Metal–Organic Framework Thin Films as Platforms for Atomic Layer

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Metal−Organic Framework Thin Films as Platforms for Atomic Layer Deposition of Cobalt Ions To Enable Electrocatalytic Water Oxidation Chung-Wei Kung,†,‡ Joseph E. Mondloch,† Timothy C. Wang,† Wojciech Bury,†,§ William Hoffeditz,†,# Benjamin M. Klahr,†,# Rachel C. Klet,† Michael J. Pellin,# Omar K. Farha,*,†,⊥ and Joseph T. Hupp*,†,# †

Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States Department of Chemical Engineering, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan § Department of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland # Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States ⊥ Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah, Saudi Arabia ‡

W Web-Enhanced Feature * S Supporting Information *

ABSTRACT: Thin films of the metal−organic framework (MOF) NU-1000 were grown on conducting glass substrates. The films uniformly cover the conducting glass substrates and are composed of free-standing sub-micrometer rods. Subsequently, atomic layer deposition (ALD) was utilized to deposit Co2+ ions throughout the entire MOF film via self-limiting surface-mediated reaction chemistry. The Co ions bind at aqua and hydroxo sites lining the channels of NU-1000, resulting in three-dimensional arrays of separated Co ions in the MOF thin film. The Co-modified MOF thin films demonstrate promising electrocatalytic activity for water oxidation.

KEYWORDS: metal−organic frameworks, water oxidation, electrocatalyst, atomic layer deposition, cobalt oxide, pyrene

1. INTRODUCTION The production of solar fuels from low-energy chemicals and visible-region sunlight is one promising avenue to ease consumption of fossil fuels and mitigate greenhouse gas production.1 A compelling example is the oxidation of water (or hydroxide) to oxygen concomitant with reduction of protons (or water) to molecular hydrogen, a storable energy carrier. Hydrogen can subsequently be used in a fuel cell with water as the only byproduct. At least on the oxidative side, the needed chemical reactions are reminiscent of those occurring during photosynthesis.2 Successful solar-fuels production requires the combination of light harvesting, charge separation, charge transfer, and a fuel-forming reaction facilitated by a catalyst. 3 One of the most important challenges for economically attractive, solar-driven water splitting is the development of highly active and robust catalysts for the oxidation of water to molecular oxygen. Significant effort has been devoted to the development of both heterogeneous and soluble homogeneous water oxidation catalysts.4−9 Although impressive progress has been made, there remains a need for new, low-cost, and high-performance electrocatalysts capable of functioning in ca. 1 M aqueous base or acid. Factoring in longterm stability as a further requirement, it seems likely that the practical embodiment of these catalysts, at least on the oxidation side, will entail heterogenization. Finally, from a © 2015 American Chemical Society

basic science perspective, well-defined heterogeneous electrocatalystscatalysts that could bridge the gap between welldefined homogeneous catalysts and ill-defined heterogeneous catalystsare an attractive goal. With the above in mind, we reasoned that appropriately chosen, water-stable metal−organic frameworks (MOFs)10−13 could play a defining role in accessing useful new electrocatalysts.14 Sometimes termed porous coordination networks, crystalline MOFs comprise molecular scale organic linkers and metal-rich nodes.15,16 Due to their regular and crystallographically well-defined nanoscale pores, predictably tunable pore sizes, and potentially extraordinarily high permanent porosity,17 MOFs have attracted attention for a wide variety of potential applications including, but not limited to, gas storage and release,18 chemical separations,19 chemical sensing,20 drug delivery,21 light harvesting,22 toxin capture and/or destruction,23−25 and catalysis.26,27 Although MOFs have been used as catalysts or photocatalysts for reactions related to solar-fuel production,28−33 in most cases the investigated materials have taken the form of solution-suspended particles that require sacrificial reagents to achieve turnover; unfortunately, in this Received: July 28, 2015 Accepted: December 4, 2015 Published: December 4, 2015 28223

DOI: 10.1021/acsami.5b06901 ACS Appl. Mater. Interfaces 2015, 7, 28223−28230

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ACS Applied Materials & Interfaces

2. EXPERIMENTAL SECTION

form, the MOF-based catalysts or photocatalysts are not readily electrochemically addressable. MOF catalyst/photocatalyst turnover or regeneration via electrochemistry clearly would be an attractive alternative to the use of sacrificial reagents.34 Electrochemical applications of MOFs constitute an emerging subfield.35 A handful of studies describe MOFs for solar fuel related electrocatalysis processes, such as oxygen evolution.36,37 In these studies, however, the MOF-derived catalysts were synthesized as microcrystalline powders and then deposited on the electrode surface by physical coating approaches, for example, drop casting. Thus, the performance of these MOF-based electrodes has suffered from stability and/ or activity issues related to loss of electrode contact. Directly chemically grown, uniform thin films of MOFs that can circumvent electrode adhesion problems have not been described for electrocatalysis related to solar fuel formation. Here we describe the chemical growth of uniform thin films composed of free-standing single-crystalline sub-micrometer rods of the high-porosity MOF NU-100038,39 on transparent fluorine-doped tin oxide (FTO) conducting glass. We subsequently use the films as supports within which to site catalytically competent Co ions (see sketches in Figure 1). The

