ARTICLE pubs.acs.org/JPCC
Photoresponse Characteristics of Archetypal MetalOrganic Frameworks Jeremy I. Feldblyum,† Elizabeth A. Keenan,‡ Adam J. Matzger,†,‡ and Stephen Maldonado*,‡,§ †
Macromolecular Science and Engineering, University of Michigan, 2300 Hayward Avenue, Ann Arbor, Michigan 48109-2136, United States ‡ Department of Chemistry, University of Michigan, 930 North University, Ann Arbor, Michigan 48109-1055, United States § Program in Applied Physics, University of Michigan, 450 Church Street, Ann Arbor, Michigan 48109-1040, United States
bS Supporting Information ABSTRACT: The photoelectrochemical responses of two archetypal metalorganic frameworks (MOFs), MOF-5 and MOF-177, have been assessed. Films of MOF-5 and MOF-177 were grown on carboxylic-acid-terminated conductive fluorine-doped tin oxide substrates. Separate analyses by powder X-ray diffraction, Raman spectroscopy, and fluorescence spectroscopy collectively indicated these films prepared via a solvothermal method in diethylformamide were free of residual impurities such as ZnO clusters and residual organics. Exposure of these films to white light illumination while immersed in acetonitrile electrolytes elicited measurable photocurrents. Wavelength-dependent analysis of the photoresponses showed that the measured photocurrents were induced by ultraviolet light and that the spectral response profiles followed closely the light absorption profiles of each respective material. Attenuation of the induced photocurrents was noted after prolonged ultraviolet light illumination and/or exposure of the films to H2O(l), indicating that the observed photoresponse properties are directly related to the structural integrity of these MOFs. The cumulative data illustrate that such MOFs have innately light-sensitive properties that are atypical in high surface area materials.
’ INTRODUCTION Low surface area coordination polymers have been intensely studied and developed as materials with tunable optical and electrical properties.13 For example, mixed metal valency networks like Fe7(CN)18 3 14H2O exhibit intense electrochromaticity.47 Networks of polycyclic aromatic hydrocarbon units coordinated to Bi3+ cations possess midsized (1 to 2 eV) optoelectronic bandgaps. 812 Ag(I)-arene networks and Cu((CH3)2-N,N0 -dicyanoquinonediimine)2 can show sheet conductivities approaching that of a metal.13,14 Conversely, the innate optoelectronic properties of high surface area coordination polymers like metalorganic frameworks (MOFs) are unclear despite the extensive data collected on the sorption,1518 separation,19 and catalytic20,21 properties of MOFs like MOF-5.22 Reports both support and refute the contention that MOFs are pure insulators like zeolites. Light-stimulated charge transfer between suspensions of MOF-5 and simple oxidizable organics,2326 computational results from a density-functional-based tight-binding method,27 and measurements of intense photoluminescence28,29 have been documented as evidence that MOF-5 is properly viewed as a material with photoresponsive properties akin to a semiconductor. In contrast, an opposing interpretation of the apparent photoluminescent properties of MOF-5 has suggested that discrete organic linkers, rather than the composite coordination polymer, are responsible for photoluminescence30 and that emission from MOFs like MOF-5 at visible wavelengths is diagnostic of structural defects or metal oxide impurities.30 This latter view intimates that no electronic r 2012 American Chemical Society
communication occurs between organic and inorganic constituents in pure MOF-5; that is, MOF-5 is an insulator under standard conditions.30 Ab initio B3LYP calculations of the electronic properties of MOF-5 have similarly suggested an insulating optoelectronic bandgap of 5 eV.31 Complications arising from differences in material preparation, propensity for material degradation under ambient conditions, and uncertainties in modeling approaches have thus far clouded the interpretations of these works. Correspondingly, the prospects and opportunities for MOFs as components in optoelectronic technologies remain ill-defined. The focus of the present report is to explore the contention that MOFs can be materials with semiconductor-like attributes; that is, their charge carrier populations can be influenced by optical excitation, and charge carriers can move appreciable distances within the lattice. Because of the ease of preparation and nondestructive nature solid/liquid contacts, photoelectrochemical heterojunctions have long been used to gauge the optoelectronic properties of crystalline organic semiconductors.32 Herein the photoelectrochemical response characteristics of two zinc-based MOFs prepared as thin films on electrode substrates are reported. Through the analysis of films grown directly on chemically modified fluorine-doped tin oxide (FTO) substrates rather than MOF films that were electrically contacted Received: July 7, 2011 Revised: November 27, 2011 Published: January 23, 2012 3112
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The Journal of Physical Chemistry C postsynthesis, conclusions can be drawn on the native optoelectronic properties of the investigated MOFs without complications from evaporated metal ohmic contacts.33 Given the extensive existing literature on the physicochemical properties of MOFs like MOF-5, the data presented here focus on MOF-522 and MOF-177.34 These two MOFs both possess high surface areas (3534 and 4746 m2 g1, respectively)35 and octahedral Zn4O(O2C)6 secondary building units but differ in the chemical identity of the linker. This report illustrates that films of MOF-5 and MOF-17734 exhibit detectable photocurrent responses upon illumination at photon fluxes of g1015 photons cm2 s1 at wavelengths absorbed by the respective material. The data indicate that the observed photoresponses cannot be attributed to the underlying substrate, that the photogenerated carriers can be collected with small but decidedly nonzero efficiency in these MOFs, and that the photoresponse characteristics are general to both of the investigated MOFs. The presented data argue against the possibility that contaminants/impurities govern the photoconductivity,30 specifically of MOF-5. Attenuation of photoresponse due to exposure to moisture or prolonged exposure to UV light is discussed in the context of elucidating design features for MOFs with improved photoresponse.
