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 plastic 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
opaque.) 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 by rapidly pouring one into another. Stir. Record
the visible absorption spectrum in a glass cuvet. (Heptane will dissolve plastic cuvets.) Discard the solution in an appropriate waste container when finished.
Extrapolate the linear portions of the lowest energy absorbance as a function of wavelength to find the band edge wavelength for your sample.
(One option is to use the band edge tab on the Beers Law template. Change the wavelength values in the colored shaded boxes to choose the ends of the linear portions of your absorbance graph. The program will least squares fit the interval and show the intersection of the two lines in the table. Another option is to transfer the wavelength and absorption data to the band edge spreadsheet and do the same analysis.)
Finding the band edge for a semiconductor.
Convert the band edge wavelength to the band gap energy, Egnano. 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.
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