- Email: [email protected]
Controlled synthesis of core-shell composites with uniform shells of a covalent organic framework
Inorganic Chemistry Communications 101 (2019) 160–163
Contents lists available at ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Short communication
Controlled synthesis of core-shell composites with uniform shells of a covalent organic framework Youjin Yaoa, Rui Zhanga, Tong Liua, Huijun Yub, , Guang Lua, ⁎
T
⁎
a
Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou 215123, China b Department of Cell Biology, School of Biology & Basic Medical Sciences, Soochow University, 199 Ren'ai Road, Suzhou 215123, China
GRAPHIC ABSTRACT
A controlled synthesis was reported to prepare well-defined core-shell Structure with uniform TAPB-DMTP-COF shells.
ARTICLE INFO
ABSTRACT
Keywords: Covalent organic framework Porous materials Hybrid materials Controlled synthesis
We report on the controlled synthesis of core-shell structured composites consisting of TAPB-DMTP-COF shells and SiO2, ZnS, or UiO-66 MOF cores. The core seeds were pre-modified with polyvinylpyrrolidone (PVP) to facilitate their dispersion in the reaction solution and with branched polyethyleneimine (BPEI) to trigger an effective heterogeneous nucleation of the imine COF. Under optimized synthetic conditions, uniform COF layer was grown on the modified seeds to yield well-defined core-shell structures with tunable shell thickness.
Covalent organic frameworks (COFs) are porous crystalline materials built by organic units via covalent bonds [1–3]. Due to their large surface area, uniform pore sizes, and high thermal and chemical stabilities, COF materials have shown great promise for a variety of applications including gas storage, separation, drug delivery, catalysis, energy conversion and storage, and photoelectronics [4–9]. The applications of COFs can be further developed or extended by integrating them with other functional materials to form hybrid composites with enhanced performances or new properties. For example, the noble metal nanoparticle-embedded COF composites could serve as catalysts ⁎
for certain chemical reactions that cannot be catalyzed by the COFs themselves [10–12]. Some metal-organic framework (MOF)/COF composites exhibited the enhanced photocatalytic activities in comparison to the constituent materials [13,14]. COF-based composites could be prepared by hybridizing the presynthesized COFs with other materials via chemical reaction [11,15] or physical processing [16] or by crystallizing COFs in the presence of nanometer- or micrometer-sized particles of other materials [12–14]. In particular, the latter strategy provides a flexible design approach to manipulating the structure configuration and thus to tuning
Corresponding authors. E-mail addresses: [email protected] (H. Yu), [email protected] (G. Lu).
https://doi.org/10.1016/j.inoche.2019.01.040 Received 20 December 2018; Received in revised form 26 January 2019; Accepted 27 January 2019 Available online 28 January 2019 1387-7003/ © 2019 Published by Elsevier B.V.
Inorganic Chemistry Communications 101 (2019) 160–163
Y. Yao et al.
Scheme 1. Schematic illustration of the controlled synthesis of core-shell structured TAPB-DMTP-COF composites containing cores of SiO2, ZnS, and UiO-66 (shown in the insets).
