Towards zero bias photoelectrochemical water splitting: onset potential improvement on a Mg:GaN modified-Ta3N5 photoanode

Tantalum nitride (Ta3N5) based photoanodes were overlaid with magnesium-doped gallium nitride (Mg:GaN) thin films by using a plasma-enhanced chemical vapor deposition (PCVD) technique, and subjected to photoelectrochemical activity tests, aiming for a negative shift of onset potential for O-2 evolution. A remarkable negative shift of the onset potential was observed after annealing Mg:GaN in N-2 gas, reaching 0 V vs. RHE, despite a lower photocurrent than that on bare Ta3N5. Mg:GaN annealed in NH3 exhibited an improvement of the photocurrent. A detailed study of the photoelectrochemical performance for various samples and a thorough characterization have revealed the effects of N-2/NH3 post annealing on Mg activation/Ta3N5 damage recovery, controlling the onset potential shift and the current density improvement. N-2 post annealing shifted the onset potential to 0 V vs. RHE but decreased the current density. On the other hand, NH3 post annealing slightly shifted the onset potential and increased the current density largely. Despite the current density loss, this onset potential shift unlocks the prospect of unassisted photoelectrochemical water splitting on Ta3N5.