Optical properties. To investigate the optical properties of the ZrN films, Tauc plots were derived from diffuse UV-Vis reflection measurements of films with varying thicknesses (Figure ). The thicker sample (Figure )) exhibits a smaller band gap energy of 2.91 eV compared to the thinner one (Figure )) with an energy of 3.50 eV. In both cases, the determined values deviate from the reported literature range for ZrN, which is typically between 2.6 to 2.8 eV. It is known that zirconium oxynitrides, which are formed after the deposition due to contact to ambient atmosphere, tend to have larger band gaps with increasing oxygen content. The disparities observed in these samples can be attributed to the known presence of oxygen and carbon ( using cyclic voltammetry (CV) with 250 mV s -1 for 25 scans. CV before and after EC cleaning was applied to evaluate changes due to electrochemical treatment (Figure )). The current in CV measurements after EC cleaning is apparently higher (more negative). This can be explained by an activation process for the ongoing HER at these potentials possibly due to removal of the oxide layer. Comparison of CV data in Ar-and N2saturated electrolyte (Figure )) showed higher currents for the latter case which could result from NRR reaction in addition to the HER. The onset potential decreases and the maximum current increases from roughly -3 to -4.5 mA cm -2 at the lower potential limit of -0.6 V. This difference could qualitatively hint towards a possible activity for the electrochemical NRR of the ZrN film. Similarly, Hanifpour et al. observed a similar current increase for polycrystalline ZrN films and also for other TMNs this was shown. Although it is compelling to attribute this difference to the NRR, this is only a qualitative indication. Proving genuine NRR activity, which is beyond the scope of this study, would require confirmation of ammonia formation. Robust electrochemical protocols have to be followed to avoid false positive results due to contaminations and other effects. Moreover, TMN are also discussed to undergo noncatalytic decomposition resulting in ammonia formation instead of genuine NRR. Thus, there is currently not a conclusive picture on the real NRR activity of TMNs, although their theoretical great catalytic potential for the NRR via the Marsvan Krevelen mechanism. We believe the synthetic capabilities of faceted ZrN growth presented in this study in conjunction with prospective further electrochemical investigation could help to shed light on the real NRR potential of TMNs.