Highly porous nanocrystalline UiO-66 thin-films via coordination

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Highly porous nanocrystalline UiO-66 thin-films via coordination modulation controlled step-by-step liquid-phase growth Anna Lisa Semrau, Suttipong Wannapaiboon, Sidharam P. Pujari, Pia Vervoorts, Bauke Albada, han zuilhof, and Roland A. Fischer Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b01719 • Publication Date (Web): 30 Nov 2018 Downloaded from http://pubs.acs.org on December 2, 2018

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Crystal Growth & Design

Highly porous nanocrystalline UiO-66 thin-films via coordination modulation controlled step-by-step liquid-phase growth A. Lisa Semrau,a,b Suttipong Wannapaiboon,a,b,c* Sidharam P. Pujari,d Pia Vervoorts,a,b Bauke Albada,d Han Zuilhof,d,e,f and Roland A. Fischera,b* a Department of Chemistry, Technische Universität München, Lichtenbergstraße 4, D-85748 Garching, Germany. b Catalysis Research Centre, Technische Universität München, Ernst-Otto-Fischer Strasse 1, D-85748 Garching, Germany. c Synchrotron Light Research Institute, 111 University Avenue, Muang District, Nakhon Ratchasima 30000, Thailand. d Laboratory of Organic Chemistry, Wageningen University, Stippeneng 4, 6708 WE Wageningen, The Netherlands. e School of Pharmaceutical Sciences and Technology, Tianjin University, 92 Weijin Road, Tianjin, P.R. China. f Department of Chemical and Materials Engineering, King Abdulaziz University, Jeddah 23218, Saudi Arabia. Metal-organic frameworks, thin-film fabrication, liquid-phase epitaxy, layer-by-layer growth, UiO-66. ABSTRACT: Metal-organic frameworks (MOF) possess exciting properties, which can be tailored by rational materials design approaches. Integration of MOFs in functional nano and mesoscale systems require selective crystallite positioning and thin-film growth techniques. Stepwise layer-by-layer liquid-phase epitaxy (LPE) emerged as one of the methods of choice to fabricate MOF@substrate systems. The layer-bylayer (lbl) approach of LPE allows a precise control over the film thickness and crystallite orientation. However, these advantages were mostly observed in cases of tetra-connected dinuclear paddle-wheel MOFs and Hoffmann-type MOFs. Higher connected MOFs (consisting of nodes with 8-12 binding sites), such as the Zr-oxo cluster based families, are notoriously hard to deposit in an acceptable quality by the stepwise liquid-phase process. Herein, we report the use of coordination modulation (CM) to assist and enhance the LPE growth of UiO-66, Zr6O4(OH)4(bdc)6 (bdc2- = 1,4-benzene-dicarboxylate) films. Highly porous and crystalline thin-films were obtained with well control of the crystallite domain size and film thickness in the nanoscale regime. The crystallinity (by GIXRD), morphology (by SEM, AFM), elemental composition (by XPS), binding properties (by IR) and adsorption capacity (by QCM) for volatile organic compounds (e.g. CH3OH) of the fabricated thin films were investigated. These results substantiate a proof-of-concept of CM-LPE of MOFs and could be the gateway to facilitate in general the deposition of chemically very robust and higher-connected MOF thin films with automatic process-controlled LPE techniques under mild synthetic conditions.

INTRODUCTION. Metal-organic frameworks (MOFs) emerged as a new field of porous crystalline materials two decades ago.1 Their three-dimensional network consists of inorganic building blocks (nodes) and multitopic organic ligands (linkers). MOFs self-assemble by coordination bond formation in solution.2 The variation of metal nodes and the choice of organic linkers allows for controlling of the network topology, the guest sorption, transport properties and the chemistry of the coordination space in the pore system; and in turn it allows the targeted design of physical and chemical properties.3 Consequently, MOFs

exhibit fascinating opportunities for many applications, such as gas storage,4–6 separation,7–9 catalysis,10–12 optics,13,14 magnetism,15,16 electronic devices,17 among others. Many of these applications such as gas separation via membranes or integration to chemical sensing and electronic devices need elaborate fabrication methods to form MOF-based composites with interfacial contact to other materials. In the area of MOF crystallite positioning and thin-film deposition, layer-by layer liquid-phase epitaxy (LPE) emerged as an important technique.18 For

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LPE, a substrate is alternatingly treated with a solution containing the metal ions for the nodes and a solution of the chosen organic linker which are separated by rinsing with solvent. This LPE approach allows to control the crystallite domain size and in turn the thin-film thickness by varying the number of deposition cycles. The LPE growth of surface mounted MOFs (so-called SURMOFs) is known to work well for the fabrication of MOF thin-films with tetra-connected nodes, such as HKUST-1 (HKUST = Hong Kong University of Science and Technology, Cu3btc2, btc3- = 1,3,5-benzene-tricarboxylate) as the main object of study.18 So far the application of LPE is mainly restricted to such comparably low-connected paddle-wheel node MOFs.18–24 Some exceptions that do not contain paddlewheel nodes but can be fabricated as SURMOFs are ZIF8,25 Hofmann-type MOFs26 and SIFSIX-1-Cu27. One of the key examples for MOFs, which is notoriously hard to deposit by any stepwise processes is the 12-connected UiO66 Zr6O4(OH)4(bdc)6 (UiO = Universitet i Oslo, bdc2- = 1,4benzene-dicarboxylate). UiO-66 and related MOFs show interesting properties such as high thermal,28 chemical29–31 and mechanical32 stability, which are promising for catalysis,33,34 gas storage35 and gas separation.36,37 So far, the investigation of UiO-66 thin film growth was restricted to electrochemical,38 solvothermal,39 atomic layer deposition (ALD)40 or vapor-assisted conversion.41 The ALD-assisted approach reported by Lausund and Nielsen40 to produce UiO-66 thin films by stepwise gas-phase deposition represents an important conceptual advance in terms of integration to existing process technologies in the microelectronic industry. Nevertheless, ALD does not provide a crystalline UiO-66 thin film directly after material deposition. The as-fabricated thin film is amorphous and only crystallizes via a post-synthetic treatment with acetic acid vapor at high temperatures. Recently, Virmani et al.41 invented the so-called vaporassisted conversion method operating at mild conditions for fabrication of UiO-66 and its related-structure MOF thin-films. Highly-oriented crystalline films were obtained from the direct mixing of the MOF precursors on the substrate surface with the assist of acetic acid vapor (as modulator). Herein, we have modified the LPE process to access the growth of high-quality UiO-66 thin-films (i.e. SURMOFs) via the stepwise bottom-up procedure by employing the addition of a coordination modulator (CM) to the LPE process (CM-LPE). UiO-66 was selected as one of the key examples for so-far inaccessible LPE grown thin-films and represents a proof-of-concept to the LPE deposition of MOF thin-films with higher connecting nodes (i.e. 8-12). This useful procedure was demonstrated by Wannapaiboon et al.42 who showed in 2017 that the addition of acidic modulator (owing the related structure with the ligands in the metal-cluster precursor) into the LPE process of the hexa-connected MOF-5 isotype Zn4O(L)3 (L=3,5-dialkyl-4-carboxypyrazolate) could much enhance the crystallinity, crystallite orientation and thus the porosity of the obtained films. This would be an alternative way to fabricate the highly-connected MOF

