1、Aligned ZnO Nanorod Arrays Grown Directly on Zinc Foils and Zinc Spheres by a Low-Temperature Oxidization Method用低温氧化方法直接在锌箔和锌颗粒培养排列有序的氧化锌纳米棒阵列ABSTRACT Vertically aligned, dense ZnO nanorod arrays were grown directly on zinc foils by a catalyst-free,low-temperature (450_500 C) oxidization method. Th
2、e zinc foils remain conductive even after the growth of ZnOnanorods on its surface. The success of this synthesis largely relies on the level of control over oxygen introduction.By replacing zinc foils with zinc microspheres, unique and sophisticated urchin-like ZnO nanorod assemblies can be readily
3、 obtained. 摘要垂直有序密排的氧化锌纳米棒阵列直接通过无催化剂，低温（450500C）氧化法在锌箔上生长。在氧化锌纳米棒生长在它表面上后，锌箔仍然具有导电性。合成能够成功在很大程度上依赖于对引导氧气控制程度。通过将锌箔更换成锌微球，独特和复杂的海胆样氧化锌纳米棒组件很容易获得。Zinc oxide (ZnO) is recognized as oneof the most important photonic materialsfor applications in the blue_ultraviolet region owing to its directwide band ga
4、p (_3.37 eV) and large excitationbinding energy (60 meV at roomtemperature).Stimulated by the recentdiscovery of belt like morphology and therealization of room-temperature UV lasingfrom ZnO nanowires,ZnO nanostructuresin the form of nanorods, nanowires, andnanobelts have attracted a great deal of a
5、ttentionfrom the research community. Especially,substantial effort has been devotedto the fabrication of vertically aligned ZnOnanowire arrays because these arrays demonstratedsuperior optical and field emissionproperties that make them promisingcandidates for applications in UV lasers,light-emittin
6、g diodes (LED),solar cells,and field emission displays.To fabricate vertically aligned ZnO nanorodarrays, three main techniques were usuallyused so far. The first technique is basedon the well-known vapor_liquid_solid(VLS) growth mechanism,10 in which goldnanoparticles were used as the catalyst todi
7、rect the nanowire growth, a-plane sapphirewhich has perfectlattice matchupwith ZnO c plane wasused as the growthsubstrates, and thegrowth was conductedat relativelyhigh temperatures of由于其直宽禁带和大的激励结合能，氧化锌是公认的应用于蓝紫外地区最重要的光子材料之一,由于带状形态发现的促进和室温紫外激光发射的实现，氧化锌纳米线，以纳米棒，纳米线，和纳米带形式存在的纳米氧化锌吸引了研究机构大量的注意力。尤其是，大量
8、努力已投入垂直有序排列的氧化锌纳米线阵列的制备中，因为这些阵列显示优越的光学和场发射性能，使他们有望应用于紫外激光器，发光二极管（发光二极管），太阳能电池，和led。为了制造垂直有序排列氧化锌纳米棒阵列，至今为止三个主要的技术通常被使用。第一种技术是在著名的气液固增长机制，其中黄金纳米粒子被用作催化剂引导纳米线的生长，蓝宝石状晶面具有完美的晶格匹配，它用氧化锌晶面被用作生长基质，并且生长在相对温度高850_1000c 的情况下进行The second techniqueis metal_organic chemical vapordeposition(MOCVD), in whichmetal
9、_organic zincprecursor (diethyl zinc,Et2Zn) was used as thezinc source andaligned ZnO nanowireswere epitaxiallysubstrates (or silicon wafers) at 400_500 C in a lowpressure MOCVD syste m.13_15第二种方法是金属有机化学气相沉积，其中金属有机锌前体被用作锌源，有序排列的纳米氧化锌在400500时在低压有机金属化学气相沉积技术培养基（或硅晶片）外延生长The third technique isbased on
10、solution method, in which ZnO nanocrystals(5_10 nm in diameter) were coated on a substrate (e.g.,silicon wafer) to act as the seeds followed by hydrothermalZnO growth in an aqueous solution of zinc nitratehydrate at 90 C.The solution process is favored forits low cost and the ease of scale-up (array
