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Imparting Catalytic Activity to a Covalent Organic Framework Material by Nanoparticle Encapsulation Xiaofei Shi,† Youjin Yao,† Yulong Xu,† Kun Liu,‡ Guangshan Zhu,*,§ Lifeng Chi,*,† and Guang Lu*,† †
Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou 215123, China ‡ State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, China § Key Laboratory of Polyoxometalate Science of the Ministry of Education, Faculty of Chemistry, Northeast Normal University, Changchun 130024, China S Supporting Information *
ABSTRACT: Integrating covalent organic frameworks (COFs) with other functional materials is a useful route to enhancing their performances and extending their applications. We report herein a simple encapsulation method for incorporating catalytically active Au nanoparticles with different sizes, shapes, and contents in a two-dimensional (2D) COF material constructed by condensing 1,3,5-tris(4-aminophenyl)benzene (TAPB) with 2,5-dimethoxyterephthaldehyde (DMTP). The encapsulation is assisted by the surface functionalization of Au nanoparticles with polyvinylpyrrolidone (PVP) and follows a mechanism based on the adsorption of nanoparticles onto surfaces of the initially formed polymeric precursor of COF. The incorporation of nanoparticles does not alter obviously the crystallinity, thermal stability, and pore structures of the framework matrices. The obtained COF composites with embedded but accessible Au nanoparticles possess large surface areas and highly open mesopores and display recyclable catalytic performance for reduction of 4-nitrophenol, which cannot be catalyzed by the pure COF material, with activities relevant to contents and geometric structures of the incorporated nanoparticles. KEYWORDS: covalent organic framework, porous materials, nanoparticles, catalysis, composites
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INTRODUCTION Covalent organic frameworks (COFs) are a new class of porous crystalline materials constructed by organic building blocks via covalent bonds.1−8 The molecular engineering-based synthesis strategy of COFs allows the flexible design for achieving unique structures and properties. Their two-dimensional (2D) or three-dimensional (3D) open network structures yield high surface areas and accessible cavities or channels with uniform sizes ranging from angstroms to nanometers, which is attractive for many applications including gas storage,9−13 separations,14−16 drug delivery,17−19 and energy conversion and storage.20−23 Built with functional moieties, COF materials can be endowed with unique optical,24−28 optoelectronic,29−31 and catalytic properties.32−34 Alternately, noninherent functionalities and extended applications of COFs can be achieved by reasonably integrating them with other functional materials. The open porous structures and outstanding stability of COFs make them excellent host matrices for many functional species such as proton carriers,35−37 electron acceptors,38−40 and enzymes.41 The cavities themselves of COF materials have also been explored as nanoreactors for chemical reactions. Based on the “confined synthesis”, metal nanoparticles can be © XXXX American Chemical Society
produced within the cavities of COFs with ultrasmall sizes and “clean surfaces”.42−46 New and enhanced performances that are relevant to the nanoparticle components have been observed for the obtained nanoparticles/COF composites in organic catalysis42−45 and hydrogen storage applications.46 Alternately, this kind of composite can be generated by encapsulating the preprepared nanoparticles during the framework synthesis. This strategy is attractive since the compositions, sizes, and shapes associated with the chemical and physical properties of nanoparticles can be feasibly tuned by their well-established synthesis.47,48 COF materials can heterogeneously nucleate and grow around the outer surfaces of some nanosized objects (e.g., carbon nanotubes with diameter of ∼50 nm,49 ZnO nanorods with diameter of ∼100 nm,50 and ∼200 nm Fe3O4 spheres51) to give the core−shell structures, just as they did on the surface of a flat substrate.52−54 Nevertheless, although the encapsulation strategy, in principle, is capable of incorporating any nanoparticles with the desired chemical or physical properties Received: December 19, 2016 Accepted: February 6, 2017
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DOI: 10.1021/acsami.6b16267 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Scheme 1. Schematic Illustration of the Encapsulation of PVP-Modified Nanoparticles in the 2D TAPB-DMTP-COF
under the optimized experimental conditions. The composites display high crystallinity, excellent thermal stability, large surface areas, and open mesopores that are substantially similar to those of the pure TAPB-DMTP-COF. In the model reaction of 4-nitrophenol reduction by NaBH4,43,59−61 the composites containing Au nanoparticles show recyclable catalytic capability, but the pure COF material does not, establishing the accessibility and activity of the encapsulated nanoparticles within the 2D COF. In addition, systematic investigation with samples incorporated with different Au nanoparticles reveals the nanoparticle-relevant catalytic performances of the composites, where higher activity (with rate constants of 0.46 and 0.36 min−1) was observed for samples containing 15 nm Au nanoparticles (0.20 and 0.10 wt %), moderate activity (with rate constant of 0.36 min−1) for that containing 25 × 100 nm Au nanorods (0.26 wt %), and lower activity (with rate constant of 0.12 min−1) for that containing 50 nm Au nanoparticles (0.38 wt %).
