Research Article pubs.acs.org/journal/ascecg
Teflon: A Decisive Additive in Directly Fabricating Hierarchical Porous Carbon with Network Structure from Natural Leaf Yeru Liang,* Qiaoying Cao, Mingtao Zheng, Haobin Huo, Hang Hu, Hanwu Dong, Yong Xiao, and Yingliang Liu* College of Materials and Energy, South China Agricultural University, 483 Wushan Road, Tianhe District, Guangzhou 510642, P. R. China S Supporting Information *
ABSTRACT: Hierarchically porous carbons are of increasing importance due to their special physicochemical properties. The state-of-the-art approaches for synthesizing hierarchical porous carbon with network structure normally suffer from specific chemistries, rigid reaction conditions, high cost, and multiple tedious steps that limit their large scale production. Herein, we present an interesting insight into the important role of Teflon additive in fabrication of hierarchical porous carbon derived from biomass and, thus, use natural Indicalamus leaves for the first time to successfully synthesize hierarchical porous carbon with a three-dimensional morphology of interconnected nanoparticle units by using a facile and post-treatment-free carbonization technique. It is surprisingly found that the addition of Teflon not only reduces the synthesis procedure by combining post-removal of silica and carbonization in a single step but also plays a decisive role in generating the hierarchical carbonaceous network structure with a specific surface area as high as 1609 m2/g without any extra activation procedures. Benefiting from the combination of well-developed porosity and valuable hierarchical porous morphology, this type of hierarchical porous carbon has demonstrated attractive liquid-phase adsorption properties toward organic molecules. KEYWORDS: Hierarchical porous carbon, Natural leaf, Activation-free, No more post-treatment, Teflon
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INTRODUCTION Porous inorganic material is one of the most active areas in materials science, not only for its fundamental scientific interest but also for its irreplaceable role in many modern-day technological applications.1−6 Among these materials, hierarchical porous carbon (HPC) is very attractive because it can provide positive synergies between each pore system and then lead to multiple benefits.7−10 These intriguing structural characteristics permit their use in a variety of applications, including energy storage, adsorption ,and catalysis.11−14 Significant progress has been attained in structural, compositional, and topological control in HPC materials enabling enhanced performance in numerous applications.15−22 Due to the decisive role of porosity in determining porous carbon’s performance in many targeted applications, the past decades have particularly witnessed the generation of a spectrum of novel HPCs with high specific surface area. In general, HPCs can be prepared through many strategies, including templating, activation, and direct carbonization methods. Among various synthesis strategies, the templating method is recognized as an efficient way for generating HPC with well controlled architecture.4,23−28 The nanostructure parameters and morphologies of the templated HPCs could be tailored easily by varying the properties of templates and controlling their © 2017 American Chemical Society
integration. Nevertheless, it is normally difficult to obtain templated HPC with a specific surface area above 1500 m2/g. Furthermore, hard templating approaches often require specific post-removal of template, which is complicated, severe, and harmful to the environment and the human body (e.g., silica template with hydrofluoric acid etching).29−32 As for soft templating, most of the soft templates are relatively costly mainly due to their multiple synthetic steps and rigorous conditions.33 Likewise, the direct carbonization technique usually involves usage of expensive carbon precursors or costly catalysts. As an environmental friendly renewable resource, biomass offers very attractive cheap raw material for synthesis of HPCs. However, the biomass-based HPCs are facing a significant bottleneck. In order to increase the porosity, activation is a general, even essential, way from biomass to high-surface-area HPCs.34−40 Activation always involves usage of high-amounts of and corrosive activators such as KOH, which require repeated post-purification treatments for removal of impurities. More importantly, it is hard to control the hierarchical pore Received: July 10, 2017 Revised: August 29, 2017 Published: August 31, 2017 9307
DOI: 10.1021/acssuschemeng.7b02318 ACS Sustainable Chem. Eng. 2017, 5, 9307−9312
