1 The Role of Surface States in the Oxygen Evolution Reaction on Hematite Beniamino Iandolo* and Anders Hellman Applied Physics Department Chalmers University of Technology Fysikgränd 3, SE‐41296 Göteborg (Sweden) E‐mail: iandolo@chalmers.se, ahell@chalmers.se Abstract: Hematite (α‐Fe2O3) is an extensively investigated semiconductor for photoelectrochemical (PEC) water oxidation from water splitting. The nature and role of surface states on the oxygen evolution reaction (OER) remain however elusive. Here, first‐principles calculations were used to investigate surface states on hematite under photoelectrochemical conditions. The density of states for two relevant hematite terminations was calculated, and in both cases the presence and the role of surface states was rationalized. Calculations also predicted a Nerstian dependence on the OER onset potential on pH, which was to a very good extent confirmed by PEC measurements on hematite model photoanodes. Impedance spectroscopy characterization confirmed that the OER takes place via the same surface states irrespective of pH. These results provide a framework for a deeper understanding of the OER when it takes place via surface states. Sunlight‐driven photoelectrochemical (PEC) water splitting carries the promise of an efficient, sustainable method for harvesting solar energy and storing it in chemical bonds in form of fuels. [1–3] In order to make PEC water splitting economically competitive, we need photoelectrodes that are inexpensive, stable against corrosion, as well as efficient in converting solar energy into chemical energy. [4] The latter requirement translates into: broad absorption of sunlight, high yield of charge carriers delivered to the appropriate semiconductor/electrolyte interface, and high catalytic activity for both hydrogen and oxygen evolution reactions (HER and OER, respectively). Since no photoelectrode has been found so far to satisfy all these requirements simultaneously, [5] research has focused on identifying materials that efficiently address some of the processes taking place in a PEC cell. In this context, hematite (α‐Fe2O3) has attracted great interest as photoanode for the OER, [6–14] being an inexpensive semiconductor, stable in electrolytic environment of pH higher than 3 [15] and capable of absorbing a significant amount of visible light thanks to its bandgap energy of around 2 eV. [16] The microscopic mechanism of the OER on hematite has not been identified yet and is the object of intense investigation. [9,17] It is generally agreed, in any case, that surface states play an important role in determining the OER efficiency. The latter is usually limited by recombination at these surface states. However, the chemical identity and role of the states are not fully understood. [18] In particular, it is not clear whether they represent intermediates in the OER, or instead participate in parasitic reactions. Herein, we used a unique combination of first‐principles calculations and PEC characterization to investigate the role of surface states on the OER on hematite. Density functional theory (DFT) calculations were employed to resolve the density of states (DOS) for hydroxyl (OH‐) terminated and oxygen (O‐) terminated hematite surfaces. The OH‐terminated surface is associated with occupied surface states extending over a broad range of energies in the bandgap, while the O‐terminated surface is characterized by occupied surface states close to the top of the valence band. The electrochemical potential at which the O‐terminated surface becomes more stable corresponds to the predicted onset potential of the OER, Vonset. The OER then takes place via the surface states whose energy is just above the top of the valence band. Moreover, Vonset is predicted to decrease linearly with increasing pH, following the Nernst equation. Such dependence of Vonset on pH is not expected if one considers only the position of the valence band edge in hematite and the OER potential, which both shift according to the same Nerstian dependence [19] (although a somewhat beneficial effect of increasing the pH from neutral to basic has been reported in terms of cathodic shift of Vonset). [20] These predictions are confirmed to a large extent by PEC characterization of model photoanodes based on hematite thin films. Impedance spectroscopy confirmed the presence of surface states that are discharged when the OER is initiated, and Vonset shifted linearly towards more cathodic potentials with a slope of 49 mV per pH unit. DFT calculations were performed as implemented in VASP, [21–23] and the computational electrochemical framework of Rossmeisl et al. was adapted [24,25] (see Experimental Section for details). Figure 1 shows the calculated DOS for bulk hematite, OH‐terminated and O‐terminated hematite (top, middle and bottom panel respectively). In order to facilitate the comparison between the three cases, the edges of the conduction band in each case are aligned. With the present theoretical approach the band‐gap in bulk hematite is 1.8 eV, which is in good agreement with the optical bandgap values of 1.9‐2.2.eV reported in various studies, depending on the utilized fabrication route. The atom resolved DOS indicates that the valence band is formed by p‐band contribution from O atoms whereas the conduction band is mostly Fe d‐band character, in excellent agreement with previous results. [26] The OH‐terminated surface is of particular interest in the context of (photo)electrochemistry, since both calculations and experiments indicate that hematite undergoes spontaneous surface hydroxylation when exposed to aqueous electrolytes. [9,27] It is clear that the OH‐ termination introduces occupied mid‐gap surface states in a broad energy range extending about 1.5 eV from the bottom of the conduction band. Surface states are also present in the O‐terminated surface (corresponding to the regime in which the OER takes place), albeit the DOS is considerably different. In particular, only the DOS peak closest