Mechanochemistry: A Green, Activation-Free and Top-Down Strategy

Sep 12, 2017 - Renewable resources (e.g., agricultural byproducts) are widely used in the production of commercial activated carbon, but the activatio...
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Letter pubs.acs.org/journal/ascecg

Mechanochemistry: A Green, Activation-Free and Top-Down Strategy to High-Surface-Area Carbon Materials Xidong Lin, Yeru Liang, Zhitao Lu, He Lou, Xingcai Zhang, Shaohong Liu, Bingna Zheng, Ruliang Liu, Ruowen Fu, and Dingcai Wu* Materials Science Institute, PCFM Lab and GDHPRC Lab, School of Chemistry, Sun Yat-sen University, 135 Xingangxi Road, Guangzhou 510275, People’s Republic of China S Supporting Information *

ABSTRACT: Renewable resources (e.g., agricultural byproducts) are widely used in the production of commercial activated carbon, but the activation procedures still have serious drawbacks. Here we develop a green, activation-free, top-down method to prepare high-surface-area carbon materials from agricultural wastes through mechanochemistry. The facile mechanochemical process can smash the monolithic agricultural wastes into tiny microparticles with abundant surfaces and bulk defects, which leads to the generation of welldeveloped hierarchical porous structures after direct carbonization. The as-obtained carbon materials simultaneously present high surface areas (1771 m2 g−1) and large pore volumes (1.88 cm3 g−1), and thus demonstrate excellent electrochemical performances as the interlayer for lithium−sulfur batteries and much superior creatinine adsorption capabilities to the medicinal charcoal tablets. These results provide a new direction for fabricating high-surface-area porous materials without any toxic reagents or complicated activation procedures, and can spur promising electrochemical and medical applications. KEYWORDS: Mechanochemistry, Green chemistry, High-surface-area carbon material

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INTRODUCTION Carbon materials, including carbon nanotubes, graphene and porous carbons, have attracted increasing attention because of their tunable surface area, good conductivity, and high chemical and physical stability.1−4 Therefore, they find utility in many practical applications, including adsorption, separation, catalysis, and energy.5−11 Among various carbon materials, activated carbons (ACs) are one of the most widely used materials today because of their high surface areas and moderate cost. Natural materials, such as coconut shells, woods, pith and nut shells, and synthetic materials like polymers, can be used as precursors of ACs. Generally, physical or chemical activation treatments are required to increase the materials’ surface area and pore volume (Figure S1).12 However, the fabrication procedures of ACs are still plagued with several problems. First, chemical activation, a way that consumes tons of harmful activating reagents such as H3PO4, KOH and ZnCl2, not only causes the corrosion of equipment but also produces large quantities of wastewater that pollute the environment. Moreover, chemical activation processes, including smashing, impregnation, carbonization and washing the activating reagents, result in low yields. In addition, physical activation, with CO2 or steam in most cases, cannot produce ACs with very large surface area.13−15 Therefore, the development of green, universal and activationfree methods to prepare high-surface-area carbon materials (HSACs) is urgently required. Green chemistry, also called sustainable chemistry, is an advancing area in present research. It focuses on the design of products and processes that minimize the utilization and generation of harmful substances.16,17 The aim of green © 2017 American Chemical Society

chemistry is not just to build the desired structure, but also to ensure the procedures and products are environmentally friendly. Spurred by the Presidential Green Chemistry Challenge Awards, the last decades have witnessed the promotion of the environmental and economic benefits of developing green chemistry. Recently, mechanochemistry, such as by grinding in high-energy ball-mills, analogous to Mjolnir (Thor’s hammer) that can smash a great quantity of things at once, has been demonstrated to be a green and powerful technique for many apparently distinct fields like extractive metallurgy, materials engineering, and medicine.17−19 Compared with traditional technological operations, mechanochemistry has notable differences, including simplification of technological procedures, elimination of tedious procedures that involve gases and reagents, and promotion of reactions. Thus, we envision that mechanochemistry would be a green and cost-effective method for carbon materials if high surface area could be obtained. Herein, we propose a green, versatile and top-down approach to develop a class of HSACs based upon mechanochemistry, which is much simpler compared with complicated activation processes and does not require any chemical or physical activating agents. The key to this novel preparation strategy is the utilization of high-energy ball-milling to produce various surfaces and bulk defects in treated solids. The overall synthesis procedure is illustrated in Figure 1. The HSACs can be Received: July 21, 2017 Revised: September 1, 2017 Published: September 12, 2017 8535

