Controlled fabrication of hierarchically branched nanopores, nanotubes, and nanowires

Abstract

Here, we report a generic synthetic approach to rationally design multiply connected and hierarchically branched nanopores inside anodic aluminum oxide templates. By using these nanochannels, we controllably fabricate a large variety of branched nanostructures, far more complex than what exists today. These nanostructures include carbon nanotubes and metallic nanowires having several hierarchical levels of multiple branching. The number and frequency of branching, dimensions, and the overall architecture are controlled precisely through pore design and templated assembly. The technique provides a powerful approach to produce nanostructures of greater morphological complexity, which could have far-reaching implications in the design of future nanoscale systems.T he design and controlled synthesis of complex nanowire (1-5) and carbon nanotubes (CNTs) (6-8) will impact developments in nanotechnology applications. The present synthesis approaches, however, limit the degree of complexity that can be controllably configured into these structures. Fabrication inside rationally designed porous templates [such as anodic aluminum oxide (AAO) templates] is ideal to produce nanowire morphologies, but this feat has been accomplished controllably only for linear (9-15) and Y-shaped (6, 7) architectures. The creation of controlled pore structure, with various levels of complexity, inside these templates provides a powerful way to produce predesigned multiply connected and branched nanowires and nanotubes. We have developed a rational approach for creating hierarchically branched nanoporous AAO templates and have fabricated a whole generation of branched nanowires and nanotubes inside these templates. As a suitable example, we detail the case of CNT structures in this work, but other material systems can also be made into similar architectures, as highlighted for the case of metallic nanowires in Materials and Methods. Materials and MethodsPreparation of AAO Templates. AAO templates were prepared by using a modified two-step anodization process (6, 16). The first-step anodization was the same for all templates. High-purity Al foils were anodized in 0.3 M oxalic acid solution at 8-10°C under a constant voltage (in the range of 40-72 V dc ) for 8 h. Then, the formed anodic aluminum layer was removed. In the second-step anodization, templates with different pore architectures underwent different processes of anodization as follows. AAO Templates with Multiple Generations of Y-Branched Pores.We reduced the anodizing voltage multiple times in the second-step anodization. Initially, the anodization was performed under the same conditions as those in the first step to create the primary stem pores; then, the anodizing voltage was reduced by a factor of 1͞ ͌ 2 to form Y-branched pores. Two-, three-, and fourgeneration Y-branched pores can be obtained by further sequential reduction of anodizing voltages. It is noted that if a subsequent anodizing voltage is Յ25 V, after any prior anodization, the samples should be washed in deionized w...

Carbon nanotubes prepared by anodic aluminum oxide template method

“…Because the nanochannels of AAO films can be either one-side open or both-side open [30,31,86], the cap structure of AAO-CNTs can be controlled with good uniformity by selecting different types of AAO templates [33,38]. [87][88][89][90][91]. Li et al [87] first synthesized well aligned Y-junction CNTs using an AAO film with Y-branched nanochannels as template.…”

“…Li et al [87] first synthesized well aligned Y-junction CNTs using an AAO film with Y-branched nanochannels as template. Meng et al [88,89] reported a method for preparing AAO template with a hierarchically branched structure (as shown in Figure 9). After the initial anodization to form the stem pores, the anodizing voltage was reduced by a factor of 1 n , transforming a linear pore into a multiply branched pore.…”

Controlled Fabrication of Noble Metal Nanomaterials via Nanosphere Lithography and Their Optical Properties

“…Progresses in PAA-LIGA have shown its abilities not only in the synthesis of the traditional high aspect ratio nanomaterials, such as nanorods (Pan, Zeng et al 2000;Lombardi, Cavallotti et al 2007;Xu, Meng et al 2009)and nanowires (Hong, Bae et al 2001;Xu, Zhang et al 2005), but also some unique nanostructures, such as porous metallic nanorods by galvanic exchange reaction (Mohl, Kumar et al 2010), Y-junction nanowires and multiply branched nanomaterials due to its flexibility in the pore structure control according to the principle 1 s V n  , where V s is the anodizing voltage for stem pores and n is the number of branched pores from that stem (Meng, Jung et al 2005;Xu, Meng et al 2009). The combination of NSL and PAA-LIGA has been further developed to create hierarchically ordered nanowire arrays, as schematically shown in Figure 13A (Chong, Zheng et al 2006).…”

Carbon Nanotube-Based Thin Films: Synthesis and Properties

“…C 60 and CNTs, as novel all-carbon -electron systems, have increasingly invited exploration for preparing their composite films from both fundamental and practical points of view. It is known that clusters of C 60 [118][119][120] and carbon nanostructures [121] can be deposited electrophoretically onto electrodes to form a film.…”

“…Shortly after the discovery of CNTs, several growth methods were developed to synthesize different forms of CNTs in a controlled manner, such as arc discharge, [55] pulsed laser deposition, [56] and catalytic CVD (CCVD). [57] For CCVD, there are several specialized versions, such as hot wire, [58] plasma-enhanced, [59] and template [60] CCVD, which are the most commonly utilized techniques today. Among those methods listed above, CCVD techniques show the great advantage that when applied on prepatterned substrates or catalyst particles, well-aligned CNT films similar to the prepatterned template can be made.…”