Electrically Conductive, Monolithic MetalâOrganic Framework

Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Electrically Conductive, Monolithic Metal−Organic Framework− Graphene (MOF@G) Composite Coatings Mohamed H. Hassan,† Rana R. Haikal,† Tawheed Hashem,‡ Julia Rinck,‡ Franz Koeniger,‡ Peter Thissen,‡ Stefan Heiβler,‡ Christof Wöll,‡ and Mohamed H. Alkordi*,† †

Center for Materials Science, Zewail City of Science and Technology, October Gardens, 6th of October, Giza 12578, Egypt Institute of Functional Interfaces (IFG), Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, Eggenstein-Leopoldshafen 76344, Germany

‡

ACS Appl. Mater. Interfaces Downloaded from pubs.acs.org by UNIV OF FLORIDA on 01/31/19. For personal use only.

S Supporting Information *

ABSTRACT: We present a novel approach to produce a composite of the HKUST-1 metal−organic framework (MOF) and graphene, which is suited for the fabrication of monolithic coatings of solid substrates. In order to avoid the degradation of graphene electrical properties resulting from chemical functionalization (e.g., oxidation yielding graphene oxide, GO), commercial, nonmodified graphene was utilized. The one-pot synthesis of the moldable composite material allows for a controllable loading of graphene and the tuning of porosity. Potentially, this facile synthesis can be transferred to other MOF systems. The monolithic coatings reported here exhibit high surface areas (1156−1078 m2/g). The electrical conductivity was high (a range of 7.6 × 10−6 S m−1to 6.4 × 10−1 S m−1) and was found to be proportional to the graphene content. The ability to readily attain different forms and shapes of the conductive, microporous composites indicates that the MOF@G system can provide a compelling approach to access various applications of MOFs, specifically in electrochemical catalysis, supercapacitors, and sensors. KEYWORDS: monolith, metal−organic framework, graphene, electrical conductivity, HKUST-1

1. INTRODUCTION Microporous materials represent a large family of solids with far reaching applications pertinent to the accessible pore systems and chemical functionality. Among the porous solids, metal−organic frameworks (MOFs) demonstrate a wide range of applications in gas sorption and separation, 1−5 in heterogeneous catalysis,6−10 as well as in energy storage applications.11−15 However, the poor electrical conductivity of MOFs, in general,16 hindered access to a variety of applications requiring charge transport, including electrochemical processes, catalysis, and sensing. This is rather unfortunate because otherwise this class of materials is highly tunable and modular17 and can be tailored by an integration of specific guest molecules or even reactive catalysts.18−20 Moreover, because MOFs are commonly produced in the form of crystalline powders, effective utilization in demanding industrial applications that require processable forms of the material, best represented by moldable forms, have remained largely unexplored. Commonly, it was perceived that the three targeted properties of high microporosity, electrical conductivity, and attainment of moldable form, from the same material are orthogonal and very difficult to realize. Here, we present a novel approach that effectively realizes the merge of © XXXX American Chemical Society

these three-targeted properties through a simple, straightforward, and transferrable pathway. Recently, the reports of successful construction of monolithic forms of the crystalline, highly microporous MOF, although still lacking enhanced electrical conductivity, highlighted the potential of maximizing the benefits and utilization of such materials in gas storage applications.21−24 On the other hand, progress in the last decade regarding composites with graphene (G) or carbon nanotubes (CNTs) marked the emergence of different families of materials with enhanced electrical properties. However, such approaches were limited in scope to nonporous polymers and supported catalysts, with poor processability. The challenge in producing processable and optimally moldable forms of such composites was recently highlighted.25 Therefore, an approach to merge the par excellence properties including high surface area, tunability, chemical functionality of MOFs, and enhanced electrical conductivity of G in a moldable and processable form is of high significance. The intrinsically poor conductivity in Received: December 4, 2018 Accepted: January 17, 2019

A

DOI: 10.1021/acsami.8b20951 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

In this approach, the MOF−G interactions can be assumed to have played a major role in enhancing the cross-linkage of the MOF seeds, formed at the early stage of the reaction. As a control, the monolithic HKUST-1 was prepared without graphene (monoHKUST-1) through the previously reported procedure.21 The complete and homogeneous coverage of the HKUST-1 over the G platelets and the monolithic topography of the sample are quite evident from SEM imaging, as shown in Figure 1. In the SEM image, the G sheets are homogenously