’ EXPERIMENTAL SECTION Materials. Zinc nitrate hexahydrate (98%), terephthalic acid (H2BDC, 99+%), allyltrimethoxysilane (97+%), acetonitrile (99.9%, extra dry), lithium perchlorate (99+%), 1,10-phenanthroline (99+%), and sodium hexafluorophosphate (98.5+%) were purchased from Acros Organics and used as received without further purification. Methanol (certified ACS, Fisher Scientific), hydrochloric acid (reagent grade, Fisher Scientific), tetraethylammonium bromide (99%, Sigma-Aldrich), and iron(II) chloride (98%, Sigma-Aldrich) were also used as received. Anhydrous toluene was obtained by passing through an activated alimuna-packed column. Water was purified to resistivity >18 MΩ cm with a Barnstead Nanopure system (Thermo Scientific). 4,40 ,400 -Benzene-1,3,5-triyl-tribenzoic acid (H3BTB), was synthesized according to a published procedure.36 Fe(phen)3(PF6)2 for chronoamperometric experiments was synthesized using a modified literature procedure.37 (See the Supporting Information.) Tetraoctylammonium tetrafluoroborate was synthesized by anion exchange from the bromide salt using conc. fluoroboric acid. Tetraoctylammonium bromide (5 g, 9.1 mmol) was dissolved in ∼50 mL of THF. Excess concentrated HBF4 (4 mL, 64 mmol) was added, and the solid was collected by precipitation with H2O and collection on a filter funnel. The precipitate was dried under vacuum at room temperature overnight to yield the tetrfluoroborate salt. Diethylformamide (DEF, >99.0%, TCI) was stored over activated carbon. Before use, DEF was poured through P5 filter paper (Fisher Scientific) to remove activated carbon and passed through a column containing silica gel. DEF purified in this manner was used within 1 month of purification or until a yellow tint, indicative of solvent decomposition, became apparent by visual inspection. Allyltrimethoxysilane, toluene, acetonitrile, and acetonitrile-based solutions were stored in a N2 glovebox until immediately before use. MOF Synthesis. For bulk MOF preparation, clear, colorless crystals of MOF-5 were obtained by dissolving 560 mg (1.88 mmol) Zn(NO3)2 3 6H2O and 100 mg (0.602 mmol) H2BDC in 15 mL of DEF, purging with dry N2 gas, and heating at 85 °C for ∼12 h.38 After cooling to room temperature, crystals were rinsed three times with pure DMF (3 15 mL). Amber-tinged
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crystals of MOF-5 were obtained similarly but in ambient atmosphere and with an extended heating time of 48 h. For MOFs grown as films, glass coated with a 400 nm thick layer of FTO (TEC 15, Rs < 12 Ω cm1, Pilkington) was cut into narrow strips (∼0.33 cm2) and used as substrates. Prior to use, FTO sections were rinsed sequentially with water, trichloroethylene, water, methanol, water, acetone, water, 10% KOH in isopropanol, and finally water.39 Following washing, FTO sections were first immersed in a 5% (v/v) solution of allyltrimethoxysilane in toluene for 1 h and subsequently rinsed with toluene, followed by methanol. Functionalized substrates were then heated at 70 °C in an aqueous solution of 0.005 M potassium permanganate, 0.195 M sodium periodate, and 0.018 M K2CO3 to oxidize surface-bound olefins, yielding a COOH-terminated surface layer.40 Substrates were rinsed sequentially with water, 0.1 M HCl, and water, and stored in water until further use. Electrode geometric surface areas were measured with a desktop scanner (Scanjet 5300C, 150 HewlettPackard). MOF films were then prepared through the procedure of Hermes et al.41 using COOH-functionalized FTO that was contacted with tinned copper wire with silver print (GC Electronics). The wire contact was sealed with epoxy (Loctite, 1C Hysol) in 6 mm inner diameter glass tubing prior to the growth of MOF films in DEF on each COOH-terminated FTO substrate. Care was taken to minimize contact between the deposition solvent and epoxy. MOF-177 layers were grown on COOHterminated FTO electrodes in a two-step process. First, solutions of 59.3 mg H3BTB and 280.5 mg Zn(NO3)2 3 6H2O hexahydrate dissolved in 15 mL of DEF were heated to 65 °C until small crystals began to grow (∼20 h). The supernatant was then cooled to room temperature and filtered through P2 filter paper (Fisher Scientific) to remove macroscopic crystals of MOF-177. Electrodes were finally immersed in supernatant solutions and heated at 65 °C for 20 h, resulting in layers of MOF-177. Material Characterization. Scanning electron micrographs were obtained using a Hitachi S3200N scanning electron microscope operating at an accelerating voltage of 15 kV. Powder X-ray diffractograms were obtained using a Bruker D8 Advance X-ray diffractometer with a Cu Kα X-ray source (λ = 1.5406 Å). Raman spectra were obtained using a Renishaw inVia Raman microscope equipped with a 514 nm laser without polarizing excitation/collection optics. Photoluminescence measurements were obtained