Green Preparation And Characterization Of Noble Metal Nanomaterials

In this paper, by using tannic acid, gallic acid and EGCG as reducing and protecting agents, we successfully made the synthesis of pure metallic silver and gold nanoparticles and bimetallic Au/Pt nanoparticles via a facile green approach. The result showed that, by simple variation of the experimental conditions, it was possible to change the sizes and morphologies of the noble metals. The as-synthesized samples were characterized by UV-Vis spectra, TEM, XRD and FTIR spectra.The main work completed in this thesis can be summarized as following:1. The gold hydrosol could be prepared by the reduction of HAuCl4 with tannic acid which existed in the cortexes and fruitages of many natural plant, such as the oak and sumach. Effects of molar ratio of TA/HAuCl4 and the tempreture on the size and shape of gold nanoparticles were investigated. Distinct transmission electron microscope (TEM) morphologies showed that the gold aggregates could be formed as TA/HAuCl4 molar ratio increased. Addition of the cationic surfactant CTAB could reverse nanoparticle aggregation into sphere nanoparticles under magnetic stiring at room temperature. The redox reaction between HAuCl4 and tannic acid would be restrained if quantitative CTAB were added into the system in advance. The anisotropic gold nanoparticles could be synthesized by the seed-mediated method. It was found that various shapes of silver nanoparticles could also be prepared by this method in the absence of adequate Na2CO3.2. A method for the synthesis of gold nanoparticles at room temperature by reducing HAuCl4 with gallic acid (GA) was presented. The particle size increased with the increase of the molar ratio of GA/HAuCl4, and the shapes of gold nanocystals changed from sphere-like nanoparticles, polygonal nanoparticles to flower-like gold aggregates. According to the preliminaryζpotential measurements, these gallic acid-coated gold nanoparticles were negatively charged, so the cationic surfactant CTAB had a strong deaggregating effect on gold nanoparticle aggregates. Heating treatment could accelerate this nanoparticle aggregation provoking process, but the process could occur only about twenty seconds if under microwave irradiation, the introduction of SDS made the colloid instable. However, PVP could not affect the morphology of the gold aggregates but only stabilize the formed nanoparticles.3. Gold nanoparticles were fabricated by EGCG reducing HAuCl4 at 40°C. Effects of molar ratio of EGCG/HAuCl4 (R) and the tempretures on the size and shape of gold nanoparticles were studied. In the range from R=1 to R=10, the maximum absorption wavelength (λmax) of gold colloid first increased with the increase of R, then decreased, and the maximumλmax occourred at about R=3.5. And with the molar ratio of EGCG/HAuCl4 increasing, the morphology of the particles was various such as spherical, cylindrical, linear, flower-shaped and sphere-like. Effects of electrolytes, such as HCl, NaCl and CaCl2 on the stability of gold colloid were examined. With addition of HCl or CaCl2 the gold particles had a tendency to aggregate obviously. It is shown that the EGCG-stabilized Au nanocrystals function as effective catalyst for the reduction of 4-nitrophenol in the presence of NaBH4.4. A facile synthesis of bimetallic Au/Pt nanoparticles with EGCG as reductant and stabilizing agent was presented. The bimetallic nanoparticles were prepared from the aqueous solutions of HAuCl4/H2PtCl6 mixtures in different molar ratios under refluxing state. With the molar ratio of HAuCl4/H2PtCl6 decreasing, the characteristic absorbance band of gold nanoparticles gradually disappeared, and the bimetallic nanoparticles showed broad absorption over the entire spectral range. The morphology changed from flower-shaped to sphere-like. The primary gold nanoparticles seemed to induce the small nanoparticles aggregation into bimetallic Au@Pt nanoparticles by simultaneous reduction. In the seed-mediated method, the gold nanoparticle served as the seed (core), and H2PtCl6 was chemically reduced on the core surface to form the shell layer. The size of the core-shell particles could be modulated by controlling the ratio of the amount of the seed and the platinum metal ion.