快速積炭法制備形貌可控的中空納米炭材料

劉洪陽，　馮振豹，　王　嘉，　刁江勇，　蘇黨生

(中國科學院金屬研究所 沈陽材料科學國家(聯合)實驗室, 遼寧 沈陽110016)

?

快速積炭法制備形貌可控的中空納米炭材料

劉洪陽，馮振豹，王嘉，刁江勇，蘇黨生

(中國科學院金屬研究所 沈陽材料科學國家(聯合)實驗室, 遼寧 沈陽110016)

介紹了一種大規模制備形貌可控的中空納米炭材料的方法。該方法以金屬氧化物為模板，利用乙苯分子在金屬氧化物表面高溫快速產生的積炭為炭源，除去模板后可以得到具有中空結構的納米炭材料。該方法簡單、高效、低成本，具有普適性，制備過程中不需要使用昂貴的表面活性劑為炭源。利用該方法，以實驗室制備的氧化鋅納米棒和商業的氧化鋅納米球為模板，分別成功的制備出中空納米炭管和中空的納米炭球。該合成方法可以進一步推廣到制備其它形貌的中空納米炭材料并用于催化和能量儲存等領域。

納米炭材料； 中空結構； 形貌可控； 模板法； 積炭反應

1　Introduction

Recently, the design and synthesis of hollow carbon nanostructures have attracted considerable attention owing to their potential applications in catalyst supports, gas storage and separation, and lithium-ion batteries[1, 2]. Up to now, the hollow carbon nanostructures have been synthesized by a template method and a hydrothermal method. Among them, the template method is more acceptable because the structure of as-prepared hollow carbon nanomaterials could be controlled by tuning the size or the morphology of the templates[3, 4]. In general, for the template method, a core-shell structure is synthesized by coating a carbon precursor on a hard template core, followed by carbonization and core removal to obtain the hollow carbon nanostructures. The template approach based on the use of solid molds also provides opportunities for synthesizing various carbon nanostructures[5-8]. However, the expensive surfactant is usually required by functionalizing or modifying the template due to the incompatibility between the template surface and shell material, resulting in a complicated fabrication process and high cost[9]. Thus, developing an efficient and low cost process to synthesize the hollow carbon nanostructures is still challenging.

Herein, we reported a facile and scalable synthesis of hollow carbon nanostructures by a template method assisted with a fast coking process at high temperatures. Through this efficient and low cost method, the hollow carbon nanotubes and hollow carbon nanospheres can be fabricated by using ZnO nanostructures as templates. The various ZnO nanostructures as template materials can be used to fabricate complex carbon nanostructures. In addition, the ZnO templates can be easily dissolved and removed in mild acids or bases[10,11]. The synthetic procedure of hollow carbon nanostructures is illustrated in Fig. 1. Firstly, the as-prepared ZnO nanorods and commercial available ZnO nanospheres were chosen as the templates. Subsequently, a carbon layer was uniformly coated on the surface of ZnO templates (C@ZnO) by a fast coking process with diluted elthylbenzene at 700 ℃ for just 2 min. Afterwards, the C@ZnO nanocompsite was treated with HCl solution at room temperature, and the hollow carbon nanotubes and hollow carbon nanospheres were obtained.

Fig. 1　A schematic illustration of the preparation of hollow carbon nanostructures.

2　Experimental

The ZnO nanorod was synthesized by a typical physical vapor deposition method under atmosphere pressure[12]. The source materials, consisting of a mixture of 0.5 g of ZnO powder and 0.5 g of active carbon, were loaded into a ceramic boat that was positioned in the middle of a quartz tube. The silica wafer was used as the substrate to collect samples, which was placed downstream the source material in the temperature zone of 700 ℃. Prior to heating, the quartz tube was purged with Ar (30 mL/min) for 30 mins. Then, it was heated up to 1 000 ℃ under the mixture of Ar (50 mL/min) and O2(10 mL/min) for 1 h. The surface of the silica substrate became white after the reaction, indicating that ZnO nanorod was deposited on it. The ZnO nanospheres with sizes of 50-100 nm were bought from Sinopharm Chemical Reagent Co., Ltd. The C@ZnO nanocomposite was prepared by a fast coking process with the ZnO template under a mixed gas flow (100 mL/min) of 2% ethylbezene balanced with He at 700 ℃ for just 2 min. After the ZnO template was removed with a 5% HCl aqueous solution for 2 h, the hollow carbon naostructures were successfully collected. Transmission electron microscopy (TEM) was performed by a Tecnai G2 F20 S-TWIN electron microscope operated at 200 kV. Raman spectroscopy was performed on a LabRam HR 800 using a 633 nm laser.

