Controllable synthesis of N-doped hollow, yolk-shell and solid carbon spheres via template-free method

Journal of Alloys and Compounds 778 (2019) 294e301

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Controllable synthesis of N-doped hollow, yolk-shell and solid carbon spheres via template-free method Lili Zhang, Lei Liu, Meng Liu, Yifeng Yu, Zepeng Hu, Beibei Liu, Haijun Lv, Aibing Chen* College of Chemical and Pharmaceutical Engineering, Hebei University of Science and Technology, Shijiazhuang, 050018, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 14 September 2018 Received in revised form 12 November 2018 Accepted 14 November 2018 Available online 15 November 2018

N-doped carbon spheres with different inner structure have attracted increasing attention due to their low density and high electrical conductivity in supercapacitors. Here, we reported controllable synthesis of hollow, yolk-shell and solid structure carbon spheres via template-free assembly method. Ethanediamine was used as nitrogen source to realize in-situ nitrogen doping and as basic catalyst to drive resorcinol-formaldehyde (RF) resin polymerization. The polymerization degree of internal RF resin tremendously depended on polymerization time, resulting in different levels of heterogeneity of the whole RF resin sphere. This fact made the structural adjustment of carbon spheres from hollow, to yolkshell and even to solid structure after dissolution of low-molecular-weight RF resin by organic solvents and annealing treatment. As electrode material for supercapacitor, carbon spheres with yolk-shell structure exhibited excellent performance with capacitance of 242 F g
1 at the current density of 1 A g
1 and outstanding cycling life stability (97.3% after 5000 cycles), which may be attributed to its unique structure, high specific surface area (1263 m2 g
1) and pore volume (0.68 cm3 g
1). The strategy of making full use of the difference of polymerization degree of resin provides a new idea for the structural engineering of functional carbon materials. © 2018 Elsevier B.V. All rights reserved.

Keywords: N-doped carbon spheres Supercapacitors Structural adjustment Polymerization degree Yolk-shell structure

1. Introduction Electric double layer capacitor (EDLC, or supercapacitor) as a new type of energy storage device is superior to traditional capacitors and rechargeable batteries, which has a plenty of advantages including cost-effectiveness, environment friendliness, high energy density, high power density, fast charge/discharge rate, and long cycle life [1e4]. Carbon materials are common used as electrode materials, whose properties are the key factor affecting the performance of supercapacitors. With advancement of supercapacitors technology, great achievements have been made in the synthesis of carbon materials with different morphologies and structures, such as activated carbon powders, carbon fibers, carbon nanotubes, and spherical carbon materials [5]. Among these carbon materials, spherical carbon materials, especially hollow carbon spheres and yolk-shell carbon spheres, are more attractive owing to their superior physical and chemical properties, including their high specific surface areas, low densities, and good electrical and

* Corresponding author. E-mail address: [email protected] (A. Chen). https://doi.org/10.1016/j.jallcom.2018.11.169 0925-8388/© 2018 Elsevier B.V. All rights reserved.

heat conduction properties [6]. It is precisely because of the unique properties and advantages of spherical carbon materials that the tremendous attention has been paid to the development of various methods for the controllable synthesis of spherical carbon materials with different inner structures. Generally, the annealing treatment of polymer spheres has demonstrated to be an effective approach for the preparation of carbon spheres. Some polymers, such as resorcinol-formaldehyde (RF) resin, saccharide, pitch, and polydopamine, have been used carbon precursors to fabricate spherical carbon materials [7e14]. Among these carbon precursors, RF resin are more frequently used due to their easy preparation, good thermal stability, easy functionalization and high char yield [15,16]. Previous studies have demonstrated that the subtle structure (e.g. hollow and yolk-shell) of spherical carbon materials has great effect on their performances in different applications. Numerous strategies have been reported for preparing spherical carbon materials, especially hollow and yolk-shell carbon spheres. Template method is very effective concerning synthesis of hollow carbon spheres and vastly investigated [17]. In a typical hard-templating approach for the preparation of hollow carbon spheres, the template is first coated with a layer of carbon precursors followed by

