This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes.
Article http://pubs.acs.org/journal/acsodf
Giant Enhancement of Carrier Mobility in Bimetallic Coordination Polymers Barun Dhara,† Vikash Kumar,† Kriti Gupta, Plawan Kumar Jha, and Nirmalya Ballav* Department of Chemistry, Indian Institute of Science Education and Research (IISER), Pune 411008, India S Supporting Information *
ABSTRACT: Electrically conductive metal−organic coordination polymers (CPs) are promising candidates for a variety of technological applications. However, poor energetic and spatial overlap between the sp-electrons of organic ligands and the delectrons of metal ion often blocks an effective charge transport (mobility) across CPs. Herein, we present a bimetallic design principle for enhancing carrier mobility in CPs. Bimetallic CPs of Fe(III) and Cr(III) ions coordinated to 1,3,5benzenetricarboxylic acid (BTC) ligand (Fe−BTC−Cr) exhibited remarkably high carrier mobility at the matching mole ratio (1:1) with enhancement factors of 102 and 104 in comparison to those of monometallic parents, Fe−BTC and Cr−BTC, respectively. The observation was substantiated by lowering of the band gap between the valence band and the conduction band upon the formation of a hybrid p−n-type structure in the bimetallic CPs. The direct current conductivity values of the CPs measured by four-probe technique were in good agreement with the alternating current conductivity values obtained from the electrochemical impedance spectroscopy. Our flexible approach of picking and choosing the appropriate combination of metal ions from the periodic table is expected to generate various CPs with desirable semiconducting properties.
1. INTRODUCTION Exploring new materials with desired functionalities are demanding in the dominance of Si-based materials upon inception in the semiconductor industry.1 Various inorganicbased semiconductors have been investigated and found to possess good electrical properties, but also accompanying brittleness. On the other hand, organic-based semiconductors are chemically customizable and mechanically flexible, but they suffer from poor electrical performances and thermal management.2 Very recently, organic−inorganic hybrids such as metal−organic coordination polymers (CPs) have come up to challenge these conventional materials by revealing a high degree of mutability3,4 and thus appealing for electronic device applications with controllable electrical conductivity.5 For example, tunable electrical conductivity in the thin-film devices of CPs was achieved by infiltration with specific guest molecules such as tetracyanoquinodimethane and ferrocene.6−9 An insulator CP was transformed into a semiconductor composite material upon incorporation of the chains of conducting polymer and such a strategy could provide platforms for various CP-based functional nanomaterials.10,11 Furthermore, a million-fold conductivity enhancement was observed on altering the metal ion from Mn(II) to Fe(II) in the CPs family MOF-74.12 Replacement of Mn(II) with Fe(II) yielded isostructural CP with a much better electrical conductivity owing to (−Fe−S)∞ chains and loosely bound β-spin electron in Fe(II). Regardless of the electrical conductivity modulation or enhancement, much attention is sacrificed over carrier mobility © 2017 American Chemical Society
and density values, also detailed discussions over the important semiconducting parameters were scarcely unveiled in the view of a simple equation: σ = ηeμ (where σ is the conductivity, η is the carrier density, and μ is the carrier mobility).13 Stimulated from the recent progress and visualizing the tuning of electrical conductivity in CPs via incorporating two metal ions rather than one metal ion, we thought to explore the emerging concept of “bimetallic” CPs, specifically in regard to the tailoring of electronic states14−18 (Figure 1b). Instead of a single metal ion (monometallic, Figure 1a), we have synthesized bimetallic CPs by employing Fe(III) and Cr(III) ions in stoichiometric ratios with organic ligand 1,3,5benzenetricarboxylic acid (BTC). Surprisingly, the bimetallic analogues (Fe−BTC−Cr) displayed remarkably higher electrical conductivity and carrier mobility values compared with monometallic parents (Fe−BTC and Cr−BTC). The Hall effect measurements show the electronic properties of semiconductor materials so as to deepen our understanding of the design principles of electrically conductive CPs, thus extending their promise in the field of electronics.
