Visible range sub band gap photoexcitation of conductive hydrogenated nanodiamonds

Hydrogenated milled nanodiamonds (H-MND) are emerging as original candidates for photocatalysis applications, demonstrating notably outstanding performances for PFOA degradation in water under solar-type illumination. However, the involved sub-band gap photoexcitation mechanisms remain poorly understood, as the material's electronic properties, such as its electron affinity, surface states, and Fermi level depend on both its surface chemistry and its interface with solvent. In this work, surface chemistry and unoccupied surface states of H-MND are investigated by X-Ray Photoelectron Spectroscopy (XPS) and Near Edge X-ray Absorption Fine Structure (NEXAFS), respectively. Their transient photoconductivity under UV and visible light is evaluated by Time-Resolved Microwave Conductivity (TRMC). Comparison with detonation hydrogenated nanodiamonds and oxidized nanodiamonds revealed that all materials exhibit surface states, but that only a hydrogenated surface combined with a high crystallinity (H-MND) induces the creation of free charge carriers under illumination. A broader look at ND band diagrams in air and in aqueous environments allowed us to deduce that ND electron affinity, coupled with the charge transfer equilibrium between ND and their environment, governs sub-band gap photoexcitation. After stabilization in aqueous colloid and creation of an adsorbate layer at the surface enhancing hole conductivity, H-MND exhibit intense photoconductivity at both 2.8 and 3.9 eV, attributed to electronic transitions from the valence band to unoccupied C-H surface states. This study provides crucial insights into the photoexcitation mechanisms of ND and confirms the use of H-MND as a new and efficient photocatalyst in water, even outperforming TiO2 under visible illumination.