Periodic mesoporous silicas and organosilicas attracted much attention for their potential applications for catalysis and separation ^{[1, 2, 3]}. The morphology,the pore architecture,and the crystallinity of the wall were intensively studied and manipulated. For the morphologies,helical nanostructures have been widely studied on the handedness control ^{[4, 5]} and the chiral pore architectures ^[6],the chirality of the walls,the formation mechanism ^[7] and potential applications ^[8]. Although the handedness of the helical nanostructures can be controlled using the self-assemblies of the low-molecular-weight gelators (LMWGs) as the templates through the sol-gel transcription approach,the synthesis of the LMWGs is tedious ^{[4, 5]}. It was reported that helical mesoporous silica and organosilica nanofibers could be prepared using achiral surfactants. The formation of the helical nanostructures was proposed ^[7] to be driven by the reduction of surface free energy and increase of entropy ^[6]. During the last decade,ordered mesoporous carbons have been also widely studied for their potential applications in the fields of water purification,catalysis,and supercapacitors. Both hard and soft templating approaches were developed for the preparation ^[9]. Recently,mesoporous carbons with spherical and flake-like morphologies have been successfully prepared ^[10]. Although mesoporous silica,organosilica,and carbon have been intensively studied,the reports on preparing mesoporous carbon/silica composites are rare ^{[11, 12]}. A simple approach for the preparation of mesoporous carbon/silica composites is the carbonization of mesoporous organosilicas ^[11]. Although the framework would shrink after carbonization,the morphologies might be retained. Here,we report the preparation of helical mesoporous carbon/ silica nanofibers with lamellar mesopores on the surfaces and twisted rod-like micropores inside using helical mesoporous 1,2- ethylene-silica nanofibers through carbonization. 2. Experimental 2.1. General methods

Transmission electron microscopy (TEM) images were obtained using an FEI TecnaiG220 at 200 kV. Field-emission scanning electron microscopy (FE-SEM) was performed using a Hitachi 4800 instrument at 800 V. Small angle X-ray diffraction (SAXRD) and wide-angle X-ray diffraction (WAXRD) patterns were obtained using an X' Pert-Pro MPD X-ray diffractometer using Cu Kα radiation with a Ni filter (1.542Å ). Specific surface area and poresize distribution were determined by the Brunauer-Emmett- Teller (BET) and Barrett-Joyner-Halon (BJH) methods using N₂ adsorption isotherm measured by a Micromeritics Tristar II 3020 instrument. 2.2. Materials

The mesoporous 1,2-ethylene-silica nanofibers were prepared according to the literature procedures ^[13]. 2.2.1. Synthetic procedures for the carbon/silica nanofibers

The mesoporous 1,2-ethylene-silica nanofibers were carbonized at 900℃ for 4.0 h with a heating rate of 3.0℃/min in Ar. The carbon/silica nanofibers were obtained by cooling the mixture to room temperature naturally. 2.2.2. Synthetic procedures for the silica nanofibers

The carbon/silica nanofibers were calcined at 300℃ for 3.0 h and 700℃ for 5.0 h with a heating rate of 3.0℃/min in air. The silica nanofibers were obtained by cooling the mixture to room temperature naturally. 3. Results and discussion

We have reported that the helical 1,2-ethylene-silica nanofibers with lamellar mesopores on the surfaces and twisted rod-like channels inside could be prepared using cetyltrimethylammonium bromide and (S)-β-citronellol ^[13]. The lamellar mesopores were formed by merging the hexagonally arranged cylinder-like silica/ surfactant micelles. After carbonization,carbon/silica nanofibers were obtained. The FE-SEM and TEM images of the carbon/silica nanofibers were shown in Fig. 1. To reveal the mesopores on the surfaces of the nanofibers,the FE-SEM images were taken using the beam deceleration method without covering metal. Left- and righthanded helical nanofibers with lamellar mesopores were identified on the surfaces of the nanofibers (Fig. 1a). These nanofibers are 0.2-2.0 mm in length,70-150 nm in diameter,and 0.5-1.0 μm in helical pitch. The TEM image shown in Fig. 1c shows several sets of fringes within a single helical nanofiber,which further indicates the pore channels are chiral and parallel to each other ^[6]. The pore diameters calculated from the TEM images are about 1.6 nm. It was reported previously that the diameters and pore diameters of the helical 1,2-ethylene-silica nanofibers were 100-200 nm and 2.8 μm,respectively ^[13]. Apparently,both of them decreased after carbonization. Several nanofibers with circular lamellar mesopores were also found (Fig. 1b). TEM indicated that hexagonal and parallel-arranged pore channels wound circularly around and were nearly perpendicular to the long axis of the nanofiber (Fig. 1d). The pore diameters calculated from the TEM images are also about 1.6 nm.

