Noble-metal nanotubes (Pt, Pd, Ag) from lyotropic mixed-surfactant liquid-crystal templates
Noble-metal nanotubes (Pt, Pd, Ag) from lyotropic mixed-surfactant liquid-crystal templates
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DOI：
10.1002/anie.200352630
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发表时间：
2004-01-01
影响因子：
16.6
通讯作者：
Uoyama, S
中科院分区：
文献类型：
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作者：
Kijima, T;Yoshimura, T;Uoyama, S
在典型的制造过程中，用肼处理摩尔比为1：1：1：60的六氯铂酸（H2 PtCl 6）、九乙二醇单十二烷基醚（C12 EO 9）、聚氧乙烯（20）脱水山梨糖醇单硬脂酸酯（Tween 60）和水的液晶相。所得黑色粉末的透射电子显微镜（TEM）图像显示纳米管材料的聚集体（图1A）。还原粉末的能量色散X射线（EDX）分析证实了铂作为主要成分的存在和氯离子的完全去除，在铂离子还原成铂金属的过程中。因此，由此获得的管状材料可以被鉴定为铂纳米管，其内径和外径分别为3和6 nm。在同一图像中观察到一些直径约为6 nm的圆形斑点，因为一些折叠的纳米管平行于电子束取向。纳米管的长度估计远大于100 nm，但不可能确定孤立纳米管的总长度，因为纳米管材料高度凝聚成致密的聚集体。还原固体的X射线衍射（XRD）图谱在2θ= 0.5-28处也显示出宽肩，这可能是由于纳米管结构（图2）。在2θ= 39.738和46.198处的两个宽峰分别归属于面心立方结构的（111）和（200）反射，其晶胞参数为0.3929 nm，与体金属的晶胞参数（0.3923 nm）非常接近。热重分析（TGA）和FTIR光谱显示，固体含有% 36重量%的剩余表面活性剂物质，这表明纳米管铂颗粒被剩余的有机物质稳定。另外的实验表明，通过用乙酸钠的乙醇溶液处理，有机物含量进一步降低至13wt%。铂纳米管的催化活性并不总是被表面活性剂或其他分子保护降低，因为被表面活性剂和溶剂稳定的各种贵金属胶体被证明对氢化有效[12]并用于Heck反应。[13]通过使用Pd（NO3）2代替H2 PtCl 6的类似反应也产生了内径和外径分别为3 nm和6 nm的钯纳米管（图1B）。使用十二烷基硫酸钠（SDS）和吐温60作为与AgNO 3的混合模板的另一种制造工艺导致形成内径和外径分别为4和7 nm的银纳米管的聚集体（图1C）。在上述三种体系中，注意到金属纳米管高度凝聚成致密的聚集体。这种聚集，以及电子束系统的稳定性，这是从放置在TEM的真空室中的金属样品蒸发的有机物种的影响，导致难以观察到一个孤立的金属纳米管的更高分辨率的图像。根据TGA，用乙醇洗涤的钯纳米管被剩余有机材料污染的程度比铂纳米管少三倍，因此前者的金属颗粒倾向于形成更致密的聚集体。这可能是
In the typical fabrication process, the liquid crystalline phase of hexachloroplatinic acid (H2PtCl6), nonaethylene glycol monododecyl ether (C12EO9), polyoxyethylene (20) sorbitan monostearate (Tween 60) and water at a molar ratio of 1: 1: 1: 60 was treated with hydrazine. The transmission electron-microscope (TEM) image of the resulting black powders showed an aggregate of nanotubular materials (Figure 1A). Energy-dispersive X-ray (EDX) analysis of the reduced powders confirmed the existence of platinum as the major component and the complete removal of chloride ions librated during the reduction of platinum ions into platinum metal. Hence, the tubular materials thus obtained can be identified as platinum nanotubes with inner and outer diameters of 3 and 6nm, respectively. Some round spots with a diameter of about 6 nm are observed in the same image because some of the folded nanotubes are oriented parallel to the electron beam. The length of nanotubes is estimated to be much greater than 100 nm, but it is impossible to determine the total length of an isolated nanotube because the nanotubular materials are highly cohered into a dense aggregate. The X-ray-diffraction (XRD) pattern of the reduced solid also exhibited a broad shoulder at 2θ= 0.5–28 that is presumably due to the nanotubular structure (Figure 2). The two broad peaks at 2θ= 39.738 and 46.198 are assigned to the (111) and (200) reflections, respectively, of the fcc structure with a unit cell parameter of 0.3929 nm, which is very close to that of the bulk metal (0.3923 nm). Thermogravimetric analysis (TGA) and FTIR spectroscopy revealed that the solid contains% 36 wt% of remaining surfactant species, which suggests that the nanotubular platinum particles are stabilized by the remaining organic species. An additional experiment showed that the organic content is further decreased to% 13 wt% by treatment with an ethanol solution of sodium acetate. The catalytic activities of platinum nanotubes would not always be lowered by surfactant or other molecular protection, as various noble metal colloids stabilized by surfactants and solvents were demonstrated to be effective for hydrogenation [12] and use in the Heck reaction.[13] A similar reaction by using Pd (NO3) 2 in place of H2PtCl6 also yielded palladium nanotubes with inner and outer diameters of 3 and 6 nm, respectively (Figure 1B). Another fabrication process that uses sodium dodecylsulfate (SDS) and Tween 60 as a mixed template with AgNO3 resulted in the formation of an aggregate of silver nanotubes with inner and outer diameters 4 and 7 nm, respectively (Figure 1C). It is noted throughout the above three systems that the metal nanotubes are highly cohered into a dense aggregate. This aggregation, together with the stability of electron-beam system, which was affected by the organic species evaporated from the metal samples placed in the vacuum chamber of TEM, led to difficulties in observing a higher resolution image of an isolated metal nanotube. According to TGA, palladium nanotubes washed with ethanol were three times less contaminated with remaining organic materials than the platinum nanotubes, and hence the former metal particles tended to form more dense aggregates. This may be the