Synthesis and characterization of iron oxide terminated diamond for solvated electrons generation

Solvated electrons (SEs) are free electrons produced in a liquid medium and surrounded by solvent molecules. They have a high reduction potential of about −2.9 V vs. Normal Hydrogen Electrode (NHE), which enables challenging reactions under mild conditions. Examples include the nitrogen reduction reaction (NRR) to ammonia, the carbon dioxide reduction reaction (CO₂RR), and the degradation of persistent halogenated organic pollutants. Optimizing the reduction of these species is a key objective for addressing global warming and environmental remediation. Diamond is a promising solid-state source for SEs photoinjection in water owing to its negative electron affinity (NEA), achievable through specific surface terminations, which allows direct electron emission under ultraviolet (UV) excitation. Hydrogen-terminated diamond (HTD) exhibits NEA but lacks long-term stability under irradiation. Emerging metal-oxide terminations promise improved surface stability through strong C–O and O–M bonds while preserving NEA. This work reports the synthesis and complete surface and electronic characterization of a new iron-monoxide-terminated diamond surface, which, to the best of our knowledge, represents the first study of this termination. The synthesis was tuned by varying FeO coverage and process conditions to investigate its behavior on polycrystalline diamond (PCD). The electronic properties showed a progressive modification with increasing FeO coverage from 0 to 1.0 MLE, where the lowest NEA values were obtained. Additional experiments assessed the water stability of the FeO termination, the behavior of iron on hydrogen-terminated PCD, and the reproducibility of the results on a model system single-crystal diamond (100) sample. Overall, these experiments provide a nearly complete characterization of iron and iron oxide behavior on diamond surfaces, offering preliminary insight into possible applications of this new termination. Interestingly, FeO termination showed well-defined states at −1.0 eV, indicating a dual effect: inducing NEA while retaining intra band-gap states potentially useful for applications. The second part of this work concerns the development of a transient absorption spectroscopy (TAS) setup to monitor SEs generation from the samples, quantify their concentration, and determine their lifetime in aqueous media. The TAS setup was designed to determine the minimum energy threshold required for SEs production by exploiting engineered intra band-gap states introduced by metal-oxide terminations. The setup was optimized using hydrogen- and oxygen-terminated diamonds, established substrates that allow comparison with previous studies. The measured SE lifetime was between 1 and 2 μs, consistent with the literature.