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

Nickel sulfides (NiS _x ) 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 _x atomic layer deposition process using an easily synthesized NiCl₂(TMPDA) precursor (TMPDA = N,N,N',N'-tetramethyl-1,3-propanediamine) with H₂S. 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 _x 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 Ni₃S₂ under HER conditions, possibly supplemented with Ni and/or Ni(OH)₂ 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 _x 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.