Sci. Bull. (2016) 61(15):1195–1201 DOI 10.1007/s11434-016-1119-6

www.scibull.com www.springer.com/scp

Article

Materials Science

Soft-to-hard templating to well-dispersed N-doped mesoporous carbon nanospheres via one-pot carbon/silica source copolymerization Qinglu Kong • Lingxia Zhang • Min Wang Mengli Li • Heliang Yao • Jianlin Shi

•

Received: 22 March 2016 / Revised: 10 May 2016 / Accepted: 18 May 2016 / Published online: 20 June 2016 Ó Science China Press and Springer-Verlag Berlin Heidelberg 2016

Abstract Here we report a new approach referred as ‘‘softto-hard templating’’ strategy via the copolymerization of carbon source (dopamine) and silica source (tetraethyl orthosilicate) for the synthesis of well dispersed N-doped mesoporous carbon nanospheres (MCNs), which exhibit high performance for electrochemical supercapacitor. This method overcomes the shortcoming of uncontrolled dispersity and complicated procedures of soft- or hard-templating methods, respectively. Moreover, the synthesized MCNs feature enriched heteroatom N-doping and easy functionalization by noble-metal nanoparticles during the one-pot synthesis. All the above characters make the asprepared MCNs a promising platform in a variety of applications. To demonstrate the applicability of the synthesized nitrogen-doped MCNs, this material has been employed as an electrode for high-performance electrochemical supercapacitor, which shows a capacitance of 223 and 140 F/g at current densities of 0.5 and 10 A/g in 1 mol/L KOH electrolyte, respectively. Keywords Mesoporous carbon  Dopamine  Ndoped  Supercapacitor

Electronic supplementary material The online version of this article (doi:10.1007/s11434-016-1119-6) contains supplementary material, which is available to authorized users. Q. Kong  L. Zhang (&)  M. Wang  M. Li  H. Yao  J. Shi (&) State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China e-mail: [email protected] J. Shi e-mail: [email protected]

1 Introduction Carbon nanospheres (CNs) have received great interest because of their great potential for applications in drug delivery, lithium batteries, oxygen reduction catalysis and nanodevices, etc. [1–14]. Owning to their large surface area, open framework structure, excellent electrical conductivity and good biocompatibility, mesoporous carbon nanospheres (MCNs) have demonstrated great advantages over other carbon materials [15–18]. Despite of many efforts made to the fabrication of MCNs [19–24], the most reported approaches are time-consuming and involve the environmental-unfriendly precursors and/or soft or hard templates [24–26]. In addition, traditional techniques also suffer from low yield and complicated procedures of separation, and often lead to reduced electric conductivity of MCNs due to the domination of sp3 hybridization in the framework structure. On the other hand, active nitrogen heteroatoms play a vital role in the electro-catalysis application of carbon materials [27]. Therefore, there is yet considerable rooms for the environmental-friendly fabrication and performance enhancement of MCNs. It is highly desirable to explore a novel and effective approach that features environmentalfriendliness using nontoxic carbon precursors instead of harmful phenol/formaldehyde and high yield in fabricating MCNs with controlled nanostructure, particle size and enriched heteroatom doping in the mesoporous framework. The soft-templating approach is usually simple but might lead to the uncontrollable morphology, unstable dispersity and interparticle aggregations; while hard-templating route provides controlled morphology but suffers from complicated multi-step. Herein, we report a facile and innoxious approach, referred as a soft-to-hard-templating strategy via the copolymerization of carbon source (dopamine) and silica source (tetraethyl orthosilicate, TEOS), for

123

1196

large-scale synthesis of well dispersed MCNs. While the template, the silica source TEOS, is ‘‘soft’’ at first, then becomes ‘‘hard’’ (SiO2) after the hydrolysis and copolymerization with dopamine. And the final removal of SiO2 gives the product of MCNs. Dopamine is a non-toxic and sustainable carbon source especially compared with the widely adopted phenol/formaldehyde. Its amino groups endow it with convenient N-doping in carbon and a great advantage of strong chelating capability with many metal ions over other carbon sources, which facilitates the loading of many functional metal ions. Specifically, the dopamine-based carbon materials have been reported containing much less sp3 hybridized C (higher electric conductivity) than widely used phenol/formaldehyde-based carbon materials [28]. This work used the obtained MCNs as an electrode for high-performance electrochemical supercapacitor, which showed rather high capacitance (223 and 140 F/g at 0.5 and 10 A/g in 1 mol/L KOH electrolyte, respectively) and excellent cyclic stability.

