A closer look at catalysts is giving researchers a better sense of how these atom-thick materials produce hydrogen.
Their findings could accelerate the development of 2D materials for energy applications, such as fuel cells.
The researchers’ technique allows them to probe through tiny “windows” created by an electron beam and measure the catalytic activity of molybdenum disulfide, a two-dimensional material that shows promise for applications that use electrocatalysis to extract hydrogen from water.
Initial tests on two variations of the material proved that most production is coming from the thin sheets’ edges.
Researchers already knew the edges of 2D materials are where the catalytic action is, so any information that helps maximize it is valuable, says Jun Lou, a professor of materials science and nanoengineering at Rice University whose lab developed the technique with colleagues at Los Alamos National Laboratory.
“We’re using this new technology to identify the active sites that have been long-predicted by theory,” Lou says. “There was some indirect proof that the edge sites are always more active than the basal planes, but now we have direct proof.”
The research opens a pathway to fast screening of potential hydrogen evolution reaction candidates among two-dimensional materials.
“The majority of the material is on the surface, and you want that to be an active catalyst, rather than just the edge,” Lou says. “If the reaction only happens at the edge, you lose the benefit of having all the surface area provided by a 2D geometry.”
The lab tested molybdenum disulfide flakes with different crystalline structures known as “1T prime” (or distorted octahedral) and 2H (trigonal prismatic). “They’re basically the same material with the same chemical composition, but the positions of their atoms are different,” Lou says. “1T prime is metallic and 2H is a semiconductor.”
He says researchers have so far experimentally proved the more conductive 1T prime was catalytic along its entire surface area, but the study proved that to be not entirely accurate.
“Our results showed the 1T prime edge is always more active than the basal plane. That was a new discovery,” he says.
After making the flakes via chemical vapor deposition, Jing Zhang, a postdoctoral researcher from Rice University, used an electron beam evaporation method to deposit electrodes to individual flakes. He then added an insulating layer of poly(methyl methacrylate), a transparent thermoplastic, and burned a pattern of “windows” in the inert material through e-beam lithography. That allowed the researchers to probe both the edges and basal planes of the 2D material, or just specific edges, at submicron resolution.
The 16 probes on the inch-square chip pulse energy into the flakes through the windows. When the material produces hydrogen, it escapes as a gas but steals an electron from the material. That creates a current that can be measured through the electrodes. Probes can be addressed individually or all at once, allowing researchers to get data for multiple sites on a single flake or from multiple flakes.
Rapid testing will help researchers alter their microscopic materials more efficiently to maximize the basal planes’ catalytic activity.
“Now there’s incentive to utilize the strength of this material—its surface area—as a catalyst,” Lou says. “This is going to be a very good screening technique to accelerate the development of 2D materials.”
The researchers reported their results this month in Advanced Materials.
Additional coauthors of the study are from Rice University and Los Alamos National Laboratory.
The Air Force Office of Scientific Research and the Welch Foundation supported the research.
Source: Rice University
Original Study DOI: 10.1002/adma.201701955