Towards nanoscaled gold phosphides: surface passivation and growth of composite nanostructures

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Gold phosphide (Au2P3) is known as a crystalline metastable phase at the macroscale. In this paper, the formation of Au2P3 nanostructures is investigated. Here, white phosphorus (P4) is used as a soluble phosphorus donor and reacted on 16 nm gold nanoparticles, in a strategy similar to the one previously used for the production of various metal phosphide nanoparticles including Ni2P, Pd5P4, PdP2, Cu3P and InP. Moderate temperature (250 °C for up to 6 h) gives a reaction limited to the surface of the nanoparticles, while the unreacted P4 stays in solution. This surface modification is then optimized by reducing the stoichiometry of P4 to Au : P = 10 : 1 and lowering the temperature to 110 °C. Interestingly, this surface modification shields the plasmon band against ligand exchange with thiols, providing more robust nanoparticles. The reaction is then conducted under harsh conditions (320 °C for 3 h) to produce crystalline Au2P3. This triggered the aggregation of the starting nanoparticles into larger nanostructures such as nanowires. Moreover, the formation of composite Au2P3–Au nanostructures is observed, where the gold phosphide domains are systematically larger than the unreacted gold nanoparticles. This suggests that gold is particularly reluctant to form gold phosphide, which relates to the metastable character of this phase.