Ultrafine Co-based Nanoparticle@Mesoporous Carbon Nanospheres

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Ultrafine Co-based nanoparticles@mesoporous carbon nanospheres toward high-performance supercapacitors Ben Liu, Lei Jin, Haoquan Zheng, Huiqin Yao, Yang Wu, Aaron Lopes, and Jie He ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11958 • Publication Date (Web): 19 Dec 2016 Downloaded from http://pubs.acs.org on December 20, 2016

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ACS Applied Materials & Interfaces

Ultrafine Co-based Nanoparticle@Mesoporous Carbon Nanospheres toward High-Performance Supercapacitors Ben Liu,a Lei Jin,a Haoquan Zheng,c Huiqin Yao,a,d Yang Wu,b Aaron Lopes,a and Jie Hea,b* a

Department of Chemistry, and b Institute of Materials Science, University of Connecticut, Storrs, CT, 06269; c School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi'an 710062, China; d Department of Chemistry, Ningxia Medical University, Yinchuan 750004, China

KEYWORDS: cobalt nanoparticles, electrode materials, mesoporous carbon, ultrafine nanoparticles, supercapacitors

ABSTRACT: A general synthetic methodology is reported to grow ultrafine cobalt-based nanoparticles (NPs, 2-7 nm) within high-surface-area mesoporous carbon (MC) frameworks. Our design strategy is based on colloidal amphiphile (CAM) templated oxidative self-polymerization of dopamine. The CAM templates consisting of a hydrophobic silica-like core and a hydrophilic PEO shell can co-assemble with dopamine and template its self-polymerization to form polydopamine (PDA) nanospheres. Given that PDA has rich binding sites such as catechol and amine to coordinate metal ions (e.g. Co2+), PDA nanospheres containing Co2+ ions can be converted into hierarchical porous carbon frameworks containing ultrafine metallic Co NPs (Co@MC)

using

high-temperature

pyrolysis.

The

CAM

templates

offer

strong

“nanoconfinements” to prevent the overgrowth of Co NPs within carbon frameworks. The yielded ultrafine Co NPs have an average size of 20 kW/kg), fast charge/discharge (within a few seconds), excellent safety, and long-term cycling ability (>105 cycles).2,4,6-10 However, the relatively low energy density extremely limits the widespread applications of supercapacitors. The rational design and controllable synthesis of novel nanomaterials is the key to solving this bottleneck. Nanostructured electrode materials such as transition metal oxides/hydroxides/sulfides have been evidenced to improve supercapacitor

performance

by

coupling

their

rich

redox

chemistries

to

harness

pseudocapacitance.3,4,9-14 Nevertheless, such materials solely suffer from poor ionic and electrical conductivity.10,15 The hybridization of transition metal-based nanomaterials with conductive carbons through, e.g. physically mixing,9,11,13 self-assembly16,17 and direct growth,18-21 is an effective method to improve the charge transfer ability. In particular, such hybrid nanomaterials, when having hierarchical porous nanostructures (i.e. macropores, mesopores and micropores), can provide

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further enhancement of supercapacitive performances by minimizing the diffusion length of ions (macropores), facilitating ion transport or charge storage (mesopores), and increasing charge accommodation (micropores).6,22 Carbon as an additive, however, can significantly lower the energy density at a high loading. Additionally, to grow nanosized transition metal-based nanoparticles (NPs) in situ within hierarchical porous carbon frameworks, it often results in the overgrowth and aggregation when increasing the loading of metal NPs up to 20 wt%.23-25 The large size of NPs is detrimental to the surface active sites for redox reactions, thus leading to poor electrochemical performance for supercapacitors. Up to now, the preparation of ultrafine transition metal-based NPs (metallic, oxide and sulfide) within porous carbon frameworks through a simple wet-chemical method remains very challenging. In this contribution, our motivation is twofold, i) how to grow aggregation-free, nanosized metal-based NPs at a high loading in the metal/carbon hybrids; and ii) how to use porous carbon frameworks to achieve the best ionic and electrical conductivity for nanosized metal NPs. We present a universal yet powerful synthetic methodology to growing ultrafine cobalt nanoparticles (Co NPs) as an example within high-surface-area conductive mesoporous carbon (MC) frameworks, in pursuit of hybrid nanomaterials for high-performance supercapacitors. Our design strategy is based on colloidal amphiphile (CAM) templated oxidative self-polymerization of dopamine. The CAM templates consist of a hydrophobic silica-like core and a hydrophilic PEO shell that can co-assemble with dopamine and template its oxidative self-polymerization to form CAM@polydopamine (CAM@PDA) nanospheres (Figure 1a). Since PDA has abundant functional groups including catechol and amine to coordinate metal ions, Co2+ ions as an example were physically adsorbed into CAM@PDA nanospheres as precursors for in situ growing metallic Co NPs (Figure 2a). After the pyrolysis of CAM@PDA nanospheres


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containing Co2+ ions at 650 oC, highly aromatic PDA became carbonized to form carbon frameworks; while Co2+ ions were converted into metallic Co NPs. During the pyrolysis, the thermally stable CAM templates evolved into mechanically strong, rigid silica (SiO2) NPs to template the formation of hierarchically porous carbon frameworks; while as-grown Co NPs were confined within carbon frameworks to prevent their overgrowth under high temperatures. It thus yielded ultrafine Co NPs with an average diameter of 2-7 nm even at a high loading of ca. 65 wt% of Co. Co NPs are covered by conductive carbon layers that prevent the dissolution of Co-based NPs and maximally enhance the interfacial conductivity of metal-carbon. Furthermore, Co NPs can be converted into oxides or sulfides in the presence of mechanically strong CAM templates. Several Co-based NPs, including metallic Co, Co oxides (CoO, CoO/Co3O4 and Co3O4), CoS2 and transition-metal doped bimetallic CoxM1-xS2 (M = Mn, Fe, Ni, and Zn) were successfully produced within MC nanospheres. Co-based hybrid porous materials exhibited excellent supercapacitive performance with outstanding long-term cycling stability, given the advantages such as ultrafine size, controllable chemical compositions, hierarchical porous structures and coverage of conductive carbons.

