Formation Mechanism of Orange Phosphorus via Photoassisted Bottom-Up Synthesis

Orange phosphorus (orange P), a theoretically predicted one-dimensional (1D) polar phosphorus allotrope characterized by repeating [P8]P2[ units, was successfully synthesized using a photoassisted bottom-up approach from a solution-processable Na₂P₁₆ (P7]P2[P7) precursor. Laser irradiation on Na₂P₁₆ film induces a phase transition from amorphous to crystalline, followed by polymerization into NaP₁₅ ([P8]P2[P3]P2[), with further rearrangement to form orange P. This material shows excellent piezoresistivity, with a gauge factor surpassing many state-of-the-art two-dimensional (2D) materials, making it an ideal candidate for high-sensitivity strain sensors. Nonetheless, the formation mechanism remains unclear. Herein, we investigate the formation mechanism by combining experiments and density functional theory (DFT) calculations. Theoretical studies suggest that polymerization proceeds via heterocoupling between Na₂P₁₆ and its photofragmented intermediates (P6]P2[P7), in which negatively charged [P7] termini are converted into neutral [P6]. The termini of resulting NaP₁₅ oligomers undergo further heterocoupling via the same mechanism, allowing polymer chain elongation. Subsequently, NaP₁₅ undergoes oxidation, where trace O₂ preferentially reacts with ionic phosphorus of [P3] subunits, triggering a rearrangement to form linear [P8]P2[. Our work provides mechanistic insights into creating the 1D polar pentagonal-phosphorus strands, enabling their potential applications in future piezo-responsive technologies.