Modular Assembly of Metal Nanoparticles/Mesoporous Carbon Two- Dimensional Nanosheets

The manipulation of polymeric micelles across extended-length scales is a key challenge in the design of integrated mesoporous materials with well-defined geometry and advanced functions. Herein, we demonstrate a modular assembly strategy to construct metal nanoparticle functionalized mesoporous carbon two-dimensional (2D) nanosheets by organizing zero-dimensional (0D) spherical monomicelle modules on the 2D supporting blocks. The modular assembly process involves two key steps: the “modularization” step is used to synthesize highly uniform metal–catecholamine (MC) complex functionalized monomicelle “modules” that can be conveniently assembled on the 2D supporting blocks (graphene oxide (GO), WS2, and MXene) in the following “assembly” step. After an annealing process, the resultant composites possess a single-layered 2D nanosheet surrounded by two single-layered mesoporous carbon at both sides and exhibit highly ordered mesostructures with large surface areas (~385 m2 g−1), tunable pore sizes (16–25 nm) and highly dispersed metal-containing nanoparticles. Due to the modularity of this assembly process, a range of metal species (Co, Fe, Ni, V, Cu, Pd, FeCo, CoNi, and FeCoNi) can be in-situ incorporated into the 2D mesoporous frameworks, which are partially embedded in the pore walls with the remaining part exposed in the pore channels. Benefiting from the unique textual structures, the resultant GO-derived functional mesoporous carbon nanosheets (Co as the functional species and being annealed at 850 °C) exhibit excellent electrocatalytic activity, long-term stability, and superior methanol tolerance for oxygen reduction reaction, which holds great potential as a catalyst for fuel cells.


Introduction
Nature is abundant of self-assembled objects with fascinating properties such as tough nacre, superhydrophobic lotus leaves and photonic butter y wings. [1][2][3] Inspired by these assemblies, tremendous efforts have been devoted to construct advanced materials with complex nanostructures by arti cial self-assembly from a toolkit of chemical building blocks such as micelles, vesicles, lms and tubes. [4][5][6] As a typical products from the assembly of polymeric micelles, ordered mesoporous materials (OMMs) have experienced a great development over the past three decades, producing a series of OMMs with unique mesostructures, various compositions and diverse architectures. [7][8][9][10][11] Particularly, the construction of two-dimensional (2D) mesoporous materials (2DMMs) have recently provided increasing opportunities for the fabrication of novel electronic materials, superior energy conversion and storage devices, and excellent catalysts due to the integrated properties from the ultrathin 2D nanosheet structure and ordered mesoporous frameworks. [12][13][14] In addition, the applicability of 2DMMs could be tremendously enhanced if some functional metal nanoparticles (NPs) with different features could be insitu con ned inside the mesopores to create materials with desired properties. [15][16][17] However, metal ions are highly electrophilic and can easily react with the nucleophiles like water, thus it is di cult to obtain a homogenous organic-inorganic mesoporous preforms through the conventional wet-chemistry approach.
In addition, the lack of suitable molecular interactions between the metal atoms and framework precursors lead to the uncontrolled growth of metal species during the calcination, which may result in the migration of NPs into the mesopores, blocking the electron/ion transport channels of the mesoporous frameworks. Therefore, a generalized bottom-up strategy for controlled in-situ incorporation of functional metal NPs in a 2D mesoporous nanostructure at a molecular self-assembly level is greatly desired but remains a great challenge.
Recently, the metal-ligand assisted self-assembly has exploited as a powerful bottom-up approach to construct various metal − organic complexes on some NPs that could be further applied as the modular "building blocks" for constructing higher-order superstructures with increased complexity and functionality. [18][19][20] Considering the modularity of coordination-driven assembly, the surface properties of a given supramolecular monomicelle core can also be readily modularized and thus have a signi cant effect on the resulting mesoporous materials. More importantly, the formation of metal-ligand bonds may work in parallel with non-covalent interactions (e.g. electrostatic attraction, hydrogen bonding, and hydrophobic interaction), which makes it possible to design materials that mimic the hierarchical ordering found in the nature system.
