New Strategies for Novel MOF-Derived Carbon Materials Based on Nanoarchitectures

Abstract

Recently, there have been many emerging strategies to boost MOF-derived nanoarchitectured carbon materials (NCMs) with mesoporous, hollow, yolk-shell, hollow/porous, and multi-dimensional structures. These new methods can be categorized as new chemistry tools utilizing SiO2, polymers, surfactants, and others (sonochemistry, salt template, and etching). Herein, we focus on the synthetic mechanisms strategically assisted by these new strategies. The relationship between the new bridges and the morphological control is discussed in detail. This review will create an important avenue for developing new carbon materials. In recent years, metal-organic framework (MOF)-derived carbon materials (CMs), known for their nanoporous structure yielding a high surface area and tunable chemical and physical properties, have drawn great interest in many fields of application, such as energy storage and conversion, environmental remediation, and catalysis. Despite the tremendous efforts involved in their development, several common drawbacks still persist during the carbonization process: (1) the intrinsic nature of micropore-dominated porous structure (limited diffusion), (2) the irreversible aggregation of metal nanoparticles, and (3) the poor control over structural evolution, which largely thwart their performance. To overcome these technical limitations, many new strategies are currently emerging to boost the development of MOF-derived nanoarchitectured CMs (NCMs). These new approaches can be considered new chemistry tools utilizing SiO2, polymers, surfactants, and others. In this review, we focus on the synthetic mechanisms of these new methods by summarizing recent findings related to MOF-derived NCMs. In recent years, metal-organic framework (MOF)-derived carbon materials (CMs), known for their nanoporous structure yielding a high surface area and tunable chemical and physical properties, have drawn great interest in many fields of application, such as energy storage and conversion, environmental remediation, and catalysis. Despite the tremendous efforts involved in their development, several common drawbacks still persist during the carbonization process: (1) the intrinsic nature of micropore-dominated porous structure (limited diffusion), (2) the irreversible aggregation of metal nanoparticles, and (3) the poor control over structural evolution, which largely thwart their performance. To overcome these technical limitations, many new strategies are currently emerging to boost the development of MOF-derived nanoarchitectured CMs (NCMs). These new approaches can be considered new chemistry tools utilizing SiO2, polymers, surfactants, and others. In this review, we focus on the synthetic mechanisms of these new methods by summarizing recent findings related to MOF-derived NCMs. Carbon-based materials (CMs) having a high surface area, electronic conductivity, and chemical stability represent promising candidates in a variety of fields such as environmental remediation,1Duan X. Sun H. Wang S. Metal-free carbocatalysis in advanced oxidation reactions.Acc. Chem. Res. 2018; 51: 678-687Crossref PubMed Scopus (160) Google Scholar, 2Hodges B.C. Cates E.L. Kim J.H. Challenges and prospects of advanced oxidation water treatment processes using catalytic nanomaterials.Nat. Nanotechnol. 2018; 13: 642-650Crossref PubMed Scopus (27) Google Scholar energy storage systems,3Zhai Y. Dou Y. Zhao D. Fulvio P.F. Mayes R.T. Dai S. Carbon materials for chemical capacitive energy storage.Adv. 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Mater. 2002; 14: 19-21Crossref Scopus (374) Google Scholar In addition, zeolite imidazolate framework (ZIF) series (one class of MOFs) have been widely used as precursors for the preparation of CMs due to its simplicity and adjustable morphology. Particularly, ZIF-8-derived CMs with a high content of nitrogen and surface area has attracted more attention. In 2016, Zhang and co-workers first reported a mesoporous silica (mSiO2)-protected-calcination approach to preparing hierarchically porous Co,N co-doped carbon nanoframework (Co,N-CNF) using a bimetallic ZIF precursor containing Zn and Co.50Shang L. Yu H. Huang X. Bian T. Shi R. Zhao Y. Waterhouse G.I. Wu L.Z. Tung C.H. Zhang T. Well-dispersed zif-derived Co,N-co-doped carbon nanoframes through mesoporous-silica-protected calcination as efficient oxygen reduction electrocatalysts.Adv. Mater. 2016; 28: 1668-1674Crossref PubMed Scopus (285) Google Scholar As shown in Figure 3A, the bimetallic ZIF NPs were prepared and the mSiO2 shell was uniformly coated on their surface through a chemical reaction of tetraethylorthosilicate (TEOS) and cetyltrimethylammonium bromide (CTAB) as the pore directing agent. Subsequently, the Co,[email protected]2 was formed from Zn,[email protected]2 by thermal treatment under N2 condition. After etching the mSiO2 shell with hydrofluoric acid (HF), the resulting nanostructured Co,N-CNF showed clear hierarchical pore structure (Figure 3B). To further understand the pore structure, they compared it with the carbonized Co,N-CNF without mSiO2 shell (denoted as A-Co,N-CNF). As shown in Figure 3C, unlike A-Co,N-CNF, the Co,N-CNF shows a type IV isotherm, implying the existence of mesopores. Thus, the mSiO2 shell is a key material to avoid the undesirable aggregation of particles in the process of pyrolysis and create MOF-derived carbon materials having a hierarchical pore structure. Later on, they successfully applied a similar strategy to synthesize monodispersed mesoporous carbon containing porphyrin-like zinc centers from ZIF-8 precursor.61Wang S. Shang L. Li L. Yu Y. Chi C. Wang K. Zhang J. Shi R. Shen H. Waterhouse G.I. et al.Metal-organic-framework-derived mesoporous carbon nanospheres containing porphyrin-like metal centers for conformal phototherapy.Adv. Mater. 