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structure because extensive activation will undoubtedly cause the collapse of their nanostructures. A major challenge still remains to develop simple and efficient methods for the production of biomass-based HPCs to date. Indicalamus leaves (ILs), as the main part of the l.latifolius plant, are widely distributed worldwide in hilly regions such as southern China. They have been widely used in food packaging, e.g., as the wrapping materials for Zongzi, due to their nontoxicity, high yield, fast growth rate, easy collection, and processability. Recently, in order to discover and recover highvalue materials from ILs, ILs were demonstrated to have very good potential application as templates to fabricate highperformance superhydrophobic biomorphic copper or zinc oxide.41,42 Despite these efforts, there is a large opportunity for further research targeting more valuable applications. Herein, we present an interesting insight into the important role of Teflon additive in fabrication of HPC derived from biomass, and thus we use natural Indicalamus leaves for the first time to successfully synthesize HPC with a three-dimensional (3D) hierarchical morphology of interconnected nanoparticle units by using a facile and post-treatment-free carbonization technique (Figure 1). It is surprisingly found that the utilization
Research Article
RESULTS AND DISCUSSION
Common l.latifolius from the southern China (Yangjiang City, Guangdong) is shown in Figure S1a. As a living plant, l.latifolius absorbs silica from soil, and the silica accumulates around cellulose microcompartments in the roots, branches, and leaves by biomineralization synthesis. In the leaves, i.e., ILs (Figure S1b), the content of the biogenetic silica depends on the variety, climate, and geographic location. In order to remove these natural silica components, post-etching with corrosive HF or NaOH or chemical activation are always needed after carbonization. Different from the conventional synthesis methods with multistep processes, our route combines the carbonization and silica removal into a single step with complete elimination of post-treatment. The ILs with silica content of 19 wt % are mixed with Teflon powder and then directly carbonized for 4 h in a furnace under protection of N2 to readily obtain the target sample HPC-1 (Figure S2). The absence of a Si peak in energy-dispersive X-ray spectroscopy (EDS) analyses of HPC-1 shows the complete removal of the natural silica component (Figure S3). For comparison, a control carbon sample is prepared through a conventional twostep strategy involving carbonization of ILs, followed by postetching of silica with HF. EDS analyses demonstrate after HF post-etching, the silica of the control sample is also removed completely (Figures S4 and S5). Besides, both HPC-1 and control sample reveal similar chemical composition and carbon framework structure (Figure S6). Taking the above results together, it is demonstrated that similar to the conventional two-step technique, our current one-step approach can remove the natural silica component effectively during carbonization and thus afford pure carbon materials. The morphologies of the obtained samples are characterized by scanning electron microscope (SEM) and transmission electron microscope (TEM) analyses. Surprisingly, it is found that just after a simple carbonization treatment, the smooth frameworks of ILs (Figure S7) are uniformly fractured on the nanometer scale to form numerous carbon nanoparticles of 10−50 nm during the high temperature pyrolysis process (Figure 2a). These nanoparticles stack each other into grapelike aggregates and then these aggregates interconnect in different directions into a 3D network structure (Figure 2b). The compact and loose aggregation of the carbon nanoparticle units leads to formation of mesopores (2 nm ≤ pore diameter ≤50 nm) and macropores (pore diameter >50 nm), respectively (Figures 2c and 2d). In addition, the carbon nanoparticle matrix contains numerous micropores (pore diameter < 2 nm), which are resulted from the emission of many noncarbon elements and carbon-containing compounds of ILs during carbonization treatment (Figures 2e and 2f). In striking contrast, from both SEM and TEM observations in Figures 2g−2i, the control sample shows a significantly poor pore structure, judging from their smooth framework surface. Significant hierarchical porous structure does not exist in the control sample. Their pore structures are further quantitatively analyzed by N2 adsorption measurement. As shown in the N2 adsorption− desorption isotherm of Figure 3a, the HPC-1 displays a steep increase of nitrogen uptake at low relative pressure (P/P0), suggesting the formation of micropores in large quantities. In addition, an obvious hysteresis loop occurs at medium P/P0, and after that, the adsorption amount increases gradually but still did not reach a plateau at the P/P0 near 1.0, demonstrating
Figure 1. Schematic illustration of comparison of the conventional two-step and our one-step synthetic routes developed here from ILs to porous carbons.
