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A Versatile Nanoemulsion Assembly Approach to Synthesize Functional Mesoporous Carbon Nanospheres with Tunable Pore Sizes and Architectures Liang Peng, Chin-Te Hung, Shuwen Wang, Xingmiao Zhang, Xiaohang Zhu, Zaiwang Zhao, Changyao Wang, Yun Tang, Wei Li, and Dongyuan Zhao J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 09 Apr 2019 Downloaded from http://pubs.acs.org on April 9, 2019
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Journal of the American Chemical Society
A Versatile Nanoemulsion Assembly Approach to Synthesize Functional Mesoporous Carbon Nanospheres with Tunable Pore Sizes and Architectures Liang Peng, Chin-Te Hung, Shuwen Wang, Xingmiao Zhang, Xiaohang Zhu, Zaiwang Zhao, Changyao Wang, Yun Tang, Wei Li*, and Dongyuan Zhao* Department of Chemistry, Laboratory of Advanced Materials, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, iChEM and State Key Laboratory of Molecular Engineering of Polymers, Fudan University, Shanghai 200433, P. R. China. ABSTRACT: Functional mesoporous carbons have attracted significant scientific and technological interests owning to their fascinating and excellent properties. However, controlled synthesis of functional mesoporous carbons with large tunable pore sizes, small particle size, well-designed functionalities and uniform morphology is still a great challenge. Herein, we report a versatile nanoemulsion assembly approach to prepare N-doped mesoporous carbon nanospheres with high uniformity and large tunable pore sizes (5-37 nm). We show that the organic molecules (e.g., 1,3,5-trimethylbenzene, TMB) not only play an important role in the evolution of pore sizes but also significantly affect the interfacial interaction between soft templates and carbon precursors. As a result, well-defined Pluronic F127/TMB/dopamine nanoemulsion can be facilely obtained in the ethanol/water system, which directs the polymerization of dopamine into highly uniform polymer nanospheres and their derived N-doped carbon nanospheres with diversely novel structures such as smooth, golf, multi-chambered and dendritic nanospheres. The resultant uniform dendritic mesoporous carbon nanospheres show the ultralarge pore size (~37 nm), small particle size (~128 nm), high surface area (~635 m2 g-1) and abundant N content (~6.8 wt%), which deliver high current density and excellent durability toward oxygen reduction reaction in alkaline solution.
INTRODUCTION:
(e.g., polystyrene-b-poly(ethylene oxide) (PS-b-PEO)) and SiO2 nanospheres as the templates.25-30 The presence of large mesopores, especially those larger than 20 nm is benefit for the enhanced application performance, which can be ascribed to the promoted mass transportation by reducing the diffusion barrier. Nevertheless, the complex lab-made and the additional etching processes are very fussy, dangerous, high cost, and thus industrial unfeasible. Meanwhile, the as-prepared large pore mesoporous carbons are generally in the form of films or irregular micro-sized particles. The long diffusion pathway still limits the mass transport to some extent. More exposed surface active sites can be accessed by decreasing the particle size of bulk mesoporous carbons to the nanoscale range.31-35 In addition, introduction of heteroatoms into the carbon frameworks, such as nitrogen, is another effective way to modify the electronic and chemical properties of carbon materials. Continuous efforts have been made to fabricate N-doped mesoporous carbons, which show outstanding performances in electrocatalysis, supercapacitors and batteries.36-43 Despite the considerable efforts have been devoted to adjusting the pore size, structure, functionality and morphology of mesoporous carbons, however, it still remains a challenge to synthesize
Carbonaceous materials, including graphene, carbon nanotubes and porous carbons have been investigated extensively in the past to explore the adaptation in many practical applications1-8 Benefiting from the merits of low density, high porosity, and large specific surface area, mesoporous carbons have attracted significant scientific and technological interests, which show great promising in various areas such as catalyst supports, adsorption, separation systems, nanoreactors and electrodes for energy application.9-19 However, most reported mesoporous carbons are of large size, irregular shape, chemical inactivity in the framework and/or small pore size. Such dimensional and natural limitations significantly restrict their applications. Exploring exceptionally large tunable pores with well-designed functionalities has been a longpursued target for development of the state-of-the-art mesoporous carbons, which may offer new opportunities in these emerging applications and further expand their application scopes.20-24 Many efforts have been carried out to synthesis of mesoporous carbons with large accessible pores, for example using high-molecular-weight block polymers
