The absolute energy level diagrams of nanodiamonds (NDs) were obtained by combining optical and photoelectron spectroscopies, Kelvin probe measurements, and energy-resolved electrochemical impedance spectroscopy. The energy levels and density of states in the bandgap of NDs are correlated with the surface chemistry and structure characterized by FTIR and Raman spectroscopy. We showed profound differences in energy band shifts (by up to 3 eV), Fermi level position (from p-type to n-type), electron affinity (from +0.5 eV to −2.2 eV), optical band gap (5.2 eV to 5.5 eV), band gap states (tail or mid-gap), and electrical conductivity depending on the high-pressure, high-temperature and detonation origin of NDs as well as on the effects of NDs' oxidation, hydrogenation, sp2/sp3 carbon phases and surface adsorbates. The obtained data were fundamental for understanding and designing NDs' optoelectrochemical functional mechanisms in diverse application areas.
