Synthetic Routes for a 2D Semiconductive Copper

Sep 3, 2018 - Varying the bridging metal species and the coordinating atom are versatile approaches to tune their intrinsic electronic properties in c...
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Synthetic Routes for a 2D Semiconductive Copper Hexahydroxybenzene Metal−Organic Framework Jihye Park,† Allison C. Hinckley,† Zhehao Huang,‡ Dawei Feng,† Andrey A. Yakovenko,§ Minah Lee,† Shucheng Chen,† Xiaodong Zou,‡ and Zhenan Bao*,† †

Department of Chemical Engineering, Stanford University, Stanford, California 94305, United States Berzelii Centre EXSELENT on Porous Materials, Department of Materials and Environmental Chemistry, Stockholm University, SE-106 91 Stockholm, Sweden § X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, United States J. Am. Chem. Soc. 2018.140:14533-14537. Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 11/15/18. For personal use only.

‡

S Supporting Information *

As representative examples of c-MOFs, the M3(C6X6)2 (X = NH, S) family (Figure 1a) has drawn increasing attention for

ABSTRACT: Conductive metal−organic frameworks (cMOFs) have shown outstanding performance in energy storage and electrocatalysis. Varying the bridging metal species and the coordinating atom are versatile approaches to tune their intrinsic electronic properties in c-MOFs. Herein we report the first synthesis of the oxygen analog of M3(C6X6)2 (X = NH, S) family using Cu(II) and hexahydroxybenzene (HHB), namely Cu-HHB [Cu3(C6O6)2], through a kinetically controlled approach with a competing coordination reagent. We also successfully demonstrate an economical synthetic approach using tetrahydroxyquinone as the starting material. Cu-HHB was found to have a partially eclipsed packing between adjacent 2D layers and a bandgap of approximately 1 eV. The addition of Cu-HHB to the family of synthetically realized M3(C6X6)2 c-MOFs will enable greater understanding of the influence of the organic linkers and metals, and further broadens the range of applications for these materials.

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ith their intrinsic electrical conductivity, permanent porosity, and integrated functionalities, conductive metal−organic frameworks (c-MOFs)1,2 have emerged as promising candidates in many applications,3−19 including electrocatalysis,7,13,14,20 energy storage,9,10,17 sensors,21−24 and electronic devices.25 The key feature that differentiates conductive MOFs from conventional, insulating MOFs is longrange electron delocalization throughout the framework. This delocalization can be achieved when the organic linker is fully conjugated and has strong orbital interaction with the metal center. As a class of crystalline, solid materials, MOFs have an outstanding degree of synthetic tunability in their structure, composition, and functionality.26,27 Unlike in insulating MOFs, the long-range electronic coupling in c-MOFs can exemplify how local changes in the building blocks affect the material’s bulk properties. Tuning the structure of c-MOFs with atomic precision is therefore a powerful strategy to study the structure−property relationship in 2D materials while simultaneously providing more candidates for electronic applications.28−31 © 2018 American Chemical Society

Figure 1. (a) Previously reported 2D MOFs in the M3(C6X6)2 family. (b) Reaction scheme of the proposed competing synthetic pathways for Cu3(C6O6)2..

their outstanding performance in energy storage and electrocatalysis, stemming from their catalytically active open metal sites and dense redox-active centers.8,9,14,16,32 The synthetic viability and high stability of these materials allow for applications in a wide range of environments. Therefore, expanding the M3(C6X6)2 family not only opens up new Received: June 25, 2018 Published: September 3, 2018 14533

