ASTROCHEMISTRY
CREDIT: NASA/JPL-CALTECH/UCLA/MPS/DLR/IDA (CRYOVOLCANO); N AT. M AT E R . (SCHEMATIC/ MICROGRAPH); NAT. ENERGY (CATHODE MATERIAL)
▸ Cryovolcano looks certain on Ceres Studies of the surface of the dwarf planet Ceres show evidence of long-postulated “cryovolcanism” in which volcanoes spew cryomagma—slurries of water, ice, and volatiles—rather than lava. Many icy moons, such as Saturn’s Enceladus and Jupiter’s Europa, have features suggesting cryovolcanism. But the identity of that geological activity hasn’t been confirmed. The new data come from NASA’s Dawn spacecraft, which has been orbiting Ceres since March 2015 as it floats in the asteroid belt between Mars and Jupiter. A set of six papers published in Science last week includes an analysis from a team led by NASA Goddard Space Flight Center scientist Ottaviano Ruesch. The researchers studied an area on Ceres known as Ahuna Mons, which they believe was formed This view of by relatively the suspected recent eruptions cryovolcano Ahuna of materials Mons on Ceres was involving carderived from a digital bonates, water terrain model and ice, and chlorine images from the salts (DOI: Dawn spacecraft. 10.1126/science. aaf4286). In addition, a team led by Debra L. Buczkowski of Johns Hopkins University’s Applied Physics Laboratory found evidence that cryovolcanism has occurred at various locations on Ceres (DOI: 10.1126/science.aaf4332). That Ceres appears to be rich in ice and water jibes with previous data sent back from Dawn and increasingly indicates that Ceres has characteristics of both a comet and an asteroid.—ELIZABETH WILSON
ENERGY STORAGE
▸ Promising zinc-ion battery cathode created The projected low cost and inherent safety of rechargeable aqueous zinc-ion batteries make energy storage devices based on the technology seem like a real winner. Large stationary versions of such batteries could
Gallium and nitrogen atoms entrench between a silicon carbide substrate and graphene to form 2-D GaN, as seen in this schematic and electron micrograph.
2-D MATERIALS
Graphene helps flatten gallium nitride Materials researchers have laid down gallium nitride as a two-dimensional material for the first time with some help from graphene sheets. Getting GaN to flatten out took some doing because the semiconductor’s 3-D crystals don’t exist naturally as stacks of atomic layers, as is the case with graphene, hexagonal boron nitride, and most other 2-D materials. A team led by Joan M. Redwing and Joshua A. Robinson of Pennsylvania State University confined gallium and nitrogen atoms in a narrow gap between a defect-rich graphene bilayer and a silicon carbide substrate to realize 2-D GaN (Nat. Mater. 2016, DOI: 10.1038/ nmat4742). Using a chemical vapor deposition system, the researchers first decomposed trimethylgallium gas to work gallium atoms into the bilayer’s defects, allowing gallium to accumulate between the graphene and silicon carbide. They then liberated nitrogen from ammonia gas, with the nitrogen migrating through the graphene to react with gallium and create discrete islands of 2-D GaN. With an electronic band gap larger than bulk GaN, 2-D GaN could provide an attractive material for electronic and optoelectronic applications, the team says. In the more immediate future, the researchers are working to create larger, continuous GaN films and to create other new 2-D materials using the graphene-capped synthesis.—MATT DAVENPORT
be particularly useful for grid storage apto synthesize ribbonlike single crystals of plications involving intermittent power Zn0.25V2O5•nH2O (n = 0.85–1.0) and fashsources such as wind turbines and solar pan- ioned cathodes from thin films of the mateels. But the batteries haven’t gotten off the rial. They paired the cathodes with metallic ground yet, mainly because of unsatisfactory zinc anodes to make test cells. Analyses cathodes, which cause based on X-ray diffracThis micrograph shows the the devices to act slugtion and other methods structure of a promising layered gishly and die quickly. show that on discharge Zn-V2O5 cathode material for University of Waterloo Zn2+ ions from the anode rechargeable aqueous zinc-ion researchers led by Linda intercalate reversibly batteries. F. Nazar have now come into the cathode, reducup with a layered vanaing the oxide and disdium oxide featuring placing water molecules. interlayer zinc ions and The team reports that water molecules that test cells exhibit kinetic seems to bypass those properties comparable problems (Nat. Energy with commercial lithi2016, DOI: 10.1038/ um-ion battery cathodes nenergy.2016.119). and retain 80% of their The team used a micharge capacity during crowave-driven hy1,000 charging cydrothermal method cles.—MITCH JACOBY SEPTEMBER 5, 2016 | CEN.ACS.ORG | C&EN
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