Valence Dependent Electrical Conductivity in a 3D Tetrahydroxyquinone Based Metal–Organic Framework

Abstract

Electrically conductive metal–organic frameworks (cMOFs) have become a topic of intense interest in recent years due to their great potential in electrochemical energy storage, electrocatalysis and sensing applications. Hitherto most reported cMOFs are 2D structures and 3D cMOFs remain rare. Herein we report a 3D cMOF, namely FeTHQ, synthesized from tetrahydroxy-1,4-quinone (THQ) and Fe(II) sulfate salt. FeTHQ exhibited a conductivity of 3.3 ± 0.55 mS cm⁻¹ at 300 K, which is high for 3D cMOFs. The conductivity for FeTHQ is valence dependent. A higher conductivity was measured with the as-prepared FeTHQ than both the air-oxidized and the sodium naphthalenide-reduced sample.

The recent developments of electrically conductive metal-organic frameworks (cMOFs)¹ have significantly increased the range of applications of MOFs in several areas, including energy storage,² electrocatalysis,³ sensors⁴ and others.⁵ Many works, in the past, have focused on expanding the library of electrically conductive MOFs, yet the majority of these reported cMOFs are layered 2D structures. Extended π-d conjugation over the organic linker and coordination units is often pursued to enable long-range electron delocalization and conductivity in cMOFs and this delocalization is facilitated by the square planar metal sites, thus making 2D cMOFs the most frequently adopted geometry in this class of materials. However, layered 2D structures can have limited surface area and unexposed metal sites due to the stacking of 2D layers. On the contrary, 3D structures may offer 3D porosity and better accessibility.^3e Currently, 3D cMOFs^3e,5a,6 remain rare species in the literature. Hence, developing new platforms for 3D cMOFs will not only enrich the library of c-MOFs, but also provide exciting opportunities for next-generation applications of conductive MOF materials. Herein, we successfully prepared a 3D cMOF (hereafter referred to as FeTHQ), using octahedral FeO₆ coordination between Fe and the ligand, tetrahydroxy-1,4-quinone (THQ). Furthermore, high conductivity (3.3 ± 0.55 mS cm⁻¹ at 300 K) is observed for this 3D cMOF. Since both Fe and the THQ ligand have more than one valence state, the main conduction mechanism for FeTHQ is hypothesized to be redox hopping, which is the origin of high conductivities in several mixed valence 3D MOFs.^5a,7

FeTHQ was prepared by a solvothermal synthesis at 80 °C, using dimethylformamide (DMF) and water as the co-solvent (Figure 1a; Supporting Information Methods). Specifically, Iron (II) sulfate and THQ were reacted to form the dark navy powders. Recently, another solvothermal synthesis of FeTHQ was demonstrated using water and diethylene glycol as co-solvent at 120°C.⁸ The structure model for FeTHQ was obtained from powder X-ray diffraction (PXRD) data and refined by Rietveld refinement (Figure 2a, Figure S1, Table S1) and the final structure (Figure 1b–e) has a cubic unit cell with axis length of 10.5457(2) Å and a space group of $\text{Pm}\overline{3}$. For each unit cell (Figure 1c–e), the chemical formula for the main framework is determined to be Fe₈(C₆O₆)₆, where C₆O₆ stands for the deprotonated THQ ligand. Slightly different from the recently reported 3D M-THQ structure,⁸ the counter cations in the pores are not identified in our structure. As a result, Fe content is obviously lower in our system (Table S2). In our reported CIF, two oxygen sites with large atomic displacement parameters, Ow1 and Ow2 (Figure S2a), which are omitted for simplicity in Figure 1, represent the diffusive electron density of solvent molecules located in the pores (Figure S2). Extended X-Ray absorption fine structure (EXAFS) was used to confirm the local coordination of FeTHQ. The fitted EXAFS (Figure 2b, Figure S4, Table S3) from our structural model agrees very well with the experimental data, thus confirming the validity of our structure model.

Figure 1.

(a) The synthetic scheme for FeTHQ; (b) the Fe octamer and (c) the unit cell structure of FeTHQ; (d) the unit cell structure of FeTHQ with extended THQ ligand; (e) expanded framework structure for FeTHQ.

Figure 2.

(a) Rietveld refinement of FeTHQ PXRD displaying the observed pattern (black) (λ = 0.4481 Å), calculated pattern (red), difference plot (gray); (b)The Fourier transform (red) and the magnitude of the real part (blue) for Fe K-edge EXAFS of FeTHQ; (c) SEM of FeTHQ powders; (d) N₂ sorption isotherm of FeTHQ, degassed at 60°C for 2 hours followed by 100°C for 8 hours.

