Abstract
The iron oxide–water interface is of interest not only in geochemical and environmental processes, but also in fields ranging from corrosion to photocatalysis. The structure of α-Fe 2O 3 (001) surfaces is not fully understood, and questions have arisen recently concerning different terminations of (001) terraces; a so-called Fe-termination is expected, but under some conditions an O-termination may also be possible. Ultra-high vacuum (UHV) scanning tunneling microscope (STM) studies report evidence for an O-termination in coexistence with an Fe-termination, but other studies find no evidence for an O-termination. Molecular mechanics studies suggest that an O-termination should be possible in an aqueous environment. An O-termination could result from dissolution; if Fe atoms were to dissolve from an Fe-termination, an O-termination would presumably result (and vice-versa). We imaged hematite (001) surfaces in air and aqueous solution using STM. To aid interpretation of the images, we use a resonant tunneling model (RTM) parameterized using ab initio calculations. Our STM and RTM results are consistent with mixed O- and Fe-terminated (001) surfaces. For acid-etched surfaces we find evidence for a periodic (with wavelength of 2.2 ± 0.2 nm) arrangement of nominal O- and Fe-terminated domains. Two different borders between domains should occur, one in which the Fe-termination is high relative to the O-termination, and the reverse. The different domain borders have significantly different heights, consistent with RTM calculations. This agreement allows us to conclude that the Fe-termination is topographically high on most terraces. Surface domains are observed in aqueous solutions at the atomic scale, and appear to be very unreactive on tens-of-seconds time scales at pH 1.
Highlights
Iron oxides, and iron oxide surface chemistry, are of importance in fields including corrosion, lubrication (Jiang et al, 1997; Ramachandran et al, 1996), magnetic properties of materials, catalysis, and geochemical processes (Brown et al, 1999; Dzombak and Morel, 1990; Henrich and Cox, 1994)
The question we wish to answer here is: are in situ scanning tunneling microscope (STM) observations consistent with this hypothesis, and if so, how do the terminations distribute themselves on (001)? In addition, can we find any evidence for dissolution along domain boundaries in in-situ images at the atomic scale?
On the natural hematite (001) surfaces, before any treatment, areas of “clean” hematite were found to be interspersed with crystallographically bounded regions covered by apparent goethite crystallites (Fig. 3) oriented preferentially along three axes separated by angles of 60°
Summary
Iron oxide surface chemistry, are of importance in fields including corrosion, lubrication (Jiang et al, 1997; Ramachandran et al, 1996), magnetic properties of materials, catalysis, and geochemical processes (Brown et al, 1999; Dzombak and Morel, 1990; Henrich and Cox, 1994). Fe3ϩ is insoluble at midrange pH and often occurs as hydr(oxide) coatings on other minerals (Stumm and Morgan, 1996; Coston et al, 1995; Hendershot and Lavkulich, 1983; Scheidegger et al, 1993), thereby influencing natural water chemistry more than its bulk abundance might suggest These influences often occur via the role of iron (hydr)oxides as sorbents for aqueous solutes (Dzomback and Morel, 1990; Kallay and Matijevic, 1985; Hug, 1997), and as an electron donor or acceptor in thermal (LaKind and Stone, 1989; Afonso et al, 1990; Suter et al, 1991), photoelectrochemical (Waite and Morel, 1984; Anderman and Kennedy, 1988; Faust et al, 1989; Sulzberger and Hug, 1994), and biologic (Lovley and Phillips, 1988; Nordstrom and Southam, 1997; Lovley, 1991) electron transfer. We discuss further adaptations of the RTM to hematite; parameterization of the RTM with ab initio calculations is described in the Appendix
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