金属钯纳米颗粒的形貌控制合成

【作者】 于艳春；

【导师】 黄涛；

【作者基本信息】 中南民族大学 ， 物理化学， 2008， 硕士

【摘要】 金属纳米粒子因其在催化、光学、微电子学、磁学、光学传感、信息存贮、生物标记等领域的广泛应用而倍受关注。金属纳米材料的性质与其颗粒大小、形貌、组成和结构紧密相关。Pd作为一种重要的铂族元素一直吸引着人们的广泛兴趣。Pd被广泛用作催化剂,其在催化方面的应用与其显著的吸氢能力密切相关。近十年来,为进一步提高其催化活性,人们致力于单分散的Pd纳米结构的尺寸和形貌控制研究,利用表面活性剂、高分子化合物等作为稳定剂制备得到了各种不同形貌的Pd纳米颗粒,但是,所得到的Pd纳米颗粒形貌不单一,大小不均匀,为多种形貌的混合物。为此,本文探索单一形貌的Pd纳米颗粒的控制合成方法。利用微波-多醇法,以H2PdCl4为前驱体,以三缩四乙二醇为溶剂和还原剂,以聚乙烯吡咯烷酮(PVP)为稳定剂,在适量KOH存在下,合成得到了正二十面体的钯纳米颗粒,并用紫外-可见光谱(UV-vis)、透射电子显微镜(TEM)、粉末X-射线衍射(XRD)以及X-射线光电子能谱(XPS)等对产物进行了表征。通过改变反应体系中前驱体H2PdCl4的浓度,可以有效地控制二十面体的平均粒径大小。采用阳离子型表面活性剂十六烷基三甲基溴化铵(CTAB)和PVP两者协同作用,以三缩四乙二醇为溶剂和还原剂,使用微波-多醇法成功地制备了平均粒径为23.8nm的立方体钯纳米结构。通过在反应体系中引入KBr、KCl和十六烷基三甲基氯化铵(CTAC)等物质考察CTAB对钯纳米结构的影响,表明Brˉ对立方体钯纳米结构的控制起主要作用。在反应之前CTAB中的Brˉ取代H2PdCl4中的Clˉ离子,不仅改变了反应前驱体的组成,而且改变了氧化还原电极电势,从而改变了还原反应速率。通过改变CTAB的浓度,可以制备出形貌单一的立方体钯纳米颗粒。利用油浴加热法,以H2PdCl4为前体,以三缩四乙二醇为溶剂和还原剂,当只采用PVP作为稳定剂时,合成的钯纳米结构为二十面体;采用CTAB和PVP协同稳定时,得到的钯纳米颗粒主要是立方八面体或棒状纳米结构;对不同形貌的钯纳米颗粒的XRD分析表明,所制备的不同形貌的Pd纳米颗粒,其晶形均为fcc结构,并且对{111}晶面择优取向的纳米结构最明显的是棒状,其次是立方八面体,最后是二十面体。更多还原

【Abstract】 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.更多还原