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Figure 1. CSA approach of the UiO-66 formation. The methacrylate ligands of the Zr6O4(OH)4(OMc)12 SBU are replaced by bdc linkers to form UiO-66.

films from the above mentioned procedures. Moreover, it also offers advantages for mild synthetic conditions (low temperature and more environmentally friendly solvents i.e. alcohols) and for a further development of composite materials such as heterostructured systems and membranes consisting of higher-connected SURMOFs using the LPE process straightforwardly. Additionally, our methods allow for the precise control of the film thickness and the fabrication of thinner films than reported in literature so far (cf. chapter Film thickness and MeOH uptake). Overall, the herein developed and reported procedure is suggested for optimizing the quality of MOF thin-films deposited via the LPE method in general. Most importantly, however, higher-connected SURMOFs (i.e. 812) should now be generally accessible. The approach that is presented involves in the 1st optimization step of the bulk phase (powder) synthesis via low temperature solvothermal reaction at conditions suitable for LPE by using a molecular secondary building unit (SBU) as a metal source (so-called controlled SBU approach) (cf. Figure 1). Then, in a 2nd step, the optimization of the thin-film deposition via LPE is done. This systematic two-step approach allows to efficiently narrowing down the huge parameter space of the SURMOF deposition protocol to a reasonable amount of settings to be tested. Additional experiments with the in-situ quartz-crystal microbalance (QCM), combined with EXAFs studies, provide insight into the chemical reaction and crystallite growth mechanism discussed hereafter. EXPERIMENTAL. In order to implement the CM-LPE technique for UiO-66 thin-films, the controlled SBU approach (CSA) was employed. Herein, the known methacrylate (McO- = H2C=C(CH3)CO2-) protected zirconium-oxo cluster, (Zr6O4(OH)4(OMc)12) was selected as the source for the zirconium node. The cluster was synthesized according to modified literature procedures (see below). The CSA approach was applied due to the facilitated MOF formation and crystallization at mild conditions during the thin-film fabrication. Synthesis of Zr6O4(OH)4(OMc)12. (Hexazirconiumtetraoxy-tetrahydroxy-dodecamethacrylate).43 The compound was synthesized according to the adapted procedure from Kickelbick and Schubert.43 In a large Schlenk flask, 1 mL of 70% w/v Zr(OPr)4 (3.1 mmol) in n-propanol was mixed at room temperature with 1 mL of


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methacrylic acid (McOH; 11.8 mmol, 5.3 eq.) under inert gas atmosphere (Ar). After two weeks, the colorless crystals were collected by filtration and washing with small quantities of n-propanol. The product was dried in vacuo for 12 h and yield a sample of 860 mg (0.51 mmol, 98 %). The characterization data are provided in the SI (cf. SI section 1). Synthesis of bulk UiO-66 via CSA mild solution approach. A sample of 25.5 mg of Zr6O4(OH)4(OMc)12 (0.015 mmol, 1 eq.), and x molar equivalents (x= 0-400) of the modulator (methacrylic acid, McOH) were dissolved in 4 mL ethanol and then the solution was sonicated (Sonorex Bandelin, 350 Watt) for 60 min. Additionally, 29.9 mg of 1,4-benzene-dicarboxylic acid (H2bdc; 0.09 mmol, 6 equiv.) was dissolved in 8 mL ethanol by sonicating for 60 min in a separated bottle. Then, the two solutions were combined in a 20 mL reaction vessel and then stored in the oven at 70 °C for 24 h to complete the crystallization process. The formed, white precipitate was washed for three times with 5 mL ethanol each. After each washing step, a centrifuge was used to separate the precipitate (7860 rpm, 5 min). The collected colorless (white) powders were dried in the oven at 40 °C. For the activation, the powder was dried in dynamic oil-pump vacuum overnight at room temperature and was stored under argon. For BET measurement the samples were activated at 150 °C for 3 h under vacuum. Fabrication of UiO-66 thin-films (SURMOFs) via CMLPE. The deposition of UiO-66 thin-films was carried out in a double-walled reaction vessel, which was heated with a silicon-oil thermostat. The solutions were pumped in and out by peristaltic pumps computer-controlled by LABview (National Instruments). For the deposition of UiO-66, QCM substrates with a silicon dioxide surface were used (Q Sense, AT cut type, Au electrode, diameter 14 mm, thickness 0.3 mm, and fundamental frequency ca. 4.95 MHz). Prior to the experiment, the substrates were activated under UV light for 60 min in order to create reactive OH groups at the substrate surface and clean it from impurities.44 The activated silica substrate was placed in the reaction vessel and alternatively treated with a 0.5 mM Zr6O4(OH)4(OMc)12 solution in ethanol (containing different amounts of McOH as modulator and 3 mM ethanol solution of H2bdc (containing different amounts of McOH as modulator and different amounts of water as additive). The experimental parameters (molar ratios of SBU, McOH and H2O) are compiled in Table 1 (also see SI 4). The substrate was immersed into each solution for 10 min. In between these reactive steps, the substrate was rinsed with pure ethanol (1000 ppm H2O, purchased from VWR, measured via Karl-Fischer-Titration) for 5 min. These deposition cycles were repeated for 80 times at 70 °C to yield the reported UiO-66 thin-film samples. Characterization methods. Powder X-ray diffraction (PXRD) and grazing incidence X-ray diffraction(GIXRD) measurements were performed on a Panalytical Empyrean instrument. For the PXRD measurements of the powder samples (referring to the 1st stage of the process development), the samples were placed on a silicon wafer cut in the [510] direction. By exchanging the optical mirror

