Student Sheet part B: Properties at the Nanoscale…
Isn’t silver always silver?
INTRODUCTION
In the last class period, you saw that we can make solutions of different colors by
reducing silver nitrate (AgNO3) in aqueous solution (in water) using different amounts of
sodium borohydride (NaBH4). Hopefully you realized that the amount of sodium
borohydride was an independent variable which was different for each group, and the
color of the solution produced depended upon the amount of NaBH4 added.
They also may have realized that the silver you made did not look like silver at all! In
fact, it was too small to be seen and a different color from the “silver” color which you
may associate with silver.
Today you’re going to read a little bit more about this reaction and answer some
questions. Then we will look at a question related to why the silver might not look silver!
THE REACTION
The full chemical reaction which occurs in the lab you did is shown below.
AgNO3(aq) + NaBH4(aq) → Ag(s) + 1/2H2(g) + 1/2B2H6(g) + NaNO3(aq)
1) Interpret this statement in words below:
Sodium borohyride
dissolved in water
Silver metal
Silver nitrate dissolved in water + _________________ react to form ____________ ,
NaBH4 (aq)
Hydrogen
Ag (s)
Sodium nitrate
________ gas, gaseous diboron hexahydride, and aqueous _______________ .
H2(g)
NaNO3 (aq)
In the reaction, you can see that silver initially has a charge of +1, and afterwards has no
charge.
Ag+ (aq) -----> Ag (s)
2) To which side of this equation must you add an electron to make it valid in terms of
conservation of charge? Why do we say silver is being reduced?
An electron must be added to Ag+, to the left side of the equation. The net ionic
reaction is Ag+ + e-  Ag. Silver is reduced because an electron is added to it.
Reduction is gaining electrons.
SILVER NANOPARTICLES
When researchers first looked at the colored solutions, there were not sure what was
inside. They took samples, let the water evaporate and looked under a light microscope,
and small very very small particles. So they used a fancier microscope, called a Scanning
Electron Microscope, so take a closer look at the materials dissolved in water in the
solutions.
A scanning electron
microscope at Carleton
College. To learn more,
go to
http://www.mos.org/sln/
SEM/seminfo.html
http://serc.carleton.edu/images/research_education/geochemsheets/techniques/UWSEM.jpg
The picture they saw looked a bit like the one shown below. These are particles made of
pure silver.
200 nm
2
0
0
n
m
Yeechi Chen, Keiko Munechika,
Jessica M. Smith, David S. Ginger
University of Washington
Department of Chemistry
Unpublished results
3) Estimate the size of the particles you see here using the scale bar. How many
particles would need to be lined up to equal the length of a meter stick? Show
your work.
Answers will vary. ~ 30 nm is my estimate.
1 particle 1nm
1m 
 9  3.3  10 7 particles or 33 million particles
30nm
10 m
4) How many silver atoms would you estimate are in one of these nanoparticles?
Assume that each silver atom occupies the volume of a cube with an edge of 0.30
nm. You can estimate the volume of the nanoparticle by assuming it’s a sphere.
The formula for the volume of a sphere is 4/3 π r3 , where r is the radius. Clearly
show your work or explain the process you used.
The volume of the nanoparticle is 4/3 π (15)3 = 14,000 nm3
The estimated volume of a silver atom is (0.3)3 = 0.027 nm3
The number of atoms in the nanoparticle is 14,000 / 0.027 = 520,000 atoms
A COLLOIDAL SOLUTION
Last class period, you saw that “bulk” silver can be made by mixing silver nitrate
and a more reactive metal. This method of production makes chunks large enough to be
seen with the naked eye. But in the synthesis you performed, something stops the
production of silver when the particles are much smaller. Because these silver particles
have sizes which are measured in nanometers, they are called silver nanoparticles,
nanocrystals, or nanoprisms.
What stops the particles from growing into larger, visible pieces of silver? If you
have studied stoichiometry and solutions you can answer this question.
5) Look at the chemical reaction on the first page. What is the molar ratio of AgNO3
to NaBH4? A different way of asking this question is do you need more AgNO3,
more NaBH4, or an equal amount of each reactant? The molar ratio is 1:1.
