Article pubs.acs.org/JPCC
Solvothermal Growth and Photophysical Characterization of a Ruthenium(II) Tris(2,2′-Bipyridine)-Doped Zirconium UiO-67 Metal Organic Framework Thin Film William A. Maza,†,§ Spencer R. Ahrenholtz,†,§ Charity C. Epley,†,§ Cynthia S. Day,‡,§ and Amanda J. Morris*,†,§ †
Department of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, United States Department of Chemistry, Wake Forest University, Winston-Salem, North Carolina 27109, United States
‡
S Supporting Information *
ABSTRACT: A thin film of a RuII(bpy)2(dcbpy)Cl2, RuDCBPY, doped metal−organic framework of Zr6O4(OH)4(bpdc)6, RuDCBPY-UiO67 (where bpy is 2,2′-bipyridine, dcbpy is 5,5′-dicarboxyphenyl-2,2′-bipyridine, and bpdc is 4,4′biphenyldicarboxylic acid), has been prepared on fluorine-doped tin oxide and glass slides solvothermally. The film is shown to be isostructural with UiO-67 and similarly doped RuDCBPY-UiO67 powders. The photophysical properties of the film show significant line broadening of the diffuse reflectance spectra, a successive red shift of the emission maxima, and biphasic kinetics with increased RuDCBPY doping of UiO-67 films above 10 mm. The two lifetime components are consistent with a dual population of RuDCBPY within the UiO-67 material: a population of RuDCBPY incorporated into the framework of UiO-67 replacing a bpdc ligand and a second population of RuDCBPY encapsulated within the octahedral cavities. The RuDCBPY dopant within the UiO-67 films interact with each other and undergo self-quenching via a resonance energy transfer mechanism. It was determined that the average distance between RuDCBPY is decreased in the film relative to similarly doped powders. This is attributed to an electrostatic effect upon formation of the framework due to increased charge at the bpdc self-assembled monolayer at the surface of the substrate.
■
Thin films composed of robust, stable, and porous MOFs are of particular interest in light of the harsh conditions necessary to carry out a variety of reactions and processes. The UiO (University of Oslo) series of zirconium-based materials are promising candidates owing to their exceptional stability under a variety of thermal, mechanical, and solvent conditions.41−43 The Zr6O4(OH)4(COO)6 clusters of the UiO-66, -67, and -68 frameworks arrange in such a way that two types of cavities are formed: a tetrahedral and larger octahedral cavity.44,45 For UiO67, in which the connecting ligand is a biphenyldicarboxylate, the diameters of the tetrahedral and octahedral pores are 11.5 and 23 Å, respectively, connected by 6.5 Å windows.44,45 Lin and co-workers have demonstrated that UiO-67 materials of molecular formula Zr6(μ3-O)4(μ3-OH)4(bpdc)6 (bpdc = biphenyldicarboxylic acid) could be doped with ruthenium(II)
INTRODUCTION
Metal−organic framework (MOF) materials continue to gain increasing attention due to their large surface areas and applicability toward catalysis, sensing, separations, and photonics. MOF engineering has, in recent years, been extended by the development of thin films, allowing for more specific/specialized applications.1−30 The current state of the art of MOF thin film fabrication and development has recently been reviewed extensively.31−33 Films consisting of MOFs have been prepared and grown on a variety of substrates by a number of different methods. Of the preparative methods employed to date, the most straightforward involves incubating the substrate along with the reaction mixture during the solvothermal process.32,34−37 Solvothermal synthesis of MOF thin films has been shown to be facilitated by prior formation of a self-assembled, oriented, organic layer terminally functionalized to mimic the connectivity of one of the native MOF ligands.32,38−40 © 2014 American Chemical Society
Received: April 7, 2014 Revised: June 8, 2014 Published: June 10, 2014 14200
dx.doi.org/10.1021/jp5034195 | J. Phys. Chem. C 2014, 118, 14200−14210
The Journal of Physical Chemistry C
Article