2. Results and Discussion 2.1 Synthesis and characterizations of MC nanospheres Our synthetic route of MC nanospheres is schematically illustrated in Figure 1a. The preparation of MC is based on using CAM-templated oxidative self-polymerization of dopamine.26-29 The CAM templates were prepared via the self-assembly of a silane-containing amphiphilic

block

copolymer

(BCP)

of

poly(ethylene

oxide)-block-poly(3-


4

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(trimethoxysiyl)propyl methacrylate) (PEO114-b-PTMSPMA133, Mn = 38 kg/mol, Mw/Mn = 1.24) in the mixture of water and ethanol (1:1, vol).30,31 The yielded CAM templates have a core-shell nanostructure consisting of a hydrophobic silica-like core and a hydrophilic PEO shell. The CAM templates are highly uniform with a core diameter of 12.8 ± 1.4 nm (Figure 1b). In a typical synthesis, an ethanol solution of the CAM templates (25 mg/mL, 40 mL) was added dropwise into an ethanol/water mixed solution of dopamine (16.7 mg/mL, 120 mL) under stirring. The oxidative self-polymerization of dopamine was catalyzed by the addition of excess ammonia solution (see details in Experimental Section). Due to the presence of the strong hydrogen bonding between the hydrophilic PEO corona of CAMs and dopamine, the occurrence of co-assembly of CAM templates with dopamine thus led to the formation of CAM@PDA composite nanospheres during the self-polymerization as given in Figure 1c.32,33 As-prepared CAM@PDA nanospheres are spherical and structurally uniform with an average diameter of 235 ± 46 nm. The CAM templates homogeneously dispersed within PDA nanospheres. The size of CAMs within PDA nanospheres is 12.3 ± 1.4 nm, comparable to that of as-made ones.


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Figure 1. The synthesis and characterizations of MC nanospheres. (a) Schematic illustration for the CAM-templated synthetic route of MC nanospheres. TEM images of (b) CAM templates, (c) as-made CAM@PDA, (d) calcined SiO2@MC and (e) MC nanospheres after the removal of silica residuals. The inset in (e) is the mesopore size distribution of MC nanospheres measured from TEM images. (f) Nitrogen sorption isotherms and (g) the corresponded pore size distribution of MC nanospheres.

The carbonization process of CAM@PDA nanospheres was further carried out via a hightemperature pyrolysis under argon at 650 oC for 2 h with a ramp of 1 oC/min. During the pyrolysis, the thermally stable CAM templates evolved into mechanically strong, rigid SiO2 NPs to support mesoporous frameworks, while highly aromatic PDA became carbonized to form carbon frameworks. The resulting nanostructures are denoted as SiO2@MC (Figure 1d). The diameter of SiO2@MC is 227 ± 37 nm, slightly smaller than that of the as-made CAM@PDA nanospheres. This was likely originated from the volume shrinkage of PDA during the high-


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temperature carbonization.31,34 No porous feature was observed for SiO2@MC, as MC nanospheres were fully filled with SiO2 NPs generated from the core of CAMs. Subsequently, after etching by NaOH to remove SiO2 residuals, the well-defined MC nanospheres were developed eventually (Figure 1e). Table 1. Physicochemical properties of nonporous C, MC and Co-based MC nanospheres. Surface Area Pore Volume Pore Size Pore Size from (m2 g-1)a (cm3 g-1)b (nm)c TEM (nm) 83 --0 Nonporous C 255 0.37 10.5 12.5 MC 275 0.77 11.5 12.3 Co@MC 196 0.37 11.1 11.1 Co3O4@MC 208 0.41 11.3 11.3 CoS2@MC Samples

a

Surface area was calculated by BET method. b The total pore volume was measured by the adsorbed amount at a relative pressure of ca. 0.99. c Pore size was calculated by the BarrettJoyner-Halenda (BJH) method based on adsorption branch of isotherms. The nanostructures of MC nanospheres were characterized using transmission electron microscopy (TEM). As given in Figure 1e, MC nanospheres have an average diameter of 225 ± 32 nm. The mesopores are homogeneously distributed throughout MC nanospheres. The average pore size is 12.5 ± 1.2 nm (the insert in Figure 1e), close to the diameter of CAM templates. The mesoporous structure was further confirmed using nitrogen adsorption-desorption isotherms. The well-developed porosity with a large mesopore was indicated by the steep and high capillary condensation steps from the type IV isotherms (Figure 1f). The mesopore size of MC nanospheres is 10.5 nm (Figure 1g), slightly smaller than that obtained from TEM images. The Brunauer-Emmett-Teller (BET) surface area of MC is 255 m2 g-1 with a total pore volume of 0.37 cm3 g-1. In addition, nonporous carbon nanospheres were also prepared in the absence of CAM templates as a control. A fairly smooth surface without any obvious porosity was found under TEM (Figure S1). The BET surface area of nonporous carbon nanospheres is 83 m2 g-1 (Table 1).


