Hittorf's phosphorus: the missing link during transformation of red phosphorus to black phosphorus

Abstract

Although orthorhombic black phosphorus (BP) single crystals have been successfully synthesized through a chemical vapor transport (CVT) reaction based on a red phosphorus/tin/iodine (RP/Sn/I₂) system, the underlying nucleation and growth mechanism is poorly understood during the transformation of RP to BP. Particularly, little is known about the critical state of BP nucleation during the CVT process. Herein, we reveal that the BP nucleation and growth proceed from the crystalline monoclinic violet or Hittorf's phosphorus (HP), which is accurately confirmed via a series of structural and optical experimental identification approaches and supported by quantum chemical studies. We thus propose a step-by-step phase-induced nucleation and growth mechanism for the growth of BP single crystals by using a CVT reaction in a RP/Sn/I₂ system. A better understanding of the transformation of RP to BP may pave the way for direct synthesis of phosphorene via a CVT method in the near future.