Black phosphorus (BP) is a layered semiconductor that has experienced increasing interest in recent years, but the reactivity in air of BP crystals remains one of the main challenges towards optoelectronic applications. In this work, exfoliated BP layers were investigated after prolonged exposures to ambient conditions in the presence of light and air containing oxygen and water. As previously reported, liquid phosphoric acid rapidly forms on the surface of the flakes [1]. However, we observed a subsequent growth of micrometer-sized crystals at the surface after few days. X-ray diffraction (XRD) and Energy-dispersive X-ray spectroscopy (EDX) reveal the presence of nitrogen atoms in the degradation compounds, which is like what was recently reported under electroreduction conditions [2]. To elucidate its origin, the BP degradation process was investigated in a controlled atmosphere containing isotopic species. The environmental conditions and mechanism behind the formation of these N defects are here discussed. This work shows that BP’s reactivity in air produces benign residues, which make it a perfectly biodegradable 2D semiconductor for widespread applications.

[1] Favron, A., Gaufrès, E., Fossard, F., Phaneuf-L’Heureux, A. L., Tang, N. Y., Lévesque, P. L., ... & Martel, R. Photooxidation and quantum confinement effects in exfoliated black phosphorus. Nature materials, 2015, 14(8), 826-832.

[2] Zhang, L., Ding, L. X., Chen, G. F., Yang, X., & Wang, H. Ammonia Synthesis Under Ambient Conditions: Selective Electroreduction of Dinitrogen to Ammonia on Black Phosphorus Nanosheets Angew. Chem. Int. Ed, 2019, 58 (9), 2528-2528.