M. Hart, E. R. White, Ji Chen, C. M. McGilvery, C. J. Pickard, A. Michaelides, A. Sella, M. S. P. Shaffer, C. G. Salzmann, Encapsulation and Polymerization of White Phosphorus Inside Single-wall Carbon Nanotubes

Angewandte Chemie International Edition 10.1002/anie.201703585

Elemental phosphorus displays an impressive number of allotropes with highly diverse chemical and physical properties. Here, we report that white phosphorus can be filled into single-wall carbon nanotubes (SWCNTs) from the liquid and thereby stabilized against the highly exothermic reaction with atmospheric oxygen. The
encapsulated tetraphosphorus molecules were visualized with transmission electron microscopy, but found to convert readily to chain structures inside the SWCNT ‘nanoreactors’. The energies of the possible chain structures were determined computationally highlighting a delicate balance between the extent of polymerization and the SWCNT diameter. Experimentally, a single-stranded zig-zag chain of phosphorus atoms was observed which represents the lowest energy structure at small confinement diameters. These onedimensional chains provide a glimpse into the very first steps of the transformation from white to red phosphorus.

SWCNTs have been filled with P4 molecules at filling yields of up to ~10 w%. The encapsulated white phosphorus is stable at room temperature in air. Yet, upon heating above 50°C, the highly exothermic reaction with oxygen takes place. This small temperature window makes our new composite material well-suited for applications where controlled releases of heat are required. A tendency for the confined P4 molecules to polymerize has been observed. According to DFT calculations, a range of possible chain structures is expected to exist including a single zig-zag chain of phosphorus atoms which has been observed in HRTEM. These polymerizations are thought to represent the very first steps in the transformation from white to red phosphorus. Future explorations of their electronic properties and chemical interactions will be of great interest. Their encapsulation provides an exciting opportunity to isolate and study highly reactive intermediates in the phase behavior of phosphorus.

Polymerization of P4 molecules inside SWCNTs. (a+b) HRTEM images of SWCNTs containing polymeric phosphorus structures. (c) Noisefiltered HRTEM image of the region highlighted in (b). (d) Simulated HRTEM image and (e) the corresponding atomic structure of a single zig-zag chain@SWCNT. Panels (c-e) are shown at the same scale.
Geometry-optimised one-dimensional phosphorus structures. As drawn, all chains contain 20 P atoms.


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