¹⁷O and ³¹P Solid-State NMR Investigations of Faceted Indium Oxide Nanomaterials

Facet engineering of oxides has been developed as an effective approach to generating the desired properties for a variety of applications, especially in catalysis. Hence, it is of great significance to unravel the difference in the surface structures of oxides with varied exposed facets in order to understand the facet–morphology–property relationships and rationally construct nanostructures with improved properties. Although electron microscopy techniques have been widely applied to visualize the facet structure of oxides, the volume of the sample tested by such methods is rather small, and the information extracted may not be representative of the whole sample. Furthermore, chemical bonding information is often unavailable. Herein, four nanostructured indium oxide (In2O3) samples with diverse exposed facets are prepared, and 17O and 31P magic angle spinning (MAS) solid-state NMR spectroscopies are applied to investigate their surface structures and acid sites. 17O and 31P NMR spectra of hexagonal In2O3 (h-In2O3) and cubic In2O3 (c-In2O3) differ significantly, indicating distinct structures and properties. For example, h-In2O3 possesses a stronger surface acidity than c-In2O3 samples, with h-In2O3 nanorods exhibiting the highest acidity. These results lead to a better understanding of the structure–property relationships of faceted In2O3 nanomaterials for catalysis.