For phasing nanocrystals, size really matters

Size is of much greater importance than previously believed in metal nanocrystals undergoing phase transformations, a finding that has important implications for the future design of hydrogen storage systems, catalysts, fuel cells and batteries.

Investigators at Lawrence Berkeley National Laboratory’s Molecular Foundry, led by Jeffrey J. Urban and Stephen Whitelam, have developed an optical probe based on luminescence that provides the first direct observations of metal nanocrystals undergoing phase transformations – changes from a solid to a liquid to a gas or plasma – during reactions with hydrogen gas. These observations reveal a surprising degree of size dependence with regard to properties of thermodynamics and kinetics.

“No one has ever directly observed phase transformations in metal nanocrystal systems before, so no one saw the size-dependence factor, which was obscured by other complicating effects,” Urban said. “The assumption had been that for nanocrystals beyond 15 nm, the thermodynamic and kinetic behavior would be essentially bulklike. However, our results show that pure size effects can be understood and productively employed over a much broader range of nanocrystal sizes than previously thought.”

While it is known that materials on the nanoscale can offer physical, chemical and mechanical properties not displayed at the microscale, knowledge about nanocrystal properties being altered during phase transformation has been lacking.

(Above left) Palladium nanocubes interacting with hydrogen gas, directly observed through in situ luminescence, reveal that size can make a much bigger difference on phase transformations than previously believed. (Above right) Stephen Whitelam, left, and Jeffrey Urban of Berkeley Lab’s Molecular Foundry. Photo courtesy of LBNL.

“Quantitative understanding of nanocrystal phase transformations has been hindered by difficulties in directly monitoring well-characterized nanoscale systems in reactive environments,” Urban said.

He and his colleagues addressed the problem with a custom-built stainless steel gas-tight cell with optical windows and heating elements connected to a high vacuum pump. They collected in situ luminescence spectra with a confocal Raman microscope as palladium nano-cubes 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 said. “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.”

Whitelam and researcher Lester Hedges led a team that developed a statistical mechanical model to quantify the observational data for palladium nanocubes of all sizes. Because of the narrow size distribution of the nanocubes, the research showed a direct correlation between luminescence and phase transitions that can be applied to other metal nanocrystal systems.

“Simple geometric arguments tell us that, under certain conditions, thermally driven solid-state phase transformations are governed by nanocrystal dimensions,” Whitelam said. “These arguments further suggest ways of optimizing hydrogen storage kinetics in a variety of metal nanocrystal systems.”

The researchers’ next step 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 that allow us to look at a number of gas-nanocrystal interactions in which controlling the thermodynamics of the interactions is paramount,” Urban said.

The study is reported in Nature Materials (doi: 10.1038/nmat3716).