Controlling the Compositional Chemistry in Single Nanoparticles for

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Controlling the Compositional Chemistry in Single Nanoparticles for Functional Hollow Carbon Nanospheres De-Shan Bin,†,‡,# Zi-Xiang Chi,†,# Yutao Li,§ Ke Zhang,‡,∥ Xinzheng Yang,‡,⊥ Yong-Gang Sun,†,‡ Jun-Yu Piao,†,‡ An-Min Cao,*,†,‡ and Li-Jun Wan*,†,‡ †

CAS Key Laboratory of Molecular Nanostructure and Nanotechnology, and CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences (CAS), Beijing 100190, People’s Republic of China ‡ University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China § Materials Science and Engineering Program and Texas Materials Institute, The University of Texas at Austin, Austin, Texas 78712, United States ∥ State Key Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China ⊥ State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China S Supporting Information *

ABSTRACT: Hollow carbon nanostructures have inspired numerous interests in areas such as energy conversion/storage, biomedicine, catalysis, and adsorption. Unfortunately, their synthesis mainly relies on template-based routes, which include tedious operating procedures and showed inadequate capability to build complex architectures. Here, by looking into the inner structure of single polymeric nanospheres, we identified the complicated compositional chemistry underneath their uniform shape, and confirmed that nanoparticles themselves stand for an effective and versatile synthetic platform for functional hollow carbon architectures. Using the formation of 3-aminophenol/ formaldehyde resin as an example, we were able to tune its growth kinetics by controlling the molecular/environmental variables, forming resin nanospheres with designated styles of inner constitutional inhomogeneity. We confirmed that this intraparticle difference could be well exploited to create a large variety of hollow carbon architectures with desirable structural characters for their applications; for example, high-capacity anode for potassium-ion battery has been demonstrated with the multishelled hollow carbon nanospheres.

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INTRODUCTION Hollow carbon nanospheres (HCNs) continue to be a focus of rapid innovations due to their unique structural and functional characters,1,2 ensuring promising potentials in many emerging fields such as energy conversion/storage, catalysis, adsorption, and biomedicine.2−7 To boost the performance of HCNs and fully exploit their potential, enormous efforts have been made in the synthesis design and structure optimization,1 highlighting key factors including pore structure,8 surface properties,9 and cavity control.10 A particular area of interest is the internal cavity of HCNs, which leaves immense room for further control of their architectures and functionalities on the optimization of their electrochemical per-formance.11,12 As far as electrode materials of rechargeable batteries are considered, the hollow structure not only offers a short diffusion distance to facilitate the mass transportation,13 but also was able to buffer the destructive volume expansion of electrode materials during charge/discharge cycling, providing an effective strategy to © 2017 American Chemical Society

mitigate the capacity fading for the development of high energy electrodes.11,14 The synthesis of HCNs has usually relied on the templatebased routes,1,2 which highlight a nanocasting procedure by depositing a suitable carbon precursor, mostly polymers, onto the removable seeds.15,16 Although this approach is conceptually straightforward in producing hollow structures, they suffer from tedious operating procedures with a low yield for desired products, limiting their potential for extensive uses.1 Notably, while the removal of the hard templates by using hazardous agents such as NaOH and HF becomes a major concern, the soft template routes are also challenged by the limited availability of suitable templates and their inadequate capability for shape control for HCNs. Therefore, from both scientific and technological points of view, it is essential to Received: July 6, 2017 Published: August 31, 2017 13492