DOI: 10.1021/acssuschemeng.7b02462 ACS Sustainable Chem. Eng. 2017, 5, 8535−8540

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ACS Sustainable Chemistry & Engineering

and carbonized at 900 °C for the desired hours in a furnace under high purity N2 flow to produce various HSAC-MCS samples. The applied carbonization temperatures were varied from 800 to 1000 °C with carbonization time from 3 to 18 h. The as-obtained HSCAs were denoted as HSAC-MCS-x-y, where x and y indicate carbonization temperature and carbonization time, respectively. In the same way, MWS or MBW (∼0.5 g) was carbonized at 900 °C for 9 h, leading to the formation of HSAC-MWS-900-9 or HSAC-MBW-900-9. SCS or BCS (∼0.5 g) was carbonized at 900 °C for 9 h to produce C-SCS-900-9 or C-BCS-900-9. Coconut shell, walnut shell or bamboo waste was washed with deionized water and then dried. Then, the purified coconut shell, walnut shell or bamboo waste (∼0.5 g) was carbonized at 900 °C for 9 h to produce C-CS-900-9, C-WS-900-9 or C-BW-900-9. Liquid Phase Adsorption Characterization. Commercial medicinal charcoal tablets (Aixite, Changtian Pharma) were washed with deionized water and then dried to remove the water-soluble film coating. 25 mg of samples were added into a conical flask, and then 40 mL of creatinine (100 mg/L) was added quickly. After that, these suspensions were shaken with a rate of 150 rpm at 30 °C for 24 h. The concentration of creatinine was measured by UV−vis spectra at wavelength of 232 nm using 20 μL supernate with 500 times dilution (20 μL was diluted to 10 mL). Material Characterizations. The structures of the samples were investigated by a Hitachi S-3400 scanning electron microscope (SEM), a Zeiss GeminiSEM500 high resolution scanning electron microscope (HRSEM) and a FEI Tecnai G2 Spirit transmission electron microscope (TEM). N2 adsorption measurement was carried out using a Micromeritics ASAP 2020 analyzer at 77 K. Dynamic light scattering (DLS) measurements (0.1 mg/mL in glycol) were carried out at 25 °C on a Brookheaven Zeta PALS Instrument with a 532 nm laser at a scattering angle of 90°. The carbonization process of the samples was monitored using a thermogravimetric analysis (TGA Q50) under nitrogen flow with a heating rate of 10 °C min−1. Raman spectra were conducted on a Renishaw in Via Laser Micro-Raman Spectrometer. Elemental analysis was performed on an Elementar Analysensysteme GmbH Vario EL analyzer. Fourier transform infrared (FTIR) spectra were measured using a Bruker Equinox 55 FTIR spectroscopy. Lengguang 722S UV−vis spectrophotometer was used to detect the adsorption performances of creatinine. Laser scattering particle analyzer (Mastersizer 2000) was used to determine the average particle size of SCS and BCS. Mercury porosimetry was carried out using a Micromeritics AutoPore IV 9500. X-ray photoelectron spectroscopy (ESCALAB 250Xi) was used for XPS spectrum. Cell Fabrication and Measurements for Li−S Battery. Carbon-based interlayers were prepared by mixing HSAC-MCS-9009 (or YP50) and polyvinylidene fluoride binder (9:1 by weight) in Nmethyl-2-pyrrolidinone (NMP). The obtained slurry was coated onto one side of a typical commercial PP separator (Celgard 2400), followed by removing NMP at 60 °C under vacuum for 12 h and cutting into a round piece with a diameter of 18 mm. Cathodes were prepared by mixing commercial sulfur powder, conductive carbon black and polyvinylidene fluoride binder (7:2:1 by weight) in NMP. The obtained slurry was spread on aluminum foil using a coater, followed by drying in a vacuum drying oven at 60 °C for 12 h. The electrolyte was composed of a 1 M bis(trifluoromethane) sulphonimide lithium salt in 1,3-dioxolane and 1,2-dimethoxyethane (1:1 by volume) with 1 wt % LiNO3 additive. CR2032 coin-type cells were assembled in an argon-filled glovebox employing lithium plate as anode, sulfur as cathode and separators with carbon-based interlayers. The galvanostatic charge−discharge tests were conducted on a LAND instrument (model CT2001A) within a voltage range of 1.7−2.8 V. The current density was 0.1 C for the first three cycles before longterm cycling tests at 0.5 C.

