The overall goal of this procedure is to prepare gold nanoparticles that are overcoated with a wide bandgap semiconductor shell. The shell material can be either cadmium or zinc sulfide, and the thickness should be tunable. This technique overcomes the large lattice mismatch between core and shell by using a four-step cation exchange technique to form the shells.
The main advantage of this technique is the ability to produce highly monodisperse gold core shell nanoparticles with excellent control of the shell thickness. The nanoparticles produced with this technique could be used as plasmonic elements that couple with absorbers in opto-electronic systems because the wide bandgap shell enables precise tuning of interaction distances. Tuning interactions in this way enables us to limit losses and maximize absorption or emission gains and provide insight into the potential of exciton plasmon coupling for a range of applications.
To begin synthesis of gold nanoparticles, work in a glove box to weigh gold salt in a vial previously cleaned with aqua regia before diluting with 100 milliliters of water in a volumetric flask. Then add 3.2 grams of solid cetyltrimethylammonium chloride, or CTAC, and heat in 25 milliliters of water to approximately 60 degrees Celsius for dissolution. Cool the mixture to room temperature and dilute to 50 milliliters with water in a volumetric flask to prepare a 0.2 molar CTAC solution.
Mix 20 milliliters of one millimolar gold solution and 20 milliliters of 0.2 molar CTAC solution inside a round-bottom boiling flask. Allow to mix in an oil bath set to 60 degrees Celsius for ten minutes. Next add 1.7 milligrams of solid borane tert-butylamine to the gold CTAC solution, and let the mixture stir for 30 minutes.
The solution should turn deep red. The resulting solution has a gold particle concentration of about 5 micromolar, and can be stored for months at a time or used immediately for the next phase of the reaction. Then, add the silver solution dropwise to the gold and ascorbic acid solution and allow the reaction to stir for two hours.
In a 70 degree Celsius oil bath, mix 10 milliliters of stock gold nanoparticles with ascorbic acid to make a 20 millimolar solution. Next, prepare a 4.0 millimolar silver nitrate solution in five milliliters of water. The reaction will turn light orange to dark orage depending on the shell thickness over the course of the reaction.
Centrifuge the nanoparticles at 21, 130 gs for ten minutes, and then re-disperse into clean water. Decant the supernatant from pelleted nanoparticles to aid in removal of bare gold nanoparticles or silver nanoparticles, which may have been formed. Weigh elemental sulfur in a 200 to one molar ratio to the silver used in the previous stage of the experiment.
For ten milliliters of gold silver core shell particles and a five nanometer shell, into ten milliliters of toluene, dissolve 1.5 milliliters of oleic acid and three milliliters of oleylamine. Once dissolved, the solution should be a light yellow. Residual water can disrupt the solubility of both the nanoparticles and the free surfactive molecules, possibly leading to irreversible aggregation of the gold naonparticles.
To ensure the removal of water during the next purification step concentrate the silver colloids via cetrifugation at 21, 130 gs for ten minutes. Disperse the silver colloids in one milliliter of water. This helps increase the efficiency of the extraction from the aqueous layer to the organic layer upon formation of the silver shell.
Add the colloids dropwise to the sulfur solution under stirring for one hour. The solution will turn dark blue for thinner shells, to purple for thicker shells, as the sulfurization goes to completion. Add the same volume of 70 percent ethanol before centrifuging.
Centrifuge the colloidal solution at 4, 000 gs for 10 minutes, after the reaction has stirred for two hours, to remove the water and unreacted sulfur from the solution. Excess oleylamine or oleic acid may fall out of solution and can be removed after this step by decanting the solution from the white solid. If necessary re-disperse the nanoparticles by sonicating in a bath sonicator for 30 seconds to one minute in order to disperse into the toluene.
Make the metal precursor by dissolving the metal nitrate into one milliliter of methanol to make a 0.2 molar solution of cadmium nitrate or zinc nitrate. Mix the metal solution with the silver-sulfide-shelled naonparticles in a one to one molar ratio with the silver. Heat to 50 degrees Celsius for cadmium shell, and 65 degree Celsius for zinc shells, under a nitrogen atmosphere.
Next, add tributylphosphine in a 500 to one molar ratio to the metal precursor. Purify via centrifugation at 21, 130 gs for ten minutes in order to remove any isolated cadmium sulfide or zinc sulfide nanoparticles which may have been formed. Disperse the pelleted nanoparticles into a clean, nonpolar solvent, such as hexane, toluene, or chloroform.
Shown here are transmission electron microscopy, or TEM images of gold nanoparticles synthesized with CTAC as the surfactant. The nanoparticles were single crystalline with a 16 nanometer diameter and a standard deviation of 0.4 nanometers. The thickness of the silver shell can be monitored through ultraviolet visible absorption spectroscopy.
In this figure silver thickness increases from black to light blue. The surface plasmon of gold nanoparticles shifts to higher energies as the thickness of the silver shell increases. TEM images of gold nanoparticles with silver shells are shown here.
Silver is grown onto the gold core to provide a template for the subsequent semiconductor shell. The shells are tuned from three to seven nanometers in radius. In the next step the silver coated nanoparticles are treated with sulfur to form a silver sulfide shell.
The resulting shells tend to be slightly larger than the previous silver shells, but uniform in distribution. Finally cadmium, in this example, or zinc can be used to replace the silver ions and produce a semiconductor shell of cadmium sulfide or zinc sulfide. The resulting shell is single crystalline with a radius corresponding to the silver radius in the first coating step.
Once mastered, this technique can be done in seven to eight hours if performed properly. While attempting this procedure, it is important to properly clean all glassware with aqua regia before use. After watching the video you should be able to coat aqueous gold nanoparticles with cadmium or zinc sulfide.
The gold nanoparticles are coated with silver, which is transformed to silver sulfide. Finally, the silver is exchanged with cadmium or zinc.