DOI: 10.1021/jacs.7b07027 J. Am. Chem. Soc. 2017, 139, 13492−13498

Article

Journal of the American Chemical Society

employed as seed materials and redispersed in deionized water, then 3aminophenol, formaldehyde solution (HCHO, 37 wt %) and ammonia aqueous solution (NH4OH, 25 wt %) were added into the deionized water and mixed with each other. After the reaction continued for 30 min at room temperature, acetone was added to selectively remove the dissolvable part. After that, resin 2S-HNs were obtained and collected by centrifugation, and then purified with distilled water. The synthesis of MS-HNs is similar to that of 2S-HNs.The detailed synthesis parameters for the MS-HNs are shown in Table S1 (see SI). After carbonization at 1200 °C for 6 h under an Ar atmosphere, the MSHCNs were obtained. Molecular Design Using Different Phenolic Monomers. Beside 3-AP, other different phenolic monomers including resorcinol, 2-AP, 4-AP, phenol and 3-nitrophenol were also employed to synthesize HCNs using the same method as described above. Considering phenol and 3-nitrophenol were less active with formaldehyde and no precipitation appeared from their reaction at room temperature, the reaction temperature and time for these two monomers were increased to 70 °C and 4 h, respectively. General Characterization. The transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were recorded on a JEOL-2100F microscope. Field emission scanning electron microscopy (FESEM) images were acquired on a SU8020 microscope. X-ray diffraction (XRD) patterns were collected on a Rigaku D.MAX-2500 with Cu Kα radiation (λ = 1.5406 Å). Raman spectra were obtained with an Thermo Fisher spectra system (EXR). N2 sorption isotherms were carried out on Quadrasorb SI-MP with samples predegassed at 300 °C. Electrochemical Measurement. All the electrochemical measurements were tested with CR2032 coin cells at room temperature for half-cell. The electrodes were composed of the active material and a binder of Polyacrylic acid (PAA) with a mass ratio of 90:10. The slurry was coated on Cu foil and dried at 80 °C for 12 h under vacuum. The electrolyte was a solution of 0.8 M KPF6 in EC: DEC (v/v 1:1). A potassium foil was used as the counter electrode and glass fiber was used as the separator. All the operations were performed in the Argonfilled glovebox. The electrochemical measurements were carried out on a LAND CT2001A battery test system at room temperature, where the voltage range was from 0.01 to 2 V versus K+/K. EIS was measured using Autolab PGSTAT 302N (Metrohm, Switzerland) over the frequency range from 100 kHz to 100 mHz.

develop new synthetic protocols that that allow a molecularlevel design and functionalization of hollow carbon architectures for advanced applications as well as more insights into the formation mechanism of nanostructured objects with a high level of functionality and complexity. The increasing demand for sustainable energy storage has inspired numerous efforts to explore new technologies with low-cost and reliable sources.17 The potassium-ion batteries (KIBs) has recently emerged as a promising candidate as an analogue to lithium-ion batteries (LIBs).18−22 The key advantage lies in the abundance and low cost of potassium when compared to lithium, which makes KIBs very attractive for massive stationary batteries. However, K+ is meanwhile known for a much larger size of 2.72 Å (1.52 Å for Li+).18 Accordingly, the K+ insertion and extraction during the charge/ discharge cycles would result in serious structural change and degradation in electrode materials, typically carbonaceous anode materials.18,20 It accordingly has become a formidable challenge for the development of reliable and stable anode materials for practical KIBs.18,20 Herein, by engineering the compositional inhomogeneity inside the single nanoparticles, we demonstrated that the polymeric nanoparticles themselves could be a versatile platform to create complicated carbon architectures. Specifically, we were able to control the growth kinetics of 3aminophenol (3-AP)/formaldehyde (3-AF) resin, forming nanospheres with uniform shape but complicate inner compositional chemistry. A location-specific distribution of different polymeric components were achieved inside the 3-AF nanospheres, which could be transformed into a large variety of HCNs structures with precise control on different key features including cavity, porosity, morphology, and architecture. The investigation of the structure evolution and formation mechanism of 3-AF particles provided a reliable tool to build different hollow carbon architectures, and to shed light on the complex nature of single nanoparticles and compositional chemistry that has barely been explored for understanding of the nanoparticle behaviors. Our preliminary results on the electrochemical performance of a representative HCNs sample, namely the three-shelled one, showed a capability to tolerate the intercalation/deintercalation of the large K+ benefiting from its unique structure with built-in cavities, and suggested a promising potential to be used as an anode material for their applications in KIBs as evidenced by its unprecedentedly high capacity and extraordinary stability.

