Direct Observation of Tantalum Nitride Photoelectrode Degradation by Environmental Transmission Electron Microscopy

Stability under realistic operating conditions remains a critical challenge for photoelectrochemical (PEC) materials in solar fuel production, where photocorrosion often leads to rapid performance degradation. In particular, the formation of insulating amorphous TaO _x /Ta-(O,N) _x surface interphases is widely recognized as a key factor limiting charge transfer and photocurrent stability in Ta₃N₅ photoanodes. Addressing this challenge requires experimental approaches capable of probing dynamic processes at the atomic level under relevant operating conditions. Here, we develop an in situ environmental transmission electron microscopy (ETEM) platform that integrates simultaneous light illumination, electrical biasing, and controlled water vapor exposure within a commercially available chip, supplemented with a customized optical add-on. This multistimulus configuration enables real-time atomic-scale imaging of PEC materials under conditions closely resembling operation. Applying this platform to Ta₃N₅, we directly capture its structural evolution under combined stimuli and reveal a previously unresolved degradation pathway involving nanocrystal detachment, migration, and inward-propagating amorphization. These processes lead to the growth of amorphous surface layers that are directly associated with performance loss in operating photoanodes. More broadly, this work establishes a mechanistic framework linking nanoscale structural transformations to macroscopic device degradation, providing concrete targets for stabilization strategies in Ta₃N₅ and related metastable PEC materials. In addition, the results demonstrate the broader potential of multistimulus ETEM for investigating complex degradation phenomena in functional energy materials.