3　Results and discussion

Fig. 2a and 2d show typical SEM and TEM images of the ZnO nanorods prepared by the traditional chemical vapor deposition method. Note that the as-synthesized ZnO nanorods are uniform in shape and size. The average length and width of the ZnO nanorods is 2-10 μm and 50-200 nm, respectively. The as-prepared ZnO nanorods were then employed as templates for the growth of the carbon layer by a fast coking process of ethylbenzene at 700 ℃ for just 2 min. Our previous study demonstrated that the coking reaction of ethylbenzene on the metal oxide catalyst surface can be quickly happened at high temperatures[13]. Fig. 2b and 2d present the TEM images of the ZnO nanorods after they are coated by the fast coking process. It can be seen that a carbon layer is uniformly coated on the middle part and even the tip of one individual ZnO nanorod. There is no gap between the carbon layer and ZnO nanorods as displayed in the HRTEM images in Fig. 2c and 2f. The average thickness of the carbon layer is around 5-10 nm. In addition, it is found that the morphology and the structure of ZnO nanorods are well maintained after being coated by the carbon layer. The TEM image in the Figure S1 provides further the evidence that the whole of one single ZnO nanorod is completely covered by a uniform carbon layer through this fast coking process.

Fig. 3a shows a representative TEM image of the finally obtained hollow carbon nanotubes after ZnO nanorods are leached away by the diluted HCl solution at room temperature. Note that all the as-prepared carbon nanostructures exhibit tubular structures (Fig. 3b). In particular, these tubular structures almost are replicated from the initial morphologies of the ZnO nanorods. The hollow carbon nanotubes resulting from the ZnO nanorods have inner diameters of 50-200 nm, and thickness of 5-10 nm with quite smooth walls. Meanwhile, there are big holes generated on the tip of some hollow carbon nanotubes (Fig. 3c). HRTEM image in Fig. 3d presents that the graphitic layers fabricated in the hollow carbon nanotubes are inconsecutive. All the above TEM results indicate that we have developed an efficient and facile way to prepare hollow carbon nanotubes though the feasible template method.

Fig. 2　(a, d) SEM and TEM images of the ZnO nanorods as the template,TEM images of the carbon layer coated on one single ZnO nanorod (b,c) in the middle part and (e-f) in the tip part.

Fig. 3　(a-c) TEM images of the hollow carbon nanotubes after the ZnO nanorods are removed by the HCl solution,(d) high-magnification TEM image of the obtained hollow carbon nanotubes.

Fig. 4　TEM images of (a) ZnO nanospheres, (b,c) ZnO nanospheres coated by the carbon layer,(d-f) as-prepared hollow carbon nanoshpheres after ZnO template removal.

The commercial available ZnO nanospheres (50-100 nm in diameter, Fig. 4a) were employed as the template to synthesize the hollow carbon nanospheres using the same process. As displayed in Fig. 2b, the coking reaction of ethylbenzene is still facilely happened on the ZnO nanoshperes. A carbon layer is evenly and tightly grown on the surface of ZnO nanospheres (Fig. 4c). The initial structure of the ZnO nanoshperes retains well after the coking process. The low-magnification TEM image of the as-prepared hollow carbon nanospheres after the ZnO nanospheres are removed is presented in Fig. 4e, which indicates that the hollow carbon nanospheres can be efficiently prepared in large scale. Meanwhile, we also found that the neighboring hollow carbon nanospheres are linked together to form an interconnected nanostructure (Fig. 4e). The HRTEM image in Fig. 4f reveals the inconsecutive graphite nature in the as-prepared hollow carbon nanospheres.

Fig. 5　Raman spectra of the (a) as-prepared hollow carbon nanotubes and (b) hollow carbon nanospheres.

Raman spectroscopy is one of effective tools to characterize the structure of hollow carbon nanostructures. It is generally considered that the intensity ratio ofDband andGband (ID/IG) represents the disorder degree of carbon materials. The band Fig. 5 at 1 363 cm-1corresponds to theDpeak arising from the breathing motion of sp2rings, and the band at 1 605 cm-1is in good agreement with theGband. The measured intensityID/IGratios of the hollow carbon nanotubes and hollow carbon nanospheres are about 3.3 and 3.5, respectively, indicating that the amorphous phase plays an important role in obtaining the carbon layer, agreeing well with the TEM observations. The nitrogen adsorption-desorption isotherms of the obtained hollow carbon nanostructures are shown in Fig. 6. The isotherm exhibits a typical IV-type curve, suggesting the presence of mesopores. The specific surface area of the hollow carbon nanotubes and hollow carbon nanospheres are calculated to be 245 and 382 m2·g-1, respectively. A broad pore size distribution (inset in Fig. 6) reveals that the as-prepared hollow carbon nanostructures have abundant porous channels that favor a high permeation and mass-transfer rate for the reactants.

Fig. 6　Nitrogen adsorption/desorption isotherms of the (a) hollow carbon nanotubes

4　Conclusions

We have proposed a template assisted approach without using surfactant for a large scale synthesis of hollow carbon nanostructures through a fast coking process with diluted ethylbenzene at high temperatures. The present process is facile, efficient and low cost. The hollow carbon nanotubes and hollow carbon nanospheres are successfully fabricated by employing as-prepared ZnO nanorods and commercial available ZnO nanospheres as the templates. These as-prepared hollow carbon nanostructures are suitable for catalytic supports or energy storage.

