Hard template fabrication

The utilization of soft templates is helpful for the construction of rare earth hydroxide nanotubes. The s)mthesis of Y(OH)3 nanotubes could be assisted by PEG (Tang et al., 2003) or grafted with PMMA (Li et al., 2004 Mo et al., 2005). Hard templates like A AO are also studied for the fabrication of rare earth hydroxide nanowires (Bocchetta et al., 2007). 

On the other hand, the soft template method involves cooperative assembly between the structure-directing agents (usually surfactants) and organic precursor species in solution. Therefore, the carbon structures obtained via soft templating are more flexible and their formation is dependent on temperature, type of solvent and ionic strength. However, there are currently only limited examples for the successful fabrication of porous carbon via the soft template method, which were reviewed recently by Wan et alP Soft template and hard template routes have been classified as endotemplate and exotemplate, respectively. 

Other Ordered Mesoporous Silicas as Hard Template Other mesoporous silicas including MCM-41, HMS, MSU-1, MSU-H, SBA-1, SBA-7, SBA-12 and SBA-16 have also been explored as hard templates to fabricate mesoporous carbons. Mesoporous silica MCM-41, which has hexagonally ordered cylindrical one-dimensional pores, was found to be unsuitable as a template for mesoporous carbon as its use yielded disordered high surface area microporous carbon. This is due to the absence of complementary micropores within the MCM-41 silica walls. However, Tian et al. have successfully produced self-supported, ordered, ultrathin carbon nanowire arrays by employing the mesoporous silica MCM-41 as template.The carbon nanowire arrays exhibit high surface areas up to 1400 m g , large pore volumes of 1.1 cm g and uniform mesopore size of ca 2.2 nm. 

A complementary approach to the fabrication of nanotubes involves the use of hard templates as tools. Hard templates are either nanofibers or porous host materials. In the former case, the nanofibers are at first coated with the waU material of the tubes or a corresponding precursor. Subsequently, the template fiber, that is, the core of the hybrid fiber thus obtained, is selectively removed so that a shell of the material initially deposited onto the template nanofiber is conserved. Template fibers can, for example, be produced in high... 

Despite these still-challenging issues, the fabrication of nanotubes using nanoporous hard templates is associated with several advantages beyond the possibility to align them. Readily available porous hosts such as self-ordered porous AAO have a narrow pore diameter (Dp) distribution and pores with Dp-values ranging from about 20 nm up to a few lOOnm. The pore depths (Tp) can be adjusted to values between about 1 im and several 100 xm. Therefore, it is easily possible to tailor the diameter and the aspect ratio of the nanotubes. However, the most important advantage is the possibility to control mesoscopic structure formation processes inside the pores. There are relatively few limitations regarding the materials that can be formed into nanotubes via hard templates. Mixtures, sols, semicrys-... 

Only few attempts have been made to rationally design the mesoscopic fine structure of nanotubes fabricated by wetting nanoporous hard templates with polymeric solutions. Chen et al. infiltrated solutions of PS-fo-PAN in DMF into AAO. As described above, the PAN was at first crosslinked and then carbonized. However, the PS domains were converted into holes, and porous amorphous carbon nanotubes could be fabricated [89]. hi a similar approach, Rodriguez et al. used a solution of PS-fo-PVP and carbohydrates associated with the PVP blocks via hydrogen bonds in DMF into AAO and obtained mesoporous amorphous carbon nanotubes with the positions of the mesopores determined by the positions of the PS domains. Solvent annealing of the BCP/hydrocarbon films in DMF/benzene vapor led to a significantly... 

Next section covers extensive discussions of various fabrication methods for conducting polymer nanomaterials in detail. This section is divided by the soft template method, hard template method, and template-free method. 

In general, template method is classified by soft and hard templates. Whereas anodic aluminum oxide (AAO) membrane, track-etched polycarbonate (PC) and zeolite can be used as hard templates, soft templates include surfactant, cyclodextrin, liquid crystal, etc. Compared with soft and hard templates, template-free method represents the fabrication technique of conducting polymer nanomaterials without the template, which is discussed in this section [115]. 

In general, PPy nanotubes have been mainly produced by the hard template method [165,172,225,226,247,248]. For example, PPy nanotube with highly imiform surface and controlled waU thickness was fabricated by one-step VDP using AAO membrane [172]. A template-mediated VDP was foimd to be a facile and effective method to fabricate polymer nanotubes. The vapor phase polymerization provides highly uniform tubular walls as well as easy control over the waU thickness. 

Recently, a facile soft template synthesis was developed for fabricating PPy nanotubes against the hard template synthesis [153,249]. PPy nanotubes could be readily produced through a cylindrical micelle templating in re-... 

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