Surfactant-Free Assembly of Mesoporous Carbon Hollow Spheres

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Surfactant-Free Assembly of Mesoporous Carbon Hollow Spheres with Large Tunable Pore Sizes Hongwei Zhang, Owen Noonan, Xiaodan Huang, Yannan Yang, Chun Xu, Liang Zhou, and Chengzhong Yu ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.6b00723 • Publication Date (Web): 06 Apr 2016 Downloaded from http://pubs.acs.org on April 8, 2016

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Surfactant-Free Assembly of Mesoporous Carbon Hollow Spheres with Large Tunable Pore Sizes Hongwei Zhang,a Owen Noonan,a Xiaodan Huang,a Yannan Yang,a Chun Xu,a Liang Zhou,b and Chengzhong Yu*,a a

Australian Institute for Bioengineering and Nanotechnology, The University of Queensland,

Brisbane, QLD 4072, Australia b

State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan

University of Technology, Wuhan 430070, Hubei, China * Address correspondence to: [email protected]

ABSTRACT: Mesoporous carbon hollow spheres (MCHS) have wide applications, including catalysis, absorption, and energy storage/conversion. Herein, we report a one-pot surfactant-free synthesis of MCHS using three molecules: resorcinol, formaldehyde and tetrapropyl orthosilicate. The co-condensation process between the in-situ generated silica primary particles and the polymer oligomers is regulated, leading to monodispersed MCHS with adjustable pore sizes from micropores to 13.9 nm. The resultant MCHS shows excellent performance for electrochemical double-layer capacitors with high capacitance (310 F g-1 at 1 A g-1), excellent rate capability (157 F g-1 at 50 A g-1), and outstanding cycling stability (98.6 % capacity retention after 10000 cycles at 10 A g-1). Our one-pot synthesis strategy is versatile and can be

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extended to fabricate metal oxide@mesoporous carbon yolk-shell structures in the absence of surfactant, paving the way towards designed synthesis of nanostructured mesoporous carbon composites for various applications.

KEYWORDS: mesoporous hollow carbon, sol-gel process, surfactant-free, supercapacitors, yolk-shell structures

Nanostructured carbon materials and their composites have attracted widespread interest across a range of fields including biomedicine, catalysis, absorption, and energy storage.1-9 Such broad applicability can be attributed to the unique characteristics of carbon materials, such as high chemical and thermal stability, good electrical conductivity, intrinsic hydrophobicity and facile modification of surface chemistry. Mesoporous carbon hollow spheres (MCHS) possess additional advantageous properties such as low density, porous shells, accessible interior space, high surface area and large pore volume when compared with microporous or non-porous materials. In general, methods for the preparation of porous carbon nanospheres can be divided into two categories, the hard-templating approach10-12 and the soft-templating approach.13-18 Recently, Dai and co-workers proposed a “silica-assisted” self-assembly strategy to prepare MCHS, which involves the polymerization of resorcinol-formaldehyde (RF) resin and tetraethyl orthosilicate (TEOS) in the presence of a cationic surfactant.19 The use of TEOS under Stöber conditions is essential for the formation of hollow cavity, however the pore size in shell is limited to 4.7 nm predominately determined by the surfactant templates. We recently reported a surfactant-free approach to synthesize hollow carbon spheres with multi-layered structures (bi- and triplelayered) and controllable morphologies (invaginated, endo-invaginated and intact spheres).20 In


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each layer carbon is microporous in nature and the mesopore volume is predominately contributed by the space in between two layers, limiting their applications where precise control over the mesopores in the wall is required. It remains a challenge to prepare monodispersed hollow carbon spheres with adjustable and large pore size using previously reported strategies, especially using a facile one-pot and surfactant-free approach. In addition to hollow carbon nanostructures, metal oxide@carbon yolk-shell structures have been extensively investigated in adsorption,21 catalysis,22-24 and batteries applications25-28. Templating method is the most efficient method to synthesize metal oxide@carbon yolk-shell structures.29 Typically, metal oxide@silica core-shell structures are firstly prepared to yield silica layers onto the surface of metal oxide cores. Subsequently, the core–shell structures are coated with carbon precursors to form core-shell-shell structures. Yolk-shell structures are obtained after carbonization and selective removal of silica layers. Obviously, the multi-step synthetic templating procedure is complicated and time consuming, not feasible for scale-up production and practical applications. In addition, previous reported methods usually result in compact carbon shells with little control over pore structures,30 which may limit their applications where mesopores are needed for mass diffusion and transport. It is desired to develop a simple approach to fabricate metal oxide@carbon yolk-shell structures with mesoporous carbon shells for various applications. Herein, we report the preparation of monodispersed MCHS with large tunable pore sizes based on the in-situ generated silica primary particles as templates in a one-pot and surfactant-free synthesis. Only three small molecules are involved as precursors in our synthesis: resorcinol, formaldehyde and tetrapropyl orthosilicate (TPOS). TPOS, which undergoes slower hydrolysis and condensation than TEOS, provides better control over silica core and primary particle


