Growth driven phase transitions in Zinc Oxide nanoparticles through machine-learning assisted simulations

TL;DR

This work addresses how ZnO nanoparticles transition from the BCT to the WRZ polymorph during bottom-up growth, despite BCT stability at small sizes under equilibrium. It leverages a PLIP+Q machine-learning interatomic potential with long-range electrostatics, atom-by-atom ZnO deposition, and SGMA-based local-order classification to capture growth-driven phase transitions and polarity effects. The key finding is that deposition induces a robust BCT→WRZ transition facilitated by a redistribution of surface ions that compensates polar facets, a process that depends on growth dynamics and long-range interactions rather than size alone. The study provides insights for designing oxide nanoparticles with targeted WRZ-rich structures and demonstrates the crucial role of long-range electrostatics in modeling oxide polarity, with potential applicability to other oxides exhibiting polar surfaces.

Abstract

This study investigates the formation of zinc oxide (ZnO) nanoparticles, a material of significant technological interest with complex structural properties, through atom-by-atom deposition modeling a process common in bottom-up synthesis. Our findings demonstrate that, although the body-centered tetragonal (BCT) structure is thermodynamically stable at equilibrium for small particle sizes, the deposition process induces a crystal-to-crystal phase transition into the more stable wurtzite (WRZ) phase. This transformation is facilitated by a specific redistribution of the nanoparticle ions, which effectively compensates the emerging polar facets at the moment of transition. These insights offer a deeper understanding of oxide nanoparticle formation, which should ultimately help the design of materials with targeted structural features.

Figures (8)

• Figure 1: Left: atomic structures of all the initial seeds considered, in the BCT or WRZ structural arrangements (gray: Zn, red: O). Right: corresponding analysis of the local atomic order using the SGMA analysis (blue: BCT, green: WRZ).
• Figure 2: Energy per atom of NPs in the BCT and WRZ phase as a function of size (number of atoms) computed with DFT and PLIP+Q.
• Figure 3: Evolution of the number of atoms in the crystalline nucleus as a function of time, averaged over the 10 independent simulations for (a) BCT-324, BCT-432 and BCT-648 initial seeds and (b) WRZ-310, WRZ-472 and WRZ-634 initial seeds, at 500 K, 700 K and 900 K.
• Figure 4: Fraction of atoms of the WRZ (green) and BCT (blue) types inside the crystalline nucleus at the end of the simulations, averaged over the 10 independent simulations, for (a) the BCT initial seeds and (b) the WRZ initial seeds at 500K, 700K and 900K.
• Figure 5: Size of the NP in number of atoms at the BCT-to-WRZ transition as a function of the initial BCT seed at 500K, 700K and 900K, averaged over the 10 independent simulations.