Maximising the amount of exposed edges on films shows potential for a variety of energy applications 

Rice University scientists have turned the semiconductor molybdenum disulphide's 2D form into a nanoporous film that can catalyse the production of hydrogen or be used for energy storage.

MoS₂ is one of a new class of thin metal/sulphide materials, known as transition metal di-chalcogenides (TMDCs) that is gaining attention because of its graphene-like qualities and ability to emit light. But the Rice lab chemist James Tour is using some different properties of MoS₂.

The versatile chemical compound is inert along its flat sides, but previous studies have determined the material's edges are highly efficient catalysts for hydrogen evolution reaction (HER), a process used in fuel cells to pull hydrogen from water. Tour and his colleagues have found a cost-effective way to create flexible films of the material that maximise the amount of exposed edge and have potential for a variety of energy-oriented applications. The Rice research appears in the journal Advanced Materials.

MoS₂ looks like graphene from above, with rows of ordered hexagons. But seen from the side, three distinct layers are revealed, with sulphur atoms in their own planes above and below the molybdenum. This crystal structure creates a more robust edge, and the more edge, the better for catalytic reactions or storage, Tour said.

"So much of chemistry occurs at the edges of materials," he said. "A 2D material is like a sheet of paper: a large plain with very little edge. But our material is highly porous. What we see in the images are short, 5- to 6-nanometer planes and a lot of edge, as though the material had bore holes drilled all the way through."

The new film was created by Tour and lead authors Yang Yang, a postdoctoral researcher; Huilong Fei, a graduate student; and their colleagues. It catalyzes the separation of hydrogen from water when exposed to a current. "Its performance as a HER generator is as good as any MoS2 structure that has ever been seen, and it's really easy to make," Tour said.

While other researchers have proposed arrays of MoS₂ sheets standing on edge, the Rice group took a different approach. First, they grew a porous molybdenum oxide film onto a molybdenum substrate through room-temperature anodisation, an electrochemical process with many uses but traditionally employed to thicken natural oxide layers on metals.

The film was then exposed to sulphur vapour at 300degC for one hour. This converted the material to molybdenum disulphide without damage to its nano-porous sponge-like structure, they reported.

The films can also serve as supercapacitors, which store energy quickly as static charge and release it in a burst. Though they don't store as much energy as an electrochemical battery, they have long lifespans and are in wide use because they can deliver far more power than a battery. The Rice lab built supercapacitors with the films; in tests, they retained 90 percent of their capacity after 10,000 charge-discharge cycles and 83 percent after 20,000 cycles.

"We see anodisation as a route to materials for multiple platforms in the next generation of alternative energy devices," Tour said. "These could be fuel cells, supercapacitors and batteries. And we've demonstrated two of those three are possible with this new material."

Co-authors of the paper are Rice graduate students Gedeng Ruan and Changsheng Xiang. 

The Peter and Ruth Nicholas Postdoctoral Fellowship of Rice's Smalley Institute for Nanoscale Science and Technology and the Air Force Office of Scientific Research Multidisciplinary University Research program supported the research.

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