2.1. Chemicals. All chemicals [benzoic acid (Aldrich, 99.5%), zirconyl chloride octahydrate (Aldrich, 98%), N,N-dimethylformamide (DMF) (Macron, 99.8%), hydrochloric acid (HCl 36.5−38.0%, Macron), acetone (Macron), sodium sulfate (Sigma, 99%), sodium hydroxide (Sigma-Aldrich, 98%), sodium bicarbonate (Sigma-Aldrich, 99.7−100.3%), dichloromethane (DCM) (Sigma-Aldrich, 99.6%), tetrabutylammonium hexafluorophosphate (TBAPF 6 ) (Fluka, 98.0%), deuterated dimethyl sulfoxide (d6-DMSO) (Cambridge, 99%), deuterated sulfuric acid (Cambridge, 96−98% solution in D2O), and bis(N,N′-di-isopropylacetamidinato)cobalt(II) (Strem Chemicals, Inc.)] were used as received without further purification. Deionized water was used throughout the work. The chemicals used for the synthesis of the 1,3,6,8-tetrakis(p-benzoic acid)pyrene (H4TBAPy) linkers41 were the same as those reported in our previous work.38 2.2. Preparation of the NU-1000 MOF Thin Film. Thin films of NU-1000 were prepared using a procedure slightly modified from the one employed for the synthesis of bulk microcrystalline powders.42 The FTO glass substrate (15 Ω/sq, Hartford Glass), with a size of 2.5 × 1.25 cm, was washed sequentially in detergent-containing water, ethanol, and acetone with the aid of sonication for 15 min. Thereafter, the substrate was dried and soaked in a solution of 0.5 mM H4TBAPy in DMF at room temperature for 12 h. The substrate was then cleaned with DMF and dried. The pretreatment of FTO described is the same as that reported for another MOF (NU-901) in our previous work.40 Benzoic acid (2.7 g) and zirconyl chloride octahydrate (105 mg) were added into 8 mL of DMF and ultrasonically dissolved in a 20 mL screw-thread sample vial (Cole-Parmer, 28 mm × 57 mm), equipped with a urea cap and PTFE foam-backed liner. Thereafter, the solution was placed into an oven at 80 °C for 2 h. After the solution had cooled to room temperature, 40 mg of H4TBAPy was added into this solution and the mixture was sonicated for 20 min. The as-prepared FTO substrate was then placed into the solution, with the conducting side facing the bottom. Thereafter, the vial was placed on the bottom of a gravity convection oven (VWR Symphony) with the temperature set at 90 °C. Positioning vials at the bottom of the oven provides a temperature gradient inside the vial, which is required for the growth of the MOF thin film.40 The vial was removed from the oven after 13 h of reaction, and the FTO substrate was removed from the vial. After all of the precipitations on the back side of the substrate had been removed, the substrate was washed with DMF; a uniform pale yellow MOF thin film could be observed on the front side of the FTO substrate (Figure S1; see also a video recording during OER). Benzoates coordinated in the obtained MOF thin film were then removed by the following activation process: 0.5 mL of 8 M hydrochloric acid aqueous solution was mixed with 13 mL of DMF; 0.05 mL of the obtained solution was then mixed with 50 mL of DMF to form the diluted acidic solution. The obtained MOF thin film was then soaked in the diluted acidic solution in a 100 °C oven for 4 days.43 The obtained film was then washed with acetone several times and soaked in acetone for 1 day. After the film had dried in air, the NU-1000 MOF thin film was then obtained. For comparison, in a few cases we also prepared thin films via electrophoretic deposition42,44,45 (EPD). In some cases catalytic sites [cobalt(II) ions] were installed prior to EPD of the MOF. In others, cobalt ions were installed after film formation; see a following section for details of Catalyst Installation. 2.3. Atomic Layer Deposition (ALD) on NU-1000 Thin Films. All ALD experiments were carried out in a Savanah S100 system (Ultratech Cambridge Nanotech) under N2. The NU-1000 thin films were directly placed into the ALD reactor preheated at 150 °C. The films were held under vacuum in the reactor for 0.5 h prior to the ALD deposition. The ALD reactions were carried out utilizing the following timing sequence (time in s): t1−t2−t3, where t1 is the precursor pulse time, t2 is the precursor exposure time (i.e., the time where the precursor is in contact with samples without applied vacuum), and t3 is the N2 purge time. First, bis(N,N′-di-isoopylacetamidinato)cobalt(II) was deposited at 150 °C utilizing 10 1−60−60 sequences. Thereafter,

