Insights into the (1×1)-to-(2×1) phase transition of the α-Fe 2O 3( [formula omitted]) surface using EELS, LEED and water TPD

Abstract

The (1×1)-to-(2×1) surface reconstruction of α-Fe 2O 3(0 1 2) (also known as the R-cut or ( 0 1 1 ̄ 2 ) surface) was examined using low energy electron diffraction (LEED), electron energy loss spectroscopy (EELS) and temperature programmed desorption (TPD). The (1×1) surface is generated by heating in 5×10 −7 Torr O 2 at 750 K, followed by cooling in O 2. The surface prepared in this manner exhibits a p(1×1) LEED pattern consistent with a bulk-terminated structure. EELS analysis of this surface shows a 2 eV bandgap consistent with that of hematite and little or no evidence for Fe 2+ surface sites. In contrast, a (2×1) LEED pattern is observed after annealing the (1×1) surface in UHV at 950 K. The EELS spectrum of the (2×1) surface exhibits a prominent loss feature at about 1 eV, which is consistent with Fe 2+ sites. The (2×1) surface prepared by annealing at 950 K does not possess a termination film of Fe 3O 4, as has been reported in the literature for the (0 0 1) surface, since the phonon spectrum remains consistent with that of α-Fe 2O 3. Both LEED and EELS detect the onset of the (1×1)-to-(2×1) reconstruction process to be at 700 K, whereas water TPD, which shows distinctively different desorption features for those two surface phases, detects the onset to occur at about 600 K. The (1×1)-to-(2×1) surface reconstruction process appears to be highly nucleated based on observations that the (2×1) LEED spots grow in during annealing without streaking and that coverage-dependent water TPD for a half (1×1)–half (2×1) surface shows simultaneous filling of both (1×1) and (2×1) binding sites. Given that the reconstruction process is accompanied by reduction of Fe 3+ surface sites to Fe 2+ sites, the preference for growth of existing (2×1) domains over initiation of new (2×1) domains implies that the kinetics for hematite reduction (i.e., Fe 2+ formation) are more favorable in the vicinity of other Fe 2+ sites than in regions rich in Fe 3+.