We report a new approach to making stable negative electron-affinity diamond surfaces by terminating diamond with amino groups (also known as amine groups,-NH 2). Previous studies have shown that negative electron affinity can be induced by terminating diamond surfaces with hydrogen, creating a surface dipole favorable toward electron emission. Here, we demonstrate that covalent tethering of positive charges in the form of protonated amino groups,-NH 3 + , also leads to negative electron affinity (NEA) and facile electron emission into vacuum and into water. Amino-terminated diamond was prepared using a very mild plasma discharge. Valence-band photoemission studies of the amino-terminated diamond samples show a characteristic "NEA" peak, demonstrating that the amino-terminated surface has NEA. Diamond's ability to emit electrons into water was evaluated using photochemical conversion of N 2 to NH 3. Time-resolved surface photovoltage studies were used to characterize charge separation at the diamond interface, and Mott-Schottky measurements were performed to characterize band-bending at the diamond-water interface. XPS studies show that the amino-terminated surfaces provide increased chemical resistance to oxidation compared with H-terminated diamond when illuminated with ultraviolet light.

Illumination of diamond with above-bandgap light results in emission of electrons into water and formation of solvated electrons. Here we characterize the materials factors that affect that dynamics of the solvated electrons produced by illumination of niobium substrates and of diamond thin films grown on niobium substrates using transient absorption spectroscopy, and we relate the solvated electron dynamics to the ability to reduce N2 to NH3. For diamond films grown on niobium substrates for different lengths of time, the initial yield of electrons is similar for the different samples, but the lifetime of the solvated electrons increases approximately 10-fold as the film grows. The time-averaged solvated electron concentration and the yield of NH3 produced from N2 both show maxima for films grown for 1-2 hours, with thicknesses of 100-200 nm. Measurements at different values of pH on boron-doped diamond films show that the instantaneous electron emission is nearly independent of pH, but the solvated electron lifetime becomes longer as the pH is increased from pH = 2 to pH = 5. Finally, we also illustrate an important caveat arising from the fact that charge neutrality requires that light-induced emission of electrons from diamond must be accompanied by corresponding oxidation reactions. In situations where the valence band holes cannot readily induce solution-phase oxidation reactions, the diamond itself can be etched by reacting with water to produce CO. Implications for other reactions such as photocatalytic CO2 reduction are discussed, along with strategies for mitigating the potential photo-etching phenomena.

Solvated electrons in water have long been of interest to chemists. While readily produced using intense multiphoton excitation of water and/or irradiation with high-energy particles, the possible role of solvated electrons in electrochemical and photoelectrochemical reactions at electrodes has been controversial. Recent studies showed that excitation of electrons to the conduction band of diamond leads to barrier-free emission of electrons into water. While these electrons can be inferred from the reactions they induce, direct detection by transient absorption measurements provides more direct evidence. Here, we present studies demonstrating direct detection of solvated electrons produced at diamond electrode surfaces and the influence of electrochemical potential and solution-phase electron scavengers. We further present a more detailed analysis of experimental conditions needed to detect solvated electrons emitted from diamond and other solid materials through transient optical absorption measurements.

Silver nanoparticles embedded into the diamond thin films enhance the optical absorption and the photocatalytic activity toward the solvated electron-initiated reduction of N to NH in water. Here, we demonstrate the formation of diamond films with embedded Ag nanoparticles <100 nm in diameter. Cross-sectional scanning electron microscopy (SEM), energy-dependent SEM, and energy-dispersive X-ray analysis demonstrate the formation of encapsulated nanoparticles. Optical absorption measurements in the visible and ultraviolet region show that the resulting films exhibit plasmonic resonances in the visible and near-ultraviolet region. Measurements of photocatalytic activity using supraband gap (λ < 225 nm) and sub-band gap (λ > 225 nm) excitation show significantly enhanced ability to convert N to NH. Incorporation of Ag nanoparticles induces a nearly 5-fold increase in activity using a sub-band gap excitation with λ > 225 nm. Our results suggest that internal photoemission, in which electrons are excited from Ag into diamond's conduction band, is an important process that extends the wavelength region beyond diamond's band gap. Other factors, including Ag-induced optical scattering and formation of graphitic impurities are also discussed.

Recent studies have demonstrated that boron-doped diamond can act as a solid-state source of electrons in water when illuminated with above-bandgap light. Excitation of electrons to diamond's conduction band leads to facile emission of electrons into water; the resulting solvated electrons are potent reducing agents able to initiate many chemical reactions such as the reduction of N 2 to NH 3 and the reduction of CO 2 to CO. Here, we report investigations of the photocatalytic activity of diamond thin films grown on Mo, Ni, and Ti substrates. Our results show that in each case, there is a maximum in the photocatalytic activity that occurs just when the diamond coalesces into a contiguous film, then decreasing as the film becomes thicker. These results suggest that electron emission arises in part from excitation of electrons in the metal substrate, followed by injection into the conduction band of the diamond film. More detailed studies on films grown on niobium show that at the open-circuit potential the films have a downward bend-bending of ~ 0.3 V, which is favorable for barrier-free electron emission. Our result suggests that metal-diamond heterostructures may provide a scalable approach to achieving difficult photocatalytic reactions.