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Younan Xia talks with
ScienceWatch.com and answers a few questions about
this month's Fast Moving Fronts paper in the field of Materials
Science. The author has also sent along images of his
work.
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Article: Shape-Controlled Synthesis of Metal
Nanocrystals: Simple Chemistry Meets Complex
Physics?
Authors: Xia, Y;Xiong, YJ;Lim, B;Skrabalak,
SE
Journal: ANGEW CHEM INT ED, 48 (1): 60-103, 2009
Addresses: Washington Univ, Dept Biomed Engn, St Louis, MO
63130 USA.
Washington Univ, Dept Biomed Engn, St Louis, MO 63130
USA.
Univ Washington, Dept Chem, Seattle, WA 98195 USA.
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Why do you think your paper is highly
cited?
Our paper provides a comprehensive review and critical assessment of
research activities centering on the syntheses and applications of metal
nanocrystals with well-controlled shapes and facets.
This article is highly cited simply because it touches upon an extremely
important subject that is being actively pursued by many research
groups.
Controlling the shape of a metal nanocrystal offers one of the most
powerful means for maneuvering its properties and enhancement of its
usefulness for a given application. For example, it has long been
recognized (mainly through theoretical modeling) that the number and
positions of surface plasmon resonance (SPR) peaks of a silver or gold
nanocrystal are strongly correlated with the shape.
A nanocube exhibits three resonance peaks while a spherical counterpart
only displays one major peak. The shape also controls how local electric
fields are distributed on the surface of a nanocrystal and thus its
efficiency in applications such as surface-enhanced Raman scattering
(SERS).
In catalysis, it is well-documented that metal nanoparticles can speed up
reactions with different activity and selectivity depending on the
crystallographic planes (i.e., facets) exposed on the surface, with the
{100} and {210} facets of platinum working the best for reactions that
involve hydrogen and carbon monoxide, respectively.
Despite the technological importance, the challenge to synthetically and
systematically control the shape of metal nanocrystals had been met with
only limited success until eight years ago when we reported a breakthrough
(Y. Sun and Y. Xia, "Shape-controlled synthesis of gold and silver
nanoparticles," Science 298: 2176-79, 2002).
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Transmission electron
microscopy
images...
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An illustration showing
how...
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Although that paper only described two case studies on cubic nanocrystals
of gold and silver, the methodologies were later extended by my and many
other groups to cover essentially all noble metals and a myriad of
different shapes. As a result, that paper has stimulated an exponential
growth of research by enabling a broad range of exciting new
demonstrations.
It is the outcomes of these research activities that form a basis for the
current review article. Based on the citation data for this review article,
it looks like the momentum created by that Science report is even
getting stronger with time, as more people are moving into the research
area of shape-controlled synthesis.
Does it describe a new discovery, methodology, or
synthesis of knowledge?
Since this is a review article, its major role is to present an
intellectual framework, including the rationale, methodology, and mechanism
for the synthesis of metal nanocrystals with well-controlled shapes.
We begin with a brief introduction to nucleation and growth mechanisms in
the context of metal nanocrystal synthesis, followed by a discussion of the
possible shapes that a metal nanocrystal might take under different
conditions.
We then focus on a variety of experimental parameters that have been
explored to manipulate both the nucleation and growth pathways in
solution-phase syntheses in an effort to generate the specific shapes.
We also elaborate on these approaches by selecting good examples in which
there is already reasonable understanding of the observed shape control or,
at least, the protocols have proven to be both reproducible and
controllable.
Towards the end, we highlight a number of applications that have been
enabled and/or enhanced by the synthesis of metal nanocrystals with
well-controlled shapes. We conclude this article with personal perspectives
on the directions toward which future research in this field might lead us.
Would you summarize the significance of your paper
in layman's terms?
This article provides the reader with an updated account of the protocols
and mechanisms with respect to the synthesis of metal nanocrystals having
well-defined and controllable shapes. In a sense, this article is a nice
combination of three major components: a complete collection of reliable
protocols, an analytical discussion of many mechanistic studies, and an
offering of insightful perspectives based on first-hand experience.
After reading through this article, the reader should be able to know
immediately what has been reported in literature; what is the best protocol
for producing nanocrystals of a specific metal and with a specific shape
(or facets); and what could be some of the potential problems.
