DOI: 10.1002/cphc.201402153 Defective a-Fe 2 O 3 (0001): An ab Initio Study Manh-Thuong Nguyen,* Nicola Seriani, and Ralph Gebauer [a] 1. Introduction Hematite (Fe 2 O 3 ) has been shown to possess great potential as a material for photocatalytic and photovoltaic applications, owing to its band gap of about 2.2 eV, its abundance, and its thermodynamic stability in nature. [1–3] It is most promising for water splitting, [1, 4] because it is not only able to operate as an anode in the oxygen-evolution reaction, but also as a material for the hydrogen-evolution reaction in photoelectrochemical cells. [5] As the surface plays a fundamental role in photocatalyt- ic applications, crystal surfaces of hematite have been subject- ed to intense studies with particular attention being devoted to the (0001) surface, a highly stable surface that is usually ex- posed in natural hematite crystals. [6, 7] Numerous experimental and theoretical studies have been carried out to determine its structure, morphology, chemical composition, and electronic properties under various conditions, ranging from fresh, briefly wetted surfaces to ones in dry and humid air, in water under bias, or covered with graphene. [8–14] The abundance of studies reflects the complexity of the behavior of this surface, which displays both O- and Fe-rich terminations that sometimes even coexist. [6] In an oxygen atmosphere, the thermodynamically stable defect-free termination is a single-Fe-terminated one in a wide range of chemical potentials of oxygen (m O ). [11] It is, however, known that point defects are often present in the bulk and at the surface of metal oxides, and they are cru- cial in modifying the physical and chemical properties of the crystal. [15, 16] It has been shown that O vacancies can dramatical- ly change the chemical reactivity of an oxide surface. [17] Also, adatoms can improve the reactivity of host surfaces with ad- sorbed molecules, [18] and even steer growth [19] and self-assem- bly [20] at surfaces. Given that vacancies and other point defects can be generated in the crystal-growing process or created at a later stage, for example by electron bombardment, it is desir- able to understand their formation, stability, and properties. Experimentally, Fe vacancies and adatoms on the hematite (0001) surface have been observed by scanning tunneling mi- croscopy. [6, 18] The presence of O vacancies on this surface has also been reported. [21] It has recently been shown that coating hematite photoanodes with a thin Al 2 O 3 layer considerably re- duces the water oxidation overpotential; [22] evidence has been presented that this is connected with the disappearance of surface electronic states that trap holes during the photocata- lytic process. These findings underline the importance of a de- tailed understanding on how structural surface modifications affect chemical and electronic properties of the surface. In this work, by using density functional theory (DFT) calcu- lations at the PBE + U level of theory, we investigate properties of hematite surfaces induced by O and Fe vacancies, Al substi- tutional impurities, and Fe and Al adatoms. We first deter- mined the formation energy of such defective sites as a func- tion of m O . The defect-induced geometry relaxation was then examined. The electronic properties and reactivity with oxygen of some defects were finally addressed. Methods and Models We employed first principles calculations by using spin-polarized plane-wave DFT as implemented in the Quantum ESPRESSO pack- age, [23] within the framework of GGA(PBE) + U formalism. [24, 25] The effective Coulomb repulsion parameter for Fe 3d orbitals of hema- tite in our calculations was set at 4.2 eV, which indeed leads to an energy gap of about 2.0 eV in hematite. The interactions between the electrons and ions were represented with ultrasoft pseudopo- tentials. [26, 27] We used a kinetic-energy cutoff of 40 Ry for the wave- function and 320 Ry for the charge density. The force convergence threshold was set at 10 3 eV 1 for structural optimizations. In the framework of the slab model, the surface free energy (g) was calculated as [Eq. (1)]: By using density functional theory calculations at the PBE + U level, we investigated the properties of hematite (0001) surfa- ces decorated with adatoms/vacancies/substituents. For the most stable surface termination over a large range of oxygen chemical potentials (m O ), the vacancy formation and adsorption energies were determined as a function of m O . Under oxygen- rich conditions, all defects are metastable with respect to the ideal surface. Under oxygen-poor conditions, O vacancies and Fe adatoms become stable. Under ambient conditions, all de- fects are metastable; in the bulk, O vacancies form more easily than Fe vacancies, whereas at the surface the opposite is true. All defects, that is, O and Fe vacancies, Fe and Al adatoms, and Al substituents, induce important modifications to the geome- try of the surface in their vicinity. Dissociative adsorption of molecular oxygen is likely to be exothermic on surfaces with Fe/Al adatoms or O vacancies. [a] Dr. M.-T. Nguyen, Dr. N. Seriani, Prof. R. Gebauer The Abdus Salam International Centre for Theoretical Physics Strada Costiera 11, 34151 Trieste (Italy) E-mail : mtnguyen@ictp.it  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemPhysChem 2014, 15, 2930 – 2935 2930 CHEMPHYSCHEM ARTICLES