MOFs is a result of the large band gap and the electronic mismatch between the molecular orbitals of the hard, carboxylate-based, linkers and the open-shell transition-metal ions, commonly utilized in MOF syntheses.16 Although this adversely affects the “through-bond” electronic conductivity of MOFs, other charge transport mechanisms including electron or hole hopping are expected to be still operative for charge transport.16 Several approaches emerged in recent literature, trying to address the poor electrical properties of MOFs, including thin layer deposition/anchoring of MOFs through epitaxial growth of thin films on conductive substrates. In this design, sufficient number of charge carriers can reach the conductive support, and thus offering a plausible pathway to access the rich electrochemical and electrical sensing applications of MOFs without altering their chemical composition, structure, or intrinsic conductivity.26−30 Another attempt was recently reported to enhance the electrical conductivity of MOF through impregnation with C60.31 Furthermore, attempts were made to enhance the intrinsic electrical conductivity of MOFs through utilization of softer Sbased organic linkers and selected metal ions, enabling longer range charge transport into the material.32 Utilizing this approach, microporous solids with high intrinsic electrical conductivity were recently experimentally reported33 and theoretically investigated.34 In this report, we describe an approach that effectively addresses: (1) the need for electronically conductive microporous solid, and (2) the ability to obtain a monolithic moldable form of such composite, simultaneously.

Figure 1. SEM image of the mono‑GHKUST-1 showing the distinctive continuum monolithic morphology of the MOF, covering the G flakes (some flakes were made visible by the sample grinding).

and continually covered by the MOF, where only a single flake of G is visible in the image (with distinctive smooth surface) that most likely resulted from sample grinding in preparation for imaging. The Fourier-transform infrared spectra (FTIR) for the two monoliths are shown in Figure 2, showing close

2. RESULTS AND DISCUSSION We report here on a fairly straightforward and widely applicable approach to access highly conductive, monolithic composites of MOFs with nanographene, which are also well suited to produce coatings. In this approach, simple incorporation of graphene platelets (G) dispersed within the reaction mixture of the MOF precursors was sufficient to result in a monolithic composite, Scheme 1. Specifically, the Scheme 1. Synthesis Scheme of the

mono‑GHKUST-1

Figure 2. FTIR spectra of the two monoliths showing the characteristic peaks of HKUST-1, in inset is the optical image of the mono‑GHKUST-1.

similarity between the two. Knowing the amount of G used in the synthesis, the G content of 20 wt % within the composite was calculated from the weight of a dry isolated material. The monoHKUST-1 demonstrated a Brunauer−Emmett−Teller (BET) surface area of 1450 m2/g while its composite with G was also microporous with a BET surface area of 1156 m2/g, as shown in Figure 3. To probe the ability for extending the synthesis to controllable loading of G, two more samples were prepared with increasing G wt % (28 and 37 wt %) and were also found to be microporous with a BET surface area of 1095 and 1078 m2/g, respectively, as shown in Figure 3. The pore size distribution in the four isolated compounds demonstrated close similarity with an average pore size of 9 Å, indicating that the porosity of the composite is mainly contributed by the

monolithic composite of graphene and the Cu-based MOF (HKUST-1) was prepared through the reaction of 1,3,5benzenetricarboxylic acid (BTC) and Cu(II) nitrate in the presence of dispersed graphene in ethanol as the solvent, at room temperature. Filtration of the reaction mixture to isolate the sol−gel, followed by drying at room temperature, was sufficient to isolate the monolithic composite mono‑GHKUST-1. This is in contrast to the previous synthetic approach to construct the monolithic HKUST-1, where centrifugation was crucial to attain the targeted product. The presence of the G platelets served to accelerate the formation of the sol−gel, previously assisted through centrifugation force. B


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MOFs), clearly demonstrating the ability to induce preferential growth of certain crystal planes through coordination interactions with the functionalized surface of the support.36 The epitaxially grown HKUST-1 on −OH or −CO2H functionalized surfaces demonstrated enhanced nucleation and growth through the (200) or the (220) planes, respectively, because of favorable formation of the Cu(II) paddlewheels on the functionalized surfaces.36 The Raman spectrum for the monolith was recorded and is shown in Figure 5. A clear contribution from the G component was

Figure 3. N2 gas sorption isotherms for the monolith and composites with various G contents collected at 77 K.

