Article pubs.acs.org/cm
Electrochemical Film Deposition of the Zirconium Metal−Organic Framework UiO-66 and Application in a Miniaturized Sorbent Trap Ivo Stassen,† Mark Styles,‡ Tom Van Assche,∥ Nicolò Campagnol,§ Jan Fransaer,§ Joeri Denayer,∥ Jin-Chong Tan,⊥ Paolo Falcaro,‡ Dirk De Vos,† and Rob Ameloot*,† †
Department of Microbial and Molecular Systems, Centre for Surface Chemistry and Catalysis, KU LeuvenUniversity of Leuven, Kasteelpark Arenberg 23, B-3001 Leuven, Belgium ‡ CSIRO Manufacturing Flagship, Clayton, Victoria 3168, Australia § Department of Metallurgy and Materials Engineering, KU LeuvenUniversity of Leuven, Kasteelpark Arenberg 44, B-3001 Leuven, Belgium ∥ Department of Chemical Engineering, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussel, Belgium ⊥ Department of Engineering Science, University of Oxford, Parks Road, Oxford OX1 3PJ, United Kingdom S Supporting Information *
ABSTRACT: Film deposition is an enabling technology for integration of novel functional materials into real-world practical applications. We report both the anodic and cathodic electrochemical film deposition of UiO-66, a prototype of highly stable, zirconium-based metal−organic frameworks, using zirconium foil as the only metal source. The fundamentally different film formation mechanisms at the cathode and anode result in a significantly different coating adhesion strength, mainly due to the formation of an oxide film serving as a bridging layer at the anode. The patterned deposition capability of the electrochemical method enables the straightforward integration of UiO-66 as a sorbent film in a miniaturized sorbent trap for online analytical sampling and concentration of dilute volatile organic compounds.
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to the substrate is often limited.24 In situ deposition generally offers an improvement over these shortcomings. Electrochemical MOF deposition has been proposed as a promising and industrially relevant approach for in situ deposition and patterning on conductive surfaces.2 Two fundamentally different electrochemical MOF deposition mechanisms have been reported: anodic deposition and cathodic deposition. In anodic deposition, MOF film formation occurs on a metal anode contacted with a ligand solution as a result of the release of a critical concentration of metal ions by anodic dissolution.25,26 In cathodic deposition, on the other hand, a solution containing a ligand, metal ions, and a so-called probase is contacted with a cathodic surface. Film deposition in this case is the result of an increase in pH near the cathodic surface, where electrochemical reduction of the probase results in local base generation and subsequent ligand deprotonation, inducing MOF formation.27,28 Even though anodic deposition of Cu(II),25,29−31 Fe(III),32 Gd(III),33 Tb(III),33 and Zn(II)34 MOFs and cathodic deposition of Zn(II),27 Tb(III),35 and Eu(III)36 MOFs have
INTRODUCTION
Metal−organic frameworks (MOFs), a class of hybrid nanoporous crystalline materials, have been studied extensively in the past decade because of their high specific surface area and their unique structural and chemical tunability.1 For applications such as gas adsorption and catalysis, MOFs are mainly used as bulk materials in the form of powders. For other applications, suitable film deposition and patterning techniques are enabling technologies.2−6 Various film deposition strategies have been reported, mostly for MOFs forming under mild conditions. However, considerable interest exists in MOFs that require harsher crystallization conditions, such as MOFs based on high-valent metal ions, as these can have superior chemical and thermal stabilities. One typical example is the Zr(IV)-based Zr6O4(OH)4(BDC)6 framework, termed UiO-66.7−9 Solvothermal synthesis,10−12 crystal structure,13,14 and various properties and potential applications in, for example, catalysis,15 photocatalysis,16 supercapacitors,17 battery technology,18 and microextraction19 have recently been studied for UiO-66 and its isoreticular analogues. However, reports on film deposition of UiO-66 are scarce and, apart from a few exceptions,20 limited to deposition of ex situ prepared particles.17,21−23 Ex situ deposited films are, however, never intergrown, and adhesion © 2015 American Chemical Society
Received: December 31, 2014 Revised: January 29, 2015 Published: February 16, 2015 1801
DOI: 10.1021/cm504806p Chem. Mater. 2015, 27, 1801−1807
Article
Chemistry of Materials
