Synthesis of CdSe Nanoparticles

1. Synthesis of CdSe Nanoparticles Hot plate heating resulted in relatively small nanoparticles (1.8 nm to 2.1 nm). A 60 minute increase in heating time showed no change in particle size. Microwave heating resulted in relatively large nanoparticles (2.3 nm to 6.3 nm). Small increases in heating time resulted in significant changes in particle size. Materials and Instrumentation Cd(NO3)2·4H2O, Selenium powder, cyclohexanone, Schlenk tube, centrifuge, CEM Discover microwave, Si Photonics CCD Array UV-Vis Spectrophotometer Methods Cd(NO3)2·4H2O (.25 g, 1 mmol) and Selenium (.08 g, 1 mmol) were added to a round bottom flask or Schlenk tube containing cyclohexanone (25 mL). Traditional heating method: Mixtures in round bottom flasks were stirred and heated using a hot plate at 95°C for 10, 30, 60, or 90 minutes in replicates of three. Microwave heating method: Mixtures in Schlenk tubes were heated using a microwave at 65°C for 30 seconds, 2.5, 5, 10, 15, or 30 minutes in replicates of three. All mixtures were allowed to cool to room temperature using an ice bath. Product mixture was centrifuged to remove excess Selenium and CdSe containing liquid was collected. Product was analyzed using a UV-Vis spectrophotometer. Materials & Methods Smaller CdSe nanoparticles (1.8 nm to 2.1 nm in diameter) were created using a hot plate, while larger nanoparticles (2.3 nm to 6.3 nm in diameter) were created using a microwave. Microwave heating is much quicker and gives a much greater range in nanoparticle size, making it a desirable heating method. However, by using a hot plate, smaller nanoparticles were created, which could be beneficial in some instances. Heating with a hot plate did not yield a large range of nanoparticle size. Smaller nanoparticles appear light to medium yellow in color, while larger nanoparticles are dark red. It is expected that under the correct heating conditions much smaller nanoparticles could be produced. Based on the trend line in Figure 5, these smaller nanoparticles would range from green to blue. Discussion Conclusions Abstract Results Atkins, Peter, et.al. Shriver & Atkins: Inorganic Chemistry 4th ed. New York: W.H. Freeman and Company, 2006. Print. Chakraborty, Tapash. Quantum Dots: A survey of the properties of artificial atoms. The Netherlands: Elsevier Science, 1999. Print. Chopra, KasturiLal, and SuhitRanjan Das. Thin Film Solar Cells. New York: Plenum Press 1983. Print. Firth, A.V.; Tao, Y.; Wang, D.; Ding, J.; Bensebaa, F. J.Mater. Chem., 2005, 15, 4367-4372. Gerbec, J.A.; Magana, D.; Washington, A.; Strouse, G.F.; J. Am. Chem. Soc. 2005, 127, 15791-15800. Jacak, Lucjan and PawelHawrylak and ArkadiuszWojs. Quantum Dots. Germany: Springer, 1998. Print. Acknowledgements All lab work was performed in the Henry Koffler building, room 550, under the supervision of EmanAkam and Dr. Tori Lockett. University of Arizona Department of Chemistry Jillian Barrow Solar cells, also known as photovoltaic cells, collect energy from the sun and convert it into usable electrical energy. Many types of solar cells currently exist, and several more are being researched and developed. Silicon is what makes up, arguably, the most well-known type of solar cell; but other materials such as cadmium selenide (CdSe) are being used and researched in order to find the most efficient solar cell, both in terms of production cost and energy use. The basis of a working solar cell is a semiconductive material, such as CdSe, which serves to absorb the light emitted from the sun. Electron-hole pairs are created in the semiconductor when a sufficient amount of energy frees an electron from the semiconductor’s crystal lattice structure. When the electron is freed, a positively charged hole where the electron once was is inevitably created. The holes and electrons can freely move throughout the material, and when two doped layers are placed close enough together, they can move from one layer to another, through diffusion. Once the electron-hole pairs are created, the electrons flow in one direction, toward the p-type semiconductor, while the holes flow in the opposite direction, towards the n-type semiconductor (such as CdSe). The result is an electric field that can be used as electrical energy. By making CdSe nanoparticles of different sizes, the available range of bandgaps increases. This leads to greater solar cell efficiency as more wavelengths of light are able to be harnessed. References The high cost and relatively low efficiency of current solar cells has lead to extensive research into the use of nanoparticles as a replacement for the more traditional silicon. Nanoparticles are a desirable replacement as they can be made in different sizes, making it possible to harness a broader range of wavelengths in sunlight. CdSe nanoparticles of various sizes were successfully prepared using both a traditional heating method (hot plate) and microwave heating. It was found that traditional heating produced relatively small particles (1.8 nm to 2.1 nm), while microwave heating resulted in relatively large particles (2.3 nm to 6.3 nm). Microwave heating allowed for more accurate control of temperature and was much quicker, while traditional heating required less expensive equipment. Figure 1—Traditional heating method. Background Information Figure 2—Microwave heating method. Figure 3—Exciton peak for a sample created using a hot plate and heated for 90 minutes at 95°C. Inset: All samples prepared using a hot plate. From left to right: 10 min., 30 min., 60 min., 90 min. Figure 4—Exciton peak for a sample created using a microwave and heated for 5 minutes at 96°C. Inset: All samples prepared using a microwave. From left to right: 30 s, 2.5 min., 5 min., 10 min., 15 min., 30 min. Figure 5—CdSe diameter graphed as a function of exciton absorbance. Inset: Table showing specific CdSe diameter values for each sample. • Exciton peaks were detected in all prepared samples. See Figures 3 and 4. • Samples prepared using a hot plate were yellow in color and showed little variation as heating time significantly increased. • The diameters of nanoparticles created using a hot plate ranged from 1.8 nm to 2.1 nm. Increasing the heat time from 30 minutes to 90 minutes did not change the size of the particle. See Figure 6. • Samples prepared using a microwave ranged from orange to dark red. • The diameters of nanoparticles created using a microwave ranged from 2.3 nm to 6.3 nm. Even a small increase in heat time from 30 seconds to 2.5 minutes resulted in a relatively large change in particle diameter, from 2.3 nm to 3.9 nm. • The diameter of the CdSe nanoparticles ranged from 1.8 nm (hot plate for 10 minutes) to 6.3 nm (microwave for 30 minutes). Figure 6—Diameters of CdSe nanoparticles.