2 Materials and methods 2.1 Synthesis 2.1.1 Chemicals and reagents All chemicals were used without further purification. Tetraethyl orthosilicate (TEOS), NaOH, ammonia solution (NH4OH, 25 %–28 %), carbon black, and poly-vinylidene fluoride (PVDF) were purchased from Sinopharm Chemical Reagent Co. Hexadecyltrimethyl ammonium chloride (CTAC, 25 wt% in water), dopamine hydrochloride (technical grade, 95 wt%), hexachloroplatinic acid hexahydrate (H2PtCl66H2O), and poly-N-vinyl-2-pyrrolidone (PVP, average molecular weights Mw = 40,000) were purchased form Sigma-Aldrich. Ethanol was obtained from Shanghai Lingfeng Chemicals Co. Milli-Q water (0.52 MX cm at 26 °C) was used in the entire experiments. 2.1.2 Fabrication of MCNs, CNs, and Pt-encapsulated MCNs The detailed fabrication process of N-doped MCNs is illustrated in Fig. 1. Dopamine and TEOS were used as organic polymer source and inorganic polymer precursor, respectively, while CTAC was employed as a soft-template. The copolymerization of organic and inorganic precursors constituted the mesoporous framework. Carbonization was then followed and silica was removed, after which MCNs were obtained. In a typical synthesis of MCNs, NH4OH (0.5 mL, 28 %) was added to a solution that contained ethanol (30 mL), deionized water (90 mL) and CTAC

123

Sci. Bull. (2016) 61(15):1195–1201

solution (1 g, 25 wt% in water) under mild stirring at room temperature for 30 min. Dopamine hydrochloride (1 g) was dissolved in deionized water (10 mL) and then directly decanted into the above solution. The color of the solution turned to pale brown immediately. Then TEOS (1.3 mL) was added to the reaction solution and the reaction was allowed to proceed for 24 h. The solid product was obtained by centrifugation and washed with deionized water for three times. After vacuum freeze-drying overnight, the resultant product was calcined in a tubular furnace at 800 °C for 2 h in N2 flow at a rate of 5 °C/min. The obtained carbon–silica composites were further treated with 1 mol/L NaOH at 80 °C to remove silica and washed with water for three times, then MCNs were obtained. CNs was prepared under the same condition without TEOS. Pt nanoparticles was synthesized as reported in Ref. [29]. 3.33 mg PVP was dissolved in a mixture of 5 mL 6 mmol/L H2PtCl6 solution and 40 mL ethanol by ultrasonic. Then the above mixture solution was refluxed in a 100 mL flask for 3 h and turned from yellow to dark brown. PVP-protected Pt nanoparticles in solution were obtained and used without further purification. Pt-encapsulated MCNs were fabricated under the same condition as MCNs except 30 mL ethanol was substituted by a mixture of 25 mL ethanol and 5 mL solution of PVP-protected Pt nanoparticles. 2.2 Characterization Transmission electron microscopy (TEM) images were recorded on a JEOL-2010F electron microscope operated at 200 kV. Fourier transform infrared spectroscopy (FTIR) was performed on a Nicolet 7000-C spectrometer in a range of wavenumber from 400 to 4,000 cm-1 by using pressed KBr tablets. X-ray photoelectron spectroscopy (XPS) signals were collected on a VG Micro MKII. Raman spectrum was obtained on a DXR Raman microscope (Thermal Scientific Co.) with a 532 nm excitation length. Dynamic light scattering (DLS) measurement was carried on Zetasizer nanoseries (Nano ZS90) in water solution at 25 °C. Nitrogen adsorption– desorption isotherms and pore size distribution were recorded on a Micrometitics Tristar 3000 system. X-ray diffraction (XRD) pattern was collected on a Rigaku D/MAX-2250 V diffractometer with graphite-monochromatized Cu Ka radiation. 2.3 Electrode preparation and electrochemical measurements Electrochemical measurements were performed with a three-electrode system on a CT2001A electrochemical work station under ambient temperature. In a three-

Sci. Bull. (2016) 61(15):1195–1201

1197

Fig. 1 (Color online) Schematic illustration of the synthesis of MCNs

electrode system, 1 mol/L KOH acted as the aqueous electrolyte while a platinum plate and the Hg/HgO electrode were used as the counter electrode and the reference electrode respectively. The working electrodes were prepared by loading a mixture of 80 wt% MCNs, 10 wt% carbon black, and 10 wt% PVDF (as a binder) on a nickel foam (1 cm2) and then pressed under 20 MPa for 1 min. The prepared working electrodes were dried at 60 °C overnight. The electrochemical impedance spectroscopy (ELS) was measured in a frequency from 10 Hz to 10 kHz with alternate current amplitude of 5 mV at an open circuit voltage. The gravimetric specific capacitance C (F/g) of the MCNs electrode in KOH (1 mol/L) was calculated on the cyclic voltammetry (CV) and galvanostatic charge/discharge curves according to the following equation C¼

I  Dt ; m  DV

ð1Þ

where I is the discharge current (A), Dt is the discharge time (s), DV is the potential window (V) in the discharge process and m is the total mass of the active materials (g).