2016; 28: 8379-8387Crossref PubMed Scopus (95) Google Scholar Recently, Li and co-workers systemically studied the formation process of the hierarchical pore structures and the thickness effect of the mSiO2 shell.62Liu C. Huang X. Wang J. Song H. Yang Y. Liu Y. Li J. Wang L. Yu C. Hollow mesoporous carbon nanocubes: rigid-interface-induced outward contraction of metal-organic frameworks.Adv. Funct. Mater. 2018; 28: 1705253Crossref Scopus (22) Google Scholar The ZIF-8 nanocubes were used as the starting precursor, and the mSiO2 layer was coated on ZIF-8, as shown in Figure 4A. Without the mSiO2 coating, thermal shrinkage (i.e., inward contraction) occurred. When the thickness of the rigid mSiO2 layer was 40 nm (denoted as [email protected]2-40), the inward contraction could be efficiently counteracted during carbonization. It is worth mentioning that the carbonization is preferentially initiated at the interface between the mSiO2 layer and the ZIF-8 surface. Subsequent outward contraction of the remaining MOFs resulted in the formation of hollow mesoporous carbon nanocubes (HMCNCs; Figures 4C–4E). On the other hand, a thin mSiO2 layer with 8 nm thickness ([email protected]2-8) could not counteract the inward contraction (Figures 4B and 4F–4H), therefore, the resulting mesoporous carbon nanocubes (SMCNCs) are without internal hollow cavity due to the deformation. To further study the formation mechanism of the HMCNCs, thermogravimetric analysis (TGA) of mSiO2, ZIF-8, and [email protected]2-40 was carried out. The major weight losses of mSiO2 and ZIF-8 at 190°C∼350°C and 600°C were 30.2% and 7.4%, respectively. A significant weight loss is observed after 600°C, due to the carbonization of ZIF-8 and the removal of Zn by evaporation at a higher temperature. For [email protected]2-40, the 17.1% weight loss at 190°C∼350°C is related to the decomposition of CTAB. However, the weight loss observed at 350°C∼600°C reaches 14.0%, while it does not exceed 2.3% in ZIF-8, suggesting that the SiO2-protected shell impacts both the contraction and thermal decomposition behaviors of ZIF-8. Through a systematic study, we proposed a mechanism of outward contraction from a rigid interface: the mSiO2-protected shell can provide a driving force for pulling the ZIF-8 precursor at carbonization “outward,” and a void space is generated by the intrinsic volume reduction of ZIF-8 after carbonization. If the mSiO2-protected shell is thick, hollow structures with mesoporous can be formed. On the contrary, SMCNCs are formed without a hollow cavity when the mSiO2 shell is too thin. The rigid interface-induced reverse contraction theory would provide a new method for controlling the preparation of CMs from MOFs for various applications. More recently, Mu and co-workers demonstrated the mSiO2-induced calcination strategy to produce ZIF-67 to grow Co- and N-co-doped carbon nanotubes (Co/N-CNTs) with a small diameter.63Zhou H. He D. Saana A.I. Yang J. Wang Z. Zhang J. Liang Q. Yuan S. Zhu J. Mu S. Mesoporous-silica induced doped carbon nanotube growth from metal-organic frameworks.Nanoscale. 2018; 10: 6147-6154Crossref PubMed Google Scholar As illustrated in Figure 5A, the prepared ZIF-67 was coated with a layer of mSiO2 and further carbonized in inert gas. During the pyrolysis, the Co ions within the ZIF-67 were firstly converted into uniform Co NPs. In the case of bare ZIF-67, the Co NPs tend to quickly aggregate as the organic scaffold is transformed into carbon layers at higher carbonization temperature. In [email protected]2, on the other hand, the mSiO2 shell prevents the Co nanocatalysts from rapid aggregation even at temperatures up to 580°C. When the temperature further increases to 650°C, a number of small-sized Co NPs catalyzes the growth of the CNTs. Such a morphological feature is also confirmed by transmission electron microscopy (TEM) studies (Figures 5B–5D). In addition, the effect of the mSiO2 shell thickness on the CNT growth was also studied. Co/N-CNTs showed more nanotube frameworks and rougher surfaces as mSiO2 increased. But, the growth of CNTs exhibited the diminishing trend when the thickness of mSiO2 shell continuously increased. These results showed that the mSiO2 shell was too thick, and they hindered the growth of nanotubes. Another SiO2-based MOF carbon with nanotube-decorated hollow structure was reported by Zhang and co-workers (Figure 6A).64Feng T. Zhang M. A mixed-ion strategy to construct CNT-decorated Co/N-doped hollow carbon for enhanced oxygen reduction.Chem. Commun. (Camb.). 2018; 54: 11570-11573Crossref PubMed Google Scholar Unlike the mSiO2 shell strategy, SiO2 spheres were utilized as a core. Then, the ZIFs-CoxZn1-x nanocrystals were grown on the SiO2 surface to form SiO2@ ZIFs-CoxZn1−x. As shown in Figure 6B, the obtained SiO2@ZIFs-Co0.23Zn0.77 exhibited a spherical structure having the ZIFs-Co0.23Zn0.77 NPs on the surface of SiO2. CNTs-Co/NHC-x was obtained, as shown in Figure 6C after the carbonization and etching steps. In this approach, SiO2 was utilized as a hard template for the fabrication of hollow architecture, and the CNTs could be precisely controlled by adjusting Co2+/Zn2+ in the precursor. It has been found that CNTs could not be produced during pyrolysis when x > 0.75, showing only retained the hollow structure (Figure 6D).Figure 6The Mixed-Ion Strategy for Preparing Carbon-Nanotube-Decorated Hollow Carbon MaterialsShow full caption(A) Schematic illustration of the synthesis of Co/N-doped hollow carbon hybrids (CNTs-Co/NHC).(B) Scanning electron microscopy (SEM) imag