| Semiconductors have been used in solar cells, photocatalysts, sensors and bioluminescence due to the unique property of photovoltaic conversion. However, wide-band-gap semiconductors such as TiO2 and ZnO can be only excited by UV light and the quick recombination of charge carriers limit the photocatalytic efficiency. In recent years, noble metals doping has been utilized to modify the wide-band-gap semiconductors. Metal/semiconductor composite nanomaterials have strong and continuous absorption in visible light region owing to LSPRs (localized surface plasmon resonances). On the other hand, noble metals act as electron traps, which reduce the recombination of charge carriers. Among a large amount of wide-band-gap semiconductors, Ag-based semiconductors have attracted increasing attention for years and a lot of researches about Ag/Ag2O and Ag/AgCl composite nanomaterials which are applied to photocatalysts and luminescence have been reported.Recently, our group has synthesized hierarchical multiple-growth-hillocks superstructures of Ag nanoparticles. But Ag nanoplates were obtained by increasing UV illumination time, which suggested the transformation of the crystallization modes. In this research, we studied the growth mechanism of Ag nanoplates. Meanwhile, three kinds of AgCl concave polyhedra and novel hierarchical microcrystals were fabricated via one-step precipitation. The detailed results are described as following:1. The morphologies of Ag nanocrystals generated on ZnO films were investigated by adjusting UV illumination time, as well as the concentration of AgNO3. Our studies demonstrated that in situ photochemically synthesized Ag nanoparticles can act as elementary units for further crystallization. When the supersaturation of in situ synthesized nanoparticles was relatively low with short UV illumination time, they crystallized into the multiple-growth-hillock-like superstructures via the screw-dislocation-driven crystallization mechanism. Increasing the nanoparticle supersaturation would switch the crystallization mode from the dislocation-dominant one to a layer-by-layer one and lead to the formation of small nanoplate-like superstructures with rough surfaces and ragged edges. Along with the well-known photoinduced structural evolution, larger nanoplates with characteristic smooth surfaces, cutting straight edges, sharp tips, and unique multi-layered structure were formed. However, Ag/Ag2O composite nanoplates were finally obtained due to the oxidation of Ag in the air.2. Three kinds of AgCl concave polyhedral were synthesized through one-step precipitation. The concentration of NH3·H2O was found to tune the eventual structures by changing the growth rates of the different sites of AgCl cubic seeds. Hopper-like cubes with concave face-centers were observed under low concentration of NH3·H2O, and octapod cubes with concave face-centers and edge-centers under middle concentration of NH3·H2O, and flower microcrystals under high concentration of NH3·H2O.3. AgCl hierarchical microcrystals containing flower-like cores and cubic shells were formed based on the flower microcrystals. The growth mechanism and the influence of the amount of Ag(NH3)4Cl were studied. It was found that the growth rate of the face-centers was the fastest at the beginning of crystallization with high concentration of NH3·H2O, which caused the formation of flower microcrystals. During the evaporation of the solvent, the concentration of NH3·H2O decreased, leading to the growth direction change. As a result, cubic shells grew along the the parallel direction of{100} planes of the flower microcrystals. Moreover, the indirect band gap of AgCl hierarchical microcrystals was 2.95 eV according to the UV-vis absorption spectrum.4. Porous Ag/AgCl hierarchical microcrystals were generated through photoreduction. And the microcrystals have strong and continuous absorption in the visible light region. At last, we explored the fluorescence of the Ag nanoclusters/AgCl hierarchical microcrystals excited by laser. |
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