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2.2. Encapsulation of cobalt NPs within MC nanospheres (Co@MC) PDA is known to have abundant functional groups including catechol and amine both of which offer the strong coordination interactions or chelation with metal ions.35,36 The as-made CAM@PDA nanospheres are expected to have rich binding sites to physically adsorb metal ions. The adsorbed metal ions within carbon supports can be further evolved into metal or metal oxide NPs for potential electrocatalytic applications.6,20,31,37-39 Using Co2+ ions to grow metallic Co NPs within MC (Co@MC) nanospheres as an example, the synthetic route of Co@MC nanospheres is shown in Figure 2a. It includes two steps, i) to adsorb Co2+ ions into CAM@PDA nanospheres and to calcine CAM@PDA nanospheres under the high temperature; resulting in the formation of metallic Co NPs while simultaneously carbonizing PDA into MC frameworks; and ii) to remove silica residuals to yield Co@MC nanospheres. In a typical experiment, the asprepared CAM@PDA nanospheres were mixed with an ethanol solution of Co(NO3)2 with a predetermined ratio. Co2+ ions were adsorbed into the CAM@PDA nanospheres by the slow evaporation of ethanol at room temperature. After pyrolyzed under argon at 650 oC for 2 h, Co2+ ions then evolved into metallic Co NPs which were well-dispersed within MC frameworks.


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Figure 2. The synthesis and characterizations of Co@MC nanospheres. (a) Schematic illustration for the synthetic route of Co@MC nanospheres. (b) SEM, (c) TEM, (d) HAADFSTEM images, (e) STEM mappings and (f) corresponding STEM-EDX spectra of Co@MC nanospheres. The sample for SEM in (b) was coated by Au. (g) The size distribution of Co NPs within Co@MC nanospheres measured from TEM images.

Homogeneous encapsulation of Co NPs within MC frameworks was first confirmed by scanning electron microscopy (SEM) and TEM. The SEM images revealed that the Co@MC have uniform spherical nanostructures (see Figure 2b and Figure S2 for more SEM images). The average diameter of Co@MC nanospheres is 229 ± 37 nm and the average pore size of mesopores is 12.3 ± 2.1 nm, fairly close to that of pure MC nanospheres. TEM images showed that Co NPs were homogeneously dispersed within MC frameworks (Figure 2c). The obtained Co NPs were highly uniform with an average diameter of 5.4 ± 0.6 nm and a standard deviation of ca. 10% (Figure 2g). The metallic Co NPs with a face-centered cubic (fcc) crystalline phase


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were verified by high-resolution TEM (HR-TEM) and X-ray diffraction (XRD) (see Figures 56). The weight percentage of Co NPs was estimated to be 31.8% from atomic absorption spectroscopy (AAS). It is interesting to note that the Co NPs were grown only within carbon frameworks other than on the surfaces or within mesopores of MC nanospheres. No aggregation and overgrowth of Co NPs was further supported by high-angle annular dark-filed scanning TEM (HAADF-STEM) (Figure 2d). The elemental distribution of Co@MC nanospheres was mapped by STEM energy-dispersive X-ray (EDX) spectroscopy. It is clear that C (blue) and N (green) distributed in the whole mesoporous frameworks, while Co (red) concentrated in the inside of MC frameworks (Figure 2e). The weight percentage of Co, C and N elements from STEM-EDX is 17.3%, 79.1% and 3.6%, respectively (Figure 2f). Meanwhile, the loading amount of Co NPs can be readily controlled by adjusting feeding ratios of CAM@PDA and Co(NO3)2. The Co loading amount of 9.7 wt%, 17.6 wt% and 64.8 wt% have been obtained using the same procedures (Figure S3). The average size of Co NPs for all samples is in the range of 4.8-6.9 nm, although a higher loading amount results in a slightly larger size of Co NPs. X-ray photoelectron spectroscopy (XPS) was used to identify the electronic state of Co and the compositions of Co@MC nanospheres. From the XPS survey spectrum, the intensity of Co 2p peaks, however, was fairly weak (Figure 3a); and the atomic ratio of Co/C is about 1.9/77.6. This implies that Co NPs are likely encapsulated deeply within MC frameworks, other than exposed on the surface; since XPS is a surface-sensitive technique with a penetration depth less than 10 nm.40 The high-resolution Co 2p region (Figure 3b) showed the two characteristic Co 2P peaks at 778.5 and 793.4 eV, assigned to metallic Co 2P3/2 and Co 2P1/2, respectively.41 Meanwhile, high-resolution N 1s spectrum (Figure 3c) revealed that Co@MC composed of three main types of N forms, including pyridinic (37.8%), pyrrolic (45.9%) and graphitic N (16.3%),


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respectively. A large proportion of pyridinic N in Co@MC possibly can enhance electrochemical performances of the resulting materials.7,8,42,43

Figure 3. Structural characterizations of Co@MC nanospheres. (a) XPS survey spectrum and (b, c) zoom-in view of Co 2p and N 1S spectrum of Co@MC nanospheres. (d) Nitrogen sorption isotherms, and (e) the pore size distribution of Co@MC nanospheres.

The mesoporous feature of Co@MC nanospheres was further examined by N2 absorptiondesorption isotherms (Figure 3d). Compared to pure MC nanospheres, the coexistence of microscopic and mesoscopic porous structures was observed for Co@MC. An obvious micropore size of 1.7 nm and a mesopore size of 11.5 nm were seen (Figure 3e). The formation of micropore with Co@MC nanospheres was possibly stemmed from the migration of adsorbed Co2+ ions to form nanosized Co NPs, which resulted in the formation of microporosity in MC