using a FluoroMax-2 spectrofluorimeter (Horiba Scientific). Crystalline powder samples immersed in DMF were transferred to a 0.7 mL quartz cuvette (Starna Cells). The cuvette was filled to the top with fresh DMF to ensure minimal exposure of MOF samples to atmospheric humidity. Emission spectra were obtained at a right angle to the excitation source. Cuvette height was adjusted to obtain a maximum fluorescence signal. Photoluminescence measurements on fresh DMF yielded no appreciable fluorescence signal in the spectral regions examined. DEF and several 1 mL DEF solutions containing H2BDC (6.6 mg), LiNO3 (9.8 mg), or terephthalic acid + LiNO3 (6.6 and 9.8 mg, respectively) were separately studied to determine the possibility that solvent decomposition products, rather than MOF defects, accounted for MOF film photoluminescence. These solutions were prepared in 4 mL Teflon-capped vials, heated to 100 °C for various periods of time, cooled to room temperature, and then analyzed in the spectrofluorimeter. During growth of MOFs on COOH-functionalized substrates, MOFs simultaneously grew on the vial walls. MOFs were collected 3113
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Figure 1. Top-down scanning electron micrographs of (a) MOF-5 and (b) MOF-177 films grown on COOH-terminated FTO substrates. Cross-section scanning electron micrographs of (c) MOF-5 and (d) MOF-177 films grown on COOH-terminated FTO substrates.
from the vial walls and rinsed three times with fresh DMF. The solvent was then replaced with CH2Cl2 four times over 3 days, after which samples were dried under vacuum (∼20 mTorr) for 10 h. Samples were transferred to and stored in a N2 glovebox until sorption analysis. Volumetric N2 isotherms were obtained with a Quantachrome NOVA 4200e using 99.999% purity N2 (Cryogenic Gasses). Electrochemical Measurements. A CHI420A (CHI Instruments) electrochemical workstation was used for electrochemical measurements. Electrochemical cells consisted of a MOFcoated working electrode, platinum wire counter electrode, and Ag/Ag+ quasi-reference electrode. For chronoamperometric measurements with Fe(phen)3(PF6)2, a 6.275 mM solution in an acetonitrile electrolyte containing 100 mM LiClO4 was used. Potential step experiments were performed in the dark by oxidizing [Fe(phen)3]2+ at a potential sufficiently positive to ensure a diffusion-limited process. The effective electrochemically active surface area was then measured using a standard protocol.42 For photoelectrochemical measurements, MOF-coated COOHterminated FTO electrodes were inserted into a home-built, ∼7 mL volume electrochemical cell filled with the appropriate electrolyte (either 100 mM lithium perchlorate, 100 mM tetraethylammonium tetrafluoroborate, or 100 mM tetraoctylammonium tetrafluoroborate) in anhydrous acetonitrile and having a quartz window for the admission of 200700 nm light. Samples examined in more than one electrolyte solution were rinsed with copious amounts of dry acetonitrile before reimmersion into the appropriate electrochemical cell. A Ag/Ag+ quasi-reference electrode and a platinum coil counter-electrode were inserted into the electrochemical cell and used for all photoelectrochemical measurements. The cell was purged with inert gas (either Ar or N2) for 10 min and sealed tightly with Teflon adapters. Polychromatic photoelectrochemical measurements were obtained using short-wavelength UV light emitted
from a hand-held UV lamp (UVGL-25, Ultraviolet Products). The custom sample cell was positioned 1 cm away from the light source, exposing electrodes to ∼2 mW/cm2 of radiation during measurements. Monochromatic photoelectrochemical measurements were obtained using a custom-built spectral response system described in detail elsewhere.43 In brief, electrodes were illuminated by light obtained from a 150 W Xe arc lamp (Newport) passed through a monochromator and chopped at a frequency of 10 Hz. A beam splitter diverted a small fraction of light to a separate Si photodiode (Newport) to measure and compensate for fluctuations in light intensity. A PAR 273 potentiostat (Princeton Applied Research) was used to record photoelectrochemical response, and an SR830 lock-in amplifier (Stanford Research Systems) was used to isolate photocurrent due to monochromatic sample illumination. Data were recorded with a custom-programmed routine in LabVIEW (National Instruments). To examine the influence of DEF decomposition products38 on MOF-5 film photocurrents, MOF-5 film electrodes were purposely soaked in the amber-colored mother liquor of MOF-5 synthesis in DEF for various amounts of time, rinsed briefly (∼1 s) with fresh acetonitrile to remove excess solution (but precluding the removal of guest species from the layer-grown MOF-5 pores), and analyzed in the electrochemical cell as described above using polychromatic UV illumination.
