Diamond as a Solvated Electron Source for Nitrogen Reductio

Diamond is a semiconductor with a wide bandgap of 5.5 eV. When diamond surface is terminated with hydrogen, the conduction-band energy lies about 1 eV above the "vacuum level". This property is termed 'negative electron affinity'. Electrons can be photoexcited with photons at energies greater than 5.5 eV and directly emitted into vacuum. Although some semiconductors can photoemit electrons into vacuum, the photocorrosion and degradation of surface modification preclude any substantial attempts to achieve such emission in water. The chemical stability of diamond enables it to act as a non-vacuum photoelectron emitter, which has not been explored previously. The high-energy photoemitted electrons are dissolved in water forming solvated electrons and then initiate reduction reactions. This allows us to bring active electrons to the reactants rather than requiring them to adsorb on catalytic surface to perform electron transfer, which could be beneficial for the reduction of non-adsorptive molecules such as CO2 and N2. The presence of solvated electrons have been confirmed by transient light absorption, an in-situ technique we developed to probe fast optical absorption. The diamond's NEA property and the electron emission from diamond to the adjacent liquid are believed to be necessary for an efficient nitrogen reduction. However, the NEA surface of diamond is vulnerable to oxidation under UV irradiation. The stability and efficiency of electron photoemission has been improved by replacing H-termination with amino termination. The enhanced stability of amino-termination is demonstrated by comparing the nitrogen and oxygen surface coverage before and after extended UV illumination. The protonated amino groups also aid the transfer of electrons to the surface and augment the chance of photoemission.;Despite the numerous known reactions of the solvated electron in the literature, reactions involving N2 are new. The mechanism of nitrogen reduction by solvated electrons is investigated via a combination of theoretical modeling and experimental verification. My collaborators and I propose that nitrogen reduction occurs via hydrogen atom addition at early reduction steps and sequential protonation / direct reduction by solvated electron at later steps. The predicted pH-dependence of ammonia yield by our computational model matches well with the experimental result.