of Teflon plays a vital factor in one-step generating the hierarchical 3D carbonaceous network structure from ILs. On one hand, Teflon is able to in situ generate hydrofluoric acid (HF) gas to etch out the natural silica component during the carbonization process, thus accomplishing carbonization and removal of silica simultaneously. That is to say, HPCs are obtained by simply mixing ILs and Teflon powder, followed by direct carbonization. Therefore, the overall strategy is very straightforward and green, not only decreasing the synthetic steps but also eliminating the usage of toxic HF or corrosive NaOH when compared with normal carbonization combined with a post-treatment strategy to porous carbon. On the other hand, the addition of Teflon is favorable to produce a unique high-surface-area 3D network morphology of interconnected nanoparticle units, since the Teflon can create additional pores by removal of volatile noncarbon elements and carboncontaining compounds upon further pyrolysis. Benefiting from their well-developed porosity and valuable hierarchical porous morphology, the obtained HPCs show attractive liquidphase adsorption properties toward organic molecules. 9308
DOI: 10.1021/acssuschemeng.7b02318 ACS Sustainable Chem. Eng. 2017, 5, 9307−9312
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pore volume is calculated to be 1.65 cm3/g according to the amount adsorbed at a relative pressure P/P0 of 0.99. In comparison, the control sample exhibits much lower nitrogen adsorption amount, and thus possesses significantly smaller BET specific surface area of 794 m2/g, Langmuir specific surface area of 1330 m2/g, and total pore volume of 0.71 cm3/g. The maximum of PSD for meso-/macropores of the control sample is 3.4 nm, which is much smaller than that of HPC-1 as well. The above results obviously demonstrate that the employment of Teflon is a key to direct formation of 3D porous network structured carbon from ILs. First of all, the addition of Teflon allows for a successful one-step synthesis of porous carbon from ILs. It is demonstrated that Teflon can in situ generate HF during a high-temperature pyrolysis process, thus accomplishing carbonization and removal of silica component simultaneously.44,45 Otherwise, for the conventional strategy to ILs-based carbon materials, two steps including the carbonization and the subsequent post-etching of silica component with HF or NaOH are essential. In addition, the addition of Teflon is a vital factor in generating the high-surface-area 3D network structure. With the help of Teflon, the silica component is completely removed at the carbonization temperature of 650 °C by releasing SiF4 gas, leading to formation of porous structure.44,46 When further increasing temperature until 900 °C and maintaining 900 °C for several hours, more and more nanopores are formed because the porous structure provides much more room for large quantities of volatile noncarbon elements and carbon-containing compounds. Meanwhile, with the release of these volatiles, the asconstructed porous carbonaceous framework is effectively melted and coalesced on the nanometer scale to form numerous carbon nanoparticles interconnected in different directions. Therefore, more and more large mesopores and macropores are generated, and then an ingenious 3D interconnected hierarchical porous carbon texture is finally obtained. While, in the case of control sample, the presence of a tight silica−carbon interface during the whole carbonization process inhibits the formation of such additional rooms for release of volatile products, resulting in a relatively low specific surface area and an absence of hierarchical network structure. The nanostructure of HPCs can be readily adjusted by altering the fabrication conditions. For example, with decreasing the Teflon/silica weight ratio from 30 to 15, the BET specific surface areas decrease from 1609 to 1323 m2/g, and the total pore volume decreases from 1.65 to 1.11 cm3/g, as shown in Table 1. The carbonization time is also found to play an important role in tuning pore structure parameters of HPCs. Extending the carbonization time from 4 to 20 h leads to a decline in BET specific surface areas and pore volumes (Table 1). The 3D nanonetwork morphology structures are well retained in various HPCs in spite of their distinct porous structures (Figures S8 and S9), indicative of good stability of nanostructure during various fabrication conditions.