DOI: 10.1021/jacs.8b06666 J. Am. Chem. Soc. 2018, 140, 14533−14537

Communication

Journal of the American Chemical Society opportunities to explore novel applications but also promotes greater understanding of structure−property relationship in cMOFs. Herein we report a new 2D MOF, namely Cu-HHB, obtained via kinetically controlled synthesis with assistance of a competing coordination reagent. Hydrothermal reaction between Cu(II) salts and hexahydroxybenzene (HHB) yielded Cu-HHB [=Cu3(C6O6)2] with a (2,3)-connected honeycomb topology. We additionally demonstrate an alternative synthetic approach using an oxidized intermediate of HHB. With a partially eclipsed packing between adjacent 2D layers, Cu3(C6O6)2 exhibits a bandgap of ∼1 eV and temperaturedependent conductivity characteristic of semiconductors. A square planar building unit, MX4 (M = Co, Ni, Cu; X = NH, O, S), has been most frequently employed to construct semiconductive/conductive MOFs.3,7−11,13,18 Changes in either the metal (M) or the coordinating atoms (X) in MX4 can significantly tune the resulting orbital interactions and thus effectively alter the electronic structures of the frameworks. Consequently, the reported MOFs that belong to the M3(C6X6)2 family (X = NH, S) have shown different electronic properties via atomic replacement of the metal node. However, expanding this family to its oxygen analog has been synthetically challenging and has not been previously reported. Varying the atom X significantly changes the nature of the coordination bond and thereby requires dramatic alteration of the synthetic procedures from the reported synthetic conditions for the nitrogen and sulfur analogs. In addition, the strong metal−ligand orbital interaction in these frameworks often leads to irreversible bond formation, resulting in poorly crystalline to amorphous products from attempts at bulk synthesis of M3(C6X6)2. Therefore, despite the ease of predicting its structure, the successful synthesis of M3(C6O6)2 has not been previously achieved. To obtain the desired 2D (2,3)-connected honeycomb structure of M3(C6O6)2, we selected hexahydroxybenzene (HHB) as the linker and Cu(II) as the bridging metal center to target the square planar coordination geometry. Direct adaptation of the reported synthetic conditions for the synthesis of M-HAB (HAB = hexaminobenzene), which used NH4OH as the base, yielded a poorly crystalline product with impurities (Figure S1).8,9,18 Considering that the presence of OH− from NH4OH often generates hydroxide and oxide byproducts, we chose ethylenediamine (en) instead to avoid using hydroxide bases. We rationalized that the use of ethylenediamine can also have two benefits aside from its role as the base (Figure 1b, Path 1). First, ethylenediamine can strongly chelate to Cu(II) to form the soluble coordination complex Cu(en)2 (Figure 1b, Path 2), which can impede the undesirable reaction between Cu(II) with the OH− present in a basic environment. Second, the formation of the Cu(en)2 complex can compete with the bond formation between Cu(II) and HHB, which would likely slow down the nucleation of the MOF to yield a more crystalline Cu-HHB product. As expected, upon addition of ethylenediamine, a much more crystalline product with a plate-like morphology was obtained (Figure 2a and Figure S4). We found that the ratio between the linker and ethylenediamine played a critical role in controlling the crystallinity and yield of the product, suggesting both kinetically and thermodynamically competing effects of ethylenediamine during MOF formation (Supporting Information Section 2 and Figure S4).

Figure 2. Structural characterizations of Cu-HHB. (a) (left) HRTEM images of Cu-HHB along [001] showing an elliptical pore packing. The inset is the fast Fourier transform of the image. Scale bar = 10 nm. (top, right) HRTEM image of the red highlighted region at high magnification. Scale bar = 2 nm. (bottom, right) Symmetry-imposed and lattice-averaged image calculated from the HRTEM image. The embedded is the partially eclipsed packing model of Cu3(C6O6)2. (b) PXRD Rietveld refinement of Cu3(C6O6)2 displaying the observed pattern (black), calculated pattern (red), difference plot (blue), and Bragg positions (black bars). Data were collected at beamline 17-BM at the Advanced Photon Source, Argonne National Laboratory (λ = 0.452 36 Å). (c) Space-filling model of the Cu3(C6O6)2 model. Red, gray, and blue spheres represent O, C, and Cu atom, respectively. Hydrogen atoms are omitted for clarity.

