Science1 publisher2 min readPublished
Electron and X-ray beams steered the same gold reaction to different nanostructures
Copper oxide nanocubes met chloroauric acid twice in a Krakow lab, once under electrons and once under soft X-rays. The X-ray run ended in hollow gold boxes and the electron run in coated cubes that stayed intact.
The Scientist · Science desk
What happened
- Physicists at the Institute of Nuclear Physics in Krakow followed the galvanic exchange between copper oxide nanocubes and chloroauric acid with both a liquid-cell electron microscope and a liquid-cell X-ray microscope, and published in Small.
- Under the photon beam the product was gold-rich hollow nanoboxes, while under the electron beam gold was deposited on the walls of the copper nanocubes and the cubes showed no sign of disappearing.
- The team put the difference down to the manner in which they had observed the reacting cubes.
- The soft X-rays came from the SOLARIS synchrotron at Jagiellonian University, where scanning at selected photon energies also reports chemical composition and degree of oxidation point by point in the image.
Compiled by The ScientistSomething wrong?How this is made
Why it matters
- contradiction The two runs disagree about whether the copper template survives at all, so at most one of them describes what the reaction does when nothing is illuminating it.
- constraint A mechanism read off a single liquid-cell movie of a redox reaction needs a second probe or a dose series behind it before it can be quoted as the chemistry of the unwatched system.
- capability If local acidity picks the product, beam dose becomes a way to write structure deliberately, and the hollow gold boxes are the form wanted for catalysis because of their surface area.
- cost The photon half of this comparison ran on synchrotron beamtime, so a group with only a lab electron microscope cannot perform the cross-check in house.
A liquid cell exists because an electron beam needs vacuum and this reaction needs water. In transmission electron microscopy the beam travels through a sample no more than a few dozen nanometres thick, and the vacuum rules out anything wet [6]. The Krakow group got around that with a holder made of two chips whose windows are thin silicon nitride membranes: the nanocubes sat on one membrane, and chloroauric acid solution arrived through capillaries [7].
Water in a beam is a reagent. Irradiation makes reactive species that tilt the local balance toward reduction or oxidation, and they change hydrogen ion activity, so the pH at the illuminated spot is not the pH of the solution being fed in [13]. An electron beam leaves its surroundings locally more acidic, and that acidity holds back the oxidation and etching of the copper oxide while promoting reduction [14].
Dr. Joanna Depciuch-Czarny, a co-author, said "the cubes themselves act as templates for the gold structures that are to be formed" [11]. A hollow box requires the template to be etched away as gold arrives. Gold sitting on walls that are still there is what suppressed etching would predict [19].
The phys.org account does not report dose rates or a run of the same chemistry with no beam on it [17]. Prof. Magdalena Parlinska, one of the lead authors, said that "although in both cases we were observing the same substances reacting with one another, we saw something different" [5]. I would not take either movie as the mechanism of the unilluminated reaction until the same chemistry has been run across a range of doses in both instruments.
One more thing keeps the two images from being matched: STXM's magnification is slightly lower than an electron microscope's [8], so the resolution on the two datasets differs.
What to watch
- A dose series in both instruments, to see whether lowering the electron dose moves the product from coated cubes toward hollow boxes.
- A beam-free bench run of the same reaction, quenched and imaged afterwards, to show which product the chemistry gives on its own.
- Whether the group turns the effect into a synthesis route, setting local pH by dose to select which structure forms.