Nanowire Arrays Electrodeposited from Liquid Crystalline Phases
Nanowire Arrays Electrodeposited from Liquid Crystalline Phases
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DOI：
10.1002/1521-4095(20020104)14:1
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发表时间：
2002-01
影响因子：
29.4
通讯作者：
Limin Huang;Huanting Wang;Zhengbao Wang;A. Mitra;K. Bozhilov;Yushan Yan
中科院分区：
文献类型：
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作者：
Limin Huang;Huanting Wang;Zhengbao Wang;A. Mitra;K. Bozhilov;Yushan Yan
金属、半导体和导电聚合物的一维纳米线和纳米线阵列因其独特的电、磁、光和力学性能以及在纳米器件中的潜在应用而受到广泛关注。虽然其他成功的方法，如气相±固相和液相±固相方法存在于硅和其他半导体纳米线中，但最广泛使用的制造纳米线和纳米线阵列的方法仍然是化学或电化学沉积，由适当的多孔hardo模板引导，如阳极氧化铝、轨迹蚀刻聚碳酸酯和云母、碳纳米管、沸石/介孔二氧化硅和二嵌段共聚物。在分子束外延制备的半导体多层材料中，新劈裂石墨的阶梯边缘和导电量子阱也被用作纳米线的生长指南。硬模板方法是制造金属、半导体和聚合物纳米线的有效途径；然而，大多数模板制作繁琐，如果需要分离单线，则需要在腐蚀性介质中溶解模板以回收纳米线。表面活性剂中间相已被证明是一种有用的、通用的纳米结构材料合成模板。例如，使用正常（油在水中）表面活性剂中间相作为模板合成多孔沸石型材料已经很好地建立起来。具有球形水微畴的反（油中水）胶束和微乳液已被用于纳米颗粒和纳米线的合成（主要是BaSO4和BaCrO4等化合物）。[21±24]上述所有合成都使用低浓度的表面活性剂，在不存在液晶相的情况下。相反，在高表面活性剂浓度下，可以得到均匀的正六方或反六方液晶相。Attard等人已经证明，正常的六方液晶可以通过电沉积的方式模板化块状多孔材料和多孔金属薄膜的合成。最近，以反六方液晶为模板，利用伽马射线辐照合成了ZnS纳米线。所得的ZnS纳米线较短（<2 lm），可能是由于反向六方液晶的随机取向所致。在这篇论文中，我们首次展示了通过电沉积从含有一维水通道的反六方液晶相中制备具有高纵横比的结晶银纳米线阵列。我们提供的证据表明，电沉积过程中的高电场有助于排列反六方液晶相，这被认为是生产高纵横比结晶金属纳米线所必需的。为了生长银纳米线，根据众所周知的三元相图，首先制备了由阴离子表面活性剂二（2-乙基己基）磺基琥珀酸钠（AOT）、对二甲苯油相和水组成的反六方液晶相。在我们的合成中，水相被0.1 M的AgNO3水溶液取代。所得混合物在偏振光显微镜下观察时具有六方液晶的双折射特性。电沉积是用恒电位器在两电极结构中进行的，两电极间距很窄（0.5±1.0 mm），前面提到的反向液晶相作为电解质。纳米线产品沉积在阴极衬底（抛光不锈钢）上，用乙醇彻底清洗。在15分钟的短沉积时间内获得高密度纳米线阵列，纳米线大致垂直于阴极表面（图1A）。纳米线阵列的轻微紊乱可能是由沉积后的洗涤过程引起的。经过2h的沉积时间，可以得到几十微米长的纳米线（图1B）。纳米线几乎与电极表面平行（不再垂直），很可能是由于沉积后的洗涤过程。高密度纳米线阵列在乙醇中超声分散后可以分离成单线（图1C），表明纳米线可以束状或单线用于纳米器件。图2显示了沉积在不锈钢衬底上的银纳米线阵列的x射线衍射（XRD）图。衍射峰的d间距分别为2.36、2.04、1.44、1.23、1.18，与面心立方（fcc） Ag金属相的衍射峰一致。能量色散x射线（EDX）分析表明，纳米线是由纯银制成的，这表明表面活性剂分子可以通过简单的洗涤过程从纳米线阵列中完全去除。纳米线的透射电镜（TEM）图像和电子衍射图如图3所示。图3A和3B分别显示了单根银纳米线片段的亮场（BF）和暗场（DF）图像。整个晶体在DF模式下是明亮的，具有工作反射（11Å1），证实了纳米线的单晶性质。单个导线要么是单晶，要么是由在{111}平面上成对的对称定向畴组成。孪晶纳米线晶体是由多个
One-dimensional (1D) nanowires and nanowire arrays of metal, semiconductor, and conductive polymers have attracted much attention because of their unique electrical, magnetic, optical, and mechanical properties, and their potential application in nanodevices. Although other successful approaches such as vapor±solid and liquid±solid phase methods exist for silicon and other semiconductor nanowires, the most widely used method for fabrication of nanowires and nanowire arrays is still chemical or electrochemical deposition guided by an appropriate porous ahardo template such as anodized alumina, track-etched polycarbonate and mica, carbon nanotubes, zeolite/mesoporous silica, and diblock copolymers. Step edges of freshly cleaved graphite and a conducting quantum well in a semiconductor multilayer prepared by molecular beam epitaxy also have been used as nanowire growth guides. The hard template approach is an effective route to metal, semiconductor, and polymer nanowires; however, most of the templates are tedious to fabricate and dissolution of the template in corrosive media is required to retrieve the nanowires if separate single wires are desired. Surfactant mesophases have proved to be a useful and versatile asofto template for the synthesis of nanostructured materials. For example, syntheses of porous zeolite-type materials using normal (oil in water) surfactant mesophases as a template are well established. Reverse (water in oil) micelles and microemulsions with spherical aqueous microdomains have been used for nanoparticle and sometimes nanowire synthesis (mainly compounds such as BaSO4 and BaCrO4). [21±24] All of the aforementioned syntheses use low surfactant concentrations