’ RESULTS Physicochemical Properties of MOF Films Grown Directly on Derivitized FTO. Figure 1 displays representative plan-
and cross-sectional-view electron micrographs of MOF-5 and MOF-177 films. These MOF films were strongly adherent to the underlying functionalized FTO interface, resistant to both 3114
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Figure 2. (red) Experimentally measured powder X-ray diffractograms of (a) MOF-5 and (b) MOF-177 films on COOH-terminated FTO. (black) Simulated powder patterns for each respective bulk crystal.
Figure 3. (a) Raman spectra for (black line) MOF-5 films on COOH-terminated FTO and (red line) ZnO powder. (b) Raman spectra for (black line) MOF-177 films on COOH-terminated FTO and (red line) ZnO powder. Raman spectra from 200 to 800 cm1 for MOF-5 and MOF-177 films are shown at 20 times the measured intensity for better comparison with ZnO. Spectra are offset vertically for clarity.
removal by solvent rinsing and to mechanical abrasion. By comparison, in the absence of initial silanization of the FTO electrode supports, no appreciable MOF film growth was observed, and any adsorbed crystals easily detached upon light rinsing with fresh DMF or acetonitrile. The films prepared on carboxylic-acid-functionalized surfaces had nominal thicknesses ranging between 10 and 100 μm. The films were polycrystalline, exhibiting nominal lateral grain sizes of 10100 μm. Void spaces were visible between large grains. To ascertain whether these regions presented either exposed COOH-terminated FTO or sections with much thinner coatings of MOFs, potential step experiments were conducted in the absence of illumination (where these MOFs all exhibited insulator behaviors) with [Fe(phen)32+] (diameter ∼1.5 nm, calculated in Accelrys Materials Studio44). The constricted pore space of MOF-5 and MOF-177 likely restricts the diffusion of [Fe(phen)32+] through the MOF pore space during the time scale of the potential step experiments (∼0.1 s), allowing primarily the uncoated COOHterminated FTO to be probed electrochemically. Apparent values of the electroactive areas of the exposed COOH-terminated FTO corresponded to up to 20% of the electrode geometric surface area, indicating that the void spaces in Figure 1 corresponded to areas where no MOF growth occurred. Note that the exposed area is an upper limit, as any diffusion of the
redox species through the MOF would allow access to the underlying substrate, artificially increasing the apparent exposed COOH-terminated FTO surface. Figure 2 shows powder X-ray diffraction (PXRD) patterns for as-grown films on COOH-terminated FTO. For each investigated MOF, the observed pattern agreed strongly with the predicted diffraction patterns for ideal MOF crystals. Other reported synthetic methods for MOF-5 crystals in the presence of water have shown evidence of the formation of discrete ZnO clusters embedded among the coordination polymers.30,45,46 Of note, neither of the collected diffraction patterns showed any reflections indicative of crystalline ZnO (Figure S3, Supporting Information). Figure 3 displays representative Raman spectra obtained for the as-deposited MOF films. The collected Raman spectra for these MOF films were compared with Raman spectra for high-quality MOF crystals grown in solution (Figure S5, Supporting Information), which demonstrated Brunauer EmmettTeller (BET) N2 isotherm surface areas of 3300 and 3500 m2 g1, respectively, consistent with predictions for the ideal (uncollapsed) crystal structures.35 For each of the investigated MOFs, the as-deposited materials exhibited spectral signatures comparable to the homogeneously grown analogs. Furthermore, as noted for the data in Figure 2, none of the Raman spectra exhibited phonon modes indicative of either 3115
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Figure 4. Comparison of (dashed lines) excitation and (solid lines) emission spectra for (a) bulk MOF-5 prepared by the method in ref 38 and (b) MOF-5 (black) and MOF-177 (red) prepared by the procedure used here for film growth on COOH-terminated FTO. The respective wavelengths used for the collection of emission in the photoluminescence excitation spectra and for the excitation in the photoluminescence emission spectra are indicated in each plot.
