Researchers at the University of Wisconsin - Madison have developed MoS2 (molybdenum disulfide) nanosheet catalysts that deliver “dramatically” enhanced hydrogen evolution reaction (HER) catalysis for the production of hydrogen gas from water—albeit still lower than platinum. However the eventual ability to use such an inexpensive, abundant alternative instead of platinum for a catalyst material would reduce the cost of hydrogen production. Their results are published as a “Just Accepted” paper online in the Journal of the American Chemical Society.
Although traditionally used as a hydrodesulfurization catalyst, molybdenum disulfide (MoS2) is also of interest as an HER catalyst that exhibits promising hydrogen evolution activity in crystalline or amorphous materials, and molecular mimics. (Earlier post.) However, the catalytic HER performance of MoS2 is currently limited by the density and reactivity of active sites, poor electrical transport, and inefficient electrical contact to the catalyst, the authors noted.
MoS2 and other 2D metal chalcogenides can exist in various polymorphs where subtle structural changes dramatically affect electrical properties. Natural MoS2 is found as the semiconducting and thermodynamically favored 2H phase which is described by two S-Mo-S layers built from edge-sharing MoS6 trigonal prisms. In contrast, the metallic 1T polymorph is described by a single S-Mo-S layer composed of edge-sharing MoS octahedra, and is not naturally found in bulk. Interesting optical and semiconducting properties drive the contemporary research on single layers of semi-conducting MX2 [where M is a metal and X is a chalcogen (sulfur, selenium, or tellurium)] isolated by mechanical or chemical exfoliation for applications in high performance devices.
Although the 1T-MX2 structure was characterized during the early exploration of 2D materials, the catalytic hydrogen evolution properties of exfoliated 1T-MoS2 nanosheets remain unexplored.
Herein we overcome the challenges limiting the catalytic performance of MoS2 by controlling the synthesis of its nanostructures and structural polymorphs using simple intercalation chemistry to make MoS2 nanostructures a highly competitive earth-abundant catalyst for the HER.—Lukowski et al.
To make the new material, the researchers deposited nanostructures of molybdenum disulfide on a disk of graphite and then applied a lithium treatment to create a different structure with different properties.
Molybdenum disulfide can be a semiconductor or a metallic phase, depending on structure. When the compound is grown on the graphite, it is a semiconductor, but it becomes metallic after the lithium treatment. The Wisconsin team discovered that the metallic phase has far greater catalytic properties.
Like graphite, which is made up of a stack of sheets that easily separate, molybdenum disulfide is made up of individual sheets that can come apart, and previous studies have shown that the catalytically active sites are located along the edges of the sheets. The lithium treatment both causes the semiconducting-to-metallic phase change and separates the sheets, creating more edges. We have taken away the limitation from molybdenum disulfide and made the active sites both more pervasive and more reactive.—Mark A. Lukowski, lead author
The research group has produced milligram quantities of the catalyst, but in principle it could be scaled up, Lukowski said.
The experiment, supported by the US Department of Energy’s Basic Energy Sciences program, is a proof of concept for a new approach for improving such catalysts.
...this represents the first application of the metallic 1T polymorph of layered metal chalcogenides in catalysis and this general approach to controlling nanostructures and polymorphism can be useful in modifying many 2D layered materials to enhance their applications in heterogeneous catalysis, solar energy, and high-performance electronics.—Lukowski et al.
Mark A. Lukowski, Andrew S. Daniel, Fei Meng, Audrey Forticaux, Linsen Li, and Song Jin (2013) Enhanced Hydrogen Evolution Catalysis from Chemically Exfoliated Metallic MoS2 Nanosheets. Journal of the American Chemical Society doi: 10.1021/ja404523s