Safe and Fast Synthesis of Black Phosphorus and Its Purification

We report a rapid and safe procedure for the synthesis of black phosphorus (BP) by the chemical transport reaction (CTR) method and a purification procedure for the as-prepared BP. Frequent explosion of glass ampules containing reactants (red P, Sn, and SnI₄) during high-temperature annealing in the CTR method is inevitable. We found that any danger from the explosions can be prevented by enclosing the glass ampule in a flange-fitted stainless-steel tube without compromising the yield and the quality of BP. In contrast to prolonged heating at approximately 823 K, the BP crystals can be synthesized by immediately cooling (100 K/h) to 400 K after heating the glass ampule to the desired highest temperature (∼823-873 K) over 2 h. The minimum time required for BP synthesis is estimated to be approximately 5 h. The as-prepared BP predominantly contains Sn and I impurities on the upper layers of the BP flakes. These impurities can be removed by exfoliating the upper layers of the flakes or by combining vacuum annealing at ∼600 K and HCl treatment.