2. RESULTS AND DISCUSSION Fe−BTC and Cr−BTC gels were prepared upon minor modifications of the reported methods, and CPs in the form of xerogels were isolated by drying the respective gels under Received: July 5, 2017 Accepted: July 26, 2017 Published: August 14, 2017 4488
DOI: 10.1021/acsomega.7b00931 ACS Omega 2017, 2, 4488−4493
ACS Omega
Article
(Figure 2c). Because the ionic radii of Fe(III) and Cr(III) are almost identical20 (∼60 and ∼62 pm, respectively), apparently Cr(III) ions did not hamper the homogeneous gelation process in the mixed phase with Fe(III) ions, namely, the formation of the Fe−BTC−Cr cuboids like Fe−BTC. A TEM image supported the FESEM studies, and the layered structure of the bimetallic Fe−BTC−Cr system could be clearly visualized (Figures 2d and S2). Also, the SAED pattern indicated the presence of a high degree of crystallinity in the bimetallic Fe−BTC−Cr system (Figure 2d, inset). Powder Xray diffraction (PXRD) patterns of the Fe−BTC, Cr−BTC, and Fe−BTC−Cr systems corroborated the FESEM/TEM observations; specifically, the bimetallic Fe−BTC−Cr system adopted the structure of the monometallic Fe−BTC system, that is, close resemblance to the previously reported singlecrystal structure, though impurity phases are inherent in polycrystalline materials like metal−organic xerogels explored here (Figure S3).21 To check the modulation of porosity in bimetallic CPs, if there is any, in comparison to monometallic parents, we recorded the N2 gas adsorption−desorption isotherms at 77 K for the Fe−BTC, Cr−BTC, and Fe−BTC−Cr (1:1) systems (Figure 3). The isotherms are typical of type I adsorption,
Figure 1. Schematics of (a) monometallic (Fe−BTC or Cr−BTC) and (b) bimetallic (Fe−BTC−Cr) CPs. Purple box represents BTC ligand and orange and sky spheres represent iron and chromium metal ions, respectively.
vacuum at room temperature.19 To synthesize the different stoichiometric ratios of bimetallic Fe−BTC−Cr gel, we mixed 1:1, 1:2, and 2:1 Fe(III)/Cr(III) nitrate salts together with BTC in ethanol solvent. The colors of all of gels/xerogels were found to be distinctively different (Figure 2a−c): Fe−BTC
Figure 3. N2 gas adsorption−desorption isotherms at 77 K of Cr− BTC (blue), Fe−BTC−Cr (red), and Fe−BTC (black) (filled symbols and open symbols indicate adsorption and desorption of N2 gas, respectively).
Figure 2. Field-emission scanning electron microscopy (FESEM) images of (a) Fe−BTC, (b) Cr−BTC, and (c) Fe−BTC−Cr. Scale bars represent a length of 2 μm in each image. Insets: photographs of respective gels and xerogels. Inset in (b) and (c): zoom-in image of Cr−BTC and 1:1 sample with 2 μm scale bar. (d) Transmission electron microscopy (TEM) image of Fe−BTC−Cr revealing layered structure (scale bar is 200 nm). Inset in (d): selected-area electron diffraction (SAED) pattern showing the crystalline nature of the bimetallic CP.
specifically adsorbing higher amount of N2 gas, as higher partial pressure and the sharp uptake of N2 near about 1 bar pressure could be due to the pore condensation effect.22,23 Interestingly, the uptake of N2 gas in the Fe−BTC−Cr system was significantly higher than that of the Fe−BTC and Cr−BTC systems, which could possibly be due to “heterogeneity” factor.16 The Brunauer−Emmett−Teller (BET) surface area of Fe−BTC−Cr was found to be ∼338 m2 g−1. The energydispersive X-ray spectroscopic (EDXS) analysis of bimetallic CPs was carried out to scrutinize the relative ratio of metal ions as well as their distribution across crystallites. The bimetallic CPs were prepared in different elemental ratios of Fe and Cr; therefore, we had to closely look into the elemental ratio across the Fe−BTC−Cr cuboids. Interestingly, at each point, the percentage of atomic composition was close to the initial feed. Also, the elemental mapping of bimetallic CPs showed a homogeneous distribution of Fe and Cr across the material for 1:1, 2:1, and 1:2 ratios (Figures 4a and S4).24 Thus, our simple experimental methodology of metal−organic gelation allowed the formation of the so-called truly/homogeneously bimetallic CPs.
(orange); Cr−BTC (dark green); and Fe−BTC−Cr (red wine). The different colors of CPs could originate from the characteristic ligand-to-metal charge transfer. In the Fourier transform infrared (FTIR) spectra, the CO stretching vibration of BTC at ∼1730 cm−1 was shifted to ∼1630 cm−1 in all of the CPs, thereby suggesting a successful coordination of the BTC ligand with Fe(III) and Cr(III) ions.16,19 In case of the bimetallic Fe−BTC−Cr, the stretching vibrations as ∼470 and ∼525 cm−1 were characteristic of Fe−O and Cr−O bonds, respectively (Figure S1). Morphological features of CPs were investigated by FESEM. Micrometer-sized irregular yet crystalline blocks were observed for Fe−BTC, whereas Cr−BTC revealed globular/fibrous patterns (Figure 2a,b). Interestingly, the FESEM images of the bimetallic Fe−BTC−Cr system showed regular cuboids and closely resembled the morphology of the Fe−BTC system 4489
DOI: 10.1021/acsomega.7b00931 ACS Omega 2017, 2, 4488−4493
ACS Omega
Article
Figure 4. (a) Elemental mapping of Fe and Cr in the bimetallic 1:1 Fe−BTC−Cr system by EDXS. (b) Fe 2p and Cr 2p X-ray photoelectron spectroscopy (XPS) spectra recorded on 1:1 Fe− BTC−Cr sample at room temperature.
