| Pacific Rim Symposium on Surfaces, Coatings and Interfaces (PacSurf 2016) | |
| Energy Harvesting & Storage | Wednesday Sessions |
| Session EH-WeP |
| Session: | Energy Harvesting & Storage Poster Session |
| Presenter: | Franz Himpsel, University of Wisconsin Madison, USA |
| Authors: | I. Boukahil, University of Wisconsin Madison, USA P.S. Johnson, University of Wisconsin Madison, USA R. Qiao, Advanced Light Source, LBNL, USA J. Bandy, University of Wisconsin Madison, USA R.J. Hamers, University of Wisconsin Madison, USA F.J. Himpsel, University of Wisconsin Madison, USA |
| Correspondent: | Click to Email |
Recently, the use of H-terminated diamond electrodes for photo-catalytic conversion of N2 to NH3 in aqueous solution was demonstrated [1]. This concept uses energetic electrons, created in diamond by UV-light and injected into the solution. The negative electron affinity of H-terminated diamond surfaces makes them efficient electron emitters [2], and their chemical inertness enables applications as electrodes in reactive environments [1]. A limiting factor was the build-up of O at the interface during the photo-catalytic reaction.
We investigated the surface electronic structure of H-terminated, polycrystalline diamond films prepared the same way as those used for ammonia synthesis in [1], before and after treatment with ozone or ammonia. X-ray absorption spectroscopy (XAS) served as atom- and bond-specific probe (compare [3] for the methodology). For ozone treatment, a sharp C1s transition to an unoccupied surface state is found at -2.5 eV below the onset of the bulk conduction band transitions (289.2 eV). A similar transition is found at -2.6 eV after ammonia treatment.
It is surprising to obtain such a well-defined surface state at complex, real-life electrode materials. That suggests a characteristic local bonding configuration. An extensive comparison with spectra from reference molecules reveals two compatible configurations for the ozone-treated surface, a keto group (>C=O) and a hydroxyl group attached to π-bonded carbon (=C-OH). Both lead to low-lying π* transitions similar to those observed at the C1s and O1s edges.
Possible roles of the surface state in the photo-injection of electrons from diamond into electrolytes are discussed. Thereby the electron-hole interaction [4] is taken into account which affects both the position of the surface state in the core level spectra and the barrier for electron injection.
[1] D. Zhu et al., Nature Materials 12, 836 (2013).
[2] F. J. Himpsel et al., Phys. Rev. B 20, 624 (1979); J. Van der Weide et al., Phys. Rev. B 50, 5803 (1994).
[3] I. Zegkinoglou et al., J. Phys. Chem. C 116, 13877 (2012).
[4] P. S. Johnson et al., J. Phys. Chem. C 120, 1366 (2016).