Fabrication and electrochemical performance of novel hollow microporous carbon nanospheres

Abstract

Development of facile synthetic procedures for the fabrication of well-defined hollow carbon nanospheres with a highly porous shell structure is still a very important but really challenging issue. Herein, we report a facile hypercrosslinking strategy to prepare hollow microporous carbon nanospheres with a BET surface area as high as 1166 m² g⁻¹. SiO₂@polystyrene core–shell nanospheres were first prepared, and then were treated through a hypercrosslinking reaction to provide the polystyrene shell with well-developed microporosity. Moreover, the as-constructed hypercrosslinked shell structure ensures a good framework carbonizability and nanostructure inheritability during the high-temperature carbonization process. The porous structure and morphology of the resulting hollow microporous carbon nanospheres can be easily tailored by varying the hypercrosslinking and carbonization conditions. Due to the rational integration of a highly microporous shell structure and a well-defined hollow spherical morphology in the nanometer range, the hollow microporous carbon nanospheres prepared here show good electrochemical performances as active electrodes of lithium ion batteries and supercapacitors.

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Introduction

Hollow carbon nanospheres (HCNSs), a class of intriguing nanoporous carbon materials, have received an increased level of attention by virtue of their special shape, low density and large interior void space fraction. These features are of great interest for a broad spectrum of applications, such as energy storage, adsorption, catalysis and drug delivery.¹ The core technology of HCNS is the design and control over the shell structure. In particular, it has been well demonstrated that incorporation of nanopores into the shell structure not only provides a large surface area for dispersion of active sites, but also serves as a favorable pathway for guest molecules to enter the void space.² Consequently, integration of a robust porous structure into the shell of HCNS is highly desirable and important for their practical applications.

Up to now, significant research has been directed towards the synthesis of HCNS with designed functionality, suitable morphology and tunable composition.^1b–d,3 Despite a great deal of effort, most of the carbonaceous shells of the HCNSs are suffering from poor nanoporosity. This could be because a small amount of pores within their carbonaceous framework are mainly generated from the burn-off of non-carbon elements and carbon-containing compounds during pyrolysis.^1k,4 These conventional HCNSs without sufficient permeable channels are limited in applications, especially in the areas that need fast mass transport. With the development of templating synthetic strategy, HCNSs with a porous shell structure have been explored. Nevertheless, many of them usually possess specific surface areas below 1000 m² g⁻¹. Although further chemical activation can generate a well-developed porous structure with high specific surface area, extensive creation of pores could easily cause the collapse of their nanostructures, especially for the nanospherical morphology.⁵ Therefore, the development of facile synthetic procedures for fabrication of well-defined HCNSs with a highly porous shell structure is still a very important but really challenging issue.

Here we report a new class of HCNSs with a well-developed microporous shell structure through a facile hypercrosslinking strategy. The overall synthetic procedure is illustrated in Fig. 1. SiO₂@polystyrene (SiO₂@PS) core–shell nanospheres are initially prepared through an emulsion polymerization procedure. Then SiO₂@PS nanospheres are treated through a facile hypercrosslinking reaction to provide the PS shell with a well-developed microporosity. More importantly, the hypercrosslinking endows the shell with a rigid hypercrosslinked structure characteristic, which ensures good framework carbonizability and nanostructure inheritability during the high-temperature carbonization process. Therefore, after the carbonization treatment and the subsequent removal of the SiO₂ component, hollow microporous carbon nanospheres (HMCNSs) with a BET surface area as high as 1166 m² g⁻¹ are obtained. Due to the rational integration of highly microporous shell structure and well-defined hollow spherical morphology in the nanometer range, the as-prepared HMCNSs show good electrochemical performances as active electrodes of supercapacitor and lithium ion battery.

Fig. 1 Schematic illustration of fabrication of hollow microporous carbon nanospheres.

Experimental section

Materials

Styrene (St, Aldrich, 99%) was purified by passing through a basic alumina column. Divinylbenzene (DVB, Aldrich, 80%), anhydrous aluminum chloride (AlCl₃, Aldrich, 99.99%), 3-(trimethoxysilyl)propyl methacrylate (MPS, Aldrich, 97%), sodium bicarbonate (NaHCO₃, Shanghai Chemical Reagent Factory, A.R.), tetraethyl orthosilicate (TEOS, Shanghai Chemical Reagent Factory, A.R.), ammonia water (NH₃·H₂O, Guangzhou Chemical Reagent Factory, 25%), hydrofluoric acid (HF, Guangzhou Chemical Reagent Factory, 40%), carbon tetrachloride (CCl₄, Tianjin Chemical Reagent Factory, A.R.), sodium dodecyl benzene sulfonate (SDBS, Tianjin Chemical Reagent Factory, A.R.) and potassium persulfate (KPS, Tianjin Chemical Reagent Factory, A.R.) were used as received.

