Hydration of magnesia cubes: a helium ion microscopy study

Area of Science:

• Materials Science
• Nanotechnology
• Surface Chemistry

Background:

• Physisorbed water significantly impacts oxide nanomaterial structure and chemistry.
• This effect is amplified when oxides contact substrates for microscopy.
• Ubiquitous water films pose challenges for nanomaterial storage and characterization.

Purpose of the Study:

• To investigate morphological changes in MgO cubes due to physisorbed water.
• To understand the role of substrates in water-induced transformations of oxide nanomaterials.
• To assess the metastability of nanomaterials under vacuum conditions.

Main Methods:

• Vapor-phase-grown MgO cubes were studied using helium ion microscopy (HIM).
• MgO cubes were annealed, pressed into indium foils, and stored under vacuum.
• Morphological changes were analyzed before and after exposure to water.

Main Results:

• Physisorbed water from indium substrates induced magnesium hydroxide layer growth on MgO cubes.
• Volume expansion and rounding of MgO cube edges were observed.
• Transformation processes included Mg(OH)2 shell formation and brucite nanosheet evolution.

Conclusions:

• Oxide nanomaterials exhibit significant metastability under ambient and vacuum conditions.
• Physisorbed water dramatically affects nanomaterial morphology and chemistry.
• Careful control of water exposure is crucial during storage and characterization of oxide nanomaterials.