Magnetic Resonance Study of Detonation Nanodiamonds with Surface Chemically Modified by Transition Metal Ions

Abstract

We report on electron magnetic resonance (EMR) and nuclear magnetic resonance (NMR) study of detonation nanodiamonds (DND) with the surface modified by copper and cobalt ions. The EMR spectrum of the pure DND sample shows an intense singlet originating from broken carbon bonds, while the spectra of copper- and cobalt-modified samples reveal additional signals with g > 2 and pronounced hyperfine structure (for copper). Increase in the Cu/Co concentration causes an increase of the corresponding EMR signals and broadening of the intense carbon-inherited singlet line. Subsequent annealing of the copper-modified samples in a hydrogen gas stream at 550 and 900°C causes narrowing of the singlet line and reduction of the Cu²⁺-related components. Applying the same annealing process to the cobalt-modified samples leads to broadening of the singlet line, reduction of Co²⁺ component and appearance of new intense low-field signals. NMR data correlate well with the EMR findings and yield information on interactions and locations of transition metal ions. ¹³C nuclear spin–lattice relaxation rate R ₁ in pure DND is driven by the interaction of nuclear spins with unpaired electron spins of broken bonds. Chemical modification of the DND surface by Cu and Co causes an increase in the relaxation rate, revealing appearance of paramagnetic Cu²⁺ and Co²⁺ complexes at the DND surface and their interaction with the carbon nuclear spins, both directly and via a coupling of Cu²⁺ and Co²⁺ electrons with those of the broken bonds. The aforementioned annealing of the Cu- and Co-DND results in an inverse process, i.e., a reduction of the relaxation rate, indicating that these complexes are destroyed and metal ions presumably join each other forming copper and cobalt nanoclusters. In the case of Co the nanoclusters are ferromagnetic, which results in the noticeable broadening of the ¹³C NMR lines.