solution was sonicated for ∼5 min to dissolve the solutes (although BPDC is only nominally soluble in DMF). Once the SAM-coated FTO or glass was added, the sealed reaction vessel was heated at 120 °C for 3 days. After cooling and drying, the slides were rinsed with DMF and then acetone and air-dried. Thin Film Characterization. X-ray powder diffraction (PXRD) pattern for the RuDCBPY-UiO67 powder was collected using a Rigaku Miniflex equipped with a Cu Kα radiation source (λ = 1.5418 Å) at a scanning speed of 0.8 min/ deg with a step size of 0.02°. Grazing incidence X-ray diffraction (GIXD) data were collected for the thin films on a Bruker D8 Discover DaVinci system with a Göbel Mirror and thin film stage using Cu Kα radiation (λ = 1.5418 Å) at a scanning speed of 1 s/step with a step size of 0.03°. Scanning electron microscopy (SEM) images were obtained using a Leo/ Zeiss 1550 Schottky field-emission scanning electron microscope. X-ray photoelectron spectra (XPS) were collected using a PHI 5300 spectrometer. Samples were irradiated using a PerkinElmer dual anode X-ray source operating with magnesium radiation with monochromatic Mg Kα radiation (hν = 1253.6 eV) at 13 kV and 250 W and a pass energy of 17.9 eV. A step size of 0.025 eV was used, and 192 sweeps were averaged for N 1s (410−390 eV) and Ru 3p (495−458 eV), 48 sweeps were averaged for C 1s (300−280 eV) and O 1s (545− 525 eV), and 144 sweeps were averaged for Zr 3d (194−174 eV). The spectra were calibrated with the C 1s peak, which is known to occur at 284.6 eV.51 FTIR spectra were obtained with a Cary 670 FTIR spectrometer equipped with a Golden Gate single reflection diamond ATR attachment. The spectra presented represent the average of 16 scans between 400 and 4000 cm−1 with a resolution of 4 cm−1. Steady-state absorption and diffuse reflectance spectra were obtained using a Cary 5000 UV−vis−near-IR spectrometer. Steady-state fluorescence spectra were obtained using a frontface geometry with a Cary Eclipse fluorescence spectrometer. Thin film samples on slides were placed diagonally into a quartz 1 cm2 fluorometer cuvette, sealed with a rubber septum and parafilm, and purged for 30−45 min with N2. The fluorescence spectra presented were reconstructed from normalized emission spectra of solid samples excited at 440, 450, and 460 nm in order to identify and correct for bands arising due to Raman scattering, as is not uncommon for solid samples (Figure S2 in the Supporting Information). All measurements were performed in the absence of any additional solvent. Fluorescence lifetime decays of the powders and films were acquired employing a front-face geometry with an Applied Photophysics model LKS.60 laser photolysis system. The third harmonic of a Continuum Surelite SLI-10 Nd:YAG laser (6−8 ns pulse width, λexc = 355 nm) was directed onto the face of sample slides, and the resulting emission signals were focused into a Spectrokinetic monochromator (Applied Photophysics, model 05-109; bandpass, 4.65 nm/mm). The signals were amplified using a photomultiplier and digitized with a HP Infinium 500 MHz digital oscilloscope (2 GS/s sampling) and recorded on a PC running the Applied Photophysics Reaction Analyzer software package. The zero-order emission was collected where the scattered incident irradiation was filtered using a bandpass filter centered at 640 nm (20 nm full width at half-maximum (fwhm)). The temperature of the sample compartment equipped with a temperature block was regulated using a VWR model 1150S refrigerated circulator. The emission lifetime of RuDCBPY-UiO67 films were obtained as a function of temperature between 5 and 35 °C.
bis(2,2′-bipyridine) mono-(2,2′-bipyridyl-5,5′-dicarboxylic acid), hereafter denoted as RuDCBPY.46,47 These doped materials displayed a propensity for oxygen sensing and a number of photocatalytic reactions.46,47 The photophysical properties of the same RuDCBPY-doped material, RuDCBPYUiO67, have been characterized as a function of doping concentration, and the dilute materials have been shown to share similar excited-state properties as those of RuDCBPY in DMF solution.48 In this report, the preparation of RuDCBPY-UiO67 thin films at various doping concentrations grown on fluorine-doped tin oxide (FTO) and glass substrates is described. The films are characterized spectroscopically and compared to the known properties of the powders. It is shown that the excited states of the films resemble those of powders prepared at similar doping concentrations. The films, though, differ from the powders in the degree of quenching and the temperature dependence of the excited-state lifetime. The increased quenching observed in the films is shown to be related to a decrease in interRuDCBPY distance in the film, which is proposed to be due to an electrostatic effect during nucleation and formation of the MOF framework.
