Chang'e 5 landed on the Moon in December 2020, where it collected a sample of the lunar regolith and returned it to Earth a few weeks later. It was the first successful lunar recovery mission since 1976. Analysis revealed traces of water and the youngest lunar rocks yet known.
Now, researchers at the Institute of Metals Research at the Chinese Academy of Sciences have discovered a new lunar sample — graphene. The material, which takes the form of two-dimensional, one-atom-thick sheets of carbon, has been making headlines for years thanks to its useful electronic and mechanical properties. It’s usually made in the lab by depositing thin sheets of carbon onto substrates — or by peeling them off from graphite pellets using tape — but natural graphene has been detected in interstellar space, meteorites, minerals, and now on the Moon.
The researchers in the new study used Raman spectrometry to analyze a small lunar sample, measuring about 2.9 x 1.6 mm, that was returned by Chang'e 5. The team identified several spots with high carbon content that appeared to be in the form of graphite. Because graphene is essentially very thin sheets of graphite, the scientists checked the sample using a higher magnification.
And indeed, some of the carbon present took the form of graphene. Some consisted of flakes two to seven layers thick, while others were part of a carbon shell surrounding other minerals. Its presence in these shells suggests that the graphene wasn't simply peeled off of graphite, the team says — instead, it likely formed through another process that required high temperatures.
Closer inspection revealed that iron compounds were only found in areas with high carbon content, suggesting that they promoted the formation of graphite and graphene, the team says. Heat from volcanic activity in the early Moon could have done the trick. Or, iron-containing minerals in the lunar soil could have captured carbon from gases in the solar wind and catalyzed it into graphene via hot plasma discharges that could be produced on the Moon's surface.
While there isn't enough graphene for future astronauts to collect and use, the team says studying how the material might form naturally could improve our own artificial efforts.
“The mineral-catalyzed formation of natural graphene sheds light on the development of low-cost scalable synthesis techniques of high-quality graphene,” the paper concluded. “Therefore, a new lunar exploration program can be stimulated and some near-future breakthroughs can be expected.”
The research was published in the journal National Science Review.
Source: Science China Press, via Phys.org