| AVS 55th International Symposium & Exhibition | |
| Surface Science | Friday Sessions |
| Session SS+AS+NC-FrM |
| Session: | Environmental Surfaces and Water Interaction with Oxide Surfaces |
| Presenter: | K.M. Rosso, Pacific Northwest National Laboratory |
| Authors: | K.M. Rosso, Pacific Northwest National Laboratory S.V. Yanina, Pacific Northwest National Laboratory S. Kerisit, Pacific Northwest National Laboratory |
| Correspondent: | Click to Email |
The semiconducting properties of a wide range of minerals are often overlooked in the study of their interfacial chemical behavior. As a case study, reductive transformation of α-Fe2O3 (hematite) in aqueous solutions is a central part of the natural iron cycle in the environment. The transformation involves reduction of surface Fe(III) to Fe(II) by electron transfer, followed by Fe(II) solubilization and precipitation of new phases. It is a long-held perception that locations of Fe(III) reduction at the interface with aqueous solution correspond directly to sites of Fe(II) release. However, hematite is a semiconductor with a propensity for moderate electron diffusivity in the surface and bulk. Hematite surfaces are also reactive with water and ions leading to surface charging behavior that is closely dependent on the crystallographic termination. Our recent focus has been on understanding how these qualities create unique conditions for the interfacial electron transfer involved in reductive transformation. We show using atomic force microscopy and surface-specific potentiometry evidence that these qualities couple interfacial electron transfer reactions at hematite (001) surfaces with those occurring at crystallographic edge terminations such as (012) via current flow through the crystal bulk. At low pH, divergent charging behavior between (001) and (012) surfaces yield a surface potential bias across the crystal of several hundred millivolts capable of biasing diffusion of charge carriers. We examined this aspect in detail with atomistic simulations of electron diffusion in bulk hematite and at (001) and (012) surfaces using a small polaron hopping model. The model supports the experimentally evident reductive transformation process of net oxidative adsorption of Fe(II) at (001) surfaces coupled by bulk charge transport to net internal reductive dissolution of Fe(III) at edge surfaces. This new paradigm for hematite reductive transformation has important implications for our understanding of the natural iron cycle in the environment. More generally, the apparent importance of chemically induced bulk crystal conduction is likely to be generalizable to a host of naturally abundant semiconducting minerals playing varied key roles in soils, sediments, and the atmosphere.