Zinc phosphide

Zinc phosphide (Zn₃P₂) is a synthetic inorganic compound that has widely divergent uses. It was originally prepared in the early 20th century by heating stoichiometric amounts of the elements. Much more recently (2013), Erik J. Luber*, Md Hosnay Mobarok, and Jillian M. Buriak* at Canada’s National Institute for Nanotechnology and the University of Alberta (both in Edmonton) prepared pure nanocrystalline Zn₃P₂ by treating tri-n-octylphosphine (TOP) with dimethylzinc at 320 °C.

Under ambient conditions, zinc phosphide has a tetragonal crystal structure (shown); it converts to cubic when heated to ≈845 °C. This brings up an unusual situation about Zn₃P₂’s melting point: Some references report that it melts at 1160 °C or higher, but others give 420 °C. The phase-change temperature clearly indicates that the higher melting point is correct.

The original use for zinc phosphide was as a rodenticide, for burrowing pests such as gophers and moles and for domestic pests such as rats and mice. It is usually combined with baits that the animals ingest. Zn₃P₂ hydrolyzes slowly in contact with water; but the acidity in the target pest’s stomach rapidly releases highly toxic phosphine gas, a respiratory poison. Phosphine was the Molecule of the Week for October 22, 2018.

The newer use of zinc phosphide is more pleasant and exciting. In 2009, Gregory M. Kimball and co-workers at Caltech (Pasadena, CA) reported that steady-state photoluminescence spectra of Zn₃P₂ wafers have a fundamental indirect band gap of 1.38 eV, close to the ideal direct band gap value of 1.5 eV. This result indicates that it may be valuable as a semiconductor in photovoltaic cells.

The work of Luber et al., mentioned above, also involved photovoltaics. Their synthetic method produced colloidal semiconducting ≈8-nm nanocrystals with the tetragonal (α-Zn₃P₂) structure. They found that the optical band gap in this form is 0.5 eV greater than that of bulk Zn₃P₂. Films prepared via deposition of the nanoparticles were used in heterojunction devices that had excellent rectification behavior. Other data, however, indicated that the particles had phosphorus-rich shells (due to the presence of elemental phosphorus [P(0)]), which hindered performance.

Subsequently, Luber, Buriak, and colleagues developed an alternative method for preparing nanocrystalline zinc phosphide that uses tris(trimethylsilyl)phosphine instead of TOP as the phosphorus source. This change greatly decreased the concentration of P(0) on the particle surfaces.