Figure 1. (a) Components and structure of NU-1000 and one possible configuration of cobalt ions on the node after AIM; (b) schematic representation of 3D arrays of separated atomic cobalt in the NU-1000 thin film.

array configuration was anticipated to greatly boost the areal density or electrode-surface concentration of catalyst sites relative to what can be achieved via conventional monolayer coating of the electrode with catalyst moieties. At the same time, the use of a porous MOF as a scaffold should permit the equivalent of hundreds of monolayers or more of catalyst sites to be readily accessed by needed counterions and reagents during electrocatalysis. Finally, on the basis of the linkercentered redox conductivity and electrochromic behavior of a closely related thin-film material that uses the same pyrenebased linker, NU-901,40 we anticipated that the framework would exhibit sufficient conductivity at water oxidation potentials to permit the full array of catalyst sites to be turned over by the underlying electrode. Such features ideally should translate into high electrocatalytic activity for water oxidation. 28224

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Research Article

ACS Applied Materials & Interfaces after 600 s of pumping, water was deposited at the same temperature utilizing 10 0.015−60−60 sequences. The Co-AIM NU-1000 thin film was then obtained. During t1 and t2, the N2 flow rate was 5 sccm, whereas during t3 the N2 flow rate was 20 sccm. 2.4. Instrumentation. Thin-film X-ray diffraction (XRD) patterns were measured on a Rigaku ATX-G thin-film diffraction workstation. 1 H nuclear magnetic resonance (NMR) spectra were recorded on a 500 MHz Varian INOVA spectrometer and referenced to the residual solvent peak. For NMR sample preparation, the MOF thin films were scraped from the FTO substrates and dissolved in a few drops of D2SO4 by sonication. The obtained solution was then mixed with d6DMSO and ready for NMR measurement. Scanning electron microscopy (SEM) images, energy dispersive X-ray spectroscopy (EDS) mapping, and EDS line scans were measured on a Hitachi SU8030 SEM, and the transmittance electron microscopy (TEM) images were collected on a Hitachi HT7700 TEM. A selected-area electron diffraction (SAED) pattern was collected by the same TEM under a low accelerating voltage of 40 kV. For inductively coupled plasma-optical emission spectroscopy (ICP-OES) experiments, two samples of the Co-AIM NU-1000 thin film were scraped from their substrates and collected into a microwave vial (4 mL); 0.25 mL of concentrated H2O2 and 0.75 mL of concentrated H2SO4 were cautiously added. The vial was capped and irradiated in a microwave oven at 150 °C for 5 min. The resultant clear solution was diluted to 25 mL with nanopure water and analyzed via ICP-OES (Varian Vista MPX instrument). Co concentration was calculated from external stock solutions and compared to the known Zr content of the MOF. UV−visible (UV−vis) spectra were measured on a Shimadzu 1601 UV−vis spectrometer. All cyclic voltammetric (CV) experiments were performed on a CHI 660 electrochemical workstation (CH Instruments, Inc., USA). A three-electrode electrochemical setup was used, with a platinum mesh and Ag/AgCl/KCl (saturatedd) electrode (homemade) as the counter electrode and reference electrode, respectively. Electrochemical data measured in aqueous solutions were adjusted to reversible hydrogen electrode (RHE) by adding (0.197 + 0.059 × pH) V to the measured potential. Overpotential for water oxidation was estimated by subtracting 1.23 V to the potential versus RHE. Linear sweep voltammetry (LSV) experiments were conducted on a ModuLab ECS potentiostat (Solartron Analytical) with the same three-electrode setup to measure the current density−potential (J−V) curves. iR corrections were made to the obtained J−V curves according to the series resistances measured on the same electrochemical setup for LSV. For the preparation of electrolyte, 0.1 M Na2SO4 aqueous solutions were titrated by NaOH aqueous solutions to prepare the Na2SO4/NaOH solutions with the pH values of 9, 10, and 11. The pH values of all solutions were measured before each measurement by using an Oakton pH meter; the 0.1 M Na2SO4 aqueous solution was found to be pH 8.2. The pH 11 buffer solution for electrochemical experiments was prepared by mixing 0.05 M NaHCO3 aqueous solution with 0.1 M NaOH. Oxygen evolution experiments were conducted in a sealed custom-designed three-electrode electrochemical cell, with the CoAIM NU-1000 thin film (1 cm2), a platinum mesh, and a Ag/AgCl/ KCl (saturated) electrode (homemade) as the working electrode, counter electrode, and reference electrode, respectively. The cell contains 15 mL of the pH 11 buffer solution under slow stirring. Nitrogen was sparged to the solution before every experiment. The dissolved oxygen concentration was measured by an Ocean Optics FOXY fluorescence probe inserted in the solution. The setup for oxygen evolution experiments is similar to that reported previously.46

Figure 2. (a) Simulated XRD pattern of NU-1000; XRD patterns of the MOF thin films (b) before activation, (c) after activation (NU1000 thin film), and (d) after ALD (Co-AIM NU-1000 thin film).