Figure 5. (a) Room temperature electrical conductivity values of Fe− BTC (red), 2:1 Fe/Cr (violet), 1:1 Fe/Cr (green), 1:2 Fe/Cr (pink), and Cr−BTC (blue). Inset: schematic of proposed band diagram causing lowering of the band gap in the bimetallic Fe−BTC−Cr system. (b) Tauc plots of Fe−BTC (red), Cr−BTC (blue), 1:1 Fe− BTC−Cr (green), and mechanical mixture (Fe−BTC + Cr−BTC) (black) recorded at room temperature. Inset: new band at ∼1.75 eV for the bimetallic Fe−BTC−Cr system.
To check the charge-transfer reaction involving Fe(III) and Cr(III) ions in the bimetallic system, we employed the XPS (Figure 4b). The appearance of Fe 2p3/2 and Cr 2p3/2 peaks at the binding energy values of 711.3 eV (with multiplet feature) and 577.4 eV, respectively, suggests the predominant presence of Fe(III) and Cr(III) ions in the 1:1 bimetallic Fe−BTC−Cr system;25 from the relative intensities of Fe 2p and Cr 2p, the elemental composition of Fe/Cr was estimated to be ∼1:1. Note that 2p3/2 and 2p1/2 doublet spectral lines are due to the spin−orbit coupling and intrinsic atomic property. The direct current (dc) electrical conductivity (σ) was recorded at the room temperature by a conventional four-probe method using the pressed pellets of various CPs. The conductivity values of the Fe−BTC and Cr−BTC systems were found to be in the range of ∼10−5−10−7 S cm−1, respectively (Figure 5a). Note that the four-probe method is recommended for CPs with resistivity exceeding 1 kΩ.26 Our recorded values on the Fe and Cr systems exactly refresh the pioneering work of Dincă and co-workers in which the role of metal ion (Fe and Mn) in influencing the electrical conductivity value was well demonstrated.12 Such an important effect could rather provide us a rational approach for tuning the electrical conductivity over a wide range (∼10−5−10−7 S cm−1 in the present study) by incorporating two different metal ions in the same CP. Indeed, the conductivity of a mechanical mixture (1:1) of Fe−BTC and Cr−BTC was noted to be ∼10−6 S cm−1 (Figure S5)clearly an intermediate of the values of the two. Surprisingly, the electrical conductivity values of the selfassembled bimetallic CPsboth 2:1 (Fe/Cr) and 1:2 (Fe/ Cr)were estimated to be in the range of ∼10−4 S cm−1. Also, for the 1:1 (Fe/Cr) bimetallic system, we observed an electrical conductivity value in the range of ∼10−3 S cm−1102-fold higher than that of Fe−BTC and 104-fold higher than that of Cr−BTC (Figure 5a). Now, the question iswhy? Usually, significant lowering of the band gap between the valence band (VB) and conduction band (CB) in the hybrid
materials is found to be the primary cause of such an enhancement in the electrical conductivity with respect to the parent compounds. Typical examples include organic-based donor−acceptor complexes27,28 and/or inorganic-based hybrid p−n semiconductors,29 which could be applicable to our bimetallic Fe−BTC−Cr system (Figure 5a; inset). We recorded the Tauc plots, namely, solid-state UV−vis absorption spectra,30 to estimate the band gap values of the Fe−BTC, Cr−BTC, and Fe−BTC−Cr systems (Figure 5b). In fact, a lowering of the band gap (∼2.4 eV) in the case of Fe−BTC−Cr was observed compared with that of both Fe−BTC (∼2.8 eV) and Cr−BTC (∼3.8 eV). Furthermore, appearance of a new band at ∼1.75 eV for the 1:1 Fe−BTC−Cr system (absent in Fe−BTC, Cr−BTC, and 1:1 mechanical mixture Fe−BTC + Cr−BTC) served as a clear signature of the hybrid charge transfer state (weak) in the bimetallic CP. Notably, a recent report provided theoretical insights into the significant modulation of electronic band structures (electrical conductivity) in bimetallic metal−organic frameworks (MOFs) in comparison with that in monometallic parents.18 Another possibility for such an enhancement in the electrical conductivity of bimetallic CPs could be the formation of a “coordination track” in the material generating a low-dimensional charge transport pathway.12,13 For example, Dincǎ and co-workers have nicely shown that the introduction of S/O substitution in the ligand in a Mn2+-based CP formed a onedimensional chain (−Mn−O/S−)∞, which increased the carrier mobility significantly vis-à-vis enhancement in the electrical conductivity up to a million-fold. Motivated by the example, we wanted to check the influence of light on the electrical conductivity values of our bimetallic CPs. However, our preliminary studies showed that upon irradiation with 1 sun 4490
