Unoccupied surface state induced by ozone and ammonia on H-terminated diamond electrodes for photocatalytic ammonia synthesis

Abstract

This work follows upon the recent demonstration of the photocatalytic conversion of N2 to NH3 in aqueous solutions via H-terminated, polycrystalline diamond electrodes. X-ray absorption spectroscopy serves as an atom- and bond-specific probe of the local bonding of oxygen and nitrogen species at the surface of the electrodes. Ozone treatment of the electrodes creates a sharp C1s transition to an unoccupied surface state at 2.5 eV below the onset of the transitions to the bulk conduction band. A similar transition occurs at 2.6 eV below the onset after ammonia treatment. It is surprising to obtain such a well-defined surface state on complex, real-life electrode materials, which suggests a characteristic local bonding configuration. An extensive comparison with spectra from reference molecules reveals keto groups (C=O) at the ozone-treated surface, with the possibility of additional tertiary alcohols (C-OH). The electron–hole interaction is taken into account for estimating the absolute energy of the surface state in the bandgap of diamond.