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RESULTS AND DISCUSSION Briefly, the 3-AF polymerization is initiated with the addition of ammonia at room temperature. Such a base-catalyzed reaction has recently been well-confirmed as a reliable process to form colloidal nanospheres of phenolic resins, which usually includes a hydrothermal reaction and results in hardly dissolvable gelled particles.23−26 Interestingly, a deviation from the widely reported high-temperature reactions to room temperature (RT) leaves us a chance to discover the complicated nature of the inner compositions inside single nanoparticles. Figure 1a shows a scanning electron microscopy (SEM) image of the 3AF particles collected after 30 min reaction at RT, which were uniform nanospheres with size around 260 nm. The transmission electron microscopy (TEM) characterization (Figure 1b) confirmed that these nanospheres were solid inside. Different from those gelled particles identified in the literature, 23−26 these nanospheres show an interesting inhomogeneous nature of their inner compositional distribution, which is indeed hard to be differentiated and visualized directly, but can be delicately manifested when treated by suitable solvents, particularly acetone as used here due to its discriminative capability on the dissolving capacity of polymers with different molecular weights. Figure 1c is the TEM image after the acetone treatment, showing the emergence of hollow nanospheres with a shell thickness of 40 nm. Notably, instead

EXPERIMENTAL SECTION

Synthesis of Single-Shelled Hollow Carbon Nanospheres (1s-HCNs). The 3-AF resin single-shelled hollow nanospheres (1SHNs) were first synthesized and then carbonized to 1S-HCNs. Typically, the resin 1S-HNs were prepared as follows: 3-aminophenol (3-AP, 0.1 g, 0.907 mmol), formaldehyde solution (HCHO (37 wt %), 0.1 mL, 1.331 mmol), and ammonia aqueous solution (NH4OH (25 wt %) as catalyst were added into 30 mL deionized water and reacted at room temperature. After the reaction continued for 30 min, 20 mL acetone was added to selectively remove the interior part of the forming solid inhomogeneous nanospheres. After that, resin 1S-HNs were collected and purified with distilled water by centrifugation. After carbonized at 1200 °C for 6h under an Ar atmosphere, the 1S-HCNs were obtained. Synthesis of Multishelled HCNs (MS-HCNs). The 3-AF resin multishelled hollow nanospheres (MS-HNs) were first synthesized and then carbonized to produced MS-HCNs. The MS-HNs were constructed by repeating the growth-and-removal cycles. For example, to obtain resin 2S-HNs, all the as-prepared resin 1S-HNs were 13493

DOI: 10.1021/jacs.7b07027 J. Am. Chem. Soc. 2017, 139, 13492−13498

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Journal of the American Chemical Society