DOI: 10.1021/acsomega.7b00931 ACS Omega 2017, 2, 4488−4493
ACS Omega
Article
intensity light, no appreciable change in the conductivity value was observed and thus photocurrent transients would be more desirable to conclude on coordination track.13 The semiconducting characteristics of Fe−BTC, Cr−BTC, and Fe−BTC−Cr were evaluated by the Hall effect measurements (Figure 6), and the important extracted parameters are
Finally, measuring and reporting the values of electrical conductivity of CPs is an important issue, as elegantly discussed by Dincǎ and co-workers in a case of Cd-based CP. Depending on the technique and external factors, the conducting value of the same CP sample can vary by more than one order of magnitude.26 Usually, four-probe method is used for lowresistance materials to remove the effect of the contact resistance. If the method is applied to high-resistance materials like most of the CPs, the conductivities calculated by the voltage difference could be overestimated.40 Therefore, apart from the four-probe method, we also explored the electrochemical impedance spectroscopy (EIS) to estimate the electrical conductivity values of the Fe−BTC, Cr−BTC, and Fe−BTC−Cr (1:1) samples. Measured data points in the Nyquist plots were fitted with a modified Randle circuit (Figure 7a−d).40,41 The electrical conductivity values of Fe−BTC, Cr−
Figure 6. Schematic diagram for the Hall effect measurement, where t is the thickness of the sample, w the width, VH the Hall voltage, B the magnetic field, l the length of the sample, and Ix is the current.
summarized in Table 1. Typically, the Hall coefficient (RH) values of nonmagnetic semiconductors are independent of the magnetic field. Because our monometallic and bimetallic CPs were both paramagnetic in nature (Figure S6), the Hall resistivity includes an additional contribution and depends on the magnetization of the samplescommonly referred as anomalous Hall effect.31 We observed a strong dependency of RH values of Fe−BTC, Cr−BTC, and Fe−BTC−Cr on the applied magnetic field (B). The data points in the plots of RH versus B could be best fitted with the equation: RH = RH0 + (C/ B), where C is a constant and RH0 is the field-independent Hall coefficient (so-called ordinary Hall coefficient)likewise modified Curie−Weiss equation for metal−organic paramagnets with the inclusion of a temperature-independent magnetization term (χ0) (Figure S7).32 Overall, it is not only the electrical conductivity values that are different for Fe−BTC and Cr−BTC but also the CPs are inherently differentthe former is a p-type semiconductor, whereas the later one is an ntype semiconductor. Thus, naturally, a p−n hybrid structure in the bimetallic Fe−BTC−Cr system is expected to take place, therefore lowering of the gap between VB and CB was observed. Remarkably, the carrier mobility value (μ) of Fe− BTC−Cr was almost 3 orders of magnitude higher than that of Fe−BTC and Cr−BTC, also the μ value of our 1:1 bimetallic Fe−BTC−Cr system is perhaps the best among the reported values of various CPs13,33−36 and relevant organic-based hybrid materials (Table S1). This tremendous increase in the mobility of bimetallic analogues is attributed to the reduction in the energy barrier for hopping of “Coulomb holes” by “Coulomb attraction” and/or formation of “two-dimensional electron gas”.37,38 Also, the presence of Fe could uniquely promote the electrical conductivity in the bimetallic CPs.39 Electrical conductivity of Fe−BTC−Cr system could be further improved by engineering the ligand containing the O/S atoms.12
Figure 7. Nyquist plots of CPs: (a) Fe−BTC, (b) Fe−BTC−Cr (1:1), and (c) Cr−BTC. Respective raw data points are fitted (black lines) with a modified Randle circuit. (d) Schematic illustration of a sample holder (upper), where WE, SE, CE, and RE are working, sense, counter, and reference electrodes, respectively. The equivalent circuit (down) used to fit the impedance spectra, where C, R, Rct, and W are capacitance, resistance, charge-transfer resistance, and Warburg element, respectively.
