MoS2 Nanostructures As Efficient, Stable, and Earth-Abundant Catalysts for Hydrogen Evolution In Acid

In order to obtain a cost effective alternative to Pt for driving the hydrogen evolution reaction (HER) in acidic media, we need to develop Earth-abundant materials that are highly efficient for the HER and remain stable over long periods of use.  Nanostructured MoS₂ has previously shown excellent HER activity with minimal overpotentials of 150-200 mV due to the presence of a large number of highly active edge sites.  Driving practically relevant geometric current densities, however, requires a high aspect ratio morphology upon which the edge sites can be vertically integrated.  We produce vertically aligned α-MoO₃ nanowires using a hot-wire chemical vapor deposition method followed by sulfidization over a range of temperatures from 150-700°C to create MoS₂ shells of varying thicknesses.  At an optimal sulfidization temperature of 200°C, these core-shell nanowires exhibit excellent activity for the HER while remaining stable over thousands of potential cycles in strong acid.  The MoS₂ shell further provides a protecting layer for the underlying MoO₃, which is otherwise unstable in strong acid, while the n-type MoO₃ provides a conductive electron pathway to the poorly conducting MoS₂.  The understanding of MoS₂ HER activity arising from edge sites provides insight into the possibility of increasing the activity of these nanowires even further.  Towards this goal, we also present work on other nanostructures of MoS₂ that also drive efficient HER.  Lastly, we explore the optical response properties of these nanowires in solution and their possibility for driving photoelectrochemical water splitting.