functionality of the resulting composites. As one of the simplest configurations of hybrid materials, core-shell structure is always attractive and has been adopted by several COF-based composites with cores of inorganic (ZnO, Fe3O4, or SiO2) or MOF materials [13,17–21]. Generally, the presence of certain functional groups on the surface of core seeds was in favor of the heterogeneous nucleation of COF materials for generating core-shell structures [13,20,21]. These functional groups were commonly introduced by chemical graft based on some specific reactions which, however, are not generally applicable to different core materials. Meanwhile, the surface properties of seeds also should be tuned properly to make them stable in the COF synthesis solutions without aggregation, which nevertheless has not received enough attention in previous studies. Herein, we demonstrate the controlled synthesis of core-shell structures consisting of shells of the high crystalline and stable TAPBDMTP-COF [7] and cores of SiO2, ZnS, and UiO-66 MOF [22] (Scheme 1). Differing from previous work [20,21], the seed surface was modified by the simultaneous physical adsorption of polyvinylpyrrolidone (PVP) and branched polyethyleneimine (BPEI). The former polymer promoted the dispersion of seeds in the COF reaction solution and the latter presented amino groups favorable to the heterogeneous nucleation of the imine COF. By optimizing the experimental conditions, uniform COF layer with tunable thickness was successfully grown on the introduced seeds, yielding well-defined core-shell structures. Monodisperse ZnS microspheres (390 nm), SiO2 microspheres (410 nm), and UiO-66 octahedral crystals (490 nm) were synthesized according to established methods [23-26] and their surfaces were
modified with BPEI and PVP after the synthesis. The procedure for growing TAPB-DMTP-COF shells was demonstrated with UiO-66 crystals as seeds. In a typical experiment, a solution of 1,3,5-tris(4-aminophenyl)benzene (TAPB) (2.6 mg), 2,5-dimethoxyterephaldehyde (DMTP) (2.2 mg), and BPEI/PVP-modified UiO-66 crystals (0.87 mg) in 4 mL of 1,4-dioxane−1-butanol (v/v, 1:1) was prepared and then mixed with 0.1 mL acetic acid. After the mixture was reacted at room temperature for 2 h, 0.26 mL acetic acid and 0.14 mL deionized water were added and the reaction was conducted further at 70 °C for 4 h. After cooling to room temperature, the product was collected by centrifugation and washed several times with tetrahydrofuran and acetone. As revealed by transmission electron microscopy (TEM) measurements, the product consisted of isolated particles that featured dark octahedral cores and bright uniform shells with an average thickness of 149 nm (Fig. 1a). The X-ray diffraction (XRD) analysis (Fig. 1b) indicated that the core-shell composite was crystalline and displayed a diffraction pattern with obvious peaks at 7.35° and 8.50° characteristic for UiO-66 [22] and at 2.79°, 4.84°, and 5.60° assignable to TAPBDMTP-COF [7]. In the thermogravimetric analysis (TGA) (Fig. S5), the composite sample showed three obvious weight losses in temperature ranges of 25–200, 200–300, and 350–600 °C, which corresponded to the volatilization of absorbed small molecules, the dehydroxylation and the elimination of monodentate modulators of UiO-66 [27], and the decomposition of COF and MOF structures, respectively. Based on the TGA data, the mass contents of MOF and COF constituents in the composite were estimated as 25% and 75%, respectively. In nitrogen sorption measurements (Fig. 1c), the composite displayed type IV
Fig. 1. a) TEM image of the core-shell structured TAPB-DMTP-COF composite prepared with 0.87 mg UiO-66 seeds. b) XRD patterns of the simulated UiO-66, coreshell composite, TAPB-DMTP-COF, and simulated TAPB-DMTP-COF. c) Nitrogen sorption isotherms of the composite, UiO-66 crystal seeds, and TAPB-DMTP-COF synthesized in the absence of seeds at 77 K up to 1 bar. 161
Inorganic Chemistry Communications 101 (2019) 160–163
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Fig. 2. TEM images of TAPB-DMTP-COF composite samples synthesized with a) unmodified UiO-66 crystal seeds and those modified with b) PVP and c) BPEI.