BTC, and Fe−BTC−Cr (1:1) measured by EIS were found to be ∼3.0 × 10−6, ∼5.0 × 10−8, and ∼1.5 × 10−4 S cm−1, respectively. Indeed, the overall trend in the electrical conductivity reflected by the EIS technique was very similar to that observed with the four-probe technique; specifically, bimetallic CPs consistently exhibited much higher electrical conductance than monometallic parent CPs. To show case our results on bimetallic CPs in the domain of electrically conductive and porous CPs, a collation of the electrical
Table 1. Summary of Hall Effect Measurements
type RH0 (cm3 C−1) ρ (Ω cm) η (cm3) μ (cm2 V−1 s−1)
Fe−BTC
Cr−BTC
1:1
1:2
2:1
p-type 1 × 103 3.3 × 104 6.2 × 1015 3 × 10−2
n-type −8 × 104 3.4 × 106 7.6 × 1013 2.4 × 10−3
n-type −1 × 104 1 × 103 4.2 × 1014 15
n-type −2 × 105 4.0 × 104 2.8 × 1013 5.5
p-type 2 × 104 3.9 × 104 2.5 × 1014 0.63
4491
DOI: 10.1021/acsomega.7b00931 ACS Omega 2017, 2, 4488−4493
ACS Omega
■
conductivity values and the BET surface area of various systems is presented in Table S2.
Article
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.7b00931. Data on FTIR; FESEM; EDXS; PXRD; electrical conductivity; magnetic properties; the Hall effect measurements (PDF)
3. CONCLUSIONS In summary, the strategy of elapsing from monometallic to bimetallic porous CPs to enhance electrical conductivity and carrier mobility is established. At matching mole ratio, the bimetallic Fe−BTC−Cr system exhibited a significantly higher porosity as well as career mobility in comparison with the monometallic Fe−BTC and Cr−BTC systems. The elevated value of mobility has been assigned to the hybrid donor− acceptor/p−n assembly. Our strategy of picking and choosing the appropriate combination of metal ions can be applied to generate a variety of electrically conducting CPs for electronic and electrochemical applications.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: +91(0)20 2590 8215. ORCID
Nirmalya Ballav: 0000-0002-7916-7334 Author Contributions †
B.D. and V.K. contributed equally.
4. MATERIALS AND METHODS 4.1. Materials and Synthesis. BTC, ferric nitrate (Fe), chromium nitrate (Cr), and ethanol were purchased from Sigma-Aldrich (India). A 0.6 mmol (242.4 mg) of ferric nitrate and 0.6 mmol (126.0 mg) BTC each were dissolved in 4 mL ethanol and subsequently mixed together with sonication to form the Fe−BTC gel.19 Similarly, the Cr−BTC gel was prepared by dissolving BTC (42.4 mg, 0.2 mmol) and chromium nitrate (80.0 mg, 0.2 mmol) in ethanol (0.5 mL each). Both the solutions were mixed together with sonication and kept at 80 °C for 4 h. The Fe−BTC−Cr gel was prepared in a similar procedure in which 0.6 mmol of mixed metal (ferric nitrate/chromium nitrate = 1:1) and 0.6 mmol of BTC were dissolve in 4 mL ethanol solvent and both solution were rapidly mixed together and kept at room temperature. 4.2. Characterization. FTIR spectra were recorded in a Nicolet 6700 spectrophotometer using KBr pellet. Solid-state UV−vis spectra were measured with a PerkinElmer Lambda 900 spectrophotometer equipped with an integrating sphere in the 250−800 nm range. The Kubelka−Munk function was used to convert the reflectance spectra into the equivalent absorption spectra using the reflectance of BaSO4 as reference. FESEM imaging and EDXS analysis were performed in a Carl-Zeiss Ultra microscope. The TEM images were recorded in Philips CM200 instrument. The XPS spectra of powder samples were obtained in Phi 5000 VersaProbe-II instrument equipped with a monochromatic Al Kα (1486.6 eV) X-ray source and a hemispherical electron analyzer. The dc conductivity values were measured by the four-probe technique using Keithley 6221 alternating current and dc current sourcemeter and 2182A Nanovoltmeter. The PXRD patterns of all xerogels were recorded from 2θ = 5 to 40° at room temperature in Bruker D8 advanced diffractometer. The magnetic measurements of powder samples were performed in MPMS XL SQUID instrument from Quantum Design. The Hall effect measurements were performed using the five-probe method in Quantum Design-PPMS-Evercool II instrument at room temperature. All of the xerogel materials were pressed into bar-shaped pellets. All of the contacts between the probe and pellet were made by the connecting gold wires. EIS measurements have been recorded in PARSTAT MC Potentiostat, PMC-2000 with a two-electrode setup. Furthermore, the pressed pellets were clipped between copper disks assembly, and the impedance measurements were performed in 1−100 MHz frequency range.
