Structure and magnetic properties of detonation nanodiamond chemically modified by copper

We report on detailed study of detonation nanodiamonds (DNDs) whose surface has been chemically modified by copper with the aid of ion exchange in water DND suspension. High resolution transmission electron microscopy, Raman, IR, electron magnetic resonance (EMR), nuclear magnetic resonance (NMR), and superconducting quantum interference device techniques were used for the characterization of DND. Carboxyl groups, appearing on the surface of a nanodiamond particle during its synthesis and purification processes, provide an effective binding of divalent copper ions to the surface. The binding results from the ion exchange between metal cations and protons of surface carboxyl groups in water solutions. IR data evidence the presence of multiple C - O - C groups in the dried copper-modified DND product. Both EMR and C-13 NMR provide direct evidences of the appearance of isolated Cu2+ ions on the surface of the 5 nm nanodiamond particles. EMR spectra reveal well-pronounced hyperfine structure due to Cu-63,Cu-65 nuclear spin I=3/2 with the spectral pattern which is typical for mononuclear axially distorted Cu2+ complexes in polycrystals. Using Cu2+ ions as paramagnetic probes two-component model of carbon inherited paramagnetic centers in DND is suggested. Magnetic susceptibility for all samples follows the Curie-Weiss law above 30 K. The concentration of magnetically observable copper ions Cu2+ (spin S=1/2) localized on the nanodiamonds surface increases up to approximately 1.5-3.5 ions per nanoparticle with increasing concentration of copper acetate in starting solutions.