To confirm that Cu-HHB has the proposed 2D Cu3(C6O6)2 structure, density functional theory (DFT) calculations were carried out. Synchrotron powder X-ray diffractions (PXRD, λ = 0.452 36 Å) were used to confirm the structure, and the Rietveld method was used to refine the structure (Figure 2b). We noticed that the layer-to-layer packing modes result in notably different diffraction patterns. Thus, both AA eclipsed and AB slipped-parallel (partially eclipsed) models were calculated. The AB slipped-parallel model with slightly slipped stacking of every two layers (Figure 2c) gives a better fit and resolves a peak at ca. 4.8° (indicated by an arrow in Figure 2b), which is unindexable with the perfectly eclipsed model (Figure S5). The Cu-HHB exhibits a C-centered orthorhombic unit cell with cell parameters of a = 13.108 Å, b = 21.592 Å, and c = 5.924 Å. The interlayer distance in Cu-HHB is ∼2.96 Å, notably shorter than what was observed in M-HAB (M = Cu, Ni) with the eclipsed packing mode.9 This further supports AB slipped-parallel model in Cu3(C6O6)2 as the slightly displaced intermolecular π−π stacking can keep the layer structure more stable in a shorter interlayer distance. To further confirm the structure, high-resolution TEM (HRTEM) images were obtained. Because of the high crystallinity of Cu-HHB, the HRTEM images clearly show the elliptical pores (Figure 2a) which match well with the AB slipped-parallel model (Figure 2c). Furthermore, a honeycomb arrangement along [001] with d110 = 11.7 Å and d020 = 10.9 Å (Figure 2a) corresponds well with the AB slipped-parallel model obtained from PXRD 14534

DOI: 10.1021/jacs.8b06666 J. Am. Chem. Soc. 2018, 140, 14533−14537

Communication

Journal of the American Chemical Society results (calculated d110 = 11.2 Å and d020 = 10.9 Å). The N2 sorption isotherm of Cu-HHB shows a BET surface area of ∼158 m2 g−1 (Figures S6−S9), indicating an intrinsic porosity that cannot be achieved with the nearly nonporous staggered structure (Figure 1b). X-ray photoelectron spectroscopy (XPS) and elemental analysis (EA) were used to determine the charges on Cu-HHB. A high-resolution XPS scan of Cu 2p3/2 exhibits a dominant peak at ∼933 eV with characteristic settelite peaks, which signifies a +2 oxidation state on Cu while a trace peak of +1 oxidation state is present (Figure S10). Both catecholate and semiquinonate states have been previously found in hydroxyl linker-based c-MOFs, indicating that the MO4 units in the framework can be either neutral or negatively charged.5,6,11 EA results suggest a chemical formula of [Cu3(C6O6)2(NH3CH2CH2NH3)1.36] for Cu-HHB, assuming that the presence of nitrogen came from the protonated ethylenediamine used in the synthesis (Table S2), which indicates nearly −1 charge at each CuO4 unit (Scheme S1). Typically, the synthesis of M3(C6X6)2 MOFs involves a reduced form of the organic linkers. However, the reduced organic linkers are often reactive, and careful protection of the starting material is required to prevent irreversible oxidation of the linkers or some complex interplay in reaction kinetics. Such complicated synthetic procedures severely impede the scale-up of c-MOFs and thus limit their employment in applications that require bulk material. Unlike its analogous linkers benzenehexathiol (BHT) and HAB, HHB has stable oxidized products, including the 2e− oxidation product tetrahydroxy1,4-quinone (THQ), the 4e− oxidation product rhodizonic acid (RA), and the 6e− oxidation product hexaketocyclohexane (HKH) (Figure 3a). In the proposed synthetic scheme, HHB undergoes the deprotonation and the serial oxidation processes to yield the Cu3(C6O6)2 product (Figure 1b). Thus, THQ can be regarded as a reaction intermediate of Cu3(C6O6)2 formation with HHB as a starting reagent. With this in mind, we hypothesized that substitution of THQ for HHB could be a synthetic “shortcut” to Cu3(C6O6)2 and attempted a synthesis via this pathway. With a slight modification from the Cu-HHB synthetic conditions, a dark blue product of Cu-THQ with the same plate-like morphology was obtained (Figure 3b). PXRD confirms that Cu-THQ has an identical Cu3(C6O6)2 structure as Cu-HHB (Figure 3c and Figure S14). The optimized synthetic conditions with THQ required a smaller amount of ethylenediamine (Figure S11) than did the synthesis with HHB, supporting our hypothesis as depicted in Figure 3b. We further attempted the syntheses with RA and HKH, which have overly oxidized state than that of the linker in the Cu3(C6O6)2 and thus RA and HKH need to be reduced to produce the final MOF form. However, the given ambient reaction environment cannot reduce these materials, and thus yielded either no solid products or precipitates with completely different PXRD patterns from Cu-HHB and Cu-THQ (Figure S13). The N2 sorption isotherm of Cu-THQ reveals a BET surface area of ∼143 m2 g−1, similar to that of Cu-HHB (Figure 3d and Figures S15−S18). It is worth noting that this is a rare example of a demonstrated c-MOF synthesis with organic linkers in different oxidation states. This result establishes the feasibility of shortening the MOF synthesis and suggests possibilities to eliminate the use of unnecessary oxidative reagents, thereby facilitating synthetic exploration for