11、s on four inchsilicon wafer and two-inch plastic substrates werereported17) but suffers from the low crystalline qualitycompared with the VLS- and MOCVD-grown nanowires.In addition, some electrical and optical applications ofthe above-mentioned ZnO nanowires remain constrainedby the expensive and/or
12、 nonconducting substrates(such as sapphire).In this paper, we report the controlled growth of verticallyaligned ZnO nanorod arrays on metal zinc foils(10 cm long by 1 cm wide) by a catalyst-free, lowtemperature(450_500 C) oxidization method. Moreamazingly, by substituting the flat zinc foils with hi
13、ghlycurved zinc microspheres, sophisticated urchin-likeZnO nanorod superstructures, such as ZnO nanorodballs/bowls whose surfaces were covered with dense,uniform ZnO nanorods, can be readily obtained. 第三种是基于溶液的方法，其中纳米氧化锌（5_10纳米直径）被涂覆在衬底（例如，硅晶片）来充当种子，随后在90c时将水热氧化锌生长在锌盐水合物溶液中.溶液生长过程成本低，易于规模化（报道的四英寸硅晶片
14、阵列和2寸塑料衬底）备受青睐，但与气液固和金属有机化学沉积法制造的纳米线相比，其结晶度低。此外，上述提及的的氧化锌纳米线一些在电光学上的应用仍然是受昂贵和/或不导电衬底制约（如蓝宝石）。在本文中，我们报告了，通过无催化剂，低温（450_500丙）氧化法，在金属锌箔（10厘米长1厘米宽）上，可控制生长垂直排列氧化锌纳米棒阵列的方法令人惊讶的是，用曲率高的的锌微球，复杂的海胆样氧化锌纳米棒的超结构，如表面覆盖着致密均匀氧化锌纳米棒氧化锌纳米棒球/碗，取代平面锌箔可以很容易获得。The growth was conducted inside a tube furnacesystem (Figure
15、1). Zinc powder was used as the sourcematerial and zinc foils (or zinc microspheres) were usedas the growth substrates. To ensure the success of thesynthesis, the introduction of oxygen needs to be carefullycontrolled.生长过程是在管式炉系统中进行（图1）。锌粉末用作源材料。锌箔（或锌微球）被用来作为生长基质。为确保合成成功，氧的引入需要小心控制。The issues relate
16、d to oxygen introductioninclude (i) the position of the oxygen tube, (ii) theoxygen flow rate, and (iii) the time when the oxygen isintroduced, which can be summarized as follows. 氧引入相关的问题包括（一）氧气管的位置，（二）氧气流量，（三）氧气导入时间，这可概括如下First, the outlet of the oxygen tube should be positioneddownstream of the s
17、ource zinc powder but rightabove the middle of the zinc foil. This avoids the oxidizationof zinc powder, while generating an efficient,wide oxidizing region over the zinc foil to facilitate theZnO nanorod growth.Second, the oxygen gas flow rate needs to be controlledbelow 5 sccm (standard cubic cent
18、imeter perminute). If the oxygen flow rate is too high, the concentrationof reactant species in the vapor will be so highthat a thick layer of byproduct (such as random ZnOnanorods, ZnO tetrapods,20 or ZnO nanocombs21,22) willbe deposited on the surface of the nanorod arrays.The optimum oxygen flow
19、rates were found to be inthe range of 3_5 sccm.Third, the oxygen gas needs to be introduced assoon as the furnace temperature reaches 600 C. Atthis temperature the zinc powder located at the furnacecenter can be efficiently evaporated to feed theZnO nanorod growth. Because of the temperature gradien
20、tof the furnace, the temperature in the growth regionwas measured to be about 450_500 C, at which3结果与讨论第一，氧气管出口应定位在锌粉源下游但锌箔的中等以上。这避免了氧化锌粉的氧化，而在氧化锌箔上产生一个有效的宽区以促进氧化锌纳米棒生长。其次，氧气流量需要被控制低于5sccm。如果氧气流量太高，在蒸气反应物浓度会很高以至于产生一层厚厚的副产品将沉积纳米棒阵列的表面上。最佳氧流量被认为是在35的范围。第三，当温度达到600 时 氧气需要迅速引入炉内 这个温度在位于炉中心锌粉可以很快的被蒸发掉，以支
