Single copper atoms drive new catalysts and switches, yet resist forming chains

Copper keeps turning up in experiments that push materials down to the scale of single atoms, but it behaves very differently depending on what it is asked to do. One study pulled apart metal wires atom by atom to see which elements form freely suspended chains of atoms as they break, and found that copper does not, even though it is one of the noble metals studied alongside gold, which does. A second study anchored individual copper atoms onto a titanium oxide catalyst and used them to split water into hydrogen with a light-conversion efficiency higher than any previously reported version of the same catalyst. A third study used clusters of copper atoms grown directly on strands of DNA to switch an electrical signal on and off within a few seconds.

None of these three results came from the same laboratory or the same technique, but together they show a common thread: what a single copper atom does depends entirely on its surroundings, whether that is a stretched metal contact, a titanium oxide crystal, or a DNA molecule.

When a very thin metal contact is slowly pulled apart, some metals do not simply snap. Instead, the last atoms stretch into a freely suspended chain, a wire only one atom wide, before the connection finally breaks. Researchers used a technique called a mechanically controllable break junction, which fractures a notched metal wire inside a cooled vacuum chamber and then lets the two clean, freshly exposed surfaces be brought back together with fine control. By recording how long the electrical conductance stays at the level expected for a single atom before dropping, they could tell whether a chain had formed: a longer plateau means a longer chain.

Gold, platinum, and iridium all formed these chains, while the chemically similar metals rhodium, palladium, and silver — copper's own neighbor — did not, breaking instead as soon as their contact reached a single atom. Gold, platinum, and iridium also share a different property: the topmost layer of atoms at their surfaces spontaneously rearranges into a denser pattern, something rhodium, palladium, and silver do not do. The researchers propose that both effects trace back to the same cause: in the heavier metals, electrons that would otherwise repel each other and space atoms apart are drawn closer to the atomic core by relativistic effects, which lets neighboring atoms bond more tightly at a surface or in a thin chain. This remains an explanation the authors put forward from the pattern across six metals, not a directly measured cause, but it accounts for why copper, like its neighbors silver and palladium, does neither.

A separate line of work put single copper atoms to work as a catalyst rather than a wire. Photocatalysts use light to drive a chemical reaction; here, titanium oxide absorbs light and generates charge carriers that can split water into hydrogen gas, but on its own it loses much of that energy to recombination before the reaction can happen. Adding a small amount of a second metal, called a cocatalyst, helps pull the useful charges away and lowers the energy needed to release hydrogen. Platinum has long been the standard cocatalyst for this job, but it is rare and costly, and researchers have been searching for a cheaper substitute that works at least as well.

The team built their catalyst from the bottom up: they first grew a metal-organic framework, a porous crystalline scaffold, then anchored copper ions directly into it before heating the material to form the final titanium oxide catalyst studded with isolated copper atoms. This approach let them load nearly 1.5 percent copper by weight while keeping the atoms spread apart rather than clumped, which mattered because loading much more copper caused the atoms to cluster into nanoparticles that blocked light from reaching the catalyst and lowered performance. The resulting catalyst produced hydrogen at 101.7 millimoles per gram per hour, edging out an equivalent platinum-loaded version at 95.3 millimoles per gram per hour, and converted 56 percent of the light hitting it at one wavelength into hydrogen-producing charge, a figure the researchers describe as exceeding all previously reported titanium oxide photocatalysts of this kind. The catalyst also kept working through six repeated test cycles and after 380 days of storage, with almost no drop in performance and only a small measured change in how much copper remained.

A third study used copper for the opposite purpose: to block, rather than help, the flow of charge. DNA can act as a molecular wire, carrying charge along the stacked, ring-shaped bases at its core, a property researchers have been trying to harness for nanoscale electronic circuits. Building a working circuit requires a way to switch that conduction on and off, which is difficult because there is no easy way to alter a single point along a DNA strand at will.

The researchers grew small clusters of copper atoms directly on a DNA strand attached to an electrode, and found that depositing these copper nanoclusters blocked the DNA's ability to carry charge, switching the wire off. Treating the same strand with a chemical that strips the copper clusters away restored conduction, switching it back on. Using an electrical potential instead of a chemical treatment, the switch could be turned off in about 2 seconds and back on within 1 second, and the cycle could be repeated multiple times, though the authors note a small amount of signal loss built up with repeated cycling.