The critical analyses running throughout this article can also serve as
useful guidelines for someone who is interested in designing and developing
a new protocol for the synthesis of metal nanocrystals with a desired
shape.
How did you become involved in this research and
were any particular problems encountered along the way?
I worked with Professor
George M. Whitesides as a Ph.D. candidate and then a
postdoctoral fellow at Harvard
University.* My thesis work
involved the development of soft lithography (See: Y. Xia and G. M.
Whitesides, "Soft lithography," Angewandte Chemie International
Edition 37: 551-75, 1998), which represents a top-down approach to
the fabrication of nanostructures.
When I launched my own research at the University of Washington in Seattle
in 1997, I wanted to explore different routes to nanostructures, and in
particular, the bottom-up approach which involves the formation of
nanostructures from building blocks with smaller dimensions, such as atomic
and molecular species. That is how I got started on the development of new
chemical methods for the synthesis of nanocrystals.
From the perspective of methodology, all chemical syntheses of nanocrystals
share the same physics: that is, nucleation and growth. However, the exact
details of these two steps can be substantially different, depending on the
materials and nanocrystals involved. That means we have to develop a
specific protocol for each type of nanocrystal, which makes it much more
complicated compared to a top-down approach, where the technique (e.g.,
electron beam writing) can often be applied to all sorts of materials and
nanostructures.
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"Our paper provides a comprehensive review and critical
assessment of research activities centering on the
syntheses and applications of metal nanocrystals with
well-controlled shapes and facets"
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For chemical synthesis, an understanding of the nucleation and growth
mechanisms becomes of paramount importance, as it allows one to see why a
specific shape is formed or not formed during a synthesis.
For example, after many years of study, we finally established that the
final shape of a metal nanocrystal is primarily determined by the number of
twin defects included in the initially formed seed, with the seed being
defined as something larger than a nucleus in which structural fluctuation
is no longer an option.
As summarized on the cover illustration accompanying this review article,
the product will be a nanocube with various degrees of truncation at
corners for a single-crystal seed (with no twin), while a right bipyramid
(or a nanobeam with a single-twinned cross-section) will be obtained from a
singly-twinned seed and a pentagonal nanorod or nanowire will be grown from
a multiply-twinned seed in a decahedral shape.
Whenever twinning becomes random, the synthesis will yield irregularly
shaped nanoparticles—a morphology that was reported again and again
in thousands of papers prior to the publication of our Science
report.
Although our results clearly illustrate the one-to-one correspondence
between the seed and the resultant nanocrystal, it is still not clear what
factor(s) determines the exact number of twin defects formed in a seed
during the nucleation process and how to control it experimentally and
reliably.
At the current stage of development, oxidative etching (a process similar
to rusting) seems to be the most effective method for selectively
eliminating different types of twinned seeds and thus generating metal
nanocrystals of a specific shape exclusively.
For example, we have clearly shown that oxidative etching based on chloride
ions and oxygen—from the air and/or dissolved in the
solvent—yields silver or palladium nanocubes with different degrees
of corner truncation, while oxidative etching with bromide ions and oxygen
generates right bipyramids together with a small portion of nanocubes.
Since the amount of chloride or bromide ions needed for the etching is so
little—typically, on the level of parts per million—all the
syntheses of metal nanocrystals are highly susceptible to ionic impurities
that could be introduced into the chemical reagents during their
manufacturing, transportation, and/or storage.
This high sensitivity to oxidative etching also explains why
shape-controlled synthesis of metal nanocrystals has been so difficult to
achieve although the first chemical synthesis of metal nanocrystals was
documented by the British scientist Michael Faraday (1791-1867) in 1847,
when he discovered that the optical properties of gold colloids differed
from those of the corresponding bulk metal.
Once we have a solid understanding of the roles played by various ionic
species and oxygen, then we have a better way to control both nucleation
and growth processes and thus, the shapes of nanocrystals, by purposely
introducing some specific ionic species into a synthesis at a
well-controlled level.
As a result of these new developments, the synthesis of metal nanocrystals
is evolving from an art into a science. This is also probably another
reason why so many people are moving into this area, as it is getting
easier to obtain metal nanocrystals with well-controlled shapes for various
applications.
