Question

Procedure:

1. A 25 mL round bottom flask equipped with a stir bar is loaded with 13 mg of CdO powder, 0.6 mL of oleic acid, and 10 mL of 1-octadecene. The reaction flask is placed into a heating mantle and surrounded with aluminum foil so that the temperature can reach 225 °C, which will take approximately 1 hr. Some hot plates may heat up faster and the experiment can be performed sooner.

2. As the CdO solution is heating, the Se solution should be prepared. Five students can share the Se solution, since only 1 mL is required for each reaction. A 25 mL round-bottom flask equipped with a stir bar is loaded with 30 mg of Se and 5 mL of 1-octadecene and allowed to stir at room temperature. To this stirred solution was added 0.4 mL of trioctylphosphine via syringe. The mixture will turn homogeneous. At this point it is ready for the next step.

3. Once the CdO solution has reached a temperature of about 225 °C, add 1 mL of the Se solution from step 2 in a rapid fashion. As soon as you add the Se solution start collecting samples of approximately 1 mL into separate test tubes continuously as the reaction will be changing colors from yellow to orange to blood red.

4. After allowing the quantum dot solutions to cool, add 1 mL of pentane to the test tube containing ~1 mL of sample. Then, pipette the samples into a glass cuvette to record their UV-Vis spectra. Use pentane as the blank. The solutions collected in test tubes can be stored overnight in the hood or drawer without decomposition if there is not enough time to record UV-vis spectra.

What reaction is occurring when the selenium dissolves after trioctylphosphine is added?

Answer 1

• This is experiment based question.
• This is the experimental method for preparing the CdSe quantum dots.
• In this method non-polar solvent such as trioctylphosphine and high temperature is used which shows the Organometallic synthesis. So, In this experiment the organometallic reaction is involved. By Organometallic synthesis, the better quality nanocrystals are obtained.
• In this experiment 13 mg CdO powder is dissolved in 0.6 mL oleic acid and forms the cadmium oleate complex in a 25 mL round bottom flask. High concentration of oleic acid can be create problem in growing crystals. CdO is the source of Cd ions. Oleic Acid is a natural surfactant and is used for the surface binding ligand to stabilizes the crystals.
• In other 25mL R.B flask, on adding TOP the solution becomes homogeneous that means Se solution is completely dissolved in TOP.
• When se solution is added in CdO solution at that time color changes takes place and it shows photoluminescence reaction.
• The Cd-oleate complex should be less stable than Cd-Se complex to synthesize the target product and this is the driving force of the reaction to proceed.
• Octadecene is used as a non-coordinating, high-boiling solvent in this experiment. It remains in liquid form at room temperature. In ODE as non-coordinating solvent with an appropriate amount of oleic acid as ligand, the size of Cd-Se crystals remains in a controllable manner.
• In the above experiment, Trioctylphosphine is acts as the coordinating solvent. Se powder solvent is used to provide control and surface for better size. After adding trioctylphosphine, se is dissolved and it provide surface for adsorption of crystals. Hence, adsorption reaction takes place here. The formation of Quantum dot crystal structure can be control by the solvent used in the reaction and temperature.
• Hence, this is all about the chemistry behind this experiment.