Herein, we report a modular assembly strategy to construct metallic NPs functionalized mesoporous carbon sandwich nanosheets from the metal-catecholamine (MC) monomicelle building blocks. This strategy involves two key steps: (i) metal-catecholamine functionalization of readily formed polystyreneblock-polyethylene oxide (PS-b-PEO) monomicelles to construct a library of spherical MC modules; (ii) mounting the modules on the 2D supporting blocks directed by the interfacial molecular interactions and inter-micelles crosslinking using metal ions. Owing to the strong chelation effect of catecholamine groups, a range of single metal (Co 2+ , Fe 2+ , Fe 3+ , Ni 2+ , V 2+ , Cu 2+ , Pd 2+ ), bi-metal (Fe 3+ /Co 2+ , Co 2+ /Ni 2+ ) and tri-metal (Fe 3+ /Co 2+ /Ni 2+ ) species can be successfully incorporated into the polymeric frameworks, and then in-situ transformed to metallic NPs followed by an annealing process under inert atmosphere.
The resultant materials not only possess well-de ned sandwich-like structures with controllable pore sizes (16-25 nm), ultrathin thicknesses (30-50 nm) and high surface areas (~ 385 m 2 g), but also exhibit uniform metal-containing NPs that are homogeneously anchored in the pore walls of the single-layered mesoporous carbon framework. Owing to the unique structure and integrated functionalities, the cobalt NPs incorporated 2D mesoporous carbon catalyst exhibits a high oxygen reduction reaction (ORR) halfwave potential of ~ 0.82 V, which is comparable to the commercial platinum/carbon (Pt/C) catalyst.
Moreover, the catalyst shows a high stability and durability under an ultrahigh concentration of menthol (2 mol L − 1 ). This study paves a promising way to controllably construct functional 2D mesoporous carbon nanosheets for electrochemical applications.

Construction of the library of MC monomicelle modules
The library of MC monomicelle modules were constructed through the metal-catecholamine functionalization of readily formed PS-b-PEO monomicelles (Fig. 1a-c). The diblock copolymer PS-b-PEO templates with various PS lengths were rst synthesized through an atom transfer radical polymerization method, which possessed a high quality ( Figure S1). The pure PS-b-PEO block copolymer monomicelles were then formed through the addition of a water/ethanol (2:1) mixture to the PS-b-PEO tetrahydrofuran (THF) solution to form a light-blue colloid solution, which exhibits a clear Tyndall phenomenon ( Figure  S2). The formation of such a colloidal solution can be explained as follows: THF is a good solvent for both the PEO and PS chains while water is a good solvent for PEO chains but a poor solvent for the PS chains. [21][22][23] Therefore, the addition of water signi cantly increases the repulsion force among the copolymers and thus induces their spontaneous micellization to reduce interface energy. By taking PS 222b-PEO 114 copolymer as an example, the transmission electron microscopy (TEM) image (Fig. 1d) clearly shows the formation of uniform spherical micelles with a particle size of ~ 20 nm. The size is slightly smaller than that measured by a dynamic light scattering (~ 22 nm, Figure S3), possibly owing to the shallow contrast of PEO shells under TEM observation. Finally, the formation of MC/PS-b-PEO monomicelle building blocks were achieved through the metal-catecholamine complex functionalization (Fig. 1a). Here, the dopamine (DA) was used as the carbon precursor because they possess abundant catechol and amine groups that can effectively bind with most inorganic and organic species. Thus, the introduction of DA and metal salts (e.g. Co(NO 3 ) 2 ) simultaneously into above solution leads to the formation of MC complexes (Fig. 1b, c), which can be deposited on the PS-b-PEO monomicelles to form the MC/PS-b-PEO composite monomicelles through the hydrogen bonding or electrostatic interaction. The formation of MC complexes was con rmed by measuring the UV-visible spectra of DA-cobalt mixture solution (Fig. 1e), which shows two clear peaks at the wavelength of 295 and 445 nm, typical for the L a -L b transition of DA and the ligand-to-metal charge transfer, respectively. 24 Owing to the high glass temperature of PS blocks, the monomicelles possess a very high stability, which acts like colloid nanoparticles. Therefore, the composite single micelles can be separated with ultracentrifugation ( Figure   S4). The scanning TEM (STEM) image of the collected composite micelles clearly displays that the spherical structure are well-retained and their corresponding energy dispersive X-ray spectroscopy (EDX) elemental mapping analyses reveal the presence of nitrogen and cobalt elements (Fig. 1f), further con rming the formation of MC/PS-b-PEO monomicelles.