crystalline or amorphous ZnO at 437 or 558 cm1, respectively.47,48 Separate Raman measurements of dilute ZnO standard samples in a KBr matrix indicate a detection limit of ∼10 parts per thousand (by mass) for ZnO by Raman analysis under the employed conditions (Figure S6, Supporting Information). The emissive properties of MOF-5 have been extensively studied2325,29,30 and have recently been used to assess the purity content of MOF-5 produced with various methodologies.30 Figure 4 shows the recorded room-temperature photoluminescence spectra for MOF-5 and MOF-177 materials specifically synthesized in this work via the well-described DEF synthetic strategy.38 Two sets of spectra are shown to highlight the differences between the established protocol for “bulk” MOF-5 synthesis and the slight variations (vide supra) used here to prepare thin films of MOF-5 in DEF. The luminescence spectrum for both MOF-5 materials recorded with excitation at 331 nm exhibited similar broadband emission centered at 445 nm, considerably blue-shifted from the typical emission observed from ZnO nanoparticles at ∼535 nm.30 To determine the extent that solvent decomposition products could contribute to “long” wavelength photoluminescence, separate photoluminescence measurements were made for DEF and DEF solutions containing terephthalic acid, LiNO3, or terephthalic acid + LiNO3 following heating to T = 100 °C for various periods of time. These solutions were then cooled to room temperature, and their luminescence properties were measured (330 nm excitation wavelength). The fluorescence spectra from these measurements are collected in Figure S7 (Supporting Information). At progressively longer times, the wavelength for maximum fluorescence red-shifted by almost 100 nm, indicating a thermally induced reaction in the mixture that produces luminescent compounds. These observations are consistent with the premise that emission at long wavelengths from MOF films in Figure 4 could arise purely from residual solvent decomposition impurities rather than structural defects/ ZnO impurities.30 The extent that any solvent decomposition impurities obstructed MOF pores was addressed through N2 adsorption isotherm measurements. For example, the measured surface areas for MOF-5 prepared in DEF in this work (3300 m2 g1 BET SA) agree with the theoretical surface area of pristine MOF-5 crystals (3390 m2 g1).16 The sensitivity of luminescence analyses, as noted elsewhere,30 is sufficient to detect residual solvent decomposition impurities at levels that do not disrupt the microporosity of the studied MOF films. Luminescence
spectra were also recorded with MOF-5 grown in DEF at extremely long times (Figure S8, Supporting Information), well in excess of times used for the materials studied here. For these materials, the center of the broadband emission was considerably red-shifted relative to the data in Figure 4, with a peak wavelength at 490 nm, consistent with the data in Figure S8 of the Supporting Information and further supporting the trace presence of solvent decomposition products as the source of long wavelength photoluminescence.30 The lack of intense long wavelength photoluminescence from MOF-177 films is suggestive of less inclusion of solvent decomposition products than MOF-5. In MOF-177, the lower synthetic temperatures, shorter crystallization times, and larger pores that facilitate easier removal through simple rinsing are all factors that could decrease the incorporation of solvent decomposition products. Photoresponse Characteristics under Polychromatic Illumination. Figure 5 illustrates the recorded photoresponses for MOF-5 and MOF-177 films immersed in deaerated 100 mM LiClO4 in acetonitrile illuminated with polychromatic illumination from a Hg arc lamp. Upon illumination, both MOF films exhibited a distinguishable increase in anodic (negative) current; that is, the hole flux at the MOF/solution interface was increased relative to that obtained in the absence of illumination. For both MOFs, the induced photocurrents were not attributable to heterogeneous charge transfer to a redox couple in solution because the solution only contained supporting electrolyte. Upon cessation of illumination, the recorded current returned to the same baseline level. Control experiments with bare FTO electrodes immersed in the same liquid electrolyte exhibited a negligible change in recorded current density. For both MOFs, the increased current density in response to illumination resulted in a fast change that was followed by a slight, slow decay. The precise shape for each cycle of the chopped optical excitations varied somewhat between samples and age of the sample. Nevertheless, the magnitude of the photocurrent densities remained consistent. Steady-state currentpotential responses recorded under chopped monochromatic illumination were used to compare the specific wavelength-dependence for each MOF with the respective film absorption profile. The as-prepared films were strongly scattering, complicating direct transmittance measurements. The top portions of Figure 6 show the wavelengthdependent diffuse reflection profiles for each MOF. These data 3116
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Figure 5. Photocurrent responses of (a) MOF-5 and (b) MOF-177 films on COOH-terminated FTO electrodes illuminated by UV-light from a handheld Hg arc lamp (∼2 mW cm2). Samples were illuminated at 10 s intervals.
Figure 6. Diffuse reflectance profiles of (a) MOF-5 films on COOH-terminated FTO and (b) MOF-177 films on COOH-terminated FTO. Measured diffuse reflectance intensities were converted through the KubelkaMunk equation to units of k/s (where k is the molar absorptivity and s is the scattering coefficient at a given wavelength). (c) Wavelength-dependent quantum yields for (closed circles) a MOF-5 film on COOH-terminated FTO and (open squares) bare FTO substrate. (d) Wavelength-dependent quantum yields for (closed circles) a MOF-177 film on COOH-terminated FTO and (open squares) bare FTO substrate.
directly report on the absorptivity of the MOFs in the dry state. The bottom portions of each plot in Figure 6 show the quantum yield (net photocurrent density divided by incident photon flux) in the spectral range between 230 to 400 nm for each MOF film type. In general, the spectral profile of the steady-current potential responses follows the diffuse reflection profiles for the respective materials but does not match the reported wavelength-dependent quantum yield values of ZnO.49 For the investigated MOFs, the apparent quantum yields were consistently in the range of 104 (on a scale of 0 to 1). No appreciable absorption was noted in the visible portion of the spectrum, and the spectral profile of the photoresponse closely followed the wavelength-dependent absorption. In particular, no photocurrents were obtained for light with long wavelengths, and the onset of photoresponse occurred at shorter wavelengths than for strong light absorbance. This latter observation is consistent with the premises that both the optical penetration of deeply penetrating photons exceeded the carrier diffusion length within the semiconductor photoelectrode and the semiconductor photoelectrode thickness was insufficient to absorb incident light fully, preventing the successful collection of photogenerated carriers at long-wavelength excitation.43 Factors that Influence the Photoresponse of MOF-5 Films. Figure 7 details the observed stability of the photoresponses of MOF-5, showing the quantum yield recorded at 260 nm as a function of time immersed in the photoelectrochemical cell. Two
Figure 7. Successive measurements of MOF-5 photoresponse under the presence (open circles) and absence (filled triangles) of continuous illumination.
