Urban and his colleagues addressed this problem with a custom-built stainless steel gas-tight cell with optical windows and heating elements and connected to a high vacuum pump. They used this experimental setup to collect insitu luminescence spectra with a confocal Raman microscope as palladium nanocubes interacted with hydrogen gas. The nanocubes were synthesized by wet-chemistry and were all clear-faceted single-crystalline objects with a narrow range in size distribution.
"Our experimental setup allowed for rapid, direct monitoring of minuscule alterations in luminescence during hydrogen sorption," Urban says. "This allowed us to uncover the size-dependence of the intrinsic thermodynamics and kinetics of hydriding and dehydriding phase transformations. We observed a dramatic decrease in luminescence as the palladium nanocubes formed hydrides. This lost luminescence was regained during dehydriding."
A statistical mechanical model whose development was led by Whitelam and co-author Hedges was then used to quantify the observational data for palladium nanocubes of all sizes. Because of the narrow size distribution of the nanocubes, Whitelam, Urban and their colleagues were able to show a direct correlation between luminescence and phase transitions that can be applied to other metal nanocrystal systems as well.
"Simple geometric arguments tell us that under certain conditions, thermally driven solid-state phase transformations are governed by nanocrystal dimensions," Whitelam says. "These arguments further suggest ways of optimizing hydrogen storage kinetics in a variety of metal nanocrystal systems."
The next step in this research will be to examine the effects of dopants on phase transformations in metal nanosystems.
"Our luminescence-probe and statistical mechanical model are a versatile combination," Urban says, "that allow us to look at a number of gas-nanocrystal interactions in which controlling the thermodynamics of the interactions is paramount."
This research was supported by
The Molecular Foundry is one of five DOE Nanoscale Science Research Centers (NSRCs), national user facilities for interdisciplinary research at the nanoscale, supported by the
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