A DFT study of the structures, stabilities and redox behaviour of the major surfaces of magnetite Fe₃O₄

Abstract

The renewed interest in magnetite (Fe3O4) as a major phase in different types of catalysts has led us to study the oxidation-reduction behaviour of its most prominent surfaces. We have employed computer modelling techniques based on the density functional theory to calculate the geometries and surface free energies of a number of surfaces at different compositions, including the stoichiometric plane, and those with a deficiency or excess of oxygen atoms. The most stable surfaces are the (001) and (111), leading to a cubic Fe3O4 crystal morphology with truncated corners under equilibrium conditions. The scanning tunnelling microscopy images of the different terminations of the (001) and (111) stoichiometric surfaces were calculated and compared with previous reports. Under reducing conditions, the creation of oxygen vacancies in the surface leads to the formation of reduced Fe species in the surface in the vicinity of the vacant oxygen. The (001) surface is slightly more prone to reduction than the (111), due to the higher stabilisation upon relaxation of the atoms around the oxygen vacancy, but molecular oxygen adsorbs preferentially at the (111) surface. In both oxidized surfaces, the oxygen atoms are located on bridge positions between two surface iron atoms, from which they attract electron density. The oxidised state is thermodynamically favourable with respect to the stoichiometric surfaces under ambient conditions, although not under the conditions when bulk Fe3O4 is thermodynamically stable with respect to Fe2O3. This finding is important in the interpretation of the catalytic properties of Fe3O4 due to the presence of oxidised species under experimental conditions.

Iron oxide surfaces

“…A recent DFT+U study [146] found that ordered Fe vacancies represent the lowest energy configuration, and reproduce the experimentally observed band gap of 2.07 eV (Figure 18). 3) and various GW calculations using the PBE+4.3 input wavefunction.…”

“…[146; 197; 198]. Most recently, Santos-Carballal et al [146] using a DFT+U approach, calculated a Wulff construction enclosed by (001) and (111) , respectively (see Figure 22). It must be noted, however, that the energies depend on the termination, and that the Fe tet termination of (001) considered does not appear to form on the basis of experimental work.…”

Nature of Nitrogen-Doped Anatase TiO2 and the Origin of Its Visible-Light Activity

“…N-doped TiO 2 is typically synthesized by calcination at 400~500 °C in air 26 . Therefore in this study a value ߤ ை ൌ െ0.8 eV is 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 4 used, which corresponds to 700 K and 0.2 atm 35,36 . …”

Importance of the Inter-Electrode Distance for the Electrochemical Synthesis of Magnetite Nanoparticles: Synthesis, Characterization, Computational Modelling, and Cytotoxicity

“…The simultaneous presence of both Fe 2+ and Fe 3+ in the octahedral sites results in a variety of interesting properties [47,[49][50][51]. Visual representations of the models used for the crystal structure of the bulk magnetite (Fe 3 O 4 ) unit cell and the primitive unit cell are provided in Fig.…”