Phase-Pure Crystalline Zinc Phosphide Nanoparticles: Synthetic Approaches and Characterization

Zinc phosphide may have potential for photovoltaic applications due to its high absorptivity of visible light and the earth abundance of its constituent elements. Two different solution-phase synthetic strategies for phase-pure and crystalline Zn3P2 nanoparticles (similar to 3-15 nm) are described here using dimethylzinc and vary with phosphorus source. Use of tri-n-octylphosphine (TOP) with ZnMe2 takes place at high temperatures (similar to 350 degrees C) and appears to proceed via rapid in situ reduction to Zn(0), followed by subsequent reaction with TOP over a period of several hours to produce Zn3P2 nanoparticles. Some degree of control over size was obtained through variance of the TOP concentration in solution; the average size of the particles decreases with increasing TOP concentration. With the more reactive phosphine, P(SiMe3)(3), lower temperatures, similar to 150 degrees C, and shorter reaction times (1 h) are required. When P(SiMe3)(3) is used, the reaction mechanism most likely proceeds via phosphido-bridged dimeric Zn(II) intermediates, and not metallic zinc species, as is the case with TOP. In all cases, the nanoparticles were characterized by a combination of X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and solution and solid-state magic-angle spinning (MAS) nuclear magnetic resonance (NMR) analyses. Surface investigation through a combination of MAS P-31 NMR and XPS analyses suggests that the particles synthesized with TOP at 350 degrees C possess a core-shell structure consisting of a crystalline Zn3P2 core and an amorphous P(0)-rich shell. Conversely, the ligand and phosphorus sources are decoupled in the P(SiMe3)(3) synthesis, resulting in significantly reduced P(0) formation.