3 Results and discussion 3.1 Structural characteristics of MCNs As illustrated in Fig. 2b, the typical TEM image shows that the resultant MCNs are uniform in size and spherical morphology. Numerous mesopores can be identified in the enlarged image of Fig. 2c, indicating the generation of mesoporous structure during the synthesis. As expected, the MCNs could be dispersed in water to form stable suspension according to DLS analysis (Fig. S1 online). And the DLS analysis result shows slightly larger particle size compared with TEM, most likely, a result of a hydration layer formed on the hydrophilic surface. In order to study deeply the role of silica source TEOS in the formation of MCNs, the experiment under the same condition without adding TEOS was performed. As shown in Fig. 2a, the

resultants CNs have smaller particle size and no mesoporous structure is formed, indicating that TEOS acts as a template precursor for the mesostructure. To further investigate the applicability of this approach, Pt nanoparticles were encapsulated in MCNs (Fig. 2d), forming a hybrid structure. Because of relatively inert property and complicated fabrication procedures of MCNs reported in literatures, it has been usually indispensible and difficult to functionalize MCNs for facilitating their application. In this study, noble-metal nanoparticles could be facilely incorporated within the MCNs, which might boost their utility in a wide range of fields. The XRD pattern of the MCNs is shown in Fig. 3a. Two characteristic peaks of the MCNs, which are located at 26° and 44° corresponding to the (002) and (101) planes of hexagonal graphite, indicate that the obtained MCNs have a hexagonal graphite structure. For Pt incorporated MCNs, XRD shows two weak peaks at 40° and 46°, ascribed to Pt(111) and (200) facets, respectively, indicating the successful introduction and small Pt particle size (Fig. S2 online). The relatively high-degree graphitization of MCNs normally implies a high electric conductivity, which is of great importance in delivering excellent capacitance. The Raman spectrum of MCNs (Fig. 3b) shows two broad peaks centered at around 1,366 and 1,592 cm-1 which are assigned to D and G bands of carbon, respectively. The intensity ratio of G and D band (IG/ID) calculated from the Raman results is as high as 1.16, revealing that the asprepared MCNs are relatively highly graphitized, which is consistent with the XRD analysis. Elemental analysis and XPS measurements were performed to identify the chemical composition of the MCNs, as shown in Table S1 and Fig. S3 (online). The results of elemental analysis and XPS measurement agree well with each other, demonstrating that N content is 3.66 at% and 4.76 at% respectively. C 1s, N 1s and O 1s peaks can be clearly seen in the XPS spectrum. The high resolution of N 1s spectrum (Fig. 4) could be fitted with three bands peaked at 398.2, 400.5 and 402.5 eV, corresponding to the pyridinic nitrogen, quaternary nitrogen, and oxidized

123

1198

Sci. Bull. (2016) 61(15):1195–1201

Fig. 2 (Color online) TEM images of the synthesized carbon nanoparticles CNs (a), MCNs (b), (c), and Pt-encapsulated MCNs (d)

Fig. 4 (Color online) The high resolution N 1s spectrum of MCNs

Fig. 3 The wide XRD pattern (a) and Raman spectrum (b) of the synthesized MCNs

123

nitrogen, respectively. The results indicate the partial conversion of N atoms from the amino groups of polydopamine into doped nitrogen in carbon during the carbonization. As demonstrated in previous reports [13, 15, 30], the heteroatom nitrogen in the carbon materials could substantially increase the electronic conductivity, and both of pyridinic and quaternary nitrogen atoms could act as the initial active sites for pseudocapacitive interactions, which makes the as-prepared MCNs promising materials for supercapacitor application. To further

