Band Engineering of Perovskite Ferrite Epitaxial Thin Films for OER Catalysis
Type of DegreePhD Dissertation
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Reliable energy from renewable sources via oxidation of water requires an efficient electrocatalyst. The high cost, low abundance, and poor stability of the current noble metal based electrocatalyst are a primary obstacle to their commercial use. Perovskite-structured transition metal oxides have demonstrated enormous potential as catalysts in oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). Particularly, LaFeO3 and LaNiO3 have shown great potential as catalysts in the oxygen evolution reaction (OER). However, their catalytic performance needs to be improved significantly for commercial applications to compete with precious metal catalysts. One primary method of optimizing electrocatalytic property is by tuning their electronic properties. In this work I show different ways to perturb the electronic structure of LaFeO3 and LaNiO3 and how that will lead to improvement in their catalytic performance. Thickness is one important parameter to tweak the electronic properties in thin films. Changing film thickness in LaFeO3 thin films grown on n-type Nb-doped SrTiO3 changes its bulk intrinsic behavior from n-type in thinner films (1-2 nm) to an intrinsic semiconductor in relatively thicker films, leading to a volcano-like trend in OER performance with thin film thickness. Cation substitution is another key parameter to tune the electronic properties. Sr substitution in LaFeO3 thin films leads to the creation of delocalized oxygen holes, facilitating electron transfer between the catalyst and electrolyte solution. As a result, a sharp increase in OER performance was observed with Sr substitution compared to LaFeO3 thin films. Similarly, creating a LaNiO3/LaFeO3 heterostructure shows a dramatic increase in OER performance of the LaFeO3 due to the interfacial charge transfer and tuning of the band alignment to produce p-type LaFeO3. Our findings in this work show that perovskite TMOs are promising materials for OER catalysis when perturbed out of their equilibrium electronic structure. They have tremendous potential to be a good substitute to the noble-metal catalyst for oxygen evolution reaction (OER) in energy storage devices.