Preparation of functionalized SiO₂ nanoparticles

The SiO₂ nanoparticles were synthesized according to the Stöber method. Typically, NH₃·H₂O (8.4 mL), water (104.1 mL) and ethanol (112.5 mL) were added into a round-bottom flask at ambient temperature; the mixture was stirred at a speed of 300 rpm. A mixture of TEOS (22.5 mL) and ethanol (202.5 mL) was poured into the flask. The reaction mixture was stirred at 30 °C for 3 h. A solution of MPS (2.25 mL) in ethanol (100 mL) was then dropped into the dispersion of the above SiO₂ nanoparticles solution within 8 h under the same speed stirring at 30 °C for 36 h. After three cycles of centrifugation (12 000 rpm, 15 min) and redispersion with ethanol, and drying at 45 °C under vacuum for 12 h, the functionalized SiO₂ nanoparticles were obtained.

Preparation of SiO₂@PS nanospheres

SiO₂@PS nanospheres were prepared by emulsion polymerization. In a typical experiment, 1.2 g of functionalized SiO₂ nanoparticles (122 nm in diameter) dispersed in 10 mL of ethanol by ultrasonication, 0.03 g of SDBS as emulsifier, 0.24 g of NaHCO₃ as buffer agent, 100 mL of water as the dispersion medium, 0.1 g of KPS as initiator, 1 mL of DVB as precrosslinking agent and 9 mL of St were placed into a four-neck flask under stirring (250 rpm). The polymerization was then carried out in an atmosphere of nitrogen for 12 h at 72 °C. The emulsion was washed in ethanol via three cycles of centrifugation (12 000 rpm, 10 min) and redispersion with ethanol and then dried at 50 °C under vacuum for 12 h.

Preparation of SiO₂@xPS nanospheres

For synthesis of SiO₂@xPS, 1.0 g of SiO₂@PS was dispersed in 60 mL of CCl₄ and swelled for 24 h, and 2.8 g of AlCl₃ was then added to the above solution. The hypercrosslinking reaction was carried out at 75 °C for designated times under stirring. Then, 60 mL HCl/actone mixture solution was added. The resulting product was filtered off, washed with HCl/actone mixture solution and then dried at 100 °C under vacuum for 12 h. Subsequently, the SiO₂@xPS nanospheres were obtained. The resulting SiO₂@xPS nanospheres were denoted as SiO₂@xPS-y, where y indicates the hypercrosslinking reaction time.

Preparation of HMCNSs

The as-prepared SiO₂@xPS nanospheres were carbonized at designated temperatures with different heating rates in N₂ flow. After using HF to remove the silica core, the HMCNSs were obtained. The resulting HMCNSs obtained at 900 °C for 3 h with a heating rate of 5 °C min⁻¹ were denoted as HMCNS-z, where z indicates the hypercrosslinking reaction time of their precursory SiO₂@xPS nanospheres.

Structural characterizations

The morphologies of the samples were observed by a JSM-6330F scanning electron microscope (SEM) and a Hitachi S-3400 transmission electron microscope (TEM). A Micromeritics ASAP 2020 surface area and porosity analyzer was used to investigate the pore structure. The BET surface area (S_BET) was analyzed by Brunauer–Emmett–Teller (BET) theory. The micropore surface area (S_mic) was determined by t-plot method and the total pore volume (V_t) was calculated from the amount adsorbed at a relative pressure P/P₀ of 0.995. The pore size distribution was analysed by original density functional theory (DFT) with non-negative regularization and medium smoothing. XRD pattern was recorded on a D-MAX 2200 VPC diffractometer using Cu Kα radiation (40 kV, 26 mA). The Raman spectrum was obtained with a Renishaw in via Raman spectrometer.

