Electroactive Ferrocene at or near the Surface of Metal–Organic

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Electroactive Ferrocene at or near the Surface of Metal−Organic Framework UiO-66 Rebecca H. Palmer,† Jian Liu,† Chung-Wei Kung,† Idan Hod,§ Omar K. Farha,†,‡,∥ and Joseph T. Hupp*,† †

Department of Chemistry and ‡Department of Chemical and Biological Engineering, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States § Department of Chemistry, Ben-Gurion University of the Negev, Beer-Sheva 8410501, Israel ∥ Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia S Supporting Information *

ABSTRACT: Here, we describe the installation of a ferrocene derivative on and within the archetypal metal−organic framework (MOF), UiO-66, by solvent-assisted ligand incorporation. Thin films of the resulting material show a redox peak characteristic of the Fc/Fc+ couple, as measured by cyclic voltammetry. Consistent with restriction of redox reactivity solely to Fc molecules sited at or near the external surfaces of MOF crystallites, chronoamperometry measurements indicate that less than 20% of the installed Fc molecules are electrochemically active. Charge-transport diffusion coefficients, DCT, of 6.1 ± 0.8 × 10−11 and 2.6 ± 0.2 × 10−9 cm2/ s were determined from potential step measurements, stepping oxidatively and reductively, respectively. The 40-fold difference in DCT values contrasts with the expectation, for simple systems, of identical values for oxidation-driven versus reduction-driven charge transport. The findings have implications for the design of MOFs suitable for delivery of redox equivalents to framework-immobilized electrocatalysts and/or delivery of charges from a chromophoric MOF film to an underlying electrode, processes that may be central to MOF-facilitated conversion of solar energy to chemical or electrical energy.

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INTRODUCTION Metal−organic frameworks (MOFs) are designable materials due to the tunability and variety of organic linkers and metal nodes used in MOF construction. Because of their porosity, along with the many ways to modify them after synthesis, MOFs have been explored for numerous candidate applications, including chemical sensing, gas storage and release, molecule-derived electronics, and catalysis.1−4 For applications such as electrocatalysis or for use in electronics, charges need to move easily.2,5,6 However, because of: (a) typically poor matching of electronic energies between linkers and nodes,7,8 (b) the redox inaccessibility of many MOF components,9 especially in the presence of solvent, (c) the typically localized nature of MOF electronic states, or equivalently (d), the typical absence of conventional electronic bands (i.e., valence and conduction bands of appreciable width, as opposed to filled and empty electronic states confined to single linkers or nodes), most MOFs are not electronically conductive under ordinary conditions. Indeed, only a handful of examples of significantly electronically conductive, porous MOFs have been reported.8,10−15 Another approach to moving charges through MOFs is to engender charge hopping by introducing energetically accessible redox centers.16−21 One example © XXXX American Chemical Society

involves the incorporation of a ruthenium tris(bipyridine)containing linker into a known MOF structure.17,22,23 Other linker-based routes to moving charges through MOFs, while retaining porosity, include the use of metalloporphyrins,18,19 tetraphenylpyrene,24 or diimides25 as linkers. Previously, we have reported on solvent-assisted ligand incorporation (SALI)26 whereby desired nonstructural molecules, offering specific functionalities, can be grafted onto suitably chosen MOF nodes.27−31 SALI enables the selection of a preferred framework, followed by functionalization, in contrast to relying on the characteristics of the MOF building blocks themselves (i.e., linkers or nodes). We reported using SALI to incorporate the well-studied, redox-active molecule, ferrocene, into the MOF NU-1000, which enabled charge transport throughout a film of this material.27,28 NU-1000 has large (30 Å diameter) channels that are able to facilitate diffusion of solvent and electrolyte throughout a film. Notably, for films of thicknesses studied thus far (i.e., microns), all of the installed ferrocenes are electrochemically addressable.27,32 Received: November 7, 2017 Revised: March 19, 2018

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Figure 1. (a) Truncated structure of UiO-66, (b) proposed binding of 1-propenoic-ferrocene to a linker vacancy on the node of UiO-66, (c) Zr6O4(OH)4 node cluster of UiO-66 with protons omitted and all linkers removed, (d) UiO-66 linker, benzene-1,4-dicarboxylate, and (e) 1propenoate-ferrocene.

