General working procedure. All reactions of air and moisture sensitive compounds were performed employing a conventional vacuum/argon line using Schlenk techniques or in an Ar-filled (AirLiquide, 99.998 % purity) MBraun Labmaster 120 glovebox. The solvents (technical grade) used for synthesis of the reported compounds were dried and purified by a MBraun solvent purification system and stored over molecular sieves (4 Å). Sample preparation for chemical analysis (e.g., NMR and TGA) was carried out in an argon filled glove box (MBraun 100). The NMR-solvents purchased from Millipore were degassed and dried over activated molecular sieves (4 Å) before using. Product characterization via NMR-spectroscopy was conducted using a Bruker Advance DPX 300 spectrometer. The spectra were referenced to the internal solvent signal (C6D6) and were analyzed with the software MestReNova v10.0.2-15465 from Mestralab Research S.L. Precursor synthesis. The precursor [Zr(guan)2(NMe2)2] (1) was synthesized following the reported procedure by Banerjee et al.. Density Functional Theory (DFT). All the calculations were performed on the basis of periodic spin-polarized density functional theory (DFT) within a plane wave basis set and projector augmented wave (PAW) formalism, as implemented in the Vienna ab initio simulation package (VASP 5.4) code. The generalized gradient approximation (GGA) with the parameterization of Perdew-Burke-Ernzerhof (PBE) is used for the exchange-correlation functional. We use 12 valence electrons for Zr, 5 for N, 4 for C, and 1 for H. The plane wave energy cutoff is set to be 400eV. The convergence of energy and forces are set to be 1×10-4eV and 1×10-3eV/Å, respectively. The precursors are put in a 30Å×30Å×30Å box and Gamma point sampling is used during the calculations. To investigate the ambient oxidation of the surface, a four-layer slab is used to model ZrN(100) substrate in a (3x3) supercell, with the vacuum region up to 15 Å. A k-point mesh of (3x3x1) was used for all the MD calculations. CVD reactor and thin film deposition. The depositions of ZrN thin films were carried out in a custom-built horizontal cold-wall CVD reactor employing the SSP (1) that was evaporated at a temperature of 135 °C. As a carrier gas, nitrogen (AirLiquide, 99.999%) was employed and 100 sccm was found to be the optimal flow rate. Prior to deposition, the 1 x 1 cm 2 Si(100) (p-type), 1.5 x 1.5 cm 2 glassy carbon and quartz glass substrates were cleaned consecutively with HPLC grade acetone, isopropanol and water in ultrasonic bath, followed by drying with argon (AirLiquide, 99.999 %). Deposition temperatures were varied between 550 °C to 850 °C and the reactor pressure was varied between 1 mbar to 10 mbar while the deposition time was varied between 5 to 60 minutes to obtain films with different thicknesses. Furthermore, the precursor concentration in the carrier gas flow was varied by variation of the precursor evaporation temperature (120 °C -140 °C) and the N2 carrier gas flow (25 -100 sccm) After fabrication, the films were cooled down to 90 °C before removing them from the MOCVD reactor tube. Thin film characterization -Methods and Instruments. X-ray diffraction was measured with a Bruker D8 advance diffractometer with Cu-Kα radiation (λ = 1.5418 nm) in Bragg-Brentano (θ -2θ) geometry over a range of 10 -60°. The applied acceleration voltage was set to 60 kV and the heating current to 30 mA. Raman was measured at a Jubin-Yvon iHR 550 spectrometer (HORIBA Jobin-Yvon, GER) with a 60x objective (Zeiss, GER). The laser was powered by 2.5 mW at a wavelength of 532 nm. To measure the samples, an exposure time of 60 s averaged over 3 spectra was used together with a grating of 1800 grooves per mm and a front opening of 1000 μm. RBS and NRA measurements were performed at the 4 MV tandem accelerator of the RUBION facility (Ruhr University Bochum, Germany). For RBS, a 2 MeV 4 He + beam (intensity 40 -50 nA) incident to the sample at a tilt angle of 7° was used. NRA analysis was performed using a 1 MeV deuteron beam at equal tilt angle. The emitted protons were detected with a silicon detector. The backscattered particles were measured at an angle of 160° by a silicon detector with a resolution of 16 keV. The stoichiometry was calculated