[meteorite-list] Nanometer-Sized Particles Change Crystal Structure When Wet

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:16:37 2004
Message-ID: <200308272052.NAA18483_at_zagami.jpl.nasa.gov>

http://www.berkeley.edu/news/media/releases/2003/08/27_change.shtml

Nanometer-sized particles change crystal structure when wet
By Robert Sanders
University of California Berkeley Press Release
August 27, 2003

BERKELEY - As scientists shrink materials down to the nanometer scale,
creating nanodots, nanoparticles, nanorods and nanotubes a few tens of atoms
across, they've found weird and puzzling behaviors that have fired their
imaginations and promised many unforeseen applications.

Now University of California, Berkeley, scientists have found another
unusual effect that could have both good and bad implications for
semiconductor devices once they've been shrunk to the nanometer scale.

The discovery also could provide a way to tell whether pieces of rock from
outer space came from planets with water.

In a paper appearing in the Aug. 28 issue of Nature, a UC Berkeley team
comprised of physicists, chemists and mineralogists reports on the unusual
behavior of a semiconducting material, zinc sulphide (ZnS), when reduced to
pieces only 3 nanometers across - clumps containing only 700 or so atoms.

They found that when the surface of a ZnS nanoparticle gets wet, its entire
crystal structure rearranges to become more ordered, closer to the structure
of a bulk piece of solid ZnS.

"People had noticed that nanoparticles often had unexpected crystal
structures and guessed it might be due to surface effects," said
post-doctoral physicist Benjamin Gilbert of UC Berkeley's Department of
Earth & Planetary Science. "This is a clear-cut demonstration that surface
effects are important in nanoparticles."

Gilbert and co-author Hengzhong Zhang, a research scientist and physical
chemist, suggest that many types of nanoparticles may be as sensitive to
water as ZnS.

"We think that, for some systems of small nanoparticles maybe 2 to 3
nanometers across, this kind of structural transition may be common," Zhang
said.

"There's a good and bad side to this," Gilbert added. "If we can control the
structure of a nanoparticle through its surface, we can expect to produce a
range of structures depending on what molecule is bound to the surface. But
this also produces unexpected effects researchers may not want."

In addition, Zhang said these effects could have implications for our
understanding of extraterrestrial materials and identification of
extraterrestrial rocks, especially when the interpretation is being done by
a robotic probe. A nanoparticle that formed in a place with water, such as
Earth, would have a more ordered surface than a nanoparticle formed in
space, where water is not present.

"Nanoparticles are probably widespread in the cosmos, and their surface
environments may vary significantly, such as water versus no water, or the
presence of organic molecules," said Jillian Banfield, professor of earth
and planetary science at UC Berkeley. Banfield has been looking at
microscopic and nanoscale particles in rocks, minerals and the environment
in general to determine what information they can provide about their
origin.

"As essentially all properties of nanoparticles - including spectroscopic
ones often measured in the identification of materials - are structure
dependent, and we now know that nanoparticle structure depends on the
surface environment, it may be important to know how phase, size, structure,
and properties relate so that spectra can be correctly interpreted."

Understanding how the characteristics of specific nanomaterials vary with
environment could also lead to their use as sensors, for example, for water.

Some nanoparticles, including ZnS, are produced by microbes as a byproduct
of metabolism. Banfield found ZnS in the form of a mineral called sphalerite
in an abandoned zinc mine in Wisconsin four years ago, a product of
sulphate-reducing bacteria. Numerous bacteria produce magnetite particles,
or iron oxide, while Banfield has found others that produce uranium oxide.
All these particles are small, ranging from nanometers to microns -
millionths of a meter - across.

The trick is to distinguish these biogenic nanoparticles from similar
nanoparticles formed by geologic processes. The importance of this issue
arose several years ago when small inclusions in a meteorite from Mars were
interpreted as being of biological origin by some and of geologic origin by
others. Similar ambiguity has arisen over how to interpret small inclusions
in rocks dating from the early years of the Earth.

Banfield studies naturally occurring nanoparticles of biologic and geologic
origin, as well as synthetic ones, in order to understand how structure,
properties, and reactivity vary with particle size. The geochemical
consequences of size-dependent phenomena may be far reaching, she said.

Zhang developed molecular dynamics models to study the reaction of ZnS
nanoparticles to surface binding, and predicted that nanoparticles grown to
around 3 nanometers across would be most sensitive to surface water. Feng
"Forrest" Huang, a post-doctoral fellow in the Banfield group, developed a
method to make ZnS nanoparticles of that size in a methanol solvent. Zhang
and Huang worked with Gilbert to observe the structural transition using
synchrotron x-ray techniques.

Within the methanol and after it had evaporated, these three-nanometer
particles were found through X-ray diffraction to have a somewhat disordered
structure at the surface, with only the core of the nanoparticle exhibiting
the regular order of bulk ZnS, which is sometimes used as a semiconductor in
specialized photoelectronics applications.

When the methanol was spiked with water, however, the ZnS particles
developed a much more ordered structure throughout. Only the immediate
surface retained a disordered crystal structure.

The UC Berkeley researchers suggest that nano-ZnS has two or more stable
structures, depending on what molecules are stuck to the surface. This is
not surprising, Gilbert said, because the surface area of nanoparticles is
so large compared to the volume that reactions at the surface are likely to
affect the entire nanoparticle. In larger materials, the surface/volume
ratio is much less, which means the surface has less effect on the interior
of the solid.

The team also was able to demonstrate that the nanoparticles undergo
reversible structure transformations at room temperature when removed from
the methanol solvent and allowed to dry out. That is, when the dry
nanoparticles are again immersed in methanol, they revert to their original
structure.

"This result demonstrates that these nanoparticles are not trapped in a
metastable state, but can respond to changes in their surface environments,"
Banfield said.

"To our knowledge, these are the first surface-driven room temperature
transitions observed in nanoparticles," she added. "In methanol, the
nanoparticles are highly distorted, but water addition removes this
distortion. If alternative ligands or solvents can be found that stabilize
alternative variants, there may be ways to generate uncommon structures
through surface binding after the nanoparticle is synthesized."

The research team plans to continue to investigate how and why the crystal
structure of ZnS nanoparticles changes with the surface environment. In
particular, they hope to find out how the crystal rearranges itself so
easily at room temperature, and how long it takes.

The work was sponsored by the National Science Foundation and the U.S.
Department of Energy.
Received on Wed 27 Aug 2003 04:52:42 PM PDT