Diamond has the unusual property of negative electron affinity, in which its conduction band lies above the vacuum level. One consequence of negative electron affinity is that illumination of diamond with above-bandgap light leads to the barrier-free emission of electrons. While facile emission of electrons from diamond into vacuum is well known, emission into liquids has not been investigated previously. We have investigated the emission of electrons into water and the subsequent reactions with small molecules such as N2 and CO2. Our results demonstrate that emission of electrons from photoilluminated diamond leads to solvated electrons that are then able to initiate reduction of N2 to NH3 in aqueous solution. Solvated electrons in the liquid phase adjacent to the diamond surface are detected directly by transient absorption spectroscopy, and successful reduction of N2 to NH3 is confirmed by isotope labeling studies. The ability to induce photocatalytic reduction of N2 is directly associated with the surface termination of diamond and the negative electron affinity of H-terminated diamond surfaces. Surprisingly, the photocatalytic reactivity is not strongly dependent on the crystalline quality and can be achieved both on high-quality diamonds and on inexpensive diamond grit. In order to elucidate the nature of the N2 reduction process, studies on diamond have been compared with similar experiments using excitation of solution-phase I- as a liquid-phase electron source. Computational studies of the many different pathways for electron reactions have lead to identification of the primary reaction pathway. The ability to induce photocatalytic reactions by directly injecting electrons into a reactant solution is a new approach toward photocatalytic reduction that does not require direct adsorption of the reactant molecules to the surface, and is therefore particularly attractive for molecules such as N2 that do not chemisorb strongly to surfaces. Results for CO2 reduction will also be presented.

Diamond photoelectron emission into water: A new approach to photocatalytic reduction of nitrogen reduction at surfaces

246th National Meeting (2013)

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