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

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.