Cubic ZnP2 nanowire growth catalysed by bismuth

Abstract

Zn₃P₂ and ZnP₂ nanowires were grown by physical vapour transport using a bismuth catalyst. Bismuth with a low surface energy and melting temperature is an ideal catalyst for the growth of Zn–P binary nanowires. The ZnP₂ nanowires were synthesized using a Zn₃P₂ source with extra phosphorus vapour supplied from a separate phosphorus source. The Zn₃P₂ nanowires had a conventional α-Zn₃P₂ tetragonal crystal structure. Surprisingly, the ZnP₂ nanowires had a cubic γ-ZnP₂ structure, which is known to be stable only at pressures higher than 1.5 GPa, and its existence is a matter of debate. We observed that Bi catalyst was the deterministic parameter for the formation of γ-ZnP₂. The lattice constants (a = 0.53578 nm) of the nanowires were determined from their d-spacing. Unlike the α-Zn₃P₂ nanowires, the γ-ZnP₂ nanowires showed a Raman band (430–470 cm⁻¹) because of the internal vibrations of infinite phosphorus chains. From the contact angle analysis, the growth of the γ-ZnP₂ nanowires when catalysed by bismuth, was dominated by centre nucleation rather than triple phase boundary nucleation. A nucleus with smaller dimensions is energetically favorable for centre nucleation, and the γ-ZnP₂ nucleus formed during the initial stage of nucleation resulted in the formation of the cubic γ-ZnP₂ nanowires. The centre nucleation model may explain the stability of γ-ZnP₂ phase even at the synthesis pressure (100–600 Torr).