Synthesis of Ligand-free CdS Nanoparticles within a Sulfur Copolymer Matrix

The overall goal of this synthesis method is to use a sulfur copolymer to produce cadmium sulfide nanoparticles. This method can provide a route for researchers to develop new nanoparticle surface modification reactions. The main advantage of this technique is that it provides a way to easily synthesize nanoparticles that do not have conventional aliphatic ligands.

This synthesis method is potentially useful in a range of fields where conventional nanoparticle ligands can be difficult to remove, exchange or block surface reactions. Demonstrating the procedure will be Trevor Martin, a graduate student from my laboratory. To prepare the molten elemental sulfur, place elemental sulfur in a 15 milliliter three-neck flask with an attached condenser and temperature probe.

Perform pump and purge cycles with vacuum and nitrogen several times. Heat under nitrogen to 150 degrees Celsius with stirring. This will cause the sulfur to become a yellow-colored liquid.

Once all of the sulfur has dissolved into the liquid, immediately inject alpha-methylstyrene into the solution. Heat the solution to 185 degrees Celsius with stirring at 500 RPM for 10 minutes. As the copolymer forms, the solution will change color from yellow to orange, finally producing a deep-red color.

Remove the solution from the heat and cool to room temperature. As it cools, the copolymer will slowly crystallize to form an orange, rubbery solid. At this stage, the copolymer can be stored at room temperature for a subsequent synthesis, or it can be used immediately.

Add cadmium acetylacetonate to the three-necked flask from the previous step so that the powder is placed evenly on top of the solid sulfur copolymer. Perform pump and purge cycles on the flask with nitrogen, and vacuum several times. It's important to ensure the cadmium precursor mixes into the liquid sulfur copolymer thoroughly and uniformly as soon as the polymer begins to melt so that the nanoparticle formation can begin homogeneously.

Heat the solution to 200 degrees Celsius under nitrogen, with stirring. The sulfur copolymer will melt and mix with the cadmium precursor, allowing the nanoparticle nucleation and growth processes to begin. Allow the nanoparticles to grow for 30 minutes before removing the solution from the heat and cooling to room temperature.

Once cooled, remove the solid nanocomposite from the flask, and store at room temperature. To remove the sulfur copolymer, place the 200 milligrams of nanocomposite in a 20 milliliter glass vial, and add 20 milliliters of chloroform. Place the vial in an ultrasonicator and sonicate for one hour to break up the nanocomposite and suspend the nanoparticles within the solution.

Following sonication, separate the solution into two 30 milliliter centrifuge tubes, and add another 20 milliliters of chloroform to each. Centrifuge the solution at 8, 736 Gs for 15 minutes. Then, decant the sulfur copolymer from the centrifuge tubes, making sure not to disturb the settled nanonparticles.

To isolate the settled nanoparticles, redisperse them by adding chloroform to each centrifuge tube and sonicating for 15 minutes. Repeat the centrifugation and redispersion three more times to ensure that all of the sulfur copolymer has been removed. Once all of the sulfur copolymer is removed, the decanted solution will no longer have an orange color.

Collect the final nanoparticles by adding two milliliters of chloroform to each centrifuge tube. Combine the collected nanoparticles in one 20 milliliter glass vial. Place the glass vial under vacuum to remove all of the chloroform and to dry the nanoparticles.

At this stage, determine the mass of the resulting nanoparticles. Compare this mass to the starting mass of the precursors to determine the yield of the reaction, using molar ratios of the starting material and product. To characterize the cadmium sulfide nanoparticles by transmission electron microscopy, or TEM, dilute 20 milligrams of the isolated nanoparticles in 20 milliliters of chloroform and ultrasonicate for one hour.

Dilute this solution in chloroform and sonicate for 15 minutes. Drop the final solution onto an ultra-thin carbon-film substrate with Holey carbon support films on a 400-mesh copper TEM grid. Place the TEM grid in a glass vial.

Hold the vial under vacuum overnight to remove any residual solvent from the sample. Once the drying is completed, acquire TEM images using a 200 kilovolt accelerating voltage, a spot size of three and an attached energy dispersive X-ray spectroscopy, or EDS detector. To characterize the cadmium sulfide nanoparticles by X-ray diffraction, dilute the isolated nanoparticles in chloroform at a concentration of 10 milligrams per milliliter.

Then, clean the lipton(mumbles)coated sodalime glass substrates by sonicating in detergent, the ionized water, acetone and isopropyl alcohol for 10 minutes in each solvent. Finally, clean the substrates in an air plasma cleaner for 10 minutes prior to drop casting. Next, drop cast the solution onto the prepared substrates in seven microliter increments.

Once the films have dried, acquire X-ray diffraction data. Collect data using 7, 000 data points at a scan rate of one data point per second, with a copper K-alpha X-ray source and an incident wavelength of 1.54059 angstroms. To characterize the cadmium sulfide nanoparticles by solution spectroscopy, disperse the isolated nanoparticles in chloroform at 0.1 milligram per milliliter and sonicate for 30 minutes.

Then, place the samples in a sealed quartz cuvette. Prepare two additional samples for analysis by dispersing the nanocomposite as well as the sulfur copolymer in formamide both at one milligram per milliliter. Stir each dispersion at 700 RPM, and heat to 70 degrees Celsius to facilitate suspension of the material.

Conduct optical absorbance measurements of all three samples using a spectrometer with a triple detector that extends across the ultraviolet, visible and near-infrared ranges. Finally, conduct photoluminescence measurements of all three samples using a fluorescence spectrophotometer with an excitation wavelength of 330 nanometers. Shown here is a TEM image showing three-to four-nanometer-sized cadmium sulfide nanoparticles forming within the sulfur copolymer at the beginning of the reaction.

This TEM image shows aggregated nanoparticles once the copolymer has been removed. The inset shows the stoichiometry of the nanoparticles from EDS spectra. Shown here is the X-ray diffraction pattern for the isolated nanoparticles, which is consistent with the formation of cadmium sulfide.

Here, spectroscopy data is shown for the isolated nanoparticles once the sulfur copolymer is removed. Once mastered, this nanoparticle synthesis can be completed in several hours. Don't forget that working with cadmium can be hazardous, and precautions such as wearing proper protective equipment should be taken while performing this procedure.

After watching this video, you should have a good understanding of how to synthesize metal sulfide nanoparticles using a sulfur copolymer. We believe that this method will help provide a way to easily tune nanoparticle surface chemistry for a variety of applications.