Temperature dependent second harmonic generation studies of materials used in energy conversion applications
McNally, Marshall Traver
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Materials in energy conversion devices often undergo a variety of degradation mechanisms. Solid oxide fuel cell cathodes materials, for example, are subject to surface compositional changes due to material segregation. The extreme operating conditions in these energy conversion devices requires the development of an operando technique that is surface and material specific to accurately probe these degradation mechanisms. Second harmonic generation (SHG) is a surface specific technique that probes the electronic structure of a material using the 2nd order polarizability. Using well characterized materials like Au, Si and NiO, we began investigating how high temperatures (260 °C) and atmospheric composition affected the surface electronic structures. To do this, a custom sample chamber dubbed TROPICS was designed and built to achieve temperature, atmospheric compositional and eventually, electrochemical control. We found that gold's SH intensity was enhanced (3.5 times) when O 2 was present in the atmosphere but this enhancement disappeared at high temperatures. Using data from titrating O 2 into a N 2 atmosphere, we concluded that a monolayer of O 2 was forming on the gold surface, providing backbonding opportunities for gold's free electrons into the partially filled O 2 pi* orbitals. Similar behavior was seen in N-type Si which also showed SH enhancement at room temperature. However, P-type and undoped Si showed no such atmospheric dependent behavior. SHG experiments done with NiO showed decoupled behavior in the electronic structure recovery between the bulk and surface. After heating to 260 °C, the SH signal did not return to pre-heating intensities but required ~60 and ~90 minutes in N 2 and air respectively. The difference in recovery time between N 2 and air could be attributed to interactions between the still paramagnetic NiO electrons and the partially filled O 2 pi* orbitals.