Unraveling the Surface Hydroxyl Network on In₂O₃ Nanoparticles with High-Field Ultrafast Magic Angle Spinning Nuclear...

Hydroxyl groups are among the major active surface sites over metal oxides. However, their spectroscopic characterizations have been challenging due to limited resolutions, especially on hydroxyl-rich surfaces where strong hydroxyl networks are present. Here, using nanostructured In₂O₃ as an example, we show significantly enhanced discrimination of the surface hydroxyl groups, owing to the high-resolution ¹H NMR spectra performed at a high magnetic field (18.8 T) and a fast magic angle spinning (MAS) of up to 60 kHz. A total of nine kinds of hydroxyl groups were distinguished and their assignments (μ₁, μ₂, and μ₃) were further identified with the assistance of ¹⁷O NMR. The spatial distribution of these hydroxyl groups was further explored via two-dimensional (2D) ¹H-¹H homonuclear correlation experiments with which the complex surface hydroxyl network was unraveled at the atomic level. Moreover, the quantitative analysis of these hydroxyl groups with such high resolution enables further investigations into the physicochemical property and catalytic performance characterizations (in CO₂ reduction) of these hydroxyl groups. This work provides insightful understanding on the surface structure/property of the In₂O₃ nanoparticles and, importantly, may prompt general applications of high-field ultrafast MAS NMR techniques in the study of hydroxyl-rich surfaces on other metal oxide materials.