| Quantum Dots (QDs), also called semiconductor nanocrystals, mainly consist of periodic group Ⅱ-Ⅵ, Ⅲ-Ⅴ or IV elements with a diameter of 1-10 nm. Because the particle size is so small, quantum dots show better physical and chemical properties than bulk counterpart materials, such as quantum size effect, small size effect, and surface effect etc. and have attracted much attention from scientists in various fields. There are strong correlations between particle size and energy gap of quantum dots, as a new fluorescent material, quantum dots have wide excitation spectra, narrow symmetrical emission spectra, high fluorescent quantum yields, high fluorescence lifetime and high stability against photo-bleaching compared with traditional organic fluorescent dyes, and already used as fluorescent probes in biomedical field. In addition, quantum dots possess the characteristic of energy gap distribution depending on particle size, when combined with TiO2, the photogenerated electron-hole separation efficiency and the visible light absorption range of TiO2 can be increased. This paper is mainly about the synthesis of water-soluble Ⅱ-Ⅵ group of binary/ternary CdTe, ZnxCd1-xSe QDs and carbon quantum dots, and their appilications in the fingerprint development and composite photocatalyst for photocatalytic degradation under visible light. Specific contents are as follows:1.The water soluble CdTe/MSA QDs are synthesized in aqueous phase with mercaptosuccinic acid (MSA) as modifier. The effects of initial pH value, the molar ratio of Cd2+ and MSA, Cd2+ and Te2- on the fluorescent spectra are studied. Compared with CdTe/TGA、CdTe/MPA QDs (TGA:thioglycolic acid; MPA:mercaptopropionic acid), the fluorescent emission peak of CdTe/MSA QDs is more quickly red-shifted to 640 nm within 5.5 h. But the fluorescent emission peaks of CdTe/TGA and CdTe/MPA QDs are about 600 nm when the reluxing time is 24 h. The result indicates that the growth of CdTe QDs is associated with the coordination between modifier and CdTe QDs, the coordination mode of single-and double-coordinated of MSA to CdTe is proposed, and they promote the growth of CdTe/MSA QDs in nucleation and Ostwald ripening process, respectively.The results of latent fingerprint development with CdTe/MSA QDs show that it is suitable for various subjects, including yellow tape, black tape, scotch tape, tinfoil, aluminum alloy and stainless steel, and only needs 5 sec. for developing clearly fingerprints. CdTe/MSA QDs is also achieved good results in fingerprint development for spraying.In the study of CdTe QDs and TiO2 composite photocatalyst, TiO2 nanosheets (TNS) with (001) crystal plane are used. In this paper, CdTe QDs/TNS composites is synthesized for the first time, and applied in visible light photocatalytic degradation of rhodamine B (RhB). The photocatalytic degradation efficiency of CdTe QDs/TNS composites, pure TNS and CdTe QDs/TiO2 nanoparticals composites are 68%,4% and 27%, respectively. The main reasons for the higher photocatalytic activity of CdTe QDs/TNS composites are as follows:Firstly, CdTe QDs can absorb visible light and excite to form electron/hole pairs. Secondly, the conduction band position of CdTe QDs is more positive than that of TiO2, the photogenerated electrons can be quickly transferred to the conduction band of TiO2, which leads the effective separation of photogenerated electrons. Thirdly, the reactivity of (001) lattice plane is much stronger than (101) lattice plane of TiO2.2.To reduce the contents of heavy metal Cd and effectively change the fluorescent emission peak, water-soluble ZnxCd1-xSe quantum dots are obtained through two step synthesis. The emission wavelength of ZnxCd1-xSe QDs can shift from 430nm to 578 nm by adjusting the x value from 1 to 0.53. The results indicate that Cd2+ is doped into the crystal lattice of ZnSe QDs, the ternary ZnxCd1-xSe QDs are successfully synthesized, and the particle size is about 5 nm; ZnxCd1-xSe QDs can clearly develop latent and blood fingerprints, especially suitable for fingerprints on tin foil; aluminum alloy; black ceramic tile, and black plastic and for old blood fingerprint development.3. C QDs with graphite structure are synthesized through an alkali-assisted electro-chemical method; the size is about 5 nm, when the excitation wavelength changes from 300 to 500 nm, the emission peak changes from 440 to 550 nm. C QDs also possess up-converted fluorescence property.In this paper, C QDs/TNS composites are firstly synthesized, and applied in visible light photocatalytic degradation of RhB. The photocatalytic degradation efficiency of C QDs/TNS composites, pure TNS and C QDs/TiO2 nanoparticals composites are 99%,4% and 45%, respectively. The main reason for the higher photocatalytic activity of C QDs/TNS composites is:firstly, C QDs can absorb visible light and excite to form electron/hole pairs; due to the up-converted fluorescence property, C QDs can absorb visible light, and then emit shorter wavelength light to excite TNS to form electron/hole pairs. Secondly, the reactivity of (001) lattice plane is much stronger than (101) lattice plane of TiO2. Because of the two aspects, the photocatalytic activity of C QDs/TNS composites is much higher.4. Water-soluble carbon nanoparticles are synthesized by hydrothermal method, using glucose as carbon source. The sizes of carbon nanoparticles are in the range of 10-25 nm; carbon nanoparticles possess up-converted fluorescence property and the emission peaks are shifted to longer wavelengths as the excitation wavelength increased. The effects of different synthesis time and temperature on the fluorescent properities are studied, the results indicate that when the synthesis time is 4 h and the temperature is 160℃, the carbon nanoparticles show highest fluorescent intensity.The C QDs/TiO2 composites photocatalyst is prepared through hydrothermal method by using glucose and TiO2 nanoparticles, and applied in visible light photocatalytic degradation of RhB. The results show that adding carbon can effectively transferred the photogenerated electrons, and then improve the photocatalytic activity. The effects of different contents of glucose and reaction time are studied, and the results show that when the amount of glucose is 0.014 g, reaction time is 15 h, the b est photocatalytic activity of C QDs/TiO2 composites is obtained. |
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