isotherm with a steep increase in nitrogen uptake at low relative pressure (< 0.01), indicating the existence of micropores. A step in the relative pressure range of 0.10–0.20 was also observed as the result of the condensation of nitrogen in mesopores of TAPB-DMTP-COF shells [7]. The isotherm evaluation gave a Brunauer-Emmett-Teller (BET) surface area of 2083 m2/g for the core-shell composite, which was slightly smaller than that of pure COF (2258 m2/g) but much larger than that of pure UiO-66 (1302 m2/g). This value is close to the BET surface area (2019 m2/g) calculated based on the mass contributions of MOF and COF constituents in the composite. The surface modification of the UiO-66 crystal seeds was critical to the subsequent growth of uniform COF shells. Without any modification, the surface of most of MOF crystals was only partially coated with COF materials (Fig. 2a). Once their surface was modified with PVP or BPEI, the coverage of deposited COF materials increased (Fig. 2b and c). However, the formed COF layers on the BPEI-modified MOF crystals were uniform in thickness whereas those on the PVP-modified counterpart displayed much rough surface morphologies. Obviously, the presence of amino groups in BPEI adsorbed on the surface of MOF crystals favored the heterogeneous nucleation of the imine TAPBDMTP-COF [13,20,21]. Without BPEI modification, COF materials were unevenly deposited on the surface of MOF crystals via the adsorption of the polymeric precursor particles of COF that initially formed in solution through homogeneous nucleation [12,28]. Unfortunately, the polycationic character of BPEI was unfavorable to the stability of seeds in the COF synthesis solution with butanol/1,4-dioxane as solvent and could cause a serious aggregation in the obtained products. In contrast,
although PVP did not assist the heterogeneous nucleation of imine-COF, its amphiphilic nature facilitated a well dispersion of MOF crystals in the reaction solution [12]. Accordingly, the BPEI/PVP-dual polymer modification endowed the MOF seed surface with favorable chemical functional groups and appropriate hydrophilic/hydrophobic property simultaneously which were both indispensable for the formation of well-defined core-shell structures. To produce core-shell structures in high efficiency, it was also necessary to suppress the homogeneous formation of COF in the reaction solution which competed with the heterogeneous growth of shells on the seed surface. This could be achieved by optimizing the concentrations of reactants and seeds in the synthesis. For the “standard precursor solution” containing 2.6 mg TAPB and 2.2 mg DMTP, 0.58 mg UiO-66 seeds were sufficient to suppress substantially the homogeneous nucleation of COF in solution (Fig. 3a) whereas a lower seed concentration (0.29 mg) would result in the formation of the core-free COF particles in the product (Fig. S6). The effective suppression of homogeneous nucleation was also beneficial for further controlling the thickness of COF shells. With the fixed concentrations of reactants, the thickness of COF shells decreased reasonably as increasing the seed concentration in the synthesis (Fig. 3b–e). For example, in the synthesis with the “standard precursor solution”, the average thickness of COF shells could be flexibly tuned from 200 nm to 30 nm by varying the concentration of UiO-66 crystal seeds from 0.58 mg to 9.28 mg (Fig. 3f). Alternately, thicker COF shells also could be produced by increasing the reactant concentrations while fixing the seed concentration in the synthesis (Fig. S7).
Fig. 3. TEM images of TAPB-DMTP-COF composites synthesized with a) 0.58 mg, b) 1.16 mg, c) 2.32 mg, d) 4.64 mg, and e) 9.28 mg UiO-66 seeds. f) The average thickness of the resulting COF shells versus the amount of MOF seeds used in the synthesis. 162
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Fig. 4. TEM images of core-shell structured TAPB-DMTP-COF composites containing cores of a) ZnS and b) SiO2.
Since PVP and BPEI can be adsorbed easily on the surface of a variety of materials, this synthesis approach should be generally applicable to other core materials as long as they are chemically and thermally stable under the current synthetic conditions of TAPB-DMTPCOF. For example, the well-defined core-shell structured ZnS/COF and SiO2/COF composites were also successfully prepared using the similar synthetic procedure (Fig. 4). In summary, we have demonstrated the controllable synthesis of core-shell structured TAPB-DMTP-COF composites based on a combination of the rational surface modification of seeds and the optimization of COF crystallization process. The facile but effective BPEI/PVP modification approach allows for the extension of core-shell structures to other core materials besides those we reported here. In addition, similar synthetic strategies also can be developed for composites constituted by other COF materials.
[10] [11] [12] [13]
[14]
[15]
Acknowledgments
[16]
This work is supported by the National Natural Science Foundation of China (grant number 21371127 and 81402279), the Collaborative Innovation Center of Suzhou Nano Science and Technology, a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and the 111 project.
[17] [18]
Appendix A. Supplementary material
[19]
More experimental details and characterizations and Fig. S1–S9. Supplementary data to this article can be found online at https://doi. org/10.1016/j.inoche.2019.01.040.