Scanning electron microscope (SEM) images (Figure 2c) and transmission electron microscope (TEM) images (Figure S5) show that FeTHQ particles have spherical morphology with the diameter ranging from 150 nm to over 1 μm with substantial interparticle growth. The N₂ adsorption isotherm of FeTHQ degassed at 100 °C (Figure 2d, Figure S6) shows a BET surface area of ~163 m² g⁻¹ and the pore size distribution matches the crystal structure (Figure S7). The relative low BET surface area is likely caused by the severe interparticle growth which would limit access to the pores and high content of absorbed solvent molecules in the pores.

With the crystal structure confirmed, the electronic structure and electrical properties of FeTHQ were further investigated. Density-functional theory (DFT) calculations were carried out to understand the band structure of this 3D MOF. The band-structure calculations were performed using the projected augmented wave (PAW) method⁹ as implemented in the Vienna Ab Initio Simulation Package (VASP)¹⁰ with further information available in Supporting Information (Methods). The results indicate that the majority of bands located near the Fermi energy is contributed by the Fe and O atoms with a non-vanishing density of states at the Fermi level. Visualization of the charge density distribution reveals localization of charge around the molecular framework with limited extension towards the porous vacuum space. Although DFT calculations often underestimate the bandgap for many materials, the simulated band structure and density of states suggest FeTHQ should be conductive with a small bandgap, or gapless.

UV-vis-NIR diffuse reflectance spectroscopy on FeTHQ gave an estimated bandgap of 0.79 eV (Figure 4a). The highest occupied molecular orbital (HOMO) level of 5.23 eV was obtained by photoemission spectroscopy in air (PESA; Figure 4b). Additionally, the conductivity of FeTHQ was measured on pressed pellets (crystallinity preserved during pressing, Figure S8) by four-point probe method. The value exhibits a positive temperature coefficient with an activation energy of 270 meV in the range of 240 to 340 K and a smaller activation energy of 150 meV from 200 to 240 K. The relatively large activation energy is ascribed to the thermally-activated hopping progress at grain boundaries, which dominates the temperature dependence of conductivity in the bulk polycrystalline pellets.^5a,5d,7,11 At 300 K, FeTHQ exhibited an averaged conductivity of 3.3 ± 0.55 mS cm⁻¹ (Figure 5a). This conductivity is quite remarkable in comparison to the reported conductivities of other 3D cMOFs, most of which are in the range of 10⁻⁶ to 1 S cm⁻¹.^5a,6 This example of using nonplanar coordination geometry to enable 3D long range electron transport in framework structures has meaningful implications for designing other 3D conductive MOFs using transition metals adopting nonplanar coordination geometries such as octahedral, tetrahedral, or even more complicated transition metal clusters.

Figure 4.

(a) Tauc plot (black) of a KBr pellet containing 0.25 wt% FeTHQ deduced from the UV–vis–NIR diffuse reflectance spectrum; (b) PESA of FeTHQ; (c) Variable-temperature conductivity for a ~200 μm thick FeTHQ pellet; (d) Arrhenius fitting of temperature dependent conductivity data.

Figure 5.

(a) Conductivity for FeTHQ samples: FeTHQ—as prepared, FeTHQ_ox—FeTHQ air oxidized for two weeks, FeTHQ_red0.5/2/4—100 mg as prepared FeTHQ reduced with 0.5/2/4 ml 0.2 M freshly prepared NaC₈H₁₀; Mössbauer spectroscopy for (b) FeTHQ, (c) FeTHQ_ox and (d) FeTHQ_red4 at 80 K.

Interestingly, the conductivity of FeTHQ decreased when exposed to ambient air over modest timescales. As shown in Figure 5a, within the same batch of sample, FeTHQ_ox (FeTHQ being exposed to air for 2 weeks) showed a much lower conductivity than the as-prepared FeTHQ. The averaged conductivity dropped from 3.3 ± 0.55 mS cm⁻¹ to 0.66 ± 0.059 mS cm⁻¹, showing a five-fold decrease. PXRD (Figure S9) shows that the diffraction pattern of FeTHQ was unchanged in the two weeks, indicating structural change is not playing an important role in the conductivity change. Since the oxidation state of Fe is expected to have great influence on the electronic properties of the MOF, X-ray photoelectron spectroscopy (XPS) and ⁵⁷Fe Mössbauer spectroscopy were utilized to examine the oxidative state of Fe.