data sets are shown, representing the temporal dependence of the quantum yield in the presence and absence of continuous illumination. For MOF-5 films, monitored under continuous illumination (open symbols), the monochromatic quantum yield decreased continuously with increasing immersion time. The photoresponse did not decay to zero within the 30 min time frame, instead asymptotically reaching a value ∼50% that of the initial photoresponse. To determine whether the decreased photoresponse was attributable to the illumination or merely 3117
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Figure 8. Raman spectrum of MOF-5 films on COOH-terminated FTO before and after brief immersion in H2O. Inset: reduction in quantum yield at 260 nm before and after brief immersion of MOF-5 film in H2O.
the exposure to the ionic electrolyte, analogous measurements were made with the samples only illuminated at t = 0 and 10 800 s but not in between. The magnitude of the photoresponse recorded at t = 10 800 s was nominally unchanged relative to the initial photoresponse signal and much larger than the response at t = 1500 s under continuous illumination. To ascertain the influence of MOF-5 structural integrity on the quantum yield for photogenerated carrier collection, MOF-5 films were briefly immersed in H2O before photoconductivity measurements. For MOF-5, exposure to aqueous solutions/wet vapors has been well-documented to disrupt the long-range order in the lattice, presumably due to hydrolysis of the ligand coordination to the metal cation centers.23,5053 Figure 8 shows the Raman spectra of a MOF-5 film prior to and after immersion in H2O. Following exposure, all vibrational features indicative of the MOF-5 lattice were no longer evident in the Raman spectrum. No Raman signatures for phonon modes of Zn-based decomposition products53 were observed. Figure 8 shows a bar graph of the recorded intensity of the quantum yield at λ = 260 nm for a MOF-5 film before and after brief immersion in H2O. A severe and immediate attenuation of the photoresponse was observed, indicating that the measured photoresponses were sensitive to the structural integrity of the MOF films. To examine further the role of framework structural properties on the measurable photoresponses, additional photoelectrochemical measurements were performed under monochromatic (λ = 260 nm) illumination in acetonitrile containing either tetraethylammonium tetrafluoroborate (TEATFB) or tetraoctylammonium tetrafluoroborate (TOATFB). The separate use of a small cation (TEA+, 8 Å ionic diameter in acetonitrile54) that can probe the microporosity of MOF-5 and a large cation (TOA+, 12 Å ionic radii in acetonitrile54) too bulky to fit inside MOF-5 (8.0 Å pore aperture)22 directly probed the premise that electrolyte permeation into MOF-5 was necessary to effect a large photoresponse. As shown in Figure S10 and Table S1 in the Supporting Information, MOF-5 film electrodes exhibited significantly larger photocurrents under illumination in solutions with TEA+ than with TOA+. A comparison of the average photoresponse magnitudes from two sets of 28 total MOF-5 film electrodes showed that the electrolytes elicited statistically different responses at the 99.62% confidence level (Student’s t test, p = 0.0038). To confirm further the influence of electrolyte cation on the
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Figure 9. Average photoresponse of three MOF-5 films illuminated by UV-light from a hand-held Hg arc lamp (∼2 mW cm2). Films were left immersed in DEF mother liquor between successive measurements. Error bars represent standard deviation of measured current density of three electrodes after different immersion times.
magnitude of photoresponse, three MOF-5 film electrodes were analyzed pairwise in both electrolytes. For these MOF-5 film electrodes, the photocurrent response was attenuated in solutions with TOA+ relative to the same electrode in TEATFB. The attenuation was not attributable to MOF-5 degradation because attenuation was observed irrespective of whether the MOF-5 film was exposed to TOA+ solutions or not. To determine what role organic impurities in the MOF films contributed to the measured photoelectrochemical behaviors, we measured the photoresponse of MOF-5 films before and after immersion into yellow DEF mother liquor from bulk MOF-5 synthesis (Figure 9). Exposure to, and incorporation of, DEF decomposition products did not lead to larger photocurrent magnitudes. Instead, exposure of MOF-5 films to DEF decomposition products led to a pronounced suppression of net photocurrent, beyond what was normally observed. The decreased photoactivity after exposure was not reversible; that is, rinsing the MOF-5 films after exposure to the DEF solution did not fully restore the original photoresponse characteristics, suggestive that purposeful inclusion of foreign materials into the MOF lattice was also detrimental to the measured photoresponse intensity.
