THE VALLEY OF NO RETURN

phoec - Ken Kuno

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30 pages

English

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THE VALLEY OF NO RETURN Written by John Tomerlin Illustrated by Michael Lacapa Teacher's Guide Created by Jan McDonald Rocky Mountain Readers

great impact on the characters
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Sujets

Metal nanoparticles
January 25, 2007
1Objectives
• To synthesize gold nanoparticles using the Turkevich method
• To synthesize gold nanoparticles using a biphasic reduction procedure
• To determine the size of the particles through dynamic light scattering
• To employ size selective precipitation to isolate nanoparticles
• To perform rudimentary ligand exchange chemistries to maker water sol-
uble Au nanoparticles
2 Introduction
Nano has been and continues to be one of the most hyped areas in science and
technology today.[1,2] It is a collective realization that interesting chemistry
and physics occurs in the perviously unexplored hinterland between the truly
molecular(thetraditionalrealmofchemistryandevenphysics)andbulkmatter
(the traditional realm of engineers). To illustrate, when gold, silver, as well as
other metals and even semiconductors are made small enough, they no longer
behave in ways that we are accustomed to seeing. For example, gold no longer
has the same bright yellow hue as the Dome outside. Instead it turns purple or
even black in color!
The same applies to semiconductors such as CdSe or CdTe. Bulk CdSe and
CdTe look black. By contrast, when they are made nano-sized their colors turn
yellow, orange and red. (Their corresponding emission also exhibits a range
of colors that span the visible part of the spectrum. See Figure 1) In fact it
is these absorption colors that give rise to the colors in medieval stained glass
windows or the yellow hues in early Renaissance paintings. Speciﬁcally, CdS
and CdSe are well known pigments in “cadmium yellow” paint. As stated in
a comprehensive treatise on the development of cadmium pigments[3], “pale
[yellow] shades could be prepared by partial precipitation from cold dilution
solutions of cadmium slats or by rapid precipitation from acid solutions... very
lighthuescouldbeattainedusingcadmiumnitrateandsodiumsulﬁdeorammo-
nium thiosulfate.” In these preparations, the color changes experienced by such
1cadmium chalcogenide compounds are almost certainly a manifestation of so
called “quantum conﬁnement” e ﬀects. This means that early 19th century pig-
ment chemists were unknowningly synthesizing nanoparticles, taking advantage
of their unique size-dependent properties.
Figure 1: Size dependent emission from a small size series of (a) CdSe and (b)
HgS NCs
In fact, we have been living with and dabbling with nanomaterials for quite
a long time, nearly 2000 years. Examples include the gold nanoparticles in
the Roman Lycurgus cup (4th century AD), the iron oxide nanoparticles in
Maya blue paint[4] (approximately 700 AD) and Michael Faraday’s colloidal
gold solutions, ﬁrst reported in 1857.[5] Notably these particles are still stable
2and are on display in the Royal Institution in Great Britain. You can visit this
display if you go abroad in your junior year. The di ﬀerence today, however, is
that we are beginning to understand how to control the optical and electrical
properties of matter through the deliberate chemical syntheses of high quality
nanomaterials. In turn, we are beginning to see new and interesting physics at
the nanoscale-physics which can potentially be exploited to make new types of
electronics and other devices.
Of course all of this takes time and money. But more importantly it takes
peoplewith the rightchemicalknowledgeand intuitiontospurfurtheradvances
in the ﬁeld. This is where you come in. By doing these experiments it is
hoped that you will begin to get a “feel” for the behavior as well as chemistry
of nanomaterials. Perhaps this will encourage you to one day contribute to
advances in this rapidly growing area.
2.1 What’s in a name
Nanomaterials go by a variety of names (nanoparticles, nanocrystals, nanocrys-
tallites, 0D materials and colloidal quantum dots). Often in the literature these
terms are used interchangeably and in some cases can result in confusion. Here
we brieﬂy describe one (our) suggested naming convention for small (spheri-
cal) nanometer-sized materials made of metals and semiconductors. In par-
ticular, we will use the term “nanoparticle” (NP) as a generic description for
either spherical metal or semiconductor particles with nanometer-sized diame-
ters. However, more often than not, we will use it to refer to metal particles.
The name “nanocrystal” (NC) or “nanocrystallite” is often used in conjunction
with semicondutor particles and as such will be reserved exclusively for these
