High-pressure chemistry of red and black phosphorus with NH3

Phosphorus is a key element for chemistry, physics, biology and Earth and planetary sciences [1]. Up to now, the most commonly used allotrope of Phosporus for industrial purposes is the toxic, unstable and highly reactive molecular form called white Phosphorus (w-P, or P4). Switching to polymeric amorphous red Phosphorus (r-P), could be of great importance in the perspective of safer synthetic routes for the obtainment of useful Phosphorus derivatives. Furthermore, a great interest has grown in the scientific community towards the synthesis, stabilization and functionalization of Phosphorene [2,3], a promising 2D platform material that is obtained from the exfoliation of layered crystalline black Phosphorus (b-P). The study of the high-pressure (HP) reactivity of Phosphorus has the twofold purpose of exploring alternative thermodynamic conditions to switch from the w-P to the more stable and less toxic r-P and b-P and of deepening the understanding about reaction mechanisms of these two allotropes, that have different properties. r-P shows a higher reactivity which is of great interest to foster chemical reactions in the presence of small N-containing molecules, in the perspective of obtaining new P-N functionalized materials. Conversely, b-P is the ideal candidate for investigating the inclusion and confined reactivity of small molecules within its layers, in the perspective of the synthesis of new Phosphorene-based materials under high density conditions. At high-pressure, two-photon (TP) absorption processes by near-UV wavelengths have been proved to be effective in inducing chemical reactivity on heterogeneous mixtures of r-P with water [4] and ethanol [5]. The efficiency of these processes is related to the dissociative character of the electronic excited states of the involved molecular systems. The generation of highly reactive species in high density conditions is able to trigger chemical reactivity with r-P and b-P against simple recombination of the molecular fragments. In this study we report the results of FTIR, Raman and XRD experiments about the high-pressure photo-induced reactivity of r- and b-P in the presence of NH3. Exploiting the dissociative character of the electronic excited states of NH3 [6], the formation of P-N bonds and the introduction of N-based functionalities in r- and b-P at room temperature and relatively low pressure (< 1.0 GPa, where NH3 is fluid) is here demonstrated, avoiding the use of solvents, catalysts and radical initiators. Our data provide evidences for the formation of a complex heterogeneous mixture of solid and fluid products. In particular, in the samples of r-P and NH3, besides H2 and PH3, two crystalline and one amorphous solid products with general PxNy and HPxNy stoichiometries were obtained. For what is concerned with b-P, a comparison of the reactivity of this allotrope and of r-P in the presence of NH3 is mandatory for probing the stability and the resistance of the layered crystalline structure of b-P with respect to the amorphous one. The presented results underline the different reactivity of the two Phosphorus allotropes, strictly related to their diverse structural features, and open perspectives for the synthesis of new materials containing P-N functionalities. Acknowledgments: Thanks are expressed to EC through the European Research Council (ERC) for funding the project PHOSFUN "Phosphorene functionalization: a new platform for advanced multifunctional materials" (Grant Agreement No. 670173) through an ERC Advanced Grant. [1] D. E. C. Corbridge, Phosphorus Chemistry, Biochemistry and Technology (CRC Press, 2013) [2] X. Ling, H. Wang, S. Huang, F. Xia, M. S. Dresselhaus, Proc. Natl. Acad. Sci. USA 2015, 112 (15), 4523. [3] L. Kou, C. Chen, S. C. Smith, J. Phys. Chem. Lett. 2015, 6 (14), 2794. [4] M. Ceppatelli, R. Bini, M. Caporali, M. Peruzzini, Angew. Chem. Int. Ed. 2013, 52, 2313. [5] M. Ceppatelli, S. Fanetti, R. Bini, J. Phys. Chem. C 2013, 117, 13129. [6] J. Urbanek, P. Vöhringer, J. Phys. Chem. B 214, 118, 265.