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formation (Step I). The silica primary particles and resorcinol-formaldehyde (RF) oligomers after polymerization co-condense onto SiO2 core particles, forming SiO2@SiO2/RF core shell structures (Step II). Carbonization is used to convert SiO2@SiO2/RF to SiO2@SiO2/C composite and after selective removal of the SiO2 component (Step III), MCHS with a large pore size of 7.5 nm is obtained. By changing the synthesis conditions such as the ratios of TEOS/TPOS or ethanol/water, the pore sizes can be precisely tuned from micropores to 13.9 nm. The obtained MCHS shows a high capacitance (310 F g-1 at 1 A g-1), excellent rate capability (157 F g-1 at 50 A g-1), and outstanding cycling stability (98.6 % capacity retention after 10000 cycles at 10 A g1

) in electrochemical double-layer capacitors (EDLCs). Our strategy can be further extended to

fabricate metal oxide@carbon yolk-shell structures with tunable structural parameters on carbon shells, which have paved the way to the designed synthesis of various functional yolk-shell structures (Figure 1b).

Figure 1 A schematic illustration of the one-pot surfactant-free synthesis of mesoporous carbon hollow spheres (a) and the extension to the synthesis of metal oxide@mesoporous carbon yolkshell structures.


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Results and Discussion Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to investigate the morphologies of SiO2@SiO2/C and MCHS obtained at different stages. The low magnification SEM image (Figure 2a) reveals that the particle size of SiO2@SiO2/C obtained after Step II and carbonization is uniform with an average size of 320 nm. Nano-sized spikes with sizes smaller than 10 nm are observed extruding on the outer surface of SiO2@SiO2/C (Figure 2b). X-ray photoelectron spectra (XPS) analysis in Figure S1 reveals the surface of SiO2@SiO2/C composite is composed of silica and carbon. After removing carbon from SiO2@SiO2/C by calcination in air, a solid core with an average diameter of 220 nm and a radial porous shell with a thickness of ~50 nm are observed (Figure 2c). The above results are in accordance with the core-shell structure of SiO2@SiO2/RF shown in Figure 1a.

Figure 2 SEM images (a and b) of SiO2@SiO2/C composite after carbonization under N2 atmosphere. TEM image (c) of SiO2@SiO2 after removing carbon by calcination in air from


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SiO2@SiO2/C composite. SEM (d and e) and TEM (f) images of MCHS after removing silica templates from SiO2@SiO2/C. Removing SiO2 from the SiO2@SiO2/C composite leads to the formation of monodispersed MCHS (Figure 2d). The mesoporous shell and open entrance on the surface is directly distinguished by SEM (Figure 2e). The hollow morphology and the radial pore channels are evidenced from the TEM image (Figure 2f). Both the TEM and SEM observations suggest the pores in the shells are disordered. The diameter of cavity and the thickness of carbon shell are similar to that of silica core and radial porous shell in Figure 2c, indicating that SiO2@SiO2/C has a dense silica core and a composite silica and carbon shell. The complementary information from Figures 2c, e and f suggests that the spikes observed in Figure 2b are radial silica shells embedded inside polymer/carbon matrix. XPS analysis (Figure S1) shows silica has been completely removed in MCHS and the major components are carbon (95.5 wt%) and oxygen (4.5 wt%). The MCHS aqueous solution exhibits obvious Tyndall effect (Figure S2a), suggesting the colloidal characteristic for MCHS. Dynamic light scattering (DLS) analysis presents a sharp peak at 390 nm in the hydrated particle size distribution of MCHS with low polydispersity index (PDI of 0.1), suggesting a uniform particle size distribution and good dispersibility of MCHS (Figure S2b). The nitrogen sorption analysis for MCHS shows type-IV adsorption isotherms (Figure S3a and additional discussion in Supporting Information). The surface area and pore volume are 1582 m2 g-1 and 2.45 cm3 g-1, respectively. The BJH pore size distribution calculated from the adsorption branch (Figure S3b) reveals a pore size of 7.5 nm. The large pore size, high surface area and pore volume of MCHS are important for their potential applications.


