Maghemite surface termination variations: Influence of models and Pt substrate

In spite of the growing interest in maghemite, its structure is not accurately known, and numerous uncertainties remain. The ongoing debate centers on its crystalline structure, whether cubic or tetragonal, and its implications for stable surface terminations. This study explores the crystalline nature of maghemite — cubic versus tetragonal — and its effects on surface stability. Using density functional theory (DFT) with Hubbard corrections, we evaluated the stability and electronic properties of maghemite’s (001) and (111) surfaces under both cubic and tetragonal configurations, while also considering the influence of a Pt substrate and strain arising from lattice mismatch. Our findings indicate that native cubic (001) surfaces are inherently more stable than tetragonal ones. However, the presence of a Pt substrate shifts this stability, favoring the cubic (111) surface presenting a higher adhesion energy. We examined the electronic properties of various cases to provide a rationalization of the observed stability order. Our study provides crucial insights into the impact of crystalline structure and Pt substrate on the stability and favored terminations of maghemite surfaces, emphasizing their prospective utility as water oxidation catalysts.