Assembly of MC monomicelle modules on 2D supporting blocks When the 2D building blocks were added, the preformed spherical composite micelles are assembled and anchored on the 2D surfaces (Fig. 2a). TEM images clearly show that the spherical micelles with a diameter of ~ 20 nm was organized into single-layered architecture with close packing structure on the GO surface ( Figure S5). The driving force for the assembly is proposed to be the electrostatic force between the PEO blocks with the oxygen-bearing groups on GO (such as -COOH and -OH). 25 Note that an ultrasonic treatment is essential for the further dispersion of GO to maintain its 2D structure (Fig. 2b,d) because the massive opposite charged solution could lead to the curling of the GO to form a nanowire structure ( Figure S6). The slight adjustment of the solution pH to 8 by the addition of ammonium hydroxide can initiate the polymerization of DA molecules (Fig. 2c). Once the polymerization occurs, the newly formed polymeric dopamine (PDA) oligomers preferentially diffuse and grow around the single micelles on the surface of GO sheets to form a cross-linked networks, afterward, resulting in a uniform coating layer on GO sheets to reduce the interfacial energies. Both the TEM and SEM images of the sample withdrawn at different reaction time reveal that the void spaces between the adjacent micelles are gradually lled out by PDA networks (Fig. 2e-g and Figure S7). With extending the reaction for 8 h, the spherical micellar structure on the GO sheets become almost invisible and the thickness of the sheet apparently increases (Figure S7a-c). The EDX mapping spectra show that the elements of carbon, oxygen, nitrogen, and cobalt are uniformly distributed on the whole sheets.

Characterization of the 2D functional mesoporous carbon nanosheets
After the removal of the template, carbonization of the polymeric framework, and crystallization of the metal species, the 2D functional mesoporous carbon nanosheets with tunable pore sizes, various metal species and different graphitization degree are obtained and the resultant nanosheets are de ned as x@mC-metal-y (x indicates the type of 2D surface; the metals can be Co, Fe, Ni, Cu, V and Pd; y represents the calcination temperature). Here, we take reduced graphene oxide (rGO) and cobalt as the example 2D surface and metal species, respectively, for the proof of concept. The SEM images of rGO@mC-Co-550 (Fig. 3a-c) reveals that the functional nanosheets retain the ultrathin 2D layered-nanosheet structure with a thickness of ~ 36 nm and possess uniform spherical mesopores with a diameter of ~ 18 nm in large domains. The thickness of the nanosheets are 2 times equal to the pore diameters, suggesting that two layers of mesoporous structure are formed. The partial overlapping of mesopores observed from TEM images (Fig. 3d, e) further con rms that the mesopores lie on both sides of the rGO, in consistent with SEM side view on the rGO@mC-Co-550 nanosheets (Fig. 3c). These results indicates that the growth of functional carbon nanosheets was directed by the adaptation of the GO sheet morphology into a sandwich-like structure. TEM images also show that uniform functional NPs with a particle size ~ 6 nm are evenly dispersed on the entire mesoporous nanosheet frameworks. High resolution TEM (HRTEM) image further reveal that the CoO nanoparticles with a d-spacing of ~ 0.35 nm that can be assigned to the lattice fringes of CoO (220) are partially embedded in the pore walls with the remaining part exposed in the mesopore channels (Fig. 3f). 26 It is speculated that this unique semi-exposure structure may not only provide an excellent con nement effect and exposed surface for reactions but also help to tightly trap the nanoparticles and prevent aggregating during the synthesis and applications. Owing to the protection effect, the content of CoO nanoparticles throughout the entire mesoporous framework can be easily tuned by changing the added cobalt nitrate to DP ratio during the synthesis. The TEM images clearly shows that as the ratio rises, the distributing density of CoO nanoparticles on the mesoporous nanosheet apparently increases while the regularity of the mesoporous framework and dispersity of the functional nanoparticles are still well kept ( Figure S8). The STEM image further con rms that the rGO@mC-Co-550 nanosheets possess a sandwich-like structure with one single rGO layer surrounded with two single-layered mesoporous carbon and many bright dots are evenly distributed in the framework (Fig. 3g). The existence of CoO nanoparticles in the mesoporous framework is evidenced by the EDX elemental mapping of Co and O (Fig. 3h-k). The elemental maps of C and N correspond to the N-doped carbon matrix of 2D nanosheets that forms due to the pyrolysis of amine groups in PDA.
