Trimethylphosphine-Assisted Surface Fingerprinting of Metal Oxide Nanoparticle by 31P Solid-State NMR: A Zinc Oxide Case Study

Nano metal oxides are becoming widely used in industrial, commercial and personal products (semiconductors, optics, solar cells, catalysts, paints, cosmetics, sun-cream lotions, etc.). However, the relationship of surface features (exposed planes, defects and chemical functionalities) with physiochemical properties is not well studied primarily due to lack of a simple technique for their characterization. In this study, solid state 31P MAS NMR is used to map surfaces on various ZnO samples with the assistance of trimethylphosphine (TMP) as a chemical probe. As similar to XRD giving structural information on a crystal, it is demonstrated that this new surface-fingerprint technique not only provides qualitative (chemical shift) but also quantitative (peak intensity) information on the concentration and distribution of cations and anions, oxygen vacancies and hydroxyl groups on various facets from a single deconvoluted 31P NMR spectrum. On the basis of this technique, a new mechanism for photocatalytic •OH radical generation from direct surface–OH oxidation is revealed, which has important implications regarding the safety of using nano oxides in personal care products.