Revealing materials’ secrets with synchrotron light

Alexander Moewes is a high-energy physicist. Literally. The University of Saskatchewan researcher doesn’t smash atoms together to study subatomic particles—the typical work of “real” high-energy physicists. Rather, Moewes radiates energy and enthusiasm when he talks about his favorite research tools—synchrotrons.

“With synchrotron radiation, you can do experiments in so many areas of science that just cannot be done in any other way,” he says excitedly.

These stadium-sized, multi-million-dollar research facilities enable scientists to reveal the structure, chemical composition, electronic properties, and other features of specimens critical to materials science, chemistry, archaeology, molecular electronics, and a host of other disciplines.

For that reason, countries around the globe have built—or are in the process of building or upgrading—about 60 synchrotrons. Most of these facilities are in Europe, North America, and Asia. But the SESAME (Synchrotron-light for Experimental Science & Applications in the Middle East) synchrotron near Amman, Jordan, the first such facility in the Middle East, is scheduled to be up and running by next year (see page 31). And last November, an international team of scientists took the first steps to establishing a synchrotron center in Africa, the only habitable continent without such a facility.

Synchrotrons produce intense radiation, ranging from radio waves to high-energy X-rays, by accelerating electrons to nearly the speed of light and driving them through a circular storage ring lined with powerful magnets.

Researchers working at experimental stations, or beamlines, which are located along the ring’s perimeter, use various types of optics to tune in precisely to the exact wavelength range needed for their studies. The high degree of tunability, especially in the X-ray range, is one of the key features of synchrotrons that isn’t available with other light sources, explains Moewes, who, together with his group, conducts research at the Canadian Light Source, in Saskatoon, Saskatchewan, and the Advanced Light Source, in Berkeley, Calif.

That capability of synchrotrons, Moewes explains, not only enables researchers to selectively probe each element in a sample individually, but also allows them to zoom in on the orbital of interest, distinguish among valencies and oxidation states, and determine an atom’s bonding geometry and coordination.

Anthony W. Van Buuren, a materials science research group leader at Lawrence Livermore National Laboratory, points to another unmatched synchrotron feature—exceptional brilliance or beam intensity. The large number of X-ray photons emitted in fleeting bursts of synchrotron light enables researchers to make ultrafast time-dependent measurements, an ideal way to capture nanosecond snapshots of proteins and other molecules undergoing rapid changes.

With recent progress in controlling synchrotron radiation, scientists can now also work with very fine beams of X-rays. This gives researchers a high degree of spatial control over their experiments, opening the door to new types of microscopy that provide nanoscale structural information along with detailed chemical maps.

All of these unique capabilities of synchrotron radiation offer unprecedented ways of peering deep into the interior of catalysts, battery materials, ancient artifacts, and other specimens without having to first isolate them from their natural or technologically relevant environments.

The wide applicability of synchrotron methods and the opportunity to use them to discover new phenomena continues to excite Moewes nearly 30 years after he first began working at these facilities.

Some adrenaline junkies get their thrills by jumping out of airplanes or scaling Mount Everest, he says. Moewes gets his fix by exploring uncharted scientific territory at the world’s most powerful light sources.

Peering into fossils and ancient artifacts

In 2003, paleontologists discovered centimeter-long fossilized eggs in northeastern Thailand that turned out to be 125 million years old. Originally, the scientists concluded that a small theropod dinosaur or perhaps a primitive bird laid the tiny eggs.

By using a powerful synchrotron imaging method to scrutinize the hidden embryonic skeletons preserved in the eggs, the team now knows the egg layer’s true identity—an anguimorph lizard, a category that includes Komodo dragons (PLOS One 2015, DOI: 10.1371/journal.pone.0128610).

According to team leader Vincent Fernandez, a beamline scientist at the European Synchrotron Radiation Facility, in Grenoble, France, the embryonic lizard skeletons are the oldest ones ever discovered in fossil eggs. Most lizards lay so-called soft-shelled eggs, says Fernandez, who was trained as a paleontologist. So the discovery that the eggs, which were the hard-shelled type, held anguimorph embryos “came as a surprise and alters our understanding of the evolution of lizard reproduction,” he says.

Synchrotron radiation was crucial to the discovery. During the initial examination of the fossils, the team observed minute embryonic bones poking out of the rocky material in some of the eggs. The fragile bones held critical clues about the species’ identity, but they were encased in rock, which prevented their extraction and presented another formidable analytical challenge.

“The problem often in paleontology is that fossil bones and the surrounding rock have similar densities,” thwarting the chance of identifying them via common nondestructive imaging methods, Fernandez says.

Ultimately, the team solved the puzzle by using a synchrotron X-ray technique known as phase-contrast microtomography, which is particularly sensitive to minute differences in the densities of a sample’s components.

Synchrotron X-ray methods are also revealing secrets hiding inside other types of fragile ancient artifacts. For example, a recent study revealed the handwritten text concealed inside rolled-up papyrus scrolls that were charred when Mount Vesuvius erupted nearly 2,000 years ago.

Another investigation determined that the secret to the strength of some Roman architectural structures is the formation of platelike crystals of strätlingite, a durable calcium aluminosilicate mineral that prevents cracks from propagating through ancient buildings’ mortar.

Analyzing new types of electronics

For decades, the electronics industry has produced one generation after another of ever-faster and more powerful electronic devices by shrinking their silicon-based components. Physical limitations have begun impeding that trend, driving scientists to look for alternative ways to squeeze more electronics into smaller spaces.

Some engineers want to turn to molecular electronics, a system in which individual molecules serve as circuit components. Others are investigating spintronics, in which digital information is controlled by electron charge—as is done today—and electron spin, which is associated with magnetism. Implementing those strategies requires understanding atomic-scale details of the underlying materials. Synchrotron-based analytical methods have helped those studies.