Figure 1. Scheme of the anodic and cathodic electrochemical deposition mechanisms demonstrated in this paper. mm2) and rinsed with acetone before use as electrode materials. 2,2′Dinitrobiphenyl-4,4′-dicarboxylic acid was prepared from dimethyl biphenyl-4,4′-dicarboxylate (Sigma-Aldrich, 99%) according to a previously published procedure,44 followed by hydrolysis with NaOH in ethanol/water and acidification with HCl. Electrochemical Deposition Setup. Film deposition was performed in an electrolytic cell consisting of a power supply unit (Thurlby Thandar Instruments TS3021S), a screw cap glass bottle (Duran pressure plus+) containing a synthesis solution, and two electrodes connected to the power source with stainless steel clips (perforating the screw cap). Heating was supplied by an oil bath. The parallelly mounted electrodes were immersed approximately 20 mm deep in solution, 15 mm from each other. Note that due to gas evolution during synthesis, an airtight cell is not recommended unless pressure resistance is ensured. High-temperature, high-pressure (HT− HP) synthesis was performed in our previously reported HT−HP electrolytic cell.32 Film Deposition Procedure. A 415 mg portion of 1,4benzenedicarboxylic acid (BDC) was dissolved in 25 mL of dimethylformamide (DMF). To this solution were added 300 μL of nitric acid 68%, 90 μL of water, and 715/1430/7150 μL of glacial acetic acid (AA). The final molar ratio of the synthesis solution was BDC:HNO3:H2O:AA:DMF = 1:2:4:5/10/50:130. The corresponding final AA concentration was respectively 0.5, 1, or 5 M. For the preparation of (NO2)2-UiO-67, a similar solution was used except for replacing BDC by 825 mg of 2,2′-dinitrobiphenyl-4,4′-dicarboxylic acid. For the preparation of ZrSQU, a previously reported synthesis solution was used.45 Zirconium foil was used both as anode and cathode material, unless stated otherwise. Before applying a voltage to the electrodes, the synthesis solution was heated to 383 K. Film deposition was performed at this temperature by applying a current of 80 mA for a specified time. A potential difference between 6 and 9 V was typically applied to maintain a constant current density. In the absence of nitric acid, no significant current could be triggered, not even at 30 V, which is the maximum potential of the power supply unit. Gas evolution could be observed (by eyes) at both electrodes during the deposition. After deposition, the synthesis solution was cooled, and the electrodes were removed, soaked subsequently in DMF and methanol, and dried under air flow. The side of the anode facing the cathode, and vice versa, was used for all analyses. Application in a Miniaturized Sorbent Trap. Device integration was performed in a similar way as our previously reported procedure.46 In summary, (1) polyether ether ketone (PEEK) adhesive tape (500 μm thick) was patterned with a meandering channel (171 mm length, 2 mm width) by precision milling and applied to the zirconium foil
been demonstrated, no reports exist on the electrochemical deposition of higher valence cation frameworks. While the oxophilic nature of Zr(IV) is a main contributor to UiO-66’s high stability,7 it also implies more challenging electrochemistry. The incorporation of Zr(IV) ions in a MOF lattice upon anodic dissolution of a zirconium electrode is in direct competition with oxide formation, more so than for the previously mentioned metals.37 Furthermore, zirconium passivation by oxide film formation on exposed metallic surfaces results in a high resistance to further oxidation, even superior to the corrosion resistance of stainless steel.38,39 Similar difficulties exist at the cathode, where deposition of zirconium hydroxide is expected via reaction of dissolved zirconium ions with cathodically generated hydroxide ions.37 Although the conversion of metal oxide and hydroxide films to MOF coatings has been demonstrated,40−42 this reaction does not seem to be feasible for Zr(IV)-based (hydr)oxides.43 These considerations underline the challenges of identifying the conditions suitable for electrochemical deposition of Zr(IV)based MOFs. Herein, we demonstrate both the anodic and cathodic film deposition of UiO-66 and related Zr(IV)-based MOFs (Figure 1). Zirconium oxide film formation on the anode, as well as the rate of cathodic deposition and the UiO-66 film morphology, is controlled via a synthesis modulation approach using acetic acid. The identification of this set of deposition conditions enables the deposition of a range of Zr-based MOFs on different conductive substrates. Film adhesion on the anode and cathode is compared by a nanoindenter scratching study. Film integration in a miniaturized flow-through device for sampling and concentration of volatile organic compounds (VOCs) is also demonstrated.
