Laser fluorescence: It's a gas

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  • Published: Oct 15, 2013
  • Author: David Bradley
  • Channels: Atomic
thumbnail image: Laser fluorescence: It's a gas

Spectroscopy off-the-shelf

Pd nanocube characterization. Data, for each set of conditions, are normalized by dividing luminescence values by the luminescence of the metal phase. Credit: Nature Materials/Bardhan et al

Using what they refer to as off-the-shelf, US researchers have developed a laser fluorescence spectroscopic technique to measure how and when nanocrystals adsorb and release hydrogen or other gases. The new investigative approach could be useful in research into lower-cost catalytic converters for vehicle exhausts, the future development of fuel cell technologies and improved gas sensors for industrial and anti-terrorism applications.

Limitations in existing methods for measuring the physical and chemical changes that take place in individual nanocrystals during the process have somewhat hindered progress in this field. Now, Rizia Bardhan of Vanderbilt University, New York, USA and colleagues - Lester Hedges, Cary Pint, Ali Javey, Stephen Whitelam and Jeffrey Urban - writing in the journal Nature Materials, describe a new laser-based technique built on conventional fluorescence spectroscopy that allows them to focus on nanocrystals in a way that was not possible before. “The new technique is simple and direct so other researchers should have no difficulty using it,” explains Bardhan. Success hinges on the fluctuations in fluorescence observed as nanocrystals adsorb and release gas molecules.

Subtle, but glowing report

"The fluorescence effect is very subtle and very sensitive to differences in nanocrystal size," Bardhan explains. "To see it you must use nanocrystals that are uniform in size." This is one reason as to why the effect has not been seen before. Fabrication techniques such as ball milling and wet-chemical approaches for making nanocrystals produce nanocrystals with a wide size distribution, which masks the effect. To test their approach, the team made uniform and stable palladium nanocrystals and used them to adsorb hydrogen gas. Their measurements showed that the size of the nanocrystals has a much greater effect on the rate of adsorption and release of hydrogen than was expected; this finding specifically has implications for the future development of safe, hydrogen storage systems.

Heading down the nano highway

The smaller the nanocrystal, the quicker the material can adsorb the gas, the more gas it can adsorb, the faster it can release it. "In the past, we thought that the size effect was limited to sizes less than 15 to 20 nanometres, but we found that it extends up to 100 nanometres," Bardhan adds. The researchers also determined that the adsorption/desorption rate was determined by just three factors: pressure, temperature, and nanocrystal size. Intriguingly, defects and strain had no significant effect on the process. They have now used their findings to build a simple computer model that allows them to predict the adsorption/desorption rates of various types and size ranges of nanocrystals with a variety of different gases. The team asserts that transformations are "unavoidably governed" by nanocrystal dimensions.

"Our results provide a general framework for understanding how nanoconfinement fundamentally impacts broad classes of thermally driven solid-state phase transformations relevant to hydrogen storage, catalysis, batteries and fuel cells," they conclude.

Related Links

Nature Mater 2013, 12, 905–912: "Uncovering the intrinsic size dependence of hydriding phase transformations in nanocrystals"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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