MOF component, as shown in Figure S2 in the Supporting Information. To confirm the construction of the targeted MOF@G composite, the powder X-ray diffraction pattern was recorded for the mono‑GHKUST-1, as shown in Figure 4. The attainment

Figure 5. Raman spectra for the G and the mono‑GHKUST-1, the green dashed line represents the components of the deconvoluted spectrum of the monolith are those ascribed to G.

evident, and the deconvolution of the spectrum allowed detailed examination of the possible mode of interaction(s) between the MOF and G. The observed bands for graphene in the composite showed detectable shift in the D, G, and 2D bands, Table 1. These observations were taken to indicate Table 1. Raman Shifts and Line Widths of the Characteristic Bands in the G and the mono‑GHKUST-1

graphene

Figure 4. Calculated X-ray diffraction (XRD) pattern for HKUST-1 (top) and the experimental pattern for the mono‑GHKUST-1 (below), asterisk denotes the diffraction peak from the G stacks.

mono‑GHKUST-1

of pure phase HKUST-1 is evident from the observed positions of the diffraction peaks, matching the calculated powder diffraction pattern calculated from the crystal structure of HKUST-1. In addition, one diffraction peak at 2θ = 26.4° is observed, which can be assigned to the stacks of graphene. Because of the random orientation of the flakes of the G support, extracting useful information about the existence of any preferred orientation for the growth of the HKUST-1 on the G is difficult by inspecting the relative intensities of the diffraction peaks. An attempt was made to deposit a thin film of HKUST-1 on a glass slide coated with graphene by alternately spin coating ethanolic solutions of the Cu(II) acetate and BTC. However, a diffraction pattern matching the data obtained for the HKUST-1 monolith solid (with no added graphene) was observed, as shown in Figure S3. Therefore, it is reasonable to assume that nondirectional, weak interactions with the G flakes are leading to isotropic nucleation and growth of the monolith. The observed diffraction pattern is similar to the recently reported data of HKUST-1 deposited on the glassy carbon electrode.35 This is in contrast to previous examples of surface-supported versions of HKUST-1 (SUR-

assignment

Raman shift (cm−1)

D G 2D D G 2D CC (BTC benzene ring) Cu(II)-carboxylate

1336 1567 2674 1346 1578 2692 998 ∼500

fwhm 74 44.7 89 74 55 71

strong interaction(s) with the graphene support by an electron-deficient species,37 the BTC rings, and the Cu(II) paddlewheels from the HKUST-1 monolith. The spectrum also demonstrates the presence of the characteristic MOF bands, with the CC stretching mode from the BTC rings observed at 998 cm−1 and Cu−O at 500 cm−1.35 The thermogravimetric analysis (TGA) of the monolith and the three composites of increasing G content are shown in Figure S4. The three composites showed increasing residual weight depending on the G content. The three composites demonstrated similar thermal stability as the monolithic HKUST-1 (ca 300 °C), suggesting that the composites can be utilized at relatively high temperatures, a particularly desirable feature especially for sensing or catalysis applications. To investigate the electrical conductivity of the obtained monoliths, impedance measurements were conducted covering the frequency range of 4 Hz to 10 kHz, as shown in Figure 6a. C


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Table 2. Electronic Conductivity Values Reported for Some Materials Including MOFs material copper graphene HKUST-1@G Ni3(HITP)2 TCNQ@Cu3(BTC)2 Mn2(DSBDC)

conductivity (S cm−1) 5

6

10 to 10 550 6.4 x 10−3 2 0.07 1.2 × 10−12

refs 39 39 this work (37 wt % G) 40 41 42

inset in Figure 6b. The transition from mostly dielectric into weakly conductive nature of the composites can also be traced in the observed impedance measurements. Although the composite with highest loading of G demonstrated frequency-independent conductivity, a weak capacitive component can be observed in the sample with 20 wt % G. We attribute this weak capacitive component to the sample testing setup, in which the bulk of the sample under investigation can be considered as a dielectric, sandwiched between two parallel conductive electrodes.

3. CONCLUSIONS Our results demonstrated that solids with enhanced electrical conductivity and maintained microporosity can simply and controllably be attained through efficient compositing of MOFs with conductive components such as graphene. Moreover, this approach provided a pathway to a moldable MOF@G composite that circumvents major challenge encountered in composites containing G or CNTs, namely processing challenges that precluded full utilization and exploitation of such composites.25 It is therefore demonstrated that this approach can result in conductive, microporous, monolithic forms, on a macroscopic scale, that are amenable for shaping and molding. The maintained microporosity of the MOF is an essential component of this approach. We demonstrated that chemical functionality, dimension/geometry of the pore systems, and electrical conductivity in the composite are not mutually exclusive, and can be tied together by the optimized combination of physical and chemical properties of the two components of the composite. The developed system is potentially transferrable to several other MOFs for the construction of conductive composites with G. Such novel microporous solid composites, with enhanced electronic conductivity and processability, are expected to have far reaching applications in electrochemical sensing, catalysis, and possibly in preferential electronically controlled or induced guest uptake characteristics. Overall, the reported approach provided insight into the essential components to attain monolithic, moldable, and processable forms of the microporous, functional, and electrically conductive composite.