Figure 1. (a) Fabrication of conventional carbon materials by carbonization of agricultural wastes. This method usually leads to carbon products with low surface areas. Thus, physical or chemical activation treatments are required to increase their surface areas. (b) Fabrication of HSACs via mechanochemical treatment and subsequent carbonization. The facile mechanochemistry can smash the monolithic agricultural wastes into tiny microparticles with abundant surfaces and bulk defects, which leads to the generation of a well-developed hierarchical porous structure upon carbonization.

obtained through a mechanochemical treatment, followed by direct carbonization. This strategy demonstrates several significant advantages. First of all, the mechanochemical preparation of HSACs from agricultural byproducts with little economic value, such as coconut shells, nutshells and bamboo wastes, is cost-effective and environmentally friendly from materials sources. Moreover, mechanochemistry introduces a clean method to prepare HSACs without using any activating agent, which is favorable to produce very pure carbons. Alkali metal ions or phosphate introduced into activation processes by other methods will be detrimental to the production of highly pure ACs and thus confines their applications.20 Last but not the least, the as-prepared HSACs present a valuable micro/ nanoscale powdery form. It is known that the micro/nanoscale forms of carbon materials are important for their processability, especially as electrode materials for supercapacitors and sulfur host materials for lithium−sulfur batteries, because they are favored for both better binding on current collector and easy electrolyte access during the charging−discharging process. Moreover, the fine carbon powders reveal promising medicinal applications for intestinal tract disease21 and hemoperfusion,22 as well as in the fields of caramel decolorization and water purification, etc. Thus, with much less waste products and side reactions, mechanochemistry will be an environmentally friendly and economical route in producing HSACs.

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EXPERIMENTAL SECTION

Synthesis of MCS, MWS and MBW. Coconut shell, walnuts shell or bamboo waste were washed with deionized water, dried and treated by a high-energy nanoball-milling machine at least 4 h (CJM-SY-B, Qinhuangdao Taiji Ring Nano-Products Co., Ltd.), producing MCS, MWS or MBW. Synthesis of SCS and BCS. Coconut shells were shredded by shredder (WJX-250, Shanghaiyuanwo) for 15 min to obtain SCS. Then SCS were further treated by ball-milling (MSK-SFM-3, MTI Corp) for 30 min; the as-prepared samples were named BCS. Synthesis of HSACs and Other Carbon Samples. MCS (∼0.5 g) was spread out evenly in the porcelain combustion boat (3 × 6 cm)

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RESULTS AND DISCUSSION Coconut shells, a class of typical agricultural wastes, are selected as raw materials of HSACs. The mechanochemical coconut shell (MCS) is prepared via high-energy ball-milling of coconut 8536

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ACS Sustainable Chemistry & Engineering shell (CS). The digital photos (the inset of Figure 2a,b) clearly show CS and MCS are monolithic and powdery, respectively.

Figure 2. Scanning electron microscope (SEM) images of (a) CS, (b) MCS and (c) HSAC-MCS-900-9; the inset digital photos show their macroscopical form. (d) High resolution scanning electron microscope (HRSEM) and (e) transmission electron microscope (TEM) images of HSAC-MCS-900-9.

Figure 3. (a) DLS particle size distribution (0.1 mg/mL in glycol) of HSAC-MCS-900-9 (red) and MCS (green). (b) N2 adsorption− desorption isotherms of HSAC-MCS-900-9 (red) and MCS (gray). (c) Density functional theory (DFT) pore size distribution of HSACMCS-900-9 (red) and MCS (gray). (d) Raman spectra of HSACMCS-900-9 (red) and C-CS-900-9 (gray). The microcrystalline planar crystal sizes La of HSAC-MCS-900-9 and C-CS-900-9 were calculated to be 1.86 and 2.06 nm, according to La = 4.35IG/ID. SBET of HSACMCS obtained at various carbonization conditions, including (e) carbonization times and (f) carbonization temperatures. (g) TG curve of MCS (orange) and CS (gray). (h) SBET of carbon products obtained from monolithic coconut shell (CS: ∼1 cm) and coconut shell powders with various average particle sizes (SCS, 257 μm; BCS, 34 μm; MCS, 2.2 μm). (i) SBET of HSAC-MBW-900-9 (green), HSACMWS-900-9 (blue) and HSAC-MCS-900-9 (red), and their mechanochemistry-free analogues (gray) (i.e., C-BW-900-9, C-WS900-9 and C-CS-900-9).

