Low capping group surface density on zinc oxide nanocrystals.

Area of Science:

• Colloidal chemistry
• Nanomaterials science
• Surface chemistry

Background:

• Ligand shells on colloidal nanocrystals influence stability and reactivity.
• Understanding ligand-surface interactions is crucial for controlling nanocrystal properties.

Purpose of the Study:

• To develop a method for quantifying the dodecylamine (DDA) capping shell on colloidal zinc oxide nanocrystals (ZnO NCs).
• To investigate the binding modes and surface density of DDA ligands on ZnO NCs.
• To elucidate the effect of annealing on DDA ligand binding.

Main Methods:

• Nuclear Magnetic Resonance (NMR) spectroscopy was employed to differentiate and quantify DDA in various binding states (strongly bound, weakly associated, free).
• Analysis of DDA surface density across different ZnO NC sizes.
• Comparison of DDA density with theoretical predictions and self-assembled monolayer coverages.

Main Results:

• Three distinct DDA binding regimes were identified: strongly bound, weakly associated, and free.
• The surface density of bound DDA on as-prepared ZnO NCs is low, significantly less than theoretical maximums.
• Annealing ZnO NCs in molten DDA increased strongly bound DDA to ~80% of a theoretical self-assembled monolayer density, while decreasing weakly associated DDA.

Conclusions:

• As-prepared ZnO NC surfaces are not densely capped and contain hydroxide groups that hinder strong DDA binding.
• Annealing effectively removes surface hydroxide groups, promoting stronger DDA ligand attachment.
• The findings provide insights into the surface chemistry of ZnO NCs and ligand binding.