also critical due to its capability to reveal the difference in the composition. Finally, a high temperature carbonization process is introduced to transfer the hollow resins into carbon materials, whose detailed analyses were shown in Figure S5 and Figure S6, revealing the formation of hollow carbon structures with the morphological characters well inherited from its polymeric precursor. Intrigued by the compositional complexity inside these single nanospheres, we carried out time-dependent experiments to track the formation of these particles. After starting the reaction, the polymerization would lead to a fast nucleation and form nuclei (t = 35 s, Figure S7a). These newly formed nuclei quickly grew into larger colloidal nanospheres (t = 1 min, Figure S7b; t = 2 min, Figure S7c) that were largely dissolvable in acetone due to their oligomeric nature. We started to observe an indissolvable surface in the following 3 min (Figure S7d), which became more prominent and got thicker with the particle growth. After 30 min, a hardened shell with its thickness around 40 nm formed and the particles became 260 nm in size (Figure S7e). Thereafter we did not observe a further size-growth of these colloids (Figure S7f). It turned out that the acetoneremovable center would shrink its territory and finally turn indissolvable after the subsequent reaction for 24 h (inset of Figure S7f), showing an inner hardening process for these oligomers. Although acetone itself is capable of revealing the inner inhomogeneity of the particles due to their different solubility, a solvent with lower dissolving capability, typically methanol, will have no obvious effect on the nanospheres (Figure S8). On the contrary, a solvent with a much stronger dissolving capability, particularly N,N-dimethylformamide (DMF), would dissolve all these colloids in the roomtemperature reaction, disclosing the ungelled nature of these particles, which is different from those gelled particles from a hydrothermal reaction.23−26 It also exemplified the importance of acetone as a suitable solvent which is able to probe the inhomogeneity of the polymeric components inside the particles. The location-specified responses to acetone revealed a localized distribution of components inside single nanospheres. Although the phenolic resin formation has been known as one of the oldest reactions, a detailed structural determination of these resins is still not possible due to their extremely complicated chemistry.31,32 We have tried a large variety of characterizations to seek a better understanding of the structural details of these 3-AF resins. Figure 2a records the Fourier transform infrared (FTIR) spectroscopy for the timedependent samples, which details a unique activity of 3-AP as both an aromatic alcohol and an aromatic amine. Typically, peaks of −CH2− (2852 cm−1)26 and C−O−C (1121 cm−1)33 revealed a phenol-formaldehyde type reaction, which includes the methylolation of 3-aminophenol together with its following condensation for ether bridges. Meanwhile, a continuous decrease of the peak intensity of Ar−NH2 at 1600 cm−1 was probably associated with the nucleophilic addition of aldehydes,34,35 which continued to form carbinolamines at the expense of −NH2. We were also able to collect the inner component and then compared its FTIR pattern to its shell counterpart (see SI for details). As shown in Figure 2b, the increase of the signal for ether bridges (C−O−C) together with the decrease in Ar−H34 and −NH2 manifested a coherent trend toward a higher reaction degree on the hardened shell. 1 H nuclear magnetic resonance (1H NMR) was used to detect the polymeric structures (Figure 2c). The signals of the

Figure 1. Morphological control of the 3-AF polymers with inner compositional inhomogeneity. (a,b) SEM and TEM image of the solid 3-AF nanopsheres. (c) TEM image of 3-AF nanospheres after their inner part were selectively dissolved. (d−i) Multishelled (from doubleshelled to seven-shelled) 3-AF hollow nanospheres through continuous growth cycles. (j) Scheme of the synthesis route for hollow 3-AF structure. The newly formed solid 3-AF nanospheres showed an inhomogeneous distribution of polymeric parts, which led to hollow structure when their inner solubility difference was utilized.

of resorting to an outside template to endure tedious operations, uniform hollow nanospheres of 3-AF resin can form through such a simple protocol when we switch the focus to the inside of single nanoparticles. As easy as it may look, the synthesis protocol endows us a high flexibility in controlling the key parameters of the hollow structure, particularly the size control of the shell and the cavity. We found that changing the volume ratios of the acetone/water could provide a tunable dissolving capability toward oligomer species, which endows us a capability to control the cavity size of the hollow nanospheres. Figure S1a−c shows that a reduced amount of acetone would result in a thicker wall together with a smaller cavity, indicating that those parts close to the surface have a higher extent of reaction thus were more stable against the dissolution. Meanwhile, the size of the nanospheres is sensitive to the reacting temperature as shown in Figure S1d−f due to the fact that higher temperature would facilitate nucleation and form more nuclei in the reaction solution, resulting in a smaller particle size of the formed particles.27−29 The simplicity of the operational protocol and the accuracy in the structural control ensure us a unique capability to form complicated structures, particularly the multishelled hollow nanostrucutres whose increased architectural complexity has introduced further difficulties in synthesis that warrant structural design.30 By using the preformed single-shelled hollow nanospheres as growth seeds as schemed in Figure S2, we can initiate a second round of growth-and-removal process (Figure S3 for nanospheres after the second-round-growth), to form monodispersed two-shelled hollow nanospheres (Figure 1d). A further expansion of the seeded growth-removal route can produce multishelled hollow nanospheres (MS-HNs) (Figure 1e−i) with precise control of key structural features such as their interspacing (Figure S4a−c). As the shell number increases to seven, the particle size is approaching 1 μm (Figure 1i), showing a reliable capability of our synthesis protocol to build complicate structures across different length scales. Figure 1j shows a brief scheme of the synthesis route with the focus on the utilization of the innate inhomogeneity of single nanospheres. Obviously, in addition to the growth habit of the 3-AF polymers, the identification of acetone as a suitable solvent is 13494