L. Zhang et al. / Journal of Alloys and Compounds 778 (2019) 294e301

annealing treatment, yielding core-shell carbon nanostructures. After removal of the templates, the core-shell carbon structures can be further converted into hollow carbon nanostructures [16]. SiO2 spheres, metal oxide and polystyrene spheres have all been used as hard templates to synthesize hollow carbon spheres [18e22]. The soft-templating route refers to the direct generation of the hollow structure by self-assembly of carbon precursors and soft templates (usually organic compounds such as sodium dodecyl sulfate, cetyltrimethylammonium, and block compound) [23e29]. Template method also has been used for the preparation of a yolk-shell structure. Liu et al. demonstrated a facile and controllable synthesis of yolk-shell structured carbon spheres by a modified “silicaassisted” route [30]. Similarly, Wang et al. also report the synthesis of uniform yolk-shell structured carbon spheres by using cationic surfactant cetyltrimethylammonium bromide as a soft template, RF as a carbon source and tetraethyl orthosilicate as an assistant poreforming agent [31]. These template strategies have achieved great success in adjusting the internal structure of spherical carbon materials, but there are also some inevitable limitations. For instance, the procedure is tedious because templates with special nanostructure need to be pre-prepared before fabrication and also need to be removed after fabrication. Furthermore, the strategies of preparing hollow and yolk-shell nanostructures are different, so templates should be prepared separately. So far, it is still a challenge to regulate the carbon sphere from hollow to yolk-shell structure via a single versatile template method. Hence, it is important to explore an easily operated, economic, versatile and adjustable approach to achieve controllable synthesis of spherical carbon materials with adjustable internal structure. It is generally believed that pure carbon spheres have highly hydrophobic surface and a limited number of specific active sites, which impede their practical application [32]. To overcome these intrinsic drawbacks, introduction of heteroatoms (especially N) into carbon spheres with different morphologies has been applied to achieve hydrophilic and active surface. Doping with nitrogen has proved to be a robust and versatile route to further boost the electronic conductivity and capacitance of carbon spheres [33]. Herein, we report a facile, green and template-free assembly approach to achieve the adjustment of internal structure of spherical carbon materials from hollow, yolk-shell to solid structure. Ethanediamine (EDA) was both used as nitrogen source and basic catalyst. High-molecular-weight peripheral RF resin formed rapidly but polymerization degree of internal RF resin gradually increases with time. Based on the heterogeneity of the whole RF resin sphere, acetone was added to selectively remove lowmolecular-weight RF oligomer, realizing the adjustment of spherical carbon materials from hollow, yolk-shell to solid structure. Compared with N-doped hollow carbon spheres (N-HCS) and Ndoped solid carbon spheres (N-SCS), N-doped yolk-shell carbon spheres (N-YSCS) exhibit better electrochemical performance, which may be ascribed to the high surface area and unique structure. 2. Materials and methods 2.1. Materials Formaldehyde solution (37%), EDA and acetone were purchased from Tianjin Yongda Chemical Corp; resorcinol was purchased from Macklin Corp. 2.2. Preparation of N-HCS, N-YSCS and N-SCS 0.1 g resorcinol, 0.1 mL formaldehyde, and 36 mL EDA as catalyst were added into 30 mL deionized water and reacted at room