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The choice of TPOS as the silica precursor is the key to obtain MCHS with a large pore size. It was reported before that using TEOS led to the formation hollow carbon spheres with microporous walls.31 We also replaced TPOS with TEOS and prepared the sample without changing other synthesis conditions (denoted as MCHS-TEOS). The nitrogen sorption analysis (Figure S3), SEM and TEM observations (Figure S4) confirm that there are only micropores on the shells. Interestingly, the pore sizes of MCHS can be further tuned between micropores and 7.5 nm by using mixed TEOS/TPOS precursors. As demonstrated in Figure S5, the particle size and shell thickness of hollow carbon spheres is around 210 nm and 15 nm, respectively, when the molar ratio of TPOS: TEOS is 0.25 (MCHS-0.25). The particles exhibit high uniformity in size and good dispersity. A few open-pore entrances can be observed on the surface from SEM image. When the ratio of TPOS: TEOS is increased to 0.5, the obtained spheres (MCHS-0.5) have a particle size of 230 nm and a shell thickness of 25 nm. More obvious open-pore entrances are observed on the surface. Further increasing the ratio of TPOS: TEOS to 0.75 leads to a particle size of 260 nm and a shell thickness of 40 nm for the obtained hollow carbon spheres (MCHS-0.75). The morphologies of the corresponding SiO2@SiO2/C and silica templates of these samples prepared from different ratio of TPOS: TEOS are shown in Figure S6. Similar with the results from the samples prepared with pure TPOS, nano-sized spikes with sizes smaller than 10 nm are observed on the surface of SiO2@SiO2/C composite and the silica templates exhibit core shell structures. It should be noted that the densities of silica primary particles increase with the increasing ratio of TPOS: TEOS, which can be explained by the presence of more silica primary particles during the co-condensation process of silicate and RF precursors when there are more TPOS in the synthesis system. Moreover, the thicknesses of silica shells are increased, consistent


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with the thicker shells of corresponding carbon spheres, which is probably due to the prolonged co-condensation process. Nitrogen sorption studies for these MCHSs show type-IV adsorption isotherms (Figure S7), similar with the result from MCHS prepared with TPOS alone. The BJH pore size distribution from adsorption branches suggests they have increased pore sizes with increasing ratio of TPOS: TEOS. When the ratio is 0.25, the pore size is around 4.75 nm. The pore sizes increase to 5.86 and 6.89 nm at the ratio of 0.5 and 0.75, respectively. This is because more silica primary particles can attach together to form larger aggregates during silica-RF oligomers self-assembly when there are more TPOS in the reaction system. Both surface area and mesopore volume increase with increasing ratio of TPOS: TEOS (Table S1). Interestingly, the ratio of EtOH: H2O also affects the pore size. The mesopore size can be enlarged to 13.9 nm when the ratio of EtOH: H2O is 6: 2 (Figure S8). The particle size and shell thickness is 230 nm and 38 nm respectively, smaller than that of MCHS prepared with the ethanol/water ratio of 7: 1 (320 nm and 50 nm). This trend is consistent with previous reports that the increased fraction of water results in smaller particle size and thinner carbon shells.19, 31 These results indicate our method is more powerful in tuning the pore sizes than the conventional soft-templating methods where the pores are controlled by the “soft” micelle size of surfactants. Nevertheless, the limitations of our method include that the pores are disordered and pore size distribution is relatively broad in MCHS, which is related to the formation mechanism of MCHS. Previously, precise control over the pore size of mesoporous carbon is generally achieved through surfactant-templating.32 In order to understand why the pore size of MCHS can be finely tuned in our design without surfactants, we systematically investigated the change of particle sizes of silica (TEOS and TPOS), RF and SiO2/RF composite as a function of reaction time


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(Figure 3a). As shown in curve I, TEOS can hydrolyze and nucleate within 15 minutes (min), forming monodispersed silica Stöber spheres. The particle size increases continuously until most silica precursors are consumed after 2 h. TPOS can also hydrolyze and nucleate within 15 min, but the particle size is smaller than that of TEOS (curve II). These spheres keep increasing in size gradually as a function of time throughout the whole reaction period (12 h). The slower polymerization kinetics of TPOS than TEOS is attributed to the steric effect of propoxy compared to relatively smaller ethoxy groups.