Interestingly, by simply changing the template with a different PS block lengths, the construction of metal containing rGO@mC nanosheets with different pore sizes can be achieved. When the PS 199 -b-PEO 114 copolymers with a shorter PS block length are used as the template, the pore size decreases to ~ 15 nm ( Figure S9a, b) while when the template with a longer PS block length are used, the pore size can be extended to ~ 24 nm (Figure S9c, d). To get more information on the pore size, the N 2 sorption isotherms of resultant nanosheets were also measured ( Figure S10 The phase purity of the annealed nanosheets at 550 ℃ is examined by the powder X-ray diffraction (XRD) analysis ( Figure S11), where two typical peaks can be observed. The broadened diffraction peaks at 25.3° can be indexed to graphite while the sharp one at 42.5° is typical for Co (220). According to the Scherrer formula, the average crystallite size is calculated to be approximately ~ 6 nm, matching well with the HRTEM result. By arising the calcination temperature, the phase and particle size of the functional nanoparticles could be changed but the ordered mesoporous framework are still maintained ( Figure S12). For further analyzing the surface chemical states of the G@mC-Co-550 nanosheets, the X-ray photoelectron spectroscopy (XPS) analysis was performed. The total survey spectrum ( Figure S13) clearly shows the presence of C, O, N and Co elements in the nanosheets compared with C and O in the GO, suggesting the formation of N and Co containing carbon materials. The N 1s XPS spectrum of G@mC-Co-550 ( Figure S14) can be deconvoluted into three peaks, which are attributed to pyridinic-N (398.9 eV), pyrrolic-N (400.8 eV) and graphitic-N (402.6 eV), respectively. 27 Nitrogen is a relatively high electronegative heteroatom which could create charged sites for carbon frameworks to improve the electrocatalytic properties of carbon material. 28,29 As for the Co element, two peaks at 782.0 eV and 770 eV could be observed, which are in consistent with Co 2p1/2 and Co 2p3/2 binding energy of CoO, respectively. It is further proved that the formation of cobalt species in the 2D nanosheet, in agreement with the HRTEM results.

Extension of modular assembly to other metals and 2D building blocks
It is worth noting that this modular assembly strategy can be facilely extended to synthesize other metal species containing 2D mesoporous carbon nanosheets such as Fe, Ni, V, Cu, and Pd. The TEM images clearly show that ordered 2D mesoporous rGO@mC-Fe-550, rGO@mC-Ni-550, rGO@mC-V-550, rGO@mC-Cu-550 and rGO@mC-Pd-550 nanosheet can be obtained with uniform spherical mesopores and evenly distributed nanoparticles (Fig. 4a-h). Owing to the difference of the property of metals, the size and crystallinity of the resultant nanoparticles could be different but all are well embedded in the mesoporous frameworks under the protection of carbon wall. More importantly, by changing GO nanosheet to other 2D materials such as WS 2 and MXene and even carbon nanotube, the metal containing mesoporous carbon single layer can still be deposited (Fig. 4i-h), holding a great promise for various applications.
