The Morphology And Crystal Phase Controlled Synthesis And Separation Of CdS Nanorods

In this paper, we prepared CdS nanorods with a certain aspect ratio distribution by solvothermal method. It is found that band-edge emission and the surface-trap emission coexisted in the as prepared NRs.Then, for the first time, CdS nanorods were sorted by length using a density gradient ultracentrifuge rate separation method. The optical properties and crystal structure of each fraction were characterizated after separation, revealing that the relative intensities of short wavelength band-edge emission and long wavelength surface-trap emission varied with the change of aspect ratio of nanorods. Inspired by the separation results, we tried to selectively synthesize monodispersed samples of short CdS nanorods with photoluminescence dominated by short wavelength band-edge emission, and long nanorods where long wavelength surface-trap emission predominates. Interestingly, we discovered that the lengths of the CdS nanorods could be controlled by varying the amount of O2 in the synthetic mixture, allowing the photoluminescence wavelengths to be tailored.It is known that CdS nanocrystal have two phases:zinc blende (ZB) and wurtzite phase. Because the (111), (220), and (311) X-ray diffraction (XRD) peaks of the ZB phase located at the same positions as the (002), (110), and (112) peaks of the wurtzite phase, and the peaks would be broadened when particle size is in nanometer-scale, it is difficult to accurately identify the crystalline phase. In this thesis, we have shown that the density gradient ultracentrifugation rate separation method could be used to sort CdS NRs according to differences in phase for the first time, and have also revealed the consequences of phase difference, including NR diameter, presence of defects, aspect ratio, and wavelength of band-edge photoluminescence. The discriminative separation thus provided further insight into the differences in the phase, structure, and optical properties between individual colloidal particles synthesized in a single batch. Similar experiments can be performed to reveal the structural and property differences between other colloidal nanoparticles. A preliminary mathematical model has been established, based on histogram data of particle size, which should assist in determining the optimum parameters for the separation of other nanostructures. It has only recently been found that O2 can also tailor the synthesis of quantum dots. Our nanoseparation work on CdS NRs has highlighted the effect of the ambient atmosphere on the morphology of CdS NRs. Then we showed that CdS NRs synthesized under N2 atmosphere is a mixture of hexagonal wurtzite and cubic zinc blende phases. Based on the above work, we hyposized the role of oxygen is the key to understanding the tailoring of the phases and morphologies of CdS NRs. In order to systematically reveal the role of oxygen, a series of experiments have been carried out to study the relationship between the synthesis conditions (e.g., the ambient atmosphere, extent of filling of the reaction vessel, precursor concentrations, and static or shaken reaction vessels) and the structure and properties of the resulting CdS NRs (e.g., phase, length, diameter, length to diameter ratio, presence of defects, photoluminescence properties, and photocatalytic activity). O2 were hidden in the empty section of autoclaves and were somewhat omitted in previous synthesis investigations. Filling degree, reagent concentration and ambient effects were understood from the new point of view. O2-depleted or reagent-rich conditions resulted in more hexagonal phase and thick polycrystalline nanorods with aspect ratio usually less than 3; while O2-sufficient or reagent-lack conditions resulted in more wurtzite phase and slim single crystalline nanorods. However, as the NR growth terminated, no more phase or morphology change would happen even treated in a highly demanding condition. A mechanism has proposed according to the above experimental results and was supported by shaking reaction experiments and tailored synthesis of ZnS nanorods. The research provided a new angle to understand and optimize the synthesis of IIB-VIA semiconductor nanocrystals.