system to a parallel beam setup, the GIXRD measurements of the thin-film samples (referring to the 2nd stage of the process development) could be performed on the same instrument. The detection was carried out with a Pixel3D detector. All measurements were performed with Cu-Kα (λ=1.54 Å) radiation and at 298 K. The XRD patterns were measured from the as-synthesized and activated powders as well as the corresponding thin-film samples. The extended X-ray absorption fine structure (EXAFS) of the synthesized powder reference samples were collected by performing X-ray absorption experiments on Zr K-edge using the transmission mode at BL1.1W, Synchrotron Light Research Institute (Public Organization), Thailand. Three consecutive EXAFS spectra of the powder samples were collected, and then combined by the ATHENA software to extract the (k) function (using the k interval from 3 to 12.5 Å-1 for Fourier transformation). Further, the EXAFS data analysis was performed using the ARTHEMIS software.45 Phase and amplitudes for all single scattering and some high-contribution multiple scattering paths in the effective range (Reff) between 1-4 Å generated from the single crystal X-ray diffraction data were used for the fitting.

Infra-red spectroscopic data (IR) of the activated samples were collected under argon by a Bruker Alpha-P FT-IR spectrometer (from 4000 cm-1 to 400 cm-1) using either a diamond attenuated total reflection (ATR) unit for powder samples or a reflection unit for thin-film samples. For the thin-film measurements uncoated silicon dioxide substrates were used for the background measurements. The scanning electron microscopic (SEM) images of the thin-film morphology of the activated samples were obtained with a JEOL JSM-7500F Field Emission Scanning Electron Microscope with the Gentle Beam mode. Note, that the samples were measured without prior gold sputtering. Atomic force microscopy (AFM) characterization: AFM images (256 × 256 pixels) were obtained with an MFP3D AFM (Asylum Research, Santa Barbara, CA). The imaging was performed in tapping mode in air using OMCL-AC240 silicon cantilevers (Olympus Corporation, Japan) with a stiffness of 1.54 N/m. Images were flattened with a third-order flattening procedure using the MFP3D software. The root-mean-square (RMS) roughness was calculated from the fluctuations of the surface height around the average height in the image. In this way the RMS value describes the topography of the surface. X-ray photoelectron spectroscopy (XPS): The X-ray photoelectron spectra at ambient temperature were obtained using a JPS-9200 photoelectron spectrometer (JEOL, Japan) for all the samples used in the study of the anti-fouling experiment, unless otherwise specified. A monochromatic Al Kα X-ray source (hν = 1486.7 eV, 12 kV and 20 mA) with an analyzer pass energy of 10 eV was used. A base pressure of 3 × 10–7 Torr was maintained in the XPS chamber during measurements and the spectra were collected at room temperature. The X-ray incidence angle and the electron acceptance angle was 10° to the surface normal. The takeoff angle φ (angle between sample and


Crystal Growth & Design detector) of 80° is defined to a precision of 1°. All XPS spectra were evaluated using the Casa XPS software (version 2.3.18). The porosities of the activated UiO-66 powders were characterized with a Micromeritrics 3Flex instrument. The measurements were performed at 77 K using N2 as adsorptive. Prior to the measurement, the samples were activated at 150 °C for 3 h in vacuo. On the other hand, the porosity of the thin-film was determined using a BELQCM4 instrument (MicrotracBEL Corp.). Prior to the sorption measurements, the films deposited on the QCM substrates were activated in situ within the measurement cell of the BEL-QCM instrument by purging with helium gas with a constant flow rate of 100 sccm for 3 h at 80 °C until the change of the QCM frequency was stable within the range of ±5 Hz in 20 min, then the thin films were regarded as activated. The masses of the MOF films (according to Sauerbrey’s equation (∆f=-(2f02)/(A(pq∙μq)0.5)∆m); where f0 = fundamental frequency of the quartz sensor, A = active crystal area, μq = density of quartz, ρq = Shear modulus of quartz, Δm = mass change and Δf = change of the oscillation frequency) were recorded. After that, methanol adsorption isotherms were collected by varying the relative vapor pressure (P/P0) of saturated methanol vapor in He gas flow at 298 K from 0.0 to 95.0%. RESULTS AND DISCUSSION. Prior to the CM-LPE experiments for SURMOF growth, the optimized conditions were identified for a mild UiO-66 solvothermal powder synthesis using the coordination modulated CSA, in order to find an appropriate window of process parameters and narrowing down the huge parameter space for CM-LPE. As the influence of the modulator (McOH) and the additive (H2O) were different for the bulk powder synthesis and the thin-film deposition experiments, the obtained experiments and results are discussed sequentially in the following sections. Synthesis of UiO-66 powders via coordination modulated CSA. The standard synthesis condition of UiO-66 employ mononuclear zirconium sources like ZrCl4 or ZrOCl2 and H2bdc as linker in DMF solution and relatively harsh reaction conditions of up to 220 °C. Additionally, acetic acid is added as coordination modulator to control nucleation and crystal growth, improve the reproducibility of the synthesis, and optimize the crystallinity and crystal shape.46 Due to the anticipated complex formation mechanism of the SBU [Zr6O4(OH)4]12+ following this chemistry and the harsh conditions (involving higher temperature and HCl formation), we selected a carboxylate-protected, preformed zirconium cluster, namely zirconium methacrylate cluster [Zr6O4(OH)4(OMc)12], as the source for the metal node and the employed ligand replacement by the desired linkers (H2bdc) at milder conditions (controlled SBU approach). The successful synthesis of UiO-66 with [Zr6O4(OH)4(OMc)12] in DMF at 100 °C was already reported by Feréy and co-workers.47 However, the crystallization of UiO-66 in DMF is very slow and therefore not suitable for the LPE growth. In addition, DMF is a nondesired, environmentally harmful solvent. Hence, the

eq. of McOH/H2O 300/2000 300/1000 Intensity / a.u.