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frameworks during calcination. The BET surface area of Co@MC is 275 m2 g-1 with a total pore volume of 0.77 cm3 g-1, slightly higher than that of pure MC nanospheres (Table 1). It should be pointed out that hierarchical pores of carbon frameworks can efficiently enhance the highperformance supercapacitors.6,22 We emphasize that the controllable growth of ultrafine metal NPs within porous carbon substrates at a high loading amount (>30 wt%) is quite challenging.19,44-46 In particular, the hightemperature thermal treatment often results in the growth and fusion of metal crystalline domains. The key here to growing ultrafine Co NPs with a nanosized diameter of ca. 5.4 nm at a high loading amount is the utilization of silane-containing CAMs as the templates.31,47 Under the high-temperature calcination, the silane-rich CAMs can convert into thermally stable and mechanically strong SiO2 NPs, which not only preserve the mesoporous nanostructures of carbon frameworks, but also act as rigid, tough supports to confine Co NPs within carbon frameworks and to inhibit the overgrowth of ultrafine Co NPs at the elevated temperatures. Importantly, the mesopores of carbon frameworks are filled with mechanically strong SiO2 NPs thoroughly during the high-temperature calcination; so that Co NPs cannot grow larger than the wall thickness of the MC framework (9.7 ± 1.9 nm). Therefore, the yielded Co NPs are likely covered with carbon and confined completely into the carbon frameworks. This is essential for many applications of these hybrid materials in electrochemical devices.25,48,49 Several control experiments were further performed to identify the role of thermally stable and mechanically strong CAM templates in the formation of ultrafine Co NPs within MC nanospheres. First, the as-obtained MC nanospheres after removing silica NPs were directly used to encapsulate Co NPs (denoted as MC-Co, Figure 4a). The calcined MC-Co under the same procedure as described above composed of much larger Co NPs with a board size distribution in


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the range of 15-50 nm. Co NPs were noted to mainly present on the surface of MC nanospheres, likely due to the migration of Co NPs during the high-temperature annealing in the absence of the CAM templates. Second, nonporous PDA nanospheres were also examined to support Co NPs (Figure S4). Likewise, the resulting Co NPs have an average size of 18.7 ± 5.9 nm, 3 times larger than that of Co@MC. The similar studies were previously reported in Co@carbon frameworks using metal organic frameworks (MOF),24,50 and other carbon supports.50,51 These findings suggest that the “nanoconfinement” effect of CAM templates is critical to grow ultrafine and nanosized Co NPs within MC nanospheres. Third, a non-silane-containing amphiphilic block copolymer of poly(ethylene oxide)-block-polystyrene (PEO114-b-PS78) was utilized as the soft templates to grow PDA33,34 and Co@MC nanospheres (denoted as Co@MC-2, see Figure 4d). Much larger Co NPs with an average diameter of 21.7 nm were observed for Co@MC-2 (Figure 4e, f). This is because of the lacking mechanical strength for non-silane-containing templates that is essential for the “nanoconfinement” effect.


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Figure 4. Synthesis and nanostructures of Co NPs encapsulated within other carbon frameworks. (a) Schematic illustration of the synthetic route and (b, c) TEM images of MC-Co. (d) Schematic illustration of the synthetic route and (e, f) TEM images of Co@MC-2. The insets in (b) and (e) are the size distributions of Co NPs.

2.3. Controllable synthesis of CoOx@MC nanospheres The controllable and selective synthesis of cobalt-based oxides with multivariable valences is one of the key parameters to determine the photo-/electrocatalytic performances of Co@MC hybrids.18,25,37,52 For example, cobalt oxides (CoOx) with various oxidation states are the promising nanocatalysts for water oxidation53-55, CO oxidation,56,57 and the electrode materials for oxygen reduction reaction,48,52 oxygen evolution reaction24,48, supercapacitors38,58-61 and Liion battery.18 In this context, metallic Co NPs encapsulated within Co-SiO2@MC nanospheres have the lowest oxidation state, which could be further oxidized gradually into CoOx by O2 through a low-temperature annealing. It thus allows us readily controlling the oxidation states of Co in the resulting hybrids. It should be emphasized that Co-SiO2@MC was chosen to synthesize CoOx, due to the “nanoconfinement” effect of mechanically strong SiO2 NPs which prevents the migration and overgrowth of as-resulted CoOx NPs.


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Figure 5. Structural evolutions of CoOx@MC nanospheres. (a) Schematic illustration for the selective synthesis of CoOx@MC nanospheres. (b) Wide-angle XRD patterns and (c) FT-IR spectroscopy of Co@MC (black), Co/CoO@MC (red), CoO/Co3O4@MC (green) and Co3O4@MC nanospheres (magenta). The sample was thermally treated under air at 250 oC for 0 h, 6 h, 24 h and 48 h, respectively.

The detailed transformation of Co oxidation states is outlined in Figure 5a. The as-prepared Co-SiO2@MC nanospheres were directly annealed at 250 oC under air. The structural transitions from Co to CoOx NPs have been studied by wide-angle X-ray diffraction (XRD) and Fourier transform infrared (FT-IR) spectroscopy. For Co@MC, the three characteristic wide-angle XRD peaks at around 44.3o, 51.4o and 75.9o for Co@MC were assigned to (111), (200) and (220)