DOI: 10.1021/jacs.7b07027 J. Am. Chem. Soc. 2017, 139, 13492−13498

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monomers, and accordingly delaying their further polymerizing into higher molecular weights. Second, the 3-AF reaction is a typical step-growth polymerization process that relies on the reactivity of multifunctional precursors to form increasingly larger oligomers.38,39Therefore, those outside layers will have a higher extent of reaction due to its step-growth nature,26 which points to an increasingly higher degree of functionality associated with an inherently higher reactivity, forming a surface layer with higher molecular weights. Notably, the polymerization slowly proceeds forward for the inner oligomers since their growing chains remain active, which finally makes the core indissolvable upon extended time as observed for these nanospheres (Figure S7f). The relevance of compositional inhomogeneity to the reaction kinetics can be further illustrated through a systematic molecular design of the phenolic monomers. For the basecatalyzed reaction, the methylolation of the phenolic aromatic ring is closely related to its electron density, which can be welltuned by different substitutes with different spatial relationships. Electron-donating groups such as −NH2 and −OH can increase the electron density of those active sites, namely the ortho- and para-positions, especially when these substituents are on the meta-site. A resonance donating effect has been recognized for both 3-AP (Figure 3a) and resorcinol (Figure

Figure 2. Characterization of the innate inhomogeneity for 3-AF resin solid nanospheres. (a) FTIR spectra for the solid samples collected at different reaction stages. The data of 3-AP monomer is also presented to show the difference after reaction. (b−d) The comparison of the FTIR spectra (b), the 1H NMR spectra (c), and the GPC spectra (d) between the interior part and its exterior counterpart for the 3-AF solid nanospheres collected at a reaction time of 30 min. In the GPC spectra, the molecules with higher molecular weight would spend less time in the column and will be elute first.

interior part exhibited an intermediate state with noticeably sharp peaks that turned into broadened ones in the spectra of the exterior part as a result of their increased mass and reduced relaxation.36,37 Meanwhile, the diminishing of the signal at 5.41 ppm (Ar−NH2) and the downfield shifting of the peak at 9.42 ppm (Ar−OH) confirmed the different conversion of these reaction groups. Such a different reaction degree can be verified by gel permeation chromatography (GPC), which is a powerful tool to determine the distribution of molecular weights and polydispersity of polymer samples. The exterior part turned into a high ratio of high-molecular-weight components with shorter retention times in the GPC spectra (Figure 2d). Table S2 lists the calculated molecular weights based on the GPS test. The exterior part shows a number-average molecular weight (Mn) of 26554, which is much higher than that of the inner part of 9532, confirming the inhomogeneous nature of the compositional distribution in the inner nanospheres. We also tested the molecular weights of the dissolved parts (the interior part) of the resin nanospheres treated by acetone/water with different volume ratios. Table S2 compared the molecular weights of the dissolved parts from this contrast experiment. The oligomers dissolved by a higher acetone concentration (acetone/water ratio of 1/1) showed a much higher average molecular weight (Mn = 16556) as compared to 9532 from the acetone/water ratio of 2/3. It means that the oligomer species with higher molecular weight in the resin nanoshperes would also be dissolved when the acetone amount is increased, which is in good agreement with the TEM observation in Figure S1a− c that a large cavity size could be achieved after increasing the acetone/water ratio. The sum of the above observations features a fast precipitation for the formation of a core−shell type configuration in the compositions. First, the initially formed oligomers would be quickly buried inside and make themselves inaccessible to the growth media, resulting in a shortage in the supply of key contributors, particularly the catalyst and the

Figure 3. Molecular design of phenolic monomers and their influence on the structural control of the corresponding polymeric resins. Different kinds of monomers have been tested for the resin synthesis. The charge distribution calculated by the DFT and NBO method is labeled on the benzene ring for all these monomers including: 3-AP (a), resorcinol (b), 2-AP (e), 4-AP (f), phenol (i), and 3-nitrophenol (j), respectively. (c,g,k) These panels showed the TEM images of the newly formed solid resin when using monomers of 3-AP (c), 4-ap (g), and phenol (k), respectively. These resins will form different structures when treated by acetone according to the TEM in (d), (h), and (l), respectively. Here, grey = C, red = O, green = N.

