The procedure shown here was adapted by Paul Hansen and George Lisensky
from Kurt Winkelmann, Thomas Noviello, and Steven Brooks, "Preparation of
CdS Nanoparticles by First-Year Undergraduates,"
Chem. Educ. (2007) 84,
709-710, which was based
on M. L. Curri, A. Agostiano, L. Manna, M. D. Monica, M. Catalano, L. Chiavarone,
V. Spagnolo and M. Lugarà, J.
Phys. Chem. B, (2000) 104, 8391-8397.
Hexadecyltrimethylammonium bromide has a long hydrophobic chain and a polar head group.
The molecule does not dissolve well in either aqueous or organic solvents. In an organic
solvent containing a small amount of water the hexadecyltrimethylammonium bromide
traps the aqueous portion in a micelle sphere with the polar heads facing in
and the non-polar tails facing out. The relative amount of pentanol cosurfactant
controls the size of the micelle.
bromide pentanol micelles of CdCl2 with similar micelles containing
nanoparticle CdS since the aqueous solution serves as a nanoreactor and the
particles cannot grow bigger than the micelle. The pentanol also acts as a
capping agent to stabilize the CdS particles. The formation of CdS nanoparticles
can be detected by spectroscopy since quantum size effects make the visible
absorption spectra different than that of bulk CdS.
Test the reagents by adding a drop of aqueous Cd+2 to a drop
of aqueous S-2. A yellow color should appear if the Na2S
solution is good. If the mixture remains clear, remake the Na2S
In a cuvet, add an equal amount of aqueous 0.012 M Cd+2 and
aqueous 0.012 M S-2. Record your observations and immediately
obtain the visible absorption spectrum (before the solution becomes too
Discard the solution in an appropriate waste container.
Add 0.20 g hexadecyltrimethylammonium bromide to a test tube. Add
a stir bar. Clamp over a magnetic stirrer.
Add 4.0 mL heptane and 1.0 mL pentanol to the hexadecyltrimethylammonium
bromide. Stir to give a suspension.
Transfer half the suspension to a second tube. Stir both solutions
to maintain the suspension.
To one test tube, add 0.1 mL (3 drops) of 0.012 M CdCl2. The
solution will clear as hexadecyltrimethylammonium bromide micelles containing
To the second test tube, add 0.1 mL (3 drops) of 0.012 M Na2S.
The solution will clear as hexadecyltrimethylammonium bromide micelles containing
Join the two solutions and mix. Record
the visible absorption spectrum in a glass cuvet. Discard the solution in an appropriate waste container.
CAUTION: Avoid physical contact with cadmium chloride and cadmium
sulfide as both are carcinogens.
Stock Solutions for hundreds of batches
0.012 M CdCl2: Dissolve 0.110 g in
50 mL distilled water. This solution keeps for months.
0.012 M Na2S.9H2O: Dissolve 0.144 g
in 50 mL distilled water. This solution does not keep well.
Is the initial product from mixing aqueous cadmium ion with aqueous sulfide ion nanosize? Why do you need to take the spectrum quickly?
Is the band gap energy of the CdS nanoparticles larger or smaller than that of bulk CdS?
What is your estimated size for the CdS nanoparticles?
The x-intercept of the linear portion of the absorbance as a function of wavelength graph is a measure of Eg. Eg = h c / λ h = 6.626x10-34 J s c = 2.998x108 m/s e = 1.602x10-19 C
ε0 = 8.854x10-12 C2/N/m2
m0 = 9.110x10-31 kg CdS
λbulk = 512 nm
ε = 5.7
me* = 0.19
mh* = 0.80
The effective mass model suggests
where r is the radius of the nanoparticle. The second term is the particle-in-a-box confinement energy for an electron-hole pair in a spherical quantum dot
and the third term is the Coulomb attraction between an electron and hole modified by the screening of charges by the crystal.
After multiplying by r2, rearranging, and using the quadratic formula,
What is the diameter of the nanoparticles?