materials. Thetermquantumdot(QD)comesfromthephysicsliteraturewhere
additional names for nanocrystals exist as described below.
In physics parlance, nanocrystals are often referred to as “zero dimensional”
(0D) materials. This is because the simplest model of a nanocrystal is the “par-
ticle in a box” problem in quantum mechanics. If you are unfamilar with this
model problem, don’t worry. You will see it soon enough. The idea is that once
you make a material physically small, the boundaries of the material begin to
“squeeze” electrons (and holes) within the object. This, in turn, changes the
electronic energy levels of the structure. So whereas electrons in bulk materials
are “free” to roam about the crystal, in nanoscale materials the physical size
of the object restricts the motion of these carriers. Thus, depending on how
many of the three Cartesian coordinates (x, y, z) the electrons are squeezed
in or conversely how many “degrees of freedom” they have, physicists will call
the system a two dimensional material (this is a thin ﬁlm with 2 degrees of
freedom for carriers within the plane of the material), a one-dimensional ma-
terial (also called a nanowire with 1 degree of freedom representing the length
of the wire) and a 0-dimensional material (a nanocrystal, it has no degrees of
freedom since the electron is completely trapped along x,y and z directions).
Finally just like chemists, physicists have other names for 0D materials. The
most common name is the “quantum dot”. So when we chemists talk about
3quantum dots we sometimes call them “colloidal quantum dots” to distinguish
them from physicist’s quantum dots, whicharemadeusingmoreexpensivema-
chinery. Alternatively, physicists sometimes call nanocrystals “artiﬁcial atoms”
because discrete atomic-like transitions are expected in their optical and elec-
trical properties.
2.2 Gold nanoparticles
In this laboratory we will be making gold nanoparticles with sizes on the order
of ∼ 10 nm. We will use two approaches to make this material to illustrate
some important chemical aspects of nanomaterials. Namely that there exists
a variety of ways for making a given material. The ﬁrst route we will use
is an aqueous preparation developed by Turkevich[6] (hence the name of the
procedure). Figure 2 illustrates several low resolution TEM images of Au NPs
made this way. Accompanying high resolution TEM images (HRTEM) of the
same particles are shown in Figure 3. The second approach is an organic phase
synthesis in toluene developed by Brust.[7]
Next, after making these materials we will illustrate that the NP surface
is very important for a number of reasons. In particular, without something
to prevent the particles from touching during growth, large uncontrolled NP
aggregates will form. This would then prevent the development of a uniform
size distribution for the resulting ensemble. In particular, two particle stabiliza-
tion schemes, steric and electrostatic stabilization, are illustrated. Next, these
ligands are important because they provide solubility to the NPs in either aque-
ous or organic media depending on the hydrophobicity or hydrophilicity of the
surface molecules.
Finally, we aim to illustrate that a good fraction of atoms reside on the NP
surface. This can be seen by calculating the fraction of surface atoms in the
particle. You may be surprised to ﬁnd that in some cases nearly 50% of the
atoms in a NP are surface atoms. Because these atoms prefer to make bonds
to other atoms, the abrupt termination of the NP makes for an odd situation
in terms of the NP optical and electrical properties. To amend this situation,
NP surface atoms often “reconstruct” in order to maximize their atomic bond-
ing. However, the organic surfactants on the NP surface can also satisfy some
of this unfulﬁlled surface bonding. These ligands therefore contribute to the
“electronic” passivation of a nanomaterial.
2.3 Nanoparticle anatomy
Figure 4 shows a cartoon depiction of a typical colloidal NP. In particular, it
consists of two main parts. The ﬁrst is the core and the second is the outer
organic stabilizing layer. The core can be made out of a variety of materials
and in this laboratory it will be made of Au. It is also the core which dictates
theopticalandelectricalpropertiesoftheNP.Asdescribedearlier, thecoreacts
as a “box” which conﬁnes electrons to the physical dimensions of the particle.
This leads to well known quantum “conﬁnement” e ﬀects dictated by quantum
4Figure 2: Low resolution TEM micrographs of Au NPs.
5Figure 3: High resolution TEM micrograph of Au NPs.
6mechanics. In conjunction with X-ray di ﬀraction measurements, it has been
determinedthatthecoreisnearlyidenticaltothecrystalstructureoftheparent
(bulk) material. As a consequence, it is more or less valid to think of the core
as a much smaller fragment of the bulk lattice. Furthermore, most o