Atomic layer deposition of nickel sulfide thin films and their thermal and electrochemical stability.

Nickel sulfides (NiS ) show promise for a range of energy and other applications, but their (in)stability under processing and operating conditions is scarcely studied. Herein, we have developed a new NiS atomic layer deposition process using an easily synthesized NiCl(TMPDA) precursor (TMPDA = ,,','-tetramethyl-1,3-propanediamine) with HS. Thin films deposited at 165-225 °C consist mostly of the β-NiS phase and display low resistivity (∼40-120 μΩ cm), high purity (<3 at% impurities), and a rough morphology. The thermal stability of the NiS thin films is studied using high-temperature X-ray diffraction, revealing that structural and compositional changes occur in reducing, inert, and oxidizing atmospheres at approximately 300-400 °C. Under electrochemical water splitting conditions, the films are unstable in acid due to dissolution, especially at oxidizing potentials. In an alkaline electrolyte, we do not observe Ni dissolution, but β-NiS transforms to NiS under HER conditions, possibly supplemented with Ni and/or Ni(OH) species. Under alkaline OER, all sulfur is lost and NiOOH is formed. In addition to offering an attractive, scalable route to the synthesis of NiS thin films, our work highlights the importance of thermal and electrochemical (in)stability of sulfides as a crucial step for understanding and engineering materials for energy and other applications.