21、持氧化锌纳米棒生长。因为该炉温度梯度，在生长区测量温度约为450_500C，在图三中可以观察到。point the surface of the zinc foil (or zinc balls) was inthe melting state and thus can be immediately oxidizedby oxygen to form a thin layer (1 _m thick) of denseZnO nanopillars (Figure 2). Like the ZnO nanocrystalseeds used in solution method,17 the
22、ZnO nanopillarsformed at this initial growth stage are believed to actas the seeds for subsequent nanorod growth. This assumptionwas verified by the following two facts.(i) 锌箔表面上的点（或锌球）是在熔融状态，因此可以立即在氧化锌表面形成薄薄的一层致密的（1 m密厚）氧化锌奈米柱（图 2）。如在溶液法中氧化锌纳米晶体种子，在这个初期增长阶段形成的氧化锌奈米柱当做随后纳米棒生长的种子。这一假设由以下两点验证。When sil
23、icon wafers or alumina plates were used as thesubstrates, only random ZnO nanorods were depositedon the substrate surfaces (in an recent report byShen et al.） quasi-aligned ZnO nanonails and nanopencilswere grown on silicon wafer surface by heatingzinc powder at 600_700 C in a tube furnace system,wh
24、ere the oxygen was probably from the system leaking).(ii) When no external zinc source was provided, thethickness of the ZnO nanopillars kept at _1 _m regardlessof the growth time. 当硅片或氧化铝板被用作衬底，只有随机氧化锌纳米棒沉积在衬底表面（据沈等人最近报告，通过在600700C碳管炉系统中加热锌粉，准定向排列氧化锌钉和纳米笔 生长在硅晶片表面，那里的氧气可能是从系统泄漏）。The later fact also
25、 indicates that the ZnO nanocrystal thin layer is seamlessly formedand firmly attached to the zinc foil surface, which protectsinner zinc from being evaporated and oxidizedduring the nanorod growth process. 随后的现象说明氧化锌纳米晶体薄层是无缝生成的并紧紧地贴在锌箔表面，这可以保护内部的锌免受蒸发和氧化。Indeed, when the foil is intentionally brok
26、en after 10_30 min growth, unreactedmetal zinc layer can be easily seen.事实上，在经过10_30分钟的生长后箔会被有意的损坏，没有反应的金属锌层能够清晰的看见。After the growth, the surface of the 10 cm _ 1 cmzinc foil was covered with a thin white-gray layer. X-raydiffraction (XRD) analysis (Figure 3) shows that the depositis wurtzite (hexag
27、onal)-structured ZnO with latticeconstants of a _ 3.249 and c _ 5.206 (JCPDS cardNo. 35-1451). 生长后，距其表面10厘米_ 1厘米表面锌箔被一层薄薄的白灰色覆盖了。X射线衍射（X射线衍射）分析（图3）显示，沉积的（六）纤锌矿结构氧化锌晶格常数是，A=3.249和c=5.206（JC PDS卡片第 35-1451）。The strong intensity of the (002) peak indicatesthat the ZnO structure has a preferential growt
28、hdirection along the c-axis orientation.Scanning electron microscopy (SEM) observationsreveal that the entire substrate surface is covered withdense, highly aligned ZnO nanorods with growth directionperpendicular to the substrate surface (Figures 4).Because of the differences in temperature (from 50
29、0 to 450 C) and oxygen concentration (from low to high)from the left end to the right end of the zinc foil, themorphologies and sizes of the as-synthesized ZnOnanorods across the 10 cm long foil vary gradually.高强度的（002）峰表明该氧化锌的结构沿c轴方向择优生长。扫描电子显微镜（扫描电镜）观察显示，整个衬底表面被垂直于衬底表面生长方向（图4）密集，高度定向氧化锌纳米棒所覆盖。因为温度
30、不同（从500 450丙）和从左到右端锌箔的氧浓度（从低到高），在长10厘米箔上合成的氧化锌纳米棒的形态和大小变化缓慢。Although there is no apparent boundary between adjacent regions, four distinctive deposition zones can be identified on the basis of the morphologies and sizes ofthe nanorods, as that labeled in Figure 4a.虽然在相邻的区域没有明显的界限,四个不同的沉积区域 可以凭借纳米棒形态
