Control of Elemental Distribution in the Nanoscale Solid-State Reaction That Produces (Ga1-xZnx)(N1-xOx),) Nanocrystals

摘要

Solid-state chemical transformations at the nanoscale can be a powerful tool for achieving compositional complexity in nanomaterials It is desirable to understand the mechanisms of such reactions and characterize the local level composition of the resulting materials. Here, we examine how reaction temperature controls the elemental distribution in (Ga1-xZnx) (N1-xOx) nano crystals (NCs) synthesized via the solid-state nitridation of a mixture of nanoscale ZnO and ZnGa2O4 NCs. (Ga1-xZnx) (N1-xOx) is a visible-light absorbing semiconductor that is of interest for applications in solar photochemistry. We couple elemental mapping using energy-dispersive X-ray spectroscopy in a scanning transmission electron microscope (STEM-EDS) with colocation analysis to study the elemental distribution and the degree of homogeneity in the (Ga1-xZnx) (N1-xOx) samples synthesized at temperatures ranging from 650 to 900 degrees C with varying ensemble compositions (i.e., x values). Over this range of temperatures, the elemental distribution ranges from highly heterogeneous at 650 degrees C, consisting of a mixture of larger particles with Ga and N enrichment near the surface and very small NCs, to uniform particles with evenly distributed constituent elements for most compositions at 800 degrees C and above. We propose a mechanism for the formation of the (Ga1-xZnx) (N1-xOx) NCs in the solid state that involves phase transformation of cubic spinel ZnGa2O4 to wurtzite (Ga1-xZnx) (N1-xOx) and diffusion of the elements along with nitrogen incorporation. The temperature-dependence of nitrogen incorporation, bulk diffusion, and vacancy-assisted diffusion processes determines the elemental distribution at each synthesis temperature. Finally, we discuss how the visible band gap of (Ga1-xZnx) (N1-xOx) NCs varies with composition and elemental distribution.