Unexpected Reduction of Eu3+ Ions to Eu2+ on a Nanodiamond Surface: An XPS, NMR, EPR, and Photoluminescence Study.

We present the results of a study of europium-grafted detonation nanodiamond using X-ray photoelectron spectroscopy (XPS), nuclear magnetic resonance (NMR), electron paramagnetic resonance (EPR), and photoluminescence techniques. The material was prepared by mixing an aqueous solution of europium(III) nitrate hexahydrate, Eu(NO3)3·6H2O, with a nanodiamond suspension, resulting in ion exchange between Eu3+ ions and protons of surface carboxyl groups and direct grafting of the nanodiamond surface with Eu3+ ions. However, after two years of storage under ambient conditions, the compound exhibits the reduction of ~13% of Eu3+ ions to the paramagnetic Eu2+ state. This unusual finding is discussed in the context of our XPS, EPR, NMR, and photoluminescence data. We propose that this effect can be associated with surface defects and sp2-hybridized graphene-like fragments that act as electron donors to Eu3+ ions. The observed spontaneous partial reduction of Eu3+ to Eu2+ during long-term storage demonstrates that the oxidation state of surface-grafted europium is not completely stable under ambient conditions. This finding is important because even a partial change in the Eu valence may modify the optical and magnetic properties of Eu-grafted nanodiamonds intended for luminescent, sensing, and quantum applications.