Figure 3. (a) HHB in different oxidation states. (b) Alternative synthetic approach for Cu3(C6O6)2. The proposed scheme that Cu3(C6O6)2 can be obtained using THQ with a shortened synthetic pathway. SEM images of Cu-HHB and Cu-THQ products. (c) PXRD patterns and (d) N2 sorption isotherm of Cu-THQ in comparison with Cu-HHB derivatives.

crystalline c-MOFs using air stable, low cost starting materials at scale (Figure S12). Having confirmed the physical structure, we further examined the electronic structure and electrical conductivity of Cu3(C6O6)2. UV−vis-NIR spectroscopy shows broad absorption bands tailing in the NIR region, corresponding to an absorption edge of ∼1 eV (Figure 4a). Using photoemission

Figure 4. (a) UV−vis-NIR spectrum. (inset) Photoelectron spectroscopy in air (PESA) curve. Photograph shows Cu-HHB powder. (b) Electrical conductivity of Cu-HHB as a function of temperature.

spectroscopy in air (PESA), the highest occupied molecular orbital (HOMO) of Cu-HHB was measured to be −5.37 eV (relative to vacuum level). A bulk electrical conductivity of 7.3 × 10−8 S cm−1 was obtained on a pressed pellet of Cu-HHB via the van der Pauw method at room temperature under a nitrogen atmosphere (Figure 4b, Figures S19−S21). The electrical conductivity of the bulk Cu-HHB was positively 14535

DOI: 10.1021/jacs.8b06666 J. Am. Chem. Soc. 2018, 140, 14533−14537

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Journal of the American Chemical Society

Source was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.

correlated with temperature, as is typical of semiconducting materials, and exhibited Arrhenius-type dependence on temperature (Figure 4b). The nearly 30× increase in conductivity over a temperature range of 80 °C is indicative of a relatively high activation energy of ∼0.46 eV, with the pellet achieving a bulk conductivity of ∼2.0 × 10−6 S cm−1 at 105 °C. The large activation energy could be attributed mostly to charge hopping between domain boundaries.33 Compared to other M3(C6X6)2 MOFs, Cu3(C6O6)2 shows a moderate conductivity which can be ascribed to weaker orbital interaction between Cu(II) and C6O6, likely due to the lower HOMO of the oxygen-containing linkers than its nitrogen and sulfur derivatives.34 Nonetheless, our results suggest that the electronic structure can indeed be tuned by modulating the orbital interaction with different linkers, facilitating the search for suitable semiconductors for potential applications. In summary, Cu-HHB was successfully obtained using a competing coordination reagent, ethylenediamine, through a kinetically controlled synthesis. We demonstrated that a 2D cMOF can be synthesized with different starting materials by understanding the mechanism of Cu3(C6O6)2 formation. Having Cu3(C6O6)2 in the M3(C6X6)2 family not only enriches the library of c-MOFs and enables fundamental study of structure−property relationship of analogs but also offers new opportunities in electronics, sensing, and energy-related applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.8b06666. Characterization details and additional experimental data (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Jihye Park: 0000-0002-8644-2103 Xiaodong Zou: 0000-0001-6748-6656 Zhenan Bao: 0000-0002-0972-1715 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J.P. acknowledges support from the Dreyfus Foundation Postdoctoral Fellowship for Environmental Chemistry. A.C.H. acknowledges support from the National Science Foundation Graduate Research Fellowship under Grant No. DGE-1147474. D.F. acknowledges support from the U.S. Department of Energy, Office of Sciences, Office of Basic Energy Sciences, to the SUNCAT Center for Interface Science and Catalysis. M.L. acknowledges partial support by the Postdoctoral Fellowship from the National Research Foundation of Korea under Grant No. NRF-2017R1A6A3A03007053. The HRTEM and PXRD structural characterization was supported by the Knut & Alice Wallenberg Foundation through the project grant 3DEM-NATUR and the Swedish Research Council (VR) through the MATsynCELL project of the Röntgen-Ångström Cluster. Use of the Advanced Photon 14536

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DOI: 10.1021/jacs.8b06666 J. Am. Chem. Soc. 2018, 140, 14533−14537