Saw-Tooth Heat-Cycling Nitridation of Metallic Cu Yields First Photoactive p‑Cu₃N for PEC Applications

Copper nitride (Cu₃N)-based binary and ternary semiconductors have the potential to significantly impact photovoltaic and photoelectrochemical applications due to their ideal and tunable band gaps and good charge carrier mobility. Yet their development has been hindered due to thermal instability, which limits process temperatures to below 200 °C, and thus the persistence of intrinsic defects has made the demonstration of photoactive Cu₃N elusive. Here, by understanding the thermal nitridation characteristics of metallic Cu in a NH₃/O₂ atmosphere by in situ X-ray diffraction (XRD), we developed a saw-tooth heat-cycling method that improves crystallinity, grain size, and morphology of Cu₃N, resulting in the first demonstration of photoactive material to date. Furthermore, this processing method stabilizes Cu₃N at temperatures as high as 550 °C, allowing for improved process parameterization. This study introduces a strategy for economically fabricated Cu₃N photocathodes and paves the way for their future integration as light absorbers in solar energy harvesting applications.