Aspects of goethite surface microtopography, structure, chemistry, and reactivity

Goethite (010), (110), and (111) growth faces and (010) cleavage surfaces o f large, natural single crystals, as well as a high surface area synthetic sample were characterized using various surface sensitive microscopies and spectroscopies. Differential interference contrast and atomic force microsc opy characterization of the natural single crystal faces showed microtopogr aphy indicative of growth, dissolution, and cleavage. Low energy electron d iffraction patterns of the goethite (010) surface exhibit sharp, intense di ffraction spots, indicating long-range order on this important surface. The se patterns have two-dimensional point group symmetry 2mm, consistent with an undistorted surface structure and unit-cell parameters a = 4.62 +/- 0.14 Angstrom and c = 2.99 +/- 0.08 Angstrom. These parameters equal the equiva lent bulk cell dimensions given the uncertainties. Ultra-high vacuum scanni ng tunneling microscopy was performed on (010) cleavage faces, although the tunneling properties of the surface were very heterogeneous. Atomic resolu tion was not obtained; however, microtopographic images are identical to th ose collected with AFM. XPS spectra from the (010) faces of two natural sam ples as well as the synthetic powder all have peak maxima for Fe (2p(3/2)) at 711.5 +/- 0.1 eV. The O(1s) line originating from the goethite can be fi t with two peaks with a chemical shift of 1.3 eV. The peak at higher bindin g energy (531.3 eV +/- 0.1 eV) represents the protonated oxygen in the stru cture, and the peak at lower binding energy (530.0 eV +/- 0.1 eV) represent s the proton-free oxygen in the structure. Ab initio and semi-empirical mod els of the (010) surface suggest that cleavage occurs through the hydroxide plane at 1/4 b in the structure. This is contrary to cleavage through the oxide plane at 1/2 b, which has been assumed in several previous studies.