[1]Lu A H, Sun T. Synthesis of discrete and dispersible hollow carbon nanospheres with high uniformity by using confined nanospace pyrolysis[J]. Angewandte Chemie International Edition, 2011, 122: 9023.

[2]Yang S B, Feng X L, Klaus M, et al. Nanographene-constructed hollow carbon spheres and their favorable electroactivity with respect to lithium storage[J]. Advanced Materials, 2010, 22: 838.

[3]Shao Q G, Tang J. Synthesis and characterization of graphene hollow spheres for application in supercapacitors[J]. Journal of Material Chemistry A, 2013, 1: 15423.

[4]Xie K, Qin X T, Hu Z, et al. Nitrogen-doped carbon nanocages as efficient metal-free electrocatalysts for oxygen reduction reaction[J]. Advanced Materials, 2012, 24: 347.

[5]Lu A H, Hao G, Sun Q, et al. Can carbon spheres be created through the st?ber method[J]. Angewandte Chemie International Edition, 2011, 50: 9023.

[6]Peng H J, Guo X F, Zhang Q, et al. Catalytic self-limited assembly at hard templates: A mesoscale approach to graphene nanoshells for lithium-sulfur batteries[J]. ACS Nano, 2014, 8: 11280.

[7]Zhu L, Peng H, Zhang Q, et al. Interconnected carbon nanotube/graphene nanosphere scaffolds as free-standingpaper electrode forhigh-rateandultra-stable lithium-sulfur batteries[J]. Nano Energy, 2015, 11: 746.

[8]Strubel P, Thieme S, Kaskel S, et al. ZnO hard templating for synthesis of hierarchical porous carbons with tailored porosity and high performance in lithium-sulfur battery[J]. Advanced Functional Materials, 2015, 25: 287.

[9]Liu R, Li C, Dai S, et al. Dopamine as a carbon source: The controlled synthesis of hollow carbon spheres and yolk-structured carbon nanocomposites[J]. Angewandte Chemie International Edition, 2011, 50: 6799.

[10]Gong W, Chen W, Zhang X J, et al. Substrate-independent and large-area synthesis of carbon nanotube thin films using ZnO nanorods as template and dopamine as carbon precursor[J]. Carbon, 2015, 83: 275.

[11]Mbuyisa P, Bhardwaj S, Cepek, P, et al. Controlled synthesis of carbon nanostructures using aligned ZnO nanorods as templates[J]. Carbon, 2015, 50: 5472.

[12]Liu H Y, Zhang L Y, Su D S, et al. Palladium nanoparticles embedded in the inner surfaces of carbon nanotubes: Synthesis, catalytic activity, and sinter resistance[J]. Angewandte Chemie International Edition, 2014, 53: 12634.

[13]Liu H Y, Su D S. A nanodiamond/CNT-SiC monolith as a novel metal free catalyst for ethylbenzene direct dehydrogenation to styrene[J]. Chemical Communications, 2014, 50: 7810.

Synthesis of hollow carbon nanostructures using a ZnO template method

LIU Hong-yang,FENG Zhen-bao,WANG Jia,DIAO Jiang-yong,SU Dang-sheng

(ShenyangNationalLaboratoryforMaterialsScience,InstituteofMetalResearch,ChineseAcademyofSciences,Shenyang110016,China)

A ZnO template approach for the large scale synthesis of hollow carbon nanostructures was developed. A diluted ethylbenzene stream was used to form a carbon layer on the template at a high temperature and a dilute HCl solution was used to etch the template. Hollow carbon nanotubes and hollow carbon nanospheres were fabricated using ZnO nanorods and nanospheres as the respective templates. The present process is simple, efficient and low cost, and may be extended to fabricate other hollow carbon nanostructures used in catalytic reactions and energy storage.

Nanocarbon; Hollow structure; Shape-controlled; Template method; Coking reaction

date: 2015-12-05;Reviseddate: 2016-01-10

National Basic Research Program of China (973 Program, 2011CBA00504); National Natural Science Foundation of China (21203214, 21133010, 21261160487, 50921004).

SU Dang-sheng, Professor. E-mail: dssu@imr.ac.cn

introduction: LIU hong-yang, Associated Professor. E-mail: liuhy@imr.ac.cn

1007-8827(2016)01-0087-05

TQ127.1+1

A

國家重點基礎研究發展計劃(973計劃，2011CBA00504)；國家自然科學基金項目(21203214，21133010，21261160487，50921004).

蘇黨生，博士，研究員. E-mail: dssu@imr.ac.cn

作者介紹： 劉洪陽，博士，副研究員. E-mail: liuhy@imr.ac.cn

10.1016/S1872-5805(16)60006-9

English edition available online ScienceDirect ( http:www.sciencedirect.comsciencejournal18725805 ).