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS Financial supports from IISER Pune, SERB (Project No. EMR/ 2016/001404), and MHRD (Project CORESUM) are thankfully acknowledged. B.D. thanks CSIR (India) for Senior Research Fellowship and V.K. thanks DST (India) for INSPIRE Fellowship. The authors thank Dr. Surjeet Singh and Prachi Telang for PPMS facility and their help during the Hall effect measurements; Dr. Sunil Nair (IISER Pune) for magnetic measurements; and Dr. Pramod Pillai (IISER Pune) for solidstate UV−vis data.
■
REFERENCES
(1) Steiner, T. D. Semiconductor Nanostructures for Optoelectronic Applications; Artech House: Boston, 2004; pp 5−36. (2) Yu, C.; Choi, K.; Yin, L.; Grunlan, J. C. Light-Weight Flexible Carbon Nanotube Based Organic Composites with Large Thermoelectric Power Factors. ACS Nano 2011, 5, 7885−7892. (3) Yaghi, O. M.; O’Keeffe, M.; Ockwig, N. W.; Chae, H. K.; Eddaoudi, M.; Kim, J. Reticular Synthesis and the Design of New Materials. Nature 2003, 423, 705−714. (4) Furukawa, H.; Cordova, K. E.; O’Keeffe, M.; Yaghi, O. M. The Chemistry and Applications of Metal−Organic Frameworks. Science 2013, 341, No. 1230444. (5) Sun, L.; Campbell, M. G.; Dincă, M. Electrically Conductive Porous Metal−Organic Frameworks. Angew. Chem., Int. Ed. 2016, 55, 3566−3579. (6) Talin, A. A.; Centrone, A.; Ford, A. C.; Foster, M. E.; Stavila, V.; Haney, P.; Kinney, R. A.; Szalai, V.; El Gabaly, F.; Yoon, H. P.; Léonard, F.; Allendorf, M. D. Tunable Electrical Conductivity in Metal−Organic Framework Thin-Film Devices. Science 2014, 343, 66−69. (7) Guo, Z.; Panda, D. K.; Maity, K.; Lindsey, D.; Parker, T. G.; Albrecht-Schmitt, T. E.; Barreda-Esparza, J. L.; Xiong, P.; Zhou, W.; Saha, S. Modulating the Electrical Conductivity of Metal-Organic Framework Films with Intercalated Guest π-Systems. J. Mater. Chem. C 2016, 4, 894−899. (8) Liu, J.; Wächter, T.; Irmler, A.; Weidler, P. G.; Gliemann, H.; Pauly, F.; Mugnaini, V.; Zharnikov, M.; Wöll, C. Electric Transport Properties of Surface-Anchored Metal−Organic Frameworks and the Effect of Ferrocene Loading. ACS Appl. Mater. Interfaces 2015, 7, 9824−9830. (9) Neumann, T.; Liu, J.; Wächter, T.; Friederich, P.; Symalla, F.; Welle, A.; Mugnaini, V.; Meded, V.; Zharnikov, M.; Wöll, C.; Wenzel, W. Superexchange Charge Transport in Loaded Metal Organic Frameworks. ACS Nano 2016, 10, 7085−7093. 4492
DOI: 10.1021/acsomega.7b00931 ACS Omega 2017, 2, 4488−4493
ACS Omega
Article
Cd2(TTFTB) as a Case Study. J. Am. Chem. Soc. 2016, 138, 14772− 14782. (27) Kirtley, J. R.; Mannhart, J. Organic Electronics: When TTF Met TCNQ. Nat. Mater. 2008, 7, 520−521. (28) Cohen, M. J.; Coleman, L. B.; Garito, A. F.; Heeger, A. J. Electrical Conductivity of Tetrathiofulvalinium Tetracyanoquinodimethan (TTF) (TCNQ). Phys. Rev. B 1974, 10, 1298−1307. (29) Lüssem, B.; Riede, M.; Leo, K. Doping of Organic Semiconductors. Phys. Status Solidi A 2013, 210, 9−43. (30) Xu, X.; Randorn, C.; Efstathiou, P.; Irvine, J. T. S. A Red Metallic Oxide Photocatalyst. Nat. Mater. 