Nanomaterials from a 3D X-ray perspective
A new X-ray technique can be used to produce high resolution images of nanocrystals in real time as they are growing, something that has not been done before. To this end, a team from DESY and the University of Hamburg has created three-dimensional cross-sectional images of nanostructures from two-dimensional projections taken through a chemical reactor.
The scientists achieved this using multi-slice X-ray ptychography, a technique in which an image is obtained computationally from the interference between coherent X-rays, that means rays that are oscillating in unison. The team is presenting its findings in the journal Scientific Reports.
Nanoparticles are used in a wide range of applications, from medicine, where they can help to treat cancer, for example, to extracting energy from sunlight. They are more effective than conventional materials in this respect, but only if their shape, size and composition are specifically adapted to the application in question.
In order to produce nanomaterials with carefully controlled properties, however, it is generally not enough to simply examine the end result of a manufacturing process. Ideally, scientists want to be able to observe every stage of the synthesis live, from the formation of the first crystals, a few nanometres across, until they assume their final form, allowing these processes to be individually controlled.
The scientists at DESY and the University of Hamburg have now taken a further step towards this goal with the help of multi-slice X-ray ptychography. "We wanted to observe nanocubes of copper oxide at the very moment they were growing inside a liquid," explains lead author Maik Kahnt, from DESY, who now works at the MAX IV laboratory in Sweden. "We had to find out whether these nanocubes could actually be resolved and imaged through the walls of the container using modern X-ray-based imaging techniques," adds co-author Lukas Grote, from the University of Hamburg. "And we have demonstrated that this is perfectly possible."
The uniquely high coherence of the X-rays at DESY's synchrotron radiation source PETRA III meant that the researchers could extract three-dimensional information about the nanoparticles growing inside from a single two-dimensional projection through a chemical reactor. "Multi-slice ptychography allows us to obtain multiple cross-sectional images through the volume of a chemical reactor, even though we only shine X-rays at it from one direction," Kahnt explains. "This shows that ultra-high-resolution images can be made inside a comparatively thick vessel, without that thickness being a limiting factor."
As a method, multi-slice ptychography therefore has the potential to track the formation of nanomaterials in real time. "To our knowledge, this is the first time this technique has been used in the context of a chemical reactor," Grote reports. "Observing nanomaterials under real conditions using X-ray imaging nevertheless poses a number of technical challenges, because they are produced in a liquid medium at high temperatures and under high pressure."
However, those involved in the collaboration between the Center for Hybrid Nanostructures (CHyN) at the University of Hamburg and DESY are confident that this will be possible in the future. There is also a huge technological significance to being able to observe the inside of chemical reactors almost in 3D, as Dorota Koziej, the CHyN group leader, emphasises: "Our ultimate goal is the controlled production of customised nanomaterials, for example for use as catalysts to produce green energy, or as inexpensive sensors for airborne pollutants."
"Realising this project with the help of X-ray microscopy, which is also becoming more and more precise, is the subject of the 'XStereoVision' project, part of the Röntgen-Ångström Cluster, a Swedish-German research collaboration," says lead scientist Christian Schroer of DESY. He sees great potential for three-dimensional, high-resolution X-ray microscopy in clarifying numerous open questions in the nanosciences.