3b), as revealed by more negative electron densities according to the calculated values of natural bond orbital (NBO) charges (see SI for computational details). Accordingly, a fast precipitation happened at room temperature, resulting in a radial difference inside the solid nanospheres (Figure 3c,d for 3AP, Figure S9a,b for resorcinol). For comparison, the substitution on either ortho- or para- positions of phenol did not have the same resonance donating effect. Both 2aminophenol (2-AP, Figure 3e) and 4-aminophenol (4-AP, Figure 3f) had less electron densities with much reduced 13495

DOI: 10.1021/jacs.7b07027 J. Am. Chem. Soc. 2017, 139, 13492−13498

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mesoporous structures by using a mixed solvent as the reaction media. For the solid nanospheres prepared in an ethanol/water (v/v 1:2) solution, their inner structure turned mesoporous rather than completely hollow upon acetone treatment (Figure 4b), unravelling the existence of nanosized domains of polymers with different molecular weights. Although a detailed molecular mechanism regulating such a microphase separation is currently unclear, it can be expected that the formed segments of oligomers will have an increased affinity and solubility in ethanol, therefore forming nanosized domains different from those grown in pure water. Such a difference in domain composition could be well exploited for the creation of a mesoporous structure (The carbonized sample had a specific surface area of 1053 m2 g−1 as shown in Figure S12), which is an exciting capability especially considering its broad potentials in a large variety of areas.1,5 Furthermore, it is convenient to construct multishelled hollow nanospheres with built-in mesopores. Figure 4c shows the TEM image of the 3-layered architecture of the mesoporous hollow nanospheres via two repetitious cycles of growth and acetone rinse operation, which produced uniform nanoparticles with well-defined hierarchical texture. Such knowledge of the complicated inner chemistry of single particles can be readily applicable to other synthesis efforts toward structural and compositional control of the hollow structures. For example, metal nanoparticles, typically Ag as shown in Figure 4d, could be incorporated into the 3-AF matrix to form composite materials with inorganic components welldispersed inside (See methods for synthetic details and Figure S13 for characterization details). Meanwhile, using the preexisting materials as seeds, it became an easy task to build yolk−shell structured composites with designated particles embedded inside, typically, SiO2 as demonstrated in Figure S14. Moreover, we found that the spatial modulation of 3-AF components was also applicable for objects other than nanospheres. With the help of different shape-directing agents for growth, typically the cationic surfactant of cetyltrimethylammonium bromide (CTAB),40 the AF polymerization would form a rod-like structure (Figure S15a), which could be converted into tubular structure (Figure S15b) by exploiting their inner chemical inhomogeneity On the basis of the above observations, we propose a scheme for the major shape evolution paths that we identified on 3-AF resin nanoparticles (Figure 5). Different from those gelled nanoparticles as reported in the literature,23−26 our control on the growth kinetics unfolded the possibility of compositional modulation inside single 3-AF particles. For a typical precipitation process, the 3-AF oligomers nucleus form first, and then grow into nanospheres. By tuning the molecular/ environmental variables, the molecular weights of the polymeric segments can be controlled and allocated into designated location inside the particles, forming different styles of spatial hardness distribution when responding to the acetone treatment (Routes 1−3). Typically, the nanospheres can exist as totally dissolvable ones (Route 1, Blue for dissolvable parts). The continuous growth could give rise to an indissolvable surface shell (Yellow for indissolvable parts) while the inside remains almost unchanged (Step 1), showing a core−shell type structure from a solubility point of view. while a repetitious operation produces multishelled hollow structures (Step 3). Meanwhile, it is also possible to build a hardened core with a high molecular weight (Route 2), with a sandwich-type polymeric structure that can then be transformed into a