’ DISCUSSION The presented data show that high surface area MOFs such as MOF-5 and MOF-177 can innately exhibit photomodulated conductivity. The data argue against the possibility that the measured photoresponse characteristics are attributable to, or are governed by, residual impurities in the lattice. Multiple materials analyses indicate that the methods used to produce the polycrystalline films in this work result in materials that are largely devoid of structural defects and ZnO impurities. The spectral profiles of the photoresponses closely follow each respective short wavelength absorption profile for each MOF, indicating that light absorption by the MOF and the changes in measured photocurrent are strongly correlated. With regards specifically to MOF-5, the premise that ZnO impurities account for the observed behaviors under illumination is inconsistent with the absence of ZnO signatures in the collected Raman spectra both before and after photoelectrochemical experiments, the lack of photoluminescent signatures at visible wavelengths indicative of ZnO,30 and X-ray diffraction data that show only diffraction peaks associated with the MOF. Separate photoelectrochemical experiments conducted after exposure to an aqueous solution or in electrolytes with cations mismatched to the pore 3118
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The Journal of Physical Chemistry C aperture of MOF-5 suggest that the photoconductivity is directly associated with the properties of the MOF film. Partial/total distortion of the long-range order of the lattice appears detrimental to the photoresponse behavior; that is, the framework structure plays a determining role in the photoresponse obtained for the MOF-5 films. The results of experiments that intentionally exposed MOF-5 films to DEF decomposition products suggest that such contaminants do not contribute to the film photoconductivity. Finally, a dependence of photoresponse on cation size was observed for MOF-5 films. The photoresponse of films in solutions with large tetraoctylammonium cations was severely reduced compared with films examined in solutions having tetraethylammonium cations. Hence, the framework structure plays a determining role in the photoresponse obtained for the MOF-5 films. In total, these observations support the contention that the photoelectrochemical experiments report on the intrinsic photoconductive properties of the selected MOFs. The measured photocurrents suggest that long-range charge migration between linker units may be a common property in crystalline MOFs. A recent report on Zn-based MOFs with visible-light absorbing linkers in the framework separately supports this premise. In that work, Kent and coworkers observed changes in the photoluminescent decay of linker units consistent with extensive charge migration between linker units in crystalline MOFs.55 Those authors did not conduct experiments to identify the role that the secondary building units have on charge transport within a MOF. We attempted additional photoelectrochemical experiments to ascertain whether visible light absorption from the metal clusters could also elicit photoresponses. Preliminary data on UMCM-150 and Mn3(BHTC)2 (BHTC = biphenyl-3,4,50 -tricarboxylate), two BHTC-based MOFs having different metal cations with and without visible light absorption, showed no difference in the wavelength-dependence of the photoresponses (Figure S11 of the Supporting Information). However, further experiments are needed to evaluate the generality of this observation because detailed analyses for these MOFs were complicated by difficulties in preparing films with coverages comparable to those in Figure 1. Nevertheless, the data shown here are consistent with the premise that the identity of the linker, more so than the metal-based secondary building units, influences the measurable photocurrent responses for the materials studied herein. MOF linkers that simultaneously promote visible light absorption, extensive charge delocalization, and open structures with high surface areas have not yet been identified. The observed photoresponse attenuation of MOF-5 under continuous UV illumination (Figure 7) is presumably due to photon-induced rearrangement or cleavage of bonds, compromising the material’s structure. The design of MOFs with more pronounced and stable photoconductivity properties will inevitably require linkers with electronic transitions that do not impact bonding within the framework. By adapting techniques that have been proven to be successful on insulators,41 we have demonstrated a useful methodology for growing MOFs on transparent conducting substrates, used here as a convenient means to identify photoconductivity in MOFs. Although no quantitative information on the charge transport properties (i.e., carrier mobilities) was obtained in this study, the small measured photocurrents are suggestive of low carrier mobilities/suboptimal long-range charge transport. The low external quantum yields reported here describe the sum photonto-electron energy conversion efficiency of the MOFs in these specific media under these particular experimental conditions. The inclusion
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of a fast outer-sphere redox mediator in the electrolyte was purposely avoided since the short-wavelength photoactivity of the MOF films overlapped with the absorbance profiles of available organic and organometallic redox couples. In addition, the ohmicity of the contact with the COOH-terminated FTO substrate was not quantitatively assessed. Unconjugated groups at electrode surfaces are tunneling barriers that can impede interfacial charge transfer.56 In the MOF films studied here, the silyl group used to functionalize FTO likely affects charge transport between the MOF and ITO substrate and could be a factor in the low measured quantum yield values. Although the possibility of further optimizing the systems studied to enhance quantum yields beyond 104 exists, the key demonstration of photoresponse has been made, and efforts are currently focused on the development of new materials that circumvent