295

temperature. Emulsion droplets are formed through the hydrogen bonding of water, resorcinol, and formaldehyde at the early stage. Then RF polymerization takes place from the inside of droplets by ethanediamine catalysis, resulting in uniform colloidal spheres. After the reaction continued for 1, 6 and 24 h, 20 mL acetone was added to selectively remove the interior part of the solid inhomogeneous spheres, forming hollow, yolk-shell and solid structure respectively. After that, N-doped carbon spheres were collected and purified with distilled water and ethanol by centrifugation. After annealing at 800  C for 3 h under N2 atmosphere, the N-HCS, NYSCS and N-SCS were obtained respectively. 2.3. Characterization Nitrogen adsorption-desorption isotherms measurements were measured on Micromeritics TriStar 3020 volumetric adsorption analyzer at 77 K. In order to calculate the specific surface area of NHCS, N-YSCS and N-SCS, the Brunauer-Emmett-Teller (BET) method was utilized. The pore size from the adsorption branches of the isotherms was calculated by Barrett-Joyner-Halenda (BJH) method. Total pore volume was estimated from the N2 amount adsorbed at a relative pressure of P/P0 ¼ 0.97. Transmission electron micrographs (TEM) were obtained on a JEOL JEM-2100 electron microscope. Xray photoelectron spectroscopy (XPS) data were measured by a Kratos Axis ULTRA X-ray photoelectron spectrometer to analyze the surface nitrogen group. 2.4. Electrochemical analysis The working electrode was prepared by pressing mixtures of carbon sample, carbon black, and polytetrafluoroethylene (weight ratio of 8:1:1) in a small amount of ethanol on Ni foam substrate. The prepared working electrode was measured on a computercontrolled electrochemical working station (CHI 760 E) at 25  C using 6 M KOH as electrolyte. The galvanostatic charge-discharge (GCD), cyclic voltammetry (CV), electrical impedance spectroscopy (EIS) and cycling stability techniques of samples were measured respectively. The CV and GCD were measured in the working voltage range of
1.0 to 0 V. For the two-electrode system, the energy density (E, Wh kg
1) and the power density (P, W kg
1) were estimated according to E ¼ C(DV)2/(2  3.6) and P ¼ 3600E/Dt, Where C is the specific capacitance of the symmetric supercapacitor calculated according to C ¼ 4IDt/DVm based on the charge/discharge curves of the two-electrode system (I was the applied current, Dt was the discharge time, DV was the potential window excluding IR drop, m was the mass of active material on the two electrodes). 3. Result and discussion As shown in Scheme 1, the internal structure of spherical carbon materials was adjusted based on different polymerization degree of the N-doped RF resin spheres. Firstly, polymerization of resorcinol and formaldehyde proceeded under catalysis of alkaline EDA, forming N-doped RF resin sphere. Here, two points should be noted. First, the polymerization rate and degree of peripheral RF resin and inner RF resin were different. The polymerization reaction of peripheral RF resin went much faster than that of inner RF resin, so the whole RF resin sphere was heterogeneous. Low polymerization degree resulted in low-molecular-weight oligomers inside which could be dissolved in organic solvents. Second, the polymerization degree of internal RF resin increased with time, so the whole heterogeneous resin sphere turned homogeneous eventually as long as polymerization time was beyond a critical period. As shown in route A, after a short period of 1 h polymerization, the

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Scheme 1. Illustration of the synthesis route for N-HCS, N-YSCS and N-SCS.

low-molecular-weight RF resin formed in the RF resin sphere, which can be removed by acetone dissolution, forming N-HCS with hollow structure. With the polymerization time extended to 6 h, the polymerization degree of inner RF resin increased, and a hard core generated which was comprised of high-molecular-weight RF polymer similar to the peripheral RF resin. Upon acetone dissolution and annealing treatment, N-YSCS with yolk-shell type architecture was obtained with an insoluble core in the center (route B). When the polymerization time was further extended to 24 h, the polymerization degree of inner RF resin further increased and a homogeneous high-molecular-weight RF resin sphere was formed, any part of which couldn't be dissolved by acetone in this case (route C), thereby N-SCS with solid structure was obtained after annealing treatment. Based on time-dependent polymerization, we have realized regulation of the structure of spherical carbon materials from hollow to yolk-shell to solid via template-free method. Fig. 1a shows TEM image of N-HCS, which exhibits uniform hollow carbon sphere with the average diameter of about 600 nm. It indicates that the polymerization degree of inner RF rein is low after the short period of polymerization (1 h). Lower-molecularweight RF oligomer can be selectively removed by acetone, forming hollow structure, which can be easily converted into N-HCS after pyrolysis. A higher resolution TEM image of the N-HCS, as shown in Fig. 1b, shows that the cavity size is about 500 nm. However, there are some residues in the innermost of the sphere, indicating that the polymerization degree of the innermost RF resin is higher than that of the rest inner RF resin. TEM image of N-YSCS (Fig. 1c) shows uniform yolk-shell particle with size of about 600 nm. It indicates higher polymerization degree of inner RF resin and formation of high-molecular-weight RF polymer core when the reaction time extends to 6 h. The TEM image in Fig. 1d further indicates a distinct yolk-shell structure with hard core of about 300 nm. However, by extending the reaction time to 24 h, solid carbon spheres are observed, as shown in Fig. 1e, indicating further polymerization of inner RF resin and homogeneous composition of the whole resin sphere. The whole resin sphere is composed of high-molecular-weight RF polymer, which can't be dissolved by acetone. The average diameter of N-SCS was about 600 nm as shown in Fig. 1f, consistent with the diameter of N-HCS and N-SCS. The consistent diameter indicates that the size of the carbon sphere is determined by the peripheral RF shell, which forms at the early stage and may doesn't change any more in later polymerization process, although polymerization of inner RF resin proceeds afterwards. As for inner RF resin, polymerization process may start from the innermost then extends outwards gradually. The surface area and pore characteristics of N-HCS, N-YSCS and N-SCS were assessed by N2 adsorption/desorption measurement.

