# Immersion Freezing Efficiency of ZnAl2O4 and MgAl2O4 Spinels, ZnO, and MgO: The Role of Oxygen Vacancies

## TL;DR

This study shows that oxygen vacancies on metal oxide surfaces can enhance ice nucleation, affecting cloud formation and climate.

## Contribution

The novel contribution is linking oxygen vacancies to enhanced ice nucleation efficiency in metal oxides like ZnAl2O4 and ZnO.

## Key findings

- Samples annealed under nitrogen promote ice nucleation at warmer temperatures compared to oxidizing atmospheres.
- ZnO nucleates ice at substantially warmer temperatures than MgO after nitrogen annealing.
- DFT calculations confirm that oxygen vacancies are more stable on Zn-containing oxides.

## Abstract

Aerosol particles that catalyze ice nucleation alter
the optical
properties and precipitation cycles of clouds. Although mineral dust
aerosol particles containing metal oxides are susceptible to the formation
of oxygen vacancies (V
O) on their surfaces,
the impact of these defects on ice nucleation activity has not been
addressed. To investigate the impact of V
O sites, we conducted a droplet immersion freezing assay on zinc aluminate
(ZnAl2O4) and magnesium aluminate (MgAl2O4) spinels annealed under air, nitrogen, and oxygen
atmospheres. We observe that samples annealed under nitrogen promote
ice nucleation at warmer temperatures compared to those treated in
oxidizing atmospheres, with the effect being most pronounced for ZnAl2O4. To further understand these results, we investigated
the immersion freezing of zinc oxide (ZnO) and magnesium oxide (MgO).
Here, we observe that ZnO nucleates ice at substantially warmer temperatures
than MgO after annealing under nitrogen. We hypothesize that the trends
in ice nucleation activity are due to the varying concentrations of V
O that form during the annealing process on
the oxide surfaces, which tend to be higher in the absence of O2. Density functional theory (DFT) calculations support our
hypothesis, indicating that V
O is more
stable on the surfaces of the Zn-containing oxides. The study suggests
that oxygen vacancies, which are common defects on metal oxide surfaces
that affect their adsorption and catalytic properties, can influence
the efficiency with which mineral dust aerosol particles activate
ice formation and affect cloud radiative forcing.