Laboratory

 

Synthesis of Nanoparticles

The Chemistry Module will teach you the syntheses of two important nanomaterials: (i) cadmium sulfide (CdS) quantum dots via reverse micelles and (ii) gold nanoparticles according to Brust. You will measure the absorbance spectra of the synthesized nanoparticles and calculate their diameter. (ADD PDF LINK)

 

Pre-lab Exercises
Be prepared to answer these problems before coming to lab.  We will discuss the answers during the first and second laboratory session.

1.   Review lecture notes from ChE 59808 Nanomaterials Course (available on blackboard): Nanoparticle synthesis_lecture_notes.pdf.
2.   What is the relationship between the wavelength and the energy of light?  What is the relationship between the frequency and the energy of light?
3.   If a substance absorbs light in the purple, blue and green sections of the visible spectrum, what color is the substance?
4.  You will prepare nanometer-sized CdS particles that are yellow.  If the particles were even smaller, what would be their color?
5.  What are the ionic charges of cadmium and sulfide in CdS?
6.  Explain the difference between micelles formed by soap in water and the CTAB micelles formed in this experiment. 
7.  Write the reaction that will occur to create CdS.
8. You will synthesize gold nanoparticles particles that are 1-3 nm in size. What will be their color?
9.   What is a phase transfer catalyst? How does it work?

 

Lab Exercises

Cadmium Sulfide Quantum Dot Synthesis

Synthesis Description:
Nanoparticles are formed by mixing a hexane solution containing CdCl2 (aq) in micelles (solution A) with a similar solution containing Na2S (aq) in micelles (solution B).  Upon combining the solutions, the CdS precipitates within the micelles but does not agglomerate. 

As you prepare each solution, continuously stir it using the stirring plate and a magnetic stirring bar.

 

Synthesis Procedure:
1.   Obtain the hexane, 1-pentanol and aqueous salt solutions.
2.   Add 2.0 mL 1-pentanol to 13 mL hexane and stir.
3.   Weigh 0.50 g CTAB to the nearest 0.01 g.  Add the CTAB to the solution prepared in step 2 and stir.  Not all the CTAB will dissolve.
4.   Add the 0.6 mL 0.012 M CdCl2 solution to the mixture from step 3.  The solution should become translucent.  Set this solution aside and label it “solution A.”

5.   Perform steps 2 through 4 again but substitute 0.6 mL 0.012 M Na2S solution for the CdCl2 solution.  Label it “solution B.”  Solution B might remain slightly cloudy even after the aqueous Na2S solution is added.  This will not affect your results.
6.   Pour solution A into solution B quickly.  Continue stirring this solution for several minutes.
7.   Measure the absorbance spectrum of your CdS nanoparticle solution.
8.   Clean up your work area.  Dispose of any CdS mixtures in the appropriate waste jar.

 

Gold Nanoparticle Synthesis

Synthesis Description:
Nanoparticles are grown in a two-phase system. The aqueous phase contains AuCl4- and N+(C8H17)4Br-, whereas the organic phase contains the dodecanethiol. AuCl4- is transferred from the aqueous phase to the organic phase by the transfer catalyst. Simultaneously, AuCl4- is reduced by the sodium borohydride and capped of by the dodecanethiol in the organic phase yielding gold nanoparticles. 

Synthesis Procedure:
1.   Mix 7.5 mL 0.03 M HAuCl4 aqueous solution with 20 mL 0.05 M N+(C8H17)4Br- toluene solution. Vigorously stir the two-phase mixture by using the stirring plate and a magnetic stirring bar until all the tetrachloroaurate is transferred into the organic layer.
2.   Add 0.0425 g (density: 0.840 g/ml, V=50 µl) C12H25SH to the organic phase.
3.   Slowly add 6 mL 0.4 M NaBH4 solution with vigorous stirring. Continue stirring for a few hours.
4.   Separate the organic phase.
5    Measure the absorption spectrum of the organic phase, which contains the gold nanoparticles

 

Post-Lab Exercises

Uses your absorbance spectrum data and any necessary Excel spreadsheet templates to perform these exercises for CdS nanoparticles.

1.   Plot your absorbance data as a function of wavelength between 380 – 530 nm.
2.   Note the region in the spectrum where the absorbance changes linearly.  Graph the linear data and obtain the equation of the line.
3.   Calculate the x-intercept.  This is the “cut-off” wavelength of the spectrum.  Convert the cut-off wavelength into units of Joules.
4.   Enter the energy value from Exercise 3 into the Excel template to calculate particle size.  Record the size of the CdS particles. 
5.   Verify that the template calculated the size correctly.  Rearrange Equation (1) in the Introduction section to solve for the particle radius.

Use your absorbance spectrum data and your NanoPartzTM Data Sheet for Au particles (available on Blackboard) to perform these exercises for Au nanoparticles.
6.   Plot your absorbance data as a function of wavelength between 500 – 560 nm.
7.   Determine the wavelength with the maximum absorption.
8.  Add the data point from exercise 8 to the “Maximum Absorption Wavelength as a Function of Nanoparticle Size” plot (Figure below) from NanoPartzTM.
9.   Can you determine the size of the Au nanoparticles you synthesized with the given correlation?  If not, discuss why it is not possible.