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microscopy (FESEM) images (Figure S1a,b) show that the as-made mesostructured polydopamine (PDA) nanospheres are quite uniform with a particle size of ~145 nm composed of continuously interweaving nanosheets (~20 nm in thickness). Transmission electron microscopy (TEM) images (Figure S1c,d) further disclose a sheet-like framework and a highly open mesostructure from the center to surface. After pyrolysis at 800 °C in N2, a reduced particle size (~128 nm) of dendritic NMCNs can be observed due to the thermal-induced structural shrinkage (Figure 1a,b). Thermogravimetric analysis (TGA) reveals that the Pluronic F127 can be totally removed before 400 °C, and the carbonization yield of the mesostructured PDA is as high as ~43 % (Figure S2). The specific nanosheet structure can be well retained without any collapse after the pyrolysis. The ultralarge mesopores are distributed on the sphere surface (Figure 1c). TEM images further show the uniform dendritic shape consisting of the nanosheet frameworks with a thickness of ~18 nm (Figure 1 d,e). The ultralarge mesopores can be clearly distinguished with a pore size of ~37 nm, and are radially oriented from the center to surface (Figure 1f). The selected-area electron diffraction (SAED) pattern (Inset of Figure 1f) shows clear diffraction rings, suggesting the partially graphitic pore walls. The ultralarge mesopores and dendritic structure can be further confirmed by the scanning TEM (STEM) image (Figure 1g). The corresponding element mapping images (Figure 1h-j) show that C, N and O atoms are uniformly distributed in the carbon framework.
mesoporous carbon nanospheres with ultralarge pore sizes, high nitrogen content and novel structures. Scheme 1. Synthesis procedure for the dendritic Ndoped mesoporous carbon nanospheres prepared by the versatile nanoemulsion assembly approach.
Herein, we report a simple and versatile nanoemulsion assembly approach to prepare N-doped mesoporous carbon nanospheres with tunable pore sizes and architectures. The key feature of this approach is the control of unique Pluronic F127/TMB/dopamine nanoemulsion by using TMB to mediate the interfacial interaction between F127 and dopamine, which directs the growth of highly uniform polymer nanospheres and their derived N-doped carbon nanospheres. Smooth nonporous nanospheres, golf nanospheres with ordered mesopores on the surface, multi-chambered mesoporous nanospheres and dendritic mesoporous nanospheres can be synthesized. Notably, the obtained uniform dendritic mesoporous carbon nanospheres possess ultralarge mesopores (~37 nm) from the center to surface, a small diameter (~128 nm), a high surface area (~635 m2 g-1), and a high N content (~6.8 wt%). We show that the resultant dendritic mesoporous nanospheres exhibit good electrocatalytic activity, methanol tolerance, and longterm stability for oxygen reduction reaction (ORR), which is much better than other carbon nanospheres due to the unique porous structures and chemical properties. This facile nanoemulsion strategy opens up new horizons for synthesis of mesoporous materials with novel structures.
Figure 1. (a-c) FESEM images; (d-f) TEM images; (g) STEM image and (h-j) EDX element mappings of the dendritic Ndoped mesoporous carbon nanospheres with ultralarge pores prepared by the versatile nanoemulsion assembly approach.
RESULTS AND DISCUSSION The dendritic N-doped mesoporous carbon nanospheres (denoted as NMCNs) with large tunable pore sizes were synthesized via a versatile nanoemulsion assembly approach by using Pluronic F127 as a template, dopamine (DA) as a nitrogen and carbon source, TMB as a mediator in the ethanol/water system, followed by carbonization in N2 (Scheme 1). Field-emission scanning electron
N2 sorption isotherms of the dendritic NMCNs show a representative type-IV curves with a sharp capillary condensation step at a high relative pressure 0.88