Shown in Figure 3a−c are plan-view SEM images of a thin film of NU-1000 at various magnifications. The film displays uniform coverage of the FTO substrate with no large-area defects in evidence (Figure 3c). Figure 3d shows a crosssectional SEM image of the FTO-supported MOF film; from the image, the film thickness is about 1.8 μm. The film is composed of free-standing rods of NU-1000 with a uniform hexagonal shape and a rod width of about 500 nm. SAED was used to investigate the crystallinity of individual sub-micrometer rods scraped from the substrate. The SAED pattern was obtained for one NU-1000 rod observed under TEM (Figure 3e) and is shown in Figure 3f. A single-crystal SAED pattern was obtained and shows the d-spacing in agreement with the simulated XRD pattern of NU-1000 (Figure 2). The films were further characterized by 1H NMR spectroscopy after digestion. The 1H NMR spectra of the MOF thin film before and after activation indicate that four benzoate ligands are coordinated to each Zr6 node before activation and that these ligands are completely removed following the activation process (Figure S2) leaving in their place terminal −OH and −OH2 ligands the primary sites for reaction with ALD molecular precursors.38,39 3.2. Catalyst Installation. We have shown elsewhere that for MOFs featuring sufficiently large (i.e., mesoporous) channels and apertures, and presenting accessible water or hydroxo ligated nodes, potentially catalytic metal ions can be introduced via a vapor-phase process akin to atomic layer deposition (ALD).38,47,48 Typically used for synthesizing thin films of metal oxides or sulfides, ALD is characterized by conformal and self-limiting film growth.49 In favorable cases, these features permit the average thicknesses of surface coatings to be controlled with close to single-atom precision.50 Notably, metal-containing precursor molecules can only be deposited at chemically active surface sitestypically surface hydroxyls. For initially hydroxyl-rich MOF nodes, the structural element corresponding to a surface coating is an atomically defined shell or cluster. In our previous work, ALD in MOFs (AIM) was used for bulk samples of NU-1000.38 Herein, ALD of bis(N,N′di-isopropylacetamidinato) cobalt(II) and water was utilized to uniformly install Co(II) ions within the NU-1000 thin films. The Co-AIM NU-1000 thin film shows the same XRD pattern

3. RESULTS AND DISCUSSION 3.1. Characterization of MOF Thin Films. As shown in Figure 1a, NU-1000 is constructed from hexa-Zr-nodes and TBAPy linkers; it is composed of one-dimensional hexagonal and triangular channels, having widths of 31 and 12 Å, respectively. The crystallinity of the MOF thin films before and after activation was examined by XRD (Figure 2). Both patterns match the simulated XRD pattern of bulk NU-1000 powders. 28225

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reversible redox peaks for the redox reaction of the TBAPy linkers can be observed, confirming that the MOF thin film is electrochemically addressable (Figure 4). Charge can be

Figure 4. CV curve of an NU-1000 film measured in 0.1 M TBAPF6 solution in CH2Cl2.52

transferred by hole hopping through the repeating pyrene units, and counterions can diffuse through the regular channels to neutralize the charges formed on the linkers. According to the estimated film mass and the charge integrated from the CV curve, >90% of TBAPy linkers in the NU-1000 film are electrochemically active. (See Supporting Information section S4.) The presence on the Zr6 node of adjacent pairs of Co ions, each presumably capable of conversion from oxidation state II to IV, suggested to us that the installed species should be capable of repetitively delivering the four oxidizing equivalents needed for conversion of water molecules or hydroxide ions to dioxygen.53 At appropriate pH values, we anticipated being able to observe via voltammetry the redox behavior of the installed cobalt ions. Figure 5 shows the response of a representative film at various pH values between 8.2 and 11, where the pH was adjusted by adding NaOH to aqueous Na2SO4. (Unfortunately, at pH values significantly above 11, the MOF film degraded.) Waves for Co(II/III) and Co(III/IV) are observable at pH 10 and 11. Notably, at the higher pH the waves shift to less positive potentialswhether measured versus the normal hydrogen electrode (NHE) or versus a pH-independent electrode. These results indicate that electron removal from the cobalt ion is, in both instances, accompanied by proton removal (presumably entailing conversion of aqua and/or hydroxo ligands to hydroxo and/or oxo ligands). Integration of the faradaic portions of the current in the chronoamperometric curve in Figure S5 reveals that only about 1% of the installed cobalt ions, corresponding to just a few MOF structural repeat units, are measurably electroactive: the total electrochemically active amount of Co in the Co-AIM NU-1000 thin film is 0.6 nmol/cm2. Given that cobalt ions are demonstrably present throughout the MOF microrods, the results point to inadequate conductivity for the MOF itself. Whereas we had anticipated that the energetic proximity of waves for oxidation of the tetraphenylpyrene-containing linker would engender the desired conductivity, we failed to anticipate that when water was used as solvent, the conductivity would be lost due to chemical (but not structural) degradation of the oxidized, that is, radical cation, form of tetraphenylpyrene. Our approaches to overcoming this problem as well as the aforementioned instability of NU-1000 at pH values above 11 will be discussed in subsequent papers.