The development of nonprecious metal catalysts with a high activity and an outstanding stability for oxygen reduction reaction (ORR) is urgent for large-scale applications of fuel cells. Here, the cobalt containing rGO@mC mesoporous carbon nanosheets annealed at different temperatures (750, 850, and 900 ℃) were evaluated for the ORR catalytic activity. Linear sweep voltammetry (LSV) result shows that the sample annealed at 850 ℃ (rGO@mC-Co-850) exhibits the largest limiting current density (5.97 mA cm − 2 ) and half-wavelength potential (~ 0.82 V), which is comparable to the commercial platinum-carbon (Pt/C) electrode (Fig. 5a). The better performance than that of pristine mesoporous carbon nanosheets (rGO@mC-850) indicates that the Co species plays an important role in promoting the ORR catalytic activity. The ORR catalytic performance of rGO@mC-Co-850 was then evaluated by cyclic voltammetry (CV) in 0.1 M KOH solution (Fig. 5b). In the N 2 saturated solution, a featureless CV curve is observed within the potential ranged from − 0.2 to + 1.2 V (vs RHE). When changed to the O 2 -saturated one, an obvious cathodic peak at 0.76 V appears, which is close to that of Pt/C electrode (0.77 V).
LSV curve of the rGO@mC-Co-850 were also recorded from 400 to 1600 rpm (Fig. 5c). As the rotation rate rises, the current density is distinctly increased. The relevant Koutecky-Levich (K-L) plots show clear linearity from 0.3 V to 0.5 V (Fig. 4D), implying a rst-order reaction kinetics for the ORR. Based on the K-L plots, the corresponding electron transfer number (n) of rGO@mC-Co-850 is estimated to be 3.6, suggesting a four-electron transfer pathway. 30 After cycling for 5000 cycles, the half-wave potential shows a negligible decrease and the current density can be maintained above 98%, indicating the superior durability of rGO@mC-Co-850 catalyst (Fig. 5e).
To date, the tolerance to the methanol crossover at cathode reactions plays a vital role in the commercialization of fuel cells. 31 When a high concentration of methanol (2 mol L − 1 ) was added, an increase of current density and then back to original was observed in the i-t curve of the rGO@mC-Co-850 electrode (Fig. 5f). The increase of the current density here possibly due to the sudden change of the concentration of reactive species after addition of massive volume of methanol. As a contrast, the current density was sharply decreased to almost zero for Pt/C catalyst, demonstrating the prominent tolerance to methanol crossover of the rGO@mC-Co-850 catalyst.
The excellent performance of the resultant rGO@mC-Co-850 can be related to its superior textual structures, such as well-de ned 2D geometry, ordered mesoporous structure, nitrogen doping and welldispersed metal nanoparticles. First, the unique sandwich-like mesoporous structure can provide short pathways for the mass transport of ORR-related species and possess more active sites for the catalytic reactions. 32 Second, the N-doping in the carbon framework increases the electron delocalization due to the good electron donating property of the N atom, forming abundant active sites for ORR. Finally, the well dispersed Co nanoparticles in a semi-exposure state may not only provide exposed metal active sites for reactions but also help to tightly trap the nanoparticles and prevent the loss of activity during the reaction applications, which may explain the excellent stability and high methanol tolerance. [33][34][35] In summary, we have reported a novel interfacial assembly strategy to in-situ construct 2D functional mesoporous carbon nanosheets based on MP/PS-b-PEO composite micelle building blocks. The resultant nanosheets possess three layers with a thin 2D nanosheet (such as, graphene, WS 2 , and MXene) surrounded by two single-layered mesoporous carbon at both sides, in which the metallic nanoparticles are partially embedded in the pore walls with the remaining part exposed in the mesopore channels. Owing to the high surface area, large pore size, and unique textual structure, the 2D G@mC-Co-850 nanosheet exhibited an excellent ORR performance, which is comparable to the commercial C/Pt electrode. More importantly, the catalyst show a high stability (no obvious change of the LSV curve after 5000 cycles) and outstanding methanol tolerance (2 mol L − 1 ), much better than that of C/Pt electrode. This study holds a promising way for the design and synthesis of 2D functional mesoporous materials for ORR catalysis.

Supplementary Files
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