The SEM images in Figure 2a,b further confirm that the continuous monolithic morphology for CS is transformed into the microparticulate morphology for MCS after the mechanochemical treatment. MCS consists of irregular particles ranging from ∼1 to ∼10 μm (Figure 2b), and MCS dispersed in glycol has a narrow and unimodal size distribution with a maximum at 2.2 μm, according to DLS analysis (Figure 3a). The elemental analysis of MCS and CS (Figure S2) reveals that the C contents are 49.2% and 50.3%, respectively, whereas the H contents are both 6.2%. Meanwhile, as shown in XPS analysis of Figure S3, MCS exhibits a stronger intensity of C−O peak at 286.5 eV23 than CS, indicating the introduction of oxygen-containing functional groups into the particle surface during the mechanochemical treatment. The MCS was then directly carbonized in a furnace under protection of N2 to readily obtain the target HSAC-MCS. HSAC-MCS-900-9 obtained at 900 °C for 9 h is also in a powdery form, but presents much smaller particle sizes, as illustrated in the SEM image (Figure 2c). Such a size decrease is ascribed to the skeleton shrinkage and possible particle breakage during carbonization. HSAC-MCS-900-9 dispersed in glycol exhibits a narrow particle size distribution with a maximum at 451 nm from DLS analysis (Figure 3a). It should be noted that ACs are often in a granular form, and are very difficult, if not impossible, to be ground into such nanoscale powders. The HRSEM image in Figure 2d and TEM image in Figure 2e highlight that HSAC-MCS-900-9 is full of hierarchical nanopores including micropores, mesopores and macropores. The hierarchical porous structure, typically obtained by tedious multiple-step construction methods, is of significant importance because it is expected to synergistically exhibit the advantages of each class of nanopores. Nitrogen adsorption experiments are employed to quantitatively probe the pore characteristics of HSAC-MCS. As shown in N2 adsorption−desorption isotherm of Figure 3b, HSACMCS-900-9 has a high adsorption uptake at low relative pressure (P/P0), indicative of the existence of plentiful micropores; and the adsorption amount goes up gradually but still does not reach a plateau near the P/P0 of 1.0,

demonstrating the presence of mesopores and macropores. The DFT pore size distribution in Figure 3c shows the micropores center at 1.3 nm, and the meso-/macropores range from 2 to 127 nm with a maximum at 5.0 nm. It should be noted that some large macropores may be too large and too open to cause capillary condensation and thus to be reflected in the DFT pore size distributions.24 So, HSAC-MCS-900-9 was further characterized by mercury porosimetry to confirm the presence of its macropores (Figure S4). The BET surface area (SBET) and pore volume of HSAC-MCS-900-9 were measured to be up to 1771 m2 g−1 and 1.88 cm3 g−1, respectively, and its micropore surface area and meso-/macropore surface area were analyzed by a t-plot method to be 713 and 1058 m2 g−1, respectively. It is worth pointing out that such an unusual pore structure of simultaneously high surface area and large pore volume makes HSACs outperform the majority of ACs and other carbon materials including carbon aerogels, carbon nanofibers, carbon nanosheets, carbon nanocages, carbon nanospheres and mesoporous carbons.25−29 In sharp contrast, its precursor MCS has a very small SBET of 5 m2 g−1 and a negligible pore volume of 0.007 cm 3 g−1 , indicative of a nonporous characteristic. These results clearly demonstrate that the facile carbonization creates a well-developed hierarchical porous structure in HSAC-MCS-900-9. Raman spectra indicates that HSAC-MCS-900-9 is an amorphous carbon with a graphite-like microcrystalline structure, which means it is composed of 8537