Figure 6. (a) Conductivity measurements for several monoliths of HKUST-1 containing increasing loadings of G, and (b) electrical conductivity measured at 4 Hz for the three monolith composites (axes plotted using the log scale), and in inset (linear scale plot).

A regular, pellet-shaped sample of the monolith was coated on two opposite faces with silver paint to ensure good contact with the leads. A linear behavior of conductivity was observed, as shown in Figure 6a, over the frequency range tested, along with observed nearly constant phase shift, as shown in Figure S5. Accordingly, a resistor equivalent circuit of the composites is suggested, with variable degree of resistivity depending on the G content. A log scale plot indicated exponential relationship between the electrical conductivity and the G content in the samples tested, as shown in Figure 6b. Using the low-frequency data, a conductivity of 6.4 × 10−1 S m−1 was recorded for the composite containing 37 wt % G, while dropping to 3 × 10−3 and 7.6 × 10−6 S m−1 in the composites containing 28 and 20 wt % G, respectively. The pristine monolith demonstrated a conductivity of 1.7 × 10−9 S m−1. It is noted that the conductivity for the monoHKUST-1 reported here is within the range of that measured previously on the HKUST-1-pressed powder sample, utilizing the 4-point direct current technique (σ < 3 × 10−9 S m−1).38 As compared to the conductivity values reported for other MOFs, Table 2, it is evident that the approach utilized here resulted in one of the most conductive forms for the MOF@G containing 37 wt % G. The observed exponential trend in conductivity scaling up with the G content in the composites indicated that electrical conductivity in the prepared composites is mostly dominated by the conductivity of the G. These observations are in good agreement with the charge percolation model,43 where the percolation threshold appears to be around the 28 wt % G,

4. EXPERIMENTAL SECTION 4.1. Synthesis of monoHKUST-1. The synthesis was adopted from the reported method by Tian et al.21 Ethanol solution of Cu(NO3)2· 2.5H2O (10 mL, 0.064 M) was mixed with another ethanol solution of BTC (10 mL, 0.062 M) for 10 min at room temperature with stirring. The mixture was centrifuged and the solid was washed with ethanol and left to dry overnight at room temperature, then dried in an isothermal oven at 100 °C for 3 h, yielding 62 mg solid. The monolith was activated by heating at 120 °C for 3 h. 4.2. Synthesis of mono‑GHKUST-1. Graphene (45 mg) was sonicated for 30 min in ethanol solution of Cu(NO3)2·2.5H2O (30 mL, 0.064 M) and mixed with another ethanol solution of BTC (30 D


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ACS Applied Materials & Interfaces mL, 0.062 M) for 10 min at room temperature with stirring. The mixture was filtered and the thick solid was washed with ethanol, collected while wet and shaped into a mold (syringe with cut end, Supporting Information), and left to dry overnight at room temperature under ethanol vapor. The system was then open to an atmosphere for 2 days before the formed pellet was dried in an isothermal oven at 100 °C for 3 h, yielding 217.5 mg solid (20 wt % G). The monolith was activated for sorption analysis by heating at 120 °C for 3 h under dynamic vacuum. Similar syntheses were conducted with increasing amount of G in the synthesis to isolate the samples with 28 and 37 wt % G.

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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.8b20951. Materials and methods, monolith optical image, pore size distribution, XRD patterns, TGA analysis, and phase shift (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Mohamed H. Hassan: 0000-0001-7960-1893 Peter Thissen: 0000-0001-7072-4109 Christof Wöll: 0000-0003-1078-3304 Mohamed H. Alkordi: 0000-0003-1807-748X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge Zewail City of Science and Technology (CMS-MA) and Karlsruhe Institute of Technology (KIT). J.R. acknowledges HEIKA, the Heidelberg-Karlsruhe Research Partnership.

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Electrically Conductive, Monolithic MetalâOrganic Framework

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