Oxidation mechanism in metal nanoclusters: Zn nanoclusters to ZnO hollow nanoclusters

TL;DR

This study investigates the long-term oxidation process of Zn nanoclusters, revealing a two-step mechanism that leads to various core-shell and hollow ZnO nanocluster structures over three years.

Contribution

It provides a detailed analysis of the oxidation mechanism of Zn nanoclusters, including morphological evolution and size-dependent structural transformations.

Findings

Zn nanoclusters rapidly form ZnO shells within days.

Larger nanoclusters retain initial shapes longer.

Smaller nanoclusters become hollow ZnO spheres over time.

Abstract

Zn nanoclusters (NCs) are deposited by Low-energy cluster beam deposition technique. The mechanism of oxidation is studied by analysing their compositional and morphological evolution over a long span of time (three years) due to exposure to ambient atmosphere. It is concluded that the mechanism proceeds in two steps. In the first step, the shell of ZnO forms over Zn NCs rapidly up to certain limiting thickness: with in few days -- depending upon the size -- Zn NCs are converted to Zn-ZnO (core-shell), Zn-void-ZnO, or hollow ZnO type NCs. Bigger than ~15 nm become Zn-ZnO (core-shell) type: among them, NCs above ~25 nm could able to retain their initial geometrical shapes (namely triangular, hexagonal, rectangular and rhombohedral), but ~25 to 15 nm size NCs become irregular or distorted geometrical shapes. NCs between ~15 to 5 nm become Zn-void-ZnO type, and smaller than ~5 nm become ZnO hollow sphere type i.e. ZnO hollow NCs. In the second step, all Zn-void-ZnO and Zn-ZnO (core-shell) structures are converted to hollow ZnO NCs in a slow and gradual process, and the mechanism of conversion proceeds through expansion in size by incorporating ZnO monomers inside the shell. The observed oxidation behaviour of NCs is compared with theory of Cabrera - Mott on low-temperature oxidation of metal.