Density functional theory study of the clean and hydrated hematite(11¯02)surfaces

Abstract

The structures of the clean and hydrated hematite ␣-Fe 2 O 3 ͑1102͒ surfaces were investigated using density functional theory, and the free energies of the surfaces in chemical equilibrium with water were calculated as a function of temperature and oxygen partial pressure using ab initio thermodynamics. At 298.15 K, the predicted lowest-energy surface, in equilibrium with 20 Torr H 2 O has a stoichiometry of ͑H 2 O͒ 2 -X-͑HO͒ 2 -Fe 2 -O 2 -R, where X denotes a vacancy of an atomic layer of Fe and R represents the bulk stoichiometric repeat ͑Fe 2 O 3 ͒. This surface stoichiometry results in three types of ͑hydr͒oxo functional groups: Fe-OH 2 ,Fe 2 -OH, and Fe 3 -O. At temperatures above 435 K, maintaining equilibrium with the same water partial pressure, the predicted lowest-energy stoichiometry is ͑HO͒ 2 -͑HO͒ 2 -Fe 2 -O 2 -Fe 2 -O 2 -R, consistent with water dissociated on the stoichiometric bulk termination. These results suggest that the experimentally observed surface termination is highly sensitive to thermal annealing and water reaction. Furthermore, differences in protonation states or the surface hydroxyl groups are shown to lead to large differences in energetic stability and layer relaxations of the oxide substrate. Prediction of the stable surface termination and its dependence on environmental variables such as water, O 2 , and thermal treatment provide a base line for understanding surface reactivity.

A Density Functional Theory Study of the Adsorption of Benzene on Hematite (α-Fe2O3) Surfaces

“…The work function, calculated as the difference between the electrostatic potential in the vacuum region and the Fermi energy of the (0001) surface, is 4.4 eV, similar to the results of Wang et al [62] and Jin et al [155]. The other surface structure studied in this work is the {011  2} surface, known to be one of the dominant growth faces exposed on natural α-Fe 2 O 3 which has been studied in the past by both experimental [156][157][158] and theoretical groups [63,69,70]. It can have a number of non-dipolar terminations, all of which we have considered, but we have used the most stable termination (see Figure 8) to investigate the adsorption properties of benzene.…”

“…Trainor et al [61] and Wang et al [62] have computed various possible structures of the hematite (0001) surface using spin-density functional theory, whereas, de Leeuw and Cooper have employed classical interatomic potential calculations to simulate the hydrated surface structures of white rust [Fe(OH) 2 ], goethite (FeOOH) and hematite (α-Fe 2 O 3 ) [63]. The structure of the {0001} and {011  2} surfaces of hematite and their interaction with water have also been reported by other theoretical groups [64][65][66][67][68][69][70][71][72]. Some of these calculations have shown that adsorbed water molecules dissociate heterolytically on the (1 × 1)-α-Fe 2 O 3 (01 1  2) surface and that the presence of surface hydroxyls results in large interlayer relaxations [64].…”

“…Theoretical calculations have, however, proven to be an indispensable complementary tool to experiments in elucidating our understanding of the surface structures of metal oxides and they have been used extensively to study the structure of the clean and hydrated hematite surfaces in the literature [63][64][65][66][67][68][69][70][71][72].…”

Density Functional Theory Study of Arsenate Adsorption onto Alumina Surfaces

“…The modeling work demonstrated that the two different terminations are both thermodynamically favorable, and that surface preparation conditions and measurement settings likely give rise to observations of the two terminations. Experimental and theoretical studies of the isostructural hematite surfaces have also been carried out, resulting in similar conclusions [24,25]. More recently, experimental and theoretical studies are building up from the mineral-water interface, in order to obtain structural information about interfacial water ordering [19,26].…”

“…Using the alpha-numeric surface stoichiometry naming scheme of Lo et al [24], we modelled arsenate on the C3, C4, and A3 terminations of the (012) surface (Figure 1). For the slab calculations, 2 × 2 supercells were used, and the k-point density was appropriately reduced to 2 × 2 × 1.…”