Palladium Nanoparticles Synthesis And Morphology Control

Considerable attention has been paid to metal nanoparticles owing to their potential applications in many fields such as catalysis, optics, microelectronics, magnetic, information storage, optical sensing, biological labeling and among others. The properties of metal nanoparticles are strongly dependent on their size and shape, as well as their composition, crystallinity and structure. As one of the noble metals, palladium has also attracted great interests because of its extraordinary properties. It can be widely used as the primary catalyst and its applications in catalysis are related to its remarkable capacity for hydrogen absorption. To improve its catalytical activity, a lot of efforts have also been devoted in the past decade to the fabrication of palladium nanostructures with monodispersive sizes and well-defined morphologies. Palladium nanoparticles with various morphologies have been prepared by using surfactants, polymers and others. However, only some mixtures of various geometric shapes of palladium were obtained generally. In this paper, shape-controlled synthesis of the monomorphological palladium nanoparticles was investigated.Palladium icosahedra were prepared in a high yield for a shorter time in tetraethylene glycol solution with H2[PdCl4] as a precursor and polyvinylpyrrolidone (PVP) as a stabilizer in the presence of an appropriate amount of KOH under microwave irradiation. The as-prepared palladium nanopartcles were characterized by UV-vis absorption spectroscopy, transmission electron microscopy, X-ray diffraction and X-ray photoelectron spectrometry. The average size of the palladium icosahedra can be controlled by changing the concentration of precursor salt.Palladium nanocubes with a mean size of about 23.8nm were also successfully synthesized with tetraethylene glycol as the reducing agent and solvent by the cooperation of cetyltrimethylammoniumbromide (CTAB) and PVP under microwave irradiation. The effect of CTAB on the shape of palladium nanopartcles was investigated by using KBr, KCl or cetyltrimethylammoniumcloride (CTAC) instead of CTAB in the reaction system. The results show that the formation of palladium nanocubes is dependent on the Brˉion. Clˉion in H2PdCl4 was substituted by Brˉion before reaction and the composition of the precursor as well as the standard electrode potential was changed. This resulted in a change of the reducing rate as well as the process of the nucleation and growth of nanoparticles. Monomorphological palladium nanocubes can be obtained by changing the concentration of CTAB.By oil-bath heating, palladium icosahedra were also obtained in a high yield with only PVP as the protecting agent. However, palladium cuboctahedra or nanorods were synthesized when PVP was cooperated with CTAB. Palladium nanopartcles with different shapes were characterized by X-ray diffraction and the results indicated that Pd nanocrystallines with the same face-centered cubic (FCC) structure prefer to a preferential {111} orientation. The order of preferential growth on this {111} face is nanorod, cuboctahetron, and then icosahedron.