Sci. Bull. (2016) 61(15):1195–1201

determine the surface chemical state of MCNs, the FTIR spectrum (Fig. S4 online) was obtained and the characteristic peaks of several functional groups were identified, which is consistent with the XPS analysis. A clear evidence for the mesoporous structure of MCNs was obtained from N2 adsorption–desorption isotherms (Fig. 5a). There is an obvious N2 adsorption step at a relative pressure (P/P0) of 0.4–1.0, indicating the presence of mesoporosity in MCNs derived from silica removal. By calculation using the Barrett–Joyner–Halenda (BJH) method, the pore size distribution (insert in Fig. 5a) can be obtained and mesopores of 3.9 nm in diameter are dominant in MCNs. The surface area of MCNs reaches as high as 976 m2/g. To further confirm the formation of mesopore structure of MCNs, the former carbonized polydopamine– silica composites were burned in air, resulting in the formation of mesoporous silica nanospheres (MSNs). Compared with the pore size distribution of MCNs, the MSNs

1199

have a relatively smaller pore size (*2.5 nm), higher surface area (1,050 m2/g) and larger particle size. On the basis of the above observations, we proposed a one-pot carbon/silica source copolymerization approach for the fabrication of MCNs. The hydrolysis/copolymerization of carbon source dopamine and silica precursor TEOS would take place in a basic mixture of ethanol, ammonia, and CTAC, which form the emulsion droplets under the assistance of hydrogen bonding among water, dopamine and silicates, resulting in organic/inorganic hybrid polydopamine–silica colloid nanospheres. It should be noted that TEOS plays an indispensible role as a template in the synthesis of MCNs. As mentioned above, in the absence of TEOS the final product was aggregated carbon nanoparticles without mesostructure. In the conventional hard-templating route, a hard template has been prepared in advance, and followed by the creation of desired components by various ways such as intra-porous intrusion, infiltration, coating, etc. However, in the present report, ‘‘soft’’ molecular silica source (TEOS) was used together with the carbon source, and the final product, MCNs, was obtained simply via a one-pot hydrolysis/copolymerization of two sources, and followed by the removal of TEOS-hydrolyzed hard silica template. Therefore, to distinguish from the general hardtemplating route, silica in the present study can be regarded as a soft-to-hard template. 3.2 Electrochemical properties of MCNs for supercapacitor

Fig. 5 (Color online) Nitrogen adsorption–desorption isotherms (inset: pore size distribution) of MCNs (a) and MSNs (b) (Tailoring pore size of 387 nitrogen-doped hollow carbon nanospheres for confining sulfur in 388 lithium -sulfur batteries), prepared from the silica etching and carbon burn-out from carbonized polydopamine– silica composites, respectively

Mesoporous structure facilitates ion transport by providing small resistance, short diffusion path and high surface area, while nitrogen-based functional groups make MCNs promising material for electrochemical supercapacitor with enhanced performance in KOH electrolyte as well. To evaluate the electrochemical behaviour of the MCNs, CV measurement was performed at varied sweep rates from 5 to 100 mV/s in a three-electrode system using 1 mol/L KOH as the electrolyte. The potential window for the cycling tests was confined between -1 and 0 V versus Hg/HgO electrode. The CV curves (Fig. 6a) of the MCNs show nearly a rectangular shape, which is well retained up to 100 mV/s, indicating the existence of double-layer capacitance behaviour. As seen in Fig. 4a, the CV curves display a slightly distorted profile, which might derive from the inherent electric resistivity of the MCNs by, e.g., doped heteroatoms in the mesoporous wall and the electrochemical hydrogen storage reaction of the MCNs in KOH. To further highlight the electrochemical performance of our samples, galvanostatic charge/discharge curves were obtained at different current densities in above three-electrode system within the same voltage windows as for the above CV tests, as shown in Fig. S5 (online). The charge/discharge plots are nearly

123

1200

Sci. Bull. (2016) 61(15):1195–1201

Fig. 6 (Color online) Electrochemical performance of MCNs in a three-electrode system using 1 mol/L KOH as electrolyte at room temperature. a CV curves of MCNs at different scan rates. b Nyquist plots of the MCNs electrode in the frequency range of 100 kHz–0.01 Hz measured by the electrochemical impedance spectroscopy (EIS) analysis at a scan rate of 5 mV/s in 1 mol/L KOH electrolyte before and after 10,000 cycles. c Cycling stability at a current density of 10 A/g up to 10,000 cycles. d Specific capacitance as a function of different current densities