Figure 3. (a−c) Plan-view SEM images of the NU-1000 thin film at various magnifications; (d) cross-sectional SEM image of the NU1000 thin film; (e) TEM image of the NU-1000 sub-microrods; (f) SAED pattern taken from the selected rod circled in panel e; (g) planview SEM image of the Co-AIM NU-1000 thin film; (h) crosssectional SEM image of the Co-AIM NU-1000 thin film, with EDS elemental mapping in the rectangular region (Zr, red; Co, green).

as the parent material (Figure 2), indicating that the overall structure of the MOF is unchanged by ALD. Plan-view and cross-sectional SEM images of the Co-AIM NU-1000 thin film are shown in Figure 3, panels g and h, respectively. These images confirm that the MOF film retains its pre-ALD morphology. Elemental mapping of the Co-AIM NU-1000 thin film was measured by EDS, as shown in the rectangular region in Figure 3h. Uniform distributions of both Zr and Co were observed, indicating that the Co atoms deposited by ALD distribute uniformly from the top to the bottom of the NU1000 thin film (see also Figure S3). The amount of Co in the film was confirmed via ICP-OES. On average, 4.3 Co atoms were found on each Zr6 node; this result is consistent with the structure proposed in Figure 1. As in other cases of metal installation on NU-1000 by AIM,38,47,48 cobalt installation was accompanied by nearly complete elimination of a sharp infrared absorption at ∼3700 cm−1. This feature is associated with an O−H stretch for the four terminal hydroxo ligands of the hexazirconium node.51 It is elimination constitutes strong evidence for cobalt siting at the node. 3.3. Electrochemistry. The CV response of the NU-1000 thin film was measured in 0.1 M TBAPF6 solution in CH2Cl2; 28226

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Figure 5. CV curves (scan rate = 25 mV/s) of the Co-AIM NU-1000 thin film measured in aqueous solutions of 0.1 M Na2SO4/NaOH adjusted to various pH values. The panel at left was obtained by plotting on a finer current-density scale data in the panel at right.

3.4. Electrocatalysis.54 The right-hand panel of Figure 5 presents cyclic voltammograms that extend into the oxygen evolution region. Note that the iR-corrected potential scale is referenced to RHE, facilitating comparisons of overpotentials at different pH values. Recall that on this scale the reversible potential for oxygen evolution is +1.23 V. The onset potential for oxygen evolution shifts to less positive values as the pH is raised from 8.2 to 9 to 10, implying that a step or steps entailing net proton loss precede the rate-determining step for O2 formation. Nevertheless, the onset potentials are large compared with a target potential of +1.63 V (i.e., 400 mV of overpotential) for oxygen evolution at a current density of 10 mA/cm2. Extension of the solution pH to 11 (unbuffered) yields a striking change in electrochemical behavior. The onset potential for catalytic current flow is favorably shifted by hundreds of millivolts. The simplest interpretation is that the species being oxidized at pH 11 is hydroxide, present at 1 mM, whereas at pH 8.2, 9, and 10 the reactant is water. The overall reactions, therefore, are 4OH− → O2 + 2H 2O + 4e−

(1)