Fig. 1

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Fig. 1.FESEM (a and b) and TEM (c and d) images of the porous carbon/silica nanofibers.

SAXRD and WAXRD patterns of the carbon/silica nanofibers were shown in Fig. 2. Before carbonization,three well-resolved diffraction peaks at 2θ of 2.01,3.45 and 3.95° were identified ^[13].The carbonization damaged the highly ordered pore architecture.Only one well-resolved diffraction peak at 2θ of 3.02° was identified in the SAXRD pattern (Fig. 2a). The d-spacing calculated from this diffraction peak is 2.9 nm. Because the pore channels within the nanofibers organized in a two-dimensional hexagonal symmetry,the distance between the adjacent pore centers is about 3.3 nm. Therefore,the thickness of the walls is about 1.7 nm. The WAXRD pattern of the carbon/silica nanofibers shows a broad peak at 2θ of 21.8°,indicating an amorphous structure (Fig. 2b). The mesoporosity of the sample was characterized using a nitrogen sorption analysis (Fig. 3a and b). The sample shows type-I-like isotherms with a H3 hysteresis loop,indicating a microporous structure with slit-like mesopores. It exhibits a nitrogen BET surface area of 476 m²/g. The BJH pore size distribution plot calculated from the desorption branch shows a peak at 4.0 nm. The diameters of the lamellar mesopores on the surfaces of the nanofibers should be about 4.0 nm.

Fig. 2

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Fig. 2.(a) SAXRD and (b) WAXRD patterns of the porous carbon/silica and silica nanofibers.

To better understand this porous structure,the carbon or the silica was selectively removed. After the carbon/silica nanofibers were calcined at 700℃ for 5.0 h in air,mesoporous silica nanofibers were obtained. Lamellar mesoporous were identified on the surfaces of the nanofibers (Fig. 4a). The two-dimensional hexagonally arranged pore channels are chiral and parallel to each other (Fig. 4b and c). Calculated from the TEM images,the pore diameters are about 1.6 nm,which are same as those of the carbon/ silica nanofibers. For the nanofibers with circularly lamellar mesopores,concentric circular micropores were identified. SAXRD and WAXRD patterns of the silica nanofibers were also shown in Fig. 2. The removal of carbon damaged the periodic arrangement of the pore channels. No diffraction peaks were identified in the SAXRD pattern (Fig. 2a). The WAXRD pattern of the silica nanofibers shows a broad peak at 2θ of 21.1°,indicating an amorphous structure (Fig. 2b). The sample shows type-I-like isotherms with a H3 hysteresis loop,indicating a microporous structure with slit-like mesopores (Fig. 3a). It exhibits a nitrogen BET surface area of 118 m²/g. The BJH pore size distribution plot calculated from the desorption branch shows a peak at 4.2 nm (Fig. 3b). The diameters of the lamellar mesopores on the surfaces of the nanofibers increased with the removal of the carbon. However,when silica was removed from the carbon/silica nanofibers using NaOH aqueous solution,carbon particles were obtained (not shown here). Therefore,the framework of the obtained carbon/silica nanofibers was constructed by cross-linked silica network and isolated carbon nanoparticles. Comparison of the isotherm plots of carbon/silica and silica nanofibers shown in Fig. 3a revealed that the removal of the carbon reduced the quantity of both micropores and mesopores. Therefore,the BET surface areas decreased from 476 m²/g to 118 m²/g. It was reported that the surface area of the 1,2-ethylene-silica nanofibers was 586 m²/g ^[13]. After carbonization,the nanofibers shrank and the pore diameters decreased (Fig. 1). As a consequence,the BET surface area decreased.

Fig. 3

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Fig. 3.(a) Nitrogen sorption isotherms and (b) BJH pore size distribution plots calculated from the desorption branch of the porous carbon/silica and silica nanofibers.

Fig. 4

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Fig. 4.FESEM (a) and TEM (b-d) images of the porous silica nanofibers.

Helical carbon/silica nanofibers with lamellar mesopores on the surfaces and twisted rod-like micropores inside were prepared by the carbonization of helical 1,2-ethylene-silica nanofibers with lamellar mesopores on the surfaces and twisted rod-like channels inside. After carbonization,both the diameters and the pore diameters of the nanofibers shrank. The framework of the obtained carbon/silica nanofibers should have been constructed by crosslinked silica network and isolated carbon nanoparticles. This kind of materials can be potentially used as catalyst supports. Moreover, single-handed helical mesoporous carbon/silica and carbonnanofibers could also be potentially prepared using the approach shown here,which might exhibit optical chirality. Acknowledgments

This work was supported by Natural Science Foundation of Jiangsu Province (No. BK2011354),the Priority Academic Program Development of Jiangsu High Education Institutions (PAPD),and the National Natural Science Foundation of China (No. 21104053,21071103 and 21074086). I