Electrochemical characterizations

The electrochemical performances of HMCNS-24 as electrode material of lithium ion battery were measured in 2032 coin cells. Li metal was used as counter/reference electrode. The working electrode was made of HMCNS (80 wt%), super P (10 wt%) and polyvinylidene fluoride (PVDF, 10 wt%) slurry coated onto a copper foil substrate. The electrodes were cut into circular disks with a diameter of 12 mm and an active material loading density of about 0.3 mg cm⁻². The electrolyte was 1 mol L⁻¹ LiPF₆ in a mixture of ethylene carbonate (EC), dimethyl carbonate (DMC) and diethyl carbonate (DEC) (1 : 1 : 1 by volume). A microporous membrane (Celgard 2300) was used as the separator. The coin cells were assembled in an argon filled glove box (Mbraun). The galvanostatic charge/discharge tests were conducted in the potential range from 0.01 to 3.0 V at different current densities. On the other hand, the electrochemical performances of HMCNS-24 as electrode material of supercapacitor were measured in 6 M KOH using a sandwich-type two-electrode testing cell at ambient condition. HMCNS electrodes in the form of round sheet were obtained by pressing a mixture film of 92 wt% HMCNS and 8 wt% polytetrafluorethylene into a nickel foam current collector. The loading density of the active material in each electrode is about 3.6 mg cm⁻². Galvanostatic charge–discharge tests were executed at different current densities over a voltage range of 0–1.0 V using Arbin BT2000 instrument. Cyclic voltammetry (CV) measurement was performed at different sweep rates with an IM6e electrochemical workstation from −1 V to 0 V. The specific capacitance (C_m) was calculated according to the equation C_m = 2 × I_mt/ΔV, where I_m, t and ΔV represent the current density, the discharge time and the discharge voltage, respectively.

Results and discussion

Synthesis of hollow microporous carbon nanospheres

As illustrated in Fig. 1, the templates of hollow cores, i.e., SiO₂ nanoparticles are prepared through an improved Stöber method. Then the surface of the SiO₂ nanoparticles is functionalized by reaction with 3-(trimethoxysilyl)propyl methacrylate to introduce C=C containing surface for an emulsion copolymerization reaction of styrene and divinylbenzene. The validity of the functionalization treatment is supported by the Fourier-transform infrared (FTIR) spectrum (Fig. S1†). Furthermore, SEM image analysis shows that the resulting functionalized SiO₂ nanoparticles have similar spherical nanostructures including average diameter (ca. 122 nm) and monodispersity to their parent SiO₂ nanoparticles, illuminating that the functionalization reaction essentially does not change the morphology (Fig. 2A, S2, S3A and S4A†).

Fig. 2 SEM images of (A) functionalized SiO₂ nanoparticles, (B) SiO₂@PS nanospheres, (D) SiO₂@xPS-24 nanospheres and (E) HMCNS-24, TEM images of (C) SiO₂@PS nanospheres and (F) HMCNS-24.

Subsequently, the divinylbenzene-precrosslinked polystyrene shells are coated onto these functionalized SiO₂ nanoparticles through an emulsion copolymerization reaction. Fig. 2B reveals that the as-prepared SiO₂@PS nanosphere presents a uniform spherical morphology, and its diameter is enlarged to be about 198 nm (Fig. S3B†). TEM image in Fig. 2C confirms the successful formation of a homogeneous polymeric shell onto the SiO₂ core, where a dark core is encapsulated in a gray layer. Dynamic light scattering (DLS) curve in Fig. S4B† shows that the SiO₂@PS nanospheres demonstrate high monodispersity characteristic with a low polydispersity index (PDI) of 0.022, indicating that the emulsion copolymerization reaction does not occur between the neighboring nanospheres.

Afterwards, the hypercrosslinking of SiO₂@PS nanospheres is carried out after adding the mixture of anhydrous aluminum chloride and carbon tetrachloride under stirring for 24 h, resulting in the core–shell structured SiO₂@xPS-24 nanospheres. SEM observation reveals that the obtained SiO₂@xPS-24 still remains the distinct and original good nanospherical morphology (Fig. 2D). The diameter of SiO₂@xPS-24 is measured to be ca. 229 nm in Fig. S3C,† which is a little larger than that of SiO₂@PS. During the hypercrosslinking process, the PS chains in the shell of SiO₂@PS are swollen in carbon tetrachloride and then undergo inter-sphere hypercrosslinking by formation of –CCl₂– crosslinking bridges (Fig. S4C and S5†). The –CCl₂– crosslinking bridges are subsequently converted to –CO– groups by hydrolysis. The formation of –CO– crosslinking bridges is confirmed by FTIR spectrum (Fig. S6†). Such a hypercrosslinking process subdivides the original solid PS shell into numerous microporous xPS shell (Table S1†). These micropores are mainly centered at 1.3 nm (Fig. S7†). Moreover, the hypercrosslinking reaction guarantees good framework carbonizability and nanostructure stability during the subsequent high-temperature carbonization process.⁶ Therefore, the subsequent high-temperature pyrolysis treatment not only transforms PS shell into carbonaceous shell, but also generates a substantial number of new 0.5 nm-sized micropores in the resulting carbonized SiO₂@xPS-24 (Fig. S8 and Table S1†).

