Synthesis Of Different ZnSe Nanocrystals: Green Chemical Approaches Toward Green Nanoproducts

With the development of science and technology, semiconductor nanocrystals are of great interest for both fundamental research and technical development. CdSe nanoparticles are the most representative material. Tuning the particles size, their optical spectra can be made to cover the whole visible range. As we know, however, the highly toxic component cadmium places them in a disadvantageous position or even in a doubtful future. ZnSe Semiconductor nanocrystals are also one of the most important members of nanomaterial families. As a direct band gap semiconductor, ZnSe is potential ideal optical material for lasers and other photoelectronic devices. And ZnSe nanocrystals are ideal substitutes for CdSe nanocrystals. In this thesis, we introduced a nontoxic, simple, cheap and reproducible strategy for synthesizing green ZnSe nanocrystals. It completely meets the standard of green chemistry. Changing the experimental condition, we have synthesized ZnSe nanoparticles, nanoflowers and nanoplates. We discuss nanoparticle's growth kinetics, explore nanoflower's growth mechanism, research nanoplate's result, characterize their optical properties, shapes, structures and so on.Under the protection of inert gas, we have synthesized largest nanoparticles (10nm) when the reaction proceeded to about 10 s.In the following period, the nanoparticles almost have a fixed particle size. But it is not a successful strategy for synthesizing high-quality nanocrystals. Increasing the amount of precursors leads to obtain relatively small nanoparticles. Thus, we provide another route to decrease the ligand/precursor molar ratio. Then the size-tuned ZnSe nanoparticles can be prepared effectively. It indicates that a better balance between nucleation and growth is achieved. By simply altering the amount of precursors, we obtain the desired balance.The synthesis of nanoflower is the same as nanoparticle, except that before the injection of Se solution, the mixture of ZnO and olive oil was kept at 330℃for increments of time. It looks like that these nanoflowers consist of some aggregated nanoparticles. The limited ligand protection mechanism is not suitable for ours. Though three different experiments, we propose a new growth mechanism of nanoflowers in our system. By heating the mixture of ZnO and olive oil, it will result to the formation of Zn complexes. When the temperature of the solution reaches 330℃, we obtain mononuclear Zn complexes. Therefore, after the Se solution is injected, ZnSe nanoparticles are synthesized. If the mixture is kept at 330℃for 15min,30min and 1h, mononuclear Zn complexes may convert to polynuclear Zn complexes with different number of Zn atoms. After the injection of Se solution, each Zn atom still grows into one ZnSe nanoparticle, but these nanoparticles closely connect and form nanoflowers because of the formation of polynuclear Zn complexes.Changing different precursor, ZnO is insteaded of ZnSt2, we obtain ZnSe nanoplates. Further experimental results show that high quality ZnSe nanoplates can be synthesized at 160℃, and the molar ratio of Zn/Se is 1:1. However, further study of the mechanism is still going on.As we know, the synthesis of highly ZnSe nanocrystals requires extremely high reaction temperatures. Boiling point of olive oil can be as high as 350℃. Because of their higher boiling point, it provides greater scope to study the high-temperature synthesis of nanocrystals.Experiments showed that, high quality ZnSe nanoparticles can not be obtained at 350℃. 330℃is the most suitable synthetic temperature.Another advantage of our experiment is that it can be carried out without inert gas. The quality of nanoparticles have not been affected under air. For example, the absorption spectra of ZnSe nanoparticles have multiresolved electronic transitions, demonstrating a narrow size distribution and highly monodispersity. It can be said that this method allows us to increase efficiency, simplify the synthetic process and reduce the waste of resources. Therefore, our strategy meets the standard of green chemistry.