Figure 2. (a, b, c) SEM images and (d, e, f) TEM images of HPC-1 at different magnification. (g, h) SEM images and (i) TEM image of the control sample.
Figure 3. (a) N2 adsorption−desorption isotherms and (b) pore size distribution curves of HPC-1 and the control sample.
the presence of mesopores and macropores. In addition, the HPC-1 exhibits an H1 type hysteresis loop, which is a characteristic of porous materials known to contain agglomerates of approximately uniform spherical particles.43 These results are well consistent with the observations from SEM and TEM images. According to the pore size distribution (PSD) curve determined by density functional theory, the micropores within the carbon framework have maximum PSD peaks at 0.7 and 1.3 nm (Figure 3b). The meso-/macropores among the 3D network range from 2 to 100 nm with a maximum peak at 34 nm. The calculated Brunauer−Emmett−Teller (BET) specific surface area and Langmuir specific surface area of HPC-1 are as high as 1609 and 2012 m2/g, respectively. The t-plot method gives its specific surface areas of micropores and external largesized pores equal to 997 and 612 m2/g, respectively. The total Table 1. Pore Structure Parameters of Typical Samples Sample
Carbonization time (h)
Teflon/silica weight ratio
SLangmuir (m2/g)
SBET (m2/g)
Sext (m2/g)
Smic (m2/g)
Vt (cm3/g)
Control sample HPC-1 HPC-2 HPC-3
4 4 4 20
0 30 15 15
1330 2012 1663 1865
794 1609 1323 1518
421 612 592 544
372 997 731 974
0.71 1.65 1.11 1.52
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Figure 4. (a) Photographs of MB solutions with various times. (b) Adsorption curves of MB for HPC-1 and the control sample. Teflon (polytetrafluoroethylene, 7A, 99.9%) was purchased from DuPont Company. Hydrofluoric acid (HF, Guangzhou Chemical Reagent Factory) and other chemicals were used as received. Synthesis of HPCs. HPCs were synthesized by direct carbonization of a mixture containing Indicalamus leaf and Teflon powder without any tedious activation procedures. In a typical protocol, different amounts of Indicalamus leaf and Teflon powder were mixed by grinding in a mortar according to the predetermined recipe shown in Table 1. The mixture was subsequently placed inside a tubular furnace and heated to 900 °C at 5 °C/min for different carbonization times under N2 flow, leading to formation of HPCs. In order to avoid the direct emission of the SiF4 gas generated during the carbonization process, an absorption device of tail gas contaning water was connected to a furnace and then the SiF4 gas can be carried away by a nitrogen stream and hydrolyzed to hydrofluorosilicic acid. Synthesis of Control Sample. Indicalamus leaf powder was placed inside a tubular furnace and then heated to 900 °C at 5 °C/min for 4 h under N2 flow. After carbonization, the as-obtained control carbon/silica composites were washed using 40 wt % hydrofluoric acid (HF) solution to remove the silica, followed by filtration off and repeated flushing with distilled water until the filtrate is neutral. After drying at 100 °C, the control sample was obtained. Structural Characterization. The microstructure of the samples was investigated with a Quanta 400F scanning electron microscope (SEM) and a FEI Tecnai G2 Spirit transmission electron microscope (TEM). TGA was performed under flowing air condition at a heating rate of 20 °C/min. N2 adsorption measurement was carried out by using a Micromeritics ASAP 2020 analyzer at 77 K. Brunauer− Emmett−Teller (BET) specific surface area (SBET) and Langmuir specific surface area (SLangmuir) were analyzed according to BET and Langmuir theories, respectively. Micropore specific surface area (Smic) was determined by the t-plot method, and then external specific surface area (Sext) was obtained by subtracting Smic from SBET. Pore size distribution was analyzed by original density functional theory (DFT) combined with non-negative regularization and medium smoothing. Total pore volume (Vt) was calculated according to the amount adsorbed at a relative pressure P/P0 of about 0.99. Liquid-Phase Adsorption Characterization. Adsorption amounts of methylene blue on carbon materials were obtained by measuring their concentrations before and after adsorption. Twenty mg of carbon sample was added into a conical flask, and then 30 mL of methylene blue solution (200 mg/L) was added quickly. After that, this suspension was stirred at room temperature. At intervals, 0.5 mL of supernate was taken out and diluted to 5 mL. The concentration of adsorbate was measured by UV−vis spectra (Youke, UV759CRT). The wavelength for methylene blue is 664 nm. The adsorption capacity (C) was calculated according to the equation C = (c0V0 − c1V1)/m, where c0, V0, c1, V1, and m represent the initial concentration, initial volume, concentration and volume after adsorption, and weight of carbon materials, respectively.