Figure 3 Particle sizes from TEOS (curve I), TPOS (curve II), pure RF particles (curve III), and TPOS+RF (curve IV) (a) and conductivity change of the solutions in TEOS and TPOS systems as a function of reaction time (b).


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We measured the conductivity of the TEOS and TPOS systems under our synthesis conditions. As shown is Figure 3b, the conductivity of TEOS system increases dramatically at the first 10 min, suggesting a large number of silica primary particles are formed within a short period of time.33 The primary particles then aggregate to form core particles, leading to the conductivity decrease.34,

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Compared to the TEOS system, the conductivity of TPOS system increases

constantly and slowly, indicating that the silica primary particles in the TPOS system have a longer lifespan and their aggregation to form core particles is slower. These observations are consistent with the changes of particle sizes in TEOS and TPOS systems in Figure 3a, providing fundamentally important knowledge for the rational choice of TPOS in the design of MCHS with tunable pore sizes. For the RF system, no obvious colloidal particle formation is observed before 2 h (curve III). After 2 h, the particle size of RF spheres rapidly increases until 6 h followed by a relatively slower increase. It is reported that silicate oligomers can interact with the hydroxyl group of phenolic resin through hydrogen bond.36 In our synthesis, whether the synthesis conditions favor the assembly between silica primary particles and RF precursors both with high concentrations is the key. In this regard, TEOS is not a good match for RF because most silica primary particles have been consumed and transformed into silica core particles at 2 h. Polymerization and condensation of RF-rich layers on the surface of silica core particles leads to only microporous hollow carbon spheres. In contrast, the concentration of silica primary particles is higher in the case of TPOS compared to TEOS after 2 h (Figure 3b), favoring the assembly of silica/RF composites. The significant difference in polymerization kinetics of TPOS and TEOS results in the final structure difference between MCHS and MCHS-TEOS.


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Our theory is further supported by the observation in TPOS+RF system in time-dependent particle size change, TEM (Figure S9) and thermo gravimetric analysis (TGA) studies (Figure S10 and Table S2). As shown in curve IV, Figure 3a, the particle size variation follows the trend of curve II (TPOS) before 2 h, afterwards the particle size at each time point is larger than that in curve II, but smaller than curve III (RF), indicating that the presence of silica has changed the homogeneous nucleation of RF spheres to heterogeneous nucleation and growth on the outer surface of preformed silica core particles. Due to the slower polymerization and condensation speed of TPOS compared to TEOS, silica primary particles are released constantly and slowly as evidenced by Figure 3b. The silica primary particles self-assemble via homogeneous nucleation to form silica core particles which are the templates for hollow cavity. When RF starts to polymerize and nucleate on the surface of silica core particles (around 2 h), there are still a large quantity of silica primary particles co-existing with the core particles (Figure 3a and 3b). The mesopore size is dependent on the silica nanoparticle size in the SiO2/RF shell (more precisely in the SiO2/carbon shell). The SiO2 nanoparticle in the shell after 24 hours’ reaction should be differentiated from the silica primary particles formed in the first place. According to a previous report, the size of silica primary particles is around 2-3 nm,34 suggesting that it is not one single silica primary particle, but the aggregate of several silica primary particles in the composite SiO2/RF shell contributes to the large mesopore formation (e.g., 7.5 nm). We propose a mechanism as follows. The co-deposition between silica primary particles and RF oligomers starts at reaction time of ~ 2 h. With prolonged reaction time, the silica primary particles with a long lifespan in the case of TPOS (Figure 3b) tend to aggregate into larger sizes due to further condensation. The silica primary particles and their aggregates interact with RF oligomers through weak hydrogen bonding and co-condense onto the silica core particles,


