Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Atomically Defined Co3O4(111) Thin Films Prepared in Ultrahigh Vacuum: Stability under Electrochemical Conditions Firas Faisal,† Manon Bertram,† Corinna Stumm,† Serhiy Cherevko,‡,§ Simon Geiger,‡ Olga Kasian,‡ Yaroslava Lykhach,† Ole Lytken,† Karl J. J. Mayrhofer,‡,§ Olaf Brummel,*,† and Jörg Libuda†,∥ †
Lehrstuhl für Physikalische Chemie II, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstraße 3, D-91058 Erlangen, Germany ‡ Department of Interface Chemistry and Surface Engineering, Max-Planck-Institut für Eisenforschung GmbH, Max-Planck-Straße 1, 40237 Düsseldorf, Germany § Helmholtz-Institute Erlangen-Nürnberg for Renewable Energy (IEK-11), Forschungszentrum Jülich GmbH, Erlangen 91058, Germany ∥ Erlangen Catalysis Resource Center, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstraße 3, D-91058 Erlangen, Germany ABSTRACT: We have explored the stability, the structure, and the chemical transformations of atomically defined Co3O4(111) thin films under electrochemical conditions. The well-ordered Co3O4(111) films were prepared on an Ir(100) single crystal under ultrahigh vacuum (UHV) conditions and subsequently transferred and characterized in the electrochemical environment by means of cyclic voltammetry (CV), scanning flow cell inductively coupled plasma mass spectrometry (SCF-ICP-MS), and electrochemical infrared reflection absorption spectroscopy (EC-IRRAS). We have found that the Co3O4(111) films are stable in phosphate buffer at pH 10 at potentials between 0.33 to 1.33 VRHE. In the corresponding potential range, the corrosion rates established by means of SCF-ICP-MS were well below 0.1 monolayer per hour. Additionally, low-energy electron diffraction (LEED) and X-ray photoelectron spectroscopy (XPS) studies have shown that the long-range order, the thickness, and the composition of the films were preserved under electrochemical conditions. Disintegration of the film and formation of holes after repeated potential cycling within the stability window were ruled out by EC-IRRAS using CO as a probe molecule. In general, the stability of the Co3O4(111) films depends critically on both the pH and electrode potential. Increasing the pH from 10 to 12 compromised the structural stability of the Co3O4(111) films due to faster redox processes at the surface. In particular, we observed accelerated oxidation of cobalt followed by the formation of oxyhydroxide during the anodic scan and accelerated reduction to cobalt hydroxides during the cathodic scan. Decreasing the pH from pH 10 to pH 8, on the other hand, led to faster dissolution, in particular at potentials below 0.2 VRHE, where the dissolution rate increased rapidly due to formation of soluble Co(II) species. Our studies demonstrate that thin well-ordered oxide films prepared in UHV can be transferred into the electrochemical environment while preserving their atomic surface structure if the conditions are chosen carefully. This opens a surface science approach to atomically defined oxide−electrolyte interfaces.
1. INTRODUCTION
Cobalt oxide is one of the most intensively studied oxides in electrocatalysis.2,4,11,12 It has been noticed early on that cobalt oxide efficiently catalyzes the OER under alkaline conditions. However, Co3O4 is also an efficient catalyst for other electrocatalytic reactions, such as the oxidation of hydrocarbon oxygenates.13,14 Noteworthy, the catalytic potential of cobalt oxide is not limited to electrocatalysis but has more recently also been recognized in heterogeneous catalysis. For instance,
Understanding electrochemical processes at oxide interfaces is essential for the development of electrocatalytic materials.1−3 On the one hand, noble and transition metal oxides are active catalysts for essential reactions in energy technology, such as the oxygen evolution reaction (OER).4−6 On the other hand, oxides may serve as modifiers or supports that improve the catalytic properties or stability of the active component.7−9 For instance, oxide matrices can help to stabilize highly dispersed noble metal aggregates and, thereby, increase the noble metal efficiency in electrocatalytic transformations.10 © XXXX American Chemical Society
Received: January 17, 2018 Revised: March 9, 2018
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DOI: 10.1021/acs.jpcc.8b00558 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Figure 1. (a) Schematic representation of the UHV transfer system; (b) sketch of the transfer process from UHV to the electrochemical environment.
some cases, the limited thickness and the higher defect density. Yet, spectroscopic and microscopic studies on atomically defined oxide surfaces have provided insight into many elementary processes in heterogeneous catalysis that could not have been obtained otherwise.24,25,27 In this work, we transfer this surface science approach to the field of electrochemistry. Our aim is to prepare atomically defined and well-characterized oxide surfaces in UHV and to transfer these into the electrochemical environment while preserving their atomic surface structure. While several groups have used surfaces prepared in UHV for electrochemical studies, most of these studies focused on metal and alloy surfaces so far (e.g., see refs 28−31). Only very few examples exist in which ordered oxide interfaces, prepared in UHV, were subsequently studied in liquid water or in liquid electrolytes.32−35 In our experiments, we use well-ordered Co3O4(111) films prepared on an Ir(100) single crystal.36 The preparation procedure was previously developed by Heinz, Hammer, and co-workers, and the structure of the films was characterized in great detail by LEED-IV and STM.37,38 Briefly, the film consists of large and atomically flat domains and exhibits a high degree of order. The surface is terminated by tetrahedral Co2+ ions of the Co3O4 spinel structure. The terminating Co2+ ions form a hexagonal lattice with an interatomic distance of 5.7 Å and are located on a close packed layer of oxygen ions (see Figure 1 and ref 36). It is noteworthy that the film can be prepared with different thickness ranging from few atomic layers to several tens of nanometers without major loss of order. In previous work, the adsorption properties and thermal behavior of the Co3O4 films were also characterized in great detail.38,39 For our planned electrochemical studies, Co3O4 is a particularly well-suited material, as it is stable over a relatively
Co3O4 is one of the reducible supports that facilitate lowtemperature CO oxidation on Au15 and, even more surprising, may become a highly active CO oxidation catalyst itself if it exposes specific crystallographic facets.16 In spite of the importance of oxide materials, it has been a long-standing challenge to understand the structure and functionality of oxide interfaces from the fundamental point of view, both in heterogeneous catalysis17 and in electrocatalysis.1−3,18 Among the main reasons is the highly dynamic interface of oxide materials under reaction conditions. The surface structure, the composition, and for reducible oxides, also the oxidation state are sensitive to the reaction environment, such as the temperature, gaseous or dissolved reactants, the pH, or electrochemical potential. In particular, the interaction with water has been a long-standing and controversially discussed topic, ranging from molecular adsorption versus dissociation19−23 to restructuring phenomena and the formation of complex phases, such as layered oxyhydroxides.2 From the microscopic point of view, the dynamically formed structures are inherently difficult to characterize and structure−reactivity relationships are, therefore, difficult to obtain. Most of the atomistic understanding available today on the interaction of oxide surfaces with gaseous reactants has been obtained in surface science studies on well-ordered oxide surfaces.24,25 Such surfaces can be prepared either from oxide single crystals or in the form of thin oxide films. Both approaches come with specific advantages and drawbacks. The advantages of oxide films grown on metal supports are their electrical and thermal conductivity (and the associated accessibility by many surface science methods) and the variability of their structure.26 Typical drawbacks of the thin film approach are the complex preparation procedures and, in B
DOI: 10.1021/acs.jpcc.8b00558 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
UV, 18.2 MΩ cm at 25 °C,