At the Canadian Light Source, in Saskatoon, Saskatchewan, for example, a research team led by University of Saskatchewan physicist Alexander Moewes has been studying iron-doped indium oxide. The material is a promising candidate for spintronics because it is semiconducting and magnetic. But the basis of its magnetism is poorly understood, which hampers efforts to design even better spintronic materials.

On the basis of X-ray absorption and X-ray scattering methods, the team found that the magnetism is linked to the 3d orbital energies of iron atoms, specifically in the Fe³⁺ state, that reside in indium lattice sites directly adjacent to a vacancy created by a missing oxygen atom (Phys. Rev. Lett. 2015, DOI: 10.1103/physrevlett.115.167401). That level of detail could only have been uncovered in a synchrotron experiment, Moewes points out. Meanwhile, Benjamin Stadtmüller and Christian Kumpf of the Jülich Research Center and coworkers used a synchrotron X-ray technique to sort out some counterintuitive findings on a molecular electronics system. The team explores the charge-transfer interplay that occurs at interfaces between metal surfaces and organic molecules.

In some of these systems, for example in optoelectronic devices, the molecules absorb light and transfer electrons through neighboring molecules to a metal electrode. In other systems, the molecules modify electric current for select applications. In all of these cases, the molecules’ bonding details strongly affect their electronic properties.

The Jülich team’s studies focused on a silver-supported film of perylene tetracarboxylic dianhydride (PTCDA) and copper(II) phthalocyanine (CuPc). When they compared the mixed-molecule film to those with just a single component, the group found that the PTCDA-silver bond elongated, suggesting it became weaker, and the CuPc-silver bond contracted, suggesting it became stronger. But analysis of the orbital energies showed the opposite was true: The bonds that elongated had become stronger, and those that shrunk became weaker.

Using a standing-wave X-ray technique and quantum calculations, the researchers determined that the bond length adjustments brought PTCDA and CuPc to nearly equal heights above the surface. That change caused CuPc to become a better electron donor and PTCDA a better acceptor, which improved charge transfer through the system—a key feature required of molecular electronic devices (Nat. Commun. 2014, DOI:10.1038/ncomms4685).

Improving solid catalysts

Many of today’s large-scale chemical processes, such as petrochemistry, pollution scrubbing, and chemical synthesis, depend on solid catalysts. High temperatures, high pressures, and other harsh working conditions eventually cause these catalysts to fail, which causes downtime and raises costs. Synchrotron methods are providing researchers with clues about catalyst failure mechanisms and strategies for avoiding them.

In one such study, a team led by Bert M. Weckhuysen of Utrecht University used an X-ray nanotomography method to examine a series of fresh and used fluid catalytic cracking catalysts. Oil refiners use such catalysts to produce gasoline and other products from crude oil. The catalysts typically consist of two active components held together by a binder: a zeolite with roughly 1-nm-wide pores and a clay with larger pores.

The team found that the catalysts fail because iron and nickel impurities from crude oil accumulate at the entrances to the clay’s large pores. That process prevents crude oil molecules from reaching the zeolites’ catalytic sites, which are located in narrow channels in the interior (Sci. Adv. 2015, DOI: 10.1126/sciadv.1400199). Weckhuysen suggests that catalyst lifetimes might be extended by coating the catalysts with a clog-resistant macroporous layer.

To sort out the details of the pore-clogging process, the Utrecht team again turned to a synchrotron method. This time they generated visual evidence for two types of clogging mechanisms. In one, iron originates from porphyrin-like crude oil molecules, and in the other, iron comes from refinery equipment. Researchers previously had proposed separate mechanisms, but the team provided the first direct visual evidence for both (ACS Catal. 2016, DOI: 10.1021/acscatal.6b00221).

Synchrotron-based X-ray diffraction has also helped with the design of catalysts that scrub nitrogen oxides (NO_x) from gas streams, including those emitted by vehicles and power plants. Researchers from Fudan University developed a nanomaterial in which NO_x-stripping catalyst particles made of vanadium oxide sit on top of hollow tungsten oxide rods. The team designed the rods to trap catalyst-poisoning alkali metals and thereby protect the catalyst. X-ray analysis of used catalyst particles showed that, indeed, the alkali atoms were trapped inside the rods.

Spying on batteries

Most people are completely dependent on batteries to power their many gadgets and devices. So it’s no surprise that top researchers everywhere are working to understand why batteries fail and how to extend battery lifetime. Synchrotron users such as John S. Okasinski, a beamline scientist at the Advanced Photon Source at Argonne National Laboratory near Chicago, approach the problem by using high-energy (>50 keV) X-rays to peer deep inside batteries while they are charging and discharging.

Okasinski explains that this type of synchrotron radiation, typically referred to as “hard” X-rays, can penetrate millimeters into materials, compared with the micrometers possible with standard laboratory X-ray sources. The extended reach enables scientists to probe a working battery’s anode, its cathode, and the layer separating the two in real time and in the presence of electrolyte solution.

Working with other Argonne scientists, Okasinski used hard X-ray methods in a series of studies on lithium-air batteries. Li-air batteries could potentially provide much more energy per weight than lithium-ion batteries used in many current electronics. But Li-air batteries tend to fail quickly.

The studies turned up multiple findings. First, trace amounts of water in the electrolyte solution, likely from electrolyte decomposition, triggered unwanted reactions. For example, the reactions caused the lithium anode to continuously decompose and form lithium hydroxide. But the news wasn’t all bad. The team also found that the lithium hydroxide layer was riddled with microscopic channels that enabled the battery to continue running—albeit weakly—until all of the lithium was consumed.

Overall, the findings suggest that a decomposition-resistant electrolyte would mitigate some of the problems and improve battery performance.