Synthesis Of The Quantum Dots And Its Application In Solar Cell And Detection

Quantum dots (QDs), a new type of semiconductor nanomaterials, possess numerical advantages, such as the adjustable color, broad excitation spectrum, photobleaching resistance, due to the special physical properties of quantum size effect, multiple exciton excitation effect and quantum confinement effect. As a result, QDs have been widely used in the fields of biomedicine, solar cell photovoltaic devices, fluorescent probes, light-emitting diode (LED) and photocatalysis. At present, organic phase synthesis, hydrothermal method and aqueous phase method are used as the main methods for the synthesis of QDs. Traditional organic synthesis method has been widely used in most commercial production, however, it suffers from the drawback of QDs undissolving in water and toxicity. Compared with it, the QDs synthesized in aqueous solution exhibit good water solubility and the preparation process is simple, which shows broad prospects in the water pollution detection and treatment. However, the fluorescence electron efficiency of the QDs is low. In addition, due to t energy shortage, the domestic and international researchers are eager to develop solar energy that is a clean energy. As the third generation solar cell, the QDs sensitized solar cells have attracted much attention because of their large extinction coefficient, the absorption spectrum adjustability and high theoretical efficiency. So far, the QDs sensitized solar cell has been mainly achieved by sensitized the Ⅱ-Ⅵ and Ⅲ-Ⅴ semiconductor (such as CdS, PbS, CdSe) to TiO2 nanotubes and ZnO particles. But due to the TiO2 nanotubes with small size, QDs easily agglomerate in the surface of the nanotubes, thus affecting the photoelectric properties of the sensitized solar cell. So it is crucial to find a simple and fast method to make the solar cell with high photoelectric conversion efficiency. Quantum dot solar cell is composed of quantum dot sensitized material, TiO2 nanostructure photoelectric pole, electrode, the oxidation and reduction of electrolyte. Its high efficiency photoelectric conversion and low cost of are related to its structure. For example, types of quantum dots, if QDs of the solar cell division was uniform on the surface and quantum dot sensitized style.In this paper, a facile synthetic approach was used to prepare CdTe and CdSe QDs via aqueous phase method. Transmission electron microscopy (TEM), X-ray powder diffraction (XRD) and FL-920 were used to characterize QDs structure morphology and fluorescence emission spectrum (PL). Besides, anodic oxidation method was used to prepare the ordered TiO2 nanotubes, and then it was sensitized by the QDs (CdTe, CdS, CdSe). The photoelectric conversion properties of the samples were studied in detail. At last, CdTe QDs were applied to detect melamine. The work was carried out as follows:(1) In aqueous solution, the CdTe QDs were synthesized successfully by microwave irradiation reaction route using 3-mercaptopropionic acid (MPA) as both a sodium tellurite reducer and a capping molecule. The optimization program of synthesis CdTe QDs with high fluorescence intensity and stability was obtained. The molar ratio of the precursor Cd/Te/MPA was 1:0.05:4.5 and the original solution pH was 10.0, the temperature of reaction is 100℃, then a maximum photoluminescence quantum yield (QY) was 63.6%. While the MSA was used as stabilizer, Na2SeO3 and Vc were as reaction precursor, the best conditions of synthesis of the water-soluble CdSe QDs were pH of 11.0 at 100℃ with the molar ratio of MSA, Vc and Se2- to be 1:0.08:1 and excellent fluorescence quantum efficiency of 21% was obtained. Finally, CdS QDs were synthesis in the aqueous phase using mercaptoacetic acid as stabilizer, sodium sulfide as sulfur source, which would be applied in quantum dot sensitized solar cells(2) Using the highly ordered anatase TiO2 nanotubes as the carrier, the CdS/TiO2 light anode was prepared by the high pressure reaction kettle. It was found that the optimum conditions were at 80℃ for 4 h with pH 10.0 and stabilizer 340μL, then the maximum photoelectric conversion efficiency of 0.64% was finally to be obtained. In addition, in this chapter, we have prepared the CdTe/CdS/TiO2 nanotube anode. The results show that the photoelectric conversion efficiency of CdTe/CdS/TiO2 nanotube was about 3 times as high as that of CdS/TiO2.(3) Successive ionic layer adsorption and reaction (SILAR) was used to study the effect of different deposition times of CdSe/TiO2 and CdSe/CdS/TiO2 nanotube solar cells. The results show that with quantum dots deposited 6 and 7 times and the test electrolyte composition for 0.25 mol/L Na2S and 0.35 mol/L Na2SO3 mixed solution, photoelectric conversion efficiency are 0.75% and 1.95%, respectively. Photoelectric conversion efficiency of the co-sensitization was improved 2.6 times as high as that of single sensitization.(4) MPA-CdTe was used as a fluorescent probe to detect the melamine. Under the conditions of solution pH of 8.0, concentration of MPA-CdTe quantum dots of 8 μg·L-1 and melamine in a range of concentration from 10 to 100 μg·L-1, the relative intensity changes of fluorescence of quantum dots showed a good linearity:Fo/F=0.0217C+0.8809, with the correlation coefficient of 0.9958 and the minimum detection limit of 2.6 μg·L-1. By applying the method of adding different concentrations of melamine in milk samples, the determination of the recovery rate is between 102% and 105%, RSD is 1.1%-1.9%.