■
EXPERIMENTAL SECTION All chemicals and solvents including RuCl3·xH2O (38−42% Ru), 2,2′-bipyridine (BPY, 99%), 2,2′-bipyridyl-5,5′-dicarboxylic acid (DCBPY, 95%), 4,4′-biphenyldicarboxylic acid (BPDC, 98%), ZrCl4 (98%), dimethylformamide (DMF, HPLC grade > 99%), KOH (85%), isopropyl alcohol, acetone, and alconox were used as obtained without further purification from either Fisher Scientific, Alfa Aesar, or Sigma-Aldrich. FTO were acquired from the Hartford Glass Co., Inc. Preparation of Ruthenium(II) Bis(2,2′-bipyridine)(2,2′bipyridyl-5,5′-dicarboxylic acid) Dichloride. Synthesis of RuDCBPY was carried out according to a procedure similar to that of Xie et al.49 Ruthenium(II) bis(2,2′-bipyridine) dichloride, Ru(bpy)2Cl2 (0.160 g, 0.33 mmol), prepared by the method of Sullivan et al.,50 and DCBPY (0.114 g, 0.46 mmol) was suspended in 60 mL of MeOH and heated at reflux under N2 at 100 °C overnight. The solvent was then removed by rotary evaporation, and the product was crystallized from MeOH/diethyl ether. Preparation of Self-Assembled Monolayer Coated FTO/Glass Slides. FTO and glass slides were cleaned by sonication in an aqueous alconox solution (5 g/(500 mL of H2O)), isopropyl alcohol, acetone, and water for ∼15 min each. After each sonication the slides were rinsed thoroughly with water and air-dried. Slides were then coated with a selfassembled monolayer (SAM) of BPDC by incubation in a DMF solution at room temperature containing 80 mg of BPDC and a stoichiometric amount of pyridine. After 24 h the slides were rinsed with fresh DMF and air-dried. Preparation of RuDCBPY-UiO67 Thin Film. RuDCBPYUiO67 thin films were prepared by modification of a procedure given by Wang et al. for synthesizing powders.46 SAM-coated slides were placed in a 6 dram scintillation vial containing 0.13 g of ZrCl4 (0.56 mmol), 0.14 g of BPDC (0.58 mmol), a known mass of RuDCBPY, and 20 mL of DMF. The amount of RuDCBPY was varied depending on the doping concentration desired within the material; at low doping concentrations, 2.2 mg of RuDCBPY was used. At the highest doping ∼50 mg was added to the ZrCl4/BPDC mixture. Before adding the SAMcoated FTO or glass, the ZrCl4/BPDC/RuDCBPY/DMF 14201
dx.doi.org/10.1021/jp5034195 | J. Phys. Chem. C 2014, 118, 14200−14210
The Journal of Physical Chemistry C
Article
Determination of RuDCBPY Loading of RuDCBPYUiO67. The degree of RuDCBPY doping within the materials was determined by soaking the films mounted on FTO or glass slides in an aqueous 0.1 M KOH solution for 5−10 min. The solutions were then filtered using a 28 mm syringe filter (0.45 μm pore size) and the absorbance obtained at 450 nm. The RuDCBPY concentration in solution was determined assuming the extinction coefficient at 450 nm is similar to that of RuBPY in water (14.6 mM−1 cm−1).52 The dry slides were weighed before and after soaking. The doping concentrations are reported in units of millimolal (mm).