some of the limitations outlined for the MOFs studied herein. Separately, because the movement of ions throughout the MOF film is necessary to compensate the charge on the MOF lattice, the transport of electrolyte through the pores of the MOFs limits the attainable photoresponse magnitudes. The observation that following immersion in decomposition-product-containing solvent MOF-5 films showed markedly lower photoresponses is consistent with the notion that pore occlusion can decrease photocurrent. Furthermore, the reduced quantum yields for MOF-5 films in acetonitrile with TOATFB indicate that the observed photoconductivity is attenuated when electrolyte cannot penetrate within the framework. The photoconductivities of MOF-5 and MOF-177 stand in contrast with the purely insulating nature of other materials with comparable porosities/accessible surface areas. Zeolites, a class of solids often used as high surface area sorbents,5760 heterogeneous catalysts,61 and catalyst supports,62,63 generally do not exhibit measurable electronic conductivity under standard conditions either in the absence or presence of illumination.2 Two notable exceptions are the cetineite-type oxoselenoantimonates6470 and metal-containing silicates.7175 Oxoselenoantimonates feature structures akin to aluminosilicate zeolites and show measurable visible-light photoconductivity.6470 However, oxoselenoantimonates are not useful as high-surface-area materials because the presence of guest species in the pores is necessary to stabilize the cetineite lattice structure, thereby precluding easily accessible microporosity. In contrast, titanosilicates like ETS-1076 are both microporous and contain light-sensitive catalytic sites for the oxidation of organics.77,78 However, the total accessible surface areas (10 100 m2 g1) of titanosilicates are still more than an order of magnitude lower than the MOFs included in this report,73,79,80 a significant percentage of their photocatalytic sites are inaccessible to reactants,81,82 and long-range photoconductivity in titanosilicate films has yet to be demonstrated despite intense interest spanning more than two decades. A related class of materials are zeolite-supported TiOx clusters,83,84 wherein high-surface-area silicious zeolites are postsynthetically modified with discrete Ti-based photoactive sites. Although these materials can retain high surface areas (1000 m2 g1),83,84 they generally lack a well-defined structure and possess a low density of discrete, randomly spaced photosensitive Ti-based reactive sites. The low density and isolated nature of the TiOx sites has thus far complicated measurement of photoconductivity properties. In contrast, the data on the photoconductivity of MOFs in this report strongly suggest that these MOFs are in fact simultaneously photoconductive and high-surface-area materials. Although the data presented herein do not provide any direct insight into the electronic structure of the investigated materials, the data implicate an optoelectronic bandgap for MOF-5 and the other MOFs studied here smaller than 5 eV.31 3119
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The Journal of Physical Chemistry C A more detailed examination of the electronic features and density of states of these MOFs is necessary to quantitatively ascertain the full electronic structure of pristine MOFs.85 If light-stimulated photoconductivity in these materials is dictated by charge hopping between organic linkers, then the simple band model used for semiconductors may not be appropriate to describe the phenomena observed here. Nevertheless, the photoconductivity of MOFs shown here suggests that applications that leverage both photoconductivity/responsivity and high surface area may be possible with judiciously designed MOFs, including energy conversion technologies or chemical sensing.
’ SUMMARY Layers of MOFs were grown on conductive substrates to assess their photoelectrochemical activity. Photocurrents were observed for layers of MOF-5 and MOF-177 that matched the absorbance profiles of these species. Characterization of these layers by powder X-ray diffraction and Raman spectroscopy as well as fluorescence measurements on similarly grown bulk material for MOF-5, attest to the purity of these films. Attenuation in photoresponse after prolonged exposure to UV light as well as after immersion in H2O suggests that the photoresponse of these materials is directly related to their structural integrity. These results show that two archetypal high-surface area MOFs exhibit innate photoconductivity and suggest that other, similar materials may also be photoactive. Judiciously designed MOFs may find utility in applications requiring photoactive, high-surface area materials. Additional work is needed to significantly extend and enhance the photoresponse of MOFs. ’ ASSOCIATED CONTENT
bS
Supporting Information. Synthesis and voltammetric characterization of Fe(phen)3(PF6)2, potential step characterization of MOF films, powder X-ray diffractograms, Raman spectra, additional photoluminescence spectra, and optical micrographs and photoresponse of UMCM-150 and Mn3(BHTC)2. This material is available free of charge via the Internet at http:// pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Phone: 734-647-4750. E-mail:
[email protected].
’ ACKNOWLEDGMENT J.I.F. recognizes a National Science Foundation graduate student fellowship and E.A.K. thanks the Seyhan Ege Summer Undergraduate Research Fellowship for support. A.J.M. acknowledges the National Science Foundation for funding (grant no. DMR-0907369). S.M. recognizes generous start-up funds provided by the University of Michigan for the support of this work. ’ REFERENCES (1) Maury, O.; Le Bozec, H. Acc. Chem. Res. 2005, 38, 691. (2) Silva, C. G.; Corma, A.; García, H. J. Mater. Chem. 2010, 20, 3141. (3) Whittell, G. R.; Hager, M. D.; Schubert, U. S.; Manners, I. Nat. Mater. 2011, 10, 176. (4) Ellis, D.; Eckhoff, M.; Neff, V. D. J. Phys. Chem. 1981, 85, 1225. (5) Itaya, K.; Shibayama, K.; Akahoshi, H.; Toshima, S. J. Appl. Phys. 1982, 53, 804.
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