2012, 11, 595−598. (31) Nagaosa, N.; Sinova, J.; Onoda, S.; MacDonald, A. H.; Ong, N. P. Anomalous Hall effect. Rev. Mod. Phys. 2010, 82, 1539−1592. (32) Dhara, B.; Tarafder, K.; Jha, P. K.; Panja, S. N.; Nair, S.; Oppeneer, P. M.; Ballav, N. Possible Room-Temperature Ferromagnetism in Self-Assembled Ensembles of Paramagnetic and Diamagnetic Molecular Semiconductors. J. Phys. Chem. Lett. 2016, 7, 4988−4995. (33) Lee, D. Y.; Kim, E.-K.; Shrestha, N. K.; Boukhvalov, D. W.; Lee, J. K.; Han, S.-H. Charge Transfer-Induced Molecular Hole Doping into Thin Film of Metal−Organic Frameworks. ACS Appl. Mater. Interfaces 2015, 7, 18501−18507. (34) Narayan, T. C.; Miyakai, T.; Seki, S.; Dincă, M. High Charge Mobility in a Tetrathiafulvalene-Based Microporous Metal−Organic Framework. J. Am. Chem. Soc. 2012, 134, 12932−12935. (35) Huang, X.; Sheng, P.; Tu, Z.; Zhang, F.; Wang, J.; Geng, H.; Zou, Y.; Di, C.-a.; Yi, Y.; Sun, Y.; Xu, W.; Zhu, D. A Two-Dimensional π-d Conjugated Coordination Polymer with Extremely High Electrical Conductivity and Ambipolar Transport Behaviour. Nat. Commun. 2015, 6, No. 7408. (36) Liu, J.; Zhou, W.; Liu, J.; Howard, I.; Kilibarda, G.; Schlabach, S.; Coupry, D.; Addicoat, M.; Yoneda, S.; Tsutsui, Y.; Sakurai, T.; Seki, S.; Wang, Z.; Lindemann, P.; Redel, E.; Heine, T.; Wöll, C. Photoinduced Charge-Carrier Generation in Epitaxial MOF Thin Films: High Efficiency as a Result of an Indirect Electronic Band Gap? Angew. Chem., Int. Ed. 2015, 54, 7441−7445. (37) Nikitenko, V. R.; Tameev, A. R.; Vannikov, A. V. Mechanism of Enhanced Mobility and Conductivity at Donor−Acceptor Organic Interfaces. Org. Electron. 2011, 12, 589−594. (38) Chen, Y. Z.; Trier, F.; Wijnands, T.; Green, R. J.; Gauquelin, N.; Egoavil, R.; Christensen, D. V.; Koster, G.; Huijben, M.; Bovet, N.; Macke, S.; He, F.; Sutarto, R.; Andersen, N. H.; Sulpizio, J. A.; Honig, M.; Prawiroatmodjo, G. E. D. K.; Jespersen, T. S.; Linderoth, S.; Ilani, S.; Verbeeck, J.; Van Tendeloo, G.; Rijnders, G.; Sawatzky, G. A.; Pryds, N. Extreme Mobility Enhancement of Two-Dimensional Electron Gases at Oxide Interfaces by Charge-Transfer-Induced Modulation Doping. Nat. Mater. 2015, 14, 801−806. (39) Sun, L.; Hendon, C. H.; Park, S. S.; Tulchinsky, Y.; Wan, R.; Wang, F.; Walsh, A.; Dincă, M. Is Iron Unique in Promoting Electrical Conductivity in MOFs? Chem. Sci. 2017, 8, 4450−4457. (40) Wang, T. C.; Hod, I.; Audu, C. O.; Vermeulen, N. A.; Nguyen, S. T.; Farha, O. K.; Hupp, J. T. Rendering High Surface Area, Mesoporous Metal−Organic Frameworks Electronically Conductive. ACS Appl. Mater. Interfaces 2017, 9, 12584−12591. (41) Jha, P. K.; Singh, S. K.; Gatla, S.; Mathon, O.; Kurungot, S.; Ballav, N. Pb2+−N Bonding Chemistry: Recycling of Polyaniline−Pb Nanocrystals Waste for Generating High-Performance Supercapacitor Electrodes. J. Phys. Chem. C 2016, 120, 911−918.
