A template method to control the shape and porosity of carbon materials
Introduction
Porous carbon materials with high surface areas and large pore volumes are widely employed in many fields of science and technology, including catalysis, purification, energy storage and separation [1], [2]. Originally they were prepared by carbonizing various hydrocarbons such as wood, phenol resin [3], a copolymer of styrene and divinylbenzene etc. Generally these carbon materials were irregular and exhibited small pores or broad pore size distributions, which limited their applications. Modern industry needs carbon materials with a definite macroscopic morphology, a suitable pore size as well as a narrow pore size distribution.
Currently there is a tendency to use a template method for producing carbon materials. In 1997 Kamegawa and Yoshida [4] prepared swelling carbon beads using silica gel as template. Kyotani and coworkers [5], [6] reported the preparation of ordered microporous carbons in zeolite nanochannels. Later Ryoo and coworkers [7], [8], [9], [10] reported the synthesis of ordered mesoporous carbons of the CMK family using ordered mesoporous silica particles such as MCM-48, SBA-1, and SBA-15 as templates. Pinnavaia and coworker [11] have prepared C-MSU-H using MSU-H as a hard template. Yoon et al. [12] synthesized carbon molecular sieves by carbonizing divinylbenzene polymer in the pores of trimethylsilyl chloride modified MCM-48. Hyeon and co-workers [13], [14] demonstrated that mesoporous carbons could be prepared by using silica sols as templates. In addition, Hyeon and coworkers [15], [16], [17] also reported the preparation of carbon foam and carbon with bimodal pores by using approximate silica templates. Due to the ordered or homogeneous structures of the silica templates, the obtained carbon materials usually exhibited narrow pore size distributions.
Though carbon materials with narrow pore size distributions have been synthesized, most of them are in the form of powder and their pore sizes cannot be freely tailored due to the silica template limitation. For the development of their actual application, it is necessary to fabricate them into various macroscopic shapes with tunable pore sizes. Recently, several groups [18], [19], [20], [21] have reported the synthesis of carbons with monolithic shape. Ryoo et al. [22] have attempted to adjust mesopore size of carbon materials. However, it is far from enough because various shaped carbons with tunable pore size in a wide range are required for different applications.
A silica monolith, which was prepared by Nakanishi and Soga [23], [24] via the combination of phase separation and sol–gel processes, exhibits both co-continuous structure and textural porosity; the co-continuous structure results from the interconnected silica skeletons and the interconnected textural pores with their size of several micrometers, while the skeleton pores exist in the silica skeletons with the pore size in the nanometer scale. Furthermore, the shape and textural pore/skeleton pore size of the silica monolith are easily controlled by the adjustment of the preparation conditions. The unique property of the silica monolith presents itself an attracting template for carbon replica.
Here, we adopted such a template and realized morphology and pore size control of carbon materials.
Section snippets
Materials and reagents
Tetramethoxysilane (TMOS) was obtained from the Chemical Factory of Wuhan University (Wuhan, China). Acetic acid, ethanol, sulfuric acid, sucrose and poly (ethylene glycol) (PEG, Mw=10,000) of analytical grade were purchased from Shanghai General Chemical Reagent Factory (Shanghai, China). Water was distilled from a quartz apparatus.
Preparation of silica monoliths
The silica monolith was prepared according to the previous reported method with modification [25]. Generally, TMOS, PEG and acetic acid were mixed together at 273 K
The composition and atomic array of the carbon monoliths
A carbon monolith was chosen as a representative for XPS analysis to determine the silica residue. XPS at all areas including the surface and the interior show signals of carbon and oxygen, and Si element can be hardly detected, indicating that the silica ingredient has been removed from the silica–carbon composite. In order to confirm the conclusion, gravimetric analysis was adopted. After calcination, the carbon was totally burned off while the possible silica would be left. By weighing the
Conclusions
A template method has been used for the preparation of carbon materials of various shapes and pore sizes. The shapes of the carbons are predetermined by the forms of the silica templates, while the skeleton pore size and textural pore size of the carbons are predicted by the skeleton wall thickness and the total skeleton size of the silica templates. By delicate template selection, carbon monoliths in cylindrical, triangular, square, loop, and pentagonal shapes can be synthesized, with their
Acknowledgements
The authors acknowledge the support of the National Nature Science Foundation of China (Grant:20275029) and the Excellent Young Teachers Program of MOE, China.
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