The new TEM from Thermo Fisher Scientific at the University of Groningen. Photo: University of Groningen.Physicists at the University of Groningen in the Netherlands have visualized hydrogen at the interface between titanium and titanium hydride with a transmission electron microscope (TEM). Using a new technique, they succeeded in visualizing both the metal and the hydrogen atoms in a single image, allowing them to test different theoretical models that describe the interface structure. The physicists report their findings in a paper in Science Advances.
To understand the properties of materials, it is often vital to observe their atomic-scale structure. But while scientists have visualized atoms with a TEM, no one has so far succeeded in producing proper images of both heavy atoms and the lightest one of all (hydrogen).
This is exactly what Bart Kooi, professor of nanostructured materials at the University of Groningen, and his colleagues have now done. Using a new TEM with advanced capabilities, they were able to produce images of both titanium atoms and hydrogen atoms at the interface between titanium and titanium hydride.
The resulting pictures show how columns of hydrogen atoms fill spaces between the titanium atoms, distorting the crystal structure. The hydrogen atoms occupy half of the spaces, which was originally predicted years ago. “In the 1980s, three different models were proposed for the position of hydrogen at the metal/metal hydride interface,” says Kooi. “We were now able to see for ourselves which model was correct.”
To create the metal/metal hydride interface, Kooi and his colleagues started out with titanium crystals, which they infused with atomic hydrogen. The hydrogen atoms penetrated the titanium in very thin wedges, forming tiny metal hydride crystals.
“In these wedges, the numbers of hydrogen and titanium atoms are the same,” Kooi explains. “The penetration of hydrogen creates a high pressure inside the crystal. The very thin hydride plates cause hydrogen embrittlement in metals, for example inside nuclear reactors.” The pressure at the interface prevents the hydrogen from escaping.
Producing images of the heavy titanium atoms and the light hydrogen atoms at the interface was quite a challenge. First, the sample was loaded with hydrogen and then viewed at a specific orientation along the interface. This was achieved by using an ion beam to cut properly aligned crystals from titanium and then to make the samples thinner – to a thickness of no more than 50nm.
The physicists were able to visualize the titanium atoms and hydrogen atoms at the same time thanks to several innovations included in the novel TEM. Heavy atoms can be visualized by the way they scatter the electrons in the microscope beam, with the scattered electrons preferably detected using high-angle detectors.
“Hydrogen is too light to cause this scattering, so for these atoms, we have to rely on constructing the image from low-angle scattering, which includes electron waves,” says Kooi. However, the material being studied causes interference in these electron waves, which has so far made identifying hydrogen atoms almost impossible.
Kooi and his colleagues detected the electron waves using a low-angle bright-field detector, which comprises a circular bright-field detector divided into four segments. By analyzing differences in the wavefronts detected in opposing segments and looking at the changes that occur when the scanning beam crosses the material, the physicists were able to filter out the interferences and visualize the very light hydrogen atoms.
“The first requirement is to have a microscope that can scan with an electron beam that is smaller than the distance between the atoms. It is subsequently the combination of the segmented bright-field detector and the analytical software that makes visualization possible,” explains Kooi, who worked in close collaboration with scientists from Thermo Fisher Scientific, the company that manufactured the TEM.
Kooi's group added various noise filters to the TEM’s software and tested them. They also performed extensive computer simulations, against which they compared the experimental images.
In this way, they were able to investigate the interaction between hydrogen and the metal, which is useful knowledge for the study of materials capable of storing hydrogen. “Metal hydrides can store more hydrogen per volume than liquid hydrogen,” says Kooi.
Furthermore, the techniques used to visualize the hydrogen could also be applied to other light atoms, such as oxygen, nitrogen or boron, which are important in many nanomaterials. “Being able to see light atoms next to heavy ones opens up all kinds of opportunities,” he adds.
This story is adapted from material from the University of Groningen, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.