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300/750 300/300 300/0 200/0 100/0 0/0 UiO-66 calc. 5

10

15

20 2 / °

25

30

Figure 2. PXRD patterns of the activated UiO-66 powders synthesized with Zr-methacrylate SBU as the metal source and different amounts of McOH (as modulator) and water (as additive).

formation and crystallization conditions for UiO-66 powder samples in ethanol were investigated and optimized (cf. Figure 2 and Table 1). Without the addition of McOH modulator to the reaction mixture only an amorphous, nevertheless porous (N2, 757 m2/g) powder material was obtained. The addition of various amounts of McOH to the reaction mixture clearly increases the crystallite domain size of the powders obtained in ethanol at 70 °C for 24 h (cf. Table 1). The addition of 300 eq. of McOH yielded nanocrystalline, highly porous (1149 m2/g) UiO-66 powders. The obtained BET surface areas (cf. Table 1) agree well with typical data for UiO-66 powder samples prepared by conventional and optimized synthetic protocols (e.g. protocol A:48 DMF, H2O, 120 °C, 24 h: 1069 m2/g; protocol B:49 DMF, H2O, 120 °C, 72 h: 1158 m2/g), and with theoretical expectations (1290 m2/g).50 Table 1. Specific surface areas (BET) of UiO-66 powders as a function of the molar ratio of McOH (as modulator) and water (as additive), N2 adsorption isotherms (cf. SI section 2) and the corresponding data of the calculated crystallite sizes (CZ) obtained from PXRD patterns (Scherrer Equation). McOH/ H2O

BET surface area

CZ/

Nm

McOH/ H2O

m2/g

BET surface area

CZ/

nm

m2/g

0/0

757

-

300/0

1149

20

100/0

749

-

300/1000

1234

38


Page 5 of 21 200/0

1018

21

300/2000

1237

39

eq. of McOH/H2O The addition of water to the reaction mixture clearly increases the crystallite domain size of the obtained materials (cf. Figure 2). The full width at half maximum (FWHM) of the 111 reflection - as a measure of the crystallite domain size - decreases from a value of 0.688° at 300 eq. McOH/0 eq. H2O) to 0.353° at 300 eq. McOH/2000 eq. H2O. According to the Scherrer equation this corresponds to a crystallite domain size of 20 nm (300 eq. McOH/0 eq. H2O) and 39 nm (300 eq. McOH/2000 eq. H2O), respectively. Importantly, the addition of water boosts the adsorption capacity (cf. Table 1) and sharpens the IR bands (cf. SI section 2). Additionally, the UiO-66 powder samples obtained via CSA were investigated via EXAFS (Zr K-edge) and the fitting results indicate that the material exhibits similar local coordination environment of Zr as in the reported single crystal XRD data of UiO-66 (CCDC 733458) (cf. SI section 3). This observation supports that the powders synthesized using lower ratio of modulators (less than 200 eq. McOH) still show the short-range order of the ZrSBU with the carboxylic linkers. By closer inspection into the first shell data in R-Space of the EXAFS data, we clearly observe the site splitting of the first shell in the cases of using more than 200 eq. McOH for the synthesis. This is explained by the fact that the local high symmetry order of the clusters is lost at the long-range scale within the UiO66 frameworks leading to the difference shift of the O species in the first shell (the distance from Zr center of the O atoms of the O2- and OH- is shortened whereas the one the of bdc linker is elongated).51 This phenomenon is only observed when the UiO-66 powders are indeed highly crystalline, which well agrees with the obtained XRD data of the powder samples. Using less than 200 eq. McOH, the site splitting is not observed as similar as in the Zr-SBU clusters (precursor), highlighting that there are rather the short-range connections between the Zr-nodes due to the incomplete replacement of the bdc linkers to the McOligands at the SBU node, and hence these sample show amorphous characteristics in the XRD patterns. Integration of Coordination Modulation into the LPE Process. In the 1st step we succeeded in the UiO-66 bulk powder formation via the CSA route at mild conditions using McOH as modulator and water as an additive. In the 2nd stage we transfer these results to the LPE deposition protocol. For the integration of coordination modulation into the LPE process (so-called CM-LPE), quite a number of parameters (more than in the solvothermal process) need to be investigated and optimized. These range from the temperature and the concentrations to the molar ratios of the preformed metal SBU, linker, modulator and additive. Additionally, the LPE method requires a specific timedependent protocol for the different deposition steps (including the washing) and the number of cycles; the choice and pre-conditioning (modification) of the

300/4000 300/3000 300/2000

Intensity / a.u.

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300/1000 300/0 200/0 0/0 UiO-66 calc. 5.0

7.5

10.0

12.5

15.0

17.5

20.0

2 / ° Figure 3. Out-of-plane GIXRD patterns of the activated, nanocrystalline UiO-66 thin-films with typical crystallite domain sizes of 10-18 nm obtained by CM-LPE using [Zr6O4(OH)4(OMc)12], H2bdc and different ratios of McOH (as modulator) and water (as additive).

substrate also plays a role. (cf. SI section 4). For the fabrication of UiO-66 thin-films, native silicon dioxide substrates (UV activated) were used instead of the thermally rather labile organo-thiol based self-assembled monolayers modified Au substrates (which are often used for investigation of LPE-fabricated MOFs thin-films). The Si/SiO2 substrates were placed in a double-walled temperature-controlled vessel (40 mL) and the vessel was set at isothermal conditions of 70 °C. Peristaltic pumps operated by LabView were used to immerse the substrate to the zirconium methacrylate cluster and the linker solution in an alternating fashion for 10 min in each step. In between the steps the substrate was rinsed with ethanol for 5 min. Like in the UiO-66 bulk powder synthesis the thin-film deposition without any modulator resulted in amorphous, but porous, coatings. To improve the thin-film quality (e.g. porosity and increase the crystallite domain size), McOH was added to the LPE process. Optimal thinfilm quality and the largest crystallite domain size was achieved by the addition of McOH into both the Zr6O4(OH)4(OMc)12 and the H2bdc solution. Water as an additive was only added to the H2dbc solution. The EtOH solution for the rinsing was used without further additives or modulators. As shown in the GIXRD patterns (cf. Figure 3) of the activated films (GIXRD for the as-synthesized thin-films cf. SI section 5), the addition of 300 eq. of McOH



in respect to the SBU and the addition of 1000 eq. of H2O resulted in the largest crystallite domains. The FWHM for the 111 reflection is between 1.27457° (for 300 eq. McOH/4000 eq. H2O) and 0.7812° (300 eq. McOH/1000 eq. H2O), and corresponds to a crystallite size of 10.9 and 17.8 nm according to the Scherrer equation, respectively. The addition of more water to the linker solution results in smaller crystallite domain sizes.

