Identification of the Surface Hydrogen Functionalities on Colloidal Zinc Oxide Nanocrystals with Multinuclear Solid-State NMR Spectroscopy

The surface hydrogen species of zinc oxide (ZnO) nanocrystals, including hydroxyl groups and water, play critical roles in determining their chemical reactivity, colloidal stability, and optoelectronic properties. However, direct structural identification of these surface species remains challenging due to their complex nature and surface disorder of the nanocrystals. Here, we present a comprehensive analysis of surface functionalities on sol-gel synthesized ZnO nanocrystals using advanced multinuclear and multidimensional solid-state nuclear magnetic resonance (NMR) spectroscopy, combined with density functional theory (DFT) theoretical calculations. By employing ¹H, ⁷Li, ¹³C, ¹⁷O, and ³¹P NMR, alongside the use of deuterated precursors, ¹⁷O isotope labeling, probe molecules, defect analysis, and electron paramagnetic resonance (EPR) spectroscopy, we resolve and assign eight distinct hydrogen species, including acetate and lithium acetate ligands, chemisorbed and physisorbed water, terminal hydroxyls (OH_T), tribridged hydroxyls (μ₃-OH), and defect-associated hydroxyls. Furthermore, ¹H-¹H homonuclear correlation SSNMR experiments elucidate the crystal facet selectivity and spatial interactions of these surface hydrogen species. Our study establishes an in-depth understanding of the rich surface hydrogen landscape on ZnO nanocrystals and offers a framework for the rational design and functional tuning of ZnO-based nanomaterials.