[20] [21]
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Contents lists available at ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Short communication
Controlled synthesis of core-shell composites with uniform shells of a covalent organic framework Youjin Yaoa, Rui Zhanga, Tong Liua, Huijun Yub, , Guang Lua, ⁎
T
⁎
a
Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou 215123, China b Department of Cell Biology, School of Biology & Basic Medical Sciences, Soochow University, 199 Ren'ai Road, Suzhou 215123, China
GRAPHIC ABSTRACT
A controlled synthesis was reported to prepare well-defined core-shell Structure with uniform TAPB-DMTP-COF shells.
ARTICLE INFO
ABSTRACT
Keywords: Covalent organic framework Porous materials Hybrid materials Controlled synthesis
We report on the controlled synthesis of core-shell structured composites consisting of TAPB-DMTP-COF shells and SiO2, ZnS, or UiO-66 MOF cores. The core seeds were pre-modified with polyvinylpyrrolidone (PVP) to facilitate their dispersion in the reaction solution and with branched polyethyleneimine (BPEI) to trigger an effective heterogeneous nucleation of the imine COF. Under optimized synthetic conditions, uniform COF layer was grown on the modified seeds to yield well-defined core-shell structures with tunable shell thickness.
Covalent organic frameworks (COFs) are porous crystalline materials built by organic units via covalent bonds [1–3]. Due to their large surface area, uniform pore sizes, and high thermal and chemical stabilities, COF materials have shown great promise for a variety of applications including gas storage, separation, drug delivery, catalysis, energy conversion and storage, and photoelectronics [4–9]. The applications of COFs can be further developed or extended by integrating them with other functional materials to form hybrid composites with enhanced performances or new properties. For example, the noble metal nanoparticle-embedded COF composites could serve as catalysts ⁎
for certain chemical reactions that cannot be catalyzed by the COFs themselves [10–12]. Some metal-organic framework (MOF)/COF composites exhibited the enhanced photocatalytic activities in comparison to the constituent materials [13,14]. COF-based composites could be prepared by hybridizing the presynthesized COFs with other materials via chemical reaction [11,15] or physical processing [16] or by crystallizing COFs in the presence of nanometer- or micrometer-sized particles of other materials [12–14]. In particular, the latter strategy provides a flexible design approach to manipulating the structure configuration and thus to tuning
Corresponding authors. E-mail addresses: [email protected] (H. Yu), [email protected] (G. Lu).
https://doi.org/10.1016/j.inoche.2019.01.040 Received 20 December 2018; Received in revised form 26 January 2019; Accepted 27 January 2019 Available online 28 January 2019 1387-7003/ © 2019 Published by Elsevier B.V.
Inorganic Chemistry Communications 101 (2019) 160–163
Y. Yao et al.
Scheme 1. Schematic illustration of the controlled synthesis of core-shell structured TAPB-DMTP-COF composites containing cores of SiO2, ZnS, and UiO-66 (shown in the insets).