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forming SiO2@SiO2/RF core-shell structures. The SiO2/RF shell is continuously increasing in thickness as evidenced by Figure 3a and S9, suggesting that the mesopore size is determined mainly by the size of aggregates. It is envisaged that the aggregated silica primary particles may have a dynamic nature and increase in size when forming the SiO2/RF shell, which explains for the relatively broad pore size distribution compared to mesoporous materials template by micelles. The overall size of silica primary particle aggregate is dependent on the concentration of silica primary particles in the reaction system and the nature of silica precursors. The TEOS system consumes the high concentration of silica primary particles formed within 10 mins to form core particles (Figure 3), leaving few silica primary particles available to interact with RF and thus a microspore dominant carbon wall is formed. The TPOS system, on the other hand, has a higher concentration of slowly released silica primary particles, which favors the formation of aggregated silica primary particles with larger sizes and consequently larger mesopores. Moreover, the co-condensation of silica primary particles and their aggregates with RF oligomers is prolonged in the TPOS system, giving rise to thicker thickness of carbon shells. Our theory can be applied to explain the observations in TPOS/TEOS systems. The increasing fraction of TPOS in the TPOS/TEOS systems increases the concentration of silica primary particles available for SiO2/RF shell formation, leading to larger silica aggregates and eventually enlarged pore sizes and increased thickness of carbon shells. In addition to the TPOS/TEOS ratio, the pore size of MCHS is also influenced by the ratio of ethanol/water. When the ratio of ethanol/water is 7:1, the co-condensation starts at around two h (also indicated by the appearance of yellow color in solution) and the pore size is 7.5 nm. With the ratio is changed to 6:2, the light yellow color appears after 1 h, corresponding to faster


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polymerization and condensation at higher amount of water. The faster homogenous nucleation of core particles with larger numbers explains for the relatively smaller particle size and shell thickness (230 and 38 nm) compared to MCHS prepared at the ethanol/water ratio of 7:1 (320 and 50 nm). Moreover, due to faster hydrolysis and condensation of silica, the concentration of silica primary particles is higher, favoring the formation of aggregated silica primary particles with larger sizes and eventually larger pore size according to our mechanism. The difference of choice of TEOS and TPOS as the silica sources for fabrication of MCHS is presented in Figure S11. Compared to our recent work,20 the MCHSs in this work do not have multi-layers in the walls. The tunable sizes of mesopores in the single-layer walls of MCHSs provide opportunities to study the correlation between pore sizes and electrochemical performance, which is important to design ideal porous carbon materials for supercapacitors application in the future. The electrochemical performance of MCHS for EDLCs was evaluated using a three-electrode configuration with 6 M KOH as the electrolyte (Figure S12). Figure 4a shows the cyclic voltammograms profiles of MCHS at various scan rates. Quasi-rectangular shaped CV curves can be obtained at scan rates up to 100 mV s-1, indicating a fast ion and electron transport during charge/discharge process. Figure 4b show representative galvanostatic charge-discharge curves of MCHS within the potential range of -1 to 0 V at various current densities from 1 to 50 A g-1. The charge-discharge curves are nearly linear and symmetric with a slight curvature, which suggests the good capacitive behavior and electrochemical reversibility.37 No obvious electrodepotential drop (IR drop) is observed, indicating low inner-pore ion transport resistance and short ion diffusion distance of the electrodes.38 The specific capacitance was determined from the discharge curve. MCHS delivers a high specific capacitance of 310.4 F g-1 at 1 A g-1 (Figure 4c),