In contrast to NU-1000, UiO-66 has apertures of ∼6 Å. UiO66 features hexa-zirconium(IV)oxo, hydroxo nodes that are nominally 12-connected via benzene dicarboxylate linkers.33 Depending on synthesis conditions, varying fractions of framework defects are present in UiO-66, with “missing linkers” typically being the dominant form of defects.34−36 These defects leave open sites on nodes where a molecule with a carboxylate tether, such as a ferrocene derivative, can bind. Given the small apertures, it is conceivable that the supporting electrolyte is largely excluded from the ferrocene-functionalized MOF crystallites and that attempts to oxidize interior ferrocene units could be constrained by the rate and/or degree to which charge-compensating anions can be drawn into the MOF. Here, we installed 1-propenoate-ferrocene (Fc-db-COO−), via SALI, into UiO-66 to investigate this idea and the closely related concept of redox hopping in this well-known and robust material (Figure 1).

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with DMF, at 2 h soaking intervals, followed by soaking the sample in DMF overnight. The solid was then washed with acetone, using the same procedure, followed by heating at 80 °C in a vacuum oven. Notably, this synthesis yields a version of UiO-66 that is rich in missing-linker type defects.35 Synthesis of 1-propenoic-ferrocene was carried out following a method reported by Galangau.38 Into a 250 mL round bottom flask, ferrocenecarboxaldehyde (1 g, 0.005 mmol), malonic acid (483 mg, 0.005 mmol), and piperidine (400 mg, 0.005 mmol) were added, with 100 mL of pyridine as the solvent. The mixture was purged with nitrogen before refluxing for 2 h. After cooling to room temperature, the mixture was diluted with 100 mL of 2 M NaOH (8 g in 100 mL of H2O) and was stirred overnight. Concentrated HCl was added slowly until a precipitate formed. Chloroform was then added, and the product was extracted. The chloroform layer was collected and then dried over MgSO4. The solvent was removed by rotovap, leaving a red powder (1H NMR, Figure S1). Ferrocenecarboxylic acid (Fc-COOH) and 1-propenoic-ferrocene (Fc-db-COOH) were installed separately into activated UiO-66 via SALI.26 A 3 mL solution of 0.07 M ferrocene derivative in DMF was placed into a 1.5 dram vial, and 32 mg of UiO-66 was added. The mixture was heated at 60° C in an oil bath for 24 h. The solid was then washed with DMF (3× or until colorless, then overnight) and acetone (3×, then overnight) with at least 2 h between washings. Film Deposition. Electrophoretic deposition, as reported by Hod and co-workers, was used to deposit UiO-66 and UiO-66-Fc onto cleaned fluorine-doped tin oxide (FTO) glass (7 Ω, 2.2 mm, Hartford Glass Company Inc.).39,40 FTO was prepared by cutting to 1.25 × 2 cm2 and then sonicating sequentially in soapy water, ethanol, and acetone for 20 min each, followed by drying under a stream of nitrogen, then placing in an oven at 90 °C for at least 10 min. For electrophoretic deposition, a suspension of 10 mg of MOF sample was sonicated in 20 mL of toluene for 5−10 min. Using alligator clips, two pieces of FTO were aligned 1 cm apart, with conductive sides facing toward each other. A power supply was used to provide a DC voltage of 136 V between the FTO pieces. Films were deposited for 3 h to achieve a uniform film. When the toluene suspension settled, the solution was again suspended by sonicating for 5 min.

