the formation of both Pt2+ and Pt4+ interfacial species on the Pt working electrode in situ. We believe this thin-film approach and the use of “tender” AP-XPS highlighted in this study is an innovative new approach to probe this key solid-liquid interface region of electrochemistry.

Insights into Electrochemical Reactions from Ambient Pressure Photoelectron Spectroscopy

“…However, the limited probing depth generally constrains the technique to the gas/solid or gas/liquid interface, while buried interfaces (solid/solid or liquid/solid) remain inaccessible. With the development of high spatial resolution instruments as well as hard (>7 keV) and "tender" (2-7 keV) X-ray photoelectron endstations, 60 it will be increasingly possible to probe buried interfaces in the near future. Recent demonstrations observed the oxidation of platinum in KF/H2O 60 and Ni in a KOH electrolyte 61 in situ, probing through 10-30 nm of electrolyte.…”

“…With the development of high spatial resolution instruments as well as hard (>7 keV) and "tender" (2-7 keV) X-ray photoelectron endstations, 60 it will be increasingly possible to probe buried interfaces in the near future. Recent demonstrations observed the oxidation of platinum in KF/H2O 60 and Ni in a KOH electrolyte 61 in situ, probing through 10-30 nm of electrolyte. Further development of hard/tender X-rays will provide valuable information on the electrochemical double layer which forms at solid/liquid interfaces.…”

Observing the Electrochemical Oxidation of Co Metal at the Solid/Liquid Interface Using Ambient Pressure X-ray Photoelectron Spectroscopy

“…Following the previously reported experimental procedures, 12 the AP-XPS measurements were performed at bending magnet beamline 9.3.1 at the Advanced Light Source of the Lawrence Berkeley National Laboratory (Berkeley, USA). Beamline 9.3.1 provides X-rays in the "tender" energy range of 2.3−5.2 keV.…”

“…By applying a small change of electrical potential, we can confirm that the electrolyte is electrically continuous through the detection of a rigid shift for the O 1s and K 2p electrolyte specific XPS peaks. 12 High resolution spectra of the Co 2p, O 1s, K 2p, and C 1s are then collected operando at −1.35, −0.4, and +0.4 V along the anodic cyclic voltammetry (CV) branch while the three electrodes are in contact with the electrolyte reservoir. All spectra are referenced to the metallic Co peak, which has a BE of 778.3 eV.…”

“…12−17 In previous studies, a "dip and pull" method was employed to create a stable nanometer-thick aqueous electrolyte on a platinum working electrode surface. 12 This ultrathin solid/liquid interface region was then probed operando during the electrochemical oxidation of the Pt electrode at an OER potential, and the formation of both Pt 2+ and Pt 4+ interfacial species was observed on the Pt foil surface. Recently, the OER reaction mechanism on a platinum electrode in an alkaline electrolyte was investigated by operando AP-XPS, and the Pt δ −OH ads was proposed to be the rate determining species for OER.…”