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carboxylate groups (brown trace); for an analogous comparison of O1s energies see Carvalho et al.52 and Wang et al.51 The film synthesized with 300 eq. of McOH has 20.9 % of free carboxylic acids and 79.1 % of bridging carboxylic acids. The addition of water to the fabrication of the UiO66 thin-film leads to an increase of free carboxylic acids (26.9 % for 4000 eq. of water; for calculation and explanation cf. SI section 7). This can be ascribed to defects

Figure 4. O1s XPS spectra of the UiO-66 thin-film fabricated with various amounts of McOH/H2O: A (0/0); B (200/0); C (300/o); D (300/1000); E (300/2000); F (300/3000) and G (300/4000). The chart shows the content of oxygen in four different chemical environments Zr-O-Zr (blue), Zr-OOC (red), Zr-OOC-2 (purple) and free COOH (green).

One series of analytical experiments were conducted and characterization techniques were applied in order to confirm the elemental composition (XPS, cf. Figure 3), crystal morphology (SEM, Figure 4; AFM, cf. SI section 10) and the porosity (BEL-QCM, methanol adsorption, cf. Figure 5) and in addition to elucidate the CM-LPE mechanism. The additional investigation of the thin-films via EXAFS was not possible due to the limitation of the Xray absorption spectroscopy (XAS) experimental set-up being operated in the transmission mode, which is not suitable for the targeted samples on non-transparent substrates like the Si/SiO2 substrates.

or to dangling carboxylates covering the outer surface of small MOF crystallites. The fourth species is observed present in the thin-film fabricated without modulator. This could originate from non-coordinating carboxylic acid in a bridging mode (cf. IR, SI section 15). Additionally, the higher amount of defects can partly explain the enhanced adsorption capacity (see data and discussion below). The presence of silicon (SI) in all the thin-film samples at first sight seemed surprising due to the surface sensitivity of the XPS method. However, as evidenced by SEM imaging (cf. Figure 5) suggested the substrates were not fully covered by UiO-66 (see discussion below).

Elemental composition and defects. XPS was used to confirm the elemental composition of the thin-films (cf. Figure 4). The ideal, defect-free stoichiometric and fully activated (adsorbent-free) UiO-66 thin-film material should contain 6.98 at% of Zr, 37.21 at% of O and 55.81 at% of C. All crystalline thin-films are composed of similar amounts of zirconium, carbon and oxygen, which are in agreement with the theoretical expectations (cf. SI section 6). The amorphous thin-film which was synthesized without adding any modulator has considerably lower zirconium and oxygen content. This can be ascribed to a higher amount of linker loading or to remaining McOH coordinated to the cluster. According to Wang et al.51 XPS spectra of oxygen can be used to determine the amount of free carboxylic acid groups in the MOF (cf. Figure 4). Free carboxylic acids (green trace) exhibit higher oxygen binding energies compared to the zirconium-bound

Morphology and thickness. Selected SEM images (cf. Figure 5) reveal that the substrates were not fully covered, rather an island structure was observed. Thus, non-covered bare silica areas exist between the crystalline UiO-66 islands. This was proven by Auger spectroscopy (cf. SI section 8). The best continuous thin-film quality with small non covered areas was obtained for the sample C (300 eq. McOH, no water). All sample images are compiled in SI section 9 (SEM) and SI section 10 (AFM). The partial coverage of the substrates can be explained by a very slow initiation of the deposition (cf. section CM-LPE mechanism, in-situ QCM monitoring (see below)). Hence, the inhibited nucleation at the Si/SiO2 substrate due to the lack of active binding functional groups on the substrate surface appears to control the obtained film morphology. The formation of dense, fully covered thin-films should be possible if the nucleation process is optimized for a more


Page 7 of 21

thorough substrate activation, modification or via the choice of different and more nucleation active substrates. However, our study was focused on the investigation of the

capacity rises. For comparison, a UiO-66 sample with a low defect content was synthesized in DMF according to Shearer et al.53 (cf. SI section 11). In the case of the CM-LPE

Figure 5. SEM images of the activated (He flow, 3 h, 80 °C), nano-crystalline UiO-66 thin-films fabricated with various amounts of McOH/H2O: A (0/0); B (200/o); C (300/o); D (300/1000); E (300/2000); F (300/3000) and G (300/4000).

grown nanocrystalline thin-films, however, the presence of water in the dosing step of the SBU decreases the crystallite domain size, and interestingly the additive water also boosts the adsorption capacity of the fabricated thin-films for MeOH absorption in a similar fashion as it was observed for the powder samples for N2 adsorption.