MATERIALS AND METHODS

Chemicals. Chemicals were received without further purification from the following. Aldrich: ferrocenecarboxylic acid (97%), terephthalic acid (H2BDC, 98%), ferrocenecarboxaldehyde (98%), malonic acid (99%), and piperidine (99%); Sigma: hydrogen peroxide (H2O2, 30% (w/w) in H2O (with stabilizer)), TCI America: tetrabutylammonium hexafluorophosphate (TBAPF6, 98%); Fisher: n,n-dimethylformamide (DMF, 99.8%), acetone (≥99.5%), hydrochloric acid (HCl, 36.5−38.0%), zirconium chloride (ZrCl4, 98%), sodium hydroxide (NaOH, ≥97%), sulfuric acid (H2SO4, 95.0− 98.0%), and potassium chloride (KCl, 99%); and Northwestern University Facilities: deionized water and nitrogen. Sample Preparation. UiO-66 was synthesized by following a literature procedure.37 Briefly, in an 8 dram vial, ZrCl4 (125 mg, 0.54 mmol) and DMF (5 mL) were mixed, followed by addition of HCl (1 mL) and 20 min of sonication. Then, H2BDC (123 mg, 0.74 mmol) was added along with DMF (10 mL) and the mixture was sonicated again, until clear. The vial was then transferred to the oven and kept at 80 °C overnight. After cooling, the supernatant was decanted and the white solid was transferred to centrifuge tubes and washed three times B

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Figure 2. SEM top view and cross-sectional view of UiO-66-Fc-db-COO films before electrochemistry.

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Safety note: Extreme caution should be taken to avoid directly contacting the pieces of FTO with each other; contact may yield sparks and, in turn, ignite the toluene. Electrochemical Measurements. Cyclic voltammetry and chronoamperometry measurements were made with a Modulab instrument (Solartron Analytical). Electrochemical measurements were made in a cell that consisted of a glass vial with a cap modified for purging with nitrogen. To the cell, 10 mL of 0.1 M KCl (aq) or 0.1 M TBAPF6 in acetonitrile was added. The solution was purged for 20 min with N2 before electrochemical measurements, as dissolved O2 is known to degrade ferrocenium.41 A silver/silver chloride reference electrode filled with an aqueous saturated KCl solution was used. Films of electrophoretically deposited UiO-66 with installed ferrocene on FTO were used as the working electrode with a platinum wire counter electrode. Current Integration. For integration of chronoamperometry signals, each current value was multiplied by the time interval measured over and summed (like a Riemann sum) to arrive at the total charge. Dividing the total charge by the Faraday constant yielded in the number moles of electroactive ferrocene. Materials Characterization. After synthesis, samples were activated on a Smart VacPrep system (Micromeritics) at 120° C under vacuum for ∼18 h. Immediately following sample activation, N2 isotherms (77 K) were measured by using a Tristar II 3020 instrument (Micromeritics) instrument. From the isotherm data, Brunauer− Emmett−Teller (BET) surface areas were obtained. X-ray photoelectron spectroscopy (XPS) measurements were carried out at the KECKII/NUANCE facility at NU on a Thermo Scientific ESCALAB 250 Xi (Al Kα radiation, hν = 1486.6 eV) equipped with an electron flood gun. XPS data were analyzed using Thermo Scientific Avantage Data System software, and all spectra were referenced to the C 1s peak (284.8 eV). Inductively coupled plasma-optical emission spectroscopy (ICP-OES) was measured on a Thermo iCAP 7600 ICP-OES instrument (Thermo Fisher Scientific) and was used to quantify the amount of ferrocene present in UiO-66 after SALI. Approximately 3 mg of each sample was individually microwave-digested in 1 mL of piranha solution, 3:1 H2SO4/H2O2 (H2O2 30 wt % in H2O). Safety note: Piranha solution is highly caustic. When working with piranha solution, care must be taken, and it is essential that proper PPE is worn (e.g., gloves, goggles, and lab coat). The digested solution was then diluted to 10 mL using Millipore water and was measured using a Thermo iCAP 7600 ICP-OES (Thermo Fisher Scientific). Wavelengths for Zr (343.823, 339.198, and 327.305) and Fe (238.204, 239.562, and 240.488) were analyzed radially for the samples and standard solutions. Scanning electron microscopy (SEM) images were obtained on either an S4800-II cFEG SEM or an SU8030 SEM (Hitachi). Prior to SEM imaging, samples were coated with approximately 8 nm of osmium by an Osmium Coater (SPI). A SmartLab (Rigaku) instrument was used to collect powder X-ray diffraction (XRD) patterns.

