ABSTRACT:Photoelectrochemical water splitting is a promising approach for renewable production of hydrogen from solar energy and requires interfacing advanced water splitting catalysts with semiconductors. Understanding the mechanism of function of such electrocatalysts at the atomic scale and under realistic working conditions is a challenging, yet important, task for advancing efficient and stable function. This is particularly true for the case of oxygen evolution catalysts and, here, we study a highly active Co 3 O 4 /Co(OH) 2 biphasic electrocatalyst on Si by means of operando ambient pressure X-ray photoelectron spectroscopy performed at the solid/liquid electrified interface. Spectral simulation and multiplet fitting reveal that the catalyst undergoes chemical-structural transformations as a function of the applied anodic potential, with complete conversion of the Co(OH) 2 and partial conversion of the spinel Co 3 O 4 phases to CoO(OH) under pre-catalytic electrochemical conditions. Furthermore, we observe new spectral features in both Co 2p and O 1s core level regions to emerge under oxygen evolution reaction conditions on CoO(OH). The operando photoelectron spectra support assignment of these newly observed features to highly active Co 4+ centers under catalytic conditions. Comparison of these results to those from a pure phase spinel Co 3 O 4 catalyst supports this interpretation and reveals that the presence of Co(OH) 2 enhances catalytic activity by promoting transformations to CoO(OH). The direct investigation of electrified interfaces presented in this work can be extended to different materials under realistic catalytic conditions, thereby providing a powerful tool for mechanism discovery and an enabling capability for catalyst design. Introduction Sustainable solutions are required to mitigate the impact of increasing world energy demand on the environment and avoid depletion of natural energy sources (1). While transduction of solar energy to electricity has already made a significant impact on the renewable energy sector, the intermittent nature of sunlight imposes critical storage challenges (2,3,4,5,6). Furthermore, addressing broader energy needs, particularly in transportation, requires new technologies for the next generation of renewable fuels. Within this context, (photo)electrochemical conversion of sunlight to hydrogen -or other chemical fuels -represents an appealing approach to both solar energy conversion and high energy density storage. An essential step in such artificial photosystems is the oxygen evolution reaction (OER), in which the protons and electrons required for the fuel formation reaction are harnessed from water (2-7, 8, 9). However, OER pathways can be complex and impose significant kinetic bottlenecks. To address this challenge, increasing efforts have been devoted to developing OER catalysts possessing high activity, long-term durability, and low cost, a combination of attributes that is most commonly obtained with first row transition metal oxides (10,11,12,13,14,15...
We have structurally, chemically and electronically characterized the most stable (010) surface of a Mo-doped BiVO 4 single crystal. Low energy electron diffraction (LEED) reveals that the surface is not significantly reconstructed from a bulk termination of the crystal. Synchrotron based X-ray spectroscopies indicate no surface enhancement of any of the crystal constituents and that the Mo dopant occupies tetrahedral sites by substituting for V at the surface. Using resonant photoemission to study the valence band structure as the V L 3 edge is scanned we observe an intra-band gap state associated with reduced vanadium formed by the Mo doping. This state is likely associated with small polaron formation at the surface. This feature is enhanced at a photon energy that is not resonant with any of the main features in the absorption spectrum of the pristine BiVO 4. This indicates that the additional electron from Mo doping likely induces further distortion of the VO 4 tetrahedral units and generates a new conduction band state either by splitting of the V dz 2 states or by hybridization of V d zx and V dz 2 states. We measure a work function of 5.15 eV for the BiVO 4 (010) surface. Measurement of the work function allows us to recast the electronic energy levels onto the normal hydrogen electrode scale for comparison to the standard reduction and oxidation potentials of water. This detailed study should provide a basis for future work aimed at a molecular level understanding of BiVO 4 /electrolyte interfaces used for photoelectrochemical water splitting.
We show atomic oxygen on an unreconstructed Ag(110) surface has a O 1s binding energy ≤ 528 eV and its stable at low coverages. Our findings point to the idea of multiple selective oxygen species in ethylene epoxidation on Ag.
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