reactivity, leading to slower precipitation. The collected solid nanospheres were still largely dissolvable in acetone even after the reaction continues for 24 h (Figure S9c,d for 2-AP, Figure 3g,h for 4-AP), showing no clue of the core−shell type compositional distribution. Besides, for the precursors of phenol (Figure 3i) or substituted phenols with electron withdrawing groups such as 3-nitrophenol (Figure 3j), they became even less active with formaldehyde and no precipitation occurred during their reaction at room temperature. An increased temperature to 70 °C for 4 h could produce a small quantity of solid precipitation for phenol precursors, which showed no irregular shapes (Figure 3k) and almost dissolved completely in acetone (Figure 3l). Furthermore, for 2,3-diaminophenol, the obtained particles before and after acetone treatment were irregular rather than spherical (Figure S10). We can speed up the above-mentioned reaction between 4AP and formaldehyde by increasing the catalyst (ammonia) concentration to enlarge the inner compositional difference, forming a hardened surface similar to 3-AF resins (Figure S11). Such a transformation embodies the complex nature of the polymerization reactions, with products that not only rely on the molecular precursors, but are also subjected to the environmental variables; it offers a reliable tool to achieve a systematic spatial modulation of 3-AF components from a synthesis perspective. Typically, compared to the acetonesoluble core, we confirmed that it was possible to build a hardened core if the growth kinetics was tuned to favor the formation of high-molecular-weight oligomers at the early stage of the polymerization, which could be achieved by using high concentrations of the monomer reactants (Figure 4a, and see Experimental Section for details). Upon acetone treatment, nanospheres with a yolk−shell type architecture showed up with an undissolved core in the center. The polymerization process can also be effectively engineered to construct

Figure 4. Representative structural control of the hollow 3-AF nanospheres on its architecture, pore size, and composition. (a) Yolk− shell structures by forming a hardened core at high reactant concentrations. (b) Mesoporous hollow 3-AF nanospheres formed by using a mixed solvent of ethanol/water (v/v 1:2) as the reaction media. (c) Three-shelled hollow 3-AF nanospheres with embedded mesopores through a repetitious growth cycle of the synthesis adopted in (b). (d) The in situ incorporation of Ag nanoparticles into the hollow nanospheres to form a composite structure of Ag/resin nanospheres. The Ag nanoparticles were well-dispersed with narrow size distribution at 2−4 nm. 13496

DOI: 10.1021/jacs.7b07027 J. Am. Chem. Soc. 2017, 139, 13492−13498

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Figure 6. Applications of HCNs for potassium-ion battery (KIB). (a) K+ discharge/charge curves at the first and the second cycle for 3SHCNs at 0.1 C. (b−d) Rate capabilities (b), EIS spectra (c), and cyclability tests at 2C (d) of representative samples including solid carbon nanospheres,1S-HCNs, 3S-HCNs, 5S-HCNs, respectively. All the carbon nanopsheres here were obtained by carbonizing the corresponding 3-AF resin nanoparticles at 1200 °C for 6 h.

curves with a high reversible capacity of 298 mAh g−1 at a current of 28 mA g−1 (around 0.1 C based on the theoretical capacity of 279 mAh g−1 for graphite). An irreversible discharged capacity loss in the first cycle, which is diminished in the second cycle, indicated a typical process probably related to the formation of a solid electrolyte interphase (SEI).18 It is noted that the HCNs showed improved battery performance when compared to the solid carbon nanospheres (Figure 6b). Such an advantage became more significant for the fast K+ insertion/extraction cycles, showing that a hollow structure was in favor of fast metal ion transportation kinetics.43,44 For the shell-controlled HCN series, we did not observe a monotonic increase of the reversible capacity and the 3S-HCNs showed the best performance especially when its rate capabilities were examined. Although a multishelled structure has been identified to contribute to higher reversible capacity, which is possibly related to surface/interface metal-ion storage mechanism,45−47 we noticed an increased charge-transfer impedance for the 5SHCNs (Figure 6c). It is therefore expected that a compromise effect of these two opposite contributions made the 3S-HCNs the highest capacity for the shell-controlled series. Importantly, the HCN sample was able to deliver a superb cycling stability as shown in Figure 6d. Besides the high capacity, the 3S-HCNs showed a capacity of 212 mAh g−1 at 2C with a capacity fading less than 5% after 100 cycles, which is the highest capacity with the least fading as far as we are aware of, showing the advantage of such an architecture control strategy on combating the structural degradation of electrode materials during K+ insertion/ extraction.