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crystalline planes (Figure 5b), indicating the metallic Co phase with the fcc structure (JCPDS 150806, black bar). Note that, the broad XRD diffraction halo at ca. 2θ = 25o for all Co-based samples is due to the amorphous carbon frameworks (Figure 5b). Several new XRD peaks gradually appeared at 36.6o, 42.9o and 61.4o for the sample when annealing under air, which are well-matched to the CoO phase (olive bars, JCPDS 43-1004), despite the co-existence of the metallic Co phase after 6 h thermal treatment. It implicated that metallic Co NPs were slowly oxidized from the surface to form CoO (indexed as Co/CoO@MC). After thermally treated for 24 h, the disappearance of the diffraction peaks for metallic Co phase was observed; while, the peak intensity of the CoO phase began to decrease. Meanwhile, the diffraction peaks of Co3O4 appeared (pink bars, JCPDS 42-1467), indicating the phase transformation from Co/CoO to CoO/Co3O4 (denoted as CoO/Co3O4@MC). Eventually, after annealing for 48 h, the typical XRD peaks of (110), (220), (311), (400) and (440) planes indexed to the spinel Co3O4 phase became clear (Co3O4@MC). Phase transformations of Co to Co3O4 NPs were further confirmed by FT-IR spectroscopy. As shown in Figure 5c, only one fairly weak absorption band at around 576 cm-1 assigned to octahedrally coordinated Co ions on the surface, was observed for Co@MC nanospheres.62 This may be due to the weak binding between Co and oxygen in the carbon frameworks or partial oxidization of Co under air. An absorption band attributed to the stretching vibrations of Co-O bonds at around 662 cm-1 gradually appeared, and the intensities at both 662 and 572 cm-1 correspondingly increased with increasing annealing periods, suggestive of the gradual oxidation of metallic Co NPs.62 Based on the results from XRD and FT-IR, we conclude that metallic Co NPs encapsulated within MC nanospheres are gradually evolved to partially oxidized mixture


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phases of Co/CoO, CoO/Co3O4 and finally to fully oxidized spinel Co3O4 NPs after annealing in air for 6h, 24h and 48 h, respectively. The structural transitions from Co to Co3O4 NPs were also monitored by HR-TEM. In Figure 6, the left column exhibits the representative TEM images of Co@MC (a), Co/CoO@MC (c), CoO/Co3O4@MC (e) and Co3O4@MC nanospheres (g) (see Figure S5 for more SEM and TEM images). TEM images showed that the distribution of CoOx NPs was not influenced by the thermal annealing and CoOx NPs were confined, highly dispersed within MC frameworks. The mesoporous nanostructure and spherical morphology of MC nanospheres as well as the size of CoOx NPs exhibited a minimum change, due to the well surface coverage of MC frameworks and “nanoconfinement” effect of the mechanically strong CAM templates. The clear lattice fringes of 0.208 and 0.137 nm were observed from Co@MC (Figure 6b), indexed to (111) and (220) planes of the fcc metallic Co. For Co/CoO@MC, mixed Co and CoO phases were observed in the HR-TEM image (right column). The d-spacings of 0.208 nm and 0.231 nm were assigned to (111) planes of metallic Co and CoO NPs, respectively (Figure 6d). Similarly, the coexistence of CoO and Co3O4 phases was observed for CoO/Co3O4@MC (Figure 6f). After annealing Co@ MC for 48 h, metallic Co NPs fully converted into Co3O4. The HRTEM image displayed in Figure 6h showed two perpendicular lattice fringes in an individual NP, corresponding to (311) and (422) planes of spinel Co3O4, respectively. With a longer thermal treatment, however, the Co oxide NPs, particularly for Co3O4@MC, were much smaller within carbon frameworks (also see Figure S6 for STEM-EDX mapping). This may respond to a oxidative dissolution mechanism of metal NPs over the surface oxidation.63 The weight percentage Co3O4 in Co3O4@MC is estimated to be 39.3% from AAS, indicative of no obvious loss of Co element during annealing.


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Figure 6. TEM (Left column) and HR-TEM (Right column) images of Co@MC (a, b), Co/CoO@MC (c, d), CoO/Co3O4@MC (e, f) and Co3O4@MC (g, h) nanospheres. The inset in (g) is the corresponding HAADF-STEM image, indicating the uniform encapsulation of Co3O4 NPs within MC frameworks.

2.4. Synthesis of CoS2@MC and bimetallic Co0.9M0.1S2@MC nanospheres Cobalt sulfide (CoSx) has been surveyed as the superior electrochemical nanomaterials for hydrogen evolution reaction39,64 and excellent electrode materials for supercapacitors21,65, because of its good electrical conductivity, and mechanical and thermal stability, as well as its rich redox activity. Meanwhile, the doped CoSx with second metal ions can further enhance its electrochemical performances by manipulating their electronic properties and surface nanostructures.64,66 In our synthetic approach, metallic Co NPs could be further transferred into CoS2 NPs (denoted as CoS2@MC) via a simple hydrothermal sulfidization treatment. The sulfidization of Co NPs was carried out using thiourea as a sulfur source at 150 oC (Figure 7a). The formation of CoS2 NPs can be seen by wide-angle XRD (Figure 7f). The diffraction peaks were well-fitted to that of cassiterite CoS2 (JCPDS 41-1471). The TEM image in Figure 7b reveals that highly dispersed CoS2 NPs are uniformly present within the spherical MC frameworks. The average diameter of CoS2@MC is 227 ± 38 nm. An obvious lattice fringe of CoS2 (200) with a d-spacing of 0.275 nm was seen from HR-TEM (Figure 7c), in good consistence with the wide-angle XRD result. The HAADF-STEM image (Figure 7d) clearly showed no aggregation or fusion of CoS2 NPs after the sulfidization. The average diameter of CoS2 NPs is 6.3 nm (Figure 7g), comparable to that of the as-made Co NPs. The STEM mapping of CoS2@MC (Figure 7e) further confirmed the distribution of Co (red) within carbon frameworks. Interesting, sulfur (blue) homogeneously distributed throughout MC frameworks, indicating the occurrence of the sulfur doping in MC frameworks as well. The doping of sulfur


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within carbon frameworks may introduce the stable sulfur-carbon active sites for enhancing their electrocatalytic performances.67-69

Figure 7. The synthesis and characterizations of CoS2@MC nanospheres. (a) Schematic illustration of the synthetic route of CoS2@MC nanospheres. (b) TEM, (c) HR-TEM, (d) HAADF-STEM images and (e) STEM-EDX mappings of CoS2@MC nanospheres. (f) Wideangle XRD of CoS2@MC. (g) The size distribution of CoS2 NPs measured from TEM images.


