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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Metallic Nanoparticles Assimilation within Metal−Organic Framework Monolith Mohamed H. Hassan,† Omar El-Basha, Rana R. Haikal, Ahmed H. Ibrahim, and Mohamed H. Alkordi* Center for Materials Science, Zewail City of Science and Technology, October Gardens, Sixth of October, Giza 12578, Egypt
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S Supporting Information *
ABSTRACT: A facile and versatile method is reported for the inclusion of gold nanoparticles (AuNPs) within a monolithic metal−organic framework (HKUST-1 MOF). Simple addition of stabilized colloidal AuNPs solution to the reaction mixture at the early stages of the formation of the MOF monolith resulted in quantitative uptake of the AuNPs within the MOF matrix. Several characterization techniques including solution and solid UV−vis spectroscopy, TEM, and XRD indicated the successful immobilization of the AuNPs. Controllable loading of AuNPs was also demonstrated, where gas sorption measurements indicated the maintained microporosity of the AuNPs-containing monoliths. This methodology has wide potential applications in demanding technologies, including sensing and catalysis, where monolithic materials of controllable physicochemical properties can be readily accessible through pore size and guest selectivity of the host MOF matrix controlling access of guest molecules to immobilized AuNPs. KEYWORDS: monolith, HKUST-1, metal−organic framework, gold nanoparticles, self-assembly
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certain types of NPs, those that are chemically compatible with the MOF matrix and of proper size to fit within the cages of the MOF. Recently, an alternative approach for inclusion of AuNPs through the successive adsorption of nanoparticles onto the continuously forming surfaces of the growing ZIF-8 MOF crystals was also demonstrated.22 In a different approach, inclusion of surface-functionalized AuNPs into porous-organic polymers through covalent linkage was also recently reported.23 Herein, we report on a direct, one-pot, methodology to incorporate the gold nanoparticles (AuNPs) into a monolith of the Cu-benzenetricarboxylate MOF known as HKUST-1. In this approach, AuNPs were adsorbed on the growing surface of the MOF sol−gel, leading to their inclusion into the solid matrix after gelation. As the AuNPs are not enclathrated within the nanocages of the MOF, this approach represents an attractive pathway for the inclusion of NPs of different composition, shape, and size within the matrix of highly microporous solids. Moreover, as the NPs are used in their preformed form, the need for chemically stable MOF against the reduction step is alleviated. In this procedure, the simple addition of citrate-stabilized colloidal solution of AuNPs to the sol−gel of the HKUST-1 monolith, in its early stages of formation, was sufficient to result in homogeneous inclusion of the AuNPs within the monolith. Three different AuNPs volumes were added to the
he family of microporous molecular solids known as metal−organic frameworks (MOFs) continue to capture scientific interest due to their hybrid composition, allowing ample space to tune their chemistry, microporosity, and pore system(s) through judicial selection of their precursors.1 In direct relation to their structural attributes, MOFs demonstrate unparalleled potentials to address a large number of demanding applications.2−4 Such applications span the range of gas storage and separation,5−9 heterogeneous catalysis,10−14 as well as for contaminant removal in water treatment.15,16 Although MOFs can be produced in several solid forms, including thin films and, most commonly, crystalline powder, the challenge of formulating a monolith form was only recently addressed.17−19 The ability to form monoliths of MOFs that can be easily molded into different shapes is highly desirable to access several applications where powder forms are faced with technical challenges in handling and utilization. The inclusion/immobilization of metallic nanoparticles in MOFs provided a pathway to mix the properties of the two, namely microporosity and/or guest selectivity of particular MOF and sensing capabilities or catalytic activities of the immobilized nanoparticles. Several approaches exist in current literature targeting the construction of NPs@MOF (NPs denotes several types of metallic nanoparticles). Common to those approaches is the two-step process of diffusing metal ions into the pores of the MOF followed by a reduction step to generate the NPs inside the cages of the MOF.20,21 Although this approach is provided for inclusion of AuNPs within the nanocages of several MOFs, it is faced by some limitations. This wetness incipient impregnation approach is limited to © XXXX American Chemical Society
Received: July 15, 2018 Accepted: September 17, 2018
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DOI: 10.1021/acsami.8b11795 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
Figure 1. TEM images of (a) monolith_AuNPs(1), (b) monolith_AuNPs(2), and (c) monolith_AuNPs(3). 1, 2, and 3 denote different AuNP loading into the HKUST-1 monoliths.
Figure 2. SEM images of the (a) monolithic HKUST-1, (b) monolith_AuNPs(1), (c) monolith_AuNPs(2), and (d) monolith_AuNPs(3).