RESULTS After installing the 1-propenoic-ferrocene by SALI, the resulting material, UiO-66-Fc-db-COO, was determined, via ICP-OES measurements, to contain an average of 1.3 ± 0.3 1-propenoicferrocene molecules per MOF node. From XPS measurements, we observed 4.5 ferrocene molecules per node (Figure S2, Table S1), indicating higher loading at the exterior surface of the UiO-66 crystallites. Loading at this level also implies a higher-than-average density of missing-linker defects, with nodes having as few as seven or eight connecting ligands (compared with 12 for an ideal, defect-free node.) The UiO-66Fc-db-COO material remained crystalline, as shown by XRD patterns (Figure S3). As expected, the Brunauer−Emmett− Teller (BET) surface area of UiO-66-Fc-db-COO (910 m2/g) is somewhat less than that of the parent material (1620 m2/g, on the basis of N2 adsorption at 77 K) due to greater crystallite mass and due to ferrocene filling micropores, indicated by a decrease in the microporous step of the N2 isotherm (see Figure S4, Table S2). Ferrocene-functionalized UiO-66 samples were electrophoretically deposited onto fluorine-doped tin oxide (FTO), as described above. Top and cross-sectional view scanning electron microscope (SEM) images of UiO-66-Fc films are shown in Figure 2. Films of electrophoretically deposited UiO66 with installed ferrocene were approximately 8 μm thick from SEM cross sections before electrochemistry and 3−6 μm thick after electrochemical measurements (Figure S5). To determine the extent to which installed ferrocene is electrochemically addressable, cyclic voltammograms (CVs) of the electrophoretically deposited films were recorded (in 0.1 M KCl (aq) at a 10 mV/s scan rate after purging with N2 to remove dissolved oxygen). As expected, unmodified UiO-66 films show no peaks over the scanned range of potentials.42 In contrast, for films of UiO-66-Fc-db-COO, CVs show redox peaks centered around 0.34 V (Figures 3 and S6), indicating that at least some of the installed ferrocene is electrochemically addressable. Curiously, the CV for UiO-66-Fc-db-COO is unsymmetrical, with the oxidation wave being flattened and extended and the reduction wave being sharpened and compressed. The asymmetry is reproducible; after 10 CV scans of a UiO-66-Fc-db-COO, no decrease in current was observed, indicating that the sample is not degrading (Figure 4a).43 Indeed, we see equivalent amounts of charge passed during oxidation and reduction processes during CV (Figure 4b). The asymmetry is reminiscent of the behavior of polyvinyl ferrocene (PVF) films;44,45 we will return to this point in a later C

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approximating the shape of the MOF crystallites as spherical, we estimate that electroactivity (at 22%) is limited to a zone just 6 nm thick and corresponding to only a few layers (i.e., pore lengths or widths) of MOF material. Thus, the vast majority of the installed ferrocene is electrochemically inaccessible. (If we consider the XPS results above, which point to ca. 4.5 ferrocenes per node, for nodes sited at the MOF crystallite exterior, the electroactive zone is estimated to be less than 2 nm thick.) We initially reasoned that the size of the needed chargecompensating anion (from the surrounding electrolyte solution) might be limiting the amount of ferrocene that can be oxidized (by sterically inhibiting incorporation of anions into MOF pores, especially after pore and/or aperture crowding by SALI-based ferrocene installation). The aperture widths for defect-free UiO-66 are ∼6 Å. (In practice, some apertures are larger, due to missing-linker or missing-node type structural defects.36,46 Similarly, some fraction of the MOF pores can be enlarged by these kinds of defects.47,48) We compared electrochemical responses of films of UiO-66-Fc-db-COO in acetonitrile solutions of lithium perchlorate (∼3.7 Å anion diameter) to the same films immersed in tetrabutylammonium tetraphenylborate (∼9 Å anion diameter). The cyclic voltammetric responses (Figure S9) showed almost no difference and provided no evidence for aperture-based size exclusion of tetraphenylborate. Evidently, sufficient defects exist in the outermost few nanometers of crystallites of UiO-66-Fcdb-COO to permit anion insertion. At the same time, the observation that only a small percentage of installed ferrocene molecules is addressable voltammetrically implies that both anions are excluded from the bulk of the crystallite interior either because of physical blocking and small apertures or because of very slow kinetics for anion uptake beyond the first 6 nm or so. For electrocatalytic applications, it is useful to know how fast charges are diffusing through (or on) UiO-66. A diffusion process is inferred due to peak currents being proportional to the scan rate (Figure S10). Values of the apparent charge diffusion coefficient, DCT, were calculated from the Cottrell equation