High resolution XPS of Fe (Figure S10a) for FeTHQ displayed a rather broad 2p_3/2 peak at ~710.0 eV, which falls between the range of Fe²⁺ (FeO ~709.5 eV) and Fe³⁺ (Fe₂O₃ ~711 eV) oxides¹² and could be indicative of mixed valence Fe in the MOF. To confirm this, ⁵⁷Fe Mössbauer spectroscopy was carried out. The Mössbauer spectrum for FeTHQ at 80 K (Figure 5b) is composed of at least three species. The dominant species (68.2%, Table S4) has an isomer shift of 0.54 mm/s and quadrupole splitting of 0.95 mm/s. These values combined with the observed magnetic hyperfine splitting in the 6 K Mössbauer spectrum (Figure S11) align with a ferric species,¹³ with the isomer shift and relatively low quadrupole splitting consistent with a S=5/2 high-spin ferric Fe site. The second fitted species (28.5%, Table S4) has an isomer shift of 1.10 mm/s and quadrupole splitting of 2.45 mm/s, consistent with an S=2 high-spin ferrous species.¹³ Another minor ferrous peak (3.3%) is also observed. The Mössbauer spectrum of the filtrate control sample (Figure S12), e.g. excess Fe in the solvothermal synthesis, indicate that the species observed in the MOF are not unreacted ferrous sulfate precursor species in the solvent. Assuming that there are no other counter ions in the as prepared framework, THQ ligand is calculated to carry 3.56 negative charges on average instead of 4 from charge neutrality, suggesting that THQ also exhibits mixed valence characteristics. Therefore, FeTHQ is identified to be a dual mixed valence system.

After air oxidation, the Fe 2p_3/2 XPS spectra (Figure S10a) shifted slightly towards higher binding energy, exhibiting a peak at 710.5 eV. This suggests that Fe in the MOF turned to higher oxidation state during the air exposure process. Again, ⁵⁷Fe Mössbauer spectroscopy was used to analyze the composition of Fe in FeTHQ_ox sample (Figure 5c). The Mössbauer spectrum for FeTHQ_ox is also fitted by three species. The dominant species with ferric parameters is 85.9% of the spectrum and the two ferrous species together are only 14.1% (Table S4). It appears that the ferrous component is converted to the ferric component in the presence of oxygen. Air oxidation lead to an even more unbalanced Fe³⁺/Fe²⁺ ratio, which makes the redox hopping less likely to happen and leads to a drop in the mobility of electrons. In addition, FeTHQ was recently reported to exhibit n-type behavior,⁸ suggesting electrons are the majority carriers in this framework. The oxidation would result in a drop of electron density in the framework. These two factors together may have caused the large drop in conductivity. While the conversion of ferrous Fe to ferric Fe is concomitant with a loss in conductivity, subtle electronic changes in the coordination environment around Fe may have contributed to this conductivity change as well. The minor differences in C1s and O1s XPS (Figure S10) between the as prepared and air oxidized FeTHQ samples indicate that the ligand is also somewhat oxidized during the oxidation.

Since redox hopping is essential for FeTHQ’s high conductivity, higher conductivity can be realized with a more balanced Fe³⁺/Fe²⁺ ratio. FeTHQ was reduced with sodium naphthalenide (Figure 5a,d; Supporting Information Methods).¹⁴ PXRD on the reduced samples showed no obvious structural change for the framework in the reduction process (Figure S13). In the most reduced sample, “FeTHQ_red4”, the ferric species is 42.8 % of the spectrum and the ferrous species is 57.2 % (Figure 5d, Table S4). Surprisingly, the conductivity of FeTHQ decreases with reduction (Figure 5a), although a more balanced Fe³⁺/Fe²⁺ ratio is achieved. The reason for the conductivity drop is likely due to the heterogenous nature of the reduction reaction (FeTHQ solid reacting with sodium naphthalenide solution) and FeTHQ’s limited pore volume and surface area. Fe sites within the particles are less accessible to reduction and the surface layer of each FeTHQ particle would get reduced first and more easily. This heterogenous reduction produced a highly reduced surface layer for FeTHQ particles, which lacks in spatial distribution the balance between different valence states and therefore does not facilitate redox hopping. These reduction studies and the reduction of FeTHQ_ox (Figure S14, 15) provide proof that redox hopping is the primary conduction mechanism for FeTHQ and a balance between different valence states that have an even intraparticle distribution is desired for high conductivity.

In this work, we presented a mixed valence cMOF, FeTHQ, with high electrical conductivity in the cubic 3D framework. Its conductivity is found to be sensitive to valence change. The high conductivity from FeTHQ demonstrates that square planar conjugation is not mandatory for achieving efficient long-range charge transport and it is possible to build 3D conductive MOFs through nonplanar coordination geometries. With high conductivity and rich redox chemistry (Figure S16), FeTHQ may demonstrate potential in electrochemical applications such as supercapacitors and batteries. The relevant studies are under way.

Figure 3.

(a) The band-structure and element-projected density of states and (b) associated charge density distribution of FeTHQ.