Indeed, the 7-day reaction without H2 under otherwise identical condition resulted in the precipitation of MoS2 in a yellow-colored aqueous solution. The color was indicative of polysulfides, dissolved sulfur chains (Sn 2-; n = 2-8) formed as a result of the reaction of H2S with elemental sulfur (S 0 ) . Additionally, the formation of S 0 was confirmed by the X-ray diffraction (XRD) measurement of MoS2 formed in the absence of H2 (SI Appendix, Fig. ). H2 reduces S 0 to H2S under hydrothermal conditions :