Figure 3. Cyclic voltammograms of EPD films of UiO-66-Fc-db-COO (navy, solid) and UiO-66 film (grey, solid) and FTO (gray, dash). Measurements were taken with a 10 mV/s scan speed in 0.1 M KCl (aq) after purging with nitrogen.

section. Briefly, however, PVF films experience both solvent swelling and anion insertion when oxidized and cooperative expulsion of both when restored to neutral form. In extreme cases, the reductive voltammetry can even be modeled as a redox-induced phase transition. To quantify the fraction of electroactive ferrocene (proportional to the amount of faradaic charge passed), current flow was integrated following potential steps between 0 and 1 V and between 1 and 0 V (vs Ag/AgCl), holding at each potential for 120 s (Figure 5a). (After chronoamperometry, films of UiO-66Fc-db-COO were digested to determine the total amount of ferrocene in each film, as well as the total amount of zirconium. From these measurements, the average loading of Fc-db-COO is 1.3 per hexa-zirconium(IV) node.) Within experimental uncertainty, stepping oxidatively, (0−1 V) yields the same percentage of electroactive Fc/Fc+ (22 ± 5%) as stepping reductively (18 ± 2%). The average diameter of the UiO-66 particles was 160 ± 50 nm, as measured by SEM (Figure S8). From particle size and unit-cell dimensions, we estimate that about 2% of the MOF nodes are sited on the exterior surface of the crystallites (details in the Supporting Information). Neglecting the possibility of higher-than-average loading for ferrocene sited at the outermost nodes (i.e., higher than 1.3 ferrocenes per node) and crudely

i = nFAC

DCT πt

(1)

Figure 4. In (a) 10 CV scans of a UiO-66-Fc-db-COO film at a scan speed of 10 mV/s in 0.1 M KCl (aq) purged with nitrogen and (b) fifth of 10 CV scans from (a), plotted current versus time, showing that charge for oxidation matches charge during reduction. D

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Figure 5. (a) Chronoamperometry of UiO-66-Fc-db-COO film in 0.1 M KCl (aq) and (b) Anson plots showing the time range used for finding DCT for chronoamperometry in (a).

where i is the current, n is the number of electrons in the process (for ferrocene, Fc → Fc+ + e−, n = 1), F is Faraday’s constant (9.65 × 104 C/mol), A is the area of the electrode in cm2, C is the concentration of ferrocene in the film (mol/cm3), and t is time (s). Here, the molar concentration is taken as an average; it was calculated as the moles of active ferrocene from chronoamperometry per volume of the film, with the latter being estimated with thickness from SEM cross sections multiplied by the geometric film area.49 When current is plotted against 1/√t, the slope is given by nFAC

DCT π

Scheme 1. Schematic Representations of (a) Electron Hopping and Coupled Anion Motion along the Exterior Surface or the Near-Surface of Ferrocene-Functionalized UiO-66 under the Low-Ion-Density Limit; and (b) Analogous Charge Transport in the High-Ion-Density Limita

, which when

solved for DCT yields the values in Table 1. Table 1. Diffusion Coefficients Calculated from Chronoamperometry Measurements in 0.1 M KCl (aq) DCT (cm2/s) voltage

UiO-66-Fc-db-COO

UiO-66-FcCOO

0−1 V 1−0 V

6.1 ± 0.8 × 10−11 2.6 ± 0.2 × 10−9

1.3 ± 0.9 × 10−9 1.0 ± 0.2 × 10−8

Additionally, we investigated ferrocenecarboxylate (FcCOO−) installed by SALI into UiO-66. For an EPD film of UiO-66-Fc-COO, a more symmetric CV is observed (Figure S11); however, the DCT value for oxidation is again smaller than for reduction (Table 1).

a

In principle, either electron hopping or counter-ion motion may determine the rate of diffusive charge transport, as encoded in the measured value of DCT. The rate of either molecular-scale process (i.e., electron hopping or counter-ion motion) may be sensitive to ion density. The sketch in (c) illustrates that extraction of electrons from a partially oxidized film (i.e., surface- and near-surface-oxidized film) entails net expulsion of charge-compensating anions from the film. In principle, the rate of this process (or the reverse process, anion uptake) may also contribute to the measured values of DCT. Furthermore, the rate of anion expulsion or insertion may depend on the depth of siting of the corresponding ferrocenium moiety.