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higher than that of MCHS-TEOS with microporous shells (202.5 F g-1 at 1 A g-1). Even at a high current density of 50 A g-1, a capacitance of 157.0 F g-1 can be maintained, corresponding to capacitance retention of 50.6 % (Figure 4c). For comparison, MCHS-TEOS can only deliver 82.3 F g-1 at 50 A g-1 with capacitance retention of 40.6 %. The electrochemical performances of other MCHS with different pore sizes are presented in Figure S13. At a low current density of 1 A g-1, the capacitances are 288, 291, and 301 F g-1 for MCHS-0.25, MCHS-0.5, MCHS-0.75, respectively. The capacitances retain 124, 135, and 145 F g-1 when the current density is increased to 50 A g-1, corresponding to 43 %, 46 %, and 48 % of capacitance retention at 1 A g-1. Combining the performances of MCHS and MCHS-TEOS in Figure 4c, it is concluded that the specific capacitances as well as the capacitance retention at high current densities increase with the increasing of pore sizes (Table S3). This observation is in accordance with previous literature reports: larger pores facilitate the electrolyte diffusion/ion transportation, resulting in higher capacitances especially at high current densities.9 Noticeably, MCHS shows an excellent cycling stability, retaining 98.6 % of its initial capacitance after 10000 cycles at 10 A g-1. The galvanostatic charge/discharge curve in the 10000th cycle overlap well with that in the initial cycle, also indicating the electrode possesses outstanding cycling stability and good charge propagation.39 The electrochemical performance in terms of high specific capacitance and good rate capability of MCHS is comparable or even better than state-of-the-art reports on carbon-based porous nanomaterials (including those with N-doping) for EDLCs (Table S4).37,

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The

superior electrochemical performance of MCHS can be attributed to the unique structural features. Firstly, MCHS possesses a highly accessible and high surface area (>1500 m² g-1), leading to a large electrode-electrolyte interface for electric double layer formation. Secondly,


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Figure 4 Cyclic voltammograms (a) and charge–discharge curves (b) of MCHS, specific capacitances of MCHS-TEOS and MCHS at various current densities (c), and cycling stability of MCHS. the large and interconnected mesopores (7.5 nm) in MCHS facilitate the electrolyte diffusion and ion transport, providing low-resistant pathways for ion diffusion. Thirdly, the hollow cavity of MCHS serves as ion-buffering reservoirs, minimizing the ion diffusion distance to the interior surfaces. Both the highly accessible surface and large mesopores of MCHS contribute the excellent electrochemical performance for EDLCs. We further demonstrate our strategy is versatile and can be used to prepare metal oxide@mesoporous carbon yolk-shell structures (Figure 1b). The TPOS/RF system can be applied to coat on the surface of core particles with various morphologies and compositions. MnO2 nanowires and spindle-like Fe2O3 nanoparticles (Figure S14) are chosen as core particles


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to demonstrate the versatility and feasibility of our coating strategy. MnO2@carbon and Fe3O4@carbon yolk-shell structures with conformal mesoporous carbon shells can be obtained via the one-pot coating strategy after carbonization and selectively removal of silica templates (Figure 5). Our strategy is promising for the fabrication of other metal oxide@carbon yolk-shell structures with various functional cores and tunable structural parameters of carbon shells for diverse applications.

Figure 5 TEM images of MnO2@C (a, b) and Fe3O4@carbon (c, d) yolk-shell structures. Conclusions In summary, we have developed a one-pot approach to synthesize mesoporous carbon hollow spheres in the absence of surfactants. The obtained MCHS have controllable mesopore size, high surface area and pore volume, and exhibit high capacitance and good rate performance for supercapacitors. We further demonstrate that our strategy can be extended to the one-pot and surfactant-free synthesis of metal oxide@mesoporous carbon yolk-shell structures. Our success


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has paved the way towards designed synthesis of hollow nanospheres with tunable pore sizes and yolk-shell structures with various morphologies and compositions for wide applications. Experimental Section Synthesis of mesoporous carbon hollow spheres. In a typical synthesis of MCHS, TPOS (3.46 ml, 12 mmol) was added to the solution containing ethanol (70 ml), H2O (10 ml) and NH3·H2O (3 ml, 25 wt%) under stirring at room temperature. After 15 minutes, resorcinol (0.4 g) and formaldehyde (0.56 ml, 37 wt%) were added to the solution and the system was kept stirring for 24 hours. The precipitates were separated by centrifugation, washed with water and ethanol, and dried at 50 °C overnight. Mesoporous porous hollow carbon spheres were obtained after carbonization at 700 °C under N2 for 5 hours and removal of silica by hydrofluoric acid (HF, 5 wt%). Other mesoporous hollow carbon spheres with different pore sizes were prepared at different molar ratio of TPOS: TEOS (the total silicon amount is fixed to 12 mmol) or different volume ratio of EtOH: H2O while the other conditions were kept unchanged. Synthesis of metal oxide@mesoporous carbon yolk-shell structures. MnO2@carbon: MnO2 nanowires were prepared according to previous report with some modification.48 In a typical synthesis, 1.0 mmol of KMnO4 and 1 mmol of NH4Cl were dissolved in 80 mL of distilled water under stirring to form a clear solution. The solution was then transferred to a 100 ml Teflon-lined autoclave, and kept h in an oven at 140 °C for 24 hours. The resulting product was collected by centrifugation, washed with distilled water and ethanol for three times, and dried in a vacuum oven at 50 °C for overnight. For the preparation of


