Template Synthesis of Carbon Nanofibers Containing Linear Mesocage Arrays
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Carbon nanofibers containing linear mesocage arrays were prepared via evaporation induced self-assembly method within AAO template with an average channel diameter of about 25 nm. The TEM results show that the mesocages have an elongated shape in the transversal direction. The results of N2 adsorption–desorption analysis indicate that the sample possesses a cage-like mesoporous structure and the average mesopore size of the sample is about 18 nm.
KeywordsTemplate synthesis Mesoporous carbon Nanofiber AAO Linear mesocage array
Mesoporous nanofibers have attracted scientific interest over the last few years because they possess the specialities of one-dimensional nanomaterials as well as the properties of mesoporous carbon materials. The current methods to prepare mesoporous nanofibers mainly focus on the use of anodized aluminum oxide (AAO) membrane combined with the evaporation induced self-assembly (EISA) method [1, 2, 3]. The EISA method is a simple but efficient way for the rapid preparation of mesoporous materials. Usually, the evaporation of ethanol concentrate the nonvolatile surfactant and precursor, thereby inducing the self-assembly of precursor-surfactant micelles and their further organization into liquid-crystalline mesophase . Mesoporous nanofibers of various materials have been prepared successfully by the aforementioned combined technique, in which mesoporous silica nanofibers have been studied extensively. The diameter of AAO channel plays an important effect on the mesostructure of nanofiber. Wu et al.  successfully prepared silica nanofibers with several kinds of mesostructure, such as stacked doughnuts, helix, core-shell, and inverted peapod forms, using AAO template with nanochannel diameters in the range 20–73 nm. A single line of aligned mesocages with an average cage size of 14 nm was obtained when the channel diameter ultimately reached 18 nm . Chen et al.  also reported the preparation of silica nanofibers containing linear arrays of mesocages based on the use of AAO template with channel diameter of 35 nm. The nanofibers containing linear mesocages arrays represent an interesting platform for various applications, such as drug delivery, sensor technology, and waveguiding devices [7, 8]. To our best knowledge, there is no report about the fabrication of carbon-based analogs of these mesostructures until now. The carbon-based analogs will possess significant advantages over these silica nanofibers, such as stability, electrical conductivity, and enhanced chemical functionality.
Recently, our group has studied the restriction effect of AAO template for the shrinkage of mesoporous polymer and prepared mesoporous carbon nanofibers with large mesopore using AAO with channel diameter in the range 50–300 nm . Herein, we report the preparation of carbon nanofibers containing linear mesocage arrays via evaporation induced self-assembly method within AAO template with average channel diameter of 25 nm.
Preparation of Anodic Aluminum Oxide Template
The AAO templates with an average pore diameter of 25 nm were prepared by anodizing aluminum foil (purity 99.5%) at 20 V in 0.5 M sulfuric acid solution. The AAO templates with an average pore diameter of 200 nm were prepared at 120 V in 0.36 M phosphoric acid, followed by chemical etching with 0.3 M phosphoric acid for 1 h.
Preparation of Carbon Nanofibers
The precursor solution used to synthesise the carbon nanofibers is similar to that reported by Meng et al. . For a typical synthesis, 1.0 g of F127 was dissolved in 20.0 g ethanol, and then 20.0 g of resol-ethanol solution containing 1.22 g of phenol and 0.78 g of formaldehyde was added. After stirring for 10 min, a homogeneous solution was obtained. Several AAO membranes with average channel diameter of 25 nm or 200 nm were added into the solution. After the evaporation of ethanol at room temperature, the AAO templates were fully infiltrated with F127/resol composite. The infiltrated AAO templates were heated at 100°C for 24 h to thermopolymerize phenolic resins. The carbonization process was conducted in N2 at 350 or 700°C for 3 h with a heating rate of 1°C·min−1. After the AAO templates were removed by using 10% aqueous HF, the products were referred as PNF-x (for 350°C heat treatment) and CNF-x (for 700°C heat treatment), respectively, where PNF is the abbreviation of polymer nanofibers, CNF is the abbreviation of carbon nanofibers, x indicates the average channel diameter of the AAO template.
The obtained products were characterized by transmission electron microscopy (TEM; FEI, TACHAI-20), and nitrogen adsorption–desorption analysis (Micromeritics, ASAP2010).
Results and Discussion
In summary, we have demonstrated the synthesis of carbon nanofibers containing linear mesocages arrays using AAO templates with an average channel diameter of 25 nm. The mesocages have an elongated shape in the transversal direction and possess a large pore size of about 18 nm.
This work was supported by Natural Science Foundation of Jiangsu Province (BK2006195).
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