Localized hydrogen evolution reaction on a monolayer MoS2 region. By controlled patterning of the PMMA layer, specific region of the material is exposed to the electrolyte and facilitate the electrochemical reaction. In this way, information (current, impedance, etc) revealed is exclusively related to the known composition of the catalyst.Scientists from Rice University and Los Alamos National Laboratory have developed a new fast-screening approach that quickly assesses how atom-thin catalyst materials produce hydrogen, in a study that could accelerate the development of 2D catalyst materials. Using a probe-bearing microchip, they explored apertures made by an electron beam to measure the catalytic activity of the molybdenum disulphide, a breakthrough that could help in applications that depend on electrocatalysis to extract hydrogen from water and the development of 2D materials for fuel cells.
As reported in Advanced Materials [Zhang et al. Adv. Mater. (2017) DOI: 10.1002/adma.201701955], the initial tests on the on-chip electrochemical device involved two variations of the material, demonstrating that most of the production comes from their edges. Although it was known the edges of 2D materials were more active than the basal planes for catalysis, the work confirmed the active sites, resolving the debate over the catalytic activities of various active sites of MoS2 for effective hydrogen evolution catalysis.
As Rice professor Jun Lou points out, “The majority of the material is on the surface, and you want that to be an active catalyst, rather than just the edge. If the reaction only happens at the edge, you lose the benefit of having all the surface area provided by a 2D geometry.”
Once nanoscale flakes had been produced using chemical vapor deposition, an electron beam evaporation method was used to deposit electrodes to individual flakes, before an insulating layer of poly(methyl methacrylate) was added to burn a configuration of “windows” in the material using e-beam lithography. The probes on the chip pulse energy into the flakes through these windows – when the hydrogen is produced, it escapes as a gas but removes an electron from the material, which makes a current that can be measured through the electrodes.
“The majority of the material is on the surface, and you want that to be an active catalyst, rather than just the edge. If the reaction only happens at the edge, you lose the benefit of having all the surface area provided by a 2D geometry.”Jun Lou
MoS2 flakes were tested with different crystalline structures known as "1T prime" (or distorted octahedral) and 2H (trigonal prismatic), which have the same chemical composition, although the positions of their atoms are different. It was thought the more conductive 1T prime was catalytic along its whole surface area, but here they demonstrated this not to be completely true, but that the 1T prime edge is always more active than the basal plane.
The team are now looking to assess other types of catalyst for hydrogen evolution reaction, including the transition metal dichalcogenides, as well as to examine other impactful electrochemical reactions to broaden the application of this fast-screening technique.