As shown in Fig. 2a, the isotherm curves of N-HCS and N-YSCS are type IV isotherms with sharp rise at the low-pressure range (P/ P0 < 0.1), indicating that existence of a certain amount of micropores in the carbon spheres. And there was an obvious hysteresis loop of isotherm curves of N-YSCS at the P/P0 range of 0.4e0.8, which is the typical characteristic of mesoporous. A sharp increase at the relative pressure range of 0.8e1 is also observed for N-HCS and N-YSCS, indicating the presence of large cavity. However, the isotherm curves of N-SCS shows type I characteristics. There is no obvious hysteresis loop at the relative pressure range of 0.4e1.0, which further proves that the N-SCS have no obvious mesoporous and hollow structures. Fig. 2b shows the pore size distribution curves of N-HCS, N-YSCS and N-SCS calculated by using the BJH model, and the curve of N-YSCS has no obvious peaks, which may be ascribed to the wide range of mesoporous pore size distributions of N-YSCS. The detailed structural parameters of different samples are listed in Table 1. Obviously, N-YSCS has the largest the BET surface area and pore volume, due to its high micropore area and large void space between core and shell. EDA, as both a base catalyst and nitrogen precursor, is added into reaction system, to achieve in-situ N doping in carbon spheres during annealing treatment. Fig. 3 exhibits the XPS results of NYSCS. The XPS survey spectrum (Fig. 3a) showed the three dominant peaks at 284.0 eV, 400.4 eV, and 532.5 eV, corresponding to C 1s, N 1s and O 1s, respectively [34]. As shown in the inset of Fig. 3a, the content of carbon, oxygen and nitrogen in the N-YSCS were 90.6, 5.9 and 3.5 wt%, respectively. These results further confirm that the N-YSCS have been successfully doped with heteroatom N. High-resolution XPS spectra for C 1s, N 1s and O 1s were performed and fitted respectively, and the corresponding forms of each element were analyzed. The fitting of N 1s spectrum of PNPEAKOH-800 (Fig. 2B) shows three broad peaks at 399.8 (pyrrolic N), 400.9 (quaternary N), and 405.4 eV (pyridine N oxides), respectively. As shown in Fig. 3b, the fitting of N1s spectrum of NYSCS shows three type peaks, corresponding to C-C at 284.6 eV, C-N at 285.4 eV, C]O at 287.4 eV, which further indicates the existence of elements N, O in the surface of N-YSCS [35e37]. The spectrum of O1s (Fig. 3c) could be deconvoluted into three type peaks with binding energies of 531.6, 533.1 and 535.5 eV that correspond to the contribution of oxygen in carboxyl groups, C]O and chemically adsorbed oxygen, respectively [38e40]. The high resolution XPS spectrum of N1s (Fig. 3d) can be further deconvoluted into three peaks at 398.3, 400.5, and 402.1 eV that correspond to the contribution of pyridinic nitrogen (398.3 eV), quaternary nitrogen (400.5 eV) and pyridine N-oxide (402.1 eV) [38]. The pyridinic nitrogen provides the active sites for electrode materials, and the quaternary nitrogen can improve the electrical conductivity of the

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Fig. 1. TEM images of N-HCS (a, b), N-YSCS (c, d) and N-SCS (e, f).

carbon materials [41,42]. The electrochemical behaviors of N-HCS, N-YSCS and N-SCS samples were evaluated using a three-electrode cell in 6 M KOH. The quasi-triangular and symmetrical GCD curves indicate that these electrodes possess typical EDLC behavior and superior charge-discharge reversibility (Fig. 4a). N-YSCS exhibits a capacitance as high as 242 F g
1 at a current density of 1 A g
1, which is obviously higher than the N-HCS (196 F g
1) and N-SCS (160 F g
1) at the same current density. The high capacitance is ascribed to the higher surface area which could provide sufficient active sites, the large void space between core and shell which can act as energy storages or nanoreactors and abundant mesoporous which shorten the diffusion distance of ions and reduce the diffusion resistance to make ions penetration into the micropores easily [43]. Then N-YSCS is further investigated by CV curves, GCD curves EIS and cycle