symmetric with a gradual slope change, and no significant voltage drop related to the internal resistance due to the polarity change can be observed, implying its perfect capacitive behaviour. Nyquist plots are shown in Fig. 6b. In low frequency region, the plots are slightly curved and indicative of the existence of pseudocapacitance in MCNsbased electrode, while they form a small semicircle in the high frequency region which implies the low electric resistivity of the MCNs. These characters, in consistence to the above characterizations, reveal the high graphitization feature of the synthesized MCNs. The specific capacitance of the MCNs decreases from 223 to 103 F/g as the current density varies from 0.5 to 20 A/g, maintaining 46 % of the initial capacitance (Fig. 6d). The high-rate supercapacitor may result from the fact that the mesoporous structure of MCNs shortens the transport length of ions and simultaneously provides a free exchange path between electrons and ions. Besides, the nitrogen-doped MCNs with high graphitization favourably offer a high electric conductivity and additional electrochemical active sites which also boost the performance for supercapacitor. To further explore the cycle stability of the MCNs, the MCNs electrode was also tested by a continuous charge–

123

discharge measurement in a 1 mol/L KOH electrolyte. In Fig. 4c, the electrode performance measured at a high density of 10 A/g is demonstrated. It can be clearly observed that the specific capacitance just shows a slight decrease at the beginning and then remains stable independent of the cycle number. As high as 93.3 % of the initial specific capacitance has been preserved after 10,000 galvanostatic charge–discharge cycles, displaying excellent cyclic stability of the MCNs electrode. Moreover, it is remarkable that there is almost no change after 10,000 cycles in the electric resistance, which further illustrates the satisfactory recyclability of the MCNs-based electrode.

4 Conclusions In summary, we proposed a facile and controllable soft-tohard templating approach via the simultaneous carbon/silica source copolymerization to synthesize nitrogen-doped MCNs using dopamine as carbon source and TEOS as initially soft template which then transforms into ‘‘hard’’ during hydrolysis/copolymerization process. This special approach features both the simplicity of soft-templating

Sci. Bull. (2016) 61(15):1195–1201

route and the controlled morphology/high-dispersity of hard-templating method for the MCNs synthesis. The nitrogen-doped MCNs have high surface area (976.44 m2/g), mesoporous structure and well graphitized wall. Deriving from the high graphitization as well as open mesostructure framework doped with nitrogen atoms, the synthesized MCNs demonstrate high performance as supercapacitor electrode with a capacitance of 223 and 140 F/g at current densities of 0.5 and 10 A/g, respectively. At the same time, the MCNs-based electrode shows excellent cyclic stability that remains 93.3 % capacitance after 10,000 cycles. Such a simple approach may be extended to the fabrication of nitrogen-based mesoporous carbon composites for other applications, such as drug delivery, gas adsorption, electrocatalysis and so on. Acknowledgments This work was supported by the National Basic Research Program of China (2013CB933200), the National High Technology Research and Development Program of China (2012AA062703), the National Natural Science Foundation of China (21177137) and the Youth Innovation Promotion Association CAS (2012200). Conflict of interest The authors declare that they have no conflict of interest.

References 1. Liu J, Wickramaratne N, Qiao S et al (2015) Molecular-based design and emerging applications of nanoporous carbon spheres. Nat Mater 14:763–774 2. Zhou W, Wang C, Zhang Q et al (2015) Tailoring pore size of nitrogen-doped hollow carbon nanospheres for confining sulfur in lithium–sulfur batteries. Adv Energy Mater 5:1401752 3. Zhai Y, Dou Y, Zhao D et al (2011) Carbon materials for chemical capacitive energy storage. Adv Mater 23:4828–4850 4. Yang T, Zhou R, Wang D et al (2015) Hierarchical mesoporous yolk–shell structured carbonaceous nanospheres for high performance electrochemical capacitive energy storage. Chem Commun 51:2518–2521 5. Yang T, Liu J, Zhou R et al (2014) N-doped mesoporous carbon spheres as the oxygen reduction reaction catalysts. J Mater Chem A 2:18139–18146 6. Wei J, Liang Y, Zhang X et al (2015) Controllable synthesis of mesoporous carbon nanospheres and Fe–N/carbon nanospheres as efficient oxygen reduction electrocatalysts. Nanoscale 7:6247–6254 7. Wang S, Li W, Hao G et al (2011) Temperature-programmed precise control over the sizes of carbon nanospheres based on benzoxazine chemistry. J Am Chem Soc 133:15304–15307 8. Schuster J, He G, Mandlmeier B et al (2012) Spherical ordered mesoporous carbon nanoparticles with high porosity for lithium– sulfur batteries. Angew Chem Int Ed 51:3591–3595 9. Ma C, Shao X, Cao D (2012) Nitrogen-doped graphene nanosheets as anode materials for lithium ion batteries: a firstprinciples study. J Mater Chem 22:8911–8915 10. Liu L, Deng Q, Agula B et al (2011) Ordered mesoporous carbon catalyst for dehydrogenation of propane to propylene. Chem Commun 47:8334–8336 11. Li C, Meng Y, Wang S et al (2015) Mesoporous carbon nanospheres featured fluorescent aptasensor for multiple diagnosis of cancer in vitro and in vivo. ACS Nano 9:12096–12103

