Controlled Synthesis Of The Core-shell Pt@SiO₂ Nanomaterials

Core-shell nanocomposite structure is an ordered assembled new structure on the nanometer scale derived from one nanomaterial coated with another through chemical bonds or other chemical interactions. Platinum ?Pt? nanoparticles have broad prospects in the field of catalysis, optics, electricity and magnetism because of its unique physical and chemical properties, which is mainly determined by the morphology and size of the nanoparticles. However, the nanoparticles are easy to be aggregated or sintered in the reaction process, especially under high temperatures, thus its application is limited. Coating the particle surface with a protective layer, such as SiO₂, can effectively solve this problem. Up to now, although the core-shell structures encapsulating nanoparticles with different kinds of morphologies can be obtained by various methods, the controlled synthesis of Pt particles and silica-coated Pt under various liquid phase reduction system needs further study. Moreover, the research concerning the synthesis of silica-coated Pt nanoparticles by the microemulsion method and other mesoporous silica structure coated particles is still scarce. So in this paper the controlled synthesis of Pt nanoparticles under a liquid phase reduction method and silica coating were investigated to gain insight into the controllable synthesis mechanism and the optimal coating conditions.Using chloroplatinum acid or platinum acetylacetonate as the platinum metal precursor, polyvinylpyrrolidone ?PVP?, myristyl trimethyl ammonium bromide ?TTAB?, oleylamine ?OAm? and oleic acid ?OA? as the surfactant, ethylene glycol ?EG?, sodium borohydride ?NaBH4?, sodium citrate ?NaCitr? and oleylamine ?OAm? as the reducing agent, the controlled synthesis of Pt nanoparticles was conducted by liquid phase reduction. The influences of the dosage of NaOH, PVP, NaBH4, OAm, OA and NaCitr, the reaction time and reaction temperature on the particle size were investigated. The results show that in hydrophilic system, the addition of NaOH and NaBH4 decreases the size of the particles, the addition of PVP increases the dispersion of the particles, the holding time and the addition of NaCitr have little effect on the particle size. Whereas in the hydrophobic system, the rising reduction temperature increases the resulted particle size, Pt metal salt precursor and the addition of nitrogen will affect the dispersion state of the particles in the system, and the holding time and the amount of surfactant added have little effect on the size and the dispersion of the particles.For the SiO₂ coating, using the tetraethylorthosilicate ?TEOS? as the silica source, Stober and microemulsion method were performed to synthesize coated nanoparticles. The influences of particle surface state, the microemulsion ratio and the amount of added TEOS on the coating were examined. The results show that PVP or TTAB has great influence on the coating in the Stober synthesis, mainly because TEOS deposited onto the particle surface requires the presence of the surfactant. The ratio of the microemulsion also has great influence on the particle dispersion, and the amount of added TEOS affect the coating state and the size of the resulting silica sphere from hydrolysis. Additionally, a number of monodisperse silica spheres was synthesized using Stober method and coated with the mesoporous silica. It was revealed that expanding the pore sizes in the mesoporous silica shells could be achieved by the increase of the CTAB amount. A SBA15 molecular sieve coated silica sphere was synthesized as well.