Where do you see your research leading in the
future?
About four or five years ago, I became interested in finding applications
for the nanomaterials my group had developed, including metal nanocrystals,
gold nanocages, and polymer nanofibers. I was particularly interested in
biomedical applications because I personally see a lot of potentials there
for nanomaterials in terms of diagnosis and therapy. It is also a research
field that probably has the strongest impact on our society. As a result, I
switched my area of concentration from chemistry to biomedical engineering
by relocating from the University of Washington in Seattle to Washington
University in St. Louis in the summer of 2007.
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"Since this is a review article, its major role is to
present an intellectual framework, including the rationale,
methodology, and mechanism for the synthesis of metal
nanocrystals with well-controlled shapes."
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Our current efforts include the development of nanofibers and other
nanostructures that can be used to control the differentiation of
stem cells and guide the outgrowth of neurites. This
will allow us to address clinical problems related to neural
regeneration, peripheral nerve repair, and spinal cord injury recovery.
Meanwhile, we are exploring the use of gold nanocages as contrast agents,
drug delivery vehicles (See: M. S. Yavuz, et al., "Gold nanocages
covered by smart polymers for controlled release with near-infrared light,"
Nature Materials 12: 935-39, 2009), and photothermal therapeutic
agents. The end applications include early cancer detection and treatment
with better efficacy and less side effects.
On the fundamental side, we are exploring new approaches to studying and
controlling cell communication. In one approach, we use gold nanocages to
deliver neurotransmitters with high spatial and temporal resolutions in
order to study how signals are received and integrated by a neuron and a
neural network.
In another approach, we use the unique photothermal properties of gold
nanocages to alter the functions of some key enzymes involved in cell
communication. Combined together, we hope we can develop a powerful new
toolset to reversibly turn on and off a signal transduction pathway.
If successful, this research will enable us to control the behavior of
cells at will. As a continuous effort from my chemistry background, part of
my group is developing more robust and efficient catalysts which are highly
sought after for various applications, including
fuel cells and catalytic converters.
Do you foresee any social or political
implications for your research?
We always want to do cutting-edge research that will have a profound impact
on society, one way or another. Regarding the nanomaterials we have
invented and developed in the past, some of them have already started to
find applications in a wide range of areas that include microelectronics,
photonics,
spectroscopy, sensing, biotechnology, medical diagnostics, catalysis, and
energy conversion/storage.
For example, our technology for silver nanowires has been licensed to
Cambrios Technologies Corp., a start-up company in the Bay Area, and is
being used to develop flexible, transparent, conductive substrates for
touch-screens and other types of display devices.
Our newly developed palladium-platinum bimetallic catalyst (See, for
example, B. Lim, et al., "Pd-Pt bimetallic nanodendrites with high
activity for oxygen reduction," Science 324: 1302-05, 2009) is
expected to cut the costs of proton-exchange membrane (PEM) fuel cells and
eventually help to commercialize this technology.
It will also help to achieve sustainability, as platinum is itself such a
rare and expensive metal that is critical for many industrial applications,
including the manufacturing of nitric acid, petroleum products, fuel cells,
and catalytic converters.
Our technology on gold nanocages is expected to radically change the way
cancer is diagnosed and treated. We are quite excited about all these
implications that our research will be able to offer.
Figures &
descriptions
Younan Xia, Ph.D.
James M. McKelvey Professor for Advanced Materials
Department of Biomedical Engineering
Washington University in St. Louis
St. Louis, MO, 63130, USA
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Web
KEYWORDS: GROWTH; METAL; NANOCRYSTAL; NUCLEATION; shape control; ENHANCED
RAMAN-SCATTERING; LARGE-SCALE SYNTHESIS; SEED-MEDIATED GROWTH; POLYHEDRAL
GOLD NANOCRYSTALS; SINGLE-CRYSTALLINE NANOWIRES; SURFACE-PLASMON RESONANCE;
UNIFORM SILVER NANOWIRES; HIGH-YIELD SYNTHESIS; STABILIZED RUTHENIUM
NANOPARTICLES; COLLOIDAL PLATINUM NANOPARTICLES.
Additional information: 
