The electronic, chemical and electrocatalytic processes and intermediates on iron oxide surfaces during photoelectrochemical water splitting

Abstract

We reassess experimental soft X-ray absorption spectra of the oxygen K-shell which we recorded operando from iron oxide during photoelectrochemical water splitting in KOH electrolyte. In particular, we refer to recently reported transitional electron hole states which originate within the charge carrier depletion layer of the iron oxide and on the iron oxide surface. For the latter we find that an intermediate oxy-peroxo species is formed on the iron oxide with increasing bias potential, which disappears upon further polarization of the electrode, concomitantly with the evolution and disappearance of the aforementioned surface state. The oxygen spectra contain also the spectroscopic signatures of the electrolyte water, the position of which changes with increasing bias potential towards lower x-ray energies, revealing the breaking and formation of hydrogen bonds in the water during the experiment. Combined with potential dependent impedance spectroscopy data we are able to sketch the molecular structure of chemical intermediates and their charge carrier dynamics.

Two-site H2O2 photo-oxidation on haematite photoanodes

“…It is noted that additional calculations (not shown here) demonstrate the same qualitative behaviour with similar features of hysteresis and multiplicity of solutions (i.e., bi-stable regimes) over a wide range of parameters spanning orders of magnitude of the respective rate constants, confirming the robustness of our model predictions. Moreover, the simple ER mechanism typically used to describe water photo-oxidation on haematite photoanodes 24 , 25 , 34 , 35 cannot yield such hysteresis because it lacks nonlinearity in the kinetic equations of its elementary steps (see Model equations and numerical computations in Methods for details).

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“…On the other hand, those intermediates can also serve as recombination centres for photo-generated charge carriers 21 . Thus, surface intermediates display intriguing and rich dynamics that have a strong influence on the PEC performance of semiconductor electrodes 14 , 22 , which on haematite photoanodes are still under debate 23 – 25 . Numerous experimental and theoretical reports have suggested the existence of a stable Fe = O intermediate species 24 – 26 .…”

Systematic Investigation of the Electronic Structure of Hematite Thin Films

“…Using UPS, information about the surface potentials including the work function is obtainable. An alternative to PES is synchrotron‐based X‐ray absorption spectroscopy (XAS) which offers the opportunity to perform in operando measurements on devices …”

Water Interaction with Sputter-Deposited Nickel Oxide on n-Si Photoanode: Cryo Photoelectron Spectroscopy on Adsorbed Water in the Frozen Electrolyte Approach

“…13,43 Recent in operando NEXAFS and EIS studies on iron oxide underline the importance of oxygen defect states for the formation of hydroxyl groups at metal oxide surfaces as a preliminary stage of water-splitting. 44 Indeed, XPS analysis indicates that upon water adsorption, predominantly the Ni 2 O 3 phase and presumably defective sites and phase boundaries are reactive. If water dissociates, hydroxylated species form (h-NiO x ).…”