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DISCUSSION In view of the observed voltammetric asymmetry and the substantial difference in the charge-transport diffusion coefficient for Fc+/Fc-based film oxidation versus film reduction, it is worth carefully considering what really is being measured in chronoamperometry (chronocoulometry) experiments. We have noted elsewhere that for high-porosity MOFs featuring large apertures, DCT can be related microscopically to the charge-hopping rate constant, ket, and that the hopping is akin to a Marcus-type electron self-exchange reaction50,51 Fc + Fc+ → Fc+ + Fc

immobilized redox couple. Depending on the chemical details of the system, DCT can reflect the kinetics of: (a) exchange-like electron hopping (as implied in eq 2), (b) counter-ion motion, or even (c) the motion of uncharged species that display differing affinities for the two forms of the redox couple.28,52 One of the assumptions for the Cottrell equation is a boundary condition of “semi-infinite” diffusion; in other words, over the course of a chronoamperometry experiment, there always exists a region away from the electrode where the

(2)

With a succession of such reactions, charge can be propagated in a diffusive fashion (Scheme 1), i.e., macroscopically transported in accord with Fick’s laws and in response to a gradient in charge concentration created in the film by delivery or removal of electrons by an underlying electrode held at a reducing or oxidizing potential relative to the MOFE

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PVF/ion/solvent redox system is not perfect. We suggest, however, that similar effects, i.e., loading-dependent couplings between electron addition/removal and ion expulsion/ insertion, are in play and that these account, at least in part, for the sizable differences in apparent charge-transport diffusion coefficients measured under conditions of oxidation versus reduction of MOF surface and near-surface redox species.

concentration of the diffusing species is unperturbed by the potential step at the electrode. With real films of finite thickness, this condition can be reasonably well satisfied by considering only data collected at early times, e.g., data collected during oxidation or reduction of, say, the first 30% or less of the electroactive sites. Here, that would mean the first 30% or less of the Fc+ and Fc sited on the exterior surface and near-surface (i.e., within ∼2 nm) of UiO-66 crystallites. It appears that in this crowded environment, the ability to move electrochemically measurable charge through the UiO66-Fc-db-COO film is strongly dependent on whether electrons are moving through an ion-rich milieu (Scheme 1b) or an iondepleted environment (Scheme 1a), the former being interrogated by early-stage oxidative chronoamperometry and the latter by early-stage reductive chronoamperometry. Indeed, the apparent charge-transport diffusion coefficient measured for early-stage reduction is ca. 40-fold greater than for early-stage oxidation. The voltammetry results point to further factors. As noted above, the oxidation wave is drawn out, especially at potentials beyond the peak in the oxidative wave. An attractive interpretation is that oxidation becomes progressively more difficult as charge-balancing anions are taken up. Ferrocenium ions sited at a crystallite surface can be charge-balanced by either anion pairing or diffuse double-layer (i.e., solution-phase) ion redistribution, whereas ferrocenium ions sited within the MOF (i.e., beneath the framework’s external surface) require anion insertion into the MOF, a process that we suggest becomes increasingly energetically and kinetically demanding as redox sites further from the MOF external surface participate. (Thus, the voltammetry and chronoamperometry measurements, in contrast to, say, steady-state redox conductivity measurements with interdigitated ultra-microelectrodes, necessarily entail uptake and release of charge-compensating ions by the MOF film (Figure 6) and not just transfer of these ions between electron-exchanging ferrocene/ferrocenium pairs.)