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Hypothetically, the second transition metal ions (Mn2+, Fe2+, Ni2+ and Zn2+) can be doped into Co@MC to form homobimetallic sulfides in MC nanospheres by stoichiometrically substituting Co2+ sites, given a similar ionic radius and electronegativity.64,70,71 Doped bimetallic NPs and their sulfides (denoted as Co0.9M0.1@MC and Co0.9M0.1S2@MC) were prepared by coordinating CAM@PDA with an ethanol solution of Co(NO3)2 and M(NO3)2 to directly grow Co0.9M0.1 NPs via the high-temperature pyrolysis (Figure S7) and then Co0.9M0.1S2 NPs via the sulfidization with thiourea (Figure 8a) (see Experimental Section for the synthetic details). First, the wide-angle XRD patterns (Figure 8f) exhibited 10% transition-metal-doped cobalt sulfides as Co0.9M0.1S2. No new peaks but the slight shift of diffraction peaks were observed, indicating that the incorporation of second metals (Mn, Fe, Ni, and Zn) did not change the intrinsic structure of cassiterite CoS2 and the second metal ions homogeneously were doped into the crystalline lattice of CoS2 by replacing Co sites. Taking bimetallic Co0.9Zn0.1S2@MC nanospheres as an example, the formation and nanostructure were thorough studied by TEM and STEM (Figures 8b and c). Co0.9Zn0.1S2 NPs with an average diameter of 6.7 nm were homogeneously distributed within MC nanospheres. The chemical composition of Co0.9Zn0.1S2 NPs was also confirmed by the STEM-EDX mapping (Figure 9d). The homogeneous distribution of Co (red), Zn (purple) and S (green) species confirmed the successful formation of bimetallic Co0.9Zn0.1 sulfides. The weight percentage of Co, Zn and S elements is 19.7%, 1.34% and 2.07%, respectively. The molar ratio of Co/Zn is 16/1, slightly higher than the addition predominant amount (9/1). Furthermore, Co0.9M0.1S2@MC nanospheres with the 10% doping of Mn, Fe and Ni were summarized in Figure S8, suggesting the universal doping strategy of the second transition metal ions.


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Figure 8. The synthesis and characterizations of Co0.9M0.1S2@MC nanospheres. (a) Schematic illustration for the synthetic route of Co0.9M0.1S2@MC nanospheres. (b, c) TEM images, (d) STEM mappings and (e) corresponding STEM-EDX spectrum of Co0.9Zn0.1S2@MC nanospheres. (f) Wide-angle XRD patterns of the Co0.9M0.1S2@MC nanospheres (M = Mn, Fe, Ni and Zn).

2.5. Evaluation of electrochemical performance of Co-based MC nanospheres The electrochemical properties of Co@MC, Co3O4@MC and CoS2@MC, as well as pure MC nanospheres as electrode nanomaterials for surpercapacitors were evaluated using a threeelectrode system in 2M KOH. The typical cyclic voltammetry (CV) curves of CoS2@MC


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nanocatalysts at various scan rates ranging from 1 to 100 mV/s in the potential range of 0-0.5 V (vs. Ag/AgCl) are given in Figure 9a. One redox pair can be seen, assigned to the Faradic redox reaction of Co2+/Co3+. All CV curves show rectangular-like shapes with distinct redox peaks, indicative of both double-layer supercapacitive and pseudocapacitive characteristics originating from the redox reactions of Co2+/Co3+.21,33,72 Similar CV curves were also observed from Co@MC and Co3O4@MC nanospheres (Figures S9 and 10), despite a much weaker current response. By contrast, the CV curves of pure MC nanospheres without Co-based NPs showed the typical double-layer supercapacitive performance (Figure S11). Electrochemical impedance spectroscopy (EIS) analysis (Figure S13) confirms that all hybrid materials have a low internal resistance (< 1.0 Ω). CoS2@MC has a slightly lower internal resistance of 0.736 Ω, compared to Co@MC (0.810 Ω) and Co3O4@MC (0.868 Ω). To compare the pseudocapacitive performance of Co@MC, Co3O4@MC, and CoS2@MC nanospheres, the typical CV curves at a scan rate of 10 mV/s and the galvanostatic charge/discharge voltage curves at 5 A/g were potted in Figures 9d and e. The CoS2@MC electrode has an oxidation peak at 338 mV (vs. Ag/AgCl) in the positive scan, corresponding to the oxidation reaction of Co2+ to Co3+. A slightly positive shift of the oxidation potential for Co@MC (346 mV) and for Co3O4@MC (355 mV) can be seen. The galvanostatic charge/discharge voltage profiles (Figure 9e) of all Co-based nanomaterials exhibited nearly triangular shapes with small plateaus at 0.25-0.2 V, in good agreement with the CV results. At 2 A/g, CoS2@MC shows the highest specific capacitance of 513 F g-1, which is approximately 2.4 times higher than that of Co@MC (214 F g-1), 4 times higher than that of Co3O4@MC (131 F g1

) and 6.5 times higher than that of pure MC (79 F g-1). The specific capacitances can be further

calculated as a function of the current density from 1 to 30 A g-1 (Figure 9f). Given the ultrafine


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size of Co-based nanomaterials within MC frameworks, the large surface electroactive area can provide abundant surface sties in an open conductive framework. This results in a much faster redox response; that is, a higher capacitance retention can be observed at a higher power density. The specific capacitance at 5, 10 and 15 A/g for CoS2@MC nanospheres was 434 F g-1, 368 F g1

, and 324 F g-1 respectively, corresponding to a 67% capacitance retention at 15 A/g compared

to that at 2 A/g. Similar capacitance retention was observed for Co@MC and Co3O4@MC at a higher charge/discharge rate as well. A specific capacitance retention of > 96% was achieved for all hybrid materials after 5 500 charge/discharge cycles at a current density of 10 A g-1 (Figure 9c), indicating the excellent cycling performances. Note that, compared to many previous reports on CoSx-based supercapacitors (Table S1),11,21,73 CoS2@MC shows superior cycling stability. The likely reason is because the dissolution of sulfide in CoS2 NPs is largely suppressed by well surface coverage of MC frameworks. Furthermore, CoS2@MC nanospheres contain only ca. 38 wt% active CoS2 NPs based on AAS measurements. When normalized to the loading mass of CoS2, the specific capacitance further reaches 1221 F g-1 at 2 A g-1 (Figure S12).