7 Methanol adsorbed / mmol g-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


6 5 4 3 2

eq. of McOH/H2O / 0/0 / 300/0 / 300/2000 / 300/4000

1 0 0

/ / /

200/0 300/1000 300/3000 / bulk

20 40 60 80 Relative vapor pressure / %

100

Figure 6. Methanol adsorption dependent on the relative methanol vapor pressure of the UiO-66 thin-films fabricated with different amounts of McOH and water, respectively. The black stars show the methanol adsorption of UiO-66 powder synthesized via the conventional route (cf. SI section 11). The adsorption and desorption are shown in solid and open symbols, respectively.

parameter space of LbL CM-LPE for achieving high crystallinity and porosity of the deposited UiO-66 material and achieving a full substrate coverage was therefore not further aimed for. Porosity measurements. The measured mass-specific methanol adsorption capacity (cf. Figure 6) depends quite significantly on the CM-LPE conditions. The solvothermal synthesized UiO-66 powder samples and the CM-LPE grown thin-films with the largest crystallite domain sizes (McOH/H2O: 300/0 eq. and 300/1000 eq., respectively) adsorb about 3.6 mmol/g of methanol (MeOH), while the thin-films fabricated with higher amounts of water (1000 eq.) adsorb up to 6.6 mmol/g. In the bulk powder synthesis of UiO-66, the addition of water increases the crystallite domain size and as well the N2 adsorption

This observation can be ascribed to the combination of effects as the formation of structural defects in the framework of UiO-66 (cf. XPS, Figure 4), the increased roughness of the films (cf. SEM, Figure 5) and the small primary crystalline domain sizes (cf. Figure 3). Additionally, residual water, which was not removed during the activation procedure can cause and increased adsorption capacity of the samples, that were synthesized with additional water. Table 2. Average thickness of UiO-66 thin-films determined by AFM and deposited mass measured via QCM. Equivalents of McOH /H2O

Average Thickness / nm

Deposited mass / ng

0/0

744

33200

200/0

390

25900

300/0

82

9700

300/1ooo

312

9800

300/2000

361

22900

300/3000

425

29100

300/4000

678

30500

CM-LPE mechanism. Comparing the average thickness and mass (Table 2), it becomes clear that the McOH facilitates the formation of smoother islands and more densely packed films (higher surface coverage).


Crystal Growth & Design Additionally, the higher amount of McOH leads to a mass decrease and thinner films. Upon increasing the water content the deposited mass increases. These observation are nicely in line with the following mechanistic considerations: during UiO-66 thin-film growth and according to equation 1, the dissolved methacrylatecapped zirconium cluster binds to the substrate surface (HL). Zr6O4(OH)4(OMc)12+HL⇌[Zr6O4(OH)4(OMc)11]L+McOH (1) H2bdc + H2O ⇌ H3O+ + Hbdc-

0

(2)

eq. of McOH/H2O

-2000 F / Hz

-1800

-4000

-1900

-6000

F / Hz

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

eq. of McOH/H2O: 100/100 H2bdc EtOH Zr6 cluster EtOH

100/100

EtOH H2bdc

-2000

EtOH

-2100

-8000

1080

0

1100

500

1120 t / min

1140

1000

1500

100/0 2000

2500

t / min Figure 7. The change of QCM oscillator frequency (ΔF) as a function of time during the fabrication of UiO-66 thin-films on Si/SiO2 substrates at 40 °C using different amounts of McOH (modulator) and water (additive).

Page 8 of 21

Upon binding, McOH is released involving a protoncoupled coordination equilibrium. Therefore, additional McOH (as modulator) shifts the equilibrium towards the dissolved cluster, resulting in a reduced nucleation density and crystal growth efficiency. Obviously, the etching of loosely bound and probably disoriented or less connected clusters improves the thin-film quality. The control of these equilibria enables the formation of highly crystalline UiO-66 thin-films. A more detailed study of the CM-LPE mechanism would need an in-situ monitoring of the growth, for example via QCM microbalance. Unfortunately, the minimum temperature of 70 °C that we found to be necessary for the deposition of highly crystalline UiO-66 thin-films samples discussed above, was still above of the maximum of the accessible temperature range of our QCM instrument (50 °C). Thus, the conditions were tested with the instrument at 40 °C and the growth curvatures were recorded. As expected, the deposition experiments only yielded in an amorphous, yet comparably porous coatings (2.8-3.1 mmol MeOH/g). We assume that the growth mechanism at 40 °C and 70 °C, are similar just the crystallite domain sizes are too small for determination via XRD techniques. Obviously, the slightly higher temperature of 70 °C significantly enhances crystal domain growth. Nevertheless, the in-situ mass tracking via QCM (cf. Figure 7) reveals that the nucleation process of UiO-66 at the given Si/SiO2 substrates takes up to 30 cycles (900 min). During that time there is no significant mass gain detected. Addition of water (cf. Figure 7, orange) to the linker solution enhances the nucleation. Nucleation is followed by a rapid growth and a stationary plateau is observed at long deposition times and large number of cycles (> 2000 min). The stationary plateau at the end can be ascribed to the formation of powder-like structures on the surface.

Figure 8. IR spectra of the UiO-66 thin-films fabricated with various molar ratios of McOH and water in respect to the amount of the metal SBU.


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The addition of more modulator (200 eq.; cf. SI section 12) reduces the formation of these structures. Just like the amount of modulator applying temperature to the deposition process has an equally important effect on the formation of crystalline UiO-66. Specifically, at 60 °C another crystalline phase forms (cf. SI section 13), which cannot be ascribed to UiO-66. The films that were fabricated at 40 °C yield amorphous, yet porous (2.8-3.1 mmol MeOH/g) thin-films (cf. SI section 14). Coordination mode of H2bdc in UiO-66 films obtained by CM-LPE. IR spectra were measured to examine the binding mode of bdc2- and the SBU. For the fabrication of UiO-66 with ALD, Lausund et al.40 reported that ALD initially resulted in an amorphous film. They investigated this material via IR spectroscopy and it became clear that the distinct coordination mode of zirconium to the oxygen of bdc2- was not visible for the amorphous film. After the post synthetic treatment with acetic acid vapor the peaks are sharpened. In this context, Verpoort et al.54 have studied the coordination mode of carboxylates and metals. They relate the difference between the peak at 1400 cm-1 and the most prominent peak in between 1500-1700 cm-1 to the coordination mode of the carboxylate to the metal. Note that these peaks correspond to the symmetric and antisymmetric stretches of the carboxyl group, respectively. According to literature, the difference of the two bands is important to distinguish between a monodentate (>200 cm-1), bidentate (50-150 cm-1) or bridging (130-200 cm-1) coordination mode of the bdc linker to the metal (cf. SI section 15). The IR spectra of the synthesized UiO-66 thin-films via CM LPE (cf. Figure 8) and the solvothermal bulk reference powders (cf. SI section 1) show that the coordination mode of the metal center and the linker sharpens upon the addition of McOH. the broader peaks of the samples, which were synthesized without any modulator can be ascribed to a mixture of different coordination modes at the same time, which are in agreement with the deviating XPS results (cf. Figure 4). All in all, the IR spectra confirmed that the CM-LPE fabricated films exhibit the expected bridging binding mode. Film thickness and MeOH uptake. Additionally to prior attempts from Wannapaiboon et al.42 and Khajavian and Ghani55 we used the CM-LPE technique to grow crystalline, 12-connected UiO-66, which could not be fabricated via the standard LPE method, yet. In comparison to other UiO-66 thin-film fabrication methods (film thicknesses of 200 nm41 – 5 μm38; cf. SI section 16), CM-LPE allows for a more precise and simple control of the film thickness even below 100 nm by the number of deposition cycles, which are applied. The crystallite domain size (18 nm) of the obtained nano-crystalline films is in the range of the reported domain sizes of 13 nm40 – 70 nm41 of other UiO-66 thin-films. Importantly, the CM-LPE fabricated UiO-66 thin-films are highly porous and can take up to 3.3-6.6 mmol MeOH/g. Unfortunately, there is no comparison for the MeOH adsorption properties of other UiO-66 thin-

