Unravelling the early oxidation mechanism of zinc phosphide (Zn3P2) surfaces by adsorbed oxygen and water: a first-principles DFT-D3 investigation

Abstract

Zinc phosphide (Zn₃P₂) is a novel earth-abundant photovoltaic material with a direct band gap of 1.5 eV. Herein, the incipient oxidation mechanism of the (001), (101), and (110) Zn₃P₂ surfaces in the presence of oxygen and water, which severely limits the fabrication of efficient Zn₃P₂-based photovoltaics, has been investigated in detail by means of dispersion-corrected density functional theory (DFT-D3) calculations. The fundamental aspects of the oxygen and water adsorption, including the initial adsorption geometries, adsorption energies, structural parameters, and electronic properties, are presented and discussed. A chemical picture and origin of the initial steps of Zn₃P₂ surface oxidation are proposed through analyses of Bader charges, partial density of states, and differential charge density isosurface contours. The results presented show that while water interacts weakly with the Zn ions on the Zn₃P₂ surfaces, molecular and dissociative oxygen species interact strongly with the (001), (101), and (110) surface species. The adsorption of oxygen is demonstrated to be characterized by a significant charge transfer from the interacting surface species, causing them to be oxidized from Zn²⁺ to Zn³⁺ formal oxidation states. Preadsorbed oxygen species are shown to facilitate the O–H bond activation of water towards its dissociation, with the adsorbed hydroxide species (OH⁻) demonstrated to draw a significant amount of charges from the interacting surface sites. Despite the fact that the semiconducting nature of the different Zn₃P₂ surfaces is preserved, we observe noticeable adsorption induced changes in their electronic structures, with the covered surface exhibiting smaller band gaps than the naked surfaces. The present study demonstrates the importance of the oxygen–water/solid interface to understand the oxidation mechanism of Zn₃P₂ in the presence of oxygen and water at the molecular level. The study also highlights the need for Zn₃P₂ nanoparticles to be protected against possible oxidation in the presence of oxygen and moisture via in situ functionalization, wherein the Zn₃P₂ nanoparticles are exposed to a vapour of organic functional molecules immediately after synthesis.