2H 2O → O2 + 4H+ + 4e−

(2)

where the protons shown in eq 2 readily combine with hydroxide ions at the pH values examined. Consistent with this interpretation, increasing the hydroxide ion concentration beyond 1 mM increases the peak voltammetric current. (Dissolution of the MOF prevented us from examining this behavior in more than a qualitative way.) Close examination of the CV at pH 10 reveals a small and irreversible peak at ca. +1.70 V versus RHE that is most reasonably attributed to catalytic oxidation of hydroxide at 0.1 mM concentration. Returning to pH 11, scanning to further positive potentials than shown in Figure 3 eventually yields larger currents assignable to direct oxidation of water. Finally, introduction of a buffer (bicarbonate) at pH 11 suppresses the peaking in the voltammetric scan, as expected if the buffer can regenerate hydroxide after local depletion (see Figure 6a). Clearly the MOF-based electrocatalyst is much more active for oxidation of OH− than H2O. With that in mind, we focused the remainder of our investigation on electrocatalytic oxygen evolution at pH 11 in buffered solution. First, to establish that the observed electrocatalysis indeed does correspond to O2 formation, we subjected a 1 cm2 sample to a constant applied (iR-uncorrected) potential of 1.85 V versus RHE, using a setup similar to what has been previously reported.46 Rapid bubble formation was observed on the surface of the Co-AIM NU-

Figure 6. (a) CV curves of bare FTO, the NU-1000 thin film, and the Co-AIM NU-1000 thin film. (b) 50-cycle CV curves (1st cycle and every 10th cycle) of the Co-AIM NU-1000 thin film. Notably, the 40th and 50th CV curves overlap completely, suggesting that a degree of film meta-stability is eventually established. All CV curves were measured in 0.05 M NaHCO3/0.1 M NaOH, pH 11, buffer solutions at a scan rate of 25 mV/s. (c) Tafel plot of the Co-AIM NU-1000 thin film measured in the same pH 11 buffer solution. Tafel data were extracted from the J−V curves (iR-corrected) measured at 1 mV/s.

1000 thin film (Figure S7). Measurements with an optical sensor confirm O2 formation and yield a rate of ∼0.14 μmol/ 28227

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cobalt(II), where each unit of the resulting array consists of, on average, four cobalt ions attached to a hexazirconium(IV) oxo/ hydroxo core (i.e., a node of NU-1000). The resulting materials display good chemical stability in aqueous solutions at pH 8.2, 9, 10, and 11, but are susceptible to dissolution at pH values >11. Electrochemical studies reveal that at pH 8.2, 9, and 10, the Co-AIM NU-1000 is only marginally catalytic for the oxidation of water to O2. At pH 11, however, the material readily electrocatalyzes the four-electron conversion of hydroxide ion to dioxygen. A reactant-diffusion-limited TOF of ca. 20 s−1 is observed at large overpotentials. Unfortunately, due to loss of redox conductivity for the MOF itself, only about 1% of the installed cobalt sites proved to be electrochemically addressable. At pH 11, accessing the full complement of cobalt sites will not increase the catalytic current beyond the observed limit of about 2.5 mA/cm2; it should, however, substantially decrease the kinetic overpotential. Indeed, the observed Tafel slope suggests that overpotentials could decrease by as much as 90 mV per factor-of-10 increase in the percentage of electrochemically addressable catalytic sites. To increase the catalytic current, higher pH values and, therefore, higher stability scaffolds will be required. We hope to report shortly on our efforts to address the challenges of achieving both framework conductivity and high-pH stability. Much of our remaining effort is centered on AIM-based installation of other catalytic metal oxide clusters, along with metal sulfides, in both thin-film and bulk metal−organic framework materials.

min (see Figure S8). Within experimental uncertainty, the faradaic yield for O2 formation is unity. Returning to Figure 6b, repetitive cyclic voltammetric scanning to ca. 2 mA/cm2 reveals slight decreases in reactivity with timepresumably due to slight MOF instability. Co-AIM NU-1000 thin films were investigated by XRD after 10 cycles of CV scans at each pH value (Figure S9); the XRD patterns indicate that the framework remains intact after electrochemical excursions. SEM images (Figure S12) likewise show that the framework remains intact. We have not examined longer term stability, but clearly this is an issue that must eventually be addressed. For now, we have limited our studies of stability to establishing the conditions under which the MOF-based catalyst is sufficiently stable for meaningful preliminary characterization and evaluation. Figure 6c shows a Tafel plot for oxygen evolution at pH 11, with a Co-AIM NU-1000 thin film as the electrocatalyst. The plot was extracted from current−density/potential data collected at slow scan rate under conditions of kinetic control (i.e., no complications due to reactant depletion). The slope of the Tafel plot is 90 mV per decade of current. (The observed slope could, in principle, be affected by uncompensated framework (MOF) resistance. If so, 90 mV per decade would be only an upper limit estimate.) Dividing 59 mV/decade by this quantity yields a transfer coefficient, β, of 0.66. This value suggests that the overall reaction rate is limited by the rate of transfer of the first of the four required oxidizing equivalents. Linear extrapolation of the Tafel plot yields overpotentials, η, of 476 and 566 mV, respectively, for hypothetical current densities of 1 and 10 mA/cm2. In practice, the overpotentials are somewhat larger due to depletion of hydroxide by oxidation, and, at 10 mA/cm2, replacement of hydroxide by water as the primary contributor to the current. (In buffered pH 11 solution, the hydroxide-derived catalytic current is concentration-limited to ∼2.5 mA/cm2.) Nevertheless, the extrapolated (hypothetical) values may be useful for purposes of comparison. Given the unexpectedly small fraction of electrochemically accessible catalyst,55 performance evaluation on a “per active site” basis, rather than on an overall basis, may also be illuminating. Although mechanistic information is lacking, we assume, for simplicity, that a pair of cobalt ions, cycling between oxidation states II and IV, constitutes one catalyst active site. On this basis, the active-site turnover frequency (TOF; four electrons per catalyst turnover) at 2 mA/cm2 is 18 s−1 (see the Supporting Information for details). Alternatively, at a benchmark overpotential of 400 mV (iR-corrected, in pH 11 buffer) the TOF is 1.4 s−1. Although these values point to promising intrinsic catalytic activity, they also underscore the need to extend the chemistry to higher pH and to gain functional access to the full array of AIM-installed catalysts, rather than only a tiny fraction. We intend to report soon on our progress on both fronts.