from the RBS and NRA data using the program SIMNRA. NRA analysis was performed using a 1 MeV deuteron beam at equal tilt angle. The emitted protons were detected with a silicon detector. X-ray photoelectron spectroscopy (XPS) studies were carried out on a ESCALAB 250 Xi instrument (Thermo Fisher) utilizing a monochromatized Al Kα photon radiation (1486.6 eV). Selected ZrN thin film samples on either GC or Si(100) substrates were analyzed by a combination of survey scans and high-resolution core level scans for peaks of interest (Zr 3d, N 1s, O 1s and C 1s). Three scans were measured for the survey with step widths of 1.0 eV, pass energy 100 eV and dwell time of 10 ms. High resolution core level spectra (5 scans each and 10 scans for N 1s spectra) were measured with step widths of 0.02 eV, pass energy of 10 eV and dwell time of 50 ms. Flood gun was used to compensate charging effects of potentially oxidized surface. Depth profiling was performed by sputtering with Ar + -ions (2 keV, 3 x 3 mm 2 ). After measurement of the surface 19 cycles for 60 s each were performed with a waiting time of 30 s before acquisition of the next spectra. The sputtering rate was determined to be 4 nm min -1 for the ZrN-Si(100) sample using a profilometer. Peak deconvolution analysis was performed by a product of Gaussian and Lorentzian peak shapes and with a Shirley background using Avantage software (version 5.9925). SEM images were measured at a Jeol JSM-7200F scanning electron microscope coupled with a SE, BSE, In-lens SE and BSE detector. For top view, the work distance was set to 5 mm and the beam current was set to 20 kV for imaging on Si(100) and GC. Specimens for TEM investigations were prepared using an FEI Helios G4 CX focused ion beam system (FIB) operating at 30 kV. In the final steps of thinning to the TEM transparency, a 5 kV voltage was applied to minimize the beam damage. TEM investigations were carried out using a JEOL JEM-ARM200F NEOARM instrument operating at 200 kV. Bright-field (BF) TEM and STEM were applied to investigate the morphology of the films. Additionally, the Zr and N distribution were investigated by STEM-EDS using the abovementioned machine. SAED results were indexed using inorganic crystal structures data from ICSD-131632 (ICSD release 2021 02 01). UV-Vis measurements of ZrN films onto quartz glass were conducted utilizing a UV-Vis spectrometer in diffuse reflection mode, employing an Ulbricht sphere, on a Cary 4000 UV-Vis. For analysis, a Tauc plot was constructed to determine the abscissa intercept of the X-axis. The band gap value reported here should be interpreted with caution because it was not corrected for interference effects due to reflection. Electrochemical NRR experiments. ZrN-GC samples were investigated in a commercial three-electrode setup (FlexCell-PTFE by Gaskatel) operated with an Autolab pontentiostat PGSTAT 128N by Metrohm and corresponding Nova 2.1 software. As reference and counter electrode, a reversible hydrogen electrode (RHE) and a coiled PtIr-wire were use, respectively. All potentials are reported versus RHE scale. The supplied N2 and Ar fed gases were purged through 5 mM sulphuric acid electrolyte and Millipore water as basic cleaning step of potential NH3 contaminants before entering the measuring cell. The experiments were performed in 0.1 M H2SO4 and room temperature. The electrochemical test protocol consists of (1) cyclic voltammetry (CV) with a scan rate of 100 mV s -1 , (2) electrochemical cleaning by several fast CV cycles (250 mV s -1 ), (3) CV measurements (100 mV s -1 ), (4) double layer capacitance measurements and (5) slow CVs with 10 mV s -1 . Double layer capacitance measurements were performed in a non-faradaic current regime by CV in a small potential window (± 0.1 V) around 0.2 V with varied scan rates (500, 200, 100, 50, 20, 10 mV s -1 ). Initially performed in Arsaturated electrolyte to evaluate the electrochemical background signal, steps (3) and ( ) were repeated in N2saturated electrolyte to evaluate potential current increase due to ongoing NRR additionally to background HER. Thus, potential NRR activity is qualitatively investigated as initial step of a more sophisticated testing protocol necessary in NRR which is beyond the scope of this study.