Benefiting from its well-developed porosity and valuable hierarchical porous morphology, HPCs fabricated in this study hold great promise for their application in many areas. Here, we focus on their liquid-phase adsorption performance organic molecules. This potential application for HPCs is evaluated by using methylene blue (MB) with a high concentration of 200 mg/L as model adsorbate. Figure 4a provides digital photos of the filtered solutions of MB after exposure to HPC-1 and the control sample for various times. For the control sample, the color of the filtered solution of MB is rather constant within 1 h. Even after long adsorption times, e.g., 24 h, it can still be observed without too much fading away, implying poor adsorption rate of the control sample toward MB. In contrast, the filtered solution after exposure to HPC-1 turns from blue to almost colorless in 1 h, and then the specific BET surface area of HPC-1 adsorbent decrease to 704 m2/g (Figure S10). This implies rapid adsorption of MB in the HPC-1, mainly because of its unique hierarchically micro-meso-macroporous structure. Further quantitative measurements show that the maximum adsorption capacity of MB on HPC-1 is as high as 293 mg/g (Figure 4b). This value is significantly higher than that on the control sample (256 mg/g) and many other reported porous carbons, 47−52 manifesting HPC-1 is very attractive in adsorption.
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CONCLUSIONS In summary, HPCs with a 3D hierarchical morphology of interconnected nanoparticle units from natural leaf are prepared by development of a facile and post-treatment-free carbonization technique. It is found that the utilization of Teflon plays a decisive role in one-step constructing the hierarchical 3D carbonaceous network structure. The obtained HPC material without any extra activation procedures exhibits BET specific surface area and pore volume as high as 1609 m2/ g and 1.65 cm3/g, respectively. By controlling the carbonization time and Teflon/silica weight ratio, the porous structure can be well tailored while retaining the 3D network architecture. Thanks to the combination of well-developed porosity and valuable hierarchical porous morphology, this type of HPC demonstrates enhanced liquid-phase adsorption properties toward organic molecules. We hope that such a work would raise concerns about the fluoropolymer additive in the facile fabrication of more unique and unusual functional materials from biomass and thus provide new opportunities in applications oriented to energy, adsorption, separation, and catalysis.
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METHODS
Materials. Indicalamus leaf (Chunze Agricultural Product Stores) was directly used as the starting material without any pretreatment. 9310
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b02318. Additional information about material characterization (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (Y.R. Liang). *E-mail:
[email protected] (Y.L. Liu). ORCID
Yeru Liang: 0000-0002-6169-9981 Mingtao Zheng: 0000-0001-8083-8724 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully acknowledge financial support from the projects of the National Natural Science Foundation of China (51602107 and U1501242) and Program for Pearl River New Star of Science and Technology in Guangzhou (201710010104).
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REFERENCES
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DOI: 10.1021/acssuschemeng.7b02318 ACS Sustainable Chem. Eng. 2017, 5, 9307−9312
Research Article
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DOI: 10.1021/acssuschemeng.7b02318 ACS Sustainable Chem. Eng. 2017, 5, 9307−9312