1201 12. Kong Q, Zhang L, Liu J et al (2014) Facile synthesis of hydrophilic multi-colour and upconversion photoluminescent mesoporous carbon nanoparticles for bioapplications. Chem Commun 50:15772–15775 13. Jeong H, Lee J, Shin W et al (2011) Nitrogen-doped graphene for high-performance ultracapacitors and the importance of nitrogendoped sites at basal planes. Nano Lett 11:2472–2477 14. Gu J, Su S, Li Y et al (2011) Hydrophilic mesoporous carbon nanoparticles as carriers for sustained release of hydrophobic anti-cancer drugs. Chem Commun 47:2101–2103 15. Li Z, Xu Z, Tan X et al (2013) Mesoporous nitrogen-rich carbons derived from protein for ultra-high capacity battery anodes and supercapacitors. Energy Environ Sci 6:871–878 16. Konicki W, Cendrowski K, Chen X et al (2013) Application of hollow mesoporous carbon nanospheres as an high effective adsorbent for the fast removal of acid dyes from aqueous solutions. Chem Eng J 228:824–833 17. Goettmann F, Fischer A, Antonietti M et al (2006) Chemical synthesis of mesoporous carbon nitrides using hard templates and their use as a metal-free catalyst for Friedel–Crafts reaction of benzene. Angew Chem Int Ed 45:4467–4471 18. Chen A, Yu Y, Zhang Y et al (2014) Aqueous-phase synthesis of nitrogen-doped ordered mesoporous carbon nanospheres as an efficient adsorbent for acidic gases. Carbon 80:19–27 19. Wan Y, Shi Y, Zhao D (2007) Supramolecular aggregates as templates: ordered mesoporous polymers and carbons. Chem Mater 20:932–945 20. Tang J, Liu J, Li C et al (2015) Synthesis of nitrogen-doped mesoporous carbon spheres with extra-large pores through assembly of diblock copolymer micelles. Angew Chem Int Ed 54:588–593 21. Ren J, Ding J, Chan K et al (2007) Dual-porosity carbon templated from monosize mesoporous silica nanoparticles. Chem Mater 19:2786–2795 22. Lu A, Li W, Hao G et al (2010) Easy synthesis of hollow polymer, carbon, and graphitized microspheres. Angew Chem Int Ed 49:1615–1618 23. Liu J, Yang T, Wang D et al (2013) A facile soft-template synthesis of mesoporous polymeric and carbonaceous nanospheres. Nat Commun 4:2798 24. Fang Y, Gu D, Zou Y et al (2010) A low-concentration hydrothermal synthesis of biocompatible ordered mesoporous carbon nanospheres with tunable and uniform size. Angew Chem Int Ed 49:7987–7991 25. Liu J, Qiao S, Liu H et al (2011) Extension of the Sto¨ber method to the preparation of monodisperse resorcinol–formaldehyde resin polymer and carbon spheres. Angew Chem Int Ed 50:5947–5951 26. Liang C, Li Z, Dai S (2008) Mesoporo¨se Kohlenstoffmaterialien: synthese und modifizierung. Angew Chem 120:3754–3776 27. Tian H, Saunders M, Dodd A et al (2016) Triconstituent coassembly synthesis of N, S-doped carbon–silica nanospheres with smooth and rough surfaces. J Mater Chem A 4:3721–3727 28. Ai K, Liu Y, Ruan C et al (2013) sp2 C-dominant N-doped carbon sub-micrometer spheres with a tunable size: a versatile platform for highly efficient oxygen-reduction catalysts. Adv Mater 25:998–1003 29. Teranishi T, Hosoe M, Tanaka T et al (1999) Size control of monodispersed Pt nanoparticles and their 2D organization by electrophoretic deposition. J Phys Chem B 103:3818–3827 30. Lim S, Li R, Ji W et al (2007) Effects of nitrogenation on singlewalled carbon nanotubes within density functional theory. Phys Rev B 76:195406

123