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the carbonization yield of the compressed monolith increases to 12%, and SBET decreases to 392 m2 g−1. We also use other agricultural byproducts including walnut shell (WS) and bamboo waste (BW) as precursors for HSACs by the same mechanochemstry and carbonization procedures. The similar micromorphologies of mechanochemical walnut shell (MWS) and mechanochemical bamboo waste (MBW) to MCS are confirmed from DLS analysis and SEM images (Figure S9 and Figure S10). Similarly, TG curves indicate that the mass loss of the MWS and MBW is obviously higher than that of WS and BW at 360 °C (Figure S9f and Figure S10f). SBET of HSAC-MWS-900-9 and HSAC-MBW-900-9 is 1302 and 1205 m2 g−1, respectively, significantly higher than that of their respective analogue without mechanochemistry (Figure 3i, Figure S11 and Figure S12). Therefore, the mechanochemistry shows its universality in producing HSACs. Our HSACs can be applied as high-performance interlayer for Li−S batteries. Recently, Li−S batteries have attracted a great deal of interest for the sake of their high theoretical capacity (1675 mAh g−1) and energy density (2567 Wh kg−1).34 However, the Li−S batteries suffer from the shuttle effect, which means active sulfur materials would turn into soluble polysulfides and migrate to the lithium anode during electrochemical cycles. The shuttle effect generates high insulated Li2S and byproducts without power output, leading to low Coulombic efficiency and poor cycling stability. One effective way to prevent the shuttle effect is to introduce carbon-based material as interlayer on the cathode side of separator, which can block polysulfides, significantly reduce the parasitic reactions and improve the cyclic stability of Li−S cells.35−37 Our HSACs, in this respect, are particularly promising as an interlayer material for a Li−S battery. Figure 4a presents galvanostatic discharge−charge curves at a low rate of 0.1 C for the Li−S cell with HSAC-MCS-900-9 interlayer. Two typical plateaus are observed after the first discharge process, indicating a two-step sulfur reduction reaction. Moreover, rate performances were evaluated from 0.1 to 3 C with 10 cycles at each rate (Figure 4b). The Li−S cell with HSAC-MCS-900-9 interlayer is found to stabilize at each rate.

turbostratic carbon sheets and their disordered packing leads to free volume and porosity (Figure 3d). By altering the carbonization time and temperature, the pore structure of HSAC-MCS can be readily adjusted. For example, when extending the carbonization time from 3 to 9 h, SBET increases notably from 819 to 1771 m2 g−1, but further increasing the carbonization time to 12 and 18 h leads to the SBET decrease to 1018 and 925 m2 g−1, respectively (Figure 3e). Moreover, SBET increases from 1339 to 1771 m2 g−1 and then drops to 1049 m2 g−1 when the carbonization temperature goes up from 800 through 900 to 1000 °C in the case of 9 h of carbonization (Figure 3f). To shed light on the carbonization process, thermogravimetric (TG) experiments are performed on the MCS and CS. Figure 3g shows the TG curves of MCS and CS. Between 200 and 500 °C, the mass loss is ascribed to the decomposition of carbohydrates, lipids, cellulose, lignin, etc (Figure S5);30 beyond 500 °C, it is the elimination of loosely bonded carbon atoms and heteroatoms such as oxygen and nitrogen, along with the densification/graphitization of carbonaceous matter. It is interesting to note that the mass loss of MCS with mechanochemistry is obviously larger than that of CS without mechanochemistry above 360 °C. As a result, the carbonization yields of HSAC-MCS-900-9 from MCS and C-CS-900-9 from CS are 4% and 17%, respectively, under the same carbonization process. Such a carbonization yield difference leads to a much lower SBET for C-CS-900-9 compared with HSAC-MCS-900-9 (313 vs 1771 m2 g−1; Figure 3h and Figure S6). In addition, CCS-900-9 is also an amorphous carbon, but has a slightly larger microcrystalline planar crystal size than HSAC-MCS-900-9 (2.06 vs 1.86 nm; Figure 3d). This result clearly illustrates that for the same carbon precursors, construction of micro/ nanoscale structures is the key to obtain carbon products with high surface areas. Normally, carbonization is largely correlated to the removal of heteroatoms, elimination of loosely bonded carbon atoms and condensation of hexagonal benzene ring to form polyhexagonal carbon layers.31 Obviously, during carbonization, CS suffers long pathways for release of volatile products from chain scission. As a result, many radical species formed upon decomposition are able to recondense into carbons before escaping, leading to high carbonization yield and low porosity. In contrast, benefiting from mechanochemical treatment, the as-constructed micro/nanoscale structures in the loose MCS powders provide quite short pathways for large quantities of noncarbon elements and carbon-containing compounds to release effectively, thus leaving numerous nanopores in the resultant HSACs. In order to prove this mechanism, CS powders ranging from dozens to hundreds of microns (Figure S7) were prepared through ball-milling (BCS, 34 μm) and shredding (SCS, 257 μm), and then carbonized under the same conditions as HSACMCS-900-9 to obtain samples C-SCS-900-9 and C-BCS-900-9, respectively. Unsurprisingly, the precursors with larger sizes trend to have higher carbonization yields and lower surface areas. For instance, the carbonization yields of C-SCS-900-9 and C-BCS-900-9 are 19% and 12%, respectively, which are much higher than that of HSAC-MCS-900-9, leading to their low SBET (592 and 862 m2 g−1, respectively) (Figure 3h). On the other hand, the MCS powders were compressed into a monolith at 2 MPa and then carbonized at the same conditions as HSAC-MCS-900-9 (Figure S8). Such confinement will suppress the escape of volatile fractions and enhance the yield of aromatization through carbonization.32,33 Just as expected,

