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The atomic structure, stability, and dynamics of defects in hexagonal boron nitride (h-BN) are investigated using an aberration-corrected transmission electron microscope operated at 80 kV between room temperature and 1000 °C. At temperatures above 700 °C, parallelogram- and hexagon-shaped defects with zigzag edges become prominent, in contrast to the triangular defects typically observed at lower temperatures. The appearance of 120° corners at defect vertices indicates the coexistence of both N- and B-terminated zigzag edges in the same defect. In situ dynamics studies show that the hexagonal holes grow by electron-induced sputtering of B-N chains, and that at high temperatures these chains can migrate from one defect corner to another. We complement the experiments with first-principles calculation which consider the thermal equilibrium formation energy of different defect configurations. It is shown that, below a critical defect size, hexagonal defects have the lowest formation energy and therefore are the more-stable configuration, and triangular defects are energetically metastable but can be "frozen in" under experimental conditions. We also discuss the possible contributions of several dynamic processes to the temperature-dependent defect formation.

Development of an efficient yet durable photoelectrode is of paramount importance for deployment of solar-fuel production. Here, we report the photoelectrochemically self-improving behaviour of a silicon/gallium nitride photocathode active for hydrogen production with a Faradaic efficiency approaching ~100%. By using a correlative approach based on different spectroscopic and microscopic techniques, as well as density functional theory calculations, we provide a mechanistic understanding of the chemical transformation that is the origin of the self-improving behaviour. A thin layer of gallium oxynitride forms on the side walls of the gallium nitride grains, via a partial oxygen substitution at nitrogen sites, and displays a higher density of catalytic sites for the hydrogen-evolving reaction. This work demonstrates that the chemical transformation of gallium nitride into gallium oxynitride leads to sustained operation and enhanced catalytic activity, thus showing promise for oxynitride layers as protective catalytic coatings for hydrogen evolution.

The efficiency of the synthesis of renewable fuels and feedstocks from electrical sources is limited, at present, by the sluggish water oxidation reaction. Single-atom catalysts (SACs) with a controllable coordination environment and exceptional atom utilization efficiency open new paradigms toward designing high-performance water oxidation catalysts. Here, using operando X-ray absorption spectroscopy measurements with calculations of spectra and electrochemical activity, we demonstrate that the origin of water oxidation activity of IrNiFe SACs is the presence of highly oxidized Ir single atom (Ir5.3+) in the NiFe oxyhydroxide under operating conditions. We show that the optimal water oxidation catalyst could be achieved by systematically increasing the oxidation state and modulating the coordination environment of the Ir active sites anchored atop the NiFe oxyhydroxide layers. Based on the proposed mechanism, we have successfully anchored Ir single-atom sites on NiFe oxyhydroxides (Ir0.1/Ni9Fe SAC) via a unique in situ cryogenic–photochemical reduction method that delivers an overpotential of 183 mV at 10 mA ⋅ cm−2 and retains its performance following 100 h of operation in 1 M KOH electrolyte, outperforming the reported catalysts and the commercial IrO2 catalysts. These findings open the avenue toward an atomic-level understanding of the oxygen evolution of catalytic centers under in operando conditions.