Atomic layer deposition of ultrathin nickel sulfide films and preliminary assessment of their performance as hydrogen evolution catalysts

Transition metal sulfides show great promise for applications ranging from catalysis to electrocatalysis to photovoltaics due to their high stability and conductivity. Nickel sulfide, particularly known for its ability to electrochemically reduce protons to hydrogen gas nearly as efficiently as expensive noble metals, can be challenging to produce with certain surface site compositions or morphologies, e.g., conformal thin films. To this end, we employed atomic layer deposition (ALD), a preeminent method to fabricate uniform and conformal films, to construct thin films of nickel sulfide (NiS_x) using bis(N,N′-di-tert-butylacetamidinato)nickel(II) (Ni(amd)₂) vapor and hydrogen sulfide gas. Effects of experimental conditions such as pulse and purge times and temperature on the growth of NiS_x were investigated. These revealed a wide temperature range, 125-225 °C, over which self-limiting NiS_x growth can be observed. In situ quartz crystal microbalance (QCM) studies revealed conventional linear growth behavior for NiS_x films, with a growth rate of 9.3 ng/cm² per cycle being obtained. The ALD-synthesized films were characterized using X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) methods. To assess the electrocatalyitic activity of NiS_x for evolution of molecular hydrogen, films were grown on conductive-glass supports. Overpotentials at a current density of 10 mA/cm² were recorded in both acidic and pH 7 phosphate buffer aqueous reaction media and found to be 440 and 576 mV, respectively, with very low NiS_x loading. These results hint at the promise of ALD-grown NiS_x materials as water-compatible electrocatalysts.