Figure 4. (a) Discharge−charge curves recorded at different cycles at 0.1 C for the Li−S cell with HSAC-MCS-900-9 interlayer; (b) rate performances for the Li−S cells with HSAC-MCS-900-9 interlayer, with YP-50 interlayer and without interlayer; (c) long-term cycle stability and Coulombic efficiency at 0.5 C for the Li−S cell with HSAC-MCS-900-9 interlayer. 8538

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envisioned that further tuning of pore structures could be accomplished by variation of precursor type and size, carbonization temperature and heating rate. We postulate that our findings will introduce a new direction in the quest for high-performance porous materials and pave the way for application breakthroughs and innovations in a spectrum of areas ranging from energy storage to environmental protection.

Discharge capacities of 1271, 1094, 921, 754, 601 and 508 mAh g−1 are obtained at 0.1, 0.2, 0.5, 1, 2 and 3 C, respectively, far exceeding the Li−S cell without HSAC-MCS-900-9 interlayer. When tuning the current density back to 0.1 C, the capacity is mostly recovered. Moreover, the Li−S cell with a commercially leading activated carbon YP-50 (SBET 1749 m2 g−1, Vt 0.89 cm−3 g−1, Figure S13) as the interlayer is also found to suffer from a serious shuttle effect and then exhibits low capacities at each rate, e.g., 695 and 98 mAh g−1 at 0.5 and 2 C, respectively. Long-term cycling performances for the Li−S cell with HSACMCS-900-9 interlayer were also tested (Figure 4c). An initial capacity of 1005 mAh g−1 is achieved at 0.5 C, and the capacity remains at 738 mAh g−1 even after 140 cycles with a high capacity retention ratio of 73%. The Coulombic efficiency is very close to 100% during the whole cycling process. The superior performances of the Li−S cell with HSAC-MCS-900-9 interlayer can be ascribed to the interlayer’s well-developed hierarchical porous structure and nanoscale particle size. The high surface area micropores of HSAC-MCS-900-9 can strongly adsorb polysulfides, whereas the small-sized nanoparticulate morphology and the externally interconnected meso-/macropores together facilitate rapid mass diffusion/transport to access the micropores. Therefore, when utilized as the interlayer, HSAC-MCS-900-9 can rapidly trap polysulfides and limit their migration to enhance the Li−S battery performances. In sharp contrast, because the isolated micropores in the conventional activated carbons mainly locate on the surface of large micro/ millimeter-scaled carbon particles (e.g., >10 μm),38 the ion transfer/diffusion pathways within activated carbons are long and tortuous and their pore surface utilization is low, leading to inferior Li−S battery performances, especially at high rate operations. It is known that the commercial medicinal charcoal tablets (Figure S14) are a classic treatment for indigestion and flatulence in the stomach or intestines, and the medicinal carbons also help reduce creatinine and uric acid levels, for the sake of their high surface areas and biocompatibility with our bodies. Activation is a typical method for the preparation of these medicinal carbons, but further purification and refinement are necessary to eliminate the harmful substances produced during activation. Here, as a highly pure carbon, we explored the adsorption performance of HSAC-MCS-900-9 toward creatinine. Thanks to its higher surface area and larger pore volume, HSAC-MCS-900-9 was found to demonstrate a significantly higher creatinine adsorption capacity (70.1 mg g−1) compared to the activated carbon obtained from the medicinal charcoal tablets with SBET of 920 m2 g−1 and Vt of 0.49 cm3 g−1 (30.0 mg g−1), which clearly confirms that the HSACs obtained here present a promising medical value.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b02462. Difference of preparation processes between AC and HSAC, digital photographs, combustion elemental analysis, XPS spectra, FT-IR spectra, N2 adsorption− desorption isotherms, pore size distributions, SEM images, TG curves, DLS particle size distributions (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Dingcai Wu. Email: [email protected]. ORCID