The targeted HMCNS is obtained by removal of SiO₂ core template from carbonized SiO₂@xPS-24. As evidenced by the SEM image in Fig. 2E and TEM image in Fig. 2F, the original good spherical morphology and distinct hollow structure are well retained in the resulting HMCNS-24 sample. It demonstrates that the hypercrosslinked PS framework is sufficiently rigid, which endows the HMCNS-24 sample with good nanostructure inheritability during the carbonization process. Simultaneously, the resulting HMCNS-24 presents good monodispersity with a low PDI of 0.005 (Fig. S4D†), suggesting no obvious aggregation of the spheres after carbonization and removal of SiO₂ core template. The overall diameter of the HMCNS-24 is reduced to about 162 nm by analysing SEM image (Fig. S3D†), indicating a framework shrinkage caused by mass loss after carbonization. Based on TEM image analysis, the typical core size and shell thickness of HMCNS-24 are calculated to be around 122 and 20 nm, respectively (Fig. S9 and S10†). High-resolution TEM image reveals that the shell of HMCNS-24 presents a three-dimensional network-type microporosity (Fig. S11†). The presence of the micropores is also confirmed by an adsorption uptake at the low relative pressure (P/P₀) in its N₂ adsorption–desorption isotherm of the HMCNS-24 (Fig. 3A). According to the DFT pore size distribution of HMCNS-24 in Fig. 3B, the size of micropores of HMCNS-24 is mainly centred at 0.5 and 1.3 nm. The macropores range from 50 to 120 nm, which may be ascribed to the hollow cores and the aggregation of carbon nanospheres. The S_BET is measured to be as high as 1166 m² g⁻¹. The external surface area and microporous surface area are calculated via the t-plot method to be 583 and 583 m² g⁻¹, respectively. The measured total pore volume is 1.36 cm³ g⁻¹.

Fig. 3 (A) N₂ adsorption–desorption isotherm and (B) DFT pore size distribution curve of HMCNS-24.

The as-constructed carbon framework of HMCNS-24 is studied by wide-angle XRD pattern and Raman spectroscopy. The XRD pattern shows broad peaks centered at 2θ ≈ 23° and 44°, demonstrating a low graphite character (Fig. S12†). The Raman spectrum shows two characteristic peaks around 1350 cm⁻¹ and 1600 cm⁻¹, which are attributed to the D (disordered) and G (graphitic) bands, respectively (Fig. S13†). The broader D band further confirms the low degree of crystallinity for carbon framework.

Control of nanostructure of hollow microporous carbon nanospheres

To vary the porous structure and morphology of the HMCNSs, a series of precursor SiO₂@xPS nanospheres with uniform morphology are prepared under different hypercrosslinking reaction times (0.25, 1, 2, 8, 24 and 48 h), as shown in Fig. S14.† It is found that the hypercrosslinking reaction time exerts a profound effect on the hollow microporous sphere structure. With the short hypercrosslinking reaction time, e.g., 0.25 and 1 h, the nanospherical morphology thoroughly disappear and the resulting HMCNS-0.25 and HMCNS-1 are mainly composed of irregular nanoparticles (Fig. S15A and B†). This result implies that the as-obtained polymeric shell framework is not rigid enough under such a low hypercrosslinking reaction time to guarantee a fine inheritability of the nanospherical structure during high temperature carbonization process.⁷ By increasing the hypercrosslinking reaction time from 2 to 48 h, the rigidity of shell framework is significantly improved and thus the nanospherical morphology can be well retained (Fig. S15C–F†). Therefore, the BET surface areas of the HMCNSs increase firstly and then decrease with extending the hypercrosslinking reaction time (Fig. S16†).