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Figure 9. Electrochemical properties of Co-based MC nanomaterials. (a) CV curves of CoS2@MC at different scan rates. (b) Galvanostatic charge/discharge curves of CoS2@MC at different current densities. (c) The long-term cycling stability of Co@MC, Co3O4@MC and CoS2@MC nanospheres. (d) The CV curves of CoS2@MC, Co@MC and Co3O4@MC at 10 mV/s. (e) Galvanostatic charge/discharge curves of CoS2@MC, Co@MC and Co3O4@MC at 5 A/g. (f) Specific capacitances of CoS2@MC, Co@MC and Co3O4@MC at different current densities.

The capacitance characteristic of all Co-based NPs originates from reversible surface or near-surface Faradic reactions for charge storage. Our results suggest that CoS2@MC is likely a promising type of pseudo-capacitive materials. In the alkaline solution, the redox reaction for CoS2 was ascribed to the Faradaic reaction of CoS2 and OH-, as CoS2 + OH- ↔ CoS2OH + e-. The high-performance of CoS2@MC electrode is attributed to a few factors, as follows: (i) mesoporosity which provides accessible surface sties to effectively minimize the diffusion paths and enhance the ion transport; (ii) nitrogen-doped carbon frameworks acting as the conductive path to facilitate electron transport between MC nanospheres and electrodes; and iii) Co-based NPs covered by conductive carbon layers that prevent the dissolution of Co-based NPs and


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enhance the cycle stability; and (iv) ultrafine Co-based NPs which efficiently shorten the transport distance of ions and enable the faster charge/discharge.

3. Conclusions We developed a facile synthetic strategy to grow ultrafine Co-based NPs within highsurface-area, conductive MC frameworks via the oxidative self-polymerization of dopamine. Given the “nanoconfinement” effect of thermally stable and mechanically strong silanecontaining CAM templates, the encapsulated Co-based NPs have an ultrafine size of < 7 nm even at a high loading amount (~65 wt%). The as-synthesized Co-based MC nanomaterials exhibited superior performances for electrochemical supercapacitors, such as the high rate capability and long-term cycling stability; due to the ultrafine size, controllable chemical compositions, hierarchical porous structures, and coverage of conductive carbons. Our synthetic method can be further extended to many other transition metal-based NPs (including metallic, oxides and sulfides) such as nickel, manganese, iron, and their bimetallic/trimetallic NPs for a broad range of device-based applications including Li-ion batteries, supercapacitors and sensors.

4. Experimental 4.1. Materials.

Dopamine, manganese nitrate tetrahydrate (Mn(NO3)2·4H2O), iron nitrate

nanohydrate (Fe(NO3)3·9H2O), cobalt nitrate hexahydrate (Co(NO3)2·6H2O), nickel nitrate hexahydrate (Ni(NO3)2·6H2O), zinc nitrate hexahydrate (Zn(NO3)2·6H2O), ammonia solution (28-30%), copper(I) bromide (CuBr), trimethylamine (TEA), monomethoxy poly(ethylene


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oxide)

with

a

molecular

weight

(Mn)

of

5

kg/mol

(PEO114),

N,N,N′,N″,N″-

pentamethyldiethylenetriamine (PMDETA), 3-(trimethoxysiyl)propyl methacrylate (TMSPMA), styrene, anisole, dichloromethane (CH2Cl2), n-hexane and ethanol were purchased from SigmaAldrich and used without further purification unless otherwise noted. Deionized water (High-Q, Inc. 103S Stills) with a resistivity of >10.0 MΩ was used in all experiments. All chemical reagents were used without further purification unless otherwise noted. 4.2. Synthesis of the amphiphilic BCPs of PEO-b-PTMSPMA and PEO-b-PS. The amphiphilic BCPs were synthesized via atom transfer radical polymerization (ATRP) as described in our previous reports.31,47,74 4.3. Synthesis of MC nanospheres and Co-based NP@MC nanospheres. 4.3.1 Synthesis of MC nanospheres.

The MC nanospheres were synthesized by the CAM-

templated oxidative polymerization of dopamine. In a typical experiment, 2 g of dopamine was dissolved in the 40 mL of ethanol and 80 mL of water, and stirred for 1 h. Then, 40 mL of ethanol containing 1 g of PEO-b-PTMSPMA (or 40 mL of THF containing 1g of PEO-b-PS) was added dropwise into the above dopamine solution under stirring at room temperature. After further stirring for 1 h, the polymerization of dopamine was initiated by the addition of ammonia aqueous solution (5 mL). The solution was kept stirring for an additional 20 h. The mixture was then concentrated and washed to obtain the as-made CAM@PDA nanospheres. The sample was calcined under Ar atmosphere at 650 oC for 2 h with a ramp of 1 oC/min and washed with 2 M hot NaOH twice, to obtain the calcined MC nanospheres. The MC nanospheres templated by PEO-b-PS and nonporous carbon nanospheres without the templates were also synthesized using


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the similar procedures, but using the amphiphilic PEO-b-PS as the template or without any template. 4.3.2 Synthesis of Co@MC nanospheres.