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EXPERIMENTAL SECTION
Chemicals and Materials. 1,4-Benzenedicarboxylic acid (SigmaAldrich, 98%), nitric acid (Fisher Scientific, 68%), acetic acid (Acros Organics, 99%), toluene (Sigma-Aldrich, 99.8%), and squaric acid (Sigma-Aldrich, 99%) were used as received without further purification. Zirconium foil (0.25 mm thick, ABCR, 99.8%), titanium foil (1.0 mm thick, ABCR, 99.2%), and titanium nitride on a silicon wafer (sputtered by physical vapor deposition) were cleaved (10 × 30 1802
DOI: 10.1021/cm504806p Chem. Mater. 2015, 27, 1801−1807
Article
Chemistry of Materials substrate. (2) The assembly was mounted in a PTFE frame with a cutout at the front to exclude reaction at the backside of the foil. Anodic film deposition was performed on the zirconium surface exposed through the PEEK pattern for 120 min (0.5 M AA concentration). After washing and drying, the sample was activated by evacuation in a vacuum oven at 373 K for 4 h. (3) Subsequently, the assembly was mounted in a home-built device designed to enable gas flow through the patterned PEEK channels while the channel temperature was controlled via heating of the zirconium foil. Downstream of the device, organics were detected using a flame ionization detector (FID). Before the adsorption experiments, the device was flushed overnight with 10 mLN min−1 He at 343 K. Next, vaporized toluene in He (200 ppmv, 10 mLN min−1) was introduced into the setup via a temperature-controlled saturator in combination with a set of mass flow controllers. After equilibration at 308 K, as indicated by FID, toluene desorption was monitored upon quickly heating the device to 343 K through the integrated heating circuit or through heating in a fast-heating (100 K min−1) convection oven. Instrumentation. Scanning electron microscopy (SEM) images were recorded using a Philips XL30 FEG. For cross-sectional imaging, the electrodes were cut. The samples were sputtered with Pt before analysis. X-ray diffraction (XRD) patterns were recorded on a STOE STADI MP instrument in Bragg−Brentano mode (2θ−θ geometry; Cu Kα1) using a linear position-sensitive detector, with a step width of 0.5° 2θ and scan rate = 1520 s per step (2θ = 5°−20°; Δ2θ = 0.01°). Powder patterns were recorded on a STOE STADI P Combi instrument in Debye−Scherrer geometry (Cu Kα1) using an IP position-sensitive detector (2θ = 0°−60°; Δ2θ = 0.03°). Backgroundcorrected XRD patterns were normalized before plotting. ATR-FTIR spectra were recorded on an Agilent Technologies Varian 670 FTIR spectrometer coupled to a Varian 620 infrared microscope equipped with a slide-on ATR tip. Nanoindenter scratch experiments31 were performed on an MTS Nanoindenter XP equipped with a Berkovich diamond indenter tip. The substrate with coated surface was laterally translated under the indenter tip for a total scratch length of 100 μm at a speed of 5 μm s−1 (0° or 180° tip orientation). The normal load was progressively increased from 0 to 10 mN at a loading rate of 0.5 mN s−1 during a ramp load run. During a pass-and-return wear test, 10 cycles, each consisting of a pass in the 0° direction and a return in the 180° direction, were performed at a normal load of 20 mN. Toluene adsorption isotherms were measured using a Micromeritics 3Flex 3500 physisorption instrument equipped with a thermostatic vapor source container. A reference (powder) sample was prepared following a previously reported solvothermal procedure.15 The samples were degassed before measurement at 343 K under vacuum (10−2 mbar) for 4 h.
addition of AA has been shown to increase the amount of linker defects and concomitantly the pore volume and surface area,47 as well as to improve crystallinity and provide control over the crystallite size and morphology.12 For anodic film deposition, an increase in AA modulator concentration from 0.5 to 1 M changes the film morphology from loosely stacked (2−5 μm) aggregates of nanocrystals, toward smaller (