stability. Charge-discharge rate performance and capacitance retention of N-YSCS were evaluated by GCD curves (Fig. 4b). It is evident that the gravimetric specific capacitance decreases with the increase of current density due to the limited diffusion of the electrolyte ions to the electrode surface during fast charging/discharging. Obviously, all of the GCD curves show quasi-triangular shapes with good symmetry, indicating excellent capacitive performance and electrochemical reversibility, even at high current densities 10 A g
1. The GCD curves exhibit small internal resistance (IR) drop (0.005) even at the current density of 10 A g
1, suggesting a rather low internal resistance and the efficient double-layer charge storage [44]. In order to investigate the influence of the current densities on the specific capacitances, the charge-discharge measurements are recorded at different current densities, as shown in Fig. 4c. As can be seen, the N-YSCS electrode reveals a specific

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Fig. 2. (a) Nitrogen adsorption-desorption isotherms of N-HCS, N-YSCS and N-SCS; (b) pore size distribution curves of N-HCS, N-YSCS and N-SCS.

Table 1 The textural parameters of N-doped carbon spheres. Samples

SBET (m2 g
1)

Vt (cm3 g
1)a

Smicro (m2 g
1)b

Vmicro (cm3g
1)c

N-HCS N-YSCS N-SCS

946 1263 501

0.51 0.68 0.29

824 1007 402

0.38 0.47 0.19

a b c

Total pore volume at P/P0~0.97. Micropore surface area determined by the t-plot. Micropore volume calculated by the t-plot method.

capacitance retention rate of 76.3%, even at a high current density of 10 A g
1, showing acceptable rate performance. In addition, the specific capacity of N-YSCS was higher than that of many other

spherical carbon materials, such as yolk-shell carbon sphere, hollow carbon spheres, mesoporous carbon spheres, and activated carbon, attributing to the combination of nitrogen functionalization and yolk-shell structure of carbon materials [45e55]. Fig. 4d is the CV curves of N-YSCS recorded at different scan rates, which still maintain a quasi-rectangular shape at low scanning rates, indicating a dominant EDLC. The slight changes of the curve shapes at high scanning rates demonstrate the ability of rapid ion migration. The fast ion response rate in N-YSCS was examined by EIS in the frequency range from 10
2e105 Hz. Fig. 4e shows the Nyquist plots of N-YSCS electrode, which exhibits a semicircle in the highfrequency and nearly straight line in the low-frequency, indicating that the sample possesses a low charge transfer resistance

Fig. 3. (a) XPS spectrum; (b) C1s spectrum; (c) O1s spectrum; (d) N1s spectrum of N-YSCS.

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Fig. 4. (a) GCD curves tested at 1 A g
1 of N-HCS, N-YSCS and N-SCS; (b) GCD curves tested at 0.5e10 A g
1 of N-YSCS; (c) The correlation of specific capacitance at various current densities for N-YSCS; (d) CV curves tested at 5e100 mV s
1 of N-YSCS in 6 M KOH solution; (e) Nyquist plot of N-YSCS; (f) Cycling performance of the N-YSCS supercapacitors for charging and discharging at a current density of 5 A g
1.

and an excellent dominance of EDLC. From the magnified region (inset of Fig. 4e) in the high frequency range, the equivalent series resistance (ESR) obtained from the intersection of the Nyquist plot at the x-axis is 0.56 U. The low resistance means a high ion transfer/ diffusion rate into the pores of electrode materials, which is crucial for enhancing rate capability and electrochemical performance. In addition, the long-time cycling stability is an important factor to identify the feasibility for practical application of the device. As shown in Fig. 4f, N-YSCS still remain 97.3% of their initial capacity after 5000 cycles of GCD at a current density of 5.0 A g
1, and there is little difference between the 1st and 5000th charge-discharge curves, indicating that the sample possesses excellent cycling ability and electrochemical reversibility. The high capacitance, benign rate capability, and excellent cyclic stability of the N-YSCS could be ascribed to particular yolk-shell structure, suitable nitrogen doped and high specific surface area. N-YSCS electrode was evaluated in two-electrode system to