500 mg of CAM@PDA nanospheres were

dispersed in 200 mL of ethanol under sonication. 10 mL of ethanol containing predominant amounts of Co(NO3)2·6H2O was added into the above solution. The mixture was stirred at room temperature until completely dry powder was collected. The Co@MC nanospheres were obtained by calcining the above power under Ar at 650 oC for 2h with a ramp of 1 oC/min and washing with 2 M hot NaOH twice. Similarly, bimetallic Co0.9M0.1@MC nanospheres were synthesized using the similar procedure, but using M(NO3)2·xH2O (M = Mn, Fe, Ni, Zn) and Co(NO3)2·6H2O with a molar ratio of 1:9 as the metal sources. As controls, the encapsulation of Co NP into nonporous carbon (pure PDA), MC after removing the CAM templates and PEO-bPS templated MC nanospheres (PEO-b-PS@PDA) was performed by following the similar procedures. 4.3.3 Synthesis of CoOx@MC nanospheres.

The CoOx@MC nanospheres were synthesized

by annealing Co-SiO2@MC at 250 oC under air with different times. Typically, Co/CoO@MC, CoO/Co3O4@MC and Co3O4@MC nanpspheres were obtained by annealing for 6 h, 24 h and 48 h, respectively. All samples were then cleaned with 2M hot NaOH to remove SiO2 residuals. 4.3.4 Synthesis of CoS2@MC nanospheres.

The sulfidation was performed by the

hydrothermal treatment of Co-SiO2@MC with thiourea at 150 oC. Typically, 50 mg of CoSiO2@MC nanospheres were added into 5 mL of water, followed by the addition of 200 mg of thiourea. The mixture was then transferred into the autoclave, and heated at 150 oC for 10 h. The obtained CoS2@MC was centrifuged and washed with water/ethanol, and etched by 2M hot


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NaOH. Similarly, bimetallic Co0.9M0.1S2@MC nanospheres were obtained from Co0.9M0.1SiO2@MC. 4.4. Electrochemical properties for supercapacitors. All electrochemical measurements were carried out on a CHI 760e electrochemical workstation in a three-electrode system with Ag/AgCl electrode as reference electrode and Pt wire as counter electrode in a 2.0 M KOH electrolyte. Active materials (90 wt%) and polyvinylidene fluoride (10 wt%) were mixed in DMF to form a uniform slurry under sonication. The working electrode was prepared by coating the slurry on the nickel foam electrode (1 × 1 cm2) which was then dried at 110 oC overnight under vacuum and was pressed before testing. Cyclic voltammetry (CV) and galvanostatic charge-discharge tests were carried out at different scan rates and current densities. The specific capacitance was calculated from the formula: C = (I∆t)/(m∆V), where I, ∆t, m, ∆V are the discharging current, discharging time, loading mass of active materials and potential window, respectively. 4.5 Characterizations. SEM was performed using an FEI Nova NanoSEM 450 with an accelerating voltage of 10 kV and a beam current of 10 mA. SEM samples were prepared by casting a suspension of the materials on silicon wafers. TEM and HR-TEM were carried out using a JEOL 2010 TEM with an accelerating voltage of 200 kV. HAADF-STEM and STEM mapping were performed using a Talos F200X Atomic Resolution Analytical Microscope. TEM and STEM samples were prepared by casting a suspension of the materials on a carbon coated copper grid (300 mesh). The wide-angle XRD patterns were recorded using a Rigaku Ultima IV diffractometer (Cu Kα radiation, λ=1.5406 Å) with an operating voltage of 40 kV and a current of 44 mA. Wide-angle


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XRD were collected over a 2θ range of 15-85o with a continuous scan rate of 0.5o/min. The Brunauer-Emmett-Teller (BET) surface areas of the catalysts were measured using a Quantachrome Autosorb-1-C automated N2 gas adsorption system. The XPS experiments were recorded on a PHI model 590 spectrometer with multi-probes using Al Kα (λ = 1486.6 eV) as the radiation source. XPS samples were prepared on carbon tape using adhesive copper tape struck to a sample stage placed in the chamber. Gel permeation chromatography (GPC) measurements were performed using a Waters GPC-1 (1515 HPLC Pump and Waters 717Plus Auto injector) equipped with a Varian 380-LC evaporative light scattering detector and a Waters 2487 dual absorbance detector, three Jordi Gel fluorinated DVB columns (1-100 K, 2-10 K and 1-500 Å). The molecular weight was calibrated using standard polystyrene samples. Proton nuclear magnetic resonance (1H NMR) spectra were recorded on a Bruker Avance 300 MHz spectrometer. FT-IR spectra were recorded by using a Bruker 66V FTIR spectrometer. AAS measurements were performed using a Varian Atomic Absorption Spectrometer AA 110. Prior to analysis, the samples (1 mg) were digested in an Aqua Regia solution (HCl : HNO3 = 3:1, by vol) for 10 h at room temperature until all solids were completely dissolved. The solutions were then diluted with DI water and analyzed by AAS. Supporting Information. More SEM, TEM and STEM characterizations and supercapacitor results of nonporous C, MC and Co-based MC nanospheres. Corresponding Author [email protected] (JH) ACKNOWLEDGMENT


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J.H. thanks the financial support of startup funds from the University of Connecticut. The SEM/TEM studies were performed using the facilities in the UConn/FEI Center for Advanced Microscopy and Materials Analysis (CAMMA). This work was also partially supported by the Green Emulsions Micelles and Surfactants (GEMS) Center, FEI Company under an FEI-UConn partnership agreement and a Research Excellence Award of the University of Connecticut.

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Ultrafine Co-based Nanoparticle@Mesoporous Carbon Nanospheres

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