Elucidating the Surface Chemistry of Zinc Phosphide Nanoparticles Through Ligand Exchange

Zn 3 P 2 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 Zn 3 P 2 nanoparticles both to elucidate the surface functionalities present after synthesis, and to enable the production of stable solutions of Zn 3 P 2 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 Zn 3 P 2 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 Zn 3 P 2 nanoparticles and the functionalized particle solutions reveals that the particle surface is terminated by Zn–CH 3 and −PH x (SiMe 3 ) 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 Zn 3 P 2 nanoparticles, in toluene, along with production of CH 4 and C 17 H 33 COOSiMe 3 as byproducts, as determined by 1 H and 13 C 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 Zn 3 P 2 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.