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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/acsami.5b06901. Additional experimental results and discussion (PDF) W Web-Enhanced Feature *

Video recording during OER.

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AUTHOR INFORMATION

Corresponding Authors

*(J.T.H.) E-mail: [email protected]. *(O.K.F.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported as part of the Argonne−Northwestern Solar Energy Research (ANSER) Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award DE-SC0001059. C.-W.K. acknowledges support from the Graduate Students Study Abroad Program sponsored by the National Science Council (Taiwan). W.B. acknowledges support from the Foundation for Polish Science through the “Kolumb” Program. J.E.M. acknowledges a DOE EERE Postdoctoral Research Award, EERE Fuel Cell Technologies Program, administered by ORISE for DOE. B.M.K. acknowledges a DOE EERE Postdoctoral Research Award, EERE Solar Program, administered by ORISE for DOE. ORISE is managed by ORAU under DOE Contract DE-AC05-060R23100. We thank Dr. Zhanyong Li for helpful discussions.

4. CONCLUSIONS In summary, adhesive and uniform MOF thin films composed of single-crystal sub-micrometer rods of NU-1000 have been grown on conducting glass electrodes. The thermal stability, large channel size, and accessible −OH and −OH2 groups of NU-1000 thin films make them amenable to catalyst installation via AIM. Using this approach, reactant-accessible 3D arrays of metal-ion-based heterogeneous catalysts can be constructed. The concept has been reduced to practice using 28228