(10) Dhara, B.; Nagarkar, S. S.; Kumar, J.; Kumar, V.; Jha, P. K.; Ghosh, S. K.; Nair, S.; Ballav, N. Increase in Electrical Conductivity of MOF to Billion-Fold upon Filling the Nanochannels with Conducting Polymer. J. Phys. Chem. Lett. 2016, 7, 2945−2950. (11) Wang, T.; Farajollahi, M.; Henke, S.; Zhu, T.; Bajpe, S. R.; Sun, S.; Barnard, J. S.; Lee, J. S.; Madden, J. D. W.; Cheetham, A. K.; Smoukov, S. K. Functional Conductive Nanomaterials via Polymerisation in Nano-Channels: PEDOT in a MOF. Mater. Horiz. 2017, 4, 64−71. (12) Sun, L.; Hendon, C. H.; Minier, M. A.; Walsh, A.; Dincă, M. Million-Fold Electrical Conductivity Enhancement in Fe2(DEBDC) versus Mn2(DEBDC) (E = S, O). J. Am. Chem. Soc. 2015, 137, 6164− 6167. (13) Sun, L.; Miyakai, T.; Seki, S.; Dincă, M. Mn 2 (2,5disulfhydrylbenzene-1,4-dicarboxylate): A Microporous Metal−Organic Framework with Infinite (−Mn−S−)∞ Chains and High Intrinsic Charge Mobility. J. Am. Chem. Soc. 2013, 135, 8185−8188. (14) Yan, L.; Shen, L.; Lv, M.; Yu, W.; Chen, J.; Wang, S.; Fu, X.; Ye, Z. Self-healing supramolecular heterometallic gels based on the synergistic effect of the constituent metal ions. Chem. Commun. 2015, 51, 17627−17629. (15) Dikhtiarenko, A.; Villanueva-Delgado, P.; Valiente, R.; García, J.; Gimeno, J. Tris(bipyridine)Metal(II)-Templated Assemblies of 3D Alkali-Ruthenium Oxalate Coordination Frameworks: Crystal Structures, Characterization and Photocatalytic Activity in Water Reduction. Polymers 2016, 8, 48−68. (16) Mahmood, A.; Xia, W.; Mahmood, N.; Wang, Q.; Zou, R. Hierarchical Heteroaggregation of Binary Metal−Organic Gels with Tunable Porosity and Mixed Valence Metal Sites for Removal of Dyes in Water. Sci. Rep. 2015, 5, No. 10556. (17) Sotnik, S. A.; Polunin, R. A.; Kiskin, M. A.; Kirillov, A. M.; Dorofeeva, V. N.; Gavrilenko, K. S.; Eremenko, I. L.; Novotortsev, V. M.; Kolotilov, S. V. Heterometallic Coordination Polymers Assembled from Trigonal Trinuclear Fe2Ni-Pivalate Blocks and Polypyridine Spacers: Topological Diversity, Sorption, and Catalytic Properties. Inorg. Chem. 2015, 54, 5169−5181. (18) Dolgopolova, E. A.; Brandt, A. J.; Ejegbavwo, O. A.; Duke, A. S.; Maddumapatabandi, T. D.; Galhenage, R. P.; Larson, B. W.; Reid, O. G.; Ammal, S. C.; Heyden, A.; Chandrashekhar, M.; Stavila, V.; Chen, D. A.; Shustova, N. B. Electronic Properties of Bimetallic Metal− Organic Frameworks (MOFs): Tailoring the Density of Electronic States through MOF Modularity. J. Am. Chem. Soc. 2017, 139, 5201− 5209. (19) Xiang, S.; Li, L.; Zhang, J.; Tan, X.; Cui, H.; Shi, J.; Hu, Y.; Chen, L.; Su, C.-Y.; James, S. L. Porous organic−inorganic hybrid aerogels based on Cr3+/Fe3+ and rigid bridging carboxylates. J. Mater. Chem. 2012, 22, 1862−1867. (20) Alloway, B. Essentials of Medical Geology: Impacts of the Natural Environment on Public Health; Elsevier/Academic Press, 2005; pp 347−372. (21) Riou-Cavellec, M.; Albinet, C.; Grenèche, J.-M.; Férey, G. Study of the iron/trimesic acid system for the hydrothermal synthesis of hybrid materials. J. Mater. Chem. 2001, 11, 3166−3171. (22) Garai, B.; Mallick, A.; Banerjee, R. Photochromic metal−organic frameworks for inkless and erasable printing. Chem. Sci. 2016, 7, 2195−2200. (23) Garai, B.; Mallick, A.; Das, A.; Mukherjee, R.; Banerjee, R. SelfExfoliated Metal−Organic Nanosheets through Hydrolytic Unfolding of Metal−Organic Polyhedra. Chem. − Eur. J. 2017, 23, 7361−7366. (24) Mao, Y.; Li, J.; Cao, W.; Ying, Y.; Hu, P.; Liu, Y.; Sun, L.; Wang, H.; Jin, C.; Peng, X. General Incorporation of Diverse Components Inside Metal−Organic Framework Thin Films at Room Temperature. Nat. Commun. 2014, 5, No. 5532. (25) Moulder, J. F.; Chastain, J. Handbook of X-ray Photoelectron Spectroscopy: A Reference Book of Standard Spectra for Identification and Interpretation of XPS Data; Physical Electronics Division, Perkin-Elmer Corporation, 1992. (26) Sun, L.; Park, S. S.; Sheberla, D.; Dincă, M. Measuring and Reporting Electrical Conductivity in Metal−Organic Frameworks: 4493
DOI: 10.1021/acsomega.7b00931 ACS Omega 2017, 2, 4488−4493