Figure 5. Schematic illustration of the different shape evolution paths for hollow structure via kinetically controlled growth. The results in literatures are related to the gelled and indissolvable nanospheres.23−26 The growth kinetics control manifests different growth routes of the 3AF polymerization with a designated allocation of different polymeric parts, showing various hardening effect of the nanospheres as listed in Routes 1−4. Different growth steps have been included in our preparation of HCNs, which typically includes the continuous growth and surface hardening of the newly formed 3-AF nanospheres (Step 1), a selective removal of the dissolvable parts (Step 2), repetitious preparation processor for multishelled structure (Step 3), and high temperature carbonization process for multishelled hollow carbon nanospheres (MS-HCNs) and mesoporous MS-HCNs (Step 4).

yolk−shell architecture after the removal of the acetone-soluble parts. Moreover, a hybrid hardening effect can be observed by controlling the reaction media (Route 3). The nanodomains of those hardened segments mixed well with the dissolvable ones, presenting a desired framework for preparing mesoporous carbon structures. It is noted that the spatial modulation of 3AF components is also applicable for objects other than nanospheres as demonstrated by the creation of tubular structures from solid nanorods (Route 4). For all the hollow 3-AF structures, a carbonization process will transform these polymer species into carbon materials with tunable porosities at designated temperatures (Step 4). The described synthetic platform for hollow carbon architectures has significant implications and applications in different fields such as energy conversion/storage, adsorption, biomedicine, and catalysis. The first of these potentials is demonstrated here by using the KIBs as an example. Our interest in these hollow carbon structures as an anode material for KIBs is inspired by the fact that the inherent cavity should be effective to buffer the volumetric expansion during the insertion/extraction of the large-size K+,41,42 which is one of the major haunted problems on KIBs. For a typical sample of the 3S-HCNs one, its charge/discharge profiles in the first two cycles were compared (Figure 6a), which showed sloping

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CONCLUSION In summary, we confirmed that the inner chemistry of single polymeric nanoparticles could be controlled toward a desired spatial modulation of their compositional distributions; through this control we were able to transform single nanoparticles into an effective synthetic platform for a large variety of hollow carbon architectures. Using the 3-AF resin as an example, we demonstrated the possibility to achieve a location-specific distribution of polymeric components inside single nano13497

DOI: 10.1021/jacs.7b07027 J. Am. Chem. Soc. 2017, 139, 13492−13498

Article

Journal of the American Chemical Society

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particles by controlling their growth kinetics, forming hollow carbon structures with precise control on their shape features. The described principles of inner chemistry control not only show potential in molecular-level design and functionalization of hollow carbon architectures for advanced applications, typically the high performance anode materials of KIBs as evidenced by the MS-HCNs, but also provide insights into the formation mechanism of nanostructured objects with a high level of functionality and complexity.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b07027. Supporting tables and figures (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] ORCID

Ke Zhang: 0000-0001-5972-5127 Xinzheng Yang: 0000-0002-2036-1220 Li-Jun Wan: 0000-0002-0656-0936 Author Contributions #

D.-S.B. and Z.-X.C. contributed equally to this work

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge funding support from the National Natural Science Foundation of China (Grant No 51672282, 21373238, 21673250), the major State Basic Research Program of China (973 program: 2013CB934000), and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDA09010101). The authors also thank Prof. John B. Goodenough from the University of Texas at Austin (USA) for revising the draft of the manuscript and Dr. Xiao Kuang from ICCAS for helpful discussion on the polymerization process.

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DOI: 10.1021/jacs.7b07027 J. Am. Chem. Soc. 2017, 139, 13492−13498