Dingcai Wu: 0000-0003-1396-0097 Author Contributions

The paper was written through contributions of all authors. All authors have given approval to the final version of the paper. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the project of the National Natural Science Foundation of China (51422307, U1601206, 51372280), National Program for Support of Top-notch Young Professionals, Guangdong Natural Science Funds for Distinguished Young Scholar (S2013050014408), Tip-top Scientific and Technical Innovative Youth Talents of Guangdong Special Support Program (2014TQ01C337), Fundamental Research Funds for the Central Universities (15lgjc17) and National Key Basic Research Program of China (2014CB932400).

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REFERENCES

(1) Yang, X.; Cheng, C.; Wang, Y.; Qiu, L.; Li, D. Liquid-Mediated Dense Integration of Graphene Materials for Compact Capacitive Energy Storage. Science 2013, 341, 534−537. (2) Liang, C.; Li, Z.; Dai, S. Mesoporous carbon materials: Synthesis and modification. Angew. Chem., Int. Ed. 2008, 47, 3696−3717. (3) Xu, F.; Tang, Z.; Huang, S.; Chen, L.; Liang, Y.; Mai, W.; Zhong, H.; Fu, R.; Wu, D. Facile synthesis of ultrahigh-surface-area hollow carbon nanospheres for enhanced adsorption and energy storage. Nat. Commun. 2015, 6, 7221. (4) Kuzmicz, D.; Prescher, S.; Polzer, F.; Soll, S.; Seitz, C.; Antonietti, M.; Yuan, J. The Colloidal Stabilization of Carbon with Carbon: Carbon Nanobubbles as both Dispersant and Glue for Carbon Nanotubes. Angew. Chem., Int. Ed. 2014, 53, 1062−1066. (5) Wu, D.; Li, Z.; Zhong, M.; Kowalewski, T.; Matyjaszewski, K. Templated Synthesis of Nitrogen- Enriched Nanoporous Carbon Materials from Porogenic Organic Precursors Prepared by ATRP. Angew. Chem., Int. Ed. 2014, 53, 3957−3960. (6) Li, Z.; Wu, D.; Liang, Y.; Fu, R.; Matyjaszewski, K. Synthesis of Well-Defined Microporous Carbons by Molecular-Scale Templating

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CONCLUSIONS In summary, we have developed a green, universal, top-down method to prepare HSACs via mechanochemistry procedures. The simple mechanochemical method is capable of breaking the monolithic coconut shells into microparticles with an average diameter of 2.2 μm. Such microparticulate morphologies promote easy release of noncarbon elements and carboncontaining compounds formed during carbonization, thus leading to formation of a well-developed hierarchical porous structure in carbon products. The as-obtained carbon materials have an average particle size of 451 nm and exhibit surface areas of up to 1771 m2 g−1. The top-down method also works well for fabricating HSACs from other agricultural byproducts. It is 8539

DOI: 10.1021/acssuschemeng.7b02462 ACS Sustainable Chem. Eng. 2017, 5, 8535−8540

Letter

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DOI: 10.1021/acssuschemeng.7b02462 ACS Sustainable Chem. Eng. 2017, 5, 8535−8540