functionality of the resulting composites. As one of the simplest configurations of hybrid materials, core-shell structure is always attractive and has been adopted by several COF-based composites with cores of inorganic (ZnO, Fe3O4, or SiO2) or MOF materials [13,17–21]. Generally, the presence of certain functional groups on the surface of core seeds was in favor of the heterogeneous nucleation of COF materials for generating core-shell structures [13,20,21]. These functional groups were commonly introduced by chemical graft based on some specific reactions which, however, are not generally applicable to different core materials. Meanwhile, the surface properties of seeds also should be tuned properly to make them stable in the COF synthesis solutions without aggregation, which nevertheless has not received enough attention in previous studies. Herein, we demonstrate the controlled synthesis of core-shell structures consisting of shells of the high crystalline and stable TAPBDMTP-COF [7] and cores of SiO2, ZnS, and UiO-66 MOF [22] (Scheme 1). Differing from previous work [20,21], the seed surface was modified by the simultaneous physical adsorption of polyvinylpyrrolidone (PVP) and branched polyethyleneimine (BPEI). The former polymer promoted the dispersion of seeds in the COF reaction solution and the latter presented amino groups favorable to the heterogeneous nucleation of the imine COF. By optimizing the experimental conditions, uniform COF layer with tunable thickness was successfully grown on the introduced seeds, yielding well-defined core-shell structures. Monodisperse ZnS microspheres (390 nm), SiO2 microspheres (410 nm), and UiO-66 octahedral crystals (490 nm) were synthesized according to established methods [23-26] and their surfaces were
modified with BPEI and PVP after the synthesis. The procedure for growing TAPB-DMTP-COF shells was demonstrated with UiO-66 crystals as seeds. In a typical experiment, a solution of 1,3,5-tris(4-aminophenyl)benzene (TAPB) (2.6 mg), 2,5-dimethoxyterephaldehyde (DMTP) (2.2 mg), and BPEI/PVP-modified UiO-66 crystals (0.87 mg) in 4 mL of 1,4-dioxane−1-butanol (v/v, 1:1) was prepared and then mixed with 0.1 mL acetic acid. After the mixture was reacted at room temperature for 2 h, 0.26 mL acetic acid and 0.14 mL deionized water were added and the reaction was conducted further at 70 °C for 4 h. After cooling to room temperature, the product was collected by centrifugation and washed several times with tetrahydrofuran and acetone. As revealed by transmission electron microscopy (TEM) measurements, the product consisted of isolated particles that featured dark octahedral cores and bright uniform shells with an average thickness of 149 nm (Fig. 1a). The X-ray diffraction (XRD) analysis (Fig. 1b) indicated that the core-shell composite was crystalline and displayed a diffraction pattern with obvious peaks at 7.35° and 8.50° characteristic for UiO-66 [22] and at 2.79°, 4.84°, and 5.60° assignable to TAPBDMTP-COF [7]. In the thermogravimetric analysis (TGA) (Fig. S5), the composite sample showed three obvious weight losses in temperature ranges of 25–200, 200–300, and 350–600 °C, which corresponded to the volatilization of absorbed small molecules, the dehydroxylation and the elimination of monodentate modulators of UiO-66 [27], and the decomposition of COF and MOF structures, respectively. Based on the TGA data, the mass contents of MOF and COF constituents in the composite were estimated as 25% and 75%, respectively. In nitrogen sorption measurements (Fig. 1c), the composite displayed type IV
Fig. 1. a) TEM image of the core-shell structured TAPB-DMTP-COF composite prepared with 0.87 mg UiO-66 seeds. b) XRD patterns of the simulated UiO-66, coreshell composite, TAPB-DMTP-COF, and simulated TAPB-DMTP-COF. c) Nitrogen sorption isotherms of the composite, UiO-66 crystal seeds, and TAPB-DMTP-COF synthesized in the absence of seeds at 77 K up to 1 bar. 161
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Fig. 2. TEM images of TAPB-DMTP-COF composite samples synthesized with a) unmodified UiO-66 crystal seeds and those modified with b) PVP and c) BPEI.
