Synthesis Of Doped ZnS Nanoparticles And The Effect Of Surface Modification On The Luminescence Properties Of Nanoparticles

The Ag+-doped ZnS (ZnS:Ag) is a type of traditional blue-light-emitting materials. The commercial ZnS:Ag microparticles, which show a emission peak around 450 nm, have been widely used in many fields, such as color picture tubes and color display tubes. Compared with their bulk materials, ZnS:Ag nanomaterials have many unique advantages, and have arrested considerable interests. However, because of the difficulties in preparation arising from the introduction of Ag+ ions to ZnS lattice, and the quenching of luminescence by surface defects, ZnS:Ag luminescent nanomaterials meet serious challenges in practical applications.Systematic studies of the chemical syntheses, luminescence properties and surface modifications of ZnS:Ag nanoparticles are presented in this dissertation, summarized as follows.1. The hydrothermal synthesis of ZnS:Ag nanoparticles. ZnS:Ag nanoparticles were prepared through a simple hydrothermal process, using zinc acetic (Zn(Ac)2), thioacetamide (TAA) and silver nitrate (AgNO3) as reagents. The effects of process details, reaction temperatures and times, the molar ratios of S/Zn and Ag/Zn during hydrothermal syntheses on luminescence properties of ZnS:Ag nanoparticles were researched. The optimum process parameters are as following:[Zn(Ac)2]= 0.1 mol/L, S/Zn (molar ratio)=1.0, Ag/Zn=1.0 at%; the filling degree of Teflon cup is 80%, and the synthesis is carried out at 200℃for 6 h. The ZnS:Ag nanoparticles obtained under the above condition are spherical with an average diameter about 30 nm. This product is well crystalline with a cubic zinc blende crystal structure. Its emission peak is around 450 nm, which is ascribed to the recombination between the sulfur vacancy-related electron trap donor having an energy level just below the conduction band and the Ag+-related hole trap acceptor above the valence band. 2. The study of luminescence properties of ZnS:Ag nanoparticles. Bimodal Gaussian fitting is applied to the emission curve of ZnS:Ag nanoparticles, and the results show that the experimental emission is mostly composed of two types of emissions:donor-acceptor pair luminescence around 443 nm and surface states luminescence around 483 nm. The fluorescence decay curve of ZnS:Ag nanoparticles is fitted with a model of stretched exponential function, and the results imply that the luminescence of ZnS matrix is replaced by the luminescence of ZnS:Ag. The emission peaks are red shifted and the lifetime is shortened by ultraviolet irradiation. The exact reasons need to be further investigated, and we propose an assumption as following. Under a long time irradiation, some unknown photochemical products may be formed on the surfaces of ZnS:Ag nanoparticles, and the Ag+ ions may be reduced to Ag metal by photons.3. The surface modification of ZnS:Ag nanoparticles. The ZnS:Ag nanoparticles synthesized by hydrothermal method were coated with inorganic shells by a chemical precipitation method. The ZnS:Ag/ZnS, ZnS:Ag/CdS, and ZnS:Ag/ZnO core-shell nanoparticles with different thickness of ZnS, CdS, and ZnO shells were prepared. The effects of shells on the luminescence properties of ZnS:Ag nanoparticles were investigated through the photoluminescence spectra and luminescence stabilities of products. In the core-shell nanoparticles involved here, the ZnO shell can most significantly enhance the luminescence of ZnS:Ag nanoparticles. The 450 nm emission intensity of ZnS:Ag/ZnO nanoparticles is up to 125% of that of ZnS:Ag nanoparticles. However, the ZnO shell hardly influences the luminescence stability under ultraviolet irradiation. The ZnS shell can only increase the luminescence of ZnS:Ag nanoparticles to some extent, but it can improve the luminescence stability under ultraviolet irradiation. Although the CdS shell can improve the luminescence stability to some extent, it quenches the luminescence of ZnS:Ag nanoparticles dramatically.4. The reverse micelle synthesis of ZnS:Ag nanoparticles. The ZnS:Ag nanoparticles were prepared in a water/AOT/n-heptane reverse micelle system, using zinc acetic (Zn(Ac)2), thioacetamide (TAA) and silver nitrate (AgNO3) as reagents and 3-Mercaptopropionic acid as a coordination agent. The effects of the molar ratio of water to surfactant (W) in reverse micelle systems, the molar ratios of MPA/Zn(Ac)2, NaOH/Zn(Ac)2, and Ag/Zn in pre-reaction solutions, and the Ostwald Ripening on luminescence properties of ZnS:Ag nanoparticles were researched. The products are identical spherical nanoparticles with average diameters about 3 nm. These nanoparticles are poorly crystalline with a cubic zinc blende crystal structure. The emission peaks are blue shifted to 435 nm, which can be attributed to a quantum size effect. This emission is still related with the recombination between the sulfur vacancy-related electron trap donor and the Ag+-related hole trap acceptor.5. The surface modification of ZnS:Ag nanoparticles in reverse micelles. A model of a spherical core-shell nanoparticle was proposed. The ZnS:Ag nanoparticles were synthesized and coated with inorganic shells in a water/AOT/n-heptane reverse system. The ZnS:Ag/ZnS, ZnS:Ag/CdS, and ZnS:Ag/ZnO core-shell nanoparticles with ZnS, CdS, and ZnO shells composed of various monolayers were prepared, and the thicknesses of shells were exactly controlled. The XRD,AES and TEM results evidence that the shells have grown around the ZnS:Ag cores. The effects of shells on the luminescence properties of ZnS:Ag nanoparticles have been investigated through the absorption spectra, photoluminescence spectra and luminescence stabilities of products. The shells significantly influence the luminescence properties of ZnS:Ag cores being about 3 nm. The CdS shell leads to the luminescence transformation from ZnS:Ag to CdS:Ag, and the luminescence between 425 nm-620 nm can be obtained by controlling the thickness of CdS shell. The ZnS and ZnO shells make the emission peaks red shift and the excitation peaks blue shift, and the emission intensities are increased to some extent. The ZnO shell can dramatically raise the luminescence stability under ultraviolet irradiation.