Yarmoff Research

Environmentally Relevant Surface Chemical Reactions One of the more exciting applications of surface science techniques is towards molecular-level studies of systems important in the environment.   The following describes our work in these areas. Reduction of Contaminants in Water by Zero-Valent Metals. In an effort to remove trace contaminants from waste and ground waters, elemental iron is being used for the reductive dechlorination of solvents and the removal of toxic trace elements. We performed fundamental investigations of the interactions of trace elements and trace element-containing compounds with single- and poly-crystalline Fe surfaces. This work helps to provide the fundamental physical and chemical understanding of these interactions that is critical for the development of cleanup techniques and procedures. We reacted selenate, chromate and uranyl compounds with Fe metal foils in solution, and used an array of techniques to probe the reacted materials. We found that contaminants are removed from solution by either reduction or adsorption. In using zero-valent Fe to remediate U, it was found that ~1 micron thick uranium oxide films grow on the surfaces. We used a large number of analytical techniques to characterize the films, evaluate the growth mechanism, and identify the mineral form of the oxide and how it evolves during annealing. The films themselves are particularly interesting as they provide a means for producing uranium oxide surfaces that can be studied in UHV without the need for extensive radiation precautions. Self-Limiting Growth of Metal Flouride Films. To address the reduction of trace metal contaminants at the most fundamental level, we investigated the adsorption of SeF6 onto Fe surfaces in UHV, and found that the Se is completely reduced, with the liberated F growing a thin film of FeF2. We further demonstrated that when a fluorine-containing molecule reacts with an Fe surface, the film growth process is self-limiting in that a particular molecule will grow to a certain film thickness and abruptly stop. Such a process has great potential for film growth in a manufacturing environment. The growth is unusual in that it initially can be described by a Mott-Cabrera mechanism, but then abruptly stops when a critical thickness is attained. We interpreted the shutdown as due to the inability of electrons from the substrate to transport to the surface and initiate dissociation of the precursor. We also extended this work to other materials, and shown that the self-limiting thickness depends on both the structure of the reactant molecule and the electronic states at the surface.  Radiation damage to nuclear material surfaces. The surface chemistry induced by the radioactive decay of particles impacts nuclear energy technologies and the storage of surplus and waste nuclear materials. We are studying the radiolysis of simple adsorbates on the surfaces of radioactive and surrogate metal oxides. The goal is to develop an understanding of the fundamental processes, such as the substrate's ability to act as a material or energy source/sink.