The structure of hematite (α-Fe2O3) (001) surfaces in aqueous media: scanning tunneling microscopy and resonant tunneling calculations of coexisting O and Fe terminations

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 ␣-Fe 2 O 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 Otermination could result from dissolution; if Fe atoms were to dissolve from an Fe-termination, an Otermination 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 Ϯ 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 Fetermination 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.

Electrochemical and photoelectrochemical investigation of water oxidation with hematite electrodes

“…The surface coverage is then calculated to be ~53%, by using the unit cell parameters for the 0001 surface of hematite, and estimating 1 active site per unit cell. 56,[63][64][65][66] This behavior is consistent with the surface intermediate decomposing bimolecularly, such as:…”

Iron oxide surfaces

“…Interestingly, α-Fe 2 O 3 (0001) single crystals have also been cleaved, and prepared by a wet-chemical method (etching in Nitric acid, washing in doubly deionized water). The samples were imaged in air [422], as well as in liquid water [423], and hexagonal lattices with 3 Å and 5 Å periodicity were observed. Although it is tempting to think these may be the O and Fe sublattices of a bulk-terminated surface, such a conclusion is probably not justified on the basis of the STM images presented, particularly in this environment.…”

In silico approach to investigating the adsorption mechanisms of short chain perfluorinated sulfonic acids and perfluorooctane sulfonic acid on hydrated hematite surface

“…The geometry optimized a-Fe 2 O 3 hexagonal unit cell Finger and Hazen, 1980)). The relaxed crystal was used to cleave the hematite (0 0 0 1) surface that was frequently observed on the naturally grown crystals and that is thermodynamically stable (Wang et al, 1998).…”

“…Scanning Tunneling Microscope (STM) (Eggleston et al, 2003), Fourier Transform Infrared Spectroscopy (FTIR) (Gao and Chorover, 2012) and X-ray scattering (Tanwar et al, 2007) methods have been used to reveal the images of surface structure and atomic arrangement in ambient conditions. In addition to these experimental techniques and methods, recently, there has been a rapid increase in publications of computational studies that can provide sufficiently detailed characterization of interface systems at the atomic level.…”