where liquid-crystalline phases are not present. In contrast, at high surfactant concentrations, homogeneous normal or reverse hexagonal liquid-crystalline phases can be obtained. Attard et al. and others have demonstrated that normal hexagonal liquid crystals can template the synthesis of bulk porous materials and porous metal films by electrodeposition. Very recently, a reverse hexagonal liquid crystal was used as a template to synthesize ZnS nanowires by gamma ray irradiation. The ZnS nanowires so obtained are fairly short (<2 lm) probably due to the random orientation of the reverse hexagonal liquid crystal. In this communication, we demonstrate for the first time fabrication of crystalline silver nanowire arrays with a high aspect ratio by electrodeposition from a reverse hexagonal liquid-crystalline phase containing one-dimensional aqueous channels. We provide evidence that a high electric field during electrodeposition helps align the reverse hexagonal liquidcrystalline phase, which is believed to be essential for production of high aspect ratio crystalline metal nanowires. For growth of silver nanowires, a reverse hexagonal liquidcrystalline phase was first prepared according to the wellknown ternary phase diagram consisting of the anionic surfactant sodium bis(2-ethylhexyl) sulfosuccinate (AOT), an oil phase of p-xylene, and water. For our synthesis the water phase was substituted by an aqueous 0.1 M AgNO3 solution. The resulting mixture has the characteristic birefringence of a hexagonal liquid crystal when viewed under a polarized light microscope. The electrodeposition was conducted by using a potentiostat in a two-electrode configuration with the two electrodes narrowly spaced (0.5±1.0 mm) and the aforementioned reverse liquid-crystalline phase as electrolyte. The nanowire product deposited on the cathode substrate (polished stainless steel) was thoroughly washed with ethanol. High-density nanowire arrays were obtained over a short deposition time of 15 min with the nanowires roughly perpendicular to the cathode surface (Fig. 1A). The slight disorder in the nanowire array may arise from the washing process after deposition. After a deposition time of 2 h, nanowires tens of micrometers long were obtained (Fig. 1B). The nanowires were almost parallel (no longer perpendicular) to the electrode surface, most probably due to the washing process after deposition. The high-density nanowire array can be separated into single wires after ultrasonic dispersion in ethanol (Fig. 1C), indicating that the nanowires can be used in a bundle form or as single wires for nanodevices. Figure 2 shows the X-ray diffraction (XRD) pattern of the silver nanowire arrays deposited on a stainless steel substrate. It shows the diffraction peaks with d-spacings of 2.36 , 2.04 , 1.44 , 1.23 , and 1.18 , which are consistent with that of face centered cubic (fcc) Ag metal phase. Energy dispersive X-ray (EDX) analysis shows that the wires are made of pure silver, suggesting that surfactant molecules can be totally removed from the nanowire arrays by a simple washing process. Transmission electron microscopy (TEM) images and electron diffraction of nanowires are shown in Figure 3. Figures 3A and 3B show a bright field (BF) and dark field (DF) image, respectively, of a fragment of a single silver nanowire. The entire crystal is bright in DF mode with operating reflection (11Å1), confirming the single crystal nature of the nanowire. The individual wire is either a single crystal or consists of symmetrically oriented domains that are twinned on {111} planes. The twinned nanowire crystals are made of multiple