<sup>17</sup>O\nand <sup>31</sup>P Solid-State NMR\nInvestigations of Faceted Indium Oxide Nanomaterials

Facet engineering of oxides has been developed as an\neffective\napproach to generating the desired properties for a variety of applications,\nespecially in catalysis. Hence, it is of great significance to unravel\nthe difference in the surface structures of oxides with varied exposed\nfacets in order to understand the facet–morphology–property\nrelationships and rationally construct nanostructures with improved\nproperties. Although electron microscopy techniques have been widely\napplied to visualize the facet structure of oxides, the volume of\nthe sample tested by such methods is rather small, and the information\nextracted may not be representative of the whole sample. Furthermore,\nchemical bonding information is often unavailable. Herein, four nanostructured\nindium oxide (In<sub>2</sub>O<sub>3</sub>) samples with diverse exposed\nfacets are prepared, and <sup>17</sup>O and <sup>31</sup>P magic angle\nspinning (MAS) solid-state NMR spectroscopies are applied to investigate\ntheir surface structures and acid sites. <sup>17</sup>O and <sup>31</sup>P NMR spectra of hexagonal In<sub>2</sub>O<sub>3</sub> (h-In<sub>2</sub>O<sub>3</sub>) and cubic In<sub>2</sub>O<sub>3</sub> (c-In<sub>2</sub>O<sub>3</sub>) differ significantly, indicating distinct structures\nand properties. For example, h-In<sub>2</sub>O<sub>3</sub> possesses\na stronger surface acidity than c-In<sub>2</sub>O<sub>3</sub> samples,\nwith h-In<sub>2</sub>O<sub>3</sub> nanorods exhibiting the highest\nacidity. These results lead to a better understanding of the structure–property\nrelationships of faceted In<sub>2</sub>O<sub>3</sub> nanomaterials\nfor catalysis.