Synthesis of Cadmium Sulfide Nanoparticles

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," J. 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.

Micelle formation is essential for the absorption of fat-soluble vitamins and complicated lipids within the human body. The use of reverse micelles or "water-in-oil" microemulsions is a simple and successful experiment that provides an effective way to introduce nanotechnology and surfactant chemistry to students in the general chemistry laboratory.

Safety:

  • Wear eye protection
  • Chemical gloves recommended

Procedure:

Step 1. 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 solution.

Step 2. 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 opaque.)
Discard the solution in an appropriate waste container.

Step 3. Add 0.20 g hexadecyltrimethylammonium bromide to a test tube. Add a stir bar. Clamp over a magnetic stirrer.

Step 4. Add 4.0 mL heptane and 1.0 mL pentanol to the hexadecyltrimethylammonium bromide. Stir to give a suspension.

Step 5. Transfer half the suspension to a second tube. Stir both solutions to maintain the suspension.

Step 6. To one test tube, add 0.1 mL (3 drops) of 0.012 M CdCl2. The solution will clear as hexadecyltrimethylammonium bromide micelles containing CdCl2 form.
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 Na2S form.

Step 7. Join the two solutions and mix. Record the visible absorption spectrum in a glass cuvet.
Discard the solution in an appropriate waste container.

Materials:

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.
  • Hexadecyltrimethylammonium bromide (Aldrich 855820 Cetyltrimethylammonium bromide)
  • Heptane
  • pentanol

Equipment

  • Two test tubes, ring stand, test tube clamps
  • Plastic dropper
  • Two 1/4" magnetic stir bars, magnetic stirrer
  • Cuvet and visible absorption spectrometer

Conclusions:

  1. Is the initial product from mixing aqueous cadmium ion with aqueous sulfide ion nanosize? Why do you need to take the spectrum quickly?
  2. Is the band gap energy of the CdS nanoparticles larger or smaller than that of bulk CdS?
  3. What is your estimated size for the CdS nanoparticles?

Calculations:

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

CdSe
λbulk = 709 nm
ε = 10.6
me* = 0.13
mh* = 0.45

ZnO
λbulk = 365 nm
ε = 8.66
me* = 0.24
mh* = 0.59

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?

YouTube Link:

http://www.youtube.com/watch?v=05FA2tLdVmM