The pore structures of HMCNSs can also be readily adjusted by altering the carbonization conditions, including carbonization temperature, time and heating rate (Fig. 4). It can be seen that S_BET increases remarkably as the carbonization temperature increases. Moreover, the carbonization time and heating rate also play important roles. S_BET significantly increases firstly and then decreases with increasing the carbonization time and heating rate. For example, S_BET increases from 551 to 1179 m² g⁻¹ when the carbonization temperature rises from 700 to 1000 °C at a heating rate of 5 °C min⁻¹. With extending the carbonization time from 1 h to 3 h, the S_BET increases notably from 820 to 1166 m² g⁻¹. When further increasing the carbonization time to 5 h, S_BET decreases to 831 m² g⁻¹. In addition, increasing the heating rate from 1 to 5 °C min⁻¹ leads to a distinct increase of S_BET from 468 to 1166 m² g⁻¹, but a further increase of the heating rate to 10 °C min⁻¹ decreases S_BET to 831 m² g⁻¹. It is worth mentioning that the nanospherical morphology is well retained in various HMCNSs in spite of their distinct porous structures (Fig. S17†), indicating the good stability of nanostructure during various carbonization treatments.

Fig. 4 S _BET of HMCNSs obtained at various carbonization conditions, including (A) carbonization temperatures, (B) carbonization times and (C) heating rates.

In order to investigate the importance of the SiO₂ core template during the carbonization process, a control carbon sample is prepared by removal of SiO₂ core template from SiO₂@xPS and subsequent carbonization. It is found that the control carbon sample can't preserve the nanospherical morphology of its polymeric precursor (Fig. S18†). Significant inter-sphere fusion occurred during the carbonization process, and the resulting control carbon sample had a very ill-defined and irregular nanomorphology (Fig. S18B and C†). This comparison result clearly confirms that the support of the SiO₂ core template is crucial to formation of the well-defined carbon nanospherical morphology.

Electrochemical performance of hollow microporous carbon nanospheres

Owing to their well-defined microporous shell and hollow carbonaceous nanospherical structure, this new type of HMCNS developed here is potential in many applications. The as-prepared HMCNS-24 can be applied as an anode of lithium-ion battery. Fig. 5A shows typical discharge–charge voltage profiles for the initial three cycles of HMCNS at a current density of 100 mA g⁻¹. The sample shows a very high first-cycle discharge capacity of 3588 mA h g⁻¹ and delivers a corresponding charge capacity of 1625 mA h g⁻¹, giving an initial coulombic efficiency of 45.3%. Afterwards, the coulombic efficiency goes up to 85.2% and 89.2% for the second and third cycles, respectively. The irreversible capacity loss could be mainly attributed to irreversible processes such as inevitable formation of solid electrolyte interface (SEI) and electrolyte decomposition, which are similar to the previous studies.⁸ Cyclic voltammetry was further carried out to elucidate the reactive process. Fig. S19A† shows the first three CV curves of HMCNS-24 in the range of 0.01–3 V at a scan rate of 0.1 mV s⁻¹. In the first cycle, two-step discharge process can be found obviously: (i) the dominant cathodic peak at 0.65 V can be ascribed to the formation of SEI layer; (ii) another cathodic peak below 0.30 V corresponds well with the reversible insertion of Li into the graphite layers and the nanocavities located at the amorphous areas. Furthermore, the disappearance of the cathodic peak at 0.65 V in the second and third cycles indicates the formation of stable SEI layer in the first cycle.

Fig. 5 (A) The galvanostatic discharge–charge curves of the first three cycles at a current density of 0.1 A g⁻¹ and (B) cycling performance and columbic efficiency at a current density of 0.1 A g⁻¹ of HMCNS-24. (C) Cyclic voltammogram curves at the sweep rate of 500 mV s⁻¹ and (D) the mass specific capacitance retention as a function of sweep rate for HMCNS-24 and YP-50.

The cycling performance of HMCNSs under a current density of 100 mA g⁻¹ is shown in Fig. 5B. The discharge capacity remains stable at around 620 mA h g⁻¹ when the test was prolonged to 50 cycles. Such high reversible capacities exceed that of many previously reported typical carbon nanospheres as anode materials (Table S2†). Moreover, the rate performance of HMCNS-24 was tested with the current densities ranging from 100 to 2000 mA g⁻¹ (Fig. S19B†). The average specific capacities are calculated to be 1610, 588, 420, 396, 340 and 279 mA h g⁻¹ at current densities of 100, 200, 500, 800, 1000 and 2000 mA g⁻¹, respectively. After the high-rate charge–discharge cycling, the specific capacity is recovered to 696 mA h g⁻¹ when the current density reduced immediately to 100 mA g⁻¹, revealing a good rate performance and reversibility of the electrode.