So we know that whichever reactant is present in a smaller molar amount is going to be
the limiting reagent. You used 1 mL of a 1.0 mM solution of AgNO3, and about 4 mL of
a 10 mM solution of NaBH4.
6) Which solution is more concentrated: the 1.0 mM solution of AgNO3 or the 10
mM solution of NaBH4? 10 mM solution of NaBH4
7) You also added more volume of the NaBH4 compared to the AgNO3. So which
reactant, the silver nitrate, or the sodium borohydride, was present in a larger
amount in terms of moles? sodium borohyride
The excess sodium borohydride is required to prevent the silver particles which form
from clumping together to form visible particles of silver. A picture of what happens
is shown below. The sodium borohydride is dissolved in water and is an ionic
compound made up of sodium ions (Na+) and borohydride polyatomic ions (BH4-).
There is an excess of BH4- ions because the sodium borohydride is present in excess.
The extra BH4- ions which do not react surround the silver nanoparticles (Ag NP in
the picture below), and prevent them from clumping together.
Solomon et. al. J. Chem. Edu. Vol 84 #2, Feb 2007 pg 322
8) Why would silver particles surrounded by BH4- ions not clump together?
The nanoparticles with the ions act as negatively charged particles. Negatively
charged particles will repel each other.
Solutions which have small charged particles, which are too small to be seen with the
eye, are called colloids. Examples of colloids include soapy water and milk.
Isn’t silver always silver?
You might remember that you added different amount of sodium borohydride,
and this led to different colors. Why is this? The answer is that adding different amounts
of sodium borohydride leads to different sizes of nanoparticles, and each nanoparticle has
its own color!
This is a crucial fact. SILVER PARTICLES WHICH ARE NANOMETER
SIZED DO NOT HAVE THE SAME PROPERTIES AS BULK SILVER!!! THE
MATERIAL PROPERTIES CHANGE BECAUSE THE SIZE OF THE PARTICLES IS
SO SMALL.
Yeechi Chen, Keiko Munechika,
Jessica M. Smith, David S. Ginger
University of Washington
Department of Chemistry
Unpublished results
In this picture, you can see that each silver nanoparticle has a slightly different color
depending on how large it is.
Why might different sized particles have different colors? It turns out that the
explanation for this phenomenon is very complicated, and is too complex for us to look
at. But we can understand why small silver particles might have some other different
properties (besides color) compared to larger silver particles by using a two dimensional
model of nanoparticles. Here’s an example.
Notice that silver atoms in the middle of the crystal make more bonds than those on the
outside. This means that the outside silver atoms have some unbonded electrons and are
not “happy”.
9) For each of the following three two dimensional silver nanoparticles, count the
number of bonds ( lines - ) and silver atoms (Ag). Then determine the number of
bonds per atoms by dividing the second number by the first.
Number of bonds: _____12_____
Number of silver atoms: _____7_____
Number of bonds per atom: __1.71_____
Number of bonds: ____42______
Number of silver atoms: ____19______
Number of bonds per atom: ___2.21_____
Number of bonds: _____90_____
Number of silver atoms: ____37______
Number of bonds per atom: ____2.43____
10) What is the trend in terms of the number of bonds per atom? Do larger crystals or
smaller crystals have more strongly bonded atoms on average?
The trend is that number of bonds per atom increases as the particles get larger.
Smaller nanoparticles have more surface atoms and so have more atoms which do
not have the maximum number of bonds possible. On average, larger crystals
have more strongly bonded atoms.
11) Based on your answer to question 10), are there any properties which you might
expect to be different for larger versus smaller silver nanoparticles? Think about
properties which depend upon the strength of bonding. State your predictions
clearly (I would expect smaller nanoparticles to be …. because … ).
Students might expect mechanical properties to differ. For example, smaller
nanoparticles might be expected to be easier to deform or less “strong” because the
atoms are held less tightly together.
Students might also expect melting points to differ. Again, since there are less bonds
on average holding an atom in a smaller nanocrystal, you might expect the smaller
nanocrystal to be easier to melt. In fact, that’s exactly what is seen experimentally.
(REFERENCE???) This is a profound result because it means that the melting point
of a pure substance is not an intrinsic material property, as we always teach. On the
nanoscale, melting point depends upon size!