HKUST-1 monolith precursor mixture (see the Supporting Information) and the resulted AuNPs@HKUST-1 monoliths are denoted as monolith_AuNP(X), X = 1−3. The UV−vis spectra for the supernatant solutions after centrifugation did not show the characteristic absorption band at 522 nm of the AuNPs, indicating successful inclusion of the AuNPs into the monolith pellets collected, Figure S1. It is also demonstrated that the amount of AuNPs inclusion can readily be controlled by controlling the amount of the colloidal AuNPs solution added to the monolith. The AuNPs loading in monolith_AuNPs(1−3) was measured through inductively coupled plasma optical emission spectrometry (ICP-OES), indicating loadings of 1.23, 3.22, and 4.98 wt %, respectively. The transmission electron microscopy (TEM) images, Figure 1, demonstrate the uniform distribution of the AuNPs within the monolith matrix. Additionally, no appreciable AuNPs aggregation/coalescence was observed due to the rapid formation of the monolith and the mild conditions utilized. The homogeneous distribution of AuNPs within the solids was further confirmed by EDX analysis, Figure S2. The monolithic nature of the isolated solids was further confirmed through the SEM images of the four samples, the monolith HKUST-1 and the three different solids containing different loading of AuNPs, Figure 2.The optical image of the HKUST-1 AuNPs (3) monolith further confirmed the findings of the SEM and TEM images, Figure S3. Thermogravimetric analysis of the isolated monoliths containing the AuNPs demonstrated a
systematic increase in residuals, 1.5−1.6 wt % increase between the three samples, in good correlation to the amount of AuNPs utilized in the synthesis, Figure S4.The UV−vis spectra of the isolated monoliths, Figure 3, demonstrated nicely the distinguished two absorption bands for the AuNPs (λmax = 541 nm) and the MOF (λmax = 702 nm). The red shift in absorption band of the AuNPs, as compared to that in the solution (λmax = 522 nm), can be explained in terms of the high dielectric constant and refractive index of the HKUST-1
Figure 3. UV−vis spectra of the AuNPs colloidal solution used and the three monolithic HKUST-1_AuNPs compounds reported. B
DOI: 10.1021/acsami.8b11795 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Letter
ACS Applied Materials & Interfaces MOF,24 and/or surface interactions with free carboxylate groups within the monolith. Similar red shift in the surface plasmon resonance of AuNPs was also previously recorded upon encapsulation within the ZIF-8 MOF solid matrix.22 The inclusion of the AuNPs in this process can be explained in similar manner observed previously, adsorption of AuNPs on the growing nuclei of the MOF.22 It can be argued that the citrate shell around the AuNPs, sharing the carboxylate functionality as the benzenetricarboxylate linkers used for the MOF, have facilitated the adsorption of the AuNPs onto the forming nuclei of the MOF. The FTIR spectra collected for the solids, Figure S5, demonstrated the HKUST-1 characteristic peaks of the (COO)− vibrational modes at 1650 and 1380 cm−1.25 In addition, the spectra indicated noticeable bands at 1540 and 1613 cm−1, previously assigned to asymmetric stretching νasy(COO)− for Au-bound citrate.26 It is noticeable that the intensity of this band increases in correlation to the amount of AuNPs present within the monolith. However, it is difficult to decipher if such observed peaks can be attributed only to the AuNP-bound citrate or if some free carboxylate groups from the BTC linker has coordinated to the surface of the AuNPs. To probe the microporosity of the resulting compounds, we measured N2 sorption isotherms, Figure 4 and Figure S6. The
noticeable peak at 2θ = 39° corresponding to Au(111) reflections.28 In conclusion, this strategy can potentially be applicable to the controlled incorporation of nanoparticles (metals, metalloids, or metal oxides, of same or different composition, same or different shapes of nanoparticles) within the MOF monolith. The maintained microporosity of the MOF monolith is an attractive attribute of this procedure where the various characteristics of NPs, including catalytic, magnetic, and optical properties, can be finely tuned through the size and functionality of the pore system(s) of the MOF monolith.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b11795. Further experimental details, synthesis, and characterization (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Omar El-Basha: 0000-0002-7465-3704 Mohamed H. Alkordi: 0000-0003-1807-748X Present Address †
M.H.H. is currently at Department of Chemistry and Biomolecular Science, Clarkson University, 8 Clarkson Avenue, Potsdam, NY 13699, USA
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge the financial support from Zewail City of Science and Technology (CMS-MA). REFERENCES
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Figure 4. N2 sorption isotherms measured at 77 K for the HKUST-1 monolith and that with the maximal AuNP loading in this study. Insert: pore size distribution histograms. Filled symbols indicate adsorption and hollow ones indicate desorption.
surface area (SA) using Brunauer−Emmett−Teller (BET) model (partial pressure range of 0.02−0.2) for the monolith was found to be 1369 m2/g, whereas for the monolith_AuNPs(1−3) samples with increasing AuNPs loadings were 1255, 1229, and 1206 m2/g, respectively. This observed trend in SA further supported the findings from the TEM for controllable inclusion of AuNPs within the matrix of the monolith. Furthermore, the pore size distribution histograms (insert in Figure 4) demonstrate essentially same PSD (calculated by the Horvath−Kawazoe method) indicating that inclusion of AuNPs was achieved within the matrix of the monolith but not inside the cages of the MOF. This is in agreement with the particle size of the AuNPs (15−20 nm) being larger than the largest cage of the HKUST-1 (∼1 nm).27 The X-ray powder diffraction pattern recorded for the three samples containing AuNPs is shown in Figure S7, showing similarity to the XRD pattern for the HKUST-1 in addition to C
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DOI: 10.1021/acsami.8b11795 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