Supercapacitors have drawn considerable attention in recent years due to their high power density, long cycle life, and short charging period. Carbon-based materials play a significant role in the development of alternative clean and high-performance supercapacitors.⁹ The good electrochemical performance is also observed in the electrochemical capacitive measurement employing HMCNS-24 as supercapacitor electrode. For comparison, a commercial activated carbon YP-50 used for supercapacitor is investigated under the same experimental conditions. As shown in Fig. 5C and S20A,† HMCNS-24 shows better rectangle-shaped cyclic voltammogram curves when compared to YP-50, especially at the high sweep rates (e.g., 500 mV s⁻¹). It indicates that HMCNS-24 facilitates more rapid ion diffusion than YP-50, especially during large current charge–discharge processes. The Nyquist plots in Fig. S20B† display that the diameter of the semicircle in correlation with the charge-transfer resistance of the electrode for HMCNS-24 (2.6 ohm) is smaller than that of YP-50 (7.5 ohm) in the high frequency region. It suggests that HMCNS-24 has lower impedance at the electrode/electrolyte interface and faster ion movement inside the nanopores. The resistance for ion diffusion critically influences the capacitive performance. As shown in Fig. 5D, no matter whether the current density is high or low, HMCNS-24 shows excellent capacitive performances when compared to YP-50. For example, the specific mass capacitance of HMCNS-24 is as high as 174 F g⁻¹ at a current density of 0.05 A g⁻¹, higher than that of YP-50 (i.e., 157 F g⁻¹). Even increasing the current density to 10 A g⁻¹, HMCNS-24 still presents a remarkably high specific capacitance of 113 F g⁻¹, which is more than 3 times as high as that of YP-50 (i.e., 37 F g⁻¹). This manifests that HMCNS-24 possesses better rate performances than YP-50. The corresponding capacitance retention ratios of HMCNS-24 are comparable with or even superior to many other high-performance carbon spheres reported previously (Table S3†). The significant superiority of HMCNS-24 over YP-50 in terms of the supercapacitive performances could be attributed to its more advanced pore structure. Generally, most of the pores in YP-50 are micropores (Fig. S21 and Table S1†). They usually locate in the micron/millimeter-scaled carbon particles and are essentially unconnected to each other (Fig. S22†). Then most of its inner micropores are difficult to be immersed by electrolyte due to absence of enough transport pathways for the ion diffusion, leading to a low rate performance. As for HMCNS-24, the micropores on the carbon shells can strongly adsorb a large quantity of electrolyte ions for high capacitances. In addition, its uniform nanospheres with hollow cores can significantly decrease the ion diffusion length (half of the shell thickness, e.g., 10 nm), greatly enhancing the availability of ion-accessible micropores and ion transport rate. HMCNS-24 exhibits a good cycling stability with barely visible degradation at a current density of 1 A g⁻¹ over 5000 cycles (Fig. S23†), indicating a robust architecture of the hollow nanospherical carbon framework of HMCNS-24.

Conclusions

In summary, we have successfully prepared a new class of HCNSs with a well-developed microporous shell structure through a facile hypercrosslinking strategy. It is well demonstrated that hypercrosslinking and SiO₂ template are the critical roles for constructing HMCNSs with good hollow nanospherical morphology, which ensure a good framework carbonizability and nanostructure inheritability during the high-temperature carbonization process. The as-prepared HMCNSs show good electrochemical performances as active electrodes of supercapacitor and lithium ion battery. We hope that the HMCNSs may trigger the potential use as advanced nanomaterials in many other applications including drug sustained-release, CO₂ capture, gas storage and sensing of toxic molecules.

Acknowledgements

The projects of Guangdong Natural Science Funds for Distinguished Young Scholar (S2013050014408), NSFC (51422307, 51372280, 51173213, 51172290, 51232005), National Program for Support of Top-notch Young Professionals, China Postdoctoral Science Foundation (2015T80932), Program for New Century Excellent Talents in University (NCET-12-0572), Program for Pearl River New Star of Science and Technology in Guangzhou (2013J2200015) and National Key Basic Research Program of China (2014CB932402) are acknowledged for funding.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Additional information about material characterization. See DOI: 10.1039/c6ra04658d

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