Researchers observe the formation of water at the nanoscale in real time

For the first time ever, researchers observed, in real time and at the molecular scale, hydrogen and oxygen atoms fusing to form tiny nanometer-sized water bubbles.

The event occurred as part of a new study from Northwestern University, in which scientists sought to understand how palladium, a rare metallic element, catalyzes the gas reaction to generate water. By observing the reaction at the nanoscale, the Northwestern team understood how the process occurs and even discovered new strategies to speed it up.

Because the reaction does not require extreme conditions, the researchers say it could be exploited as a practical solution for rapidly generating water in arid environments, including on other planets.

The research is published in the Proceedings of the National Academy of Sciences.

“By directly visualizing water generation at the nanoscale, we were able to identify optimal conditions for rapid water generation under ambient conditions,” said Vinayak Dravid of Northwestern, lead author of the study. “These findings have significant implications for practical applications, such as the rapid generation of water in deep space environments using gases and metal catalysts, without requiring extreme reaction conditions.

“Think of Matt Damon’s character, Mark Watney, in the movie “The Martian.” He burned rocket fuel to extract the hydrogen, then added oxygen from his oxygenator. Our process is analogous, except we bypass the need for fire and other extreme conditions We simply mixed palladium and gases.

Dravid is the Abraham Harris Professor of Materials Science and Engineering at Northwestern’s McCormick School of Engineering and the founding director of the Experimental Center for Atomic and Nanoscale Characterization (NUANCE) at Northwestern University, where the study was conducted. He is also Director of Global Initiatives at the International Institute of Nanotechnology.

New technology enables discovery

Since the early 1900s, researchers have known that palladium can act as a catalyst to quickly generate water. But exactly how this reaction occurs remains a mystery.

“It’s a known phenomenon, but it has never been fully understood,” said Yukun Liu, first author of the study and a Ph.D. candidate in Dravid’s laboratory. “Because you really need to be able to combine direct visualization of water generation and analysis of the structure at the atomic scale in order to understand what is happening with the reaction and how to optimize it.”

But observing the process with atomic precision was simply impossible – until nine months ago. In January 2024, Dravid’s team unveiled a new method for analyzing gas molecules in real time. Dravid and his team developed an ultra-thin glassy membrane that traps gas molecules in honeycomb-shaped nanoreactors, so they can be viewed in transmission electron microscopes under high vacuum.

With the new technique, previously published in Scientific advancesResearchers can examine gas samples at atmospheric pressure at a resolution of just 0.102 nanometers, compared to a resolution of 0.236 nanometers using other cutting-edge tools. The technique also allowed, for the first time, simultaneous analysis of spectral and reciprocal information.

“With the ultrathin membrane, we get more information from the sample itself,” said Kunmo Koo, first author of the Science Advances paper and a research associate at the NUANCE Center, where he is mentored by research associate professor Xiaobing Hu. “Otherwise, the information from the thick container interferes with the analysis.”

The smallest bubble ever seen

Using the new technology, Dravid, Liu and Koo examined the reaction of palladium. First, they saw hydrogen atoms penetrating the palladium, expanding its square lattice. But when they saw tiny water bubbles forming on the surface of the palladium, the researchers couldn’t believe their eyes.

“We think this may be the smallest bubble ever formed that has been seen directly,” Liu said. “It’s not what we expected. Luckily we were recording it so we could prove to others that we weren’t crazy.”

“We were skeptical,” Koo added. “We needed to do further research to prove that it was indeed water that had formed.”

The team implemented a technique called electron energy loss spectroscopy to analyze the bubbles. By examining the energy loss of the scattered electrons, the researchers identified oxygen-binding characteristics unique to water, confirming that the bubbles were indeed water. The researchers then cross-checked this result by heating the bubble to assess the boiling point.

“It’s a nanoscale analogue of the Chandrayaan-1 lunar rover experiment, which was looking for evidence of water in lunar soil,” Koo said. “When studying the Moon, he used spectroscopy to analyze and identify molecules in the atmosphere and on the surface. We took a similar spectroscopic approach to determine whether the product generated was indeed water. “

Optimization recipe

After confirming that the palladium reaction generated water, the researchers then sought to optimize the process. They added hydrogen and oxygen separately at different times or mixed them to determine which sequence of events generated water at the fastest rate.

Dravid, Liu and Koo found that adding hydrogen first, followed by oxygen, led to the fastest reaction rate. Because the hydrogen atoms are so small, they can squeeze between the palladium atoms, causing the metal to expand. After filling the palladium with hydrogen, the researchers added oxygen gas.

“Oxygen atoms are energetically favorable for adsorption on palladium surfaces, but they are too large to enter the lattice,” Liu said. “When we first circulated oxygen, its dissociated atoms covered the entire surface of the palladium, so hydrogen could not adsorb on the surface to start the reaction. But when we first stored the hydrogen in the palladium, then added oxygen, the reaction started. The hydrogen comes out of the palladium to react with the oxygen, the palladium contracts and returns to its original state.

Sustainable system for deep space

The Northwestern team imagines that others, in the future, could potentially prepare hydrogen-filled palladium before traveling to space. Then, to produce drinking water or to water plants, travelers will only have to add oxygen. Although the study focused on investigating bubble generation at the nanoscale, larger palladium sheets would generate much larger amounts of water.

“Palladium may seem expensive, but it is recyclable,” Liu said. “Our process does not consume any. The only thing consumed is gas, and hydrogen is the most abundant gas in the universe. After the reaction, we can reuse the palladium platform again and again.”