films reported in literature (cf. SI section 16). However, we show that the specific uptake of methanol for conventionally synthesized UiO-66 powder samples and the obtained thin-films is equal. The results show that the CM-LPE technique is a powerful tool in the growth of so far inaccessible MOF thin-films. CONCLUSION. We demonstrate a systematic approach that combines continuous modulation (CM) solvothermal bulk powder synthesis (1st step) with transfer to CM-LPE for film deposition (2nd step). This way we effectively reduce the parameter space for the 2nd step and obtain meaningful reference data for the related thin-film materials. The influence of methacrylic acid as the modulator, water as an additional additive to facilitate proton-dependent coordination equilibria, and the effect of temperature on UiO-66 thin-film formation were all eq. of McOH/H2O: 300/4000 300/3000 300/2000

Intensity / a.u.

Page 9 of 21

300/1000 300/0 200/0 0/0

4000

3500

3000 2500 2000 1500 wave number / cm-1

1000

500

studied in detail. The accessibility of crystalline UiO-66 thin-films via the CM-LPE method will help to develop new composite materials, such as heterostructured systems and membranes consisting of related-robust, higher-connected SURMOFs and further integration into devices via our LPE-based method. Additionally, the LPE process allows to fabricate UiO-66 thin films at mild conditions. We currently investigate the application of CM-LPE for thinfilm fabrication of notoriously hard to grow SURMOFs of higher connectivity (i.e. 8-12 fold) such as other Zr-based systems and the MIL family of metal-organic frameworks.

ASSOCIATED CONTENT Supporting Information Supporting Information include the full characterization (IR, PXRD, N2 adsorption, EXAFS) of the bulk UiO-66 powders and the XPS, AFM and SEM images of the thin-films and further information. This material is available free of charge via the Internet at http://pubs.acs.org.



AUTHOR INFORMATION

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Corresponding Author *[email protected] *[email protected], [email protected]

ORCID ID Anna Lisa Semrau: 0000-0001-7087-932X Suttipong Wannapaiboon: 0000-0002-6765-9809 Sidharam Pujari: 0000-0003-0479-8884 Pia Vervoorts: 0000-0002-0399-7016 Bauke Albada: 0000-0003-3659-2434 Han Zuilhof: 0000-0001-5773-8506 Roland A. Fischer: 0000-0002-7532-5286

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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

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ACKNOWLEDGMENT ALS thanks the DFG Priority Program 1928 ‘Coordination Networks: Building Blocks for Functional Systems’. BL1.1W at the Synchrotron Light Research Institute (Public Organization), Thailand is acknowledged for the facilities and the XAS beamtime. We thank Dr. Chatree Saiyasombat and Dr. Prae Chirawatkul for fruitful discussion on the EXAFS data analysis.

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ABBREVIATIONS ALD, Atomic layer deposition; AFM, Atomic force microscopy; H2bdc, 1,4-Benzene-dicarboxylic acid; CM, coordination modulation; GIXRD, Grazing incidence X-ray diffraction; IR, Infrared spectroscopy; LPE, liquid phase epitaxy; McOH, Methacrylic acid; MOF, metal-organic framework; PXRD, Powder X-ray diffraction; QCM, Quartz crystal microbalance, SEM, Scanning Electron Microscopy; UiO-66, [Zr6O4(OH)4(bdc)6].