further examine its capacitive performances for real supercapacitor. The GCD curves of N-YSCS with different current density were displayed in Fig. 5a. Even at high current density, the GCD curves of the capacitor maintain the shape of quasi-triangular. At a current density of 0.2 A g
1, the specific capacitance of the N-YSCS was calculated to be 224 F g
1. Moreover, the N-YSCS electrode still remains as high as 150 F g
1 at a high current density of 10 A g
1 with a high capacity retention rate of 67% (inset of Fig. 5b), indicating that the microporous/mesoporous/yolk-shell structure provided more transport and diffusion channels for electrolyte ions. As shown in Fig. 5b, the energy densities and power densities of NYSCS electrode were calculated at different current densities. The energy densities of N-YSCS accordingly decreased from 19.3 to 11.8 Wh kg
1 when power densities increased from 0.3 to 15.2 kW kg
1, demonstrating again that the unique nanostructure of N-YSCS led to the superior performance.

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Fig. 5. (a) GCD curves of N-YSCS at difference current density in two-electrode system; (b) Ragone plots of N-YSCS and the correlation of specific capacitance at various current densities for N-YSCS in two-electrode system.

4. Conclusion In summary, depending on the changed polymerization degree of RF resin, we have demonstrated a controllable template-free assembly route to prepare nitrogen doped carbon spheres with variable inner structures (hollow, yolk-shell and solid). The carbon spheres with yolk-shell structure possess high surface areas (1263 m2 g
1), large pore volumes (0.68 cm3 g
1) and suitable nitrogen doping and manifests remarkable supercapacitors performance with high capacitance, favorable capacitance retention and excellent cycling stability. The structural adjustment of spherical carbon materials by the template-free strategy makes full use of the difference of polymerization degree of resin, which may provide insights into the formation mechanism of polymer nanostructure and guide the formation of carbon nanomaterials with high functionality and variable structures. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (21676070), Hebei One HundredExcellent Innovative Talent Program (III) (SLRC2017034), Hebei Science and Technology Project (17214304D, 16214510D), The Excellent Going Abroad Experts' Training Program in Hebei Province. Beijing National Laboratory for Molecular Sciences. References [1] C. Merlet, B. Rotenberg, P.A. Madden, P.L. Taberna, P. Simon, Y. Gogotsi, M. Salanne, On the molecular origin of supercapacitance in nanoporous carbon electrodes, Nat. Mater. 11 (2012) 306e310. [2] H. Jiang, P.S. Lee, C.Z. Li, 3D carbon based nanostructures for advanced supercapacitors, Energy Environ. Sci. 6 (2012) 41e53. [3] Y.H. Lu, Y. Huang, M.J. Zhang, Y.S. Chen, Nitrogen-doped graphene materials for supercapacitor applications, J. Nanosci. Nanotechnol. 14 (2014) 1134e1144. [4] Z.J. Fan, J. Yan, T. Wei, L.J. Zhi, G.Q. Ning, T.Y. Li, Asymmetric supercapacitors based on graphene/MnO2 and activated carbon nanofiber electrodes with high power and energy density, Adv. Funct. Mater. 21 (2011) 2366e2375. [5] C. Liu, J. Wang, J.S. Li, M.L. Zeng, R. Luo, J.Y. Shen, X.Y. Sun, W.Q. Han, L.J. Wang, Synthesis of N-doped hollow-structured mesoporous carbon spheres for highperformance supercapacitors, ACS Appl. Mater. Interfaces 8 (2016) 7194e7204. [6] Q. Wu, W. Li, J. Tan, S.X. Liu, Flexible cage-like carbon spheres with ordered mesoporous structures prepared via a soft-template/hydrothermal process from carboxymethylcellulose, RSC Adv. 4 (2014) 61518e61524. [7] X.M. Sun, Y.D. Li, Colloidal carbon spheres and their core/shell structures with noble-metal nanoparticles, Angew. Chem. Int. Ed. 116 (2004) 607e611. [8] R.J. White, K. Tauer, M. Antonietti, M.M. Titirici, Functional hollow carbon spheres by latex templating, J. Am. Chem. Soc. 132 (2010) 17360e17363. [9] B.B. Hu, K. Wang, L.H. Wu, S.H. Yu, M. Antonietti, M.M. Titirici, Engineering carbon materials from the hydrothermal carbonization process of biomass,

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