Electron Spin Dynamics of Single and Paired Methyl Radicals at the ZnO Nanoparticle Surface

The photoinduced paramagnetic behavior exhibited by ZnO nanoparticles (NPs) grown by hydrothermal methods is investigated by means of continuous and pulse electron paramagnetic resonance (EPR) as a function of temperature and microwave power. Aside of the usual light-induced signals of methyl radicals (^•CH₃) and core defects (CDs), some other intermediate lines are often detected, which are here demonstrated to originate from radical pairs (^•CH₃)–(^•CH₃) coupled by an almost isotropic Heisenberg exchange interaction. Continuous and pulsed EPR experiments show that methyl radicals undergo irreversible chemical reactions above 40–50 K and reversible rotation hindrance below 30–40 K. On the contrary, radical pairs do not disappear up to 110 K, with no decay after illumination is removed. These surface methyl radical pairs are the most chemically and thermally stable photogenerated species. The paired spins have longer spin–lattice and spin–spin relaxation times, with a phase memory time of about 4.3 μs at 40 K.