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Framework Films via a Postsynthetic Ligand Exchange. Chem. Commun. 2015, 51, 66–69. Lausund, K. B.; Nilsen, O. All-Gas-Phase Synthesis of UiO66 through Modulated Atomic Layer Deposition. Nat. Commun. 2016, 7, 1–9. Virmani, E.; Rotter, J. M.; Mähringer, A.; Von Zons, T.; Godt, A.; Bein, T.; Wuttke, S.; Medina, D. D. On-Surface Synthesis of Highly Oriented Thin Metal-Organic Framework Films through Vapor-Assisted Conversion. J. Am. Chem. Soc. 2018, 140, 4812–4819. Wannapaiboon, S.; Sumida, K.; Dilchert, K.; Tu, M.; Kitagawa, S.; Fukukawa, S.; Fischer, R. Enhanced Properties of Metal-Organic Framework Thin-Films via Integration of Coordination Modulation with Layer-by-Layer Fabrication Processes. J. Adv. Mater. Chem. A 2017, 5, 13665–13673. Kickelbick, G.; Schubert, U. Oxozirconium Methacrylate Clusters: Zr6(OH)4O4(OMc)12 and Zr4O2(OMc)12 (OMc = Methacrylate). Chem. Ber. Recl. 1997, 6, 473–477. Vig, J. R.; W. Lebus, J. UV/Ozone Cleaning of Surfaces. IEEE Trans. Parts Hybrids Packag. 1977, 12, 365–370. Ravel, R.; Newville, M. ATHENA, ARTEMIS, HEPHAESTUS: Data Analysis for X-Ray Absorption Spectroscopy Using IFEFFIT. J. Synchrotron Radiat. 2005, 12, 537–541. Schaate, A.; Roy, P.; Godt, A.; Lippke, J.; Waltz, F.; Wiebcke, M.; Behrens, P. Modulated Synthesis of Zr-Based MetalOrganic Frameworks: From Nano to Single Crystals. Chem. A Eur. J. 2011, 17, 6643–6651. Guillerm, V.; Gross, S.; Serre, C.; Devic, T.; Bauer, M.; Férey, G. A Zirconium Methacrylate Oxocluster as Precursor for the Low-Temperature Synthesis of Porous Zirconium(IV) Dicarboxylates. Chem. Commun. 2010, 46, 767–769. Valenzano, L.; Civalleri, B.; Chavan, S.; Bordiga, S.; Nilsen, M. H.; Jakobsen, S.; Lillerud, K. P.; Lamberti, C. Disclosing the Complex Structure of UiO-66 Metal Organic Framework: A Synergic Combination of Experiment and Theory. Chem. Mater. 2011, 23, 1700–1718. Shearer, G. C.; Chavan, S.; Bordiga, S.; Svelle, S.; Olsbye, U.; Lillerud, K. P. Defect Engineering: Tuning the Porosity and Composition of the Metal-Organic Framework UiO-66 via Modulated Synthesis. Chem. Mater. 2016, 28, 3749–3761. Gómez-Gualdrón, D. A.; Moghadam, P. Z.; Hupp, J. T.; Farha, O. K.; Snurr, R. Q. Application of Consistency Criteria to Calculate BET Areas of Micro- and Mesoporous MetalOrganic Frameworks. J. Am. Chem. Soc. 2016, 138, 215–224. Wang, Y.; Li, L.; Dai, P.; Yan, L.; Cao, L.; Gu, X.; Zhao, X. The Missing-Nodes Directed Synthesis of Hierarchical Pores on a Zirconium Metal-Organic Framework with Tunable Porosity and Enhanced Surface Acidity via Microdroplet Flow Reaction. J. Mater. Chem. A 2017, 5, 22372–22379. Carvalho, R. R.; Pujari, S. P.; Vrouwe, E. X.; Zuilhof, H. Mild and Selective C-H Activation of COC Microfluidic Channels Allowing Covalent Multifunctional Coatings. ACS Appl. Mater. Interfaces 2017, 9, 16644–16650. Shearer, G. C.; Chavan, S.; Ethiraj, J.; Vitillo, J. G.; Svelle, S.; Olsbye, U.; Lamberti, C.; Bordiga, S.; Lillerud, K. P. Tuned to Perfection: Ironing out the Defects in Metal-Organic Framework UiO-66. Chem. Mater. 2014, 26, 4068–4071. Verpoort, F.; Haemers, T.; Roose, P.; Maes, J. P. Characterization of a Surface Coating Formed from Carboxylic Acid-Based Coolants. Appl. Spectrosc. 1999, 53, 1528–1534. Khajavian, R.; Ghani, K. Fabrication of [Cu2(Bdc)2(Bpy)]n Thin Films Using Coordination Modulation-Assisted Layerby-Layer Growth. CrystEngComm 2018, 20, 1546–1552.



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For Table of Contents use only Highly porous nanocrystalline UiO-66 thin-films via coordination modulation controlled step-by-step liquid-phase growth A. Lisa Semrau, Suttipong Wannapaiboon, Sidharam P. Pujari, Pia Vervoorts, Bauke Albada, Han Zuilhof, and Roland A. Fischer

Herein, we report the use of coordination modulation (CM) liquid phase epitaxy (LPE) to grow UiO-66, Zr6O4(OH)4(bdc)6 (bdc2- = 1,4-benzene-dicarboxylate) films. Highly porous and crystalline thin-films were obtained with high control of the crystallite domain sizes and film thickness in the nanoscale regime.


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Figure 1. CSA approach of the UiO-66 formation. The methacrylate ligands of the Zr6O4(OH)4(OMc)12 SBU are replaced by bdc linkers to form UiO-66. 279x90mm (150 x 150 DPI)



Figure 2. PXRD patterns of the activated UiO-66 powders synthesized with Zr-methacrylate SBU as the metal source and different amounts of McOH (as modulator) and water (as additive). 117x119mm (300 x 300 DPI)


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Figure 4. O1s XPS spectra of the UiO-66 thin-film fabricated with various amounts of McOH/H2O: A (0/0); B (200/0); C (300/0); D (300/1000); E (300/2000); F (300/3000) and G(300/4000). The chart shows the content of oxygen in four different chemical environments Zr-O-Zr (blue), Zr-OOC (red), Zr-OOC-2 (purple) and free COOH (green). 208x106mm (150 x 150 DPI)



Figure 5. SEM images of the activated (He flow, 3 h, 80 °C), nano-crystalline UiO-66 thin-films fabricated with various amounts of McOH/H2O: A (0/0); B (200/0); C (300/0); D (300/1000); E (300/2000); F (300/3000) and G (300/4000). 176x67mm (220 x 220 DPI)


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Figure 6. Methanol adsorption dependent on the relative methanol vapor pressure of the UiO-66 thin-films fabricated with different amounts of McOH and water, respectively. The black stars show the methanol adsorption of UiO-66 powder synthesized via the conventional route (cf. SI section 11). The adsorption and desorption are shown in solid and open symbols, respectively. 220x180mm (300 x 300 DPI)



Figure 3. Out-of-plane GIXRD patterns of the activated, nanocrystalline UiO-66 thin-films with typical crystallite domain sizes of 10-18 nm obtained by CM-LPE using [Zr6O4(OH)4(OMc)12], H2bdc and different ratios of McOH (as modulator) and water (as additive). 231x312mm (300 x 300 DPI)


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Figure 7. The change of QCM oscillator frequency (ΔF) as a function of time during the fabrication of UiO-66 thin-films on Si/SiO2 substrates at 40 °C using different amounts of McOH (modulator) and water (additive). 84x78mm (150 x 150 DPI)



Figure 8. IR spectra of the UiO-66 thin-films fabricated with various molar ratios of McOH and water in respect to the amount of the metal SBU. 90x92mm (300 x 300 DPI)


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Table of Contents graphic 279x143mm (150 x 150 DPI)


Highly porous nanocrystalline UiO-66 thin-films via coordination

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