Elucidating the Surface Chemistry of Zinc Phosphide Nanoparticles Through Ligand Exchange

Zn3P2 nanoparticles, a potential earth abundant nanomaterial for photovoltaic applications, are prepared via a solution-based synthesis and end up capped with weakly bound phosphine ligands. These ligands are easily displaced from the nanoparticle surface, leading to an irreversible aggregation of particles. In this work, we elaborate the chemistry of Zn3P2 nanoparticles both to elucidate the surface functionalities present after synthesis, and to enable the production of stable solutions of Zn3P2 colloidal solutions. Three different types of anionic type ligands, formed from their neutral precursors of oleic acid, n-decylphosphonic acid, and 1-octadecanethiol, were shown to be effective in yielding soluble functionalized nanoparticles. The functionalized Zn3P2 nanoparticles were thoroughly characterized by electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction analyses, and FTIR spectroscopy. A combination of FTIR and multinuclear solution NMR spectroscopic studies on the starting agglomerated Zn3P2 nanoparticles and the functionalized particle solutions reveals that the particle surface is terminated by Zn-CH3 and -PHx(SiMe3)(3-x) groups. Using oleic acid as the workhorse ligand, it was shown that addition of oleic acid to agglomerated nanoparticles led to a homogeneous dispersion of Zn3P2 nanoparticles, in toluene, along with production of CH4 and C17H33COOSiMe3 as byproducts, as determined by H-1 and C-13 NMR spectroscopy. FTIR spectroscopy of the ligand-exchanged particles indicated oleate coordination, along with the appearance of what has been assigned as a P-H stretch. Similar reaction chemistry was observed during ligand exchange with n-decylphosphonic acid and 1-octadecanethiol. On the basis of these data, a general mechanism for ligand exchange chemistry on the Zn3P2 nanoparticle surface was proposed to enable both the production of zinc phosphide nanoparticle solutions and the determination of various routes to surface functionalization of this material.