Shallow Oxygen Substitution Defect to Deeper Defect Transformation Mechanism in Ta₃N₅ under Light Irradiation

Defects are ubiquitous in semiconductors and critical to photo(electro)chemical performance, but the change of defect properties under light irradiation remains poorly understood. Herein, we studied defect change properties of Ta₃N₅ with transient absorption (TA) spectroscopy. A broad transient absorption (>650 nm) was observed and attributed to trapped electrons in oxygen impurities (substitution oxygen at nitrogen sites, O_N), and two bleach signals at 510 and 580 nm were obtained and ascribed to free holes of Ta₃N₅. The charge recombination between the trapped electrons and the free holes is sensitively related to O_N defects. The trap-detrapping recombination is retarded by increase of excitation intensity, which is contrary to the normal dependence of charge dynamics on excitation intensity. This abnormal dependence indicates that shallow O_N^• (singly positive charge states) defects of Ta₃N₅ transform to deeper O_N^× (neutral charge states) defects under strong light irradiation. The defect transformation results in long-lived free holes in Ta₃N₅ for photo(electro)catalysis.