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(21) Taylor-Pashow, K. M. L.; Della Rocca, J.; Xie, Z. G.; Tran, S.; Lin, W. B. Postsynthetic Modifications of Iron-Carboxylate Nanoscale Metal−Organic Frameworks for Imaging and Drug Delivery. J. Am. Chem. Soc. 2009, 131, 14261−14263. (22) So, M. C.; Wiederrecht, G. P.; Mondloch, J. E.; Hupp, J. T.; Farha, O. K. Metal−Organic Framework Materials for LightHarvesting and Energy Transfer. Chem. Commun. 2015, 51, 3501− 3510. (23) DeCoste, J. B.; Peterson, G. W. Metal−Organic Frameworks for Air Purification of Toxic Chemicals. Chem. Rev. 2014, 114, 5695− 5727. (24) Katz, M. J.; Mondloch, J. E.; Totten, R. K.; Park, J. K.; Nguyen, S. T.; Farha, O. K.; Hupp, J. T. Simple and Compelling Biomimetic Metal−Organic Framework Catalyst for the Degradation of Nerve Agent Simulants. Angew. Chem., Int. Ed. 2014, 53, 497−501. (25) Van Humbeck, J. F.; McDonald, T. M.; Jing, X.; Wiers, B. M.; Zhu, G.; Long, J. R. Ammonia Capture in Porous Organic Polymers Densely Functionalized with Brønsted Acid Groups. J. Am. Chem. Soc. 2014, 136, 2432−2440. (26) Lee, J.; Farha, O. K.; Roberts, J.; Scheidt, K. A.; Nguyen, S. T.; Hupp, J. T. Metal−Organic Framework Materials as Catalysts. Chem. Soc. Rev. 2009, 38, 1450−1459. (27) Ma, L. Q.; Falkowski, J. M.; Abney, C.; Lin, W. B. A Series of Isoreticular Chiral Metal−Organic Frameworks as a Tunable Platform for Asymmetric Catalysis. Nat. Chem. 2010, 2, 838−846. (28) Wang, C.; Xie, Z. G.; deKrafft, K. E.; Lin, W. B. Doping Metal− Organic Frameworks for Water Oxidation, Carbon Dioxide Reduction, and Organic Photocatalysis. J. Am. Chem. Soc. 2011, 133, 13445− 13454. (29) Silva, C. G.; Luz, I.; Xamena, F. X. L. I.; Corma, A.; García, H. Water Stable Zr−Benzenedicarboxylate Metal−Organic Frameworks as Photocatalysts for Hydrogen Generation. Chem. - Eur. J. 2010, 16, 11133−11138. (30) Wang, C.; deKrafft, K. E.; Lin, W. B. Pt Nanoparticles@ Photoactive Metal−Organic Frameworks: Efficient Hydrogen Evolution via Synergistic Photoexcitation and Electron Injection. J. Am. Chem. Soc. 2012, 134, 7211−7214. (31) Pullen, S.; Fei, H. H.; Orthaber, A.; Cohen, S. M.; Ott, S. Enhanced Photochemical Hydrogen Production by a Molecular Diiron Catalyst Incorporated into a Metal−Organic Framework. J. Am. Chem. Soc. 2013, 135, 16997−17003. (32) Zhou, T. H.; Du, Y. H.; Borgna, A.; Hong, J. D.; Wang, Y. B.; Han, J. Y.; Zhang, W.; Xu, R. Post-Synthesis Modification of a Metal− Organic Framework to Construct a Bifunctional Photocatalyst for Hydrogen Production. Energy Environ. Sci. 2013, 6, 3229−3234. (33) Fu, Y. H.; Sun, D. R.; Chen, Y. J.; Huang, R. K.; Ding, Z. X.; Fu, X. Z.; Li, Z. H. An Amine-Functionalized Titanium Metal−Organic Framework Photocatalyst with Visible-Light-Induced Activity for CO2 Reduction. Angew. Chem., Int. Ed. 2012, 51, 3364−3367. (34) Albeit involving chlorocarbon reduction rather than water oxidation or reduction, an instructive early example of electrochemical regeneration of an electrode-immobilized MOF catalyst has recently been reported: Ahrenholtz, S. R.; Epley, C. C.; Morris, A. J. Solvothermal Preparation of an Electrocatalytic Metalloporphyrin MOF Thin Film and its Redox Hopping Charge-Transfer Mechanism. J. Am. Chem. Soc. 2014, 136, 2464−2472. (35) Morozan, A.; Jaouen, F. Metal Organic Frameworks for Electrochemical Applications. Energy Environ. Sci. 2012, 5, 9269−9290. (36) Babu, K. F.; Kulandainathan, M. A.; Katsounaros, I.; Rassaei, L.; Burrows, A. D.; Raithby, P. R.; Marken, F. Electrocatalytic Activity of Basolite F300 Metal-Organic-Framework Structures. Electrochem. Commun. 2010, 12, 632−635. (37) Gong, Y.; Shi, H. F.; Hao, Z.; Sun, J. L.; Lin, J. H. Two Novel Co(II) Coordination Polymers Based on 1,4-bis(3pyridylaminomethyl)benzene as Electrocatalysts for Oxygen Evolution from Water. Dalton Trans. 2013, 42, 12252−12259. (38) Mondloch, J. E.; Bury, W.; Fairen-Jimenez, D.; Kwon, S.; Demarco, E. J.; Weston, M. H.; Sarjeant, A. A.; Nguyen, S. T.; Stair, P. C.; Snurr, R. Q.; Farha, O. K.; Hupp, J. T. Vapor-Phase Metalation by

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DOI: 10.1021/acsami.5b06901 ACS Appl. Mater. Interfaces 2015, 7, 28223−28230

Research Article

ACS Applied Materials & Interfaces

mass spectrometry of this reaction mixture using 18O-labeled water revealed that the majority of the O2 formed was a result of oxone degradation, not water oxidation. (55) To confirm that the observed electrocatalytic activity is indeed due to cobalt ions cited within the MOF, we also examined films that had been formed electrophoretically from free crystalline NU-1000 material that had been preloaded with cobalt via bulk-phase AIM. Section S9 of the Supporting Information contains representative results. Briefly, however, the alternative synthesis yields films that are electrocatalytic for oxygen evolution. Sample-to-sample variations are observed, but under conditions where conventionally prepared catalytic films yield 2 mA/cm2 of catalytic current, films prepared by the alternative method yielded 2−4 times less catalytic current. We have not explored the reasons for the difference, but note that thin films prepared by the alternative method are less adhesive than solvothermally grown films; this difference conceivably could lead to less effective electrochemical coupling between the MOF and FTO.

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DOI: 10.1021/acsami.5b06901 ACS Appl. Mater. Interfaces 2015, 7, 28223−28230