Photoemission from diamond films and substrates into water: dynamics of solvated electrons and implications for diamond photoelectrochemistry

Abstract

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.

Beyond fossil fuel–driven nitrogen transformations

“…Photochemical reduction of N 2 to NH 3 has also been reported, and there may be guidance in these studies relevant to electrochemistry (45, 46). Photocatalytic routes to NH 3 have been summarized recently (47).…”

Experimental Studies of Electron Affinity and Work Function from Aluminium on Oxidized Diamond (100) and (111) Surfaces

“…[ 5 ] Similarly, low‐work‐function diamond devices have potential for applications such as photoelectrochemical CO 2 conversion. [ 6 ]…”

Amino-terminated diamond surfaces: Photoelectron emission and photocatalytic properties

“…Most previous studies formed solvated electrons by using high-energy arcs, energetic laser pulses, or high-energy radioactive particles [10]. In recent work, we showed that diamond's NEA and high chemical stability allows it to be used as a solid-state source of solvated electrons in water, and that these electrons can induce novel reactions such as the reduction of N 2 to NH 3 [14,15], the reduction of H + to neutral atomic hydrogen (H• ) [16], and the one-electron reduction of CO 2 to its radical anion [17]. However, we also found that the electron-emissive properties of H-terminated diamond decreased over a duration of several hours due to oxidation of the surface [14].…”