Topological insulators (TIs) are of particular interest in the recent solid‐state research because of their exceptional features stemming from the conducting, topologically protected surface states. The exotic properties include the occurrence of novel quantum phenomena and make them promising materials for spintronics and quantum computing applications. Theoretical studies have provided a vast amount of valuable predictions and proposals, whose experimental observation and implementation, to date, are often hindered by an insufficient sample quality. The effect of even a relatively low concentration of defects can make the access to purely topological surface states impossible. This points out the need of high‐quality bulk‐insulating materials with ultra‐clean surfaces/interfaces, which requires sophisticated sample/device preparations as well as special precautions during the measurements. Herein, the challenging work on 3D TI thin films with a focus on is reported. It covers the optimization of the molecular beam epitaxy growth process, the in situ characterization of surface states and transport properties, the influence of exposure to ambient gases and of capping layers, as well as the effect of interfacing TI thin film with magnetic materials.

The ternary compound Hf 2 B 2 Ir 5 was assessed as an electrocatalyst for the oxygen evolution reaction (OER) in 0.1 M H 2 SO 4 . The oxidative environment restructures the studied material in the near-surface region, creating cavities in which agglomerates of IrO x (OH) y (SO 4 ) z particles are incorporated. These in situ generated particles result from the oxidation of secondary phases in the matrix as well as from self-controlled near-surface oxidation of the ternary compound itself. The oxidation is controlled by the structural and chemical bonding features of Hf 2 B 2 Ir 5 . The cage-like motif, exhibiting mostly ionic interactions between positively charged Hf atoms and a covalently bonded Ir−B network, selectively controls the extent and kinetics of the transformation process induced during the operation of the electrocatalyst. The resulting self-optimized composite material, formed by a Hf 2 B 2 Ir 5 matrix surrounding IrO x (OH) y (SO 4 ) z particles, was used in the OER over 240 h at 100 mA cm −2 current density. The chemical changes, as well as the OER performance, were studied via a combination of bulk-and surface-sensitive experimental techniques as well as by employing a quantum-chemical bonding analysis.