in COF materials, the nanoparticle-relevant catalysis applications of the resulting COF composites have not been reported yet. For 2D COF-based composites, the orientation of channels and possible crystal defects (e.g., amorphous portion and stacking faults) in the framework matrices may also raise doubts about the accessibility and catalytic activity of the enshrouded nanoparticles. In addition, for the systematic investigation on the catalytic performance of the resulting composites, it is highly desirable to develop new encapsulation methods that enable the easy incorporation of smaller nanoparticles (99%) could be achieved for polymer spheres using the current method. The contents of nanoparticles can be tuned by changing the concentration of nanoparticles in the stock solutions (Table S1). The fraction of polymer spheres containing single Au nanoparticle in the products was observed to increase as lowering the concentration of nanoparticles used in the synthesis, however, at the expense of a slight decrease in the encapsulation efficiency (Figure S3). The final sizes of polymer spheres do not change obviously on tuning nanoparticle concentrations or even in the absence of nanoparticles in the synthesis, suggesting that the presence of nanoparticles does not alter substantially the homogeneous nucleation and subsequent growth of polymer spheres under current synthetic conditions. The encapsulation process was further investigated by time-dependent TEM analysis (Figure S4). The adsorption of Au nanoparticles onto surfaces of the gradually formed oligomeric polymer precursors (with irregular shapes), instead of the nanoparticle@polymer core−shell morphology, was observed commonly in the early time (∼15 min) of the encapsulation process (Figure S4b). The above results suggest that the encapsulation of nanoparticles does not follow predominantly the heterogeneous nucleation mechanism. We speculate that in the current solution system the hydrophilic/ hydrophobic interactions between PVP molecules adsorbed on the nanoparticle surfaces and the initially formed tiny oligomeric polymer precursors (with high surface energy) are responsible for the adsorption of nanoparticles on the surfaces of polymeric precursor of COF, resulting in the final encapsulation of nanoparticles as polymer spheres subsequently growing. In fact, the PVP-assisted encapsulation strategy has been adopted previously for incorporating nanoparticles in other materials such as silica62 and metal−organic frameworks (MOFs)63 based on either heterogeneous nucleation or nanoparticle adsorption. In an additional control experiment with excess free PVP intentionally added in the reaction, severe agglomeration of polymer spheres was observed in the product (Figure S5), suggesting again the strong interactions between PVP and polymer of spheres. In principle, this encapsulation method is applicable to any nanoparticles that are chemically stable in the acidic solution system used in current work since the surface modification with PVP is easy to operate and has proved to work well for various nanoparticles differed in compositions, sizes, and shapes. In fact, composites containing 50 nm Au nanoparticles, 25 × 100 nm Au nanorods, and 3.8 nm Pt nanoparticles were also successfully prepared using this method (Figure 2a−c). The agglomeration of polymer spheres observed in the Pt
Figure 1. Microscopy characterizations of COF composite containing 0.20 wt % 15 nm Au nanoparticles: (a) low-magnification SEM image, (b) high-magnification SEM image, (c) low-resolution TEM image, and (d) high-resolution TEM image.
exhibit rough surfaces on which convex domains with sizes of 50−100 nm could be observed clearly (Figure 1b). Transmission electron microscopy (TEM) measurements reveal that each (brighter) polymer sphere contains either single or several (darker) Au nanoparticles in their central regions (Figure 1c). Essentially, no unencapsulated nanoparticles were observed outside the spheres or on their outer surfaces. Close observation at the edges of spheres under higher magnification C
DOI: 10.1021/acsami.6b16267 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
consistent with that of TAPB-DMTP-COF previously reported,33 suggesting that the TAPB-DMTP-COF materials synthesized using current protocol are highly crystalline and that the incorporation of nanoparticles does not degrade obviously the crystallinity of framework matrices. The diffraction peaks assignable to nanoparticles are too weak to be observed clearly in the patterns of hybrid composites presumably due to their low contents (and small sizes). However, energy-dispersive X-ray microanalysis (EDX) (Figure S9) and inductively coupled plasma analysis (ICP) confirm explicitly their existence in the composites. Thermogravimetric analysis (TGA) (Figure S11) shows that the nanoparticle/COF composites are stable in nitrogen up to 400 °C without obvious weight loss, as the pure TAPB-DMTP-COF does. The porosity of evacuated composites was investigated by nitrogen-sorption measurements. All composites display type IV isotherms (Figure 4a), as pure TAPB-DMTP-COF sample
Figure 2. TEM images of COF composites containing (a) 0.38 wt % 50 nm Au nanoparticles, (b) 0.26 wt % 25 × 100 nm Au nanorods, (c) 3.8 nm Pt nanoparticles, and (d) 15 and 50 nm Au nanoparticles.
nanoparticle-containing product is presumably due to the existence of a small amount of free PVP in the nanoparticle stock solution even after intensive washing process. In addition, the simultaneous encapsulation of different types of nanoparticles within the composites can also be achieved easily. Although we just demonstrated this flexibility using 15 and 50 nm Au nanoparticles (Figure 2d), multifunctional composites could be prepared by encapsulating more than one types of nanoparticles with judicious selection. The crystallinity of the obtained composites was investigated by powder X-ray diffraction (XRD) measurements. As shown in Figure 3 and Figure S7, all the samples display an intense peak at 2.79°, moderate peaks at 4.84°, 5.60°, and 7.39°, and weak peaks at 9.73° and 25.42° in their diffraction patterns, which is
Figure 4. (a) Nitrogen-sorption isotherms for the activated COF and COF composites individually containing 0.10 and 0.20 wt % 15 nm Au nanoparticles, 0.38 wt % 50 nm Au nanoparticles, and 0.26 wt % 25 × 100 nm Au nanorods at 77 K up to 1 bar. (b) Pore-size distributions for COF and COF composite containing 0.20 wt % 15 nm Au nanoparticles.
does, with steep increase in nitrogen uptake at low relative pressure (