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MnO2@carbon, 30 mg of MnO2 was dispersed in a solution containing ethanol (70 ml), distilled water (10 ml) and ammonia water (3 ml, 25 wt%) by sonication. Afterwards, TPOS (0.5 ml), resorcinol (0.1 g) and formaldehyde (0.14 ml, 37 wt%) were added to the solution and the system was kept stirring for 24 hours. The precipitates were separated by centrifugation, washed with water and ethanol, and dried at 50 °C overnight. MnO2@carbon yolk-shell structures were obtained after carbonization at 700 °C under N2 for 5 hours and removal of silica by NaOH solution (4M). Fe3O4@carbon: Spindle-like Fe2O3 was prepared according to previous report with minor modification.49 FeCl3 (2 mmol) and sodium phosphate (NaH2PO4, 0.02 mmol) were dissolved in 100 ml of a EtOH/H2O solution (1: 1 in volume) under stirring. Afterwards, the mixture was hydrothermal treatment at 100 °C for 48 h. the solid products were collected by centrifugation, washed with distilled water and ethanol, and dried at 50 °C overnight. The coating procedures are the same as MnO2@C except that the obtained Fe2O3 nanoparticles were used as the cores. Structural Characterizations The morphology and structure of the samples were investigated by field emission scanning electron microscope (FESEM, JEOL 7001) operated at 15 kV and transmission electron microscope (TEM, JEOL 2100) at 200 kV. X-ray photoelectron spectra (XPS) were recorded on a Kratos Axis ULTRA X-ray photoelectron spectrometer using a monochromatic Al Kα (1486.6 eV) X-ray source and a 165 mm hemispherical electron energy analyzer. Nitrogen adsorption isotherms were measured at 77 K using a TriStar II Surface Area and Porosity analyzer (Micromeritics). The samples were degassed under vacuum at 180 °C for 6 hours before analysis. Thermo gravimetric analysis (TGA) was performed on a TGA/DSC1 STARe System in


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air or nitrogen (25−800 °C, 2 °C min-1). Dynamic light scattering (DLS) measurement was carried out at 25 °C on a Malvern Zetasizer Nano ZS instrument. Electrochemical evaluation A three-electrode configuration was used to evaluate the electrochemical performances of MCHS (Figure S12). A slurry composing of active materials (MCHS), carbon black, and polytetrafluoroethylene (PTFE) in a weight ratio of 8:1:1 was prepared with water as the solvent under stirring. After stirring for 24 hours, the slurry was spread onto a Ni form chip and dried in an oven at 100 °C under vacuum overnight. The device was fabricated with working electrode, Hg/HgO electrode (reference electrode), Ni foam (counter electrode) and 6 M KOH as the electrolyte in a glass container, as shown in Figure S12. Electrochemical measurements were carried out on Solartron 1480 Multistat with the potential window from -1 to 0 V versus the Hg/HgO electrode. The specific gravimetric capacitance of the electrode was determined according to the following equation: ூ×∆௧

‫ܥ‬௠ = ௠×∆௏ where Cm is the specific capacitance (F g-1), m is the mass of the electrode (g), I is the discharge current (A), ∆t is the discharge time (s), and ∆V is the potential window of the discharge (V). ASSOCIATED CONTENT Supporting Information Available. Other characterizations along with additional supporting data. This material is available free of charge via the Internet at http://pubs.acs.org AUTHOR INFORMATION


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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no conflict of interest. ACKNOWLEDGMENT The authors acknowledge the financial support from the Australian Research Council, the Queensland Government, the CAS/SAFEA International Partnership Program for Creative Research Teams, the Australian National Fabrication Facility and the Australian Microscopy and Microanalysis Research Facility at the Centre for Microscopy and Microanalysis, The University of Queensland. REFERENCES 1.

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Table of Contents Graphic and Synopsis


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Surfactant-Free Assembly of Mesoporous Carbon Hollow Spheres

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