■
RESULTS AND DISCUSSION Synthesis and Structure Characterization. Incubation of a BPDC SAM-coated substrate (FTO or glass) results in the growth of a crystalline thin film of RuDCBPY-UiO67 on the surface of the substrate as evidenced by SEM (Figure 1). The thickness of the films were deduced from the SEM images taken from the side of the film at the film/FTO interface and were found, on average, to be approximately 10 μm. The surface of the RuDCBPY-UiO67 films appeared rugged with obvious crystalline features. However, the crystalline size and shape of the RuDCBPY-UiO67 films were not as defined as for the undoped UiO-67 films, whose fairly congruent crystalline shapes were tetragonal bipyramidal (Supporting Information Figure S1). The GIXD pattern of the film on FTO and the PXRD pattern of the same film scraped off the substrate are consistent with that of the undoped UiO-67 powder (not shown), RuDCBPY-UiO67 powder, and the simulated pattern of UiO67 generated from the structural data (Figure 2). The GIXD and PXRD patterns for the film and the RuDCBPY-UiO67 powder show the characteristic strong UiO-67 (111) and (002) peaks at 2θ values of ∼5.7° and 6.5° indicative that the RuDCBPY-UiO67 films are isostructural with UiO-67. The FTIR spectrum of the RuDCBPY-UiO67/FTO displays features common to the RuDCBPY-UiO67 powder and BPDC ligand (Figure 2). The band assignments of UiO-67 and the doped RuDCBPY-UiO67 are consistent with previously reported results.41,48,53−55 Small shifts (10 mm). This trend has also been observed with increased RuDCBPY doping concentrations in RuDCBPY-UiO67 powders.48 However, the doping concentrations at which this was observed in the films are somewhat lower (20 mm).48 Emission lifetime decays obtained for films at higher doping could adequately be fit to the sum of two discrete exponential functions as well as a stretched exponential function (also known as the Kohlrausch exponential function or the Kohlrausch−Williams−Watts, KWW, decay function).66,67 The latter function describes a distribution of lifetimes by a characteristic parameter β which corresponds to the shape of the distribution. The lifetime value, τ, obtained from the KWW function is the lifetime at the distribution maxima. From τ and β, the average lifetime, ⟨τ⟩, and decay rate constant, ⟨k⟩, can be obtained.68 Results from the biexponential and stretched exponential decay fits are summarized in Table 1 and represent the average of three separate trials for each film. The results obtained from the stretched exponential fits for RuDCBPY-UiO67/FTO films at doping concentrations above 6 mm indicate that the β and ⟨τ⟩ display some sensitivity to an increase in doping. Specifically, a general decrease was observed in ⟨τ⟩ with increased doping concentration, whereas β was observed to increase from ∼0.5 to ∼0.7. From the biexponential fits, it was found that the fractional contributions of the fast (τ1 ∼ 20 ns) and slow (τ2 ∼ 150−200 ns) components were insensitive to an
dx.doi.org/10.1021/jp5034195 | J. Phys. Chem. C 2014, 118, 14200−14210
The Journal of Physical Chemistry C
Article
dipole−dipole mechanism should not affect the magnitude of the observed emission lifetime, impurities, crystal lattice defects, or resonant vibrations inherent to the films may couple to the formed 3MLCT providing energy sinks which quench the emission lifetime.87−93 Upon increased doping, the lifetime of RuDCBPY was found to decrease in a manner similar to that observed in RuDCBPY-UiO67 powders. Three RET regimes have been identified and are dependent on the magnitude of the electronic coupling between the transition moments of the donor and acceptor species; these are the strong coupling, weak coupling, and very weak coupling regimes.68,94,95 An r3 donor−acceptor intermolecular distance dependence is typically observed in systems displaying strong and weak coupling, which are excitonic in nature, whereas the very weak coupling regime is characterized by an r6 distance dependence.68,94−97 The underlying theories describing these regimes imply a discontinuous region between the weak and very weak coupling regime, in which the distance dependence is ill-defined. However, Kenkre and Knox have proposed that the interaction potential along and between these regimes may be described by a continuous single function.97 The order of the distance dependence on the quenching of RuDCBPY in the RuDCBPY-UiO67 films was determined using a general model developed by Inokuti and Hirayami, which relates the time dependence of the emission with the power dependence of the donor−acceptor distance for RET reactions,98 eq 1.
increase in doping concentration; the lifetime of the fast component was also found to be insensitive to doping concentration. The lifetimes obtained for the slow components, however, were quite sensitive to doping concentration. Residuals obtained for the biexponential and stretched exponential function fits to the data were slightly better for the former function (Figure S3 in the Supporting Information). In all, the general trends observed here match those observed for RuDCBPY-UiO67 powders at various degrees of doping.48 Origin of Quenching Behavior. Photoinduced oxidation of RuBPY derivatives doped onto SnO2 and TiO2 dispersions and films has been well established and could explain the quenched lifetime of RuDCBPY in UiO-67/FTO.69−81 Therefore, to ascertain the role of the FTO substrate as a potential quencher of the RuDCBPY 3MLCT, films were also grown on conventional SiO2-based glass slides since SiO2 is known to be insulating and, consequently, does not participate in electron transfer with adsorbed RuBPY.82−84 It should be noted that RuDCBPY-UiO67/glass films loaded below 10 mm were difficult to obtain (even under synthetic conditions in which RuDCBPY mole ratios were
