Nh4 2mos4

All chemicals were used as received without further purification. In a typical synthesis, 10 mg (NH₄)₂MoS₄ (99.97%, Sigma-Aldrich) was added to 6 ml DMF (anhydrous, 99.8%, Sigma-Aldrich) followed by stirring for 15 min under ambient condition. The resulting solution was transferred into a 10 ml microwave reaction vessel, which was then heated to 240 °C at the fast ramp and the temperature was maintained for 2 h in a microwave reactor (CEM corporation) operated under the sealed vessel mode. The reaction solution was cooled to room temperature with pressurized nitrogen flow and the resulting black product was collected via centrifugation (8,000 r.p.m. for 5 min). The precipitate was washed with distilled water and absolute ethanol for at least four times to remove ions and possible remnants, followed by drying at 60 °C in an oven for 4 h. For the time-dependent experiment, the temperature was set at 240 °C while reactions stopped at different times; for the temperature-dependent experiment, the reaction time was 2 h while the reaction temperatures were altered intentionally, all the corresponding products were collected for further studies.

The synthesis of Ni-doped MoS₂ using nickel particles (particle size: 10 μm, 99.9%, Sigma-Aldrich) followed a previously reported method.²⁴ Prior to synthesis, 200 g of Ni particles were sequentially prepared and sonicated in water, 0.1 M HCl, and acetone for 30 min. Subsequently, the dried nickel particles were immersed in 800 mL of N,N-dimethylformamide (DMF, anhydrous, 99.8%, Sigma-Aldrich) solution containing 320 mg of ammonium tetrathiomolybdate ((NH₄)₂MoS₄, 99.97%, Sigma-Aldrich), as shown in Fig. 1. They were heated at 200 °C in a Teflon autoclave (1000 mL) for 12 h during hydrothermal synthesis with a stirring at an rpm of 200. The pressure was maintained above 15 bar. After rinsing with water and ethanol, more than 200 g of homogeneous Ni-doped MoS₂ powder was obtained. As-received MoS₂ (99.97%, Sigma-Aldrich) was also prepared for comparison.