31、尺寸上的差异区分开来,如图四所示.From theleft end to the right end, the morphologies of the productschange from nanoneedles (zone 1; Figure 4b), touniform nanorods (zone 2; Figure 4c,d), nanonails (zone3; Figure 4e), and finally micronails (zone 4; Figure 4f,g).While the lengths of the nanorods formed in differentz
32、ones are almost the same (_5 _m), the average diametersof the nanorods increase significantly from about50 nm of the nanoneedles (Figure 4b) to about 200 nmof the uniform nanorods (Figure 4d) and nanonails (Figure4e), and to about 500 nm of the micronails (Figure4f,g; the diameters of the nail caps
33、are up to 1 _m). Eventhough the growth temperature has some influenceon the diameters of the nanorods, the major contributionis believed to come from the difference in oxygenconcentration and thus the difference in ZnO reactantvapor concentration. Since the growth was conductedunder 500 Torr of flow
34、ing argon gas and the outlet ofthe oxygen tube is located right above the boundary ofzone 2 and zone 3, the oxygen concentration (andthus the ZnO reactant vapor concentration) over zone3 and zone 4 should be much higher than that overzone 2 and zone 1 (due to back diffusion), resulting inthe formati
35、on of larger and nail-like nanorods at thedownstream region.从左端到右端，产品的形态从纳米针，均匀的纳米棒,纳米钉，最后到微米钉。不同区纳米棒形成的长度几乎是相同的（_5 _m），该纳米棒平均直径从约50纳米的纳米针（图）增加到200纳米均匀纳米棒（图）和纳米钉（图详情），最后是大约500纳米的微米钉. 虽然生长温度对纳米棒直径有一定的影响,氧浓度差异被认为是起了很大的作用.因此，氧化锌反应水汽浓度也不同。由于生长在500托流动氩气体中, 氧气管出口也位于上述2区和3 区边界，在区 3、4区氧气浓度（因此氧化锌蒸汽反应物浓度）要远高于
36、在2区和1 区（因背部扩散），这导致在下游形成较大和钉子般纳米棒The cross-sectional image of the nanorod arrayshown in Figure 4c reveals that the aligned ZnO nanorodsgrow perpendicularly from a ZnO thin film. Closeobservation (Figure 4d) clearly shows that the film iscomposed of dense ZnO nanocrystals and that thegrowth of ZnO nan
37、orods was initiated from thesenanocrystals. 图4ZnO纳米棒阵列横断面图像的显示，定向排列的氧化锌纳米棒从氧化锌薄膜垂直生长。进一步观察清楚地表明，薄膜由致密的纳米晶体组成并且氧化锌纳米棒的生长源于这些纳米晶体。This observation confirms our assumptionon the seeding function of the nanocrystals.这一观察证实了我们在纳米晶的播种功能的假设。Thetop view images of the aligned ZnO nanorods (inset inFigure 4c,
38、 and Figure 4g) display perfect hexagonalcross section, indicating the preferential _0001_growth direction of the nanorods, which is in goodagreement with the XRD result (Figure 3).Besides the growth on flat zinc foils, our lowtemperatureoxidization method can also be used togrow dense ZnO nanorods
39、on highly curved zinc surface. 顶端观察到定向排列的氧化锌纳米棒（插图图4 和图 4）的图像，显示完美的六边形截面，表明完美的沿0001生长的纳米棒，这是很好符合XRD 结果（图3）。除了在平面锌箔上生长，我们的低温氧化法也可以用来在曲率很大锌表面培养高密度纳米棒。By replacing zinc foils with zinc microspheres andkeeping the growth conditions unchanged, we wereable to grow sophisticated urchin-like ZnO nanorod superstructures.Like the growth on zinc foils, the morphologiesand sizes of the ZnO nanorods grown on zincmicrospheres vary with the deposition regions. 通过用锌微球替换锌箔,并保持生长条件不变，我们能够培养复杂的海胆样氧化锌纳米棒超结构。就像生长在锌箔一样，生长在锌微球氧化锌纳米棒的形貌和大小的随沉积区而改变。Accordingly,spherical ZnO assemblies based on ZnO nanoneedles,