isotherm with a steep increase in nitrogen uptake at low relative pressure (< 0.01), indicating the existence of micropores. A step in the relative pressure range of 0.10–0.20 was also observed as the result of the condensation of nitrogen in mesopores of TAPB-DMTP-COF shells [7]. The isotherm evaluation gave a Brunauer-Emmett-Teller (BET) surface area of 2083 m2/g for the core-shell composite, which was slightly smaller than that of pure COF (2258 m2/g) but much larger than that of pure UiO-66 (1302 m2/g). This value is close to the BET surface area (2019 m2/g) calculated based on the mass contributions of MOF and COF constituents in the composite. The surface modification of the UiO-66 crystal seeds was critical to the subsequent growth of uniform COF shells. Without any modification, the surface of most of MOF crystals was only partially coated with COF materials (Fig. 2a). Once their surface was modified with PVP or BPEI, the coverage of deposited COF materials increased (Fig. 2b and c). However, the formed COF layers on the BPEI-modified MOF crystals were uniform in thickness whereas those on the PVP-modified counterpart displayed much rough surface morphologies. Obviously, the presence of amino groups in BPEI adsorbed on the surface of MOF crystals favored the heterogeneous nucleation of the imine TAPBDMTP-COF [13,20,21]. Without BPEI modification, COF materials were unevenly deposited on the surface of MOF crystals via the adsorption of the polymeric precursor particles of COF that initially formed in solution through homogeneous nucleation [12,28]. Unfortunately, the polycationic character of BPEI was unfavorable to the stability of seeds in the COF synthesis solution with butanol/1,4-dioxane as solvent and could cause a serious aggregation in the obtained products. In contrast,
although PVP did not assist the heterogeneous nucleation of imine-COF, its amphiphilic nature facilitated a well dispersion of MOF crystals in the reaction solution [12]. Accordingly, the BPEI/PVP-dual polymer modification endowed the MOF seed surface with favorable chemical functional groups and appropriate hydrophilic/hydrophobic property simultaneously which were both indispensable for the formation of well-defined core-shell structures. To produce core-shell structures in high efficiency, it was also necessary to suppress the homogeneous formation of COF in the reaction solution which competed with the heterogeneous growth of shells on the seed surface. This could be achieved by optimizing the concentrations of reactants and seeds in the synthesis. For the “standard precursor solution” containing 2.6 mg TAPB and 2.2 mg DMTP, 0.58 mg UiO-66 seeds were sufficient to suppress substantially the homogeneous nucleation of COF in solution (Fig. 3a) whereas a lower seed concentration (0.29 mg) would result in the formation of the core-free COF particles in the product (Fig. S6). The effective suppression of homogeneous nucleation was also beneficial for further controlling the thickness of COF shells. With the fixed concentrations of reactants, the thickness of COF shells decreased reasonably as increasing the seed concentration in the synthesis (Fig. 3b–e). For example, in the synthesis with the “standard precursor solution”, the average thickness of COF shells could be flexibly tuned from 200 nm to 30 nm by varying the concentration of UiO-66 crystal seeds from 0.58 mg to 9.28 mg (Fig. 3f). Alternately, thicker COF shells also could be produced by increasing the reactant concentrations while fixing the seed concentration in the synthesis (Fig. S7).
Fig. 3. TEM images of TAPB-DMTP-COF composites synthesized with a) 0.58 mg, b) 1.16 mg, c) 2.32 mg, d) 4.64 mg, and e) 9.28 mg UiO-66 seeds. f) The average thickness of the resulting COF shells versus the amount of MOF seeds used in the synthesis. 162
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Fig. 4. TEM images of core-shell structured TAPB-DMTP-COF composites containing cores of a) ZnS and b) SiO2.
Since PVP and BPEI can be adsorbed easily on the surface of a variety of materials, this synthesis approach should be generally applicable to other core materials as long as they are chemically and thermally stable under the current synthetic conditions of TAPB-DMTPCOF. For example, the well-defined core-shell structured ZnS/COF and SiO2/COF composites were also successfully prepared using the similar synthetic procedure (Fig. 4). In summary, we have demonstrated the controllable synthesis of core-shell structured TAPB-DMTP-COF composites based on a combination of the rational surface modification of seeds and the optimization of COF crystallization process. The facile but effective BPEI/PVP modification approach allows for the extension of core-shell structures to other core materials besides those we reported here. In addition, similar synthetic strategies also can be developed for composites constituted by other COF materials.
[10] [11] [12] [13]
[14]
[15]
Acknowledgments
[16]
This work is supported by the National Natural Science Foundation of China (grant number 21371127 and 81402279), the Collaborative Innovation Center of Suzhou Nano Science and Technology, a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and the 111 project.
[17] [18]
Appendix A. Supplementary material
[19]
More experimental details and characterizations and Fig. S1–S9. Supplementary data to this article can be found online at https://doi. org/10.1016/j.inoche.2019.01.040.
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