Two-dimensional (2D) inorganic transition metal boride nanosheets are emerging as promising post-graphene materials in energy research due to their unique properties. State-of-the-art processing strategies are based on chemical etching of bulk material synthesized via solid-state reaction at temperatures above 1000 °C. Here, we report the direct formation of MoB MBene domains in a MoAlB thin film by Al deintercalation from MoAlB in the vicinity of AlO_x regions. Hence, based on these results a straightforward processing pathway for the direct formation of MoB MBene-AlO_x heterostructures without employing chemical etching is proposed here.

Here, we report on atomic scale characterization of various defects in a MoAlB (MAB) phase thin film grown by DC sputtering at a synthesis temperature of 700 °C. Aberration-corrected scanning transmission electron microscopy reveals the formation of an intergrown metastable Mo₃Al₂B₄ phase accompanied by thermally stable 90° twist boundaries, coexisting within the pristine MoAlB matrix. The concurrent formation of these structural defects in the MoAlB matrix can be rationalized based on minute differences in formation enthalpies as shown via density functional theory calculations. The specific structural nature of both the twist boundary and compositional defect (Mo₃Al₂B₄) in a MoAlB matrix is hitherto unreported in literature. Apart from these defects, faceted grain boundaries are observed. In the vicinity of amorphous AlO_x regions, Al is deintercalated and a 2D MoB MBene phase is formed as reported before. Besides these amorphous AlO_x regions, a few nanometer-sized 3D MoB clusters are found. The advancement of aberration-corrected scanning transmission electron microscopy significantly improves characterization from 1D to 3D defects which is important for thin film materials design for the moderate synthesis temperature range. The reported defects might play an important role in the formation of 2D MoB MBenes.

MoAlB is a potential candidate for high-temperature application since a dense, adherent alumina scale is formed. While, based on X-ray diffraction investigations, the formation of phase pure orthorhombic MoAlB coatings is observed, energy dispersive X-ray spectroscopy carried out in a scanning transmission electron microscope reveals the presence of Al-rich and O-rich regions within the MoAlB matrix. The oxidation kinetics of coatings and bulk is similar to the scale thickness formed on the MoAlB coating after oxidation at 1200 °C for 30 min is similar to the one extrapolated for bulk MoAlB. Furthermore, the oxidation kinetics of MoAlB coatings is significantly lower than the one reported for bulk Ti₂AlC. Finally, the elastic properties measured for the as-deposited coatings are consistent ab initio predictions.

Molybdenum disulfide (MoS₂₎ nanostructures are promising catalysts for proton‐exchange‐membrane (PEM) electrolyzers to replace expensive noble metals. Their large‐scale application demands high activity for the hydrogen evolution reaction (HER) as well as robust durability. Doping is commonly applied to enhance the HER activity of MoS₂‐based nanocatalysts, but the effect of dopants on the electrochemical and structural stability is yet to be discussed. Herein, operando electrochemical measurements to the structural evolution of the materials down to the nanometric scale are correlated by identical location electron microscopy and spectroscopy. The range of stable operation for MoS₂ nanocatalysts with and without rhenium doping is experimentally defined. The responsible degradation mechanisms at first electrolyte contact, open circuit stabilization, and HER conditions are experimentally identified and confirmed with the calculated Pourbaix diagram of Re‐doped MoS₂. Doping MoS₂‐based nanocatalysts is validated as a promising strategy for continuing the improvement of high‐performance and durable PEM electrolyzers.

Various compositional defects such as Mo₃Al₂B₄, Mo₄Al₃B₄, Mo₆Al₅B₆ and Al₃Mo, together with MoB MBene, are observed to be coexisting in a MoAlB MAB phase thin film grown at 800 °C by high-power pulsed magnetron sputtering.