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CONCLUSIONS UiO-66, when intentionally prepared in a defect-rich form (i.e., one to two missing linkers per node), can be uniformly functionalized by the SALI technique redox-active species; here, ferrocene molecules presenting pendant carboxylates (node binding groups). Electrophoretic deposition of the functionalized material on conductive glass permits redox accessibility as well as the dynamics of ferrocene-functionalized-film oxidation and re-reduction to be interrogated electrochemically. Chronocoulometry measurements reveal that only about 20% of the installed ferrocene is electrochemically addressable. We attribute the observed electroactivity to those ferrocene molecules attached at or near the external surface of UiO-66. For the ∼160 nm MOF crystallites comprising the film, 22% of the ferrocene molecules reside within roughly the outermost 2 nm. Cyclic voltammetry measurements point to an asymmetry in the dynamics of UiO-66-Fc-db-COO film oxidation versus rereduction. Chronoamperometry measurements can be used to determine the apparent diffusion coefficient, DCT, for charge transport across the films. The value obtained on the basis of Fc+ reduction to Fc (potential step from +1 to 0 V) is 40-fold larger than the value obtained by oxidizing Fc to Fc+ (potential step from 0 to +1 V). The unexpected difference (typically, values are identical...) reflects the chemical complexity of both site-to-site hopping and net charging or discharging of the film, with the DCT values reflecting some combination of rates for electron hopping, coupled anion motion in the opposite direction, and net insertion or expulsion of charge-compensating electrolyte-derived anions from the films (or surface pairing and unpairing with outermost Fc+/0 units) during the transient chronoamperometry measurements. We anticipate that these findings will inform our ongoing work on electrocatalysis by molecules or clusters sited within redox-conductive MOFs (both inherently redox-conductive MOFs and MOFs that are rendered redox conductive by appending suitable molecular species).

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Figure 6. Illustration of counter-ion uptake coupled with oxidation of ferrocene. Uptake becomes increasingly difficult as increasing amounts of ferrocene are oxidized and as the oxidation involves sites further from the MOF external surface.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.7b03846. Materials characterization; cyclic voltammetry of ferrocene derivatives, films with different electrolytes, electrolyte after cyclic voltammetry, dependence on the scan rate; many chronoamperometric step experiment; details on estimating external nodes and ferrocene (PDF)

In their studies of PVF film electrochemistry, including electrochemical quartz crystal microbalance (EQCM) interrogation of changes in film mass, Bruckenstein and co-workers have shown that coupled chemical processes (especially solvent swelling) can lag far behind ferrocene oxidation to ferrocenium. They also have shown that depending on the direction (oxidative versus reductive) and the extent of electrochemical perturbation, overall rate control can be exerted by different steps and that transient oxidation and reduction responses need not mirror each other.53 EQCM measurements are beyond the scope of our study, and the analogy between the systems studied here and the

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AUTHOR INFORMATION

Corresponding Author

*[email protected]. ORCID

Jian Liu: 0000-0002-5024-1879 F

DOI: 10.1021/acs.langmuir.7b03846 Langmuir XXXX, XXX, XXX−XXX

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Chung-Wei Kung: 0000-0002-5739-1503 Idan Hod: 0000-0003-4837-8793 Omar K. Farha: 0000-0002-9904-9845 Joseph T. Hupp: 0000-0003-3982-9812 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge support from the U.S. Department of Energy, Office of Science, Basic Energy Sciences (Grant No. DE-FG02-87ER13808) and Northwestern University. R.H.P. acknowledges support from the National Science Foundation Graduate Research Fellowship program under Grant No. DGE1324585. This work made use of the EPIC facility of the NUANCE Center at Northwestern University, which has received support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF NNCI-1542205); the MRSEC program (NSF DMR-1121262) at the Materials Research Center; the International Institute for Nanotechnology (IIN); the Keck Foundation; and the State of Illinois, through the IIN. This work also made use of the J.B. Cohen Xray Diffraction Facility supported by the MRSEC program of the National Science Foundation (DMR-1121262) at the Materials Research Center of Northwestern University.

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DOI: 10.1021/acs.langmuir.7b03846 Langmuir XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.langmuir.7b03846 Langmuir XXXX, XXX, XXX−XXX