A team of researchers has found that nanoripples in graphene can make it a strong catalyst, contrary to predictions that the carbon sheet is as chemically inert as bulk graphite.
Led by Professor Andre Geim from the National Graphene Institute (NGI), the researchers found that nanorippled graphene can accelerate hydrogen splitting as well as the best metallic-based catalysts. The findings were unexpected, with previous research predicting that graphene would be as chemically inert as the bulk graphite from which it is obtained. The effect is likely to be present in all two-dimensional materials, which inherently are all non-flat.
The work, ‘Unexpected catalytic activity of nanorippled graphene,’ is published in the Proceedings of the National Academy of Sciences (PNAS).
The non-flatness of graphene makes it a strong catalyst
In collaboration with researchers from China and the US, the team from the National Graphene Institute conducted a series of experiments to show that the non-flatness of graphene makes it a strong catalyst. They demonstrated that graphene’s nanoscale corrugations were linked to its chemical reactivity with molecular hydrogen (H2) by using ultrasensitive gas flow measurements and Raman spectroscopy. It was also found that the activation energy for its dissociation into atomic hydrogen (H) was relatively small.
The team evaluated whether this reactivity is enough to make the material an efficient catalyst, by using a mixture of hydrogen and deuterium gases (D2). They discovered that graphene does behave as a powerful catalyst, converting H2 and D2 into HD. This greatly contrasted with the behaviour of graphite and other carbon-based materials under the same conditions.
The analysis of the gas showed that the amount of HD generated by monolayer graphene was around the same as for the known hydrogen catalysts, such as zirconia, magnesium oxide, and copper. However, graphene was only required in tiny quantities, less than 100 times the latter catalysts.
The results have implications for the use of other 2D materials as catalysts
“Our paper shows that freestanding graphene is quite different from both graphite and atomically flat graphene that are chemically extremely inert. We have also proved that nanoscale corrugations are more important for catalysis than the ‘usual suspects’ such as vacancies, edges, and other defects on graphene’s surface,” said Dr Pengzhan Sun, first author of the paper.
Lead author of the paper, Professor Geim, added: “As nanorippling naturally occurs in all atomically thin crystals, because of thermal fluctuations and unavoidable local mechanical strain, other 2D materials may also show similarly enhanced reactivity. As for graphene, we can certainly expect it to be catalytically and chemically active in other reactions, not only those involving hydrogen.
“2D materials are most often perceived as atomically flat sheets, and effects caused by unavoidable nanoscale corrugations have so far been overlooked. Our work shows that those effects can be dramatic, which has important implications for the use of 2D materials. For example, bulk molybdenum sulphide and other chalcogenides are often employed as 3D catalysts. Now we should wonder if they could be even more active in their 2D form.”