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.

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.

Mixing hexadecyltrimethylammonium bromide pentanol micelles of CdCl₂ with similar micelles containing Na₂S produces 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.

Procedure

Wear eye protection

Chemical gloves recommended

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.

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.)

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 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.

Join the two solutions and mix. Record the visible absorption spectrum in a glass cuvet.

Materials

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.

CAUTION: Avoid physical contact with cadmium chloride and cadmium sulfide as both are carcinogens.

hexadecyltrimethylammonium bromide (Aldrich 855820 Cetyltrimethylammonium bromide)
heptane
pentanol

Calculations

λbulk = 512 nm Eg = h c / λ		The x-intercept of the linear portion of the absorbance as a function of wavelength graph is a measure of Eg.
e = 1.60x10-19 C ε0 = 8.85x10-12 C2/N/m2 ε = 5.7 h = 6.63x10-34 J s c = 2.998x108 m/s me* = 1.73x10-31 kg mh* = 7.29x10-31 kg		The effective mass model suggests where r is the radius of the nanoparticle. After multiplying by r2, rearranging and using the quadratic formula, What is the diameter of the CdS nanoparticles?

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